Caveolae--from ultrastructure to molecular mechanisms

CAV3 alleviates diabetic cardiomyopathy via inhibiting NDUFA10-mediated mitochondrial dysfunction

BACKGROUND

The progression of diabetic cardiomyopathy (DCM) is noticeably influenced by mitochondrial dysfunction. Variants of caveolin 3 (CAV3) play important roles in cardiovascular diseases. However, the potential roles of CAV3 in mitochondrial function in DCM and the related mechanisms have not yet been elucidated.

METHODS

Cardiomyocytes were cultured under high-glucose and high-fat (HGHF) conditions in vitro, and db/db mice were employed as a diabetes model in vivo. To investigate the role of CAV3 in DCM and to elucidate the molecular mechanisms underlying its involvement in mitochondrial function, we conducted Liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis and functional experiments.

RESULTS

Our findings demonstrated significant downregulation of CAV3 in the cardiac tissue of db/db mice, which was found to be associated with cardiomyocyte apoptosis in DCM. Importantly, cardiac-specific overexpression of CAV3 effectively inhibited the progression of DCM, as it protected against cardiac dysfunction and cardiac remodeling associated by alleviating cardiomyocyte mitochondrial dysfunction. Furthermore, mass spectrometry analysis and immunoprecipitation assays indicated that CAV3 interacted with NDUFA10, a subunit of mitochondrial complex I. CAV3 overexpression reduced the degradation of lysosomal pathway in NDUFA10, restored the activity of mitochondrial complex I and improved mitochondrial function. Finally, our study demonstrated that CAV3 overexpression restored mitochondrial function and subsequently alleviated DCM partially through NDUFA10.

CONCLUSIONS

The current study provides evidence that CAV3 expression is significantly downregulated in DCM. Upregulation of CAV3 interacts with NDUFA10, inhibits the degradation of lysosomal pathway in NDUFA10, a subunit of mitochondrial complex I, restores the activity of mitochondrial complex I, ameliorates mitochondrial dysfunction, and thereby protects against DCM. These findings indicate that targeting CAV3 may be a promising approach for the treatment of DCM.

Molecular Mechanisms of Glaucoma Pathogenesis with Implications to Caveolin Adaptor Protein and Caveolin-Shp2 Axis

Glaucoma is a common retinal disorder characterized by progressive optic nerve damage, resulting in visual impairment and potential blindness. Elevated intraocular pressure (IOP) is a major risk factor, but some patients still experience disease progression despite IOP-lowering treatments. Genome-wide association studies have linked variations in the Caveolin1/2 (CAV-1/2) gene loci to glaucoma risk. Cav-1, a key protein in caveolae membrane invaginations, is involved in signaling pathways and its absence impairs retinal function. Recent research suggests that Cav-1 is implicated in modulating the BDNF/TrkB signaling pathway in retinal ganglion cells, which plays a critical role in retinal ganglion cell (RGC) health and protection against apoptosis. Understanding the interplay between these proteins could shed light on glaucoma pathogenesis and provide potential therapeutic targets.

The New General Biological Property of Stem-like Tumor Cells (Part II: Surface Molecules, Which Belongs to Distinctive Groups with Particular Functions, Form a Unique Pattern Characteristic of a Certain Type of Tumor Stem-like Cells)

An ability of poorly differentiated cells of different genesis, including tumor stem-like cells (TSCs), to internalize extracellular double-stranded DNA (dsDNA) fragments was revealed in our studies. Using the models of Krebs-2 murine ascites carcinoma and EBV-induced human B-cell lymphoma culture, we demonstrated that dsDNA internalization into the cell consists of several mechanistically distinct phases. The primary contact with cell membrane factors is determined by electrostatic interactions. Firm contacts with cell envelope proteins are then formed, followed by internalization into the cell of the complex formed between the factor and the dsDNA probe bound to it. The key binding sites were found to be the heparin-binding domains, which are constituents of various cell surface proteins of TSCs-either the C1q domain, the collagen-binding domain, or domains of positively charged amino acids. These results imply that the interaction between extracellular dsDNA fragments and the cell, as well as their internalization, took place with the involvement of glycocalyx components (proteoglycans/glycoproteins (PGs/GPs) and glycosylphosphatidylinositol-anchored proteins (GPI-APs)) and the system of scavenger receptors (SRs), which are characteristic of TSCs and form functional clusters of cell surface proteins in TSCs. The key provisions of the concept characterizing the principle of organization of the "group-specific" cell surface factors of TSCs of various geneses were formulated. These factors belong to three protein clusters: GPs/PGs, GIP-APs, and SRs. For TSCs of different tumors, these clusters were found to be represented by different members with homotypic functions corresponding to the general function of the cluster to which they belong.

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

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

Myostatin: Basic biology to clinical application

Myostatin is a member of the transforming growth factor (TGF)-β superfamily. It is expressed by animal and human skeletal muscle cells where it limits muscle growth and promotes protein breakdown. Its effects are influenced by complex mechanisms including transcriptional and epigenetic regulation and modulation by extracellular binding proteins. Due to its actions in promoting muscle atrophy and cachexia, myostatin has been investigated as a promising therapeutic target to counteract muscle mass loss in experimental models and patients affected by different muscle-wasting conditions. Moreover, growing evidence indicates that myostatin, beyond to regulate skeletal muscle growth, may have a role in many physiologic and pathologic processes, such as obesity, insulin resistance, cardiovascular and chronic kidney disease. In this chapter, we review myostatin biology, including intracellular and extracellular regulatory pathways, and the role of myostatin in modulating physiologic processes, such as muscle growth and aging. Moreover, we discuss the most relevant experimental and clinical evidence supporting the extra-muscle effects of myostatin. Finally, we consider the main strategies developed and tested to inhibit myostatin in clinical trials and discuss the limits and future perspectives of the research on myostatin.

