GLUT4 is retained by an intracellular cycle of vesicle formation and fusion with endosomes

Caveolin-2 palmitoylation turnover facilitates insulin receptor substrate-1-directed lipid metabolism by insulin receptor tyrosine kinase

Here, we show that insulin induces palmitoylation turnover of Caveolin-2 (Cav-2) in adipocytes. Acyl protein thioesterases-1 (APT1) catalyzes Cav-2 depalmitoylation, and zinc finger DHHC domain-containing protein palmitoyltransferase 21 (ZDHHC21) repalmitoylation of the depalmitoylated Cav-2 for the turnover, thereby controlling insulin receptor (IR)-Cav-2-insulin receptor substrate-1 (IRS-1)-Akt-driven signaling. Insulin-induced palmitoylation turnover of Cav-2 facilitated glucose uptake and fat storage through induction of lipogenic genes. Cav-2-, APT1-, and ZDHHC21-deficient adipocytes, however, showed increased induction of lipolytic genes and glycerol release. In addition, white adipose tissues from insulin sensitive and resistant obese patients exhibited augmented expression of LYPLA1 (APT1) and ZDHHC20 (ZDHHC20). Our study identifies the specific enzymes regulating Cav-2 palmitoylation turnover, and reveals a new mechanism by which insulin-mediated lipid metabolism is controlled in adipocytes.

Akt may associate with insulin-responsive vesicles via interaction with sortilin

Insulin-responsive vesicles (IRVs) deliver the glucose transporter Glut4 to the plasma membrane in response to activation of the insulin signaling cascade: insulin receptor-IRS-PI3 kinase-Akt-TBC1D4-Rab10. Previous studies have shown that Akt, TBC1D4, and Rab10 are compartmentalized on the IRVs. Although functionally significant, the mechanism of Akt association with the IRVs remains unknown. Using pull-down assays, immunofluorescence microscopy, and cross-linking, we have found that Akt may be recruited to the IRVs via the interaction with the juxtamembrane domain of the cytoplasmic C terminus of sortilin, a major IRV protein. Overexpression of full-length sortilin increases insulin-stimulated phosphorylation of TBC1D4 and glucose uptake in adipocytes, while overexpression of the cytoplasmic tail of sortilin has the opposite effect. Our findings demonstrate that the IRVs represent both a scaffold and a target of insulin signaling.

Regulated dynamic subcellular GLUT4 localization revealed by proximal proteome mapping in human muscle cells

Regulation of glucose transport, which is central for control of whole-body metabolism, is determined by the amount of GLUT4 glucose transporter (also known as SLC2A4) in the plasma membrane (PM) of fat and muscle cells. Physiologic signals [such as activated insulin receptor or AMP-activated protein kinase (AMPK)] increase PM GLUT4. Here, we show that the distribution of GLUT4 between the PM and interior of human muscle cells is dynamically maintained, and that AMPK promotes PM redistribution of GLUT4 by regulating exocytosis and endocytosis. Stimulation of exocytosis by AMPK is mediated by Rab10 and the Rab GTPase-activating protein TBC1D4. APEX2 proximity mapping reveals that GLUT4 traverses both PM-proximal and PM-distal compartments in unstimulated muscle cells, further supporting retention of GLUT4 by a constitutive retrieval mechanism. AMPK-stimulated translocation involves GLUT4 redistribution among the same compartments traversed in unstimulated cells, with a significant recruitment of GLUT4 from the Golgi and trans-Golgi network compartments. Our comprehensive proximal protein mapping provides an integrated, high-density, whole-cell accounting of the localization of GLUT4 at a resolution of ∼20 nm that serves as a structural framework for understanding the molecular mechanisms regulating GLUT4 trafficking downstream of different signaling inputs in a physiologically relevant cell type.

Methods to Quantify the Dynamic Recycling of Plasma Membrane Channels

Store-operated Ca2+ entry (SOCE) is a ubiquitous Ca2+ signaling modality mediated by Orai Ca2+ channels at the plasma membrane (PM) and the endoplasmic reticulum (ER) Ca2+ sensors STIM1/2. At steady state, Orai1 constitutively cycles between an intracellular compartment and the PM. Orai1 PM residency is modulated by its endocytosis and exocytosis rates. Therefore, Orai1 trafficking represents an important regulatory mechanism to define the levels of Ca2+ influx. Here, we present a protocol using the dually tagged YFP-HA-Orai1 with a cytosolic YFP and extracellular hemagglutinin (HA) tag to quantify Orai1 cycling rates. For measuring Orai1 endocytosis, cells expressing YFP-HA-Orai1 are incubated with mouse anti-HA antibody for various periods of time before being fixed and stained for surface Orai1 with Cy5-labeled anti-mouse IgG. The cells are fixed again, permeabilized, and stained with Cy3-labeled anti-mouse IgG to reveal anti-HA that has been internalized. To quantify Orai1 exocytosis rate, cells are incubated with anti-HA antibody for various incubation periods before being fixed, permeabilized, and then stained with Cy5-labeled anti-mouse IgG. The Cy5/YFP ratio is plotted over time and fitted with a mono-exponential growth curve to determine exocytosis rate. Although the described assays were developed to measure Orai1 trafficking, they are readily adaptable to other PM channels. Key features Detailed protocols to quantify endocytosis and exocytosis rates of Orai1 at the plasma membrane that can be used in various cell lines. The endocytosis and exocytosis assays are readily adaptable to study the trafficking of other plasma membrane channels.

GLUT4 dynamic subcellular localization is controlled by AMP kinase activation as revealed by proximal proteome mapping in human muscle cells

Regulation of glucose transport into muscle and adipocytes, central for control of whole-body metabolism, is determined by the amount of GLUT4 glucose transporter in the plasma membrane ( PM ). Physiologic signals (activated insulin receptor or AMP kinase [ AMPK ]), acutely increase PM GLUT4 to enhance glucose uptake. Here we show in kinetic studies that intracellular GLUT4 is in equilibrium with the PM in unstimulated cultured human skeletal muscle cells, and that AMPK promotes GLUT4 redistribution to the PM by regulating both exocytosis and endocytosis. AMPK-stimulation of exocytosis requires Rab10 and Rab GTPase activating protein TBC1D4, requirements shared with insulin control of GLUT4 in adipocytes. Using APEX2 proximity mapping, we identify, at high-density and high-resolution, the GLUT4 proximal proteome, revealing GLUT4 traverses both PM proximal and distal compartments in unstimulated muscle cells. These data support intracellular retention of GLUT4 in unstimulated muscle cells by a dynamic mechanism dependent on the rates of internalization and recycling. AMPK promoted GLUT4 translocation to the PM involves redistribution of GLUT4 among the same compartments traversed in unstimulated cells, with a significant redistribution of GLUT4 from the PM distal Trans Golgi Network Golgi compartments. The comprehensive proximal protein mapping provides an integrated, whole cell accounting of GLUT4's localization at a resolution of ∼20 nm, a structural framework for understanding the molecular mechanisms regulating GLUT4 trafficking downstream of different signaling inputs in physiologically relevant cell type and as such, sheds new light on novel key pathways and molecular components as potential therapeutic approaches to modulate muscle glucose uptake.

