Caveolin-1 Interacts Directly with Dynamin-2

Caveolin-3 negatively regulates endocytic recycling of cardiac KATP channels

Sarcolemmal ATP-sensitive potassium (KATP) channels play a vital role in cardioprotection. Cardiac KATP channels are enriched in caveolae and physically interact with the caveolae structural protein caveolin-3 (Cav3). Disrupting caveolae impairs the regulation of KATP channels through several signaling pathways. However, the direct functional effect of Cav3 on KATP channels is still poorly understood. Here, we used the cardiac KATP channel subtype, Kir6.2/SUR2A, and showed that Cav3 greatly reduced KATP channel surface density and current amplitude in a caveolae-independent manner. A screen of Cav3 functional domains revealed that a 25 amino acid region in the membrane attachment domain of Cav3 is the minimal effective segment (MAD1). The peptide corresponding to the MAD1 segment decreased KATP channel current in a concentration-dependent manner with an IC50 of ∼5 μM. The MAD1 segment prevented KATP channel recycling, thus decreasing KATP channel surface density and abolishing the cardioprotective effect of ischemic preconditioning. Our research identified the Cav3 MAD1 segment as a novel negative regulator of KATP channel recycling, providing pharmacological potential in the treatment of diseases with KATP channel trafficking defects.NEW & NOTEWORTHY Cardiac KATP channels physically interact with caveolin-3 in caveolae. In this study, we investigated the functional effect of caveolin-3 on KATP channel activity and identified a novel segment (MAD1) in the C-terminus domain of Caveolin-3 that negatively regulates KATP channel surface density and current amplitude by impairing KATP channel recycling. The peptide corresponding to the MAD1 segment abolished the cardioprotective effect of ischemic preconditioning.

Dynamin2 functions as an accessory protein to reduce the rate of caveola internalization

Caveolae are small membrane invaginations that generally are stably attached to the plasma membrane. Their release is believed to depend on the GTPase dynamin 2 (Dyn2), in analogy with its role in fission of clathrin-coated vesicles. The mechanistic understanding of caveola fission is, however, sparse. Here, we used microscopy-based tracking of individual caveolae in living cells to determine the role of Dyn2 in caveola dynamics. We report that Dyn2 stably associated with the bulb of a subset of caveolae, but was not required for formation or fission of caveolae. Dyn2-positive caveolae displayed longer plasma membrane duration times, whereas depletion of Dyn2 resulted in shorter duration times and increased caveola fission. The stabilizing role of Dyn2 was independent of its GTPase activity and the caveola stabilizing protein EHD2. Thus, we propose that, in contrast to the current view, Dyn2 is not a core component of the caveolae machinery, but rather functions as an accessory protein that restrains caveola internalization.

A universal method to analyze cellular internalization mechanisms via endocytosis without non-specific cross-effects

Endocytosis is an essential biological process for nutrient absorption and intercellular communication; it can also be used to accelerate the cellular internalization of drug delivery carriers. Clarifying the cellular uptake mechanisms of unidentified endogenous and exogenous molecules and designing new effective drug delivery systems require an accurate, specific endocytosis analysis methodology. Therefore, we developed a method to specifically evaluate cellular internalization via three main endocytic pathways: clathrin- and caveolae-mediated endocytosis, and macropinocytosis. We first revealed that most known endocytosis inhibitors had no specific inhibitory effect or were cytotoxic. Second, we successfully established an alternative method using small interfering RNA to knock down dynamin-2 and caveolin-1, which are necessary for clathrin- and caveolae-mediated endocytosis, in HeLa cells. Third, we established another method to specifically analyze macropinocytosis using rottlerin on A431 cells. Finally, we validated the proposed methods by testing the cellular internalization of a biological molecule (insulin) and carriers (nanoparticles and cell-penetrating peptides). Through this study, we established versatile methods to precisely and specifically evaluate endocytosis of newly developed biopharmaceuticals or drug delivery systems.

The molecular organization of differentially curved caveolae indicates bendable structural units at the plasma membrane

Caveolae are small coated plasma membrane invaginations with diverse functions. Caveolae undergo curvature changes. Yet, it is unclear which proteins regulate this process. To address this gap, we develop a correlative stimulated emission depletion (STED) fluorescence and platinum replica electron microscopy imaging (CLEM) method to image proteins at single caveolae. Caveolins and cavins are found at all caveolae, independent of curvature. EHD2 is detected at both low and highly curved caveolae. Pacsin2 associates with low curved caveolae and EHBP1 with mostly highly curved caveolae. Dynamin is absent from caveolae. Cells lacking dynamin show no substantial changes to caveolae, suggesting that dynamin is not directly involved in caveolae curvature. We propose a model where caveolins, cavins, and EHD2 assemble as a cohesive structural unit regulated by intermittent associations with pacsin2 and EHBP1. These coats can flatten and curve to enable lipid traffic, signaling, and changes to the surface area of the cell.

LAMTOR1 degrades MHC-II via the endocytic in hepatocellular carcinoma

Tumor cell surface antigen recognition is a major hallmark of cancer therapy, and loss of major histocompatibility complex class I (MHC-I) is the most common mechanism that impairs tumor cell surface antigen processing and expression. In addition to this, MHC-II regulates antigen presentation in CD4+ T cell immune responses involved in tumor killing by CD8+ T cells, whereas the regulation of endocytosis regulating MHC-II antigen presentation has not been reported. Therefore, the regulation of the endocytosis pathway on the expression of MHC-II surface level and antitumor T cell response remains to be explored. In this experiment, we found that LAMTOR1 regulates the endocytic pathway through the GTPase domain of DNM2 and triggers the formation of autophagosomes. We performed flow cytometry and western blotting analyses, which revealed that the expression of MHC-II molecules on the surface of cells is influenced by LAMTOR1 through the endocytic pathway. We showed that the expression of MHC-II molecules, which recognize CD4+ T cells on the surface of cells, was regulated by LAMTOR1 through an endocytic pathway. By coculture experiments, we showed that CD8+/CD4+ T cells exhibit substantially higher levels of tumor cell apoptosis than those observed when hepatocellular carcinoma (HCC) cells were cocultured with CD8+ T cells alone. This study revealed that LAMTOR1 decreases the expression levels of MHC-II on cell surfaces in order to reduce antigen expression, leading to a decrease in antitumor T cell responses.

Protective role of Cav-1 in pneumolysin-induced endothelial barrier dysfunction

Pneumolysin (PLY) is a bacterial pore forming toxin and primary virulence factor of Streptococcus pneumonia, a major cause of pneumonia. PLY binds cholesterol-rich domains of the endothelial cell (EC) plasma membrane resulting in pore assembly and increased intracellular (IC) Ca2+ levels that compromise endothelial barrier integrity. Caveolae are specialized plasmalemma microdomains of ECs enriched in cholesterol. We hypothesized that the abundance of cholesterol-rich domains in EC plasma membranes confers cellular susceptibility to PLY. Contrary to this hypothesis, we found increased PLY-induced IC Ca2+ following membrane cholesterol depletion. Caveolin-1 (Cav-1) is an essential structural protein of caveolae and its regulation by cholesterol levels suggested a possible role in EC barrier function. Indeed, Cav-1 and its scaffolding domain peptide protected the endothelial barrier from PLY-induced disruption. In loss of function experiments, Cav-1 was knocked-out using CRISPR-Cas9 or silenced in human lung microvascular ECs. Loss of Cav-1 significantly enhanced the ability of PLY to disrupt endothelial barrier integrity. Rescue experiments with re-expression of Cav-1 or its scaffolding domain peptide protected the EC barrier against PLY-induced barrier disruption. Dynamin-2 (DNM2) is known to regulate caveolar membrane endocytosis. Inhibition of endocytosis, with dynamin inhibitors or siDNM2 amplified PLY induced EC barrier dysfunction. These results suggest that Cav-1 protects the endothelial barrier against PLY by promoting endocytosis of damaged membrane, thus reducing calcium entry and PLY-dependent signaling.

