Signaling networks: the origins of cellular multitasking

[Confocal laser scanning microscopy to study molecular mechanism of mast cell activation]

In the immune system, mast cells are a key cell type in the pathogenesis of immunoglobulin E (IgE)-dependent hypersensitivity reactions. Engagement of the high-affinity IgE receptors by multivalent antigens initiates the downstream activation of signal-transducing enzymes and evokes degranulation and cytokine production via an increase in the intracellular Ca2+ concentration. In addition, mast cells also play a prominent role in non-IgE-mediated hypersensitivity reactions. Mast cells are closely apposed to nerves in vivo and are likely to be regulated functionally by nerves. However, the molecular mechanisms for mast cell activation in an IgE-dependent and -independent manner have not been fully clarified. Confocal laser scanning microscopy has played an essential role in cell biology by allowing visualization of specific intracellular signaling molecules with high spatiotemporal resolution in living cells. We have studied intracellular movements of Ca2+ using a specific fluorescent probe and several types of signaling molecules using derivatives of green fluorescent protein in a living single mast cell using a microscopic strategy. We here describe our imaging analysis of the calcium signals to the nucleus, the movement of secretory granules in the degranulation process, and the nucleocytoplasmic shuttling of mitogen-activated protein kinase in mast cells. Further, we demonstrate that direct communication between mast cells and nerves occurs. These findings provide useful information from a new perspective to understand the molecular mechanisms of allergic reaction and inflammation.

Identification of growth factor-regulated proteins using 2D electrophoresis and mass spectrometry

Proteomic technology has recently emerged as a powerful tool for detecting both qualitative and quantitative changes of proteins that occur upon activation of complex signaling pathways. In the present study, comparison of the protein profile of platelet-derived growth factor (PDGF), epidermal growth factor (EGF) and nerve growth factor (NGF)-stimulated and unstimulated cells with two-dimensional electrophoresis followed by mass spectrometric analysis led to the identification of a number of proteins, several of which had not been previously shown to be regulated by receptor-tyrosine kinases. Using subcellular fractionation, our approach was able to identify not only changes due to altered gene transcription, but also due to intracellular protein translocation or modification. One of the proteins that was identified among other PDGF-regulated molecules was prohibitin, a potential tumour suppressor previously implicated in cell cycle regulation and protection of mitochondrial proteins from degradation. Further analysis confirmed that mitochondria-associated prohibitin translocates to an insoluble perinuclear compartment. This study demonstrates the utility of proteomic strategies in identifying potential growth factor-regulated effectors.

Formation of regulatory patterns during signal propagation in a Mammalian cellular network

We developed a model of 545 components (nodes) and 1259 interactions representing signaling pathways and cellular machines in the hippocampal CA1 neuron. Using graph theory methods, we analyzed ligand-induced signal flow through the system. Specification of input and output nodes allowed us to identify functional modules. Networking resulted in the emergence of regulatory motifs, such as positive and negative feedback and feedforward loops, that process information. Key regulators of plasticity were highly connected nodes required for the formation of regulatory motifs, indicating the potential importance of such motifs in determining cellular choices between homeostasis and plasticity.

The GTPase dMiro Is Required for Axonal Transport of Mitochondria to Drosophila Synapses

We have identified EMS-induced mutations in Drosophila Miro (dMiro), an atypical mitochondrial GTPase that is orthologous to human Miro (hMiro). Mutant dmiro animals exhibit defects in locomotion and die prematurely. Mitochondria in dmiro mutant muscles and neurons are abnormally distributed. Instead of being transported into axons and dendrites, mitochondria accumulate in parallel rows in neuronal somata. Mutant neuromuscular junctions (NMJs) lack presynaptic mitochondria, but neurotransmitter release and acute Ca2+ buffering is only impaired during prolonged stimulation. Neuronal, but not muscular, expression of dMiro in dmiro mutants restored viability, transport of mitochondria to NMJs, the structure of synaptic boutons, the organization of presynaptic microtubules, and the size of postsynaptic muscles. In addition, gain of dMiro function causes an abnormal accumulation of mitochondria in distal synaptic boutons of NMJs. Together, our findings suggest that dMiro is required for controlling anterograde transport of mitochondria and their proper distribution within nerve terminals.

Discovery and in vitro evaluation of potent kinase inhibitors: Pyrido[1′,2′:1,5]pyrazolo[3,4-d]pyrimidines

The discovery, synthesis, potential binding mode, and in vitro kinase profile of several pyrido[1',2':1,5]pyrazolo[3,4-d]pyrimidines as potent kinase inhibitors are discussed.

Toward predictive models of mammalian cells

Progress in experimental and theoretical biology is likely to provide us with the opportunity to assemble detailed predictive models of mammalian cells. Using a functional format to describe the organization of mammalian cells, we describe current approaches for developing qualitative and quantitative models using data from a variety of experimental sources. Recent developments and applications of graph theory to biological networks are reviewed. The use of these qualitative models to identify the topology of regulatory motifs and functional modules is discussed. Cellular homeostasis and plasticity are interpreted within the framework of balance between regulatory motifs and interactions between modules. From this analysis we identify the need for detailed quantitative models on the basis of the representation of the chemistry underlying the cellular process. The use of deterministic, stochastic, and hybrid models to represent cellular processes is reviewed, and an initial integrated approach for the development of large-scale predictive models of a mammalian cell is presented.

