Caenorhabditis elegans LET-502 is related to Rho-binding kinases and human myotonic dystrophy kinase and interacts genetically with a homolog of the regulatory subunit of smooth muscle myosin phosphatase to affect cell shape

Conditional nmy-1 and nmy-2 alleles establish that nonmuscle myosins are required for late Caenorhabditis elegans embryonic elongation

The elongation of Caenorhabditis elegans embryos allows examination of mechanical interactions between adjacent tissues. Muscle contractions during late elongation induce the remodeling of epidermal circumferential actin filaments through mechanotransduction. Force inputs from the muscles deform circumferential epidermal actin filament, which causes them to be severed, eventually reformed, and shortened. This squeezing force drives embryonic elongation. We investigated the possible role of the nonmuscle myosins NMY-1 and NMY-2 in this process using nmy-1 and nmy-2 thermosensitive alleles. Our findings show these myosins act redundantly in late elongation, since double nmy-2(ts); nmy-1(ts) mutants immediately stop elongation when raised to 25°C. Their inactivation does not reduce muscle activity, as measured from epidermis deformation, suggesting that they are directly involved in the multistep process of epidermal remodeling. Furthermore, NMY-1 and NMY-2 inactivation is reversible when embryos are kept at the nonpermissive temperature for a few hours. However, after longer exposure to 25°C double mutant embryos fail to resume elongation, presumably because NMY-1 was seen to form protein aggregates. We propose that the two C. elegans nonmuscle myosin II act during actin remodeling either to bring severed ends or hold them.

Actomyosin-mediated apical constriction promotes physiological germ cell death in C. elegans

Germ cell apoptosis in Caenorhabditis elegans hermaphrodites is a physiological process eliminating around 60% of all cells in meiotic prophase to maintain tissue homeostasis. In contrast to programmed cell death in the C. elegans soma, the selection of germ cells undergoing apoptosis is stochastic. By live-tracking individual germ cells at the pachytene stage, we found that germ cells smaller than their neighbors are selectively eliminated through apoptosis before differentiating into oocytes. Thus, cell size is a strong predictor of physiological germ cell death. The RAS/MAPK and ECT/RHO/ROCK pathways together regulate germ cell size by controlling actomyosin constriction at the apical rachis bridges, which are cellular openings connecting the syncytial germ cells to a shared cytoplasmic core. Enhancing apical constriction reduces germ cell size and increases the rate of cell death while inhibiting the actomyosin network in the germ cells prevents their death. We propose that actomyosin contractility at the rachis bridges of the syncytial germ cells amplifies intrinsic disparities in cell size. Through this mechanism, the animals can adjust the balance between physiological germ cell death and oocyte differentiation.

TES-1/Tes and ZYX-1/Zyxin protect junctional actin networks under tension during epidermal morphogenesis in the C. elegans embryo

LIM-domain-containing repeat (LCR) proteins are recruited to strained actin filaments within stress fibers in cultured cells,1,2,3 but their roles at cell-cell junctions in living organisms have not been extensively studied. Here, we show that the Caenorhabditis elegans LCR proteins TES-1/Tes and ZYX-1/Zyxin are recruited to apical junctions during embryonic elongation when junctions are under tension. In genetic backgrounds in which embryonic elongation fails, junctional recruitment is severely compromised. The two proteins display complementary patterns of expression: TES-1 is expressed in lateral (seam) epidermal cells, whereas ZYX-1 is expressed in dorsal and ventral epidermal cells. tes-1 and zyx-1 mutant embryos display junctional F-actin defects. The loss of either protein strongly enhances morphogenetic defects in hypomorphic mutant backgrounds for cadherin/catenin complex (CCC) components. The LCR regions of TES-1 and ZYX-1 are recruited to stress fiber strain sites (SFSSs) in cultured vertebrate cells. Together, these data establish TES-1 and ZYX-1 as components of a multicellular, tension-sensitive system that stabilizes the junctional actin cytoskeleton during embryonic morphogenesis.

Orchestrating nonmuscle myosin II filament assembly at the onset of cytokinesis

Contractile forces in the actomyosin cortex are required for cellular morphogenesis. This includes the invagination of the cell membrane during division, where filaments of nonmuscle myosin II (NMII) are responsible for generating contractile forces in the cortex. However, how NMII heterohexamers form filaments in vivo is not well understood. To quantify NMII filament assembly dynamics, we imaged the cortex of Caenorhabditis elegans embryos at high spatial resolution around the time of the first division. We show that during the assembly of the cytokinetic ring, the number of NMII filaments in the cortex increases and more NMII motors are assembled into each filament. These dynamics are influenced by two proteins in the RhoA GTPase pathway, the RhoA-dependent kinase LET-502 and the myosin phosphatase MEL-11. We find that these two proteins differentially regulate NMII activity at the anterior and at the division site. We show that the coordinated action of these regulators generates a gradient of free NMII in the cytoplasm driving a net diffusive flux of NMII motors toward the cytokinetic ring. Our work highlights how NMII filament assembly and disassembly dynamics are orchestrated over space and time to facilitate the up-regulation of cortical contractility during cytokinesis.

Tissue-specific regulation of epidermal contraction during C. elegans embryonic morphogenesis

Actin and myosin mediate the epidermal cell contractions that elongate the Caenorhabditis elegans embryo from an ovoid to a tubular-shaped worm. Contraction occurs mainly in the lateral epidermal cells, while the dorsoventral epidermis plays a more passive role. Two parallel pathways trigger actinomyosin contraction, one mediated by LET-502/Rho kinase and the other by PAK-1/p21 activated kinase. A number of genes mediating morphogenesis have been shown to be sufficient when expressed either laterally or dorsoventrally. Additional genes show either lateral or dorsoventral phenotypes. This led us to a model where contractile genes have discrete functions in one or the other cell type. We tested this by examining several genes for either lateral or dorsoventral sufficiency. LET-502 expression in the lateral cells was sufficient to drive elongation. MEL-11/Myosin phosphatase, which antagonizes contraction, and PAK-1 were expected to function dorsoventrally, but we could not detect tissue-specific sufficiency. Double mutants of lethal alleles predicted to decrease lateral contraction with those thought to increase dorsoventral force were previously shown to be viable. We hypothesized that these mutant combinations shifted the contractile force from the lateral to the dorsoventral cells and so the embryos would elongate with less lateral cell contraction. This was tested by examining 10 single and double mutant strains. In most cases, elongation proceeded without a noticeable alteration in lateral contraction. We suggest that many embryonic elongation genes likely act in both lateral and dorsoventral cells, even though they may have their primary focus in one or the other cell type.

LET-502/ROCK Regulates Endocytic Recycling by Promoting Activation of RAB-5 in a Distinct Subpopulation of Sorting Endosomes

To explore the mechanism of Rab5/RAB-5 activation during endocytic recycling, we perform a genome-wide RNAi screen and identify a recycling regulator, LET-502/ROCK. LET-502 preferentially interacts with RAB-5(GDP) and activates RABX-5 GEF activity toward RAB-5, presumably by disrupting the self-inhibiting conformation of RABX-5. Furthermore, we find that the concomitant loss of LET-502 and another CED-10 effector, TBC-2/RAB-5-GAP, results in an endosomal buildup of RAB-5, indicating that CED-10 directs TBC-2-mediated RAB-5 inactivation and re-activates RAB-5 via LET-502 afterward. Then, we compare the functional position of LET-502 with that of RME-6/RAB-5-GEF. Loss of LET-502-RABX-5 module or RME-6 leads to diminished RAB-5 presence in spatially distinct endosome groups. We conclude that in the intestine of C. elegans, RAB-5 resides in discrete endosome subpopulations. Under the oversight of CED-10, LET-502 synergizes with RABX-5 to revitalize RAB-5 on a subset of endosomes in the deep cytosol, ensuring the progress of basolateral recycling.

