Salt inducible kinases control osteoblast maturation in alveolar bone

Abstract

Alveolar bone serves to support and anchor teeth. It differs from most other skeletal tissue due to its unique origin from neural crest-derived mesenchymal cells and its formation through intramembranous ossification. The development of alveolar bone occurs coupled with tooth eruption through various signaling networks, including the parathyroid-related protein (PTHrP) pathway. Salt inducible kinases (SIKs) are important downstream regulators of PTH/PTHrP signaling in long bones, growth plate chondrocytes, and kidneys. Previous studies reported that the deletion of SIKs increases bone turnover and trabecular bone mass in long bones. However, the role of SIKs in alveolar bone remains unknown. To address this question, we used Ubq-CreERt2; Sik2f/f; Sik3f/f mice to perform tamoxifen-inducible global deletion in 3 different mouse models: development, adult onset, and following molar extraction. Tamoxifen-induced global deletion of SIK2/SIK3 was performed at postnatal day 3 in the developmental model, at postnatal week 12 in adult mice, and in some adult mice tooth extraction was performed 10 weeks post-tamoxifen injection. Surprisingly, compared to wild-type controls, SIK2/SIK3 mutant mice showed reduced alveolar bone mass in all models. Micro-CT demonstrated less bone volume and mineralization in the mutant alveolar bone lacking SIK2 and SIK3. Reduced alveolar bone in SIK2/SIK3 mutants was associated with reduced bone formation as assessed by calcein labeling. Furthermore, TRAP staining failed to show increased osteoclast activity, indicating that the alveolar bone loss is primarily caused by reduced bone formation rather than increased bone resorption. In regions of absent alveolar bone, increased alkaline phosphatase with reduced osteocalcin expression was noted, suggesting increased immature osteoblasts in the mutant mice lacking SIK2/SIK3. At 4 weeks post-tooth extraction, control maxillary first molar sockets displayed complete bone healing, while SIK2/SIK3 mutant mice exhibited absent bone healing in the tooth socket. The mutant extraction socket was filled with fibrous-like tissue showing increased alkaline phosphatase staining with decreased osteocalcin expression, consistent with the developmental model result. Our results demonstrate that the absence of SIK2/SIK3 impairs terminal osteoblast maturation in alveolar bone. Taken together, these findings emphasize the importance of SIKs in alveolar bone osteoblast differentiation, maturation, and alveolar bone formation.