Alternative routes to the cell surface underpin insulin-regulated membrane trafficking of GLUT4

Extract of Phyllanthus emblica L. fruit stimulates basal glucose uptake and ameliorates palmitate-induced insulin resistance through AMPK activation in C2C12 myotubes

BACKGROUND

The fruit of Phyllanthus emblica L., a traditional medicine in China and India, is used to treat diabetes mellitus. Its water extract (WEPE) has demonstrated hypoglycemic effects in diabetic rats, but its mechanisms on glucose utilization and insulin resistance in skeletal muscle remain unclear. Therefore, this study aims to investigate the effects and underlying mechanisms of WEPE on glucose utilization and insulin resistance using C2C12 myotubes.

METHODS

Effects of WEPE on glucose uptake, GLUT4 translocation, and AMPK and AKT phosphorylation were investigated in C2C12 myotubes and palmitate-treated myotubes. An AMPK inhibitor and siRNA were used to explore the mechanisms of WEPE. Glucose uptake was determined using a 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino)-2-deoxyglucose (2-NBDG) uptake assay, and protein expression and GLUT4 translocation were assessed via western blotting.

RESULTS

In normal myotubes, WEPE significantly stimulated glucose uptake and GLUT4 translocation to the plasma membrane at concentrations of 125 and 250 µg/mL. This was accompanied by an increase in the phosphorylation of AMPK and its downstream targets. However, both compound C and AMPK siRNA blocked the WEPE-induced GLUT4 translocation and glucose uptake. Moreover, pretreatment with STO-609, a calcium/calmodulin-dependent protein kinase kinase β (CaMKKβ) inhibitor, inhibited WEPE-induced AMPK phosphorylation and attenuated the WEPE-stimulated glucose uptake and GLUT4 translocation. In myotubes treated with palmitate, WEPE prevented palmitate-induced insulin resistance by enhancing insulin-mediated glucose uptake and AKT phosphorylation. It also restored the insulin-mediated translocation of GLUT4 from cytoplasm to membrane. However, these effects of WEPE on glucose uptake and GLUT4 translocation were blocked by pretreatment with compound C.

CONCLUSIONS

WEPE significantly stimulated basal glucose uptake though CaMKKβ/AMPK pathway and markedly ameliorated palmitate-induced insulin resistance by activating the AMPK pathway in C2C12 myotubes.

Cellugyrin (synaptogyrin-2) dependent pathways are used by bacterial cytolethal distending toxin and SARS-CoV-2 virus to gain cell entry

Aggregatibacter actinomycetemcomitans cytolethal distending toxin (Cdt) is capable of intoxicating lymphocytes macrophages, mast cells and epithelial cells. Following Cdt binding to cholesterol, in the region of membrane lipid rafts, the CdtB and CdtC subunits are internalized and traffic to intracellular compartments. These events are dependent upon, cellugyrin, a critical component of synaptic like microvesicles (SLMVCg+). Target cells, such as Jurkat cells, rendered unable to express cellugyrin are resistant to Cdt-induced toxicity. Similar to Cdt, SARS-CoV-2 entry into host cells is initiated by binding to cell surface receptors, ACE-2, also associated with cholesterol-rich lipid rafts; this association leads to fusion and/or endocytosis of viral and host cell membranes and intracellular trafficking. The similarity in internalization pathways for both Cdt and SARS-CoV-2 led us to consider the possibility that cellugyrin was a critical component in both processes. Cellugyrin deficient Calu-3 cells (Calu-3Cg-) were prepared using Lentiviral particles containing shRNA; these cells were resistant to infection by VSV/SARS-CoV-2-spike pseudotype virus and partially resistant to VSV/VSV-G pseudotype virus. Synthetic peptides representing various regions of the cellugyrin protein were prepared and assessed for their ability to bind to Cdt subunits using surface plasmon resonance. Cdt was capable of binding to a region designated the middle outer loop (MOL) which corresponds to a region extending into the cytoplasmic surface of the SLMVCg+. SARS-CoV-2 spike proteins were assessed for their ability to bind to cellugyrin peptides; SARS-CoV-2 full length spike protein preferentially binds to a region within the SLMVCg+ lumen, designated intraluminal loop 1A. SARS-CoV-2-spike protein domain S1, which contains the receptor binding domains, binds to cellugyrin N-terminus which extends out from the cytoplasmic surface of SLMV. Binding specificity was further analyzed using cellugyrin scrambled peptide mutants. We propose that SLMVCg+ represent a component of a common pathway that facilitates pathogen and/or pathogen-derived toxins to gain host cell entry.

