Phosphorylation and inhibition of type III adenylyl cyclase by calmodulin-dependent protein kinase II in vivo

The structure, catalytic mechanism and regulation of adenylyl cyclase

The recent structure determinations of the mammalian effector enzyme adenylyl cyclase reveal the structure of its catalytic core, provide new insights into its catalytic mechanism and suggest how diverse signaling molecules regulate its activity.

Chronic morphine augments adenylyl cyclase phosphorylation: relevance to altered signaling during tolerance/dependence

Despite the demonstration that chronic morphine increases phosphorylation of multiple substrate proteins, their identity has, for the most part, remained elusive. Thus far, chronic morphine has not been shown to increase the phosphorylation of any identified effector protein. This is the first demonstration that persistent activation of opioid receptors has profound effects on phosphorylation of adenylyl cyclase (AC). A dramatic increase in phosphorylation of AC (type II family) was observed in ileum longitudinal muscle myenteric plexus preparations obtained from chronic morphine-treated guinea pigs. Analogous results were obtained when AC was immunoprecipitated using two differentially directed AC antibodies. The magnitude of the augmented AC phosphorylation was substantially attenuated by chelerythrine, a protein kinase C-selective inhibitor. These results suggest the potential relevance of increased phosphorylation (protein kinase C-mediated) of AC to opioid tolerant/dependent mechanisms. Because phosphorylation of AC isoforms (type II family) can significantly increase their stimulatory responsiveness to Gsalpha and Gbetagamma, this mechanism could underlie, in part, the predominance of opioid AC stimulatory signaling observed in opioid tolerant/dependent tissue. Moreover, in light of the fact that many G protein-coupled receptors signal through common effector proteins, this effect provides a mechanism for divergent consequences of chronic morphine treatment and could explain the well documented complexity of changes that accompany the opioid tolerant/dependent state.

Identification of a Gialpha binding site on type V adenylyl cyclase

The stimulatory G protein alpha subunit Gsalpha binds within a cleft in adenylyl cyclase formed by the alpha1-alpha2 and alpha3-beta4 loops of the C2 domain. The pseudosymmetry of the C1 and C2 domains of adenylyl cyclase suggests that the homologous inhibitory alpha subunit Gialpha could bind to the analogous cleft within C1. We demonstrate that myristoylated guanosine 5'-3-O-(thio)triphosphate-Gialpha1 forms a stable complex with the C1 (but not the C2) domain of type V adenylyl cyclase. Mutagenesis of the membrane-bound enzyme identified residues whose alteration either increased or substantially decreased the IC50 for inhibition by Gialpha1. These mutations suggest binding of Gialpha within the cleft formed by the alpha2 and alpha3 helices of C1, analogous to the Gsalpha binding site in C2. Adenylyl cyclase activity reconstituted by mixture of the C1 and C2 domains of type V adenylyl cyclase was also inhibited by Gialpha. The C1b domain of the type V enzyme contributed to affinity for Gialpha, but the source of C2 had little effect. Mutations in this soluble system faithfully reflected the phenotypes observed with the membrane-bound enzyme. The pseudosymmetrical structure of adenylyl cyclase permits bidirectional regulation of activity by homologous G protein alpha subunits.

Neurotensin enhances agonist-induced cAMP accumulation in PC3 cells via Ca2+ -dependent adenylyl cyclase(s)

A human prostate cancer cell line (PC3) with abundant neurotensin (NT) receptors was used to demonstrate that NT potentiated 3',5'-cyclic adenosine monophate (cAMP) accumulation in response to a variety of stimuli, including both direct forskolin (F) and indirect (prostaglandin, (PGE2), isoproterenol (ISO) and cholera toxin (CTx)) activators of adenylyl cyclase. Several mechanisms were investigated and our results indicated an effect on the rate of cAMP formation and not on degradation or extrusion. For each stimulus, NT enhanced efficacy without altering EC50. The effect of NT did not involve stimulatory G-protein (Gs)-activation or interference with a tonic inhibitory G-protein (Gi)-mediated inhibition. A similar response was obtained when NT was added with the stimulus or given as a two minute pulse which was removed prior to addition of stimulus. The potentiating activity disappeared with a t1,2 of approximately 15 min. NT transiently elevated cellular [Ca2+]i and its effects on cAMP could be mimicked by [Ca2+]i-elevating agents (uridine triphosphate (UTP), thapsigargin and ionomycin). Buffering cellular [Ca2+]i with 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM) inhibited cAMP responses to ISO and F in presence and absence of NT. These data support the idea that NT potentiated cAMP formation in response to a variety of stimuli by facilitating the activation of Ca2+ -dependent adenylyl cyclases.

