Compromised junctional integrity phenocopies age-dependent renal dysfunction in Drosophila Snakeskin mutants

Transporting epithelia provide a protective barrier against pathogenic insults while allowing the controlled exchange of ions, solutes and water with the external environment. In invertebrates, these functions depend on formation and maintenance of 'tight' septate junctions (SJs). However, the mechanism by which SJs affect transport competence and tissue homeostasis, and how these are modulated by ageing, remain incompletely understood. Here, we demonstrate that the Drosophila renal (Malpighian) tubules undergo an age-dependent decline in secretory capacity, which correlates with mislocalisation of SJ proteins and progressive degeneration in cellular morphology and tissue homeostasis. Acute loss of the SJ protein Snakeskin in adult tubules induced progressive changes in cellular and tissue architecture, including altered expression and localisation of junctional proteins with concomitant loss of cell polarity and barrier integrity, demonstrating that compromised junctional integrity is sufficient to replicate these ageing-related phenotypes. Taken together, our work demonstrates a crucial link between epithelial barrier integrity, tubule transport competence, renal homeostasis and organismal viability, as well as providing novel insights into the mechanisms underpinning ageing and renal disease.

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.

DNA transposons mediate duplications via transposition-independent and -dependent mechanisms in metazoans

Despite long being considered as "junk", transposable elements (TEs) are now accepted as catalysts of evolution. One example is Mutator-like elements (MULEs, one type of terminal inverted repeat DNA TEs, or TIR TEs) capturing sequences as Pack-MULEs in plants. However, their origination mechanism remains perplexing, and whether TIR TEs mediate duplication in animals is almost unexplored. Here we identify 370 Pack-TIRs in 100 animal reference genomes and one Pack-TIR (Ssk-FB4) family in fly populations. We find that single-copy Pack-TIRs are mostly generated via transposition-independent gap filling, and multicopy Pack-TIRs are likely generated by transposition after replication fork switching. We show that a proportion of Pack-TIRs are transcribed and often form chimeras with hosts. We also find that Ssk-FB4s represent a young protein family, as supported by proteomics and signatures of positive selection. Thus, TIR TEs catalyze new gene structures and new genes in animals via both transposition-independent and -dependent mechanisms.

The novel membrane protein Hoka regulates septate junction organization and stem cell homeostasis in the Drosophila gut

Smooth septate junctions (sSJs) regulate the paracellular transport in the intestinal tract in arthropods. In Drosophila, the organization and physiological function of sSJs are regulated by at least three sSJ-specific membrane proteins: Ssk, Mesh and Tsp2A. Here, we report a novel sSJ membrane protein, Hoka, which has a single membrane-spanning segment with a short extracellular region, and a cytoplasmic region with Tyr-Thr-Pro-Ala motifs. The larval midgut in hoka mutants shows a defect in sSJ structure. Hoka forms a complex with Ssk, Mesh and Tsp2A, and is required for the correct localization of these proteins to sSJs. Knockdown of hoka in the adult midgut leads to intestinal barrier dysfunction and stem cell overproliferation. In hoka-knockdown midguts, aPKC is upregulated in the cytoplasm and the apical membrane of epithelial cells. The depletion of aPKC and yki in hoka-knockdown midguts results in reduced stem cell overproliferation. These findings indicate that Hoka cooperates with the sSJ proteins Ssk, Mesh and Tsp2A to organize sSJs, and is required for maintaining intestinal stem cell homeostasis through the regulation of aPKC and Yki activities in the Drosophila midgut.

A glucose-supplemented diet enhances gut barrier integrity in Drosophila

Dietary intervention has received considerable attention as an approach to extend lifespan and improve aging. However, questions remain regarding optimal dietary regimes and underlying mechanisms of lifespan extension. Here, we asked how an increase of glucose in a chemically defined diet extends the lifespan of adult Drosophila melanogaster We showed that glucose-dependent lifespan extension is not a result of diminished caloric intake, or changes to systemic insulin activity, two commonly studied mechanisms of lifespan extension. Instead, we found that flies raised on glucose-supplemented food increased the expression of cell-adhesion genes, delaying age-dependent loss of intestinal barrier integrity. Furthermore, we showed that chemical disruption of the gut barrier negated the lifespan extension associated with glucose treatment, suggesting that glucose-supplemented food prolongs adult viability by enhancing the intestinal barrier. We believe our data contribute to understanding intestinal homeostasis, and may assist efforts to develop preventative measures that limit effects of aging on health.

