2 Ca2+-sensitive adenylyl cyclases

Involvement of calcium channels in the regulation of adipogenesis

As an important second messenger in adipocytes, calcium ions (Ca²⁺) are essential in regulating various intracellular signalling pathways that control critical cellular functions. Calcium channels show selective permeability to Ca²⁺ and facilitate Ca²⁺ entry into the cytoplasm, which are normally located in the plasmatic and intracellular membranes. The increase of cytosolic Ca²⁺ modulates a variety of signalling pathways and results in the transcription of target genes that contribute to adipogenesis, a key cellular event includes proliferation and differentiation of adipocyte. In the past decades, the involvement of some Ca²⁺-permeable ion channels, such as Ca²⁺ release-activated Ca²⁺ channels, transient receptor potential channels, voltage-gated calcium channels and others, in adipogenesis has been extensively explored. In the present review, we provided a summary of the expression and contributions of these Ca²⁺-permeable channels in mediating Ca²⁺ influxes that drive adipogenesis. Moreover, we discussed their potentials as future therapeutic targets.

Sonic hedgehog enhances calcium oscillations in hippocampal astrocytes

Sonic hedgehog (SHH) is important for organogenesis during development. Recent studies have indicated that SHH is also involved in the proliferation and transformation of astrocytes to the reactive phenotype. However, the mechanisms underlying these are unknown. Involvement of SHH signaling in calcium (Ca) signaling has not been extensively studied. Here, we report that SHH and Smoothened agonist (SAG), an activator of the signaling receptor Smoothened (SMO) in the SHH pathway, activate Ca oscillations in cultured murine hippocampal astrocytes. The response was rapid, on a minute time scale, indicating a noncanonical pathway activity. Pertussis toxin blocked the SAG effect, indicating an involvement of a G_i coupled to SMO. Depletion of extracellular ATP by apyrase, an ATP-degrading enzyme, inhibited the SAG-mediated activation of Ca oscillations. These results indicate that SAG increases extracellular ATP levels by activating ATP release from astrocytes, resulting in Ca oscillation activation. We hypothesize that SHH activates SMO-coupled Gi in astrocytes, causing ATP release and activation of G_q/11-coupled P2 receptors on the same cell or surrounding astrocytes. Transcription factor activities are often modulated by Ca patterns; therefore, SHH signaling may trigger changes in astrocytes by activating Ca oscillations. This enhancement of Ca oscillations by SHH signaling may occur in astrocytes in the brain in vivo because we also observed it in hippocampal brain slices. In summary, SHH and SAG enhance Ca oscillations in hippocampal astrocytes, G_i mediates SAG-induced Ca oscillations downstream of SMO, and ATP-permeable channels may promote the ATP release that activates Ca oscillations in astrocytes.

NMDA Receptor Dependent Long-term Potentiation in Chronic Pain

Since the discovery of NMDA receptor (NMDAR) dependent long-term potentiation (LTP) in the hippocampus, many studies have demonstrated that NMDAR dependent LTP exists throughout central synapses, including those involved in sensory transmission and perception. NMDAR LTP has been reported in spinal cord dorsal horn synapses, anterior cingulate cortex and insular cortex. Behavioral, genetic and pharmacological studies show that inhibiting or reducing NMDAR LTP produced analgesic effects in animal models of chronic pain. Investigation of signalling mechanisms for NMDAR LTP may provide novel targets for future treatment of chronic pain.

Cilia have high cAMP levels that are inhibited by Sonic Hedgehog-regulated calcium dynamics

Protein kinase A (PKA) phosphorylates Gli proteins, acting as a negative regulator of the Hedgehog pathway. PKA was recently detected within the cilium, and PKA activity specifically in cilia regulates Gli processing. Using a cilia-targeted genetically encoded sensor, we found significant basal PKA activity. Using another targeted sensor, we measured basal ciliary cAMP that is fivefold higher than whole-cell cAMP. The elevated basal ciliary cAMP level is a result of adenylyl cyclase 5 and 6 activity that depends on ciliary phosphatidylinositol (3,4,5)-trisphosphate (PIP₃), not stimulatory G protein (Gα_s), signaling. Sonic Hedgehog (SHH) reduces ciliary cAMP levels, inhibits ciliary PKA activity, and increases Gli1. Remarkably, SHH regulation of ciliary cAMP and downstream signals is not dependent on inhibitory G protein (Gα_i/o) signaling but rather Ca²⁺ entry through a Gd³⁺-sensitive channel. Therefore, PIP₃ sustains high basal cAMP that maintains PKA activity in cilia and Gli repression. SHH activates Gli by inhibiting cAMP through a G protein-independent mechanism that requires extracellular Ca²⁺ entry.

Contribution of synaptic plasticity in the insular cortex to chronic pain

Animal and human studies have consistently demonstrated that cortical regions are important for pain perception and pain-related emotional changes. Studies of the anterior cingulate cortex (ACC) have shown that adult cortical synapses can be modified after peripheral injuries, and long-term changes at synaptic level may contribute to long-lasting suffering in patients. It also explains why chronic pain is resistant to conventional analgesics that act by inhibiting synaptic transmission. Insular cortex (IC), another critical cortical area, is found to be highly plastic and can undergo long-term potentiation (LTP) after injury. Inhibiting IC LTP reduces behavioral sensitization caused by injury. LTP of glutamatergic transmission in pain related cortical areas serves as a key mechanism for chronic pain.

Pancreatic Beta Cell G-Protein Coupled Receptors and Second Messenger Interactions: A Systems Biology Computational Analysis

Insulin secretory in pancreatic beta-cells responses to nutrient stimuli and hormonal modulators include multiple messengers and signaling pathways with complex interdependencies. Here we present a computational model that incorporates recent data on glucose metabolism, plasma membrane potential, G-protein-coupled-receptors (GPCR), cytoplasmic and endoplasmic reticulum calcium dynamics, cAMP and phospholipase C pathways that regulate interactions between second messengers in pancreatic beta-cells. The values of key model parameters were inferred from published experimental data. The model gives a reasonable fit to important aspects of experimentally measured metabolic and second messenger concentrations and provides a framework for analyzing the role of metabolic, hormones and neurotransmitters changes on insulin secretion. Our analysis of the dynamic data provides support for the hypothesis that activation of Ca²⁺-dependent adenylyl cyclases play a critical role in modulating the effects of glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and catecholamines. The regulatory properties of adenylyl cyclase isoforms determine fluctuations in cytoplasmic cAMP concentration and reveal a synergistic action of glucose, GLP-1 and GIP on insulin secretion. On the other hand, the regulatory properties of phospholipase C isoforms determine the interaction of glucose, acetylcholine and free fatty acids (FFA) (that act through the FFA receptors) on insulin secretion. We found that a combination of GPCR agonists activating different messenger pathways can stimulate insulin secretion more effectively than a combination of GPCR agonists for a single pathway. This analysis also suggests that the activators of GLP-1, GIP and FFA receptors may have a relatively low risk of hypoglycemia in fasting conditions whereas an activator of muscarinic receptors can increase this risk. This computational analysis demonstrates that study of second messenger pathway interactions will improve understanding of critical regulatory sites, how different GPCRs interact and pharmacological targets for modulating insulin secretion in type 2 diabetes.

Nucleotide-mediated airway clearance

A thin layer of airway surface liquid (ASL) lines the entire surface of the lung and is the first point of contact between the lung and the environment. Surfactants contained within this layer are secreted in the alveolar region and are required to maintain a low surface tension and to prevent alveolar collapse. Mucins are secreted into the ASL throughout the respiratory tract and serve to intercept inhaled pathogens, allergens and toxins. Their removal by mucociliary clearance (MCC) is facilitated by cilia beating and hydration of the ASL by active ion transport. Throughout the lung, secretion, ion transport and cilia beating are under purinergic control. Pulmonary epithelia release ATP into the ASL which acts in an autocrine fashion on P2Y(2) (ATP) receptors. The enzymatic network describes in Chap. 2 then mounts a secondary wave of signaling by surface conversion of ATP into adenosine (ADO), which induces A(2B) (ADO) receptor-mediated responses. This chapter offers a comprehensive description of MCC and the extensive ramifications of the purinergic signaling network on pulmonary surfaces.

