Cyclic AMP regulation of messenger RNA level of the stimulatory GTP-binding protein Gs alpha. Isoproterenol, forskolin and 8-bromoadenosine 3':5'-cyclic monophosphate increase the level of Gs alpha mRNA in cultured astroglial cells

FSH mediated cAMP signalling upregulates the expression of Gα subunits in pubertal rat Sertoli cells

Follicle Stimulating Hormone (FSH) acts via FSH-Receptor (FSH-R) by employing cAMP as the dominant secondary messenger in testicular Sertoli cells (Sc) to support spermatogenesis. Binding of FSH to FSH-R, results the recruitment of the intracellular GTP binding proteins, either stimulatory Gαs or inhibitory Gαi that in turn regulate cAMP production in Sc. The cytosolic concentration of cAMP being generated by FSH-R thereafter critically determines the downstream fate of the FSH signalling. The pleiotropic action of FSH due to differential cAMP output during functional maturation of Sc has been well studied. However, the developmental and cellular regulation of the Gα proteins associated with FSH-R is poorly understood in Sc. In the present study, we report the differential transcriptional modulation of the Gα subunit genes by FSH mediated cAMP signalling in neonatal and pubertal rat Sc. Our data suggested that unlike in neonatal Sc, both the basal and FSH/forskolin induced expression of Gαs, Gαi-1, Gαi-2 and Gαi-3 transcripts was significantly (p < 0.05) up-regulated in pubertal Sc. Further investigations involving treatment of Sc with selective Gαi inhibitor pertussis toxin, confirmed the elevated expression of Gi subunits in pubertal Sc. Collectively our results indicated that the high level of Gαi subunits serves as a negative regulator to optimize cAMP production in pubertal Sc.

Low levels of Gαs and Ric8b in testicular sertoli cells may underlie restricted FSH action during infancy in primates

FSH acts via testicular Sertoli cells (Sc) bearing FSH receptor (FSH-R) for regulating male fertility. Despite an adult-like FSH milieu in infant boys and monkeys, spermatogenesis is not initiated until the onset of puberty. We used infant and pubertal monkey Sc to reveal the molecular basis underlying developmental differences of FSH-R signaling in them. Unlike pubertal Sc, increasing doses of FSH failed to augment cAMP production by infant Sc. The expression of Gαs subunit and Ric8b, which collectively activate adenylyl cyclase (AC) for augmenting cAMP production and gene transcription, were significantly low in infant Sc. However, forskolin, which acts directly on AC bypassing FSH-R, augmented cAMP production and gene transcription uniformly in both infant and pubertal Sc. FSH-induced Gαs mRNA expression was higher in pubertal Sc. However, Gαi-2 expression was down-regulated by FSH in pubertal Sc, unlike infant Sc. FSH failed, but forskolin or 8-Bromoadenosine 3',5'-cyclic monophosphate treatment to infant Sc significantly augmented the expression of transferrin, androgen binding protein, inhibin-β-B, stem cell factor, and glial-derived neurotropic factor, which are usually up-regulated by FSH in pubertal Sc during spermatogenic onset. This suggested that lack of FSH mediated down-regulation of Gαi-2 expression and limited expression of Gαs subunit as well as Ric8b may underlie limited FSH responsiveness of Sc during infancy. This study also divulged that intracellular signaling events downstream of FSH-R are in place and can be activated exogenously in infant Sc. Additionally, this information may help in the proper diagnosis and treatment of infertile individuals having abnormal G protein-coupled FSH-R.

Neuropathology of bipolar disorder

The literature on the neuropathology of bipolar disorder (BD) is reviewed. Postmortem findings in the areas of pathomorphology, signal transduction, neuropeptides, neurotransmitters, cell adhesion molecules, and synaptic proteins are considered. Decreased glial numbers and density in both BD and major depressive disorder (MDD) have been reported, whereas cortical neuron counts were not different in BD (in Brodmann's areas [BAs] 9 and 24). In contrast, MDD patients showed reductions in neuronal size and density (BA 9, BA 47). There are a number of findings of alterations in neuropeptides and monoamines in BD brains. Norepinephrine turnover was increased in several cortical regions and thalamus, whereas the serotonin metabolite, 5-hydroxyindoleacetic acid, and the serotonin transporter were reduced in the cortex. Several reports further implicated both cyclic adenosine monophosphate and phosphatidylinositol (PI) cascade abnormalities. G protein concentrations and activity increases were found in the occipital, prefrontal, and temporal cortices in BD. In the PI signal cascade, alterations in PKC activity were found in the prefrontal cortex. In the occipital cortex, PI hydrolysis was decreased. Two isoforms of the neural cell adhesion molecules were increased in the hippocampus of BD, whereas the synaptic protein marker, synaptophysin, was not changed. The findings of glial reduction, excess signal activity, neuropeptide abnormalities, and monoamine alterations suggest distinct imbalances in neurochemical regulation. Possible alterations in pathways involving ascending projections from the brain stem are considered. Larger numbers of BD brains are needed to further refine the conceptual models that have been proposed, and to develop coherent models of the pathophysiology of BD.

