Recycling endosomes can serve as intermediates during transport from the Golgi to the plasma membrane of MDCK cells

The function of the intermediate compartment in pre-Golgi trafficking involves its stable connection with the centrosome

Because the functional borders of the intermediate compartment (IC) are not well defined, the spatial map of the transport machineries operating between the endoplasmic reticulum (ER) and the Golgi apparatus remains incomplete. Our previous studies showed that the IC consists of interconnected vacuolar and tubular parts with specific roles in pre-Golgi trafficking. Here, using live cell imaging, we demonstrate that the tubules containing the GTPase Rab1A create a long-lived membrane compartment around the centrosome. Separation of this pericentrosomal domain of the IC from the Golgi ribbon, due to centrosome motility, revealed that it contains a distinct pool of COPI coats and acts as a temperature-sensitive way station in post-ER trafficking. However, unlike the Golgi, the pericentrosomal IC resists the disassembly of COPI coats by brefeldin A, maintaining its juxtaposition with the endocytic recycling compartment, and operation as the focal point of a dynamic tubular network that extends to the cell periphery. These results provide novel insight into the compartmental organization of the secretory pathway and Golgi biogenesis. Moreover, they reveal a direct functional connection between the IC and the endosomal system, which evidently contributes to unconventional transport of the cystic fibrosis transmembrane conductance regulator to the cell surface.

SH3TC2/KIAA1985 protein is required for proper myelination and the integrity of the node of Ranvier in the peripheral nervous system

Charcot-Marie-Tooth disease type 4C (CMT4C) is an early-onset, autosomal recessive form of demyelinating neuropathy. The clinical manifestations include progressive scoliosis, delayed age of walking, muscular atrophy, distal weakness, and reduced nerve conduction velocity. The gene mutated in CMT4C disease, SH3TC2/KIAA1985, was recently identified; however, the function of the protein it encodes remains unknown. We have generated knockout mice where the first exon of the Sh3tc2 gene is replaced with an enhanced GFP cassette. The Sh3tc2(DeltaEx1/DeltaEx1) knockout animals develop progressive peripheral neuropathy manifested by decreased motor and sensory nerve conduction velocity and hypomyelination. We show that Sh3tc2 is specifically expressed in Schwann cells and localizes to the plasma membrane and to the perinuclear endocytic recycling compartment, concordant with its possible function in myelination and/or in regions of axoglial interactions. Concomitantly, transcriptional profiling performed on the endoneurial compartment of peripheral nerves isolated from control and Sh3tc2(DeltaEx1/DeltaEx1) animals uncovered changes in transcripts encoding genes involved in myelination and cell adhesion. Finally, detailed analyses of the structures composed of compact and noncompact myelin in the peripheral nerve of Sh3tc2(DeltaEx1/DeltaEx1) animals revealed abnormal organization of the node of Ranvier, a phenotype that we confirmed in CMT4C patient nerve biopsies. The generated Sh3tc2 knockout mice thus present a reliable model of CMT4C neuropathy that was instrumental in establishing a role for Sh3tc2 in myelination and in the integrity of the node of Ranvier, a morphological phenotype that can be used as an additional CMT4C diagnostic marker.

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.

Pollen tube growth: a delicate equilibrium between secretory and endocytic pathways

Although pollen tube growth is a prerequisite for higher plant fertilization and seed production, the processes leading to pollen tube emission and elongation are crucial for understanding the basic mechanisms of tip growth. It was generally accepted that pollen tube elongation occurs by accumulation and fusion of Golgi-derived secretory vesicles (SVs) in the apical region, or clear zone, where they were thought to fuse with a restricted area of the apical plasma membrane (PM), defining the apical growth domain. Fusion of SVs at the tip reverses outside cell wall material and provides new segments of PM. However, electron microscopy studies have clearly shown that the PM incorporated at the tip greatly exceeds elongation and a mechanism of PM retrieval was already postulated in the mid-nineteenth century. Recent studies on endocytosis during pollen tube growth showed that different endocytic pathways occurred in distinct zones of the tube, including the apex, and led to a new hypothesis to explain vesicle accumulation at the tip; namely, that endocytic vesicles contribute substantially to V-shaped vesicle accumulation in addition to SVs and that exocytosis does not involve the entire apical domain. New insights suggested the intriguing hypothesis that modulation between exo- and endocytosis in the apex contributes to maintain PM polarity in terms of lipid/protein composition and showed distinct degradation pathways that could have different functions in the physiology of the cell. Pollen tube growth in vivo is closely regulated by interaction with style molecules. The study of endocytosis and membrane recycling in pollen tubes opens new perspectives to studying pollen tube-style interactions in vivo.

Membrane proteins follow multiple pathways to the basolateral cell surface in polarized epithelial cells

Newly synthesized apical and basolateral membrane proteins are sorted from one another in polarized epithelial cells. The trans-Golgi network participates in this sorting process, but some basolateral proteins travel from the Golgi to recycling endosomes (REs) before their surface delivery. Using a novel system for pulse–chase microscopy, we have visualized the postsynthetic route pursued by a newly synthesized cohort of Na,K-ATPase. We find that the basolateral delivery of newly synthesized Na,K-ATPase occurs via a pathway distinct from that pursued by the vesicular stomatitis virus G protein (VSV-G). Na,K-ATPase surface delivery occurs at a faster rate than that observed for VSV-G. The Na,K-ATPase does not pass through the RE compartment en route to the plasma membrane, and Na,K-ATPase trafficking is not regulated by the same small GTPases as other basolateral proteins. Finally, Na,K-ATPase and VSV-G travel in separate post-Golgi transport intermediates, demonstrating directly that multiple routes exist for transport from the Golgi to the basolateral membrane in polarized epithelial cells.

