Rho GEFs and GAPs: emerging integrators of extracellular matrix signaling

Abstract

Investigating cell migration in 3D settings has revealed that specific extracellular matrix environments require differential activities of the Rho GTPases for efficient migration. However, it is largely unknown how the activities of specific Rho GTPases are modulated to direct cell migration in response to different extracellular matrix cues. We have recently reported that extracellular matrix-dependent regulation of a specific Rho GEF is a fundamental mechanism governing cell migration in different microenvironments, providing a direct mechanism for extracellular matrix-specific regulation of Rho GTPase activity directing cell motility. We discovered that the Rho GEF βPix has a unique function during cell migration in fibrillar collagen environments by restraining RhoA signaling through a conserved signaling axis involving Cdc42 and the Rho GAP srGAP1. In this Commentary, we expand upon this new pathway and discuss potential mechanotransductive and therapeutic applications. Additionally, we speculate on a generalized role for Rho GEFs and GAPs in providing localized, context-dependent responses to the cellular microenvironment during cell migration and other cellular processes.

Keywords: 3D migration; ECM, extracellular matrix; FRET, Fluorescence resonance energy transfer; GAP, GTPase activating protein; GEF, Guanine nucleotide exchange factor; HUVEC, Human umbilical cord vein endothelial cell; T526, Threonine 526; extracellular matrix; rho GAP; rho GEF; rho GTPase.

Figure 1.

Summary model of the extracellular matrix-specific regulation and function of βPix during cell migration. During migration in fibronectin-rich environments, βPix is phosphorylated at threonine 526 and localizes to focal adhesions through assocations with GIT1 and paxillin. Conversely, binding of α₂β₁ to fibrillar collagen leads, through PP2A, to loss of phosphorylation at T526 on βPix, βPix localization to the plasma membrane, and associations with srGAP1 and Cdc42. βPix/Cdc42/srGAP1 act to locally suppress RhoA activity at the leading edge of the cell, establishing inversely polarized gradients of Cdc42 and RhoA activity that are required for efficient cell migration in fibrillar collagen environments.

Figure 2.

Primary human foreskin fibroblasts immunostained for actin (green, phalloidin) during migration in 3D collagen matrix (left, magneta collagen fibers) and 3D cell-derived matrix (right, magneta fibronectin fibers). Cells in both 3D matrix environments can migrate with similar morphologies and dynamics, yet a requirement for βPix/Cdc42/srGAP1 signaling does not exist in the fibronectin-rich cell-derived matrix. This highlights the importance of understanding how cells respond to diverse ECM environments and appropriately regulate their migratory signaling. Nuclei (blue) visualized by DAPI.