Caveolin-1 induces formation of membrane tubules that sense actomyosin tension and are inhibited by polymerase I and transcript release factor/cavin-1

Role of Caveolae family-related proteins in the development of breast cancer

Breast cancer has become the most significant malignant tumor threatening women's lives. Caveolae are concave pits formed by invagination of the plasma membrane that participate in many biological functions of the cell membrane, such as endocytosis, cell membrane assembly, and signal transduction. In recent years, Caveolae family-related proteins have been found to be closely related to the occurrence and development of breast cancer. The proteins associated with the Caveolae family-related include Caveolin (Cav) and Cavins. The Cav proteins include Cav-1, Cav-2 and Cav-3, among which Cav-1 has attracted the most attention as a tumor suppressor and promoting factor affecting the proliferation, apoptosis, migration, invasion and metastasis of breast cancer cells. Cav-2 also has dual functions of inhibiting and promoting cancer and can be expressed in combination with Cav-1 or play a regulatory role alone. Cav-3 has been less studied in breast cancer, and the loss of its expression can form an antitumor microenvironment. Cavins include Cavin-1, Cavin-2, Cavin-3 and Cavin-4. Cavin-1 inhibits Cav-1-induced cell membrane tubule formation, and its specific role in breast cancer remains controversial. Cavin-2 acts as a breast cancer suppressor, inhibiting breast cancer progression by blocking the transforming growth factor (TGF-β) signaling pathway. Cavin-3 plays an anticancer role in breast cancer, but its specific mechanism of action is still unclear. The relationship between Cavin-4 and breast cancer is unclear. In this paper, the role of Caveolae family-related proteins in the occurrence and development of breast cancer and their related mechanisms are discussed in detail to provide evidence supporting the further study of Caveolae family-related proteins as potential targets for the diagnosis and treatment of breast cancer.

PV1 in Caveolae Controls Lung Endothelial Permeability

Caveolae are prominent plasmalemmal invaginations in endothelial cells, especially in the lung vasculature, which comprises a vast surface area. PV1 (plasmalemmal vesicle-associated protein-1), a 60-kD glycoprotein expressed in endothelial cells, is essential for generating spoke-like diaphragmatic structures that span the neck region of endothelial caveolae. However, their role in caveolae-mediated uptake and endothelial-barrier function is unknown. Here, we generated mice with endothelial cell-specific deletion of PV1 through tamoxifen-induced Cdh5.Cre.ERT2 (endothelial-specific vascular cadherin.Cre.estrogen receptor 2)-mediated excision of the floxed PV1 allele. We observed that loss of PV1 specifically in endothelial cells increased lung vascular permeability of fluid and protein, indicating that PV1 is required for maintenance of lung vascular-barrier integrity. Endothelial-specific PV1 deletion also increased caveolae-mediated uptake of tracer albumin compared with controls, promoted Au-albumin accumulation in the bulb of caveolae, and induced caveolar swelling. In addition, we observed the progressive loss of plasma proteins from the circulation and reduced arterial pressure resulting from transudation of water and protein as well as edema formation in multiple tissues, including lungs. These changes seen after endothelial-specific PV1 deletion occurred in the absence of disruption of endothelial junctions. We demonstrated that exposure of wild-type mice to endotoxin, which is known to cause acute lung injury and increase protein permeability, also significantly reduced PV1 protein expression. We conclude that the key function of PV1 is to regulate lung endothelial permeability through its ability to restrict the entry of plasma proteins such as albumin into caveolae and their transport through the endothelial barrier.

