Compensation is not a Cure: an Example Involving Blood Pressure

Near the entire corpus of present day medicine for age-related disease, even the comparatively successful treatments, is essentially compensatory in nature. It fails to address in any meaningful way the underlying causes of aging and disease. "Comparatively successful" is presently measured against doing nothing, rather than against the goal of a cure, of controlling the aging process. By that latter standard, there is no such thing as successful medicine for age-related disease. Yet.

The research noted here is one small demonstration of the point that compensatory efforts fail because they do not address the root causes of the problem: the underlying pathology marches on, overwhelms the bounds of possible compensatory efforts, and patients decline and die as a result. Blood pressure rises with age because blood vessels stiffen, because of persistent cross-links in the extracellular matrix, and because of calcification encouraged by the presence of senescent cells, and because of related dysfunctions in the signaling mechanisms that coordinate vascular contractions and reactions to pressure. Current pharmaceuticals that do reliably lower blood pressure do nothing for the roots of the issue.

Hypertension affects about 40% of those aged over 25 and is a major risk factor for heart disease, stroke and kidney failure. An interdisciplinary group of scientists found that conventional medication aimed at reducing high blood pressure restored normal vascular rhythms only in the largest blood vessels but not the smallest ones. "It is clear that current anti-hypertensive treatments, while successfully controlling blood pressure, do not restore microvascular function."

Based on a networks physiology approach, the researchers compared a group aged in their twenties and two older groups aged around 70 - one with no history of hypertension and the other taking medications for high blood pressure. In the older group being treated for high blood pressure the drug treatment restored normal function at the level of arterioles and larger vessels. But when the researchers studied the nonlinear dynamical properties of the smallest blood vessels in the body, they found differences between the two older groups.

"Specifically, current hypertensive treatment did not fully restore the coherence or the strength of coupling between oscillations in the heart rate, respiration, and vascular rhythms (vasomotion). These are thought to be important in the efficient and adaptive behaviour of the cardiovascular system. Indeed, one aspect of ageing is the progressive physiological weakening of these links that keep the cardiovascular system reactive and functional. The results have not only confirmed previous observations of progressive impairment with age of the underlying mechanisms of coordination between cardiac and microvascular activity, but for the first time have revealed that these effects are exacerbated in hypertension. Current antihypertensive treatment is evidently unable to correct this dysfunction."


Outliers Such as Mole Rats Break and Enhance the Models of Aging and Metabolism

Models and trends established across collections of species are used as a tool to try to understand the complex relationship between metabolism and aging, meaning how exactly the natural variations between individuals and species arise from the behavior of cells and interaction with the surrounding environment. This is something of a sideshow to the main business of rejuvenation research, but since the scientific impulse is to map and understand, there is much more of the sideshow taking place than actual efforts to repair the causes of aging. In this slow and expensive business of deciphering the detailed progression of aging, the greatest insight can arise from the outlying examples that do not fit into the models and hypotheses that manage to explain most observations. Some of the various long-lived mole-rat species provide good examples of the type, as illustrated by this open access paper.

Reproduction is an energetically expensive process that supposedly impairs somatic integrity in the long term, because resources are limited and have to be allocated between reproduction and somatic maintenance, as predicted by the life history trade-off model. The consequence of reduced investment in somatic maintenance is a gradual deterioration of function, i.e. senescence. However, this classical trade-off model gets challenged by an increasing number of contradicting studies that show no negative effect of high metabolic rate on lifespan, or even a positive association. Consequently, more research is needed to gather representative data from animals with different life histories, to gain a comprehensive understanding of how life history trade-offs influence lifespan.

Ansell's mole-rats (Fukomys anselli) are subterranean rodents with an extraordinary long lifespan, 22 years being the maximum recorded age thus far. They live in multigenerational families where typically only the founder pair (breeders) reproduces. Most of the offspring (non-breeders) forego reproduction and remain in the natal family. A clear contradiction to the classic trade-off model has been shown in this species: breeding individuals live up to twice as long as their non-breeding counterparts, a feature which is unique amongst mammals. Previous studies showed that daily activity between breeders and non-breeders does not show differences, and social rank does not influence life expectancy. Hence, extrinsic factors like aggression, fighting and higher workload in non-breeders are not likely to influence the lifespan difference. Here, we test the hypothesis that breeders and non-breeders of Ansell's mole-rats differ in their mass specific resting metabolic rate (msRMR), as a possible approach to understand the bimodal aging pattern.

