Tango1 coordinates the formation of endoplasmic reticulum/Golgi docking sites to mediate secretory granule formation

The FAM114A proteins are adaptors for the recycling of Golgi enzymes

Golgi-resident enzymes remain in place while their substrates flow through from the endoplasmic reticulum to elsewhere in the cell. COPI-coated vesicles bud from the Golgi to recycle Golgi residents to earlier cisternae. Different enzymes are present in different parts of the stack, and one COPI adaptor protein, GOLPH3, acts to recruit enzymes into vesicles in part of the stack. Here, we used proximity biotinylation to identify further components of intra-Golgi vesicles and found FAM114A2, a cytosolic protein. Affinity chromatography with FAM114A2, and its paralogue FAM114A1, showed that they bind to Golgi-resident membrane proteins, with membrane-proximal basic residues in the cytoplasmic tail being sufficient for the interaction. Deletion of both proteins from U2OS cells did not cause substantial defects in Golgi function. However, a Drosophila orthologue of these proteins (CG9590/FAM114A) is also localised to the Golgi and binds directly to COPI. Drosophila mutants lacking FAM114A have defects in glycosylation of glue proteins in the salivary gland. Thus, the FAM114A proteins bind Golgi enzymes and are candidate adaptors to contribute specificity to COPI vesicle recycling in the Golgi stack.

Anaplasma phagocytophilum effector EgeA facilitates infection by hijacking TANGO1 and SCFD1 from ER-Golgi exit sites to pathogen-occupied inclusions

The obligatory intracellular bacterium Anaplasma phagocytophilum causes human granulocytic anaplasmosis, an emerging zoonosis. Anaplasma has limited biosynthetic and metabolic capacities, yet it effectively replicates inside of inclusions/vacuoles of eukaryotic host cells. Here, we describe a unique Type IV secretion system (T4SS) effector, ER-Golgi exit site protein of Anaplasma (EgeA). In cells infected by Anaplasma, secreted native EgeA, EgeA-GFP, and the C-terminal half of EgeA (EgeA-C)-GFP localized to Anaplasma-containing inclusions. In uninfected cells, EgeA-C-GFP localized to cis-Golgi, whereas the N-terminal half of EgeA-GFP localized to the ER. Pull-down assays identified EgeA-GFP binding to a transmembrane protein in the ER, Transport and Golgi organization protein 1 (TANGO1). By yeast two-hybrid analysis, EgeA-C directly bound Sec1 family domain-containing protein 1 (SCFD1), a host protein of the cis-Golgi network that binds TANGO1 at ER-Golgi exit sites (ERES). Both TANGO1 and SCFD1 localized to the Anaplasma inclusion surface. Furthermore, knockdown of Anaplasma EgeA or either host TANGO1 or SCFD1 significantly reduced Anaplasma infection. TANGO1 and SCFD1 prevent ER congestion and stress by facilitating transport of bulky or unfolded proteins at ERES. A bulky cargo collagen and the ER-resident chaperon BiP were transported into Anaplasma inclusions, and several ER stress marker genes were not up-regulated in Anaplasma-infected cells. Furthermore, EgeA transfection reduced collagen overexpression-induced BiP upregulation. These results suggest that by binding to the two ERES proteins, EgeA redirects the cargo-adapted ERES to pathogen-occupied inclusions and reduces ERES congestion, which facilitates Anaplasma nutrient acquisition and reduces ER stress for Anaplasma survival and proliferation.

Emerging cell and molecular targets for treating mucus hypersecretion in asthma

Mucus provides a protective barrier that is crucial for host defense in the lungs. However, excessive or abnormal mucus can have pathophysiological consequences in many pulmonary diseases, including asthma. Patients with asthma are treated with agents that relax airway smooth muscle and reduce airway inflammation, but responses are often inadequate. In part, this is due to the inability of existing therapeutic agents to directly target mucus. Accordingly, there is a critical need to better understand how mucus hypersecretion and airway plugging are affected by the epithelial cells that synthesize, secrete, and transport mucus components. This review highlights recent advances in the biology of mucin glycoproteins with a specific focus on MUC5AC and MUC5B, the chief macromolecular components of airway mucus. An improved mechanistic understanding of key steps in mucin production and secretion will help reveal novel potential therapeutic strategies.

