CRMP4 Inhibits Bone Formation by Negatively Regulating BMP and RhoA Signaling

We identified the neuroprotein collapsing response mediator protein-4 (CRMP4) as a noncanonical osteogenic factor that regulates the differentiation of mouse bone marrow skeletal stem cells (bone marrow stromal stem cells [mBMSCs]) into osteoblastic cells. CRMP4 is the only member of the CRMP1–CRMP5 family to be expressed by mBMSCs and in osteoprogenitors of both adult mouse and human bones. In vitro gain-of-function and loss-of-function of CRMP4 in murine stromal cells revealed its inhibitory effect on osteoblast differentiation. In addition, Crmp4-deficient mice (Crmp4^–/–) displayed a 40% increase in bone mass, increased mineral apposition rate, and bone formation rate, compared to wild-type controls. Increased bone mass in Crmp4^–/– mice was associated with enhanced BMP2 signaling and BMP2-induced osteoblast differentiation in Crmp4^–/– osteoblasts (OBs). Furthermore, Crmp4^–/– OBs exhibited enhanced activation of RhoA/focal adhesion kinase (FAK) signaling that led to cytoskeletal changes with increased cell spreading. In addition, Crmp4^–/– OBs exhibited increased cell proliferation that was mediated via inhibiting cyclin-dependent kinase inhibitor 1B, p27^Kip1 and upregulating cyclin D1 expression which are targets of RhoA signaling pathway. Our findings identify CRMP4 as a novel negative regulator of osteoblast differentiation.