Pertussis toxin treatment of whole blood. A novel approach to assess G protein function in congestive heart failure

β2-Adrenergic Signaling Modulates Mitochondrial Function and Morphology in Skeletal Muscle in Response to Aerobic Exercise

The molecular mechanisms underlying skeletal muscle mitochondrial adaptations induced by aerobic exercise (AE) are not fully understood. We have previously shown that AE induces mitochondrial adaptations in cardiac muscle, mediated by sympathetic stimulation. Since direct sympathetic innervation of neuromuscular junctions influences skeletal muscle homeostasis, we tested the hypothesis that β₂-adrenergic receptor (β₂-AR)-mediated sympathetic activation induces mitochondrial adaptations to AE in skeletal muscle. Male FVB mice were subjected to a single bout of AE on a treadmill (80% Vmax, 60 min) under β₂-AR blockade with ICI 118,551 (ICI) or vehicle, and parameters of mitochondrial function and morphology/dynamics were evaluated. An acute bout of AE significantly increased maximal mitochondrial respiration in tibialis anterior (TA) isolated fiber bundles, which was prevented by β₂-AR blockade. This increased mitochondrial function after AE was accompanied by a change in mitochondrial morphology towards fusion, associated with increased Mfn1 protein expression and activity. β₂-AR blockade fully prevented the increase in Mfn1 activity and reduced mitochondrial elongation. To determine the mechanisms involved in mitochondrial modulation by β₂-AR activation in skeletal muscle during AE, we used C2C12 myotubes, treated with the non-selective β-AR agonist isoproterenol (ISO) in the presence of the specific β₂-AR antagonist ICI or during protein kinase A (PKA) and Gα_i protein blockade. Our in vitro data show that β-AR activation significantly increases mitochondrial respiration in myotubes, and this response was dependent on β₂-AR activation through a Gα_s-PKA signaling cascade. In conclusion, we provide evidence for AE-induced β₂-AR activation as a major mechanism leading to alterations in mitochondria function and morphology/dynamics. β₂-AR signaling is thus a key-signaling pathway that contributes to skeletal muscle plasticity in response to exercise.

Increased Gsα within blood cell membrane lipid microdomains in some depressive disorders: an exploratory study

The stimulatory guanine nucleotide binding protein Gs couples many cellular receptors to adenylate cyclase, and the Gsα subunit activates all 9 isoforms of the adenylate cyclase catalytic unit to produce the enzyme product cyclicAMP or cAMP. In prefrontal cortex and cerebellum of unipolar depressive suicides, Rasenick and colleagues have found increased concentrations of Gsα in membrane lipid microdomains (Donati et al., 2008), where the ensconced Gsα is less likely to activate adenylate cyclase by receptor and postreceptor pathways (Allen et al., 2005, 2009). We report that a group of 7 depressed patients (DP-1) had (1) reduced activation of platelet receptor-stimulated adenylate cyclase by both prostaglandins E2 and D2 compared to controls, and (2) reduced postreceptor stimulation of adenylate cyclase by aluminum fluoride ion in both platelets and mononuclear leukocytes when compared to both another group of depressed patients (DP-2, n = 17) and to controls (n = 21). Our observations in the blood cells of the group DP-1 support the findings of Donati et al. (2008), and they reflect the importance of this interaction between the activated Gsα subunit and membrane lipid microdomains in the pathophysiology and treatment of some major depressive disorders.

Hypofunctionality of Gi proteins as aetiopathogenic mechanism for migraine and cluster headache

The involvement of Gi proteins in the modulation of pain perception has been widely established, and mutations in G-proteins have already been identified as the aetiopathological cause of human diseases. The aim of the present study was to determine whether a deficiency or a hypofunctionality of the Gi proteins occurred in primary headache. The functionality and the level of expression of Gi proteins were investigated in lymphocytes from migraine without aura, migraine with aura and cluster headache sufferers. A reduced capability to inhibit forskolin-stimulated adenylyl cyclase activity in headache patients was observed. Migraine patients also showed basal adenosine cAMP levels about four times higher than controls. The reduced activity of Gi proteins seems not to be related to a reduction of protein levels since no significant reduction of the Gialpha subunits was observed. These results indicate Gi protein hypofunctionality as an aetiopathogenic mechanism in migraine and cluster headache.

