Lipid rafts mediate internalization of beta1-integrin in migrating intestinal epithelial cells

Altered endocytosis in cellular senescence

Cellular senescence occurs in response to diverse stresses (e.g., telomere shortening, DNA damage, oxidative stress, oncogene activation). A growing body of evidence indicates that alterations in multiple components of endocytic pathways contribute to cellular senescence. Clathrin-mediated endocytosis (CME) and caveolae-mediated endocytosis (CavME) represent major types of endocytosis that are implicated in senescence. More recent research has also identified a chromatin modifier and tumor suppressor that contributes to the induction of senescence via altered endocytosis. Here, molecular regulators of aberrant endocytosis-induced senescence are reviewed and discussed in the context of their capacity to serve as senescence-inducing stressors or modifiers.

Clathrin-independent endocytosis, retrograde trafficking, and cell polarity

Several mechanisms allow for cargo internalization into cells within membrane-bound endocytic carriers. How these internalization processes couple to specific pathways of intracellular distribution remains poorly explored. Here, we review uptake reactions that are independent of the conventional clathrin machinery. We discuss how these link to retrograde trafficking from endosomes to the Golgi apparatus and exemplify biological situations in which the polarized secretion capacity of the Golgi apparatus allows for retrograde cargoes to be delivered to specialized areas of the plasma membrane, such as the leading edge of migratory cells or the immunological synapse of immune cells. We also address the evidence that allows to position apicobasal polarity of epithelial cells in this context. The underlying theme is thereby the functional coupling between specific types of endocytosis to intracellular retrograde trafficking for protein cargoes that need to be localized in a highly polarized and dynamic manner to plasmalemmal subdomains.

Roles of Membrane Domains in Integrin-Mediated Cell Adhesion

The composition and organization of the plasma membrane play important functional and regulatory roles in integrin signaling, which direct many physiological and pathological processes, such as development, wound healing, immunity, thrombosis, and cancer metastasis. Membranes are comprised of regions that are thick or thin owing to spontaneous partitioning of long-chain saturated lipids from short-chain polyunsaturated lipids into domains defined as ordered and liquid-disorder domains, respectively. Liquid-ordered domains are typically 100 nm in diameter and sometimes referred to as lipid rafts. We posit that integrin β senses membrane thickness and that mechanical force on the membrane regulates integrin activation through membrane thinning. This review examines what we know about the nature and mechanism of the interaction of integrins with the plasma membrane and its effects on regulating integrins and its binding partners.

Lipid Rafts in Exosome Biogenesis

Exosomes (secreted extracellular vesicles formed in the intracellular vesicular transport system) play a crucial role in distant cell-cell communication. Exosomes transfer active forms of various biomolecules; the molecular composition of the exosomal cargo is a result of targeted selection and depends on the type of producer cells. The mechanisms underlying exosome formation and cargo selection are poorly understood. It is believed that there are several pathways for exosome biogenesis, although the questions about their independence and simultaneous coexistence in the cell still remain open. The least studied topic is the recently discovered mechanism of exosome formation associated with lipid rafts, or membrane lipid microdomains. Here, we present modern concepts and basic hypotheses on the mechanisms of exosome biogenesis and secretion and summarize current data on the involvement of lipid rafts and their constituent molecules in these processes. Special attention is paid to the analysis of possible role in the exosome formation of raft-forming proteins of the SPFH family, components of planar rafts, and caveolin, the main component of caveolae.

Protein kinase A activity is regulated by actomyosin contractility during cell migration and is required for durotaxis

Dynamic subcellular regulation of protein kinase A (PKA) activity is important for the motile behavior of many cell types, yet the mechanisms governing PKA activity during cell migration remain largely unknown. The motility of SKOV-3 epithelial ovarian cancer (EOC) cells has been shown to be dependent both on localized PKA activity and, more recently, on mechanical reciprocity between cellular tension and extracellular matrix rigidity. Here, we investigated the possibility that PKA is regulated by mechanical signaling during migration. We find that localized PKA activity in migrating cells rapidly decreases upon inhibition of actomyosin contractility (specifically, of myosin ATPase, Rho kinase, or myosin light-chain kinase activity). Moreover, PKA activity is spatially and temporally correlated with cellular traction forces in migrating cells. Additionally, PKA is rapidly and locally activated by mechanical stretch in an actomyosin contractility-dependent manner. Finally, inhibition of PKA activity inhibits mechanically guided migration, also known as durotaxis. These observations establish PKA as a locally regulated effector of cellular mechanotransduction and as a regulator of mechanically guided cell migration.

Lipid rafts regulate the lamellipodia formation of melanoma A375 cells via actin cytoskeleton-mediated recruitment of β1 and β3 integrin

Lipid rafts, distinct liquid-ordered plasma membrane microdomains, have been shown to regulate tumor cell migration by internalizing and recycling cell-surface proteins. The present study reports that lipid rafts are a prerequisite for lamellipodia formation, which is the first step in the processes of tumor cell migration. The results from the wound-healing assay and immunostaining indicated that lipid rafts were asymmetrically distributed to the leading edge of migrating melanoma A375 cells during lamellipodia formation. When the integrity of lipids rafts was disrupted, lamellipodia formation was inhibited. The investigation of possible molecular mechanisms indicated that lipid rafts recruited β1 and β3 integrins, two important adhesion proteins for cell migration, to the lamellipodia. However, the different distribution characteristics of β1 and β3 integrins implied disparate functions in lamellipodia formation. Further immunostaining experiments showed that the actin cytoskeleton was responsible for lipid raft-mediated β1 and β3 integrin distribution in the lamellipodia. Together, these findings provide novel insights into the regulation of lipid rafts in lamellipodia formation, and suggest that lipid rafts may be novel and attractive targets for cancer therapy.

Lipids in the cell: organisation regulates function

Lipids are fundamental building blocks of all cells and play important roles in the pathogenesis of different diseases, including inflammation, autoimmune disease, cancer, and neurodegeneration. The lipid composition of different organelles can vary substantially from cell to cell, but increasing evidence demonstrates that lipids become organised specifically in each compartment, and this organisation is essential for regulating cell function. For example, lipid microdomains in the plasma membrane, known as lipid rafts, are platforms for concentrating protein receptors and can influence intra-cellular signalling. Lipid organisation is tightly regulated and can be observed across different model organisms, including bacteria, yeast, Drosophila, and Caenorhabditis elegans, suggesting that lipid organisation is evolutionarily conserved. In this review, we summarise the importance and function of specific lipid domains in main cellular organelles and discuss recent advances that investigate how these specific and highly regulated structures contribute to diverse biological processes.