A genetic screen for temperature-sensitive morphogenesis-defective Caenorhabditis elegans mutants

Morphogenesis involves coordinated cell migrations and cell shape changes that generate tissues and organs, and organize the body plan. Cell adhesion and the cytoskeleton are important for executing morphogenesis, but their regulation remains poorly understood. As genes required for embryonic morphogenesis may have earlier roles in development, temperature-sensitive embryonic-lethal mutations are useful tools for investigating this process. From a collection of ∼200 such Caenorhabditis elegans mutants, we have identified 17 that have highly penetrant embryonic morphogenesis defects after upshifts from the permissive to the restrictive temperature, just prior to the cell shape changes that mediate elongation of the ovoid embryo into a vermiform larva. Using whole genome sequencing, we identified the causal mutations in seven affected genes. These include three genes that have roles in producing the extracellular matrix, which is known to affect the morphogenesis of epithelial tissues in multicellular organisms: the rib-1 and rib-2 genes encode glycosyltransferases, and the emb-9 gene encodes a collagen subunit. We also used live imaging to characterize epidermal cell shape dynamics in one mutant, or1219ts, and observed cell elongation defects during dorsal intercalation and ventral enclosure that may be responsible for the body elongation defects. These results indicate that our screen has identified factors that influence morphogenesis and provides a platform for advancing our understanding of this fundamental biological process.

Epithelial morphogenesis, tubulogenesis and forces in organogenesis

As multi-cellular organisms evolved from small clusters of cells to complex metazoans, biological tubes became essential for life. Tubes are typically thought of as mainly playing a role in transport, with the hollow space (lumen) acting as a conduit to distribute nutrients and waste, or for gas exchange. However, biological tubes also provide a platform for physiological, mechanical, and structural functions. Indeed, tubulogenesis is often a critical aspect of morphogenesis and organogenesis. C. elegans is made up of tubes that provide structural support and protection (the epidermis), perform the mechanical and enzymatic processes of digestion (the buccal cavity, pharynx, intestine, and rectum), transport fluids for osmoregulation (the excretory system), and execute the functions necessary for reproduction (the germline, spermatheca, uterus and vulva). Here we review our current understanding of the genetic regulation, molecular processes, and physical forces involved in tubulogenesis and morphogenesis of the epidermal, digestive and excretory systems in C. elegans.

Redox signaling modulates Rho activity and tissue contractility in the Caenorhabditis elegans spermatheca

Actomyosin-based contractility in smooth muscle and nonmuscle cells is regulated by signaling through the small GTPase Rho and by calcium-activated pathways. We use the myoepithelial cells of the Caenorhabditis elegans spermatheca to study the mechanisms of coordinated myosin activation in vivo. Here, we show that redox signaling modulates RHO-1/Rho activity in this contractile tissue. Exogenously added as well as endogenously generated hydrogen peroxide decreases spermathecal contractility by inhibition of RHO-1, which depends on a conserved cysteine in its nucleotide binding site (C20). Further, we identify an endogenous gradient of H₂O₂ across the spermathecal tissue, which depends on the activity of cytosolic superoxide dismutase, SOD-1. Collectively, we show that SOD-1-mediated H₂O₂ production regulates the redox environment and fine tunes Rho activity across the spermatheca through oxidation of RHO-1 C20.

Game of Tissues: How the Epidermis Thrones C. elegans Shape

The versatility of epithelial cell structure is universally exploited by organisms in multiple contexts. Epithelial cells can establish diverse polarized axes within their tridimensional structure which enables them to flexibly communicate with their neighbors in a 360° range. Hence, these cells are central to multicellularity, and participate in diverse biological processes such as organismal development, growth or immune response and their misfunction ultimately impacts disease. During the development of an organism, the first task epidermal cells must complete is the formation of a continuous sheet, which initiates its own morphogenic process. In this review, we will focus on the C. elegans embryonic epithelial morphogenesis. We will describe how its formation, maturation, and spatial arrangements set the final shape of the nematode C. elegans. Special importance will be given to the tissue-tissue interactions, regulatory tissue-tissue feedback mechanisms and the players orchestrating the process.

The C. elegans BRCA2-ALP/Enigma Complex Regulates Axon Regeneration via a Rho GTPase-ROCK-MLC Phosphorylation Pathway

The ability of specific neurons to regenerate their axons after injury is governed by cell-intrinsic regeneration pathways. However, the mechanisms regulating axon regeneration are not well understood. Here, we identify the brc-2 gene encoding a homolog of the mammalian BRCA2 tumor suppressor as a regulator of axon regeneration in Caenorhabditis elegans motor neurons. We show that the RHO-1/Rho GTPase-LET-502/ROCK (Rho-associated coiled-coil kinase)-regulatory non-muscle myosin light-chain (MLC-4/MLC) phosphorylation signaling pathway regulates axon regeneration. BRC-2 functions between RHO-1 and LET-502, suggesting that BRC-2 is required for the activation of LET-502 by RHO-1-GTP. We also find that one component that interacts with BRC-2, the ALP (α-actinin-associated LIM protein)/Enigma protein ALP-1, is required for regeneration and acts between LET-502 and MLC-4 phosphorylation. Furthermore, we demonstrate that ALP-1 associates with LET-502 and MLC-4. Thus, ALP-1 serves as a platform to activate MLC-4 phosphorylation mediated by the RHO-1-LET-502 signaling pathway.

Mypt1 regulates Bmp signaling to promote embryonic exocrine pancreas growth in zebrafish

Myosin phosphatase targeting subunit 1 (Mypt1) is the regulatory subunit of myosin phosphatase which dephosphorylates the light chain of myosin II to inhibit its contraction. Although biochemical properties of Mypt1 have been characterized in detail, its biological functions in organisms are not well understood. The zebrafish mypt1 ^sq181 allele was found defective in the ventral pancreatic bud and extrapancreatic duct development, resulting in dysplasia of exocrine pancreas. In mypt1 ^sq181 mutant, the early growth of the ventral pancreatic bud was initiated but failed to expand due to impaired cell proliferation and increased cell apoptosis. As Mypt1 is essential for cell migration, the loss-of-function of Mypt1 in the mutant disrupted the lateral plate mesoderm migration during gut looping, therefore, altering the Bmp2a expression pattern within it, and eventually leading to impaired Bmp signaling in the adjacent exocrine pancreas. Overexpression of bmp2a could rescue the development of exocrine pancreas, suggesting that the impaired Bmp2a signaling is responsible for the pancreatic development defects. Bmp2a has been reported to promote the early specification of the ventral pancreatic bud, and our study reveals that it continues to serve as a cell proliferation/survival signal to ensure pancreatic bud growth properly in zebrafish.

Physics of growing biological tissues: the complex cross-talk between cell activity, growth and resistance

For many organisms, shapes emerge from growth, which generates stresses, which in turn can feedback on growth. In this review, theoretical methods to analyse various aspects of morphogenesis are discussed with the aim to determine the most adapted method for tissue mechanics. We discuss the need to work at scales intermediate between cells and tissues and emphasize the use of finite elasticity for this. We detail the application of these ideas to four systems: active cells embedded in tissues, brain cortical convolutions, the cortex of Caenorhabditis elegans during elongation and finally the proliferation of epithelia on extracellular matrix. Numerical models well adapted to inhomogeneities are also presented. This article is part of the theme issue 'Rivlin's legacy in continuum mechanics and applied mathematics'.

