Fas2EB112: a tale of two chromosomes

The cell-cell adhesion molecule Fasciclin II (Fas2) has long been studied for its evolutionarily conserved role in axon guidance. It is also expressed in the follicular epithelium, where together with a similar protein, Neuroglian (Nrg), it helps to drive the reintegration of cells born out of the tissue plane. Remarkably, one Fas2 protein null allele, Fas2G0336, demonstrates a mild reintegration phenotype, whereas work with the classic null allele Fas2EB112 showed more severe epithelial disorganization. These observations raise the question of which allele (if either) causes a bona fide loss of Fas2 protein function. The problem is not only relevant to reintegration but fundamentally important to understanding what this protein does and how it works: Fas2EB112 has been used in at least 37 research articles, and Fas2G0336 in at least three. An obvious solution is that one of the two chromosomes carries a modifier that either suppresses (Fas2G0336) or enhances (Fas2EB112) phenotypic severity. We find not only the latter to be the case, but identify the enhancing mutation as Nrg14, also a classic null allele.

Fas2EB112: A Tale of Two Chromosomes

The cell-cell adhesion molecule Fasciclin II (Fas2) has long been studied for its evolutionarily-conserved role in axon guidance. It is also expressed in the follicular epithelium, where together with a similar protein, Neuroglian (Nrg), it helps to drive the reintegration of cells born out of the tissue plane. Remarkably, one Fas2 protein null allele, Fas2G0336, demonstrates a mild reintegration phenotype, whereas work with the classic null allele Fas2EB112 showed more severe epithelial disorganization. These observations raise the question of which allele (if either) causes a bona fide loss of Fas2 protein function. The problem is not only relevant to reintegration but fundamentally important to understanding what this protein does and how it works: Fas2EB112 has been used in at least 37 research articles, and Fas2G0336 in at least three. An obvious solution is that one of the two chromosomes carries a modifier that either suppresses (Fas2G0336) or enhances (Fas2EB112) phenotypic severity. We find not only the latter to be the case, but identify the enhancing mutation as Nrg14, also a classic null allele.

Requirement of Snakeskin for normal functions of midgut and Malpighian tubules in Henosepilachna vigintioctopunctata

Septate junctions (SJs) are located between epithelial cells and play crucial roles in epithelial barrier formation and epithelia cell homeostasis. Nevertheless, the molecular constituents, especially those related to smooth SJs (sSJs), have not been well explored in non-Drosophilid insects. A putative integral membrane protein Snakeskin (Ssk) was identified in a Coleoptera foliar pest Henosepilachna vigintioctopunctata. RNA interference-aided knockdown of Hvssk at the third-instar larval stage arrested larval development. Most resultant larvae failed to shed larval exuviae until their death. Silence of Hvssk at the fourth-instar larvae inhibited the growth and reduced foliage consumption. Dissection and microscopic observation revealed that compromised expression of Hvssk caused obvious phenotypic defects in the midgut. A great number of morphologically abnormal columnar epithelial cells accumulated throughout the midgut lumen. Moreover, numerous vesicles were observed in the malformed cells of the Malpighian tubules (Mt). All the Hvssk depleted larvae remained as prepupae; they gradually darkened and eventually died. Furthermore, depletion of Hvssk at the pupal stage suppressed adult feeding and shortened adult lifespan. These findings demonstrated that Ssk plays a vital role in the integrity and function of both midguts and Mt, and established the conservative roles of Ssk in the formation of epithelial barrier and the homeostasis of epithelial cells in H. vigintioctopunctata.

Coordination of tissue homeostasis and growth by the Scribble-α-Catenin-Septate junction complex

Maintaining apicobasal polarity (ABP) is crucial for epithelial integrity and homeostasis during tissue development. Although intracellular mechanisms underlying ABP establishment have been well studied, it remains to be addressed how the ABP coordinates tissue growth and homeostasis. By studying Scribble, a key ABP determinant, we address molecular mechanisms underlying ABP-mediated growth control in the Drosophila wing imaginal disc. Our data reveal that genetic and physical interactions between Scribble, Septate junction complex and α-Catenin appear to be key for sustaining ABP-mediated growth control. Cells with conditional scribble knockdown instigate the loss of α-Catenin, ultimately leading to the formation of neoplasia accompanying with activation of Yorkie. In contrast, cells expressing wild type scribble progressively restore ABP in scribble hypomorphic mutant cells in a non-autonomous manner. Our findings provide unique insights into cellular communication among optimal and sub-optimal cells to regulate epithelial homeostasis and growth.

