Regulation of Sec16 levels and dynamics links proliferation and secretion

ER exit in physiology and disease

The biosynthetic secretory pathway is comprised of multiple steps, modifications and interactions that form a highly precise pathway of protein trafficking and secretion, that is essential for eukaryotic life. The general outline of this pathway is understood, however the specific mechanisms are still unclear. In the last 15 years there have been vast advancements in technology that enable us to advance our understanding of this complex and subtle pathway. Therefore, based on the strong foundation of work performed over the last 40 years, we can now build another level of understanding, using the new technologies available. The biosynthetic secretory pathway is a high precision process, that involves a number of tightly regulated steps: Protein folding and quality control, cargo selection for Endoplasmic Reticulum (ER) exit, Golgi trafficking, sorting and secretion. When deregulated it causes severe diseases that here we categorise into three main groups of aberrant secretion: decreased, excess and altered secretion. Each of these categories disrupts organ homeostasis differently, effecting extracellular matrix composition, changing signalling events, or damaging the secretory cells due to aberrant intracellular accumulation of secretory proteins. Diseases of aberrant secretion are very common, but despite this, there are few effective therapies. Here we describe ER exit sites (ERES) as key hubs for regulation of the secretory pathway, protein quality control and an integratory hub for signalling within the cell. This review also describes the challenges that will be faced in developing effective therapies, due to the specificity required of potential drug candidates and the crucial need to respect the fine equilibrium of the pathway. The development of novel tools is moving forward, and we can also use these tools to build our understanding of the acute regulation of ERES and protein trafficking. Here we review ERES regulation in context as a therapeutic strategy.

Endoplasmic Reticulum Exit Sites scale with somato-dendritic size in neurons

Nervous systems exhibit dramatic diversity in cell morphology and size. How neurons regulate their biosynthetic and secretory machinery to support such diversity is not well understood. Endoplasmic reticulum exit sites (ERESs) are essential for maintaining secretory flux, and are required for normal dendrite development, but how neurons of different size regulate secretory capacity remains unknown. In Caenorhabditis elegans, we find that the ERES number is strongly correlated with the size of a neuron's dendritic arbor. The elaborately branched sensory neuron, PVD, has especially high ERES numbers. Asymmetric cell division provides PVD with a large initial cell size critical for rapid establishment of PVD's high ERES number before neurite outgrowth, and these ERESs are maintained throughout development. Maintenance of ERES number requires the cell fate transcription factor MEC-3, C. elegans TOR (ceTOR/let-363), and nutrient availability, with mec-3 and ceTOR/let-363 mutant PVDs both displaying reductions in ERES number, soma size, and dendrite size. Notably, mec-3 mutant animals exhibit reduced expression of a ceTOR/let-363 reporter in PVD, and starvation reduces ERES number and somato-dendritic size in a manner genetically redundant with ceTOR/let-363 perturbation. Our data suggest that both asymmetric cell division and nutrient sensing pathways regulate secretory capacities to support elaborate dendritic arbors.

The Early Secretory Pathway Is Crucial for Multiple Aspects of the Hepatitis C Virus Life Cycle

Although most of the early events of the hepatitis C virus (HCV) life cycle are well characterized, our understanding of HCV egress is still unclear. Some reports implicate the conventional endoplasmic reticulum (ER)-Golgi route, while some propose noncanonical secretory routes. Initially, the envelopment of HCV nucleocapsid occurs by budding into the ER lumen. Subsequently, the HCV particle exit from the ER is assumed to be mediated by coat protein complex II (COPII) vesicles. COPII vesicle biogenesis also involves the recruitment of cargo to the site of vesicle biogenesis via interaction with COPII inner coat proteins. We investigated the modulation and the specific role of the individual components of the early secretory pathway in HCV egress. We observed that HCV inhibits cellular protein secretion and triggers the reorganization of the ER exit sites and ER-Golgi intermediate compartments (ERGIC). Gene-specific knockdown of the components of this pathway such as SEC16A, TFG, ERGIC-53, and COPII coat proteins demonstrated the functional significance of these components and the distinct role played by these proteins in various aspects of the HCV life cycle. SEC16A is essential for multiple steps in the HCV life cycle, whereas TFG is specifically involved in HCV egress and ERGIC-53 is crucial for HCV entry. Overall, our study establishes that the components of the early secretory pathway are essential for HCV propagation and emphasize the importance of the ER-Golgi secretory route in this process. Surprisingly, these components are also required for the early stages of the HCV life cycle due to their role in overall intracellular trafficking and homeostasis of the cellular endomembrane system. IMPORTANCE The virus life cycle involves entry into the host, replication of the genome, assembly of infectious progeny, and their subsequent release. Different aspects of the HCV life cycle, including entry, genome replication, and assembly, are well characterized; however, our understanding of the HCV release is still not clear and subject to debate due to varied findings. Here, we attempted to address this controversy and enhance our understanding of HCV egress by evaluating the role of the different components of the early secretory pathway in the HCV life cycle. To our surprise, we found that the components of the early secretory pathway are not only essential for HCV release but also contribute to many other earlier events of the HCV life cycle. This study emphasizes the importance of the early secretory pathway for the establishment of productive HCV infection in hepatocytes.