Low msRMR is a common trait in bathyergid rodents interpreted as an adaptation to the subterranean habitat, and our measurements generally confirm previous studies. However, our finding that long-lived breeders of F. anselli have higher metabolic rates compared to shorter-lived non-breeders is novel. This aspect is most interesting since investment in reproduction was long thought to impair somatic maintenance according to the classical trade-off model, but recent findings refer to the trade-off model as being too simplistic. Especially in terms of female reproduction, a meta-analysis from different homeothermic vertebrates has shown that in intraspecific comparisons between breeders and non-breeders, breeders had lower levels of oxidative damage in certain tissues.

This effect could be attributed to upregulation of antioxidant defense mechanisms, such as glutathione or superoxide dismutase activity, which shows a tissue-dependent upregulation in several species during reproduction. This oxidative shielding hypothesis, even if not consistent across different studies, suggests a reproduction-induced protection of mothers and offspring. Ansell's mole-rats are continuously reproducing once they achieve the reproductive status. Oxidative shielding might protect the animals from detrimental pregnancy effects due to a higher energy turnover in female breeders compared to non-breeders. However, the bimodal lifespan in Ansell's mole-rats is not sex-dependent, indicating a general effect in terms of reproductive status, msRMR, and lifespan rather than just a pregnancy effect restricted to females.

Oxidative stress as a main factor contributing to life history trade-offs is getting challenged by increasing contradictory studies. The uncoupling-to-survive hypothesis complements simplistic theories of senescence by explaining apparent exceptions. It suggests that elevated oxygen consumption, a measure for msRMR in the present study, could be also observed due to uncoupling of proton flux in the mitochondria. This process, also referred to as inducible proton-leak, is facilitated by uncoupling proteins and increases RMR. On the other hand, inducible proton-leak is known to reduce ROS production by reducing mitochondrial membrane potentials. Hence the higher msRMR measured in breeders of Ansell's mole-rats could be due to higher rates of mitochondrial uncoupling compared to non-breeders.

Several studies found higher rates of uncoupling in those laboratory mice that lived longer compared to other individuals with shorter lifespans. However, in the case of mole-rats this model should be considered carefully, since in naked mole-rats, surprisingly high levels of oxidative damage to DNA, lipids and proteins were found, which contrasts with the proposed benefit of mitochondrial uncoupling. In general, our finding stresses the complexity of currently discussed aging mechanisms.


The 2017 Winter SENS Rejuvenation Research Fundraiser: Become a SENS Patron, and Your Donations are Matched

This year's SENS Research Foundation winter fundraiser launches today, with a target of $250,000. Donations will support ongoing rejuvenation research programs at the SENS Research Foundation Research Center, as well as in laboratories at Yale, the Buck Institute, the Babraham Institute, and Oxford. The SENS Research Foundation continues to carefully unblock important but neglected fields of research that are relevant to repairing the cell and tissue damage that causes aging - you might take a look at the SENS timeline to see the past and presently ongoing success stories, in which charitable donations were used to move promising research from idea to demonstration to commercial development. A range of important research programs are still in the early stages or the middle of this process, and thus the more that we support these efforts, the faster the progress towards a comprehensive suite of rejuvenation therapies capable of turning back aging and age-related disease.

Following last year's model, Josh Triplett, Christophe and Dominique Cornuejols, and Fight Aging! have put together a $36,000 challenge fund for SENS Patrons. We will match the next year of donations for anyone who becomes a SENS Patron by signing up as a new monthly donor at the SENS Research Foundation between now and December 31st of this year. I invite you all to please put your best foot forward and help out. The SENS Research Foundation is a 501(c)(3) charity, and donations are tax deductible, even in much of Europe, though the details are a little more complex, and vary by country. Tell a friend. Print out and put up one of our posters. Set up a fundraising exercise of your own - there are many ways to help out.

I might be just a touch biased on this topic, but to my eyes supporting this cause is truly effective altruism. Not just because aging is the greatest cause of pain, suffering, and death in the world - by a large margin, and the poor suffer the most, as is always the case - but because the SENS Research Foundation, and the Methuselah Foundation before it, have a proven track record when it comes to turning philanthropic donations for SENS research programs into concrete progress towards human rejuvenation.

