Direct interaction of Rab4 with syntaxin 4

Drosophila transmembrane protein 214 (dTMEM214) regulates midgut glucose uptake and systemic glucose homeostasis

The availability of glucose transporter in the small intestine critically determines the capacity for glucose uptake and consequently systemic glucose homeostasis. Hence a better understanding of the physiological regulation of intestinal glucose transporter is pertinent. However, the molecular mechanisms that regulate sodium-glucose linked transporter 1 (SGLT1), the primary glucose transporter in the small intestine, remain incompletely understood. Recently, the Drosophila SLC5A5 (dSLC5A5) has been found to exhibit properties consistent with a dietary glucose transporter in the Drosophila midgut, the equivalence of the mammalian small intestine. Hence, the fly midgut could serve as a suitable model system for the study of the in vivo molecular underpinnings of SGLT1 function. Here, we report the identification, through a genetic screen, of Drosophila transmembrane protein 214 (dTMEM214) that acts in the midgut enterocytes to regulate systemic glucose homeostasis and glucose uptake. We show that dTMEM214 resides in the apical membrane and cytoplasm of the midgut enterocytes, and that the proper subcellular distribution of dTMEM214 in the enterocytes is regulated by the Rab4 GTPase. As a corollary, Rab4 loss-of-function phenocopies dTMEM214 loss-of-function in the midgut as shown by a decrease in enterocyte glucose uptake and an alteration in systemic glucose homeostasis. We further show that dTMEM214 regulates the apical membrane localization of dSLC5A5 in the enterocytes, thereby revealing dTMEM214 as a molecular regulator of glucose transporter in the midgut.

Targeting Small GTPases and Their Prenylation in Diabetes Mellitus

A fundamental role of pancreatic β-cells to maintain proper blood glucose level is controlled by the Ras superfamily of small GTPases that undergo post-translational modifications, including prenylation. This covalent attachment with either a farnesyl or a geranylgeranyl group controls their localization, activity, and protein–protein interactions. Small GTPases are critical in maintaining glucose homeostasis acting in the pancreas and metabolically active tissues such as skeletal muscles, liver, or adipocytes. Hyperglycemia-induced upregulation of small GTPases suggests that inhibition of these pathways deserves to be considered as a potential therapeutic approach in treating T2D. This Perspective presents how inhibition of various points in the mevalonate pathway might affect protein prenylation and functioning of diabetes-affected tissues and contribute to chronic inflammation involved in diabetes mellitus (T2D) development. We also demonstrate the currently available molecular tools to decipher the mechanisms linking the mevalonate pathway’s enzymes and GTPases with diabetes.

Excess Rab4 rescues synaptic and behavioral dysfunction caused by defective HTT-Rab4 axonal transport in Huntington's disease

Huntington's disease (HD) is characterized by protein inclusions and loss of striatal neurons which result from expanded CAG repeats in the poly-glutamine (polyQ) region of the huntingtin (HTT) gene. Both polyQ expansion and loss of HTT have been shown to cause axonal transport defects. While studies show that HTT is important for vesicular transport within axons, the cargo that HTT transports to/from synapses remain elusive. Here, we show that HTT is present with a class of Rab4-containing vesicles within axons in vivo. Reduction of HTT perturbs the bi-directional motility of Rab4, causing axonal and synaptic accumulations. In-vivo dual-color imaging reveal that HTT and Rab4 move together on a unique putative vesicle that may also contain synaptotagmin, synaptobrevin, and Rab11. The moving HTT-Rab4 vesicle uses kinesin-1 and dynein motors for its bi-directional movement within axons, as well as the accessory protein HIP1 (HTT-interacting protein 1). Pathogenic HTT disrupts the motility of HTT-Rab4 and results in larval locomotion defects, aberrant synaptic morphology, and decreased lifespan, which are rescued by excess Rab4. Consistent with these observations, Rab4 motility is perturbed in iNeurons derived from human Huntington's Disease (HD) patients, likely due to disrupted associations between the polyQ-HTT-Rab4 vesicle complex, accessory proteins, and molecular motors. Together, our observations suggest the existence of a putative moving HTT-Rab4 vesicle, and that the axonal motility of this vesicle is disrupted in HD causing synaptic and behavioral dysfunction. These data highlight Rab4 as a potential novel therapeutic target that could be explored for early intervention prior to neuronal loss and behavioral defects observed in HD.

Trafficking and other regulatory mechanisms for organic anion transporting polypeptides and organic anion transporters that modulate cellular drug and xenobiotic influx and that are dysregulated in disease

