BIOCHEMICAL PROPERTIES OF GABA (B) RECEPTORS (BACLOFEN, ADENYLATE CYCLASE, CYCLIC-AMP, PHOSPHOLIPASE, PROTEIN KINASE C)

(gamma)-Aminobutyric acid (GABA), a neurotransmitter in the mammalian central nervous system, influences neuronal activity by interacting with at least two pharmacologically and functionally distinct receptors. GABA(,A) receptors are sensitive to blockade by bicuculline, are associated with benzodiazepine and barbiturate binding sites, and mediate chloride flux. The biochemical and pharmacolocal properties of GABA(,B) receptors, which are stereoselectively activated by (beta)-p-chlorophenyl GABA (baclofen), are less well understood. The aim of this study was to define these features of GABA(,B) receptors, with particular emphasis on their possible relationship to the adenylate cyclase system in brain.^ By themselves, GABA agonists have no effect on cAMP accumulation in rat brain slices. However, some GABA agonists markedly enhance the cAMP accumulation that results from exposure to norepinephrine, adenosine, VIP, and cholera toxin. Evidence that this response is mediated by the GABA(,B) system is provided by the finding that it is bicuculline-insensitive, and by the fact that only those agents that interact with GABA(,B) binding sites are active in this regard. GABA(,B) agonists are able to enhance neurotransmitter-stimulated cAMP accumulation in only certain brain regions, and the response is not influenced by phosphodiesterase inhibitors, although is totally dependent on the availability of extracellular calcium. Furthermore, data suggest that inhibition of phospholipase A(,2), a calcium-dependent enzyme, decreases the augmenting response to baclofen, although inhibitors of arachidonic acid metabolism are without effect. These findings indicate that either arachidonic acid or lysophospholipid, products of PLA(,2)-mediated degradation of phospholipids, mediates the augmentation. Moreover, phorbol esters, compounds which directly activate protein kinase C, were also found to enhance neurotransmitter-stimulated cAMP accumulation in rat brain slices. Since this enzyme is known to be stimulated by unsaturated fatty acids such as arachidonate, it is proposed that GABA(,B) agonists enhance cAMP accumulation by fostering the production of arachidonic acid which stimulates protein kinase C, leading to the phosphorylation of some component of the adenylate cyclase system. Thus, GABA, through an interaction with GABA(,B) receptors, modulates neurotransmitter receptor responsiveness in brain. The pharmocological manipulation of this response could lead to the development of therapeutic agents having a more subtle influence than current drugs on central nervous system function. ^

Sheep macrophages express at least two distinct receptors for IgG which have similar affinity for homologous IgG1 and IgG2.

Ovine bone marrow-derived macrophages (BMM) may express several IgG receptor (Fc gamma receptor; FcR) subsets. To study this, model particles (opsonized erythrocytes; EA), which are selectively handled by certain FcR subsets of human macrophages were used in cross-inhibition studies and found to react in a similar manner with FcR subsets of sheep macrophages. In experiments with monoclonal antibodies against subsets of human FcR, human erythrocytes (E) treated with human anti-D-IgG (anti-D-EAhu) and sheep E treated with bovine IgG1 (Bo1-EAs) were handled selectively by human macrophage FcRI and FcRII, respectively. Rabbit-IgG-coated sheep E (Rb-EAs) were recognized by FcRI, FcRII and possibly also by FcRIII of human macrophages. Anti-D-EAhu, Bo1-EAs and Rb-EAs were also ingested by sheep BMM. Competitive inhibition tests, using various homologous and heterologous IgG isotypes as fluid phase inhibitors and the particles used as FcR-specific tools in man (anti-D-EAhu and Bo1-EAs), revealed a heterogeneity of FcR also in sheep BMM. Thus, ingestion of anti-D-EAhu by ovine BMM was inhibited by low concentrations of competitor IgG from rabbit or man in the fluid phase, but not at all by bovine IgG1, whereas ingestion of Bo1-EAs was inhibited by bovine IgG1. This suggested that anti-D-EAhu were recognized by a FcR subset distinct from that recognizing bovine-IgG1. It was concluded that sheep BMM express functional analogs of human macrophage FcRI and FcRII and that Bo1-EAs and anti-D-EAhu are handled by distinct subsets of BMM FcR. All EAhu tested (EAhu treated with anti-D, sheep IgG1 or sheep IgG2) were ingested to a lower degree than EAs. This inefficient phagocytosis could be enhanced by treatment of EAhu with antiglobulin from the rabbit, suggesting that it is caused by a low degree of activity of opsonizing antibodies rather than special properties of the erythrocytes themselves. Several lines of evidence suggested that both FcR subsets of ovine BMM recognize both ovine IgG1 and IgG2. In contrast, bovine IgG1 reacts with one FcR subset and bovine IgG2 interacts inefficiently with all FcR of ovine BMM.

Interactions between Siglec-7/9 receptors and ligands influence NK cell-dependent tumor immunosurveillance

Abstract Alteration of the surface glycosylation pattern on malignant cells potentially affects tumor immunity by directly influencing interactions with glycan-binding proteins (lectins) on the surface of immunomodulatory cells. The sialic acid-binding Ig-like lectins Siglec-7 and -9 are MHC class I-independent inhibitory receptors on human NK cells that recognize sialic acid-containing carbohydrates. Here, we found that the presence of Siglec-9 defined a subset of cytotoxic NK cells with a mature phenotype and enhanced chemotactic potential. Interestingly, this Siglec-9+ NK cell population was reduced in the peripheral blood of cancer patients. Broad analysis of primary tumor samples revealed that ligands of Siglec-7 and -9 were expressed on human cancer cells of different histological types. Expression of Siglec-7 and -9 ligands was associated with susceptibility of NK cell-sensitive tumor cells and, unexpectedly, of presumably NK cell-resistant tumor cells to NK cell-mediated cytotoxicity. Together, these observations have direct implications for NK cell-based therapies and highlight the requirement to consider both MHC class I haplotype and tumor-specific glycosylation.