α-catenin is a key mechanosensor that forms force-dependent interactions with F-actin, thereby coupling the cadherin-catenin complex to the actin cytoskeleton at adherens junctions (AJs). However, the molecular mechanisms by which α-catenin engages F-actin under tension remained elusive. Here we show that the α1-helix of the α-catenin actin-binding domain (αcat-ABD) is a mechanosensing motif that regulates tension-dependent F-actin binding and bundling. αcat-ABD containing an α1-helix-unfolding mutation (H1) shows enhanced binding to F-actin in vitro. Although full-length α-catenin-H1 can generate epithelial monolayers that resist mechanical disruption, it fails to support normal AJ regulation in vivo. Structural and simulation analyses suggest that α1-helix allosterically controls the actin-binding residue V796 dynamics. Crystal structures of αcat-ABD-H1 homodimer suggest that α-catenin can facilitate actin bundling while it remains bound to E-cadherin. We propose that force-dependent allosteric regulation of αcat-ABD promotes dynamic interactions with F-actin involved in actin bundling, cadherin clustering, and AJ remodeling during tissue morphogenesis.

Uterine leiomyomas are extremely common estrogen and progesterone-dependent tumors of the myometrium and cause irregular uterine bleeding, severe anemia, and recurrent pregnancy loss in 15-30% of reproductive-age women. Each leiomyoma is thought to arise from a single mutated myometrial smooth muscle stem cell. Leiomyoma side-population (LMSP) cells comprising 1% of all tumor cells and displaying tumor-initiating stem cell characteristics are essential for estrogen-and progesterone-dependent in vivo growth of tumors, although they have remarkably lower estrogen/progesterone receptor levels than mature myometrial or leiomyoma cells. However, how estrogen/progesterone regulates the growth of LMSP cells via mature neighboring cells is unknown. Here, we demonstrate a critical paracrine role of the wingless-type (WNT)/β-catenin pathway in estrogen/progesterone-dependent tumorigenesis, involving LMSP and differentiated myometrial or leiomyoma cells. Estrogen/progesterone treatment of mature myometrial cells induced expression of WNT11 and WNT16, which remained constitutively elevated in leiomyoma tissues. In LMSP cells cocultured with mature myometrial cells, estrogen-progesterone selectively induced nuclear translocation of β-catenin and induced transcriptional activity of its heterodimeric partner T-cell factor and their target gene AXIN2, leading to the proliferation of LMSP cells. This effect could be blocked by a WNT antagonist. Ectopic expression of inhibitor of β-catenin and T-cell factor 4 in LMSP cells, but not in mature leiomyoma cells, blocked the estrogen/ progesterone-dependent growth of human tumors in vivo. We uncovered a paracrine role of the WNT/β-catenin pathway that enables mature myometrial or leiomyoma cells to send mitogenic signals to neighboring tissue stem cells in response to estrogen and progesterone, leading to the growth of uterine leiomyomas.WNT/β-catenin signaling | paracrine signaling | tumor biology

α-Catenin (α-cat) is an actin-binding protein required for cell-cell cohesion. Although this adhesive function for α-cat is well appreciated, cells contain a substantial amount of nonjunctional α-cat that may be used for other functions. We show that α-cat is a nuclear protein that can interact with β-catenin (β-cat) and T-cell factor (TCF) and that the nuclear accumulation of α-cat depends on β-cat. Using overexpression, knockdown, and chromatin immunoprecipitation approaches, we show that α-cat attenuates Wnt/β-cat-responsive genes in a manner that is downstream of β-cat/TCF loading on promoters. Both β-cat-and actin-binding domains of α-cat are required to inhibit Wnt signaling. A nuclear-targeted form of α-cat induces the formation of nuclear filamentous actin, whereas cells lacking α-cat show altered nuclear actin properties. Formation of nuclear actin filaments correlates with reduced RNA synthesis and altered chromatin organization. Conversely, nuclear extracts made from cells lacking α-cat show enhanced general transcription in vitro, an activity that can be partially rescued by restoring the C-terminal actin-binding region of α-cat. These data demonstrate that α-cat may limit gene expression by affecting nuclear actin organization.alpha-catenin | beta-catenin

Hypercapnia (HC), elevation of the partial pressure of CO2 in blood and tissues, is a risk factor for mortality in patients with severe acute and chronic lung diseases. We previously showed that HC inhibits multiple macrophage and neutrophil antimicrobial functions and increases the mortality of bacterial pneumonia in mice. In this study, we show that normoxic HC increases viral replication, lung injury, and mortality in mice infected with influenza A virus (IAV). Elevated CO2 increased IAV replication and inhibited antiviral gene and protein expression in macrophages in vivo and in vitro. HC potentiated IAV-induced activation of Akt, whereas specific pharmacologic inhibition or short hairpin RNA knockdown of Akt1 in alveolar macrophages blocked HC’s effects on IAV growth and the macrophage antiviral response. Our findings suggest that targeting Akt1 or the downstream pathways through which elevated CO2 signals could enhance macrophage antiviral host defense and improve clinical outcomes in hypercapnic patients with advanced lung disease.

Wood et al. show that α-catenin homodimers, but not monomers, selectively bind phosphatidylinositol-3,4,5-trisphosphate–containing vesicles with high affinity in vitro in a cadherin-independent manner and that cadherin-free α-catenin is recruited to the leading edge of migrating cells in a phosphatidylinositol 3-kinase–dependent manner to promote adhesion and migration.