Homeostatic Regulation of ROS-Triggered Hippo-Yki Pathway via Autophagic Clearance of Ref(2)P/p62 in the Drosophila Intestine

Homeostasis of intestinal epithelia is maintained by coordination of the proper rate of regeneration by stem cell division with the rate of cell loss. Regeneration of host epithelia is normally quiescent upon colonization of commensal bacteria; however, the epithelia often develop dysplasia in a context-dependent manner, the cause and underlying mechanism of which remain unclear. Here, we show that in Drosophila intestine, autophagy lowers the sensitivity of differentiated enterocytes to reactive oxygen species (ROS) that are produced in response to commensal bacteria. We find that autophagy deficiency provokes ROS-dependent excessive regeneration and subsequent epithelial dysplasia and barrier dysfunction. Mechanistically, autophagic substrate Ref(2)P/p62, which co-localizes and physically interacts with Dachs, a Hippo signaling regulator, accumulates upon autophagy deficiency and thus inactivates Hippo signaling, resulting in stem cell over-proliferation non-cell autonomously. Our findings uncover a mechanism whereby suppression of undesirable regeneration by autophagy maintains long-term homeostasis of intestinal epithelia.

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.

Candidate genetic determinants of intraspecific variation in pea aphid susceptibility to RNA interference

RNA interference (RNAi) plays a key role in insect defense against viruses and transposable elements, and it is being applied as an experimental tool and for insect pest control. However, RNAi efficiency is highly variable for some insects, notably the pea aphid Acyrthosiphon pisum. In this study, we used natural variation in RNAi susceptibility of pea aphids to identify genes that influence RNAi efficiency. Susceptibility to orally-delivered dsRNA against the gut aquaporin gene AQP1 (ds-AQP1) varied widely across a panel of 83 pea aphid genotypes, from zero to total mortality. Genome-wide association between aphid performance on ds-AQP1 supplemented diet and aphid genetic variants yielded 103 significantly associated single nucleotide polymorphisms (SNPs), including variants in 55 genes, at the 10^-4 probability cut-off. When ds-AQP1 was co-administered with dsRNA against six candidate genes, aphid mortality was reduced for three (50%) genes: the orthologs of the Drosophila genes trachealess (CG42865), headcase (CG15532) and a gene coding a peritrophin-A domain (CG8192), indicating that these genes function to promote RNAi efficiency against AQP1 in the pea aphid. Aphid susceptibility (quantified as mortality) to ds-AQP1 was correlated with RNAi against a further gene, snakeskin with essential gut function unrelated to AQP1, for some but not all aphid genotypes tested, suggesting that the determinants of RNAi efficiency may be partly gene-specific. This study demonstrates high levels of natural variation in susceptibility to RNAi and demonstrates the value of harnessing this variation to identify genes influencing RNAi efficiency.

Ageing, metabolism and the intestine

The intestinal epithelium serves as a dynamic barrier to the environment and integrates a variety of signals, including those from metabolites, commensal microbiota, immune responses and stressors upon ageing. The intestine is constantly challenged and requires a high renewal rate to replace damaged cells in order to maintain its barrier function. Essential for its renewal capacity are intestinal stem cells, which constantly give rise to progenitor cells that differentiate into the multiple cell types present in the epithelium. Here, we review the current state of research of how metabolism and ageing control intestinal stem cell function and epithelial homeostasis. We focus on recent insights gained from model organisms that indicate how changes in metabolic signalling during ageing are a major driver for the loss of stem cell plasticity and epithelial homeostasis, ultimately affecting the resilience of an organism and limiting its lifespan. We compare findings made in mouse and Drosophila and discuss differences and commonalities in the underlying signalling pathways and mechanisms in the context of ageing.