Identification of the genes encoding candidate septate junction components expressed during early development of the sea urchin, Strongylocentrotus purpuratus, and evidence of a role for Mesh in the formation of the gut barrier

Septate junctions (SJs) evolved as cell-cell junctions that regulate the paracellular barrier and integrity of epithelia in invertebrates. Multiple morphological variants of SJs exist specific to different epithelia and/or phyla but the biological significance of varied SJ morphology is unclear because the knowledge of the SJ associated proteins and their functions in non-insect invertebrates remains largely unknown. Here we report cell-specific expression of nine candidate SJ genes in the early life stages of the sea urchin Strongylocentrotus purpuratus. By use of in situ RNA hybridization and single cell RNA-seq we found that the expression of selected genes encoding putatively SJ associated transmembrane and cytoplasmic scaffold molecules was dynamically regulated during epithelial development in the embryos and larvae with different epithelia expressing different cohorts of SJ genes. We focused a functional analysis on SpMesh, a homolog of the Drosophila smooth SJ component Mesh, which was highly enriched in the endodermal epithelium of the mid- and hindgut. Functional perturbation of SpMesh by both CRISPR/Cas9 mutagenesis and vivo morpholino-mediated knockdown shows that loss of SpMesh does not disrupt the formation of the gut epithelium during gastrulation. However, loss of SpMesh resulted in a severely reduced gut-paracellular barrier as quantitated by increased permeability to 3-5 ​kDa FITC-dextran. Together, these studies provide a first look at the molecular SJ physiology during the development of a marine organism and suggest a shared role for Mesh-homologous proteins in forming an intestinal barrier in invertebrates. Results have implications for consideration of the traits underlying species-specific sensitivity of marine larvae to climate driven ocean change.

Cell polarity signalling at the birth of multicellularity: What can we learn from the first animals

The innovation of multicellularity has driven the unparalleled evolution of animals (Metazoa). But how is a multicellular organism formed and how is its architecture maintained faithfully? The defining properties and rules required for the establishment of the architecture of multicellular organisms include the development of adhesive cell interactions, orientation of division axis, and the ability to reposition daughter cells over long distances. Central to all these properties is the ability to generate asymmetry (polarity), coordinated by a highly conserved set of proteins known as cell polarity regulators. The cell polarity complexes, Scribble, Par and Crumbs, are considered to be a metazoan innovation with apicobasal polarity and adherens junctions both believed to be present in all animals. A better understanding of the fundamental mechanisms regulating cell polarity and tissue architecture should provide key insights into the development and regeneration of all animals including humans. Here we review what is currently known about cell polarity and its control in the most basal metazoans, and how these first examples of multicellular life can inform us about the core mechanisms of tissue organisation and repair, and ultimately diseases of tissue organisation, such as cancer.

Serial passage in an insect host indicates genetic stability of the human probiotic Escherichia coli Nissle 1917

Background and objectives

The probiotic Escherichia coli strain Nissle 1917 (EcN) has been shown to effectively prevent and alleviate intestinal diseases. Despite the widespread medical application of EcN, we still lack basic knowledge about persistence and evolution of EcN outside the human body. Such knowledge is important also for public health aspects, as in contrast to abiotic therapeutics, probiotics are living organisms that have the potential to evolve. This study made use of experimental evolution of EcN in an insect host, the red flour beetle Tribolium castaneum, and its flour environment.

Methodology

Using a serial passage approach, we orally introduced EcN to larvae of T.castaneum as a new host, and also propagated it in the flour environment. After eight propagation cycles, we analyzed phenotypic attributes of the passaged replicate EcN lines, their effects on the host in the context of immunity and infection with the entomopathogen Bacillus thuringiensis, and potential genomic changes using WGS of three of the evolved lines.

