TANGO1 marshals the early secretory pathway for cargo export

Mucins and Their Roles in Asthma

Mucus is a crucial component of airway host defense. For optimal protection, its chief components-the mucins MUC5AC and MUC5B-need to be tightly regulated. Their expression localizes to specific secretory epithelial cell types capable of producing and secreting massive glycopolymers. In asthma, abnormal mucus is an important clinical problem that is effectively treated with therapies that directly target mucins. This review summarizes what is known about how mucin gene regulation, protein synthesis, and secretion are regulated in healthy and asthmatic lungs. Ultimately, a better understanding of these processes could help identify novel ways of preventing or reversing airway mucus dysfunction.

The life cycle of type IV collagen

Type IV collagen is a large triple helical molecule that forms a covalently cross-linked network within basement membranes (BMs). Type IV collagen networks play key roles in mechanically supporting tissues, shaping organs, filtering blood, and cell signaling. To ensure tissue health and function, all aspects of the type IV collagen life cycle must be carried out accurately. However, the large triple helical structure and complex life-cycle of type IV collagen, poses many challenges to cells and tissues. Type IV collagen predominantly forms heterotrimers and to ensure proper construction, expression of the distinct α-chains that comprise a heterotrimer needs tight regulation. The α-chains must also be accurately modified by several enzymes, some of which are specific to collagens, to build and stabilize the triple helical trimer. In addition, type IV collagen is exceptionally long (400nm) and thus the packaging and trafficking of the triple helical trimer from the ER to the Golgi must be modified to accommodate the large type IV collagen molecule. During ER-to-Golgi trafficking, as well as during secretion and transport in the extracellular space type IV collagen also associates with specific chaperone molecules that maintain the structure and solubility of collagen IV. Type IV collagen trimers are then delivered to BMs from local and distant sources where they are integrated into BMs by interactions with cell surface receptors and many diverse BM resident proteins. Within BMs type IV collagen self-associates into a network and is crosslinked by BM resident enzymes. Finally, homeostatic type IV collagen levels in BMs are maintained by poorly understood mechanisms involving proteolysis and endocytosis. Here, we provide an overview of the life cycle of collagen IV, highlighting unique mechanisms and poorly understood aspects of type IV collagen regulation.

License to drive: Receptor-mediated ER exit of proteins and lipids

The secretory pathway, which begins at the endoplasmic reticulum (ER) through the COPII complex, is responsible for transporting proteins and lipid carriers to various destined cellular compartments or extracellular space. The fundamental mechanism by which the COPII operates is evolutionarily conserved. Nevertheless, the vast diversity of mammalian cargos poses significant challenges to the secretory pathway, especially considering the intricate physiology in vivo. Particularly, certain physiologically essential cargos, including procollagen and lipoproteins, appear to be oversized for these canonical carriers, implying the need for additional sophisticated regulation at the onset step so-called ER exit. Emerging evidence highlights the critical role of cargo receptors in selective sorting for ER export, illuminating the complex biology of the trafficking dynamics, which holds broad implications for human health and diseases.

Tango1L but not Tango1S, Tali and cTAGE5 is required for export of type II collagen in medaka fish

Newly synthesized proteins destined for the secretory pathway are folded and assembled in the endoplasmic reticulum (ER) and then transported to the Golgi apparatus via COPII vesicles, which are normally 60-90 nm. COPII vesicles must accordingly be enlarged to accommodate proteins larger than 90 nm, such as long-chain collagen. Key molecules involved in this enlargement are Tango1 and Tango1-like (Tali), which are transmembrane proteins in the ER encoded by the MIA3 and MIA2 genes, respectively. Interestingly, two splicing variants are expressed from each of these two genes: Tango1L and Tango1S from the MIA3 gene, and Tali and cTAGE5 from the MIA2 gene. Here, we constructed Tango1L-knockout (KO), Tango1S-KO, Tali-KO, and cTAGE5-KO separately in medaka fish, a vertebrate model organism, and characterized them. Results showed that only Tango1L-KO conferred a lethal phenotype to medaka fish. Only Tango1L-KO medaka fish exhibited a shorter tail than wild-type (WT) fish and showed the defects in the export of type II collagen from the ER, contrary to the previous reports analyzing Tango1-KO or Tali-KO mice and the results of knockdown experiments in human cultured cells. Medaka fish may employ a simpler system than mammals for the export of large molecules from the ER.Key words: intracellular transport, COPII vesicles, enlargement, endoplasmic reticulum, Golgi apparatus.

