Summary

A novel skeletal stem cell which gives rise to all layers of articular chondrocytes

Abstract

Background

The mature articular cartilage does not exhibit regenerative capacity. Additionally, identity of the true endogenous articular cartilage stem cell is unknown, and this significantly constrains the cartilage repair paradigm. In this context, despite second- and third-generation improvements in chondral repair techniques, they still retain significant rates of failure with formation of sub-par fibrocartilage. The orthopaedic surgery field has been implementing cellular techniques in the absence of a refined understanding of reparative mechanisms, in spite of paradoxical findings in the role of the mature articular chondrocyte in chondral repair. Building upon our recent investigations into skeletal stem cells (SSCs), we have identified a novel skeletal stem cell giving rise to articular chondrocytes (termed acSSCs). Our studies demonstrate that acSSCs possess the ability to generate articular cartilage both in transplantation settings and during normal joint development, representing significant potential for cartilage repair.

Methods

Regeneration in the rest of the skeleton is driven by activation of skeletal stem cells to produce mature cell types mediating new bone formation. These SSCs express a cell surface marker complement fundamentally different from that of mesenchymal stromal cells. Embarking in a new direction building upon our expertise in the discovery of skeletal stem cell subsets, we utilized two adipose tissue cell surface markers, CD24 (a cell adhesion molecule) and CD29 (an integrin) that were found to be upregulated in a specific subset of SSCs based on bulk RNA expression analysis. Using similar cell surface markers as previously described, a CD24+CD29+ fraction of cells expressing SSC markers were found to display stemness and self-renewal capacity via gold-standard serial transplantation studies in mice. These cells were found to be non-overlapping with all previously studied populations.

Results

Only these candidate articular cartilage SSCs and not other subsets appear to give rise to the articular cartilage following syngeneic transplantation, contributing to all layers of the articular cartilage from deep to the superficial zones. Graft-derived cartilage was found to be persistent, and acSSCs also demonstrate multipotency giving rise to the epiphyseal osteoblasts, adipocytes, and physeal chondrocytes. This represents the first demonstration of a cell population that can directly give rise to morphologically normal hyaline articular cartilage after isolation and syngeneic orthotopic transplantation. Flow cytometric data, alongside label retention and lineage tracing data formulated a novel differentiation sequence for the acSSC as a (at minimum) tripotent articular cartilage stem cell. In preclinical models of OA induced by destabilization of the medial meniscus (DMM), acSSC grafts not only improved histological features of cartilage degeneration but also enhanced functional outcomes as assessed by gait analysis.

Conclusions

This work represents the first discovery of the articular cartilage skeletal stem cell. These findings represent a significant advancement in our understanding of articular cartilage generation, regeneration and offer potential targets for therapeutic intervention. Ultimately, insights that arise from studying how acSSCs function, and why they fail in adults, should lead to the development of effective therapies that circumvent current impediments to the treatment of articular injury and osteoarthritis.