Fight Aging! provides a weekly digest of news and commentary for thousands of subscribers interested in the latest longevity science: progress towards the medical control of aging in order to prevent age-related frailty, suffering, and disease, as well as improvements in the present understanding of what works and what doesn't work when it comes to extending healthy life. Expect to see summaries of recent advances in medical research, news from the scientific community, advocacy and fundraising initiatives to help speed work on the repair and reversal of aging, links to online resources, and much more.
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- SENS Patron Fundraiser for 2017: Additional Challenge Fund Donors Sought
- Loss of Lipid Chaperones Mimics Some Aspects of Calorie Restriction
- POT1 is a Second Shelterin Component that Influences Aspects of Aging
- Immune Cell Telomeres and Senescence in the Context of Viral Infection and Aging
- Researchers Generate Decellularized Livers, Ready for New Cells and Transplantation
- Rejuvenation Therapies will Grant Additional Healthy Years, Not Years of Disability
- The Prospect of Engineering Better Gut Bacteria
- A Small Molecule Drug that Selectively Induces Apoptosis in Cancer Cells
- The Genre of Popular Science Articles on Treating Aging that Fail to Mention SENS Rejuvenation Research Programs
- Cellular Senescence in Chronic Kidney Disease
- There Will be Many More Approaches to the Destruction of Senescent Cells
- Healthier Older People have a Gut Microbiome More Like that of Younger People
- Bubr1 and Brain Aging
- No Great Surprises in a Recent Study of the Causes of Variation in Human Lifespan
- The Roles of mTOR in Aging
SENS Patron Fundraiser for 2017: Additional Challenge Fund Donors Sought
As has become the custom, the SENS Research Foundation will be running a year-end fundraiser in the last few months of 2017, the proceeds going to support work on the foundations of real, working rejuvenation therapies. We of the Fight Aging! community will do our part to help make it a success. Expect it to start up at the end of October or the first week of November. Last year Josh Triplett, Christophe and Dominique Cornuejols, and Fight Aging! joined forces to put up 36,000 challenge fund that matched the first year of donations made by anyone who signed up as a SENS Patron by setting up recurring monthly donations to the SENS Research Foundation. We all think it important to help build up the core of supporters who supply the regular donations that help to fund SENS research projects: the more regular donors, the less uncertain the flow of funding, something that is always a challenge for non-profit organizations. On this topic, you might recall that the initial success of the Methuselah Foundation, launched nearly fifteen years ago now, was built atop the funds and support provided by the Methuselah 300, a group of monthly donors.
For the SENS Patron challenge, we hit 85% of our target in 2016, encouraged the participation of numerous new monthly donors, and I'm pleased to say we'll be back again for another try this year, with another 36,000 pledged to challenge the community. But why stop at 36,000? If you want to make a difference to the future of human health and longevity, and want your contributions to make a sizable impact, then why not help our community fundraiser by joining in to expand the SENS Patron challenge fund? The SENS year end fundraisers have a great record when it comes to the use of challenge funds to attract new donations. Further, the SENS rejuvenation research programs that our donations have supported over the past decade or more, first at the Methuselah Foundation, and then at the SENS Research Foundation, have a proven track record of enabling active clinical development of therapies, such as in areas relating to mitochondrial damage, senescent cells, and clearance of cellular waste. More of that sort of thing will be rolling out later this year and early next year, so keep an eye out for new announcements on that front.
While very welcome, this concrete progress enabled by our support does only cover the first set of tasks, the first set of research areas and potential therapies to be unblocked and to reach fruition. There is more yet to be done, and philanthropy is still needed to accomplish these goals - to rescue and expand areas of research that are still languishing, but that have the potential to be just as exciting and influential in the treatment of aging as, say, the clearance of senescent cells has become in recent years. Six years ago senescent cells were a disregarded backwater for everyone except SENS advocates and a few determined research teams who struggled to find funding. Change can be rapid when it finally takes off, and our support for the SENS Research Foundation is a very important, necessary foundation for that change. There are few other areas of philanthropy where one can help to generate such enormous, important changes in the capacity of medical science. The defeat of aging lies somewhere ahead, a improvement in the human condition more profound than any other yet achieved through medicine. We are helping to make it happen.
So give it some thought: there is a warm welcome waiting for any new challenge fund donors willing to step up and grow this year's SENS Patron fundraiser - please contact us with questions or offers of support.
Loss of Lipid Chaperones Mimics Some Aspects of Calorie Restriction
The research I'll note today involves genetic knockout of fatty acid-binding proteins in mice, something that appears to slow the development of metabolic disorders associated with excess fat tissue and aging - there is a lot more funding for investigation of the former cause as opposed to the latter cause, sadly. The work is, I think, chiefly interesting for mimicking some of the cellular effects of calorie restriction, while preventing some degree of the metabolic decline that accompanies aging, but achieving all of this without either extending life or improving the other usual functional measures of aging: loss of strength, cognitive decline, and so forth. In principle that sort of result should be quite hard to achieve, and indeed I can think of few lines of research in which this happens with any reliability in short-lived species such as mice. They are sensitive to environmental and genetic interventions, with very plastic life spans in comparison to those of longer-lived species such as our own. Anything that constitutes a significant improvement to health should also extend life.
Extending the duration of measures of health without extending life span is hard precisely because aging is determined by cell and tissue damage, a consequence of that damage, just like the decline of any complex machinery. There are only a few options when it comes to how to proceed: fix the root cause damage, try to compensate for loss of function by adding more capacity, or try to prevent secondary effects that result from the primary damage. Medicine to date has focused on the latter two options, which is precisely why it produces only marginal, incremental benefits. Making a damaged machine work well without repairing the damage is exactly as challenging as it sounds.
The genetic intervention carried out by the researchers in this paper has the look of a method of preventing secondary effects, some of those resulting from weight gain and fat tissue dysfunction in aging, by interfering in the processing of fats. That is no doubt an overly simplistic consideration. For example, we know that simple surgical removal of visceral fat significantly extends life span in mice, and yet the genetic approach here, that reduces weight gain, has no such outcome. A first thought is that it is possible that removal of fatty acid-binding proteins is causing harm in other areas of biochemistry, and thus shortening life even as it helps on the metabolic front. So while the researchers discuss their data as evidence of a decoupling of metabolic health and life span, and make a fair case, it may or may not be what is happening under the hood.
Targeting 'lipid chaperones' may hold promise for lifelong preservation of metabolic health
Scientists found that mice that lack fatty acid-binding proteins (FABPs) exhibit substantial protection against obesity, inflammation, insulin resistance, type 2 diabetes, and fatty liver disease as they age compared with mice that have FABPs. However, this remarkable extension of metabolic health was not found to lengthen lifespan. FABPs are escort proteins or "lipid chaperones" that latch onto fat molecules, transport them within cells, and dictate their biological effects. Previous work found that when FABP-deficient mice were fed high-fat or high-cholesterol-containing diets, they did not develop type 2 diabetes, fatty liver, or heart disease.
Metabolic health typically deteriorates with age, and researchers believe that this contributes to age-associated chronic diseases and mortality. Studies have shown that high-calorie diets impair metabolism and accelerate aging; conversely, calorie restriction has been shown to prevent age-related metabolic diseases and extend lifespan. In the new study, researchers examined metabolic function in multiple cohorts of FABP-deficient mice throughout their life. They found that FABP deficiency markedly reduced age-related weight gain, inflammation, deterioration of glucose tolerance, insulin sensitivity, and other metabolic malfunctions. This effect was more strongly observed in female than male mice. Surprisingly however, they did not find any improvement to lifespan or preservation of muscular, cognitive, or cardiac functions with age.
The researchers saw striking similarities between the alterations in tissue gene expression and metabolite signatures in the genetic model of FABP-deficiency developed for this study and the alterations that occur due to calorie restriction. The findings suggest that it may be possible to mimic part of the metabolic benefits of calorie restriction by targeting FABPs. In addition, by examining the molecular differences between these models, it may also be possible to identify other pathways that contribute to longer life span or alternative strategies to prevent metabolic diseases.
