A Rac-cGMP signaling pathway

The small GTPase Rac and the second messenger cGMP (guanosine 3',5'-cyclic monophosphate) are critical regulators of diverse cell functions. When activated by extracellular signals via membrane signaling receptors, Rac executes its functions through engaging downstream effectors such as p21-activated kinase (PAK), a serine/threonine protein kinase. However, the molecular mechanism by which membrane signaling receptors regulate cGMP levels is not known. Here we have uncovered a signaling pathway linking Rac to the increase of cellular cGMP. We show that Rac uses PAK to directly activate transmembrane guanylyl cyclases (GCs), leading to increased cellular cGMP levels. This Rac/PAK/GC/cGMP pathway is involved in platelet-derived growth factor-induced fibroblast cell migration and lamellipodium formation. Our findings connect two important regulators of cellular physiological functions and provide a general mechanism for diverse receptors to modulate physiological responses through elevating cellular cGMP levels.     

Compartmentalization in T-cell signalling: membrane microdomains and polarity orchestrate signalling and morphology

Lymphocyte function is regulated by complex signalling responses to diverse extracellular inputs, and a cell will often receive multiple, conflicting signals at one time. The mechanisms by which a lymphocyte integrates these signals into a single cellular response are not well understood. An important factor in the integration of signals likely involves the regulation of access of signalling molecules to cell surface receptors and of receptor signals to morphological determinants within the cell. Recent studies have led to important advances in our understanding of both the mechanisms by which signals are compartmentalized in T cells and the physiological role played by such compartmentalization. We review progress in the field, with a particular focus on membrane microdomains or lipid rafts and on cell polarity.     

Regulation of interferon-gamma receptor (INF-gammaR) chains: a peculiar way to rule the life and death of human lymphocytes

Interferon-gamma (IFN-gamma) is a lymphokine produced by activated T lymphocytes and NK cells, that plays an important role in host defense mechanisms by exerting pleiotropic activities on a wide range of cell types. Cellular responses to IFN-gamma are mediated by its heterodimeric cell surface receptor (IFN-gammaR), which activates downstream signal transduction cascades, ultimately leading to the regulation of gene expression. Several observations suggest that the signals resulting from the binding of IFN-gamma to its receptor depend on the number of surface receptors transducing the IFN-gamma signal. This review summarizes recent advances in the understanding of the fine regulation of the response of human lymphocytes to IFN-gamma through an interplay between surface expression of IFN-gammaR and a variety of environmental factors that combine to control their fate.     

Short Forms of Membrane Receptors: Generation and Role in Hormonal Signal Transduction

This review highlights the generation of various types of short forms of membrane hormonal receptors and the mode of regulation of their tissue-specific patterns. The short forms of membrane receptors are classified on the basis of localization of missing functional fragments. The review provides examples of tissues for which expression of short forms may serve as a marker of changes of ontogenetic stage, physiological state, or the development of pathological process. The short forms of receptors are shown to participate in determining tissue-specificity and efficacy of hormonal signal transduction, as well as in transport of hormones within cell, through physiological barriers, and in blood circulation. Peculiarities of signal transduction pathways for short receptor forms and potential physiological significance of these forms are analyzed. It is concluded that the ratio of long and short receptor forms may serve as a key marker of dynamic changes of differentiation stage and alterations of metabolic and proliferative activity of tissues under normal and pathologic conditions, and thus to be an important indicator of therapeutic effect for many pathological processes.     

The cytoskeletal protein zyxin—A universal regulator of cell adhesion and gene expression

The attachment of a cell to the extracellular matrix or the surface of another cells affects not only the cell motility, but also gene expression. In view of this, an important problem is to establish the molecular mechanisms of signal transduction from the receptors of cell adhesion to the nucleus, in particular, to identify and investigate the protein transducers of these signals. One of these transducers, the LIM domain protein zyxin, is predominantly localized at the sites of cell adhesion, where it participates in the assembly of actin filaments. Owing to its location near the inner surface of the membrane, zyxin can interact with the transmembrane receptors of some signaling cascades and affect the signal transduction from the extracellular ligands of these receptors. Furthermore, under certain conditions, zyxin moves from the sites of cell contacts to the nucleus, where it directly participates in the regulation of gene expression. Of particular interest is the function of zyxin as a possible coordinator of gene expression and morphogenetic movements in embryogenesis. The published data discussed in the present review indicate the important role of zyxin in transmitting information from the regions of cell contacts to the genetic apparatus of the cell.     