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.

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Snakeskin and Mesh are septate junction proteins essential for intestinal homeostasis

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

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

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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.

A cell atlas of the adult Drosophila midgut

Studies of the adult Drosophila midgut have led to many insights in our understanding of cell-type diversity, stem cell regeneration, tissue homeostasis, and cell fate decision. Advances in single-cell RNA sequencing provide opportunities to identify new cell types and molecular features. We used single-cell RNA sequencing to characterize the transcriptome of midgut epithelial cells and identified 22 distinct clusters representing intestinal stem cells, enteroblasts, enteroendocrine cells (EEs), and enterocytes. This unbiased approach recovered most of the known intestinal stem cells/enteroblast and EE markers, highlighting the high quality of the dataset, and led to insights on intestinal stem cell biology, cell type-specific organelle features, the roles of new transcription factors in progenitors and regional variation along the gut, 5 additional EE gut hormones, EE hormonal expression diversity, and paracrine function of EEs. To facilitate mining of this rich dataset, we provide a web-based resource for visualization of gene expression in single cells. Altogether, our study provides a comprehensive resource for addressing functions of genes in the midgut epithelium.

The septate junction protein Mesh is required for epithelial morphogenesis, ion transport, and paracellular permeability in the Drosophila Malpighian tubule

Septate junctions (SJs) are occluding cell-cell junctions that have roles in paracellular permeability and barrier function in the epithelia of invertebrates. Arthropods have two types of SJs, pleated SJs and smooth SJs (sSJs). In Drosophila melanogaster, sSJs are found in the midgut and Malpighian tubules, but the functions of sSJs and their protein components in the tubule epithelium are unknown. Here we examined the role of the previously identified integral sSJ component, Mesh, in the Malpighian tubule. We genetically manipulated mesh specifically in the principal cells of the tubule at different life stages. Tubules of flies with developmental mesh knockdown revealed defects in epithelial architecture, sSJ molecular and structural organization, and lack of urine production in basal and kinin-stimulated conditions, resulting in edema and early adult lethality. Knockdown of mesh during adulthood did not disrupt tubule epithelial and sSJ integrity but decreased the transepithelial potential, diminished transepithelial fluid and ion transport, and decreased paracellular permeability to 4-kDa dextran. Drosophila kinin decreased transepithelial potential and increased chloride permeability, and it stimulated fluid secretion in both control and adult mesh knockdown tubules but had no effect on 4-kDa dextran flux. Together, these data indicate roles for Mesh in the developmental maturation of the Drosophila Malpighian tubule and in ion and macromolecular transport in the adult tubule.

The transcription factor Hey and nuclear lamins specify and maintain cell identity

The inability of differentiated cells to maintain their identity is a hallmark of age-related diseases. We found that the transcription factor Hey supervises the identity of differentiated enterocytes (ECs) in the adult Drosophila midgut. Lineage tracing established that Hey-deficient ECs are unable to maintain their unique nuclear organization and identity. To supervise cell identity, Hey determines the expression of nuclear lamins, switching from a stem-cell lamin configuration to a differentiated lamin configuration. Moreover, continued Hey expression is required to conserve large-scale nuclear organization. During aging, Hey levels decline, and EC identity and gut homeostasis are impaired, including pathological reprograming and compromised gut integrity. These phenotypes are highly similar to those observed upon acute targeting of Hey or perturbation of lamin expression in ECs in young adults. Indeed, aging phenotypes were suppressed by continued expression of Hey in ECs, suggesting that a Hey-lamin network safeguards nuclear organization and differentiated cell identity.

