It is Quite Possible to Create a Senescent Cell Clearance Therapy that is Too Good

Therapies for senescent cell clearance as a treatment for aging are going to be an ongoing concern within the next few years. Multiple different methods have been demonstrated to selectively kill senescent cells in mice, including the genetic engineering approach used a few years ago and the various senolytic drug candidates discovered more recently. These have variable effectiveness in different tissues, with some tissue types retaining all of their senescent cells, suggesting that no initial clinical treatment is going to be perfect. Even these prototypes are, however, clearing as much as a quarter of senescent cells in some tissues. In the case of senolytic drugs this is enough for old mice to display lasting health benefits after even a single treatment.

Why is the destruction of senescent cells an important goal? In short because cellular senescence is a contributing cause of aging. When damaged or faced with a toxic, stressed environment, cells tend to become senescent. A senescent cell stops replicating and secretes signals that both adjust the behavior of surrounding cells, making them more likely to become senescent, and make the senescent cell itself a target for destruction by the immune system. This is probably a defense against cancer, removing from play those cells most likely to become cancerous. Evolution likes reuse, and senescent cells are also transiently involved in wound healing and structural control over embryonic development. Nonetheless, having too many senescent cells is a bad thing, and that is exactly what happens with advancing age: senescent cells that evade destruction linger indefinitely, and their numbers grow over time, especially once the immune system starts to decline in old age. In large numbers senescent cells cause chronic inflammation and their collective signaling actively harms tissue structure and function. Their presence contributes to all of the common age-related diseases via these and a range of other, similar mechanisms. Periodic removal of senescent cells would solve all of these problems.

Senescent cell clearance treatments could be made much more efficient than the prototypes demonstrated so far in mice. That seems inevitable, based on some combination of innovation in delivery methods and innovation in kill mechanisms. We should always expect the first approaches to be weak in comparison to those that come later, with the benefit of more funding and attention. It is, however, quite possible for a therapy to be too good at killing senescent cells. Consider the study from some years back that showed as many as 20% of the skin cells in old baboons exhibited the signature for senescence. Now, what do you imagine would happen to you if 20% of the cells throughout your body died in a matter of a day? It wouldn't be pleasant. Clearing cells isn't a magical clean sweep of a process: a dead cell leaves behind debris and lot of frantic signaling in its last moments, and in volume that can be far worse than just leaving the cells alone. This is a well known problem in the cancer research community, a section of the medical establishment very focused on selectively killing cells. The condition that can result from having a significant number of cells die in a short period of time as the result of treatment is known as tumor lysis syndrome. At the mild end of the spectrum the outcome is sickness and metabolic dysregulation, while severe cases bring kidney failure and death, the systems of blood filtration utterly overwhelmed by a flood of cell debris and toxins.

Thus, naively, a hypothetical highly efficient senescent cell clearance therapy might work just fine in a 40-something adult, with tissues containing comparatively few senescent cells, while having a strong chance of killing patients in their 70s, with tissues containing many more senescent cells and also possessed of less resilient organs. Fortunately this issue is well understood in the research community, so no such highly efficient therapy is ever going to be produced. Approaches that could be this efficient in theory will be diluted or otherwise limited and delivered over a number of spaced treatments, producing a steady or stepped destruction of senescent cells at a safe pace. What that safe pace will turn out to be in humans is an open question, to be answered by experimentation, trials, and further studies, but the mouse data suggests it can be fairly rapid - just not all at once, immediately.

Comments

Yes, but it looks like cells reaching the Hayflick limit is important in Baboons whereas it is apparently not important in humans:

"For replicative senescence, the most important biomarker is telomere dysfunction-induced foci, or TIFs. Presence of these structures signals that the protective chromosome caps called telomeres have dwindled enough to halt cell division. ... What they found: The number of senescent cells increased exponentially with age. TIF-positive cells made up about 4 percent of the connective tissue cell population in 5-year-olds. In 30-year-olds, that number rose as high as 20 percent."

So maybe a 30 year old Baboon has higher numbers of senescent cells than a 90 year old human?