The G alpha(o/i)-coupled cannabinoid receptor-mediated neurite outgrowth involves Rap regulation of Src and Stat3

The study of the signaling pathways regulating neurite outgrowth in culture is important because of their potential role in neuronal differentiation in vivo. We have previously shown that the G alpha(o/i)-coupled CB1 cannabinoid receptor (CB1R) activates Rap1 to induce neurite outgrowth. G alpha(o/i) also activates the Src-Stat3 pathway. Here, we studied the relationship between the G alpha(o/i)-Rap1 and Src-Stat3 pathways and the role of these signaling pathways in CB1R-mediated neurite outgrowth in Neuro-2A cells. The CB1 agonist HU-210 induced pertussis toxin-sensitive Src and Stat3 phosphorylation. Dominant negative (DN) mutants of Src and Stat3 blocked CB1R-induced neurite outgrowth. Constitutively active Rap 1B and Ral-activated Src and CB1R-induced Src phosphorylation was inhibited by Rap1-DN and Ral-DN, indicating that both Rap1 and Ral mediate downstream signaling from G alpha(o/i) for Src activation. Rap1-activated Ral and Ral-DN blocked Rap-induced Src phosphorylation. G alpha(o)-induced Stat3 activation was blocked by Ral-DN, whereas v-Src-induced Stat3 activation was not inhibited by Ral-DN, indicating that the CB1R, through G alpha(o), mediates the sequential activation of Rap1 to Ral to Src to Stat3 in Neuro-2A cells. Downstream of Src, the CB1R also activated Rac1 and JNK, which enhanced CBR1-mediated Stat3 activation. Rac-DN blocked CB1R-induced activation of JNK. Pharmacological inhibition of JNK blocked Src and CB1R activation of Stat3, indicating that Rac and JNK are also involved in CB1R-mediated neurite outgrowth. Overall, this study demonstrated that G alpha(o/i)-coupled CB1R triggers neurite outgrowth in Neuro-2A through the activation of a signaling network containing two pathways that bifurcate at Src and converge at Stat3.

Binding of activated alpha2-macroglobulin to its cell surface receptor GRP78 in 1-LN prostate cancer cells regulates PAK-2-dependent activation of LIMK

Two characteristics of highly malignant cells are their increased motility and secretion of proteinases allowing these cells to penetrate surrounding basement membranes and metastasize. Activation of 21-kDa activated kinases (PAKs) is an important mechanism for increasing cell motility. Recently, we reported that binding of receptor-recognized forms of the proteinase inhibitor alpha2-macroglobulin (alpha2M*) to GRP78 on the cell surface of 1-LN human prostate cancer cells induces mitogenic signaling and cellular proliferation. In the current study, we have examined the ability of alpha2M* to activate PAK-1 and PAK-2. Exposure of 1-LN cells to alpha2M* caused a 2- to 3-fold increase in phosphorylated PAK-2 and a similar increase in its kinase activity toward myelin basic protein. By contrast, the phosphorylation of PAK-1 was only negligibly affected. Silencing the expression of the GRP78 gene, using either of two different mRNA sequences, greatly attenuated the appearance of phosphorylated PAK-2 in alpha2M*-stimulated cells. Treatment of 1-LN cells with alpha2M* caused translocation of PAK-2 in association with NCK to the cell surface as evidenced by the co-immunoprecipitation of PAK-2 and NCK in the GRP78 immunoprecipitate from plasma membranes. alpha2M*-induced activation of PAK-2 was inhibited by prior incubation of the cells with specific inhibitors of tyrosine kinases and phosphatidylinositol 3-kinase. PAK-2 activation was accompanied by significant increases in the levels of phosphorylated LIMK and phosphorylated cofilin. Silencing the expression of the PAK-2 gene greatly attenuated the phosphorylation of LIMK. In conclusion, we show for the first time the activation of PAK-2 in 1-LN prostate cancer cells by a proteinase inhibitor, alpha2-macroglobulin. These studies suggest a mechanism by which alpha2M* enhances the metastatic potential of these cells.

Computational approaches for modeling regulatory cellular networks

Cellular components interact with each other to form networks that process information and evoke biological responses. A deep understanding of the behavior of these networks requires the development and analysis of mathematical models. In this article, different types of mathematical representations for modeling signaling networks are described, and the advantages and disadvantages of each type are discussed. Two experimentally well-studied signaling networks are then used as examples to illustrate the insight that could be gained through modeling. Finally, the modeling approach is expanded to describe how signaling networks might regulate cellular machines and evoke phenotypic behaviors.

Transduction of receptor signals by beta-arrestins

The transmission of extracellular signals to the interior of the cell is a function of plasma membrane receptors, of which the seven transmembrane receptor family is by far the largest and most versatile. Classically, these receptors stimulate heterotrimeric G proteins, which control rates of generation of diffusible second messengers and entry of ions at the plasma membrane. Recent evidence, however, indicates another previously unappreciated strategy used by the receptors to regulate intracellular signaling pathways. They direct the recruitment, activation, and scaffolding of cytoplasmic signaling complexes via two multifunctional adaptor and transducer molecules, beta-arrestins 1 and 2. This mechanism regulates aspects of cell motility, chemotaxis, apoptosis, and likely other cellular functions through a rapidly expanding list of signaling pathways.

Reactive species-mediated regulation of cell signaling and the cell cycle: the role of MAPK

Cardiovascular disease development is significantly influenced by the effects of reactive species (RS). By virtue of their controlled production, regulation, and reactive nature, RS play important roles in the modulation of cellular signaling, growth, and death in the vasculature. Concentration gradients are important in determining the effects of RS. Low to moderate concentrations of RS act as mediators in signaling cascades and gene regulation, whereas high levels of RS cause cellular damage and death. Because a dual redox regulation state seems to exist in several signaling cascades, e.g., RS often induce upstream initiating events, whereas downstream events are reliant on reductive processes, alterations in cellular redox states influence the activation/inactivation of signaling events and transcription factors. In this review, the relationships between RS, specific signal transduction pathways, and aspects of cell-cycle control are discussed.

Teaching resources. G-protein effectors

This Teaching Resource provides lecture notes and slides for a class covering two aspects of G protein-mediated signaling and is part of the course "Cell Signaling Systems: A Course for Graduate Students." The lecture begins with a discussion of the regulation of receptor-G protein coupling and then proceeds to describe the specificity of the response achieved through regulation of specific isoforms of effectors.