Caveolin-2 in association with nuclear lamina controls adipocyte hypertrophy

Here, we identify that Caveolin-2 (Cav-2), an integral membrane protein, controls adipocyte hypertrophy in association with nuclear lamina. In the hypertrophy stage of adipogenesis, pY19-Cav-2 association with lamin A/C facilitated the disengagement of CCAAT/enhancer-binding protein α (C/EBPα) and peroxisome proliferator-activated receptor γ (PPARγ) from lamin A/C and repressed Cav-2 promoter at the nuclear periphery for epigenetic activation of Cav-2, and thereby promoted C/EBPα and PPARγ-induced adipocyte hypertrophy. Stable expression of Cav-2 was required and retained by phosphorylation, deubiquitination, and association with lamin A/C for the adipocyte hypertrophy. However, obese adipocytes exhibited augmented Cav-2 stability resulting from the up-regulation of lamin A/C over lamin B1, protein tyrosine phosphatase 1B (PTP1B), and nuclear deubiquitinating enzyme (DUB), Uchl5. Our findings show a novel epigenetic regulatory mechanism of adipocyte hypertrophy by Cav-2 at the nuclear periphery.

Genetic marker identification of SEC13 gene for milk production traits in Chinese holstein

SEC13 homolog, nuclear pore and COPII coat complex component (SEC13) is the core component of the cytoplasmic COPII complex, which mediates material transport from the endoplasmic reticulum to the Golgi complex. Our preliminary work found that SEC13 gene was differentially expressed in dairy cows during different stages of lactation, and involved in metabolic pathways of milk synthesis such as citric acid cycle, fatty acid, starch and sucrose metabolisms, so we considered that the SEC13 might be a candidate gene affecting milk production traits. In this study, we detected the polymorphisms of SEC13 gene and verified their genetic effects on milk yield and composition traits in a Chinese Holstein cow population. By sequencing the whole coding and partial flanking regions of SEC13, we found four single nucleotide polymorphisms (SNPs). Subsequent association analysis showed that these four SNPs were significantly associated with milk yield, fat yield, protein yield or protein percentage in the first and second lactations (p ≤.0351). We also found that two SNPs in SEC13 formed one haplotype block by Haploview4.2, and the block was significantly associated with milk yield, fat yield, fat percentage, protein yield or protein percentage (p ≤ .0373). In addition, we predicted the effect of SNP on 5'region on transcription factor binding sites (TFBSs), and found that the allele A of 22:g.54362761A>G could bind transcription factors (TFs) GATA5, GATA3, HOXD9, HOXA10, CDX1 and Hoxd13; and further dual-luciferase reporter assay verified that the allele A of this SNP inhibited the fluorescence activity. We speculate that the A allele of 22:g.54362761A>G might inhibit the transcriptional activity of SEC13 gene by binding the TFs, which may be a cause mutation affecting the formation of milk production traits in dairy cows. In summary, we proved that SEC13 has a significant genetic effect on milk production traits and the identified significant SNPs could be used as candidate genetic markers for GS SNP chips development; on the other hand, we verified the transcriptional regulation of 22:g.54362761A>G on SEC13 gene, providing research direction for further function validation tests.

A high-content endogenous GLUT4 trafficking assay reveals new aspects of adipocyte biology

Insulin-induced GLUT4 translocation to the plasma membrane in muscle and adipocytes is crucial for whole-body glucose homeostasis. Currently, GLUT4 trafficking assays rely on overexpression of tagged GLUT4. Here we describe a high-content imaging platform for studying endogenous GLUT4 translocation in intact adipocytes. This method enables high fidelity analysis of GLUT4 responses to specific perturbations, multiplexing of other trafficking proteins and other features including lipid droplet morphology. Using this multiplexed approach we showed that Vps45 and Rab14 are selective regulators of GLUT4, but Trarg1, Stx6, Stx16, Tbc1d4 and Rab10 knockdown affected both GLUT4 and TfR translocation. Thus, GLUT4 and TfR translocation machinery likely have some overlap upon insulin-stimulation. In addition, we identified Kif13A, a Rab10 binding molecular motor, as a novel regulator of GLUT4 traffic. Finally, comparison of endogenous to overexpressed GLUT4 highlights that the endogenous GLUT4 methodology has an enhanced sensitivity to genetic perturbations and emphasises the advantage of studying endogenous protein trafficking for drug discovery and genetic analysis of insulin action in relevant cell types.

Epigenetic regulation of Cebpb activation by pY19-Caveolin-2 at the nuclear periphery in association with the nuclear lamina

Here, we show that Caveolin-2 (Cav-2) is an epigenetic regulator for adipogenesis. Upon adipogenic stimulation, inner nuclear membrane (INM)-targeted pY19-Cav-2 interacted with lamin A/C to disengage the repressed Cebpb promoter from lamin A/C, which facilitated the Cebpb promoter association with lamin B1. Consequently, pY19-Cav-2 recruited lysine demethylase 4b (KDM4b) for demethylation of histone H3 lysine 9 trimethylation (H3K9me3) and histone acetyltransferase GCN5 for acetylation of H3K27, and subsequently RNA polymerase II (Pol II) on Cebpb promoter for epigenetic activation of Cebpb, to initiate adipogenesis. Cav-2 knock-down abrogated the Cebpb activation and blocked the Pparg2 and Cebpa activation. Re-expression of Cav-2 restored Cebpb activation and adipogenesis in Cav-2-deficient preadipocytes. Our data identify a new mechanism by which the epigenetic activation of Cebpb is controlled at the nuclear periphery to promote adipogenesis.

Knockout of Syntaxin-4 in 3T3-L1 adipocytes reveals new insight into GLUT4 trafficking and adiponectin secretion

Adipocytes are key to metabolic regulation, exhibiting insulin-stimulated glucose transport that is underpinned by the insulin-stimulated delivery of glucose transporter type 4 (SLC2A4, also known and hereafter referred to as GLUT4)-containing vesicles to the plasma membrane where they dock and fuse, and increase cell surface GLUT4 levels. Adipocytokines, such as adiponectin, are secreted via a similar mechanism. We used genome editing to knock out syntaxin-4, a protein reported to mediate fusion between GLUT4-containing vesicles and the plasma membrane in 3T3-L1 adipocytes. Syntaxin-4 knockout reduced insulin-stimulated glucose transport and adiponectin secretion by ∼50% and reduced GLUT4 levels. Ectopic expression of haemagglutinin (HA)-tagged GLUT4 conjugated to GFP showed that syntaxin-4-knockout cells retain significant GLUT4 translocation capacity, demonstrating that syntaxin-4 is dispensable for insulin-stimulated GLUT4 translocation. Analysis of recycling kinetics revealed only a modest reduction in the exocytic rate of GLUT4 in knockout cells, and little effect on endocytosis. These analyses demonstrate that syntaxin-4 is not always rate limiting for GLUT4 delivery to the cell surface. In sum, we show that syntaxin-4 knockout results in reduced insulin-stimulated glucose transport, depletion of cellular GLUT4 levels and inhibition of adiponectin secretion but has only modest effects on the translocation capacity of the cells.

