Fight Aging! Newsletter, April 1st 2024

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Predicting the Order of Arrival of the First Rejuvenation Therapies

It has been going on eight years since I last speculated on the order of arrival of the first rejuvenation therapies. Tempus fugit, and time for an updated version! Eight years is a long enough span of time for the first of those rejuvenation therapies to now exist, albeit in a prototypical form, arguably proven in principle but not concretely. The world progresses but my biases remain much the same: the first rejuvenation therapies to work well enough to merit the name will be based on the SENS vision, that aging is at root caused by a few classes of accumulated cell and tissue damage, and biotechnologies that either repair that damage or render it irrelevant will as a result produce rejuvenation. The number of groups aiming to produce these therapies has grown considerably. A longevity industry now exists, scores of biotech and pharmaceutical companies where eight years ago there were only a handful. We are eight years further into the grand transition across decades that is taking place in the medical life sciences, in which the treatment of aging will grow to become the majority of medicine.

Age-related diseases are age-related precisely because they are caused by the same processes of damage that cause aging. The only distinctions between aging and disease are the names given to various collections of symptoms. All of frailty, disease, weakness, pain, and suffering in aging is the result of accumulated damage and consequent dysfunction at the level of cells and protein machinery inside those cells. Once the medical community becomes firmly set on the goal of repairing that damage, humanity will be well on its way to controlling and managing aging as a chronic condition, preventing it from causing harm to the patient by periodically repairing and removing the causes of aging before they rise to the level of producing symptoms and dysfunction. The therapies of the increasingly near future will be very different from those of the past. The full rejuvenation toolkit of the next few decades will consist of a range of different treatments, each targeting a different type of molecular damage in cells and tissues. What follows is a list of potential (and existing!) rejuvenation therapies in a speculative order of arrival.

1) Clearance of Senescent Cells

A good number of companies are presently developing a wide range of senolytic approaches to selectively destroying senescent cells, thereby removing their contribution to degenerative aging. A wide range of small molecules provoke apoptosis in senescent cells in a wide range of different ways, while other approaches train the immune system to better destroy lingering senescent cells. Studies in mice continue to demonstrate rapid, sizable reversal of aspects of aging and many different age-related diseases following the use of senolytic therapies. The most advanced of the first generation senolytic therapies are in phase 2 clinical trials. Given another decade there will be multiple novel senolytic drugs approved for use in the clinic, and no doubt increasingly used off-label by older individuals.

I'll make the argument that the first effective senolytic therapy to be tested in animals, the dasatinib and quercetin combination, is both legitimately a rejuvenation therapy and also readily available to anyone who wants to undergo the treatment. The use of dasatinib and quercetin in combination has been demonstrated to clear senescent cells from some human tissues to a similar degree as it does in mice. It is prescribed off-label by more adventurous physicians and anti-aging practices. It will likely be decades before we know the degree to which any senolytic affects human life expectancy - no-one seems much incentivized to run the sort of long-term human trial that would be required. Nonetheless, the burden of senescent cells is in principle a component of aging, supported by a great deal of animal data. Dasatinib and quercetin removes some of that burden, and this is rejuvenation to my eyes, even if broader benefits to health remain to be demonstrated comprehensively in human trials.

2) Restoration of a Youthful Gut Microbiome

The gut microbiome ages in the sense that the distribution of microbial populations changes in harmful ways: more pro-inflammatory microbes and fewer microbes capable of generating beneficial metabolites. Animal studies suggest that the state of the gut microbiome is at least as consequential to long-term health as diet and exercise. As is the case for senolytics, proven ways to reverse age-related changes in gut microbiome exist, but are not widely used, and it will most likely remain unclear for decades to come as to exactly how much of an effect such a rejuvenation has on human long-term health and life span.

The most obvious and cost-effective intervention for rejuvenation of the aged gut microbiome, the results demonstrated in animal studies and a few small human trials, is fecal microbiota transplant from a young individual. This produces a lasting reset of the gut microbe, and can be readily carried out by anyone willing to put in the work. There are even services that sell screened fecal material from young donors. A second approach is immunization with flagellin to encourage the immune system to clear unwanted microbes, those equipped with flagellae. These microbes are largely harmful, causing chronic inflammation, as well as diminishing the populate size of beneficial species by outcompeting them.

Unlike senolytics, there is no rush to commercialize forms of fecal microbiota transplant for the treatment of age-related conditions. There is one FDA-approved fecal microbiota transplantation therapy, for a condition in which the intestine is overrun with pathological bacteria, but that is about it. Thus it seems unlikely that concrete data will emerge any time soon on the degree to which gut microbime rejuvenation improves health and life span. While we can say that it is evidently rejuvenation, and animal data supports that assertion, whether it is rejuvenation to a practical degree in humans remains to be proven.

3) Clearance of the First Few Types of Amyloid

There are about twenty different types of amyloid in the human body, misfolded proteins that form solid deposits. Not all are robustly associated with age-related dysfunction, but of those that are, some progress has been made towards effective therapies based on either direct clearance or interfering in the pace of creation of altered proteins. In the matter of the amyloid-β associated with Alzheimer's disease, there are now several immunotherapies that have demonstrated effective clearance of amyloid-β from the brain. The side-effect profile leaves much to be desired, and it has become clear that late stage Alzheimer's is past the point at which clearing amyloid-β helps all that much. It may well be a useful preventative strategy, however, assuming that the treatments for clearance can be made more benign.

Transthyretin amyloid is associated with heart disease, and is thought to be the primary cause of death in supercentenarians. There are now FDA-approved therapies based on interfering in the creation of altered transthyretin. Some are applicable to the wild-type rather than genetic condition of transthyretin amyloidosis. Arguably every older person should be using these intermittently, assuming a mild side-effect profile, but it will take some time for costs to fall to the point at which this is practical.

This sentiment applies to any therapy targeting forms of amyloid - and there are many more forms to be addressed. To the degree that these treatments are effective and safe, everyone much over the age of 40 should be undergoing a course of treatment every few years. Should we expect more such treatments to emerge over the next decade? Perhaps. More attention is being given to the amyloid called medin, for example, drawing attention to its contributions to the pathology of degenerative aging. It may be that developers will turn their attention to this and other amyloids, but it is hard to predict how fashion and happenstance steers the choice of investment into specific avenues of medical development.

4) A Robust Cure for Cancer

If asked a decade ago, a universal cure for cancer looked fairly distant. There was clearly work on telomeres and telomerase relevant to cancer, but it didn't have the look of programs ready to make the leap to the clinic. All cancers depend absolutely on the ability to continually lengthen telomeres, and so avoid the Hayflick limit on cell replication. Telomere lengthening occurs through the activity of telomerase or the less well understood alternative lengthening of telomeres (ALT) mechanisms. If telomerase and ALT can both be blocked, temporarily and either globally throughout the body or selectively in cancerous tissue, then cancer will wither and become controllable. This is too fundamental a part of cellular biochemistry for the rapid mutational evolution of cancer cells to work around. Stem cell populations will suffer while telomerase activity is blocked, as they require telomere lengthening for self-renewal, but that is a lesser problem when compared to cancer and one that the stem cell research community will become increasingly able to address in the years ahead.