Uncoupling of Metabolic Health from Longevity through Genetic Alteration of Adipose Tissue Lipid-Binding Proteins
In this study, we have shown that the lipid chaperones FABP4/FABP5 are critical intermediate factors in the deterioration of metabolic systems during aging. Consistent with their roles in chronic inflammation and insulin resistance in young prediabetic mice, we found that FABPs promote the deterioration of glucose homeostasis; metabolic tissue pathologies, particularly in white and brown adipose tissue and liver; and local and systemic inflammation associated with aging. A systematic approach, including lipidomics and pathway-focused transcript analysis, revealed that calorie restriction (CR) and Fabp4/5 deficiency result in similar changes to the adipose tissue metabolic state, specifically enhanced expression of genes driving de novo lipogenesis and non-esterified fatty acids accumulation. Furthermore, CR was associated with reduced FABP4 in circulation, providing a potential molecular mechanism underlying its metabolic benefit.
The extension of metabolic health by Fabp deficiency is long-lasting even in aged female mice. However, despite the remarkable protection in glycemic control, insulin sensitivity, inflammation, and tissue steatosis in Fabp-deficient mice, we did not observe any change in the lifespan curves. We also did not detect preservation of cardiac, muscular, and cognitive functions. In females, there was even a mild decline in cardiomuscular function associated with Fabp deficiency during aging. These observations support the concept that, in higher organisms, significant improvements in metabolic tissue inflammation, metabolic tissue integrity, and systemic metabolic homeostasis may not necessarily lead to increased longevity.
Our studies with Fabp-deficient mice now provide genetic evidence in animal models that prolonged metabolic health, particularly glucose and lipid homeostasis, may be uncoupled from lifespan and maintenance of cardiac, muscular, and cognitive systems, which partially recapitulates the human pathophysiology observed during intensive glycemic control. Furthermore, it is intriguing that there is a considerable overlap between the unique lipidomic profile, especially in adipose tissue, of Fabp-deficient animals with those that have been subject to CR. Future studies exploring the similarities and distinctions between these models in multiple sites may provide additional insights into specific pathways and their regulation of healthspan and lifespan. Further exploration of the disconnect between metabolic health and longevity may also shed light on alternative therapeutic approaches against diabetes and possibly other metabolic diseases that are associated with aging as a risk factor.
POT1 is a Second Shelterin Component that Influences Aspects of Aging
You might recall that researchers recently demonstrated that increased levels of TRF1, a component of the shelterin protein complex, could modestly extend healthy (but not overall) life span in mice. The effect is likely mediated through raised levels of stem cell activity in older individuals, somewhat turning back the usual trend towards declining tissue maintenance. The paper I'll point out today makes an good companion piece, in that it examines the shelterin component POT1, finding that increased levels of this protein also help to maintain stem cell activity. Both POT1 and TRF1 decline with advancing age, and the argument made by some researchers is that shelterin activity is one of the more relevant mechanisms in stem cell aging. That, of course, says comparatively little about where this fits in the chain of cause and effect. If there is less POT1 and TRF1, what caused that? I'm inclined to think that changes in protein expression, and the epigenetic alterations needed to increase or decrease production of proteins from their genetic blueprints, are reactions to more fundamental cell and tissue damage.
What is shelterin and what does it do? This complex is involved in defending telomeres, the repeated DNA sequences found at the end of chromosomes, from various DNA repair and other processes that would cheerfully and destructively cut them short at any moment. Telomere length is an important part of the mechanisms that permit or restrict cell replication: a little of their length is lost with each cell division, and when too short a cell either becomes senescent or self-destructs. The vast majority of cells in the body are restricted in the number of divisions they can carry out, on a countdown to destruction, and this is the foundation of all of the methods used by complex organisms such as mammals to suppress cancer to a sufficient degree to get by. Only a small number of cells, the germline and the stem cells responsible for tissue maintenance, use telomerase to lengthen their telomeres and thus replicate indefinitely. Keeping only a small number of cells privileged in this way greatly reduces the risk of one of them becoming damaged in a way that causes it to run rampant, the seed for a cancer. Too little shelterin and stem cells start to fail in their self-renewal, becoming inactive, senescent, or destroying themselves, because they progressively fail to maintain their long telomeres. More shelterin produces the opposite effect, making stem cell populations better maintained and more active in older individuals.
Stem cells have evolved to decline with age. The current consensus is that this, like a very large number of line items in cellular biochemistry, involves resistance to cancer. Evolutionary pressures lead to a species that attains a certain life span, but how exactly that life span is achieved by cell biochemistry may vary. Our species, long-lived in comparison to our nearest primate cousins, appears to have achieved a large enough resistance to cancer to obtain those additional years at the cost of a slow decline into frailty and organ failure. It doesn't have to be that way - one can look at elephants, for example, who achieved sufficient resistance to cancer to live as long as they do via much more efficient cancer suppression mechanisms. In this context, each species' biochemistry ends up where it does through the forces of natural selection favoring a certain life span, interacting with the happenstance of moving from point A to point B in the biochemistry of cells through evolutionary time. Changes in the availability of shelterin over a lifetime are just one small part of this picture.
The telomere binding protein Pot1 maintains haematopoietic stem cell activity with age
Appropriate regulation of haematopoietic stem cell (HSC) self-renewal is critical for the maintenance of life long hematopoiesis. However, long-term repeated cell divisions induce the accumulation of DNA damage, which, along with replication stress, significantly compromises HSC function. This sensitivity to stress-induced DNA-damage is a primary obstacle to establishing robust protocols for the ex vivo expansion of functional HSCs. Telomeres are particularly sensitive to such damage because they are fragile sites in the genome. As HSCs lose telomeric DNA with each cell division, which ultimately limits their replicative potential, HSCs therefore require a protective mechanism to prevent DNA damage response (DDR) at telomeres in order to maintain their function.
The shelterin complex - which contains six subunit proteins, TRF1, TRF2, POT1, TIN2, TPP1, and RAP1 - has a crucial role in the regulation of telomere length and loop structure, as well as in the protection of telomeres from DDR signaling pathways such as ATR. Protection of telomeres 1 (POT1) binds to telomeric single-stranded DNA (ssDNA) and thereby prevents ATR signaling. Human shelterin contains a single POT1 protein, whereas the mouse genome has two POT1 orthologs, Pot1a and Pot1b, which have different functions at telomeres. Pot1a is required for the repression of DDR at telomeres. In contrast, Pot1b is involved in the maintenance of telomere terminus structure. It has recently been shown that shelterin components TRF1, Pot1b, and Tpp1 critically regulate HSC activity and survival. However, due to embryonic lethality in Pot1a knockout mice, the role of Pot1a in maintaining HSC function is still unclear and it is not known if POT1/Pot1a has a non-telomeric role in HSC regulation and maintenance.
Here, we found that Pot1a regulates HSC activity by inhibiting ATR-dependent telomeric DNA damage, and thereby protecting cells from associated apoptosis. These results indicate that the formation of the shelterin complex at the telomeric region is important to Pot1a mediated maintenance of HSC activity. However, in addition to this telomeric role we have also identified a novel non-telomeric role, preventing the production of reactive oxygen species (ROS). Due to these protective functions, we find that treatment with exogenous Pot1a maintains HSC self-renewal and function ex vivo and improves the activity of aged HSCs. This new non-telomeric role is particularly interesting since reduction of ROS is thought to be crucial in inhibiting global DNA damage in HSC in culture.
