Identification and functional analysis of splice variants of the germ cell soluble adenylyl cyclase

Soluble adenylyl cyclase, the cell-autonomous member of the family

Soluble adenylyl cyclase (sAC) is the evolutionarily most ancient of a set of 10 adenylyl cyclases (Adcys). While Adcy1 to Adcy9 are cAMP-producing enzymes that are activated by G-protein coupled receptors (GPCRs), Adcy10 (sAC) is an intracellular adenylyl cyclase. sAC plays a pivotal role in numerous cellular processes, ranging from basic physiological functions to complex signaling cascades. As a distinct member of the adenylyl cyclase family, sAC is not activated by GPCRs and stands apart due to its unique characteristics, regulation, and localization within cells. This minireview aims to honour Ulli Brandt, the outgoing Executive Editor of our journal, Biochimica Biophysica Acta (BBA), and longstanding Executive Editor of the BBA section Bioenergetics. We will therefore focus this review on bioenergetic aspects of sAC and, in addition, review some important recent general developments in the field of research on sAC.

The Role of Soluble Adenylyl Cyclase in the Regulation of Flagellar Motility in Ascidian Sperm

Flagellar motility in sperm is activated and regulated by factors related to the eggs at fertilization. In the ascidian Ciona intestinalis, a sulfated steroid called the SAAF (sperm activating and attracting factor) induces both sperm motility activation and chemotaxis. Cyclic AMP (cAMP) is one of the most important intracellular factors in the sperm signaling pathway. Adenylyl cyclase (AC) is the key enzyme that synthesizes cAMP at the onset of the signaling pathway in all cellular functions. We previously reported that both transmembrane AC (tmAC) and soluble AC (sAC) play important roles in sperm motility in Ciona. The tmAC plays a major role in the SAAF-induced activation of sperm motility. On the other hand, sAC is involved in the regulation of flagellar beat frequency and the Ca2+-dependent chemotactic movement of sperm. In this study, we focused on the role of sAC in the regulation of flagellar motility in Ciona sperm chemotaxis. The immunochemical analysis revealed that several isoforms of sAC protein were expressed in Ciona sperm, as reported in mammals and sea urchins. We demonstrated that sAC inhibition caused strong and transient asymmetrization during the chemotactic turn, and then sperm failed to turn toward the SAAF. In addition, real-time Ca2+ imaging in sperm flagella revealed that sAC inhibition induced an excessive and prolonged Ca2+ influx to flagella. These results indicate that sAC plays a key role in sperm chemotaxis by regulating the clearance of [Ca2+]i and by modulating Ca2+-dependent flagellar waveform conversion.

Lactate modulates iron metabolism by binding soluble adenylyl cyclase

Overproduction of lactate (LA) can occur during exercise and in many diseases such as cancers. Individuals with hyperlactatemia often display anemia, decreased serum iron, and elevated hepcidin, a key regulator of iron metabolism. However, it is unknown whether and how LA regulates hepcidin expression. Here, we show LA binds to soluble adenylyl cyclase (sAC) in normal hepatocytes and affects systemic iron homeostasis in mice by increasing hepcidin expression. Comprehensive in vitro, in vivo, and in silico experiments show that the LA-sAC interaction raises cyclic adenosine monophosphate (cAMP) levels, which activates the PKA-Smad1/5/8 signaling pathway to increase hepcidin transcription. We verified this regulatory axis in wild-type mice and in mice with disordered iron homeostasis. LA also regulates hepcidin in humans at rest and subjected to extensive exercise that produce elevated LA. Our study links hyperlactatemia to iron deficiency, offering a mechanistic explanation for anemias seen in athletes and patients with lactic acidosis.

Alkaline Dilution Alters Sperm Motility in Dairy Goat by Affecting sAC/cAMP/PKA Pathway Activity

In dairy goat farming, increasing the female kid rate is beneficial to milk production and is, therefore, economically beneficial to farms. Our previous study demonstrated that alkaline incubation enriched the concentration of X-chromosome-bearing sperm; however, the mechanism by which pH affects the motility of X-chromosome-bearing sperm remains unclear. In this study, we explored this mechanism by incubating dairy goat sperm in alkaline dilutions, examining the pattern of changes in sperm internal pH and Ca2+ concentrations and investigating the role of the sAC/cAMP/PKA pathway in influencing sperm motility. The results showed that adding a calcium channel inhibitor during incubation resulted in a concentration-dependent decrease in the proportion of spermatozoa with forward motility, and the sperm sAC protein activity was positively correlated with the calcium ion concentration (r = 0.9972). The total motility activity, proportion of forward motility, and proportion of X-chromosome-bearing sperm decreased (p < 0.05) when cAMP/PKA protease activity was inhibited. Meanwhile, the enrichment of X-chromosome-bearing sperm by pH did not affect the sperm capacitation state. These results indicate that alkaline dilution incubation reduces Ca2+ entry into X-sperm and the motility was slowed down through the sAC/cAMP/PKA signaling pathway, providing a theoretical foundation for further optimization of the sex control method.

Assessing potency and binding kinetics of soluble adenylyl cyclase (sAC) inhibitors to maximize therapeutic potential

In mammalian cells, 10 different adenylyl cyclases produce the ubiquitous second messenger, cyclic adenosine monophosphate (cAMP). Amongst these cAMP-generating enzymes, bicarbonate (HCO3 -)-regulated soluble adenylyl cyclase (sAC; ADCY10) is uniquely essential in sperm for reproduction. For this reason, sAC has been proposed as a potential therapeutic target for non-hormonal contraceptives for men. Here, we describe key sAC-focused in vitro assays to identify and characterize sAC inhibitors for therapeutic use. The affinity and binding kinetics of an inhibitor can greatly influence in vivo efficacy, therefore, we developed improved assays for assessing these efficacy defining features.