So a decade ago the fundamental research was progressing, but not all that rapidly. Still, all it takes is one innovative approach to produce good enough animal data, and a clinical program will rapidly arise. At present the drug called THIO is in clinical trials after an accelerated program of development at Maia Biotechnology. THIO is metabolized and utilized by telomerase, then incorporated into telomeres to cause disruptive consequences leading to cell death. Since near all cancer cells aggressively utilize telomerase, these are the cells that die when THIO is introduced. It should work for the 90% of cancers that do not evolve to make use of ALT, and will be widely used off-label following clinical approval for any one type of cancer. From this starting point, we might expect a great deal more effort in the decades to come to focus on telomere lengthening as a primary target in cancer - and at some point a group with a novel approach will swoop in to deal with the remaining ALT part of the problem.

5) Thymic Rejuvenation to Increase the Supply of Immune Cells

Another possible approach to partially restore lost function in the aging immune system is to increase the pace at which new immune cells are created. This is a very slow pace indeed in older people, due in large part to the age-related decline of the thymus. The thymus acts as a nursery for the maturation of T cells, and its atrophy thus restricts the rate at which new cells enter circulation. Over the 2010s, there was some progress towards engineering of replacement active thymus tissue, as well as methods of providing signal proteins that instruct the old thymus to regenerate and begin to act in a more youthful manner. Transplants of young thymus organs into old mice demonstrated that this class of approach can produce a meaningful improvement in immune function, and thereby extend healthy life.

A decade ago, this was one of a number of regenerative approaches that were on the verge, just waiting for someone to join the final two dots together, found a biotech company, and get moving. That now seems to be happening. The approach of providing signal proteins has proven to be hard, but there are now a few biotech companies, some quite well funded and connected, focused on either (a) cell therapies using cell types that naturally home to the thymus, such as Thymmune Therapeutics, or (b) looking for small molecules that cleverly interfere in the regulation of thymus growth while avoiding the pitfalls associated with past efforts, such as Thymofox.

Further, Intervene Immune has run clinical trials of a growth hormone approach, producing data to suggest a modest degree of thymic regrowth over a year or more of treatment; interestingly data from the CALERIE trial of calorie restriction indicates a similar gain from a few years of slight calorie restriction, implying thymic involution to be perhaps a more dynamic process than suspected. Meanwhile, some interesting advances are taking place in the research community, such as the possibility of gene therapy delivery system that can in fact effectively target the thymus following intravenous delivery. Exciting times! The state of the field looks promising for some form of effective thymus rejuvenation strategy to emerge in the decade ahead.

6) Mitochondrial Repair

Mitochondria, the power plants of the cell, are herds of bacteria-like organelles that bear their own DNA. This DNA becomes damaged in the course of normal cellular processes, and certain forms of mitochondrial DNA damage - to the thirteen genes needed for oxidative phosphorylation - produce malfunctioning mitochondria that can overtake their cells. Further, epigenetic changes characteristic of aging disrupt the dynamics of mitochondria, disrupting the quality control process of mitophagy. This also allows poorly functioning mitochondria to replicate and prosper, but occurs in cells throughout the body.

There are numerous possible approaches to the problem of dysfunctional mitochondrial in aged tissues: upregulation of existing repair mechanisms of mitophagy; delivery of replacement mitochondrial DNA or whole mitochondria; partial reprogramming of cells to restore normal gene expression relating to mitochondrial dynamics and mitophagy; and so forth. Of these, the closest to the clinic are mitochondrial transfusion therapies and the various approaches to adjusting repair and propagation of damaged states in mitochondria, trying to tilt the balance to favor better function. The development of partial reprogramming therapies has enormous funding at present, but arguably much larger challenges must be solved before it can be broadly applied to tissues across the body. It remains hard to say how effective these approaches will be relevative to one another. How long will they last before fading in an aged tissue environment? Also unknown.

In the case of mitochondrial transfusion, two companies (cellvie and Mitrix Bio) are working towards clinical programs. Their challenge is near entirely the development of the processes by which enough mitochondria for human use can be manufactured. From the present starting point that is a tough scaling problem. In the case of altering mitochondrial dynamics or the regulation of mitophagy, there are range of possibilities already sold in the supplement market, such as SkQ1 and MitoQ, none of which are all that impressive when compared to the effects of exercise. Will companies like Stealth Bio do any better than this? That remains to be seen. There is a market for small molecule and supplement-like products that are not as good as exercise, but it is probably not a market that should interest us.

The SENS approach is somewhat more radical, involving gene therapy to introduce copies of the thirteen genes into the cell nucleus, altered to ensure that the proteins produced can migrate back to the mitochondria where they are needed. Mitochondria will thus have the necessary protein machinery for correct function regardless of the state of their DNA. This has been demonstrated for three of the thirteen genes of interest, numbers two and three in 2016, and getting that far took the better part of ten years at a low level of funding. A company, Gensight Biologics has championed this approach in clinical trials for one of the genes, in the treatment of a rare genetic disorder, but little further or broader development has taken place outside of academia. Will it be useful to have therapies that fix half the problem, moving six or seven genes to the cell nucleus? Will that reduce the impact on aging by half? It is hard to say until that is possible and demonstrated in mice. A decade ago, it seemed plausible that researchers would get there by now - but this has not happened. There is still too little funding and support for this approach, and one might well argue that backing mitochondrial transfusion seems a better wager at this point, even given the unknowns.

7) Reversing Stem Cell Aging

The stem cell industry remains massively funded, and is ultimately on a collision course with stem cell aging. Most of the conditions that one would want to use stem cell therapies to treat are age-related conditions. Researchers must thus work towards ensuring that the altered cellular environment, the damage of aging, doesn't prevent these treatments from working - that pristine cells can integrate and work well, not immediately die or decline in response to an age-damaged stem cell niche. Despite some progress over the past decade, particularly around the question of whether cellular senescence is degrading cell therapy outcomes, it is fair to say that the research community isn't engaging aggressively with this goal, however. Possible reasons for this include the fact that most stem cell treatments, even without addressing issues of the aged tissue environment, represent a considerable improvement in the scope of what is possible to achieve through modern medicine. So the incentive to go further is perhaps not as strong as it might otherwise be.

Stem cell populations become damaged by age, falling into quiescence or declining in overall numbers. They should be replaced with new populations, but while simple in concept, and even achieved for some cell types, such as the blood stem cells that produce immune cells, this is easier said than done for the body as a whole. Every tissue type is its own special case. There are hundreds of types of cell in the body. Each supporting stem cell population has so far required specific methodologies to be developed, and specific behaviors and biochemistry to be laboriously mapped. It isn't even entirely clear that researchers have found all of the stem cell or stem-like cell populations of interest. There is an enormous amount of work to be done here, and at the moment the field is still largely in the phase of getting the basics, the maps, and the reliable therapeutic methods sorted out for a few of the better understood tissue types, bone marrow and muscles in particular. All in all this has the look of a long-term, incremental prospect, despite the high levels of funding for this line of medical research and development.