Energy and Dynamics of Caveolae Trafficking

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

Caveolin1 Tyrosine-14 Phosphorylation: Role in Cellular Responsiveness to Mechanical Cues

The plasma membrane is a dynamic lipid bilayer that engages with the extracellular microenvironment and intracellular cytoskeleton. Caveolae are distinct plasma membrane invaginations lined by integral membrane proteins Caveolin1, 2, and 3. Caveolae formation and stability is further supported by additional proteins including Cavin1, EHD2, Pacsin2 and ROR1. The lipid composition of caveolar membranes, rich in cholesterol and phosphatidylserine, actively contributes to caveolae formation and function. Post-translational modifications of Cav1, including its phosphorylation of the tyrosine-14 residue (pY14Cav1) are vital to its function in and out of caveolae. Cells that experience significant mechanical stress are seen to have abundant caveolae. They play a vital role in regulating cellular signaling and endocytosis, which could further affect the abundance and distribution of caveolae at the PM, contributing to sensing and/or buffering mechanical stress. Changes in membrane tension in cells responding to multiple mechanical stimuli affects the organization and function of caveolae. These mechanical cues regulate pY14Cav1 levels and function in caveolae and focal adhesions. This review, along with looking at the mechanosensitive nature of caveolae, focuses on the role of pY14Cav1 in regulating cellular mechanotransduction.

Caveolin-1 Ablation Imparts Partial Protection Against Inner Retinal Injury in Experimental Glaucoma and Reduces Apoptotic Activation

Retinal ganglion cell degeneration is a characteristic feature of glaucoma, and accordingly, protection of these cells constitutes a major therapeutic objective in the disease. Here, we demonstrate the key influence of caveolin (Cav) in regulating the inner retinal homeostasis in two models of experimentally elevated intraocular pressure (IOP). Two groups of Cav-1^-/- and wild-type mice were used in the study. Animals were subjected to experimentally induced chronic and acutely elevated IOP and any changes in their retinal function were assessed by positive scotopic threshold response recordings. TUNEL and cleaved caspase-3 assays were performed to evaluate apoptotic changes in the retina while Brn3a immunostaining was used as a marker to assess and quantify ganglion cell layer (GCL) changes. H&E staining was carried out on retinal sections to evaluate histological differences in retinal laminar structure. Cav-1 ablation partially protected the inner retinal function in both chronic and acute models of elevated IOP. The protective effects of Cav-1 loss were also evident histologically by reduced loss of GCL density in both models. The phenotypic protection in Cav-1^-/- glaucoma mice paralleled with increased TrkB phosphorylation and reduced endoplasmic reticulum stress markers and apoptotic activation in the inner retinas. This study corroborated previous findings of enhanced Shp2 phosphorylation in a chronic glaucoma model and established a novel role of Cav-1 in mediating activation of this phosphatase in the inner retina in vivo. Collectively, these findings highlight the critical involvement of Cav-1 regulatory mechanisms in ganglion cells in response to increased IOP, implicating Cav-1 as a potential therapeutic target in glaucoma.

Caveolin-3 deficiency associated with the dystrophy P104L mutation impairs skeletal muscle mitochondrial form and function

BACKGROUND

Caveolin-3 (Cav3) is the principal structural component of caveolae in skeletal muscle. Dominant pathogenic mutations in the Cav3 gene, such as the Limb Girdle Muscular Dystrophy-1C (LGMD1C) P104L mutation, result in substantial loss of Cav3 and myopathic changes characterized by muscle weakness and wasting. We hypothesize such myopathy may also be associated with disturbances in mitochondrial biology. Herein, we report studies assessing the effects of Cav3 deficiency on mitochondrial form and function in skeletal muscle cells.

METHODS

L6 myoblasts were stably transfected with Cav3^P104L or expression of native Cav3 repressed by shRNA or CRISPR/Cas9 genome editing prior to performing fixed/live cell imaging of mitochondrial morphology, subcellular fractionation and immunoblotting, or analysis of real time mitochondrial respiration. Skeletal muscle from wild-type and Cav3^-/- mice was processed for analysis of mitochondrial proteins by immunoblotting.

RESULTS

Caveolin-3 was detected in mitochondrial-enriched membranes isolated from mouse gastrocnemius muscle and L6 myoblasts. Expression of Cav3^P104L in L6 myoblasts led to its targeting to the Golgi and loss of native Cav3 (>95%), including that associated with mitochondrial membranes. Cav3^P104L reduced mitochondrial mass and induced fragmentation of the mitochondrial network that was associated with significant loss of proteins involved in mitochondrial biogenesis, respiration, morphology, and redox function [i.e. PGC1α, succinate dehyrdogenase (SDHA), ANT1, MFN2, OPA1, and MnSOD). Furthermore, Cav3^P104L myoblasts exhibited increased mitochondrial cholesterol and loss of cardiolipin. Consistent with these changes, Cav3^P104L expression reduced mitochondrial respiratory capacity and increased myocellular superoxide production. These morphological, biochemical, and functional mitochondrial changes were phenocopied in myoblasts in which Cav3 had been silenced/knocked-out using shRNA or CRISPR. Reduced mitochondrial mass, PGC1α, SDHA, ANT1, and MnSOD were also demonstrable in Cav3^-/- mouse gastrocnemius. Strikingly, Cav3 re-expression in Cav3KO myoblasts restored its mitochondrial association and facilitated reformation of a tubular mitochondrial network. Significantly, re-expression also mitigated changes in mitochondrial superoxide, cholesterol, and cardiolipin content and recovered cellular respiratory capacity.

CONCLUSIONS

Our results identify Cav3 as an important regulator of mitochondrial homeostasis and reveal that Cav3 deficiency in muscle cells associated with the Cav3^P104L mutation invokes major disturbances in mitochondrial respiration and energy status that may contribute to the pathology of LGMD1C.

From Dynamics to Membrane Organization: Experimental Breakthroughs Occasion a "Modeling Manifesto"

New experimental techniques, especially in the context of observing molecular dynamics, reveal the plasma membrane to be heterogeneous and "scale rich," from nanometers to microns and from microseconds to seconds. This is critical information, which shows that scale-dependent transport governs the molecular encounters that underlie cellular signaling. The data are rich and reaffirm the importance of the cortical cytoskeleton, protein aggregates, and lipidomic complexity on the statistics of molecular encounters. Moreover, the data demand simulation approaches with a particular set of features, hence the "manifesto." Together with the experimental data, simulations that satisfy these requirements hold the promise of a deeper understanding of membrane spatiotemporal organization. Several experimental breakthroughs in measuring molecular membrane dynamics are reviewed, the constraints that they place on simulations are discussed, and the status of simulation approaches that aim to meet them are detailed.

