Transient Membrane Localization of SPV-1 Drives Cyclical Actomyosin Contractions in the C. elegans Spermatheca

SummaryBackgroundActomyosin contractility is the major cellular force driving changes in cell and tissue shape. A principal regulator of contractility is the small GTPase RhoA. External mechanical forces have been shown to impact RhoA activity and cellular contractility. However, the mechanotransduction pathway from external forces to actomyosin contractility is poorly understood.ResultsHere, we show that actomyosin contractility in the C. elegans spermatheca is under control of RHO-1/RhoA, which, in turn, is regulated by the F-BAR and RhoGAP protein SPV-1. In the relaxed spermatheca, SPV-1 localizes through its F-BAR domain to the apical membrane, where it inhibits RHO-1/RhoA activity through its RhoGAP domain. Oocyte entry forces the spermatheca cells to stretch, and subsequently SPV-1 detaches from the membrane, permitting RHO-1 activity to increase. The increase in RHO-1 activity facilitates spermatheca contraction and expulsion of the newly fertilized embryo into the uterus, leading to relaxation of the spermatheca, SPV-1 membrane localization, and initiation of a new cycle.ConclusionsOur results demonstrate how transient membrane localization of a novel F-BAR domain, likely via specific binding to curved membranes, coupled to a RhoGAP domain, can provide feedback between a mechanical signal (membrane stretching) and actomyosin contractility. We anticipate this to be a widely utilized feedback mechanism used to balance actomyosin forces in the face of externally applied forces, as well as intrinsic processes involving cell deformation, from single-cell migration to tissue morphogenesis

The C. elegans Zonula Occludens Ortholog Cooperates with the Cadherin Complex to Recruit Actin during Morphogenesis

SummaryThe dramatic cell-shape changes necessary to form a multicellular organism require cell-cell junctions to be both pliable and strong. The zonula occludens (ZO) subfamily of membrane-associated guanylate kinases (MAGUKs) are scaffolding molecules thought to regulate cell-cell adhesion [1–3], but there is little known about their roles in vivo. To elucidate the functional role of ZO proteins in a living embryo, we have characterized the sole C. elegans ZO family member, ZOO-1. ZOO-1 localizes with the cadherin-catenin complex during development, and its junctional recruitment requires the transmembrane proteins HMR-1/E-cadherin and VAB-9/claudin, but surprisingly, not HMP-1/α-catenin or HMP-2/β-catenin. zoo-1 knockdown results in lethality during elongation, resulting in the rupture of epidermal cell-cell junctions under stress and failure of epidermal sheet sealing at the ventral midline. Consistent with a role in recruiting actin to the junction in parallel to the cadherin-catenin complex, zoo-1 loss of function reduces the dynamic recruitment of actin to junctions and enhances the severity of actin filament defects in hypomorphic alleles of hmp-1 and hmp-2. These results show that ZOO-1 cooperates with the cadherin-catenin complex to dynamically regulate strong junctional anchorage to the actin cytoskeleton during morphogenesis

Glycosyl Phosphatidylinositol Anchor Biosynthesis Is Essential for Maintaining Epithelial Integrity during Caenorhabditis elegans Embryogenesis

10.1371/journal.pgen.1005082PLoS Genetics113e100508

An optogenetic tool for the activation of endogenous diaphanous-related formins induces thickening of stress fibers without an increase in contractility: Photo-activation of Diaphanous-related Formins

We have developed an optogenetic technique for the activation of diaphanous related formins. Our approach is based on fusion of the Light-Oxygen-Voltage 2 domain of Avena sativa Phototrophin1 to an isolated Diaphanous Autoregulatory Domain from mDia1. This “caged” diaphanous autoregulatory domain was inactive in the dark, but in the presence of blue light rapidly activated endogenous diaphanous related formins. Using an F-actin reporter we observed filopodia and lamellipodia formation as well as a steady increase in F-actin along existing stress fibers, starting within minutes of photo-activation. Interestingly, we did not observe the formation of new stress fibers. Remarkably, a 1.9 fold increase in F-actin was not paralleled by an increase in myosin II along stress fibers and the amount of tension generated by the fibers, as judged by focal adhesion size, appeared unchanged. Our results suggest a decoupling between F-actin accumulation and contractility in stress fibers and demonstrate the utility of photoactivatable diaphanous autoregulatory domain for the study of diaphanous related formin function in cells