Molecular characterization of the insecticidal activity of double-stranded RNA targeting the smooth septate junction of western corn rootworm (Diabrotica virgifera virgifera)

The western corn rootworm (WCR, Diabrotica virgifera virgifera) gene, dvssj1, is a putative homolog of the Drosophila melanogaster gene, snakeskin (ssk). This gene encodes a membrane protein associated with the smooth septate junction (SSJ) which is required for the proper barrier function of the epithelial lining of insect intestines. Disruption of DVSSJ integrity by RNAi technique has been shown previously to be an effective approach for corn rootworm control, by apparent suppression of production of DVSSJ1 protein leading to growth inhibition and mortality. To understand the mechanism that leads to the death of WCR larvae by dvssj1 double-stranded RNA, we examined the molecular characteristics associated with SSJ functions during larval development. Dvssj1 dsRNA diet feeding results in dose-dependent suppression of mRNA and protein; this impairs SSJ formation and barrier function of the midgut and results in larval mortality. These findings suggest that the malfunctioning of the SSJ complex in midgut triggered by dvssj1 silencing is the principal cause of WCR death. This study also illustrates that dvssj1 is a midgut-specific gene in WCR and its functions are consistent with biological functions described for ssk.

Septate junction in the distal ileac plexus of larval lepidopteran Trichoplusia ni: alterations in paracellular permeability during ion transport reversal

The Malpighian tubules (MTs) and hindgut together act as the functional kidney in insects. MTs of caterpillars are notably complex and consist of several regions that display prominent differences in ion transport. The distal ileac plexus (DIP) is a region of Malpighian tubule that is of particular interest because it switches from ion secretion to ion reabsorption in larvae fed on ion-rich diets. The pathways of solute transport in the DIP are not well understood, but one potential route is the paracellular pathway between epithelial cells. This pathway is regulated by the septate junctions (SJs) in invertebrates, and in this study, we found regional and cellular heterogeneity in expression of several integral SJ proteins. DIP of larvae fed ion-rich diets demonstrated a reduction in paracellular permeability, coupled with alterations in both SJ morphology and the abundance of its molecular components. Similarly, treatment in vitro with helicokinin (HK), an antidiuretic hormone identified by previous studies, altered mRNA abundance of many SJ proteins and reduced paracellular permeability. HK was also shown to target a secondary cell-specific SJ protein Tsp2A. Taken together, our data suggest that dietary ion loading, known to cause ion transport reversal in the DIP of larval T. ni, leads to alterations in the paracellular permeability, SJ morphology and its molecular component abundance. The results suggest that HK is an important endocrine factor that co-regulates ion transport, water transport and paracellular permeability in MTs of larval lepidopterans. We propose that co-regulation of all three components of the MT function in larval lepidopterans allows for safe toggling between ion secretion and reabsorption in the DIP in response to variations in dietary ion availability.

Septate junctions regulate gut homeostasis through regulation of stem cell proliferation and enterocyte behavior in Drosophila

Smooth septate junctions (sSJs) contribute to the epithelial barrier, which restricts leakage of solutes through the paracellular route of epithelial cells in the Drosophila midgut. We previously identified three sSJ-associated membrane proteins, Ssk, Mesh, and Tsp2A, and showed that these proteins were required for sSJ formation and intestinal barrier function in the larval midgut. Here, we investigated the roles of sSJs in the Drosophila adult midgut. Depletion of any of the sSJ-proteins from enterocytes resulted in remarkably shortened lifespan and intestinal barrier dysfunction in flies. Interestingly, the sSJ-protein-deficient flies showed intestinal hypertrophy accompanied by accumulation of morphologically abnormal enterocytes. The phenotype was associated with increased stem cell proliferation and activation of the MAP kinase and Jak-Stat pathways in stem cells. Loss of cytokines Unpaired2 and Unpaired3, which are involved in Jak-Stat pathway activation, reduced the intestinal hypertrophy, but not the increased stem cell proliferation, in flies lacking Mesh. The present findings suggest that SJs play a crucial role in maintaining tissue homeostasis through regulation of stem cell proliferation and enterocyte behavior in the Drosophila adult midgut.

How do oral insecticidal compounds cross the insect midgut epithelium?

