Destabilization of plasma membrane structure by prevention of actin polymerization. Microtubule-dependent tubulation of the plasma membrane

Protein Delivery to Cytosol by Cell-Penetrating Peptide Bearing Tandem Repeat Penetration-Accelerating Sequence

Pas2r12 is comprised of a repeat of the penetration-accelerating sequence (Pas) (Pas2: FFLIG-FFLIG) and D-form dodeca-arginine (r12), a cell-penetrating peptide. Pas2r12 significantly enhances cytosolic delivery of cargo proteins, including enhanced green fluorescent protein and immunoglobulin G. Simply incubating Pas2r12 with cargo leads to their cytosolic tranlsocation. Cytosolic delivery of cargo by Pas2r12 involves caveolae-mediated endocytosis. In this chapter, we describe methods of cytosolic delivery of cargo using Pas2r12 and provide methods for investigating the cellular uptake pathway of cargo by Pas2r12.

Oligoarginine-Bearing Tandem Repeat Penetration-Accelerating Sequence Delivers Protein to Cytosol via Caveolae-Mediated Endocytosis

To facilitate the cytosolic delivery of larger molecules such as proteins, we developed a new cell-penetrating peptide sequence, named Pas2r12, consisting of a repeated Pas sequence (FFLIG-FFLIG) and d-dodeca-arginine (r12). This peptide significantly enhanced the cellular uptake and cytosolic release of enhanced green fluorescent protein and immunoglobulin G as cargos. We found that simply mixing Pas2r12 with cargos could generate cytosolic introducible forms. The cytosolic delivery of cargos by Pas2r12 was found to be an energy-requiring process, to rely on actin polymerization, and to be suppressed by caveolae-mediated endocytosis inhibitors (genistein and methyl-β-cyclodextrin) and small interfering RNA against caveolin-1. These results suggest that Pas2r12 enhances membrane penetration of cargos without the need for cross-linking and that caveolae-mediated endocytosis may be the route by which cytosolic delivery is enhanced.

Mold Alkaloid Cytochalasin D Modifies the Morphology and Secretion of fMLP-, LPS-, or PMA-Stimulated Neutrophils upon Adhesion to Fibronectin

Neutrophils play an essential role in innate immunity due to their ability to migrate into infected tissues and kill microbes with bactericides located in their secretory granules. Neutrophil transmigration and degranulation are tightly regulated by actin cytoskeleton. Invading pathogens produce alkaloids that cause the depolymerization of actin, such as the mold alkaloid cytochalasin D. We studied the effect of cytochalasin D on the morphology and secretion of fMLP-, LPS-, or PMA-stimulated human neutrophils upon adhesion to fibronectin. Electron microscopy showed that the morphology of the neutrophils adherent to fibronectin in the presence of various stimuli differed. But in the presence of cytochalasin D, all stimulated neutrophils exhibited a uniform nonspread shape and developed thread-like membrane tubulovesicular extensions (cytonemes) measuring 200 nm in diameter. Simultaneous detection of neutrophil secretory products by mass spectrometry showed that all tested stimuli caused the secretion of MMP-9, a key enzyme in the neutrophil migration. Cytochalasin D impaired the MMP-9 secretion but initiated the release of cathepsin G and other granular bactericides, proinflammatory agents. The release of bactericides apparently occurs through the formation, shedding, and lysis of cytonemes. The production of alkaloids which modify neutrophil responses to stimulation via actin depolymerization may be part of the strategy of pathogen invasion.

Microtubule motors power plasma membrane tubulation in clathrin-independent endocytosis

How the plasma membrane is bent to accommodate clathrin-independent endocytosis remains uncertain. Recent studies suggest Shiga and cholera toxin induce membrane curvature required for their uptake into clathrin-independent carriers by binding and cross-linking multiple copies of their glycosphingolipid receptors on the plasma membrane. But it remains unclear if toxin-induced sphingolipid crosslinking provides sufficient mechanical force for deforming the plasma membrane, or if host cell factors also contribute to this process. To test this, we imaged the uptake of cholera toxin B-subunit into surface-derived tubular invaginations. We found that cholera toxin mutants that bind to only one glycosphingolipid receptor accumulated in tubules, and that toxin binding was entirely dispensable for membrane tubulations to form. Unexpectedly, the driving force for tubule extension was supplied by the combination of microtubules, dynein and dynactin, thus defining a novel mechanism for generating membrane curvature during clathrin-independent endocytosis.

