E-Cadherin/HMR-1 Membrane Enrichment Is Polarized by WAVE-Dependent Branched Actin.

Polarized epithelial cells adhere to each other at apical junctions that connect to the apical F-actin belt. Regulated remodeling of apical junctions supports morphogenesis, while dysregulated remodeling promotes diseases such as cancer. We have documented that branched actin regulator, WAVE, and apical junction protein, Cadherin, assemble together in developing C. elegans embryonic junctions. If WAVE is missing in embryonic epithelia, too much Cadherin assembles at apical membranes, and yet apical F-actin is reduced, suggesting the excess Cadherin is not fully functional. We proposed that WAVE supports apical junctions by regulating the dynamic accumulation of Cadherin at membranes. To test this model, here we examine if WAVE is required for Cadherin membrane enrichment and apical-basal polarity in a maturing epithelium, the post-embryonic C. elegans intestine. We find that larval and adult intestines have distinct apicobasal populations of Cadherin, each with distinct dependence on WAVE branched actin. In vivo imaging shows that loss of WAVE components alters post-embryonic E-cadherin membrane enrichment, especially at apicolateral regions, and alters the lateral membrane. Analysis of a biosensor for PI(4,5)P2 suggests loss of WAVE or Cadherin alters the polarity of the epithelial membrane. EM (electron microscopy) illustrates lateral membrane changes including separations. These findings have implications for understanding how mutations in WAVE and Cadherin may alter cell polarity.

WAVE/SCAR promotes endocytosis and early endosome morphology in polarized C. elegans epithelia.

Cells can use the force of actin polymerization to drive intracellular transport, but the role of actin in endocytosis is not clear. Studies in single-celled yeast demonstrate the essential role of the branched actin nucleator, Arp2/3, and its activating nucleation promoting factors (NPFs) in the process of invagination from the cell surface through endocytosis. However, some mammalian studies have disputed the need for F-actin and Arp2/3 in Clathrin-Mediated Endocytosis (CME) in multicellular organisms. We investigate the role of Arp2/3 during endocytosis in Caenorhabditis elegans, a multicellular organism with polarized epithelia. Arp2/3 and its NPF, WAVE/SCAR, are essential for C. elegans embryonic morphogenesis. We show that WAVE/SCAR and Arp2/3 regulate endocytosis and early endosome morphology in diverse tissues of C. elegans. Depletion of WAVE/SCAR or Arp2/3, but not of the NPF Wasp, severely disrupts the distribution of molecules proposed to be internalized via CME, and alters the subcellular enrichment of the early endosome regulator RAB-5. Loss of WAVE/SCAR or of the GEFs that regulate RAB-5 results in similar defects in endocytosis in the intestine and coelomocyte cells. This study in a multicellular organism supports an essential role for branched actin regulators in endocytosis, and identifies WAVE/SCAR as a key NPF that promotes Arp2/3 endocytic function in C. elegans.

Arp2/3 promotes junction formation and maintenance in the Caenorhabditis elegans intestine by regulating membrane association of apical proteins.

It has been proposed that Arp2/3, which promotes nucleation of branched actin, is needed for epithelial junction initiation but is less important as junctions mature. We focus here on how Arp2/3 contributes to the Caenorhabditis elegans intestinal epithelium and find important roles for Arp2/3 in the maturation and maintenance of junctions in embryos and adults. Electron microscope studies show that embryos depleted of Arp2/3 form apical actin-rich microvilli and electron-dense apical junctions. However, whereas apical/basal polarity initiates, apical maturation is defective, including decreased apical F-actin enrichment, aberrant lumen morphology, and reduced accumulation of some apical junctional proteins, including DLG-1. Depletion of Arp2/3 in adult animals leads to similar intestinal defects. The DLG-1/AJM-1 apical junction proteins, and the ezrin-radixin-moesin homologue ERM-1, a protein that connects F-actin to membranes, are required along with Arp2/3 for apical F-actin enrichment in embryos, whereas cadherin junction proteins are not. Arp2/3 affects the subcellular distribution of DLG-1 and ERM-1. Loss of Arp2/3 shifts both ERM-1 and DLG-1 from pellet fractions to supernatant fractions, suggesting a role for Arp2/3 in the distribution of membrane-associated proteins. Thus, Arp2/3 is required as junctions mature to maintain apical proteins associated with the correct membranes.

Requirements for F-BAR proteins TOCA-1 and TOCA-2 in actin dynamics and membrane trafficking during Caenorhabditis elegans oocyte growth and embryonic epidermal morphogenesis.