Posted by: Jim at January 11th, 2016 4:30 PM

@Jim

Hi Jim!
I believe senescent cell in humans has importance but mostly for disease initiation and progression. I'm not saying senescent cell is 'the be all end all' of diseases; but they do contribute greatly because they secrete the important factors that create this chronic inflammation.

Senescent cells are important for baboons but also humans; both mostly for diseases. While TIF is, as you quote, is replicative-based, less inducible-based. Since baboons have a shorter lifespan than the MLSP of a human, it would make sense that they are replicatively limited because their telomeres drop down further and created more TIFs faster; TIFs are telomere DNA damage and due to the problem of replication being an imperfect job. Our somatic cells don't have telomerase or very little; even then, as stated before, telomerase is shunted away from the nucleus (during oxidative stress) as we age (because oxidative stress rises during diseases that become prevalent/visible with age, but oxidative stress can rise too as the telomeres shortens (telomere shortening creates chromosomal unstability and dysfunction, leading to oxidative stress susceptibility increase). Cells reaching the replicative Hayflick limit is also important for humans because that is something no therapy has a solution for (Telomerase effect is unkown at this point and damage removal could be just disease post-poning rather actual age reversal (replicative senescence shows that the body is Highly Tuned to make Little damage and allow a human to reach 120 years; so what damages could we remove that would have such an impact on intrinsic aging ? AGEs, Carbonyls, Isoprostanes, Mitochondrial Lesions/Deletions ? How much are they 'impacting' in terms of weight; possibly not as much as we believe for intrinsic aging but for disease initiation/progession they matter a lot because they promote inflammation, a major culprit of rapid inducible senescence). Removing replicately senescent cells (late in life) could possibly solve that but the amount will be massive and as Reason, states dramatic removal could have dramatic fatal results; so this would need to be into stepped removal; even then, I think the amount of replicatively senescent cells will be too high - at that very late point in life - to go around and play in them through total removal - without creating dramatic problems in frail very aged or near dead bodies. This means, we must stop inducible senescence and replicative lifespan senescence right at young age; old people will be very hard to anything about. Only people at 40 or below will gain, sadly. But concentrating also on other health improvements therapies in elders is what is needed if inducible-senescence cell clearance (disease progression) is not working as planned.

Posted by: CANanonymity at January 11th, 2016 8:27 PM

@CANanonymity: I don't think it is anywhere near that dire. I'd expect even very old people can be near completely cleared of senescent cells given a decently efficient therapy, just that it must proceed more gradually over time than for their younger counterparts. I'm thinking the difference between a one-time treatment and a six month set of treatment, as an example of the likely range.

Posted by: Reason at January 11th, 2016 8:36 PM

@Reason

Hi Reason,

You are right , a very slow gradual thing could possibly make it with little side effects. It definitely will have to be gradual and with care to not have any unintended deadly side effect.

After rethinking it,
I think I will backtrack on saying there is no therapy that could twarth replicative senescence. Removing damages Will slow aging, it is assured, the effect may be milder than what we hoped (AGEs, protein carbamylation, protein carbonyls, mtDNA deletions, mtDNA lesions (8-oxo-dG), SSBs, DSBs, and the whole paraphernalia when are removed they make the animal lived longer lifespans; they have correlation but also causation; it's just they are so numerous in types, that - altogether - impact 'each' their 'small percentage' in aging) Flies that have AGEs or carbonyls reduced have increased MLSP, same goes for mice through calorie restriction that can show MLSP increase. As the animals lives dramatically longer, the damage accumulation theory fades into less important and impacting on aging).

But there is one therapy if it ever comes to fruition that could (LysoSENS, removal of lipofuscin through proteasomal and lysososome/autophagosomes increased formation and nanorobots lipofuscin degrading-enzyme filled targeting the proteasome and lipofuscin granules).

Lipofuscin, much like calorie restriction, as been shown time and again, it is causal to intrinsic aging and cell Replicative Senescence.

