VIP21-Caveolin, a protein of the trans-Golgi network and caveolae

Emerging Insights into the Molecular Architecture of Caveolin-1

Caveolins are an unusual family of membrane proteins whose primary biological function is to build small invaginated membrane structures at the surface of cells known as caveolae. Caveolins and caveolae regulate numerous signaling pathways, lipid homeostasis, intracellular transport, cell adhesion, and cell migration. They also serve as sensors and protect the plasma membrane from mechanical stress. Despite their many important functions, the molecular basis for how these 50-100 nm "little caves" are assembled and regulate cell physiology has perplexed researchers for 70 years. One major impediment to progress has been the lack of information about the structure of caveolin complexes that serve as building blocks for the assembly of caveolae. Excitingly, recent advances have finally begun to shed light on this long-standing question. In this review, we highlight new developments in our understanding of the structure of caveolin oligomers, including the landmark discovery of the molecular architecture of caveolin-1 complexes using cryo-electron microscopy.

Tumor-stroma biomechanical crosstalk: a perspective on the role of caveolin-1 in tumor progression

Tumor stiffening is a hallmark of malignancy that actively drives tumor progression and aggressiveness. Recent research has shed light onto several molecular underpinnings of this biomechanical process, which has a reciprocal crosstalk between tumor cells, stromal fibroblasts, and extracellular matrix remodeling at its core. This dynamic communication shapes the tumor microenvironment; significantly determines disease features including therapeutic resistance, relapse, or metastasis; and potentially holds the key for novel antitumor strategies. Caveolae and their components emerge as integrators of different aspects of cell function, mechanotransduction, and ECM-cell interaction. Here, we review our current knowledge on the several pivotal roles of the essential caveolar component caveolin-1 in this multidirectional biomechanical crosstalk and highlight standing questions in the field.

Deciphering the roles of caveolin in neurodegenerative diseases: The good, the bad and the importance of context

Neurodegenerative diseases (NDDs), which contribute to progressive and irreversible impairments of both the structure and function of the nervous system, pose a substantial socioeconomic burden on society. Mitochondrial dysfunction, oxidative stress, membrane damage, DNA damage, and abnormal protein degradation pathways play pivotal roles in the etiology of NDDs. Recently, growing evidence has demonstrated that caveolins are important in the pathology of NDDs due to their cellular functions in signal transduction, endocytosis, transcytosis, cholesterol transport, and lipid homeostasis. Given the significance of caveolins, here we review the literature to clarify their molecular mechanisms and roles in NDDs. We first briefly introduce the general background on caveolins. Next, we focus on the various important functions of caveolins in the brain. Finally, we emphasize recent progress regarding caveolins, especially Cav-1, which exert both benefit and unfavorable effects in NDDs such as AD and PD. Collectively, the data presented here should advance the investigation of caveolins for the future development of innovative strategies for the treatment of NDDs.

Cellular microdomains for nitric oxide signaling in endothelium and red blood cells

There is accumulating evidence that biological membranes are not just homogenous lipid structures, but are highly organized in microdomains, i.e. compartmentalized areas of protein and lipid complexes, which facilitate necessary interactions for various signaling pathways. Each microdomain exhibits unique composition, membrane location and dynamics, which ultimately shape their functional characteristics. In the vasculature, microdomains are crucial for organizing and compartmentalizing vasodilatory signals that contribute to blood pressure homeostasis. In this review we aim to describe how membrane microdomains in both the endothelium and red blood cells allow context-specific regulation of the vasodilatory signal nitric oxide (NO) and its corresponding metabolic products, and how this results in tightly controlled systemic physiological responses. We will describe (1) structural characteristics of microdomains including lipid rafts and caveolae; (2) endothelial cell caveolae and how they participate in mechanosensing and NO-dependent mechanotransduction; (3) the myoendothelial junction of resistance arterial endothelial cells and how protein-protein interactions within it have profound systemic effects on blood pressure regulation, and (4) putative/proposed NO microdomains in RBCs and how they participate in control of systemic NO bioavailability. The sum of these discussions will provide a current view of NO regulation by cellular microdomains.

The caveolar membrane system in endothelium: From cell signaling to vascular pathology

Caveolae are 50- to 100-nm cholesterol and glycosphingolipid-rich flask-shaped invaginations commonly observed in many terminally differentiated cells. These organelles have been described in many cell types and are particularly abundant in endothelial cells, where they have been involved in the regulation of certain signaling pathways. Specific scaffolding proteins termed caveolins, along with the more recently discovered members of the cavin family, represent the major protein components during caveolae biogenesis. In addition, multiple studies aimed to investigate the expression and the regulation of these proteins significantly contributed to elucidate the role of caveolae and caveolins in endothelial cell physiology and disease. The aim of this review is to survey recent evidence of the involvement of the caveolar network in endothelial cell biology and endothelial cell dysfunction.

Caveolae and Caveolin-1 Integrate Reverse Cholesterol Transport and Inflammation in Atherosclerosis

Lipid disorder and inflammation play critical roles in the development of atherosclerosis. Reverse cholesterol transport is a key event in lipid metabolism. Caveolae and caveolin-1 are in the center stage of cholesterol transportation and inflammation in macrophages. Here, we propose that reverse cholesterol transport and inflammation in atherosclerosis can be integrated by caveolae and caveolin-1.

Regulation of Phagolysosomal Digestion by Caveolin-1 of the Retinal Pigment Epithelium Is Essential for Vision

Caveolin-1 associates with the endo/lysosomal machinery of cells in culture, suggesting that it functions at these organelles independently of its contribution to cell surface caveolae. Here we explored mice lacking caveolin-1 specifically in the retinal pigment epithelium (RPE). The RPE supports neighboring photoreceptors via diurnal phagocytosis of spent photoreceptor outer segment fragments. Like mice lacking caveolin-1 globally, (RPE)CAV1(-/-) mice developed a normal RPE and neural retina but showed reduced rod photoreceptor light responses, indicating that lack of caveolin-1 affects photoreceptor function in a non-cell-autonomous manner. (RPE)CAV1(-/-) RPE in situ showed normal particle engulfment but delayed phagosome clearance and reversed diurnal profiles of levels and activities of lysosomal enzymes. Therefore, eliminating caveolin-1 specifically impairs phagolysosomal degradation by the RPE in vivo. Endogenous caveolin-1 was recruited to maturing phagolysosomes in RPE cells in culture. Consistent with these in vivo data, a moderate increase (to ∼ 2.5-fold) or decrease (by half) of caveolin-1 protein levels in RPE cells in culture was sufficient to accelerate or impair phagolysosomal digestion, respectively. A mutant form of caveolin-1 that fails to reach the cell surface augmented degradation like wild-type caveolin-1. Acidic lysosomal pH and increased protease activity are essential for digestion. We show that halving caveolin-1 protein levels significantly alkalinized lysosomal pH and decreased lysosomal enzyme activities. Taken together, our results reveal a novel role for intracellular caveolin-1 in modulating phagolysosomal function. Moreover, they show, for the first time, that organellar caveolin-1 significantly affects tissue functionality in vivo.