Impact of interorganelle coordination between the conventional early secretory pathway and autophagy in cellular homeostasis and stress response

The conventional early secretory pathway and autophagy are two essential interconnected cellular processes that are crucial for maintaining cellular homeostasis. The conventional secretory pathway is an anabolic cellular process synthesizing and delivering proteins to distinct locations, including different organelles, the plasma membrane, and the extracellular media. On the other hand, autophagy is a catabolic cellular process that engulfs damaged organelles and aberrant cytosolic constituents into the double autophagosome membrane. After fusion with the lysosome and autolysosome formation, this process triggers digestion and recycling. A growing list of evidence indicates that these anabolic and catabolic processes are mutually regulated. While knowledge about the molecular actors involved in the coordination and functional cooperation between these two processes has increased over time, the mechanisms are still poorly understood. This review article summarized and discussed the most relevant evidence about the key molecular players implicated in the interorganelle crosstalk between the early secretory pathway and autophagy under normal and stressful conditions.

Stress-induced phase separation of ERES components into Sec bodies precedes ER exit inhibition in mammalian cells

Phase separation of components of ER exit sites (ERES) into membraneless compartments, the Sec bodies, occurs in Drosophila cells upon exposure to specific cellular stressors, namely, salt stress and amino acid starvation, and their formation is linked to the early secretory pathway inhibition. Here, we show Sec bodies also form in secretory mammalian cells upon the same stress. These reversible and membraneless structures are positive for ERES components, including both Sec16A and Sec16B isoforms and COPII subunits. We find that Sec16A, but not Sec16B, is a driver for Sec body formation, and that the coalescence of ERES components into Sec bodies occurs by fusion. Finally, we show that the stress-induced coalescence of ERES components into Sec bodies precedes ER exit inhibition, leading to their progressive depletion from ERES that become non-functional. Stress relief causes an immediate dissolution of Sec bodies and the concomitant restoration of ER exit. We propose that the dynamic conversion between ERES and Sec body assembly, driven by Sec16A, regulates protein exit from the ER during stress and upon stress relief in mammalian cells, thus providing a conserved pro-survival mechanism in response to stress.

Mechanical strain stimulates COPII-dependent secretory trafficking via Rac1

Cells are constantly exposed to various chemical and physical stimuli. While much has been learned about the biochemical factors that regulate secretory trafficking from the endoplasmic reticulum (ER), much less is known about whether and how this trafficking is subject to regulation by mechanical signals. Here, we show that subjecting cells to mechanical strain both induces the formation of ER exit sites (ERES) and accelerates ER-to-Golgi trafficking. We found that cells with impaired ERES function were less capable of expanding their surface area when placed under mechanical stress and were more prone to develop plasma membrane defects when subjected to stretching. Thus, coupling of ERES function to mechanotransduction appears to confer resistance of cells to mechanical stress. Furthermore, we show that the coupling of mechanotransduction to ERES formation was mediated via a previously unappreciated ER-localized pool of the small GTPase Rac1. Mechanistically, we show that Rac1 interacts with the small GTPase Sar1 to drive budding of COPII carriers and stimulates ER-to-Golgi transport. This interaction therefore represents an unprecedented link between mechanical strain and export from the ER.

