Polarized Th1 and Th2 cells are less responsive to negative feedback by receptors coupled to the AC/cAMP system compared to freshly isolated T cells

1. The adenylyl cyclase (AC)/cyclic adenosine monophosphate (cAMP) system is known to negatively regulate transcriptional activity of T cells, thereby possibly modulating T-cell-mediated responses at the sites of inflammation. Effects of cAMP have been widely studied in freshly isolated T cells and T-cell clones; yet, effects in differentiated Th1 and Th2 cells are largely unknown. 2. To obtain differentiated T helper cells, we activated naive T cells for 1 week in the presence of IL-12 plus alpha-IL-4 to generate Th1-type cells and in the presence of IL-4 plus alpha-IL-12 to generate Th2-type cells. 3. We demonstrate that, in contrast to freshly isolated T cells, the production of Th1 (IFN-gamma) and Th2 (IL-4, IL-5) cytokines in polarized T helper cells is not strictly controlled by the activation of AC/cAMP-linked beta(2)-adrenergic and prostaglandin (PG)E(2) receptors. 4. In Th2 cells, PGE(2) could still activate the G(s) protein-coupled AC/cAMP system and subsequently induce CREB phosphorylation, whereas PGE(2) was unable to activate the cAMP-dependent pathway in Th1 cells. In both Th1 and Th2 cells, the induction of CREB phosphorylation by beta(2)-agonist fenoterol was impaired. 5. The loss of control over cytokine production by cAMP elevating agents in differentiated Th1 and Th2 subsets may have important implications for the regulation of Th1- and Th2-mediated diseases, in particular those associated with the ongoing immune responses.

The N-acetyltransferase RimJ responds to environmental stimuli to repress pap fimbrial transcription in Escherichia coli

In uropathogenic Escherichia coli, P pili (Pap) facilitate binding to host epithelial cells and subsequent colonization. Whereas P pili can be produced at 37 degrees C, the expression of these fimbriae is suppressed at 23 degrees C. Previously, insertion mutations in rimJ, a gene encoding the N-terminal acetyltransferase of ribosomal protein S5, were shown to disrupt this thermoregulatory response, allowing papBA transcription at low temperature. In this study, we created an in-frame deletion of rimJ. This deletion relieved the repressive effects not only of low temperature but also of rich (Luria-Bertani [LB]) medium and glucose on papBA transcription, indicating that RimJ modulates papBA transcription in response to multiple environmental stimuli. papI transcription was also shown to be regulated by RimJ. papBA transcription is also controlled by a phase variation mechanism. We demonstrated that the regulators necessary to establish a phase ON state--PapI, PapB, Dam, Lrp, and cyclic AMP-CAP-are still required for papBA transcription in a rimJ mutant strain. rimJ mutations increase the rate at which bacteria transition into the phase ON state, indicating that RimJ inhibits the phase OFF-->ON transition. A DeltarimJ hns651 mutant is viable on LB medium but not on minimal medium. This synthetic lethality, along with transcriptional analyses, indicates that RimJ and H-NS work through separate pathways to control papBA transcription. Mutations in rimJ do not greatly influence the transcription of the fan, daa, or fim operon, suggesting that RimJ may be a pap-specific regulator. Overexpression of rimJ under conditions repressive for papBA transcription complements the DeltarimJ mutation but has little effect on transcription under activating conditions, indicating that the ability of RimJ to regulate transcription is environmentally controlled.

The N-acetyltransferase RimJ responds to environmental stimuli to repress pap fimbrial transcription in Escherichia coli

In uropathogenic Escherichia coli, P pili (Pap) facilitate binding to host epithelial cells and subsequent colonization. Whereas P pili can be produced at 37 degrees C, the expression of these fimbriae is suppressed at 23 degrees C. Previously, insertion mutations in rimJ, a gene encoding the N-terminal acetyltransferase of ribosomal protein S5, were shown to disrupt this thermoregulatory response, allowing papBA transcription at low temperature. In this study, we created an in-frame deletion of rimJ. This deletion relieved the repressive effects not only of low temperature but also of rich (Luria-Bertani [LB]) medium and glucose on papBA transcription, indicating that RimJ modulates papBA transcription in response to multiple environmental stimuli. papI transcription was also shown to be regulated by RimJ. papBA transcription is also controlled by a phase variation mechanism. We demonstrated that the regulators necessary to establish a phase ON state--PapI, PapB, Dam, Lrp, and cyclic AMP-CAP-are still required for papBA transcription in a rimJ mutant strain. rimJ mutations increase the rate at which bacteria transition into the phase ON state, indicating that RimJ inhibits the phase OFF-->ON transition. A DeltarimJ hns651 mutant is viable on LB medium but not on minimal medium. This synthetic lethality, along with transcriptional analyses, indicates that RimJ and H-NS work through separate pathways to control papBA transcription. Mutations in rimJ do not greatly influence the transcription of the fan, daa, or fim operon, suggesting that RimJ may be a pap-specific regulator. Overexpression of rimJ under conditions repressive for papBA transcription complements the DeltarimJ mutation but has little effect on transcription under activating conditions, indicating that the ability of RimJ to regulate transcription is environmentally controlled.

Genetic analysis of the role of G protein-coupled receptor signaling in electrotaxis

Cells display chemotaxis and electrotaxis by migrating directionally in gradients of specific chemicals or electrical potential. Chemotaxis in Dictyostelium discoideum is mediated by G protein-coupled receptors. The unique Gbeta is essential for all chemotactic responses, although different chemoattractants use different receptors and Galpha subunits. Dictyostelium amoebae show striking electrotaxis in an applied direct current electric field. Perhaps electrotaxis and chemotaxis share similar signaling mechanisms? Null mutation of Gbeta and cAMP receptor 1 and Galpha2 did not abolish electrotaxis, although Gbeta-null mutations showed suppressed electrotaxis. By contrast, G protein signaling plays an essential role in chemotaxis. G protein-coupled receptor signaling was monitored with PHcrac-green fluorescent protein, which translocates to inositol phospholipids at the leading edge of cells during chemotaxis. There was no intracellular gradient of this protein during electrotaxis. However, F-actin was polymerized at the leading edge of cells during electrotaxis. We conclude that reception and transduction of the electrotaxis signal are largely independent of G protein-coupled receptor signaling and that the pathways driving chemotaxis and electrotaxis intersect downstream of heterotrimeric G proteins to invoke cytoskeletal elements.

A cAMP receptor protein, SYCRP1, is responsible for the cell motility of Synechocystis sp. PCC 6803

Disruption of the sycrp1 gene encoding a cyanobacterial cAMP receptor protein makes cells of Synechocystis sp. PCC 6803 non-motile. Electron microscopy showed that the sycrp1-deficient strain had a reduced number of thick pili on the cell surface compared with the wild-type strain. It is suggested that cAMP-SYCRP1 complex controls the biogenesis of pili.

Stoichiometry and structural effect of the cyclic nucleotide binding to cyclic AMP receptor protein

Cyclic AMP receptor protein (CRP) is a homodimeric protein, which is activated by cAMP binding to function as a transcriptional regulator of many genes in prokaryotes. Until now, the actual number of cAMP molecules that can be bound by CRP in solution has been ambiguous. In this work, we performed a nuclear magnetic resonance study on CRP to investigate the stoichiometry of cyclic nucleotide binding to CRP. A series of (1)H-(15)N heteronuclear single quantum coherence (HSQC) spectra of the protein in the absence and in the presence of cAMP or cGMP were analyzed. The addition of cAMP to CRP induced a biphasic spectral change up to 4 equivalents, whereas the cGMP addition made a monophasic change up to 2 equivalents. Altogether, the results not only established for the first time that CRP possesses two cyclic AMP-binding sites in each monomer, even in a solution without DNA, but also suggest that the syn-cAMP binding sites of the CRP dimer can be formed by an allosteric conformational change of the protein upon the binding of two anti-cAMPs at the N-terminal domain. In addition, a residue-specific inspection of the spectral changes provides some new structural information about the cAMP-induced allosteric activation of CRP.