Tetracyclines increase lipid phosphate phosphatase expression on plasma membranes and turnover of plasma lysophosphatidate

Extracellular lysophosphatidate and sphingosine 1-phosphate (S1P) are important bioactive lipids, which signal through G-protein-coupled receptors to stimulate cell growth and survival. The lysophosphatidate and S1P signals are terminated partly by degradation through three broad-specificity lipid phosphate phosphatases (LPPs) on the cell surface. Significantly, the expression of LPP1 and LPP3 is decreased in many cancers, and this increases the impact of lysophosphatidate and S1P signaling. However, relatively little is known about the physiological or pharmacological regulation of the expression of the different LPPs. We now show that treating several malignant and nonmalignant cell lines with 1 μg/ml tetracycline, doxycycline, or minocycline significantly increased the extracellular degradation of lysophosphatidate. S1P degradation was also increased in cells that expressed high LPP3 activity. These results depended on an increase in the stabilities of the three LPPs and increased expression on the plasma membrane. We tested the physiological significance of these results and showed that treating rats with doxycycline accelerated the clearance of lysophosphatidate, but not S1P, from the circulation. However, administering 100 mg/kg/day doxycycline to mice decreased plasma concentrations of lysophosphatidate and S1P. This study demonstrates a completely new property of tetracyclines in increasing the plasma membrane expression of the LPPs.     

Genetics and cell biology of lysophosphatidic acid receptor-mediated signaling during cortical neurogenesis

Lysophosphatidic acid (LPA) is a small lysophospholipid that signals through G-protein coupled receptors (GPCRs) to mediate diverse cellular responses. Two LPA receptors, LPA(1) and LPA(2), show gene expression profiles in mouse embryonic cerebral cortex, suggesting roles for LPA signaling in cerebral cortical development. Here, we review loss-of-function and gain-of-function models that have been used to examine LPA signaling. Genetic deletion of lpa(1) or both lpa(1) and lpa(2) in mice results in 50-65% neonatal lethality, but not obvious cortical phenotypes in survivors, suggesting that compensatory signaling systems exist for regulating cortical development. A gain-of-function model, approached by increasing receptor activation through exogenous delivery of LPA, shows that LPA signaling regulates cerebral cortical growth and anatomy by affecting proliferation, differentiation and cell survival during embryonic development.     

Recent advances on proteins of plant terminal membranes.

Since the beginning of the 1990s, our knowledge of the protein equipment of plant membranes progresses at an accelerating pace, owing to the irruption of molecular biology tools and genetics strategies in plant biology. Map-based cloning strategies and exploration of EST databases rapidly enrich the catalog of cDNA or gene sequences expected to code for membrane proteins. The accumulation of 'putative' membrane proteins reinforces the need for structural, functional and physiological information. Indeed, ambiguities often exist concerning the association to a membrane, the membrane identity and the topology of the protein inserted in the membrane. The combination of directed mutagenesis and heterologous expression of plant genes in various systems and plant reverse genetics has opened the possibility to study molecular and physiological functions. This review will emphasize how these tools have been essential for the exciting recent discoveries on plant terminal membrane proteins. These discoveries concern a variety of transport systems for ions, organic solutes including auxin, water channels, a large collection of systems suspected to act as receptors of chemical signals, proteins thought to control vesicle trafficking and enzymatic systems.     

Laminin 5 processing and its integration into the ECM.

Laminins are a family of multi-functional basement membrane proteins. Their C-terminal domain binds to cell surface receptors and is thereby responsible for cell anchorage and the initiation of specific outside-in and inside-out signals. With their N-terminal parts, laminins interact with proteins of the extracellular matrix scaffold to secure the basement membrane to the underlying mesenchymal tissue. Laminins 5A (alpha3Abeta3gamma2), 5B (alpha3Bbeta3gamma2) and 6 (alpha3Abeta1gamma1) are isoforms specific of the basement membrane underneath the epidermis and they undergo a sequential series of extracellular proteolytic changes, which might successively turn on and off one or several of their biological and mechanical functions. Under physiological conditions, such as in adult human skin, epithelial laminins have lost part of the C- and N-terminal domains of the alpha3 and gamma2 chains, respectively. In contrast, in cylindromatosis, a rare inherited disease characterised by major ultrastructural alterations of the basement membrane and altered expression/distribution of integrin receptors, laminin processing has not been completed. Together, these results suggest that laminin processing may regulate signalling pathways and the architecture of the basement membrane by restricting the repertoire of interactions with cell surface receptors and extracellular matrix components.     

