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

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.

Preferential Expression of Ca2+-Stimulable Adenylyl Cyclase III in the Supraventricular Area, Including Arrhythmogenic Pulmonary Vein of the Rat Heart

Ectopic excitability in pulmonary veins (PVs) is the major cause of atrial fibrillation. We previously reported that the inositol trisphosphate receptor in rat PV cardiomyocytes cooperates with the Na+-Ca2+ exchanger to provoke ectopic automaticity in response to norepinephrine. Here, we focused on adenylyl cyclase (AC) as another effector of norepinephrine stimulation. RT-PCR, immunohistochemistry, and Western blotting revealed that the abundant expression of Ca2+-stimulable AC3 was restricted to the supraventricular area, including the PVs. All the other AC isotypes hardly displayed any region-specific expressions. Immunostaining of isolated cardiomyocytes showed an enriched expression of AC3 along the t-tubules in PV myocytes. The cAMP-dependent response of L-type Ca2+ currents in the PV and LA cells is strengthened by the 0.1 mM intracellular Ca2+ condition, unlike in the ventricular cells. The norepinephrine-induced automaticity of PV cardiomyocytes was reversibly suppressed by 100 µM SQ22536, an adenine-like AC inhibitor. These findings suggest that the specific expression of AC3 along t-tubules may contribute to arrhythmogenic automaticity in rat PV cardiomyocytes.

Sensory Transduction in Photoreceptors and Olfactory Sensory Neurons: Common Features and Distinct Characteristics

The past decades have seen tremendous progress in our understanding of the function of photoreceptors and olfactory sensory neurons, uncovering the mechanisms that determine their properties and, ultimately, our ability to see and smell. This progress has been driven to a large degree by the powerful combination of physiological experimental tools and genetic manipulations, which has enabled us to identify the main molecular players in the transduction cascades of these sensory neurons, how their properties affect the detection and discrimination of stimuli, and how diseases affect our senses of vision and smell. This review summarizes some of the common and unique features of photoreceptors and olfactory sensory neurons that make these cells so exciting to study.

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.

Transduction and Adaptation Mechanisms in the Cilium or Microvilli of Photoreceptors and Olfactory Receptors From Insects to Humans

Sensing changes in the environment is crucial for survival. Animals from invertebrates to vertebrates use both visual and olfactory stimuli to direct survival behaviors including identification of food sources, finding mates, and predator avoidance. In primary sensory neurons there are signal transduction mechanisms that convert chemical or light signals into an electrical response through ligand binding or photoactivation of a receptor, that can be propagated to the olfactory and visual centers of the brain to create a perception of the odor and visual landscapes surrounding us. The fundamental principles of olfactory and phototransduction pathways within vertebrates are somewhat analogous. Signal transduction in both systems takes place in the ciliary sub-compartments of the sensory cells and relies upon the activation of G protein-coupled receptors (GPCRs) to close cyclic nucleotide-gated (CNG) cation channels in photoreceptors to produce a hyperpolarization of the cell, or in olfactory sensory neurons open CNG channels to produce a depolarization. However, while invertebrate phototransduction also involves GPCRs, invertebrate photoreceptors can be either ciliary and/or microvillar with hyperpolarizing and depolarizing responses to light, respectively. Moreover, olfactory transduction in invertebrates may be a mixture of metabotropic G protein and ionotropic signaling pathways. This review will highlight differences of the visual and olfactory transduction mechanisms between vertebrates and invertebrates, focusing on the implications to the gain of the transduction processes, and how they are modulated to allow detection of small changes in odor concentration and light intensity over a wide range of background stimulus levels.

Intranasal sodium citrate in quantitative and qualitative olfactory dysfunction: results from a prospective, controlled trial of prolonged use in 60 patients

Objectives

We have previously shown that treatment with intranasal sodium citrate may be beneficial in post-infectious olfactory dysfunction. Sodium citrate reduces free intranasal calcium and is, therefore, thought to prevent calcium-mediated feedback inhibition at the level of the olfactory receptor. We aimed to determine whether treatment with a 2-week course of intranasal sodium citrate improves quantitative olfactory function in patients with post-infectious impairment. We also aimed to determine whether sodium citrate is beneficial in treating qualitative olfactory dysfunction.

Methods

We performed a prospective, controlled study. Patients applied intranasal sodium citrate solution to the right nasal cavity for 2 weeks. The left nasal cavity was untreated and, therefore, acted as an internal control. Monorhinal olfactory function was assessed using the “Sniffin’ Sticks” composite ‘TDI’ score, before and after treatment. The presence of parosmia and phantosmia was also assessed.

Results

Overall, there was a significant increase in TDI after treatment (using the best of right and left sides). Treatment with sodium citrate did not significantly improve quantitative olfactory function, compared to control. The proportion of patients reporting parosmia did not change significantly after treatment. However, there was a significant reduction in the proportion of patients reporting phantosmia, at the end of the study period.

Conclusions

Treatment with intranasal sodium citrate for a period of 2 weeks does not appear to improve quantitative olfactory function in patients with post-infectious impairment, compared to control. It may, however, be beneficial in treating phantosmia, which should be further addressed in future work.

The cyclic AMP signaling pathway in the rodent main olfactory system

Odor perception begins with the detection of odorant molecules by the main olfactory epithelium located in the nasal cavity. Odorant molecules bind to and activate a large family of G-protein-coupled odorant receptors and trigger a cAMP-mediated transduction cascade that converts the chemical stimulus into an electrical signal transmitted to the brain. Morever, odorant receptors and cAMP signaling plays a relevant role in olfactory sensory neuron development and axonal targeting to the olfactory bulb. This review will first explore the physiological response of olfactory sensory neurons to odorants and then analyze the different components of cAMP signaling and their different roles in odorant detection and olfactory sensory neuron development.

Sensory primary cilium is a responsive cAMP microdomain in renal epithelia

Primary cilia are hair-like cellular extensions that sense microenvironmental signals surrounding cells. The role of adenylyl cyclases in ciliary function has been of interest because the product of adenylyl cyclase activity, cAMP, is relevant to cilia-related diseases. In the present study, we show that vasopressin receptor type-2 (V2R) is localized to cilia in kidney epithelial cells. Pharmacologic inhibition of V2R with tolvaptan increases ciliary length and mechanosensory function. Genetic knockdown of V2R, however, does not have any effect on ciliary length, although the effect of tolvaptan on ciliary length is dampened. Our study reveals that tolvaptan may have a cilia-specific effect independent of V2R or verapamil-sensitive calcium channels. Live-imaging of single cilia shows that V2R activation increases cilioplasmic and cytoplasmic cAMP levels, whereas tolvaptan mediates cAMP changes only in a cilia-specific manner. Furthermore, fluid-shear stress decreases cilioplasmic, but not cytoplasmic cAMP levels. Our data indicate that cilioplasmic and cytoplasmic cAMP levels are differentially modulated. We propose that the cilium is a critical sensor acting as a responsive cAMP microcompartment during physiologically relevant stimuli.