Uncoordinated 119 protein controls trafficking of Lck via the Rab11 endosome and is critical for immunological synapse formation

The activation of T cells through the TCR is essential for development of the adaptive immune response. TCR does not have any enzymatic activity and relies on the plasma membrane-associated lymphocyte-specific protein tyrosine kinase (Lck) for initiation of signaling. Here we uncover a mechanism that is responsible for plasma membrane targeting of Lck. We show that Lck is transported to the membrane via a specific endosomal compartment. The transport depends on the adaptor protein Uncoordinated 119 (Unc119), on the GTPase rat brain 11 (Rab11), and on the actin cytoskeleton. Unc119 regulates the activation of Rab11. Consequently, Unc119 orchestrates the recruitment of the actin-based motor protein, myosin 5B, and the organization of multiprotein complexes on endosomes. The Unc119-regulated pathway is essential for immunological synapse formation and T cell activation.

Clustering and lateral concentration of raft lipids by the MAL protein

MAL, a compact hydrophobic, four-transmembrane-domain apical protein that copurifies with detergent-resistant membranes is obligatory for the machinery that sorts glycophosphatidylinositol (GPI)-anchored proteins and others to the apical membrane in epithelia. The mechanism of MAL function in lipid-raft-mediated apical sorting is unknown. We report that MAL clusters formed by two independent procedures-spontaneous clustering of MAL tagged with the tandem dimer DiHcRED (DiHcRED-MAL) in the plasma membrane of COS7 cells and antibody-mediated cross-linking of FLAG-tagged MAL-laterally concentrate markers of sphingolipid rafts and exclude a fluorescent analogue of phosphatidylethanolamine. Site-directed mutagenesis and bimolecular fluorescence complementation analysis demonstrate that MAL forms oligomers via xx intramembrane protein-protein binding motifs. Furthermore, results from membrane modulation by using exogenously added cholesterol or ceramides support the hypothesis that MAL-mediated association with raft lipids is driven at least in part by positive hydrophobic mismatch between the lengths of the transmembrane helices of MAL and membrane lipids. These data place MAL as a key component in the organization of membrane domains that could potentially serve as membrane sorting platforms.

Roles of endosomal trafficking in neurite outgrowth and guidance

Membrane trafficking and cargo delivery are essential for axonal and dendritic growth and guidance. Neurons have numerous diverse post-Golgi vesicles and recent advances have clarified their identity and regulation. Combinatorial approaches using in vivo imaging of 'intracellular cargo address labels' and functional perturbation have provided insight into these processes. In particular, the UNC-51 kinase regulates the trafficking of early endosomes and their axon guidance molecular cargos in several types of neurons in multiple organisms. Vesicular compartments bearing features of recycling endosomes, late endosomes or lysosomes also contribute to membrane addition and protein trafficking during neurite outgrowth and extension. New work shows that ubiquitylation of cargos and Rab effectors further specifies the trafficking routes of post-Golgi vesicles. These findings have begun to provide a more detailed view of the molecular mechanisms involved in neurite outgrowth and guidance. Additionally, high-resolution light microscopy imaging promises greater temporal and spatial understanding of vesicular exchange and maturation in neurons in the near future.

Monomeric IgG is neuroprotective via enhancing microglial recycling endocytosis and TNF-alpha

In brain, monomeric immunoglobin G (IgG) is regarded as quiescent and only poised to initiate potentially injurious inflammatory reactions via immune complex formation associated with phagocytosis and tumor necrosis factor alpha (TNF-alpha) production in response to disease. Using rat hippocampal slice and microglial cultures, here we show instead that physiological levels (i.e., 0.2-20 microg/ml) of monomeric IgG unassociated with disease triggered benign low-level proinflammatory signaling that was neuroprotective against CA1 area excitotoxicity and followed a U-shaped or hormetic dose-response. The data indicate that physiological IgG levels activated microglia by enhancing recycling endocytosis plus TNF-alpha release from these cells to produce the neuroprotection. Minocycline, known for its anti-inflammatory and neuroprotective effects when given after disease onset, abrogated IgG-mediated neuroprotection and related microglial effects when given before injury. In contrast, E-prostanoid receptor subtype 2 (EP2) activation, which served as an exemplary paracrine stimulus like the one expected from neuronal activity, amplified IgG-mediated increased microglial recycling endocytosis and TNF-alpha production. Furthermore, like monomeric IgG these EP2 related effects took days to be effective, suggesting both were adaptive anabolic effects consistent with those seen from other long-term preconditioning stimuli requiring de novo protein synthesis. The data provide the first evidence that brain monomeric IgG at physiological levels can have signaling function via enhanced recycling endocytosis/TNF-alpha production from microglia unassociated with disease and that these IgG-mediated changes may be a means by which paracrine signaling from neuronal activity influences microglia to evoke neuroprotection. The data provide further support that low-level proinflammatory neural immune signaling unassociated with disease enhances brain function.

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.

Sdmg1 is a component of secretory granules in mouse secretory exocrine tissues

Sdmg1 is a conserved eukaryotic transmembrane protein that is mainly expressed in the gonads where it may have a role in mediating signaling between somatic cells and germ cells. In this study we demonstrate that secretory exocrine cells in the pancreas, salivary gland, and mammary gland also express Sdmg1. Furthermore, we show that Sdmg1 expression is up-regulated during pancreas development when regulated secretory granules start to appear, and that Sdmg1 colocalizes with secretory granule markers in adult pancreatic acinar cells. In addition, we show that Sdmg1 co-purifies with secretory granules during subcellular fractionation of the pancreas and that Sdmg1 and the secretory granule marker Vamp2 are localized to distinct subdomains in the secretory granule membrane. These data suggest that Sdmg1 is a component of regulated secretory granules in exocrine secretory cells and that the developmental regulation of Sdmg1 expression is related to a role for Sdmg1 in post-Golgi membrane trafficking.