Ability of E. coli cyclic AMP receptor protein to differentiate cyclic nucelotides: effects of single site mutations

Escherichia coli cyclic AMP receptor protein (CRP) is a global transcriptional regulator which controls the expression of many different genes. Although different cyclic nucleotides can bind to CRP with almost equal affinity, only in the presence of cAMP could wild-type CRP bind to specific DNA sequences. Molecular genetic studies have identified a class of mutants, CRP*, which either do not require exogenous cAMP for activation or can be activated by cGMP. Thus, these mutants might aid in identifying the structural elements that are involved in the modulation of CRP to correctly differentiate the messages embedded in cyclic nucleotides. In this in vitro study, five CRP* mutants, namely, D53H, S62F, G141Q, G141K, and L148R, were tested for their abilities to bind the lac promoter sequence and the effects of cyclic nucleotides in modulating DNA sequence recognition. For comparison, non-CRP* mutants K52N, T127L, H159L, and K52N/H159L were studied. cCMP and cGMP can replace cAMP as an allosteric effector in all of these CRP mutants except S62F and non-CRP* mutants. The D53H, G141Q, G141K, and L148R mutants exhibit significantly higher affinity for the lac promoter sequence than wild-type CRP while S62F and the non-CRP* mutants exhibit reduced affinity. To probe the pathway of communication, the energetics of subunit assembly in these mutants were monitored by sedimentation equilibrium, and the conformational states of these mutants were probed by proteolysis and accessibility of Cys178 to chemical modifications. Results from these studies imply that signals due to mutations are mostly transmitted through the subunit interface. Thus, residues in CRP outside of the cyclic nucleotide binding site modulate the ability of CRP to differentiate these three cyclic nucleotides through long-range communication. Furthermore, this study shows that CRP* mutations do not impart any unique properties to CRP except that the DNA binding constants are shifted to a regime of higher affinity.

Extracellular cAMP inhibits proximal reabsorption: are plasma membrane cAMP receptors involved?

Glucagon binding to hepatocytes has been known for a long time to not only stimulate intracellular cAMP accumulation but also, intriguingly, induce a significant release of liver-borne cAMP in the blood. Recent experiments have shown that the well-documented but ill-understood natriuretic and phosphaturic actions of glucagon are actually mediated by this extracellular cAMP, which inhibits the reabsorption of sodium and phosphate in the renal proximal tubule. The existence of this "pancreato-hepatorenal cascade" indicates that proximal tubular reabsorption is permanently influenced by extracellular cAMP, the concentration of which is most probably largely dependent on the insulin-to-glucagon ratio. The possibility that renal cAMP receptors may be involved in this process is supported by the fact that cAMP has been shown to bind to brush-border membrane vesicles. In other cell types (i.e., adipocytes, erythrocytes, glial cells, cardiomyocytes), cAMP eggress and/or cAMP binding have also been shown to occur, suggesting additional paracrine effects of this nucleotide. Although not yet identified in mammals, cAMP receptors (cARs) are already well characterized in lower eukaryotes. The amoeba Dictyostelium discoideum expresses four different cARs during its development into a multicellular organism. cARs belong to the superfamily of seven transmembrane domain G protein-coupled receptors and exhibit a modest homology with the secretin receptor family (which includes PTH receptors). However, the existence of specific cAMP receptors in mammals remains to be demonstrated. Disturbances in the pancreato-hepatorenal cascade provide an adequate pathophysiological understanding of several unexplained observations, including the association of hyperinsulinemia and hypertension, the hepatorenal syndrome, and the hyperfiltration of diabetes mellitus. The observations reviewed in this paper show that cAMP should no longer be regarded only as an intracellular second messenger but also as a first messenger responsible for coordinated hepatorenal functions, and possibly for paracrine regulations in several other tissues.

tGRAP, the G-protein coupled receptors mutant database

The searchable mutant database tGRAP (previously called tinyGRAP) at the University of Tromsø contains data on mutated G-protein coupled receptors (GPCRs). All data have been extracted from scientific papers and entered manually into the database. The current version of the tGRAP mutant database (tGRAP.uit.no, release 10, April 2001) contains around 10 500 mutants extracted from almost 1400 research papers containing mutant data on five families of GPCRs, i.e. Family A, rhodopsin-like; Family B, secretin-like; Family C, metabotropic glutamate-like; Family D, pheromone; Family E, cAMP receptors. A query form provides rapid and simple access to relevant mutant information. In addition to this query form, a tool that enables the user to access mutation data via sequence alignments has been introduced. The ability to access mutant data from such alignments increases the usefulness of the mutant database and facilitates comparison of mutagenesis data between receptors. Moreover, this tool allows the construction of tailor-made sequence alignment views from any combination of receptors belonging to the same class. The database is available at http://tGRAP.uit.no/.

A simple mechanism for co-dependence on two activators at an Escherichia coli promoter

The Escherichia coli melAB promoter is co-dependent upon two transcription activators, MelR and the cyclic AMP receptor protein, CRP. In this study we demonstrate positive co-operativity between the binding of MelR and CRP at the melAB promoter, which provides a simple mechanism for its co-dependence. MelR binds to four sites, centred at positions -42.5, -62.5, -100.5 and -120.5 relative to the melAB transcription start point. When MelR is pre-bound, CRP is able to bind to a target located between MelR at positions -62.5 and -100.5. This increases the occupation of the two downstream sites for MelR, which is essential for transcription activation. We have identified residues within activating region 1 (AR1) of CRP that are important in transcription activation of the melAB promoter. At simple CRP-dependent promoters, the surface of CRP containing these residues is involved in contacting the RNA polymerase alpha subunit. Our results show that, at the melAB promoter, the surface of CRP containing AR1 contacts MelR rather than RNA polymerase. Thus, MelR and CRP activate transcription by a novel mechanism in which they bind co-operatively to adjacent sites and form a bacterial enhanceosome.

Propagating chemoattractant waves coordinate periodic cell movement inDictyosteliumslugs

Migration and behaviour of Dictyostelium slugs results from coordinated movement of its constituent cells. It has been proposed that cell movement is controlled by propagating waves of cAMP as during aggregation and in the mound. We report the existence of optical density waves in slugs; they are initiated in the tip and propagate backwards. The waves reflect periodic cell movement and are mediated by cAMP, as injection of cAMP or cAMP phosphodiesterase disrupts wave propagation and results in effects on cell movement and, therefore, slug migration. Inhibiting the function of the cAMP receptor cAR1 blocks wave propagation, showing that the signal is mediated by cAR1. Wave initiation is strictly dependent on the tip; in decapitated slugs no new waves are initiated and slug movement stops until a new tip regenerates. Isolated tips continue to migrate while producing waves. We conclude from these observations that the tip acts as a pacemaker for cAMP waves that coordinate cell movement in slugs.Movies available on-line

Single-molecule analysis of chemotactic signaling in Dictyostelium cells

Single-molecule imaging techniques were used to reveal the binding of individual cyclic adenosine 3',5'-monophosphate molecules to heterotrimeric guanine nucleotide-binding protein coupled receptors on the surface of living Dictyostelium discoideum cells. The binding sites were uniformly distributed and diffused rapidly in the plane of the membrane. The probabilities of individual association and dissociation events were greater for receptors at the anterior end of the cell. Agonist-induced receptor phosphorylation had little effect on any of the monitored properties, whereas G protein coupling influenced the binding kinetics. These observations illustrate the dynamic properties of receptors involved in gradient sensing and suggest that these may be polarized in chemotactic cells.

