The role of G proteins in transmembrane signalling

The Impact of Membrane Protein Diffusion on GPCR Signaling

Spatiotemporal signal shaping in G protein-coupled receptor (GPCR) signaling is now a well-established and accepted notion to explain how signaling specificity can be achieved by a superfamily sharing only a handful of downstream second messengers. Dozens of Gs-coupled GPCR signals ultimately converge on the production of cAMP, a ubiquitous second messenger. This idea is almost always framed in terms of local concentrations, the differences in which are maintained by means of spatial separation. However, given the dynamic nature of the reaction-diffusion processes at hand, the dynamics, in particular the local diffusional properties of the receptors and their cognate G proteins, are also important. By combining some first principle considerations, simulated data, and experimental data of the receptors diffusing on the membranes of living cells, we offer a short perspective on the modulatory role of local membrane diffusion in regulating GPCR-mediated cell signaling. Our analysis points to a diffusion-limited regime where the effective production rate of activated G protein scales linearly with the receptor–G protein complex’s relative diffusion rate and to an interesting role played by the membrane geometry in modulating the efficiency of coupling.

Supramolecularly regulated artificial transmembrane signal transduction for 'ON/OFF'-switchable enzyme catalysis

An artificial signal transduction model with a supramolecular recognition headgroup, a membrane anchoring group, and a pro-enzyme catalysis endgroup was constructed. The transmembrane translocation of the transducer can be reversibly regulated by competitive host-guest complexations as an input signal to control an enzyme reaction inside the lipid vesicles.

Neuroprotective Effects of Guanosine in Ischemic Stroke-Small Steps towards Effective Therapy

Guanosine (Guo) is a nucleotide metabolite that acts as a potent neuromodulator with neurotrophic and regenerative properties in neurological disorders. Under brain ischemia or trauma, Guo is released to the extracellular milieu and its concentration substantially raises. In vitro studies on brain tissue slices or cell lines subjected to ischemic conditions demonstrated that Guo counteracts destructive events that occur during ischemic conditions, e.g., glutaminergic excitotoxicity, reactive oxygen and nitrogen species production. Moreover, Guo mitigates neuroinflammation and regulates post-translational processing. Guo asserts its neuroprotective effects via interplay with adenosine receptors, potassium channels, and excitatory amino acid transporters. Subsequently, guanosine activates several prosurvival molecular pathways including PI3K/Akt (PI3K) and MEK/ERK. Due to systemic degradation, the half-life of exogenous Guo is relatively low, thus creating difficulty regarding adequate exogenous Guo distribution. Nevertheless, in vivo studies performed on ischemic stroke rodent models provide promising results presenting a sustained decrease in infarct volume, improved neurological outcome, decrease in proinflammatory events, and stimulation of neuroregeneration through the release of neurotrophic factors. In this comprehensive review, we discuss molecular signaling related to Guo protection against brain ischemia. We present recent advances, limitations, and prospects in exogenous guanosine therapy in the context of ischemic stroke.

Neuromodulatory Effects of Guanine-Based Purines in Health and Disease

The function of guanine-based purines (GBPs) is mostly attributed to the intracellular modulation of heteromeric and monomeric G proteins. However, extracellular effects of guanine derivatives have also been recognized. Thus, in the central nervous system (CNS), a guanine-based purinergic system that exerts neuromodulator effects, has been postulated. The thesis that GBPs are neuromodulators emerged from in vivo and in vitro studies, in which neurotrophic and neuroprotective effects of these kinds of molecules (i.e., guanosine) were demonstrated. GBPs induce several important biological effects in rodent models and have been shown to reduce seizures and pain, stabilize mood disorder behavior and protect against gliomas and diseases related with aging, such as ischemia or Parkinson and Alzheimer diseases. In vitro studies to evaluate the protective and trophic effects of guanosine, and of the nitrogenous base guanine, have been fundamental for understanding the mechanisms of action of GBPs, as well as the signaling pathways involved in their biological roles. Conversely, although selective binding sites for guanosine have been identified in the rat brain, GBP receptors have not been still described. In addition, GBP neuromodulation may depend on the capacity of GBPs to interact with well-known membrane proteins in glutamatergic and adenosinergic systems. Overall, in this review article, we present up-to-date GBP biology, focusing mainly on the mechanisms of action that may lead to the neuromodulator role of GBPs observed in neurological disorders.

Guanosine: a Neuromodulator with Therapeutic Potential in Brain Disorders

Guanosine is a purine nucleoside with important functions in cell metabolism and a protective role in response to degenerative diseases or injury. The past decade has seen major advances in identifying the modulatory role of extracellular action of guanosine in the central nervous system (CNS). Evidence from rodent and cell models show a number of neurotrophic and neuroprotective effects of guanosine preventing deleterious consequences of seizures, spinal cord injury, pain, mood disorders and aging-related diseases, such as ischemia, Parkinson’s and Alzheimer’s diseases. The present review describes the findings of in vivo and in vitro studies and offers an update of guanosine effects in the CNS. We address the protein targets for guanosine action and its interaction with glutamatergic and adenosinergic systems and with calcium-activated potassium channels. We also discuss the intracellular mechanisms modulated by guanosine preventing oxidative damage, mitochondrial dysfunction, inflammatory burden and modulation of glutamate transport. New and exciting avenues for future investigation into the protective effects of guanosine include characterization of a selective guanosine receptor. A better understanding of the neuromodulatory action of guanosine will allow the development of therapeutic approach to brain diseases.

STORAGE OF ACTIVITY OF THE HUMAN CHORIONIC GONADOTROPIN HORMONE IN SOLUTION AT ADDITION OF ORGANIC COMPOUNDS TO THE PHARMACOLOGICAL COMPOSITION

The more stable among the tested samples were samples with saccharose in the concentration of 50–75 mg per cm3. While adding of L- lysine to samples the most stable activity was discovered in the experimental series of samples with the content of lysine of 10 mg per cm3 – activity increased by 54 % as compared to theoretical initial activity of HCG during 8 weeks. While storing gonadotropin with L-glycine fluctuations of hormone activity in all series of samples were observed. Adding of 0.2 mg per cm3 of L-glycine had a more expressed stabilizing effect. Adding of 0.2 mg per cm3 of L-methionine produced relatively high and stable activity of gonadotropin during the 6 weeks storage. Adding of 0.25 mg / cm3 of L- glycine and 75.50 mg / cm3 of saccharose to experimental samples during 2 weeks at 40 °C provided 69.8 % and 60.7 % saving activity of hCG respectively. Activity of gonadotropin in a series of samples with the addition of L- glycine and mannitol was significantly lower and at the end of the study was at an appropriate rate with the control series models. The highest activity of gonadotropin was detected while adding fillers – 10 mg / cm3 L-lysine and 75 mg / cm3 saccharose and mannitol – to recipes as a stabilizer.

Antagonists show GTP-sensitive high-affinity binding to the sigma-1 receptor

BACKGROUND AND PURPOSE

Sigma-1 receptors are atypical receptors with potentially two transmembrane domains. Antagonists require doses significantly higher than their published affinities to have biological effects. We have reassessed the binding characteristics of these ligands and found antagonists bind to high- and low-affinity states not distinguished by agonists.

