Insulin regulates Glut4 confinement in plasma membrane clusters in adipose cells

GLUT4 dispersal at the plasma membrane of adipocytes: a super-resolved journey

In adipose tissue, insulin stimulates glucose uptake by mediating the translocation of GLUT4 from intracellular vesicles to the plasma membrane. In 2010, insulin was revealed to also have a fundamental impact on the spatial distribution of GLUT4 within the plasma membrane, with the existence of two GLUT4 populations at the plasma membrane being defined: (1) as stationary clusters and (2) as diffusible monomers. In this model, in the absence of insulin, plasma membrane-fused GLUT4 are found to behave as clusters. These clusters are thought to arise from exocytic events that retain GLUT4 at their fusion sites; this has been proposed to function as an intermediate hub between GLUT4 exocytosis and re-internalisation. By contrast, insulin stimulation induces the dispersal of GLUT4 clusters into monomers and favours a distinct type of GLUT4-vesicle fusion event, known as fusion-with-release exocytosis. Here, we review how super-resolution microscopy approaches have allowed investigation of the characteristics of plasma membrane-fused GLUT4 and further discuss regulatory step(s) involved in the GLUT4 dispersal machinery, introducing the scaffold protein EFR3 which facilitates localisation of phosphatidylinositol 4-kinase type IIIα (PI4KIIIα) to the cell surface. We consider how dispersal may be linked to the control of transporter activity, consider whether macro-organisation may be a widely used phenomenon to control proteins within the plasma membrane, and speculate on the origin of different forms of GLUT4-vesicle exocytosis.

Potentilla fulgens upregulate GLUT4, AMPK, AKT and insulin in alloxan-induced diabetic mice: an in vivo and in silico study

OBJECTIVE

This study was designed to investigate whether the glucose lowering effects of Potentilla fulgens acts by modulating GLUT4, AKT2 and AMPK expression in the skeletal muscle and liver tissues.

METHODOLOGY

Alloxan-induced diabetic mice treated with Potentilla fulgens was assessed for their blood glucose and insulin level, mRNA and protein expression using distinguished methods. Additionally, GLUT4, AKT2 and AMPK were docked with catechin, epicatechin, kaempferol, metformin, quercetin and ursolic acid reportedly present in Potentilla fulgens.

RESULTS

Potentilla fulgens ameliorates hyperglycaemia and insulin sensitivity via activation of AKT2 and AMPK, increases the expression of GLUT4, AKT2, AMPKα1 and AMPKα2 whose levels are reduced under diabetic condition. Molecular docking revealed interacting residues and their binding affinities (-4.56 to -8.95 Kcal/mol).

CONCLUSIONS

These findings provide more clarity vis-avis the mechanism of action of the phytoceuticals present in Potentilla fulgens extract which function through their action on GLUT4, PKB and AMPK.

Benchmarking Thiolate-Driven Photoswitching of Cyanine Dyes

Carbocyanines are among the best performing dyes in single-molecule localization microscopy (SMLM), but their performance critically relies on optimized photoswitching buffers. Here, we study the versatile role of thiols in cyanine photoswitching at varying intensities generated in a single acquisition by a microelectromechanical systems (MEMS) mirror placed in the excitation path. The key metrics we have analyzed as a function of the thiolate concentration are photon budget, on-state and off-state lifetimes and the corresponding impact on image resolution. We show that thiolate acts as a concentration bandpass filter for the maximum achievable resolution and determine a minimum of ∼1 mM is necessary to facilitate SMLM measurements. We also identify a concentration bandwidth of 1-16 mM in which the photoswitching performance can be balanced between high molecular brightness and high off-time to on-time ratios. Furthermore, we monitor the performance of the popular oxygen scavenger system based on glucose and glucose oxidase over time and show simple measures to avoid acidification during prolonged measurements. Finally, the impact of buffer settings is quantitatively tested on the distribution of the glucose transporter protein 4 within the plasma membrane of adipocytes. Our work provides a general strategy for achieving optimal resolution in SMLM with relevance for the development of novel buffers and dyes.

