A novel mechanism for adenylyl cyclase inhibition from the crystal structure of its complex with catechol estrogen

The pseudokinase domain in receptor guanylyl cyclases

Cyclic GMP is produced by enzymes called guanylyl cyclases, of which the membrane-associated forms contain an intracellular pseudokinase domain that allosterically regulates the C-terminal guanylyl cyclase domain. Ligand binding to the extracellular domain of these single transmembrane-spanning domain receptors elicits an increase in cGMP levels in the cell. The pseudokinase domain (or kinase-homology domain) in these receptors appears to be critical for ligand-mediated activation. While the pseudokinase domain does not possess kinase activity, biochemical evidence indicates that the domain can bind ATP and thereby allosterically regulate the catalytic activity of these receptors. The pseudokinase domain also appears to be the site of interaction of regulatory proteins, as seen in the retinal guanylyl cyclases that are involved in visual signal transduction. In the absence of structural information on the pseudokinase-guanylyl cyclase domain organization of any member of this family of receptors, biochemical evidence has provided clues to the physical interaction of the pseudokinase and guanylyl cyclase domain. An α-helical linker region between the pseudokinase domain and the guanylyl cyclase domain regulates the basal activity of these receptors in the absence of a stimulatory ligand and is important for stabilizing the structure of the pseudokinase domain that can bind ATP. Here, we present an overview of salient features of ATP-mediated regulation of receptor guanylyl cyclases and describe biochemical approaches that allow a clearer understanding of the intricate interplay between the pseudokinase domain and catalytic domain in these proteins.

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.

A Potent N-(piperidin-4-yl)-1H-pyrrole-2-carboxamide Inhibitor of Adenylyl Cyclase of G. lamblia: Biological Evaluation and Molecular Modelling Studies

In this work, we report a derivative of N-(piperidin-4-yl)-1H-pyrrole-2-carboxamide as a new inhibitor for adenylyl cyclase of Giardia lamblia which was obtained from a study using structural data of the nucleotidyl cyclase 1 (gNC1) of this parasite. For such a study, we developed a model for this specific enzyme by using homology techniques, which is the first model reported for gNC1 of G. lamblia. Our studies show that the new inhibitor has a competitive mechanism of action against this enzyme. 2-Hydroxyestradiol was used as the reference compound for comparative studies. Results in this work are important from two points of view. on the one hand, an experimentally corroborated model for gNC1 of G. lamblia obtained by molecular modelling is presented; on the other hand, the new inhibitor obtained is an undoubtedly excellent starting structure for the development of new metabolic inhibitors for G. lamblia.

Effect of Soluble Adenylyl Cyclase (ADCY10) Inhibitors on the LH-Stimulated cAMP Synthesis in Mltc-1 Leydig Cell Line

In contrast to all transmembrane adenylyl cyclases except ADCY9, the cytosolic soluble adenylyl cyclase (ADCY10) is insensitive to forskolin stimulation and is uniquely modulated by calcium and bicarbonate ions. In the present paper, we focus on ADCY10 localization and a kinetic analysis of intracellular cAMP accumulation in response to human LH in the absence or presence of four different ADCY10 inhibitors (KH7, LRE1, 2-CE and 4-CE) in MTLC-1 cells. ADCY10 was immuno-detected in the cytoplasm of MLTC-1 cells and all four inhibitors were found to inhibit LH-stimulated cAMP accumulation and progesterone level in MLTC-1 and testosterone level primary Leydig cells. Interestingly, similar inhibitions were also evidenced in mouse testicular Leydig cells. In contrast, the tmAC-specific inhibitors ddAdo3′ and ddAdo5′, even at high concentration, exerted weak or no inhibition on cAMP accumulation, suggesting an important role of ADCY10 relative to tmACs in the MLTC-1 response to LH. The strong synergistic effect of HCO₃⁻ under LH stimulation further supports the involvement of ADCY10 in the response to LH.

Distinct effects on cAMP signaling of carbamazepine and its structural derivatives do not correlate with their clinical efficacy in epilepsy

The antiepileptic sodium channel blocker, carbamazepine, has long been known to be able to attenuate cAMP signals. This could be of clinical importance since cAMP signaling has been shown to be involved in epileptogenesis and seizures. However, no information on the ability to affect cAMP signaling is available for the marketed structural derivatives, oxcarbazepine and eslicarbazepine acetate or their dominating metabolite, licarbazepine. Thus, we employed a HEK293 cell line stably expressing a cAMP biosensor to assess the effect of these two drugs on cAMP accumulation. We find that oxcarbazepine does not affect cAMP accumulation whereas eslicarbazepine acetate, surprisingly, is able to enhance cAMP accumulation. Since the transcription of ADCY8 (adenylyl cyclase isoform 8; AC8) has been found to be elevated in epileptic tissue from patients, we subsequently expressed AC8 in the HEK293 cells. In the AC8-expressing cells, oxcarbazepine was now able to attenuate whereas eslicarbazepine maintained its ability to increase cAMP accumulation. However, at all concentrations tested, licarbazepine demonstrated no effect on cAMP accumulation. Thus, we conclude that the effects exerted by carbamazepine and its derivatives on cAMP accumulation do not correlate with their clinical efficacy in epilepsy. However, this does not disqualify cAMP signaling per se as a potential disease-modifying drug target for epilepsy since more potent and selective inhibitors may be of therapeutic value.

Synergistic mutations in soluble guanylyl cyclase (sGC) reveal a key role for interfacial regions in the sGC activation mechanism

Soluble guanylyl cyclase (sGC) is the main receptor for nitric oxide (NO) and a central component of the NO-cGMP pathway, critical to cardiovascular function. NO binding to the N-terminal sensor domain in sGC enhances the cyclase activity of the C-terminal catalytic domain. Our understanding of the structural elements regulating this signaling cascade is limited, hindering structure-based drug design efforts that target sGC to improve the management of cardiovascular diseases. Conformational changes are thought to propagate the NO-binding signal throughout the entire sGC heterodimer, via its coiled-coil domain, to reorient the catalytic domain into an active conformation. To identify the structural elements involved in this signal transduction cascade, here we optimized a cGMP-based luciferase assay that reports on heterologous sGC activity in Escherichia coli and identified several mutations that activate sGC. These mutations resided in the dorsal flaps, dimer interface, and GTP-binding regions of the catalytic domain. Combinations of mutations from these different elements synergized, resulting in even greater activity and indicating a complex cross-talk among these regions. Molecular dynamics simulations further revealed conformational changes underlying the functional impact of these mutations. We propose that the interfacial residues play a central role in the sGC activation mechanism by coupling the coiled-coil domain to the active site via a series of hot spots. Our results provide new mechanistic insights not only into the molecular pathway for sGC activation but also for other members of the larger nucleotidyl cyclase family.

