Activated ezrin promotes cell migration through recruitment of the GEF Dbl to lipid rafts and preferential downstream activation of Cdc42

AKAPs in lipid rafts are required for optimal antigen presentation by dendritic cells

Dendritic cell (DC) maturation and antigen presentation are regulated by activation of protein kinase A (PKA) signaling pathways, through unknown mechanisms. We have recently shown that interfering with PKA signaling through the use of anchoring inhibitor peptides hinders antigen presentation and DC maturation. These experiments provide evidence that DC maturation and antigen presentation are regulated by A-kinase anchoring proteins (AKAPs). Herein, we determine that the presence of AKAPs and PKA in lipid rafts regulates antigen presentation. Using a combination of western blotting and immuno-cytochemistry, we illustrate the presence of AKAP149, AKAP79, Ezrin and the regulatory subunits of PKA in DC lipid rafts. Incubation of DCs with the type II anchoring inhibitor, AKAP-in silico (AKAP-IS), removes Ezrin and RII from the lipid raft without disrupting raft formation. Addition of a lipid raft disruptor, methyl-β-cyclodextrin, blocks the efficacy of AKAP-IS, suggesting that the lipid raft must be intact for AKAP-IS to inhibit antigen presentation. Ezrin and AKAP79 are present in the lipid raft of stimulated KG1 cells, but Ezrin is not present in the lipid raft of unstimulated KG1 cells and AKAP79 levels are greatly diminished, suggesting that Ezrin and AKAP79 may be the key AKAPs responsible for regulating antigen presentation.

An integrative approach to the identification of Arabidopsis and rice genes involved in xylan and secondary wall development

Xylans constitute the major non-cellulosic component of plant biomass. Xylan biosynthesis is particularly pronounced in cells with secondary walls, implying that the synthesis network consists of a set of highly expressed genes in such cells. To improve the understanding of xylan biosynthesis, we performed a comparative analysis of co-expression networks between Arabidopsis and rice as reference species with different wall types. Many co-expressed genes were represented by orthologs in both species, which implies common biological features, while some gene families were only found in one of the species, and therefore likely to be related to differences in their cell walls. To predict the subcellular location of the identified proteins, we developed a new method, PFANTOM (plant protein family information-based predictor for endomembrane), which was shown to perform better for proteins in the endomembrane system than other available prediction methods. Based on the combined approach of co-expression and predicted cellular localization, we propose a model for Arabidopsis and rice xylan synthesis in the Golgi apparatus and signaling from plasma membrane to nucleus for secondary cell wall differentiation. As an experimental validation of the model, we show that an Arabidopsis mutant in the PGSIP1 gene encoding one of the Golgi localized candidate proteins has a highly decreased content of glucuronic acid in secondary cell walls and substantially reduced xylan glucuronosyltransferase activity.

LKB1 tumor suppressor protein regulates actin filament assembly through Rho and its exchange factor Dbl independently of kinase activity

BACKGROUND

Germline mutations in LKB1 result in Peutz-Jeghers Syndrome characterized by intestinal hamartomas and increased incidence of epithelial cancers. LKB1 encodes a serine/threonine kinase that plays an important role in regulating energy metabolism through the AMPK/mTOR signaling pathway. In addition, LKB1 is homologous to PAR-4, a polarity protein first described in C. elegans, while activation of LKB1 in mammalian epithelial cells induces the polarized assembly of actin filaments.

RESULTS

To explore the mechanism by which LKB1 interacts with the actin cytoskeleton, we introduced LKB1 into HeLa cells that lack endogenous LKB1. This results in activation of the small GTPase Rho and the assembly of linear actin filaments associated with focal adhesions. These effects on the actin cytoskeleton are attenuated by siRNA-mediated depletion of the guanine nucleotide exchange factor Dbl. Co-expression of the LKB1 with the adaptor protein STRAD induces actin filament puncta associated with phospho-ezrin.

CONCLUSIONS

This study reveals that LKB1 regulates the actin cytoskeleton through a Dbl/Rho pathway.

