Rab10 regulates membrane transport through early endosomes of polarized Madin-Darby canine kidney cells

A role for Rab10 in von Willebrand factor release discovered by an AP-1 interactor screen in C. elegans

BACKGROUND

Endothelial von Willebrand factor (VWF) mediates platelet adhesion and acts as a protective chaperone to clotting factor VIII. Rapid release of highly multimerized VWF is particularly effective in promoting hemostasis. To produce this protein, an elaborate biogenesis is required, culminating at the trans-Golgi network (TGN) in storage within secretory granules called Weibel-Palade bodies (WPB). Failure to correctly form these organelles can lead to uncontrolled secretion of low-molecular-weight multimers of VWF. The TGN-associated adaptor AP-1 and its interactors clathrin, aftiphilin and γ-synergin are essential to initial WPB formation at the Golgi apparatus, and thus to VWF storage and secretion.

OBJECTIVES

To identify new proteins implicated in VWF storage and/or secretion.

METHODS

A genomewide RNA interference (RNAi) screen was performed in the Nematode C. elegans to identify new AP-1 genetic interactors.

RESULTS

The small GTPase Rab10 was found to genetically interact with a partial loss of function of AP-1 in C. elegans. We investigated Rab10 in human primary umbilical vein endothelial cells (HUVECs). We report that Rab10 is enriched at the Golgi apparatus, where WPB are formed, and that in cells where Rab10 expression has been suppressed by siRNA, VWF secretion is altered: the amount of rapidly released VWF was significantly reduced. We also found that Rab8A has a similar function.

CONCLUSION

Rab10 and Rab8A are new cytoplasmic factors implicated in WPB biogenesis that play a role in generating granules that can rapidly respond to secretagogue.

Pre-sorting endosomal transport of the GPI-anchored protein, CD59, is regulated by EHD1

EHD1 regulates the trafficking of multiple receptors from the endocytic recycling compartment (ERC) to the plasma membrane. However, the potential role of EHD1 in regulating the family of glycosylphosphatidylinositol-anchored proteins (GPI-APs) has not been determined. Here we demonstrate a novel role for EHD1 in regulating the trafficking of CD59, an endogenous GPI-AP, at early stages of trafficking through the endocytic pathway. EHD1 displays significant colocalization with newly internalized CD59. Upon EHD1 depletion, there is a rapid Rab5-independent coalescence of CD59 in the ERC region. However, expression of an active Arf6 mutant (Q67L), which traps internalized pre-sorting endosomal cargo in phosphatidylinositol(4,5)-bisphosphate enriched vacuoles, prevents this coalescence. It is of interest that sustained PKC activation leads to a similar coalescence of CD59 at the ERC, and treatment of EHD1-depleted cells with a PKC inhibitor (Go6976) blocked this rapid relocation of CD59. However, unlike sustained PKC activation, EHD1 depletion does not induce the translocation of PKCα to ERC. The results presented herein provide evidence that EHD1 is involved in the control of CD59 transport from pre-sorting endosomes to the ERC in a PKC-dependent manner. However, the mechanisms of EHD1-induced coalescence of CD59 at the ERC differ from those induced by sustained PKC activation.

Quantitative analysis of synaptic vesicle Rabs uncovers distinct yet overlapping roles for Rab3a and Rab27b in Ca2+-triggered exocytosis

Rab GTPases are molecular switches that orchestrate protein complexes before membrane fusion reactions. In synapses, Rab3 and Rab5 proteins have been implicated in the exo-endocytic cycling of synaptic vesicles (SVs), but an involvement of additional Rabs cannot be excluded. Here, combining high-resolution mass spectrometry and chemical labeling (iTRAQ) together with quantitative immunoblotting and fluorescence microscopy, we have determined the exocytotic (Rab3a, Rab3b, Rab3c, and Rab27b) and endocytic (Rab4b, Rab5a/b, Rab10, Rab11b, and Rab14) Rab machinery of SVs. Analysis of two closely related proteins, Rab3a and Rab27b, revealed colocalization in synaptic nerve terminals, where they reside on distinct but overlapping SV pools. Moreover, whereas Rab3a readily dissociates from SVs during Ca(2+)-triggered exocytosis, and is susceptible to membrane extraction by Rab-GDI, Rab27b persists on SV membranes upon stimulation and is resistant to GDI-coupled Rab retrieval. Finally, we demonstrate that selective modulation of the GTP/GDP switch mechanism of Rab27b impairs SV recycling, suggesting that Rab27b, probably in concert with Rab3s, is involved in SV exocytosis.

Recycling endosomes in apical plasma membrane domain formation and epithelial cell polarity

Recycling endosomes have taken central stage in the intracellular sorting and polarized trafficking of apical and basolateral plasma membrane components. Molecular players in the underlying mechanisms are now emerging, including small GTPases, class V myosins and adaptor proteins. In particular, defects in the expression or function of these recycling endosome-associated and endosome-regulating proteins have been implicated in cell surface polarity defects and diseases, including microvillus inclusion disease, arthrogryposis-renal dysfunction-cholestasis syndrome, and virus susceptibility. Key findings are that recycling endosomes recruit and deliver core polarity proteins to lateral cell surfaces and initiate the biogenesis of apical plasma membrane domains and epithelial cell polarity. Here, we review recent data that implicate recycling endosomes in the establishment and maintenance of epithelial cell polarity.