Exercise activates the phosphatidylinositol 3-kinase pathway

Physical exercise is known to enhance psychological well-being and coping capacity. Voluntary physical exercise in rats also robustly and rapidly up-regulates hippocampal brain-derived neurotrophic factor (BDNF) mRNA levels, which are potentiated following a regimen of chronic antidepressant treatment. Increased BDNF levels are associated with enhanced activity of cyclic AMP response element binding protein (CREB). So far, relatively little is known about the intracellular signaling mechanisms mediating this effect of exercise. We wished to explore the possibility that exercise and/or antidepressant treatment activate the hippocampal phosphatidylinositol-3 (PI-3) kinase pathway, which mediates cellular survival. In young male Sprague-Dawley rats, we examined the effects of 2 weeks of daily voluntary wheel-running activity and/or tranylcypromine (n = 7 per group) on the levels of the active forms of protein-dependent kinase-1 (PDK-1), PI-3 kinase, phospho-thr308-Akt, phospho-ser473-Akt, and phospho-glycogen synthase kinase-3beta (GSK3beta; inactive form), as well as BDNF, activated CREB, and the phospho-Trk receptor, in the rat hippocampus, and compared these with sedentary saline-treated controls. Immunoblotting analyses revealed that in exercising rats, there was a significant increase in PI-3 kinase expression (4.61 times that of controls, P = 0.0161) and phosphorylation of PDK-1 (2.73 times that of controls, P = 0.0454), thr308-Akt (2.857 times that of controls, P = 0.0082), CREB (60.27 times that of controls, P = 0.05), and Trk (35.3 times that of controls, P < 0.0001) in the hippocampi of exercising animals; BDNF was also increased (3.2 times that of controls), but this was not statistically significant. In rats receiving both exercise and tranylcypromine, BDNF (4.51 times that of controls, P = 0.0068) and PI-3 kinase (4.88 times that of controls, P = 0.0103), and the phospho- forms of Trk (13.67 times that of controls, P = 0.0278), thr308-Akt (3.644 times that of controls, P = 0.0004), GSK-3beta (2.93 times that of controls, P = 0.026), and CREB (88.97 times that of controls, P = 0.0053) were significantly increased. These results suggest that the exercise-induced expression of BDNF is associated with the increased expression of several key intermediates of the PI-3 kinase/Akt pathway, which is known for its role in enhancing neuronal survival.

Differential effects of regulator of G protein signaling (RGS) proteins on serotonin 5-HT1A, 5-HT2A, and dopamine D2 receptor-mediated signaling and adenylyl cyclase activity

Regulator of G protein signaling (RGS) proteins function as GTPase accelerating proteins (GAP) for Galpha subunits, attenuating G-protein-coupled receptor signal transduction. The present study tested the ability of members of different subfamilies of RGS proteins to modulate both G-protein-dependent and -independent signaling in mammalian cells. RGS4, RGS10, and RGSZ1 significantly attenuated Galphai-mediated signaling by 5-HT1A, but not by dopamine D2, receptor-expressing cells. Additionally, RGS4 and RGS10 significantly inhibited forskolin-stimulated cAMP production in both cell lines. In contrast, RGS2, RGS7, and RGSZ1 had no effect on forskolin-stimulated cAMP production in these cells. RGS2 and RGS7 significantly decreased Galphaq-mediated signaling by 5-HT2A receptors, confirming that the RGS4 and RGS10 effects on forskolin-stimulated cAMP production were specific, and not simply due to overexpression. Interestingly, similar expression levels of RGS4 protein resulted in greater inhibition of G-protein-independent cAMP production compared to G-protein-dependent GAP activity. Our results suggest specificity and selectivity of RGS proteins on G-protein-dependent and -independent signaling in mammalian cells.

Introduction to the concept of functioning-dependent structures in living cells

The assembly of proteins into larger structures may confer advantages such as increased resistance to hydrolytic enzymes. metabolite channelling, and reduction of the number of proteins or other active molecules required for cell functioning. We propose the term functioning-dependent structures (FDSs) for those associations of proteins that are created and maintained by their action in accomplishing a function, as reported in many experiments. Here we model the simplest possible cases of two-partner FDSs in which the associations either catalyse or inhibit reactions. We show that FDSs may display regulatory properties (e.g., a sigmoidal response or a linear kinetic behaviour over a large range of substrate concentrations) even when the individual proteins are enzymes of the Michaelis-Menten type. The possible involvement of more complicated FDSs or of FDS networks in real living systems is discussed. From the thermodynamic point of view, FDS formation and decay are responsible for an extra production of entropy, which may be considered characteristic of living systems.

Teaching resources. Introduction: Overview of pathways and networks and GPCR signaling

This Teaching Resource provides the overview to the course "Cell Signaling Systems: A Course for Graduate Students" and lays out the general principles that can be deduced from the current understanding of the organization of cell signaling pathways and networks and how information flows through these pathways and networks. In addition, the lecture provides an overview of G protein-coupled receptor (GPCR) signaling. A description of the lecture, along with a set of slides (http://stke.sciencemag.org/cgi/content/full/sigtrans;2005/270/tr4/DC1) used to present this information, is provided.

Postsynaptic signaling networks: cellular cogwheels underlying long-term plasticity

Learning depends on positive or negative changes in synaptic transmission that are synapse-specific and sustained. Synaptic signals can be directly measured and respond to certain kinds of stimulation by becoming persistently enhanced (long-term potentiation, LTP) or decreased (long-term depression, LTD). Studying LTP and LTD opens a window on to the molecular mechanisms of memory. Although changes in both pre- and postsynaptic strength have been implicated in LTP and LTD, most attention has been focused on changes in postsynaptic glutamate receptor density. This is controlled by intracellular Ca(2+) ions via a network of signaling molecules. Changes in postsynaptic Ca(2+) concentration depend on the coincidence of appropriate synaptic signals, as is found in learning situations. The long-term persistence of LTP and LTD requires gene transcription and translation. It is posited that local translation at the synapse, in a self-sustaining manner, mediates the persistence of long-term changes despite constant turnover of the synaptic components.