p24-Tango1 interactions ensure ER-Golgi interface stability and efficient transport

The eukaryotic p24 family, consisting of α-, β-, γ- and δ-p24 subfamilies, has long been known to be involved in regulating secretion. Despite increasing interest in these proteins, fundamental questions remain about their role. Here, we systematically investigated Drosophila p24 proteins. We discovered that members of all four p24 subfamilies are required for general secretion and that their localizations between ER exit site (ERES) and Golgi are interdependent in an α→βδ→γ sequence. We also found that localization of p24 proteins and ERES determinant Tango1 requires interaction through their respective GOLD and SH3 lumenal domains, with Tango1 loss sending p24 proteins to the plasma membrane and vice versa. Finally, we show that p24 loss expands the COPII zone at ERES and increases the number of ER-Golgi vesicles, supporting a restrictive role of p24 proteins on vesicle budding for efficient transport. Our results reveal Tango1-p24 interplay as central to the generation of a stable ER-Golgi interface.

TANGO1 Dances to Export of Procollagen from the Endoplasmic Reticulum

The endoplasmic reticulum (ER) to Golgi secretory pathway is an elegantly complex process whereby protein cargoes are manufactured, folded, and distributed from the ER to the cisternal layers of the Golgi stack before they are delivered to their final destinations. The export of large bulky cargoes such as procollagen and its trafficking to the Golgi is a sophisticated mechanism requiring TANGO1 (Transport ANd Golgi Organization protein 1. It is also called MIA3 (Melanoma Inhibitory Activity protein 3). TANGO1 has two prominent isoforms, TANGO1-Long and TANGO1-Short, and each isoform has specific functions. On the luminal side, TANGO1-Long has an HSP47 recruitment domain and uses this protein to collect collagen. It can also tether its paralog isoforms cTAGE5 and TALI and along with these proteins enlarges the vesicle to accommodate procollagen. Recent studies show that TANGO1-Long combines retrograde membrane flow with anterograde cargo transport. This complex mechanism is highly activated in fibrosis and promotes the excessive deposition of collagen in the tissues. The therapeutic targeting of TANGO1 may prove successful in the control of fibrotic disorders. This review focuses on TANGO1 and its complex interaction with other procollagen export factors that modulate increased vesicle size to accommodate the export of procollagen.

Unique asymmetric distribution of phosphatidylserine and phosphatidylethanolamine in Toxoplasma gondii revealed by nanoscale analysis

Toxoplasma gondii is a highly prevalent obligate apicomplexan parasite that is important in clinical and veterinary medicine. It is known that glycerophospholipids phosphatidylserine (PtdSer) and phosphatidylethanolamine (PtdEtn), especially their expression levels and flip-flops between cytoplasmic and exoplasmic leaflets, in the membrane of T. gondii play important roles in efficient growth in host mammalian cells, but their distributions have still not been determined because of technical difficulties in studying intracellular lipid distribution at the nanometer level. In this study, we developed an electron microscopy method that enabled us to determine the distributions of PtdSer and PtdEtn in individual leaflets of cellular membranes by using quick-freeze freeze-fracture replica labeling. Our findings show that PtdSer and PtdEtn are asymmetrically distributed, with substantial amounts localized at the luminal leaflet of the inner membrane complex (IMC), which comprises flattened vesicles located just underneath the plasma membrane (see Figs. 2B and 7). We also found that PtdSer was absent in the cytoplasmic leaflet of the inner IMC membrane, but was present in considerable amounts in the cytoplasmic leaflet of the middle IMC membrane, suggesting a barrier-like mechanism preventing the diffusion of PtdSer in the cytoplasmic leaflets of the two membranes. In addition, the expression levels of both PtdSer and PtdEtn in the luminal leaflet of the IMC membrane in the highly virulent RH strain were higher than those in the less virulent PLK strain. We also found that the amount of glycolipid GM3, a lipid raft component, was higher in the RH strain than in the PLK strain. These results suggest a correlation between lipid raft maintenance, virulence, and the expression levels of PtdSer and PtdEtn in T. gondii.

