The mobile receptor hypothesis revisited: a mechanistic role for hormone receptor lateral mobility in signal transduction

Quantitation of F-actin in cytoskeletal reorganization: Context, methodology and implications

The actin cytoskeleton is involved in a large number of cellular signaling events in addition to providing structural integrity to the cell. Actin polymerization is a key event during cellular signaling. Although the role of actin cytoskeleton in cellular processes such as trafficking and motility has been extensively studied, the reorganization of the actin cytoskeleton upon signaling has been rarely explored due to lack of suitable assays. Keeping in mind this lacuna, we developed a confocal microscopy based approach that relies on high magnification imaging of cellular F-actin, followed by image reconstruction using commercially available software. In this review, we discuss the context and relevance of actin quantitation, followed by a detailed hands-on approach of the methodology involved with specific points on troubleshooting and useful precautions. In the latter part of the review, we elucidate the method by discussing applications of actin quantitation from our work in several important problems in contemporary membrane biology ranging from pathogen entry into host cells, to GPCR signaling and membrane-cytoskeleton interaction. We envision that future discovery of cell-permeable novel fluorescent probes, in combination with genetically encoded actin-binding reporters, would allow real-time visualization of actin cytoskeleton dynamics to gain deeper insights into active cellular processes in health and disease.

Insights into cellular signaling from membrane dynamics

Biological membranes allow morphological compartmentalization of cells and represent complex micro-heterogeneous fluids exhibiting a range of dynamics. The plasma membrane occupies a central place in cellular signaling which allows the cell to perform a variety of functions. In this review, we analyze cellular signaling in a dynamic biophysical framework guided by the "mobile receptor hypothesis". We describe a variety of examples from literature in which lateral diffusion of signaling membrane proteins acts as an important determinant in the efficiency of signaling. A major focus in our review is on membrane-embedded G protein-coupled receptors (GPCRs) which act as cellular signaling hubs for diverse cellular functions. Taken together, we describe a dynamics-based signaling paradigm with chosen examples from literature to elucidate how such a paradigm helps us understand signaling by GPCRs, maintenance of cellular polarity in yeast and infection by pathogens. We envision that with further technological advancement, it would be possible to explore cellular signaling more holistically as cells undergo development, differentiation and aging, thereby providing us a robust window into the dynamics of the cellular interior and its functional correlates.

Dynamics of the actin cytoskeleton mediates receptor cross talk: An emerging concept in tuning receptor signaling

Recent evidence implicates the actin cytoskeleton in the control of receptor signaling. This may be of particular importance in the context of immune receptors, such as the B cell receptor, where dysregulated signaling can result in autoimmunity and malignancy. Here, we discuss the role of the actin cytoskeleton in controlling receptor compartmentalization, dynamics, and clustering as a means to regulate receptor signaling through controlling the interactions with protein partners. We propose that the actin cytoskeleton is a point of integration for receptor cross talk through modulation of protein dynamics and clustering. We discuss the implication of this cross talk via the cytoskeleton for both ligand-induced and low-level constitutive (tonic) signaling necessary for immune cell survival.

A mechanistic role for polypeptide hormone receptor lateral mobility in signal transduction

Lateral diffusion of membrane-integral receptors within the plane of the membrane has been postulated to be mechanistically important for signal transduction. Direct measurement of polypeptide hormone receptor lateral mobility using fluorescence photobleaching recovery techniques indicates that tyrosine kinase receptors are largely immobile at physiological temperatures. This is presumably due to their signal transduction mechanism which requires intermolecular autophosphorylation through receptor dimerization and thus immobilization for activation. In contrast, G-protein coupled receptors must interact with other membrane components to effect signal transduction, and consistent with this, the phospholipase C-activating vasopressin V1- and adenylate cyclase activating V2-receptors are highly laterally mobile at 37°C. Modulation of the V2-receptor mobile fraction (f) has demonstrated a direct correlation between f and receptor-agonist-dependent maximal cAMP productionin vivo at 37°C. This indicates that f is a key parameter in hormone signal transduction especially at physiological hormone concentrations, consistent with mobile receptors being required to effect V2-agonist-dependent activation of G-proteins. Measurements using a V2-specific antagonist show that antagonist-occupied receptors are highly mobile at 37°C, indicating that receptor immobilization is not the basis of antagonism. In contrast to agonist-occupied receptor however, antagonistoccupied receptors are not immobilized prior to endocytosis and down-regulation. Receptors may thus be freely mobile in the absence of agonistic ligand; stimulation by hormone agonist results in receptor association with other proteins, probably including cytoskeletal components, and immobilization. Receptor immobilization may be one of the important steps of desensitization subsequent to agonistic stimulation, through terminating receptor lateral movement which is instrumental in generating and amplifying the initial stimulatory signal within the plane of the membrane.

