Regulation of type I adenylyl cyclase by calmodulin kinase IV in vivo

Multiple Roles of cAMP in Vertebrate Retina

cAMP is a key regulatory molecule that controls many important processes in the retina, including phototransduction, cell development and death, growth of neural processes, intercellular contacts, retinomotor effects, and so forth. The total content of cAMP changes in the retina in a circadian manner following the natural light cycle, but it also shows local and even divergent changes in faster time scales in response to local and transient changes in the light environment. Changes in cAMP might also manifest or cause various pathological processes in virtually all cellular components of the retina. Here we review the current state of knowledge and understanding of the regulatory mechanisms by which cAMP influences the physiological processes that occur in various retinal cells.

Ca2+-stimulated adenylyl cyclases as therapeutic targets for psychiatric and neurodevelopmental disorders

As the main secondary messengers, cyclic AMP (cAMP) and Ca2+ trigger intracellular signal transduction cascade and, in turn, regulate many aspects of cellular function in developing and mature neurons. The group I adenylyl cyclase (ADCY, also known as AC) isoforms, including ADCY1, 3, and 8 (also known as AC1, AC3, and AC8), are stimulated by Ca2+ and thus functionally positioned to integrate cAMP and Ca2+ signaling. Emerging lines of evidence have suggested the association of the Ca2+-stimulated ADCYs with bipolar disorder, schizophrenia, major depressive disorder, post-traumatic stress disorder, and autism. In this review, we discuss the molecular and cellular features as well as the physiological functions of ADCY1, 3, and 8. We further discuss the recent therapeutic development to target the Ca2+-stimulated ADCYs for potential treatments of psychiatric and neurodevelopmental disorders.

Cross-Talk Between the Adenylyl Cyclase/cAMP Pathway and Ca2+ Homeostasis

Cyclic AMP and Ca²⁺ are the first second or intracellular messengers identified, unveiling the cellular mechanisms activated by a plethora of extracellular signals, including hormones. Cyclic AMP generation is catalyzed by adenylyl cyclases (ACs), which convert ATP into cAMP and pyrophosphate. By the way, Ca²⁺, as energy, can neither be created nor be destroyed; Ca²⁺ can only be transported, from one compartment to another, or chelated by a variety of Ca²⁺-binding molecules. The fine regulation of cytosolic concentrations of cAMP and free Ca²⁺ is crucial in cell function and there is an intimate cross-talk between both messengers to fine-tune the cellular responses. Cancer is a multifactorial disease resulting from a combination of genetic and environmental factors. Frequent cases of cAMP and/or Ca²⁺ homeostasis remodeling have been described in cancer cells. In those tumoral cells, cAMP and Ca²⁺ signaling plays a crucial role in the development of hallmarks of cancer, including enhanced proliferation and migration, invasion, apoptosis resistance, or angiogenesis. This review summarizes the cross-talk between the ACs/cAMP and Ca²⁺ intracellular pathways with special attention to the functional and reciprocal regulation between Orai1 and AC8 in normal and cancer cells.

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Adenylyl cyclases (ACs) generate the second messenger cAMP from ATP. Mammalian cells express nine transmembrane AC (mAC) isoforms (AC1-9) and a soluble AC (sAC, also referred to as AC10). This review will largely focus on mACs. mACs are activated by the G-protein Gαs and regulated by multiple mechanisms. mACs are differentially expressed in tissues and regulate numerous and diverse cell functions. mACs localize in distinct membrane compartments and form signaling complexes. sAC is activated by bicarbonate with physiologic roles first described in testis. Crystal structures of the catalytic core of a hybrid mAC and sAC are available. These structures provide detailed insights into the catalytic mechanism and constitute the basis for the development of isoform-selective activators and inhibitors. Although potent competitive and noncompetitive mAC inhibitors are available, it is challenging to obtain compounds with high isoform selectivity due to the conservation of the catalytic core. Accordingly, caution must be exerted with the interpretation of intact-cell studies. The development of isoform-selective activators, the plant diterpene forskolin being the starting compound, has been equally challenging. There is no known endogenous ligand for the forskolin binding site. Recently, development of selective sAC inhibitors was reported. An emerging field is the association of AC gene polymorphisms with human diseases. For example, mutations in the AC5 gene (ADCY5) cause hyperkinetic extrapyramidal motor disorders. Overall, in contrast to the guanylyl cyclase field, our understanding of the (patho)physiology of AC isoforms and the development of clinically useful drugs targeting ACs is still in its infancy.

Activation of Phosphatidylinositol-Linked Dopamine Receptors Induces a Facilitation of Glutamate-Mediated Synaptic Transmission in the Lateral Entorhinal Cortex

The lateral entorhinal cortex receives strong inputs from midbrain dopamine neurons that can modulate its sensory and mnemonic function. We have previously demonstrated that 1 µM dopamine facilitates synaptic transmission in layer II entorhinal cortex cells via activation of D1-like receptors, increased cAMP-PKA activity, and a resulting enhancement of AMPA-receptor mediated currents. The present study assessed the contribution of phosphatidylinositol (PI)-linked D1 receptors to the dopaminergic facilitation of transmission in layer II of the rat entorhinal cortex, and the involvement of phospholipase C activity and release of calcium from internal stores. Whole-cell patch-clamp recordings of glutamate-mediated evoked excitatory postsynaptic currents were obtained from pyramidal and fan cells. Activation of D1-like receptors using SKF38393, SKF83959, or 1 µM dopamine induced a reversible facilitation of EPSCs which was abolished by loading cells with either the phospholipase C inhibitor U-73122 or the Ca2+ chelator BAPTA. Neither the L-type voltage-gated Ca2+ channel blocker nifedipine, nor the L/N-type channel blocker cilnidipine, blocked the facilitation of synaptic currents. However, the facilitation was blocked by blocking Ca2+ release from internal stores via inositol 1,4,5-trisphosphate (InsP3) receptors or ryanodine receptors. Follow-up studies demonstrated that inhibiting CaMKII activity with KN-93 failed to block the facilitation, but that application of the protein kinase C inhibitor PKC(19-36) completely blocked the dopamine-induced facilitation. Overall, in addition to our previous report indicating a role for the cAMP-PKA pathway in dopamine-induced facilitation of synaptic transmission, we demonstrate here that the dopaminergic facilitation of synaptic responses in layer II entorhinal neurons also relies on a signaling cascade dependent on PI-linked D1 receptors, PLC, release of Ca2+ from internal stores, and PKC activation which is likely dependent upon both DAG and enhanced intracellular Ca2+. These signaling pathways may collaborate to enhance sensory and mnemonic function in the entorhinal cortex during tonic release of dopamine.

