The Tricellular Junction Protein Sidekick Regulates Vertex Dynamics to Promote Bicellular Junction Extension

Comparative analysis of testicular fusion in Spodoptera litura (cutworm) and Bombyx mori (silkworm): Histological and transcriptomic insights

Spodoptera litura commonly known as the cutworm, is among the most destructive lepidopteran pests affecting over 120 plants species. The powerful destructive nature of this lepidopteran is attributable to its high reproductive capacity. The testicular fusion that occurs during metamorphosis from larvae to pupa in S.litura positively influences the reproductive success of the offspring. In contrast, Bombyx mori, the silkworm, retains separate testes throughout its life and does not undergo this fusion process. Microscopic examination reveals that during testicular fusion in S.litura, the peritoneal sheath becomes thinner and more translucent, whereas in B.mori, the analogous region thickens. The outer basement membrane in S.litura exhibits fractures, discontinuity, and uneven thickness accompanied by a significant presence of cellular secretions, large cell size, increased vesicles, liquid droplets, and a proliferation of rough endoplasmic reticulum and mitochondria. In contrast, the testicular peritoneal sheath of B.mori at comparable developmental stage exhibits minimal change. Comparative transcriptomic analysis of the testicular peritoneal sheath reveals a substantial difference in gene expression between the two species. The disparity in differential expressed genes (DEGs) is linked to an enrichment of numerous transcription factors, intracellular signaling pathways involving Ca2+ and GTPase, as well as intracellular protein transport and signaling pathways. Meanwhile, structural proteins including actin, chitin-binding proteins, membrane protein fractions, cell adhesion, extracellular matrix proteins are predominantly identified. Moreover, the study highlights the enrichment of endopeptidases, serine proteases, proteolytic enzymes and matrix metalloproteins, which may play a role in the degradation of the outer membrane. Five transcription factors-Slforkhead, Slproline, Slcyclic, Slsilk, and SlD-ETS were identified, and their expression pattern were confirmed by qRT-PCR. they are candidates for participating in the regulation of testicular fusion. Our findings underscore significant morphological and trancriptomic variation in the testicular peritoneal sheath of S.litura compared to the silkworm, with substantial changes at the transcriptomic level coinciding with testicular fusion. The research provides valuable clues for understanding the complex mechanisms underlying this unique phenomenon in insects.

Optical inactivation of intracellular molecules by fast-maturating photosensitizing fluorescence protein, HyperNova

Photosensitizing fluorescence protein is a promising tool for chromophore-assisted light inactivation (CALI) that enables specific oxidation and inactivation of intracellular molecules. However, a commonly used monomeric photosensitizing fluorescent protein, SuperNova, shows a low CALI efficiency due to its insufficient maturation at 37 °C, thereby limiting the application of CALI to various molecules, especially in mammalian cells. Here, we present a photosensitizing fluorescence protein, HyperNova, with markedly improved maturation at 37 °C, leading to greatly enhanced CALI efficiency. Exploiting this quality, HyperNova enables the application of CALI to variety of molecules such as a mitotic kinase and transcriptional factors that were highly challenging with conventional SuperNova. To further demonstrate the utility of HyperNova, we have also succeeded in developing novel CALI techniques for MAP kinases by HyperNova. Our findings suggest that HyperNova has the potential to expand the molecular toolbox for manipulating biological events in living cells, providing new avenues for investigating cellular signaling pathways.

Chromophore-Assisted Light Inactivation for Protein Degradation and Its Application in Biomedicine

The functional investigation of proteins holds immense significance in unraveling physiological and pathological mechanisms of organisms as well as advancing the development of novel pharmaceuticals in biomedicine. However, the study of cellular protein function using conventional genetic manipulation methods may yield unpredictable outcomes and erroneous conclusions. Therefore, precise modulation of protein activity within cells holds immense significance in the realm of biomedical research. Chromophore-assisted light inactivation (CALI) is a technique that labels photosensitizers onto target proteins and induces the production of reactive oxygen species through light control to achieve precise inactivation of target proteins. Based on the type and characteristics of photosensitizers, different excitation light sources and labeling methods are selected. For instance, KillerRed forms a fusion protein with the target protein through genetic engineering for labeling and inactivates the target protein via light activation. CALI is presently predominantly employed in diverse biomedical domains encompassing investigations into protein functionality and interaction, intercellular signal transduction research, as well as cancer exploration and therapy. With the continuous advancement of CALI technology, it is anticipated to emerge as a formidable instrument in the realm of life sciences, yielding more captivating outcomes for fundamental life sciences and precise disease diagnosis and treatment.