Exploring the mechanistic insights of Cas scaffolding protein family member 4 with protein tyrosine kinase 2 in Alzheimer's disease by evaluating protein interactions through molecular docking and dynamic simulations

Cas scaffolding protein family member 4 and protein tyrosine kinase 2 are signaling proteins, which are involved in neuritic plaques burden, neurofibrillary tangles, and disruption of synaptic connections in Alzheimer's disease. In the current study, a computational approach was employed to explore the active binding sites of Cas scaffolding protein family member 4 and protein tyrosine kinase 2 proteins and their significant role in the activation of downstream signaling pathways. Sequential and structural analyses were performed on Cas scaffolding protein family member 4 and protein tyrosine kinase 2 to identify their core active binding sites. Molecular docking servers were used to predict the common interacting residues in both Cas scaffolding protein family member 4 and protein tyrosine kinase 2 and their involvement in Alzheimer's disease-mediated pathways. Furthermore, the results from molecular dynamic simulation experiment show the stability of targeted proteins. In addition, the generated root mean square deviations and fluctuations, solvent-accessible surface area, and gyration graphs also depict their backbone stability and compactness, respectively. A better understanding of CAS and their interconnected protein signaling cascade may help provide a treatment for Alzheimer's disease. Further, Cas scaffolding protein family member 4 could be used as a novel target for the treatment of Alzheimer's disease by inhibiting the protein tyrosine kinase 2 pathway.

Influence of cholesterol/caveolin-1/caveolae homeostasis on membrane properties and substrate adhesion characteristics of adult human mesenchymal stem cells

BACKGROUND

Adult mesenchymal stem cells (MSCs) are an important resource for tissue growth, repair, and regeneration. To utilize MSCs more effectively, a clear understanding of how they react to environmental cues is essential. Currently, relatively little is known about how the composition of the plasma membranes affects stem cell phenotype and properties. The presence of lipid molecules, including cholesterol in particular, in the plasma membrane plays a crucial role in regulating a variety of physiological processes in cells. In this study, we examined the effects of perturbations in cholesterol/caveolin-1 (CAV-1)/caveolae homeostasis on the membrane properties and adhesive characteristics of MSCs. Findings from this study will contribute to the understanding of how cholesterol/CAV-1/caveolae regulates aspects of the cell membrane important to cell adhesion, substrate sensing, and microenvironment interaction.

METHODS

We generated five experimental MSC groups: 1) untreated MSCs; 2) cholesterol-depleted MSCs; 3) cholesterol-supplemented MSCs; 4) MSCs transfected with control, nonspecific small interfering (si)RNA; and 5) MSCs transfected with CAV-1 siRNA. Each cell group was analyzed for perturbation of cholesterol status and CAV-1 expression by performing Amplex Red cholesterol assay, filipin fluorescence staining, and real-time polymerase chain reaction (PCR). The membrane fluidity in the five experimental cell groups were measured using pyrene fluorescence probe staining followed by FACS analysis. Cell adhesion to collagen and fibronectin as well as cell surface integrin expression were examined.

RESULTS

Cholesterol supplementation to MSCs increased membrane cholesterol, and resulted in decreased membrane fluidity and localization of elevated numbers of caveolae and CAV-1 to the cell membrane. These cells showed increased expression of α1, α4, and β1 integrins, and exhibited higher adhesion rates to fibronectin and collagen. Conversely, knockdown of CAV-1 expression or cholesterol depletion on MSCs caused a parallel decrease in caveolae content and an increase in membrane fluidity due to decreased delivery of cholesterol to the cell membrane. Cells with depleted CAV-1 expression showed decreased cell surface integrin expression and slower adhesion to different substrates.

CONCLUSIONS

Our results demonstrate that perturbations in cholesterol/CAV-1 levels significantly affect the membrane properties of MSCs. These findings suggest that modification of membrane cholesterol and/or CAV-1 and caveolae may be used to manipulate the biological activities of MSCs.

Effects of host cell sterol composition upon internalization of Yersinia pseudotuberculosis and clustered β1 integrin

Yersinia pseudotuberculosis is a foodborne pathogenic bacterium that causes acute gastrointestinal illness, but its mechanisms of infection are incompletely described. We examined how host cell sterol composition affected Y. pseudotuberculosis uptake. To do this, we depleted or substituted cholesterol in human MDA-MB-231 epithelial cells with various alternative sterols. Decreasing host cell cholesterol significantly reduced pathogen internalization. When host cell cholesterol was substituted with various sterols, only desmosterol and 7-dehydrocholesterol supported internalization. This specificity was not due to sterol dependence of bacterial attachment to host cells, which was similar with all sterols studied. Because a key step in Y. pseudotuberculosis internalization is interaction of the bacterial adhesins invasin and YadA with host cell β1 integrin, we compared the sterol dependence of wildtype Y. pseudotuberculosis internalization with that of Δinv, ΔyadA, and ΔinvΔyadA mutant strains. YadA deletion decreased bacterial adherence to host cells, whereas invasin deletion had no effect. Nevertheless, host cell sterol substitution had a similar effect on internalization of these bacterial deletion strains as on the wildtype bacteria. The ΔinvΔyadA double mutant adhered least to cells and so was not significantly internalized. The sterol structure dependence of Y. pseudotuberculosis internalization differed from that of endocytosis, as monitored using antibody-clustered β1 integrin and previous studies on other proteins, which had a more permissive sterol dependence. This study suggests that agents could be designed to interfere with internalization of Yersinia without disturbing endocytosis.

The distribution of vinculin to lipid rafts plays an important role in sensing stiffness of extracellular matrix

Extracellular matrix (ECM) stiffness regulates cell differentiation, survival, and migration. Our previous study has shown that the interaction of the focal adhesion protein vinculin with vinexin α plays a critical role in sensing ECM stiffness and regulating stiffness-dependent cell migration. However, the mechanism how vinculin-vinexin α interaction affects stiffness-dependent cell migration is unclear. Lipid rafts are membrane microdomains that are known to affect ECM-induced signals and cell behaviors. Here, we show that vinculin and vinexin α can localize to lipid rafts. Cell-ECM adhesion, intracellular tension, and a rigid ECM promote vinculin distribution to lipid rafts. The disruption of lipid rafts with Methyl-β-cyclodextrin impaired the ECM stiffness-mediated regulation of vinculin behavior and rapid cell migration on rigid ECM. These results indicate that lipid rafts play an important role in ECM-stiffness regulation of cell migration via vinculin.