Gα(i/o)-coupled receptor-mediated sensitization of adenylyl cyclase: 40 years later

Heterologous sensitization of adenylyl cyclase (also referred to as superactivation, sensitization, or supersensitization of adenylyl cyclase) is a cellular adaptive response first described 40 years ago in the laboratory of Dr. Marshall Nirenberg. This apparently paradoxical cellular response occurs following persistent activation of Gαi/o-coupled receptors and causes marked enhancement in the activity of adenylyl cyclases, thereby increasing cAMP production. Since our last review in 2005, significant progress in the field has led to a better understanding of the relevance of, and the cellular biochemical processes that occur during the development and expression of heterologous sensitization. In this review we will discuss the recent advancements in the field and the mechanistic hypotheses on heterologous sensitization.

Inhibition of Ca²⁺/calmodulin-dependent protein kinase kinase 2 stimulates osteoblast formation and inhibits osteoclast differentiation

Bone remodeling, a physiological process characterized by bone formation by osteoblasts (OBs) and resorption of preexisting bone matrix by osteoclasts (OCs), is vital for the maintenance of healthy bone tissue in adult humans. Imbalances in this vital process result in pathological conditions including osteoporosis. Owing to its initial asymptomatic nature, osteoporosis is often detected only after the patient has sustained significant bone loss or a fracture. Hence, anabolic therapeutics that stimulate bone accrual is in high clinical demand. Here we identify Ca²⁺/calmodulin (CaM)-dependent protein kinase kinase 2 (CaMKK2) as a potential target for such therapeutics because its inhibition enhances OB differentiation and bone growth and suppresses OC differentiation. Mice null for CaMKK2 possess higher trabecular bone mass in their long bones, along with significantly more OBs and fewer multinuclear OCs. In vitro, although Camkk2⁻/⁻ mesenchymal stem cells (MSCs) yield significantly higher numbers of OBs, bone marrow cells from Camkk2⁻/⁻ mice produce fewer multinuclear OCs. Acute inhibition of CaMKK2 by its selective, cell-permeable pharmacological inhibitor STO-609 also results in increased OB and diminished OC formation. Further, we find phospho-protein kinase A (PKA) and Ser¹³³ phosphorylated form of cyclic adenosine monophosphate (cAMP) response element binding protein (pCREB) to be markedly elevated in OB progenitors deficient in CaMKK2. On the other hand, genetic ablation of CaMKK2 or its pharmacological inhibition in OC progenitors results in reduced pCREB as well as significantly reduced levels of its transcriptional target, nuclear factor of activated T cells, cytoplasmic (NFATc1). Moreover, in vivo administration of STO-609 results in increased OBs and diminished OCs, conferring significant protection from ovariectomy (OVX)-induced osteoporosis in adult mice. Overall, our findings reveal a novel function for CaMKK2 in bone remodeling and highlight the potential for its therapeutic inhibition as a valuable bone anabolic strategy that also inhibits OC differentiation in the treatment of osteoporosis.

Muscarinic receptors stimulate AC2 by novel phosphorylation sites, whereas Gβγ subunits exert opposing effects depending on the G-protein source

Direct phosphorylation of AC2 (adenylyl cyclase 2) by PKC (protein kinase C) affords an opportunity for AC2 to integrate signals from non-canonical pathways to produce the second messenger, cyclic AMP. The present study shows that stimulation of AC2 by pharmacological activation of PKC or muscarinic receptor activation is primarily the result of phosphorylation of Ser⁴⁹⁰ and Ser⁵⁴³, as opposed to the previously proposed Thr¹⁰⁵⁷. A double phosphorylation-deficient mutant (S490/543A) of AC2 was insensitive to PMA (phorbol myristic acid) and CCh (carbachol) stimulation, whereas a double phosphomimetic mutant (S490/543D) mimicked the activity of PKC-activated AC2. Putative Gβγ-interacting sites are in the immediate environment of these PKC phosphorylation sites (Ser⁴⁹⁰ and Ser⁵⁴³) that are located within the C1b domain of AC2, suggesting a significant regulatory importance of this domain. Consequently, we examined the effect of both G_q-coupled muscarinic and G_i-coupled somatostatin receptors. Employing pharmacological and FRET (fluorescence resonance energy transfer)-based real-time single cell imaging approaches, we found that Gβγ released from the G_q-coupled muscarinic receptor or G_i-coupled somatostatin receptors exert inhibitory or stimulatory effects respectively. These results underline the sophisticated regulatory capacities of AC2, in not only being subject to regulation by PKC, but also and in an opposite manner to Gβγ subunits, depending on their source.

New paradigms in cAMP signalling

Signalling through adenosine 3'5' monophosphate (cAMP) is known to be important in virtually every cell. The mapping of the human genome over the past two decades has revealed an unexpected complexity of cAMP signalling, which is shared from insects to mammals. A more recent technical advance is the ability to monitor intracellular cAMP levels at subcellular spatial resolution within the time-domains of fast biochemical reactions. Thus, new light has been shed on old paradigms, some of which turn out to be multiple new ones. The novel aspects of cAMP signalling are highlighted here: (1) agonist induced plasticity - showing how the repertory of cAMP signalling genes supports homeostatic adaptation; (2) sustained cAMP signalling after endocytosis; (3) pre-assembled receptor-Gs-adenylyl cyclase complexes. Finally, a hypothetical model of propagating neuronal cAMP signals travelling form dendrites to the cell body is presented.

Interactions between intracellular free Ca2+ and cyclic AMP in neuroendocrine cells

Calcium ions and cyclic adenosine monophosphate (cAMP) are virtually ubiquitous intracellular signaling molecules in mammalian cells. This paper will focus on the cross-talk between Ca(2+) and cAMP mobilizing signaling pathways and summarize the underlying molecular mechanisms. Subsequently, workings of adenohypophyseal corticotrope cells will be reviewed to highlight the physiological relevance of a Ca(2+) cAMP interactions in neuroendocrinology.

