Distinct transport vesicles mediate the delivery of plasma membrane proteins to the apical and basolateral domains of MDCK cells

Distinct roles for luminal acidification in apical protein sorting and trafficking in zebrafish

Epithelial cell physiology critically depends on the asymmetric distribution of channels and transporters. However, the mechanisms targeting membrane proteins to the apical surface are still poorly understood. Here, we performed a visual forward genetic screen in the zebrafish intestine and identified mutants with defective apical targeting of membrane proteins. One of these mutants, affecting the vacuolar H⁺-ATPase gene atp6ap1b, revealed specific requirements for luminal acidification in apical, but not basolateral, membrane protein sorting and transport. Using a low temperature block assay combined with genetic and pharmacologic perturbation of luminal pH, we monitored transport of newly synthesized membrane proteins from the TGN to apical membrane in live zebrafish. We show that vacuolar H⁺-ATPase activity regulates sorting of O-glycosylated proteins at the TGN, as well as Rab8-dependent post-Golgi trafficking of different classes of apical membrane proteins. Thus, luminal acidification plays distinct and specific roles in apical membrane biogenesis.

Basolateral delivery of the type I transforming growth factor beta receptor is mediated by a dominant-acting cytoplasmic motif

A novel motif within the cytoplasmic tail of the type I TGF-β receptor (TβRI) controls basolateral delivery. While this element functions independent of TβRI recycling and heteromeric TGF-β receptor trafficking, it can dominantly direct an apically expressed receptor to the basolateral membrane in polarized epithelial cells.

Delivery of biomolecules to the correct subcellular locales is critical for proper physiological function. To that end, we have previously determined that type I and II transforming growth factor beta (TGF-β) receptors (TβRI and TβRII, respectively) localize to the basolateral domain in polarized epithelia. While TβRII targeting was shown to be regulated by sequences between amino acids 529 and 538, the analogous region(s) within TβRI is unknown. To address that question, sequential cytoplasmic TβRI truncations and point mutations identified a targeting motif between residues 158 and 163 (VxxEED) required for basolateral TβRI expression. Further studies documented that receptor internalization, down-regulation, direct recycling, or Smad signaling were unaffected by motif mutations that caused TβRI mislocalization. However, inclusion of amino acids 148–217 containing the targeting motif was able to direct basolateral expression of the apically sorted nerve growth factor receptor (NGFR, p75; extracellular and transmembrane regions) in a dominant manner. Finally, coexpression of apically targeted type I and type II TGF-β receptors mediated Smad3 signaling from the apical membrane of polarized epithelial cells. These findings demonstrate that the absence of apical TGF-β signaling in normal epithelia is primarily a reflection of domain-specific receptor expression and not an inability to couple with the signaling machinery.

The Continuing Mystery of Lipid Rafts

Since its initial formalization nearly 20 years ago, the concept of lipid rafts has generated a tremendous amount of attention and interest and nearly as much controversy. The controversy is perhaps surprising because the notion itself is intuitive: compartmentalization in time and space is a ubiquitous theme at all scales of biology, and therefore, the partitioning of cellular membranes into lateral subdivision should be expected. Nevertheless, the physicochemical principles responsible for compartmentalization and the molecular mechanisms by which they are functionalized remain nearly as mysterious today as they were two decades ago. Herein, we review recent literature on this topic with a specific focus on the major open questions in the field including: (1) what are the best tools to assay raft behavior in living membranes? (2) what is the function of the complex lipidome of mammalian cells with respect to membrane organization? (3) what are the mechanisms that drive raft formation and determine their properties? (4) how can rafts be modulated? (5) how is membrane compartmentalization integrated into cellular signaling? Despite decades of intensive research, this compelling field remains full of fundamental questions.

Iterative sorting of apical and basolateral cargo in Madin-Darby canine kidney cells

A novel assay quantitatively distinguishes different cargo pairs by their degree of colocalization at the TGN and the evolution of colocalization during their TGN-to-surface transport. Apical NTRp75 and basolateral VSVG in MDCK cells undergo continuous sorting between TGN exit and surface arrival.

For several decades, the trans-Golgi network (TGN) was considered the most distal stop and hence the ultimate protein-sorting station for distinct apical and basolateral transport carriers that reach their respective surface domains in the direct trafficking pathway. However, recent reports of apical and basolateral cargoes traversing post-Golgi compartments accessible to endocytic ligands before their arrival at the cell surface and the post-TGN breakup of large pleomorphic membrane fragments that exit the Golgi region toward the surface raised the possibility that compartments distal to the TGN mediate or contribute to biosynthetic sorting. Here we describe the development of a novel assay that quantitatively distinguishes different cargo pairs by their degree of colocalization at the TGN and by the evolution of colocalization during their TGN-to-surface transport. Keys to the high resolution of our approach are 1) conversion of perinuclear organelle clustering into a two-dimensional microsomal spread and 2) identification of TGN and post-TGN cargo without the need for a TGN marker that universally cosegregates with all cargo. Using our assay, we provide the first evidence that apical NTRp75 and basolateral VSVG in Madin–Darby canine kidney cells still undergo progressive sorting after they exit the TGN toward the cell surface.

Rafting through traffic: Membrane domains in cellular logistics

The intricate and tightly regulated organization of eukaryotic cells into spatially and functionally distinct membrane-bound compartments is a defining feature of complex organisms. These compartments are defined by their lipid and protein compositions, with their limiting membrane as the functional interface to the rest of the cell. Thus, proper segregation of membrane proteins and lipids is necessary for the maintenance of organelle identity, and this segregation must be maintained despite extensive, rapid membrane exchange between compartments. Sorting processes of high efficiency and fidelity are required to avoid potentially deleterious mis-targeting and maintain cellular function. Although much molecular machinery associated with membrane traffic (i.e. membrane budding/fusion/fission) has been characterized both structurally and biochemically, the mechanistic details underlying the tightly regulated distribution of membranes between subcellular locations remain to be elucidated. This review presents evidence for the role of ordered lateral membrane domains known as lipid rafts in both biosynthetic sorting in the late secretory pathway, as well as endocytosis and recycling to/from the plasma membrane. Although such evidence is extensive and the involvement of membrane domains in sorting is definitive, specific mechanistic details for raft-dependent sorting processes remain elusive.

