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

Entry of nanoparticles into cells and tissues: status and challenges

In this article we discuss how nanoparticles (NPs) of different compositions may interact with and be internalized by cells, and the consequences of that for cellular functions. A large number of NPs are made with the intention to improve cancer treatment, the goal being to increase the fraction of injected drug delivered to the tumor and thereby improve the therapeutic effect and decrease side effects. Thus, we discuss how NPs are delivered to tumors and some challenges related to investigations of biodistribution, pharmacokinetics, and excretion. Finally, we discuss requirements for bringing NPs into clinical use and aspects when it comes to usage of complex and slowly degraded or nondegradable NPs.

Lipid compartments and lipid metabolism as therapeutic targets against coronavirus

Lipids perform a series of cellular functions, establishing cell and organelles' boundaries, organizing signaling platforms, and creating compartments where specific reactions occur. Moreover, lipids store energy and act as secondary messengers whose distribution is tightly regulated. Disruption of lipid metabolism is associated with many diseases, including those caused by viruses. In this scenario, lipids can favor virus replication and are not solely used as pathogens' energy source. In contrast, cells can counteract viruses using lipids as weapons. In this review, we discuss the available data on how coronaviruses profit from cellular lipid compartments and why targeting lipid metabolism may be a powerful strategy to fight these cellular parasites. We also provide a formidable collection of data on the pharmacological approaches targeting lipid metabolism to impair and treat coronavirus infection.

Redox Regulation of Signaling Complex between Caveolin-1 and Neuronal Calcium Sensor Recoverin

Caveolin-1 is a cholesterol-binding scaffold protein, which is localized in detergent-resistant membrane (DRM) rafts and interacts with components of signal transduction systems, including visual cascade. Among these components are neuronal calcium sensors (NCSs), some of which are redox-sensitive proteins that respond to calcium signals by modulating the activity of multiple intracellular targets. Here, we report that the formation of the caveolin-1 complex with recoverin, a photoreceptor NCS serving as the membrane-binding regulator of rhodopsin kinase (GRK1), is a redox-dependent process. Biochemical and biophysical in vitro experiments revealed a two-fold decreased affinity of recoverin to caveolin-1 mutant Y14E mimicking its oxidative stress-induced phosphorylation of the scaffold protein. At the same time, wild-type caveolin-1 demonstrated a 5-10-fold increased affinity to disulfide dimer of recoverin (dRec) or its thiol oxidation mimicking the C39D mutant. The formation of dRec in vitro was not affected by caveolin-1 but was significantly potentiated by zinc, the well-known mediator of redox homeostasis. In the MDCK cell model, oxidative stress indeed triggered Y14 phosphorylation of caveolin-1 and disulfide dimerization of recoverin. Notably, oxidative conditions promoted the accumulation of phosphorylated caveolin-1 in the plasma membrane and the recruitment of recoverin to the same sites. Co-localization of these proteins was preserved upon depletion of intracellular calcium, i.e., under conditions reducing membrane affinity of recoverin but favoring its interaction with caveolin-1. Taken together, these data suggest redox regulation of the signaling complex between recoverin and caveolin-1. During oxidative stress, the high-affinity interaction of thiol-oxidized recoverin with caveolin-1/DRMs may disturb the light-induced translocation of the former within photoreceptors and affect rhodopsin desensitization.

Pore formation-independent cell death induced by a β-barrel pore-forming toxin

Vibrio cholerae cytolysin (VCC) is a β-barrel pore-forming toxin (β-PFT). It exhibits potent hemolytic activity against erythrocytes that appears to be a direct outcome of its pore-forming functionality. However, VCC-mediated cell-killing mechanism is more complicated in the case of nucleated mammalian cells. It induces apoptosis in the target nucleated cells, mechanistic details of which are still unclear. Furthermore, it has never been explored whether the ability of VCC to trigger programmed cell death is stringently dependent on its pore-forming activity. Here, we show that VCC can evoke hallmark features of the caspase-dependent apoptotic cell death even in the absence of the pore-forming ability. Our study demonstrates that VCC mutants with abortive pore-forming hemolytic activity can trigger apoptotic cell death responses and cytotoxicity, similar to those elicited by the wild-type toxin. VCC as well as its pore formation-deficient mutants display prominent propensity to translocate to the target cell mitochondria and cause mitochondrial membrane damage. Therefore, our results for the first time reveal that VCC, despite being an archetypical β-PFT, can kill target nucleated cells independent of its pore-forming functionality. These findings are intriguing for a β-PFT, whose destination is generally expected to remain limited on the target cell membranes, and whose mode of action is commonly attributed to the membrane-damaging pore-forming ability. Taken together, our study provides critical new insights regarding distinct implications of the two important virulence functionalities of VCC for the V. cholerae pathogenesis process: hemolytic activity for iron acquisition and cytotoxicity for tissue damage by the bacteria.

Impact of Caveolin-Mediated Endocytosis on the Trafficking of HIV within the Colonic Barrier

In the United States, most new cases of human immunodeficiency virus (HIV) belong to the at-risk group of gay and bisexual men. Developing therapies to reverse viral latency and prevent spread is paramount for the HIV cure agenda. In gay and bisexual men, a major, yet poorly characterized, route of HIV entry is via transport across the colonic epithelial barrier. While colonic tears and paracellular transport contribute to infection, we hypothesize that HIV entry through the colonic mucosa proceeds via a process known as transcytosis, involving (i) virion binding to the apical surface of the colonic epithelium, (ii) viral endocytosis, (iii) transport of virions across the cell, and (iv) HIV release from the basolateral membrane. Using Caco-2 colonic epithelial cells plated as a polarized monolayer in transwells, we characterized the mechanism of HIV transport. After exposing the monolayer to HIV apically, reverse transcription quantitative PCR (RT-qPCR) of the viral genome present in the basolateral chamber revealed that transport is dose dependent, cooperative, and inefficient, with released virus first detectable at 12 h. Inefficiency may be associated with >50% decline in detectable intracellular virus that correlates temporally with increased association of the virion with lysosomal-associated membrane protein 1 (LAMP-1+) endosomes. Microscopy revealed green fluorescent protein (GFP)-labeled HIV within the confines of the epithelial monolayer, with no virus detectable between cells, suggesting that viral transport is transcellular. Treatment of the monolayer with endocytosis inhibitors, cholesterol reducing agents, and small interfering RNA (siRNA) to caveolin showed that viral endocytosis is mediated by caveolin-coated endosomes contained in lipid rafts. These results indicate that HIV transport across the intestinal epithelial barrier via transcytosis is a viable mechanism for viral spread and a potential therapeutic target.

