Rab6 is required for rapid, cisternal-specific, intra-Golgi cargo transport

Molecular mechanism for recognition of the cargo adapter Rab6GTP by the dynein adapter BicD2

Rab6 is a key modulator of protein secretion. The dynein adapter Bicaudal D2 (BicD2) recruits the motors cytoplasmic dynein and kinesin-1 to Rab6GTP-positive vesicles for transport; however, it is unknown how BicD2 recognizes Rab6. Here, we establish a structural model for recognition of Rab6GTP by BicD2, using structure prediction and mutagenesis. The binding site of BicD2 spans two regions of Rab6 that undergo structural changes upon the transition from the GDP- to GTP-bound state, and several hydrophobic interface residues are rearranged, explaining the increased affinity of the active GTP-bound state. Mutations of Rab6GTP that abolish binding to BicD2 also result in reduced co-migration of Rab6GTP/BicD2 in cells, validating our model. These mutations also severely diminished the motility of Rab6-positive vesicles in cells, highlighting the importance of the Rab6GTP/BicD2 interaction for overall motility of the multi-motor complex that contains both kinesin-1 and dynein. Our results provide insights into trafficking of secretory and Golgi-derived vesicles and will help devise therapies for diseases caused by BicD2 mutations, which selectively affect the affinity to Rab6 and other cargoes.

Movement of the endoplasmic reticulum is driven by multiple classes of vesicles marked by Rab-GTPases

Peripheral endoplasmic reticulum (ER) tubules move along microtubules to interact with various organelles through membrane contact sites (MCS). Traditionally, ER moves by either sliding along stable microtubules via molecular motors or attaching to the plus ends of dynamic microtubules through tip attachment complexes (TAC). A recently discovered third process, hitchhiking, involves motile vesicles pulling ER tubules along microtubules. Previous research showed that ER hitchhikes on Rab5- and Rab7-marked endosomes, but it is uncertain if other Rab-vesicles can do the same. In U2OS cells, we screened Rabs for their ability to cotransport with ER tubules and found that ER hitchhikes on post-Golgi vesicles marked by Rab6 (isoforms a and b). Rab6-ER hitchhiking occurs independently of ER-endolysosome contacts and TAC-mediated ER movement. Disrupting either Rab6 or the motility of Rab6-vesicles reduces overall ER movement. Conversely, relocating these vesicles to the cell periphery causes peripheral ER accumulation, indicating that Rab6-vesicle motility is crucial for a subset of ER movements. Proximal post-Golgi vesicles marked by TGN46 are involved in Rab6-ER hitchhiking, while other post-Golgi vesicles (Rabs 8/10/11/13/14) are not essential for ER movement. Our further analysis finds that ER to Golgi vesicles marked by Rab1 are also capable of driving a subset of ER movements. Taken together, our findings suggest that ER hitchhiking on Rab-vesicles is a significant mode of ER movement.

SIGNIFICANCE STATEMENT

Peripheral endoplasmic reticulum tubules move on microtubules by either attaching to motors (cargo adaptor-mediated), dynamic microtubule-plus ends (tip attachment complexes) or motile vesicles (hitchhiking) but the prevalence of each mode is not clearPost-Golgi vesicles marked by Rab6/TGN46 and ER to Golgi vesicles marked by Rab1 drive ER movementsER hitchhiking on multiple classes of vesicles (endolysosomal, post-Golgi and ER to Golgi) marked by Rabs plays a prominent role in ER movement.

Structural basis for Rab6 activation by the Ric1-Rgp1 complex

Rab GTPases act as molecular switches to regulate organelle homeostasis and membrane trafficking. Rab6 plays a central role in regulating cargo flux through the Golgi and is activated via nucleotide exchange by the Ric1-Rgp1 protein complex. Ric1-Rgp1 is conserved throughout eukaryotes but the structural and mechanistic basis for its function has not been established. Here we report the cryoEM structure of a Ric1-Rgp1-Rab6 complex representing a key intermediate of the nucleotide exchange reaction. This structure reveals the overall architecture of the complex and enabled us to identify interactions critical for proper recognition and activation of Rab6 on the Golgi membrane surface. Ric1-Rgp1 interacts with the nucleotide-binding domain of Rab6 using an uncharacterized helical domain, which we establish as a novel RabGEF domain by identifying residues required for Rab6 nucleotide exchange. Unexpectedly, the complex uses an arrestin fold to interact with the Rab6 hypervariable domain, indicating that interactions with the unstructured C-terminal regions of Rab GTPases may be a common specificity mechanism used by their activators. Collectively, our findings provide a detailed mechanistic understanding of regulated Rab6 activation at the Golgi.