This article has an associated First Person interview with Hannah L. Black and Rachel Livingstone, joint first authors of the paper.

Summary: Syntaxin-4 knockout reduces insulin-stimulated glucose transport, depletes levels of cellular GLUT4 and inhibits secretion of adiponectin but only modestly affects the translocation capacity of the cells.

Microtubules in insulin action: what's on the tube?

Microtubules (MT) have a role in the intracellular response to insulin stimulation and subsequent glucose transport by glucose transporter 4 (GLUT4), which resides in specialized storage vesicles that travel through the cell. Before GLUT4 is inserted into the plasma membrane for glucose transport, it undergoes complex trafficking through the cell via the integration of cytoskeletal networks. In this review, we highlight the importance of MT elements in insulin action in adipocytes through a summary of MT depolymerization studies, MT-based GLUT4 movement, molecular motor proteins involved in GLUT4 trafficking, as well as MT-related phenomena in response to insulin and links between insulin action and MT-associated proteins.

Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression

Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.

Insulin action in adipocytes, adipose remodeling, and systemic effects

On this 100th anniversary of the discovery of insulin, we recognize the critical role that adipocytes, which are exquisitely responsive to insulin, have played in determining the mechanisms for insulin action at the cellular level. Our understanding of adipose tissue biology has evolved greatly, and it is now clear that adipocytes are far more complicated than simple storage depots for fat. A growing body of evidence documents how adipocytes, in response to insulin, contribute to the control of whole-body nutrient homeostasis. These advances highlight adipocyte plasticity, heterogeneity, and endocrine function, unique features that connect adipocyte metabolism to the regulation of other tissues important for metabolic homeostasis (e.g., liver, muscle, pancreas).

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.

Insulin-promoted mobilization of GLUT4 from a perinuclear storage site requires RAB10

Insulin controls glucose uptake into muscle and fat cells by inducing a net redistribution of glucose transporter 4 (GLUT4) from intracellular storage to the plasma membrane (PM). The TBC1D4-RAB10 signaling module is required for insulin-stimulated GLUT4 translocation to the PM, although where it intersects GLUT4 traffic was unknown. Here we demonstrate that TBC1D4-RAB10 functions to control GLUT4 mobilization from a trans-Golgi network (TGN) storage compartment, establishing that insulin, in addition to regulating the PM proximal effects of GLUT4-containing vesicles docking to and fusion with the PM, also directly regulates the behavior of GLUT4 deeper within the cell. We also show that GLUT4 is retained in an element/domain of the TGN from which newly synthesized lysosomal proteins are targeted to the late endosomes and the ATP7A copper transporter is translocated to the PM by elevated copper. Insulin does not mobilize ATP7A nor does copper mobilize GLUT4, and RAB10 is not required for copper-elicited ATP7A mobilization. Consequently, GLUT4 intracellular sequestration and mobilization by insulin is achieved, in part, through utilizing a region of the TGN devoted to specialized cargo transport in general rather than being specific for GLUT4. Our results define the GLUT4-containing region of the TGN as a sorting and storage site from which different cargo are mobilized by distinct signals through unique molecular machinery.

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.

Thirty sweet years of GLUT4

A pivotal metabolic function of insulin is the stimulation of glucose uptake into muscle and adipose tissues. The discovery of the insulin-responsive glucose transporter type 4 (GLUT4) protein in 1988 inspired its molecular cloning in the following year. It also spurred numerous cellular mechanistic studies laying the foundations for how insulin regulates glucose uptake by muscle and fat cells. Here, we reflect on the importance of the GLUT4 discovery and chronicle additional key findings made in the past 30 years. That exocytosis of a multispanning membrane protein regulates cellular glucose transport illuminated a novel adaptation of the secretory pathway, which is to transiently modulate the protein composition of the cellular plasma membrane. GLUT4 controls glucose transport into fat and muscle tissues in response to insulin and also into muscle during exercise. Thus, investigation of regulated GLUT4 trafficking provides a major means by which to map the essential signaling components that transmit the effects of insulin and exercise. Manipulation of the expression of GLUT4 or GLUT4-regulating molecules in mice has revealed the impact of glucose uptake on whole-body metabolism. Remaining gaps in our understanding of GLUT4 function and regulation are highlighted here, along with opportunities for future discoveries and for the development of therapeutic approaches to manage metabolic disease.

The CCT chaperonin is a novel regulator of Ca2+ signaling through modulation of Orai1 trafficking

Store-operated Ca²⁺ entry (SOCE) encodes a range of cellular responses downstream of Ca²⁺ influx through the SOCE channel Orai1. Orai1 recycles at the plasma membrane (PM), with ~40% of the total Orai1 pool residing at the PM at steady state. The mechanisms regulating Orai1 recycling remain poorly understood. We map the domains in Orai1 that are required for its trafficking to and recycling at the PM. We further identify, using biochemical and proteomic approaches, the CCT [chaperonin-containing TCP-1 (T-complex protein 1)] chaperonin complex as a novel regulator of Orai1 recycling by primarily regulating Orai1 endocytosis. We show that Orai1 interacts with CCT through its intracellular loop and that inhibition of CCT-Orai1 interaction increases Orai1 PM residence. This increased residence is functionally significant as it results in prolonged Ca²⁺ signaling, early formation of STIM1-Orai1 puncta, and more rapid activation of NFAT (nuclear factor of activated T cells) downstream of SOCE. Therefore, the CCT chaperonin is a novel regulator of Orai1 trafficking and, as such, a modulator of Ca²⁺ signaling and effector activation kinetics.