Choreographing the adenylyl cyclase signalosome: sorting out the partners and the steps

Adenylyl cyclases are a ubiquitous family of enzymes and are critical regulators of metabolic and cardiovascular function. Multiple isoforms of the enzyme are expressed in a range of tissues. However, for many processes, the adenylyl cyclase isoforms have been thought of as essentially interchangeable, with their impact more dependent on their common actions to increase intracellular cyclic adenosine monophosphate content regardless of the isoform involved. It has long been appreciated that each subfamily of isoforms demonstrate a specific pattern of "upstream" regulation, i.e., specific patterns of ion dependence (e.g., calcium-dependence) and specific patterns of regulation by kinases (protein kinase A (PKA), protein kinase C (PKC), raf). However, more recent studies have suggested that adenylyl cyclase isoform-selective patterns of signaling are a wide-spread phenomenon. The determinants of these selective signaling patterns relate to a number of factors, including: (1) selective coupling of specific adenylyl cyclase isoforms with specific G protein-coupled receptors, (2) localization of specific adenylyl cyclase isoforms in defined structural domains (AKAP complexes, caveolin/lipid rafts), and (3) selective coupling of adenylyl cyclase isoforms with specific downstream signaling cascades important in regulation of cell growth and contractility. The importance of isoform-specific regulation has now been demonstrated both in mouse models as well as in humans. Adenylyl cyclase has not been viewed as a useful target for therapeutic regulation, given the ubiquitous expression of the enzyme and the perceived high risk of off-target effects. Understanding which isoforms of adenylyl cyclase mediate distinct cellular effects would bring new significance to the development of isoform-specific ligands to regulate discrete cellular actions.

Adenylate cyclase and calmodulin-dependent kinase have opposite effects on osteoclastogenesis by regulating the PKA-NFATc1 pathway

Nuclear factor of activated T cells c1 (NFATc1) is a transcription factor crucial for the differentiation of osteoclasts. In this study we discovered new signaling pathways involving cAMP regulators that modulate NFATc1 during osteoclastogenesis. The osteoclast differentiation factor receptor activator of NF-κB ligand (RANKL) increased the expression of adenylate cyclase 3 (AC3), accompanied by a rise in the intracellular cAMP level in osteoclasts. The knockdown of AC3 enhanced in vitro osteoclastogenesis and in vivo bone resorption, whereas cAMP-elevating agents showed opposite effects. The antiosteoclastogenic effect of the AC3-cAMP pathway was mediated by the inhibition of NFATc1 nuclear translocation and its autoamplification via a protein kinase A (PKA)-dependent mechanism. RANKL has been shown to activate Ca(2+) /calmodulin-dependent protein kinases (CaMKs). Knockdown or catalytic inhibition of CaMKs elevated intracellular cAMP levels in RANKL-treated osteoclast precursors and suppressed the activation of NFATc1. Taken together, our results demonstrate a pivotal role for the cAMP-PKA-NFATc1 signaling pathway during osteoclast differentiation, suggesting a mechanism by which osteoclastogenesis is fine-tuned by a balance between AC3 and CaMKs activities.

Regulation by Ca2+-signaling pathways of adenylyl cyclases

Interplay between the signaling pathways of the intracellular second messengers, cAMP and Ca(2+), has vital consequences for numerous essential physiological processes. Although cAMP can impact on Ca(2+)-homeostasis at many levels, Ca(2+) either directly, or indirectly (via calmodulin [CaM], CaM-binding proteins, protein kinase C [PKC] or Gβγ subunits) may also regulate cAMP synthesis. Here, we have evaluated the evidence for regulation of adenylyl cyclases (ACs) by Ca(2+)-signaling pathways, with an emphasis on verification of this regulation in a physiological context. The effects of compartmentalization and protein signaling complexes on the regulation of AC activity by Ca(2+)-signaling pathways are also addressed. Major gaps are apparent in the interactions that have been assumed, revealing a need to comprehensively clarify the effects of Ca(2+) signaling on individual ACs, so that the important ramifications of this critical interplay between Ca(2+) and cAMP are fully appreciated.

Physiological roles for G protein-regulated adenylyl cyclase isoforms: insights from knockout and overexpression studies

Cyclic AMP is a universal second messenger, produced by a family of adenylyl cyclase (AC) enzymes. The last three decades have brought a wealth of new information about the regulation of cyclic AMP production by ACs. Nine hormone-sensitive, membrane-bound AC isoforms have been identified in addition to a tenth isoform that lacks membrane spans and more closely resembles the cyanobacterial AC enzymes. New model systems for purifying and characterizing the catalytic domains of AC have led to the crystal structure of these domains and the mapping of numerous interaction sites. However, big hurdles remain in unraveling the roles of individual AC isoforms and their regulation in physiological systems. In this review we explore the latest on AC knockout and overexpression studies to better understand the roles of G protein regulation of ACs in the brain, olfactory bulb, and heart.

Genetic evidence for the requirement of adenylyl cyclase 1 in synaptic scaling of forebrain cortical neurons

Homeostatic plasticity is important to stabilize the activity level of neuronal circuits. Molecular mechanisms underlying neuronal homeostatic plasticity in response to activity deprivation are not completely understood. We found that prolonged alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor blockade by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) resulted in larger, faster miniature excitatory postsynaptic current (mEPSC) events with enhanced frequency in cultured forebrain cortical neurons. Furthermore, GluR1 protein level and CREB-dependent transcription were up-regulated. Blockade of L-type Ca(2+) channels but not kainate receptors produced similar effects to the AMPA receptor blockade. Genetic deletion of AC1 (adenylyl cyclase 1), but not AC8, a key neuronal adenylyl cyclase, significantly reduced inactivity-induced GluR1 changes. Our results indicate the synthesis of homomeric GluR1 AMPA receptors and their possible insertion into synapses due to synaptic inactivity in the cortex. AC1 plays a subtype selective role in this process by coupling signals from L-type Ca(2+) channels to downstream signalling pathways.

