Imaging of insulin signaling in skeletal muscle of living mice shows major role of T-tubules

Measurements of basal d-glucose transport through GLUT1 across the intact plasma membrane of isolated segments from single fast- and slow-twitch skeletal muscle fibres of rat

Aim

To develop a method for direct measurement of the fluorescent d‐glucose analogue 2‐NBDG transport across the plasma membrane of single skeletal muscle fibres and derive the theoretical framework for determining the kinetic parameters for d‐glucose transport under basal conditions.

Methods

A novel method is described for measuring free 2‐NBDG transport across plasma membrane of single rat muscle fibres at rest. The 2‐NBDG uptake was >90% suppressed by 100 µM cytochalasin B in both fast‐twitch and slow‐twitch fibres, indicating that the 2‐NBDG transport is GLUT‐mediated. Fibres were identified as fast‐twitch or slow‐twitch based on the differential sensitivity of their contractile apparatus to Sr²⁺.

Results

The time course of 2‐NBDG uptake in the presence of 50 µM 2‐NBDG follows a one‐phase exponential plateau curve and is faster in fast‐twitch (rate constant 0.053 ± 0.0024 s^‐1) than in slow‐twitch fibres (rate constant 0.031 ± 0.0021 s^‐1). The rate constants were markedly reduced in the presence of 20 mM d‐glucose to 0.0082 ± 0.0004 s^‐1 and 0.0056 ± 0.0002 s^‐1 in fast‐twitch and slow‐twitch fibres respectively. 2‐NBDG transport was asymmetric, consistent with GLUT1 being the major functional GLUT isoform transporting 2‐NBDG in muscle fibres at rest. The parameters describing the transport kinetics for both 2‐NBDG and d‐glucose (dissociation constants, Michaelis–Menten constants, maximal rates of uptake and outflow) were calculated from the measurements made with 2‐NBDG.

Conclusion

Free 2‐NBDG and d‐glucose transport across the plasma membrane of single rat muscle fibres at rest is fast, conclusively showing that the rate‐limiting step in d‐glucose uptake in skeletal muscle is not necessarily the GLUT‐mediated transport of d‐glucose.

Activation of the NLRP3 Inflammasome Increases the IL-1β Level and Decreases GLUT4 Translocation in Skeletal Muscle during Insulin Resistance

Low-grade chronic inflammation plays a pivotal role in the pathogenesis of insulin resistance (IR), and skeletal muscle has a central role in this condition. NLRP3 inflammasome activation pathways promote low-grade chronic inflammation in several tissues. However, a direct link between IR and NLRP3 inflammasome activation in skeletal muscle has not been reported. Here, we evaluated the NLRP3 inflammasome components and their role in GLUT4 translocation impairment in skeletal muscle during IR. Male C57BL/6J mice were fed with a normal control diet (NCD) or high-fat diet (HFD) for 8 weeks. The protein levels of NLRP3, ASC, caspase-1, gasdermin-D (GSDMD), and interleukin (IL)-1β were measured in both homogenized and isolated fibers from the flexor digitorum brevis (FDB) or soleus muscle. GLUT4 translocation was determined through GLUT4myc-eGFP electroporation of the FBD muscle. Our results, obtained using immunofluorescence, showed that adult skeletal muscle expresses the inflammasome components. In the FDB and soleus muscles, homogenates from HFD-fed mice, we found increased protein levels of NLRP3 and ASC, higher activation of caspase-1, and elevated IL-1β in its mature form, compared to NCD-fed mice. Moreover, GSDMD, a protein that mediates IL-1β secretion, was found to be increased in HFD-fed-mice muscles. Interestingly, MCC950, a specific pharmacological NLRP3 inflammasome inhibitor, promoted GLUT4 translocation in fibers isolated from the FDB muscle of NCD- and HFD-fed mice. In conclusion, we found increased NLRP3 inflammasome components in adult skeletal muscle of obese insulin-resistant animals, which might contribute to the low-grade chronic metabolic inflammation of skeletal muscle and IR development.

NLRP3 Inflammasome: Potential Role in Obesity Related Low-Grade Inflammation and Insulin Resistance in Skeletal Muscle

Among multiple mechanisms, low-grade inflammation is critical for the development of insulin resistance as a feature of type 2 diabetes. The nucleotide-binding oligomerization domain-like receptor family (NOD-like) pyrin domain containing 3 (NLRP3) inflammasome has been linked to the development of insulin resistance in various tissues; however, its role in the development of insulin resistance in the skeletal muscle has not been explored in depth. Currently, there is limited evidence that supports the pathological role of NLRP3 inflammasome activation in glucose handling in the skeletal muscle of obese individuals. Here, we have centered our focus on insulin signaling in skeletal muscle, which is the main site of postprandial glucose disposal in humans. We discuss the current evidence showing that the NLRP3 inflammasome disturbs glucose homeostasis. We also review how NLRP3-associated interleukin and its gasdermin D-mediated efflux could affect insulin-dependent intracellular pathways. Finally, we address pharmacological NLRP3 inhibitors that may have a therapeutical use in obesity-related metabolic alterations.

Alternative signaling pathways from IGF1 or insulin to AKT activation and FOXO1 nuclear efflux in adult skeletal muscle fibers

Muscle atrophy is regulated by the balance between protein degradation and synthesis. FOXO1, a transcription factor, helps to determine this balance by activating pro-atrophic gene transcription when present in muscle fiber nuclei. Foxo1 nuclear efflux is promoted by AKT-mediated Foxo1 phosphorylation, eliminating FOXO1's atrophy-promoting effect. AKT activation can be promoted by insulin-like growth factor 1 (IGF1) or insulin via a pathway including IGF1 or insulin, phosphatidylinositol 3-kinase, and AKT. We used confocal fluorescence time-lapse imaging of FOXO1-GFP in adult isolated living muscle fibers maintained in culture to explore the effects of IGF1 and insulin on FOXO1-GFP nuclear efflux with and without pharmacological inhibitors. We observed that although AKT inhibitor blocks the IGF1- or insulin-induced effect on FOXO1 nuclear efflux, phosphatidylinositol 3-kinase inhibitors, which we show to be effective in these fibers, do not. We also found that inhibition of the protein kinase ACK1 or ATM contributes to the suppression of FOXO1 nuclear efflux after IGF1. These results indicate a novel pathway that has been unexplored in the IGF1- or insulin-induced regulation of FOXO1 and present information useful both for therapeutic interventions for muscle atrophy and for further investigative areas into insulin insensitivity and type 2 diabetes.

Heterogeneity in subcellular muscle glycogen utilisation during exercise impacts endurance capacity in men

KEY POINTS

When muscle biopsies first began to be used routinely in research on exercise physiology five decades ago, it soon become clear that the muscle content of glycogen is an important determinant of exercise performance. Glycogen particles are stored in distinct pools within the muscles, but the role of each pool during exercise and how this is affected by diet is unknown. Here, the effects of diet and exercise on these pools, as well as their relation to endurance during prolonged cycling were examined. We demonstrate here that an improved endurance capacity with high carbohydrate loading is associated with a temporal shift in the utilisation of the distinct stores of glycogen pools and is closely linked to the content of the glycogen pool closest to actin and myosin (intramyofibrillar glycogen). These findings highlight the functional importance of distinguishing between different subcellular microcompartments of glycogen in individual muscle fibres.

ABSTRACT

In muscle cells, glycogen is stored in three distinct subcellular pools: between or within myofibrils (inter- and intramyofibrillar glycogen, respectively) or beneath the sarcolemma (subsarcolemmal glycogen) and these pools may well have different functions. Here, we investigated the effect of diet and exercise on the content of these distinct pools and their relation to endurance capacity in type 1 and 2 muscle fibres. Following consumption of three different diets (normal, mixed diet = MIX, high in carbohydrate = HIGH, or low in carbohydrate = LOW) for 72 h, 11 men cycled at 75% of V ̇ O 2 max until exhaustion. The volumetric content of the glycogen pools in muscle biopsies obtained before, during, and after exercise were quantified by transmission electron micrographs. The mean (SD) time to exhaustion was 150 (30), 112 (22), and 69 (18) minutes in the HIGH, MIX and LOW trials, respectively (P < 0.001). As shown by multiple regression analyses, the intramyofibrillar glycogen content in type 1 fibres, particularly after 60 min of exercise, correlated most strongly with time to exhaustion. In the HIGH trial, intramyofibrillar glycogen was spared during the initial 60 min of exercise, which was associated with levels and utilisation of subsarcolemmal glycogen above normal. In all trials, utilisation of subsarcolemmal and intramyofibrillar glycogen was more pronounced than that of intermyofibrillar glycogen in relative terms. In conclusion, the muscle pool of intramyofibrillar glycogen appears to be the most important for endurance capacity in humans. In addition, a local abundance of subsarcolemmal glycogen reduces the utilisation of intramyofibrillar glycogen during exercise.