Phosphorylation and inhibition of olfactory adenylyl cyclase by CaM kinase II in Neurons: a mechanism for attenuation of olfactory signals

Acute desensitization of olfactory signaling is a critical property of the olfactory system that allows animals to detect and respond to odorants. Correspondingly, an important feature of odorant-stimulated cAMP increases is their transient nature, a phenomenon that may be attributable to the unique regulatory properties of the olfactory adenylyl cyclase (AC3). AC3 is stimulated by receptor activation and inhibited by Ca2+ through Ca2+/calmodulin kinase II (CaMKII) phosphorylation at Ser-1076. Since odorant-stimulated cAMP increases are accompanied by elevated intracellular Ca2+, CaMKII inhibition of AC3 may contribute to termination of olfactory signaling. To test this hypothesis, we generated a polyclonal antibody specific for AC3 phosphorylated at Ser-1076. A brief exposure of mouse olfactory cilia or primary olfactory neurons to odorants stimulated phosphorylation of AC3 at Ser-1076. This phosphorylation was blocked by inhibitors of CaMKII, which also ablated cAMP decreases associated with odorant-stimulated cAMP transients. These data define a novel mechanism for termination of olfactory signaling that may be important in olfactory responses.

Signal transduction and regulation of lung endothelial cell permeability. Interaction between calcium and cAMP

Pulmonary endothelium forms a semiselective barrier that regulates fluid balance and leukocyte trafficking. During the course of lung inflammation, neurohumoral mediators and oxidants act on endothelial cells to induce intercellular gaps permissive for transudation of proteinaceous fluid from blood into the interstitium. Intracellular signals activated by neurohumoral mediators and oxidants that evoke intercellular gap formation are incompletely understood. Cytosolic Ca2+ concentration ([Ca2+]i) and cAMP are two signals that importantly dictate cell-cell apposition. Although increased [Ca2+]i promotes disruption of the macrovascular endothelial cell barrier, increased cAMP enhances endothelial barrier function. Furthermore, during the course of inflammation, elevated endothelial cell [Ca2+]i decreases cAMP to facilitate intercellular gap formation. Given the significance of both [Ca2+]i and cAMP in mediating cell-cell apposition, this review addresses potential sites of cross talk between these two intracellular signaling pathways. Emerging data also indicate that endothelial cells derived from different vascular sites within the pulmonary circulation exhibit distinct sensitivities to permeability-inducing stimuli; that is, elevated [Ca2+]i promotes macrovascular but not microvascular barrier disruption. Thus this review also considers the roles of [Ca2+]i and cAMP in mediating site-specific alterations in endothelial permeability.

Adenylyl cyclase activity and gene expression during mesodermal differentiation of the P19 embryonal carcinoma cells

DMSO-primed P19 pluripotent cells, which recapitulate the first stages of mammalian cardiogenesis and endodermal formation, were used as an in vitro model to analyze the variations in activity and expression of the different adenylyl cyclase (AC) isoforms during the early events of embryonic cell differentiation. Here, we show that the total AC activity, which increases up to 10-fold after differentiation of P19 cells, is mainly associated with increases in AC2, AC5, and AC6 mRNA levels. Particularly, the marked increase in AC5 mRNA correlates with the appearance of beating cardiomyocytes and with the transcription of the atrial myosin light chain (MLC1A) gene which encodes a protein specifically involved in the cardiac muscle cell contractile phenotype. Together, the results strongly suggest that 1) a rise in cyclic AMP (cAMP) may be associated with cardiomyocyte and endodermal cell differentiation during mammalian embryogenesis; and 2) AC5 gene expression starts very early during normal mouse cardiogenesis and correlates with the differentiation of cardiomyocytes.

