Rab9 and retromer regulate retrograde trafficking of luminal protein required for epithelial tube length control

Microtubule-dependent balanced cell contraction and luminal-matrix modification accelerate epithelial tube fusion

Connection of tubules into larger networks is the key process for the development of circulatory systems. In Drosophila development, tip cells of the tracheal system lead the migration of each branch and connect tubules by adhering to each other and simultaneously changing into a torus-shape. We show that as adhesion sites form between fusion cells, myosin and microtubules form polarized bundles that connect the new adhesion site to the cells' microtubule-organizing centres, and that E-cadherin and retrograde recycling endosomes are preferentially deposited at the new adhesion site. We demonstrate that microtubules help balancing tip cell contraction, which is driven by myosin, and is required for adhesion and tube fusion. We also show that retrograde recycling and directed secretion of a specific matrix protein into the fusion-cell interface promote fusion. We propose that microtubule bundles connecting these cell–cell interfaces coordinate cell contractility and apical secretion to facilitate tube fusion.

During tracheal tube fusion in Drosophila, a pair of tip cells form an adherens junction and then fuse their plasma membranes. Here the authors show that a balanced pulling force mediated by myosin and microtubules, as well as localized deposition of matrix, promotes plasma membrane fusion.

Retromer-Mediated Protein Sorting and Vesicular Trafficking

Retromer is an evolutionarily conserved multimeric protein complex that mediates intracellular transport of various vesicular cargoes and functions in a wide variety of cellular processes including polarized trafficking, developmental signaling and lysosome biogenesis. Through its interaction with the Rab GTPases and their effectors, membrane lipids, molecular motors, the endocytic machinery and actin nucleation promoting factors, retromer regulates sorting and trafficking of transmembrane proteins from endosomes to the trans-Golgi network (TGN) and the plasma membrane. In this review, I highlight recent progress in the understanding of retromer-mediated protein sorting and vesicle trafficking and discuss how retromer contributes to a diverse set of developmental, physiological and pathological processes.

Retromer and Rab2-dependent trafficking mediate PS1 degradation by proteasomes in endocytic disturbance

Accumulating evidence suggests that endocytic pathway deficits are involved in Alzheimer's disease pathogenesis. Several reports show that endocytic disturbance affects β-amyloid peptide (Aβ) cleavage from β-amyloid precursor protein (APP). Presenilin-1 (PS1) is the catalytic core of the γ-secretase complex required for Aβ generation. Previously, we showed that aging induces endocytic disturbance, resulting in the accumulation of Aβ and APP in enlarged endosomes. It remains unclear, however, whether PS1 localization and function are affected with endocytic disturbance. Here, we report that in endocytic disturbance, PS1 is transported from endosomes to ER/Golgi compartments via retromer trafficking, and that PS1 interacts with vacuolar protein sorting-associated protein 35 both in vitro and in vivo. Moreover, PS1 is degraded by proteasomes via a Rab2-dependent trafficking pathway, only during endocytic disturbance. These findings suggest that PS1 levels and localization in endosomes are regulated by retromer trafficking and ER-associated degradation system, even if endocytic disturbance significantly induces the endosomal accumulation of APP and β-site APP-cleaving enzyme 1. Results of this study also suggest that retromer deficiency can affect PS1 localization in endosomes, where Aβ cleavage mainly occurs, possibly leading to enhanced Aβ pathology. We proposed the following mechanism for intracellular transport of presenilin-1 (PS1). When endosome/lysosome trafficking is disturbed, PS1 is transported from endosome to endoplasmic reticulum (ER)/Golgi compartments via retromer and Rab2-mediated trafficking, and then degraded by endoplasmic reticulum-associated degradation (ERAD). Perturbations in this trafficking can cause abnormal endosomal accumulation of PS1, and then may lead to exacerbated Aβ pathology. Cover Image for this issue: doi: 10.1111/jnc.13318.

Subversion of Retrograde Trafficking by Translocated Pathogen Effectors

Intracellular bacterial pathogens subvert the endocytic bactericidal pathway to form specific replication-permissive compartments termed pathogen vacuoles or inclusions. To this end, the pathogens employ type III or type IV secretion systems, which translocate dozens, if not hundreds, of different effector proteins into their host cells, where they manipulate vesicle trafficking and signaling pathways in favor of the intruders. While the distinct cocktail of effectors defines the specific processes by which a pathogen vacuole is formed, the different pathogens commonly target certain vesicle trafficking routes, including the endocytic or secretory pathway. Recently, the retrograde transport pathway from endosomal compartments to the trans-Golgi network emerged as an important route affecting pathogen vacuole formation. Here, we review current insight into the host cell's retrograde trafficking pathway and how vacuolar pathogens of the genera Legionella, Coxiella, Salmonella, Chlamydia, and Simkania employ mechanistically distinct strategies to subvert this pathway, thus promoting intracellular survival and replication.