GLUT4 translocation and dispersal operate in multiple cell types and are negatively correlated with cell size in adipocytes

The regulated translocation of the glucose transporter, GLUT4, to the surface of adipocytes and muscle is a key action of insulin. This is underpinned by the delivery and fusion of GLUT4-containing vesicles with the plasma membrane. Recent studies have revealed that a further action of insulin is to mediate the dispersal of GLUT4 molecules away from the site of GLUT4 vesicle fusion with the plasma membrane. Although shown in adipocytes, whether insulin-stimulated dispersal occurs in other cells and/or is exhibited by other proteins remains a matter of debate. Here we show that insulin stimulates GLUT4 dispersal in the plasma membrane of adipocytes, induced pluripotent stem cell-derived cardiomyocytes and HeLa cells, suggesting that this phenomenon is specific to GLUT4 expressed in all cell types. By contrast, insulin-stimulated dispersal of TfR was not observed in HeLa cells, suggesting that the mechanism may be unique to GLUT4. Consistent with dispersal being an important physiological mechanism, we observed that insulin-stimulated GLUT4 dispersal is reduced under conditions of insulin resistance. Adipocytes of different sizes have been shown to exhibit distinct metabolic properties: larger adipocytes exhibit reduced insulin-stimulated glucose transport compared to smaller cells. Here we show that both GLUT4 delivery to the plasma membrane and GLUT4 dispersal are reduced in larger adipocytes, supporting the hypothesis that larger adipocytes are refractory to insulin challenge compared to their smaller counterparts, even within a supposedly homogeneous population of cells.

EFR3 and phosphatidylinositol 4-kinase IIIα regulate insulin-stimulated glucose transport and GLUT4 dispersal in 3T3-L1 adipocytes

Insulin stimulates glucose transport in muscle and adipocytes. This is achieved by regulated delivery of intracellular glucose transporter (GLUT4)-containing vesicles to the plasma membrane where they dock and fuse, resulting in increased cell surface GLUT4 levels. Recent work identified a potential further regulatory step, in which insulin increases the dispersal of GLUT4 in the plasma membrane away from the sites of vesicle fusion. EFR3 is a scaffold protein that facilitates localization of phosphatidylinositol 4-kinase type IIIα to the cell surface. Here we show that knockdown of EFR3 or phosphatidylinositol 4-kinase type IIIα impairs insulin-stimulated glucose transport in adipocytes. Using direct stochastic reconstruction microscopy, we also show that EFR3 knockdown impairs insulin stimulated GLUT4 dispersal in the plasma membrane. We propose that EFR3 plays a previously unidentified role in controlling insulin-stimulated glucose transport by facilitating dispersal of GLUT4 within the plasma membrane.

Mechanism of INSR clustering with insulin activation and resistance revealed by super-resolution imaging

Insulin receptor (INSR) is a key protein in the INSR signaling pathway and plays a critical role in biological processes, especially in the regulation of glucose homeostasis. Many metabolic diseases are often accompanied by abnormal INSR signaling. However, the specific effector mechanisms regulating insulin resistance and the distribution patterns of INSR during cell membrane activation remain unclear. Here, we investigated the changes in the distribution of INSR during activation using super-resolution imaging. By observing the connection between INSR activation and its distribution, we found that insulin resistance inhibits its receptor clustering. More importantly, we found that INSR has a highly co-localized relationship with the skeletal protein βII-spectrin. Specific knockout of βII-spectrin inhibited the interaction of INSR with GLUT4 and affected the normal metabolism of glucose. Our work elucidates the effects of insulin activation and insulin resistance on INSR distribution and reveals a potential relationship between INSR and cytoskeleton at the single molecule level, which promotes a deeper understanding of the roles associated with insulin signaling and insulin resistance.

Contributive Role of Hyperglycemia and Hypoglycemia Towards the Development of Alzheimer's Disease

Alzheimer's disease (AD) is one of the causes of dementia that results from several infections/biological conditions leading to either cell disruption or loss of neuronal communication. Studies have documented the accumulation of two proteins, beta-amyloid (Aβ), which accumulates on the exteriors of neurons, and tau (Tau), which assembles at the interiors of brain cells and is chiefly liable for the progression of the disease. Several molecular and cellular pathways account for the accumulation of amyloid-β and the formation of neurofibrillary tangles, which are phosphorylated variants of Tau protein. Moreover, research has revealed a potential connection between AD and diabetes. It has also been demonstrated that both hypoglycemia and hyperglycemia have a significant role in the development of AD. In addition, SUMO (small ubiquitin-like modifier protein) plays a crucial role in the pathogenesis of AD. SUMOylation is the process by which modification of amyloid precursor protein (APP) and Tau takes place. Furthermore, Drosophila melanogaster has proven to be an efficient model organism in studies to establish the relationship between AD and variations in blood glucose levels. In addition, the review successfully identifies the common pathway that links the effects of fluctuations in glucose levels on AD pathogenesis and advancements.

