The role of osteocrin and its interaction with the natriuretic system in bone growth

Using a viral-based signal-trap strategy targeting the bone "secretome" we identified a novel gene coding for a small secreted protein we termed "osteocrin" (Ostn). A 1280-bp mRNA encodes Ostn producing a mature protein of 103 amino acids with a molecular mass of 11.4 kDa. Tissue Northern blots and in situ hybridization showed Ostn was highly expressed in osteoblasts and immunohistochemistry on adult mouse bone further confirmed this. Subsequently Ostn has also been identified in other mesenchymal tissues such as fat, muscle, tendons, and ligaments. Interestingly, the only homology Ostn showed with any known proteins was with members of the natriuretic peptide (NP) family. We therefore hypothesized that Ostn could interact with the NP-receptors, thereby modulating NP actions on the skeleton. We showed that Ostn binds specifically and saturably to the NPR-C receptor with a K~5 nM with no binding to the GC-A or GC-B receptors. Deletion of several of the residues deemed important for NP binding to NPR-C led to abolition of Ostn binding, confirming the presence of a "natriuretic motif." Functionally, Ostn was able to augment CNPstimulated cGMP production in both pre-chondrocytic (ATDC5) and osteoblastic (UMR106) cells suggesting increased NP levels due to attenuation of NPR-C-associated NP-clearance. Bone-specific overexpression of Ostn in transgenic mice resulted in elongated bones and a marked kyphosis. This was associated with elevated bone cGMP levels suggesting elevated natriuretic peptide activity contributes to the increased bone length. An increase in growth plate chondrocyte proliferation was at least partially responsible for the bone overgrowth. This was similar to other published mouse models with enhanced NP function that also displayed a bone overgrowth phenotype. Thus we have demonstrated that Ostn is a naturally occurring ligand of the NPR-C clearance receptor and may act to locally modulate the actions of the natriuretic system in bone by blocking the clearance action of NPR-C thus locally elevating levels of CNP