Nicotine causes age-dependent changes in gene expression in the adolescent female rat brain

Humans often start smoking during adolescence. Recent results suggest that rodents may also be particularly vulnerable to nicotine dependence during adolescence. We examined the effect of chronic nicotine exposure on gene expression profiles during adolescence in female rats, who were dosed with nicotine (and control animals were dosed with saline) via subcutaneously implanted osmotic minipumps. Brain samples were collected at four ages: before puberty (postnatal day 25), at about the time of puberty in females (postnatal day 35), and after puberty (postnatal days 45 and 55). The expression of 7931 genes in three brain areas was measured using DNA microarrays. Quantitative RT-PCR was also employed to confirm the expression patterns of selected genes. We used a novel clustering technique (principal cluster analysis) to classify 162 nicotine-regulated genes into five clusters, of which only one (cluster A) showed similar patterns of gene expression across all three brain areas (ventral striatum, prefrontal cortex, and hippocampus). Three clusters of genes (A, B, and C) showed dramatic peaks in their nicotine responses at the same age (p35). The other two clusters (D1 and D2) showed smaller peaks and/or valleys in their nicotine responses at p35 and p45. Thus, the age of maximal gene expression response to nicotine in female rats corresponds approximately to the age of maximal behavioral response and the age of puberty.

Molecular details of cAMP generation in mammalian cells: a tale of two systems

The second messenger cAMP has been extensively studied for half a century, but the plethora of regulatory mechanisms controlling cAMP synthesis in mammalian cells is just beginning to be revealed. In mammalian cells, cAMP is produced by two evolutionary related families of adenylyl cyclases, soluble adenylyl cyclases (sAC) and transmembrane adenylyl cyclases (tmAC). These two enzyme families serve distinct physiological functions. They share a conserved overall architecture in their catalytic domains and a common catalytic mechanism, but they differ in their sub-cellular localizations and responses to various regulators. The major regulators of tmACs are heterotrimeric G proteins, which transduce extracellular signals via G protein-coupled receptors. sAC enzymes, in contrast, are regulated by the intracellular signaling molecules bicarbonate and calcium. Here, we discuss and compare the biochemical, structural and regulatory characteristics of the two mammalian AC families. This comparison reveals the mechanisms underlying their different properties but also illustrates many unifying themes for these evolutionary related signaling enzymes.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

Galphaq-coupled receptor signaling enhances adenylate cyclase type 6 activation

Calcium signaling robustly inhibits AC6 activity in membrane preparations and in intact cells via capacitative calcium entry (CCE). However, the release of intracellular calcium has not been demonstrated to robustly alter AC6 signaling and activation of Galpha(q)-coupled receptors in tissues that express AC6 enhances cyclic AMP accumulation. To specifically examine the ability of Galpha(q)-coupled receptors to modulate AC6 signaling in intact cells, we used stably transfected HEK-AC6 cells. We demonstrate that AC6 activation is potentiated by activation of endogenous muscarinic receptors expressed in HEK293 cells. Muscarinic receptor activation failed to potentiate the activation of the closely related AC5 isoform. Expression of recombinant Galpha(q)-coupled muscarinic or serotonin receptors, or constitutively active Galpha(q), also potentiated drug-stimulated cyclic AMP accumulation in HEK-AC6 cells. Muscarinic receptor-mediated potentiation of AC6 activation was not due to activation of PKC or modulation of Galpha(i/o)-mediated inhibition of AC6. We demonstrate that calcium chelation or inhibition of calmodulin attenuates the effect of carbachol on AC6 activation. These data support the hypothesis that Galpha(q)-coupled receptor-mediated calcium signaling potentiates AC6 activation in intact cells.

Why Calcium-Stimulated Adenylyl Cyclases?

The Ca2+/calmodulin-stimulated adenylyl cyclases, AC1 and AC8, play a critical role in several forms of neuroplasticity, including long-lasting long-term potentiation (L-LTP) and long-term memory (LTM). By coupling neuronal activity and Ca2+increases to the production of cAMP, AC1 and AC8 activate cAMP-dependent signal transduction and transcriptional pathways critical for L-LTP and LTM.

Increased mitogen-activated protein kinase activity and TNF-alpha production associated with Mycobacterium smegmatis- but not Mycobacterium avium-infected macrophages requires prolonged stimulation of the calmodulin/calmodulin kinase and cyclic AMP/protein kinase A pathways

Previous studies have shown the mitogen-activated protein kinases (MAPKs) to be activated in macrophages upon infection with Mycobacterium, and that expression of TNF-alpha and inducible NO synthase by infected macrophages was dependent on MAPK activation. Additional analysis demonstrated a diminished activation of p38 and extracellular signal-regulated kinase (ERK)1/2 in macrophages infected with pathogenic strains of Mycobacterium avium compared with infections with the fast-growing, nonpathogenic Mycobacterium smegmatis and Mycobacterium phlei. However, the upstream signals required for MAPK activation and the mechanisms behind the differential activation of the MAPKs have not been defined. In this study, using bone marrow-derived macrophages from BALB/c mice, we determined that ERK1/2 activation was dependent on the calcium/calmodulin/calmodulin kinase II pathway in both M. smegmatis- and M. avium-infected macrophages. However, in macrophages infected with M. smegmatis but not M. avium, we observed a marked increase in cAMP production that remained elevated for 8 h postinfection. This M. smegmatis-induced cAMP production was also dependent on the calmodulin/calmodulin kinase pathway. Furthermore, stimulation of the cAMP/protein kinase A pathway in M. smegmatis-infected cells was required for the prolonged ERK1/2 activation and the increased TNF-alpha production observed in these infected macrophages. Our studies are the first to demonstrate an important role for the calmodulin/calmodulin kinase and cAMP/protein kinase A pathways in macrophage signaling upon mycobacterial infection and to show how cAMP production can facilitate macrophage activation and subsequent cytokine production.