Characterization of lipoprotein lipase storage vesicles in 3T3-L1 adipocytes

Lipoprotein lipase (LPL) is a secreted triglyceride lipase involved in the clearance of very-low-density lipoproteins and chylomicrons from circulation. LPL is expressed primarily in adipose and muscle tissues and transported to the capillary lumen. LPL secretion is regulated by insulin in adipose tissue; however, few studies have examined the regulatory and trafficking steps involved in secretion. Here, we describe the intracellular localization and insulin-dependent trafficking of LPL in 3T3-L1 adipocytes. We compared LPL trafficking to the better characterized trafficking pathways taken by leptin and GLUT4 (also known as SLC2A4). We show that the LPL trafficking pathway shares some characteristics of these other pathways, but that LPL subcellular localization and trafficking are distinct from those of GLUT4 and leptin. LPL secretion occurs slowly in response to insulin and rapidly in response to the Ca2+ ionophore ionomycin. This regulated trafficking is dependent on Golgi protein kinase D and the ADP-ribosylation factor GTPase ARF1. Together, these data give support to a new trafficking pathway for soluble cargo that is active in adipocytes.

GLUT4 On the move

Insulin rapidly stimulates GLUT4 translocation and glucose transport in fat and muscle cells. Signals from the occupied insulin receptor are translated into downstream signalling changes in serine/threonine kinases within timescales of seconds, and this is followed by delivery and accumulation of the glucose transporter GLUT4 at the plasma membrane. Kinetic studies have led to realisation that there are distinct phases of this stimulation by insulin. There is a rapid initial burst of GLUT4 delivered to the cell surface from a subcellular reservoir compartment and this is followed by a steady-state level of continuing stimulation in which GLUT4 recycles through a large itinerary of subcellular locations. Here, we provide an overview of the phases of insulin stimulation of GLUT4 translocation and the molecules that are currently considered to activate these trafficking steps. Furthermore, we suggest how use of new experimental approaches together with phospho-proteomic data may help to further identify mechanisms for activation of these trafficking processes.

Fluorescence Microscopy-Based Quantitation of GLUT4 Translocation

Postprandial insulin-stimulated glucose uptake into target tissue is crucial for the maintenance of normal blood glucose homeostasis. This step is rate-limited by the number of facilitative glucose transporters type 4 (GLUT4) present in the plasma membrane. Since insulin resistance and impaired GLUT4 translocation are associated with the development of metabolic disorders such as type 2 diabetes, this transporter has become an important target of antidiabetic drug research. The application of screening approaches that are based on the analysis of GLUT4 translocation to the plasma membrane to identify substances with insulinomimetic properties has gained global research interest in recent years. Here, we review methods that have been implemented to quantitate the translocation of GLUT4 to the plasma membrane. These methods can be broadly divided into two sections: microscopy-based technologies (e.g., immunoelectron, confocal or total internal reflection fluorescence microscopy) and biochemical and spectrometric approaches (e.g., membrane fractionation, photoaffinity labeling or flow cytometry). In this review, we discuss the most relevant approaches applied to GLUT4 thus far, highlighting the advantages and disadvantages of these approaches, and we provide a critical discussion and outlook into new methodological opportunities.

Molecular identification of glucose transporter 4: The responsiveness to starvation, glucose, insulin and glucagon on glucose transporter 4 in common carp (Cyprinus carpio L.)

Glucose transporter 4 (GLUT4) is comprehensively investigated in mammals, while the comparative research of GLUT4 in common carp is deficient. To investigate the function of GLUT4, carp glut4 was first isolated. The open reading frame of carp glut4 was 1518 bp in length, encoding 505 amino acids. A high-sequence homology was identified in carp and teleost, and the phylogenetic tree displayed that the carp GLUT4 was clustered with the teleost. A high level of glut4 mRNA was analysed in fat, red muscle and white muscle. After fasting treatment, glut4 mRNA expression was increased significantly in muscle. In the oral glucose tolerance test experiment, glut4 mRNA was also significantly elevated in muscle, gut and fat. Furthermore, intraperitoneal injection of insulin resulted in the upregulation of glut4 gene expression significantly in white muscle, gut and fat. On the contrary, the glut4 mRNA level in the white muscle, gut and fat was markedly downregulated after glucagon injection. These results suggest that GLUT4 might play important roles in food intake and could be regulated by nutrient condition, insulin and glucagon in common carp. Our study is the first to report on GLUT4 in common carp. These data provide a basis for further study on fish GLUT4.

