Guanyl cyclase activity of human blood lymphocytes

Targeting Phosphodiesterases-Towards a Tailor-Made Approach in Multiple Sclerosis Treatment

Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system (CNS) characterized by heterogeneous clinical symptoms including gradual muscle weakness, fatigue, and cognitive impairment. The disease course of MS can be classified into a relapsing-remitting (RR) phase defined by periods of neurological disabilities, and a progressive phase where neurological decline is persistent. Pathologically, MS is defined by a destructive immunological and neuro-degenerative interplay. Current treatments largely target the inflammatory processes and slow disease progression at best. Therefore, there is an urgent need to develop next-generation therapeutic strategies that target both neuroinflammatory and degenerative processes. It has been shown that elevating second messengers (cAMP and cGMP) is important for controlling inflammatory damage and inducing CNS repair. Phosphodiesterases (PDEs) have been studied extensively in a wide range of disorders as they breakdown these second messengers, rendering them crucial regulators. In this review, we provide an overview of the role of PDE inhibition in limiting pathological inflammation and stimulating regenerative processes in MS.

Nitric oxide reduces T lymphocyte adhesion to human brain microvessel endothelial cells via a cGMP-dependent pathway

The entry of lymphocytes into the brain is normally limited by the blood-brain barrier, however, during inflammation prominent lymphocytic infiltration occurs. In this study, we investigated the effects of nitric oxide (NO) on the adhesion of T cells to cultured human brain microvessel endothelial cells. T cell adhesion to unstimulated or tumor necrosis factor-alpha (TNF-alpha)-treated cells was quantified by counting the number of lymphocytes bound to the monolayer by light microscopy. TNF-alpha increased T cell adhesion in a time-dependent manner. Incubation of monolayers with NO donors decreased adhesion. This effect was blocked by a guanylyl cyclase inhibitor and mimicked by a cGMP agonist, and was thus dependent on the generation of cGMP. NO did not modulate adhesion molecule expression in the endothelial cells, suggesting an action on the T cells. Pre-treatment of T cells with NO or a cGMP agonist decreased binding to recombinant endothelial adhesion molecules. These findings suggest that NO can modulate the adhesion of T cells to human brain microvessel endothelial cells via a cGMP-dependent mechanism, and may thus regulate lymphocyte traffic during central nervous system inflammation.

Are cyclic nucleotides involved in the initiation of mitogenic activation of human lymphocytes?

In order to obtain more insight into the possible role of cyclic AMP or cyclic GMP in modulating the initial cellular processes following activation of lymphocytes, we measured the effects of the T-cell mitogen concanavalin A and other substances including hormones on the cyclic nucleotide levels in human peripheral blood lymphocytes. The enzyme activities of the corresponding nucleotide cyclases, adenylate cyclase and guanylate cyclase were measured in both isolated plasma membranes or the cytosol of resting or concanavalin A stimulated rabbit thymocytes. Concanavalin A in a mitogenic concentration of about 5-10 micrograms/ml caused small, but consistent increases in cAMP but no changes in cGMP levels during the first hour of activation. Concomitantly, the specific activity of plasma membrane-bound adenylate cyclase was always increased at least 1.5-fold 30 min after stimulation of rabbit thymocytes with concanavalin A, but no effect could be detected on the specific activities of plasma membrane-bound or soluble guanylate cyclase. At high, supraoptimal concentrations of concanavalin A (more than 20 micrograms/ml) cAMP levels dramatically increased in human lymphocytes within minutes, but cGMP levels again were unaffected. Forskolin and beta-adrenergic hormones elevated cAMP in human lymphocytes, whereas cGMP levels were increased by the addition of sodium nitroprusside or alpha-adrenergic hormones. Sodium nitroprusside, in concentrations which elevated cGMP in human lymphocytes, had no influence on the incorporation of [3H]uridine into RNA of resting or concanavalin A stimulated human lymphocytes. Addition of forskolin resulted in an increase of cAMP levels and a dose-dependent decrease of [3H]uridine incorporation into RNA of concanavalin A-stimulated lymphocytes with no effect on resting lymphocytes. The data suggest that cGMP does not play a role in the initial phase of mitogenic activation of lymphocytes, whereas cAMP may be involved in the blast transformation process as an inhibitory signal.