Characterization of a loss-of-function NSF attachment protein beta mutation in monozygotic triplets affected with epilepsy and autism using cortical neurons from proband-derived and CRISPR-corrected induced pluripotent stem cell lines

We investigated whether a homozygous recessive genetic variant of NSF attachment protein beta (NAPB) gene inherited by monozygotic triplets contributed to their phenotype of early-onset epilepsy and autism. Induced pluripotent stem cell (iPSC) lines were generated from all three probands and both parents. The NAPB genetic variation was corrected in iPSC lines from two probands by CRISPR/Cas9 gene editing. Cortical neurons were produced by directed, in vitro differentiation from all iPSC lines. These cell line-derived neurons enabled us to determine that the genetic variation in the probands causes exon skipping and complete absence of NAPB protein. Electrophysiological and transcriptomic comparisons of cortical neurons derived from parents and probands cell lines indicate that loss of NAPB function contributes to alterations in neuronal functions and likely contributed to the impaired neurodevelopment of the triplets.

Pan-cancer analysis of SYNGR2 with a focus on clinical implications and immune landscape in liver hepatocellular carcinoma

BACKGROUND

Synaptogyrin-2 (SYNGR2), as a member of synaptogyrin gene family, is overexpressed in several types of cancer. However, the role of SYNGR2 in pan-cancer is largely unexplored.

METHODS

From the TCGA and GEO databases, we obtained bulk transcriptomes, and clinical information. We examined the expression patterns, prognostic values, and diagnostic value of SYNGR2 in pan-cancer, and investigated the relationship of SYNGR2 expression with tumor mutation burden (TMB), microsatellite instability (MSI), immune infiltration, and immune checkpoint (ICP) genes. The gene set enrichment analysis (GSEA) software was used to perform pathway analysis. Besides, we built a nomogram of liver hepatocellular carcinoma patients (LIHC) and validated its prediction accuracy.

RESULTS

SYNGR2 was highly expressed in most cancers. The high expression of SYNGR2 significantly reduced the overall survival (OS), disease-specific survival (DSS), disease-free interval (DFI), and progression-free interval (PFI) in multiple types of cancer. Also, receiver operating characteristic (ROC) curve analysis demonstrated that SYNGR2 showed high accuracy in distinguishing cancerous tissues from normal ones. Moreover, SYNGR2 expression was correlated with TMB, MSI, immune scores, and immune cell infiltrations. We also analyzed the association of SYNGR2 with immunotherapy response in LIHC. Finally, a nomogram including SYNGR2 and pathologic T, N, M stage was built and exhibited good predictive power for the OS, DSS, and PFI of LIHC patients.

CONCLUSION

Overall, SYNGR2 is a critical oncogene in various tumors. SYNGR2 participates in the carcinogenic progression, and may contribute to the immune infiltration in tumor microenvironment. Our study suggests that SYNGR2 can serve as a predictor related to prognosis in pan-cancer, especially LIHC.

Oral Subacute Exposure to Cadmium LOAEL Dose Induces Insulin Resistance and Impairment of the Hormonal and Metabolic Liver-Adipose Axis in Wistar Rats