In addition to its role in protecting against stress we also found that Pot1a has a central role in regulating stem cell activity during aging. We observed that expression of Pot1a is lost during aging, and this loss results in the accumulation of DNA damage, alterations in metabolism, and an increase in ROS production, which in turn compromises aged HSC function. However, we observed that this decline is reversible: remarkably ex vivo treatment of aged HSCs with recombinant POT1a is able to re-activate aged HSCs. Since Pot1a overexpression inhibited the expression of Mtor and Rptor in aged HSCs, the regulation of mTOR signaling by Pot1a may participate in this re-activation of aged HSC function.
Although the precise mechanisms by which this functional improvement occurs have yet to be fully determined, our results indicate that exogenous Pot1a can both prevent telomeric and non-telomeric DNA damage and inhibit ROS production, thereby inducing a more potent immature phenotype in aged HSCs upon ex vivo culture. It will be interesting to clarify how these mechanisms are related to one another and determine, for example, whether telomere insufficiencies precede metabolic changes and ROS production or vice versa.
Immune Cell Telomeres and Senescence in the Context of Viral Infection and Aging
It is considered that a sizable component of the disarray of the aged immune system is caused by cytomegalovirus infection, and here I thought I'd note a couple of recent papers that touch on the intersection between this topic and the measurement of telomere length. The herpesvirus cytomegalovirus cannot be cleared from the body by the immune system; it lurks and reappears again and again, but causes few or no obvious issues in the vast majority of individuals beyond this one long-term problem. It is pervasive, and more than 90% of the population is infected by the time they reach old age. Ever more immune cells become specialized to attack cytomegalovirus, that number expanding rapidly in later life. The immune system operates with only a low rate of replacement cells, which makes it act very much like a space-limited system, with a ceiling on the number of cells it can support at once. Too much of its limited count of cells becomes taken up by cytomegalovirus-specific cells that are incapable of performing all the other necessary tasks, such as destroying cancerous cells, or attacking novel, unrecognized pathogens.
At present telomere length is usually measured in immune cells taken from a blood sample. Considered over a population, average telomere length via this measure tends to trend down over the course of a lifetime. Individuals can vary considerably, however, and average length bounces up and down quite dynamically with health changes and other short-term environmental factors. It isn't much use as a metric for any sort of individual assessment. What does telomere length even signify? Well, every time a cell divides, telomeres shorten a little. When they get too short, the cell self-destructs or becomes senescent, ceases to divide, and is then usually destroyed by the immune system. Stem cells, however, maintain long telomeres via use of telomerase, and carry out their task of tissue maintenance by delivering a supply of new daughter cells with long telomeres. So average telomere length in any cell population is a smeared-out metric that reflects something of cell division rates and something of stem cell activity rates. We know that stem cell activity declines with age, and this would be enough for us to expect some sort of fall in average telomere length.
Immune cells division rates are greatly influenced by many factors that are not relevant in other cell types: the presence of pathogens; the degree to which tissues are generating inflammatory signals; and so forth. In particular, we would expect persistent pathogens such as herpesviruses and HIV to push the immune system into greater replication, shorter telomeres, high rates of senescence, and general exhaustion as a result - which appears to be the case. What can be done about the issue, however? The most promising line of attack for cytomegalovirus, a mostly harmless pathogen aside from its decades-long grinding down of the immune system, appears not to be to tackle the virus itself, but to periodically destroy and replace all of the problem immune cells. Getting rid of cytomegalovirus would be a nice bonus on top of that, but not of any great use in and of itself for old people. The damage has already been done. Immune destruction and recreation isn't pie in the sky: it is already being accomplished in the context of curing serious autoimmune conditions. However, the therapeutic approaches used are presently fairly damaging, akin to chemotherapy in impact on the patient. Given better and more gentle methodologies of selective cell destruction - such as those under development at Oisin Biotechnologies, among others - then this will become a very plausible prospect.
Telomere Dynamics in Immune Senescence and Exhaustion Triggered by Chronic Viral Infection
The progressive loss of immunological memory during aging correlates with a reduced proliferative capacity and shortened telomeres of T cells. Growing evidence suggests that this phenotype is recapitulated during chronic viral infection. The antigenic volume imposed by persistent and latent viruses exposes the immune system to unique challenges that lead to host T-cell exhaustion, characterized by impaired T-cell functions. These dysfunctional memory T cells lack telomerase, the protein capable of extending and stabilizing chromosome ends, imposing constraints on telomere dynamics.
Unlike normal memory T cells, which persist due to the levels of interleukin-7 (IL-7) and IL-15, exhausted T cells only require the presence of viral antigen to continue proliferating. This is partly due to losses in interleukin-2 receptor-β (CD122) and interleukin-7 receptor (CD127) that limit generation of virus specific T cells. Because viral antigen is intermittently or constantly supplied to these cells, viral specific T cells never cease proliferating. Depending on the length of infection, this could result in progressively shorter telomeres and an age-related decline in T-cell responses.
A deleterious consequence of excessive telomere shortening is the premature induction of replicative senescence of CD8+ T cells. While senescent cells are unable to expand, they can survive for extended periods of time, occupying immunological space where functional immune cells could exist. The accumulation of senescent CD8+ T cells has been proposed to play a role in failed immune surveillance and in facilitating the development of metastasis of certain cancer types. Interestingly, some studies proposed that it may be possible to reverse this phenotype by reactivating telomerase expression.
Evidence is mounting that high levels of antigen stimulation result in excessive proliferation, driving cells into a state of replicative senescence due to telomere attrition. The benefits for addressing viral T-cell exhaustion and immune senescence in patients with chronic viral infections and chronic inflammatory or auto-immune diseases are great so as to finally eradicate the chronic virus. Therefore, it is relevant to the ongoing efforts to develop therapeutic vaccines aimed at stimulating CD8+ T-cell responses and current immunotherapy based on adoptive transfer of expanded virus-specific CD8+ T cells.
There are still many questions when it comes to the therapeutic potential of blocking T-cell exhaustion. One concern is whether fully exhausted T cells can be reactivated. If exhausted T cells have reached a state of terminal differentiation, they may have undergone permanent cell cycle arrest and irreversible cellular senescence. In this case, it is important that anti-exhaustion therapy (such as drugs to block immune inhibitory markers) be given at the proper time, before the cells become permanently differentiated. In the latter case, it would then be imperative to target these cells for removal through enhanced cell death, since reactivation is not possible.
Telomere Shortening, Inflammatory Cytokines, and Anti-Cytomegalovirus Antibody Follow Distinct Age-Associated Trajectories in Humans
Chronic infection with cytomegalovirus (CMV) has a profound impact on the immune system and is considered one of the causes of immunosenescence in the elderly. The serum titer of CMV-specific IgG has been widely used as an indicator of CMV infection status, but its significance in immunosenescence is less well defined. Age-associated increase in anti-CMV IgG has been reported from cross-sectional studies, but its trajectory has not been analyzed in longitudinal studies. Although a recent study found no difference of telomere length in subjects between CMV seropositive and negative from a cross-sectional analysis, it is unknown whether the rates of changes of these age-associated biomarkers in vivo are correlated or independent.
In this study, we sought to measure the in vivo changes of telomere length, inflammation-related cytokine and anti-CMV antibody titer with age and to determine the trajectory of these age-associated immune changes and their inter-relationship using longitudinal analysis over an average of 13 years. Specifically, we assessed the individual longitudinal trajectories of peripheral blood mononuclear cell (PBMC) telomere length, eight pro-inflammatory cytokines, and anti-CMV IgG titer in 456 subjects. Strikingly, aging-associated changes in these variables occur with a distinct trajectory in each individual. Thus, immune aging is a heterogeneous process across individuals, and an assessment of immunosenescence requires a combinatorial evaluation of multiple age-associated biomarkers.
Although aging affects multiple organs and tissues, the rates of age-related changes display a remarkable degree of variation within the human population. Our previous longitudinal studies of aging of the immune system assessed immune cell composition and telomere length, and demonstrated highly individualized changes. Overall, the results of our longitudinal studies suggest that the manifestations of aging-associated changes in the immune system are multifaceted and exhibit independent trajectories. These findings suggest that there is no dominant integrator among the three classes of age-related change studied here: telomere attrition in PBMCs, increased circulating IFN-γ and IL-6, and increased titers of anti-CMV IgG.
