IPMK-1 (inositol phosphate multikinase) is required for optimal integrin adhesion complex assembly in C. elegans muscle

Abstract

Integrin adhesion complexes (IACs) are a network of many proteins including the transmembrane protein integrin that anchor cells to the extracellular matrix (ECM). In C. elegans muscle IACs exist at the bases of dense bodies and M-lines, and at the boundaries of adjacent muscle cells (MCBs). We have reported that the proper assembly of IACs requires the Rac GEF PIX-1 and members of the PIX-1 pathway. Here we report that further investigation into the original Million Mutation Project strain that led us to uncover the role of PIX-1 in muscle, revealed a genetic enhancer of the PIX-1 phenotype. Genetic mapping shows that the enhancer is a recessive mutation in a single gene, ipmk-1. ipmk-1, encodes inositol phosphate multikinase and converts PIP2 to PIP3, IP3 to IP4, and IP4 to IP5. Loss of function of pix-1 results in reduced accumulation of IAC proteins at the MCB, and ipmk-1; pix-1 double mutants additionally display a large gap between adjacent muscle cells. ipmk-1 similarly enhances other members of the PIX pathway including PAK-1, and the RacGAP RRC-1. Lack of ipmk-1 itself results in abnormal clumping of IAC components at the MCB, decreased whole animal locomotion, and mild sarcomere disorganization. Analysis of ipmk-1; ipp-5 double mutants suggest that the MCB defect of ipmk-1 is due to decreased PIP3 in the cell membrane rather than the decreased IP3 and Ca2+ signaling. Overall, these studies provide in vivo evidence, specifically in muscle, that the level of PIP3 and PIP2 influence the proper assembly of IACs. Article SummaryC. elegans muscle have integrin adhesion complexes (IACs) at dense bodies, M-lines, and boundaries between adjacent muscle cells (MCBs). The Benian lab reported that assembly of these IACs requires the PIX-1 pathway. Here they report ipmk-1 is a genetic enhancer of the PIX pathway. ipmk-1encodes inositol phosphate multikinase converting PIP2 to PIP3. pix-1 shows reduced accumulation of IAC components; ipmk-1; pix-1 double mutants additionally have large gaps between muscle cells. ipmk-1 mutants display abnormal clumping of IAC components. This is the first report of the importance of these lipids in assembly of IACs in vivo, and in particular, in muscle.