African horse sickness virus infects BSR cells through macropinocytosis

Cellular pathways involved in cell entry by African horse sickness virus (AHSV), a member of the Orbivirus genus within the Reoviridae family, have not yet been determined. Here, we show that acidic pH is required for productive infection of BSR cells by AHSV-4, suggesting that the virus is likely internalized by an endocytic pathway. We subsequently analyzed the major endocytic routes using specific inhibitors and determined the consequences for AHSV-4 entry into BSR cells. The results indicated that virus entry is dynamin dependent, but clathrin- and lipid raft/caveolae-mediated endocytic pathways were not used by AHSV-4 to enter and infect BSR cells. Instead, binding of AHSV-4 to BSR cells stimulated uptake of a macropinocytosis-specific cargo and inhibition of Na(+)/H(+) exchangers, actin polymerization and cellular GTPases and kinases involved in macropinocytosis significantly inhibited AHSV-4 infection. Altogether, the data suggest that AHSV-4 infects BSR cells by utilizing macropinocytosis as the primary entry pathway.

Implications of thermogenic adipose tissues for metabolic health

Excess and ectopic fat accumulation in obesity is a major risk factor for developing hyperlipidemia, type 2 diabetes and cardiovascular disease. The activation of brown and/or beige adipocytes is a promising target for the treatment of metabolic disorders as the combustion of excess energy by these thermogenic adipocytes may help losing weight and improving plasma parameters including triglyceride, cholesterol and glucose levels. The regulation of heat production by thermogenic adipose tissues is based on a complex crosstalk between the autonomous nervous system, intracellular and secreted factors. This multifaceted alignment regulates thermogenic demands to environmental circumstances in dependence on available energy resources. This review summarizes the current knowledge how thermogenic tissues can be targeted to combat the burden of diseases with a special focus on lipid metabolism and diseases related to lipoprotein metabolism.

Multidrug resistance protein 1 localization in lipid raft domains and prostasomes in prostate cancer cell lines

BACKGROUND

One of the problems in prostate cancer (CaP) treatment is the appearance of the multidrug resistance phenotype, in which ATP-binding cassette transporters such as multidrug resistance protein 1 (MRP1) play a role. Different localizations of the transporter have been reported, some of them related to the chemoresistant phenotype.

AIM

This study aimed to compare the localization of MRP1 in three prostate cell lines (normal, androgen-sensitive, and androgen-independent) in order to understand its possible role in CaP chemoresistance.

METHODS

MRP1 and caveolae protein markers were detected using confocal microscopy, performing colocalization techniques. Lipid raft isolation made it possible to detect these proteins by Western blot analysis. Caveolae and prostasomes were identified by electron microscopy.

RESULTS

We show that MRP1 is found in lipid raft fractions of tumor cells and that the number of caveolae increases with malignancy acquisition. MRP1 is found not only in the plasma membrane associated with lipid rafts but also in cytoplasmic accumulations colocalizing with the prostasome markers Caveolin-1 and CD59, suggesting that in CaP cells, MRP1 is localized in prostasomes.

CONCLUSION

We hypothesize that the presence of MRP1 in prostasomes could serve as a reservoir of MRP1; thus, taking advantage of the release of their content, MRP1 could be translocated to the plasma membrane contributing to the chemoresistant phenotype. The presence of MRP1 in prostasomes could serve as a predictor of malignancy in CaP.

Lipid raft disarrangement as a result of neuropathological progresses: a novel strategy for early diagnosis?

Lipid rafts are the preferential site of numerous membrane signaling proteins which are involved in neuronal functioning and survival. These proteins are organized in multiprotein complexes, or signalosomes, in close contact with lipid classes particularly represented in lipid rafts (i.e. cholesterol, sphingolipids and saturated fatty acids), which may contribute to physiological responses leading to neuroprotection. Increasing evidence indicates that alteration of lipid composition in raft structures as a consequence of neuropathologies, such as Alzheimer's disease (AD) and Parkinson's disease (PD), causes a dramatic increase in lipid raft order. These phenomena may correlate with perturbation of signalosome activities, likely contributing to neurodegenerative progression. Interestingly, significant disruption of stable raft microenvironments has been already observed in the first stages of either AD or PD, suggesting that these alterations may represent early events in the neuropathological development. In this regard, the search for biochemical markers, such as specific metabolic products altered in the brain at the first steps of the disease, presently represents an important challenge for early diagnostic strategies. Alterations of these biomarkers may be reflected in either plasma or cerebrospinal fluid, thus representing a potential strategy to predict an accurate diagnosis. We propose that pathologically-linked lipid raft markers may be interesting candidates to be explored at this level, although it has not been studied so far to what extent alteration of different signalosome components may be reflected in peripheral fluids. In this mini-review, we will discuss on relevant aspects of lipid rafts that contribute to the modulation of neuropathological events related to AD and PD. An interesting hypothesis is that anomalies on raft biomarkers measured at peripheral fluids might mirror the lipid raft pathology observed in early stages of AD and PD.

Endothelial membrane reorganization during leukocyte extravasation

Leukocyte trafficking from the bloodstream to inflamed tissues across the endothelial barrier is an essential response in innate immunity. Leukocyte adhesion, locomotion, and diapedesis induce signaling in endothelial cells and this is accompanied by a profound reorganization of the endothelial cell surfaces that is only starting to be unveiled. Here we review the current knowledge on the leukocyte-mediated alterations of endothelial membrane dynamics and their role in promoting leukocyte extravasation. The formation of protein- and lipid-mediated cell adhesion nanodomains at the endothelial apical surface, the extension of micrometric apical membrane docking structures, which are derived from microvilli and embrace adhered leukocytes, as well as the vesicle-trafficking pathways that are required for efficient leukocyte diapedesis, are discussed. The coordination between these different endothelial membrane-remodeling events probably provides the road map for transmigrating leukocytes to find exit points in the vessel wall, in a context of severe mechanical and inflammatory stress. A better understanding of how vascular endothelial cells respond to immune cell adhesion should enable new therapeutic strategies to be developed that can abrogate uncontrolled leukocyte extravasation in inflammatory diseases.