Nucleoside-Diphosphate-Kinase of P. gingivalis is Secreted from Epithelial Cells In the Absence of a Leader Sequence Through a Pannexin-1 Interactome

Nucleoside-diphosphate-kinases (NDKs) are leaderless, multifunctional enzymes. The mode(s) of NDK secretion is currently undefined, while extracellular translocation of bacterial NDKs is critical for avoidance of host pathogen clearance by opportunistic pathogens such as Porphyromonas gingivalis. P. gingivalis-NDK during infection inhibits extracellular-ATP (eATP)/P2X₇-receptor mediated cell death in gingival epithelial cells (GECs) via eATP hydrolysis. Furthermore, depletion of pannexin-1-hemichannel (PNX1) coupled with P2X₇-receptor blocks the infection-induced eATP release in GECs, and P. gingivalis-NDK impacts this pathway. Ultrastructural and confocal microscopy of P. gingivalis-co-cultured GECs or green-fluorescent-protein (GFP)-P. gingivalis-NDK transfected GECs revealed a perinuclear/cytoplasmic localization of NDK. eATP stimulation induced NDK recruitment to the cell periphery. Depletion of PNX1 by siRNA or inhibition by probenecid resulted in significant blocking of extracellular NDK activity and secretion using ATPase and ELISA assays. Co-immunoprecipitation-coupled Mass-spectrometry method revealed association of P. gingivalis-NDK to the myosin-9 motor molecule. Interestingly, inhibition of myosin-9, actin, and lipid-rafts, shown to be involved in PNX1-hemichannel function, resulted in marked intracellular accumulation of NDK and decreased NDK secretion from infected GECs. These results elucidate for the first time PNX1-hemichannels as potentially main extracellular translocation pathway for NDKs from an intracellular pathogen, suggesting that PNX1-hemichannels may represent a therapeutic target for chronic opportunistic infections.

Chloride channel-3 promotes tumor metastasis by regulating membrane ruffling and is associated with poor survival

The chloride channel-3 (ClC-3) protein is known to be a component of Cl⁻ channels involved in cell volume regulation or acidification of intracellular vesicles. Here, we report that ClC-3 was highly expressed in the cytoplasm of metastatic carcinomatous cells and accelerated cell migration in vitro and tumor metastasis in vivo. High-grade expression of cytoplasmic ClC-3 predicted poor survival in cancer patients. We found that independent of its volume-activated Cl⁻ channel properties, ClC-3 was able to promote cell membrane ruffling, required for tumor metastasis. ClC-3 mediated membrane ruffling by regulating keratin 18 phosphorylation to control β1 Integrin recycling. Therefore, cytoplasmic ClC-3 plays an active and key role in tumor metastasis and may be a valuable prognostic biomarker and a therapeutic target to prevent tumor spread.

Probiotic-derived polyphosphate improves the intestinal barrier function through the caveolin-dependent endocytic pathway

Probiotics exhibit beneficial functions for host homeostasis maintenance. We herein investigated the mechanism by which Lactobacillus brevis-derived poly P exhibited a beneficial function. Immunostaining indicated that poly P was captured in the plasma membrane via integrin β1 in Caco2/bbe cells. The uptake of poly P was reduced by the inhibition of integrin β1 as well as caveolin-1, a major component of lipid rafts. The function of poly P, including the induction of HSP27 and enhancement of the intestinal barrier function, was suppressed by the inhibition of caveolin-1, illustrating that the function of poly P was mediated by the endocytic pathway. High-throughput sequencing revealed that poly P induced tumor necrosis factor alpha-induced protein 3, which contributes to cytoprotection, including upregulation of the intestinal barrier function. The present study demonstrates a novel host-probiotic interaction through the uptake of bacterial substance into host cells, which is distinct from pattern recognition receptor pathways.

Plasma membrane mechanisms in a preclinical rat model of chronic pain

We have recently shown that the prolongation of prostaglandin E2 hyperalgesia in a preclinical model of chronic pain-hyperalgesic priming-is mediated by release of cyclic adenosine monophosphate from isolectin B4-positive nociceptors and its metabolism by ectonucleotidases to produce adenosine. The adenosine, in turn, acts in an autocrine mechanism at an A1 adenosine receptor whose downstream signaling mechanisms in the nociceptor are altered to produce nociceptor sensitization. We previously showed that antisense against an extracellular matrix molecule, versican, which defines the population of nociceptors involved in hyperalgesic priming, eliminated the prolongation of prostaglandin E2 hyperalgesia. To further evaluate the mechanisms at the interface between the extracellular matrix and the nociceptor's plasma membrane involved in hyperalgesia prolongation, we interrupted a plasma membrane molecule involved in versican signaling, integrin β1, with an antisense oligodeoxynucleotide. Integrin β1 antisense eliminated mechanical hyperalgesia induced by an adenosine A1 receptor agonist, cyclopentyladenosine, in the primed rat. We also disrupted a molecular complex of signaling molecules that contains integrin β1, lipid rafts, with methyl-β-cyclodextrin, which attenuated the prolongation without affecting the acute phase of prostaglandin E2 hyperalgesia, while having no effect on cyclopentyladenosine hyperalgesia. Our findings help to define the plasma membrane mechanisms involved in a preclinical model of chronic pain.

PERSPECTIVE

The present study contributes to a further understanding of mechanisms involved in the organization of messengers at the plasma membrane that participate in the transition from acute to chronic pain.

Invasion of epithelial cells by Campylobacter jejuni is independent of caveolae

Caveolae are 25–100 nm flask-like membrane structures enriched in cholesterol and glycosphingolipids. Researchers have proposed that Campylobacter jejuni require caveolae for cell invasion based on the finding that treatment of cells with the cholesterol-depleting compounds filipin III or methyl-β-cyclodextrin (MβCD) block bacterial internalization in a dose-dependent manner. The purpose of this study was to determine the role of caveolae and caveolin-1, a principal component of caveolae, in C. jejuni internalization. Consistent with previous work, we found that the treatment of HeLa cells with MβCD inhibited C. jejuni internalization. However, we also found that the treatment of HeLa cells with caveolin-1 siRNA, which resulted in greater than a 90% knockdown in caveolin-1 protein levels, had no effect on C. jejuni internalization. Based on this observation we performed a series of experiments that demonstrate that MβCD acts broadly, disrupting host cell lipid rafts and C. jejuni-induced cell signaling. More specifically, we found that MβCD inhibits the cellular events necessary for C. jejuni internalization, including membrane ruffling and Rac1 GTPase activation. We also demonstrate that MβCD disrupted the association of the β₁ integrin and EGF receptor, which are required for the maximal invasion of epithelial cells. In agreement with these findings, C. jejuni were able to invade human Caco-2 cells, which are devoid of caveolae, at a level equal to that of HeLa cells. Taken together, the results of our study demonstrate that C. jejuni internalization occurs in a caveolae-independent manner.