Organic anion transporters (OATs) and organic anion-transporting polypeptides (OATPs), encoded by a number of solute carrier (SLC)22A and SLC organic anion (SLCO) genes, mediate the absorption and distribution of drugs and other xenobiotics. The regulation of OATs and OATPs is complex, comprising both transcriptional and post-translational mechanisms. Plasma membrane expression is required for cellular substrate influx by OATs/OATPs. Thus, interest in post-translational regulatory processes, including membrane targeting, endocytosis, recycling and degradation of transporter proteins, is increasing because these are critical for plasma membrane expression. After being synthesized, transporters undergo N-glycosylation in the endoplasmic reticulum and Golgi apparatus and are delivered to the plasma membrane by vesicular transport. Their expression at the cell surface is maintained by de novo synthesis and recycling, which occurs after clathrin- and/or caveolin-dependent endocytosis of existing protein. Several studies have shown that phosphorylation by signalling kinases is important for the internalization and recycling processes, although the transporter protein does not appear to be directly phosphorylated. After internalization, transporters that are targeted for degradation undergo ubiquitination, most likely on intracellular loop residues. Epigenetic mechanisms, including methylation of gene regulatory regions and transcription from alternate promoters, are also significant in the regulation of certain SLC22A/SLCO genes. The membrane expression of OATs/OATPs is dysregulated in disease, which affects drug efficacy and detoxification. Several transporters are expressed in the cytoplasmic subcompartment in disease states, which suggests that membrane targeting/internalization/recycling may be impaired. This article focuses on recent developments in OAT and OATP regulation, their dysregulation in disease and the significance for drug therapy.

Role of Rab GTPases and their interacting proteins in mediating metabolic signalling and regulation

The vesicular transport pathways, which shuttle materials to and from the cell surface and within the cell, and the metabolic (growth factor and nutrient) signalling pathways, which integrate a variety of extracellular and intracellular signals to mediate growth, proliferation or survival, are both important for cellular physiology. There is evidence to suggest that the transport and metabolic signalling pathways intersect-vesicular transport can affect the regulation of metabolic signals and vice versa. The Rab family GTPases regulate the specificity of vesicular transport steps in the cell. Together with their interacting proteins, Rabs would likely constitute the points of intersection between vesicular transport and metabolic signalling pathways. Examples of these points would include growth factor signalling, glucose and lipid metabolism, as well as autophagy. Many of these processes involve mechanistic/mammalian target of rapamycin (mTOR) complex 1 (mTORC1) in downstream cascades, or are regulated by TORC signalling. A general functionality of the vesicular transport processes controlled by the Rabs is also important for spatial and temporal regulation of the transmission of metabolic signals between the cell surface and the nucleus. In other cases, specific Rabs and their interacting proteins are known to function in recruiting metabolism-related proteins to target membranes, or may compete with other factors in the TORC signalling pathway as a means of metabolic regulation. We review and discuss herein examples of how Rabs and their interacting proteins can mediate metabolic signalling and regulation in cells.

Small GTPase Rab17 regulates the surface expression of kainate receptors but not α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in hippocampal neurons via dendritic trafficking of Syntaxin-4 protein

Glutamate receptors are fundamental for control synaptic transmission, synaptic plasticity, and neuronal excitability. However, many of the molecular mechanisms underlying their trafficking remain elusive. We previously demonstrated that the small GTPase Rab17 regulates dendritic trafficking in hippocampal neurons. Here, we investigated the role(s) of Rab17 in AMPA receptor (AMPAR) and kainate receptor (KAR) trafficking. Although Rab17 knockdown did not affect surface expression of the AMPAR subunit GluA1 under basal or chemically induced long term potentiation conditions, it significantly reduced surface expression of the KAR subunit GluK2. Rab17 co-localizes with Syntaxin-4 in the soma, dendritic shaft, the tips of developing hippocampal neurons, and in spines. Rab17 knockdown caused Syntaxin-4 redistribution away from dendrites and into axons in developing hippocampal neurons. Syntaxin-4 knockdown reduced GluK2 but had no effect on GluA1 surface expression. Moreover, overexpression of constitutively active Rab17 promoted dendritic surface expression of GluK2 by enhancing Syntaxin-4 translocation to dendrites. These data suggest that Rab17 mediates the dendritic trafficking of Syntaxin-4 to selectively regulate dendritic surface insertion of GluK2-containing KARs in rat hippocampal neurons.

Engagement of the small GTPase Rab31 protein and its effector, early endosome antigen 1, is important for trafficking of the ligand-bound epidermal growth factor receptor from the early to the late endosome

Rab31 is a member of the Rab5 subfamily of Rab GTPases. Although localized largely to the trans-Golgi network, it shares common guanine nucleotide exchange factors and effectors with other Rab5 subfamily members that have been implicated in endocytic membrane traffic. We investigated whether Rab31 also has a role in the trafficking of the ligand-bound EGF receptor (EGFR) internalized through receptor-mediated endocytosis. We found that loss of Rab31 inhibits, but overexpression enhances, EGFR trafficking to the late endosomes and that the effect of Rab31 silencing could be specifically rescued by overexpression of a silencing-resistant form of Rab31. Rab31 was found to interact with the EGFR by coimmunoprecipitation and affinity pulldown analyses, and the primarily trans-Golgi network-localized Rab31 has increased colocalization with the EGFR in A431 cells 30 min after pulsing with EGF. A glycerol gradient sedimentation assay suggested that Rab31 is sequestered into a high molecular weight complex after stimulation with EGF, as was early endosome antigen 1 (EEA1), a factor responsible for endosomal tethering and fusion events. We found that loss of EEA1 reduced the interaction between Rab31 and the EGFR and abrogated the effect of Rab31 overexpression on the trafficking of the EGFR. Likewise, loss of GAPex5, a Rab31 guanine nucleotide exchange factor that has a role in ubiquitination and degradation of the EGFR, reduced the interaction of Rab31 with the EGFR and its effect on EGFR trafficking. Taken together, our results suggest that Rab31 is an important regulator of endocytic trafficking of the EGFR and functions in an EGFR trafficking complex that includes EEA1 and GAPex5.