AS160 associates with the Na+,K+-ATPase and mediates the adenosine monophosphate-stimulated protein kinase-dependent regulation of sodium pump surface expression

The sodium pump interacts with AS160, a protein that regulates the trafficking of the GLUT4 glucose transporter. This interaction drives the internalization of the sodium pump from the cell surface, and this process is in turn controlled by the energy-sensing kinase adenosine monophosphate-stimulated protein kinase.

The Na⁺,K⁺-ATPase is the major active transport protein found in the plasma membranes of most epithelial cell types. The regulation of Na⁺,K⁺-ATPase activity involves a variety of mechanisms, including regulated endocytosis and recycling. Our efforts to identify novel Na⁺,K⁺-ATPase binding partners revealed a direct association between the Na⁺,K⁺-ATPase and AS160, a Rab-GTPase-activating protein. In COS cells, coexpression of AS160 and Na⁺,K⁺-ATPase led to the intracellular retention of the sodium pump. We find that AS160 interacts with the large cytoplasmic NP domain of the α-subunit of the Na⁺,K⁺-ATPase. Inhibition of the activity of the adenosine monophosphate-stimulated protein kinase (AMPK) in Madin-Darby canine kidney cells through treatment with Compound C induces Na⁺,K⁺-ATPase endocytosis. This effect of Compound C is prevented through the short hairpin RNA-mediated knockdown of AS160, demonstrating that AMPK and AS160 participate in a common pathway to modulate the cell surface expression of the Na⁺,K⁺-ATPase.

Family-wide characterization of the DENN domain Rab GDP-GTP exchange factors

Target or substrate Rab GTPases are identified for 17 proteins with DENN domains.

A key requirement for Rab function in membrane trafficking is site-specific activation by GDP-GTP exchange factors (GEFs), but the majority of the 63 human Rabs have no known GEF. We have performed a systematic characterization of the 17 human DENN domain proteins and demonstrated that they are specific GEFs for 10 Rabs. DENND1A/1B localize to clathrin patches at the plasma membrane and activate Rab35 in an endocytic pathway trafficking Shiga toxin to the trans-Golgi network. DENND2 GEFs target to actin filaments and control Rab9-dependent trafficking of mannose-6-phosphate receptor to lysosomes. DENND4 GEFs target to a tubular membrane compartment adjacent to the Golgi, where they activate Rab10, which suggests a function in basolateral polarized sorting in epithelial cells that compliments the non-DENN GEF Sec2 acting on Rab8 in apical sorting. DENND1C, DENND3, DENND5A/5B, MTMR5/13, and MADD activate Rab13, Rab12, Rab39, Rab28, and Rab27A/27B, respectively. Together, these findings provide a basis for future studies on Rab regulation and function.

Rab GEFs and GAPs

Rabs are GTP-binding proteins with conserved functions in membrane trafficking. They are regulated by a diverse group of structurally unrelated GDP-GTP exchange factors (GEFs), and a family of GTP-hydrolysis activating proteins (GAPs) containing the conserved TBC domain. Recent structural and cell biological studies shed new light on the mechanisms of Rab GEF and GAP action, and the cellular trafficking pathways they act in.

Rab10 associates with primary cilia and the exocyst complex in renal epithelial cells

Rab10, a mammalian homolog of the yeast Sec4p protein, has previously been associated with endocytic recycling and biosynthetic membrane transport in cultured epithelia and with Glut4 translocation in adipocytes. Here, we report that Rab10 associates with primary cilia in renal epithelia in culture and in vivo. In addition, we find that Rab10 also colocalizes with exocyst proteins at the base of nascent cilia, and physically interacts with the exocyst complex, as detected with anti-Sec8 antibodies. These data suggest that membrane transport to the primary cilum may be mediated by interactions between Rab10 and an exocyst complex located at the cilium base.

EHBP-1 functions with RAB-10 during endocytic recycling in Caenorhabditis elegans

Caenorhabditis elegans RAB-10 functions in endocytic recycling in polarized cells, regulating basolateral cargo transport in the intestinal epithelia and postsynaptic cargo transport in interneurons. Here we show binding of RAB-10 to EHBP-1, a CH-domain protein, and demonstrate a requirement for EHBP-1 in RAB-10–regulated transport in both of these tissues.

Caenorhabditis elegans RAB-10 functions in endocytic recycling in polarized cells, regulating basolateral cargo transport in the intestinal epithelia and postsynaptic cargo transport in interneurons. A similar role was found for mammalian Rab10 in MDCK cells, suggesting that a conserved mechanism regulates these related pathways in metazoans. In a yeast two-hybrid screen for binding partners of RAB-10 we identified EHBP-1, a calponin homology domain (CH) protein, whose mammalian homolog Ehbp1 was previously shown to function during endocytic transport of GLUT4 in adipocytes. In vivo we find that EHBP-1-GFP colocalizes with RFP-RAB-10 on endosomal structures of the intestine and interneurons and that ehbp-1 loss-of-function mutants share with rab-10 mutants specific endosome morphology and cargo localization defects. We also show that loss of EHBP-1 disrupts transport of membrane proteins to the plasma membrane of the nonpolarized germline cells, a defect that can be phenocopied by codepletion of RAB-10 and its closest paralog RAB-8. These results indicate that RAB-10 and EHBP-1 function together in many cell types and suggests that there are differences in the level of redundancy among Rab family members in polarized versus nonpolarized cells.