Cell Replicative Senescence is completely abrogated when Lipofuscin residue granules are removed. Immortal stem gonadal cells or cancerous cells never accumulate lipofuscin like we do. In fact, an old study from 1980s, showed through FACS fluorescence that Werner fibroblasts cells (accelerated aging progeroid fibroblasts) that cultured to replicative senescence reached about 20 PDs, but when there was retroviral SV-40 (Simian Virus-40) transfection in their fibroblasts it created something special. The infected Werner fibroblasts were culture to an incredible 'telomerazed-like' effect of 275 PDs (275 population doublings, which is by standards semi-immortal), the virus was capable of maintaining the telomeres completely frozen despite replicative life 'telomere-loss end-problem' continuing its course (or possibly increasing hTERT telomerase but normally these are somatic cells with no telomerase, possibly hTERT was derepressed by the virus, it would make sense), thus the cells went on to continue dividing way above the normal human fibroblats (60 PDs maximum only). This study used FACS fluorescence to see the effect of SV40 infection, the lipofuscin granules were observed in the cells (normal, Werner and Werner-Sv40 infected), the normal human fibroblasts had a abated progession of granules (since they live longer lifespans than Werner progeroid syndrome people); the werner fibroblasts were loaded with granules in less than 20 PDs, and the werner-SV40 fibroblasts were pristine; they kept on a state of nothingness; nearly no granules anywhere until by 200 PDs were there was a little bit starting to show; only until the last 50 PDs (from 225-275PDs) did it start to show up. Each and everyone of these cells showed the exact same amount of 'total lipofuscin' that killed the cell. Meaning 100% of lipofuscin werner happens very quickly (20 PDs), 100% lipofuscin in humans in 60 PDs, and 100% lipofuscin in semi-immortal werner-SV40 in 275 PDs.

This proved clearly that lipofuscin is ''The Final Damage'' composed of all other damages (in the proteasome) and is not just correlative - but also causative to intrinsic aging and a possible shot at immortality (without telomerase therapy need) since it would solve the replicative problem. Proteasome inhibition creates an accelerated accumulation of lipofuscin and increase in oxidative stress. Excess oxidative stress accelerates telomere loss which will later on trigger replicative senescence. Linking proteasome to lipofuscin formation to replicative senescence (over the long term, whereas in short term it's mostly inducible senescence and disease creation) and thus to telomere loss. And lipofuscin is important in both senescence and apoptosis, causing both.

Proteasome inhibition by lipofuscin/ceroid during **postmitotic aging of fibroblasts***
1. http://www.ncbi.nlm.nih.gov/pubmed/10928983

Proteasome inhibition enhances lipofuscin formation
2. http://www.ncbi.nlm.nih.gov/pubmed/12485885

Aggregates of oxidized proteins (lipofuscin) induce apoptosis through proteasome inhibition and dysregulation of proapoptotic proteins
3. http://www.sciencedirect.com/science/article/pii/S0891584905000055

''Proteasomal inhibition blocks NFϰB activation and leads among others to increased susceptibility to oxidative stress and apoptosis ''

Lipofuscin: formation, effects and role of macroautophagy
4. http://www.sciencedirect.com/science/article/pii/S221323171300027X

Posted by: CANanonymity at January 11th, 2016 9:49 PM

@CAN - I can see you are trying, but you are still falling into the trap of writing almost unreadable walls of text when you write.

You could try - summarizing your points into numbered single sentences before your main paragraph.

Or even just writing the concluding sentence of your argument and then writing the supporting facts as bullet points.

For example in your first reply your conclusion seems to be "Only people at 40 or below will gain, sadly" but then the supporting facts are lost in a jumble of words above that.

Here is how you could have written it:

I believe only people aged less than 40 will benefit from senescent cell removal.

Why?
- There is something called 'inducible senescence', and this will mean that senescent cell removal will be ineffective past 40.
- There is something called 'intrinsic aging', and this will mean that senescent cell removal will be ineffective past 40.

Posted by: Jim at January 12th, 2016 1:14 AM

Any future senotherapeutic approach should definitely be taken with caution, especially since the senescent state may play a role in wound healing/regeneration and embryonic/placental development. Senotherapeutics will likely be more beneficial as a preventative measure rather than a treatment. The senescent state may be beneficial over apoptosis because they maintain tissue integrity that would otherwise be lost if cells underwent apoptosis. Therefore, mass removal of senescent cell may certainly be counter productive, but could potentially be balanced out with stem cell administration to replace lost tissue simultaneous.