Soluble adenylyl cyclase-mediated cAMP signaling and the putative role of PKA and EPAC in cerebral mitochondrial function

Mitochondria produce the bulk of the ATP in most cells, including brain cells. Regulating this complex machinery to match the energetic needs of the cell is a complicated process that we have yet to understand in its entirety. In this context, 3',5'-cyclic AMP (cAMP) has been suggested to play a seminal role in signaling-metabolism coupling and regulation of mitochondrial ATP production. In cells, cAMP signals may affect mitochondria from the cytosolic side but more recently, a cAMP signal produced within the matrix of mitochondria by soluble adenylyl cyclase (sAC) has been suggested to regulate respiration and thus ATP production. However, little is known about these processes in brain mitochondria, and the effectors of the cAMP signal generated within the matrix are not completely clear since both protein kinase A (PKA) and exchange protein activated by cAMP 1 (EPAC1) have been suggested to be involved. Here, we review the current knowledge and relate it to brain mitochondria. Further, based on measurements of respiration, membrane potential, and ATP production in isolated mouse brain cortical mitochondria we show that inhibitors of sAC, PKA, or EPAC affect mitochondrial function in distinct ways. In conclusion, we suggest that brain mitochondria do regulate their function via sAC-mediated cAMP signals and that both PKA and EPAC could be involved downstream of sAC. Finally, due to the role of faulty mitochondrial function in a range of neurological diseases, we expect that the function of sAC-cAMP-PKA/EPAC signaling in brain mitochondria will likely attract further attention.

The inhibitors of soluble adenylate cyclase 2-OHE, KH7, and bithionol compromise mitochondrial ATP production by distinct mechanisms

Soluble adenylate cyclase (sAC) is a non-plasma membrane-bound isoform of the adenylate cyclases signaling via the canonical second messenger, 3',5'-cyclic AMP (cAMP). sAC is involved in key physiological processes such as insulin release, sperm motility, and energy metabolism. Thus, sAC has attracted interest as a putative drug target and attempts have been made to develop selective inhibitors. Since sAC has a binding constant for its substrate, ATP, in the millimolar range, reductions in mitochondrial ATP production may be part of the mechanism-of-action of sAC inhibitors and the potential of these compounds to study the physiological outcomes of inhibition of sAC might be severely hampered by this. Here, we evaluate the effects of two commonly employed inhibitors, 2-OHE and KH7, on mitochondrial ATP production and energy metabolism. For comparison, we included a recently identified inhibitor of sAC, bithionol. Employing mitochondria isolated from mouse brain, we show that all three compounds are able to curb ATP production albeit via distinct mechanisms. Bithionol and KH7 mainly inhibit ATP production by working as a classical uncoupler whereas 2-OHE mainly works by decreasing mitochondrial respiration. These findings were corroborated by investigating energy metabolism in acute brain slices from mice. Since all three sAC inhibitors are shown to curb mitochondrial ATP production and affect energy metabolism, caution should be exercised when employed to study the physiological roles of sAC or for validating sAC as a drug target.

Mitochondrial cAMP and Ca2+ metabolism in adrenocortical cells

The biological effects of physiological stimuli of adrenocortical glomerulosa cells are predominantly mediated by the Ca²⁺ and the cAMP signal transduction pathways. The complex interplay between these signalling systems fine-tunes aldosterone secretion. In addition to the well-known cytosolic interactions, a novel intramitochondrial Ca²⁺-cAMP interplay has been recently recognised. The cytosolic Ca²⁺ signal is rapidly transferred into the mitochondrial matrix where it activates Ca²⁺-sensitive dehydrogenases, thus enhancing the formation of NADPH, a cofactor of steroid synthesis. Quite a few cell types, including H295R adrenocortical cells, express the soluble adenylyl cyclase within the mitochondria and the elevation of mitochondrial [Ca²⁺] activates the enzyme, thus resulting in the Ca²⁺-dependent formation of cAMP within the mitochondrial matrix. On the other hand, mitochondrial cAMP (mt-cAMP) potentiates the transfer of cytosolic Ca²⁺ into the mitochondrial matrix. This cAMP-mediated positive feedback control of mitochondrial Ca²⁺ uptake may facilitate the rapid hormonal response to emergency situations since knockdown of soluble adenylyl cyclase attenuates aldosterone production whereas overexpression of the enzyme facilitates steroidogenesis in vitro. Moreover, the mitochondrial Ca²⁺-mt-cAMP-Ca²⁺ uptake feedback loop is not a unique feature of adrenocortical cells; a similar signalling system has been described in HeLa cells as well.

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.

Mitochondrial cAMP exerts positive feedback on mitochondrial Ca2+ uptake via the recruitment of Epac1

We have previously demonstrated in H295R adrenocortical cells that the Ca²⁺-dependent production of mitochondrial cAMP (mt-cAMP) by the matrix soluble adenylyl cyclase (sAC; encoded by ADCY10) is associated with enhanced aldosterone production. Here, we examined whether mitochondrial sAC and mt-cAMP fine tune mitochondrial Ca²⁺ metabolism to support steroidogenesis. Reduction of mt-cAMP formation resulted in decelerated mitochondrial Ca²⁺ accumulation in intact cells during K⁺-induced Ca²⁺ signalling and also in permeabilized cells exposed to elevated perimitochondrial [Ca²⁺]. By contrast, treatment with the membrane-permeable cAMP analogue 8-Br-cAMP, inhibition of phosphodiesterase 2 and overexpression of sAC in the mitochondrial matrix all intensified Ca²⁺ uptake into the organelle. Identical mt-cAMP dependence of mitochondrial Ca²⁺ uptake was also observed in HeLa cells. Importantly, the enhancing effect of mt-cAMP on Ca²⁺ uptake was independent from both the mitochondrial membrane potential and Ca²⁺ efflux, but was reduced by Epac1 (also known as RAPGEF3) blockade both in intact and in permeabilized cells. Finally, overexpression of sAC in the mitochondrial matrix potentiated aldosterone production implying that the observed positive feedback mechanism of mt-cAMP on mitochondrial Ca²⁺ accumulation may have a role in the rapid initiation of steroidogenesis.This article has an associated First Person interview with the first author of the paper.