Modeling cell signaling networks

Cell signaling pathways interact with one another to form networks in mammalian systems. Such networks are complex in their organization and exhibit emergent properties such as bistability and ultrasensitivity. Analysis of signaling networks requires a combination of experimental and theoretical approaches including the development and analysis of models. This review focuses on theoretical approaches to understanding cell signaling networks. Using heterotrimeric G protein pathways an example, we demonstrate how interactions between two pathways can result in a network that contains a positive feedback loop and function as a switch. Different mathematical approaches that are currently used to model signaling networks are described, and future challenges including the need for databases as well as enhanced computing environments are discussed.

Common and distinct elements in cellular signaling via EGF and FGF receptors

Signaling pathways that are activated by epidermal growth factor (EGF) or fibroblast growth factor (FGF) receptors have been identified and compared (detailed Connections Maps are available at Science's Signal Transduction Knowledge Environment). Both receptors stimulate a similar complement of intracellular signaling pathways. However, whereas activated EGF receptors (EGFRs) function as the main platform for recruitment of signaling proteins, signaling through the FGF receptors (FGFRs) is mediated primarily by assembly of a multidocking protein complex. Moreover, FGFR signaling is subject to additional intracellular and extracellular control mechanisms that do not affect EGFR signaling. The differential circuitry of the intracellular networks that are activated by EGFR and FGFR may affect signal specificity and physiological responses.

Biological robustness

Robustness is a ubiquitously observed property of biological systems. It is considered to be a fundamental feature of complex evolvable systems. It is attained by several underlying principles that are universal to both biological organisms and sophisticated engineering systems. Robustness facilitates evolvability and robust traits are often selected by evolution. Such a mutually beneficial process is made possible by specific architectural features observed in robust systems. But there are trade-offs between robustness, fragility, performance and resource demands, which explain system behaviour, including the patterns of failure. Insights into inherent properties of robust systems will provide us with a better understanding of complex diseases and a guiding principle for therapy design.

From Transcription Profile to Expression: The Signaling Repertoire of the SK-N-MC Neuroepithelioma Cell-Line

SK-N-MC neuroepithelioma cells are routinely cultured and widely used as a model system in biochemical and pharmacological experiments. To clarify the gene expression patterns of SK-N-MC cells with respect to G protein-coupled receptors and signaling network components, we describe in this report the transcription profile of the cell line. Following the traditional pathway from genome to proteome, selected examples are further examined at the level of protein expression and by functional assays. cRNA targets derived from total RNA extracts were hybridized to Affymetrix Human Genome U133A GeneChip arrays, and the data were analyzed and grouped according to functional aspects. Results obtained for neuropeptide Y (NPY) Y1, Y5, and orexin Ox1 receptors were confirmed by RT-PCR. It is surprising that we found the presence of both NPY receptor subtypes and the absence of the orexin receptor at the mRNA level. Receptor-binding experiments confirmed NPY binding of the Y1 receptor in the nanomolar range but gave no evidence for high expression levels of Y5 receptor subtypes on the cell surface. Protein expression was assayed with immunoblots by using antibodies directed against selected Galpha protein subunits. The presence of at least Galphas, Galphai3, and Galphai2 subunits was indicated.

The nuclear receptor co-repressor (N-CoR) utilizes repression domains I and III for interaction and co-repression with ETO

The acute human leukemias are associated with the presence of chimeric gene products that arise from spontaneous chromosomal translocations. The t(8;21) translocation gene product led to the discovery of the Eight Twenty-One (ETO) gene. When fused to RUNX1, ETO is thought to mediate the formation of a repressive complex at RUNX1-dependent genes. ETO has also been found to act as a co-repressor of the promyelocytic zinc finger and Bcl-6 oncoproteins, suggesting that it may play a common role as a transcriptional co-repressor leading to human disease. An analysis of ETO-mediated repression revealed that one of the key binding partners of ETO is the nuclear receptor co-repressor (N-CoR). It is shown that two highly conserved domains of ETO interact with repression domains I and III of N-CoR. One of the ETO domains displays significant homology to Drosophila TAF(II)110, whereas the other is a predicted zinc binding motif that engages a conserved PPLXP motif in repression domain III of N-CoR. Together, these domains of ETO cooperate in repression with N-CoR and the binding sites in N-CoR overlap with those for other repressive factors. Thus, ETO has the potential to participate in a number of repressive complexes, which can be distinguished by their binding partners and target genes.

Quantitative information management for the biochemical computation of cellular networks

Understanding complex protein networks within cells requires the ability to develop quantitative models and to numerically compute the properties and behavior of the networks. To carry out such computational analysis, it is necessary to use modeling tools and information management systems (IMSs) where the quantitative data, associated to its biological context, can be stored, curated, and reliably retrieved. We have focused on the biochemical computation of cellular interactions and developed an IMS that stores both quantitative information on the cellular components and their interactions, and the basic reactions governing those interactions. This information can be used to construct pathways and eventually large-scale networks. This system, SigPath, is available on the Internet (http://www.sigpath.org). Key features of the approach include (i) the use of background information (for example, names of molecules, aliases, and accession codes) to ease data submission and link this quantitative database with other qualitative databases, (ii) a strategy to allow refinement of information over time by multiple users, (iii) the development of a data representation that stores both qualitative and quantitative information, and (iv) features to assist contributors and users in assembling custom quantitative models from the information stored in the IMS. Currently, models assembled in SigPath can be automatically exported to several computing environments, such as Kinetikit/Genesis, Virtual Cell, Jarnac/JDesigner, and JSim. We anticipate that, when appropriately populated, such a system will be useful for large-scale quantitative studies of cell-signaling networks and other cellular networks. SigPath is distributed under the GNU General Public License.

