mTORC1 Plays an Important Role in Skeletal Development by Controlling Preosteoblast Differentiation

Abstract

The mammalian target of rapamycin complex 1 (mTORC1) is activated by extracellular factors that control bone accrual. However, the direct role of this complex in osteoblast biology remains to be determined. To investigate this question, we disrupted mTORC1 function in preosteoblasts by targeted deletion of Raptor (Rptor) in Osterix-expressing cells. Deletion of Rptor resulted in reduced limb length that was associated with smaller epiphyseal growth plates in the postnatal skeleton. Rptor deletion caused a marked reduction in pre-and postnatal bone accrual, which was evident in skeletal elements derived from both intramembranous and endochondrial ossification. The decrease in bone accrual, as well as the associated increase in skeletal fragility, was due to a reduction in osteoblast function. In vitro, osteoblasts derived from knockout mice display a reduced osteogenic potential, and an assessment of bone-developmental markers in Rptor knockout osteoblasts revealed a transcriptional profile consistent with an immature osteoblast phenotype suggesting that osteoblast differentiation was stalled early in osteogenesis. Metabolic labeling and an assessment of cell size of Rptor knockout osteoblasts revealed a significant decrease in protein synthesis, a major driver of cell growth. These findings demonstrate that mTORC1 plays an important role in skeletal development by regulating mRNA translation during preosteoblast differentiation.

Osteocalcin‐dependent regulation of glucose metabolism and fertility: Skeletal implications for the development of insulin resistance

“…Mice in which Raptor , an essential component of mTORC1, is deleted in OBs ( Rptor OB −/− ) are severely osteopenic and Raptor null OBs fail to undergo mineralization (Chen & Long, ; Fitter et al, ); bone phenotypes that are similar to InsR OB−/− and Gprc6a −/− mice (Fulzele et al, ; Pi et al, ). Transcriptomic analyses revealed that raptor null OBs have an immature bone phenotype, suggesting that mTORC1 signaling is crucial for the differentiation of pre‐OBs into mature OBs (Chen & Long, ; Fitter et al, ). Importantly, total OCN levels are significantly decreased in Rptor OB −/− mice (Fitter et al, ).…”

“…Transcriptomic analyses revealed that raptor null OBs have an immature bone phenotype, suggesting that mTORC1 signaling is crucial for the differentiation of pre‐OBs into mature OBs (Chen & Long, ; Fitter et al, ). Importantly, total OCN levels are significantly decreased in Rptor OB −/− mice (Fitter et al, ). While this may be secondary to the stall in osteogenesis, these data provide evidence that mTORC1 in OBs is important for OCN expression.…”

“…While the function of mTORC1 in OBs requires further investigation, mTORC1‐dependent regulation of protein synthesis is thought to play a key role. Conditional deletion of Raptor in the developing mesenchyme ( Prx‐1‐Cre ; (Chen & Long, )) and pre‐osteoblasts ( Osx‐Cre ; (Fitter et al, )) causes a significant reduction in protein synthesis, which may account for the defects in limb length and bone mineralization observed in these mice. In support of this, a significant reduction in mTORC1 activity and protein synthesis was observed in mice in which Glut1, a glucose transporter, was deleted in pre‐osteoblasts ( Glut osx −/− ) (Wei et al, ).…”

Mesenchymal Stem Cell-Specific and Preosteoblast-Specific Ablation of TSC1 in Mice Lead to Severe and Slight Spinal Dysplasia, Respectively

“…As a vital regulator of cell metabolism and proliferation, mTORC1 is ubiquitously expressed in all mammalian cell types [20]. A key role of mTORC1 in bone formation was originally identified in almost all skeleton-related cell lineages.…”

Loss of EfnB1 in the osteogenic lineage compromises their capacity to support hematopoietic stem/progenitor cell maintenance

“…Primary stromal cells were isolated and cultured from 4-week-old EfnB1 fl/fl mice as described previously [32]. Cells from EfnB1 fl/fl mice were infected with a lentivirus containing the tamoxifen-inducible self-deleting Cre recombinase LEGO-CreERT2-iG2) in the presence of Polybrene (4 mg/mL) as described previously [61,62]. Stably transduced cells were selected by FACS based on enhanced green fluorescent protein expression.…”