The cell biology of systemic insulin function

Insulin is the paramount anabolic hormone, promoting carbon energy deposition in the body. Its synthesis, quality control, delivery, and action are exquisitely regulated by highly orchestrated intracellular mechanisms in different organs or "stations" of its bodily journey. In this Beyond the Cell review, we focus on these five stages of the journey of insulin through the body and the captivating cell biology that underlies the interaction of insulin with each organ. We first analyze insulin's biosynthesis in and export from the β-cells of the pancreas. Next, we focus on its first pass and partial clearance in the liver with its temporality and periodicity linked to secretion. Continuing the journey, we briefly describe insulin's action on the blood vasculature and its still-debated mechanisms of exit from the capillary beds. Once in the parenchymal interstitium of muscle and adipose tissue, insulin promotes glucose uptake into myofibers and adipocytes, and we elaborate on the intricate signaling and vesicle traffic mechanisms that underlie this fundamental function. Finally, we touch upon the renal degradation of insulin to end its action. Cellular discernment of insulin's availability and action should prove critical to understanding its pivotal physiological functions and how their failure leads to diabetes.

Alternative translation initiation of Caveolin-2 desensitizes insulin signaling through dephosphorylation of insulin receptor by PTP1B and causes insulin resistance

Insulin resistance, defined as attenuated sensitivity responding to insulin, impairs insulin action. Direct causes and molecular mechanisms of insulin resistance have thus far remained elusive. Here we show that alternative translation initiation (ATI) of Caveolin-2 (Cav-2) regulates insulin sensitivity. Cav-2β isoform yielded by ATI desensitizes insulin receptor (IR) via dephosphorylation by protein-tyrosine phosphatase 1B (PTP1B), and subsequent endocytosis and lysosomal degradation of IR, causing insulin resistance. Blockage of Cav-2 ATI protects against insulin resistance by preventing Cav-2β-PTP1B-directed IR desensitization, thereby normalizing insulin sensitivity and glucose uptake. Our findings show that Cav-2β is a negative regulator of IR signaling, and identify a mechanism causing insulin resistance through control of insulin sensitivity via Cav-2 ATI.

A long-term mechanistic computational model of physiological factors driving the onset of type 2 diabetes in an individual

A computational model of the physiological mechanisms driving an individual's health towards onset of type 2 diabetes (T2D) is described, calibrated and validated using data from the Diabetes Prevention Program (DPP). The objective of this model is to quantify the factors that can be used for prevention of T2D. The model is energy and mass balanced and continuously simulates trajectories of variables including body weight components, fasting plasma glucose, insulin, and glycosylated hemoglobin among others on the time-scale of years. Modeled mechanisms include dynamic representations of intracellular insulin resistance, pancreatic beta-cell insulin production, oxidation of macronutrients, ketogenesis, effects of inflammation and reactive oxygen species, and conversion between stored and activated metabolic species, with body-weight connected to mass and energy balance. The model was calibrated to 331 placebo and 315 lifestyle-intervention DPP subjects, and one year forecasts of all individuals were generated. Predicted population mean errors were less than or of the same magnitude as clinical measurement error; mean forecast errors for weight and HbA1c were ~5%, supporting predictive capabilities of the model. Validation of lifestyle-intervention prediction is demonstrated by synthetically imposing diet and physical activity changes on DPP placebo subjects. Using subject level parameters, comparisons were made between exogenous and endogenous characteristics of subjects who progressed toward T2D (HbA1c > 6.5) over the course of the DPP study to those who did not. The comparison revealed significant differences in diets and pancreatic sensitivity to hyperglycemia but not in propensity to develop insulin resistance. A computational experiment was performed to explore relative contributions of exogenous versus endogenous factors between these groups. Translational uses to applications in public health and personalized healthcare are discussed.

Downregulation of a GPCR by β-Arrestin2-Mediated Switch from an Endosomal to a TGN Recycling Pathway

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone involved in nutrient homeostasis. GIP receptor (GIPR) is constitutively internalized and returned to the plasma membrane, atypical behavior for a G-protein-coupled receptor (GPCR). GIP promotes GIPR downregulation from the plasma membrane by inhibiting recycling without affecting internalization. This transient desensitization is achieved by altered intracellular trafficking of activated GIPR. GIP stimulation induces a switch in GIPR recycling from a rapid endosomal to a slow trans-Golgi network (TGN) pathway. GPCR kinases and β-arrestin2 are required for this switch in recycling. A coding sequence variant of GIPR, which has been associated with metabolic alterations, has altered post-activation trafficking characterized by enhanced downregulation and prolonged desensitization. Downregulation of the variant requires β-arrestin2 targeting to the TGN but is independent of GPCR kinases. The single amino acid substitution in the variant biases the receptor to promote GIP-stimulated β-arrestin2 recruitment without receptor phosphorylation, thereby enhancing downregulation.

Adenovirus Protein E4-ORF1 Activation of PI3 Kinase Reveals Differential Regulation of Downstream Effector Pathways in Adipocytes

Insulin activation of phosphatidylinositol 3-kinase (PI3K) regulates metabolism, including the translocation of the Glut4 glucose transporter to the plasma membrane and inactivation of the FoxO1 transcription factor. Adenoviral protein E4-ORF1 stimulates cellular glucose metabolism by mimicking growth-factor activation of PI3K. We have used E4-ORF1 as a tool to dissect PI3K-mediated signaling in adipocytes. E4-ORF1 activation of PI3K in adipocytes recapitulates insulin regulation of FoxO1 but not regulation of Glut4. This uncoupling of PI3K effects occurs despite E4-ORF1 activating PI3K and downstream signaling to levels achieved by insulin. Although E4-ORF1 does not fully recapitulate insulin's effects on Glut4, it enhances insulin-stimulated insertion of Glut4-containing vesicles to the plasma membrane independent of Rab10, a key regulator of Glut4 trafficking. E4-ORF1 also stimulates plasma membrane translocation of ubiquitously expressed Glut1 glucose transporter, an effect that is likely essential for E4-ORF1 to promote an anabolic metabolism in a broad range of cell types.

Metabolism and acetylation contribute to leucine-mediated inhibition of cardiac glucose uptake

High plasma leucine levels strongly correlate with type 2 diabetes. Studies of muscle cells have suggested that leucine alters the insulin response for glucose transport by activating an insulin-negative feedback loop driven by the mammalian target of rapamycin/p70 ribosomal S6 kinase (mTOR/p70S6K) pathway. Here, we examined the molecular mechanism involved in leucine's action on cardiac glucose uptake. Leucine was indeed able to curb glucose uptake after insulin stimulation in both cultured cardiomyocytes and perfused hearts. Although leucine activated mTOR/p70S6K, the mTOR inhibitor rapamycin did not prevent leucine's inhibitory action on glucose uptake, ruling out the contribution of the insulin-negative feedback loop. α-Ketoisocaproate, the first metabolite of leucine catabolism, mimicked leucine's effect on glucose uptake. Incubation of cardiomyocytes with [¹³C]leucine ascertained its metabolism to ketone bodies (KBs), which had a similar negative impact on insulin-stimulated glucose transport. Both leucine and KBs reduced glucose uptake by affecting translocation of glucose transporter 4 (GLUT4) to the plasma membrane. Finally, we found that leucine elevated the global protein acetylation level. Pharmacological inhibition of lysine acetyltransferases counteracted this increase in protein acetylation and prevented leucine's inhibitory action on both glucose uptake and GLUT4 translocation. Taken together, these results indicate that leucine metabolism into KBs contributes to inhibition of cardiac glucose uptake by hampering the translocation of GLUT4-containing vesicles via acetylation. They offer new insights into the establishment of insulin resistance in the heart.NEW & NOTEWORTHY Catabolism of the branched-chain amino acid leucine into ketone bodies efficiently inhibits cardiac glucose uptake through decreased translocation of glucose transporter 4 to the plasma membrane. Leucine increases protein acetylation. Pharmacological inhibition of acetylation reverses leucine's action, suggesting acetylation involvement in this phenomenon.Listen to this article's corresponding podcast at http://ajpheart.podbean.com/e/leucine-metabolism-inhibits-cardiac-glucose-uptake/.