Plasma membrane--cortical cytoskeleton interactions: a cell biology approach with biophysical considerations

From a biophysical standpoint, the interface between the cell membrane and the cytoskeleton is an intriguing site where a "two-dimensional fluid" interacts with an exceedingly complex three-dimensional protein meshwork. The membrane is a key regulator of the cytoskeleton, which not only provides docking sites for cytoskeletal elements through transmembrane proteins, lipid binding-based, and electrostatic interactions, but also serves as the source of the signaling events and molecules that control cytoskeletal organization and remolding. Conversely, the cytoskeleton is a key determinant of the biophysical and biochemical properties of the membrane, including its shape, tension, movement, composition, as well as the mobility, partitioning, and recycling of its constituents. From a cell biological standpoint, the membrane-cytoskeleton interplay underlies--as a central executor and/or regulator--a multitude of complex processes including chemical and mechanical signal transduction, motility/migration, endo-/exo-/phagocytosis, and other forms of membrane traffic, cell-cell, and cell-matrix adhesion. The aim of this article is to provide an overview of the tight structural and functional coupling between the membrane and the cytoskeleton. As biophysical approaches, both theoretical and experimental, proved to be instrumental for our understanding of the membrane/cytoskeleton interplay, this review will "oscillate" between the cell biological phenomena and the corresponding biophysical principles and considerations. After describing the types of connections between the membrane and the cytoskeleton, we will focus on a few key physical parameters and processes (force generation, curvature, tension, and surface charge) and will discuss how these contribute to a variety of fundamental cell biological functions.

Mechanisms underlying the confined diffusion of cholera toxin B-subunit in intact cell membranes

Multivalent glycolipid binding toxins such as cholera toxin have the capacity to cluster glycolipids, a process thought to be important for their functional uptake into cells. In contrast to the highly dynamic properties of lipid probes and many lipid-anchored proteins, the B-subunit of cholera toxin (CTxB) diffuses extremely slowly when bound to its glycolipid receptor GM(1) in the plasma membrane of living cells. In the current study, we used confocal FRAP to examine the origins of this slow diffusion of the CTxB/GM(1) complex at the cell surface, relative to the behavior of a representative GPI-anchored protein, transmembrane protein, and fluorescent lipid analog. We show that the diffusion of CTxB is impeded by actin- and ATP-dependent processes, but is unaffected by caveolae. At physiological temperature, the diffusion of several cell surface markers is unchanged in the presence of CTxB, suggesting that binding of CTxB to membranes does not alter the organization of the plasma membrane in a way that influences the diffusion of other molecules. Furthermore, diffusion of the B-subunit of another glycolipid-binding toxin, Shiga toxin, is significantly faster than that of CTxB, indicating that the confined diffusion of CTxB is not a simple function of its ability to cluster glycolipids. By identifying underlying mechanisms that control CTxB dynamics at the cell surface, these findings help to delineate the fundamental properties of toxin-receptor complexes in intact cell membranes.

Human melanoma cells express FGFR/Src/Rho signaling that entails an adhesion-independent caveolin-1 membrane association