Insights into the Regulatory Properties of Human Adenylyl Cyclase Type 9

Membrane-bound adenylyl cyclase (AC) isoforms have distinct regulatory mechanisms that contribute to their signaling specificity and physiologic roles. Although insight into the physiologic relevance of AC9 has progressed, the understanding of AC9 regulation is muddled with conflicting studies. Currently, modes of AC9 regulation include stimulation by Gαs, protein kinase C (PKC) βII, or calcium-calmodulin kinase II (CaMKII) and inhibition by Gαi/o, novel PKC isoforms, or calcium-calcineurin. Conversely, the original cloning of human AC9 reported that AC9 is insensitive to Gαi inhibition. The purpose of our study was to clarify which proposed regulators of AC9 act directly or indirectly, particularly with respect to Gαi/o. The proposed regulators, including G proteins (Gαs, Gαi, Gαo, Gβγ), protein kinases (PKCβII, CaMKII), and forskolin, were systematically evaluated using classic in vitro AC assays and cell-based cAMP accumulation assays in COS-7 cells. Our studies show that AC9 is directly regulated by Gαs with weak conditional activation by forskolin; other modes of proposed regulation either occur indirectly or possibly require additional scaffolding proteins to facilitate regulation. We also show that AC9 contributes to basal cAMP production; knockdown or knockout of endogenous AC9 reduces basal AC activity in COS-7 cells and splenocytes. Importantly, although AC9 is not directly inhibited by Gαi/o, it can heterodimerize with Gαi/o-regulated isoforms, AC5 and AC6.

Simultaneous Loss of NCKX4 and CNG Channel Desensitization Impairs Olfactory Sensitivity

In vertebrate olfactory sensory neurons (OSNs), Ca²⁺ plays key roles in both mediating and regulating the olfactory response. Ca²⁺ enters OSN cilia during the response through the olfactory cyclic nucleotide-gated (CNG) channel and stimulates a depolarizing chloride current by opening the olfactory Ca²⁺-activated chloride channel to amplify the response. Ca²⁺ also exerts negative regulation on the olfactory transduction cascade, through mechanisms that include reducing the CNG current by desensitizing the CNG channel via Ca²⁺/calmodulin (CaM), to reduce the response. Ca²⁺ is removed from the cilia primarily by the K⁺-dependent Na⁺/Ca²⁺ exchanger 4 (NCKX4), and the removal of Ca²⁺ leads to closure of the chloride channel and response termination. In this study, we investigate how two mechanisms conventionally considered negative regulatory mechanisms of olfactory transduction, Ca²⁺ removal by NCKX4, and desensitization of the CNG channel by Ca²⁺/CaM, interact to regulate the olfactory response. We performed electro-olfactogram (EOG) recordings on the double-mutant mice, NCKX4^-/-;CNGB1^ΔCaM, which are simultaneously lacking NCKX4 (NCKX4^-/-) and Ca²⁺/CaM-mediated CNG channel desensitization (CNGB1^ΔCaM). Despite exhibiting alterations in various response attributes, including termination kinetics and adaption properties, OSNs in either NCKX4^-/- mice or CNGB1^ΔCaM mice show normal resting sensitivity, as determined by their unchanged EOG response amplitude. We found that OSNs in NCKX4^-/-;CNGB1^ΔCaM mice displayed markedly reduced EOG amplitude accompanied by alterations in other response attributes. This study suggests that what are conventionally considered negative regulatory mechanisms of olfactory transduction also play a role in setting the resting sensitivity in OSNs.

SIGNIFICANCE STATEMENT

Sensory receptor cells maintain high sensitivity at rest. Although the mechanisms responsible for setting the resting sensitivity of sensory receptor cells are not well understood, it has generally been assumed that the sensitivity is set primarily by how effectively the components in the activation cascade of sensory transduction can be stimulated. Our findings in mouse olfactory sensory neurons suggest that mechanisms that are primarily responsible for terminating the olfactory response are also critical for proper resting sensitivity.

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.

The effect of intranasal sodium citrate on olfaction in post-infectious loss: results from a prospective, placebo-controlled trial in 49 patients

OBJECTIVES

Free calcium plays an integral role in peripheral olfactory processing, including feedback inhibition. It has therefore been suggested that reduction of intranasal free calcium with buffer solutions such as sodium citrate may improve olfactory function in patients with smell impairment. Several previous studies have supported this hypothesis, particularly in post-infectious olfactory loss. We therefore aimed to determine whether treatment with intranasal sodium citrate improves olfactory function in patients with post-infectious impairment.

DESIGN

Prospective, single-blind, placebo-controlled trial.

SETTING

Interdisciplinary Smell and Taste Clinic, TU Dresden (tertiary referral centre).

PARTICIPANTS

Forty-nine adult participants with post-infectious olfactory impairment (M : F = 11 : 38, mean age 58.71 ± 11.03 years).

MAIN OUTCOME MEASURES

Olfactory function (odour threshold and identification) before and after treatment as determined using "Sniffin' Sticks". Patients were treated monorhinally with 1 mL sodium citrate solution. The contralateral nasal cavity was treated with 1 mL physiological sodium chloride solution, which acted as internal control. Clinical improvement was assumed where threshold or identification score increased by ≥2.5 or 3 points, respectively, or ≥5.5 points together.

RESULTS

We demonstrated a statistically significant improvement in composite threshold + identification scores following treatment with sodium citrate, compared with placebo. This was true for all patients (mean improvement 0.87 ± 2.68 points, P = 0.04), and on subgroup analysis in those with hyposmia (mean improvement 1.15 ± 2.37 points, P = 0.02). However, the effect size did not reach clinical significance.

CONCLUSIONS

Further basic and clinical work is required to fully delineate the effect of intranasal sodium citrate in the treatment of post-infectious olfactory loss.

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.

Deep sequencing of the murine olfactory receptor neuron transcriptome

The ability of animals to sense and differentiate among thousands of odorants relies on a large set of olfactory receptors (OR) and a multitude of accessory proteins within the olfactory epithelium (OE). ORs and related signaling mechanisms have been the subject of intensive studies over the past years, but our knowledge regarding olfactory processing remains limited. The recent development of next generation sequencing (NGS) techniques encouraged us to assess the transcriptome of the murine OE. We analyzed RNA from OEs of female and male adult mice and from fluorescence-activated cell sorting (FACS)-sorted olfactory receptor neurons (ORNs) obtained from transgenic OMP-GFP mice. The Illumina RNA-Seq protocol was utilized to generate up to 86 million reads per transcriptome. In OE samples, nearly all OR and trace amine-associated receptor (TAAR) genes involved in the perception of volatile amines were detectably expressed. Other genes known to participate in olfactory signaling pathways were among the 200 genes with the highest expression levels in the OE. To identify OE-specific genes, we compared olfactory neuron expression profiles with RNA-Seq transcriptome data from different murine tissues. By analyzing different transcript classes, we detected the expression of non-olfactory GPCRs in ORNs and established an expression ranking for GPCRs detected in the OE. We also identified other previously undescribed membrane proteins as potential new players in olfaction. The quantitative and comprehensive transcriptome data provide a virtually complete catalogue of genes expressed in the OE and present a useful tool to uncover candidate genes involved in, for example, olfactory signaling, OR trafficking and recycling, and proliferation.