Regulation of the COPII secretory machinery via focal adhesions and extracellular matrix signaling

Proteins that enter the secretory pathway are transported from their place of synthesis in the endoplasmic reticulum to the Golgi complex by COPII-coated carriers. The networks of proteins that regulate these components in response to extracellular cues have remained largely elusive. Using high-throughput microscopy, we comprehensively screened 378 cytoskeleton-associated and related proteins for their functional interaction with the coat protein complex II (COPII) components SEC23A and SEC23B. Among these, we identified a group of proteins associated with focal adhesions (FERMT2, MACF1, MAPK8IP2, NGEF, PIK3CA, and ROCK1) that led to the downregulation of SEC23A when depleted by siRNA. Changes in focal adhesions induced by plating cells on ECM also led to the downregulation of SEC23A and decreases in VSVG transport from ER to Golgi. Both the expression of SEC23A and the transport defect could be rescued by treatment with a focal adhesion kinase inhibitor. Altogether, our results identify a network of cytoskeleton-associated proteins connecting focal adhesions and ECM-related signaling with the gene expression of the COPII secretory machinery and trafficking.

Unscrambling exit site patterns on the endoplasmic reticulum as a quenched demixing process

The endoplasmic reticulum (ER) is a vital organelle in mammalian cells with a complex morphology. Consisting of sheet-like cisternae in the cell center, the peripheral ER forms a vast tubular network on which a dispersed pattern of a few hundred specialized domains (ER exit sites (ERESs)) is maintained. Molecular details of cargo sorting and vesicle formation at individual ERESs, fueling the early secretory pathway, have been studied in some detail. The emergence of spatially extended ERES patterns, however, has remained poorly understood. Here, we show that these patterns are determined by the underlying ER morphology, suggesting ERESs to emerge from a demixing process that is quenched by the ER network topology. In particular, we observed fewer but larger ERESs when transforming the ER network to more sheet-like morphologies. In contrast, little to no changes with respect to native ERES patterns were observed when fragmenting the ER, indicating that hampering the diffusion-mediated coarse graining of domains is key for native ERES patterns. Model simulations support the notion of effective diffusion barriers impeding the coarse graining and maturation of ERES patterns. We speculate that tuning a simple demixing mechanism by the ER topology allows for a robust but flexible adaption of ERES patterns, ensuring a properly working early secretory pathway in a variety of conditions.

Export Control: Post-transcriptional Regulation of the COPII Trafficking Pathway

The coat protein complex II (COPII) mediates forward trafficking of protein and lipid cargoes from the endoplasmic reticulum. COPII is an ancient and essential pathway in all eukaryotes and COPII dysfunction underlies a range of human diseases. Despite this broad significance, major aspects of COPII trafficking remain incompletely understood. For example, while the biochemical features of COPII vesicle formation are relatively well characterized, much less is known about how the COPII system dynamically adjusts its activity to changing physiologic cues or stresses. Recently, post-transcriptional mechanisms have emerged as a major mode of COPII regulation. Here, we review the current literature on how post-transcriptional events, and especially post-translational modifications, govern the COPII pathway.

The TRAPP complex mediates secretion arrest induced by stress granule assembly

The TRAnsport Protein Particle (TRAPP) complex controls multiple membrane trafficking steps and is strategically positioned to mediate cell adaptation to diverse environmental conditions, including acute stress. We have identified the TRAPP complex as a component of a branch of the integrated stress response that impinges on the early secretory pathway. The TRAPP complex associates with and drives the recruitment of the COPII coat to stress granules (SGs) leading to vesiculation of the Golgi complex and arrest of ER export. The relocation of the TRAPP complex and COPII to SGs only occurs in cycling cells and is CDK1/2-dependent, being driven by the interaction of TRAPP with hnRNPK, a CDK substrate that associates with SGs when phosphorylated. In addition, CDK1/2 inhibition impairs TRAPP complex/COPII relocation to SGs while stabilizing them at ER exit sites. Importantly, the TRAPP complex controls the maturation of SGs. SGs that assemble in TRAPP-depleted cells are smaller and are no longer able to recruit RACK1 and Raptor, two TRAPP-interactive signaling proteins, sensitizing cells to stress-induced apoptosis.