Concomitant regulation of Ca2+ mobilization and G13 expression in human erythroleukemia cells

In human erythroleukemia (HEL) cells, stimulation of alpha2-adrenoceptors by adrenaline or neuropeptide Y Y1 receptors by neuropeptide Y, concomitantly inhibit cAMP accumulation and stimulate mobilization of Ca2+ from intracellular stores via pertussis toxin-sensitive G-proteins. Treatment of HEL cells in chemically-defined, serum-free medium with 1.25% dimethylsulfoxide (DMSO) for 4 days, increased alpha2-adrenoceptor number by 120%, while the neuropeptide Y receptor number was not significantly changed. In DMSO-treated HEL cells, Ca2+ elevations by adrenaline or neuropeptide Y were significantly reduced by 28% and 57%, respectively, while basal Ca2+ and elevations by thrombin or thapsigargin were not significantly altered. Adrenaline and neuropeptide Y-induced inhibition of forskolin-stimulated cAMP accumulation was not significantly altered upon DMSO treatment. While immunodetectable alpha-subunits of Gi2 were not significantly changed by DMSO treatment, those of Gi3 were reduced by 27%. Inactivation of pertussis toxin substrates by pertussis toxin treatment and inhibition of adrenaline or neuropeptide Y stimulated Ca2+ elevations were linearly correlated. These data are compatible with the idea that, in HEL cells, alpha2-adrenoceptors and neuropeptide Y receptors couple to inhibition of adenylyl cyclase via Gi2 while they couple to Ca2+ elevations via Gi3.

Signal transduction by platelet adenylate cyclase: alterations in depressed patients may reflect impairment in the coordinated integration of cellular signals (coincidence detection)

BACKGROUND

Adenylate cyclase (AC) responds to distinct but coincident signals from the agonist-stimulated G-protein Gs and the inhibitory G-protein Gi by generating a greater output signal-to-noise ratio--i.e., agonist-stimulated to basal ratio (fold-stimulation)--through coincidence detection than that generated by a single input (Gs) alone. Such coincidence detection by murine brain AC was found to be enhanced during chronic antidepressant treatment with imipramine.

METHODS

We examined and compared the basal, agonist-stimulated, and guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) or AlF4 ion postreceptor-stimulated AC activities in mononuclear leukocytes and platelets from the same blood specimens obtained from depressed patients (n = 27) and control subjects (n = 19).

RESULTS

In all subjects, the differences (delta GTP gamma S or delta AlF4) between postreceptor measures of AC in mononuclear leukocytes (where AC is regulated by Gs but not by Gi) and platelets (where AC is regulated by both Gs and Gi) were highly significant. In controls, the relationships between delta GTP gamma S or delta AlF4 and basal, agonist-stimulated, and the fold-stimulation of agonist-stimulated platelet AC resembled the regulation of AC by Gi in model-membrane systems. Comparable relationships between delta GTP gamma S or delta AlF4 and basal, agonist-stimulated, and the fold-stimulation of agonist-stimulated platelet AC activities were not observed in depressed patients.

CONCLUSIONS

Our results suggest that in controls, platelet AC enzyme activity is determined (in part) by the coordinated integration of signals from Gs and Gi through coincidence detection, while such coincidence detection by platelet AC may be impaired in patients with depressive disorders.