Functional Significance of the Adcy10-Dependent Intracellular cAMP Compartments

Mounting evidence confirms the compartmentalized structure of evolutionarily conserved 3'⁻5'-cyclic adenosine monophosphate (cAMP) signaling, which allows for simultaneous participation in a wide variety of physiological functions and ensures specificity, selectivity and signal strength. One important player in cAMP signaling is soluble adenylyl cyclase (sAC). The intracellular localization of sAC allows for the formation of unique intracellular cAMP microdomains that control various physiological and pathological processes. This review is focused on the functional role of sAC-produced cAMP. In particular, we examine the role of sAC-cAMP in different cellular compartments, such as cytosol, nucleus and mitochondria.

Molecular, Enzymatic, and Cellular Characterization of Soluble Adenylyl Cyclase From Aquatic Animals

The enzyme soluble adenylyl cyclase (sAC) is the most recently identified source of the messenger molecule cyclic adenosine monophosphate. sAC is evolutionarily conserved from cyanobacteria to human, is directly stimulated by [Formula: see text] ions, and can act as a sensor of environmental and metabolic CO₂, pH, and [Formula: see text] levels. sAC genes tend to have multiple alternative promoters, undergo extensive alternative splicing, be translated into low mRNA levels, and the numerous sAC protein isoforms may be present in various subcellular localizations. In aquatic organisms, sAC has been shown to mediate various functions including intracellular pH regulation in coral, blood acid/base regulation in shark, heart beat rate in hagfish, and NaCl absorption in fish intestine. Furthermore, sAC is present in multiple other species and tissues, and sAC protein and enzymatic activity have been reported in the cytoplasm, the nucleus, and other subcellular compartments, suggesting even more diverse physiological roles. Although the methods and experimental tools used to study sAC are conventional, the complexity of sAC genes and proteins requires special considerations that are discussed in this chapter.

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.

Characterization of a novel signal transducer element intrinsic to class IIIa/b adenylate cyclases and guanylate cyclases

Adenylate cyclases (ACs) are signaling proteins that produce the second messenger cAMP. Class III ACs comprise four groups (class IIIa-d) of which class IIIa and IIIb ACs have been identified in bacteria and eukaryotes. Many class IIIa ACs are anchored to membranes via hexahelical domains. In eukaryotic ACs, membrane anchors are well conserved, suggesting that this region possesses important functional characteristics that are as yet unknown. To address this question, we replaced the hexahelical membrane anchor of the mycobacterial AC Rv1625c with the hexahelical quorum-sensing receptor from Legionella, LqsS. Using this chimera, we identified a novel 19-amino-acid cyclase transducer element (CTE) located N-terminally to the catalytic domain that links receptor stimulation to effector activation. Coupling of the receptor to the AC was possible at several positions distal to the membrane exit, resulting in stimulatory or inhibitory responses to the ligand Legionella autoinducer-1. In contrast, on the AC effector side functional coupling was only successful when starting with the CTE. Bioinformatics approaches established that distinct CTEs are widely present in class IIIa and IIIb ACs and in vertebrate guanylate cyclases. The data suggest that membrane-delimited receiver domains transduce regulatory signals to the downstream catalytic domains in an engineered AC model system. This may suggest a previously unknown mechanism for cellular cAMP regulation.

Discovery of LRE1 as a specific and allosteric inhibitor of soluble adenylyl cyclase

The prototypical second messenger cAMP regulates a wide variety of physiological processes. It can simultaneously mediate diverse functions by acting locally in independently regulated microdomains. In mammalian cells, two types of adenylyl cyclase generate cAMP: G-protein-regulated transmembrane adenylyl cyclases and bicarbonate-, calcium- and ATP-regulated soluble adenylyl cyclase (sAC). Because each type of cyclase regulates distinct microdomains, methods to distinguish between them are needed to understand cAMP signaling. We developed a mass-spectrometry-based adenylyl cyclase assay, which we used to identify a new sAC-specific inhibitor, LRE1. LRE1 bound to the bicarbonate activator binding site and inhibited sAC via a unique allosteric mechanism. LRE1 prevented sAC-dependent processes in cellular and physiological systems, and it will facilitate exploration of the therapeutic potential of sAC inhibition.

Structural insight into photoactivation of an adenylate cyclase from a photosynthetic cyanobacterium

Cyclic-AMP is one of the most important second messengers, regulating many crucial cellular events in both prokaryotes and eukaryotes, and precise spatial and temporal control of cAMP levels by light shows great promise as a simple means of manipulating and studying numerous cell pathways and processes. The photoactivated adenylate cyclase (PAC) from the photosynthetic cyanobacterium Oscillatoria acuminata (OaPAC) is a small homodimer eminently suitable for this task, requiring only a simple flavin chromophore within a blue light using flavin (BLUF) domain. These domains, one of the most studied types of biological photoreceptor, respond to blue light and either regulate the activity of an attached enzyme domain or change its affinity for a repressor protein. BLUF domains were discovered through studies of photo-induced movements of Euglena gracilis, a unicellular flagellate, and gene expression in the purple bacterium Rhodobacter sphaeroides, but the precise details of light activation remain unknown. Here, we describe crystal structures and the light regulation mechanism of the previously undescribed OaPAC, showing a central coiled coil transmits changes from the light-sensing domains to the active sites with minimal structural rearrangement. Site-directed mutants show residues essential for signal transduction over 45 Å across the protein. The use of the protein in living human cells is demonstrated with cAMP-dependent luciferase, showing a rapid and stable response to light over many hours and activation cycles. The structures determined in this study will assist future efforts to create artificial light-regulated control modules as part of a general optogenetic toolkit.