Caveolin-1 inhibits TrkA-induced cell death by influencing on TrkA modification associated with tyrosine-490 phosphorylation

Caveolin-1, a main structural protein constituent of caveolae, plays an important role in the signal transduction, endocytosis, and cholesterol transport. In addition, caveolin-1 has conflictive role in the regulation of cell survival and death depending on intracellular signaling pathways. The receptor tyrosine kinase TrkA has been known to interact with caveolin-1, and exploits multiple functions such as cell survival, death and differentiation. In this report, we investigated how TrkA-induced cell death signaling is regulated by caveolin-1 in both TrkA and caveolin-1 overexpressing stable U2OS cells. Here we show that TrkA co-localizes with caveolin-1 mostly as a large aggresome around nucleus by confocal immunofluorescence microscopy. Interestingly, TrkA-mediated Bak cleavage was suppressed by caveolin-1, indicating an inhibition of TrkA-induced cell death signaling by caveolin-1. Moreover, caveolin-1 altered TrkA modification including tyrosine-490 phosphorylation and unidentified cleavage(s), resulting in the inhibition of TrkA-induced apoptotic cell death. Our results suggest that caveolin-1 could suppress TrkA-mediated pleiotypic effects by altering TrkA modification via functional interaction.

Endothelial caveolae and caveolin-1 as key regulators of atherosclerosis

This commentary discusses the role of caveolin-1 in atherosclerosis.

Localisation of endothelin B receptor variants to plasma membrane microdomains and its effects on downstream signalling

The endothelin B (ET(B)) receptor can undergo a proteolytic cleavage resulting in an unglycosylated N-terminally truncated receptor. We investigated whether ET(B) receptor processing affects caveolar localisation and mitogenic signalling. Distinct subcellular localisations of ET(B) receptor constructs and epidermal growth factor (EGF) receptor ligands were analysed performing detergent-free caveolae preparations and total internal reflection fluorescence microscopy. ET(B) receptor-induced transactivation of the EGF receptor and its downstream signalling was investigated performing shedding assays and ERK1/2 phosphorylation analyses. In COS7 cells, the N-terminally truncated but not the full-length or glycosylation-deficient ET(B) receptor localised to caveolae. In caveolae-free HEK293 cells, only ET(B) receptor constructs fused to caveolin-2 localised to membrane microdomains. A caveolar accumulation of the ET(B) receptor disfavoured EGF receptor ligand shedding. Nonetheless, the activation of ERK1/2 was efficient and long-lasting. In HEK293 cells, the shedding activity was also impaired by N-terminal truncation. The subsequent ERK1/2 phosphorylation was long-lasting only for the full-length ET(B) receptor. We conclude that the ET(B) receptor localisation might depend on the presence of caveolae within the cell investigated. The data further suggest that caveolar enrichment of ET(B) receptors does not facilitate the release of EGF receptor ligands. However, independent of their localisation, ET(B) receptors are able to induce an ERK1/2 phosphorylation.

The extracellular domain of the TGFβ type II receptor regulates membrane raft partitioning

Cell-surface TGFβ (transforming growth factor β) receptors partition into membrane rafts and the caveolin-positive endocytic compartment by an unknown mechanism. In the present study, we investigated the determinant in the TGFβ type II receptor (TβRII) that is necessary for membrane raft/caveolar targeting. Using subcellular fractionation and immunofluorescence microscopy techniques, we demonstrated that the extracellular domain of TβRII mediates receptor partitioning into raft and caveolin-positive membrane domains. Pharmacological perturbation of glycosylation using tunicamycin or the mutation of Mgat5 [mannosyl(α-1,6)-glycoprotein β-1,6-N-acetylglucosaminyltransferase V] activity interfered with the raft partitioning of TβRII. However, this was not due to the glycosylation state of TβRII, as a non-glycosylated TβRII mutant remained enriched in membrane rafts. This suggested that other cell-surface glycoproteins associate with the extracellular domain of TβRII and direct their partitioning in membrane raft domains. To test this we analysed a GMCSF (granulocyte/macrophage colony-stimulating factor)–TβRII chimaeric receptor, which contains a glycosylated GMCSF extracellular domain fused to the transmembrane and intracellular domains of TβRII. This chimaeric receptor was found to be largely excluded from membrane rafts and caveolin-positive structures. Our results indicate that the extracellular domain of TβRII mediates receptor partitioning into membrane rafts and efficient entrance into caveolin-positive endosomes.

Expression of endothelial phosphorylated caveolin-1 is increased in brain injury

AIMS

Increased endothelial caveolae leading to transcytosis of plasma proteins is associated with blood-brain barrier (BBB) breakdown and cerebral oedema in brain injury. Increased expression of caveolin-1alpha (Cav-1), an integral caveolar membrane protein, was reported in endothelium of arterioles and veins with BBB breakdown to fibronectin post injury. In this study the phosphorylation state of Cav-1 and its association with BBB breakdown was determined in the rat cortical cold injury model over a period of days 0.5-6 post lesion.

METHODS

Expression of phosphorylated Cav-1 was determined by immunoblotting and dual labelling immunofluorescence for phosphorylated caveolin-1 and fibronectin, a marker of BBB breakdown. A phospho-specific monoclonal antibody that selectively recognizes only tyrosine 14-phosphorylated Cav-1 (PY14Cav-1) was used.

RESULTS

Immunoblots showed constitutive expression of PY14Cav-1 in cortex of control rats and a significant increase in PY14Cav-1 expression at the lesion site up to day 4 post lesion. PY14Cav-1 immunostaining was observed in the endothelium of lesion vessels at days 0.5-4 post lesion, in neutrophils at days 0.5 and 2 and in macrophages at day 6 post lesion. Dual labelling showed that 100% of vessels with BBB breakdown to fibronectin showed endothelial PY14Cav-1 on day 0.5, the percentage decreasing to 62% on day 4. On day 6, none of the vessels showed endothelial phosphorylated Cav-1.