A distant TANGO1 family member promotes vitellogenin export from the ER in C. elegans

Vitellogenin is thought to share a common ancestor with human apolipoprotein B (ApoB) for systemic lipid transport. In Caenorhabditis elegans, although a general route for inter-tissue vitellogenin transport has been described, the full mechanism that underlies its intracellular trafficking within the intestine remains obscure. In humans, the TANGO1 family of proteins generates membrane carriers to accommodate bulky ApoB-containing lipoprotein particles for their endoplasmic reticulum (ER) export. TANGO1 orthologs have hitherto been discovered in most metazoans, except nematodes. Here, we report the C. elegans TNGL-1 as a mediator of vitellogenin export from the ER. Depletion of TNGL-1 causes the retention of vitellogenin in the ER lumen. Furthermore, the TNGL-1 C-terminal unstructured domain and its luminal globular domain are required for its proper localization and cargo engagement, respectively. Our findings support TNGL-1 as a distant TANGO1 family member and point to the universal requirement of TANGO1-based mechanisms for the secretion of specific metazoan proteins.

TMEM39A and TMEM131 facilitate bulk transport of ECM proteins through large COPII vesicle formation

The growth of Caenorhabditis elegans involves multiple molting processes, during which old cuticles are shed and new cuticles are rapidly formed. This process requires the regulated bulk secretion of cuticle components. The transmembrane protein-39 (TMEM-39) mutant exhibits distinct dumpy and ruptured phenotypes characterized by notably thin cuticles. TMEM-39 primarily co-localizes with the coat protein II complex (COPII) in large vesicles rather than small COPII vesicles. These TMEM-39-associated large vesicles (TMEM-39-LVs) form robustly during the molting period and co-localize with various extracellular matrix components, including BLI-1 collagen, BLI-3 dual oxidase, and carboxypeptidases. Through immunoprecipitation using TMEM39A-FLAG and proteomics analysis in human sarcoma cells, we identify TMEM39A-associated proteins, including TMEM131. Knockdown of TMEM131 results in reduced TMEM39A-LV formation and collagen secretion in both C. elegans and human sarcoma cells, indicating a cooperative role between TMEM39A and TMEM131 in the secretion of extracellular components through the formation of large COPII vesicles. Given the conservation of TMEM39A and its associated proteins between C. elegans and humans, TMEM39A-LVs may represent a fundamental machinery for rapid and extensive secretion across metazoans.

Endoplasmic reticulum exit sites are segregated for secretion based on cargo size

TANGO1, TANGO1-Short, and cTAGE5 form stable complexes at the endoplasmic reticulum exit sites (ERES) to preferably export bulky cargoes. Their C-terminal proline-rich domain (PRD) binds Sec23A and affects COPII assembly. The PRD in TANGO1-Short was replaced with light-responsive domains to control its binding to Sec23A in U2OS cells (human osteosarcoma). TANGO1-ShortΔPRD was dispersed in the ER membrane but relocated rapidly, reversibly, to pre-existing ERES by binding to Sec23A upon light activation. Prolonged binding between the two, concentrated ERES in the juxtanuclear region, blocked cargo export and relocated ERGIC53 into the ER, minimally impacting the Golgi complex organization. Bulky collagen VII and endogenous collagen I were collected at less than 47% of the stalled ERES, whereas small cargo molecules were retained uniformly at almost all the ERES. We suggest that ERES are segregated to handle cargoes based on their size, permitting cells to traffic them simultaneously for optimal secretion.

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.

Protein condensates in the the secretory pathway: Unraveling biophysical interactions and function

The emergence of phase separation phenomena among macromolecules has identified biomolecular condensates as fundamental cellular organizers. These condensates concentrate specific components and accelerate biochemical reactions without relying on membrane boundaries. Although extensive studies have revealed a large variety of nuclear and cytosolic membraneless organelles, we are witnessing a surge in the exploration of protein condensates associated with the membranes of the secretory pathway, such as the endoplasmic reticulum and the Golgi apparatus. This review focuses on protein condensates in the secretory pathway and discusses their impact on the organization and functions of this cellular process. Moreover, we explore the modes of condensate-membrane association and the biophysical and cellular consequences of protein condensate interactions with secretory pathway membranes.

Mea6/cTAGE5 cooperates with TRAPPC12 to regulate PTN secretion and white matter development

Mutations of TRAPPC12 are associated with progressive childhood encephalopathy including abnormal white matter. However, the underlying pathogenesis is still unclear. Here, we found that Trappc12 deficiency in CG4 and oligodendrocyte progenitor cells (OPCs) affects their differentiation and maturation. In addition, TRAPPC12 interacts with Mea6/cTAGE5, and Mea6/cTAGE5 ablation in OPCs affects their proliferation and differentiation, leading to marked hypomyelination, compromised synaptic functionality, and aberrant behaviors in mice. We reveal that TRAPPC12 is associated with COPII components at ER exit site, and Mea6/cTAGE5 cKO disrupts the trafficking pathway by affecting the distribution and/or expression of TRAPPC12, SEC13, SEC31A, and SAR1. Moreover, we observed marked disturbances in the secretion of pleiotrophin (PTN) in Mea6-deficient OPCs. Notably, exogenous PTN supplementation ameliorated the differentiation deficits of these OPCs. Collectively, our findings indicate that the association between TRAPPC12 and MEA6 is important for cargo trafficking and white matter development.

