How Cellular Senescence Is Slowly Killing You — and How to Fix It

1. Introduction

When your body ages, it’s not actually your body aging. The cells in your body age, and as they do, not all of them die. Some of them enter “senescence,” which is essentially your cells throwing a tantrum and entering dysfunction — at the same time recruiting the cells around them to do the same.

2. Cellular Senescence

2.1 Definition

Put simply, cellular senescence is a permanent state of cell cycle arrest. Senescent cells can never re-enter the cell cycle no matter the physical stimuli, yet they are very resistant to the apoptotic death signal, so they refuse to die and are just sort of hanging around, looking for trouble.

2.2 Stages of Senescence

1) Regulators activate the senescence response.

2) The senescent cell exits the cell cycle. The cycle may be blocked in the G1 phase to prevent the initiation of DNA replication, or it may be blocked in the G2 phase to prevent mitosis — this causes it to completely bypass mitosis and directly return to G1, where it stays.

3) The senescent cell increases in size due to various factors, including the increase in lysosome activity. It also gains apoptosis resistance (it cannot die) and starts expressing inflammatory SASPs (discussed later).

4) The senescent cell contributes to chronic inflammation.

A little confused over parts 3 and 4? Read on.

2.3 Purpose

The primary use of senescent cells is as a defense mechanism. When cells become cancerous, they grow rapidly and create deadly tumours, so the body makes them senescent — unable to replicate.

Unfortunately, as you will read later in the article, senescent cells later in life actually cause tumours and other health problems, so this begs the question — why did natural selection (the “selection of the fittest” process where healthier animals reproduce more and are thus able to better pass down their genes) end up “selecting” for senescent cells?

The answer lies in the antagonistic pleiotropy theory, which states that aging is caused by “pleiotropic” genes. These are genes that enhance fitness in early life but harm fitness later in life. Realize that natural selection is much stronger early in life, before reproduction. Why? Remember that natural selection is hinged on reproduction. The more you reproduce, the more you pass down your genes, and the more your genes are “naturally selected” for. So once you’ve passed the age of reproduction, it doesn’t matter when you die and doesn’t matter what your genes might do to you — natural selection won’t care.

If that sounds confusing, think about it this way. Let’s pretend you have a gene A, and your friend has a gene B. Gene A is an antagonistic pleiotropic gene, while gene B is not. You both parent 5 children, and then once you’re maybe 50, gene A and gene B kick in. So you die a bit earlier, and your friend lives longer. But you’ve both produced 5 children who have your genes, and the same happens for the next generation. Thus, the antagonistic pleiotropic genes live, while the genes that might harm an organism before reproduction are naturally selected against.

This theory was further confirmed when scientists discovered that senescent cells do a significant job preventing the creation of tumours during the rapid growth of embryonic cells early in life.

3. Root Causes

Note: The causes below all link back to the activation of tumor suppressors, which initiate pathways that promote senescence (more rarely, it may be other pathways)

3.1 Replicative Senescence

Every time DNA duplicates in a cell before mitosis or meiosis, telomeres shorten by a few base pairs. The enzyme telomerase reduces telomere shortening, but this is rare and is often only found in cancer cells, embryonic stem cells, and other cells that need to constantly replicate. They keep shortening with every replication after they reach what is called the “Hayflick limit,” where the cell can no longer duplicate without accumulating DNA damage.

This elicits the DNA damage response (DDR), which initiates a pathway that causes senescence. DNA damage agents like UV light also cause the DDR, which is considered “therapy-induced senescence.”

3.2 Stress-Induced Premature Senescence

This form of senescence covers almost all forms of senescence outside of replicative senescence and is considered “abnormal” since telomere shortening is more “natural.”

The main form of stress-induced premature senescence is oxidative stress and other DNA stresses.

Oxidative damage starts with free radical production. Typically, electrons are flowing down the electron transport chain during cellular processes like cellular respiration and photosynthesis, but during free radical production, they escape this transport chain. Once escaped, electrons react with free oxygen or free nitrogen, producing a “reactive oxygen species,” also called an ROS. An ROS then goes through the cell with the power to oxidize lipids, proteins, mitochondrial DNA (mtDNA), and nuclear DNA (nDNA), which we call oxidative damage.

In proteins, ROS forms extra protein linkages or oxidizes protein backbones, causing protein fragmentation and aggregation of protein clumps, which harms the function of these proteins. In DNA, ROS causes the DNA strands to break as well as causing point mutations — mutations of single nucleotides. Even more unfortunately, ROS is extremely soluble in lipid bilayers, which allows it to sneak through these bilayers very easily. And even even more unfortunately, the cell membrane is made up of a phospholipid bilayer, which means that they are extremely vulnerable to oxidative damage.

