A tension-induced mechanotransduction pathway promotes epithelial morphogenesis

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.

The Caenorhabditis elegans cuticle and precuticle: a model for studying dynamic apical extracellular matrices in vivo

Apical extracellular matrices (aECMs) coat the exposed surfaces of animal bodies to shape tissues, influence social interactions, and protect against pathogens and other environmental challenges. In the nematode Caenorhabditis elegans, collagenous cuticle and zona pellucida protein-rich precuticle aECMs alternately coat external epithelia across the molt cycle and play many important roles in the worm's development, behavior, and physiology. Both these types of aECMs contain many matrix proteins related to those in vertebrates, as well as some that are nematode-specific. Extensive differences observed among tissues and life stages demonstrate that aECMs are a major feature of epithelial cell identity. In addition to forming discrete layers, some cuticle components assemble into complex substructures such as ridges, furrows, and nanoscale pillars. The epidermis and cuticle are mechanically linked, allowing the epidermis to sense cuticle damage and induce protective innate immune and stress responses. The C. elegans model, with its optical transparency, facilitates the study of aECM cell biology and structure/function relationships and all the myriad ways by which aECM can influence an organism.

C. elegans Afadin is required for epidermal morphogenesis and functionally interfaces with the cadherin-catenin complex and RhoGAP PAC-1/ARHGAP21

During epithelial morphogenesis, the apical junctions connecting cells must remodel as cells change shape and make new connections with their neighbors. In the C. elegans embryo, new apical junctions form when epidermal cells migrate and seal with one another to encase the embryo in skin ('ventral enclosure'), and junctions remodel when epidermal cells change shape to squeeze the embryo into a worm shape ('elongation'). The junctional cadherin-catenin complex (CCC), which links epithelial cells to each other and to cortical actomyosin, is essential for C. elegans epidermal morphogenesis. RNAi genetic enhancement screens have identified several genes encoding proteins that interact with the CCC to promote epidermal morphogenesis, including the scaffolding protein Afadin (AFD-1), whose depletion alone results in only minor morphogenesis defects. Here, by creating a null mutation in afd-1, we show that afd-1 provides a significant contribution to ventral enclosure and elongation on its own. Unexpectedly, we find that afd-1 mutant phenotypes are strongly modified by diet, revealing a previously unappreciated parental nutritional input to morphogenesis. We identify functional interactions between AFD-1 and the CCC by demonstrating that E-cadherin is required for the polarized distribution of AFD-1 to cell contact sites in early embryos. Finally, we show that afd-1 promotes the enrichment of polarity regulator, and CCC-interacting protein, PAC-1/ARHGAP21 to cell contact sites, and we identify genetic interactions suggesting that afd-1 and pac-1 regulate epidermal morphogenesis at least in part through parallel mechanisms. Our findings reveal that C. elegans AFD-1 makes a significant contribution to epidermal morphogenesis and functionally interfaces with core and associated CCC proteins.

Cyclic muscle contractions reinforce the actomyosin motors and mediate the full elongation of C. elegans embryo

The paramount importance of mechanical forces in morphogenesis and embryogenesis is widely recognized, but understanding the mechanism at the cellular and molecular level remains challenging. Because of its simple internal organization, Caenorhabditis elegans is a rewarding system of study. As demonstrated experimentally, after an initial period of steady elongation driven by the actomyosin network, muscle contractions operate a quasi-periodic sequence of bending, rotation, and torsion, that leads to the final fourfold size of the embryos before hatching. How actomyosin and muscles contribute to embryonic elongation is investigated here theoretically. A filamentary elastic model that converts stimuli generated by biochemical signals in the tissue into driving forces, explains embryonic deformation under actin bundles and muscle activity, and dictates mechanisms of late elongation based on the effects of energy conversion and dissipation. We quantify this dynamic transformation by stretches applied to a cylindrical structure that mimics the body shape in finite elasticity, obtaining good agreement and understanding of both wild-type and mutant embryos at all stages.

A PAK family kinase and the Hippo/Yorkie pathway modulate WNT signaling to functionally integrate body axes during regeneration

Successful regeneration of missing tissues requires seamless integration of positional information along the body axes. Planarians, which regenerate from almost any injury, use conserved, developmentally important signaling pathways to pattern the body axes. However, the molecular mechanisms which facilitate cross talk between these signaling pathways to integrate positional information remain poorly understood. Here, we report a p21-activated kinase (smed-pak1) which functionally integrates the anterior-posterior (AP) and the medio-lateral (ML) axes. pak1 inhibits WNT/β-catenin signaling along the AP axis and, functions synergistically with the β-catenin-independent WNT signaling of the ML axis. Furthermore, this functional integration is dependent on warts and merlin-the components of the Hippo/Yorkie (YKI) pathway. Hippo/YKI pathway is a critical regulator of body size in flies and mice, but our data suggest the pathway regulates body axes patterning in planarians. Our study provides a signaling network integrating positional information which can mediate coordinated growth and patterning during planarian regeneration.

Targeting Nuclear Mechanics Mitigates the Fibroblast Invasiveness in Pathological Dermal Scars Induced by Matrix Stiffening

Pathological dermal scars such as keloids present significant clinical challenges lacking effective treatment options. Given the distinctive feature of highly stiffened scar tissues, deciphering how matrix mechanics regulate pathological progression can inform new therapeutic strategies. Here, it is shown that pathological dermal scar keloid fibroblasts display unique metamorphoses to stiffened matrix. Compared to normal fibroblasts, keloid fibroblasts show high sensitivity to stiffness rather than biochemical stimulation, activating cytoskeletal-to-nuclear mechanosensing molecules. Notably, keloid fibroblasts on stiff matrices exhibit nuclear softening, concomitant with reduced lamin A/C expression, and disrupted anchoring of lamina-associated chromatin. This nuclear softening, combined with weak adhesion and high contractility, facilitates the invasive migration of keloid fibroblasts through confining matrices. Inhibiting lamin A/C-driven nuclear softening, via lamin A/C overexpression or actin disruption, mitigates such invasiveness of keloid fibroblasts. These findings highlight the significance of the nuclear mechanics of keloid fibroblasts in scar pathogenesis and propose lamin A/C as a potential therapeutic target for managing pathological scars.

