The Rho-GEF PIX-1 directs assembly or stability of lateral attachment structures between muscle cells

Jasmine C Moody,Eric A Ortlund,Hiroshi Qadota,Courtney J Christian,C Denise Okafor,April R Reedy,Niveda Shanmugan,Yohei Matsunaga,Guy M Benian

doi:10.1038/s41467-020-18852-4

Abstract

PIX proteins are guanine nucleotide exchange factors (GEFs) that activate Rac and Cdc42, and are known to have numerous functions in various cell types. Here, we show that a PIX protein has an important function in muscle. From a genetic screen in C. elegans, we found that pix-1 is required for the assembly of integrin adhesion complexes (IACs) at borders between muscle cells, and is required for locomotion of the animal. A pix-1 null mutant has a reduced level of activated Rac in muscle. PIX-1 localizes to IACs at muscle cell boundaries, M-lines and dense bodies. Mutations in genes encoding proteins at known steps of the PIX signaling pathway show defects at muscle cell boundaries. A missense mutation in a highly conserved residue in the RacGEF domain results in normal levels of PIX-1 protein, but a reduced level of activated Rac in muscle, and abnormal IACs at muscle cell boundaries.

Highlights

PIX proteins are guanine nucleotide exchange factors (GEFs) that activate Rac and Cdc[42], and are known to have numerous functions in various cell types
We screened the Million Mutation Project (MMP)[12] mutant strains for defects in integrin adhesion complex organization
We found that in the nonsense mutant, pix-1(gk299374), other integrin adhesion complexes (IACs) components are missing from the muscle cell boundaries, including UNC-52 in the extracellular matrix (ECM), PAT-3 (β-integrin) at the muscle cell membrane, UNC-112, and UNC-95 (Fig. 1d)

Summary

Introduction

PIX proteins are guanine nucleotide exchange factors (GEFs) that activate Rac and Cdc[42], and are known to have numerous functions in various cell types. We understand the steps involved in the formation of IACs7–9, we do not know how the composition of an IAC is determined, and we do not know what determines where an IAC forms This question is especially important for skeletal muscle cells, in which one type of IAC (the costamere) forms at regular intervals and anchors the peripherally located myofibrils to the sarcolemma and ECM at the level of Z-disks. As compared to wild type, a pix-1 null mutant and a pix-1 missense mutant show reduced levels of activated (GTP bound) Rac in muscle Both deficiency and overexpression of wild-type PIX-1 protein result in disrupted muscle cell boundaries and decreased whole nematode locomotion, suggesting that the level of PIX-1 protein needs to be tightly regulated. Our results demonstrate that the PIX signaling pathway has an important function in muscle

Methods

Results

Conclusion