Cyclic AMP regulates G(omicronalpha) protein and mRNA levels by modulating the transcriptional rate of G(omicronalpha) gene

In rat astroglial cells, four G(omicronalpha) transcripts were found: G(omicron2alpha) mRNA (5.7 kb) and three G(omicron1alpha) mRNAs (4.0, 3.0 and 2.3 kb). However, G(omicron2alpha) but little G(omicronalpha1) proteins were present in membrane-enriched fractions. Culturing astroglial cells with forskolin (10 microM) or isoproterenol (10 microM) a beta-adrenergic agonist increased transiently in a time-dependent manner the levels of G(omicronalpha) proteins. The degradation rate of G(omicronalpha) proteins was slightly decreased by the cAMP treatment. In parallel, forskolin (10 microM) treatment increased transiently the amounts of both G(omicron1alpha) and G(omicron2alpha) mRNAs. The relative transcription rate of G(omicronalpha) gene was increased by 1.7-fold in forskolin-treated cells whereas the half-lives of G(omicron1alpha) and G(omicron2alpha) mRNAs were not significantly changed. These results suggest that cAMP regulates the transcription rate of G(omicronalpha) gene and this is compatible with the existence of a cAMP responsive element in the promoter of the G(omicronalpha) gene.

Galpha(i2)-mRNA and -protein regulation as a mechanism for heterologous sensitization and desensitization of insulin secretion

Prolonged exposure of cells to an agonist of a G-protein-coupled receptor usually results in an attenuation of the cellular response. To elucidate the cellular mechanisms of sensitization or desensitization in an insulin secretory cell system (INS-1 cells), we investigated a regulatory link between G-protein alpha(s)- and alpha(i2)-subunits mRNA, their protein levels and insulin secretion as the biological effect using various compounds. Incubation with epinephrine (50 microM) for 8 h decreased alpha(s)- and alpha(i2)-mRNA levels to 58% and 72%, respectively, which is reversed after a longer incubation. From results using isoprenaline and the alpha2-agonist UK 14,304 epinephrine is shown to mediate its actions via alpha2- but not beta-adrenoceptors. The insulin inhibitory neuropeptide galanin (50 nM) caused a decrease of alpha(s)- and alpha(i2)-mRNA levels, whereas insulinotropic compounds (incretin hormones) such as GIP or GLP-1 (both 10 nM) led to an increase of alpha(s)- and alpha(i2)-mRNA levels. By using the Ca2+ channel blocker verapamil (50 microM) alpha(i2)-mRNA changes clearly depend on Ca2+ influx. The effects on alpha(i2)-mRNA were accompanied by a parallel, albeit weaker effect on the protein level (only GIP and UK 14,304 were investigated). The changes in alpha(i2)-mRNA levels by either compound were paralleled by inverse changes in insulin secretion: preincubation with UK 14,304 for 8 h led to an increased insulin secretion when challenged by either GLP-1, GIP or glucose (8.3 mM). This was similar for galanin, another potent inhibitor of insulin release. On the other hand, exposure to the incretins GIP or GLP-1 for 8 h induced a smaller insulin release when challenged afterwards by either UK 14,304, galanin, GIP, GLP-1, or glucose. Thus the influence on insulin secretion of various compounds is reciprocal to the regulation of alpha(i2)-mRNA levels but not alpha(s)-mRNA levels. There is, therefore, evidence from all the manoeuvres used that alpha(i2)-mRNA regulation may play a role in heterologous sensitization and desensitization of insulin secretion.