The TRIP6/LATS1 complex constitutes the tension sensor of α-catenin/vinculin at both bicellular and tricellular junctions

Cell-cell mechanotransduction regulates tissue development and homeostasis. α-catenin, the core component of adherens junctions, functions as a tension sensor and transducer by recruiting vinculin and transducing signals that influence cell behaviors. α-catenin/vinculin complex-mediated mechanotransduction regulates multiple pathways, such as Hippo pathway. However, their associations with the α-catenin-based tension sensors at cell junctions are still not fully addressed. Here, we uncovered the TRIP6/LATS1 complex co-localizes with α-catenin/vinculin at both bicellular junctions (BCJs) and tricellular junctions (TCJs). The localization of TRIP6/LATS1 complex to both TCJs and BCJs requires ROCK1 and α-catenin. Treatment by cytochalasin B, Y-27632 and blebbistatin all impaired the BCJ and TCJ junctional localization of TRIP6/LATS1, indicating that the junctional localization of TRIP6/LATS1 is mechanosensitive. The α-catenin/vinculin/TRIP6/LATS1 complex strongly localized to TCJs and exhibited a discontinuous button-like pattern on BCJs. Additionally, we developed and validated an α-catenin/vinculin BiFC-based mechanosensor that co-localizes with TRIP6/LATS1 at BCJs and TCJs. The mechanosensor exhibited a discontinuous distribution and motile signals at BCJs. Overall, our study revealed that TRIP6 and LATS1 are novel compositions of the tension sensor, together with the core complex of α-catenin/vinculin, at both the BCJs and TCJs.

Damage control of epithelial barrier function in dynamic environments

Epithelial tissues cover the surfaces and lumens of the internal organs of multicellular animals and crucially contribute to internal environment homeostasis by delineating distinct compartments within the body. This vital role is known as epithelial barrier function. Epithelial cells are arranged like cobblestones and intricately bind together to form an epithelial sheet that upholds this barrier function. Central to the restriction of solute and fluid diffusion through intercellular spaces are occluding junctions, tight junctions in vertebrates and septate junctions in invertebrates. As part of epithelial tissues, cells undergo constant renewal, with older cells being replaced by new ones. Simultaneously, the epithelial tissue undergoes relative rearrangement, elongating, and shifting directionally as a whole. The movement or shape changes within the epithelial sheet necessitate significant deformation and reconnection of occluding junctions. Recent advancements have shed light on the intricate mechanisms through which epithelial cells sustain their barrier function in dynamic environments. This review aims to introduce these noteworthy findings and discuss some of the questions that remain unanswered.

Adherens junctions as molecular regulators of emergent tissue mechanics

Tissue and organ development during embryogenesis relies on the collective and coordinated action of many cells. Recent studies have revealed that tissue material properties, including transitions between fluid and solid tissue states, are controlled in space and time to shape embryonic structures and regulate cell behaviours. Although the collective cellular flows that sculpt tissues are guided by tissue-level physical changes, these ultimately emerge from cellular-level and subcellular-level molecular mechanisms. Adherens junctions are key subcellular structures, built from clusters of classical cadherin receptors. They mediate physical interactions between cells and connect biochemical signalling to the physical characteristics of cell contacts, hence playing a fundamental role in tissue morphogenesis. In this Review, we take advantage of the results of recent, quantitative measurements of tissue mechanics to relate the molecular and cellular characteristics of adherens junctions, including adhesion strength, tension and dynamics, to the emergent physical state of embryonic tissues. We focus on systems in which cell-cell interactions are the primary contributor to morphogenesis, without significant contribution from cell-matrix interactions. We suggest that emergent tissue mechanics is an important direction for future research, bridging cell biology, developmental biology and mechanobiology to provide a holistic understanding of morphogenesis in health and disease.

Chromophore-assisted light inactivation of target proteins for singularity biology

Singularity phenomena are rare events that occur only with a probability of one in tens of thousands and yet play an important role in the fate of the entire system. Recently, an ultra-wide-field microscopy imaging systems, AMATERAS, have been developed to reliably capture singularity phenomena. However, to determine whether a rare phenomenon captured by microscopy is a true singularity phenomenon-one with a significant impact on the entire system-, causal analysis is required. In this section, we introduce the CALI method, which uses light to inactivate molecules as one of the techniques enabling causal analysis. In addition, we discuss the technical innovations of the CALI method that are required to contribute to the future development of singularity biology.

Assembly, dynamics and remodeling of epithelial cell junctions throughout development

Cell junctions play key roles in epithelial integrity. During development, when epithelia undergo extensive morphogenesis, these junctions must be remodeled in order to maintain mechanochemical barriers and ensure the cohesion of the tissue. In this Review, we present a comprehensive and integrated description of junctional remodeling mechanisms in epithelial cells during development, from embryonic to adult epithelia. We largely focus on Drosophila, as quantitative analyses in this organism have provided a detailed characterization of the molecular mechanisms governing cell topologies, and discuss the conservation of these mechanisms across metazoans. We consider how changes at the molecular level translate to tissue-scale irreversible deformations, exploring the composition and assembly of cellular interfaces to unveil how junctions are remodeled to preserve tissue homeostasis during cell division, intercalation, invagination, ingression and extrusion.