The holy grail of metabolic disease: brown adipose tissue

PURPOSE OF REVIEW

The finding that brown adipose tissue (BAT) is present in adults brought BAT physiology into the focus of many researchers interested in energy metabolism. Here, we review recent insight into how BAT develops, functions and might help to treat metabolic disorders in humans.

RECENT FINDINGS

BAT is under control of the nervous system, and several pathways have been identified that allow direct manipulation of BAT biology. In addition, some brown adipocytes arise from a distinct subset of white adipocyte precursors and studies were performed that characterize the development of these 'brite' adipocytes. Importantly, progress has been made in understanding how BAT takes up and dissipates nutrients that in metabolic disorders are present in excess. Finally, as it seems that BAT activity declines with age and obesity, we review findings that might shed light on how humans could sustain or increase BAT activity, thus preventing or treating obesity, hyperlipidemia and type 2 diabetes.

SUMMARY

BAT is a powerful organ that controls the development of metabolic disease. These powers are boosted by mechanisms that turn white into brown fat and enhance lipid flux into BAT. However, in humans, it remains unclear what was the first: metabolic disease or decreased BAT activity.

Caveolins in rhabdomyosarcoma

Caveolins are scaffolding proteins that play a pivotal role in numerous processes, including caveolae biogenesis, vesicular transport, cholesterol homeostasis and regulation of signal transduction. There are three different isoforms (Cav-1, -2 and -3) that form homo- and hetero-aggregates at the plasma membrane and modulate the activity of a number of intracellular binding proteins. Cav-1 and Cav-3, in particular, are respectively expressed in the reserve elements (e.g. satellite cells) and in mature myofibres of skeletal muscle and their expression interplay characterizes the switch from muscle precursors to differentiated elements. Recent findings have shown that caveolins are also expressed in rhabdomyosarcoma, a group of heterogeneous childhood soft-tissue sarcomas in which the cancer cells seem to derive from progenitors that resemble myogenic cells. In this review, we will focus on the role of caveolins in rhabdomyosarcomas and on their potential use as markers of the degree of differentiation in these paediatric tumours. Given that the function of Cav-1 as tumour conditional gene in cancer has been well-established, we will also discuss the relationship between Cav-1 and the progression of rhabdomyosarcoma.

Signalosomes in the brain: relevance in the development of certain neuropathologies such as Alzheimer's disease

Emerging data suggest that compartmentalization of signaling molecules into particular membrane compartments, or lipid rafts, may be at the basis of numerous activities related to neuronal preservation against different pathologies. These signaling platforms (signalosomes) are formed by complex lipid and protein that may interact to develop a plethora of different physiological responses upon activation by different extracellular stimuli, thereby contributing to neuroprotection. One of the first studied signalosomes involved in neuroprotection against Alzheimer's disease (AD) is constituted by estrogen receptor (ER), in association with scaffolding caveolin-1 and a voltage-dependent anion channel (VDAC). In this complex, ER plays a neuroprotective role partially through the modulation of VDAC activation, a porin involved in amyloid-beta-induced toxicity. Interestingly, ER and VDAC interactions appear to be altered in lipid rafts of AD brains, a phenomenon that may contribute to neuronal impairment. Alterations in lipid components of these subdomains may contribute to destabilization of this macrocomplex. These recent advances in the relevance of signaling platforms related to brain preservation, in particular against AD, are discussed in this work.

Molecular profiling of the human nasal epithelium: A proteomics approach

A comprehensive proteomic profiling of nasal epithelium (NE) is described. This study relies on simple subcellular fractionation used to obtain soluble- and membrane-enriched fractions followed by 2-dimensional liquid chromatography (2D-LC) separation and tandem mass spectrometry (MS/MS). The cells were collected using a brushing technique applied on NE of clinically evaluated volunteers. Subsequently, the soluble- and the membrane-protein enriched fractions were prepared and analyzed in parallel using 2D-LC-MS/MS. In a set of 1482 identified proteins, 947 (63.9%) proteins were found to be associated to membrane fraction. Grand average hydropathy value index (GRAVY) analysis, the transmembrane protein mapping and annotations of primary location deposited in the Human Protein Reference Database (HPRD) confirmed an enrichment of hydrophobic proteins on this dataset. Ingenuity Pathway Analysis (IPA) of soluble fraction revealed an enrichment of molecular and cellular functions associated with cell death, protein folding and drug metabolism while in membrane fraction showed an enrichment of functions associated with molecular transport, protein trafficking and cell-to-cell signaling and interaction. The IPA showed similar enrichment of functions associated with cellular growth and proliferation in both soluble and membrane subproteomes. This finding was in agreement with protein content analysis using exponentially modified protein abundance index (emPAI). A comparison of our data with previously published studies focusing on respiratory tract epithelium revealed similarities related to identification of proteins associated with physical barrier function and immunological defence. In summary, we extended the NE molecular profile by identifying and characterizing proteins associated to pivotal functions of a respiratory epithelium, including the control of fluid volume and ionic composition at the airways' surface, physical barrier maintenance, detoxification and immunological defence. The extent of similarities supports the applicability of a less invasive analysis of NE to assess prognosis and treatment response of lung diseases such as asthma, cystic fibrosis and chronic obstructive pulmonary disease.