Caveolae have been indicated as a center of cytoskeleton regulation for Src kinase/Rho GTPase signaling. In addition, Src recruitment on intact cortical actin cytoskeleton appears to be required for bFGF/FGFR signal activation. Recently, we established a relationship between caveolin-1 (Cav-1) expression and cell migration in human malignant melanoma, constitutively activated by a bFGF autoregulatory loop. This work intends to investigate whether caveolae's asset, through bFGF/FGFR/c-Src/Rho signaling, could be related to melanoma cell anchorage. Accordingly, we revealed the existence of a FGFR/Src kinase pathway in Cav-1 enriched detergent-resistant membranes (DRMs) of Me665/1 metastatic melanoma cells, as confirmed by FGFR silencing. Moreover, we determined the expression and phosphorylation levels of Cav-1/Src/Erk signal pathway as a function of FGFR activation and cell density. A sucrose density gradient ultracentrifugation was employed to monitor Cav-1 membrane association and buoyancy in Me665/1 cells treated for actin fragmentation or for altered phosphorylation signals. As a result, melanoma cells show remarkable resistance to Cav-1 disassembly, together with persisting cell signal activity, being Src and Cav-1 crucial modulators of Rho GTPases. In conclusion, our study primarily highlights, in a metastatic melanoma cell line expressing caveolin, the circumstances whereby caveola structural and functional endurance enables the FGFR/Src/Rho GTPases pathway to keep on cell progression.

Melanin-concentrating hormone facilitates migration of preadipocytes

Adipose tissue develops from differentiating preadipocytes that expand and migrate. 3T3-L1 preadipocytes respond to melanin-concentrating hormone (MCH) by increasing leptin production. Here, we investigate whether MCH elicits remodeling of the actin cytoskeleton and whether this translates into altered migratory capacity of these cells. Incubation with MCH resulted in a loss of actin stress fibers accompanied by a change in morphology from a stretched-out fibroblast to a rounded cell. PMC-3881-PI, a MCH receptor 1 antagonist blocked the effect, confirming this receptor is solely responsible for MCH-mediated actin rearrangements. Both a pharmacological activator and inhibitor of phospholipase C were used to demonstrate this molecule's importance to the signaling pathway. Finally, MCH was shown to facilitate preadipocyte migration into a scratch wound, revealing a previously unknown role for MCH in the regulation of cellular migration. We conclude that MCH could influence the expansion of adipose tissue through its ability to enhance preadipocyte migration.

Mouse hepatitis virus type 2 enters cells through a clathrin-mediated endocytic pathway independent of Eps15

It has recently been shown that cell entry of mouse hepatitis virus type 2 (MHV-2) is mediated through endocytosis (Z. Qiu et al., J. Virol. 80:5768-5776, 2006). However, the molecular mechanism underlying MHV-2 entry is not known. Here we employed multiple chemical and molecular approaches to determine the molecular pathways for MHV-2 entry. Our results showed that MHV-2 gene expression and infectivity were significantly inhibited when cells were treated with chemical and physiologic blockers of the clathrin-mediated pathway, such as chlorpromazine and hypertonic sucrose medium. Furthermore, viral gene expression was significantly inhibited when cells were transfected with a small interfering RNA specific to the clathrin heavy chain. However, these treatments did not affect the infectivity and gene expression of MHV-A59, demonstrating the specificity of the inhibitions. In addition, overexpression of a dominant-negative mutant of caveolin 1 did not have any effect on MHV-2 infection, while it significantly blocked the caveolin-dependent uptake of cholera toxin subunit B. These results demonstrate that MHV-2 utilizes the clathrin- but not caveolin-mediated endocytic pathway for entry. Interestingly, when the cells transiently overexpressed a dominant-negative form (DIII) of Eps15, which is thought to be an essential component of the clathrin pathway, viral gene expression and infectivity were unaffected, although DIII expression blocked transferrin uptake and vesicular stomatitis virus infection, which are dependent on clathrin-mediated endocytosis. Thus, MHV-2 entry is mediated through clathrin-dependent but Eps15-independent endocytosis.

Update on BK virus entry and intracellular trafficking

BK virus (BKV) is a small, non-enveloped, double-stranded DNA virus and a member of the Polyomaviridae family. As the recently recognized etiologic agent of polyomavirus-associated nephropathy, the events involved in BKV invasion of host cells are an important area of study. Using cell culture models, the mechanism by which BKV infects permissive hosts to gain access to the replication machinery within these cells is beginning to unfold. BKV uses an N-linked glycoprotein containing an alpha(2,3)-linked sialic acid as a receptor. After this initial attachment, BKV enters cells through caveolae-mediated endocytosis. Intracellular trafficking via cellular cytoskeletal components follows this relatively slow and cholesterol-dependent internalization. BKV must reach the nucleus for viral transcription and replication to occur. Elucidating the steps of the early viral lifecycle would provide clues to help explain the infectious spread and pathology of this human pathogen.

