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- The Urge to Radical Life Extension
- Mir-294 Awakens an Embryonic Proliferation Behavior in Heart Cells, Spurring Regeneration Following Heart Attack
- Evidence for Senescent Cells to Cause Aortic Aneurysms
- Rejuvenation Biotechnology Companies Presenting at Biotech Investing in Longevity, in San Francisco May 2019
- The Cosmological Noocene
- Mesenchymal Stem Cells Improve Heart Regeneration via Macrophage Polarization
- The Goal of Kidney Rejuvenation
- Efficiently Reprogramming Fibroblasts into Cardiomyocytes for Heart Regeneration
- Amyloid-β Causes Pericyte Dysfunction and Reduced Blood Flow in the Aging Brain
- Suggesting that Cytomegalovirus Infection Contributes to Metabolic Syndrome
- A Better Understanding of the Mechanisms Surrounding Thymic Involution
- Temporary β-catenin Inhibition Attenuates Effects of Aging on Bone Regeneration
- A Discussion of Mitochondrial DNA Damage and Aging
- Higher Protein Intake Correlates with Lower Risk of Frailty in Old Age
- Mitochondrial Oxidative Stress in Neurodegenerative Disease
The Urge to Radical Life Extension
Those portions of the modern longevity community interested in bringing an end to aging and extending healthy human life span indefinitely tend to be the older portions, people who have been a part of the broader movement for quite some time. Newcomers tend to be more moderate, aiming at lesser goals. Perhaps this is a result of the successful projects, such as the SENS Research Foundation and Methuselah Foundation, tending to moderate their rhetoric as they attract a broader and larger base of support. I think that this road to moderation might be a problem, and that there is thus a continued role for those who loudly declaim that the goal is to control aging absolutely, via new medical technology, and that the natural consequence of that control is healthy, active, youthful life that extends for centuries or more.
If the goals that our movement works towards are broadly watered down from radical life extension of centuries to just adding a few more years, then marginal projects that can do no more than add a few more years will come to dominate the field to the exclusion of everything else. We are already more or less in this situation, in that that the vast majority of funding goes towards discovery and development of small molecules that tinker with the operation of an aged metabolism to make it a little more resilient to the underlying causes of aging. If that is all that is done, then we’ll all age and die on basically the same schedule as our parents and grandparents. It will be a grand waste of opportunity, given that we have the knowledge and the means to do far better, such as by following the SENS agenda for rejuvenation biotechnologies based on repairing the root causes of aging.
This popular media article looks at a few of the people who do make no bones about aiming at radical life extension. It isn’t terrible, thankfully, though it doesn’t quite manage to escape the straitjacket of conformity, the author suggesting that it is somehow strange to want to live for a long time in good health, or strange to want to avoid a slow, crumbling, painful death. There is no present status quo so terrible that it will not have its defenders, and for whatever reason the status quo of aging and suffering and omnipresent death and loss are aggressively defended. But setting that aside, the article manages to capture the present state of development and the viewpoints of its subjects quite well, which is a change over past years of media attention.
How to live forever: meet the extreme life-extensionists
In 2016, an American real-estate investor named James Strole established the Coalition for Radical Life Extension, a nonprofit based in Arizona which aims to galvanise mainstream support for science that might one day significantly prolong human life. Standards in modern medicine are allowing us to live longer now than ever before. But that is not Strole’s concern. What good are a few more measly years? He is interested in extending life not by days and weeks, but by decades and even centuries, to the degree that mortality becomes optional – an end to The End. He isn’t alone. Life extensionists have become a fervent and increasingly vocal bunch. Famously, the community includes venture capitalists and Silicon Valley billionaires, non-gerontologists all, and nearly all men, who consider death undesirable.
The current life-extensionist strategy is twofold. First, achieve a “wellness foundation,” Strole says. Second, stay alive until the coming gerontological breakthrough. All that is required is to “live long enough for the next innovation,” and presuming you do, “You can buy another 20 years.” Twenty years here, 20 years there, it all adds up, and suddenly you’re 300. This is a common view. Last year the British billionaire Jim Mellon, who has written a book on longevity, titled Juvenescence, said: “If you can stay alive for another 10 to 20 years, if you aren’t yet over 75 and if you remain in reasonable health for your age, you have an excellent chance of living to more than 110.” To most, 110 seems a modest target. Why not forever? “It’s not some big quantum leap,” Strole says, by way of explanation. He invokes the analogy of a ladder: “step by step by step” to unlimited life. In 2009 the American futurist Ray Kurzweil coined a similar metaphor, referring instead to “bridges to immortality”.
Aubrey de Grey, a serious scientist, considers life extension a health issue, which is perhaps the field’s most convincing argument. Gerontologists are not hoping to end death, he says. Instead, “We’re interested in people not getting sick when they get old.” No matter how much society rails against the concept of immortality, nobody really wants to suffer through Alzheimer’s, or suddenly fall foul of cardiovascular disease. Gerontology is the act of developing treatments for age-related diseases, de Grey argues – of reducing the causes of death, not death itself. “The benefits of living longer are not the point. The benefits are not having Alzheimer’s disease.” For de Grey, indefinite life is a by-product, not a goal.
Are we anywhere near to a breakthrough? So far, research has produced modest yields. Gerontologists speak prophetically of potential, but most warn a significant human development remains somewhere far off in the distance – almost in sight but not quite. Richard Hodes, the director of the National Institute of Aging, a US government agency, told me that, though research in animals has led to “dramatic increases in lifespan”, some of them multi-fold, “There has been far less quantitative effect as those models have moved towards mammalian species.” The biologist Laura Deming, who in 2011 established the Longevity Fund, a venture capital firm that supports “high-potential longevity companies”, told me that startups continue to successfully root out biological markers of ageing – inefficient cells, mitochondrial decline – but that, in humans, “We really don’t know right now what will work and what won’t.”
Much of gerontology focuses on identifying types of damage that accumulate with age and developing ways to halt or reverse that accumulation. It has been discovered, for example, that as we grow older, certain cells become senescent and harmful but nevertheless stick around, getting in the way like comatose guests at the end of a house party. Removing those cells have helped mice have longer, healthier lifespans. Similar forms of genetic engineering have been successful in other animal models. But to reach the mainstream, gerontologists must convince government agencies to support human adoption, a complicated and long-winded task, given the general view that death is a normal human process.
Mir-294 Awakens an Embryonic Proliferation Behavior in Heart Cells, Spurring Regeneration Following Heart Attack
That microRNA-294 (mir-294) beneficially affects heart regeneration was discovered via its presence in embryonic stem cell exosomes. Exosomes are extracellular vesicles, membrane-bound packages of molecules that cells pass between one another. They are interesting to the research community because it is in principle much easier to construct a therapy based on delivery of exosomes harvested from stem cells than it is to deliver those same stem cells. Thus most of the present generation of stem cell therapies may well be replaced in the near future by the delivery of extracellular vesicles, and many research groups are testing exosomes from stem cells to see how well they work to spur greater regeneration.