The microtubule plus end tracking protein Orbit/MAST/CLASP acts downstream of the tyrosine kinase Abl in mediating axon guidance

Axon guidance requires coordinated remodeling of actin and microtubule polymers. Using a genetic screen, we identified the microtubule-associated protein Orbit/MAST as a partner of the Abelson (Abl) tyrosine kinase. We find identical axon guidance phenotypes in orbit/MAST and Abl mutants at the midline, where the repellent Slit restricts axon crossing. Genetic interaction and epistasis assays indicate that Orbit/MAST mediates the action of Slit and its receptors, acting downstream of Abl. We find that Orbit/MAST protein localizes to Drosophila growth cones. Higher-resolution imaging of the Orbit/MAST ortholog CLASP in Xenopus growth cones suggests that this family of microtubule plus end tracking proteins identifies a subset of microtubules that probe the actin-rich peripheral growth cone domain, where guidance signals exert their initial influence on cytoskeletal organization. These and other data suggest a model where Abl acts as a central signaling node to coordinate actin and microtubule dynamics downstream of guidance receptors.

Emerging classes of protein–protein interaction inhibitors and new tools for their development

Protein-protein interactions play a key role in the signal transduction pathways that regulate cellular function. Three years ago, few descriptions of small molecule protein-protein interaction inhibitors (SMPPIIs) existed in the literature. Today, the number of examples of both the biology and chemistry of such interaction inhibitors is growing rapidly. This growth occurs at the convergence of medicinal chemistry, signaling biology and novel assay technology for profiling emerging compound classes and modes of action. Protein translocation assays provide a unique new tool for identifying, profiling, and optimizing SMPPIIs. This review summarizes recent work in the field, and outlines future developments we can anticipate.

Protein phosphatase 2A as a new target for morphogenetic studies in the chick limb

The family of ser/thr protein phosphatases 2A (PP2A) is a major regulator of cell proliferation and cell death and is critically involved in the maintenance of homeostasis. In order to analyse the importance of PP2A proteins in apoptotic and developmental processes, this review focuses on previous studies concerning the role of PP2A in morphogenesis. We first analyse wing formation in Drosophila, a model for invertebrates, then chick limb bud, a model for vertebrates. We also present a pioneer experiment to illustrate the potential relevance of PP2A studies in BMP signalling during chicken development and we finally discuss the BMP downstream signalling pathways.

Signaling control of mRNA translation in cancer pathogenesis

The regulation of translation and the control of ribosome biogenesis are essential cellular processes whose impact on cell growth and proliferation is manifested at a number of specific levels. Disruption in one or more of the steps that control protein biosynthesis has been associated with alterations in the regulation of cell growth and cell cycle progression. Consistent with this, tumor suppressors and proto-oncogenes have been found to act on these functions and may therefore regulate malignant progression by affecting the protein synthetic machinery. Although many studies have correlated deregulation of protein biosynthesis with cancer, it remains to be established whether this process is necessary and/or sufficient for neoplastic transformation and metastasis.

Robust global sensitivity in multiple enzyme cascade system explains how the downstream cascade structure may remain unaffected by cross-talk

A steady-state framework was applied to the ubiquitous tricyclic enzyme cascade structure, as seen in the mitogen-activated protein (MAP) kinase system, to analyze the effect of upstream kinase concentrations on final output response. The results suggest that signal amplification achieved by the cascade structure ensured that the modifying enzymes at various steps of the cascade were nearly saturated. Thus, there was no change in the response sensitivity with increasing upstream kinase concentration. Analysis was also extended to branching of a signaling pathway as an example of cross-talk. It was observed that the cascade structure confers a larger share of the signal transduction properties to its last kinase. This phenomenon in enzyme cascades may explain how the response of the terminal MAP kinase is unaffected by cross-talk of upstream kinases.

Opposing effects of PI3 kinase pathway activation on human myeloid and erythroid progenitor cell proliferation and differentiation in vitro

OBJECTIVE

To investigate 1) the effects of lineage-specific cytokines (G-CSF and EPO) combined with ligands for different classes of cytokine receptors (common beta chain, gp130, and tyrosine kinase) on proliferation by human myeloid and erythroid progenitor cells; and 2) the signal transduction pathways associated with combinatorial cytokine actions.

PATIENTS AND METHODS

CFU-GM and BFU-E were cloned in vitro. Secondary colony formation by replated CFU-GM and subcolony formation by BFU-E provided measures of progenitor cell proliferation. Studies were performed in the presence of cytokine combinations with and without signal transduction inhibitors.

RESULTS

Proliferation by CFU-GM and BFU-E was enhanced synergistically when common beta chain receptor cytokines (IL-3 or GM-CSF) were combined with G-CSF or EPO, but not with gp130 receptor cytokines (LIF or IL-6) or tyrosine kinase receptor cytokines (SCF, HGF, Flt-3 ligand, or PDGF). Delayed addition studies with G-CSF+IL-3 and EPO+IL-3 demonstrated that synergy required the presence of both cytokines from the initiation of the culture. The Jak2-specific inhibitor, AG490, abrogated the effect of combining IL-3 with EPO but had no effect on the enhanced CFU-GM proliferation stimulated by IL-3+G-CSF. The PI3 kinase inhibitors LY294002 and wortmannin substituted for G-CSF in combination with IL-3 since proliferation in the presence of LY294002/wortmannin+IL-3 was enhanced to the same extent as in the presence of G-CSF+IL-3. In contrast, LY294002 and wortmannin inhibited proliferation in the presence of EPO and in the presence of EPO+IL-3.

CONCLUSION

1) IL-3 may activate different signal transduction pathways when combined with G-CSF and when combined with EPO; 2) different signal transducing intermediates regulate erythroid and myeloid progenitor cell proliferation; and 3) inhibition of the PI3 kinase pathway suppresses myeloid progenitor cell differentiation and thereby increases proliferation.

Visualization of Molecular Activities Inside Living Cells with Fluorescent Labels

The major task of modern cell biology is to identify the function and relation of the many different gene products, discovered by genomics and proteomics approaches, in the context of the living cell. To achieve this goal, an increasing toolbox of custom-designed biosensors based on fluorescent labels is available to study the molecular activities of the cellular machinery. An overview of the current status of the young field of molecular-cellular physiology is presented that includes the application of fluorescent labels in the design of biosensors and the major detection schemes used to extract their sensing information. In particular, the use of the photophysical phenomenon of Förster resonance energy transfer (FRET) as a powerful indicator of cellular biochemical events is discussed. In addition, we will point out the challenges and directions of the field and project the short-term future for the application of fluorescence-based biosensors in biology.

