Reduction in the stability of the Gs-catalytic unit complex of adenylate cyclase in isoproterenol-induced heterologous desensitization

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.

Anomalous regulation of β-adrenoceptor signaling in brain regions of the newborn rat

Desensitization, an essential homeostatic response to excessive or continued beta-adrenoceptor (betaAR) stimulation, is deficient in immature cells. To determine the mechanisms underlying anomalous betaAR responses in newborn rats, we administered terbutaline, a beta2AR agonist, on postnatal day 2 and evaluated signaling through adenylyl cyclase (AC) in cell membrane preparations 4 h later. Although a small decrement in isoproterenol-stimulated AC was obtained in the forebrain, robust sensitization was seen in the brainstem and cerebellum, in association with heterologous increases in AC catalytic activity: increased basal, dopamine-stimulated and forskolin-stimulated AC. Superimposed on this global increase, there was a small degree of betaAR and dopamine receptor desensitization, characterized by decreases in the isoproterenol/forskolin and dopamine/forskolin AC activity ratios. Our results indicate that, in some immature brain regions, betaAR desensitization is masked by more substantial increases in the activity of signaling elements downstream from the receptors, resulting in sustained responses in the face of continued receptor stimulation. These effects are likely responsible for the maintenance of betaAR activity associated with neurotrophic input during synaptogenesis but may also contribute to adverse effects of betaAR agonists used in preterm labor.

Are developing beta-adrenoceptors able to desensitize? Acute and chronic effects of beta-agonists in neonatal heart and liver

During fetal and neonatal development, beta-adrenergic receptors (beta-ARs) appear to be resistant to desensitization by beta-agonist drugs. To determine the mechanisms underlying the regulatory differences between adults and neonates, we administered isoproterenol, a mixed beta(1)/beta(2)-AR agonist, and terbutaline, a beta(2)-selective agonist. Effects were examined in the ensuing 4 h after a single injection, or after the last of four daily injections. We prepared cell membranes from heart (predominantly beta(1)-ARs) and liver (predominantly beta(2)-ARs) and assessed signal transduction in the adenylyl cyclase (AC) pathway. In the first few hours after a single administration of isoproterenol to adult rats, cardiac beta-ARs showed activation of G proteins (elevated AC response to forskolin) and desensitization of beta-AR-mediated responses; after the fourth injection, heterologous desensitization emerged, characterized by a loss of signaling mediated either through beta-ARs or glucagon receptors. Terbutaline evoked an increase in the forskolin response but no desensitization of receptor-mediated responses. When we gave the same treatments to neonatal rats, we observed cardiac G protein activation, but there was neither homologous nor heterologous desensitization of beta-ARs or glucagon receptors. In the adult liver, isoproterenol and terbutaline both failed to evoke desensitization, regardless of whether the drugs were given once or for 4 days. In neonates, however, acute or chronic treatment elicited homologous desensitization of beta-AR-mediated AC signaling, while sensitizing the response to glucagon. These results show that neonatal beta-ARs are inherently capable of desensitization in some, but not all, cell types; cellular responses can be maintained through heterologous sensitization of signaling proteins downstream from the receptor. Differences from adult patterns of response are highly tissue selective and are likely to depend on ontogenetic differences in subtypes of beta-ARs and AC.