The RhoGAP RRC-1 is required for the assembly or stability of integrin adhesion complexes and is a member of the PIX pathway in muscle

GTPases cycle between active GTP bound and inactive GDP bound forms. Exchange of GDP for GTP is catalyzed by guanine nucleotide exchange factors (GEFs). GTPase activating proteins (GAPs) accelerate GTP hydrolysis, to promote the GDP bound form. We reported that the RacGEF, PIX-1, is required for assembly of integrin adhesion complexes (IAC) in striated muscle of Caenorhabditis elegans. In C. elegans, IACs are found at the muscle cell boundaries (MCBs), and bases of sarcomeric M-lines and dense bodies (Z-disks). Screening C. elegans mutants in proteins containing RhoGAP domains revealed that loss of function of rrc-1 results in loss of IAC components at MCBs, disorganization of M-lines and dense bodies, and reduced whole animal locomotion. RRC-1 localizes to MCBs, like PIX-1. The localization of RRC-1 at MCBs requires PIX-1, and the localization of PIX-1 requires RRC-1. Loss of function of CED-10 (Rac) shows lack of PIX-1 and RRC-1 at MCBs. RRC-1 exists in a complex with PIX-1. Transgenic rescue of rrc-1 was achieved with wild type RRC-1 but not RRC-1 with a missense mutation in a highly conserved residue of the RhoGAP domain. Our results are consistent with RRC-1 being a RhoGAP for the PIX pathway in muscle.

Longevity interventions modulate mechanotransduction and extracellular matrix homeostasis in C. elegans

Dysfunctional extracellular matrices (ECM) contribute to aging and disease. Repairing dysfunctional ECM could potentially prevent age-related pathologies. Interventions promoting longevity also impact ECM gene expression. However, the role of ECM composition changes in healthy aging remains unclear. Here we perform proteomics and in-vivo monitoring to systematically investigate ECM composition (matreotype) during aging in C. elegans revealing three distinct collagen dynamics. Longevity interventions slow age-related collagen stiffening and prolong the expression of collagens that are turned over. These prolonged collagen dynamics are mediated by a mechanical feedback loop of hemidesmosome-containing structures that span from the exoskeletal ECM through the hypodermis, basement membrane ECM, to the muscles, coupling mechanical forces to adjust ECM gene expression and longevity via the transcriptional co-activator YAP-1 across tissues. Our results provide in-vivo evidence that coordinated ECM remodeling through mechanotransduction is required and sufficient to promote longevity, offering potential avenues for interventions targeting ECM dynamics.

Mechanisms of lineage specification in Caenorhabditis elegans

The studies of cell fate and lineage specification are fundamental to our understanding of the development of multicellular organisms. Caenorhabditis elegans has been one of the premiere systems for studying cell fate specification mechanisms at single cell resolution, due to its transparent nature, the invariant cell lineage, and fixed number of somatic cells. We discuss the general themes and regulatory mechanisms that have emerged from these studies, with a focus on somatic lineages and cell fates. We next review the key factors and pathways that regulate the specification of discrete cells and lineages during embryogenesis and postembryonic development; we focus on transcription factors and include numerous lineage diagrams that depict the expression of key factors that specify embryonic founder cells and postembryonic blast cells, and the diverse somatic cell fates they generate. We end by discussing some future perspectives in cell and lineage specification.

The long protostomic-type cytoplasmic intermediate filament (cIF) protein in Branchiostoma supports the phylogenetic transition between the protostomic- and the chordate-type cIFs

We identified 23 and 20 cytoplasmic IF (cIF) genes in the two Branchiostoma belcheri and Branchiostoma lanceolatum cephalochordates, respectively. Combining these results with earlier data on the related Branchiostoma floridae, the following conclusions can be drawn. First, the Branchiostoma N4 protein with a long lamin-like coil 1B segment is the only protostomic-type cIF found so far in any analysed chordate or vertebrate organism. Second, Branchiostoma is the only organism known so far containing both the long protostomic- and the short chordate-prototypes of cIFs. This finding provides so far missing molecular evidence for the phylogenetic transition between the protostomic- and the chordate-type IF sequences at the base of the cephalochordates and vertebrates. Third, this finding also brings some support for another hypothesis, that the long protostomic-type cIF is subjected to evolutionary constraints in order to preclude inappropriate interactions with lamin and that the latter complexes might be prevented by a several heptad-long rod deletion, which released the selective constraints on it and promoted, at least in part, its expansion in nematodes, cephalochordates, and in vertebrates. Finally, here-presented data confirmed our previous results that cephalochordates do not have any vertebrate type III or type IV IF homolog.

Cell contractility in early animal evolution

Tissue deformation mediated by collective cell contractility is a signature characteristic of animals. In most animals, fast and reversible contractions of muscle cells mediate behavior, while slow and irreversible contractions of epithelial or mesenchymal cells play a key role in morphogenesis. Animal tissue contractility relies on the activity of the actin/myosin II complex (together referred to as 'actomyosin'), an ancient and versatile molecular machinery that performs a broad range of functions in development and physiology. This review synthesizes emerging insights from morphological and molecular studies into the evolutionary history of animal contractile tissue. The most ancient functions of actomyosin are cell crawling and cytokinesis, which are found in a wide variety of unicellular eukaryotes and in individual metazoan cells. Another contractile functional module, apical constriction, is universal in metazoans and shared with choanoflagellates, their closest known living relatives. The evolution of animal contractile tissue involved two key innovations: firstly, the ability to coordinate and integrate actomyosin assembly across multiple cells, notably to generate supracellular cables, which ensure tissue integrity but also allow coordinated morphogenesis and movements at the organism scale; and secondly, the evolution of dedicated contractile cell types for adult movement, belonging to two broad categories respectively defined by the expression of the fast (striated-type) and slow (smooth/non-muscle-type) myosin II paralogs. Both contractile cell types ancestrally resembled generic contractile epithelial or mesenchymal cells and might have played a versatile role in both behavior and morphogenesis. Modern animal contractile cells span a continuum between unspecialized contractile epithelia (which underlie behavior in modern placozoans), epithelia with supracellular actomyosin cables (found in modern sponges), epitheliomuscular tissues (with a concentration of actomyosin cables in basal processes, for example in sea anemones), and specialized muscle tissue that has lost most or all epithelial properties (as in ctenophores, jellyfish and bilaterians). Recent studies in a broad range of metazoans have begun to reveal the molecular basis of these transitions, powered by the elaboration of the contractile apparatus and the evolution of 'core regulatory complexes' of transcription factors specifying contractile cell identity.