The use of oral insecticidal molecules (small molecules, peptides, dsRNA) via spray or plant mediated applications represents an efficient way to manage damaging insect species. With the exception of Bt toxins that target the midgut epithelium itself, most of these compounds have targets that lie within the hemocoel (body) of the insect. Because of this, one of the greatest factors in determining the effectiveness of an oral insecticidal compound is its ability to traverse the gut epithelium and enter the hemolymph. However, for many types of insecticidal compounds, neither the pathway taken across the gut nor the specific genes which influence uptake are fully characterized. Here, we review how different types of insecticidal compounds enter or cross the midgut epithelium through passive (diffusion) or active (transporter based, endocytosis) routes. A deeper understanding of how insecticidal molecules cross the gut will help to best utilize current insecticides and also provide for more rational design of future ones.

Intestinal Snakeskin Limits Microbial Dysbiosis during Aging and Promotes Longevity

Intestinal barrier dysfunction is an evolutionarily conserved hallmark of aging, which has been linked to microbial dysbiosis, altered expression of occluding junction proteins, and impending mortality. However, the interplay between intestinal junction proteins, age-onset dysbiosis, and lifespan determination remains unclear. Here, we show that altered expression of Snakeskin (Ssk), a septate junction-specific protein, can modulate intestinal homeostasis, microbial dynamics, immune activity, and lifespan in Drosophila. Loss of Ssk leads to rapid and reversible intestinal barrier dysfunction, altered gut morphology, dysbiosis, and dramatically reduced lifespan. Remarkably, restoration of Ssk expression in flies showing intestinal barrier dysfunction rescues each of these phenotypes previously linked to aging. Intestinal up-regulation of Ssk protects against microbial translocation following oral infection with pathogenic bacteria. Furthermore, intestinal up-regulation of Ssk improves intestinal barrier function during aging, limits dysbiosis, and extends lifespan. Our findings indicate that intestinal occluding junctions may represent prolongevity targets in mammals.

Identification of the septate junction protein gliotactin in the mosquito Aedes aegypti: evidence for a role in increased paracellular permeability in larvae

Septate junctions (SJs) regulate paracellular permeability across invertebrate epithelia. However, little is known about the function of SJ proteins in aquatic invertebrates. In this study, a role for the transmembrane SJ protein gliotactin (Gli) in the osmoregulatory strategies of larval mosquito (Aedes aegypti) was examined. Differences in gli transcript abundance were observed between the midgut, Malpighian tubules, hindgut and anal papillae of A. aegypti, which are epithelia that participate in larval mosquito osmoregulation. Western blotting of Gli revealed its presence in monomer, putative dimer and alternatively processed protein forms in different larval mosquito organs. Gli localized to the entire SJ domain between midgut epithelial cells and showed a discontinuous localization along the plasma membranes of epithelial cells of the rectum as well as the syncytial anal papillae epithelium. In the Malpighian tubules, Gli immunolocalization was confined to SJs between the stellate and principal cells. Rearing larvae in 30% seawater caused an increase in Gli protein abundance in the anterior midgut, Malpighian tubules and hindgut. Transcriptional knockdown of gli using dsRNA reduced Gli protein abundance in the midgut and increased the flux rate of the paracellular permeability marker, polyethylene glycol (molecular weight 400 Da; PEG-400). Data suggest that in larval A. aegypti, Gli participates in the maintenance of salt and water balance and that one role for Gli is to participate in the regulation of paracellular permeability across the midgut of A. aegypti in response to changes in environmental salinity.

Salinity alters snakeskin and mesh transcript abundance and permeability in midgut and Malpighian tubules of larval mosquito, Aedes aegypti