Results

We observed weak phenotypic differences between the ancestral EcN and both, beetle and flour passaged EcN lines, in motility and growth at 30°C, but neither any genetic changes, nor the expected increased persistence of the beetle-passaged lines. One of these lines displayed distinct morphological and physiological characteristics.

Conclusions and implications

Our findings suggest that EcN remains rather stable during serial passage in an insect. Weak phenotypic changes in growth and motility combined with a lack of genetic changes indicate a certain degree of phenotypic plasticity of EcN.

Lay Summary

For studying adaptation of the human probiotic Escherichia coli strain Nissle 1917, we introduced it to a novel insect host system and its environment using a serial passage approach. After passage, we observed weak phenotypic changes in growth and motility but no mutations or changes in persistence inside the host.

Interaction between Discs large and Pins/LGN/GPSM2: a comparison across species

The orientation of the mitotic spindle determines the direction of cell division, and therefore contributes to tissue shape and cell fate. Interaction between the multifunctional scaffolding protein Discs large (Dlg) and the canonical spindle orienting factor GPSM2 (called Pins in Drosophila and LGN in vertebrates) has been established in bilaterian models, but its function remains unclear. We used a phylogenetic approach to test whether the interaction is obligate in animals, and in particular whether Pins/LGN/GPSM2 evolved in multicellular organisms as a Dlg-binding protein. We show that Dlg diverged in C. elegans and the syncytial sponge Opsacas minuta and propose that this divergence may correspond with differences in spindle orientation requirements between these organisms and the canonical pathways described in bilaterians. We also demonstrate that Pins/LGN/GPSM2 is present in basal animals, but the established Dlg-interaction site cannot be found in either Placozoa or Porifera. Our results suggest that the interaction between Pins/LGN/GPSM2 and Dlg appeared in Cnidaria, and we therefore speculate that it may have evolved to promote accurate division orientation in the nervous system. This work reveals the evolutionary history of the Pins/LGN/GPSM2-Dlg interaction and suggests new possibilities for its importance in spindle orientation during epithelial and neural tissue development.

The Evolutionary Assembly of Neuronal Machinery

Neurons are highly specialized cells equipped with a sophisticated molecular machinery for the reception, integration, conduction and distribution of information. The evolutionary origin of neurons remains unsolved. How did novel and pre-existing proteins assemble into the complex machinery of the synapse and of the apparatus conducting current along the neuron? In this review, the step-wise assembly of functional modules in neuron evolution serves as a paradigm for the emergence and modification of molecular machinery in the evolution of cell types in multicellular organisms. The pre-synaptic machinery emerged through modification of calcium-regulated large vesicle release, while the postsynaptic machinery has different origins: the glutamatergic postsynapse originated through the fusion of a sensory signaling module and a module for filopodial outgrowth, while the GABAergic postsynapse incorporated an ancient actin regulatory module. The synaptic junction, in turn, is built around two adhesion modules controlled by phosphorylation, which resemble septate and adherens junctions. Finally, neuronal action potentials emerged via a series of duplications and modifications of voltage-gated ion channels. Based on these origins, key molecular innovations are identified that led to the birth of the first neuron in animal evolution.

Mechanical processes underlying precise and robust cell matching

During the development of complicated multicellular organisms, the robust formation of specific cell-cell connections (cell matching) is required for the generation of precise tissue structures. Mismatches or misconnections can lead to various diseases. Diverse mechanical cues, including differential adhesion and temporally varying cell contractility, are involved in regulating the process of cell-cell recognition and contact formation. Cells often start the process of cell matching through contact via filopodia protrusions, mediated by specific adhesion interactions at the cell surface. These adhesion interactions give rise to differential mechanical signals that can be further perceived by the cells. In conjunction with contractions generated by the actomyosin networks within the cells, this differentially coded adhesion information can be translated to reposition and sort cells. Here, we review the role of these different cell matching components and suggest how these mechanical factors cooperate with each other to facilitate specificity in cell-cell contact formation.