Most vesicles contain a wide variety of molecules, but in the case of embryonic stem cell exosomes and the injured heart, researchers found that near all of the therapeutic effect was mediated by mir-294. Thus they could go a step further and discard the exosomes as well as the cells. The results of that line of work are noted in today’s publicity materials and paper. Applying mir-294 causes adult heart muscle cells to regress into a state more like that of embryonic cells, provoking greater replication and thus greater regeneration. This sort of in-situ reprogramming of cell behavior is growing in popularity in the research community, see the work of Turn.bio for example, though it remains to be seen whether or not it can be made safe enough to be the basis for a near future regenerative therapies.
Embryonic MicroRNA Fuels Heart Cell Regeneration, Temple Researchers Show
By adulthood, the heart is no longer able to replenish injured or diseased cells. As a result, heart disease or an event like a heart attack can be disastrous, leading to massive cell death and permanent declines in function. A new study is the first to show that a very small RNA molecule known as miR-294, when introduced into heart cells, can reactivate heart cell proliferation and improve heart function in mice that have suffered the equivalent of a heart attack in humans. “In previous work, we discovered that miR-294 actively regulates the cell cycle in the developing heart. But shortly after birth miR-294 is no longer expressed. The heart is very proliferative when miR-294 is expressed in early life. We wanted to see if reintroducing it into adult heart cells would turn them back to an embryonic-like state, allowing them to make new heart cells.”
The researchers tested their idea in mice that had myocardial infarction (heart attack). Mice were treated with miR-294 continuously for two weeks after sustaining myocardial injury. Two months following treatment, the researchers observed noticeable improvements in heart function and a decrease in the area of damaged tissue. Examination of treated heart cells revealed evidence of cell cycle reentry, indicating that the cells had been reactivated, regaining the ability to produce new cells. “The miR-294 treatment reawakened an embryonic signaling program in the adult heart cells. Because of this, the old heart cells were no longer quite like adult cells, but neither were they fully embryonic. In this in-between state, however, they had the ability to make new cells.”
Transient Introduction of miR-294 in the Heart Promotes Cardiomyocyte Cell Cycle Reentry After Injury
Embryonic heart is characterized of rapidly dividing cardiomyocytes required to build a working myocardium. Cardiomyocytes retain some proliferative capacity in the neonates but lose it in adulthood. Consequently, a number of signaling hubs including microRNAs are altered during cardiac development that adversely impacts regenerative potential of cardiac tissue. Embryonic stem cell cycle miRs are a class of microRNAs exclusively expressed during developmental stages; however, their effect on cardiomyocyte proliferation and heart function in adult myocardium has not been studied previously.
In this study, we determine whether transient reintroduction of embryonic stem cell cycle miR-294 promotes cardiomyocyte cell cycle reentry enhancing cardiac repair after myocardial injury. A doxycycline-inducible AAV9-miR-294 vector was delivered to mice for activating miR-294 in myocytes for 14 days continuously after myocardial infarction. miR-294-treated mice significantly improved left ventricular functions together with decreased infarct size and apoptosis 8 weeks after MI. Myocyte cell cycle reentry increased in miR-294 hearts parallel to increased small myocyte number in the heart. Isolated adult myocytes from miR-294 hearts showed upregulation of cell cycle markers and miR-294 targets 8 weeks after MI. Thus ectopic transient expression of miR-294 recapitulates developmental signaling and phenotype in cardiomyocytes promoting cell cycle reentry that leads to augmented cardiac function in mice after myocardial infarction.
Evidence for Senescent Cells to Cause Aortic Aneurysms
Cells enter a senescent state in response to molecular damage, a toxic environment, reaching the Hayflick limit on replication, or to aid in wound healing, among other reasons. A senescent cell halts replication and begins to secrete a mix of inflammatory signals, growth factors, and other molecules that influence surrounding cells. This is useful and beneficial when it occurs in potentially cancerous, damaged cells, or as a part of the wound healing process. Normally these cells quickly self-destruct or are destroyed by the immune system. It is when senescent cells evade destruction and linger for the long term that the problems begin. The signals that are beneficial in the short term become destructive to tissue function and structure, additionally producing chronic inflammation and all of its accompanying problems.
In recent years, the research community has finally adopted the SENS Research Foundation view of aging in the matter of senescent cells – fifteen years late to the party, but better late than never. Meaningful progress requires more scientists and sources of funding to be involved than was the case a decade ago, so it is good that this is happening. Researchers have now demonstrated that growing numbers of senescent cells contribute to a wide range of age-related conditions, and are likely the primary cause for some of them, such as arthritis. In animal studies, selectively destroying a sizable fraction of senescent cells can extend healthy life spans, and reverse the progression of age-related diseases. Senescent cells are in effect actively maintaining a disrupted, dysfunctional state of tissue function and metabolism. Removing them turns back these consequences, producing a narrow form of rejuvenation. Aging is itself an accumulation of damage, and these senescent cells are a form of damage.
The evidence for cellular senescence to be a significant contributing cause of specific age-related conditions continues to accumulate, and ever faster as more funding pours into this part of the field. The research results noted here are an example of the type, new discoveries in the relevance of senescence to age-related disease that are announced every few months. The more that is discovered, the better for all of our futures, given that work continues on ever better ways to remove senescent cells from old tissues. That the catastrophic thinning and structural failure of aorta walls involves senescent cells is one more potential benefit to be realized by senolytic therapies capable of clearing senescent cells.
Scientists find potential way to defuse ‘time bomb’ of cardiology
Ascending aortic aneurysms grow for decades without any warning signs and can be fatal once they rupture. It is known that these aneurysms are caused by the thinning of the aortic wall which weakens it and makes it silently grow like a balloon over time without any symptoms. If caught early enough, they can be surgically repaired at low risk, but if they go undetected, which many do, they will eventually rupture or cause a tear in the wall of the aorta, called an aortic dissection. While the phenomenon is well documented, the medical community previously had little evidence to understand the mechanisms causing it to occur or how to prevent it.
Now, researchers have shown that a process that is recognized in cancer biology is causing the cells to become destructive and eat away at the surrounding muscle tissue, weakening the aortic wall. “We discovered that within the wall of the aorta, a small proportion of the muscle cells have entered into a state called senescence. Rather than die, these senescent cells become destructive, secreting enzymes that chew the area around them. There are select research groups around the world that are coming up with compounds that have shown promise in clearing out senescent cells. They are thinking about it for certain aging-related diseases, but it could be positioned for this important problem as well.”