TANGO1 interacts with NRTN to promote hepatocellular carcinoma progression by regulating the PI3K/AKT/mTOR signaling pathway

Transport and Golgi organization 1 (TANGO1) also known as MIA3, belongs to the melanoma inhibitory activity gene (MIA) family together with MIA, MIA2 and OTOR; these members play different roles in different tumors, but the mechanism underlying TANGO1s effect on hepatocellular carcinoma (HCC) is unclear. Our study confirmed that TANGO1 is a promoter of HCC, In HCC cells, TANGO1 can promote proliferation, inhibit apoptosis, promote EMT. These changes were reversed after TANGO1 inhibition. We explored the molecular mechanism of TANGO1 and HCC and found that the promoting effect of TANGO1 on HCC related to neurturin (NRTN) and the PI3K/AKT/mTOR signaling pathway based on RNA-seq results. NRTN is not only related to neuronal growth, differentiation and maintenance but is also involved in a variety of tumorigenic processes, and PI3K/AKT/mTOR signaling pathway has been shown to be involved in HCC progression. We verified that TANGO1 interacts with NRTN in HCC cells using endogenous Co-IP and confocal localization, and both promote HCC progression by activating the PI3K/AKT/mTOR signaling pathway. Our results reveal the mechanism by which TANGO1 promotes HCC progression, suggesting that the TANGO1/NRTN axis may be a potential therapeutic target for HCC worthy of further investigation.

Characterization of a fold in TANGO1 evolved from SH3 domains for the export of bulky cargos

Bulky cargos like procollagens, apolipoproteins, and mucins exceed the size of conventional COPII vesicles. During evolution a process emerged in metazoans, predominantly governed by the TANGO1 protein family, that organizes cargo at the exit sites of the endoplasmic reticulum and facilitates export by the formation of tunnel-like connections between the ER and Golgi. Hitherto, cargo-recognition appeared to be mediated by an SH3-like domain. Based on structural and dynamic data as well as interaction studies from NMR spectroscopy and microscale thermophoresis presented here, we show that the luminal cargo-recognition domain of TANGO1 adopts a new functional fold for which we suggest the term MOTH (MIA, Otoraplin, TALI/TANGO1 homology) domain. These MOTH domains, as well as an evolutionary intermediate found in invertebrates, constitute a distinct domain family that emerged from SH3 domains and acquired the ability to bind collagen.

Acute manipulation and real-time visualization of membrane trafficking and exocytosis in Drosophila

Intracellular trafficking of secretory proteins plays key roles in animal development and physiology, but so far, tools for investigating the dynamics of membrane trafficking have been limited to cultured cells. Here, we present a system that enables acute manipulation and real-time visualization of membrane trafficking through the reversible retention of proteins in the endoplasmic reticulum (ER) in living multicellular organisms. By adapting the "retention using selective hooks" (RUSH) approach to Drosophila, we show that trafficking of GPI-linked, secreted, and transmembrane proteins can be controlled with high temporal precision in intact animals and cultured organs. We demonstrate the potential of this approach by analyzing the kinetics of ER exit and apical secretion and the spatiotemporal dynamics of tricellular junction assembly in epithelia of living embryos. Furthermore, we show that controllable ER retention enables tissue-specific depletion of secretory protein function. The system is broadly applicable to visualizing and manipulating membrane trafficking in diverse cell types in vivo.

The small GTPase Sar1, control centre of COPII trafficking

Sar1 is a small GTPase of the ARF family. Upon exchange of GDP for GTP, Sar1 associates with the endoplasmic reticulum (ER) membrane and recruits COPII components, orchestrating cargo concentration and membrane deformation. Many aspects of the role of Sar1 and regulation of its GTP cycle remain unclear, especially as complexity increases in higher organisms that secrete a wider range of cargoes. This review focusses on the regulation of GTP hydrolysis and its role in coat assembly, as well as the mechanism of Sar1-induced membrane deformation and scission. Finally, we highlight the additional specialisation in higher eukaryotes and the outstanding questions on how Sar1 functions are orchestrated.