SNARE proteins are essential in the potentiation of NMDA receptors by group II metabotropic glutamate receptors

The group II metabotropic glutamate receptors (group II mGluRs) have emerged as the new drug targets for the treatment of mental disorders like schizophrenia. To understand the potential mechanisms underlying the antipsychotic effects of group II mGluRs, we examined their impact on NMDA receptors (NMDARs), since NMDAR hypofunction has been implicated in schizophrenia. The activation of group II mGluRs caused a significant enhancement of NMDAR currents in cortical pyramidal neurons, which was associated with increased NMDAR surface expression and synaptic localization. We further examined whether these effects of group II mGluRs are through the regulation of NMDAR exocytosis via SNARE proteins, a family of proteins involved in vesicle fusion. We found that the enhancing effect of APDC, a selective agonist of group II mGluRs, on NMDAR currents was abolished when botulinum toxin was delivered into the recorded neurons to disrupt the SNARE complex. Inhibiting the function of two key SNARE proteins, SNAP-25 and syntaxin 4, also eliminated the effect of APDC on NMDAR currents. Moreover, the application of APDC increased the activity of Rab4, a small Rab GTPase mediating fast recycling from early endosomes to the plasma membrane, and enhanced the interaction between syntaxin 4 and Rab4. Knockdown of Rab4 or expression of dominant-negative Rab4 attenuated the effect of APDC on NMDAR currents. Taken together, these results have identified key molecules involved in the group II mGluR-induced potentiation of NMDAR exocytosis and function.

Using multifocal plane microscopy to reveal novel trafficking processes in the recycling pathway

A major outstanding issue in cell biology is the lack of understanding of the contribution of tubulovesicular transport carriers (TCs) to intracellular trafficking pathways within 3D cellular environments. This is primarily due to the challenges associated with the use of microscopy techniques to track these highly motile, small compartments. In the present study we have used multifocal plane microscopy with localized photoactivation to overcome these limitations. Using this approach, we have characterized individual components constituting the recycling pathway of the receptor FcRn. Specifically, several different pathways followed by TCs that intersect with larger, relatively static sorting endosomes have been defined. These pathways include a novel 'looping' process in which TCs leave and return to the same sorting endosome. Significantly, TCs with different itineraries can be identified by associations with distinct complements of Rab GTPases, APPL1 and SNX4. These studies provide a framework for further analyses of the recycling pathway.

Interaction of α-taxilin localized on intracellular components with the microtubule cytoskeleton

Intracellular vesicle traffic plays an essential role in the establishment and maintenance of organelle identity and biosynthetic transport. We have identified α-taxilin as a binding partner of the syntaxin family, which is involved in intracellular vesicle traffic. Recently, we have found that α-taxilin is over-expressed in malignant tissues including hepatocellular carcinoma and renal cell carcinoma. However, a precise role of α-taxilin in intracellular vesicle traffic and carcinogenesis remains unclear. Then, we first investigated here the intracellular distribution of α-taxilin in Hela cells. Immunofluorescence studies showed that α-taxilin distributes throughout the cytoplasm and exhibits a tubulo-vesicular pattern. Biochemical studies showed that α-taxilin is abundantly localized on intracellular components as a peripheral membrane protein. Moreover, we found that α-taxilin distributes in microtubule-dependent and syntaxin-independent manners, that α-taxilin directly binds to polymerized tubulin in vitro, and that N-ethylmaleimide but not brefeldin A affects the intracellular distribution of α-taxilin. These results indicate that α-taxilin is localized on intracellular components in a syntaxin-independent manner and that the α-taxilin-containing intracellular components are associated with the microtubule cytoskeleton and suggest that α-taxilin functions as a linker protein between the α-taxilin-containing intracellular components and the microtubule cytoskeleton.

Subcellular trafficking of the substrate transporters GLUT4 and CD36 in cardiomyocytes

Cardiomyocytes use glucose as well as fatty acids for ATP production. These substrates are transported into the cell by glucose transporter 4 (GLUT4) and the fatty acid transporter CD36. Besides being located at the sarcolemma, GLUT4 and CD36 are stored in intracellular compartments. Raised plasma insulin concentrations and increased cardiac work will stimulate GLUT4 as well as CD36 to translocate to the sarcolemma. As so far studied, signaling pathways that regulate GLUT4 translocation similarly affect CD36 translocation. During the development of insulin resistance and type 2 diabetes, CD36 becomes permanently localized at the sarcolemma, whereas GLUT4 internalizes. This juxtaposed positioning of GLUT4 and CD36 is important for aberrant substrate uptake in the diabetic heart: chronically increased fatty acid uptake at the expense of glucose. To explain the differences in subcellular localization of GLUT4 and CD36 in type 2 diabetes, recent research has focused on the role of proteins involved in trafficking of cargo between subcellular compartments. Several of these proteins appear to be similarly involved in both GLUT4 and CD36 translocation. Others, however, have different roles in either GLUT4 or CD36 translocation. These trafficking components, which are differently involved in GLUT4 or CD36 translocation, may be considered novel targets for the development of therapies to restore the imbalanced substrate utilization that occurs in obesity, insulin resistance and diabetic cardiomyopathy.

