Collagen VIα2 chain deficiency causes trabecular bone loss by potentially promoting osteoclast differentiation through enhanced TNFα signaling

Type VI collagen is well known for its role in muscular disorders, however its function in bone is still not well understood. To examine its role in bone we analyzed femoral and vertebral bone mass by micro-computed tomography analysis, which showed lower bone volume/total volume and trabecular number in Col6 alpha 2-KO mice compared with WT. Dynamic histomorphometry showed no differences in trabecular bone formation between WT and Col6 alpha 2-KO mice based on the mineral appositional rate, bone formation rate, and mineralizing perimeter. Femoral sections were assessed for the abundance of Tartrate Resistant Acid Phosphatase-positive osteoclasts, which revealed that mutant mice had more osteoclasts compared with WT mice, indicating that the primary effect of Col6a2 deficiency is on osteoclastogenesis. When bone marrow stromal cells (BMSCs) from WT and Col6 alpha 2-KO mice were treated with rmTNF alpha protein, the Col6 alpha 2-KO cells expressed higher levels of TNF alpha mRNA compared with WT cells. This was accompanied by higher levels of p-p65, a down-stream target of TNF alpha, suggesting that BMSCs from Col6 alpha 2-KO mice are highly sensitive to TNF alpha signaling. Taken together, our data imply that Col6a2 deficiency causes trabecular bone loss by enhancing osteoclast differentiation through enhanced TNF alpha signaling.