N-cadherin association with lipid rafts regulates its dynamic assembly at cell-cell junctions in C2C12 myoblasts

Adherens junctions organize size-selective proteolytic hotspots critical for Notch signalling

Adherens junctions (AJs) create spatially, chemically and mechanically discrete microdomains at cellular interfaces. Here, using a mechanogenetic platform that generates artificial AJs with controlled protein localization, clustering and mechanical loading, we find that AJs also organize proteolytic hotspots for γ-secretase with a spatially regulated substrate selectivity that is critical in the processing of Notch and other transmembrane proteins. Membrane microdomains outside of AJs exclusively organize Notch ligand-receptor engagement (LRE microdomains) to initiate receptor activation. Conversely, membrane microdomains within AJs exclusively serve to coordinate regulated intramembrane proteolysis (RIP microdomains). They do so by concentrating γ-secretase and primed receptors while excluding full-length Notch. AJs induce these functionally distinct microdomains by means of lipid-dependent γ-secretase recruitment and size-dependent protein segregation. By excluding full-length Notch from RIP microdomains, AJs prevent inappropriate enzyme-substrate interactions and suppress spurious Notch activation. Ligand-induced ectodomain shedding eliminates size-dependent segregation, releasing Notch to translocate into AJs for processing by γ-secretase. This mechanism directs radial differentiation of ventricular zone-neural progenitor cells in vivo and more broadly regulates the proteolysis of other large cell-surface receptors such as amyloid precursor protein. These findings suggest an unprecedented role of AJs in creating size-selective spatial switches that choreograph γ-secretase processing of multiple transmembrane proteins regulating development, homeostasis and disease.

Advanced DNA Zipper Probes for Detecting Cell Membrane Lipid Domains

The cell membrane is a complex mixture of lipids, proteins, and other components. By forming dynamic lipid domains, different membrane molecules can selectively interact with each other to control cell signaling. Herein, we report several new types of lipid-DNA conjugates, termed as "DNA zippers", which can be used to measure cell membrane dynamic interactions and the formation of lipid domains. Dependent on the choice of lipid moieties, cholesterol- and sphingomyelin-conjugated DNA zippers specifically locate in and detect membrane lipid-ordered domains, while in contrast, a tocopherol-DNA zipper can be applied for the selective imaging of lipid-disordered phases. These versatile and programmable probes can be further engineered into membrane competition assays to simultaneously detect multiple types of membrane dynamic interactions. These DNA zipper probes can be broadly used to study the correlation between lipid domains and various cellular processes, such as the epithelial-mesenchymal transition.

Chondroitin sulfate E downregulates N-cadherin and suppresses myotube formation

Myogenesis, the formation of muscle fibers, is affected by certain glycoproteins, including chondroitin sulfate (CS), which are involved in various cellular processes. We aimed to investigate the mechanism underlying CS-E-induced suppression of myotube formation using the myoblast cell line C2C12. Differentiated cells treated with 0.1 mg/ml CS-E for nine days showed multinucleated and rounded myotubes with myosin heavy chain positivity. No difference was found between the CS-E-treated group with rounded myotubes and CS (−) controls with elongated myotubes in the levels of phospho-cofilin, a protein involved in the dynamics of actin cytoskeleton. Interestingly, N-cadherin, which is involved in the gene expression of myoblast fusion factors (myomaker and myomixer), was significantly downregulated at both the mRNA and protein levels following CS-E treatment. These results suggest that N-cadherin downregulation is one of the mechanisms underlying the CS-E-induced suppression of myotube formation.

Liquid-liquid phase separation as a common organizing principle of intracellular space and biomembranes providing dynamic adaptive responses

This work is devoted to the phenomenon of liquid-liquid phase separation (LLPS), which has come to be recognized as fundamental organizing principle of living cells. We distinguish separation processes with different dimensions. Well-known 3D-condensation occurs in aqueous solution and leads to membraneless organelle (MLOs) formation. 2D-films may be formed near membrane surfaces and lateral phase separation (membrane rafts) occurs within the membranes themselves. LLPS may also occur on 1D structures like DNA and the cyto- and nucleoskeleton. Phase separation provides efficient transport and sorting of proteins and metabolites, accelerates the assembly of metabolic and signaling complexes, and mediates stress responses. In this work, we propose a model in which the processes of polymerization (1D structures), phase separation in membranes (2D structures), and LLPS in the volume (3D structures) influence each other. Disordered proteins and whole condensates may provide membrane raft separation or polymerization of specific proteins. On the other hand, 1D and 2D structures with special composition or embedded IDRs can nucleate condensates. We hypothesized that environmental change may trigger a LLPS which can propagate within the cell interior moving along the cytoskeleton or as an autowave. New phase propagation quickly and using a low amount of energy adjusts cell signaling and metabolic systems to new demands. Cumulatively, the interconnected phase separation phenomena in different dimensions represent a previously unexplored system of intracellular communication and regulation which cannot be ignored when considering both physiological and pathological cell processes.

In vivo Neural Crest Cell Migration Is Controlled by "Mixotaxis"

Directed cell migration is essential all along an individual's life, from embryogenesis to tissue repair and cancer metastasis. Thus, due to its biomedical relevance, directed cell migration is currently under intense research. Directed cell migration has been shown to be driven by an assortment of external biasing cues, ranging from gradients of soluble (chemotaxis) to bound (haptotaxis) molecules. In addition to molecular gradients, gradients of mechanical properties (duro/mechanotaxis), electric fields (electro/galvanotaxis) as well as iterative biases in the environment topology (ratchetaxis) have been shown to be able to direct cell migration. Since cells migrating in vivo are exposed to a challenging environment composed of a convolution of biochemical, biophysical, and topological cues, it is highly unlikely that cell migration would be guided by an individual type of "taxis." This is especially true since numerous molecular players involved in the cellular response to these biasing cues are often recycled, serving as sensor or transducer of both biochemical and biophysical signals. In this review, we confront literature on Xenopus cephalic neural crest cells with that of other cell types to discuss the relevance of the current categorization of cell guidance strategies. Furthermore, we emphasize that while studying individual biasing signals is informative, the hard truth is that cells migrate by performing a sort of "mixotaxis," where they integrate and coordinate multiple inputs through shared molecular effectors to ensure robustness of directed cell motion.

Principles and Applications of Biological Membrane Organization

Many critical biological events, including biochemical signaling, membrane traffic, and cell motility, originate at membrane surfaces. Each such event requires that members of a specific group of proteins and lipids rapidly assemble together at a specific site on the membrane surface. Understanding the biophysical mechanisms that stabilize these assemblies is critical to decoding and controlling cellular functions. In this article, we review progress toward a quantitative biophysical understanding of the mechanisms that drive membrane heterogeneity and organization. We begin from a physical perspective, reviewing the fundamental principles and key experimental evidence behind each proposed mechanism. We then shift to a biological perspective, presenting key examples of the role of heterogeneity in biology and asking which physical mechanisms may be responsible. We close with an applied perspective, noting that membrane heterogeneity provides a novel therapeutic target that is being exploited by a growing number of studies at the interface of biology, physics, and engineering.