Out, in and back again: PtdIns(4,5)P(2) regulates cadherin trafficking in epithelial morphogenesis

The morphogenesis of epithelial cells in the tissue microenvironment depends on the regulation of the forces and structures that keep cells in contact with their neighbours. The formation of cell-cell contacts is integral to the establishment and maintenance of epithelial morphogenesis. In epithelial tissues, the misregulation of the signalling pathways that control epithelial polarization induces migratory and invasive cellular phenotypes. Many cellular processes influence cadherin targeting and function, including exocytosis, endocytosis and recycling. However, the localized generation of the lipid messenger PtdIns(4,5)P(2) is emerging as a fundamental signal controlling all of these processes. The PtdIns(4,5)P(2)-generating enzymes, PIPKs (phosphatidylinositol phosphate kinases) are therefore integral to these pathways. By the spatial and temporal targeting of PIPKs via the actions of its functional protein associates, PtdIns(4,5)P(2) is generated at discrete cellular locales to provide the cadherin-trafficking machinery with its required lipid messenger. In the present review, we discuss the involvement of PtdIns(4,5)P(2) and the PIPKs in the regulation of the E-cadherin (epithelial cadherin) exocytic and endocytic machinery, the modulation of actin structures at sites of adhesion, and the direction of cellular pathways which determine the fate of E-cadherin and cell-cell junctions. Recent work is also described that has defined phosphoinositide-mediated E-cadherin regulatory pathways by the use of organismal models.

Galectin-4-regulated delivery of glycoproteins to the brush border membrane of enterocyte-like cells

We have previously reported that silencing of galectin-4 expression in polarized HT-29 cells perturbed apical biosynthetic trafficking and resulted in a phenotype similar to the inhibitor of glycosylation, 1-benzyl-2-acetamido-2-deoxy-beta-d-galactopyranoside (GalNAcalpha-O-bn). We now present evidence of a lipid raft-based galectin-4-dependent mechanism of apical delivery of glycoproteins in these cells. First, galectin-4 recruits the apical glycoproteins in detergent-resistant membranes (DRMs) because these glycoproteins were depleted in DRMs isolated from galectin-4-knockdown (KD) HT-29 5M12 cells. DRM-associated glycoproteins were identified as ligands for galectin-4. Structural analysis showed that DRMs were markedly enriched in a series of complex N-glycans in comparison to detergent-soluble membranes. Second, in galectin-4-KD cells, the apical glycoproteins still exit the Golgi but accumulated inside the cells, showing that their recruitment within lipid rafts and their apical trafficking required the delivery of galectin-4 at a post-Golgi level. This lectin that is synthesized on free cytoplasmic ribosomes is externalized from HT-29 cells mostly in the apical medium and follows an apical endocytic-recycling pathway that is required for the apical biosynthetic pathway. Together, our data show that the pattern of N-glycosylation of glycoproteins serves as a recognition signal for endocytosed galectin-4, which drives the raft-dependent apical pathway of glycoproteins in enterocyte-like HT-29 cells.

Clathrin adaptor AP-1 complex excludes multiple postsynaptic receptors from axons in C. elegans

Neurons are highly polarized cells with morphologically and molecularly distinct axonal and dendritic compartments. It is not well understood how postsynaptic receptors are selectively enriched in dendrites in vivo. We investigated the molecular mechanisms of dendritically polarized localization of a glutamate receptor, an acetylcholine receptor, and a ROR-type receptor tyrosine kinase in the interneuron RIA in C. elegans. We found that the clathrin adaptor AP-1 complex mu1 subunit UNC-101 functions cell autonomously to maintain the correct localization of these receptors in a dynamin-dependent manner. In unc-101 mutants, instead of being dendritically enriched, all 3 receptors are evenly distributed in the axonal and dendritic compartments. Surprisingly, UNC-101 predominantly localizes to the axonal compartment, suggesting a possible transcytosis model for the dendritic targeting of neurotransmitter receptors.

Epithelial cell surface polarity: the early steps

Establishment and maintenance of epithelial cell surface polarity is of vital importance for the correct function of transporting epithelia. To maintain normal cell function, the distribution of apical and basal-lateral proteins is highly regulated and defects in expression levels or plasma membrane targeting can have severe consequences. It has been shown recently that initiation of cell-surface polarity occurs immediately upon cell-cell contact, and requires components of the lateral targeting patch, the Exocyst and the lateral SNARE complex to specify delivery of basolateral proteins to the site of cell-cell adhesion. The Exocyst and SNARE complex are present in the cytoplasm in single epithelial cells before adhesion. Upon initial cell-cell adhesion, E-cadherin accumulates at the forming contact between cells. Shortly hereafter, components of the lateral targeting patch, the Exocyst and the lateral SNARE complex, co-localize with E-cadherin at the forming contact, where they function in specifying the delivery of basal-lateral.

Effects of colchicine on localization of alkaline phosphatase in McA-RH 7777 rat hepatoma cells

We investigated the changes caused by microtubule disruption in cell contact-induced translocation of alkaline phosphatase (ALP) from the Golgi area to the plasma membrane in McA-RH 7777 cells. When the cells were treated with colchicine, the tubular structure of microtubules in the cytoplasm was lost. Colchicine treatment also resulted in the appearance of numerous dots containing mannosidase II (man II) throughout the cytoplasm. Moreover, ALP was distributed in small dots throughout the cytoplasm, as well as in all regions of the plasma membrane, although it was most concentrated at sites of intercellular contact. On the other hand, when the cells were incubated in basal medium after colchicine treatment, large spots containing ALP reappeared in the perinuclear cytoplasm more quickly than the accumulation of small dots containing man II. These findings suggest that colchicine causes disassembly of the Golgi complex into fragments, which scatter throughout the cytoplasm, but that it does not interfere with translocation of ALP to the plasma membrane. Furthermore, cytoplasmic ALP may be localized at sites other than the Golgi complex.

