Osteoblast-Specific Deletion of Pkd2 Leads to Low-Turnover Osteopenia and Reduced Bone Marrow Adiposity

Abstract

Polycystin-1 (Pkd1) interacts with polycystin-2 (Pkd2) to form an interdependent signaling complex. Selective deletion of Pkd1 in the osteoblast lineage reciprocally regulates osteoblastogenesis and adipogenesis. The role of Pkd2 in skeletal development has not been defined. To this end, we conditionally inactivated Pkd2 in mature osteoblasts by crossing Osteocalcin (Oc)-Cre;Pkd2 +/null mice with floxed Pkd2 (Pkd2 flox/flox) mice. Oc-Cre;Pkd2 flox/null (Pkd2 Oc-cKO) mice exhibited decreased bone mineral density, trabecular bone volume, cortical thickness, mineral apposition rate and impaired biomechanical properties of bone. Pkd2 deficiency resulted in diminished Runt-related transcription factor 2 (Runx2) expressions in bone and impaired osteoblastic differentiation ex vivo. Expression of osteoblast-related genes, including, Osteocalcin, Osteopontin, Bone sialoprotein (Bsp), Phosphate-regulating gene with homologies to endopeptidases on the X chromosome (Phex), Dentin matrix protein 1 (Dmp1), Sclerostin (Sost), and Fibroblast growth factor 23 (FGF23) were reduced proportionate to the reduction of Pkd2 gene dose in bone of Oc-Cre;Pkd2 flox/+ and Oc-Cre;Pkd2 flox/null mice. Loss of Pkd2 also resulted in diminished peroxisome proliferator-activated receptor γ (PPARγ) expression and reduced bone marrow fat in vivo and reduced adipogenesis in osteoblast culture ex vivo. Transcriptional co-activator with PDZ-binding motif (TAZ) and Yes-associated protein (YAP), reciprocally acting as co-activators and co-repressors of Runx2 and PPARγ, were decreased in bone of Oc-Cre;Pkd2 flox/null mice. Thus, Pkd1 and Pkd2 have coordinate effects on osteoblast differentiation and opposite effects on adipogenesis, suggesting that Pkd1 and Pkd2 signaling pathways can have independent effects on mesenchymal lineage commitment in bone.

Polycystin-1 interacts with TAZ to stimulate osteoblastogenesis and inhibit adipogenesis

“…Ex vivo adenoviral Cre-mediated deletion of Pkd1 in primary osteoblasts isolated from Pkd1 fl/fl mice also resulted in impaired osteoblast differentiation and increased adipogenic markers, whereas adenoviral Cre-mediated deletion of Pkd2 resulted in a concordant decrease in both osteogenic and adipogenic markers. These cell culture studies are consistent with our prior finding that conditional deletion of Pkd1 in the osteoblast lineage in mice results in inhibition of osteoblastogenesis and stimulation of adipogenesis in vivo, whereas conditional deletion of Pkd2 results in concordant inhibition of both osteoblastogenesis and adipogenesis (30,32).…”

“…, and MC3T3-E1 osteoblastic cell lines (ATCC) were cultured in α-MEM containing 10% FBS and 1% P/S as previously described (31,32). For intracellular calcium measurements, Fluo-4 or Fluo-4 AM (Invitrogen) was used according to the manufacturer's instructions (25).…”

“…The distal femoral metaphyses were also scanned using a Scanco μCT 40 (Scanco Medical AG). A 3D image analysis was done to determine bone volume and cortical thickness as previously described (30)(31)(32).…”

“…In this regard, osteoblastspecific deletion of Pkd1 or Pkd2 resulted in osteopenia, reduced runt-related transcription factor 2-dependent (Runx2-dependent) osteoblastogenesis, and impaired bone mechanosensing responses to in vivo mechanical loading in mice (30)(31)(32). While loss of Pkd1 and Pkd2 have concordant effects on osteoblast-mediated bone formation, they exhibit opposite effects on bone marrow adipogenesis (30)(31)(32). Indeed, loss of Pkd1 increases PPARγ-dependent adipogenesis, leading to increased bone marrow fat, while loss of Pkd2 results in decreased adipogenesis and suppression of both Runx2 and PPARγ.…”

“…Indeed, loss of Pkd1 increases PPARγ-dependent adipogenesis, leading to increased bone marrow fat, while loss of Pkd2 results in decreased adipogenesis and suppression of both Runx2 and PPARγ. The discordant effects on adipogenesis suggest that PC1 and PC2 functions can be uncoupled and that alternative, PC2-independent pathways link PC1 to adipogenesis (30)(31)(32).…”