Temporal separation of insulin-stimulated GLUT4/IRAP vesicle plasma membrane docking and fusion in 3T3L1 adipocytes

Discovery of New Nanomolar Selective IRAP Inhibitors

Among the M1 family of oxytocinase aminopeptidases, insulin-regulated aminopeptidase IRAP, is an emerging drug target implicated in various biological pathways and particularly in MHC-I antigen presentation through amino-terminal trimming of exogenous cross-presented peptides. A few series of inhibitors inspired either by angiotensin IV, one of IRAP substrates, or by bestatin a pan aminopeptidase inhibitor, have been disclosed. However, the variety and number of chemotypes remains relatively limited. Here we disclose the design and optimization of a series of hydroxamic acids IRAP inhibitors bearing a 5-substituted indole. Docking studies of the best compound 43 (BDM_92499), a single-digit nanomolar and selective inhibitor of IRAP, suggest an original binding mode and highlight the substituent on the indole and a primary amide as groups driving selectivity. Several inhibitors in the series displayed IRAP-dependent inhibition of antigen cross-presentation. These results pave the way to the development of novel therapeutic agents targeting IRAP.

TUSC5 regulates insulin-mediated adipose tissue glucose uptake by modulation of GLUT4 recycling

Objective

Failure to properly dispose of glucose in response to insulin is a serious health problem, occurring during obesity and is associated with type 2 diabetes development. Insulin-stimulated glucose uptake is facilitated by the translocation and plasma membrane fusion of vesicles containing glucose transporter 4 (GLUT4), the rate-limiting step of post-prandial glucose disposal.

Methods

We analyzed the role of Tusc5 in the regulation of insulin-stimulated Glut4-mediated glucose uptake in vitro and in vivo. Furthermore, we measured Tusc5 expression in two patient cohorts.

Results

Herein, we report that TUSC5 controls insulin-stimulated glucose uptake in adipocytes, in vitro and in vivo. TUSC5 facilitates the proper recycling of GLUT4 and other key trafficking proteins during prolonged insulin stimulation, thereby enabling proper protein localization and complete vesicle formation, processes that ultimately enable insulin-stimulated glucose uptake. Tusc5 knockout mice exhibit impaired glucose disposal and TUSC5 expression is predictive of glucose tolerance in obese individuals, independent of body weight. Furthermore, we show that TUSC5 is a PPARγ target and in its absence the anti-diabetic effects of TZDs are significantly blunted.

Conclusions

Collectively, these findings establish TUSC5 as an adipose tissue-specific protein that enables proper protein recycling, linking the ubiquitous vesicle traffic machinery with tissue-specific insulin-mediated glucose uptake into adipose tissue and the maintenance of a healthy metabolic phenotype in mice and humans.

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Tusc5 regulates glucose uptake in adipose tissue by modulating the GSV recycling machinery.

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Tusc5 knockout mice develop insulin resistance due to impaired adipose tissue glucose uptake.

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Rosiglitazone improves glucose homeostasis in part through the induction of Tusc5.

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Tusc5 is a novel adipose specific adaptor protein linking Glut4 trafficking to the ubiquitous machinery.

Cooling reduces cAMP-stimulated exocytosis and adiponectin secretion at a Ca2+-dependent step in 3T3-L1 adipocytes

We investigated the effects of temperature on white adipocyte exocytosis (measured as increase in membrane capacitance) and short-term adiponectin secretion with the aim to elucidate mechanisms important in regulation of white adipocyte stimulus-secretion coupling. Exocytosis stimulated by cAMP (included in the pipette solution together with 3 mM ATP) in the absence of Ca2+ (10 mM intracellular EGTA) was equal at all investigated temperatures (23°C, 27°C, 32°C and 37°C). However, the augmentation of exocytosis induced by an elevation of the free cytosolic [Ca2+] to ~1.5 μM (9 mM Ca2+ + 10 mM EGTA) was potent at 32°C or 37°C but less distinct at 27°C and abolished at 23°C. Adiponectin secretion stimulated by 30 min incubations with the membrane permeable cAMP analogue 8-Br-cAMP (1 mM) or a combination of 10 μM forskolin and 200 μM IBMX was unaffected by a reduction of temperature from 32°C to 23°C. At 32°C, cAMP-stimulated secretion was 2-fold amplified by inclusion of the Ca2+ ionophore ionomycin (1μM), an effect that was not observed at 23°C. We suggest that cooling affects adipocyte exocytosis/adiponectin secretion at a Ca2+-dependent step, likely involving ATP-dependent processes, important for augmentation of cAMP-stimulated adiponectin release.

