Control of Gliotactin localization and levels by tyrosine phosphorylation and endocytosis is necessary for survival of polarized epithelia

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

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

Overexpressed Gliotactin activates BMP signaling through interfering with the Tkv-Dad association

Epithelial junctions ensure cell-cell adhesion and establish permeability barriers between cells. At the corners of epithelia, the tricellular junction (TCJ) is formed by three adjacent epithelial cells and generates a functional barrier. In Drosophila, a key TCJ protein is Gliotactin (Gli) where loss of Gli disrupts barrier formation and function. Conversely, overexpressed Gli spreads away from the TCJ and triggers apoptosis, delamination, and cell migration. Thus, Gli protein levels are tightly regulated and by two mechanisms, at the protein levels by tyrosine phosphorylation and endocytosis and at the mRNA level through microRNA-184. Regulation of Gli mRNA is mediated through a Gli-BMP-miR184 feedback loop. Excessive Gli triggers BMP signaling pathway through the activation of Tkv type-I BMP receptor and Mad. Elevated level of pMad induces micrRNA-184 expression which in turn targets the Gli 3'UTR and mRNA degradation. Gli activation of Tkv is not through its ligand Dpp but rather through the inhibition of Dad, an inhibitory-Smad. Here, we show that ectopic expression of Gli interferes with Tkv-Dad association by sequestering Dad away from Tkv. The reduced inhibitory effect of Dad on Tkv results in the increased Tkv-pMad signaling activity, and this effect is continuous through larval and pupal wing formation.

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.

Organelle aging: Lessons from model organisms

Most cellular processes descend into failure during aging. While a large collection of longevity pathways has been identified in the past decades, the mechanism for age-related decline of cellular homeostasis and organelle function remains largely unsolved. It is known that many organelles undergo structural and functional changes during normal aging, which significantly contributes to the decline of tissue function at old ages. Since recent studies have revealed an emerging role of organelles as regulatory hubs in maintaining cellular homeostasis, understanding of organelle aging will provide important insights into the cellular basis of organismal aging. Here we review current progress on the characterization of age-dependent structural and functional alterations in the more well-studied organelles, as well as the known mechanisms governing organelle aging in model organisms, with a special focus on the fruit fly Drosophila melanogaster.

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.

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.

The Drosophila tricellular junction protein Gliotactin regulates its own mRNA levels through BMP-mediated induction of miR-184

Epithelial bicellular and tricellular junctions are essential for establishing and maintaining permeability barriers. Tricellular junctions are formed by the convergence of three bicellular junctions at the corners of neighbouring epithelia. Gliotactin, a member of the Neuroligin family, is located at theDrosophilatricellular junction, and is crucial for the formation of tricellular and septate junctions, as well as permeability barrier function. Gliotactin protein levels are tightly controlled by phosphorylation at tyrosine residues and endocytosis. Blocking endocytosis or overexpressing Gliotactin results in the spread of Gliotactin from the tricellular junction, resulting in apoptosis, delamination and migration of epithelial cells. We show that Gliotactin levels are also regulated at the mRNA level by micro (mi)RNA-mediated degradation and that miRNAs are targeted to a short region in the 3'UTR that includes a conserved miR-184 target site. miR-184 also targets a suite of septate junction proteins, including NrxIV, coracle and Mcr. miR-184 expression is triggered when Gliotactin is overexpressed, leading to activation of the BMP signalling pathway. Gliotactin specifically interferes with Dad, an inhibitory SMAD, leading to activation of the Tkv type-I receptor and activation of Mad to elevate the biogenesis and expression of miR-184.

AP-2-complex-mediated endocytosis of Drosophila Crumbs regulates polarity by antagonizing Stardust

Maintenance of epithelial polarity depends on the correct localization and levels of polarity determinants. The evolutionarily conserved transmembrane protein Crumbs is crucial for the size and identity of the apical membrane, yet little is known about the molecular mechanisms controlling the amount of Crumbs at the surface. Here, we show that Crumbs levels on the apical membrane depend on a well-balanced state of endocytosis and stabilization. The adaptor protein 2 (AP-2) complex binds to a motif in the cytoplasmic tail of Crumbs that overlaps with the binding site of Stardust, a protein known to stabilize Crumbs on the surface. Preventing endocytosis by mutation of AP-2 causes expansion of the Crumbs-positive plasma membrane domain and polarity defects, which can be partially rescued by removing one copy of crumbs. Strikingly, knocking down both AP-2 and Stardust leads to the retention of Crumbs on the membrane. This study provides evidence for a molecular mechanism, based on stabilization and endocytosis, to adjust surface levels of Crumbs, which are essential for maintaining epithelial polarity.