Role of Rab GTPases in membrane traffic and cell physiology

Intracellular membrane traffic defines a complex network of pathways that connects many of the membrane-bound organelles of eukaryotic cells. Although each pathway is governed by its own set of factors, they all contain Rab GTPases that serve as master regulators. In this review, we discuss how Rabs can regulate virtually all steps of membrane traffic from the formation of the transport vesicle at the donor membrane to its fusion at the target membrane. Some of the many regulatory functions performed by Rabs include interacting with diverse effector proteins that select cargo, promoting vesicle movement, and verifying the correct site of fusion. We describe cascade mechanisms that may define directionality in traffic and ensure that different Rabs do not overlap in the pathways that they regulate. Throughout this review we highlight how Rab dysfunction leads to a variety of disease states ranging from infectious diseases to cancer.

Bioinformatics for the NuGO proof of principle study: analysis of gene expression in muscle of ApoE3*Leiden mice on a high-fat diet using PathVisio

Insulin resistance is a characteristic of type-2 diabetes and its development is associated with an increased fat consumption. Muscle is one of the tissues that becomes insulin resistant after high fat (HF) feeding. The aim of the present study is to identify processes involved in the development of HF-induced insulin resistance in muscle of ApOE3*Leiden mice by using microarrays. These mice are known to become insulin resistant on a HF diet. Differential gene expression was measured in muscle using the Affymetrix mouse plus 2.0 array. To get more insight in the processes, affected pathway analysis was performed with a new tool, PathVisio. PathVisio is a pathway editor customized with plug-ins (1) to visualize microarray data on pathways and (2) to perform statistical analysis to select pathways of interest. The present study demonstrated that with pathway analysis, using PathVisio, a large variety of processes can be investigated. The significantly regulated genes in muscle of ApOE3*Leiden mice after 12 weeks of HF feeding were involved in several biological pathways including fatty acid beta oxidation, fatty acid biosynthesis, insulin signaling, oxidative stress and inflammation.

Regulation of D6 chemokine scavenging activity by ligand- and Rab11-dependent surface up-regulation

The decoy receptor D6 plays a nonredundant role in the control of inflammatory processes through scavenging of inflammatory chemokines. However it remains unclear how it is regulated. Here we show that D6 scavenging activity relies on unique trafficking properties. Under resting conditions, D6 constitutively recycled through both a rapid wortmannin (WM)-sensitive and a slower brefeldin A (BFA)-sensitive pathway, maintaining low levels of surface expression that required both Rab4 and Rab11 activities. In contrast to "conventional" chemokine receptors that are down-regulated by cognate ligands, chemokine engagement induced a dose-dependent BFA-sensitive Rab11-dependent D6 re-distribution to the cell membrane and a corresponding increase in chemokine degradation rate. Thus, the energy-expensive constitutive D6 cycling through Rab11 vesicles allows a rapid, ligand concentration-dependent increase of chemokine scavenging activity by receptor redistribution to the plasma membrane. D6 is not regulated at a transcriptional level in a variety of cellular contexts, thus ligand-dependent optimization of its scavenger performance represents a rapid and unique mechanism allowing D6 to control inflammation.

Tyrosine phosphorylation of Munc18c regulates platelet-derived growth factor-stimulated glucose transporter 4 translocation in 3T3L1 adipocytes

Platelet-derived growth factor (PDGF) stimulation of skeletal muscle, cultured myotubes, and 3T3L1 adipocytes results in glucose transporter 4 (Glut4) translocation, albeit to a reduced level compared with insulin. To address the mechanism of PDGF action, we have determined that the Syntaxin 4 negative regulatory protein, Munc18c, undergoes PDGF-stimulated phosphorylation on tyrosine residue 521. The tyrosine phosphorylation of Munc18c on Y521 occurred concomitant with the dissociation of the Munc18c protein from Syntaxin 4 in a time frame consistent with Glut4 translocation. Moreover, expression of the wild-type Munc18c protein did not inhibit PDGF-induced Glut4 translocation, whereas expression of Y521A-Munc18c mutant was inhibitory and failed to dissociate from Syntaxin 4. In contrast, expression of either wild-type Munc18c or the Y521A-Munc18c mutant both resulted in a marked inhibition of insulin-stimulated Glut4 translocation. Together, these data demonstrate that one mechanism accounting for the PDGF induction of Glut4 translocation is the suppression of the Munc18c negative regulation of Syntaxin 4 function.

Small G proteins in insulin action: Rab and Rho families at the crossroads of signal transduction and GLUT4 vesicle traffic

Insulin stimulates glucose uptake into muscle and adipose tissues through glucose transporter 4 (GLUT4). GLUT4 cycles between the intracellular compartments and the plasma membrane. GLUT4 traffic-regulating insulin signals are largely within the insulin receptor-insulin receptor substrate-phosphatidylinositol 3-kinase (IR-IRS-PI3K) axis. In muscle cells, insulin signal bifurcates downstream of the PI3K into one arm leading to the activation of the Ser/Thr kinases Akt and atypical protein kinase C, and another leading to the activation of Rho family protein Rac1 leading to actin remodelling. Activated Akt inactivates AS160, a GTPase-activating protein for Rab family small G proteins. Here we review the roles of Rab and Rho proteins, particularly Rab substrates of AS160 and Rac1, in insulin-stimulated GLUT4 traffic. We discuss: (1) how distinct steps in GLUT4 traffic may be regulated by discrete Rab proteins, and (2) the importance of Rac1 activation in insulin-induced actin remodelling in muscle cells, a key element for the net gain in surface GLUT4.