Prior exercise in humans redistributes intramuscular GLUT4 and enhances insulin-stimulated sarcolemmal and endosomal GLUT4 translocation

Objective

Exercise is a cornerstone in the management of skeletal muscle insulin-resistance. A well-established benefit of a single bout of exercise is increased insulin sensitivity for hours post-exercise in the previously exercised musculature. Although rodent studies suggest that the insulin-sensitization phenomenon involves enhanced insulin-stimulated GLUT4 cell surface translocation and might involve intramuscular redistribution of GLUT4, the conservation to humans is unknown.

Methods

Healthy young males underwent an insulin-sensitizing one-legged kicking exercise bout for 1 h followed by fatigue bouts to exhaustion. Muscle biopsies were obtained 4 h post-exercise before and after a 2-hour hyperinsulinemic-euglycemic clamp.

Results

A detailed microscopy-based analysis of GLUT4 distribution within seven different myocellular compartments revealed that prior exercise increased GLUT4 localization in insulin-responsive storage vesicles and T-tubuli. Furthermore, insulin-stimulated GLUT4 localization was augmented at the sarcolemma and in the endosomal compartments.

Conclusions

An intracellular redistribution of GLUT4 post-exercise is proposed as a molecular mechanism contributing to the insulin-sensitizing effect of prior exercise in human skeletal muscle.

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Intramyocellular GLUT4 is redistributed 4 h after exercise in humans.

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GLUT4 content is increased in GLUT4 storage vesicles and T-tubuli post-exercise.

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Prior exercise + insulin increases sarcolemmal and endosomal GLUT4.

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GLUT4 redistribution may thus contribute to post-exercise muscle insulin-sensitization.

The Emerging Roles of Nicotinamide Adenine Dinucleotide Phosphate Oxidase 2 in Skeletal Muscle Redox Signaling and Metabolism

Significance: Skeletal muscle is a crucial tissue to whole-body locomotion and metabolic health. Reactive oxygen species (ROS) have emerged as intracellular messengers participating in both physiological and pathological adaptations in skeletal muscle. A complex interplay between ROS-producing enzymes and antioxidant networks exists in different subcellular compartments of mature skeletal muscle. Recent evidence suggests that nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are a major source of contraction- and insulin-stimulated oxidants production, but they may paradoxically also contribute to muscle insulin resistance and atrophy. Recent Advances: Pharmacological and molecular biological tools, including redox-sensitive probes and transgenic mouse models, have generated novel insights into compartmentalized redox signaling and suggested that NOX2 contributes to redox control of skeletal muscle metabolism. Critical Issues: Major outstanding questions in skeletal muscle include where NOX2 activation occurs under different conditions in health and disease, how NOX2 activation is regulated, how superoxide/hydrogen peroxide generated by NOX2 reaches the cytosol, what the signaling mediators are downstream of NOX2, and the role of NOX2 for different physiological and pathophysiological processes. Future Directions: Future research should utilize and expand the current redox-signaling toolbox to clarify the NOX2-dependent mechanisms in skeletal muscle and determine whether the proposed functions of NOX2 in cells and animal models are conserved into humans.

Electroporated GLUT4-7myc-GFP detects in vivo glucose transporter 4 translocation in skeletal muscle without discernible changes in GFP patterns

NEW FINDINGS

What is the central question of this study? Resolving the mechanism(s) leading to glucose transporter 4 (GLUT4) translocation to the muscle surface membrane has great therapeutic potential. However, the measurement of GLUT4 translocation is technically challenging. Here, we asked whether electroporation of GLUT4-7myc-GFP into skeletal muscle could be used as a tool to study GLUT4 translocation in vivo. What is the main finding and its importance? By acutely inducing GLUT4-7myc-GFP expression in skeletal muscle, we verified that in vivo exercise and AICAR stimulation increased the GLUT4 presence in the sarcolemma measured as myc signal. Importantly, the increased myc signal in the sarcolemma was not accompanied by major visual changes in the distribution of the GFP signal.

ABSTRACT

Insulin and exercise lead to translocation of the glucose transporter 4 (GLUT4) to the surface membrane of skeletal muscle fibres. This process is pivotal for facilitating glucose uptake into skeletal muscle. To study this, a robust assay is needed to measure the translocation of GLUT4 in adult skeletal muscle directly. Here, we aimed to validate a simple GLUT4 translocation assay using a genetically encoded biosensor in mouse skeletal muscle. We transfected GLUT4-7myc-GFP into mouse muscle to study live GLUT4 movement and to evaluate GLUT4 insertion in the muscle surface membrane after in vivo running exercise and pharmacological activation of AMP-activated protein kinase (AMPK). Transfection led to expression of GLUT4-7myc-GFP that was dynamic in live flexor digitorum brevis fibres and which, upon insulin stimulation, exposed the myc epitope extracellularly. Running exercise, in addition to AMPK activation by 5-aminoimidazole-4-carboxamide ribonucleotide, induced ∼125 and ∼100% increase, respectively, in extracellularly exposure of GLUT4 in the surface membrane of tibialis anterior muscle. Interestingly, the clear increase in surface-exposed GLUT4 content induced by insulin, exercise or AMPK activation was not accompanied by any discernible reorganization of the GLUT4-GFP signal. In conclusion, we provide a detailed description of an easy-to-use translocation assay to study GLUT4 accumulation at the surface membrane induced by exercise and exercise-mimicking stimuli. Notably, our analyses revealed that increased GLUT4 surface membrane accumulation was not accompanied by a discernible change in the GLUT4 localization pattern.

Molecular imaging of diabetes and diabetic complications: Beyond pancreatic β-cell targeting

Diabetes is a chronic non-communicable disease affecting over 400 million people worldwide. Diabetic patients are at a high risk of various complications, such as cardiovascular, renal, and other diseases. The pathogenesis of diabetes (both type 1 and type 2 diabetes) is associated with a functional impairment of pancreatic β-cells. Consequently, most efforts to manage and prevent diabetes have focused on preserving β-cells and their function. Advances in imaging techniques, such as magnetic resonance imaging, magnetic resonance spectroscopy, positron emission tomography, and single-photon-emission computed tomography, have enabled noninvasive and quantitative detection and characterization of the population and function of β-cells in vivo. These advantages aid in defining and monitoring the progress of diabetes and determining the efficacy of anti-diabetic therapies. Beyond β-cell targeting, molecular imaging of biomarkers associated with the development of diabetes, e.g., lymphocyte infiltration, insulitis, and metabolic changes, may also be a promising strategy for early detection of diabetes, monitoring its progression, and occurrence of complications, as well as facilitating exploration of new therapeutic interventions. Moreover, molecular imaging of glucose uptake, production and excretion in specified tissues is critical for understanding the pathogenesis of diabetes. In the current review, we summarize and discuss recent advances in noninvasive imaging technologies for imaging of biomarkers beyond β-cells for early diagnosis of diabetes, investigation of glucose metabolism, and precise diagnosis and monitoring of diabetic complications for better management of diabetic patients.

Electrical pulse stimulation induces GLUT4 translocation in C2C12 myotubes that depends on Rab8A, Rab13, and Rab14

The signals mobilizing GLUT4 to the plasma membrane in response to muscle contraction are less known than those elicited by insulin. This disparity is undoubtedly due to lack of suitable in vitro models to study skeletal muscle contraction. We generated C₂C₁₂ myotubes stably expressing HA-tagged GLUT4 (C2C12-GLUT4 HA) that contract in response to electrical pulse stimulation (EPS) and investigated molecular mechanisms regulating GLUT4 HA. EPS (60 min, 20 V, 1 Hz, 24-ms pulses at 976-ms intervals) elicited a gain in surface GLUT4 HA (GLUT4 translocation) comparably to insulin or 5-amino imidazole-4-carboxamide ribonucleotide (AICAR). A myosin II inhibitor prevented EPS-stimulated myotube contraction and reduced surface GLUT4 by 56%. EPS stimulated AMPK and CaMKII phosphorylation, and EPS-stimulated GLUT4 translocation was reduced in part by small interfering (si)RNA-mediated AMPKα1/α2 knockdown, compound C, siRNA-mediated Ca²⁺/calmodulin-dependent protein kinase (CaMKII)δ knockdown, or CaMKII inhibitor KN93. Key regulatory residues on the Rab-GAPs AS160 and TBC1D1 were phosphorylated in response to EPS. Stable expression of an activated form of the Rab-GAP AS160 (AS160-4A) diminished EPS- and insulin-stimulated GLUT4 translocation, suggesting regulation of GLUT4 vesicle traffic by Rab GTPases. Knockdown of each Rab8a, Rab13, or Rab14 reduced, in part, GLUT4 translocation induced by EPS, whereas only Rab8a, or Rab14 knockdown reduced the AICAR response. In conclusion, EPS involves Rab8a, Rab13, and Rab14 to elicit GLUT4 translocation but not Rab10; moreover, Rab10 and Rab13 are not engaged by AMPK activation alone. C2C12-GLUT4 HA cultures constitute a valuable in vitro model to investigate molecular mechanisms of contraction-stimulated GLUT4 translocation.