EXPERIMENTAL APPROACH

The affinities of sigma-1 receptor ligands was assessed using radioligand saturation and competition binding of [³H]-(+)-pentazocine to permeabilized MDA-MB-468 cells. This was compared with the effect of ligands on metabolic activity using an MTS-based assay and calcium signalling using cells loaded with the calcium dye, Fura-2.

KEY RESULTS

Sigma-1 receptor antagonists, but not agonists, show GTP- and suramin-sensitive high-affinity binding. Functional responses (calcium signalling and metabolic activity), while associated with sigma-1 receptor binding, required binding to an unidentified, low-affinity target.

CONCLUSIONS AND IMPLICATIONS

Sigma-1 receptors are coupled to G proteins. This interaction is only observed when analysing antagonist binding. The identity of the G protein remains to be resolved. The concept of agonist and antagonist at the sigma-1 receptor needs to be revisited.

Proposal of a guanine-based purinergic system in the mammalian central nervous system

Guanine-based purines have been traditionally studied as modulators of intracellular processes, mainly G-protein activity. However, they also exert several extracellular effects not related to G proteins, including modulation of glutamatergic activity, trophic effects on neural cells, and behavioral effects. In this article, the putative roles of guanine-based purines on the nervous system are reviewed, and we propose a specific guanine-based purinergic system in addition to the well-characterized adenine-based purinergic system. Current evidence suggest that guanine-based purines modulate glutamatergic parameters, such as glutamate uptake by astrocytes and synaptic vesicles, seizures induced by glutamatergic agents, response to ischemia and excitotoxicity, and are able to affect learning, memory and anxiety. Additionally, guanine-based purines have important trophic functions affecting the development, structure, or maintenance of neural cells. Although studies addressing the mechanism of action (receptors and second messenger systems) of guanine-based purines are still insufficient, these findings point to the guanine-based purines (nucleotides and guanosine) as potential new targets for neuroprotection and neuromodulation.

Peptide insertion, positioning, and stabilization in a membrane: insight from an all-atom molecular dynamics simulation

Peptide insertion, positioning, and stabilization in a model membrane are probed via an all-atom molecular dynamics (MD) simulation. One peptide (WL5) is simulated in each leaflet of a solvated dimyristoylglycero-3-phosphate (DMPC) membrane. Within the first 5 ns, the peptides spontaneously insert into the membrane and then stabilize during the remaining 70 ns of simulation time. In both leaflets, the peptides localize to the membrane interface, and this localization is attributed to the formation of peptide-lipid hydrogen bonds. We show that the single tryptophan residue in each peptide contributes significantly to these hydrogen bonds; specifically, the nitrogen heteroatom of the indole ring plays a critical role. The tilt angles of the indole rings relative to the membrane normal in the upper and lower leaflets are approximately 26 degrees and 54 degrees , respectively. The tilt angles of the entire peptide chain are 62 degrees and 74 degrees . The membrane induces conformations of the peptide that are characteristic of beta-sheets, and the peptide enhances the lipid ordering in the membrane. Finally, the diffusion rate of the peptides in the membrane plane is calculated (based on experimental peptide concentrations) to be approximately 6 A(2)/ns, thus suggesting a 500 ns time scale for intermolecular interactions.

Can a GDP-liganded G-protein be active?

The replacement of GDP bound to the alpha-subunit of a G-protein by GTP is generally considered a crucial step in the activation of effectors by a G-protein. New data by Uğur et al. (2005) (p. 720) raise the possibility that for the heterotrimeric G-protein Gs, GDP-liganded Gs is able to activate the effector adenylyl cyclase as potently and effectively as when Gs is in its GTP bound form. We summarize here the evidence that GTP is necessary for effector activation by G-proteins and discuss potential implications and limitations of data to the contrary.

Temperature-dependent interaction of the bovine hippocampal serotonin(1A) receptor with G-proteins

The ligand binding and G-protein coupling of the bovine hippocampal 5-HT1A receptor as a function of temperature was monitored. There is an almost complete and irreversible loss in agonist binding at 50 degrees C. However, the antagonist binding is reduced only by 50%, and this could be reversed if the temperature is lowered to 25 degrees C. Interestingly, the agonist binding of the 5-HT1A receptor in membranes exposed to 50 degrees C is inhibited to a much lesser extent by GTP-gamma-S, a non-hydrolysable analogue of GTP, indicating uncoupling of the 5-HT1A receptor to G-proteins at 50 degrees C. We propose that high temperature selectively and irreversibly inactivates G-proteins thereby affecting G-protein-receptor interaction and agonist binding of the 5-HT1A receptor.

Trimeric G proteins and vesicle formation

Among the proteins regulating vesicular traffic, the small, Ras-like GTPases have received particular attention. Several recent reports indicate that another class of GTP-binding (G) protein, the heterotrimeric G proteins, also participates in the regulation of vesicular traffic. Thus, studies using transfected cells and cell-free systems show that a pertussis toxin-sensitive trimeric G protein, G(i3), is involved in the formation of secretory vesicles from the Golgi complex. These results raise the intriguing possibility that signal transduction processes across intracellular membranes play a role in vesicle formation, and provide important clues about the molecular machinery involved in this process.

Aberrant G protein signaling in nervous system tumors

Object. Guanosine triphosphate (GTP)—binding proteins, also known as G proteins, play important roles in the regulation of cell growth and differentiation by transmitting intracellular signals from cell surface receptors. In this paper, the authors review G protein signaling in general and its aberrations in four human nervous system tumors.

Methods. In the nervous system, four tumor types have been associated with aberrant G protein signaling. The first tumor type includes astrocytomas, which have increased levels of the activated form of the small G protein, p21-ras, without primary oncogenic p21-ras mutations. The likely source for increased p21-ras activity in sporadically occurring astrocytomas is overexpressed or constitutively activated growth factor receptors, whereas in neurofibromatosis Type 1 (NF1)—associated astrocytomas, the source is a loss of expression of neurofibromin, a major inactivator of p21-ras (ras—GTPase activating protein [GAP]). The second type of tumor associated with aberrant G protein signaling includes sporadic and NF1-associated neurofibromas and malignant peripheral nerve sheath tumors, which also have increased p21-ras activity due to a loss of neurofibromin expression. The third tumor type includes subependymal giant cell astrocytomas as part of the tuberous sclerosis complex (TSC). These tumors display a loss of tuberin expression due to germline mutations in the TSC2 gene. Tuberin functions as an inactivator of the small G protein rap1B (rap1-GAP) and, hence, loss of its expression could lead to increased rap1B activity. In addition to TSC-associated tumors, the authors demonstrate that the majority of sporadically occurring astrocytomas display either loss of tuberin or overexpression of rap1B. This suggests that increased rap1B activity, which can augment p21-ras—mediated signals, also contributes to G protein—mediated aberrant signaling in sporadically occurring astrocytomas. The fourth tumor type includes a significant subset of pituitary adenomas that show constitutive activation of the Gα subunit of the large heterotrimeric Gs protein, which is involved in hormone receptor signaling. The net result of this aberrant activation is increased cyclic adenosine monophosphate and mitogenic tumor-promoting signals.