Salidroside-Mediated Autophagic Targeting of Active Src and Caveolin-1 Suppresses Low-Density Lipoprotein Transcytosis across Endothelial Cells

Subendothelial retention of apolipoprotein B100-containing lipoprotein, such as low-density lipoprotein (LDL), is the initial step of atherogenesis. Activation of autophagy exhibits beneficial effects for the treatment of atherosclerosis. In our previous study, we demonstrated that hyperglycemia suppressed autophagic degradation of caveolin-1, which in turn resulted in acceleration of caveolae-mediated LDL transcytosis across endothelial cells and lipid retention. Therefore, targeting the crossed pathway in autophagy activation and LDL transcytosis interruption may be a promising antiatherosclerotic strategy. In metabolic diseases, including atherosclerosis, salidroside, a phenylpropanoid glycoside compound (3,5-dimethoxyphenyl) methyl-β-glucopyranoside), is the most important compound responsible for the therapeutic activities of Rhodiola. However, whether salidroside suppresses LDL transcytosis to alleviate atherosclerosis has not yet been elucidated. In the present study, we demonstrated that salidroside significantly decreased LDL transcytosis across endothelial cells. Salidroside-induced effects were dramatically blocked by AMPK (adenosine monophosphate-activated protein kinase) inhibitor (compound c, AMPKα siRNA) and by overexpression of exogenous tyrosine-phosphorylated caveolin-1 using transfected cells with phosphomimicking caveolin-1 on tyrosine 14 mutant plasmids (Y14D). Furthermore, we observed that salidroside promoted autophagosome formation via activating AMPK. Meanwhile, the interaction between caveolin-1 and LC3B-II, as well as the interaction between active Src (indicated by the phosphorylation of Src on tyrosine 416) and LC3B-II, was significantly increased, upon stimulation with salidroside. In addition, both bafilomycin A₁ (a lysosome inhibitor) and an AMPK inhibitor (compound c) markedly prevented salidroside-induced autophagic degradation of p-Src and caveolin-1. Moreover, the phosphorylation of caveolin-1 on tyrosine 14 was disrupted due to the downregulation of p-Src and caveolin-1, thereby directly decreasing LDL transcytosis by attenuating the number of caveolae on the cell membrane and by preventing caveolae-mediated LDL endocytosis released from the cell membrane. In ApoE^−/− mice, salidroside significantly delayed the formation of atherosclerotic lesions. Meanwhile, a significant increase in LC3B, accompanied by attenuated accumulation of the autophagy substrate SQSTM1, was observed in aortic endothelium of ApoE^−/− mice. Taken together, our findings demonstrated that salidroside protected against atherosclerosis by inhibiting LDL transcytosis through enhancing the autophagic degradation of active Src and caveolin-1.

Cells respond to deletion of CAV1 by increasing synthesis of extracellular matrix

A range of cellular functions have been attributed to caveolae, flask-like invaginations of the plasma membrane. Here, we have used RNA-seq to achieve quantitative transcriptional profiling of primary embryonic fibroblasts from caveolin 1 knockout mice (CAV1-/- MEFs), and thereby to gain hypothesis-free insight into how these cells respond to the absence of caveolae. Components of the extracellular matrix were decisively over-represented within the set of genes displaying altered expression in CAV1-/- MEFs when compared to congenic wild-type controls. This was confirmed biochemically and by imaging for selected examples. Up-regulation of components of the extracellular matrix was also observed in a second cell line, NIH-3T3 cells genome edited to delete CAV1. Up-regulation of components of the extracellular matrix was detected in vivo by assessing collagen deposition and compliance of CAV1-/- lungs. We discuss the implications of these findings in terms of the cellular function of caveolae.