Class I myosins in B-cell physiology: functions in spreading, immune synapses, motility, and vesicular traffic

Myosins comprise a family of motor proteins whose role in muscle contraction and motility in a large range of eukaryotic cells has been widely studied. Although these proteins have been characterized extensively and much is known about their function in different cellular compartments, little is known about these molecules in hematopoietic cells. Myosins expressed by cells from the immune response are involved in maintaining plasma membrane tension, moving and secreting vesicles, endo- and exocytotic processes, and promoting the adhesion and motility of cells. Herein, we summarize our current understanding of class I myosins in B cells, with an emphasis on the emerging roles of these molecular motors in immune functions.

Mature VLDL triggers the biogenesis of a distinct vesicle from the trans-Golgi network for its export to the plasma membrane

Post-Golgi trafficking of mature VLDL (very-low-density lipoprotein) is crucial in maintaining normal TAG (triacylglycerol) homoeostasis of hepatocytes; however, the mechanism that regulates the exit of mature VLDL from the TGN (trans-Golgi network) is not known. We developed an in vitro TGN-budding assay that allowed us to examine the formation of secretory vesicles from the TGN in primary rat hepatocytes. We isolated TAG-rich PG-VTVs (post-TGN VLDL transport vesicles) using a continuous sucrose density gradient. PG-VTVs were distributed in low-density fractions, whereas protein transport vesicles were present in relatively higher-density fractions of the same sucrose gradient. EM revealed large intact PG-VTVs ranging 300-350 nm in size. The biogenesis of PG-VTVs from the TGN required cytosol, ATP, GTP hydrolysis and incubation at 37°C. PG-VTVs concentrated the VLDL proteins: apolipoproteins apoB100, apoAIV, apoAI and apoE, but did not contain either albumin or transferrin. Proteinase K treatment did not degrade VLDL core proteins, suggesting that PG-VTVs were sealed. PG-VTVs were able to fuse with and deliver VLDL to the PM (plasma membrane) in a vectorial manner. We conclude that we have identified a new TGN-derived vesicle, the PG-VTV, which specifically transports mature VLDL from the TGN to the PM.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Vasopressin induces apical expression of caveolin in rat kidney collecting duct principal cells

Caveolin (Cav)1 is expressed in the basolateral membrane domain of renal collecting duct (CD) principal cells (PCs), where it is associated with caveolae. To reveal any potential involvement of Cav1 in vasopressin signaling, we used specific monoclonal and polyclonal antibodies to examine its localization in CD PCs of Brattleboro (BB) rats treated with vasopressin (DDAVP). Compared with controls, immunofluorescence revealed a time-dependent increase in Cav1 expression in the apical membrane domain of PCs, where it overlapped with aquaporin-2 (AQP2). After 24 h of DDAVP treatment, Cav1 was visible as an increased number of small apical spots. The staining gradually became more extensive, and, after 2 wk of DDAVP, it occupied the majority of the apical membrane domain of many PCs. Cav1 also assumed an apical localization in PCs of DDAVP-treated Sprague-Dawley and Long-Evans rats. Similarly, Cav2 appeared at the apical pole of PCs after DDAVP treatment of BB, Sprague-Dawley, and Long-Evans rats. Immunogold electron microscopy confirmed bipolar Cav1 membrane expression in DDAVP-treated BB rats, whereas caveolae were only detected on the basolateral membrane. Immunoblot analysis of BB rat whole kidney homogenates revealed no significant increase in Cav1 levels in DDAVP-treated rats, suggesting that DDAVP induces Cav1 relocalization or modifies its targeting. We conclude that Cav1 and Cav2 trafficking and membrane localization are dramatically altered by the action of DDAVP. Importantly, the absence of apical caveolae indicates that while Cavs may have an as yet undetermined role in vasopressin-regulated signaling processes, this is probably unrelated to AQP2 internalization by caveolae.

A new class of carriers that transport selective cargo from the trans Golgi network to the cell surface

We have isolated a membrane fraction enriched in a class of transport carriers that form at the trans Golgi network (TGN) and are destined for the cell surface in HeLa cells. Protein kinase D (PKD) is required for the biogenesis of these carriers that contain myosin II, Rab6a, Rab8a, and synaptotagmin II, as well as a number of secretory and plasma membrane-specific cargoes. Our findings reveal a requirement for myosin II in the migration of these transport carriers but not in their biogenesis per se. Based on the cargo secreted by these carriers we have named them CARTS for CARriers of the TGN to the cell Surface. Surprisingly, CARTS are distinct from the carriers that transport vesicular stomatitis virus (VSV)-G protein and collagen I from the TGN to the cell surface. Altogether, the identification of CARTS provides a valuable means to understand TGN to cell surface traffic.

Quantitative analysis of the lipidomes of the influenza virus envelope and MDCK cell apical membrane

Analysis of the lipid composition of influenza virus–infected cells provides support for the membrane raft-based biogenesis model.

The influenza virus (IFV) acquires its envelope by budding from host cell plasma membranes. Using quantitative shotgun mass spectrometry, we determined the lipidomes of the host Madin–Darby canine kidney cell, its apical membrane, and the IFV budding from it. We found the apical membrane to be enriched in sphingolipids (SPs) and cholesterol, whereas glycerophospholipids were reduced, and storage lipids were depleted compared with the whole-cell membranes. The virus membrane exhibited a further enrichment of SPs and cholesterol compared with the donor membrane at the expense of phosphatidylcholines. Our data are consistent with and extend existing models of membrane raft-based biogenesis of the apical membrane and IFV envelope.

N-Glycosylation instead of cholesterol mediates oligomerization and apical sorting of GPI-APs in FRT cells

In contrast to MDCK cells, in FRT cells oligomerization and apical sorting of GPI-APs are mediated by N-glycosylation independent of cholesterol and raft association.

Sorting of glycosylphosphatidyl-inositol–anchored proteins (GPI-APs) in polarized epithelial cells is not fully understood. Oligomerization in the Golgi complex has emerged as the crucial event driving apical segregation of GPI-APs in two different kind of epithelial cells, Madin–Darby canine kidney (MDCK) and Fisher rat thyroid (FRT) cells, but whether the mechanism is conserved is unknown. In MDCK cells cholesterol promotes GPI-AP oligomerization, as well as apical sorting of GPI-APs. Here we show that FRT cells lack this cholesterol-driven oligomerization as apical sorting mechanism. In these cells both apical and basolateral GPI-APs display restricted diffusion in the Golgi likely due to a cholesterol-enriched membrane environment. It is striking that N-glycosylation is the critical event for oligomerization and apical sorting of GPI-APs in FRT cells but not in MDCK cells. Our data indicate that at least two mechanisms exist to determine oligomerization in the Golgi leading to apical sorting of GPI-APs. One depends on cholesterol, and the other depends on N-glycosylation and is insensitive to cholesterol addition or depletion.