Functional Characterization of Facilitative Glucose Transporter 4 With a Delay Responding to Plasma Glucose Level in Blunt Snout Bream (Megalobrama amblycephala)

Facilitative glucose transporter 4 (GLUT4) plays a central role in mediating insulin function to increase glucose uptake in glucose metabolism homeostasis. In this study, the function and localization of GLUT4 in blunt snout bream (Megalobrama amblycephala) were first investigated, and then, the response measured as carbohydrate level, was analyzed. The results showed that the cDNA sequence of GLUT4 in blunt snout bream (MaGLUT4, GenBank accession no: MT447093) was 2868 bp in length, and the corresponding mRNA contained a 5'-UTR region of 513 bp and a 3'-UTR region of 837 bp. MaGLUT4 had an open reading frame of 1518 bp and was encoded by 505 amino acids. Its theoretical isoelectric point and molecular weight was 6.41 and 55.47 kDa, respectively. A comparison of these characteristics with BLASTP results from the NCBI database showed that MaGLUT4 had the highest homology with Cypriniformes fish, with MaGLUT4 and GLUT4 of other Cypriniformes clustered in the phylogenetic tree with other GLUT1-4 amino acid sequences. Compared with the results from the homo_sapiens and mus_musculus data sets, some mutations were observed in the GLUT4 amino acid sequence of these aquatic animals, including an FQQI mutation to FQQL, LL mutation to MM, and TELEY mutation to TELDY. MaGLUT4 was constitutively expressed in the muscle, intestine, and liver, with the highest mRNA level observed in muscle. Furthermore, the predicted tertiary structure and results of immunohistochemical staining showed that MaGLUT4 was a transmembrane protein primarily located in the plasma membrane, where it accounts for 60.9% of the total expressed, according to an analysis of subcellular localization. Blood glucose level peaked within 1 h, and the insulin level peaked at 6 h, while the mRNA and protein levels of GLUT4 showed an upward trend with an increase in feeding time and decreased sharply after 12 h. These results confirmed that MaGLUT4 was mainly distributed in muscles and crosses the cell membrane. The changes in the insulin, mRNA, and protein levels of MaGUT4 lagged far behind changes in blood glucose levels. This delay in insulin level changes and GLUT4 activation might be the important reasons for glucose intolerance of this fish species.

Super-Resolution Live Cell Microscopy of Membrane-Proximal Fluorophores

Here, we present a simple and robust experimental setup for the super-resolution live cell microscopy of membrane-proximal fluorophores, which is comparably easy to perform and to implement. The method is based on Structured Illumination Microscopy (SIM) with a switchable spatial light modulator (SLM) and exchangeable objective lenses for epi-illumination and total internal reflection fluorescence (TIRF) microscopy. While, in the case of SIM (upon epi-illumination), cell layers of about 1-2 µm in close proximity to the plasma membrane can be selected by software, layers in the 100 nm range are assessed experimentally by TIRF-SIM. To show the applicability of this approach, both methods are used to measure the translocation of the glucose transporter 4 (GLUT4) from intracellular vesicles to the plasma membrane upon stimulation by insulin or insulin-mimetic compounds, with a lateral resolution of around 100 nm and an axial resolution of around 200 nm. While SIM is an appropriate method to visualize the intracellular localization of GLUT4 fused with a green fluorescent protein, TIRF-SIM permits the quantitative evaluation of its fluorescence in the plasma membrane. These imaging methods are discussed in the context of fluorescence lifetime kinetics, providing additional data for the molecular microenvironment.

Multiorgan Metabolomics and Lipidomics Provide New Insights Into Fat Infiltration in the Liver, Muscle Wasting, and Liver-Muscle Crosstalk Following Burn Injury

Burn injury mediated hypermetabolic syndrome leads to increased mortality among severe burn victims, due to liver failure and muscle wasting. Metabolic changes may persist up to 2 years following the injury. Thus, understanding the underlying mechanisms of the pathology is crucially important to develop appropriate therapeutic approaches. We present detailed metabolomic and lipidomic analyses of the liver and muscle tissues in a rat model with a 30% body surface area burn injury located at the dorsal skin. Three hundred and thirty-eight of 1587 detected metabolites and lipids in the liver and 119 of 1504 in the muscle tissue exhibited statistically significant alterations. We observed excessive accumulation of triacylglycerols, decreased levels of S-adenosylmethionine, increased levels of glutamine and xenobiotics in the liver tissue. Additionally, the levels of gluconeogenesis, glycolysis, and tricarboxylic acid cycle metabolites are generally decreased in the liver. On the other hand, burn injury muscle tissue exhibits increased levels of acyl-carnitines, alpha-hydroxyisovalerate, ophthalmate, alpha-hydroxybutyrate, and decreased levels of reduced glutathione. The results of this preliminary study provide compelling observations that liver and muscle tissues undergo distinctly different changes during hypermetabolism, possibly reflecting liver-muscle crosstalk. The liver and muscle tissues might be exacerbating each other's metabolic pathologies, via excessive utilization of certain metabolites produced by each other.