The Drosophila septate junctions beyond barrier function: Review of the literature, prediction of human orthologs of the SJ-related proteins and identification of protein domain families

The involvement of Septate Junctions (SJs) in critical cellular functions that extend beyond their role as diffusion barriers in the epithelia and the nervous system has made the fruit fly an ideal model for the study of human diseases associated with impaired Tight Junction (TJ) function. In this study, we summarized current knowledge of the Drosophila melanogaster SJ-related proteins, focusing on their unconventional functions. Additionally, we sought to identify human orthologs of the corresponding genes as well as protein domain families. The systematic literature search was performed in PubMed and Scopus databases using relevant key terms. Orthologs were predicted using the DIOPT tool and aligned protein regions were determined from the Pfam database. 3-D models of the smooth SJ proteins were built on the Phyre2 and DMPFold protein structure prediction servers. A total of 30 proteins were identified as relatives to the SJ cellular structure. Key roles of these proteins, mainly in the regulation of morphogenetic events and cellular signalling, were highlighted. The investigation of protein domain families revealed that the SJ-related proteins contain conserved domains that are required not only for cell-cell interactions and cell polarity but also for cellular signalling and immunity. DIOPT analysis of orthologs identified novel human genes as putative functional homologs of the fruit fly SJ genes. A gap in our knowledge was identified regarding the domains that occur in the proteins encoded by eight SJ-associated genes. Future investigation of these domains is needed to provide functional information.

An Axon-Pathfinding Mechanism Preserves Epithelial Tissue Integrity

Epithelial tissues form the boundaries of organs, where they perform a range of functions, including secretion, absorption, and protection. These tissues are commonly composed of discrete cell layers-sheets of cells that are one-cell thick. In multiple systems examined, epithelial cells round up and move in the apical direction before dividing, likely in response to neighbor-cell crowding [1-6]. Because of this movement, daughter cells may be born displaced from the tissue layer. Reintegration of these displaced cells supports tissue growth and maintains tissue architecture [4]. Two conserved IgCAMs (immunoglobulin superfamily cell adhesion molecules), neuroglian (Nrg) and fasciclin 2 (Fas2), participate in cell reintegration in the Drosophila follicular epithelium [4]. Like their vertebrate orthologs L1CAM and NCAM1/2, respectively, Nrg and Fas2 are cell adhesion molecules primarily studied in the context of nervous system development [7-10]. Consistent with this, we identify another neural IgCAM, Fasciclin 3 (Fas3), as a reintegration factor. Nrg, Fas2, and Fas3 are components of the insect septate junction, the functional equivalent of the vertebrate tight junction, but proliferating follicle cells do not have mature septate junctions, and we find that the septate junction protein neurexin IV does not participate in reintegration [11, 12]. Here, we show that epithelial reintegration works in the same way as IgCAM-mediated axon growth and pathfinding; it relies not only on extracellular adhesion but also mechanical coupling between IgCAMs and the lateral spectrin-based membrane skeleton. Our work indicates that reintegration is mediated by a distinct epithelial adhesion assembly that is compositionally and functionally equivalent to junctions made between axons.

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'.

The Snakeskin-Mesh Complex of Smooth Septate Junction Restricts Yorkie to Regulate Intestinal Homeostasis in Drosophila

Tight junctions in mammals and septate junctions in insects are essential for epithelial integrity. We show here that, in the Drosophila intestine, smooth septate junction proteins provide barrier and signaling functions. During an RNAi screen for genes that regulate adult midgut tissue growth, we found that loss of two smooth septate junction components, Snakeskin and Mesh, caused a hyperproliferation phenotype. By examining epitope-tagged endogenous Snakeskin and Mesh, we demonstrate that the two proteins are present in the cytoplasm of differentiating enteroblasts and in cytoplasm and septate junctions of mature enterocytes. In both enteroblasts and enterocytes, loss of Snakeskin and Mesh causes Yorkie-dependent expression of the JAK-STAT pathway ligand Upd3, which in turn promotes proliferation of intestinal stem cells. Snakeskin and Mesh form a complex with each other, with other septate junction proteins and with Yorkie. Therefore, the Snakeskin-Mesh complex has both barrier and signaling function to maintain stem cell-mediated tissue homeostasis.