Exosomal 2',3'-CNP from mesenchymal stem cells promotes hippocampus CA1 neurogenesis/neuritogenesis and contributes to rescue of cognition/learning deficiencies of damaged brain

Mesenchymal stem cells (MSCs) have been used in clinical studies to treat neurological diseases and damage. However, implanted MSCs do not achieve their regenerative effects by differentiating into and replacing neural cells. Instead, MSC secretome components mediate the regenerative effects of MSCs. MSC-derived extracellular vesicles (EVs)/exosomes carry cargo responsible for rescuing brain damage. We previously showed that EP₄ antagonist-induced MSC EVs/exosomes have enhanced regenerative potential to rescue hippocampal damage, compared with EVs/exosomes from untreated MSCs. Here we show that EP₄ antagonist-induced MSC EVs/exosomes promote neurosphere formation in vitro and increase neurogenesis and neuritogenesis in damaged hippocampi; basal MSC EVs/exosomes do not contribute to these regenerative effects. 2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNP) levels in EP₄ antagonist-induced MSC EVs/exosomes are 20-fold higher than CNP levels in basal MSC EVs/exosomes. Decreasing elevated exosomal CNP levels in EP₄ antagonist-induced MSC EVs/exosomes reduced the efficacy of these EVs/exosomes in promoting β3-tubulin polymerization and in converting toxic 2',3'-cAMP into neuroprotective adenosine. CNP-depleted EP₄ antagonist-induced MSC EVs/exosomes lost the ability to promote neurogenesis and neuritogenesis in damaged hippocampi. Systemic administration of EV/exosomes from EP₄ -antagonist derived MSC EVs/exosomes repaired cognition, learning, and memory deficiencies in mice caused by hippocampal damage. In contrast, CNP-depleted EP₄ antagonist-induced MSC EVs/exosomes failed to repair this damage. Exosomal CNP contributes to the ability of EP₄ antagonist-elicited MSC EVs/exosomes to promote neurogenesis and neuritogenesis in damaged hippocampi and recovery of cognition, memory, and learning. This experimental approach should be generally applicable to identifying the role of EV/exosomal components in eliciting a variety of biological responses.

Discoidin domain receptor 1 deficiency in vascular smooth muscle cells leads to mislocalisation of N-cadherin contacts

N-cadherin mediates cell–cell contacts in vascular smooth muscle cells (VSMCs), and regulates VSMC behaviours including migration and proliferation. Discoidin domain receptor 1 (DDR1) is a collagen binding receptor also implicated in these processes. Previous studies have shown that both N-cadherin and DDR1 are upregulated after vascular injury, but it is not known whether there is a relationship between the two molecules. In the current study we found that N-cadherin was mislocalised from cell–cell junctions in the absence of DDR1. This occurred in spite of the fact that there was no significant difference in total cell lysate levels of N-cadherin between DDR1+/+ and DDR1−/− VSMCs. Analysis of lipid raft fractions revealed decreased N-cadherin and associated junctional complex catenins in DDR1−/− compared to DDR1+/+ VSMCs. Treatment with cholesterol oxidase or methyl-β-cyclodextrin to disrupt lipid rafts removed N-cadherin and DDR1 from the raft fractions. Reciprocal co-immunoprecipitations suggested the association of DDR1 and N-cadherin. Importantly, transfection of DDR1−/− cells with full-length DDR1b rescued the formation of N-cadherin junctions. Together, these data reveal that N-cadherin cell–cell contacts in VSMCs are regulated through interactions with DDR1 and both molecules are located in lipid rafts.

Summary: Here we show for the first time that discoidin domain receptor 1 associates with and stabilizes N-cadherin cell–cell contacts in vascular smooth muscle cells.

Cell number in mesenchymal stem cell aggregates dictates cell stiffness and chondrogenesis

Background

Although mesenchymal stem/stromal cell (MSC) chondrogenic differentiation has been thoroughly investigated, the rudiments for enhancing chondrogenesis have remained largely dependent on external cues. Focus to date has been on extrinsic variables such as soluble signals, culture conditions (bioreactors), and mechanical stimulation. However, the role of intrinsic mechanisms of MSC programming-based mechanobiology remains to be explored. Since aggregation of MSCs, a prerequisite for chondrogenesis, generates tension within the cell agglomerate, we inquired if the initial number of cells forming the aggregate (aggregate cell number (ACN)) can impact chondrogenesis.

Methods

Aggregates of varying ACN were formed using well-established centrifugation approach. Progression of chondrogenic differentiation in the aggregates was assessed over 3 weeks in presence and absence of transforming growth factor-beta 1 (TGF-β1). Mechanical properties of the cells were characterized using high-throughput real-time deformability cytometry (RT-DC), and gene expression was analyzed using Affymetrix gene array. Expression of molecular markers linked to chondrogenesis was assessed using western blot and immunofluorescence.

Results

Reducing ACN from 500 k to 70 k lead to activation and acceleration of the chondrogenic differentiation, independent of soluble chondro-inductive factors, which involves changes to β-catenin-dependent TCF/LEF transcriptional activity and expression of anti-apoptotic protein survivin. RT-DC analysis revealed that stiffness and size of cells within aggregates are modulated by ACN. A direct correlation between progression of chondrogenesis and emergence of stiffer cell phenotype was found. Affymetrix gene array analysis revealed a downregulation of genes associated with lipid synthesis and regulation, which could account for observed changes in cell stiffness. Immunofluorescence and western blot analysis revealed that increasing ACN upregulates the expression of lipid raft protein caveolin-1, a β-catenin binding partner, and downregulates the expression of N-cadherin. As a demonstration of the relevance of these findings in MSC-based strategies for skeletal repair, it is shown that implanting aggregates within collagenous matrix not only decreases the necessity for high cell numbers but also leads to marked improvement in the quality of the deposited tissue.

Conclusions

This study presents a simple and donor-independent strategy to enhance the efficiency of MSC chondrogenic differentiation and identifies changes in cell mechanics coincident with MSC chondrogenesis with potential translational applications.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1103-y) contains supplementary material, which is available to authorized users.