Seno-destructive smooth muscle cells in the ascending aorta of patients with bicuspid aortic valve disease
We undertook in situ analysis of ascending aortas from 68 patients, seeking potentially damaging cellular senescence cascades. Aortas were assessed for senescence-associated-ß-galactosidase activity, p16Ink4a, and p21 expression, and double-strand DNA breaks. The senescence-associated secretory phenotype (SASP) of cultured-aged bicuspid aortic valve (BAV) aortic smooth muscle cells (SMCs) was evaluated by transcript profiling and consequences probed by combined immunofluorescence and circular polarization microscopy. The contribution of p38 MAPK signaling was assessed by immunostaining and blocking strategies.
Herein, we report that senescent SMCs accumulate in aneurysmal ascending aortas associated with bicuspid and tricuspid aortic valves. Moreover, we identified a particular predisposition to SMC senescence in BAV aortopathy, indicated by the presence of senescent SMCs in non-aneurysmal BAV aortas, enrichment of cellular senescence at the aortic convexity, and multivariable analysis of potential aneurysm risk factors. We further show that senescent aortic SMCs have a pronounced collagenolytic SASP, a destructive profile that is controlled by p38 MAPK. The findings identify a cellular aging cascade in human BAV disease and a “seno-destructive” SMC phenotype that may underlie the aortic wall degeneration.
Rejuvenation Biotechnology Companies Presenting at Biotech Investing in Longevity, in San Francisco May 2019
Aikora Health and Foresight Institute recently collaborated to host a gathering of investors, entrepreneurs, and supporters from the core rejuvenation biotechnology community. The event was held in San Francisco, and I attended to present a summary of ongoing work at Repair Biotechnologies. It was an interesting mix of local folk and visitors from across the US, a chance to catch up with fellow travelers from other companies and some of our investors. As you probably know, the SENS Research Foundation and a number of influential aging research institutions, such as the Buck Institute, are based in the Bay Area. It has long been the case that the venture and technology communities in California include many people sympathetic to the SENS goal of bringing aging under medical control – it isn’t a coincidence that the SENS Research Foundation set up their research center in this part of the world.
The presentations were recorded, and in the video here see my outline in addition to those by principals at the Methuselah Fund, Leucadia Therapeutics, and Turn.bio. As you might recall, Repair Biotechnologies is working on reversal of atherosclerotic lesions, aiming to prevent the contribution of this condition to late life mortality, and regrowth of the thymus, so as to restore the pace of creation of T cells, and improve immune function in later life. We recently raised our seed round, so we’re hard at work in the lab at the moment.
The other two biotech companies are working on very interesting projects, and I’ve mentioned both in the past here at Fight Aging! In the case of Leucadia, you might look at the presentation given by Doug Ethell at Undoing Aging 2018 for a good overview of the company and its approach. It is exactly the sort of radically different, cost-effective approach to Alzheimer’s disease that we’d like to see more of. Turn.bio is equally radical in the goal of a href=”https://www.fightaging.org/archives/2019/03/turn-bio-transiently-reprogramming-cells-to-near-pluripotence-as-a-therapy-for-aging/”>transiently reprogramming cells in vivo, spurring them into the improvements observed in the reprogramming of somatic cells into induced pluripotent cells: repair of mitochondrial function, possibly repair of other molecular damage, and reversion of epigenetic markers of aging. There should be more of this sort of ambition in evidence in the biotechnology community.
Methuselah Fund, Leucadia Therapeutics, Turn Biotechnologies and Repair Biotechnologies
Four presentations at “Biotech Investing in Longevity” on 1st May 2019 in San Francisco: Sergio Ruiz – Methuselah Fund; Doug Ethell – Leucadia Therapeutics; Vittorio Sebastiano – Turn Bio; Reason – Repair Biotechnologies.
The Methuselah Fund is designed to accelerate results in the longevity field, extending the healthy human lifespan. They measure their success not just by financial return-on-investments but also by what they call return-on-mission. Their DNA stems from The Methuselah Foundation, which has been working hard during the last 18 years to make 90 the new 50 by 2030.
Leucadia Therapeutics is determined to end Alzheimer’s disease with Arethusta, a first-in-class treatment for mild cognitive impairment associated with Alzheimer’s disease. Cerebrospinal fluid (CSF) clears toxic metabolites from intercellular spaces in the brain, much as the lymphatic system does in the rest of the body. The first regions of the brain to be impacted by Alzheimer’s disease are cleared by CSF that drains across a porous bone called the cribriform plate. Aging and life events can occlude the cribriform plate and reduce the CSF-mediated clearance of toxic metabolites from those regions of the brain, thereby causing plaques and tangles formation. Leucadia’s patented Arethusta technology restores CSF flow across the cribriform plate, improving the clearance of toxic metabolites from the earliest regions of the brain to be affected by Alzheimer’s disease.
Turn.bio develops a transient reprogramming protocol that has demonstrated a youthful reversion of eight of the nine hallmarks of aging. Reversion of the ninth is being currently being developed. They technology has already been proven to rejuvenate five different tissue types of the human body with more being evaluated. Osteoarthritis, skin damage, and sarcopenia are all proven targets of the technology, with other indications soon to be tested.
Repair Biotechnologies is a longevity company with the mission to develop and bring to the clinic therapies that significantly improve human healthspan through targeting the causes of age-related diseases and aging itself. The company currently runs two preclinical development programs: the first for thymus regeneration and immune system restoration, and the second for reversal of atherosclerosis.
Aikora Health and Foresight Institute joined forces to organize a series of talks on biotech investment and longevity. They gathered a curated group of entrepreneurs, scientists, and investors to discuss exciting projects that seek to extend human healthspan, surveying a diversity of novel approaches, and discussing which ambitious goals are realistically within our reach.
Aikora Health connect investors with companies, founders and scientists in the health tech, genomics, and regenerative medicine sectors. Our key focus is on longevity tech with the potential to transform healthcare and human aging. We offer insight and information regarding the biotech and increasingly important longevity space, in addition to matching founders of biotech and longevity companies with funding and strategic partnerships.
Foresight Institute is a leading think tank and public interest organization focused on emerging world-shaping technologies. It was founded in 1986 on a vision of coming revolutions in technology that will bring extraordinary opportunities, as well as unprecedented challenges. Foresight’s mission is to steer towards positive futures, futures of Existential Hope.
The Cosmological Noocene
Here is a sketch of the future, without any specific dates assigned to its milestones. The molecular biochemistry of living beings is fully mapped and understood. The human mind is reverse engineered. It is run in software. A million variants and improvements are constructed. Molecular nanotechnology is established and becomes a mature industry, available to everyone. Anything and everything can be built efficiently and at next to zero cost given the raw materials and a specification. All disease is abolished, and aging is defeated: these are problems that boil down to control over molecules, just another form of maintaining a machine to remove wear. The future stretches out indefinitely for all living entities, whether biological or otherwise. Given that, is perhaps never too early for at least a little long-term thinking, even though we’re still here in the present, working away at the very first rung of this tall ladder to the future.