The GTPase Rab4 interacts with Chlamydia trachomatis inclusion membrane protein CT229

Chlamydiae, which are obligate intracellular bacteria, replicate in a nonlysosomal vacuole, termed an inclusion. Although neither the host nor the chlamydial proteins that mediate the intracellular trafficking of the inclusion have been clearly identified, several enhanced green fluorescent protein (GFP)-tagged Rab GTPases, including Rab4A, are recruited to chlamydial inclusions. GFP-Rab4A associates with inclusions in a species-independent fashion by 2 h postinfection by mechanisms that have not yet been elucidated. To test whether chlamydial inclusion membrane proteins (Incs) recruit Rab4 to the inclusion, we screened a collection of chlamydial Incs for their ability to interact with Rab4A by using a yeast two-hybrid assay. From our analysis, we identified a specific interaction between Rab4A and Chlamydia trachomatis Inc CT229, which is expressed during the initial stages of infection. CT229 interacts with only wild-type Rab4A and the constitutively active GTPase-deficient Rab4AQ67L but not with the dominant-negative GDP-restricted Rab4AS22N mutant. To confirm the interaction between CT229 and Rab4A, we demonstrated that DsRed-CT229 colocalized with GFP-Rab4A in HeLa cells and more importantly wild-type and constitutively active GFP-Rab4A colocalized with CT229 at the inclusion membrane in C. trachomatis serovar L2-infected HeLa cells. Taken together, these data suggest that CT229 interacts with and recruits Rab4A to the inclusion membrane and therefore may play a role in regulating the intracellular trafficking or fusogenicity of the chlamydial inclusion.

Identification and characterization of a novel variant in the highly conserved catalytic center of Rab11a

Small GTPases of the Rab family regulate vesicular traffic and distribution of proteins in different cell types. Rab11a is a member of this GTP hydrolyzing protein class and acts as a mediator of insulin stimulated translocation of the glucose transporter GLUT4 in peripheral tissues including heart and skeletal muscle. Here we report on Rab11a Q70R, a mutation in the catalytic center of Rab11a, observed in the cardiomyoblast cell line H9c2. Analysis of GTPase activity showed that Rab11a Q70L acts as a classical constitutive active mutant. Interestingly, the GTPase activity of Rab11a Q70R was not significantly different from the enzymatic activity of the Rab11a Q70 wild type protein. We therefore conclude that the glutamine residue of Rab11a at position 70 is not strictly essential for GTPase activity of this protein in contrast to Ras and other Rab proteins.

Rab4GTPase modulates CFTR function by impairing channel expression at plasma membrane

Cystic fibrosis (CF), an autosomal recessive disorder, is caused by the disruption of biosynthesis or the function of a membrane cAMP-activated chloride channel, CFTR. CFTR regulatory mechanisms include recruitment of channel proteins to the cell surface from intracellular pools and by protein-protein interactions. Rab proteins are small GTPases involved in regulated trafficking controlling vesicle docking and fusion. Rab4 controls recycling events from endosome to the plasma membrane, fusion, and degradation. The colorectal cell line HT-29 natively expresses CFTR and responds to cAMP stimulation with an increase in CFTR-mediated currents. Rab4 over-expression in HT-29 cells inhibits both basal and cAMP-stimulated CFTR-mediated currents. GTPase-deficient Rab4Q67L and GDP locked Rab4S22N both inhibit channel activity, which appears characteristically different. Active status of Rab4 was confirmed by GTP overlay assay, while its expression was verified by Western blotting. The pull-down and immunoprecipitation experiments suggest that Rab4 physically interacts with CFTR through protein-protein interaction. Biotinylation with cell impermeant NHS-Sulfo-SS-Biotin implies that Rab4 impairs CFTR expression at cell surface. The enhanced cytosolic CFTR indicates that Rab4 expression restrains CFTR appearance at the cell membrane. The study suggests that Rab4 regulates the channel through multiple mechanisms that include protein-protein interaction, GTP/GDP exchange, and channel protein trafficking. We propose that Rab4 is a dynamic molecule with a significant role in CFTR function.

Rab4 GTP/GDP modulates amiloride-sensitive sodium channel (ENaC) function in colonic epithelia

The sodium-selective amiloride-sensitive epithelial sodium channel (ENaC) mediates electrogenic sodium re-absorption in tight epithelia. ENaC expression at the plasma membrane requires regulated transport, processing, and macromolecular assembly of subunit proteins in a defined and highly compartmentalized manner. Ras-related Rab GTPases monitor these processes in a highly regulated sequence of events. In order to evaluate the role of Rab proteins in ENaC function, Rab4 wild-type (WT), the GTPase-deficient mutant Rab4Q67L, and the dominant negative GDP-locked mutant Rab4S22N were over-expressed in the colon cancer cell line, HT-29 and amiloride-sensitive currents were recorded. Rab4 over-expression inhibited amiloride-sensitive currents. The effect was reversed by introducing Rab4-neutralizing antibody and Rab4 specific SiRNA. The GDP-locked Rab4 mutant inhibited, while GTPase-deficient mutant moderately stimulated amiloride-sensitive currents. Active status of Rab4 was confirmed by GTP overlay assay, while its expression was verified by Western blotting. Immunoprecipitation and pull-down assay suggest protein-protein interaction between Rab4 and ENaC. In addition, the functional modulation coincides with concomitant changes in ENaC expression at the cell surface and in intracellular pool. We propose that Rab4 is a critical element that regulates ENaC function by mechanisms that include GTP-GDP status, recycling, and expression level. Our observations imply that channel expression in apical membranes of epithelial cell system incorporates RabGTPase as an essential determinant of channel function and adds an exciting paradigm to ENaC therapeutics.