Environmental signals controlling production of hemagglutinin/protease in Vibrio cholerae

Vibrio cholerae hemagglutinin/protease (Hap) was induced upon nutrient limitation and strongly repressed by glucose. Hap was not produced in a mutant defective in the cyclic AMP (cAMP) receptor protein, suggesting that intracellular cAMP levels mediate Hap expression. No difference was found in Hap production between an rpoS deletion mutant and its isogenic wild-type precursor, indicating that the alternate sigma(s) factor is not essential for Hap expression. Based on these and previous results, we discuss the role of Hap in the pathogenesis of cholera.

RasC is required for optimal activation of adenylyl cyclase and Akt/PKB during aggregation

Disruption of Dictyostelium rasC, encoding a Ras subfamily protein, generated cells incapable of aggregation. While rasC expression is enriched in a cell type-specific manner during post-aggregative development, the defect in rasC(-) cells is restricted to aggregation and fully corrected by application of exogenous cAMP pulses. cAMP is not produced in rasC(-) cells stimulated by 2'-deoxy-cAMP, but is produced in response to GTPgammaS in cell lysates, indicating that G-protein-coupled cAMP receptor activation of adenylyl cyclase is regulated by RasC. However, cAMP-induced ERK2 phosphorylation is unaffected in rasC(-) cells, indicating that RasC is not an upstream activator of the mitogen-activated protein kinase required for cAMP relay. rasC(-) cells also exhibit reduced chemotaxis to cAMP during early development and delayed response to periodic cAMP stimuli produced by wild-type cells in chimeric mixtures. Furthermore, cAMP-induced Akt/PKB phosphorylation through a phosphatidylinositide 3-kinase (PI3K)-dependent pathway is dramatically reduced in rasC(-) cells, suggesting that G-protein-coupled serpentine receptor activation of PI3K is regulated by RasC. Cells lacking the RasGEF, AleA, exhibit similar defects as rasC(-) cells, suggesting that AleA may activate RasC.

A cell number-counting factor regulates group size in Dictyostelium by differentially modulating cAMP-induced cAMP and cGMP pulse sizes

A secreted counting factor (CF), regulates the size of Dictyostelium discoideum fruiting bodies in part by regulating cell-cell adhesion. Aggregation and the expression of adhesion molecules are mediated by relayed pulses of cAMP. Cells also respond to cAMP with a short cGMP pulse. We find that CF slowly down-regulates the cAMP-induced cGMP pulse by inhibiting guanylyl cyclase activity. A 1-min exposure of cells to purified CF increases the cAMP-induced cAMP pulse. CF does not affect the cAMP receptor or its interaction with its associated G proteins or the translocation of the cytosolic regulator of adenylyl cyclase to the membrane in response to cAMP. Pulsing streaming wild-type cells with a high concentration of cAMP results in the formation of small groups, whereas reducing cAMP pulse size with exogenous cAMP phosphodiesterase during stream formation causes cells to form large groups. Altering the extracellular cAMP pulse size does not phenocopy the effects of CF on the cAMP-induced cGMP pulse size or cell-cell adhesion, indicating that CF does not regulate cGMP pulses and adhesion via CF's effects on cAMP pulses. The results suggest that regulating cell-cell adhesion, the cGMP pulse size, or the cAMP pulse size can control group size and that CF regulates all three of these independently.

The phosphorylated C-terminus of cAR1 plays a role in cell-type-specific gene expression and STATa tyrosine phosphorylation

cAMP receptors mediate some signaling pathways via coupled heterotrimeric G proteins, while others are G-protein-independent. This latter class includes the activation of the transcription factors GBF and STATa. Within the cellular mounds formed by aggregation of Dictyostelium, micromolar levels of cAMP activate GBF function, thereby inducing the transcription of postaggregative genes and initiating multicellular differentiation. Activation of STATa, a regulator of culmination and ecmB expression, results from cAMP receptor-dependent tyrosine phosphorylation and nuclear localization, also in mound-stage cells. During mound development, the cAMP receptor cAR1 is in a low-affinity state and is phosphorylated on multiple serine residues in its C-terminus. This paper addresses possible roles of cAMP receptor phosphorylation in the cAMP-mediated stimulation of GBF activity, STATa tyrosine phosphorylation, and cell-type-specific gene expression. To accomplish this, we have expressed cAR1 mutants in a strain in which the endogenous cAMP receptors that mediate postaggregative gene expression in vivo are deleted. We then examined the ability of these cells to undergo morphogenesis and induce postaggregative and cell-type-specific gene expression and STATa tyrosine phosphorylation. Analysis of cAR1 mutants in which the C-terminal tail is deleted or the ligand-mediated phosphorylation sites are mutated suggests that the cAR1 C-terminus is not essential for GBF-mediated postaggregative gene expression or STATa tyrosine phosphorylation, but may play a role in regulating cell-type-specific gene expression and morphogenesis. A mutant receptor, in which the C-terminal tail is constitutively phosphorylated, exhibits constitutive activation of STATa tyrosine phosphorylation in pulsed cells in suspension and a significantly impaired ability to induce cell-type-specific gene expression. The constitutively phosphorylated receptor also exerts a partial dominant negative effect on multicellular development when expressed in wild-type cells. These findings suggest that the phosphorylated C-terminus of cAR1 may be involved in regulating aspects of receptor-mediated processes, is not essential for GBF function, and may play a role in mediating subsequent development.

Receptor-mediated activation of heterotrimeric G-proteins in living cells

Receptor-mediated activation of heterotrimeric GTP-binding proteins (G-proteins) was visualized in living Dictyostelium discoideum cells by monitoring fluorescence resonance energy transfer (FRET) between alpha- and beta- subunits fused to cyan and yellow fluorescent proteins. The G-protein heterotrimer rapidly dissociated and reassociated upon addition and removal of chemoattractant. During continuous stimulation, G-protein activation reached a dose-dependent steady-state level. Even though physiological responses subsided, the activation did not decline. Thus, adaptation occurs at another point in the signaling pathway, and occupied receptors, whether or not they are phosphorylated, catalyze the G-protein cycle. Construction of similar energy-transfer pairs of mammalian G-proteins should enable direct in situ mechanistic studies and applications such as drug screening and identifying ligands of newly found G-protein-coupled receptors.

Modeling the model organism Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum is a fascinating organism, not only for biologists, but also for physicists. Since the Belousov-Zhabotinskii reaction pattern, a well-known non-linear phenomenon in chemistry, was observed during aggregation of Dictyostelium amoebae, Dictyostelium has been one of the major subjects of non-linear dynamics studies. Macroscopic theory, such as continuous cell density approximation, has been a common approach to studying pattern formation since the pioneering work of Turing. Recently, promising microscopic approaches, such as the cellular dynamics method, have emerged. They have shown that Dictyostelium is useful as a model system in biology, The synchronization mechanism of oscillatory production of cyclic adenosine 3',5'-monophosphate in Dictyostelium is discussed in detail to show how it is a universal feature that can explain synchronization in other organisms.

Neuronal plasticity and survival in mood disorders

Studies at the basic and clinical levels demonstrate that neuronal atrophy and cell death occur in response to stress and in the brains of depressed patients. Although the mechanisms have yet to be fully elucidated, progress has been made in characterizing the signal transduction cascades that control neuronal atrophy and programmed cell death and that may be involved in the action of antidepressant treatment. These pathways include the cyclic adenosine monophosphate and neurotrophic factor signal transduction cascades. It is notable that these same pathways have been demonstrated to play a pivotal role in cellular models of neural plasticity. This overlap of plasticity and cell survival pathways, together with studies demonstrating that neuronal activity enhances cell survival, suggests that neuronal atrophy and death could result from a disruption of the mechanisms underlying neural plasticity. The role of these pathways and failure of neuronal plasticity in stress-related mood disorders are discussed.