Identification of nucleolin as a lipid-raft-dependent β1-integrin-interacting protein in A375 cell migration

Lipid rafts are related to cell surface receptor function. Integrin is a major surface receptor protein in cell adhesion and migration on the extracellular matrix (ECM). Here, we showed that lipid rafts played a critical role in human melanoma A375 cell spreading and migration on fibronectin; an important component of the ECM that interacts with β1 integrin. We found that the disruption of lipid rafts did not markedly inhibit the expression and activation of β1 integrin. By coimmunoprecipitation and mass spectrometry, we investigated the influence of lipid rafts on the β1 integrin complex and identified nucleolin as a potential lipid-raft-dependent β1-integrin-interacting protein. Upon confirmation of the interaction between β1 integrin and nucleolin, further studies revealed that nucleolin colocalized with β1 integrin in lipid rafts and raft disruption interrupted their association. In addition, knockdown of nucleolin markedly attenuated A375 cell spreading and migration on fibronectin. Taken together, we demonstrated that nucleolin is a critical lipid-raft-dependent β1-integrin-interacting protein in A375 cell spreading and migration on fibronectin.

α-Lipoic acid suppresses migration and invasion via downregulation of cell surface β1-integrin expression in bladder cancer cells

Our previous study showed α-lipoic acid (LA) downregulated cell surface β1-integrin expression of v-H-ras-transformed derivative of rat fibroblast with amelioration of their malignant phenotype. Here, we evaluated the ameliorating effect of LA on the malignant characters in H-ras-transformed bladder cancer cells. H-ras mutated bladder cancer line, T24 cells were incubated with LA to evaluate the inhibitory effect on proliferation, migration, invasion and β1-integrin expression. Fluorescence staining of F-actin and western blotting analyses of the related signaling pathways were also performed. LA inhibited the proliferation of T24 cells. Cell adhesion to collagen IV and fibronectin was strikingly inhibited by LA treatment accompanied by downregulation of cell surface but not whole cell β1-integrin expression. LA clearly inhibited cell migration and invasion of T24 cells, which were mimicked by extracellular signal-regulated kinase (ERK) and Akt pathway inhibition. Actually, LA significantly downregulated the phosphorylated ERK and Akt levels. Moreover, LA downregulated phosphorylated focal adhesion kinase level with disappearance of stress fiber formation. Finally, although LA induced the internalization of cell surface β1-integrin, disruption of the raft did not affect the action of LA. Taken together, LA is a promising agent to improve malignant character of bladder cancer cells through regulation of cellular β1-integrin localization.

A conserved C-terminal sequence of high-risk cutaneous beta-human papillomavirus E6 proteins alters localization and signalling of β1-integrin to promote cell migration

Beta-human papillomaviruses (β-HPV) infect cutaneous epithelia, and accumulating evidence suggests that the virus may act as a co-factor with UV-induced DNA damage in the development and progression of non-melanoma skin cancer, although the molecular mechanisms involved are poorly understood. The E6 protein of cutaneous β-HPV types encodes functions consistent with a role in tumorigenesis, and E6 expression can result in papilloma formation in transgenic animals. The E6 proteins of high-risk α-HPV types, which are associated with the development of anogenital cancers, have a conserved 4 aa motif at their extreme C terminus that binds to specific PDZ domain-containing proteins to promote cell invasion. Likewise, the high-risk β-HPVs HPV5 and HPV8 E6 proteins also share a conserved C-terminal motif, but this is markedly different from that of α-HPV types, implying functional differences. Using binding and functional studies, we have shown that β-HPV E6 proteins target β1-integrin using this C-terminal motif. E6 expression reduced membrane localization of β1-integrin, but increased overall levels of β1-integrin protein and its downstream effector focal adhesion kinase in human keratinocytes. Altered β1-integrin localization due to E6 expression was associated with actin cytoskeleton rearrangement and increased cell migration that was abolished by point mutations in the C-terminal motif of E6. We concluded that modulation of β1-integrin signalling by E6 proteins may contribute towards the pathogenicity of these β-HPV types.

Calpain-dependent cytoskeletal rearrangement exploited for anthrax toxin endocytosis

The protective antigen component of Bacillus anthracis toxins can interact with at least three distinct proteins on the host cell surface, capillary morphogenesis gene 2 (CMG2), tumor endothelial marker 8, and β1-integrin, and, with the assistance of other host proteins, enters targeted cells by receptor-mediated endocytosis. Using an antisense-based phenotypic screen, we discovered the role of calpains in this process. We show that functions of a ubiquitous Ca(2+)-dependent cysteine protease, calpain-2, and of the calpain substrate talin-1 are exploited for association of anthrax toxin and its principal receptor, CMG2, with higher-order actin filaments and consequently for toxin entry into host cells. Down-regulated expression of calpain-2 or talin-1, or pharmacological interference with calpain action, did not affect toxin binding but reduced endocytosis and increased the survival of cells exposed to anthrax lethal toxin. Adventitious expression of wild-type talin-1 promoted toxin endocytosis and lethality, whereas expression of a talin-1 mutant (L432G) that is insensitive to calpain cleavage did not. Disruption of talin-1, which links integrin-containing focal adhesion complexes to the actin cytoskeleton, facilitated association of toxin bound to its principal cell-surface receptor, CMG2, with higher-order actin filaments undergoing dynamic disassembly and reassembly during endocytosis. Our results reveal a mechanism by which a bacterial toxin uses constitutively occurring calpain-mediated cytoskeletal rearrangement for internalization.