Rap2 function requires palmitoylation and recycling endosome localization

Rap2A, Rap2B, and Rap2C are Ras-like small G proteins. The role of their post-translational processing has not been investigated due to the lack of information on their downstream signaling. We have recently identified the Traf2- and Nck-interacting kinase (TNIK), a member of the STE20 group of mitogen-activated protein kinase kinase kinase kinases, as a specific Rap2 effector. Here we report that, in HEK293T cells, Rap2A (farnesylated) and Rap2C (likely farnesylated), but not Rap2B (geranylgeranylated), require palmitoylation for membrane-association and TNIK activation, whereas all Rap2 proteins, including Rap2B, require palmitoylation for induction of TNIK-mediated phenotype, the suppression of cell spreading. Furthermore, we report for the first time that, in COS-1 cells, Rap2 proteins localize, and recruit TNIK, to the recycling endosomes, but not the Golgi nor the endoplasmic reticulum, in a palmitoylation-dependent manner. These observations implicate the involvement of palmitoylation and recycling endosome localization in cellular functions of Rap2 proteins.

Polarized traffic of LRP1 involves AP1B and SNX17 operating on Y-dependent sorting motifs in different pathways

Low-density lipoprotein receptor-related protein 1 (LRP1) is an endocytic recycling receptor with two cytoplasmic tyrosine-based basolateral sorting signals. Here we show that during biosynthetic trafficking LRP1 uses AP1B adaptor complex to move from a post-TGN recycling endosome (RE) to the basolateral membrane. Then it recycles basolaterally from the basolateral sorting endosome (BSE) involving recognition by sorting nexin 17 (SNX17). In the biosynthetic pathway, Y(29) but not N(26) from a proximal NPXY directs LRP1 basolateral sorting from the TGN. A N(26)A mutant revealed that this NPXY motif recognized by SNX17 is required for the receptor's exit from BSE. An endocytic Y(63)ATL(66) motif also functions in basolateral recycling, in concert with an additional endocytic motif (LL(86,87)), by preventing LRP1 entry into the transcytotic apical pathway. All this sorting information operates similarly in hippocampal neurons to mediate LRP1 somatodendritic distribution regardless of the absence of AP1B in neurons. LRP1 basolateral distribution results then from spatially and temporally segregation steps mediated by recognition of distinct tyrosine-based motifs. We also demonstrate a novel function of SNX17 in basolateral/somatodendritic recycling from a different compartment than AP1B endosomes.

Roles for the recycling endosome, Rab8, and Rab11 in hantavirus release from epithelial cells

Hantavirus structural proteins are believed to localize to intracellular membranes often identified as Golgi membranes, in virus-infected cells. After virus budding into the Golgi luminal space, virus-containing vesicles are transported to the plasma membrane via trafficking pathways that are not well defined. Using the New World hantavirus, Andes virus, we have investigated the role of various Rab proteins in the release of hantavirus particles from infected cells. Rabs 8 and 11 were found to colocalize with Andes virus proteins in virus infected cells and when expressed from cDNA, implicating the recycling endosome as an organelle important for hantavirus infection. Small interfering RNA-mediated downregulation of Rab11a alone or Rab11a and Rab11b together resulted in a decrease in infectious virus particle secretion from infected cells. Downregulation of Rab8a did not alter infectious virus release but reduction of both isoforms did. These data implicate the recycling endosome and the Rab proteins associated with vesicular transport to or from this intracellular organelle as an important pathway for hantavirus trafficking to the plasma membrane.

Coordinated protein sorting, targeting and distribution in polarized cells

The polarized distribution of functions in polarized cells requires the coordinated interaction of three machineries that modify the basic mechanisms of intracellular protein trafficking and distribution. First, intrinsic protein-sorting signals and cellular decoding machineries regulate protein trafficking to plasma membrane domains; second, intracellular signalling complexes define the plasma membrane domains to which proteins are delivered; and third, proteins that are involved in cell-cell and cell-substrate adhesion orientate the three-dimensional distribution of intracellular signalling complexes and, accordingly, the direction of membrane traffic. The integration of these mechanisms into a complex and dynamic network is crucial for normal tissue function and is often defective in disease states.

Role of Mammalian coronin 7 in the biosynthetic pathway

Most coronin proteins rely on interaction with actin in their functions. Mammalian coronin 7 has not been shown to interact with actin, but rather to bind to the outer side of Golgi complex membranes. Targeting of coronin 7 to Golgi membranes requires the activity of Src kinase and integrity of AP-1 adaptor protein complex. Coronin 7 further physically interacts with both AP-1 and Src in vivo and in vitro and is phosphorylated by Src. Depletion of coronin 7 by RNAi results in Golgi breakdown and accumulation of arrested cargo proteins, suggesting the protein functions in the later stages of cargo sorting and export from the Golgi complex. We suggest that coronin 7 acts as a mediator of cargo vesicle formation at the trans-Golgi network (TGN) downstream of AP-1 interaction with cargo but upstream of protein kinase D dependent membrane fission.

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.

