INPP5K controls the dynamic structure and signaling of wild-type and mutated, leukemia-associated IL-7 receptors.

The downstream signaling of the interleukin-7 (IL-7) receptor (IL-7R) plays important physiological and pathological roles, including the differentiation of lymphoid cells and proliferation of acute lymphoblastic leukemia cells. Gain-of-function mutations in the IL-7Rα chain, the specific component of the receptor for IL-7, result in constitutive, IL-7-independent signaling and trigger acute lymphoblastic leukemia. Here, we show that the loss of the phosphoinositide 5-phosphatase INPP5K is associated with increased levels of the INPP5K substrate phosphatidylinositol 4,5-bisphosphate (PtdIns[4,5]P2) and causes an altered dynamic structure of the IL-7 receptor. We discovered that the IL-7Rα chain contains a very conserved positively charged polybasic amino acid sequence in its cytoplasmic juxtamembrane region; this region establish stronger ionic interactions with negatively charged PtdIns(4,5)P2 in the absence of INPP5K, freezing the IL-7Rα chain structure. This dynamic structural alteration causes defects in IL-7R signaling, culminating in decreased expressions of EBF1 and PAX5 transcription factors, in microdomain formation, cytoskeletal reorganization, and bone marrow B-cell differentiation. Similar alterations after the reduced INPP5K expression also affected mutated, constitutively activated IL-7Rα chains that trigger leukemia development, leading to reduced cell proliferation. Altogether, our results indicate that the lipid 5-phosphatase INPP5K hydrolyzes PtdIns(4,5)P2, allowing the requisite conformational changes of the IL-7Rα chain for optimal signaling.

The phosphoinositide 5-phosphatase INPP5K: From gene structure to in vivo functions.

INPP5K (Inositol Polyphosphate 5-Phosphatase K, or SKIP (for Skeletal muscle and Kidney enriched Inositol Phosphatase) is a member of the phosphoinositide 5-phosphatases family. Its protein structure is comprised of a N-terminal catalytic domain which hydrolyses both PtdIns(4,5)P2 and PtdIns(3,4,5)P3, followed by a SKICH domain at the C-terminus which is responsible for protein-protein interactions and subcellular localization of INPP5K. Strikingly, INPP5K is mostly concentrated in the endoplasmic reticulum, although it is also detected at the plasma membrane, in the cytosol and the nucleus. Recently, mutations in INPP5K have been detected in patients with a rare form of autosomal recessive congenital muscular dystrophy with cataract, short stature and intellectual disability. INPP5K functions extend from control of insulin signaling, endoplasmic reticulum stress response and structural integrity, myoblast differentiation, cytoskeleton organization, cell adhesion and migration, renal osmoregulation, to cancer. The goal of this review is thus to summarize and comment recent and less recent data in the literature on INPP5K, in particular on the structure, expression, intracellular localization, interactions and functions of this specific member of the 5-phosphatases family.

Altered chondrocyte differentiation, matrix mineralization and MEK-Erk1/2 signaling in an INPPL1 catalytic knock-out mouse model of opsismodysplasia.

Opsismodysplasia (OPS) is a rare but severe autosomal recessive skeletal chondrodysplasia caused by inactivating mutations in the Inppl1/Ship2 gene. The molecular mechanism leading from Ship2 gene inactivation to OPS is currently unknown. Here, we used our Ship2Δ/Δ mouse expressing reduced amount of a catalytically-inactive SHIP2 protein and a previously reported SHIP2 inhibitor to investigate growth plate development and mineralization in vivo, ex vivo and in vitro. First, as observed in OPS patients, catalytic inactivation of SHIP2 in mouse leads to reduced body length, shortening of long bones, craniofacial dysmorphism, reduced height of the hyperthrophic chondrocyte zone and to defects in growth plate mineralization. Second, intrinsic Ship2Δ/Δ bone defects were sufficient to induce the characteristic OPS alterations in bone growth, histology and mineralization ex vivo. Third, expression of osteocalcin was significantly increased in SHIP2-inactivated chondrocyte cultures whereas production of mineralized nodules was markedly decreased. Targeting osteocalcin mRNA with a specific shRNA increased the production of mineralized nodules. Fourth, levels of p-MEK and p-Erk1/2 were significantly increased in SHIP2-inactivated chondrocytes in response to serum and IGF-1, but not to FGF2, as compared to control chondrocytes. Treatment of chondrocytes and bones in culture with a MEK inhibitor partially rescued the production of mineralized nodules, the size of the hypertrophic chondrocyte zone and bone growth, raising the possibility of a treatment that could partially reduce the phenotype of this severe condition. Altogether, our results indicate that Ship2Δ/Δ mice represent a relevant model for human OPS. They also highlight the important role of SHIP2 in chondrocytes during endochondral ossification and its different differentiation steps. Finally, we identified a role of osteocalcin in mineralized nodules production and for the MEK-Erk1/2 signaling pathway in the OPS phenotype.