Cadmium is a nonessential transition metal considered one of the more hazardous environmental contaminants. The population is chronically exposed to this metal at low concentrations, designated as the LOAEL (lowest observable adverse effect level) dose. We aimed to investigate whether oral subacute exposure to cadmium LOAEL disrupts hormonal and metabolic effects of the liver-adipose axis in Wistar rats. Fifty male Wistar rats were separated into two groups: control (standard normocalorie diet + water free of cadmium) and cadmium (standard normocalorie diet + drinking water with 32.5 ppm CdCl2). After 1 month, zoometry, a serum lipid panel, adipokines, and proinflammatory cytokines were evaluated. Tests of glucose and insulin tolerance (ITT) and insulin resistance were performed. Histological studies on structure, triglyceride distribution, and protein expression of the insulin pathway were performed in the liver and retroperitoneal adipose tissue. In both tissues, the cadmium, triglyceride, glycogen, and proinflammatory cytokine contents were also quantified. The cadmium group developed dyslipidemia, glucose intolerance, hyperinsulinemia, hyperleptinemia, inflammation, and selective insulin resistance in the liver and adipose tissue. In the liver, glycogen synthesis was diminished, while de novo lipogenesis increased, which was associated with low GSK3β-pS9 and strong expression of SREBP-1c. Dysfunctional adipose tissue was observed with hypertrophy and lipolysis, without changes in SREBP-1c expression and low glycogen synthesis. Both tissues accumulated cadmium and developed inflammation. In conclusion, oral subacute cadmium LOAEL dose exposure induces inflammation, insulin signaling modifications, an early insulin resistance stage (insensibility), and impairment of the hormonal and metabolic liver-adipose axis in Wistar rats.

SYNGR2 serves as a prognostic biomarker and correlates with immune infiltrates in esophageal squamous cell carcinoma

BACKGROUND

Synaptogyrin-2 (SYNGR2) plays an important role in regulating membrane traffic in nonneuronal cells. However, the role of SYNGR2 in esophageal squamous cell carcinoma (ESCC) remains unclear.

METHODS

All original data were downloaded from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases and integrated via R 3.5.3. SYNGR2 expression was explored in the TCGA and GEO databases. The correlations between SYNGR2 and cancer immune characteristics were analyzed via the TIMER and TISIDB databases.

RESULTS

In general, SYNGR2 was predominantly overexpressed and had reference value in the diagnosis and prognostic estimation of ESCC. Upregulated SYNGR2 was associated with poorer overall survival, poorer disease-specific survival and T stage in ESCC. Mechanistically, we identified hub genes that included a total of 38 SYNGR2-related genes, which were tightly associated with the protein polyubiquitination pathway in ESCC patients. SYNGR2 expression was negatively related to the infiltrating levels of T helper cells. SYNGR2 methylation was positively correlated with the expression of chemokines (CCL2 and CXCL12), chemokine receptors (CCR1 and CCR2), immunoinhibitors (CXCL12 and TNFRSF4) and immunostimulators (CSF1R and PDCD1LG2) in ESCC.

CONCLUSION

SYNGR2 may be used as a biomarker for determining prognosis and immune infiltration in ESCC.

Ubiquitin-like processing of TUG proteins as a mechanism to regulate glucose uptake and energy metabolism in fat and muscle

In response to insulin stimulation, fat and muscle cells mobilize GLUT4 glucose transporters to the cell surface to enhance glucose uptake. Ubiquitin-like processing of TUG (Aspscr1, UBXD9) proteins is a central mechanism to regulate this process. Here, recent advances in this area are reviewed. The data support a model in which intact TUG traps insulin-responsive "GLUT4 storage vesicles" at the Golgi matrix by binding vesicle cargoes with its N-terminus and matrix proteins with its C-terminus. Insulin stimulation liberates these vesicles by triggering endoproteolytic cleavage of TUG, mediated by the Usp25m protease. Cleavage occurs in fat and muscle cells, but not in fibroblasts or other cell types. Proteolytic processing of intact TUG generates TUGUL, a ubiquitin-like protein modifier, as the N-terminal cleavage product. In adipocytes, TUGUL modifies a single protein, the KIF5B kinesin motor, which carries GLUT4 and other vesicle cargoes to the cell surface. In muscle, this or another motor may be modified. After cleavage of intact TUG, the TUG C-terminal product is extracted from the Golgi matrix by the p97 (VCP) ATPase. In both muscle and fat, this cleavage product enters the nucleus, binds PPARγ and PGC-1α, and regulates gene expression to promote fatty acid oxidation and thermogenesis. The stability of the TUG C-terminal product is regulated by an Ate1 arginyltransferase-dependent N-degron pathway, which may create a feedback mechanism to control oxidative metabolism. Although it is now clear that TUG processing coordinates glucose uptake with other aspects of physiology and metabolism, many questions remain about how this pathway is regulated and how it is altered in metabolic disease in humans.