Characterization of the Role of the Rab GTPase-activating Protein AS160 in Insulin-regulated GLUT4 Trafficking

Insulin stimulates the translocation of the glucose transporter GLUT4 from intracellular vesicles to the plasma membrane. In the present study we have conducted a comprehensive proteomic analysis of affinity-purified GLUT4 vesicles from 3T3-L1 adipocytes to discover potential regulators of GLUT4 trafficking. In addition to previously identified components of GLUT4 storage vesicles including the insulin-regulated aminopeptidase insulin-regulated aminopeptidase and the vesicle soluble N-ethylmaleimide factor attachment protein (v-SNARE) VAMP2, we have identified three new Rab proteins, Rab10, Rab11, and Rab14, on GLUT4 vesicles. We have also found that the putative Rab GTPase-activating protein AS160 (Akt substrate of 160 kDa) is associated with GLUT4 vesicles in the basal state and dissociates in response to insulin. This association is likely to be mediated by the cytosolic tail of insulin-regulated aminopeptidase, which interacted both in vitro and in vivo with AS160. Consistent with an inhibitory role of AS160 in the basal state, reduced expression of AS160 in adipocytes using short hairpin RNA increased plasma membrane levels of GLUT4 in an insulin-independent manner. These findings support an important role for AS160 in the insulin regulated trafficking of GLUT4.

High resolution crystal structures of human Rab4a in its active and inactive conformations

The Ras-related human GTPase Rab4a is involved in the regulation of endocytosis through the sorting and recycling of early endosomes. Towards further insight, we have determined the three-dimensional crystal structure of human Rab4a in its GppNHp-bound state to 1.6 Angstroms resolution and in its GDP-bound state to 1.8 Angstroms resolution, respectively. Despite the similarity of the overall structure with other Rab proteins, Rab4a displays significant differences. The structures are discussed with respect to the recently determined structure of human Rab5a and its complex with the Rab5-binding domain of the bivalent effector Rabaptin-5. The Rab4 specific residue His39 modulates the nucleotide binding pocket giving rise to a reduced rate for nucleotide hydrolysis and exchange. In comparison to Rab5, Rab4a has a different GDP-bound conformation within switch 1 region and displays shifts in position and orientation of the hydrophobic triad. The observed differences at the S2-L3-S3 region represent a new example of structural plasticity among Rab proteins and may provide a structural basis to understand the differential binding of similar effector proteins.

Selective expression of a sec1/munc18 member in sea urchin eggs and embryos

Regulated secretion is mediated by SNAREs (soluble NSF attachment receptors) and their regulators and effectors, which include the SM (sec1/munc18) family of proteins. Homologs of the SNAREs have been identified in sea urchins, associated with cortical granule exocytosis at fertilization, with membranes of the cleavage furrow, and in secretory cells later in development. To contribute to the understanding of regulated secretion in sea urchins we have cloned the single SM protein homolog from two species of sea urchin, Lytechinus variegatus and Strongylocentrotus purpuratus. In oocytes and eggs, we find that it localizes to the plasma membrane and the cortical region of the egg, consistent with a role in one of the steps leading to cortical granule exocytosis. The protein is also expressed throughout development, enriched in membranes of the cleavage furrow in early embryos, and in cells of the gut in advanced embryos. Furthermore, we find that sec1/munc18 co-localizes with its cognate binding partner syntaxin. Finally, our biochemical analysis shows that the protein associates with rab3 in high molecular weight complexes, suggesting that the exocytotic machinery functions as a multi-protein subunit to mediate regulated secretion in sea urchins. These results will be instrumental in the future to functionally test the SNARE regulators associated with multiple membrane fusion events.

Regulated membrane trafficking of the insulin-responsive glucose transporter 4 in adipocytes

Since the discovery of insulin roughly 80 yr ago, much has been learned about how target cells receive, interpret, and respond to this peptide hormone. For example, we now know that insulin activates the tyrosine kinase activity of its cell surface receptor, thereby triggering intracellular signaling cascades that regulate many cellular processes. With respect to glucose homeostasis, these include the function of insulin to suppress hepatic glucose production and to increase glucose uptake in muscle and adipose tissues, the latter resulting from the translocation of the glucose transporter 4 (GLUT4) to the cell surface membrane. Although simple in broad outline, elucidating the molecular intricacies of these receptor-signaling pathways and membrane-trafficking processes continues to challenge the creative ingenuity of scientists, and many questions remain unresolved, or even perhaps unasked. The identification and functional characterization of specific molecules required for both insulin signaling and GLUT4 vesicle trafficking remain key issues in our pursuit of developing specific therapeutic agents to treat and/or prevent this debilitating disease process. To this end, the combined efforts of numerous research groups employing a range of experimental approaches has led to a clearer molecular picture of how insulin regulates the membrane trafficking of GLUT4.