Are there ways other than complete replacement of cell populations that might enable reinvigoration of aged stem cell populations? It seems that there might be, though we can argue about the degree to which these approaches are in any way affecting aging per se. There are ways to adjust the regulation of stem cell behavior that improve tissue function even given that one is working with aged stem cell populations. To pick one example, Ship of Theseus develops an approach based on Hox family transcription factors that appears to provoke greater muscle stem cell activity. As another example, look at the work of Mogling Bio, building on demonstrations showing CASIN to improve stem cell function in a number of populations, particularly the hematopoietic stem cells of the bone marrow. These and other, analogous approaches will find their way to clinic long before replacement of stem cell populations is a going concern.

8) Clearance of Cross-Links, Glucosepane-Based and Others

Clearance of cross-links in the extracellular matrix of tissues is, like senescent cell destruction, one of the most exciting of early rejuvenation therapies. It is a single target that influences a great many aspects of aging: if we look at just the cross-link-induced loss of elasticity in blood vessels alone, that has a major influence on mortality through hypertension and consequent impact on cardiovascular health. It is also a single target in the sense that near all persistent cross-links important to aging in humans so far appear to be based on one compound, glucosepane. Thus all that is needed is one drug candidate.

The attention given to glucosepane cross-link clearance remains anemic, despite considerable efforts to create a toolkit and unblock the research community conducted by the SENS Research Foundation and their allies in the research community, including a method of cheaply and reliably synthesizing glucosepane. Work in the mid-2010s conducted in the Spiegel Lab, as well as other parallel lines of research, led to the formation of Revel Pharmaceuticals to commercialize glucosepane cross-link breakers. Unfortunately, little further progress has occurred - while still being a sizable step forward over the state of the field a decade ago, this remains a narrow effort pioneered by few researchers.

A small number of other narrow programs have emerged focused on the lens of the eye, where different forms of cross-linking are involved in stiffening the lens to the point at which muscle strength is inadequate to focus correctly. A lipoic acid choline ester approach looked promising, but failed in Phase 2 trials. Another company, Lento Bio, is now in the early stages of working on another approach to the cross-links that stiffen the lens. Again, there are few groups in this space, and more are needed to ensure a good chance of progress towards the clinic in the near future.

9) Partial Reprogramming

Ten years ago, one could mount a good argument for the epigenetic change characteristic of aging to be distant from the root causes of aging, a downstream effect. Since then, evidence has mounted for some of this epigenetic change to be a direct result of forms of DNA damage and repair, tying it directly to one of the root causes of aging, the stochastic DNA damage that takes place constantly in all cells. At the same time, researchers have demonstrated that the reprogramming techniques based on exposure to Yamanaka factors, initially used in efforts to produce induced pluripotent stem cells for research and cell therapy development, reversed epigenetic aging long before they started to change cell fate. Short-term exposure to reprogramming, now known as partial reprogramming, is potentially a way to reset the epigenetic changes characteristic of aging and restore many forms of cell function. Clearly partial reprgramming cannot help with existing mutational damage to nuclear DNA, nor with the presence of persistent molecules that even youthful cells cannot effectively break down. But it is demonstrated to restore lost mitochondrial function, to pick one example.

When might we expect the first therapies based on partial reprogramming to reach the clinic? This part of the field has become enormously well funded in recent years. Given the the 3 billion investment in Altos Labs alone, never mind the other few biotech startup companies with more than 100M in initial funding each, such as Retro Biosciences and NewLimit, there will be no shortage of effort put into preclinical and clinical development. Nonetheless, the challenges are sizable. Different cell types and tissues require different exposures and balances of reprogramming factors for best effect. Too much reprogramming produces cancer via induced pluripotency. Too much reprogramming in the liver and intestines seems very detrimental to health in animal studies. The most likely path to near term therapies is to restrict them to isolated organs, such as the retina and optic nerve. Another possibility is for the groups working on small molecules capable of triggering expression of reprogramming factors to land on something that is more useful for systemic delivery than the gene therapy approaches, but development of small molecule reprogramming is still in the very early stages.

So it remains to be seen as to how matters will unfold in the years ahead. The advent of reprogramming as the leading, widely recognized approach to rejuvenation, and the degree to which funding poured into this project, caught a lot of people by surprise. It seems likely that the surprises will continue, given that we are only a few years into the development of this part of the field.

10) Immune System Destruction and Restoration

The destruction and recreation of the immune system is not a widespread technique, but it has been demonstrated successfully in human clinical trials and animal studies in a variety of contexts over the past twenty years. Researchers and clinicians have used chemotherapy to destroy immune cells and the hematopoietic cells that create them, followed by hematopoietic stem cell transplant (HSCT) to reconstruct the immune system. This approach has resulted in effective cures for multiple sclerosis patients, and has been attempted with varying degrees of success for a number of other autoimmune conditions. The catch here is that chemotherapy and HSCT are not trivial undertakings. The costs and risks are significant, both immediately and in terms of impact on later health and life expectancy. It only makes sense for people who are otherwise on their way to an early death or disability, as is the case for multiple sclerosis patients. However, there are a number of approaches on the way to practical realization that will make chemotherapy obsolete for the selective destruction of immune cells and stem cells - approaches with minimal or no side-effects. See a combined approach targeting c-kit and CD47, for example. Sophisticated cell targeting systems such as the gene therapy approach developed for senescent cell clearance by Oisin Biotechnologies could also be turned to stem cell or immune cell destruction, given suitable markers of cell chemistry. There are quite a few of these, any one of which would be good enough.

Replacing the taxing procedures of chemotherapy and HSCT with a safe, side-effect-free treatment would mean that the field of immune system restoration could immediately expand to assess its merits as a treatment for immunosenescence, the age-related failure of the immune system. This decay is in part a problem of configuration: a lifetime of exposure to persistent pathogens such as herpesviruses leaves too much of the immune system uselessly devoted to specific targets that it cannot effectively clear from the body, and too little left ready to fight new threats and destroy malfunctioning cells. Then there are various forms of autoimmunity that become prevalent in older people, not all of which are in any way fully understood - consider the comparatively recent discovery of type 4 diabetes, for example. Clearing out the entire immune system, all of its memory and quirks, and restarting it fresh with a new supply of stem cells is a good approach to many of the issues in the aged immune system. Not all of them, but many of them, and considering the broad influence immune function has over many other aspects of health and tissue function, it seems a worthwhile goal.