Proteomics Analysis of Rat Brain Postsynaptic Density

The postsynaptic density contains multiple protein complexes that together relay the presynaptic neurotransmitter input to the activation of the postsynaptic neuron. In the present study we took two independent proteome approaches for the characterization of the protein complement of the postsynaptic density, namely 1) two-dimensional gel electrophoresis separation of proteins in conjunction with mass spectrometry to identify the tryptic peptides of the protein spots and 2) isolation of the trypsin-digested sample that was labeled with isotope-coded affinity tag, followed by liquid chromatography-tandem mass spectrometry for the partial separation and identification of the peptides, respectively. Functional grouping of the identified proteins indicates that the postsynaptic density is a structurally and functionally complex organelle that may be involved in a broad range of synaptic activities. These proteins include the receptors and ion channels for glutamate neurotransmission, proteins for maintenance and modulation of synaptic architecture, sorting and trafficking of membrane proteins, generation of anaerobic energy, scaffolding and signaling, local protein synthesis, and correct protein folding and breakdown of synaptic proteins. Together, these results imply that the postsynaptic density may have the ability to function (semi-) autonomously and may direct various cellular functions in order to integrate synaptic physiology.

Somatic activation of oncogenic Kras in hematopoietic cells initiates a rapidly fatal myeloproliferative disorder

RAS mutations are common in myeloid malignancies; however, it is not known whether oncogenic RAS can initiate leukemia. We show that expressing mutant K-Ras(G12D) protein from the endogenous murine locus rapidly induces a fatal myeloproliferative disorder with 100% penetrance characterized by tissue infiltration, hypersensitivity to growth factors, and hyperproliferation. Hematopoietic cells from diseased mice demonstrated increased levels of Ras-GTP, but effector kinases were not constitutively phosphorylated and responded normally to growth factors. Oncogenic RAS is sufficient to initiate myeloid leukemogenesis in mice, and this provides an in vivo system for biologic and preclinical studies.

Mechanical regulation of mitogen-activated protein kinase signaling in articular cartilage

Articular chondrocytes respond to mechanical forces by alterations in gene expression, proliferative status, and metabolic functions. Little is known concerning the cell signaling systems that receive, transduce, and convey mechanical information to the chondrocyte interior. Here, we show that ex vivo cartilage compression stimulates the phosphorylation of ERK1/2, p38 MAPK, and SAPK/ERK kinase-1 (SEK1) of the JNK pathway. Mechanical compression induced a phased phosphorylation of ERK consisting of a rapid induction of ERK1/2 phosphorylation at 10 min, a rapid decay, and a sustained level of ERK2 phosphorylation that persisted for at least 24 h. Mechanical compression also induced the phosphorylation of p38 MAPK in strictly a transient fashion, with maximal phosphorylation occurring at 10 min. Mechanical compression stimulated SEK1 phosphorylation, with a maximum at the relatively delayed time point of 1 h and with a higher amplitude than ERK1/2 and p38 MAPK phosphorylation. These data demonstrate that mechanical compression alone activates MAPK signaling in intact cartilage. In addition, these data demonstrate distinct temporal patterns of MAPK signaling in response to mechanical loading and to the anabolic insulin-like growth factor-I. Finally, the data indicate that compression coactivates distinct signaling pathways that may help define the nature of mechanotransduction in cartilage.

hSef inhibits PC-12 cell differentiation by interfering with Ras-mitogen-activated protein kinase MAPK signaling

Growth factor signaling by receptor tyrosine kinases regulates several cell fates, such as proliferation and differentiation. Sef was genetically identified as a negative regulator of fibroblast growth factor (FGF) signaling. Using bioinformatic methods and rapid amplification of cDNA ends-PCR, we isolated both the mouse and the human Sef genes, which encoded the Sef protein and Sef-S isoform that was generated through alternative splicing. We provide evidence that the Sef gene products were located mainly on the cell membrane. Co-immunoprecipitation and immunostaining experiments indicate that hSef interacts with FGFR1 and FGFR2 but not FGFR3. Our results demonstrated that stably expressed hSef strongly inhibits FGF2- or nerve growth factor-induced PC-12 cell differentiation. The intracellular domain of hSef is necessary for the inhibitory effect on FGF2-induced PC-12 cell differentiation. Furthermore, our data suggested Sef exerted the negative effect on FGF2-induced PC-12 cell differentiation through the prevention of Ras-mitogen-activated protein kinase signaling, possibly functioning upstream of the Ras molecule. These findings suggest that Sef may play an important role in the regulation of PC-12 cell differentiation.

Transcriptome fingerprints distinguish hallucinogenic and nonhallucinogenic 5-hydroxytryptamine 2A receptor agonist effects in mouse somatosensory cortex

Most neuropharmacological agents and many drugs of abuse modulate the activity of heptahelical G-protein-coupled receptors. Although the effects of these ligands result from changes in cellular signaling, their neurobehavioral activity may not correlate with results of in vitro signal transduction assays. 5-Hydroxytryptamine 2A receptor (5-HT2AR) partial agonists that have similar pharmacological profiles differ in the behavioral responses they elicit. In vitro studies suggest that different agonists acting at the same receptor may establish distinct patterns of signal transduction. Testing this hypothesis in the brain requires a global signal transduction assay that is applicable in vivo. To distinguish the cellular effects of the different 5-HT2AR agonists, we developed an assay for global signal transduction on the basis of high throughput quantification of rapidly modulated transcripts. Study of the responses to agonists in human embryonic kidney 293 cells stably expressing 5-HT2ARs demonstrated that each agonist elicits a distinct transcriptome fingerprint. We therefore studied behavioral and cortical signal transduction responses in wild-type and 5-HT2AR null-mutant mice. The hallucinogenic chemicals (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) and lysergic acid diethylamide (LSD) stimulated a head-twitch behavioral response that was not observed with the nonhallucinogenic lisuride hydrogen maleate (LHM) and was absent in receptor null-mutant mice. We also found that DOI, LSD, and LHM each induced distinct transcriptome fingerprints in somatosensory cortex that were absent in 5-HT2AR null-mutants. Moreover, DOI and LSD showed similarities in the transcriptome fingerprints obtained that were not observed with the behaviorally inactive drug LHM. Our results demonstrate that chemicals acting at the 5-HT2AR induce specific cellular response patterns in vivo that are reflected in unique changes in the somatosensory cortex transcriptome.