MDM2-related responses in 3T3-L1 adipocytes exposed to cooling and subsequent rewarming

Insulin-like growth factor-I and insulin induce the production of phospho-Ser-166 MDM2, a target of Akt, and influence the formation of the MDM2 complex. The glycolipid hormone insulin differentially activates phosphatidylinositol 3-kinase (PI3K)/Akt pathways in 3T3-L1 (L1) adipocytes incubated at 19 °C. Responses of L1 adipocytes to different temperature changes and their regulatory mechanisms are poorly understood. We exposed L1 adipocytes to cooling and subsequent rewarming in the presence or absence of wortmannin, a PI3K inhibitor, or mithramycin A, a transcription inhibitor, and examined the induction of phospho-Ser-166 MDM2 and MDM2 and the subcellular formation of the MDM2 complex using western blot analysis. Exposure to 28 and 18 °C induced phospho-MDM2 in cells and increased the level of MDM2 in the plasma membrane of cells. These temperatures did not affect the total MDM2 level. Similar results were obtained when the cells were treated with insulin. Exposure to 4 °C increased the total MDM2 level and did not induce phospho-MDM2, which was induced by rewarming at 37 °C after cooling at 4°C without any alteration in the protein level. Mithramycin A (10 μM) did not alter the increase in protein level induced at 4 °C. The induction of phospho-molecules at 28 and 18 °C was impaired slightly by 1 μM of wortmannin but not by 0.1 μM of wortmannin. This low concentration of wortmannin completely blocked the induction of phospho-MDM2 by rewarming. Our results indicate that temperature changes induce MDM2-related responses, including those that are stimulated by receptor responses and dependent on a kinase inhibitor, in L1 adipocytes.

Sub-cellular localization of insulin-regulated membrane aminopeptidase, IRAP to vesicles in neurons

Angiotensin IV and LVV-hemorphin 7 promote robust enhancing effects on learning and memory. These peptides are also competitive inhibitors of the insulin-regulated membrane aminopeptidase, suggesting that the biological actions of these peptides may result from inhibition of IRAP activity. However, the normal function of IRAP in the brain is yet to be determined. The present study investigated the sub-cellular distribution of IRAP in four neuronal cell lines and in the mouse brain. Using sub-cellular fractionation, IRAP was found to be enriched in low density microsomes, while lower levels of IRAP were also present in high density microsomes, plasma membrane and mitochondrial fractions. Dual-label immunohistochemistry confirmed the presence of IRAP in vesicles co-localized with the vesicular maker VAMP2, in the trans Golgi network co-localized with TGN 38 and in endosomes co-localized with EEA1. Finally using electron microscopy, IRAP specific immunoreactivity was predominantly associated with large 100-200 nm vesicles in hippocampal neurons. The location, appearance and size of these vesicles are consistent with neurosecretory vesicles. IRAP precipitate was also detected in intracellular structures including the rough endoplasmic reticulum, Golgi stack and mitochondrial membranes. The sub-cellular localization of IRAP in neurons demonstrated in the present study bears striking parallels with distribution of IRAP in insulin responsive cells, where the enzyme plays a role in insulin-regulated glucose uptake. Therefore, we propose that the function of IRAP in neurons may be similar to that in insulin responsive cells.

Insulin stimulates membrane fusion and GLUT4 accumulation in clathrin coats on adipocyte plasma membranes

