Abstract
The intestinal epithelial barrier (IEB) depends on stable interepithelial protein complexes such as tight junctions (TJ), adherens junctions (AJ), and the actin cytoskeleton. During inflammation, the IEB is compromised due to TJ protein internalization and actin remodeling. An important actin regulator is the actin-related protein 2/3 (Arp2/3) complex, which induces actin branching. Activation of Arp2/3 by nucleation-promoting factors is required for the formation of epithelial monolayers, but little is known about the relevance of Arp2/3 inhibition and endogenous Arp2/3 inhibitory proteins for IEB regulation. We found that the recently identified Arp2/3 inhibitory protein arpin was strongly expressed in intestinal epithelial cells. Arpin expression decreased in response to tumor necrosis factor (TNF)α and interferon (IFN)γ treatment, whereas the expression of gadkin and protein interacting with protein C-kinase α-subunit 1 (PICK1), other Arp2/3 inhibitors, remained unchanged. Of note, arpin coprecipitated with the TJ proteins occludin and claudin-1 and the AJ protein E-cadherin. Arpin depletion altered the architecture of both AJ and TJ, increased actin filament content and actomyosin contractility, and significantly increased epithelial permeability, demonstrating that arpin is indeed required for maintaining IEB integrity. During experimental colitis in mice, arpin expression was also decreased. Analyzing colon tissues from ulcerative colitis patients by Western blot, we found different arpin levels with overall no significant changes. However, in acutely inflamed areas, arpin was significantly reduced compared to non-inflamed areas. Importantly, patients receiving mesalazine had significantly higher arpin levels than untreated patients. As arpin depletion (theoretically meaning more active Arp2/3) increased permeability, we wanted to know whether Arp2/3 inhibition would show the opposite. Indeed, the specific Arp2/3 inhibitor CK666 ameliorated TNFα/IFNγ-induced permeability in established Caco-2 monolayers by preventing TJ disruption. CK666 treatment also attenuated colitis development, colon tissue damage, TJ disruption, and permeability in dextran sulphate sodium (DSS)-treated mice. Our results demonstrate that loss of arpin triggers IEB dysfunction during inflammation and that low arpin levels can be considered a novel hallmark of acute inflammation.
Highlights
The intestinal epithelium is a single layer of cells lining the gut lumen that provides a physical barrier against luminal bacteria and antigens and regulates absorption and diffusion of water, nutrients, and ions
Either 5 mg/kg of CK666 or DMSO alone were i.p. injected daily starting on day 3 of dextran sulphate sodium (DSS) treatment when colitis symptoms started to manifest
No alterations in disease activity index (DAI) were observed in the control groups that received water and were injected with DMSO or CK666 (Figure 6A)
Summary
The intestinal epithelium is a single layer of cells lining the gut lumen that provides a physical barrier against luminal bacteria and antigens and regulates absorption and diffusion of water, nutrients, and ions. Wiskott–Aldrich syndrome protein (WASP)-family verprolinhomologous protein (WAVE) activates Arp2/3 at lamellipodia, where it is antagonized by arpin (Dang et al, 2013). The importance of the Arp2/3 complex and its activator WAVE in epithelial barrier formation has been well studied (Verma et al, 2004, 2012; Zhou et al, 2013, 2015). While the importance of arpin has been shown in different cancer types (Liu et al, 2016; Lomakina et al, 2016; Li T. et al, 2017; Li Y. et al, 2017; Zhang et al, 2019), nothing is known about arpin functions in intestinal epithelium and in epithelial barrier regulation during inflammatory disorders including UC
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