Computational modeling of amylin-induced calcium dysregulation in rat ventricular cardiomyocytes

Hyperamylinemia is a condition that accompanies obesity and precedes type II diabetes, and it is characterized by above-normal blood levels of amylin, the pancreas-derived peptide. Human amylin oligomerizes easily and can deposit in the pancreas [1], brain [2], and heart [3], where they have been associated with calcium dysregulation. In the heart, accumulating evidence suggests that human amylin oligomers form moderately cation-selective [4,5] channels that embed in the cell sarcolemma (SL). The oligomers increase membrane conductance in a concentration-dependent manner [5], which is correlated with elevated cytosolic Ca²⁺. These findings motivate our core hypothesis that non-selective inward Ca²⁺ conduction afforded by human amylin oligomers increase cytosolic and sarcoplasmic reticulum (SR) Ca²⁺ load, which thereby magnifies intracellular Ca²⁺ transients. Questions remain however regarding the mechanism of amylin-induced Ca²⁺ dysregulation, including whether enhanced SL Ca²⁺ influx is sufficient to elevate cytosolic Ca²⁺ load [6], and if so, how might amplified Ca²⁺ transients perturb Ca²⁺-dependent cardiac pathways. To investigate these questions, we modified a computational model of cardiomyocytes Ca²⁺ signaling to reflect experimentally-measured changes in SL membrane permeation and decreased sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) function stemming from acute and transgenic human amylin peptide exposure. With this model, we confirmed the hypothesis that increasing SL permeation alone was sufficient to enhance Ca²⁺ transient amplitudes. Our model indicated that amplified cytosolic transients are driven by increased Ca²⁺ loading of the SR and that greater fractional release may contribute to the Ca²⁺-dependent activation of calmodulin, which could prime the activation of myocyte remodeling pathways. Importantly, elevated Ca²⁺ in the SR and dyadic space collectively drive greater fractional SR Ca²⁺ release for human amylin expressing rats (HIP) and acute amylin-exposed rats (+Amylin) mice, which contributes to the inotropic rise in cytosolic Ca²⁺ transients. These findings suggest that increased membrane permeation induced by oligomeratization of amylin peptide in cell sarcolemma contributes to Ca²⁺ dysregulation in pre-diabetes.

Myocyte membrane and microdomain modifications in diabetes: determinants of ischemic tolerance and cardioprotection

Cardiovascular disease, predominantly ischemic heart disease (IHD), is the leading cause of death in diabetes mellitus (DM). In addition to eliciting cardiomyopathy, DM induces a ‘wicked triumvirate’: (i) increasing the risk and incidence of IHD and myocardial ischemia; (ii) decreasing myocardial tolerance to ischemia–reperfusion (I–R) injury; and (iii) inhibiting or eliminating responses to cardioprotective stimuli. Changes in ischemic tolerance and cardioprotective signaling may contribute to substantially higher mortality and morbidity following ischemic insult in DM patients. Among the diverse mechanisms implicated in diabetic impairment of ischemic tolerance and cardioprotection, changes in sarcolemmal makeup may play an overarching role and are considered in detail in the current review. Observations predominantly in animal models reveal DM-dependent changes in membrane lipid composition (cholesterol and triglyceride accumulation, fatty acid saturation vs. reduced desaturation, phospholipid remodeling) that contribute to modulation of caveolar domains, gap junctions and T-tubules. These modifications influence sarcolemmal biophysical properties, receptor and phospholipid signaling, ion channel and transporter functions, contributing to contractile and electrophysiological dysfunction, cardiomyopathy, ischemic intolerance and suppression of protective signaling. A better understanding of these sarcolemmal abnormalities in types I and II DM (T1DM, T2DM) can inform approaches to limiting cardiomyopathy, associated IHD and their consequences. Key knowledge gaps include details of sarcolemmal changes in models of T2DM, temporal patterns of lipid, microdomain and T-tubule changes during disease development, and the precise impacts of these diverse sarcolemmal modifications. Importantly, exercise, dietary, pharmacological and gene approaches have potential for improving sarcolemmal makeup, and thus myocyte function and stress-resistance in this ubiquitous metabolic disorder.

Heterotypic endosomal fusion as an initial trigger for insulin-induced glucose transporter 4 (GLUT4) translocation in skeletal muscle

KEY POINTS

Comprehensive imaging analyses of glucose transporter 4 (GLUT4) behaviour in mouse skeletal muscle was conducted. Quantum dot-based single molecule nanometry revealed that GLUT4 molecules in skeletal myofibres are governed by regulatory systems involving 'static retention' and 'stimulus-dependent liberation'. Vital imaging analyses and super-resolution microscopy-based morphometry demonstrated that insulin liberates the GLUT4 molecule from its static state by triggering acute heterotypic endomembrane fusion arising from the very small GLUT4-containing vesicles in skeletal myofibres. Prior exposure to exercise-mimetic stimuli potentiated this insulin-responsive endomembrane fusion event involving GLUT4-containing vesicles, suggesting that this endomembranous regulation process is a potential site related to the effects of exercise.

ABSTRACT

Skeletal muscle is the major systemic glucose disposal site. Both insulin and exercise facilitate translocation of the glucose transporter glucose transporter 4 (GLUT4) via distinct signalling pathways and exercise also enhances insulin sensitivity. However, the trafficking mechanisms controlling GLUT4 mobilization in skeletal muscle remain poorly understood as a resuly of technical limitations. In the present study, which employs various imaging techniques on isolated skeletal myofibres, we show that one of the initial triggers of insulin-induced GLUT4 translocation is heterotypic endomembrane fusion arising from very small static GLUT4-containing vesicles with a subset of transferrin receptor-containing endosomes. Importantly, pretreatment with exercise-mimetic stimuli potentiated the susceptibility to insulin responsiveness, as indicated by these acute endomembranous activities. We also found that AS160 exhibited stripe-like localization close to sarcomeric α-actinin and that insulin induced a reduction of the stripe-like localization accompanying changes in its detergent solubility. The results of the present study thus provide a conceptual framework indicating that GLUT4 protein trafficking via heterotypic fusion is a critical feature of GLUT4 translocation in skeletal muscles and also suggest that the efficacy of the endomembranous fusion process in response to insulin is involved in the benefits of exercise.

Neurological and Motor Disorders: TRPC in the Skeletal Muscle

Transient receptor potential canonical (TRPC) channels belong to the large family of TRPs that are mostly nonselective cation channels with a great variety of gating mechanisms. TRPC are composed of seven members that can all be activated downstream of agonist-induced phospholipase C stimulation, but some members are also stretch-activated and/or are part of the store-operated Ca²⁺ entry (SOCE) pathway. Skeletal muscles generate contraction via an explosive increase of cytosolic Ca²⁺ concentration resulting almost exclusively from sarcoplasmic reticulum Ca²⁺ channel opening. Even if neglected for a long time, it is now commonly accepted that Ca²⁺ entry via SOCE and other routes is essential to sustain contractions of the skeletal muscle. In addition, Ca²⁺ influx is required during muscle regeneration, and alteration of the influx is associated with myopathies. In this chapter, we review the implication of TRPC channels at different stages of muscle regeneration, in adult muscle fibers, and discuss their implication in myopathies.