Serum Phospholipid Profile Changes in Gaucher Disease and Parkinson's Disease

Alterations in the levels of serum sphingolipids and phospholipids have been reported in Gaucher disease and in Parkinson's disease, suggesting a potential role of these lipids as biomarkers. This project's objective is to detect novel associations and novel candidate biomarkers in the largest Spanish Gaucher and Parkinson diseases of the Iberian Peninsula. For that, 278 participants were included: 100 sporadic Parkinson's patients, 70 Gaucher patients, 15 GBA1-mutation-carrier Parkinson's patients and 93 controls. A serum lipidomics array including 10 phospholipid groups, 368 species, was performed using high-performance liquid chromatography-mass spectrometry. Lipid levels were compared between groups via multiple-regression analyses controlling for clinical and demographic parameters. Additionally, lipid levels were compared within the Gaucher and Parkinson's groups controlling for medication and/or disease severity. Results were controlled for robustness by filtering of non-detectable lipid values. There was an increase in the levels of phosphatidylcholine, with a simultaneous decrease in lyso-phosphatidylcholine, in the Gaucher, Parkinson's and GBA1-mutation-carrier Parkinson's patients vs. controls. Phosphatidylethanolamine, lyso- and plasmalogen-phosphatidylethanolamine were also increased in Gaucher and Parkinson's. Gaucher patients also showed an increase in lyso-phosphatidylserine and phosphatidylglycerol. While in the Gaucher and Parkinson's groups, velaglucerase alpha and dopamine agonists, respectively, showed positive associations with the lipid changes, miglustat treatment in Gaucher patients normalized the altered phosphatidylcholine/lyso-phosphatidylcholine ratio. In conclusion, Gaucher and Parkinson's patients showed changes in various serum phospholipid levels when compared with healthy controls, further supporting the role of such lipids in disease development and, possibly, as putative biomarkers. This hypothesis was reinforced by the normalizing effect of miglustat, and by controlling for data robustness, even though the limited number of participants, especially in the sub-distribution by treatment groups in GD requires validation in a larger number of patients.

Strategies to safely target widely expressed soluble adenylyl cyclase for contraception

In humans, the prototypical second messenger cyclic AMP is produced by 10 adenylyl cyclase isoforms, which are divided into two classes. Nine isoforms are G protein coupled transmembrane adenylyl cyclases (tmACs; ADCY1-9) and the 10th is the bicarbonate regulated soluble adenylyl cyclase (sAC; ADCY10). This review details why sAC is uniquely druggable and outlines ways to target sAC for novel forms of male and female contraception.

Molecular mechanisms of sperm motility are conserved in an early-branching metazoan

Reef-building corals are the keystone species of the world’s most biodiverse yet threatened marine ecosystems. Coral reproduction, critical for reef resilience, requires that coral sperm swim through the water column to reach the egg. However, little is known about the mechanisms that regulate coral sperm motility. We found here that coral sperm motility is pH dependent and that activation of motility requires signaling via the pH-sensing enzyme soluble adenylyl cyclase. This study reveals the deep conservation of a sperm activation pathway from humans to corals, presenting the first comprehensive examination of the molecular mechanisms regulating sperm motility in an early-diverging animal. These results are critical for understanding the resilience of this sensitive life stage to a changing marine environment.

Efficient and targeted sperm motility is essential for animal reproductive success. Sperm from mammals and echinoderms utilize a highly conserved signaling mechanism in which sperm motility is stimulated by pH-dependent activation of the cAMP-producing enzyme soluble adenylyl cyclase (sAC). However, the presence of this pathway in early-branching metazoans has remained unexplored. Here, we found that elevating cytoplasmic pH induced a rapid burst of cAMP signaling and triggered the onset of motility in sperm from the reef-building coral Montipora capitata in a sAC-dependent manner. Expression of sAC in the mitochondrial-rich midpiece and flagellum of coral sperm support a dual role for this molecular pH sensor in regulating mitochondrial respiration and flagellar beating and thus motility. In addition, we found that additional members of the homologous signaling pathway described in echinoderms, both upstream and downstream of sAC, are expressed in coral sperm. These include the Na⁺/H⁺ exchanger SLC9C1, protein kinase A, and the CatSper Ca²⁺ channel conserved even in mammalian sperm. Indeed, the onset of motility corresponded with increased protein kinase A activity. Our discovery of this pathway in an early-branching metazoan species highlights the ancient origin of the pH-sAC-cAMP signaling node in sperm physiology and suggests that it may be present in many other marine invertebrate taxa for which sperm motility mechanisms remain unexplored. These results emphasize the need to better understand the role of pH-dependent signaling in the reproductive success of marine animals, particularly as climate change stressors continue to alter the physiology of corals and other marine invertebrates.

Optimization of lead compounds into on-demand, nonhormonal contraceptives: leveraging a public-private drug discovery institute collaboration†

Efforts to develop new male or female nonhormonal, orally available contraceptives assume that to be effective and safe, targets must be (1) essential for fertility; (2) amenable to targeting by small-molecule inhibitors; and (3) restricted to the germline. In this perspective, we question the third assumption and propose that despite its wide expression, soluble adenylyl cyclase (sAC: ADCY10), which is essential for male fertility, is a valid target. We hypothesize that an acute-acting sAC inhibitor may provide orally available, on-demand, nonhormonal contraception for men without adverse, mechanism-based effects. To test this concept, we describe a collaboration between academia and the unique capabilities of a public-private drug discovery institute.

Molecular and biochemical characterization of the bicarbonate-sensing soluble adenylyl cyclase from a bony fish, the rainbow trout Oncorhynchus mykiss

Soluble adenylyl cyclase (sAC) is a HC O 3   - -stimulated enzyme that produces the ubiquitous signalling molecule cAMP, and deemed an evolutionarily conserved acid-base sensor. However, its presence is not yet confirmed in bony fishes, the most abundant and diverse of vertebrates. Here, we identified sAC genes in various cartilaginous, ray-finned and lobe-finned fish species. Next, we focused on rainbow trout sAC (rtsAC) and identified 20 potential alternative spliced mRNAs coding for protein isoforms ranging in size from 28 to 186 kDa. Biochemical and kinetic analyses on purified recombinant rtsAC protein determined stimulation by HC O 3   - at physiologically relevant levels for fish internal fluids (EC₅₀ ∼ 7 mM). rtsAC activity was sensitive to KH7, LRE1, and DIDS (established inhibitors of sAC from other organisms), and insensitive to forskolin and 2,5-dideoxyadenosine (modulators of transmembrane adenylyl cyclases). Western blot and immunocytochemistry revealed high rtsAC expression in gill ion-transporting cells, hepatocytes, red blood cells, myocytes and cardiomyocytes. Analyses in the cell line RTgill-W1 suggested that some of the longer rtsAC isoforms may be preferentially localized in the nucleus, the Golgi apparatus and podosomes. These results indicate that sAC is poised to mediate multiple acid-base homeostatic responses in bony fishes, and provide cues about potential novel functions in mammals.