B-cell receptor: from resting state to activate

B-cell activation is triggered by the binding of antigen to the B-cell receptor (BCR). The early molecular events triggered by BCR binding of ligand have been well-characterized both biochemically and using optical microscopy techniques to visualize B-cell activation as it happens. However, we understand much less about the BCR before activation. For this reason, this review will address recent advances in our view of the structure, organization and dynamics of the resting, unstimulated BCR. These parameters have important implications for our understanding of the initiation of B-cell activation and will be discussed in the context of current models for BCR activation. These models include the conformation-induced oligomerization model, in which binding of antigen to monomeric BCR induces a pulling or twisting force causing conformational unmasking of a clustering interface in the Cμ4 domain. Conversely, the dissociation activation model proposes that BCRs exist in auto-inhibitory oligomers on the resting B-cell surface and binding of antigen promotes the dissociation of the BCR oligomer exposing phosphorylation residues within Igα/Igβ. Finally, the collision coupling model suggests that BCR are segregated from activating co-receptors or kinases and activation is associated with changes in BCR mobility on the cell surface, which allows for the functional interaction of these elements.

Differential binding of VEGF isoforms to VEGF receptor 2 in the presence of neuropilin-1: a computational model

A comprehensive, biophysically accurate, computational model of vascular endothelial growth factor (VEGF) family member interactions with endothelial cell surface receptors was developed to study angiogenesis. Neuropilin-1 (NRP1) and the signaling VEGF receptor, VEGFR2, do not interact directly but are bridged by one VEGF isoform, VEGF165. Using the model and published experimental data, we estimated the kinetic rate of this VEGFR2-NRP1 coupling in vitro. With the use of this rate, our model gives predictions in good quantitative agreement with several independent in vitro experiments involving VEGF121and VEGF165isoforms, confirming that VEGFR2-NRP1 coupling through VEGF165can fully explain the observed differences in receptor binding and phosphorylation in response to these isoforms. Model predictions also determine the mechanism of action of a commonly used NRP1 antibody and predict the results of potential future experiments. This is the first model to include VEGF isoforms or NRPs, and it is a necessary step toward a quantitative molecular level description of VEGF that can be extended to in vivo situations. The model has applications for both proangiogenic and antiangiogenic therapies, such as for heart disease and cancer, as well as in tissue engineering.

Signal- and importin-dependent nuclear targeting of the kidney anion exchanger 1-binding protein kanadaptin

Kanadaptin (kidney anion exchanger adaptor protein) has recently been identified as a protein with binding activity to the cytoplasmic domain of the kidney Na(+)-independent Cl(-)/HCO(-)(3) anion exchanger 1 (kAE1). Since it is widely expressed in tissues devoid of kAE1, however, kanadaptin is likely to have additional cellular roles. This is supported by its multidomain structure, and possession of three clusters of basic amino acids exhibiting similarity to known nuclear localization sequences (NLSs). In the present study, we use immunofluorescence and subcellular fractionation approaches to demonstrate that kanadaptin is localized within the nuclei of various epithelial and non-epithelial cultured cell types. The role of the different NLSs is examined in transfection studies using plasmids encoding full-length kanadaptin with or without green fluorescent protein (GFP) as a fusion tag, as well as truncation derivatives thereof. Strong nuclear localization of fusion proteins containing amino acids 140-230 of kanadaptin, which include the sequence AVSRKRKA(193) (NLS1) was observed. Substitution of Arg(191) with a threonine residue resulted in a cytoplasmic location of the expressed protein, while NLS1 proved sufficient to target an otherwise cytoplasmically localized beta-galactosidase-GFP fusion protein to the nucleus. Using a direct binding assay we show that a fusion protein containing kanadaptin amino acids 1-231 (but not the Thr(191) substituted derivative) is recognized with nM affinity by the conventional NLS-binding importin alpha/beta heterodimer. Nuclear import studies on microinjected and permeabilized rat hepatoma cells demonstrated functionality of the NLS in nuclear targeting, with inhibition by antibodies demonstrating the requirement of both importin alpha and beta for nuclear import of kanadaptin. That kanadaptin possesses a functional importin-alpha/beta-recognized NLS explains the nuclear localization of kanadaptin in various cultured cell types, and opens up the possibility that kanadaptin may have a signalling role in the nucleus.