Cavin-1 regulates caveolae-mediated LDL transcytosis: crosstalk in an AMPK/eNOS/ NF-κB/Sp1 loop

Caveolae are specialized lipid rafts structure in the cell membrane and critical for regulating endothelial functions, e.g. transcytosis of macromolecules like low density lipoprotein (LDL) etc. Specifically, the organization and functions of caveolae are mediated by structure protein (caveolin-1) and adapter protein (cavin-1). The pathogenic role of caveolin-1 is well studied; nevertheless, mechanisms whereby cavin-1 regulates signaling transduction remain poorly understood. The aim of this study was designed to explore the role of cavin-1 in caveolae-mediated LDL transcytosis across endothelial cells. We reported here that cavin-1 knockdown mediated by small interfering RNA (siRNA) caused a significant decrease of LDL transcytosis. Moreover, cavin-1 knockdown increased the activity of endothelial nitric oxide synthase (eNOS) and the production of nitric oxide (NO). Consequently, an eNOS inhibitor, N-Nitro-L-Arginine Methyl Ester (L-NAME), not only suppressed the activity of specificity protein (Sp1) and nuclear factor kappa B (NF-κB), but also inhibited both activities via activating adenosine 5‘-monophosphate- activated protein kinase (AMPK). In conclusion, we proposed an AMPK/eNOS/NF-κB/Sp1 circuit loop was formed to regulate caveolae residing proteins’ expression, e.g. LDL receptor (LDLR), caveolin-1, eNOS, thereby to regulate caveolae-mediated LDL transcytosis in endothelial cells.

Caveolins and cavins in the trafficking, maturation, and degradation of caveolae: implications for cell physiology

Caveolins (Cavs) are ~20 kDa scaffolding proteins that assemble as oligomeric complexes in lipid raft domains to form caveolae, flask-shaped plasma membrane (PM) invaginations. Caveolae ("little caves") require lipid-lipid, protein-lipid, and protein-protein interactions that can modulate the localization, conformational stability, ligand affinity, effector specificity, and other functions of proteins that are partners of Cavs. Cavs are assembled into small oligomers in the endoplasmic reticulum (ER), transported to the Golgi for assembly with cholesterol and other oligomers, and then exported to the PM as an intact coat complex. At the PM, cavins, ~50 kDa adapter proteins, oligomerize into an outer coat complex that remodels the membrane into caveolae. The structure of caveolae protects their contents (i.e., lipids and proteins) from degradation. Cellular changes, including signal transduction effects, can destabilize caveolae and produce cavin dissociation, restructuring of Cav oligomers, ubiquitination, internalization, and degradation. In this review, we provide a perspective of the life cycle (biogenesis, degradation), composition, and physiologic roles of Cavs and caveolae and identify unanswered questions regarding the roles of Cavs and cavins in caveolae and in regulating cell physiology.¹.

High cell-surface density of HER2 deforms cell membranes

Breast cancers (BC) with HER2 overexpression (referred to as HER2 positive) progress more aggressively than those with normal expression. Targeted therapies against HER2 can successfully delay the progression of HER2-positive BC, but details of how this overexpression drives the disease are not fully understood. Using single-molecule biophysical approaches, we discovered a new effect of HER2 overexpression on disease-relevant cell biological changes in these BC. We found HER2 overexpression causes deformation of the cell membranes, and this in turn disrupts epithelial features by perturbing cell-substrate and cell-cell contacts. This membrane deformation does not require receptor signalling activities, but results from the high levels of HER2 on the cell surface. Our finding suggests that early-stage morphological alterations of HER2-positive BC cells during cancer progression can occur in a physical and signalling-independent manner.

Cavin family proteins and the assembly of caveolae

Caveolae are an abundant feature of the plasma membrane in many cells. Until recently, they were generally considered to be membrane invaginations whose formation primarily driven by integral membrane proteins called caveolins. However, the past decade has seen the emergence of the cavin family of peripheral membrane proteins as essential coat components and regulators of caveola biogenesis. In this Commentary, we summarise recent data on the role of cavins in caveola formation, highlighting structural studies that provide new insights into cavin coat assembly. In mammals, there are four cavin family members that associate through homo- and hetero-oligomerisation to form distinct subcomplexes on caveolae, which can be released into the cell in response to stimuli. Studies from several labs have provided a better understanding of cavin stoichiometry and the molecular basis for their oligomerisation, as well as identifying interactions with membrane phospholipids that may be important for caveola function. We propose a model in which coincident, low-affinity electrostatically controlled protein-protein and protein-lipid interactions allow the formation of caveolae, generating a meta-stable structure that can respond to plasma membrane stress by release of cavins.