A diffusible factor involved in MAP-kinase ERK2-regulated development of Dictyostelium

Mitogen-activated protein (MAP)-kinase extracellular signal regulated kinase (ERK2) is essential for regulation of the intracellular cyclic adenosine monophosphate (cAMP) level in Dictyostelium. The mutant lacking ERK2, erk2-null, is arrested at the pre-aggregation stage, but develops into a fruiting body in a mixed population of wild-type and mutant cells. This fact implies that wild-type cells provide a certain factor that is missing in erk2-null. It was clarified that both wild-type strains KAx3 and Ax2 secreted a diffusible factor that enables erk2-null to develop. The fruiting body formed from erk2-null cells was smaller than that formed by the wild-type cells and consisted of a small sorus supported by a slender stalk with a single row of vacuolated stalk cells. The resulting spores were able to germinate and multiply on a bacterial lawn, but they were unable to develop unless the factor was provided. After 8 h of starvation, wild-type cells started to secrete the factor, which had a molecular mass of less than 3 kDa and was heat stable. The effect of this factor could not be mimicked by either cAMP or folate. Adenylyl cyclase A and cell surface cAMP receptors cAR1 and cAR3 were all indispensable components for the factor to function. Considering the molecular mass and the mode of action, this factor could be a novel one. Possible targets of this factor are discussed in terms of cAMP-dependent protein kinase activation.

The novel tyrosine kinase ZAK1 activates GSK3 to direct cell fate specification

Inhibition of GSK3 by 7-TM Wnt/wg receptor signaling is critical for specifying embryonic cell fate patterns. In Dictyostelium, the 7-TM cAMP receptors regulate GSK3 by parallel, antagonistic pathways to establish a developmental body plan. We describe here a novel tyrosine kinase, ZAK1, downstream of 7-TM cAMP receptor signaling that is required for GSK3 activation during development. zak1-nulls have reduced GSK3 activity and are defective in GSK3-regulated developmental pathways. Moreover, recombinant ZAK1 phosphorylates and activates GSK3 in vitro. We propose that ZAK1 is a positive regulator of GSK3 activity required for cell pattern formation in Dictyostelium and speculate that similar mechanisms exist to antagonize Wnt/wg signaling for metazoan cell fate specification.

Regulation of signal transduction pathways and gene expression by mood stabilizers and antidepressants

OBJECTIVE

To determine whether the currently available evidence supports the hypothesis that antidepressants and mood stabilizers may bring about some of their long-term therapeutic effects by regulating signal transduction pathways and gene expression in the central nervous system.

METHODS

To address this question, we reviewed the evidence showing that chronic administration of antidepressants and mood stabilizers involves alterations in signaling pathways and gene expression in the central nervous system.

RESULTS

A large body of data has shown that lithium and valproate exert effects on the protein kinase C signaling pathway and the activator protein 1 family of transcription factors; in contrast, antidepressants affect the cyclic adenosine monophosphate pathway and may bring about their therapeutic effects by modulating cyclic adenosine monophosphate-regulated gene expression in the central nervous system.

CONCLUSIONS

Given the key roles of these signaling cascades in the amplification and integration of signals in the central nervous system, the findings have clear implications not only for research into the etiology and pathophysiology of the severe mood disorders but also for the development of novel and innovative treatment strategies.

Effect of mutations at the monomer-monomer interface of cAMP receptor protein on specific DNA binding

To determine the thermodynamic role of binding of an operon to cAMP receptor protein (CRP) in the activation of transcription, isothermal titration calorimetry measurements were performed on the binding of three 40-base pair DNA sequences to the cyclic nucleoside complexes of CRP and its mutants at 296 K. The three 40-base pair sequences consisted of a consensus DNA (conDNA) duplex derived from the CRP-binding site sequences of the operons activated by CRP and two DNA sequences based on the CRP-binding site sequences of the lac operon (lacDNA) and of the gal operon (galDNA). The mutants of CRP consisted of a T127L mutant, a S128A mutant, and a mutant containing both mutations (CRP*) which not only alter the transcriptional activity of the CRP complexes but also are involved in the monomer-monomer interfacial interactions of the CRP dimer. The binding reactions of the DNA duplexes to the fully cNMP-ligated CRP-mutant complexes were endothermic with binding constants as high as 6.6 +/- 1.1 x 10(6) M-1 (conDNA.CRP(cAMP)2). ConDNA binding to the unligated T127L and CRP* mutants was observed as well as conDNA and lacDNA binding to CRP with cAMP bound to only one monomer. The reduction of the binding constants with increase in KCl concentration indicated the formation of two ion pairs for the cAMP-ligated CRP and S128A complexes and four ion pairs for the cAMP-ligated T127L and CRP* complexes. Reduction of the DNA binding constants upon substitution of D2O for H2O in the buffer, the large heat capacity changes, and the enthalpy-entropy compensation exhibited by the binding reactions indicate the importance of dehydration in the binding reaction. Small angle neutron scattering measurements on the lacDNA.CRP(cAMP)2 complex in D2O/H2O mixtures show that the DNA is bent around the cAMP-ligated protein in solution.

[Repression of catabolites in gram-positive and gram-negative bacteria]

Analyzes the mechanism of catabolite repression of grampositive and gramnegative bacteria. The role of cyclic adenosine monophosphate and CRP protein, forming a complex, is shown. Contribution of ATP kinase to manifestation of the catabolic repression phenomenon in grampositive bacteria is discussed.

Interactions between the Escherichia coli cAMP receptor protein and the C-terminal domain of the alpha subunit of RNA polymerase at class I promoters

The Escherichia coli cAMP receptor protein (CRP) is a factor that activates transcription at over 100 target promoters. At Class I CRP-dependent promoters, CRP binds immediately upstream of RNA polymerase and activates transcription by making direct contacts with the C-terminal domain of the RNA polymerase alpha subunit (alphaCTD). Since alphaCTD is also known to interact with DNA sequence elements (known as UP elements), we have constructed a series of semi-synthetic Class I CRP-dependent promoters, carrying both a consensus DNA-binding site for CRP and a UP element at different positions. We previously showed that, at these promoters, the CRP-alphaCTD interaction and the CRP-UP element interaction contribute independently and additively to transcription initiation. In this study, we show that the two halves of the UP element can function independently, and that, in the presence of the UP element, the best location for the DNA site for CRP is position -69.5. This suggests that, at Class I CRP-dependent promoters where the DNA site for CRP is located at position -61.5, the two alphaCTDs of RNA polymerase are not optimally positioned. Two experiments to test this hypothesis are presented.

Efflux of cyclic adenosine monophosphate from cells: mechanisms and physiological implications

Although cyclic nucleotides are hydrophilic compounds, extracellular cAMP (cAMPo) rapidly accumulates during the activation of adenylate cyclase. This review considers the kinetic characteristics of cAMP transport through the plasma membrane and its physiological implications. The influx and efflux of cAMP occur via different carriers. At physiological concentrations of cAMPo, the influx of cAMP does not significantly contribute to regulation of the intracellular content of the cyclic nucleotide, but it is responsible for the accumulation of cAMPi in experiments at [cAMP]o approximately 1 mM. In contrast, the high rate of cAMP efflux is mainly responsible for normalization of [cAMP]i during long-term activation of adenylate cyclase. The possible involvement of ATP-binding cassette proteins (ABC proteins) in the efflux of cAMP from the cell is considered. In procaryotes cAMPo is a signal molecule during the generation of cell colonies, acting on special receptors that interact with GTP-binding proteins. Such receptors have not been found in vertebrates, and in most cases the signal functions of cAMPo are mediated by its degradation by extracellular enzymes with subsequent activation of adenosine receptors.

Glycogen synthase kinase-3 (GSK-3) is regulated during Dictyostelium development via the serpentine receptor cAR3

Glycogen synthase kinase-3 (GSK-3) is required during metazoan development to mediate the effects of the extracellular signal wingless/Wnt-1 and hence is necessary for correct cell type specification. GSK-3 also regulates cell fate during Dictyostelium development, but in this case it appears to mediate the effects of extracellular cAMP. By direct measurement of GSK-3 kinase activity during Dictyostelium development, we find that there is a rise in activity at the initiation of multicellular development which can be induced by cAMP. The timing of the rise correlates with the requirement for the Dictyostelium homologue of GSK-3, GSKA, to specify cell fate. We show that loss of the cAMP receptor cAR3 almost completely abolishes the rise in kinase activity and causes a mis-specification of cell fate that is equivalent to that seen in a gskA- mutant. The phenotype of a cAR3(−) mutant however is less severe than loss of gskA and ultimately gives rise to an apparently wild-type fruiting body. These results indicate that in Dictyostelium extracellular cAMP acts via cAR3 to cause a rise in GSKA kinase activity which regulates cell type patterning during the initial stages of multicellularity.

