Impaired intramembranous bone formation during bone repair in the absence of tumor necrosis factor-alpha signaling

Tumor necrosis factor-alpha (TNF-alpha) is known to mediate bone resorption ; however, its role in osteogenesis has not been fully elucidated. In order to investigate the direct role of TNF-alpha signaling in the recruitment a nd differentiation of osteoblasts, two separate models of bone repair were used, marrow ablation and simple transverse fractures. These models were ca rried out in the tibiae of both wild-type and knock-out mice in which both TNF-alpha receptors (p55(-/-)/p75(-/-)) had been ablated. Marrow ablation i s a unique model in which robust intramembranous bone formation is induced without an endochondral component, followed by remodeling and restoration o f the original trabecular architecture of the bone marrow. In contrast, fra cture repair proceeds concurrently through both endochondral and intramembr anous processes of new bone tissue formation. In both models of bone repair , healing was delayed in the TNF-alpha receptor (p55(-/-)/p75(-/-)) deficie nt mice. In the marrow ablation model, young osteoblasts were recruited int o the marrow space by day three in the wild-type mice, while the TNF-alpha (p55(-/-)/p75(-/-)) mice had only granulation tissue in the marrow cavity. Type I collagen and osteocalcin mRNA expressions were reduced similar to 30 and similar to 50%. respectively, of the control values in the TNF-alpha r eceptor ablated mice. In the fracture repair model there was almost a compl ete absence of the initial intramembranous bone formation on the periosteal surface in the TNF-alpha (p55(-/-)/p75(-/-)) mice. As healing progressed h owever, the callus tissues were greatly enlarged, and there was a delay in hypertrophy of the chondrocytes and the resorption of cartilage tissue. Whi le during the initial period of fracture repair there was a marked reductio n in the expression of both type I collagen and osteocalcin mRNAs in the TN F-alpha (p55(-/-)/p75(-/-)) mice, levels of these mRNAs were elevated by si milar to 10-20% over the wild type at the later time points in the absence of endochondral resorption of the callus. The lack of inhibition of osteoge nesis during endochondral resorption suggests that a different set of signa ls are involved in the recruitment of osteogenic cells during endochondral repair then during intramembranous bone formation. Go-culture of chondrocyt es with a mesenchymal stem cell line was carried out to examine if chondroc ytes themselves produced paracrine factors that promote osteogenic differen tiation. These experiments demonstrated that chondrocytes do indeed produce factors that promoted osteogenic differentiation. In summary, the results presented here suggest that TNF-alpha plays a crucial role in promoting pos tnatal bone repair through the induction of osteoprogenitor cell recruitmen t or osteogenic cell activation in the context of intramembranous bone form ation. These results further suggest that the signals that promote osteogen esis during endochondral bone formation are different from those involved i n intramembranous bone formation. Copyright (C) 2001 S. Karger AG, Basel.