Reduced SIRPα phosphorylation and concommitant SHP-2–PI3K–Akt2 signaling decrease osteoblast differentiation

• Normal differentiation of bone forming osteoblasts is a prerequisite for maintenance of skeletal health and is dependent on an intricate cellular signaling including the essential transcription factor Runx2. The cell surface glycoprotein CD47 and its receptor signal regulatory protein alpha (SIRPα) are suggested to regulate bone cell differentiation. In the present study, we investigated osteoblastic differentiation of bone marrow stromal cells from SIRPα mutant mice lacking the cytoplasmic signaling domain of SIRPα. Moreover, we compared downstream signaling events of SIRPα in wild-type and CD47-deficient mouse bone marrow stromal cells. SIRPα-mutant stromal cells showed significantly less expression of Runx2, Sp7 (osterix), Bglap (osteocalcin), and Akp1 (alkaline phosphatase) mRNA compared to stromal cells from wild-type mice. An impaired osteoblastogenesis in SIRPα-mutant cell cultures was demonstrated by lower alkaline phosphatase activity and less mineral formation compared to wild-type cultures. Western blot analyses showed that CD47 expression was required for Src homology-2 domain containing protein tyrosine phosphatase- 2 (SHP-2) to associate with SIRPa. As a result, SHP-2 and Akt2 in stromal cells from CD47 deficient mice were less phosphorylated, as compared to that in wild-type stromal cells. In conclusion, we here show that CD47-dependent SIRPα signaling through SHP-2–PI3K–Akt2 strongly influences osteogenic differentiation of bone marrow stromal cells.

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cellforskning

cell research

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vet (ämneskategori)

ovr (ämneskategori)