I prefer the potential approach of preventing senescent induction altogether. For example, quiescent cells with DNA damage appear to only convert to a senescent state when induced to proliferate required for cell turnover. In fact, the elevation in senescent cell numbers with age may be a consequence of elevated quiescent to senescent conversion as our cells increasingly need to be replaced. Eliminating DNA damage in quiescent cells before there convert to senescence may be one attractive senotherapeutic approach worth considering.

Posted by: DGAB at January 12th, 2016 6:47 AM

MMTP is currently in consultation with SENS and others regarding our Senolytics program. It could be possible that removing too many senescent cells too fast is bad news but we can experiment with differing doses and frequency in our mouse testing in order to establish these things. We will keep you folks posted and keep an eye on our website for news.

http://www.majormouse.org

Posted by: Steve H at January 12th, 2016 10:42 AM

@Jim

I apologize for the lengthy response and writing style, it's because my thought process is a trickle-like thing.

@DGAB

Hi DGAB !

'' ...Therefore, mass removal of senescent cell may certainly be counter productive, but could potentially be balanced out with stem cell administration to replace lost tissue simultaneous... Eliminating DNA damage in quiescent cells before there convert to senescence may be one attractive senotherapeutic approach worth considering.''

Interesting take. I think stem cell injection hits a certain limit because mice studies that had mesenchymal stem cell injection had health improvement but mostly the same lifespan (it means it reduced inducible senescence rather than replicative one).

The reason for that is as you say DNA damages (in quiescent cells), this DNA damage is mainly caused by replicative end-problem (and other pathways such as oncogenic damage and random dna damage), it is also the cause of replicative senescence. Telomerase is one answer as it capable of reducing telomeric DNA damage by recreating repeats on the damage telomeres (oxidative damages to telomere equals accelerated telomeric DNA nucleotide basepairs repeat loss;

replication problem is that during replication (since it is imperfect) telomeres lose DNA content anyway - damages or not - this replication problem/imperfection - is - a major damage itself to the telomere that drives late replicative senescence activation).
Telomerase therapy (as done by BioViva company) could solve this, so is stem cell injection (but it has a limit in terms of rejuvenation effect it seems) plus non-gonadal stem cells (have no or little telomerase) lose telomeres too; so they enter replicative senescence, same as fibroblasts and other somatic
no-telomerase cells for example. As such, continuously self-generating stem cells in the lab and injecting them continuously could face the problem that one day or another, aging still wins and takes over (in what i call a 'catch-up/mop-up game', playing catch-up is hard to win).

So that leaves one great therapy that could solve this big riddle; and that's removal of the final pigment of intrinsic aging (lipofuscin) that is constituted of mix-match of residue (dead mitochondria, oxidized lipids, oxidized sugars, broken DNA, AGEs, carbonyls, Malondialdehyde, TBARS, prostanes, ceroid, all packed into one fun pigment that contributes to all of them). Studies showed that replicative senescence can be twarthed by Telomerase, ALT recombination or by one study that showed lipofuscin is causal to replicative senescence. As such LysoSENS (lysosomal/autophagic removal of lipofuscin) could solve this. Telomere protection were the base of all of this (for replicative senescence is telomeric DNA loss-based).

Posted by: CANanonymity at January 12th, 2016 11:59 AM

@CANanonymity

Thanks for the information. What do you think of taking currently available proteasome pathway activators as a lifespan extention strategy?

Posted by: Santi at January 12th, 2016 1:30 PM

@CANanonymity

From my understandings, lipofuscin is a consequence of cellular senescence and not a cause. So you would expect to observe liposuscin earlier in cells undergoing premature senescence such as Werner Syndrome fibroblasts and absent in immortalized cells that do not undergo senescence.

Also, senescent cells, particularly fibroblasts, develop a pro-survival phenotype and do not die whether lipofuscin is present or not. It is this pro-survival phenotype that likely causes senescent cells to persist in tissues if the immune system fails to remove them. Under optimal conditions, senescent fibroblasts can be maintained in culture for years.