From an economic perspective those who thrive in the era of molecular manufacturing and comprehension of mind are those who make the most efficient use of the matter that they own, and those who gain control over the most matter: quality versus quantity shift back and forth in the degree of advantage as the ability to accurately and rapidly manipulate large masses of matter at the atomic level and lower grows. Matter is most economically efficient when incorporated as the workings of an intelligent entity. The end state here is a continuum of thinking matter, and there are countless arrangements by which matter might be made aware. Our evolved biology is among the least intricate and least capable of these possibilities. At the most efficient we might envisage space- and matter-efficient computational processors plus the necessary workings for support: communication, energy, repair, and so forth. The higher the fraction of that mix that can be devoted to data processing and transfer, the more economically effective the entities who use that system as a substrate.
Evolved intelligences are not rational actors in search of growth above all other goals. We have parks and entertainment industries, for example. There is no reason why constructed or augmented intelligences should be any different, but equally they have one important quality: they can change themselves and their progeny in defined ways to achieve defined outcomes in mental state. The alterations and experiments that provide economic advantages will prosper. Entities who choose to incorporate an urge to growth will become the majority. At some point the value of an asteroid, a moon, a planetary crust, or a star in its natural state falls below the value of the same matter dismantled and used as raw materials for computational processing. After that it is just a matter of time before this solar system, a wilderness at present, and a collection of parks in ages ahead, is transitioned into a more efficient arrangement of matter in which near every portion of the whole is intelligent. This change will propagate outward to other stellar systems, without end, driven by simple economic considerations. A sea of cultures of a complexity and scope beyond our imaginings, and our world today the tiniest mote of a seed, that could be emulated by the smallest discrete material unit of computational processing in that future substrate.
So it is less a matter of manifest destiny that we will convert our entire future light cone into intelligence, and more a matter of economic inevitability, the destination at the end of the random walk of choice simply because some classes of choice will be made more frequently than others. The outcome of human action writ large, for a very expansive definition of the word human. All of this, however, indicates that there is something very important that we at present do not understand about the nature of reality. Nothing in our present situation as a species appears to be exceptional: stars are everywhere in vast numbers, planets also, and complex organic molecules are seen wherever we have the ability to observe them. Thus intelligence should arise elsewhere. The age of the universe is very long in comparison to the time taken for our spontaneous generation, yet we see no evidence that any other intelligence has come before us. This is often expressed as the Fermi Paradox, but is perhaps best thought of as the Wilderness Paradox, which is to ask why everything we observe, out to the very limits of the visible universe, is apparently natural and unaltered. Where are the signs of what we know is possible and inevitable for an intelligent evolved species, the conversion of matter to more efficient forms on a vast scale?
The only self-consistent solution to the Fermi Paradox that does not require some new and presently missing piece of scientific understanding is the Simulation Argument: that we are in a box and walled off from the real world, whatever that might be, created by some demiurge for purposes guessable but ultimately unknowable through any action on our part. Prosaically that demiurge might be a descendant of a past humanity similar to ours, an entity that is running one of countless ancestor simulations for scientific reasons. Far less prosaic options are also possible, in which the demiurge is simulating from first principles a radically different cosmology from its own and thus its nature and motivations are inscrutable. These possibilities of the Simulation Argument are dissatisfying to explore, however, for all the same reasons as the brain in a jar thought experiment is a dead end. Best to assume it is not true, as it if is there is nothing useful you can do about it, individually or collectively. It is Pascal’s Wager turned inside-out.
It is more interesting to speculate on what it is that we don’t understand at present about the nature of reality. There are numerous candidates, and most present thinking is directed towards those related to enforcing our rarity, often expressed as the Great Filter, one or more enormously unlikely steps that lie between the origin of a barren world scattered with a few organic compounds and the destination of an intelligent species engaged in repurposing of raw materials on a vast scale. All proposed Great Filters are very speculative; there is a great deal of room to argue about odds when you only have one example to work with, or events of the distant past must be reconstructed from theory, or future development of the species considered in detail rather than at a very high level, all which makes coming to any sort of rigorous conclusion next to impossible. All that is practical to achieve is to build the shape of the argument that would be sufficient if the actual numbers and proposed events in fact exist in reality. Given this uncertainty, any proposed Great Filter becomes an ever less satisfying answer the further we look outward and the more galaxies we see without any sign of massive engineering. It only serves to argue for our uniqueness, which is implausible given what we presently know and the isotropic nature of all other observed aspects of the natural universe across vast spans of distance and time.
Per our present understanding of physics and intelligent economic activity, we will turn every part of that great span, stars and all, into our descendants if not diverted or stopped by some outside influence. The cosmological noocene, an ocean of intelligence of breathtaking scope and grandeur. That the natural universe remains as it is to be used by us indicates that something is awry, however, that some vital and important understanding is missing. We as a species are still in the act of making the first fumbling explorations of the bounds of the possible with regards to what it is that we don’t know.
Mesenchymal Stem Cells Improve Heart Regeneration via Macrophage Polarization
It is well known that the most commonly available forms of stem cell therapy produce benefits via signaling on the part of the transplanted cells, which soon die, rather than via any sort of integration of these cells into tissues. These treatments use varieties of what are called mesenchymal stem cells, which is actually a poorly defined, broad category. One clinic’s mesenchymal stem cells are usually meaningfully different from those of the next. Nonetheless, these therapies fairly reliably reduce chronic inflammation. This can allow for improved regeneration in patients, but that outcome is much less reliable in practice.
The innate immune cells known as macrophages are important in the complex dance of tissue regeneration. In recent years researchers have become increasingly interested in deciphering and altering macrophage behavior, switching more of these cells from the aggressive and inflammatory M1 polarization, responsible for hunting pathogens, to the pro-regenerative M2 polarization. It is thought that aging is characterized by too much of a bias towards M1, and the balance might be forced back to M2 via the application of suitable therapies. It is perhaps not surprising that we should find that some existing therapies that can modulate inflammation and improve regeneration act through this mechanism.
Myocardial infarction (MI) is a major cause of coronary heart disease (CHD). More and more studies have shown that stem cells can play an important role in tissue repair and anti-inflammation. In particular, mesenchymal stem cells (MSCs) have shown anti-inflammatory and immunological functions. Indeed, MSCs have also been shown to have the potential to enhance the recovery and regeneration of the infarcted myocardium. The current belief on the role of MSCs in myocardial regeneration is their synthesis and secretion of cytokines and other trophic growth factors to signal to the injured myocardial cells, which may also involve anti-aging effects.