That said, has there been much meaningful progress in this part of the field over the past ten years? Not really. Some research has moved forward incrementally, such as on the topic of age-associated B cells and their depletion, or clearance of microglia in the brain. In the broader field, good number of immunomodulatory approaches to therapy are under development at various stages, some even explicitly aimed at restoring better immune function in the old, but very few target clearance of immune cell populations. The only one that springs to mind is related to the aforementioned clearance of microglia in the brain, since there are existing drugs and a simple mechanism - CSF1R inhibition - by which this can be achieved. The early stage venture Glionics would like to use this clearance and recovery as a way to delivery drugs throughout the brain, rather than specifically to achieve benefits resulting from clearance.

11) Clearance of Other Amyloids, Aggregates, and Sundry Lysosomal Garbage

A good portion of aging is driven by the accumulation of waste products, either because they are hard for our biochemistry to break down, is the case for glucosepane cross-links and many of the components of lipofuscin that degrade lysosomal function in long-lived cells, or because clearance systems fail over time, as appears likely to be the case for the amyloid-β involved in Alzheimer's disease. There are a lot of these compounds: a score of amyloids, any number of lipofuscin constituents, the altered tau that shows up in tauopathies, and so on and so forth. In many cases there isn't even a good defensible link between a specific waste compound and specific age-related diseases: the waste is one contribution buried in many contributions, and the research community won't start putting numbers to relative importance until it is possible to clear out these contributions one by one and observe the results.

A range of research groups are picking away at individual forms of waste, some with large amounts of funding, some with very little funding, but this is a similar situation to that I outlined above for stem cell aging. There is a huge amount of work to accomplish because there are many targets to address, and with few exceptions, such as amyloid-β, it is unclear which of the targets are the most important. They will all have to be addressed, in some order, but there are only so many researchers and only so much funding. We can hope that as the first effective therapies make it into the clinic, most likely for the clearance of forms of amyloid, there will be a growing enthusiasm for work on ways to remove other types of metabolic waste.

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Interesting Epidemiological Results for Time Restricted Feeding

Epidemiology is prone to difficulties that arise when attempting to interpret limited data sets, packed with information, but never quite all of the information that would be necessary to see the real story. Try to look at the health differences between people who drink and people who don't, and the sample of those who don't is polluted by former problem drinkers with problematic lives. Try to look at the health differences between obese and non-obese patients, and the non-obese cohort will be polluted by formerly obese people who have lost weight due to poor health and advanced disease. Try to base considerations on body mass index (BMI), and the high BMI groups are polluted by heavily muscled former athletes. And so forth.

Some of these problems are now evident in hindsight and sufficiently debated for researchers to have come to a resolution. No-one uses BMI when height and waist circumference are available. Researchers seek weight history rather than looking only at snapshots in time. But each of these advances can require ten to fifteen years to percolate through the epidemiological community. Even after that point, the old data sets remain the old data sets, lacking modern conveniences. Further, there are always new ways to provoke debate in the framing of data, and provoking debate is a great way to have people cite your paper. So the incentives tend to line up for more of the same in the future.

So to today's epidemiological results, which fly in the face of everything that is known about intermittent fasting and calorie restriction. To grossly oversimplify decades of research, it is starting to look like time spent hungry is beneficial regardless of calorie intake, and all mild forms of calorie restriction, fasting, and time-restricted feeding appear both beneficial and safe. So what might be happening here? How does one find a correlation between time-restricted feeding and greatly increased risk of cardiovascular mortality? If one looks at what the researchers say about the data, it occurs to me that selecting for time-restricted feeding habits in the general population is selecting for people who have been prompted into that course by the perception of needing to lose weight or having received physician guidance to lose weight. Both of those options correlate with being overweight or obese. Thus selecting for unusual dietary habits in this study population may be a good proxy for excess weight.

8-hour time-restricted eating linked to a 91% higher risk of cardiovascular death

Time-restricted eating, a type of intermittent fasting, involves limiting the hours for eating to a specific number of hours each day, which may range from a 4- to 12-hour time window in 24 hours. Many people who follow a time-restricted eating diet follow a 16:8 eating schedule, where they eat all their foods in an 8-hour window and fast for the remaining 16 hours each day, the researchers noted. Previous research has found that time-restricted eating improves several cardiometabolic health measures, such as blood pressure, blood glucose, and cholesterol levels.

In this study, researchers investigated the potential long-term health impact of following an 8-hour time-restricted eating plan. They reviewed information about dietary patterns for participants in the annual 2003-2018 National Health and Nutrition Examination Surveys (NHANES) in comparison to data about mortality the U.S., from 2003 through December 2019, from the Centers for Disease Control and Prevention's National Death Index database.

The analysis found: (a) people who followed a pattern of eating all of their food across less than 8 hours per day had a 91% higher risk of death due to cardiovascular disease; (b) the increased risk of cardiovascular death was also seen in people living with heart disease or cancer; (c) among people with existing cardiovascular disease, an eating duration of no less than 8 but less than 10 hours per day was also associated with a 66% higher risk of death from heart disease or stroke; (d) time-restricted eating did not reduce the overall risk of death from any cause; (e) An eating duration of more than 16 hours per day was associated with a lower risk of cancer mortality among people with cancer.

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Loss of Anti-Microbial Peptides as a Mechanism for Age-Related Changes in Gut Microbiome Composition

The balance of microbial populations making up the gut microbiome changes with age in ways that are damaging to long-term health. Firstly the proportion of pro-inflammatory microbes grows, provoking the immune system into greater degrees of unresolved inflammation. This state of inflammaging observed in older individuals is disruptive to tissue structure and function throughout the body, and contributes meaningfully to the onset and progression of many age-related conditions. Secondly, the proportion of microbes generating beneficial metabolites decreases, leading to other forms of dysfunction. For example, butyrate production leads to BDNF expression, regulating important mechanisms such as neurogenesis in the brain. This production of butyrate by the gut microbiome diminishes with age.

Why does the population of the gut microbiome shift with aging? It is widely thought that immune system aging is important, in that (a) the immune system is responsible for gardening the gut microbiome, suppressing problem species, and (b) the immune system becomes less effective with age. In today's open access paper, researchers explore one of the less frequently considered aspects of innate immunity, the production of antimicrobial peptides, small molecules that can kill many types of microbe. Working in mice, the researchers show that decreased production of these peptides in intestinal tissues correlates directly with the increase of harmful bacterial species in the gut microbiome. This points the way to novel classes of therapy that might beneficially adjust the gut microbiome, restoring it to a more youthful balance of microbial populations.

Evidence that the loss of colonic anti-microbial peptides may promote dysbiotic Gram-negative inflammaging-associated bacteria in aging mice

Aging studies in humans and mice have played a key role in understanding the intestinal microbiome and an increased abundance of "inflammaging" Gram-negative (Gn) bacteria. The mechanisms underlying this inflammatory profile in the aging microbiome are unknown. We tested the hypothesis that an aging-related decrease in colonic crypt epithelial cell anti-microbial peptide (AMP) gene expression could promote colonic microbiome inflammatory Gn dysbiosis and inflammaging.

