Pacsin 2 is recruited to caveolae and functions in caveolar biogenesis

News from the caves: update on the structure and function of caveolae

Recent data from the study of the cell biology of caveolae have provided insights both into how these flask-shaped invaginations of the plasma membrane are formed and how they may function in different contexts. This review discusses experiments that analyse the composition and ultrastructural distribution of protein complexes responsible for generating caveolae, that suggest functions for caveolae in response to mechanical stress or damage to the plasma membrane, that show that caveolae may have an important role during the signalling events for regulation of metabolism, and that imply that caveolae can act as endocytic vesicles at the plasma membrane. We also highlight unexpected roles for caveolar proteins in regulating circadian rhythms and new insights into the way in which caveolae may be involved in fatty acid uptake in the intestine. Current outstanding questions in the field are emphasised.

Clathrin-independent pathways of endocytosis

There are many pathways of endocytosis at the cell surface that apparently operate at the same time. With the advent of new molecular genetic and imaging tools, an understanding of the different ways by which a cell may endocytose cargo is increasing by leaps and bounds. In this review we explore pathways of endocytosis that occur in the absence of clathrin. These are referred to as clathrin-independent endocytosis (CIE). Here we primarily focus on those pathways that function at the small scale in which some have distinct coats (caveolae) and others function in the absence of specific coated intermediates. We follow the trafficking itineraries of the material endocytosed by these pathways and finally discuss the functional roles that these pathways play in cell and tissue physiology. It is likely that these pathways will play key roles in the regulation of plasma membrane area and tension and also control the availability of membrane during cell migration.

Endocytic crosstalk: cavins, caveolins, and caveolae regulate clathrin-independent endocytosis

Caveolar proteins and caveolae negatively regulate a second clathrin-independent endocytic CLIC/GEEC pathway; caveolin-1 affects membrane diffusion properties of raft-associated CLIC cargo, and the scaffolding domain of caveolin-1 is required and sufficient for endocytic inhibition.

Several studies have suggested crosstalk between different clathrin-independent endocytic pathways. However, the molecular mechanisms and functional relevance of these interactions are unclear. Caveolins and cavins are crucial components of caveolae, specialized microdomains that also constitute an endocytic route. Here we show that specific caveolar proteins are independently acting negative regulators of clathrin-independent endocytosis. Cavin-1 and Cavin-3, but not Cavin-2 or Cavin-4, are potent inhibitors of the clathrin-independent carriers/GPI-AP enriched early endosomal compartment (CLIC/GEEC) endocytic pathway, in a process independent of caveola formation. Caveolin-1 (CAV1) and CAV3 also inhibit the CLIC/GEEC pathway upon over-expression. Expression of caveolar protein leads to reduction in formation of early CLIC/GEEC carriers, as detected by quantitative electron microscopy analysis. Furthermore, the CLIC/GEEC pathway is upregulated in cells lacking CAV1/Cavin-1 or with reduced expression of Cavin-1 and Cavin-3. Inhibition by caveolins can be mimicked by the isolated caveolin scaffolding domain and is associated with perturbed diffusion of lipid microdomain components, as revealed by fluorescence recovery after photobleaching (FRAP) studies. In the absence of cavins (and caveolae) CAV1 is itself endocytosed preferentially through the CLIC/GEEC pathway, but the pathway loses polarization and sorting attributes with consequences for membrane dynamics and endocytic polarization in migrating cells and adult muscle tissue. We also found that noncaveolar Cavin-1 can act as a modulator for the activity of the key regulator of the CLIC/GEEC pathway, Cdc42. This work provides new insights into the regulation of noncaveolar clathrin-independent endocytosis by specific caveolar proteins, illustrating multiple levels of crosstalk between these pathways. We show for the first time a role for specific cavins in regulating the CLIC/GEEC pathway, provide a new tool to study this pathway, identify caveola-independent functions of the cavins and propose a novel mechanism for inhibition of the CLIC/GEEC pathway by caveolin.