Desensitization of G-protein-coupled receptors. agonist-induced phosphorylation of the chemoattractant receptor cAR1 lowers its intrinsic affinity for cAMP

Agonist-induced phosphorylation of G-protein-coupled receptors has been shown to facilitate the desensitization processes, such as receptor internalization, decreased efficiency of coupling to G-proteins, or decreased ligand affinity. The lowered affinity may be an intrinsic property of the phosphorylated receptor or it may be the result of altered interactions between the modified receptor and downstream components such as G-proteins or arrestins. To address this issue, we purified cAR1, the major chemoattractant receptor of Dictyostelium discoideum by a strategy that is independent of the ligand binding capacity of the receptor. To our knowledge, this represents the first successful purification of a chemoattractant receptor. The hexyl-histidine-tagged receptor was solubilized from a highly enriched plasma membrane preparation and purified by Ni2+-chelating chromatography. The protocol offers a simple way to purify 100-500 micrograms of a G-protein coupled receptor that can be targeted to the plasma membrane of D. discoideum. The Kd value for the purified cAR1 was about 200 nM, consistent with that of receptors that are not coupled to G-proteins in intact cells. In contrast, the affinity of phosphorylated cAR1, purified from desensitized cells, was about three times lower. Treatment of the phosphorylated receptor with protein phosphatases caused dephosphorylation and parallel restoration of higher affinity. We propose that ligand-induced phosphorylation of G-protein-coupled receptors causes a decrease in intrinsic affinity and may be useful in maintaining the receptor's sensitivity at high agonist levels. This affinity decrease may precede other processes such as receptor internalization or uncoupling from G-proteins.

A molecular network that produces spontaneous oscillations in excitable cells of Dictyostelium

A network of interacting proteins has been found that can account for the spontaneous oscillations in adenylyl cyclase activity that are observed in homogenous populations of Dictyostelium cells 4 h after the initiation of development. Previous biochemical assays have shown that when extracellular adenosine 3',5'-cyclic monophosphate (cAMP) binds to the surface receptor CAR1, adenylyl cyclase and the MAP kinase ERK2 are transiently activated. A rise in the internal concentration of cAMP activates protein kinase A such that it inhibits ERK2 and leads to a loss-of-ligand binding by CAR1. ERK2 phosphorylates the cAMP phosphodiesterase REG A that reduces the internal concentration of cAMP. A secreted phosphodiesterase reduces external cAMP concentrations between pulses. Numerical solutions to a series of nonlinear differential equations describing these activities faithfully account for the observed periodic changes in cAMP. The activity of each of the components is necessary for the network to generate oscillatory behavior; however, the model is robust in that 25-fold changes in the kinetic constants linking the activities have only minor effects on the predicted frequency. Moreover, constant high levels of external cAMP lead to attenuation, whereas a brief pulse of cAMP can advance or delay the phase such that interacting cells become entrained.

The sequential mechanism of guanidine hydrochloride-induced denaturation of cAMP receptor protein from Escherichia coli. A fluorescent study using 8-anilino-1-naphthalenesulfonic acid

cAMP receptor protein (CRP) regulates expression of a number of genes in Escherichia coli. The protein is a homodimer and each monomer is folded into two structural domains. The biological activation of CRP upon cAMP binding may involve the subunit realignment as well as reorientation between the domains within each subunit. In order to study the interactions between the subunits or domains, we performed stopped-flow measurements of the guanidine hydrochloride (GuHCI)-induced denaturation of CRP. The changes in CRP structure induced by GuHCl were monitored using both intrinsic Trp fluorescence as well as the fluorescence of an extrinsic probe, 8-anilino-1-Naphthalenesulfonic acid (ANS). Results of CRP denaturation using Trp fluorescence detection are consistent with a two-step model [Malecki, and Wasylewski, (1997), Eur. J. Biochem. 243, 660], where the dissociation of dimer into subunits is followed by the monomer unfolding. The denaturation of CRP monitored by ANS fluorescence reveals the existence of two additional processes. One occurs before the dissociation of CRP into subunits, whereas the second takes place after the dissociation, but prior to proper subunit unfolding. These additional processes suggest that CRP denaturation is described by a more complicated mechanism than a simple three-state equilibrium and may involve additional changes in both inter- and intrasubunit interactions. We also report the effect of cAMP on the kinetics of CRP subunit unfolding and refolding.

Dissection of the cAMP induced cytosolic calcium response in Dictyostelium discoideum: the role of cAMP receptor subtypes and G protein subunits

The cAMP signaling cascade leading to changes in [Ca2+]i in Dictyostelium discoideum was analyzed using cell lines overexpressing single cAMP receptor subtypes (cAR1-cAR3) or lacking the G(alpha2) or G(beta) subunit of the G protein. Imaging of fura2-dextran-loaded amoebae revealed cAMP-induced [Ca2+]i changes characteristic for each receptor subtype activated. Cells expressing distinct subtypes sort to defined zones during multicellular development suggesting involvement of the specific [Ca2+]i transients in patterning processes. Whereas generation of the [Ca2+]i increase was G(alpha2)-independent, only few cells devoid of G(beta) displayed a [Ca2+]i change after stimulation indicating its participation in the regulation of the calcium homeostasis.

Selection of gbeta subunits with point mutations that fail to activate specific signaling pathways in vivo: dissecting cellular responses mediated by a heterotrimeric G protein in Dictyostelium discoideum

In Dictyostelium discoideum, a unique Gbeta subunit is required for a G protein-coupled receptor system that mediates a variety of cellular responses. Binding of cAMP to cAR1, the receptor linked to the G protein G2, triggers a cascade of responses, including activation of adenylyl cyclase, gene induction, actin polymerization, and chemotaxis. Null mutations of the cAR1, Galpha2, and Gbeta genes completely impair all these responses. To dissect specificity in Gbetagamma signaling to downstream effectors in living cells, we screened a randomly mutagenized library of Gbeta genes and isolated Gbeta alleles that lacked the capacity to activate some effectors but retained the ability to regulate others. These mutant Gbeta subunits were able to link cAR1 to G2, to support gene expression, and to mediate cAMP-induced actin polymerization, and some were able to mediate to chemotaxis toward cAMP. None was able to activate adenylyl cyclase, and some did not support chemotaxis. Thus, we separated in vivo functions of Gbetagamma by making point mutations on Gbeta. Using the structure of the heterotrimeric G protein displayed in the computer program CHAIN, we examined the positions and the molecular interactions of the amino acids substituted in each of the mutant Gbetas and analyzed the possible effects of each replacement. We identified several residues that are crucial for activation of the adenylyl cyclase. These residues formed an area that overlaps but is not identical to regions where bovine Gtbetagamma interacts with its regulators, Galpha and phosducin.