Immunofluorescence as a Method to Study Golgi Organization in Larval Salivary Glands of Drosophila melanogaster

Immunofluorescence is an important research tool in cell biology that reveals structural organization of subcellular organelles by detecting their associated constituents. Here, we describe an antibody staining method to detect Golgi-associated proteins in Drosophila larval salivary glands, using the cis-Golgi protein Lava lamp and the clathrin adaptor AP-1 as a suitable example. Golgi bodies immunostained using this protocol can be visualized using confocal or structured illumination microscopy.

In vivo characterization of Drosophila golgins reveals redundancy and plasticity of vesicle capture at the Golgi apparatus

The Golgi is the central sorting station in the secretory pathway and thus the destination of transport vesicles arriving from the endoplasmic reticulum and endosomes and from within the Golgi itself. Cell viability, therefore, requires that the Golgi accurately receives multiple classes of vesicle. One set of proteins proposed to direct vesicle arrival at the Golgi are the golgins, long coiled-coil proteins localized to specific parts of the Golgi stack. In mammalian cells, three of the golgins, TMF, golgin-84, and GMAP-210, can capture intra-Golgi transport vesicles when placed in an ectopic location. However, the individual golgins are not required for cell viability, and mouse knockout mutants only have defects in specific tissues. To further illuminate this system, we examine the Drosophila orthologs of these three intra-Golgi golgins. We show that ectopic forms can capture intra-Golgi transport vesicles, but strikingly, the cargo present in the vesicles captured by each golgin varies between tissues. Loss-of-function mutants show that the golgins are individually dispensable, although the loss of TMF recapitulates the male fertility defects observed in mice. However, the deletion of multiple golgins results in defects in glycosylation and loss of viability. Examining the vesicles captured by a particular golgin when another golgin is missing reveals that the vesicle content in one tissue changes to resemble that of a different tissue. This reveals a plasticity in Golgi organization between tissues, providing an explanation for why the Golgi is sufficiently robust to tolerate the loss of many of the individual components of its membrane traffic machinery.

Regulated Restructuring of Mucins During Secretory Granule Maturation In Vivo

Mucins are large, highly glycosylated transmembrane and secreted proteins that line and protect epithelial surfaces. However, the details of mucin biosynthesis and packaging in vivo are largely unknown. Here, we demonstrate that multiple distinct mucins undergo intragranular restructuring during secretory granule maturation in vivo, forming unique structures that are spatially segregated within the same granule. We further identify temporally-regulated genes that influence mucin restructuring, including those controlling pH (Vha16-1), Ca²⁺ ions (fwe) and Cl^- ions (Clic and ClC-c). Finally, we show that altered mucin glycosylation influences the dimensions of these structures, thereby affecting secretory granule morphology. This study elucidates key steps and factors involved in intragranular, rather than intergranular segregation of mucins through regulated restructuring events during secretory granule maturation. Understanding how multiple distinct mucins are efficiently packaged into and secreted from secretory granules may provide insight into diseases resulting from defects in mucin secretion.

Drosophila Glue: A Promising Model for Bioadhesion

The glue produced by Drosophila larvae to attach themselves to a substrate for several days and resist predation until the end of metamorphosis represents an attractive model to develop new adhesives for dry environments. The adhesive properties of this interesting material have been investigated recently, and it was found that it binds as well as strongly adhesive commercial tapes to various types of substrates. This glue hardens rapidly after excretion and is made of several proteins. In D. melanogaster, eight glue proteins have been identified: four are long glycosylated mucoproteins containing repeats rich in prolines, serines and threonines, and four others are shorter proteins rich in cysteines. This protein mix is produced by the salivary glands through a complex packaging process that is starting to be elucidated. Drosophila species have adapted to stick to various substrates in diverse environmental conditions and glue genes appear to evolve rapidly in terms of gene number, number of repeats and sequence of the repeat motifs. Interestingly, besides its adhesive properties, the glue may also have antimicrobial activities. We discuss future perspectives and avenues of research for the development of new bioadhesives mimicking Drosophila fly glue.