Total internal reflection fluorescence (TIRF) microscopy reveals highly mobile structures containing enhanced green fluorescent protein-tagged glucose transporter 4 (GLUT4) within a zone about 100 nm beneath the plasma membrane of 3T3-L1 adipocytes. We developed a computer program (Fusion Assistant) that enables direct analysis of the docking/fusion kinetics of hundreds of exocytic fusion events. Insulin stimulation increases the fusion frequency of exocytic GLUT4 vesicles by approximately 4-fold, increasing GLUT4 content in the plasma membrane. Remarkably, insulin signaling modulates the kinetics of the fusion process, decreasing the vesicle tethering/docking duration prior to membrane fusion. In contrast, the kinetics of GLUT4 molecules spreading out in the plasma membrane from exocytic fusion sites is unchanged by insulin. As GLUT4 accumulates in the plasma membrane, it is also immobilized in punctate structures on the cell surface. A previous report suggested these structures are exocytic fusion sites (Lizunov et al., J. Cell Biol. 169:481-489, 2005). However, two-color TIRF microscopy using fluorescent proteins fused to clathrin light chain or GLUT4 reveals these structures are clathrin-coated patches. Taken together, these data show that insulin signaling accelerates the transition from docking of GLUT4-containing vesicles to their fusion with the plasma membrane and promotes GLUT4 accumulation in clathrin-based endocytic structures on the plasma membrane.

A Specific Dileucine Motif Is Required for the GGA-dependent Entry of Newly Synthesized Insulin-responsive Aminopeptidase into the Insulin-responsive Compartment

In muscle and adipose cells, the insulin-responsive aminopeptidase (IRAP) is localized to intracellular storage sites and undergoes insulin-dependent redistribution to the cell surface. Following expression, the newly synthesized IRAP protein traffics to the perinuclear insulin-sensitive compartment and acquires insulin sensitivity 6-9 h following biosynthesis. Knockdown of GGA1 by RNA interference prevented IRAP from entering, but not exiting, the insulin-responsive compartment. Mutation of the dileucine motif at positions 76 and 77 (EGFP-IRAP/AA(76,77)), but not the dileucine motif at positions 53 and 54, resulted in the rapid default of the reporter to the cell surface beginning at 3 h following biosynthesis. Alanine substitution of 9 residues amino- or carboxyl-terminal to LL(76,77) did not perturb basal intracellular sequestration or abrogate insulin-stimulated IRAP translocation. Moreover, a dominant interfering GGA mutant (VHS-GAT) potently inhibited insulin-stimulated translocation of EGFP-IRAP/WT but did not block the constitutive exocytotic trafficking of EGFP-IRAP/AA(76,77). In addition, the EGFP-IRAP/WT and EGFP-IRAP/AA(76,77) constructs occupied morphologically distinct tubulovesicular compartments in the perinuclear region. Taken together, these data indicate that LL(76,77) functions during the GGA-dependent sorting of newly made IRAP into the insulin-responsive storage compartment.

Initial entry of IRAP into the insulin-responsive storage compartment occurs prior to basal or insulin-stimulated plasma membrane recycling

To examine the acquisition of insulin sensitivity after the initial biosynthesis of the insulin-responsive aminopeptidase (IRAP), 3T3-L1 adipocytes were transfected with an enhanced green fluorescent protein-IRAP (EGFP-IRAP) fusion protein. In the absence of insulin, IRAP was rapidly localized (1-3 h) to secretory membranes and retained in these intracellular membrane compartments with little accumulation at the plasma membrane. However, insulin was unable to induce translocation to the plasma membrane until 6-9 h after biosynthesis. This was in marked contrast to another type II membrane protein (syntaxin 3) that rapidly defaulted to the plasma membrane 3 h after expression. In parallel with the time-dependent acquisition of insulin responsiveness, the newly synthesized IRAP protein converted from a brefeldin A-sensitive to a brefeldin A-insensitive state. The initial trafficking of IRAP to the insulin-responsive compartment was independent of plasma membrane endocytosis, as expression of a dominant-interfering dynamin mutant (Dyn/K44A) inhibited transferrin receptor endocytosis but had no effect on the insulin-stimulated translocation of the newly synthesized IRAP protein.