Functional promiscuity of gene regulation by serpentine receptors in Dictyostelium discoideum

Serpentine receptors such as smoothened and frizzled play important roles in cell fate determination during animal development. In Dictyostelium discoideum, four serpentine cyclic AMP (cAMP) receptors (cARs) regulate expression of multiple classes of developmental genes. To understand their function, it is essential to know whether each cAR is coupled to a specific gene regulatory pathway or whether specificity results from the different developmental regulation of individual cARs. To distinguish between these possibilities, we measured gene induction in car1 car3 double mutant cell lines that express equal levels of either cAR1, cAR2, or cAR3 under a constitutive promoter. We found that all cARs efficiently mediate both aggregative gene induction by cAMP pulses and induction of postaggregative and prespore genes by persistent cAMP stimulation. Two exceptions to this functional promiscuity were observed. (i) Only cAR1 can mediate adenosine inhibition of cAMP-induced prespore gene expression, a phenomenon that was found earlier in wild-type cells. cAR1's mediation of adenosine inhibition suggests that cAR1 normally mediates prespore gene induction. (ii) Only cAR2 allows entry into the prestalk pathway. Prestalk gene expression is induced by differentiation-inducing factor (DIF) but only after cells have been prestimulated with cAMP. We found that DIF-induced prestalk gene expression is 10 times higher in constitutive cAR2 expressors than in constitutive cAR1 or cAR3 expressors (which still have endogenous cAR2), suggesting that cAR2 mediates induction of DIF competence. Since in wild-type slugs cAR2 is expressed only in anterior cells, this could explain the so far puzzling observations that prestalk cells differentiate at the anterior region but that DIF levels are actually higher at the posterior region. After the initial induction of DIF competence, cAMP becomes a repressor of prestalk gene expression. This function can again be mediated by cAR1, cAR2, and cAR3.

Temperature-sensitive Gbeta mutants discriminate between G protein-dependent and -independent signaling mediated by serpentine receptors

Deletion of the single gene for the Dictyostelium G protein beta-subunit blocks development at an early stage. We have now isolated temperature-sensitive alleles of Gbeta to investigate its role in later development. We show that Gbeta is directly required for adenylyl cyclase A activation and for morphogenetic signaling during the entire developmental program. Gbeta was also essential for induction of aggregative gene expression by cAMP pulses, a process that is mediated by serpentine cAMP receptors (cARs). However, Gbeta was not required for cAR-mediated induction of prespore genes and repression of stalk genes, and neither was Gbeta needed for induction of prestalk genes by the differentiation inducing factor (DIF). cAMP induction of prespore genes and repression of stalk genes is mediated by the protein kinase GSK-3. GSK-3 also determines cell-type specification in insects and vertebrates and is regulated by the wingless/wnt morphogens that are detected by serpentine fz receptors. The G protein-dependent and -independent modes of cAR-mediated signaling reported here may also exist for the wingless/wnt signaling pathways in higher organisms.

Expression and distribution of parotid secretory proteins in experimental diabetes

Previous studies of experimental diabetes have demonstrated changes in the levels of specific salivary proteins. The present study is part of a larger effort aimed at elucidating the mechanism(s) by which insulin regulates salivary protein expression in the rat parotid gland. Diabetes was induced in 2-3-month-old male Fischer 344 rats by injection of streptozotocin (STZ). After 30 days one group of rats was given insulin for 7 days. Untreated rats served as controls. As previously observed, parotid acinar cells from diabetic rats accumulated lipid and contained occasional crystalloid lysosomes. Quantitative immunogold labeling of secretory granules in diabetic glands revealed decreases of 30-60% for proline-rich-proteins (PRPs), amylase and parotid secretory protein (PSP), but labeling for acidic epididymal glycoprotein (AEG) was unchanged. The response to insulin treatment was variable: amylase and PSP labeling were partly restored, but PRP and AEG labeling showed little change. Photoaffinity labeling of cyclic AMP receptor proteins (cARP) showed changes in several tissues including a consistent increase in the diabetic parotid gland. Immunogold labeling of secretory granules with antibody to cARP was similar in control and diabetic parotids, but nuclear and cytoplasmic label was decreased in diabetic acinar cells. These results indicate that STZ-diabetes and insulin reconstitution cause variable changes in the expression of parotid secretory proteins. Changes in cARP levels suggest that the insulin and cyclic AMP pathways may interact in regulating expression of salivary secretory proteins.

Switching of chemoattractant receptors programs development and morphogenesis in Dictyostelium: receptor subtypes activate common responses at different agonist concentrations

One of the common functional features among G-protein coupled receptors is the occurrence of multiple subtypes involved in similar signal transduction events. The cAMP chemoattractant receptor family of Dictyostelium discoideum is composed of four receptors (cAR1-cAR4), which are expressed sequentially throughout the developmental transition from a unicellular to a multicellular organism. The receptors differ in affinity for cAMP and in the sequences of their C-terminal domains. In this study, we constitutively expressed cAR1, cAR2, and cAR3 as well as a series of chimeric and mutant receptors and assessed the capacity of each to mediate chemotaxis, activation of adenylyl cyclase and actin polymerization, and rescue the developmental defect of car1-/car3- cells. We found that various receptors and mutants sense different concentration ranges of cAMP but all can mediate identical responses during the aggregation stage of development. The responses displayed very similar kinetics, suggesting no major differences in regulatory properties attributable to the C-terminal domains. We speculate that switching of receptor subtypes during development enables the organism to respond to the changing concentrations of the chemoattractant and thereby program morphogenesis appropriately.

Positive and negative transcriptional regulation of the Escherichia coli gluconate regulon gene gntT by GntR and the cyclic AMP (cAMP)-cAMP receptor protein complex

The gntT gene of Escherichia coli is specifically induced by gluconate and repressed via catabolite repression. Thus, gluconate is both an inducer and a repressor of gntT expression since gluconate is a catabolite-repressing sugar. In a gntR deletion mutant, the expression of a chromosomal gntT::lacZ fusion is both high and constitutive, confirming that GntR is the negative regulator of gntT. Indeed, GntR binds to two consensus gnt operator sites; one overlaps the -10 region of the gntT promoter, and the other is centered at +120 with respect to the transcriptional start site. The binding of GntR to these sites was proven in vitro by gel redardation assays and in vivo by site-directed mutagenesis of the binding sites. Binding of GntR to the operators is eliminated by gluconate and also by 6-phosphogluconate at a 10-fold-higher concentration. Interestingly, when gntR deletion strains are grown in the presence of gluconate, there is a twofold decrease in gntT expression which is independent of catabolite repression and binding of GntR to the operator sites. This novel response of gntR mutants to the inducer is termed ultrarepression. Transcription of gntT is activated by binding of the cyclic AMP (cAMP)-cAMP receptor protein (CRP) complex to a CRP binding site positioned at -71 upstream of the gntT transcription start site.

Spacing requirements for interactions between the C-terminal domain of the alpha subunit of Escherichia coli RNA polymerase and the cAMP receptor protein

During transcription initiation at bacterial promoters, the C-terminal domain of the RNA polymerase alpha subunit (alphaCTD) can interact with DNA-sequence elements (known as UP elements) and with activator proteins. We have constructed a series of semi-synthetic promoters carrying both an UP element and a consensus DNA-binding site for the Escherichia coli cAMP receptor protein (CRP; a factor that activates transcription by making direct contacts with alphaCTD). At these promoters, the UP element was located at a variety of distances upstream of the CRP-binding site, which was fixed at position -41.5 bp upstream of the transcript start. At some positions, the UP element caused enhanced promoter activity whereas, at other positions, it had very little effect. In no case was the CRP-dependence of the promoter relieved. DNase I and hydroxyl-radical footprinting were used to study ternary RNA polymerase-CRP-promoter complexes formed at two of the most active of these promoters, and co-operativity between the binding of CRP and purified alpha subunits was studied. The footprints show that alphaCTD binds to the UP element as it is displaced upstream but that this displacement does not prevent alphaCTD from being contacted by CRP. Models to account for this are discussed.

MAP kinase function in amoeboid chemotaxis

Mutants lacking the MAP kinase DdERK2 show reduced chemotactic responses to folate and cAMP. Analysis of cAMP chemotaxis shows that Dderk2- cells are defective in chemotaxis to high concentrations of cAMP. This defect is due to an inability to repolarize in the continued presence of high concentrations of cAMP. Under these conditions, the speed of movement of mutant cells remains low. Instead of generating a leading pseudopod, mutant cells generate transient crown-like structures over multiple regions of the cell surface. These structures differ from pseudopods in that they contain myosin II as well as F actin and coronin. These studies identify a role for MAP kinases in coordinating the formation of cell projections generated in response to chemoattractants.

