Osteoblast derived-neurotrophin‑3 induces cartilage removal proteases and osteoclast-mediated function at injured growth plate in rats

Abstract Faulty bony repair causes dysrepair of injured growth platecartilage and bone growth defects in children; however, the underlying mechanisms are unclear. Recently, we observed the prominent induction of neurotrophin‑3(NT-3) and its important roles as an osteogenic and angiogenic factor promoting the bony repair. The current study investigated its roles in regulating injury site remodelling. In a rat tibial growth platedrill-hole injury repair model, NT-3 was expressed prominently in osteoblasts at the injury site. Recombinant NT-3 (rhNT-3) systemic treatment enhanced, but NT-3 immunoneutralization attenuated, expression of cartilage-removal proteases (MMP-9 and MMP-13), presence of bone-resorbing osteoclastsand expression of osteoclastprotease cathepsin K, and remodelling at the injury site. NT-3 was also highly induced in cultured mineralizing rat bone marrow stromal cells, and the conditioned medium augmented osteoclastformation and resorptive activity, an ability that was blocked by presence of anti-NT-3 antibody. Moreover, NT-3 and receptor TrkC were induced during osteoclastogenesis, and rhNT-3 treatment activated TrkC downstream kinase Erk1/2 in differentiating osteoclastsalthough rhNT-3 alone did not affect activation of osteoclastogenic transcription factors NF-κB or NFATin RAW 264.7 osteoclastprecursor cells. Furthermore, rhNT-3 treatment increased, but NT-3 neutralization reduced, expression of osteoclastogenic cytokines ( RANKL, TNF-α, and IL-1) in mineralizing osteoblasts and in growth plateinjury site, and rhNT-3 augmented the induction of these cytokines caused by RANKLtreatment in RAW 264.7 cells. Thus, injury site osteoblast-derived NT-3 is important in promoting growth plateinjury site remodelling, as it induces cartilage proteases for cartilage removal and augments osteoclastogenesis and resorption both directly (involving activing Erk1/2 and substantiating RANKL-induced increased expression of osteoclastogenic signals in differentiating osteoclasts) and indirectly (inducing osteoclastogenic signals in osteoblasts).