Akt Activation Is Required at a Late Stage of Insulin-Induced GLUT4 Translocation to the Plasma Membrane

Insulin stimulates the translocation of glucose transporter GLUT4 from intracellular vesicles to the plasma membrane (PM). This involves multiple steps as well as multiple intracellular compartments. The Ser/Thr kinase Akt has been implicated in this process, but its precise role is ill defined. To begin to dissect the role of Akt in these different steps, we employed a low-temperature block. Upon incubation of 3T3-L1 adipocytes at 19 C, GLUT4 accumulated in small peripheral vesicles with a slight increase in PM labeling concomitant with reduced trans-Golgi network labeling. Although insulin-dependent translocation of GLUT4 to the PM was impaired at 19 C, we still observed movement of vesicles toward the surface. Strikingly, insulin-stimulated Akt activity, but not phosphatidylinositol 3 kinase activity, was blocked at 19 C. Consistent with a multistep process in GLUT4 trafficking, insulin-stimulated GLUT4 translocation could be primed by treating cells with insulin at 19 C, whereas this was not the case for Akt activation. These data implicate two insulin-regulated steps in GLUT4 translocation: 1) redistribution of GLUT4 vesicles toward the cell cortex—this process is Akt-independent and is not blocked at 19 C; and 2) docking and/or fusion of GLUT4 vesicles with the PM—this process may be the major Akt-dependent step in the insulin regulation of glucose transport.

Separation of insulin signaling into distinct GLUT4 translocation and activation steps

GLUT4 (glucose transporter 4) plays a pivotal role in insulin-induced glucose uptake to maintain normal blood glucose levels. Here, we report that a cell-permeable phosphoinositide-binding peptide induced GLUT4 translocation to the plasma membrane without inhibiting IRAP (insulin-responsive aminopeptidase) endocytosis. However, unlike insulin treatment, the peptide treatment did not increase glucose uptake in 3T3-L1 adipocytes, indicating that GLUT4 translocation and activation are separate events. GLUT4 activation can occur at the plasma membrane, since insulin was able to increase glucose uptake with a shorter time lag when inactive GLUT4 was first translocated to the plasma membrane by pretreating the cells with this peptide. Inhibition of phosphatidylinositol (PI) 3-kinase activity failed to inhibit GLUT4 translocation by the peptide but did inhibit glucose uptake when insulin was added following peptide treatment. Insulin, but not the peptide, stimulated GLUT1 translocation. Surprisingly, the peptide pretreatment inhibited insulin-induced GLUT1 translocation, suggesting that the peptide treatment has both a stimulatory effect on GLUT4 translocation and an inhibitory effect on insulin-induced GLUT1 translocation. These results suggest that GLUT4 requires translocation to the plasma membrane, as well as activation at the plasma membrane, to initiate glucose uptake, and both of these steps normally require PI 3-kinase activation.

Entry of newly synthesized GLUT4 into the insulin-responsive storage compartment is GGA dependent

Following biosynthesis, both GLUT1 and VSV-G proteins appear rapidly (2-3 h) at the plasma membrane, whereas GLUT4 is retained in intracellular membrane compartments and does not display any significant insulin responsiveness until 6-9 h. Surprisingly, the acquisition of insulin responsiveness did not require plasma membrane endocytosis, as expression of a dominant-interfering dynamin mutant (Dyn/K44A) had no effect on the insulin-stimulated GLUT4 translocation. Furthermore, expression of endocytosis-defective GLUT4 mutants or continuous surface labeling with an exofacial specific antibody demonstrated that GLUT4 did not transit the cell surface prior to the acquisition of insulin responsiveness. The expression of a dominant-interfering GGA mutant (VHS-GAT) had no effect on the trafficking of newly synthesized GLUT1 or VSV-G protein to the plasma membrane, but completely blocked the insulin-stimulated translocation of newly synthesized GLUT4. Furthermore, in vitro budding of GLUT4 vesicles but not GLUT1 or the transferrin receptor was inhibited by VHS-GAT. Together, these data demonstrate that following biosynthesis, GLUT4 directly sorts and traffics to the insulin-responsive storage compartment through a specific GGA-sensitive process.

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

The intracellularly stored GLUT4 glucose transporter is rapidly translocated to the cell surface upon insulin stimulation. Regulation of GLUT4 distribution is key for the maintenance of whole body glucose homeostasis. We find that GLUT4 is excluded from the plasma membrane of adipocytes by a dynamic retention/retrieval mechanism. Our kinetic studies indicate that GLUT4-containing vesicles continually bud and fuse with endosomes in the absence of insulin and that these GLUT4 vesicles are 5 times as likely to fuse with an endosome as with the plasma membrane. We hypothesize that this intracellular cycle of vesicle budding and fusion is an element of the active mechanism by which GLUT4 is retained. The GLUT4 trafficking pathway does not extensively overlap with that of furin, indicating that the trans-Golgi network, a compartment in which furin accumulates, is not a significant storage reservoir of GLUT4. An intact microtubule cytoskeleton is required for insulin-stimulated recruitment to the cell surface, although it is not required for the basal budding/fusion cycle. Nocodazole disruption of the microtubule cytoskeleton reduces the insulin-stimulated exocytosis of GLUT4, accounting for the reduced insulin-stimulated translocation of GLUT4 to the cell surface.