@neurIST complex information processing toolchain for the integrated management of cerebral aneurysms

Cerebral aneurysms are a multi-factorial disease with severe consequences. A core part of the European project @neurIST was the physical characterization of aneurysms to find candidate risk factors associated with aneurysm rupture. The project investigated measures based on morphological, haemodynamic and aneurysm wall structure analyses for more than 300 cases of ruptured and unruptured aneurysms, extracting descriptors suitable for statistical studies. This paper deals with the unique challenges associated with this task, and the implemented solutions. The consistency of results required by the subsequent statistical analyses, given the heterogeneous image data sources and multiple human operators, was met by a highly automated toolchain combined with training. A testimonial of the successful automation is the positive evaluation of the toolchain by over 260 clinicians during various hands-on workshops. The specification of the analyses required thorough investigations of modelling and processing choices, discussed in a detailed analysis protocol. Finally, an abstract data model governing the management of the simulation-related data provides a framework for data provenance and supports future use of data and toolchain. This is achieved by enabling the easy modification of the modelling approaches and solution details through abstract problem descriptions, removing the need of repetition of manual processing work.

A role for caveolin 1 in assembly and budding of the paramyxovirus parainfluenza virus 5

Caveolin 1 (Cav-1) is an integral membrane protein that forms the coat structure of plasma membrane caveolae and regulates caveola-dependent functions. Caveolae are enriched in cholesterol and sphingolipids and are related to lipid rafts. Many studies implicate rafts as sites of assembly and budding of enveloped virus. We show that Cav-1 colocalizes with the paramyxovirus parainfluenza virus 5 (PIV-5) nucleocapsid (NP), matrix (M), and hemagglutinin-neuraminidase (HN) proteins. Moreover, electron microscopy shows that Cav-1 is clustered at sites of viral budding. HN, M, and F(1)/F(2) are associated with detergent-resistant membranes, and these proteins float on sucrose gradients with Cav-1-rich fractions. A complex containing Cav-1 with M, NP, and HN from virus-infected cells and a complex containing Cav-1 and M from M-transfected cells were found on coimmunoprecipitation. A role of Cav-1 in the PIV-5 life cycle was investigated by utilizing MCF-7 human breast cancer cells that stably express Cav-1 (MCF-7/Cav-1). PIV-5 entry into MCF-7 and MCF-7/Cav-1 was found to be Cav-1 independent. However, the interaction between HN and M proteins was dramatically reduced in the Cav-1 null MCF-7 cells, and PIV-5 grown in MCF-7 cells had a reduced infectivity. Similarly, when PIV-5 was grown in MDCK cells that stably expressed dominant negative Cav-1 (MDCK/P132LCav-1), the virus showed a reduced infectivity. Virions lacking Cav-1 were defective and contained high levels of host cellular proteins and reduced levels of HN and M. These data suggest that Cav-1 affects assembly and/or budding, and this is supported by the finding that Cav-1 is incorporated into mature viral particles.

TRIM72 negatively regulates myogenesis via targeting insulin receptor substrate-1

Lipid rafts have been known to be platforms to initiate cellular signal transduction of insulin-like growth factor (IGF) inducing skeletal muscle differentiation and hypertrophy. Here, tripartite motif 72 (TRIM72), with a really interesting new gene (RING)-finger domain, a B-box, two coiled-coil domains, and a SPRY (SPla and RYanodine receptor) domain, was revealed to be predominantly expressed in the sarcolemma lipid rafts of skeletal and cardiac muscles. Adenoviral TRIM72 overexpression prevented but RNAi-mediated TRIM72 silencing enhanced C2C12 myogenesis by modulating the IGF-induced insulin receptor substrate-1 (IRS-1) activation through the molecular association of TRIM72 with IRS-1. Furthermore, myogenic activity was highly enhanced with increased IGF-induced Akt activation in the satellite cells of TRIM72(-/-) mice, compared to those of TRIM72+/+ mice. Because TRIM72 promoter analysis shows that two proximal E-boxes in TRIM72 promoter were essential for MyoD- and Akt-dependent TRIM72 transcription, we can conclude that TRIM72 is a novel antagonist of IRS-1, and is essential as a negative regulator of IGF-induced muscle differentiation.

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

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

Lipid raft-mediated regulation of G-protein coupled receptor signaling by ligands which influence receptor dimerization: a computational study

G-protein coupled receptors (GPCRs) are the largest family of cell surface receptors; they activate heterotrimeric G-proteins in response to ligand stimulation. Although many GPCRs have been shown to form homo- and/or heterodimers on the cell membrane, the purpose of this dimerization is not known. Recent research has shown that receptor dimerization may have a role in organization of receptors on the cell surface. In addition, microdomains on the cell membrane termed lipid rafts have been shown to play a role in GPCR localization. Using a combination of stochastic (Monte Carlo) and deterministic modeling, we propose a novel mechanism for lipid raft partitioning of GPCRs based on reversible dimerization of receptors and then demonstrate that such localization can affect GPCR signaling. Modeling results are consistent with a variety of experimental data indicating that lipid rafts have a role in amplification or attenuation of G-protein signaling. Thus our work suggests a new mechanism by which dimerization-inducing or inhibiting characteristics of ligands can influence GPCR signaling by controlling receptor organization on the cell membrane.

"Clickable" polymer-caged nanobins as a modular drug delivery platform

Modularly clickable polymer-caged nanobins (PCNs) were prepared from liposome templates using a drop-in cholesterol-modified poly(acrylic acid) reagent followed by cross-linking with alkyne-functionalized diamine linker that allows for the conjugation of azide-modified targeting ligands via click ligation. These PCNs possess pH-responsive characteristics that can be used to trigger the release of encapsulated doxorubicin (DXR) payload inside the liposomal core under mild acidic conditions. After click-conjugation with azide-modified folate as an active targeting ligand, the resulting folate-conjugated, DXR-loaded PCNs (f-PCN(DXR)) demonstrated enhanced potency to folate receptor (FR)-positive tumor cells such as KB and OvCa432 over FR-negative MCF7 cells. f-PCN(DXR) can readily discriminate FR-positive tumor cells as a function of the level of cellular FR-expression, showing different degrees of potentiation in each cell. With both targeting functionalities and pH-sensitive drug-releasing triggers, f-PCN(DXR) was fifty-times more potent than the untargeted agent toward cancer cells that overexpress the folate target receptors.