Kinase-mediated quasi-dimers of EGFR

Ligand-induced dimerization of the epidermal growth factor receptor (ErbB-1/EGFR) involves conformational changes that expose an extracellular dimerization interface. Subsequent alterations within the cytoplasmic kinase domain, which culminate in tyrosine phosphorylation, are less understood. Our study addressed this question by using two strategies: a chimeric receptor approach employed ErbB-3, whose defective kinase domain was replaced by the respective part of EGFR. The implanted full-length kinase, unlike its subdomains, conferred dimerization and catalysis. The data infer that the kinase function of EGFR is restrained by the carboxyl tail; once grafted distally to the ectopic tail of ErbB-3, the kinase domain acquires quasi-dimerization and activation. In an attempt to alternatively refold the cytoplasmic tail, our other approach employed kinase inhibitors. Biophysical measurements and covalent cross-linking analyses showed that inhibitors targeting the active conformation of EGFR, in contrast to a compound recognizing the inactive conformation, induce quasi-dimers in a manner similar to the chimeric ErbB-3 molecule. Collectively, these observations unveil kinase domain-mediated quasi-dimers, which are regulated by an autoinhibitory carboxyl tail. On the basis of these observations, we propose that quasi-dimers precede formation of ligand-induced, fully active dimers, which are stabilized by both extracellular and intracellular receptor-receptor interactions.

Differential effects of ceramide and sphingosine 1-phosphate on ERM phosphorylation: probing sphingolipid signaling at the outer plasma membrane

ERM proteins are regulated by phosphorylation of the most C-terminal threonine residue, switching them from an activated to an inactivated form. However, little is known about the control of this regulation. Previous work in our group demonstrated that secretion of acid sphingomyelinase acts upstream of ERM dephosphorylation, suggesting the involvement of sphingomyelin (SM) hydrolysis in ERM regulation. To define the role of specific lipids, we employed recombinant bacterial sphingomyelinase (bSMase) as a direct probe of SM metabolism at the plasma membrane. bSMase induced a rapid dose- and time-dependent decrease in ERM dephosphorylation. ERM dephosphorylation was driven by ceramide generation and not by sphingomyelin depletion, as shown using recombinant sphingomyelinase D. The generation of ceramide at the plasma membrane was sufficient for ERM regulation, and no intracellular SM hydrolysis was required, as was visualized using Venus-tagged lysenin probe, which specifically binds SM. Interestingly, hydrolysis of plasma membrane bSMase-induced ceramide using bacterial ceramidase caused ERM hyperphosphorylation and formation of cell surface protrusions. The effects of plasma membrane ceramide hydrolysis were due to sphingosine 1-phosphate formation, as ERM phosphorylation was blocked by an inhibitor of sphingosine kinase and induced by sphingosine 1-phosphate. Taken together, these results demonstrate a new regulatory mechanism of ERM phosphorylation by sphingolipids with opposing actions of ceramide and sphingosine 1-phosphate. The approach also defines a tool kit to probe sphingolipid signaling at the plasma membrane.

MicroRNA-205 promotes keratinocyte migration via the lipid phosphatase SHIP2

microRNA-205 (miR-205) and miR-184 coordinately regulate the lipid phosphatase SHIP2 for Akt survival signaling in keratinocytes. As the PI3K-Akt pathway has also been implicated in regulating the actin cytoskeleton and cell motility, we investigated the role that these 2 miRNAs play in keratinocyte migration. We used antagomirs (antago) to reduce the levels of miR-205 and miR-184 in primary human epidermal keratinocytes (HEKs) and corneal epithelial keratinocytes (HCEKs) as well as direct SHIP2 silencing using siRNA oligos. Treatment of HEKs and HCEKs with antago-205 increased SHIP2 levels and impaired the ability of these cells to seal linear scratch wounds compared with untreated or irrelevant-antago treatments. In contrast, AKT signaling was enhanced and wounds sealed faster in HCEKs where miR-184 was suppressed, enabling miR-205 to inhibit SHIP2. Similar increases in migration were observed following direct SHIP2 silencing in HEKs. Furthermore, down-regulation of miR-205 resulted in an increase in Rho-ROCKI activity, phosphorylation of the actin severing protein cofilin, and a corresponding diminution of filamentous actin. The connection among miR-205, RhoA-ROCKI-cofilin inactivation, and the actin cytoskeleton represents a novel post-translational mechanism for the regulation of normal human keratinocyte migration.