MT1-MMP targeting to endolysosomes is mediated by upregulation of flotillins

Tumor cell invasion and metastasis formation are the major cause of death in cancer patients. These processes rely on extracellular matrix (ECM) degradation mediated by organelles termed invadopodia, to which the transmembrane matrix metalloproteinase MT1-MMP (also known as MMP14) is delivered from its reservoir, the RAB7-containing endolysosomes. How MT1-MMP is targeted to endolysosomes remains to be elucidated. Flotillin-1 and -2 are upregulated in many invasive cancers. Here, we show that flotillin upregulation triggers a general mechanism, common to carcinoma and sarcoma, which promotes RAB5-dependent MT1-MMP endocytosis and its delivery to RAB7-positive endolysosomal reservoirs. Conversely, flotillin knockdown in invasive cancer cells greatly reduces MT1-MMP accumulation in endolysosomes, its subsequent exocytosis at invadopodia, ECM degradation and cell invasion. Our results demonstrate that flotillin upregulation is necessary and sufficient to promote epithelial and mesenchymal cancer cell invasion and ECM degradation by controlling MT1-MMP endocytosis and delivery to the endolysosomal recycling compartment.

Sphingolipid de novo biosynthesis is essential for intestine cell survival and barrier function

Serine palmitoyltransferase (SPT) is the rate-limiting enzyme for sphingolipid biosynthesis. SPT has two major subunits, SPTLC1 and SPTLC2. We previously found that liver Sptlc2 deficiency in early life impairs the development of adherens junctions. Here, we investigated the role of Sptlc2 deficiency in intestine. We treated Sptlc2-Flox/villin-Cre-ERT2 mice with tamoxifen (days 1, 2, and 3) to ablate Sptlc2 specifically in the intestine. At day 6 after tamoxifen treatment, Sptlc2-deficient mice had significantly decreased body weight with concurrent diarrhea and rectal bleeding. The number of goblet cells was reduced in both large and small intestine of Sptlc2-deficient mice compared with controls. Sptlc2 deficiency suppressed the level of mucin2 in the colon and increased circulating lipopolysaccharides, suggesting that SPT activity has a housekeeping function in the intestine. All Sptlc2-deficient mice died 7-10 days after tamoxifen treatment. Notably, supplementation with antibiotics and dexamethasone reduced lethality by 70%. We also found that colon specimens from patients with inflammatory bowel diseases had significantly reduced Sptlc2 expression, SPTLC2 staining, and goblet cell numbers. SPT activity is crucial for intestinal cell survival and barrier function.

Early events in the assembly of E-cadherin adhesions

E-cadherin is a calcium dependent cell adhesion molecule that is key to the organization of cells in the epithelial tissue. It is a multidomain, trans-membrane protein in which the extracellular domain forms the homotypic, adhesive interaction while the intracellular domain interacts with the actin cytoskeleton through the catenin family of adaptor proteins. A number of recent studies have provided novel insights into the mechanism of adhesion formation by this class of adhesion proteins. Here, we describe an updated view of the process of E-cadherin adhesion formation with an emphasis on the role of molecular mobility, clustering, and active cellular processes.

Liver serine palmitoyltransferase activity deficiency in early life impairs adherens junctions and promotes tumorigenesis

Serine palmitoyltransferase is the key enzyme in sphingolipid biosynthesis. Mice lacking serine palmitoyltransferase are embryonic lethal. We prepared liver-specific mice deficient in the serine palmitoyltransferase long chain base subunit 2 gene using an albumin-cyclization recombination approach and found that the deficient mice have severe jaundice. Moreover, the deficiency impairs hepatocyte polarity, attenuates liver regeneration after hepatectomy, and promotes tumorigenesis. Importantly, we show that the deficiency significantly reduces sphingomyelin but not other sphingolipids in hepatocyte plasma membrane; greatly reduces cadherin, the major protein in adherens junctions, on the membrane; and greatly induces cadherin phosphorylation, an indication of its degradation. The deficiency affects cellular distribution of β-catenin, the central component of the canonical Wnt pathway. Furthermore, such a defect can be partially corrected by sphingomyelin supplementation in vivo and in vitro.

CONCLUSION

The plasma membrane sphingomyelin level is one of the key factors in regulating hepatocyte polarity and tumorigenesis. (Hepatology 2016;64:2089-2102).

N-cadherin-functionalized polymer-tethered multi-bilayer: a cell surface-mimicking substrate to probe cellular mechanosensitivity

Fate and function of anchorage-dependent cells depend on a variety of environmental cues, including those of mechanical nature. Previous progress in the understanding of cellular mechanosensitivity has been closely linked to the availability of artificial cell substrates of adjustable viscoelasticity, allowing for a direct correlation between substrate stiffness and cell response. Exemplary, polymeric gel substrates with polymer-conjugated cell-substrate linkers provided valuable insight into the role of mechanical signals during cell migration in an extracellular matrix environment. In contrast, less is known about the role of external mechanical signals across cell-cell interfaces, in part, due to the limitations of traditional polymeric substrates to mimic the remarkable dynamics of cell-cell linkages. To overcome this shortcoming, we introduce a cell surface-mimicking cell substrate of adjustable stiffness, which is comprised of a polymer-tethered lipid multi-bilayer stack with N-cadherin linkers. Unlike traditional polymeric cell substrates with polymer-conjugated linkers, this novel artificial cell substrate is able to replicate the dynamic assembly/disassembly of cadherin linkers into linker clusters and the long-range movements of cadherin-based cell-substrate linkages observed at cell-cell interfaces. Moreover, substrate stiffness can be changed by adjusting the number of bilayers in the multi-bilayer stack, thus enabling the analysis of cellular mechanosensitivity in the presence of artificial cell-cell linkages. The presented biomembrane-mimicking cell substrate provides a valuable tool to explore the functional role of mechanical cues from neighboring cells.

Proteomic Analysis of Lipid Raft-Like Detergent-Resistant Membranes of Lens Fiber Cells

PURPOSE

Plasma membranes of lens fiber cells have high levels of long-chain saturated fatty acids, cholesterol, and sphingolipids-key components of lipid rafts. Thus, lipid rafts are expected to constitute a significant portion of fiber cell membranes and play important roles in lens biology. The purpose of this study was to characterize the lens lipid raft proteome.

METHODS

Quantitative proteomics, both label-free and iTRAQ methods, were used to characterize lens fiber cell lipid raft proteins. Detergent-resistant, lipid raft membrane (DRM) fractions were isolated by sucrose gradient centrifugation. To confirm protein localization to lipid rafts, protein sensitivity to cholesterol removal by methyl-β-cyclodextrin was quantified by iTRAQ analysis.

RESULTS

A total of 506 proteins were identified in raft-like detergent-resistant membranes. Proteins identified support important functions of raft domains in fiber cells, including trafficking, signal transduction, and cytoskeletal organization. In cholesterol-sensitivity studies, 200 proteins were quantified and 71 proteins were strongly affected by cholesterol removal. Lipid raft markers flotillin-1 and flotillin-2 and a significant fraction of AQP0, MP20, and AQP5 were found in the DRM fraction and were highly sensitive to cholesterol removal. Connexins 46 and 50 were more abundant in nonraft fractions, but a small fraction of each was found in the DRM fraction and was strongly affected by cholesterol removal. Quantification of modified AQP0 confirmed that fatty acylation targeted this protein to membrane raft domains.