Integrated Proteomic and Transcriptomic Analysis of Gonads Reveal Disruption of Germ Cell Proliferation and Division, and Energy Storage in Glycogen in Sterile Triploid Pacific Oysters (Crassostrea gigas)

Triploid oysters have poor gonadal development, which can not only bring higher economic benefits but also have a potential application in the genetic containment of aquaculture. However, the key factors that influence germ cell development in triploid oysters remain unclear. In this study, data-independent acquisition coupled to transcriptomics was applied to identify genes/proteins related to sterility in triploid Crassostrea gigas. Eighty-four genes were differentially expressed at both the protein and mRNA levels between fertile and sterile females. For male oysters, 207 genes were differentially expressed in the transcriptomic and proteomic analysis. A large proportion of downregulated genes were related to cell division, which may hinder germ cell proliferation and cause apoptosis. In sterile triploid females, a primary cause of sterility may be downregulation in the expression levels of certain mitotic cell cycle-related genes. In sterile triploid males, downregulation of genes related to cell cycle and sperm motility indicated that the disruption of mitosis or meiosis and flagella defects may be linked with the blocking of spermatogenesis. Additionally, the genes upregulated in sterile oysters were mainly associated with the biosynthesis of glycogen and fat, suggesting that sterility in triploids stimulates the synthesis of glycogen and energy conservation in gonad tissue.

Genetic evidence for an inhibitory role of tomosyn in insulin-stimulated GLUT4 exocytosis

Exocytosis is a vesicle fusion process driven by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). A classic exocytic pathway is insulin-stimulated translocation of the glucose transporter type 4 (GLUT4) from intracellular vesicles to the plasma membrane in adipocytes and skeletal muscles. The GLUT4 exocytic pathway plays a central role in maintaining blood glucose homeostasis and is compromised in insulin resistance and type 2 diabetes. A candidate regulator of GLUT4 exocytosis is tomosyn, a soluble protein expressed in adipocytes. Tomosyn directly binds to GLUT4 exocytic SNAREs in vitro but its role in GLUT4 exocytosis was unknown. In this work, we used CRISPR-Cas9 genome editing to delete the two tomosyn-encoding genes in adipocytes. We observed that both basal and insulin-stimulated GLUT4 exocytosis was markedly elevated in the double knockout (DKO) cells. By contrast, adipocyte differentiation and insulin signaling remained intact in the DKO adipocytes. In a reconstituted liposome fusion assay, tomosyn inhibited all the SNARE complexes underlying GLUT4 exocytosis. The inhibitory activity of tomosyn was relieved by NSF and α-SNAP, which act in concert to remove tomosyn from GLUT4 exocytic SNAREs. Together, these studies revealed an inhibitory role for tomosyn in insulin-stimulated GLUT4 exocytosis in adipocytes. We suggest that tomosyn-arrested SNAREs represent a reservoir of fusion capacity that could be harnessed to treat patients with insulin resistance and type 2 diabetes.

Rapid Enzyme-Mediated Biotinylation for Cell Surface Proteome Profiling

Cell surface is the primary site for sensing extracellular stimuli. The knowledge of the transient changes on the surfaceome upon a perturbation is very important as the initial changed proteins could be driving molecules for some phenotype. In this study, we report a fast cell surface labeling strategy based on peroxidase-mediated oxidative tyrosine coupling strategy, enabling efficient and selective cell surface labeling within seconds. With a labeling time of 1 min, 2684 proteins, including 1370 (51%) cell surface-annotated proteins (cell surface/plasma membrane/extracellular), 732 transmembrane proteins, and 81 cluster of differentiation antigens, were identified from HeLa cells. By comparison with the negative control experiment using quantitative proteomics, 500 (68%) out of the 731 significantly enriched proteins (p-value < 0.05, ≥2-fold) in positive experimental samples were cell surface-annotated proteins. Finally, this technology was applied to track the dynamic changes of the surfaceome upon insulin stimulation at two time points (5 min and 2 h) in HepG2 cells. Thirty-two proteins, including INSR, CTNNB1, TFRC, IGF2R, and SORT1, were found to be significantly regulated (p-value < 0.01, ≥1.5-fold) after insulin exposure by different mechanisms. We envision that this technique could be a powerful tool to analyze the transient changes of the surfaceome with a good time resolution and to delineate the temporal and spatial regulation of cellular signaling.