How to Avoid a No-Deal ER Exit

Efficiency and fidelity of protein secretion are achieved thanks to the presence of different steps, located sequentially in time and space along the secretory compartment, controlling protein folding and maturation. After entering into the endoplasmic reticulum (ER), secretory proteins attain their native structure thanks to specific chaperones and enzymes. Only correctly folded molecules are allowed by quality control (QC) mechanisms to leave the ER and proceed to downstream compartments. Proteins that cannot fold properly are instead retained in the ER to be finally destined to proteasomal degradation. Exiting from the ER requires, in most cases, the use of coated vesicles, departing at the ER exit sites, which will fuse with the Golgi compartment, thus releasing their cargoes. Protein accumulation in the ER can be caused by a too stringent QC or by ineffective transport: these situations could be deleterious for the organism, due to the loss of the secreted protein, and to the cell itself, because of abnormal increase of protein concentration in the ER. In both cases, diseases can arise. In this review, we will describe the pathophysiology of protein folding and transport between the ER and the Golgi compartment.

LTK is an ER-resident receptor tyrosine kinase that regulates secretion

The endoplasmic reticulum (ER) is a major regulator of cellular proteostasis. However, only little is known about signaling molecules resident to this organelle. Centonze et al. identify LTK as the first ER-resident receptor tyrosine kinase and show that it stimulates secretory trafficking out of the ER.

The endoplasmic reticulum (ER) is a key regulator of cellular proteostasis because it controls folding, sorting, and degradation of secretory proteins. Much has been learned about how environmentally triggered signaling pathways regulate ER function, but only little is known about local signaling at the ER. The identification of ER-resident signaling molecules will help gain a deeper understanding of the regulation of ER function and thus of proteostasis. Here, we show that leukocyte tyrosine kinase (LTK) is an ER-resident receptor tyrosine kinase. Depletion of LTK as well as its pharmacologic inhibition reduces the number of ER exit sites and slows ER-to-Golgi transport. Furthermore, we show that LTK interacts with and phosphorylates Sec12. Expression of a phosphoablating mutant of Sec12 reduces the efficiency of ER export. Thus, LTK-to-Sec12 signaling represents the first example of an ER-resident signaling module with the potential to regulate proteostasis.

COPII vesicles can affect the activity of antisense oligonucleotides by facilitating the release of oligonucleotides from endocytic pathways

RNase H1-dependent, phosphorothioate-modified antisense oligonucleotides (PS-ASOs) can enter cells through endocytic pathways and need to be released from the membrane-enclosed organelles, a limiting step for antisense activity. Accumulating evidence has suggested that productive PS-ASO release mainly occurs from late endosomes (LEs). However, how PS-ASOs escape from LEs is not well understood. Here, we report that upon PS-ASO incubation, COPII vesicles, normally involved in ER-Golgi transport, can re-locate to PS-ASO-containing LEs. Reduction of COPII coat proteins significantly decreased PS-ASO activity, without affecting the levels of PS-ASO uptake and early-to-late endosome transport, but caused slower PS-ASO release from LEs. COPII co-localization with PS-ASOs at LEs does not require de novo assembly of COPII at ER. Interestingly, reduction of STX5 and P115, proteins involved in tethering and fusion of COPII vesicles with Golgi membranes, impaired COPII re-localization to LEs and decreased PS-ASO activity. STX5 can re-locate to LEs upon PS-ASO incubation, can bind PS-ASOs, and the binding appears to be required for this pathway. Our study reveals a novel release pathway in which PS-ASO incubation causes LE re-localization of STX5, which mediates the recruitment of COPII vesicles to LEs to facilitate endosomal PS-ASO release, and identifies another key PS-ASO binding protein.