CONCLUSIONS

These data represent the first comprehensive profile of the lipid raft proteome of lens fiber cells and provide information on membrane protein organization in these cells.

Cholesterol lowering: role in cancer prevention and treatment

The accumulation of cholesterol is a general feature of cancer tissue, and recent evidence suggests that cholesterol plays critical roles in the progression of cancers, including breast, prostate, and colorectal cancers. The dysregulation of metabolic pathways, including those involved in cholesterol biosynthesis, is implicated in tumor development and cancer progression. Lipid rafts are highly dynamic cholesterol-enriched domains of the cell membrane, involved in various cellular functions, including the regulation of transmembrane signaling at the cell surface. It was recently demonstrated that lipid rafts also play critical roles in cancer cell adhesion and migration. This review focuses on our current understanding of how cholesterol regulation, lipid rafts, and dysregulated cholesterol biosynthesis contribute to cancer development and progression, and the therapeutic potential of cholesterol lowering for cancer prevention and treatment.

YES oncogenic activity is specified by its SH4 domain and regulates RAS/MAPK signaling in colon carcinoma cells

Members of the SRC family of tyrosine kinases (SFK) display important functions in human cancer, but their specific role in tumorigenesis remains unclear. We previously demonstrated that YES regulates a unique oncogenic signaling important for colorectal cancer (CRC) progression that is not shared with SRC. Here, we addressed the underlying mechanism involved in this process. We show that YES oncogenic signaling relies on palmitoylation of its SH4 domain that controls YES localization in cholesterol-enriched membrane micro-domains. Specifically, deletion of the palmitoylation site compromised YES transforming activity, while addition of a palmitoylation site in the SH4 domain of SRC was sufficient for SRC to restore the transforming properties of cells in which YES had been silenced. Subsequently, SILAC phosphoproteomic analysis revealed that micro-domain-associated cell adhesive components and receptor tyrosine kinases are major YES substrates. YES also phosphorylates upstream regulators of RAS/MAPK signaling, including EGFR, SHC and SHP2, which were not targeted by SRC due to the absence of palmitoylation. Accordingly, EGFR-induced MAPK activity was attenuated by YES down-regulation, while increased RAS activity significantly restored cell transformation that was lost upon YES silencing. Collectively, these results uncover a critical role for the SH4 domain in the specification of SFK oncogenic activity and a selective role for YES in the induction of RAS/MAPK signaling in CRC cells.

Identification of novel adipokines differential regulated in C57BL/Ks and C57BL/6

Visceral adiposity is associated with metabolic disorders, but little is known on the underlying pathophysiological mechanism. One possible link might be the release of various signalling and mediator proteins, named adipokines. Our hypothesis was that dependent on genetic background factors are released which might trigger a primary disease susceptibility. This study characterizes the adipokines released from visceral adipose tissue from two metabolic healthy mouse strains, i.e. C57BL/Ks (BKS) and C57BL/6 (C57), of which the former genetic background is more sensitive to develop diabetes following metabolic challenge. Using liquid chromatography (LC)-electrospray ionization (ESI)-MS/MS, a reference map comprising 597 adipokines was generated (http://www.diabesityprot.org). Thirty-five adipokines, including six not previously described ones, were differentially released between the mouse strains. Most notable is the reduced release of the adiponectin-binding protein T-Cadherin (CAD13) in BKS mice. This observation highlights the importance of secretome profiling in unravelling the complex interplay between genetic diversity and lifestyle.

Flotillins in intercellular adhesion - from cellular physiology to human diseases

Flotillin 1 and 2 are ubiquitous and highly conserved proteins. They were initially discovered in 1997 as being associated with specific caveolin-independent cholesterol- and glycosphingolipid-enriched membrane microdomains and as being expressed during axon regeneration. Flotillins have a role in a large number of physiopathological processes, mainly through their function in membrane receptor clustering and in the regulation of clathrin-independent endocytosis. In this Commentary, we summarize the research performed so far on the role of flotillins in cell-cell adhesion. Recent studies have demonstrated that flotillins directly regulate the formation of cadherin complexes. Indeed, flotillin microdomains are required for the dynamic association and stabilization of cadherins at cell-cell junctions and also for cadherin signaling. Moreover, because flotillins regulate endocytosis and also the actin cytoskeleton, they could have an indirect role in the assembly and stabilization of cadherin complexes. Because it has also recently been shown that flotillins are overexpressed during neurodegenerative diseases and in human cancers, where their upregulation is associated with metastasis formation and poor prognosis, understanding to what extent flotillin upregulation participates in the development of such pathologies is thus of particular interest, as well as how, at the molecular level, it might affect cell adhesion processes.

Mesoscale computational studies of membrane bilayer remodeling by curvature-inducing proteins

Biological membranes constitute boundaries of cells and cell organelles. These membranes are soft fluid interfaces whose thermodynamic states are dictated by bending moduli, induced curvature fields, and thermal fluctuations. Recently, there has been a flood of experimental evidence highlighting active roles for these structures in many cellular processes ranging from trafficking of cargo to cell motility. It is believed that the local membrane curvature, which is continuously altered due to its interactions with myriad proteins and other macromolecules attached to its surface, holds the key to the emergent functionality in these cellular processes. Mechanisms at the atomic scale are dictated by protein-lipid interaction strength, lipid composition, lipid distribution in the vicinity of the protein, shape and amino acid composition of the protein, and its amino acid contents. The specificity of molecular interactions together with the cooperativity of multiple proteins induce and stabilize complex membrane shapes at the mesoscale. These shapes span a wide spectrum ranging from the spherical plasma membrane to the complex cisternae of the Golgi apparatus. Mapping the relation between the protein-induced deformations at the molecular scale and the resulting mesoscale morphologies is key to bridging cellular experiments across the various length scales. In this review, we focus on the theoretical and computational methods used to understand the phenomenology underlying protein-driven membrane remodeling. Interactions at the molecular scale can be computationally probed by all atom and coarse grained molecular dynamics (MD, CGMD), as well as dissipative particle dynamics (DPD) simulations, which we only describe in passing. We choose to focus on several continuum approaches extending the Canham - Helfrich elastic energy model for membranes to include the effect of curvature-inducing proteins and explore the conformational phase space of such systems. In this description, the protein is expressed in the form of a spontaneous curvature field. The approaches include field theoretical methods limited to the small deformation regime, triangulated surfaces and particle-based computational models to investigate the large-deformation regimes observed in the natural state of many biological membranes. Applications of these methods to understand the properties of biological membranes in homogeneous and inhomogeneous environments of proteins, whose underlying curvature fields are either isotropic or anisotropic, are discussed. The diversity in the curvature fields elicits a rich variety of morphological states, including tubes, discs, branched tubes, and caveola. Mapping the thermodynamic stability of these states as a function of tuning parameters such as concentration and strength of curvature induction of the proteins is discussed. The relative stabilities of these self-organized shapes are examined through free-energy calculations. The suite of methods discussed here can be tailored to applications in specific cellular settings such as endocytosis during cargo trafficking and tubulation of filopodial structures in migrating cells, which makes these methods a powerful complement to experimental studies.