Epigenomic damage is the other factor. When this occurs, the chromatin organization of the cells’ DNA relaxes and forms structures called senescence associated heterochromatin foci (SAHF), which are condensed regions of chromatin.

3.3 Oncogenes

Oncogenes are the group of genes that promote cell growth and often lead to tumour development. When these genes are expressed, they trigger altered DNA replication that initiates DDR responses and leads to tumour suppression, causing senescence.

4. Identification of Senescence

There are several ways senescence is identified:

1) The primary way scientists identify senescent cells is by looking for the activity of senescence-associated beta-galactosidase, which is a compound detectable at a pH of 6. This is effective for identifying senescent cells both in vitro (in the lab) and in vivo (within an organism).

2) The cell also has an abnormal size with a smoothed, enlarged, and flattened shape.

3) The cell has bigger lysosomes to accommodate for increasing amounts of lipofuscin (a product found in aging cells) in the cytosol.

4) The cell is arbitrarily oriented.

5. How Senescent Cells Behave in the Body

There are several broad impacts of senescent cells in the body:

1) Not only do senescent cells stay metabolically active, they have increased glycolysis and mTOR (pathway) activity because of its increased size, which puts stress on the body.

2) Senescent cells secrete the senescence-associated secretory phenotype (SASP), which includes proinflammatory cytokines (proteins crucial for cell signalling) causing inflammation. The SASP has many effects on the body, such as affecting cell proliferation, modifying cell niches, etc., but it has a key link to aging discussed later.

3) Senescent cells induce other cells into senescence. They deplete a catabolic enzyme (an enzyme that breaks down substrates) called glycogen phosphorylase, which causes the accumulation of glycogen which is correlated to more senescence induction. Moreover, mitochondria do not function properly in the early stage of senescence, so ROS is overproduced in senescent cells, leading to further oxidative damage that causes more senescence in a dangerous positive feedback loop.

6. Aging and Senescence

Aging animals have been found to gradually accumulate more senescent cells, and the inflammation that this causes ends up contributing to every major age-related disease.

Importantly, the SASP can “communicate” with immune cells such as NK cells, macrophages, and T cells to worsen inflammation. There are several examples of harms that stem from this:

1) The SASP secretes matrix metalloproteinases (MMPs) which can disrupt normal tissue structures in breast epithelial cells such disrupting milk protein production and cause dermal and epidermal thinning as well as a loss of collagen (an important structural protein in tissues).

2) Senescent pulmonary artery smooth muscle cells caused thickening and medial hypertrophy (expansion of the middle layer) of pulmonary arteries, which can cause pulmonary hypertension (high blood pressure).

3) The SASP in astrocytes can promote age-related neurodegeneration that leads to diseases like Alzheimer’s or Parkinson’s.

These studies were further proven when scientists found that transplanting senescent cells into young, healthy animals caused the impaired physical functions listed above. Furthermore, senescent cells are found to accumulate at sites of age-related disease — for instance, it is higher in the adipose tissue of old women with physical dysfunction than healthy elderly women. In mice, senescent cells have been shown to cause genomic instability, which led to faster aging.

7. Treatment

7.1 Senolytics

The aim of senolytics is to kill senescent cells, and there are a few popular methods.

1) Targeting senescent cell anti-apoptotic pathways (SCAPs). These pathways is similar to those that protect cancer cells from dying, and they are upregulated in senescent cells, preventing apoptosis. Scientists have already tried targeting SCAPs and their proteins as well as looking for inhibitors of anti-apoptotic proteins. So far, analyses identified 46 compounds that can be senolytic by targeting SCAPs. Let’s discuss 3 examples!

• D+Q: D represents dasatinib, which is an anti-cancer drug that inhibits cell proliferation and migration, as well as inducing apoptosis. Q represents quercetin, which is a flavonoid (a class of chemicals found in plants) that has many potential medicinal applications. When taken together as an oral dose, D+Q was shown to alleviate dysfunction in patients with idiopathic pulmonary fibrosis, and it has entered clinical trials for treatment of other age-related diseases.
• BCL-2 family inhibitors: The BCL-2 family of proteins help resist apoptosis in senescent cells, so scientists have been looking for BCL-2 inhibitors that can downregulate these proteins. Many inhibitors have been found and were shown to be effective in mice.
• P53: P53 is a transcription factor that plays a role in DNA repair, cell growth, apoptosis, and other forms of senescence maintenance, so scientists have been looking for compounds that target the p53 pathway. For instance, they designed a FOXO4-D-retro-inverso peptide to disrupt the interaction of FOXO4 and p53, which reduce p53’s impact on senescent cells, allowing them to undergo apoptosis.