Role of macromolecular hydration in the binding of the Escherichia coli cyclic AMP receptor to DNA

The osmotic stress technique was used to measure the changes in macromolecular hydration that accompany binding of the Escherichia coli CAP protein to its transcription-regulatory site (C1) in the lactose promoter and that accompany the transfer of CAP from site C1 to nonspecific genomic DNA. Formation of the C1 complex is accompanied by the net release of 79 +/- 11 water molecules. If all water molecules were released from macromolecular surfaces, this result would be consistent with a net reduction of solvent-accessible surface area of 711 +/- 189 A2. This area is only slightly smaller than the solvent-inaccessible macromolecular interface in crystalline CAP-DNA complexes. The transfer of CAP from site C1 to nonspecific sites is accompanied by the net uptake of 56 +/- 10 water molecules. Taken with the water stoichiometry of sequence-specific binding, this value implies that formation of a nonspecific complex is accompanied by the net release of 2-44 water molecules. The enhanced stabilities of CAP-DNA complexes with increased osmolality (decreased water activity) may contribute to the ability of E.coli cells to tolerate dehydration and/or high external salt concentrations.

Mapping conformational changes in a protein: application of a protein footprinting technique to cAMP-induced conformational changes in cAMP receptor protein

We have used protein footprinting [Heyduk, E., & Heyduk, T. (1994) Biochemistry 33, 9643] to detect and map ligand-induced conformational changes in cAMP receptor protein (CRP). The binding of cAMP to CRP dramatically increases the specific DNA binding activity of the protein and, as has been previously shown, induces conformational changes in the protein. Protein footprinting experiments with the free CRP, the CRP-cAMP complex, and the CRP-cGMP complex were analyzed quantitatively. Binding of cAMP produced measurable differences in the susceptibility of CRP to the cleavage by Fe-EDTA. Almost all of these changes occurred in the C-terminal domain (DNA binding domain) of the protein. Additional changes were observed at the ends of the C alpha-helix, which is involved in intersubunit contacts in the CRP dimer, and in the hinge peptide, connecting N-terminal and C-terminal domains of the protein. The boundaries of the regions in the C-terminal domain, which exhibited changes in susceptibility to Fe-EDTA cleavage, almost exactly corresponded to D, E, and F alpha-helices which are involved directly in the recognition of DNA. The F alpha-helix, which provides all base-specific contacts in the CRP-DNA complex, became hypersensitive to Fe-EDTA-mediated cleavage, whereas the solvent exposure of D and E alpha-helices was decreased upon binding of cAMP. These results suggest that a significant part of cAMP-induced conformational change in CRP involves a movement of secondary structure elements in the C-terminal domain of the protein so that the recognition F alpha-helix becomes exposed to the solvent. In contrast to cAMP, binding of cGMP produced insignificant changes in susceptibility to Fe-EDTA-mediated cleavage. This is consistent with the inability of cGMP to induce functional conformational changes in CRP. The protein footprinting technique appears to be sufficiently sensitive for detection and mapping of ligand-induced conformational changes in proteins.

Autonomous and nonautonomous regulation of axis formation by antagonistic signaling via 7-span cAMP receptors and GSK3 in Dictyostelium

Early during Dictyostelium development a fundamental cell-fate decision establishes the anteroposterior (prestalk/prespore) axis. Signaling via the 7-transmembrane cAMP receptor CAR4 is essential for creating and maintaining a normal pattern; car4-null alleles have decreased levels of prestalk-specific mRNAs but enhanced expression of prespore genes. car4- cells produce all of the signals required for prestalk differentiation but lack an extracellular factor necessary for prespore differentiation of wild-type cells. This secreted factor decreases the sensitivity of prespore cells to inhibition by the prestalk morphogen DIF-1. At the cell autonomous level, CAR4 is linked to intracellular circuits that activate prestalk but inhibit prespore differentiation. The autonomous action of CAR4 is antagonistic to the positive intracellular signals mediated by another cAMP receptor, CAR1 and/or CAR3. Additional data indicate that these CAR-mediated pathways converge at the serine/threonine protein kinase GSK3, suggesting that the anterior (prestalk)/posterior (prespore) axis of Dictyostelium is regulated by an ancient mechanism that is shared by the Wnt/Fz circuits for dorsoventral patterning during early Xenopus development and establishing Drosophila segment polarity.

Phosphorylation of the G protein alpha-subunit, G alpha 2, of Dictyostelium discoideum requires a functional and activated G alpha 2

The G alpha 2-subunit of Dictyostelium discoideum is essential to the initial stage of the cell's developmental life cycle. In response to the extracellular chemoattractant, cAMP, G alpha 2 is activated and transiently phosphorylated on serine-113 [Chen et al. (1994): J Biol Chem 269:20925-20930]. The role of G alpha 2 phosphorylation remains elusive; cells expressing the S113A, nonphosphorylated mutation of G alpha 2 appear to proceed through the developmental phase normally. To gain insight into the function of G alpha 2 phosphorylation, the conditions for G alpha 2 phosphorylation were examined using a variety of alpha-subunit point mutations and chimeras. Mutations that block the G protein activation cycle prior to or at the hydrolysis of GTP (G alpha 2-S45A, G alpha 2-G207A, and G alpha 2-Q208L) preclude G alpha 2 phosphorylation in vivo. Phosphorylation of the G alpha 2-Q208L mutation does however occur in an in vitro phosphorylation assay. It appears that G alpha 2 phosphorylation, shown previously in vivo to require the cAMP receptor, also requires signaling through the G2 pathway. Results from the in vitro assay suggest that the substrate for phosphorylation is the alpha-subunit monomer.

Maximization of transcription of the serC (pdxF)-aroA multifunctional operon by antagonistic effects of the cyclic AMP (cAMP) receptor protein-cAMP complex and Lrp global regulators of Escherichia coli K-12

The arrangement of the Escherichia coli serC (pdxF) and aroA genes into a cotranscribed multifunctional operon allows coregulation of two enzymes required for the biosynthesis of L-serine, pyridoxal 5'-phosphate, chorismate, and the aromatic amino acids and vitamins. RNase T2 protection assays revealed two major transcripts that were initiated from a promoter upstream from serC (pdxF). Between 80 to 90% of serC (pdxF) transcripts were present in single-gene mRNA molecules that likely arose by Rho-independent termination between serC (pdxF) and aroA. serC (pdxF)-aroA cotranscripts terminated at another Rho-independent terminator near the end of aroA. We studied operon regulation by determining differential rates of beta-galactosidase synthesis in a merodiploid strain carrying a single-copy lambda[phi(serC [pdxF]'-lacZYA)] operon fusion. serC (pdxF) transcription was greatest in bacteria growing in minimal salts-glucose medium (MMGlu) and was reduced in minimal salts-glycerol medium, enriched MMGlu, and LB medium. serC (pdxF) transcription was increased in cya or crp mutants compared to their cya+ crp+ parent in MMGlu or LB medium. In contrast, serC (pdxF) transcription decreased in an lrp mutant compared to its lrp+ parent in MMGlu. Conclusions obtained by using the operon fusion were corroborated by quantitative Western immunoblotting of SerC (PdxF), which was present at around 1,800 dimers per cell in bacteria growing in MMGlu. RNase T2 protection assays of serC (pdxF)-terminated and serC (pdxF)-aroA cotranscript amounts supported the conclusion that the operon was regulated at the transcription level under the conditions tested. Results with a series of deletions upstream of the P(serC (pdxF)) promoter revealed that activation by Lrp was likely direct, whereas repression by the cyclic AMP (cAMP) receptor protein-cAMP complex (CRP-cAMP) was likely indirect, possibly via a repressor whose amount or activity was stimulated by CRP-cAMP.