Bicarbonate, carbon dioxide and pH sensing via mammalian bicarbonate-regulated soluble adenylyl cyclase

Soluble adenylyl cyclase (sAC; ADCY10) is a bicarbonate (HCO₃ ^-)-regulated enzyme responsible for the generation of cyclic adenosine monophosphate (cAMP). sAC is distributed throughout the cell and within organelles and, as such, plays a role in numerous cellular signalling pathways. Carbonic anhydrases (CAs) nearly instantaneously equilibrate HCO₃ ^-, protons and carbon dioxide (CO₂); because of the ubiquitous presence of CAs within cells, HCO₃ ^--regulated sAC can respond to changes in any of these factors. Thus, sAC can function as a physiological HCO₃ ^-/CO₂/pH sensor. Here, we outline examples where we have shown that sAC responds to changes in HCO₃ ^-, CO₂ or pH to regulate diverse physiological functions.

Soluble adenylyl cyclase regulates the cytosolic NADH/NAD+ redox state and the bioenergetic switch between glycolysis and oxidative phosphorylation

The evolutionarily conserved soluble adenylyl cyclase (sAC, ADCY10) mediates cAMP signaling exclusively in intracellular compartments. Because sAC activity is sensitive to local concentrations of ATP, bicarbonate, and free Ca²⁺, sAC is potentially an important metabolic sensor. Nonetheless, little is known about how sAC regulates energy metabolism in intact cells. In this study, we demonstrated that both pharmacological and genetic suppression of sAC resulted in increased lactate secretion and decreased pyruvate secretion in multiple cell lines and primary cultures of mouse hepatocytes and cholangiocytes. The increased extracellular lactate-to-pyruvate ratio upon sAC suppression reflected an increased cytosolic free [NADH]/[NAD⁺] ratio, which was corroborated by using the NADH/NAD⁺ redox biosensor Peredox-mCherry. Mechanistic studies in permeabilized HepG2 cells showed that sAC inhibition specifically suppressed complex I of the mitochondrial respiratory chain. A survey of cAMP effectors revealed that only selective inhibition of exchange protein activated by cAMP 1 (Epac1), but not protein kinase A (PKA) or Epac2, suppressed complex I-dependent respiration and significantly increased the cytosolic NADH/NAD⁺ redox state. Analysis of the ATP production rate and the adenylate energy charge showed that inhibiting sAC reciprocally affects ATP production by glycolysis and oxidative phosphorylation while maintaining cellular energy homeostasis. In conclusion, our study shows that, via the regulation of complex I-dependent mitochondrial respiration, sAC-Epac1 signaling regulates the cytosolic NADH/NAD⁺ redox state, and coordinates oxidative phosphorylation and glycolysis to maintain cellular energy homeostasis. As such, sAC is effectively a bioenergetic switch between aerobic glycolysis and oxidative phosphorylation at the post-translational level.

Everything you ever wanted to know about PKA regulation and its involvement in mammalian sperm capacitation

The 3', 5'-cyclic adenosine monophosphate (cAMP) dependent protein kinase (PKA) is a tetrameric holoenzyme comprising a set of two regulatory subunits (PKA-R) and two catalytic (PKA-C) subunits. The PKA-R subunits act as sensors of cAMP and allow PKA-C activity. One of the first signaling events observed during mammalian sperm capacitation is PKA activation. Thus, understanding how PKA activity is restricted in space and time is crucial to decipher the critical steps of sperm capacitation. It is widely accepted that PKA specificity depends on several levels of regulation. Anchoring proteins play a pivotal role in achieving proper localization signaling, subcellular targeting and cAMP microdomains. These multi-factorial regulation steps are necessary for a precise spatio-temporal activation of PKA. Here we discuss recent understanding of regulatory mechanisms of PKA in mammalian sperm, such as post-translational modifications, in the context of its role as the master orchestrator of molecular events conducive to capacitation.

The basics of mitochondrial cAMP signalling: Where, when, why

Cytosolic cAMP signalling in live cells has been extensively investigated in the past, while only in the last decade the existence of an intramitochondrial autonomous cAMP homeostatic system began to emerge. Thanks to the development of novel tools to investigate cAMP dynamics and cAMP/PKA-dependent phosphorylation within the matrix and in other mitochondrial compartments, it is now possible to address directly and in intact living cells a series of questions that until now could be addressed only by indirect approaches, in isolated organelles or through subcellular fractionation studies. In this contribution we discuss the mechanisms that regulate cAMP dynamics at the surface and inside mitochondria, and its crosstalk with organelle Ca²⁺ handling. We then address a series of still unsolved questions, such as the intramitochondrial localization of key elements of the cAMP signaling toolkit, e.g., adenylate cyclases, phosphodiesterases, protein kinase A (PKA) and Epac. Finally, we discuss the evidence for and against the existence of an intramitochondrial PKA pool and the functional role of cAMP increases within the organelle matrix.

The C-terminal region affects the activity of photoactivated adenylyl cyclase from Oscillatoria acuminata

Photoactivated adenylyl cyclase (PAC) is a unique protein that, upon blue light exposure, catalyzes cAMP production. The crystal structures of two PACs, from Oscillatoria acuminata (OaPAC) and Beggiatoa sp. (bPAC), have been solved, and they show a high degree of similarity. However, the photoactivity of OaPAC is much lower than that of bPAC, and the regulatory mechanism of PAC photoactivity, which induces the difference in activity between OaPAC and bPAC, has not yet been clarified. Here, we investigated the role of the C-terminal region in OaPAC, the length of which is the only notable difference from bPAC. We found that the photoactivity of OaPAC was inversely proportional to the C-terminal length. However, the deletion of more than nine amino acids did not further increase the activity, indicating that the nine amino acids at the C-terminal critically affect the photoactivity. Besides, absorption spectral features of light-sensing domains (BLUF domains) of the C-terminal deletion mutants showed similar light-dependent spectral shifts as in WT, indicating that the C-terminal region influences the activity without interacting with the BLUF domain. The study characterizes new PAC mutants with modified photoactivities, which could be useful as optogenetics tools.

