Polycystin-1 (PC1) may play an important role in skeletogenesis through regulation of the bone-specific transcrip-tion factor Runx2-II. In the current study, we found that PC1 co-localizes with the calcium channel polycystin-2 (PC2) in primary cilia of MC3T3-E1 osteoblasts. To establish the role of Runx2-II in medi-ating PC1 effects on bone, we crossed het-erozygous Pkd1m1Bei and Runx2-II mice to create double heterozygous mice (Pkd1+/m1Bei/ Runx2-II+/-) deficient in both PC1 and Runx2-II. Pkd1+/m1Bei/ Runx2-II+/- mice exhibited additive reductions in Runx2-II expression that was associated with impaired endochondral bone devel-opment, defective osteoblast-mediated bone formation and osteopenia. In addi-tion, we found that basal intracellular calcium levels were reduced in homozy-gous Pkd1m1Bei osteoblasts. In contrast, overexpression of a PC1 C-tail construct increased intracellular calcium and selec-tively stimulated Runx2-II P1 promoter activity in osteoblasts through a calcium-dependent mechanism. Site-directed mutagenesis of critical amino acids in the coiled-coil domain of PC1 required for coupling to PC2 abolished PC1-mediated Runx2-II P1 promoter activity. Addi-tional promoter analysis mapped the PC1 responsive region to the "osteoblast-specific" enhancer element between -420 and -350bp that contains NFI and AP-1 binding sites. Chromatin immunoprecipi-tation assays confirmed the calcium-dependent binding of NFI to this region. These findings indicate that PC1 regu-lates osteoblast function through intracel-lular calcium-dependent control of Runx2-II expression. The overall function of the primary cilium/polycystin complex may be to sense and transduce environ-mental clues into signals regulating os-teoblast differentiation and bone devel-opment.