Andrographis paniculata Improves Insulin Resistance in High-Fat Diet-Induced Obese Mice and TNFα-Treated 3T3-L1 Adipocytes

Pro-inflammatory cytokines interfere with blood glucose homeostasis, which leads to hyperglycemia. Andrographis paniculata (AP) has been shown to possess anti-inflammatory activity and to reduce blood glucose levels in diabetes. The two major bioactive diterpenoids in AP, andrographolide (AND) and 14-deoxy-11,12-didehydroandrographolide (deAND), have potent anti-inflammatory activity. We studied whether APE (an ethanolic extract of AP), AND, and deAND could improve a high-fat diet (HFD)-induced hyperglycemia in vivo and TNF[Formula: see text]-induced impairment of insulin signaling in vitro. Male C57BL/6JNarl mice were fed a normal diet (ND) or the HFD, and the fatty mice were treated with APE, deAND, or AND for 16 weeks. 3T3-L1 cells were used to study the underlying mechanisms by which APE, deAND, or AND attenuated TNF[Formula: see text]-induced insulin resistance. The HFD significantly induced obesity, hyperglycemia, insulin resistance, and inflammation, whereas APE and deAND significantly ameliorated HFD-induced obesity, hyperglycemia, insulin resistance, and TNF[Formula: see text] production. The HFD significantly impaired insulin signaling by decreasing the protein expression of p-IRS1 tyr632 and p-AKT ser473, as well as the membrane translocation of GLUT4 in response to insulin stimulation in epididymal adipose tissue. HFD-impaired the membrane translocation of GLUT4 was significantly reversed by deAND and APE. In addition, deAND and APE markedly reversed the detrimental effect of TNF[Formula: see text] on the insulin signaling pathway and glucose uptake in 3T3-L1 cells. Despite no significant positive effect on p-AS160, a trend for recovery by deAND and APE was observed. These results suggest that deAND and APE protect against HFD-induced insulin resistance by ameliorating inflammation-driven impairment of insulin sensitivity.

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.

Subcutaneous adipose tissue imaging of human obesity reveals two types of adipocyte membranes: Insulin-responsive and -nonresponsive

In adipose tissue, resistance to insulin's ability to increase glucose uptake can be induced by multiple factors, including obesity. Impaired insulin action may take place at different spatial loci at the cellular or subcellular level. To begin to understand the spatial response to insulin in human subcutaneous adipose tissue (hSAT), we developed a quantitative imaging method for activation of a major signaling node in the glucoregulatory insulin signaling pathway. After treatment with insulin or control media, biopsied tissues were immunostained for Akt phosphorylation at Thr-308/9 (pAkt) and then imaged by confocal fluorescence microscopy automated to collect a large grid of high resolution fields. In hSAT from 40 men and women with obesity, substantial heterogeneity of pAkt densities in adipocyte membranes were quantified in each image mosaic using a spatial unit of at least twice the size of the point spread function. Statistical analysis of the distribution of pAkt spatial units was best fit as the weighted sum of two separate distributions, corresponding to either a low or high pAkt density. A "high pAkt fraction" metric was calculated from the fraction of high pAkt distributed units over the total units. Importantly, upon insulin stimulation, tissues from the same biopsy showed either a minimal or a substantial change in the high pAkt fraction. Further supporting a two-state response to insulin stimulation, subjects with similar insulin sensitivity indices are also segregated into either of two clusters identified by the amount of membrane-localized pAkt.

RalA controls glucose homeostasis by regulating glucose uptake in brown fat

Insulin increases glucose uptake into adipose tissue and muscle by increasing trafficking of the glucose transporter Glut4. In cultured adipocytes, the exocytosis of Glut4 relies on activation of the small G protein RalA by insulin, via inhibition of its GTPase activating complex RalGAP. Here, we evaluate the role of RalA in glucose uptake in vivo with specific chemical inhibitors and by generation of mice with adipocyte-specific knockout of RalGAPB. RalA was profoundly activated in brown adipose tissue after feeding, and its inhibition prevented Glut4 exocytosis. RalGAPB knockout mice with diet-induced obesity were protected from the development of metabolic disease due to increased glucose uptake into brown fat. Thus, RalA plays a crucial role in glucose transport in adipose tissue in vivo.