Roles of calcium/calmodulin-dependent kinase II in long-term memory formation in crickets

Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a key molecule in many systems of learning and memory in vertebrates, but roles of CaMKII in invertebrates have not been characterized in detail. We have suggested that serial activation of NO/cGMP signaling, cyclic nucleotide-gated channel, Ca²⁺/CaM and cAMP signaling participates in long-term memory (LTM) formation in olfactory conditioning in crickets, and here we show participation of CaMKII in LTM formation and propose its site of action in the biochemical cascades. Crickets subjected to 3-trial conditioning to associate an odor with reward exhibited memory that lasts for a few days, which is characterized as protein synthesis-dependent LTM. In contrast, animals subjected to 1-trial conditioning exhibited memory that lasts for only several hours (mid-term memory, MTM). Injection of a CaMKII inhibitor prior to 3-trial conditioning impaired 1-day memory retention but not 1-hour memory retention, suggesting that CaMKII participates in LTM formation but not in MTM formation. Animals injected with a cGMP analogue, calcium ionophore or cAMP analogue prior to 1-trial conditioning exhibited 1-day retention, and co-injection of a CaMKII inhibitor impaired induction of LTM by the cGMP analogue or that by the calcium ionophore but not that by the cAMP analogue, suggesting that CaMKII is downstream of cGMP production and Ca²⁺ influx and upstream of cAMP production in biochemical cascades for LTM formation. Animals injected with an adenylyl cyclase (AC) activator prior to 1-trial conditioning exhibited 1-day retention. Interestingly, a CaMKII inhibitor impaired LTM induction by the AC activator, although AC is expected to be a downstream target of CaMKII. The results suggest that CaMKII interacts with AC to facilitate cAMP production for LTM formation. We propose that CaMKII serves as a key molecule for interplay between Ca²⁺ signaling and cAMP signaling for LTM formation, a new role of CaMKII in learning and memory.

Emerging role of CaMKII in neuropsychiatric disease

Although it has been known for decades that hippocampal calcium/calmodulin (CaM)-dependent protein kinase II (CaMKII) plays an essential role in learning and memory consolidation, the roles of CaMKII in other brain regions are only recently being explored in depth. A series of recent studies suggest that CaMKII dysfunction throughout the brain may underlie myriad neuropsychiatric disorders, including drug addiction, schizophrenia, depression, epilepsy, and multiple neurodevelopmental disorders, perhaps through maladaptations in glutamate signaling and neuroplasticity. I review here the structure, function, subcellular localization, and expression patterns of CaMKII isoforms, as well as recent advances demonstrating that disturbances in these properties may contribute to psychiatric disorders.

Impacts of hypersaline acclimation on the acute toxicity of the organophosphate chlorpyrifos to salmonids

Acclimation to hypersaline conditions enhances the acute toxicity of certain thioether organophosphate and carbamate pesticides in some species of euryhaline fish. As the organophosphate chlorpyrifos is commonly detected in salmonid waterways, the impacts of hypersaline conditions on its toxicity were examined. In contrast to other previously examined pesticides, time to death by chlorpyrifos was more rapid in freshwater than in hypersaline water (16ppth). The median lethal time (LT50) after 100μg/L chlorpyrifos exposure was 49h (95% CI: 31-78) and 120h (95% CI: 89-162) for rainbow trout (Oncorhynchus mykiss) in freshwater and those acclimated to hypersaline conditions, respectively. Previous studies with hypersaline acclimated fish indicated induction of xenobiotic metabolizing enzymes that may detoxify chlorpyrifos. In the current study, chlorpyrifos metabolism was unaltered in liver and gill microsomes of freshwater and hypersaline acclimated fish. Acetylcholinesterase inhibition in brain and bioavailability of chlorpyrifos from the aqueous exposure media were also unchanged. In contrast, mRNA expression of neurological targets: calcium calmodulin dependent protein kinase II delta, chloride intracellular channel 4, and G protein alpha i1 were upregulated in saltwater acclimated fish, consistent with diminished neuronal signaling which may protect animals from cholinergic overload associated with acetylcholinesterase inhibition. These results indicate targets other than acetylcholinesterase may contribute to the altered toxicity of chlorpyrifos in salmonids under hypersaline conditions.

PKCβII acts downstream of chemoattractant receptors and mTORC2 to regulate cAMP production and myosin II activity in neutrophils

mTORC2 has been shown to be involved in cytoskeletal regulation, but the mechanisms by which this takes place are poorly understood. This study shows that PKCβII is specifically required for mTORC2-dependent activation of adenylyl cyclase 9 and back retraction during neutrophil chemotaxis to chemoattractants.

Chemotaxis is a process by which cells polarize and move up a chemical gradient through the spatiotemporal regulation of actin assembly and actomyosin contractility, which ultimately control front protrusions and back retractions. We previously demonstrated that in neutrophils, mammalian target of rapamycin complex 2 (mTORC2) is required for chemoattractant-mediated activation of adenylyl cyclase 9 (AC9), which converts ATP into cAMP and regulates back contraction through MyoII phosphorylation. Here we study the mechanism by which mTORC2 regulates neutrophil chemotaxis and AC9 activity. We show that inhibition of protein kinase CβII (PKCβII) by CPG53353 or short hairpin RNA knockdown severely inhibits chemoattractant-induced cAMP synthesis and chemotaxis in neutrophils. Remarkably, PKCβII-inhibited cells exhibit specific and severe tail retraction defects. In response to chemoattractant stimulation, phosphorylated PKCβII, but not PKCα, is transiently translocated to the plasma membrane, where it phosphorylates and activates AC9. mTORC2-mediated PKCβII phosphorylation on its turn motif, but not its hydrophobic motif, is required for membrane translocation of PKCβII. Inhibition of mTORC2 activity by Rictor knockdown not only dramatically decreases PKCβII activity, but it also strongly inhibits membrane translocation of PKCβII. Together our findings show that PKCβII is specifically required for mTORC2-dependent AC9 activation and back retraction during neutrophil chemotaxis.

Non-raft adenylyl cyclase 2 defines a cAMP signaling compartment that selectively regulates IL-6 expression in airway smooth muscle cells: differential regulation of gene expression by AC isoforms

Adenylyl cyclase (AC) isoforms differ in their tissue distribution, cellular localization, regulation, and protein interactions. Most cell types express multiple AC isoforms. We hypothesized that cAMP produced by different AC isoforms regulates unique cellular responses in human bronchial smooth muscle cells (BSMC). Overexpression of AC2, AC3, or AC6 had distinct effects on forskolin (Fsk)-induced expression of a number of known cAMP-responsive genes. These data show that different AC isoforms can differentially regulate gene expression. Most notable, overexpression and activation of AC2 enhanced interleukin 6 (IL-6) expression, but overexpression of AC3 or AC6 had no effect. IL-6 production by BSMC was induced by Fsk and select G protein-coupled receptor (GPCR) agonists, though IL-6 levels did not directly correlate with global cAMP levels. Treatment with PKA selective 6-Bnz-cAMP or Epac selective 8-CPT-2Me-cAMP cAMP analogs revealed a predominant role for PKA in cAMP-mediated induction of IL-6. IL-6 promoter mutations demonstrated that AP-1 and CRE transcription sites were required for Fsk to stimulate IL-6 expression. Our present study defines an AC2 cAMP signaling compartment that specifically regulates IL-6 expression in BSMC via Epac and PKA and demonstrates that other AC isoforms are excluded from this pool.