A high-content imaging approach to profile C. elegans embryonic development

The Caenorhabditis elegans embryo is an important model for analyzing mechanisms of cell fate specification and tissue morphogenesis. Sophisticated lineage-tracing approaches for analyzing embryogenesis have been developed but are labor intensive and do not naturally integrate morphogenetic readouts. To enable the rapid classification of developmental phenotypes, we developed a high-content method that employs two custom strains: a Germ Layer strain that expresses nuclear markers in the ectoderm, mesoderm and endoderm/pharynx; and a Morphogenesis strain that expresses markers labeling epidermal cell junctions and the neuronal cell surface. We describe a procedure that allows simultaneous live imaging of development in 80-100 embryos and provide a custom program that generates cropped, oriented image stacks of individual embryos to facilitate analysis. We demonstrate the utility of our method by perturbing 40 previously characterized developmental genes in variants of the two strains containing RNAi-sensitizing mutations. The resulting datasets yielded distinct, reproducible signature phenotypes for a broad spectrum of genes that are involved in cell fate specification and morphogenesis. In addition, our analysis provides new in vivo evidence for MBK-2 function in mesoderm fate specification and LET-381 function in elongation.

The RhoGAP SPV-1 regulates calcium signaling to control the contractility of the Caenorhabditis elegans spermatheca during embryo transits

Contractility of the nonmuscle and smooth muscle cells that comprise biological tubing is regulated by the Rho-ROCK (Rho-associated protein kinase) and calcium signaling pathways. Although many molecular details about these signaling pathways are known, less is known about how they are coordinated spatiotemporally in biological tubes. The spermatheca of the Caenorhabditis elegans reproductive system enables study of the signaling pathways regulating actomyosin contractility in live adult animals. The RhoGAP (GTPase--activating protein toward Rho family small GTPases) SPV-1 was previously identified as a negative regulator of RHO-1/Rho and spermathecal contractility. Here, we uncover a role for SPV-1 as a key regulator of calcium signaling. spv-1 mutants expressing the calcium indicator GCaMP in the spermatheca exhibit premature calcium release, elevated calcium levels, and disrupted spatial regulation of calcium signaling during spermathecal contraction. Although RHO-1 is required for spermathecal contractility, RHO-1 does not play a significant role in regulating calcium. In contrast, activation of CDC-42 recapitulates many aspects of spv-1 mutant calcium signaling. Depletion of cdc-42 by RNA interference does not suppress the premature or elevated calcium signal seen in spv-1 mutants, suggesting other targets remain to be identified. Our results suggest that SPV-1 works through both the Rho-ROCK and calcium signaling pathways to coordinate cellular contractility.

Regulation of Actin Dynamics in the C. elegans Somatic Gonad

The reproductive system of the hermaphroditic nematode C. elegans consists of a series of contractile cell types—including the gonadal sheath cells, the spermathecal cells and the spermatheca–uterine valve—that contract in a coordinated manner to regulate oocyte entry and exit of the fertilized embryo into the uterus. Contraction is driven by acto-myosin contraction and relies on the development and maintenance of specialized acto-myosin networks in each cell type. Study of this system has revealed insights into the regulation of acto-myosin network assembly and contractility in vivo.

The RhoGAP HUM-7/Myo9 integrates signals to modulate RHO-1/RhoA during embryonic morphogenesis in Caenorhabditiselegans

During embryonic morphogenesis, cells and tissues undergo dramatic movements under the control of F-actin regulators. Our studies of epidermal cell migrations in developing Caenorhabditiselegans embryos have identified multiple plasma membrane signals that regulate the Rac GTPase, thus regulating WAVE and Arp2/3 complexes, to promote branched F-actin formation and polarized enrichment. Here, we describe a pathway that acts in parallel to Rac to transduce membrane signals to control epidermal F-actin through the GTPase RHO-1/RhoA. RHO-1 contributes to epidermal migration through effects on underlying neuroblasts. We identify signals to regulate RHO-1-dependent events in the epidermis. HUM-7, the C. elegans homolog of human MYO9A and MYO9B, regulates F-actin dynamics during epidermal migration. Genetics and biochemistry support that HUM-7 behaves as a GTPase-activating protein (GAP) for the RHO-1/RhoA and CDC-42 GTPases. Loss of HUM-7 enhances RHO-1-dependent epidermal cell behaviors. We identify SAX-3/ROBO as an upstream signal that contributes to attenuated RHO-1 activation through its regulation of HUM-7/Myo9. These studies identify a new role for RHO-1 during epidermal cell migration, and suggest that RHO-1 activity is regulated by SAX-3/ROBO acting on the RhoGAP HUM-7.

Syncytial germline architecture is actively maintained by contraction of an internal actomyosin corset

Syncytial architecture is an evolutionarily-conserved feature of the germline of many species and plays a crucial role in their fertility. However, the mechanism supporting syncytial organization is largely unknown. Here, we identify a corset-like actomyosin structure within the syncytial germline of Caenorhabditis elegans, surrounding the common rachis. Using laser microsurgery, we demonstrate that actomyosin contractility within this structure generates tension both in the plane of the rachis surface and perpendicular to it, opposing membrane tension. Genetic and pharmacological perturbations, as well as mathematical modeling, reveal a balance of forces within the gonad and show how changing the tension within the actomyosin corset impinges on syncytial germline structure, leading, in extreme cases, to sterility. Thus, our work highlights a unique tissue-level cytoskeletal structure, and explains the critical role of actomyosin contractility in the preservation of a functional germline.

Germline cells in many species are fused to form a syncytium but the mechanics behind the maintenance of these structures are poorly defined. Here, the authors propose an inner contractile actomyosin corset provides a supportive framework to maintain germline architecture in C. elegans.

A Genetically Encoded Biosensor Strategy for Quantifying Non-muscle Myosin II Phosphorylation Dynamics in Living Cells and Organisms

Complex cell behaviors require dynamic control over non-muscle myosin II (NMMII) regulatory light chain (RLC) phosphorylation. Here, we report that RLC phosphorylation can be tracked in living cells and organisms using a homotransfer fluorescence resonance energy transfer (FRET) approach. Fluorescent protein-tagged RLCs exhibit FRET in the dephosphorylated conformation, permitting identification and quantification of RLC phosphorylation in living cells. This approach is versatile and can accommodate several different fluorescent protein colors, thus enabling multiplexed imaging with complementary biosensors. In fibroblasts, dynamic myosin phosphorylation was observed at the leading edge of migrating cells and retracting structures where it persistently colocalized with activated myosin light chain kinase. Changes in myosin phosphorylation during C. elegans embryonic development were tracked using polarization inverted selective-plane illumination microscopy (piSPIM), revealing a shift in phosphorylated myosin localization to a longitudinal orientation following the onset of twitching. Quantitative analyses further suggested that RLC phosphorylation dynamics occur independently from changes in protein expression.

Tissue-Specific Functions of fem-2/PP2c Phosphatase and fhod-1/formin During Caenorhabditis elegans Embryonic Morphogenesis

The cytoskeleton is the basic machinery that drives many morphogenetic events. Elongation of the C. elegans embryo from a spheroid into a long, thin larva initially results from actomyosin contractility, mainly in the lateral epidermal seam cells, while the corresponding dorsal and ventral epidermal cells play a more passive role. This is followed by a later elongation phase involving muscle contraction. Early elongation is mediated by parallel genetic pathways involving LET-502/Rho kinase and MEL-11/MYPT myosin phosphatase in one pathway and FEM-2/PP2c phosphatase and PAK-1/p21 activated kinase in another. While the LET-502/MEL-11 pathway appears to act primarily in the lateral epidermis, here we show that FEM-2 can mediate early elongation when expressed in the dorsal and ventral epidermis. We also investigated the early elongation function of FHOD-1, a member of the formin family of actin nucleators and bundlers. Previous work showed that FHOD-1 acts in the LET-502/MEL-11 branch of the early elongation pathway as well as in muscle for sarcomere organization. Consistent with this, we found that lateral epidermal cell-specific expression of FHOD-1 is sufficient for elongation, and FHOD-1 effects on elongation appear to be independent of its role in muscle. Also, we found that fhod-1 encodes long and short isoforms that differ in the presence of a predicted coiled-coil domain. Based on tissue-specific expression constructions and an isoform-specific CRISPR allele, the two FHOD-1 isoforms show partially specialized epidermal or muscle function. Although fhod-1 shows only impenetrant elongation phenotypes, we were unable to detect redundancy with other C. elegans formin genes.