Regulatory effects of protein S-acylation on insulin secretion and insulin action

Post-translational modifications (PTMs) such as phosphorylation and ubiquitination are well-studied events with a recognized importance in all aspects of cellular function. By contrast, protein S-acylation, although a widespread PTM with important functions in most physiological systems, has received far less attention. Perturbations in S-acylation are linked to various disorders, including intellectual disability, cancer and diabetes, suggesting that this less-studied modification is likely to be of considerable biological importance. As an exemplar, in this review, we focus on the newly emerging links between S-acylation and the hormone insulin. Specifically, we examine how S-acylation regulates key components of the insulin secretion and insulin response pathways. The proteins discussed highlight the diverse array of proteins that are modified by S-acylation, including channels, transporters, receptors and trafficking proteins and also illustrate the diverse effects that S-acylation has on these proteins, from membrane binding and micro-localization to regulation of protein sorting and protein interactions.

An Allosteric Binding Site on Sortilin Regulates the Trafficking of VLDL, PCSK9, and LDLR in Hepatocytes

ApoB lipoproteins (apo B-Lp) are produced in hepatocytes, and their secretion requires the cargo receptor sortilin. We examined the secretion of apo B-Lp-containing very low-density lipoprotein (VLDL), an LDL progenitor. Sortilin also regulates the trafficking of the subtilase PCSK9, which when secreted binds the LDL receptor (LDLR), resulting in its endocytosis and destruction at the lysosome. We show that the site 2 binding compound (cpd984) has multiple effects in hepatocytes, including (1) enhanced Apo-Lp secretion, (2) increased cellular PCSK9 retention, and (3) augmented levels of LDLR at the plasma membrane. We postulate that cpd984 enhances apo B-Lp secretion in part through binding the lipid phosphatidylinositol 3,4,5-trisphosphate (PIP₃), which is present at higher levels on circulating VLDL form fed rats relative to after fasting. We attribute the enhanced VLDL secretion to its increased binding affinity for sortilin site 1 induced by cpd984 binding site 2. This hinders PCSK9 binding and secretion, which would subsequently prevent its binding to LDLR leading to its degradation. This suggests that site 2 is an allosteric regulator of site 1 binding. This effect is not limited to VLDL, as cpd984 augments binding of the neuropeptide neurotensin (NT) to sortilin site 1. Molecular dynamics simulations demonstrate that the C-terminus of NT (Ct-NT) stably binds site 1 through an electrostatic interaction. This was bolstered by the ability of Ct-NT to disrupt lower-affinity interactions between sortilin and the site 1 ligand PIP₃. Together, these data show that binding cargo at sortilin site 1 is allosterically regulated through site 2 binding, with important ramifications for cellular lipid homeostasis involving proteins such as PCSK9 and LDLR.

Motor neuron translatome reveals deregulation of SYNGR4 and PLEKHB1 in mutant TDP-43 amyotrophic lateral sclerosis models