Restoration of Glucose-Stimulated Cdc42-Pak1 Activation and Insulin Secretion by a Selective Epac Activator in Type 2 Diabetic Human Islets

Glucose metabolism stimulates cell division control protein 42 homolog (Cdc42)-p21-activated kinase (Pak1) activity and initiates filamentous actin (F-actin) cytoskeleton remodeling in pancreatic β-cells so that cytoplasmic secretory granules can translocate to the plasma membrane where insulin exocytosis occurs. Since glucose metabolism also generates cAMP in β-cells, the cross talk of cAMP signaling with Cdc42-Pak1 activation might be of fundamental importance to glucose-stimulated insulin secretion (GSIS). Previously, the type-2 isoform of cAMP-regulated guanine nucleotide exchange factor 2 (Epac2) was established to mediate a potentiation of GSIS by cAMP-elevating agents. Here we report that nondiabetic human islets and INS-1 832/13 β-cells treated with the selective Epac activator 8-pCPT-2'-O-Me-cAMP-AM exhibited Cdc42-Pak1 activation at 1 mmol/L glucose and that the magnitude of this effect was equivalent to that which was measured during stimulation with 20 mmol/L glucose in the absence of 8-pCPT-2'-O-Me-cAMP-AM. Conversely, the cAMP antagonist Rp-8-Br-cAMPS-pAB prevented glucose-stimulated Cdc42-Pak1 activation, thereby blocking GSIS while also increasing cellular F-actin content. Although islets from donors with type 2 diabetes had profound defects in glucose-stimulated Cdc42-Pak1 activation and insulin secretion, these defects were rescued by the Epac activator so that GSIS was restored. Collectively, these findings indicate an unexpected role for cAMP as a permissive or direct metabolic coupling factor in support of GSIS that is Epac2 and Cdc42-Pak1 regulated.

Shedding light on the role of cAMP in mammalian sperm physiology

Mammalian fertilization relies on sperm finding the egg and penetrating the egg vestments. All steps in a sperm's lifetime crucially rely on changes in the second messenger cAMP (cyclic adenosine monophosphate). In recent years, it has become clear that signal transduction in sperm is not a continuum, but rather organized in subcellular domains, e.g. the sperm head and the sperm flagellum, with the latter being further separated into the midpiece, principal piece, and endpiece. To understand the underlying signaling pathways controlling sperm function in more detail, experimental approaches are needed that allow to study sperm signaling with spatial and temporal precision. Here, we will give a comprehensive overview on cAMP signaling in mammalian sperm, describing the molecular players involved in these pathways and the sperm functions that are controlled by cAMP. Furthermore, we will highlight recent advances in analyzing and manipulating sperm signaling with spatio-temporal precision using light.

Pharmacological modulation of the CO2/HCO3-/pH-, calcium-, and ATP-sensing soluble adenylyl cyclase

Cyclic AMP (cAMP), the prototypical second messenger, has been implicated in a wide variety of (often opposing) physiological processes. It simultaneously mediates multiple, diverse processes, often within a single cell, by acting locally within independently-regulated and spatially-restricted microdomains. Within each microdomain, the level of cAMP will be dependent upon the balance between its synthesis by adenylyl cyclases and its degradation by phosphodiesterases (PDEs). In mammalian cells, there are many PDE isoforms and two types of adenylyl cyclases; the G protein regulated transmembrane adenylyl cyclases (tmACs) and the CO₂/HCO₃^-/pH-, calcium-, and ATP-sensing soluble adenylyl cyclase (sAC). Discriminating the roles of individual cyclic nucleotide microdomains requires pharmacological modulators selective for the various PDEs and/or adenylyl cyclases. Such tools present an opportunity to develop therapeutics specifically targeted to individual cAMP dependent pathways. The pharmacological modulators of tmACs have recently been reviewed, and in this review, we describe the current status of pharmacological tools available for studying sAC.

Mammalian Nucleotidyl Cyclases and Their Nucleotide Binding Sites

Mammalian membranous and soluble adenylyl cyclases (mAC, sAC) and soluble guanylyl cyclases (sGC) generate cAMP and cGMP from ATP and GTP, respectively, as substrates. mACs (nine human isoenzymes), sAC, and sGC differ in their overall structures owing to specific membrane-spanning and regulatory domains but consist of two similarly folded catalytic domains C1 and C2 with high structure-based homology between the cyclase species. Comparison of available crystal structures - VC1:IIC2 (a construct of domains C1a from dog mAC5 and C2a from rat mAC2), human sAC and sGC, mostly in complex with substrates, substrate analogs, inhibitors, metal ions, and/or modulators - reveals that especially the nucleotide binding sites are closely related. An evolutionarily well-conserved catalytic mechanism is based on common binding modes, interactions, and structural transformations, including the participation of two metal ions in catalysis. Nucleobase selectivity relies on only few mutations. Since in all cases the nucleoside moiety is embedded in a relatively spacious cavity, mACs, sAC, and sGC are rather promiscuous and bind nearly all purine and pyrimidine nucleotides, including CTP and UTP, and many of their derivatives as inhibitors with often high affinity. By contrast, substrate specificity of mammalian adenylyl and guanylyl cyclases is high due to selective dynamic rearrangements during turnover.

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.

Ocean acidification stimulates alkali signal pathway: A bicarbonate sensing soluble adenylyl cyclase from oyster Crassostrea gigas mediates physiological changes induced by CO2 exposure

Ocean acidification (OA) has been demonstrated to have severe effects on marine organisms, especially marine calcifiers. However, the impacts of OA on the physiology of marine calcifiers and the underlying mechanisms remain unclear. Soluble adenylyl cyclase (sAC) is an acid-base sensor in response to [HCO₃^-] and an intracellular source of cyclic AMP (cAMP). In the present study, an ortholog of sAC was identified from pacific oyster Crassostrea gigas (designated as CgsAC) and the catalytic region of CgsAC was cloned and expressed. Similar to the native CgsAC from gill tissues, the recombinant CgsAC protein (rCgsAC) exhibited [HCO₃^-] mediated cAMP-forming activity, which could be inhibited by a small molecule KH7. After 16days of CO₂ exposure (pH=7.50), the mRNA transcripts of CgsAC increased in muscle, mantle, hepatopancreas, gill, male gonad and haemocytes, and two truncated CgsAC forms of 45kD and 20kD were produced. Cytosolic CgsAC could be translocated from the cytoplasm and nuclei to the membrane in response to CO₂ exposure. Besides, CO₂ exposure could increase the production of cAMP and intracellular pH of haemocytes, which was regulated by CgsAC (p<0.05), suggesting the existence of a [HCO₃^-]/CgsAC/cAMP signal pathway in oyster. The elevated CO₂ could induce an increase of ROS level (p<0.05) and a decrease of phagocytic rate of haemocytes (p<0.05), which could be inhibited by KH7. The results collectively suggest that CgsAC is an important acid-base sensor in oyster and the [HCO₃^-]/CgsAC/cAMP signal pathway might be responsible for intracellular alkalization effects on oxidative phosphorylation and innate immunity under CO₂ exposure. The changes of intracellular pH, ROS, and phagocytosis mediated by CgsAC might help us to further understand the effects of ocean acidification on marine calcifiers.