Fluorescence lifetime endoscopy using TCSPC for the measurement of FRET in live cells

Development of remote imaging for diagnostic purposes has progressed dramatically since endoscopy began in the 1960's. The recent advent of a clinically licensed intensity-based fluorescence micro-endoscopic instrument has offered the prospect of real-time cellular resolution imaging. However, interrogating protein-protein interactions deep inside living tissue requires precise fluorescence lifetime measurements to derive the Förster resonance energy transfer between two tagged fluorescent markers. We developed a new instrument combining remote fiber endoscopic cellular-resolution imaging with TCSPC-FLIM technology to interrogate and discriminate mixed fluorochrome labeled beads and expressible GFP/TagRFP tags within live cells. Endoscopic-FLIM (e-FLIM) data was validated by comparison with data acquired via conventional FLIM and e-FLIM was found to be accurate for both bright bead and dim live cell samples. The fiber based micro-endoscope allowed remote imaging of 4 microm and 10 microm beads within a thick Matrigel matrix with confident fluorophore discrimination using lifetime information. More importantly, this new technique enabled us to reliably measure protein-protein interactions in live cells embedded in a 3D matrix, as demonstrated by the dimerization of the fluorescent protein-tagged membrane receptor CXCR4. This cell-based application successfully demonstrated the suitability and great potential of this new technique for in vivo pre-clinical biomedical and possibly human clinical applications.

The SUMO modification pathway is involved in the BRCA1 response to genotoxic stress

Mutations in BRCA1 are associated with a high risk of breast and ovarian cancer. BRCA1 participates in the DNA damage response and acts as a ubiquitin ligase. However, its regulation remains poorly understood. Here we report that BRCA1 is modified by small ubiquitin-like modifier (SUMO) in response to genotoxic stress, and co-localizes at sites of DNA damage with SUMO1, SUMO2/3 and the SUMO-conjugating enzyme Ubc9. PIAS SUMO E3 ligases co-localize with and modulate SUMO modification of BRCA1, and are required for BRCA1 ubiquitin ligase activity in cells. In vitro SUMO modification of the BRCA1/BARD1 heterodimer greatly increases its ligase activity, identifying it as a SUMO-regulated ubiquitin ligase (SRUbL). Further, PIAS SUMO ligases are required for complete accumulation of double-stranded DNA (dsDNA) damage-repair proteins subsequent to RNF8 accrual, and for proficient double-strand break repair. These data demonstrate that the SUMOylation pathway plays a significant role in mammalian DNA damage response.

Visualisation of signalling in immune cells

Currently, a great number of approaches are employed in investigation of the immune system. These range from experiments in live animals and biochemical techniques to investigate whole organs or cell populations down to single cell and molecular techniques to look at dynamics in specific cell-cell interactions. It is the latter approach that this chapter focusses on. The use of Förster resonance energy transfer (FRET) techniques to probe protein-protein interactions that are involved in receptor signalling to the cytoskeleton in intact cells is now well established. Various FRET biosensors are available to visualise several critical cell processes, giving information about activity and the location of key signalling molecules. As a specific set of examples in this chapter, we have generated variants of the original Rho, Rac and Cdc42 "Raichu" probes and improved their fluorophore combination to make them suitable for FLIM. These were employed in a number of assays to determine signal dynamics in T and NK cells. Specific protocols of how to use these probes and technical notes are described.

Ezrin mediates c-Myc actions in prostate cancer cell invasion

The forced overexpression of c-Myc in mouse prostate and in normal human prostate epithelial cells results in tumor transformation with an invasive phenotype. How c-Myc regulates cell invasion is poorly understood. In this study, we have investigated the interplay of c-Myc and androgens in the regulation of prostate cancer cell invasion. We found that c-Myc induces cell invasion and anchorage-independent growth by regulating ezrin protein expression in the presence of androgens. The activity of the ezrin promoter is controlled by androgens through c-Myc, which binds to a phylogenetically conserved E-Box located in the proximal promoter region. Besides, we also show that ezrin is an important regulator of c-Myc protein levels. These effects are achieved through androgen-induced changes in ezrin phosphorylation, which results in the regulation of downstream signals. These downstream signals involve the modulation of Akt and GSK-3beta activity resulting in increased c-Myc protein synthesis and inhibition of its degradation. In summary, we have shown a key role for ezrin as a mediator of c-Myc-induced tumorigenesis in prostate cancer cells.