Conclusions. The authors' review of G protein signaling and aberrations in this process is made with the long-term view that increased understanding of relevant signaling pathways will eventually lead to novel biological targeted therapies against these tumors.

Studies on the relevance of the glycan at Asn-52 of the alpha-subunit of human chorionic gonadotropin in the alphabeta dimer

Glycosylation of Asn-52 of the alpha-subunit (alphaAsn-52) is required for bioactivity of the alphabeta-dimeric human chorionic gonadotropin (hCG), although at a molecular level the effect of the glycan at alphaAsn-52 is not yet understood. To study the role of this glycan for heterodimer stability, the beta-subunit was recombined in solution with either the alpha-subunit or the alpha-subunit enzymically deglycosylated at alphaAsn-52. Enzymic deglycosylation avoids modification of the glycans at alphaAsn-78 and disturbing the protein folding. The efficiency of recombination after 16 h is 80%, independent of whether alphaAsn-52 is glycosylated or not. The dissociation constant of the hCG complex, with or without the glycan at alphaAsn-52, is less than 1 x 10(-5) s(-1), indicating that the glycan at alphaAsn-52 does not contribute significantly to the stability of the dimer. CD and NMR spectra indicate a local conformational difference between both alphabeta-dimeric hCG variants, most probably involving amino acids of the hCG beta-subunit close to the glycan at alphaAsn-52. These data explain the native-like receptor-binding abilities of hCG lacking the glycan at alphaAsn-52. It is proposed that for bioactivity the glycan at alphaAsn-52 is necessary for inducing and stabilizing a conformational change in hCG upon binding to the receptor, resulting in activation of the signal-transduction pathway.

The effects of lithium on ex vivo cytokine production

BACKGROUND

Studies suggest that lithium may have profound immunomodulatory effects in animal models as well as in humans.

METHODS

In this study, whole blood cultures from normal control subjects were established for 5 days and the effects of lithium on cytokine production were investigated. Because many of lithium's actions have been postulated to be modulated through phosphoinositide (PI), protein kinase C (PKC) and cyclic adenosine monophosphate (c-AMP) signaling pathways, the effects of myo-inositol and prostaglandin E(2), alone or in combination with lithium, were also investigated.

RESULTS

We found that lithium caused an increase in interleukin-4 and interleukin-10 levels, traditionally classified as T-helper lymphocyte type-2 cytokines, and a decrease in interleukin-2 and interferon-gamma levels, traditionally classified as T-helper lymphocyte type-1 (TH-1) cytokines. This shift cannot be fully explained by lithium's actions on the PI, PKC, or c-AMP messenger systems.

CONCLUSIONS

Monocytes exposed to lithium in the presence of a mitogen for 5 days produced a shift toward the production of TH-2 cytokines and away from the production of TH-1 cytokines. The study suggests that lithium may have complex time-dependent effects on immune function.

New thoughts on the role of the beta-gamma subunit in G-protein signal transduction

Heterotrimeric G proteins are involved in numerous biological processes, where they mediate signal transduction from agonist-bound G-protein-coupled receptors to a variety of intracellular effector molecules and ion channels. G proteins consist of two signaling moieties: a GTP-bound alpha subunit and a beta-gamma heterodimer. The beta-gamma dimer, recently credited as a significant modulator of G-protein-mediated cellular responses, is postulated to be a major determinant of signaling fidelity between G-protein-coupled receptors and downstream effectors. In this review we have focused on the role of beta-gamma signaling and have included examples to demonstrate the heterogeneity in the heterodimer composition and its implications in signaling fidelity. We also present an overview of some of the effectors regulated by beta-gamma and draw attention to the fact that, although G proteins and their associated receptors play an instrumental role in development, there is rather limited information on beta-gamma signaling in embryogenesis.

Changes in binding of iodomelatonin to membranes of Leydig cells and steroidogenesis after prolonged in vitro exposure to melatonin

The aim of the present study was to investigate the effects of prolonged exposure to melatonin (MLT) on the binding of iodomelatonin to membranes of rat Leydig cells and the subsequent modulation of testosterone and cyclic adenocine monophosphate (cAMP) secretion from these cells by MLT itself. Leydig cells were Percoll-purified from adult rats and cultured in vitro with MLT (1--100 nmol/L) for 16 h. Binding assays with 2(125I)iodomelatonin were then performed; moreover, testosterone and cAMP secretion during an acute challenge with lutenizing hormone (LH) (20 mIU/mL for 3 h) was assayed by RIA. As a result of prolonged MLT administration, a decrease in maximum binding density (Bmax) and equilibrium dissociation constant (Kd) of the binding of 2(125I)iodomelatonin to purified cell membranes was noted. Higher testosterone and cAMP secretion during LH challenge were recorded in cells pre-incubated with MLT; notwithstanding, the inhibitory effect of acutely administered MLT on LH-challenged secretions was not only retained but also reinforced, as the IC50 was 30% lower in cells pre-treated with the higher concentration of MLT (100 nM). Cycloheximide administration (10 microg/mL for 16 h) did not prevent hyper-sensitization to LH challenge or to acute MLT administration on LH challenge. Pertussis toxin (180 ng/mL for 16 h) prevented hyper-sensitization to LH, but not to acutely administered MLT. Forskolin (10 nmol/L) administration abolished either phenomena. In conclusion, prolonged exposure to MLT modulates the secretion of testosterone by cultured rat Leydig cells. Although MLT receptors were reduced, hyper-sensitization to LH challenge and to acutely administered MLT on LH challenge were observed with the higher concentration of MLT. Reduction in intracellular cAMP as a result of prolonged administration of MLT, could be the primary cause of both phenomena. On the one hand, reduced cAMP could start re-arrangement of the G-proteins and thus LH-dependent adenylate cyclase sensitization. On the other hand, reduced cAMP could render the Leydig cells more responsive to MLT itself through a mechanism which does not involve G-protein re-arrangement.

Serotonergic gene expression and depression: implications for developing novel antidepressants

The development and configuration of several neural networks is dependent on the actions of serotonin (5-HT) acting through multiple hetero- and autoreceptor subtypes. During early brain development 5-HT modulates morphogenetic activities, such as neural differentiation, axon outgrowth, and synaptic modeling. In the adult brain, midbrain raphe serotonergic neurons project to a variety of brain regions and modulate a wide range of physiological functions. Several lines of evidence indicate that genetically determined variability in serotonergic gene expression, as it has been documented for the 5-HT transporter, influences temperamental traits and may lead to psychopathological conditions with increased anxiety, depression, and aggression. Investigation of the regulation of serotonergic gene transcription and its impact on neuronal development, synaptic plasticity, and neurogenesis spur interest to identify serotonergic gene-related molecular factors underlying disease states and to develop more effective antidepressant treatment strategies. Gene targeting strategies have increasingly been integrated into investigations of brain function and along with the fading dogma of a limited capacity of neurons for regeneration and reproducibility, it is realized that gene transfer techniques using efficient viral vectors in conjunction with neuron-selective transcriptional control systems may also be applicable to complex disorders of the brain. Given the fact that the 5-HT system continues to be an important target for drug development and production, novel strategies aiming toward the modification of 5-HT function at the level of gene expression are likely to be exploited by enterprises participating actively in the introduction of alternative therapeutic approaches.

