Biomarkers for hypertrophic chondrocyte differentiation are associated with spatial cellular organisation and suggest endochondral ossification-like processes in osteoarthritic cartilage: An exploratory study.

Background: In healthy articular cartilage, chondrocytes are found along arcades of collagen fibers as Single Strings. With onset of cartilage degeneration this pattern changes to Double Strings. In the course of osteoarthritis Small Clusters, and finally Big Clusters form. In highly degenerated articular cartilage, another poorly understood pattern is found where chondrocyte morphology differs considerably, and the distribution of cells is diffuse. Progression of osteoarthritis is accompanied by key processes such as chondrocyte proliferation, apoptosis, hypertrophic differentiation, inflammation, and angiogenesis. The aim of this exploratory study was to identify biomarkers for these processes in the context of spatial cellular organizational changes in articular cartilage.

Methods: Cartilage explants (n = 166 patients) were sorted according to their predominant cellular pattern. Quantitative or semi-quantitative analysis of 39 biomarkers were performed by multiplex assay (31) or ELISA (8), and qualitative analysis on 12 immunohistochemical markers.

Results: Hypertrophic differentiation (e.g. type-X collagen, osteopontin, osteocalcin and interleukin-6) and angiogenesis were associated with changes in chondrocyte organisation. First changes take place already at the transition from Single Strings to Double Strings. Drastic changes in the appearance of numerous biomarkers are found at the transition from Big Clusters to Diffuse.

Conclusion: Key processes in osteoarthritis and their biomarkers seem to depend on the spatial distribution of chondrocytes in articular cartilage. Abrupt changes in biomarker occurrence were observed between Big Clusters and Diffuse insinuating that the Diffuse pattern is composed of a different cell population or at least a different form of chondrocyte morphology.

The Translational Potential Of This Article: In situ identification of the different spatial chondrocyte patterns by fluorescence microscopy has already been established in the recent past. Analysing human in-situ cartilage explants rather than isolated OA chondrocytes closes the gap between in vitro and in vivo studies and as such, stretches a big step towards translation of the observed findings. The direct association between tissue biomarker profile and cellular arrangements representing different states of OA sheds new light on the molecular and cellular physiopathology, especially in the context of larger processes such as angiogenesis, cellular proliferation, differentiation, and apoptosis. This also opens an interesting perspective for future investigation of such biomarkers and processes in clinical studies.