Dynamics of progenitor cells in the formation and repair of synovial joints

Abstract

Synovial joints connect skeletal elements providing mobility. The knee joint is the most complex of synovial joints, comprising several distinct structures that include articular cartilage (AC), cruciate ligaments and meniscus. The interzone marks the early stage of joint formation, and Gdf5 as an interzone gene marks the progenitor cell population that forms the structures of the synovial joint. Further, a recent study suggests mesenchymal cells peripheral to the interzone are continuously recruited in an expansion of the Gdf5 cell pool. Thus, their relative contribution to the different structures is dependent at the time of recruitment and their location within the developing joint.

We have previous identified Lgr5 as a novel interzone marker with decedent cells contributing to all structure of a synovial joint. In this study, expression patterns of Scx, a tendon/ligament progenitor cell marker, and Collagen XXII (ColXXII), superficial zone AC marker, were closely investigated relative to Lgr5 during synovial joint development. Three subpopulations were identified, consisting of (1) Lgr5+/ColXXII+/Scx- marking presumptive superficial zone AC, (2) Lgr5+/ColXXII-/Scx+ marking presumptive cruciate ligament, and (3) Lgr5+/ColXXII+/Scx+ marking presumptive meniscus. Interestingly, ColXXII marks a broad interzone region, but its expression is restricted at the superficial zone of both AC and meniscus, supporting a single layer of cells at the superficial surface. Cells within this ColXXII-containing layer interact with the extracellular matrix mediated by β1 integrin, with α11 as the potential partner.

Articular cartilage defects affect millions worldwide. It possesses minimal repair capability, and the reason is not clear. Autologous chondrocytes and mesenchymal progenitor cells have been used as treatment with limited success, as these are not the right cells, often leading to fibrocartilage formation and further cartilage degradation. We propose that Lgr5+ interzone cells are poised with the greatest potential to become articular cartilage chondrocytes for cartilage repair. Indeed, our preliminary study using interzone tissue/cells implanted into a cartilage defect showed efficient repair within a short period forming juvenile AC-like tissue filling the whole defect. Importantly, there is a reestablishment of the ColXXII and CILP1 containing superficial layer, integrating with the host tissue and superficial layer.

To provide further insights into the molecular signature of the Lgr5+ interzone cells, binding motifs analysis was performed on a group of transcription factors (TFs) that are differentially expressed in Lgr5+ interzone. Gene ontology analysis on common binding motifs reveals their association with cell adhesion, cell motility, and ECM. With validation from single cell transcriptome data, genes including AC superficial zone markers, Col22a1, Itga11, Cd73, Cilp1, Thbs4, Dkk3, and Sulf1 were identified, suggesting possible regulation of superficial zone precursor cells before cavitation. Pak1 and Chd13 with known interaction with collagen stiffness and dorsal-ventral positioning of limb mesenchyme respectively were also identified, suggesting cell orientation and motility is tightly regulated through TFs and their possible contribution for joint cavitation. This, together with clearer understanding of the molecular controls regulating the differentiation process will allow the in vitro generation of these Lgr5+ progenitor cells for cartilage repair, preventing degeneration that can lead to early onset osteoarthritis.