Brief Notes on Senolytics

Senowhat?

Senolytics are therapies that help remove senescent cells. Senescent cells are cells that have stopped dividing. They produce a harmful pro-inflammatory state called the senescence-associated secretory phenotype (SASP). As we age, senescent cells accumulate. The resulting SASP contributes to age-related deterioration. 

Example "D and Q"

One well-known senolytic approach is combining dasatinib (a chemotherapy drug) with quercetin (an antioxidant found in apple peel). This drug combination (D+Q) has yielded promising results in both mouse and clinical trials. Mouse models for osteoporosis experienced increased bone mass after D + Q administration. D+Q also increased cardiac function in both old mice and atherosclerosis (hardening of arteries) models. "D and Q" administration to patients with idiopathic pulmonary fibrosis caused enough improvement to warrant a more expansive trial.

Aquaporins in the Senescence of Tendons

Senescence, a hallmark of aging, is a state in which cells stop dividing. Once a cell is senescent, it secretes signals called the Senescence Associated Secretory Phenotype or SASP. Unfortunately, while SASP has its purpose, it also results in inflammation and tissue dysfunction[1].

Senescence can affect many different tissue types including connective tissue like tendons and muscles. Tendons are what attaches bone to muscle. Tendon maintenance is dependent on tendon stem and progenitor cells (TSPCs). As they age, these cells undergo a decrease in their ability to self-renew, differentiate, and migrate to their proper location[2].

Researchers investigated how senescence might be controlled in TSPCs. One suspected cause is changes in the number of aquaporins. Aquaporins are membrane protein channels that transport water in and out of the cell. There is some evidence that aquaporin activity might have an effect on aging. For example, we know that aged skin has less aquaporins than young skin. Some researchers also hypothesize that aquaporins can regulate stem cells. A decrease in one type of aquaporin, AQPI, for example, is associated with senescence of TSPCs[2]. 

To look at the affects of decreased AQPI directly, researchers analyzed old and young rat TSPCs. They observed a decrease in AQPI (expression) and JAK/STAT signalling in old rat TSPCs compared to young. JAK/STAT signalling is a complicated pathway in which multiple protein players act on each other. So, I won’t go into detail on that right now. Just remember 

that JAK/STAT controls gene expression[2].

These observations only make the clear that there is a correlation between old age and decreased AQPI and JAK/STAT signalling. So far, no causation is implied here yet. However,  researchers then increased the expression of AQP1 in rats. They then saw an increase in differentiation capacity, self-renewal, and migration in TSPCs. Essentially, the senescence phenotype of the TSPCs was restored to a more healthy and normal phenotype. Pretty cool. 

 So it looks like AQP1 plays a role in regulating TSPCs senescence[2]. But what do we do about this? Is messing with aquaporins to restore senescent cells a good therapy route?

We should acknowledge that this study is only one paper, and that parsing out cause and effect on a molecular scale is extremely hard (other factors could be involved: molecular pathways are complicated). Also, this was only done in mice. Additionally, a little common (biological) sense is needed: Even though increased stem cell function might seem beneficial, we should be careful. 

Always look at the restoration of senescence cells with a critical eye. Senescence is not just the result of wear and tear with no biological purpose in the body.  Yes, in aging, it is harmful. However, acutely, senescence plays an important role in preventing cancer. When a cell’s DNA gets damaged it has the choice of becoming senescent rather than replicating out of control into a tumor. This means we need to be wary of any therapy where we seek to block senescence. This is especially the case when we are already studying means to remove cells that have have become senescent with senolytics.

Source

1. AQP1 and Cellular Senescence in the Aging of Tendons. (2020, March 28). Retrieved September 04, 2020, from https://www.fightaging.org/archives/2020/04/aqp1-and-cellular-senescence-in-the-aging-of-tendons/

2. Chen, M., Li, Y., Xiao, L., Dai, G., Lu, P., Wang, Y., & Rui, Y. (2020). AQP1 modulates tendon stem/progenitor cells senescence during tendon aging. Cell Death & Disease, 11(3). doi:10.1038/s41419-020-2386-3

Mitophagy in Antiaging Therapies

Intro

Today's article will be a quick review of a biological cleanup process called mitophagy and why it is important for healthy aging. Mitophagy might be the reason why some antiaging therapies work in model organisms.

But first a review on mitochondria...