TTC17 is an endoplasmic reticulum resident TPR-containing adaptor protein

Protein folding, quality control, maturation, and trafficking are essential processes for proper cellular homeostasis. Around one-third of the human proteome is targeted to the endoplasmic reticulum (ER), the organelle that serves as entrance into the secretory pathway. Successful protein trafficking is paramount for proper cellular function and to that end there are many ER resident proteins that ensure efficient secretion. Here, biochemical and cell biological analysis was used to determine that TTC17 is a large, soluble, ER-localized protein that plays an important role in secretory trafficking. Transcriptional analysis identified the predominantly expressed protein isoform of TTC17 in various cell lines. Further, TTC17 localizes to the ER and interacts with a wide variety of chaperones and cochaperones normally associated with ER protein folding, quality control, and maturation processes. TTC17 was found to be significantly upregulated by ER stress and through the creation and use of TTC17-/- cell lines, quantitative mass spectrometry identified secretory pathway wide trafficking defects in the absence of TTC17. Notably, trafficking of insulin-like growth factor type 1 receptor, glycoprotein nonmetastatic melanoma protein B, clusterin, and UDP-glucose:glycoprotein glucosyltransferase 1 were significantly altered in H4 neuroglioma cells. This study defines a novel ER trafficking factor and provides insight into the protein-protein assisted trafficking in the early secretory pathway.

DYRK3 enables secretory trafficking by maintaining the liquid-like state of ER exit sites

The dual-specificity kinase DYRK3 controls the formation and dissolution of multiple biomolecular condensates, regulating processes including stress recovery and mitotic progression. Here, we report that DYRK3 functionally interacts with proteins associated with endoplasmic reticulum (ER) exit sites (ERESs) and that inhibition of DYRK3 perturbs the organization of the ERES-Golgi interface and secretory trafficking. DYRK3-mediated regulation of ERES depends on the N-terminal intrinsically disordered region (IDR) of the peripheral membrane protein SEC16A, which co-phase separates with ERES components to form liquid-like condensates on the surface of the ER. By modulating the liquid-like properties of ERES, we show that their physical state is essential for functional cargo trafficking through the early secretory pathway. Our findings support a mechanism whereby phosphorylation by DYRK3 and its reversal by serine-threonine phosphatases regulate the material properties of ERES to create a favorable physicochemical environment for directional membrane traffic in eukaryotic cells.

HDAC-Specific Inhibitors Induce the Release of Porcine Epidemic Diarrhea Virus via the COPII-Coated Vesicles

Porcine epidemic diarrhea virus (PEDV) is an alpha-coronavirus causing acute diarrhea and high mortality in neonatal suckling piglets, resulting in huge economic losses for the global swine industry. The replication, assembly and cell egression of PEDV, an enveloped RNA virus, are mediated via altered intracellular trafficking. The underlying mechanisms of PEDV secretion are poorly understood. In this study, we found that the histone deacetylase (HDAC)-specific inhibitors, trichostatin A (TSA) and sodium butyrate (NaB), facilitate the secretion of infectious PEDV particles without interfering with its assembly. We found that PEDV N protein and its replicative intermediate dsRNA colocalize with coat protein complex II (COPII)-coated vesicles. We also showed that the colocalization of PEDV and COPII is enhanced by the HDAC-specific inhibitors. In addition, ultrastructural analysis revealed that the HDAC-specific inhibitors promote COPII-coated vesicles carrying PEDV virions and the secretion of COPII-coated vesicles. Consistently, HDAC-specific inhibitors-induced PEDV particle secretion was abolished by Sec24B knockdown, implying that the HDAC-specific inhibitors-mediated COPII-coated vesicles are required for PEDV secretion. Taken together, our findings provide initial evidence suggesting that PEDV virions can assemble in the endoplasmic reticulum (ER) and bud off from the ER in the COPII-coated vesicles. HDAC-specific inhibitors promote PEDV release by hijacking the COPII-coated vesicles.