Calculation of diffusion-limited kinetics for the reactions in collision coupling and receptor cross-linking

Both enzyme (e.g., G-protein) activation via a collision coupling model and the formation of cross-linked receptors by a multivalent ligand involve reactions between two molecules diffusing in the plasma membrane. The diffusion of these molecules is thought to play a critical role in these two early signal transduction events. In reduced dimensions, however, diffusion is not an effective mixing mechanism; consequently, zones in which the concentration of particular molecules (e.g., enzymes, receptors) becomes depleted or enriched may form. To examine the formation of these depletion/ accumulation zones and their effect on reaction rates and ultimately the cellular response, Monte Carlo techniques are used to simulate the reaction and diffusion of molecules in the plasma membrane. The effective reaction rate at steady state is determined in terms of the physical properties of the tissue and ligand for both enzyme activation via collision coupling and the generation of cross-linked receptors. The diffusion-limited reaction rate constant is shown to scale with the mean square displacement of a receptor-ligand complex. The rate constants determined in the simulation are compared with other theoretical predictions as well as experimental data.

Effects of intracellular angiotensin II in vascular smooth muscle cells

Angiotensin (Ang) II is present inside vascular smooth muscle cells (VSMCs); however, its intracellular functions, if any, are unknown. We tested the hypothesis that intracellular Ang II exerts effects on cytosolic Ca2+ ([Ca2+]i) in VSMCs. Ang II was administered via microinjection. Intracellular Ang II localization was demonstrated by fluorescein-labeled Ang II and electron microscopy. [Ca2+]i was monitored by confocal microscopy with fluo 3. Ang II was identified in endosomes and in the nucleus by both localizing techniques. Microinjection of Ang II (10(-10) mol/L) led to a rapid increase in [Ca2+]i in the cytosol and in the nucleus. The [Ca2+]i increase was due to the influx of extracellular Ca2+ ions. The intracellular Ang II effect was totally inhibited by the concomitant injection of the Ang II antagonist CV-11947. Desensitization of extracellular Ang II receptors, on the other hand, did not influence the intracellular effects, nor did extracellular CV-11947. The increase in [Ca2+]i was observed not only in the microinjected cell but also in directly adjacent VSMCs. In contrast to the microinjected cells, the [Ca2+]i increase in the adjacent cells was mostly due to release from intracellular stores. Pretreatment with thapsigargin abolished the Ang II response in adjacent cells. Microinjection of inositol tris-phosphate induced a [Ca2+]i response in adjacent cells that was similar to the Ang II-induced effects. Preincubation of VSMCs with the uncoupling substances dimethyl sulfoxide and heptanol did not decrease the Ang II response but instead prevented a [Ca2+]i surge in adjacent cells. We conclude that intracellular Ang II binds to intracellular Ang II receptors and elicits an increased [Ca2+]i in the injected cell and, thereafter, cells in the immediate neighborhood. Cell-cell contact is necessary for the Ang II-mediated effects. The data suggest that intracellular Ang II may stimulate a cluster of VSMCs from a single cell via the release of second messengers.

Imaging fluorescence correlation spectroscopy: nonuniform IgE distributions on planar membranes

Fluorescence correlation spectroscopy is useful for detecting and characterizing molecular clusters that are smaller than or approximately equal to optical resolution in size. Here, we report the development of an approach in which the pixel-to-pixel fluorescence fluctuations from a single fluorescence image are spatially autocorrelated. In these measurements, tetramethylrhodamine-labeled, anti-trinitrophenyl IgE antibodies were specifically bound to substrate-supported planar membranes composed of trinitrophenyl-aminocaproyldipalmitoylphosphatidylethanolamine and dipalmitoylphosphatidylcholine. The antibody-coated membranes were illuminated with the evanescent field from a totally internally reflected laser beam, and the fluorescence arising from the IgE-coated membranes was recorded with a cooled CCD camera. The image was corrected for the elliptical Gaussian shape of the evanescent illumination after background subtraction. The spatial autocorrelation functions of the resulting images generated two useful parameters: the extrapolated initial values, which were related to the average cluster intensity and density; and the correlation distances, which were related to the average cluster size. These parameters varied with the IgE density, and unlabeled polyclonal anti-IgE enhanced the nonuniform IgE distributions. The autocorrelation functions calculated from images of planar membranes containing fluorescently labeled lipids rather than bound, labeled IgE demonstrated that the spatial nonuniformities were prominent only in the presence of IgE. Fluorescent beads were used to demonstrate the principles and the methods.

