Cartilage oligomeric matrix protein (COMP) forms part of the connective tissue of normal human hair follicles

Abstract:  Hair follicle cycling is driven by epithelial–mesenchymal interactions (EMI), which require extracellular matrix (ECM) modifications to control the crosstalk between key epithelial‐ and mesenchymal‐derived growth factors and cytokines. The exact roles of these ECM modifications in hair cycle‐associated EMI are still unknown. Here, we used differential microarray analysis of laser capture‐microdissected human scalp hair follicles (HF) to identify new ECM components that distinguish fibroblasts from the connective tissue sheath (CTS) from those of the follicular dermal papilla (DP). These analyses provide the first evidence that normal human CTS fibroblasts are characterized by the selective in situ‐transcription of cartilage oligomeric matrix protein (COMP). Following this up on the protein level, COMP was found to be hair cycle‐dependent, suggesting critical role in this process: COMP is expressed during telogen and early anagen at regions of EMI and is degraded during catagen (only the CTS adjacent to the bulge remains COMP+ during catagen). Notably, COMP gene expression in vitro suggests direct correlation with the expression of TGFβ2 in CTS fibroblasts. This raises the question whether COMP expression undergoes regulation by transforming growth factor, beta (TGFβ) signalling. The intrafollicular COMP expression suggests to be functionally important and deserves further scrutiny in hair biology as indicated by the fact that altered COMP expression might be associated with scant fine hair in the case of some chondrodysplasia and scleroderma patients. Together these results reveal for the first time that COMP is part of the ECM and suggests its important role in normal human HF biology.