Osteoarthritis and Cellular Senescence
A brief discussion on the scientific bases of the correlation between accumulation of senescent cell and development of Osteoarthritis
When we get old and weary, we retire. Funny enough, our cells like to do the same. Cellular senescence is essentially when the cell retired. Unlike your grandpa though, retired cells are not the kind of retiree you want to have by your side. They are noisy, producing inflammatory agent that disrupt tissue function, and they are unproductive. To a further extent, too much of those retired cells in your tissue are a main factor in aging and age related pathologies. In this article, I would like to briefly discuss the correlation between having too much senescent cells, and one of the most prominent age related pathology: Osteoarthritis.
A brief readers manual:
This article is divided into two general section.
The first is a more concise, less tech heavy narrative on the topic. This is suited for readers who wish to have a general understanding on the phenomenon but not yet wish to dive through the maze of scientific literature and technical terms. The writing will be in the most simple terms and analogy; my goal is for everyone to have basic understanding of the concept.
The second is a research report based and with extensive reference to several published studies prominent in the field. This is suited for readers who wish to discuss the topic on a technical level, understanding the mechanism behind the phenomenon. I have attached a list of definition for technical terms for more convenient reading experience, please enjoy from here the pleasure of knowledge.
*Citation and reference to be located at the end of the text.
Section 1: The Narrative
1.1 Conclusion first, does senescent cell cause osteoarthritis?
No, there is not yet any scientific agreement on the causation of Osteoarthritis by the accumulation of senescent cells. What we do have, though, are evidence suggesting a correlation.
The several evidence suggesting this correlation are as below: Firstly, senescent cell accumulation are seen in larger numbers in sample of human cartilage with osteoarthritis. Secondly, senescent cell is identified to increase after experimentally induced joint injury; a potential cause of Osteoarthritis. Thirdly, senescent cells are known to be able to destroy the lubricating liquid inside our joint. And lastly, repairing mechanical injury in the joint does not stop the disease from further developing, indicating potential factor like senescent cell may exist.
So, you would ask, isn’t the evidence pretty strong that Osteoarthritis is caused by senescent cells? Well, yes, but no. This might seem like irrelevant scientific squabble between two bearded old man in white coat, but in fact it is important. To understand why, let me first talk briefly about what Osteoarthritis is, and then what argument stands against the causation thesis.
1.2 Osteoarthritis
80% of us humans over the age of 65 are effected by osteoarthritis. It is the most common joint disease for the older folks and ladies. Generally, patient of osteoarthritis feels immense joint pain, swelling, and in more extreme cases, mechanical failure to movement. It is very debilitating, stripping away millions of people’s ability to function on a daily basis.
The cause of osteoarthritis can be multiple, but the pathological process of the disease is simple. The smooth layer of lubrication (synovium, a type of lymph fluid) in your joints wears off, the cartilage are stuck between two hard bone smashing against each other in your every movement. For me at least, thats quite gruesome to picture.
A further progression of the disease may see the cells on the exterior layer of your connective tissues (to be specific, the subchondral bone, the layer beneath the cartilage) calcifies, and form into hardened object impeding movement. This causes some horrific pain and debilitation.
1.3 So why isn’t senescent cell indeed the cause?
Well, now this is a little more complicated. In simple terms, cells in the joint, especially the cartilage, doesn’t like to replicate a whole lot. The most common means of formation of senescent cells, however, are replication. The Hayflick limit, a fancy ways of speaking, is the upper most times a cell can replicate before the genetic material is in danger of being damaged. The general number for this limit is around 50 replication. After the cell reaches the limit, it either under go apoptosis (essentially cellular suicide), or under go senescence (thus retire). Without much replication, there must be another factor causing senescence.
The most prominent potential factor identified is oxidative stress. When reactive oxygen series (ROS), as name sake, oxygen that react with molecules in and around cells, accumulate, oxidation reaction becomes more likely. This process of oxidation have potential in damaging the cells genetic sequence. Any damage to the genetic sequence will likely lead to the cell to respond with stopping all its activity thus senescence, lest it mutate and develop into cancer and other horrible stuff.
So why is this related? Well, as it turns out, when your joint develops osteoarthritis, it releases reactive oxygen series. Ah. Now this is a loop, isn’t it. So is it the development of osteoarthritis inducing the oxidation which caused the senescent cells, thus explaining why senescent cells are seen at sight of osteoarthritis? Or is it the senescent cells that cause the Osteoarthritis in the first place? Well, we don’t know. Or best guess is, however, a strong correlation exsist.