The myosin light-chain kinase MLCK-1 relocalizes during Caenorhabditis elegans ovulation to promote actomyosin bundle assembly and drive contraction

We identify the Caenorhabditis elegans myosin light-chain kinase, MLCK-1, required for contraction of spermathecae. During contraction, MLCK-1 moves from the apical cell boundaries to the basal actomyosin bundles, where it stabilizes myosin downstream of calcium signaling. MLCK and ROCK act in distinct subsets of cells to coordinate the timing of contraction.

The interplay of stiffness and force anisotropies drives embryo elongation

The morphogenesis of tissues, like the deformation of an object, results from the interplay between their material properties and the mechanical forces exerted on them. The importance of mechanical forces in influencing cell behaviour is widely recognized, whereas the importance of tissue material properties, in particular stiffness, has received much less attention. Using Caenorhabditis elegans as a model, we examine how both aspects contribute to embryonic elongation. Measuring the opening shape of the epidermal actin cortex after laser nano-ablation, we assess the spatiotemporal changes of actomyosin-dependent force and stiffness along the antero-posterior and dorso-ventral axis. Experimental data and analytical modelling show that myosin-II-dependent force anisotropy within the lateral epidermis, and stiffness anisotropy within the fiber-reinforced dorso-ventral epidermis are critical in driving embryonic elongation. Together, our results establish a quantitative link between cortical tension, material properties and morphogenesis of an entire embryo.

DOI:http://dx.doi.org/10.7554/eLife.23866.001

Animals come in all shapes and size, from ants to elephants. In all cases, the tissues and organs in the animal’s body acquire their shape as the animal develops. Cells in developing tissues squeeze themselves or push and pull on one another, and the resulting forces generate the final shape. This process is called morphogenesis and it is often studied in a worm called Caenorhabditis elegans. This worm’s simplicity makes it easy to work with in the laboratory. Yet processes that occur in C. elegans also take place in other animals, including humans, and so the discoveries made using this worm can have far-reaching implications.

As they develop, the embryos of C. elegans transform from a bean-shaped cluster of cells into the characteristic long shape of a worm, with the head at one end and the tail at the other. The force required to power this elongation is provided by the outer layer of cells of the embryo, known as the epidermis. In these cells, motor-like proteins called myosins pull against a mesh-like scaffold within the cell called the actin cytoskeleton; this pulling is thought to squeeze the embryo all around and cause it to grow longer.

Six strips of cells, running from the head to the tail, make up the epidermis of a C. elegans embryo. Myosin is mostly active in two strips of cells that run along the two sides of the embryo. In the strips above and below these strips (in other words, those on the upper and lower sides of the worm), the myosins are much less active. However, it is not fully understood how this distribution of myosin causes worms to elongate only along the head-to-tail axis.

Vuong-Brender et al. have now mapped the forces exerted in the cells of the worm’s epidermis. The experiments show that, in the strips of cells on the sides of the embryo, myosin’s activity causes the epidermis to constrict around the embryo, akin to a boa constrictor tightening around its prey. At the same time, the actin filaments in the other strips form rigid bundles oriented along the circumference that stiffen the cells in these strips. This prevents the constriction from causing the embryo to inflate at the top and bottom strips. As such, the only direction the embryo can expand is along the axis that runs from its head to its tail.

Together, these findings suggest that a combination of oriented force and stiffness ensure that the embryo only elongates along the head-to-tail axis. The next step is to understand how this orientation and the coordination between cells are controlled at the molecular level.

DOI:http://dx.doi.org/10.7554/eLife.23866.002

Mechanical forces drive neuroblast morphogenesis and are required for epidermal closure

Tissue morphogenesis requires myosin-dependent events such as cell shape changes and migration to be coordinated between cells within a tissue, and/or with cells from other tissues. However, few studies have investigated the simultaneous morphogenesis of multiple tissues in vivo. We found that during Caenorhabditis elegans ventral enclosure, when epidermal cells collectively migrate to cover the ventral surface of the embryo, the underlying neuroblasts (neuronal precursor cells) also undergo morphogenesis. We found that myosin accumulates as foci along the junction-free edges of the ventral epidermal cells to form a ring, whose closure is myosin-dependent. We also observed the accumulation of myosin foci and the adhesion junction proteins E-cadherin and α-catenin in the underlying neuroblasts. Myosin may help to reorganize a subset of neuroblasts into a rosette-like pattern, and decrease their surface area as the overlying epidermal cells constrict. Since myosin is required in the neuroblasts for ventral enclosure, we propose that mechanical forces in the neuroblasts influence constriction of the overlying epidermal cells. In support of this model, disrupting neuroblast cell division or altering their fate influences myosin localization in the overlying epidermal cells. The coordination of myosin-dependent events and forces between cells in different tissues could be a common theme for coordinating morphogenetic events during metazoan development.

Two distinct myosin II populations coordinate ovulatory contraction of the myoepithelial sheath in the Caenorhabditis elegans somatic gonad

In the nematode somatic gonad, nonmuscle myosin and muscle myosin form distinct filaments and coordinate ovulatory contraction of the myoepithelial sheath. Nonmuscle myosin regulatory light chain is phosphoregulated, and its phosphorylation and dephosphorylation are critical for successful ovulation.

The myoepithelial sheath in the somatic gonad of the nematode Caenorhabditis elegans has nonstriated contractile actomyosin networks that produce highly coordinated contractility for ovulation of mature oocytes. Two myosin heavy chains are expressed in the myoepithelial sheath, which are also expressed in the body-wall striated muscle. The troponin/tropomyosin system is also present and essential for ovulation. Therefore, although the myoepithelial sheath has smooth muscle–like contractile apparatuses, it has a striated muscle–like regulatory mechanism through troponin/tropomyosin. Here we report that the myoepithelial sheath has a distinct myosin population containing nonmuscle myosin II isoforms, which is regulated by phosphorylation and essential for ovulation. MLC-4, a nonmuscle myosin regulatory light chain, localizes to small punctate structures and does not colocalize with large, needle-like myosin filaments containing MYO-3, a striated-muscle myosin isoform. RNA interference of MLC-4, as well as of its upstream regulators, LET-502 (Rho-associated coiled-coil forming kinase) and MEL-11 (a myosin-binding subunit of myosin phosphatase), impairs ovulation. Expression of a phosphomimetic MLC-4 mutant mimicking a constitutively active state also impairs ovulation. A striated-muscle myosin (UNC-54) appears to provide partially compensatory contractility. Thus the results indicate that the two spatially distinct myosin II populations coordinately regulate ovulatory contraction of the myoepithelial sheath.

FBN-1, a fibrillin-related protein, is required for resistance of the epidermis to mechanical deformation during C. elegans embryogenesis

During development, biomechanical forces contour the body and provide shape to internal organs. Using genetic and molecular approaches in combination with a FRET-based tension sensor, we characterized a pulling force exerted by the elongating pharynx (foregut) on the anterior epidermis during C. elegans embryogenesis. Resistance of the epidermis to this force and to actomyosin-based circumferential constricting forces is mediated by FBN-1, a ZP domain protein related to vertebrate fibrillins. fbn-1 was required specifically within the epidermis and FBN-1 was expressed in epidermal cells and secreted to the apical surface as a putative component of the embryonic sheath. Tiling array studies indicated that fbn-1 mRNA processing requires the conserved alternative splicing factor MEC-8/RBPMS. The conserved SYM-3/FAM102A and SYM-4/WDR44 proteins, which are linked to protein trafficking, function as additional components of this network. Our studies demonstrate the importance of the apical extracellular matrix in preventing mechanical deformation of the epidermis during development.