Characterization of alpha-soluble N-ethylmaleimide-sensitive fusion attachment protein in alveolar type II cells: implications in lung surfactant secretion

N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment protein (alpha-SNAP) are thought to be soluble factors that transiently bind and disassemble SNAP receptor complex during exocytosis in neuronal and endocrine cells. Lung surfactant is secreted via exocytosis of lamellar bodies from alveolar epithelial type II cells. However, the secretion of lung surfactant is a relatively slow process, and involvement of SNAP receptor and its cofactors (NSF and alpha-SNAP) in this process has not been demonstrated. In this study, we investigated a possible role of alpha-SNAP in surfactant secretion. alpha-SNAP was predominantly associated with the membranes in alveolar type II cells as determined by Western blot and immunocytochemical analysis using confocal microscope. Membrane-associated alpha-SNAP was not released from the membrane fraction when the cells were lyzed in the presence of Ca2+ or Mg2+ATP. The alkaline condition (0.1 M Na2CO3, pH 12), known to extract peripheral membrane proteins also failed to release it from the membrane. Phase separation using Triton X-114 showed that alpha-SNAP partitioned into both aqueous and detergent phases. NSF had membrane-bound characteristics similar to alpha-SNAP in type II cells. Permeabilization of type II cells with beta-escin resulted in a partial loss of alpha-SNAP from the cells, but cellular NSF was relatively unchanged. Addition of exogenous alpha-SNAP to the permeabilized cells increased surfactant secretion in a dose-dependent manner, whereas exogenous NSF has much less effects. An alpha-SNAP antisense oligonucleotide decreased its protein level and inhibited surfactant secretion. Our results suggest a role of alpha-SNAP in lung surfactant secretion.

Cellular munc18c levels can modulate glucose transport rate and GLUT4 translocation in 3T3L1 cells

Munc18c has been shown to bind syntaxin 4 and to play a role in GLUT4 translocation and glucose transport, although this role is as yet poorly defined. In the present study, the effects of modulating the available level of munc18c on glucose transport and GLUT4 translocation were examined. Over-expression of munc18c in 3T3L1 adipocytes inhibited insulin-stimulated glucose transport by approximately 50%. Basal glucose transport rates were also decreased by approximately 25%. In contrast, microinjection of a munc18c polyclonal antibody stimulated GLUT4 translocation by approximately 60% over basal levels without affecting insulin-stimulated GLUT4 levels. Microinjection of a control antibody had no effect. These data are consistent with the likelihood that antibody microinjection sequesters munc18c enabling translocation/fusion of GLUT4 vesicles. Mutagenesis of a potential proline-directed kinase phosphorylation site in munc18c, T569, that in previous studies of its neuronal counterpart munc18a caused its dissociation from its complex with syntaxin 1a, had no effect on munc18c's association with syntaxin 4 or its inhibition of glucose transport, indicative that phosphorylation of this residue is not important for insulin regulation of glucose transport. The over-expression and microinjection sequestration data support an inhibitory role for munc18c on translocation/fusion of GLUT4 vesicles. They further show that altering the level of available munc18c in 3T3L1 cells can modulate glucose transport rates, indicating its potential as a target for therapeutics in diabetes.

An adipocentric view of signaling and intracellular trafficking

Adipocytes have traditionally been considered to be the primary site for whole body energy storage mainly in the form of triglycerides and fatty acids. This occurs through the ability of insulin to markedly stimulate both glucose uptake and lipogenesis. Conventional wisdom held that defects in fuel partitioning into adipocytes either because of increased adipose tissue mass and/or increased lipolysis and circulating free fatty acids resulted in dyslipidemia, obesity, insulin resistance and perhaps diabetes. However, it has become increasingly apparent that loss of adipose tissue (lipodystrophies) in both animal models and humans also leads to metabolic disorders that result in severe states of insulin resistance and potential diabetes. These apparently opposite functions can be resolved by the establishment of adipocytes not only as a fuel storage depot but also as a critical endocrine organ that secretes a variety of signaling molecules into the circulation. Although the molecular function of these adipocyte-derived signals are poorly understood, they play a central role in the maintenance of energy homeostasis by regulating insulin secretion, insulin action, glucose and lipid metabolism, energy balance, host defense and reproduction. The diversity of these secretory factors include enzymes (lipoprotein lipase (LPL) and adipsin), growth factors [vascular endothelial growth factor (VEGF)], cytokines (tumor necrosis factor-alpha, interleukin 6) and several other hormones involved in fatty acid and glucose metabolism (leptin, Acrp30, resistin and acylation stimulation protein). Despite the large number of molecules secreted by adipocytes, our understanding of the pathways and mechanisms controlling intracellular trafficking and exocytosis in adipocytes is poorly understood. In this article, we will review the current knowledge of the trafficking and secretion processes that take place in adipocytes, focusing our attention on two of the best characterized adipokine molecules (leptin and adiponectin) and on one of the most intensively studied regulated membrane proteins, the GLUT4 glucose transporter.