MR1 antigen presentation to MAIT cells and other MR1-restricted T cells

MHC antigen presentation plays a fundamental role in adaptive and semi-invariant T cell immunity. Distinct MHC molecules bind antigens that differ in chemical structure, origin and location and present them to specialized T cells. MHC class I-related protein 1 (MR1) presents a range of small molecule antigens to MR1-restricted T (MR1T) lymphocytes. The best studied MR1 ligands are derived from microbial metabolism and are recognized by a major class of MR1T cells known as mucosal-associated invariant T (MAIT) cells. Here, we describe the MR1 antigen presentation pathway: the known types of antigens presented by MR1, the location where MR1-antigen complexes form, the route followed by the complexes to the cell surface, the mechanisms involved in termination of MR1 antigen presentation and the accessory cellular proteins that comprise the MR1 antigen presentation machinery. The current road map of the MR1 antigen presentation pathway reveals potential strategies for therapeutic manipulation of MR1T cell function and provides a foundation for further studies that will lead to a deeper understanding of MR1-mediated immunity.

RAB6A functions as a critical modulator of the stem-like subsets in cholangiocarcinoma

RAB6A is a member of RAB GTPase family and plays an important role in the targeted transport of neurotrophic receptors and inflammatory cytokines. RAB6A-mediated secretory pathway is involved in many physiological and pathological processes. Defects in RAB6A-mediated secretory pathway may lead to the development of many diseases, including cancer. However, its role in cholangiocarcinoma (CCA) has not yet been revealed. We explored the regulatory role of RAB6A in the stem-like subsets of CCA. We showed that RAB6A knockdown (KD) impedes cancer stem cells (CSCs) properties and epithelial-mesenchymal transition in vitro and that suppression of RAB6A inhibits tumor growth in vivo. We screened target cargos of RAB6A in CCA cells and identified a extracellular matrix component as the target cargo. RAB6A binds directly to OPN, and RAB6A KD suppressed OPN secretion and inhibited the interaction between OPN and αV integrin receptor. Moreover, RAB6A KD inhibited the AKT signaling pathway, which is a downstream effector of the integrin receptor signaling. In addition, shRNA targeting OPN blocked endogenous expression of OPN and consequently weakened CSCs properties in RAB6A-formed spheres. Similarly, inhibitor of AKT signaling, MK2206 also impedes oncogenic function of RAB6A in the stem-like subsets of CCA cells. In conclusion, our findings showed that RAB6A sustains CSCs phenotype maintenance by modulating the secretion of OPN and consequentially activating the downstream AKT signaling pathway. Targeting the RAB6A/OPN axis may be an effective strategy for CCA therapy.

TMED10 mediates the loading of neosynthesised Sonic Hedgehog in COPII vesicles for efficient secretion and signalling

The morphogen Sonic Hedgehog (SHH) plays an important role in coordinating embryonic development. Short- and long-range SHH signalling occurs through a variety of membrane-associated and membrane-free forms. However, the molecular mechanisms that govern the early events of the trafficking of neosynthesised SHH in mammalian cells are still poorly understood. Here, we employed the retention using selective hooks (RUSH) system to show that newly-synthesised SHH is trafficked through the classical biosynthetic secretory pathway, using TMED10 as an endoplasmic reticulum (ER) cargo receptor for efficient ER-to-Golgi transport and Rab6 vesicles for Golgi-to-cell surface trafficking. TMED10 and SHH colocalized at ER exit sites (ERES), and TMED10 depletion significantly delays SHH loading onto ERES and subsequent exit leading to significant SHH release defects. Finally, we utilised the Drosophila wing imaginal disc model to demonstrate that the homologue of TMED10, Baiser (Bai), participates in Hedgehog (Hh) secretion and signalling in vivo. In conclusion, our work highlights the role of TMED10 in cargo-specific egress from the ER and sheds light on novel important partners of neosynthesised SHH secretion with potential impact on embryonic development.