Lipid rafts control human melanoma cell migration by regulating focal adhesion disassembly

Tumor cell migration is a crucial step in the metastatic cascade, and interruption of this step is considered to be logically effective in preventing tumor metastasis. Lipid rafts, distinct liquid ordered plasma membrane microdomains, have been shown to influence cancer cell migration, but the underlying mechanisms are still not well understood. Here, we report that lipid rafts regulate the dynamics of actin cytoskeleton and focal adhesion in human melanoma cell migration. Disrupting the integrity of lipid rafts with methyl-β cyclodextrin enhances actin stress fiber formation and inhibits focal adhesion disassembly, accompanied with alterations in cell morphology. Furthermore, actin cytoskeleton, rather than microtubules, mediates the lipid raft-dependent focal adhesion disassembly by regulating the dephosphorylation of focal adhesion proteins and the internalization of β3 integrin. We also show that Src-RhoA-Rho kinase signaling pathway is responsible for lipid raft disruption-induced stress fiber formation. Taken together, these observations provide a new mechanism to further explain how lipid rafts regulate the migration of melanoma cell and suggest that lipid rafts may be novel and attractive targets for cancer therapy.

Lipid raft regulates the initial spreading of melanoma A375 cells by modulating β1 integrin clustering

Cell adhesion and spreading require integrins-mediated cell-extracellular matrix interaction. Integrins function through binding to extracellular matrix and subsequent clustering to initiate focal adhesion formation and actin cytoskeleton rearrangement. Lipid raft, a liquid ordered plasma membrane microdomain, has been reported to play major roles in membrane motility by regulating cell surface receptor function. Here, we identified that lipid raft integrity was required for β1 integrin-mediated initial spreading of melanoma A375 cells on fibronectin. We found that lipid raft disruption with methyl-β-cyclodextrin led to the inability of focal adhesion formation and actin cytoskeleton rearrangement by preventing β1 integrin clustering. Furthermore, we explored the possible mechanism by which lipid raft regulates β1 integrin clustering and demonstrated that intact lipid raft could recruit and modify some adaptor proteins, such as talin, α-actinin, vinculin, paxillin and FAK. Lipid raft could regulate the location of these proteins in lipid raft fractions and facilitate their binding to β1 integrin, which may be crucial for β1 integrin clustering. We also showed that lipid raft disruption impaired A375 cell migration in both transwell and wound healing models. Together, these findings provide a new insight for the relationship between lipid raft and the regulation of integrins.

Endocytosis of gene delivery vectors: from clathrin-dependent to lipid raft-mediated endocytosis

The ideal nonviral vector delivers its nucleic acid cargo to a specific intracellular target. Vectors enter cells mainly through endocytosis and are distributed to various intracellular organelles. Recent advances in microscopy, lipidomics, and proteomics confirm that the cell membrane is composed of clusters of lipids, organized in the form of lipid raft domains, together with non-raft domains that comprise a generally disordered lipid milieu. The binding of a nonviral vector to either region can determine the pathway for its endocytic uptake and subsequent intracellular itinerary. Given this model of the cell membrane structure, endocytic pathways should be reclassified in relation to lipid rafts. In this review, we attempt to assess the currently recognized endocytic pathways in mammalian cells. The endocytic pathways are classified in relation to the membrane regions that make up the primary endocytic vesicles. This review covers the well-recognized clathrin-mediated endocytosis (CME), phagocytosis, and macropinocytosis in addition to the less addressed pathways that take place in lipid rafts. These include caveolae-mediated, flotillin-dependent, GTPase regulator associated with focal adhesion kinase-1 (GRAF1)-dependent, adenosine diphosphate-ribosylation factor 6 (Arf6)-dependent, and RhoA-dependent endocytic pathways. We summarize the regulators associated with each uptake pathway and methods for interfering with these regulators are discussed. The fate of endocytic vesicles resulting from each endocytic uptake pathway is highlighted.

Role of caveolin-1 in fibrotic diseases

Fibrosis underlies the pathogenesis of numerous diseases and leads to severe damage of vital body organs and, frequently, to death. Better understanding of the mechanisms resulting in fibrosis is essential for developing appropriate treatment solutions and is therefore of upmost importance. Recent evidence suggests a significant antifibrotic potential of an integral membrane protein, caveolin-1. While caveolin-1 has been widely studied for its role in the regulation of cell signaling and endocytosis, its possible implication in fibrosis remains largely unclear. In this review we survey involvement of caveolin-1 in various cellular processes and highlight different aspects of its antifibrotic activity. We hypothesize that caveolin-1 conveys a homeostatic function in the process of fibrosis by (a) regulating TGF-β1 and its downstream signaling; (b) regulating critical cellular processes involved in tissue repair, such as migration, adhesion and cellular response to mechanical stress; and (c) antagonizing profibrotic processes, such as proliferation. Finally, we consider this homeostatic function of caveolin-1 as a possible novel approach in treatment of fibroproliferative diseases.

Annexin A2 regulates β1 integrin internalization and intestinal epithelial cell migration

The gastrointestinal epithelium functions as an important barrier that separates luminal contents from the underlying tissue compartment and is vital in maintaining mucosal homeostasis. Mucosal wounds in inflammatory disorders compromise the critical epithelial barrier. In response to injury, intestinal epithelial cells (IECs) rapidly migrate to reseal wounds. We have previously observed that a membrane-associated, actin binding protein, annexin A2 (AnxA2), is up-regulated in migrating IECs and plays an important role in promoting wound closure. To identify the mechanisms by which AnxA2 promotes IEC movement and wound closure, we used a loss of function approach. AnxA2-specific shRNA was utilized to generate IECs with stable down-regulation of AnxA2. Loss of AnxA2 inhibited IEC migration while promoting enhanced cell-matrix adhesion. These functional effects were associated with increased levels of β1 integrin protein, which is reported to play an important role in mediating the cell-matrix adhesive properties of epithelial cells. Because cell migration requires dynamic turnover of integrin-based adhesions, we tested whether AnxA2 modulates internalization of cell surface β1 integrin required for forward cell movement. Indeed, pulse-chase biotinylation experiments in IECs lacking AnxA2 demonstrated a significant increase in cell surface β1 integrin that was accompanied by decreased β1 integrin internalization and degradation. These findings support an important role of AnxA2 in controlling dynamics of β1 integrin at the cell surface that in turn is required for the active turnover of cell-matrix associations, cell migration, and wound closure.

