Tissue accumulation of p16INK4a-positive senescent cells is associated with age-related disorders, such as osteoarthritis (OA). These cell-cycle arrested cells affect tissue function through a specific secretory phenotype. The links between OA onset and senescence remain poorly described. Using experimental OA protocol and transgenic Cdkn2a+/luc and Cdkn2aluc/luc mice, we found that the senescence-driving p16INK4a is a marker of the disease, expressed by the synovial tissue, but is also an actor: its somatic deletion partially protects against cartilage degeneration. We test whether by becoming senescent, the mesenchymal stromal/stem cells (MSCs), found in the synovial tissue and sub-chondral bone marrow, can contribute to OA development. We established an in vitro p16INK4a-positive senescence model on human MSCs. Upon senescence induction, their intrinsic stem cell properties are altered. When co-cultured with OA chondrocytes, senescent MSC show also a seno-suppressive properties impairment favoring tissue degeneration. To evaluate in vivo the effects of p16INK4a-senescent MSC on healthy cartilage, we rely on the SAMP8 mouse model of accelerated senescence that develops spontaneous OA. MSCs isolated from these mice expressed p16INK4a. Intra-articular injection in 2-month-old C57BL/6JRj male mice of SAMP8-derived MSCs was sufficient to induce articular cartilage breakdown. Our findings reveal that senescent p16INK4a-positive MSCs contribute to joint alteration.

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Over the past years, through in vitro studies and unique animal models, biologists and clinicians have demonstrated that cellular senescence is at the root of numerous age-related chronic diseases including osteoarthritis and osteoporosis. This non-proliferative cellular syndrome can modify other surrounding tissue-resident cells through the establishment of a deleterious catabolic and inflammatory microenvironment. Targeting these deleterious cells through local or systemic seno-therapeutic agent delivery in pre-clinical models improves dramatically clinical signs and extends health span. In this review, we will summarize the current knowledge on cellular senescence, list the different strategies for identifying senosuppressive therapeutic agents and their translations to rheumatic diseases.

Our societies are facing with the emergence of an exponential number of patients with age-related degenerative chronic diseases such as osteoarthritis or osteoporosis. The "better" aging will thus be at the center of the next medical challenges in order to delay the loss of independence of the elderly and reduce costs of our health services. Over the last 5 years, based on innovative mouse models or in vitro studies, several research teams have demonstrated that many age-related degenerative diseases have in common a deleterious accumulation of so-called senescent cells in their respective deficient tissues. Thus, under the concept of senolysis, it has been proposed to target pharmacologically in vivo these cells to eliminate them and thus delay the emergence of these chronic diseases of the elderly subject. We propose here to summarize the recent strategies applied for the identification of novel senolytics and their uses in osteoarthritis and osteoporosis therapies.

BackgroundTissue accumulation of p16INK4A-positive senescent cells is associated with age-related disorders such as osteoarthritis (OA). These senescent cells induce a tissue loss of function through a particular secretory phenotype called SASP (senescence-associated secretory phenotype).ObjectivesLinks between OA onset and cellular senescence remain poorly detailed. We wanted to determine the localization of articular senescent cells in in vivo OA mouse models and study the involvement of mesenchymal stem cells (MSC) senescence in OA pathogenesis.MethodsWild-type mice C57BL/6, SAMP8/R1 (senescence accelerated mouse-prone and resistant), transgenic p16INK4A +/Luc and p16INK4A Luc/Luc were used. Experimental OA was induced by intraarticular injections of collagenase (CIOA). Cartilage, synovial tissue and subchondral bone were analyzed by histology, RT-qPCR and micro-tomography. MSCs come from healthy human donors and primary chondrocytes from OA patients.Results(1) CIOA was induced in senescence-driven luciferase transgenic mice. Under CCD camera, a peak in luminescence was detected at day 24 post-injection revealing the presence of senescent cells in the joint. Remarkably, articular senescence is not only a marker of the pathology but contributes to OA onset: mice deficient in p16INK4a, a main senescence-driving known cell cycle inhibitor, were partially protected against CIOA. These results were confirmed in C57Bl/6 mice after CIOA by showing an increase in gene expression for senescence, catabolic and inflammatory markers in the synovial tissue preceding cartilage degradation.(2) MSCs found in synovial, cartilage, fat pad and bone marrow participate in joint homeostasis. Because MSC are proposed to be at the root of OA development, we hypothesize that cellular senescence onset in these progenitor cells would be a possible etiological factor for OA. We have established an in vitro p16INK4A-induced senescence model on human primary MSC: their intrinsic properties such as self-renewing are altered during senescence onset. Furthermore, in co-culture conditions with chondrocytes from OA patients, senescent MSC lost their extrinsic chondroprotective properties.(3) To in vivo challenge these findings, we rely on the mouse model of accelerated senescence SAMP8, which develop spontaneous OA at the age of 6 months with cartilage degradation, synovial hypertrophy, osteophytosis and subchondral bone remodeling associated to meniscal calcification. Isolated MSC from these mice express senescence but non-inflammatory markers (p16INK4a, p21waf1, MMP13, TGF-β1). Remarkably, intra-articular injection of these isolated SAMP8-derived MSC compared to SAMR1-derived control MSC, in young wid-type C57Bl/6 mice, was suffisant by its own, to induce significant articular cartilage degradation (OA score of 12.2 ± 1.5 vs 6.1 ± 3.5 for SAMP8 and SAMR1 MSC respectively. p <0.05).Conclusionp16INK4A-induced cellular senescence in MSC played a causative role in cartilage loss of function and OA pathogeny. In vitro, senescent MSC show altered...