Underlying mechanisms that ensure actomyosin-mediated directional remodeling of cell-cell contacts for multicellular movement: Tricellular junctions and negative feedback as new aspects underlying actomyosin-mediated directional epithelial morphogenesis: Tricellular junctions and negative feedback as new aspects underlying actomyosin-mediated directional epithelial morphogenesis

Actomyosin (actin-myosin II complex)-mediated contractile forces are central to the generation of multifaceted uni- and multi-cellular material properties and dynamics such as cell division, migration, and tissue morphogenesis. In the present article, we summarize our recent researches addressing molecular mechanisms that ensure actomyosin-mediated directional cell-cell junction remodeling, either shortening or extension, driving cell rearrangement for epithelial morphogenesis. Genetic perturbation clarified two points concerning cell-cell junction remodeling: an inhibitory mechanism against negative feedback in which actomyosin contractile forces, which are well known to induce cell-cell junction shortening, can concomitantly alter actin dynamics, oppositely leading to perturbation of the shortening; and tricellular junctions as a point that organizes extension of new cell-cell junctions after shortening. These findings highlight the notion that cells develop underpinning mechanisms to transform the multi-tasking property of actomyosin contractile forces into specific and proper cellular dynamics in space and time.

Attachment and detachment of cortical myosin regulates cell junction exchange during cell rearrangement in the Drosophila wing epithelium

Epithelial cells remodel cell adhesion and change their neighbors to shape a tissue. This cellular rearrangement proceeds in three steps: the shrinkage of a junction, exchange of junctions, and elongation of the newly generated junction. Herein, by combining live imaging and physical modeling, we showed that the formation of myosin-II (myo-II) cables around the cell vertices underlies the exchange of junctions in the Drosophila wing epithelium. The local and transient detachment of myo-II from the cell cortex is regulated by the LIM domain-containing protein Jub and the tricellular septate junction protein M6. Moreover, we found that M6 shifts to the adherens junction plane on jub RNAi and that Jub is persistently retained at reconnecting junctions in m6 RNAi cells. This interplay between Jub and M6 can depend on the junction length and thereby couples the detachment of cortical myo-II cables and the shrinkage/elongation of the junction during cell rearrangement. Furthermore, we developed a mechanical model based on the wetting theory and clarified how the physical properties of myo-II cables are integrated with the junction geometry to induce the transition between the attached and detached states and support the unidirectionality of cell rearrangement. Collectively, this study elucidates the orchestration of geometry, mechanics, and signaling for exchanging junctions.

Src42A is required for E-cadherin dynamics at cell junctions during Drosophila axis elongation

Src kinases are important regulators of cell adhesion. Here, we have explored the function of Src42A in junction remodelling during Drosophila gastrulation. Src42A is required for tyrosine phosphorylation at bicellular (bAJ) and tricellular (tAJ) junctions in germband cells, and localizes to hotspots of mechanical tension. The role of Src42A was investigated using maternal RNAi and CRISPR-Cas9-induced germline mosaics. We find that, during cell intercalations, Src42A is required for the contraction of junctions at anterior-posterior cell interfaces. The planar polarity of E-cadherin is compromised and E-cadherin accumulates at tricellular junctions after Src42A knockdown. Furthermore, we show that Src42A acts in concert with Abl kinase, which has also been implicated in cell intercalations. Our data suggest that Src42A is involved in two related processes: in addition to establishing tension generated by the planar polarity of MyoII, it may also act as a signalling factor at tAJs to control E-cadherin residence time.

Epithelial Cell Polarity During Drosophila Midgut Development

The adult Drosophila midgut epithelium is derived from a group of stem cells called adult midgut precursors (AMPs) that are specified during the migration of the endoderm in early embryogenesis. AMPs are maintained and expanded in AMP nests that lie on the basal side of the larval midgut throughout the larval development. During metamorphosis, the larval midgut undergoes histolysis and programmed cell death, while the central cells in the AMP nests form the future adult midgut and the peripheral cells form the transient pupal midgut. Here we review what is known about how cells polarise in the embryonic, larval, pupal and adult midgut, and discuss the open questions about the mechanisms that control the changes in cell arrangements, cell shape and cell polarity during midgut development.

Inhibition of negative feedback for persistent epithelial cell-cell junction contraction by p21-activated kinase 3

Actin-mediated mechanical forces are central drivers of cellular dynamics. They generate protrusive and contractile dynamics, the latter of which are induced in concert with myosin II bundled at the site of contraction. These dynamics emerge concomitantly in tissues and even each cell; thus, the tight regulation of such bidirectional forces is important for proper cellular deformation. Here, we show that contractile dynamics can eventually disturb cell–cell junction contraction in the absence of p21-activated kinase 3 (Pak3). Upon Pak3 depletion, contractility induces the formation of abnormal actin protrusions at the shortening junctions, which causes decrease in E-cadherin levels at the adherens junctions and mislocalization of myosin II at the junctions before they enough shorten, compromising completion of junction shortening. Overexpressing E-cadherin restores myosin II distribution closely placed at the junctions and junction contraction. Our results suggest that contractility both induces and perturbs junction contraction and that the attenuation of such perturbations by Pak3 facilitates persistent junction shortening.