A conserved signal and GTPase complex are required for the ciliary transport of polycystin-1

Primary cilia regulate epithelial differentiation and organ function. Failure of mutant polycystins to localize to cilia abolishes flow-stimulated calcium signaling and causes autosomal dominant polycystic kidney disease. We identify a conserved amino acid sequence, KVHPSST, in the C-terminus of polycystin-1 (PC1) that serves as a ciliary-targeting signal. PC1 binds a multimeric protein complex consisting of several GTPases (Arf4, Rab6, Rab11) and the GTPase-activating protein (GAP), ArfGAP with SH3 domain, ankyrin repeat and PH domain 1 (ASAP1) in the Golgi, which facilitates vesicle budding and Golgi exocytosis. A related N-terminal ciliary-targeting sequence in polycystin-2 similarly binds Arf4. Deletion of the extreme C-terminus of PC1 ablates Arf4 and ASAP1 binding and prevents ciliary localization of an integral membrane CD16.7-PC1 chimera. Interactions are confirmed for chimeric and endogenous proteins through quantitated in vitro and cell-based approaches. PC1 also complexes with Rab8; knockdown of trafficking regulators Arf4 or Rab8 functionally blocks CD16.7-PC1 trafficking to cilia. Mutations in rhodopsin disrupt a similar signal and cause retinitis pigmentosa, while Bardet-Biedl syndrome, primary open-angle glaucoma, and tumor cell invasiveness are linked to dysregulation of ASAP1 or Rab8 or its effectors. In this paper, we provide evidence for a conserved GTPase-dependent ciliary-trafficking mechanism that is shared between epithelia and neurons, and is essential in ciliary-trafficking and cell homeostasis.

Architecture of the mammalian Golgi

Since its first visualization in 1898, the Golgi has been a topic of intense morphological research. A typical mammalian Golgi consists of a pile of stapled cisternae, the Golgi stack, which is a key station for modification of newly synthesized proteins and lipids. Distinct stacks are interconnected by tubules to form the Golgi ribbon. At the entrance site of the Golgi, the cis-Golgi, vesicular tubular clusters (VTCs) form the intermediate between the endoplasmic reticulum and the Golgi stack. At the exit site of the Golgi, the trans-Golgi, the trans-Golgi network (TGN) is the major site of sorting proteins to distinct cellular locations. Golgi functioning can only be understood in light of its complex architecture, as was revealed by a range of distinct electron microscopy (EM) approaches. In this article, a general concept of mammalian Golgi architecture, including VTCs and the TGN, is described.

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.

Role of the terminal domains in sodium channel localization

Voltage-gated sodium channels are membrane proteins that initiate action potentials in neurons following membrane depolarization. Members of this family show differential distribution at the subcellular level. The mechanisms underlying the targeting of these isoforms are not understood. However, their specificity is important because the isoforms can change the excitability of the membrane due to differences in their electrophysiological properties. In this study, chimeras generated between Na(V)1.2 and Na(V)1.6 were used to test channel domains for sequence that would allow Na(V)1.2 to localize to unmyelinated axons when Na(V)1.6 could not. We show that the N-terminal 202 amino acids of the Na(V)1.2 channel can mediate membrane domain-specific sorting in polarized epithelial cells and are necessary but not sufficient for localizing the isoform to the axons of cultured neurons. The domain-sorting signal is in the region between amino acids 110-202 of the Na(V)1.2 channel. The C-terminal 451 amino acids of Na(V)1.2 likely contain determinants that interact with neuron-specific factors to direct Na(V)1.2 to the axon.

Segregation of sphingolipids and sterols during formation of secretory vesicles at the trans-Golgi network

The trans-Golgi network (TGN) is the major sorting station in the secretory pathway of all eukaryotic cells. How the TGN sorts proteins and lipids to generate the enrichment of sphingolipids and sterols at the plasma membrane is poorly understood. To address this fundamental question in membrane trafficking, we devised an immunoisolation procedure for specific recovery of post-Golgi secretory vesicles transporting a transmembrane raft protein from the TGN to the cell surface in the yeast Saccharomyces cerevisiae. Using a novel quantitative shotgun lipidomics approach, we could demonstrate that TGN sorting selectively enriched ergosterol and sphingolipid species in the immunoisolated secretory vesicles. This finding, for the first time, indicates that the TGN exhibits the capacity to sort membrane lipids. Furthermore, the observation that the immunoisolated vesicles exhibited a higher membrane order than the late Golgi membrane, as measured by C-Laurdan spectrophotometry, strongly suggests that lipid rafts play a role in the TGN-sorting machinery.

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.

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.

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.

Distinct v-SNAREs regulate direct and indirect apical delivery in polarized epithelial cells

SNARE [soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor] proteins control the membrane-fusion events of eukaryotic membrane-trafficking pathways. Specific vesicular and target SNAREs operate in specific trafficking routes, but the degree of specificity of SNARE functions is still elusive. Apical fusion requires the polarized distribution at the apical surface of the t-SNARE syntaxin 3, and several v-SNAREs including TI-VAMP and VAMP8 operate at the apical plasma membrane in polarized epithelial cells. It is not known, however, whether specific v-SNAREs are involved in direct and indirect routes to the apical surface. Here, we used RNAi to assess the role of two tetanus-neurotoxin-insensitive v-SNAREs, TI-VAMP/VAMP7 and VAMP8, in the sorting of raft- and non-raft-associated apical markers that follow either a direct or a transcytotic delivery, respectively, in FRT or Caco2 cells. We show that TI-VAMP mediates the direct apical delivery of both raft- and non-raft-associated proteins. By contrast, sorting by means of the transcytotic pathway is not affected by TI-VAMP knockdown but does appear to be regulated by VAMP8. Together with the specific role of VAMP3 in basolateral transport, our results demonstrate a high degree of specificity in v-SNARE function in polarized cells.

