Conditional deletion ofPkd1in osteocytes disrupts skeletal mechanosensing in mice

Abstract

We investigated whether polycystin-1 is a bone mechanosensor. We conditionally deleted Pkd1 in mature osteoblasts/osteocytes by crossing Dmp1-Cre with Pkd1(flox/m1Bei) mice, in which the m1Bei allele is nonfunctional. We assessed in wild-type and Pkd1-deficient mice the response to mechanical loading in vivo by ulna loading and ex vivo by measuring the response of isolated osteoblasts to fluid shear stress. We found that conditional Pkd1 heterozygotes (Dmp1-Cre;Pkd1(flox/+)) and null mice (Pkd1(Dmp1-cKO)) exhibited a ∼ 40 and ∼ 90% decrease, respectively, in functional Pkd1 transcripts in bone. Femoral bone mineral density (12 vs. 27%), trabecular bone volume (32 vs. 48%), and cortical thickness (6 vs. 17%) were reduced proportionate to the reduction of Pkd1 gene dose, as were mineral apposition rate (MAR) and expression of Runx2-II, Osteocalcin, Dmp1, and Phex. Anabolic load-induced periosteal lamellar MAR (0.58 ± 0.14; Pkd1(Dmp1-cKO) vs. 1.68 ± 0.34 μm/d; control) and increases in Cox-2, c-Jun, Wnt10b, Axin2, and Runx2-II gene expression were significantly attenuated in Pkd1(Dmp1-cKO) mice compared with controls. Application of fluid shear stress to immortalized osteoblasts from Pkd1(null/null) and Pkd1(m1Bei/m1Bei)-derived osteoblasts failed to elicit the increments in cytosolic calcium observed in wild-type controls. These data indicate that polycystin-1 is essential for the anabolic response to skeletal loading in osteoblasts/osteocytes.

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

“…Similarly, transfection of PC1-CTT, TAZ, or Pparg individually suppresses aP2 promoter activity, and cotransfection of all 3 constructs results in additive suppression of aP2 gene transcription in C3H10T1/2 cells. We also found that both hard matrix and mechanical stretch stimulated the polycystins/ TAZ mechanosensing signaling complex in vitro, consistent with the possibility that this complex may integrate the response to different physical forces in the bone microenvironment (31,33). Since these in vitro methods of mechanical loading are nonphysiological, additional studies to explore the response to mechanical loading in compound Pkd1 and TAZ heterozygous mutant mouse models are needed to further test our hypothesis in vivo.…”

“…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).…”

A paradigm shift for bone quality in dentistry: A literature review

“…It has been reported that osteocytes respond to fluid flow with ciliadependent increases in cyclooxygenase-2 gene (COX-2) expression and prostaglandin E2 production [142]. Moreover, conditional deletion of polycystic kidney disease 1 (Pkd1), which is linked to the primary cilium, impaired skeletal mechanosensing in mouse osteoblasts and osteocytes [143,144]. Therefore, the primary cilium in osteocytes may also play a role as a mechanosensor.…”

High-resolution 3D imaging of osteocytes and computational modelling in mechanobiology: insights on bone development, ageing, health and disease

“…Lee and colleagues have shown that fluid flow induces an increase in Ca 2+ signalling . This hypothesis is supported by studies in mice, which show that deletion or disruption of ciliary genes leads to a disruption of mechanosensation (Xiao et al, 2011) or reduced bone formation (Qiu et al, 2012). Other studies have provided evidence that the deflection of primary cilia by fluid flow induces osteogenic and bone resorptive responses (Malone et al, 2007).…”