In vivo models of mucin biosynthesis and function

The secreted mucus layer that lines and protects epithelial cells is conserved across diverse species. While the exact composition of this protective layer varies between organisms, certain elements are conserved, including proteins that are heavily decorated with N-acetylgalactosamine-based sugars linked to serines or threonines (O-linked glycosylation). These heavily O-glycosylated proteins, known as mucins, exist in many forms and are able to form hydrated gel-like structures that coat epithelial surfaces. In vivo studies in diverse organisms have highlighted the importance of both the mucin proteins as well as their constituent O-glycans in the protection and health of internal epithelia. Here, we summarize in vivo approaches that have shed light on the synthesis and function of these essential components of mucus.

A time to heal: microRNA and circadian dynamics in cutaneous wound repair

Many biological systems have evolved circadian rhythms based on the daily cycles of daylight and darkness on Earth. Such rhythms are synchronised or entrained to 24-h cycles, predominantly by light, and disruption of the normal circadian rhythms has been linked to elevation of multiple health risks. The skin serves as a protective barrier to prevent microbial infection and maintain homoeostasis of the underlying tissue and the whole organism. However, in chronic non-healing wounds such as diabetic foot ulcers (DFUs), pressure sores, venous and arterial ulcers, a variety of factors conspire to prevent wound repair. On the other hand, keloids and hypertrophic scars arise from overactive repair mechanisms that fail to cease in a timely fashion, leading to excessive production of extracellular matrix (ECM) components such as such as collagen. Recent years have seen huge increases in our understanding of the functions of microRNAs (miRNAs) in wound repair. Concomitantly, there has been growing recognition of miRNA roles in circadian processes, either as regulators or targets of clock activity or direct responders to external circadian stimuli. In addition, miRNAs are now known to function as intercellular signalling mediators through extracellular vesicles (EVs). In this review, we explore the intersection of mechanisms by which circadian and miRNA responses interact with each other in relation to wound repair in the skin, using keratinocytes, macrophages and fibroblasts as exemplars. We highlight areas for further investigation to support the development of translational insights to support circadian medicine in the context of these cells.

Native proline-rich motifs exploit sequence context to target actin-remodeling Ena/VASP protein ENAH

The human proteome is replete with short linear motifs (SLiMs) of four to six residues that are critical for protein-protein interactions, yet the importance of the sequence surrounding such motifs is underexplored. We devised a proteomic screen to examine the influence of SLiM sequence context on protein-protein interactions. Focusing on the EVH1 domain of human ENAH, an actin regulator that is highly expressed in invasive cancers, we screened 36-residue proteome-derived peptides and discovered new interaction partners of ENAH and diverse mechanisms by which context influences binding. A pocket on the ENAH EVH1 domain that has diverged from other Ena/VASP paralogs recognizes extended SLiMs and favors motif-flanking proline residues. Many high-affinity ENAH binders that contain two proline-rich SLiMs use a noncanonical site on the EVH1 domain for binding and display a thermodynamic signature consistent with the two-motif chain engaging a single domain. We also found that photoreceptor cilium actin regulator (PCARE) uses an extended 23-residue region to obtain a higher affinity than any known ENAH EVH1-binding motif. Our screen provides a way to uncover the effects of proteomic context on motif-mediated binding, revealing diverse mechanisms of control over EVH1 interactions and establishing that SLiMs can’t be fully understood outside of their native context.

ER exit sites in Drosophila display abundant ER-Golgi vesicles and pearled tubes but no megacarriers

Secretory cargos are collected at endoplasmic reticulum (ER) exit sites (ERES) before transport to the Golgi apparatus. Decades of research have provided many details of the molecular events underlying ER-Golgi exchanges. Essential questions, however, remain about the organization of the ER-Golgi interface in cells and the type of membrane structures mediating traffic from ERES. To investigate these, we use transgenic tagging in Drosophila flies, 3D-structured illumination microscopy (SIM), and focused ion beam scanning electron microscopy (FIB-SEM) to characterize ERES-Golgi units in collagen-producing fat body, imaginal discs, and imaginal discs overexpressing ERES determinant Tango1. Facing ERES, we find a pre-cis-Golgi region, equivalent to the vertebrate ER-Golgi intermediate compartment (ERGIC), involved in both anterograde and retrograde transport. This pre-cis-Golgi is continuous with the rest of the Golgi, not a separate compartment or collection of large carriers, for which we find no evidence. We observe, however, many vesicles, as well as pearled tubules connecting ERES and Golgi.

