A study of the H2-receptor for histamine stimulating adenylate cyclase in homogenates of guinea-pig lung parenchyma

Role of A-Kinase Anchoring Protein 12 in the Central Nervous System

A-kinase anchoring protein (AKAP) 12 is a scaffolding protein that anchors various signaling proteins to the plasma membrane. These signaling proteins include protein kinase A, protein kinase C, protein phosphatase 2B, Src-family kinases, cyclins, and calmodulin, which regulate their respective signaling pathways. AKAP12 expression is observed in the neurons, astrocytes, endothelial cells, pericytes, and oligodendrocytes of the central nervous system (CNS). Its physiological roles include promoting the development of the blood-brain barrier, maintaining white-matter homeostasis, and even regulating complex cognitive functions such as long-term memory formation. Under pathological conditions, dysregulation of AKAP12 expression levels may be involved in the pathology of neurological diseases such as ischemic brain injury and Alzheimer's disease. This minireview aimed to summarize the current literature on the role of AKAP12 in the CNS.

Real-time analysis of the inside-out regulation of lymphocyte function-associated antigen-1 revealed similarities to and differences from very late antigen-4

Ten years ago, we introduced a fluorescent probe that shed light on the inside-out regulation of one of the major leukocyte integrins, very late antigen-4 (VLA-4, CD49d/CD29). Here we describe the regulation of another leukocyte integrin, lymphocyte function-associated antigen-1 (LFA-1, CD11a/CD18) using a novel small fluorescent probe in real time on live cells. We found that multiple signaling mechanisms regulate LFA-1 conformation in a manner analogous to VLA-4. LFA-1 can be rapidly activated by Gα_i-coupled G protein-coupled receptors (GPCRs) and deactivated by Gα_s-coupled GPCRs. The effects of Gα_s-coupled GPCR agonists can be reversed in real time by receptor-specific antagonists. The specificity of the fluorescent probe binding has been assessed in a competition assay using the natural LFA-1 ligand ICAM-1 and the LFA-1-specific α I allosteric antagonist BIRT0377. Similar to VLA-4 integrin, modulation of the ligand dissociation rate can be observed for different LFA-1 affinity states. However, we also found a striking difference in the binding of the small fluorescent ligand. In the absence of inside-out activation ligand, binding to LFA-1 is extremely slow, at least 10 times slower than expected for diffusion-limited binding. This implies that an additional structural mechanism prevents ligand binding to inactive LFA-1. We propose that such a mechanism explains the inability of LFA-1 to support cell rolling, where the absence of its rapid engagement by a counterstructure in the inactive state leads to a requirement for a selectin-mediated rolling step.

Galphas-coupled receptor signaling actively down-regulates alpha4beta1-integrin affinity: a possible mechanism for cell de-adhesion

BACKGROUND

Activation of integrins in response to inside-out signaling serves as a basis for leukocyte arrest on endothelium, and migration of immune cells. Integrin-dependent adhesion is controlled by the conformational state of the molecule (i.e. change in the affinity for the ligand and molecular unbending (extension)), which is regulated by seven-transmembrane Guanine nucleotide binding Protein-Coupled Receptors (GPCRs). alpha4beta1-integrin (CD49d/CD29, Very Late Antigen-4, VLA-4) is expressed on leukocytes, hematopoietic stem cells, hematopoietic cancer cells, and others. Affinity and extension of VLA-4 are both rapidly up-regulated by inside-out signaling through several Galphai-coupled GPCRs. The goal of the current report was to study the effect of Galphas-coupled GPCRs upon integrin activation.

RESULTS

Using real-time fluorescent ligand binding to assess affinity and a FRET based assay to probe alpha4beta1-integrin unbending, we show that two Galphas-coupled GPCRs (H2-histamine receptor and beta2-adrenergic receptor) as well as several cAMP agonists can rapidly down modulate the affinity of VLA-4 activated through two Galphai-coupled receptors (CXCR4 and FPR) in U937 cells and primary human peripheral blood monocytes. This down-modulation can be blocked by receptor-specific antagonists. The Galphas-induced responses were not associated with changes in the expression level of the Galphai-coupled receptors. In contrast, the molecular unbending of VLA-4 was not significantly affected by Galphas-coupled GPCR signaling. In a VLA-4/VCAM-1-specific myeloid cell adhesion system, inhibition of the VLA-4 affinity change by Galphas-coupled GPCR had a statistically significant effect upon cell aggregation.

CONCLUSION

We conclude that Galphas-coupled GPCRs can rapidly down modulate the affinity state of VLA-4 binding pocket through a cAMP dependent pathway. This plays an essential role in the regulation of cell adhesion. We discuss several possible implications of this described phenomenon.

Efficacy of a selective histamine H2 receptor agonist, dimaprit, in experimental models of endotoxin shock and hepatitis in mice