An actin filament population defined by the tropomyosin Tpm3.1 regulates glucose uptake

Actin has an ill-defined role in the trafficking of GLUT4 glucose transporter vesicles to the plasma membrane (PM). We have identified novel actin filaments defined by the tropomyosin Tpm3.1 at glucose uptake sites in white adipose tissue (WAT) and skeletal muscle. In Tpm 3.1-overexpressing mice, insulin-stimulated glucose uptake was increased; while Tpm3.1-null mice they were more sensitive to the impact of high-fat diet on glucose uptake. Inhibition of Tpm3.1 function in 3T3-L1 adipocytes abrogates insulin-stimulated GLUT4 translocation and glucose uptake. In WAT, the amount of filamentous actin is determined by Tpm3.1 levels and is paralleled by changes in exocyst component (sec8) and Myo1c levels. In adipocytes, Tpm3.1 localizes with MyoIIA, but not Myo1c, and it inhibits Myo1c binding to actin. We propose that Tpm3.1 determines the amount of cortical actin that can engage MyoIIA and generate contractile force, and in parallel limits the interaction of Myo1c with actin filaments. The balance between these actin filament populations may determine the efficiency of movement and/or fusion of GLUT4 vesicles with the PM.

Aegeline from Aegle marmelos stimulates glucose transport via Akt and Rac1 signaling, and contributes to a cytoskeletal rearrangement through PI3K/Rac1

Aegeline is an alkaloidal-amide, isolated from the leaves of Aegle marmelos and have shown antihyperglycemic as well as antidyslipidemic activities in the validated animal models of type 2 diabetes mellitus. Here we delineate, aegeline enhanced GLUT4 translocation mediated 2-deoxy-glucose uptake in both time and concentration-dependent manner. 2-deoxy-glucose uptake was completely stymied by the transport inhibitors (wortmannin and genistein) in C2C12 myotubes. Pharmacological inhibition of Akt (also known as protein kinase B) and Ras-related C3 botulinum toxin substrate 1 (Rac1) suggest that both Akt and Rac1 operate aegeline-stimulated glucose transport via distinct parallel pathways. Moreover, aegeline activates p21 protein-activated kinase 1 (PAK1) and cofilin (an actin polymerization regulator). Rac1 inhibitor (Rac1 inhib II) and PAK1 inhibitor (IPA-3) completely blocked aegeline-induced phosphorylation of cofilin and p21 protein-activated kinase 1 (PAK1). In summary, these findings suggest that aegeline stimulates the glucose transport through Akt and Rac1 dependent distinct parallel pathways and have cytoskeletal roles via stimulation of the PI3-kinase-Rac1-PAK1-cofilin pathway in the skeletal muscle cells. Therefore, multiple targets of aegeline in the improvement of insulin sensitivity of the skeletal muscle cells may be suggested.

EGF as a New Therapeutic Target for Medulloblastoma Metastasis

Medulloblastoma (MB) is a malignant pediatric brain tumor known for its aggressive metastatic potential. Despite the well-documented migration of MB cells to other parts of the brain and spinal column, MB chemotaxis is poorly understood. Herein, we examined the in vitro migratory and cellular responses of MB-derived cells to external signaling of Epidermal Growth Factor (EGF), hepatocyte growth factor (HGF), platelet-derived growth factor (PDGF-BB), and the stromal cell-derived factors 1-alpha (SDF-1). Experiments utilized transwell assays and immunocytochemistry to identify receptor activation in MB migration, and used a microfluidic platform to examine directionality, trajectory, and gradient-dependence of motile cells. Data illustrates that MB-derived cells respond strongly to EGF in a dosage and gradient-dependent manner with increased EGF-R activation, and show that high EGF gradient fields cause an increased number of cells to migrate longer directed distances. Our results provide evidence that EGF and its receptor play an important role than previously documented in MB chemotactic migration than previously documented and should be considered for developing migration-target therapies against MB metastasis.

Differential Role of Insulin/IGF-1 Receptor Signaling in Muscle Growth and Glucose Homeostasis

Insulin and insulin-like growth factor 1 (IGF-1) are major regulators of muscle protein and glucose homeostasis. To determine how these pathways interact, we generated mice with muscle-specific knockout of IGF-1 receptor (IGF1R) and insulin receptor (IR). These MIGIRKO mice showed >60% decrease in muscle mass. Despite a complete lack of insulin/IGF-1 signaling in muscle, MIGIRKO mice displayed normal glucose and insulin tolerance. Indeed, MIGIRKO mice showed fasting hypoglycemia and increased basal glucose uptake. This was secondary to decreased TBC1D1 resulting in increased Glut4 and Glut1 membrane localization. Interestingly, overexpression of a dominant-negative IGF1R in muscle induced glucose intolerance in MIGIRKO animals. Thus, loss of insulin/IGF-1 signaling impairs muscle growth, but not whole-body glucose tolerance due to increased membrane localization of glucose transporters. Nonetheless, presence of a dominant-negative receptor, even in the absence of functional IR/IGF1R, induces glucose intolerance, indicating that interactions between these receptors and other proteins in muscle can impair glucose homeostasis.

Visualization and quantitation of GLUT4 translocation in human skeletal muscle following glucose ingestion and exercise

Insulin- and contraction-stimulated increases in glucose uptake into skeletal muscle occur in part as a result of the translocation of glucose transporter 4 (GLUT4) from intracellular stores to the plasma membrane (PM). This study aimed to use immunofluorescence microscopy in human skeletal muscle to quantify GLUT4 redistribution from intracellular stores to the PM in response to glucose feeding and exercise. Percutaneous muscle biopsy samples were taken from the m. vastus lateralis of ten insulin-sensitive men in the basal state and following 30 min of cycling exercise (65% VO_2 max). Muscle biopsy samples were also taken from a second cohort of ten age-, BMI- and VO_2 max-matched insulin-sensitive men in the basal state and 30 and 60 min following glucose feeding (75 g glucose). GLUT4 and dystrophin colocalization, measured using the Pearson's correlation coefficient, was increased following 30 min of cycling exercise (baseline r = 0.47 ± 0.01; post exercise r = 0.58 ± 0.02; P < 0.001) and 30 min after glucose ingestion (baseline r = 0.42 ± 0.02; 30 min r = 0.46 ± 0.02; P < 0.05). Large and small GLUT4 clusters were partially depleted following 30 min cycling exercise, but not 30 min after glucose feeding. This study has, for the first time, used immunofluorescence microscopy in human skeletal muscle to quantify increases in GLUT4 and dystrophin colocalization and depletion of GLUT4 from large and smaller clusters as evidence of net GLUT4 translocation to the PM.

Quantitative immunofluorescence microscopy of subcellular GLUT4 distribution in human skeletal muscle: effects of endurance and sprint interval training

Increases in insulin‐mediated glucose uptake following endurance training (ET) and sprint interval training (SIT) have in part been attributed to concomitant increases in glucose transporter 4 (GLUT4) protein content in skeletal muscle. This study used an immunofluorescence microscopy method to investigate changes in subcellular GLUT4 distribution and content following ET and SIT. Percutaneous muscle biopsy samples were taken from the m. vastus lateralis of 16 sedentary males in the overnight fasted state before and after 6 weeks of ET and SIT. An antibody was fully validated and used to show large (> 1 μm) and smaller (<1 μm) GLUT4‐containing clusters. The large clusters likely represent trans‐Golgi network stores and the smaller clusters endosomal stores and GLUT4 storage vesicles (GSVs). Density of GLUT4 clusters was higher at the fibre periphery especially in perinuclear regions. A less dense punctate distribution was seen in the rest of the muscle fibre. Total GLUT4 fluorescence intensity increased in type I and type II fibres following both ET and SIT. Large GLUT4 clusters increased in number and size in both type I and type II fibres, while the smaller clusters increased in size. The greatest increases in GLUT4 fluorescence intensity occurred within the 1 μm layer immediately adjacent to the PM. The increase in peripheral localisation and protein content of GLUT4 following ET and SIT is likely to contribute to the improvements in glucose homeostasis observed after both training modes.

e12085

This paper first describes the development of a novel confocal immunofluorescence microscopy method that allows quantitation of GLUT4 content in the plasma membrane and 1 µm layers below it in a muscle fibre‐type specific manner. Skeletal muscle biopsies obtained from sedentary young men before and after 6 weeks of traditional endurance training (ET) and sprint interval training (SIT) were then analysed to show for the first time increases in large and small GLUT4 clusters with greater increases in the layer within 1 µm of the plasma membrane, which is the layer from which most GLUT4 fusion events emanate. This training‐induced redistribution is likely to contribute to the increase in insulin sensitivity seen following both ET and SIT.