1.4 My Reasons (hypothesis) and rebuttal
I promise, the next technical term you will hear is the last you will need to get through this article. SASP, the senescence associated secretory phenotype, or in comprehensible english the behaviour of senescent cells are a crucial component to our body’s immunoresponse to physical injury. Senescent cells cause inflammation, as we mentioned before. Inflammation is generally good clearing out the debris in a injured cite. Thus, when the immunoresponse cells (macrophages) initially arrive at the cite of the impact, they trigger a response that causes SASP function.
In the case of osteoarthritis, when the injury (or aging related cause) in the joint occurs, our bodies immunoresponse attract the senescent cells over to the cite. However, instead of behaving like what they're suppose to and be productive, the senescent cells instead starts to break down the lubricant in the joint (the reason is yet unknown why). This cause more friction in the joint, more hard bone bashing against each other (with cartilage innocently stuck in between), and thus more injury. The negative loop causes further progression of osteoarthritis. Now this again is a hypothesis basing on current result, but I still wish it to be presented here.
Another ignored factor is, what if the cell in the bone, cartilage and everything in between goes senescent? Well, studies found that in initial development of osteoarthritis, the cells making up the surface of the cartilage and connective tissue (subchondral bone) begin to behave differently. Instead of maintaining the structural integrity, they instead start actively destroying the structure, and forming into hard boney lumps (sclerosis). Generally, when behaviour of cells change so drastic, we can suspect some functional changes occurred on the genetic level (well phenotype in this case). This stands in osteoarthritis, as it is believe that the senescence in the cells composing the bone that caused this behaviour.
1.5 Why this topic again?
Well, whether the cause or not, from the current information, we can comfortably say reducing the senescent cells in the joint will help patients affected by osteoarthritis.
In a experiment, two mouse, which are identical twins, were tested for the affect in removing senescent cells from joints. One of the mouse have senescent cells in their joints systematically removed every 12 month of age (they age differently [quicker] than human) until its death from natural cause, while the other is used as a control. The mouse with its senescent cells removed have joint structure similar to young healthy mouse at the moment of its death, while the controlled mouse seen the normal decay in structure and pathological development.
As we know accumulation of senescent cells play a factor in the development of osteoarthritis, clearing it out will help reduce the effect. As osteoarthritis is the most prominent joint disease from aging, affecting 80% of all people over 65 years old, the opening up of a new treatment pathway, or perhaps even prevention pathway, is sure to impact millions. Lets wait and behold what unravels in this field in the foreseeable future.
This is the first of a series of article I will be writing on senescent cells, age related pathologies, and recent innovation in human longevity. Please follow me for more content like this. Please also leave your comment on what you like and what you wish is better, my sincere gratitude for your input and advice!
Section 2: The research report
2.1 Interest in Senescent cell and Osteoarthritis: Where from?
The roll of senescent cell (SNC) in osteoarthritis, had sparks interest in development of senotherapies, and treatments. One major senotherapeutic strategy is senolysis, in which senolytic (drugs) are used to specifically kill SNCs. Common strategies include inhibiting pro-survival adaptation of SNCs used to resist apoptosis, and inhibition of senescence-associated secretory phenotype (SASP). (Nature; Childs, 2017)
Define:
1. Osteoarthritis: Most common form of arthritis, occurs when protective cartilage cushioning end of bones wear down over time. Mostly affect and damage joint in hands, knee, hips and spine.
2. Senolytic: class of small molecules under research with ability to selectively induce death of senescent cells.
2.2 Brief overview of Osteoarthritis
Osteoarthritis are the most common joint disease of late life, affecting 80% of individual over age 65. Patient may develop OA after joint trauma, manifesting as joint pain, swelling and mechanical failure. Cartilage lining the surface of articulating joints provides low-friction surface, allowing painless movement. Membranous structure encloses entire joint known as synovial membrane, or synovium producing synovial fluid that bathes the joint, also serving as nutrients and mechanical lubricant. (Jeon, 2018)
Define:
1. OA: Gradual cartilage loss by wear and tear, with more complex causes identified.
2. Synovial membrane: Synovial cavity consist of synovial membrane heavily infiltrated by lymphocytes and plasma cells, and synovial fluid.
3. Synovial fluid: also called synovia, a viscous, non-Newtonian fluid, used by synovium and coats tendons in tendon sheaths.
4. Tendon: A tough ban of fibrous connective tissue connecting muscle to bone and capable of withstanding tension. Made with lot of strong collagen fiber within.