Expression of adenylyl cyclase mRNAs in the denervated and in the developing mouse skeletal muscle

Changes in the activity and in the expression of adenylyl cyclase (AC) were examined in mouse skeletal muscle after denervation and during development. Four isoforms of AC (AC2, AC6, AC7, and AC9) were detected by Northern blot analysis in gastrocnemius muscle, AC9 being the most abundant. After denervation, the levels of AC2 and AC9 mRNA decreased, whereas those of AC6 and AC7 increased. AC activity in response to several neurotransmitters was increased after denervation. During development, AC activity was high in fetus and neonate and declined in the adult; the sensitivity of AC activity to various neurotransmitters was the highest on the third postnatal day. The levels of AC6 and AC7 mRNAs were high on the third postnatal day and then decreased in adult, paralleling the decline in AC activity. All the characteristics of AC expression and activity in fetus and neonate resembled those observed in denervated adult muscle. These results indicate that changes in AC activity and AC mRNAs play an important role in the various physiopathological states of skeletal muscle, especially during muscle atrophy.

Cloning, chromosomal mapping, and regulatory properties of the human type 9 adenylyl cyclase (ADCY9)

The type 9 adenylyl cyclase (AC9) is a widely distributed adenylyl cyclase that was originally cloned from a mouse cDNA library. Here we report the cloning, chromosomal mapping, and regulatory properties of human AC9 (HGMW-approved symbol ADCY9). Although the human AC9 sequence shows 86% homology with mouse AC9, divergence at the C2a/C2b junction results in an alternative C2b amino acid sequence. In situ hybridization localized the human AC9 gene to both human and mouse chromosomes 16. AC9 mRNA is present in all tissues examined, with the highest levels found in skeletal muscle, heart, and brain. To characterize the regulatory properties of human AC9 in vivo, the enzyme was expressed in HEK-293 cells. Human AC9 is stimulated by beta-adrenergic receptor activation but is insensitive to forskolin, Ca2+ and somatostatin. In contrast to mouse AC9, the activity of human AC9 is unaffected by inhibitors of calcineurin. These data emphasize the importance of determining the regulatory properties of human adenylyl cyclases.

The olfactory adenylyl cyclase III is expressed in rat germ cells during spermiogenesis

To identify the adenylyl cyclase (AC) genes expressed in mammalian germ cells, RT-PCR of testis and germ cell RNA was performed using degenerated primers based on the homologous region of the AC catalytic domain. This strategy yielded high-frequency amplification of a complementary DNA (cDNA) identical to type III AC (ACIII), a form previously identified as the major adenylyl cyclase expressed in the olfactory system. Ribonuclease protection studies confirmed that ACIII transcripts are present in germ cells, appear during the meiotic prophase, and accumulate during spermiogenesis. A Northern blot analysis performed on total testis RNA demonstrated the presence of a predominant transcript of 7.5 kb, suggesting that the ACIII expressed in germ cells may derive from a splicing variant different from the 4.5 kb transcripts expressed in somatic cells. To determine whether these RNAs are translated into a protein, Western blot analysis was performed using an antibody specific for the carboxyl terminus of ACIII. An immunoreactive protein of 170 kDa was detected in extracts from total testis and from germ cells. Immunofluorescence localization of this protein in the seminiferous tubules showed that ACIII was predominantly expressed in postmeiotic germ cells from round spermatids in the cap phase to maturing elongating spermatids. The ACIII antigen was located mostly on the acrosomal membrane rather than on the plasma membrane of developing spermatids. The spatial and temporal expression of ACIII in germ cells indicates a role of this AC in the acrosome formation. Together with the observation that members of the olfactory receptor family and an olfactory phosphodiesterase are expressed in spermatids, these findings suggest that a signal transduction system used in olfaction is also used during gamete development.

Capacitative Ca2+ entry is involved in cAMP synthesis in mouse parotid acini

Muscarinic receptor interaction leading to augmentation of isoproterenol-stimulated cAMP accumulation in mouse parotid acini involves Ca2+ (28). The effectiveness of capacitative Ca2+ entry and intracellular Ca2+ release on this response was determined in time course studies by using three independent tools to manipulate the free intracellular Ca2+ concentration: the muscarinic agonist carbachol, thapsigargin, and ionomycin. Time course studies revealed that Ca2+ release from intracellular stores by carbachol produced an early rapid increase (0.25-0.5 min) in stimulated cAMP levels, whereas capacitative Ca2+ entry resulted in a sustained increase in stimulated cAMP levels that was blocked by La3+. Capacitative Ca2+ entry, alone, was involved in thapsigargin and ionomycin augmentation of stimulated cAMP accumulation. The inability of phosphodiesterase inhibitors, 3-isobutyl-1-methylxanthine and milrinone, to prevent agonist augmentation of cAMP levels, as well as the finding that the type VIII adenylyl cyclase (ACVIII) is expressed in parotid acini, suggests that capacitative Ca2+ entry augments stimulated cAMP accumulation, at least in part, via activation of this adenylyl cyclase isoenzyme.