Paxillin and focal adhesion kinase colocalise in human skeletal muscle and its associated microvasculature

Focal adhesion kinase (FAK) and paxillin are functionally linked hormonal- and mechano-sensitive proteins. We aimed to describe paxillin's subcellular distribution using widefield and confocal immunofluorescence microscopy and test the hypothesis that FAK and paxillin colocalise in human skeletal muscle and its associated microvasculature. Percutaneous muscle biopsies were collected from the m. vastus lateralis of seven healthy males, and 5-μm cryosections were stained with anti-paxillin co-incubated with anti-dystrophin to identify the sarcolemma, anti-myosin heavy chain type I for fibre-type differentiation, anti-dihydropyridine receptor to identify T-tubules, lectin UEA-I to identify the endothelium of microvessels and anti-α-smooth muscle actin to identify vascular smooth muscle cells (VSMC). Colocalisation of anti-paxillin with anti-dystrophin or anti-FAK was quantified using Pearson's correlation coefficient on confocal microscopy images. Paxillin was primarily present in (sub)sarcolemmal regions of skeletal muscle fibres where it colocalised with dystrophin (r = 0.414 ± 0.026). The (sub)sarcolemmal paxillin immunofluorescence intensity was ~2.4-fold higher than in sarcoplasmic regions (P < 0.001) with sarcoplasmic paxillin immunofluorescence intensity ~10 % higher in type I than in type II fibres (P < 0.01). In some longitudinally orientated fibres, paxillin formed striations that corresponded to the I-band region. Paxillin immunostaining was highest in endothelial and VSMC and distributed heterogeneously in both cell types. FAK and paxillin colocalised at (sub)sarcolemmal regions and within the microvasculature (r = 0.367 ± 0.036). The first images of paxillin in human skeletal muscle suggest paxillin is present in (sub)sarcolemmal and I-band regions of muscle fibres and within the microvascular endothelium and VSMC. Colocalisation of FAK and paxillin supports their suggested role in hormonal and mechano-sensitive signalling.

Insulin- and contraction-induced glucose transporter 4 traffic in muscle: insights from a novel imaging approach

Insulin- and contraction-mediated glucose transporter 4 (GLUT4) trafficking have different kinetics in mature skeletal muscle. Intravital imaging indicates that insulin-stimulated GLUT4 trafficking differs between t-tubules and sarcolemma. In contrast, contraction-induced GLUT4 trafficking does not differ between membrane surfaces. This distinction likely is caused by differences in the underlying signaling pathways regulating GLUT4 vesicle depletion, GLUT4 membrane fusion, and GLUT4 reinternalization.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

Exercise, GLUT4, and skeletal muscle glucose uptake

Glucose is an important fuel for contracting muscle, and normal glucose metabolism is vital for health. Glucose enters the muscle cell via facilitated diffusion through the GLUT4 glucose transporter which translocates from intracellular storage depots to the plasma membrane and T-tubules upon muscle contraction. Here we discuss the current understanding of how exercise-induced muscle glucose uptake is regulated. We briefly discuss the role of glucose supply and metabolism and concentrate on GLUT4 translocation and the molecular signaling that sets this in motion during muscle contractions. Contraction-induced molecular signaling is complex and involves a variety of signaling molecules including AMPK, Ca(2+), and NOS in the proximal part of the signaling cascade as well as GTPases, Rab, and SNARE proteins and cytoskeletal components in the distal part. While acute regulation of muscle glucose uptake relies on GLUT4 translocation, glucose uptake also depends on muscle GLUT4 expression which is increased following exercise. AMPK and CaMKII are key signaling kinases that appear to regulate GLUT4 expression via the HDAC4/5-MEF2 axis and MEF2-GEF interactions resulting in nuclear export of HDAC4/5 in turn leading to histone hyperacetylation on the GLUT4 promoter and increased GLUT4 transcription. Exercise training is the most potent stimulus to increase skeletal muscle GLUT4 expression, an effect that may partly contribute to improved insulin action and glucose disposal and enhanced muscle glycogen storage following exercise training in health and disease.

Fatty acid transport proteins chronically relocate to the transverse-tubules in muscle from obese Zucker rats but are resistant to further insulin-induced translocation

OBJECTIVES

Recently, we have demonstrated that FA transport proteins are located within the t-tubule fraction of rodent muscle, and that insulin stimulation causes their translocation to this membrane fraction. Chronic relocation of the FA transport protein FAT/CD36 to the sarcolemma is observed in obese rodents and humans, and correlates with intramuscular lipid accumulation and insulin resistance. It is not known whether in an obese, insulin resistant state FA transporters also chronically relocate to the t-tubules. Furthermore, it is not known whether the insulin-stimulated translocation of the various FA transport proteins to the t-tubules is impaired in insulin resistance.

METHODS

Sarcolemmal and t-tubule membrane fractions were isolated via differential centrifugation from muscles of lean and obese female Zucker rats during basal or insulin stimulated conditions. FA transport proteins were measured via western blot on both membrane fractions.

RESULTS

Our results demonstrate that in muscle from insulin resistant Zucker rats, FAT/CD36, FABPpm and FATP1 are all increased on the t-tubules in the basal state (+72%, +120%, and +69%, respectively), potentially contributing to the accumulation of intramuscular lipids. Insulin failed to increase the content of the FA transport proteins on either the t-tubule or sarcolemma above the elevated basal levels, analogous to the well characterized impairment of insulin-stimulated GLUT4 translocation to both membrane domains in obesity.

CONCLUSION

FA transport proteins chronically relocate to the t-tubule domain in insulin resistant muscle, potentially contributing to lipid accumulation. Further translocation of the FA transport proteins to this domain during insulin stimulation, however, is impaired.

Contraction and AICAR stimulate IL-6 vesicle depletion from skeletal muscle fibers in vivo

Recent studies suggest that interleukin 6 (IL-6) is released from contracting skeletal muscles; however, the cellular origin, secretion kinetics, and signaling mechanisms regulating IL-6 secretion are unknown. To address these questions, we developed imaging methodology to study IL-6 in fixed mouse muscle fibers and in live animals in vivo. Using confocal imaging to visualize endogenous IL-6 protein in fixed muscle fibers, we found IL-6 in small vesicle structures distributed throughout the fibers under basal (resting) conditions. To determine the kinetics of IL-6 secretion, intact quadriceps muscles were transfected with enhanced green fluorescent protein (EGFP)-tagged IL-6 (IL-6-EGFP), and 5 days later anesthetized mice were imaged before and after muscle contractions in situ. Contractions decreased IL-6-EGFP-containing vesicles and protein by 62% (P < 0.05), occurring rapidly and progressively over 25 min of contraction. However, contraction-mediated IL-6-EGFP reduction was normal in muscle-specific AMP-activated protein kinase (AMPK) α2-inactive transgenic mice. In contrast, the AMPK activator AICAR decreased IL-6-EGFP vesicles, an effect that was inhibited in the transgenic mice. In conclusion, resting skeletal muscles contain IL-6-positive vesicles that are expressed throughout myofibers. Contractions stimulate the rapid reduction of IL-6 in myofibers, occurring through an AMPKα2-independent mechanism. This novel imaging methodology clearly establishes IL-6 as a contraction-stimulated myokine and can be used to characterize the secretion kinetics of other putative myokines.

Rac1 signaling is required for insulin-stimulated glucose uptake and is dysregulated in insulin-resistant murine and human skeletal muscle

The actin cytoskeleton-regulating GTPase Rac1 is required for insulin-stimulated GLUT4 translocation in cultured muscle cells. However, involvement of Rac1 and its downstream signaling in glucose transport in insulin-sensitive and insulin-resistant mature skeletal muscle has not previously been investigated. We hypothesized that Rac1 and its downstream target, p21-activated kinase (PAK), are regulators of insulin-stimulated glucose uptake in mouse and human skeletal muscle and are dysregulated in insulin-resistant states. Muscle-specific inducible Rac1 knockout (KO) mice and pharmacological inhibition of Rac1 were used to determine whether Rac1 regulates insulin-stimulated glucose transport in mature skeletal muscle. Furthermore, Rac1 and PAK1 expression and signaling were investigated in muscle of insulin-resistant mice and humans. Inhibition and KO of Rac1 decreased insulin-stimulated glucose transport in mouse soleus and extensor digitorum longus muscles ex vivo. Rac1 KO mice showed decreased insulin and glucose tolerance and trended toward higher plasma insulin concentrations after intraperitoneal glucose injection. Rac1 protein expression and insulin-stimulated PAK(Thr423) phosphorylation were decreased in muscles of high fat-fed mice. In humans, insulin-stimulated PAK activation was decreased in both acute insulin-resistant (intralipid infusion) and chronic insulin-resistant states (obesity and diabetes). These findings show that Rac1 is a regulator of insulin-stimulated glucose uptake and a novel candidate involved in skeletal muscle insulin resistance.