We have recently shown that the effects of transplantation of CD146+ MSCs on myocardial regeneration after MI exceeds the effects of transplantation of MSCs, likely resulting from reduction of aging-associated cellular reactive oxygen species in injured cardiac muscle cells (CMCs). Many effects of MSCs on tissue repair and cell regeneration are conducted through their crosstalk with macrophages. It is traditionally thought that macrophage are deemed to be white blood cells with a major functionality of swallowing and ingesting wastes, dying or dead cells, and impurities. Nevertheless, recently studies have shown that macrophages have much more functions other than phagocytosis. Therefore, a more complicated classification of macrophages has been applied, in which 2 subtypes of macrophages are distinguished by two phenotypes. One was named as “M1” macrophages, while the other alternatively polarized one was named as “M2” macrophages, which function in regulation of humoral immunity and promotion of tissue repair.
Since the role of macrophages in the MSC-mediated recovery of heart function after MI remains unclear, this question was thus addressed in the current study. We found that transplantation of MSCs did not alter the total number of the macrophages in the injured heart, but induced their polarization towards a M2-phenotype. Moreover, administration of TNFα into MSC-transplanted mice, which prevented M2-polarization of macrophages, abolished the effects of MSCs on recovery of heart function and on the reduction of infarcted cardiac tissue. Thus, our data suggest that MSCs may rejuvenate CMCs after ischemic injury at least partially through induction of M2-polarization of macrophages.
The Goal of Kidney Rejuvenation
The authors of this open access paper review the aging of the kidney and consider the prospects for using factors from young blood as a means of rejuvenation. This is a fairly narrow view, as there are many other approaches that should produce rejuvenation of the aged kidney, ranging from those close to realization, such as senolytic therapies to clear senescent cells, or various approaches to stem cell therapy, to those yet to be achieved, meaning much of the rest of the SENS agenda of rejuvenation biotechnologies to repair the damage that causes aging. Nonetheless, after so many years of trying to persuade the research community to open up on the topic of addressing the mechanisms of aging as a means of therapy, it is very pleasant to see so many publications in the literature doing just that. The present open discourse is a sea change in comparison to the silence of a decade or two ago, in which few researchers were willing to speak in public about treating aging. The science was always valid and promising, it is the culture that has changed for the better.
It is well established that aging is associated with structural and functional renal changes. With the possible exception of the lung, the changes in kidney function with normal aging are the most dramatic of any human organ or organ system. The normal kidney loses about 25% of its mass during aging, with the loss involving both cortical glomeruli and tubules. Functionally, the aging kidney has a parallel decline in both glomerular and tubular function. The Baltimore longitudinal study demonstrated an average of 0.75 mL/min/year decline in glomerular filtration rate (GFR) in 254 men without hypertension or kidney disease. The GFR loss rate is tripled in subjects over 40 as compared with those under 40.
Cellular senescence describes an everlasting growth arrest of still viable and metabolically active cells. The cell-cycle regulators and tumor suppressors p16Ink4a and p19ARF are involved in cellular senescence. The expression of p16 Ink4a in the kidney has been known to increase with age and could be found in a variety of renal cell types. Renal p16Ink4a expression has been suggested as an ideal marker for renal aging and shown to foresee transplant outcome. In normal human glomerular, p16Ink4a expression is increased with age and in all resident cell types. Studies in a transgenic mouse model confirmed that ablating p16Ink4a positive senescent cells not only prolongs the lifespan, but also attenuates glomerulosclerosis in aging kidney and decreasse blood urea nitrogen levels. Furthermore, depletion of p16Ink4a resulted in reduction of renal interstitial fibrosis and nephron atrophy in mice after ischemia-reperfusion injury, indicating inhibition of senescence provides a protective effect on the development of fibrosis.
Considering the increase of the aging population, it is extremely urgent to identify a way to retard the aging process or rejuvenate the community. To test the effects of young blood on aged organ, young blood infusion or parabiosis may be used. Parabiosis is an experimental model aiming to join the circulatory system of two animals. Heterochronic parabiosis is used to connect an aged partner to a young partner, and can be used to demonstrate the effects of young blood on aged organs, and vice versa. With this model, rejuvenation in the aged heterochronic parabiont has been shown in different organs such as muscle, liver, brain, and heart.
In aged kidneys, a recent study showed that young blood environment enhances the autophagy of aged kidney through down-regulation of aging-related protein p16Ink4a and SA-β-gal, up-regulation of autophagy factors Atg5 and LC3BII, and down-regulation of autophagic degradation protein p62. Moreover, recent studies provided evidence that young systemic milieu may alleviate renal ischemia-reperfusion injury in elderly mice probably through reduction of oxidative stress, inflammation, apoptosis, and enhancement of autophagy in the injured aged kidney. Although evidence showed that young blood can attenuate renal aging and injury induced by ischemia-reperfusion injury in elderly mice, it will be important to identify and study the effects of specific blood-borne rejuvenating factors in the young blood or aging factors in the old blood in addition to put efforts into delineate the mechanisms underlying the renal cell senescence. This information will provide novel ideas to turn back the clock of the aging kidneys.
Efficiently Reprogramming Fibroblasts into Cardiomyocytes for Heart Regeneration
The heart regenerates only very poorly, and responds to injury by producing scar tissue, a process that involves fibroblast cells. Additionally, the age-related disruption of regenerative processes produced by senescent cells and chronic inflammation tends to empower fibroblasts to produce fibrosis in the heart even in the absence of injury. One potential approach to the challenge of poor heart regeneration and growing fibrosis is to reprogram the fibroblasts of scar tissue into functional heart muscle cells, cardiomyocytes. Given recent demonstrations of in situ cell reprogramming, it is plausible to think that this can be accomplished. The challenge is to do so without disrupting the vital structural and electrical properties of heart tissue.
A heart attack leaves damaged scar tissue on the heart and limits its ability to beat efficiently. But what if scientists could reprogram scar tissue cells called fibroblasts into healthy heart muscle cells called cardiomyocytes? Researchers have made great strides on this front with lab experiments and research in mice, but human cardiac reprogramming has remained a great challenge. Now, for the first time, researchers have developed a stable, reproducible, minimalistic platform to reprogram human fibroblast cells into cardiomyocytes.
The researchers introduced a cocktail of three genes – Mef2c, Gata4, and Tbx5 – to human cardiac fibroblast cells with a specific optimized dose. To increase efficiency, they performed a screen of supplementary factors and identified MIR-133, a small RNA molecule that when added to the three-gene cocktail – and with further in-culture modifications – reprogrammed human cardiac fibroblast cells into cardiomyocytes at an efficiency rate of 40 to 60 percent.
Analysis identified a critical point during the reprogramming process when a cell has to “decide” between progressing into a cardiomyocyte or regressing to their previous fibroblast cell fate. Once that process begins, a suite of signaling molecules and proteins launch the cells onto different molecular routes that dictate their cell type development. The researchers also created a unique cell fate index to quantitatively assess the progress of reprogramming. Using this index, they determined that human cardiac reprogramming progresses at a much slower pace than that of the previously well-described mouse reprogramming, revealing key differences across species and reprogramming conditions.