5. Tendon sheaths: Layer of synovial membrane around a tendon, protects tendon in bumpy parts with bones such as wrist and food. A layer of connective tissue in short. (NCBI, 2018)
Cartilage tissue fractures at sites of high mechanical stress, a process termed fibrillation. In cartilage tissue and in fibrillation, the loss of extracellular matrix (ECM) reduces proteoglycans and collagen fiber. Beyond these changes, subchondral bone forms osteophytes, fibrocartilage capped bony outgrowth present on joint margins, and dysfunctional synovial fluid compromises lubrication.
Define:
1. Extracellular matrix (ECM): three-dimensional network of extracellular (outside of cells) macromolecules, such as collagen, enzymes and glycoproteins providing structural biochemical support to surrounding cells.
2. Proteoglycans: Protein heavily glycosylates.
3. Glycosylated: Reaction in which carbohydrate, the glycosyl donor, is attached to hydroxyl or other functional glycosyl acceptor. [require deeper dive into biochemistry]
4. Collagen fiber: Fiber consisting of collagen, the most plentiful protein in human body, providing structure to skin and helping with blood clot. Building block for tendons and ligament.
5. Subchondral bone: layer of bone just below cartilage in a joint.
6. Osteophytes: bony lumps (bone spurs) that grow on bones of spines or around joints. Generally, form next to joints affected by osteoarthritis.
7. Fibrocartilage: Transition tissue with a blend between hyaline cartilage and dense fibrous connective tissue.
8. Fibrous connective tissue: surrounded by blood vessels and provides support for internal organs.
Osteoarthritis develop in a process where fibroblast-like cell in periosteum differentiate into chondrocytes and deposit cartilage matrix, serving as template of new bone. Synovium thickens and frequently inflames; menisci may be displacing into joint space.
Define:
1. Fibroblast: Cell that synthesizes extracellular matrix and collagen.
2. Periosteum: Dense layer of vascular connective tissue enveloping the bones except at surface of joints.
3. Chondrocytes: Only cell found in healthy cartilage, produce and maintain cartilaginous matrix, consist mainly of collagen and proteoglycans.
2.2.2 Senescent cell and association
Several in vitro discoveries found link between inflammation and joint trauma; differential expression of inflammatory molecules and pathways, however, was not found in chondrocytes isolated from normal human OA cartilage. Further, repairing mechanical injury or instability does not prevent PTOA (Posttraumatic OA) development, suggesting additional biological mechanism beyond simple mechanical wear and tear or purely inflammatory factor contribute to clinical disease.
Cellular senescence is a potential molecular mechanism promoting age-related OA and PTOA. SnCs with their robust proinflammatory secretome entails secretion of extracellular proteases, proinflammatory cytokines, chemokines and growth factor. Although cell and context dependent, the effect is generally negative on surrounding tissue.
Define:
1. Chemokines: A family of small cytokines, signaling proteins secreted by cells. Have the ability to induce chemotaxis in nearby cells.
2. Chemotaxis: Movement of organism in response to chemical stimulus in the general direction of increasing or decreasing concentration of particular substances.
Oncogene induced senescent human cells generally share certain SASP protein overlap with radiation induced senescence in same cell (genotoxic stress induced), but SASPs induced by either have distinct features. This outline the potential difference in between distinct senescent cell, though this will not be covered in this report. (further reading: Jeon, 2018)
2.3 Pathological connection
Inflammation and degradation of tissue in OA could be mediated by SASP associated factors. Accumulation of senescent cell and the corresponding buildup of inflammatory agent are thought to have impact on development of OA. p16INK4A and p21 expression are found in multiple tissues in arthritic joints. The connection in the role of the tissue specific SASP and progression over time is not yet elucidated.
2.3.1 Spotlight: Cartilage
Articular cartilage primarily populated by chondrocytes made of collagen and aggrecan and functions in maintenance of ECM level and remodeling MMPs. During the initial development of OA, chondrocytes near the superficial layer begin clustering and increases anabolic activity. The chondrocytes phenotype is alternated toward ECM destruction and inflammatory cytokine production. This happens primarily through collagen degrading MMP-13 and aggrecan-degrading ADAMTS-5. Proteoglycans becomes depleted and collagen network is eroded, chondrocytes become hypertrophy-like and leading to calcified cartilage.
Define:
1. Articular cartilage: Layer of cartilage connecting the bone and the muscle.
2. ECM: Extracellular matrix, three-dimensional network, such as collagen and glycoprotein providing structural and biochemical support to surrounding cells.
