A Drosophila homolog of cadherin associated with armadillo and essential for embryonic cell-cell adhesion

Dachsous and Fat coordinately repress the Dachs-Dlish-Approximated complex to control growth

Two protocadherins, Dachsous and Fat, regulate organ growth in Drosophila via the Hippo pathway. Dachsous and Fat bind heterotypically to regulate the abundance and subcellular localization of a "core complex" consisting of Dachs, Dlish, and Approximated. This complex localizes to the junctional cortex where it represses Warts. Dachsous is believed to promote growth by recruiting and stabilizing this complex, while Fat represses growth by promoting its degradation. Here, we examine the functional relationships between the intracellular domains of Dachsous and Fat and the core complex. While Dachsous promotes the accumulation of core complex proteins in puncta, it is not required for their assembly. Indeed, the core complex accumulates maximally in the absence of both Dachsous and Fat. Furthermore, Dachsous represses growth in the absence of Fat by removing the core complex from the junctional cortex. Fat similarly recruits core complex components but promotes their degradation. Our findings reveal that Dachsous and Fat coordinately constrain tissue growth by repressing the core complex.

Surrounding tissue morphogenesis with disrupted posterior midgut invagination during Drosophila gastrulation

Gastrulation involves multiple, physically-coupled tissue rearrangements. During Drosophila gastrulation, posterior midgut (PMG) invagination promotes both germband extension and hindgut invagination, but whether the normal epithelial rearrangement of PMG invagination is required for morphogenesis of the connected tissues has been unclear. In steppke mutants, epithelial organization of the PMG primordium is strongly disrupted. Despite this disruption, germband extension and hindgut invagination are remarkably effective, and involve myosin network inductions known to promote their wild-type remodelling. Known tissue-autonomous signaling could explain the planar-polarized, junctional myosin networks of the germband, but pushing forces from PMG invagination have been implicated in inducing apical myosin networks of the hindgut primordium. To confirm that the wave of hindgut primordium myosin accumulations is due to mechanical effects, rather than diffusive signalling, we analyzed α-catenin RNAi embryos, in which all of the epithelial tissues initially form but then lose cell-cell adhesion, and observed strongly diminished hindgut primordium myosin accumulations. Thus, alternate mechanical changes in steppke mutants seem to circumvent the lack of normal PMG invagination to induce hindgut myosin networks and invagination. Overall, both germband extension and hindgut invagination are robust to experimental disruption of the PMG invagination, and, although the processes occur with some abnormalities in steppke mutants, there is remarkable redundancy in the multi-tissue system. Such redundancy could allow complex morphogenetic processes to change over evolutionary time.

Drosophila anterior midgut internalization via collective epithelial-mesenchymal transition at the embryo surface and enclosure by surrounding tissues

Internal organ development requires cell internalization, which can occur individually or collectively. The best characterized mode of collective internalization is epithelial invagination. Alternate modes involving collective mesenchymal behaviours at the embryo surface have been documented, but their prevalence is unclear. The Drosophila embryo has been a major model for the study of epithelial invaginations. However, internalization of the Drosophila anterior midgut primordium is incompletely understood. Here, we report that an epithelial-mesenchymal transition (EMT) occurs across the internalizing primordium when it is still at the embryo surface. At the earliest internalization stage, the primordium displays less junctional DE-cadherin than surrounding tissues but still exhibits coordinated epithelial structure as it invaginates with the ventral furrow. This initial invagination is transient, and its loss correlates with the activation of an associated mitotic domain. Activation of a subsequent mitotic domain across the broader primordium results in cell divisions with mixed orientations that deposit some cells within the embryo. However, cell division is non-essential for primordium internalization. Post-mitotically, the surface primordium displays hallmarks of EMT: loss of adherens junctions, loss of epithelial cell polarity, and gain of cell protrusions. Primordium cells extend over each other as they internalize asynchronously as individuals or small groups, and the primordium becomes enclosed by the reorganizations of surrounding epithelial tissues. We propose that collective EMT at the embryo surface promotes anterior midgut internalization through both inwardly-directed divisions and movements of its cells, and that the latter process is facilitated by surrounding tissue remodeling.

Dachsous and Fat coordinately repress the Dachs-Dlish-Approximated complex to control growth

Two protocadherins, Dachsous (Ds) and Fat (Ft), regulate organ growth in Drosophila via the Hippo pathway. Ds and Ft bind heterotypically to regulate the abundance and subcellular localization of a 'core complex' consisting of Dachs, Dlish and Approximated. This complex localizes to the junctional cortex where it promotes growth by repressing the pathway kinase Warts. Ds is believed to promote growth by recruiting and stabilizing the core complex at the junctional cortex, while Ft represses growth by promoting degradation of core complex components. Here, we examine the functions of intracellular domains of Ds and Ft and their relationship to the core complex. While Ds promotes accumulation of the core complex proteins in cortical puncta, it is not required for core complex assembly. Indeed, the core complex assembles maximally in the absence of both Ds and Ft. Furthermore, while Ds promotes growth in the presence of Ft, it represses growth in the absence of Ft by removing the core complex from the junctional cortex. Ft similarly recruits core complex components, however it normally promotes their degradation. Our findings reveal that Ds and Ft constrain tissue growth by repressing the default 'on' state of the core complex.

E-cadherin tunes tissue mechanical behavior before and during morphogenetic tissue flows

Adhesion between epithelial cells enables the remarkable mechanical behavior of epithelial tissues during morphogenesis. However, it remains unclear how cell-cell adhesion influences mechanics in both static and dynamically flowing confluent epithelial tissues. Here, we systematically modulate E-cadherin-mediated adhesion in the Drosophila embryo and study the effects on the mechanical behavior of the germband epithelium before and during dramatic tissue remodeling and flow associated with body axis elongation. Before axis elongation, we find that increasing E-cadherin levels produces tissue comprising more elongated cells and predicted to be more fluid-like, providing reduced resistance to tissue flow. During axis elongation, we find that the dominant effect of E-cadherin is tuning the speed at which cells proceed through rearrangement events. Before and during axis elongation, E-cadherin levels influence patterns of actomyosin-dependent forces, supporting the notion that E-cadherin tunes tissue mechanics in part through effects on actomyosin. Notably, the effects of ∼4-fold changes in E-cadherin levels on overall tissue structure and flow are relatively weak, suggesting that the system is tolerant to changes in absolute E-cadherin levels over this range where an intact tissue is formed. Taken together, these findings reveal dual-and sometimes opposing-roles for E-cadherin-mediated adhesion in controlling tissue structure and dynamics in vivo, which result in unexpected relationships between adhesion and flow in confluent tissues.

Confinement promotes nematic alignment of spindle-shaped cells during Drosophila embryogenesis

Tissue morphogenesis is often controlled by actomyosin networks pulling on adherens junctions (AJs), but junctional myosin levels vary. At an extreme, the Drosophila embryo amnioserosa forms a horseshoe-shaped strip of aligned, spindle-shaped cells lacking junctional myosin. What are the bases of amnioserosal cell interactions and alignment? Compared with surrounding tissue, we find that amnioserosal AJ continuity has lesser dependence on α-catenin, the mediator of AJ-actomyosin association, and greater dependence on Bazooka/Par-3, a junction-associated scaffold protein. Microtubule bundles also run along amnioserosal AJs and support their long-range curvilinearity. Amnioserosal confinement is apparent from partial overlap of its spindle-shaped cells, its outward bulging from surrounding tissue and from compressive stress detected within the amnioserosa. Genetic manipulations that alter amnioserosal confinement by surrounding tissue also result in amnioserosal cells losing alignment and gaining topological defects characteristic of nematically ordered systems. With Bazooka depletion, confinement by surrounding tissue appears to be relatively normal and amnioserosal cells align despite their AJ fragmentation. Overall, the fully elongated amnioserosa appears to form through tissue-autonomous generation of spindle-shaped cells that nematically align in response to confinement by surrounding tissue.