Using a novel coculture model to dissect the role of intramuscular lipid load on skeletal muscle insulin responsiveness under reduced estrogen conditions

Reductions in estrogen function lead to adiposity and peripheral insulin resistance. Significant metabolic changes have been found in adipocytes and skeletal muscle with disruptions in the estrogen-signaling axis; however, it is unclear if intercellular communication exists between these tissues. The purpose of this study was to examine the impact of isolated adipocytes cocultured with single adult skeletal muscle fibers (SMF) collected from control female (SHAM) and ovariectomized female (OVX) mice. In addition, a second purpose was to compare differential effects of primary adipocytes from omental and inguinal adipose depots on SMF from these same groups. OVX SMF displayed greater lipid content, impaired insulin signaling, and lower insulin-induced glucose uptake compared with SHAM SMF without coculture. In the SHAM group, regardless of the adipose depot of origin, coculture induced greater intracellular lipid content compared with control SHAM SMF. The increased lipid in the SMF was associated with impaired insulin-induced glucose uptake when adipocytes were of omental, but not inguinal, origin. Coculture of OVX SMF with omental or inguinal adipocytes resulted in higher lipid content but no further reduction in insulin-induced glucose uptake compared with control OVX SMF. The data indicate that, in the OVX condition, there is a threshold for lipid accumulation in skeletal muscle beyond which there is no further impairment in insulin responsiveness. These results also demonstrate depot-specific effects of adipocyte exposure on skeletal muscle glucose uptake and further implicate a role for increased intracellular lipid storage in the pathogenesis of insulin resistance when estrogen levels are reduced.

Insulin and contraction-induced movement of fatty acid transport proteins to skeletal muscle transverse-tubules is distinctly different than to the sarcolemma

Fatty acid (FA) transport proteins are known to exist on the sarcolemma of skeletal muscle. However, it is unknown whether the t-tubules, which comprise ~60% of the cell surface, also harbor these proteins. We examined FA transport proteins from both membrane fractions in unstimulated, insulin-stimulated and contracted skeletal muscle. Sarcolemmal and t-tubule membrane fractions were isolated from the same muscle homogenate using a discontinuous sucrose gradient. Our results demonstrate that the relative content of FA transport proteins within the two fractions and the magnitude to which they increase when stimulated were distinctly different. In unstimulated muscle FAT/CD36, FATP4, and FABPpm are abundant on the sarcolemma (3-, 8-, and 10-fold greater than t-tubule, respectively), whereas FATP1 resides primarily within the t-tubule fraction (1- to 2-fold greater than the sarcolemma). With both stimuli, in terms of absolute increase, FAT/CD36 predominantly translocated to the sarcolemma and FATP1 to the t-tubules. There are clear differences in the profile of FA transport proteins and the response to stimuli of the sarcolemma and t-tubules. FATP1, a variable and unresponsive protein on the sarcolemma, appears to reside primarily in the t-tubules where it is responsive to stimuli.

Immunofluorescent visualisation of focal adhesion kinase in human skeletal muscle and its associated microvasculature

Within animal skeletal muscle, focal adhesion kinase (FAK) has been associated with load-dependent molecular and metabolic adaptation including the regulation of insulin sensitivity. This study aimed to generate the first visual images of the localisation of FAK within human skeletal muscle fibres and its associated microvasculature using widefield and confocal immunofluorescence microscopy. Percutaneous muscle biopsies, taken from five lean, active males, were frozen and 5-μm cryosections were incubated with FAK antibodies for visualisation in muscle fibres and the microvasculature. Anti-myosin heavy chain type I was used for fibre-type differentiation. Muscle sections were also incubated with anti-dihydropyridine receptor (DHPR) to investigate co-localisation of FAK with the t-tubules. FITC-conjugated Ulex europaeus Agglutinin I stained the endothelium of the capillaries, whilst anti-smooth muscle actin stained the vascular smooth muscle of arterioles. Fibre-type differences in the intensity of FAK immunofluorescence were determined with image analysis software. In transversely and longitudinally orientated fibres, FAK was localised at the sarcolemmal regions. In longitudinally orientated fibres, FAK staining also showed uniform striations across the fibre and co-staining with DHPR suggests FAK associates with the t-tubules. There was no fibre-type difference in sarcoplasmic FAK content. Within the capillary endothelium and arteriolar smooth muscle, FAK was distributed heterogeneously as clusters. This is the first study to visualise FAK in human skeletal muscle microvasculature and within the (sub)sarcolemmal and t-tubule regions using immunofluorescence microscopy. This technique will be an important tool for investigating the role of FAK in the intracellular signalling of human skeletal muscle and the endothelium of its associated microvasculature.

Insulin stimulates fusion, but not tethering, of GLUT4 vesicles in skeletal muscle of HA-GLUT4-GFP transgenic mice

Insulin regulates glucose uptake into fat and muscle by modulating the subcellular distribution of GLUT4 between the cell surface and intracellular compartments. However, quantification of these translocation processes in muscle by classical subcellular fractionation techniques is confounded by contaminating microfibrillar protein; dynamic studies at the molecular level are almost impossible. In this study, we introduce a muscle-specific transgenic mouse model in which HA-GLUT4-GFP is expressed under the control of the MCK promoter. HA-GLUT4-GFP was found to translocate to the plasma membrane and T-tubules after insulin stimulation, thus mimicking endogenous GLUT4. To investigate the dynamics of GLUT4 trafficking in skeletal muscle, we quantified vesicles containing HA-GLUT4-GFP near the sarcolemma and T-tubules and analyzed insulin-stimulated exocytosis at the single vesicle level by total internal reflection fluorescence and confocal microscopy. We found that only 10% of the intracellular GLUT4 pool comprised mobile vesicles, whereas most of the GLUT4 structures remained stationary or tethered at the sarcolemma or T-tubules. In fact, most of the insulin-stimulated exocytosis emanated from pretethered vesicles, whereas the small pool of mobile GLUT4 vesicles was not significantly affected by insulin. Our data strongly suggest that the mobile pool of GLUT4 vesicles is not a major site of insulin action but rather locally distributed. Most likely, pretethered GLUT4 structures are responsible for the initial phase of insulin-stimulated exocytosis.

Regulated exocytosis: novel insights from intravital microscopy

Regulated exocytosis is a fundamental process that every secretory cell uses to deliver molecules to the cell surface and the extracellular space by virtue of membranous carriers. This process has been extensively studied using various approaches such as biochemistry, electrophysiology and electron microscopy. However, recent developments in time-lapse light microscopy have made possible imaging individual exocytic events, hence, advancing our understanding of this process at a molecular level. In this review, we focus on intravital microscopy (IVM), a light microscopy-based approach that enables imaging subcellular structures in live animals, and discuss its recent application to study regulated exocytosis. IVM has revealed differences in regulation and modality of regulated exocytosis between in vitro and in vivo model systems, unraveled novel aspects of this process that can be appreciated only in in vivo settings and provided valuable and novel information on its molecular machinery. In conclusion, we make the case for IVM being a mature technique that can be used to investigate the molecular machinery of several intracellular events under physiological conditions.

Extracellular movement of signaling molecules

Extracellular signaling molecules have crucial roles in development and homeostasis, and their incorrect deployment can lead to developmental defects and disease states. Signaling molecules are released from sending cells, travel to target cells, and act over length scales of several orders of magnitude, from morphogen-mediated patterning of small developmental fields to hormonal signaling throughout the organism. We discuss how signals are modified and assembled for transport, which routes they take to reach their targets, and how their range is affected by mobility and stability.

