PTH signaling in Ctsk+ cells is critical for skeletal homeostasis and tooth formation and eruption

Abstract

PTH signaling is of primordial clinical importance in the regulation of skeletal development and homeostasis, as well as in tooth formation and eruption. Both skeletal and dental mesenchymal cells express PTH1r and are target of PTH signaling. Our lab has been investigating the function of a recently identified periosteal stem cell (PSC) population labeled by Cathepsin K (Ctsk) in the regulation of cortical bone homeostasis. Ctsk+ lineage PSCs, which fulfil stemness criteria, express high levels of the PTH1r and respond to iPTH treatment. Ctsk is also expressed in dental pulp cells, dental follicle cells, and the periodontal ligament which are known to play a key role in tooth development and eruption. Whether PTH signaling in Ctsk+ lineage cells is required for proper periosteal bone formation and tooth development and eruption is not known. We, therefore, undertook a study to investigate whether PTH signaling in the Ctsk expressing PSCs and dental mesenchymal cells regulates these processes. To this end, we generated mice lacking PTH1r, specifically in Ctsk+ cells, using the CtskCre mice and analysed their cortical bone and teeth. Eight-week-old CtskCre;PTH1rfl/fl (CtskPTH1r) male and female mice are significantly smaller than their control littermates (PTH1rfl/fl). microCT and bone histomorphometry analyses revealed a significant decrease in cortical bone volume (%), cortical thickness, and periosteal MAR in CtskPTH1r mice compared to PTH1rfl/fl littermates. Notably, CtskPTH1r male and female mice present with failure of molar eruption and impaired incisor eruption. microCT analyses and histological examination revealed several abnormalities in 8-week-old CtskPTH1r mice, including truncated molar roots, loss of the periodontal ligament, root ankylosis, reduced cementoblasts, and markedly decreased alveolar bone. Severe dental anomalies werealso seen in CtskPTH1rmice at P12 and P19. Confirming that both the skeletal and the tooth phenotype are a consequence of deletion of the PTH1r in the mesenchymal cell lineage, Lys2Cre mice, widely used to deleted genes of interest in osteoclasts, do not present with any skeletal and dental phenotype. While our findings confirm the significance of PTH signaling in both periosteal bone formation and tooth development and eruption, they reveal for the first time a crucial role for Ctsk+ lineage cell- dependent PTH signaling within the periosteum and dental mesenchyme. The periosteum is a significant source of stem cells and progenitors contributing to bone growth and homeostasis, regeneration and response to anabolic drugs. Investigating the signaling molecules and pathways regulating periosteal stem cells offers an opportunity to advance our understanding of the mechanisms involved in these processes and may open novel and targeted therapeutic approaches for human diseases associated with bone fragility and impaired bone regeneration. Similarly, a comprehensive understanding of distinct subsets of dental mesenchymal cell populations and unravelling their regulation is of significance for the effective pursuit of novel dental regenerative strategies.