E-cadherin tunes tissue mechanical behavior before and during morphogenetic tissue flows

Adhesion between epithelial cells enables the remarkable mechanical behavior of epithelial tissues during morphogenesis. However, it remains unclear how cell-cell adhesion influences mechanics in static as well as in dynamically flowing epithelial tissues. Here, we systematically modulate E-cadherin-mediated adhesion in the Drosophila embryo and study the effects on the mechanical behavior of the germband epithelium before and during dramatic tissue remodeling and flow associated with body axis elongation. Before axis elongation, we find that increasing E-cadherin levels produces tissue comprising more elongated cells and predicted to be more fluid-like, providing reduced resistance to tissue flow. During axis elongation, we find that the dominant effect of E-cadherin is tuning the speed at which cells proceed through rearrangement events, revealing potential roles for E-cadherin in generating friction between cells. Before and during axis elongation, E-cadherin levels influence patterns of actomyosin-dependent forces, supporting the notion that E-cadherin tunes tissue mechanics in part through effects on actomyosin. Taken together, these findings reveal dual-and sometimes opposing-roles for E-cadherin-mediated adhesion in controlling tissue structure and dynamics in vivo that result in unexpected relationships between adhesion and flow.

The journey of a generation: advances and promises in the study of primordial germ cell migration

The germline provides the genetic and non-genetic information that passes from one generation to the next. Given this important role in species propagation, egg and sperm precursors, called primordial germ cells (PGCs), are one of the first cell types specified during embryogenesis. In fact, PGCs form well before the bipotential somatic gonad is specified. This common feature of germline development necessitates that PGCs migrate through many tissues to reach the somatic gonad. During their journey, PGCs must respond to select environmental cues while ignoring others in a dynamically developing embryo. The complex multi-tissue, combinatorial nature of PGC migration is an excellent model for understanding how cells navigate complex environments in vivo. Here, we discuss recent findings on the migratory path, the somatic cells that shepherd PGCs, the guidance cues somatic cells provide, and the PGC response to these cues to reach the gonad and establish the germline pool for future generations. We end by discussing the fate of wayward PGCs that fail to reach the gonad in diverse species. Collectively, this field is poised to yield important insights into emerging reproductive technologies.

Steroid hormone signaling synchronizes cell migration machinery, adhesion and polarity to direct collective movement

Migratory cells - either individually or in cohesive groups - are critical for spatiotemporally regulated processes such as embryonic development and wound healing. Their dysregulation is the underlying cause of formidable health problems such as congenital abnormalities and metastatic cancers. Border cell behavior during Drosophila oogenesis provides an effective model to study temporally regulated, collective cell migration in vivo. Developmental timing in flies is primarily controlled by the steroid hormone ecdysone, which acts through a well-conserved, nuclear hormone receptor complex. Ecdysone signaling determines the timing of border cell migration, but the molecular mechanisms governing this remain obscure. We found that border cell clusters expressing a dominant-negative form of ecdysone receptor extended ineffective protrusions. Additionally, these clusters had aberrant spatial distributions of E-cadherin (E-cad), apical domain markers and activated myosin that did not overlap. Remediating their expression or activity individually in clusters mutant for ecdysone signaling did not restore proper migration. We propose that ecdysone signaling synchronizes the functional distribution of E-cadherin, atypical protein kinase C (aPKC), Discs large (Dlg1) and activated myosin post-transcriptionally to coordinate adhesion, polarity and contractility and temporally control collective cell migration.

Deciphering internal and external factors influencing intestinal junctional complexes

The intestinal barrier, an indispensable guardian of gastrointestinal health, mediates the intricate exchange between internal and external environments. Anchored by evolutionarily conserved junctional complexes, this barrier meticulously regulates paracellular permeability in essentially all living organisms. Disruptions in intestinal junctional complexes, prevalent in inflammatory bowel diseases and irritable bowel syndrome, compromise barrier integrity and often lead to the notorious "leaky gut" syndrome. Critical to the maintenance of the intestinal barrier is a finely orchestrated network of intrinsic and extrinsic factors that modulate the expression, composition, and functionality of junctional complexes. This review navigates through the composition of key junctional complex components and the common methods used to assess intestinal permeability. It also explores the critical intracellular signaling pathways that modulate these junctional components. Lastly, we delve into the complex dynamics between the junctional complexes, microbial communities, and environmental chemicals in shaping the intestinal barrier function. Comprehending this intricate interplay holds paramount importance in unraveling the pathophysiology of gastrointestinal disorders. Furthermore, it lays the foundation for the development of precise therapeutic interventions targeting barrier dysfunction.

Complex dynamics of coral gene expression responses to low pH across species

Coral capacity to tolerate low pH affects coral community composition and, ultimately, reef ecosystem function. Low pH submarine discharges ('Ojo'; Yucatán, México) represent a natural laboratory to study plasticity and acclimatization to low pH in relation to ocean acidification. A previous >2-year coral transplant experiment to ambient and low pH common garden sites revealed differential survivorship across species and sites, providing a framework to compare mechanistic responses to differential pH exposures. Here, we examined gene expression responses of transplants of three species of reef-building corals (Porites astreoides, Porites porites and Siderastrea siderea) and their algal endosymbiont communities (Symbiodiniaceae) originating from low pH (Ojo) and ambient pH native origins (Lagoon or Reef). Transplant pH environment had the greatest effect on gene expression of Porites astreoides hosts and symbionts and P. porites hosts. Host P. astreoides Ojo natives transplanted to ambient pH showed a similar gene expression profile to Lagoon natives remaining in ambient pH, providing evidence of plasticity in response to ambient pH conditions. Although origin had a larger effect on host S. siderea gene expression due to differences in symbiont genera within Reef and Lagoon/Ojo natives, subtle effects of low pH on all origins demonstrated acclimatization potential. All corals responded to low pH by differentially expressing genes related to pH regulation, ion transport, calcification, cell adhesion and stress/immune response. This study demonstrates that the magnitude of coral gene expression responses to pH varies considerably among populations, species and holobionts, which could differentially affect acclimatization to and impacts of ocean acidification.

Dynamics and functions of E-cadherin complexes in epithelial cell and tissue morphogenesis

Cell-cell adhesion is at the center of structure and dynamics of epithelial tissue. E-cadherin-catenin complexes mediate Ca2+-dependent trans-homodimerization and constitute the kernel of adherens junctions. Beyond the basic function of cell-cell adhesion, recent progress sheds light the dynamics and interwind interactions of individual E-cadherin-catenin complex with E-cadherin superclusters, contractile actomyosin and mechanics of the cortex and adhesion. The nanoscale architecture of E-cadherin complexes together with cis-interactions and interactions with cortical actomyosin adjust to junctional tension and mechano-transduction by reinforcement or weakening of specific features of the interactions. Although post-translational modifications such as phosphorylation and glycosylation have been implicated, their role for specific aspects of in E-cadherin function has remained unclear. Here, we provide an overview of the E-cadherin complex in epithelial cell and tissue morphogenesis focusing on nanoscale architectures by super-resolution approaches and post-translational modifications from recent, in particular in vivo, studies. Furthermore, we review the computational modelling in E-cadherin complexes and highlight how computational modelling has contributed to a deeper understanding of the E-cadherin complexes.