Microtubule motors power plasma membrane tubulation in clathrin-independent endocytosis

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

Tagging strategies strongly affect the fate of overexpressed caveolin-1

Caveolin-1 (Cav1) is the primary scaffolding protein of caveolae, flask-shaped invaginations of the plasma membrane thought to function in endocytosis, mechanotransduction, signaling and lipid homeostasis. A significant amount of our current knowledge about caveolins and caveolae is derived from studies of transiently overexpressed, C-terminally tagged caveolin proteins. However, how different tags affect the behavior of ectopically expressed Cav1 is still largely unknown. To address this question, we performed a comparative analysis of the subcellular distribution, oligomerization state and detergent resistance of transiently overexpressed Cav1 labeled with three different C-terminal tags (EGFP, mCherry and myc). We show that addition of fluorescent protein tags enhances the aggregation and/or degradation of both wild-type Cav1 and an oligomerization defective P132L mutant. Strikingly, complexes formed by overexpressed Cav1 fusion proteins excluded endogenous Cav1 and Cav2, and the properties of native caveolins were largely preserved even when abnormal aggregates were present in cells. These findings suggest that differences in tagging strategies may be a source of variation in previously published studies of Cav1 and that overexpressed Cav1 may exert functional effects outside of caveolae. They also highlight the need for a critical re-evaluation of current knowledge based on transient overexpression of tagged Cav1.

mRNA profiling reveals determinants of trastuzumab efficiency in HER2-positive breast cancer

Intrinsic and acquired resistance to the monoclonal antibody drug trastuzumab is a major problem in the treatment of HER2-positive breast cancer. A deeper understanding of the underlying mechanisms could help to develop new agents. Our intention was to detect genes and single nucleotide polymorphisms (SNPs) affecting trastuzumab efficiency in cell culture. Three HER2-positive breast cancer cell lines with different resistance phenotypes were analyzed. We chose BT474 as model of trastuzumab sensitivity, HCC1954 as model of intrinsic resistance, and BTR50, derived from BT474, as model of acquired resistance. Based on RNA-Seq data, we performed differential expression analyses on these cell lines with and without trastuzumab treatment. Differentially expressed genes between the resistant cell lines and BT474 are expected to contribute to resistance. Differentially expressed genes between untreated and trastuzumab treated BT474 are expected to contribute to drug efficacy. To exclude false positives from the candidate gene set, we removed genes that were also differentially expressed between untreated and trastuzumab treated BTR50. We further searched for SNPs in the untreated cell lines which could contribute to trastuzumab resistance. The analysis resulted in 54 differentially expressed candidate genes that might be connected to trastuzumab efficiency. 90% of 40 selected candidates were validated by RT-qPCR. ALPP, CALCOCO1, CAV1, CYP1A2 and IGFBP3 were significantly higher expressed in the trastuzumab treated than in the untreated BT474 cell line. GDF15, IL8, LCN2, PTGS2 and 20 other genes were significantly higher expressed in HCC1954 than in BT474, while NCAM2, COLEC12, AFF3, TFF3, NRCAM, GREB1 and TFF1 were significantly lower expressed. Additionally, we inferred SNPs in HCC1954 for CAV1, PTGS2, IL8 and IGFBP3. The latter also had a variation in BTR50. 20% of the validated subset have already been mentioned in literature. For half of them we called and analyzed SNPs. These results contribute to a better understanding of trastuzumab action and resistance mechanisms.