C. elegans Hemidesmosomes Sense Collagen Damage to Trigger Innate Immune Response in the Epidermis

The collagens are an enormous family of extracellular matrix proteins that play dominant roles in cell adhesion, migration and tissue remodeling under many physiological and pathological conditions. However, their function mechanisms in regulating innate immunity remain largely undiscovered. Here we use C. elegans epidermis as the model to address this question. The C. elegans epidermis is covered with a collagen-rich cuticle exoskeleton and can produce antimicrobial peptides (AMPs) against invading pathogens or physical injury. Through an RNAi screen against collagen-encoding genes, we found that except the previously reported six DPY collagens and the BLI-1 collagen, the majority of collagens tested appear unable to trigger epidermal immune defense when damaged. Further investigation suggests that the six DPY collagens form a specific substructure, which regulates the interaction between BLI-1 and the hemidesmosome receptor MUP-4. The separation of BLI-1 with MUP-4 caused by collagen damage leads to the detachment of the STAT transcription factor-like protein STA-2 from hemidesmosomes and the induction of AMPs. Our findings uncover the mechanism how collagens are organized into a damage sensor and how the epidermis senses collagen damage to mount an immune defense.

Mechanotransduction through hemidesmosomes during aging and longevity

Hemidesmosomes are structural protein complexes localized at the interface of tissues with high mechanical demand and shear forces. Beyond tissue anchoring, hemidesmosomes have emerged as force-modulating structures important for translating mechanical cues into biochemical and transcriptional adaptation (i.e. mechanotransduction) across tissues. Here, we discuss the recent insights into the roles of hemidesmosomes in age-related tissue regeneration and aging in C. elegans, mice and humans. We highlight the emerging concept of preserved dynamic mechanoregulation of hemidesmosomes in tissue maintenance and healthy aging.

Alterations of Cytoskeleton Networks in Cell Fate Determination and Cancer Development

Cytoskeleton proteins have been long recognized as structural proteins that provide the necessary mechanical architecture for cell development and tissue homeostasis. With the completion of the cancer genome project, scientists were surprised to learn that huge numbers of mutated genes are annotated as cytoskeletal or associated proteins. Although most of these mutations are considered as passenger mutations during cancer development and evolution, some genes show high mutation rates that can even determine clinical outcomes. In addition, (phospho)proteomics study confirms that many cytoskeleton-associated proteins, e.g., β-catenin, PIK3CA, and MB21D2, are important signaling mediators, further suggesting their biofunctional roles in cancer development. With emerging evidence to indicate the involvement of mechanotransduction in stemness formation and cell differentiation, mutations in these key cytoskeleton components may change the physical/mechanical properties of the cells and determine the cell fate during cancer development. In particular, tumor microenvironment remodeling triggered by such alterations has been known to play important roles in autophagy, metabolism, cancer dormancy, and immune evasion. In this review paper, we will highlight the current understanding of how aberrant cytoskeleton networks affect cancer behaviors and cellular functions through mechanotransduction.

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.

Bidirectional regulation of structural damage on autophagy in the C. elegans epidermis

A variety of disturbances such as starvation, organelle damage, heat stress, hypoxia and pathogen infection can influence the autophagic process. However, how the macroautophagy/autophagy machinery is regulated intrinsically by structural damage of the cell remains largely unknown. In this work, we utilized the C. elegans epidermis as the model to address this question. Our results showed that structural damage by mechanical wounding exerted proximal inhibitory effect and distant promotional effect on autophagy within the same epidermal cell. By disrupting individual mechanical supporting structures, we found that only damage of the basal extracellular matrix or the underlying muscle cells activated a distinct autophagic response in the epidermis. On the contrary, structural disruption of the epidermal cells at the apical side inhibited autophagy activation caused by different stress factors. Mechanistic studies showed that the basal promotional effect of structural damage on epidermal autophagy was mediated by a mechanotransduction pathway going through the basal hemidesmosome receptor and LET-363/MTOR, while the apical inhibitory effect was mostly carried out by activation of calcium signaling. Elevated autophagy in the epidermis played a detrimental rather than a beneficial role on cell survival against structural damage. The results obtained from these studies will not only help us better understand the pathogenesis of structural damage- and autophagy-related diseases, but also provide insight into more generic rules of autophagy regulation by the structural and mechanical properties of cells across species.Abbreviations : ATG: autophagy related; BLI-1: BLIstered cuticle 1; CeHDs: C. elegans hemidesmosomes; COL-19: COLlagen 19; DPY-7: DumPY 7; ECM: extracellular matrix; EPG-5: ectopic PGL granules 5; GFP: green fluorescent protein; GIT-1: GIT1 (mammalian G protein-coupled receptor kinase InTeractor 1) homolog; GTL-2: Gon-Two Like 2 (TRP subfamily); HIS-58, HIStone 58; IFB-1: Intermediate Filament, B 1; LET: LEThal; LGG-1: LC3, GABARAP and GATE-16 family 1; MTOR: mechanistic target of rapamycin; MTORC1: MTOR complex 1; MUP-4: MUscle Positioning 4; NLP-29: Neuropeptide-Like Protein 29; PAT: Paralyzed Arrest at Two-fold; PIX-1: PIX (PAK (p21-activated kinase) Interacting eXchange factor) homolog 1; RFP: red fluorescent protein; RNAi: RNA interference; SQST-1: SeQueSTosome related 1; UNC: UNCoordinated; UV: ultraviolet; VAB-10: variable ABnormal morphology 10; WT: wild type.

