The polycystic kidney disease 1 (Pkd1) gene is required for the responses of osteochondroprogenitor cells to midpalatal suture expansion in mice

Abstract

Mechanical stress is known to modulate postnatal skeletal growth and development. However, the mechanisms underlying the mechanotransduction are not fully understood. Polycystin-1 (PC1) is a promising candidate among proteins that may play a role in the process as it has been shown to function as a flow sensor in renal epithelium and it is known to be important for skeletal development. To investigate whether PC1 is involved in mechanotransduction in skeletal tissues, mice with a conditional deficiency for PC1 in neural crest cells, osteoblasts or chondrocytes were subjected to midpalatal suture expansion. Dynamic bone labeling revealed that new bone formation in response to expansion was significantly reduced in Wnt1Cre;Pkd1 mice, as the suture area containing new bone was 14.0±3.4% in mutant mice versus 65.0±3.8% in control mice at 2 weeks (p<0.001). In contrast, stress-induced new bone formation was not affected in OsxCre;Pkd1 mice. The increase in cell proliferation and differentiation into osteoblasts, seen in wild-type mice 1 day after force delivery, was not observed until 14 days in Wnt1Cre;Pkd1 mice. TUNEL labeling showed a significant increase in apoptotic suture cells at days 1 and 3 (from 7.0±0.5% to 13.5±1.4% at day 1 and from 4.6±1.1% to 10.5±1.7% at day 3, p<0.05). Abnormal ossification of nasal cartilage of Wnt1Cre;Pkd1 mice was accelerated upon suture expansion. Such ossification was also observed, but to a lesser extent in Col2a1-ER*Cre;Pkd1 mice. Transcript levels of Runx2 and MMP13 were significantly increased in the nasal cartilage of Wnt1Cre;Pkd1 mice compared to controls (p<0.05 and p<0.001, respectively), and in mutant mice with expansion versus without expansion (p<0.05 and p<0.001, respectively). Lack of PC1 in chondroprogenitor cells also resulted in increased cell apoptosis and an altered arrangement of chondrocytes in nasal cartilage. These results indicate that PC1 plays a critical role in the response of osteochondroprogenitor cells to the mechanical tissue stress induced by midpalatal suture expansion. They also suggest that the combination of an in vivo mechanical model, such as midpalatal suture expansion, with conditional deficiency for proteins that play a role in mechanotransduction, represents a powerful experimental strategy to explore underlying mechanisms.

Role of polycystin-1 in bone remodeling:Orthodontic tooth movement study in mutant mice

“…Current studies [15][16][17][18][19] specific to the effect of primary cilia on skeletal growth and mechanical stress indeed showed promising results in support of the PC1 sensor role hypothesis.…”

“…Based on the analysis of the skull phenotype observed in Pkd1-deficient mice, Kolpakova-Hart et al 17 hypothesized that tensile force within the growing viscerocranium is essential for skeletal growth and that PC1 is an important mediator of this process, as it controls proliferation of mesenchymal cells at the osteogenic fronts. Later on, Hou et al 18 demonstrated that these mutant mice exhibited an impaired response to tensile force. Our findings support these previous studies, as we observed impaired tooth movement in the mutant mice.…”

Polycystins and mechanotransduction: From physiology to disease

“…Furthermore, abnormal bone development and osteopenia was observed upon loss of Pkd1 function in mice due to impaired osteoblast differentiation [14] . Another study of a mouse model with midpalatal suture expansion demonstrated that proliferation and differentiation of periosteal osteochondroprogenitor cells that were mechanically stimulated requires Pkd1 [35] . This is in concert with a recent study from our group, exploring PC1 involvement in mechanical load (stretching)induced signaling pathways in human pre osteoblasts.…”

Craniofacial Development and Growth in Polycystic Kidney Disease

“…Cellular responses have been shown to vary dependent on the magnitude of mechanical force, and its frequency or duration (33,34,35). In addition, the stage of cellular differentiation is also likely to be involved in cellular responses to mechanical force (32). At a physiological level of mechanical stress, Pkd1 negatively regulates the differentiation of osteoprogenitor cells to osteoblasts, whereas it accelerates differentiation to matured osteoblasts.…”

“…Pkd1 is expressed in prechondrocytes, osteoblasts and osteocytes (30,31). In order to clarify the interaction between Pkd1 and mechanical force on ossification in sutures, midpalatal suture expansion has been studied as a mechanical force (32) and shown to lead to new bone formation. Osteogenic response to tensile stress is significantly reduced due to reduced proliferation, delayed differentiation and increased apoptosis of osteochondroprogenitor cells in Pkd1 mutants.…”