Type B γ-Aminobutyric Acid Receptors Modulate the Function of the Extracellular Ca2+-Sensing Receptor and Cell Differentiation in Murine Growth Plate Chondrocytes

Extracellular calcium-sensing receptors (CaRs) and metabotropic or type B γ-aminobutyric acid receptors (GABA-B-Rs), two closely related members of family C of the G protein-coupled receptor superfamily, dimerize in the formation of signaling and membrane-anchored receptor complexes. We tested whether CaRs and two GABA-B-R subunits (R1 and R2) are expressed in mouse growth plate chondrocytes (GPCs) by PCR and immunocytochemistry and whether interactions between these receptors influence the expression and function of the CaR and extracellular Ca2+-mediated cell differentiation. Both CaRs and the GABA-B-R1 and -R2 were expressed in the same zones of the growth plate and extensively colocalized in intracellular compartments and on the membranes of cultured GPCs. The GABA-B-R1 coimmunoprecipitated with the CaR, confirming a physical interaction between the two receptors in GPCs. In vitro knockout of GABA-B-R1 genes, using a Cre-lox recombination strategy, blunted the ability of high extracellular Ca2+ concentration to activate phospholipase C and ERK1/2, suppressed cell proliferation, and enhanced apoptosis in cultured GPCs. In GPCs, in which the GABA-B-R1 was acutely knocked down, there was reduced expression of early chondrocyte markers, aggrecan and type II collagen, and increased expression of the late differentiation markers, type X collagen and osteopontin. These results support the idea that physical interactions between CaRs and GABA-B-R1s modulate the growth and differentiation of GPCs, potentially by altering the function of CaRs.