Muscular hydraulics drive larva-polyp morphogenesis

Development is a highly dynamic process in which organisms often experience changes in both form and behavior, which are typically coupled to each other. However, little is known about how organismal-scale behaviors such as body contractility and motility impact morphogenesis. Here, we use the cnidarian Nematostella vectensis as a developmental model to uncover a mechanistic link between organismal size, shape, and behavior. Using quantitative live imaging in a large population of developing animals, combined with molecular and biophysical experiments, we demonstrate that the muscular-hydraulic machinery that controls body movement also drives larva-polyp morphogenesis. We show that organismal size largely depends on cavity inflation through fluid uptake, whereas body shape is constrained by the organization of the muscular system. The generation of ethograms identifies different trajectories of size and shape development in sessile and motile animals, which display distinct patterns of body contractions. With a simple theoretical model, we conceptualize how pressures generated by muscular hydraulics can act as a global mechanical regulator that coordinates tissue remodeling. Altogether, our findings illustrate how organismal contractility and motility behaviors can influence morphogenesis.

A transient apical extracellular matrix relays cytoskeletal patterns to shape permanent acellular ridges on the surface of adult C. elegans

Epithelial cells secrete apical extracellular matrices to form protruding structures such as denticles, ridges, scales, or teeth. The mechanisms that shape these structures remain poorly understood. Here, we show how the actin cytoskeleton and a provisional matrix work together to sculpt acellular longitudinal alae ridges in the cuticle of adult C. elegans. Transient assembly of longitudinal actomyosin filaments in the underlying lateral epidermis accompanies deposition of the provisional matrix at the earliest stages of alae formation. Actin is required to pattern the provisional matrix into longitudinal bands that are initially offset from the pattern of longitudinal actin filaments. These bands appear ultrastructurally as alternating regions of adhesion and separation within laminated provisional matrix layers. The provisional matrix is required to establish these demarcated zones of adhesion and separation, which ultimately give rise to alae ridges and their intervening valleys, respectively. Provisional matrix proteins shape the alae ridges and valleys but are not present within the final structure. We propose a morphogenetic mechanism wherein cortical actin patterns are relayed to the laminated provisional matrix to set up distinct zones of matrix layer separation and accretion that shape a permanent and acellular matrix structure.

Mechanotransduction in Skin Inflammation

In the process of mechanotransduction, the cells in the body perceive and interpret mechanical stimuli to maintain tissue homeostasis and respond to the environmental changes. Increasing evidence points towards dysregulated mechanotransduction as a pathologically relevant factor in human diseases, including inflammatory conditions. Skin is the organ that constantly undergoes considerable mechanical stresses, and the ability of mechanical factors to provoke inflammatory processes in the skin has long been known, with the Koebner phenomenon being an example. However, the molecular mechanisms and key factors linking mechanotransduction and cutaneous inflammation remain understudied. In this review, we outline the key players in the tissue's mechanical homeostasis, the available data, and the gaps in our current understanding of their aberrant regulation in chronic cutaneous inflammation. We mainly focus on psoriasis as one of the most studied skin inflammatory diseases; we also discuss mechanotransduction in the context of skin fibrosis as a result of chronic inflammation. Even though the role of mechanotransduction in inflammation of the simple epithelia of internal organs is being actively studied, we conclude that the mechanoregulation in the stratified epidermis of the skin requires more attention in future translational research.

The roles of inter-tissue adhesion in development and morphological evolution

The study of how neighboring tissues physically interact with each other, inter-tissue adhesion, is an emerging field at the interface of cell biology, biophysics and developmental biology. Inter-tissue adhesion can be mediated by either cell-extracellular matrix adhesion or cell-cell adhesion, and both the mechanisms and consequences of inter-tissue adhesion have been studied in vivo in numerous vertebrate and invertebrate species. In this Review, we discuss recent progress in understanding the many functions of inter-tissue adhesion in development and evolution. Inter-tissue adhesion can couple the motion of adjacent tissues, be the source of mechanical resistance that constrains morphogenesis, and transmit tension required for normal development. Tissue-tissue adhesion can also create mechanical instability that leads to tissue folding or looping. Transient inter-tissue adhesion can facilitate tissue invasion, and weak tissue adhesion can generate friction that shapes and positions tissues within the embryo. Lastly, we review studies that reveal how inter-tissue adhesion contributes to the diversification of animal morphologies.

Generic self-stabilization mechanism for biomolecular adhesions under load

Mechanical loading generally weakens adhesive structures and eventually leads to their rupture. However, biological systems can adapt to loads by strengthening adhesions, which is essential for maintaining the integrity of tissue and whole organisms. Inspired by cellular focal adhesions, we suggest here a generic, molecular mechanism that allows adhesion systems to harness applied loads for self-stabilization through adhesion growth. The mechanism is based on conformation changes of adhesion molecules that are dynamically exchanged with a reservoir. Tangential loading drives the occupation of some states out of equilibrium, which, for thermodynamic reasons, leads to association of further molecules with the cluster. Self-stabilization robustly increases adhesion lifetimes in broad parameter ranges. Unlike for catch-bonds, bond rupture rates can increase monotonically with force. The self-stabilization principle can be realized in many ways in complex adhesion-state networks; we show how it naturally occurs in cellular adhesions involving the adaptor proteins talin and vinculin.

Cellular adhesions have the remarkable property that they adapt their stability to the applied mechanical load. Here, authors describe a generic physical mechanism that explains self-stabilization of idealized adhesion systems under shear.