Levels of Gsα(short and long), Gα(olf) and Gβ(common) subunits, and calcium-sensitive adenylyl cyclase isoforms (1, 5/6, 8) in post-mortem human brain caudate and cortical membranes: comparison with rat brain membranes and potential stoichiometric relationships

The levels of expression of Gsα(short and long), Gα(olf) and Gβ(common) subunits, and calcium-sensitive adenylyl cyclases isoforms (AC1, 5/6, and 8) in human brain cortical and caudate membranes were quantified by western blot analysis in order to establish their contribution to the patterns of AC functioning. Both areas expressed Gsα(long) (52 kDa) with values ranging from about 1400 ng/mg of membrane protein in cerebral cortex to close to 600 ng/mg of membrane protein in caudate nucleus. In contrast, Gsα(short) and Gsα(olf) were expressed separately, Gsα(short) in cortical membranes with values around 500 ng/mg of membrane protein and Gα(olf) in caudate membranes with values around 1300 ng/mg of membrane protein. Quantitative measurements of Gβ, revealed a similar expression level in cortical and caudate membranes (5444±732 versus 5511±394 ng/mg protein; p=0.966). The B(max) values of GTPγS-dependent [(3)H]-forskolin binding show the following descending order: rat striatal membranes>rat cortical membranes=human caudate membranes>human cortical membranes. Therefore, as measured immunochemically and by [(3)H]-forskolin binding, there seems to be a vast excess of Gsα subunits over catalytic units of AC. The highest levels of AC5/6 expression were detected in caudate membranes. AC8 was little expressed, and there were no significant differences in the relative values between both human brain regions. Finally, the levels of the AC1 isoform were significantly lower in caudate than in cortical membranes. It is concluded that these stoichiometric data contribute nonetheless to explain the significant differences observed in signalling capacities through the AC system in both human brain regions.

alpha-Tocopherol decreases the somatostatin receptor-effector system and increases the cyclic AMP/cyclic AMP response element binding protein pathway in the rat dentate gyrus

Neuronal survival has been shown to be enhanced by alpha-tocopherol and modulated by cyclic AMP (cAMP). Somatostatin (SST) receptors couple negatively to adenylyl cyclase (AC), thus leading to decreased cAMP levels. Whether alpha-tocopherol can stimulate neuronal survival via regulation of the somatostatinergic system, however, is unknown. The aim of this study was to investigate the effects of alpha-tocopherol on the SST signaling pathway in the rat dentate gyrus. To that end, 15-week-old male Sprague-Dawley rats were treated daily for 1 week with (+)-alpha-tocopherol or vehicle and sacrificed on the day following the last administration. No changes in either SST-like immunoreactivity (SST-LI) content or SST mRNA levels were detected in the dentate gyrus as a result of alpha-tocopherol treatment. A significant decrease in the density of the SST binding sites and an increase in the dissociation constant, however, were detected. The lower SST receptor density in the alpha-tocopherol-treated rats correlated with a significant decrease in the protein levels of the SST receptor subtypes SSTR1-SSTR4, whereas the corresponding mRNA levels were unaltered. G-protein-coupled-receptor kinase 2 expression was decreased by alpha-tocopherol treatment. This vitamin induced a significant increase in both basal and forskolin-stimulated AC activity, as well as a decrease in the inhibitory effect of SST on AC. Whereas the protein levels of AC type V/VI were not modified by alpha-tocopherol administration, ACVIII expression was significantly enhanced, suggesting it might account for the increase in AC activity. In addition, this treatment led to a reduction in Gialpha1-3 protein levels and in Gi functionality. alpha-Tocopherol did not affect the expression of the regulator of G-protein signaling 6/7 (RGS6/7). Finally, alpha-tocopherol induced an increase in the levels of phosphorylated cAMP response element binding protein (p-CREB) and total CREB in the dentate gyrus. Since CREB synthesis and phosphorylation promote the survival of many cells, including neurons, whereas SST inhibits the cAMP-PKA pathway, which is known to be involved in CREB phosphorylation, the alpha-tocopherol-induced reduction of SSTR observed here might possibly contribute, via increased cAMP levels and CREB activity, to the mechanism by which this vitamin promotes the survival of newborn neurons in the dentate gyrus.

Structural basis for inhibition of mammalian adenylyl cyclase by calcium

Type V and VI mammalian adenylyl cyclases (AC5, AC6) are inhibited by Ca(2+) at both sub- and supramicromolar concentration. This inhibition may provide feedback in situations where cAMP promotes opening of Ca(2+) channels, allowing fine control of cardiac contraction and rhythmicity in cardiac tissue where AC5 and AC6 predominate. Ca(2+) inhibits the soluble AC core composed of the C1 domain of AC5 (VC1) and the C2 domain of AC2 (IIC2). As observed for holo-AC5, inhibition is biphasic, showing "high-affinity" (K(i) = approximately 0.4 microM) and "low-affinity" (K(i) = approximately 100 microM) modes of inhibition. At micromolar concentration, Ca(2+) inhibition is nonexclusive with respect to pyrophosphate (PP(i)), a noncompetitive inhibitor with respect to ATP, but at >100 microM Ca(2+), inhibition appears to be exclusive with respect to PP(i). The 3.0 A resolution structure of Galphas.GTPgammaS/forskolin-activated VC1:IIC2 crystals soaked in the presence of ATPalphaS and 8 microM free Ca(2+) contains a single, loosely coordinated metal ion. ATP soaked into VC1:IIC2 crystals in the presence of 1.5 mM Ca(2+) is not cyclized, and two calcium ions are observed in the 2.9 A resolution structure of the complex. In both of the latter complexes VC1:IIC2 adopts the "open", catalytically inactive conformation characteristic of the apoenzyme, in contrast to the "closed", active conformation seen in the presence of ATP analogues and Mg(2+) or Mn(2+). Structures of the pyrophosphate (PP(i)) complex with 10 mM Mg(2+) (2.8 A) or 2 mM Ca(2+) (2.7 A) also adopt the open conformation, indicating that the closed to open transition occurs after cAMP release. In the latter complexes, Ca(2+) and Mg(2+) bind only to the high-affinity "B" metal site associated with substrate/product stabilization. Ca(2+) thus stabilizes the inactive conformation in both ATP- and PP(i)-bound states.

Insights into the residence in lipid rafts of adenylyl cyclase AC8 and its regulation by capacitative calcium entry

Adenylyl cyclases (ACs) are a family of critically important signaling molecules that are regulated by multiple pathways. Adenylyl cyclase 8 (AC8) is a Ca(2+) stimulated isoform that displays a selective regulation by capacitative Ca(2+) entry (CCE), the process whereby the entry of Ca(2+) into cells is triggered by the emptying of intracellular stores. This selectivity was believed to be achieved through the localization of AC8 in lipid raft microdomains, along with components of the CCE apparatus. In the present study, we show that an intact leucine zipper motif is required for the efficient N-linked glycosylation of AC8, and that this N-linked glycosylation is important to target AC8 into lipid rafts. Disruption of the leucine zipper by site-directed mutagenesis results in the elimination of N-glycosylated forms and their exclusion from lipid rafts. Mutants of AC8 that cannot be N-glycosylated are not demonstrably associated with rafts, although they can still be regulated by CCE; however, raft integrity is required for the regulation of these mutants. These findings suggest that raft localized proteins in addition to AC8 are needed to mediate its regulation by CCE.

