Insulin-regulated release from the endosomal recycling compartment is regulated by budding of specialized vesicles

Subcellular localization and activity of multidrug resistance proteins

The multidrug resistance (MDR) phenotype is associated with the overexpression of members of the ATP-binding cassette family of proteins. These MDR transporters are expressed at the plasma membrane, where they are thought to reduce the cellular accumulation of toxins over time. Our data demonstrate that members of this family are also expressed in subcellular compartments where they actively sequester drugs away from their cellular targets. The multidrug resistance protein 1 (MRP1), P-glycoprotein, and the breast cancer resistance protein are each present in a perinuclear region positive for lysosomal markers. Fluorescence-activated cell sorting analysis suggests that these three drug transporters do little to reduce the cellular accumulation of the anthracycline doxorubicin. However, whereas doxorubicin enters cells expressing MDR transporters, this drug is sequestered away from the nucleus, its subcellular target, in vesicles expressing each of the three drug resistance proteins. Using a cell-impermeable inhibitor of MRP1 activity, we demonstrate that MRP1 activity on intracellular vesicles is sufficient to confer a drug resistance phenotype, whereas disruption of lysosomal pH is not. Intracellular localization and activity for MRP1 and other members of the MDR transporter family may suggest different strategies for chemotherapeutic regimens in a clinical setting.

Push/pull mechanisms of GLUT4 traffic in muscle cells

AIMS

Understanding the mechanisms by which insulin regulates glucose transporter 4 (GLUT4) traffic in skeletal muscle has been a major challenge since the discoveries of glucose transporter's translocation and the cloning of GLUT4. Here we summarize our work of the past 5 years on the regulation of GLUT4 traffic in skeletal muscle cells.

METHODS

L6 cells overexpressing GLUT4 harbouring an exofacial myc epitope gave us the opportunity to perform dynamic assessments of GLUT4 exocytosis, endocytosis, as well as a means to follow GLUT4 molecules along their journey through intracellular compartments.

RESULTS

We found that insulin stimulation, which results in the expected gain in surface GLUT4, is mostly attributed to enhanced GLUT4 exocytosis, and does not significantly affect the initial rate of internalization. Two mechanisms by which insulin enhances GLUT4 exocytosis are described: 'Pull' relates to actin remodelling-based segregation of the insulin signalling molecules and the directed recruitment of GLUT4/VAMP2 containing vesicles. 'Push' is the accelerated inter-endosomal transit of endocytosed GLUT4 molecules through the recycling endosome. The interface between the two types of regulatory input by insulin is suggested to be the budding of GLUT4 from the transferrin receptor (TfR)-containing, recycling endosome.

CONCLUSIONS

We propose a model on the identity of the GLUT4 pools responsible for GLUT4 recruitment to the plasma membrane in the basal state, or following insulin or hyperosmolarity stimuli.

Supported planar bilayers in studies on immune cell adhesion and communication

Supported planar bilayers have been used extensively in immunology to study molecular interactions at interfaces as a model for cell-cell interaction. Examples include Fc receptor-mediated adhesion and signaling and formation of the immunological synapse between T cells and antigen-presenting cells. The advantage of the supported planar bilayer system is control of the bilayer composition and the optical advantages of imaging the cell-bilayer or bilayer-bilayer interface by various types of trans-, epi- and total internal reflection illumination. Supported planar bilayers are simple to form by liposome fusion and recent advances in micro- and nanotechnology greatly extend the power of supported bilayers to address key questions in immunology and cell biology.

Movement of plasma-membrane-associated clathrin spots along the microtubule cytoskeleton

The current understanding of the role of plasma- membrane-associated clathrin suggests that clathrin-coated pits form at the sites of activated receptors and then, following internalization, the clathrin coat is rapidly shed. Utilizing total internal reflection fluorescence microscopy (TIR-FM), we have documented linear lateral motion of cell-surface-associated dsRed-clathrin spots parallel to the plasma membrane. Clathrin spot motility was observed in multiple cell lines (MDCK, CHO, Cos-7 and HeLa). In MDCK cells dsRed-clathrin spots moved along linear pathways up to 4 micro m in length with rates of approximately 0.8 micro m/s. Spots did not generally undergo internalization during movement. The motion of these puncta was coincident with the microtubule cytoskeleton, and depolymerization of microtubules reduced spot motility over 10-fold. Over-expression of the microtubule-associated protein tau-EGFP decreased spot run length by 40% without affecting the rate of movement. Thus dsRed-clathrin puncta move along the microtubule cytoskeleton parallel to the cell surface.