A novel function for Rab1 and Rab11 during secretory granule maturation

Regulated exocytosis is an essential process whereby specific cargo proteins are secreted in a stimulus-dependent manner. Cargo-containing secretory granules are synthesized in the trans-Golgi network (TGN); after budding from the TGN, granules undergo modifications, including an increase in size. These changes occur during a poorly understood process called secretory granule maturation. Here, we leverage the Drosophila larval salivary glands as a model to characterize a novel role for Rab GTPases during granule maturation. We find that secretory granules increase in size ∼300-fold between biogenesis and release, and loss of Rab1 or Rab11 reduces granule size. Surprisingly, we find that Rab1 and Rab11 localize to secretory granule membranes. Rab11 associates with granule membranes throughout maturation, and Rab11 recruits Rab1. In turn, Rab1 associates specifically with immature granules and drives granule growth. In addition to roles in granule growth, both Rab1 and Rab11 appear to have additional functions during exocytosis; Rab11 function is necessary for exocytosis, while the presence of Rab1 on immature granules may prevent precocious exocytosis. Overall, these results highlight a new role for Rab GTPases in secretory granule maturation.

TANGO1 marshals the early secretory pathway for cargo export

TANGO1 protein facilitates the endoplasmic reticulum (ER) export of large cargoes that cannot be accommodated in 60 nm transport vesicles. It assembles into a ring in the plane of the ER membrane to create a distinct domain. Its lumenal portion collects and sorts folded cargoes while its cytoplasmic domains collar COPII coats, recruit retrograde COPI-coated membranes that fuse within the TANGO1 ring, thus opening a tunnel for cargo transfer from the ER into a growing export conduit. This mode of cargo transfer bypasses the need for vesicular intermediates and is used to export the most abundant and bulky cargoes. The evolution of TANGO1 and its activities defines the difference between yeast and animal early secretory pathways.

Maturing secretory granules: Where secretory and endocytic pathways converge

Secretory granules (SGs) are specialized organelles responsible for the storage and regulated release of various biologically active molecules from the endocrine and exocrine systems. Thus, proper SG biogenesis is critical to normal animal physiology. Biogenesis of SGs starts at the trans-Golgi network (TGN), where immature SGs (iSGs) bud off and undergo maturation before fusing with the plasma membrane (PM). How iSGs mature is unclear, but emerging studies have suggested an important role for the endocytic pathway. The requirement for endocytic machinery in SG maturation blurs the line between SGs and another class of secretory organelles called lysosome-related organelles (LROs). Therefore, it is important to re-evaluate the differences and similarities between SGs and LROs.

Interorganelle communication and membrane shaping in the early secretory pathway

Biomolecules in the secretory pathway use membrane trafficking for reaching their final intracellular destination or for secretion outside the cell. This highly dynamic and multipartite process involves different organelles that communicate to one another while maintaining their identity, shape, and function. Recent studies unraveled new mechanisms of interorganelle communication that help organize the early secretory pathway. We highlight how the spatial proximity between endoplasmic reticulum (ER) exit sites and early Golgi elements provides novel means of ER-Golgi communication for ER export. We also review recent findings on how membrane contact sites between the ER and the trans-Golgi membranes can sustain anterograde traffic out of the Golgi complex.

Tunnels for Protein Export from the Endoplasmic Reticulum

The functions of coat protein complex II (COPII) coats in cargo packaging and the creation of vesicles at the endoplasmic reticulum are conserved in eukaryotic protein secretion. Standard COPII vesicles, however, cannot handle the secretion of metazoan-specific cargoes such as procollagens, apolipoproteins, and mucins. Metazoans have thus evolved modules centered on proteins like TANGO1 (transport and Golgi organization 1) to engage COPII coats and early secretory pathway membranes to engineer a novel mode of cargo export at the endoplasmic reticulum.