Apical cargo traverses endosomal compartments on the passage to the cell surface

Epithelial polarity is based on intracellular sorting machinery that maintains the asymmetric distribution of lipids and proteins to the cell surface. Dependent on their lipid raft affinity, newly synthesized apical polypeptides are segregated into distinct vesicle populations subsequent to the passage through the Golgi apparatus. Using a combined fluorescence microscopic and biochemical approach, we found that lipid raft-associated sucrase-isomaltase (SI) as well as non-raft-associated lactase-phlorizin hydrolase (LPH) traverse endosomal compartments before entering the apical membrane. Fluorescent fusion proteins of both hydrolases were co-stained with Rab4-, Rab8- and Rab11-positive endosomes in polarized Madin-Darby canine kidney and non-polarized COS-1 cells. Immunoisolation of post-Golgi vesicles subsequent to different times of TGN release revealed that LPH and SI navigate in chronological order through Rab4-, Rab8- and Rab11-positive endosomes. Thereafter, the two hydrolases are segregated into distinct vesicle populations. In addition, apical membrane traffic could be significantly inhibited by RNA interference-mediated depletion of these guanosine triphosphatases. These results suggest that in epithelial cells, lipid raft-dependent and -independent apical cargo follow a transendosomal route.

Clathrin-dependent and independent endocytic pathways in tobacco protoplasts revealed by labelling with charged nanogold

Positively charged nanogold was used as a probe to trace the internalization of plasma membrane (PM) domains carrying negatively charged residues at an ultrastructural level. The probe revealed distinct endocytic pathways within tobacco protoplasts and allowed the morphology of the organelles involved in endocytosis to be characterized in great detail. Putative early endosomes with a tubulo-vesicular structure, similar to that observed in animal cells, are described and a new compartment, characterized by interconnected vesicles, was identified as a late endosome using the Arabidopsis anti-syntaxin family Syp-21 antibody. Endocytosis dissection using Brefeldin A (BFA), pulse chase, temperature- and energy-dependent experiments combined with quantitative analysis of nanogold particles in different compartments, suggested that recycling to the PM predominated with respect to degradation. Further experiments using ikarugamycin (IKA), an inhibitor of clathrin-dependent endocytosis, and negatively charged nanogold confirmed that distinct endocytic pathways coexist in tobacco protoplasts.

Differential sorting and Golgi export requirements for raft-associated and raft-independent apical proteins along the biosynthetic pathway

Sorting signals for apically destined proteins are highly diverse and can be present within the luminal, membrane-associated, and cytoplasmic domains of these proteins. A subset of apical proteins partition into detergent-resistant membranes, and the association of these proteins with glycolipid-enriched microdomains or lipid rafts may be important for their proper targeting. Recently, we observed that raft-associated and raft-independent apical proteins take different routes to the apical surface of polarized Madin-Darby canine kidney cells (Cresawn, K. O., Potter, B. A., Oztan, A., Guerriero, C. J., Ihrke, G., Goldenring, J. R., Apodaca, G., and Weisz, O. A. (2007) EMBO J. 26, 3737-3748). Here we reconstituted in vitro the export of raft-associated and raft-independent markers staged intracellularly at 19 degrees C. Surprisingly, whereas release of the raft-associated protein influenza hemagglutinin was dependent on the addition of an ATP-regenerating system and cytosol, release of a yellow fluorescent protein (YFP)-tagged raft-independent protein (the 75-kDa neurotrophin receptor; YFP-p75) was efficient even in the absence of these constituents. Subsequent studies suggested that YFP-p75 is released from the trans-Golgi network in fragile tubules that do not withstand isolation procedures. Moreover, immunofluorescence analysis revealed that hemagglutinin and YFP-p75 segregate into distinct subdomains of the Golgi complex at 19 degrees C. Our data suggest that raft-associated and raft-independent proteins accumulate at distinct intracellular sites upon low temperature staging, and that upon warming, they exit these compartments in transport carriers that have very different membrane characteristics and morphologies.

Active Rab11 and functional recycling endosome are required for E-cadherin trafficking and lumen formation during epithelial morphogenesis

The correct targeting and trafficking of the adherens junction protein epithelial cadherin (E-cadherin) is a major determinant for the acquisition of epithelial cell polarity and for the maintenance of epithelial integrity. The compartments and trafficking components required to sort and transport E-cadherin to the basolateral cell surface remain to be fully defined. On the basis of previous data, we know that E-cadherin is trafficked via the recycling endosome (RE) in nonpolarized and newly polarized cells. Here we explore the role of the RE throughout epithelial morphogenesis in MDCK monolayers and cysts. Time-lapse microscopy in live cells, altering RE function biochemically, and expressing a dominant-negative form of Rab11 (DN-Rab11), each showed that the RE is always requisite for E-cadherin sorting and trafficking. The RE remained important for E-cadherin trafficking in MDCK cells from a nonpolarized state through to fully formed, polarized epithelial monolayers. During the development of epithelial cysts, DN-Rab11 disrupted E-cadherin targeting and trafficking, the subapical localization of pERM and actin, and cyst lumen formation. This final effect demonstrated an early and critical interdependence of Rab11 and the RE for E-cadherin targeting, apical membrane formation, and cell polarity in cysts.

Endocytic traffic in animal cell cytokinesis

Cytokinesis is the final step of mitosis whereby two daughter cells physically separate. It is initiated by the assembly of an actomyosin contractile ring at the mitotic cell equator, which constricts the cytoplasm between the two reforming nuclei resulting in the formation of a narrow intercellular bridge filled with central spindle microtubule bundles. Cytokinesis terminates with the cleavage of the intercellular bridge in a poorly understood process called abscission. Recent work has highlighted the importance of membrane trafficking events occurring from membrane compartments flanking the bridge to the central midbody region. In particular, polarized delivery of endocytic recycling membranes is essential for completion of animal cell cytokinesis. Why endocytic traffic occurs within the intercellular bridge remains largely mysterious and its significance for cytokinesis will be discussed.