Fibrous stroma: Driver and passenger in cancer development

Cumulative evidence shows that fibrogenic stroma and stiff extracellular matrix (ECM) not only result from tumor growth but also play pivotal roles in cellular transformation and tumor initiation. This emerging concept may largely account for the increased cancer risk associated with environmental fibrogenic agents, such as asbestos and silica, and with chronic conditions that are fibrogenic, such as obesity and diabetes. It may also contribute to poor outcomes in patients treated with certain chemotherapeutics that can promote fibrosis, such as bleomycin and methotrexate. Although the mechanistic details of this phenomenon are still being unraveled, we provide an overview of the experimental evidence linking fibrogenic stroma and tumor initiation. In this Review, we will summarize the causes and consequences of fibrous stroma and how this stromal cue is transmitted to the nuclei of parenchymal cells through a physical continuum from the ECM to chromatin, as well as ECM-dependent biochemical signaling that contributes to cellular transformation.

Microgravity Modifies the Phenotype of Fibroblast and Promotes Remodeling of the Fibroblast-Keratinocyte Interaction in a 3D Co-Culture Model

Microgravity impairs tissue organization and critical pathways involved in the cell–microenvironment interplay, where fibroblasts have a critical role. We exposed dermal fibroblasts to simulated microgravity by means of a Random Positioning Machine (RPM), a device that reproduces conditions of weightlessness. Molecular and structural changes were analyzed and compared to control samples growing in a normal gravity field. Simulated microgravity impairs fibroblast conversion into myofibroblast and inhibits their migratory properties. Consequently, the normal interplay between fibroblasts and keratinocytes were remarkably altered in 3D co-culture experiments, giving rise to several ultra-structural abnormalities. Such phenotypic changes are associated with down-regulation of α-SMA that translocate in the nucleoplasm, altogether with the concomitant modification of the actin-vinculin apparatus. Noticeably, the stress associated with weightlessness induced oxidative damage, which seemed to concur with such modifications. These findings disclose new opportunities to establish antioxidant strategies that counteract the microgravity-induced disruptive effects on fibroblasts and tissue organization.

KIAA0319 influences cilia length, cell migration and mechanical cell-substrate interaction

Following its association with dyslexia in multiple genetic studies, the KIAA0319 gene has been extensively investigated in different animal models but its function in neurodevelopment remains poorly understood. We developed the first human cellular knockout model for KIAA0319 in RPE1 retinal pigment epithelia cells via CRISPR-Cas9n to investigate its role in processes suggested but not confirmed in previous studies, including cilia formation and cell migration. We observed in the KIAA0319 knockout increased cilia length and accelerated cell migration. Using Elastic Resonator Interference Stress Microscopy (ERISM), we detected an increase in cellular force for the knockout cells that was restored by a rescue experiment. Combining ERISM and immunostaining we show that RPE1 cells exert highly dynamic, piconewton vertical pushing forces through actin-rich protrusions that are surrounded by vinculin-rich pulling sites. This protein arrangement and force pattern has previously been associated to podosomes in other cells. KIAA0319 depletion reduces the fraction of cells forming these actin-rich protrusions. Our results suggest an involvement of KIAA0319 in cilia biology and cell-substrate force regulation.

Eph/Ephrin-Based Protein Complexes: The Importance of cis Interactions in Guiding Cellular Processes

Although intracellular signal transduction is generally represented as a linear process that transmits stimuli from the exterior of a cell to the interior via a transmembrane receptor, interactions with additional membrane-associated proteins are often critical to its success. These molecules play a pivotal role in mediating signaling via the formation of complexes in cis (within the same membrane) with primary effectors, particularly in the context of tumorigenesis. Such secondary effectors may act to promote successful signaling by mediating receptor-ligand binding, recruitment of molecular partners for the formation of multiprotein complexes, or differential signaling outcomes. One signaling family whose contact-mediated activity is frequently modulated by lateral interactions at the cell surface is Eph/ephrin (EphA and EphB receptor tyrosine kinases and their ligands ephrin-As and ephrin-Bs). Through heterotypic interactions in cis, these molecules can promote a diverse range of cellular activities, including some that are mutually exclusive (cell proliferation and cell differentiation, or adhesion and migration). Due to their broad expression in most tissues and their promiscuous binding within and across classes, the cellular response to Eph:ephrin interaction is highly variable between cell types and is dependent on the cellular context in which binding occurs. In this review, we will discuss interactions between molecules in cis at the cell membrane, with emphasis on their role in modulating Eph/ephrin signaling.

A Maternal-Effect Toxin Affects Epithelial Differentiation and Tissue Mechanics in Caenorhabditis elegans

Selfish genetic elements that act as post-segregation distorters cause lethality in non-carrier individuals after fertilization. Two post-segregation distorters have been previously identified in Caenorhabditis elegans, the peel-1/zeel-1 and the sup-35/pha-1 elements. These elements seem to act as modification-rescue systems, also called toxin/antidote pairs. Here we show that the maternal-effect toxin/zygotic antidote pair sup-35/pha-1 is required for proper expression of apical junction (AJ) components in epithelia and that sup-35 toxicity increases when pathways that establish and maintain basal epithelial characteristics, die-1, elt-1, lin-26, and vab-10, are compromised. We demonstrate that pha-1(e2123) embryos, which lack the antidote, are defective in epidermal morphogenesis and frequently fail to elongate. Moreover, seam cells are frequently misshaped and mispositioned and cell bond tension is reduced in pha-1(e2123) embryos, suggesting altered tissue material properties in the epidermis. Several aspects of this phenotype can also be induced in wild-type embryos by exerting mechanical stress through uniaxial loading. Seam cell shape, tissue mechanics, and elongation can be restored in pha-1(e2123) embryos if expression of the AJ molecule DLG-1/Discs large is reduced. Thus, our experiments suggest that maternal-effect toxicity disrupts proper development of the epidermis which involves distinct transcriptional regulators and AJ components.