Vesicle formation and endocytosis: function, machinery, mechanisms, and modeling

Vesicle formation provides a means of cellular entry for extracellular substances and for recycling of membrane constituents. Mechanisms governing the two primary endocytic pathways (i.e., caveolae- and clathrin-mediated endocytosis, as well as newly emerging vesicular pathways) have become the focus of intense investigation to improve our understanding of nutrient, hormone, and drug delivery, as well as opportunistic invasion of pathogens. In this review of endocytosis, we broadly discuss the structural and signaling proteins that compose the molecular machinery governing endocytic vesicle formation (budding, invagination, and fission from the membrane), with some regard for the specificity observed in certain cell types and species. Important biochemical functions of endocytosis and diseases caused by their disruption also are discussed, along with the structures of key components of endocytic pathways and their known mechanistic contributions. The mechanisms by which principal components of the endocytic machinery are recruited to the plasma membrane, where they interact to induce vesicle formation, are discussed, together with computational approaches used to simulate simplified versions of endocytosis with the hope of clarifying aspects of vesicle formation that may be difficult to determine experimentally. Finally, we pose several unanswered questions intended to stimulate further research interest in the cell biology and modeling of endocytosis.

Inhibition of myostatin with emphasis on follistatin as a therapy for muscle disease

In most cases, pharmacologic strategies to treat genetic muscle disorders and certain acquired disorders, such as sporadic inclusion body myositis, have produced modest clinical benefits. In these conditions, inhibition of the myostatin pathway represents an alternative strategy to improve functional outcomes. Preclinical data that support this approach clearly demonstrate the potential for blocking the myostatin pathway. Follistatin has emerged as a powerful antagonist of myostatin that can increase muscle mass and strength. Follistatin was first isolated from the ovary and is known to suppress follicle-stimulating hormone. This raises concerns for potential adverse effects on the hypothalamic-pituitary-gonadal axis and possible reproductive capabilities. In this review we demonstrate a strategy to bypass off-target effects using an alternatively spliced cDNA of follistatin (FS344) delivered by adeno-associated virus (AAV) to muscle. The transgene product is a peptide of 315 amino acids that is secreted from the muscle and circulates in the serum, thus avoiding cell-surface binding sites. Using this approach our translational studies show increased muscle size and strength in species ranging from mice to monkeys. Adverse effects are avoided, and no organ system pathology or change in reproductive capabilities has been seen. These findings provide the impetus to move toward gene therapy clinical trials with delivery of AAV-FS344 to increase size and function of muscle in patients with neuromuscular disease.

Caveolin-1 and Rac regulate endothelial capillary-like tubular formation and fenestral contraction in sinusoidal endothelial cells

BACKGROUND/AIMS

Rho guanidine triphosphatases (GTPases) are major regulators of cell migration. We investigated the cytoskeleton and Rho GTPases during cell migration and morphogenesis processes in isolated rat liver sinusoidal endothelial cells (LSECs) cultured on Matrigel while stimulated by the vascular endothelial growth factor (VEGF).

METHODS

To obtain primary monolayers, LSECs were cultured on Matrigel for 5-17 h with or without VEGF. Sinusoidal endothelial fenestrae (SEF) morphology was observed using scanning electron microscopy and transmission electron microscopy. RhoA, Rac1 and phosphorylated myosin light-chain kinase, Rho-binding domain of Rhotekin and the p21-binding domain of p21-activated protein kinase were analysed using Western blotting.

RESULTS

The LSECs showed cellular protrusions and or cords of aligned cells resembling primitive capillary-like structures, with SEF contraction. Time course analyses of Rac1 activation matched specific morphological changes. Rac1 activity increased progressively to 17 h in cells cultured without VEGF, but markedly increased at 7 h in the presence of VEGF. RhoA activity was slightly elevated at 5 h. The levels of endogenous caveolin-1 (CAV-1) expression increased in a time-dependent manner, reaching a peak at 7 h. CAV-1 expression occurred immediately before the formation of the capillary-like tube. Moreover, treatment with VEGF regulated CAV-1 expression in LSECs.

CONCLUSIONS

Spatial activation of Rac1 is involved in the formation of a capillary-like tubular network accompanying SEF contraction in LSECs, implying that endothelial migration and adhesion are necessary for LSECs tubular formation in the liver. CAV-1 might play an important positive role in the regulation of LSEC tubular formation.

Myocardial interstitial Cajal-like cells (ICLC) in caveolin-1 KO mice

Abstract We compared, by transmission electron microscopy (TEM), the ultrastructure of interstitial Cajal-like cells (ICLC) in normal mammalian myocardium versus caveolin-1 null mice. TEM showed that myocardial ICLCs of caveolin-1-deficient mice retain their main ultrastructural characteristics, for example, location among cardiomyocytes, close vicinity to nerves and/or blood capillaries, specialized cell-to-cell junctions, presence of 2-3 typical processes, which are very long (several tens of micrometres), but are very thin (0.1-0.2 microm) and moniliform. However, the most striking modification of myocardial ICLC in caveolin-1 KO mice was the absence of caveolae. Beyond this main observation, three other findings could be reported: (1) the absence of caveolae in capillary endothelium, (2) persistence of (some) caveolae at the level of cardiomyocte sarcolemma or vascular smooth muscle cell sarcolemma and (3) (un)expected ultrastructural modifications such as increased thickness of capillary basement membrane and increased autophagy of several cardiomyocytes.

The Na-K-ATPase and calcium-signaling microdomains

The Na-K-ATPase is an energy-transducing ion pump that converts the free energy of ATP into transmembrane ion gradients. It also serves as a functional receptor for cardiotonic steroids such as ouabain and digoxin. Binding of ouabain to the Na-K-ATPase can activate calcium signaling in a cell-specific manner. The exquisite calcium modulation via the Na-K-ATPase is achieved by the ability of the pump to integrate signals from numerous protein and non-protein molecules, including ion transporters, channels, protein kinases/phosphatases, as well as cellular Na+. This review focuses on the unique properties of the Na-K-ATPase and its role in the formation of different calcium-signaling microdomains.