Identification of Novel Signaling Complexes by Functional Proteomics

The rapid development of proteomic technologies, combined with the completion of the Human Genome Map, has enabled the compiling of an unprecedented inventory of cellular proteins. Functional proteomics is an emerging field that aims to utilize the enormous amount of information provided by these proteomic technologies to understand the functions of cellular proteins. The utility of functional proteomics has been recently exploited to elucidate cellular mechanisms in numerous fields, of particular salience in the area of signal transduction. This review presents a functional proteomic approach for the study of cardiac cell signaling. It illustrates the strategies by which the subproteome of a targeted signaling system is characterized in an unbiased fashion, the manner in which the biochemical functions of this subproteome are assessed using established molecular and protein chemistry methods, and the challenges associated with these studies.

Transcriptome fingerprints distinguish hallucinogenic and nonhallucinogenic 5-hydroxytryptamine 2A receptor agonist effects in mouse somatosensory cortex

Most neuropharmacological agents and many drugs of abuse modulate the activity of heptahelical G-protein-coupled receptors. Although the effects of these ligands result from changes in cellular signaling, their neurobehavioral activity may not correlate with results of in vitro signal transduction assays. 5-Hydroxytryptamine 2A receptor (5-HT2AR) partial agonists that have similar pharmacological profiles differ in the behavioral responses they elicit. In vitro studies suggest that different agonists acting at the same receptor may establish distinct patterns of signal transduction. Testing this hypothesis in the brain requires a global signal transduction assay that is applicable in vivo. To distinguish the cellular effects of the different 5-HT2AR agonists, we developed an assay for global signal transduction on the basis of high throughput quantification of rapidly modulated transcripts. Study of the responses to agonists in human embryonic kidney 293 cells stably expressing 5-HT2ARs demonstrated that each agonist elicits a distinct transcriptome fingerprint. We therefore studied behavioral and cortical signal transduction responses in wild-type and 5-HT2AR null-mutant mice. The hallucinogenic chemicals (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) and lysergic acid diethylamide (LSD) stimulated a head-twitch behavioral response that was not observed with the nonhallucinogenic lisuride hydrogen maleate (LHM) and was absent in receptor null-mutant mice. We also found that DOI, LSD, and LHM each induced distinct transcriptome fingerprints in somatosensory cortex that were absent in 5-HT2AR null-mutants. Moreover, DOI and LSD showed similarities in the transcriptome fingerprints obtained that were not observed with the behaviorally inactive drug LHM. Our results demonstrate that chemicals acting at the 5-HT2AR induce specific cellular response patterns in vivo that are reflected in unique changes in the somatosensory cortex transcriptome.

Identification of an IQGAP1/AKAP79 complex in beta-cells

IQGAP1, is a recently discovered scaffold protein proposed to regulate membrane cytoskeleton events through protein-protein interactions with F-actin, E-cadherin, beta-catenin, and CLIP170. The binding of IQGAP1 to its partners is regulated by calcium/calmodulin (Ca(++)/CaM) and the small molecular weight guanine nucleotide triphosphatases (GTPases), Cdc42, and Rac1. Here we identify a novel IQGAP1 scaffolding function by isolating the cyclic AMP dependent kinase (PKA) with IQGAP1. IQGAP1 was co-purified with PKA using 5'-cyclic AMP (cAMP) affinity chromatography and PKA activity was co-immunoprecipitated with IQGAP1 using an anti-IQGAP1 antibody. The association of IQGAP1 with PKA was shown to occur through a direct interaction between A kinase anchoring protein 79 (AKAP79) and the carboxyl-terminal domain of IQGAP1. This suggests that cAMP/PKA may be coupled with Ca(++)/CaM and GTPases through an IQGAP1/AKAP79 complex.

A constitutively active somatic mutation of the human lutropin receptor found in Leydig cell tumors activates the same families of G proteins as germ line mutations associated with Leydig cell hyperplasia

Using a Leydig tumor cell line (MA-10) transiently transfected with the human lutropin receptor (hLHR) and mutants thereof, we examined the identity of the G proteins activated by the agonist-engaged hLHR-wild type (wt) and by three of its naturally occurring constitutively active mutants. Two of the mutants examined, L457R in transmembrane helix 3 and D578Y in transmembrane helix 6, are germ-line mutations found in boys with Leydig cell hyperplasia and precocious puberty. The third, D578H, is a somatic mutation found in Leydig cell tumors in boys with precocious puberty. We show that the hLHR-wt and the three mutants activate the G(s), G(i/o), and G(q/11), but not the G(12/13), families of G proteins. The activation of these G proteins by the hLHR-wt occurs only when engaged by agonist, but their activation by the L457R, D578Y, and D578H mutants occurs independently of agonist stimulation. We conclude that the G proteins activated by constitutively active mutants of the hLHR associated with Leydig cell hyperplasia or tumors are identical and are the same as those activated by the agonist-engaged hLHR-wt. If there was preferential activation of some G protein families by the somatic D578H mutation found in Leydig cell tumors as opposed to the germ line mutations found in Leydig cell hyperplasia, then one could envision mechanisms by which the D578H mutant would be oncogenic. The data presented here suggest that such mechanisms do not need to be considered.

