Connexin43 deficiency causes delayed ossification, craniofacial abnormalities, and osteoblast dysfunction

Connexin(Cx)43 is the major gap junction protein present in osteoblasts. We have shown that overexpression of Cx45 in osteoblasts expressing endogenou s Cx43 leads to decreased cell-cell communication (Koval, M., S.T. Geist, E .M. Westphale, A.E. Kemendy, R. Civitelli, E.C. Beyer, and T.H. Steinberg. 1995. J. Cell Biol. 130:987-995) and transcriptional downregulation of seve ral osteoblastic differentiation markers (Lecanda, E, D.A. Towler, K. Ziamb aras, S.-L. Cheng, M. Koval, T.H. Steinberg, and R. Civitelli. 1998. Mel. B iol. Cell 9:2249-2258). Here, using the Cx43-null mouse model, we determine d whether genetic deficiency of Cx43 affects skeletal development in vivo. Both intramembranous and endochondral ossification of the cranial vault wer e delayed in the mutant embryos, and cranial bones originating from migrato ry neural crest cells were also hypoplastic, leaving an open foramen at bir th. Cx43-deficient animals also exhibited retarded ossification of the clav icles, ribs, vertebrae, and limbs, demonstrating that skeletal abnormalitie s are not restricted to a neural crest defect. However, the axial and appen dicular skeleton of Cx43-null animals were essentially normal at birth. Cel l to cell diffusion of calcein was poor among Cx43-deficient osteoblasts, w hose differentiated phenotypic profile and mineralization potential were gr eatly impaired, compared with wild-type cells. Therefore, in addition to th e reported neural crest cell defect, lack of Cx43 also causes a generalized osteoblast dysfunction, leading to delayed mineralization and skull abnorm alities. Cell to cell signaling, mediated by Cx43 gap junctions, was critic al for normal osteogenesis, craniofacial development, and osteoblastic func tion.