Brief exposure to copper induces apoptosis and alters mediators of olfactory signal transduction in coho salmon

Pacific salmon are particularly susceptible to copper (Cu)-induced olfactory injuries that can ultimately inhibit neurobehaviors critical to survival. However, the molecular mechanisms underlying Cu-mediated olfactory impairment remain poorly understood. In the present study, we conducted a short-term Cu exposure at levels relevant to urban runoff (5, 25 and 50 ppb) , and investigated the roles of impaired olfactory signal transduction and induced apoptosis as underlying mechanisms of olfactory injury. Increased cell death in the olfactory epithelium was evident in coho receiving 4h exposures to 25 and 50 ppb Cu. Expression of olfactory marker protein (omp), a marker of mature olfactory sensory neurons, also decreased at 50 ppb Cu. Immunohistochemical analysis of coho olfactory epithelium demonstrated a loss of type 3 adenylate cyclase (ACIII) in the apical olfactory epithelium cilia at all levels of Cu exposure, suggesting an inhibitory effect of Cu in olfactory signaling. Accompanying the loss of ACIII in Cu-exposed coho were reduced intracellular cyclic guanosine monophosphate (cGMP) levels in the olfactory rosettes. Collectively, these results support a linkage among the initial steps of olfactory signaling in Cu-induced salmon olfactory injury, and suggesting that monitoring olfactory cGMP levels may aid in the assessment of salmon olfactory injury.

Stimulation of electro-olfactogram responses in the main olfactory epithelia by airflow depends on the type 3 adenylyl cyclase

Cilia of olfactory sensory neurons are the primary sensory organelles for olfaction. The detection of odorants by the main olfactory epithelium (MOE) depends on coupling of odorant receptors to the type 3 adenylyl cyclase (AC3) in olfactory cilia. We monitored the effect of airflow on electro-olfactogram (EOG) responses and found that the MOE of mice can sense mechanical forces generated by airflow. The airflow-sensitive EOG response in the MOE was attenuated when cAMP was increased by odorants or by forskolin suggesting a common mechanism for airflow and odorant detection. In addition, the sensitivity to airflow was significantly impaired in the MOE from AC3(-/-) mice. We conclude that AC3 in the MOE is required for detecting the mechanical force of airflow, which in turn may regulate odorant perception during sniffing.

Phosphorylation of adenylyl cyclase III at serine1076 does not attenuate olfactory response in mice

Feedback inhibition of adenylyl cyclase III (ACIII) via Ca(2+)-induced phosphorylation has long been hypothesized to contribute to response termination and adaptation of olfactory sensory neurons (OSNs). To directly determine the functional significance of this feedback mechanism for olfaction in vivo, we genetically mutated serine(1076) of ACIII, the only residue responsible for Ca(2+)-induced phosphorylation and inhibition of ACIII (Wei et al., 1996, 1998), to alanine in mice. Immunohistochemistry and Western blot analysis showed that the mutation affects neither the cilial localization nor the expression level of ACIII in OSNs. Electroolfactogram analysis showed no differences in the responses between wild-type and mutant mice to single-pulse odorant stimulations or in several stimulation paradigms for adaptation. These results suggest that phosphorylation of ACIII on serine(1076) plays a far less important role in olfactory response attenuation than previously thought.

Phosphorylation of adenylyl cyclase III at serine1076 does not attenuate olfactory response in mice

Feedback inhibition of adenylyl cyclase III (ACIII) via Ca(2+)-induced phosphorylation has long been hypothesized to contribute to response termination and adaptation of olfactory sensory neurons (OSNs). To directly determine the functional significance of this feedback mechanism for olfaction in vivo, we genetically mutated serine(1076) of ACIII, the only residue responsible for Ca(2+)-induced phosphorylation and inhibition of ACIII (Wei et al., 1996, 1998), to alanine in mice. Immunohistochemistry and Western blot analysis showed that the mutation affects neither the cilial localization nor the expression level of ACIII in OSNs. Electroolfactogram analysis showed no differences in the responses between wild-type and mutant mice to single-pulse odorant stimulations or in several stimulation paradigms for adaptation. These results suggest that phosphorylation of ACIII on serine(1076) plays a far less important role in olfactory response attenuation than previously thought.

Adenylyl cyclase 2 selectively couples to E prostanoid type 2 receptors, whereas adenylyl cyclase 3 is not receptor-regulated in airway smooth muscle

Adenylyl cyclases (ACs) are important regulators of airway smooth muscle function, because β-adrenergic receptor (βAR) agonists stimulate AC activity and cAMP production. We have previously shown in a number of cell types that AC6 selectively couples to βAR and these proteins are coexpressed in lipid rafts. We overexpressed AC2, AC3, and AC6 in mouse bronchial smooth muscle cells (mBSMCs) and human embryonic kidney (HEK)-293 cells by using recombinant adenoviruses and assessed their localization and regulation by various G protein-coupled receptors (GPCRs). AC3 and AC6 were expressed primarily in caveolin-rich fractions, whereas AC2 expression was excluded from these domains. AC6 expression enhanced cAMP production in response to isoproterenol but did not increase responses to butaprost, reflecting the colocalization of AC6 with β(2)AR but not E prostanoid type 2 receptor (EP(2)R) in lipid raft fractions. AC2 expression enhanced butaprost-stimulated cAMP production but had no effect on the β(2)AR-mediated response. AC3 did not couple to any GPCR tested. Forskolin-induced arborization of mBSMCs was assessed as a functional readout of cAMP signaling. Arborization was enhanced by overexpression of AC6 and AC3, but AC2 had no effect. GPCR-stimulated arborization mirrored the selective coupling observed for cAMP production. With the addition of the phosphodiesterase 4 (PDE4) inhibitor rolipram AC2 accelerated forskolin-stimulated arborization. Thus, AC2 selectively couples to EP(2)R, but signals from this complex are limited by PDE4 activity. AC3 does not seem to couple to GPCR in either mBSMCs or HEK-293 cells, so it probably exists in a distinct signaling domain in these cells.

Development and distribution of neuronal cilia in mouse neocortex

Neuronal primary cilia are not generally recognized, but they are considered to extend from most, if not all, neurons in the neocortex. However, when and how cilia develop in neurons are not known. This study used immunohistochemistry for adenylyl cyclase III (ACIII), a marker of primary cilia, and electron microscopic analysis to describe the development and maturation of cilia in mouse neocortical neurons. Our results indicate that ciliogenesis is initiated in late fetal stages after neuroblast migration, when the mother centriole docks with the plasma membrane, becomes a basal body, and grows a cilia bud that we call a procilium. This procilium consists of a membranous protrusion extending from the basal body but lacking axonemal structure and remains undifferentiated until development of the axoneme and cilia elongation starts at about postnatal day 4. Neuronal cilia elongation and final cilia length depend on layer position, and the process extends for a long time, lasting 8-12 weeks. We show that, in addition to pyramidal neurons, inhibitory interneurons also grow cilia of comparable length, suggesting that cilia are indeed present in all neocortical neuron subtypes. Furthermore, the study of mice with defective ciliogenesis suggested that failed elongation of cilia is not essential for proper neuronal migration and laminar organization or establishment of neuronal polarity. Thus, the function of this organelle in neocortical neurons remains elusive.