Agonist-induced down-regulation of endogenous protein kinase c α through an endolysosomal mechanism

Protein kinase C (PKC) isozymes undergo down-regulation upon sustained stimulation. Previous studies have pointed to the existence of both proteasome-dependent and -independent pathways of PKCα processing. Here we demonstrate that these down-regulation pathways are engaged in different subcellular compartments; proteasomal degradation occurs mainly at the plasma membrane, whereas non-proteasomal processing occurs in the perinuclear region. Using cholesterol depletion, pharmacological inhibitors, RNA interference, and dominant-negative mutants, we define the mechanisms involved in perinuclear accumulation of PKCα and identify the non-proteasomal mechanism mediating its degradation. We show that intracellular accumulation of PKCα involves at least two clathrin-independent, cholesterol/lipid raft-mediated pathways that do not require ubiquitination of the protein; one is dynamin-dependent and likely involves caveolae, whereas the other is dynamin- and small GTPase-independent. Internalized PKCα traffics through endosomes and is delivered to the lysosome for degradation. Supportive evidence includes (a) detection of the enzyme in EEA1-positive early endosomes, Rab7-positive late endosomes/multivesicular bodies, and LAMP1-positive lysosomes and (b) inhibition of its down-regulation by lysosome-disrupting agents and leupeptin. Only limited dephosphorylation of PKCα occurs during trafficking, with fully mature enzyme being the main target for lysosomal degradation. These studies define a novel and widespread mechanism of desensitization of PKCα signaling that involves endocytic trafficking and lysosome-mediated degradation of the mature, fully phosphorylated protein.

Lysophosphatidic acid promotes cell migration through STIM1- and Orai1-mediated Ca2+(i) mobilization and NFAT2 activation

Lysophosphatidic acid (LPA) enhances cell migration and promotes wound healing in vivo, but the intracellular signaling pathways regulating these processes remain incompletely understood. Here we investigated the involvement of agonist-induced Ca²⁺ entry and STIM1 and Orai1 proteins in regulating nuclear factor of activated T cell (NFAT) signaling and LPA-induced keratinocyte cell motility. As monitored by Fluo-4 imaging, stimulation with 10 μℳ LPA in 60 μℳ Ca²⁺_o evoked Ca²⁺_i transients owing to store release, whereas addition of LPA in physiological 1.2 mℳ Ca²⁺_o triggered store release coupled to extracellular Ca²⁺ entry. Store-operated Ca²⁺ entry (SOCE) was blocked by the SOCE inhibitor diethylstilbestrol (DES), STIM1 silencing using RNA interference (RNAi), and expression of dominant/negative Orai1^R91W. LPA induced significant NFAT activation as monitored by nuclear translocation of green fluorescent protein-tagged NFAT2 and a luciferase reporter assay, which was impaired by DES, expression of Orai1^R91W, and inhibition of calcineurin using cyclosporin A (CsA). By using chemotactic migration assays, LPA-induced cell motility was significantly impaired by STIM1, CsA, and NFAT2 knockdown using RNAi. These data indicate that in conditions relevant to epidermal wound healing, LPA induces SOCE and NFAT activation through Orai1 channels and promotes cell migration through a calcineurin/NFAT2-dependent pathway.

Aquaporin 2 promotes cell migration and epithelial morphogenesis

The aquaporin 2 (AQP2) water channel, expressed in kidney collecting ducts, contributes critically to water homeostasis in mammals. Animals lacking or having significantly reduced levels of AQP2, however, have not only urinary concentrating abnormalities but also renal tubular defects that lead to neonatal mortality from renal failure. Here, we show that AQP2 is not only a water channel but also an integrin-binding membrane protein that promotes cell migration and epithelial morphogenesis. AQP2 expression modulates the trafficking and internalization of integrin β1, facilitating its turnover at focal adhesions. In vitro, disturbing the interaction between AQP2 and integrin β1 by mutating the RGD motif led to reduced endocytosis, retention of integrin β1 at the cell surface, and defective cell migration and tubulogenesis. Similarly, in vivo, AQP2-null mice exhibited significant retention of integrin β1 at the basolateral membrane and had tubular abnormalities. In summary, these data suggest that the water channel AQP2 interacts with integrins to promote renal epithelial cell migration, contributing to the structural and functional integrity of the mammalian kidney.

Recent advances in small bowel diseases: Part I

As is the case in all parts of gastroenterology and hepatology, there have been many advances in our knowledge and understanding of small intestinal diseases. Over 1000 publications were reviewed for 2008 and 2009, and the important advances in basic science as well as clinical applications were considered. In Part I of this Editorial Review, seven topics are considered: intestinal development; proliferation and repair; intestinal permeability; microbiotica, infectious diarrhea and probiotics; diarrhea; salt and water absorption; necrotizing enterocolitis; and immunology/allergy. These topics were chosen because of their importance to the practicing physician.

Dynamin 2 mutations in Charcot-Marie-Tooth neuropathy highlight the importance of clathrin-mediated endocytosis in myelination

Mutations in dynamin 2 (DNM2) lead to dominant intermediate Charcot-Marie-Tooth neuropathy type B, while a different set of DNM2 mutations cause autosomal dominant centronuclear myopathy. In this study, we aimed to elucidate the disease mechanisms in dominant intermediate Charcot-Marie-Tooth neuropathy type B and to find explanations for the tissue-specific defects that are associated with different DNM2 mutations in dominant intermediate Charcot-Marie-Tooth neuropathy type B versus autosomal dominant centronuclear myopathy. We used tissue derived from Dnm2-deficient mice to establish an appropriate peripheral nerve model and found that dominant intermediate Charcot-Marie-Tooth neuropathy type B-associated dynamin 2 mutants, but not autosomal dominant centronuclear myopathy mutants, impaired myelination. In contrast to autosomal dominant centronuclear myopathy mutants, Schwann cells and neurons from the peripheral nervous system expressing dominant intermediate Charcot-Marie-Tooth neuropathy mutants showed defects in clathrin-mediated endocytosis. We demonstrate that, as a consequence, protein surface levels are altered in Schwann cells. Furthermore, we discovered that myelination is strictly dependent on Dnm2 and clathrin-mediated endocytosis function. Thus, we propose that altered endocytosis is a major contributing factor to the disease mechanisms in dominant intermediate Charcot-Marie-Tooth neuropathy type B.

Degradation of internalized αvβ5 integrin is controlled by uPAR bound uPA: effect on β1 integrin activity and α-SMA stress fiber assembly

Myofibroblasts (Mfs) that persist in a healing wound promote extracellular matrix (ECM) accumulation and excessive tissue contraction. Increased levels of integrin αvβ5 promote the Mf phenotype and other fibrotic markers. Previously we reported that maintaining uPA (urokinase plasminogen activator) bound to its cell-surface receptor, uPAR prevented TGFβ-induced Mf differentiation. We now demonstrate that uPA/uPAR controls integrin β5 protein levels and in turn, the Mf phenotype. When cell-surface uPA was increased, integrin β5 levels were reduced (61%). In contrast, when uPA/uPAR was silenced, integrin β5 total and cell-surface levels were increased (2–4 fold). Integrin β5 accumulation resulted from a significant decrease in β5 ubiquitination leading to a decrease in the degradation rate of internalized β5. uPA-silencing also induced α-SMA stress fiber organization in cells that were seeded on collagen, increased cell area (1.7 fold), and increased integrin β1 binding to the collagen matrix, with reduced activation of β1. Elevated cell-surface integrin β5 was necessary for these changes after uPA-silencing since blocking αvβ5 function reversed these effects. Our data support a novel mechanism by which downregulation of uPA/uPAR results in increased integrin αvβ5 cell-surface protein levels that regulate the activity of β1 integrins, promoting characteristics of the persistent Mf.