Microtubules and actin microfilaments regulate lipid raft/caveolae localization of adenylyl cyclase signaling components

Microtubules and actin filaments regulate plasma membrane topography, but their role in compartmentation of caveolae-resident signaling components, in particular G protein-coupled receptors (GPCR) and their stimulation of cAMP production, has not been defined. We hypothesized that the microtubular and actin cytoskeletons influence the expression and function of lipid rafts/caveolae, thereby regulating the distribution of GPCR signaling components that promote cAMP formation. Depolymerization of microtubules with colchicine (Colch) or actin microfilaments with cytochalasin D (CD) dramatically reduced the amount of caveolin-3 in buoyant (sucrose density) fractions of adult rat cardiac myocytes. Colch or CD treatment led to the exclusion of caveolin-1, caveolin-2, beta1-adrenergic receptors (beta1-AR), beta2-AR, Galpha(s), and adenylyl cyclase (AC)5/6 from buoyant fractions, decreasing AC5/6 and tyrosine-phosphorylated caveolin-1 in caveolin-1 immunoprecipitates but in parallel increased isoproterenol (beta-AR agonist)-stimulated cAMP production. Incubation with Colch decreased co-localization (by immunofluorescence microscopy) of caveolin-3 and alpha-tubulin; both Colch and CD decreased co-localization of caveolin-3 and filamin (an F-actin cross-linking protein), decreased phosphorylation of caveolin-1, Src, and p38 MAPK, and reduced the number of caveolae/mum of sarcolemma (determined by electron microscopy). Treatment of S49 T-lymphoma cells (which possess lipid rafts but lack caveolae) with CD or Colch redistributed a lipid raft marker (linker for activation of T cells (LAT)) and Galpha(s) from lipid raft domains. We conclude that microtubules and actin filaments restrict cAMP formation by regulating the localization and interaction of GPCR-G(s)-AC in lipid rafts/caveolae.

Caveolin regulates microtubule polymerization in the vascular smooth muscle cells

Microtubule and caveolin have common properties in intracellular trafficking and the regulation of cellular growth. Overexpression of caveolin in vascular smooth muscle cells increased the polymer form of microtubule without changing in the total amount of tubulin, and downregulation of caveolin decreased the polymer form of microtubule. Fractionation of cellular proteins followed by immunodetection as well as immunostaining of caveolin and microtubule revealed that caveolin and a portion of microtubule were co-localized in caveolar fractions. A caveolin scaffolding domain peptide, which mimics caveolin function, did not alter the polymerization of microtubule in vitro, but dramatically inhibited the depolymerization of microtubule induced by stathmin, a microtubule destabilizing protein, which was also found in caveolar fractions. Accordingly, it is most likely that caveolin increased the polymer form of microtubule through the inhibition of a microtubule destabilizer, stathmin, suggesting a novel role of caveolin in regulating cellular network and trafficking.

Caveolae-mediated internalization of extracellular HIV-1 tat fusion proteins visualized in real time

The Tat protein from HIV-1, when fused with heterologous proteins or peptides, can traverse cell membranes. This ability has generated great interest due to potential therapeutic applications. However, the relevant cellular pathway and its dynamics have not been elucidated yet. Here we unravel the intracellular fate of exogenously added Tat fused with green fluorescent protein (GFP) in live HeLa and CHO cells, from the early interaction with the plasma membrane up to the long-term accumulation in the perinuclear region. We demonstrate that the internalization process of full-length Tat and of heterologous proteins fused to the transduction domain of Tat exploits a caveolar-mediated pathway and is inhibited at 4 degrees C. Remarkably, a slow linear movement toward the nucleus of individual GFP-tagged Tat-filled caveolae with an average velocity of 3 micro m/h was observed. No fluorescence was observed in the nucleus, possibly suggesting that Tat fusion protein unfolding is required for nuclear translocation. In addition, early sensitivity to cytochalasin-D treatment indicates the essential role of the actin cytoskeleton in the displacement of Tat vesicles toward the nucleus. Our results imply that HIV-1 Tat mediates the internalization of protein cargos in a slow and temperature-dependent manner by exploiting the caveolar pathway.