Amyotrophic lateral sclerosis (ALS) is an incurable neurological disease with progressive loss of motor neuron (MN) function in the brain and spinal cord. Mutations in TARDBP, encoding the RNA-binding protein TDP-43, are one cause of ALS, and TDP-43 mislocalization in MNs is a key pathological feature of >95% of ALS cases. While numerous studies support altered RNA regulation by TDP-43 as a major cause of disease, specific changes within MNs that trigger disease onset remain unclear. Here, we combined translating ribosome affinity purification (TRAP) with RNA sequencing to identify molecular changes in spinal MNs of TDP-43-driven ALS at motor symptom onset. By comparing the MN translatome of hTDP-43A315T mice to littermate controls and to mice expressing wild type hTDP-43, we identified hundreds of mRNAs that were selectively up- or downregulated in MNs. We validated the deregulated candidates Tex26, Syngr4, and Plekhb1 mRNAs in an independent TRAP experiment. Moreover, by quantitative immunostaining of spinal cord MNs, we found corresponding protein level changes for SYNGR4 and PLEKHB1. We also observed these changes in spinal MNs of an independent ALS mouse model caused by a different patient mutant allele of TDP-43, suggesting that they are general features of TDP-43-driven ALS. Thus, we identified SYNGR4 and PLEKHB1 to be deregulated in MNs at motor symptom onset in TDP-43-driven ALS models. This spatial and temporal pattern suggests that these proteins could be functionally important for driving the transition to the symptomatic phase of the disease.

Building GLUT4 Vesicles: CHC22 Clathrin's Human Touch

Insulin stimulates glucose transport by triggering regulated delivery of intracellular vesicles containing the GLUT4 glucose transporter to the plasma membrane. This process is defective in diseases such as type 2 diabetes (T2DM). While studies in rodent cells have been invaluable in understanding GLUT4 traffic, evolutionary plasticity must be considered when extrapolating these findings to humans. Recent work has identified species-specific distinctions in GLUT4 traffic, notably the participation of a novel clathrin isoform, CHC22, in humans but not rodents. Here, we discuss GLUT4 sorting in different species and how studies of CHC22 have identified new routes for GLUT4 trafficking. We further consider how different sorting-protein complexes relate to these routes and discuss other implications of these pathways in cell biology and disease.

Internalization and Intoxication of Human Macrophages by the Active Subunit of the Aggregatibacter actinomycetemcomitans Cytolethal Distending Toxin Is Dependent Upon Cellugyrin (Synaptogyrin-2)

The Aggregatibacter actinomycetemcomitans cytolethal distending toxin (Cdt) is a heterotrimeric AB₂ toxin capable of inducing cell cycle arrest and apoptosis in lymphocytes and other cell types. Recently, we have demonstrated that human macrophages are resistant to Cdt-induced apoptosis but are susceptible to toxin-induced pro-inflammatory cytokine response involving activation of the NLRP3 inflammasome. Exposure to Cdt results in binding to the cell surface followed by internalization and translocation of the active subunit, CdtB, to intracellular compartments. Internalization involves hijacking of retrograde pathways; treatment of cells with Retro-2 leads to a decrease in CdtB-Golgi association. These events are dependent upon toxin binding to cholesterol in the context of lipid rich membrane microdomains often referred to as lipid rafts. We now demonstrate that within 1 h of exposure of macrophages to Cdt, CdtB is internalized and found primarily within lipid rafts; concurrently, cellugyrin (synaptogyrin-2) also translocates into lipid rafts. Further analysis by immunoprecipitation indicates that CdtB associates with complexes containing both cellugyrin and Derlin-2. Moreover, a human macrophage cell line deficient in cellugyrin expression (THP-1^Cg-) challenged with Cdt failed to internalize CdtB and was resistant to the Cdt-induced pro-inflammatory response. We propose that lipid rafts along with cellugyrin play a critical role in the internalization and translocation of CdtB to critical intracellular target sites in human macrophages. These studies provide the first evidence that cellugyrin is expressed in human macrophages and plays a critical role in Cdt toxicity of these cells.

Characterisation of GLUT4 trafficking in HeLa cells: comparable kinetics and orthologous trafficking mechanisms to 3T3-L1 adipocytes

Insulin-stimulated glucose transport is a characteristic property of adipocytes and muscle cells and involves the regulated delivery of glucose transporter (GLUT4)-containing vesicles from intracellular stores to the cell surface. Fusion of these vesicles results in increased numbers of GLUT4 molecules at the cell surface. In an attempt to overcome some of the limitations associated with both primary and cultured adipocytes, we expressed an epitope- and GFP-tagged version of GLUT4 (HA–GLUT4–GFP) in HeLa cells. Here we report the characterisation of this system compared to 3T3-L1 adipocytes. We show that insulin promotes translocation of HA–GLUT4–GFP to the surface of both cell types with similar kinetics using orthologous trafficking machinery. While the magnitude of the insulin-stimulated translocation of GLUT4 is smaller than mouse 3T3-L1 adipocytes, HeLa cells offer a useful, experimentally tractable, human model system. Here, we exemplify their utility through a small-scale siRNA screen to identify GOSR1 and YKT6 as potential novel regulators of GLUT4 trafficking in human cells.