Amyloid-β Causes Pericyte Dysfunction and Reduced Blood Flow in the Aging Brain
The brain is an energy-hungry organ, and the supply of oxygen and nutrients to brain tissue is vital to its function. This is one of the reasons why cardiovascular disease contributes to neurodegeneration. Researchers know that cells that wrap small blood vessels in the brain, called pericytes, tend to become dysfunctional or die in later life, another of the cellular casualties of the damage of aging. This causes greater constriction of the blood vessels, reducing the blood flow to tissues. Researchers here provide evidence for this to be a consequence of the aggregation of amyloid-β, characteristic of the early stages of Alzheimer’s disease. This is an intriguing addition to what is known of the issues caused by protein aggregation in neurodegenerative conditions.
A new study looked at the role of pericytes, cells wrapped around capillaries that have the ability to contract and regulate blood flow. Researchers examined capillaries in Alzheimer’s-affected human brain tissue and in mice bred to develop Alzheimer’s pathology, and found that they were squeezed by pericytes. They also applied amyloid beta protein (which accumulates in the brains of people with Alzheimer’s) to slices of healthy brain tissue, and found that the capillaries were squeezed as a result. They calculated that the constriction was severe enough to halve blood flow, which is comparable to the decrease in blood flow found in parts of the brain affected by Alzheimer’s.
“Our study has, for the first time, identified the underlying mechanism behind the reduction of brain blood flow in Alzheimer’s disease. Since reduced blood flow is the first clinically detectable sign of Alzheimer’s, our research generates new leads for possible treatments in the early phase of the disease. Damage to synapses and neurons in Alzheimer’s is usually attributed to the actions of amyloid and tau proteins accumulating in the brain. Our research raises the question of what fraction of the damage is a consequence of the decrease in energy supply that amyloid produces by constricting the brain’s finer blood vessels. In clinical trials, drugs that clear amyloid beta from the brain have not succeeded in slowing mental decline at a relatively late phase of the disease. We now have a new avenue for therapies intervening at an earlier stage.”
Suggesting that Cytomegalovirus Infection Contributes to Metabolic Syndrome
Metabolic syndrome is the precursor to type 2 diabetes, and is caused by the presence of excess visceral fat tissue. Age is a factor, however, in that people become more susceptible to the harmful consequences of being overweight in later life. Why is this the case? Recent evidence points towards the creation of additional lingering senescent cells as an important mechanism linking fat tissue to chronic inflammation and disruption of metabolism. Cellular senescence is an age-related mechanism.
The open access paper here proposes an intriguing additional process relating to persistent cytomegalovirus (CMV) infection. CMV is very prevalent, and the consensus on its effects is that its presence is an important contribution to the decline of the immune system with advancing age. Too many immune cells become specialized to tackling CMV, leaving too few for other tasks. In the context in which the supply of new immune cells is greatly diminished, due to loss of thymic tissue, and declining activity of hematopoietic stem cells, this is a big problem.
Cytomegalovirus (CMV) is a ubiquitous herpesvirus infecting most of the world’s population. CMV has been rigorously investigated for its impact on lifelong immunity and potential complications arising from lifelong infection. A rigorous adaptive immune response mounts during progression of CMV infection from acute to latent states. CD8 T cells, in large part, drive this response and have very clearly been demonstrated to take up residence in the salivary gland and lungs of infected mice during latency. However, the role of tissue resident CD8 T cells as an ongoing defense mechanism against CMV has not been studied in other anatomical locations.
Therefore, we sought to identify additional locations of anti-CMV T cell residency and the physiological consequences of such a response. Through RT-qPCR we found that mouse CMV (mCMV) infected the visceral adipose tissue and that this resulted in an expansion of leukocytes in situ. We further found, through flow cytometry, that adipose tissue became enriched in cytotoxic CD8 T cells that are specific for mCMV antigens from day 7 post infection through the lifespan of an infected animal and that carry markers of tissue residence. Furthermore, we found that inflammatory cytokines are elevated alongside the expansion of CD8 T cells. Finally, we show a correlation between the inflammatory state of adipose tissue in response to mCMV infection and the development of hyperglycemia in mice.
Overall, this study identifies adipose tissue as a location of viral infection leading to a sustained and lifelong adaptive immune response mediated by CD8 T cells that correlates with hyperglycemia. This data potentially provides a mechanistic link between metabolic syndrome and chronic infection.
A Better Understanding of the Mechanisms Surrounding Thymic Involution
Researchers here report on their exploration of the protein interactions involved in the loss of active thymic tissue with age, a process called thymic involution. Since the thymus is where T cells mature, this loss contributes to the age-related decline of the adaptive immune system. Historically, this sort of investigation has focused on FOXN1 as the master regulator of thymic growth and activity. Upstream of FOXN1 is BMP4, however, and the paper here discusses the ways in which BMP4 is dysregulated with age. This discussion should probably be read in the context of other work that strongly suggests chronic inflammation is the driver of changes leading to thymic involution. Nothing happens in isolation in aging tissues, and there are usually deeper causes to be considered.
Thymic epithelial cells (TECs) are essential for the establishment of the specialized microenvironment that orchestrates the development of naive, self-tolerant T cells from hematopoietic precursors. They are supported by non-epithelial thymic stromal cells (TSCs), such as fibroblasts and endothelial cells, in an extracellular matrix-rich three-dimensional (3D) scaffold structure. TECs can be broadly divided into functionally and spatially distinct cortical (cTEC) and medullary (mTEC) subsets. Thymic epithelial progenitor cells (TEPC) support the development of both cTECs and mTECs during thymus organogenesis.
Deterioration of thymus function occurs naturally during aging and ultimately constrains the host immune repertoire. It is characterized by a reduction in total thymic cellularity and naive T cell production. Reduced TEC turnover and diminished levels of transcription factor forkhead-box N1 (FOXN1), a master regulator of TEC lineage specification, have been observed in the aged thymus. An increase in steroidal hormone production at puberty has also been implicated in age-related thymus involution, with androgen deprivation (AD) inducing the recovery of naive T cell production and bone marrow function in aged male mice and in humans. However, the mechanisms and signaling pathways causing the post-pubertal loss of specific TECs and underpinning AD-induced thymocyte regeneration remain unclear.
In this study, we examined numeric, phenotypic, and transcriptomic alterations in TEC and non-TEC (non-epithelial stromal cells, fibroblasts, and endothelial cells) stromal subsets during age-related thymic involution, and following transient thymic recovery via AD. We identify two major phases of thymic epithelial cell (TEC) loss during aging: a block in mature TEC differentiation from the pool of immature precursors, occurring at the onset of puberty, followed by impaired TEC progenitor differentiation and depletion of cTEC and mTEC lineage-specific precursors. We reveal that an increase in follistatin production by aging TECs contributes to their own demise. TEC loss occurs primarily through the antagonism of activin A signaling, which we show is required for TEC maturation and acts in dissonance to BMP4, which promotes the maintenance of TEC progenitors. These results support a model in which an imbalance of activin A and BMP4 signaling underpins the degeneration of postnatal TEC maintenance during aging, and its reversal enables the transient replenishment of mature TECs.