Caveolin-1 expression and cavin stability regulate caveolae dynamics in adipocyte lipid store fluctuation

Adipocytes specialized in the storage of energy as fat are among the most caveolae-enriched cell types. Loss of caveolae produces lipodystrophic diabetes in humans, which cannot be reversed by endothelial rescue of caveolin expression in mice, indicating major importance of adipocyte caveolae. However, how caveolae participate in fat cell functions is poorly understood. We investigated dynamic conditions of lipid store fluctuations and demonstrate reciprocal regulation of caveolae density and fat cell lipid droplet storage. We identified caveolin-1 expression as a crucial step in adipose cell lines and in mice to raise the density of caveolae, to increase adipocyte ability to accommodate larger lipid droplets, and to promote cell expansion by increased glucose utilization. In human subjects enrolled in a trial of 8 weeks of overfeeding to promote fattening, adipocyte expansion response correlated with initial caveolin-1 expression. Conversely, lipid mobilization in cultured adipocytes to induce lipid droplet shrinkage led to biphasic response of cavin-1 with ultimate loss of expression of cavin-1 and -3 and EHD2 by protein degradation, coincident with caveolae disassembly. We have identified the key steps in cavin/caveolin interplay regulating adipocyte caveolae dynamics. Our data establish that caveolae participate in a unique cell response connected to lipid store fluctuation, suggesting lipid-induced mechanotension in adipocytes.

Caveolin and cavin family members: dual roles in cancer

Caveolae are specialized plasma membrane subdomains with distinct lipid and protein compositions, which play an essential role in cell physiology through regulation of trafficking and signaling functions. The structure and functions of caveolae have been shown to require the proteins caveolins. Recently, members of the cavin protein family were found to be required, in concert with caveolins, for the formation and function of caveolae. Caveolins have a paradoxical role in the development of cancer formation. They have been involved in both tumor suppression and oncogenesis, depending on tumor type and progress stage. High expression of caveolins and cavins leads to inhibition of cancer-related pathways, such as growth factor signaling pathways. However, certain cancer cells that express caveolins and cavins have been shown to be more aggressive and metastatic because of their increased potential for anchorage-independent growth. Here, we will survey the functional roles of caveolins and of different cavin family members in cancer regulation.

Emerging role of polymerase-1 and transcript release factor (PTRF/ Cavin-1) in health and disease

Polymerase-1 and release transcript factor (PTRF) was initially reported to be involved in the termination of the transcription process. More recently, it has been implicated in the formation of caveolae, cave-like structures in the plasma membrane. The effects of PTRF related to caveolae suggest that this protein may play important roles in health and disease. PTRF is highly expressed in various cells, including adipocytes, osteoblasts and muscle (cardiac, skeletal and smooth) cells. The role of PTRF in prostate cancer has been recently reviewed but there is growing evidence that PTRF is involved in other physiological processes such as cell repair and the regulation of glucose and lipid metabolism and, furthermore, altered expression of PTRF may be associated with disease. This review discusses the emerging role of PTRF in health and disease.

The spatiotemporal dynamics and membranous features of the Plasmodium liver stage tubovesicular network

For membrane-bound intracellular pathogens, the surrounding vacuole is the portal of communication with the host cell. The parasitophorous vacuole (PV) harboring intrahepatocytic Plasmodium parasites satisfies the parasites' needs of nutrition and protection from host defenses to allow the rapid parasite growth that occurs during the liver stage of infection. In this study, we visualized the PV membrane (PVM) and the associated tubovesicular network (TVN) through fluorescent tagging of two PVM-resident Plasmodium berghei proteins, UIS4 and IBIS1. This strategy revealed previously unrecognized dynamics with which these membranes extend throughout the host cell. We observed dynamic vesicles, elongated clusters of membranes and long tubules that rapidly extend and contract from the PVM in a microtubule-dependent manner. Live microscopy, correlative light-electron microscopy and fluorescent recovery after photobleaching enabled a detailed characterization of these membranous features, including velocities, the distribution of UIS4 and IBIS1, and the connectivity of PVM and TVN. Labeling of host cell compartments revealed association of late endosomes and lysosomes with the elongated membrane clusters. Moreover, the signature host autophagosome protein LC3 was recruited to the PVM and TVN and colocalized with UIS4. Together, our data demonstrate that the membranes surrounding intrahepatic Plasmodium are involved in active remodeling of host cells.