Sphingomyelinase activates GLUT4 translocation via a cholesterol-dependent mechanism

A basis for the insulin mimetic effect of sphingomyelinase on glucose transporter isoform GLUT4 translocation remains unclear. Because sphingomyelin serves as a major determinant of plasma membrane cholesterol and a relationship between plasma membrane cholesterol and GLUT4 levels has recently become apparent, we assessed whether GLUT4 translocation induced by sphingomyelinase resulted from changes in membrane cholesterol content. Exposure of 3T3-L1 adipocytes to sphingomyelinase resulted in a time-dependent loss of sphingomyelin from the plasma membrane and a concomitant time-dependent accumulation of plasma membrane GLUT4. Degradation products of sphingomyelin did not mimic this stimulatory action. Plasma membrane cholesterol amount was diminished in cells exposed to sphingomyelinase. Restoration of membrane cholesterol blocked the stimulatory effect of sphingomyelinase. Increasing concentrations of methyl-beta-cyclodextrin, which resulted in a dose-dependent reversible decrease in membrane cholesterol, led to a dose-dependent reversible increase in GLUT4 incorporation into the plasma membrane. Although increased plasma membrane GLUT4 content by cholesterol extraction with concentrations of methyl-beta-cyclodextrin above 5 mM most likely reflected decreased GLUT4 endocytosis, translocation stimulated by sphingomyelinase or concentrations of methyl-beta-cyclodextrin below 2.5 mM occurred without any visible changes in the endocytic retrieval of GLUT4. Furthermore, moderate loss of cholesterol induced by sphingomyelinase or low concentrations of methyl-beta-cyclodextrin did not alter membrane integrity or increase the abundance of other plasma membrane proteins such as the GLUT1 glucose transporter or the transferrin receptor. Regulation of GLUT4 translocation by moderate cholesterol loss did not involve known insulin-signaling proteins. These data reveal that sphingomyelinase enhances GLUT4 exocytosis via a novel cholesterol-dependent mechanism.

The exocytotic trafficking of TC10 occurs through both classical and nonclassical secretory transport pathways in 3T3L1 adipocytes

To examine the structural determinants necessary for TC10 trafficking, localization, and function in adipocytes, we generated a series of point mutations in the carboxyl-terminal targeting domain of TC10. Wild-type TC10 (TC10/WT) localized to secretory membrane compartments and caveolin-positive lipid raft microdomains at the plasma membrane. Expression of a TC10/C206S point mutant resulted in a trafficking and localization pattern that was indistinguishable from that of TC10/WT. In contrast, although TC10/C209S or the double TC10/C206,209S mutant was plasma membrane localized, it was excluded from both the secretory membrane system and the lipid raft compartments. Surprisingly, inhibition of Golgi membrane transport with brefeldin A did not prevent plasma membrane localization of TC10 or H-Ras. Moreover, inhibition of trans-Golgi network exit with a 19 degrees C temperature block did not prevent the trafficking of TC10 or H-Ras to the plasma membrane. These data demonstrate that TC10 and H-Ras can both traffic to the plasma membrane by at least two distinct transport mechanisms in adipocytes, one dependent upon intracellular membrane transport and another independent of the classical secretory membrane system. Moreover, the transport through the secretory pathway is necessary for the localization of TC10 to lipid raft microdomains at the plasma membrane.