Deciphering the relationship between caveolae-mediated intracellular transport and signalling events

The caveolae-mediated transport across polarized epithelial cell barriers has been largely deciphered in the last decades and is considered the second essential intracellular transfer mechanism, after the clathrin-dependent endocytosis. The basic cell biology knowledge was supplemented recently, with the molecular mechanisms beyond caveolae generation implying the key contribution of the lipid-binding proteins (the structural protein Caveolin and the adapter protein Cavin), along with the bulb coat stabilizing molecules PACSIN-2 and Eps15 homology domain protein-2. The current attention is focused also on caveolae architecture (such as the bulb coat, the neck, the membrane funnel inside the bulb, and the associated receptors), and their specific tasks during the intracellular transport of various cargoes. Here, we resume the present understanding of the assembly, detachment, and internalization of caveolae from the plasma membrane lipid raft domains, and give an updated view on transcytosis and endocytosis, the two itineraries of cargoes transport via caveolae. The review adds novel data on the signalling molecules regulating caveolae intracellular routes and on the transport dysregulation in diseases. The therapeutic possibilities offered by exploitation of Caveolin-1 expression and caveolae trafficking, and the urgent issues to be uncovered conclude the review.

Sterolight as imaging tool to study sterol uptake, trafficking and efflux in living cells

Information about cholesterol subcellular localization and transport pathways inside cells is essential for understanding and treatment of cholesterol-related diseases. However, there is a lack of reliable tools to monitor it. This work follows the fate of Sterolight, a BODIPY-labelled sterol, within the cell and demonstrates it as a suitable probe for visualization of sterol/lipid trafficking. Sterolight enters cells through an energy-independent process and knockdown experiments suggest caveolin-1 as its potential cellular carrier. Intracellular transport of Sterolight is a rapid process, and transfer from ER and mitochondria to lysosomes and later to lipid droplets requires the participation of active microtubules, as it can be inhibited by the microtubule disruptor nocodazole. Excess of the probe is actively exported from cells, in addition to being stored in lipid droplets, to re-establish the sterol balance. Efflux occurs through a mechanism requiring energy and may be selectively poisoned with verapamil or blocked in cells with mutated cholesterol transporter NPC1. Sterolight is efficiently transferred within and between different cell populations, making it suitable for monitoring numerous aspects of sterol biology, including the live tracking and visualization of intracellular and intercellular transport.

Recruitment of TNF ligands to lipid rafts is mediated by their physical association with caveolin-1

Activities of the tumour necrosis factor (TNF) family members are associated with their targeting to lipid rafts, specialised regions of the plasma membrane. Herein, we investigated the physical association of TNF and its family members cluster of differentiation 40 ligand (CD40L) and tumour necrosis factor-related apoptosis-inducing ligand with caveolin-1, a lipid raft resident protein. We discovered that the intracellular domains of TNF and CD40L interact with caveolin-1, and the membrane proximal region of TNF is required for the binding of caveolin-1 domains. Full-length TNF can form a complex with caveolin-1 in membrane rafts of HeLa cells, and caveolin-1 knockdown leads to impaired TNF transport to rafts. These findings provide the first evidence of a direct interaction between TNF, CD40L and caveolin-1 and suggest that caveolin-1 may be responsible for recruiting TNF to lipid rafts.

New insights into exosome mediated tumor-immune escape: Clinical perspectives and therapeutic strategies

Recent advances in extracellular vesicle biology have uncovered a substantial role in maintaining cell homeostasis in health and disease conditions by mediating intercellular communication, thus catching the scientific community's attention worldwide. Extracellular microvesicles, some called exosomes, functionally transfer biomolecules such as proteins and non-coding RNAs from one cell to another, influencing the local environment's biology. Although numerous advancements have been made in treating cancer patients with immune therapy, controlling the disease remains a challenge in the clinic due to tumor-driven interference with the immune response and inability of immune cells to clear cancer cells from the body. The present review article discusses the recent findings and knowledge gaps related to the role of exosomes derived from tumors and the tumor microenvironment cells in tumor escape from immunosurveillance. Further, we highlight examples where exosomal non-coding RNAs influence immune cells' response within the tumor microenvironment and favor tumor growth and progression. Therefore, exosomes can be used as a therapeutic target for the treatment of human cancers.

Energy and Dynamics of Caveolae Trafficking

Caveolae are 70–100 nm diameter plasma membrane invaginations found in abundance in adipocytes, endothelial cells, myocytes, and fibroblasts. Their bulb-shaped membrane domain is characterized and formed by specific lipid binding proteins including Caveolins, Cavins, Pacsin2, and EHD2. Likewise, an enrichment of cholesterol and other lipids makes caveolae a distinct membrane environment that supports proteins involved in cell-type specific signaling pathways. Their ability to detach from the plasma membrane and move through the cytosol has been shown to be important for lipid trafficking and metabolism. Here, we review recent concepts in caveolae trafficking and dynamics. Second, we discuss how ATP and GTP-regulated proteins including dynamin and EHD2 control caveolae behavior. Throughout, we summarize the potential physiological and cell biological roles of caveolae internalization and trafficking and highlight open questions in the field and future directions for study.

The Potential Contribution of Caveolin 1 to HIV Latent Infection

Combinatorial antiretroviral therapy (cART) suppresses HIV replication to undetectable levels and has been effective in prolonging the lives of HIV infected individuals. However, cART is not capable of eradicating HIV from infected individuals mainly due to HIV’s persistence in small reservoirs of latently infected resting cells. Latent infection occurs when the HIV-1 provirus becomes transcriptionally inactive and several mechanisms that contribute to the silencing of HIV transcription have been described. Despite these advances, latent infection remains a major hurdle to cure HIV infected individuals. Therefore, there is a need for more understanding of novel mechanisms that are associated with latent infection to purge HIV from infected individuals thoroughly. Caveolin 1(Cav-1) is a multifaceted functional protein expressed in many cell types. The expression of Cav-1 in lymphocytes has been controversial. Recent evidence, however, convincingly established the expression of Cav-1 in lymphocytes. In lieu of this finding, the current review examines the potential role of Cav-1 in HIV latent infection and provides a perspective that helps uncover new insights to understand HIV latent infection.