DOI:http://dx.doi.org/10.7554/eLife.06565.001

For an animal embryo to develop, its cells must organize themselves into tissues and organs. For example, skin and the lining of internal organs—such as the lungs and gut—are made from cells called epithelial cells, which are tightly linked to form flat sheets.

In a microscopic worm called Caenorhabditis elegans, the outermost layer of epithelial cells (called the epidermis) forms over the surface of the embryo early on in embryonic development. Shortly afterwards, the embryonic epidermis experiences powerful contractions along the surface of the embryo. The force generated by these contractions converts the embryo from an oval shape to a roughly cylindrical form. These contractions also squeeze the internal tissues and organs, which correspondingly elongate along with the epidermis.

It has been known for decades that such ‘mechanical’ forces are important for the normal development of embryos. However, it remains poorly understood how these forces generate tissues and organs of the proper shape—partly because it is difficult to measure forces in living embryos. It is also not clear how the mechanical properties of specific tissues are controlled.

Now, Kelley, Yochem, Krieg et al. have analyzed the development of C. elegans' embryos and discovered a novel mechanical interplay between the feeding organ (called the pharynx) and the worm's epidermis. The experiments involved studying several mutant worms that perturb epidermal contractions and disrupt the attachment of the pharynx to the epidermis. These studies suggested that the pharynx exerts a strong inward pulling force on the epidermis during development. Using recently developed methods, Kelley, Yochem, Krieg et al. then measured mechanical forces within intact worm embryos and demonstrated that greater forces were experienced in cells that were being pulled by the pharynx.

Kelley, Yochem, Krieg et al. further analyzed how the epidermis normally resists this pulling force from the pharynx and implicated a protein called FBN-1. This worm protein is structurally related to a human protein that is affected in people with a disorder called Marfan Syndrome. Worm embryos without the FBN-1 protein become severely deformed because they are unable to withstand mechanical forces at the epidermis. FBN-1 is normally synthesized and then transported to the outside of the worm embryo by epidermal cells, where it is thought to assemble into a meshwork of long fibers. This provides a strong scaffold that attaches to the epidermis to prevent the epidermis from undergoing excessive deformation while it experiences mechanical forces.

The work of Kelley, Yochem, Krieg et al. provides an opportunity to understand how FBN-1 and other fiber-forming proteins are produced and transported to the cell surface. Moreover, these findings may have implications for human diseases and birth defects that result from an inability of tissues to respond appropriately to mechanical forces.

DOI:http://dx.doi.org/10.7554/eLife.06565.002

α integrin cytoplasmic tails can rescue the loss of Rho-family GTPase signaling in the C. elegans somatic gonad

Integrin signaling relies on multiple, distinct pathways to impact a diverse set of cell behaviors. The Rho family of GTPases are well-established downstream signaling partners of integrins that regulate cell shape, polarity, and migration. The nematode C. elegans provides a simple in vivo system for studying both integrins and the Rho family. Our previous work showed that the C. elegans α integrin cytoplasmic tails have tissue-specific functions during development. Here, we use chimeric α integrins to show that the cytoplasmic tails can rescue the loss of the Rho family of GTPases in three cell types in the somatic gonad. Knockdown of rho-1 by RNAi causes defects in sheath cell actin organization, ovulation, and vulva morphology. Chimeric α integrin ina-1 with the pat-2 cytoplasmic tail can rescue both actin organization and ovulation after rho-1 RNAi, yet cannot restore vulva morphology. Knockdown of cdc-42 by RNAi causes defects in sheath cell actin organization, ovulation, vulva morphology, and distal tip cell migration. Chimeric α integrin pat-2 with the ina-1 cytoplasmic tail can rescue vulva morphology defects and distal tip cell migration after cdc-42 RNAi, yet cannot restore sheath cell actin organization or ovulation. Disruption of Rac yields the same phenotype in distal tip cells regardless of α integrin cytoplasmic tail composition. Taken together, the cytoplasmic tails of α integrins can bypass signaling from members of the Rho family of GTPases during development.

A bioinformatic and computational study of myosin phosphatase subunit diversity

Variability in myosin phosphatase (MP) subunits may provide specificity in signaling pathways that regulate muscle tone. We utilized public databases and computational algorithms to investigate the phylogenetic diversity of MP regulatory (PPP1R12A-C) and inhibitory (PPP1R14A-D) subunits. The comparison of exonic coding sequences and expression data confirmed or refuted the existence of isoforms and their tissue-specific expression in different model organisms. The comparison of intronic and exonic sequences identified potential expressional regulatory elements. As examples, smooth muscle MP regulatory subunit (PPP1R12A) is highly conserved through evolution. Its alternative exon E24 is present in fish through mammals with two invariant features: 1) a reading frame shift generating a premature termination codon and 2) a hexanucleotide sequence adjacent to the 3' splice site hypothesized to be a novel suppressor of exon splicing. A characteristic of the striated muscle MP regulatory subunit (PPP1R12B) locus is numerous and phylogenetically variable transcriptional start sites. In fish this locus only codes for the small (M21) subunit, suggesting the primordial function of this gene. Inhibitory subunits show little intragenic variability; their diversity is thought to have arisen by expansion and tissue-specific expression of different gene family members. We demonstrate differences in the regulatory landscape between smooth muscle enriched (PPP1R14A) and more ubiquitously expressed (PPP1R14B) family members and identify deeply conserved intronic sequence and predicted transcriptional cis-regulatory elements. This bioinformatic and computational study has uncovered a number of attributes of MP subunits that supports selection of ideal model organisms and testing of hypotheses regarding their physiological significance and regulated expression.

High-throughput capturing and characterization of mutations in essential genes of Caenorhabditis elegans

BACKGROUND

Essential genes are critical for the development of all organisms and are associated with many human diseases. These genes have been a difficult category to study prior to the availability of balanced lethal strains. Despite the power of targeted mutagenesis, there are limitations in identifying mutations in essential genes. In this paper, we describe the identification of coding regions for essential genes mutated using forward genetic screens in Caenorhabditis elegans. The lethal mutations described here were isolated and maintained by a wild-type allele on a rescuing duplication.

RESULTS

We applied whole genome sequencing to identify the causative molecular lesion resulting in lethality in existing C. elegans mutant strains. These strains are balanced and can be easily maintained for subsequent characterization. Our method can be effectively used to analyze mutations in a large number of essential genes. We describe here the identification of 64 essential genes in a region of chromosome I covered by the duplication sDp2. Of these, 42 are nonsense mutations, six are splice signal mutations, one deletion, and 15 are non-synonymous mutations. Many of the essential genes in this region function in cell cycle, transcriptional regulation, and RNA processing.

CONCLUSIONS

The essential genes identified here are represented by mutant strains, many of which have more than one mutant allele. The genetic resource can be utilized to further our understanding of essential gene function and will be applicable to the study of C. elegans development, conserved cellular function, and ultimately lead to improved human health.