Bithionol Potently Inhibits Human Soluble Adenylyl Cyclase through Binding to the Allosteric Activator Site

The signaling molecule cAMP regulates functions ranging from bacterial transcription to mammalian memory. In mammals, cAMP is synthesized by nine transmembrane adenylyl cyclases (ACs) and one soluble AC (sAC). Despite similarities in their catalytic domains, these ACs differ in regulation. Transmembrane ACs respond to G proteins, whereas sAC is uniquely activated by bicarbonate. Via bicarbonate regulation, sAC acts as a physiological sensor for pH/bicarbonate/CO2, and it has been implicated as a therapeutic target, e.g. for diabetes, glaucoma, and a male contraceptive. Here we identify the bisphenols bithionol and hexachlorophene as potent, sAC-specific inhibitors. Inhibition appears mostly non-competitive with the substrate ATP, indicating that they act via an allosteric site. To analyze the interaction details, we solved a crystal structure of an sAC·bithionol complex. The structure reveals that the compounds are selective for sAC because they bind to the sAC-specific, allosteric binding site for the physiological activator bicarbonate. Structural comparison of the bithionol complex with apo-sAC and other sAC·ligand complexes along with mutagenesis experiments reveals an allosteric mechanism of inhibition; the compound induces rearrangements of substrate binding residues and of Arg(176), a trigger between the active site and allosteric site. Our results thus provide 1) novel insights into the communication between allosteric regulatory and active sites, 2) a novel mechanism for sAC inhibition, and 3) pharmacological compounds targeting this allosteric site and utilizing this mode of inhibition. These studies provide support for the future development of sAC-modulating drugs.

Protein Kinase A Governs Oxidative Phosphorylation Kinetics and Oxidant Emitting Potential at Complex I

The mitochondrial electron transport system (ETS) is responsible for setting and maintaining both the energy and redox charges throughout the cell. Reversible phosphorylation of mitochondrial proteins, particularly via the soluble adenylyl cyclase (sAC)/cyclic AMP (cAMP)/Protein kinase A (PKA) axis, has recently been revealed as a potential mechanism regulating the ETS. However, the governance of cAMP/PKA signaling and its implications on ETS function are incompletely understood. In contrast to prior reports using exogenous bicarbonate, we provide evidence that endogenous CO₂ produced by increased tricarboxylic acid (TCA) cycle flux is insufficient to increase mitochondrial cAMP levels, and that exogenous addition of membrane permeant 8Br-cAMP does not enhance mitochondrial respiratory capacity. We also report important non-specific effects of commonly used inhibitors of sAC which preclude their use in studies of mitochondrial function. In isolated liver mitochondria, inhibition of PKA reduced complex I-, but not complex II-supported respiratory capacity. In permeabilized myofibers, inhibition of PKA lowered both the K_m and V_max for complex I-supported respiration as well as succinate-supported H₂O₂ emitting potential. In summary, the data provided here improve our understanding of how mitochondrial cAMP production is regulated, illustrate a need for better tools to examine the impact of sAC activity on mitochondrial biology, and suggest that cAMP/PKA signaling contributes to the governance of electron flow through complex I of the ETS.

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.

Requirement of cAMP signaling for Schwann cell differentiation restricts the onset of myelination

Isolated Schwann cells (SCs) respond to cAMP elevation by adopting a differentiated post-mitotic state that exhibits high levels of Krox-20, a transcriptional enhancer of myelination, and mature SC markers such as the myelin lipid galactocerebroside (O1). To address how cAMP controls myelination, we performed a series of cell culture experiments which compared the differentiating responses of isolated and axon-related SCs to cAMP analogs and ascorbate, a known inducer of axon ensheathment, basal lamina formation and myelination. In axon-related SCs, cAMP induced the expression of Krox-20 and O1 without a concomitant increase in the expression of myelin basic protein (MBP) and without promoting axon ensheathment, collagen synthesis or basal lamina assembly. When cAMP was provided together with ascorbate, a dramatic enhancement of MBP expression occurred, indicating that cAMP primes SCs to form myelin only under conditions supportive of basal lamina formation. Experiments using a combination of cell permeable cAMP analogs and type-selective adenylyl cyclase (AC) agonists and antagonists revealed that selective transmembrane AC (tmAC) activation with forskolin was not sufficient for full SC differentiation and that the attainment of an O1 positive state also relied on the activity of the soluble AC (sAC), a bicarbonate sensor that is insensitive to forskolin and GPCR activation. Pharmacological and immunological evidence indicated that SCs expressed sAC and that sAC activity was required for morphological differentiation and the expression of myelin markers such as O1 and protein zero. To conclude, our data indicates that cAMP did not directly drive myelination but rather the transition into an O1 positive state, which is perhaps the most critical cAMP-dependent rate limiting step for the onset of myelination. The temporally restricted role of cAMP in inducing differentiation independently of basal lamina formation provides a clear example of the uncoupling of signals controlling differentiation and myelination in SCs.

Pivotal role of extended linker 2 in the activation of Gα by G protein-coupled receptor

G protein-coupled receptors (GPCRs) relay extracellular signals mainly to heterotrimeric G-proteins (Gαβγ) and they are the most successful drug targets. The mechanisms of G-protein activation by GPCRs are not well understood. Previous studies have revealed a signal relay route from a GPCR via the C-terminal α5-helix of Gα to the guanine nucleotide-binding pocket. Recent structural and biophysical studies uncover a role for the opening or rotating of the α-helical domain of Gα during the activation of Gα by a GPCR. Here we show that β-adrenergic receptors activate eight Gαs mutant proteins (from a screen of 66 Gαs mutants) that are unable to bind Gβγ subunits in cells. Five of these eight mutants are in the αF/Linker 2/β2 hinge region (extended Linker 2) that connects the Ras-like GTPase domain and the α-helical domain of Gαs. This extended Linker 2 is the target site of a natural product inhibitor of Gq. Our data show that the extended Linker 2 is critical for Gα activation by GPCRs. We propose that a GPCR via its intracellular loop 2 directly interacts with the β2/β3 loop of Gα to communicate to Linker 2, resulting in the opening and closing of the α-helical domain and the release of GDP during G-protein activation.