However, there remain issues with SCAP-targeting senolytics. For instance, in some senescent cells, there may be multiple SCAPs, so targeting a single one will not work — in this case, we would need multiple compounds (e.g. having D+Q instead of only D or only Q). Moreover, some of the treatments used are considered “single SCAP node targets,” which induce apoptosis in a restricted range of senescent cell types and target a single pathway. However, these may have a greater risk of toxicity because higher dosage is needed to fully suppress a single node. On the other hand, using substances that lightly impact many nodes might be less dangerous but more difficult to do — it may require merging many substances into one treatment.

2) Some scientists believe that organisms have intrinsic immunosurveillance against senescent cells. This means that seenscent cells may already be being eliminated by mechanisms that the immune system uses to eliminate other threats. Thus, perhaps the best way to eliminate senescent cells is to boost the immune system’s ability to do this.

There are certain cell surface proteins that are highly expressed by senescent cells, and we may be able to harness the immune system’s existing defenses to target these proteins. For instance, scientists are developing engineered chimeric antigen receptor (CAR) T cells and vaccines to target these proteins. There has also been study about the interaction between NK and senescent cells. NK cells use perforin-mediated granule exocytosis (essentially, they secrete granules containing perforin to form pores on the target cell) rather than using the death receptor ligands (these ligands induce cell death by binding to cell receptors) that most other cells use. This method is likely better because senescent cells express decoy receptor 2 (DC2) which prevents targeting by death ligands.

Unfortunately, many of these techniques also affect non-senescent cells, and there is no way to switch it off like other senolytics. Moreover, it is extremely expensive and needs to be specifically made for each individual patient.

3) Proteolysis targeting chimera senolytics (PROTAC) is a targeted protein degradation method that uses a ligand specific to a target protein, an E3 ubiquitin ligase (you don’t need to know what that is, just know it exists), and a “linker.” The target protein is brought together with the E3 ligase, causing “ubiquitination” of the protein that leads to the degradation of this protein. This specific targeting can help destroy proteins critical to cellular senescence, such as the BCL-2 family. PROTAC also requires less drug exposure which allows for reduced toxicity.

7.2 Senomorphics

Senomorphics, also known as senostatics, suppress senescence without inducing apoptosis. Rather, it interferes with the regulators of the SASP, such as by using inhibitors of the mTOR pathway, reducing the harmful secretions of senescent cells. Unlike senolytics, this requires continuous administration of the drug. Let’s discuss two main examples!

• Rapamycin (also known as sirolimus): This was initially used as an anti-fungal agent until scientists discovered its immunosuppresive and anti-proliferative properties. It was used to prevent organ rejection during kidney transplantation, but now scientist have discovered that it can suppress SASPs and was proven to do so in mouse, rat, and human cells by inhibiting TORC1 activity in the mTOR pathway. However, it has side effects like thrombocytopenia.
• Metformin: This was originally used to treat type 2 diabetes but now has been found to downregulate the expression of SASP factors in many senescent cells.

7.3 Senoverters

Senoverters attempt to get senescent cells to reenter the cell cycle, but this appears to have less potential than the previous 2 methods. Scientists found that when some pathways’ signalling were suppressed, human dermal cells were able to reenter the cell cycle. Scientists also managed to reprogram senescent cells into pluripotent stem cells (cells that have yet to specialize).

7.4 Reactivating Telomerase

Reactivating telomerase was shown to reverse tissue degeneration in mice with telomere dysfunction, but the potential in humans appears limited.

7.5 Systemic v.s. Local Administration

Although senescent cells tend to gather in areas of physical dysfunction, they can also spread and gradually induce systemic dysfunction through the SASP. Thus, instead of administering drugs at global sites, scientists are working on administering them globally. This is particularly important for older individuals because they tend to have senescent cells accumulated in many areas.

7.6 Challenges

There are three main challenges discussed below.

1) Senescent cells are meant as a cancer-preventing technique, thus drugs need to be safety-tested extremely well. They must not prevent the formation of senescent cells, otherwise cancer cells could become rampant.

2) There are many different types of cells (e.g. muscle cells, skin cells, etc.) that may be responsive to different drugs, so scientists need to do single-cell tests.

3) Companies currently running senescent cell tests tend to be vague in their objectives, so it is difficult to fully grasp the point research is at.

8. Takeaways

If you forget everything else, make sure to remember these 3 key takeaways.

1) Senescent cells are permanently removed from the cell cycle and arise as a tumour-prevention method.

2) Senescent cells secrete senescence-associated secretory phenotype (SASPs), which cause aging.

3) Senescent cells use the senescent cell anti-apoptotic pathway (SCAP) to cause aging. To kill or reduce the harms of senescent cells, scientists target the SCAP.

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