Distinct Akt phosphorylation states are required for insulin regulated Glut4 and Glut1-mediated glucose uptake

Insulin, downstream of Akt activation, promotes glucose uptake into fat and muscle cells to lower postprandial blood glucose, an enforced change in cellular metabolism to maintain glucose homeostasis. This effect is mediated by the Glut4 glucose transporter. Growth factors also enhance glucose uptake to fuel an anabolic metabolism required for tissue growth and repair. This activity is predominantly mediated by the Glut1. Akt is activated by phosphorylation of its kinase and hydrophobic motif (HM) domains. We show that insulin-stimulated Glut4-mediated glucose uptake requires PDPK1 phosphorylation of the kinase domain but not mTORC2 phosphorylation of the HM domain. Nonetheless, an intact HM domain is required for Glut4-mediated glucose uptake. Whereas, Glut1-mediated glucose uptake also requires mTORC2 phosphorylation of the HM domain, demonstrating both phosphorylation-dependent and independent roles of the HM domain in regulating glucose uptake. Thus, mTORC2 links Akt to the distinct physiologic programs related to Glut4 and Glut1-mediated glucose uptake.

DOI:http://dx.doi.org/10.7554/eLife.26896.001

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.

Disruption of Adipose Rab10-Dependent Insulin Signaling Causes Hepatic Insulin Resistance

Insulin controls glucose uptake into adipose and muscle cells by regulating the amount of GLUT4 in the plasma membrane. The effect of insulin is to promote the translocation of intracellular GLUT4 to the plasma membrane. The small Rab GTPase, Rab10, is required for insulin-stimulated GLUT4 translocation in cultured 3T3-L1 adipocytes. Here we demonstrate that both insulin-stimulated glucose uptake and GLUT4 translocation to the plasma membrane are reduced by about half in adipocytes from adipose-specific Rab10 knockout (KO) mice. These data demonstrate that the full effect of insulin on adipose glucose uptake is the integrated effect of Rab10-dependent and Rab10-independent pathways, establishing a divergence in insulin signal transduction to the regulation of GLUT4 trafficking. In adipose-specific Rab10 KO female mice, the partial inhibition of stimulated glucose uptake in adipocytes induces insulin resistance independent of diet challenge. During euglycemic-hyperinsulinemic clamp, there is no suppression of hepatic glucose production despite normal insulin suppression of plasma free fatty acids. The impact of incomplete disruption of stimulated adipocyte GLUT4 translocation on whole-body glucose homeostasis is driven by a near complete failure of insulin to suppress hepatic glucose production rather than a significant inhibition in muscle glucose uptake. These data underscore the physiological significance of the precise control of insulin-regulated trafficking in adipocytes.

The first intracellular loop of GLUT4 contains a retention motif

Glucose transporter GLUT4 plays a major role in glucose homeostasis and is efficiently retained intracellularly in adipocytes and myocytes. To simplify the analysis of its retention, various intracellular GLUT4 domains were fused individually to reporter molecules. Of the four short cytoplasmic loops of GLUT4, only the first nine-residue-long loop conferred intracellular retention of truncated forms of the transferrin receptor and CD4 in adipocytes. In contrast, the same loop of GLUT1 was without effect. The reporter molecules to which the first loop of GLUT4 was fused localized, unlike GLUT4, to the TGN, possibly explaining why these molecules did not respond to insulin. The retention induced by the GLUT4 loop was specific to adipocytes as it did not induce retention in preadipocytes. Of the SQWLGRKRA sequence that constitutes this loop, mutation of either the tryptophan or lysine residue abrogated reporter retention. Mutation of these residues individually into alanines in the full-length GLUT4 molecule resulted in a decreased retention for GLUT4-W105A. We conclude that the first intracellular loop of GLUT4 contains retention motif WLGRK, in which Trp105 plays a prominent role.

Insulin promotes Rip11 accumulation at the plasma membrane by inhibiting a dynamin- and PI3-kinase-dependent, but Akt-independent, internalisation event

Rip11 is a Rab11 effector protein that has been shown to be important in controlling the trafficking of several intracellular cargoes, including the fatty acid transporter FAT/CD36, V-ATPase and the glucose transporter GLUT4. We have previously demonstrated that Rip11 translocates to the plasma membrane in response to insulin and here we examine the basis of this regulated phenomenon in more detail. We show that Rip11 rapidly recycles between the cell interior and surface, and that the ability of insulin to increase the appearance of Rip11 at the cell surface involves an inhibition of Rip11 internalisation from the plasma membrane. By contrast the hormone has no effect on the rate of Rip11 translocation towards the plasma membrane. The ability of insulin to inhibit Rip11 internalisation requires dynamin and class I PI3-kinases, but is independent of the activation of the protein kinase Akt; characteristics which are very similar to the mechanism by which insulin inhibits GLUT4 endocytosis.

A STIM1-dependent 'trafficking trap' mechanism regulates Orai1 plasma membrane residence and Ca²⁺ influx levels

The key proteins mediating store-operated Ca(2+) entry (SOCE) are the endoplasmic reticulum (ER) Ca(2+) sensor STIM1 and the plasma membrane Ca(2+)-selective channel Orai1. Here, we quantitatively dissect Orai1 trafficking dynamics and show that Orai1 recycles rapidly at the plasma membrane (Kex≃0.1 min(-1)), with ∼40% of the total Orai1 pool localizing to the plasma membrane at steady state. A subset of intracellular Orai1 localizes to a sub-plasmalemal compartment. Store depletion is coupled to Orai1 plasma membrane enrichment in a STIM1-dependent fashion. This is due to trapping of Orai1 into cortical ER STIM1 clusters, leading to its removal from the recycling pool and enrichment at the plasma membrane. Interestingly, upon high STIM1 expression, Orai1 is trapped into STIM1 clusters intracellularly, thus preventing its plasma membrane enrichment following store depletion. Consistent with this, STIM1 knockdown prevents trapping of excess Orai1 into limiting STIM1 clusters in the cortical ER. SOCE-dependent Ca(2+) influx shows a similar biphasic dependence on the Orai1:STIM1 ratio. Therefore, a STIM1-dependent Orai1 'trafficking trap' mechanism controls Orai1 plasma membrane enrichment and SOCE levels, thus modulating the SOCE 'bandwidth' for downstream signaling.

