Transcriptomic insights into nerve growth factor-associated appositional bone formation in the reparative zone in...

In osteonecrosis of the femoral head, appositional bone formation occurs in the reparative zone, where new trabeculae surround necrotic bone, leading to trabecular thickening. However, the underlying molecular mechanisms remain unclear. We previously demonstrated a significant correlation between trabecular thickening and nerve growth factor (NGF) expression in the reparative zone of stage 3 osteonecrosis of the femoral head. This study explored NGF's molecular relationship with appositional bone formation.

RNA sequencing was performed on reparative zone samples from four stage 3 osteonecrosis of the femoral head femoral heads. One to three cylindrical samples (6 mm in diameter) were harvested per head. As controls, three samples were collected from the reparative zone of a single stage 1 osteonecrosis of the femoral head without histological evidence of appositional bone formation. Differentially expressed genes were identified using the DESeq2. Enrichment analysis was performed using the Gene Ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases via DAVID. NGF expression was evaluated using immunohistochemistry.

Stage 3 samples showed the upregulation of bone formation-related genes, including COL1A1, RUNX2, BMP2, BGLAP, and NGF. KEGG analysis revealed the enrichment of bone-related signaling pathways, such as Wnt, TGF-β, and PI3K-Akt. NGF expression was observed in the reparative zone of stage 3 femoral heads, which exhibited appositional bone formation, but not in stage 1.

These findings indicate that the reparative zone exhibiting appositional bone formation shows transcriptional activation of bone formation, accompanied by increased NGF expression. NGF may be involved in the molecular mechanisms underlying appositional bone formation in osteonecrosis of the femoral head, and serve as a potential regulator of this process.