The TOCA family of F-BAR-containing proteins bind to and remodel lipid bilayers via their conserved F-BAR domains, and regulate actin dynamics via their N-Wasp binding SH3 domains. Thus, these proteins are predicted to play a pivotal role in coordinating membrane traffic with actin dynamics during cell migration and tissue morphogenesis. By combining genetic analysis in Caenorhabditis elegans with cellular biochemical experiments in mammalian cells, we showed that: i) loss of CeTOCA proteins reduced the efficiency of Clathrin-mediated endocytosis (CME) in oocytes. Genetic interference with CeTOCAs interacting proteins WSP-1 and WVE-1, and other components of the WVE-1 complex, produced a similar effect. Oocyte endocytosis defects correlated well with reduced egg production in these mutants. ii) CeTOCA proteins localize to cell-cell junctions and are required for proper embryonic morphogenesis, to position hypodermal cells and to organize junctional actin and the junction-associated protein AJM-1. iii) Double mutant analysis indicated that the toca genes act in the same pathway as the nematode homologue of N-WASP/WASP, wsp-1. Furthermore, mammalian TOCA-1 and C. elegans CeTOCAs physically associated with N-WASP and WSP-1 directly, or WAVE2 indirectly via ABI-1. Thus, we propose that TOCA proteins control tissues morphogenesis by coordinating Clathrin-dependent membrane trafficking with WAVE and N-WASP-dependent actin-dynamics.

The WAVE/SCAR complex promotes polarized cell movements and actin enrichment in epithelia during C. elegans embryogenesis.

The WAVE/SCAR complex promotes actin nucleation through the Arp2/3 complex, in response to Rac signaling. We show that loss of WVE-1/GEX-1, the only C. elegans WAVE/SCAR homolog, by genetic mutation or by RNAi, has the same phenotype as loss of GEX-2/Sra1/p140/PIR121, GEX-3/NAP1/HEM2/KETTE, or ABI-1/ABI, the three other components of the C. elegans WAVE/SCAR complex. We find that the entire WAVE/SCAR complex promotes actin-dependent events at different times and in different tissues during development. During C. elegans embryogenesis loss of CED-10/Rac1, WAVE/SCAR complex components, or Arp2/3 blocks epidermal cell migrations despite correct epidermal cell differentiation. 4D movies show that this failure occurs due to decreased membrane dynamics in specific epidermal cells. Unlike myoblasts in Drosophila, epidermal cell fusions in C. elegans can occur in the absence of WAVE/SCAR or Arp2/3. Instead we find that subcellular enrichment of F-actin in epithelial tissues requires the Rac-WAVE/SCAR-Arp2/3 pathway. Intriguingly, we find that at the same stage of development both F-actin and WAVE/SCAR proteins are enriched apically in one epithelial tissue and basolaterally in another. We propose that temporally and spatially regulated actin nucleation by the Rac-WAVE/SCAR-Arp2/3 pathway is required for epithelial cell organization and movements during morphogenesis.

The Arp2/3 activators WAVE and WASP have distinct genetic interactions with Rac GTPases in Caenorhabditis elegans axon guidance.

In the developing nervous system, axons are guided to their targets by the growth cone. Lamellipodial and filopodial protrusions from the growth cone underlie motility and guidance. Many molecules that control lamellipodia and filopodia formation, actin organization, and axon guidance have been identified, but it remains unclear how these molecules act together to control these events. Experiments are described here that indicate that, in Caenorhabditis elegans, two WH2-domain-containing activators of the Arp2/3 complex, WVE-1/WAVE and WSP-1/WASP, act redundantly in axon guidance and that GEX-2/Sra-1 and GEX-3/Kette, molecules that control WAVE activity, might act in both pathways. WAVE activity is controlled by Rac GTPases, and data are presented here that suggest WVE-1/WAVE and CED-10/Rac act in parallel to a pathway containing WSP-1/WASP and MIG-2/RhoG. Furthermore, results here show that the CED-10/WVE-1 and MIG-2/WSP-1 pathways act in parallel to two other molecules known to control lamellipodia and filopodia and actin organization, UNC-115/abLIM and UNC-34/Enabled. These results indicate that at least three actin-modulating pathways act in parallel to control actin dynamics and lamellipodia and filopodia formation during axon guidance (WASP-WAVE, UNC-115/abLIM, and UNC-34/Enabled).