Cnot7-Null Mice Exhibit High Bone Mass Phenotype and Modulation of BMP Actions

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Abstract
Cnot7 is a recently identified regulator of spermatogenesis in adult mice. Because Cnot7 binds to Tob, a BMP inhibitor shown to be involved in bone metabolism, we examined whether Cnot7 is involved in bone mass regulation by using adult Cnot7 deficient mice. Cnot7−/− mice exhibited a high bone mass phenotype. This was associated with an increase in bone formation rate but not with any alteration in bone resorption parameters. On BMP treatment, Cnot7−/− cells expressed higher levels of alkaline phosphatase compared with control cells. Direct BMP2 injection induced larger bone mass in Cnot7−/− calvaria than control in vivo. These observations revealed that Cnot7 is an endogenous suppressor of bone mass and inhibits BMP actions in osteoblasts. Introduction: The molecular mechanisms involved in the determination of bone mass have been gradually understood based on recent analyses. Cnot7 (Ccr4-Not complex 7) is a component of transcriptional Ccr4-Not complex, is conserved from yeast to human, and binds to Tob, but its function in bone is not understood. Materials and Methods: To elucidate the role of involvement of Cnot7 in bone mass determination, we examined the bone of adult male Cnot7-null and heterozygous mice based on μCT analyses, histomorphometry, cell cultures, and in vivo BMP assays. Results: Cnot7−/− mice showed an increase in bone mass levels by >50% compared with controls. Analyses of the histomorphometric parameters indicated that bone formation activity in Cnot7−/− mice was enhanced, whereas bone resorption activity was not altered. These effects on osteoblasts were cell autonomous because mineralized nodule formation was enhanced in the cultures of bone marrow cells prepared from Cnot7−/− mice. In vitro analyses to elucidate Cnot7 effects revealed that BMP-induced expression of alkaline phosphatase in Cnot7−/− calvaria-derived osteoblastic cells was enhanced compared with controls. Moreover, BMP injection—induced new bone formation in vivo was enhanced in Cnot7−/− mice. Conclusions: These observations indicated that Cnot7 is an endogenous suppressor of bone mass in adult mice and inhibits BMP actions.