Mitochondria are an organelle within cells that produce ATP, the energy currency of the cell. Their structure is controlled by dynamic processes based on the internal conditions of the cell. Mitochondria switch back and forth between fusion (fusing together into tubular structures) and fission (splitting apart)[1]. 

Transmission Electron Microscope Image of Mitochondria Public Domain. Author: Louisa Howard

Mitochondrial dysfunction is a hallmark of aging. As we age, the numbers of malfunctioned or broken mitochondria increase. With advancing age, mitochondria populations also tend to participate in fusion more than fission. This results in larger structures that are harder to break down with mitophagy.

 Mitophagy is a special subclass of autophagy, a process of swallowing up and breaking down damaged organelles and proteins, that specifically targets mitochondria. As we age, this process decreases. This results in an accumulation of dysfunctional mitochondria which causes problems in cell and tissue function[1].

Mitophagy and Anti-Aging

A lot of popular life-extending regimens (in lab animals), might work partially because they enhance mitophagy. Both exercise and calorie restriction (reduction of calories by 40% of normal intake without malnutrition) are known to be beneficial to healthy aging and to promote longer lifespans. (For people, the benefit of calorie restriction is still being explored. Exercise is universally recommended, however.) Both of these processes activate the AMPK-Ulk1 complex, a pathway that enhances mitophagy downstream. 

There are also a few drugs that seem to work partially due to their mitophagy enhancing abilities. For example, Urolitin A, which increases fitness in aging C .elegans and muscle function in mice, works by increasing autophagy in general. You might have also heard about different drug approaches to increase levels of NAD+ in the body. It turns out that that also enhances mitophagy.

C. elegans Public Domain. Author: S.Kbradnam

 Probably one of the most famous aging drugs of all, rapamycin, also promotes mitophagy. Rapamycin has increased the lifespan in a slew of organisms including yeast, worms, flies, and mice. It works by inhibiting mTOR, which normally would inhibit autophagy. So by enhancing autophagy altogether rapamycin also increases levels of mitophagy[2].

Conclusion

• Mitophagy is a process that breaks down damaged mitochondria.
• As we age, mitochondria tend to fuse together forming long structures. This makes it harder to break down mitochondria with mitophagy.
• Both calorie restriction and exercise can induce mitophagy in model organisms
• An assortment of antiaging drugs also promote autophagy and mitophagy.

1. The Importance of Mitophagy in Aging. Fight Aging! https://www.fightaging.org/archives/2020/04/the-importance-of-mitophagy-in-aging/ (2020).

2. Chen, G., Kroemer, G. & Kepp, O. Mitophagy: An Emerging Role in Aging and Age-Associated Diseases. Front. Cell Dev. Biol. 8, (2020).

Microbiome Transplants Between Young and Old

Aging and intestinal dysfunction are correlated in lots of organisms spanning from insects, to mammals, to humans. This is characterized by increased permeability in the intestinal barrier and a decrease in microbial diversity. This loss of microbiome diversity has several causes including diet and dysfunction in the immune system[1].

The amount of intestinal dysfunction with age varies between individuals. Based on this, researchers have explored whether there are differences in the intestinal microbiota of long-lived people. There is some evidence for this. In one study, researchers analyzed the microbiomes of long-lived Chinese and Italians. They had greater microbiota diversity and more operational taxonomic units (a measure of the number of species)[2].

This inspires the question of whether the microbiome from a person exhibiting good health and longevity could be beneficial on its own. We do know that the microbiomes of young organisms are beneficial compared to old microbiomes. For example, a fecal transplant on a 10-day old fly into another 10-day old fly results in a longer lifespan and less intestinal permeability than a transplant from a 30-day old fly into a 10-day old fly[3]. And it's not just flies. You see similar results in other organisms like killifish[4].

So, what about transplants from those who are aging well? To find out, researchers took fecal samples from long-lived humans and transplanted them into mice. This resulted in an increase in microbiota diversity. Moreover, there was also an increase in species considered to be beneficial like short fatty acid chain producers. They saw a reduction in some age indices, markers we see with aging, including a decrease in lipofuscin and beta-galactosidase[5].

Based on these results, it would be beneficial to characterize the species existing in long-lived people. Fecal transplants are rarely used and are not the goal of this research. Beyond the general ick factor, there is a lot of risk of introducing pathogens through it. However, research like this could spearhead the design of new probiotics that might promote healthier aging. Once we know what makes a microbiome pro-longevity, this might be possible.