EHD1 mediates vesicle trafficking required for normal muscle growth and transverse tubule development

EHD proteins have been implicated in intracellular trafficking, especially endocytic recycling, where they mediate receptor and lipid recycling back to the plasma membrane. Additionally, EHDs help regulate cytoskeletal reorganization and induce tubule formation. It was previously shown that EHD proteins bind directly to the C2 domains in myoferlin, a protein that regulates myoblast fusion. Loss of myoferlin impairs normal myoblast fusion leading to smaller muscles in vivo but the intracellular pathways perturbed by loss of myoferlin function are not well known. We now characterized muscle development in EHD1-null mice. EHD1-null myoblasts display defective receptor recycling and mislocalization of key muscle proteins, including caveolin-3 and Fer1L5, a related ferlin protein homologous to myoferlin. Additionally, EHD1-null myoblast fusion is reduced. We found that loss of EHD1 leads to smaller muscles and myofibers in vivo. In wildtype skeletal muscle EHD1 localizes to the transverse tubule (T-tubule), and loss of EHD1 results in overgrowth of T-tubules with excess vesicle accumulation in skeletal muscle. We provide evidence that tubule formation in myoblasts relies on a functional EHD1 ATPase domain. Moreover, we extended our studies to show EHD1 regulates BIN1 induced tubule formation. These data, taken together and with the known interaction between EHD and ferlin proteins, suggests that the EHD proteins coordinate growth and development likely through mediating vesicle recycling and the ability to reorganize the cytoskeleton.

Identification of caveolin-1 as a potential causative factor in the generation of trastuzumab resistance in breast cancer cells

The oncogenic tyrosine kinase receptor ErbB2 is a prognostic factor and target for breast cancer therapeutics. In contrast with the other ErbB receptors, ErbB2 is hardly internalized by ligand induced mechanisms, indicating a prevalent surface expression. Elevated levels of ErbB2 in tumor cells are associated with its defective endocytosis and down regulation. Here we show that caveolin-1 expression in breast cancer derived SKBR-3 cells (SKBR-3/Cav-1) facilitates ligand induced ErbB2 endocytosis using an artificial peptide ligand EC-eGFP. Similarly, stimulation with humanized anti ErbB2 antibody Trastuzumab (Herceptin) was found to be internalized and co-localized with caveolin-1 in SKBR-3/Cav-1 cells. Internalized EC-eGFP and Trastuzumab in SKBR-3/Cav-1 cells were then delivered via caveolae to the caveolin-1 containing early endosomes. Consequently, attenuated Fc receptor mediated ADCC functions were observed when exposed to Trastuzumab and EC-Fc (EC-1 peptide conjugated to Fc part of human IgG). On the other hand, this caveolae dependent endocytic synergy was not observed in parental SKBR-3 cells. Therefore, caveolin-1 expression in breast cancer cells could be a predictive factor to estimate how cancer cells are likely to respond to Trastuzumab treatment.

Overexpression of caveolin-1 is sufficient to phenocopy the behavior of a disease-associated mutant