High-Pressure Freezing Followed by Freeze Substitution: An Optimal Electron Microscope Technique to Study Golgi Apparatus Organization and Membrane Trafficking

A major goal of structural biologists is to preserve samples as close to their living state as possible. High-pressure freezing (HPF) is a state-of-art technique that freezes the samples at high pressure (~2100 bar) and low temperature (-196 °C) within milliseconds. This ultrarapid fixation enables simultaneous immobilization of all cellular components and preserves the samples in a near-native state. This facilitates the study of dynamic processes in Golgi apparatus organization and membrane trafficking. The work in our laboratory shows that high-pressure freezing followed by freeze substitution (FS), the introduction of organic solvents at low temperature prior to plastic embedding, can better preserve the structure of Golgi apparatus and Golgi-associated vesicles. Here, we present a protocol for freezing monolayer cell cultures on sapphire disks followed by freeze substitution. We were able to use this protocol to successfully study Golgi organization and membrane trafficking in HeLa cells. The protocol gives decidedly better preservation of Golgi apparatus and associated vesicles than conventional chemically fixed preparation and as a plastic embedded preparation can be readily extended to 3D electron microscopy imaging through sequential block face-scanning electron microscopy. The 3D imaging of a multi-micron thick organelle such as the Golgi apparatus located near the cell nucleus is greatly facilitated relative to hydrated sample imaging techniques such as cryo-electron microscopy.

Proteomic profiling of exosomes derived from pancreatic beta-cells cultured under hyperglycemia

Introduction

Cargo carried by extracellular vesicles (EVs) is considered a promising diagnostic marker, especially proteins. EVs can be divided according to their size and way of biogenesis into exosomes (diameter < 200 nm) and ectosomes (diameter > 200 nm). Exosomes are considered to be of endocytic origin, and ectosomes are produced by budding and shedding from the plasma membrane [1].

Methods

The first step of this study was a characterization of the exosome sample. Using Tunable Resistive Pulse Sensing (qNano) size distribution and concentration were measured. The mean size of exosomes was 120±9.17 nm. In the present study, a nano liquid chromatography coupled with tandem mass spectrometry (nanoLCMS/MS) was used to compare protein profiles of exosomes secreted by pancreatic beta cells (1.1B4) grown under normal glucose (NG, 5 mM D-glucose) and high glucose (HG, 25 mM D-glucose) conditions. The EV samples were lysed, and proteins were denatured, digested, and analyzed using a Q-Exactive mass spectrometer coupled with the UltiMate 3000 RSLC nano system. The nanoLC-MS/MS data were searched against the SwissProt Homo sapiens database using MaxQuant software and protein quantitation was done by the MaxLFQ algorithm. Statistical analysis was carried out with Perseus software. Further bioinformatic analysis was performed using the FunRich 3.1.4 software with the UniProt protein database and String [2].

Results

As a result of the nanoLC-MS/MS analysis more than 1,000 proteins were identified and quantified in each sample. The average number of identified proteins in exosomes was 1,397. Label-free quantitative analysis showed that exosome composition differed significantly between those isolated under NG and HG conditions. Many pathways were down-regulated in HG, particularly the ubiquitin-proteasome pathway. In addition, a significant up-regulation of the Ras-proteins pathway was observed in HG.

Conclusion

Our description of exosomes protein content and its related functions provides the first insight into the EV interactome and its role in glucose intolerance development and diabetic complications. The results also indicate the applicability of EV proteins for further investigation regarding their potential as circulating in vivo biomarkers.

Maintaining Golgi Homeostasis: A Balancing Act of Two Proteolytic Pathways

The Golgi apparatus is a central hub for cellular protein trafficking and signaling. Golgi structure and function is tightly coupled and undergoes dynamic changes in health and disease. A crucial requirement for maintaining Golgi homeostasis is the ability of the Golgi to target aberrant, misfolded, or otherwise unwanted proteins to degradation. Recent studies have revealed that the Golgi apparatus may degrade such proteins through autophagy, retrograde trafficking to the ER for ER-associated degradation (ERAD), and locally, through Golgi apparatus-related degradation (GARD). Here, we review recent discoveries in these mechanisms, highlighting the role of the Golgi in maintaining cellular homeostasis.