Stim1, an endoplasmic reticulum Ca2+ sensor, negatively regulates 3T3-L1 pre-adipocyte differentiation

Ca(2+) plays a complex role in the differentiation of committed pre-adipocytes into mature, fat laden adipocytes. Stim1 is a single pass transmembrane protein that has an essential role in regulating the influx of Ca(2+) ions through specific plasma membrane store-operated Ca(2+) channels. Stim1 is a sensor of endoplasmic reticulum Ca(2+) store content and when these stores are depleted ER-localized Stim1 interacts with molecular components of store-operated Ca(2+) channels in the plasma membrane to activate these channels and induce Ca(2+) influx. To investigate the potential role of Stim1 in Ca(2+)-mediated adipogenesis, we investigated the expression of Stim1 during adipocyte differentiation and the effects of altering Stim1 expression on the differentiation process. Western blotting revealed that Stim1 was expressed at low levels in 3T3-L1 pre-adipocytes and was upregulated 4 days following induction of differentiation. However, overexpression of Stim1 potently inhibited their ability to differentiate and accumulate lipid, and reduced the expression of C/EBP alpha and adiponectin. Stim1-mediated differentiation was shown to be dependent on store-operated Ca(2+) entry, which was increased upon overexpression of Stim1. Overexpression of Stim1 did not disrupt cell proliferation, mitotic clonal expansion or subsequent growth arrest. siRNA-mediated knockdown of endogenous Stim1 had the opposite effect, with increased 3T3-L1 differentiation and increased expression of C/EBP alpha and adiponectin. We thus demonstrate for the first time the presence of store-operated Ca(2+) entry in 3T3-L1 adipocytes, and that Stim1-mediated Ca(2+) entry negatively regulates adipocyte differentiation. We suggest that increased expression of Stim1 during 3T3-L1 differentiation may act, through its ability to modify the level of Ca(2+) influx through store-operated channels, to balance the level of differentiation in these cells in vitro.

TRPC channels and store-operated Ca(2+) entry

Store-operated calcium entry (SOCE) is a major mechanism for Ca(2+) influx. Since SOCE was first proposed two decades ago many techniques have been used in attempting to identify the nature of store-operated Ca(2+) (SOC) channels. The first identified and best-characterised store-operated current is I(CRAC), but a number of other currents activated by Ca(2+) store depletion have also been described. TRPC proteins have long been proposed as SOC channel candidates; however, whether any of the TRPCs function as SOC channels remains controversial. This review attempts to provide an overview of the arguments in favour and against the role of TRPC proteins in the store-operated mechanisms of agonist-activated Ca(2+) entry.

Heterogeneity of acid secretion induced by carbachol and histamine along the gastric gland axis and its relationship to [Ca2+]i

The gastric glands of the mammalian fundic mucosa are constituted by different cell types. Gastric fluid is a mixture of acid, alkali, ions, enzymes, and mucins secreted by parietal, chief, and mucous cells. We studied activation of acid secretion using LysoSensor Yellow/Blue in conjunction with fluo 3 to measure changes in pH and Ca(2+) in isolated rabbit gastric glands. We evidenced a spatial heterogeneity in the amplitude of acid response along the gland axis under histamine and cholinergic stimulation. Carbachol induced a transitory pH increase before acidification. This relative alkalinization may be related to granule release from other cell types. Omeprazole inhibited the acid component but not the rise in pH. Histamine stimulated acid secretion without increase of lumen pH. We studied the relationship between Ca(2+) release and/or entry and H(+) secretion in glands stimulated by carbachol. Ca(2+) release was associated with a fast and transient components of H(+) secretion. We found a linear relationship between Ca(2+) release and H(+) secretion. Ca(2+) entry was associated with a second slow and larger component of acid secretion. The fast component may be the result of activation of Cl(-) and K(+) channels and hence H(+)/K(+) pumps already present in the membrane, whereas the slow component might be associated with translocation of H(+)/K(+) pumps to the canaliculi. In conclusion, with cholinergic stimulation, gastric glands secrete a mixture of acid and other product(s) with a pH above 4.2, both triggered by Ca(2+) release. Maintenance of acid secretion depends on Ca(2+) entry and perhaps membrane fusion.

Functional consequences of activating store-operated CRAC channels

Store-operated CRAC channels, which are activated by the emptying of the endoplasmic reticulum Ca(2+) stores, are an important and widespread route for triggering rises in cytoplasmic Ca(2+). The cellular responses that are activated in response to Ca(2+) entry through CRAC channels are being dissected out, and recent evidence has established that CRAC channels can induce both short-term (safeguarding the Ca(2+) content of the endoplasmic reticulum, maintenance of cytoplasmic Ca(2+) oscillations, enzyme activation, secretion) and long-term (gene expression) changes in cells. CRAC channel activation is therefore capable of evoking a range of temporally distinct responses, highlighting the versatility of this ubiquitous Ca(2+) entry pathway.

Calcium-mediated, purinergic stimulation and polarized localization of calcium-sensitive adenylyl cyclase isoforms in human airway epithelia

Purinergic stimulation of human airway epithelia results in a prolonged increase in ciliary beat frequency that depends on calcium-mediated cAMP production [Lieb, T., Wijkstrom Frei, C., Frohock, J.I., Bookman, R.J. and Salathe, M. (2002) Prolonged increase in ciliary beat frequency after short-term purinergic stimulation in human airway epithelial cells. J. Physiol. (Lond.) 538, 633-646]. Here, fully differentiated human airway epithelial cells in culture are shown to express calcium-stimulated transmembrane adenylyl cyclase (tmAC) isoforms (types 1, 3, and 8) by reverse transcription polymerase chain reaction. Immunohistochemistry of tracheal sections and fully differentiated airway epithelial cell cultures revealed polarized expression of these tmACs, with types 1 and 8 localized to the apical membrane and thus at the position required for ciliary regulation. Real-time, ciliated-cell specific cAMP production by tmACs upon apical, purinergic stimulation with UTP was confirmed using fluorescent energy resonance transfer between fluorescently tagged PKA subunits.

Higher-order organization and regulation of adenylyl cyclases

There is increasing awareness of the compartmentalization of cAMP signalling--the means by which cAMP levels change in discrete domains of the cell with discrete local consequences. Current developments in understanding the organization of adenylyl cyclases in the plasma membrane are illuminating how the earliest part of cAMP compartmentalization could occur. This review focuses on recent findings regarding three levels of adenylyl cyclase organization--oligomerization, positioning to lipid rafts and participation in multiprotein signalling complexes. This organization, coupled with the role of scaffolding proteins in arranging the downstream effectors of cAMP, helps to identify complexes that greatly facilitate the translation of enzyme activation into local consequences.

Store-Operated Calcium Channels

In electrically nonexcitable cells, Ca2+influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and proliferation, and apoptosis. The major Ca2+entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+stores activates Ca2+influx (store-operated Ca2+entry, or capacitative Ca2+entry). Several biophysically distinct store-operated currents have been reported, but the best characterized is the Ca2+release-activated Ca2+current, ICRAC. Although it was initially considered to function only in nonexcitable cells, growing evidence now points towards a central role for ICRAC-like currents in excitable cells too. In spite of intense research, the signal that relays the store Ca2+content to CRAC channels in the plasma membrane, as well as the molecular identity of the Ca2+sensor within the stores, remains elusive. Resolution of these issues would be greatly helped by the identification of the CRAC channel gene. In some systems, evidence suggests that store-operated channels might be related to TRP homologs, although no consensus has yet been reached. Better understood are mechanisms that inactivate store-operated entry and hence control the overall duration of Ca2+entry. Recent work has revealed a central role for mitochondria in the regulation of ICRAC, and this is particularly prominent under physiological conditions. ICRACtherefore represents a dynamic interplay between endoplasmic reticulum, mitochondria, and plasma membrane. In this review, we describe the key electrophysiological features of ICRACand other store-operated Ca2+currents and how they are regulated, and we consider recent advances that have shed insight into the molecular mechanisms involved in this ubiquitous and vital Ca2+entry pathway.