The adipocyte plasma membrane caveolin functional/structural organization is necessary for the efficient endocytosis of GLUT4

It is well established that insulin stimulation of glucose uptake requires the translocation of intracellular localized GLUT4 protein to the cell surface membrane. This plasma membrane-redistributed GLUT4 protein was partially co-localized with caveolin as determined by confocal fluorescent microscopy but was fully excluded from lipid rafts based upon Triton X-100 extractability. Cholesterol depletion with methyl-beta-cyclodextrin, filipin, or cholesterol oxidase resulted in an insulin-independent increase in the amount of plasma membrane-localized GLUT4 that was fully reversible by cholesterol replenishment. This basal accumulation of cell surface GLUT4 occurred due to an inhibition of GLUT4 endocytosis. However, this effect was not specific since cholesterol extraction also resulted in a dramatic inhibition of clathrin-mediated endocytosis as assessed by transferrin receptor internalization. To functionally distinguish between caveolin- and clathrin-dependent endocytic processes, we took advantage of a dominant-interfering caveolin 1 mutant (Cav1/S80E) that specifically disrupts caveolae organization. Expression of Cav1/S80E, but not the wild type (Cav1/WT) or Cav1/S80A mutant, inhibited cholera toxin B internalization without any significant effect on transferrin receptor endocytosis. In parallel, Cav1/S80E expression increased the amount of plasma membrane-localized GLUT4 protein in an insulin-independent manner. Although Cav1/S80E also decreased GLUT4 endocytosis, the extent of GLUT4 internalization was only partially reduced ( approximately 40%). In addition, expression of Cav1/WT and Cav1/S80A enhanced GLUT4 endocytosis by approximately 20%. Together, these data indicate that the endocytosis of GLUT4 requires clathrin-mediated endocytosis but that the higher order structural organization of plasma membrane caveolin has a significant influence on this process.

GLUT4 recycles via a trans-Golgi network (TGN) subdomain enriched in Syntaxins 6 and 16 but not TGN38: involvement of an acidic targeting motif

Insulin stimulates glucose transport in fat and muscle cells by triggering exocytosis of the glucose transporter GLUT4. To define the intracellular trafficking of GLUT4, we have studied the internalization of an epitope-tagged version of GLUT4 from the cell surface. GLUT4 rapidly traversed the endosomal system en route to a perinuclear location. This perinuclear GLUT4 compartment did not colocalize with endosomal markers (endosomal antigen 1 protein, transferrin) or TGN38, but showed significant overlap with the TGN target (t)-soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) Syntaxins 6 and 16. These results were confirmed by vesicle immunoisolation. Consistent with a role for Syntaxins 6 and 16 in GLUT4 trafficking we found that their expression was up-regulated significantly during adipocyte differentiation and insulin stimulated their movement to the cell surface. GLUT4 trafficking between endosomes and trans-Golgi network was regulated via an acidic targeting motif in the carboxy terminus of GLUT4, because a mutant lacking this motif was retained in endosomes. We conclude that GLUT4 is rapidly transported from the cell surface to a subdomain of the trans-Golgi network that is enriched in the t-SNAREs Syntaxins 6 and 16 and that an acidic targeting motif in the C-terminal tail of GLUT4 plays an important role in this process.

Real-time analysis of clathrin-mediated endocytosis during cell migration

Simultaneous dual-color total-internal-reflection fluorescence microscopy (TIR-FM) was performed to analyze the internalization and distribution of markers for clathrin-mediated endocytosis (clathrin, dynamin1, dynamin2 and transferrin) in migrating cells. In MDCK cells, which endogenously express dynamin2, the dynamin2-EGFP fluorescence demonstrated identical spatial and temporal behavior as clathrin both prior to and during internalization. By contrast, in the same cells, the neuronal dynamin1 only localized with clathrin just prior to endocytosis. In migrating cells, each endocytic marker was polarized towards the leading edge, away from the lagging edge. These observations suggest a re-evaluation of the functional differences between dynamin1 and dynamin2, and of the role of clathrin-mediated endocytosis in cell migration.