Mutations and alterations in caveolin-1 expression levels have been linked to a number of human diseases. How misregulation of caveolin-1 contributes to disease is not fully understood, but has been proposed to involve the intracellular accumulation of mutant forms of the protein. To better understand the molecular basis for trafficking defects that trap caveolin-1 intracellularly, we compared the properties of a GFP-tagged version of caveolin-1 P132L, a mutant form of caveolin-1 previously linked to breast cancer, with wild-type caveolin-1. Unexpectedly, wild-type caveolin-1-GFP also accumulated intracellularly, leading us to examine the mechanisms underlying the abnormal localization of the wild type and mutant protein in more detail. We show that both the nature of the tag and cellular context impact the subcellular distribution of caveolin-1, demonstrate that even the wild-type form of caveolin-1 can function as a dominant negative under some conditions, and identify specific conformation changes associated with incorrectly targeted forms of the protein. In addition, we find intracellular caveolin-1 is phosphorylated on Tyr14, but phosphorylation is not required for mistrafficking of the protein. These findings identify novel properties of mistargeted forms of caveolin-1 and raise the possibility that common trafficking defects underlie diseases associated with overexpression and mutations in caveolin-1.

Introduction of caveolae structural proteins into the protozoan Toxoplasma results in the formation of heterologous caveolae but not caveolar endocytosis

Present on the plasma membrane of most metazoans, caveolae are specialized microdomains implicated in several endocytic and trafficking mechanisms. Caveolins and the more recently discovered cavins are the major protein components of caveolae. Previous studies reported that caveolar invaginations can be induced de novo on the surface of caveolae-negative mammalian cells upon heterologous expression of caveolin-1. However, it remains undocumented whether other components in the transfected cells participate in caveolae formation. To address this issue, we have exploited the protozoan Toxoplasma as a heterologous expression system to provide insights into the minimal requirements for caveogenesis and caveolar endocytosis. Upon expression of caveolin-1, Toxoplasma accumulates prototypical exocytic caveolae 'precursors' in the cytoplasm. Toxoplasma expressing caveolin-1 alone, or in conjunction with cavin-1, neither develops surface-located caveolae nor internalizes caveolar ligands. These data suggest that the formation of functional caveolae at the plasma membrane in Toxoplasma and, by inference in all non-mammalian cells, requires effectors other than caveolin-1 and cavin-1. Interestingly, Toxoplasma co-expressing caveolin-1 and cavin-1 displays an impressive spiraled network of membranes containing the two proteins, in the cytoplasm. This suggests a synergistic activity of caveolin-1 and cavin-1 in the morphogenesis and remodeling of membranes, as illustrated for Toxoplasma.

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.

Co-regulation of cell polarization and migration by caveolar proteins PTRF/Cavin-1 and caveolin-1

Caveolin-1 and caveolae are differentially polarized in migrating cells in various models, and caveolin-1 expression has been shown to quantitatively modulate cell migration. PTRF/cavin-1 is a cytoplasmic protein now established to be also necessary for caveola formation. Here we tested the effect of PTRF expression on cell migration. Using fluorescence imaging, quantitative proteomics, and cell migration assays we show that PTRF/cavin-1 modulates cellular polarization, and the subcellular localization of Rac1 and caveolin-1 in migrating cells as well as PKCα caveola recruitment. PTRF/cavin-1 quantitatively reduced cell migration, and induced mesenchymal epithelial reversion. Similar to caveolin-1, the polarization of PTRF/cavin-1 was dependent on the migration mode. By selectively manipulating PTRF/cavin-1 and caveolin-1 expression (and therefore caveola formation) in multiple cell systems, we unveil caveola-independent functions for both proteins in cell migration.

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.

Caveolin targeting to late endosome/lysosomal membranes is induced by perturbations of lysosomal pH and cholesterol content

Caveolin-1 traffics to late endosomal/lysosomal membranes in response to manipulations of the cholesterol content of cells, suggesting that caveolin functions in the egress of cholesterol from this organelle. Cavicles associate with the periphery of the lysosome as they do with caveosomes, but these are separate organelles.