The Triple-Repeat Protein Anakonda Controls Epithelial Tricellular Junction Formation in Drosophila

In epithelia, specialized tricellular junctions (TCJs) mediate cell contacts at three-cell vertices. TCJs are fundamental to epithelial biology and disease, but only a few TCJ components are known, and how they assemble at tricellular vertices is not understood. Here we describe a transmembrane protein, Anakonda (Aka), which localizes to TCJs and is essential for the formation of tricellular, but not bicellular, junctions in Drosophila. Loss of Aka causes epithelial barrier defects associated with irregular TCJ structure and geometry, suggesting that Aka organizes cell corners. Aka is necessary and sufficient for accumulation of Gliotactin at TCJs, suggesting that Aka initiates TCJ assembly by recruiting other proteins to tricellular vertices. Aka's extracellular domain has an unusual tripartite repeat structure that may mediate self-assembly, directed by the geometry of tricellular vertices. Conversely, Aka's cytoplasmic tail is dispensable for TCJ localization. Thus, extracellular interactions, rather than TCJ-directed intracellular transport, appear to mediate TCJ assembly.

The Drosophila blood-brain barrier: development and function of a glial endothelium

The efficacy of neuronal function requires a well-balanced extracellular ion homeostasis and a steady supply with nutrients and metabolites. Therefore, all organisms equipped with a complex nervous system developed a so-called blood-brain barrier, protecting it from an uncontrolled entry of solutes, metabolites or pathogens. In higher vertebrates, this diffusion barrier is established by polarized endothelial cells that form extensive tight junctions, whereas in lower vertebrates and invertebrates the blood-brain barrier is exclusively formed by glial cells. Here, we review the development and function of the glial blood-brain barrier of Drosophila melanogaster. In the Drosophila nervous system, at least seven morphologically distinct glial cell classes can be distinguished. Two of these glial classes form the blood-brain barrier. Perineurial glial cells participate in nutrient uptake and establish a first diffusion barrier. The subperineurial glial (SPG) cells form septate junctions, which block paracellular diffusion and thus seal the nervous system from the hemolymph. We summarize the molecular basis of septate junction formation and address the different transport systems expressed by the blood-brain barrier forming glial cells.

Molecular organization of tricellular tight junctions

When the apicolateral border of epithelial cells is compared with a polygon, its sides correspond to the apical junctional complex, where cell adhesion molecules assemble from the plasma membranes of two adjacent cells. On the other hand, its vertices correspond to tricellular contacts, where the corners of three cells meet. Vertebrate tricellular contacts have specialized structures of tight junctions, termed tricellular tight junctions (tTJs). tTJs were identified by electron microscopic observations more than 40 years ago, but have been largely forgotten in epithelial cell biology since then. The identification of tricellulin and angulin family proteins as tTJ-associated membrane proteins has enabled us to study tTJs in terms of not only the paracellular barrier function but also unknown characteristics of epithelial cell corners via molecular biological approaches.

Modulation of Anopheles stephensi gene expression by nitroquine, an antimalarial drug against Plasmodium yoelii infection in the mosquito

Background

Antimalarial drugs may impact mosquito’s defense against Plasmodium parasites. Our previous study showed nitroquine significantly reduced infection of Anopheles stephensi by Plasmodium yoelii, but the underlying mechanism remains unclear. In order to understand how transmission capacity of An. stephensi was affected by nitroquine, we explored the transcriptome of adult females after different treatments, examined changes in gene expression profiles, and identified transcripts affected by the drug and parasite.

Methodology/Principal Findings

We extended massively parallel sequencing and data analysis (including gene discovery, expression profiling, and function prediction) to An. stephensi before and after Plasmodium infection with or without nitroquine treatment. Using numbers of reads assembled into specific contigs to calculate relative abundances (RAs), we categorized the assembled contigs into four groups according to the differences in RA values infection induced, infection suppressed, drug induced, and drug suppressed. We found both nitroquine in the blood meal and Plasmodium infection altered transcription of mosquito genes implicated in diverse processes, including pathogen recognition, signal transduction, prophenoloxidase activation, cytoskeleton assembling, cell adhesion, and oxidative stress. The differential gene expression may have promoted certain defense responses of An. stephensi against the parasite and decreased its infectivity.

Conclusions/Significance

Our study indicated that nitroquine may regulate several immune mechanisms at the level of gene transcription in the mosquito against Plasmodium infection. This highlights the need for better understanding of antimalarial drug’s impact on parasite survival and transmission. In addition, our data largely enriched the existing sequence information of An. stephensi, an epidemiologically important vector species.