This study examined the distribution and localization of the septate junction (SJ) proteins snakeskin (Ssk) and mesh in osmoregulatory organs of larval mosquito (Aedes aegypti), as well as their response to altered environmental salt levels. Ssk and mesh transcripts and immunoreactivity were detected in tissues of endodermal origin such as the midgut and Malpighian tubules of A. aegypti larvae, but not in ectodermally derived hindgut and anal papillae. Immunolocalization of Ssk and mesh in the midgut and Malpighian tubules indicated that both proteins are concentrated at regions of cell-cell contact between epithelial cells. Transcript abundance of ssk and mesh was higher in the midgut and Malpighian tubules of brackish water (BW, 30% SW) reared A. aegypti larvae when compared with freshwater (FW) reared animals. Therefore, [³H]polyethylene glycol (MW 400Da, PEG-400) flux was examined across isolated midgut and Malpighian tubule preparations as a measure of their paracellular permeability. It was found that PEG-400 flux was greater across the midgut of BW versus FW larvae while the Malpighian tubules of BW-reared larvae had reduced PEG-400 permeability in conjunction with increased Cl^- secretion compared to FW animals. Taken together, data suggest that Ssk and mesh are found in smooth SJs (sSJs) of larval A. aegypti and that their abundance alters in association with changes in epithelial permeability when larvae reside in water of differing salt content. This latter observation suggests that Ssk and mesh play a role in the homeostatic control of salt and water balance in larval A. aegypti.

Discovery of midgut genes for the RNA interference control of corn rootworm

RNA interference (RNAi) is a promising new technology for corn rootworm control. This paper presents the discovery of new gene targets - dvssj1 and dvssj2, in western corn rootworm (WCR). Dvssj1 and dvssj2 are orthologs of the Drosophila genes snakeskin (ssk) and mesh, respectively. These genes encode membrane proteins associated with smooth septate junctions (SSJ) which are required for intestinal barrier function. Based on bioinformatics analysis, dvssj1 appears to be an arthropod-specific gene. Diet based insect feeding assays using double-stranded RNA (dsRNA) targeting dvssj1 and dvssj2 demonstrate targeted mRNA suppression, larval growth inhibition, and mortality. In RNAi treated WCR, injury to the midgut was manifested by "blebbing" of the midgut epithelium into the gut lumen. Ultrastructural examination of midgut epithelial cells revealed apoptosis and regenerative activities. Transgenic plants expressing dsRNA targeting dvssj1 show insecticidal activity and significant plant protection from WCR damage. The data indicate that dvssj1 and dvssj2 are effective gene targets for the control of WCR using RNAi technology, by apparent suppression of production of their respective smooth septate junction membrane proteins located within the intestinal lining, leading to growth inhibition and mortality.

A de novo transcriptome of the Malpighian tubules in non-blood-fed and blood-fed Asian tiger mosquitoes Aedes albopictus: insights into diuresis, detoxification, and blood meal processing

Background. In adult female mosquitoes, the renal (Malpighian) tubules play an important role in the post-prandial diuresis, which removes excess ions and water from the hemolymph of mosquitoes following a blood meal. After the post-prandial diuresis, the roles that Malpighian tubules play in the processing of blood meals are not well described.

Methods. We used a combination of next-generation sequencing (paired-end RNA sequencing) and physiological/biochemical assays in adult female Asian tiger mosquitoes (Aedes albopictus) to generate molecular and functional insights into the Malpighian tubules and how they may contribute to blood meal processing (3–24 h after blood ingestion).

Results/Discussion. Using RNA sequencing, we sequenced and assembled the first de novo transcriptome of Malpighian tubules from non-blood-fed (NBF) and blood-fed (BF) mosquitoes. We identified a total of 8,232 non-redundant transcripts. The Malpighian tubules of NBF mosquitoes were characterized by the expression of transcripts associated with active transepithelial fluid secretion/diuresis (e.g., ion transporters, water channels, V-type H⁺-ATPase subunits), xenobiotic detoxification (e.g., cytochrome P450 monoxygenases, glutathione S-transferases, ATP-binding cassette transporters), and purine metabolism (e.g., xanthine dehydrogenase). We also detected the expression of transcripts encoding sodium calcium exchangers, G protein coupled-receptors, and septate junctional proteins not previously described in mosquito Malpighian tubules. Within 24 h after a blood meal, transcripts associated with active transepithelial fluid secretion/diuresis exhibited a general downregulation, whereas those associated with xenobiotic detoxification and purine catabolism exhibited a general upregulation, suggesting a reinvestment of the Malpighian tubules’ molecular resources from diuresis to detoxification. Physiological and biochemical assays were conducted in mosquitoes and isolated Malpighian tubules, respectively, to confirm that the transcriptomic changes were associated with functional consequences. In particular, in vivo diuresis assays demonstrated that adult female mosquitoes have a reduced diuretic capacity within 24 h after a blood meal. Moreover, biochemical assays in isolated Malpighian tubules showed an increase in glutathione S-transferase activity and the accumulation of uric acid (an end product of purine catabolism) within 24 h after a blood meal. Our data provide new insights into the molecular physiology of Malpighian tubules in culicine mosquitoes and reveal potentially important molecular targets for the development of chemical and/or gene-silencing insecticides that would disrupt renal function in mosquitoes.