Moesin orchestrates cortical polarity of melanoma tumour cells to initiate 3D invasion

Tumour cell dissemination through corporal fluids (blood, lymph and body cavity fluids) is a distinctive feature of the metastatic process. Tumour cell transition from fluid to adhesive conditions involves an early polarization event and major rearrangements of the submembrane cytoskeleton that remain poorly understood. As regulation of cortical actin-membrane binding might be important in this process, we investigated the role of ezrin and moesin, which are key crosslinking proteins of the ERM (ezrin, radixin, moesin) family. We used short interfering RNA (siRNA) to show that moesin is crucial for invasion by melanoma cells in 3D matrices and in early lung colonization. Using live imaging, we show that following initial adhesion to the endothelium or 3D matrices, moesin is redistributed away from the region of adhesion, thereby generating a polarized cortex: a stable cortical actin dome enriched in moesin and an invasive membrane domain full of blebs. Using Lifeact-GFP, a 17-amino-acid peptide that binds F-actin, we show the initial symmetry breaking of cortical actin cytoskeleton during early attachment of round cells. We also demonstrated that ezrin and moesin are differentially distributed during initial invasion of 3D matrices, and, specifically, that moesin controls adhesion-dependent activation of Rho and subsequent myosin II contractility. Our results reveal that polarized moesin plays a role in orienting Rho activation, myosin II contractility, and cortical actin stability, which is crucial for driving directional vertical migration instead of superficial spreading on the fluid-to-solid tissue interface. We propose that this mechanism of cortical polarization could sustain extravasation of fluid-borne tumour cells during the process of metastasis.

The potential of optical proteomic technologies to individualize prognosis and guide rational treatment for cancer patients

Genomics and proteomics will improve outcome prediction in cancer and have great potential to help in the discovery of unknown mechanisms of metastasis, ripe for therapeutic exploitation. Current methods of prognosis estimation rely on clinical data, anatomical staging and histopathological features. It is hoped that translational genomic and proteomic research will discriminate more accurately than is possible at present between patients with a good prognosis and those who carry a high risk of recurrence. Rational treatments, targeted to the specific molecular pathways of an individual's high-risk tumor, are at the core of tailored therapy. The aim of targeted oncology is to select the right patient for the right drug at precisely the right point in their cancer journey. Optical proteomics uses advanced optical imaging technologies to quantify the activity states of and associations between signaling proteins by measuring energy transfer between fluorophores attached to specific proteins. Förster resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM) assays are suitable for use in cell line models of cancer, fresh human tissues and formalin-fixed paraffin-embedded tissue (FFPE). In animal models, dynamic deep tissue FLIM/FRET imaging of cancer cells in vivo is now also feasible. Analysis of protein expression and post-translational modifications such as phosphorylation and ubiquitination can be performed in cell lines and are remarkably efficiently in cancer tissue samples using tissue microarrays (TMAs). FRET assays can be performed to quantify protein-protein interactions within FFPE tissue, far beyond the spatial resolution conventionally associated with light or confocal laser microscopy. Multivariate optical parameters can be correlated with disease relapse for individual patients. FRET-FLIM assays allow rapid screening of target modifiers using high content drug screens. Specific protein-protein interactions conferring a poor prognosis identified by high content tissue screening will be perturbed with targeted therapeutics. Future targeted drugs will be identified using high content/throughput drug screens that are based on multivariate proteomic assays. Response to therapy at a molecular level can be monitored using these assays while the patient receives treatment: utilizing re-biopsy tumor tissue samples in the neoadjuvant setting or by examining surrogate tissues. These technologies will prove to be both prognostic of risk for individuals when applied to tumor tissue at first diagnosis and predictive of response to specifically selected targeted anticancer drugs. Advanced optical assays have great potential to be translated into real-life benefit for cancer patients.

Myelin, DIGs, and membrane rafts in the central nervous system

Over the past 40 years our understanding of the organization of cell membranes has changed dramatically. Membranes are no longer viewed as a homogenous sea of phospholipids studded with randomly positioned islands of proteins. Our current view of the membrane involves the formation of small lipid clusters, comprised mainly of cholesterol and sphingolipids, known as membrane rafts. These lipid clusters apparently include and exclude specific proteins leading to the hypothesis that these domains (1) regulate cellular polarity and compartmentalization through trafficking and sorting, (2) provide platforms for cellular signaling and adhesion, and (3) function as cellular gate keepers. Tremendous controversy surrounds the concept of membrane rafts primarily because these small, highly dynamic entities are too small to be observed with traditional microscopic methods and the most utilized approach for raft analysis relies on poorly quantified, inconsistent biochemical extractions. New analytical approaches are being developed and applied to the study of membrane rafts and these techniques provide great promise for furthering our understanding of these enigmatic domains. In this review we will provide a brief summary of the current understanding of membrane rafts, utilizing the CNS myelin literature for illustrative purposes, and present caveats that should be considered when studying these domains.