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.

Structural analysis of human soluble adenylyl cyclase and crystal structures of its nucleotide complexes-implications for cyclase catalysis and evolution

The ubiquitous second messenger cAMP regulates a wide array of functions, from bacterial transcription to mammalian memory. It is synthesized by six evolutionarily distinct adenylyl cyclase (AC) families. In mammals, there are two AC types: nine transmembrane ACs (tmACs) and one soluble AC (sAC). Both AC types belong to the widespread cyclase class III, which has members in numerous organisms from archaeons to mammals. Class III also contains all known guanylyl cyclases (GCs), which synthesize the cAMP-related messenger cGMP in many eukaryotes and possibly some prokaryotes. Among mammalian ACs, sAC is uniquely regulated by bicarbonate, and has been proposed to be more closely related to a bacterial AC subfamily than to mammalian ACs, on the basis of sequence comparisons. Here, we used crystal structures of human sAC catalytic domains to analyze its relationships with other class III ACs and GCs, and to study its substrate selection mechanisms. Structural comparisons revealed a similarity within an sAC-like subfamily but no family-specific structure elements, and an unexpected sAC similarity to eukaryotic GCs and a potential bacterial GC. We further solved novel crystal structures of sAC catalytic domains in complex with a substrate analog, unprocessed ATP substrate, and product after soaking with ATP or GTP. The structures show a novel ATP-binding conformation, and suggest mechanisms for substrate association and recognition. Our results could explain the limited substrate specificity of sAC, suggest how specificity is increased in other cyclases, and indicate evolutionary relationships among class III enzymes, with sAC being close to a putative 'ancestor' cyclase.

DATABASE

Coordinates and structure factors for the novel sAC-cat structures described have been deposited with the Worldwide PDB (www.pdb.org): ApCpp soak (entry 4usu), ATP + Ca(2+) soak (entry 4usv), GTP + Mg(2+) soak (entry 4ust), ATP soak (entry 4usw).

Soluble adenylyl cyclase of sea urchin spermatozoa

Fertilization, a key step in sexual reproduction, requires orchestrated changes in cAMP concentrations. It is notable that spermatozoa (sperm) are among the cell types with extremely high adenylyl cyclase (AC) activity. As production and consumption of this second messenger need to be locally regulated, the discovery of soluble AC (sAC) has broadened our understanding of how such cells deal with these requirements. In addition, because sAC is directly regulated by HCO(3)(-) it is able to translate CO₂/HCO(3)(-)/pH changes into cAMP levels. Fundamental sperm functions such as maturation, motility regulation and the acrosome reaction are influenced by cAMP; this is especially true for sperm of the sea urchin (SU), an organism that has been a model in the study of fertilization for more than 130 years. Here we summarize the discovery and properties of SU sperm sAC, and discuss its involvement in sperm physiology. 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.

Expression, purification, crystallization and preliminary X-ray diffraction analysis of a mammalian type 10 adenylyl cyclase

The second messenger cAMP is synthesized in mammals by ten differently regulated adenylyl cyclases (AC1-10). These ACs are grouped into nucleotidyl cyclase class III based on homologies in their catalytic domains. The catalytic domain of AC10 is unique, however, in being activated through direct interaction with calcium and bicarbonate. Here, the production, crystallization and X-ray diffraction analysis of the catalytic domain of human AC10 are described as a basis for structural studies of regulator binding sites and mechanisms. The recombinant protein had high specific AC activity, and crystals of AC10 in space group P63 diffracted to ∼2.0 Å resolution on a synchrotron beamline. A complete diffraction data set revealed unit-cell parameters a = b = 99.65, c = 98.04 Å, indicating one AC10 catalytic domain per asymmetric unit, and confirmed that the obtained crystals are suitable for structure solution and mechanistic studies.

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.

In Silico Prediction of Tyrosinase and Adenylyl Cyclase Inhibitors from Natural Compounds

Although many herbal medicines are effective in the treatment of hyperpigmentation, the potency of different constituents remains unknown. In this work, more than 20,000 herbal ingredients from 453 herbs were docked into the crystal structures of adenylyl cyclase and a human homology tyrosinase model using Surflex-Dock. These two enzymes are responsible for melanin production and inhibition of them may attain a skin-whitening effect superior to currently available agents. The essential drug properties for topical formulation of the herbal ingredients, including skin permeability, sensitization, irritation, corrosive and carcinogenic properties were predicted by Dermwin, Skin Sensitization Alerts (SSA), Skin Irritation Corrosion Rules Estimation Tool (SICRET) and Benigni/Bossa rulebase module of Toxtree. Moreover, similarity ensemble and pharmacophore mapping approaches were used to forecast other potential targets for these herbal compounds by the software, SEArch and PharmMapper. Overall, this study predicted seven compounds to have advanced drug-like properties over the well-known effective tyrosinase inhibitors, arbutin and kojic acid. These seven compounds have the highest potential for further in vitro and in vivo investigation with the aim of developing safe and high-efficacy skin-whitening agents.

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.

Mitochondrial Ca²⁺ uptake induces cyclic AMP generation in the matrix and modulates organelle ATP levels

While the role of mitochondrial Ca²⁺ homeostasis in cell pathophysiology is widely accepted, the possibility that cAMP regulates mitochondrial functions has only recently received experimental support. The site of cAMP production, its targets, and its functions in the organelles remain uncertain. Using a variety of genetic/pharmacological tools, we here demonstrate that the mitochondrial inner membrane is impermeable to cytosolic cAMP, while an autonomous cAMP signaling toolkit is expressed in the matrix. We demonstrate that rises in matrix Ca²⁺ powerfully stimulate cAMP increases within mitochondria and that matrix cAMP levels regulate their ATP synthesizing efficiency. In cardiomyocyte cultures, mitochondrial cAMP can be increased by treatments that augment the frequency and amplitude of Ca²⁺ oscillations within the cytosol and organelles, revealing that mitochondria can integrate an oscillatory Ca²⁺ signal to increase cAMP in their matrix. The present data reveal the existence, within mitochondria, of a hitherto unknown crosstalk between Ca²⁺ and cAMP.

