mTOR (mammalian target of rapamycin) is a serine-threonine kinase that plays a central role in several key cellular pathways that have been implicated in bone formation. mTOR mediates these roles by forming the multi-protein complexes mTORC1 and mTORC2, which are defined by the unique proteins raptor and rictor, respectively.
Studies from our laboratory have demonstrated that inhibition of mTORC1 increases the osteoblastic potential of MSCs while simultaneously inhibiting adipogenic differentiation, suggesting the potential for mTORC1 as a therapeutic target for the treatment of osteoporosis-related bone disease. To determine the effects of mTORC1 on the formation of the skeleton, we utilised the Cre-loxP system to generate mice with targeted deletion of raptor in pre-osteoblast cells. This was achieved by crossing mice expressing Cre recombinase under the control of the pre-osteoblast-specific osterix promoter with mice harbouring floxed raptor genes.
This study examined the in vivo effect of osteoblast-specific knockout of raptor on postnatal skeletal development. Histological and CT analyses were used to assess changes in skeletal development in male and female OBRaptor-/-, OBRaptor-/+ and wildtype (WT) littermate controls at 4, 8 and 12 weeks of age. OBRaptor-/- and OBRaptor-/+ animals display a stunted phenotype, when compared with WT controls, with a significant reduction in weight, body length and tibial length at 4 weeks which normalised during later development. Histological analyses indicate that this growth lag is coupled with a decrease in the width of the tibial growth plate. Analysis of the tibial micro architecture by CT indicates a disruption of trabecular bone formation during development of the OBRaptor-/- and OBRaptor-/+ animals. Furthermore, CT analysis demonstrated a decrease in calvarial bone volume in OBRaptor-/- and OBRaptor-/+ mice, when compared with WT controls, at all time points examined. These findings implicate a role for mTORC1 in OB maturation and function in postnatal skeletal development.