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.

Human bronchial smooth muscle cells express adenylyl cyclase isoforms 2, 4, and 6 in distinct membrane microdomains

Adenylyl cyclases (AC) are important regulators of airway smooth muscle function, because β-adrenergic receptor (AR) agonists stimulate AC activity and increase airway diameter. We assessed expression of AC isoforms in human bronchial smooth muscle cells (hBSMC). Reverse transcriptase-polymerase chain reaction and immunoblot analyses detected expression of AC2, AC4, and AC6. Forskolin-stimulated AC activity in membranes from hBSMC displayed Ca(2+)-inhibited and G(βγ)-stimulated AC activity, consistent with expression of AC6, AC2, and AC4. Isoproterenol-stimulated AC activity was inhibited by Ca(2+) but unaltered by G(βγ), whereas butaprost-stimulated AC activity was stimulated by G(βγ) but unaffected by Ca(2+) addition. Using sucrose density centrifugation to isolate lipid raft fractions, we found that only AC6 localized in lipid raft fractions, whereas AC2 and AC4 localized in nonraft fractions. Immunoisolation of caveolae using caveolin-1 antibodies yielded Ca(2+)-inhibited AC activity (consistent with AC6 expression), whereas the nonprecipitated material displayed G(βγ)-stimulated AC activity (consistent with expression of AC2 and/or AC4). Overexpression of AC6 enhanced cAMP production in response to isoproterenol and beraprost but did not increase responses to prostaglandin E(2) or butaprost. β(2)AR, but not prostanoid EP(2) or EP(4) receptors, colocalized with AC5/6 in lipid raft fractions. Thus, particular G protein-coupled receptors couple to discreet AC isoforms based, in part, on their colocalization in membrane microdomains. These different cAMP signaling compartments in airway smooth muscle cells are responsive to different hormones and neurotransmitters and can be regulated by different coincident signals such as Ca(2+) and G(βγ).

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.

Serotonin stimulation of cAMP-dependent plasticity in Aplysia sensory neurons is mediated by calmodulin-sensitive adenylyl cyclase

Calmodulin (CaM)-sensitive adenylyl cyclase (AC) in sensory neurons (SNs) in Aplysia has been proposed as a molecular coincidence detector during conditioning. We identified four putative ACs in Aplysia CNS. CaM binds to a sequence in the C1b region of AC-AplA that resembles the CaM-binding sequence in the C1b region of AC1 in mammals. Recombinant AC-AplA was stimulated by Ca(2+)/CaM. AC-AplC is most similar to the Ca(2+)-inhibited AC5 and AC6 in mammals. Recombinant AC-AplC was directly inhibited by Ca(2+), independent of CaM. AC-AplA and AC-AplC are expressed in SNs, whereas AC-AplB and AC-AplD are not. Knockdown of AC-AplA demonstrated that serotonin stimulation of cAMP-dependent plasticity in SNs is predominantly mediated by this CaM-sensitive AC. We propose that the coexpression of a Ca(2+)-inhibited AC in SNs, together with a Ca(2+)/CaM-stimulated AC, would enhance the associative requirement for coincident Ca(2+) influx and serotonin for effective stimulation of cAMP levels and initiation of plasticity mediated by AC-AplA.

CFTR-adenylyl cyclase I association responsible for UTP activation of CFTR in well-differentiated primary human bronchial cell cultures

Chloride secretion by airway epithelial cells is defective in cystic fibrosis (CF). The conventional paradigm is that CFTR is activated through cAMP and protein kinase A (PKA), whereas the Ca(2+)-activated chloride channel (CaCC) is activated by Ca(2+) agonists like UTP. We found that most chloride current elicited by Ca(2+) agonists in primary cultures of human bronchial epithelial cells is mediated by CFTR by a mechanism involving Ca(2+) activation of adenylyl cyclase I (AC1) and cAMP/PKA signaling. Use of selective inhibitors showed that Ca(2+) agonists produced more chloride secretion from CFTR than from CaCC. CFTR-dependent chloride secretion was reduced by PKA inhibition and was absent in CF cell cultures. Ca(2+) agonists produced cAMP elevation, which was blocked by adenylyl cyclase inhibition. AC1, a Ca(2+)/calmodulin-stimulated adenylyl cyclase, colocalized with CFTR in the cell apical membrane. RNAi knockdown of AC1 selectively reduced UTP-induced cAMP elevation and chloride secretion. These results, together with correlations between cAMP and chloride current, suggest that compartmentalized AC1-CFTR association is responsible for Ca(2+)/cAMP cross-talk. We further conclude that CFTR is the principal chloride secretory pathway in non-CF airways for both cAMP and Ca(2+) agonists, providing a novel mechanism to link CFTR dysfunction to CF lung disease.

The calmodulin-stimulated adenylate cyclase ADCY8 sets the sensitivity of zebrafish retinal axons to midline repellents and is required for normal midline crossing

The chemokine SDF1 activates a cAMP-mediated signaling pathway that antagonizes retinal responses to the midline repellent slit. We show that knocking down the calmodulin-activated adenylate cyclase ADCY8 makes retinal axons insensitive to SDF1. Experiments in vivo using male and female zebrafish (Danio rerio) confirm a mutual antagonism between slit signaling and ADCY8-mediated signaling. Unexpectedly, knockdown of ADCY8 or another calmodulin-activated cyclase, ADCY1, induces ipsilateral misprojections of retinal axons that would normally cross the ventral midline. We demonstrate a cell-autonomous requirement for ADCY8 in retinal neurons for normal midline crossing. These findings are the first to show that ADCY8 is required for axonal pathfinding before axons reach their targets. They support a model in which ADCY8 is an essential component of a signaling pathway that opposes repellent signaling. Finally, they demonstrate that ADCY8 helps regulate retinal sensitivity to midline guidance cues.

Somatostatin signaling in neuronal cilia is critical for object recognition memory

Most neurons possess a single, nonmotile cilium that projects out from the cell surface. These microtubule-based organelles are important in brain development and neurogenesis; however, their function in mature neurons is unknown. Cilia express a complement of proteins distinct from other neuronal compartments, one of which is the somatostatin receptor subtype SST(3). We show here that SST(3) is critical for object recognition memory in mice. sst3 knock-out mice are severely impaired in discriminating novel objects, whereas they retain normal memory for object location. Further, systemic injection of an SST(3) antagonist (ACQ090) disrupts recall of familiar objects in wild-type mice. To examine mechanisms of SST(3), we tested synaptic plasticity in CA1 hippocampus. Electrically evoked long-term potentiation (LTP) was normal in sst3 knock-out mice, while adenylyl cyclase/cAMP-mediated LTP was impaired. The SST(3) antagonist also disrupted cAMP-mediated LTP. Basal cAMP levels in hippocampal lysate were reduced in sst3 knock-out mice compared with wild-type mice, while the forskolin-induced increase in cAMP levels was normal. The SST(3) antagonist inhibited forskolin-stimulated cAMP increases, whereas the SST(3) agonist L-796,778 increased basal cAMP levels in hippocampal slices but not hippocampal lysate. Our results show that somatostatin signaling in neuronal cilia is critical for recognition memory and suggest that the cAMP pathway is a conserved signaling motif in cilia. Neuronal cilia therefore represent a novel nonsynaptic compartment crucial for signaling involved in a specific form of synaptic plasticity and in novelty detection.