Vacuolar membrane dynamics revealed by GFP-AtVam3 fusion protein

BACKGROUND

The plant vacuole is a multifunctional organelle that has various physiological functions. The vacuole dynamically changes its function and shape, dependent on developmental and physiological conditions. Our current understanding of the dynamic processes of vacuolar morphogenesis has suffered from the lack of a marker for observing these processes in living cells.

RESULTS

We have developed transgenic Arabidopsis thaliana expressing a vacuolar syntaxin-related molecule (AtVam3/SYP22) fused with green fluorescent protein (GFP). Observations using confocal laser scanning microscopy demonstrated that the plant vacuole contained a dynamic membrane system that underwent a complex architectural remodelling. Three-dimensional reconstitution and time-lapse analysis of GFP-fluorescence images revealed that cylindrical and sheet-like structures were present in the vacuolar lumen and were moving dynamically. The movement, but not the structure itself, was abolished by cytochalasin D, an inhibitor of actin polymerization. This moving structure, which sometimes penetrated through the vacuolar lumen, possessed a dynamic membrane architecture similar to the previously recognized "transvacuolar strand."

CONCLUSION

We propose two possible models for the formation of the vacuolar lumenal structure. Membrane structures including protruding tubules and reticular networks have recently been recognized in many other organelles, and may be actively involved in intra- and/or inter-organelle signalling.

Morphology and dynamics of the endocytic pathway in Dictyostelium discoideum

Dictyostelium discoideum is a genetically and biochemically tractable social amoeba belonging to the crown group of eukaryotes. It performs some of the tasks characteristic of a leukocyte such as chemotactic motility, macropinocytosis, and phagocytosis that are not performed by other model organisms or are difficult to study. D. discoideum is becoming a popular system to study molecular mechanisms of endocytosis, but the morphological characterization of the organelles along this pathway and the comparison with equivalent and/or different organelles in animal cells and yeasts were lagging. Herein, we used a combination of evanescent wave microscopy and electron microscopy of rapidly frozen samples to visualize primary endocytic vesicles, vesicular-tubular structures of the early and late endo-lysosomal system, such as multivesicular bodies, and the specialized secretory lysosomes. In addition, we present biochemical and morphological evidence for the existence of a micropinocytic pathway, which contributes to the uptake of membrane along side macropinocytosis, which is the major fluid phase uptake process. This complex endosomal compartment underwent continuous cycles of tubulation/vesiculation as well as homo- and heterotypic fusions, in a way reminiscent of mechanisms and structures documented in leukocytes. Finally, egestion of fluid phase from the secretory lysosomes was directly observed.

Caveolae are highly immobile plasma membrane microdomains, which are not involved in constitutive endocytic trafficking

To investigate whether caveolae are involved in constitutive endocytic trafficking, we expressed N- and C- terminally green fluorescent protein (GFP)-tagged caveolin- 1 fusion proteins in HeLa, A431, and Madin-Darby canine kidney cells. The fusion proteins were shown by immunogold labeling to be sorted correctly to caveolae. By using confocal microscopy and photobleaching techniques, it was found that although intracellular structures labeled with GFP-tagged caveolin were dynamic, GFP-labeled caveolae were very immobile. However, after incubation with methyl- beta-cyclodextrin, distinct caveolae disappeared and the mobility of GFP-tagged caveolin in the plasma membrane increased. Treatment of cells with cytochalasin D caused lateral movement and aggregation of GFP-labeled caveolae. Therefore, both cholesterol and an intact actin cytoskeleton are required for the integrity of GFP-labeled caveolae. Moreover, stimulation with okadaic acid caused increased mobility and internalization of the labeled caveolae. Although the calculated mobile fraction (for t = infinity) of intracellular, GFP-tagged caveolin- associated structures was 70-90%, GFP-labeled caveolae in unstimulated cells had a mobile fraction of <20%, a value comparable to that previously reported for E-cadherin in junctional complexes. We therefore conclude that caveolae are not involved in constitutive endocytosis but represent a highly stable plasma membrane compartment anchored by the actin cytoskeleton.