Retromer in Polarized Protein Transport

Retromer is an evolutionary conserved protein complex required for endosome-to-Golgi retrieval of receptors for lysosomal hydrolases. It is constituted by a heterotrimer encoded by the vacuolar protein sorting (VPS) gene products Vps26, Vps35, and Vps29, which selects cargo, and a dimer of phosphoinositide-binding sorting nexins, which deforms the membrane. Recent progress in the mechanism of retromer assembly and functioning has strengthened the link between sorting at the endosome and cytoskeleton dynamics. Retromer is implicated in endosomal sorting of many cargos and plays an essential role in plant and animal development. Although it is best known for endosome sorting to the trans-Golgi network, it also intervenes in recycling to the plasma membrane. In polarized cells, such as epithelial cells and neurons, retromer may also be utilized for transcytosis and long-range transport. Considerable evidence implicates retromer in establishment and maintenance of cell polarity. That includes sorting of the apical polarity module Crumbs; regulation of retromer function by the basolateral polarity module Scribble; and retromer-dependent recycling of various cargoes to a certain surface domain, thus controlling polarized location and cell homeostasis. Importantly, altered retromer function has been linked to neurodegeneration, such as in Alzheimer's or Parkinson's disease. This review will underline how alterations in retromer localization and function may affect polarized protein transport and polarity establishment, thereby causing developmental defects and disease.

Spatiotemporal Resolution of Rab9 and CI-MPR Dynamics in the Endocytic Pathway

Rab9 is a small GTPase that localizes to the trans-Golgi Network (TGN) and late endosomes. Its main function has long been connected to the recycling of mannose-6-phosphate receptors (MPRs). However, recent studies link Rab9 also to autophagy and lysosome biogenesis. In this paper, using confocal imaging, we characterize for the first time the live dynamics of the Rab9 constitutively active mutant, Rab9Q66L. We find that it localizes predominantly to late endosomes and that its expression in HeLa cells disperses TGN46 and cation-independent (CI-MPR) away from the Golgi yet, has no effect on the retrograde transport of CI-MPR. We also show that CI-MPR and Rab9 enter the endosomal pathway together at the transition stage between early, Rab5-positive, and late, Rab7a-positive, endosomes. CI-MPR localizes transiently to separate domains on these endosomes, where vesicles carrying CI-MPR attach and detach within seconds. Taken together, our results demonstrate that Rab9 mediates the delivery of CI-MPR to the endosomal pathway, entering the maturing endosome at the early-to-late transition.

Molecular Control of Actin Dynamics In Vivo: Insights from Drosophila

The actin cytoskeleton provides mechanical support for cells and generates forces to drive cell shape changes and cell migration in morphogenesis. Molecular understanding of actin dynamics requires a genetically traceable model system that allows interdisciplinary experimental approaches to elucidate the regulatory network of cytoskeletal proteins in vivo. Here, we will discuss some examples of how advances in Drosophila genetics and high-resolution imaging techniques contribute to the discovery of new actin functions, signaling pathways, and mechanisms of actin regulation in vivo.

Tissue-specific tagging of endogenous loci in Drosophila melanogaster

Fluorescent protein tags have revolutionized cell and developmental biology, and in combination with binary expression systems they enable diverse tissue-specific studies of protein function. However these binary expression systems often do not recapitulate endogenous protein expression levels, localization, binding partners and/or developmental windows of gene expression. To address these limitations, we have developed a method called T-STEP (tissue-specific tagging of endogenous proteins) that allows endogenous loci to be tagged in a tissue specific manner. T-STEP uses a combination of efficient CRISPR/Cas9-enhanced gene targeting and tissue-specific recombinase-mediated tag swapping to temporally and spatially label endogenous proteins. We have employed this method to GFP tag OCRL (a phosphoinositide-5-phosphatase in the endocytic pathway) and Vps35 (a Parkinson's disease-implicated component of the endosomal retromer complex) in diverse Drosophila tissues including neurons, glia, muscles and hemocytes. Selective tagging of endogenous proteins allows, for the first time, cell type-specific live imaging and proteomics in complex tissues.