Desmosome assembly and disassembly are membrane raft-dependent

Strong intercellular adhesion is critical for tissues that experience mechanical stress, such as the skin and heart. Desmosomes provide adhesive strength to tissues by anchoring desmosomal cadherins of neighboring cells to the intermediate filament cytoskeleton. Alterations in assembly and disassembly compromise desmosome function and may contribute to human diseases, such as the autoimmune skin blistering disease pemphigus vulgaris (PV). We previously demonstrated that PV auto-antibodies directed against the desmosomal cadherin desmoglein 3 (Dsg3) cause loss of adhesion by triggering membrane raft-mediated Dsg3 endocytosis. We hypothesized that raft membrane microdomains play a broader role in desmosome homeostasis by regulating the dynamics of desmosome assembly and disassembly. In human keratinocytes, Dsg3 is raft associated as determined by biochemical and super resolution immunofluorescence microscopy methods. Cholesterol depletion, which disrupts rafts, prevented desmosome assembly and adhesion, thus functionally linking rafts to desmosome formation. Interestingly, Dsg3 did not associate with rafts in cells lacking desmosomal proteins. Additionally, PV IgG-induced desmosome disassembly occurred by redistribution of Dsg3 into raft-containing endocytic membrane domains, resulting in cholesterol-dependent loss of adhesion. These findings demonstrate that membrane rafts are required for desmosome assembly and disassembly dynamics, suggesting therapeutic potential for raft targeting agents in desmosomal diseases such as PV.

Lipid rafts and detergent-resistant membranes in epithelial keratinocytes

Our understanding of the plasma membrane has markedly increased since Singer and Nicolson proposed the fluid mosaic model in 1972. While their revolutionary theory of the lipid bilayer remains largely valid, it is now known that lipids and proteins are not randomly dispersed throughout the plasma membrane but instead may be organized within membrane microdomains, commonly referred to as lipid rafts. Lipid rafts are highly dynamic, detergent resistant, and enriched with both cholesterol and glycosphingolipids. The two main types are flotillin-rich planar lipid rafts and caveolin-rich caveolae. It is proposed that flotillin and caveolin proteins regulate cell communication by compartmentalizing and interacting with signal transduction proteins within their respective lipid microdomains. Consequently, membrane rafts play an important role in vital cellular functions including migration, invasion, and signaling; thus, alterations in their microenvironment can initiate signaling pathways that affect cellular function and behavior. Therefore, the identification of lipid rafts and their associated proteins is integral to the study of transmembrane signaling. Here, we review the current standard protocols and biochemical approaches used to isolate and define raft proteins from epithelial cells and tissues. Furthermore, in Section 3 of this chapter, detailed protocols are offered for isolating lipid rafts by subjection to detergent and sucrose density centrifugation, as well as an approach for selectively isolating caveolae. Methods to manipulate rafts with treatments such as methyl-β-cyclodextrin and flotillin III are also described.

Flotillins directly interact with γ-catenin and regulate epithelial cell-cell adhesion

Flotillin-1 and flotillin-2 are two homologous, membrane raft associated proteins. Although it has been reported that flotillins are involved in cell adhesion processes and play a role during breast cancer progression, thus making them interesting future therapeutic targets, their precise function has not been well elucidated. The present study investigates the function of these proteins in cell-cell adhesion in non-malignant cells. We have used the non-malignant epithelial MCF10A cells to study the interaction network of flotillins within cell-cell adhesion complexes. RNA interference was used to examine the effect of flotillins on the structure of adherens junctions and on the association of core proteins, such as E-cadherin, with membrane rafts. We here show that the cadherin proteins of the adherens junction associate with flotillin-2 in MCF10A cells and in various human cell lines. In vitro, flotillin-1 and flotillin-2 directly interact with γ-catenin which is so far the only protein known to be present both in the adherens junction and the desmosome. Mapping of the interaction domain within the γ-catenin sequence identified the Armadillo domains 6-8, especially ARM domain 7, to be important for the association with flotillins. Furthermore, depletion of flotillins significantly influenced the morphology of the adherens junction in human epithelial MCF10A cells and altered the association of E-cadherin and γ-catenin with membrane rafts. Taken together, these observations suggest a functional role for flotillins, especially flotillin-2, in cell-cell adhesion in non-malignant epithelial cells.

Flotillin microdomains stabilize cadherins at cell-cell junctions

Cadherins are essential in many fundamental processes and assemble at regions of cell-cell contact in large macromolecular complexes named adherens junctions. We have identified flotillin 1 and 2 as new partners of the cadherin complexes. We show that flotillins are localised at cell-cell junctions (CCJs) in a cadherin-dependent manner. Flotillins and cadherins are constitutively associated at the plasma membrane and their colocalisation at CCJ increases with CCJ maturation. Using three-dimensional structured illumination super-resolution microscopy, we found that cadherin and flotillin complexes are associated with F-actin bundles at CCJs. The knockdown of flotillins dramatically affected N- and E-cadherin recruitment at CCJs in mesenchymal and epithelial cell types and perturbed CCJ integrity and functionality. Moreover, we determined that flotillins are required for cadherin association with GM1-containing plasma membrane microdomains. This allows p120 catenin binding to the cadherin complex and its stabilization at CCJs. Altogether, these data demonstrate that flotillin microdomains are required for cadherin stabilization at CCJs and for the formation of functional CCJs.

Regulation of pre-fusion events: recruitment of M-cadherin to microrafts organized at fusion-competent sites of myogenic cells

Background

Previous research indicates that the membrane ruffles and leading edge of lamellipodia of myogenic cells contain presumptive fusion sites. A micrometer-sized lipid raft (microraft) is organized at the presumptive fusion site of mouse myogenic cells in a cell-contact independent way and serves as a platform tethering adhesion proteins that are relevant to cell fusion. However, the mechanisms underlying recruitment of adhesion proteins to lipid rafts and microraft organization remain unknown.

Results

Here we show that small G-protein Rac1 was required for microraft organization and subsequent cell fusion. However, Rac1 activity was unnecessary for recruitment of M-cadherin to lipid rafts. We found that p120 catenin (p120) binds to M-cadherin exclusively in lipid rafts of differentiating myogenic cells. The Src kinase inhibitor SU6656 prevented p120 binding to M-cadherin and their recruitment to lipid rafts, then suppressed microraft organization, membrane ruffling, and myogenic cell fusion. Suppression of membrane ruffling in SU6656-treated cells was partially restored by pretreatment with the protein tyrosine phosphatase inhibitor vanadate. The present analyses using an antibody to tyrosine phosphorylated p120 suggest that Src family kinases play a role in binding of p120 to M-cadherin and the recruitment of M-cadherin to lipid rafts through phosphorylation of putative substrates other than p120.