VEGF/VEGFR-2 system exerts neuroprotection against Phoneutria nigriventer spider envenomation through PI3K-AKT-dependent pathway

This study was undertaken to elucidate why VEGF/VEGFR-2 is elevated in the hippocampus of rats injected with Phoneutria nigriventer spider venom (PNV). PNV delays Na⁺ channels inactivation; blocks Ca²⁺ and K⁺ channels, increases glutamate release, causes blood-brain breakdown (BBBb), brain edema and severe excitotoxicity. Analytical FT-IR spectroscopy showed profound alteration in molecular biochemical state, with evidences for VEGFR-2 (KDR/Flk-1) signaling mediation. By blocking VEGF/VEGFR-2 binding via pre-treatment with itraconazole we demonstrated that animals' condition was deteriorated soon at 1-2 h post-PNV exposure concurrently with decreased expression of VEGF, BBB-associated proteins, ZO-1, β-catenin, laminin, P-gp (P-glycoprotein), Neu-N (neuron's viability marker) and MAPKphosphorylated-p38, while phosphorylated-ERK and Src pathways were increased. At 5 h and coinciding with incipient signs of animals' recuperation, the proteins associated with protection (HIF-1α, VEGF, VEGFR-1, VEGFR-2, Neu-N, occludin, β-catenin, laminin, P-gp efflux protein, phosphorylated-p38) increased thus indicating p38 pathway activation together with paracellular route strengthening. However, the BBB transcellular trafficking and caspase-3 increased (pro-apoptotic pathway activation). At 24 h, the transcellular route reestablished physiological state but the pro-survival pathway PI3K/(p-Akt) dropped in animals underwent VEGF/VEGFR-2 binding inhibition, whereas it was significantly activated at matched interval in PNV group without prior itraconazole; these results demonstrate impaired VEGF' survival effects at 24 h. The inhibition of VEGF/VEGFR-2 binding identified 5 h as turning point at which multi-level dynamic interplay was elicited to reverse hippocampal damage. Collectively, the data confirmed VEGFR-2 signaling via serine-threonine kinase Akt as neuroprotective pathway against PNV-induced damage. Further studies are needed to elucidate mechanisms underlying PNV effects.

Keeping in touch with the membrane; protein- and lipid-mediated confinement of caveolae to the cell surface

Caveolae are small Ω-shaped invaginations of the plasma membrane that play important roles in mechanosensing, lipid homeostasis and signaling. Their typical morphology is characterized by a membrane funnel connecting a spherical bulb to the membrane. Membrane funnels (commonly known as necks and pores) are frequently observed as transient states during fusion and fission of membrane vesicles in cells. However, caveolae display atypical dynamics where the membrane funnel can be stabilized over an extended period of time, resulting in cell surface constrained caveolae. In addition, caveolae are also known to undergo flattening as well as short-range cycles of fission and fusion with the membrane, requiring that the membrane funnel closes or opens up, respectively. This mini-review considers the transition between these different states and highlights the role of the protein and lipid components that have been identified to control the balance between surface association and release of caveolae.

Passing the Vascular Barrier: Endothelial Signaling Processes Controlling Extravasation

A central function of the vascular endothelium is to serve as a barrier between the blood and the surrounding tissue of the body. At the same time, solutes and cells have to pass the endothelium to leave or to enter the bloodstream to maintain homeostasis. Under pathological conditions, for example, inflammation, permeability for fluid and cells is largely increased in the affected area, thereby facilitating host defense. To appropriately function as a regulated permeability filter, the endothelium uses various mechanisms to allow solutes and cells to pass the endothelial layer. These include transcellular and paracellular pathways of which the latter requires remodeling of intercellular junctions for its regulation. This review provides an overview on endothelial barrier regulation and focuses on the endothelial signaling mechanisms controlling the opening and closing of paracellular pathways for solutes and cells such as leukocytes and metastasizing tumor cells.

Disparate Entry of Adenoviruses Dictates Differential Innate Immune Responses on the Ocular Surface

Human adenovirus infection of the ocular surface is associated with severe keratoconjunctivitis and the formation of subepithelial corneal infiltrates, which may persist and impair vision for months to years following infection. Long term pathology persists well beyond the resolution of viral replication, indicating that the prolonged immune response is not virus-mediated. However, it is not clear how these responses are sustained or even initiated following infection. This review discusses recent work from our laboratory and others which demonstrates different entry pathways specific to both adenovirus and cell type. These findings suggest that adenoviruses may stimulate specific pattern recognition receptors in an entry/trafficking-dependent manner, leading to distinct immune responses dependent on the virus/cell type combination. Additional work is needed to understand the specific connections between adenoviral entry and the stimulation of innate immune responses by the various cell types present on the ocular surface.

Pathophysiological Role of Caveolae in Hypertension

Caveolae, flask-shaped cholesterol-, and glycosphingolipid-rich membrane microdomains, contain caveolin 1, 2, 3 and several structural proteins, in particular Cavin 1-4, EHD2, pacsin2, and dynamin 2. Caveolae participate in several physiological processes like lipid uptake, mechanosensitivity, or signaling events and are involved in pathophysiological changes in the cardiovascular system. They serve as a specific membrane platform for a diverse set of signaling molecules like endothelial nitric oxide synthase (eNOS), and further maintain vascular homeostasis. Lack of caveolins causes the complete loss of caveolae; induces vascular disorders, endothelial dysfunction, and impaired myogenic tone; and alters numerous cellular processes, which all contribute to an increased risk for hypertension. This brief review describes our current knowledge on caveolae in vasculature, with special focus on their pathophysiological role in hypertension.

Caveolin-1 and dynamin-2 overexpression is associated with the progression of bladder cancer

Caveolae-mediated endocytosis regulates cell adhesion and growth in an anchorage-dependent manner. Studies of the endocytic function of caveolae have suggested a wide-ranging list of cargoes, including a number of receptors and extracellular proteins, ligands and nutrients from the extracellular matrix. Disruption of the processes of caveolae-mediated endocytosis mediated by signaling proteins is critical to cellular integrity. Caveolin-1 and dynamin-2 are the 2 major proteins associated with endocytotic function. Mechanistically, dynamin-2 has a co-equal role with caveolin-1 in terms of caveolae-derived endosome formation. Recent studies have revealed the pathological outcomes associated with the dysregulation of caveolin-1 and dynamin-2 expression. Increased expression levels of the gene for caveolin, Cav-1, resulting in augmented cellular metastasis and invasion, have been demonstrated in various types of cancer, and overexpression of the gene for dynamin-2, DNM2, has been associated with tumorigenesis in cervical, pancreatic and lung cancer. An increased expression of Cav-1 and DNM2 is known to be associated with the invasive behavior of cancer cells, and with cancer progression. Furthermore, it has been previously demonstrated that, in caveolar assembly and caveolae mediated endocytosis, Cav-1 interacts directly with DNM2 during the processes. Altered expression of the 2 genes is critical for the normal function of the cell. The expression patterns of Cav-1 and DNM2 have been previously examined in bladder cancer cell lines, and were each demonstrated to be overexpressed. In the present study, the expression levels of these 2 genes in bladder cancer samples were quantified. The gene expression levels of Cav-1 and DNM2 were identified to be increased 8.88- and 8.62-fold, respectively, in tumors compared with the normal controls. Furthermore, high-grade tumors exhibited significantly increased expression levels of Cav-1 and DNM2 (both P<0.0001) compared with the low-grade tumors. In addition, compared with normal control samples, the expression of the 2 genes in tumor samples was observed to be highly significant (P<0.0001), with a marked positive correlation identified for the tumors (Pearson's correlation coefficient, r=0.80 for the tumor samples vs. r=0.32 in the normal control samples). Taken together, the results of the present study demonstrated that the overexpression of Cav-1 and DNM2 genes, and a determination of their correlation coefficients, may be a potential risk factor for bladder cancer, in addition to other clinical factors.