IMPORTANCE Despite the success of combination antiretroviral therapy in suppressing HIV replication and the emergence and effectiveness of PrEP-based prevention strategies, in 2018, 37,968 people in the United States received a new HIV diagnosis, accompanied by 15,820 deaths. While the annual number of new diagnoses decreased 7% from 2014 to 2018, 14% of people with HIV did not know they were infected. Gay and bisexual men accounted for 69% of all HIV diagnoses and 83% of diagnoses among males. Due to the scope of the HIV epidemic, determining and understanding precise routes of infection and the mechanisms of viral spread are paramount to ending the epidemic. Since transcellular transport of HIV across an intact colonic epithelial barrier is poorly understood, our overall goal is to characterize the molecular events involved in HIV transcytosis across the intestinal epithelial cell.

Evaluation of Cardiotonic Steroid Modulation of Cellular Cholesterol and Phospholipid

We have previously shown that 21-benzylidene digoxin (21-BD) increases the total cholesterol and phospholipid content on the membrane of HeLa cells. Lipid modulation caused by cardiotonic steroids (CTS) is still unexplored. Therefore, the aim of the present study was to evaluate the cholesterol and phospholipid modulation of the cell membrane caused by ouabain and 21-BD and the possible involvement of the caveolae on this modulation. For this, one cell line containing caveolae (HeLa) and other not containing (Caco-2) were used. The modulation of the lipid profile was evaluated by total cholesterol and phospholipids measurements, and identification of membrane phospholipids by HPTLC. The cholesterol distribution was evaluated by filipin staining. The caveolin-1 expression was evaluated by Western Blotting. Ouabain had no effect on the total membrane lipid content in both cell lines. However, 21-BD increased total membrane phospholipid content and had no effect on the membrane cholesterol content in Caco-2 cells. CTS were not able to alter the specific phospholipids content. In the filipin experiments, 21-BD provoked a remarkable redistribution of cholesterol to the perinuclear region of HeLa cells. In Caco-2 cells, it was observed only a slight increase in cholesterol, especially as intracellular vesicles. The caveolin-1 expression was not altered by any of the compounds. Our data mainly show different effects of two cardiotonic steroids. Ouabain had no effect on the lipid profile of cells, whereas 21-BD causes important changes in cholesterol and phospholipid content. Therefore, the modulation of cholesterol content in the plasma membrane of HeLa cells is not correlated with the expression of caveolin-1.

Novel mechanisms regulating endothelial barrier function in the pulmonary microcirculation

The pulmonary epithelial and vascular endothelial cell layers provide two sequential physical and immunological barriers that together form a semi-permeable interface and prevent alveolar and interstitial oedema formation. In this review, we focus specifically on the continuous endothelium of the pulmonary microvascular bed that warrants strict control of the exchange of gases, fluid, solutes and circulating cells between the plasma and the interstitial space. The present review provides an overview of emerging molecular mechanisms that permit constant transcellular exchange between the vascular and interstitial compartment, and cause, prevent or reverse lung endothelial barrier failure under experimental conditions, yet with a clinical perspective. Based on recent findings and at times seemingly conflicting results we discuss emerging paradigms of permeability regulation by altered ion transport as well as shifts in the homeostasis of sphingolipids, angiopoietins and prostaglandins.

Clathrin-independent endocytosis: an increasing degree of complexity

This article aims at providing an update on the complexity of clathrin-independent endocytosis. It is now almost 30 years since we first wrote a review about its existence; at that time many people believed that with the exception of macropinocytosis, which will only be briefly mentioned in this review, all uptake could be accounted for by clathrin-dependent endocytosis. Now it is generally accepted that there are different clathrin-independent mechanisms, some of them regulated by ligands and membrane lipid composition. They can be both dynamin-dependent and -independent, meaning that the uptake cannot be accounted for by caveolae and other dynamin-dependent processes such as tubular structures that can be induced by toxins, e.g. Shiga toxin, or the fast endophilin mediated endocytosis recently described. Caveolae seem to be mostly quite stable structures with other functions than endocytosis, but evidence suggests that they may have cell-type dependent functions. Although several groups have been working on endocytic mechanisms for years, and new advanced methods have improved our ability to study mechanistic details, there are still a number of important questions we need to address, such as: How many endocytic mechanisms does a cell have? How quantitatively important are they? What about the complexity in polarized cells where clathrin-independent endocytosis is differentially regulated on the apical and basolateral poles? These questions are not easy to answer since one and the same molecule may contribute to more than one process, and manipulating one mechanism can affect another. Also, several inhibitors of endocytic processes commonly used turn out to be less specific than originally thought. We will here describe the current view of clathrin-independent endocytic processes and the challenges in studying them.

HIV internalization into oral and genital epithelial cells by endocytosis and macropinocytosis leads to viral sequestration in the vesicles

Recently, we showed that HIV-1 is sequestered, i.e., trapped, in the intracellular vesicles of oral and genital epithelial cells. Here, we investigated the mechanisms of HIV-1 sequestration in vesicles of polarized tonsil, foreskin and cervical epithelial cells. HIV-1 internalization into epithelial cells is initiated by multiple entry pathways, including clathrin-, caveolin/lipid raft-associated endocytosis and macropinocytosis. Inhibition of HIV-1 attachment to galactosylceramide and heparan sulfate proteoglycans, and virus endocytosis and macropinocytosis reduced HIV-1 sequestration by 30-40%. T-cell immunoglobulin and mucin domain 1 (TIM-1) were expressed on the apical surface of polarized tonsil, cervical and foreskin epithelial cells. However, TIM-1-associated HIV-1 macropinocytosis and sequestration were detected mostly in tonsil epithelial cells. Sequestered HIV-1 was resistant to trypsin, pronase, and soluble CD4, indicating that the sequestered virus was intracellular. Inhibition of HIV-1 intraepithelial sequestration and elimination of vesicles containing virus in the mucosal epithelium may help in the prevention of HIV-1 mucosal transmission.