3. MMPs: Matrix metalloproteinase, calcium dependent zinc-containing endopeptidases. Holds important role in tissue remodeling associated with various physiological and pathological process, such as: tissue repair, cirrhosis, arthritis…
4. Anabolic activity: Process of building organs and tissues. Produce growth and differentiation of cells and increase of body size.
5. Collagen: component of fascia, cartilage, ligaments, tendons, bone and skin. Have great tensile strength.
2.3.1.2 Factor of correlation with senescence
Senescent accumulation is seen at higher level in human OA cartilage compared with age-matched healthy cartilage. Localization of senescent chondrocytes near osteoarthritis lesions, but not healthy tissues. “The positive correlation between presence and degree of senescence and disease severity remain controversial.” (Newest result should validate/invalidate this hypothesis.)
2.3.1.3 replicative senescence vs. other senescence pathway
Chondrocytes rarely replicate in cartilage homeostasis; thus, other senescence responses rather than replicative senescence are likely the factor. Nonetheless, SnCs secrete SASP factors, including inflammatory mediators, and enzymes capable of digesting ECM. Likely candidate to the root cause might be senescence induced by oxidative stress, mitochondrial dysfunction, genomic and epigenomic damage.
Define:
1. Homeostasis: The ability to remain relative stable internal state in human body that persists despite external changes.
2. Inflammatory mediators: A messenger acting on blood vessels and/or cells to promote an inflammatory response. (Further reading on inflammation: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5993766/)
ROS may damage telomeres independent of cell replication. This promotes senescence and loss of ECM, and further lead to chondrocytes apoptosis. In chondrocytes, ROS induced mitochondrial dysfunction can amplify chondrocyte inflammatory and matrix catabolic responses to IL-1beta and TNF, mediated by NF-kappaB. Mechanoresponsive nature of chondrocytes indicate, excessive loading of articular surface and acute joint trauma resulting from it led to release of ROS, thus increasing oxidative stress on chondrocytes.
Define:
1. ROS: Reactive oxygen species, natural biproduct of normal aerobic metabolism of oxygen, plays important roles in modulation of cell survival and cell death. Elevated level intracellularly (within cell) constitute oxidative stress. (Further reading on ROS: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5921829/)
2. Mitochondrial dysfunction: occurs when mitochondria no longer function in optimal state due to disease of conditions.
3. Catabolic: set of metabolic pathways that breaks down molecules into smaller units, either oxidized to release energy or used in other anabolic reaction.
4. IL-1Beta: Interleukin 1 beta. Pro-inflammatory cytokine.
5. TNF: Tumor necrosis a cytokine (cell signaling protein), capable of inducing fever, apoptotic cell death, and inhibit tumorigenesis. Respond to sepsis via IL-1 and IL-6 producing cells.
2.3.2 Spotlight: Subchondral bone
Subchondral bone is connected mechanically, physical and biologically with cartilage. Subchondral bone sclerosis is a hallmark of OA progression. Bone change in OA joint may be the cause or the consequence of articular cartilage dysfunction. Thinning or loss of volume of subchondral bone is the early sign of disease, follow by sclerotic and stiffens in pathological progression. TGF-Beta activity is present in subchondral bone at onset of ACLT induced PTOA. p16INK4A-positive SnCs are present in subchondral BM of aged mice in ACLT-induced OA model.
Define:
1. Subchondral bone: The layer of bone right below cartilage in a joint. (chondral referring to cartilage)
2. Sclerotic: Stiffening of a tissue or anatomical feature.
3. TGF-beta: Transforming growth factor beta, a multifunctional cytokine produced by all white blood cell lineages. Able to activate downstream substrates and regulatory proteins, induce transcription of different target genes that function in differentiation, chemotaxis, proliferation and activation of immune cells.
4. ACL: Anterior cruciate ligament, key ligament stabilising knee joint. ACLT; Anterior cruciate ligament trauma.
2.3.3 Spotlight: IPFPs and senescence
IPFPs contain immune cells that can respond to and/or secrete inflammatory cytokines, including SASP-associated TNF and IL-6. IPFPs may contribute to inflammation in joint, either in response to SnCs in conjunction tissues or owing their harboring of SnCs.