Novel regulatory properties of human type 9 adenylate cyclase

Nine membrane-bound members of the mammalian adenylate cyclase family have been identified. The least characterized and most divergent in sequence of the nine adenylate cyclase isoforms is AC9. Stimulation by Galpha(s) and inhibition by Ca2+/calcineurin are two modes of regulation that have been reported for AC9. We explored the possibility of additional modes of regulation of human AC9. We now report that quinpirole activation of the inhibitory G protein-coupled D2L dopamine receptor inhibits Galpha(s) stimulation of AC9 by approximately 50%. The effects of quinpirole were reversed by the D2 antagonist spiperone and by pertussis toxin pretreatment. We also report the first evidence for regulation of AC9 by protein kinase C (PKC). Specifically, phorbol ester activation of PKC significantly attenuated (approximately 50%) Galpha(s)-stimulated AC9 activity. The effect of PKC activation on AC9 was reversed by the PKC inhibitor bisindolylmaleimide. Galpha(s)-stimulated cyclic accumulation was reduced more by simultaneous addition of both quinpirole and phorbol 12-myristate 13-acetate than by either drug alone. Additional studies investigated the role of glycosylation on AC9 activity. The results show that blocking glycosylation of AC9 significantly attenuates Galpha(s) stimulation. In contrast, the ability of PKC and Galpha(i/o) to negatively regulate AC9 did not seem to be affected by the glycosylation state of AC9. These observations demonstrate the diverse regulatory features of AC9 and the ability of AC9 to integrate multiple signals.

Regulation and organization of adenylyl cyclases and cAMP

Adenylyl cyclases are a critically important family of multiply regulated signalling molecules. Their susceptibility to many modes of regulation allows them to integrate the activities of a variety of signalling pathways. However, this property brings with it the problem of imparting specificity and discrimination. Recent studies are revealing the range of strategies utilized by the cyclases to solve this problem. Microdomains are a consequence of these solutions, in which cAMP dynamics may differ from the broad cytosol. Currently evolving methodologies are beginning to reveal cAMP fluctuations in these various compartments.

Isoform-specific regulation of adenylyl cyclase: a potential target in future pharmacotherapy

Adenylyl cyclase (AC) is a target enzyme of multiple G-protein-coupled receptors (GPCRs). In the past decade, the cloning, structure and biochemical properties of nine AC isoforms were reported, and each isoform of AC shows distinct patterns of tissue distribution and biochemical/pharmacological properties. In addition to the conventional regulators of this enzyme, such as calmodulin (CaM) or PKC, novel regulators, for example, caveolin, have been identified. Most importantly, these regulators work on AC in an isoform dependent manner. Recent studies have demonstrated that certain classic AC inhibitors, i.e., P-site inhibitors, show an isoform-dependent inhibition of AC. The side chain modifications of forskolin, a diterpene extract from Coleus forskolii, markedly enhance its isoform selectivity. When taken together, these findings suggest that it is feasible to develop new pharmacotherapeutic agents that target AC isoforms to regulate various neurohormonal signals in a highly tissue-/organ-specific manner.

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.

Calmodulin-regulated adenylyl cyclases: cross-talk and plasticity in the central nervous system

Gene disruption studies have shown that the Ca(2+)-stimulated adenylyl cyclases, AC1 and AC8, are critical for some forms of synaptic plasticity, including long-term potentiation as well as long-term memory formation (LTM). It is hypothesized that these enzymes are required for LTM to support the increased expression of a family of genes regulated through the cAMP/Ca(2+) response element-binding protein/cAMP response element transcriptional pathway. In contrast to AC1 and AC8, AC3 is a Ca(2+)-inhibited adenylyl cyclase that plays an essential role in olfactory signal transduction. Coupling of odorant receptors to AC3 stimulates cAMP transients that function as the major second messenger for olfactory signaling. These cAMP transients are caused, at least in part, by Ca(2+) inhibition of AC3, which is mediated through calmodulin-dependent protein kinase II. The unique structure and regulatory properties of these adenylyl cyclases make them attractive drug target sites for modulation of a number of physiological processes including memory formation and olfaction.

Ghrelin and growth hormone (GH) secretagogues potentiate GH-releasing hormone (GHRH)-induced cyclic adenosine 3',5'-monophosphate production in cells expressing transfected GHRH and GH secretagogue receptors

GHRH stimulates GH secretion from somatotroph cells of the anterior pituitary via a pathway that involves GHRH receptor activation of adenylyl cyclase and increased cAMP production. The actions of GHRH to release GH can be augmented by the synthetic GH secretagogues (GHS), which bind to a distinct G protein-coupled receptor to activate phospholipase C and increase production of the second messengers calcium and diacylglycerol. The stomach peptide ghrelin represents an endogenous ligand for the GHS receptor, which does not activate the cAMP signaling pathway. This study investigates the effects of GHS and ghrelin on GHRH-induced cAMP production in a homogenous population of cells expressing the cloned GHRH and GHS receptors. Each epitope-tagged receptor was shown to be appropriately expressed and to functionally couple to its respective second messenger pathway in this heterologous cell system. Although activation of the GHS receptor alone had no effect on cAMP production, coactivation of the GHS and GHRH receptors produced a cAMP response approximately twice that observed after activation of the GHRH receptor alone. This potentiated response is dose dependent with respect to both GHRH and GHS, is dependent on the expression of both receptors, and was observed with a variety of peptide and nonpeptide GHS compounds as well as with ghrelin-(1-5). Pharmacological inhibition of signaling molecules associated with GHS receptor activation, including G protein betagamma-subunits, phospholipase C, and protein kinase C, had no effect on GHS potentiation of GHRH-induced cAMP production. Importantly, the potentiation appears to be selective for the GHRH receptor. Treatment of cells with the pharmacological agent forskolin elevated cAMP levels, but these levels were not further increased by GHS receptor activation. Similarly, activation of two receptors homologous to the GHRH receptor, the vasoactive intestinal peptide and secretin receptors, increased cAMP levels, but these levels were not further increased by GHS receptor activation. Based on these findings, we speculate that direct interactions between the GHRH and GHS receptors may explain the observed effects on signal transduction.