Adaptor proteins of blood platelets

Adaptor proteins play a pivotal role in the regulation of signal transduction events elicited after the engagement of cell surface receptors. Platelets exhibit a number of integral membrane receptors capable of initiating a cellular response. These include collagen receptors, von Willebrand factor receptors, the fibrinogen receptor, and a number of G-protein coupled receptors, such as those for thrombin and ADP. The primary function of platelet receptors is the translation of externally applied signals into appropriate responses leading to platelet activation being a prerequisite for normal hemostasis. Multitude of signalling pathways described in platelets is based on the interaction of compounds of many different categories, such as transmembrane receptors, protein kinases, protein phoshatases, G-proteins, transmembrane and cytosolic adaptor proteins, phosphoinositides, cyclic AMP or GMP. Adaptor proteins lack intrinsic effector function, but contain distinct molecular domains, which mediate protein-protein and protein-lipid interactions. These molecules thus serve as a scaffolding, around which effectors and their substrates are assembled into three-dimensional signaling complexes. Adaptor proteins integrate receptor-mediated signals at intracellular levels and couple signaling receptors to cytosolic signaling pathways. While the function of adaptor proteins is well established in immune cells, the knowledge about their role in platelet activation is still at the onset Over the last decade numerous adaptor proteins have been identified in platelets and shown to be involved in accurate assembly of intracellular signaling complexes. Collagen-induced platelet intracellular signaling through GPVI resembles the functional response of B- and T-cell antigen receptors and is the best described in the literature. This review focuses on the structure and functional role of the most extensively studied adaptor proteins during platelet activation induced by physiological agonists.     

E-cadherin roles in animal biology: A perspective on thyroid hormone-influence

The establishment, remodeling and maintenance of tissular architecture during animal development, and even across juvenile to adult life, are deeply regulated by a delicate interplay of extracellular signals, cell membrane receptors and intracellular signal messengers. It is well known that cell adhesion molecules (cell-cell and cell-extracellular matrix) play a critical role in these processes. Particularly, adherens junctions (AJs) mediated by E-cadherin and catenins determine cell-cell contact survival and epithelia function. Consequently, this review seeks to encompass the complex and prolific knowledge about E-cadherin roles during physiological and pathological states, particularly focusing on the influence exerted by the thyroid hormone (TH).     

Beta-glucans as conductors of immune symphonies

The use of immunostimulants has received increased attention due to the discovery of Toll-like receptors (TLR) or/and pattern recognition receptors (PRR). These receptors have been found to bind molecules from a range of pathogens including self-molecules. When cell damage has occurred many of the released molecular structures act as so-called "danger" signals possessing pathogen-associated molecular patterns (PAMP). These danger signals often consist of repeating molecular moieties yielding high molecular weight compounds. Examples are beta-glucans and CpG containing DNA, but some danger signals possess low molecular weight structures. It has been found that the PRR bind unit structures of PAMP, and that PAMP-binding involves several other humoral and cell membrane proteins, exemplified by the more or less simultaneous LPS recognition displayed by MD-2, CD-14 and TLR4 on the cell membrane. Also, the binding of beta-glucans has been shown to include several different cell membrane receptors. Several immunostimulants are commercially exploited in aquaculture as feed additives. This applies to beta-glucans, alginates and nucleotides. Despite their use as feed additives no targeted approach has been conducted to include PAMP as adjuvants in fish vaccines. Interestingly, most of the PAMP studied activate antigen-presenting cells together with na?ve T cells into dendritic cells and Th1 or Th2 cells [1]. In turn, this may activate Th1 and Th2 immune responses with production of Th1 or Th2 signature molecules such as IFN-gamma and IL-4, respectively [2-4]. This review will mainly focus on binding characteristics of beta-glucans, their effects on T helper cell differentiation, effects on functional levels, gene expression profiles and application of the commonly used ss-glucan in the aquaculture sector. In addition, ss-glucans show promises in shrimp aquaculture by inducing disease resistance, this review will also highlight the use and the effects of beta-glucans in experimental models.     

Signal perception and transduction: the role of protein kinases

Cells can react to environmental changes by transduction of extracellular signals, to produce intracellular responses. Membrane-impermeable signal molecules are recognized by receptors, which are localized on the plasma membrane of the cell. Binding of a ligand can result in the stimulation of an intrinsic enzymatic activity of its receptor or the modulation of a transducing protein. The modulation of one or more intracellular transducing proteins can finally lead to the activation or inhibition of a so-called 'effector protein'. In many instances, this also results in altered gene expression. Phosphorylation by protein kinases is one of the most common and important regulatory mechanisms in signal transmission. This review discusses the non-channel transmembrane receptors and their downstream signaling, with special focus on the role of protein kinases.     

Solution NMR study of integral membrane proteins

Signals between a cell and its environment are often transmitted through membrane proteins; therefore, many membrane proteins, including G protein-coupled receptors (GPCRs) and ion channels, are important drug targets. Structural information about membrane proteins remains limited owing to challenges in protein expression, purification and the selection of membrane-mimicking systems that will retain protein structure and function. This review describes recent advances in solution NMR applied to the structural study of integral membrane proteins. The examples herein demonstrate that solution NMR spectroscopy will play a unique role not only in structural analysis, but also drug discovery of membrane proteins.