The novel Rab11-FIP/Rip/RCP family of proteins displays extensive homo- and hetero-interacting abilities

The Rab11-FIP/Rip/RCP proteins are a recently described novel protein family, whose members interact with Rab GTPases that function in endosomal recycling. To date, five such proteins have been described in humans, all of which interact with Rab11, and one (RCP) also interacts with Rab4. Here, we characterise several of these proteins with respect to their ability to interact with Rab4, as well as their ability to self-interact, and to interact with each other. We now demonstrate that two of the family members-pp75/Rip11 and Rab11-FIP3 do not bind Rab4 and show that several members of the family can self-interact and interact with each other. These interactions primarily involve their C-terminal end which includes the Rab binding domain (RBD) that is contained within a predicted coiled-coil, or ERM motif. We identify a new (sixth) member of the protein family, which we propose to name Rab11-FIP4, and report the family evolutionary complexity and chromosomal distribution. Furthermore, we propose that the ability of these proteins to bind each other will be important in effecting membrane trafficking events by forming protein 'platforms,' regulated by Rab11 and/or Rab4 activity.

Rab coupling protein (RCP), a novel Rab4 and Rab11 effector protein

Rab4 and Rab11 are small GTPases belonging to the Ras superfamily. They both function as regulators along the receptor recycling pathway. We have identified a novel 80-kDa protein that interacts specifically with the GTP-bound conformation of Rab4, and subsequent work has shown that it also interacts strongly with Rab11. We name this protein Rab coupling protein (RCP). RCP is predominantly membrane-bound and is expressed in all cell lines and tissues tested. It colocalizes with early endosomal markers including Rab4 and Rab11 as well as with the transferrin receptor. Overexpression of the carboxyl-terminal region of RCP, which contains the Rab4- and Rab11-interacting domain, results in a dramatic tubulation of the transferrin compartment. Furthermore, expression of this mutant causes a significant reduction in endosomal recycling without affecting ligand uptake or degradation in quantitative assays. RCP is a homologue of Rip11 and therefore belongs to the recently described Rab11-FIP family.

Regulated transport of the glucose transporter GLUT4

In muscle and fat cells, insulin stimulates the delivery of the glucose transporter GLUT4 from an intracellular location to the cell surface, where it facilitates the reduction of plasma glucose levels. Understanding the molecular mechanisms that mediate this translocation event involves integrating our knowledge of two fundamental processes--the signal transduction pathways that are triggered when insulin binds to its receptor and the membrane transport events that need to be modified to divert GLUT4 from intracellular storage to an active plasma membrane shuttle service.

Identification of protein kinase Calpha as an essential, but not sufficient, cytosolic factor for Ca2+-induced alpha- and dense-core granule secretion in platelets

Upon activation, platelets release many active substances. Here, we have analyzed the mechanism governing Ca(2+)-induced secretion of von Willebrand factor stored in alpha-granules and 5-hydroxytryptamine in dense-core granules in permeabilized human platelets. Both secretions were dependent on ATP and cytosol. An essential factor for both granule secretions was purified from rat brain cytosol and identified to be protein kinase Calpha (PKCalpha) by partial amino acid sequencing. Purified PKCalpha efficiently stimulated both secretions in the presence of cytosol, whereas PKCalpha alone did not support the secretion of either type of granules, suggesting that PKCalpha is not a sufficient factor. Finally, in human platelet cytosol fractionated by a gel filtration column, the stimulatory activity for dense-core granule secretion paralleled with the concentration of PKC, suggesting that PKC could also be such a stimulatory factor in platelet cytosol. Thus, we identified PKCalpha as an essential, but not sufficient, cytosolic factor for the Ca(2+)-induced secretions of both alpha- and dense-core granules in platelets.

The role of small G-proteins in the regulation of glucose transport (review)

Insulin increases the rate of glucose transport into fat and muscle cells by stimulating the translocation of intracellular Glut 4-containing vesicles to the plasma membrane. This results in a marked increase in the amount of the facilitative glucose transporter Glut 4 at the cell surface, allowing for an enhanced glucose uptake. This process requires a continuous cycling through the early endosomes, a Glut 4 specific storage compartment and the plasma membrane. The main effect of insulin is to increase the rate of Glut 4 trafficking from its specific storage compartment to the plasma membrane. The whole phenomenon involves signal transduction from the insulin receptor, vesicle trafficking (sorting and fusion processes) and actin cytoskeleton modifications, which are all supposed to require small GTPases. This review describes the potential role of the various members of the Ras, Rad, Rho, Arf and Rab families in the traffic of the Glut 4-containing vesicles.

Expression of a prenylation-deficient Rab4 inhibits the GLUT4 translocation induced by active phosphatidylinositol 3-kinase and protein kinase B

The small GTPase Rab4 has been shown to participate in the subcellular distribution of GLUT4 under both basal and insulin-stimulated conditions in adipocytes. In the present work, we have characterized the effect of Rab4 DeltaCT, a prenylation-deficient and thus cytosolic form of Rab4, in this process. We show that the expression of Rab4 DeltaCT in freshly isolated adipocytes inhibits insulin-induced GLUT4 translocation, but only when this protein is in its GTP-bound active form. Further, it not only blocks the effect of insulin, but also that of a hyperosmotic shock, but does not interfere with the effect of zinc ions on GLUT4 translocation. Rab4 DeltaCT was then shown to prevent GLUT4 translocation induced by the expression of an active form of phosphatidylinositol 3-kinase or of protein kinase B, without altering the activities of the enzymes. Our results are consistent with a role of Rab4 DeltaCT acting as a dominant negative protein towards Rab4, possibly by binding to Rab4 effectors.