Sensitization by dexamethasone of lymphocyte cyclic AMP formation: evidence for increased function of the adenylyl cyclase catalyst

1. Glucocorticoids and elevations of intracellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) may affect lymphocyte activation, proliferation and effector functions in similar ways. Therefore, we have investigated the effects of the glucocorticoid, dexamethasone, on human lymphocyte cyclic AMP formation. 2. Treatment of resting human lymphocytes with the glucocorticoid, dexamethasone, sensitized prostaglandin E2-stimulated cyclic AMP accumulation in a time- and concentration-dependent manner. 3. In membranes of lymphocytes treated for 24 h with 100 nM dexamethasone, maximal adenylyl cyclase activity stimulated by prostaglandin E2, isoprenaline, guanosine 5'-triphosphate (GTP), forskolin and MnCl2 was significantly enhanced; the EC50 for these agents was not significantly altered. 4. beta 2-Adrenoceptor density, immunodetectable alpha-subunits of the G-proteins Gs and Gi, and pertussis toxin-substrates were not significantly altered by dexamethasone treatment. 5. In dexamethasone-treated lymphocytes, prostaglandin E2-mediated inhibition of concanavalin A-induced Ca2+ elevations was doubled compared to control cells. 6. Based on these data and the observation that enhancement of forskolin- and MnCl2-stimulated adenylyl cyclase activity could quantitatively account for the enhancement of prostaglandin E2-, isoprenaline- or GTP-stimulated adenylyl cyclase activity, we conclude that dexamethasone treatment sensitizes cyclic AMP formation in resting human lymphocytes by altering the adenylyl cyclase catalyst rather than G-proteins or hormone receptors. This results in an enhanced capability of cyclic AMP generating agonists to inhibit early steps of lymphocyte activation.

Changes in cAMP formation in mononuclear leukocytes of heart and renal transplant recipients

In mononuclear leukocytes (MNL) of renal transplant recipients treated with cyclosporine A and prednisone, an increase of basal cAMP generation has been observed. In order to characterize the mechanisms underlying changes of cAMP generation in patients who were treated with immunosuppressives following heart transplantation, we investigated the beta-adrenoceptor--G protein--adenylate cyclase signal transduction cascade in heart transplant recipients and for comparison in renal transplant recipients as well as controls. Basal cAMP formation in MNL was elevated in heart transplant recipients by 272% and in renal transplant recipients by 148% compared to controls. Following beta-adrenoceptor stimulation with isoprenaline, cAMP formation in MNL of heart transplant recipients was similar to the controls, but was enhanced in renal transplant recipients to 138%. Investigation of beta-adrenoceptor density on MNL as a possible cause for increased cAMP formation revealed similar receptor numbers in controls and in cardiac or renal transplant recipients. Furthermore, the increase of the beta-adrenoceptor density on MNL, which is observed following infusion of isoprenaline, was similar in controls and heart transplant recipients. The amount of pertussis- and cholera toxin substrates was the same in heart transplant recipients as in controls. In contrast, MNL of renal transplant recipients showed a marked increase of Gs alpha by 45% and a smaller albeit significant increase of Gi alpha by 15%, as judged by cholera toxin and pertussis toxin labeling, respectively. Investigation of inotropic parameters by echocardiography under control conditions and during the infusion of increasing concentrations of isoprenaline revealed no difference in the basal contractility and the inotropic response to beta-adrenergic stimulation in controls and heart transplant recipients. It is concluded that changes of G-protein expression are involved in the increase of the cAMP-generation in MNL of heart transplant recipients. These alterations in MNL cannot be taken as a model of cellular function in the transplanted heart, but it is reasonable to suggest that elevations of cAMP formation in MNL may contribute to the immunosuppressive effects of the treatment with cyclosporine A or corticosteroids, the mechanism of which could be an alteration of Gs alpha or the catalyst in renal transplant recipients and the catalyst in heart transplant recipients which occurs without any changes of beta-adrenoceptors.