•

Snakeskin and Mesh are septate junction proteins essential for intestinal homeostasis

•

Snakeskin and Mesh act in enteroblasts and enterocytes to regulate stem cell division

•

Snakeskin and Mesh form a complex with and restrict the activity of Yorkie

•

Loss of Snakeskin and Mesh allows Yorkie to promote Upd3 expression and growth

The septate junction protein Tetraspanin 2A is critical to the structure and function of Malpighian tubules in Drosophila melanogaster

Tetraspanin-2A (Tsp2A) is an integral membrane protein of smooth septate junctions in Drosophila melanogaster. To elucidate its structural and functional roles in Malpighian tubules, we used the c42-GAL4/UAS system to selectively knock down Tsp2A in principal cells of the tubule. Tsp2A localizes to smooth septate junctions (sSJ) in Malpighian tubules in a complex shared with partner proteins Snakeskin (Ssk), Mesh, and Discs large (Dlg). Knockdown of Tsp2A led to the intracellular retention of Tsp2A, Ssk, Mesh, and Dlg, gaps and widening spaces in remaining sSJ, and tumorous and cystic tubules. Elevated protein levels together with diminished V-type H⁺-ATPase activity in Tsp2A knockdown tubules are consistent with cell proliferation and reduced transport activity. Indeed, Malpighian tubules isolated from Tsp2A knockdown flies failed to secrete fluid in vitro. The absence of significant transepithelial voltages and resistances manifests an extremely leaky epithelium that allows secreted solutes and water to leak back to the peritubular side. The tubular failure to excrete fluid leads to extracellular volume expansion in the fly and to death within the first week of adult life. Expression of the c42-GAL4 driver begins in Malpighian tubules in the late embryo and progresses upstream to distal tubules in third instar larvae, which can explain why larvae survive Tsp2A knockdown and adults do not. Uncontrolled cell proliferation upon Tsp2A knockdown confirms the role of Tsp2A as tumor suppressor in addition to its role in sSJ structure and transepithelial transport.

Molecular organization and function of vertebrate septate-like junctions

Septate-like junctions display characteristic ladder-like ultrastructure reminiscent of the invertebrate epithelial septate junctions and are present at the paranodes of myelinated axons. The paranodal junctions where the myelin loops attach to the axon at the borders of the node of Ranvier provide both a paracellular barrier to ion diffusion and a lateral fence along the axonal membrane. The septate-like junctions constrain the proper distribution of nodal Na⁺ channels and juxtaparanodal K⁺ channels, which are required for the safe propagation of the nerve influx and rapid saltatory conduction. The paranodal cell adhesion molecules have been identified as target antigens in peripheral demyelinating autoimmune diseases and the pathogenic mechanisms described. This review aims at presenting the recent knowledge on the molecular and structural organization of septate-like junctions, their formation and stabilization during development, and how they are involved in demyelinating diseases.

The Drosophila Blood-Brain Barrier Adapts to Cell Growth by Unfolding of Pre-existing Septate Junctions

The blood-brain barrier is crucial for nervous system function. It is established early during development and stays intact during growth of the brain. In invertebrates, septate junctions are the occluding junctions of this barrier. Here, we used Drosophila to address how septate junctions grow during larval stages when brain size increases dramatically. We show that septate junctions are preassembled as long, highly folded strands during embryonic stages, connecting cell vertices. During subsequent cell growth, these corrugated strands are stretched out and stay intact during larval life with very little protein turnover. The G-protein coupled receptor Moody orchestrates the continuous organization of junctional strands in a process requiring F-actin. Consequently, in moody mutants, septate junction strands cannot properly stretch out during cell growth. To compensate for the loss of blood-brain barrier function, moody mutants form interdigitating cell-cell protrusions, resembling the evolutionary ancient barrier type found in primitive vertebrates or invertebrates such as cuttlefish.