Functional consequence of targeting protein kinase B/Akt to GLUT4 vesicles

We have investigated the role of protein kinase B (Akt) in the insulin-stimulated translocation of vesicles containing the insulin-responsive isoform of glucose transporter (GLUT4) to the plasma membrane of adipocytes. Previous reports have suggested that protein kinase B can bind to intracellular GLUT4 vesicles in an insulin-dependent manner, but the functional consequence of this translocation is not known. In this study we have artificially targeted constitutively active and kinase-inactive mutants of protein kinase B to intracellular GLUT4 vesicles by fusing them with the N-terminus of GLUT4 itself. We examined the effect of these mutants on the insulin-dependent translocation of the insulin-responsive amino peptidase IRAP(a bona fide GLUT4-vesicle-resident protein). A kinase-inactive protein kinase B targeted to GLUT4 vesicles was an extremely effective dominant-negative inhibitor of insulin-stimulated IRAP translocation to the plasma membrane. By contrast, a kinase-inactive protein kinase B expressed in the cytoplasm did not have an effect. The results suggest that protein kinase B has an important functional role at, or in the vicinity of, GLUT4 vesicles in the insulin-dependent translocation of those vesicles to the plasma membrane of adipocytes.

Insulin and isoproterenol have opposing roles in the maintenance of cytosol pH and optimal fusion of GLUT4 vesicles with the plasma membrane

Insulin treatment of rat adipocytes increases both cytoplasmic alkalinity and glucose transport activity. Both processes are blocked by the phosphatidylinositol 3-kinase inhibitor wortmannin. Isoproterenol pre-treatment reverses the alkalinizing effects of insulin and leads to attenuation of insulin-stimulated glucose transport activity and exposure of GLUT4 to photolabeling reagents at the cell surface. These effects of isoproterenol are mimicked by acid loading and are reversed by cell-alkalinizing conditions. However, neither isoproterenol nor acid loading alters the total level of GLUT4 at the plasma membrane as revealed by Western blotting of plasma membrane fractions or immunodetection of GLUT4 in plasma membrane lawns. GLUT4 is therefore occluded from participation in glucose transport catalysis by a pH-sensitive process. To examine the kinetics of trafficking that lead to these changes in cell surface GLUT4 occlusion, we have utilized a new biotinylated photolabel, GP15. This reagent has a 70-atom spacer between the biotin and the photolabeling diazirine group, and this allows quenching of the surface signal of biotinylated GLUT4 by extracellular avidin. The rates of GLUT4 internalization are only slightly altered by isoproterenol or acidification, mainly due to reduced recycling over long internalization times. By contrast, insulin stimulation of GLUT4 exocytosis is slowed by isoproterenol or acidification pre-treatments. Biphasic time courses are evident, with an initial burst of exposure at the cell surface followed by a slow phase. It is hypothesized that the burst kinetics are a consequence of a two-phase fusion reaction that is rapid in the presence of insulin but slowed by cytosol acidification.

Insulin-regulated trafficking of dual-labeled glucose transporter 4 in primary rat adipose cells

In isolated rat adipose cells, physiologically relevant insulin target cells, glucose transporter 4 (GLUT4) subcellular trafficking can be assessed by transfection of exofacially HA-tagged GLUT4. To simultaneously visualize the transfected GLUT4, we fused GFP with HA-GLUT4. With the resulting chimeras, GFP-HA-GLUT4 and HA-GLUT4-GFP, we were able to visualize for the first time the cell-surface localization, total expression, and intracellular distribution of GLUT4 in a single cell. Confocal microscopy reveals that the intracellular proportions of both GFP-HA-GLUT4 and HA-GLUT4-GFP are properly targeted to the insulin-responsive aminopeptidase-positive vesicles. Dynamic studies demonstrate close similarities in the trafficking kinetics between the two constructs and with native GLUT4. However, while the basal subcellular distribution of HA-GLUT4-GFP and the response to insulin are indistinguishable from those of HA-GLUT4 and endogenous GLUT4, most of the GFP-HA-GLUT4 is targeted to the plasma membrane with little further insulin response. Thus, HA-GLUT4-GFP will be useful to study GLUT4 trafficking in vivo while GFP on the N-terminus interferes with intracellular retention.