Beta-adrenoceptors in cardiac disease

The human heart contains both beta 1 and beta 2-adrenoceptors; both mediate positive inotropic and chronotropic effects. In chronic heart failure, beta-adrenoceptor number is reduced, presumably, by down-regulation by endogenous noradrenaline which is elevated due to increased sympathetic activity. Since the human heart contains only a few spare receptors for beta-adrenoceptor-mediated positive inotropic effects and the amount of spare receptors declines in chronic heart failure, it is not surprising that the reduced beta-adrenoceptor number is accompanied by decreased contractile responses to beta-adrenoceptor agonists (including endogenous catecholamines), and the extent of decrease in maximal inotropic response is more pronounced as the disease becomes more advanced. Moreover, in chronic heart failure myocardial G(i)-protein, which inhibits cAMP formation, is increased, which might further contribute to the reduction in beta-adrenoceptor-mediated effects. It appears that, at present, the best therapy for severe heart failure is a successful heart transplant, since in the transplanted heart beta-adrenoceptor number and function seems to be normalized. Moreover, the data currently available do not suggest any development of super- or subsensitivity of postsynaptic cardiac beta-adrenoceptors in the transplanted human heart.

Distribution of myocardial beta-adrenoceptor subtypes and coupling to the adenylate cyclase in children with congenital heart disease and implications for treatment

In congestive heart failure, down-regulation of myocardial beta-adrenoceptors (beta-AR) due to an elevated sympathetic tone is well known. In infancy and childhood, heart failure is usually related to congenital heart disease (CHD). Therefore, 71 samples of right atrial tissue of infants and children with CHD undergoing cardiac surgery were studied for beta-adrenoceptor density and distribution of the beta 1-/beta 2-AR subtypes. In 49 cases, the coupling of the beta-AR to the adenylate cyclase (AC) was examined. In a further study of 19 myocardial samples, AC was selectively stimulated with beta 1- or beta 2-AR whereas the other subtype was blocked by an antagonist. The following results were obtained: (1) Infants and children with severe acyanotic or cyanotic CHD had severely reduced beta-AR densities. (2) In most of the cases, the beta-AR down regulation is beta 1-subtype selective, but in critically ill newborns with congenital aortic valve stenosis or transposition of the great arteries, there is additional significant beta 2-AR down-regulation. In Fallot patients treated with the beta-antagonist propranolol, a significant increased beta-AR number compared with untreated Fallot patients was found. (3) beta-Adrenoceptor reduction in CHD is correlated with elevated noradrenaline plasma levels, thus proving a sympathetic dysregulation. (4) In CHD with moderate hemodynamic load, beta 2-AR coupling to AC was markedly more efficient than beta 1-AR coupling. The small number of myocardial beta 2-AR produced most of the cyclic adenosine monophosphate. (5) In severe acyanotic and cyanotic CHD, a partial decoupling of the beta 2-AR to the AC occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue- and subunit-specific regulation of G-protein expression by hypo- and hyperthyroidism

Thyroid hormone status has profound effects on signal transduction in various tissues throughout the body. Therefore, we quantified the signal transducing G-proteins in the rat heart, cerebral cortex, vas deferens and liver by immunoblotting and pertussis toxin labeling in response to chemically induced hypothyroidism (treatment with propylthiouracil) and hyperthyroidism (treatment with triiodothyronine). Levels of the pertussis toxin (PTX) substrates Gi alpha and Go alpha in the heart and vas deferens were inversely correlated with thyroid hormone levels, i.e. Gi alpha and Go alpha were decreased or unchanged in hyperthyroid rats and increased in hypothyroid rats compared to control animals. The cerebral cortex and liver expression of PTX substrates Gi alpha and Go alpha was not affected by changes in thyroid hormone. Regulation of Gs alpha protein was more complex in that Gs alpha was unaffected in the other tissues tested. Expression of G-protein beta-subunits was not affected by thyroid status in the heart, liver, or cerebral cortex. Our results suggest that tissue- and G-protein-specific factors are involved in the regulation of G-protein subunits by thyroid hormone. Moreover, cardiac expression of Gs alpha is upregulated by increases or decreases in the normal level of thyroid hormone.