Integrating receptor signal inputs that influence small Rho GTPase activation dynamics at the immunological synapse

The Rho GTPase Cdc42 regulates cytoskeletal changes at the immunological synapse (IS) that are critical to T-cell activation. By imaging fluorescent activity biosensors (Raichu) using fluorescence lifetime imaging microscopy, Cdc42 activation was shown to display kinetics that are conditional on the specific receptor input (through two IS-associated receptors, CD3 and beta1 integrin). CD3-triggered Cdc42 activity is dependent on the cyto-2 (NPIY) motif of the beta1 integrin cytoplasmic domain. Perturbations of the ezrin-radixin-moesin (ERM) function blocked CD3- and beta1-dependent increases in Cdc42 activity. Both IS-associated receptors probably lie on a serial molecular pathway and transduce signals through the ERM-dependent machinery that is responsible for the remodeling and stabilization of the synapse. Cdc42 activity is impaired in beta1 integrin-deficient T cells that form conjugates with antigen-presenting cells but is partially restored in the context of an antigen-specific synapse. This restoration of Cdc42 activity is due, at least in part, to the recruitment and activation of beta2 integrin.

Establishment and characterization of 4 new human pancreatic cancer cell lines: evidences of different tumor phenotypes

OBJECTIVES

Pancreatic cancer still remains a challenge for its biological complexity and lack of effective therapeutic strategies. Establishing new pancreatic cancer cell lines is therefore of paramount importance to clarify its biology.

METHODS

We established and characterized 4 new pancreatic cancer cell lines (PP78, PP109, PP117, and PP161) according to their genetic (K-Ras, TP53, CDKN2A, and MADH4; DNA fingerprinting; karyotype), cytostructural (cytokeratins 7, 8, 18, and 19 vimentin, and ezrin), and functional profiles (doubling time; migration assay).

RESULTS

K-Ras, TP53, and CDKN2A gene alterations were detected in all 4 of them. Each cell line had a unique DNA profile revealed by DNA fingerprinting. A complex karyotype with numerous structural and numeric chromosomal abnormalities was present in each cell line. All 4 cell lines showed positivity for cytokeratins 7, 8, and 18. All but PP78 expressed cytokeratin 19, whereas vimentin was expressed only in PP117 and PP78 cells. A different ezrin cellular distribution was noticed in PP78 and PP117, being mostly located at membrane ruffles. This peculiar distribution was associated with the strongest migratory capability.

CONCLUSIONS

Our results seem to confirm the pancreatic ductal adenocarcinoma heterogeneity; in fact, the same genetic abnormalities (K-Ras, TP53, and CDKN2A) may have different effects on tumor biology depending on cellular differentiation.

Spatial distribution of protein kinase A activity during cell migration is mediated by A-kinase anchoring protein AKAP Lbc

Protein kinase A (PKA) has been suggested to be spatially regulated in migrating cells due to its ability to control signaling events that are critical for polarized actin cytoskeletal dynamics. Here, using the fluorescence resonance energy transfer-based A-kinase activity reporter (AKAR1), we find that PKA activity gradients form with the strongest activity at the leading edge and are restricted to the basal surface in migrating cells. The existence of these gradients was confirmed using immunocytochemistry using phospho-PKA substrate antibodies. This observation holds true for carcinoma cells migrating randomly on laminin-1 or stimulated to migrate on collagen I with lysophosphatidic acid. Phosphodiesterase inhibition allows the formation of PKA activity gradients; however, these gradients are no longer polarized. PKA activity gradients are not detected when a non-phosphorylatable mutant of AKAR1 is used, if PKA activity is inhibited with H-89 or protein kinase inhibitor, or when PKA anchoring is perturbed. We further find that a specific A-kinase anchoring protein, AKAP-Lbc, is a major contributor to the formation of these gradients. In summary, our data show that PKA activity gradients are generated at the leading edge of migrating cells and provide additional insight into the mechanisms of PKA regulation of cell motility.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

PIM-1-specific mAb suppresses human and mouse tumor growth by decreasing PIM-1 levels, reducing Akt phosphorylation, and activating apoptosis