Differentiation of cultured brown adipocytes is associated with a selective increase in the short variant of g(s)alpha protein. Evidence for higher functional activity of g(s)alphaS

In order to examine whether the differentiation process in brown adipocytes cultivated in primary culture is associated with substantial alterations in the complement of G proteins, the levels of these proteins were investigated with immuno-electrophoretic techniques in membrane preparations from proliferating and differentiated cultured mouse brown adipocytes. We observed that differentiation was associated with a dramatic (more than threefold) increase in the short variant of G(s)alpha protein (G(s)alphaS). The long variant of G(s)alpha (G(s)alphaL), as well as G(i)1alpha, G(i)2alpha, G(q)alpha, G(11)alpha and Gbeta subunit proteins remained unchanged whereas G(i)3alpha protein was decreased. These changes were accompanied by marked increase in isoprenaline-, forskolin- as well as manganese-stimulated adenylyl cyclase. Thus, the marked increase in beta-adrenergic responsiveness of fully differentiated confluent brown adipocytes (day 8-9), as compared with that of proliferating undifferentiated cells of 'fibroblast phenotype' (day 3-4), is associated with a significant increase in the relative proportion between the short and long variants of G(s)alpha (the G(s)alphaS/G(s)alphaL ratio) along with a decrease in G(i)3alpha protein. These data also suggest that the short variant of G(s)alpha exhibits higher functional activity than the long variant of this G protein.

Lithium regulates PKC-mediated intracellular cross-talk and gene expression in the CNS in vivo

It has become increasingly appreciated that the long-term treatment of complex neuropsychiatric disorders like bipolar disorder (BD) involves the strategic regulation of signaling pathways and gene expression in critical neuronal circuits. Accumulating evidence from our laboratories and others has identified the family of protein kinase C (PKC) isozymes as a shared target in the brain for the long-term action of both lithium and valproate (VPA) in the treatment of BD. In rats chronically treated with lithium at therapeutic levels, there is a reduction in the levels of frontal cortical and hippocampal membrane-associated PKC alpha and PKC epsilon. Using in vivO microdialysis, we have investigated the effects of chronic lithium on the intracellular cross-talk between PKC and the cyclic AMP (cAMP) generating system in vivo. We have found that activation of PKC produces an increase in dialysate cAMP levels in both prefrontal cortex and hippocampus, effects which are attenuated by chronic lithium administration. Lithium also regulates the activity of another major signaling pathway the c-Jun N-terminal kinase pathway--in a PKC-dependent manner. Both Li and VPA, at therapeutically relevant concentrations, increase the DNA binding of activator protein 1 (AP-1) family of transcription factors in cultured cells in vitro, and in rat brain ex vivo. Furthermore, both agents increase the expression of an AP-1 driven reporter gene, as well as the expression of several endogenous genes known to be regulated by AP-1. Together, these results suggest that the PKC signaling pathway and PKC-mediated gene expression may be important mediators of lithium's long-term therapeutic effects in a disorder as complex as BD.

Signalling pathways in the brain: cellular transduction of mood stabilisation in the treatment of manic-depressive illness

The long-term treatment of manic-depressive illness (MDI) likely involves the strategic regulation of signalling pathways and gene expression in critical neuronal circuits. Accumulated evidence has identified signalling pathways, in particular the family of protein kinase C (PKC) isozymes, as targets for the long-term action of lithium. Chronic lithium administration produces a reduction in the expression of PKC alpha and epsilon, as well as a major PKC substrate, MARCKS, which has been implicated in long-term neuroplastic events in the developing and adult brain. More recently, studies have demonstrated robust effects of lithium on another kinase system, GSK-3beta, and on neuroprotective/neurotrophic proteins in the brain. Given the key roles of these signalling cascades in the amplification and integration of signals in the central nervous system, these findings have clear implications not only for research into the neurobiology of MDI, but also for the future development of novel and innovative treatment strategies.

Assessment of striatal D1 and D2 dopamine receptor-G protein coupling by agonist-induced [35S]GTP gamma S binding

The dopamine receptor-mediated modulation of guanosine 5'-O-(gamma-[35S]thio)triphosphate ([35S]GTP gamma S) binding has been characterized in rat striatal membranes. In optimized experimental conditions, the potency of dopamine was 4.47 microM [3.02-6.61 microM] and a maximal response representing 54.8 +/- 4.5% increase above basal level was observed. Data obtained with different agonists and antagonists clearly revealed that the most important fraction of this response was reflecting D2 receptor activation. Further analysis with specific antagonists also supported evidence for the involvement of D1 dopamine receptors. The potencies of compounds interacting with D1 and D2 receptors were deduced from [35S]GTP gamma S binding experiments and compared with their binding affinities for these receptors measured in similar experimental conditions. A good correlation between these parameters was observed, supporting the applicability of this technique for the study of dopamine receptors in the central nervous system.

In vivo modulation of G proteins and opioid receptor function by antisense oligodeoxynucleotides

The work in our laboratory has been designed to characterize the transducer mechanisms coupled to neurotransmitter receptors in the plasma membrane. Particular attention has been paid to the physiological/pharmacological effects mediated by the opioid system. Antisense oligodeoxynucleotides have proved useful in correlating opioid receptor clones with those defined pharmacologically. The involvement of the cloned opioid receptors mu, delta, and kappa in analgesia has been determined by means of in vivo injection of ODNs directed to the receptor mRNAs. Using this strategy the classes of G-transducer proteins regulated by each type/subtype of opioid receptor in the promotion of antinociception have also been characterized. After displaying different patterns of binding to their receptors, opioids trigger a variety of intracellular signals. The physiological implications and therapeutic potential of these findings merit consideration.

Ziskind-Somerfeld Research Award. Protein kinase C signaling in the brain: molecular transduction of mood stabilization in the treatment of manic-depressive illness

Understanding the biology of the pharmacological stabilization of mood will undoubtedly serve to provide significant insight into the pathophysiology of manic-depressive illness (MDI). Accumulating evidence from our laboratories and those of other researchers has identified the family of protein kinase C isozymes as a shared target in the brain for the long-term action of both lithium and valproate. In rats chronically treated with lithium, there is a reduction in the hippocampus of the expression of two protein kinase isozymes, alpha and epsilon, as well as a reduction in the expression of a major PKC substrate, MARCKS, which has been implicated in long-term neuroplastic events in the developing and adult brain. In addition, we have been investigating the down-stream impact of these mood stabilizers on another kinase system, GSK-3 beta and on the AP-1 family of transcription factors. Further studies have generated promising preliminary data in support of the antimanic action of tamoxifen, and antiestrogen that is also a PKC inhibitor. Future studies must address the therapeutic relevance of these protein targets in the brain using innovative strategies in both animal and clinical investigations to ultimately create opportunities for the discovery of the next generations of mood stabilizers for the treatment of MDI.