Ontogeny of G-protein expression: control by beta-adrenoceptors

Cardiac cell homeostasis is maintained in the face of excessive beta-adrenoceptor stimulation through the process of desensitization. Desensitization is not an inherent property of these cells but rather is acquired during development; neonates given beta-agonists actually show heterologous sensitization, involving changes in the expression and catalytic activity of adenylyl cyclase (AC) as well as an increased receptor/G-protein coupling. The current study examines the role of specific G-protein components, G(s)alpha and G(i)alpha, in the ontogeny of beta-adrenoceptor responses and in the transition from agonist-induced sensitization to desensitization. Between postnatal days (PN) 6 and 15 there was a significant decrease in the 52 kDa isoform of G(s)alpha with no accompanying change of the 45 kDa form; over the same period, G(i)alpha3 also declined substantially. In contrast, the 45 kDa isoform of G(s)alpha and G(i)alpha1,2 remained fairly constant over the same period and fluoride-stimulated AC activity increased. Treatment with isoproterenol on PN2-5 did not result in any significant changes in G(s)alpha expression but robustly decreased G(i)alpha1,2. These changes were accompanied by heterologous sensitization of AC activity at the level of AC itself, evidenced by equivalent increases in the enzymatic response to fluoride and forskolin-Mn2+. Isoproterenol given to older animals (PN11-14) also caused specific loss of G(i) protein, in this case targeting G(i)alpha3, whereas G(s)alpha again was unchanged; in contrast to the younger group, the older animals displayed heterologous desensitization of AC at the level of G-protein function (specific loss of the fluoride response). These results indicate that the normal ontogenetic increase of cardiac beta-adrenoceptor coupling to AC is not dependent on the absolute amount of G-proteins, nor on the relative balance of stimulatory (G(s)) and inhibitory (G(i)) subunits. However, the ability of receptor stimulation to downregulate G(i)alpha1,2, an event which is specific to immature cardiac cells, is likely to be an important component of the resistance of the fetal/neonatal heart to agonist-induced desensitization and hypertrophy. The maintenance of cardiac beta-adrenoceptor signaling in the face of intense stimulation is likely to play an important role in the physiologic adaptations necessary to the perinatal transition.

Ontogeny of beta-adrenoceptor/adenylyl cyclase desensitization mechanisms: the role of neonatal innervation

The ability of adrenergic stimulation to elicit desensitization of the beta-receptor/adenylyl cyclase signaling cascade is not an inherent property of cells but rather is acquired during the period in which sympathetic innervation develops. This study examines whether innervation provides the signal that enables target cardiac and hepatic cells to learn to desensitize their responses. Neonatal rats were sympathectomized with 6-OHDA on postnatal day 1 and were treated at various ages with a regimen of isoproterenol known to elicit desensitization in adults. In control rats, desensitization first appeared between days 6 and 15. Desensitization was heterologous, involving changes in the efficiency of G-protein coupling, as there were parallel decreases in isoproterenol-stimulated adenylyl cyclase activity, basal activity and fluoride-stimulated activity (maximal G-protein activation) without changes in forskolin-Mn2+-stimulated activity (total cyclase catalytic activity). The lesioned animals showed a delay in the onset of desensitization as isoproterenol did not evoke decreased responsiveness until day 25 in the heart; the liver did not display agonist-induced desensitization even at day 25. The effects of lesioning on development of desensitization were entirely separable from those on regulation of beta-receptors themselves: agonist-induced decreases in receptor binding appeared by day 15 in both control and lesioned animals. Uniquely in the youngest animals (6 days old), isoproterenol treatment produced heterologous sensitization of adenylyl cyclase responses rather than desensitization, with a parallel increase in basal, isoproterenol-, fluoride- and forskolin-Mn2+-stimulated activity; the latter indicates induction of total catalytic activity as the primary mechanism of sensitization. The lesioned neonates did not show sensitization, despite the fact that during this period, sympathetic pathways are not functionally competent. Our results indicate that innervation provides a timing signal for the onset of desensitization capabilities of sympathetic target cells, but is not absolutely required for the cells to learn how to desensitize. Prior to the onset of desensitization, agonists induce sensitization that may be important in preserving physiological responsiveness during ontogenetic surges of adrenergic activity. The absence of sensitization in lesioned animals implies that, before physiological function is completely established, early pioneer synapses provide a trophic signal that enables cells to increase their sensitivity to stimulation during the perinatal transition period.

Atypical regulation of hepatic adenylyl cyclase and adrenergic receptors during a critical developmental period: agonists evoke supersensitivity accompanied by failure of receptor down-regulation