Real-time imaging of sodium glucose transporter (SGLT1) trafficking and activity in single cells

The processes controlling targeting of glucose transporters to apical and basolateral membranes of polarized cells are complex and not-well understood. We have engineered SGLT1 and GLUT4 constructs linked to fluorescent proteins to highlight the differences in transporter expression and trafficking, in real time, in different cell types. Activity was assessed in parallel using a FRET glucose sensor. In COS cells and HEK cells, SGLT1 was distributed between the plasma membrane and intracellular compartments, but there was little expression in CHO cells. Trafficking was investigated using the lysosome inhibitors NH₄Cl (10 mmol/L) and chloroquine (150 μmol/L) and the proteasome inhibitors MG-262 (1 μmol/L) and lactacystin (5 μmol/L). Lysosome inhibitors caused SGLT1 accumulation into intracellular bodies, whereas proteasome inhibitors induced SGLT1 accumulation in the plasma membrane, even in CHO cells. Our data suggest that a fraction of SGLT1 is rapidly degraded by lysosomes and never reached the plasma membrane; another fraction reaches the membrane and is subsequently degraded by lysosomes following internalization. The latter process is regulated by the ubiquitin/proteasome pathway, acting at a late stage of the lysosomal pathway. Using the cholesterol inhibitor MβCD (3 mmol/L), a dominant negative dynamin (K44A) and caveolin, we showed that SGLT1 internalization is lipid raft-mediated, but caveolin-independent. In contrast, GLUT4 internalization is dynamin-dependent, but cholesterol-independent. The physiological relevance of these data is discussed in terms of differential membrane compartmentalization of the transporters and expression under stress conditions.

Cell-free formation and interactome analysis of caveolae

Caveolae are linked to signaling protein regulation through interactions with caveolins. We describe a cell-free system for the biogenesis of caveolae and show phosphorylated-caveolins preferentially bind signaling proteins. Our validation in vivo shows that phosphorylated CAV1 recruits TRAF2 to the endosome to form a signaling platform.

Caveolae have been linked to the regulation of signaling pathways in eukaryotic cells through direct interactions with caveolins. Here, we describe a cell-free system based on Leishmania tarentolae (Lt) extracts for the biogenesis of caveolae and show its use for single-molecule interaction studies. Insertion of expressed caveolin-1 (CAV1) into Lt membranes was analogous to that of caveolin in native membranes. Electron tomography showed that caveolins generate domains of precise size and curvature. Cell-free caveolae were used in quantitative assays to test the interaction of membrane-inserted caveolin with signaling proteins and to determine the stoichiometry of interactions. Binding of membrane-inserted CAV1 to several proposed binding partners, including endothelial nitric-oxide synthase, was negligible, but a small number of proteins, including TRAF2, interacted with CAV1 in a phosphorylation-(CAV1^Y14)–stimulated manner. In cells subjected to oxidative stress, phosphorylated CAV1 recruited TRAF2 to the early endosome forming a novel signaling platform. These findings lead to a novel model for cellular stress signaling by CAV1.

Impairment of Fas-ligand-caveolin-1 interaction inhibits Fas-ligand translocation to rafts and Fas-ligand-induced cell death

Fas-ligand/CD178 belongs to the TNF family proteins and can induce apoptosis through death receptor Fas/CD95. The important requirement for Fas-ligand-dependent cell death induction is its localization to rafts, cholesterol- and sphingolipid-enriched micro-domains of membrane, involved in regulation of different signaling complexes. Here, we demonstrate that Fas-ligand physically associates with caveolin-1, the main protein component of rafts. Experiments with cells overexpressing Fas-ligand revealed a FasL N-terminal pre-prolin-rich region, which is essential for the association with caveolin-1. We found that the N-terminal domain of Fas-ligand bears two caveolin-binding sites. The first caveolin-binding site binds the N-terminal domain of caveolin-1, whereas the second one appears to interact with the C-terminal domain of caveolin-1. The deletion of both caveolin-binding sites in Fas-ligand impairs its distribution between cellular membranes, and attenuates a Fas-ligand-induced cytotoxicity. These results demonstrate that the interaction of Fas-ligand and caveolin-1 represents a molecular basis for Fas-ligand translocation to rafts, and the subsequent induction of Fas-ligand-dependent cell death. A possibility of a similar association between other TNF family members and caveolin-1 is discussed.

Mechanistic understanding the bioeffects of ultrasound-driven microbubbles to enhance macromolecule delivery

Ultrasound-driven microbubbles can trigger reversible membrane perforation (sonoporation), open interendothelial junctions and stimulate endocytosis, thereby providing a temporary and reversible time-window for the delivery of macromolecules across biological membranes and endothelial barriers. This time-window is related not only to cavitation events, but also to biological regulatory mechanisms. Mechanistic understanding of the interaction between cavitation events and cells and tissues, as well as the subsequent cellular and molecular responses will lead to new design strategies with improved efficacy and minimized side effects. Recent important progress on the spatiotemporal characteristics of sonoporation, cavitation-induced interendothelial gap and endocytosis, and the spatiotemporal bioeffects and the preliminary biological mechanisms in cavitation-enhanced permeability, has been made. On the basis of the summary of this research progress, this Review outlines the underlying bioeffects and the related biological regulatory mechanisms involved in cavitation-enhanced permeability; provides a critical commentary on the future tasks and directions in this field, including developing a standardized methodology to reveal mechanism-based bioeffects in depth, and designing biology-based treatment strategies to improve efficacy and safety. Such mechanistic understanding the bioeffects that contribute to cavitation-enhanced delivery will accelerate the translation of this approach to the clinic.

Escherichia coli K1 utilizes host macropinocytic pathways for invasion of brain microvascular endothelial cells

Eukaryotic cells utilize multiple endocytic pathways for specific uptake of ligands or molecules, and these pathways are commonly hijacked by pathogens to enable host cell invasion. Escherichia coli K1, a pathogenic bacterium that causes neonatal meningitis, invades the endothelium of the blood-brain barrier, but the entry route remains unclear. Here, we demonstrate that the bacteria trigger an actin-mediated uptake route, stimulating fluid phase uptake, membrane ruffling and macropinocytosis. The route of uptake requires intact lipid rafts as shown by cholesterol depletion. Using a variety of perturbants we demonstrate that small Rho GTPases and their downstream effectors have a significant effect on bacterial invasion. Furthermore, clathrin-mediated endocytosis appears to play an indirect role in E. coli K1 uptake. The data suggest that the bacteria effect a complex interplay between the Rho GTPases to increase their chances of uptake by macropinocytosis into human brain microvascular endothelial cells.

Caveolin-1 Influences LFA-1 Redistribution upon TCR Stimulation in CD8 T Cells

TCR stimulation by peptide-MHC complexes on APCs requires precise reorganization of molecules into the area of cellular contact to form an immunological synapse from where T cell signaling is initiated. Caveolin (Cav)1, a widely expressed transmembrane protein, is involved in the regulation of membrane composition, cellular polarity and trafficking, and the organization of signal transduction pathways. The presence of Cav1 protein in T cells was identified only recently, and its function in this context is not well understood. We show that Cav1-knockout CD8 T cells have a reduction in membrane cholesterol and sphingomyelin, and upon TCR triggering they exhibit altered morphology and polarity, with reduced effector function compared with Cav1 wild-type CD8 T cells. In particular, redistribution of the β₂ integrin LFA-1 to the immunological synapse is compromised in Cav1-knockout T cells, as is the ability of LFA-1 to form high-avidity interactions with ICAM-1. Our results identify a role for Cav1 in membrane organization and β₂ integrin function in primary CD8 T cells.