Diffusion of Exit Sites on the Endoplasmic Reticulum: A Random Walk on a Shivering Backbone

Major parts of the endoplasmic reticulum (ER) in eukaryotic cells are organized as a dynamic network of membrane tubules connected by three-way junctions. On this network, self-assembled membrane domains, called ER exit sites (ERES), provide platforms at which nascent cargo proteins are packaged into vesicular carriers for subsequent transport along the secretory pathway. Although ERES appear stationary and spatially confined on long timescales, we show here via single-particle tracking that they exhibit a microtubule-dependent and heterogeneous anomalous diffusion behavior on short and intermediate timescales. By quantifying key parameters of their random walk, we show that the subdiffusive motion of ERES is distinct from that of ER junctions, i.e., ERES are not tied to junctions but rather are mobile on ER tubules. We complement and corroborate our experimental findings with model simulations that also indicate that ERES are not actively moved by microtubules. Altogether, our study shows that ERES perform a random walk on the shivering ER backbone, indirectly powered by microtubular activity. Similar phenomena can be expected for other domains on subcellular structures, setting a caveat for the interpretation of domain-tracking data.

Modulation of the secretory pathway by amino-acid starvation

As a major anabolic pathway, the secretory pathway needs to adapt to the demands of the surrounding environment and responds to different exogenous signals and stimuli. In this context, the transport in the early secretory pathway from the endoplasmic reticulum (ER) to the Golgi apparatus appears particularly regulated. For instance, protein export from the ER is critically stimulated by growth factors. Conversely, nutrient starvation also modulates functions of the early secretory pathway in multiple ways. In this review, we focus on amino-acid starvation and how the function of the early secretory pathway is redirected to fuel autophagy, how the ER exit sites are remodeled into novel cytoprotective stress assemblies, and how secretion is modulated in vivo in starving organisms. With the increasingly exciting knowledge on mechanistic target of rapamycin complex 1 (mTORC1), the major nutrient sensor, it is also a good moment to establish how the modulation of the secretory pathway by amino-acid restriction intersects with this major signaling hub.

The link between autophagy and secretion: a story of multitasking proteins

The secretory and autophagy pathways can be thought of as the biosynthetic (i.e., anabolic) and degradative (i.e., catabolic) branches of the endomembrane system. In analogy to anabolic and catabolic pathways in metabolism, there is mounting evidence that the secretory and autophagy pathways are intimately linked and that certain regulatory elements are shared between them. Here we highlight the parallels and points of intersection between these two evolutionarily highly conserved and fundamental endomembrane systems. The intersection of these pathways may play an important role in remodeling membranes during cellular stress.

Sec16 in conventional and unconventional exocytosis: Working at the interface of membrane traffic and secretory autophagy?

Sec16 is classically perceived to be a scaffolding protein localized to the transitional endoplasmic reticulum (tER) or the ER exit sites (ERES), and has a conserved function in facilitating coat protein II (COPII) complex-mediated ER exit. Recent findings have, however, pointed toward a role for Sec16 in unconventional exocytosis of certain membrane proteins, such as the Cystic fibrosis transmembrane conductance regulator (CFTR) in mammalian cells, and possibly also α-integrin in certain contexts of Drosophila development. In this regard, Sec16 interacts with components of a recently deciphered pathway of stress-induced unconventional exocytosis, which is dependent on the tether protein Golgi reassembly stacking proteins (GRASPs) and the autophagy pathway. Intriguingly, Sec16 also appears to be post-translationally modified by autophagy-related signaling processes. Sec16 is known to be phosphorylated by the atypical extracellular signal regulated kinase 7 (Erk7) upon serum and amino acid starvation, both represent conditions that trigger autophagy. Recent work has also shown that Sec16 is phosphorylated, and thus regulated by the prominent autophagy-initiating Unc-51-like autophagy activating kinase 1 (Ulk1), as well as another autophagy modulator Leucine-rich repeat kinase 2 (Lrrk2). The picture emerging from Sec16's network of physical and functional interactors allows the speculation that Sec16 is situated (and may in yet undefined ways function) at the interface between COPII-mediated exocytosis of conventional vesicular traffic and the GRASP/autophagy-dependent mode of unconventional exocytosis.