Insulin regulates alveolar epithelial function by inducing Na+/K+-ATPase translocation to the plasma membrane in a process mediated by the action of Akt

Stimulation of Na(+)/K(+)-ATPase translocation to the cell surface increases active Na(+) transport, which is the driving force of alveolar fluid reabsorption, a process necessary to keep the lungs free of edema and to allow normal gas exchange. Here, we provide evidence that insulin increases alveolar fluid reabsorption and Na(+)/K(+)-ATPase activity by increasing its translocation to the plasma membrane in alveolar epithelial cells. Insulin-induced Akt activation is necessary and sufficient to promote Na(+)/K(+)-ATPase translocation to the plasma membrane. Phosphorylation of AS160 by Akt is also required in this process, whereas inactivation of the Rab GTPase-activating protein domain of AS160 promotes partial Na(+)/K(+)-ATPase translocation in the absence of insulin. We found that Rab10 functions as a downstream target of AS160 in insulin-induced Na(+)/K(+)-ATPase translocation. Collectively, these results suggest that Akt plays a major role in Na(+)/K(+)-ATPase intracellular translocation and thus in alveolar fluid reabsorption.

Apical trafficking in epithelial cells: signals, clusters and motors

In the early days of epithelial cell biology, researchers working with kidney and/or intestinal epithelial cell lines and with hepatocytes described the biosynthetic and recycling routes followed by apical and basolateral plasma membrane (PM) proteins. They identified the trans-Golgi network and recycling endosomes as the compartments that carried out apical-basolateral sorting. They described complex apical sorting signals that promoted association with lipid rafts, and simpler basolateral sorting signals resembling clathrin-coated-pit endocytic motifs. They also noticed that different epithelial cell types routed their apical PM proteins very differently, using either a vectorial (direct) route or a transcytotic (indirect) route. Although these original observations have generally held up, recent studies have revealed interesting complexities in the routes taken by apically destined proteins and have extended our understanding of the machinery required to sustain these elaborate sorting pathways. Here, we critically review the current status of apical trafficking mechanisms and discuss a model in which clustering is required to recruit apical trafficking machineries. Uncovering the mechanisms responsible for polarized trafficking and their epithelial-specific variations will help understand how epithelial functional diversity is generated and the pathogenesis of many human diseases.

The early endosome: a busy sorting station for proteins at the crossroads

Endocytosis marks the entry of internalized receptors into the complex network of endocytic trafficking pathways. Endocytic vesicles are rapidly targeted to a distinct membrane-bound endocytic organelle referred to as the early endosome. Despite the existence of numerous internalization routes, early endosomes (EE) serve as a focal point of the endocytic pathway. Sorting events initiated at this compartment determine the subsequent fate of internalized proteins and lipids, destining them either for recycling to the plasma membrane, degradation in lysosomes or delivery to the trans-Golgi network. Sorting of endocytic cargo to the latter compartments is accomplished through the formation of distinct microdomains within early endosomes, through the coordinate recruitment and assembly of the sorting machinery. An elaborate network of interactions between endocytic regulatory proteins ensures synchronized sorting of cargo to microdomains followed by morphological changes at the early endosomal membranes. Consequently, the cargo targeted either for recycling back to the plasma membrane, or for retrograde transport to the trans-Golgi network, localizes to newly-formed tubular membranes. With a high ratio of membrane surface to lumenal volume, these tubules effectively concentrate the recycling cargo, ensuring efficient transport out of the EE. Conversely, receptors sorted for degradation cluster at the flat clathrin lattices involved in invaginations of the limiting membrane, associating with newly formed intralumenal vesicles. In this review we will discuss the characteristics of early endosomes, their role in the regulation of endocytic transport, and their aberrant function in a variety of diseases.

Amot recognizes a juxtanuclear endocytic recycling compartment via a novel lipid binding domain

Polarity proteins promote the asymmetric organization of cells by orienting intracellular sorting mechanisms, such as protein trafficking and cytoskeletal assembly. The localization of individual polarity proteins in turn is often determined by association with factors that mediate contact with other cells or the substratum. This arrangement for the Par and Crb apical polarity complexes at the tight junction is disrupted by the adaptor protein Amot. Amot directly binds the scaffolding proteins Patj and Mupp1 and redistributes them and their binding partners from the plasma membrane to endosomes. However, the mechanism by which Amot is targeted to endosomes is unknown. Here, a novel lipid binding domain within Amot is shown to selectively bind with high affinity to membranes containing monophosphorylated phosphatidylinositols and cholesterol. With similar lipid specificity, Amot inserts into and tubulates membranes in vitro and enlarges perinuclear endosomal compartments in cells. Based on the similar distribution of Amot with cholesterol, Rab11, and Arf6, such membrane interactions are identified at juxtanuclear endocytic recycling compartments. Taken together, these findings indicate that Amot is targeted along with associated apical polarity proteins to the endocytic recycling compartment via this novel membrane binding domain.