Endosomal SNARE proteins regulate CFTR activity and trafficking in epithelial cells

The Cystic Fibrosis Transmembrane conductance Regulator (CFTR) protein is a chloride channel localized at the apical plasma membrane of epithelial cells. We previously described that syntaxin 8, an endosomal SNARE (Soluble N-ethylmaleimide-sensitive factor Attachment protein REceptor) protein, interacts with CFTR and regulates its trafficking to the plasma membrane and hence its channel activity. Syntaxin 8 belongs to the endosomal SNARE complex which also contains syntaxin 7, vti1b and VAMP8. Here, we report that these four endosomal SNARE proteins physically and functionally interact with CFTR. In LLC-PK1 cells transfected with CFTR and in Caco-2 cells endogenously expressing CFTR, we demonstrated that endosomal SNARE protein overexpression inhibits CFTR activity but not swelling- or calcium-activated iodide efflux, indicating a specific effect upon CFTR activity. Moreover, co-immunoprecipitation experiments in LLC-PK1-CFTR cells showed that CFTR and SNARE proteins belong to a same complex and pull-down assays showed that VAMP8 and vti1b preferentially interact with CFTR N-terminus tail. By cell surface biotinylation and immunofluorescence experiments, we evidenced that endosomal SNARE overexpression disturbs CFTR apical targeting. Finally, we found a colocalization of CFTR and endosomal SNARE proteins in Rab11-positive recycling endosomes, suggesting a new role for endosomal SNARE proteins in CFTR trafficking in epithelial cells.

Rab8 regulates basolateral secretory, but not recycling, traffic at the recycling endosome

Rab8 is a monomeric GTPase that regulates the delivery of newly synthesized proteins to the basolateral surface in polarized epithelial cells. Recent publications have demonstrated that basolateral proteins interacting with the mu1-B clathrin adapter subunit pass through the recycling endosome (RE) en route from the TGN to the plasma membrane. Because Rab8 interacts with these basolateral proteins, these findings raise the question of whether Rab8 acts before, at, or after the RE. We find that Rab8 overexpression during the formation of polarity in MDCK cells, disrupts polarization of the cell, explaining how Rab8 mutants can disrupt basolateral endocytic and secretory traffic. However, once cells are polarized, Rab8 mutants cause mis-sorting of newly synthesized basolateral proteins such as VSV-G to the apical surface, but do not cause mis-sorting of membrane proteins already at the cell surface or in the endocytic recycling pathway. Enzymatic ablation of the RE also prevents traffic from the TGN from reaching the RE and similarly results in mis-sorting of newly synthesized VSV-G. We conclude that Rab8 regulates biosynthetic traffic through REs to the plasma membrane, but not trafficking of endocytic cargo through the RE. The data are consistent with a model in which Rab8 functions in regulating the delivery of TGN-derived cargo to REs.

Crag regulates epithelial architecture and polarized deposition of basement membrane proteins in Drosophila

The polarized architecture of epithelia relies on an interplay between the cytoskeleton, the trafficking machinery, and cell-cell and cell-matrix adhesion. Specifically, contact with the basement membrane (BM), an extracellular matrix underlying the basal side of epithelia, is important for cell polarity. However, little is known about how BM proteins themselves achieve a polarized distribution. In a genetic screen in the Drosophila follicular epithelium, we identified mutations in Crag, which encodes a conserved protein with domains implicated in membrane trafficking. Follicle cells mutant for Crag lose epithelial integrity and frequently become invasive. The loss of Crag leads to the anomalous accumulation of BM components on both sides of epithelial cells without directly affecting the distribution of apical or basolateral membrane proteins. This defect is not generally observed in mutants affecting epithelial integrity. We propose that Crag plays a unique role in organizing epithelial architecture by regulating the polarized secretion of BM proteins.

Accumulation of AMPA receptors in autophagosomes in neuronal axons lacking adaptor protein AP-4

AP-4 is a member of the adaptor protein complexes, which control vesicular trafficking of membrane proteins. Although AP-4 has been suggested to contribute to basolateral sorting in epithelial cells, its function in neurons is unknown. Here, we show that disruption of the gene encoding the beta subunit of AP-4 resulted in increased accumulation of axonal autophagosomes, which contained AMPA receptors and transmembrane AMPA receptor regulatory proteins (TARPs), in axons of hippocampal neurons and cerebellar Purkinje cells both in vitro and in vivo. AP-4 indirectly associated with the AMPA receptor via TARPs, and the specific disruption of the interaction between AP-4 and TARPs caused the mislocalization of endogenous AMPA receptors in axons of wild-type neurons. These results indicate that AP-4 may regulate proper somatodendritic-specific distribution of its cargo proteins, including AMPA receptor-TARP complexes and the autophagic pathway in neurons.

Sdmg1 is a conserved transmembrane protein associated with germ cell sex determination and germline-soma interactions in mice

In mammals, the supporting cell lineage in an embryonic gonad communicates the sex-determining decision to various sexually dimorphic cell types in the developing embryo, including the germ cells. However, the molecular nature of the sex-determining signals that pass from the supporting cells to the germ cells is not well understood. We have identified a conserved transmembrane protein, Sdmg1, owing to its male-specific expression in mouse embryonic gonads. Sdmg1 is expressed in the Sertoli cells of embryonic testes from 12.5 dpc, and in granulosa cells of growing follicles in adult ovaries. In Sertoli cells, Sdmg1 is localised to endosomes, and knock-down of Sdmg1 in Sertoli cell lines causes mis-localisation of the secretory SNARE Stx2 and defects in membrane trafficking. Upregulation of Sdmg1 appears to be part of a larger programme of changes to membrane trafficking pathways in embryonic Sertoli cells, and perturbing secretion in male embryonic gonads in organ culture causes male-to-female germ cell sex reversal. These data suggest that changes that occur in the cell biology of embryonic Sertoli cells may facilitate the communication of male sex-determining decisions to the germ cells during embryonic development.