In contrast to the disassociation among age-related changes in telomere length of PBMCs, circulating inflammatory cytokines, and titer of anti-CMV IgG, the changes of various inflammatory cytokines with age show a number of positive correlations. The rate of increase in IL-6 is positively correlated with the rate of change in IL-4, and the rate of IL-1β is positively correlated with the rates of IL-13, IL-12p70, and IL-2. Although not all these cytokines displayed statistically significant age-associated changes, this suggests that the expression of these multiple pro-inflammatory cytokines may be regulated by common stimulators and/or that these cytokines may regulate one another in an autocrine and paracrine fashion. This mutual enhancement of inflammatory cytokine expression may explain why the increase in pro-inflammatory cytokines with age is rarely limited to a single cytokine.
Researchers Generate Decellularized Livers, Ready for New Cells and Transplantation
Decellularization is the most promising near term approach to generating patient-matched organs for transplantation. It is a fairly simple concept at root: researchers remove all of the cells from an organ, leaving the scaffold of the extracellular matrix with all of its intricate details and chemical cues. The challenge lies in building a reliable methodology that can be scaled up for widespread use. Much of the work on decellularization to date has focused on hearts and lungs, but in the paper noted here, researchers outline a method for reliably decellularizing whole livers.
Decellularization does of course require a donor organ as a starting point, unfortunately, but that can include a significant fraction of the potential donor organs that would normally be rejected by the medical community for one reason or another, as well as organs from other species, such as pigs. Given suitably genetically engineered pigs, a decellularized pig organ repopulated with human cells should contain no proteins that will provoke significantly harmful responses following transplantation. This and other options should roll out into availability in the years ahead, ahead of the range of more ambitious tissue engineering projects that aim to grow entire organs from a patient cell sample.
Decellularization is ahead of other methodologies for the creation for patient-matched organs because the research community has yet to produce a good method of generating the intricate networks of tiny blood vessels that are needed to support tissue much larger than a millimeter or two in depth - the distance that nutrients can perfuse in the absence of capillaries. Yet over the past few years many research groups have demonstrated the production of organoids, tiny sections of complex, functional organ tissue, for a variety of organs. Thus the actual production of organs from patient cells will be a going concern just as soon as the blood vessel question is figured out. Unfortunately, this has been the state of the field for years now, with many promising leads but no definitive end in sight. Meaningful progress in bringing decellularization to the medical community is to be welcomed in the meanwhile.
Decellularization of Whole Human Liver Grafts Using Controlled Perfusion for Transplantable Organ Bioscaffolds
The only therapy for liver cirrhosis is liver transplantation, but the shortage of organ donors imposes a severe limit to the number of patients who benefit from this therapy. With increasing shortage of donor organs and decrease of their quality, the development of novel procedures and alternatives for organ transplantation becomes essential. Thus, organ engineering, which involves the repopulation of acellular matrices, was explored with the use of polymeric scaffolds or three-dimensional (3D) printing of liver tissue to make scaffolds that can be seeded with hepatocytes or other cell types.
Although these are powerful tools worth exploring, it remains difficult to design and create artificial, yet functional liver tissue with functional vascular and biliary trees for clinical use. Alternatively, removal of cells from an existing organ, leaving a complex mixture of structural and functional proteins that constitute the extracellular matrix (ECM), may provide a natural habitat for reseeding with an appropriate population of cells, and connected to the blood stream and biliary system.
Ideally, ECM is cell free, but remains the interlocking mesh of fibrous proteins (collagen, elastin, fibronectin, and laminin) and glycosaminoglycans (GAGs). Evidence from rodent models shows the feasibility of decellularization of whole liver organs that provides an excellent scaffold for reseeding liver (stem) cells for graft engineering. Also, porcine and sheep liver have been successfully decellularized to obtain ECM for transplantation. However, so far, there is very limited experience with decellularization of whole livers from humans.
Recently, researchers demonstrated efficient decellularization of a whole liver and partial livers to generate small cubes of human liver scaffold. Different decellularization methods have been described among which are physical force (freeze/thaw, sonication, and mechanical agitation), enzymatic agents (trypsin, endonucleases, and exonucleases), and/or chemical agents (ionic, nonionic, and zwitterionic detergents). Usually, combinations of these methods are used. In larger organs, such as human or porcine liver, perfusion through the intrinsic vascular beds is the favorable route to be able to reach all cells. So far, most experimental decellularization protocols include the use of sodium dodecyl sulfate (SDS) to generate full freedom of cells and translucency, but this also progressively destroys the ECM and hampers clinical translation.
In this study, we report successful decellularization of human livers to obtain transplantable whole organ scaffolds. We show proof of concept that these scaffolds can serve as feasible resources for future tissue-engineering purposes. Using a controlled perfusion system, a complete 3D acellular human liver scaffold was generated on a clinically relevant scale and free of allo-antigens. We present the feasibility of systematically upscaling the decellularization process to discarded human livers. Eleven human livers were efficiently decellularized by nonionic detergents by machine perfusion. A careful choice of the decellularization methodology is of great importance as methods described for decellularization may be well suitable for other organs than the liver, but may damage the composition of the matrix proteins.
Repopulation of a complex organ such as the liver poses numerous challenges. Using the extracellular matrix of the native liver obviously helps to create the most optimal niche for cells to repopulate, but the types of cells to be infused to create fully functional liver tissue remains to be elucidated. In addition to the liver-specific matrix proteins, the still present vascular and biliary system may also provide entry routes for the different cell types needed. Obviously, efficient recellularization is a complex process in which hepatocytes or other parenchymal cells need to pass the remnant basement membrane of the decellularized blood vessels or bile ducts to enter the parenchyma after vascular or biliary administration, respectively. In addition, cell numbers that are required for efficient recellularization are highly dependent on cell type and volume of the scaffold.
Reendothelialization is a pivotal step to prevent thrombosis as a result of the massive collagen contact surface that blood will encounter upon reperfusion, and which cannot be prevented by coating with heparin. We demonstrated, like others did in animal models, that matrix sections can be reseeded with endothelial cells and these cells end up at the location of the decellularized blood vessels and pave the basal membrane. In our studies, HUVEC were used as a source of endothelial cells, as in most studies in rodents and pigs, but other sources such as endothelial progenitor cells are also used and show similar results. The next hurdle to be taken toward clinical application is to choose a cell source for liver parenchyma repopulation. An adult liver contains ∼150-350 billion cells of which the largest part (70%-85%) is made up by hepatocytes. However, adult primary hepatocytes of high quality are scarce and therefore limit tissue-engineering applications. Ideally, autologous cells, isolated from the patients themselves, are used as these cells will have a low risk to trigger an immune response. Alternatively, (autologous) pluripotent stem cells that self-renew and are able to differentiate into all cell types needed could be seeded.
In summary, human cadaveric livers can be successfully decellularized using machine perfusion and nonionic detergents, and can be repopulated with endothelial cells. The next steps toward clinical application involve finding a cell source or combinations of cell types to reseed the matrix, including the vascular and biliary system, to gain functional liver tissue.
Rejuvenation Therapies will Grant Additional Healthy Years, Not Years of Disability
When it comes to persuading the public to support work on the development of rejuvenation therapies, it sometimes seems that, even after years of effort, we're still somewhat stuck at the point of convincing people that therapies to push back aging and extend life will result in more healthy years rather than an extended period of ever-increasing decrepitude. The knee-jerk response to the goal of life extension is to imagine an eking out of the period of pain, suffering, and ill-health at the end of life. Obviously, this isn't all that attractive a prospect. Yet it was never the goal: therapies that successfully treat the causes of aging, repairing the accumulated cell and tissue damage that lies at the root of aging, will produce rejuvenation and additional healthy years. Despite a couple of decades of messaging from the scientific community and advocates for healthy life extension, all telling the public that extended health and youth is the goal, we still run headlong into this false expectation of an extended old age of sickness and diminishment.