Occluding junctions as novel regulators of tissue mechanics during wound repair

Simple epithelial repair is mediated by the contraction of an actomyosin cable and cellular rearrangements at the wound edge. Carvalho et al. show that occluding junctions are required for epithelial repair by regulating these cellular rearrangements and tissue mechanical properties.

In epithelial tissues, cells tightly connect to each other through cell–cell junctions, but they also present the remarkable capacity of reorganizing themselves without compromising tissue integrity. Upon injury, simple epithelia efficiently resolve small lesions through the action of actin cytoskeleton contractile structures at the wound edge and cellular rearrangements. However, the underlying mechanisms and how they cooperate are still poorly understood. In this study, we combine live imaging and theoretical modeling to reveal a novel and indispensable role for occluding junctions (OJs) in this process. We demonstrate that OJ loss of function leads to defects in wound-closure dynamics: instead of contracting, wounds dramatically increase their area. OJ mutants exhibit phenotypes in cell shape, cellular rearrangements, and mechanical properties as well as in actin cytoskeleton dynamics at the wound edge. We propose that OJs are essential for wound closure by impacting on epithelial mechanics at the tissue level, which in turn is crucial for correct regulation of the cellular events occurring at the wound edge.

Evolution of Animal Neural Systems

Nervous systems are among the most spectacular products of evolution. Their provenance and evolution have been of interest and often the subjects of intense debate since the late nineteenth century. The genomics era has provided researchers with a new set of tools with which to study the early evolution of neurons, and recent progress on the molecular evolution of the first neurons has been both exciting and frustrating. It has become increasingly obvious that genomic data are often insufficient to reconstruct complex phenotypes in deep evolutionary time because too little is known about how gene function evolves over deep time. Therefore, additional functional data across the animal tree are a prerequisite to a fuller understanding of cell evolution. To this end, we review the functional modules of neurons and the evolution of their molecular components, and we introduce the idea of hierarchical molecular evolution.

Getting Nervous: An Evolutionary Overhaul for Communication

The evolution of a nervous system as a control system of the body's functions is a key innovation of animals. Its fundamental units are neurons, highly specialized cells dedicated to fast cell-cell communication. Neurons pass signals to other neurons, muscle cells, or gland cells at specialized junctions, the synapses, where transmitters are released from vesicles in a Ca²⁺-dependent fashion to activate receptors in the membrane of the target cell. Reconstructing the origins of neuronal communication out of a more simple process remains a central challenge in biology. Recent genomic comparisons have revealed that all animals, including the nerveless poriferans and placozoans, share a basic set of genes for neuronal communication. This suggests that the first animal, the Urmetazoan, was already endowed with neurosecretory cells that probably started to connect into neuronal networks soon afterward. Here, we discuss scenarios for this pivotal transition in animal evolution.

The repertoire of epithelial morphogenesis on display: Progressive elaboration of Drosophila egg structure

Epithelial structures are foundational for tissue organization in all metazoans. Sheets of epithelial cells form lateral adhesive junctions and acquire apico-basal polarity perpendicular to the surface of the sheet. Genetic analyses in the insect model, Drosophila melanogaster, have greatly advanced our understanding of how epithelial organization is established, and how it is modulated during tissue morphogenesis. Major insights into collective cell migrations have come from analyses of morphogenetic movements within the adult follicular epithelium that cooperates with female germ cells to build a mature egg. Epithelial follicle cells progress through tightly choreographed phases of proliferation, patterning, reorganization and migrations, before they differentiate to form the elaborate structures of the eggshell. Distinct structural domains are organized by differential adhesion, within which lateral junctions are remodeled to further shape the organized epithelia. During collective cell migrations, adhesive interactions mediate supracellular organization of planar polarized macromolecules, and facilitate crawling over the basement membrane or traction against adjacent cell surfaces. Comparative studies with other insects are revealing the diversification of morphogenetic movements for elaboration of epithelial structures. This review surveys the repertoire of follicle cell morphogenesis, to highlight the coordination of epithelial plasticity with progressive differentiation of a secretory epithelium. Technological advances will keep this tissue at the leading edge for interrogating the precise spatiotemporal regulation of normal epithelial reorganization events, and provide a framework for understanding pathological tissue dysplasia.