Actin and myosin operate at cell–cell junctions during junctional shortening. Here the authors show that prolonged actomyosin contractility can compromise junctional shortening, and that Pak3 is required for attenuation of abnormal active protrusive structure and thus keeps junction contraction, appropriate E-cadherin distribution, and junction shortening in Drosophila.

Sidekick dynamically rebalances contractile and protrusive forces to control tissue morphogenesis

Contractile actomyosin and protrusive branched F-actin networks interact in a dynamic balance, repeatedly contracting and expanding apical cell contacts to organize the epithelium of the developing fly retina. Previously we showed that the immunoglobulin superfamily protein Sidekick (Sdk) contributes to contraction by recruiting the actin binding protein Polychaetoid (Pyd) to vertices. Here we show that as tension increases during contraction, Sdk progressively accumulates at vertices, where it toggles to recruit the WAVE regulatory complex (WRC) to promote actin branching and protrusion. Sdk alternately interacts with the WRC and Pyd using the same C-terminal motif. With increasing protrusion, levels of Sdk and the WRC decrease at vertices while levels of Pyd increase paving the way for another round of contraction. Thus, by virtue of dynamic association with vertices and interchangeable associations with contractile and protrusive effectors, Sdk is central to controlling the balance between contraction and expansion that shapes this epithelium.

Tricellulin secures the epithelial barrier at tricellular junctions by interacting with actomyosin

The epithelial cell sheet functions as a barrier to prevent invasion of pathogens. It is necessary to eliminate intercellular gaps not only at bicellular junctions, but also at tricellular contacts, where three cells meet, to maintain epithelial barrier function. To that end, tight junctions between adjacent cells must associate as closely as possible, particularly at tricellular contacts. Tricellulin is an integral component of tricellular tight junctions (tTJs), but the molecular mechanism of its contribution to the epithelial barrier function remains unclear. In this study, we revealed that tricellulin contributes to barrier formation by regulating actomyosin organization at tricellular junctions. Furthermore, we identified α-catenin, which is thought to function only at adherens junctions, as a novel binding partner of tricellulin. α-catenin bridges tricellulin attachment to the bicellular actin cables that are anchored end-on at tricellular junctions. Thus, tricellulin mobilizes actomyosin contractility to close the lateral gap between the TJ strands of the three proximate cells that converge on tricellular junctions.

[Optical inactivation of molecular functions in vivo by chromophore-assisted light inactivation]

Many biological phenomena have spatio-temporal characteristics, such as the expression of molecular activity locally or at a limited time. Such phenomena have been observed in various organisms from slime mold to mammals, and are considered to be one of the basic patterns in biological reactions. Live imaging studies using the fluorescent protein GFP and fluorescence microscopy have become a standard technique in the life sciences to reveal the dynamics of these characteristic biological phenomena. On the other hand, the characteristic behaviors of molecules and cells captured by microscopy only correlate with life phenomena, and the causal relationship of whether they really matter is unknown. It is unclear whether they are really important or not. Therefore, to elucidate their physiological significance, it is important to introduce spatiotemporal manipulation techniques to manipulate molecules and cells locally and at arbitrary timing, and to perform causal analysis in vivo. The chromophore-assisted light inactivation (CALI) method, which uses light to inactivate molecular functions, is an optical technology that enables such spatiotemporal manipulation, and has recently been used in vivo in various model organisms, attracting widespread attention. In this section, we will review the principle of the CALI method, actual research examples, in particular, its in vivo application, and future prospects.

Adhesion-regulated junction slippage controls cell intercalation dynamics in an Apposed-Cortex Adhesion Model

Cell intercalation is a key cell behaviour of morphogenesis and wound healing, where local cell neighbour exchanges can cause dramatic tissue deformations such as body axis extension. Substantial experimental work has identified the key molecular players facilitating intercalation, but there remains a lack of consensus and understanding of their physical roles. Existing biophysical models that represent cell-cell contacts with single edges cannot study cell neighbour exchange as a continuous process, where neighbouring cell cortices must uncouple. Here, we develop an Apposed-Cortex Adhesion Model (ACAM) to understand active cell intercalation behaviours in the context of a 2D epithelial tissue. The junctional actomyosin cortex of every cell is modelled as a continuous viscoelastic rope-loop, explicitly representing cortices facing each other at bicellular junctions and the adhesion molecules that couple them. The model parameters relate directly to the properties of the key subcellular players that drive dynamics, providing a multi-scale understanding of cell behaviours. We show that active cell neighbour exchanges can be driven by purely junctional mechanisms. Active contractility and cortical turnover in a single bicellular junction are sufficient to shrink and remove a junction. Next, a new, orthogonal junction extends passively. The ACAM reveals how the turnover of adhesion molecules regulates tension transmission and junction deformation rates by controlling slippage between apposed cell cortices. The model additionally predicts that rosettes, which form when a vertex becomes common to many cells, are more likely to occur in actively intercalating tissues with strong friction from adhesion molecules.