O-GlcNAc Dynamics: The Sweet Side of Protein Trafficking Regulation in Mammalian Cells

The transport of proteins between the different cellular compartments and the cell surface is governed by the secretory pathway. Alternatively, unconventional secretion pathways have been described in mammalian cells, especially through multivesicular bodies and exosomes. These highly sophisticated biological processes rely on a wide variety of signaling and regulatory proteins that act sequentially and in a well-orchestrated manner to ensure the proper delivery of cargoes to their final destination. By modifying numerous proteins involved in the regulation of vesicular trafficking, post-translational modifications (PTMs) participate in the tight regulation of cargo transport in response to extracellular stimuli such as nutrient availability and stress. Among the PTMs, O-GlcNAcylation is the reversible addition of a single N-acetylglucosamine monosaccharide (GlcNAc) on serine or threonine residues of cytosolic, nuclear, and mitochondrial proteins. O-GlcNAc cycling is mediated by a single couple of enzymes: the O-GlcNAc transferase (OGT) which catalyzes the addition of O-GlcNAc onto proteins, and the O-GlcNAcase (OGA) which hydrolyses it. Here, we review the current knowledge on the emerging role of O-GlcNAc modification in the regulation of protein trafficking in mammalian cells, in classical and unconventional secretory pathways.

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.

Pancreatic beta cell ER export in health and diabetes

In the secretory pathway of the pancreatic beta cell, proinsulin and other secretory granule proteins are first produced in the endoplasmic reticulum (ER). Beta cell ER homeostasis is vital for normal beta cell functions and is maintained by the delicate balance between protein synthesis, folding, export and degradation. Disruption of ER homeostasis leads to beta cell death and diabetes. Among the four components to maintain ER homeostasis, the role of ER export in insulin biogenesis or beta cell survival was not well-understood. COPII (coat protein complex II) dependent transport is a conserved mechanism for most cargo proteins to exit ER and transport to Golgi apparatus. Emerging evidence began to reveal a critical role of COPII-dependent ER export in beta cells. In this review, we will first discuss the basic components of the COPII transport machinery, the regulation of cargo entry and COPII coat assembly in mammalian cells, and the general concept of receptor-mediated cargo sorting in COPII vesicles. On the basis of these general discussions, the current knowledge and recent developments specific to the beta cell COPII dependent ER export are summarized under normal and diabetic conditions.

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.

Vesicle trafficking and vesicle fusion: mechanisms, biological functions, and their implications for potential disease therapy

Intracellular vesicle trafficking is the fundamental process to maintain the homeostasis of membrane-enclosed organelles in eukaryotic cells. These organelles transport cargo from the donor membrane to the target membrane through the cargo containing vesicles. Vesicle trafficking pathway includes vesicle formation from the donor membrane, vesicle transport, and vesicle fusion with the target membrane. Coat protein mediated vesicle formation is a delicate membrane budding process for cargo molecules selection and package into vesicle carriers. Vesicle transport is a dynamic and specific process for the cargo containing vesicles translocation from the donor membrane to the target membrane. This process requires a group of conserved proteins such as Rab GTPases, motor adaptors, and motor proteins to ensure vesicle transport along cytoskeletal track. Soluble N-ethyl-maleimide-sensitive factor (NSF) attachment protein receptors (SNARE)-mediated vesicle fusion is the final process for vesicle unloading the cargo molecules at the target membrane. To ensure vesicle fusion occurring at a defined position and time pattern in eukaryotic cell, multiple fusogenic proteins, such as synaptotagmin (Syt), complexin (Cpx), Munc13, Munc18 and other tethering factors, cooperate together to precisely regulate the process of vesicle fusion. Dysfunctions of the fusogenic proteins in SNARE-mediated vesicle fusion are closely related to many diseases. Recent studies have suggested that stimulated membrane fusion can be manipulated pharmacologically via disruption the interface between the SNARE complex and Ca²⁺ sensor protein. Here, we summarize recent insights into the molecular mechanisms of vesicle trafficking, and implications for the development of new therapeutics based on the manipulation of vesicle fusion.

The Golgi Apparatus and its Next-Door Neighbors

The Golgi apparatus represents a central compartment of membrane traffic. Its apparent architecture, however, differs considerably among species, from unstacked and scattered cisternae in the budding yeast Saccharomyces cerevisiae to beautiful ministacks in plants and further to gigantic ribbon structures typically seen in mammals. Considering the well-conserved functions of the Golgi, its fundamental structure must have been optimized despite seemingly different architectures. In addition to the core layers of cisternae, the Golgi is usually accompanied by next-door compartments on its cis and trans sides. The trans-Golgi network (TGN) can be now considered as a compartment independent from the Golgi stack. On the cis side, the intermediate compartment between the ER and the Golgi (ERGIC) has been known in mammalian cells, and its functional equivalent is now suggested for yeast and plant cells. High-resolution live imaging is extremely powerful for elucidating the dynamics of these compartments and has revealed amazing similarities in their behaviors, indicating common mechanisms conserved along the long course of evolution. From these new findings, I would like to propose reconsideration of compartments and suggest a new concept to describe their roles comprehensively around the Golgi and in the post-Golgi trafficking.