79 replies to this topic

[johnhemming]

The metric of reversing senescence is a reduction in minimum CRP.

[Zarathrustra]

Yes, I I said: CRP is not only a good proxy for senescent cell reduction if the theory is correct, but in its own right. If you Google for causes of inflammation, you will not find cellular senescence in the top references.

 

The more I read about cellular senescence, the less I think there's any milage in the theory, and the less useful the idea is.

 

There's no research that backs up your assertion that reversing senescence reduces CRP.

 

Having said all that, there's enough well-established measurable ageing metrics to be going on with.

 

Even if the theory is true, it won't address the issue of age-reversal.

[johnhemming]

We obviously disagree. There is a lot of research that points to IL-6 being part of SASP and IL-6 stimulating CRP.  However, there is no reason why we have to agree and I am quite happy to allow you to have the last word on this if you have any further comment.  My view is that one of the two key causes of age based deterioration and the failure to express proteins with long genes is the senescent cell load.  Hence if you reduce the senescent cell load (and I measure it by the minimum CRP level over a number of measurements - that gives background IL-6) then that reduces the harm done to the body.

 

In the end, however, people have different views.  I think I am right, but I may be wrong.  You have a right to have a different view and after this post I will not respond to any further comment as we won't have a meeting of the minds.

[Zarathrustra]

Sure John.

 

I think your view begs the question on several levels: you presume what has not, and seemingly cannot, be shown - that there is a senescent load. How do you know such a thing exists, when no-one has shown it does? Without any measure of total senescent cells, we cannot know whether it increases, whether it matters, whether lifestyle changes alter it. It may be that having lots of senescent cells is not a problem. It may be that eating grapefruit (or any other of an infinite number of lifestyle events) gets rid of tham. It may be that they are of short-term burden when we are ill. Etc.

 

Meanwhile, there are many other well proven ageing metrics which can me altered by lifetyle changes, resulting in performing as well as in younger years. It may even turn out that getting rid of senescent cell load worsens other age metrics (tho' I do doubt that).

 

Attending to established ageing metrics enables one to slow ageing, and extend life now. Meanwhile, we await the development of cellular modification that may actually reverse ageing and hope we're alive still when such an approach is proven and available.

 

Good luck. And thanks for the discussion.

[Rocket]

But how do you know that senescent cells have any effect on ageing or illnesses when you cannot measure them?

Why not just stick to inflammation?

Senolytics is a major field of research, based on the assumption that any such stuff would reduce senescence. But if you cannot measure senescence, how would you know?

I'm not against the theory of senescence being an important part of ageing and illnesses - just that it is an unproven theory. A theory that seems to be taken for granted as being true. Not good science. In fact, not science at all.

By the chemicals they secrete. Yes its that simple.

[Zarathrustra]

Thanks, I wasn't aware of that.

 

Can you please give some references?

 

I assume there are unique to senescent cell chemicals, that can be assessed for the whole body? That they've been measured over time? And for different ages?

[CynthesisToday]

A) doi:10.1038/s41591-018-0092-9 "Senolytics improve physical function and increase lifespan in old age" (2019) 

"Physical function declines in old age, portending disability, increased health expenditures, and mortality. Cellular senescence,

leading to tissue dysfunction, may contribute to these consequences of aging, but whether senescence can directly drive age-
related pathology and be therapeutically targeted is still unclear. Here we demonstrate that transplanting relatively small
numbers of senescent cells into young mice is sufficient to cause persistent physical dysfunction, as well as to spread cellular
senescence to host tissues. Transplanting even fewer senescent cells had the same effect in older recipients and was accom-
panied by reduced survival, indicating the potency of senescent cells in shortening health- and lifespan. The senolytic cocktail,
dasatinib plus quercetin, which causes selective elimination of senescent cells, decreased the number of naturally occurring
senescent cells and their secretion of frailty-related proinflammatory cytokines in explants of human adipose tissue. Moreover,
intermittent oral administration of senolytics to both senescent cell–transplanted young mice and naturally aged mice allevi-
ated physical dysfunction and increased post-treatment survival by 36% while reducing mortality hazard to 65%. Our study
provides proof-of-concept evidence that senescent cells can cause physical dysfunction and decreased survival even in young
mice, while senolytics can enhance remaining health- and lifespan in old mice."