Define:
1. IPFPs: located inside and near joint capsule, potential source of proinflammatory cytokines and chemokines in OA.
2.3.4 Spotlight: Synovium and senescence
It’s unclear whether change in Synovium are primary or occur secondary to joint inflammation and cartilage breakdown. SnCs in nearby joint tissues may impact the synovium through SASPs, whose chemokines and cytokines can attract and modulate resident and migrating immune cells in synovium. Additionally, synovial fibroblasts secrete less synovial fluid, rich in hyaluronan and lubricin, leading to increased cartilage degradation. Fibroblast also unregulated catabolic genes and proinflammatory cytokines, leading onto joint destruction and OA pain. Cell in synovium proliferate and become more susceptible to senescence. p16INK4Apositive senescent cell are present in synovium in ACLT-induced OA.
2.4 Senescence and OA, Causation?
Senescent chondrocytes are associated with both posttraumatic and age-related OA, yet causation is currently unclear. Experiment identified SnCs increased in joint over 2-week period after PTOA inducing ACLT surgery, later decreasing to a steady level above baseline. Expression of SASP factors MMP-13, IL-6 and IL-1B evoke similar changes in joint.
Impact of SnCs clearance on age related OA was tested in another transgenic mouse model, allowing selective killing of SnCs using drug promoting caspase-dependent apoptosis. Animals with SnCs systematically removed starting at 12 months of age until natural death had structural and matrix appearance similar to young healthy animals.
The immune system is guardian of tissue integrity. SnCs and SASP are a link to immune system and therefore tissue damage response. Joint trauma initiates immune response by sending DAMPs signal. The DAMPs signals include alarmins such as HMGB1 and product of cartilage breakdown. The DAMPs release into synovial fluid generate proinflammatory and catabolic mediator by signaling through pattern recognition receptors on chondrocytes. These receptors include TLRs and RAGE receptor. TLR1-TLR7 and TLR9 are activate in synovium of OA patient. These in turn triggers MyD88-dependent signaling activates NF-kappaB, the proinflammatory stress-related transcription factor, and secretion of SASP-affiliated cytokines.
Define:
1. HMGB1: nuclear protein that binds to DNA and acts as architectural chromatin binding factor. In extracellular form, it binds to inflammatory RAGE receptor.
2. Receptor: special structures found on cell membranes. Activate when bind (attach) to specialised molecules.
3. TLRs: Toll-like receptors, plays key role in innate immune system. Usually express as single pass membrane spanning receptor found on sentinel cells such as macrophages and dendritic cells.
Macrophages, first innate immune cell to migrate to wounds, may acquire proinflammatory, proregenerative or profibrotic phenotypes. Proinflammatory macrophages produce TNF-alfa, IL-1 and IL-6 to remove debris. Later, proinflammatory macrophages transition toward proregenerative and profibrotic to promote tissue repair in response to IL-4, IL-10 and IL-13 or other immunoreceptors agent.
Define:
1. IL-x: Interleukin-x family cytokines, plays central role in regulation of immune and inflammatory responses to infection or sterile insult.
This is the first of a series of article I will be writing on senescent cells, age related pathologies, and recent innovation in human longevity. Please follow me for more content like this. Please also leave your comment on what you like and what you wish is better, my sincere gratitude for your input and advice!
Reference:
Childs, B., Gluscevic, M., Baker, D., Laberge, R., Marquess, D., Dananberg, J., & Deursen, J. (2017, July 21). Senescent cells: An emerging target for diseases of ageing. Retrieved November 18, 2020, from https://www.nature.com/articles/nrd.2017.116
Jeon, O., David, N., Campisi, J., & Elisseeff, J. (2018, April 2). Senescent cells and osteoarthritis: A painful connection. Retrieved November 18, 2020, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5873863/
IQWiG. NCBI. (2018, July 26). What are tendons and tendon sheaths? Retrieved November 15, 2020, from https://www.ncbi.nlm.nih.gov/books/NBK525770/
Pignolo, R., Passos, J., & Khosla, S. (2020, July 10). Reducing senescent cell burden in aging and disease. Retrieved November 11, 2020, from https://www.cell.com/trends/molecular-medicine/fulltext/S1471-4914(20)30077-0
Song, S., Lam, E., Tchkonia, T., Kirkland, J., & Sun, Y. (2020, April 06). Senescent Cells: Emerging Targets for Human Aging and Age-Related Diseases. Retrieved November 12, 2020, from https://www.sciencedirect.com/science/article/pii/S0968000420300852
Research:
Cell Signalling technology. (2020). Cellular Senescence. Retrieved November 15, 2020, from https://www.cellsignal.com/contents/_/cellular-senescence/overview-of-cellular-senescence