Identification of detergent-resistant plasma membrane microdomains in dictyostelium: enrichment of signal transduction proteins

Unlike most other cellular proteins, the chemoattractant receptor, cAR1, of Dictyostelium is resistant to extraction by the zwitterionic detergent, CHAPS. We exploited this property to isolate a subcellular fraction highly enriched in cAR1 by flotation of CHAPS lysates of cells in sucrose density gradients. Immunogold electron microscopy studies revealed a homogeneous preparation of membrane bilayer sheets. This preparation, designated CHAPS-insoluble floating fraction (CHIEF), also contained a defined set of 20 other proteins and a single uncharged lipid. Cell surface biotinylation and preembedding immunoelectron microscopy both confirmed the plasma membrane origin of this preparation. The cell surface phosphodiesterase (PDE) and a downstream effector of cAR1, adenylate cyclase (ACA), were specifically localized in these structures, whereas the cell adhesion molecule gp80, most of the major cell surface membrane proteins, cytoskeletal components, the actin-binding integral membrane protein ponticulin, and G-protein alpha- and beta-subunits were absent. Overall, CHIFF represents about 3-5% of cell externally exposed membrane proteins. All of these results indicate that CHIFF is derived from specialized microdomains of the plasma membrane. The method of isolation is analogous to that of caveolae. However, we were unable to detect distinct caveolae-like structures on the cell surface associated with cAR1, which showed a diffuse staining profile. The discovery of CHIFF facilitates the purification of cAR1 and related signaling proteins and the biochemical characterization of receptor-mediated processes such as G-protein activation and desensitization. It also has important implications for the "fluid mosaic" model of the plasma membrane structures.

Catabolite repression by glucose 6-phosphate, gluconate and lactose in Escherichia coli

While catabolite repression by glucose has been studied extensively and is understood in large detail in Enterobacteriaceae, catabolite repression by carbohydrates that are not transported by the phosphotransferase system (PTS) has always remained an enigma. Examples of non-PTS carbohydrates that cause catabolite repression in Escherichia coli are gluconate, lactose and glucose 6-phosphate. In this article it is shown that enzyme IIA(Glc) of the PTS is not involved in catabolite repression by these carbon sources. Carbon sources that caused strong catabolite repression of beta-galactosidase lowered the concentration of both cAMP and the cAMP receptor protein (CRP). A strong correlation was found between the amounts of cAMP and CRP and the strength of the repression. The levels of cAMP and CRP were modulated in various ways. Neither overproduction of CRP nor an increased cAMP concentration could completely relieve the repression by glucose 6-phosphate, lactose and gluconate. Simultaneously increasing the cAMP and the CRP levels was lethal for the cells. In a mutant expressing a constant amount of cAMP-independent CRP* protein, catabolite repression was absent. The same was found in a mutant in which lac transcription is independent of cAMP/CRP. These results, combined with the fact that both the cAMP and the CRP levels are lowered by glucose 6-phosphate, lactose and gluconate, lead to the conclusion that the decreased cAMP and CRP levels are the cause of catabolite repression by these non-PTS carbon sources.

Stability and kinetics of unfolding and refolding of cAMP receptor protein from Escherichia coli

cAMP receptor protein (CRP) is involved in regulation of expression of several genes in Escherichia coli. The protein is a homodimer and each monomer is folded into two distinct structural domains. The mechanism of the biological activity of the protein may involve the interaction between the subunits and domains. In order to determine the interaction between the subunits or domains of CRP, we have studied the reversible denaturation of the protein by guanidine hydrochloride. The unfolding and refolding kinetics of CRP was monitored using stopped-flow fluorescence spectroscopy at 20 degrees C and pH 7.9. The results of CRP denaturation indicate that the transition can be described by a three-state model: (CRP native)2<=> 2 (CRP native)<=>2 (CRP denatured). The faster process, characterized by the relaxation time tau 2 = 80 +/- 3 ms, corresponds to the dissociation of CRP dimer into monomers. The slower process has the relaxation time tau t = 1.9 +/- 0.1 s and corresponds to the cooperative unfolding of CRP monomer. The free energy change in the absence of denaturant upon CRP dissociation is delta G dis degrees = 46.9 +/- 2.5 kJ/mol and for monomer unfolding delta G unf degrees = 30.9 +/- 1.3 kJ/mol. The thermal unfolding of CRP was studied by circular dichroism and fluorescence spectroscopy at various guanidine hydrochloride concentrations. It has been found that the native protein is maximally stable at about 21 +/- 0.3 degrees C and is denatured upon heating and cooling from this temperature. The apparent free energy change for CRP unfolding at 21 degrees C is equal to 30.5 +/- 0.4 kJ/mol and the apparent specific heat change is equal to delta Cp, app = 10.7 +/- 0.7 kJ mol-1 K-1. The predicted values of cold denaturation midpoint is equal to tau G = -18.8 +/- 1.5 degrees C and for high-temperature transition tau G = 63.1 +/- 1.5 degrees C. The predicted midpoint of high-temperature unfolding transition is about the same as determined experimentally.

Influence of the location of the cAMP receptor protein binding site on the geometry of a transcriptional activation complex in Escherichia coli

The interactions between the cAMP receptor protein (CRP) and RNA polymerase during transcriptional activation at the Escherichia coli malT promoter have been analyzed using a combination of footprinting methods. We show that a closed complex is formed at this promoter in the absence of activator and that CRP merely stabilizes the open complex. The alpha-subunits of the RNA polymerase are involved in this effect as shown by KMnO4 footprinting. The open complex formed in the presence of CRP is structurally identical to the one found at a CRP-independent promoter up-mutant. UV-laser footprinting yields distinct signals for the different protein-DNA interactions within the complex and for interactions between CRP and RNA polymerase. We monitor these signals in promoter variants that place the CRP binding site at different distances upstream of the start site of transcription. Signals within the core promoter region, as well as those located just upstream of the -35 hexamer, are unaffected by the position of the CRP binding site. Contacts of RNA polymerase with the upstream promoter region change in a mutant RNA polymerase containing a truncated alpha-subunit. We conclude that at least one of the alpha-subunits of RNA polymerase binds to DNA upstream of the -35 hexamer and that this interaction is unaffected by the position of the CRP binding site. We discuss models that account for the different activities of CRP in transcriptional activation as a function of promoter geometry.

A cAMP response element and an Ets motif are involved in the transcriptional regulation of flt-1 tyrosine kinase (vascular endothelial growth factor receptor 1) gene

The flt-1 gene encodes a transmembrane tyrosine kinase, Flt-1, a receptor for vascular endothelial growth factor. The expression of flt-1 gene is restricted to endothelial cells in vivo. To understand the molecular mechanism underlying endothelial-specific expression of this gene, we studied the functional significance of transcriptional motifs in the 200-base pair region of the human flt-1 gene promoter, which has been identified to confer cell type specificity. By point mutation analysis using chloramphenicol acetyltransferase plasmids in 293E1 cells, which express significant levels of flt-1 mRNA, we found that an Ets motif, E4, at -54 to -51 and a cAMP response element (CRE) at -83 to -76 are involved in the transcriptional regulation of this gene. Disruption of either this CRE or E4 within the promoter sequence of 90 base pairs resulted in a decrease in chloramphenicol acetyltransferase activity of 90%, indicating that co-existence of both of CRE and Ets motif E4 is necessary for transcription of the flt-1 gene. Co-transfection of an expression vector containing c-ets-1, c-ets-2, or c-erg cDNA with this 90-base pair sequence yielded a 5-8-fold elevation of chloramphenicol acetyltransferase activity, further supporting the idea that Ets family protein(s) participates in the regulation of the flt-1 gene. Gel shift assays using nuclear extracts of 293E1 and endothelial cells demonstrated the existence of protein factor(s) that specifically binds to CRE and Ets motif E4, respectively. Taken together, our results strongly suggest cooperation of a CRE and an Ets motif for the function of the flt-1 gene promoter.