Coordinated Mechanosensitivity of Membrane Rafts and Focal Adhesions

Endothelial cells sense mechanical forces of blood flow through mechanisms that involve focal adhesions (FAs). The mechanosensitive pathways that originate from FA-associated integrin activation may involve membrane rafts, small cholesterol- and sphigolipid-rich domains that are either immobilized, by virtue of their attachment to the cytoskeleton, or highly mobile in the plane of the plasma membrane. In this study, we fluorescently labeled non-mobile and mobile populations of GM1, a ganglioside associated with lipid rafts, and transfected cells with the red fluorescent protein-(RFP-) talin, an indicator of integrin activation at FAs, in order to determine the kinetics and sequential order of raft and talin mechanosensitivity. Cells were imaged under confocal microscopy during mechanical manipulation of a FA induced by a fibronectin (FN)-functionalized nanoelectrode with feedback control of position. First, FA deformation led to long range deformation of immobile rafts followed by active recoil of a subpopulation of displaced rafts. Second, initial adhesion between the FN-probe and the cell induced rapid accumulation of GM1 at the probe site with a time constant of 1.7 s. Talin accumulated approximately 20 s later with a time constant of 0.6 s. Third, a 1 μm deformation of the FA lead to immediate (0.3 s) increase in GM1 fluorescence and a later (6 s) increase in talin. Fourth, long term deformation of FAs led to continual GM1 accumulation at the probe site that was reversed upon removal of the deformation. These results demonstrate that rafts are directly mechanosensitive and that raft mobility may enable the earliest events related to FA mechanosensing and reinforcement upon force application.

The role of cholesterol in UV light B-induced apoptosis

Modification of major lipid raft components, such as cholesterol and ceramide, plays a role in regulation of programmed cell death under various stimuli. However, the relationship between cholesterol level modification and the activation of apoptotic signaling cascades upon UVB light has not been established. In this report, we demonstrate that upon UVB irradiation cholesterol levels in membrane rafts of skin cells increase, which leads to Fas-receptor (Fas) aggregation in the rafts. Utilizing a continuous velocity floatation technique, we show that Fas accumulated in the lipid rafts of human melanoma M624 cells after UVB irradiation. The subsequent events of death-inducing signaling complex formation were also detected in the lipid raft fractions. Depletion of cholesterol by methyl-β-cyclodextrin reduces Fas aggregation, while overloading increases. Disruption of lipid rafts also prevents Fas death domain-associated protein (Daxx) from dissociating from Fas in the lipid rafts, which is accompanied with a reduced apoptotic, but increased nonapoptotic death of UVB-irradiated human keratinocytes, HaCaT cells. Results indicate that cholesterol located in the plasma membrane of skin cells is required for lipid raft domain formation and activation of UVB-induced apoptosis.

Gambogic acid inhibits tumor cell adhesion by suppressing integrin β1 and membrane lipid rafts-associated integrin signaling pathway

Cell adhesion plays an important role in the steps of cancer metastasis. Regulation of cell-cell (intercellular) and cell-matrix adhesion is a promising strategy for cancer progression. Gambogic acid is a xanthone derived from the resin of the Chinese plant Garciania hanburyi, with potent anti-metastasis activity on highly metastatic cells. The aim of this study was to investigate the function and mechanism of gambogic acid on tumor adhesion. We found that gambogic acid strongly inhibited the adhesion of human cancer cells to fibronectin. This inhibition was associated with the deformation of focal adhesion complex, which was mediated by suppressing the expression of integrin β1 and integrin signaling pathway. In vitro, cell lipid rafts clustering was inhibited following treatment of gambogic acid, which induced the suppression of integrin β1 and focal adhesion complex proteins colocalization within rafts. Moreover, gambogic acid significantly decreased cellular cholesterol content, whereas cholesterol replenishment lessened the inhibitory effect of gambogic acid on cell adhesion. Real-time PCR analysis showed that gambogic acid reduced mRNA levels of hydroxymethylglutaryl-CoA reductase and sterol regulatory element binding protein-2, while increased acetyl-CoA acetyltransferase-1/2. Taken together, these results demonstrate that gambogic acid inhibits cell adhesion via suppressing integrin β1 abundance and cholesterol content as well as the membrane lipid raft-associated integrin function, which provide new evidence for the anti-cancer activity of gambogic acid.

Mechanisms of amyloid-Beta Peptide uptake by neurons: the role of lipid rafts and lipid raft-associated proteins

A hallmark pathological feature of Alzheimer's disease (AD) is the accumulation of extracellular plaques composed of the amyloid-beta (Aβ) peptide. Thus, classically experiments were designed to examine Aβ toxicities within the central nervous system (CNS) from the extracellular space. However, a significant amount of evidence now suggests that intraneuronal accumulation of Aβ is neurotoxic and may play an important role in the disease progression of AD. One of the means by which neurons accumulate intracellular Aβ is through uptake of extracellular Aβ peptides, and this process may be a potential link between Aβ generation, synaptic dysfunction, and AD pathology. Recent studies have found that neuronal internalization of Aβ involves lipid rafts and various lipid raft-associated receptor proteins. Uptake mechanisms independent of lipid rafts have also been implicated. The aim of this paper is to summarize these findings and discuss their significance in the pathogenesis of AD.

Clathrin-independent carriers form a high capacity endocytic sorting system at the leading edge of migrating cells

Although the importance of clathrin- and caveolin-independent endocytic pathways has recently emerged, key aspects of these routes remain unknown. Using quantitative ultrastructural approaches, we show that clathrin-independent carriers (CLICs) account for approximately three times the volume internalized by the clathrin-mediated endocytic pathway, forming the major pathway involved in uptake of fluid and bulk membrane in fibroblasts. Electron tomographic analysis of the 3D morphology of the earliest carriers shows that they are multidomain organelles that form a complex sorting station as they mature. Proteomic analysis provides direct links between CLICs, cellular adhesion turnover, and migration. Consistent with this, CLIC-mediated endocytosis of key cargo proteins, CD44 and Thy-1, is polarized at the leading edge of migrating fibroblasts, while transient ablation of CLICs impairs their ability to migrate. These studies provide the first quantitative ultrastructural analysis and molecular characterization of the major endocytic pathway in fibroblasts, a pathway that provides rapid membrane turnover at the leading edge of migrating cells.

