Are Mitochondrial Mutations Really All That Important?
Prompted by attention given to a recent study claiming to cast doubt on the primary role of damaged mitochondria in aging, here is a lengthy and detailed article from the SENS Research Foundation on what is known of mitochondrial DNA damage and aging. It is worth bearing in mind when reading the scientific literature that any single study, especially if claiming to overthrow the consensus, should always be weighed against the rest of the recent literature in a given field:
The study was of fibroblasts, which are a kind of skin cell. It is interesting and contributes to a long-standing debate in this field about the frequency of specific mitochondrial DNA mutations with age and tissue type, and whether they contribute to specific diseases. It is clear at this point that mitochondrial dysfunction occurs with age and that damage in the form of mutations to mitochondria contributes to the diseases and disabilities of aging. We don't believe that this particular study is actually a challenge to scientists' existing understanding about how changes in mitochondria with age both drive and are driven by cellular and molecular damage, and the diseases and disabilities of aging.What is actually known about the frequency and impact of specifically age-related mitochondrial mutations? First, in line with the ability of dividing cells to dilute out structural damage, multiple studies in aging rodents and humans report that the mutations in mitochondria that persist in cells and thus accumulate with age are confined almost entirely to cell types that don't divide during adulthood (e.g., brain neurons, heart muscle cells, and skeletal muscle). Second, those mutations are quite surprisingly rare: even in tissues that are actually affected by mitochondrial mutations with age, fewer than 1% - and perhaps as few as 0.1% - of cells are found to be affected.
Still, the evidence suggesting that this damage drives degenerative aging is powerful. The level of oxidative damage to mitochondrial DNA, the rate of accumulation of mitochondrial DNA mutations with age, and the structural vulnerability to such mutations are collectively robustly correlated with species maximum lifespan (the strongest integrative measure of the overall rate of aging in a species). Remarkably, this has recently been demonstrated even in rockfish, whose senescence is nearly negligible: lifespan in rockfish species was found to correlate negatively with the rate of mutation of their mitochondrial, but not nuclear, genomes - a relationship that the investigators' analysis suggested was not likely to be an artifact of tradeoffs with fecundity or the rate of germline DNA replication.
Calorie restriction (the most robust intervention that slows the rate of aging in mammals) lowers the rate of accumulation of mitochondrial deletion mutations with age. And when mice are given a transgene that directs a form of the antioxidant catalase directly to their mitochondria - an enzyme that complements the existing antioxidant machinery in the mitochondria in a way that reduces total mitochondrial DNA oxidative damage, including but not limited to deletion mutations - it extends their mean and maximal lifespan and ameliorates multiple pathologies of aging. Yet no such effects are observed when the same enzyme is directed to sites outside of the mitochondria, or when other antioxidant enzymes are expressed elsewhere in the cell, or even when non-complementary enzymes are sent to the mitochondria.
The apparent paradox in all of this is the strong link between mitochondrial DNA deletions and the rate of degenerative aging in the face of the rarity of such mutations. There are two broad kinds of resolution to this paradox. The first is the tissue-specific one. Although cells overtaken by mitochondria bearing DNA deletions are rare, they can have powerful effects on health in tissues where they are unusually enriched in critical cell types, particularly if relatively few of those cells exist in the first place. Such is the case for the key dopamine-producing neurons in an area of the brain known as the substantia nigra pars compacta (SNc). SNc dopaminergic neurons are much more vulnerable to being overtaken by mitochondria bearing large deletions in their DNA than are other cell types in the brain, and such mutations clearly drive dysfunction, including being tightly liked to Parkinson's disease. The same high regional vulnerability to mitochondrial DNA deletions occurs in people suffering with non-Parkinson movement disorders and even in "normal" aging brains, albeit at a lower rate and yet the finding has no parallel in the smaller and less harmful point mutations.
The other kind of tissue-specific effect relates more to the unique properties of the affected cell type itself, with the cardinal case in this category being skeletal muscle. Unlike most cell types, skeletal muscle "cells" are not isolated from all of their neighbors by a membrane. Instead, the long stretches of skeletal muscle fibers are comprised of multiple segments, each of which contains its own nucleus, which is in turn supported by a local population of mitochondria, with additional mitochondria in the membrane-bound space outside the fiber itself. Mitochondrial DNA deletions not only accumulate with age at a faster pace in skeletal muscle than in many other aging tissues, but because of that structure their effects are much more catastrophic. When a local nucleus' mitochondrial population is overtaken by deletion mutations, the segment first atrophies at that point, and then fails, leading the fiber to split or break locally and ultimately causing the loss of the entire fiber. These processes - loss of energy production and the splitting and loss of fibers - are a key driver of sarcopenia, the age-related loss of skeletal muscle mass and function that occurs even in lifelong master athletes.
