Osteocyte TSC1 promotes sclerostin secretion to restrain osteogenesis in mice

Area of Science:

• Bone Biology
• Cell Signaling
• Osteocyte Function

Background:

• Osteocytes regulate bone formation via sclerostin secretion.
• Regulation of sclerostin production in osteocytes is not fully understood.
• Tuberous sclerosis complex 1 (TSC1) is implicated in cellular regulation.

Purpose of the Study:

• To investigate the role of TSC1 in osteocyte sclerostin secretion.
• To elucidate the mechanism linking TSC1, mTORC1, Sirt1, and sclerostin production.
• To assess the impact of TSC1 disruption on bone formation and osteoclastogenesis.

Main Methods:

• Generation of DMP1-Cre-directed Tsc1 conditional knockout (CKO) mice.
• In vitro studies using MLO-Y4 osteocyte-like cells with TSC1 knockdown or rapamycin treatment.
• Analysis of sclerostin, RANKL, and mTORC1 signaling pathways.
• Assessment of bone formation and osteoclast activity in vivo.

Main Results:

• Osteocyte TSC1 deletion (Tsc1 CKO) led to increased osteoclast formation but reduced sclerostin production, causing severe osteosclerosis and enhanced bone formation.
• TSC1 knockdown in osteocyte-like cells activated mTORC1 and decreased sclerostin.
• Rapamycin treatment inhibited mTORC1 and increased sclerostin expression.
• Mechanical loading activated mTORC1 and suppressed sclerostin expression in osteocytes.

Conclusions:

• TSC1 promotes sclerostin secretion and inhibits osteogenesis by suppressing mTORC1 and Sirt1.
• TSC1/mTORC1 signaling regulates osteocyte sclerostin secretion and bone formation in response to mechanical stimuli.
• Targeting TSC1 is a potential therapeutic strategy for increasing osteogenesis and preventing bone loss.