Light modulates the melanophore response to alpha-MSH in Xenopus laevis: an analysis of the signal transduction crosstalk mechanisms involved

Melanin granule (melanosome) dispersion within Xenopus laevis melanophores is evoked either by light or alpha-MSH. We have previously demonstrated that the initial biochemical steps of light and alpha-MSH signaling are distinct, since the increase in cAMP observed in response to alpha-MSH was not seen after light exposure. cAMP concentrations in response to alpha-MSH were significantly lower in cells pre-exposed to light as compared to the levels in dark-adapted melanophores. Here we demonstrate the presence of an adenylyl cyclase (AC) in the Xenopus melanophore, similar to the mammalian type IX which is inhibited by Ca(2+)-calmodulin-activated phosphatase. This finding supports the hypothesis that the cyclase could be negatively modulated by a light-promoted Ca(2+) increase. In fact, the activity of calcineurin PP2B phosphatase was increased by light, which could result in AC IX inhibition, thus decreasing the response to alpha-MSH. St-Ht31, a disrupting agent of protein kinase A (PKA)-anchoring kinase A protein (AKAP) complex totally blocked the melanosome dispersing response to alpha-MSH, but did not impair the photo-response in Xenopus melanophores. Sequence comparison of a melanophore AKAP partial clone with GenBank sequences showed that the anchoring protein was a gravin-like adaptor previously sequenced from Xenopus non-pigmentary tissues. Co-immunoprecipitation of Xenopus AKAP and the catalytic subunit of PKA demonstrated that PKA is associated with AKAP and it is released in the presence of alpha-MSH. We conclude that in X. laevis melanophores, AKAP12 (gravin-like) contains a site for binding the inactive PKA thus compartmentalizing PKA signaling and also possesses binding sites for PKC. Light diminishes alpha-MSH-induced increase of cAMP by increasing calcineurin (PP2B) activity, which in turn inhibits adenylyl cyclase type IX, and/or by activating PKC, which phosphorylates the gravin-like molecule, thus destabilizing its binding to the cell membrane.

Evolutionary conservation of the signaling proteins upstream of cyclic AMP-dependent kinase and protein kinase C in gastropod mollusks

The protein kinase C (PKC) and the cAMP-dependent kinase (protein kinase A; PKA) pathways are known to play important roles in behavioral plasticity and learning in the nervous systems of a wide variety of species across phyla. We briefly review the members of the PKC and PKA family and focus on the evolution of the immediate upstream activators of PKC and PKA i.e., phospholipase C (PLC) and adenylyl cyclase (AC), and their conservation in gastropod mollusks, taking advantage of the recent assembly of the Aplysiacalifornica and Lottia gigantea genomes. The diversity of PLC and AC family members present in mollusks suggests a multitude of possible mechanisms to activate PKA and PKC; we briefly discuss the relevance of these pathways to the known physiological activation of these kinases in Aplysia neurons during plasticity and learning. These multiple mechanisms of activation provide the gastropod nervous system with tremendous flexibility for implementing neuromodulatory responses to both neuronal activity and extracellular signals.

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.

Organization and Ca2+Regulation of Adenylyl Cyclases in cAMP Microdomains

The adenylyl cyclases are variously regulated by G protein subunits, a number of serine/threonine and tyrosine protein kinases, and Ca2+. In some physiological situations, this regulation can be readily incorporated into a hormonal cascade, controlling processes such as cardiac contractility or neurotransmitter release. However, the significance of some modes of regulation is obscure and is likely only to be apparent in explicit cellular contexts (or stages of the cell cycle). The regulation of many of the ACs by the ubiquitous second messenger Ca2+provides an overarching mechanism for integrating the activities of these two major signaling systems. Elaborate devices have been evolved to ensure that this interaction occurs, to guarantee the fidelity of the interaction, and to insulate the microenvironment in which it occurs. Subcellular targeting, as well as a variety of scaffolding devices, is used to promote interaction of the ACs with specific signaling proteins and regulatory factors to generate privileged domains for cAMP signaling. A direct consequence of this organization is that cAMP will exhibit distinct kinetics in discrete cellular domains. A variety of means are now available to study cAMP in these domains and to dissect their components in real time in live cells. These topics are explored within the present review.

Introduction of bisecting GlcNAc in N-glycans of adenylyl cyclase III enhances its activity

Adenylyl cyclases (ACs) catalyze the synthesis of cAMP in response to extracellular and intracellular signals and are responsible for a wide variety of biological activities including cell growth, differentiation, and metabolism. There are nine, currently known, isoforms of transmembrane ACs, and the primary structure of the catalytic unit and the potential N-glycosylation sites are highly conserved among them. The enzyme beta1,4-N-acetylglucosaminyltransferase III (GnT-III) catalyzes the addition of a bisecting N-acetylglucosamine (GlcNAc) to N-glycans. We have been studying the function of GnT-III on signaling molecules. In this study, we report on the effects of a bisecting GlcNAc on AC signaling. We established GnT-III stable expressing cell lines of Neuro-2a mouse neuroblastoma cells and B16 mouse melanoma cells. Forskolin-induced AC activation and downstream signaling, such as the synthesis of cAMP and the phosphorylation of transcriptional factor CRE-binding protein were upregulated in the GnT-III transfectants compared with mock transfectants or a dominant negative mutant of GnT-III-transfected cells. Since endogenous AC expression levels in Neuro-2a and B16 cells were too low to permit the glycosylation status to be examined, AC type III (ACIII) was overexpressed in a stable expression system using Flp-In-293 cells. The N-glycans of ACIII in the GnT-III transfectants were confirmed to be modified by the introduction of a bisecting GlcNAc, and AC activity was found to be significantly up-regulated in the GnT-III transfectants. Thus, the structure of N-glycans of ACIII regulates its enzymatic activity and downstream signaling.