Differential involvement of endocytic compartments in the biosynthetic traffic of apical proteins

Newly synthesized basolateral markers can traverse recycling endosomes en route to the surface of Madin-Darby canine kidney cells; however, the routes used by apical proteins are less clear. Here, we functionally inactivated subsets of endocytic compartments and examined the effect on surface delivery of the basolateral marker vesicular stomatitis virus glycoprotein (VSV-G), the raft-associated apical marker influenza hemagglutinin (HA), and the non-raft-associated protein endolyn. Inactivation of transferrin-positive endosomes after internalization of horseradish peroxidase (HRP)-containing conjugates inhibited VSV-G delivery, but did not disrupt apical delivery. In contrast, inhibition of protein export from apical recycling endosomes upon expression of dominant-negative constructs of myosin Vb or Sec15 selectively perturbed apical delivery of endolyn. Ablation of apical endocytic components accessible to HRP-conjugated wheat germ agglutinin (WGA) disrupted delivery of HA but not endolyn. However, delivery of glycosylphosphatidylinositol-anchored endolyn was inhibited by >50% under these conditions, suggesting that the biosynthetic itinerary of a protein is dependent on its targeting mechanism. Our studies demonstrate that apical and basolateral proteins traverse distinct endocytic intermediates en route to the cell surface, and that multiple routes exist for delivery of newly synthesized apical proteins.

Variable actin dynamics requirement for the exit of different cargo from thetrans-Golgi network

Efficient post-Golgi trafficking depends on microtubules, but actin filaments and actin-associated proteins are also postulated. Here we examined, by inverse fluorescence recovery after photobleaching, the role of actin dynamics in the exit from the TGN of fluorescent-tagged apical or basolateral and raft or non-raft-associated cargoes. Either the actin-stabilizing jasplakinolide or the actin-depolymerising latrunculin B variably but significantly inhibited post-Golgi traffic of non-raft associated apical p75NTR and basolateral VSV-G cargoes. The TGN-exit of the apical-destined VSV-G mutant was impaired only by latrunculin B. Strikingly, the raft-associated GPI-anchor protein was not affected by either actin toxin. Results indicate that actin dynamics participates in the TGN egress of both apical- and basolateral-targeted proteins but is not needed for apical raft-associated cargo.

Oligomerization Is a Specific Requirement for Apical Sorting of Glycosyl-Phosphatidylinositol-Anchored Proteins but Not for Non-Raft-Associated Apical Proteins

Protein apical sorting in polarized epithelial cells is mediated by two different mechanisms, raft dependent and raft independent. In Madin-Darby canine kidney (MDCK) cells, an essential step for apical sorting of glycosyl-phosphatidylinositol (GPI)-anchored proteins (GPI-APs) is their coalescence into high-molecular-weight (HMW) oligomers. Here we show that this mechanism is also functional in Fischer rat thyroid cells, which possess a different sorting phenotype compared with MDCK cells. We demonstrate that, as in MDCK cells, both apical and basolateral GPI-APs associate with detergent-resistant microdomains, but that only the apical proteins are able to oligomerize into HMW complexes during their passage through the medial Golgi. We also show that oligomerization is a specific requirement for apical sorting of GPI-APs and is not used by transmembrane, non-raft-associated apical proteins.

GPI-anchored proteins are directly targeted to the apical surface in fully polarized MDCK cells

The polarity of epithelial cells is dependent on their ability to target proteins and lipids in a directional fashion. The trans-Golgi network, the endosomal compartment, and the plasma membrane act as sorting stations for proteins and lipids. The site of intracellular sorting and pathways used for the apical delivery of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are largely unclear. Using biochemical assays and confocal and video microscopy in living cells, we show that newly synthesized GPI-APs are directly delivered to the apical surface of fully polarized Madin-Darby canine kidney cells. Impairment of basolateral membrane fusion by treatment with tannic acid does not affect the direct apical delivery of GPI-APs, but it does affect the organization of tight junctions and the integrity of the monolayer. Our data clearly demonstrate that GPI-APs are directly sorted to the apical surface without passing through the basolateral membrane. They also reinforce the hypothesis that apical sorting of GPI-APs occurs intracellularly before arrival at the plasma membrane.

Vectorial Secretion of Perivitelline Membrane Glycoprotein ZPC of Japanese Quail (Coturnix japonica) in Polarized Madin-Darby Canine Kidney Cells

The avian perivitelline membrane (PL), which is an investment homologous to the mammalian zona pellucida, is found between the surface of the oocyte and the apical surface of ovarian granulosa cells. Our previous study demonstrated that ZPC, one of the components of PL, is synthesized in ovarian granulosa cells. However, how the secretion of ZPC is regulated in the cells has been insufficiently investigated. We studied the secretion of quail ZPC expressed in polarized Madin-Darby canine kidney (MDCK) cells in a dual-chamber apparatus. Western blot analyses of the conditioned medium demonstrated that the majority of the secreted ZPC were distributed in the apical compartment. When ZPC lacking N-linked oligosaccharides was transfected into the cells, the 31-kDa immunoreactive band was detected in both the apical and the basolateral medium. Interestingly, immunohistochemical observations of the follicular wall demonstrated that the predominant intracellular form of ZPC in the cells localized in the apical side of the perinuclear region apposed to the PL, but not the basolateral side, indicating the possibility that ZPC could be selectively transported toward the apical surface in vivo. Taken together, these results indicated that ZPC expressed in MDCK cells are selectively released to the apical compartment, and that the N-linked carbohydrates might possess information that causes the efficient transport of ZPC to the apical surface of the cells.

Galectin-4 and sulfatides in apical membrane trafficking in enterocyte-like cells

We have previously reported that 1-benzyl-2-acetamido-2-deoxy-alpha-D-galactopyranoside (GalNAc alpha-O-bn), an inhibitor of glycosylation, perturbed apical biosynthetic trafficking in polarized HT-29 cells suggesting an involvement of a lectin-based mechanism. Here, we have identified galectin-4 as one of the major components of detergent-resistant membranes (DRMs) isolated from HT-29 5M12 cells. Galectin-4 was also found in post-Golgi carrier vesicles. The functional role of galectin-4 in polarized trafficking in HT-29 5M12 cells was studied by using a retrovirus-mediated RNA interference. In galectin-4-depleted HT-29 5M12 cells apical membrane markers accumulated intracellularly. In contrast, basolateral membrane markers were not affected. Moreover, galectin-4 depletion altered the DRM association characteristics of apical proteins. Sulfatides with long chain-hydroxylated fatty acids, which were also enriched in DRMs, were identified as high-affinity ligands for galectin-4. Together, our data propose that interaction between galectin-4 and sulfatides plays a functional role in the clustering of lipid rafts for apical delivery.