The PAF1 complex is involved in embryonic epidermal morphogenesis in Caenorhabditis elegans

The PAF1 complex (PAF1C) is an evolutionarily conserved protein complex involved in transcriptional regulation and chromatin remodeling. How the PAF1C is involved in animal development is still not well understood. Here, we report that, in the nematode Caenorhabditis elegans, the PAF1C is involved in epidermal morphogenesis in late embryogenesis. From an RNAi screen we identified the C. elegans ortholog of a component of the PAF1C, CTR-9, as a gene whose depletion caused various defects during embryonic epidermal morphogenesis, including epidermal cell positioning, ventral enclosure and epidermal elongation. RNAi of orthologs of other four components of the PAF1C (PAFO-1, LEO-1, CDC-73 and RTFO-1) caused similar epidermal defects. In these embryos, whereas the number and cell fate determination of epidermal cells were apparently unaffected, their position and shape were severely disorganized. PAFO-1::mCherry, mCherry::LEO-1 and GFP::RTFO-1 driven by the authentic promoters were detected in the nuclei of a wide range of cells. Nuclear localization of GFP::RTFO-1 was independent of other PAF1C components, while PAFO-1::mCherry and mCherry::LEO-1 dependent on other components except RTFO-1. Epidermis-specific expression of mCherry::LEO-1 rescued embryonic lethality of the leo-1 deletion mutant. Thus, although the PAF1C is universally expressed in C. elegans embryos, its epidermal function is crucial for the viability of this animal.

The minus-end actin capping protein, UNC-94/tropomodulin, regulates development of the Caenorhabditis elegans intestine

BACKGROUND

Tropomodulins are actin-capping proteins that regulate the stability of the slow-growing, minus-ends of actin filaments. The C. elegans tropomodulin homolog, UNC-94, has sequence and functional similarity to vertebrate tropomodulins. We investigated the role of UNC-94 in C. elegans intestinal morphogenesis.

RESULTS

In the embryonic C. elegans intestine, UNC-94 localizes to the terminal web, an actin- and intermediate filament-rich structure that underlies the apical membrane. Loss of UNC-94 function results in areas of flattened intestinal lumen. In worms homozygous for the strong loss-of-function allele, unc-94(tm724), the terminal web is thinner and the amount of F-actin is reduced, pointing to a role for UNC-94 in regulating the structure of the terminal web. The non-muscle myosin, NMY-1, also localizes to the terminal web, and we present evidence that increasing actomyosin contractility by depleting the myosin phosphatase regulatory subunit, mel-11, can rescue the flattened lumen phenotype of unc-94 mutants.

CONCLUSIONS

The data support a model in which minus-end actin capping by UNC-94 promotes proper F-actin structure and contraction in the terminal web, yielding proper shape of the intestinal lumen. This establishes a new role for a tropomodulin in regulating lumen shape during tubulogenesis.

The role of the formin gene fhod-1 in C. elegans embryonic morphogenesis

During the second half of embryogenesis, the ellipsoidal Caenorhabditis elegans embryo elongates into a long, thin worm. This elongation requires a highly organized cytoskeleton composed of actin microfilaments, microtubules and intermediate filaments throughout the epidermis of the embryo. This architecture allows the embryonic epidermal cells to undergo a smooth muscle-like actin/myosin-based contraction that is redundantly controlled by LET- 502/Rho kinase and MEL-11/myosin phosphatase in one pathway and FEM-2/PP2c phosphatase and PAK-1/p21-activated kinase in a parallel pathway(s). Although actin microfilaments surround the embryo, the force for contraction is generated mainly in the lateral (seam) epidermal cells whose actin microfilaments appear qualitatively different from those in their dorsal/ventral neighbors. We have identified FHOD-1, a formin family actin nucleator, which acts in the lateral epidermis. fhod-1 mutants show microfilament defects in the embryonic lateral epidermal cells and FHOD-1 protein is detected only in those cells. fhod-1 genetic interactions with let-502, mel-11, fem-2 and pak-1 indicate that fhod-1 preferentially regulates those microfilaments acting with let-502 and mel-11, and in parallel to fem-2 and pak-1. Thus, FHOD-1 may contribute to the qualitative differences in microfilaments found in the contractile lateral epidermal cells and their non-contractile dorsal and ventral neighbors. Different microfilament populations may be involved in the different contractile pathways.

Filamin and phospholipase C-ε are required for calcium signaling in the Caenorhabditis elegans spermatheca

The Caenorhabditis elegans spermatheca is a myoepithelial tube that stores sperm and undergoes cycles of stretching and constriction as oocytes enter, are fertilized, and exit into the uterus. FLN-1/filamin, a stretch-sensitive structural and signaling scaffold, and PLC-1/phospholipase C-ε, an enzyme that generates the second messenger IP₃, are required for embryos to exit normally after fertilization. Using GCaMP, a genetically encoded calcium indicator, we show that entry of an oocyte into the spermatheca initiates a distinctive series of IP₃-dependent calcium oscillations that propagate across the tissue via gap junctions and lead to constriction of the spermatheca. PLC-1 is required for the calcium release mechanism triggered by oocyte entry, and FLN-1 is required for timely initiation of the calcium oscillations. INX-12, a gap junction subunit, coordinates propagation of the calcium transients across the spermatheca. Gain-of-function mutations in ITR-1/IP₃R, an IP₃-dependent calcium channel, and loss-of-function mutations in LFE-2, a negative regulator of IP₃ signaling, increase calcium release and suppress the exit defect in filamin-deficient animals. We further demonstrate that a regulatory cassette consisting of MEL-11/myosin phosphatase and NMY-1/non-muscle myosin is required for coordinated contraction of the spermatheca. In summary, this study answers long-standing questions concerning calcium signaling dynamics in the C. elegans spermatheca and suggests FLN-1 is needed in response to oocyte entry to trigger calcium release and coordinated contraction of the spermathecal tissue.

During organism development and normal physiological function cells sense, integrate, and respond to a variety of cues or signals including biochemical and mechanical stimuli. In this study we used Caenorhabditis elegans, a small transparent worm, to study filamin (FLN-1), a structural protein that may act as a molecular strain gauge. The C. elegans spermatheca is a contractile tube that is stretched during normal function, making it an ideal candidate for study of how cells respond to stretch. Oocytes are ovulated into the spermatheca, fertilized, and then pushed into the uterus by constriction of the spermatheca. The ability of the spermatheca to constrict depends on inositol 1,4,5-triphosphate (IP₃), a signaling molecule produced by the enzyme phospholipase C (PLC-1) that triggers calcium release within cells. In animals with mutated FLN-1 or PLC-1 the spermathecal cells fail to constrict. Using genetic analysis and a calcium-sensitive fluorescent protein, we show that FLN-1 functions with PLC-1 to regulate IP₃ production, calcium release, and contraction of the spermatheca. Filamin may function to sense stretch caused by entering oocytes and to trigger constriction. These findings establish a link between filamin and calcium signaling that may apply to similar signaling pathways in other systems.

Coordinated lumen contraction and expansion during vulval tube morphogenesis in Caenorhabditis elegans

Morphogenesis is a developmental phase during which cell fates are executed. Mechanical forces shaping individual cells play a key role during tissue morphogenesis. By investigating morphogenesis of the Caenorhabditis elegans hermaphrodite vulva, we show that the force-generating actomyosin network is differentially regulated by NOTCH and EGFR/RAS/MAPK signaling to shape the vulval tube. NOTCH signaling activates expression of the RHO kinase LET-502 in the secondary cell lineage through the ETS-family transcription factor LIN-1. LET-502 induces actomyosin-mediated contraction of the apical lumen in the secondary toroids, thereby generating a dorsal pushing force. In contrast, MAPK signaling in the primary lineage downregulates LET-502 RHO kinase expression to prevent toroid contraction and allow the gonadal anchor cell to expand the dorsal lumen of the primary toroids. The antagonistic action of the MAPK and NOTCH pathways thus controls vulval tube morphogenesis linking cell fate specification to morphogenesis.