As a model of aging, C57BL/6J mice fecal (colonic) microbiota and isolated colonic crypt epithelial cell gene expression were assessed at 2 months, 15 months, and 25 months. Fecal microbiota exhibited a significantly increased relative abundance of pro-inflammatory Gn bacteria with aging. Colonic crypt epithelial cell gene expression analysis showed significant age-related downregulation of key AMP genes that repress the growth of Gn bacteria. The aging-related decrease in AMP gene expressions is significantly correlated with an increased abundance in Gn bacteria (dysbiosis), loss of colonic barrier gene expression, and senescence- and inflammation-related gene expression.

This study supports the proposed model that aging-related loss of colonic crypt epithelial cell AMP gene expression can promote increased relative abundances of Gn inflammaging-associated bacteria and gene expression markers of colonic inflammaging. These data may support new targets for aging-related therapies based on intestinal genes and microbiomes.

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A Skeptical View of the Role of Nuclear DNA Damage in Aging

It is evident and settled that stochastic nuclear DNA damage contributes to cancer. The more of it that you have, the worse your risk. What is still very much debated is whether nuclear DNA damage contributes meaningfully to degenerative aging, and how it does so. Most mutational damage to DNA occurs in regions that are inactive, in cells that have comparatively few divisions remaining before reaching the Hayflick limit. Even if damage alters the function of such a cell, in some non-cancerous way, it is unclear as to how this could amount to a meaningful contribution to loss of tissue function.

The one school of thought is focused on somatic mosaicism, the spread of mutations throughout a tissue when mutational damage occurs in stem cells. In this case subtle dysfunctions could accumulate and interact with the spread of mutated cells over time. While there is evidence for somatic mosaicism to contribute to the risk of some forms of cancer, evidence is still lacking for it to meaningfully affect tissue function to the degree that aging does.

A second school of thought is focused on unexpected consequences of the repeated operation of DNA repair machinery. Double strand break repair can apparently deplete factors needed for maintaining the correct structure and epigenetic control of nuclear DNA, leading to age-related changes in epigenetics and gene expression. This is a comparatively recent discovery, and not yet fully digested, replicated, and risen to the status of consensus. It is an attractive proposition, however, a way to explain how stochastic mutational damage to inactive regions of the genome can somehow produce a consistent, systemic outcome throughout tissue, while forms of mutational damage other than double strand breaks can occur in greater amounts without strongly impacting age-related degeneration.

Somatic mutations in human ageing: New insights from DNA sequencing and inherited mutations

Taken together, recent DNA sequencing experiments focused on quantifying mutations with age reveal a gradual increase in mutations, and widespread evidence of clonal expansion of rapidly dividing mutant clones. These observations are consistent with the age-related increase in cancer observed in most tissues. However, the levels of mutations reported so far are difficult to reconcile with most ageing phenotypes. Whether and how somatic mutations in ageing tissues, affecting mostly non-coding regions and overwhelmingly different genes in different cells, can cause dysfunction is unclear. Likewise, while clonal expansion may be a factor in ageing and result in tissue dysfunction, so far this is not directly supported by experimental data and remains an open question.

As such, there is a stark contrast between cancer and ageing: while cancer can originate from mutations in a single cell and subsequent clonal expansion, shown empirically to occur, age-related dysfunction would need, we suggest, many mutations in a very large number of cells in a tissue. Evolutionarily this has led others to suggest that the evolutionary pressure to prevent cancer will result in levels of somatic mutations in tissues across the lifespan that will be lower than the number of mutations needed to cause most other age-related conditions.

Recent evidence from inherited mutations in patients with increased somatic mutation burden and no symptoms of accelerated ageing also cast doubt on the role of somatic mutations in most ageing phenotypes - even if it is not well understood why hypermutator phenotypes sometimes do and sometimes do not result in progeroid phenotypes. Perhaps other forms of DNA damage and/or genome instability may accumulate at much greater rates in human tissues, but these have not been studied in detail and have thus far limited empirical support. The impact of clonal expansion, somatic copy-number alteration (SCNAs), and structural variations (SVs) on ageing phenotypes, in fact, remains to be further investigated. Advances in genome sequencing technology together with the development of computational methods to reliably detect large-scale structural alterations at a single-cell level should shed light on the potential role of SVs and SCNAs on human ageing.

After the idea that somatic mutations could be the main cause of ageing was first proposed in the 1950's, Maynard-Smith questioned it by arguing that the number of mutations necessary would be too high to be consistent with the data available at the time. Decades and numerous technological advances in genetics and genomics later, which have produced quantitative data on mutation load in aged tissues, and yet we are no closer to empirically showing a role for somatic mutations in ageing and, in fact, have grounds to question it.

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Problematic B Cells Accumulate in Visceral Fat and Indirectly Provoke Inflammation

The authors of today's open access paper present an interesting and novel way in which visceral fat tissue provokes chronic inflammation. It has been noted that dysfunctional B cells accumulate with age. Here, dysfunctional B cells of a specific subtype are shown to accumulate in aged visceral fat tissue, acting to provoke other immune cells in visceral fat tissue, such as macrophages, into a more pro-inflammatory state. The researchers demonstrate that removing the B cell population helps to reduce the age-related inflammation generated by visceral fat by removing the contribution to inflammatory macrophage behavior.

Of note, B cells regenerate quite rapidly following clearance, and it seems that using pharmacological means or gene therapies to clear out B cells in aged individuals would improve a number of issues. Targeted clearance of specific immune cells (such as microglia in the brain), or indeed the immune system as a whole, is an underdeveloped area of medical research, and one that could in principle produce therapies capable of reversing a number of aspects of immune aging.

Age-associated accumulation of B cells promotes macrophage inflammation and inhibits lipolysis in adipose tissue during sepsis

Aging is accompanied by an increase in visceral adiposity, immune cell activation, and decreased ability of visceral white adipose tissue (vWAT) to maintain homeostatic functions such as lipolysis that is required for the generation of free fatty acids. Lipolysis is activated via canonical (catecholamine) or non-canonical pathways (bacteria or inflammatory cytokines). The chronic inflammatory activation of macrophages and B cells seen during aging suppresses catecholamine-stimulated lipolysis by limiting the bioavailability of catecholamines, but it is unclear whether these vWAT immune cells from older organisms would enhance or suppress the stimulated lipolysis in the context of sepsis and inflammation.

Canonical lipolysis induced by catecholamines declines during aging due to factors including an expansion of lymphocytes, pro-inflammatory macrophage polarization, and an increase in chronic low-grade inflammation; however, the extent to which the non-canonical pathway of lipolysis is active and impacted by immune cells during aging remains unclear.

Therefore, we aimed to define the extent to which immune cells from old mice influence non-canonical lipolysis during sepsis. We identified age-associated impairments of non-canonical lipolysis and an accumulation of dysfunctional B1 B cells in the visceral white adipose tissue (vWAT) of old mice. B cells can be classified as innate-like B1 B cells that acutely produce non-specific natural antibodies during bacterial infections or adaptive B2 B cells that become memory B cells and produce high-affinity antibodies. These subsets expand equally with age and can be distinguished by the expression of CD11b on the B1 subset but not the B2 subset.

