Calcium regulation of the soluble adenylyl cyclase expressed in mammalian spermatozoa

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

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

Inhibition of human sperm motility by contraceptive anti-eppin antibodies from infertile male monkeys: effect on cyclic adenosine monophosphate

Epididymal protease inhibitor (eppin [official symbol, SPINLW1]) is of interest as a male contraceptive target because of its specificity and location on the human sperm surface. We have examined the effect of anti-eppin antibodies from infertile male monkeys and the effect of recombinant human semenogelin on human sperm motility. Anti-eppin antibodies significantly decreased the progressive motility of human spermatozoa as measured by decreased total distance traveled, decreased straight-line distance, and decreased velocity. Anti-eppin treatment of spermatozoa significantly increased the amount of cAMP present in nonprogressive spermatozoa; however, approximately 25% of antibody-treated spermatozoa could be rescued by the addition of cAMP-acetoxymethyl ester, indicating that anti-eppin-treated spermatozoa have a compromised ability to utilize cAMP. Addition of recombinant human semenogelin has a concentration-dependent inhibitory effect on progressive motility (increased tortuosity and decreased velocity). We tested the hypothesis that anti-eppin antibodies bound to eppin would subsequently block semenogelin binding to eppin. Anti-eppin antibodies from infertile monkeys inhibited eppin from binding to semenogelin. Addition of affinity-purified antibodies made to the dominant C-terminal epitope of eppin had an inhibitory effect on progressive motility (increased tortuosity, decreased velocity, and straight distance). Our results suggest that the eppin-semenogelin binding site is critical for the removal of semenogelin in vivo during semen liquefaction and for the initiation of progressive motility. We conclude that the eppin-semenogelin binding site on the surface of human spermatozoa is an ideal target for a nonsteroidal male contraceptive.

Somatic 'soluble' adenylyl cyclase isoforms are unaffected in Sacy tm1Lex/Sacy tm1Lex 'knockout' mice

Background

Mammalian Soluble adenylyl cyclase (sAC, Adcy10, or Sacy) represents a source of the second messenger cAMP distinct from the widely studied, G protein-regulated transmembrane adenylyl cyclases. Genetic deletion of the second through fourth coding exons in Sacy^tm1Lex/Sacy^tm1Lex knockout mice results in a male sterile phenotype. The absence of any major somatic phenotype is inconsistent with the variety of somatic functions identified for sAC using pharmacological inhibitors and RNA interference.

Principal Findings

We now use immunological and molecular biological methods to demonstrate that somatic tissues express a previously unknown isoform of sAC, which utilizes a unique start site, and which ‘escapes’ the design of the Sacy^tm1Lex knockout allele.

Conclusions/Significance

These studies reveal increased complexity at the sAC locus, and they suggest that the known isoforms of sAC play a unique function in male germ cells.

Glucose and GLP-1 stimulate cAMP production via distinct adenylyl cyclases in INS-1E insulinoma cells

In β cells, both glucose and hormones, such as GLP-1, stimulate production of the second messenger cAMP, but glucose and GLP-1 elicit distinct cellular responses. We now show in INS-1E insulinoma cells that glucose and GLP-1 produce cAMP with distinct kinetics via different adenylyl cyclases. GLP-1 induces a rapid cAMP signal mediated by G protein–responsive transmembrane adenylyl cyclases (tmAC). In contrast, glucose elicits a delayed cAMP rise mediated by bicarbonate, calcium, and ATP-sensitive soluble adenylyl cyclase (sAC). This glucose-induced, sAC-dependent cAMP rise is dependent upon calcium influx and is responsible for the glucose-induced activation of the mitogen-activated protein kinase (ERK1/2) pathway. These results demonstrate that sAC-generated and tmAC-generated cAMP define distinct signaling cascades.