A novel proneural function of Asense is integrated with the sequential actions of Delta-Notch, L'sc and Su(H) to promote the neuroepithelial to neuroblast transition

In order for neural progenitors (NPs) to generate distinct populations of neurons at the right time and place during CNS development, they must switch from undergoing purely proliferative, self-renewing divisions to neurogenic, asymmetric divisions in a tightly regulated manner. In the developing Drosophila optic lobe, neuroepithelial (NE) cells of the outer proliferation center (OPC) are progressively transformed into neurogenic NPs called neuroblasts (NBs) in a medial to lateral proneural wave. The cells undergoing this transition express Lethal of Scute (L'sc), a proneural transcription factor (TF) of the Acheate Scute Complex (AS-C). Here we show that there is also a peak of expression of Asense (Ase), another AS-C TF, in the cells neighboring those with transient L'sc expression. These peak of Ase cells help to identify a new transitional stage as they have lost NE markers and L'sc, they receive a strong Notch signal and barely exhibit NB markers. This expression of Ase is necessary and sufficient to promote the NE to NB transition in a more robust and rapid manner than that of l'sc gain of function or Notch loss of function. Thus, to our knowledge, these data provide the first direct evidence of a proneural role for Ase in CNS neurogenesis. Strikingly, we found that strong Delta-Notch signaling at the lateral border of the NE triggers l'sc expression, which in turn induces ase expression in the adjacent cells through the activation of Delta-Notch signaling. These results reveal two novel non-conventional actions of Notch signaling in driving the expression of proneural factors, in contrast to the repression that Notch signaling exerts on them during classical lateral inhibition. Finally, Suppressor of Hairless (Su(H)), which seems to be upregulated late in the transitioning cells and in NBs, represses l'sc and ase, ensuring their expression is transient. Thus, our data identify a key proneural role of Ase that is integrated with the sequential activities of Delta-Notch signaling, L'sc, and Su(H), driving the progressive transformation of NE cells into NBs.

The cell adhesion molecule Echinoid promotes tissue survival and separately restricts tissue overgrowth in Drosophila imaginal discs

The interactions that cells in Drosophila imaginal discs have with their neighbors are known to regulate their ability to survive. In a screen of genes encoding cell surface proteins for gene knockdowns that affect the size or shape of mutant clones, we found that clones of cells with reduced levels of echinoid (ed) are fewer, smaller, and can be eliminated during development. In contrast, discs composed mostly of ed mutant tissue are overgrown. We find that ed mutant tissue has lower levels of the anti-apoptotic protein Diap1 and has increased levels of apoptosis which is consistent with the observed underrepresentation of ed mutant clones and the slow growth of ed mutant tissue. The eventual overgrowth of ed mutant tissue results not from accelerated growth, but from prolonged growth resulting from a failure to arrest growth at the appropriate final size. Ed has previously been shown to physically interact with multiple Hippo-pathway components and it has been proposed to promote Hippo pathway signaling, to exclude Yorkie (Yki) from the nucleus, and restrain the expression of Yki-target genes. We did not observe changes in Yki localization in ed mutant tissue and found decreased levels of expression of several Yorkie-target genes, findings inconsistent with the proposed effect of Ed on Yki. We did, however, observe increased expression of several Yki-target genes in wild-type cells neighboring ed mutant cells, which may contribute to elimination of ed mutant clones. Thus, ed has two distinct functions: an anti-apoptotic function by maintaining Diap1 levels, and a function to arrest growth at the appropriate final size. Both of these are unlikely to be explained by a simple effect on the Hippo pathway.

Hedgehog morphogen gradient is robust towards variations in tissue morphology in Drosophila

During tissue development, gradients of secreted signaling molecules known as morphogens provide cells with positional information. The mechanisms underlying morphogen spreading have been widely studied, however, it remains largely unexplored whether the shape of morphogen gradients is influenced by tissue morphology. Here, we developed an analysis pipeline to quantify the distribution of proteins within a curved tissue. We applied it to the Hedgehog morphogen gradient in the Drosophila wing and eye-antennal imaginal discs, which are flat and curved tissues, respectively. Despite a different expression profile, the slope of the Hedgehog gradient was comparable between the two tissues. Moreover, inducing ectopic folds in wing imaginal discs did not affect the slope of the Hedgehog gradient. Suppressing curvature in the eye-antennal imaginal disc also did not alter the Hedgehog gradient slope but led to ectopic Hedgehog expression. In conclusion, through the development of an analysis pipeline that allows quantifying protein distribution in curved tissues, we show that the Hedgehog gradient is robust towards variations in tissue morphology.

The role of intercellular junction proteins in the penetration resistance of Drosophila larvae to avermectin

Insecticide resistance has become an increasingly serious challenge for agriculture in the world. To reveal the mechanisms of insecticide resistance, majority of studies have been carried out on the insensitivity of insecticide targets and the metabolism of insecticides. However, the mechanism of the insecticide penetration resistance in insects remains unclear. This study aimed to reveal the mechanism underlying the penetration resistance of Drosophila larvae to insecticide avermectin (AVM). Levels of intercellular junction proteins (IJPs) in the larvae were determined by Western blotting analysis and immunofluorescence assay. The result showed that the expression of IJPs septate junction and adherens junction proteins increased in the AVM-resistant insects compared with those in the AVM-susceptible ones, and the upregulation of the IJPs was mediated by the activation of protein kinase C (PKC) pathway. That AVM induced the activation of PKC was found not only in the Drosophila larvae but also in Drosophila S2 cells. These findings revealed that AVM could activate PKC pathway in Drosophila larvae, which mediated the upregulation of the IJPs and then led to the resistance to AVM, suggesting that the chemicals that can disrupt PKC activation may potentially be used to circumvent the resistance to AVM in insects.

Notch Missense Mutations in Drosophila Reveal Functions of Specific EGF-like Repeats in Notch Folding, Trafficking, and Signaling

Notch signaling plays various roles in cell-fate specification through direct cell-cell interactions. Notch receptors are evolutionarily conserved transmembrane proteins with multiple epidermal growth factor (EGF)-like repeats. Drosophila Notch has 36 EGF-like repeats, and while some play a role in Notch signaling, the specific functions of most remain unclear. To investigate the role of each EGF-like repeat, we used 19 previously identified missense mutations of Notch with unique amino acid substitutions in various EGF-like repeats and a transmembrane domain; 17 of these were identified through a single genetic screen. We assessed these mutants' phenotypes in the nervous system and hindgut during embryogenesis, and found that 10 of the 19 Notch mutants had defects in both lateral inhibition and inductive Notch signaling, showing context dependency. Of these 10 mutants, six accumulated Notch in the endoplasmic reticulum (ER), and these six were located in EGF-like repeats 8-10 or 25. Mutations with cysteine substitutions were not always coupled with ER accumulation. This suggests that certain EGF-like repeats may be particularly susceptible to structural perturbation, resulting in a misfolded and inactive Notch product that accumulates in the ER. Thus, we propose that these EGF-like repeats may be integral to Notch folding.

Morphogenetic forces planar polarize LGN/Pins in the embryonic head during Drosophila gastrulation

Spindle orientation is often achieved by a complex of Partner of Inscuteable (Pins)/LGN, Mushroom Body Defect (Mud)/Nuclear Mitotic Apparatus (NuMa), Gαi, and Dynein, which interacts with astral microtubules to rotate the spindle. Cortical Pins/LGN recruitment serves as a critical step in this process. Here, we identify Pins-mediated planar cell polarized divisions in several of the mitotic domains of the early Drosophila embryo. We found that neither planar cell polarity pathways nor planar polarized myosin localization determined division orientation; instead, our findings strongly suggest that Pins planar polarity and force generated from mesoderm invagination are important. Disrupting Pins polarity via overexpression of a myristoylated version of Pins caused randomized division angles. We found that disrupting forces through chemical inhibitors, depletion of an adherens junction protein, or blocking mesoderm invagination disrupted Pins planar polarity and spindle orientation. Furthermore, directional ablations that separated mesoderm from mitotic domains disrupted spindle orientation, suggesting that forces transmitted from mesoderm to mitotic domains can polarize Pins and orient division during gastrulation. To our knowledge, this is the first in vivo example where mechanical force has been shown to polarize Pins to mediate division orientation.