Pathways and mechanisms of endocytic recycling

Endocytic recycling is coordinated with endocytic uptake to control the composition of the plasma membrane. Although much of our understanding of endocytic recycling has come from studies on the transferrin receptor, a protein internalized through clathrin-dependent endocytosis, increased interest in clathrin-independent endocytosis has led to the discovery of new endocytic recycling systems. Recent insights into the regulatory mechanisms that control endocytic recycling have focused on recycling through tubular carriers and the return to the cell surface of cargoes that enter cells through clathrin-independent mechanisms. Recent work emphasizes the importance of regulated recycling in processes as diverse as cytokinesis, cell adhesion, morphogenesis, cell fusion, learning and memory.

The GTPase Rab3b/3c-positive recycling vesicles are involved in cross-presentation in dendritic cells

Antigen cross-presentation in dendritic cells is a complex intracellular membrane transport process, but the underlying molecular mechanisms remain to be thoroughly investigated. In this study, we examined the effect of siRNA-mediated knockdown of 57 Rab GTPases, the key regulators of membrane trafficking, on antigen cross-presentation. Twelve Rab GTPases were identified to be associated with antigen cross-presentation, and Rab3b/3c was indicated to be colocalized with MHC class I molecules at perinuclear tubular structure. Tracing with fluorescence protein-tagged beta(2)-microglobulin demonstrated that the MHC class I molecules were internalized from the plasma membrane to Rab3b/3c-positive compartments, which were also colocalized with the internalized transferrin. Moreover, depletion of Rab3b/3c strongly reduced the fast phase recycling rate of transferrin receptors. Furthermore, the Rab3b/3c-positive compartments were colocalized with a fraction of Rab27a at a juxtaposition of phagosomes. Together, these data demonstrate that Rab3b/3c-positive recycling vesicles are involved in and may constitute one of the recycling compartments in exogenous antigen cross-presentation.

Taking the scenic route: biosynthetic traffic to the plasma membrane in polarized epithelial cells

The maintenance of epithelial cell function requires the establishment and continuous renewal of differentiated apical and basolateral plasma membrane domains with distinct lipid and protein compositions. Newly synthesized proteins destined for either surface domain are processed along the biosynthetic pathway and segregated into distinct subsets of transport carriers emanating from the trans-Golgi network. Recent studies have illuminated additional complexities in the subsequent delivery of these proteins to the cell surface. In particular, multiple routes to the apical and basolateral cell surfaces have been uncovered, and many of these involve indirect passage through endocytic compartments. This review summarizes our current understanding of these routes and discusses open issues that remain to be clarified.

[Identification of proteins in complexes with alpha-latrotoxin receptors]

A thorough analysis of proteins capable of interacting with presynaptic receptors of alpha-latrotoxin was carried out. The protein components of receptor complexes were isolated from rat brain membranes by affinity chromatography on immobilized alpha-latrotoxin and antibodies to the cytoplasmic moiety of the calcium-independent receptor of alpha-latrotoxin (CIRL) followed by analysis by mass spectrometry. Several proteins were identified, with structural proteins, intracellular signal proteins, and proteins involved in the endocytosis and transport of synaptic vesicles being among them.

Macrophage NADPH oxidase flavocytochrome B localizes to the plasma membrane and Rab11-positive recycling endosomes

Flavocytochrome b(558), the catalytic core of the phagocytic NADPH oxidase, mediates the transfer of electrons from NADPH to molecular oxygen to generate superoxide for host defense. Flavocytochrome b is a membrane heterodimer consisting of a large subunit gp91(phox) (NOX2) and a smaller subunit, p22(phox). Although in neutrophils flavocytochrome b has been shown to localize to the plasma membrane and specific granules, little is known about its distribution in macrophages. Using immunofluorescent staining and live cell imaging of fluorescently tagged gp91(phox) and p22(phox), we demonstrate in a Chinese hamster ovary cell model system and in RAW 264.7 and primary murine bone marrow-derived macrophages that flavocytochrome b is found in the Rab11-positive recycling endocytic compartment, as well as in Rab5-positive early endosomes and plasma membrane. Additionally, we show that unassembled p22(phox) and gp91(phox) subunits localize to the endoplasmic reticulum, which redistribute to the cell surface and endosomal compartments following heterodimer formation. These studies show for the first time that flavocytochrome b localizes to intracellular compartments in macrophages that recycle to the plasma membrane, which may act as a reservoir to deliver flavocytochrome b to the cell surface and phagosome membranes.