Nanoparticles and targeted drug delivery in cancer therapy

Surgery, chemotherapy, radiotherapy, and hormone therapy are the main common anti-tumor therapeutic approaches. However, the non-specific targeting of cancer cells has made these approaches non-effective in the significant number of patients. Non-specific targeting of malignant cells also makes indispensable the application of the higher doses of drugs to reach the tumor region. Therefore, there are two main barriers in the way to reach the tumor area with maximum efficacy. The first, inhibition of drug delivery to healthy non-cancer cells and the second, the direct conduction of drugs into tumor site. Nanoparticles (NPs) are the new identified tools by which we can deliver drugs into tumor cells with minimum drug leakage into normal cells. Conjugation of NPs with ligands of cancer specific tumor biomarkers is a potent therapeutic approach to treat cancer diseases with the high efficacy. It has been shown that conjugation of nanocarriers with molecules such as antibodies and their variable fragments, peptides, nucleic aptamers, vitamins, and carbohydrates can lead to effective targeted drug delivery to cancer cells and thereby cancer attenuation. In this review, we will discuss on the efficacy of the different targeting approaches used for targeted drug delivery to malignant cells by NPs.

Intestinal epithelial cell caveolin 1 regulates fatty acid and lipoprotein cholesterol plasma levels

Caveolae and their structural protein caveolin 1 (CAV1) have roles in cellular lipid processing and systemic lipid metabolism. Global deletion of CAV1 in mice results in insulin resistance and increases in atherogenic plasma lipids and cholesterol, but protects from diet-induced obesity and atherosclerosis. Despite the fundamental role of the intestinal epithelia in the regulation of dietary lipid processing and metabolism, the contributions of CAV1 to lipid metabolism in this tissue have never been directly investigated. In this study the cellular dynamics of intestinal Cav1 were visualized in zebrafish and the metabolic contributions of CAV1 were determined with mice lacking CAV1 in intestinal epithelial cells (CAV1^IEC-KO). Live imaging of Cav1–GFP and fluorescently labeled caveolae cargos shows localization to the basolateral and lateral enterocyte plasma membrane (PM), suggesting Cav1 mediates transport between enterocytes and the submucosa. CAV1^IEC-KO mice are protected from the elevation in circulating fasted low-density lipoprotein (LDL) cholesterol associated with a high-fat diet (HFD), but have increased postprandial LDL cholesterol, total free fatty acids (FFAs), palmitoleic acid, and palmitic acid. The increase in circulating FAs in HFD CAV1^IEC-KO mice is mirrored by decreased hepatic FAs, suggesting a non-cell-autonomous role for intestinal epithelial cell CAV1 in promoting hepatic FA storage. In conclusion, CAV1 regulates circulating LDL cholesterol and several FA species via the basolateral PM of enterocytes. These results point to intestinal epithelial cell CAV1 as a potential therapeutic target to lower circulating FFAs and LDL cholesterol, as high levels are associated with development of type II diabetes and cardiovascular disease.

Summary: Caveolin 1, which forms caveolae, localizes to the basolateral membrane of zebrafish intestinal epithelial cells and regulates circulating murine fatty acid and lipoprotein cholesterol levels.

Clostridium difficile Toxin A Undergoes Clathrin-Independent, PACSIN2-Dependent Endocytosis

Clostridium difficile infection affects a significant number of hospitalized patients in the United States. Two homologous exotoxins, TcdA and TcdB, are the major virulence factors in C. difficile pathogenesis. The toxins are glucosyltransferases that inactivate Rho family-GTPases to disrupt host cellular function and cause fluid secretion, inflammation, and cell death. Toxicity depends on receptor binding and subsequent endocytosis. TcdB has been shown to enter cells by clathrin-dependent endocytosis, but the mechanism of TcdA uptake is still unclear. Here, we utilize a combination of RNAi-based knockdown, pharmacological inhibition, and cell imaging approaches to investigate the endocytic mechanism(s) that contribute to TcdA uptake and subsequent cytopathic and cytotoxic effects. We show that TcdA uptake and cellular intoxication is dynamin-dependent but does not involve clathrin- or caveolae-mediated endocytosis. Confocal microscopy using fluorescently labeled TcdA shows significant colocalization of the toxin with PACSIN2-positive structures in cells during entry. Disruption of PACSIN2 function by RNAi-based knockdown approaches inhibits TcdA uptake and toxin-induced downstream effects in cells indicating that TcdA entry is PACSIN2-dependent. We conclude that TcdA and TcdB utilize distinct endocytic mechanisms to intoxicate host cells.

Clostridium difficile is a bacterial pathogen that causes nearly half a million infections each year in the United States. It infects the human colon and causes diarrhea, colitis and, in some cases, death. C. difficile infection is mediated by the action of two large homologous toxins, TcdA and TcdB. Disruption of host cell function by these toxins requires entry into cells. There are multiple ways for pathogens and virulence factors such as viruses and toxins to enter host cells. The entry mechanism is often directed by a cell surface receptor and can impact the trafficking and virulence properties of the pathogenic factor. Investigating the internalization strategy can provide critical insight into the mechanism of action for specific pathogens and virulence factors. In our current study, we sought to determine the strategy utilized by TcdA to enter host cells. We show that TcdA uptake occurs by a clathrin- and caveolae-independent endocytic mechanism that is mediated by PACSIN2 and dynamin. We also show that TcdA and TcdB can utilize different routes of entry, which may have implications regarding their cytotoxic mechanisms. In summary, our results provide new insights into the mechanism of cellular intoxication by TcdA and the role of PACSIN2 in endocytosis.

Caveolin-3 plays a critical role in autophagy after ischemia-reperfusion

Autophagy is a dynamic recycling process responsible for the breakdown of misfolded proteins and damaged organelles, providing nutrients and energy for cellular renovation and homeostasis. Loss of autophagy is associated with cardiovascular diseases. Caveolin-3 (Cav-3), a muscle-specific isoform, is a structural protein within caveolae and is critical to stress adaptation in the heart. Whether Cav-3 plays a role in regulating autophagy to modulate cardiac stress responses remains unknown. In the present study, we used HL-1 cells, a cardiac muscle cell line, with stable Cav-3 knockdown (Cav-3 KD) and Cav-3 overexpression (Cav-3 OE) to study the impact of Cav-3 in regulation of autophagy. We show that traditional stimulators of autophagy (i.e., rapamycin and starvation) result in upregulation of the process in Cav-3 OE cells while Cav-3 KD cells have a blunted response. Cav-3 coimmunoprecipitated with beclin-1 and Atg12, showing an interaction of caveolin with autophagy-related proteins. In the heart, autophagy may be a major regulator of protection from ischemic stress. We found that Cav-3 KD cells have a decreased expression of autophagy markers [beclin-1, light chain (LC3-II)] after simulated ischemia and ischemia-reperfusion (I/R) compared with WT, whereas OE cells showed increased expression. Moreover, Cav-3 KD cells showed increased cell death and higher level of apoptotic proteins (cleaved caspase-3 and cytochrome c) with suppressed mitochondrial function in response to simulated ischemia and I/R, whereas Cav-3 OE cells were protected and had preserved mitochondrial function. Taken together, these results indicate that autophagy regulates adaptation to cardiac stress in a Cav-3-dependent manner.

