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Newfound Enthusiasm in Mining Senescent Cells for Mechanisms Relevant to Therapy

Cellular senescence is one of the root causes of aging. Nonetheless, the study of cellular senescence used to be a comparative backwater in aging research, and as a topic it was mostly of interest to cancer researchers seeking ways to better shut down the replication of cancerous cells. But there is nothing quite like having a company raise $300 million in venture funding for rejuvenation therapies based on manipulation and destruction of senescent cells to bring a little excitement to this part of fundamental aging research. Who knows how many useful, exploitable mechanisms are yet to be found in senescent cells and the signals they generate? Each one is a potential lottery ticket for the discovering institution and research group.

This was in fact always the case, and for decades a compelling set of evidence has strongly suggested that the accumulation of senescent cells is a significant contribution to aging. Yet next to no-one was funding or working on it seriously until the high-profile proof of concept study in mice reported in 2011, in which senescent cells were eliminated and health and life span improved as a consequence. After that point, the avalanche started, leading to today's crop of first generation senolytic therapies capable of selectively destroying a fraction of the senescent cells present in older individuals.

Today I thought I'd point out a couple of examples of the sort of paper that results from an influx of funding and interest to the study of the fundamental biochemistry of senescent cells. The research community is mining for gold. The first explores the harmful signals secreted by senescent cells, the major way in which they cause tissue dysfunction in aging and age-related disease. A faction within the research community is more comfortable interfering in these signals rather than destroying senescent cells, despite it likely being a far worse and more challenging approach to therapy. The second paper is one of many in which researchers explore the role of mitochondrial activity in senescence, in search of approaches that might modulate the activity in beneficial ways. Both papers are quite different in focus, but they emerge from the same newfound interest in senescence as a cause of aging.

Small extracellular vesicles and their miRNA cargo are anti-apoptotic members of the senescence-associated secretory phenotype

Senescent cells lose their cell type specific functionality and replicative potential required for tissue regeneration and acquire a senescence-associated secretory phenotype (SASP). The SASP is characterized by the secretion of growth factors, pro-inflammatory cytokines and chemokines, as well as extracellular matrix (ECM) remodeling enzymes. These SASP factors are considered to over-proportionally exert negative effects on tissue homeostasis and regeneration in vivo if chronically present by acting in a paracrine manner on the neighboring cells and ECM. Attenuation of the negative effects of the SASP have been shown to restore the formation of functional human skin equivalents and has been suggested as a putative target in preventing age-associated diseases and frailty.

Recently, extracellular vesicles (EVs) and their cargo have been reported to act in a similar manner as hormones or cytokines during intercellular communication. They are secreted by many, if not all cells, and by encapsulation of their cargo, they transport proteins, mRNAs, lipids, and non-coding RNAs, specifically miRNAs, over short or long distances. Thus, although many protein based SASP factors have been identified, miRNAs and EVs are under suspicion to be part of the SASP. However, a systematic catalogue of SASP-miRNAs has not yet been established and their selective secretion during senescence has not been studied so far. Here, we confirm that EVs and their miRNA cargo are indeed part of the SASP (EV-SASP) and identified a set of selectively retained and secreted miRNAs after the onset of senescence. In addition, senescent cell derived EVs might contribute to an anti-apoptotic environment in tissues where senescent cells have accumulated.

Mitochondrial peptides modulate mitochondrial function during cellular senescence

Mitochondria play important roles in cellular energy production, metabolism, and cellular signaling. These organelles have their own genomes, the mitochondrial DNA (mtDNA). Epigenetic modification of mitochondrial DNA, including DNA methylation, is still controversial. The overall mitochondrial DNA methylation occurs at a lower frequency compared to nuclear DNA, but specific locations have been found to be differentially methylated in certain cellular conditions or in different biological samples.

Humanin is a 24-amino acid peptide encoded within the mtDNA. It is secreted in response to cellular stress and has broad cytoprotective and neuroprotective effects. MOTS-c is a 16-amino acid peptide encoded within the mtDNA that improves metabolic functions. Among the basic processes that are known to drive aging phenotypes and pathology are genomic instability, epigenetic alterations, mitochondrial dysfunction, and cellular senescence. Although humanin and MOTS-c have protective roles in multiple age-associated diseases, the roles of these peptides in cellular senescence have not been explored.

Senescent cells are metabolically active, producing energy-consuming effectors of senescence, despite the loss of proliferative activity. Depending on the inducer, senescent cells show higher levels of glycolysis, fatty acid oxidation, and mitochondrial respiration. Manipulating bioenergetic status can induce senescence and a SASP, suggesting that bioenergetics play a role in the senescence phenotype. Thus, altering the metabolic status of senescence cells may be an important strategy for eliminating the deleterious effects of senescence. In this study, we investigate mitochondrial energetics and mtDNA methylation in senescent cells, and evaluate the potential of humanin and MOTS-c as novel senolytics or SASP modulators that can alleviate symptoms of frailty and extend health span by targeting mitochondrial bioenergetics.

Comments

The problem I believe in that that we have too many basic scientists and gerontologists who merely study ageing and too few biomedical engineers who do real work -- repair damage. In other words we should switch our focus from ageing research to rejuvenation engineering. There are many areas where we know enough to make working therapies. Basic researchers do their work quite well -- they are trained to investigate new nature laws, not solve engineering problems. Since ageing is an engineering problem, then from the basic researchers and gerontologists' point of view it looks like "not my job" to reverse it.

Posted by: Ariel at June 19th, 2018 6:15 AM

All I can say is - thank god for Oisin Biotechnology. It will be far quicker to just remove a decent percentage of senescent cells and then look for the effects than all this research into trying to modify the downstream signalling of senescent cells.

Also, I kind of have a suspicion that Unity's small molecule drugs will have limiting side effects. Although Oisin's Fusogenix lipid delivery is still unproven in humans.

Given the size of the COPD market and that nebulized lipid lung gene therapy is almost out of the clinic and availble (see the UK Cystic Fibrosis Gene Therapy Consortium) I'm surprized Oisin haven't considered targeting COPD. The constortium is focused at present on getting a modified lentivirus into clinical trials, but say they have some comercial interest for their plasmid in lipid technology from their previous trial. I can't be the only person thinking that they should try Oisin's fusogenix lipid technology?

Posted by: Jim at June 19th, 2018 8:51 AM

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