Endocytosis is the process that allows cells to take up molecules from the environment. Several endocytic pathways exist in mammalian cells. While the best understood endocytic pathway uses clathrin, recent years have seen a great increase in our understanding of clathrin-independent endocytic pathways. Here we characterize the crosstalk between caveolae, flask-shaped specialized microdomains present at the plasma membrane, and a second clathrin-independent pathway, the CLIC/GEEC Cdc42-regulated endocytic pathway. These pathways are segregated in migrating cells with caveolae at the rear and CLIC/GEEC endocytosis at the leading edge. Here we find that specific caveolar proteins, caveolins and cavins, can also negatively regulate the CLIC/GEEC pathway. With the help of several techniques, including quantitative electron microscopy analysis and real-time live-cell imaging, we demonstrate that expression of caveolar proteins affects early carrier formation, causes cellular lipid changes, and changes the activity of the key regulator of the CLIC/GEEC pathway, Cdc42. The functional consequences of loss of caveolar proteins on the CLIC/GEEC pathway included inhibition of polarized cell migration and increased endocytosis in tissue explants.

Live imaging of prions reveals nascent PrPSc in cell-surface, raft-associated amyloid strings and webs

Mammalian prions refold host glycosylphosphatidylinositol-anchored PrP(C) into β-sheet-rich PrP(Sc). PrP(Sc) is rapidly truncated into a C-terminal PrP27-30 core that is stable for days in endolysosomes. The nature of cell-associated prions, their attachment to membranes and rafts, and their subcellular locations are poorly understood; live prion visualization has not previously been achieved. A key obstacle has been the inaccessibility of PrP27-30 epitopes. We overcame this hurdle by focusing on nascent full-length PrP(Sc) rather than on its truncated PrP27-30 product. We show that N-terminal PrP(Sc) epitopes are exposed in their physiological context and visualize, for the first time, PrP(Sc) in living cells. PrP(Sc) resides for hours in unexpected cell-surface, slow moving strings and webs, sheltered from endocytosis. Prion strings observed by light and scanning electron microscopy were thin, micrometer-long structures. They were firmly cell associated, resisted phosphatidylinositol-specific phospholipase C, aligned with raft markers, fluoresced with thioflavin, and were rapidly abolished by anti-prion glycans. Prion strings and webs are the first demonstration of membrane-anchored PrP(Sc) amyloids.

Syndapin 3 modulates fusion pore expansion in mouse neuroendocrine chromaffin cells

Adrenal neuroendocrine chromaffin cells receive excitatory synaptic input from the sympathetic nervous system and secrete hormones into the peripheral circulation. Under basal sympathetic tone, modest amounts of freely soluble catecholamine are selectively released through a restricted fusion pore formed between the secretory granule and the plasma membrane. Upon activation of the sympathoadrenal stress reflex, elevated stimulation drives fusion pore expansion, resulting in increased catecholamine secretion and facilitating release of copackaged peptide hormones. Thus regulated expansion of the secretory fusion pore is a control point for differential hormone release of the sympathoadrenal stress response. Previous work has shown that syndapin 1 deletion alters transmitter release and that the dynamin 1-syndapin 1 interaction is necessary for coupled endocytosis in neurons. Dynamin has also been shown to be involved in regulation of fusion pore expansion in neuroendocrine chromaffin cells through an activity-dependent association with syndapin. However, it is not known which syndapin isoform(s) contributes to pore dynamics in neuroendocrine cells. Nor is it known at what stage of the secretion process dynamin and syndapin associate to modulate pore expansion. Here we investigate the expression and localization of syndapin isoforms and determine which are involved in mediating fusion pore expansion. We show that all syndapin isoforms are expressed in the adrenal medulla. Mutation of the SH3 dynamin-binding domain of all syndapin isoforms shows that fusion pore expansion and catecholamine release are limited specifically by mutation of syndapin 3. The mutation also disrupts targeting of syndapin 3 to the cell periphery. Syndapin 3 exists in a persistent colocalized state with dynamin 1.