Human skeletal muscle glycogen utilization in exhaustive exercise: role of subcellular localization and fibre type

Although glycogen is known to be heterogeneously distributed within skeletal muscle cells, there is presently little information available about the role of fibre types, utilization and resynthesis during and after exercise with respect to glycogen localization. Here, we tested the hypothesis that utilization of glycogen with different subcellular localizations during exhaustive arm and leg exercise differs and examined the influence of fibre type and carbohydrate availability on its subsequent resynthesis. When 10 elite endurance athletes (22 ± 1 years, VO2 max = 68 ± 5 ml kg-1 min-1, mean ± SD) performed one hour of exhaustive arm and leg exercise, transmission electron microscopy revealed more pronounced depletion of intramyofibrillar than of intermyofibrillar and subsarcolemmal glycogen. This phenomenon was the same for type I and II fibres, although at rest prior to exercise, the former contained more intramyofibrillar and subsarcolemmal glycogen than the latter. In highly glycogen-depleted fibres, the remaining small intermyofibrillar and subsarcolemmal glycogen particles were often found to cluster in groupings. In the recovery period, when the athletes received either a carbohydrate-rich meal or only water the impaired resynthesis of glycogen with water alone was associated primarily with intramyofibrillar glycogen. In conclusion, after prolonged high-intensity exercise the depletion of glycogen is dependent on subcellular localization. In addition, the localization of glycogen appears to be influenced by fibre type prior to exercise, as well as carbohydrate availability during the subsequent period of recovery. These findings provide insight into the significance of fibre type-specific compartmentalization of glycogen metabolism in skeletal muscle during exercise and subsequent recovery. .

Intravital microscopy as a tool to study drug delivery in preclinical studies

The technical developments in the field of non-linear microscopy have made intravital microscopy one of the most successful techniques for studying physiological and pathological processes in live animals. Intravital microscopy has been utilized to address many biological questions in basic research and is now a fundamental tool for preclinical studies, with an enormous potential for clinical applications. The ability to dynamically image cellular and subcellular structures combined with the possibility to perform longitudinal studies have empowered investigators to use this discipline to study the mechanisms of action of therapeutic agents and assess the efficacy on their targets in vivo. The goal of this review is to provide a general overview of the recent advances in intravital microscopy and to discuss some of its applications in preclinical studies.

Kinetics of contraction-induced GLUT4 translocation in skeletal muscle fibers from living mice

OBJECTIVE

Exercise is an important strategy for the treatment of type 2 diabetes. This is due in part to an increase in glucose transport that occurs in the working skeletal muscles. Glucose transport is regulated by GLUT4 translocation in muscle, but the molecular machinery mediating this process is poorly understood. The purpose of this study was to 1) use a novel imaging system to elucidate the kinetics of contraction-induced GLUT4 translocation in skeletal muscle and 2) determine the function of AMP-activated protein kinase alpha2 (AMPKalpha2) in this process.

RESEARCH DESIGN AND METHODS

Confocal imaging was used to visualize GLUT4-enhanced green fluorescent protein (EGFP) in transfected quadriceps muscle fibers in living mice subjected to contractions or the AMPK-activator AICAR.

RESULTS

Contraction increased GLUT4-EGFP translocation from intracellular vesicle depots to both the sarcolemma and t-tubules with similar kinetics, although translocation was greater with contractions elicited by higher voltage. Re-internalization of GLUT4 did not begin until 10 min after contractions ceased and was not complete until 130 min after contractions. AICAR increased GLUT4-EGFP translocation to both sarcolemma and t-tubules with similar kinetics. Ablation of AMPKalpha2 activity in AMPKalpha2 inactive transgenic mice did not change GLUT4-EGFP's basal localization, contraction-stimulated intracellular GLUT4-EGFP vesicle depletion, translocation, or re-internalization, but diminished AICAR-induced translocation.

CONCLUSIONS

We have developed a novel imaging system to study contraction-stimulated GLUT4 translocation in living mice. Contractions increase GLUT4 translocation to the sarcolemma and t-tubules with similar kinetics and do not require AMPKalpha2 activity.

Measuring GLUT4 translocation in mature muscle fibers

Skeletal muscle is the major tissue for postprandial glucose disposal. Facilitated glucose uptake into muscle fibers is mediated by increases in surface membrane levels of the glucose transporter GLUT4 via insulin- and/or muscle contraction-mediated GLUT4 translocation. However, the regulatory mechanisms controlling GLUT4 translocation in skeletal muscle have been difficult to characterize at the cell biology level due to muscle tissue complexity. Muscle cell culture models have improved our understanding of GLUT4 translocation and glucose transport regulation, but in vitro muscle models lack many of the characteristics of mature muscle fibers. Thus, the molecular and cellular details of GLUT4 translocation in mature skeletal muscle are deficient. The objective of this review is to highlight how advances in recent experimental approaches translate into an enhanced understanding of the regulation of GLUT4 translocation and glucose transport in mature skeletal muscle.

Imaging of protein translocation in situ in skeletal muscle of living mice

Skeletal muscle plays a key role in regulating whole body glucose homeostasis and severe dysfunction in insulin-mediated glucose uptake is the hallmark of insulin-resistant states and type II diabetes. Therefore it is highly pathophysiologically relevant to perform detailed studies of insulin signaling inside skeletal muscle cells in order to elucidate the specific molecular events during both normal and insulin-resistant conditions. So far, cell biology imaging techniques have been limited to in vitro cultured muscle originating from primary or cell line-based myoblasts. However, these types of cultured muscle lack many characteristics of fully differentiated muscle cells. By performing intravital protein translocation analysis directly in situ in living animals, we have been able to give a high-resolution account of the spatial and temporal details during insulin signaling in vivo in muscle that does not have the limitations of in vitro cultures. We have shown that after i.v. insulin injection, PI3-kinase activation and, in turn, GLUT4 translocation are initiated at the plasma membrane proper, the sarcolemma. Then insulin signaling progresses into the t-tubules with a velocity corresponding to the diffusion of sulforhodamine B-conjugated insulin molecules. By using intravital confocal time-lapse analysis we have revealed that the t-tubules are the membrane surface where the majority of the insulin signaling is located.

Intravital microscopy: a novel tool to study cell biology in living animals

Intravital microscopy encompasses various optical microscopy techniques aimed at visualizing biological processes in live animals. In the last decade, the development of non-linear optical microscopy resulted in an enormous increase of in vivo studies, which have addressed key biological questions in fields such as neurobiology, immunology and tumor biology. Recently, few studies have shown that subcellular processes can be imaged dynamically in the live animal at a resolution comparable to that achieved in cell cultures, providing new opportunities to study cell biology under physiological conditions. The overall aim of this review is to give the reader a general idea of the potential applications of intravital microscopy with a particular emphasis on subcellular imaging. An overview of some of the most exciting studies in this field will be presented using resolution as a main organizing criterion. Indeed, first we will focus on those studies in which organs were imaged at the tissue level, then on those focusing on single cells imaging, and finally on those imaging subcellular organelles and structures.

Crucial role of the small GTPase Rac1 in insulin-stimulated translocation of glucose transporter 4 to the mouse skeletal muscle sarcolemma

The Rho family GTPase Rac1 has been implicated in the regulation of glucose uptake in myoblast cell lines. However, no evidence for the role of Rac1 has been provided by a mouse model. The purpose of this study is to test the involvement of Rac1 in insulin action in mouse skeletal muscle. Intravenous administration of insulin indeed elicited Rac1 activation in gastrocnemius muscle, suggesting the involvement of Rac1 in this signaling pathway. We then examined whether insulin-stimulated translocation of the facilitative glucose transporter GLUT4 from its storage sites to the skeletal muscle sarcolemma depends on Rac1. We show that ectopic expression of constitutively activated Rac1, as well as intravenous administration of insulin, caused translocation of GLUT4 to the gastrocnemius muscle sarcolemma, as revealed by immunofluorescent staining of a transiently expressed exofacial epitope-tagged GLUT4 reporter. Of particular note, insulin-dependent, but not constitutively activated Rac1-induced, GLUT4 translocation was markedly suppressed in skeletal muscle-specific rac1-knockout mice compared to control mice. Immunogold electron microscopic analysis of endogenous GLUT4 gave similar results. Collectively, we propose a critical role of Rac1 in insulin-dependent GLUT4 translocation to the skeletal muscle sarcolemma, which has heretofore been predicted solely by cell culture studies.

Membrane Fatty Acid Transporters as Regulators of Lipid Metabolism: Implications for Metabolic Disease

Long-chain fatty acids and lipids serve a wide variety of functions in mammalian homeostasis, particularly in the formation and dynamic properties of biological membranes and as fuels for energy production in tissues such as heart and skeletal muscle. On the other hand, long-chain fatty acid metabolites may exert toxic effects on cellular functions and cause cell injury. Therefore, fatty acid uptake into the cell and intracellular handling need to be carefully controlled. In the last few years, our knowledge of the regulation of cellular fatty acid uptake has dramatically increased. Notably, fatty acid uptake was found to occur by a mechanism that resembles that of cellular glucose uptake. Thus, following an acute stimulus, particularly insulin or muscle contraction, specific fatty acid transporters translocate from intracellular stores to the plasma membrane to facilitate fatty acid uptake, just as these same stimuli recruit glucose transporters to increase glucose uptake. This regulatory mechanism is important to clear lipids from the circulation postprandially and to rapidly facilitate substrate provision when the metabolic demands of heart and muscle are increased by contractile activity. Studies in both humans and animal models have implicated fatty acid transporters in the pathogenesis of diseases such as the progression of obesity to insulin resistance and type 2 diabetes. As a result, membrane fatty acid transporters are now being regarded as a promising therapeutic target to redirect lipid fluxes in the body in an organ-specific fashion.