Curculigoside and polyphenol-rich ethyl acetate fraction of Molineria latifolia rhizome improved glucose uptake via potential mTOR/AKT activated GLUT4 translocation

Adipose tissue is one of the major organs responsible for rapid restoration of postprandial glucose fluxes. Being the major isoform of glucose transporter in adipose tissue, regulations of insulin-dependent GLUT4 trafficking have always been of research interest. The present study aimed to examine the molecular mechanisms underlying the efficacy of curculigoside and polyphenol-rich ethyl acetate fraction (EAF) of Molineria latifolia rhizome in triggering glucose uptake. We assessed the adipogenic potential and glucose uptake stimulatory activity of curculigoside and EAF by employing a murine 3T3-L1 adipocyte model. The transcriptional and translational expressions of selected intermediates in the insulin signalling pathway were evaluated. While curculigoside neither promoted adipogenesis nor activated peroxisome proliferator activated receptor gamma, treatment with polyphenol-rich EAF resulted otherwise. However, both treatments enhanced insulin-stimulated uptake of glucose. This was coupled with increased availability of GLUT4 at the plasma membrane of the differentiated adipocytes although the total GLUT4 protein level was unaffected. In addition, the treatment increased the phosphorylation of both AKT and mTOR, which have been reported to be associated with GLUT4 translocation. The present findings proposed that curculigoside and EAF increased glucose transport activity of 3T3-L1 adipocytes via GLUT4 translocation as a result of potential mTOR/AKT activation. The more potent efficacy observed with EAF suggested potential synergistic and multi-targeted action.

Total Internal Reflection Fluorescence Microscopy to Study GLUT4 Trafficking

Total internal reflection fluorescence (TIRF) microscopy is a powerful method that allows examination of plasma membrane close events in real time. The last decade, the method has successfully been used to explore GLUT4 translocation in adipocytes. Here, we describe the procedure for studying GLUT4 trafficking using TIRF microscopy in isolated primary adipocytes.

A systems biology analysis connects insulin receptor signaling with glucose transporter translocation in rat adipocytes

Type 2 diabetes is characterized by insulin resistance, which arises from malfunctions in the intracellular insulin signaling network. Knowledge of the insulin signaling network is fragmented, and because of the complexity of this network, little consensus has emerged for the structure and importance of the different branches of the network. To help overcome this complexity, systems biology mathematical models have been generated for predicting both the activation of the insulin receptor (IR) and the redistribution of glucose transporter 4 (GLUT4) to the plasma membrane. Although the insulin signal transduction between IR and GLUT4 has been thoroughly studied with modeling and time-resolved data in human cells, comparable analyses in cells from commonly used model organisms such as rats and mice are lacking. Here, we combined existing data and models for rat adipocytes with new data collected for the signaling network between IR and GLUT4 to create a model also for their interconnections. To describe all data (>140 data points), the model needed three distinct pathways from IR to GLUT4: (i) via protein kinase B (PKB) and Akt substrate of 160 kDa (AS160), (ii) via an AS160-independent pathway from PKB, and (iii) via an additional pathway from IR, e.g. affecting the membrane constitution. The developed combined model could describe data not used for training the model and was used to generate predictions of the relative contributions of the pathways from IR to translocation of GLUT4. The combined model provides a systems-level understanding of insulin signaling in rat adipocytes, which, when combined with corresponding models for human adipocytes, may contribute to model-based drug development for diabetes.

Synthesis and Live-Cell Imaging of Fluorescent Sterols for Analysis of Intracellular Cholesterol Transport

Cellular cholesterol homeostasis relies on precise control of the sterol content of organelle membranes. Obtaining insight into cholesterol trafficking pathways and kinetics by live-cell imaging relies on two conditions. First, one needs to develop suitable analogs that resemble cholesterol as closely as possible with respect to their biophysical and biochemical properties. Second, the cholesterol analogs should have good fluorescence properties. This interferes, however, often with the first requirement, such that the imaging instrumentation must be optimized to collect photons from suboptimal fluorophores, but good cholesterol mimics, such as the intrinsically fluorescent sterols, cholestatrienol (CTL) or dehydroergosterol (DHE). CTL differs from cholesterol only in having two additional double bonds in the ring system, which is why it is slightly fluorescent in the ultraviolet (UV). In the first part of this protocol, we describe how to synthesize and image CTL in living cells relative to caveolin, a structural component of caveolae. In the second part, we explain in detail how to perform time-lapse experiments of commercially available BODIPY-tagged cholesterol (TopFluor-cholesterol^®; TF-Chol) in comparison to DHE. Finally, using two-photon time-lapse imaging data of TF-Chol, we demonstrate how to use our imaging toolbox SpatTrack for tracking sterol rich vesicles in living cells over time.