Conclusions

The present study showed that the procedure establishing fusion-competent sites consists of two sequential events: recruitment of adhesion complexes to lipid rafts and organization of microrafts. The recruitment of M-cadherin to lipid rafts depended on interaction with p120 catenin, whereas the organization of microrafts was controlled by a small G protein, Rac1.

Stiffened lipid platforms at molecular force foci

How mechanical forces are sensed remains largely mysterious. The forces that gate prokaryotic and several eukaryotic channels were found to come from the lipid membrane. Our survey of animal cells found that membrane force foci all have cholesterol-gathering proteins and are reinforced with cholesterol. This result is evident in overt force sensors at the tips of stereocilia for vertebrate hearing and the touch receptor of Caenorhabditis elegans and mammalian neurons. For less specialized cells, cadherins sustain the force between neighboring cells and integrins between cells and matrix. These tension bearers also pass through and bind to a cholesterol-enriched platform before anchoring to cytoskeleton through other proteins. Cholesterol, in alliance with sphingomyelin and specialized proteins, enforces a more ordered structure in the bilayer. Such a stiffened platform can suppress mechanical noise, redirect, rescale, and confine force. We speculate that such platforms may be dynamic. The applied force may allow disordered-phase lipids to enter the platform-staging channel opening in the thinner mobile neighborhood. The platform may also contain specialized protein/lipid subdomains enclosing mechanosensitive channels to open with localized tension. Such a dynamic stage can mechanically operate structurally disparate channels or enzymes without having to tie them directly to cadherin, integrin, or other protein tethers.

Electrophysiology of single and aggregate Cx43 hemichannels

Connexin43 (Cx43) is the most ubiquitous gap junction protein in the human body and is essential for cell-to-cell communication in a variety of organs and organ systems. As a result, Cx43 is responsible for mediating both electrical and chemical signals, passing dissolved solutes and small signaling molecules between cells in a coordinated fashion. Here, we explore the electrophysiological properties of hemichannels formed from Cx43 and Cx43 fused to eGFP (Cx43eGFP) and their interactions in a planar lipid membrane (BLM). Unlike in vivo patch clamp experiments, Cx43 was purified and isolated from other membrane constituents allowing elucidation of individual protein responses to various electrical and chemical stimuli. Using this system, we examined hemichannel electrophysiology and the roles of several well-known gap junction blockers, namely: lanthanum, heptanol, carbenoxalone and lindane. We also observed a critical number of hemichannels required for an accelerated conductance increase, an emergent electrical signature indicative of plaque formation.

Galectin-3 protein regulates mobility of N-cadherin and GM1 ganglioside at cell-cell junctions of mammary carcinoma cells

Galectin-3 binding to cell surface glycoproteins, including branched N-glycans generated by N-acetylglucosaminyltransferase V (Mgat5) activity, forms a multivalent, heterogeneous, and dynamic lattice. This lattice has been shown to regulate integrin and receptor tyrosine kinase signaling promoting tumor cell migration. N-cadherin is a homotypic cell-cell adhesion receptor commonly overexpressed in tumor cells that contributes to cell motility. Here we show that galectin-3 and N-cadherin interact and colocalize with the lipid raft marker GM1 ganglioside in cell-cell junctions of mammary epithelial cancer cells. Disruption of the lattice by deletion of Mgat5, siRNA depletion of galectin-3, or competitive inhibition with lactose stabilizes cell-cell junctions. It also reduces, in a p120-catenin-dependent manner, the dynamic pool of junctional N-cadherin. Proteomic analysis of detergent-resistant membranes (DRMs) revealed that the galectin lattice opposes entry of many proteins into DRM rafts. N-cadherin and catenins are present in DRMs; however, their DRM distribution is not significantly affected by lattice disruption. Galectin lattice integrity increases the mobile fraction of the raft marker, GM1 ganglioside binding cholera toxin B subunit Ctb, at cell-cell contacts in a p120-catenin-independent manner, but does not affect the mobility of either Ctb-labeled GM1 or GFP-coupled N-cadherin in nonjunctional regions. Our results suggest that the galectin lattice independently enhances lateral molecular diffusion by direct interaction with specific glycoconjugates within the adherens junction. By promoting exchange between raft and non-raft microdomains as well as molecular dynamics within junction-specific raft microdomains, the lattice may enhance turnover of N-cadherin and other glycoconjugates that determine junctional stability and rates of cell migration.

Rac1 and RhoA GTPases have antagonistic functions during N-cadherin-dependent cell-cell contact formation in C2C12 myoblasts

BACKGROUND INFORMATION

N-cadherin, a member of the Ca(2+)-dependent cell-cell adhesion molecule family, plays an essential role in the induction of the skeletal muscle differentiation programme. However, the molecular mechanisms which govern the formation of N-cadherin-dependent cell-cell contacts in myoblasts remain unexplored.

RESULTS

In the present study, we show that N-cadherin-dependent cell contact formation in myoblasts is defined by two stages. In the first phase, N-cadherin is highly mobile in the lamellipodia extensions between the contacting cells. The second stage corresponds to the formation of mature N-cadherin-dependent cell contacts, characterized by the immobilization of a pool of N-cadherin which appears to be clustered in the interdigitated membrane structures that are also membrane attachment sites for F-actin filaments. We also demonstrated that the formation of N-cadherin-dependent cell-cell contacts requires a co-ordinated and sequential activity of Rac1 and RhoA. Rac1 is involved in the first stage and facilitates N-cadherin-dependent cell-cell contact formation, but it is not absolutely required. Conversely, RhoA is necessary for N-cadherin-dependent cell contact formation, since, via ROCK (Rho-associated kinase) signalling and myosin 2 activation, it allows the stabilization of N-cadherin at the cell-cell contact sites.

CONCLUSIONS

We have shown that Rac1 and RhoA have opposite effects on N-cadherin-dependent cell-cell contact formation in C2C12 myoblasts and act sequentially to allow its formation.

