Novel aspects of chondrocyte biology and changes in the cartilage ECM revealed by proteomic analysis of mouse cartilage development in vivo

Within the zones of growth plate cartilage, coordinated changes in gene expression and chondrocyte morphology/organization are essential for normal bone growth. Disease-causing mutations in humans and modification of target genes in mice have revealed roles for specific ECM components and more recently, integrins and cytoskeletal elements. Our whole-genome microarray profiling of microdissected growth plate zones has shed light on cartilage development and pathological mechanisms. However differential proteomic studies are challenging due to the limited material available from microdissected cartilage zones. To redress this, we compared mouse femoral head cartilage at two developmental stages; one with a high proportion of reserve and proliferating chondrocytes (P3), the other with a high proportion of hypertrophic chondrocytes (P21). Analysis of sequential cartilage extracts by label-free quantitative mass spectrometry (1) identified >700 proteins, of which ~250 were differentially abundant (p<0.05). Proteins enriched in P21 cartilage reflected formation of the secondary ossification centre (collagen X, alkaline phosphatase) and development of the articular surface (lubricin, lactadherin). Other significant changes included proteins involved in cell adhesion, cytoskeletal remodelling, calcium homeostasis, glycolysis and chromatin organisation. Differential extractability of several proteins (chondroadherin, fibronectin, annexin 5) revealed a transition from readily soluble (NaCl-extracted) at P3 to poorly soluble (GuHCl-extracted) at P21, suggesting integration/association with the developing ECM. In addition, novel matrix proteins (vitrin, urb, SRPX and SRPX2) are being investigated by immunohistochemistry to identify region-specific expression at P21. A major finding was a selective shift in the abundance of ER resident chaperones. While Hsp47 and BiP were robustly expressed at P3 and P21 stages, the glycoprotein-specific ERp57/calnexin complex appeared dramatically reduced in P21 cartilage. In several chondrodysplasias, the result of protein misfolding is ER stress. However the differential abundance of specific chaperone systems may render different chondrocyte sub-populations more or less susceptible to ER stress.