Proteomic analysis of dendritic cell-derived exosomes: a secreted subcellular compartment distinct from apoptotic vesicles

Dendritic cells constitutively secrete a population of small (50-90 nm diameter) Ag-presenting vesicles called exosomes. When sensitized with tumor antigenic peptides, dendritic cells produce exosomes, which stimulate anti-tumor immune responses and the rejection of established tumors in mice. Using a systematic proteomic approach, we establish the first extensive protein map of a particular exosome population; 21 new exosomal proteins were thus identified. Most proteins present in exosomes are related to endocytic compartments. New exosomal residents include cytosolic proteins most likely involved in exosome biogenesis and function, mainly cytoskeleton-related (cofilin, profilin I, and elongation factor 1alpha) and intracellular membrane transport and signaling factors (such as several annexins, rab 7 and 11, rap1B, and syntenin). Importantly, we also identified a novel category of exosomal proteins related to apoptosis: thioredoxin peroxidase II, Alix, 14-3-3, and galectin-3. These findings led us to analyze possible structural relationships between exosomes and microvesicles released by apoptotic cells. We show that although they both represent secreted populations of membrane vesicles relevant to immune responses, exosomes and apoptotic vesicles are biochemically and morphologically distinct. Therefore, in addition to cytokines, dendritic cells produce a specific population of membrane vesicles, exosomes, with unique molecular composition and strong immunostimulating properties.

Induction of genotoxic effects and modulation of the intracellular calcium level in syrian hamster embryo (SHE) fibroblasts caused by ochratoxin A

The mycotoxin ochratoxin A (OTA) is a naturally occuring contaminant of food. The genotoxic status of OTA is still controversial because contradictory results were obtained in various microbial and mammalian gene mutation assays. In this study, OTA was investigated to examine its potency to induce micronuclei (MN) in SHE cells. The SHE-micronucleus assay revealed that OTA induces MN in a dose- and time-dependent manner. The results of kinetochore analysis revealed that mainly clastogenic events are involved in OTA genotoxicity. Induction of mitotic disturbances can be closely related to changes of the intracellular calcium concentration ([Ca2+]i). The investigated time course of OTA-induced [Ca2+]i changes revealed that the obtained signal is a short spike signal resembling physiological responses. In the absence of extracellular calcium, a long-lasting signal indicates possible damage to intracellular calcium stores or channels. Our data show that the OTA-induced [Ca2+]i rise is caused by Ca2+ -release from intracellular stores as well as Ca2+ influx from extracellular area. Finally, the influence of the changed intracellular calcium level on the actin cytoskeleton was investigated. Visualization of the actin filaments revealed time- and concentration-dependent effects. Cell shrinkage and depolymerized filaments were observed. We conclude that OTA disrupts actin filaments by a direct irreversible binding to actin.

Polymerizing microtubules activate site-directed F-actin assembly in nerve growth cones

We identify an actin-based protrusive structure in growth cones termed "intrapodium." Unlike filopodia, intrapodia are initiated exclusively within lamellipodia and elongate in a continuous (nonsaltatory) manner parallel to the plane of the dorsal plasma membrane causing a ridge-like protrusion. Intrapodia resemble the actin-rich structures induced by intracellular pathogens (e.g., Listeria) or by extracellular beads. Cytochalasin B inhibits intrapodial elongation and removal of cytochalasin B produced a burst of intrapodial activity. Electron microscopic studies revealed that lamellipodial intrapodia contain both short and long actin filaments oriented with their barbed ends toward the membrane surface or advancing end. Our data suggest an interaction between microtubule endings and intrapodia formation. Disruption of microtubules by acute nocodazole treatment decreased intrapodia frequency, and washout of nocodazole or addition of the microtubule-stabilizing drug Taxol caused a burst of intrapodia formation. Furthermore, individual microtubule ends were found near intrapodia initiation sites. Thus, microtubule ends or associated structures may regulate these actin-dependent structures. We propose that intrapodia are the consequence of an early step in a cascade of events that leads to the development of F-actin-associated plasma membrane specializations.