Identification of a haem domain in human soluble adenylate cyclase

The second messengers cAMP and cGMP mediate a multitude of physiological processes. In mammals, these cyclic nucleotides are formed by related Class III nucleotidyl cyclases, and both ACs (adenylate cyclases) and GCs (guanylate cyclases) comprise transmembrane receptors as well as soluble isoforms. Whereas sGC (soluble GC) has a well-characterized regulatory HD (haem domain) that acts as a receptor for the activator NO (nitric oxide), very little is known about the regulatory domains of the ubiquitous signalling enzyme sAC (soluble AC). In the present study, we identify a unique type of HD as a regulatory domain in sAC. The sAC-HD (sAC haem domain) forms a larger oligomer and binds, non-covalently, one haem cofactor per monomer. Spectral analyses and mutagenesis reveal a 6-fold co-ordinated haem iron atom, probably with non-typical axial ligands, which can bind both NO and CO (carbon monoxide). Splice variants of sAC comprising this domain are expressed in testis and skeletal muscle, and the HD displays an activating effect on the sAC catalytic core. Our results reveal a novel mechanism for regulation of cAMP signalling and suggest a need for reanalysis of previous studies on mechanisms of haem ligand effects on cyclic nucleotide signalling, particularly in testis and skeletal muscle.

Biological roles of cAMP: variations on a theme in the different kingdoms of life

Cyclic AMP (cAMP) plays a key regulatory role in most types of cells; however, the pathways controlled by cAMP may present important differences between organisms and between tissues within a specific organism. Changes in cAMP levels are caused by multiple triggers, most affecting adenylyl cyclases, the enzymes that synthesize cAMP. Adenylyl cyclases form a large and diverse family including soluble forms and others with one or more transmembrane domains. Regulatory mechanisms for the soluble adenylyl cyclases involve either interaction with diverse proteins, as happens in Escherichia coli or yeasts, or with calcium or bicarbonate ions, as occurs in mammalian cells. The transmembrane cyclases can be regulated by a variety of proteins, among which the α subunit and the βγ complex from G proteins coupled to membrane receptors are prominent. cAMP levels also are controlled by the activity of phosphodiesterases, enzymes that hydrolyze cAMP. Phosphodiesterases can be regulated by cAMP, cGMP or calcium-calmodulin or by phosphorylation by different protein kinases. Regulation through cAMP depends on its binding to diverse proteins, its proximal targets, this in turn causing changes in a variety of distal targets. Specifically, binding of cAMP to regulatory subunits of cAMP-dependent protein kinases (PKAs) affects the activity of substrates of PKA, binding to exchange proteins directly activated by cAMP (Epac) regulates small GTPases, binding to transcription factors such as the cAMP receptor protein (CRP) or the virulence factor regulator (Vfr) modifies the rate of transcription of certain genes, while cAMP binding to ion channels modulates their activity directly. Further studies on cAMP signalling will have important implications, not only for advancing fundamental knowledge but also for identifying targets for the development of new therapeutic agents.

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.

Characterization of Plasmodium falciparum adenylyl cyclase-β and its role in erythrocytic stage parasites

The most severe form of human malaria is caused by the parasite Plasmodium falciparum. The second messenger cAMP has been shown to be important for the parasite's ability to infect the host's liver, but its role during parasite growth inside erythrocytes, the stage responsible for symptomatic malaria, is less clear. The P. falciparum genome encodes two adenylyl cyclases, the enzymes that synthesize cAMP, PfACα and PfACβ. We now show that one of these, PfACβ, plays an important role during the erythrocytic stage of the P. falciparum life cycle. Biochemical characterization of PfACβ revealed a marked pH dependence, and sensitivity to a number of small molecule inhibitors. These inhibitors kill parasites growing inside red blood cells. One particular inhibitor is selective for PfACβ relative to its human ortholog, soluble adenylyl cyclase (sAC); thus, PfACβ represents a potential target for development of safe and effective antimalarial therapeutics.

Role of catecholestrogens on ovarian prostaglandin secretion in vitro in the catfish Heteropneustes fossilis and possible mechanism of regulation

Seasonal, periovulatory and 2-hydroxyestradiol-17β (2-OHE(2))-induced changes on ovarian prostaglandin (PG) E(2) and F(2α) were investigated under in vivo or in vitro in the female catfish Heteropneustes fossilis. Both PGE(2) and PGF(2α) increased significantly during ovarian recrudescence with the peak levels in spawning phase. The PGs showed periovulatory changes with the peak levels at 16 h after the hCG treatment. Incubation of postvitellogenic ovary fragments with estradiol-17β (E(2)), 2-OHE(2) or 2-methoxyE(2) produced concentration-dependent increases in PG levels; 2-OHE(2) was more effective. In order to identify the receptor mechanism involved in the 2-OHE(2)-induced PG stimulation, the ovarian pieces were incubated with phentolamine (an α-adrenergic antagonist), propranolol (a β-adrenergic antagonist) or tamoxifen (an estrogen receptor blocker) alone or in combination with 2-OHE(2). The incubation of the tissues with the receptor blockers alone did not produce any significant effect on basal PG levels. However, co- and pre-incubation of the tissues with the blockers resulted in inhibition of the stimulatory effect of 2-OHE(2) on the PGs. Phentolamine was more effective than propranolol. The signal transduction pathway(s) involved in the 2-OHE(2)-induced PG secretion was investigated. The incubation of the ovarian pieces with 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor), chelerythrine (a protein kinase C inhibitor) and PD098059 (a mitogen-activated protein kinase inhibitor) significantly lowered the basal secretion of PGF(2α) and PGE(2). In contrast, H89 (a protein kinase A inhibitor) increased the basal secretion of PGs at 1 and 5 μM concentration and decreased it at 10 μM concentration. The co- or pre-incubation with IBMX, H89, chelerythrine and PD098059 significantly inhibited the stimulatory effect of 2-OHE(2) on PGF(2α) and PGE(2) levels. The inhibition was higher in the pre-incubation groups. Chelerythrine was the most effective followed by PD098059, IBMX and H89. The results suggest that 2-OHE(2) may employ both adrenergic and estrogen receptors, or a novel receptor mechanism having properties of both adrenergic and estrogen receptors.