Retromer and sorting nexins in endosomal sorting

The evolutionarily conserved endosomal retromer complex rescues transmembrane proteins from the lysosomal degradative pathway and facilitates their recycling to other cellular compartments. Retromer functions in conjunction with numerous associated proteins, including select members of the sorting nexin (SNX) family. In the present article, we review the molecular architecture and cellular roles of retromer and its various functional partners. The endosomal network is a crucial hub in the trafficking of proteins through the cellular endomembrane system. Transmembrane proteins, here termed cargos, enter endosomes by endocytosis from the plasma membrane or by trafficking from the trans-Golgi network (TGN). Endosomal cargo proteins face one of the two fates: retention in the endosome, leading ultimately to lysosomal degradation or export from the endosome for reuse ('recycling'). The balance of protein degradation and recycling is crucial to cellular homoeostasis; inappropriate sorting of proteins to either fate leads to cellular dysfunction. Retromer is an endosome-membrane-associated protein complex central to the recycling of many cargo proteins from endosomes, both to the TGN and the plasma membrane (and other specialized compartments, e.g. lysosome-related organelles). Retromer function is reliant on a number of proteins from the SNX family. In the present article, we discuss this inter-relationship and how defects in retromer function are increasingly being linked with human disease.

The knottin-like Blufensin family regulates genes involved in nuclear import and the secretory pathway in barley-powdery mildew interactions

Plants have evolved complex regulatory mechanisms to control a multi-layered defense response to microbial attack. Both temporal and spatial gene expression are tightly regulated in response to pathogen ingress, modulating both positive and negative control of defense. BLUFENSINs, small knottin-like peptides in barley, wheat, and rice, are highly induced by attack from fungal pathogens, in particular, the obligate biotrophic fungus, Blumeria graminis f. sp. hordei (Bgh), causal agent of barley powdery mildew. Previous research indicated that Blufensin1 (Bln1) functions as a negative regulator of basal defense mechanisms. In the current report, we show that BLN1 and BLN2 can both be secreted to the apoplast and Barley stripe mosaic virus (BSMV)-mediated overexpression of Bln2 increases susceptibility of barley to Bgh. Bimolecular fluorescence complementation (BiFC) assays signify that BLN1 and BLN2 can interact with each other, and with calmodulin. We then used BSMV-induced gene silencing to knock down Bln1, followed by Barley1 GeneChip transcriptome analysis, to identify additional host genes influenced by Bln1. Analysis of differential expression revealed a gene set enriched for those encoding proteins annotated to nuclear import and the secretory pathway, particularly Importin α1-b and Sec61 γ subunits. Further functional analysis of these two affected genes showed that when silenced, they also reduced susceptibility to Bgh. Taken together, we postulate that Bln1 is co-opted by Bgh to facilitate transport of disease-related host proteins or effectors, influencing the establishment of Bgh compatibility on its barley host.

Wash functions downstream of Rho1 GTPase in a subset of Drosophila immune cell developmental migrations

Drosophila immune cells undergo four stereotypical developmental migrations to populate the embryo. Wash is a downstream effector of Rho1 and establishes Rho1>Wash>Arp2/3 as the regulatory pathway controlling the cytoskeleton during one of these developmental hemocyte migrations in a WASH regulatory complex–independent manner.

Drosophila immune cells, the hemocytes, undergo four stereotypical developmental migrations to populate the embryo, where they provide immune reconnoitering, as well as a number of non–immune-related functions necessary for proper embryogenesis. Here, we describe a role for Rho1 in one of these developmental migrations in which posteriorly located hemocytes migrate toward the head. This migration requires the interaction of Rho1 with its downstream effector Wash, a Wiskott–Aldrich syndrome family protein. Both Wash knockdown and a Rho1 transgene harboring a mutation that prevents Wash binding exhibit the same developmental migratory defect as Rho1 knockdown. Wash activates the Arp2/3 complex, whose activity is needed for this migration, whereas members of the WASH regulatory complex (SWIP, Strumpellin, and CCDC53) are not. Our results suggest a WASH complex–independent signaling pathway to regulate the cytoskeleton during a subset of hemocyte developmental migrations.