A Glut4-vesicle marker protein, insulin-responsive aminopeptidase, is localized in a novel vesicular compartment in PC12 cells

Glut4-containing vesicles represent a regulated recycling compartment in insulin-sensitive fat and skeletal muscle cells, the nature and origin of which are not fully understood. In addition to Glut4 itself, these vesicles compartmentalize a number of proteins, at least one of which, insulin-responsive aminopeptidase, or IRAP, is completely colocalized with Glut4 in insulin-sensitive tissues. However, unlike Glut4, IRAP is expressed in a variety of other tissues and cell lines. Here, we explored the intracellular localization of IRAP in the rat pheochromocytoma cell line PC12. We found that this protein is present in a distinct population of slowly recycling light vesicles. By gradient centrifugations, immunoadsorption and double immunofluorescent staining, these vesicles are different from transferrin-containing endosomes, small synaptic vesicles and secretory granules and may thus represent a novel compartment in PC12 cells. Glut4-GFP chimera transiently expressed in PC12 cells is targeted to IRAP-containing vesicles indicating that cotargeting of Glut4 and IRAP is not specific for adipocytes and myocytes, but is faithful in a foreign cell type. We suggest that PC12 cells may possess a novel type of a vesicular carrier that may represent the homolog of Glut4-vesicles.

Differentiated 3T3L1 adipocytes are composed of heterogenous cell populations with distinct receptor tyrosine kinase signaling properties

Various studies have demonstrated that the platelet-derived growth factor (PDGF) receptor in adipocytes can activate PI 3-kinase activity without affecting insulin-responsive glucose transporter (GLUT4) translocation. To investigate this phenomenon of receptor signaling specificity, we utilized single cell analysis to determine the cellular distribution and signaling properties of PDGF and insulin in differentiated 3T3L1 adipocytes. The insulin receptor was highly expressed in a large percentage of the cell population (>95%) that also expressed caveolin 2 and GLUT4 with very low levels of the PDGF receptor. In contrast, the PDGF receptor was only expressed in approximately 10% of the differentiated 3T3L1 cell population with relatively low levels of the insulin receptor, caveolin 2, and GLUT4. Consistent with this observation, insulin stimulated the phosphorylation of Akt in the caveolin 2- and GLUT4-positive cells, whereas PDGF primarily stimulated Akt phosphorylation in the caveolin 2- and GLUT4-negative cell population. Furthermore, transfection of the PDGF receptor in the insulin receptor-, GLUT4-, and caveolin 2-positive cells resulted in the ability of PDGF to stimulate GLUT4 translocation. These data demonstrate that differentiated 3T3L1 adipocytes are not a homogeneous population of cells, and the lack of PDGF receptor expression in the GLUT4-positive cell population accounts for the inability of the endogenous PDGF receptor to activate GLUT4 translocation.

Munc18c regulates insulin-stimulated glut4 translocation to the transverse tubules in skeletal muscle

To examine the intracellular trafficking and translocation of GLUT4 in skeletal muscle, we have generated transgenic mouse lines that specifically express a GLUT4-EGFP (enhanced green fluorescent protein) fusion protein under the control of the human skeletal muscle actin promoter. These transgenic mice displayed EGFP fluorescence restricted to skeletal muscle and increased glucose tolerance characteristic of enhanced insulin sensitivity. The GLUT4-EGFP protein localized to the same intracellular compartment as the endogenous GLUT4 protein and underwent insulin- and exercise-stimulated translocation to both the sarcolemma and transverse-tubule membranes. Consistent with previous studies in adipocytes, overexpression of the syntaxin 4-binding Munc18c isoform, but not the related Munc18b isoform, in vivo specifically inhibited insulin-stimulated GLUT4-EGFP translocation. Surprisingly, however, Munc18c inhibited GLUT4 translocation to the transverse-tubule membrane without affecting translocation to the sarcolemma membrane. The ability of Munc18c to block GLUT4-EGFP translocation to the transverse-tubule membrane but not the sarcolemma membrane was consistent with substantially reduced levels of syntaxin 4 in the transverse-tubule membrane. Together, these data demonstrate that Munc18c specifically functions in the compartmentalized translocation of GLUT4 to the transverse-tubules in skeletal muscle. In addition, these results underscore the utility of this transgenic model to directly visualize GLUT4 translocation in skeletal muscle.