 

From the headings in the Results section:

 

- "Transplanting small numbers of senescent cells is sufficient to induce physical dysfunction in young mice."

- "Aging and high-fat diet exacerbate effects of senescent cell transplantation."

- "Dasatinib plus quercetin reduces senescent cell burden and decreases proinflammatory cytokine secretion in human adipose tissue."

- "Eliminating senescent cells both prevents and alleviates physical dysfunction induced by senescent cell transplantation."

- "Clearance of senescent cells alleviates physical dysfunction and increases late-life survival without extending morbidity in aged mice."

 

https://www.cell.com...8924(20)30100-8 "Classical and Nonclassical Intercellular Communication in Senescence and Ageing" (2020) "Intercellular communication refers to the different ways through which cells communicate with each other and transfer a variety of messages. These communication methods involve a number of different processes that occur individually or simultaneously, which change depending on the physiological or pathological context. The best characterized means of intercellular communication is the release of soluble factors that affect the function of neighboring cells. However, there are many other ways by which cells can communicate with each other. Here, we review the different means of intercellular communication including soluble factors in the context of senescence, ageing, and age-related diseases."

 

The effect of senescence is context specific. The "Classical and Nonclassical..." review paper describes some of the various contexts (e.g. senescence is required in a successful pregnancy), and different SASP molecules depending on context. Some nice summary tables, diagrams, and "cartoons", too. Includes references you could follow up on.

 

B) https://www.nature.c...les/nature10600 (2011) "Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders" From the last sentence in the abstract: "These data indicate that cellular senescence is causally implicated in generating age-related phenotypes and that removal of senescent cells can prevent or delay tissue dysfunction and extend healthspan." You can get the full text through scholar.google search of the title.

 

C) https://www.pnas.org...pnas.92.20.9363 "A biomarker that identifies senescent human cells in culture and in aging skin in vivo" (1995) From the last two sentences of the abstract: " In skin samples from human donors of different age, there was an age-dependent increase in this marker in dermal fibroblasts and epidermal keratinocytes. This marker provides in situ evidence that senescent cells may exist and accumulate with age in vivo."

 

D) Local clearance of senescent cells attenuates the development of post-traumatic osteoarthritis and creates a pro-regenerative environment (2017) 

 

E) https://www.ncbi.nlm...les/PMC4993332/ "Aging of mice is associated with p16(Ink4a)‐ and β‐galactosidase‐ positive macrophage accumulation that can be induced in young mice by senescent cells" (2016)

 

If you scholar.google search on the above original research articles (A, B, C, D, and E) through the "cited by" and use the "search within cited articles" tick box with "-review" you can find a lot of original research that developed from these original research articles.

Edited by CynthesisToday, 13 April 2023 - 08:29 PM.

[Zarathrustra]

But neither of your references says anything about (a) whole body burden of senescent cells; (b) whether that can even be measured.

 

It is entirely plausable that in general there is an increase in senescent cells with age. What is not known is whether this is true. Many plausable theories have been disproved.

 

It may be, and I suspect it is so, that the main effect of senescent cells is increasing inflammation, which then adversly effects many other biological pprocesses. It may well be that it is the inflammation that is the mediator of all degenerative processes, irrespective of what increases the inflammation. However, the body may well have ways of reducing the senescent cell burden, as it does inflammaton, by normal lifestyle factors - exercise, nutrition, etc. But without a metric for the senescent cell burden, we cannot know. But we do already know how to reduce inflammation by changes of lifestyle. The senescent cell burden theory seems unproven and unnecessary. But I may be wrong. I await the emprical research, which has a prior requrement of an assay for this concept.