CD8+ T cell-mediated enhancement of tumour necrosis factor-alpha (TNF-alpha) production and HIV-1 LTR-driven gene expression in human monocytic cells is pertussis toxin-sensitive

HIV replication and LTR-mediated gene expression can be modulated by CD8+ T cells in a cell type-dependent manner. We have previously shown that supernatants of activated CD8+ T cells of HIV-infected individuals greatly enhanced p24 levels in human macrophages infected with NSI or SI primary isolates of HIV-1. Here we have examined the effect of culture with CD8+ T cell supernatants on HIV-1 LTR-mediated gene expression in monocytic cells. CD8+ T cell supernatants enhanced LTR-mediated gene expression in U38 cells activated with Tat in the absence or presence of phorbol myristate acetate (PMA) and ionomycin or TNF-alpha. Further, enhancement of LTR-mediated gene expression and virus replication in U38 cells and U1 cells, respectively, was pertussis toxin-sensitive. The enhancement of gene expression and virus replication was associated with increased levels of TNF-alpha and was significantly abrogated by antibody to TNF-alpha. In contrast, the suppression of LTR-mediated gene expression by CD8+ T cell supernatants in Jurkat T cells was not pertussis toxin-sensitive and TNF-alpha levels were not affected. These results demonstrate that factors produced by CD8+ T cells utilize different cellular pathways to mediate their effects on HIV transcription and replication in different cell types.

Alpha2-adrenergic agonist modulation of [35S]GTPgammaS binding to guanine-nucleotide-binding-proteins in human platelet membranes

Alpha2-adrenoceptor (alpha2-AR)-regulated binding of the labelled GTP analog, guanosine 5'-O-(3-[35S]thiotriphosphate) ([35S]GTPgammaS), to guanine-nucleotide-binding proteins (G proteins) was studied in human platelet membranes. Under optimal conditions, the potent alpha2-AR agonist, 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304), increased the binding of [35S]GTPgammaS up to approximately 1.8 fold, with half-maximal increase at 60 nM and was competitively inhibited by the alpha2-AR antagonist Rauwolscine. The actions of both UK 14304 and Rauwolscine were modulated by monovalent and divalent cation levels, as well as by the concentrations of GDP. [35S]GTPgammaS binding induced by UK 14304 had a Kd value of 4.5 +/- 0.3 nM and a Bmax value of 4.15 +/- 0.40 pmol/mg protein. The rank order of potencies of adrenergic ligands tested in stimulating [3S]GTPgammaS binding and inhibiting forskolin-stimulated c-AMP accumulation was UK 14304> Guanabenz acetate> Oxymetazoline hydrochloride> B-HT 920 dihydrochloride> p-Aminoclonidine hydrochloride> Clonidine hydrochloride. The data presented indicate that enhancement of [35S]GTPgammaS binding by alpha2-AR in human platelet membranes provides a simple functional measure for receptor activation and can be used for determination of potencies and efficacies of ligands at the alpha2-AR.

Solution structure of the alpha-subunit of human chorionic gonadotropin

The three-dimensional solution structure of the alpha-subunit in the alpha, beta heterodimeric human chorionic gonadotropin (hCG), deglycosylated with endo-beta-N-acetylglucosaminidase-B (dg-alpha hCG), was determined using 2D homonuclear and 2D heteronuclear 1H, 13C NMR spectroscopy at natural abundance in conjunction with the program package XPLOR. The distance geometry/simulated annealing protocol was modified to allow for the efficient modelling of the cystine knot motif present in alpha hCG. The protein structure was modelled with 620 interproton distance restraints and the GlcNAc residue linked to Asn78 was modelled with 30 protein-carbohydrate and 3 intraresidual NOEs. The solution structure of dg-alpha hCG is represented by an ensemble of 27 structures. In comparison to the crystal structure of the dimer, the solution structure of free dg-alpha hCG exhibits: (a) an increased structural disorder (residues 33-57); (b) a different backbone conformation near Val76 and Glu77; and (c) a larger flexibility. These differences are caused by the absence of the interactions with the beta-subunit. Consequently, in free dg-alpha hCG, compared to the intact dimer, the two hairpin loops 20-23 and 70-74 are arranged differently with respect to each other. The beta-GlcNAc(78) is tightly associated with the hydrophobic protein-core in between the beta-hairpins. This conclusion is based on the NOEs from the axial H1, H3, H5 atoms and the N-acetyl protons of beta-GlcNAc(78) to the protein-core. The hydrophobic protein-core between the beta-hairpins is thereby shielded from the solvent.

Pseudo-noncompetitive antagonism of M1, M3, and M5 muscarinic receptor-mediated Ca2+ mobilization by muscarinic antagonists

Muscarinic receptors M1, M3, and M5 were expressed in Sf9 cells. Three different patterns of inhibition of Ca2+ elevations could be resolved for the subtype nonselective muscarinic receptor antagonists: (i) a right shift of the agonist dose-response curve, (ii) a right shift of the agonist dose-response curve and a depression of the maximum signal, and (iii) an intermediate pattern where the antagonist apparently behaved more competitively at higher concentrations. A simulation performed assuming that these differences are due to differences in the dissociation rates of the antagonists reproduced all three different modes of inhibition; the novel intermediate pattern (iii) is suggested to be caused by an intermediate antagonist dissociation rate. A direct correlation between the type of inhibition and the measured dissociation rate of the antagonists was also observed. Functional selectivity between receptor subtypes based on the dissociation constants is suggested based on the results.

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.

Agonist and antagonist modulation of [35S]-GTP gamma S binding in transfected CHO cells expressing the neurotensin receptor

1. The functional interaction of the cloned rat neurotensin receptor with intracellular G-proteins was investigated by studying the binding of the radiolabelled guanylyl nucleotide analogue [35S]-GTP gamma S induced by neurotensin to membranes prepared from transfected Chinese hamster ovary (CHO) cells. 2. The agonist-induced binding of [35S]-GTP gamma S was only detected in the presence of NaCl in the incubation buffer. However, it was also demonstrated that the binding of [3H]-neurotensin to its receptor was inhibited by NaCl. In the presence of 50 mM NaCl, the binding of the labelled nucleotide was about 2 fold increased by stimulation with saturating concentrations of neurotensin (EC50 value of 2.3 +/- 0.9 nM). 3. The stimulation of [35S]-GTP gamma S binding by neurotensin was mimicked by the stable analogue of neurotensin, JMV-449 (EC50 value of 1.7 +/- 0.4 nM) and the neurotensin related peptide neuromedin N (EC50 value of 21 +/- 6 nM). 4. The NT-induced [35S]-GTP gamma S binding was competitively inhibited by SR48692 (pA2 value of 9.55 +/- 0.28), a non-peptide neurotensin receptor antagonist. SR48692 alone had no effect on the specific binding of [35S]-GTP gamma S. 5. The response to neurotensin was found to be inhibited by the aminosteroid U-73122, a putative inhibitor of phospholipase C-dependent processes, indicating that this drug may act at the G-protein level. 6. Taken together, these results constitute the first characterization of the exchange of guanylyl nucleotides at the G-protein level that is induced by the neuropeptide neurotensin after binding to its receptor.