The Lateral Epidermis Actively Counteracts Pulling by the Amnioserosa During Dorsal Closure

Dorsal closure is a prominent morphogenetic process during Drosophila embryogenesis, which involves two epithelial tissues, that is, the squamous amnioserosa and the columnar lateral epidermis. Non-muscle myosin II-driven constriction in the amnioserosa leads to a decrease in the apical surface area and pulls on the adjacent lateral epidermis, which subsequently moves dorsally. The pull by the amnioserosa becomes obvious in an elongation of the epidermal cells, especially of those in the first row. The contribution of the epidermal cell elongation has remained unclear to dorsal closure. Cell elongation may be a mere passive consequence or an active response to the pulling by the amnioserosa. Here, we found that the lateral epidermis actively responds. We analyzed tensions within tissues and cell junctions by laser ablation before and during dorsal closure, the elliptical and dorsal closure stages, respectively. Furthermore, we genetically and optochemically induced chronic and acute cell contraction, respectively. In this way, we found that tension in the epidermis increased during dorsal closure. A correspondingly increased tension was not observed at individual junctions, however. Junctional tension even decreased during dorsal closure in the epidermis. We strikingly observed a strong increase of the microtubule amount in the epidermis, while non-muscle myosin II increased in both tissues. Our data suggest that the epidermis actively antagonizes the pull from the amnioserosa during dorsal closure and the increased microtubules might help the epidermis bear part of the mechanical force.

Myosin V facilitates polarised E-cadherin secretion

E-cadherin has a fundamental role in epithelial tissues by providing cell-cell adhesion. Polarised E-cadherin exocytosis to the lateral plasma membrane is central for cell polarity and epithelial homeostasis. Loss of E-cadherin secretion compromises tissue integrity and is a prerequisite for metastasis. Despite this pivotal role of E-cadherin secretion, the transport mechanism is still unknown. Here we identify Myosin V as the motor for E-cadherin secretion. Our data reveal that Myosin V and F-actin are required for the formation of a continuous apicolateral E-cadherin belt, the zonula adherens. We show by live imaging how Myosin V transports E-cadherin vesicles to the plasma membrane, and distinguish two distinct transport tracks: an apical actin network leading to the zonula adherens and parallel actin bundles leading to the basal-most region of the lateral membrane. E-cadherin secretion starts in endosomes, where Rab11 and Sec15 recruit Myosin V for transport to the zonula adherens. We also shed light on the endosomal sorting of E-cadherin by showing how Rab7 and Snx16 cooperate in moving E-cadherin into the Rab11 compartment. Thus, our data help to understand how polarised E-cadherin secretion maintains epithelial architecture and prevents metastasis. This article is protected by copyright. All rights reserved.

SCAR/WAVE complex recruitment to a supracellular actomyosin cable by myosin activators and a junctional Arf-GEF during Drosophila dorsal closure

Expansive Arp2/3 actin networks and contractile actomyosin networks can be spatially and temporally segregated within the cell, but the networks also interact closely at various sites, including adherens junctions. However, molecular mechanisms coordinating these interactions remain unclear. We found that the SCAR/WAVE complex, an Arp2/3 activator, is enriched at adherens junctions of the leading edge actomyosin cable during Drosophila dorsal closure. Myosin activators were both necessary and sufficient for SCAR/WAVE accumulation at leading edge junctions. The same myosin activators were previously shown to recruit the cytohesin Arf-GEF Steppke to these sites, and mammalian studies have linked Arf small G protein signaling to SCAR/WAVE activation. During dorsal closure, we find that Steppke is required for SCAR/WAVE enrichment at the actomyosin-linked junctions. Arp2/3 also localizes to adherens junctions of the leading edge cable. We propose that junctional actomyosin activity acts through Steppke to recruit SCAR/WAVE and Arp2/3 for regulation of the leading edge supracellular actomyosin cable during dorsal closure.

Ion Channels in Epithelial Dynamics and Morphogenesis

Mechanosensitive ion channels mediate the neuronal sensation of mechanical signals such as sound, touch, and pain. Recent studies point to a function of these channel proteins in cell types and tissues in addition to the nervous system, such as epithelia, where they have been little studied, and their role has remained elusive. Dynamic epithelia are intrinsically exposed to mechanical forces. A response to pull and push is assumed to constitute an essential part of morphogenetic movements of epithelial tissues, for example. Mechano-gated channels may participate in sensing and responding to such forces. In this review, focusing on Drosophila, we highlight recent results that will guide further investigations concerned with the mechanistic role of these ion channels in epithelial cells.

Germline soma communication mediated by gap junction proteins regulates epithelial morphogenesis

Gap junction (GJ) proteins, the primary constituents of GJ channels, are conserved determinants of patterning. Canonically, a GJ channel, made up of two hemi-channels contributed by the neighboring cells, facilitates transport of metabolites/ions. Here we demonstrate the involvement of GJ proteins during cuboidal to squamous epithelial transition displayed by the anterior follicle cells (AFCs) from Drosophila ovaries. Somatically derived AFCs stretch and flatten when the adjacent germline cells start increasing in size. GJ proteins, Innexin2 (Inx2) and Innexin4 (Inx4), functioning in the AFCs and germline respectively, promote the shape transformation by modulating calcium levels in the AFCs. Our observations suggest that alterations in calcium flux potentiate STAT activity to influence actomyosin-based cytoskeleton, possibly resulting in disassembly of adherens junctions. Our data have uncovered sequential molecular events underlying the cuboidal to squamous shape transition and offer unique insight into how GJ proteins expressed in the neighboring cells contribute to morphogenetic processes.

Shape transitions between different subtypes of epithelial cells i.e., cuboidal, squamous and columnar are ubiquitous and are essential during organogenesis across animal kingdom. We demonstrate that heteromeric combination of gap junction proteins, Drosophila Innexin2 and Drosophila Innexin 4 (also known as Zero population growth or Zpg), expressed in the soma and germline of fly egg respectively, mediates the shape transition of cuboidal follicle cells to squamous fate. Interestingly, the two gap junction proteins likely participate as constituents of a calcium channel. Further, we show that somatic follicle cells and germline nurse cells communicate through calcium fluxes that activates STAT in the follicle cells. Activated STAT modulates the levels/ activity of junctional complexes thus aiding shape transition of cuboidal cells to squamous fate. These findings provide novel insights into how communication between different cell types with distinct origins achieve shape transitions essential for proper organ assemblies.

Structural variability and dynamics in the ectodomain of an ancestral-type classical cadherin revealed by AFM imaging

Type III cadherin represents the ancestral form of classical cadherin in bilaterian metazoans. Drosophila possesses type III and type IVa cadherins, known as DN- and DE-cadherins, respectively. Mature DN- and DE-cadherins have 15 and 7 extracellular cadherin domain (EC) repeats, respectively, with DN-cadherin EC6–EC11 homologous to DE-cadherin EC1–EC6. These EC repeats contain predicted complete or partial Ca²⁺-free inter-EC linkers that potentially contribute to adhesion. Comparative structure–function studies of DN- and DE-cadherins may help us understand the ancestral and derived states of classical cadherin-mediated adhesion mechanisms. Here, using bead aggregation assays, we found that DN-cadherin EC1–EC11 and DE-cadherin EC1–EC6 exhibit Ca²⁺-dependent adhesive properties. Using high-speed atomic force microscopy (HS-AFM) imaging in solution, we show that both DN- and DE-cadherin ectodomains share a common morphological framework consisting of a strand-like and a globule-like portion. Furthermore, the DN-cadherin EC repeats are highly variable, flexible in morphology and have at least three bendable sites, one of which is located in EC6–EC11 and can act as a flexible hinge. Our findings provide insights into diversification of classical cadherin-mediated adhesion mechanisms.