Caveolin-1 regulates TCR signal strength and regulatory T-cell differentiation into alloreactive T cells

Caveolin-1 (Cav-1) is a key organizer of membrane specializations and a scaffold protein that regulates signaling in multiple cell types. We found increased Cav-1 expression in human and murine T cells after allogeneic hematopoietic cell transplantation. Indeed, Cav-1(-/-)donor T cells caused less severe acute graft-versus-host disease (GVHD) and yielded higher numbers of regulatory T cells (Tregs) compared with controls. Depletion of Tregs from the graft abrogated this protective effect. Correspondingly, Treg frequencies increased when Cav-1(-/-)T cells were exposed to transforming growth factor-β/T-cell receptor (TCR)/CD28 activation or alloantigen stimulation in vitro compared with wild-type T cells. Mechanistically, we found that the phosphorylation of Cav-1 is dispensable for the control of T-cell fate by using a nonphosphorylatable Cav-1 (Y14F/Y14F) point-mutation variant. Moreover, the close proximity of lymphocyte-specific protein tyrosine kinase (Lck) to the TCR induced by TCR-activation was reduced in Cav-1(-/-)T cells. Therefore, less TCR/Lck clustering results in suboptimal activation of the downstream signaling events, which correlates with the preferential development into a Treg phenotype. Overall, we report a novel role for Cav-1 in TCR/Lck spatial distribution upon TCR triggering, which controls T-cell fate toward a regulatory phenotype. This alteration translated into a significant increase in the frequency of Tregs and reduced GVHD in vivo.

Caveolae: One Function or Many?

Caveolae are small, bulb-shaped plasma membrane invaginations. Mutations that ablate caveolae lead to diverse phenotypes in mice and humans, making it challenging to uncover their molecular mechanisms. Caveolae have been described to function in endocytosis and transcytosis (a specialized form of endocytosis) and in maintaining membrane lipid composition, as well as acting as signaling platforms. New data also support a model in which the central function of caveolae could be related to the protection of cells from mechanical stress within the plasma membrane. We present evidence for these diverse roles and consider in vitro and in vivo experiments confirming a mechanoprotective role. We conclude by highlighting current gaps in our knowledge of how mechanical signals may be transduced by caveolae.

Host cell virus entry mediated by Australian bat lyssavirus G envelope glycoprotein occurs through a clathrin-mediated endocytic pathway that requires actin and Rab5

Background

Australian bat lyssavirus (ABLV), a rhabdovirus of the genus Lyssavirus which circulates in both pteropid fruit bats and insectivorous bats in mainland Australia, has caused three fatal human infections, the most recent in February 2013, manifested as acute neurological disease indistinguishable from clinical rabies. Rhabdoviruses infect host cells through receptor-mediated endocytosis and subsequent pH-dependent fusion mediated by their single envelope glycoprotein (G), but the specific host factors and pathways involved in ABLV entry have not been determined.

Methods

ABLV internalization into HEK293T cells was examined using maxGFP-encoding recombinant vesicular stomatitis viruses (rVSV) that express ABLV G glycoproteins. A combination of chemical and molecular approaches was used to investigate the contribution of different endocytic pathways to ABLV entry. Dominant negative Rab GTPases were used to identify the endosomal compartment utilized by ABLV to gain entry into the host cell cytosol.

Results

Here we show that ABLV G-mediated entry into HEK293T cells was significantly inhibited by the dynamin-specific inhibitor dynasore, chlorpromazine, a drug that blocks clathrin-mediated endocytosis, and the actin depolymerizing drug latrunculin B. Over expression of dominant negative mutants of Eps15 and Rab5 also significantly reduced ABLV G-mediated entry into HEK293T cells. Chemical inhibitors of caveolae-dependent endocytosis and macropinocytosis and dominant negative mutants of Rab7 and Rab11 had no effect on ABLV entry.

Conclusions

The predominant pathway utilized by ABLV for internalization into HEK293T cells is clathrin-and actin-dependent. The requirement of Rab5 for productive infection indicates that ABLV G-mediated fusion occurs within the early endosome compartment.

Regulation of endothelial nitric oxide synthase activity by protein-protein interaction

Endothelial nitric oxide synthase (eNOS) is expressed in vascular endothelial cells and plays an important role in the regulation of vascular tone, platelet aggregation and angiogenesis. Protein-protein interactions represent an important posttranslational mechanism for eNOS regulation. eNOS has been shown to interact with a variety of regulatory and structural proteins which provide fine tuneup of eNOS activity and eNOS protein trafficking between plasma membrane and intracellular membranes in a number of physiological and pathophysiological processes. eNOS interacts with calmodulin, heat shock protein 90 (Hsp90), dynamin-2, β-actin, tubulin, porin, high-density lipoprotein (HDL) and apolipoprotein AI (ApoAI), resulting in increases in eNOS activity. The negative eNOS interacting proteins include caveolin, G protein-coupled receptors (GPCR), nitric oxide synthase-interacting protein (NOSIP), and nitric oxide synthase trafficking inducer (NOSTRIN). Dynamin-2, NOSIP, NOSTRIN, and cytoskeleton are also involved in eNOS trafficking in endothelial cells. In addition, eNOS associations with cationic amino acid transporter-1 (CAT-1), argininosuccinate synthase (ASS), argininosuccinate lyase (ASL), and soluble guanylate cyclase (sGC) facilitate directed delivery of substrate (L-arginine) to eNOS and optimizing NO production and NO action on its target. Regulation of eNOS by protein-protein interactions would provide potential targets for pharmacological interventions in NO-compromised cardiovascular diseases.

Caveolae internalization repairs wounded cells and muscle fibers

Rapid repair of plasma membrane wounds is critical for cellular survival. Muscle fibers are particularly susceptible to injury, and defective sarcolemma resealing causes muscular dystrophy. Caveolae accumulate in dystrophic muscle fibers and caveolin and cavin mutations cause muscle pathology, but the underlying mechanism is unknown. Here we show that muscle fibers and other cell types repair membrane wounds by a mechanism involving Ca²⁺-triggered exocytosis of lysosomes, release of acid sphingomyelinase, and rapid lesion removal by caveolar endocytosis. Wounding or exposure to sphingomyelinase triggered endocytosis and intracellular accumulation of caveolar vesicles, which gradually merged into larger compartments. The pore-forming toxin SLO was directly visualized entering cells within caveolar vesicles, and depletion of caveolin inhibited plasma membrane resealing. Our findings directly link lesion removal by caveolar endocytosis to the maintenance of plasma membrane and muscle fiber integrity, providing a mechanistic explanation for the muscle pathology associated with mutations in caveolae proteins.

DOI:http://dx.doi.org/10.7554/eLife.00926.001

Cells must be able to rapidly repair damage to their outer membranes. This is particularly important in the case of muscle cells, which are vulnerable to damage, and the failure of these cells to repair their outer membranes leads to the muscle wastage seen in muscular dystrophy. Researchers do not fully understand how cells repair membrane, but one popular theory is that they use the membranes of specialized vesicles to ‘patch’ areas that have been damaged.

A group of proteins called caveolins have also been implicated in membrane repair but, again, the details have not been worked out. These proteins are best known for their role in the formation of caveolae — small pouches formed by invaginated sections of the plasma membrane. Now, Corrotte et al. have obtained evidence that membrane repair relies not on patching, but on endocytosis (the process by which substances are taken into the cell in small vesicles that ‘pinch’ from the plasma membrane) of these caveolae pouches.