Intracellular trafficking pathways in the assembly of connexins into gap junctions

Trafficking pathways underlying the assembly of connexins into gap junctions were examined using living COS-7 cells expressing a range of connexin-aequorin (Cx-Aeq) chimeras. By measuring the chemiluminescence of the aequorin fusion partner, the translocation of oligomerized connexins from intracellular stores to the plasma membrane was shown to occur at different rates that depended on the connexin isoform. Treatment of COS-7 cells expressing Cx32-Aeq and Cx43-Aeq with brefeldin A inhibited the movement of these chimera to the plasma membrane by 84 +/- 4 and 88 +/- 4%, respectively. Nocodazole treatment of the cells expressing Cx32-Aeq and Cx43-Aeq produced 29 +/- 16 and 4 +/- 7% inhibition, respectively. In contrast, the transport of Cx26 to the plasma membrane, studied using a construct (Cx26/43T-Aeq) in which the short cytoplasmic carboxyl-terminal tail of Cx26 was replaced with the extended carboxyl terminus of Cx43, was inhibited 89 +/- 5% by nocodazole and was minimally affected by exposure of cells to brefeldin A (17 +/-11%). The transfer of Lucifer yellow across gap junctions between cells expressing wild-type Cx32, Cx43, and the corresponding Cx32-Aeq and Cx43-Aeq chimeras was reduced by nocodazole treatment and abolished by brefeldin A treatment. However, the extent of dye coupling between cells expressing wild-type Cx26 or the Cx26/43T-Aeq chimeras was not significantly affected by brefeldin A treatment, but after nocodazole treatment, transfer of dye to neighboring cells was greatly reduced. These contrasting effects of brefeldin A and nocodazole on the trafficking properties and intercellular dye transfer are interpreted to suggest that two pathways contribute to the routing of connexins to the gap junction.

Platelet activating factor receptor (PAF-R) is found in a large endosomal compartment in human umbilical vein endothelial cells

In previous studies, we have localized the platelet activating factor receptor (PAF-R) in situ on the surface of the endothelium in a number of microvascular beds without providing information on its intracellular location. In the present study, we used human umbilical vein cells (HUVECs) as a model to immunolocalize PAF-R by light and electron microscopic procedures. We raised two different polyclonal antibodies against synthetic peptides of the C- and N-terminal of PAF-R and used them for immunolocalization studies. By immunofluorescence, we found that the anti-C-terminal antibody (CPAF-R) stains an extensive intracellular tubular network. By electron microscopy, using a preembedding staining procedure, we detected PAF-R on the surface of the plasmalemma in a staining pattern similar to that described on microvascular endothelia in situ, but at a considerably lower density. Immunogold labeling of thin frozen sections revealed the presence of PAF-R on the plasmalemma, and especially in an extensive network of tubular-vesicular elements and vesicles associated with it. No detectable amounts of PAF-R were found in the endoplasmic reticulum (ER) or in Golgi cisternae. Double immunofluorescence labeling with antibodies for compartment marker proteins and PAF-R revealed that PAF-R localizes in an endosomal compartment. Confocal microscopy showed that PAF-R colocalizes in this compartment together with the transferrin receptor (Tf-R) and the thrombin receptor (TH-R), but it also showed that the colocalization was partial rather than complete. These findings suggest that the endosomal network is either discontinuous or, conversely, that the proteins in its membrane do not have a fully randomized distribution.