On a separate note, I think too much emphasis and assumptions on cell senescence in living organisms is based on replicative senescence. The truth is that we know very little about senescent cells in tissues or the primary cause of their induction. At this point it can only be speculated.

Posted by: DGAB at January 12th, 2016 6:51 PM

@Santi

Hi Santi ! Thanks for that ! I think they are a definitive help and are worth it.
Anything that can help the proteasome should help out to clear debris/residues/unfolded proteins and other junk prepped for degradation. Some studies using proteasome activators in mice showed a nice effect, nothing miraculous though, it was about a Calorie Restriciton (CR) health-like effect (in fact, studies showed that CR, which detoxifies body and removes toxins/crap, is dependent on proateasome and autophagy for part of its rejuvenating effects (if proteasome/autophagy is blocked while doing CR, CR benefits are again mostly all abrogated; showing proteasome, lysosome and autophagic process are needed in the equation for the whole of CR to work). One caution though, activating the proteasome is not always good, in certain states,too much of a good thing is bad. It is overactive in certain disease states and - itself- can contribute to inflammation and disease progression (excessive excitatory processes, synaptic exocytosis/endocytosis). I like to think of it as a big junkyard/scrapyard that must be kept efficient and low-adequate 'in the background' 'active', but never be in a sort of'extreme mode' of degradation and overtaking things (it could degrade good stuff and implode from excitatory overdegrading); it has to be kept in balance, not too little or not so much either; the proteasome works in fine-crafed balance with everything else during cell cycle; messing this balance can have harmful consequence.

@DGAB

Thanks for your response !

'' Their tubular epithelia showed the deposition of lipofuscin and the presence of senescence-associated beta-galactosidase (SA-beta-GAL). However, no tubular cells were atrophic. In electron microscopy, SMP30-KO mice showed markedly enlarged lysosomes containing an electron dense substance. These are convincing hallmarks of senescence. ''

The SMP30-KO mice are a type of accelerated aging/death mice. Just like progeria in progeroid syndromes of accelerated aging (Werner, Hutchinson-Gilford's, Trisomy 21, Down Syndrome's,..). The fact that these mices and these people die younger is quite revealing. Replicative senescence is linked to telomeric DNA loss, as it triggers onset of replicative senescence through p53 oncogene inhibitor mediator, activating final p21; which increases ROS production.

I too had some doubt for a while, if lipofuscin was correlative, only or causative too. After careful consideration, I changed my mind and from all the data it really seems more towards causality than just a random 'appearing pigment' correlate produce of aging process.

''Also, senescent cells, particularly fibroblasts, develop a pro-survival phenotype and do not die whether lipofuscin is present or not. It is this pro-survival phenotype that likely causes senescent cells to persist in tissues if the immune system fails to remove them. Under optimal conditions, senescent fibroblasts can be maintained in culture for years.''

You are right. But we can't hide from the fact that these humans and mice showing accelerated senescence die too. Replicative senescence is not viable with eternal lifespan sadly. In lab they may keep them going on eternally senescent; it just doesn't work in humans (as a biological process during aging). Some cells must fall to apoptosis or senescence, but clearly it is not good at all; telomeric loss is bound to reach telomeric M1 crisis point (5-6 kb, replicative senescence) which could go on for a little while to reach second M2 final point (2 kb) and there nothing goes further (in studies, the entity/organism is already dead); otherwise transformation happens and this is cancerous immortalization (though hTERT highjacking or ALT recombination). It means, your telomeres which control genetic aging phenotype, by replicative senescence-bound process, tell us the finality and the causality of lipofuscin; because when lipofuscin is injected it blocks the proteasome, increasing oxidative stress and resulting in accelerated telomere loss - and accelerated replicative lifespan shortening; thus quicker organismal death. I am not sure how senescent cell maintenance could work/be viable (they secrete pro-inflammatory molecules and cytokines that help drive inflammation) with immortality. Or how long can a human be kept 'senescent' and in a fragile decrepit state...not very long (from real life), at least not eternally; one day or another inflammation would make the body overtly fragile and succomb (to some disease complication). Lipofuscin wins on us anyways, correlative or causative or not it is a marker of aging and telomere rate-loss (a universal one, and we can't circumvent its presence, that is why nanorobots degrading enzymes-filled is a solution).