 

If that's the best references you can find, then this has not contradicted my assertion: there is no method of measuring the total senescent cells in a human body. Without this, you cannot say (a) whether it (the senescent cell burden) increases with age; (b) whether this burden correlates with degenerative diseases or any other illnesses; © whether lifestyle effects this burden. The latter point is critical and appears to be a problem for those advocating senolytics as a rejuvinating hope, as if lifestyle can so reduce the burden without the possible side-effects of some touted senolytics, then that would be preferable.

 

 

[Zarathrustra]

Another example where, without any evidence, longevity newscasters make spurious claims: "We know that the number of senescent cells in tissues grows with age" Fight Aging! Newsletter, April 17th 2023. This despite them quoting research that directly contradicts this:

"Researchers found more senescent cells in the old skin compared with young skin samples. However, in the samples from old individuals, the number of senescent cells did not increase as individuals got progressively older, suggesting that some type of mechanism kicks in to keep them in check."

 

It may well be that lifestyle factors that slow ageing (exercise, nutrition, calorie restriction, et al) as well as other beneficial effects (lower resting heart rate, reversing sarcopenia, etc) may also reduce any senescent cell burden. Indeed, the latter may not even be a burden. This has yet to shown.

 

If this unproven theory of senescent cells being a problem is to ever become useful, there needs to be a direct measure of total body senescen cells.

[hplus]

There are several methods to measure the burden of senescent cells in a tissue. Here are a few commonly used techniques:

1. Senescence-associated beta-galactosidase (SA-β-gal) assay: This is a widely used method to detect senescent cells in vitro and in vivo. The SA-β-gal assay measures the activity of beta-galactosidase, which is a lysosomal enzyme that becomes active in senescent cells. The cells are stained with a substrate that is cleaved by beta-galactosidase, producing a blue color that can be visualized under a microscope.

2. p16INK4a expression: p16INK4a is a protein that is upregulated in senescent cells. Immunohistochemical staining can be used to detect the expression of p16INK4a in tissue samples.

3. Flow cytometry: Flow cytometry can be used to quantify the number of senescent cells in a tissue sample. This method involves labeling cells with fluorescent antibodies that recognize specific markers of senescence, such as p16INK4a or senescence-associated heterochromatic foci (SAHF).

4. Quantitative PCR (qPCR): qPCR can be used to measure the expression levels of genes that are known to be upregulated in senescent cells, such as p16INK4a and p21CIP1.

5. Senolytic treatment: Senolytic drugs are compounds that selectively kill senescent cells. By treating a tissue sample with a senolytic drug and measuring the reduction in senescent cell burden, the initial burden can be inferred.

[Zarathrustra]

Thanks. All you have said is what I know already: it is possible to determine at one time, in one tissue sample, the amount of senescent cells present.

 

The issue is not determing the presence at any one time in a particular organ. That is well established now.

 

The issue is (a) whether the total of senescent cells in a body can be ascertained; (b) whether, once we can measure this (and not until), can we determine whether it (1) increases as a person ages; (2) whether this total is affected by one's lifestyle or anything else (such as senolytics); (3) whether any reduction has any noticeable change in one's ageing or even reversal. Without this metric, all is empty speculation.

 

We know that inflammation, as measured by hsCRP tends to increase, on average, as people age. That inflammation is thought to be partly deterrmined by the amount of senescent cells, amongst many other things (for instance, disease, fat). That inflammation is affected by lifestyle, and an older person's increase can be reversed - but without, seemingly, any obvious reversal of ageing - which may hence be true of 

[hplus]

I don't think that the contribution of senescent cells to the aging process is speculative. There are countless papers out there that prove this.

 

If you can measure the burden of senescent cells in organs, you can measure it for the entire body. You just have to add the numbers. So I don't see your point.