p21-Activated kinase 1 (PAK1) in aging and longevity: An overview

The p21-activated kinases (PAKs) belong to serine/threonine kinases family, regulated by ∼21 kDa small signaling G proteins RAC1 and CDC42. The mammalian PAK family comprises six members (PAK1-6) that are classified into two groups (I and II) based on their domain architecture and regulatory mechanisms. PAKs are implicated in a wide range of cellular functions. PAK1 has recently attracted increasing attention owing to its involvement in oncogenesis, tumor progression, and metastasis as well as several life-limiting diseases and pathological conditions. In Caenorhabditis elegans, PAK1 functions limit the lifespan under basal conditions by inhibiting forkhead transcription factor DAF-16. Interestingly, PAK depletion extended longevity and attenuated the onset of age-related phenotypes in a premature-aging mouse model and delayed senescence in mammalian fibroblasts. These observations implicate PAKs as not only oncogenic but also aging kinases. Therefore, PAK-targeting genetic and/or pharmacological interventions, particularly PAK1-targeting, could be a viable strategy for developing cancer therapies with relatively no side effects and promoting healthy longevity. This review describes PAK family proteins, their biological functions, and their role in regulating aging and longevity using C. elegans. Moreover, we discuss the effect of small-molecule PAK1 inhibitors on the lifespan and healthspan of C. elegans.

Identifying Plectin Isoform Functions through Animal Models

Plectin, a high-molecular-weight cytoskeletal linker protein, binds with high affinity to intermediate filaments of all types and connects them to junctional complexes, organelles, and inner membrane systems. In addition, it interacts with actomyosin structures and microtubules. As a multifunctional protein, plectin has been implicated in several multisystemic diseases, the most common of which is epidermolysis bullosa simplex with muscular dystrophy (EBS-MD). A great part of our knowledge about plectin’s functional diversity has been gained through the analysis of a unique collection of transgenic mice that includes a full (null) knockout (KO), several tissue-restricted and isoform-specific KOs, three double KOs, and two knock-in lines. The key molecular features and pathological phenotypes of these mice will be discussed in this review. In summary, the analysis of the different genetic models indicated that a functional plectin is required for the proper function of striated and simple epithelia, cardiac and skeletal muscle, the neuromuscular junction, and the vascular endothelium, recapitulating the symptoms of humans carrying plectin mutations. The plectin-null line showed severe skin and muscle phenotypes reflecting the importance of plectin for hemidesmosome and sarcomere integrity; whereas the ablation of individual isoforms caused a specific phenotype in myofibers, basal keratinocytes, or neurons. Tissue-restricted ablation of plectin rendered the targeted cells less resilient to mechanical stress. Studies based on animal models other than the mouse, such as zebrafish and C. elegans, will be discussed as well.

Regulation of hemidesmosome dynamics and cell signaling by integrin α6β4

Hemidesmosomes (HDs) are specialized multiprotein complexes that connect the keratin cytoskeleton of epithelial cells to the extracellular matrix (ECM). In the skin, these complexes provide stable adhesion of basal keratinocytes to the underlying basement membrane. Integrin α6β4 is a receptor for laminins and plays a vital role in mediating cell adhesion by initiating the assembly of HDs. In addition, α6β4 has been implicated in signal transduction events that regulate diverse cellular processes, including proliferation and survival. In this Review, we detail the role of α6β4 in HD assembly and beyond, and we discuss the molecular mechanisms that regulate its function.

How Do Mechanics Guide Fibroblast Activity? Complex Disruptions during Emphysema Shape Cellular Responses and Limit Research

The emphysema death toll has steadily risen over recent decades, causing the disease to become the third most common cause of death worldwide in 2019. Emphysema is currently incurable and could be due to a genetic condition (Alpha-1 antitrypsin deficiency) or exposure to pollutants/irritants, such as cigarette smoke or poorly ventilated cooking fires. Despite the growing burden of emphysema, the mechanisms behind emphysematous pathogenesis and progression are not fully understood by the scientific literature. A key aspect of emphysematous progression is the destruction of the lung parenchyma extracellular matrix (ECM), causing a drastic shift in the mechanical properties of the lung (known as mechanobiology). The mechanical properties of the lung such as the stiffness of the parenchyma (measured as the elastic modulus) and the stretch forces required for inhalation and exhalation are both reduced in emphysema. Fibroblasts function to maintain the structural and mechanical integrity of the lung parenchyma, yet, in the context of emphysema, these fibroblasts appear incapable of repairing the ECM, allowing emphysema to progress. This relationship between the disturbances in the mechanical cues experienced by an emphysematous lung and fibroblast behaviour is constantly overlooked and consequently understudied, thus warranting further research. Interestingly, the failure of current research models to integrate the altered mechanical environment of an emphysematous lung may be limiting our understanding of emphysematous pathogenesis and progression, potentially disrupting the development of novel treatments. This review will focus on the significance of emphysematous lung mechanobiology to fibroblast activity and current research limitations by examining: (1) the impact of mechanical cues on fibroblast activity and the cell cycle, (2) the potential role of mechanical cues in the diminished activity of emphysematous fibroblasts and, finally, (3) the limitations of current emphysematous lung research models and treatments as a result of the overlooked emphysematous mechanical environment.

Neuron tracing and quantitative analyses of dendritic architecture reveal symmetrical three-way-junctions and phenotypes of git-1 in C. elegans

Complex dendritic trees are a distinctive feature of neurons. Alterations to dendritic morphology are associated with developmental, behavioral and neurodegenerative changes. The highly-arborized PVD neuron of C. elegans serves as a model to study dendritic patterning; however, quantitative, objective and automated analyses of PVD morphology are missing. Here, we present a method for neuronal feature extraction, based on deep-learning and fitting algorithms. The extracted neuronal architecture is represented by a database of structural elements for abstracted analysis. We obtain excellent automatic tracing of PVD trees and uncover that dendritic junctions are unevenly distributed. Surprisingly, these junctions are three-way-symmetrical on average, while dendritic processes are arranged orthogonally. We quantify the effect of mutation in git-1, a regulator of dendritic spine formation, on PVD morphology and discover a localized reduction in junctions. Our findings shed new light on PVD architecture, demonstrating the effectiveness of our objective analyses of dendritic morphology and suggest molecular control mechanisms.

