SIK3 is essential for chondrocyte hypertrophy during skeletal development in mice

Abstract

SUMMARYChondrocyte hypertrophy is crucial for endochondral ossification, but the mechanism underlying this process is not fully understood. We report that salt-inducible kinase 3 (SIK3) deficiency causes severe inhibition of chondrocyte hypertrophy in mice. SIK3-deficient mice showed dwarfism as they aged, whereas body size was unaffected during embryogenesis. Anatomical and histological analyses revealed marked expansion of the growth plate and articular cartilage regions in the limbs, accumulation of chondrocytes in the sternum, ribs and spine, and impaired skull bone formation in SIK3-deficient mice. The primary phenotype in the skeletal tissue of SIK3-deficient mice was in the humerus at E14.5, where chondrocyte hypertrophy was markedly delayed. Chondrocyte hypertrophy was severely blocked until E18.5, and the proliferative chondrocytes occupied the inside of the humerus. Consistent with impaired chondrocyte hypertrophy in SIK3-deficient mice, native SIK3 expression was detected in the cytoplasm of prehypertrophic and hypertrophic chondrocytes in developing bones in embryos and in the growth plates in postnatal mice. HDAC4, a crucial repressor of chondrocyte hypertrophy, remained in the nuclei in SIK3-deficient chondrocytes, but was localized in the cytoplasm in wild-type hypertrophic chondrocytes. Molecular and cellular analyses demonstrated that SIK3 was required for anchoring HDAC4 in the cytoplasm, thereby releasing MEF2C, a crucial facilitator of chondrocyte hypertrophy, from suppression by HDAC4 in nuclei. Chondrocyte-specific overexpression of SIK3 induced closure of growth plates in adulthood, and the SIK3-deficient cartilage phenotype was rescued by transgenic SIK3 expression in the humerus. These results demonstrate an essential role for SIK3 in facilitating chondrocyte hypertrophy during skeletogenesis and growth plate maintenance.

A pathway to bone: signaling molecules and transcription factors involved in chondrocyte development and maturation

“…5B). Indeed, loss of Sik3 causes severe inhibition of chondrocyte hypertrophy in mice (Sasagawa et al, 2012). Taken together, these observations suggest that Sik3-induced phosphorylation of Hdac4 liberates both Mef2 and Runx family members from interaction with Hdac4, and thus permits the induction of chondrocyte hypertrophy target genes by these key transcriptional regulators.…”

“…Structural proteins: aggrecan (Acan) (Inada et al, 1999), Col2a1 (de Frutos et al, 2007), Col10a1 (de Frutos et al, 2007), Mmp9 and Mmp13 (Mmp9/13) (Inada et al, 1999), and Spp1 (Inada et al, 1999). Others: Hdac4 (Vega et al, 2004), Sik3 (Sasagawa et al, 2012) and reactive oxygen species (ROS) (Morita et al, 2007). For a comprehensive review of the expression pattern of BMP and TGFβ ligands and receptors, see Minina et al (2005).…”

Development of the Endochondral Skeleton

“…PTHrP induces dephosphorylation of HDAC4 and promotes its nuclear translocation and repression of MEF2 transcriptional activity (Kozhemyakina et al 2009). Conversely, salt-inducible kinase 3 (SIK3) is required for the exclusion of HDAC4 from the nuclei so that hypertrophy could occur, as Sik3 -/ -mice exhibited a marked delay in chondrocyte hypertrophy (Sasagawa et al 2012). Thus, the HDAC4 -MEF2C complex appears to be a central node of regulation for chondrocyte hypertrophy.…”

Regulation of SIK1 abundance and stability is critical for myogenesis

“…In myoblasts, SIK1 directly phosphorylates and inhibits class II histone deacetylase proteins (HDACs), which leads to de-repression of MEF2 activity and full expression of terminal myogenic genes including desmin (14). Intriguingly, the SIK/AMPK-HDAC-MEF2 pathway is evolutionarily conserved in Caenorhabditis elegans sensory neurons (18) and mediates distinct physiologic functions in mammalian striatal neurons (19), chondrocytes (20,21), and hepatocytes (22,23).…”