SHIP2 controls matrix mineralization by regulation of the RhoA/ROCK pathway and remodeling of the actin cytoskeleton

Mutations in INPPL1, the gene coding for SH2 Domain-Containing Inositol 5-Phosphatase 2 (SHIP2), cause Opsismodysplasia, a severe chondrodysplasia characterized by delayed bone maturation. The mechanism by which the loss of an inositol phosphatase causes a major skeletal developmental defect is unclear. To investigate the role of SHIP2 in mineralization, the INPPL1 gene was deleted in vitro in chondrocyte and osteoblast differentiation models and the effect of the loss of SHIP2 on cell differentiation, subsequent mineralization, and on actin cytoskeleton formation was investigated. The loss of SHIP2 does not impact differentiation but, consistent with the disease phenotype, induces a significant reduction in extracellular matrix mineralization in both cell types. Absence of SHIP2 also altered the actin cytoskeleton to increase cell adhesion and focal adhesion formation. Furthermore, inhibition of actin polymerization in SHIP2-deficient cells rescued the mineralization phenotype. RhoA/ROCK, Cdc42 and Rac1 are the three main RhoGTPases responsible for actin cytoskeleton regulation in bone cells. Specific inhibitors of these RhoGTPases were used to determine the pathways involved in SHIP2-mediated mineralization. Since only the ROCK pathway inhibitor rescued the mineralization phenotype, it is concluded that SHIP2 regulates actin cytoskeleton remodeling and consequently extracellular matrix mineralization by inhibiting the RhoA/ROCK pathway.