Antigen-stimulated trafficking from the recycling compartment to the plasma membrane in RBL mast cells

Binding of fluorescein isothiocyanate (FITC)-conjugated cholera toxin B subunit to ganglioside GM1 on RBL-2H3 cells at 37 degrees C results in labeling of the plasma membrane as well as a pool of perinuclear intracellular membranes identified as the endosomal recycling compartment. Antigen-mediated activation of IgE receptor signaling causes rapid, sustained outward trafficking of these labeled endosomes, that is monitored as an increase in FITC fluorescence due to relief of quenching in the acidic endosomes upon delivery to the plasma membrane. Stimulation of this process depends on the integrity of cholesterol-dependent lipid rafts and occurs in response to Ca2+-mobilizing thapsigargin as well as antigen. Inhibitors of some early signaling enzymes stimulated by FcepsilonRI, including Syk tyrosine kinase and phosphoinositide 3-kinase, have little or no effect on this trafficking response. Other signaling pathways, including activation of phospholipase C and Ca2+ influx, do not appear to be necessary for the initiation of the outward trafficking response, but they contribute to maintaining the sustained phase of this process. Consistent with this, antigen-stimulated ruffles are labeled with FITC-cholera toxin B in a Ca2+-dependent manner. Thus, this trafficking response provides a mechanism by which an internal membrane pool can contribute to plasma membrane remodeling during stimulated membrane ruffling, cell motility, and phagocytosis.

Membrane proximal lysosomes are the major vesicles responsible for calcium-dependent exocytosis in nonsecretory cells

Similar to its role in secretory cells, calcium triggers exocytosis in nonsecretory cells. This calcium-dependent exocytosis is essential for repair of membrane ruptures. Using total internal reflection fluorescence microscopy, we observed that many organelles implicated in this process, including ER, post-Golgi vesicles, late endosomes, early endosomes, and lysosomes, were within 100 nm of the plasma membrane (in the evanescent field). However, an increase in cytosolic calcium led to exocytosis of only the lysosomes. The lysosomes that fused were predominantly predocked at the plasma membrane, indicating that calcium is primarily responsible for fusion and not recruitment of lysosomes to the cell surface.

GLUT4 retention in adipocytes requires two intracellular insulin-regulated transport steps

Insulin regulates glucose uptake into fat and muscle by modulating the distribution of the GLUT4 glucose transporter between the surface and interior of cells. The GLUT4 trafficking pathway overlaps with the general endocytic recycling pathway, but the degree and functional significance of the overlap are not known. In this study of intact adipocytes, we demonstrate, by using a compartment-specific fluorescence-quenching assay, that GLUT4 is equally distributed between two intracellular pools: the transferrin receptor-containing endosomes and a specialized compartment that excludes the transferrin receptor. These pools of GLUT4 are in dynamic communication with one another and with the cell surface. Insulin-induced redistribution of GLUT4 to the surface requires mobilization of both pools. These data establish a role for the general endosomal system in the specialized, insulin-regulated trafficking of GLUT4. Trafficking through the general endosomal system is regulated by rab11. Herein, we show that rab11 is required for the transport of GLUT4 from endosomes to the specialized compartment and for the insulin-induced translocation to the cell surface, emphasizing the importance of the general endosomal pathway in the specialized trafficking of GLUT4. Based on these findings we propose a two-step model for GLUT4 trafficking in which the general endosomal recycling compartment plays a specialized role in the insulin-regulated traffic of GLUT4. This compartment-based model provides the framework for understanding insulin-regulated trafficking at a molecular level.

Calcium-dependent transferrin receptor recycling in bovine chromaffin cells

The release of regulated secretory granules is known to be calcium dependent. To examine the Ca2+-dependence of other exocytic fusion events, transferrin recycling in bovine chromaffin cells was examined. Internalised 125I-transferrin was released constitutively from cells with a half-time of about 7 min. Secretagogues that triggered catecholamine secretion doubled the rate of 125I-transferrin release, the time courses of the two triggered secretory responses being similar. The triggered 125I-transferrin release came from recycling endosomes rather than from sorting endosomes or a triggered secretory vesicle pool. Triggered 125I-transferrin release, like catecholamine secretion from the same cells, was calcium dependent but the affinities for calcium were very different. The extracellular calcium concentrations that gave rise to half-maximal evoked secretion were 0.1 mm for 125I-transferrin and 1.0 mm for catecholamine, and the intracellular concentrations were 0.1 microm and 1 microm, respectively. There was significant 125I-transferrin recycling in the virtual absence of intracellular Ca2+, but the rate increased when Ca2+ was raised above 1 nm, and peaked at 1 microm when the rate had doubled. Botulinum toxin type D blocked both transferrin recycling and catecholamine secretion. These results indicate that a major component of the vesicular transport required for the constitutive recycling of transferrin in quiescent cells is calcium dependent and thus under physiological control, and also that some of the molecular machinery involved in transferrin recycling/fusion processes is shared with that for triggered neurosecretion.

Fat targets for insulin signaling

Adipose cells control flux of fatty acids to peripheral tissues by storing and hydrolyzing triglyceride under hormonal control. New data reveal that insulin may regulate this process in part by promoting membrane trafficking of intracellular fatty acid transporters FATP1 and FATP4 to the plasma membrane.

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.