Transport Mechanism of Coumarin 6 Nanocrystals with Two Particle Sizes in MDCKII Monolayer and Larval Zebrafish

Nanocrystals (NCs) were utilized as oral formulations in commercial products to deliver lipophilic drug, but their transport mechanisms are not fully understood. This study aimed to explore the transport mechanism of NCs using in vitro Madin-Darby canine kidney II (MDCK II) cells and in vivo larval zebrafish models. Coumarin 6 (C6) was formulated into NCs with particle size of 67.5 ± 5.2 and 190 ± 9.2 nm. In vitro studies showed that 70 nm NCs accumulated in lysosome and endoplasmic reticulum (ER) as destinations. Lipid raft pathways mediated the endocytosis, while lipid raft, ER/Golgi, and Golgi/plasma membrane pathways were involved in exocytosis and transcytosis process. However, 200 nm NCs accumulated more in a lysosome, where lipid raft pathways were also involved in the endocytosis process. In vivo studies in larval zebrafish model further confirmed that the above network plays an important role in the absorption and distribution of C6-NCs.

Mammalian cell growth on gold nanoparticle-decorated substrates is influenced by the nanoparticle coating

In this work, we study epithelial cell growth on substrates decorated with gold nanorods that are functionalized either with a positively charged cytotoxic surfactant or with a biocompatible polymer exhibiting one of two different end groups, resulting in a neutral or negative surface charge of the particle. Upon observation of cell growth for three days by live cell imaging using optical dark field microscopy, it was found that all particles supported cell adhesion while no directed cell migration and no significant particle internalization occurred. Concerning cell adhesion and spreading as compared to cell growth on bare substrates after 3 days of incubation, a reduction by 45% and 95%, respectively, for the surfactant particle coating was observed, whereas the amino-terminated polymer induced a reduction by 30% and 40%, respectively, which is absent for the carboxy-terminated polymer. Furthermore, interface-sensitive impedance spectroscopy (electric cell-substrate impedance sensing, ECIS) was employed in order to investigate the micromotility of cells added to substrates decorated with various amounts of surfactant-coated particles. A surface density of 65 particles/µm(2) (which corresponds to 0.5% of surface coverage with nanoparticles) diminishes micromotion by 25% as compared to bare substrates after 35 hours of incubation. We conclude that the surface coating of the gold nanorods, which were applied to the basolateral side of the cells, has a recognizable influence on the growth behavior and thus the coating should be carefully selected for biomedical applications of nanoparticles.

Elevated cAMP increases aquaporin-3 plasma membrane diffusion

Regulated urine concentration takes place in the renal collecting duct upon arginine vasopressin (AVP) stimulation, where subapical vesicles containing aquaporin-2 (AQP2) are inserted into the apical membrane instantly increasing water reabsorption and urine concentration. The reabsorped water exits via basolateral AQP3 and AQP4. Upon long-term stimulation with AVP or during thirst, expression levels of both AQP2 and AQP3 are increased; however, there is so far no evidence for short-term AVP regulation of AQP3 or AQP4. To facilitate the increase in transepithelial water transport, AQP3 may be short-term regulated via changes in protein-protein interactions, incorporation into lipid rafts, and/or changes in steady-state turnover, which could result in changes in the diffusion behavior of AQP3. Thus we measured AQP3 diffusion coefficients upon stimulation with the AVP mimic forskolin to reveal if AQP3 could be short-term regulated by AVP. k-Space image correlation spectroscopy (kICS) analysis of time-lapse image sequences of basolateral enhanced green fluorescent protein-tagged AQP3 (AQP3-EGFP) revealed that the forskolin-mediated elevation of cAMP increased the diffusion coefficient by 58% from 0.0147 ± 0.0082 μm(2)/s (control) to 0.0232 ± 0.0085 μm(2)/s (forskolin, P < 0.05). Quantum dot-conjugated antibody labeling also revealed a significant increase in AQP3 diffusion upon forskolin treatment by 44% [0.0104 ± 0.0040 μm(2)/s (control) vs. 0.0150 ± 0.0016 μm(2)/s (forskolin, P < 0.05)]. Immunoelectron microscopy showed no obvious difference in AQP3-EGFP expression levels or localization in the plasma membrane upon forskolin stimulation. Thus AQP3-EGFP diffusion is altered upon increased cAMP, which may correspond to basolateral adaptations in response to the increased apical water readsorption.

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

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

Epstein-Barr virus transcytosis through polarized oral epithelial cells

Although Epstein-Barr virus (EBV) is an orally transmitted virus, viral transmission through the oropharyngeal mucosal epithelium is not well understood. In this study, we investigated how EBV traverses polarized human oral epithelial cells without causing productive infection. We found that EBV may be transcytosed through oral epithelial cells bidirectionally, from both the apical to the basolateral membranes and the basolateral to the apical membranes. Apical to basolateral EBV transcytosis was substantially reduced by amiloride, an inhibitor of macropinocytosis. Electron microscopy showed that virions were surrounded by apical surface protrusions and that virus was present in subapical vesicles. Inactivation of signaling molecules critical for macropinocytosis, including phosphatidylinositol 3-kinases, myosin light-chain kinase, Ras-related C3 botulinum toxin substrate 1, p21-activated kinase 1, ADP-ribosylation factor 6, and cell division control protein 42 homolog, led to significant reduction in EBV apical to basolateral transcytosis. In contrast, basolateral to apical EBV transcytosis was substantially reduced by nystatin, an inhibitor of caveolin-mediated virus entry. Caveolae were detected in the basolateral membranes of polarized human oral epithelial cells, and virions were detected in caveosome-like endosomes. Methyl β-cyclodextrin, an inhibitor of caveola formation, reduced EBV basolateral entry. EBV virions transcytosed in either direction were able to infect B lymphocytes. Together, these data show that EBV transmigrates across oral epithelial cells by (i) apical to basolateral transcytosis, potentially contributing to initial EBV penetration that leads to systemic infection, and (ii) basolateral to apical transcytosis, which may enable EBV secretion into saliva in EBV-infected individuals.