Changes in membrane lipid composition cause alterations in epithelial cell-cell adhesion structures in renal papillary collecting duct cells

In epithelial tissues, adherens junctions (AJ) mediate cell-cell adhesion by using proteins called E-cadherins, which span the plasma membrane, contact E-cadherin on other cells and connect with the actin cytoskeleton inside the cell. Although AJ protein complexes are inserted in detergent-resistant membrane microdomains, the influence of membrane lipid composition in the preservation of AJ structures has not been extensively addressed. In the present work, we studied the contribution of membrane lipids to the preservation of renal epithelial cell-cell adhesion structures. We biochemically characterized the lipid composition of membranes containing AJ complexes. By using lipid membrane-affecting agents, we found that such agents induced the formation of new AJ protein-containing domains of different lipid composition. By using both biochemical approaches and fluorescence microscopy we demonstrated that the membrane phospholipid composition plays an essential role in the in vivo maintenance of AJ structures involved in cell-cell adhesion structures in renal papillary collecting duct cells.

Localisation to lipid rafts correlates with increased function of the Gal/GalNAc lectin in the human protozoan parasite, Entamoeba histolytica

Entamoeba histolytica is the causative agent of dysentery and liver abscess and is prevalent in developing countries. Adhesion to the host is critical to infection and is mediated by amoebic surface receptors. One such receptor, the Gal/GalNAc lectin, binds to galactose or N-acetylgalactosamine residues on host components and consists of heavy (Hgl), light (Lgl) and intermediate (Igl) subunits. The mechanism by which the lectin assembles into a functional complex is not known. The parasite also relies on cholesterol-rich domains (lipid rafts) for adhesion. Therefore, it is conceivable that rafts regulate the assembly or function of the lectin. To test this, amoebae were loaded with cholesterol and lipid rafts were purified and characterised. Western blotting showed that cholesterol loading resulted in co-compartmentalisation of all three subunits in rafts. This co-compartmentalisation was accompanied by an increase in the ability of the amoebae to bind to host cells in a galactose-specific manner, suggesting that there is a correlation between location and function of the Gal/GalNAc lectin. Cholesterol loading did not increase the surface levels of the lectin subunits. Therefore, the cholesterol-induced increase in adhesion was not the result of externalisation of an internal pool of subunits. A mutant cell line that modestly responded to cholesterol with a slight increase in adhesion exhibited only a slight enrichment of Hgl and Lgl in rafts. This supports the connection between location and function of the Gal/GalNAc lectin. Actin can also influence the interaction of proteins with rafts. Therefore, the sub-membrane distribution of the lectin subunits was also assessed after treatment with an actin depolymerising agent, cytochalasin D. Cytochalasin D-treatment had no effect on the submembrane distribution of the subunits, suggesting that actin does not prevent the association of lectin subunits with rafts in this system. Together, these data provide insight into the molecular mechanisms regulating the location and function of this adhesin.

Adenosine monophosphate-activated kinase alpha1 promotes endothelial barrier repair

The vascular endothelium responds to damage through activation of multiple signaling events that restore cell-cell adhesion and vascular integrity. However, the molecular mechanisms that integrate these events are not clearly defined. Herein, we identify a previously unexpected role for adenosine monophosphate-activated protein kinase (AMPK) in pulmonary microvascular endothelial cell (PMVEC) repair. PMVECs selectively express the AMPKα1 catalytic subunit, pharmacological and short hairpin RNA-mediated inhibition of which attenuates Ca(2+) entry in these cells induced by the inflammatory Ca(2+)-signaling mimetic thapsigargin. We find that AMPKα1 activity is required for the formation of PMVEC cell-cell networks in a prorepair environment and for monolayer resealing after wounding. Decreasing AMPKα1 expression reduces barrier resistance in PMVEC monolayers, results consistent with a role for AMPKα1 in cell-cell adhesion. AMPKα1 colocalizes and coimmunoprecipitates with the adherens junction protein N-cadherin and cofractionates with proteins selectively expressed in caveolar membranes. Assessment of permeability, by measuring the filtration coefficient (K(f)) in isolated perfused lungs, confirmed that AMPK activation contributes to barrier repair in vivo. Our findings thus provide novel evidence for AMPKα1 in Ca(2+) influx-mediated signaling and wound repair in the endothelium.

Cadherins at cell-autonomous membrane contacts control macropinocytosis

Cadherins aggregate and stabilize cell-cell junctions through interactions with adjacent cells. In addition, N-cadherin and E-cadherin concentrate at free edges or at the lamellipodia of migrating cells and are found within large vesicles called macropinosomes, which develop from membrane ruffles. The binding properties of cadherins have not previously been associated with the localization of cadherins at membrane ruffles; however, we report that the dorsal, ventral and lateral membrane contacts that occur as a result of the overlap of membrane ruffles aggregate N-cadherin, and that both N-cadherin and E-cadherin promote macropinosome closure and fluid-phase uptake in macropinosomes. These data reveal a previously unsuspected function for cadherin-mediated cell-cell adhesion molecules in the closure of cell-autonomous membrane contacts at membrane ruffles, resulting in macropinocytosis.

Filamin A is a phosphorylation target of membrane but not cytosolic adenylyl cyclase activity

Transmembrane adenylyl cyclase (AC) generates a cAMP pool within the subplasma membrane compartment that strengthens the endothelial cell barrier. This cAMP signal is steered toward effectors that promote junctional integrity and is inactivated before it accesses microtubules, where the cAMP signal causes phosphorylation of tau, leading to microtubule disassembly and barrier disruption. During infection, Pseudomonas aeruginosa uses a type III secretion system to inject a soluble AC, ExoY, into the cytosol of pulmonary microvascular endothelial cells. ExoY generates a cAMP signal that disrupts the endothelial cell barrier. We tested the hypothesis that this ExoY-dependent cAMP signal causes phosphorylation of tau, without inducing phosphorylation of membrane effectors that strengthen endothelial barrier function. To approach this hypothesis, we first discerned the membrane compartment in which endogenous transmembrane AC6 resides. AC6 was resolved in caveolin-rich lipid raft fractions with calcium channel proteins and the cell adhesion molecules N-cadherin, E-cadherin, and activated leukocyte adhesion molecule. VE-cadherin was excluded from the caveolin-rich fractions and was detected in the bulk plasma membrane fractions. The actin binding protein, filamin A, was detected in all membrane fractions. Isoproterenol activation of ACs promoted filamin phosphorylation, whereas thrombin inhibition of AC6 reduced filamin phosphorylation within the membrane fraction. In contrast, ExoY produced a cAMP signal that did not cause filamin phosphorylation yet induced tau phosphorylation. Hence, our data indicate that cAMP signals are strictly compartmentalized; whereas cAMP emanating from transmembrane ACs activates barrier-enhancing targets, such as filamin, cAMP emanating from soluble ACs activates barrier-disrupting targets, such as tau.