Deletion of cavin genes reveals tissue-specific mechanisms for morphogenesis of endothelial caveolae

Caveolae are abundant in endothelial cells and are thought to have important roles in endothelial cell biology. The cavin proteins are key components of caveolae, and are expressed at varied amounts in different tissues. Here we use knockout mice to determine the roles of cavins 2 and 3 in caveolar morphogenesis in vivo. Deletion of cavin 2 causes loss of endothelial caveolae in lung and adipose tissue, but has no effect on the abundance of endothelial caveolae in heart and other tissues. Changes in the morphology of endothelium in cavin 2 null mice correlate with changes in caveolar abundance. Cavin 3 is not required for making caveolae in the tissues examined. Cavin 2 determines the size of cavin complexes, and acts to shape caveolae. Cavin 1, however, is essential for normal oligomerization of caveolin 1. Our data reveal that endothelial caveolae are heterogeneous, and identify cavin 2 as a determinant of this heterogeneity.

Cavin proteins are key components of mammalian caveolae and are expressed from four genes in a tissue-specific manner. Gram Hansen et al. demonstrate that caveolae in the endothelia of different tissues are remarkably heterogeneous, and reveal a role for cavin 2 in determining the apparent size of cavin complexes.

Scratching the surface: actin' and other roles for the C-terminal Eps15 homology domain protein, EHD2

The C-terminal Eps15 homology domain-containing (EHD) proteins participate in multiple aspects of endocytic membrane trafficking. Of the four mammalian EHD proteins, EHD2 appears to be the most disparate, both in terms of sequence homology, and in subcellular localization/function. Since its initial description as a plasma membrane-associated protein, the precise function of EHD2 has remained enigmatic. Various reports have suggested roles for EHD2 at the plasma membrane, within the endocytic transport system, and even in the nucleus. For example, EHD2 facilitates membrane fusion/repair in muscle cells. Recently the focus has shifted to the role of EHD2 in regulating caveolae. Indeed, EHD2 is highly expressed in tissues rich in caveolae, including fat, muscle and blood vessels. This review highlights cumulative evidence linking EHD2 to actin-rich structures at the plasma membrane, where the plasma membrane-associated phospholipid phosphatidylinositol 4,5-bisphosphate controls EHD2 recruitment. Herein we examine the key pathways where EHD2 might function, and address its potential involvement in these processes.

Role of phosphatidylinositol 4,5-bisphosphate in regulating EHD2 plasma membrane localization

The four mammalian C-terminal Eps15 homology domain-containing proteins (EHD1-EHD4) play pivotal roles in endocytic membrane trafficking. While EHD1, EHD3 and EHD4 associate with intracellular tubular/vesicular membranes, EHD2 localizes to the inner leaflet of the plasma membrane. Currently, little is known about the regulation of EHD2. Thus, we sought to define the factors responsible for EHD2’s association with the plasma membrane. The subcellular localization of endogenous EHD2 was examined in HeLa cells using confocal microscopy. Although EHD partner proteins typically mediate EHD membrane recruitment, EHD2 was targeted to the plasma membrane independent of two well-characterized binding proteins, syndapin2 and EHBP1. Additionally, the EH domain of EHD2, which facilitates canonical EHD protein interactions, was not required to direct overexpressed EHD2 to the cell surface. On the other hand, several lines of evidence indicate that the plasma membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) plays a crucial role in regulating EHD2 subcellular localization. Pharmacologic perturbation of PIP2 metabolism altered PIP2 plasma membrane distribution (as assessed by confocal microscopy), and caused EHD2 to redistribute away from the plasma membrane. Furthermore, overexpressed EHD2 localized to PIP2-enriched vacuoles generated by active Arf6. Finally, we show that although cytochalasin D caused actin microfilaments to collapse, EHD2 was nevertheless maintained at the plasma membrane. Intriguingly, cytochalasin D induced relocalization of both PIP2 and EHD2 to actin aggregates, supporting a role of PIP2 in controlling EHD2 subcellular localization. Altogether, these studies emphasize the significance of membrane lipid composition for EHD2 subcellular distribution and offer new insights into the regulation of this important endocytic protein.