Mechanisms of EHD/RME-1 protein function in endocytic transport

The evolutionarily conserved Eps15 homology domain (EHD)/receptor-mediated endocytosis (RME)-1 family of C-terminal EH domain proteins has recently come under intense scrutiny because of its importance in intracellular membrane transport, especially with regard to the recycling of receptors from endosomes to the plasma membrane. Recent studies have shed new light on the mode by which these adenosine triphosphatases function on endosomal membranes in mammals and Caenorhabditis elegans. This review highlights our current understanding of the physiological roles of these proteins in vivo, discussing conserved features as well as emerging functional differences between individual mammalian paralogs. In addition, these findings are discussed in light of the identification of novel EHD/RME-1 protein and lipid interactions and new structural data for proteins in this family, indicating intriguing similarities to the Dynamin superfamily of large guanosine triphosphatases.

Alternative splicing in class V myosins determines association with Rab10

Rab proteins influence vesicle trafficking pathways through the assembly of regulatory protein complexes. Previous investigations have documented that Rab11a and Rab8a can interact with the tail region of myosin Vb and regulate distinct trafficking pathways. We have now determined that a related Rab protein, Rab10, can interact with myosin Va, myosin Vb, and myosin Vc. Rab10 localized to a system of tubules and vesicles that have partially overlapping localization with Rab8a. Both Rab8a and Rab10 were mislocalized by the expression of dominant-negative myosin V tails. Interaction with Rab10 was dependent on the presence of the alternatively spliced exon D in myosin Va and myosin Vb and the homologous region in myosin Vc. Yeast two-hybrid assays and fluorescence resonance energy transfer studies confirmed that Rab10 binding to myosin V tails in vivo required the alternatively spliced exon D. In contrast to our previous work, we found that Rab11a can interact with both myosin Va and myosin Vb tails independent of their splice isoform. These results indicate that Rab GTPases regulate diverse endocytic trafficking pathways through recruitment of multiple myosin V isoforms.

Rab13 regulates membrane trafficking between TGN and recycling endosomes in polarized epithelial cells

To maintain polarity, epithelial cells continuously sort transmembrane proteins to the apical or basolateral membrane domains during biosynthetic delivery or after internalization. During biosynthetic delivery, some cargo proteins move from the trans-Golgi network (TGN) into recycling endosomes (RE) before being delivered to the plasma membrane. However, proteins that regulate this transport step remained elusive. In this study, we show that Rab13 partially colocalizes with TGN38 at the TGN and transferrin receptors in RE. Knockdown of Rab13 with short hairpin RNA in human bronchial epithelial cells or overexpression of dominant-active or dominant-negative alleles of Rab13 in Madin-Darby canine kidney cells disrupts TGN38/46 localization at the TGN. Moreover, overexpression of Rab13 mutant alleles inhibits surface arrival of proteins that move through RE during biosynthetic delivery (vesicular stomatitis virus glycoprotein [VSVG], A-VSVG, and LDLR-CT27). Importantly, proteins using a direct route from the TGN to the plasma membrane are not affected. Thus, Rab13 appears to regulate membrane trafficking between TGN and RE.

Use of fluorescence-activated vesicle sorting for isolation of Naked2-associated, basolaterally targeted exocytic vesicles for proteomics analysis

By interacting with the cytoplasmic tail of a Golgi-processed form of transforming growth factor-alpha (TGFalpha), Naked2 coats TGFalpha-containing exocytic vesicles and directs them to the basolateral corner of polarized epithelial cells where the vesicles dock and fuse in a Naked2 myristoylation-dependent manner. These TGFalpha-containing Naked2-associated vesicles are not directed to the subapical Sec6/8 exocyst complex as has been reported for other basolateral cargo, and thus they appear to represent a distinct set of basolaterally targeted vesicles. To identify constituents of these vesicles, we exploited our finding that myristoylation-deficient Naked2 G2A vesicles are unable to fuse at the plasma membrane. Isolation of a population of myristoylation-deficient, green fluorescent protein-tagged G2A Naked2-associated vesicles was achieved by biochemical enrichment followed by flow cytometric fluorescence-activated vesicle sorting. The protein content of these plasma membrane de-enriched, flow-sorted fluorescent G2A Naked2 vesicles was determined by LC/LC-MS/MS analysis. Three independent isolations were performed, and 389 proteins were found in all three sets of G2A Naked2 vesicles. Rab10 and myosin IIA were identified as core machinery, and Na(+)/K(+)-ATPase alpha1 was identified as an additional cargo within these vesicles. As an initial validation step, we confirmed their presence and that of three additional proteins tested (annexin A1, annexin A2, and IQGAP1) in wild-type Naked2 vesicles. To our knowledge, this is the first large scale protein characterization of a population of basolaterally targeted exocytic vesicles and supports the use of fluorescence-activated vesicle sorting as a useful tool for isolation of cellular organelles for comprehensive proteomics analysis.