GABA inhibition of cyclic AMP production in immortalized GnRH neurons is mediated by calcineurin-dependent dephosphorylation of adenylyl cyclase 9

The neurotransmitter gamma-aminobutyric acid (GABA) is an important modulator of gonadotropin-releasing hormone (GnRH), and consequently of reproduction. GABA, acting via ionotropic GABAA receptors, exerts a biphasic effect on GnRH secretion in immortalized GnRH cells. The initial increase in GnRH secretion is triggered by a sharp rise in [Ca2+]i, while the progressive decline of GnRH levels that follows is paralleled by reduced levels of intracellular cAMP. The experiments described here were designed to explore the potential signaling pathways involved in this novel GABAA ionotropic inhibition of cAMP synthesis in GT1-7 cells. Using RT-PCR and real-time PCR, we found that GT1-7 cells express 8 of 9 known membrane adenylyl cyclase (AC) isoforms, including a large proportion of AC3 and AC9, as well as AC5 and AC6, all of which are negatively regulated by increases in [Ca2+]i. In contrast, isoforms of AC that are positively regulated by [Ca2+]i were barely detectable (AC1) or undetectable (AC8). Pharmacological activation of L-type voltage-operated calcium channels with BayK 8644 produced a decrease in [cAMP]i similar to that induced by GABA, while blocking these calcium channels with verapamil reversed the effect of GABA on cAMP synthesis. Furthermore, blocking calcineurin with deltamethrin, FK-506 or cyclosporin A blocked the inhibitory effect of GABA on [cAMP]i, supporting the involvement of AC9 in this effect. In addition, blocking Ca2+/calmodulin-dependent protein kinase II (CamKII) with KN-62 partially reversed the action of GABA, suggesting that AC3 may also be involved in this effect. Finally, GABA increased phosphatase activity in a calcium-dependent manner, an effect blocked by calcineurin inhibitors. Collectively, our results show that the ionotropic action of GABA via the activation of GABAA receptors can decrease AC activity in immortalized GnRH neurons, and that the effect of GABA appears to be mediated by a transient increase in [Ca2+]i followed by activation of calcineurin and CamKII, leading to dephosphorylation of AC9 and phosphorylation of AC3, respectively, and subsequently reducing the synthesis of cAMP.

Fast adaptation in mouse olfactory sensory neurons does not require the activity of phosphodiesterase

Vertebrate olfactory sensory neurons rapidly adapt to repetitive odorant stimuli. Previous studies have shown that the principal molecular mechanisms for odorant adaptation take place after the odorant-induced production of cAMP, and that one important mechanism is the negative feedback modulation by Ca²⁺-calmodulin (Ca²⁺-CaM) of the cyclic nucleotide-gated (CNG) channel. However, the physiological role of the Ca²⁺-dependent activity of phosphodiesterase (PDE) in adaptation has not been investigated yet. We used the whole-cell voltage-clamp technique to record currents in mouse olfactory sensory neurons elicited by photorelease of 8-Br-cAMP, an analogue of cAMP commonly used as a hydrolysis-resistant compound and known to be a potent agonist of the olfactory CNG channel. We measured currents in response to repetitive photoreleases of cAMP or of 8-Br-cAMP and we observed similar adaptation in response to the second stimulus. Control experiments were conducted in the presence of the PDE inhibitor IBMX, confirming that an increase in PDE activity was not involved in the response decrease. Since the total current activated by 8-Br-cAMP, as well as that physiologically induced by odorants, is composed not only of current carried by Na⁺ and Ca²⁺ through CNG channels, but also by a Ca²⁺-activated Cl⁻ current, we performed control experiments in which the reversal potential of Cl⁻ was set, by ion substitution, at the same value of the holding potential, −50 mV. Adaptation was measured also in these conditions of diminished Ca²⁺-activated Cl⁻ current. Furthermore, by producing repetitive increases of ciliary's Ca²⁺ with flash photolysis of caged Ca²⁺, we showed that Ca²⁺-activated Cl⁻ channels do not adapt and that there is no Cl⁻ depletion in the cilia. All together, these results indicate that the activity of ciliary PDE is not required for fast adaptation to repetitive stimuli in mouse olfactory sensory neurons.

Odorant receptors directly activate phospholipase C/inositol-1,4,5-trisphosphate coupled to calcium influx in Odora cells

Mechanisms by which odorants activate signaling pathways in addition to cAMP are hard to evaluate in heterogeneous mixtures of primary olfactory neurons. We used single cell calcium imaging to analyze the response to odorant through odorant receptor (OR) U131 in the olfactory epithelial cell line Odora (Murrell and Hunter 1999), a model system with endogenous olfactory signaling pathways. Because adenylyl cyclase levels are low, agents activating cAMP formation do not elevate calcium, thus unmasking independent signaling mediated by OR via phospholipase C (PLC), inositol-1,4,5-trisphosphate (IP(3)), and its receptor. Unexpectedly, we found that extracellular calcium is required for odor-induced calcium elevation without the release of intracellular calcium, even though the latter pathway is intact and can be stimulated by ATP. Relevant signaling components of the PLC pathway and G protein isoforms are identified by western blot in Odora cells as well as in olfactory sensory neurons (OSNs), where they are localized to the ciliary zone or cell bodies and axons of OSNs by immunohistochemistry. Biotinylation studies establish that IP(3) receptors type 2 and 3 are at the cell surface in Odora cells. Thus, individual ORs are capable of elevating calcium through pathways not directly mediated by cAMP and this may provide another avenue for odorant signaling in the olfactory system.

Dependence of electrical activity and calcium influx-controlled prolactin release on adenylyl cyclase signaling pathway in pituitary lactotrophs

Pituitary lactotrophs in vitro fire extracellular Ca2+-dependent action potentials spontaneously through still unidentified pacemaking channels, and the associated voltage-gated Ca2+influx (VGCI) is sufficient to maintain basal prolactin (PRL) secretion high and steady. Numerous plasma membrane channels have been characterized in these cells, but the mechanism underlying their pacemaking activity is still not known. Here we studied the relevance of cyclic nucleotide signaling pathways in control of pacemaking, VGCI, and PRL release. In mixed anterior pituitary cells, both VGCI-inhibitable and -insensitive adenylyl cyclase (AC) subtypes contributed to the basal cAMP production, and soluble guanylyl cyclase was exclusively responsible for basal cGMP production. Inhibition of basal AC activity, but not soluble guanylyl cyclase activity, reduced PRL release. In contrast, forskolin stimulated cAMP and cGMP production as well as pacemaking, VGCI, and PRL secretion. Elevation in cAMP and cGMP levels by inhibition of phosphodiesterase activity was also accompanied with increased PRL release. The AC inhibitors attenuated forskolin-stimulated cyclic nucleotide production, VGCI, and PRL release. The cell-permeable 8-bromo-cAMP stimulated firing of action potentials and PRL release and rescued hormone secretion in cells with inhibited ACs in an extracellular Ca2+-dependent manner, whereas 8-bromo-cGMP and 8-(4-chlorophenylthio)-2'-O-methyl-cAMP were ineffective. Protein kinase A inhibitors did not stop spontaneous and forskolin-stimulated pacemaking, VGCI, and PRL release. These results indicate that cAMP facilitates pacemaking, VGCI, and PRL release in lactotrophs predominantly in a protein kinase A- and Epac cAMP receptor-independent manner.