Specific conserved C-terminal amino acids of Caenorhabditis elegans HMP-1/α-catenin modulate F-actin binding independently of vinculin

Stable intercellular adhesions formed through the cadherin-catenin complex are important determinants of proper tissue architecture and help maintain tissue integrity during morphogenetic movements in developing embryos. A key regulator of this stability is α-catenin, which connects the cadherin-catenin complex to the actin cytoskeleton. Although the C-terminal F-actin-binding domain of α-catenin has been shown to be crucial for its function, a more detailed in vivo analysis of discrete regions and residues required for actin binding has not been performed. Using Caenorhabditis elegans as a model system, we have characterized mutations in hmp-1/α-catenin that identify HMP-1 residues 687-742 and 826-927, as well as amino acid 802, as critical to the localization of junctional proximal actin during epidermal morphogenesis. We also find that the S823F transition in a hypomorphic allele, hmp-1(fe4), decreases actin binding in vitro. Using hmp-1(fe4) animals in a mutagenesis screen, we were then able to identify 11 intragenic suppressors of hmp-1(fe4) that revert actin binding to wild-type levels. Using homology modeling, we show that these amino acids are positioned at key conserved sites within predicted α-helices in the C terminus. Through the use of transgenic animals, we also demonstrate that HMP-1 residues 315-494, which correspond to a putative mechanotransduction domain that binds vinculin in vertebrate αE-catenin, are not required during epidermal morphogenesis but may aid efficient recruitment of HMP-1 to the junction. Our studies are the first to identify key conserved amino acids in the C terminus of α-catenin that modulate F-actin binding in living embryos of a simple metazoan.

Cadherin complexity: recent insights into cadherin superfamily function in C. elegans

Cadherin superfamily proteins mediate cell-cell adhesion during development. The C. elegans embryo is a powerful system for analyzing how cadherins function in highly stereotyped morphogenetic events. In the embryo, the classical cadherin HMR-1 acts along with the Rac pathway and SAX-7/L1CAM during gastrulation. As adherens junctions mature, PAR complex proteins differentially regulate cadherin complex localization, and SRGP-1/Slit/Robo GAP aids adhesion by promoting membrane bending. Once adherens junctions form, actin is linked to the cell surface via HMP-1/α-catenin, whose actin binding activity is regulated in novel ways. FMI-1/Flamingo and CDH-4/Fat-like regulate axonal morphology of both pioneer and follower neurons. C. elegans thus continues to be useful for uncovering precise functions for cadherin superfamily proteins and their associates in a simple metazoan.

RHGF-2 is an essential Rho-1 specific RhoGEF that binds to the multi-PDZ domain scaffold protein MPZ-1 in Caenorhabditis elegans

RhoGEF proteins activate the Rho family of small GTPases and thus play a key role in regulating fundamental cellular processes such as cell morphology and polarity, cell cycle progression and gene transcription. We identified a Caenorhabditis elegans RhoGEF protein, RHGF-2, as a binding partner of the C. elegans multi-PDZ domain scaffold protein MPZ-1 (MUPP1 in mammals). RHGF-2 exhibits significant identity to the mammalian RhoGEFs PLEKHG5/Tech/Syx and contains a class I C-terminal PDZ binding motif (SDV) that interacts most strongly to MPZ-1 PDZ domain eight. RHGF-2 RhoGEF activity is specific to the C. elegans RhoA homolog RHO-1 as determined by direct binding, GDP/GTP exchange and serum response element-driven reporter activity. rhgf-2 is an essential gene since rhgf-2 deletion mutants do not elongate during embryogenesis and hatch as short immobile animals that arrest development. Interestingly, the expression of a functional rhgf-2::gfp transgene appears to be exclusively neuronal and rhgf-2 overexpression results in loopy movement with exaggerated body bends. Transient expression of RHGF-2 in N1E-115 neuroblastoma cells prevents neurite outgrowth similar to constitutive RhoA activation in these cells. Together, these observations indicate neuronally expressed RHGF-2 is an essential RHO-1 specific RhoGEF that binds most strongly to MPZ-1 PDZ domain eight and is required for wild-type C. elegans morphology and growth.

Cell architecture: surrounding muscle cells shape gland cell morphology in the Caenorhabditis elegans pharynx

The acquisition and maintenance of shape is critical for the normal function of most cells. Here we investigate the morphology of the pharyngeal glands of Caenorhabditis elegans. These unicellular glands have long cellular processes that extend discrete lengths through the pharyngeal musculature and terminate at ducts connected to the pharyngeal lumen. From a genetic screen we identified several mutants that affect pharyngeal gland morphology. The most severe such mutant is an allele of sma-1, which encodes a β-spectrin required for embryonic elongation, including elongation of the pharynx. In sma-1 mutants, gland projections form normally but become increasingly abnormal over time, acquiring additional branches, outgrowths, and swelling, suggestive of hypertrophy. Rather than acting in pharyngeal glands, sma-1 functions in the surrounding musculature, suggesting that pharyngeal muscles play a critical role in maintenance of gland morphology by restricting their growth, and analysis of other mutants known to affect pharyngeal muscles supports this hypothesis. We suggest that gland morphology is maintained by a balance of forces from the muscles and the glands.

The RHO-1 RhoGTPase modulates fertility and multiple behaviors in adult C. elegans

The Rho family of small GTPases are essential during early embryonic development making it difficult to study their functions in adult animals. Using inducible transgenes expressing either a constitutively active version of the single C. elegans Rho ortholog, RHO-1, or an inhibitor of endogenous Rho (C3 transferase), we demonstrate multiple defects caused by altering Rho signaling in adult C. elegans. Changes in RHO-1 signaling in cholinergic neurons affected locomotion, pharyngeal pumping and fecundity. Changes in RHO-1 signaling outside the cholinergic neurons resulted in defective defecation, ovulation, and changes in C. elegans body morphology. Finally both increased and decreased RHO-1 signaling in adults resulted in death within hours. The multiple post-developmental roles for Rho in C. elegans demonstrate that RhoA signaling pathways continue to be used post-developmentally and the resulting phenotypes provide an opportunity to further study post-developmental Rho signaling pathways using genetic screens.

The C. elegans homolog of Drosophila Lethal giant larvae functions redundantly with PAR-2 to maintain polarity in the early embryo

Polarity is essential for generating cell diversity. The one-cell C. elegans embryo serves as a model for studying the establishment and maintenance of polarity. In the early embryo, a myosin II-dependent contraction of the cortical meshwork asymmetrically distributes the highly conserved PDZ proteins PAR-3 and PAR-6, as well as an atypical protein kinase C (PKC-3), to the anterior. The RING-finger protein PAR-2 becomes enriched on the posterior cortex and prevents these three proteins from returning to the posterior. In addition to the PAR proteins, other proteins are required for polarity in many metazoans. One example is the conserved Drosophila tumor-suppressor protein Lethal giant larvae (Lgl). In Drosophila and mammals, Lgl contributes to the maintenance of cell polarity and plays a role in asymmetric cell division. We have found that the C. elegans homolog of Lgl, LGL-1, has a role in polarity but is not essential. It localizes asymmetrically to the posterior of the early embryo in a PKC-3-dependent manner, and functions redundantly with PAR-2 to maintain polarity. Furthermore, overexpression of LGL-1 is sufficient to rescue loss of PAR-2 function. LGL-1 negatively regulates the accumulation of myosin (NMY-2) on the posterior cortex, representing a possible mechanism by which LGL-1 might contribute to polarity maintenance.

Tissue morphogenesis: how multiple cells cooperate to generate a tissue

Genetic analysis in model organisms has recently achieved a detailed molecular description of many key cellular processes controlling embryonic morphogenesis. To understand higher order tissue morphogenesis, we now need to define how these processes become integrated across different cell groups and cell layers. Here, we review progress in this fast moving area, which was to a large degree made possible by novel imaging methods and the increasingly frequent use of modeling. Discussing examples from Caenorhabditis elegans and Drosophila embryos, two powerful and simple models, we highlight novel principles relying in part on mechanical tension, and outline the role of junctions as signal integrators.