TANGO1 recruits Sec16 to coordinately organize ER exit sites for efficient secretion

Mammalian endoplasmic reticulum (ER) exit sites export a variety of cargo molecules including oversized cargoes such as collagens. However, the mechanisms of their assembly and organization are not fully understood. TANGO1L is characterized as a collagen receptor, but the function of TANGO1S remains to be investigated. Here, we show that direct interaction between both isoforms of TANGO1 and Sec16 is not only important for their correct localization but also critical for the organization of ER exit sites. The depletion of TANGO1 disassembles COPII components as well as membrane-bound ER-resident complexes, resulting in fewer functional ER exit sites and delayed secretion. The ectopically expressed TANGO1 C-terminal domain responsible for Sec16 binding in mitochondria is capable of recruiting Sec16 and other COPII components. Moreover, TANGO1 recruits membrane-bound macromolecular complexes consisting of cTAGE5 and Sec12 to the ER exit sites. These data suggest that mammalian ER exit sites are organized by TANGO1 acting as a scaffold, in cooperation with Sec16 for efficient secretion.

Regulation of EGFR surface levels by COPII-dependent trafficking

Farhan discusses Scharaw et al.’s study about how the COPII machinery is used to replenish EGFR at the cell surface.

Cell surface levels of epidermal growth factor receptors (EGFRs) are thought to be controlled mainly by endocytic trafficking, with biosynthetic EGFR trafficking presumed to be a constitutive and unregulated process. However, Scharaw et al. (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201601090) demonstrate a role for inducible COPII trafficking in controlling EGFR surface levels.

The endosomal transcriptional regulator RNF11 integrates degradation and transport of EGFR

Maintenance of EGFR plasma membrane levels is critical for cell functioning. Scharaw et al. demonstrate that endosomal RNF11 is required for transcriptional up-regulation of COPII components, specifically facilitating EGFR transport in response to its lysosomal degradation after EGF stimulation.

Stimulation of cells with epidermal growth factor (EGF) induces internalization and partial degradation of the EGF receptor (EGFR) by the endo-lysosomal pathway. For continuous cell functioning, EGFR plasma membrane levels are maintained by transporting newly synthesized EGFRs to the cell surface. The regulation of this process is largely unknown. In this study, we find that EGF stimulation specifically increases the transport efficiency of newly synthesized EGFRs from the endoplasmic reticulum to the plasma membrane. This coincides with an up-regulation of the inner coat protein complex II (COPII) components SEC23B, SEC24B, and SEC24D, which we show to be specifically required for EGFR transport. Up-regulation of these COPII components requires the transcriptional regulator RNF11, which localizes to early endosomes and appears additionally in the cell nucleus upon continuous EGF stimulation. Collectively, our work identifies a new regulatory mechanism that integrates the degradation and transport of EGFR in order to maintain its physiological levels at the plasma membrane.

Sec16 alternative splicing dynamically controls COPII transport efficiency

The transport of secretory proteins from the endoplasmic reticulum (ER) to the Golgi depends on COPII-coated vesicles. While the basic principles of the COPII machinery have been identified, it remains largely unknown how COPII transport is regulated to accommodate tissue- or activation-specific differences in cargo load and identity. Here we show that activation-induced alternative splicing of Sec16 controls adaptation of COPII transport to increased secretory cargo upon T-cell activation. Using splice-site blocking morpholinos and CRISPR/Cas9-mediated genome engineering, we show that the number of ER exit sites, COPII dynamics and transport efficiency depend on Sec16 alternative splicing. As the mechanistic basis, we suggest the C-terminal Sec16 domain to be a splicing-controlled protein interaction platform, with individual isoforms showing differential abilities to recruit COPII components. Our work connects the COPII pathway with alternative splicing, adding a new regulatory layer to protein secretion and its adaptation to changing cellular environments.

The transport of secretory proteins from the endoplasmic reticulum to the Golgi depends on COPII-coated vesicles. Here, the authors show that activation-induced alternative splicing of Sec16 controls adaptation of COPII transport to increased secretory cargo upon T cell activation.

Genome-wide association study identifies novel susceptibility loci for cutaneous squamous cell carcinoma

Cutaneous squamous cell carcinoma represents the second most common cutaneous malignancy, affecting 7–11% of Caucasians in the United States. The genetic determinants of susceptibility to cutaneous squamous cell carcinoma remain largely unknown. Here we report the results of a two-stage genome-wide association study of cutaneous squamous cell carcinoma, totalling 7,404 cases and 292,076 controls. Eleven loci reached genome-wide significance (P<5 × 10⁻⁸) including seven previously confirmed pigmentation-related loci: MC1R, ASIP, TYR, SLC45A2, OCA2, IRF4 and BNC2. We identify an additional four susceptibility loci: 11q23.3 CADM1, a metastasis suppressor gene involved in modifying tumour interaction with cell-mediated immunity; 2p22.3; 7p21.1 AHR, the dioxin receptor involved in anti-apoptotic pathways and melanoma progression; and 9q34.3 SEC16A, a putative oncogene with roles in secretion and cellular proliferation. These susceptibility loci provide deeper insight into the pathogenesis of squamous cell carcinoma.