Because deletion mutations in mitochondrial DNA are core molecular lesions driving these diseases, repair of these mutations will be central to their prevention, arrest, and reversal. But you can't tell that from a study of skin cells.
Does the fact that muscle fibers disappear with age as a result of mitochondrial degradation a fact or a theory? Many processes could contribute to the loss of fibres, including disuse. We know that older men (80+) who take hormones can maintain large muscles...
We have recently learned that a simple hormone (oxytocine) could rejuvinate muscle cells (in mice). For all we know, such in vivo renewal could prevent and maybe reverse sarcopenia. Is oxytocine related to mitochondrial damage and if so, how?
Myostatin gene therapy also rejuvenates muscle so there are multiple routes to the problem.
That is a very well written article by Michael Rae on a convoluted topic. I can't leave a comment on the SENSRF website, but just wanted to say well done for taking the time to write it.
The prior article linked to is also very good:
http://sens.org/research/research-blog/nothin-gonna-hold-me-back-clearance-senescent-cells-tissue-rejuvenation
One question I have is that in one of the recent news articles on the SENRF's work on allotopic expression of Mitochondria there was a throwaway comment that "the cells with the mitochondrial genes epxressed in the nucleus only grow about half as fast as regular cells without a mitochondrial gene deletion in the same circumstances". I'm assuming these circumstances are feeding the cells on an energy source that can only be broken down with the gene in question.
Do you know anything about this Reason? Is it a potential roadblock?
I can't find the news article video, although it is not the recent BBC Horizons video.
Daniel Lemire wrote: Does the fact that muscle fibers disappear with age as a result of mitochondrial degradation a fact or a theory?
It's a fact: Judd Aiken, in particular, has shown it very elegantly with a series of painstaking (literally and figuratively) microdissection studies in CR and AL rats, then nonhuman primates, and recently aging male humans (see citations in the article).
Daniel Lemire wrote: Many processes could contribute to the loss of fibres, including disuse. We know that older men (80+) who take hormones can maintain large muscles...
We also know that the larger muscles of older, hormone-taking men are disproportionately weak, with some men showing no strength gains despite an increase in lean mass. It's important to understand that studies in exercising rats and in lifelong human master athletes show that things like exercise and hormones promote hypertrophy of the surviving fibers: they do little or nothing to sustain existing fibers, and cannot replace fibers once they are severed.
Daniel Lemire wrote: We have recently learned that a simple hormone (oxytocine) could rejuvinate muscle cells (in mice). For all we know, such in vivo renewal could prevent and maybe reverse sarcopenia. Is oxytocine related to mitochondrial damage and if so, how?
I know that it's hard to keep track of all of this, but there's a lot of different things wrong with old muscle tissue, including degeneration of neuromuscular junctions, mitochondrial impairments, infiltration and replacement of muscle tissue with adipose and collagen, etc. The fiber loss is driven largely by mitochondrial DNA deletions (along with acute injury, which of course isn't part of aging per se but still adds to the burden of damage to the muscles over the lifetime). When Conboy's lab reports that parabiosis or oxytocin restores youthful muscle "regeneration" of the muscle, they're talking about one specific process: the ability of aging satellite cells (muscle tissue-specific stem cells) to mobilize out of the niche in response to injury. This provides fresh myonuclei to reinforce the surviving, damaged fibers — but it doesn't re-grow the animal a whole new skeletal muscle fiber after it's been severed. There are no interventions that do that, and only CR is known to slow down the rate at which muscle fibers are lost with age (this being, evidently, due to slowing the rate of accumulation of mitochondrial DNA deletions within the muscle fiber segements). As with most kinds of cell loss in the brain, we just can't look to endogenous repair systems to fix this: we must perforce introduce new fibers exogenously using cell therapy.
Steve H wrote: Myostatin gene therapy also rejuvenates muscle so there are multiple routes to the problem.
Myostatin does not rejuvenate aging muscle: it merely increases mass, in a way that is greater in magnitude but no better than and possibly not as good as hormone therapy. Even in young, healthy animals, inhibition of myostatin leads to bigger but proportionately weaker muscles, apparently in part because it causes myocyte hypertrophy but fails to recruit myoblasts, leading to muscles that can't be fully recruited to generate force. When you then consider doing this to biologically aged people with sarcopenia or its beginnings, you have to remember all of the additional structural damage that aging muscles suffer. As a result, old muscle, like muscle grown through myostatin inhibition, is disproportionately weak to its mass compared to young muscle. Just adding intrinsically dysfunctional mass onto a degenerating foundation is not a good solution.
Jim wrote: in one of the recent news articles on the SENRF's work on allotopic expression of Mitochondria there was a throwaway comment that "the cells with the mitochondrial genes epxressed in the nucleus only grow about half as fast as regular cells without a mitochondrial gene deletion in the same circumstances". I'm assuming these circumstances are feeding the cells on an energy source that can only be broken down with the gene in question.