Whenever the topic of increasing human lifespan is discussed the concern is sometimes raised that a longer life would mean a life spent frail and decrepit. This is sometimes known as the Tithonus error and shows a fundamental misunderstanding of the aims of rejuvenation biotechnology. Tithonus, as the story goes, was granted immortality by Zeus, but the father of the gods had not also granted eternal youth. Tithonus never died, but he kept aging like any other mortal; eventually, he was so decrepit, disease-ridden, and demented that his life had become unbearable.
This type of concern is sometimes raised by those who don't have a clear picture of rejuvenation biotechnologies and fear that an extended period of frailty and decrepitude may be what scientists are after. Thankfully, quite the opposite is true, and, in fact, Tithonus' grim fate is physically impossible. In the fanciful realm of gods and myths, anything goes and the impossible becomes mundane, but in the real world, neither Zeus nor anyone else could make you live forever without eliminating or obviating the aging process. This is because death is nothing but the result of a critical failure of your inner workings - if you died, it means something crucial in your body stopped functioning properly and thus triggered a cascade of failure whose ultimate consequence was your death.
In particular, in the case of death by old age, the critical failure is caused by one or several pathologies resulting from a life-long process of damage accumulation. This process is slow but insidious, and it starts speeding up considerably after middle age. Frailty, weakness, and all the notorious diseases of old age are its primary consequences and are due to the fact that accumulated damage prevents your body from functioning at its best; when the damage is extensive enough, your body cannot function at all anymore. Living forever while aging forever would thus be equivalent to a human-made machine still functioning despite all of its mechanisms being eventually completely broken, which is a contradiction in terms.
A very small-scale version of Tithonus' myth does actually take place as a consequence of present-day geriatric medicine. Geriatric medicine focuses on treating the symptoms of age-related diseases rather than their causes, with the result of modestly improving patient health and lifespan - in other words, although with the best intentions, geriatrics does prolong the time patients spend in decrepitude. They live a little longer because mitigating the symptoms slightly postpones the inevitable, but as age-related damage keeps accumulating, eventually the point of no return is reached. It's a bit like trying to empty a river using a coffee mug.
Interventions for different types of age-related damage - such as senolytics for senescent cell clearance, enzyme replacement therapy to dispose of intracellular waste, and AGE-breaking molecules to eliminate extracellular cross-links - are currently being developed, and some are even undergoing human clinical trials. The aim of rejuvenation biotechnology is neither extending frailty nor achieving a modest amelioration of an elderly patient's health; rather, the goal is to comprehensively address age-related damage to allow people to maintain youthful levels of health for as long as they live, however long that may be.
The Prospect of Engineering Better Gut Bacteria
Scientists are finding that gut bacteria have some influence on natural variations in longevity; not as much as exercise or calorie intake - though they they may mediate some of those benefits - but enough to be interesting to the research community. The distribution of species changes with age, for example, and gut bacteria interact with the immune system to produce inflammation and other effects. A range of other specific mechanisms will probably emerge in the years ahead as more research teams join the investigation. I suspect that this is one of those parts of the field that will diminish in importance as rejuvenation therapies after the SENS model take off: the size of the effects are just not all that interesting in a world in which it becomes possible to reliably add ten or more healthy years with treatments to clear senescent cells, remove glucosepane cross-links, and so forth. Still, I think you'll have to agree that the prospect of engineering better, more beneficial gut bacteria, as outlined here, is certainly interesting from a technical and future scope of options perspective, setting aside the question of the size of likely near-term benefits for a moment.
We have a symbiotic relationship with the trillions of bacteria that live in our bodies - they help us, we help them. It turns out that they even speak the same language. And new research suggests these newly discovered commonalities may open the door to "engineered" gut flora who can have therapeutically beneficial effects on disease. In a double-barreled discovery, researchers found that gut bacteria and human cells, though different in many ways, speak what is basically the same chemical language, based on molecules called ligands. Building on that, they developed a method to genetically engineer the bacteria to produce molecules that have the potential to treat certain disorders by altering human metabolism. In a test of their system on mice, the introduction of modified gut bacteria led to reduced blood glucose levels and other metabolic changes in the animals.
The method involves the lock-and-key relationship of ligands, which bind to receptors on the membranes of human cells to produce specific biological effects. In this case, the bacteria-derived molecules are mimicking human ligands that bind to a class of receptors known as GPCRs, for G-protein-coupled receptors. Many of the GPCRs are implicated in metabolic diseases and are the most common targets of drug therapy. And they're conveniently present in the gastrointestinal tract, where the gut bacteria are also found. The researchers engineered gut bacteria to produce specific ligands, N-acyl amides, that bind with a specific human receptor, GPR 119, that is known to be involved in the regulation of glucose and appetite, and has previously been a therapeutic target for the treatment of diabetes and obesity. The bacterial ligands they created turned out to be almost identical structurally to the human ligands.
Among the advantages of working with bacteria is that their genes are easier to manipulate than human genes and much is already known about them. "All the genes for all the bacteria inside of us have been sequenced at some point. The biggest change in thought in this field over the last 20 years is that our relationship with these bacteria isn't antagonistic. They are a part of our physiology. What we're doing is tapping into the native system and manipulating it to our advantage. This is a first step in what we hope is a larger-scale, functional interrogation of what the molecules derived from microbes can do."
A Small Molecule Drug that Selectively Induces Apoptosis in Cancer Cells
This cancer research is interesting for the strong resemblance it bears to current senolytic strategies to destroy senescent cells by forcing them into the programmed cell death process of apoptosis: these cells are primed for that fate, but fail to reach it on their own. The therapies used affect normal cells as well as the targeted senescent cells, but cause little impact in the healthy cells that should be spared. This same type of approach is here applied to cancerous cells, using a close relative of the pro-apoptosis targets employed for senescent cells. Considered at the high level, this makes an interesting counterpoint to the trend towards the development of precision targeting in cancer research: any method of killing cells is useful if it can be restrained to only the cells that should be killed. Therein lies the challenge, of course.
In principle it seems possible to produce a therapy that can be globally applied throughout the body but only does harm to cancerous cells - though in practice such selectivity is a sliding scale, and nothing is perfect. Causing less harm to the patient than the current standard of chemotherapy is a low bar, but conversely the effectiveness of this first attempt seems marginal. It is only slowing down cancer a little rather than fixing the problem. Still, it is only the starting point for a whole new area of exploration.
Scientists have discovered the first compound that directly makes cancer cells commit suicide while sparing healthy cells. The new treatment approach was directed against acute myeloid leukemia (AML) cells but may also have potential for attacking other types of cancers. The newly discovered compound combats cancer by triggering apoptosis - an important process that rids the body of unwanted or malfunctioning cells. Apoptosis trims excess tissue during embryonic development, for example, and some chemotherapy drugs indirectly induce apoptosis by damaging DNA in cancer cells.
Apoptosis occurs when BAX - the "executioner protein" in cells - is activated by "pro-apoptotic" proteins in the cell. Once activated, BAX molecules home in on and punch lethal holes in mitochondria, the parts of cells that produce energy. But all too often, cancer cells manage to prevent BAX from killing them. They ensure their survival by producing copious amounts of "anti-apoptotic" proteins that suppress BAX and the proteins that activate it. "Our novel compound revives suppressed BAX molecules in cancer cells by binding with high affinity to BAX's activation site. BAX can then swing into action, killing cancer cells while leaving healthy cells unscathed."