Rab18 and Rab43 have key roles in ER-Golgi trafficking

Rabs and Arfs/Arls are Ras-related small GTPases of particular relevance to membrane trafficking. It is thought that these proteins regulate specific pathways through interactions with coat, motor, tether and SNARE proteins. We screened a comprehensive list of Arf/Arl/Rab proteins, previously identified on purified Golgi membranes by a proteomics approach (37 in total), for Golgi or intra-Golgi localization, dominant-negative and overexpression phenotypes. Further analysis of two of these proteins, Rab18 and Rab43, strongly indicated roles in ER-Golgi trafficking. Rab43-T32N redistributed Golgi elements to ER exit sites without blocking trafficking of the secretory marker VSVG-GFP from ER to cell surface. Wild-type Rab43 redistributes the p150Glued subunit of dynactin, consistent with a specific role in regulating association of pre-Golgi intermediates with microtubules. Overexpression of wild-type GFP-Rab18 or incubation with any of three siRNAs directed against Rab18 severely disrupts the Golgi complex and reduces secretion of VSVG. Rab18 mutants specifically enhance retrograde Golgi-ER transport of the COPI-independent cargo β-1,4-galactosyltransferase (Galtase)-YFP but not the COPI-dependent cargo p58-YFP from the Golgi to ER in a photobleach assay. Rab18-S22N also potentiated brefeldin-A-induced ER-Golgi fusion. This study is the first comprehensive application of large-scale proteomics to the cell biology of small GTPases of the secretory pathway.

A role for autophagy in the extension of lifespan by dietary restriction in C. elegans

In many organisms, dietary restriction appears to extend lifespan, at least in part, by down-regulating the nutrient-sensor TOR (Target Of Rapamycin). TOR inhibition elicits autophagy, the large-scale recycling of cytoplasmic macromolecules and organelles. In this study, we asked whether autophagy might contribute to the lifespan extension induced by dietary restriction in C. elegans. We find that dietary restriction and TOR inhibition produce an autophagic phenotype and that inhibiting genes required for autophagy prevents dietary restriction and TOR inhibition from extending lifespan. The longevity response to dietary restriction in C. elegans requires the PHA-4 transcription factor. We find that the autophagic response to dietary restriction also requires PHA-4 activity, indicating that autophagy is a transcriptionally regulated response to food limitation. In spite of the rejuvenating effect that autophagy is predicted to have on cells, our findings suggest that autophagy is not sufficient to extend lifespan. Long-lived daf-2 insulin/IGF-1 receptor mutants require both autophagy and the transcription factor DAF-16/FOXO for their longevity, but we find that autophagy takes place in the absence of DAF-16. Perhaps autophagy is not sufficient for lifespan extension because although it provides raw material for new macromolecular synthesis, DAF-16/FOXO must program the cells to recycle this raw material into cell-protective longevity proteins.

A role for EHD4 in the regulation of early endosomal transport

All four of the C-terminal Eps15 homology domain (EHD) proteins have been implicated in the regulation of endocytic trafficking. However, the high level of amino acid sequence identity among these proteins has made it challenging to elucidate the precise function of individual EHD proteins. We demonstrate here with specific peptide antibodies that endogenous EHD4 localizes to Rab5-, early embryonic antigen 1 (EEA1)- and Arf6-containing endosomes and colocalizes with internalized transferrin in the cell periphery. Knock-down of EHD4 expression by both small interfering RNA and short hairpin RNA leads to the generation of enlarged early endosomal structures that contain Rab5 and EEA1 as well as internalized transferrin or major histocompatibility complex class I molecules. In addition, cargo destined for degradation, such as internalized low-density lipoprotein, also accumulates in the enlarged early endosomes in EHD4-depleted cells. Moreover, we have demonstrated that these enlarged early endosomes are enriched in levels of the activated GTP-bound Rab5. Finally, we show that endogenous EHD4 and EHD1 interact in cells, suggesting coordinated involvement in the regulation of receptor transport along the early endosome to endocytic recycling compartment axis. The results presented herein provide evidence that EHD4 is involved in the control of trafficking at the early endosome and regulates exit of cargo toward both the recycling compartment and the late endocytic pathway.

KCNQ1 and KCNE1 K+ channel components are involved in early left-right patterning in Xenopus laevis embryos

Several ion transporters have been implicated in left-right (LR) patterning. Here, we characterize a new component of the early bioelectrical circuit: the potassium channel KCNQ1 and its accessory subunit KCNE1. Having cloned the native Xenopus versions of both genes, we show that both are asymmetrically localized as maternal proteins during the first few cleavages of frog embryo development in a process dependent on microtubule and actin organization. Molecular loss-of-function using dominant negative constructs demonstrates that both gene products are required for normal LR asymmetry. We propose a model whereby these channels provide an exit path for K(+) ions brought in by the H(+),K(+)-ATPase. This physiological module thus allows the obligate but electroneutral H(+),K(+)-ATPase to generate an asymmetric voltage gradient on the left and right sides. Our data reveal a new, bioelectrical component of the mechanisms patterning a large-scale axis in vertebrate embryogenesis.

Improving Signal Levels in Intravital Multiphoton Microscopy using an Objective Correction Collar

Multiphoton microscopy has enabled biologists to collect high-resolution images hundreds of microns into biological tissues, including tissues of living animals. While the depth of imaging exceeds that possible from any other form of light microscopy, multiphoton microscopy is nonetheless generally limited to depths of less than a millimeter. Many of the advantages of multiphoton microscopy for deep tissue imaging accrue from the unique nature of multiphoton fluorescence excitation. However, the quadratic relationship between illumination level and fluorescence excitation makes multiphoton microscopy especially susceptible to factors that degrade the illumination focus. Here we examine the effect of spherical aberration on multiphoton microscopy in fixed kidney tissues and in the kidneys of living animals. We find that spherical aberration, as evaluated from axial asymmetry in the point spread function, can be corrected by adjustment of the correction collar of a water immersion objective lens. Introducing a compensatory positive spherical aberration into the imaging system decreased the depth-dependence of signal levels in images collected from living animals, increasing signal by up to 50%.