Regulation of Integrin β 1 recycling to lipid rafts by Rab1a to promote cell migration

Rab1a is a member of the Rab family of small GTPases with a well characterized function in the regulation of vesicle trafficking from the endoplasmic reticulum to the Golgi apparatus and within Golgi compartments. The integrin family heterodimeric transmembrane proteins serve as major receptors for extracellular matrix proteins, which play essential roles in cell adhesion and migration. Although effects on intracellular trafficking of integrins or other key cargos by Rab1a could influence cell migration, the regulatory mechanisms linking Rab1a to cell migration are not well understood. Here, we report identification of Rab1a as a novel regulator of cell migration using an unbiased RNAi screen targeting GTPases. Inhibition of Rab1a reduced integrin-mediated cell adhesion and spreading on fibronectins, reduced integrin β1 localization to lipid rafts, and decreased recycling of integrin β1 to the plasma membrane. Analysis of Rab1a effector molecules showed that p115 mediated Rab1a regulation of integrin recycling and lipid raft localization in cell migration. Taken together, these results suggest a novel function for Rab1a in the regulation of cell migration through controlling integrin β1 recycling and localization to lipid rafts via a specific downstream effector pathway.

Keratin 8/18 modulation of protein kinase C-mediated integrin-dependent adhesion and migration of liver epithelial cells

Hepatocyte and hepatoma cell IFs are made solely of keratins 8/18 (K8/K18). Cell adhesion and migration involve integrin interactions with focal adhesion kinase (FAK) and protein kinase C (PKC). Here we report a new regulatory function for K8/K18 IFs in the PKC-mediated integrin/FAK-dependent adhesion and migration of simple epithelial cells.

Keratins are intermediate filament (IF) proteins of epithelial cells, expressed as pairs in a lineage/differentiation manner. Hepatocyte and hepatoma cell IFs are made solely of keratins 8/18 (K8/K18), the hallmark of all simple epithelia. Cell attachment/spreading (adhesion) and migration involve the formation of focal adhesions at sites of integrin interactions with extracellular matrix, actin adaptors such as talin and vinculin, and signaling molecules such as focal adhesion kinase (FAK) and member(s) of the protein kinase C (PKC) family. Here, we identify the novel PKCδ as mediator of the K8/K18 modulation of hepatoma cell adhesion and migration. We also demonstrate a K8/K18-dependent relationship between PKCδ and FAK activation through an integrin/FAK-positive feedback loop, in correlation with a reduced FAK time residency at focal adhesions. Notably, a K8/K18 loss results to a time course modulation of the receptor of activated C-kinase-1, β1-integrin, plectin, PKC, and c-Src complex formation. Although the K8/K18 modulation of hepatocyte adhesion also occurs through a PKC mediation, these differentiated epithelial cells exhibit minimal migrating ability, in link with marked differences in protein partner content and distribution. Together, these results uncover a key regulatory function for K8/K18 IFs in the PKC-mediated integrin/FAK-dependent adhesion and migration of simple epithelial cells.

Dynamin 2 and human diseases

Dynamin 2 (DNM2) mutations cause autosomal dominant centronuclear myopathy, a rare form of congenital myopathy, and intermediate and axonal forms of Charcot-Marie-Tooth disease, a peripheral neuropathy. DNM2 is a large GTPase mainly involved in membrane trafficking through its function in the formation and release of nascent vesicles from biological membranes. DNM2 participates in clathrin-dependent and clathrin-independent endocytosis and intracellular membrane trafficking (from endosomes and Golgi apparatus). Recent studies have also implicated DNM2 in exocytosis. DNM2 belongs to the machinery responsible for the formation of vesicles and regulates the cytoskeleton providing intracellular vesicle transport. In addition, DNM2 tightly interacts with and is involved in the regulation of actin and microtubule networks, independent from membrane trafficking processes. We summarize here the molecular, biochemical, and functional data on DNM2 and discuss the possible pathophysiological mechanisms via which DNM2 mutations can lead to two distinct neuromuscular disorders.

Tumor suppressor scribble regulates assembly of tight junctions in the intestinal epithelium

Formation of the epithelial barrier and apico-basal cell polarity represent two characteristics and mutually dependent features of differentiated epithelial monolayers. They are controlled by special adhesive structures, tight junctions (TJs), and polarity protein complexes that define the apical and the basolateral plasma membrane. The functional interplay between TJs and polarity complexes remains poorly understood. We investigated the role of Scribble, a basolateral polarity protein and known tumor suppressor, in regulating TJs in human intestinal epithelium. Scribble was enriched at TJs in T84 and SK-CO15 intestinal epithelial cell monolayers and sections of normal human colonic mucosa. siRNA-mediated knockdown of Scribble in SK-CO15 cells attenuated development of epithelial barrier and inhibited TJ reassembly independently of other basolateral polarity proteins Lgl-1 and Dlg-1. Scribble selectively co-imunoprecipitated with TJ protein ZO-1, and ZO-1 was important for Scribble recruitment to intercellular junctions and TJ reassembly. Lastly, Scribble was mislocalized from TJs and its expression down-regulated in interferon-gamma-treated T84 cell monolayers and inflamed human intestinal mucosa in vivo. We conclude that Scribble is an important regulator of TJ functions and plasticity in the intestinal epithelium. Down-regulation of Scribble may mediate mucosal barrier breakdown during intestinal inflammation.

The role of vesicle trafficking in epithelial cell motility

Cell motility is important for many physiological and pathological processes including organ development, wound healing, cancer metastasis and correct immune responses. In particular, epithelial wound healing is both a medically relevant topic and a common experimental model. Mechanisms underlying generation of a polarized cell and maintenance of a motile phenotype during steady-state migration are not well understood. Polarized trafficking of bulk membrane and cell adhesion molecules has been implicated in regulation of cell motility. The present review focuses on the role of different trafficking pathways in epithelial cell migration, including clathrin-mediated endocytosis, caveolar endocytosis, exocytosis of biosynthetic cargo, ‘short-loop’ and ‘long-loop’ endosomal recycling.