Depletion of apical transport proteins perturbs epithelial cyst formation and ciliogenesis

Epithelial cells are vital for maintaining the complex architecture and functions of organs in the body. Directed by cues from the extracellular matrix, cells polarize their surface into apical and basolateral domains, and connect by extensive cell-cell junctions to form tightly vowen epithelial layers. In fully polarized cells, primary cilia project from the apical surface. Madin-Darby canine kidney (MDCK) cells provide a model to study organization of cells as monolayers and also in 3D in cysts. In this study retrovirus-mediated RNA interference (RNAi) was used to generate a series of knockdowns (KDs) for proteins implicated in apical transport: annexin-13, caveolin-1, galectin-3, syntaxin-3, syntaxin-2 and VIP17 and/or MAL. Cyst cultures were then employed to study the effects of these KDs on epithelial morphogenesis. Depletion of these proteins by RNAi stalled the development of the apical lumen in cysts and resulted in impaired ciliogenesis. The most severe ciliary defects were observed in annexin-13 and syntaxin-3 KD cysts. Although the phenotypes demonstrate the robustness of the formation of the polarized membrane domains, they indicate the important role of apical membrane biogenesis in epithelial organization.

Spermatocyte cytokinesis requires rapid membrane addition mediated by ARF6 on central spindle recycling endosomes

The dramatic cell shape changes during cytokinesis require the interplay between microtubules and the actomyosin contractile ring, and addition of membrane to the plasma membrane. Numerous membrane-trafficking components localize to the central spindle during cytokinesis, but it is still unclear how this machinery is targeted there and how membrane trafficking is coordinated with cleavage furrow ingression. Here we use an arf6 null mutant to show that the endosomal GTPase ARF6 is required for cytokinesis in Drosophila spermatocytes. ARF6 is enriched on recycling endosomes at the central spindle, but it is required neither for central spindle nor actomyosin contractile ring assembly, nor for targeting of recycling endosomes to the central spindle. However, in arf6 mutants the cleavage furrow regresses because of a failure in rapid membrane addition to the plasma membrane. We propose that ARF6 promotes rapid recycling of endosomal membrane stores during cytokinesis, which is critical for rapid cleavage furrow ingression.

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.

Morphogenesis of post-Golgi transport carriers

The trans-Golgi network (TGN) is one of the main, if not the main, sorting stations in the process of intracellular protein trafficking. It is therefore of central importance to understand how the key players in the TGN-based sorting and delivery process, the post-Golgi carriers (PGCs), form and function. Over the last few years, modern morphological approaches have generated new insights into the questions of PGC biogenesis, structure and dynamics. Here, we present a view by which the “lifecycle” of a PGC consists of several distinct stages: the formation of TGN tubular export domains (where different cargoes are segregated from each other and from the Golgi enzymes); the docking of these tubular domains onto molecular motors and their extrusion towards the cell periphery along microtubules; the fission of the forming PGC from the donor membrane; and the delivery of the newly formed PGC to its specific acceptor organelle. It is now important to add the many molecular machineries that have been described as operating at the TGN to this “morphofunctional map” of the TGN export process.

Epithelial polarity requires septin coupling of vesicle transport to polyglutamylated microtubules

In epithelial cells, polarized growth and maintenance of apical and basolateral plasma membrane domains depend on protein sorting from the trans-Golgi network (TGN) and vesicle delivery to the plasma membrane. Septins are filamentous GTPases required for polarized membrane growth in budding yeast, but whether they function in epithelial polarity is unknown. Here, we show that in epithelial cells septin 2 (SEPT2) fibers colocalize with a subset of microtubule tracks composed of polyglutamylated (polyGlu) tubulin, and that vesicles containing apical or basolateral proteins exit the TGN along these SEPT2/polyGlu microtubule tracks. Tubulin-associated SEPT2 facilitates vesicle transport by maintaining polyGlu microtubule tracks and impeding tubulin binding of microtubule-associated protein 4 (MAP4). Significantly, this regulatory step is required for polarized, columnar-shaped epithelia biogenesis; upon SEPT2 depletion, cells become short and fibroblast-shaped due to intracellular accumulation of apical and basolateral membrane proteins, and loss of vertically oriented polyGlu microtubules. We suggest that septin coupling of the microtubule cytoskeleton to post-Golgi vesicle transport is required for the morphogenesis of polarized epithelia.

Mint3/X11gamma is an ADP-ribosylation factor-dependent adaptor that regulates the traffic of the Alzheimer's Precursor protein from the trans-Golgi network

Beta-amyloid peptides (Abeta) are the major component of plaques in brains of Alzheimer's patients, and are they derived from the proteolytic processing of the beta-amyloid precursor protein (APP). The movement of APP between organelles is highly regulated, and it is tightly connected to its processing by secretases. We proposed previously that transport of APP within the cell is mediated in part through its sorting into Mint/X11-containing carriers. To test our hypothesis, we purified APP-containing vesicles from human neuroblastoma SH-SY5Y cells, and we showed that Mint2/3 are specifically enriched and that Mint3 and APP are present in the same vesicles. Increasing cellular APP levels increased the amounts of both APP and Mint3 in purified vesicles. Additional evidence supporting an obligate role for Mint3 in traffic of APP from the trans-Golgi network to the plasma membrane include the observations that depletion of Mint3 by small interference RNA (siRNA) or mutation of the Mint binding domain of APP changes the export route of APP from the basolateral to the endosomal/lysosomal sorting route. Finally, we show that increased expression of Mint3 decreased and siRNA-mediated knockdowns increased the secretion of the neurotoxic beta-amyloid peptide, Abeta(1-40). Together, our data implicate Mint3 activity as a critical determinant of post-Golgi APP traffic.