Researchers first described the structure and shape of BAX's activation site in 2008, and have since looked for small molecules that can activate BAX strongly enough to overcome cancer cells' resistance to apoptosis. The team initially used computers to screen more than one million compounds to reveal those with BAX-binding potential. The most promising 500 compounds were then evaluated in the laboratory. A compound dubbed BTSA1 (short for BAX Trigger Site Activator 1) proved to be the most potent BAX activator, causing rapid and extensive apoptosis when added to several different human AML cell lines. The researchers next tested BTSA1 in blood samples from patients with high-risk AML. Strikingly, BTSA1 induced apoptosis in the patients' AML cells but did not affect patients' healthy blood-forming stem cells.
Finally, the researchers generated animal models of AML by grafting human AML cells into mice. BTSA1 was given to half the AML mice while the other half served as controls. On average, the BTSA1-treated mice survived significantly longer (55 days) than the control mice (40 days), with 43 percent of BTSA1-treated AML mice alive after 60 days and showing no signs of AML. Importantly, the mice treated with BTSA1 showed no evidence of toxicity.
The Genre of Popular Science Articles on Treating Aging that Fail to Mention SENS Rejuvenation Research Programs
This popular science article on efforts to treat aging as a medical condition is a particularly good example of the type that fail to mention SENS rejuvenation research and any related efforts that involve repair of the cell and tissue damage that causes aging. This one even omits any mention of senolytics, the rapidly broadening efforts to clear senescent cells that are supported by increasingly robust evidence, which has to be a deliberate omission in any overview of the current state of the field. The rise of senolytics and the current enthusiasm for study of senescent cells is very hard to miss. Why do authors do this? What is the prejudice that leads them to focus on marginal, challenging efforts that haven't made significant progress towards practical therapies, such as work on calorie restriction and calorie restriction mimetics? This author is clearly capable of finding sensible things to say about many of the topics that are covered, which makes it more of a mystery.
As researchers work to develop and test ways to slow aging, they will first look to create treatments intended for people in their 50s and 60s, when chronic diseases often start to set in. Studies evaluating those treatments, some of which are already planned (most notably the trial for metformin), should only take a few months or years, measuring secondary indicators like frailty instead of death itself to ensure their efficacy. Eventually, there might be drugs for people to start taking when they're even younger. But giving pharmaceuticals to healthy people is a hard sell. Without extensive long-term clinical trials, it's impossible to anticipate how the decades-long use of an anti-aging drug will affect other aspects of long-term health. There will almost inevitably be some side effects, and the public will have to wade through discussions of whether or not it's worth it. There are people who question whether the clinical trials needed to prove the safety and efficacy of such therapies are even ethical.
These issues hint at a deeper ideological hurdle stopping anti-aging treatments from becoming commonplace. For now, our medical system is designed to address medical conditions as they arise. Putting interventions to treat aging on the market would mean a fundamental shift in our medical system, towards preventative medicine. "We've been trained in biomedicine to focus on sickness rather than health, so that paradigm shift will take time." And to move from success in the lab to having an actual impact on human wellbeing, you need to have public opinion on your side. Social acceptance of aging interventions could pave the way for the medical shift. The field of anti-aging research suffers from a reputation problem. For decades, products running the gamut from skin creams to herbal supplements have claimed to have "anti-aging" properties, with virtually no science to back them up. "People associate our field with snake oil. That only adds to that perception that it's not rigorous." What's more, people in general are reluctant to talk about getting old and dying.
For now, researchers are still trying to get the U.S. Food and Drug Administration (FDA) onboard. As it stands now, the FDA only approves treatments for a specific medical condition. Now researchers in the field of aging are trying to convince the agency to make a separate designation for preventative medicine. From the FDA's perspective, the field of medicine built around combating aging is still in its infancy. "A question not yet answered is how many aging-related but otherwise independent diseases (coronary artery disease, dementia, sarcopenia, etc.) would need to be improved for us to consider the therapeutic effect an 'anti-aging' effect, rather than an effect on specific diseases. It is worth noting again that a drug that improved any of these conditions would be very valuable," an FDA spokesperson said. It's also still a challenge to figure out how to measure whether or not these interventions are effective.
"If the field of aging is going to move forward in having drugs to treat aging in humans, we're going to have to have an FDA-approved pipeline to do so." Having that framework in place will drive innovation, researchers claim - more research money can be allocated towards prevention, and pharmaceutical companies will work to develop new drugs that could potentially be used by the entire adult population. Though researchers don't believe there will be a special designation for anti-aging interventions anytime soon, a clear FDA pathway, plus more frank public discourse, could give the field a reputation to match the rigorous science already underway. And it seems increasingly likely that some intervention or another will emerge to keep people healthy for longer. "20 years ago, I would have said finding a way to extend the health span had a .5 percent chance of working. It's up to a 25 percent chance now, and every year it's going up."
Cellular Senescence in Chronic Kidney Disease
There is good evidence for the growing number of senescent cells present in old tissues to be an important root cause of fibrosis, the breakdown of normal regenerative processes that results in scar-like structures in place of functional tissue. Chronic kidney disease is one of a number of age-related condition driven by fibrosis, all of which presently lack effective forms of treatment, capable of significantly turning back the progression of fibrosis. Fortunately, change is coming: researchers are exploring the link between fibrotic diseases and cellular senescence with an eye to producing new classes of treatment. Numerous approaches to the targeted removal of senescent cells are presently under development. The first and simplest of them are already entering human trials. I expect to see considerable progress in the treatment of fibrosis in the years ahead.
The continuous accumulation of senescent cells leads to the age-related deterioration of vital organs and thus constitutes an organism's ageing process. Correspondingly the therapeutic removal of senescent cells can improve health and prolong lifespan. Compared with young people, the elderly population not only is more susceptible to kidney damage but also shows more severe clinical manifestations and a lower likelihood of recovery of renal function. Chronic kidney disease (CKD) is increasingly being accepted as a type of renal ageing. Along with the process of ageing, the kidney shows certain types of changes for which specific findings are lacking. The ageing kidney and CKD share a great number of similarities in both structural and functional changes.
CKD is a frequent independent risk factor for renal failure and other age-related diseases. CKD is a complex pathological process mainly involving oxidative stress, inflammation, autophagy, apoptosis, and epigenetics. Recently, cellular senescence has become an increasingly popular and extensively studied topic because of its role in the occurrence and development of CKD. In CKD, the expression levels of senescence-associated β-galactosidase (SA-β-gal) and cell cycle inhibitor p16 protein were significantly increased in the glomeruli, tubules and interstitium, suggesting that the process of cellular senescence occurs in CKD. Many factors involved in the progress of CKD, such as urinary toxins, infections, dialysis treatment, and excessive activation of the renin-angiotensin system, can cause diverse types of DNA damage response (DDR) and accelerate the ageing process. The role of cellular senescence in CKD cannot be ignored.
When cells become senescent, they remain metabolically active and undergo widespread gene expression changes, secreting certain factors and changing the surrounding environment. This is the senescence-associated secretory phenotype (SASP), consisting of all types of cytokines, chemokines, growth factors, and proteases. In the course of kidney diseases, several cell types in the kidney experience cellular senescence and secrete a large number of factors that are collectively defined as the CKD-associated secretory phenotype (CASP). It has been demonstrated that CASP and SASP have prominent similarities, which may act as an essential medium mediating the interaction between CKD and cellular senescence.
Although there is a striking resemblance between SASP and CASP in terms of their features of up-regulation and the species involved, there remain many gaps in the understanding of the complex role of cellular senescence and SASP in CKD and other age-related diseases. It is beneficial to establish their mechanisms in the pathogenesis and progression of CKD. Therefore, the common process of cellular senescence and SASP is considered a treatment target for CKD and other age-related diseases.
There Will be Many More Approaches to the Destruction of Senescent Cells
Targeted removal of senescent cells is a narrow form of rejuvenation, reversing one of the causes of degenerative aging. A variety of different approaches are in clinical development: targeting standard cell destruction techniques based on gene expression inside cells, as illustrated by the Oisin Biotechnologies method; various antibodies that bind to surface characteristics of senescent cells to induce immune cells to destroy them; and numerous small molecule drug candidates to target portions of the cellular mechanisms that either encourage or prevent cell self-destruction.