Inhibition of cholesterol biosynthesis impairs insulin secretion and voltage-gated calcium channel function in pancreatic beta-cells

Insulin secretion from pancreatic beta-cells is mediated by the opening of voltage-gated Ca2+ channels (CaV) and exocytosis of insulin dense core vesicles facilitated by the secretory soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein machinery. We previously observed that beta-cell exocytosis is sensitive to the acute removal of membrane cholesterol. However, less is known about the chronic changes in endogenous cholesterol and its biosynthesis in regulating beta-cell stimulus-secretion coupling. We examined the effects of inhibiting endogenous beta-cell cholesterol biosynthesis by using the squalene epoxidase inhibitor, NB598. The expression of squalene epoxidase in primary and clonal beta-cells was confirmed by RT-PCR. Cholesterol reduction of 36-52% was observed in MIN6 cells, mouse and human pancreatic islets after a 48-h incubation with 10 mum NB598. A similar reduction in cholesterol was observed in the subcellular compartments of MIN6 cells. We found NB598 significantly inhibited both basal and glucose-stimulated insulin secretion from mouse pancreatic islets. CaV channels were markedly inhibited by NB598. Rapid photolytic release of intracellular caged Ca2+ and simultaneous measurements of the changes in membrane capacitance revealed that NB598 also inhibited exocytosis independently from CaV channels. These effects were reversed by cholesterol repletion. Our results indicate that endogenous cholesterol in pancreatic beta-cells plays a critical role in regulating insulin secretion. Moreover, chronic inhibition of cholesterol biosynthesis regulates the functional activity of CaV channels and insulin secretory granule mobilization and membrane fusion. Dysregulation of cellular cholesterol may cause impairment of beta-cell function, a possible pathogenesis leading to the development of type 2 diabetes.

Control of cardiac rate by "funny" channels in health and disease

Activation of the "funny" (pacemaker, I f) current during the diastolic depolarization phase of an action potential is the main mechanism underlying spontaneous, rhythmic activity of cardiac pacemaker cells. In the past three decades, a wealth of evidence elucidating the function of the funny current in the generation and modulation of cardiac pacemaker activity has been gathered. The slope of early diastolic depolarization, and thus the heart rate, is controlled precisely by the degree of I f activation during diastole. I f is also accurately and rapidly modulated by changes of the cytosolic concentration of the second messenger cAMP, operated by the autonomous nervous system through beta-adrenergic, mainly beta2, and in the opposite way by muscarinic receptor, stimulation. Recently, novel in vivo data, both in animal models and humans, have been collected that confirm the key role of I f in pacemaking. In particular, an inheritable point mutation in the cyclic nucleotide-binding domain of human HCN4, the main hyperpolarization-activated cyclic nucleotide (HCN) isoform contributing to native funny channels of the sinoatrial node, was shown to be associated with sinus bradycardia in a large family. Because of their properties, funny channels have long been a major target of classical pharmacological research and are now target of innovative gene/cell-based therapeutic approaches aimed to exploit their function in cardiac rate control.

Membrane microdomains and metabolic pathways that define anandamide and 2-arachidonyl glycerol biosynthesis and breakdown

Anandamide (AEA) and 2-arachidonyl glycerol (2-AG), endogenous ligands for the CB1 and CB2 cannabinoid receptors, are referred to as endocannabinoids because they mimic the actions of delta9-tetrahydrocannabinol (Delta9-THC), a plant-derived cannabinoid. The processes by which AEA and 2-AG are biosynthesized, released, taken up by cells and hydrolyzed have been of much interest as potential therapeutic targets. In this review we will discuss the progress that has been made to characterize the primary pathways for AEA and 2-AG formation and breakdown as well as the role that specialized membrane microdomains known as lipid rafts play in these processes. Furthermore we will review the recent advances made to track and detect AEA in biological matrices.

Filamin A is a novel caveolin-1-dependent target in IGF-I-stimulated cancer cell migration

Caveolin-1 is an essential structural constituent of caveolae which is involved in regulation of mitogenic signaling and oncogenesis. Caveolin-1 has been implicated in cell migration but its exact role and mechanism of action in this process remained obscure. We have previously reported that expression of caveolin-1 in stably transfected MCF-7 human breast cancer (MCF-7/Cav1) cells up-regulates phosphorylation of a putative Akt substrate protein, designated pp340 [D. Ravid, S. Maor, H. Werner, M. Liscovitch, Caveolin-1 inhibits cell detachment-induced p53 activation and anoikis by upregulation of insulin-like growth factor-I receptors and signaling, Oncogene 24 (2005) 1338-1347.]. We now show, using differential detergent extraction, SDS-PAGE and mass spectrometry, that the major protein in the pp340 band is the actin filament cross-linking protein filamin A. The identity of pp340 as filamin A was confirmed by immunoprecipitation of pp340 with specific filamin A antibodies. RT-PCR, flow cytometry and Western blot analyses show that filamin A mRNA and protein levels are respectively 3.5- and 2.5-fold higher in MCF-7/Cav1 cells than in MCF-7 cells. Basal filamin A phosphorylation on Ser-2152, normalized to total filamin A levels, is 7.8-fold higher in MCF-7/Cav1 than in MCF-7 cells. Insulin-like growth factor-I (IGF-I) stimulates phosphorylation of filamin A on Ser-2152 in MCF-7 cells and further enhances Ser-2152 phosphorylation over its already high basal level in MCF-7/Cav1 cells. The effect of IGF-I is inhibited by the PI3K inhibitor wortmannin, indicating that IGF-I-stimulated phosphorylation of filamin A occurs via the PI3K/Akt pathway. Co-immunoprecipitation experiments have confirmed a previous report showing that filamin A and caveolin-1 co-exist in a complex and have revealed the presence of active phospho-Akt in this complex. Ser-2152 phosphorylation of filamin A has been implicated in cancer cell migration. Accordingly, caveolin-1 expression dramatically enhances IGF-I-dependent MCF-7 cell migration. These data indicate that caveolin-1 specifies filamin A as a novel target for Akt-mediated filamin A Ser-2152 phosphorylation thus mediating the effects of caveolin-1 on IGF-I-induced cancer cell migration.