Lifelong deficiency of B cells in mice results in restored non-canonical lipolysis and reductions in pro-inflammatory macrophage populations. This data indicates that age-related accumulation of B cells promotes a pro-inflammatory macrophage phenotype including the upregulation of NLRP3 inflammasome activation, supporting a model in which B cells indirectly suppress non-canonical lipolysis by promoting macrophage inflammation.

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Estrogen-Related Receptor Agonists as Exercise Mimetic Drugs

Just as the research community is interested in finding pharmaceutical ways to provoke some the beneficial reactions to calorie restriction, there is also considerable effort devoted to the search for drug candidates that can mimic some of the benefits of exercise. If the history of calorie restriction mimetic drug development is any guide, this will be a slow process, and the resulting compounds will produce lesser benefits than actual exercise, as they will only touch on a small subset of the processes involved. Still, there is no shortage of programs in this space, and here is one example.

Exercise benefits both mind and body. A drug that can mimic these effects could offset the muscle atrophy and weakness that can occur as people age or are affected by cancer, certain genetic conditions, or other reasons they are unable to carry out regular physical activity. The metabolic changes associated with exercise kick off with the activation of specialized proteins, known as estrogen-related receptors (ERRs), which come in three forms: ERRα, ERRβ, and ERRγ. After about a decade of work, researchers developed a compound named SLU-PP-332, which activates all three forms, including the most challenging target, ERRα. This type of ERR regulates exercise-induced stress adaptation and other important physiological processes in muscle. In experiments with mice, the team found this compound increased a fatigue-resistant type of muscle fiber while also improving the animals' endurance when they ran on a rodent treadmill.

To identify SLU-PP-332, the researchers scrutinized the structure of the ERRs and how they bind to molecules that activate them. Then, to improve upon their discovery and develop variations that could be patented, the team designed new molecules to strengthen the interaction with the receptors and thus provoke a stronger response than SLU-PP-332 can provide. When developing the new compounds, the team also optimized the molecules for other desirable characteristics, such as stability and low potential for toxicity. Research using SLU-PP-332 suggests targeting ERRs could be useful against specific diseases. Studies in animals with this preliminary compound indicate that it could have a benefit against obesity, heart failure, or a decline in kidney function with age.

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Thoughts on Micronutrient Intake During Calorie Restriction

Despite the considerable attention given to calorie restriction and intermittent fasting in the research community, there is very little formalism applied to the practice in humans. The few clinical trials conducted to date have had to pick their own protocols, and it is only comparatively recently that the fasting mimicking diet was developed to plant a flag on one specific implementation. The challenge here is that it is difficult to monetize calorie restriction and fasting, and thus there is no push towards standardization or more detailed assessment of variant protocols from any of the usual parties who might otherwise be doing so. It is unlikely that the fasting mimicking diet or any of the existing fasting and calorie restriction trial protocols happen to be the best approach, and also unlikely that much work will be done to improve on this situation.

Caloric restriction (CR) or energy restriction, when carefully designed, monitored, and implemented in self-motivated and compliant individuals, proves to be a viable non-pharmacologic strategy for human weight control and obesity management. Beyond its role in weight management, CR has the potential to impede responses involved not only in the pathogenesis of various diseases but also in the aging process in adults, thereby being proposed to promote a healthier and longer life. The core objective of implementing caloric restriction is to establish a balance between energy intake and expenditure, typically involving a reduction in intake and an increase in expenditure - a negative balance at least initially. It may transition toward and maintain a more desired equilibrium over time.

However, it is essential to note that CR may lead to a proportional reduction in micronutrient intake unless corresponding supplementation is provided. Historical human case reports on CR have consistently maintained adequate intakes (AI) or recommended dietary allowances (RDA) for essential micronutrients, including vitamins and minerals. Similarly, longevity studies involving non-human primates have upheld micronutrient consumption levels comparable to control groups or baseline measures. Recent randomized controlled trials (RCTs) have also endorsed daily supplementation of multivitamins and minerals to meet micronutrient needs. However, aside from these human case reports, limited human trials, and primate experiments, there remains a notable gap in human research specifically addressing precise micronutrient requirements during CR.

While adhering to AI or RDA for minerals and vitamins appears sensible in the current practice, it's important to recognize that these guidelines are formulated for generally healthy populations under standard circumstances. The adequacy of these guidelines in the setting of prolonged and profound negative energy balance remains unclear. From perspectives of evidence-based medicine and precision nutrition, this field necessitates comprehensive exploration to uncover the intricacies of absorption, utilization, and metabolism and the requirement of each hydrophilic and lipophilic vitamin and mineral during these special periods. Such investigations are crucial to determine whether existing daily dietary recommendations for micronutrients are quantitatively inadequate, excessive, or appropriate when energy balance remains negative over extended durations.

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MKK4 Inhibition Provokes Greater Liver Regeneration

Researchers here report on an approach to meaningfully stimulate the regenerative capacity of the liver. The liver is one of the few organs capable of significant regrowth in mammals, and the way in which it does so is quite different from the regenerative response found in other tissues. Thus while the results here are quite impressive, they don't apply to other organs. This is purely a way to manipulate the regulation of liver regeneration.

One key feature of acute and chronic liver diseases, and after extended liver resections, is the inability of hepatocytes to sufficiently regenerate and restore or maintain a critical functional liver mass. Although healthy livers harbor a nearly unlimited regenerative potential, damage-associated changes in the hepatic microenvironment of acutely or chronically injured livers diminish the hepatocytes' regenerative capacity. Unfortunately, the underlying molecular mechanisms are poorly understood.

We recently reported on the discovery of the dual specific kinase MKK4 as a master regulator of hepatocyte regeneration. MKK4 is a MAP2 kinase and part of the stress-activated protein kinase (SAPK)/mitogen-activated protein kinase (MAPK) signaling networks. MKK4 can be activated upon a cell's exposure to different stress stimuli. Short hairpin RNA (shRNA) mediated silencing of MKK4 was found to unlock endogenous regenerative capacity of hepatocytes in acutely or chronically injured livers via derouting SAPK signaling predominantly through MKK7 and JNK1 toward a downstream pro-regenerative transcriptional program mediated by ATF2 and ELK1. Unfortunately, no small molecule inhibitors for selective MKK4 inhibition are available.

We here report on the development and in vitro and in vivo characterization of first-in-class small molecule inhibitors of the dual specific kinase MKK4 (MKK4i). MKK4i increased liver regeneration upon hepatectomy in murine and porcine models. Strikingly, treatment with the clinical candidate HRX215 prevented post-hepatectomy-liver-failure (PHLF) and allowed for the survival of pigs in a lethal 85% hepatectomy model, suggesting that boosted liver regeneration by HRX215 might represent a viable treatment option for human PHLF and the pathogenetically related small for-size syndrome (SFSS) after liver transplantation. Testing of HRX215 in a phase I trial in 48 healthy volunteers revealed excellent safety, tolerability, and pharmacokinetics (PKs) of HRX215. Clinical trials to probe HRX215 as a therapeutic option to prevent/treat liver failure after extensive oncological liver resections or after transplantation of small liver grafts are warranted.

