Control of alveolar bone development, homeostasis, and socket healing by salt inducible kinases.

Alveolar bone supports and anchors teeth. The parathyroid hormone-related protein (PTHrP) pathway plays a key role in alveolar bone biology. Salt inducible kinases (SIKs) are important downstream regulators of PTH/PTHrP signaling in the appendicular skeleton where SIK inhibition increases bone formation and trabecular bone mass. However, the function of these kinases in alveolar bone remains unknown. Here, we report a critical role for SIK2/SIK3 in alveolar bone development, homeostasis, and socket healing after tooth extraction. Inducible SIK2/SIK3 deletion led to dramatic alveolar bone defects without changes in tooth eruption. Ablating these kinases impairs alveolar bone formation due to disrupted osteoblast maturation, a finding associated with ectopic periostin expression by fibrous cells in regions of absent alveolar bone at steady state and following molar extraction. Distinct phenotypic consequences of SIK2/SIK3 deletion in appendicular versus craniofacial bones prompted us to identify a specific transcriptomic signature in alveolar versus long bone osteoblasts. Thus, SIK2/SIK3 deletion illuminates a key role for these kinases in alveolar bone biology and highlights the emerging concept that different osteoblast subsets utilize unique genetic programs. Summary statement: SIK2/SIK3 deletion in alveolar bone reduces bone formation and mass by impairing osteoblast maturation, unlike in long bones, where it increases bone formation and mass.

A parathyroid hormone/salt-inducible kinase signaling axis controls renal vitamin D activation and organismal calcium homeostasis.

The renal actions of parathyroid hormone (PTH) promote 1,25-vitamin D generation; however, the signaling mechanisms that control PTH-dependent vitamin D activation remain unknown. Here, we demonstrated that salt-inducible kinases (SIKs) orchestrated renal 1,25-vitamin D production downstream of PTH signaling. PTH inhibited SIK cellular activity by cAMP-dependent PKA phosphorylation. Whole-tissue and single-cell transcriptomics demonstrated that both PTH and pharmacologic SIK inhibitors regulated a vitamin D gene module in the proximal tubule. SIK inhibitors increased 1,25-vitamin D production and renal Cyp27b1 mRNA expression in mice and in human embryonic stem cell-derived kidney organoids. Global- and kidney-specific Sik2/Sik3 mutant mice showed Cyp27b1 upregulation, elevated serum 1,25-vitamin D, and PTH-independent hypercalcemia. The SIK substrate CRTC2 showed PTH and SIK inhibitor-inducible binding to key Cyp27b1 regulatory enhancers in the kidney, which were also required for SIK inhibitors to increase Cyp27b1 in vivo. Finally, in a podocyte injury model of chronic kidney disease-mineral bone disorder (CKD-MBD), SIK inhibitor treatment stimulated renal Cyp27b1 expression and 1,25-vitamin D production. Together, these results demonstrated a PTH/SIK/CRTC signaling axis in the kidney that controls Cyp27b1 expression and 1,25-vitamin D synthesis. These findings indicate that SIK inhibitors might be helpful for stimulation of 1,25-vitamin D production in CKD-MBD.

Partial prevention of glucocorticoid-induced osteocyte deterioration in young male mice with osteocrin gene therapy.

Glucocorticoid excess suppresses osteocyte remodeling of surrounding bone minerals, causes apoptosis of osteoblasts and osteocytes, and disrupts bone remodeling, eventually, leading to glucocorticoid-induced osteoporosis and bone fragility. Preventing apoptosis and preserving osteocyte morphology could be an effective means of preventing bone loss during glucocorticoid treatment. We hypothesized that osteocrin, which preserves osteocyte viability and morphology in Sp7-deficient mice, could prevent osteocyte death and dysfunction in a glucocorticoid excess model. We used adeno-associated virus (AAV8) to induce osteocrin overexpression in mice one week before implantation with prednisolone or placebo pellets. After 28 days, prednisolone caused the expected reduction in cortical bone thickness and osteocyte canalicular length in control AAV8-treated mice, and these effects were blunted in mice receiving AAV8-osteocrin. Glucocorticoid-induced changes in cortical porosity, trabecular bone mass, and gene expression were not prevented by osteocrin. These findings support a modest therapeutic potential for AAV8-osteocrin in preserving osteocyte morphology during disease.

Dual targeting of salt inducible kinases and CSF1R uncouples bone formation and bone resorption.

Bone formation and resorption are typically coupled, such that the efficacy of anabolic osteoporosis treatments may be limited by bone destruction. The multi-kinase inhibitor YKL-05-099 potently inhibits salt inducible kinases (SIKs) and may represent a promising new class of bone anabolic agents. Here, we report that YKL-05-099 increases bone formation in hypogonadal female mice without increasing bone resorption. Postnatal mice with inducible, global deletion of SIK2 and SIK3 show increased bone mass, increased bone formation, and, distinct from the effects of YKL-05-099, increased bone resorption. No cell-intrinsic role of SIKs in osteoclasts was noted. In addition to blocking SIKs, YKL-05-099 also binds and inhibits CSF1R, the receptor for the osteoclastogenic cytokine M-CSF. Modeling reveals that YKL-05-099 binds to SIK2 and CSF1R in a similar manner. Dual targeting of SIK2/3 and CSF1R induces bone formation without concomitantly increasing bone resorption and thereby may overcome limitations of most current anabolic osteoporosis therapies.