Angulin-1 seals tricellular contacts independently of tricellulin and claudins

Tricellular tight junctions (tTJs) are specialized tight junctions (TJs) that seal the intercellular space at tricellular contacts (TCs), where the vertices of three epithelial cells meet. Tricellulin and angulin family membrane proteins are known constituents of tTJs, but the molecular mechanism of tTJ formation remains elusive. Here, we investigated the roles of angulin-1 and tricellulin in tTJ formation in MDCK II cells by genome editing. Angulin-1-deficient cells lost the plasma membrane contact at TCs with impaired epithelial barrier function. The C terminus of angulin-1 bound to the TJ scaffold protein ZO-1, and disruption of their interaction influenced the localization of claudins at TCs, but not the tricellular sealing. Strikingly, the plasma membrane contact at TCs was formed in tricellulin- or claudin-deficient cells. These findings demonstrate that angulin-1 is responsible for the plasma membrane seal at TCs independently of tricellulin and claudins.

Feeling the force: Multiscale force sensing and transduction at the cell-cell interface

A universal principle of all living cells is the ability to sense and respond to mechanical stimuli which is essential for many biological processes. Recent efforts have identified critical mechanosensitive molecules and response pathways involved in mechanotransduction during development and tissue homeostasis. Tissue-wide force transmission and local force sensing need to be spatiotemporally coordinated to precisely regulate essential processes during development such as tissue morphogenesis, patterning, cell migration and organogenesis. Understanding how cells identify and interpret extrinsic forces and integrate a specific response on cell and tissue level remains a major challenge. In this review we consider important cellular and physical factors in control of cell-cell mechanotransduction and discuss their significance for cell and developmental processes. We further highlight mechanosensitive macromolecules that are known to respond to external forces and present examples of how force responses can be integrated into cell and developmental programs.

Effect of cellular rearrangement time delays on the rheology of vertex models for confluent tissues

Large-scale tissue deformation during biological processes such as morphogenesis requires cellular rearrangements. The simplest rearrangement in confluent cellular monolayers involves neighbor exchanges among four cells, called a T1 transition, in analogy to foams. But unlike foams, cells must execute a sequence of molecular processes, such as endocytosis of adhesion molecules, to complete a T1 transition. Such processes could take a long time compared to other timescales in the tissue. In this work, we incorporate this idea by augmenting vertex models to require a fixed, finite time for T1 transitions, which we call the "T1 delay time". We study how variations in T1 delay time affect tissue mechanics, by quantifying the relaxation time of tissues in the presence of T1 delays and comparing that to the cell-shape based timescale that characterizes fluidity in the absence of any T1 delays. We show that the molecular-scale T1 delay timescale dominates over the cell shape-scale collective response timescale when the T1 delay time is the larger of the two. We extend this analysis to tissues that become anisotropic under convergent extension, finding similar results. Moreover, we find that increasing the T1 delay time increases the percentage of higher-fold coordinated vertices and rosettes, and decreases the overall number of successful T1s, contributing to a more elastic-like-and less fluid-like-tissue response. Our work suggests that molecular mechanisms that act as a brake on T1 transitions could stiffen global tissue mechanics and enhance rosette formation during morphogenesis.

The morphogenetic changes that lead to cell extrusion in development and cell competition

Cell extrusion is a morphogenetic process in which unfit or dying cells are eliminated from the tissue at the interface with healthy neighbours in homeostasis. This process is also highly associated with cell fate specification followed by differentiation in development. Spontaneous cell death occurs in development and inhibition of this process can result in abnormal development, suggesting that survival or death is part of cell fate specification during morphogenesis. Moreover, spontaneous somatic mutations in oncogenes or tumour suppressor genes can trigger new morphogenetic events at the interface with healthy cells. Cell competition is considered as the global quality control mechanism for causing unfit cells to be eliminated at the interface with healthy neighbours in proliferating tissues. In this review, I will discuss variations of cell extrusion that are coordinated by unfit cells and healthy neighbours in relation to the geometry and topology of the tissue in development and cell competition.

Transient opening of tricellular vertices controls paracellular transport through the follicle epithelium during Drosophila oogenesis

Paracellular permeability is regulated to allow solute transport or cell migration across epithelial or endothelial barriers. However, how cell-cell junction dynamics controls paracellular permeability is poorly understood. Here, we describe patency, a developmentally regulated process in Drosophila oogenesis, during which cell vertices in the follicular epithelium open transiently to allow paracellular transport of yolk proteins for uptake by the oocyte. We show that the sequential removal of E-cadherin, N-cadherin, NCAM/Fasciclin 2, and Sidekick from vertices precedes their basal-to-apical opening, while the subsequent assembly of tricellular occluding junctions marks the termination of patency and seals the paracellular barrier. E-cadherin-based adhesion is required to limit paracellular channel size, whereas stabilized adherens junctions, prolonged NCAM/Fasciclin 2 expression, blocked endocytosis, or increased actomyosin contractility prevent patency. Our findings reveal a key role of cell vertices as gateways controlling paracellular transport and demonstrate that dynamic regulation of adhesion and actomyosin contractility at vertices governs epithelial barrier properties.