A consensus cAMP-dependent protein kinase (PK-A) site in place of the CcN motif casein kinase II site simian virus 40 large T-antigen confers PK-A-mediated regulation of nuclear import

The regulation of nuclear protein transport by phosphorylation plays a central role in gene expression in eukaryotic cells. We previously showed that nuclear import of SV40 large tumor antigen (T-ag) fusion proteins is regulated by the CcN motif, comprising phosphorylation sites for casein kinase II and the cyclin-dependent kinase cdc2, together with the nuclear localization signal. Regulation of nuclear uptake by CcN motif kinase sites also holds true for the yeast transcription factor SWI5 and the Xenopus nuclear phosphoprotein nucleoplasmin. To test directly whether a kinase site other than those of the CcN motif could regulate nuclear import of T-ag, the CcN motif casein kinase II site, which markedly increases the rate of T-ag nuclear import, was replaced by a consensus site for the cAMP-dependent protein kinase (PK-A) using site-directed mutagenesis. The resultant fusion protein could be specifically phosphorylated by PK-A in vitro and in cell extracts. Nuclear import of the fluorescently labeled protein was analyzed in the HTC rat hepatoma cell line both in vivo (microinjected cells) and in vitro (mechanically perforated cells) in the presence and the absence of cAMP and/or PK-A catalytic subunit using confocal laser scanning microscopy. In vitro PK-A-prephosphorylated protein was also tested. All results indicated that the rate of nuclear import was increased by phosphorylation at the PK-A site (2-5-fold), demonstrating that kinases other than those of the CcN motif can regulate nuclear import in response to stimulatory signals. The phosphorylation-regulated nuclear localization signal derived here represents an important first step toward developing a signal conferring inducible nuclear targeting of molecules of interest.

Beta 1 integrins signal lipid second messengers required during cell adhesion

Clustering of integrin receptors during cell adhesion stimulates signal transduction across the cell membrane. Second messengers are generated, activating cytosolic proteins and causing cytoskeletal assembly and rearrangement. HeLa cell adhesion to a collagen substrate has been shown to initiate an arachidonic acid-mediated signaling pathway, leading to the activation of protein kinase C (PKC) and cell spreading. To determine the role of integrin receptors in triggering this signaling pathway, monoclonal antibodies to beta 1 integrins were used to either cluster integrins on the cell surface or to provide an integrin-dependent substrate for cell adhesion. Using this approach, we have defined a pathway required for cell spreading that can be initiated by the ligation of integrins and leads to the activation of PKC. Specifically, our results indicate that clustering beta 1 integrins results in the activation of phospholipase A2 leading to the production of arachidonic acid and the activation of PKC.

Cyclin-dependent kinase site-regulated signal-dependent nuclear localization of the SW15 yeast transcription factor in mammalian cells

Control over the nuclear transport of transcription factors (TFs) represents a level of gene regulation integral to cellular processes such as differentiation, transformation and signal transduction. The Saccharomyces cerevisiae TF SWI5 is excluded from the nucleus in a cell cycle-dependent fashion, mediated by phosphorylation by the cyclin-dependent kinase (cdk) CDC28. Nuclear entry occurs in G1. beta-galactosidase fusion proteins carrying SWI5 amino acids 633-682, including the nuclear localization sequence (NLS: Lys-Lys-Tyr-Glu-Asn-Val-Val-Ile-Lys-Arg-Ser-Pro-Arg-Lys-Arg-Gly-Arg-Pro- Arg-Lys655) were analyzed for subcellular localization in appropriate temperature-sensitive yeast strains blocked in G1 or G2/M using indirect immunofluorescence, and for nuclear import kinetics in living rat hepatoma or Vero African green monkey kidney cells microinjected with fluorescently labeled bacterially expressed protein and quantitative confocal laser microscopy. Cell cycle-dependent nuclear localization in yeast was both NLS and cdk site-dependent, whereby mutation of the cdk site serines (Ser646 and Ser664) to alanine resulted in constitutive nuclear localization. In mammalian cells, the SWI5 fusion proteins were similarly transported to the nucleus in an NLS-dependent fashion, while the mutation to Ala of the cdk site serines increased the maximal level of nuclear accumulation from about 1- to over 8-fold. We suggest that phosphorylation at the cdk sites inhibits nuclear transport of SWI5, consistent with our previous observations for the inhibition of SV40 large tumor antigen nuclear transport by phosphorylation by the cdk cdc2. The results indicate for the first time that a yeast NLS and, fascinatingly, its regulatory mechanisms are functional in higher eukaryotes, implying the universal nature of regulatory signals for protein transport to the nucleus.