How do polymeric micelles cross epithelial barriers?

Non-parenteral delivery of drugs using nanotechnology-based delivery systems is a promising non-invasive way to achieve effective local or systemic drug delivery. The efficacy of drugs administered non-parenterally is limited by their ability to cross biological barriers, and epithelial tissues particularly present challenges. Polymeric micelles can achieve transepithelial drug delivery because of their ability to be internalized into cells and/or cross epithelial barriers, thereby delivering drugs either locally or systematically following non-parenteral administration. This review discusses the particular characteristics of various epithelial barriers and assesses their potential as non-parenteral routes of delivery. The material characteristics of polymeric micelles (e.g., size, surface charge, and surface decoration) and of unimers dissociated from polymeric micelles determine their interactions (non-specific and/or specific) with mucus and epithelial cells as well as their intracellular fate. This paper outlines the mechanisms governing the major modes of internalization of polymeric micelles into epithelial cells, with an emphasis on specific recent examples of the transport of drug-loaded polymeric micelles across epithelial barriers.

Caveolin-1 is a novel regulator of K-RAS-dependent migration in colon carcinogenesis

Caveolin-1 is an essential component of membrane caveolae. It is an important regulator of cellular processes such as signal transduction and endocytosis. We report here, for the first time, that caveolin-1 is a target of the K-RAS oncogene in colon carcinogenesis. Caveolin-1 is induced in colon cancer cells and in human colon tumor samples, in response to K-RAS activating mutations. An activated K-RAS oncogene transcriptionally induces caveolin-1 expression in human colon cancer cells and this effect is not restricted to the type of activating K-RAS mutation. Inhibition of the P-I3 Kinase-AKT pathway, but not the ERK MAPK pathway, both important K-RAS effectors, leads to a decrease in caveolin-1 expression indicating that the AKT pathway is involved in caveolin-1 expression in response to an activated K-RAS. Increased AKT signaling induces caveolin-1 expression by increasing the activity of the transcription factor, Sp1. Interestingly; caveolin-1 depletion alters K-RAS-dependent signaling by decreasing Grb2-SOS activity. Consistent with these finding, caveolin-1-depleted cells shows decreased migration in vitro. However, caveolin-1 overexpression by itself does not increase migration whereas an activated Src can increase migration in a caveolin-1-dependent manner. This increased migration is highly dependent on the RhoA GTPase, indicating that an activated K-RAS modulates migration in part via caveolin-1 induction, and increasing RhoA activity via phospho-caveolin-1. Our findings indicate that K-RAS regulates both caveolin-1 expression and other factors affecting caveolin-1 functions in colon cancer-derived cell migration.

Constitutive formation of caveolae in a bacterium

Caveolin plays an essential role in the formation of characteristic surface pits, caveolae, which cover the surface of many animal cells. The fundamental principles of caveola formation are only slowly emerging. Here we show that caveolin expression in a prokaryotic host lacking any intracellular membrane system drives the formation of cytoplasmic vesicles containing polymeric caveolin. Vesicle formation is induced by expression of wild-type caveolins, but not caveolin mutants defective in caveola formation in mammalian systems. In addition, cryoelectron tomography shows that the induced membrane domains are equivalent in size and caveolin density to native caveolae and reveals a possible polyhedral arrangement of caveolin oligomers. The caveolin-induced vesicles or heterologous caveolae (h-caveolae) form by budding in from the cytoplasmic membrane, generating a membrane domain with distinct lipid composition. Periplasmic solutes are encapsulated in the budding h-caveola, and purified h-caveolae can be tailored to be targeted to specific cells of interest.

Infectious spleen and kidney necrosis virus (a fish iridovirus) enters Mandarin fish fry cells via caveola-dependent endocytosis

Infectious spleen and kidney necrosis virus (ISKNV) is the type species of the genus Megalocytivirus from the family Iridoviridae. Megalocytiviruses have been implicated in more than 50 fish species infections and currently threaten the aquaculture industry, causing great economic losses in China, Japan, and Southeast Asia. However, the cellular entry mechanisms of megalocytiviruses remain largely uncharacterized. In this study, the main internalization mechanism of ISKNV was investigated by using mandarin fish fry (MFF-1) cells. The progression of ISKNV infection is slow, and infection is not inhibited when the cells are treated with ammonium chloride (NH(4)Cl), chloroquine, sucrose, and chlorpromazine, which are inhibitors of clathrin-dependent endocytosis. The depletion of cellular cholesterol by methyl-β-cyclodextrin results in the significant inhibition of ISKNV infection; however, the infection is resumed with cholesterol replenishment. Inhibitors of caveolin-1-involved signaling events, including phorbol 12-myristate 13-acetate (PMA), genistein, and wortmannin, impair ISKNV entry into MFF-1 cells. Moreover, ISKNV entry is dependent on dynamin and the microtubule cytoskeleton. Cofraction analysis of ISKNV and caveolin-1 showed that ISKNV colocates with caveolin-1 during virus infection. These results indicate that ISKNV entry into MFF-1 cells proceeds via classical caveola-mediated endocytosis and is dependent on the microtubules that serve as tracks along which motile cavicles may move via a caveola-caveosome-endoplasmic reticulum (ER) pathway. As a fish iridovirus, ISKNV entry into MFF-1 cells is different from the clathrin-mediated endocytosis of frog virus 3 entry into mammalian cells (BHK-21) at 28°C, which has been recognized as a model for iridoviruses. Thus, our work may help further the understanding of the initial steps of iridovirus infection.

Coordinated regulation of caveolin-1 and Rab11a in apical recycling compartments of polarized epithelial cells

Recent studies have identified caveolin-1, a protein best known for its functions in caveolae, in apical endocytic recycling compartments in polarized epithelial cells. However, very little is known about the regulation of caveolin-1 in the endocytic recycling pathway. To address this question, in the current study we compared the relationship between compartments enriched in sub-apical caveolin-1 and Rab11a, a well-defined marker of apical recycling endosomes, using polarized MDCK cells as a model. We show that caveolin-1-containing vesicles define a compartment that partially overlaps with Rab11a, and that the distribution of subapical caveolin-1 and Rab11a shows a similar dependence on microtubule disruption. Mutants of the Rab11a effector, Rab11-FIP2 also altered the localization of caveolin-1. These findings indicate that caveolin-1 is coordinately regulated with Rab11a within the apical recycling system of polarized epithelial cells, suggesting that the two proteins are components of the same pathway.