This article has an associated First Person interview with the first author of the paper.

Summary: Atomic force microscopy imaging reveals that the ectodomain of an ancestral-type classical cadherin has a flexibly bendable strand-like portion responsible for homophilic adhesion.

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.

Planar Cell Polarity and E-Cadherin in Tissue-Scale Shape Changes in Drosophila Embryos

Planar cell polarity and anisotropic cell behavior play critical roles in large-scale epithelial morphogenesis, homeostasis, wound repair, and regeneration. Cell-Cell communication and mechano-transduction in the second to minute scale mediated by E-cadherin complexes play a central role in the coordination and self-organization of cellular activities, such as junction dynamics, cell shape changes, and cell rearrangement. Here we review the current understanding in the interplay of cell polarity and cell dynamics during body axis elongation and dorsal closure in Drosophila embryos with a focus on E-cadherin dynamics in linking cell and tissue polarization and tissue-scale shape changes.

Differential cell adhesion implemented by Drosophila Toll corrects local distortions of the anterior-posterior compartment boundary

Maintaining lineage restriction boundaries in proliferating tissues is vital to animal development. A long-standing thermodynamics theory, the differential adhesion hypothesis, attributes cell sorting phenomena to differentially expressed adhesion molecules. However, the contribution of the differential adhesion system during tissue morphogenesis has been unsubstantiated despite substantial theoretical support. Here, we report that Toll-1, a transmembrane receptor protein, acts as a differentially expressed adhesion molecule that straightens the fluctuating anteroposterior compartment boundary in the abdominal epidermal epithelium of the Drosophila pupa. Toll-1 is expressed across the entire posterior compartment under the control of the selector gene engrailed and displays a sharp expression boundary that coincides with the compartment boundary. Toll-1 corrects local distortions of the boundary in the absence of cable-like Myosin II enrichment along the boundary. The reinforced adhesion of homotypic cell contacts, together with pulsed cell contraction, achieves a biased vertex sliding action by resisting the separation of homotypic cell contacts in boundary cells. This work reveals a self-organizing system that integrates a differential adhesion system with pulsed contraction of cells to maintain lineage restriction boundaries.

Detecting New Allies: Modifier Screen Identifies a Genetic Interaction Between Imaginal disc growth factor 3 and combover, a Rho-kinase Substrate, During Dorsal Appendage Tube Formation in Drosophila

Biological tube formation underlies organ development and, when disrupted, can cause severe birth defects. To investigate the genetic basis of tubulogenesis, we study the formation of Drosophila melanogaster eggshell structures, called dorsal appendages, which are produced by epithelial tubes. Previously we found that precise levels of Drosophila Chitinase-Like Proteins (CLPs), encoded by the Imaginal disc growth factor (Idgf) gene family, are needed to regulate dorsal-appendage tube closure and tube migration. To identify factors that act in the Idgf pathway, we developed a genetic modifier screen based on the finding that overexpressing Idgf3 causes dorsal appendage defects with ∼50% frequency. Using a library of partially overlapping heterozygous deficiencies, we scanned chromosome 3L and found regions that enhanced or suppressed the Idgf3-overexpression phenotype. Using smaller deletions, RNAi, and mutant alleles, we further mapped five regions and refined the interactions to 58 candidate genes. Importantly, mutant alleles identified combover (cmb), a substrate of Rho-kinase (Rok) and a component of the Planar Cell Polarity (PCP) pathway, as an Idgf3-interacting gene: loss of function enhanced while gain of function suppressed the dorsal appendage defects. Since PCP drives cell intercalation in other systems, we asked if cmb/+ affected cell intercalation in our model, but we found no evidence of its involvement in this step. Instead, we found that loss of cmb dominantly enhanced tube defects associated with Idgf3 overexpression by expanding the apical area of dorsal appendage cells. Apical surface area determines tube volume and shape; in this way, Idgf3 and cmb regulate tube morphology.

Collectively stabilizing and orienting posterior migratory forces disperses cell clusters in vivo

Individual cells detach from cohesive ensembles during development and can inappropriately separate in disease. Although much is known about how cells separate from epithelia, it remains unclear how cells disperse from clusters lacking apical-basal polarity, a hallmark of advanced epithelial cancers. Here, using live imaging of the developmental migration program of Drosophila primordial germ cells (PGCs), we show that cluster dispersal is accomplished by stabilizing and orienting migratory forces. PGCs utilize a G protein coupled receptor (GPCR), Tre1, to guide front-back migratory polarity radially from the cluster toward the endoderm. Posteriorly positioned myosin-dependent contractile forces pull on cell-cell contacts until cells release. Tre1 mutant cells migrate randomly with transient enrichment of the force machinery but fail to separate, indicating a temporal contractile force threshold for detachment. E-cadherin is retained on the cell surface during cell separation and augmenting cell-cell adhesion does not impede detachment. Notably, coordinated migration improves cluster dispersal efficiency by stabilizing cell-cell interfaces and facilitating symmetric pulling. We demonstrate that guidance of inherent migratory forces is sufficient to disperse cell clusters under physiological settings and present a paradigm for how such events could occur across development and disease.

Role of Armadillo repeat 2 and kinesin-II motor subunit Klp64D for wingless signaling in Drosophila

Armadillo (Arm) is crucial for transducing Wingless (Wg) signaling. Previously, we have shown that Klp64D, a motor subunit of Drosophila kinesin-II, interacts with Arm for Wg signaling. Molecular basis for this interaction has remained unknown. Here we identify a critical Arm repeat (AR) required for binding Klp64D and Wg signaling. Arm/\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\varvec{\beta}}$$\end{document}β-catenin family proteins contain a conserved domain of 12 Arm repeats (ARs). Five of these ARs can interact with Klp64D, but only the second AR (AR2) binds to the cargo/tail domain of Klp64D. Overexpression of AR2 in wing imaginal disc is sufficient to cause notched wing margin. This phenotype by AR2 is enhanced or suppressed by reducing or increasing Klp64D expression, respectively. AR2 overexpression inhibits Wg signaling activity in TopFlash assay, consistent with its dominant-negative effects on Klp64D-dependent Wg signaling. Overexpression of the Klp64D cargo domain also results in dominant-negative wing notching. Genetic rescue data indicate that both AR2 and Klp64D cargo regions are required for the function of Arm and Klp64D, respectively. AR2 overexpression leads to an accumulation of Arm with GM130 Golgi marker in Klp64D knockdown. This study suggests that Wg signaling for wing development is regulated by specific interaction between AR2 and the cargo domain of Klp64D.

Prostaglandins regulate invasive, collective border cell migration

While prostaglandins (PGs), short-range lipid signals, regulate single cell migration, their roles in collective migration remain unclear. To address this, we use Drosophila border cell migration, an invasive, collective migration that occurs during Stage 9 of oogenesis. Pxt is the Drosophila cyclooxygenase-like enzyme responsible for PG synthesis. Loss of Pxt results in both delayed border cell migration and elongated clusters, whereas somatic Pxt knockdown causes delayed migration and compacted clusters. These findings suggest PGs act in both the border cells and nurse cells, the substrate on which the border cells migrate. As PGs regulate the actin bundler Fascin, and Fascin is required for on-time migration, we assessed whether PGs regulate Fascin to promote border cell migration. Coreduction of Pxt and Fascin results in delayed migration and elongated clusters. The latter may be due to altered cell adhesion, as loss of Pxt or Fascin, or coreduction of both, decreases integrin levels on the border cell membranes. Conversely, integrin localization is unaffected by somatic knockdown of Pxt. Together these data lead to the model that PG signaling controls Fascin in the border cells to promote migration and in the nurse cells to maintain cluster cohesion.