Temporary β-catenin Inhibition Attenuates Effects of Aging on Bone Regeneration
For all that it reports a success, the open access paper here is an excellent example of the prevalent, inferior approach to the development of therapies for age-related conditions. Instead of looking for causes of the problem in question, the slow and dysfunction regeneration of bone fractures in older individuals, the scientists find a way to tinker with the dysfunctional state of aged metabolism, overriding one of the detrimental regulatory changes to some degree. As a strategy this will always be far less effective than tackling the underlying root causes of age-related dysfunction: trying to tinker a damaged engine into continued operation without fixing the damage is always going to be a challenging task. Nonetheless, this strategy remains much more popular in the research community, more is the pity.
Almost a third of humans will fracture a bone, and most often these injuries go on to successfully heal. However, various environmental and biological factors can hinder this regenerative process. With age, the pace of fracture repair slows, and the risk of non-union increases. This slower pace of repair in older individuals is responsible for increased morbidity and even mortality.
While there are many factors that could impair fracture healing with aging, the pace of fracture repair can be rejuvenated by circulating factors present in young animals. Recent data suggests that factors produced by young macrophage cells pay a critical role in the rejuvenation process. While these factors regulate the pace of repair, their effects on mesenchymal cells differentiating to osteoblasts are mediated by signaling pathways such as β-catenin.
β-catenin signaling plays different roles in mesenchymal differentiation at different repair stages. In the initial phase of repair, the level of β-catenin needs to be precisely regulated as levels that are too high or too low will prevent undifferentiated mesenchymal cells from becoming osteochondral progenitors and will inhibit fracture healing. Once the cells differentiate to osteochondral progenitors, higher levels of β-catenin stimulate osteogenesis and enhance fracture repair.
Here we examined the ability of pharmacologic agents that target β-catenin to improve the quality of fracture repair in old mice. 20 month old mice were treated with Nefopam or the tankyrase inhibitor XAV939 after a tibia fracture. Fractures were examined 21 days later by micro-CT and histology, and 28 days later using mechanical testing. Daily treatment with Nefopam for three or seven days but not ten days improved the amount of bone present at the fracture site, inhibited β-catenin protein level, and increased colony forming units osteoblastic from bone marrow cells. This data supports the notion that high levels of β-catenin in the early phase of fracture healing in old animals slows osteogenesis, and suggests a pharmacologic approach that targets β-catenin to improve fracture repair in the elderly.
A Discussion of Mitochondrial DNA Damage and Aging
Every cell in the body contains a swarm of mitochondria, responsible for packaging the chemical energy store molecule ATP that is used to power cellular processes. Mitochondria are the distant descendants of symbiotic bacteria, and retain a remnant of the original DNA. This mitochondrial DNA is unfortunately less well protected and maintained than DNA in the cell nucleus. It is thought that a sizable fraction of the declines of aging are caused by accumulated damage to mitochondrial DNA, coupled with progressive failure of the quality control mechanisms responsible for recycling dysfunctional mitochondria. Reducing the functional capacity of cells via a reduction in available energy can produce all sorts of detrimental effects, and while much of the relevant research is focused on the energy hungry tissues of brain and muscle, this is a problem in all tissues.
The mitochondrial organelle, a double-membraned organelle with its evolutionary origins in the eubacterial kingdom, is the central factor in the energy metabolism of the eukaryotic cell. It is responsible for the vast majority of the ATP (adenosine triphosphate) produced in the cell (~90% under normal circumstances), which it produces through oxidative phosphorylation (OXPHOS) by way of a multi-subunit complex called the electron transport chain (ETC). The mitochondrion possesses its own small, circular genome (mtDNA) that encodes key RNA and proteins required for OXPHOS.
The mtDNA mutation rate depends on many factors, including the extent of oxidative stress and the fidelity of the mitochondrial DNA polymerase (POLG). The production of reactive oxygen species (ROS) is an inevitable outcome of the oxidative phosphorylation process that occurs within the mitochondrion, and these chemical byproducts are, by their nature, damaging to DNA. Thus, given its proximity to the source of ROS production, mtDNA experiences a high rate of ROS-induced mutation. ROS production is also increased by pre-existing mtDNA damage, excess calories, regional mtDNA genetic variations, and alterations in nuclear DNA expression of stress response genes, creating a vicious cycle where increased ROS production can encourage the occurrence of even more ROS production over time.
There is a well-established correlation between aging and a decline in mitochondrial function, which likely contributes to age-related senescence and geriatric disease. PolgD257A “mutator” mice – which exhibit increased mtDNA mutation rates – show an accelerated aging phenotype, suggesting that the accumulation of mtDNA mutations over time may be a crucial factor driving the aging of mammals. The age-related decline in mitochondrial function is likely caused, in large part, by the gradual accumulation of somatic mtDNA mutations due to ROS damage and DNA replication errors. This accumulation of mtDNA mutations promotes even more ROS production, establishing a vicious cycle and accelerating the aging process. Although a serious mtDNA mutation can be acquired early in life, for most people it will take several decades to acquire one or more disease-causing mutations and have them reach a sufficiently high level of heteroplasmy to cause serious health issues. This gradual accumulation of mutations and increase in heteroplasmy may help explain the time-dependent decline in function that occurs with age.
Somatically-acquired mtDNA mutations can occur anywhere in mtDNA and may even include large deletions or duplications. Loss of mtDNA integrity (by altered mtDNA copy number or increased mutations) has been implicated in cellular dysfunction with aging. Deletion mutations are an insidious risk because the reduced size of mtDNA molecules carrying large deletions (ΔmtDNAs) gives them a replicative advantage over normal mtDNA.
Any mtDNA molecules containing a deleterious mutation in a coding gene, combined with a D-loop mutation that creates a strong proliferative advantage, would have the potential to create severe issues in old age, particularly if they cause the cell to take on neoplastic properties. Furthermore, this ubiquitous and steadily accumulating mutational load in the mtDNA population will likely create general problems for the ETC function and mitochondrial health, thus contributing to age-related senescence. For these reasons, we believe there is great merit in the argument that mitochondrial defects play a significant role in the development of many common diseases of age. We hypothesize that the diversity of mutations in mtDNA could be decisive for the variability of clinical phenotypes, such as age of disease onset.