Unpolarized release of vaccinia virus and HIV antigen by colchicine treatment enhances intranasal HIV antigen expression and mucosal humoral responses

The induction of a strong mucosal immune response is essential to building successful HIV vaccines. Highly attenuated recombinant HIV vaccinia virus can be administered mucosally, but even high doses of immunization have been found unable to induce strong mucosal antibody responses. In order to solve this problem, we studied the interactions of recombinant HIV vaccinia virus Tiantan strain (rVTT-gagpol) in mucosal epithelial cells (specifically Caco-2 cell layers) and in BALB/c mice. We evaluated the impact of this virus on HIV antigen delivery and specific immune responses. The results demonstrated that rVTT-gagpol was able to infect Caco-2 cell layers and both the nasal and lung epithelia in BALB/c mice. The progeny viruses and expressed p24 were released mainly from apical surfaces. In BALB/c mice, the infection was limited to the respiratory system and was not observed in the blood. This showed that polarized distribution limited antigen delivery into the whole body and thus limited immune response. To see if this could be improved upon, we stimulated unpolarized budding of the virus and HIV antigens by treating both Caco-2 cells and BALB/c mice with colchicine. We found that, in BALB/c mice, the degree of infection and antigen expression in the epithelia went up. As a result, specific immune responses increased correspondingly. Together, these data suggest that polarized budding limits antigen delivery and immune responses, but unpolarized distribution can increase antigen expression and delivery and thus enhance specific immune responses. This conclusion can be used to optimize mucosal HIV vaccine strategies.

Scaffolding proteins mediating membrane-initiated extra-nuclear actions of estrogen receptor

Estrogen mediates biological effects on cell proliferation, differentiation, and homeostasis through estrogen receptor (ER). In addition to functioning as a ligand-activated nuclear transcription factor to directly regulate gene transcription, ER also mediates rapid activation of signaling pathways independent of its transcriptional activity. A subpopulation of ER localized to the cell membrane or cytoplasm has been proposed to mediate ER activation of signaling pathways. This review focuses on recent advances in our understanding of mechanisms responsible for ER cytoplasm/membrane localization, where rapid extra-nuclear signaling is initiated. These mechanisms include lipid modification of the receptor (palmitoylation) and interactions with membrane and cytoplasmic adaptor proteins including caveolins, striatin, p130Cas, Shc, HPIP, MTA-1s, and MNAR/PELP1. While it is clear that ER mediates rapid extra-nuclear signaling resulting in activation of signaling pathways such as Src/MAPK and PI-3 kinase/Akt, how ER extra-nuclear signaling influences overall ER/estrogen physiology is still not well understood. Future studies defining physiological roles of ER extra-nuclear actions and crosstalk with its nuclear counterparts will be important to our overall understanding of estrogen and ER biological functions.

Endocytosis of nanomedicines

Novel nanomaterials are being developed to improve diagnosis and therapy of diseases through effective delivery of drugs, biopharmaceutical molecules and imaging agents to target cells in disease sites. Such diagnostic and therapeutic nanomaterials, also termed "nanomedicines", often require site-specific cellular entry to deliver their payload to sub-cellular locations hidden beneath cell membranes. Nanomedicines can employ multiple pathways for cellular entry, which are currently insufficiently understood. This review, first, classifies various mechanisms of endocytosis available to nanomedicines including phagocytosis and pinocytosis through clathrin-dependent and clathrin-independent pathways. Second, it describes the current experimental tools to study endocytosis of nanomedicines. Third, it provides specific examples from recent literature and our own work on endocytosis of nanomedicines. Finally, these examples are used to ascertain 1) the role of particle size, shape, material composition, surface chemistry and/or charge for utilization of a selected pathway(s); 2) the effect of cell type on the processing of nanomedicines; and 3) the effect of nanomaterial-cell interactions on the processes of endocytosis, the fate of the nanomedicines and the resulting cellular responses. This review will be useful to a diverse audience of students and scientists who are interested in understanding endocytosis of nanomedicines.

Caveolin regulation of neuronal intracellular signaling

Caveolin proteins physically interact with and compartmentalize membrane-localized signaling proteins to facilitate high-fidelity intracellular signaling. Though primarily studied outside the nervous system, recent investigations have revealed that caveolin proteins are key modulators of a variety of neuronal intracellular signaling pathways. Through both protein aggregation and segregation, caveolin proteins can exert positive and negative influences on intracellular signaling. This review will detail recent findings regarding caveolin function in the brain.

Transport of iron chelators and chelates across MDCK cell monolayers: implications for iron excretion during chelation therapy

Iron chelators are effective at removing iron from the body in iron overload, but little is known about the handling of iron chelates by the kidney. We studied the transport of deferoxamine, deferasirox, and three hydroxypyridones, and their iron chelates, in polarized renal epithelial MDCK cells growing on Transwell inserts. Directional iron efflux was also studied in (59)Fe-loaded cells. The chelators were transported at comparable rates in the apical and basolateral directions and moved faster than their corresponding chelates, except for deferoxamine, which did not move from the basolateral to the apical side. In contrast, the chelates were transported faster in the apical-to-basolateral direction. More permeable chelators were more efficient at removing iron from iron-loaded cells compared with deferoxamine. Iron is preferentially removed from the basolateral side, and kinetic modeling suggests facilitated diffusion of chelates in some cases. Basolateral iron efflux is temperature-dependent and partially sensitive to ATP depletion. Polarized transport of chelates suggests the kidney may be involved in reabsorption of iron bound to chelators, with a temperature-sensitive facilitated removal of some iron complexes from the basolateral side. Further studies are warranted to determine if these processes may contribute to the observed nephrotoxicity of some iron chelators.

Endocytosis unplugged: multiple ways to enter the cell

Endocytosis occurs at the cell surface and involves internalization of the plasma membrane (PM) along with its constituent membrane proteins and lipids. Endocytosis is involved in sampling of the extracellular milieu and also serves to regulate various processes initiated at the cell surface. These include nutrient uptake, signaling from cell-surface receptors, and many other processes essential for cell and tissue functioning in metazoans. It is also central to the maintenance of PM lipid and protein homeostasis. There are multiple means of internalization that operate concurrently, at the cell surface. With advancement in high-resolution visualization techniques, it is now possible to track multiple endocytic cargo at the same time, revealing a remarkable diversity of endocytic processes in a single cell. A combination of live cell imaging and efficient genetic manipulations has also aided in understanding the functional hierarchy of molecular players in these mechanisms of internalization. Here we provide an account of various endocytic routes, their mechanisms of operation and occurrence across phyla.