Sources

1. Transplanting Gut Microbes from Long-Lived Humans into Mice to Assess the Outcomes. Fight Aging! https://www.fightaging.org/archives/2020/04/transplanting-gut-microbes-from-long-lived-humans-into-mice-to-assess-the-outcomes/ (2020).

2. Kong, F. et al. Gut microbiota signatures of longevity. Curr. Biol. 26, R832–R833 (2016).

3. Clark, R. I. et al. Distinct Shifts in Microbiota Composition during Drosophila Aging Impair Intestinal Function and Drive Mortality. Cell Rep. 12, 1656–1667 (2015).

4. Smith, P. et al. Regulation of life span by the gut microbiota in the short-lived African turquoise killifish. eLife 6, (2017).

5. Chen, Y. et al. Transplant of microbiota from long-living people to mice reduces aging-related indices and transfers beneficial bacteria. Aging 12, 4778–4793 (2020).

Evidence for Gut Microbiome’s Influence on Azheimer’s Disease

The Connection between your Gut and Brain

There is mounting evidence that changes in the gut microbiome (changes in the diversity and quantity of species in the intestines) contributes to Alzheimer's disease. 

At first glance, this sounds like a logical jump.  Gut bacteria are secluded in the intestines.  The brain is separated from the rest of the body by the blood-brain barrier.  So what could bacteria in the gut have to do with the brain? It suggests an interaction between two (seemingly) heavily guarded parties. Here, I’ll try to convince you of the current evidence that has led researchers to connect the two(1,2).

But first, a background on Alzheimer's Disease

Alzheimer's is characterized by inflammation and the formation of amyloid-beta and tau deposits in the brain. These deposits are misfolded, aggregated proteins.  In a sort of chicken or the egg scenario , it's unclear whether these deposits are the cause of problems or whether they are the result of the brain trying to cope with something else.  For example, studies that have looked at the removal of amyloid-beta haven’t had the most promising results(3).  If amyloid-beta is causing dysfunction, why is removal ineffective? This question remains an issue(1, 2).

What we do know about Alzheimer's is it is associated with several risk factors.  A risk factor is something that seems to increase the likelihood of a particular disease. It is not the same as saying that it directly causes the disease. Instead, think of a risk factor as something associated or correlated with a disease that may or may not be causational (no guarantees). It is generally a good practice to avoid risk factors if you are trying to stay healthy.

For Alzheimer's, diet affects your risk.  Japan, which experienced dietary changes due to globalization, is seeing increases in Alzheimer's rates.  Sleep and dysfunction of the circadian rhythm also contribute. Our levels of activity (the lack thereof) also put us at risk.  Even things as random as too much noise too frequently seem to have an affect (1,2).  

The Microbiome Connection

One fairly strong risk factor that is just starting to be explored might be in the gut. People with Alzheimer's have very different microbiomes (different proportions of species) than healthy people. In the normal gut, there are about 1,000 species of microbes. The gut is composed of a mix of both helpful (lactobacillus, and bifidobacterium) and pathogenic bacteria. As we age, the composition of our microbiome changes (1, 2).

This is a big deal because the composition of microbiomes has connections with a lot of common pathologies including diabetes, obesity, heart disease, and of course, Alzheimer's. We are just now working out why the Alzheimer's connection exists (1, 2).

One possible reason is that when the microbiome changes, it changes both the chemicals secreted in the gut and the response of the immune system. A change in microbiome composition can cause gut bacteria to move into the lymphoid system. That, in turn, increases the permeability of the intestine and the blood brain barrier. This means that the gut and the brain are not impenetrably separated. Bacteria in the gut and their secretions can reach the brain.  And in the case of Alzheimer's, they do (1, 2).

Once we accept that bacteria are invading other areas of the body, we need to think about what they secrete. They make amyloid peptides, proteins that contribute to making biofilms, cell-to-cell adhesion, and other interactions. If “amyloid” sounds familiar, it should. The bacterial amyloid is fairly similar to the problematic human beta-amyloid we see in Alzheimer's disease.

We also know Alzheimer's is characterized by inflammation. Bacteria secrete lipopolysaccharides which contribute to neuroinflammation when they are in the bloodstream.  Alzheimer’s patients have lipopolysaccharides levels three times greater than healthy people(1, 2).