A tetraspanin regulates septate junction formation in Drosophila midgut

Septate junctions (SJs) are membrane specializations that restrict the free diffusion of solutes through the paracellular pathway in invertebrate epithelia. In arthropods, two morphologically different types of septate junctions are observed; pleated (pSJs) and smooth (sSJs), which are present in ectodermally and endodermally derived epithelia, respectively. Recent identification of sSJ-specific proteins, Mesh and Ssk, in Drosophila indicates that the molecular compositions of sSJs and pSJs differ. A deficiency screen based on immunolocalization of Mesh identified a tetraspanin family protein, Tsp2A, as a newly discovered protein involved in sSJ formation in Drosophila Tsp2A specifically localizes at sSJs in the midgut and Malpighian tubules. Compromised Tsp2A expression caused by RNAi or the CRISPR/Cas9 system was associated with defects in the ultrastructure of sSJs, changed localization of other sSJ proteins, and impaired barrier function of the midgut. In most Tsp2A mutant cells, Mesh failed to localize to sSJs and was distributed through the cytoplasm. Tsp2A forms a complex with Mesh and Ssk and these proteins are mutually interdependent for their localization. These observations suggest that Tsp2A cooperates with Mesh and Ssk to organize sSJs.

Transcriptional cellular responses in midgut tissue of Aedes aegypti larvae following intoxication with Cry11Aa toxin from Bacillus thuringiensis

Background

Although much is known about the mechanism of action of Bacillus thuringiensis Cry toxins, the target tissue cellular responses to toxin activity is less understood. Previous transcriptomic studies indicated that significant changes in gene expression occurred during intoxication. However, most of these studies were done in organisms without a sequenced and annotated reference genome. A reference genome and transcriptome is available for the mosquito Aedes aegypti, and its importance as a disease vector has positioned its biological control as a primary health concern. Through RNA sequencing we sought to determine the transcriptional changes observed during intoxication by Cry11Aa in A. aegypti and to analyze possible defense and recovery mechanisms engaged after toxin ingestion.

Results

In this work the changes in the transcriptome of 4^th instar A. aegypti larvae exposed to Cry11Aa toxin for 0, 3, 6, 9, and 12 h were analyzed. A total of 1060 differentially expressed genes after toxin ingestion were identified with two bioconductoR packages: DESeq2 and EdgeR. The most important transcriptional changes were observed after 9 or 12 h of toxin exposure. GO enrichment analysis of molecular function and biological process were performed as well as Interpro protein functional domains and pBLAST analyses. Up regulated processes include vesicular trafficking, small GTPase signaling, MAPK pathways, and lipid metabolism. In contrast, down regulated functions are related to transmembrane transport, detoxification mechanisms, cell proliferation and metabolism enzymes. Validation with RT-qPCR showed large agreement with Cry11Aa intoxication since these changes were not observed with untreated larvae or larvae treated with non-toxic Cry11Aa mutants, indicating that a fully functional pore forming Cry toxin is required for the observed transcriptional responses.

Conclusions

This study presents the first transcriptome of Cry intoxication response in a fully sequenced insect, and reveals possible conserved cellular processes that enable larvae to contend with Cry intoxication in the disease vector A. aegypti. We found some similarities of the mosquito responses to Cry11Aa toxin with previously observed responses to other Cry toxins in different insect orders and in nematodes suggesting a conserved response to pore forming toxins. Surprisingly some of these responses also correlate with transcriptional changes observed in Bti-resistant and Cry11Aa-resistant mosquito larvae.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2240-7) contains supplementary material, which is available to authorized users.