Using fluorescent sphingolipid analogs to study intracellular lipid trafficking

Sphingolipids (SLs), including glycosphingolipids, are found on the plasma membrane where they play important roles in a wide variety of cell functions, including cell-cell communication, cell growth and differentiation, host-pathogen interactions, and cell-signaling events. This unit illustrates the use of fluorescent SL analogs to identify the mechanisms underlying SL endocytosis and subsequent intracellular trafficking. Techniques used to study SL domain formation at the plasma membrane, endocytic mechanisms and intracellular transport steps are highlighted. The use of biochemical treatments and dominant-negative protein expression to block specific steps in lipid trafficking are also discussed.

Lipid microdomains and k(+) channel compartmentation: detergent and non-detergent-based methods for the isolation and characterisation of cholesterol-enriched lipid rafts

The traditional view of the plasma membrane as a uniform cellular envelope formed from a homogenous mixture of lipids has been refined in recent years to reflect the heterogeneity of its composite lipids. The membrane can consist of upwards of 500 different types of lipids, which exhibit complex and dynamic interactions. Cholesterol and sphingolipids, in particular, partition away from the bulk of bilayer to form distinct microdomains or 'rafts'. Although controversial, lipid rafts have attracted considerable attention over recent years, firstly because of their apparent ability to selectively aggregate interacting signalling molecules, including ion channels and receptors, and secondly because of the implication that they may be involved in the spatial organisation of signalling pathways. Here we describe methods to isolate lipid rafts, assess their purity and determine the distribution of potassium channel proteins between raft and non-raft fractions.

Vesicular trafficking: molecular tools and targets

Intracellular trafficking of membrane-coated vesicles represents a fundamental process that controls the architecture of different intracellular compartments and communication between the cell and its environment. Major trafficking pathways consist of an inward flux of endocytic vesicles from the plasma membrane and an outward flux of exocytic vesicles to the plasma membrane. This overview describes a number of molecular biology tools commonly used to analyze endocytic and exocytic pathways. The overall emphasis is on major proteins responsible for vesicle formation, recognition, and fusion. These include components of vesicle coats, adaptor complexes, SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, and Rab guanosine 5'-triphosphatases (GTPases), which represent attractive targets for genetic manipulation aimed at unraveling mechanisms of endocytosis and exocytosis.

Agonist treatment did not affect association of mu opioid receptors with lipid rafts and cholesterol reduction had opposite effects on the receptor-mediated signaling in rat brain and CHO cells

Lipid rafts are small cholesterol- and glycosphingolipid-enriched membrane subdomains. Here we compared the mu opioid receptor (MOR)-lipid rafts relationship in the rat brain, where neurons have non-caveolae rafts, and in CHO cells stably transfected with HA-rat MOR (CHO-HA-rMOR), which are enriched in caveolae. Membranes of rat caudate putamen (CPu) and thalamus or CHO-HA-rMOR cells were homogenized, sonicated in a detergent-free 0.5 M Na(2)CO(3) buffer and fractionated through sucrose density gradients. Western blot and [(3)H]diprenorphine binding showed that approximately 70% of MOR in CHO-HA-rMOR was present in low-density (5-20% sucrose) fractions enriched in cholesterol and/or ganglioside M1 (GM1) (lipid rafts) in plasma membranes, whereas about 70% and 45% of MOR in CPu and thalamus, respectively, were associated with lipid rafts. Incubation with a saturating concentration of etorphine or morphine at 37 degrees C for 30 min failed to change the MOR location in rafts in CHO-HA-rMOR, indicating that the internalized MOR does not move out of rafts, in contrast to the delta opioid receptor. In vivo, rafts association of MOR in CPu and thalamus was not affected significantly in rats implanted with two 75-mg morphine pellets for 72 h. In addition, cholesterol reduction by methyl-beta-cyclodextrin (MCD) disrupted rafts and shifted MOR to higher density fractions in both CHO-HA-rMOR and CPu membranes. However, MCD treatment had opposite impacts on MOR signaling in the two tissues: it attenuated MOR-mediated [(35)S]GTPgammaS binding in CPu but enhanced it in CHO-HA-rMOR.

Pathways of clathrin-independent endocytosis

There are numerous ways that endocytic cargo molecules may be internalized from the surface of eukaryotic cells. In addition to the classical clathrin-dependent mechanism of endocytosis, several pathways that do not use a clathrin coat are emerging. These pathways transport a diverse array of cargoes and are sometimes hijacked by bacteria and viruses to gain access to the host cell. Here, we review our current understanding of various clathrin-independent mechanisms of endocytosis and propose a classification scheme to help organize the data in this complex and evolving field.

Podocin participates in the assembly of tight junctions between foot processes in nephrotic podocytes

The predominant type of cellular junction between normal podocyte foot processes is the slit diaphragm. Under nephrotic conditions,however, foot process effacement leads to the loss of slit diaphragms and the new formationof tight junctions composed of the proteins coxsackievirus and adenovirus receptor (CAR) and zonula occludens 1 (ZO-1). Podocin, a protein that plays a key role in maintaining the integrity of the slit diaphragm, has also been localized to these tight junctions, but its function at this site is unknown. In this study, we confirmed that podocin colocalizes with CAR and ZO-1 at the tight junction between foot processes in nephrotic rats. Using primary cultures of rat podocytes, as well as cell lines that co-expressed podocin and CAR, we observed that podocin was recruited to sites of cell-cell contact and that it co-localized with CAR and ZO-1. Immunoprecipitation suggested that these three junctional proteins from a multi-protein complex. Consistent with this, we found that podociin facilitated the coalescence of preassembled lipid rafts containing CAR and restricted their lateral mobility, the latter likely a result of dynamic actin reorganization and subsequent tethering of CAR-podocin complexes to the cytoskeleton. In conclusion, in addition to serving as a structural protein of the slit diaphragm of normal podocytes, our data suggest that podocin may also serve as a scaffold that links tight junction proteins to the actin cytoskeleton in nephrotic foot processes.