CONCLUSIONS

The presence of phosphorylated Cav-1 in endothelium of vessels showing BBB breakdown suggests that phosphorylated Cav-1 signalling may be one of the factors associated with early BBB breakdown and brain oedema in brain injury.

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

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

Caveolae structure and function

Studies on the structure and function of caveolae have revealed how this versatile subcellular organelle can influence numerous signalling pathways. This brief review will discuss a few of the key features of caveolae as it relates to signalling and disease processes.

Subcellular distribution of swine vesicular disease virus proteins and alterations induced in infected cells: a comparative study with foot-and-mouth disease virus and vesicular stomatitis virus

The intracellular distribution of swine vesicular disease virus (SVDV) proteins and the induced reorganization of endomembranes in IBRS-2 cells were analyzed. Fluorescence to new SVDV capsids appeared first upon infection, concentrated in perinuclear circular structures and colocalized to dsRNA. As in foot-and-mouth disease virus (FMDV)-infected cells, a vesicular pattern was predominantly found in later stages of SVDV capsid morphogenesis that colocalized with those of non-structural proteins 2C, 2BC and 3A. These results suggest that assembly of capsid proteins is associated to the replication complex. Confocal microscopy showed a decreased fluorescence to ER markers (calreticulin and protein disulfide isomerase), and disorganization of cis-Golgi gp74 and trans-Golgi caveolin-1 markers in SVDV- and FMDV-, but not in vesicular stomatitis virus (VSV)-infected cells. Electron microscopy of SVDV-infected cells at an early stage of infection revealed fragmented ER cisternae with expanded lumen and accumulation of large Golgi vesicles, suggesting alterations of vesicle traffic through Golgi compartments. At this early stage, FMDV induced different patterns of ER fragmentation and Golgi alterations. At later stages of SVDV cytopathology, cells showed a completely vacuolated cytoplasm containing vesicles of different sizes. Cell treatment with brefeldin A, which disrupts the Golgi complex, reduced SVDV (approximately 5 log) and VSV (approximately 4 log) titers, but did not affect FMDV growth. Thus, three viruses, which share target tissues and clinical signs in natural hosts, induce different intracellular effects in cultured cells.

Productive entry of type C foot-and-mouth disease virus into susceptible cultured cells requires clathrin and is dependent on the presence of plasma membrane cholesterol

We have characterized the entry leading to productive infection of a type C FMDV in two cell lines widely used for virus growth, BHK-21 and IBRS-2. Inhibition of clathrin-mediated endocytosis by sucrose treatment decreased both cell entry and virus multiplication. Evidence of a direct requirement of clathrin for productive viral entry was obtained using BHK21-tTA/anti-CHC cells, which showed a significant reduction of viral entry and infection when the synthesis and functionality of clathrin heavy chain was inhibited (Tet- cells). This was also observed for vesicular stomatitis virus (VSV) productive entry. The effect of NH(4)Cl and concanamycin A on FMDV entry and infection was consistent with the requirement of acidic compartments for decapsidation and virus replication. As expected from its higher stability at acidic pH, this requirement was higher for VSV. Since BHK-21 and IBRS-2 cells expressed caveolin-1, we explored the effect on productive virus entry of drugs that interfere with caveolae-mediated endocytosis. Treatment with nystatin did not reduce entry and infection of FMDV or VSV, while cholesterol depletion with MbetaCD significantly inhibited both steps of the FMDV cycle, indicating that plasma membrane cholesterol is required for virus productive entry.

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.

Caveolins bind to (Na+, K+)/H+ exchanger NHE7 by a novel binding module

NHE7 was identified as the first mammalian organelle-membrane type (Na+, K+)/H+ exchanger that may contribute to the ion homeostasis in the trans-Golgi network (TGN) and endosomes. Here we show that caveolins directly bind to the C-terminal extension of NHE7 by an unconventional binding-module. NHE7 is partly associated with caveolae/lipid raft fractions, and heterologous expression of caveolin dominant-negative mutants as well as cholesterol depriving drugs diminished such associations. In contrast to the wild type NHE7, a deletion mutant lacking the C-terminal extension was predominantly detected in non-caveolae/lipid rafts. We further show that a small fraction of NHE7 is targeted to the cell surface and subsequently internalized. Endocytosis of NHE7 was efficiently inhibited by pharmacological maneuvers that block clathrin-dependent endocytosis, whereas dominant-negative caveolin mutants or methyl beta-cyclodextrin did not affect NHE7-internalization. Thus, NHE7 associates with both caveolae/lipid rafts and non-caveolae/lipid raft, and the two pools likely exhibit separate dynamics.

Antibody-specific detection of caveolin-1 in subapical compartments of MDCK cells

Caveolin-1 is the major structural component of caveolae and is also found in the Golgi complex of many cell types. Occasionally, caveolin-1 has been observed in additional intracellular compartments, including recycling endosomes. Why caveolin-1 expression is detected at these sites only infrequently is not clear. In this study, we test the hypothesis that non-caveolar, non-Golgi pools of caveolin-1 display unique and/or fixation-dependent epitopes. We compared the ability of a panel of antibodies raised against various domains of caveolin-1 to detect distinct subcellular pools of the protein by immunofluorescence microscopy in Madin-Darby canine kidney (MDCK) cells, a cell line where the subcellular localization of caveolin-1 has been extensively characterized. We show that three antibodies directed to the N-terminus of caveolin-1 recognize a previously undetected pool of caveolin-1 in the subapical region of MDCK cells, a localization characteristic of endosomal recycling compartments. The antibodies vary in their ability to label caveolin-1 at the cell surface, and the epitopes detected by each are highly fixation dependent. Our findings suggest that no single caveolin antibody or staining condition is capable of detecting all the caveolin-1 in a cell simultaneously. Consequently, the subcellular distribution of caveolin-1 may be much broader than currently believed.