The immunophilin Zonda controls regulated exocytosis in endocrine and exocrine tissues

Exocytosis is a fundamental process in physiology, that ensures communication between cells, organs and even organisms. Hormones, neuropeptides and antibodies, among other cargoes are packed in exocytic vesicles that need to reach and fuse with the plasma membrane to release their content to the extracellular milieu. Hundreds of proteins participate in this process and several others in its regulation. We report here a novel component of the exocytic machinery, the Drosophila transmembrane immunophilin Zonda (Zda), previously found to participate in autophagy. Zda is highly expressed in secretory tissues, and regulates exocytosis in at least three of them: the ring gland, insulin-producing cells and the salivary gland. Using the salivary gland as a model system, we found that Zda is required at final steps of the exocytic process for fusion of secretory granules to the plasma membrane. In a genetic screen we identified the small GTPase RalA as a crucial regulator of secretory granule exocytosis that is required, similarly to Zda, for fusion between the secretory granule and the plasma membrane.

Mistargeting of secretory cargo in retromer-deficient cells

Intracellular trafficking is a basic and essential cellular function required for delivery of proteins to the appropriate subcellular destination; this process is especially demanding in professional secretory cells, which synthesize and secrete massive quantities of cargo proteins via regulated exocytosis. The Drosophila larval salivary glands are composed of professional secretory cells that synthesize and secrete mucin proteins at the onset of metamorphosis. Using the larval salivary glands as a model system, we have identified a role for the highly conserved retromer complex in trafficking of secretory granule membrane proteins. We demonstrate that retromer-dependent trafficking via endosomal tubules is induced at the onset of secretory granule biogenesis, and that recycling via endosomal tubules is required for delivery of essential secretory granule membrane proteins to nascent granules. Without retromer function, nascent granules do not contain the proper membrane proteins; as a result, cargo from these defective granules is mistargeted to Rab7-positive endosomes, where it progressively accumulates to generate dramatically enlarged endosomes. Retromer complex dysfunction is strongly associated with neurodegenerative diseases, including Alzheimer's disease, characterized by accumulation of amyloid β (Aβ). We show that ectopically expressed amyloid precursor protein (APP) undergoes regulated exocytosis in salivary glands and accumulates within enlarged endosomes in retromer-deficient cells. These results highlight recycling of secretory granule membrane proteins as a critical step during secretory granule maturation and provide new insights into our understanding of retromer complex function in secretory cells. These findings also suggest that missorting of secretory cargo, including APP, may contribute to the progressive nature of neurodegenerative disease.

Complementary and divergent functions of zebrafish Tango1 and Ctage5 in tissue development and homeostasis

Coat protein complex II (COPII) factors mediate cargo export from the endoplasmic reticulum (ER), but bulky collagens and lipoproteins are too large for traditional COPII vesicles. Mammalian CTAGE5 and TANGO1 have been well characterized individually as specialized cargo receptors at the ER that function with COPII coats to facilitate trafficking of bulky cargoes. Here, we present a genetic interaction study in zebrafish of deletions in ctage5, tango1, or both to investigate their distinct and complementary potential functions. We found that Ctage5 and Tango1 have different roles related to organogenesis, collagen versus lipoprotein trafficking, stress-pathway activation, and survival. While disruption of both ctage5 and tango1 compounded phenotype severity, mutation of either factor alone revealed novel tissue-specific defects in the building of heart, muscle, lens, and intestine, in addition to previously described roles in the development of neural and cartilage tissues. Together, our results demonstrate that Ctage5 and Tango1 have overlapping functions, but also suggest divergent roles in tissue development and homeostasis.