The utilization of pathogen-like cellular trafficking by single chain block copolymer

Amphiphilic triblock copolymer, poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide), Pluronic P85, is unexpectedly shown to utilize sophisticated cellular trafficking mechanisms and enter brain microvessel endothelial cells and primary neurons that are poorly penetrable. Though caveolae serve as a primary entry site for the copolymer single chains, in cells devoid of caveolae, the copolymer can still exploit caveolae- and clathrin-independent routes. This parallels the copolymer's trafficking itinerary with that of biological pathogens. The similarity is reinforced since both bypass early endosomes/lysosomes and transport to the endoplasmic reticulum. The copolymer finally reaches the mitochondrion that serves as its final destination. Notably, it also succeeds to gain entry in brain microvessel endothelial cells through caveolae and in primary neurons through caveolae- and clathrin-independent pathway. In neurons the copolymer accumulates in the cell body followed by anterograde trafficking towards the axons/dendrites. Overall, dissecting the trafficking of a synthetic polymer in multiple cell types triggers development of novel delivery systems that can selectively target intracellular compartments and provide entry in cells currently considered impenetrable.

The exploitation of differential endocytic pathways in normal and tumor cells in the selective targeting of nanoparticulate chemotherapeutic agents

Polymeric micelles with cross-linked ionic cores of poly(methacrylic acid) and nonionic shell of poly(ethylene oxide) (cl-micelles) are shown here to readily internalize in epithelial cancer cells but not in normal epithelial cells that form tight junctions (TJ). The internalization of such cl-micelles in the cancer cells proceeded mainly through caveolae-mediated endocytosis. In confluent normal epithelial cells this endocytosis route was absent at the apical side and the cl-micelles sequestered in TJ regions of the cell membrane without entering the cells for at least 24h. Disruption of the TJ by calcium deprivation resulted in redistribution of cl-micelles inside the cells. In cancer cells following initial cellular entry the cl-micelles bypassed the early endosomes and reached the lysosomes within 30min. This allowed designing cl-micelles with cytotoxic drug, doxorubicin, linked via pH-sensitive hydrazone bonds, which were cleaved in the acidic environment of lysosomes resulting in accumulation of the drug in the nucleus after 5h. Such pH-sensitive cl-micelles displayed selective toxicity to cancer cells but were non-toxic to normal epithelial cells. In conclusion, we describe major difference in interactions of cl-micelles with cancer and normal cells that can lead to development of novel drug delivery system with reduced side effects and higher efficacy in cancer chemotherapy.

Loss of myosin VI no insert isoform (NoI) induces a defect in clathrin-mediated endocytosis and leads to caveolar endocytosis of transferrin receptor

Myosin VI is a motor protein that moves toward the minus end of actin filaments. It is involved in clathrin-mediated endocytosis and associates with clathrin-coated pits/vesicles at the plasma membrane. In this article the effect of the loss of myosin VI no insert isoform (NoI) on endocytosis in nonpolarized cells was examined. The absence of myosin VI in fibroblasts derived from the Snell's waltzer mouse (myosin VI knock-out) gives rise to defective clathrin-mediated endocytosis with shallow clathrin-coated pits and a strong reduction in the internalization of clathrin-coated vesicles. To compensate for this defect in clathrin-mediated endocytosis, plasma membrane receptors such as the transferrin receptor (TfR) are internalized by a caveola-dependent pathway. Moreover the clathrin adaptor protein, AP-2, necessary for TfR internalization, follows the receptor and relocalizes in caveolae in Snell's waltzer fibroblasts.

Clinical and translational implications of the caveolin gene family: lessons from mouse models and human genetic disorders

Here we review the clinical and translational implications of the caveolin gene family for understanding the pathogenesis of human diseases, including breast and prostate cancers, pulmonary hypertension, cardiomyopathy, diabetes, and muscular dystrophy. Detailed phenotypic analysis of caveolin knockout mice has served to highlight the crucial role of a caveolin deficiency in the pathogenesis of many human disease processes. Mutations in the human caveolin genes are associated with a number of established genetic disorders (such as breast cancer, lipodystrophy, muscular dystrophy, and cardiomyopathy), making the caveolins important and novel targets for drug development. The implementation of new strategies for caveolin replacement therapy-including caveolin mimetic peptides-is ongoing.

Cystic fibrosis transmembrane conductance regulator and caveolin-1 regulate epithelial cell internalization of Pseudomonas aeruginosa

Patients with cystic fibrosis (CF) exhibit defective innate immunity and are susceptible to chronic lung infection with Pseudomonas aeruginosa. To investigate the molecular bases for the hypersusceptibility of CF patients to P. aeruginosa, we used the IB3-1 cell line with two defective CF transmembrane conductance regulator (CFTR) genes (DeltaF508/W1282X) to generate isogenic stable, clonal lung epithelial cells expressing wild-type (WT)-CFTR with an NH(2)-terminal green fluorescent protein (GFP) tag. GFP-CFTR exhibited posttranslational modification, subcellular localization, and anion transport function typical of WT-CFTR. P. aeruginosa internalization, a component of effective innate immunity, required functional CFTR and caveolin-1, as shown by: 1) direct correlation between GFP-CFTR expression levels and P. aeruginosa internalization; 2) enhanced P. aeruginosa internalization by aminoglycoside-induced read through of the CFTR W1282X allele in IB3-1 cells; 3) decreased P. aeruginosa internalization following siRNA knockdown of GFP-CFTR or caveolin-1; and 4) spatial association of P. aeruginosa with GFP-CFTR and caveolin-1 at the cell surface. P. aeruginosa internalization also required free lateral diffusion of GFP-CFTR, allowing for bacterial coclustering with GFP-CFTR and caveolin-1 at the plasma membrane. Thus efficient initiation of innate immunity to P. aeruginosa requires formation of an epithelial "internalization platform" involving both caveolin-1 and functional, laterally mobile CFTR.

Liver X Receptor Activation Induces the Uptake of Cholesteryl Esters From High Density Lipoproteins in Primary Human Macrophages

Objective— Liver X receptors (LXRs) are oxysterol-activated nuclear receptors regulating reverse cholesterol transport, in part by modulating cholesterol efflux from macrophages to apoAI and HDL via the ABCA1 and ABCG1/ABCG4 pathways. Moreover, LXR activation increases intracellular cholesterol trafficking via the induction of NPC1 and NPC2 expression. However, implication of LXRs in the selective uptake of cholesteryl esters from lipoproteins in human macrophages has never been reported.