Past charitable donors have seen a number of strategic investments in promising but underfunded research turn their donations into active commercial development efforts. For example: work on preventing the consequences of mitochondrial DNA damage, one of the root causes of aging, through allotopic expression of mitochondrial genes was funded with modest support starting back in 2008. That gave rise to Gensight, a company that now puts tens of millions of dollars into developing this technology. SENS programs that mined bacteria for enzymes capable of safely breaking down age-related metabolic waste have resulted in candidates for drug development that are licensed out to the LysoClear program to tackle age-related macular degeneration, and to for efforts to break down some of the harmful compounds that contribute to atherosclerosis. Further, efforts to remove the transthyretin amyloid connected to heart disease, using catalytic antibodies, have moved into a company for commercial development. More is on the way. This year, work on an important component of a universal cancer therapy, achieved through suppression of telomere lengthening, is being spun out, along with promising work on glucosepane cross-link breaking - one of the more important causes of the loss of tissue elasticity that damages skin and, more importantly, blood vessels.

The SENS Research Foundation has also funded research in cellular senescence in aging, and SENS advocates has persistently and actively fought for more funding for this line of development for fifteen years. This helped to bring to an end the long period during which the research community rejected this very important field of research. As senolytic therapies to clear senescent cells have finally blossomed into a suddenly popular area of development, the SENS Research Foundation helped to seed fund the startup Oisin Biotechnologies, working on a gene therapy approach to selectively destroy senescent cells and cancerous cells with minimal side-effects. That company is presently raising a new round of funding to take their work to the clinic.

As a final item to consider, remember that all of this exciting progress towards the end goal of effective human rejuvenation was built atop a modest starting point, that being the small, simple decisions of a few thousand people just like you and I: people who gave a small amount of money every month, such as the members of the Methuselah 300. It is because of these people, their conversations, and their dedication and vision, that the first SENS rejuvenation research programs took place at all. It is because of these people that high net worth individuals such as Peter Thiel, Michael Greve, and Jim Mellon have been drawn to the field to provide significant material support. We make a difference. We are the leaders, we are the people carrying the lantern to light the way. Because of our efforts, the world will be a better place tomorrow, one in which being old doesn't have to mean being sick, frail, and faltering.

Considering Common Mechanisms in Alzheimer's Disease and Osteoporosis

It has been observed that Alzheimer's disease and osteoporosis appear to be correlated to a larger degree than one would expect simply because both emerge, after a long chain of cause and effect, from the root causes of aging. That they are correlated in this way suggests that they share in common some parts of the middle of that long chain. Given that osteoporosis is a condition of the bones, a disruption of the balance between cells that create bone and cells that destroy bone, and Alzheimer's is a condition of the brain, in which aggregated proteins overwhelm cells, what could these two very different outcomes of aging have in common? This open access paper looks at some of the current evidence and hypotheses.

Accumulation of abnormally folded amyloid beta peptide (Aβ) in cerebral amyloid plaques is the pathologic hallmark of Alzheimer's disease (AD). Aβ originates from the amyloid precursor protein (APP), a membrane protein expressed in many tissues and synapses of neurons with unknown function. A group of specific enzymes named secretases cleave APP into distinct fragments. APP cleavage by β-secretase and then γ-secretase leads to pathological Aβ oligomers. Oligomers are the units which form protofibrils and later fibrils and plaques. Genetic models of AD are typically established by excessively expressing Aβ protein and the current hypothesis of AD etiology centers around amyloid plaques.

Unlike the complexity and controversy in AD, the pathogenesis of osteoporosis is known as an imbalance between bone formation and mineralization. Hyperparathyroidism, Vitamin D deficiency, and steroid use are common causes of osteoporosis. Osteoporosis is mostly asymptomatic until minor trauma or falls lead to fractures. Bone formation involves bone matrix production and mineralization, whereas bone resorption is a biological erosive process mediated by osteoclasts. When the balance leans toward bone resorption, bone mineral density (BMD) decreases and osteoporosis develops.