Modification of fluid lipid and mobile protein fractions of reticulocyte plasma membranes affects agonist-stimulated adenylate cyclase. Application of the percolation theory

The technique of fluorescence recovery after photobleaching was used to measure the lateral mobility of membrane integral proteins in reticulocyte plasma membranes which were treated to modify the 'fluid' lipid or immobilized protein fractions, hence increasing the relative prevalence of obstacles to protein lateral motion. This was achieved by either: (1) treating the plasma membranes with phospholipase A2 followed by extraction of the hydrolysis products using fatty-acid-free bovine serum albumin, resulting in a decrease in the membrane 'fluid' lipid portion; or (2) preincubating the plasma membranes with polylysines, resulting in plasma membrane protein aggregation and immobilization. As the prevalence of obstacles to lateral motion increased in plasma membranes through the treatments described above, the mobility of the membrane integral proteins diminished. Experimental results for the dependence of protein mobility on the prevalence of obstacles to lateral motion were compared to theoretical data in order to verify the applicability of the percolation theory to reticulocyte plasma membranes. The influence of a decrease in the 'fluid' lipid and an increase in the immobilized membrane protein fractions upon the hormone-stimulated adenylate cyclase activity has been studied as well. As the 'solid' lipid and immobilized membrane protein fractions decreased, both the hormone-stimulated adenylate cyclase activity and the fraction of beta-adrenergic receptors with high affinity to hormone diminished. It was shown that this correlation can be caused by a decrease in membrane fraction accessible to the movement of the interacting proteins of the adenylate cyclase complex. Hormonal stimulation of adenylate cyclase is discussed in terms of the percolation theory.

An irregularity in the transmembrane domain helix correlates with the rate of insulin receptor internalization

Internalization of insulin and its receptor via receptor-mediated endocytosis is an important step in insulin-induced signal transduction. To investigate the structural determinants underlying the enhanced internalization rate observed for the insulin receptor transmembrane mutant Gly933-Pro934-->Ala-Ala (GP-->AA), we have designed and chemically synthesized two peptides, IR(TM)-GP and IR-(TM)-AA, corresponding respectively to the N-terminal portion of the wild-type and the mutant insulin receptor TM segment containing these sites. Conformational studies by circular dichroism (CD) spectroscopy on these two peptides in their monomeric states revealed that peptide IR(TM)-GP forms an irregular helix in the membrane-mimetic environments of sodium dodecyl sulfate (SDS) micelles with a possible "kink" in the helix imposed by its Gly-Pro sequence, while peptide IR(TM)-AA assumes largely classical alpha-helical structure under corresponding conditions. The helical pattern of peptide IR(TM)-AA was maintained at elevated temperatures, while the shape of the CD curve for peptide IR(TM)-GP was found to alter as a function of temperature. At higher concentrations, both peptides formed high molecular weight aggregates in SDS micelles, as demonstrated by SDS-PAGE gels, but peptide IR(TM)-AA was shown to aggregate more readily and more extensively than peptide IR(TM)-GP. Fluorescent dye-leakage experiments indicated that peptide IR(TM)-GP produces an enhanced disruption of the membrane bilayer in phosphatidylglycerol vesicles vs that induced by IR(TM)-AA.(ABSTRACT TRUNCATED AT 250 WORDS)

Lateral mobility of the antagonist-occupied V2 vasopressin receptor in membranes of renal epithelial cells

The lateral mobility of membrane integral receptors has been implicated as playing a significant role in signal transduction. The adenylate cyclase-coupled vasopressin V2 receptor has been shown to be highly laterally mobile in membranes of LLC-PK1 renal epithelial cells at physiological temperature using a fluorescent vasopressin agonist, with lateral mobility of the V2 receptor proposed to play a role in both adenylate cyclase activation and ligand induced receptor internalization and down-regulation. This study reports the synthesis and characterization of two new fluorescent antagonists [(beta-mercapto-beta,beta-cyclopentamethylene propionic acid)1,D-Tyr2,Ile4,Lys9(N6-fluoresceinylaminothiocarbonyl )]AVP (FL-AVP-anta) and [(beta-mercapto-beta,beta-cyclopentamethylene propionic acid)1,D-Tyr2,Ile4,Lys9(N6-tetramethylrhodamylaminothioca rbonyl)]AVP (TR-AVP-anta) for the V2 receptor. The latter was used to determine the parameters of lateral mobility of the V2 receptor in the non-activated antagonist-occupied form. Using fluorescence photobleaching techniques, results were largely comparable to those for agonist-occupied receptor, indicating high mobility at 37 degrees C. Antagonistic properties of the V2 receptor ligands are apparently not related to decreased receptor lateral mobility. Photobleaching measurements, however, did show that in contrast to V2 agonist, V2 antagonist did not induce receptor immobilization due to aggregation with time at 37 degrees C, indicating that this could be of mechanistic importance in the internalization process.