Diverse roles of the actin cytoskeleton in striated muscle

In addition to the highly specialized contractile apparatus, it is becoming increasingly clear that there is an extensive actin cytoskeleton which underpins a wide range of functions in striated muscle. Isoforms of cytoskeletal actin and actin-associated proteins (non-muscle myosins, cytoskeletal tropomyosins, and cytoskeletal alpha-actinins) have been detected in a number of regions of striated muscle: the sub-sarcolemmal costamere, the Z-disc and the T-tubule/sarcoplasmic reticulum membranes. As the only known function of these proteins is through association with actin filaments, their presence in striated muscles indicates that there are spatially and functionally distinct cytoskeletal actin filament systems in these tissues. These filaments are likely to have important roles in mechanical support, ion channel function, myofibrillogenenous and vesicle trafficking.

Functional role of neuroendocrine-specific protein-like 1 in membrane translocation of GLUT4

OBJECTIVE

In skeletal muscles, dantrolene inhibits the exercise-induced membrane translocation of GLUT4. It has been postulated that the inhibitory action of dantrolene on Ca(2+) release from the sarcoplasmic reticulum (SR) causes inhibition of exercise-induced glucose uptake; however, the precise mechanism has not been adequately studied.

RESEARCH DESIGN AND METHODS

We discovered that dantrolene can bind to skeletal-type neuroendocrine-specific protein-like 1 (sk-NSPl1) with photoreactive dantrolene derivatives. In sk-NSPl1-deficient muscles, we examined the change in glucose uptake and the membrane translocation of GLUT4. In addition, we examined the change in blood glucose and also measured the glycogen level in both isolated and in situ skeletal muscles after electrical stimulation using our mutant mouse.

RESULTS

In sk-NSPl1-deficient muscles, exercise-induced glucose uptake was totally abolished with no change in insulin-induced glucose uptake. The Ca(2+) release mechanism and its inhibition by dantrolene were completely preserved in these muscles. The expression of GLUT4 in the mutant muscles also appeared unchanged. Confocal imaging and results using the membrane isolation method showed that exercise/contraction did not enhance GLUT4 translocation in these sk-NSPl1-deficient muscles under conditions of adequate muscle glycogen consumption. The blood glucose level in normal mice was reduced by electrical stimulation of the hind limbs, but that in mutant mice was unchanged.

CONCLUSIONS

sk-NSPl1 is a novel dantrolene receptor that plays an important role in membrane translocation of GLUT4 induced by contraction/exercise. The 23-kDa sk-NSPl1 may also be involved in the regulation of glucose levels in the whole body.

In vivo imaging of GLUT4 translocationThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process

In skeletal muscle, both insulin and muscle contractions mediate translocation of glucose transporter GLUT4 to the plasma membrane proper, the sarcolemma, and the specialized membrane channel network, the transverse (t)-tubules. Despite the fact that skeletal muscle glucose uptake plays a major role in normal conditions, in insulin resistance, and type II diabetes, the details of GLUT4 translocation and the intracellular signalling involved have not been fully described. A main reason is that the majority of experiments have been carried out in muscle cultures in vitro. In vitro cultured muscle is not fully differentiated and, therefore, diverges from real muscle, in that it has lower expression of GLUT4, an underdeveloped or nonexistent t-tubule network, and a reduced or nonexistent response to insulin. Thus, experiments carried out in cultured muscle cell systems might give misleading results on how GLUT4 translocation and the signalling involved takes place. To address this problem, a confocal imaging technique has been developed that allows delineation of the spartial and spatial distribution of GFP-tagged GLUT4 (GLUT4-GFP) translocation in living muscle fibers in situ in anesthetized mice. The effects of stimuli with insulin or in situ muscle contractions in fully differentiated muscle fibers can now be studied before, during, and after applying stimuli. Initial analysis of insulin-stimulated GLUT4-GFP translocation showed a delay in maximal translocation between the sarcolemma and t-tubules. Corresponding to the delay, we found that fluorescent tagged insulin reaches the sarcolemma first and then, with a delay, diffuses into the t-tubule system, enabling interaction with local insulin receptors and, in turn, triggering local insulin signalling and local GLUT4 translocation. In parallel, we showed that the majority of GLUT4 depot vesicles do not move long distances but are depleted locally in the sarcolemma or t-tubule regions. Analysis of GLUT4 translocation in insulin-resistant muscle showed that, primarily, GLUT4 recruitment in the t-tubule region is affected. We have now analysed the kinetics of contraction-mediated GLUT4 translocation and reinternalization, as well as dilineated some of the key signalling points involved in these processes.

Regulation of glucose transporter 4 traffic by energy deprivation from mitochondrial compromise

Skeletal muscle is the major store and consumer of fatty acids and glucose. Glucose enters muscle through glucose transporter 4 (GLUT4). Upon insufficient oxygen availability or energy compromise, aerobic metabolism of glucose and fatty aids cannot proceed, and muscle cells rely on anaerobic metabolism of glucose to restore cellular energy status. An increase in glucose uptake into muscle is a key response to stimuli requiring rapid energy supply. This chapter analyses the mechanisms of the adaptive regulation of glucose transport that rescue muscle cells from mitochondrial uncoupling. Under these conditions, the initial drop in ATP recovers rapidly, through a compensatory increase in glucose uptake. This adaptive response involves AMPK activation by the initial ATP drop, which elevates cell surface GLUT4 and glucose uptake. The gain in surface GLUT4 involves different signals and routes of intracellular traffic compared with those engaged by insulin. The hormone increases GLUT4 exocytosis through phosphatidylinositol 3-kinase and Akt, whereas energy stress retards GLUT4 endocytosis through AMPK and calcium inputs. Given that energy stress is a component of muscle contraction, and that contraction activates AMPK and raises cytosolic calcium, we hypothesize that the increase in glucose uptake during contraction may also involve a reduction in GLUT4 endocytosis.

A transgenic mouse model to study glucose transporter 4myc regulation in skeletal muscle

Skeletal muscle is the major site for dietary glucose disposal, taking up glucose via glucose transporter 4 (GLUT4). Although subcellular fractionation studies demonstrate that insulin increases GLUT4 density in sarcolemma and transverse tubules, fractionation cannot discern GLUT4 vesicle-membrane association from insertion and exofacial exposure. Clonal muscle cultures expressing exofacially tagged GLUT4 have allowed quantification of GLUT4 exposure at the cell surface, its exocytosis, endocytosis, and partner proteins. We hypothesized that transgenic expression of GLUT4myc in skeletal muscles would provide a useful model to investigate GLUT4 biology in vivo. A homozygous mouse colony was generated expressing GLUT4myc driven by the muscle creatine kinase (MCK) promoter. GLUT4 protein levels were about 3-fold higher in hindlimb muscles of MCK-GLUT4myc transgenic mice compared with littermates (P < 0.05). Insulin (12 nm, 30 min) induced a 2.1-fold increase in surface GLUT4myc detected by immunofluorescence of the exofacial myc epitope in nonpermeabilized muscle fiber bundles (P < 0.05). Glucose uptake and surface GLUT4myc levels were 3.5- and 3-fold higher, respectively, in giant membrane vesicles blebbed from hindlimb muscles of insulin-stimulated transgenic mice compared with unstimulated counterparts (P < 0.05). Muscle contraction also elevated both parameters, an effect partially additive to insulin's. GLUT4myc immunoprecipitation with anti-myc antibodies avoids interfering with associated intracellular binding proteins. Tether, containing a UBX domain, for GLUT4 coimmunoprecipitated with GLUT4myc and insulin stimulation significantly decreased such association (P < 0.05). MCK-GLUT4myc transgenic mice are thus useful to quantify exofacial GLUT4 exposure at the sarcolemma and GLUT4 binding partners in skeletal muscle, essential elements in the investigation of muscle GLUT4 regulation in physiological and pathological states in vivo.