Development of a new model system to dissect isoform specific Akt signalling in adipocytes

Our study describes the development and validation of a new model system that allows for acute control of signalling by specific Akt isoforms. This model system revealed new insights into the role of Akt kinases in glucose transport and adipogenesis.

Protein kinase B (Akt) kinases are critical signal transducers mediating insulin action. Genetic studies revealed that Akt1 and Akt2 signalling differentially contribute to sustain lipid and glucose homoeostasis; however Akt isoform-specific effectors remain elusive due to the lack of a suitable model system to mechanistically interrogate Akt isoform-specific signalling. To overcome those technical limitations we developed a novel model system that provides acute and specific control of signalling by Akt isoforms. We generated mutants of Akt1 and Akt2 resistant to the allosteric Akt inhibitor MK-2206. We then developed adipocyte cell lines, in which endogenous Akt1 or Akt2 has been replaced by their corresponding drug-resistant Akt mutant. Treatment of those cells with MK-2206 allowed for acute and specific control of either Akt1 or Akt2 function. Our data showed that Akt1^W80A and Akt2^W80A mutants are resistant to MK-2206, dynamically regulated by insulin and able to signal to Akt downstream effectors. Analyses of insulin action in this cellular system showed that Akt1 and Akt2 are both able to mediate insulin regulation of the transcription factor forkhead box O1 (FoxO1) and the glucose transporter 4 (GLUT4), revealing a redundant role for these Akt kinases in the control of glucose transport into fat cells. In contrast, Akt1 signalling is uniquely required for adipogenesis, by controlling the mitotic clonal expansion (MCE) of pre-adipocytes that precedes white adipose cell differentiation. Our data provide new insights into the role of Akt kinases in glucose transport and adipogenesis and support our model system as a valuable tool for the biochemical characterization of signalling by specific Akt isoforms.

Fatty acylated caveolin-2 is a substrate of insulin receptor tyrosine kinase for insulin receptor substrate-1-directed signaling activation

Here, we demonstrate that insulin receptor (IR) tyrosine kinase catalyzes Tyr-19 and Tyr-27 phosphorylation of caveolin-2 (cav-2), leading to stimulation of signaling proteins downstream of IR, and that the catalysis is dependent on fatty acylation status of cav-2, promoting its interaction with IR. Cav-2 is myristoylated at Gly-2 and palmitoylated at Cys-109, Cys-122, and Cys-145. The fatty acylation deficient mutants are unable to localize in the plasma membrane and not phosphorylated by IR tyrosine kinase. IR interacts with the C-terminal domain of cav-2 containing the cysteines for palmitoylation. IR mutants, Y999F and K1057A, but not W1220S, fail interaction with cav-2. Insulin receptor substrate-1 (IRS-1) is recruited to interact with the IR-catalyzed phospho-tyrosine cav-2, which facilitates IRS-1 association with and activation by IR to initiate IRS-1-mediated downstream signaling. Cav-2 fatty acylation and tyrosine phosphorylation are necessary for the IRS-1-dependent PI3K-Akt and ERK activations responsible for glucose uptake and cell survival and proliferation. In conclusion, fatty acylated cav-2 is a new substrate of IR tyrosine kinase, and the fatty acylation and phosphorylation of cav-2 present novel mechanisms by which insulin signaling is activated.

A naturally occurring GIP receptor variant undergoes enhanced agonist-induced desensitization, which impairs GIP control of adipose insulin sensitivity

Glucose-dependent insulinotropic polypeptide (GIP), an incretin hormone secreted from gastrointestinal K cells in response to food intake, has an important role in the control of whole-body metabolism. GIP signals through activation of the GIP receptor (GIPR), a G-protein-coupled receptor (GPCR). Dysregulation of this pathway has been implicated in the development of metabolic disease. Here we demonstrate that GIPR is constitutively trafficked between the plasma membrane and intracellular compartments of both GIP-stimulated and unstimulated adipocytes. GIP induces a downregulation of plasma membrane GIPR by slowing GIPR recycling without affecting internalization kinetics. This transient reduction in the expression of GIPR in the plasma membrane correlates with desensitization to the effects of GIP. A naturally occurring variant of GIPR (E354Q) associated with an increased incidence of insulin resistance, type 2 diabetes, and cardiovascular disease in humans responds to GIP stimulation with an exaggerated downregulation from the plasma membrane and a delayed recovery of GIP sensitivity following cessation of GIP stimulation. This perturbation in the desensitization-resensitization cycle of the GIPR variant, revealed in studies of cultured adipocytes, may contribute to the link of the E354Q variant to metabolic disease.

Reciprocal regulation of endocytosis and metabolism

The cellular uptake of many nutrients and micronutrients governs both their cellular availability and their systemic homeostasis. The cellular rate of nutrient or ion uptake (e.g., glucose, Fe(3+), K(+)) or efflux (e.g., Na(+)) is governed by a complement of membrane transporters and receptors that show dynamic localization at both the plasma membrane and defined intracellular membrane compartments. Regulation of the rate and mechanism of endocytosis controls the amounts of these proteins on the cell surface, which in many cases determines nutrient uptake or secretion. Moreover, the metabolic action of diverse hormones is initiated upon binding to surface receptors that then undergo regulated endocytosis and show distinct signaling patterns once internalized. Here, we examine how the endocytosis of nutrient transporters and carriers as well as signaling receptors governs cellular metabolism and thereby systemic (whole-body) metabolite homeostasis.

