Description
Transmembrane glycoprotein NMB is a protein that in humans is encoded by the GPNMB gene. Two transcript variants encoding 560 and 572 amino acid isoforms have been characterized for this gene in humans. The mouse and rat orthologues of GPNMB are known as DC-HIL and Osteoactivin (OA), respectively. GPNMB is a type I transmembrane glycoprotein which shows homology to the pmel17 precursor, a melanocyte-specific protein. In osteoblast progenitor cells, Osteoactivin works as a positive regulator of osteoblast differentiation during later stages of matrix maturation and mineralization [9] that is mediated at least in part by Bone morphogenetic protein 2 in a SMAD1 dependent manner to promote osteoblast differentiation. In addition, using a rat fracture model, Osteoactivin (OA) enhances the repairing process in bone fracture, demonstrated by its high expression during chondrogenesis (soft callus) and osteogenesis (hard callus) compared to the intact femurs that is why Osteoactivin (OA) could be a novel therapeutic agent used to treat generalized osteoporosis or localized osteopenia during fracture repair by stimulating bone growth and regeneration. Similarly, Osteoactivin expression increases during osteoclast differentiation and it is functionally implicated in this process, possibly by promoting the fusion of osteoclast progenitor cells