Brian Wu

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.

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, in this study 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 drop 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.

Purpose of Review Axial spondyloarthritis (axSpA) is a chronic multi-organ rheumatic disease affecting various tissues, and recent key advancements have provided significant insight into understanding its immunopathology. Recent Findings Expanded CD8⁺ T cell subsets bearing unique T cell receptor (TCR) motifs (TRAV21/TRBV9) in HLA-B27-positive axSpA patients or those with acute anterior uveitis were recently identified. These T cells can bind to self- and bacterial-derived peptides, supporting the “arthritogenic peptide” hypothesis. Monoclonal antibody-mediated elimination of these expanded T cells has shown potential as a novel treatment for axSpA in a phase II clinical trial, offering a groundbreaking therapeutic approach. In addition to T cells, neutrophils have emerged as important mediators of both inflammation and new bone formation in axSpA, potentially contributing to HLA-B27-independent immune pathomechanisms. Key neutrophil-derived molecules—such as macrophage migration inhibitory factor, hypoxia-inducible factor 1-alpha, caspase recruitment domain-containing protein 9, and neutrophil extracellular traps (NETs)—have been shown in preclinical models to promote Th17-mediated inflammation and reduce regulatory T cell numbers. These mechanisms may be relevant to axSpA pathogenesis and represent potential new therapeutic targets. Summary In recent years, significant progress has been made in understanding the immunopathology of axSpA through both HLA-B27-dependent and -independent mechanisms. However, key questions remain. The pathogenic HLA-B27:05-bound peptides driving CD8⁺ T cell expansion remain unidentified. Moreover, due to the diversity of TCR motifs that recognize self- and microbial peptides, therapies targeting a single TCR (e.g., TRBV9) may have limited efficacy. Further research is needed to clarify axSpA pathogenesis across both pathways.