Cutaneous squamous cell carcinoma is the second most common type of skin cancer. In this genome-wide association study, which includes over 7,000 cases, the authors identify 4 new susceptibility loci for this cancer and also provide independent replication of 9 previously reported susceptibility loci.

SEC16A is a RAB10 effector required for insulin-stimulated GLUT4 trafficking in adipocytes

Sec16A is known to be required for COPII vesicle formation from the ER. Here, Bruno et al. show that, independent of its role at the ER, Sec16A is a RAB10 effector involved in the insulin-stimulated formation of specialized transport vesicles that ferry the GLUT4 glucose transporter to the plasma membrane of adipocytes.

RAB10 is a regulator of insulin-stimulated translocation of the GLUT4 glucose transporter to the plasma membrane (PM) of adipocytes, which is essential for whole-body glucose homeostasis. We establish SEC16A as a novel RAB10 effector in this process. Colocalization of SEC16A with RAB10 is augmented by insulin stimulation, and SEC16A knockdown attenuates insulin-induced GLUT4 translocation, phenocopying RAB10 knockdown. We show that SEC16A and RAB10 promote insulin-stimulated mobilization of GLUT4 from a perinuclear recycling endosome/TGN compartment. We propose RAB10–SEC16A functions to accelerate formation of the vesicles that ferry GLUT4 to the PM during insulin stimulation. Because GLUT4 continually cycles between the PM and intracellular compartments, the maintenance of elevated cell-surface GLUT4 in the presence of insulin requires accelerated biogenesis of the specialized GLUT4 transport vesicles. The function of SEC16A in GLUT4 trafficking is independent of its previously characterized activity in ER exit site formation and therefore independent of canonical COPII-coated vesicle function. However, our data support a role for SEC23A, but not the other COPII components SEC13, SEC23B, and SEC31, in the insulin stimulation of GLUT4 trafficking, suggesting that vesicles derived from subcomplexes of COPII coat proteins have a role in the specialized trafficking of GLUT4.

Systems biology of the secretory pathway: what have we learned so far?

Several RNAi screens were performed in search for regulators of the secretory pathway. These screens were performed in different organisms and cell lines and relied on different readouts. Therefore, they have only little overlap among their hits, leading to the question of what we have learned from this approach so far and how these screens contributed towards an integrative understanding of the endomembrane system. The aim of this review is to revisit these screens and discuss their strengths and weaknesses as well as potential reasons for their failure to overlap with each other. As with secretory trafficking, RNAi screens were also performed on other cellular processes such as cell migration and autophagy, both of which were shown to be intimately linked to secretion. Another aim of this review is to compare the outcome of the RNAi screens on secretion, autophagy and cell migration and ask whether the functional genomic approaches have uncovered potential mechanistic insights into the links between these processes.

Phospholipase C γ1 regulates early secretory trafficking and cell migration via interaction with p115

The role of early secretory trafficking in the regulation of cell motility remains incompletely understood. Here we used a small interfering RNA screen to monitor the effects on structure of the Golgi apparatus and cell migration. Two major Golgi phenotypes were observed-fragmented and small Golgi. The latter exhibited a stronger correlation with a defect in cell migration. Among the small Golgi hits, we focused on phospholipase C γ1 (PLCγ1). We show that PLCγ1 regulates Golgi structure and cell migration independently of its catalytic activity but in a manner that depends on interaction with the tethering protein p115. PLCγ1 regulates the dynamics of p115 in the early secretory pathway, thereby controlling trafficking from the endoplasmic reticulum to the Golgi. Our results uncover a new function of PLCγ1 that is independent of its catalytic function and link early secretory trafficking to the regulation of cell migration.