The cytosolic domains of Ca2+-sensitive adenylyl cyclases dictate their targeting to plasma membrane lipid rafts

Lipid rafts are specialized, cholesterol-rich domains of the plasma membrane that are enriched in certain signaling proteins, including Ca(2+)-sensitive adenylyl cyclases. This restrictive localization plays a key role in the regulation of the Ca(2+)-stimulable AC8 and the Ca(2+)-inhibitable AC6 by capacitative calcium entry. Interestingly, AC7, a Ca(2+)-insensitive AC, is found in the plasma membrane but is excluded from lipid rafts (Smith, K. E., Gu, C., Fagan, K. A., Hu, B., and Cooper, D. M. F. (2002) J. Biol. Chem. 277, 6025-6031). The mechanisms governing the specific membrane targeting of adenylyl cyclase isoforms remain unknown. To address this issue, a series of chimeras were produced between the raft-targeted AC5 and the non-raft-targeted AC7, involving switching of their major domains. The AC5-AC7 chimeras were expressed in HEK 293 cells and lipid rafts were isolated from the bulk plasma membrane by either detergent-based or non-detergent-based fractionation methods. Additionally, confocal imaging was used to investigate the precise cellular targeting of the chimeras. Surprisingly, the two tandem six-transmembrane domains of AC5 were not required for localization to lipid rafts. Rather, AC5 localization depended on the complete cytoplasmic loops (C1 and C2); constructs with mixed domains were either retained in the endoplasmic reticulum or degraded. Similar conclusions are drawn for the lipid raft localization of the Ca(2+)/calmodulin-stimulable AC8; again, the C1 and C2 domains are critical. Thus, protein-protein interactions may be more important than protein-lipid interactions in targeting these calcium-sensitive enzymes to lipid rafts.

Effects of age and spatial learning on adenylyl cyclase mRNA expression in the mouse hippocampus

Adenylyl cyclase (AC) subtypes have been implicated in memory processes and synaptic plasticity. In the present study, the effects of aging and learning on Ca2+/calmodulin-stimulable AC1, Ca2+-insensitive AC2 and Ca2+/calcineurin-inhibited AC9 mRNA level were compared in the dorsal hippocampus of young-adult and aged C57BL/6 mice using in situ hybridization. Both AC1 and AC9 mRNA expression were downregulated in aged hippocampus, whereas AC2 mRNA remained unchanged, suggesting differential sensitivities to the aging process. We next examined AC mRNA expression in the hippocampus after spatial learning in the Morris water maze. Acquisition of the spatial task was associated with an increase of AC1 and AC9 mRNA levels in both young-adult and aged groups, suggesting that Ca2+-sensitive ACs are oppositely regulated by aging and learning. However, aged-trained mice had reduced AC1 and AC9, but greater AC2, mRNA levels relative to young-trained mice and age-related learning impairments were correlated with reduced AC1 expression in area CA1. We suggest that reduced levels of hippocampal AC1 mRNA may greatly contribute to age-related defects in spatial memory.

The class III adenylyl cyclases: multi-purpose signalling modules

cAMP serves as a second messenger in virtually all organisms. The most wide-spread class of cAMP-generating enzymes are the class III adenylyl cyclases. Most class III adenylyl cyclases are multi-domain proteins. The catalytic domains exclusively work as dimers, catalysis proceeds at the dimer interface, so that both monomers provide catalytic residues to each catalytic center. Inspection of amino acid sequence profiles suggests a division of the class III adenylyl cyclases in to four subclasses, class IIIa-IIId. Genome projects and postgenomic analysis have provided novel aspects in terms of catalysis and regulation. Alterations in the canonical catalytic residues occur in all four subclasses suggesting a plasticity of the catalytic mechanisms. The vast variety of additional, probably regulatory modules found in class III adenylyl cyclases obviously reflects a large collection of regulatory inputs the catalytic domains have adapted to. The large versatility of class III adenylyl cyclase catalytic domains remains a major scientific challenge.

Kinetic properties of "soluble" adenylyl cyclase. Synergism between calcium and bicarbonate

"Soluble" adenylyl cyclase (sAC) is a widely expressed source of cAMP in mammalian cells that is evolutionarily, structurally, and biochemically distinct from the G protein-responsive transmembrane adenylyl cyclases. In contrast to transmembrane adenylyl cyclases, sAC is insensitive to heterotrimeric G protein regulation and forskolin stimulation and is uniquely modulated by bicarbonate ions. Here we present the first report detailing kinetic analysis and biochemical properties of purified recombinant sAC. We confirm that bicarbonate regulation is conserved among mammalian sAC orthologs and demonstrate that bicarbonate stimulation is consistent with an increase in the V(max) of the enzyme with little effect on the apparent K(m) for substrate, ATP-Mg(2+). Bicarbonate can further increase sAC activity by relieving substrate inhibition. We also identify calcium as a direct modulator of sAC activity. In contrast to bicarbonate, calcium stimulates sAC activity by decreasing its apparent K(m) for ATP-Mg(2+). Because of their different mechanisms, calcium and bicarbonate synergistically activate sAC; therefore, small changes of either calcium or bicarbonate will lead to significant changes in cellular cAMP levels.

Short-term plasticity of cyclic adenosine 3',5'-monophosphate signaling in anterior pituitary corticotrope cells: the role of adenylyl cyclase isotypes

Anterior pituitary corticotropes show a wide repertory of responses to hypothalamic neuropeptides and adrenal corticosteroids. The hypothesis that plasticity of the cAMP signaling system underlies this adaptive versatility was investigated. In dispersed rat anterior pituitary cells, depletion of intracellular Ca2+ stores with thapsigargin combined with ryanodine or caffeine enhanced the corticotropin releasing-factor (CRF)-evoked cAMP response by 4-fold, whereas reduction of Ca2+ entry alone had no effect. CRF-induced cAMP was amplified 15-fold by arginine-vasopressin (AVP) or phorbol-dibutyrate ester. In the presence of inhibitors of cyclic nucleotide phosphodiesterases and phorbol-dibutyrate ester, the depletion of Ca2+ stores had no further effect on CRF-induced cAMP accumulation. Adenohypophysial expression of mRNAs for the Ca2+-inhibited adenylyl cyclases (ACs) VI and IX, and the protein kinase C-stimulated ACs II and VII was demonstrated. ACIX was detected in corticotropes by immunocytochemistry, whereas ACII and ACVI were not present. The data show negative feedback regulation of CRF-induced cAMP levels by Ca2+ derived from ryanodine receptor-operated intracellular stores. Stimulation of protein kinase C by AVP enhances Ca2+-independent cAMP synthesis, thus changing the characteristics of intracellular Ca2+ feedback. It is proposed that the modulation of intracellular Ca2+ feedback in corticotropes by AVP is an important element of physiological control.

A critical interplay between Ca2+ inhibition and activation by Mg2+ of AC5 revealed by mutants and chimeric constructs

Adenylyl cyclase type 5 (AC5) is sensitive to both high and low affinity inhibition by Ca(2+). This property provides a sensitive feedback mechanism of the Ca(2+) entry that is potentiated by cAMP in sources where AC5 is commonly expressed (e.g. myocardium). Remarkably little is known about the molecular mechanism whereby Ca(2+) inhibits AC5. Because previous studies had showed that Ca(2+) antagonized the activation of adenylyl cyclase brought about by Mg(2+), we have now evaluated the Mg(2+)-binding domain in the catalytic site as the potential site of the interaction, using a number of mutations of AC5 with impaired Mg(2+) activation. Mg(2+) activation exerted contrasting effects on the high and low affinity Ca(2+) inhibition. In both wild type and mutants, activation by Mg(2+) decreased the absolute amount of high affinity inhibition without affecting the K(i) value, whereas the K(i) value for low affinity inhibition was decreased. These effects were directly proportional to the sensitivity of the mutants to Mg(2+). Parallel changes were noted in the efficacies of Ca(2+), Sr(2+), and Ba(2+) in the mutant species, suggesting a simple mutation in a shared domain. Strikingly, forskolin, which activates by a mechanism different from Mg(2+), did not modify inhibition by Ca(2+). Deletion of the N terminus and the C1b domain of AC5 and a chimera formed with AC2 confirmed that the catalytic domain alone was responsible for high affinity inhibition. We therefore conclude that both low and high affinity inhibition by Ca(2+) are exerted on different conformations of the Mg(2+)-binding sites in the catalytic domain of AC5.