A fat body-derived apical extracellular matrix enzyme is transported to the tracheal lumen and is required for tube morphogenesis in Drosophila

The apical extracellular matrix plays a central role in epithelial tube morphogenesis. In the Drosophila tracheal system, Serpentine (Serp), a secreted chitin deacetylase expressed by the tracheal cells plays a key role in regulating tube length. Here, we show that the fly fat body, which is functionally equivalent to the mammalian liver, also contributes to tracheal morphogenesis. Serp was expressed by the fat body, and the secreted Serp was taken up by the tracheal cells and translocated to the lumen to functionally support normal tracheal development. This process was defective in rab9 and shrub/vps32 mutants and in wild-type embryos treated with a secretory pathway inhibitor, leading to an abundant accumulation of Serp in the fat body. We demonstrated that fat body-derived Serp reached the tracheal lumen after establishment of epithelial barrier function and was retained in the lumen in a chitin synthase-dependent manner. Our results thus reveal that the fat body, a mesodermal organ, actively contributes to tracheal development.

Rab proteins and the compartmentalization of the endosomal system

Of the approximately 70 human Rab GTPases, nearly three-quarters are involved in endocytic trafficking. Significant plasticity in endosomal membrane transport pathways is closely coupled to receptor signaling and Rab GTPase-regulated scaffolds. Here we review current literature pertaining to endocytic Rab GTPase localizations, functions, and coordination with regulatory proteins and effectors. The roles of Rab GTPases in (1) compartmentalization of the endocytic pathway into early, recycling, late, and lysosomal routes; (2) coordination of individual transport steps from vesicle budding to fusion; (3) effector interactomes; and (4) integration of GTPase and signaling cascades are discussed.

Rab proteins and the compartmentalization of the endosomal system

Of the approximately 70 human Rab GTPases, nearly three-quarters are involved in endocytic trafficking. Significant plasticity in endosomal membrane transport pathways is closely coupled to receptor signaling and Rab GTPase-regulated scaffolds. Here we review current literature pertaining to endocytic Rab GTPase localizations, functions, and coordination with regulatory proteins and effectors. The roles of Rab GTPases in (1) compartmentalization of the endocytic pathway into early, recycling, late, and lysosomal routes; (2) coordination of individual transport steps from vesicle budding to fusion; (3) effector interactomes; and (4) integration of GTPase and signaling cascades are discussed.

In vivo evidence of pathogenicity of VPS35 mutations in the Drosophila

Mutations of VPS35, a component of the retromer complex have been associated with late onset familial Parkinson's disease. The D620N mutation in VPS35 appears to be most prevalent, however, P316S was found in two cases within the same family and a control, whereas L774M was identified in 6 cases and 1 control. In vivo evidence of their pathogenicity is lacking. Here we investigated the in vivo effects of P316S, D620N and L774M using Drosophila as a model. We generated transgenic human VPS35-expressing mutations and demonstrated that VPS35 D620N transgenic flies led to late-onset loss of TH-positive DA neurons, poor mobility, shortened lifespans and increased sensitivity to rotenone, a PD-linked environmental toxin, with some of these phenotypes observed for P316S but not in L774M transgenic flies. We conclude that D620N and to a smaller extent P316S are associated with pathogenicity in PD.

The Scribble module regulates retromer-dependent endocytic trafficking during epithelial polarization

Scribble (Scrib) module proteins are major regulators of cell polarity, but how they influence membrane traffic is not known. Endocytosis is also a key regulator of polarity through roles that remain unclear. Here we link Scrib to a specific arm of the endocytic trafficking system. Drosophila mutants that block AP-2-dependent endocytosis share many phenotypes with Scrib module mutants, but Scrib module mutants show intact internalization and endolysosomal transport. However, defective traffic of retromer pathway cargo is seen, and retromer components show strong genetic interactions with the Scrib module. The Scrib module is required for proper retromer localization to endosomes and promotes appropriate cargo sorting into the retromer pathway via both aPKC-dependent and -independent mechanisms. We propose that the Scrib module regulates epithelial polarity by influencing endocytic itineraries of Crumbs and other retromer-dependent cargo.

Seamless tube shape is constrained by endocytosis-dependent regulation of active Moesin

Most tubes have seams (intercellular or autocellular junctions that seal membranes together into a tube), but "seamless" tubes also exist. In Drosophila, stellate-shaped tracheal terminal cells make seamless tubes, with single branches running through each of dozens of cellular extensions. We find that mutations in braided impair terminal cell branching and cause formation of seamless tube cysts. We show that braided encodes Syntaxin7 and that cysts also form in cells deficient for other genes required either for membrane scission (shibire) or for early endosome formation (Rab5, Vps45, and Rabenosyn-5). These data define a requirement for early endocytosis in shaping seamless tube lumens. Importantly, apical proteins Crumbs and phospho-Moesin accumulate to aberrantly high levels in braided terminal cells. Overexpression of either Crumbs or phosphomimetic Moesin induced lumenal cysts and decreased terminal branching. Conversely, the braided seamless tube cyst phenotype was suppressed by mutations in crumbs or Moesin. Indeed, mutations in Moesin dominantly suppressed seamless tube cyst formation and restored terminal branching. We propose that early endocytosis maintains normal steady-state levels of Crumbs, which recruits apical phosphorylated (active) Moe, which in turn regulates seamless tube shape through modulation of cortical actin filaments.