Gastrulation in Drosophila melanogaster: Genetic control, cellular basis and biomechanics

Gastrulation is generally understood as the morphogenetic processes that result in the spatial organization of the blastomere into the three germ layers, ectoderm, mesoderm and endoderm. This review summarizes our current knowledge of the morphogenetic mechanisms in Drosophila gastrulation. In addition to the events that drive mesoderm invagination and germband elongation, we pay particular attention to other, less well-known mechanisms including midgut invagination, cephalic furrow formation, dorsal fold formation, and mesoderm layer formation. This review covers topics ranging from the identification and functional characterization of developmental and morphogenetic control genes to the analysis of the physical properties of cells and tissues and the control of cell and tissue mechanics of the morphogenetic movements in the gastrula.

Fascin regulates protrusions and delamination to mediate invasive, collective cell migration in vivo

BACKGROUND

The actin bundling protein Fascin is essential for developmental cell migrations and promotes cancer metastasis. In addition to bundling actin, Fascin has several actin-independent roles; how these other functions contribute to cell migration remains unclear. Border cell migration during Drosophila oogenesis provides an excellent model to study Fascin's various roles during invasive, collective cell migration.

RESULTS

On-time border cell migration during Stage 9 requires Fascin (Drosophila Singed). Fascin functions not only within the migrating border cells, but also within the nurse cells, the substrate for this migration. Fascin genetically interacts with the actin elongation factor Enabled to promote on-time Stage 9 migration and overexpression of Enabled suppresses the defects seen with loss of Fascin. Loss of Fascin results in increased, shorter and mislocalized protrusions during migration. Additionally, loss of Fascin inhibits border cell delamination and increases E-Cadherin (Drosophila Shotgun) adhesions on both the border cells and nurse cells.

CONCLUSIONS

Overall, Fascin promotes on-time border cell migration during Stage 9 and contributes to multiple aspects of this invasive, collective cell migration, including both protrusion dynamics and delamination. These findings have implications beyond Drosophila, as border cell migration has emerged as a model to study mechanisms mediating cancer metastasis.

Collective Migrations of Drosophila Embryonic Trunk and Caudal Mesoderm-Derived Muscle Precursor Cells

Mesoderm migration in the Drosophila embryo is a highly conserved, complex process that is required for the formation of specialized tissues and organs, including the somatic and visceral musculature. In this FlyBook chapter, we will compare and contrast the specification and migration of cells originating from the trunk and caudal mesoderm. Both cell types engage in collective migrations that enable cells to achieve new positions within developing embryos and form distinct tissues. To start, we will discuss specification and early morphogenetic movements of the presumptive mesoderm, then focus on the coordinate movements of the two subtypes trunk mesoderm and caudal visceral mesoderm, ending with a comparison of these processes including general insights gained through study.

Cellular homeostasis in the Drosophila retina requires the lipid phosphatase Sac1

The complex functions of cellular membranes, and thus overall cell physiology, depend on the distribution of crucial lipid species. Sac1 is an essential, conserved, ER-localized phosphatase whose substrate, phosphatidylinositol 4-phosphate (PI4P), coordinates secretory trafficking and plasma membrane function. PI4P from multiple pools is delivered to Sac1 by oxysterol-binding protein and related proteins in exchange for other lipids and sterols, which places Sac1 at the intersection of multiple lipid distribution pathways. However, much remains unknown about the roles of Sac1 in subcellular homeostasis and organismal development. Using a temperature-sensitive allele (Sac1^ts), we show that Sac1 is required for structural integrity of the Drosophila retinal floor. The β_ps-integrin Myospheroid, which is necessary for basal cell adhesion, is mislocalized in Sac1^ts retinas. In addition, the adhesion proteins Roughest and Kirre, which coordinate apical retinal cell patterning at an earlier stage, accumulate within Sac1^ts retinal cells due to impaired endo-lysosomal degradation. Moreover, Sac1 is required for ER homeostasis in Drosophila retinal cells. Together, our data illustrate the importance of Sac1 in regulating multiple aspects of cellular homeostasis during tissue development.

Casein kinase 1α decreases β-catenin levels at adherens junctions to facilitate wound closure in Drosophila larvae

Skin wound repair is essential to restore barrier function and prevent infection after tissue damage. Wound-edge epidermal cells migrate as a sheet to close the wound. However, it is still unclear how cell-cell junctions are regulated during wound closure (WC). To study this, we examined adherens junctions during WC in Drosophila larvae. β-Catenin is reduced at the lateral cell-cell junctions of wound-edge epidermal cells in the early healing stages. Destruction complex components, including Ck1α, GSK3β and β-TrCP, suppress β-catenin levels in the larval epidermis. Tissue-specific RNAi targeting these genes also caused severe WC defects. The Ck1αRNAi -induced WC defect is related to adherens junctions because loss of either β-catenin or E-cadherin significantly rescued this WC defect. In contrast, TCFRNAi does not rescue the Ck1αRNAi -induced WC defect, suggesting that Wnt signaling is not related to this defect. Direct overexpression of β-catenin recapitulates most of the features of Ck1α reduction during wounding. Finally, loss of Ck1α also blocked junctional E-cadherin reduction around the wound. Our results suggest that Ck1α and the destruction complex locally regulate cell adhesion to facilitate efficient wound repair.

Scribble and Discs Large mediate tricellular junction formation

Junctional complexes that mediate cell adhesion are key to epithelial integrity, cell division and permeability barrier formation. In Drosophila, the scaffolding proteins Scribble (Scrib) and Discs Large (Dlg) are key regulators of epithelial polarity, proliferation, assembly of junctions and protein trafficking. We found that Scrib and Dlg are necessary for the formation of the tricellular junction (TCJ), a unique junction that forms in epithelia at the point of convergence of three neighboring cells. Scrib and Dlg are in close proximity with the TCJ proteins Gliotactin (Gli) and Bark Beetle (Bark), and both are required for TCJ protein recruitment. Loss of Bark or Gli led to basolateral spread of the TCJ complex at the cell corners. Loss of the septate junction proteins Nrx-IV and the Na⁺/K⁺ ATPase also resulted in basolateral spread of the entire TCJ complex at the cell corners. The Scrib PDZ1-2 domains and the Dlg GUK domain are necessary for Bark and Gli localization to the TCJ. Overall, we propose a model in which Scrib and Dlg are key components of the TCJ, and form a complex with Bark and Gli.

The Drosophila Hedgehog receptor component Interference hedgehog (Ihog) mediates cell-cell interactions through trans-homophilic binding

Hedgehog (Hh) signaling is crucial for establishing complex cellular patterns in embryonic tissues and maintaining homeostasis in adult organs. In Drosophila, Interference hedgehog (Ihog) or its close paralogue Brother of Ihog (Boi) forms a receptor complex with Patched to mediate intracellular Hh signaling. Ihog proteins (Ihog and Boi) also contribute to cell segregation in wing imaginal discs through an unknown mechanism independent of their role in transducing the Hh signal. Here, we report a molecular mechanism by which the Ihog proteins mediate cell-cell interactions. We found that Ihog proteins are enriched at the site of cell-cell contacts and engage in trans-homophilic interactions in a calcium-independent manner. The region that we identified as mediating the trans-Ihog-Ihog interaction overlaps with the Ihog-Hh interface on the first fibronectin repeat of the extracellular domain of Ihog. We further demonstrate that Hh interferes with Ihog-mediated homophilic interactions by competing for Ihog binding. These results, thus, not only reveal a mechanism for Ihog-mediated cell-cell interactions but also suggest a direct Hh-mediated regulation of both intracellular signaling and cell adhesion through Ihog.