Ordinarily, beta-adrenergic receptors and responses linked to the receptors increase with development but in the liver, beta-receptors are higher in the fetus and neonate than in adulthood. We examined how hepatic beta-receptor signaling mediated through adenylyl cyclase is regulated in rats of different ages. In each case, animals were pretreated with isoproterenol for 4 d, and on the 5th d, hepatic membrane preparations were examined for adenylyl cyclase activity and receptor binding capabilities. Uniquely in 6-d-old animals, the cyclase response to isoproterenol was enhanced by chronic pretreatment, caused by heterologous sensitization mediated through effects on total catalytic activity (increased response to forskolin-Mn2+) and on G-protein coupling (enhanced effect of fluoride and increased GTP dependence of basal activity). Isoproterenol pretreatment failed to cause beta-receptor down-regulation in 6-d-old animals, but by 15 d of age, down-regulation was detected along with slight desensitization of the cyclase response. However, at 25 d, neither effect was present. In adulthood, repeated isoproterenol administration failed to cause cyclase desensitization but did reduce beta-receptor numbers; the loss of receptors was still unusual in that beta-receptor down-regulation could be achieved with either isoproterenol or with methoxamine, an alpha-receptor agonist. The results indicate that, early in development, hepatic beta-receptor-mediated responses are enhanced, not desensitized, after chronic stimulation. These effects would foster responsiveness of hepatic gluconeogenesis in the face of the massive adrenergic stimulation associated with the transition from fetal to neonatal life. In adulthood, when receptor numbers are far lower than in the neonate, the inability to desensitize the signaling cascade despite receptor down-regulation would serve to maintain the response to catecholamines.

Beta-adrenoceptor control of cardiac adenylyl cyclase during development: agonist pretreatment in the neonate uniquely causes heterologous sensitization, not desensitization

In the adult, increased stimulation of postsynaptic receptor sites produces compensatory desensitization that reduces tissue responsiveness. During development, however, responses in most systems increase with age and with the maturation of neuronal inputs. In the current study, we examined whether agonist-induced desensitization of cardiac beta-adrenergic receptor signaling mediated through adenylyl cyclase could be elicited in 6-, 15- and 25-day-old rats, and in adults. In each case, animals were pretreated with isoproterenol daily for four days preceding the experiment, and on the fifth day, cardiac membrane preparations were examined. Fifteen and 25-day-old animals and adults all exhibited desensitization, as demonstrated by a diminished cyclase response to isoproterenol in vitro. However, in 6-day-old animals, the enzymatic response to isoproterenol was enhanced by chronic pretreatment. Measurements of the G-protein-sensitive component of cyclase (decrement in activity obtained with deletion of GTP from the reaction mixture, stimulatory response to fluoride) indicated heterologous desensitization in the older animals, evidenced by diminished dependence on GTP and reduced response to fluoride; the 6-day-old animals showed enhanced GTP dependence and augmentation of the fluoride response. Uniquely in 6-day-old animals, the total catalytic activity of adenylyl cyclase, measured with forskolin-Mn2+, was markedly elevated by chronic isoproterenol pretreatment, whereas it was unaffected in older animals. These data suggest that regulation of receptor signaling is completely different early in neonatal life. Instead of producing desensitization of responses, agonist exposure promotes receptor signaling by enhancing expression and/or catalytic efficiency of adenylyl cyclase. In older animals, the predominant effect is heterologous desensitization mediated at the level of G-proteins. These developmental differences are likely to be important in the maintenance of tissue responsiveness during the period in which innervation develops, as well as in the ability of neurotrophic input to 'program' the responsiveness of target tissues.

Difference in sensitivity to alkaline phosphatase treatment between rat reticulocyte membranes in which beta-adrenoceptor desensitization was induced by isoproterenol, dibutyryl cAMP and phorbol ester

The effect of alkaline phosphatase (3.1.3.1) on desensitization of beta-adrenoceptor-responsive adenylate cyclase and the role of phosphorylation in desensitization were examined. Treatment of rat reticulocytes with isoproterenol, dibutyryl cAMP and tetradecanoyl phorbol acetate (TPA) caused the desensitization of beta-adrenoceptor-coupled adenylate cyclase. When the membranes from dibutyryl cAMP- and TPA-desensitized cells were incubated with alkaline phosphatase for 60 min at 30 degrees C, pH 8.0, the desensitization of isoproterenol-stimulated adenylate cyclase was markedly attenuated in both preparations. When the membranes from isoproterenol-desensitized cells were treated with alkaline phosphatase under the same conditions, the attenuation of the desensitization of alkaline phosphatase was less than in the case of treatment with dibutyryl cAMP or TPA. In other words, isoproterenol-induced desensitization was more resistant to alkaline phosphatase treatment. Isoproterenol- and dibutyryl cAMP-induced desensitization of NaF-stimulated adenylate cyclase were also attenuated by alkaline phosphatase treatment. Although the stability of the Gs-catalytic unit complex of adenylate cyclase was reduced by isoproterenol treatment, the reduction of stability was also decreased by alkaline phosphatase treatment.