Dimaprit, a selective histamine H2 receptor agonist, was examined in experimental models of endotoxin shock and hepatitis in mice. Injection of lipopolysaccharide (8 mg/kg i.v.) into Balb/c mice resulted in an elevation of plasma tumor necrosis factor-alpha (TNF-alpha), reaching the maximal level at 1 h post-lipopolysaccharide (1147 U/ml). Oral administration of dimaprit 200 mg/kg, 1 h prior to lipopolysaccharide challenge, inhibited the increase in plasma TNF-alpha by 71% and also the survival rate was increased to 62.5% from 8.3% in the disease control. In a mouse hepatitis model, simultaneous injection of galactosamine (700 mg/kg i.v.) and lipopolysaccharide (3 micrograms/kg i.v.) into Balb/c mice caused an increase in plasma TNF-alpha, peaking at 1 h, followed by an elevation of L-alanine aminotransferase (E.C.2.6.1.2) activity at 4 h onward. Oral administration of dimaprit 200 mg/kg, 1 h prior to galactosamine and lipopolysaccharide, reduced the increase in plasma TNF-alpha by 99% and L-alanine aminotransferase by 82%. In vitro, dimaprit dose dependently inhibited the production of TNF-alpha in mouse peritoneal macrophages and human peripheral blood monocytes stimulated with lipopolysaccharide with IC50 values of 1 microM. The decrease in TNF-alpha production by dimaprit was reversed by cimetidine, a histamine H2 receptor antagonist. Dimaprit dose dependently suppressed TNF-alpha mRNA in human peripheral blood monocytes. These results suggest that activation of the histamine H2 receptor downregulates the production of TNF-alpha, and that histamine may be an important regulator in pathological conditions in which TNF-alpha plays an important role.

Molecular pharmacological aspects of histamine receptors

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

GABAB receptors and their significance in mammalian pharmacology

Much progress has been made in GABAB receptor pharmacology since the discovery of this receptor in 1980. Selective agonists and antagonists have been developed and a functional role for the receptor as a mediator of slow inhibitory postsynaptic potentials in many brain regions has emerged. In this article, Norman Bowery discusses the evidence for heterogeneity of GABAB receptors, their possible physiological and pathological roles and the therapeutic potential of GABAB receptor agonists and antagonists.

Role of phosphoinositide metabolism in functional antagonism of airway smooth muscle contraction by beta-adrenoceptor agonists

Histamine and the muscarinic agonists, methacholine, oxotremorine, and McN-A-343, were used to contract guinea-pig tracheal smooth muscle preparations. Cumulative dose-relaxation curves with isoprenaline were performed subsequently. In addition, the concentration-dependent induction of phosphoinositide metabolism by the contractile agonists was measured in bovine tracheal smooth muscle. All agonists were found to induce a decrease of the apparent affinity of isoprenaline and a loss of relaxation, depending on the concentration and type of contractile agonist used. The differential effects of the contractile agonists, especially at higher and supramaximal concentrations, on these beta-adrenergic parameters could be explained by differences in phosphoinositide metabolism.

The lack of involvement of cyclic nucleotides in the smooth muscle relaxant action of BRL 34915

1. The relaxant effect of BRL 34915 has been examined on four isolated preparations, the bovine retractor penis (BRP), guinea-pig taenia coli, guinea-pig trachea and rabbit aortic strip contracted by either histamine, carbachol, noradrenaline or 10 mM KCl. Even though the probability of the involvement of external calcium entering through voltage-operated channels in these tissues varied, there was little corresponding variation in sensitivity to BRL 34915. 2. The relaxant effect of BRL 34915 on the BRP and guinea-pig taenia coli was unaffected by haemoglobin 3.3 microM or Apamin 0.5 microM, concentrations which blocked completely the relaxant effect of non-adrenergic, non-cholinergic (NANC) nerve stimulation in these tissues. 3. BRL 34915 in doses causing maximum relaxation did not increase the levels of either cyclic AMP or cyclic GMP in the BRP, although the appropriate enzymes were present and could be stimulated by forskolin or sodium nitroprusside. 4. In the BRP isoprenaline 30 microM acting through beta-receptors caused maximum relaxation but did not raise the levels of cyclic AMP, even though lower doses of 2 microM did raise the levels of this nucleotide in the rabbit uterus. 5. These results provide some indirect evidence that membrane hyperpolarization may not be the only cause of the smooth muscle relaxation induced by BRL 34915. However, neither a rise in cyclic AMP nor cyclic GMP are satisfactory alternative mechanisms.

Modulation of guinea-pig lung adenylate cyclase by ovalbumin sensitization

1. The regulation of lung adenylate cyclase was investigated in guinea pigs sensitized with high dose (300-500 micrograms kg-1) ovalbumin to raise IgG(I) and low dose (2.8-4.0 micrograms kg-1) to raise to IgG/IgE (II) antibodies. 2. Basal activity of sensitized II (IgG/IgE antibodies) lung adenylate cyclase was approximately double that of the control values. 3. Guanosine 5'-triphosphate (GTP) was a potent activator of adenylate cyclase from the sensitized II (IgG/IgE) lung membranes. 4. The sensitivity of lung membrane adenylate cyclase to stimulation by beta-adrenoceptor agonists or VIP was reduced whereas no significant difference was observed with histamine or bradykinin on the sensitized II membranes. 5. Possible mechanisms involved in the increase in basal activity and in the decreased sensitivity to beta-adrenoceptor- and VIP-mediated stimulation of adenylate cyclase by ovalbumin sensitization of guinea-pig lung membranes are discussed.

Intracellular effectors and modulators of GABA-A and GABA-B receptors: a commentary

The inhibitory neurotransmitter GABA activates two receptor subtypes that can be distinguished by their pharmacology. The GABA-A site is competitively antagonized by bicuculline and exclusively coupled to a chloride channel. The GABA-B receptor, for which baclofen is the only specific agonist, is resistant to bicuculline inhibition and, depending upon its localization, will activate K currents and/or inhibit Ca currents. Both electrophysiological and biochemical approaches have been applied to the study of each receptor. The membrane and intracellular components that to date have been implicated in GABA-B activation are discussed: G proteins, adenylate cyclase and intracellular calcium levels. This latter factor is also discussed with respect to GABA-A receptor action.