Abl and Canoe/Afadin mediate mechanotransduction at tricellular junctions

Epithelial structure is generated by the dynamic reorganization of cells in response to mechanical forces. Adherens junctions transmit forces between cells, but how cells sense and respond to these forces in vivo is not well understood. We identify a mechanotransduction pathway involving the Abl tyrosine kinase and Canoe/Afadin that stabilizes cell adhesion under tension at tricellular junctions in the Drosophila embryo. Canoe is recruited to tricellular junctions in response to actomyosin contractility, and this mechanosensitivity requires Abl-dependent phosphorylation of a conserved tyrosine in the Canoe actin-binding domain. Preventing Canoe tyrosine phosphorylation destabilizes tricellular adhesion, and anchoring Canoe at tricellular junctions independently of mechanical inputs aberrantly stabilizes adhesion, arresting cell rearrangement. These results identify a force-responsive mechanism that stabilizes tricellular adhesion under tension during epithelial remodeling.

Cell Junction Mechanics beyond the Bounds of Adhesion and Tension

Cell-cell junctions, in particular adherens junctions, are major determinants of tissue mechanics during morphogenesis and homeostasis. In attempts to link junctional mechanics to tissue mechanics, many have utilized explicitly or implicitly equilibrium approaches based on adhesion energy, surface energy, and contractility to determine the mechanical equilibrium at junctions. However, it is increasingly clear that they have significant limitations, such as that it remains challenging to link the dynamics of the molecular components to the resulting physical properties of the junction, to its remodeling ability, and to its adhesion strength. In this perspective, we discuss recent attempts to consider the aspect of energy dissipation at junctions to draw contact points with soft matter physics where energy loss plays a critical role in adhesion theories. We set the grounds for a theoretical framework of the junction mechanics that bridges the dynamics at the molecular scale to the mechanics at the tissue scale.

Optical manipulation of molecular function by chromophore-assisted light inactivation

In addition to simple on/off switches for molecular activity, spatiotemporal dynamics are also thought to be important for the regulation of cellular function. However, their physiological significance and in vivo importance remain largely unknown. Fluorescence imaging technology is a powerful technique that can reveal the spatiotemporal dynamics of molecular activity. In addition, because imaging detects the correlations between molecular activity and biological phenomena, the technique of molecular manipulation is also important to analyze causal relationships. Recent advances in optical manipulation techniques that artificially perturb molecules and cells via light can address this issue to elucidate the causality between manipulated target and its physiological function. The use of light enables the manipulation of molecular activity in microspaces, such as organelles and nerve spines. In this review, we describe the chromophore-assisted light inactivation method, which is an optical manipulation technique that has been attracting attention in recent years.

Apical stress fibers enable a scaling between cell mechanical response and area in epithelial tissue

Biological systems tailor their properties and behavior to their size throughout development and in numerous aspects of physiology. However, such size scaling remains poorly understood as it applies to cell mechanics and mechanosensing. By examining how the Drosophila pupal dorsal thorax epithelium responds to morphogenetic forces, we found that the number of apical stress fibers (aSFs) anchored to adherens junctions scales with cell apical area to limit larger cell elongation under mechanical stress. aSFs cluster Hippo pathway components, thereby scaling Hippo signaling and proliferation with area. This scaling is promoted by tricellular junctions mediating an increase in aSF nucleation rate and lifetime in larger cells. Development, homeostasis, and repair entail epithelial cell size changes driven by mechanical forces; our work highlights how, in turn, mechanosensitivity scales with cell size.

Neuronal immunoglobulin superfamily cell adhesion molecules in epithelial morphogenesis: insights from Drosophila

In this review, we address the function of immunoglobulin superfamily cell adhesion molecules (IgCAMs) in epithelia. Work in the Drosophila model system in particular has revealed novel roles for calcium-independent adhesion molecules in the morphogenesis of epithelial tissues. We review the molecular composition of lateral junctions with a focus on their IgCAM components and reconsider the functional roles of epithelial lateral junctions. The epithelial IgCAMs discussed in this review have well-defined roles in the nervous system, particularly in the process of axon guidance, suggesting functional overlap and conservation in mechanism between that process and epithelial remodelling. We expand on the hypothesis that epithelial occluding junctions and synaptic junctions are compositionally equivalent and present a novel hypothesis that the mechanism of epithelial cell (re)integration and synaptic junction formation are shared. We highlight the importance of considering non-cadherin-based adhesion in our understanding of the mechanics of epithelial tissues and raise questions to direct future work. This article is part of the discussion meeting issue 'Contemporary morphogenesis'.