Neurofibromin binds to caveolin-1 and regulates ras, FAK, and Akt

Neurofibromin (Nf1) is an approximately 280 kDa protein having tumor suppressor function, presumably by virtue of its GTPase activating domain, but little is known regarding molecular aspects of its effector pathways. Caveolin-1 (Cav-1) regulates diverse signaling molecules and has itself been implicated as a tumor suppressor. Here we demonstrate that Nf1 binds to Cav-1's scaffolding domain and co-immunoprecipitates with Cav-1. Analysis of Nf1's primary structure reveals four potential caveolin binding domains, and interestingly, in individuals with neurofibromatosis I, missense mutations occur with high frequency in 3 of the 4 putative domains. We show that Nf1 modulates ras, Akt, and focal adhesion kinase pathways, thereby affecting cytoskeletal organization; moreover, Nf1's effects on signaling are altered when lipid rafts and caveolae are disrupted by cholesterol depletion. These novel findings provide insight into possible signaling mechanisms of Nf1 and suggest that together Nf1 and Cav-1 may coordinately regulate cell growth and differentiation.

Structure of caveolae

The introduction of the electron microscope to the study of the biological materials in the second half of the last century has dramatically expanded our view and understanding of the inner workings of cells by enabling the discovery and study of subcellular organelles. A population of flask-shaped or spherical invaginations of the plasma membrane were described and named plasmalemmal vesicles or caveolae. Until the discovery of caveolin-1 as their first molecular marker in early 1990s, the study of caveolae was the exclusive domain of electron microscopists that demonstrated caveolae at different surface densities in most mammalian cells with few exceptions. Electron microscopy techniques in combination with other approaches have also revealed the structural features of caveolae as well as some of their protein and lipid residents. This review summarizes the data on the structure and components of caveolae and their stomatal diaphragms.

Caveolin; different roles for insulin signal?

Caveolae, discovered by electron microscope in the 1950s, are membrane invaginations that accommodate various molecules that are involved in cellular signaling. Caveolin, a major protein component of caveolae identified in 1990s, has been known to inhibit the function of multiple caveolar proteins, such as kinases, which are involved in cell growth and proliferation, and thus considered to be a general growth signal inhibitor. Recent studies using transgenic mouse models have suggested that insulin signal may be exempted from this inhibition, which rather requires the presence of caveolin for proper signaling. Caveolin may stabilize insulin receptor protein or directly stimulate insulin receptors. Other studies have demonstrated that caveolae provide the TC10 complex with cellular microdomains for glucose transportation through Glut4. These findings suggest that caveolin plays an important role in insulin signal to maintain glucose metabolism in intact animals. However, the role of caveolin in insulin signal may differ from that in other transmembrane receptor signals.

Endothelial vesiculo-vacuolar organelles, pockets and multi-layered fenestrated lamellae in the capillaries of the mouse carotid body

Fenestrated capillaries represent the basic structural unit in the carotid body. They mediate a characteristic hyperpermeability state in this organ. Endothelial fenestrae and plasmalemmal vesicles are of particular importance in this respect. The present electron microscopic study of the capillaries of the mouse carotid body demonstrates prominent endothelial cell structures that are suggested to be closely related to endothelial fenestrae and plasmalemmal vesicles. These structures include: (1) Vesiculo-vacuolar organelles formed by fusion and intercommunication of vesicles and vacuoles of variable dimensions. (2) Pockets in the form of fenestrated membrane-bound vacuoles that communicate either with the capillary lumen, pericapillary space or both via multiple apertures or fenestrae. (3) Multi-layered fenestrated Lamellae where the endothelial cytoplasm is divided into multiple attenuated sheets provided with several fenestrae. The latter two structures are preferentially located in the thick perinuclear region of the endothelial cell. Their fenestrae are always distributed in linear series and show close similarity to the usual chains of fenestrae in the attenuated periphery of the endothelial cells. The individual apertures of the fenestrated vacuoles and multi-layered fenestrated lamellae are closely similar to the stomata of fully opened plasmalemmal vesicles suggesting a relationship between them. Morphological and morphometrical analysis of a series of fenestrae belonging to these structures revealed that they are identical to the usual chains of fenestrae in the attenuated periphery of the endothelial cells.

Caveolin-1 as a prime modulator of aging: a new modality for phenotypic restoration?

Aging can be characterized by structural changes and functional deterioration during the lifetime, for which hundreds of explanations have been put forward. Recently, we have proposed the gate theory of aging, in which gatekeeper molecules at the membrane level would play the prime role in determining the senescent phenotype. Caveolin-1 would be a prime candidate for such a role as a major determinant of the aging process. Caveolin-1 can associate with a variety of molecules, involved in signal transduction, endocytosis and transcytosis, cytoskeletal arrangement, etc. The level of caveolin-1 is strictly regulated to maintain cellular integrity, leading to cellular transformation if depleted, and to the senescent phenotype if overexpressed. In case of senescent cells, the functional and physiological responses to the mitogenic stimuli can be restored and the morphological shape can be resumed by simple adjustment of caveolin-1 status. Therefore, it is suggested that prime modulator molecules, represented by caveolin-1, play a key role in determining the senescent phenotype, either as a physiological response or altered morphology.

Analysis of the interaction between respiratory syncytial virus and lipid-rafts in Hep2 cells during infection

The assembly of respiratory syncytial virus (RSV) in lipid-rafts was examined in Hep2 cells. Confocal and electron microscopy showed that during RSV assembly, the cellular distribution of the complement regulatory proteins, decay accelerating factor (CD55) and CD59, changes and high levels of these cellular proteins are incorporated into mature virus filaments. The detergent-solubility properties of CD55, CD59, and the RSV fusion (F) protein were found to be consistent with each protein being located predominantly within lipid-raft structures. The levels of these proteins in cell-released virus were examined by immunoelectronmicroscopy and found to account for between 5% and 15% of the virus attachment (G) glycoprotein levels. Collectively, our findings suggest that an intimate association exists between RSV and lipid-raft membranes and that significant levels of these host-derived raft proteins, such as those regulating complement activation, are subsequently incorporated into the envelope of mature virus particles.

Identification of caveolae-like structures on the surface of intact cells using scanning force microscopy

Caveolae are small, functionally important membrane invaginations found on the surface of many different cell types. Using electron microscopy, caveolae can be unequivocally identified in cell membranes by virtue of their size and the presence of caveolin/VIP22 proteins in the caveolar coat. In this study we have applied for the first time scanning force microscopy (SFM), to visualize caveolae on the surface of living and fixed cells. By scanning the membranes of Chinese hamster ovary cells (CHO), using the tapping mode of the SFM in fluid, we could visualize small membrane pits on the cell membranes of living and fixed cells. Two populations of pits with mean diameters of around 100 nm and 200 nm were present. In addition, the location of many pits visualized with the SFM was coincident with membrane spots fluorescently labeled with a green fluorescent protein-caveolin-1 fusion protein. Scanning force microscopy on cells treated with methyl-beta-cyclodextrin, an agent that sequesters cholesterol and disrupts caveolae, abolished pits with a measured diameter of 100 nm but left pits of around 200 nm diameter intact. Thus, the smallest membrane pits measured with the SFM in CHO cells were indeed very likely to be identical to caveolae. These experiments show for the first time that SFM can be used to visualize caveolae in intact cells.