Synaptic concentration of type-I adenylyl cyclase in cerebellar neurons

Specific subcellular targeting and spatial arrangement of signaling molecules are important for efficient signal transduction. The neuro-specific type-I adenylyl cyclase (AC1) is stimulated by Ca2+, and plays an essential role in neurodevelopment and neuroplasticity. We generated hemagglutinin (HA)-tagged AC1 to study its subcellular localization in cultured neurons. The HA-tagged AC1 has similar enzymatic activity and regulatory properties to that of non-tagged protein. HA-AC1 targeted to both apical and basolateral domains in the epithelial Madin-Darby canine kidney (MDCK) cells, and it was found in both axons and dendrites in cultured hippocampal neurons as well as in cerebellar granule neurons. Interestingly, AC1 showed a distinct punctate form of immunostaining in MDCK cells and transfected neurons, suggesting it targets to specific subcellular domains. By immunostaining with different synaptic markers, we found that AC1 puncta were located at the excitatory synapses in cerebellar granule neurons. Our data provide a possible cellular mechanism for the physiological role of AC1 in neuroplasticity.

Regulation of angiotensin II-induced G protein signaling by phosducin-like protein

Phosducin-like protein (PhLP) is a broadly expressed member of the phosducin (Pd) family of G protein betagamma subunit (Gbetagamma)-binding proteins. Though PhLP has been shown to bind Gbetagamma in vitro, little is known about its physiological function. In the present study, the effect of PhLP on angiotensin II (Ang II) signaling was measured in Chinese hamster ovary cells expressing the type 1 Ang II receptor and various amounts of PhLP. Up to 3.6-fold overexpression of PhLP had no effect on Ang II-stimulated inositol trisphosphate (IP(3)) formation, whereas further increases caused an abrupt decrease in IP(3) production with half-maximal inhibition occurring at 6-fold PhLP overexpression. This threshold level for inhibition corresponds to the cellular concentration of cytosolic chaperonin complex, a recently described binding partner that preferentially binds PhLP over Gbetagamma. Results of pertussis toxin sensitivity, GTPgammaS binding, and immunoprecipitation experiments suggest that PhLP inhibits phospholipase Cbeta activation by dual mechanisms: (i) steric blockage of Gbetagamma activation of PLCbeta and (ii) interference with Gbetagamma-dependent cycling of G(q)alpha by the receptor. These results suggest that G protein signaling may be regulated through controlling the cellular concentration of free PhLP by inducing its expression or by regulating its binding to the chaperonin.

Co-requirement of cyclic AMP- and calcium-dependent protein kinases for transcriptional activation of cholecystokinin gene by protein hydrolysates

Little is known about the mechanisms by which protein-derived nutrients regulate hormone gene expression in the intestine. We have previously reported that protein hydrolysates (i.e. peptones), which are representative of the protein fraction in the lumen, increased cholecystokinin (CCK) gene transcription in the STC-1 enteroendocrine cell line. In the present work, we examined the intracellular events evoked by peptones to stimulate CCK gene transcription. In STC-1 cells, peptones stimulated cyclic AMP production and protein kinase A (PKA) activity. This was associated with a nuclear translocation of the PKA catalytic subunit and with a PKA-dependent phosphorylation of the CRE-binding protein (CREB) at Ser(133). Using transient transfection experiments and reporter luciferase assays, we show that peptone-stimulated transcriptional activity of the CCK gene promoter was significantly decreased when the PKA pathway was inhibited. Furthermore, the intracellular calcium chelator 1,2-bis-(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-tetra(acetoxymethyl)ester completely inhibited peptone-induced stimulation of the CCK gene promoter activity, phosphorylation of CREB, and PKA activity. Peptones increased, in a calcium-dependent manner, the phosphorylation of extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) and the MEK inhibitor PD98059 decreased the peptone-induced stimulation of CCK gene promoter activity. This stimulation was also reduced by 30% in the presence of the calcium/calmodulin-dependent protein kinase (CaMK) inhibitor KN-93. Total inhibition was obtained when the PKA, ERK, and CaMK pathways were simultaneously blocked with appropriate inhibitors to these pathways. These results demonstrate the simultaneous involvement of cAMP- and calcium-dependent protein kinases in the stimulation of intestinal CCK gene transcription by protein-derived nutrients.

Protein kinase C inhibits type VI adenylyl cyclase by phosphorylating the regulatory N domain and two catalytic C1 and C2 domains

We previously showed that phosphorylation of Ser(10) of the N terminus domain of the type VI adenylyl cyclase (ACVI) partly mediated protein kinase C (PKC)-induced inhibition of ACVI. We now report that phosphorylation of the other two cytosolic domains (C1 and C2), which form the catalytic core complex of ACVI, also contributes to PKC-mediated inhibition. In vitro phosphorylation by PKC of the recombinant C1a and C2 domains, and of the synthetic peptides representing potential PKC phosphorylation sites, suggests that Ser(568) and Ser(674) of the C1 domain and Thr(931) of the C2 domain might act as substrates for PKC. We next created several full-length ACVI mutants in which one or more of the four likely PKC phosphorylation sites (Ser(10), Ser(568), Ser(674), and Thr(931)) were mutated to alanine. Simultaneous mutation of at least two of the three likely residues located in the N and C1 domains (Ser(10), Ser(568), and Ser(674)) was required to render ACVI variants completely insensitive to PKC treatment. In contrast, a single mutation of Thr(931) was sufficient to create a functional ACVI mutant that exhibited no detectable PKC-mediated inhibition, demonstrating the essentiality of Thr(931) to PKC-mediated regulation. Based on these results, we propose that the three cytosolic domains of ACVI might form a regulatory complex. Phosphorylation of this regulatory complex at different sites might induce a fine-tuning of the catalytic core complex and subsequently lead to alternation in the catalytic activity of ACVI.

An important role of neural activity-dependent CaMKIV signaling in the consolidation of long-term memory

Calcium/calmodulin-dependent protein kinase IV (CaMKIV) has been implicated in the regulation of CRE-dependent transcription. To investigate the role of this kinase in neuronal plasticity and memory, we generated transgenic mice in which the expression of a dominant-negative form of CaMKIV (dnCaMKIV) is restricted to the postnatal forebrain. In these transgenic mice, activity-induced CREB phosphorylation and c-Fos expression were significantly attenuated. Hippocampal late LTP (L-LTP) was also impaired, whereas basic synaptic function and early LTP (E-LTP) were unaffected. These deficits correlated with impairments in long-term memory, specifically in its consolidation/retention phase but not in the acquisition phase. These results indicate that neural activity-dependent CaMKIV signaling in the neuronal nucleus plays an important role in the consolidation/retention of hippocampus-dependent long-term memory.