Electron Microscopic Cytochemistry of Adenylyl Cyclase Activity in Mouse Spermatozoa

We investigated adenylyl cyclase activity of mouse spermatozoa by electron microscopic cytochemistry. Subcellular localization of enzyme activity was determined in the presence and absence of bicarbonate ions. Results confirm the existence in sperm of a bicarbonate-regulated adenylyl cyclase, which suggests microdomain signaling. (J Histochem Cytochem 52:833–836, 2004)

Sodium-hydrogen exchanger NHA1 and NHA2 control sperm motility and male fertility

Our previous work identified NHA1, a testis-specific sodium–hydrogen exchanger, is specifically localized on the principal piece of mouse sperm flagellum. Our subsequent study suggested that the number of newborns and fertility rate of NHA1-vaccinated female mice are significantly stepped down. In order to define the physiological function of NHA1 in spermatozoa, we generated Nha1^Fx/Fx, Zp3-Cre (hereafter called Nha1 cKO) mice and found that Nha1 cKO males were viable and subfertile with reduced sperm motility. Notably, cyclic AMP (cAMP) synthesis by soluble adenylyl cyclase (sAC) was attenuated in Nha1 cKO spermatozoa and cAMP analogs restored sperm motility. Similar to Nha1 cKO males, Nha2^Fx/Fx, Zp3-Cre (hereafter called Nha2 cKO) male mice were subfertile, indicating these two Nha genes may be functionally redundant. Furthermore, we demonstrated that male mice lacking Nha1 and Nha2 genes (hereafter called Nha1/2 dKO mice) were completely infertile, with severely diminished sperm motility owing to attenuated sAC-cAMP signaling. Importantly, principal piece distribution of NHA1 in spermatozoa are phylogenetically conserved in spermatogenesis. Collectively, our data revealed that NHA1 and NHA2 function as a key sodium–hydrogen exchanger responsible for sperm motility after leaving the cauda epididymidis.

Calcium-dependent mitochondrial cAMP production enhances aldosterone secretion

Glomerulosa cells secrete aldosterone in response to agonists coupled to Ca(2+) increases such as angiotensin II and corticotrophin, coupled to a cAMP dependent pathway. A recently recognized interaction between Ca(2+) and cAMP is the Ca(2+)-induced cAMP formation in the mitochondrial matrix. Here we describe that soluble adenylyl cyclase (sAC) is expressed in H295R adrenocortical cells. Mitochondrial cAMP formation, monitored with a mitochondria-targeted fluorescent sensor (4mtH30), is enhanced by HCO3(-) and the Ca(2+) mobilizing agonist angiotensin II. The effect of angiotensin II is inhibited by 2-OHE, an inhibitor of sAC, and by RNA interference of sAC, but enhanced by an inhibitor of phosphodiesterase PDE2A. Heterologous expression of the Ca(2+) binding protein S100G within the mitochondrial matrix attenuates angiotensin II-induced mitochondrial cAMP formation. Inhibition and knockdown of sAC significantly reduce angiotensin II-induced aldosterone production. These data provide the first evidence for a cell-specific functional role of mitochondrial cAMP.

Endothelial CD99 signals through soluble adenylyl cyclase and PKA to regulate leukocyte transendothelial migration

CD99 is a critical regulator of leukocyte transendothelial migration (TEM). Watson et al. describe the CD99 signaling pathway responsible. This involves a complex of CD99 with the A-kinase anchoring protein ezrin and soluble adenylyl cyclase that activates protein kinase A during leukocyte TEM.

CD99 is a critical regulator of leukocyte transendothelial migration (TEM). How CD99 signals during this process remains unknown. We show that during TEM, endothelial cell (EC) CD99 activates protein kinase A (PKA) via a signaling complex formed with the lysine-rich juxtamembrane cytoplasmic tail of CD99, the A-kinase anchoring protein ezrin, and soluble adenylyl cyclase (sAC). PKA then stimulates membrane trafficking from the lateral border recycling compartment to sites of TEM, facilitating the passage of leukocytes across the endothelium. Pharmacologic or genetic inhibition of EC sAC or PKA, like CD99 blockade, arrests neutrophils and monocytes partway through EC junctions, in vitro and in vivo, without affecting leukocyte adhesion or the expression of relevant cellular adhesion molecules. This is the first description of the CD99 signaling pathway in TEM as well as the first demonstration of a role for sAC in leukocyte TEM.

A soluble adenylyl cyclase form targets to axonemes and rescues beat regulation in soluble adenylyl cyclase knockout mice

Ciliary beating is important for effective mucociliary clearance. Soluble adenylyl cyclase (sAC) regulates ciliary beating, and a roughly 50-kD sAC variant is expressed in axonemes. Normal human bronchial epithelial (NHBE) cells express multiple sAC splice variants: full-length sAC; variants with catalytic domain 1 (C1) deletions; and variants with partial C1. One variant, sACex5v2-ex12v2, contains two alternative splices creating new exons 5 (ex5v2) and 12 (ex12v2), encoding a roughly 45-kD protein. It is therefore similar in size to ciliary sAC. The variant increases in expression upon ciliogenesis during differentiation at the air-liquid interface. When expressed in NHBE cells, this variant was targeted to cilia. Exons 5v2-7 were important for ciliary targeting, whereas exons 2-4 prevented it. In vitro, cytoplasmic sACex2-ex12v2 (containing C1 and C2) was the only variant producing cAMP. Ciliary sACex5v2-ex12v2 was not catalytically active. Airway epithelial cells isolated from wild-type mice revealed sAC-dependent ciliary beat frequency (CBF) regulation, analogous to NHBE cells: CBF rescue from HCO3(-)/CO2-mediated intracellular acidification was sensitive to the sAC inhibitor, KH7. Compared with wild type, sAC C2 knockout (KO) mice revealed lower CBF baseline, and the HCO3(-)/CO2-mediated CBF decrease was not inhibited by KH7, confirming lack of functional sAC. Human sACex5v2-ex12v2 was targeted to cilia and sACex2-ex12v2 to the cytoplasm in these KO mice. Introduction of the ciliary sACex5v2-ex12v2 variant, but not the cytoplasmic sACex2-ex12v2, restored functional sAC activity in C2 KO mice. Thus, we show, for the first time, a mammalian axonemal targeting sequence that localizes a sAC variant to cilia to regulate CBF.