Reciprocal regulation of calcium dependent and calcium independent cyclic AMP hydrolysis by protein phosphorylation

The hydrolysis of cyclic nucleotide second messengers takes place through multiple cyclic nucleotide phosphodiesterases (PDEs). The significance of this diversification is not fully understood. Here we report the differential regulation of low K(m) Ca2+-activated (PDE1C) and Ca2+-independent, rolipram-sensitive (PDE4) PDEs by protein phosphorylation in the neuroendocrine cell line AtT20. Incubation of cells with 8-(4-chlorophenylthio)-cyclic AMP (CPT-cAMP) enhanced PDE4 and reduced PDE1C activity. These effects were blocked by H89 indicating mediation by cAMP-dependent protein kinase (PKA), furthermore in broken cell preparations PKA produced the same reciprocal changes of PDE activities. Calyculin A, an inhibitor of protein phosphatases 1 and 2 A, stimulated PDE4 and enhanced the inhibitory effect of CPT-cAMP on PDE1C. The reduction of PDE1C activity was characterized by a marked attenuation of the activation by Ca2+/calmodulin. Stimulation of PDE4 activity by CPT-cAMP or calyculin A was attributable to PDE4D3 and these effects could also be reproduced in human embryonic kidney cells expressing epitope-tagged PDE4D3. Together, these data show reciprocal regulation of PDE1C and PDE4D by PKA, which represents a novel scheme for plasticity in intracellular signalling.

Dimerization of mammalian adenylate cyclases

Mammalian adenylate cyclases are predicted to possess complex topologies, comprising two cassettes of six transmembrane-spanning motifs followed by a cytosolic, catalytic ATP-binding domain. Recent studies have begun to provide insights on the tertiary assembly of these proteins; crystallographic analysis has revealed that the two cytosolic domains dimerize to form a catalytic core, while more recent biochemical and cell biological analysis shows that the two transmembrane cassettes also associate to facilitate the functional assembly and trafficking of the enzyme. The older literature had suggested that adenylate cyclases might form higher order aggregates, although the methods used did not necessarily provide convincing evidence of biologically relevant events. In the present study, we have pursued this question by a variety of approaches, including rescue or suppression of function by variously modified molecules, coimmunoprecipitation and fluorescence resonance energy transfer (FRET) analysis between molecules in living cells. The results strongly suggest that adenylate cyclases dimerize (or oligomerize) via their hydrophobic domains. It is speculated that this divalent property may allow adenylate cyclases to participate in multimeric signaling assemblies.

In vivo assessment of local phosphodiesterase activity using tailored cyclic nucleotide-gated channels as cAMP sensors

Phosphodiesterases (PDEs) catalyze the hydrolysis of the second messengers cAMP and cGMP. However, little is known about how PDE activity regulates cyclic nucleotide signals in vivo because, outside of specialized cells, there are few methods with the appropriate spatial and temporal resolution to measure cyclic nucleotide concentrations. We have previously demonstrated that adenovirus-expressed, olfactory cyclic nucleotide-gated channels provide real-time sensors for cAMP produced in subcellular compartments of restricted diffusion near the plasma membrane (Rich, T.C., K.A. Fagan, H. Nakata, J. Schaack, D.M.F. Cooper, and J.W. Karpen. 2000. J. Gen. Physiol. 116:147-161). To increase the utility of this method, we have modified the channel, increasing both its cAMP sensitivity and specificity, as well as removing regulation by Ca(2)+-calmodulin. We verified the increased sensitivity of these constructs in excised membrane patches, and in vivo by monitoring cAMP-induced Ca(2)+ influx through the channels in cell populations. The improved cAMP sensors were used to monitor changes in local cAMP concentration induced by adenylyl cyclase activators in the presence and absence of PDE inhibitors. This approach allowed us to identify localized PDE types in both nonexcitable HEK-293 and excitable GH4C1 cells. We have also developed a quantitative framework for estimating the K(I) of PDE inhibitors in vivo. The results indicate that PDE type IV regulates local cAMP levels in HEK-293 cells. In GH4C1 cells, inhibitors specific to PDE types I and IV increased local cAMP levels. The results suggest that in these cells PDE type IV has a high K(m) for cAMP, whereas PDE type I has a low K(m) for cAMP. Furthermore, in GH4C1 cells, basal adenylyl cyclase activity was readily observable after application of PDE type I inhibitors, indicating that there is a constant synthesis and hydrolysis of cAMP in subcellular compartments near the plasma membrane. Modulation of constitutively active adenylyl cyclase and PDE would allow for rapid control of cAMP-regulated processes such as cellular excitability.

Regulation and role of adenylyl cyclase isoforms

At least nine closely related isoforms of adenylyl cyclases (ACs), the enzymes responsible for the synthesis of cyclic AMP (cAMP) from ATP, have been cloned and characterized in mammals. Depending on the properties and the relative levels of the isoforms expressed in a tissue or a cell type at a specific time, extracellular signals received through the G-protein-coupled receptors can be differentially integrated. The present review deals with various aspects of such regulations, emphasizing the role of calcium/calmodulin in activating AC1 and AC8 in the central nervous system, the potential inhibitory effect of calcium on AC5 and AC6, and the changes in the expression pattern of the isoforms during development. A particular emphasis is given to the role of cAMP during drug and ethanol dependency and to some experimental limitations (pitfalls in the interpretation of cellular transfection, scarcity of the invalidation models, existence of complex macromolecular structures, etc).

Persistent interactions between the two transmembrane clusters dictate the targeting and functional assembly of adenylyl cyclase

Adenylyl cyclases possess complex structures like those of the ATP binding cassette (ABC) transporter family, which includes the cystic fibrosis transmembrane regulator, the P-glycoprotein, and ATP-sensitive K(+) channels [1-4]. These structures comprise a cytosolic N terminus followed by two tandem six-transmembrane cassettes, each associated with a highly homologous (ATP binding) cytosolic loop [5-8]. The catalytic domains, which are located in the two large cytoplasmic loops, are highly conserved and well studied. The crystal structure of these domains has even been described recently [9, 10]. However, nothing is known of the function or organization of the 12 transmembrane segments. In the present study we adopted a range of strategies including live-cell fluorescence resonance energy transfer (FRET) microscopy, coimmunoprecipitation, and functional assays of various truncated and substituted, fluorescently-tagged molecules to analyze the trafficking and activity of this molecule. When expressed as individual peptides, the two transmembrane domains - largely independently of any cytosolic region - formed a tight complex that was delivered to the plasma membrane. This cooperation between the two intact transmembrane domains was essential and sufficient to target the enzyme to the plasma membrane of the cell. The extracellular loop between the ninth and tenth transmembrane segments, which contains an N-glycosylation site, was also necessary. Furthermore, the interaction between the two transmembrane clusters played a critical role in bringing together the cytosolic catalytic domains to express functional adenylyl cyclase activity in the intact cell.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

Characterisation of human adenylyl cyclase IX reveals inhibition by Ca(2+)/Calcineurin and differential mRNA plyadenylation

The functional diversity of adenylyl cyclases provides for different modes of cyclic AMP signalling in mammals. This study reports the cloning and functional characterisation of a cDNA encoding human adenylyl cyclase IX (ACIX). The data show that human ACIX is a Ca(2+)/calcineurin-inhibited adenylyl cyclase prominently expressed in vital organs, including brain, heart, and pancreas. ACIX mRNA was detected in several brain regions, including neocortex, hippocampus, striatum, and cerebellum. By in situ hybridisation, ACIX mRNA was localised to pyramidal and granule cells of the hippocampus, indicating that it is expressed predominantly in nerve cells. Further analysis of ACIX mRNA expression revealed two major forms of ACIX mRNA that arose through tissue-specific differential mRNA polyadenylation. Taken together, the data show that (a) human ACIX is under inhibitory control by Ca(2+) through calcineurin, (b) ACIX may be involved in higher brain functions, and (c) post-transcriptional regulation of ACIX gene expression is a species-specific control mechanism that may enhance the versatility of cyclic AMP signalling in humans.