Methods and Results— Our results show that (1) selective CE uptake from HDL 3 is highly efficient in human monocyte-derived macrophages; (2) surprisingly, HDL 3 -CE uptake is strongly increased by LXR activation despite antiatherogenic effects of LXRs; (3) HDL 3 -CE uptake increase is not linked to SR-BI expression modulation but it is dependent of proteoglycan interactions; (4) HDL 3 -CE uptake increase is associated with increased expression and secretion of apoE and LPL, two proteins interacting with proteoglycans; (5) HDL 3 -CE uptake increase depends on the integrity of raft domains and is associated with an increased caveolin-1 expression.

Conclusions— Our study identifies a new role for LXRs in the control of cholesterol homeostasis in human macrophages. LXR activation results in enhanced dynamic intracellular cholesterol fluxes through an increased CE uptake from HDL and leads to an increased cholesterol availability to efflux to apoAI and HDL.

Caveolin proteins and estrogen signaling in the brain

Best described outside the nervous system, caveolins are structural proteins that form caveolae, functional microdomains at the plasma membrane that cluster related signaling molecules. Caveolin-associated proteins include G protein-coupled receptors and G proteins, receptor tyrosine kinases, as well as protein kinases, ion channels and various other signaling enzymes. Not surprisingly, a wide array of biological disorders are thought to be rooted in caveolin dysfunction. In addition, caveolins traffic and cluster estrogen receptors to caveolae. Interactions between the estrogen receptors ERalpha and ERbeta with caveolins appear critical in many non-neuronal cell types, e.g., disruption of normal function may underlie many forms of breast cancer. Recent findings suggest caveolins may also play an essential role in membrane estrogen receptor function in the nervous system. Not only are they expressed in neurons and glia, but different caveolin isoforms also appear necessary to generate distinct functional signaling complexes. With membrane estrogen receptors responsible for the efficient activation of a multitude of intracellular signaling pathways, which in turn influence a wide variety of nervous system functions, caveolin proteins are poised to act as the central coordinators of these processes.

Activated K-RAS increases polyamine uptake in human colon cancer cells through modulation of caveolar endocytosis

Endocytic pathways have been implicated in polyamine transport in mammalian cells, but specific mechanisms have not been described. We have shown that expression of a dominant negative (DN) form of the GTPase Dynamin, but not Eps15, diminished polyamine uptake in colon cancer cells indicating a caveolar and nonclathrin uptake mode. Polyamines co-sediment with lipid raft/caveolin-1 rich fractions, of the plasma membrane in a sucrose density gradient. Knock down of caveolin-1 significantly increased polyamine uptake. Conversely, ectopic expression of this protein resulted in diminished polyamine uptake. We also found that presence of an activated K-RAS oncogene significantly increased polyamine uptake by colon cancer cells. This effect is through an increase in caveolin-1 phosphorylation at tyrosine residue 14. Caveolin-1 is a negative regulator of caveolar endocytosis and phosphorylation in a K-RAS dependent manner leads to an increase in caveolar endocytosis. In cells expressing wild type K-RAS, addition of exogenous uPA was sufficient to stimulate caveolar endocytosis of polyamines. This effect was abrogated by the addition of a SRC kinase inhibitor. These data indicate that polyamine transport follows a dynamin-dependent and clathrin-independent endocytic uptake route, and this route is positively regulated by the oncogenic expression of K-RAS in a caveolin-1 dependent manner.

Clathrin-independent endocytosis: from nonexisting to an extreme degree of complexity

Today it is generally accepted that there are several endocytic mechanisms, both the clathrin-dependent one and mechanisms which operate without clathrin and with different requirements when it comes to dynamin, small GTP-binding proteins of the Rho family and specific lipids. It should be noted that clathrin-independent endocytosis can occur even when the cholesterol level in the membrane has been reduced to so low levels that caveolae are gone and clathrin-coated membrane areas are flat. Although new investigators in the field take it for granted that there is a multitude of entry mechanisms, it has taken a long time for this to become accepted. However, more work needs to be done, because one can still ask the question: How many endocytic mechanisms does a cell have, what are their function, and how are they regulated? This article describes some of the history of endocytosis research and attempts to give an overview of the complexity of the mechanisms and their regulation.

Coxsackievirus entry across epithelial tight junctions requires occludin and the small GTPases Rab34 and Rab5

The major group B coxsackievirus (CVB) receptor is a component of the epithelial tight junction (TJ), a protein complex that regulates the selective passage of ions and molecules across the epithelium. CVB enters polarized epithelial cells from the TJ, causing a transient disruption of TJ integrity. Here we show that CVB does not induce major reorganization of the TJ, but stimulates the specific internalization of occludin-a TJ integral membrane component-within macropinosomes. Although occludin does not interact directly with virus, depletion of occludin prevents CVB entry into the cytoplasm and inhibits infection. Both occludin internalization and CVB entry require caveolin but not dynamin; both are blocked by inhibitors of macropinocytosis and require the activity of Rab34, Ras, and Rab5, GTPases known to regulate macropinocytosis. Thus, CVB entry depends on occludin and occurs by a process that combines aspects of caveolar endocytosis with features characteristic of macropinocytosis.

Molecular determinants of endothelial transcytosis and their role in endothelial permeability

Caveolae transcytosis with its diverse mechanisms-fluid phase, adsorptive, and receptor-mediated-plays an important role in the continuous exchange of molecules across the endothelium. We will discuss key features of endothelial transcytosis and caveolae that have been studied recently and have increased our understanding of caveolae function in transcytosis at the molecular level. During transcytosis, caveolae "pinch off" from the plasma membrane to form discrete vesicular carriers that shuttle to the opposite front of endothelial cells, fuse with the plasma membrane, and discharge their cargo into the perivascular space. Endothelial transcytosis exhibits distinct properties, the most important being rapid and efficient coupling of endocytosis to exocytosis on opposite plasma membrane. We address herein the membrane fusion-fission reactions that underlie transcytosis. Caveolae move across the endothelial cells with their cargo predominantly in the fluid phase through an active process that bypasses the lysosomes. Endothelial transcytosis is a constitutive process of vesicular transport. Recent studies show that transcytosis can be upregulated in response to pathological stimuli. Transcytosis via caveolae is an important route for the regulation of endothelial barrier function and may participate in different vascular diseases.