# MRCKα Is a Suppressor of GEF-H1/RhoA/MRTF Signaling in Tubular Cells

## TL;DR

MRCKα suppresses GEF-H1/RhoA signaling in kidney cells and may help prevent kidney fibrosis by controlling fibrotic gene activity.

## Contribution

MRCKα is identified as a novel suppressor of GEF-H1/RhoA/MRTF signaling in tubular cells during kidney fibrosis.

## Key findings

- MRCKα interacts with GEF-H1 and suppresses RhoA activation and stress fiber formation.
- MRCKα depletion increases TGFβ1-induced GEF-H1 activity and fibrotic gene expression.
- MRCKα expression is reduced in a mouse model of kidney fibrosis.

## Abstract

What are the main findings?
Myotonic Dystrophy Kinase-related Cdc42-binding kinase (MRCK)α is a new interactor of GEF-H1 that suppresses GEF-H1/RhoA signaling and modulates actin remodeling and phospho-MLC in tubular cells.MRCKα is a suppressor of MRTF-dependent fibrotic genes in tubular cells, and its expression is reduced in a kidney fibrosis mouse model.

Myotonic Dystrophy Kinase-related Cdc42-binding kinase (MRCK)α is a new interactor of GEF-H1 that suppresses GEF-H1/RhoA signaling and modulates actin remodeling and phospho-MLC in tubular cells.

MRCKα is a suppressor of MRTF-dependent fibrotic genes in tubular cells, and its expression is reduced in a kidney fibrosis mouse model.

What are the implications of the main findings?
MRCKα may exert crucial negative feedback on stimulus-induced GEF-H1 activation to suppress GEF-H1/RhoA signaling.Our study implicates MRCKα as a potential new suppressor of tubular reprogramming in kidney fibrosis.

MRCKα may exert crucial negative feedback on stimulus-induced GEF-H1 activation to suppress GEF-H1/RhoA signaling.

Our study implicates MRCKα as a potential new suppressor of tubular reprogramming in kidney fibrosis.

Tubule-derived pro-fibrotic mediators are central for the development of kidney fibrosis. We previously showed that fibrotic stimuli activate and elevate GEF-H1 (ARHGEF2) in tubular cells, leading to RhoA-dependent fibrotic reprogramming. In search of new mechanisms of GEF-H1 regulation, here we used immunoprecipitation and proximity ligation assay to show interaction between GEF-H1 and Myotonic Dystrophy Kinase-related Cdc42-binding kinase (MRCK)α in tubular cells. MRCKα silencing elevated GEF-H1 activity, and induced GEF-H1-dependent RhoA activation, stress fibre formation and myosin light chain phosphorylation. MRCKα depletion also elevated phospho-cofilin levels in a RhoA-dependent manner. The fibrogenic cytokine TGFβ1 rapidly increased binding between GEF-H1 and MRCKα, while MRCKα silencing augmented TGFβ1-induced GEF-H1 activation, suggesting a negative feedback loop. An mRNA array detecting fibrogenic genes revealed increase in a subset of basal and TGFβ1-induced genes following MRCKα depletion. MRCKα silencing promoted nuclear translocation of the profibrotic transcriptional co-activator Myocardin-Related Transcription Factor (MRTF), and MRTF-A+B depletion prevented increase in ACTA2 (α-smooth muscle actin), a key marker of fibrotic reprogramming. Finally, total MRCKα mRNA was reduced in a murine kidney fibrosis model, and immunohistochemistry revealed a decrease in tubular MRCKα. Taken together, we identified MRCKα as a new suppressor of GEF-H1/RhoA/MRTF signaling. Reduced MRCKα expression in kidney fibrosis may promote tubular fibrotic gene expression.