Heme and FLVCR-related transporter families SLC48 and SLC49

Heme is critical for a variety of cellular processes, but excess intracellular heme may result in oxidative stress and membrane injury. Feline leukemia virus subgroup C receptor (FLVCR1), a member of the SLC49 family of four paralogous genes, is a cell surface heme exporter, essential for erythropoiesis and systemic iron homeostasis. Disruption of FLVCR1 function blocks development of erythroid progenitors, likely due to heme toxicity. Mutations of SLC49A1 encoding FLVCR1 are noted in patients with a rare neurodegenerative disorder: posterior column ataxia with retinitis pigmentosa. FLVCR2 is highly homologous to FLVCR1 and may function as a cellular heme importer. Mutations of SLC49A2 encoding FLVCR2 are observed in Fowler syndrome, a rare proliferative vascular disorder of the brain. The functions of the remaining members of the SLC49 family, MFSD7 and DIRC2 (encoded by the SLC49A3 and SLC49A4 genes), are unknown, although the latter is implicated in hereditary renal carcinomas. SLC48A1 (heme responsive gene-1, HRG-1), the sole member of the SLC48 family, is associated with the endosome and appears to transport heme from the endosome into the cytosol.

Gliotactin and Discs-large are co-regulated to maintain epithelial integrity

Establishment and maintenance of permeability barriers is one of the most important functions of epithelial cells. Tricellular junctions (TCJs) maintain the permeability barriers at the contact site of three epithelial cells. Gliotactin (Gli), a member of the Neuroligin family, is the only known Drosophila protein exclusively localized to the TCJ and is necessary for maintenance of the permeability barrier. Over-expression triggers the spread of Gliotactin away from the TCJ and causes epithelial cells to delaminate, migrate and die. Furthermore, excess Gli at the cell membrane results in an extensive down regulation of Dlg at the septate junctions. The intracellular domain of Gli contains two highly conserved tyrosine residues, and a PDZ binding motif. We previously found that phosphorylation of the tyrosine residues are necessary to control the level of Gliotactin at the TCJ. In this study we demonstrate that the phenotypes associated with excess Gliotactin is due to a functional interaction between Gliotactin and Dlg that is dependent on both tyrosine phosphorylation as well as the PDZ binding motif. We further show that elevated levels of Dlg strongly enhance Gliotactin over-expression phenotypes to the point where tissue over-growth is observed. The exhibition of these phenotypes require phosphorylation of Dlg on serine 797, a known Par1 phosphorylation target. Blocking this phosphorylation completely suppresses the cell invasiveness and apoptotic phenotypes associated with Gliotactin overexpression. Additionally, we show that Drosophila JNK acts downstream of Gliotactin and Dlg to mediate the overgrowth and apoptosis caused by the functional interaction of Gliotactin and Dlg.

Drosophila Src regulates anisotropic apical surface growth to control epithelial tube size

Networks of epithelial and endothelial tubes are essential for the function of organs such as the lung, kidney and vascular system. The sizes and shapes of these tubes are highly regulated to match their individual functions. Defects in tube size can cause debilitating diseases such as polycystic kidney disease and ischaemia. It is therefore critical to understand how tube dimensions are regulated. Here we identify the tyrosine kinase Src as an instructive regulator of epithelial-tube length in the Drosophila tracheal system. Loss-of-function Src42 mutations shorten tracheal tubes, whereas Src42 overexpression elongates them. Surprisingly, Src42 acts distinctly from known tube-size pathways and regulates both the amount of apical surface growth and, with the conserved formin dDaam, the direction of growth. Quantitative three-dimensional image analysis reveals that Src42- and dDaam-mutant tracheal cells expand more in the circumferential than the axial dimension, resulting in tubes that are shorter in length-but larger in diameter-than wild-type tubes. Thus, Src42 and dDaam control tube dimensions by regulating the direction of anisotropic growth, a mechanism that has not previously been described.

Analysis of protein dynamics within the septate junction reveals a highly stable core protein complex that does not include the basolateral polarity protein Discs large

Barrier junctions prevent pathogen invasion and restrict paracellular leakage across epithelial sheets. To understand how one barrier junction, the septate junction (SJ), is regulated in vivo, we used fluorescence recovery after photobleaching (FRAP) to examine SJ protein dynamics in Drosophila. Most SJ-associated proteins, including Coracle, Neurexin IV and Nervana 2, displayed similar, extremely immobile kinetics. Loss of any of these components resulted in dramatically increased mobility of all others, suggesting that they form a single, highly interdependent core complex. Immobilization of SJ core components coincided with formation of the morphological SJ but occurred after their known role in maintaining epithelial polarity, suggesting that these functions are independent. In striking contrast to the core components, the tumor suppressor protein Discs large was much more mobile and its loss did not affect mobility of core SJ proteins, suggesting that it is not a member of this complex, even though it colocalizes with the SJ. Similarly, disruption of endocytosis affected localization of SJ core components, but did not affect their mobility. These results indicate that formation of a stable SJ core complex is separable from its proper subcellular localization, and provide new insights into the complex processes that regulate epithelial polarity and assembly of the SJ.