Regulation of membrane trafficking in polarized epithelial cells

Polarized epithelial cells continuously sort transmembrane proteins to either apical or basolateral plasma membrane domains. Research in recent years has made tremendous progress in understanding the molecular mechanisms of the major pathways to either basolateral or apical domain. This understanding will help us elucidating how these pathways are interconnected in ensuring maintenance of cell polarity and integrity of epithelial monolayers.

RAB-10 regulates glutamate receptor recycling in a cholesterol-dependent endocytosis pathway

Regulated endocytosis of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors (AMPARs) is critical for synaptic plasticity. However, the specific combination of clathrin-dependent and -independent mechanisms that mediate AMPAR trafficking in vivo have not been fully characterized. Here, we examine the trafficking of the AMPAR subunit GLR-1 in Caenorhabditis elegans. GLR-1 is localized on synaptic membranes, where it regulates reversals of locomotion in a simple behavioral circuit. Animals lacking RAB-10, a small GTPase required for endocytic recycling of intestinal cargo, are similar in phenotype to animals lacking LIN-10, a postsynaptic density 95/disc-large/zona occludens-domain containing protein: GLR-1 accumulates in large accretions and animals display a decreased frequency of reversals. Mutations in unc-11 (AP180) or itsn-1 (Intersectin 1), which reduce clathrin-dependent endocytosis, suppress the lin-10 but not rab-10 mutant phenotype, suggesting that LIN-10 functions after clathrin-mediated endocytosis. By contrast, cholesterol depletion, which impairs lipid raft formation and clathrin-independent endocytosis, suppresses the rab-10 but not the lin-10 phenotype, suggesting that RAB-10 functions after clathrin-independent endocytosis. Animals lacking both genes display additive GLR-1 trafficking defects. We propose that RAB-10 and LIN-10 recycle AMPARs from intracellular endosomal compartments to synapses along distinct pathways, each with distinct sensitivities to cholesterol and the clathrin-mediated endocytosis machinery.

Melanophilin, a novel aldosterone-induced gene in mouse cortical collecting duct cells

The molecular mechanisms of aldosterone-regulated Na+ transport are not entirely clear. The goal of this study was to identify aldosterone-induced genes potentially involved in the trafficking of the epithelial Na+ channel (ENaC). We report that the transcript levels of melanophilin (MLPH), a protein involved in vesicular trafficking in melanocytes, are rapidly increased by aldosterone in cortical collecting duct (CCD) cells. This effect was near maximal at physiological aldosterone concentrations, indicating that it is mediated by the mineralocorticoid receptor. De novo protein synthesis is not required for the induction of MLPH mRNA by aldosterone. To determine whether this induction has functional consequences on transepithelial Na+ current, we generated clonal CCD cell lines that express a tetracycline-inducible MLPH. Induction of MLPH in these cells led to a relatively modest, but statistically significant, increase in amiloride-sensitive Na+ current, suggesting the MLPH may be involved in ENaC trafficking. MyosinVc, the epithelial-specific class V myosin that is highly homologous to MyosinVa, another component of the melanosome trafficking complex, has putative consensus sites for serum and glucocorticoid-induced kinase 1 (SGK1), an early aldosterone-induced kinase that mediates some of aldosterone's effects on Na+ transport. Our results indicate that MyosinVc is phosphorylated by endogenous SGK1, suggesting that this complex may be involved in the aldosterone-regulated trafficking of ENaC in the CCD. These results suggest potential mechanisms by which aldosterone may regulate Na+ transport both directly, by increasing the abundance of MLPH, and indirectly by increasing the transcription of SGK1, which in turn regulates the activity of MyosinVc.

Characterization of immunoisolated human gastric parietal cells tubulovesicles: identification of regulators of apical recycling

Gastric parietal cells possess an amplified apical membrane recycling system dedicated to regulated apical recycling of H-K-ATPase. While amplified in parietal cells, apical recycling is critical to polarized secretory processes in most epithelial cells. To clarify putative regulators of apical recycling, we prepared immunoisolated parietal cell H-K-ATPase-containing recycling membranes from human stomachs and analyzed protein contents by tryptic digestion and mass spectrometry. We identified and validated by Western blots many of the proteins previously identified on immunoisolated rabbit tubulovesicles, including Rab11, Rab25, syntaxin 3, secretory carrier membrane proteins (SCAMPs), and vesicle-associated membrane protein (VAMP)2. In addition, we detected several previously unrecognized proteins, including Rab10, VAMP8, syntaxin 7, and syntaxin 12/13. We also identified the K(+) channel component KCNQ1. Immunostaining of human gastric mucosal sections confirmed the presence of each of these proteins in parietal cells and their colocalization with H-K-ATPase on tubulovesicles. To investigate the role of the identified soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins in apical recycling, we transfected them as DsRed2 fusions into an enhanced green fluorescent protein (EGFP)-Rab11a-expressing Madin-Darby canine kidney (MDCK) cell line. Syntaxin 12/13 and VAMP8 caused a collapse of the EGFP-Rab11a compartment, whereas a less dramatic effect was observed in cells transfected with syntaxin 3, syntaxin 7, or VAMP2. The five DsRed2-SNARE chimeras were also transfected into a MDCK cell line overexpressing Rab11-FIP2(129-512). All five of the chimeras were drawn into the collapsed apical recycling system. This study, which represents the first proteomic analysis of an immunoisolated vesicle population from native human tissue, demonstrates the diversity of putative regulators of the apical recycling system.

