The mechanisms of joint morphogenesis are still elusive. By simultaneously exploring joint developmental and homeostatic mechanisms, our laboratory has identified that TGF-beta type II receptor (TBRII) signaling operates an active developmental and homeostatic machinery that prevents joints from the detrimental effects of cytokines. We first indicated that the conditional inactivation of TBRII signaling in embryonic mesenchyme limb progenitors (TBRIIPrx1KO) leads to failure of joint development and its inducible postnatal inactivation (TBRII-ER-Prx1KO) leads to progressive osteoarthritis (OA). Then by developing a TBRII reporter mouse, we characterized that the TBRII expressing cells have the anatomical, ontogenic, slow-cycling trait and in-vivo and ex-vivo expression profiles of progenitor joint cells and can express markers of differentiated joint elements (articular cartilage, tenocytes and synoviocytes markers) while maintaining joint progenitor marker expressions. Furthermore, TBRII progenitor cells localized in specific niches such the synovial-enthesial-articular cartilage complex. In mice subjected to surgical destabilization of medial meniscus (DMM) to induce post-traumatic OA, slow-proliferating-TBRII progenitors responded to DMM-induced OA by becoming fast-proliferating cells. To characterize the genomic profile of joint interzone forming cells, we combined laser capture microdissection for RNA sampling with microarray analysis. We determined that the setting in which joint-interzone forming interzone cells develop is distinct from adjacent growth plate chondrocytes and is characterized by down-regulation of cytokines. Of a total of 337 genes classified in “Inflammation mediated by chemokine and cytokine signaling pathway”, 227 were down-regulated more than 2 fold. Notably, a large number of these cytokines have been shown to have a role in OA development. Furthermore, such regulation was deranged in the presumptive joint of TBRIIPrx1KO mice that showed an increase of monocyte-chemoattractant protein-5 (MCP-5) of more than 100-fold and interleukin-36 alpha (IL36alfa) of more than 40-fold compared to control interzone joint cells. In-vivo and ex-vivo blockade of the sole MCP-5 receptor, CCR2, in TBRIIPrx1KO led to rescue of joint development. Moreover, in DMM-induced OA, MCP-5 was found to be increased and MCP-5 signaling blockage, at early stage of OA development, prevented the subchondral bone reaction induced by DMM. TBRII-ER-Prx1KO mice that develop OA, showed a consistent increase of IL36-alfa and intra-articular injection of IL36Ra (receptor antagonist) halted the OA progression. In aging mice that spontaneously develop OA, IL36alfa increased consistently with OA progression while TBRII expression was found to simultaneously declining. Analyses of IL36alfa and TBRII expression in histological knee sections of human subjects with different stages of histological knee OA (from none to severe OA) revealed a consistent increase of IL36alfa expression within the OA lesions that was associated with a parallel decrease of TBRII expression. Results were confirmed by RT-PCR analyses of isolated RNA from shaved cartilage from subjects from none to severe histological knee OA. Our findings represent a paradigm shift in the joint biology research field that presently perceives joints as the passive victims of inflammatory cytokines that cause their degeneration. Instead, our studies demonstrate that joints hold an active developmental mechanism that is operated by the TBRII signaling to tightly regulate the expression of cytokines such as MCP-5 and IL36alfa and such mechanism is essential for joint morphogenesis and to prevent OA degeneration.
Disclosure of Interest None declared