Provirus integration site for Moloney murine leukemia virus (PIM1) is a proto-oncogene that encodes a serine/threonine kinase with multiple cellular functions. Overexpression of PIM-1 plays a critical role in progression of prostatic and hematopoietic malignancies. Here we describe the generation of a mAb specific for GST-PIM-1, which reacted strongly with most human and mouse cancer tissues and cell lines of prostate, breast, and colon origin but only weakly (if at all) with normal tissues. The mAb binds to PIM-1 in the cytosol and nucleus as well as to PIM-1 on the surface of human and murine cancer cells. Treatment of human and mouse prostate cancer cell lines with the PIM-1-specific mAb resulted in disruption of PIM-1/Hsp90 complexes, decreased PIM-1 and Hsp90 levels, reduced Akt phosphorylation at Ser473, reduced phosphorylation of Bad at Ser112 and Ser136, and increased cleavage of caspase-9, an indicator of activation of the mitochondrial cell death pathway. The mAb induced cancer cell apoptosis and synergistically enhanced antitumor activity when used in combination with cisplatin and epirubicin. In tumor models, the PIM-1-specific mAb substantially inhibited growth of the human prostate cancer cell line DU145 in SCID mice and the mouse prostate cancer cell TRAMP-C1 in C57BL/6 mice. These findings are important because they provide what we believe to be the first in vivo evidence that treatment of prostate cancer may be possible by targeting PIM-1 using an Ab-based therapy.

In Vitro and in Vivo Characterization of Molecular Interactions between Calmodulin, Ezrin/Radixin/Moesin, and L-selectin

L-selectin is a cell adhesion molecule that tethers leukocytes to the luminal walls of venules during inflammation and enables them to roll under the force of blood flow. Clustering of L-selectin during rolling is thought to promote outside-in signals that lead to integrin activation and chemokine receptor expression, ultimately contributing to leukocyte arrest. Several studies have underscored the importance of the L-selectin cytoplasmic tail in functionally regulating adhesion and signaling. Interestingly, the L-selectin tail comprises only 17 amino acids, and yet it is thought to bind simultaneously to several proteins. For example, constitutive association of calmodulin (CaM) and ezrin/radixin/moesin (ERM) to L-selectin confers resistance to proteolysis and microvillar positioning, respectively. In this report we found that recombinant purified CaM and ERM bound non-competitively to the same tail of L-selectin. Furthermore, molecular modeling supported the possibility that CaM, L-selectin, and moesin could form a heterotrimeric complex. Finally, using fluorescence lifetime imaging microscopy to measure fluorescence resonance energy transfer, it was shown that CaM, L-selectin, and ERM could interact simultaneously in vivo. Moreover, L-selectin clustering promoted CaM/ERM interaction in cis (i.e. derived from neighboring L-selectin tails). These results highlight a novel intracellular event that occurs as a consequence of L-selectin clustering, which could participate in transducing signals that promote the transition from rolling to arrest.

Multiphoton time-domain fluorescence lifetime imaging microscopy: practical application to protein–protein interactions using global analysis

Förster resonance energy transfer (FRET) detected via fluorescence lifetime imaging microscopy (FLIM) and global analysis provide a way in which protein–protein interactions may be spatially localized and quantified within biological cells. The FRET efficiency and proportion of interacting molecules have been determined using bi-exponential fitting to time-domain FLIM data from a multiphoton time-correlated single-photon counting microscope system. The analysis has been made more robust to noise and significantly faster using global fitting, allowing higher spatial resolutions and/or lower acquisition times. Data have been simulated, as well as acquired from cell experiments, and the accuracy of a modified Levenberg–Marquardt fitting technique has been explored. Multi-image global analysis has been used to follow the epidermal growth factor-induced activation of Cdc42 in a short-image-interval time-lapse FLIM/FRET experiment. Our implementation offers practical analysis and time-resolved-image manipulation, which have been targeted towards providing fast execution, robustness to low photon counts, quantitative results and amenability to automation and batch processing.

CagA of Helicobacter pylori alters the expression and cellular distribution of host proteins involved in cell signaling

To expand our knowledge of Helicobacter pylori virulence mechanisms, we used iTRAQ (isobaric tagging reagents for relative and absolute quantification)-based proteomic analysis to investigate the effect of H. pylori on gastric AGS tissue culture cells. In particular, we were interested in finding out which effects of H. pylori were dependent on the cytotoxins CagA and VacA. Protein analysis was restricted to detergent-resistant membranes (DRMs), because both toxins were described previously to localize in lipid raft-like domains. Using H. pylori wild type and two isogenic mutants, DeltacagA and DeltavacA, we identified a total of 21 proteins that were either increased or decreased in the DRMs due to bacterial infection. The effect on three of these proteins, ezrin, syndecan-4 and Rab11-FIP1, were furthermore dependent on CagA. Because these proteins have been implicated in cell migration, adhesion and polarity, they might act as important mediators of CagA cytotoxicity.