Structure and Functions of Sidekicks

Many of the immunoglobulin superfamily (IgSF) molecules play pivotal roles in cell communication. The Sidekick (Sdk) gene, first described in Drosophila, encodes the single-pass transmembrane protein, Sdk, which is one of the largest among IgSF membrane proteins. Sdk first appeared in multicellular animals during the Precambrian age and later evolved to Sdk1 and Sdk2 in vertebrates by gene duplication. In flies, a single Sdk is involved in positioning photoreceptor neurons and their axons in the visual system and is responsible for dynamically rearranging cell shapes by strictly populating tricellular adherens junctions in epithelia. In vertebrates, Sdk1 and Sdk2 are expressed by unique sets of cell types and distinctively participate in the formation and/or maintenance of neural circuits in the retina, indicating that they are determinants of synaptic specificity. These functions are mediated by specific homophilic binding of their ectodomains and by intracellular association with PDZ scaffold proteins. Recent human genetic studies as well as animal experiments implicate that Sdk genes may influence various neurodevelopmental and psychiatric disorders, such as autism spectrum disorders, attention-deficit hyperactivity disorder, addiction, and depression. The gigantic Sdk1 gene is susceptible to erratic gene rearrangements or mutations in both somatic and germ-line cells, potentially contributing to neurological disorders and some types of cancers. This review summarizes what is known about the structure and roles of Sdks.

Interplay between Anakonda, Gliotactin, and M6 for Tricellular Junction Assembly and Anchoring of Septate Junctions in Drosophila Epithelium

In epithelia, tricellular junctions (TCJs) serve as pivotal sites for barrier function and integration of both biochemical and mechanical signals [1-3]. In Drosophila, TCJs are composed of the transmembrane protein Sidekick at the adherens junction (AJ) level, which plays a role in cell-cell contact rearrangement [4-6]. At the septate junction (SJ) level, TCJs are formed by Gliotactin (Gli) [7], Anakonda (Aka) [8, 9], and the Myelin proteolipid protein (PLP) M6 [10, 11]. Despite previous data on TCJ organization [12-14], TCJ assembly, composition, and links to adjacent bicellular junctions (BCJs) remain poorly understood. Here, we have characterized the making of TCJs within the plane of adherens junctions (tricellular adherens junction [tAJ]) and the plane of septate junctions (tricellular septate junction [tSJ]) and report that their assembly is independent of each other. Aka and M6, whose localizations are interdependent, act upstream to localize Gli. In turn, Gli stabilizes Aka at tSJ. Moreover, tSJ components are not only essential at vertex, as we found that loss of tSJ integrity induces micron-length bicellular SJ (bSJ) deformations. This phenotype is associated with the disappearance of SJ components at tricellular contacts, indicating that bSJs are no longer connected to tSJs. Reciprocally, SJ components are required to restrict the localization of Aka and Gli at vertex. We propose that tSJs function as pillars to anchor bSJs to ensure the maintenance of tissue integrity in Drosophila proliferative epithelia.

Cell intercalation in a simple epithelium

Cell intercalation is a key topological transformation driving tissue morphogenesis, homeostasis and diseases such as cancer cell invasion. In recent years, much work has been undertaken to better elucidate the fundamental mechanisms controlling intercalation. Cells often use protrusions to propel themselves in between cell neighbours, resulting in topology changes. Nevertheless, in simple epithelial tissues, formed by a single layer of densely packed prism-shaped cells, topology change takes place in an astonishing fashion: cells exchange neighbours medio-laterally by conserving their apical-basal architecture and by maintaining an intact epithelial layer. Medio-lateral cell intercalation in simple epithelia is thus an exemplary case of both robustness and plasticity. Interestingly, in simple epithelia, cells use a combinatory set of mechanisms to ensure a topological transformation at the apical and basal sides. This article is part of the discussion meeting issue 'Contemporary morphogenesis'.

Control of cell shape during epithelial morphogenesis: recent advances

Morphogenesis is an essential process by which a given tissue, organ or organism acquires its final shape. A select number of mechanisms are used in order to drive epithelial morphogenesis, including cell shape changes as well as cell death or cell division. A cell's shape results from the combination of intrinsic properties of the actomyosin and microtubule (MTs) cytoskeletons, and extrinsic properties due to physical interactions with the neighbouring environment. While we now have a good understanding of the genetic pathways and some of the signalling pathways controlling cell shape changes, the mechanical properties of cells and their role in morphogenesis remain largely unexplored. Recent improvements in microscopy techniques and the development of modelling and quantitative methods have enabled a better understanding of the bio-mechanical events controlling cell shape during morphogenesis. This review aims to highlight recent findings elegantly unravelling and quantifying the contribution of mechanical forces during morphogenesis.