The role of soluble adenylyl cyclase in neurite outgrowth

Axon regeneration in the mature central nervous system is limited by extrinsic inhibitory signals and a postnatal decline in neurons' intrinsic growth capacity. Neuronal levels of the second messenger cAMP are important in regulating both intrinsic growth capacity and neurons' responses to extrinsic factors. Approaches which increase intracellular cAMP in neurons enhance neurite outgrowth and facilitate regeneration after injury. Thus, understanding the factors which affect cAMP in neurons is of potential therapeutic importance. Recently, soluble adenylyl cyclase (sAC, ADCY10), the ubiquitous, non-transmembrane adenylyl cyclase, was found to play a key role in neuronal survival and axon growth. sAC is activated by bicarbonate and cations and may translate physiologic signals from metabolism and electrical activity into a neuron's decision to survive or regenerate. Here we critically review the literature surrounding sAC and cAMP signaling in neurons to further elucidate the potential role of sAC signaling in neurite outgrowth and regeneration. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Established and potential physiological roles of bicarbonate-sensing soluble adenylyl cyclase (sAC) in aquatic animals

Soluble adenylyl cyclase (sAC) is a recently recognized source of the signaling molecule cyclic AMP (cAMP) that is genetically and biochemically distinct from the classic G-protein-regulated transmembrane adenylyl cyclases (tmACs). Mammalian sAC is distributed throughout the cytoplasm and it may be present in the nucleus and inside mitochondria. sAC activity is directly stimulated by HCO3(-), and sAC has been confirmed to be a HCO3(-) sensor in a variety of mammalian cell types. In addition, sAC can functionally associate with carbonic anhydrases to act as a de facto sensor of pH and CO2. The two catalytic domains of sAC are related to HCO3(-)-regulated adenylyl cyclases from cyanobacteria, suggesting the cAMP pathway is an evolutionarily conserved mechanism for sensing CO2 levels and/or acid/base conditions. Reports of sAC in aquatic animals are still limited but are rapidly accumulating. In shark gills, sAC senses blood alkalosis and triggers compensatory H(+) absorption. In the intestine of bony fishes, sAC modulates NaCl and water absorption. And in sea urchin sperm, sAC may participate in the initiation of flagellar movement and in the acrosome reaction. Bioinformatics and RT-PCR results reveal that sAC orthologs are present in most animal phyla. This review summarizes the current knowledge on the physiological roles of sAC in aquatic animals and suggests additional functions in which sAC may be involved.

Intracellular pH in sperm physiology

Intracellular pH (pHi) regulation is essential for cell function. Notably, several unique sperm ion transporters and enzymes whose elimination causes infertility are either pHi dependent or somehow related to pHi regulation. Amongst them are: CatSper, a Ca(2+) channel; Slo3, a K(+) channel; the sperm-specific Na(+)/H(+) exchanger and the soluble adenylyl cyclase. It is thus clear that pHi regulation is of the utmost importance for sperm physiology. This review briefly summarizes the key components involved in pHi regulation, their characteristics and participation in fundamental sperm functions such as motility, maturation and the acrosome reaction.

Central role of soluble adenylyl cyclase and cAMP in sperm physiology

Cyclic adenosine 3',5'-monophosphate (cAMP), the first second messenger to be described, plays a central role in cell signaling in a wide variety of cell types. Over the last decades, a wide body of literature addressed the different roles of cAMP in cell physiology, mainly in response to neurotransmitters and hormones. cAMP is synthesized by a wide variety of adenylyl cyclases that can generally be grouped in two types: transmembrane adenylyl cyclase and soluble adenylyl cyclases. In particular, several aspects of sperm physiology are regulated by cAMP produced by a single atypical adenylyl cyclase (Adcy10, aka sAC, SACY). The signature that identifies sAC among other ACs, is their direct stimulation by bicarbonate. The essential nature of cAMP in sperm function has been demonstrated using gain of function as well as loss of function approaches. This review unifies state of the art knowledge of the role of cAMP and those enzymes involved in cAMP signaling pathways required for the acquisition of fertilizing capacity of mammalian sperm. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Soluble adenylyl cyclase in health and disease

The second messenger cAMP is integral for many physiological processes. Soluble adenylyl cyclase (sAC) was recently identified as a widely expressed intracellular source of cAMP in mammalian cells. sAC is evolutionary, structurally, and biochemically distinct from the G-protein-responsive transmembranous adenylyl cyclases (tmAC). The structure of the catalytic unit of sAC is similar to tmAC, but sAC does not contain transmembranous domains, allowing localizations independent of the membranous compartment. sAC activity is stimulated by HCO(3)(-), Ca²⁺ and is sensitive to physiologically relevant ATP fluctuations. sAC functions as a physiological sensor for carbon dioxide and bicarbonate, and therefore indirectly for pH. Here we review the physiological role of sAC in different human tissues with a major focus on the lung. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease, guest edited by J. Buck and L.R. Levin.

Crystal structure of human soluble adenylate cyclase reveals a distinct, highly flexible allosteric bicarbonate binding pocket

Soluble adenylate cyclases catalyse the synthesis of the second messenger cAMP through the cyclisation of ATP and are the only known enzymes to be directly activated by bicarbonate. Here, we report the first crystal structure of the human enzyme that reveals a pseudosymmetrical arrangement of two catalytic domains to produce a single competent active site and a novel discrete bicarbonate binding pocket. Crystal structures of the apo protein, the protein in complex with α,β-methylene adenosine 5′-triphosphate (AMPCPP) and calcium, with the allosteric activator bicarbonate, and also with a number of inhibitors identified using fragment screening, all show a flexible active site that undergoes significant conformational changes on binding of ligands. The resulting nanomolar-potent inhibitors that were developed bind at both the substrate binding pocket and the allosteric site, and can be used as chemical probes to further elucidate the function of this protein.