Differential roles of C-terminal Eps15 homology domain proteins as vesiculators and tubulators of recycling endosomes

Endocytic recycling involves the return of membranes and receptors to the plasma membrane following their internalization into the cell. Recycling generally occurs from a series of vesicular and tubular membranes localized to the perinuclear region, collectively known as the endocytic recycling compartment. Within this compartment, receptors are sorted into tubular extensions that later undergo vesiculation, allowing transport vesicles to move along microtubules and return to the cell surface where they ultimately undergo fusion with the plasma membrane. Recent studies have led to the hypothesis that the C-terminal Eps15 homology domain (EHD) ATPase proteins are involved in the vesiculation process. Here, we address the functional roles of the four EHD proteins. We developed a novel semipermeabilized cell system in which addition of purified EHD proteins to reconstitute vesiculation allows us to assess the ability of each protein to vesiculate MICAL-L1-decorated tubular recycling endosomes (TREs). Using this assay, we show that EHD1 vesiculates membranes, consistent with enhanced TRE generation observed upon EHD1 depletion. EHD4 serves a role similar to that of EHD1 in TRE vesiculation, whereas EHD2, despite being capable of vesiculating TREs in the semipermeabilized cells, fails to do so in vivo. Surprisingly, the addition of EHD3 causes tubulation of endocytic membranes in our semipermeabilized cell system, consistent with the lack of tubulation observed upon EHD3 depletion. Our novel vesiculation assay and in vitro electron microscopy analysis, combined with in vivo data, provide evidence that the functions of both EHD1 and EHD4 are primarily in TRE membrane vesiculation, whereas EHD3 is a membrane-tubulating protein.

Molecular composition and ultrastructure of the caveolar coat complex

The single protein caveolar coat complex comprises only cavins and caveolins, coats the caveolar bulb, and is probably responsible for creating caveolae.

Caveolae are an abundant feature of the plasma membrane of many mammalian cell types, and have key roles in mechano-transduction, metabolic regulation, and vascular permeability. Caveolin and cavin proteins, as well as EHD2 and pacsin 2, are all present in caveolae. How these proteins assemble to form a protein interaction network for caveolar morphogenesis is not known. Using in vivo crosslinking, velocity gradient centrifugation, immuno-isolation, and tandem mass spectrometry, we determine that cavins and caveolins assemble into a homogenous 80S complex, which we term the caveolar coat complex. There are no further abundant components within this complex, and the complex excludes EHD2 and pacsin 2. Cavin 1 forms trimers and interacts with caveolin 1 with a molar ratio of about 1∶4. Cavins 2 and 3 compete for binding sites within the overall coat complex, and form distinct subcomplexes with cavin 1. The core interactions between caveolin 1 and cavin 1 are independent of cavin 2, cavin 3, and EHD2 expression, and the cavins themselves can still interact in the absence of caveolin 1. Using immuno-electron microscopy as well as a recently developed protein tag for electron microscopy (MiniSOG), we demonstrate that caveolar coat complexes form a distinct coat all around the caveolar bulb. In contrast, and consistent with our biochemical data, EHD2 defines a different domain at the caveolar neck. 3D electron tomograms of the caveolar coat, labeled using cavin-MiniSOG, show that the caveolar coat is composed of repeating units of a unitary caveolar coat complex.

Caveolae are flask-shaped invaginations in the plasma membrane of many mammalian cell types, and are particularly abundant in fat cells, muscle cells, and the cells that line blood vessels. Although caveolae are likely to be important for cellular responses to mechanical stress, intracellular trafficking, and signaling events, we still lack an understanding of the precise molecular mechanisms for how they form and carry out these functions. Here we address the question of how caveolae are made. Recent years have seen a considerable expansion of the catalogue of known protein components present in caveolae. Our study shows that the main protein components, cavins and caveolins, assemble into one specific complex. We reveal how different amounts of two caveolar proteins, cavin 2 and cavin 3, may be incorporated into this single type of complex, thereby potentially conferring different functional properties on caveolae. Using electron microscopy, we demonstrate that the protein complex is distributed all around the membrane bulb of caveolae, and so can be truly described as the caveolar coat. The caveolar coat excludes the protein EHD2, which regulates the dynamics of caveolae—this protein has a distinct distribution at the caveolar neck. These findings provide the basis for a more complete understanding of the network of protein interactions that produces caveolae.