Morituri te salutant? Olfactory signal transduction and the role of phosphoinositides

During the past 150 years, researchers have investigated the cellular, physiological, and molecular mechanisms underlying the sense of smell. Based on these efforts, a conclusive model of olfactory signal transduction in the vertebrate's nose is now available, spanning from G-protein-mediated odorant receptors to ion channels, which are linked by a cyclic adenosine 3',5'-monophosphate-mediated signal transduction cascade. Here we review some historical milestones in the chronology of olfactory research, particularly emphasising the role of cyclic nucleotides and inositol trisphosphate as alternative second messengers in olfactory cells. We will describe the functional anatomy of the nose, outline the cellular composition of the olfactory epithelium, and describe the discovery of the molecular backbone of the olfactory signal transduction cascade. We then summarize our current model, in which cyclic adenosine monophosphate is the sole excitatory second messenger in olfactory sensory neurons. Finally, a possible significance of microvillous olfactory epithelial cells and inositol trisphosphate in olfaction will be discussed.

Molecular identification and functional characterization of an adenylyl cyclase from the honeybee

Cyclic AMP (cAMP) serves as an important messenger in virtually all organisms. In the honeybee (Apis mellifera), cAMP-dependent signal transduction has been implicated in behavioural processes as well as in learning and memory. Key components of cAMP-signalling cascades are adenylyl cyclases. However, the molecular identities and biochemical properties of adenylyl cyclases are completely unknown in the honeybee. We have cloned a cDNA (Amac3) from honeybee brain that encodes a membrane-bound adenylyl cyclase. The Amac3 gene is an orthologue of the Drosophila ac39E gene. The corresponding proteins share an overall amino acid similarity of approximately 62%. Phylogenetically, AmAC3 belongs to group 1 adenylyl cyclases. Heterologously expressed AmAC3 displays basal enzymatic activity and efficient coupling to endogenous G protein signalling pathways. Stimulation of beta-adrenergic receptors induces AmAC3 activity with an EC(50) of about 3.1 microm. Enzymatic activity is also increased by forskolin (EC(50) approximately 15 microm), a specific agonist of membrane-bound adenylyl cyclases. Similar to certain biogenic amine receptor genes of the honeybee, Amac3 transcripts are expressed in many somata of the brain, especially in mushroom body neurones. These results suggest that the enzyme serves in biogenic amine signal transduction cascades and in higher brain functions that contribute to learning and memory of the bee.

Comprehensive analysis of the expression patterns of the adenylate cyclase gene family in the developing and adult mouse brain

Adenylate cyclases (Adcys) are components of several developmentally, neurophysiologically, and pharmacologically relevant signaling pathways. A prominent feature of Adcys is their ability to integrate multiple signaling pathways into a single second messenger pathway, the production of cAMP. Nine isoforms of membrane-bound Adcys are known, each encoded by a distinct gene. These isoforms differ in their response to regulatory upstream pathways as well as in their distribution in the brain and elsewhere. Use of various detection methods and animal species has, however, hampered a direct comparison of expression patterns, so the potential contribution of single isoforms to Adcy activity in different brain regions remains unclear. We have determined the expression patterns of all nine Adcy genes in the embryonic, postnatal day 7, and adult mouse brain by nonradioactive robotic in situ hybridization (ISH). Here we describe the salient features of these patterns. Regional colocalization of Adcy transcripts encoding isoforms with different regulatory properties was detected in the cortex, subregions of the hippocampus, olfactory bulb, thalamus, and striatum. Hence, our expression data support models for modulation of cAMP signaling by combinatorial action of multiple Adcy isoforms. However, in several instances, the expression domains of genes encoding isoforms with similar regulatory properties spatially exclude each other, which is most evident in not previously described expression domains of the embryonic midbrain roof. This is suggestive of functional specialization.

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.

Calmodulin is required for vasopressin-stimulated increase in cyclic AMP production in inner medullary collecting duct

Calmodulin plays a critical role in regulation of renal collecting duct water permeability by vasopressin. However, specific targets for calmodulin action have not been thoroughly addressed. In the present study, we investigated whether Ca2+/calmodulin regulates adenylyl cyclase activity in the renal inner medullary collecting duct. Rat inner medullary collecting duct suspensions were incubated in the presence or absence of 0.1 nM vasopressin and the calmodulin inhibitors, monodansylcadaverine, W-7, and trifluoperazine, followed by measurement of cAMP. Vasopressin-stimulated cAMP elevation was significantly attenuated in the presence of calmodulin inhibitors. Analysis of transglutaminase 2 knock-out mice confirmed that these compounds were not acting through inhibition of transglutaminase 2 activity. Calmodulin inhibitors also blocked both cholera toxin- and forskolin-stimulated cAMP accumulation. In isolated perfused tubules, W-7 reversibly blocked vasopressin-stimulated urea permeability, a process that requires a rise in intracellular cAMP but does not appear to involve protein trafficking to the apical plasma membrane. These results suggest that calmodulin is required for vasopressin-stimulated adenylyl cyclase activity in the intact inner medullary collecting duct. Reverse transcription-PCR, immunoblotting, and immunohistochemistry revealed the presence of the calmodulin-sensitive adenylyl cyclase type 3 in the rat collecting duct, an isoform previously not known to be expressed in the collecting duct. Long-term treatment of Brattleboro rats with a vasopressin analog markedly decreased adenylyl cyclase type 3 protein abundance, providing an explanation for long-term down-regulation of vasopressin response in the collecting duct. These studies demonstrate the importance of calmodulin in the regulation of collecting duct adenylyl cyclase activity and transport function.

Long lasting inhibition of adenylyl cyclase selectively mediated by inositol 1,4,5-trisphosphate-evoked calcium release

In A7r5 smooth muscle cells, vasopressin stimulates release of Ca2+ from intracellular stores and Ca2+ entry, and it inhibits adenylyl cyclase (AC) activity. Inhibition of AC is prevented by inhibition of phospholipase C or when the increase in cytosolic [Ca2+] is prevented by the Ca2+ buffer, BAPTA. It is unaffected by pertussis toxin, inhibition of protein kinase C, or L-type Ca2+ channels or by removal of extracellular Ca2+. The independence of extracellular Ca2+ occurs despite inhibition of AC by vasopressin persisting for at least 15 min, whereas the cytosolic [Ca2+] returns to its basal level within 1-2 min in Ca2+-free medium. Although capacitative Ca2+ entry (CCE), activated by emptying stores with thapsigargin, inhibits AC, Ca2+ entry via CCE or L-type Ca2+ channels activated by vasopressin is ineffective. Temporally separating vasopressin-evoked Ca2+ release from the assessment of AC activity revealed that the transient Ca2+ signal resulting from Ca2+ mobilization causes a long lasting inhibition of AC. By contrast, inhibition of AC by thapsigargin-evoked CCE reverses rapidly after removal of extracellular Ca2+. Inhibition of AC by vasopressin is prevented by inhibition of Ca2+-calmodulin-dependent protein kinase II. We conclude that persistent inhibition of AC (probably AC-3) by vasopressin is mediated by inositol trisphosphate-evoked Ca2+ release causing activation of Ca2+-calmodulin-dependent protein kinase II. Our results establish that an important interaction between two ubiquitous signaling pathways is tuned selectively to Ca2+ release via inositol trisphosphate receptors and that the interaction transduces a transient Ca2+ signal into a long lasting inhibition of AC.