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Apigenin, Sleep, and Aging

For those following research into efforts to upregulate NAD+ levels to improve mitochondrial function, this paper is an interesting sidebar. Some degree of loss of NAD+ emerges from increased activity of CD38. Apigenin is a dietary supplement that can modestly influence both sleep and pace of aging, the latter in short-lived laboratory species at least. Apigenin can increase NAD+ levels by inhibiting CD38 activity. Like much of metabolism, this is all very interesting, but the effect sizes are nothing to write home about. If upregulating NAD+ levels is the goal, you'll do better by exercising. The fundamental flaw in so much of medical development, particularly in the supplement industry, is that few of the people involved seem to pay any attention to effect size. It is crazy! Effect size should be the first thing anyone looks for. Then side-effects. So much effort is, to my eyes, wasted on development programs for products with effects that are significantly worse than the results of exercise.

NAD+, a pivotal coenzyme central to metabolism, exhibits a characteristic decline with age. In mice, NAD+ levels can be elevated via treatment with apigenin, a natural flavonoid that inhibits the NAD+-consuming glycoprotein CD38. In animal models, apigenin positively impacts both sleep and longevity. For example, apigenin improves learning and memory in older mice, reduces tumor proliferation in a mouse xenograft model of triple-negative breast cancer, and induces sedative effects in mice and rats. Moreover, apigenin elongates survival in fly models of neurodegenerative disease and apigenin glycosides increase lifespan in worms.

Apigenin's therapeutic potential is underscored by human clinical studies using chamomile extract, which contains apigenin as an active ingredient. Collectively, chamomile extract has been reported to alleviate anxiety, improve mood, and relieve pain. Furthermore, dietary apigenin intake positively correlates with sleep quality in a large cohort of adults. Apigenin's electron-rich flavonoid structure gives it strong bonding capacity to diverse molecular structures across receptors and enzymes. The effects of apigenin extend beyond CD38 inhibition, encompassing agonistic and antagonistic modulation of various targets, including GABA and inflammatory pathways. Cumulatively, a large body of evidence positions apigenin as a unique molecule capable of influencing both aging and sleep. Further studies are warranted to better understand apigenin's nuanced mechanisms and clinical potential.

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More Commentary on the Role of PF4 in Reducing Brain Inflammation

Platelets in the blood are not just involved in clotting. Near every aspect of our biology has evolved many different functions, and the complexity of our biochemistry is still far from fully explored. Increased platelet factor 4 (PF4) shows up as a feature in a number of different interventions known to reduce inflammation in the aging brain. Researchers are now moving in the direction of developing therapies for neurodegenerative conditions based on the upregulation of PF4 or the delivery of recombinant PF4.

Platelets are a component of blood best known for their role in clotting, but research in recent years has illuminated other functions beyond the traditional part they play in healing wounds. Each platelet contains more than 1,000 bioactive molecules, known as platelet factors, that get released in different combinations when platelets are activated.

Several years ago, researchers discovered some of the first hints that one of those hundreds of molecules, PF4, might be particularly important. The group was trying to explain why exercise increases neurogenesis in the hippocampus, a brain region important for learning and memory. Hypothesizing that something happens in the blood after exercise that affects the brain, researchers first screened the blood of adult mice after they started running on running wheels placed in their cages. They found about 80 factors whose abundance in the blood changed significantly after exercise, and PF4 was among those that rose the most.

When the researchers injected PF4 into the bloodstream of older mice through their tails on a schedule of one injection every third day for 24 days, the old mice behaved much more like young mice in the avoidance task and other memory tests. The findings echoed the cognitive benefits researchers have seen with exercise, which included increased neurogenesis in the brains of the older mice treated with PF4.

While this group was conducting its research linking exercise to PF4 and PF4 to cognition, another team of researchers was trying to identify individual components in young blood that might explain its rejuvenating effects. Inflammation is known to increase with age. In the brain, inflammation activates a type of immune cells called microglia, ultimately leading to neuronal dysfunction and neurodegeneration. Previous studies have shown that injecting old blood into young mice increases inflammation in the young hippocampus. The new study was the first to show that young blood attenuates neuroinflammation and, in turn, reduces the activity of microglia. The injection of platelets alone was enough to have these effects. And the most common protein in the platelet solution, analysis showed, was PF4.

Although PF4 didn't appear to cross the blood-brain barrier, data showed that PF4 affects the peripheral immune system, reducing the number of pro-aging immune factors in circulation, decreasing neuroinflammation and enhancing cognitive function. That sequence of events suggests that both circulating immune factors and the peripheral immune system could be targets of future therapeutics.

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Lipid Droplets in Microglia Involved in Alzheimer's Pathology

Microglia are innate immune cells resident in the central nervous system. Microglial dysfunction is clearly a contributing factor in the onset and progression of age-related neurodegenerative conditions, including Alzheimer's disease, as well as the accompanying chronic inflammation of brain tissue. As to why microglia become problematic and inflammatory, there are any number of possible contributing mechanisms to consider. Cellular senescence, mitochondrial dysfunction, reactions to cell debris or the presence of persistent viral infections, and more. In this vein, researchers here discuss excessive lipid accumulation in microglia as a possible contributing cause of Alzheimer's disease.

Several genetic risk factors for Alzheimer's disease implicate genes involved in lipid metabolism and many of these lipid genes are highly expressed in glial cells. However, the relationship between lipid metabolism in glia and Alzheimer's disease pathology remains poorly understood. Through single-nucleus RNA sequencing of brain tissue in Alzheimer's disease, we have identified a microglial state defined by the expression of the lipid droplet-associated enzyme ACSL1 with ACSL1-positive microglia being most abundant in patients with Alzheimer's disease having the APOE4/4 genotype.

In human induced pluripotent stem cell-derived microglia, fibrillar amyloid-β induces ACSL1 expression, triglyceride synthesis, and lipid droplet accumulation in an APOE-dependent manner. Additionally, conditioned media from lipid droplet-containing microglia lead to Tau phosphorylation and neurotoxicity in an APOE-dependent manner. Our findings suggest a link between genetic risk factors for Alzheimer's disease with microglial lipid droplet accumulation and neurotoxic microglia-derived factors, potentially providing therapeutic strategies for Alzheimer's disease.

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Inhibiting P16 in Microglia Reduces Amyloid Plaques in Mice

Researchers here show that targeting microglia in a mouse model of Alzheimer's disease to suppress p16 expression can reduce amyloid-β plaques. This appears to be a way to interfere in a maladaptive reaction to amyloid-β on the part of microglia, innate immune cells responsible for clearing molecular debris from brain tissue. P16 is a marker of cellular senescence, though may also be characteristic of non-senescent but still problematic, pro-inflammatory microglia. There is a good amount of evidence to suggest that both senescent and overly active microglia are important to the progression of neurodegenerative conditions such as Alzheimer's disease. Senescent cells can be cleared by senolytic therapies, and evidence in animal studies suggests that this should help Alzheimer's patients. Dealing with non-senescent, activated and problematic microglia will require a different strategy, however.