Multiphoton microscopy and fluorescence lifetime imaging microscopy (FLIM) to monitor metastasis and the tumor microenvironment

Cancer metastasis involves complex cell behavior and interaction with the extracellular matrix by metabolically active cells. To observe invasion and metastasis with sub-cellular resolution in vivo, multiphoton microscopy (MPM) allows imaging more deeply into tissues with less toxicity, compared with other optical imaging methods. MPM can be combined with second harmonic generation (SHG), fluorescent lifetime imaging microscopy (FLIM), and spectral-lifetime imaging microscopy (SLIM). SHG facilitates imaging of stromal collagen and tumor-stroma interactions, including the architecture and remodeling of the tumor microenvironment. FLIM allows characterization of exogenous and endogenous fluorophores, such as the metabolites FAD and NADH to score for metabolic state and provide optical biomarkers. SLIM permits additional identification and separation of endogenous and exogenous fluorophores by simultaneously collecting their spectra and lifetime, producing an optical molecular "fingerprint". Both FLIM and SLIM also serve as an improved method for the assessment of Förster (or fluorescence) resonance energy transfer (FRET). Hence, the use and further development of these approaches strongly enhances the visualization and quantification of tumor progression, invasion, and metastasis. Herein, we review recent developments of multiphoton FLIM and SLIM to study 2D and 3D cell migration, invasion into the tumor microenvironment, and metastasis.

Rac regulates the interaction of fascin with protein kinase C in cell migration

Fascin is an actin-bundling protein that is low or absent in normal epithelia; its upregulation correlates with poor prognosis in many human carcinomas. We have recently demonstrated in mouse xenograft models that fascin contributes to tumour development and metastasis through its dual actin-bundling and active PKC-binding activities. Rac was implicated as a regulator of fascin-dependent colon carcinoma cell migration in vitro. Here, we tested the hypothesis that Rac regulates the interaction of fascin with active PKC. The major conventional PKC in colon carcinoma cells is protein kinase Cγ (PKCγ). Endogenous PKCγ, fascin and Rac1 colocalised at lamellipodial margins of migrating cells. Colocalisation of fascin and PKCγ depended on Rac activity, and inhibition of Rac decreased PKCγ activity in cell extracts but not in vitro. Fluorescence resonance energy transfer/fluorescence lifetime imaging microscopy uncovered that fascin and PKCγ interact in protrusions and filopodia of migrating cells. Mechanistically, the interaction depended on phosphorylated fascin, active PKCγ and active Rac, but not on active Cdc42. The activity of Rac on the fascin/PKC complex was mediated in part by Pak. Elucidation of this novel pathway for regulation of the fascin/PKCγ complex in migrating carcinoma cells suggests novel targets for therapeutic intervention in metastasis.

Remodeling of cellular cytoskeleton by the acid sphingomyelinase/ceramide pathway

The chemotherapeutic agent cisplatin is widely used in treatment of solid tumors. In breast cancer cells, cisplatin produces early and marked changes in cell morphology and the actin cytoskeleton. These changes manifest as loss of lamellipodia/filopodia and appearance of membrane ruffles. Furthermore, cisplatin induces dephosphorylation of the actin-binding protein ezrin, and its relocation from membrane protrusions to the cytosol. Because cisplatin activates acid sphingomyelinase (ASMase), we investigate here the role of the ASMase/ceramide (Cer) pathway in mediating these morphological changes. We find that cisplatin induces a transient elevation in ASMase activity and its redistribution to the plasma membrane. This translocation is blocked upon overexpression of a dominant-negative (DN) ASMase^S508A mutant and by a DN PKCδ. Importantly; knockdown of ASMase protects MCF-7 cells from cisplatin-induced cytoskeletal changes including ezrin dephosphorylation. Reciprocally, exogenous delivery of D-e-C₁₆-Cer, but not dihydro-C₁₆-Cer, recapitulates the morphotropic effects of cisplatin. Collectively, these results highlight a novel tumor suppressor property for Cer and a function for ASMase in cisplatin-induced cytoskeletal remodeling.