Gx/z is regulated by mu but not delta opioid receptors in the stimulation of the low Km GTPase activity in mouse periaqueductal grey matter

High affinity low K(m) GTPase activity was measured in membrane preparations of adult mouse mesencephalic periaqueductal grey matter (PAG). Opioids displaying selectivity towards mu- or delta-opioid receptors (OR) activated the enzyme in a concentration-dependent manner. Antibodies to mu-OR greatly impaired the potential of mu-agonists, [D-Ala2,N-MePhe4,Gly-ol5]-enkephalin (DAMGO) and morphine, to increase hydrolysis of GTP. The same antibodies had little effect on [D-Pen2,5]enkephalin (DPDPE) and [D-Ala2]deltorphin II, both agonists at delta-OR. Stimulation of GTPase by DPDPE and [D-Ala2]deltorphin II - but not by morphine or DAMGO - was diminished by antibodies to delta-OR. The blockade of G(i2)alpha subunits by specific antibodies impaired the activation of G alpha-related GTPase by all opioids. Antibodies in vitro, and oligodeoxynucleotides in vivo, prepared against Gx/z alpha subunits, reduced the release of Pi promoted by DAMGO and morphine. The impairment of Gx/z proteins also slightly reduced the effect of the delta2 agonist [D-Ala2]deltorphin II. At delta1 receptors, DPDPE fully expressed its activation of GTPase. These results indicate that in the PAG, mu-OR and delta-OR couple with Gi2 transducer proteins. Notably, mu-OR also regulates the pertussis toxin-insensitive G-protein Gx/z, an effect poorly exhibited by delta-OR in this tissue.

Signal transduction in gastrointestinal smooth muscle

Signal transduction in gastric and intestinal smooth muscle is mediated by receptors coupled via distinct G proteins to various effector enzymes, including PI-specific PLC-beta 1 and PLC-beta 3, and phosphatidylcholine (PC)-specific PLC, PLD and PLA2. Activation of these enzymes is different in circular and longitudinal muscle cells, generating Ca(2+)-mobilizing (IP3, AA, cADPR) and other (DAG) messengers responsible for the initial and sustained phases of contraction, respectively. IP3-dependent Ca2+ release occurs only in circular muscle. Ca2+ mobilization in longitudinal muscle involves a cascade initiated by agonist-induced transient activation of PLA2 and formation of AA, AA-dependent depolarization of the plasma membrane and opening of voltage-sensitive Ca2+ channels. The influx of Ca2+ induces Ca2+ release by activating sarcoplasmic ryanodine receptor/Ca2+ channel and stimulates cADPR formation which enhances Ca(2+)-induced Ca2+ release. The initial [Ca2+]i transient in both muscle cell types results in Ca2+/calmodulin-dependent activation of MLC kinase, phosphorylation of MLC20 and interaction of actin and myosin. The sustained phase is mediated by a Ca(2+)-independent isoform of PKC, PKC-epsilon DAG for this process is generated by PLC- and PLD-mediated hydrolysis of PC. Relaxation is mediated by cAMP-and/or cGMP-dependent protein kinase which inhibit the initial [Ca2+]i transient and reduce the sensitivity of MLC kinase to [Ca2+]i. Relaxation induced by the main neurotransmitters, VIP and PACAP, involves two cascades, one of which reflects activation of adenylyl cyclase. A distinct cascade involves G-protein-dependent stimulation of Ca2+ influx leading to Ca2+/calmodulin-dependent activation of a constitutive eNOS in muscle cells; the generation of NO activates soluble guanylyl cyclase. The resultant activation of PKA and PKG is jointly responsible for muscle relaxation.

Mechanistic model of G-protein signal transduction. Determinants of efficacy and effect of precoupled receptors

Tissue-specific characteristics (e.g. receptor number) and agonist-specific characteristics (e.g. agonist binding kinetics) play roles in determining cellular response. The roles that these characteristics play are quantified by models of signal transduction. We examined signal transduction through G-protein-linked receptors, using a model based on the collision coupling model but including interconverting receptor states and the precoupling of receptors with G-proteins prior to the addition of agonist. Reaction and diffusion of molecules within the plasma membrane were simulated using Monte Carlo techniques. The G-protein activation produced by our model was compared with that produced by the collision coupling model. We quantitatively examined how the parameters characteristic of the tissue and agonist determine the midpoint and maximal response of the dose-response curve. Activation through agonist binding to precoupled receptors can produce significantly higher activation rates than does collision coupling. Tissue and agonist characteristics have qualitatively similar effects but quantitatively distinct effects on activation for the two models. Using standard experimental techniques, it may be possible to exploit these differences to determine the mechanism of G-protein activation in a specific cell system. A quantitative comparison of model predictions with published data on the beta-adrenergic receptor system (Stickle D and Barber R, Mole Pharmacol 40: 276-288, 1991) also is presented.

Pharmacological and biochemical evidence for the simultaneous expression of CCKB/gastrin and CCKA receptors in the pig pancreas

1. In the pig, the secretory response of the pancreas is not inhibited by the antagonist MK329 suggesting that cholecystokininA (CCKA) receptors are not involved. 2. Membranes were isolated from the pancreas of 6 Large White pigs to characterize their CCK receptors. 3. The binding of [125I]-BH-[Thr, Nle]CCK-9 was dependent on pH, maximal after a 90 min incubation period, saturable and reversible. Saturation analysis of the binding demonstrated a single class of high affinity sites (Kd = 0.22 +/- 0.02 nM) and a binding capacity, Bmax = 110.64 +/- 12.50 fmol mg-1 protein. 4. Competition binding by agonists and antagonists of CCKA and CCKB/gastrin receptors demonstrated the presence of two distinct binding components, sites presenting a high affinity for [Thr, Nle]CCK-9, gastrin, PD 135158, L-365, 260 and a low affinity for MK329, SR 27897, and sites presenting a high affinity for [Thr, Nle]CCK-9, MK329, SR 27897 and a low affinity for gastrin, PD 135158, L-365,260. 5. These pharmacological data demonstrate the presence of both CCKA and CCKB/gastrin receptors in the pig pancreas, the latter being predominant. 6. Two distinct membrane proteins (50 and 85-100 kDa, respectively) display pharmacological features of CCKB/gastrin and CCKA receptors. 7. In pigs, as in calves and humans, CCKB/gastrin receptors are predominant in the pancreas.

Guanine nucleotide binding proteins in opioid-dependent patients

Guanine nucleotide binding (G) protein levels and functioning in the platelets of 19 methadone-maintained patients were compared to age and sex matched, normal controls. We found that in the methadone patients, G alpha s-levels were significantly higher, while the levels of G alpha i 1/2 and pertussis toxin catalyzed [32P]ADP ribosylation were significantly lower compared to control subjects in platelet membranes. We have further found that when all three of these biochemical indicators were combined in a discriminant function analysis, 79% of the methadone patients were correctly classified and 83% of the controls were correctly classified.