Cholesterol is required for the polarized secretion of erythropoietin in Madin-Darby canine kidney cells

It has already been reported that stably expressed exogenous human wild-type EPO (wtEPO) is preferentially secreted to the apical side and one of the three N-linked carbohydrate chains critically acts as an apical sorting determinant in Madin-Darby canine kidney (MDCK) cells. It has been suggested that lipid rafts are involved in the apical sorting of membrane and secretory proteins. To investigate the involvement of lipid rafts in the apical sorting of wtEPO, we examined the effect of cholesterol depletion with methyl-beta-cyclodextrin on the secretion polarity of EPO and analyzed Triton X-100 insoluble cell extracts by sucrose density gradients centrifugation in MDCK cells. We found that wtEPO was shifted in non-polarized direction by cholesterol depletion. Most of the wtEPO was not detectable in the raft fractions by sucrose density gradients centrifugation analysis. These results indicate that apical secretion of EPO involves a cholesterol-dependent mechanism probably not involving lipid rafts.

Effect of estradiol on mouse uterine epithelial cell transepithelial resistance (TER)

PROBLEM

The effects of estradiol on epithelial cell function in the uterus may either be direct or indirect through the paracrine effects of underlying stromal cells. The aim of this study was to test whether estradiol-17beta (E(2)) acts directly to regulate uterine epithelial cell monolayer integrity.

METHODS OF STUDY

Mouse uterine epithelial cells were isolated and grown on cell culture inserts to form confluent, polarized monolayers, as indicated by the development of high transepithelial resistance (TER).

RESULTS

When polarized epithelial cells were treated with E(2), TER was significantly decreased within 24 hr of exposure. Epithelial cells remained hormonally responsive in culture for at least 10 days. In contrast to estradiol, incubation with progesterone, cortisol, aldosterone, and DHT had no effect on uterine epithelial cell TER. The ability of E(2) to decrease TER was inhibited following co-incubation with ICI 182,780, a pure estrogen receptor antagonist. To further investigate the mechanism involved in estradiol-induced decreases in TER, we tested the effect of TAPI-0, an inhibitor of matrix metalloproteinases. Our findings indicate that TAPI-0 reversed the inhibitory effect of E(2) on TER.

CONCLUSIONS

These studies demonstrate that epithelial monolayer integrity is directly influenced by E(2) and ER mediated. Further, it suggests that the mechanism through which estradiol decreases TER is mediated by matrix metalloproteinases.

Protein oligomerization modulates raft partitioning and apical sorting of GPI-anchored proteins

An essential but insufficient step for apical sorting of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) in epithelial cells is their association with detergent-resistant microdomains (DRMs) or rafts. In this paper, we show that in MDCK cells both apical and basolateral GPI-APs associate with DRMs during their biosynthesis. However, only apical and not basolateral GPI-APs are able to oligomerize into high molecular weight complexes. Protein oligomerization begins in the medial Golgi, concomitantly with DRM association, and is dependent on protein-protein interactions. Impairment of oligomerization leads to protein missorting. We propose that oligomerization stabilizes GPI-APs into rafts and that this additional step is required for apical sorting of GPI-APs. Two alternative apical sorting models are presented.

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

The AP-1B clathrin adaptor complex is responsible for the polarized transport of many basolateral membrane proteins in epithelial cells. Localization of AP-1B to recycling endosomes (REs) along with other components (exocyst subunits and Rab8) involved in AP-1B-dependent transport suggested that RE might be an intermediate between the Golgi and the plasma membrane. Although the involvement of endosomes in the secretory pathway has long been suspected, we now present direct evidence using four independent methods that REs play a role in basolateral transport in MDCK cells. Newly synthesized AP-1B-dependent cargo, vesicular stomatitis virus glycoprotein G (VSV-G), was found by video microscopy, immunoelectron microscopy, and cell fractionation to enter transferrin-positive REs within a few minutes after exit from the trans-Golgi network. Although transient, RE entry appears essential because enzymatic inactivation of REs blocked VSV-G delivery to the cell surface. Because an apically targeted VSV-G mutant behaved similarly, these results suggest that REs not only serve as an intermediate but also as a common site for polarized sorting on the endocytic and secretory pathways.

Assembly and budding of influenza virus

Influenza viruses are causative agents of an acute febrile respiratory disease called influenza (commonly known as "flu") and belong to the Orthomyxoviridae family. These viruses possess segmented, negative stranded RNA genomes (vRNA) and are enveloped, usually spherical and bud from the plasma membrane (more specifically, the apical plasma membrane of polarized epithelial cells). Complete virus particles, therefore, are not found inside infected cells. Virus particles consist of three major subviral components, namely the viral envelope, matrix protein (M1), and core (viral ribonucleocapsid [vRNP]). The viral envelope surrounding the vRNP consists of a lipid bilayer containing spikes composed of viral glycoproteins (HA, NA, and M2) on the outer side and M1 on the inner side. Viral lipids, derived from the host plasma membrane, are selectively enriched in cholesterol and glycosphingolipids. M1 forms the bridge between the viral envelope and the core. The viral core consists of helical vRNP containing vRNA (minus strand) and NP along with minor amounts of NEP and polymerase complex (PA, PB1, and PB2). For viral morphogenesis to occur, all three viral components, namely the viral envelope (containing lipids and transmembrane proteins), M1, and the vRNP must be brought to the assembly site, i.e. the apical plasma membrane in polarized epithelial cells. Finally, buds must be formed at the assembly site and virus particles released with the closure of buds. Transmembrane viral proteins are transported to the assembly site on the plasma membrane via the exocytic pathway. Both HA and NA possess apical sorting signals and use lipid rafts for cell surface transport and apical sorting. These lipid rafts are enriched in cholesterol, glycosphingolipids and are relatively resistant to neutral detergent extraction at low temperature. M1 is synthesized on free cytosolic polyribosomes. vRNPs are made inside the host nucleus and are exported into the cytoplasm through the nuclear pore with the help of M1 and NEP. How M1 and vRNPs are directed to the assembly site on the plasma membrane remains unclear. The likely possibilities are that they use a piggy-back mechanism on viral glycoproteins or cytoskeletal elements. Alternatively, they may possess apical determinants or diffuse to the assembly site, or a combination of these pathways. Interactions of M1 with M1, M1 with vRNP, and M1 with HA and NA facilitate concentration of viral components and exclusion of host proteins from the budding site. M1 interacts with the cytoplasmic tail (CT) and transmembrane domain (TMD) of glycoproteins, and thereby functions as a bridge between the viral envelope and vRNP. Lipid rafts function as microdomains for concentrating viral glycoproteins and may serve as a platform for virus budding. Virus bud formation requires membrane bending at the budding site. A combination of factors including concentration of and interaction among viral components, increased viscosity and asymmetry of the lipid bilayer of the lipid raft as well as pulling and pushing forces of viral and host components are likely to cause outward curvature of the plasma membrane at the assembly site leading to bud formation. Eventually, virus release requires completion of the bud due to fusion of the apposing membranes, leading to the closure of the bud, separation of the virus particle from the host plasma membrane and release of the virus particle into the extracellular environment. Among the viral components, M1 contains an L domain motif and plays a critical role in budding. Bud completion requires not only viral components but also host components. However, how host components facilitate bud completion remains unclear. In addition to bud completion, influenza virus requires NA to release virus particles from sialic acid residues on the cell surface and spread from cell to cell. Elucidation of both viral and host factors involved in viral morphogenesis and budding may lead to the development of drugs interfering with the steps of viral morphogenesis and in disease progression.