Molecular diversity of adenylyl cyclases in human and rat myometrium. Correlation with global adenylyl cyclase activity during mid- and term pregnancy

Expression and regulation of myometrial adenylyl cyclases (AC) were studied during pregnancy. Hybridization of poly(A)+ RNA with specific cDNA probes for enzyme types I-IX indicated 1) the presence of transcripts encoding types II-VI and type IX in rat and human, and type VII in rat and 2) the absence of detectable mRNA for types I and VIII in both species. No substantial change was observed in the amount of specific mRNA and basal AC activity from mid-pregnancy to term. However, activation of the alpha2-adrenergic receptor/Gi protein pathway resulted in potentiation of Gs-stimulated AC activity at mid-pregnancy but not at term (Mhaouty, S., Cohen-Tannoudji, J., Bouet-Alard, R., Limon-Boulez, I., Maltier, J. P., and Legrand, C. (1995) J. Biol. Chem. 270, 11012-11016). We demonstrate in the present work that betagamma scavengers transducin-alpha and QEHA peptide abolished this positive input. On the other hand, increasing submicromolar concentrations of free Ca2+, a situation that mimics late term, reduced the forskolin-stimulated AC activity with an IC50 of 3.9 microM. Thus, the presence in myometrium of AC II family (types II, IV, VII) confers ability to G inhibitory proteins to stimulate enzyme activity via betagamma complexes at mid-pregnancy, whereas expression of AC III, V, and VI isoforms confers to the myometrial AC system a high sensitivity to inhibition by Ca2+-dependent processes at term. These data suggest that in the pregnant myometrium, the expression of different species of AC with distinct regulatory properties provides a mechanism for integrating positively or negatively the responses to various hormonal inputs existing either during pregnancy or in late term.

Modulation by calcium/calmodulin-dependent protein kinase II of functional response of delta opioid receptor in neuroblastoma x glioma hybrid (NG108-15) cells

The potential modulation of opioid receptor signaling by calcium/calmodulin-dependent protein kinase II (CaMKII) has been investigated in NG108-15 cells. Both CaMKII specific inhibitors used, KN62 and KN93, time- and dose-dependently blocked inhibition of cAMP accumulation by [D-Pen2, D-Pen5]enkephalin (DPDPE), with an IC50 of about 1.2 microM and 0.8 microM, respectively. In the presence of 1 microM KN62 or KN93, the DPDPE dose-response curve shifted to the right (IC50 from 0.7 to 20 nM for KN62 and from 0.65 to 10 nM for KN93, respectively), and the maximal response was also significantly reduced. KN92, an inactive analogue of KN93, showed no significant impact, while ionomycin, an activator of CaMKII, greatly potentiated the opioid receptor response, suggesting that the effects of KN62, KN93 and ionomycin were likely mediated through CaMKII. In addition, KN62 did not affect ligand binding, receptor/Gi coupling, or basal and forskolin-stimulated adenylyl cyclase activity, suggesting its possible interference in the Gi/adenylyl cyclase interaction. Furthermore, a CaMKII inhibitor potently blocked the functional responses of other Gi-coupled receptors (m4 muscarinic and alpha2 adrenergic receptors) tested, but not that of Gs-coupled receptors (prostaglandin E1 and adenosine receptors). Our results clearly demonstrate that CaMKII modulates the signaling of opioid receptor and other Gi-coupled receptors.