Adenylyl cyclase 3 mediates prostaglandin E(2)-induced growth inhibition in arterial smooth muscle cells

Arterial smooth muscle cell (SMC) proliferation contributes to a number of vascular pathologies. Prostaglandin E(2) (PGE(2)), produced by the endothelium and by SMCs themselves, acts as a potent SMC growth inhibitor. The growth-inhibitory effects of PGE(2) are mediated through activation of G-protein-coupled membrane receptors, activation of adenylyl cyclases (ACs), formation of cAMP, and subsequent inhibition of mitogenic signal transduction pathways in SMCs. Of the 10 different mammalian AC isoforms known today, seven isoforms (AC2-7 and AC9) are expressed in SMCs from various species. We show that, despite the presence of several different AC isoforms, the principal AC isoform activated by PGE(2) in human arterial SMCs is a calmodulin kinase II-inhibited AC with characteristics similar to those of AC3. AC3 is expressed in isolated human arterial SMCs and in intact aorta. We further show that arterial SMCs isolated from AC3-deficient mice are resistant to PGE(2)-induced growth inhibition. In summary, AC3 is the principal AC isoform activated by PGE(2) in arterial SMCs, and AC3 mediates the growth-inhibitory effects of PGE(2). Because AC3 activity is inhibited by intracellular calcium through calmodulin kinase II, AC3 may serve as an important integrator of growth-inhibitory signals that stimulate cAMP formation and growth factors that increase intracellular calcium.

Ca(2+)-dependent sensitization of adenylyl cyclase activity

It has been shown that intracellular Ca(2+) concentrations have multiple modulatory influences on hormone-stimulated adenylyl cyclase activity. Here, we report that increasing intracellular Ca(2+) concentration by treating cells with the Ca(2+) ionophore A23187 leads to a sensitization of the beta-adrenoceptor-stimulated adenylyl cyclase activity in Ltk(-) cells expressing the human beta(2)-adrenoceptor. The ionophore treatment led to a 20+/-5% increase of the maximal isoproterenol-stimulated cyclase activity that could be prevented by chelation of the extracellular Ca(2+) using EGTA. A similar Ca(2+)-mediated sensitization (20+/-6%) was observed when the adenylyl cyclase was directly stimulated by the diterpene forskolin indicating that the catalytic activity of the enzyme itself is modulated by the change in Ca(2+) concentration. Sensitization of both the isoproterenol- and forskolin-stimulated adenylyl cyclase activities were completely blocked by treatment with KN-62[1-[N,O-bis-(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine], an inhibitor of the Ca(2+)-calmodulin-dependent protein kinase (CamKinase). Taken together, our data reveal the existence of a CamKinase-dependent sensitization process acting at the level of the adenylyl cyclase catalytic moiety.

Molecular biological approaches to unravel adenylyl cyclase signaling and function

Signal transduction through the cell membrane requires the participation of one or more plasma membrane proteins. For many transmembrane signaling events adenylyl cyclases (ACs) are the final effector enzymes which integrate and interpret divergent signals from different pathways. The enzymatic activity of adenylyl cyclases is stimulated or inhibited in response to the activation of a large number of receptors in virtually all cells of the human body. To date, ten different mammalian isoforms of adenylyl cyclase (AC) have been cloned and characterized. Each isoform has its own distinct tissue distribution and regulatory properties, providing possibilities for different cells to respond diversely to similar stimuli. The product of the enzymatic reaction catalyzed by ACs, cyclic AMP (cAMP) has been shown to play a crucial role for a variety of fundamental physiological cell functions ranging from cell growth and differentiation, to transcriptional regulation and apoptosis. In the past, investigations into the regulatory mechanisms of ACs were limited by difficulties associated with their purification and the availability of the proteins in any significant amount. Moreover, nearly every cell expresses several AC isoforms. Therefore, it was difficult to perform biochemical characterization of the different AC isoforms and nearly impossible to assess the physiological roles of the individual isoforms in intact cells, tissue or organisms. Recently, however, different molecular biological approaches have permitted several breakthroughs in the study of ACs. Recombinant technologies have allowed biochemical analysis of adenylyl cyclases in-vitro and the development of transgenic animals as well as knock-out mice have yielded new insights in the physiological role of some AC isoforms. In this review, we will focus mainly on the most novel approaches and concepts, which have delineated the mechanisms regulating AC and unravelled novel functions for this enzyme.

CaM kinase IV regulates lineage commitment and survival of erythroid progenitors in a non-cell-autonomous manner

Developmental functions of calmodulin-dependent protein kinase IV (CaM KIV) have not been previously investigated. Here, we show that CaM KIV transcripts are widely distributed during embryogenesis and that strict regulation of CaM KIV activity is essential for normal primitive erythropoiesis. Xenopus embryos in which CaM KIV activity is either upregulated or inhibited show that hematopoietic precursors are properly specified, but few mature erythrocytes are generated. Distinct cellular defects underlie this loss of erythrocytes: inhibition of CaM KIV activity causes commitment of hematopoietic precursors to myeloid differentiation at the expense of erythroid differentiation, on the other hand, constitutive activation of CaM KIV induces erythroid precursors to undergo apoptotic cell death. These blood defects are observed even when CaM KIV activity is misregulated only in cells that do not contribute to the erythroid lineage. Thus, proper regulation of CaM KIV activity in nonhematopoietic tissues is essential for the generation of extrinsic signals that enable hematopoietic stem cell commitment to erythroid differentiation and that support the survival of erythroid precursors.

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).

Peptide signaling paths related to intoxication, memory and addiction

Abstract Many peptides bind to G protein-coupled receptors and activate intracellular signaling paths for adaptive cellular responses. The components of these paths can be affected by signals from other neurotransmitters to produce overall integrated results not easily predicted from customary a priori considerations. This intracellular cross-talk among signaling paths provides a "filter" through which long-term tonic signals affect short-term phasic signals as they progress toward the nucleus and induce long-term adaptation of gene expression which provide enduring attributes of acquired memories and addictions. Peptides of the PACAP family provide intracellular signaling that involves kinases, scaffolding interactions, Ca2 + mobilization, and gene expression to facilitate development of tolerance to alcohol and development of associative memories. The peptide-induced enhancement of NMDA receptor responses to extracellular glutamate also may increase behavioral sensitization to the low doses of alcohol that occur at the onset of each bout of drinking. Because many gene products participate in each signaling path, each behavioral response to alcohol is a polygenic process of many steps with no single gene product sufficient to interpret fully the adaptive response to alcohol. Different susceptibility of individuals to alcohol addiction may be a cumulative result of small differences among the many signaling components. Understanding this network of signals may help interpret future "magic bullets" proposed to treat addiction.