Balance between apical membrane growth and luminal matrix resistance determines epithelial tubule shape

The morphological stability of biological tubes is crucial for the efficient circulation of fluids and gases. Failure of this stability causes irregularly shaped tubes found in multiple pathological conditions. Here, we report that Drosophila mutants of the ESCRT III component Shrub/Vps32 exhibit a strikingly elongated sinusoidal tube phenotype. This is caused by excessive apical membrane synthesis accompanied by the ectopic accumulation and overactivation of Crumbs in swollen endosomes. Furthermore, we demonstrate that the apical extracellular matrix (aECM) of the tracheal tube is a viscoelastic material coupled with the apical membrane. We present a simple mechanical model in which aECM elasticity, apical membrane growth, and their interaction are three vital parameters determining the stability of biological tubes. Our findings demonstrate a mechanical role for the extracellular matrix and suggest that the interaction of the apical membrane and an elastic aECM determines the final morphology of biological tubes independent of cell shape.

Retromer: a master conductor of endosome sorting

The endosomal network comprises an interconnected network of membranous compartments whose primary function is to receive, dissociate, and sort cargo that originates from the plasma membrane and the biosynthetic pathway. A major challenge in cell biology is to achieve a thorough molecular description of how this network operates, and in so doing, how defects contribute to the etiology and pathology of human disease. We discuss the increasing body of evidence that implicates an ancient evolutionary conserved complex, termed "retromer," as a master conductor in the complex orchestration of multiple cargo-sorting events within the endosomal network.

Novel mechanisms of tube-size regulation revealed by the Drosophila trachea

The size of various tubes within tubular organs such as the lung, vascular system and kidney must be finely tuned for the optimal delivery of gases, nutrients, waste and cells within the entire organism. Aberrant tube sizes lead to devastating human illnesses, such as polycystic kidney disease, fibrocystic breast disease, pancreatic cystic neoplasm and thyroid nodules. However, the underlying mechanisms that are responsible for tube-size regulation have yet to be fully understood. Therefore, no effective treatments are available for disorders caused by tube-size defects. Recently, the Drosophila tracheal system has emerged as an excellent in vivo model to explore the fundamental mechanisms of tube-size regulation. Here, we discuss the role of the apical luminal matrix, cell polarity and signaling pathways in regulating tube size in Drosophila trachea. Previous studies of the Drosophila tracheal system have provided general insights into epithelial tube morphogenesis. Mechanisms that regulate tube size in Drosophila trachea could be well conserved in mammalian tubular organs. This knowledge should greatly aid our understanding of tubular organogenesis in vertebrates and potentially lead to new avenues for the treatment of human disease caused by tube-size defects.

Control of airway tube diameter and integrity by secreted chitin-binding proteins in Drosophila

The transporting function of many branched tubular networks like our lungs and circulatory system depend on the sizes and shapes of their branches. Understanding the mechanisms of tube size control during organ development may offer new insights into a variety of human pathologies associated with stenoses or cystic dilations in tubular organs. Here, we present the first secreted luminal proteins involved in tube diametric expansion in the Drosophila airways. obst-A and gasp are conserved among insect species and encode secreted proteins with chitin binding domains. We show that the widely used tracheal marker 2A12, recognizes the Gasp protein. Analysis of obst-A and gasp single mutants and obst-A; gasp double mutant shows that both genes are primarily required for airway tube dilation. Similarly, Obst-A and Gasp control epidermal cuticle integrity and larval growth. The assembly of the apical chitinous matrix of the airway tubes is defective in gasp and obst-A mutants. The defects become exaggerated in double mutants indicating that the genes have partially redundant functions in chitin structure modification. The phenotypes in luminal chitin assembly in the airway tubes are accompanied by a corresponding reduction in tube diameter in the mutants. Conversely, overexpression of Obst-A and Gasp causes irregular tube expansion and interferes with tube maturation. Our results suggest that the luminal levels of matrix binding proteins determine the extent of diametric growth. We propose that Obst-A and Gasp organize luminal matrix assembly, which in turn controls the apical shapes of adjacent cells during tube diameter expansion.