NPY-stimulated Ca2+ mobilization in SK-N-MC cells is enhanced after isoproterenol treatment

Neuropeptide Y (NPY) receptors in the SK-N-MC human neuroblastoma cell line couple to mobilization of intracellular Ca2+ and inhibition of adenylylcyclase. Pretreatment of SK-N-MC cells with isoproterenol enhanced the NPY-stimulated Ca2+ mobilization, mainly by increasing the maximal response to NPY. The enhancement was time-(maximal after 24 h) and concentration-dependent (maximal at 10 microM isoproterenol), blocked by the beta-adrenergic antagonist propranolol, and mimicked by forskolin. Concomitant treatment with cycloheximide prevented the enhancing effect of isoproterenol, suggesting the involvement of protein synthesis. Isoproterenol treatment did not alter the number or affinity of 125I-labeled NPY binding sites, the amount of pertussis toxin substrates, or NPY-mediated inhibition of cAMP accumulation. Similarly, isoproterenol treatment had no effect on basal intracellular Ca2+ and on Ca2+ increases elicited by carbachol, caffeine, or ionomycin. We conclude that isoproterenol treatment can sensitize NPY receptor responsiveness in a way that is specific for Ca2+ mobilization mechanisms used by this hormone.

Lithium administration modulates platelet Gi in humans

Platelet G proteins were assessed in 7 normal volunteers before and after 14 days of lithium administration at therapeutic plasma levels. Cholera and pertussis toxin catalyzed ADP-ribosylation of platelet membrane proteins were measured by SDS-PAGE. Immunoblotting with specific antibodies was used to measure platelet membrane alpha i content. There was a statistically significant 37% increase in pertussis toxin mediated ADP-ribosylation of a 40,000 Mr protein in platelet membranes after lithium administration, but cholera toxin mediated ADP-ribosylation of a 45,000 Mr protein and alpha i immunoblotting were unchanged by lithium. Increased pertussis toxin stimulated ADP-ribosylation in the absence of changes in alpha i content could be explained by a shift in platelet Gi in favor of its undissociated, inactive form. This would be consistent with increased platelet adenylyl cyclase activity found in these same subjects after lithium.

Beta-adrenoceptor density on mononuclear leukocytes and right atrial myocardium in infants and children with congenital heart disease

Sympathetic regulation of myocardial performance has been shown to be altered in congestive heart failure. Right atrial tissue of children with severe acyanotic and cyanotic congenital heart disease (CHD) showed a significantly lower beta-receptor density than that of children with less severe defects. Since mononuclear leukocytes (MNL) contain a homogeneous population of beta 2-adrenoceptors which have similar properties to those of cardiac beta 2-adrenoceptors, they are frequently used for studying the beta-adrenergic system. In a group of 37 children with CHD of different types and severity who underwent cardiac surgery, we compared the MNL beta-adrenoceptor density to the type and severity of CHD and looked for a possible relationship to plasma catecholamine levels and to the right atrial beta-adrenoceptor density. Membranes of MNL and myocardial cells were radiolabeled with (-)3-[125I]Iodocyanopindolol [( 125I]ICYP). A significantly higher beta-adrenoceptor density on MNL was found in patients with moderate acyanotic CHD (group I) than in those with severe acyanotic (group II) and cyanotic CHD (group III). Patients of group I showed approximately 50% higher myocardial beta-receptor density than those of groups II and III. ICI 118.551-[125I]ICYP competition studies revealed that in groups II and III significantly lower proportions and densities of beta 1-receptors were found compared to group I. Noradrenaline (NA) plasma levels in group II and group III were significantly higher than those in group I. The adrenaline plasma levels were found to be very high in all children with CHD.(ABSTRACT TRUNCATED AT 250 WORDS)

G-protein coupling and signalling of Y1-like neuropeptide Y receptors in SK-N-MC cells