Senescent cells are primed for the programmed cell death process of apoptosis, and the overwhelming majority follow that path. The few that linger are the problem, but there are many points in the mechanisms of apoptosis that might be targeted to push them over the edge. A few have been discovered and demonstrated, such as the Bcl-2 family, the interaction between FOXO4 and p53, and HSP90, but the research community has only started in earnest on this line of work in the past couple of years. Initial successes to date will encourage greater efforts in the years ahead. The research here is an example of the type, in that it is a more detailed consideration of how cells choose between continued senescence and self-destruction that points out a new potential target by which that choice can be swayed in either direction.
DNA damage is a threat to genome integrity and its protection relies on the tumor protein, p53, signaling pathway response to the threat. The activity of the p53 pathway involves several feedback loops that control phosphorylated p53 concentration levels and can influence in different ways the expression of gene sets that lead to specific cell fates. In general, positive feedback loops are associated with cell fate stabilization and negative feedback loops with reversible cell fates. Under DNA damage the cell cycle is arrested at checkpoints activating the p53 pathway dynamics, in the case of light DNA damage an oscillatory dynamics is observed while for heavy damage, senescence (permanently cell cycle arrested cells) or apoptosis pathways are triggered.
Experimental and theoretical attempts to describe the oscillatory and apoptotic phenotypes are in progress, but in the case of senescence more investigations are required. Recently, an experiment confirmed a correlation between the DNA damage level induced by the anti-cancer drug etoposide with a switch in the p53 pathway behavior. For low concentrations of the drug culture cells present an oscillatory phenotype and few cell deaths, while for high concentrations there are arrested cells, no oscillations, and many cell deaths.
The onset of senescence is associated mainly with the upregulation of the cell cycle inhibitors pRB, p21, and/or the senescence DNA locus CDKN2A. MicroRNAs (miRNAs) can also regulate the cell cycle. For example, microRNAs can form feedback loops with p53. MiRNAs are small noncoding regulatory RNA molecules that target specific messenger RNAs (mRNAs) to repress their translation. A recent experimental study confirmed that miR-16, whose expression is regulated by p53, mediates the fate between senescence or apoptosis through p21. By changing miR-16 expression level the authors observed a phenotype change from senescence to apoptosis in cells. These experimental observations provide a basis for understanding how the p53 pathway dynamics is determined by repairable or irreparable DNA damage, and how perturbations of miR-16 can allow the control of cell fate.
Healthier Older People have a Gut Microbiome More Like that of Younger People
The research community has amassed a fair amount of evidence to show that the composition of the gut microbiome changes with aging and has some influence over the pace of aging. Consider interactions between gut bacteria and the immune system, and the degree to which it promotes chronic inflammation, for example. Other mechanisms by which our gut microbes influence systems and organs are also being uncovered of late. Just how much the gut microbiome contributes to natural variations in human life span remains an open question: is it on a par with exercise and calorie intake, or a lesser influence? Further, are changes in the gut microbiome a consequence of lifestyle choices or are they a more independent factor? Research such as the program noted here attempts to put some bounds to the possible range of answers.
In one of the largest microbiota studies conducted in humans, researchers have shown a potential link between healthy aging and a healthy gut. The researchers studied the gut bacteria in a cohort of more than 1,000 Chinese individuals in a variety of age-ranges from 3 to over 100 years-old who were self-selected to be extremely healthy with no known health issues and no family history of disease. The results showed a direct correlation between health and the microbes in the intestine. "It begs the question - if you can stay active and eat well, will you age better, or is healthy aging predicated by the bacteria in your gut?"
The study showed that the overall microbiota composition of the healthy elderly group was similar to that of people decades younger, and that the gut microbiota differed little between individuals from the ages of 30 to over 100. "The main conclusion is that if you are ridiculously healthy and 90 years old, your gut microbiota is not that different from a healthy 30 year old in the same population." Whether this is cause or effect is unknown, but the study authors point out that it is the diversity of the gut microbiota that remained the same through their study group. "This demonstrates that maintaining diversity of your gut as you age is a biomarker of healthy aging, just like low-cholesterol is a biomarker of a healthy circulatory system." The researchers suggest that resetting an elderly microbiota to that of a 30-year-old might help promote health.
Bubr1 and Brain Aging
In mice, loss of Bubr1 produces high levels of DNA damage, cancer, and the appearance of accelerated aging. The proteins produced by this gene are an important part of the mechanisms controlling cell division, and their absence results in all sorts of harm to chromosomal structures. As is true of many such progeroid mechanisms related to DNA damage, it remains an open question as to whether Bubr1 is also relevant in normal aging. Interestingly, the production of artificially increased levels of Bubr1 in mice does modestly slow some measures of aging - but the effects on life span may be due to a reduction in cancer incidence rather than any other effect on the processes of aging. It is much harder than you might think to peel apart the various influences and causes in studies of this nature. One of the areas of focus in the study of Bubr1 and aging is the brain and its loss of function, particularly the declining rate at which new neurons are created; here is a short overview of recent research on this topic.
The hippocampus is one neurogenic region in the adult mammalian brain that continues to produce neurons well into adulthood. This process of neurogenesis occurs in the subgranular zone (SGZ) of the hippocampal dentate gyrus that harbors neural stem cells (NSCs). These actively participate in a sequential process where they proliferate, migrate and mature into neurons that are functionally integrated into the hippocampal circuitry. This is a highly plastic process that affords the hippocampus roles in memory formation, learning, and mood regulation. However, it is also an age-dependent one where the number of NSCs decline with age. Age-related cognitive disability is one example of the functional implications of deficits in this process. A molecular understanding of this course has so far eluded the field. Recent evidence has demonstrated that BubR1, a mitotic checkpoint kinase, decreases with natural aging and induces progeroid features and aging-related central nervous system (CNS) abnormalities. In our recent study we sought to address if BubR1 played a role in age-related hippocampal changes.
In this study, we show BubR1 is expressed in the radial-glia like NSCs (RGC), and its expression is reduced in an age-dependent manner. We used progeroid BubR1H/H mice with reduced hippocampal BubR1 levels to show significantly reduced proliferation. Progenitor cell types vulnerable to BubR1 insufficiency included significant reductions in activated RGCs, intermediate progenitor cells, and neuroblasts. Such changes in cellular proliferation were exacerbated in BubR1 H/H mice in an age-dependent manner. Next, we sought to address if BubR1 played a role in maturation of the surviving neurons. An in vitro analysis using post-mitotic neurons derived from adult NSCs showed BubR1 localization in the dendrites and the cytoplasm. BubR1H/H mice showed a significant increase in the portion of immature neurons with a concurrent decrease in mature neurons, indicating delayed neuronal maturation in BubR1H/H mice. Importantly, these morphological alterations were significantly rescued in BubR1-overexpression mice, suggesting a critical post-mitotic role of BubR1 in newborn neurons.
This study expands on the varied and emerging functions of BubR1 and implicates it as a key regulator in the age-dependent changes in adult hippocampal neurogenesis. In addition, while BubR1 is primarily known as a key component for mitosis, our study is the first to delineate the critical post-mitotic role for BubR1 in neuronal maturation. However, this study does not yet provide the mechanistic link or elucidation of the molecular machinery that occurs between BubR1 decrease and significant reductions in proliferation and maturation of newborn hippocampal neurons. Recent studies from our lab have identified involvement of Wnt signaling as a novel molecular regulator to this process. Furthermore, it remains to be understood if sustained BubR1 levels during aging process may have a protective role in the aged brain, and thus represent a novel therapeutic target for age-related cognitive declines. This is a future direction that can shed further light on BubR1 and aging.