Higher Protein Intake Correlates with Lower Risk of Frailty in Old Age
Physical weakness is a sizable component of age-related frailty. The loss of muscle mass and strength that accompanies aging, known as
sarcopenia, has many potential contributing causes, with varying degrees of accompanying evidence. One of these is dietary, a lower protein intake in older individuals, and dysfunction in processing of amino acids such as leucine. That this correlates with frailty, as illustrated here, doesn’t necessarily mean that it is an important cause, however. The alternative is that the factors that lead to reduced dietary intake in later life are distinct issues that arise from similar root causes to those of sarcopenia: consider something as simple as increased difficulty when swallowing, for example. When it comes to mechanisms, there is better evidence for chronic inflammation produced by cellular senescence or declining stem cell function to be primary causes in sarcopenia, and these are not particularly related to dietary protein intake.
Frailty can be defined as a state of augmented sensitivity and vulnerability to external stressors in old age. The Fried frailty phenotype classifies frailty as the presence of three or more of the following five components: weakness, slowness, low physical activity, exhaustion, and weight loss, and prefrailty as the presence of one or two of the Fried phenotype criteria. In a recent systematic review carried out in 61,500 individuals aged 65 and older, the overall prevalence of frailty was estimated to be 10.7%, and 41.6% were prefrail with one or two components of Fried frailty phenotype.
There is growing support for the concept that greater protein intake may preserve physical function in older adults. The anabolic response to amino acid intake may be blunted in older people, particularly if they have low intakes of protein. In addition, animal protein intake may be associated with muscle strength in older adults, which may be associated with a lower risk of frailty; whereas, plant-based protein sources may have limited potential to stimulate the skeletal muscle anabolic response. The exact reasons are not understood but it might be that plant proteins are considered to have a lower content of essential amino acids compared to animal protein sources. Nevertheless, knowledge regarding the association between adequate protein intakes, according to recommendations, and sources of protein intake with frailty are limited.
We hypothesized that the prevalence of frailty and prefrailty was lower among older women consuming ≥ 1.1 protein/kg body weight (BW) compared to those with lower intakes. Participants were 440 women aged 65─72 years enrolled in the Osteoporosis Risk Factor and Prevention-Fracture Prevention Study. Protein intake g/kg BW and g/d was calculated using a 3-day food record at baseline. At the 3-year follow-up, frailty phenotype was defined as the presence of three or more, and prefrailty as the presence of one or two, of the Fried criteria. At the 3-year follow-up, 36 women were frail and 206 women were prefrail. Higher protein intake ≥ 1.1 g/kg BW was associated with a lower likelihood of prefrailty (odds ratio = 0.45) and frailty (odds ratio = 0.09) when compared to protein intake of less than 1.1 g/kg BW at the 3-year follow-up. Women in the higher tertile of animal protein intake, but not plant protein, had a lower prevalence of frailty. Thus protein intake ≥ 1.1 g/kg BW and higher intake of animal protein may be beneficial to prevent the onset of frailty in older women.
Mitochondrial Oxidative Stress in Neurodegenerative Disease
Aging is characterized by increasing dysfunction in mitochondria, the power plants of the cell, responsible for packaging chemical energy store molecules, ATP, to power cellular operations. Mitochondrial decline in aging is most studied in energy hungry tissues such as the muscles and brain, and it is widely accepted that mitochondrial dysfunction is an important feature of neurodegenerative diseases. Mitochondrial dysfunction is likely caused by damage to mitochondrial DNA and loss of quality control mechanisms responsible for destroying malfunctioning mitochondria. It manifests as a reduced supply of ATP and increased generation of oxidative molecules.
Oxidative reactions that disrupt cellular machinery occur constantly even in youth, and cells possess many antioxidant and repair mechanisms to keep this damage under control. Oxidative damage is even used as necessary signaling in processes such as the beneficial response to exercise. Consistently raised amounts of oxidative molecules are harmful in many ways, however, not just damaging internal cellular operations, but also producing downstream issues such as altered forms of cholesterol that contribute to the progression of atherosclerosis.
Aging is the primary risk factor for a number of human diseases, as well as neurodegenerative disorders. A growing body of evidence highlights bioenergetic impairments as well as alterations in the reduction-oxidation (redox) homeostasis in the brain with the increasing of the age. The brain is composed by highly differentiated cells that populate different anatomical regions and requires about 20% of body basal oxygen for its functions. Thus, it is not surprising that alterations in brain energy metabolisms lead to neurodegeneration.
Cellular energy is mainly produced via oxidative phosphorylation taking place within mitochondria, which are crucial organelles for numerous cellular processes, such as energy metabolism, calcium homeostasis, lipid biosynthesis, and apoptosis. Glucose oxidation is the most relevant source of energy in the brain, because of its high rate of ATP generation needed to maintain neuronal energy demands. Thus, neurons rely almost exclusively on the mitochondria, which produce the energy required for most of the cellular processes, including synaptic plasticity and neurotransmitter synthesis.
Reactive oxygen species (ROS) are normally produced in the cell of living organisms as a result of normal cellular metabolism and are fundamental in the maintenance of cellular homeostasis. When an imbalance between ROS production and detoxification occurs, ROS production may overwhelm antioxidant defenses, leading to the generation of a noxious condition called oxidative stress and overall to the impairment of the cellular functions. This phenomenon is observed in many pathological cases involving mitochondrial dysfunction, as well as in aging. The brain is particularly vulnerable to oxidative stress and damage, because of its high oxygen consumption, low antioxidants defenses, and high content of polyunsaturated fats very prone to be oxidized.
Mitochondrial dysfunction is one of the main features of the aging process, particularly in organs requiring a high-energy source such as the heart, muscles, brain, or liver. Although a large amount of data support the role of mitochondrial ROS production in aging, it has also recently been demonstrated the involvement of the mitochondrial permeability transition in the mechanisms of aging. The age-associated decrease in mitochondrial membrane potential correlated with reduced ATP synthesis in tissues of old animals. The mitochondrial permeability transition is due to a nonspecific pore called the mitochondrial permeability transition pore (mPTP) occurring when mitochondria become overloaded with calcium. Indeed, it is well known that aging alters cytosolic calcium pick-up and the sensitivity of the mPTP to calcium enhanced under oxidative stress conditions.
Neurons are postmitotic highly differentiated cells with a lifespan similar to that of the whole organism. These excitable cells are more sensitive to the accumulation of oxidative damages compared to dividing cells and are more prone to accumulating defective mitochondria during aging. Thus, it is not surprising the importance of protecting systems, including antioxidant defenses, to maintain neuronal integrity and survival. All the neurodegenerative disorders share several common features, such as the accumulation of abnormally aggregated proteins and the involvement of oxidative damage and mitochondrial dysfunction. Many of the genes associated with Parkinson’s disease or ALS are linked to mitochondria. In addition, all aggregated misfolded proteins (β-amyloid, tau, and α-synuclein) are known to inhibit mitochondrial function and induce oxidative stress. Therefore, the identification of common mechanisms underlying neurodegenerative diseases, including mitochondrial dysfunction, will increase our understanding of the essential requirements for neuronal survival that can inform future neuroprotective therapies.