Gene expression profiles reveal that chondrogenic progenitor cells and synovial cells are closely related

Abstract

We showed previously that chondrogenic progenitor cells (CPCs) from the superficial zone of articular cartilage respond vigorously to cartilage wounding by responding chemotactically to cell debris, but the physiologic functions of CPCs remain unclear. To help bridge this knowledge gap we undertook a comparative analysis of gene expression in bovine CPCs, chondrocytes, synovial fibroblasts (synoviocytes), and cells isolated from synovial fluid (SFCs). Analysis of microarrays parsed the four cell types into two distinct groups, one composed only of chondrocytes and the other of CPCs, synoviocytes, and SFCs. The groups differed with respect to metalloendopeptidase, collagen, and cytokine gene expression. Quantitative PCR showed that, relative to chondrocytes, all other cells under-expressed cartilage matrix genes. CPCs significantly over-expressed genes encoding the chemokines interleukin 8 (IL8), and C-C motif ligand 2, while synoviocytes over-expressed the chemokine C-X-C motif Ligand 12. Sulfated glycosaminoglycan deposition in pellet cultures by CPCs was intermediate between chondrocytes and synoviocytes/SFCs. These results indicate that the CPC phenotype more closely resembles synoviocytes and SFCs than chondrocytes. CPCs show a tendency to over-express chemokines that promote immune cell chemotaxis, suggesting they mediate inflammation in response to cartilage wounding.

Keywords: chondrocytes; chondrogenic progenitor cells (CPCs); gene expression; microarray.

Figure 1.

Global heat map and hierarchical cluster analysis of chondrocytes, CPCs, synoviocytes and SFCs. The 5-fold change based global heat map revealed the enormous difference between chondrocytes and CPCs as well as to show the similarities among CPCs, synoviocytes and SFCs (specific region was enlarged in B). The hierarchical cluster analysis showed the similarities among four cell types and directly divided all cell types into two major categories (chondrocytes and the other combined cell types) (A).

Figure 2.

Annotated heat maps based on various gene functions. Five gene function based annotated heat maps (A. metalloendopeptidase, B. collagen, C. cytokine, D. inflammatory, E. extracellular) exhibited the differences among four cell types.

Figure 3.

Matrix forming gene expression analysis. qPCR showed substantially higher expression of all matrix forming genes (Collagen ΙΙ, Aggrecan, Link protein and COMP) in chondrocytes than in other three cell types. (*: p<0.05; **: p<0.01; ***: p<0.001)

Figure 4.

Inflammatory and transcriptional gene expression analysis. qPCR exhibited dramatically lower expression of most inflammatory related genes (IL8, CCL2 and CXCL12) in chondrocytes than in the other three cell types. Chondrocytes showed significantly up-regulated expression over synoviocytes and SFCs and significantly down-regulated expression of synoviocytes and SFCs in the context of SOX9 and RUNX2, respectively. (*: p<0.05; **: p<0.01; ***: p<0.001)

Figure 5.

sGAG assay of chondrocytes, CPCs, SFCs and synoviocytes. sGAG assay legibly demonstrated the glycosaminoglycan contents in each cell type, revealing that chondrocytes contain the highest amount, and significant differences exist when comparing chondrocytes to the other cell types. (*: p<0.05; **: p<0.01; ***: p<0.001)