The Ezrin-radixin-moesin family member ezrin regulates stable microtubule formation and retroviral infection

We recently identified the cytoskeletal regulatory protein moesin as a novel gene that inhibits retroviral replication prior to reverse transcription by downregulation of stable microtubule formation. Here, we provide evidence that overexpression of ezrin, another closely related ezrin-radixin-moesin (ERM) family member, also blocks replication of both murine leukemia viruses and human immunodeficiency virus type 1 (HIV-1) in Rat2 fibroblasts before reverse transcription, while knockdown of endogenous ezrin increases the susceptibility of human cells to HIV-1 infection. Together, these results suggest that ERM proteins may be important determinants of retrovirus susceptibility through negative regulation of stable microtubule networks.

Quantification of integrin receptor agonism by fluorescence lifetime imaging

Both spatiotemporal analyses of adhesion signalling and the development of pharmacological inhibitors of integrin receptors currently suffer from the lack of an assay to measure integrin-effector binding and the response of these interactions to antagonists. Indeed, anti-integrin compounds have failed in the clinic because of secondary side effects resulting from agonistic activity. Here, we have expressed integrin-GFP and effector-mRFP pairs in living cells and quantified their association using fluorescence lifetime imaging microscopy (FLIM) to measure fluorescence resonance energy transfer (FRET). Association of talin with beta1 integrin and paxillin with alpha4 integrin was dependent on both the ligand and receptor activation state, and was sensitive to inhibition with small molecule RGD and LDV mimetics, respectively. An adaptation of the assay revealed the agonistic activity of these small molecules, thus demonstrating that these compounds may induce secondary effects in vivo via integrin activation. This study provides insight into the dependence of the activity of small molecule anti-integrin compounds upon receptor conformation, and provides a novel quantitative assay for the validation of potential integrin antagonists.

Integrin regulation of caveolin function

Caveolae are unique organelles that are found in the plasma membrane of many cell types. They participate in various processes such as lipid recycling, cellular signalling and endocytosis. A variety of signalling molecules localize to caveolae in response to various stimuli, providing a potential mechanism for the spatial regulation of signal transduction pathways. Caveolin-1, a constitutive protein of caveolae, has been implicated in the regulation of cell growth, lipid trafficking, endocytosis and cell migration. Phosphorylation of caveolin-1 on Tyr 14 is involved in integrin-regulated caveolae trafficking and also in signalling at focal adhesions in migrating cells. In this review, we focus on recent studies that describe the role of caveolin-1 in integrin signal transduction, and how this interplay links extracellular matrix anchorage to cell proliferation, polarity and directional migration.

Positive feedback between Cdc42 activity and H+ efflux by the Na-H exchanger NHE1 for polarity of migrating cells

A fundamental feature of cell polarity in response to spatial cues is asymmetric amplification of molecules generated by positive feedback signaling. We report a positive feedback loop between the guanosine triphosphatase Cdc42, a central determinant in eukaryotic cell polarity, and H⁺ efflux by Na-H⁺ exchanger 1 (NHE1), which is necessary at the front of migrating cells for polarity and directional motility. In response to migratory cues, Cdc42 is not activated in fibroblasts expressing a mutant NHE1 that lacks H⁺ efflux, and wild-type NHE1 is not activated in fibroblasts expressing mutationally inactive Cdc42-N17. H⁺ efflux by NHE1 is not necessary for release of Cdc42–guanosine diphosphate (GDP) from Rho GDP dissociation inhibitor or for the membrane recruitment of Cdc42 but is required for GTP binding by Cdc42 catalyzed by a guanine nucleotide exchange factor (GEF). Data indicate that GEF binding to phosphotidylinositol 4,5–bisphosphate is pH dependent, suggesting a mechanism for how H⁺ efflux by NHE1 promotes Cdc42 activity to generate a positive feedback signal necessary for polarity in migrating cells.

Arf6 and microtubules in adhesion-dependent trafficking of lipid rafts

Integrin-mediated adhesion regulates membrane binding sites for Rac1 within lipid rafts. Detachment of cells from the substratum triggers the clearance of rafts from the plasma membrane through caveolin-dependent internalization. The small GTPase Arf6 and microtubules also regulate Rac-dependent cell spreading and migration, but the mechanisms are poorly understood. Here we show that endocytosis of rafts after detachment requires F-actin, followed by microtubule-dependent trafficking to recycling endosomes. When cells are replated on fibronectin, rafts exit from recycling endosomes in an Arf6-dependent manner and return to the plasma membrane along microtubules. Both of these steps are required for the plasma membrane targeting of Rac1 and for its activation. These data therefore define a new membrane raft trafficking pathway that is crucial for anchorage-dependent signalling.