Sodium saccharin inhibits adenylyl cyclase activity in non-taste cells

We have studied the in vitro effect of sodium saccharin (NaSacch) on the rat adipocyte adenylyl cyclase complex. NaSacch (2.5-50 mM) inhibited significantly in a dose-dependent manner basal and isoproterenol-stimulated cAMP accumulation on isolated rat adipocytes. Similarly, NaSacch (2.5-50 mM) inhibited forskolin-stimulated adenylyl cyclase activity measured in the presence of Mg(2+)-ATP on adipocyte, astrocyte and thyrocyte membrane fractions. In contrast, NaSacch did not inhibit but slightly increased the forskolin-stimulated adenylyl cyclase activity measured in the presence of Mn(2+)-ATP and GDP beta S, a stable GDP analogue. The effect of NaSacch was not mediated through either the A1-adenosine receptor (A1R) or the alpha 2-adrenergic receptor (alpha 2AR). The inhibitory effect of NaSacch was additive to that of A1R agonist and was not blocked by the addition of the alpha 2AR antagonist RX 821002. Pretreatment of adipocytes with pertussis toxin slightly attenuated but did not abolish the inhibitory effect of NaSacch on forskolin-stimulated adenylyl cyclase activity on membrane fractions. These data suggest that the inhibitory effect of NaSacch on forskolin stimulated-adenylyl cyclase in adipocytes does not imply only Gi protein but also other direct or indirect inhibitory pathway(s) which remain to be determined.

Gene usage and regulation of Gsα gene expression in thyroid cells

The TSH receptor is a G-protein-coupled seven transmembrane segment receptor. The interaction between TSH and its receptor mediates signal transduction by activating adenylyl cyclase through Gsα. There are four forms of Gsα (two short [45 kDa] and two large [52 kDa]), arising from alternative splicing of exon 3 of the Gsα gene. Gsα-1 and -2 contain exon 3, whereas exon 3 is spliced out in Gsα-3 and -4. The inclusion of a serine residue at the 3' splice junction of exon 3 distinguishes Gsα-2 and -4 from Gsα-1 and -3. The expression of different Gsα forms appears to be tissue-specific. In this study, we have examined the Gsα splice variants in 26 human thyroid tumor specimens and rat thyroid tissues as well as a rat FRTL-5 cell line. Furthermore, we have studied the regulation of the Gsα gene expression by TSH and cAMP in FRTL-5 cells. We found that Gsα-1 and -4 mRNA were present in both human and rat thyroid cells, although Gsα-4 was more abundant in human thyroid cells as compared to rat thyroid and FRTL-5 cells. The Gsα mRNA can be easily amplified by RT-PCR regardless of tumor type and stage, suggesting that Gsα gene expression in thyroid tumors may not be markedly affected by dedifferentiation of thyroid cells.Both TSH and 8-bromo-cAMP, a cAMP analog, can stimulate the Gsα gene expression in FRTL-5 cells with maximal effect by 6 h and 1 h, respectively. The addition of cycloheximide to the culture of FRTL-5 cells abolished the effect of bTSH, but not that of 8-bromo-cAMP, on the expression of the Gsα gene. Cellular cAMP measurements showed that bTSH-stimulated cAMP production was significantly reduced to the basal level after addition of cycloheximide. These results suggest that regulation of the Gsα gene expression by TSH is mediated by a cAMP-dependent process and requires new protein synthesis.

The short and long forms of the alpha subunit of the stimulatory guanine-nucleotide-binding protein are unequally redistributed during (-)-isoproterenol-mediated desensitization of intact S49 lymphoma cells

We report here that desensitization of the beta-adrenergic receptor-triggered transmembrane signalling in S49 wild-type lymphoma cells, induced by (-)-isoproterenol (1 microM), results in unequal intracellular redistribution of the splicing variants of the alpha subunit of the stimulatory guanine-nucleotide-binding regulatory (Gs alpha) protein (Gs alpha-short and Gs alpha-long) and alters the functional characteristics of the membrane-associated signal transduction complex. We found that two cellular pools of membranes, light-density membranes and plasma membranes prepared by sucrose-density-gradient centrifugation of cell homogenates differed in their content of Gs alpha splicing subforms and, moreover, that prolonged activation of the beta-adrenergic pathway induced intermembrane redistribution of the splicing variants of Gs alpha. Short (10 min) as well as prolonged (1 h) (-)-isoproterenol treatment of the cells shifted Gs alpha-short from light-density membranes to plasma membranes and increased the total amount of light-density membrane-bound Gs alpha-long; in parallel, the maximal (-)-isoproterenol-stimulated or AlF4(-)-stimulated adenylyl cyclase activities measured in the plasma membrane pools prepared from treated cells decreased. The functional characteristics of the membrane-bound Gs alpha pools were examined by a cyc(-)-reconstitutive adenylyl cyclase assay where extracts of the plasma membrane and light-density-membrane pools, respectively, were mixed with plasma membranes derived from the mutant S49 cell line, cyc-, lacking Gs alpha. The maximal cyc(-)-reconstitutive activities of the extracts prepared from light-density membranes of short-term as well as long-term desensitized cells increased compared to control cells. These findings may indicate differences in the functioning of the splicing variants of Gs alpha.