Caveolin-3 expression during early chicken development

Caveolin-3, a protein that is correlated with caveolae, is found in muscle cells, especially during their differentiation. Although the distribution of caveolin-3 has been studied in cases such as adult and late embryonic mammalians, the expression of caveolin-3 has not been clearly defined during chicken development. In this study, we detected intense caveolin-3 immunoreactivity (IR) as early as embryonic day 4 (E4), most of the signals were localized within the neural tube and myotome. While IRs in the brain occurred in radial glia at E6, these intensities were reduced to an almost undetectable level at E8. In the case of muscle cells, the exclusive localization of caveolin-3 in the cytoplasmic membrane was detected even at E11, much earlier than in mammalian muscle tissues. Although the caveolin-3 IR pattern was similar to that reported by previous studies, we found some interesting mismatches in the case of avian tissues. Although we are unable to explain caveolin-3 expression patterns in the early embryonic stages, this study could provide a basis for further study on the function of caveolin-3 in avian embryogenesis.

Interaction of phosphoinositolglycan(-peptides) with plasma membrane lipid rafts of rat adipocytes

Insulin receptor-independent activation of the insulin signal transduction cascade in insulin-responsive target cells by phosphoinositolglycans (PIG) and PIG-peptides (PIG-P) is accompanied by redistribution of glycosylphosphatidylinositol (GPI)-anchored plasma membrane proteins (GPI proteins) and dually acylated nonreceptor tyrosine kinases from detergent/carbonate-resistant glycolipid-enriched plasma membrane raft domains of high-cholesterol content (hcDIGs) to rafts of lower cholesterol content (lcDIGs). Here we studied the nature and localization of the primary target of PIG(-P) in isolated rat adipocytes. Radiolabeled PIG-P (Tyr-Cys-Asn-NH-(CH(2))(2)-O-PO(OH)O-6Manalpha1(Manalpha1-2)-2Manalpha1-6Manalpha1-4GluN1-6Ino-1,2-(cyclic)-phosphate) prepared by chemical synthesis or a radiolabeled lipolytically cleaved GPI protein from Saccharomyces cerevisiae, which harbors the PIG-P moiety, bind to isolated hcDIGs but not to lcDIGs. Binding is saturable and abolished by pretreatment of intact adipocytes with trypsin followed by NaCl or with N-ethylmaleimide, indicating specific interaction of PIG-P with a cell surface protein. A 115-kDa polypeptide released from the cell surface by the trypsin/NaCl-treatment is labeled by [(14)C]N-ethylmaleimide. The labeling is diminished upon incubation of adipocytes with PIG-P which can be explained by direct binding of PIG-P to the 115-kDa protein and concomitant loss of its accessibility to N-ethylmaleimide. Binding of PIG-P to hcDIGs is considerably increased after pretreatment of adipocytes with (glycosyl)phosphatidylinositol-specific phospholipases compatible with lipolytic removal of endogenous ligands, such as GPI proteins/lipids. These data demonstrate that in rat adipocytes synthetic PIG(-P) as well as lipolytically cleaved GPI proteins interact specifically with hcDIGs. The interaction depends on the presence of a trypsin/NaCl/NEM-sensitive 115-kDa protein located at hcDIGs which thus represents a candidate for a binding protein for exogenous insulin-mimetic PIG(-P) and possibly endogenous GPI proteins/lipids.

Dynamics of plasma membrane microdomains and cross-talk to the insulin signalling cascade

The critical role of the heterogeneous nature of cellular plasma membranes in transmembrane signal transduction has become increasingly appreciated during the past decade. Areas of relatively disordered, loosely packed phospholipids are disrupted by hydrophobic detergent/carbonate-insoluble glycolipid-enriched raft microdomains (DIGs) of highly ordered (glyco)sphingolipids and cholesterol. DIGs exhibit low buoyant density and are often enriched in glycosylphosphatidylinositol-anchored plasma membrane proteins (GPI proteins), dually acylated signalling proteins, such as non-receptor tyrosine kinases (NRTKs), and caveolin. At least two types of DIGs, hcDIGs and lcDIGs, can be discriminated on basis of higher and lower content, respectively, of these typical DIGs components. In quiescent differentiated cells, GPI proteins and NRTKs are mainly associated with hcDIGs, however, in adipose cells certain insulin-mimetic stimuli trigger redistribution of subsets of GPI proteins and NRTKs from hcDIGs to lcDIGs. Presumably, these stimuli induce displacement of GPI proteins from a GPI receptor located at hcDIGs whereas simultaneously NRTKs dissociate from a complex with caveolin located at hcDIGs, too. NRTKs are thereby activated and, in turn, modulate intracellular signalling pathways, such as stimulation of metabolic insulin signalling in insulin-sensitive cells. The apparent dynamics of DIGs may provide a target mechanism for regulating the activity of lipid-modified signalling proteins by small drug molecules, as exemplified by the sulfonylurea, glimepiride, which lowers blood glucose in an insulin-independent fashion, in part.

Respiratory syncytial virus assembly occurs in GM1-rich regions of the host-cell membrane and alters the cellular distribution of tyrosine phosphorylated caveolin-1

We have previously shown that respiratory syncytial virus (RSV) assembly occurs within regions of the host-cell surface membrane that are enriched in the protein caveolin-1 (cav-1). In this report, we have employed immunofluorescence microscopy to further examine the RSV assembly process. Our results show that RSV matures at regions of the cell surface that, in addition to cav-1, are enriched in the lipid-raft ganglioside GM1. Furthermore, a comparison of mock-infected and RSV-infected cells by confocal microscopy revealed a significant change in the cellular distribution of phosphocaveolin-1 (pcav-1). In mock-infected cells, pcav-1 was located at regions of the cell that interact with the extracellular matrix, termed focal adhesions (FA). In contrast, RSV-infected cells showed both a decrease in the levels of pcav-1 associated with FA and the appearance of pcav-1-containing cytoplasmic vesicles, the latter being absent in mock-infected cells. These cytoplasmic vesicles were clearly visible between 9 and 18 h post-infection and coincided with the formation of RSV filaments, although we did not observe a direct association of pcav-1 with mature virus. In addition, we noted a strong colocalization between pcav-1 and growth hormone receptor binding protein-7 (Grb7), within these cytoplasmic vesicles, which was not observed in mock-infected cells. Collectively, these findings show that the RSV assembly process occurs within specialized lipid-raft structures on the host-cell plasma membrane, induces the cellular redistribution of pcav-1 and results in the formation of cytoplasmic vesicles that contain both pcav-1 and Grb7.