The statement is accurate, certainly with our allotopically-expressed (AE) genes to date, and you're right to infer that this is on a medium with a fuel source (glucose) that requires the gene in question (because it requires a functioning electron transport chain (ETS), which in turn requires having all of the individual subunits of the chain to be synthesized, including the AE genes as backups for the mutated native mitochondrial one).
It's reasonable to expect that as we refine our protocols, we'll do a better job of restoring ETS activity, and that future iterations of AE will do even better.
However, it's important to understand that we don't need to fully restore energy production in these cells to prevent and reverse most of the effects of mitochondrial mutations, especially during the first few decades of "escape velocity" following the availability of the first comprehensive panel of rejuvenation biotechnologies. In skeletal muscle, maintaining any amount of ETS activity should prevent the clonal expansion of mutant mitochondria from the fiber segment where they originate into neighboring segments, and thereby the eventual breakage of the fiber. In the SNc, AE should at least prevent the actual death of dopaminergic neurons, which will allow some ongoing SNc-to-striatum dopaminergic innervation and regulated feedback while cell therapy for PD is still using relatively crude intra-striatal protocols; meanwhile, it will keep the existing circuitry intact and ready to accommodate reinforcement by more advanced, future orthotopic cell transplant protocols. And in these locations and everywhere else, maintaining even low levels of ETS activity should abrogate the entry of cells that have been taken over by deletion-bearing mitochondria into the abnormal metabolic state that then (in the scenario proposed in Dr. de Grey's thesis) propagates oxidative stress throughout the rest of the body and leads to aberrant mitochondrial function even in cells with intact mitochondrial genomes.
(Correction: the fuel source we use to force the cells to rely on the ETS is galactose, not glucose).
Thanks for taking the time for some excellent replies Michael. I really hope you guys' allotopic expression of mitochondrial genes comes through in a mouse model soonish.
I also really hope that Judith Campisi, Tamara Tchkonia and James L. Kirkland figure out how to ablate senescent preadipocytes and reverse or partially reverse type 2 diabetes. That would be an amazing breakthrough.
"Myostatin does not rejuvenate aging muscle: it merely increases mass, in a way that is greater in magnitude but no better than and possibly not as good as hormone therapy ..... As a result, old muscle, like muscle grown through myostatin inhibition, is disproportionately weak to its mass compared to young muscle. Just adding intrinsically dysfunctional mass onto a degenerating foundation is not a good solution."
See here:
http://www.acceleronpharma.com/products/ace-083/
they say:
"ACE-083 is an investigational protein therapeutic that has been designed for local administration to increase muscle mass and strength in specific muscles and muscle groups. Acceleron is developing ACE-083 for diseases in which improved muscle strength may provide a clinical benefit, such as inclusion body myositis and certain forms of muscular dystrophy."
... "can increase muscle mass and strength" ... seems to be a soluble myostatin receptor.
Steve H wrote: Myostatin gene therapy also rejuvenates muscle so there are multiple routes to the problem.
Michael wrote: Myostatin does not rejuvenate aging muscle [...]
(First, a slight correction: I mistakenly echoed Steve's misstatement back to him: we meant, of course, inhibition of myostatin).
Adrian Crisan wrote: "ACE-083 is an investigational protein therapeutic that has been designed for local administration to increase muscle mass and strength [... and] may provide a clinical benefit [in diseases] such as inclusion body myositis and certain forms of muscular dystrophy.
So, first, notice that they're talking about its use in IBM and some forms of muscular dystrophy, and that it may be beneficial in some muscle groups. Even if it's shown to be effective in these disorders, that doesn't mean that it's going to be useful in the muscle of people with muscle degeneration driven by degenerative aging.
Second, even if it does improve muscle mass or strength, that doesn't rebut my statement: if you add enough poor-quality muscle, you're eventually going to get a net increase in strength. However, that isn't the same thing as adding in more fully-functional, youthful muscle, nor of undoing all the other damage wrought in the existing muscle tissue.
There's a similar case with bisphosphonates for osteoporosis: yes, you increase bone mass, and yes, you reduce the risk of at least some fractures for at least five years in patients with outright osteoporosis However, it lowers the quality of the bone, because it works by inhibiting the turnover of old, damaged, low-quality bone. The result is that the benefit is not observed in people who only have osteopenia (who still usually have more youthful, intact bone and are at lower risk in the first place), and not only does the risk reduction disappear over the course of ten years, but you actually get an increase in the risk of "atypical" fractures, such as femoral shaft, subtrochanteric, atypical femur, and possibly diaphyseal fractures.
We should not be jumping after agents that add more poor-quality muscle into our remaining old, degenerating tissue: our mission is nothing less than rejuvenation.