Bone Abstracts

Mammalian target of rapamycin (mTOR) is a serine–threonine kinase that plays a central role in a number of key cellular pathways that have been previously implicated in bone formation. mTOR mediates these diverse roles by forming two multi-protein complexes, mTORC1 and mTORC2, each of which is defined by unique proteins raptor and rictor respectively.

Studies from our laboratory have previously demonstrated that inhibition of mTORC1 increases the osteoblastic potential of MSCs and increases mineral production while simultaneously inhibiting adipogienic differentiation, suggesting the potential for mTORC1 as a therapeutic target for osteoperosis-related bone disease. To determine the effect of mTORC1 on the formation of the skeleton, we have utilised the Cre-loxP system to generate mice with targeted deletion of raptor in pre-osteoblast cells. This was achieved by crossing mice expressing the Cre recombinase under control of the pre-osteoblast specific osterix promoter with mice harboring floxed raptor genes.

This study examined the in vivo effect of osteoblast specific knockout of raptor on postnatal skeletal development. Male and female OB^{Raptor−/−} (hom), OB^Raptor−/+ (het) and wildtype (WT) littermate controls were harvested at 4, 8 and 12 weeks old. Histological and μCT analyses were used to assess changes in skeletal development. When compared to WT, hom and het animals display a stunted phenotype with a significant reduction in weight and height at 4, 8 and 12 weeks of age. Analysis of the tibial micro-architecture by μCT indicates a disruption of trabecular bone formation during development in the hom and het animals. Histological analyses show that this is coupled with a decrease in width of the tibial growth plate at 4 weeks. Furthermore, μCT images of the calvaria demonstrate a decrease in mineral thickness and impaired suture formation in both het and hom mice compared to WT at all time points examined. These findings implicate mTORC1 in osteoblast maturation and function in postnatal skeletal development.