Inducible Exoc7/Exo70 knockout reveals a critical role of the exocyst in insulin-regulated GLUT4 exocytosis

Insulin promotes glucose uptake by triggering the translocation of glucose transporter type 4 (GLUT4) from intracellular vesicles to the plasma membrane through exocytosis. GLUT4 exocytosis is a vesicle fusion event involving fusion of GLUT4-containing vesicles with the plasma membrane. For GLUT4 vesicle fusion to occur, GLUT4 vesicles must first be tethered to the plasma membrane. A key tethering factor in exocytosis is a heterooctameric protein complex called the exocyst. The role of the exocyst in GLUT4 exocytosis, however, remains incompletely understood. Here we first systematically analyzed data from a genome-scale CRISPR screen in HeLa cells that targeted virtually all known genes in the human genome, including 12 exocyst genes. The screen recovered only a subset of the exocyst genes, including exocyst complex component 7 (Exoc7/Exo70). Other exocyst genes, however, were not isolated in the screen, likely because of functional redundancy. Our findings suggest that selection of an appropriate exocyst gene is critical for genetic studies of exocyst functions. Next we developed an inducible adipocyte genome editing system that enabled Exoc7 gene deletion in adipocytes without interfering with adipocyte differentiation. We observed that insulin-stimulated GLUT4 exocytosis was markedly inhibited in Exoc7 KO adipocytes. Insulin signaling, however, remained intact in these KO cells. These results indicate that the exocyst plays a critical role in insulin-stimulated GLUT4 exocytosis in adipocytes. We propose that the strategy outlined in this work could be instrumental in genetically dissecting other membrane-trafficking pathways in adipocytes.

GLUT4 Storage Vesicles: Specialized Organelles for Regulated Trafficking

Fat and muscle cells contain a specialized, intracellular organelle known as the GLUT4 storage vesicle (GSV). Insulin stimulation mobilizes GSVs, so that these vesicles fuse at the cell surface and insert GLUT4 glucose transporters into the plasma membrane. This example is likely one instance of a broader paradigm for regulated, non-secretory exocytosis, in which intracellular vesicles are translocated in response to diverse extracellular stimuli. GSVs have been studied extensively, yet these vesicles remain enigmatic. Data support the view that in unstimulated cells, GSVs are present as a pool of preformed small vesicles, which are distinct from endosomes and other membrane-bound organelles. In adipocytes, GSVs contain specific cargoes including GLUT4, IRAP, LRP1, and sortilin. They are formed by membrane budding, involving sortilin and probably CHC22 clathrin in humans, but the donor compartment from which these vesicles form remains uncertain. In unstimulated cells, GSVs are trapped by TUG proteins near the endoplasmic reticulum - Golgi intermediate compartment (ERGIC). Insulin signals through two main pathways to mobilize these vesicles. Signaling by the Akt kinase modulates Rab GTPases to target the GSVs to the cell surface. Signaling by the Rho-family GTPase TC10α stimulates Usp25m-mediated TUG cleavage to liberate the vesicles from the Golgi. Cleavage produces a ubiquitin-like protein modifier, TUGUL, that links the GSVs to KIF5B kinesin motors to promote their movement to the cell surface. In obesity, attenuation of these processes results in insulin resistance and contributes to type 2 diabetes and may simultaneously contribute to hypertension and dyslipidemia in the metabolic syndrome.