Tip-to-tip interaction in the crystal packing of PACSIN 2 is important in regulating tubulation activity

The F-BAR domain containing proteins PACSINs are cytoplasmic phosphoproteins involved in various membrane deformations, such as actin reorganization, vesicle transport and microtubule movement. Our previous study shows that all PACSINs are composed of crescent shaped dimers with two wedge loops, and the wedge loop-mediated lateral interaction between neighboring dimers is important for protein packing and tubulation activity. Here, from the crystal packing of PACSIN 2, we observed a tight tip-to-tip interaction, in addition to the wedge loop-mediated lateral interaction. With this tip-to-tip interaction, the whole packing of PACSIN 2 shows a spiral-like assembly with a central hole from the top view. Elimination of this tip-to-tip connection inhibited the tubulation function of PACSIN 2, indicating that tip-to-tip interaction plays an important role in membrane deformation activity. Together with our previous study, we proposed a packing model for the assembly of PACSIN 2 on membrane, where the proteins are connected by tip-to-tip and wedge loop-mediated lateral interactions on the surface of membrane to generate various diameter tubules.

Syndapin--a membrane remodelling and endocytic F-BAR protein

Syndapin [also called PACSIN (protein kinase C and casein kinase II interacting protein)] is an Fes-CIP4 homology Bin-amphiphysin-Rvs161/167 (F-BAR) and Src-homology 3 domain-containing protein. Three genes give rise to three main isoforms in mammalian cells. They each function in different endocytic and vesicle trafficking pathways and provide critical links between the cytoskeletal network in different cellular processes, such as neuronal morphogenesis and cell migration. The membrane remodelling activity of syndapin via its F-BAR domain and its interaction partners, such as dynamin and neural Wiskott-Aldrich syndrome protein binding to its Src-homology 3 domain, are important with respect to its function. Its various partner proteins provide insights into its mechanism of action, as well as its differential roles in these cellular processes. Signalling pathways leading to the regulation of syndapin function by phosphorylation are now contributing to our understanding of the broader functions of this family of proteins.

Cooperation of MICAL-L1, syndapin2, and phosphatidic acid in tubular recycling endosome biogenesis

MICAL-L1 and the BAR-domain protein syndapin2 bind to phosphatidic acid (PA), a novel lipid component of recycling endosomes (REs). Interactions between these proteins stabilize their association with membranes and allow nucleation of tubules by syndapin2. A new role is highlighted for PA in recycling, suggesting a mechanism for tubular RE formation.

Endocytic transport necessitates the generation of membrane tubules and their subsequent fission to transport vesicles for sorting of cargo molecules. The endocytic recycling compartment, an array of tubular and vesicular membranes decorated by the Eps15 homology domain protein, EHD1, is responsible for receptor and lipid recycling to the plasma membrane. It has been proposed that EHD dimers bind and bend membranes, thus generating recycling endosome (RE) tubules. However, recent studies show that molecules interacting with CasL-Like1 (MICAL-L1), a second, recently identified RE tubule marker, recruits EHD1 to preexisting tubules. The mechanisms and events supporting the generation of tubular recycling endosomes were unclear. Here, we propose a mechanism for the biogenesis of RE tubules. We demonstrate that MICAL-L1 and the BAR-domain protein syndapin2 bind to phosphatidic acid, which we identify as a novel lipid component of RE. Our studies demonstrate that direct interactions between these two proteins stabilize their association with membranes, allowing for nucleation of tubules by syndapin2. Indeed, the presence of phosphatidic acid in liposomes enhances the ability of syndapin2 to tubulate membranes in vitro. Overall our results highlight a new role for phosphatidic acid in endocytic recycling and provide new insights into the mechanisms by which tubular REs are generated.