Tissue specificity and physiological relevance of various isoforms of adenylyl cyclase

The present review focuses on the potential physiological regulations involving different isoforms of adenylyl cyclase (AC), the enzymatic activity responsible for the synthesis of cAMP from ATP. Depending on the properties and the relative level of the isoforms expressed in a tissue or a cell type at a specific time, extracellular signals received by the G protein-coupled receptors can be differently integrated. We report here on various aspects of such regulations, emphasizing the role of Ca(2+)/calmodulin in activating AC1 and AC8 in the central nervous system, the potential inhibitory effect of Ca(2+) on AC5 and AC6, and the changes in the expression pattern of the isoforms during development. A particular emphasis is given to the role of cAMP during drug dependence. Present experimental limitations are also underlined (pitfalls in the interpretation of cellular transfection, scarcity of the invalidation models, and so on).

Cyclic nucleotide analogs as biochemical tools and prospective drugs

Cyclic AMP (cAMP) and cyclic GMP (cGMP) are key second messengers involved in a multitude of cellular events. From the wealth of synthetic analogs of cAMP and cGMP, only a few have been explored with regard to their therapeutic potential. Some of the first-generation cyclic nucleotide analogs were promising enough to be tested as drugs, for instance N(6),O(2)'-dibutyryl-cAMP and 8-chloro-cAMP (currently in clinical Phase II trials as an anticancer agent). Moreover, 8-bromo and dibutyryl analogs of cAMP and cGMP have become standard tools for investigations of biochemical and physiological signal transduction pathways. The discovery of the Rp-diastereomers of adenosine 3',5'-cyclic monophosphorothioate and guanosine 3',5'-cyclic monophosphorothioate as competitive inhibitors of cAMP- and cGMP-dependent protein kinases, as well as subsequent development of related analogs, has proven very useful for studying the molecular basis of signal transduction. These analogs exhibit a higher membrane permeability, increased resistance against degradation, and improved target specificity. Furthermore, better understanding of signaling pathways and ligand/protein interactions has led to new therapeutic strategies. For instance, Rp-8-bromo-adenosine 3',5'-cyclic monophosphorothioate is employed against diseases of the immune system. This review will focus mainly on recent developments in cyclic nucleotide-related biochemical and pharmacological research, but also highlights some historical findings in the field.

Cyclic nucleotide-gated channels colocalize with adenylyl cyclase in regions of restricted cAMP diffusion

Cyclic AMP is a ubiquitous second messenger that coordinates diverse cellular functions. Current methods for measuring cAMP lack both temporal and spatial resolution, leading to the pervasive notion that, unlike Ca(2+), cAMP signals are simple and contain little information. Here we show the development of adenovirus-expressed cyclic nucleotide-gated channels as sensors for cAMP. Homomultimeric channels composed of the olfactory alpha subunit responded rapidly to jumps in cAMP concentration, and their cAMP sensitivity was measured to calibrate the sensor for intracellular measurements. We used these channels to detect cAMP, produced by either heterologously expressed or endogenous adenylyl cyclase, in both single cells and cell populations. After forskolin stimulation, the endogenous adenylyl cyclase in C6-2B glioma cells produced high concentrations of cAMP near the channels, yet the global cAMP concentration remained low. We found that rapid exchange of the bulk cytoplasm in whole-cell patch clamp experiments did not prevent the buildup of significant levels of cAMP near the channels in human embryonic kidney 293 (HEK-293) cells expressing an exogenous adenylyl cyclase. These results can be explained quantitatively by a cell compartment model in which cyclic nucleotide-gated channels colocalize with adenylyl cyclase in microdomains, and diffusion of cAMP between these domains and the bulk cytosol is significantly hindered. In agreement with the model, we measured a slow rate of cAMP diffusion from the whole-cell patch pipette to the channels (90% exchange in 194 s, compared with 22-56 s for substances that monitor exchange with the cytosol). Without a microdomain and restricted diffusional access to the cytosol, we are unable to account for all of the results. It is worth noting that in models of unrestricted diffusion, even in extreme proximity to adenylyl cyclase, cAMP does not reach high enough concentrations to substantially activate PKA or cyclic nucleotide-gated channels, unless the entire cell fills with cAMP. Thus, the microdomains should facilitate rapid and efficient activation of both PKA and cyclic nucleotide-gated channels, and allow for local feedback control of adenylyl cyclase. Localized cAMP signals should also facilitate the differential regulation of cellular targets.

Cyclic adenosine monophosphate signaling in the rat vomeronasal organ: role of an adenylyl cyclase type VI

The present study indicates that male rat urinary components in female rat vomeronasal organ microvillar preparations not only induce a rapid and transient IP(3) signal, but in addition, the level of cAMP decreases with a delayed and sustained time course. This decrease seems to be a consequence of the preceding activation of the phosphoinositol pathway rather than the result of an enhanced phosphodiesterase activity or an inhibition of adenylyl cyclase (AC) via Galpha(i) or Galpha(o). This notion is supported by the finding that activation of the endogenous protein kinase C suppresses basal as well as forskolin-induced cAMP formation. Furthermore, it was observed that elevated levels of calcium inhibit cAMP formation in rat VNO microvillar preparations. These properties of cAMP signaling in the VNO of rats may be mediated by a calcium- and protein kinase C-inhibited AC VI subtype, which is localized in microvillar preparations of the VNO.

The type 8 adenylyl cyclase is critical for Ca2+ stimulation of cAMP accumulation in mouse parotid acini

Capacitative Ca(2+) entry stimulates cAMP synthesis in mouse parotid acini, suggesting that one of the Ca(2+)-sensitive adenylyl cyclases (AC1 or AC8) may play an important role in the regulation of parotid function (Watson, E. L., Wu, Z., Jacobson, K. L., Storm, D. R., Singh, J. C., and Ott, S. M. (1998) Am. J. Physiol. 274, C557-C565). To evaluate the role of AC1 and AC8 in Ca(2+) stimulation of cAMP synthesis in parotid cells, acini were isolated from AC1 mutant (AC1-KO) and AC8 mutant (AC8-KO) mice and analyzed for Ca(2+) stimulation of intracellular cAMP levels. Although Ca(2+) stimulation of intracellular cAMP levels in acini from AC1-KO mice was indistinguishable from wild type mice, acini from AC8-KO mice showed no Ca(2+)-stimulated cAMP accumulation. This indicates that AC8, but not AC1, plays a major role in coupling Ca(2+) signals to cAMP synthesis in parotid acini. Interestingly, treatment of acini from AC8-KO mice with agents, i.e. carbachol and thapsigargin that increase intracellular Ca(2+), lowered cAMP levels. This decrease was dependent upon Ca(2+) influx and independent of phosphodiesterase activation. Immunoblot analysis revealed that AC5/6 and AC3 are expressed in parotid glands. Inhibition of calmodulin (CaM) kinase II with KN-62, or inclusion of the CaM inhibitor, calmidazolium, did not prevent agonist-induced inhibition of stimulated cAMP accumulation. In vitro studies revealed that Ca(2+), independently of CaM, inhibited isoproterenol-stimulated AC. Data suggest that agonist augmentation of stimulated cAMP levels is due to activation of AC8 in mouse parotid acini, and strongly support a role for AC5/6 in the inhibition of stimulated cAMP levels.

Molecular diversity of cyclic AMP signalling

Several neuroendocrine control systems are prominently controlled by G-protein coupled receptors that activate the cAMP signal transduction pathway. The discovery of multiple genes that encode the molecular machinery of cAMP metabolism has revolutionized our knowledge of cAMP mediated processes. This perhaps all too familiar second messenger can be generated by nine different membrane enzymes in the context of varied levels of activation of G proteins as well as Ca(2+)- and protein kinase C-dependent processes. The amplitude, length and subcellular distribution of the cAMP signal are further modulated by over twenty functionally distinct isotypes of cAMP-degrading phosphodiesterases in a cell- and stimulus-specific manner. The present review summarizes the key properties of the molecular machinery that generates the cAMP signal and highlights how it is deployed in neuroendocrine systems.