 

It is also very likely that different organs are affected in different ways. So knowing the burden for the entire body is perhaps not that important. To extend the lifespan, you want to know which vital organ will fail first and intervene there. If a person dies of old age, it is usually not because the entire body fails.

[Zarathrustra]

In the absence of a measure of the whole body burden, upon which the theory is based (that the total increase of senescent cells inceases with age, causing incrreased inflammatioin and burdening the immune system whilst detracting being as effective in younger ages), we have no way of knowing if this is true.

 

It is begging the question to say this burden is a major determinant of degerative diseases, when we do not know if this burden does increse with age - because we have not measured it.

 

It has been shown that some older organs have lots of senescent cells. What has not been shown is that they of an increasing quantity over timie in any one individual (but if I'm wrong on this, I'd appreciate the research reference). The quote I gave on 15th April specifically said that the total did not increase with age in older people in their skin. Maybe it even reduces in other orgains? Or exercising reduces it. Its all speculation.

 

I like the theory, but at the moment it is unfounded and even has some evidence against it.

 

Meanwhile, there are other biometrics (CRP, eGFR, resting heart rate, etc) which do show a worsening with age; but, more importantly, can be reversed with lifestyle changes. Why wouldn't that be true for senescent cells?

 

There may be other problems with the theory, but we cannot know, as there's no empirical test for it - which any scientific theory requires.

 

 

[hplus]

Measuring the burden of senescent cells for the entire can be done easily, and the number of senescent cells increases in all organs is established. I read many papers in this field every week and don't remember one paper that would doubt this.

 

We also know that organs can age at different speeds. This can be measured with epigenetic clocks. The same can be said for the accumulation of senescent cells. However, I have never read a paper claiming that senescent cell burdens did not increase at all in a particular organ.

 

However, lifestyle changes or supplementation with senolycitcs can decrease the burden on senescent cells, and this usually also comes with a reduction of the signs of aging (aging phenotype).

 

This is a recent paper that I found interesting: Local senolysis in aged mice only partially replicates the benefits of systemic senolysis

[Zarathrustra]

Thanks for the rerference, but it doesn't address my concerns about the whole-body senescent celll burden. Alas, all the papers on this subject take for granted that the total senescent cell burden increases, without ever referencing any research that has proven this. I too was taken in by this casual assumption. 

 

You may read many papers about senescent cells, but each and every one I have traced never mentions measuring the whole-body senescent cell burden. Perhaps I missed those. So a reference to where this is established would be appreciated.

 

Whilst I have read papers about the senescent cell burden in particular organs, I have yet to read one where this is done over time - that is, noted that there was an increase with age; only that some older people have more senescent cells in a specific organ.

 

The paper I referenced earlier, which noted "The researchers found more senescent cells in the old skin compared with young skin samples. However, in the samples from old individuals, the number of senescent cells did not increase as individuals got progressively older, suggesting that some type of mechanism kicks in to keep them in check" can be found Boosting-the-bodys-anti-viral-immune-response-may-eliminate-aging-cells (massgeneral.org)

 

Epigenetic clocks are notoriously unreliable to date. Different researches come up with differing results for the same person. In the end of the day, slowing ageing or reversing it has to be reflected in the actual person's age-related abilities (muscle-mass, resting heart rate, frailty reversal, running speed, etc). Just producing a figure that purports to 'show' a person has reversed ageing without accomplishing such macro-bodily improvements is worthless.

[hplus]

It is might true that senescent cells stop accumulating at some point. However, we need more research here. Nevertheless, there is plenty of research out there that shows that senescent cells accumulate in many different organs. The skin is just one of many examples.

 

Considering the theory behind senescent cells, it would be a big surprise if some organs were excluded here. The number one cause is the Hayflick limit. This applies to almost all cells in the body (for instance, neurons are an exception). Another cause is DNA double-strand breaks. This also applies to all cells in the body. The immune system removes most senescent cells, but as it gets weaker with age, more and more senescent cells accumulate. This also applies to the entire body.