Summary

Here are the main facts covered to suggest a connection between microbiomes and Alzheimer's disease:

• Alzheimer's patients have abnormal microbiomes.
• Microbiome changes can cause bacteria to move into the lymphoid system giving it a path to reach and influence the brain.
• Bacteria has been shown to reach the brains of Alzheimer's patients.
• Bacteria can secrete amyloid similar to the amyloid-beta we find in Alzheimer's Disease.
• Bacteria can secrete lipopolysaccharides, which cause neuroinflammation. Neuroinflammation is one characteristic of Alzheimer's. Alzheimer's patients have high levels of lipopolysaccharides .

Sources

1. Age-Related Changes in the Gut Microbiome and Alzheimer’s Disease. Fight Aging! https://www.fightaging.org/archives/2020/04/age-related-changes-in-the-gut-microbiome-and-alzheimers-disease/ (2020).
2. Askarova, S. et al. The Links Between the Gut Microbiome, Aging, Modern Lifestyle and Alzheimer’s Disease. Front. Cell. Infect. Microbiol. 10, (2020).
3. Makin, S. The amyloid hypothesis on trial. Nature 559, S4–S7 (2018).

Welcome

Greetings! Welcome to an informal course on human aging.

Why did I write this?
I’m writing this blog to encapsulate what I have learned about the science of aging. I want to create an easy to follow reference on aging science for the public. Although many resources exist for this already, (notably, the Life Extension Advocacy Foundation and Fight Aging) I believe this site could serve as a useful adjunct. Essentially, the goal of this blog is to explain high level concepts using simplified (non-PhD level) language. Once you have the basics, it will be easier to understand the constantly accumulating literature.

Who am I?
I'm currently a Master's student in biology with a B.S. in Biomolecular Engineering and a minor in Bioinformatics. Longevity science is the focus of my career and I intend it to be my life's work.

Snake Oil, Potions, and the Fountain of Youth
The first thing that has to be understood about aging research is that it is inherently controversial. To date, no real treatment, supplement, drug, or fountain has been able to strongly enhance human lifespan. Yes, there are interesting and perhaps hopeful avenues of research on it that could perhaps go in that direction. I would not be writing this blog if that weren't the case. However, as things are at the moment, the best longevity health advice out there is basically to keep a healthy diet and exercise.

If you’ve heard headlines on longevity research you might ask:

“But wait, haven’t many studies altered lifespan in different animals?”

That’s absolutely true, but we must be careful. What works for a mouse does not always work in humans. We know this from many failed drug trials. So, we need to be careful about taking the results seen in rats or mice and assuming they work in people. Aging researchers, of course, are already privy to this. That’s why clinical human trials exist after animal models have been tested.

However, the general public and investors need to watch out. Those that lack scruples can easily advertise to an uneducated public certain youth treatments with no scientific proof that it works in humans.

Moreover, those in aging research have to work to mitigate the possible side effects or harm caused by treatments. This is one inherent problem of aging research in humans. When it comes to bioethics, our clinical duties are to cause no unnecessary harm to patients. Aging is a harm in itself, but is considered an inevitable harm. Therefore, using a drug with risks on an aging person (when they can be allowed to age naturally) is ethically problematic to study. Most of the medical and scientific community does not consider aging to be a disease.

With that in mind, it is hard to support clinical trials in humans. Would you support a study that used a potentially harmful drug that might hurt someone to cure something that is not a disease, and is perhaps an inevitability? Can you live with the potential that you might cause harm with no benefit? It is a tough dilemma. It is one of the reasons why it is so hard to really progress in aging research.

I do not say this to knock longevity research or to say that it should not be pursued. Rather, I think it is important that we are extremely transparent about this and call out the legitimate science from the charlatans. The charlatans will sell you untested drugs that received okay results in mice. The true researchers are holding back and working hard. Their goal is a noble one, even if its final outcome cannot be reached. Even if human lifespan cannot be extended to any significant level aging research is still basic science. By studying aging, we are also studying the major outcomes of aging: heart disease, cancer, Alzheimer's, and many other diseases. Aging is a primary factor in many common causes of death. I like to imagine that a trickle down effect for all these diseases exists when the root cause, aging, is studied. To me, this makes studying aging well worth the pursuit.