Structures, substrates, and regulators of Mammalian sirtuins - opportunities and challenges for drug development

Sirtuins are NAD⁺-dependent protein deacetylases regulating metabolism, stress responses, and aging processes. Mammalia have seven Sirtuin isoforms, Sirt1–7, which differ in their substrate specificities and subcellular localizations. The physiological functions of Sirtuins make them interesting therapeutic targets, which has stimulated extensive efforts on development of small molecule Sirtuin modulators. Yet, most Sirtuin inhibitors show limited potency and/or isoform specificity, and the mechanism of Sirtuin activation by small molecules remains obscure. Accumulating information on Sirtuin substrates, structures, and regulation mechanisms offer new opportunities for the challenging task to develop potent and specific small molecule modulators for mammalian Sirtuins for in vivo studies and therapeutic applications. We therefore recapitulate advances in structural and mechanistic studies on substrate recognition and deacetylation by Sirtuins, and in the characterization of compounds and molecular mechanisms regulating their activity. We then discuss challenges and opportunities from these findings for Sirtuin-targeted drug development efforts.

In vitro effects of 2-hydroxyestradiol-17β on ovarian follicular steroid secretion in the catfish Heteropneustes fossilis and identification of the receptor and signaling mechanisms

Ovarian pieces containing postvitellogenic follicles were incubated in vitro with different concentrations of the catecholestrogen 2-hydroxyestradiol-17β (2-OHE(2)) to evaluate its effects on steroid production and germinal vesicle breakdown (GVBD) in the catfish Heteropneustes fossilis. The incubation with 2-OHE(2) induced a shift in steroidogenic pattern: the C(19) and C(18) steroids testosterone (T) and estradiol-17β (E(2)), respectively were significantly decreased with a concomitant significant increase in the C(21) steroids progesterone (P(4)), 17-hydroxyprogesterone (17-OHP), 17,20β-dihydroxy-4-pregnen-3-one (17,20β-DP), 17,20α-dihydroxy-4-pregnen-3-one (17,20α-DP) and cortisol (F). Concomitantly, the catecholestrogen induced dose-dependently GVBD response, the first sign of meiosis resumption. The co- and pre-incubations of the ovarian pieces with 2-OHE(2), and adrenergic (phentolamine, α-blocker and propranolol, β-blocker) or estrogen (tamoxifen) receptor blockers resulted in inhibition of the stimulatory effect of the catecholestrogen on C(21) steroids and reversed the inhibition of testosterone and E(2). The α-blocker was more effective than the β-blocker. Our results suggest that 2-OHE(2) appears to employ both adrenergic (α-type) and estrogen receptor mechanisms in mediating the effects. The co- or pre-incubation of ovarian pieces with IBMX (a cAMP elevating drug), H89 (a protein kinase A inhibitor), and PD098059 (a MAP kinase kinase inhibitor) significantly inhibited the stimulatory effect of 2-OHE(2) on the C(21) steroids. The effect of chelerythrine (a protein kinase C inhibitor), on the other hand, varied with the incubation condition. In the co-incubation, the steroids showed varied effects: 17,20β-DP, testosterone and E(2) were elevated, and P(4) and 17-OHP were decreased. In the pre-incubation set up, all the steroids were inhibited except E(2). The inhibition by the blockers was higher in the pre-incubation groups. Taken together, the data suggest the involvement cAMP-protein kinase A, protein kinase C and MAP kinase pathways in the modulation of the steroidogenic activity.

Crystal structure and regulation mechanisms of the CyaB adenylyl cyclase from the human pathogen Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic bacterial pathogen and a major cause of healthcare-associated infections. While the organism's intrinsic and acquired resistance to most antibiotics hinders treatment of P. aeruginosa infections, the regulatory networks controlling its virulence provide novel targets for drug development. CyaB, a key regulator of P. aeruginosa virulence, belongs to the Class III adenylyl cyclase (AC) family of enzymes that synthesize the second messenger cyclic adenosine 3',5'-monophosphate. These enzymes consist of a conserved catalytic domain fused to one or more regulatory domains. We describe here the biochemical and structural characterization of CyaB and its inhibition by small molecules. We show that CyaB belongs to the Class IIIb subfamily, and like other subfamily members, its activity is stimulated by inorganic carbon. CyaB is also regulated by its N-terminal MASE2 (membrane-associated sensor 2) domain, which acts as a membrane anchor. Using a genetic screen, we identified activating mutations in CyaB. By solving the crystal structure of the CyaB catalytic domain, we rationalized the effects of these mutations and propose that CyaB employs regulatory mechanisms similar to other Class III ACs. The CyaB structure further indicates subtle differences compared to other Class III ACs in both the active site and the inhibitor binding pocket. Consistent with these differences, we observed a unique inhibition profile, including identification of a CyaB selective compound. Overall, our results reveal mechanistic details of the physiological and pharmacological regulation of CyaB and provide the basis for its exploitation as a therapeutic drug target.

Role of soluble adenylyl cyclase in the heart

This review discusses the potential place of soluble adenylyl cyclase (sAC) in the framework of signaling in the cardiovascular system. cAMP has been studied as a critical and pleiotropic second messenger in cardiomyocytes, endothelial cells, and smooth muscle vascular cells for many years. It is involved in the transduction of signaling by catecholamines, prostaglandins, adenosine, and glucagon, just to name a few. These hormones can act via cAMP by binding to a G protein-coupled receptor on the plasma membrane with subsequent activation of a heterotrimeric G protein and its downstream effector, transmembrane adenylyl cyclase. This has long been the canonical standard for cAMP production in a cell. However, the relatively recent discovery of a unique source of cAMP, sAC, creates the potential for a shift in this signaling paradigm. In fact, sAC has been shown to play a role in apoptosis in coronary endothelial cells and cardiomyocytes. Additionally, it links nutrient utilization with ATP production in the liver and brain, which suggests one of many potential roles for sAC in cardiac function. The possibility of producing cAMP from a source distal to the plasma membrane provides a critical new building block for reconstructing the cellular signaling infrastructure.