Super-resolution microscopy reveals the insulin-resistance-regulated reorganization of GLUT4 on plasma membranes

GLUT4 (also known as SLC2A4) is essential for glucose uptake in skeletal muscles and adipocytes, which play central roles in whole-body glucose metabolism. Here, using direct stochastic optical reconstruction microscopy (dSTORM) to investigate the characteristics of plasma-membrane-fused GLUT4 at the single-molecule level, we have demonstrated that insulin and insulin resistance regulate the spatial organization of GLUT4 in adipocytes. Stimulation with insulin shifted the balance of GLUT4 on the plasma membrane toward a more dispersed configuration. In contrast, insulin resistance induced a more clustered distribution of GLUT4 and increased the mean number of molecules per cluster. Furthermore, our data demonstrate that the F⁵QQI motif and lipid rafts mediate the maintenance of GLUT4 clusters on the plasma membrane. Mutation of F⁵QQI (F⁵QQA-GLUT4) induced a more clustered distribution of GLUT4; moreover, destruction of lipid rafts in adipocytes expressing F⁵QQA-GLUT4 dramatically decreased the percentage of large clusters and the mean number of molecules per cluster. In conclusion, our data clarify the effects of insulin stimulation or insulin resistance on GLUT4 reorganization on the plasma membrane and reveal new pathogenic mechanisms of insulin resistance.

Fluorogenic probes reveal a role of GLUT4 N-glycosylation in intracellular trafficking

Glucose transporter 4 (GLUT4) is an N-glycosylated protein that maintains glucose homeostasis by regulating the protein translocation. To date, it has been unclear whether the N-glycan of GLUT4 contributes to its intracellular trafficking. Here, to clarify the role of the N-glycan, we developed fluorogenic probes that label cytoplasmic and plasma-membrane proteins for multicolor imaging of GLUT4 translocation. One of the probes, which is cell impermeant, selectively detected exocytosed GLUT4. Using this probe, we verified the 'log' of the trafficking, in which N-glycan-deficient GLUT4 was transiently translocated to the cell membrane upon insulin stimulation and was rapidly internalized without retention on the cell membrane. The results strongly suggest that the N-glycan functions in the retention of GLUT4 on the cell membrane. This study showed the utility of the fluorogenic probes and indicated that this imaging tool will be applicable for research on various membrane proteins that show dynamic changes in localization.

Resin embedded multicycle imaging (REMI): a tool to evaluate protein domains

Protein complexes associated with cellular processes comprise a significant fraction of all biology, but our understanding of their heterogeneous organization remains inadequate, particularly for physiological densities of multiple protein species. Towards resolving this limitation, we here present a new technique based on resin-embedded multicycle imaging (REMI) of proteins in-situ. By stabilizing protein structure and antigenicity in acrylic resins, affinity labels were repeatedly applied, imaged, removed, and replaced. In principle, an arbitrarily large number of proteins of interest may be imaged on the same specimen with subsequent digital overlay. A series of novel preparative methods were developed to address the problem of imaging multiple protein species in areas of the plasma membrane or volumes of cytoplasm of individual cells. For multiplexed examination of antibody staining we used straightforward computational techniques to align sequential images, and super-resolution microscopy was used to further define membrane protein colocalization. We give one example of a fibroblast membrane with eight multiplexed proteins. A simple statistical analysis of this limited membrane proteomic dataset is sufficient to demonstrate the analytical power contributed by additional imaged proteins when studying membrane protein domains.