The interaction of JRAB/MICAL-L2 with Rab8 and Rab13 coordinates the assembly of tight junctions and adherens junctions

The assembly of tight junctions (TJs) and adherens junctions (AJs) is regulated by the transport of integral TJ and AJ proteins to and/or from the plasma membrane (PM) and it is tightly coordinated in epithelial cells. We previously reported that Rab13 and a junctional Rab13-binding protein (JRAB)/molecule interacting with CasL-like 2 (MICAL-L2) mediated the endocytic recycling of an integral TJ protein occludin and the formation of functional TJs. Here, we investigated the role of Rab13 and JRAB/MICAL-L2 in the transport of other integral TJ and AJ proteins claudin-1 and E-cadherin to the PM by using a Ca(2+)-switch model. Although knockdown of Rab13 specifically suppressed claudin-1 and occludin but not E-cadherin transport, knockdown of JRAB/MICAL-L2 and expression of its Rab13-binding domain (JRAB/MICAL-L2-C) inhibited claudin-1, occludin, and E-cadherin transport. We then identified Rab8 as another JRAB/MICAL-L2-C-binding protein. Knockdown of Rab8 inhibited the Rab13-independent transport of E-cadherin to the PM. Rab8 and Rab13 competed with each other for the binding to JRAB/MICAL-L2 and functionally associated with JRAB/MICAL-L2 at the perinuclear recycling/storage compartments and PM, respectively. These results suggest that the interaction of JRAB/MICAL-L2 with Rab8 and Rab13 coordinates the assembly of AJs and TJs.

Par3 functions in the biogenesis of the primary cilium in polarized epithelial cells

Par3 is a PDZ protein important for the formation of junctional complexes in epithelial cells. We have identified an additional role for Par3 in membrane biogenesis. Although Par3 was not required for maintaining polarized apical or basolateral membrane domains, at the apical surface, Par3 was absolutely essential for the growth and elongation of the primary cilium. The activity reflected its ability to interact with kinesin-2, the microtubule motor responsible for anterograde transport of intraflagellar transport particles to the tip of the growing cilium. The Par3 binding partners Par6 and atypical protein kinase C interacted with the ciliary membrane component Crumbs3 and we show that the PDZ binding motif of Crumbs3 was necessary for its targeting to the ciliary membrane. Thus, the Par complex likely serves as an adaptor that couples the vectorial movement of at least a subset of membrane proteins to microtubule-dependent transport during ciliogenesis.

How are the cellular functions of myosin VI regulated within the cell?

This review, dedicated to the memory of Professor Setsuro Ebashi, focuses on our current work investigating the cellular functions and regulation of the unique unconventional motor, myosin VI. This myosin, unlike all the other myosins so far studied, moves towards the minus end of actin filaments and has been implicated in a wide range of cellular processes such as endocytosis, exocytosis, cell migration, cell division and cytokinesis. Myosin VI’s involvement in these cellular pathways is mediated by its interaction with specific adaptor proteins and is regulated by multiple regulatory signals and modifications such as calcium ions, PtdIns(4,5)P₂ (PIP₂) and phosphorylation. Understanding the functions of myosin VI within the cell and how it is regulated is now of utmost importance given the recent observations that it is associated with a number of human disorders such as deafness and cancers.

Antibody to AP1B adaptor blocks biosynthetic and recycling routes of basolateral proteins at recycling endosomes

The epithelial-specific adaptor AP1B sorts basolateral plasma membrane (PM) proteins in both biosynthetic and recycling routes, but the site where it carries out this function remains incompletely defined. Here, we have investigated this topic in Fischer rat thyroid (FRT) epithelial cells using an antibody against the medium subunit micro1B. This antibody was suitable for immunofluorescence and blocked the function of AP1B in these cells. The antibody blocked the basolateral recycling of two basolateral PM markers, Transferrin receptor (TfR) and LDL receptor (LDLR), in a perinuclear compartment with marker and functional characteristics of recycling endosomes (RE). Live imaging experiments demonstrated that in the presence of the antibody two newly synthesized GFP-tagged basolateral proteins (vesicular stomatitis virus G [VSVG] protein and TfR) exited the trans-Golgi network (TGN) normally but became blocked at the RE within 3-5 min. By contrast, the antibody did not block trafficking of green fluorescent protein (GFP)-LDLR from the TGN to the PM but stopped its recycling after internalization into RE in approximately 45 min. Our experiments conclusively demonstrate that 1) AP1B functions exclusively at RE; 2) TGN-to-RE transport is very fast and selective and is mediated by adaptors different from AP1B; and 3) the TGN and AP1B-containing RE cooperate in biosynthetic basolateral sorting.

Myosin VI and its interacting protein LMTK2 regulate tubule formation and transport to the endocytic recycling compartment

Myosin VI is an actin-based retrograde motor protein that plays a crucial role in both endocytic and secretory membrane trafficking pathways. Myosin VI's targeting to and function in these intracellular pathways is mediated by a number of specific binding partners. In this paper we have identified a new myosin-VI-binding partner, lemur tyrosine kinase 2 (LMTK2), which is the first transmembrane protein and kinase that directly binds to myosin VI. LMTK2 binds to the WWY site in the C-terminal myosin VI tail, the same site as the endocytic adaptor protein Dab2. When either myosin VI or LMTK2 is depleted by siRNAs, the transferrin receptor (TfR) is trapped in swollen endosomes and tubule formation in the endocytic recycling pathway is dramatically reduced, showing that both proteins are required for the transport of cargo, such as the TfR, from early endosomes to the endocytic recycling compartment.