Insulin action on GLUT4 traffic visualized in single 3T3-l1 adipocytes by using ultra-fast microscopy

A novel imaging technology, high-speed microscopy, has been used to visualize the process of GLUT4 translocation in response to insulin in single 3T3-L1 adipocytes. A key advantage of this technology is that it requires extremely low light exposure times, allowing the quasi-continuous capture of information over 20-30 min without photobleaching or photodamage. The half-time for the accumulation of GLUT4-eGFP (enhanced green fluorescent protein) at the plasma membrane in a single cell was found to be of 5-7 min at 37 degrees C. This half-time is substantially longer than that of exocytic vesicle fusion in neuroendocrine cells, suggesting that additional regulatory mechanisms are involved in the stimulation of GLUT4 translocation by insulin. Analysis of four-dimensional images (3-D over time) revealed that, in response to insulin, GLUT4-eGFP-enriched vesicles rapidly travel from the juxtanuclear region to the plasma membrane. In nontransfected adipocytes, impairment of microtubule and actin filament function inhibited insulin-stimulated glucose transport by 70 and 50%, respectively. When both filament systems were impaired insulin-stimulated glucose transport was completely inhibited. Taken together, the data suggest that the regulation of long-range motility of GLUT4-containing vesicles through the interaction with microtubule- and actin-based cytoskeletal networks plays an important role in the overall effect of insulin on GLUT4 translocation.

Insulin recruits GLUT4 from specialized VAMP2-carrying vesicles as well as from the dynamic endosomal/trans-Golgi network in rat adipocytes

Insulin treatment of fat cells results in the translocation of the insulin-responsive glucose transporter type 4, GLUT4, from intracellular compartments to the plasma membrane. However, the precise nature of these intracellular GLUT4-carrying compartments is debated. To resolve the nature of these compartments, we have performed an extensive morphological analysis of GLUT4-containing compartments, using a novel immunocytochemical technique enabling high labeling efficiency and 3-D resolution of cytoplasmic rims isolated from rat epididymal adipocytes. In basal cells, GLUT4 was localized to three morphologically distinct intracellular structures: small vesicles, tubules, and vacuoles. In response to insulin the increase of GLUT4 at the cell surface was compensated by a decrease in small vesicles, whereas the amount in tubules and vacuoles was unchanged. Under basal conditions, many small GLUT4 positive vesicles also contained IRAP (88%) and the v-SNARE, VAMP2 (57%) but not markers of sorting endosomes (EEA1), late endosomes, or lysosomes (lgp120). A largely distinct population of GLUT4 vesicles (56%) contained the cation-dependent mannose 6-phosphate receptor (CD-MPR), a marker protein that shuttles between endosomes and the trans-Golgi network (TGN). In response to insulin, GLUT4 was recruited both from VAMP2 and CD-MPR positive vesicles. However, while the concentration of GLUT4 in the remaining VAMP2-positive vesicles was unchanged, the concentration of GLUT4 in CD-MPR-positive vesicles decreased. Taken together, we provide morphological evidence indicating that, in response to insulin, GLUT4 is recruited to the plasma membrane by fusion of preexisting VAMP2-carrying vesicles as well as by sorting from the dynamic endosomal-TGN system.

Discrimination of GLUT4 vesicle trafficking from fusion using a temperature-sensitive Munc18c mutant

To examine the temporal relationship between pre- and post-docking events, we generated a Munc18c temperature-sensitive mutant (Munc18c/TS) by substitution of arginine 240 with a lysine residue. At the permissive temperature (23 degrees C), overexpression of both the wild type (Munc18c/WT) and the R240K mutant inhibited insulin-stimulated GLUT4/IRAP vesicle translocation. However, at the non-permissive temperature (37 degrees C) only Munc18c/WT inhibited GLUT4/IRAP translocation whereas Munc18c/TS was without effect. Moreover, Munc18c/WT bound to syntaxin 4 at both 23 and 37 degrees C whereas Munc18c/TS bound syntaxin 4 only at 23 degrees C. This was due to a temperature-dependent conformational change in Munc18c/TS, as its ability to bind syntaxin 4 and effects on GLUT4 translocation were rapidly reversible while protein expression levels remained unchanged. Furthermore, insulin stimulation of Munc18c/TS-expressing cells at 23 degrees C followed by temperature shift to 37 degrees C resulted in an increased rate of GLUT4 translocation compared with cells stimulated at 37 degrees C. To date, this is the first demonstration that the rate-limiting step for insulin-stimulated GLUT4 translocation is the trafficking of GLUT4 vesicles and not their fusion with the plasma membrane.