Intracellular cAMP signaling by soluble adenylyl cyclase

Soluble adenylyl cyclase (sAC) is a recently identified source of the ubiquitous second messenger cyclic adenosine 3',5' monophosphate (cAMP). sAC is distinct from the more widely studied source of cAMP, the transmembrane adenylyl cyclases (tmACs); its activity is uniquely regulated by bicarbonate anions, and it is distributed throughout the cytoplasm and in cellular organelles. Due to its unique localization and regulation, sAC has various functions in a variety of physiological systems that are distinct from tmACs. In this review, we detail the known functions of sAC, and we reassess commonly held views of cAMP signaling inside cells.

Physiological carbon dioxide, bicarbonate, and pH sensing

In biological systems, carbon dioxide exists in equilibrium with bicarbonate and protons. The individual components of this equilibrium (i.e., CO₂, HCO₃⁻, and H(+)), which must be sensed to be able to maintain cellular and organismal pH, also function as signals to modulate multiple physiological functions. Yet, the molecular sensors for CO₂/HCO₃⁻/pH remained unknown until recently. Here, we review recent progress in delineating molecular and cellular mechanisms for sensing CO₂, HCO₃⁻, and pH.

Decreased soluble adenylyl cyclase activity in cystic fibrosis is related to defective apical bicarbonate exchange and affects ciliary beat frequency regulation

Human airway cilia contain soluble adenylyl cyclase (sAC) that produces cAMP upon HCO(3)(-)/CO(2) stimulation to increase ciliary beat frequency (CBF). Because apical HCO(3)(-) exchange depends on cystic fibrosis transmembrane conductance regulator (CFTR), malfunctioning CFTR might impair sAC-mediated CBF regulation in cells from patients with cystic fibrosis (CF). By Western blot, sAC isoforms are equally expressed in normal and CF airway epithelial cells, but CBF decreased more in CF than normal cells upon increased apical HCO(3)(-)/CO(2) exposure in part because of greater intracellular acidification from unbalanced CO(2) influx (estimated by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) fluorescence). Importantly, ciliated cell-specific cAMP production (estimated by FRET fluorescence ratio changes of tagged cAMP-dependent protein kinase (PKA) subunits expressed under a ciliated cell-specific promoter) in response to increased apical HCO(3)(-)/CO(2) perfusion was higher in normal compared with CF cells. Inhibition of bicarbonate influx via CFTR (CFTR(inh)172) and inhibition of sAC (KH7) and PKA activation (H89) led to larger CBF declines in normal cells, now comparable with changes seen in CF cells. These inhibitors also reduced FRET changes in normal cells to the level of CF cells with the expected exception of H89, which does not prevent dissociation of the fluorescently tagged PKA subunits. Basolateral permeabilization and subsequent perfusion with HCO(3)(-)/CO(2) rescued CBF and FRET changes in CF cells to the level of normal cells. These results suggest that CBF regulation by sAC-produced cAMP could be impaired in CF, thereby possibly contributing to mucociliary dysfunction in this disease, at least during disease exacerbations when airway acidification is common.

Modulation of NaCl absorption by [HCO(3)(-)] in the marine teleost intestine is mediated by soluble adenylyl cyclase

Intestinal HCO(3)(-) secretion and NaCl absorption are essential for counteracting dehydration in marine teleost fish. We investigated how these two processes are coordinated in toadfish. HCO(3)(-) stimulated a luminal positive short-circuit current (I(sc)) in intestine mounted in Ussing chamber, bathed with the same saline solution on the external and internal sides of the epithelium. The I(sc) increased proportionally to the [HCO(3)(-)] in the bath up to 80 mM NaHCO(3), and it did not occur when NaHCO(3) was replaced with Na(+)-gluconate or with NaHCO(3) in Cl(-)-free saline. HCO(3)(-) (20 mM) induced a approximately 2.5-fold stimulation of I(sc), and this [HCO(3)(-)] was used in all subsequent experiments. The HCO(3)(-)-stimulated I(sc) was prevented or abolished by apical application of 10 muM bumetanide (a specific inhibitor of NKCC) and by 30 microM 4-catechol estrogen [CE; an inhibitor of soluble adenylyl cyclase (sAC)]. The inhibitory effects of bumetanide and CE were not additive. The HCO(3)(-)-stimulated I(sc) was prevented by apical bafilomycin (1 microM) and etoxolamide (1 mM), indicating involvement of V-H(+)-ATPase and carbonic anhydrases, respectively. Immunohistochemistry and Western blot analysis confirmed the presence of an NKCC2-like protein in the apical membrane and subapical area of epithelial intestinal cells, of Na(+)/K(+)-ATPase in basolateral membranes, and of an sAC-like protein in the cytoplasm. We propose that sAC regulates NKCC activity in response to luminal HCO(3)(-), and that V-H(+)-ATPase and intracellular carbonic anhydrase are essential for transducing luminal HCO(3)(-) into the cell by CO(2)/HCO(3)(-) hydration/dehydration. This mechanism putatively coordinates HCO(3)(-) secretion with NaCl and water absorption in toadfish intestine.

Bicarbonate-sensing soluble adenylyl cyclase is an essential sensor for acid/base homeostasis

pH homeostasis is essential for life, yet it remains unclear how animals sense their systemic acid/base (A/B) status. Soluble adenylyl cyclase (sAC) is an evolutionary conserved signaling enzyme that produces the second messenger cAMP in response to bicarbonate ions (HCO(3)(-)). We cloned the sAC ortholog from the dogfish, a shark that regulates blood A/B by absorbing and secreting protons (H(+)) and HCO(3)(-) at its gills. Similar to mammalian sAC, dogfish soluble adenylyl cyclase (dfsAC) is activated by HCO(3)(-) and can be inhibited by two structurally and mechanistically distinct small molecule inhibitors. dfsAC is expressed in the gill epithelium, where the subset of base-secreting cells resides. Injection of inhibitors into animals under alkaline stress confirmed that dfsAC is essential for maintaining systemic pH and HCO(3)(-) levels in the whole organism. One of the downstream effects of dfsAC is to promote the insertion of vacuolar proton pumps into the basolateral membrane to absorb H(+) into the blood. sAC orthologs are present throughout metazoans, and mammalian sAC is expressed in A/B regulatory organs, suggesting that systemic A/B sensing via sAC is widespread in the animal kingdom.