Template-based mapping of dynamic motifs in tissue morphogenesis

Tissue morphogenesis relies on repeated use of dynamic behaviors at the levels of intracellular structures, individual cells, and cell groups. Rapidly accumulating live imaging datasets make it increasingly important to formalize and automate the task of mapping recurrent dynamic behaviors (motifs), as it is done in speech recognition and other data mining applications. Here, we present a "template-based search" approach for accurate mapping of sub- to multi-cellular morphogenetic motifs using a time series data mining framework. We formulated the task of motif mapping as a subsequence matching problem and solved it using dynamic time warping, while relying on high throughput graph-theoretic algorithms for efficient exploration of the search space. This formulation allows our algorithm to accurately identify the complete duration of each instance and automatically label different stages throughout its progress, such as cell cycle phases during cell division. To illustrate our approach, we mapped cell intercalations during germband extension in the early Drosophila embryo. Our framework enabled statistical analysis of intercalary cell behaviors in wild-type and mutant embryos, comparison of temporal dynamics in contracting and growing junctions in different genotypes, and the identification of a novel mode of iterative cell intercalation. Our formulation of tissue morphogenesis using time series opens new avenues for systematic decomposition of tissue morphogenesis.

Cell-cell junctions as sensors and transducers of mechanical forces

Epithelial and endothelial monolayers are multicellular sheets that form barriers between the 'outside' and 'inside' of tissues. Cell-cell junctions, made by adherens junctions, tight junctions and desmosomes, hold together these monolayers. They form intercellular contacts by binding their receptor counterparts on neighboring cells and anchoring these structures intracellularly to the cytoskeleton. During tissue development, maintenance and pathogenesis, monolayers encounter a range of mechanical forces from the cells themselves and from external systemic forces, such as blood pressure or tissue stiffness. The molecular landscape of cell-cell junctions is diverse, containing transmembrane proteins that form intercellular bonds and a variety of cytoplasmic proteins that remodel the junctional connection to the cytoskeleton. Many junction-associated proteins participate in mechanotransduction cascades to confer mechanical cues into cellular responses that allow monolayers to maintain their structural integrity. We will discuss force-dependent junctional molecular events and their role in cell-cell contact organization and remodeling.

Immunoglobulin superfamily beat-Ib mediates intestinal regeneration induced by reactive oxygen species in Drosophila

Reactive oxygen species (ROS) often injure intestinal epithelia that cause loss of damaged cells, which is mainly repaired by proliferation of intestinal stem cells (ISCs). To maintain the homeostatic state, coordination of sensing of the ROS injury and the subsequent epithelial cell loss with the replenishment by cell renewal is crucial. However, little is known about how gut epithelial cells initiate regenerative responses against ROS to maintain the tissue integrity. Here, we carried out a genome-wide screen, by which we identify immunoglobulin superfamily beaten path Ib (beat-Ib) as an essential gene for provoking ISC proliferation against ROS in Drosophila intestine. Interestingly, the beat-Ib function is required in differentiated enterocytes, the main targeted cells by ROS in the intestinal tract, but is dispensable in the stem cells. Moreover, beat-Ib is not involved in enterocyte apoptosis at ROS injury. These findings indicate the essential role of beat-Ib in Drosophila midgut enterocytes for initiating the non-cell-autonomous induction of ISC division in response to environmental ROS stresses.

The tricellular vertex-specific adhesion molecule Sidekick facilitates polarised cell intercalation during Drosophila axis extension

In epithelia, tricellular vertices are emerging as important sites for the regulation of epithelial integrity and function. Compared to bicellular contacts, however, much less is known. In particular, resident proteins at tricellular vertices were identified only at occluding junctions, with none known at adherens junctions (AJs). In a previous study, we discovered that in Drosophila embryos, the adhesion molecule Sidekick (Sdk), well-known in invertebrates and vertebrates for its role in the visual system, localises at tricellular vertices at the level of AJs. Here, we survey a wide range of Drosophila epithelia and establish that Sdk is a resident protein at tricellular AJs (tAJs), the first of its kind. Clonal analysis showed that two cells, rather than three cells, contributing Sdk are sufficient for tAJ localisation. Super-resolution imaging using structured illumination reveals that Sdk proteins form string-like structures at vertices. Postulating that Sdk may have a role in epithelia where AJs are actively remodelled, we analysed the phenotype of sdk null mutant embryos during Drosophila axis extension using quantitative methods. We find that apical cell shapes are abnormal in sdk mutants, suggesting a defect in tissue remodelling during convergence and extension. Moreover, adhesion at apical vertices is compromised in rearranging cells, with apical tears in the cortex forming and persisting throughout axis extension, especially at the centres of rosettes. Finally, we show that polarised cell intercalation is decreased in sdk mutants. Mathematical modelling of the cell behaviours supports the notion that the T1 transitions of polarised cell intercalation are delayed in sdk mutants, in particular in rosettes. We propose that this delay, in combination with a change in the mechanical properties of the converging and extending tissue, causes the abnormal apical cell shapes in sdk mutant embryos.

This study identifies the adhesion molecule Sidekick as a resident protein of tricellular vertices between cells, at the level of adherens junctions. A combination of quantitative methods and modelling provides evidence that Sidekick facilitates polarised cell intercalation during Drosophila axis extension.