Structure and function of endothelial caveolae

Caveolae are spherical invaginations of the plasma membrane and associated vesicles that are found at high surface densities in most cells, endothelia included. Their structural framework has been shown to consist of oligomerized caveolin molecules interacting with cholesterol and sphingolipids. Caveolae have been involved in many cellular functions such as endocytosis, signal transduction, mechano-transduction, potocytosis, and cholesterol trafficking. Some confusion still persists in the field with respect to the relationship between caveolae and the lipid rafts, which have been involved in many of the above functions. In addition to all these, endothelial caveolae have been involved in capillary permeability by their participation in the process of transcytosis. This short review will focus on their structure and components, methods used to determine these components, and the role of caveolae in the transendothelial exchanges between blood plasma and the interstitial fluid.

Caveolin-1 is incorporated into mature respiratory syncytial virus particles during virus assembly on the surface of virus-infected cells

We have employed immunofluorescence microscopy and transmission electron microscopy to examine the assembly and maturation of respiratory syncytial virus (RSV) in the Vero cell line C1008. RSV matures at the apical cell surface in a filamentous form that extends from the plasma membrane. We observed that inclusion bodies containing viral ribonucleoprotein (RNP) cores predominantly appeared immediately below the plasma membrane, from where RSV filaments form during maturation at the cell surface. A comparison of mock-infected and RSV-infected cells by confocal microscopy revealed a significant change in the pattern of caveolin-1 (cav-1) fluorescence staining. Analysis by immuno-electron microscopy showed that RSV filaments formed in close proximity to cav-1 clusters at the cell surface membrane. In addition, immuno-electron microscopy showed that cav-1 was closely associated with early budding RSV. Further analysis by confocal microscopy showed that cav-1 was subsequently incorporated into the envelope of RSV filaments maturing on the host cell membrane, but was not associated with other virus structures such as the viral RNPs. Although cav-1 was incorporated into the mature virus, it was localized in clusters rather than being uniformly distributed along the length of the viral filaments. Furthermore, when RSV particles in the tissue culture medium from infected cells were examined by immuno-negative staining, the presence of cav-1 on the viral envelope was clearly demonstrated. Collectively, these findings show that cav-1 is incorporated into the envelope of mature RSV particles during egress.

Cell biology of caveolae and caveolin

Originally described in the 1950s caveolae are morphologically identifiable as small omega-shaped plasma membrane invaginations present in most cell types. Caveolae are particularly abundant in adipocytes, fibroblasts, type 1 pneumocytes, endothelial and epithelial cells as well as in smooth and striated muscle cells. The first proposed function for caveolae was that of mediating the internalisation and transendothelial trafficking of solutes. Caveolae have been the object of intense research since the discovery of a biochemical marker protein, caveolin, in the early 1990s. Three genes encoding for caveolins have been characterised in mammals. Caveolins (18-24 kDa) are integral membrane proteins that constitute the major protein component of caveolar membrane in vivo. In addition to a structural role of caveolins in the formation of caveolae, caveolin protein interacts directly, and in a regulated manner, with a number of signalling molecules. We present here a general overview of the current knowledge on the structural role of caveolin in caveolae formation, and implication of caveolin in the control of cell signalling.

Caveolae: from basic trafficking mechanisms to targeting transcytosis for tissue-specific drug and gene delivery in vivo

Continuous endothelium and epithelium create formidable barriers to endogenous molecules as well as targeted drug and gene therapies in vivo. Caveolae represent a possible vesicular trafficking pathway through cell barriers. Here we discuss recent discoveries regarding the basic function of caveolae in transport including transcellular trafficking, intracellular trafficking to distinct endosomes, and molecular mechanisms mediating their budding, docking and fusion (dynamin and SNARE machinery). New technologies to purify and map caveolae as well as generate new probes selectively targeting caveolae in vivo provide valuable tools not only for investigating caveolar endocytosis/transcytosis but also elucidating new potential applications for site-directed treatment of many diseases. Vascular targeting of the caveolar trafficking pathway may be a useful strategy for achieving tissue-specific pharmacodelivery that also overcomes key, normally restrictive cell barriers which greatly reduce the efficacy of many therapies in vivo.

The role of caveolae and caveolin in vesicle-dependent and vesicle-independent trafficking

Caveolae can mediate endocytosis, transcytosis, and potocytosis. Our understanding of these processes as well as the elucidation of the molecular machinery involved has greatly expanded. In addition, caveolin, a 22 kDa protein often associated with caveolae, can promote the trafficking of sterol through the cytoplasm independent of vesicles. Caveolin also influences the formation, morphology, and function of caveolae. The ability of caveolae and caveolin to mediate macromolecular transport directly impacts a variety of physiological and pathophysiological processes.

Caveolin-1 and caveolin-2 expression in mouse macrophages. High density lipoprotein 3-stimulated secretion and a lack of significant subcellular co-localization

Evidence for caveolin expression in macrophages is scarce and conflicting. We therefore examined caveolin-1 and caveolin-2 expression in resident and thioglycollate-elicited mouse peritoneal macrophages (tg-MPM) and in the J774 mouse macrophage cell line by RT-PCR, ribonuclease protection assay, immunoblotting, and immunofluorescence. We found that relative to 3T3 cells, resident MPM and tg-MPM express low amounts of caveolin-1 (45 and 15% of those in 3T3 fibroblasts, respectively), while J774.A1 cells do not express any. Caveolin-2, on the other hand, is expressed in all cells examined, with highest expression in tg-MPM and the lowest in J774 cells. The relative levels of caveolin expression in the various cells correspond well with their respective mRNA levels, as measured by ribonuclease protection assay. Caveolin-1, present primarily on the cell surface, does not co-localize significantly with caveolin-2, which is present primarily in the Golgi compartment in all macrophages studied. Loading of tg-MPM with cholesterol or variations in unesterified cholesterol content appear to have little effect on the level of caveolin-1 or -2 expression or their distribution. Stimulation of cholesterol efflux by HDL(3) leads to caveolin-1 and caveolin-2 secretion to the cell culture medium, a process not detected in the absence of HDL(3). The lack of significant co-localization of the two caveolin isoforms in primary macrophages and their secretion in the presence of HDL(3) provides an interesting and physiologically relevant model system to study additional aspects of caveolin function.