Bone resorption is driven by the receptor activator nuclear factor-kappa B ligand (RANKL) / receptor activator nuclear factor-kappa B (RANK) signaling network, a signaling complex with multiple downstream pathways. The binding of RANKL to RANK triggers the cascade. Amyloid deposition in the brain and RANKL signaling are two seemingly independent pathways leading to AD and osteoporosis. The possible linkage between these two pathways has been investigated by measuring osteoclast activities in a transgenic mice model of AD. Both in vitro and in vivo examinations showed enhanced Aβ expression in bone, together with increased adipose tissue formation in the marrow space, analogous to osteoporotic bones. The abnormally expressed amyloid deposition appears to interfere with the RANKL signaling cascade and in turn the balance between bone formation and bone resorption. Similar findings extend to human studies.

Previous observational studies have reported the increased frequency of comorbid osteoporosis in AD. The relationship between these two diseases is more likely one of shared etiology than one condition causing the other. The overexpression of Aβ may take place in both brain and bone, interfering with the RANKL signaling cascade, enhancing osteoclast activities, and leading to osteoporosis. There is a growing body of evidence from in vitro and in vivo studies that the AD pathology in the brain can be reflected by examining the bone. Future investigation will focus on assessing biomarkers of cognitive aging in patients with osteoporosis and looking into the bone microstructure of patients with AD.


Short-Term Calorie Restriction Boosts Innate Immunity in Flies

Calorie restriction slows aging, with the current consensus being that this is largely mediated through increased autophagy, the housekeeping processes that clear out and recycle broken components within the cell. Calorie restriction does, however, change more or less everything there is to be measured in cellular metabolism, so it is certainly possible that other mechanisms are relevant. In this context, researchers here present evidence to show that, at least in flies, the defense against infection mounted by the innate immune system is enhanced by short term calorie restriction. It is also worth considering that this sort of effect may explain some of the degree to which calorie restriction reduces the burden of cellular senescence and cancer risk over the long term, by incrementally improving the ability of the immune system to remove harmful and potentially harmful cells.

Studies of dietary restriction, a reduction in nutrient intake without malnutrition, in a diverse array of organisms have revealed it to be an effective way to extend lifespan and promote broad-spectrum improvement in health during aging. Early work focused on total caloric intake as the driving force behind these beneficial effects, but studies that have comprehensively examined the effects of individual macronutrients on lifespan underscore the importance of protein-to-carbohydrate ratio. In the fruit fly, Drosophila melanogaster, yeast restriction has been used as an alternative to wholesale dilution of the diet to effectively extend female fly lifespan. These effects have also been observed in mammals, where protein restriction increased rodent lifespan. Together, these studies establish that the life-extending benefits associated with dietary restriction can be achieved without reducing total caloric intake when the relative consumption of protein to carbohydrates is low.

A striking feature of the effects of dietary restriction is its acute nature, yielding beneficial outcomes with short-term application. In Drosophila, a switch to a restricted diet reduced short-term mortality risk within 48 hr, and in mice, 1 week of protein starvation decreased tissue damage caused by temporary blockage of blood flow during surgical operation, greatly improving survival following renal ischemic injury. Even ad libitum feeding of low-protein, high-carbohydrate diets for 8 weeks resulted in metabolic improvement in mice compared to those fed high-protein, low-carbohydrate diets.

A significant threat to global health is infectious diseases. Acute preventative strategies that strengthen immunity prior to such procedures are therefore of strong interest. To answer the questions of whether, similar to general health and aging, innate immune function is acutely modulated by individual nutrients, we executed a comprehensive analysis of the effects of dietary composition on survival following pathogenic infection in Drosophila. Although lacking adaptive immunity, insects are equipped with innate immunity, which is an ancient first-line defense mechanism that recognizes the pattern of invading microorganisms as well as their virulence factors. Drosophila innate immunity has humoral and cellular components, and this innate immune response is highly conserved between Drosophila and mammals.

Here, we present evidence that yeast restriction, but not carbohydrate restriction, substantially improves fly survival following bacterial infection through several components of innate immunity. We find that yeast-restriction-mediated enhancement of innate immunity is orchestrated by components of the target of rapamycin (TOR) signaling network, in which reduced TOR signaling results in a stabilization of the transcription factor Myc through its suppressor protein phosphatase 2A. Myc in turn mediates a sustained induction of genes that encode antimicrobial peptides, which are effective bacterial killers. These results implicate a function for protein phosphatase 2A (PP2A) and Myc as signaling molecules that serve to potentiate the immune response in yeast-restricted animals following pathogenic infection.


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.

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.


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.


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.

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.


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 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.


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.

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.


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.


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.