Crystal structures of human soluble adenylyl cyclase reveal mechanisms of catalysis and of its activation through bicarbonate

cAMP is an evolutionary conserved, prototypic second messenger regulating numerous cellular functions. In mammals, cAMP is synthesized by one of 10 homologous adenylyl cyclases (ACs): nine transmembrane enzymes and one soluble AC (sAC). Among these, only sAC is directly activated by bicarbonate (HCO3(-)); it thereby serves as a cellular sensor for HCO3(-), carbon dioxide (CO2), and pH in physiological functions, such as sperm activation, aqueous humor formation, and metabolic regulation. Here, we describe crystal structures of human sAC catalytic domains in the apo state and in complex with substrate analog, products, and regulators. The activator HCO3(-) binds adjacent to Arg176, which acts as a switch that enables formation of the catalytic cation sites. An anionic inhibitor, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, inhibits sAC through binding to the active site entrance, which blocks HCO3(-) activation through steric hindrance and trapping of the Arg176 side chain. Finally, product complexes reveal small, local rearrangements that facilitate catalysis. Our results provide a molecular mechanism for sAC catalysis and cellular HCO3(-) sensing and a basis for targeting this system with drugs.

Role of the bicarbonate-responsive soluble adenylyl cyclase in pH sensing and metabolic regulation

The evolutionarily conserved soluble adenylyl cyclase (sAC, adcy10) was recently identified as a unique source of cAMP in the cytoplasm and the nucleus. Its activity is regulated by bicarbonate and fine-tuned by calcium. As such, and in conjunction with carbonic anhydrase (CA), sAC constitutes an HCO(-) 3/CO(-) 2/pH sensor. In both alpha-intercalated cells of the collecting duct and the clear cells of the epididymis, sAC is expressed at significant level and involved in pH homeostasis via apical recruitment of vacuolar H(+)-ATPase (VHA) in a PKA-dependent manner. In addition to maintenance of pH homeostasis, sAC is also involved in metabolic regulation such as coupling of Krebs cycle to oxidative phosphorylation via bicarbonate/CO2 sensing. Additionally, sAC also regulates CFTR channel and plays an important role in regulation of barrier function and apoptosis. These observations suggest that sAC, via bicarbonate-sensing, plays an important role in maintaining homeostatic status of cells against fluctuations in their microenvironment.

Adenylyl cyclases in the digestive system

Adenylyl cyclases (ACs) are a group of widely distributed enzymes whose functions are very diverse. There are nine known transmembrane AC isoforms activated by Gαs. Each has its own pattern of expression in the digestive system and differential regulation of function by Ca(2+) and other intracellular signals. In addition to the transmembrane isoforms, one AC is soluble and exhibits distinct regulation. In this review, the basic structure, regulation and physiological roles of ACs in the digestive system are discussed.

pH sensing via bicarbonate-regulated "soluble" adenylyl cyclase (sAC)

Soluble adenylyl cyclase (sAC) is a source of the second messenger cyclic adenosine 3′, 5′ monophosphate (cAMP). sAC is directly regulated by bicarbonate (HCO⁻₃) ions. In living cells, HCO⁻₃ ions are in nearly instantaneous equilibrium with carbon dioxide (CO₂) and pH due to the ubiquitous presence of carbonic anhydrases. Numerous biological processes are regulated by CO₂, HCO⁻₃, and/or pH, and in a number of these, sAC has been shown to function as a physiological CO₂/HCO₃/pH sensor. In this review, we detail the known pH sensing functions of sAC, and we discuss two highly-studied, pH-dependent pathways in which sAC might play a role.

CO2/HCO3(-)- and calcium-regulated soluble adenylyl cyclase as a physiological ATP sensor

The second messenger molecule cAMP is integral for many physiological processes. In mammalian cells, cAMP can be generated from hormone- and G protein-regulated transmembrane adenylyl cyclases or via the widely expressed and structurally and biochemically distinct enzyme soluble adenylyl cyclase (sAC). sAC activity is uniquely stimulated by bicarbonate ions, and in cells, sAC functions as a physiological carbon dioxide, bicarbonate, and pH sensor. sAC activity is also stimulated by calcium, and its affinity for its substrate ATP suggests that it may be sensitive to physiologically relevant fluctuations in intracellular ATP. We demonstrate here that sAC can function as a cellular ATP sensor. In cells, sAC-generated cAMP reflects alterations in intracellular ATP that do not affect transmembrane AC-generated cAMP. In β cells of the pancreas, glucose metabolism generates ATP, which corresponds to an increase in cAMP, and we show here that sAC is responsible for an ATP-dependent cAMP increase. Glucose metabolism also elicits insulin secretion, and we further show that sAC is necessary for normal glucose-stimulated insulin secretion in vitro and in vivo.

Pharmacological distinction between soluble and transmembrane adenylyl cyclases

The second messenger cAMP is involved in a number of cellular signaling pathways. In mammals, cAMP is produced by either the hormonally responsive, G protein-regulated transmembrane adenylyl cyclases (tmACs) or by the bicarbonate- and calcium-regulated soluble adenylyl cyclase (sAC). To develop tools to differentiate tmAC and sAC signaling, we determined the specificity and potency of commercially available adenylyl cyclase inhibitors. In cellular systems, two inhibitors, KH7 and catechol estrogens, proved specific for sAC, and 2',5'-dideoxyadenosine proved specific for tmACs. These tools provide a means to define the specific contributions of the different families of adenylyl cyclases in cells and tissues, which will further our understanding of cell signaling.

Analysis of signaling pathways controlling flagellar movements in mammalian spermatozoa

The mammalian sperm flagellum is an example of organelles where sensory and signaling functions are integrated with motility. Structurally related to other ciliary appendages, it has unique cytoskeletal structures that serve to assemble signaling complexes as well as components of metabolic pathways. Flagellar motility is regulated by signaling events that control sperm ion milieu, energy production, and classical second messenger-dependent phosphorylation cascades. Here, we will concentrate on the cAMP signaling pathway associated with flagellar motility. We will describe methods to analyze the properties of a unique adenylyl cyclase termed sAC (gene name Sacy, Adcy10), which plays an essential function in cAMP accumulation in spermatozoa. This soluble adenylyl cyclase (sAC) is a sensor for bicarbonate, pH, and Ca(2+) and is likely involved in coupling energy homeostasis with signaling events. We will also describe methods to investigate the macromolecular complexes that bring together sAC and other signaling molecules.