Corrotte et al. treated cells with streptolysin O, a toxin that forms pores in the membrane that cannot be repaired using patches, and found that this led to the formation of small membrane-derived vesicles that looked just like caveolae. Further tests confirmed that these vesicles contained caveolar proteins, and that they removed the toxin from the plasma membrane by endocytosis. Similar effects were seen in response to mechanical damage caused by tiny glass beads. Moreover, blocking the expression of caveolar genes prevented cells from repairing membrane damage.

Based on their findings, Corrotte et al. propose an alternative model for the repair process; namely that cellular damage triggers an influx of calcium ions, which causes vesicles called lysosomes to release chemicals that promote the formation of caveolae. These then remove the damaged area through endocytosis, restoring the integrity of the membrane. The results offer new insights into why mutations in caveolar proteins are associated with muscle disorders, including muscular dystrophy and cardiac dysfunction.

DOI:http://dx.doi.org/10.7554/eLife.00926.002

Next-generation sequencing analysis of gene regulation in the rat model of retinopathy of prematurity

PURPOSE

The purpose of this study was to identify the genes, biochemical signaling pathways, and biological themes involved in the pathogenesis of retinopathy of prematurity (ROP).

METHODS

Next-generation sequencing (NGS) was performed on the RNA transcriptome of rats with the Penn et al. (Pediatr Res 36:724-731, 1994) oxygen-induced retinopathy model of ROP at the height of vascular abnormality, postnatal day (P) 19, and normalized to age-matched, room-air-reared littermate controls. Eight custom-developed pathways with potential relevance to known ROP sequelae were evaluated for significant regulation in ROP: The three major Wnt signaling pathways, canonical, planar cell polarity (PCP), and Wnt/Ca(2+); two signaling pathways mediated by the Rho GTPases RhoA and Cdc42, which are, respectively, thought to intersect with canonical and non-canonical Wnt signaling; nitric oxide signaling pathways mediated by two nitric oxide synthase (NOS) enzymes, neuronal (nNOS) and endothelial (eNOS); and the retinoic acid (RA) signaling pathway. Regulation of other biological pathways and themes was detected by gene ontology using the Kyoto Encyclopedia of Genes and Genomes and the NIH's Database for Annotation, Visualization, and Integrated Discovery's GO terms databases.

RESULTS

Canonical Wnt signaling was found to be regulated, but the non-canonical PCP and Wnt/Ca(2+) pathways were not. Nitric oxide signaling, as measured by the activation of nNOS and eNOS, was also regulated, as was RA signaling. Biological themes related to protein translation (ribosomes), neural signaling, inflammation and immunity, cell cycle, and cell death were (among others) highly regulated in ROP rats.

CONCLUSIONS

These several genes and pathways identified by NGS might provide novel targets for intervention in ROP.

Endocytic pathways involved in filovirus entry: advances, implications and future directions

Detailed knowledge of the host-virus interactions that accompany filovirus entry into cells is expected to identify determinants of viral virulence and host range, and to yield targets for the development of antiviral therapeutics. While it is generally agreed that filovirus entry into the host cytoplasm requires viral internalization into acidic endosomal compartments and proteolytic cleavage of the envelope glycoprotein by endo/lysosomal cysteine proteases, our understanding of the specific endocytic pathways co-opted by filoviruses remains limited. This review addresses the current knowledge on cellular endocytic pathways implicated in filovirus entry, highlights the consensus as well as controversies, and discusses important remaining questions.

Evaluating caveolin interactions: do proteins interact with the caveolin scaffolding domain through a widespread aromatic residue-rich motif?

Caveolins are coat proteins of caveolae, small flask-shaped pits of the plasma membranes of most cells. Aside from roles in caveolae formation, caveolins recruit, retain and regulate many caveolae-associated signalling molecules. Caveolin-protein interactions are commonly considered to occur between a ∼20 amino acid region within caveolin, the caveolin scaffolding domain (CSD), and an aromatic-rich caveolin binding motif (CBM) on the binding partner (фXфXXXXф, фXXXXфXXф or фXфXXXXфXXф, where ф is an aromatic and X an unspecified amino acid). The CBM resembles a typical linear motif - a short, simple sequence independently evolved many times in different proteins for a specific function. Here we exploit recent improvements in bioinformatics tools and in our understanding of linear motifs to critically examine the role of CBMs in caveolin interactions. We find that sequences conforming to the CBM occur in 30% of human proteins, but find no evidence for their statistical enrichment in the caveolin interactome. Furthermore, sequence- and structure-based considerations suggest that CBMs do not have characteristics commonly associated with true interaction motifs. Analysis of the relative solvent accessible area of putative CBMs shows that the majority of their aromatic residues are buried within the protein and are thus unlikely to interact directly with caveolin, but may instead be important for protein structural stability. Together, these findings suggest that the canonical CBM may not be a common characteristic of caveolin-target interactions and that interfaces between caveolin and targets may be more structurally diverse than presently appreciated.

Reovirus uses multiple endocytic pathways for cell entry

Entry of reovirus virions has been well studied in several tissue culture systems. After attachment to junctional adhesion molecule A (JAM-A), virions undergo clathrin-mediated endocytosis followed by proteolytic disassembly of the capsid and penetration to the cytoplasm. However, during in vivo infection of the intestinal tract, and likely in the tumor microenvironment, capsid proteolysis (uncoating) is initiated extracellularly. We used multiple approaches to determine if uncoated reovirus particles, called intermediate subviral particles (ISVPs), enter cells by directly penetrating the limiting membrane or if they take advantage of endocytic pathways to establish productive infection. We found that entry and infection by reovirus ISVPs was inhibited by dynasore, an inhibitor of dynamin-dependent endocytosis, as well as by genistein and dominant-negative caveolin-1, which block caveolar endocytosis. Inhibition of caveolar endocytosis also reduced infection by reovirus virions. Extraction of membrane cholesterol with methyl-β-cyclodextrin inhibited infection by virions but had no effect when infection was initiated with ISVPs. We found this pathway to be independent of both clathrin and caveolin. Together, these data suggest that reovirus virions can use both dynamin-dependent and dynamin-independent endocytic pathways during cell entry, and they reveal that reovirus ISVPs can take advantage of caveolar endocytosis to establish productive infection.

Regulation of dynamin-2 assembly-disassembly and function through the SH3A domain of intersectin-1s

Intersectin-1s (ITSN-1s), a five Src homology 3 (SH3) domain-containing protein, is critically required for caveolae and clathrin-mediated endocytosis (CME), due to its interactions with dynamin (dyn). Of the five SH3A-E domains, SH3A is unique because of its high affinity for dyn and potent inhibition of CME. However, the molecular mechanism by which SH3A integrates in the overall function of ITSN-1s to regulate the endocytic process is not understood. Using biochemical and functional approaches as well as high-resolution electron microscopy, we show that SH3A exogenously expressed in human lung endothelial cells caused abnormal endocytic structures, distorted caveolae clusters, frequent staining-dense rings around the caveolar necks and 60% inhibition of caveolae internalization. In vitro studies further revealed that SH3A, similar to full-length ITSN-1s stimulates dyn2 oligomerization and guanosine triphosphatase (GTP)ase activity, effects not detected when other SH3 domains of ITSN-1s were used as controls. Strikingly, in the presence of SH3A, dyn2-dyn2 interactions are stabilized and despite continuous GTP hydrolysis, dyn2 oligomers cannot disassemble. SH3A may hold up caveolae release from the plasma membrane and formation of free-transport vesicles, by prolonging the lifetime of assembled dyn2. Altogether, our results indicate that ITSN-1s, via its SH3A has the unique ability to regulate dyn2 assembly-disassembly and function during endocytosis.