Caveolin-1 is an integral membrane protein of plasma membrane caveolae. Here we report that caveolin-1 collects at the cytosolic surface of lysosomal membranes when cells are serum starved. This is due to an elevation of the intralysosomal pH, since ionophores and proton pump inhibitors that dissipate the lysosomal pH gradient also trapped caveolin-1 on late endosome/lysosomes. Accumulation is both saturable and reversible. At least a portion of the caveolin-1 goes to the plasma membrane upon reversal. Several studies suggest that caveolin-1 is involved in cholesterol transport within the cell. Strikingly, we find that blocking cholesterol export from lysosomes with progesterone or U18666A or treating cells with low concentrations of cyclodextrin also caused caveolin-1 to accumulate on late endosome/lysosomal membranes. Under these conditions, however, live-cell imaging shows cavicles actively docking with lysosomes, suggesting that these structures might be involved in delivering caveolin-1. Targeting of caveolin-1 to late endosome/lysosomes is not observed normally, and the degradation rate of caveolin-1 is not altered by any of these conditions, indicating that caveolin-1 accumulation is not a consequence of blocked degradation. We conclude that caveolin-1 normally traffics to and from the cytoplasmic surface of lysosomes during intracellular cholesterol trafficking.

Cell surface dynamics – how Rho GTPases orchestrate the interplay between the plasma membrane and the cortical cytoskeleton

Small GTPases are known to regulate hundreds of cell functions. In particular, Rho family GTPases are master regulators of the cytoskeleton. By regulating actin nucleation complexes, Rho GTPases control changes in cell shape, including the extension and/or retraction of surface protrusions and invaginations. Protrusion and invagination of the plasma membrane also involves the interaction between the plasma membrane and the cortical cytoskeleton. This interplay between membranes and the cytoskeleton can lead to an increase or decrease in the plasma membrane surface area and its tension as a result of the fusion (exocytosis) or internalization (endocytosis) of membranous compartments, respectively. For a long time, the cytoskeleton and plasma membrane dynamics were investigated separately. However, studies from many laboratories have now revealed that Rho GTPases, their modulation of the cytoskeleton, and membrane traffic are closely connected during the dynamic remodeling of the cell surface. Arf- and Rab-dependent exocytosis of specific vesicles contributes to the targeting of Rho GTPases and their regulatory factors to discrete sites of the plasma membrane. Rho GTPases regulate the tethering of exocytic vesicles and modulate their subsequent fusion. They also have crucial roles in the different forms of endocytosis, where they participate in the sorting of membrane domains as well as the sculpting and sealing of membrane flasks and cups. Here, we discuss how cell surface dynamics depend on the orchestration of the cytoskeleton and the plasma membrane by Rho GTPases.

The F-BAR domain protein PACSIN2 associates with Rac1 and regulates cell spreading and migration

The Rac1 GTPase controls cytoskeletal dynamics and is a key regulator of cell spreading and migration mediated by signaling through effector proteins, such as the PAK kinases and the Scar and WAVE proteins. We previously identified a series of regulatory proteins that associate with Rac1 through its hypervariable C-terminal domain, including the Rac1 activator β-Pix (also known as Rho guanine-nucleotide-exchange factor 7) and the membrane adapter caveolin-1. Here, we show that Rac1 associates, through its C-terminus, with the F-BAR domain protein PACSIN2, an inducer of membrane tubulation and a regulator of endocytosis. We show that Rac1 localizes with PACSIN2 at intracellular tubular structures and on early endosomes. Active Rac1 induces a loss of PACSIN2-positive tubular structures. By contrast, Rac1 inhibition results in an accumulation of PACSIN2-positive tubules. In addition, PACSIN2 appears to regulate Rac1 signaling; siRNA-mediated loss of PACSIN2 increases the levels of Rac1-GTP and promotes cell spreading and migration in a wound healing assay. Moreover, ectopic expression of PACSIN2 reduces Rac1-GTP levels in a fashion that is dependent on the PACSIN2-Rac1 interaction, on the membrane-tubulating capacity of PACSIN2 and on dynamin. These data identify the BAR-domain protein PACSIN2 as a Rac1 interactor that regulates Rac1-mediated cell spreading and migration.

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

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