Heterologous desensitization mediated by G protein-specific binding to caveolin

We examined the notion that sequestration of G protein subunits by binding to caveolin impedes G protein reassociation and leads to transient, G protein-specific desensitization of response in dispersed smooth muscle cells. Cholecystokinin octapeptide (CCK-8) and substance P (SP) were used to activate G(q/11), cyclopentyl adenosine (CPA) was used to activate G(i3), and acetylcholine (ACh) was used to activate both G(q/11) and G(i3) via m3 and m2 receptors, respectively. CCK-8 and SP increased only Galpha(q/11), and CPA increased only Galpha(i3) in caveolin immunoprecipitates; caveolin and other G proteins were not increased. ACh increased both Galpha(q/11) and Galpha(i3) in a time- and concentration-dependent fashion: only Galpha(q/11) was increased in the presence of an m2 antagonist, and only Galpha(i3) was increased in the presence of an m3 antagonist. To determine whether transient G protein binding to caveolin affected subsequent responses mediated by the same G protein, PLC-beta activity was measured in cells stimulated sequentially with two different agonists that activate either the same or a different G protein. After treatment of the cells with ACh and an m2 antagonist, the phospholipase C-beta (PLC-beta) response to CCK-8 and SP, but not CPA, was decreased; conversely, after treatment of the cells with ACh and an m3 antagonist, the PLC-beta response to CPA, but not CCK-8 or SP, was decreased. Similarly, after treatment with CCK-8 or SP, the PLC-beta response mediated by G(q/11) only was decreased, whereas after treatment with CPA, the PLC-beta response mediated by G(i3) only was decreased. A caveolin-binding Galpha(q/11) fragment blocked the binding of activated Galpha(q/11) but not Galpha(i3) to caveolin-3 and prevented desensitization of the PLC-beta response mediated only by other G(q/11)-coupled receptors. A caveolin-binding Galpha(i3) fragment had the reverse effect. Thus, transient binding of receptor-activated G protein subunits to caveolin impedes reassociation of the heterotrimeric species and leads to desensitization of response mediated by other receptors coupled to the same G protein.

Targeting of membrane transporters in renal epithelia: when cell biology meets physiology

Epithelial cells in the kidney have highly specialized transport mechanisms that differ among the many tubule segments, and among the different cell types that are present in some regions. The purpose of this brief review is to examine some of the major intracellular mechanisms by which the membrane proteins that participate in these differentiated cellular functions are addressed, sorted, and delivered to specific membrane domains of epithelial cells. Unraveling these processes is important not only for our understanding of normal cellular function but is also critical for the interpretation of pathophysiological dysfunction in the context of newly generated molecular and cellular information concerning hereditary and acquired transporter abnormalities. Among the topics covered are sorting signals on proteins, role of the cytoskeleton, vesicle coat proteins, the fusion machinery, and exo- and endocytosis of recycling proteins. Examples of these events in renal epithelial cells are highlighted throughout this review and are related to the physiology of the kidney.

Identification of caveolae and caveolin in C6 glioma cells

Caveolae (CAV) constitute a novel subcellular transport vesicle that has received special attention based on its proven and postulated participation in transcytosis, potocytosis, and in cell signaling events. One of the principal components of CAV are caveolin protein isoforms. Here, we have undertaken the immunochemical identification of CAV and the known caveolin isoforms (1alpha, 1beta, 2 and 3) in cultured rat C6 glioma cells. Immunoblot analysis revealed that particulate fractions from rat C6 glioma cells express caveolin-1 and caveolin-2. The relative detergent-insolubility of these caveolin isoforms was also determined by Western blot analysis. Indirect immunofluorescence analysis with caveolin-1 and -2 antibodies revealed staining patterns typical of CAV's known subcellular distribution and localization. For both caveolin isoforms immunocytochemical staining was characterized by intensely fluorescent puncta throughout the cytoplasm and diffuse micropatches at the level of the plasmalemma. Perinuclear staining was also detected, consistent and suggestive of caveolin's localization in the trans Golgi region. The caveolin-1 and -2 immunoreactivity seen in Western blots and immunocytochemically is related to structurally relevant CAV as supported by the isolation of caveolin-enriched membrane complexes using two different methods. Light-density, Triton X-100-insoluble caveolin-1- and caveolin-2-enriched fractions were obtained after fractionation of rat C6 glioma cells and their separation over 5-40% discontinuous sucrose-density gradients. Similar fractions were obtained using a detergent-free, sodium carbonate-based fractionation method. These results further support the localization of CAV and caveolins in glial cells. In addition, they demonstrate that cultured C6 glioma cells can be useful as a model system to study the role of CAV and caveolins in subcellular transport and signal transduction events in glial cells and the brain.

Sphingolipid depletion increases formation of the scrapie prion protein in neuroblastoma cells infected with prions

Sphingolipid-rich rafts play an essential role in the posttranslational (Borchelt, D. R., Scott, M., Taraboulos, A., Stahl, N., and Prusiner, S. B. (1990) J. Cell Biol. 110, 743-752)) formation of the scrapie prion protein PrP(Sc) from its normal conformer PrP(C) (Taraboulos, A., Scott, M., Semenov, A., Avrahami, D., Laszlo, L., Prusiner, S. B., and Avraham, D. (1995) J. Cell Biol. 129, 121-132). We investigated the importance of sphingolipids in the metabolism of the PrP isoforms in scrapie-infected ScN2a cells. The ceramide synthase inhibitor fumonisin B(1) (FB(1)) reduced both sphingomyelin (SM) and ganglioside GM1 in cells by up to 50%, whereas PrP(Sc) increased by 3-4-fold. Whereas FB(1) profoundly altered the cell lipid composition, the raft residents PrP(C), PrP(Sc), caveolin 1, and GM1 remained insoluble in Triton X-100. Metabolic radiolabeling demonstrated that PrP(C) production was either unchanged or slightly reduced in FB(1)-treated cells, whereas PrP(Sc) formation was augmented by 3-4-fold. To identify the sphingolipid species the decrease of which correlates with increased PrP(Sc), we used two other reagents. When cells were incubated with sphingomyelinase for 3 days, SM levels decreased, GM1 was unaltered, and PrP(Sc) increased by 3-4-fold. In contrast, the glycosphingolipid inhibitor PDMP reduced PrP(Sc) while increasing SM. Thus, PrP(Sc) seems to correlate inversely with SM levels. The effects of SM depletion contrasted with those previously obtained with the cholesterol inhibitor lovastatin, which reduced PrP(Sc) and removed it from detergent-insoluble complexes.