Asymmetric interactions between phosphorylation pathways regulating ciliary beat frequency in human nasal respiratory epithelium in vitro

1. The effects of the sequential stimulation of ciliary beat frequency (CBF) via two different phosphorylation cascades (dependent on protein kinase A (PKA) and calmodulin, respectively) were determined using video microscopy applied to a perfused preparation of human nasal respiratory epithelium in vitro. Dibutyryl cyclic AMP (db-cAMP) (10(-3) M) was used to stimulate PKA and the calcium ionophore 4-Br-A23187 (10(-5) M) was used to stimulate calmodulin-dependent phosphorylation. 2. Perfusion with db-cAMP (10(-3) M) alone showed an early rise in CBF (15.0 +/- 4%, mean +/- S.E.M., P < 0.05) by 10 min which remained elevated for 35 min; in contrast, the highest CBF response to 4-Br-A23187 (10(-5) M) alone was not achieved until 35 min (16.1 +/- 1.8%, P < 0.05). 3. When a db-cAMP stimulus was applied to cells which had been pre-incubated with 4-Br-A23187 for 30 min, a further rise in CBF (maximal at 20 min, 14.3 +/- 2%, P < 0.05) was observed. Reversing the sequence of perfusions, cells pre-incubated with db-cAMP showed no further rise in response to stimulation with 4-Br-A23187. 4. We hypothesized that PKA inhibited the response to the 4-Br-A23187. This notion was supported by the restoration of the CBF response (22.8 +/- 4%, P < 0.05) to 4-Br-A23187 when the cells were pre-incubated with the protein kinase inhibitor 1-(5-isoquinolinyl-sulphonyl)-2-methylpiperazine (10(-3) M), before the sequential perfusions with db-cAMP and 4-Br-A23187. We conclude that the A23187-dependent pathway, which regulates intrinsic CBF, is inhibited by db-cAMP but not vice versa.

Role of the prenyl group on the G protein gamma subunit in coupling trimeric G proteins to A1 adenosine receptors

The coupling of receptors to heterotrimeric G proteins is determined by interactions between the receptor and the G protein alpha subunits and by the composition of the betagamma dimers. To determine the role of the gamma subunit prenyl modification in this interaction, the CaaX motifs in the gamma1 and gamma2 subunits were altered to direct modification with different prenyl groups, recombinant betagamma dimers expressed in the baculovirus/Sf9 insect cell system, and the dimers purified. The activity of the betagamma dimers was compared in two assays: formation of the high affinity agonist binding conformation of the A1 adenosine receptor and receptor-catalyzed exchange of GDP for GTP on the alpha subunit. The beta1gamma1 dimer (modified with farnesyl) was significantly less effective than beta1gamma2 (modified with geranylgeranyl) in either assay. The beta1gamma1-S74L dimer (modified with geranylgeranyl) was nearly as effective as beta1gamma2 in either assay. The beta1gamma2-L71S dimer (modified with farnesyl) was significantly less active than beta1gamma2. Using 125I-labeled betagamma subunits, it was determined that native and altered betagamma dimers reconstituted equally well into Sf9 membranes containing A1 adenosine receptors. These data suggest that the prenyl group on the gamma subunit is an important determinant of the interaction between receptors and G protein gamma subunits.

Surface plasmon resonance spectroscopy studies of membrane proteins: transducin binding and activation by rhodopsin monitored in thin membrane films

Surface plasmon resonance (SPR) spectroscopy can provide useful information regarding average structural properties of membrane films supported on planar solid substrates. Here we have used SPR spectroscopy for the first time to monitor the binding and activation of G-protein (transducin or Gt) by bovine rhodopsin incorporated into an egg phosphatidylcholine bilayer deposited on a silver film. Rhodopsin incorporation into the membrane, performed by dilution of a detergent solution of the protein, proceeds in a saturable manner. Before photolysis, the SPR data show that Gt binds tightly (Keq approximately equal to 60 nM) and with positive cooperativity to rhodopsin in the lipid layer to form a closely packed film. A simple multilayer model yields a calculated average thickness of about 57 A, in good agreement with the structure of Gt. The data also demonstrate that Gt binding saturates at a Gt/rhodopsin ratio of approximately 0.6. Moreover, upon visible light irradiation, characteristic changes occur in the SPR spectrum, which can be modeled by a 6 A increase in the average thickness of the lipid/protein film caused by formation of metarhodopsin II (MII). Upon subsequent addition of GTP, further SPR spectral changes are induced. These are interpreted as resulting from dissociation of the alpha-subunit of Gt, formation of new MII-Gt complexes, and possible conformational changes of Gt as a consequence of complex formation. The above results clearly demonstrate the ability of SPR spectroscopy to monitor interactions among the proteins associated with signal transduction in membrane-bound systems.

Vasopressin stimulates Ca2+ spiking activity in A7r5 vascular smooth muscle cells via activation of phospholipase A2

[Arg8]-vasopressin (AVP) is both a potent vasoconstrictor and a mitogen for vascular smooth muscle cells. AVP binds to a single class of receptors (V1a) in the A7r5 rat aortic smooth muscle cell line (Kd approximately 2 nmol/L). Stimulation of these cells with AVP results in an increase in cytoplasmic free Ca2+ concentration ([Ca2+]i) by releasing intracellular Ca2+ stores and increasing Ca2+ influx; the EC50 for these effects is approximately 5 nmol/L. AVP has recently been reported to stimulate arachidonic acid release in primary cultures of rat aortic smooth muscle over a much lower concentration range (EC50 approximately 0.05 nmol/L). The present study examined the effects of varying concentrations of AVP on spontaneous Ca2+ spiking activity in fura 2-loaded A7r5 cells. Frequency of CA2+ spiking increased with increasing [AVP] in the range of 10 to 500 pmol/L. Higher concentrations of AVP inhibited spiking but elicited the characteristic [Ca2+]i changes ascribed to the release of Ca2+ stores and increased Ca2+ entry. The effects of both low and high concentrations of AVP were inhibited by [1-(beta-mercapto-beta,beta,-pentamethylenepropionic acid),2-0-methyltyrosine]arginine vasopressin, a selective V1a vasopressin antagonist. Nimodipine (50 nmol/L), a blocker of L-type voltage-sensitive Ca2+ channels, abolished the Ca(2+)-spiking activity without inhibiting a maximal [Ca2+]i response to AVP (1 mumol/L). AVP-stimulated Ca2+ spiking, but not release of intracellular Ca2+ stores, was also abolished by ONO-RS-082 (1 mumol/L), an inhibitor of phospholipase A2. These results suggest that occupation of a small fraction of V1a vasopressin receptors by AVP results in stimulation of phospholipase A2 and leads to increased Ca(2+)-spiking activity. This effect may be important for fine tuning of vascular tone, whereas maximal stimulation by AVP (full receptor occupancy) may be required for more vigorous or sustained vasoconstriction or mitogenesis.