Regulation of adenylyl cyclase in the central nervous system

Adenylyl cyclases (ACs) are a family of enzymes that synthesize one of the major second messengers, cAMP, upon stimulation. Since the report of the first adenylyl cyclase (AC) gene in 1989, tremendous efforts have been devoted to identifying and characterizing more AC isozymes. In the past decade, significant knowledge regarding the basic structure, tissue distribution, and regulation of AC isozymes has been accumulated. Because members of the AC superfamily are tightly controlled by various signals, one of the most important impacts of these AC isozymes is their contribution to the complexity and fine-tuning of cellular signalling, especially in the central nervous system (CNS) where multiple signals constantly occur. This review focuses on recent progress toward understanding the physiological roles of ACs in the CNS.

Activation of CA(2+)/calmodulin-dependent protein kinase IV in cultured rat hippocampal neurons

Ca(2+)/calmodulin-dependent protein kinase IV (CaM kinase IV) is a multifunctional enzyme that is abundantly present in the nuclei of neurons. We report the properties of phosphorylation and activation of CaM kinase IV in comparison to CaM kinase II in cultured rat hippocampal neurons. Phosphorylation and activity of CaM kinase IV as well as CaM kinase II were increased by treatment of neurons either with glutamate or high K(+). Glutamate-induced phosphorylation and activity of CaM kinase IV were blocked by N-methyl-D-asparate (NMDA) antagonists, and NMDA application instead of glutamate did increase CaM kinase IV phosphorylation. CaM kinase IV phosphorylation was also increased by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), and was blocked by an inhibitor of NMDA receptor. The AMPA-induced phosphorylation was blocked by tetrodotoxin, a Na(+) channel blocker, that was expected to block endogenous glutamate transmission indirectly. On the other hand, high K(+)-induced phosphorylation and activation were not blocked by inhibitors of glutamate receptors, and effectively blocked by nifedipine, an L-type Ca(2+) channel blocker. These properties were similar between CaM kinase IV and CaM kinase II.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

The N terminus domain of type VI adenylyl cyclase mediates its inhibition by protein kinase C

Previous results from our laboratory have shown that phosphorylation of type VI adenylyl cyclase (ACVI) by protein kinase C (PKC) caused suppression of adenylyl cyclase activity. In the present study, we investigated the role of the N terminus cytosolic domain of ACVI in this PKC-mediated inhibition of ACVI. Removal of amino acids 1 to 86 of ACVI or mutation of Ser(10) (a potential PKC phosphorylation site) into alanine significantly relieved the PKC-mediated inhibition and markedly reduced the PKC-evoked protein phosphorylation. PKC also effectively phosphorylated a recombinant N terminus cytosolic domain (amino acids 1-160) protein of ACVI and a synthetic peptide representing Ser(10). In addition, the amino acids 1 to 86 truncated mutant exhibited kinetic properties similar to those of the wild type. Taken together, these data demonstrate that the highly variable N terminus cytoplasmic domain of ACVI is a regulatory domain with a critical role in PKC-mediated suppression, which is a hallmark of this adenylyl cyclase isozyme. In addition, Ser(10) was found to serve as an acceptor for the PKC-mediated phosphorylating transfer of ACVI.

Immunological evidence that the beta isoform of Ca2+/calmodulin-dependent protein kinase IV is a cerebellar granule cell-specific product of the CaM kinase IV gene

Ca2+/calmodulin-dependent protein kinase IV (CaM kinase IV) exists as two monomeric isoforms, alpha and beta. In this study, we raised an antibody against the beta isoform and provided immunohistochemical evidence for specific expression of the beta isoform in cerebellar granule cells as a single gene-derived translational product distinct from the alpha isoform. Immunohistochemical examination showed that the beta-immunoreactivity was confined to the nuclei of the cerebellar granule cells, in contrast to the more widespread immunoreactivity for the alpha isoform in both nuclei and cytoplasm of the cerebellar granule cells and many other neurons with dominant nuclear localization. In developing cerebella, the beta-immunoreactivity gradually appeared in the internal granule cells during the postnatal 2nd and 3rd weeks, while the alpha-immunoreactivity had already appeared in the internal granule cells in the 1st postnatal week. Unlike the alpha isoform, beta-immunoreactivity was not detected in the Purkinje cells at any developmental stages. The differential expression of the alpha and beta isoforms suggests that each isoform may be involved in different cerebellar functions.

The Ca-calmodulin-dependent protein kinase cascade

The Ca2+-calmodulin-dependent protein kinase (CaM kinase) cascade includes three kinases: CaM-kinase kinase (CaMKK); and the CaM kinases CaMKI and CaMKIV, which are phosphorylated and activated by CaMKK. Members of this cascade respond to elevation of intracellular Ca2+ levels and are particularly abundant in brain and in T cells. CaMKK and CaMKIV localize both to the nucleus and to the cytoplasm, whereas CaMKI is only cytosolic. Nuclear CaMKIV regulates transcription through phosphorylation of several transcription factors, including CREB. In the cytoplasm, there is extensive cross-talk between CaMKK, CaMKIV and other signaling cascades, including those that involve the cAMP-dependent kinase (PKA), MAP kinases and protein kinase B (PKB; also known as Akt). Activation of PKB by CaMKK appears to be important in protection of neurons from programmed cell death during development.

The interactions of adenylate cyclases with P-site inhibitors

Recent kinetic, binding and crystallographic studies using P-site inhibitors of mammalian adenylate bases provide new insights into the catalytic mechanism of these highly regulated enzymes. Here, Carmen Dessauer and colleagues discuss the conformational states of adenylate cyclase, the structural determinants of inhibitor binding and the potential uses of these inhibitors as pharmacological agents.