Human adipose cells in vitro are either refractory or responsive to insulin, reflecting host metabolic state

While intercellular communication processes are frequently characterized by switch-like transitions, the endocrine system, including the adipose tissue response to insulin, has been characterized by graded responses. Yet here individual cells from adipose tissue biopsies are best described by a switch-like transition between the basal and insulin-stimulated states for the trafficking of the glucose transporter GLUT4. Two statistically-defined populations best describe the observed cellular heterogeneity, representing the fractions of refractive and responsive adipose cells. Furthermore, subjects exhibiting high systemic insulin sensitivity indices (SI) have high fractions of responsive adipose cells in vitro, while subjects exhibiting decreasing SI have increasing fractions of refractory cells in vitro. Thus, a two-component model best describes the relationship between cellular refractory fraction and subject SI. Since isolated cells exhibit these different response characteristics in the presence of constant culture conditions and milieu, we suggest that a physiological switching mechanism at the adipose cellular level ultimately drives systemic SI.

Trapping by clusters of channels, receptors, and transporters: quantitative description

Various membrane functional units such as receptors, transporters, and channels, whose action necessarily involves capturing diffusing molecules, are often organized into multimeric complexes forming clusters on the cell and organelle membranes. These functional units themselves are usually oligomers of several integral proteins, which have their own symmetry. Depending on the symmetry, they form clusters on different packing lattices. Moreover, local membrane inhomogeneities, e.g., the so-called membrane domains, rafts, stalks, etc., lead to different patterns even within the structures on the same packing lattice. Units in the cluster compete for diffusing molecules and screen each other. Here we propose a general approach that allows one to quantify the screening effects. The approach is used to derive simple approximate formulas giving the trapping rates of diffusing molecules by clusters of absorbers on lattices of different packing symmetries. The obtained results describe smooth variation of the trapping rate from the sum of the rates of individual absorbers forming the cluster to the effective collective rate. The latter shows how the trapping efficiency of an individual absorber decreases as the number of absorbers in the cluster increases and/or the inter-absorber distance decreases. Numerical tests demonstrate good agreement between the rates predicted by the theory and obtained from Brownian dynamics simulations for clusters of different shapes and sizes.

Novel effects of Brefeldin A (BFA) in signaling through the insulin receptor (IR) pathway and regulating FoxO1-mediated transcription

Brefeldin A (BFA) is a fungal metabolite best known for its ability to inhibit activation of ADP-ribosylation factor (Arf) and thereby inhibit secretory traffic. BFA also appears to regulate the trafficking of the GLUT4 glucose transporter by inducing its relocation from intracellular stores to the cell surface. Such redistribution of GLUT4 is normally regulated by insulin-mediated signaling. Hence, we tested whether BFA may intersect with the insulin pathway. We report that BFA causes the activation of the insulin receptor (IR), IRS-1, Akt-2, and AS160 components of the insulin pathway. The response is mediated through phosphoinositol-3-kinase (PI3K) and Akt kinase since the PI3K inhibitor wortmannin and the Akt inhibitors MK2206 and perifosine inhibit the BFA effect. BFA-mediated activation of the insulin pathway results in Akt-mediated phosphorylation of the insulin-responsive transcription factor FoxO1. This leads to nuclear exclusion of FoxO1 and a decrease in transcription of the insulin-responsive gene SIRT-1. Our findings suggest novel effects for BFA in signaling and transcription, and imply that BFA has multiple intracellular targets and can be used to regulate diverse cellular responses that include vesicular trafficking, signaling and transcription.

Impaired tethering and fusion of GLUT4 vesicles in insulin-resistant human adipose cells

Systemic glucose homeostasis is profoundly influenced by adipose cell function. Here we investigated GLUT4 dynamics in living adipose cells from human subjects with varying BMI and insulin sensitivity index (Si) values. Cells were transfected with hemagglutinin (HA)-GLUT4-green fluorescent protein (GFP)/mCherry (red fluorescence), and were imaged live using total internal reflection fluorescence and confocal microscopy. HA-GLUT4-GFP redistribution to the plasma membrane (PM) was quantified by surface-exposed HA epitope. In the basal state, GLUT4 storage vesicle (GSV) trafficking to and fusion with the PM were invariant with donor subject Si, as was total cell-surface GLUT4. In cells from insulin-sensitive subjects, insulin augmented GSV tethering and fusion approximately threefold, resulting in a corresponding increase in total PM GLUT4. However, with decreasing Si, these effects diminished progressively. All insulin-induced effects on GLUT4 redistribution and trafficking correlated strongly with Si and only weakly with BMI. Thus, while basal GLUT4 dynamics and total cell-surface GLUT4 are intact in human adipose cells, independent of donor Si, cells from insulin-resistant donors show markedly impaired GSV tethering and fusion responses to insulin, even after overnight culture. This altered insulin responsiveness is consistent with the hypothesis that adipose cellular dysfunction is a primary contributor to systemic metabolic dysfunction.