Chanez-Paredes, S. D. | bioRxiv Science

bioRxiv2023

Arpin deficiency increases actomyosin contractility and vascular permeability

Arpin was discovered as an inhibitor of the Arp2/3 complex localized at the lamellipodial tip of fibroblasts, where it regulated migration steering. Recently, we showed that arpin stabilizes the epithelial barrier in an Arp2/3-dependent manner. However, expression and functions of arpin in endothelial cells (EC) have not yet been described. Arpin mRNA and protein are expressed in EC and downregulated by pro-inflammatory cytokines. Arpin depletion in HUVEC causes the formation of actomyosin stress fibers leading to increased permeability in an Arp2/3-independent manner. Instead, inhibitors of ROCK1 and ZIPK, kinases involved in the generation of stress fibers, normalize the loss-of-arpin effects on actin filaments and permeability. Arpin-deficient mice are viable but show a characteristic vascular phenotype in the lung including edema, microhemorrhage and vascular congestion, increased F-actin levels and vascular permeability. Our data show that, apart from being an Arp2/3 inhibitor, arpin is also a regulator of actomyosin contractility and endothelial barrier integrity. SUMMARYThe expression and functions of arpin in endothelial cells are unknown. We show that arpin controls actomyosin contractility and endothelial barrier integrity in an Arp2/3-independent manner via ROCK1/ZIPK. Arpin-deficient mice are viable, but also show increased basal and induced vascular permeability.

cell biology↗

bioRxiv2022

Selective perijunctional MLCK1 recruitment in Crohn's disease: Identification of essential structural domains

Intestinal epithelia express two long myosin light chain kinase (MLCK) splice variants, MLCK1 and MLCK2. Unlike MLCK2, MLCK1 is concentrated at the perijunctional actomyosin ring and this localization is enhanced by tumor necrosis factor (TNF) signaling. Here we sought to identify and characterize the domain(s) that direct basal and TNF-induced MLCK1 subcellular localization. Quantitative morphometry demonstrated specific increases in MLCK1 expression and perijunctional localization in Crohns disease patient biopsies, relative to controls. TNF induced perijunctional recruitment of MLCK1-EGFP but did not affect localization of MLCK2-EGFP, which was predominantly associated with basal stress fibers. Recombinant N-terminal MLCK1 and MLCK2 regions accelerated actin polymerization in vitro but were not different from one another. In contrast, the affinity of N-terminal MLCK1 binding to F-actin was greater than that of MLCK2. Perijunctional MLCK1 and MLCK2 domain recruitment in intestinal epithelial cells paralleled in vitro F-actin binding. The unique MLCK1 Ig3 domain was necessary, but not sufficient, for both F-actin binding and perijunctional recruitment, but, nevertheless, displaced perijunctional MLCK1, enhanced steady-state barrier function, and limited TNF-induced MLCK1 recruitment and barrier loss. These data demonstrate selective perijunctional MLCK1 recruitment in Crohns disease, suggest that F-actin binding contributes to perijunctional recruitment, and show that Ig3 can act as a dominant negative effector that limits TNF-induced MLCK1 recruitment and barrier loss. These results data provide key mechanistic detail that will enable development of therapeutics that target Ig3, or its intercellular binding partners, to reverse inflammation-induced barrier loss and limit disease progression.

cell biology↗