Insulin-regulated Glut4 translocation: membrane protein trafficking with six distinctive steps

The trafficking kinetics of Glut4, the transferrin (Tf) receptor, and LRP1 were quantified in adipocytes and undifferentiated fibroblasts. Six steps were identified that determine steady state cell surface Glut4: (i) endocytosis, (ii) degradation, (iii) sorting, (iv) sequestration, (v) release, and (vi) tethering/docking/fusion. Endocytosis of Glut4 is 3 times slower than the Tf receptor in fibroblasts (ken = 0.2 min(-1) versus 0.6 min(-1)). Differentiation decreases Glut4 ken 40% (ken = 0.12 min(-1)). Differentiation also decreases Glut4 degradation, increasing total and cell surface Glut4 3-fold. In fibroblasts, Glut4 is recycled from endosomes through a slow constitutive pathway (kex = 0.025-0.038 min(-1)), not through the fast Tf receptor pathway (kex = 0.2 min(-1)). The kex measured in adipocytes after insulin stimulation is similar (kex = 0.027 min(-1)). Differentiation decreases the rate constant for sorting into the Glut4 recycling pathway (ksort) 3-fold. In adipocytes, Glut4 is also sorted from endosomes into a second exocytic pathway through Glut4 storage vesicles (GSVs). Surprisingly, transfer from endosomes into GSVs is highly regulated; insulin increases the rate constant for sequestration (kseq) 8-fold. Release from sequestration in GSVs is rate-limiting for Glut4 exocytosis in basal adipocytes. AS160 regulates this step. Tethering/docking/fusion of GSVs to the plasma membrane is regulated through an AS160-independent process. Insulin increases the rate of release and fusion of GSVs (kfuseG) 40-fold. LRP1 cycles with the Tf receptor and Glut4 in fibroblasts but predominantly with Glut4 after differentiation. Surprisingly, AS160 knockdown accelerated LRP1 exocytosis in basal and insulin-stimulated adipocytes. These data indicate that AS160 may regulate trafficking into as well as release from GSVs.

Dynamic GLUT4 sorting through a syntaxin-6 compartment in muscle cells is derailed by insulin resistance-causing ceramide

GLUT4 constitutively recycles between the plasma membrane and intracellular depots. Insulin shifts this dynamic equilibrium towards the plasma membrane by recruiting GLUT4 to the plasma membrane from insulin-responsive vesicles. Muscle is the primary site for dietary glucose deposition; however, how GLUT4 sorts into insulin-responsive vesicles, and if and how insulin resistance affects this process, is unknown. In L6 myoblasts stably expressing myc-tagged GLUT4, we analyzed the intracellular itinerary of GLUT4 as it internalizes from the cell surface and examined if such sorting is perturbed by C2-ceramide, a lipid metabolite causing insulin resistance. Surface-labeled GLUT4myc that internalized for 30 min accumulated in a Syntaxin-6 (Stx6)- and Stx16-positive perinuclear sub-compartment devoid of furin or internalized transferrin, and displayed insulin-responsive re-exocytosis. C2-ceramide dispersed the Stx6-positive sub-compartment and prevented insulin-responsive re-exocytosis of internalized GLUT4myc, even under conditions not affecting insulin-stimulated signaling towards Akt. Microtubule disruption with nocodazole prevented pre-internalized GLUT4myc from reaching the Stx6-positive perinuclear sub-compartment and from undergoing insulin-responsive exocytosis. Removing nocodazole allowed both parameters to recover, suggesting that the Stx6-positive perinuclear sub-compartment was required for GLUT4 insulin-responsiveness. Accordingly, Stx6 knockdown inhibited by ∼50% the ability of internalized GLUT4myc to undergo insulin-responsive re-exocytosis without altering its overall perinuclear accumulation. We propose that Stx6 defines the insulin-responsive compartment in muscle cells. Our data are consistent with a model where ceramide could cause insulin resistance by altering intracellular GLUT4 sorting.

Development of a quantitative Correlative Light Electron Microscopy technique to study GLUT4 trafficking

Correlative Light Electron Microscopy (CLEM) combines advantages of light microscopy and electron microscopy in one experiment to deliver information above and beyond the capability of either modality alone. There are many different CLEM techniques, each having its own special advantages but also its technical challenges. It is however the biological question that (should) drive(s) the development and application of a specific CLEM technique in order to provide the answer. Here we describe the development of a CLEM technique that is based on the Tokuyasu cryo immuno-gold labelling technique that has allowed us to quantitatively study GLUT4 trafficking.

Cellular regulation of glucose uptake by glucose transporter GLUT4

GLUT4 is regulated by its intracellular localization. In the absence of insulin, GLUT4 is efficiently retained intracellularly within storage compartments in muscle and fat cells. Upon insulin stimulation (and contraction in muscle), GLUT4 translocates from these compartments to the cell surface where it transports glucose from the extracellular milieu into the cell. Its implication in insulin-regulated glucose uptake makes GLUT4 not only a key player in normal glucose homeostasis but also an important element in insulin resistance and type 2 diabetes. Nevertheless, how GLUT4 is retained intracellularly and how insulin acts on this retention mechanism is largely unclear. In this review, the current knowledge regarding the various molecular processes that govern GLUT4 physiology is discussed as well as the questions that remain.

Conventional knockout of Tbc1d1 in mice impairs insulin- and AICAR-stimulated glucose uptake in skeletal muscle

In the obesity-resistant SJL mouse strain, we previously identified a naturally occurring loss-of-function mutation in the gene for Tbc1d1. Characterization of recombinant inbred mice that carried the Tbc1d1(SJL) allele on a C57BL/6J background indicated that loss of TBC1D1 protects from obesity, presumably by increasing the use of fat as energy source. To provide direct functional evidence for an involvement of TBC1D1 in energy substrate metabolism, we generated and characterized conventional Tbc1d1 knockout mice. TBC1D1-deficient mice showed moderately reduced body weight, decreased respiratory quotient, and an elevated resting metabolic rate. Ex vivo analysis of intact isolated skeletal muscle revealed a severe impairment in insulin- and AICAR-stimulated glucose uptake in glycolytic extensor digitorum longus muscle and a substantially increased rate of fatty acid oxidation in oxidative soleus muscle. Our results provide direct evidence that TBC1D1 plays a major role in glucose and lipid utilization, and energy substrate preference in skeletal muscle.

Insulin triggers surface-directed trafficking of sequestered GLUT4 storage vesicles marked by Rab10

Understanding how glucose transporter isoform 4 (GLUT4) redistributes to the plasma membrane during insulin stimulation is a major goal of glucose transporter research. GLUT4 molecules normally reside in numerous intracellular compartments, including specialized storage vesicles and early/recycling endosomes. It is unclear how these diverse compartments respond to insulin stimulation to deliver GLUT4 molecules to the plasma membrane. For example, do they fuse with each other first or remain as separate compartments with different trafficking characteristics? Our recent live cell imaging studies are helping to clarify these issues. Using Rab proteins as specific markers to distinguish between storage vesicles and endosomes containing GLUT4, we demonstrate that it is primarily internal GLUT4 storage vesicles (GSVs) marked by Rab10 that approach and fuse at the plasma membrane and GSVs don’t interact with endosomes on their way to the plasma membrane. These new findings add strong support to the model that GSV release from intracellular retention plays a major role in supplying GLUT4 molecules onto the PM under insulin stimulation.

Insulin responsiveness of glucose transporter 4 in 3T3-L1 cells depends on the presence of sortilin

Insulin-dependent translocation of Glut4 to the plasma membrane of fat and skeletal muscle cells plays the key role in postprandial clearance of blood glucose. In undifferentiated cells, insulin responsiveness of Glut4 depends on the presence of sortilin, whereas sortilin responds to insulin regardless of Glut4 expression.