By reasoning rapidly, we can infer it is more causative, by accumulation, toxicity and clogging the proteasome it is causative, if it wasn't causative it wouldn't do anything to the proteasome when inserted in it; but it does (and as shown in CR studies, protaesome/autophagy KO abrogates life extension by CR; as such proteasome function is causal in replicative senescence and the proteasome/lysosomes take care in part of lipofuscin). The fact that a senescent prone mice dies younger and has faster lipofuscin accumulation, is not just a make-up decoration but a part of the accelerated aging of this mice and its proteasomal dysfunction.

'' ..... The tissue proteasome activity was significantly lower in SAMP10 (Senescence-accelerated prone mice-10) mice at ...''

''Rats were perfused and brains were examined immunohistochemically at 2 days, 7 days and 1 month after injection. Alz-50, a monoclonal antibody against abnormally phosphorylated tau proteins, stained neurons along the cortical needle track at 2 but not 7 days after injection of amyloid or lipofuscin.
Marked neuronal loss was never observed at any time after lipofuscin injection.''

Here it shows that there is not marked neuronal loss by lipofuscin - but there is, some, and that's the problem. Lipofuscin is a lonngg term affair over an entire lifespan. A few dead neurons here and there, braink shrinkage and as lipofuscin overtakes the cell compartment, the cell is suffocated and clogged, the proteasome and autophagy lysosomal mass increase and becomes utterly blocked, leading the senescence or apoptosis/death. Lipofuscin causes both pathways.

Accelerated tubular cell senescence in SMP30 knockout mice.
1. http://www.ncbi.nlm.nih.gov/pubmed/16874657

Senescence-accelerated Mice (SAMs) as a Model for Brain Aging and Immunosenescence.
2. http://pubmedcentralcanada.ca/pmcc/articles/PMC3295080/

Role of p14ARF in replicative and induced senescence of human fibroblasts.
3. http://www.ncbi.nlm.nih.gov/pubmed/11564860?dopt=Abstract

Lipofuscin accumulation in proliferating fibroblasts in vitro: an indicator of oxidative stress.
4. http://www.ncbi.nlm.nih.gov/pubmed/11250119?dopt=Abstract

Effects of Injected Alzheimer Beta-amyloid cores in the rat brain.
5. http://www.jstor.org/stable/2357939?seq=1#page_scan_tab_contents

A2E, a component of lipofuscin, is pro-angiogenic in vivo.
6. http://www.ncbi.nlm.nih.gov/pubmed/19418485

Subcellular aging markers of retinal pigment epithelium in Japanese quail Coturnix japonica
7. http://link.springer.com/article/10.3103%2FS0096392513040093

Table: Neurotoxic Effects of Lipofuscin. Toxic Effects of Lipofuscin.
8. http://biofoundations.org/wp-content/uploads/2015/08/BIoFoundationsLipopigmentsTable.pdf

Lipofuscin and aging: a matter of toxic waste.
9. http://www.ncbi.nlm.nih.gov/pubmed/15689603

Posted by: CANanonymity at January 13th, 2016 9:27 AM

I should first note that the issues raised by Reason in this thoughtful post apply to medical intervention in aging generally: the interventions themselves, as well as ancillary treatments to enable them and the process of removing damage from tissues, can be expected to take some toll on the body, and a progressively-older body can be expected to be progressively less able to endure, recover from, and benefit from the rigors of therapy. This is why — although rejuvenation biotechnologies have the advantage of clearing existing cellular and molecular damage out of aging tissues, and therefore being effective in persons in whom substantial aging damage has already accrued — they none the less do have a limited therapeutic window, as illustrated here. As people reach truly advanced ages, the total burden of aging damage will rise and the homeostatic capacities of the organism will decline, making any therapy less be less and less able to hold back and reverse the cascading effects of the lifelong burden of mutually-synergistic aging processes.