Caveolae as plasma membrane sensors, protectors and organizers

Caveolae are submicroscopic, plasma membrane pits that are abundant in many mammalian cell types. The past few years have seen a quantum leap in our understanding of the formation, dynamics and functions of these enigmatic structures. Caveolae have now emerged as vital plasma membrane sensors that can respond to plasma membrane stresses and remodel the extracellular environment. Caveolae at the plasma membrane can be removed by endocytosis to regulate their surface density or can be disassembled and their structural components degraded. Coat proteins, called cavins, work together with caveolins to regulate the formation of caveolae but also have the potential to dynamically transmit signals that originate in caveolae to various cellular destinations. The importance of caveolae as protective elements in the plasma membrane, and as membrane organizers and sensors, is highlighted by links between caveolae dysfunction and human diseases, including muscular dystrophies and cancer.

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.

Seg1 controls eisosome assembly and shape

Seg1 establishes a platform for the assembly of eisosomes and is important for determining their length.

Eisosomes are stable domains at the plasma membrane of the budding yeast Saccharomyces cerevisiae and have been proposed to function in endocytosis. Eisosomes are composed of two main cytoplasmic proteins, Pil1 and Lsp1, that form a scaffold around furrow-like plasma membrane invaginations. We show here that the poorly characterized eisosome protein Seg1/Ymr086w is important for eisosome biogenesis and architecture. Seg1 was required for efficient incorporation of Pil1 into eisosomes and the generation of normal plasma membrane furrows. Seg1 preceded Pil1 during eisosome formation and established a platform for the assembly of other eisosome components. This platform was further shaped and stabilized upon the arrival of Pil1 and Lsp1. Moreover, Seg1 abundance controlled the shape of eisosomes by determining their length. Similarly, the Schizosaccharomyces pombe Seg1-like protein Sle1 was necessary to generate the filamentous eisosomes present in fission yeast. The function of Seg1 in the stepwise biogenesis of eisosomes reveals striking architectural similarities between eisosomes in yeast and caveolae in mammals.

Impaired contractility and detrusor hypertrophy in cavin-1-deficient mice

Caveolae are membrane invaginations present in a variety of cell types. Formation of caveolae depends on caveolins and on the more recently discovered family of proteins known as the cavins. Genetic ablation of caveolin-1 was previously shown to give rise to a number of urogenital alterations, but the effects of cavin-1 deletion on urogenital function remain unknown. Here we characterised detrusor contractility and structure in cavin-1-deficient mice. Electron microscopy demonstrated essentially complete lack of caveolae in the knock-out detrusor, and immunoblotting disclosed reduced levels of cavin-3 and of all caveolin proteins. Bladder weight was increased in male knock-out mice, and length-tension relationships demonstrated a reduction in depolarisation-induced contraction. Contractility in response to muscarinic receptor activation was similarly reduced. Despite these functional changes, micturition patterns were similar in conscious and freely moving animals and diuresis was unchanged. Our breeding additionally disclosed that the number of knock-out mice generated in heterozygous crosses was lower than expected, suggesting embryonic/perinatal lethality. In conclusion, this is the first study to show that cavin-1 is critical for detrusor caveolae and for the overall contractility and structure of the urinary bladder.

Stressing caveolae new role in cell mechanics

It has been almost 60 years since caveolae were first visualized by Eichi Yamada and George Palade. Nevertheless, these specialized invaginations of the plasma membrane remain without clear and recognized physiological function. The recent identification of new caveolar components and the ability to probe cell mechanics with sophisticated opticophysical devices have shed new light on this fascinating organelle. Early studies from the 1970s suggested that caveolae could participate in the regulation of membrane dynamics. Recent data have established caveolae as mechanosensors that respond immediately to mechanical stress by flattening into the plasma membrane. Here, we focus on the molecular consequences that result from the caveolar disassembly/reassembly cycle induced by membrane tension variations at the surface of the cell under physiological and pathological conditions.