Effect of Lithium on the Mechanism of Glucose Transport in Skeletal Muscles

While lithium is known to stimulate glucose transport into skeletal muscle, the mechanisms of the increased glucose transport by lithium in skeletal muscle are not well defined yet. We excised epitrochlearis muscles from male Wistar rats and measured the transport rates of a glucose analog into lithium-, insulin-, and muscular contraction-stimulated skeletal muscle tissue and we also analyzed the levels of cell surface glucose transport 4 using a photolabeling and multicolor immunofluorescence method. In addition, we generated a cell line that stably expresses myc-tagged GLUT4 to measure the rates of GLUT4 internalization and externalization. Lithium significantly increased 2-DG glucose transport rate in skeletal muscles; however, it was significantly lower than the stimulation induced by the maximum concentration of insulin or tetanic contraction. But co-treatment of lithium with insulin or tetanic contraction increased glucose transport rate by ∼200% more than lithium alone. When skeletal muscle tissues were treated with lithium, insulin, and muscular contraction, the levels of cell surface GLUT4 protein contents were increased similarly by ∼6-fold compared with the basal levels. When insulin or lithium stimuli were maintained, the rate of GLUT4myc internalization was significantly lower, and lithium was found to suppress the internalization of GLUT4myc more strongly. The lithium-induced increase in glucose uptake of skeletal muscles appears to increase in cell surface GLUT4 levels caused by decreased internalization of GLUT4. It is concluded that co-treatment of lithium with insulin and muscular contraction had a synergistic effect on glucose transport rate in skeletal muscle.

Internalization of the Active Subunit of the Aggregatibacter actinomycetemcomitans Cytolethal Distending Toxin Is Dependent upon Cellugyrin (Synaptogyrin 2), a Host Cell Non-Neuronal Paralog of the Synaptic Vesicle Protein, Synaptogyrin 1

The Aggregatibacter actinomycetemcomitans cytolethal distending toxin (Cdt) is a heterotrimeric AB₂ toxin capable of inducing lymphocytes, and other cell types, to undergo cell cycle arrest and apoptosis. Exposure to Cdt results in binding to the cell surface followed by internalization and translocation of the active subunit, CdtB, to intracellular compartments. These events are dependent upon toxin binding to cholesterol in the context of lipid rich membrane microdomains often referred to as lipid rafts. We now demonstrate that, in addition to binding to the plasma membrane of lymphocytes, another early and critical event initiated by Cdt is the translocation of the host cell protein, cellugyrin (synaptogyrin-2) to the same cholesterol-rich microdomains. Furthermore, we demonstrate that cellugyrin is an intracellular binding partner for CdtB as demonstrated by immunoprecipitation. Using CRISPR/cas9 gene editing we established a Jurkat cell line deficient in cellugyrin expression (Jurkat^Cg−); these cells were capable of binding Cdt, but unable to internalize CdtB. Furthermore, Jurkat^Cg− cells were not susceptible to Cdt-induced toxicity; these cells failed to exhibit blockade of the PI-3K signaling pathway, cell cycle arrest or cell death. We propose that cellugyrin plays a critical role in the internalization and translocation of CdtB to critical intracellular target sites. These studies provide critical new insight into the mechanism by which Cdt, and in particular, CdtB is able to induce toxicity.

Update on GLUT4 Vesicle Traffic: A Cornerstone of Insulin Action

Glucose transport is rate limiting for dietary glucose utilization by muscle and fat. The glucose transporter GLUT4 is dynamically sorted and retained intracellularly and redistributes to the plasma membrane (PM) by insulin-regulated vesicular traffic, or 'GLUT4 translocation'. Here we emphasize recent findings in GLUT4 translocation research. The application of total internal reflection fluorescence microscopy (TIRFM) has increased our understanding of insulin-regulated events beneath the PM, such as vesicle tethering and membrane fusion. We describe recent findings on Akt-targeted Rab GTPase-activating proteins (GAPs) (TBC1D1, TBC1D4, TBC1D13) and downstream Rab GTPases (Rab8a, Rab10, Rab13, Rab14, and their effectors) along with the input of Rac1 and actin filaments, molecular motors [myosinVa (MyoVa), myosin1c (Myo1c), myosinIIA (MyoIIA)], and membrane fusion regulators (syntaxin4, munc18c, Doc2b). Collectively these findings reveal novel events in insulin-regulated GLUT4 traffic.