Wnt signaling establishes the microtubule polarity in neurons through regulation of Kinesin-13

Neuronal polarization is facilitated by the formation of axons with parallel arrays of plus-end-out and dendrites with the nonuniform orientation of microtubules. In C. elegans, the posterior lateral microtubule (PLM) neuron is bipolar with its two processes growing along the anterior-posterior axis under the guidance of Wnt signaling. Here we found that loss of the Kinesin-13 family microtubule-depolymerizing enzyme KLP-7 led to the ectopic extension of axon-like processes from the PLM cell body. Live imaging of the microtubules and axonal transport revealed mixed polarity of the microtubules in the short posterior process, which is dependent on both KLP-7 and the minus-end binding protein PTRN-1. KLP-7 is positively regulated in the posterior process by planar cell polarity components of Wnt involving rho-1/rock to induce mixed polarity of microtubules, whereas it is negatively regulated in the anterior process by the unc-73/ced-10 cascade to establish a uniform microtubule polarity. Our work elucidates how evolutionarily conserved Wnt signaling establishes the microtubule polarity in neurons through Kinesin-13.

Calcium signaling mediates a biphasic mechanoadaptive response of endothelial cells to cyclic mechanical stretch

The vascular system is precisely regulated to adjust blood flow to organismal demand, thereby guaranteeing adequate perfusion under varying physiological conditions. Mechanical forces, such as cyclic circumferential stretch, are among the critical stimuli that dynamically adjust vessel distribution and diameter, but the precise mechanisms of adaptation to changing forces are unclear. We find that endothelial monolayers respond to cyclic stretch by transient remodeling of the vascular endothelial cadherin–based adherens junctions and the associated actomyosin cytoskeleton. Time-resolved proteomic profiling reveals that this remodeling is driven by calcium influx through the mechanosensitive Piezo1 channel, triggering Rho activation to increase actomyosin contraction. As the mechanical stimulus persists, calcium signaling is attenuated through transient down-regulation of Piezo1 protein. At the same time, filamins are phosphorylated to increase monolayer stiffness, allowing mechanoadaptation to restore junctional integrity despite continuing exposure to stretch. Collectively, this study identifies a biphasic response to cyclic stretch, consisting of an initial calcium-driven junctional mechanoresponse, followed by mechanoadaptation facilitated by monolayer stiffening.

Intermediate filaments: New insights are bublin up

Cytoplasmic intermediate filaments affect cell shape and tissue integrity, and mutations in the proteins that make up these filaments contribute to many human diseases. A new study has identified a conserved protein, BBLN-1/bublin, that is important for intermediate filament organization.

Transparent Touch: Insights From Model Systems on Epidermal Control of Somatosensory Innervation

Somatosensory neurons (SSNs) densely innervate our largest organ, the skin, and shape our experience of the world, mediating responses to sensory stimuli including touch, pressure, and temperature. Historically, epidermal contributions to somatosensation, including roles in shaping innervation patterns and responses to sensory stimuli, have been understudied. However, recent work demonstrates that epidermal signals dictate patterns of SSN skin innervation through a variety of mechanisms including targeting afferents to the epidermis, providing instructive cues for branching morphogenesis, growth control and structural stability of neurites, and facilitating neurite-neurite interactions. Here, we focus onstudies conducted in worms (Caenorhabditis elegans), fruit flies (Drosophila melanogaster), and zebrafish (Danio rerio): prominent model systems in which anatomical and genetic analyses have defined fundamental principles by which epidermal cells govern SSN development.

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.

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.

A Compression Engine to Coordinate Tissue Elongation in the Embryo

How tissue remodelling is coordinated during morphogenesis is still an open question. In this issue of Developmental Cell, Xiong et al. (2020) reveals the regulation of coordinated tissue elongation during avian embryonic development by inter-tissue mechanical interactions acting as a compression engine.

Multi-tissue patterning drives anterior morphogenesis of the C. elegans embryo

Complex structures derived from multiple tissue types are challenging to study in vivo, and our knowledge of how cells from different tissues are coordinated is limited. Model organisms have proven invaluable for improving our understanding of how chemical and mechanical cues between cells from two different tissues can govern specific morphogenetic events. Here we used Caenorhabditis elegans as a model system to show how cells from three different tissues are coordinated to give rise to the anterior lumen. While some aspects of pharyngeal morphogenesis have been well-described, it is less clear how cells from the pharynx, epidermis and neuroblasts coordinate to define the location of the anterior lumen and supporting structures. Using various microscopy and software approaches, we define the movements and patterns of these cells during anterior morphogenesis. Projections from the anterior-most pharyngeal cells (arcade cells) provide the first visible markers for the location of the future lumen, and facilitate patterning of the surrounding neuroblasts. These neuroblast patterns control the rate of migration of the anterior epidermal cells, whereas the epidermal cells ultimately reinforce and control the position of the future lumen, as they must join with the pharyngeal cells for their epithelialization. Our studies are the first to characterize anterior morphogenesis in C. elegans in detail and should lay the framework for identifying how these different patterns are controlled at the molecular level.

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.

The plastic cell: mechanical deformation of cells and tissues

Epithelial cells possess the ability to change their shape in response to mechanical stress by remodelling their junctions and their cytoskeleton. This property lies at the heart of tissue morphogenesis in embryos. A key feature of embryonic cell shape changes is that they result from repeated mechanical inputs that make them partially irreversible at each step. Past work on cell rheology has rarely addressed how changes can become irreversible in a complex tissue. Here, we review new and exciting findings dissecting some of the physical principles and molecular mechanisms accounting for irreversible cell shape changes. We discuss concepts of mechanical ratchets and tension thresholds required to induce permanent cell deformations akin to mechanical plasticity. Work in different systems has highlighted the importance of actin remodelling and of E-cadherin endocytosis. We also list some novel experimental approaches to fine-tune mechanical tension, using optogenetics, magnetic beads or stretching of suspended epithelial tissues. Finally, we discuss some mathematical models that have been used to describe the quantitative aspects of accounting for mechanical cell plasticity and offer perspectives on this rapidly evolving field.