Lipid rafts, caveolae and GPI-linked proteins

Lipid rafts and caveolae are specialized membrane microdomains enriched in sphingolipids and cholesterol. They function in a variety of cellular processes including but not limited to endocytosis, transcytosis, signal transduction and receptor recycling. Here, we outline the similarities and differences between lipid rafts and caveolae as well as discuss important components and functions of each.

Pacsin 2 is recruited to caveolae and functions in caveolar biogenesis

The pacsin (also termed syndapin) protein family is well characterised structurally. They contain F-BAR domains associated with the generation or maintenance of membrane curvature. The cell biology of these proteins remains less understood. Here, we initially confirm that EHD2, a protein previously shown biochemically to be present in caveolar fractions and to bind to pacsins, is a caveolar protein. We go on to report that GFP-pacsin 2 can be recruited to caveolae, and that endogenous pacsin 2 partially colocalises with caveolin 1 at the plasma membrane. Analysis of the role of pacsin 2 in caveolar biogenesis using small interfering RNA (siRNA) reveals that loss of pacsin 2 function results in loss of morphologically defined caveolae and accumulation of caveolin proteins within the plasma membrane. Overexpression of the F-BAR domain of pacsin 2 (but not the related F-BAR domains of CIP4 and FBP17) disrupts caveolar morphogenesis or trafficking, implying that pacsin 2 interacts with components required for these processes. We propose that pacsin 2 has an important role in the formation of plasma membrane caveolae.

Caveolin-1 orchestrates TCR synaptic polarity, signal specificity, and function in CD8 T cells

TCR engagement triggers the polarized recruitment of membrane, actin, and transducer assemblies within the T cell-APC contact that amplify and specify signaling cascades and T effector activity. We report that caveolin-1, a scaffold that regulates polarity and signaling in nonlymphoid cells, is required for optimal TCR-induced actin polymerization, synaptic membrane raft polarity, and function in CD8, but not CD4, T cells. In CD8(+) T cells, caveolin-1 ablation selectively impaired TCR-induced NFAT-dependent NFATc1 and cytokine gene expression, whereas caveolin-1 re-expression promoted NFATc1 gene expression. Alternatively, caveolin-1 ablation did not affect TCR-induced NF-κB-dependent Iκbα expression. Cav-1(-/-) mice did not efficiently promote CD8 immunity to lymphocytic choriomeningitis virus, nor did cav-1(-/-) OT-1(+) CD8(+) T cells efficiently respond to Listeria monocytogenes-OVA after transfer into wild-type hosts. Therefore, caveolin-1 is a T cell-intrinsic orchestrator of TCR-mediated membrane polarity and signal specificity selectively employed by CD8 T cells to customize TCR responsiveness.

Essential role of PACSIN2/syndapin-II in caveolae membrane sculpting

Caveolae are flask-shaped invaginations of the plasma membrane that are associated with tumor formation, pathogen entry and muscular dystrophy, through the regulation of lipids, signal transduction and endocytosis. Caveolae are generated by the fusion of caveolin-1-containing vesicles with the plasma membrane, which then participate in endocytosis via dynamin. Proteins containing membrane-sculpting F-BAR (or EFC) domains organize the membrane in clathrin-mediated endocytosis. Here, we show that the F-BAR protein PACSIN2 sculpts the plasma membrane of the caveola. The PACSIN2 F-BAR domain interacts directly with caveolin-1 by unmasking autoinhibition of PACSIN2. Furthermore, the membrane invaginations induced by the PACSIN2 F-BAR domain contained caveolin-1. Knockdown of PACSIN2 resulted in abnormal morphology of caveolin-1-associated plasma membranes, presumably as a result of decreased recruitment of dynamin-2 to caveolin-1. These results indicate that PACSIN2 mediates membrane sculpting by caveolin-1 in caveola morphology and recruits dynamin-2 for caveola fission.

Dynamin activates NO production in rat renal inner medullary collecting ducts via protein-protein interaction with NOS1

We hypothesized that nitric oxide synthase (NOS) isoforms may be regulated by dynamin (DNM) in the inner medullary collecting duct (IMCD). The aims of this study were to determine which DNM isoforms (DNM1, DNM2, DNM3) are expressed in renal IMCDs, whether DNM interacts with NOS, whether a high-salt diet alters the interaction of DNM and NOS, and whether DNM activates NO production. DNM2 and DNM3 are highly expressed in the rat IMCD, while DNM1 is localized outside of the IMCD. We found that DNM1 interacts with NOS1α, NOS1β, and NOS3 in the inner medulla of male Sprague-Dawley rats on a 0.4% salt diet. DNM2 interacts with NOS1α, while DNM3 interacts with both NOS1α and NOS1β. DNM2 and DNM3 do not interact with NOS3 in the rat inner medulla. We did not observe any change in the DNM/NOS interactions with rats on a 4% salt diet after 7 days. Furthermore, NOS1α interacts with DNM2 in mIMCD3 and COS7 cells transfected with NOS1α and DNM2-GFP constructs and the NOS1 reductase domain is necessary for the interaction. Finally, COS7 cells expressing NOS1α or NOS1α/DNM2-GFP had significantly higher nitrite production compared with DNM2-GFP only. Nitrite production was blocked by the DNM inhibitor dynasore or the dominant negative DNM2K44A. Ionomycin stimulation further increased nitrite production in the NOS1α/DNM2-GFP cells compared with NOS1α only. In conclusion, DNM and NOS1 interact in the rat renal IMCD and this interaction leads to increased NO production, which may influence NO production in the renal medulla.

The cytoskeletal protein Ndel1 regulates dynamin 2 GTPase activity

Cytoskeleton dynamics, membranes trafficking and positioning are essential for the proper functioning of any mammalian cell. The identification of the molecules and mechanisms that allow these cellular processes to interface is vital for understanding cell behaviors. Ndel1, the mammalian homolog of the Aspergillus nidulans NudE, organizes the cytoskeleton and regulates molecular motors, thereby impacting on the positioning of membranes. Hypothetically, Ndel1 can act in concert with enzymes controlling membrane trafficking (vesicle-mediated transport) per se, but this idea has never been investigated. We now report that a pool of Ndel1 associates directly with Dynamin 2 (Dyn2), a large cytosolic GTPase involved in the trafficking of the AMPA receptor subunit GluR1. In vitro, Ndel1 enhances Dyn2 GTPase activity in its unassembled and assembled forms, without promoting oligomerization of the enzyme. In cells, gain and loss of function of Ndel1 recapitulate the effects of overexpression of Dyn2 and Dyn2 dominant negative with reduced GTPase activity on the intracellular localization of GluR1, respectively, without affecting the stability of microtubules. Together, these results indicate that Ndel1 regulates Dyn2 GTPase activity and impacts GluR1-containing membranes distribution in a manner reminiscent of Dyn2.