Escargot maintains stemness and suppresses differentiation in Drosophila intestinal stem cells

Snail family transcription factors are expressed in various stem cell types, but their function in maintaining stem cell identity is unclear. In the adult Drosophila midgut, the Snail homolog Esg is expressed in intestinal stem cells (ISCs) and their transient undifferentiated daughters, termed enteroblasts (EB). We demonstrate here that loss of esg in these progenitor cells causes their rapid differentiation into enterocytes (EC) or entero-endocrine cells (EE). Conversely, forced expression of Esg in intestinal progenitor cells blocks differentiation, locking ISCs in a stem cell state. Cell type-specific transcriptome analysis combined with Dam-ID binding studies identified Esg as a major repressor of differentiation genes in stem and progenitor cells. One critical target of Esg was found to be the POU-domain transcription factor, Pdm1, which is normally expressed specifically in differentiated ECs. Ectopic expression of Pdm1 in progenitor cells was sufficient to drive their differentiation into ECs. Hence, Esg is a critical stem cell determinant that maintains stemness by repressing differentiation-promoting factors, such as Pdm1.

Molecular organization and function of invertebrate occluding junctions

Septate junctions (SJs) are specialized intercellular junctions that function as permeability barriers to restrict the free diffusion of solutes through the paracellular routes in invertebrate epithelia. SJs are subdivided into several morphological types that vary among different animal phyla. In several phyla, different types of SJ have been described in different epithelia within an individual. Arthropods have two types of SJs: pleated SJs (pSJs) and smooth SJs (sSJs), found in ectodermally and endodermally derived epithelia, respectively. Several lines of Drosophila research have identified and characterized a large number of pSJ-associated proteins. Two sSJ-specific proteins have been recently reported. Molecular dissection of SJs in Drosophila and animals in other phyla will lead to a better understanding of the functional differences among SJ types and of evolutionary aspects of these permeability barriers.

A dynamic paracellular pathway serves diuresis in mosquito Malpighian tubules

Female mosquitoes gorge on vertebrate blood, a rich nutrient source for developing eggs, but gorging meals increase the risk of predation. Mosquitoes are quick to reduce the flight payload with a potent diuresis. Diuretic peptides of the insect kinin family induce a tenfold reduction in the paracellular resistance of Malpighian tubules and increase the paracellular permeation of Cl(-), the counterion of the transepithelial secretion of Na(+) and K(+). As a result, the transepithelial secretion of NaCl and KCl and water increases. Insect kinins signal the opening of the paracellular pathway via G protein-coupled receptors and the elevation of intracellular [Ca(2+)], which leads to the reorganization of the cytoskeleton associated with the septate junction (SJ). The reorganization may affect the septate junctional proteins that control the barrier and permselectivity properties of the paracellular pathway. The proteins involved in the embryonic formation of the SJ and in epithelial polarization are largely known for ectodermal epithelia, but the proteins that form and mediate the dynamic functions of the SJ in Malpighian tubules remain to be determined.

A novel protein complex, mesh-ssk, is required for septate junction formation in drosophila midgut

Septate junctions (SJs) are specialized intercellular junctions that restrict the free diffusion of solutes through the paracellular route in invertebrate epithelia. In arthropods, two morphologically different types of SJs have been reported: pleated SJs and smooth SJs (sSJs), which are found in ectodermally and endodermally derived epithelia, respectively. However, the molecular and functional differences between these SJ types have not been elucidated. Here we report that a novel sSJ-specific component, a single-pass transmembrane protein, termed ‘Mesh’ is highly concentrated in Drosophila sSJs. Compromised mesh expression causes defects in the organization of sSJs, in the localizations of other sSJ proteins, and in the barrier function of the midgut. Ectopic expression of Mesh in cultured cells induces cell-cell adhesion. Mesh forms a complex with Ssk, another sSJ-specific protein, and these proteins are mutually interdependent for their localization. Thus, a novel protein complex comprising Mesh and Ssk plays a significant role in sSJ formation and in intestinal barrier function in Drosophila.