Desmosome assembly and cell-cell adhesion are membrane raft-dependent processes

The aim of our study was to investigate the association of desmosomal proteins with cholesterol-enriched membrane domains, commonly called membrane rafts, and the influence of cholesterol on desmosome assembly in epithelial Madin-Darby canine kidney cells (clone MDc-2). Biochemical analysis proved an association of desmosomal cadherin desmocollin 2 (Dsc2) in cholesterol-enriched fractions that contain membrane raft markers caveolin-1 and flotillin-1 and the novel raft marker ostreolysin. Cold detergent extraction of biotinylated plasma membranes revealed that ∼60% of Dsc2 associates with membrane rafts while the remainder is present in nonraft and cholesterol-poor membranes. The results of immunofluorescence microscopy confirmed colocalization of Dsc2 and ostreolysin. Partial depletion of cholesterol with methyl-β-cyclodextrin disturbs desmosome assembly, as revealed by sequential recordings of live cells. Moreover, cholesterol depletion significantly reduces the strength of cell-cell junctions and partially releases Dsc2 from membrane rafts. Our data indicate that a pool of Dsc2 is associated with membrane rafts, particularly with the ostreolysin type of membrane raft, and that intact membrane rafts are necessary for desmosome assembly. Taken together, these data suggest cholesterol as a potential regulator that promotes desmosome assembly.

Resveratrol protects against protease inhibitor-induced reactive oxygen species production, reticulum stress and lipid raft perturbation

OBJECTIVE

HIV protease inhibitors have been successfully used in highly active antiretroviral therapy of HIV-1 infection, but their benefits are compromised by a number of clinically important adverse side-effects. Several studies showed that protease inhibitors induce sarco/endoplasmic reticulum stress and overproduction of reactive oxygen species (ROS), but the hierarchy of these events was never established in protease inhibitor-treated cells. Our objective was to determine whether ROS production and sarco/endoplasmic reticulum stress were co-induced by protease inhibitors in human primary skeletal myotubes and whether antioxidant treatment with resveratrol could protect against protease inhibitor-induced cellular damages.

DESIGN AND METHODS

We analyzed in cultures of primary human skeletal myotubes the effects of four protease inhibitors (atazanavir, lopinavir, ritonavir and saquinavir) on ROS production (by measuring the reduction of nitro blue tetrazolium and by fluorescence microscopy using CM-H2DCFDA), on sarco/endoplasmic reticulum stress induction (by immunofluorescence and electron microscopy) and on the expression and localization at lipid rafts of Caveolin 3 and Flotillin 1, two major components of lipid rafts (by immunoblotting and biochemical experiments). Cells were co-incubated with resveratrol to assess its protective effects.

RESULTS

In myotubes, protease inhibitors increased ROS production, altered sarco/endoplasmic reticulum morphology, increased expression of C/EBP homologous protein, a sarco/endoplasmic reticulum stress marker, and decreased expression and localization at lipid rafts of Caveolin 3 and Flotillin 1. Importantly, these deleterious protease inhibitor effects were inhibited by the antioxidant resveratrol.

CONCLUSION

Our findings demonstrate a protective effect of resveratrol against protease inhibitor-induced sarco/endoplasmic reticulum stress in human myotubes.

Src family tyrosine kinases-driven colon cancer cell invasion is induced by Csk membrane delocalization

The nonreceptor tyrosine kinases of the Src family (SFK) are frequently deregulated in human colorectal cancer (CRC), and they have been implicated in tumour growth and metastasis. How SFK are activated in this cancer has not been clearly established. Here, we show that the SFK-dependent invasion is induced by inactivation of the negative regulator C-terminal Src kinase, Csk. While the level of Csk was inconsistent with SFK activity in colon cancer cells, its membrane translocation, needed for efficient regulation of membrane-localized SFK activity, was impaired. Accordingly, Csk downregulation did not affect SFK oncogenic activity in these cells, whereas expression of a membrane-localized form of this kinase affected their invasive activity. Downregulation of the transmembrane and rafts-localized Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched microdomain (PAG), was instrumental for the cytoplasmic accumulation of Csk. Re-expression of PAG in cells from late-stage CRC inhibited SFK invasive activity in a Csk-dependent manner. Conversely, inactivation of its residual expression in early-stage CRC cells promoted SFK invasive activity. Finally, this mechanism was specific to CRC as Csk coupling to SFK was readily detected in breast cancer cells. Therefore, Csk mis-localization defines a novel mechanism for SFK oncogenic activation in CRC cells.

N-cadherin/p120 catenin association at cell-cell contacts occurs in cholesterol-rich membrane domains and is required for RhoA activation and myogenesis

p120 catenin is a major regulator of cadherin stability at cell-cell contacts and a modulator of Rho GTPase activities. In C2C12 myoblasts, N-cadherin is stabilized at cell contacts through its association with cholesterol-rich membrane domains or lipid rafts (LR) and acts as an adhesion-activated receptor that activates RhoA, an event required for myogenesis induction. Here, we report that association of p120 catenin with N-cadherin at cell contacts occurs specifically in LR. We demonstrate that interaction of p120 catenin with N-cadherin is required for N-cadherin association with LR and for its stabilization at cell contacts. LR disruption inhibits myogenesis induction and N-cadherin-dependent RhoA activation as does the perturbation of the N-cadherin-p120 catenin complex after p120 catenin knockdown. Finally, we observe an N-cadherin-dependent accumulation of RhoA at phosphatidylinositol 4,5-bisphosphate-enriched cell contacts which is lost after LR disruption. Thus, a functional N-cadherin-catenin complex occurs in cholesterol-rich membrane microdomains which allows the recruitment of RhoA and the regulation of its activity during myogenesis induction.

Cytoskeleton-membrane interactions in membrane raft structure

Cell membranes are structurally heterogeneous, composed of discrete domains with unique physical and biological properties. Membrane domains can form through a number of mechanisms involving lipid-lipid and protein-lipid interactions. One type of membrane domain is the cholesterol-dependent membrane raft. How rafts form remains a current topic in membrane biology. We review here evidence of structuring of rafts by the cortical actin cytoskeleton. This includes evidence that the actin cytoskeleton associates with rafts, and that many of the structural and functional properties of rafts require an intact actin cytoskeleton. We discuss the mechanisms of the actin-dependent raft organization, and the properties of the actin cytoskeleton in regulating raft-associated signaling events. We end with a discussion of membrane rafts and the actin cytoskeleton in T cell activation, which function synergistically to initiate the adaptive immune response.

Mechanisms for the formation of membranous nanostructures in cell-to-cell communication

Cells interact by exchanging material and information. Two methods of cell-to-cell communication are by means of microvesicles and by means of nanotubes. Both microvesicles and nanotubes derive from the cell membrane and are able to transport the contents of the inner solution. In this review, we describe two physical mechanisms involved in the formation of microvesicles and nanotubes: curvature-mediated lateral redistribution of membrane components with the formation of membrane nanodomains; and plasmamediated attractive forces between membranes. These mechanisms are clinically relevant since they can be affected by drugs. In particular, the underlying mechanism of heparin's role as an anticoagulant and tumor suppressor is the suppression of microvesicluation due to plasma-mediated attractive interaction between membranes.