The Yeast G Protein α Subunit Gpa1 Transmits a Signal through an RNA Binding Effector Protein Scp160

In yeast Saccharomyces cerevisiae the G protein betagamma subunits (Ste4/Ste18) have long been known to transmit the signal required for mating. Here we demonstrate that GTPase-deficient mutants of Galpha (Gpa1) directly activate the mating response pathway. We also show that signaling by activated Gpa1 requires direct coupling to an RNA binding protein Scp160. These findings suggest an additional role for Gpa1 and reveal Scp160 as a component of the mating response pathway in yeast.

PKA and PKC content in the honey bee central brain differs in genotypic strains with distinct foraging behavior

Selection of honey bees for pollen storage resulted in high and low pollen-hoarding strains differing in foraging behavior traits including resource choice and quality, load size, sucrose responsiveness, age of foraging initiation, and learning performance. To determine how these genotypic differences correlate with changes at the level of proteins involved in neuronal function, we measured the content of protein kinase A, protein kinase C, and synapsin in the brains of high- and low-strain bees. In the central brain protein kinase A and protein kinase C levels were greater in high-strain bees and increased from emergence to 5 days in both strains. By 15 days, high-strain bees retained significantly higher levels of protein kinase C than low-strain bees, but overall protein kinase C content decreased in both strains. Synapsin levels increased from emergence to 5 days but did not differ between the two strains. In contrast to the protein kinase A content in the central brain, the basal protein kinase A activity did not differ between the strains or between the two age groups. This provides first evidence that the two genetic strains of honey bees show characteristic differences in the regulation of protein expression that may contribute to the behavioral differences between them.

IQGAP1 as signal integrator: Ca2+, calmodulin, Cdc42 and the cytoskeleton

A family of proteins known as IQGAPs have been identified in yeast, amebas and mammals. IQGAPs are multidomain molecules that contain several protein-interacting motifs which mediate binding to target proteins. Mammalian IQGAP1 is a component of signaling networks that are integral to maintaining cytoskeletal architecture and cell-cell adhesion. Published data suggest that IQGAP1 is a scaffolding protein that modulates cross-talk among diverse pathways in complex regulatory circuits. These pathways include modulating the actin cytoskeleton, mediating signaling by Rho family GTPases and calmodulin, regulating E-cadherin and beta-catenin function and organizing microtubules.

TGF-beta and the Smad signal transduction pathway

Transforming growth factor beta (TGF-beta) superfamily members are important regulators of many diverse developmental and homeostatic processes and disruption of their activity has been implicated in a variety of human diseases ranging from cancer to chondrodysplasias and pulmonary hypertension. TGF-beta family members signal through transmembrane Ser-Thr kinase receptors that directly regulate the intracellular Smad pathway. Smads are a unique family of signal transduction molecules that can transmit signals directly from the cell surface receptors to the nucleus, where they regulate transcription by interacting with DNA binding partners as well as transcriptional coactivators and corepressors. In addition, more recent evidence indicates that Smads can also function both as substrates and adaptors for ubiquitin protein ligases, which mediate the targeted destruction of intracellular proteins. Smads have thus emerged as multifunctional transmitters of TGF-beta family signals that play critical roles in the development and homeostasis of metazoans.

Doxorubicin and vinorelbine act independently via p53 expression and p38 activation respectively in breast cancer cell lines

In the treatment of breast cancer, combination chemotherapy is used to overcome drug resistance. Combining doxorubicin and vinorelbine in the treatment of patients with metastatic breast cancer has shown high response rates; even single-agent vinorelbine in patients previously exposed to anthracyclines results in significant remission. Alterations in protein kinase-mediated signal transduction and p53 mutations may play a role in drug resistance with cross-talk between signal transduction and p53 pathways. The aim of this study was to establish the effects of doxorubicin and vinorelbine, as single agents, in combination, and as sequential treatments, on signal transduction and p53 in the breast cancer cell lines MCF-7 and MDA-MB-468. In both cell lines, increased p38 activity was demonstrated following vinorelbine but not doxorubicin treatment, whether vinorelbine was given prior to or simultaneously with doxorubicin. Mitogen-activated protein kinase (MAPK) activity and p53 expression remained unchanged following vinorelbine treatment. Doxorubicin treatment resulted in increased p53 expression, without changes in MAPK or p38 activity. These findings suggest that the effect of doxorubicin and vinorelbine used in combination may be achieved at least in part through distinct mechanisms. This additivism, where doxorubicin acts via p53 expression and vinorelbine through p38 activation, may contribute to the high clinical response rate when the two drugs are used together in the treatment of breast cancer.

Age-related changes in vascular adrenergic signaling: clinical and mechanistic implications

A large and growing segment of the general population are age 65 or older, and this percentage will continue to rise. Primary care of this population has, and is becoming a priority for clinicians. Hypertension, orthostatic hypotension, arterial insufficiency, and atherosclerosis are common disorders in the elderly that lead to significant morbidity and mortality. One common factor to these conditions is an age-related decline in beta-adrenergic receptor (beta-AR)-mediated function and subsequent cAMP generation. Presently, there is no single cellular factor that can explain this age-related decline, and thus the primary cause of this homeostatic imbalance is yet to be identified. However, the etiology is clearly associated with an age-related change in the ability of beta-AR receptor to respond to agonist at the cellular level. This article will review what is presently understood regarding the molecular and biochemical basis of age-impaired beta-AR receptor-mediated signaling. A fundamental understanding of why beta-AR-mediated vasorelaxation is impaired with age will provide new insights and innovative strategies for the management of the multiple clinical disorders that effect older people.

Histone and chromatin cross-talk

Chromatin is the physiologically relevant substrate for all genetic processes inside the nuclei of eukaryotic cells. Dynamic changes in the local and global organization of chromatin are emerging as key regulators of genomic function. Indeed, a multitude of signals from outside and inside the cell converges on this gigantic signaling platform. Numerous post-translational modifications of histones, the main protein components of chromatin, have been documented and analyzed in detail. These 'marks' appear to crucially mediate the functional activity of the genome in response to upstream signaling pathways. Different layers of cross-talk between several components of this complex regulatory system are emerging, and these epigenetic circuits are the focus of this review.