Analysis of the Drosophila Compound Eye with Light and Electron Microscopy

The Drosophila compound eye is composed of about 750 units, called ommatidia, which are arranged in a highly regular pattern. Eye development proceeds in a stereotypical fashion, where epithelial cells of the eye imaginal discs are specified, recruited, and differentiated in a sequential order that leads to the highly precise structure of an adult eye. Even small perturbations, for example in signaling pathways that control proliferation, cell death, or differentiation, can impair the regular structure of the eye, which can be easily detected and analyzed. In addition, the Drosophila eye has proven to be an ideal model for studying the genetic control of neurodegeneration, since the eye is not essential for viability. Several human neurodegeneration diseases have been modeled in the fly, leading to a better understanding of the function/misfunction of the respective gene. In many cases, the genes involved and their functions are conserved between flies and human. More strikingly, when ectopically expressed in the fly eye some human genes, even those without a Drosophila counterpart, can induce neurodegeneration, detectable by aberrant phototaxis, impaired electrophysiology, or defects in eye morphology and retinal histology. These defects are often rather subtle alteration in shape, size, or arrangement of the cells, and can be easily scored at the ultrastructural level. This chapter aims to provide an overview regarding the analysis of the retina by light and electron microscopy.

Rapid Disruption of Dishevelled Activity Uncovers an Intercellular Role in Maintenance of Prickle in Core Planar Polarity Protein Complexes

Planar polarity, the coordinated polarization of cells in the plane of a tissue, is important for normal tissue development and function. Proteins of the core planar polarity pathway become asymmetrically localized at the junctions between cells to form intercellular complexes that coordinate planar polarity between cell neighbors. Here, we combine tools to rapidly disrupt the activity of the core planar polarity protein Dishevelled, with quantitative measurements of protein dynamics and levels, and mosaic analysis, to investigate Dishevelled function in maintenance of planar polarity. We provide mechanistic insight into the hierarchical relationship of Dishevelled with other members of the core planar polarity complex. Notably, we show that removal of Dishevelled in one cell causes rapid release of Prickle into the cytoplasm in the neighboring cell. This release of Prickle generates a self-propagating wave of planar polarity complex destabilization across the tissue. Thus, Dishevelled actively maintains complex integrity across intercellular junctions.

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Inducible genetic tools can efficiently disrupt Dishevelled activity in vivo

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Dishevelled activity continuously promotes core planar polarity complex stability

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Prickle is maintained in intercellular complexes cell non-autonomously by Dishevelled

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Unbound Prickle results in intercellular propagation of complex destabilization

Bi-allelic Loss-of-Function Variants in DNMBP Cause Infantile Cataracts

Infantile and childhood-onset cataracts form a heterogeneous group of disorders; among the many genetic causes, numerous pathogenic variants in additional genes associated with autosomal-recessive infantile cataracts remain to be discovered. We identified three consanguineous families affected by bilateral infantile cataracts. Using exome sequencing, we found homozygous loss-of-function variants in DNMBP: nonsense variant c.811C>T (p.Arg271^∗) in large family F385 (nine affected individuals; LOD score = 5.18 at θ = 0), frameshift deletion c.2947_2948del (p.Asp983^∗) in family F372 (two affected individuals), and frameshift variant c.2852_2855del (p.Thr951Metfs^∗41) in family F3 (one affected individual). The phenotypes of all affected individuals include infantile-onset cataracts. RNAi-mediated knockdown of the Drosophila ortholog still life (sif), enriched in lens-secreting cells, affects the development of these cells as well as the localization of E-cadherin, alters the distribution of septate junctions in adjacent cone cells, and leads to a ∼50% reduction in electroretinography amplitudes in young flies. DNMBP regulates the shape of tight junctions, which correspond to the septate junctions in invertebrates, as well as the assembly pattern of E-cadherin in human epithelial cells. E-cadherin has an important role in lens vesicle separation and lens epithelial cell survival in humans. We therefore conclude that DNMBP loss-of-function variants cause infantile-onset cataracts in humans.

FGF controls epithelial-mesenchymal transitions during gastrulation by regulating cell division and apicobasal polarity

To support tissue and organ development, cells transition between epithelial and mesenchymal states. Here, we have investigated how mesoderm cells change state in Drosophila embryos and whether fibroblast growth factor (FGF) signaling plays a role. During gastrulation, presumptive mesoderm cells invaginate, undergo an epithelial-to-mesenchymal state transition (EMT) and migrate upon the ectoderm. Our data show that EMT is a prolonged process in which adherens junctions progressively decrease in number throughout the migration of mesoderm cells. FGF influences adherens junction number and promotes mesoderm cell division, which we propose decreases cell-cell attachments to support slow EMT while retaining collective cell movement. We also found that, at the completion of migration, cells form a monolayer and undergo a reverse mesenchymal-to-epithelial transition (MET). FGF activity leads to accumulation of β-integrin Myospheroid basally and cell polarity factor Bazooka apically within mesoderm cells, thereby reestablishing apicobasal cell polarity in an epithelialized state in which cells express both E-Cadherin and N-Cadherin. In summary, FGF plays a dynamic role in supporting mesoderm cell development to ensure collective mesoderm cell movements, as well as proper differentiation of mesoderm cell types.

Midgut-derived neuropeptide F controls germline stem cell proliferation in a mating-dependent manner

Stem cell maintenance is established by neighboring niche cells that promote stem cell self-renewal. However, it is poorly understood how stem cell activity is regulated by systemic, tissue-extrinsic signals in response to environmental cues and changes in physiological status. Here, we show that neuropeptide F (NPF) signaling plays an important role in the pathway regulating mating-induced germline stem cell (GSC) proliferation in the fruit fly Drosophila melanogaster. NPF expressed in enteroendocrine cells (EECs) of the midgut is released in response to the seminal-fluid protein sex peptide (SP) upon mating. This midgut-derived NPF controls mating-induced GSC proliferation via ovarian NPF receptor (NPFR) activity, which modulates bone morphogenetic protein (BMP) signaling levels in GSCs. Our study provides a molecular mechanism that describes how a gut-derived systemic factor couples stem cell behavior to physiological status, such as mating, through interorgan communication.

Communication between different organs is essential to respond quickly to environmental cues or changes in the physiological status of an organism. Recent studies have shown the existence of humoral factors or hormones, which are transported by the circulatory system to different organs and achieve coordination between them. Here, we have analyzed the communication mechanism between organs that regulates proliferation of germline stem cells (GSCs) in the ovary of the fruit fly Drosophila melanogaster. We show that a peptide hormone called neuropeptide F (NPF) is a key player in this process. This peptide is produced in both the brain and the midgut, and, remarkably, we find that only NPF released from the midgut is crucial for controlling post-mating GSC proliferation. Our data suggest that mating stimulates the release of NPF from the endocrine cells of the midgut stimulated by the presence of a seminal peptide. Midgut-derived NPF is then transduced through an NPF-specific G-protein–coupled receptor expressed in the ovary, and this triggers GSC proliferation. Our study identifies an essential interaction between the digestive system and the ovary that regulates the size of stem cell populations in flies depending on mating.

The phosphoinositide phosphatase Sac1 regulates cell shape and microtubule stability in the developing Drosophila eye

Epithelial patterning in the developing Drosophila melanogaster eye requires the Neph1 homolog Roughest (Rst), an immunoglobulin family cell surface adhesion molecule expressed in interommatidial cells (IOCs). Here, using a novel temperature-sensitive (ts) allele, we show that the phosphoinositide phosphatase Sac1 is also required for IOC patterning. Sac1ts mutants have rough eyes and retinal patterning defects that resemble rst mutants. Sac1ts retinas exhibit elevated levels of phosphatidylinositol 4-phosphate (PI4P), consistent with the role of Sac1 as a PI4P phosphatase. Indeed, genetic rescue and interaction experiments reveal that restriction of PI4P levels by Sac1 is crucial for normal eye development. Rst is delivered to the cell surface in Sac1ts mutants. However, Sac1ts mutant IOCs exhibit severe defects in microtubule organization, associated with accumulation of Rst and the exocyst subunit Sec8 in enlarged intracellular vesicles upon cold fixation ex vivo Together, our data reveal a novel requirement for Sac1 in promoting microtubule stability and suggest that Rst trafficking occurs in a microtubule- and exocyst-dependent manner.