Expression of caveolin-1 and polarized formation of invaginated caveolae in Caco-2 and MDCK II cells

We have studied caveolin-1 expression and the frequency and distribution of typical invaginated caveolae as they are identified by electron microscopy in the polarized epithelial cell lines MDCK II and Caco-2. In wild-type MDCK II cells caveolin expression is high and more than 400 caveolae/mm filter were observed at the basolateral membrane. No caveolae were found at the apical surface. By contrast, wild-type Caco-2 cells do not express caveolin-1 and have extremely few, if any caveolae. Caco-2 cells were stably transfected with the gene for caveolin-1 in order to investigate if the formation of caveolae is polarized also in these cells. We have isolated Caco-2 clones expressing different levels of caveolin-1, where the level of expression varies from 10–100% of the endogenous level in MDCK II cells. Caveolin-1 expression in Caco-2 cells gives rise to a marked immunofluorescense labeling mainly at the lateral plasma membrane. By electron microscopy an increase from less than 4 caveolae/mm filter in wild-type Caco-2 cells to 21–76 caveolae/mm filter in Caco-2 clones transfected with caveolin-1 was revealed and these caveolae were exclusively localized to the basolateral membrane. Thus expression of heterologous caveolin-1 in Caco-2 cells leads to polarized formation of caveolae, but there is a lack of correlation between the amount of caveolin expressed in the cells and the number of caveolae, suggesting that factors in addition to caveolin are required for generation of caveolae.

ADP-ribosylation factor 6 regulates a novel plasma membrane recycling pathway

ADP-ribosylation factor (ARF) 6 localizes to the plasma membrane (PM) in its GTP state and to a tubulovesicular compartment in its GDP state in HeLa cells that express wild-type or mutant forms of this GTPase. Aluminum fluoride (AlF) treatment of ARF6-transfected cells redistributes ARF6 to the PM and stimulates the formation of actin-rich surface protrusions. Here we show that cytochalasin D (CD) treatment inhibited formation of the AlF-induced protrusions and shifted the distribution of ARF6 to a tubular membrane compartment emanating from the juxtanuclear region of cells, which resembled the compartment where the GTP-binding defective mutant of ARF6 localized. This membrane compartment was distinct from transferrin-positive endosomes, could be detected in the absence of ARF6 overexpression or CD treatment, and was accessible to loading by PM proteins lacking clathrin/AP-2 cytoplasmic targeting sequences, such as the IL-2 receptor alpha subunit Tac. ARF6 and surface Tac moved into this compartment and back out to the PM in the absence of pharmacologic treatment. Whereas AlF treatment blocked internalization, CD treatment blocked the recycling of wild-type ARF6 and Tac back to the PM; these blocks were mimicked by expression of ARF6 mutants Q67L and T27N, which were predicted to be in either the GTP- or GDP-bound state, respectively. Thus, the ARF6 GTP cycle regulates this membrane traffic pathway. The delivery of ARF6 and membrane to defined sites along the PM may provide components necessary for remodeling the cell surface and the underlying actin cytoskeleton.

Evidence of a tubular system for transendothelial transport in arterial capillaries of the rete mirabile

The arterial endothelial cells of the rete capillaries of the eel were examined by transmission electron microscopy on thin sections, on freeze-fracture replicas, by scanning electron microscopy, after cytochemical osmium impregnation and perfusion with peroxidase. The study revealed the existence of membrane-bound tubules and vesicles that open at both the luminal and abluminal poles of the cell and at the level of the intercellular space. The tubules are straight or present successive dilations and constrictions. They branch in various directions and intrude deeply into the cell cytoplasm, forming a complex tubular network within the cell. Immunocytochemical techniques were applied on immersion-fixed tissues and on perfusion of the capillaries with albumin and insulin. These demonstrated that the tubular-vesicular system is involved in the transport of circulating proteins. Furthermore, protein A-gold immunocytochemistry has revealed the association of actin with the membranes of this system. On the basis of these results, we suggest that the transendothelial transport of serum proteins takes place by a transcytotic process through a membrane-bound tubular-vesicular system and is equivalent to the large pore system presumed from functional studies.