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.

Adseverin, an actin-binding protein, modulates hypertrophic chondrocyte differentiation and osteoarthritis progression.

In osteoarthritis (OA), a disease characterized by progressive articular cartilage degradation and calcification, the articular chondrocyte phenotype changes and this correlates with actin cytoskeleton alterations suggesting that it regulates gene expression essential for proper phenotype. This study reports that OA is associated with the loss of adseverin, an actin capping and severing protein. Adseverin deletion (Adseverin-/-) in mice compromised articular chondrocyte function, by reducing F-actin and aggrecan expression and increasing apoptosis, Indian hedgehog, Runx2, MMP13, and collagen type X expression, and cell proliferation. This led to stiffer cartilage and decreased hyaline and increased calcified cartilage thickness. Together, these changes predisposed the articular cartilage to enhanced OA severity in Adseverin-/- mice who underwent surgical induction of OA. Adseverin-/- chondrocyte RNA sequencing and in vitro studies together suggests that adseverin modulates cell viability and prevents mineralization. Thus, adseverin maintains articular chondrocyte phenotype and cartilage tissue homeostasis by preventing progression to hypertrophic differentiation in vivo. Adseverin may be chondroprotective and a potential therapeutic target.

Adseverin, an actin binding protein, regulates articular chondrocyte phenotype.

Chondrocytes dedifferentiate as a result of monolayer culture for cell number expansion. This is associated with the development of an elongated shape, increased actin polymerization, development of stress fibres, and expression of contractile molecules. Given the changes in actin status with dedifferentiation, the hypothesis of this study was that adseverin, an actin severing and capping protein, plays a role in regulating chondrocyte phenotype and function. This study reports that serial passaging of articular chondrocytes in monolayer culture resulted in loss of adseverin protein expression as early as Day 14 of culture and remained repressed in Passage 2 (P2) cells. Knockdown of adseverin by siRNA in primary chondrocytes promoted an increase in cell size and an elongated shape, actin stress fibres, decreased G-/F-actin ratio, and increased number of actin-free barbed ends. The cells also showed increased expression of the contractile genes and proteins, vinculin and α-smooth muscle actin, and increased ability to contract collagen gels. These are all features of dedifferentiation. These effects were due to adseverin as adseverin overexpression following transfection of the green fluorescent protein-adseverin plasmid partially reversed all of these changes in P2 chondrocytes. Furthermore, sox9 and aggrecan chondrogenic gene expression was upregulated, and collagen type I genes expression was downregulated with adseverin overexpression. The change in aggrecan mRNA expression had functional consequence as these cells exhibited increased total proteoglycan synthesis. These findings demonstrate that adseverin regulates features indicative of redifferentiation in passaged articular chondrocytes through modulation of the actin cytoskeleton status and potentially may regulate the maintenance of phenotype in primary chondrocytes.