Molecular mechanisms of membrane polarity in renal epithelial cells

Exciting discoveries in the last decade have cast light onto the fundamental mechanisms that underlie polarized trafficking in epithelial cells. It is now clear that epithelial cell membrane asymmetry is achieved by a combination of intracellular sorting operations, vectorial delivery mechanisms and plasmalemma-specific fusion and retention processes. Several well-defined signals that specify polarized segregation, sorting, or retention processes have, now, been described in a number of proteins. The intracellular machineries that decode and act on these signals are beginning to be described. In addition, the nature of the molecules that associate with intracellular trafficking vesicles to coordinate polarized delivery, tethering, docking, and fusion are also becoming understood. Combined with direct visualization of polarized sorting processes with new technologies in live-cell fluorescent microscopy, new and surprising insights into these once-elusive trafficking processes are emerging. Here we provide a review of these recent advances within an historically relevant context.

ADP-ribosylation factor 1-independent protein sorting and export from the trans-Golgi network

Polarized epithelial cells efficiently sort newly synthesized apical and basolateral proteins into distinct transport carriers that emerge from the trans-Golgi network (TGN), and this sorting is recapitulated in nonpolarized cells. While the targeting signals of basolaterally destined proteins are generally cytoplasmically disposed, apical sorting signals are not typically accessible to the cytosol, and the transport machinery required for segregation and export of apical cargo remains largely unknown. Here we investigated the molecular requirements for TGN export of the apical marker influenza hemagglutinin (HA) in HeLa cells using an in vitro reconstitution assay. HA was released from the TGN in intact membrane-bound compartments, and export was dependent on addition of an ATP-regenerating system and exogenous cytosol. HA release was inhibited by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) as well as under conditions known to negatively regulate apical transport in vivo, including expression of the acid-activated proton channel influenza M2. Interestingly, release of HA was unaffected by depletion of ADP-ribosylation factor 1, a small GTPase that has been implicated in the recruitment of all known adaptors and coat proteins to the Golgi complex. Furthermore, regulation of HA release by GTPgammaS or M2 expression was unaffected by cytosolic depletion of ADP-ribosylation factor 1, suggesting that HA sorting remains functionally intact in the absence of the small GTPase. These data suggest that TGN sorting and export of influenza HA does not require classical adaptors involved in the formation of other classes of exocytic carriers and thus appears to proceed via a novel mechanism.

Differential trafficking of transforming growth factor-beta receptors and ligand in polarized epithelial cells

Epithelial cells in vivo form tight cell-cell associations that spatially separate distinct apical and basolateral domains. These domains provide discrete cellular processes essential for proper tissue and organ development. Using confocal imaging and selective plasma membrane domain activation, the type I and type II transforming growth factor-beta (TGFbeta) receptors were found to be localized specifically at the basolateral surfaces of polarized Madin-Darby canine kidney (MDCK) cells. Receptors concentrated predominantly at the lateral sites of cell-cell contact, adjacent to the gap junctional complex. Cytoplasmic domain truncations for each receptor resulted in the loss of specific lateral domain targeting and dispersion to both the apical and basal domains. Whereas receptors concentrate basolaterally in regions of direct cell-cell contact in nonpolarized MDCK cell monolayers, receptor staining was absent from areas of noncell contact. In contrast to the defined basolateral polarity observed for the TGFbeta receptor complex, TGFbeta ligand secretion was found to be from the apical surfaces. Confocal imaging of MDCK cells with an antibody to TGFbeta1 confirmed a predominant apical localization, with a stark absence at the basal membrane. These findings indicate that cell adhesion regulates the localization of TGFbeta receptors in polarized epithelial cultures and that the response to TGFbeta is dependent upon the spatial distribution and secretion of TGFbeta receptors and ligand, respectively.

Apical accumulation of the Drosophila PDGF/VEGF receptor ligands provides a mechanism for triggering localized actin polymerization

Epithelial tissue functions depend largely on a polarized organization of the individual cells. We examined the roles of the Drosophila PDGF/VEGF receptor (PVR) in polarized epithelial cells, with specific emphasis on the wing disc epithelium. Although the receptor is broadly distributed in this tissue, two of its ligands, PVF1 and PVF3 are specifically deposited within the apical extracellular space, implying that polarized apical activation of the receptor takes place. The apical localization of the ligands involves a specialized secretion pathway. Clones for null alleles of Pvr or expression of RNAi constructs showed no phenotypes in the wing disc or pupal wing, suggesting that Pvr plays a redundant role in this tissue. However, when uniform expression of a constitutively dimerizing receptor was induced, loss of epithelial polarity, formation of multiple adherens and septate junctions, and tumorous growth were observed in the wing disc. Elevation of the level of full-length PVR also gave rise to prominent phenotypes, characterized by higher levels of actin microfilaments at the basolateral areas of the cells and irregular folding of the tissue. Together, these results suggest that polarized PVR activation is necessary for the proper organization of the wing disc epithelium, by regulating the apical assembly of the actin cytoskeleton.

Membrane dynamics and the regulation of epithelial cell polarity

Plasma membranes of epithelial cells consist of two domains, an apical and a basolateral domain, the surfaces of which differ in composition. The separation of these domains by a tight junction and the fact that specific transport pathways exist for intracellular communication between these domains and distinct intracellular compartments relevant to cell polarity development, have triggered extensive research on issues that focus on how the polarity is generated and maintained. Apart from proper assembly of tight junctions, their potential functioning as landmark for the transport machinery, cell-cell adhesion is obviously instrumental in barrier formation. In recent years, distinct endocytic compartments, defined as subapical compartment or common endosome, were shown to play a prominent role in regulating membrane trafficking to and from polarized membrane domains. Sorting devices remain to be determined but likely include distinct rab proteins, and evidence is accumulating to indicate that signaling events may direct intracellular membrane transport, intimately involved in the biogenesis and maintenance of polarized membrane domains and hence the development of cell polarity.