CGEF-1 and CHIN-1 regulate CDC-42 activity during asymmetric division in the Caenorhabditis elegans embryo

A fluorescent biosensor reports the localization of CDC-42 activity in the C. elegans embryo and was used to identify regulators of CDC-42 activity, one of which is involved in a novel regulatory loop that maintains cortical PAR polarity. CDC-42 activity regulates myosin II recruitment during the maintenance phase via the kinase MRCK-1.

The anterior–posterior axis of the Caenorhabditis elegans embryo is elaborated at the one-cell stage by the polarization of the partitioning (PAR) proteins at the cell cortex. Polarization is established under the control of the Rho GTPase RHO-1 and is maintained by the Rho GTPase CDC-42. To understand more clearly the role of the Rho family GTPases in polarization and division of the early embryo, we constructed a fluorescent biosensor to determine the localization of CDC-42 activity in the living embryo. A genetic screen using this biosensor identified one positive (putative guanine nucleotide exchange factor [GEF]) and one negative (putative GTPase activating protein [GAP]) regulator of CDC-42 activity: CGEF-1 and CHIN-1. CGEF-1 was required for robust activation, whereas CHIN-1 restricted the spatial extent of CDC-42 activity. Genetic studies placed CHIN-1 in a novel regulatory loop, parallel to loop described previously, that maintains cortical PAR polarity. We found that polarized distributions of the nonmuscle myosin NMY-2 at the cell cortex are independently produced by the actions of RHO-1, and its effector kinase LET-502, during establishment phase and CDC-42, and its effector kinase MRCK-1, during maintenance phase. CHIN-1 restricted NMY-2 recruitment to the anterior during maintenance phase, consistent with its role in polarizing CDC-42 activity during this phase.

Continuum model of epithelial morphogenesis during Caenorhabditis elegans embryonic elongation

The purpose of this work is to provide a biomechanical model to investigate the interplay between cellular structures and the mechanical force distribution during the elongation process of Caenorhabditis elegans embryos. Epithelial morphogenesis drives the elongation process of an ovoid embryo to become a worm-shaped embryo about four times longer and three times thinner. The overall anatomy of the embryo is modelled in the continuum mechanics framework from the structural organization of the subcellular filaments within epithelial cells. The constitutive relationships consider embryonic cells as homogeneous materials with an active behaviour, determined by the non-muscle myosin II molecular motor, and a passive viscoelastic response, related to the directional properties of the filament network inside cells. The axisymmetric elastic solution at equilibrium is derived by means of the incompressibility conditions, the continuity conditions for the overall embryo deformation and the balance principles for the embryonic cells. A particular analytical solution is proposed from a simplified geometry, demonstrating the mechanical role of the microtubule network within epithelial cells in redistributing the stress from a differential contraction of circumferentially oriented actin filaments. The theoretical predictions of the biomechanical model are discussed within the biological scenario proposed through genetic analysis and pharmacological experiments.

Myosin II regulation during C. elegans embryonic elongation: LET-502/ROCK, MRCK-1 and PAK-1, three kinases with different roles

Myosin II plays a central role in epithelial morphogenesis; however, its role has mainly been examined in processes involving a single cell type. Here we analyze the structure, spatial requirement and regulation of myosin II during C. elegans embryonic elongation, a process that involves distinct epidermal cells and muscles. We developed novel GFP probes to visualize the dynamics of actomyosin remodeling, and found that the assembly of myosin II filaments, but not actin microfilaments, depends on the myosin regulatory light chain (MLC-4) and essential light chain (MLC-5, which we identified herein). To determine how myosin II regulates embryonic elongation, we rescued mlc-4 mutants with various constructs and found that MLC-4 is essential in a subset of epidermal cells. We show that phosphorylation of two evolutionary conserved MLC-4 serine and threonine residues is important for myosin II activity and organization. Finally, in an RNAi screen for potential myosin regulatory light chain kinases, we found that the ROCK, PAK and MRCK homologs act redundantly. The combined loss of ROCK and PAK, or ROCK and MRCK, completely prevented embryonic elongation, but a constitutively active form of MLC-4 could only rescue a lack of MRCK. This result, together with systematic genetic epistasis tests with a myosin phosphatase mutation, suggests that ROCK and MRCK regulate MLC-4 and the myosin phosphatase. Moreover, we suggest that ROCK and PAK regulate at least one other target essential for elongation, in addition to MLC-4.

The N- or C-terminal domains of DSH-2 can activate the C. elegans Wnt/beta-catenin asymmetry pathway

Dishevelleds are modular proteins that lie at the crossroads of divergent Wnt signaling pathways. The DIX domain of dishevelleds modulates a beta-catenin destruction complex, and thereby mediates cell fate decisions through differential activation of Tcf transcription factors. The DEP domain of dishevelleds mediates planar polarity of cells within a sheet through regulation of actin modulators. In Caenorhabditis elegans asymmetric cell fate decisions are regulated by asymmetric localization of signaling components in a pathway termed the Wnt/beta-catenin asymmetry pathway. Which domain(s) of Disheveled regulate this pathway is unknown. We show that C. elegans embryos from dsh-2(or302) mutant mothers fail to successfully undergo morphogenesis, but transgenes containing either the DIX or the DEP domain of DSH-2 are sufficient to rescue the mutant phenotype. Embryos lacking zygotic function of SYS-1/beta-catenin, WRM-1/beta-catenin, or POP-1/Tcf show defects similar to dsh-2 mutants, including a loss of asymmetry in some cell fate decisions. Removal of two dishevelleds (dsh-2 and mig-5) leads to a global loss of POP-1 asymmetry, which can be rescued by addition of transgenes containing either the DIX or DEP domain of DSH-2. These results indicate that either the DIX or DEP domain of DSH-2 is capable of activating the Wnt/beta-catenin asymmetry pathway and regulating anterior-posterior fate decisions required for proper morphogenesis.

The C. elegans zonula occludens ortholog cooperates with the cadherin complex to recruit actin during morphogenesis

The 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/alpha-catenin or HMP-2/beta-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.

Epithelial morphogenesis in embryos: asymmetries, motors and brakes

Epithelial cells play a central role in many embryonic morphogenetic processes, during which they undergo highly coordinated cell shape changes. Here, we review some common principles that have recently emerged through genetic and cellular analyses performed mainly with invertebrate genetic models, focusing on morphogenetic processes involving epithelial sheets. All available data argue that myosin II is the main motor that induces cell shape changes during morphogenesis. We discuss the control of myosin II activity during epithelial morphogenesis, as well as the recently described involvement of microtubules in this process. Finally, we examine how forces unleashed by myosin II can be measured, how embryos use specific brakes to control molecular motors and the potential input of mechano-sensation in morphogenesis.

The Fat-like cadherin CDH-4 controls axon fasciculation, cell migration and hypodermis and pharynx development in Caenorhabditis elegans

Cadherins are one of the major families of adhesion molecules with diverse functions during embryonic development. Fat-like cadherins form an evolutionarily conserved subgroup characterized by an unusually large number of cadherin repeats in the extracellular domain. Here we describe the role of the Fat-like cadherin CDH-4 in Caenorhabditis elegans development. Cdh-4 mutants are characterized by hypodermal defects leading to incompletely penetrant embryonic or larval lethality with variable morphogenetic defects. Independently of the morphogenetic defects cdh-4 mutant animals also exhibit fasciculation defects in the ventral and dorsal cord, the major longitudinal axon tracts, as well as migration defects of the Q neuroblasts. In addition CDH-4 is essential for establishing and maintaining the attachment between the buccal cavity and the pharynx. Cdh-4 is expressed widely in most affected cells and tissues during embryogenesis suggesting that CDH-4 functions to ensure that proper cell contacts are made and maintained during development.