 

I recommend searching PubMed to learn how senescent cells accumulate in different organs. Here is a recent meta-study: Cellular senescence and chronological age in various human tissues: A systematic review and meta-analysis

[Zarathrustra]

You perhaps did not get round to reading all my contricutions here. It was because I had spent many hours trawling PubMed (and others) that I came to the conclusion that started this thread.

 

Your reference bears out my point. And nowhere does it addess the point that there is no metric of total senescent burden.

 

It seems it is at present difficult to measure senescent cells, even in small samples of specific organs - and this varies a lot.

 

But as I understand it, the problems caused by senescent cells (if there are any) is mediated by inflammation (the SASP effect) - apparantly caused by the immune system having to try dealing with these cells. If that is it, and given that inflammation is a known problem in itself, however cause, then that is the metric we should be concerned about. And about which we have abundant evidence, both relating to its variaition with age and with lifestyle. Probably in reducing inflammation we are dealing with whatever problems senescent cells may be causing too. Hence, no need to both about senescent cells.

 

To my mind, a much more fruitful research area has just opened up: Gene transcription ageing the root of overall ageing

[hplus]

I guess you didn't find a paper about the total burden of senescent cells because this metric wouldn't be that interesting. We want to know if the accumulation of senescent cells causes aging and how different tissues are affected. We also want to understand the mechanisms. That senescent cells contribute to aging is beyond any doubt. Calculating the total burden for an organism wouldn't contribute anything meaningful here.

 

Maybe someone calculated this number in one of the countless papers about senescent cells as an academic diligence task. But what for? It is already evident that it can't serve as a metric for the biological age. It is like calculating the average income of different countries as a metric of how rich a country is. The US has the most billionaires, and no other industrialized county has so many homeless people. So how much information would you gain by calculating the average income?

 

This sentence is critical in the paper I cited earlier: 

 

 

While senescence is higher in aged tissue samples, the magnitude of senescence varies considerably depending upon tissue type, tissue section, and marker used to detect senescence.

 

[Zarathrustra]

If the theory that senescent cells are a (major?) cause of ageing doesn't equate to knowing the total, then I don't know what the theroy is about. Certainly senolytics if based on that idea - taking such supposed supplements is on the basis of them being systemic (that is, the work on the whole body).

 

If knowing the total isn't interesting, why is knowing any other whole-body metric (inflammation, musclemass),  interesting? I'm afraid your comment makes no sense to me.

 

The reason I require such a metric to test the theory is that only with such a metric can we track whether it changes with age (the whole basis of the theory), or changes with lifestyle (so by altering one's lifestyle one can see whether one is improving one's age metric).

 

"Senescence (/sɪˈnɛsəns/) or biological aging is the gradual deterioration of functional characteristics in living organisms. The word senescence can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence involves an increase in death rates and/or a decrease in fecundity with increasing age, at least in the latter part of an organism's life cycle." Wikipedia

 

We do not yet know whether increrases happen in particular organs inone person over time and is effected by lifestyle, let alone the whole body. 

 

"That senescent cells contribute to aging is beyond any doubt" Yes it is. Because that is just what we do not know. Where is your proof of that?

[osris]

I have now spent the last two weeks searching and reading many papers relating to cellular senescence and senolytics, and give here my summary along with successive documents of my research (most previously attached, but here in one place for ease of reference), along with my own longevity references. Thank you colleagues for your references and critiques; these prompted me to do the work required to sort things out.