Ca(2+), Sr(2+), and Ba(2+) identify distinct regulatory sites on adenylyl cyclase (AC) types VI and VIII and consolidate the apposition of capacitative cation entry channels and Ca(2+)-sensitive ACs

Ca(2+)-sensitive adenylyl cyclases may act as early integrators of the two major second messenger-signaling pathways mediated by Ca(2+) and cAMP. Ca(2+) stimulation of adenylyl cyclase type I (ACI) and adenylyl cyclase type VIII (ACVIII) is mediated by calmodulin and the site on these adenylyl cyclases that interacts with calmodulin has been defined. By contrast, the mechanism whereby Ca(2+) inhibits adenylyl cyclase type V (ACV) and adenylyl cyclase type VI (ACVI) is unknown. In this study, Ca(2+), Sr(2+), and Ba(2+) were compared to probe the involvement of E-F hand-like domains in both Ca(2+) stimulation and inhibition of ACVIII and ACVI, respectively. HEK 293 cells transfected with ACVIII cDNA and C6-2B glioma cells (where the endogenous adenylyl cyclases is predominantly ACVI) were used to compare the effects of these three cations in in vitro and in vivo measurements. The in vitro data identified two Ca(2+) regulatory sites for both ACVIII and ACVI. Strikingly different potency series for these cations at mediating high affinity stimulation and inhibition of ACVIII and ACVI, respectively, effectively rule out the possibility that calmodulin or proteins utilizing similar Ca(2+)-binding motifs mediate inhibition of ACVI. On the other hand, the low affinity inhibition that is common to both ACVIII and ACVI showed virtually identical potency profiles for the IIa cation series, indicating a common site of action. Remarkably, whereas Sr(2+) was rather ineffective at regulating these cyclases (particularly ACVI) in vitro, adequate concentrations accumulated in the vicinity of these enzymes as a consequence of capacitative cation entry to partially regulate both of these activities in vivo. This latter finding consolidates earlier observations that Ca(2+)-sensitive adenylyl cyclases detect and respond to capacitative cation entry rather than global cytosolic cation concentrations.

Inhibition by calcium of mammalian adenylyl cyclases

Ca(2+) regulates mammalian adenylyl cyclases in a type-specific manner. Stimulatory regulation is moderately well understood. By contrast, even the concentration range over which Ca(2+) inhibits adenylyl cyclases AC5 and AC6 is not unambiguously defined; even less so is the mechanism of inhibition. In the present study, we compared the regulation of Ca(2+)-stimulable and Ca(2+)-inhibitable adenylyl cyclases expressed in Sf9 cells with tissues that predominantly express these activities in the mouse brain. Soluble forms of AC5 containing either intact or truncated major cytosolic domains were also examined. All adenylyl cyclases, except AC2 and the soluble forms of AC5, displayed biphasic Ca(2+) responses, suggesting the presence of two Ca(2+) sites of high ( approximately 0.2 microM) and low affinity ( approximately 0.1 mM). With a high affinity, Ca(2+) (i) stimulated AC1 and cerebellar adenylyl cyclases, (ii) inhibited AC6 and striatal adenylyl cyclase, and (iii) was without effect on AC2. With a low affinity, Ca(2+) inhibited all adenylyl cyclases, including AC1, AC2, AC6, and both soluble forms of AC5. The mechanism of both high and low affinity inhibition was revealed to be competition for a stimulatory Mg(2+) site(s). A remarkable selectivity for Ca(2+) was displayed by the high affinity site, with a K(i) value of approximately 0.2 microM, in the face of a 5000-fold excess of Mg(2+). The present results show that high and low affinity inhibition by Ca(2+) can be clearly distinguished and that the inhibition occurs type-specifically in discrete adenylyl cyclases. Distinction between these sites is essential, or quite spurious inferences may be drawn on the nature or location of high affinity binding sites in the Ca(2+)-inhibitable adenylyl cyclases.

Discriminating between capacitative and arachidonate-activated Ca(2+) entry pathways in HEK293 cells

We have recently questioned whether the capacitative or store-operated model for receptor-activated Ca(2+) entry can account for the influx of Ca(2+) seen at low agonist concentrations, such a those typically producing [Ca(2+)](i) oscillations. Instead, we have identified an arachidonic acid-regulated, noncapacitative Ca(2+) entry mechanism that appears to be specifically responsible for the receptor-activated entry of Ca(2+) under these conditions. However, it is unclear whether these two systems reflect the activity of distinct entry pathways or simply different mechanisms of regulating a common pathway. We therefore used the known selectivity of the Ca(2+)-stimulated type VIII adenylyl cyclase for Ca(2+) entry occurring via the capacitative pathway (Fagan, K. A., Mahey, R., and Cooper, D. M. F. (1996) J. Biol. Chem. 271, 12438-12444) to attempt to discriminate between these two entry mechanisms in HEK293 cells. Consistent with the earlier reports, we found that thapsigargin induced an approximate 3-fold increase in adenylyl cyclase activity that was unrelated to global changes in [Ca(2+)](i) or to the release of Ca(2+) from internal stores but was specifically dependent on the induced capacitative entry of Ca(2+). In marked contrast, the arachidonate-induced entry of Ca(2+) completely failed to affect adenylyl cyclase activity despite producing a substantially greater rate of entry than that induced by thapsigargin. These data demonstrate that the arachidonate-activated entry of Ca(2+) occurs via an entirely distinct influx pathway.

Molecular identification of adenylyl cyclase 3 in bovine corpus luteum and its regulation by prostaglandin F2alpha-induced signaling pathways

The involvement of cAMP in various aspects of ovarian steroidogenic cells functions has been extensively studied. However, the adenylyl cyclase (AC) types expressed in ovarian cells, of any species, are not yet determined. The present study was undertaken to identify AC types present in bovine luteal cells and their regulation by various stimuli. AC isoforms 2, 3, 5, 6, 7, 8, and 9 were detected in the bovine brain by Northern blotting analysis, whereas the bovine corpus luteum (CL) only expressed AC3 and 6 mRNAs, with AC3 being more abundant than AC6. The use of AC3-specific primers in RT-PCR reaction verified the presence of AC3 mRNA in both bovine and rat CL tissue as well as in bovine steroidogenic luteal cells. Because these two AC isoforms, AC3 and 6, exhibit distinct regulatory patterns we have next examined the effects of various signaling pathways on AC activity in luteal cells. These studies have shown that: 1) prostaglandin (PG) F2alpha and phorbol 12-myristate 13-acetate markedly elevated agonist-stimulated cAMP synthesis (these effects were inhibited by addition of highly specific PKC inhibitor, bisindolylmaleimide); 2) depletion of Ca2+ from the incubation medium inhibited AC activity; 3) physiological concentrations of Ca2+ ions (up to 5 mM) significantly stimulated cAMP production in luteal cells; and 4) the effects of Ca2+ on cAMP synthesis were evident only in the presence of forskolin. These regulatory characteristics of AC activity are consistent with the molecular identification of ACs indicating the presence of AC3 in luteal cells. The reported data may delineate the cross-talk between physiological activators of AC in the CL (such as LH, PGE2, and PGI2) and other ligands (such as PGF2alpha and endothelin-1), which indirectly modulate AC activity. Therefore, the identification of AC isoforms present in luteal cells is an important step toward understanding the mode of action of a wide array of hormones regulating ovarian cells.

