A variety of receptors coupled to GTP-binding regulatory proteins (G proteins) initiate signals that culminate in activation of the mitogen-activated protein kinases ERK1 and ERK2. We demonstrate here that the human 5-HT1A receptor expressed in Chinese hamster ovary cells similarly promotes activation of ERK1 and ERK2, but that the pathway used does not conform entirely to those proposed previously for G protein-coupled receptors. Activation of ERK2 by the 5-HT1A receptor-selective agonist 8-hydroxy-N,N-dipropyl-2-aminotetralin hydrobromide (8-OH-DPAT) was inhibited completely by pertussis toxin and substantially by prolonged treatment of cells with phorbol 12-myristate 13-acetate. The implied requirement for protein kinase C, however, was negated in studies with bisindolylmaleimide and Ro-31-8220, which, although completely inhibiting activation of ERK2 by phorbol ester, had no impact on activation by 8-OH-DPAT. The anticipated inhibition by the tyrosine kinase inhibitors genistein and herbimycin A, moreover, was marginal at best. As expected for a Gi-coupled receptor, the inhibitors of phosphatidylinositol 3-kinase wortmannin and LY294002 inhibited activation of ERK2, albeit only partly (70%). Of significance, an inhibitor of a phosphatidylcholine-specific phospholipase C, tricyclodecan-9-yl-xanthogenate (D609), caused a similar degree of inhibition. When the two types of inhibitors were combined, an almost complete inhibition was achieved. Our data suggest that phosphatidylinositol 3-kinase and phosphatidylcholine-specific phospholipase C represent components of different, but partly overlapping pathways that can account almost entirely for the activation of ERK2 by the 5-HT1A receptor.

The roles of 3¢,5¢-cyclic adenosine monophosphate (cAMP) and protein kinase A in 5-hydroxytryptamine (5-HT) 7 receptormediated activation of extracellular-regulated kinase (ERK) were studied in cultured hippocampal neurons and transfected PC12 cells. Activation of ERK by neuronal G s -coupled receptors has been thought to proceed through a protein kinase A-dependent pathway. In fact we identified coupling of 5-HT 7 receptors to activation of adenylyl cyclase and protein kinase A. However, no inhibition of agonist-stimulated ERK activation was found when cells were treated with H-89 and KT5720 at concentrations sufficient to completely inhibit activation of protein kinase A. However, activation of ERK was found to be sensitive to the adenylyl cyclase inhibitor 9-(tetrahydrofuryl)-adenine, suggesting a possible role for a cAMP-guanine nucleotide exchange factor (cAMP-GEF). Co-treatment of cells with 8-(4-chlorophenylthio)-2¢-O-methyladenosine 3¢,5¢-cyclic monophosphate, a direct activator of the cAMP-GEFs Epac1 and 2, reversed the inhibition of agonist-stimulated ERK activation induced by adenylyl cyclase inhibition. Additionally, over-expression of Epac1 enhanced 5-HT 7 receptor-mediated activation of ERK. These results demonstrate that the activation of ERK mediated by neuronal G s -coupled receptors can proceed through cAMP-dependent pathways that utilize cAMP-GEFs rather than protein kinase A.

We have examined the ability of extracellular ATP to elicit intracellular Ca2" mobilization in a broad range of human leukocytes at particular stages of hematopoietic differentiation.