Neuronal expression of soluble adenylyl cyclase in the mammalian brain

Cyclic 3',5'-adenosine monophosphate (cAMP) is a critical and ubiquitous second messenger involved in a multitude of signaling pathways. Soluble adenylyl cyclase (sAC) is a novel source of cAMP subject to unique localization and regulation. It was originally discovered in mammalian testis and found to be activated by bicarbonate and calcium. sAC has been implicated in diverse processes, including astrocyte-neuron metabolic coupling and axonal outgrowth of neurons. However, despite these functional studies, demonstration of sAC protein expression outside of testis has been controversial. Recently, we showed sAC immunoreactivity in astrocytes, but the question of neuronal expression of sAC remained. We now describe the generation of a second sAC knockout mouse model (C2KO) designed to more definitively address questions of sAC expression, and we demonstrate conclusively using immune-electron microscopy that sAC is expressed in neuronal profiles in the central nervous system.

Novel approach for the detection of the vestiges of testicular mRNA splicing errors in mature spermatozoa of Japanese Black bulls

There is a serious problem with the reduction of male reproductive performance of the livestock in the world. We have a hypothesis that the splicing error-caused derivation of aberrant sperm motility-related proteins may be one of its causal factors. It is thought that fresh testicular tissues are necessary for the detection of splicing errors of the mRNA. However, it is difficult to obtain testicular tissues from a number of agriculturally important bulls by surgical methods, because such procedures may have deleterious effects on bulls’ reproductive performance. The aim of this study was to examine the usefulness of mRNA fragments collected from ejaculated spermatozoa as alternative analytical samples for detection of the splicing errors. In the first experiment, we characterized the alternative splicing and splicing error of bull testicular ADCY10 mRNA which coded the synthase of the regulatory molecule for sperm motility “cAMP”. In testes, the exon 11-lacking variant coding the truncated ADCY10 was derived by alternative splicing. However, splicing errors, which accompanied the frame shift in the second cyclase domain, were occasionally observed in the exon 11-lacking variant. This aberrant variant retained intronic nucleotides (4 bases, CCAG) connecting the initial part of exon 10 due to splicing errors and consequently yielded the cleavage site for a restriction enzyme (Cac8I) which recognized the nucleotide sequences (GCNNGC). In the second experiment, we recovered residual testicular mRNA fragments from ejaculated spermatozoa and observed the splicing error-caused derivation of the aberrant variant of ADCY 10. Ejaculated spermatozoa conserved mRNA fragments of the exon 11-lacking variant coding exons 9, 10, 12 and 13. Moreover, the above-mentioned aberrant variant of ADCY10 mRNA fragment was detectable by Cac8I digestion treatment using the sperm mRNAs. These results indicate the utility of sperm mRNA fragments for the detection of splicing errors in bull testicular mRNAs.

Metabolic communication between astrocytes and neurons via bicarbonate-responsive soluble adenylyl cyclase

Astrocytes are proposed to participate in brain energy metabolism by supplying substrates to neurons from their glycogen stores and from glycolysis. However, the molecules involved in metabolic sensing and the molecular pathways responsible for metabolic coupling between different cell types in the brain are not fully understood. Here we show that a recently cloned bicarbonate (HCO₃⁻) sensor, soluble adenylyl cyclase (sAC), is highly expressed in astrocytes and becomes activated in response to HCO₃⁻ entry via the electrogenic NaHCO₃ cotransporter (NBC). Activated sAC increases intracellular cAMP levels, causing glycogen breakdown, enhanced glycolysis, and the release of lactate into the extracellular space, which is subsequently taken up by neurons for use as an energy substrate. This process is recruited over a broad physiological range of [K⁺](ext) and also during aglycemic episodes, helping to maintain synaptic function. These data reveal a molecular pathway in astrocytes that is responsible for brain metabolic coupling to neurons.

Soluble adenylyl cyclase activity is necessary for retinal ganglion cell survival and axon growth

cAMP is a critical second messenger mediating activity-dependent neuronal survival and neurite growth. We investigated the expression and function of the soluble adenylyl cyclase (sAC, ADCY10) in CNS retinal ganglion cells (RGCs). We found sAC protein expressed in multiple RGC compartments including the nucleus, cytoplasm and axons. sAC activation increased cAMP above the level seen with transmembrane adenylate cyclase (tmAC) activation. Electrical activity and bicarbonate, both physiologic sAC activators, significantly increased survival and axon growth, whereas pharmacologic or siRNA-mediated sAC inhibition dramatically decreased RGC survival and axon growth in vitro, and survival in vivo. Conversely, RGC survival and axon growth were unaltered in RGCs from AC1/AC8 double knock-out mice or after specifically inhibiting tmACs. These data identify a novel sAC-mediated cAMP signaling pathway regulating RGC survival and axon growth, and suggest new neuroprotective or regenerative strategies based on sAC modulation.

Activation of soluble adenylyl cyclase protects against secretagogue stimulated zymogen activation in rat pancreaic acinar cells

An early feature of acute pancreatitis is activation of zymogens, such as trypsinogen, within the pancreatic acinar cell. Supraphysiologic concentrations of the hormone cholecystokinin (CCK; 100 nM), or its orthologue cerulein (CER), induce zymogen activation and elevate levels of cAMP in pancreatic acinar cells. The two classes of adenylyl cyclase, trans-membrane (tmAC) and soluble (sAC), are activated by distinct mechanisms, localize to specific subcellular domains, and can produce locally high concentrations of cAMP. We hypothesized that sAC activity might selectively modulate acinar cell zymogen activation. sAC was identified in acinar cells by PCR and immunoblot. It localized to the apical region of the cell under resting conditions and redistributed intracellularly after treatment with supraphysiologic concentrations of cerulein. In cerulein-treated cells, pre-incubation with a trans-membrane adenylyl cyclase inhibitor did not affect zymogen activation or amylase secretion. However, treatment with a sAC inhibitor (KH7), or inhibition of a downstream target of cAMP, protein kinase A (PKA), significantly enhanced secretagogue-stimulated zymogen activation and amylase secretion. Activation of sAC with bicarbonate significantly inhibited secretagogue-stimulated zymogen activation; this response was decreased by inhibition of sAC or PKA. Bicarbonate also enhanced secretagogue-stimulated cAMP accumulation; this effect was inhibited by KH7. Bicarbonate treatment reduced secretagogue-stimulated acinar cell vacuolization, an early marker of pancreatitis. These data suggest that activation of sAC in the pancreatic acinar cell has a protective effect and reduces the pathologic activation of proteases during pancreatitis.