Functional properties of Ca2+-inhibitable type 5 and type 6 adenylyl cyclases and role of Ca2+ increase in the inhibition of intracellular cAMP content

Among the different adenylyl cyclase (AC) isoforms, type 5 and type 6 constitute a subfamily which has the remarkable property of being inhibited by submicromolar Ca2+ concentrations in addition to Galphai-mediated processes. These independent and cumulative negative regulations are associated to a low basal enzymatic activity which can be strongly activated by Galphas-mediated interactions or forskolin. These properties ensure possible wide changes of cAMP synthesis. Regulation of cAMP synthesis by Ca2+ was studied in cultured or native cells which express naturally type 5 and/or type 6 AC, including well-defined renal epithelial cells. The results underline two characteristics of the inhibition due to agonist-elicited increase of intracellular Ca2+: i) Ca2+ rises achieved through capacitive Ca2+ entry or intracellular Ca2+ release can inhibit AC to a similar extent; and ii) in a same cell type, different agonists inducing similar overall Ca2+ rises elicit a variable inhibition of AC activity. The results suggest that a high efficiency of AC regulation by Ca2+ is linked to a requisite close localization of AC enzyme and Ca2+ rises.

Control of cAMP in lung endothelial cell phenotypes. Implications for control of barrier function

Pulmonary microvascular endothelial cells (PMVECs) form a more restrictive barrier to macromolecular flux than pulmonary arterial endothelial cells (PAECs); however, the mechanisms responsible for this intrinsic feature of PMVECs are unknown. Because cAMP improves endothelial barrier function, we hypothesized that differences in enzyme regulation of cAMP synthesis and/or degradation uniquely establish an elevated content in PMVECs. PMVECs possessed 20% higher basal cAMP concentrations than did PAECs; however, increased content was accompanied by 93% lower ATP-to-cAMP conversion rates. In PMVECs, responsiveness to beta-adrenergic agonist (isoproterenol) or direct adenylyl cyclase (forskolin) activation was attenuated and responsiveness to phosphodiesterase inhibition (rolipram) was increased compared with those in PAECs. Although both types of endothelial cells express calcium-inhibited adenylyl cyclase, constitutive PMVEC cAMP accumulation was not inhibited by physiological rises in cytosolic calcium, whereas PAEC cAMP accumulation was inhibited 30% by calcium. Increasing either PMVEC calcium entry by maximal activation of store-operated calcium entry or ATP-to-cAMP conversion with rolipram unmasked calcium inhibition of adenylyl cyclase. These data indicate that suppressed calcium entry and low ATP-to-cAMP conversion intrinsically influence calcium sensitivity. Adenylyl cyclase-to-cAMP phosphodiesterase ratios regulate cAMP at elevated levels compared with PAECs, which likely contribute to enhanced microvascular barrier function.

Modulation of a pacemaker current through Ca(2+)-induced stimulation of cAMP production

Brief increases in [Ca2+]i can result in prolonged changes in neuronal properties. A Ca(2+)-dependent modulation of the hyperpolarization-activated cation current (Ih) controls the slow recurrence of synchronized thalamocortical activity. Here we show that the persistent activation of Ih is initiated by rapidly increased [Ca2+]i and subsequent production of cAMP. The modulation is maintained via a facilitated interaction of cAMP with open (voltage-gated) h-channels, inducing prolonged activation of Ih that may outlast the presence of increased free [Ca2+]i and [cAMP]i. This persistent Ih activation may control the presence and periodicity of both normal and abnormal synchronized thalamocortical rhythms.

Calmodulin-binding sites on adenylyl cyclase type VIII

Ca2+ stimulation of adenylyl cyclase type VIII (ACVIII) occurs through loosely bound calmodulin. However, where calmodulin binds in ACVIII and how the binding activates this cyclase have not yet been investigated. We have located two putative calmodulin-binding sites in ACVIII. One site is located at the N terminus as revealed by overlay assays; the other is located at the C terminus, as indicated by mutagenesis studies. Both of these calmodulin-binding sites were confirmed by synthetic peptide studies. The N-terminal site has the typical motif of a Ca2+-dependent calmodulin-binding domain, which is defined by a characteristic pattern of hydrophobic amino acids, basic and aromatic amino acids, and a tendency to form amphipathic alpha-helix structures. Functional, mutagenesis studies suggest that this binding makes a minor contribution to the Ca2+ stimulation of ACVIII activity, although it might be involved in calmodulin trapping by ACVIII. The primary structure of the C-terminal site resembles another calmodulin-binding motif, the so-called IQ motif, which is commonly Ca2+-independent. Mutagenesis and functional assays indicate that this latter site is a calcium-dependent calmodulin-binding site, which is largely responsible for the Ca2+ stimulation of ACVIII. Removal of this latter calmodulin-binding region from ACVIII results in a hyperactivated enzyme state and a loss of Ca2+ sensitivity. Thus, Ca2+/calmodulin regulation of ACVIII may be through a disinhibitory mechanism, as is the case for a number of other targets of Ca2+/calmodulin.

The relationship between the free concentrations of Ca2+ and Ca2+-calmodulin in intact cells

Using stably expressed fluorescent indicator proteins, we have determined for the first time the relationship between the free Ca2+ and Ca2+-calmodulin concentrations in intact cells. A similar relationship is obtained when the free Ca2+ concentration is externally buffered or when it is transiently increased in response to a Ca2+-mobilizing agonist. Below a free Ca2+ concentration of 0.2 microM, no Ca2+-calmodulin is detectable. A global maximum free Ca2+-calmodulin concentration of approximately 45 nM is produced when the free Ca2+ concentration exceeds 3 microM, and a half-maximal concentration is produced at a free Ca2+ concentration of 1 microM. Data for fractional saturation of the indicators suggest that the total concentration of calmodulin-binding proteins is approximately 2-fold higher than the total calmodulin concentration. We conclude that high-affinity calmodulin targets (Kd </= 10 nM) are efficiently activated throughout the cell, but efficient activation of low-affinity targets (Kd >/= 100 nM) occurs only where free Ca2+-calmodulin concentrations can be locally enhanced.

G protein regulation of adenylate cyclase

Adenylate cyclase integrates positive and negative signals that act through G protein-coupled cell-surface receptors with other extracellular stimuli to finely regulate levels of cAMP within the cell. Recently, the structures of the cyclase catalytic core complexed with the plant diterpene forskolin, and a cyclase-forskolin complex bound to an activated form of the stimulatory G protein subunit Gs alpha have been solved by X-ray crystallography. These structures provide a wealth of detail about how different signals could converge at the core cyclase domains to regulate catalysis. In this article, William Simonds reviews recent advances in the molecular and structural biology of this key regulatory enzyme, which provide new insight into its ability to integrate multiple signals in diverse cellular contexts.

Simple fluctuation of Ca2+ elicits the complex circadian dynamics of cyclic AMP and cyclic GMP in Paramecium

The circadian dynamics of cyclic adenosine 3′,5′-monophosphate (cAMP) and cyclic guanosine 3′,5′-monophosphate (cGMP) were simulated in Paramecium multimicronucleatum. The mathematical functions determined closely mimic the Ca2+ dependence of adenylate cyclase (AC) and guanylate cyclase (GC) activities as documented in P. tetraurelia. Patterns of cAMP concentration ([cAMP]), cGMP concentration ([cGMP]), and the ratio [cGMP]/[cAMP] were calculated with respect to Ca2+ concentrations ([Ca2+]) fluctuating sinusoidally with a period of 24 hours at three different levels: low, medium, and high. The functions displayed varying patterns of [cAMP] characteristic for [Ca2+] fluctuating at each level, while patterns of [cGMP] and [cGMP]/[cAMP] almost paralleled [Ca2+] fluctuations. Similar patterns were observed for actual [cAMP] and [cGMP] measured during the light/dark cycle in P. multimicronucleatum, grown in axenic media additionally containing [Ca2+] at 25 (low), 100 (medium), or 400 (high) microM, respectively. The coincidence between simulated and measured fluctuations of [cAMP] and [cGMP] suggests that the circadian fluctuations of intracellular [Ca2+] primarily stimulate activities of AC and GC via their different degrees of Ca2+ dependence, which are ultimately responsible for the circadian spatiotemporal organization of various physiological functions in Paramecium.