How Daughters Tell Their Mums to Behave

Most tissues include several cell types, which generally develop or get repaired synchronously so as to remain properly organized. In a recent Cell Stem Cell article, Ning et al. (2020) reveals how the tensile state of the skin suprabasal cells non-autonomously regulate stem cell behavior in the basal layer.

Epithelial colonies in vitro elongate through collective effects

Epithelial tissues of the developing embryos elongate by different mechanisms, such as neighbor exchange, cell elongation, and oriented cell division. Since autonomous tissue self-organization is influenced by external cues such as morphogen gradients or neighboring tissues, it is difficult to distinguish intrinsic from directed tissue behavior. The mesoscopic processes leading to the different mechanisms remain elusive. Here, we study the spontaneous elongation behavior of spreading circular epithelial colonies in vitro. By quantifying deformation kinematics at multiple scales, we report that global elongation happens primarily due to cell elongations, and its direction correlates with the anisotropy of the average cell elongation. By imposing an external time-periodic stretch, the axis of this global symmetry breaking can be modified and elongation occurs primarily due to orientated neighbor exchange. These different behaviors are confirmed using a vertex model for collective cell behavior, providing a framework for understanding autonomous tissue elongation and its origins.

Periodic subcellular structures undergo long-range synchronized reorganization during C. elegans epidermal development

Periodic pattern formation on the cellular and tissue scale is an important process and has been extensively studied. However, periodic pattern formation at the subcellular level still remains poorly understood. The C. elegans epidermis displays a highly ordered parallel stripe pattern as part of its subcellular structure, making it an ideal model to study the formation and reorganization of periodic patterns within cells. Here, we show that the initial formation of periodic striped patterns in the C. elegans epidermis is dependent on actin and spectrin, and requires the apical membrane attachment structures for maintenance. The periodic subcellular structures do not accommodate cell growth by continuously making new stripes. Instead, they increase the number of stripes by going through one round of uniform duplication, which is independent of the increasing epidermal length or the developmental cycles. This long-range synchronized reorganization of subcellular structures is achieved by physical links established by extracellular collagens together with extension forces generated from epidermal cell growth. Our studies uncover a novel strategy employed by evenly spaced and interlinked subcellular structures to maintain their integrity and equidistribution during cell growth and tissue development.

C. elegans Apical Extracellular Matrices Shape Epithelia

Apical extracellular matrices (aECMs) coat exposed surfaces of epithelia to shape developing tissues and protect them from environmental insults. Despite their widespread importance for human health, aECMs are poorly understood compared to basal and stromal ECMs. The nematode Caenorhabditis elegans contains a variety of distinct aECMs, some of which share many of the same types of components (lipids, lipoproteins, collagens, zona pellucida domain proteins, chondroitin glycosaminoglycans and proteoglycans) with mammalian aECMs. These aECMs include the eggshell, a glycocalyx-like pre-cuticle, both collagenous and chitin-based cuticles, and other understudied aECMs of internal epithelia. C. elegans allows rapid genetic manipulations and live imaging of fluorescently-tagged aECM components, and is therefore providing new insights into aECM structure, trafficking, assembly, and functions in tissue shaping.

The Rho-GEF PIX-1 directs assembly or stability of lateral attachment structures between muscle cells

PIX proteins are guanine nucleotide exchange factors (GEFs) that activate Rac and Cdc42, and are known to have numerous functions in various cell types. Here, we show that a PIX protein has an important function in muscle. From a genetic screen in C. elegans, we found that pix-1 is required for the assembly of integrin adhesion complexes (IACs) at borders between muscle cells, and is required for locomotion of the animal. A pix-1 null mutant has a reduced level of activated Rac in muscle. PIX-1 localizes to IACs at muscle cell boundaries, M-lines and dense bodies. Mutations in genes encoding proteins at known steps of the PIX signaling pathway show defects at muscle cell boundaries. A missense mutation in a highly conserved residue in the RacGEF domain results in normal levels of PIX-1 protein, but a reduced level of activated Rac in muscle, and abnormal IACs at muscle cell boundaries.

In Full Force. Mechanotransduction and Morphogenesis during Homeostasis and Tissue Regeneration

The interactions of form and function have been the focus of numerous studies in the context of development and more recently regeneration. Our understanding on how cells, tissues and organs sense and interpret external cues, such as mechanical forces, is becoming deeper as novel techniques in imaging are applied and the relevant signaling pathways emerge. These cellular responses can be found from bacteria to all multicellular organisms such as plants and animals. In this review, we focus on hemodynamic flow and endothelial shear stress during cardiovascular development and regeneration, where the interactions of morphogenesis and proper function are more prominent. In addition, we address the recent literature on the role of extracellular matrix and fibrotic response during tissue repair and regeneration. Finally, we refer to examples where the integration of multi-disciplinary approaches to understand the biomechanics of cellular responses could be utilized in novel medical applications.

Mechanosensing in embryogenesis

Mechanical forces generated by living cells at the molecular level propagate to the cellular and organismal level and have profound consequences for embryogenesis. A direct result of force application is movement, as occurs in chromosome separation, cell migration, or tissue folding. A less direct, but equally important effect of force, is the activation of mechanosensitive signaling, which allows cells to probe their mechanical surrounding and communicate with each other over short and long distances. In this review, we focus on forces as a means of conveying information and affecting cell behavior during embryogenesis. We discuss four developmental processes that demonstrate the involvement of force in cell fate determination, growth, morphogenesis, and organogenesis, in a variety of model organisms. Finally, a generalizable pathway of mechanosensing and mechanotransduction in vivo is described, and we highlight similarities between morphogens and forces in patterning of embryos.