C-terminal Src kinase (Csk) regulates the tricellular junction protein Gliotactin independent of Src

The tricellular junction (TCJ) forms at the convergence of three neighboring epithelia. The targeting of Gliotactin, an essential TCJ protein, to the TCJ is controlled by phosphorylation and endocytosis. C-terminal Src kinase controls endocytosis of Gliotactin in an Src-independent manner.

Tricellular junctions (TCJs) are uniquely placed permeability barriers formed at the corners of polarized epithelia where tight junctions in vertebrates or septate junctions (SJ) in invertebrates from three cells converge. Gliotactin is a Drosophila TCJ protein, and loss of Gliotactin results in SJ and TCJ breakdown and permeability barrier loss. When overexpressed, Gliotactin spreads away from the TCJs, resulting in disrupted epithelial architecture, including overproliferation, cell delamination, and migration. Gliotactin levels are tightly controlled at the mRNA level and at the protein level through endocytosis and degradation triggered by tyrosine phosphorylation. We identified C-terminal Src kinase (Csk) as a tyrosine kinase responsible for regulating Gliotactin endocytosis. Increased Csk suppresses the Gliotactin overexpression phenotypes by increasing endocytosis. Loss of Csk causes Gliotactin to spread away from the TCJ. Although Csk is known as a negative regulator of Src kinases, the effects of Csk on Gliotactin are independent of Src and likely occur through an adherens junction associated complex. Overall, we identified a new Src-independent role for Csk in the control of Gliotactin, a key tricellular junction protein.

Exome sequencing and network analysis identifies shared mechanisms underlying spinocerebellar ataxia

The autosomal dominant cerebellar ataxias, referred to as spinocerebellar ataxias in genetic nomenclature, are a rare group of progressive neurodegenerative disorders characterized by loss of balance and coordination. Despite the identification of numerous disease genes, a substantial number of cases still remain without a genetic diagnosis. Here, we report five novel spinocerebellar ataxia genes, FAT2, PLD3, KIF26B, EP300, and FAT1, identified through a combination of exome sequencing in genetically undiagnosed families and targeted resequencing of exome candidates in a cohort of singletons. We validated almost all genes genetically, assessed damaging effects of the gene variants in cell models and further consolidated a role for several of these genes in the aetiology of spinocerebellar ataxia through network analysis. Our work links spinocerebellar ataxia to alterations in synaptic transmission and transcription regulation, and identifies these as the main shared mechanisms underlying the genetically diverse spinocerebellar ataxia types.

Dendritic space-filling requires a neuronal type-specific extracellular permissive signal in Drosophila

Neurons sometimes completely fill available space in their receptive fields with evenly spaced dendrites to uniformly sample sensory or synaptic information. The mechanisms that enable neurons to sense and innervate all space in their target tissues are poorly understood. Using Drosophila somatosensory neurons as a model, we show that heparan sulfate proteoglycans (HSPGs) Dally and Syndecan on the surface of epidermal cells act as local permissive signals for the dendritic growth and maintenance of space-filling nociceptive C4da neurons, allowing them to innervate the entire skin. Using long-term time-lapse imaging with intact Drosophila larvae, we found that dendrites grow into HSPG-deficient areas but fail to stay there. HSPGs are necessary to stabilize microtubules in newly formed high-order dendrites. In contrast to C4da neurons, non-space-filling sensory neurons that develop in the same microenvironment do not rely on HSPGs for their dendritic growth. Furthermore, HSPGs do not act by transporting extracellular diffusible ligands or require leukocyte antigen-related (Lar), a receptor protein tyrosine phosphatase (RPTP) and the only known Drosophila HSPG receptor, for promoting dendritic growth of space-filling neurons. Interestingly, another RPTP, Ptp69D, promotes dendritic growth of C4da neurons in parallel to HSPGs. Together, our data reveal an HSPG-dependent pathway that specifically allows dendrites of space-filling neurons to innervate all target tissues in Drosophila.

Evolutionary origin of type IV classical cadherins in arthropods

Background

Classical cadherins are a metazoan-specific family of homophilic cell-cell adhesion molecules that regulate morphogenesis. Type I and type IV cadherins in this family function at adherens junctions in the major epithelial tissues of vertebrates and insects, respectively, but they have distinct, relatively simple domain organizations that are thought to have evolved by independent reductive changes from an ancestral type III cadherin, which is larger than derived paralogs and has a complicated domain organization. Although both type III and type IV cadherins have been identified in hexapods and branchiopods, the process by which the type IV cadherin evolved is still largely unclear.

Results

Through an analysis of arthropod genome sequences, we found that the only classical cadherin encoded in chelicerate genomes was the type III cadherin and that the two type III cadherin genes found in the spider Parasteatoda tepidariorum genome exhibited a complex yet ancestral exon-intron organization in arthropods. Genomic and transcriptomic data from branchiopod, copepod, isopod, amphipod, and decapod crustaceans led us to redefine the type IV cadherin category, which we separated into type IVa and type IVb, which displayed a similar domain organization, except type IVb cadherins have a larger number of extracellular cadherin (EC) domains than do type IVa cadherins (nine versus seven). We also showed that type IVa cadherin genes occurred in the hexapod, branchiopod, and copepod genomes whereas only type IVb cadherin genes were present in malacostracans. Furthermore, comparative characterization of the type IVb cadherins suggested that the presence of two extra EC domains in their N-terminal regions represented primitive characteristics. In addition, we identified an evolutionary loss of two highly conserved cysteine residues among the type IVa cadherins of insects.

Conclusions

We provide a genomic perspective of the evolution of classical cadherins among bilaterians, with a focus on the Arthropoda, and suggest that following the divergence of early arthropods, the precursor of the insect type IV cadherin evolved through stepwise reductive changes from the ancestral type III state. In addition, the complementary distributions of polarized genomic characters related to type IVa/IVb cadherins may have implications for our interpretations of pancrustacean phylogeny.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0991-2) contains supplementary material, which is available to authorized users.

Transmission of cytokinesis forces via E-cadherin dilution and actomyosin flows

During epithelial cytokinesis, the remodelling of adhesive cell-cell contacts between the dividing cell and its neighbours has profound implications for the integrity, arrangement and morphogenesis of proliferative tissues. In both vertebrates and invertebrates, this remodelling requires the activity of non-muscle myosin II (MyoII) in the interphasic cells neighbouring the dividing cell. However, the mechanisms that coordinate cytokinesis and MyoII activity in the neighbours are unknown. Here we show that in the Drosophila notum epithelium, each cell division is associated with a mechanosensing and transmission event that controls MyoII dynamics in neighbouring cells. We find that the ring pulling forces promote local junction elongation, which results in local E-cadherin dilution at the ingressing adherens junction. In turn, the reduction in E-cadherin concentration and the contractility of the neighbouring cells promote self-organized actomyosin flows, ultimately leading to accumulation of MyoII at the base of the ingressing junction. Although force transduction has been extensively studied in the context of adherens junction reinforcement to stabilize adhesive cell-cell contacts, we propose an alternative mechanosensing mechanism that coordinates actomyosin dynamics between epithelial cells and sustains the remodelling of the adherens junction in response to mechanical forces.