Characterization and subcellular localization of nucleotide cyclases in calf thymus lymphocytes

The role of cyclic nucleotides in the regulation of lymphocyte growth and differentiation remains controversial, as an adequate characterization of the key enzymes, adenylate cyclase and guanylate cyclase, in the plasma membrane of lymphocytes is still lacking. In this study, calf thymus lymphocytes were disrupted by nitrogen cavitation and various cellular fractions were isolated by differential centrifugation and subsequent sucrose density ultracentrifugation. As revealed by the chemical composition and the activities of some marker enzymes, the plasma membrane fraction proved to be highly purified. Nucleotide cyclases were present in the plasma membranes in high specific activities, basal activities of adenylate cyclase being 13.7 pmol/mg protein per min and 34.0 pmol/mg protein per min for the guanylate cyclase, respectively. Adenylate cyclase could be stimulated by various effectors added directly to the enzyme assay, including NaF, GTP, 5'-guanylyl imidodiphosphate, Mn2+ and molybdate. Addition of beta-adrenergic agonists only showed small stimulating effects on the enzyme activity in isolated plasma membranes. Basal activity of adenylate cyclase as well as activities stimulated by NaF or 5'-guanylyl imidodiphosphate exhibited regular Michaelis-Menten kinetics. Activation by both agents only marginally affected the Km values, but largely increased Vmax. The activity of the plasma membrane-bound guanylate cyclase was about 10-fold enhanced by the nonionic detergent Triton X-100 and high concentrations of lysophosphatidylcholine, but was slightly decreased upon addition of the alpha-cholinergic agonist carbachol. Basal guanylate cyclase indicated to be an allosteric enzyme, as analyzed by the Hill equation with an apparent Hill coefficient close to 2. In contrast, Triton X-100 solubilized enzyme showed regular substrate kinetics with increasing Vmax but unaffected Km values. Thus the lymphocyte plasma membrane contains both adenylate cyclase and guanylate cyclase at high specific activities, with properties characteristic for hormonally stimulated enzymes.

Zinc localization in taste bud membranes

Zinc was measured by flame aspiration atomic absorption spectrophotometry in homogenates and in enriched fractions and subfractions from bovine taste bud membranes and from surrounding control tissues that contained no taste buds. Zinc was found in significantly higher concentrations in tissues containing taste buds and increased in concentration as biochemical and electron microscopic purity increased. The role of zinc in taste bud membranes could relate to its role in membrane stabilization or to its activity in alkaline phosphatase, a zinc-dependent enzyme whose specific activity increased in taste bud membranes in the same manner as did zinc concentration.

Manganese and magnesium dependent properties and inner plasma membrane surface localization of guanylate cyclase from murine plasmocytoma cells

The particulate fraction from murine plasmocytoma cells contained 90 per cent of the total guanylate cyclase activity. Triton X-100 produced a 6 fold stimulation of guanylate cyclase activity in plasma membrane enriched fractions obtained by zonal centrifugation. Isolated inside out (10) vesicles contained 9 times more activity than rightside out (RSO) vesicles. This difference was abolished by Triton X-100 treatment of the vesicles indicating that the catalytic site of guanylate cyclase is located on the inner face of the plasma membrane. Kinetic studies of membranous guanylate cyclase showed that optimal activity was found with manganese. Only 20 per cent of this activity was obtained with magnesium. The Km for GTP with magnesium (1.4 mM) was about 7 fold greater than with manganese (0.2 mM). Positive cooperativity was obtained in both cases and the Hill coefficients were 1.8 for manganese and 1.6 for magnesium. Physiological concentrations of ATP were found to inhibit both manganese and magnesium supported activities indicating a possible regulatory mechanism for this nucleotide in vivo.

Patterns of cyclic nucleotides in normal and leukaemic human leucocytes

Because recent observations indicate that metabolism of cyclic nucleotides may be altered in neoplastic cells, the intracellular levels of cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) were measured in mononuclear leukaemic and normal human leucocytes. The activities of adenylate cyclase, guanylate cyclase and cyclic nucleotide phosphodiesterases were also determined. Under basal conditions, cAMP levels were always higher in the normal leucocytes, whilst cGMP levels were of the same order of magnitude in both normal and leukaemic cells, causing the cAMP/cGMP ratios to be significantly lower in leukaemic leucocytes. Leukaemic cells significantly increased cyclic nucleotide levels in response to theophylline, but did not respond to serotonin, carbamylcholine or D,L-isoproterenol. Preincubation of these leucocytes with theophylline produced a detectable cAMP response to D,L-isoproterenol but no cGMP response to serotonin or carbamylcholine was found. Adenylate cyclase and guanylate cyclase were significantly lower in leukaemic than in normal cells, which could largely explain the abnormal cyclic nucleotide pattern found in human leukaemic leucocytes. In our experiments, cAMP phosphodiesterase activity was comparable in normal and leukaemic cells, whereas cGMP phosphodiesterase activity was undetectable inall mononuclear-leucocyte preparations with the methods used.