No Great Surprises in a Recent Study of the Causes of Variation in Human Lifespan
A recent study of human life expectancy uses a novel approach but the results offer no real surprises, confirming most of the current consensus associations. As a tour of the high level points, it is worth skimming. There are few genetic relationships that are large enough to be seen, and those that are visible are small in comparison to the impact of lifestyle choices. Excess fat tissue is just about as harmful as smoking for the obese: two months of life expectancy lost for every kilogram of excess weight. This all confirms the long-standing common wisdom when it comes to maintaining health for the long term - but also shows that the scope of the possible in the absence of rejuvenation therapies is very limited. You can move your life expectancy a few years up or a good many years down given the tools and techniques of yesterday. For more than that, we must look to the SENS research programs and similar efforts to repair the cell and tissue damage that causes aging.
Longevity is of interest to us all, and philosophers have long speculated on the extent to which it is pre-determined by fate. Here we focus on a narrower question - the extent and nature of its genetic basis and how this inter-relates with that of health and disease traits. In what follows, we shall use longevity as an umbrella term. We shall also more specifically refer to lifespan (the duration of life) and long-livedness (living to extreme old age, usually defined by a threshold, such as 90 years). Up to 25% of the variability in human lifespan has been estimated to be genetic, but genetic variation at only three loci (near APOE, FOXO3A and CHRNA3/5) have so far been demonstrated to be robustly associated with lifespan.
Prospective genomic studies of lifespan have been hampered by the fact that subject participation is often only recent, allowing insufficient follow-up time for a well-powered analysis of participant survival. On the other hand, case-control studies of long-livedness have had success and some technical appeal (focusing on the truly remarkable), but such studies can be limited and costly in their recruitment. We recently showed that the extension of the kin-cohort method to parental lifespans, beyond age 40, of genotyped subjects could be used to detect genetic associations with lifespan with some power in genomically British participants in UK Biobank (UKB).
Here we extend that approach in a genome-wide association meta-analysis (GWAMA) to discovery across UKB European- and African-ancestry populations and 24 further population studies (LifeGen), mainly from Europe, Australia and North America, to search for further genetic variants influencing longevity. We then use those GWAMA results to measure genetic correlations and carry out Mendelian randomisation (MR) between other traits and lifespan seeking to elucidate the underlying effects of disease and socio-economic traits on longevity, in a framework less hampered by confounding and reverse causality than observational epidemiology.
We replicated previous findings of genome-wide significant associations between longevity and variants at CHRNA3/5 and APOE and discovered two further associations, at LPA and HLA-DQA1/DRB1, with replication of the further associations in a long-livedness study. We found no evidence of association between lifespan and the other 10 loci previously found to suggestively associate with lifespan, despite apparent power to do so. We showed strong negative genetic correlation between coronary artery disease (CAD), smoking, and type 2 diabetes and lifespan, while education and openness to experience were positively genetically correlated. Using MR, we found that moving from the 25th to 75th percentile of cigarettes per day, systolic blood pressure, fasting insulin and body mass index (BMI) causally reduced lifespan by 5.3, 5.2, 4.1 and 3.8 years, respectively, and similarly moving from the 25th to 75th percentile of educational attainment causally extended lifespan by 4.7 years.
Our finding that a reduction in one BMI unit leads to a 7-month extension of life expectancy, appears broadly consistent with those recently published by the Global BMI Mortality Collaboration, where great effort was made to exclude confounding and reverse causalit7. We also found each year longer spent in education translates into approximately a year longer lifespan. When compared using the interquartile distance, risk factors generally exhibited stronger effects on mortality than disease susceptibility. Although both CAD and cigarette smoking show a very similar genetic correlation with lifespan, the measured effect of smoking is twice as large as that of CAD, perhaps because smoking influences mortality through multiple pathways.
Our results show that longevity is partly determined by the predisposition to common diseases and, to an even greater extent, by modifiable risk factors. The genetic architecture of lifespan appears complex and diverse and there appears to be no single genetic elixir of long life.
The Roles of mTOR in Aging
Next to insulin signaling, the biochemistry surrounding mechanistic target of rapamycin (mTOR) is probably the greatest point of study for that part of the mainstream research community interested in modestly slowing aging through pharmaceuticals, researchers who generally show little interest in the alternative approach of repairing the causes of aging to produce rejuvenation. Drugs and drug candidates to slow aging are largely intended to adjust the operation of cellular metabolism involved in nutrient sensing to mimic some of the beneficial response to calorie restriction, such as increased autophagy. mTOR is, as one might imagine, the primary target for the action of rapamycin, and similar pharmaceuticals known as rapalogs, that inhibit mTOR and have been shown to slow aging in mice. The paper here is a good summary of present knowledge on the subject.
The most studied and best understood longevity pathways govern metabolism according to available nutrient levels. The fundamental mechanisms from signaling cascades to protein complexes are conserved across phyla. A controlling hub at the center of nutrient sensing and signaling is the mechanistic target of rapamycin (mTOR) that governs cellular growth, protein synthesis, and degradation. mTOR acts upstream of several transcription factors, such as TFEB, FOXO, FOXA, and Nrf, that are essential for lifespan-extending strategies such as dietary restriction. These transcription factors also control autophagy, a cellular process that clears proteins and dysfunctional organelles, and reduces proteotoxic and oxidative stress while maintaining a pool of amino acids for protein synthesis. mTOR responds to amino acids, a pathway modulated by proteins such as sestrins.
Here we will review the current knowledge on the best-known longevity pathways across animal models, namely insulin/insulin-like signaling and its downstream transcription factor FOXO, and transcription factor FOXA-dependent signaling. We consider how FOXO and FOXA are regulated by mTOR, and what role autophagy plays in the lifespan extension they confer. We also consider additional longevity mechanisms that rely on lipid signaling and the proteasome. We conclude with a discussion of how advancements in technologies such as induced pluripotent stem cells can enable the study of longevity-regulating mechanisms in human systems, and how emerging ideas on nuclear-cytoplasmic compartmentalization and its loss could contribute to our understanding of transcriptional dysregulation of nutrient-sensing pathways in aging.
The mechanism through which mTOR accelerates cellular and organismal aging is still unclear, but causative elements discussed include increased oxidative and proteotoxic stress associated with mTOR-mediated mRNA translation and inhibition of autophagy resulting in the accumulation of defective organelles, including mitochondria. It is important to emphasize the complexity of the pathway: mTOR regulates metabolic transcription factors and can be regulated by the same transcription factors, such as TFEB and FOXO, and mTOR is able to regulate nuclear morphology and induce epigenetic changes by which it is affected.
Several components of the mTOR pathway have still not been investigated in the context of aging and longevity. It is possible that differential expression or activity of TOR-regulating proteins can be part of the age-associated changes in the base level of mTOR signaling, associated also with a decline in protein turnover and autophagy, and increase in protein aggregation. Another possible way the regulation of these longevity-driving processes could deteriorate over time is the loss of nucleocytoplasmic compartmentalization, as seen in progeria, and also in healthy aged individuals, whose cells show evidence of increased nuclear membrane blebbing and progerin buildup. In addition to the recorded effects of this loss on DNA damage and promotion of cellular senescence, further aggravated with simultaneously increased mTOR signaling, this could possibly disable the highly controlled localization of transcription factors, including those regulating processes related to aging, feeding into a vicious cycle of perturbed metabolism and homeostasis.
Rapamycin has recently been shown to alleviate some aging phenotypes while exacerbating others. These results could be due at least in part to attenuated mTORC2 activity, the loss of which has been shown to reduce longevity in Caenorhabditis elegans and in liver-specific mTORC2 knockout mice, while inhibition of mTORC1 is largely viewed as advantageous. Development of new drugs targeting the amino acid sensing pathway may increase selectivity to mTORC1 and enable assessments of longevity changes upon pharmacological complex-specific mTOR inhibition.