Age-dependent accumulation of amyloid plaques in patients with sporadic Alzheimer's disease (AD) is associated with reduced amyloid clearance. Older microglia have a reduced ability to phagocytose amyloid, so phagocytosis of amyloid plaques by microglia could be regulated to prevent amyloid accumulation. Furthermore, considering the aging-related disruption of cell cycle machinery in old microglia, we hypothesize that regulating their cell cycle could rejuvenate them and enhance their ability to promote more efficient amyloid clearance.

First, we used gene ontology analysis of microglia from young and old mice to identify differential expression of cyclin-dependent kinase inhibitor 2A (p16ink4a), a cell cycle factor related to aging. We found that p16ink4a expression was increased in microglia near amyloid plaques in brain tissue from patients with AD and 5XFAD mice, a model of AD. In the BV2 microglia cell line, small interfering RNA (siRNA)-mediated p16ink4a downregulation transformed microglia with enhanced amyloid phagocytic capacity through regulated the cell cycle and increased cell proliferation.

To regulate microglial phagocytosis by gene transduction, we used poly (D,L-lactic-co-glycolic acid) (PLGA) nanoparticles, which predominantly target microglia, to deliver the siRNA and to control microglial reactivity. Nanoparticle-based delivery of p16ink4a siRNA reduced amyloid plaque formation and the number of aged microglia surrounding the plaque and reversed learning deterioration and spatial memory deficits. We propose that downregulation of p16ink4a in microglia is a promising strategy for the treatment of Alzheimer's disease.

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Further Considering the Altered Transcription of Longer Genes with Age

The machinery of gene expression changes with age. In recent years, it has been noted that the length of gene sequences correlates with the degree to which transcription of gene sequences into RNA molecules changes over the course of aging. Later work has started to examine the proximate causes of these changes, various fine detail mechanisms buried in the depths of transcription. The research community is not yet at the point of being able to conclusively demonstrate that altered transcription of longer genes produces meaningful downstream consequences in degenerative aging, as interventions specifically targeting just this process of transcriptional change have yet to be established. This is worth keeping an eye on, however.

Recent studies of aging organisms have identified a systematic phenomenon, characterized by a negative correlation between gene length and their expression in various cell types, species, and diseases. We term this phenomenon gene-length-dependent transcription decline (GLTD) and suggest that it may represent a bottleneck in the transcription machinery and thereby significantly contribute to aging as an etiological factor.

Currently, it is yet to be understood whether GLTD is only a marker of aging or whether it also actively plays a role in the aging process itself. We consider this to be the greatest outstanding question on GLTD. To measure the potential impact of GLTD on aging, it would be necessary to identify interventions or experimental schemes that only affect GLTD. However, no such intervention or experimental scheme is currently known.

To quantify the magnitude of impact of any such intervention, we may further need to first apply them to animal models. Even in such studies, it would remain challenging to attribute effects toward aging to GLTD rather than any single subsets of genes that change their transcription as part of GLTD. Nevertheless, we cautiously suspect a causal contribution.

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Regulatory T Cells Contribute to Reduced Myelination in the Aging Brain

Myelin surrounds the axons that connect neurons to one another, and is required for the transmission of electrical impulses. This myelin sheath is maintained by oligodendrocytes. These cells do not carry out their work in isolation; a great many factors are involved in determining the size and capabilities of the oligodendrocyte population. Aging is disruptive to the myelination carried out by oligodendrocytes. The consequences are not as bad as the profound loss of myelin that takes place in demyelinating diseases such as multiple sclerosis, but age-related loss of myelination does appear to degrade cognitive function. Researchers are thus interested in understanding the mechanisms involved, in search of ways to restore a youthful capacity for myelination in the aging brain.

Myelin regeneration (remyelination) is essential to prevent neurodegeneration in demyelinating diseases such as Multiple Sclerosis, however, its efficiency declines with age. Regulatory T cells (Treg) recently emerged as critical players in tissue regeneration, including remyelination. However, the effect of ageing on Treg-mediated regenerative processes is poorly understood.

Here, we show that expansion of aged Treg does not rescue age-associated remyelination impairment due to an intrinsically diminished capacity of aged Treg to promote oligodendrocyte differentiation and myelination in male and female mice. This decline in regenerative Treg functions can be rescued by a young environment. We identified Melanoma Cell Adhesion Molecule 1 (MCAM1) and Integrin alpha 2 (ITGA2) as candidates of Treg-mediated oligodendrocyte differentiation that decrease with age.

Our findings demonstrate that ageing limits the neuroregenerative capacity of Treg, likely limiting their remyelinating therapeutic potential in aged patients, and describe two mechanisms implicated in Treg-driven remyelination that may be targetable to overcome this limitation.

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Tristetraprolin Upregulation Reduces Frailty and Increases Bone Density in Old Mice

Researchers here report that life-long tristetraprolin (TTP) upregulation leads to reduced frailty and improved bone mineral density in aged mice. One of the functions of TTP is that it suppresses expression of the pro-inflammatory TNF-α cytokine, so a reduced degree of age-related inflammation would be the first place to look for an explanation of the outcome noted here. It is an open question as to whether TTP upregulation produces a more nuanced and reactive reduction of TNF-α signaling than is the case for the blunt, across the board inhibition achieved by present anti-TNF-α therapies, and is thus a mechanism that interferes less in the necessary immune response to injury and infection.

Researchers have presented results from a study aiming to use a novel transgenic mouse model (TTP knock-in - TTPKI) that has a moderate elevation of tristetraprolin (TTP) systemically to understand if there is a long-term benefit for bone health. The study performed body composition, physical performance assessments, and frailty assessments on the 6-month-old and 22-month-old TTPKI and C57BL/6N wild-type male and female mice.

Microcomputed tomography (µCT) and decalcified sections of the tibia were used to determine static bone histomorphometric parameters and bone histomorphometry, respectively. Immunophenotypic analysis of bone marrow (BM), spleen, and mesenteric lymph nodes were analyzed by flow cytometry for myeloid and lymphocyte populations. Myeloid population BM osteoclastogenic potential was assessed.

Body composition with aged control and TTPKI mice revealed significant sex and genotype differences. Aged TTPKI mice displayed decreased frailty scores and increased quality of life compared to control similarly aged mice. The tibia from aged TTPKI mice exhibited higher bone mineral density (BMD) than aged control mice. Age-related decline in immune cell composition was partially reversed in aged TTPKI mice. In an osteoclast differentiation assay, BM myeloid progenitors from TTPKI mice exhibited fewer osteoclasts with reduced eroded bone surface area. Improved functional capacity, BMD, and immune cell composition indicate that enhanced expression of TTP can promote a healthier phenotype during aging.

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