Pannexin 3 and connexin 43 modulate skeletal development through their distinct functions and expression patterns

Pannexin 3 (Panx3) and connexin 43 (Cx43; also known as GJA1) are two major gap junction proteins expressed in osteoblasts. Here, we studied their functional relationships in skeletal formation by generating Panx3^-/- and Panx3^-/-;Cx43^-/- mice and comparing their skeletal phenotypes with Cx43^-/- mice. Panx3^-/- mice displayed defects in endochondral and intramembranous ossification, resulting in severe dwarfism and reduced bone density. The skeletal abnormalities of Panx3^-/-;Cx43^-/- mice were similar to those in Panx3^-/- mice. The gross appearance of newborn Cx43^-/- skeletons showed no obvious abnormalities, except for less mineralization of the skull. In Panx3^-/- mice, proliferation of chondrocytes and osteoblasts increased and differentiation of these cells was inhibited. Panx3 promoted expression of osteogenic proteins such as ALP and Ocn (also known as ALPL and BGLAP, respectively), as well as Cx43, by regulating Osx (also known as SP7) expression. Panx3 was induced in the early differentiation stage and reduced during the maturation stage of osteoblasts, when Cx43 expression increased in order to promote mineralization. Furthermore, only Panx3 functioned as an endoplasmic reticulum (ER) Ca²⁺ channel to promote differentiation, and it could rescue mineralization defects in Cx43^-/- calvarial cells. Our findings reveal that Panx3 and Cx43 have distinct functions in skeletal formation.