Identification of an extracellular motif involved in the binding of guanine nucleotides by a glutamate receptor

The chick cerebellar kainate (KA) binding protein (KBP), a member of the family of ionotropic glutamate receptors, harbours a glycine-rich (GxGxxG) motif known to be involved in the binding of ATP and GTP to kinases and G proteins respectively. Here, we report that guanine, but not adenine, nucleotides interact with KBP by inhibiting [3H]KA binding in a competitive-like manner, displaying IC50 values in the micromolar range. To locate the GTP binding site, KBP was photoaffinity labelled with [alpha-32P]GTP. The reaction was blocked by KA, glutamate, 6-cyano-7-nitroquinoxaline-2,3-dione and antibodies raised against a peptide containing the glycine-rich motif. Site-directed mutagenesis of residues K72 and Y73 within the glycine-rich motif followed by the expression of the KBP mutants at the surface of HEK 293 cells showed a decrease in GTP binding affinity by factors of 10 and 100 respectively. The binding of [3H]KA to the K72A/T KBP mutants was not affected but binding to the Y73I KBP mutant was decreased by a factor of 10. Accordingly, we propose that the glycine-rich motif of KBP forms part of a guanine nucleotide binding site. We further suggest that the glycine-rich motif is the binding site at which guanine nucleotides inhibit the glutamate-mediated responses of various members of the subfamily of glutamate ionotropic receptors.

Effects of repeated doses of azapirones on rat brain 5-HT1A receptors and plasma corticosterone levels

Chronic buspirone or ipsapirone (3 mg/kg, twice daily) administration to rats for 10 days decreased the sensitivity of inhibition of single-unit activity of serotonergic dorsal raphe neurons to a challenge by each drug. The ED50 for buspirone was increased from 0.1 mg/kg to 1.8 mg/kg, and the ED50 for ipsapirone was increased from 0.7 mg/kg to 1.2 mg/kg. The binding properties (Kd and Bmax) of [3H]8-OH-DPAT to membranes of cerebral cortex and hippocampus were unaffected by chronic administration of either buspirone or ipsapirone. Chronic buspirone or ipsapirone administration increased the tolerance of the hypothalamic-pituitary-adrenal axis (HPAA) following a challenge by each drug. The ED50 for elevation of plasma corticosterone levels was increased from 4.0 mg/kg to 7.6 mg/kg for buspirone and 6.2 mg/kg to 8.0 mg/kg for ipsapirone. Chronic buspirone administration decreased the basal activity of the HPAA by 63%. Chronic buspirone administration did not alter the plasma corticosterone response of the HPAA to a 1-min episode of rotational stress. (Mg2+)-ATPase, (Na+ + K+)-ATPase, (Ca2+ + Mg2+)-ATPase and calmodulin-stimulated (Ca2+ + Mg2+)-ATPase activities of erythrocyte plasma membrane were unaffected by either chronic or acute buspirone treatment, or by the addition of the drug to the in vitro assay systems.

GTP and guanosine synergistically enhance NGF-induced neurite outgrowth from PC12 cells

Six per cent of rat pheochromocytoma (PC12) cells extended neurites (processes greater than one cell diameter in length) in the presence of 300 microM extracellular GTP or 300 microM guanosine for 48 hr, compared to only 2.5% of cells in control cultures. In the presence of 40 ng/ml of 2.5S NGF, about 20-35% of PC12 cells had neurites after 48 hr, and the addition of 300 microM guanosine or GTP together with NGF synergistically increased the proportion of cells with neurites to 40-65%. GTP and guanosine also increased the average number of branches per neurite, from 0.6 in NGF-treated cultures to 1.2 (guanosine) or 1.5 (GTP). Neurites formed after exposure to NGF alone had axonal characteristics as determined by immunocytochemistry with antibody, SMI-31, against axonal-specific polyphosphorylated neurofilament epitopes. Neurites generated with the addition of both guanosine or GTP had the same characteristics. GTP probably did not exert its effects via the P2X or P2Y purinoceptors because the adenine nucleotides ATP, ATP gamma S, ADP beta S, and ADP, which are all agonists of these receptors, inhibited rather than enhanced, NGF-induced neurite outgrowth. UTP also enhanced the proportion of cells with neurites, although not to the same degree as did GTP. This may indicate activity through a P2U-like nucleotide receptor. However, the response profile obtained, GTP > UTP >> ATP, does not fit the profile of any known P2Y, P2X or P2U receptor. The poorly hydrolyzable GTP analogues, GTP gamma S and GDP beta s were also unable to enhance the proportion of cells with neurites. This implied that GTP may produce its effects through a GTP-specific ectoenzyme or kinase. This idea was supported by results showing that another poorly hydrolyzable analogue, GMP-PCP, competitively inhibited the effects of GTP on neurite outgrowth. GTP did not exert its effects after hydrolysis to guanosine since the metabolic intermediates GDP and GMP were also ineffective in enhancing the proportion of cells with neurites. Moreover, the effects of GTP and guanosine were mutually additive, implying that these two purines utilized different signal transduction mechanisms. The effects of guanosine were not affected by the nucleoside uptake inhibitors nitrobenzylthioinosine (NBTI) and dipyridamole, indicating that a transport mechanism was not involved. Guanosine also did not activate the purinergic P1 receptors, because the A2 receptor antagonists, 1,3-dipropyl-7-methylxanthine (DPMX) or CGS15943, and the A1 receptor antagonist, 1,3-dipropyl-8-(2-amino-4-chloro)xanthine (PACPX) did not inhibit its reaction. Therefore guanosine enhanced neurite outgrowth by a signal transduction mechanism that does not include the activation of the P1 purinoceptors. The enhancement of the neuritogenic effects of NGF by GTP and guanosine may have physiological implications in sprouting and functional recovery after neuronal injury in the CNS, due to the high levels of nucleosides and nucleotides released from dead or injured cells.

Characterisation of the interaction between guanyl nucleotides and AMPA receptors in rat brain

Guanyl nucleotides inhibit the binding of the AMPA receptor agonists [3H]fluorowillardiine and [3H]AMPA and the competitive antagonist [3H]CNQX to rat brain cerebrocortical membranes. The rank order of inhibition for each of the radioligands tested was GTP > GDP > GMP. The nucleotides CTP and ATP showed no effect. GTP inhibition was unaffected by the presence or absence of NaCl and MgCl2. Pre-treatment of the membranes with GTP, and its removal before addition of radioligand, did not inhibit binding. Quantitative autoradiography demonstrated that GTP inhibition occurred throughout the brain. These results are consistent with guanyl nucleotides acting at an extracellular site present on all AMPA receptor subunits, occupation of which inhibits agonist and antagonist binding.

Perspectives for biophysicochemical modifications of enzymes

This article reviews the strategies and successes of modifying enzymes by means of biophysicochemical transformations. By judicious choice of methods, it has been possible to modify enzymes through physical interactions, chemical reactions and/or mutagenesis to alter a very wide range of properties ranging from stability and solubility on the one hand to catalytic activity and selectivity on the other.