Subcellular trafficking of exogenously expressed interferon-? in Madin-Darby canine kidney cells

We have recently demonstrated that when IFN-beta was exogenously expressed in epithelial cells, transiently expressed IFN-beta was predominantly secreted from the cell side to which the transfection was performed, while stably expressed one was almost equally secreted to the apical and basolateral sides. In the present study, we analyzed the subcellular transport of IFN-beta using confocal imaging with green fluorescent protein (GFP)-tagged IFN-beta in Madin-Darby canine kidney (MDCK) cells. Stably expressed and transiently expressed human IFN-beta (HuIFN-beta)-GFPs were seen in upper regions of the nucleus. In stable HuIFN beta-GFP-producing transformants, transiently expressed mouse IFN-beta (MuIFN-beta) was apparently co-localized with the bulk of the constitutive HuIFN beta-GFP proteins at TGN, and a significant quantity of them then appeared to pass into distinct post-TGN vesicles, accepting either type of IFN. Meanwhile, when cells were co-transfected with both expression vectors, transiently expressed both IFNs tended to co-localize not only at TGN but in post-TGN vesicles. These results suggest that stably and transiently expressed IFN-betas, albeit co-localized at TGN, were transported through apparently discriminated post-TGN routes.

Uncovering multiple axonal targeting pathways in hippocampal neurons

Neuronal polarity is, at least in part, mediated by the differential sorting of membrane proteins to distinct domains, such as axons and somata/dendrites. We investigated the pathways underlying the subcellular targeting of NgCAM, a cell adhesion molecule residing on the axonal plasma membrane. Following transport of NgCAM kinetically, surprisingly we observed a transient appearance of NgCAM on the somatodendritic plasma membrane. Down-regulation of endocytosis resulted in loss of axonal accumulation of NgCAM, indicating that the axonal localization of NgCAM was dependent on endocytosis. Our data suggest the existence of a dendrite-to-axon transcytotic pathway to achieve axonal accumulation. NgCAM mutants with a point mutation in a crucial cytoplasmic tail motif (YRSL) are unable to access the transcytotic route. Instead, they were found to travel to the axon on a direct route. Therefore, our results suggest that multiple distinct pathways operate in hippocampal neurons to achieve axonal accumulation of membrane proteins.

1-benzyl-2-acetamido-2-deoxy-alpha-D-galactopyranoside blocks the apical biosynthetic pathway in polarized HT-29 cells

In previous work we reported that long term treatment of polarized HT-29 cells by 1-benzyl-2-acetamido-2-deoxy-alpha-d-galactopyranoside (GalNAcalpha-O-bn) induced undersialylation and intracellular distribution of apical glycoproteins such as dipeptidyl peptidase IV (DPP-IV), and we suggested therefore that sialylation could act as an apical targeting signal. In this work, the apical direct biosynthetic route was studied after transfection of polarized enterocyte-like HT-29 5M12 cloned cells with a murine cDNA coding for a soluble form of DPP-IV, which was secreted into the apical medium. A 24-h treatment of transfected cells by GalNAcalpha-O-bn markedly inhibited the apical secretion and the sialylation of this soluble murine DPP-IV, which became blocked inside the cell. A similar short GalNAcalpha-O-bn treatment also induced an intracellular distribution of both endogenous transmembrane DPP-IV and proteins involved in the regulation of the apical trafficking such as the apical t-SNARE syntaxin-3 and the raft-associated protein annexin XIIIb, whereas the basolateral t-SNARE syntaxin-4 kept its normal localization. These apical membrane proteins moved efficiently from trans-Golgi network to apical carrier vesicles but failed to be transported from carrier vesicles to the apical plasma membrane. Isolation of membrane microdomains showed that GalNAcalpha-O-bn induced the formation of abnormal lipid-rich microdomains in comparison to normal rafts, as shown by their lower buoyant density and their depletion in annexin XIIIb. In conclusion, GalNAcalpha-O-bn blocks the anterograde traffic to the apical surface of polarized HT-29 cells at the transport level or docking/fusion level of carrier vesicles.

Effect of mouse uterine stromal cells on epithelial cell transepithelial resistance (TER) and TNFalpha and TGFbeta release in culture

Recognizing that uterine stromal cells regulate several uterine epithelial cell function(s), the current study was undertaken to more fully define cell-cell communication in the uterus and to examine the role of uterine stromal cells in regulating epithelial cell monolayer integrity and cytokine release. Uterine epithelial and stromal cells from adult intact mice were isolated and cultured separately on cell culture inserts and/or in culture plates. Epithelial cells, which reach confluence as indicated by high transepithelial resistance (TER > 1000 ohms/well), preferentially release transforming growth factor-beta (TGFbeta) into the basolateral chamber ( approximately 70% > apical) and tumor necrosis factor-alpha (TNFalpha) into the apical compartment ( approximately 30% > basolateral). When epithelial cells on cell culture inserts were transferred to plates containing stromal cells, coculture for 24-48 h increased epithelial cell TER ( approximately 70% higher than control) and decreased TNFalpha release into both the apical and basolateral chambers ( approximately 30%-50%). In contrast, TGFbeta release was not affected by the presence of stromal cells. In other studies, the effects of stromal cells on epithelial cell TER and TNFalpha release persisted for 5-7 days following the removal of stromal cells and were also seen in coculture studies in which conditioned stromal media (CSM) was placed in the basolateral chamber. These studies indicate that uterine stromal cells produce a soluble factor(s) that regulates epithelial cell TER and release of TNFalpha without effecting TGFbeta release. These results suggest that uterine stromal cells communicate with epithelial cells via a soluble factor(s) to maintain uterine integrity and epithelial secretory function.

Organelle proteomics: implications for subcellular fractionation in proteomics

Functional proteome analysis is not restricted to the sequence information but includes the broad spectrum of structural modifications and quantitative changes of proteins to which they are subjected in different tissues and cell organelles and during the development of an organism. Cell biology has provided the means required for the analysis of the composition and properties of purified cellular elements. Subcellular fractionation is an approach universal across all cell types and tissues, including cardiac and vascular system. Subcellular fractionation and proteomics form an ideal partnership when it comes to enrichment and analysis of intracellular organelles and low abundant multiprotein complexes. Subcellular fractionation is a flexible and adjustable approach resulting in reduced sample complexity and is most efficiently combined with high-resolution 2D gel/mass spectrometry analysis as well as with gel-independent techniques. In this study we introduce state of the art subcellular fractionation techniques and discuss their suitability, advantages, and limitations for proteomics research.