 

Summary:

The cellular senescence theory as one of the major causes of ageing is unproven. None of the underlying theory has been substantiated, all the way up from the Hayflick limit, through the human body accumulating senescent cells as the human ages, to proving that clearing away senescent cells reverses ageing or reduces degenerative diseases. There is no metric for the burden of senescent cells. Any supposed effect of an increasing amount of senescent cells is via increases of inflammation. Any supposed effect of supposed Senolytics is independent of the unproven theory – tho’ the empirical effect of the Senolytics is possibly real. The latter has not been tracked over sufficient time yet to detect any deleterious side-effects. Meanwhile, there is already established evidence that (a) various lifestyles directly effect longevity; (b) the effectiveness of the lifestyle components can be measured by established metrics and shown to correlate with extended age. My own use of senolytic supplements shows differing correlations with my biometrics, underlying the uncertainties of such products – even if they may be true for some people, they may not be for others.

 

Whilst the theory that there is increasing amounts of senescent cells as a body ages, and that this causes the problems of ageing, is plausible. That some senescent cells cause these problems by increasing the amount of inflammation is also plausible. And that clearing away such cells will reduce the amount of illnesses of ageing people. Further, that using this theory to discover means of clearing away such cells will cause rejuvenation may be successful; indeed, there is evidence it is so. However, this latter sentence is not itself proof that the underlying theory (as laid out in the earlier sentences) is correct.

 

First, the very basis of senescent cells is questionable – the Hayflick limit.

 

“In the simulation, the default Hayflick limit of a normal stem cell is 72 as an approximation of the realistic number between 50 and 70 (Shay and Wright, 2000).” It seems this is for cells in vitro, not vivo. It is not true for stem cells.

 

“It seemed to decrease with the age of the cell donor and, as a form of cell senescence, was thought to underlie the aging process. More discriminating analysis, however, showed that the fibroblasts decayed in a stochastic manner from the time of their explantation, at a rate that increased with the number of population doublings in culture. There was no consistent relation to the age of the donor.” Promise and problems in relating cellular senescence in ...

 

We do not know if this cell limit is true of humans – it may be more or less; different for different types of cells; that telomerase (which effects when the Hayflick limit is reached) may be more available for some cells and not others. We also do not know why there is a variation of the Hayflick limit. It is probable that there is a great variation in the speed and amount of senescent cells in a human, nor what proportion is cleared away by the normal biological processes (apoptosis; immune cells’ clearance).

 

ChatGPT agrees with you:

 

"Your argument raises valid points and concerns about the current state of research on senescence and senolytics. Let's break down some key aspects of your argument:

 

Lack of Empirical Basis and Measurement for Senescence:

 

• You argue that there is no empirical basis for various theories related to senescence and senolutics.
• You highlight the absence of a reliable method to assess the senescent burden in a human body, making it challenging to evaluate the effectiveness of proposed interventions.

 

Uncertainty About Senescent Burden and Health Impact:

 

• You express skepticism about the assumed connection between senescent burden and mortality, frailty, and illnesses.
• The lack of empirical evidence linking senescence reduction to improved health or increased lifespan is a concern.

 

Unproven Nature of Senescence Theory:

 

• You emphasize that the theory of senescence as a root cause of aging is speculative and not proven.
• The argument suggests that it is premature to base extensive research and applications on this theory without solid evidence.

 

Focus on Inflammation as a Central Target:

 

• You propose that inflammation should be the primary focus for both illness and longevity research, given its known association with deleterious effects.
• The argument questions why researchers do not concentrate solely on inflammation as a measurable and central metric.

 

Critique of Senolytics Research:

 

• You raise concerns about the validity of senolytics research, as it assumes that reducing senescence would have positive effects without a reliable way to measure senescence.

 

Call for Caution in Accepting Unproven Theories:

 

• The overall message is a call for caution in accepting the theory of senescence as a significant factor in aging and illness without robust evidence.

 

In summary, your argument highlights the need for rigorous scientific evidence and measurement methods before widely accepting and investing in theories related to senescence. It underscores the importance of avoiding premature and potentially harmful interventions based on speculative grounds. This perspective aligns with a cautious and evidence-based approach in scientific research."