The endogenous cardiac sarcoplasmic reticulum Ca2+/calmodulin-dependent kinase is activated in response to beta-adrenergic stimulation and becomes Ca2+-independent in intact beating hearts

We investigated the effects of beta-adrenergic stimulation on the activity of the endogenous cardiac sarcoplasmic reticulum Ca2+/calmodulin-dependent protein kinase (SRCaM kinase) in Langendorff-perfused rat hearts. We found that isoproterenol induced generation of autonomous (Ca2+-independent) SRCaM kinase activity to 28 +/- 4.4% of the total activity. Moreover, dephosphorylation of the autonomous SRCaM kinase with protein phosphatase 2A resulted in an enzyme that was again dependent on Ca2+ and calmodulin for its activity. Activation of SRCaM kinase was coupled to phospholamban phosphorylation and activation of the cAMP-signaling system. Our results suggest that the cardiac SRCaM kinase is activated in response to beta-adrenoceptor stimulation. This activation stimulates autophosphorylation at its regulatory domain and converts it to an active Ca2+-independent species that may be the basis for potentiation of Ca2+ transients in the heart.

Calmodulin-regulated adenylyl cyclases and neuromodulation

Coincidence detection and crosstalk between signal transduction systems play very important regulatory roles in the nervous system, particularly in the regulation of transcription. Coupling of the Ca2+ and cAMP regulatory systems by calmodulin-regulated adenylyl cyclases is hypothesized to be important for some forms of synaptic plasticity, neuroendocrine function, and olfactory detection. Recent studies of a mutant mouse deficient in type I calmodulin-sensitive adenylyl cyclase have provided the first evidence that adenylyl cyclases are important for synaptic plasticity, as well as for learning and memory in vertebrates.

Protein kinase C inhibits adenylyl cyclase type VI activity during desensitization of the A2a-adenosine receptor-mediated cAMP response

We have previously reported that phosphorylation of adenylyl cyclase type VI (AC6) may result in the suppression of adenylyl cyclase activity during desensitization of the A2a-adenosine receptor-mediated cAMP response (A2a desensitization) in rat pheochromocytoma PC12 cells. In the present study, we demonstrate that protein kinase C (PKC) is responsible for the phosphorylation and inhibition of AC6 during A2a desensitization. Inhibition of PKC by several independent methods markedly blocked the suppression of AC6 during A2a desensitization. Purified PKC from rat brain directly phosphorylated and inhibited recombinant AC6 expressed in Sf21 cells. Substantially lower AC6 activities were also observed in PC12 cells overexpressing PKCdelta or PKCepsilon. Stimulation of A2a-R in PC12 cells under the same conditions as those required for A2a desensitization resulted in an increase in Ca2+-independent PKC activity. Most importantly, exogenous PKC did not further suppress AC6 activity in A2a-desensitized membranes. In vitro PKC phosphorylation of AC6 isolated from A2a-desensitized cells was also profoundly lower than that from control cells, suggesting a specific role for PKC in regulating AC6 during A2a desensitization in PC12 cells. Taken together, our data demonstrate that a calcium-independent, novel PKC inhibits AC6 activity during A2a desensitization in PC12 cells. Independent regulation of AC6 by calcium-independent PKC and by Ca2+ provides an exquisite mechanism for integrating signaling pathways to fine-tune cAMP synthesis.

Differential regulation of type I and type VIII Ca2+-stimulated adenylyl cyclases by Gi-coupled receptors in vivo

Coupling of intracellular Ca2+ to cAMP increases may be important for some forms of synaptic plasticity. The type I adenylyl cyclase (I-AC) is a neural-specific, Ca2+-stimulated enzyme that couples intracellular Ca2+ to cAMP increases. Since optimal cAMP levels may be crucial for some types of synaptic plasticity, mechanisms for inhibition of Ca2+-stimulated adenylyl cyclases may also be important for neuroplasticity. Here we report that Ca2+ stimulation of I-AC is inhibited by activation of Gi-coupled somatostatin and dopamine D2L receptors. This inhibition is due primarily to Gialpha and not betagamma subunits since coexpression of betagamma-binding proteins with I-AC did not affect somatostatin inhibition. However, betagamma released from Gs did inhibit I-AC, indicating that the enzyme can be inhibited by betagamma in vivo. Interestingly, type VIII adenylyl cyclase (VIII-AC), another Ca2+-stimulated adenylyl cyclase, was not inhibited by Gi-coupled receptors. These data indicate that I-AC and VIII-AC are differentially regulated by Gi-coupled receptors and provide distinct mechanisms for interactions between the Ca2+ and cAMP signal transduction systems. We propose that I-AC may be particularly important for synaptic plasticity that depends upon rapid and transient cAMP increases, whereas VIII-AC may contribute to transcriptional-dependent synaptic plasticity that is dependent upon prolonged, Ca2+-stimulated cAMP increases.