Distinct cytoskeletal tracks direct individual vesicle populations to the apical membrane of epithelial cells

A key aspect in the structure of epithelial and neuronal cells is the maintenance of a polarized organization based on highly specific sorting machinery at the exit site of the trans Golgi network (TGN). Epithelial cells sort protein and lipid components into different sets of carriers for the apical or basolateral plasma membrane. The two intestinal proteins lactase-phlorizin hydrolase (LPH) and sucrase-isomaltase (SI) are delivered to the apical plasma membrane of epithelial cells with high fidelity but differ in their affinity to detergent-insoluble, glycolipid-enriched complexes (DIGs). Using a two-color labeling technique, we have recently characterized two post-Golgi vesicle populations that direct LPH and SI separately to the apical cell surface. Here, we investigated the structure and identification of protein components in these vesicle populations and assessed the role of cytoskeletal post-Golgi transport routes for apical cargo. Apart from the central role of microtubules in vesicle transport, we demonstrate that the transport of SI-carrying apical vesicles (SAVs) occurs along actin tracks in the cellular periphery, whereas LPH-carrying apical vesicles (LAVs) are transferred in an actin-independent fashion to the apical membrane. Our data further indicate that myosin 1A is the actin-associated motor protein that drives SAVs along actin filaments to the apical cell surface.

Stability of protein production from recombinant mammalian cells

One of the most important criteria for successful generation of a therapeutic protein from a recombinant cell is to obtain a cell line that maintains stability of production. If this is not achieved it can generate problems for process yields, effective use of time and money, and for regulatory approval of products. However, selection of a cell line that sustains stability of production over the required time period may be difficult to achieve during development of a therapeutic protein. There are several studies in the literature that have reported on the instability of protein production from recombinant cell lines. The causes of instability of production are varied and, in many cases, the exact molecular mechanisms are unknown. The production of proteins by cells is modulated by molecular events at levels ranging from transcription, posttranscriptional processing, translation, posttranslational processing, to secretion. There is potential for regulation of stability of protein production at many or all of these stages. In this study we review published information on stability of protein production for three industrially important cell lines: hybridoma, Chinese hamster ovary (CHO), and nonsecreting (NS0) myeloma cell lines. We highlight the most likely molecular loci at which instability may be engendered and indicate other areas of protein production that may affect stability from mammalian cells. We also outline approaches that could help to overcome the problems associated with unpredictable expression levels and maximized production, and indicate the consequences these might have for stability of production.

Involvement of caveolin-2 in caveolar biogenesis in MDCK cells

Caveolins have been identified as key components of caveolae, specialized cholesterol-enriched raft domains visible as small flask-shaped invaginations of the plasma membrane. In polarized MDCK cells caveolin-1 and -2 are found together on basolateral caveolae whereas the apical membrane, where only caveolin-1 is present, lacks caveolae. Expression of a caveolin mutant prevented the formation of the large caveolin-1/-2 hetero-oligomeric complexes, and led to intracellular retention of caveolin-2 and disappearance of caveolae from the basolateral membrane. Correspondingly, in MDCK cells over-expressing caveolin-2 the basolateral membrane exhibited an increased number of caveolae. These results indicate the involvement of caveolin-2 in caveolar biogenesis.

Asparagine-linked oligosaccharide-independent secretion of egg envelope glycoprotein ZPC of the Japanese quail (Coturnix japonica)

In avian species, a glycoprotein homologous to mammalian ZPC is synthesized in the granulosa cells of developing follicles. We have previously reported that the newly synthesized ZPC (proZPC) in the granulosa cells is cleaved at the consensus furin cleavage site to generate mature ZPC prior to secretion. In the present study, we examined the role of asparagine (N)-linked oligosaccharides in the proteolytic processing of proZPC and the subsequent secretion of ZPC by using site-directed mutagenesis of the consensus sequence for N-glycosylation, and tunicamycin, an inhibitor for N-glycosylation of glycoprotein. Western blot analysis demonstrated that tunicamycin did not block either proteolytic cleavage of proZPC or the subsequent ZPC secretion. Moreover, a site-directed mutant that possesses a mutated sequence for N-glycosylation was efficiently secreted from the cells. These results indicate that proteolytic cleavage of proZPC, and the subsequent ZPC secretion occur in the absence of N-linked oligosaccharides. Therefore, the addition of N-glycans to ZPC polypeptide is not required for quail ZPC secretion.

Trans-Golgi network and subapical compartment of HepG2 cells display different properties in sorting and exiting of sphingolipids

In HepG2 cells, the subapical compartment (SAC) is involved in the biogenesis of membrane polarity. By contrast, direct apical transport originating from the trans-Golgi network (TGN), which may contribute to polarity establishment, has been poorly defined in these cells. Thus, although newly synthesized sphingolipids can be directly transported from the TGN to the apical membrane, numerous apical resident proteins are traveling via the transcytotic route. Here, we developed an in vitro transport assay and compared the molecular sorting of 6-[N-(7-nitrobenz-2-oxa-1,3 diazol-4-yl)amino] hexanoyl-sphingomyelin (C(6)NBD-SM) and C(6)NBD-glucosylceramide (C(6)NBD-GlcCer) in TGN and SAC. SM is released from both TGN and SAC in the lumenal leaflet of transport vesicles. This holds also for GlcCer released from the SAC but not for a substantial fraction that departed from the Golgi. Distinct transport vesicles, enriched in either SM or GlcCer are released from SAC, consistent with their rigid sorting in this compartment. Different vesicle populations could not be recovered from TGN, although in situ experiments reveal that GlcCer is preferentially transported to the apical membrane, reflecting different transport mechanisms. The results indicate that in HepG2 cells sphingolipids are mainly sorted in the SAC membrane and that the release of SM from SAC and TGN is differentially regulated.

Membrane and cytoskeleton dynamics during axonal elongation and stabilization

Proper nervous activities are gradually developing events. Reflecting this, embryonic neurons start differentiation by sprouting multiple extensions, neurites, which do not bear clear axonal or dendritic structural and molecular characteristics. Later in development one of these multiple neurites elongates further, generating a morphologically polarized neuron with a single long axon and many short dendrites. Still, despite such morphological differences these processes can switch destiny, further reflecting their immaturity. Final and irreversible axonal and dendritic commitment occurs after both axons and dendrites have elongated considerably. Recent evidence suggests that the transition from axonal immaturity to maturity reflects changes in the mechanisms used by neurons to control the precise membrane and cytoskeleton polarization. This chapter provides an overview of how these mechanisms contribute to the formation of an axon.