Insulin-dependent translocation of glucose transporter 4 (Glut4) to the plasma membrane of fat and skeletal muscle cells plays the key role in postprandial clearance of blood glucose. Glut4 represents the major cell-specific component of the insulin-responsive vesicles (IRVs). It is not clear, however, whether the presence of Glut4 in the IRVs is essential for their ability to respond to insulin stimulation. We prepared two lines of 3T3-L1 cells with low and high expression of myc₇-Glut4 and studied its translocation to the plasma membrane upon insulin stimulation, using fluorescence-assisted cell sorting and cell surface biotinylation. In undifferentiated 3T3-L1 preadipocytes, translocation of myc₇-Glut4 was low regardless of its expression levels. Coexpression of sortilin increased targeting of myc₇-Glut4 to the IRVs, and its insulin responsiveness rose to the maximal levels observed in fully differentiated adipocytes. Sortilin ectopically expressed in undifferentiated cells was translocated to the plasma membrane regardless of the presence or absence of myc₇-Glut4. AS160/TBC1D4 is expressed at low levels in preadipocytes but is induced in differentiation and provides an additional mechanism for the intracellular retention and insulin-stimulated release of Glut4.

Single point mutations result in the miss-sorting of Glut4 to a novel membrane compartment associated with stress granule proteins

Insulin increases cellular glucose uptake and metabolism in the postprandial state by acutely stimulating the translocation of the Glut4 glucose transporter from intracellular membrane compartments to the cell surface in muscle and fat cells. The intracellular targeting of Glut4 is dictated by specific structural motifs within cytoplasmic domains of the transporter. We demonstrate that two leucine residues at the extreme C-terminus of Glut4 are critical components of a motif (IRM, insulin responsive motif) involved in the sorting of the transporter to insulin responsive vesicles in 3T3L1 adipocytes. Light microscopy, immunogold electron microscopy, subcellular fractionation, and sedimentation analysis indicate that mutations in the IRM cause the aberrant targeting of Glut4 to large dispersed membrane vesicles that are not insulin responsive. Proteomic characterization of rapidly and slowly sedimenting membrane vesicles (RSVs and SSVs) that were highly enriched by immunoadsorption for either wild-type Glut4 or an IRM mutant revealed that the major vesicle fraction containing the mutant transporter (IRM-RSVs) possessed a relatively small and highly distinct protein population that was enriched for proteins associated with stress granules. We suggest that the IRM is critical for an early step in the sorting of Glut4 to insulin-responsive subcellular membrane compartments and that IRM mutants are miss-targeted to relatively large, amorphous membrane vesicles that may be involved in a degradation pathway for miss-targeted or miss-folded proteins or represent a transitional membrane compartment that Glut4 traverses en route to insulin responsive storage compartments.

Posttranslational modifications of GLUT4 affect its subcellular localization and translocation

The facilitative glucose transporter type 4 (GLUT4) is expressed in adipose and muscle and plays a vital role in whole body glucose homeostasis. In the absence of insulin, only ~1% of cellular GLUT4 is present at the plasma membrane, with the vast majority localizing to intracellular organelles. GLUT4 is retained intracellularly by continuous trafficking through two inter-related cycles. GLUT4 passes through recycling endosomes, the trans Golgi network and an insulin-sensitive intracellular compartment, termed GLUT4-storage vesicles or GSVs. It is from GSVs that GLUT4 is mobilized to the cell surface in response to insulin, where it increases the rate of glucose uptake into the cell. As with many physiological responses to external stimuli, this regulated trafficking event involves multiple posttranslational modifications. This review outlines the roles of posttranslational modifications of GLUT4 on its function and insulin-regulated trafficking.

Insulin regulates Glut4 confinement in plasma membrane clusters in adipose cells

Insulin-stimulated delivery of glucose transporter-4 (GLUT4) to the plasma membrane (PM) is the hallmark of glucose metabolism. In this study we examined insulin’s effects on GLUT4 organization in PM of adipose cells by direct microscopic observation of single monomers tagged with photoswitchable fluorescent protein. In the basal state, after exocytotic delivery only a fraction of GLUT4 is dispersed into the PM as monomers, while most of the GLUT4 stays at the site of fusion and forms elongated clusters (60–240 nm). GLUT4 monomers outside clusters diffuse freely and do not aggregate with other monomers. In contrast, GLUT4 molecule collision with an existing cluster can lead to immediate confinement and association with that cluster. Insulin has three effects: it shifts the fraction of dispersed GLUT4 upon delivery, it augments the dissociation of GLUT4 monomers from clusters ∼3-fold and it decreases the rate of endocytic uptake. All together these three effects of insulin shift most of the PM GLUT4 from clustered to dispersed states. GLUT4 confinement in clusters represents a novel kinetic mechanism for insulin regulation of glucose homeostasis.

A role for Rab14 in the endocytic trafficking of GLUT4 in 3T3-L1 adipocytes

Insulin enhances the uptake of glucose into adipocytes and muscle cells by promoting the redistribution of the glucose transporter isoform 4 (GLUT4) from intracellular compartments to the cell surface. Rab GTPases regulate the trafficking itinerary of GLUT4 and several have been found on immunopurified GLUT4 vesicles. Specifically, Rab14 has previously been implicated in GLUT4 trafficking in muscle although its role, if any, in adipocytes is poorly understood. Analysis of 3T3-L1 adipocytes using confocal microscopy demonstrated that endogenous GLUT4 and endogenous Rab14 exhibited a partial colocalisation. However, when wild-type Rab14 or a constitutively-active Rab14Q70L mutant were overexpressed in these cells, the colocalisation with both GLUT4 and IRAP became extensive. Interestingly, this colocalisation was restricted to enlarged 'ring-like' vesicular structures (mean diameter 1.3 µm), which were observed in the presence of overexpressed wild-type Rab14 and Rab14Q70L, but not an inactive Rab14S25N mutant. These enlarged vesicles contained markers of early endosomes and were rapidly filled by GLUT4 and transferrin undergoing endocytosis from the plasma membrane. The Rab14Q70L mutant reduced basal and insulin-stimulated cell surface GLUT4 levels, probably by retaining GLUT4 in an insulin-insensitive early endosomal compartment. Furthermore, shRNA-mediated depletion of Rab14 inhibited the transit of GLUT4 through early endosomal compartments towards vesicles and tubules in the perinuclear region. Given the previously reported role of Rab14 in trafficking between endosomes and the Golgi complex, we propose that the primary role of Rab14 in GLUT4 trafficking is to control the transit of internalised GLUT4 from early endosomes into the Golgi complex, rather than direct GLUT4 translocation to the plasma membrane.