Receptor complexes cotransported via polarized endocytic pathways form clusters with distinct organizations

Previously, FRET confocal microscopy has shown that polymeric IgA-receptor (pIgA-R) is distributed in a clustered manner in apical endosomes. To test whether different membrane-bound components form clusters during membrane trafficking, live-cell quantitative FRET was used to characterize the organization of pIgA-R and transferrin receptor (TFR) in endocytic membranes of polarized MDCK cells upon internalization of donor- and acceptor-labeled ligands. We show that pIgA-R and TFR complexes form increasingly organized clusters during cotransport from basolateral to perinuclear endosomes. The organization of these receptor clusters in basolateral versus perinuclear/apical endosomes is significantly different; the former showing a mixed random/clustered distribution while the latter highly organized clusters. Our results indicate that although both perinuclear and apical endosomes comprise pIgA-R and TFR clusters, their E% levels are significantly different suggesting that these receptors are packed into clusters in a distinct manner. The quantitative FRET-based assay presented here suggests that different receptor complexes form clusters, with diverse levels of organization, while being cotransported via the polarized endocytic pathways.

Rab10, a target of the AS160 Rab GAP, is required for insulin-stimulated translocation of GLUT4 to the adipocyte plasma membrane

GLUT4 trafficking to the plasma membrane of muscle and fat cells is regulated by insulin. An important component of insulin-regulated GLUT4 distribution is the Akt substrate AS160 rab GTPase-activating protein. Here we show that Rab10 functions as a downstream target of AS160 in the insulin-signaling pathway that regulates GLUT4 translocation in adipocytes. Overexpression of a mutant of Rab10 defective for GTP hydrolysis increased GLUT4 on the surface of basal adipocytes. Rab10 knockdown resulted in an attenuation of insulin-induced GLUT4 redistribution to the plasma membrane and a concomitant 2-fold decrease in GLUT4 exocytosis rate. Re-expression of a wild-type Rab10 restored normal GLUT4 translocation. The basal increase in plasma-membrane GLUT4 due to AS160 knockdown was partially blocked by knocking down Rab10 in the same cells, further indicating that Rab10 is a target of AS160 and a positive regulator of GLUT4 trafficking to the cell surface upon insulin stimulation.

Rabs 8A and 14 are targets of the insulin-regulated Rab-GAP AS160 regulating GLUT4 traffic in muscle cells

Insulin causes translocation of glucose transporter GLUT4 to the membrane of muscle and fat cells, a process requiring Akt activation. The Rab GTPase-activating protein (Rab-GAP) AS160 is inhibited upon phosphorylation by insulin-activated Akt, thereby allowing GLUT4 translocation. Although several Rab proteins are detected on GLUT4 vesicles, the target Rabs of AS160 involved in the GLUT4 translocation have not been identified. We test whether Rabs 8A, 10, and 14 (in vitro targets of AS160) rescue the inhibition of GLUT4 translocation caused by 'constitutively active' 4P-AS160 in L6 muscle cells. Coexpression of GFP-tagged Rabs 8A or Rab14 with 4P-AS160 prevented the inhibition of GLUT4 translocation imposed by 4P-AS160. GFP-tagged, constitutively active Rab8A also elicited this rescue. In contrast, neither wild-type nor constitutively active GFP-tagged Rab10 restored GLUT4 translocation. These results suggest that Rab8A and possibly Rab14 may be targets of AS160 leading to GLUT4 translocation in L6 muscle cells.

Rab10 is involved in basolateral transport in polarized Madin-Darby canine kidney cells

The sorting of newly synthesized membrane proteins to the cell surface is an important mechanism of cell polarity. To identify more of the molecular machinery involved, we investigated the function of the small GTPase Rab10 in polarized epithelial Madin-Darby canine kidney cells. We find that GFP-tagged Rab10 localizes primarily to the Golgi during early cell polarization. Expression of an activated Rab10 mutant inhibits biosynthetic transport from the Golgi and missorts basolateral cargo to the apical membrane. Depletion of Rab10 by RNA interference has only mild effects on biosynthetic transport and epithelial polarization, but simultaneous inhibition of Rab10 and Rab8a more strongly impairs basolateral sorting. These results indicate that Rab10 functions in trafficking from the Golgi at early stages of epithelial polarization, is involved in biosynthetic transport to the basolateral membrane and may co-operate with Rab8.