Oligomers of the ATPase EHD2 confine caveolae to the plasma membrane through association with actin

Caveolae are plasma membrane microdomains that play important roles in signalling and endocytosis. The ATPase EHD2 shuttles on and off the static population of caveolae in an ATPase cycledependent manner and links caveolae to actin filaments confining them to the plasma membrane.

Caveolae are specialized domains present in the plasma membrane (PM) of most mammalian cell types. They function in signalling, membrane regulation, and endocytosis. We found that the Eps-15 homology domain-containing protein 2 (EHD2, an ATPase) associated with the static population of PM caveolae. Recruitment to the PM involved ATP binding, interaction with anionic lipids, and oligomerization into large complexes (60–75S) via interaction of the EH domains with intrinsic NPF/KPF motifs. Hydrolysis of ATP was essential for binding of EHD2 complexes to caveolae. EHD2 was found to undergo dynamic exchange at caveolae, a process that depended on a functional ATPase cycle. Depletion of EHD2 by siRNA or expression of a dominant-negative mutant dramatically increased the fraction of mobile caveolar vesicles coming from the PM. Overexpression of EHD2, in turn, caused confinement of cholera toxin B in caveolae. The confining role of EHD2 relied on its capacity to link caveolae to actin filaments. Thus, EHD2 likely plays a key role in adjusting the balance between PM functions of stationary caveolae and the role of caveolae as vesicular carriers.

Cholesterol depletion in adipocytes causes caveolae collapse concomitant with proteosomal degradation of cavin-2 in a switch-like fashion

Caveolae, little caves of cell surfaces, are enriched in cholesterol, a certain level of which is required for their structural integrity. Here we show in adipocytes that cavin-2, a peripheral membrane protein and one of 3 cavin isoforms present in caveolae from non-muscle tissue, is degraded upon cholesterol depletion in a rapid fashion resulting in collapse of caveolae. We exposed 3T3-L1 adipocytes to the cholesterol depleting agent methyl-β-cyclodextrin, which results in a sudden and extensive degradation of cavin-2 by the proteasome and a concomitant movement of cavin-1 from the plasma membrane to the cytosol along with loss of caveolae. The recovery of cavin-2 at the plasma membrane is cholesterol-dependent and is required for the return of cavin-1 from the cytosol to the cell surface and caveolae restoration. Expression of shRNA directed against cavin-2 also results in a cytosolic distribution of cavin-1 and loss of caveolae. Taken together, these data demonstrate that cavin-2 functions as a cholesterol responsive component of caveolae that is required for cavin-1 localization to the plasma membrane, and caveolae structural integrity.

Caveolar domain organization and trafficking is regulated by Abl kinases and mDia1

The biology of caveolin-1 (Cav1)/caveolae is intimately linked to actin dynamics and adhesion receptors. Caveolar domains are organized in hierarchical levels of complexity from curved or flattened caveolae to large, higher-order caveolar rosettes. We report that stress fibers controlled by Abl kinases and mDia1 determine the level of caveolar domain organization, which conditions the subsequent inward trafficking of caveolar domains induced upon loss of cell adhesion from the extracellular matrix. Abl-deficient cells have fewer stress fibers, a smaller pool of stress-fiber co-aligned Cav1 and increased clustering of Cav1/caveolae at the cell surface. Defective caveolar linkage to stress fibers prevents the formation of big caveolar rosettes upon loss of cell adhesion, correlating with a lack of inward trafficking. Live imaging of stress fibers and Cav1 showed that the actin-linked Cav1 pool loses its spatial organization in the absence of actin polymerization and is dragged and clustered by depolymerizing filaments. We identified mDia1 as the actin polymerization regulator downstream of Abl kinases that controls the stress-fiber-linked Cav1 pool. mDia1 knockdown results in Cav1/caveolae clustering and defective inward trafficking upon loss of cell adhesion. By contrast, cell elongation imposed by the excess of stress fibers induced by active mDia1 flattens caveolae. Furthermore, active mDia1 rescues the actin co-aligned Cav1 pool and Cav1 inward trafficking upon loss of adhesion in Abl-deficient cells. Thus, caveolar domain organization and trafficking are tightly coupled to adhesive and stress fiber regulatory pathways.