Evolution of complexity in the integrin adhesome

Integrin-mediated adhesion is as ancient as multicellularity, but it was not always as complex as it is in humans. Here, I examine the extent of conservation of 192 adhesome proteins across the genomes of nine model organisms spanning one and a half billion years of evolution. The work reveals that Rho GTPases, lipid- and serine/threonine-kinases, and phosphatases existed before integrins, but tyrosine phosphorylation developed concomitant with integrins. The expansion of specific functional groups such as GAPs, GEFs, adaptors, and receptors is demonstrated, along with the expansion of specific protein domains, such as SH3, PH, SH2, CH, and LIM. Expansion is due to gene duplication and creation of families of paralogues. Apparently, these paralogues share few partners and create new sets of interactions, thus increasing specificity and the repertoire of integrin-mediated signaling. Interestingly, the average number of interactions positively correlates with the evolutionary age of proteins. While shedding light on the evolution of adhesome complexity, this analysis also highlights the relevance and creates a framework for studying integrin-mediated adhesion in simpler model organisms

Structured Illumination Microscopy Reveals Focal Adhesions are Composed of Linear Subunits

The ability to mechanically interact with the extracellular matrix is a fundamental feature of adherent eukaryotic cells. Cell-matrix adhesion in many cell types is mediated by protein complexes called focal adhesions (FAs). Recent progress in super resolution microscopy revealed FAs possess an internal organization, yet such methods do not enable observation of the formation and dynamics of their internal structure in living cells. Here, we combine structured illumination microscopy (SIM) with total internal reflection fluorescence microscopy (TIRF) to show that the proteins inside FA patches are distributed along elongated subunits, typically 300 6 100 nm wide, separated by 400 6 100 nm, and individually connected to actin cables. We further show that the formation and dynamics of these linear subunits are intimately linked to radial actin fiber formation and actomyosin contractility. We found FA growth to be the result of nucleation of new linear subunits and their coordinated elongation. Taken together, this study reveals that the basic units of mature focal adhesion are 300-nm-wide elongated, dynamic structures. We anticipate this ultrastructure to be relevant to investigation of the function of FAs and their behavior in response to mechanical stress. V C 2015 Wiley Periodicals, Inc

Plastin increases cortical connectivity to facilitate robust polarization and timely cytokinesis.

The cell cortex is essential to maintain animal cell shape, and contractile forces generated within it by nonmuscle myosin II (NMY-2) drive cellular morphogenetic processes such as cytokinesis. The role of actin cross-linking proteins in cortical dynamics is still incompletely understood. Here, we show that the evolutionarily conserved actin bundling/cross-linking protein plastin is instrumental for the generation of potent cortical actomyosin contractility in the Caenorhabditis elegans zygote. PLST-1 was enriched in contractile structures and was required for effective coalescence of NMY-2 filaments into large contractile foci and for long-range coordinated contractility in the cortex. In the absence of PLST-1, polarization was compromised, cytokinesis was delayed or failed, and 50% of embryos died during development. Moreover, mathematical modeling showed that an optimal amount of bundling agents enhanced the ability of a network to contract. We propose that by increasing the connectivity of the F-actin meshwork, plastin enables the cortex to generate stronger and more coordinated forces to accomplish cellular morphogenesis

The F-BAR domain of SRGP-1 facilitates cell–cell adhesion during C. elegans morphogenesis

SRGP-1 activity leads to outward bending and projections of membranes at cell–cell junctions, promoting robust adhesion between cells during embryonic closure events

Pre-metazoan origins and evolution of the cadherin adhesome

Vertebrate adherens junctions mediate cell–cell adhesion via a “classical” cadherin–catenin “core” complex, which is associated with and regulated by a functional network of proteins, collectively named the cadherin adhesome (“cadhesome”). The most basal metazoans have been shown to conserve the cadherin–catenin “core”, but little is known about the evolution of the cadhesome. Using a bioinformatics approach based on both sequence and structural analysis, we have traced the evolution of this larger network in 26 organisms, from the uni-cellular ancestors of metazoans, through basal metazoans, to vertebrates. Surprisingly, we show that approximately 70% of the cadhesome, including proteins with similarity to the catenins, predate metazoans. We found that the transition to multicellularity was accompanied by the appearance of a small number of adaptor proteins, and we show how these proteins may have helped to integrate pre-metazoan sub-networks via PDZ domain–peptide interactions. Finally, we found the increase in network complexity in higher metazoans to have been driven primarily by expansion of paralogs. In summary, our analysis helps to explain how the complex protein network associated with cadherin at adherens junctions first came together in the first metazoan and how it evolved into the even more complex mammalian cadhesome