Local clearance of senescent cells attenuates the development of post-traumatic osteoarthritis and creates a pro-regenerative environment

Abstract

Senescent cells (SnCs) accumulate in many vertebrate tissues with age and contribute to age-related pathologies1–3, presumably through their secretion of factors contributing to the senescence-associated secretory phenotype (SASP)4–6. Removal of SnCs delays several pathologies7–9 and increases healthy lifespan8. Aging and trauma are risk factors for the development of osteoarthritis (OA)10, a chronic disease characterized by degeneration of articular cartilage leading to pain and physical disability. Senescent chondrocytes are found in cartilage tissue isolated from patients undergoing joint replacement surgery11–14, yet their role in disease pathogenesis is unknown. To test the idea that SnCs might play a causative role in OA, we used the p16-3MR transgenic mouse, which harbors a p16INK4a (Cdkn2a) promoter driving the expression of a fusion protein containing synthetic Renilla luciferase and monomeric red fluorescent protein domains, as well as a truncated form of herpes simplex virus 1 thymidine kinase (HSV-TK)15,16. This mouse strain allowed us to selectively follow and remove SnCs after anterior cruciate ligament transection (ACLT). We found that SnCs accumulated in the articular cartilage and synovium after ACLT, and selective elimination of these cells attenuated the development of post-traumatic OA, reduced pain and increased cartilage development. Intra-articular injection of a senolytic molecule that selectively killed SnCs validated these results in transgenic, non-transgenic and aged mice. Selective removal of the SnCs from in vitro cultures of chondrocytes isolated from patients with OA undergoing total knee replacement decreased expression of senescent and inflammatory markers while also increasing expression of cartilage tissue extracellular matrix proteins. Collectively, these findings support the use of SnCs as a therapeutic target for treating degenerative joint disease.

Expression of p16INK4a is a biomarker of chondrocyte aging but does not cause osteoarthritis

“…Applying our genetic approach to a joint injury model was also motivated by the demonstration that p16 INK4a expression increases with transection of the anterior cruciate ligament and that elimination of senescent cells can mitigate post‐traumatic OA (Jeon et al., 2017). DMM surgery was performed on littermate cohorts of Acan tm1(cre/ERT2)Crm p16 INK4a+/+ and Acan tm1(cre/ERT2)Crm p16 L/L male mice at 12 months of age and we assessed cartilage degradation 8 weeks after surgery.…”

“…We found that the somatic loss of p16 INK4a in chondrocytes did not protect against either age‐related or post‐traumatic OA. This finding is interesting in light of the beneficial effect of selectively eliminating senescent cells from murine hindlimbs after anterior cruciate ligament transection (ACLT) (Jeon et al., 2017). ACLT is a more severe model than DMM (Fang & Beier, 2014), but the more likely explanation for the discrepancy is that our approach eliminated p16 INK4a without affecting the SASP, whereas the senolytic compound reduced the production of SASP factors from the joint.…”

“…Senolytics are particularly attractive for OA because they have the potential to clear multiple cell types that secrete SASP factors into the joint space. Importantly, the periodic clearance of senescent cells from the joint was shown to have a long‐term benefit after ACLT despite a short half‐life of the compound (Jeon et al., 2017), indicating that sustained pharmacologic activity is not required. Intriguingly, this study also presented evidence that clearing p16 INK4a ‐high cells may limit the development of age‐related spontaneous OA in addition to the more extensive studies in the context of post‐traumatic OA (Jeon et al., 2017).…”

“…The functional role of p16 INK4a in cartilage aging and OA is less clear, although knockdown and overexpression studies in cultured chondrocytes have confirmed that p16 INK4a expression is associated with the production of catabolic factors involved in dedifferentiation and OA (Philipot et al., 2014; Zhou, Lou & Zhang, 2004). Further evidence that non‐cell‐autonomous effects of senescence contribute to OA has come from a recent study showing that selective elimination of senescent cells can mitigate the development of OA (Jeon et al., 2017). This “senolytic” approach seeks to target senescent cells that are marked by high expression of p16 INK4a , in line with findings from related murine models showing that the depletion of p16 INK4a ‐high cells in old animals can ameliorate features of aging and extend lifespan (Baker et al., 2011, 2016; Kirkland, Tchkonia, Zhu, Niedernhofer & Robbins, 2017).…”

Oxidation resistance 1 is a novel senolytic target

“…This hypothesis is supported by recent studies demonstrating that the genetic clearance of SCs prolongs the lifespan of mice and delays the onset of several age‐related diseases and disorders in both progeroid and naturally aged mice (Baker et al., 2011, 2016). Therefore, the pharmacological clearance of SCs with a small molecule, a senolytic agent that can selectively kill SCs, is potentially a novel anti‐aging strategy and a new treatment for chemotherapy‐ and radiotherapy‐induced side effects (Baar et al., 2017; Chang et al., 2016; Childs et al., 2016; Demaria et al., 2017; Jeon et al., 2017; Ogrodnik et al., 2017; Pan et al., 2017; Schafer et al., 2017; Yosef et al., 2016; Zhu et al., 2015). …”