Isolation, cloning, and localization of rat PV-1, a novel endothelial caveolar protein

By using an immunoisolation procedure (Stan, R.-V., W.G. Roberts, K. Ihida, D. Predescu, L. Saucan, L. Ghitescu, and G.E. Palade. 1997. Mol. Biol. Cell. 8:595-605) developed in our laboratory, we have isolated a caveolar subfraction from rat lung endothelium and we have partially characterized the proteins of this subfraction which include an apparently caveolae-specific glycoprotein we propose to call PV-1 (formerly known as gp68). The isolation and partial sequencing of PV-1, combined with the cloning of the full length PV-1 cDNA led to the following conclusions: (a) PV-1 is a novel single span type II integral membrane protein (438 amino acids long) which forms homodimers in situ; (b) the transmembrane domain of PV-1 is near the NH2 terminus defining a short cytoplasmic endodomain and a large COOH-terminal ectodomain exposed to the blood plasma; (c) PV-1 is N-glycosylated and its glycan antennae bear terminal nonreducing galactosyl residues in alpha1-3 linkage. PV-1 is expressed mostly in the lung but both the messenger RNA and the protein can be detected at lower levels also in kidney, spleen, liver, heart, muscle, and brain. No signal could be detected in testis and two lower molecular weight forms were detected in brain. Immunocytochemical studies carried out by immunodiffusion on rat lung with an anti-PV-1 polyclonal antibody directed against a COOH-terminal epitope reveal a specific localization of PV-1 to the stomatal diaphragms of rat lung endothelial caveolae and confirm the extracellular orientation of the PV-1 COOH terminus.

Detergent-insoluble glycosphingolipid/cholesterol-rich membrane domains, lipid rafts and caveolae (review)

Within the cell membrane glycosphingolipids and cholesterol cluster together in distinct domains or lipid rafts, along with glycosyl-phosphatidylinositol (GPI)-anchored proteins in the outer leaflet and acylated proteins in the inner leaflet of the bilayer. These lipid rafts are characterized by insolubility in detergents such as Triton X-100 at 4 degrees C. Studies on model membrane systems have shown that the clustering of glycosphingolipids and GPI-anchored proteins in lipid rafts is an intrinsic property of the acyl chains of these membrane components, and that detergent extraction does not artefactually induce clustering. Cholesterol is not required for clustering in model membranes but does enhance this process. Single particle tracking, chemical cross-linking, fluorescence resonance energy transfer and immunofluorescence microscopy have been used to directly visualize lipid rafts in membranes. The sizes of the rafts observed in these studies range from 70-370 nm, and depletion of cellular cholesterol levels disrupts the rafts. Caveolae, flask-shaped invaginations of the plasma membrane, that contain the coat protein caveolin, are also enriched in cholesterol and glycosphingolipids. Although caveolae are also insoluble in Triton X-100, more selective isolation procedures indicate that caveolae do not equate with detergent-insoluble lipid rafts. Numerous proteins involved in cell signalling have been identified in caveolae, suggesting that these structures may function as signal transduction centres. Depletion of membrane cholesterol with cholesterol binding drugs or by blocking cellular cholesterol biosynthesis disrupts the formation and function of both lipid rafts and caveolae, indicating that these membrane domains are involved in a range of biological processes.

Role of plasmalemmal caveolae in signal transduction

Caveolae are specialized plasmalemmal microdomains originally studied in numerous cell types for their involvement in the transcytosis of macromolecules. They are enriched in glycosphingolipids, cholesterol, sphingomyelin, and lipid-anchored membrane proteins, and they are characterized by a light buoyant density and resistance to solubilization by Triton X-100 at 4 degreesC. Once the identification of the marker protein caveolin made it possible to purify this specialized membrane domain, it was discovered that caveolae also contain a variety of signal transduction molecules. This includes G protein-coupled receptors, G proteins and adenylyl cyclase, molecules involved in the regulation of intracellular calcium homeostasis, and their effectors including the endothelial isoform of nitric oxide synthase, multiple components of the tyrosine kinase-mitogen-activated protein kinase pathway, and numerous lipid signaling molecules. More recent work has indicated that caveolae further serve to compartmentalize, modulate, and integrate signaling events at the cell surface. This specialized plasmalemmal domain warrants direct consideration in future investigations of both normal and pathological signal transduction in pulmonary cell types.

The caveolae membrane system

The cell biology of caveolae is a rapidly growing area of biomedical research. Caveolae are known primarily for their ability to transport molecules across endothelial cells, but modern cellular techniques have dramatically extended our view of caveolae. They form a unique endocytic and exocytic compartment at the surface of most cells and are capable of importing molecules and delivering them to specific locations within the cell, exporting molecules to extracellular space, and compartmentalizing a variety of signaling activities. They are not simply an endocytic device with a peculiar membrane shape but constitute an entire membrane system with multiple functions essential for the cell. Specific diseases attack this system: Pathogens have been identified that use it as a means of gaining entrance to the cell. Trying to understand the full range of functions of caveolae challenges our basic instincts about the cell.

Recent advances in intestinal macromolecular drug delivery via receptor-mediated transport pathways

Receptor-mediated transport mechanisms provide a pathway for the trafficking of extracellular macromolecules into (endocytosis) and across (transcytosis) the cell. This comprises the binding of a ligand to a specific cell-surface receptor, clustering of the ligand-receptor complexes in endocytotic vesicles and vesicular sorting. This review focuses on recent advances in cellular and molecular biology pertaining to receptor-mediated endocytosis. A concise overview is presented of current and potential future applications of targeting to RME mechanisms to improve oral macromolecular drug delivery.