We have studied [125I]neuropeptide Y-binding sites and neuropeptide Y-mediated second messenger responses in human SK-N-MC neuroblastoma cells with special reference to the role of G-proteins. Neuropeptide Y stimulated two second messenger responses in SK-N-MC cells, inhibition of cAMP accumulation and mobilization of Ca2+ from intracellular stores. Both effects were completely abolished by pretreatment with pertussis toxin. Binding of [125I]neuropeptide Y to intact cells or SK-N-MC cell membranes was rapid, reversible, characterized by high affinity and low capacity, and had pharmacological characteristics of a homogeneous population of Y1-like neuropeptide Y receptors. In permeabilized cells, [125I] neuropeptide Y binding was inhibited by GTP gamma S in a concentration-dependent manner. Saturation experiments in the absence and presence of GTP gamma S demonstrated a reduction in the number of high-affinity [125I]neuropeptide Y-binding sites without a decrease in affinity of the remaining sites. Pretreatment of intact cells with pertussis toxin completely abolished the inhibition of [125I]neuropeptide Y binding by GTP gamma S. Moreover, pertussis toxin treatment reduced the number of high-affinity [125I]neuropeptide Y binding sites. We conclude that the agonist ligand [125I]neuropeptide Y identifies functional neuropeptide Y receptors in SK-N-MC cells; however, the number of specific [125I]neuropeptide Y-binding sites may not necessarily reflect the number of neuropeptide Y receptors, because the former is affected by the functional state of cellular G-proteins.

Immunodetectable levels of the inhibitory guanine nucleotide-binding regulatory proteins in failing human heart: discordance with measurements of adenylate cyclase activity and levels of pertussis toxin substrate

Human hearts with idiopathic dilated cardiomyopathy have diminished adenylate cyclase activity and increased amounts of the alpha-subunit of the inhibitory guanine nucleotide-binding regulatory protein (alpha Gi) as measured by pertussis toxin catalyzed ADP-ribosylation. We utilized specific antisera against synthetic peptides corresponding to amino sequences deduced from cDNA's encoding the three alpha Gi subspecies to compare the immunologic and bioactivity levels of Gi in failing and non-failing human hearts. The various antisera detected three peptides with Mr 42,000, 38,000, and 37,000. Only the Mr 42,000 peptide co-migrated with the pertussis toxin substrate. Although functional activity of alpha Gi was increased in the particulate fractions of the failing heart as measured by inhibition of guanine nucleotide-stimulated adenylate cyclase activity and the quantity of pertussis toxin substrate was also increased, there were not associated changes in the levels of immunodetectable Gi. Therefore, the increased functional activity of Gi in the failing human heart as assessed by adenylate cyclase measurements cannot be explained by a relative increase in the among of Gi protein.

Mitigation of beta 1- and/or beta 2-adrenoceptor function in human heart failure

1. Patients with congestive heart failure (CHF) have an elevated activity of the sympatho-adrenal system. We have investigated several aspects of beta-adrenoceptor desensitization in such patients. 2. The positive inotropic response to isoprenaline was attenuated in CHF patients, and the pD2-values for isoprenaline's positive inotropic effect gradually decreased in more severe forms of the disease. Stimulation of adenylate cyclase by isoprenaline was also mitigated in cardiac membranes from patients with CHF. 3. We then studied the density of cardiac beta 1- and beta 2-adrenoceptors in order to understand the mechanism of beta-adrenoceptor desensitization in these patients. Our data show that cardiac beta 1-adrenoceptors are down-regulated in all forms of severe CHF, but that cardiac beta 2-adrenoceptor density decreases only in some forms of CHF including ischaemic cardiomyopathy and mitral valve disease. 4. In circulating mononuclear leucocytes (MNL) obtained from CHF patients at rest, isoprenaline- and prostaglandin E1-stimulated cAMP generation as well as cholera toxin and pertussis toxin catalyzed ADP ribosylation were similar to those in MNL from control patients. However, pretreatment of intact MNL with pertussis toxin enhanced cAMP generation in CHF patients but not in healthy control subjects, suggesting a tonic inhibitory effect of Gi in such patients. 5. We conclude that alterations of adrenoceptors and of their signal transduction might contribute to the desensitization of beta-adrenergic responses in CHF.