SummarySalmonella has developed ways to modulate host cellular response in order to survive. Although the steps required for such modulation have been incompletely characterized, there is increasing evidence for a role for SptP, a type III secretion protein. In part, the actions of SptP are thought to be mediated through its reported inhibition of the extracellular-regulated kinase (ERK) MAP kinase pathway. In the present studies, a series of transfections were performed in which various constitutively activated components of the MAP kinase pathway were co-transfected with SptP in order to determine the mechanism by which SptP inhibits this MAP kinase activation. SptP was found to inhibit the activation of ERK stimulated by both a constitutively active form of Ras and a partially activated form of Raf-1 containing a phospho-mimetic mutation (Raf Y340D). In contrast, the activation of ERK by constitutively active forms of MAP kinase kinase (MEK) was not inhibited, suggesting that the actions of SptP were mediated by Raf-1. In order to determine how SptP might interfere with activation of Raf, we utilized a membrane-localized form of Raf. Constitutive membrane-localization of Raf (Raf-CAAX), resulting in partial activation, did not prevent inhibition by SptP. However, introduction of an additional, partially activating (Y340D) phospho-mimetic mutation, to RafCAAX, dramatically reduced the ability of SptP to inhibit Raf action. Comparison of SptP mutants, lacking either GTPase-activating protein (GAP) activity or tyrosine phosphatase activity, further suggested that SptP inhibits both the membrane localization and subsequent phosphorylation required for activation of Raf. Both tyrosine phosphatase activity and GAP activity were responsible for SptP inhibition of Raf(Y340D)-induced ERK activation, but only GAP activity was responsible for inhibition of the membrane localized forms of Raf-1. To assess the biological significance of SptP, we examined tumour necrosis factor (TNF)-a a a a induction following Salmonella infection. SptP gene deletion enhanced the capacity of Salmonella to induce TNF-a a a a secretion following infection of J774A.1 macrophage cells.

5-HT 1A receptors have been hypothesized to mediate some of the neuronal plasticity and behavioral responses stimulated by serotonin selective reuptake inhibitors. Although the cellular signaling pathways required for inducing these actions have not yet been determined, roles for the neuroprotective extracellular-regulated kinase (ERK) mitogenactivated protein (MAP) kinase and Akt pathways have been suggested. In the current studies we have utilized primary cultures to directly determine whether hippocampal 5-HT 1A receptors couple to activation of Akt and ERK. We found that E18 hippocampal neurons exhibit a twofold activation of Akt when exposed to nanomolar concentrations of 5-HT. The 5-HT 1/7 receptor-selective agonist 5-carboxamidotryptamine maleate (5-CT) and the 5-HT 1A/7 receptor-selective agonist 8-hydroxy-N,N-dipropyl-aminotetralin (8-OH-DPAT) maleate were found to activate Akt with equal efficacy, and similar potency, to 5-HT. p-MPPI and WAY-100635, antagonists selective for 5-HT 1A receptors, completely inhibited 5-CT-stimulated Akt activation. Activation of Akt was also inhibited by pretreatment with pertussis toxin as well as the phosphatidylinositol 3-kinase inhibitors, wortmannin and LY294002. In contrast, the 5-HT selective antagonist, SB269970, caused no inhibition. Although the density of 5-HT 1A receptors expressed by cultured neurons was sufficient to activate Akt, no activation of ERK was observed. These findings suggest that Akt, and not ERK, may be relevant to previous reports of hippocampal 5-HT 1A receptors mediating neurotrophic responses.

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Brain-derived neurotrophic factor (BDNF) and insulin-like growth factor-1 (IGF-1) seem to play key roles in mediating neuronal plasticity in the hippocampus. In the current studies, we have used cultured hippocampal neurons to study possible interactions between the two growth factors in modulating neuronal signaling pathways. BDNF and IGF-1 were found to each effectively activate the neuroprotective Akt pathway, with the magnitude of activation being at least additive when cultures were simultaneously treated with supramaximal concentrations of peptides. Likewise, a cumulative inhibitory Akt-dependent phosphorylation of proapoptotic glycogen synthase kinase-3 was observed. Immunofluorescent studies demonstrated that a single population of neurons responded to BDNF and IGF-1. In contrast, the magnitude of BDNF-stimulated extracellular signal-regulated kinase (ERK) activation was found to be much greater than that of IGF-1-stimulated ERK, such that the difference in magnitude stimulated by BDNF in the presence and absence of IGF-1 did not reach statistical significance. Consistent with the observed agonist-stimulated activation of Akt, BDNF and IGF-1 were both found to act as neurotrophins, enhancing neuronal survival under low-insulin culture conditions. Maximal survival was achieved when both growth factors were present. These findings provide insight into the significance of multiple growth factors stimulating activation of ERK and Akt in the central nervous system. In some cases, the magnitude of activation required to elicit biological responses may be achieved only with a combination of compounds.