However, in the specific case of senescent cell ablation, we can actually move beyond speculation, as we now have three separate published reports of extensive in vivo clearance of senescent cells from the tissues not only of relatively old wild-type mice (> 24 months-old in the Kirkland "senolytic" report — which is roughly equivalent, by coincidence, to a 70-year-old human), but also from the tissues of animal models with quite severe, artificially-accelerated burdens of senescent cell accumulation (whole-body irradiation and BubR hypomorphic mutations). In addition to these, we have yet-unpublished results from Julie Anderson from the Buck Institute, who presented thrilling results using Judith Campisi's p16-3MR mouse system in a Parkinson's disease mouse model at SENS Research Foundation's Rejuvenation Biotechnology 2014 conference, and the system has now been shown to prevent or reverse a range of diseases of aging modeled in transgenic mice.

Despite a very high burden of senescent cells, clearance of same from these animals' tissues has not only failed to result in any apparent treatment-related ill effects, but has substantially improved the tissue integrity, health, and functionality, despite being burdened with substantial amounts of additional tissue pathology inflicted by "normal" aging, irradiation, or mutations.

Additionally, it's worth remembering that there's no reason we have to completely clear senescent cells from aging tissues to effect functional rejuvenation: the burden simply has to be brought down to levels similar to those in young or middle-aged individuals, when tissues are still quite capable of supporting normal function and homeostasis. And because senescent cells accumulate slowly over a lifetime — and don't themselves replicate, unlike cancer cells — there's no reason that we should feel bound to reach our ideal therapeutic target all at once: if toxicity similar to tumor lysis syndrome arises, we could establish clearance protocols that spread the process out over several rounds of submaximal treatment so as to reach tissue targets over an acceptable timeframe while still minimizing any side-effects from the therapy itself or the death of cells.

All of this suggests that the therapeutic window for the application of senescent cell-clearing/"senolytic" therapies in aging humans will be quite wide. And clearance of senescent cells from local tissues will, in turn, open the therapeutic window for other rejuvenation biotechnologies a crack, by helping normalize the systemic milieu and putting a halt to pathological local tissue remodeling.

@Jim: careful. Don't assume that TIF-positive senescent cells got that way as a result of normal replicative aging. First, TIFs can also be induced by telomere damage resulting from oxidative damage, with only much lower background erosion from replication. Second, of course, some cells do become senescent as a result of having eroded their telomeres during accelerated cellular replication as part of the cancer process, only to be shut down by the senescence machinery after having reached the functional limits of their telomeres without evolving a mutation that hijacks the telomere-maintenance machinery, the one indispensable feature shared by all cancer cells. This may strictly speaking still be replicative senescence, but it is not the result of normal cell replication as envisioned when the "Hayflick limit" was discovered in vitro. A more ambiguous case can arise during tissue fibrosis and/or diseases of local tissue hyperproliferation driven by chronic inflammation or remodeling.

@DGAB: Senescence does indeed play a role in wound healing, which is yet another reason why ablation of senescent cells is preferable to modulation (inhibition or reversal) of the senescence machinery itself. Clearance can still lead to problems if initiated why it's important not to rely on interventions that interfere with the senescence machinery itself. The fact that senescent cells play a role in the prevention of fibrosis during the resolution of wound healing means that it will be wise to schedule rounds of therapy when a person is not recovering from such wounds, including surgery, and could also be another reason to spread rounds of submaximal clearance out over time as laid out above.

Embryonic/placental development would be more of a concern, and would likely have to be delayed in pregnant women until they had given birth — or, alternatively, pregnancy could be delayed if treatment is a therapeutic imperative for the mother.

The senescent state may well be favored by evolution over apoptosis in some cases for the reasons you suggest (as argued years ago by Magnus Lynch in a paper presented at the inaugural SENS Conference, IABG 10); this is yet another advantage of the "damage-repair" approach, as we are at liberty to replenish the tissues using cell therapy and tissue engineering without the need to induce additional rounds of replication in local stem cell niches.

Posted by: Michael at January 19th, 2016 5:55 PM

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