Internalization of UT-A1 urea transporter is dynamin dependent and mediated by both caveolae- and clathrin-coated pit pathways

Dynamin is a large GTPase involved in several distinct modes of cell endocytosis. In this study, we examined the possible role of dynamin in UT-A1 internalization. The direct relationship of UT-A1 and dynamin was identified by coimmunoprecipitation. UT-A1 has cytosolic NH(2) and COOH termini and a large intracellular loop. Dynamin specifically binds to the intracellular loop of UT-A1, but not the NH(2) and COOH termini. In cell surface biotinylation experiments, coexpression of dynamin and UT-A1 in HEK293 cells resulted in a decrease of UT-A1 cell surface expression. Conversely, cells expressing dynamin mutant K44A, which is deficient in GTP binding, showed an increased accumulation of UT-A1 protein on the cell surface. Cell plasma membrane lipid raft fractionation experiments revealed that blocking endocytosis with dynamin K44A causes UT-A1 protein accumulation in both the lipid raft and nonlipid raft pools, suggesting that both caveolae- and clathrin-mediated mechanisms may be involved in the internalization of UT-A1. This was further supported by 1) small interfering RNA to knock down either caveolin-1 or μ2 reduced UT-A1 internalization in HEK293 cells and 2) inhibition of either the caveolae pathway by methyl-β-cyclodextrin or the clathrin pathway by concanavalin A caused UT-A1 cell membrane accumulation. Functionally, overexpression of dynamin, caveolin, or μ2 decreased UT-A1 urea transport activity and decreased UT-A1 cell surface expression. We conclude that UT-A1 endocytosis is dynamin-dependent and mediated by both caveolae- and clathrin-coated pit pathways.

Caveolin-3 regulates protein kinase A modulation of the Ca(V)3.2 (alpha1H) T-type Ca2+ channels

Voltage-gated T-type Ca(2+) channel Ca(v)3.2 (α(1H)) subunit, responsible for T-type Ca(2+) current, is expressed in different tissues and participates in Ca(2+) entry, hormonal secretion, pacemaker activity, and arrhythmia. The precise subcellular localization and regulation of Ca(v)3.2 channels in native cells is unknown. Caveolae containing scaffolding protein caveolin-3 (Cav-3) localize many ion channels, signaling proteins and provide temporal and spatial regulation of intracellular Ca(2+) in different cells. We examined the localization and regulation of the Ca(v)3.2 channels in cardiomyocytes. Immunogold labeling and electron microscopy analysis demonstrated co-localization of the Ca(v)3.2 channel and Cav-3 relative to caveolae in ventricular myocytes. Co-immunoprecipitation from neonatal ventricular myocytes or transiently transfected HEK293 cells demonstrated that Ca(v)3.1 and Ca(v)3.2 channels co-immunoprecipitate with Cav-3. GST pulldown analysis confirmed that the N terminus region of Cav-3 closely interacts with Ca(v)3.2 channels. Whole cell patch clamp analysis demonstrated that co-expression of Cav-3 significantly decreased the peak Ca(v)3.2 current density in HEK293 cells, whereas co-expression of Cav-3 did not alter peak Ca(v)3.1 current density. In neonatal mouse ventricular myocytes, overexpression of Cav-3 inhibited the peak T-type calcium current (I(Ca,T)) and adenovirus (AdCa(v)3.2)-mediated increase in peak Ca(v)3.2 current, but did not affect the L-type current. The protein kinase A-dependent stimulation of I(Ca,T) by 8-Br-cAMP (membrane permeable cAMP analog) was abolished by siRNA directed against Cav-3. Our findings on functional modulation of the Ca(v)3.2 channels by Cav-3 is important for understanding the compartmentalized regulation of Ca(2+) signaling during normal and pathological processes.

The Arabidopsis dynamin-related protein2 family is essential for gametophyte development

Clathrin-mediated membrane trafficking is critical for multiple stages of plant growth and development. One key component of clathrin-mediated trafficking in animals is dynamin, a polymerizing GTPase that plays both regulatory and mechanical roles. Other eukaryotes use various dynamin-related proteins (DRP) in clathrin-mediated trafficking. Plants are unique in the apparent involvement of both a family of classical dynamins (DRP2) and a family of dynamin-related proteins (DRP1) in clathrin-mediated membrane trafficking. Our analysis of drp2 insertional mutants demonstrates that, similar to the DRP1 family, the DRP2 family is essential for Arabidopsis thaliana development. Gametophytes lacking both DRP2A and DRP2B were inviable, arresting prior to the first mitotic division in both male and female gametogenesis. Mutant pollen displayed a variety of defects, including branched or irregular cell plates, altered Golgi morphology and ectopic callose deposition. Ectopic callose deposition was also visible in the pollen-lethal drp1c-1 mutant and appears to be a specific feature of pollen-defective mutants with impaired membrane trafficking. However, drp2ab pollen arrested at earlier stages in development than drp1c-1 pollen and did not accumulate excess plasma membrane or display other gross defects in plasma membrane morphology. Therefore, the DRP2 family, but not DRP1C, is necessary for cell cycle progression during early gametophyte development. This suggests a possible role for DRP2-dependent clathrin-mediated trafficking in the transduction of developmental signals in the gametophyte.

Regulation of endothelial permeability via paracellular and transcellular transport pathways

The endothelium functions as a semipermeable barrier regulating tissue fluid homeostasis and transmigration of leukocytes and providing essential nutrients across the vessel wall. Transport of plasma proteins and solutes across the endothelium involves two different routes: one transcellular, via caveolae-mediated vesicular transport, and the other paracellular, through interendothelial junctions. The permeability of the endothelial barrier is an exquisitely regulated process in the resting state and in response to extracellular stimuli and mediators. The focus of this review is to provide a comprehensive overview of molecular and signaling mechanisms regulating endothelial barrier permeability with emphasis on the cross-talk between paracellular and transcellular transport pathways.

Caveolae, caveolins, and cavins: complex control of cellular signalling and inflammation

Caveolae are specialized lipid rafts that form flask-shaped invaginations of the plasma membrane. They are involved in cell signalling and transport and have been shown critically regulate vascular reactivity and blood pressure. The organization and functions of caveolae are mediated by coat proteins (caveolins) and support or adapter proteins (cavins). The caveolins, caveolin-1, -2, and -3, form the structural backbone of caveolae. These proteins are also highly integrated into caveolae function and have their own activity independent of caveolae. The cavins, cavins 1-4, are involved in regulation of caveolae and modulate the function of caveolins by promoting the membrane remodelling and trafficking of caveolin-derived structures. The relationships between these different proteins are complex and intersect with many aspects of cell function. Caveolae have also been implicated in chronic inflammatory conditions and other pathologies including atherosclerosis, inflammatory bowel disease, muscular dystrophy, and generalized dyslipidaemia. The pathogenic role of the caveolins is an emerging area, however, the roles of cavins in disease is just beginning to be explored. This review will examine the relationship between caveolins and cavins and explore the role of caveolae in inflammatory signalling mechanisms.

Exploring the caves: cavins, caveolins and caveolae

Caveolae are ampullate (flask-shaped) invaginations that are abundant in the plasma membrane of many mammalian cell types. Although caveolae are implicated in a wide range of processes including endothelial transcytosis, lipid homeostasis and cellular signalling, a detailed molecular picture of many aspects of their function has been elusive. Until recently, the only extensively characterised protein components of caveolae were the caveolins. Recently, data from several laboratories have demonstrated that a family of four related proteins, termed cavins 1-4, plays key roles in caveolar biogenesis and function. Salient properties of the cavin family include their propensity to form complexes with each other and their different but overlapping tissue distribution. This review summarises recent data on the cavins, and sets them in the context of open questions on the construction and function of caveolae. The discovery of cavins implies that caveolae might have unexpectedly diverse structural properties, in accord with the wide range of functions attributed to these 'little caves'.