Tales of the ER-Golgi Frontier: Drosophila-Centric Considerations on Tango1 Function

In the secretory pathway, the transfer of cargo from the ER to the Golgi involves dozens of proteins that localize at specific regions of the ER called ER exit sites (ERES), where cargos are concentrated preceding vesicular transport to the Golgi. Despite many years of research, we are missing crucial details of how this highly dynamic ER-Golgi interface is defined, maintained and functions. Mechanisms allowing secretion of large cargos such as the very abundant collagens are also poorly understood. In this context, Tango1, discovered in the fruit fly Drosophila and widely conserved in animal evolution, has received a lot of attention in recent years. Tango1, an ERES-localized transmembrane protein, is the single fly member of the MIA/cTAGE family, consisting in humans of TANGO1 and at least 14 different related proteins. After its discovery in flies, a specific role of human TANGO1 in mediating secretion of collagens was reported. However, multiple studies in Drosophila have demonstrated that Tango1 is required for secretion of all cargos. At all ERES, through self-interaction and interactions with other proteins, Tango1 aids ERES maintenance and tethering of post-ER membranes. In this review, we discuss discoveries on Drosophila Tango1 and put them in relation with research on human MIA/cTAGE proteins. In doing so, we aim to offer an integrated view of Tango1 function and the nature of ER-Golgi transport from an evolutionary perspective.

A physical mechanism of TANGO1-mediated bulky cargo export

The endoplasmic reticulum (ER)-resident protein TANGO1 assembles into a ring around ER exit sites (ERES), and links procollagens in the ER lumen to COPII machinery, tethers, and ER-Golgi intermediate compartment (ERGIC) in the cytoplasm (Raote et al., 2018). Here, we present a theoretical approach to investigate the physical mechanisms of TANGO1 ring assembly and how COPII polymerization, membrane tension, and force facilitate the formation of a transport intermediate for procollagen export. Our results indicate that a TANGO1 ring, by acting as a linactant, stabilizes the open neck of a nascent COPII bud. Elongation of such a bud into a transport intermediate commensurate with bulky procollagens is then facilitated by two complementary mechanisms: (i) by relieving membrane tension, possibly by TANGO1-mediated fusion of retrograde ERGIC membranes and (ii) by force application. Altogether, our theoretical approach identifies key biophysical events in TANGO1-driven procollagen export.

TANGO1 membrane helices create a lipid diffusion barrier at curved membranes

We have previously shown TANGO1 organises membranes at the interface of the endoplasmic reticulum (ER) and ERGIC/Golgi (Raote et al., 2018). TANGO1 corrals retrograde membranes at ER exit sites to create an export conduit. Here the retrograde membrane is, in itself, an anterograde carrier. This mode of forward transport necessitates a mechanism to prevent membrane mixing between ER and the retrograde membrane. TANGO1 has an unusual membrane helix organisation, composed of one membrane-spanning helix (TM) and another that penetrates the inner leaflet (IM). We have reconstituted these membrane helices in model membranes and shown that TM and IM together reduce the flow of lipids at a region of defined shape. We have also shown that the helices align TANGO1 around an ER exit site. We suggest this is a mechanism to prevent membrane mixing during TANGO1-mediated transfer of bulky secretory cargos from the ER to the ERGIC/Golgi via a tunnel.

Biallelic TANGO1 mutations cause a novel syndromal disease due to hampered cellular collagen secretion

The transport and Golgi organization 1 (TANGO1) proteins play pivotal roles in the secretory pathway. Full length TANGO1 is a transmembrane protein localised at endoplasmic reticulum (ER) exit sites, where it binds bulky cargo within the ER lumen and recruits membranes from the ER Golgi intermediate compartment to create an exit route for their export. Here we report the first TANGO1-associated syndrome in humans. A synonymous substitution that results in exon eight skipping in most mRNA molecules, ultimately leading to a truncated TANGO1 protein was identified as disease-causing mutation. The four homozygously affected sons of a consanguineous family display severe dentinogenesis imperfecta, short stature, various skeletal abnormalities, insulin-dependent diabetes mellitus, sensorineural hearing loss, and mild intellectual disability. Functional studies in HeLa and U2OS cells revealed that the corresponding truncated TANGO1 protein is dispersed in the ER and its expression in cells with intact endogenous TANGO1 impairs cellular collagen I secretion.