The BAR Domain Superfamily Proteins from Subcellular Structures to Human Diseases

Eukaryotic cells have complicated membrane systems. The outermost plasma membrane contains various substructures, such as invaginations and protrusions, which are involved in endocytosis and cell migration. Moreover, the intracellular membrane compartments, such as autophagosomes and endosomes, are essential for cellular viability. The Bin-Amphiphysin-Rvs167 (BAR) domain superfamily proteins are important players in membrane remodeling through their structurally determined membrane binding surfaces. A variety of BAR domain superfamily proteins exist, and each family member appears to be involved in the formation of certain subcellular structures or intracellular membrane compartments. Most of the BAR domain superfamily proteins contain SH3 domains, which bind to the membrane scission molecule, dynamin, as well as the actin regulatory WASP/WAVE proteins and several signal transduction molecules, providing possible links between the membrane and the cytoskeleton or other machineries. In this review, we summarize the current information about each BAR superfamily protein with an SH3 domain(s). The involvement of BAR domain superfamily proteins in various diseases is also discussed.

EHD2 regulates caveolar dynamics via ATP-driven targeting and oligomerization

Eps15 homology domain-containing 2 (EHD2) belongs to the EHD-containing protein family of dynamin-related ATPases involved in membrane remodeling in the endosomal system. EHD2 dimers oligomerize into rings on highly curved membranes, resulting in stimulation of the intrinsic ATPase activity. In this paper, we report that EHD2 is specifically and stably associated with caveolae at the plasma membrane and not involved in clathrin-mediated endocytosis or endosomal recycling, as previously suggested. EHD2 interacts with pacsin2 and cavin1, and ordered membrane assembly of EHD2 is dependent on cavin1 and caveolar integrity. While the EHD of EHD2 is dispensable for targeting, we identified a loop in the nucleotide-binding domain that, together with ATP binding, is required for caveolar localization. EHD2 was not essential for the formation or shaping of caveolae, but high levels of EHD2 caused distortion and loss of endogenous caveolae. Assembly of EHD2 stabilized and constrained caveolae to the plasma membrane to control turnover, and depletion of EHD2, resulting in endocytic and more dynamic and short-lived caveolae. Thus, following the identification of caveolin and cavins, EHD2 constitutes a third structural component of caveolae involved in controlling the stability and turnover of this organelle.

Synergistic BAR-NPF interactions in actin-driven membrane remodeling

Cell and organelle shape can profoundly influence proper cellular function. In recent years, two machineries have emerged as major regulators of membrane shape: Bin-Amphiphysin-Rvs161/167 (BAR) domain-containing proteins, which induce membrane invaginations or protrusions, and nucleation promoting factors (NPFs), which activate the Arp2/3 complex and are thus responsible for the generation of branched actin networks that push on membranes. Several BAR-NPF interactions have been shown to induce various types of protrusions, such as lamellipodia or filopodia, or invaginations, including trafficking organelles such as caveolae, endosomes and clathrin-coated pits (CCPs). This review focuses on how collaboration between these two interacting machineries, which emerges as a unified mechanism of membrane remodeling, accounts for such a variety of membrane shapes.

Membrane trafficking and signaling: two sides of the same coin

Recent findings on clathrin-dependent and non clathrin-dependent endocytic routes are currently changing our classical view of endocytosis. Originally seen as a way for the cell to internalize membrane, receptors or various soluble molecules, this process is in fact directly linked to complex signaling pathways. Here, we review new insights in endocytosis and present latest development in imaging techniques that allow us to visualize and follow the dynamics of membrane-associated signaling events at the plasma membrane and other intracellular compartments. The immune synapse is taken as an illustration of the importance of membrane reorganization and proteins clustering to initiate and maintain signaling. Future challenges include understanding the crosslink between traffic and signaling and how all compartmentalized signals are integrated inside the cell at a higher level.