Soluble adenylyl cyclase is required for activation of sperm but does not have a direct effect on hyperactivation

Soluble adenylyl cyclase (SACY) is an essential component of cAMP-signalling cascades that activate sperm motility and capacitate sperm. SACY activity is stimulated by HCO(3)(-) and Ca(2+). Sperm from Sacy(-/-) (null) mice were immotile or weakly motile, but cAMP analogues N(6),2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (dbcAMP) and adenosine 3',5'-cyclic monophosphate acetoxymethyl ester (cAMP-AM) activated motility. Null sperm activated by dbcAMP quickly developed hairpin bends at the junction of the midpiece and principal piece, which could be prevented by omitting HCO(3)(-). Treating Sacy(-/-) sperm with thimerosal or NH(4)Cl to raise flagellar cytoplasmic Ca(2+) could not substitute for cAMP analogues in activating motility; however, sperm activated with cAMP-AM hyperactivated after thimerosal treatment. Treating activated wild-type sperm with SACY inhibitor KH7 did not prevent hyperactivation from developing during capacitation in vitro, although high doses impaired motility. These results indicate that, while the SACY/cAMP signalling pathway is required for motility activation, it is not directly involved in triggering hyperactivation.

Bicarbonate-induced phosphorylation of p270 protein in mouse sperm by cAMP-dependent protein kinase

Signaling by cAMP-dependent protein kinase (PKA) plays an important role in the regulation of mammalian sperm motility. However, it has not been determined how PKA signaling leads to changes in motility, and specific proteins responsible for these changes have not yet been identified as PKA substrates. Anti-phospho-(Ser/Thr) PKA substrate antibodies detected a sperm protein with a relative molecular weight of 270,000 (p270), which was phosphorylated within 1 min after incubation in a medium supporting capacitation. Phosphorylation of p270 was induced by bicarbonate or a cAMP analog, but was blocked by the PKA inhibitor H-89, indicating that p270 is likely a PKA substrate in sperm. In addition, phosphorylation of p270 was inhibited by stearated peptide st-Ht31, suggesting that p270 is phosphorylated by PKA associated with an A-kinase anchoring protein (AKAP). AKAP4 is the major fibrous sheath protein of mammalian sperm and tethers regulatory subunits of PKA to localize phosphorylation events. Phosphorylation of p270 occurred in sperm lacking AKAP4, suggesting that AKAP4 is not involved directly in the phosphorylation event. Phosphorylated p270 was enriched in fractionated sperm tails and appeared to be present in multiple compartments including a detergent-resistant membrane fraction. PKA phosphorylation of p270 within 1 min of incubation under capacitation conditions suggests that this protein may have an important role in the initial signaling events that lead to the activation and subsequent hyperactivation of sperm motility.

Effects of extracellular adenosine 5'-triphosphate on human sperm motility

Extracellular adenosine 5'-triphosphate (ATP) previously has been shown to increase the fertilization percentage in human in vitro fertilization (IVF) performed for male factor infertility. The objective of this study is to determine the effects of extracellular adenosine 5'-triphosphate (ATPe) on human sperm function by examining its effects on end points of sperm capacitation. Sperm obtained from healthy volunteers with normal semen parameters, asthenozoospermic men, and cryopreserved samples were incubated in medium with or without 2.5 mM ATPe. The effects of ATPe on acrosomal exocytosis, protein tyrosine phosphorylation, and sperm motility parameters were quantified. Although ATPe did not affect acrosomal exocytosis or protein tyrosine phosphorylation in sperm from healthy donors, it significantly altered several motility parameters, with the largest effects manifested in increased curvilinear velocity and percentage hyperactivation. ATPe similarly affected sperm selected for poor motility and thawed cryopreserved sperm but to a lesser extent than its effects on sperm with normal motility. ATPe increased straight-line velocity and linearity of sperm obtained from asthenozoospermic men. Human sperm motility characteristics are altered by ATPe; this finding may explain its previously reported beneficial effect on human IVF. These results suggest that ATPe could constitute a new therapeutic modality in the treatment of male infertility.

Control of basal CFTR gene expression by bicarbonate-sensitive adenylyl cyclase in human pulmonary cells

The CFTR protein, encoded by the gene whose mutations induce Cystic Fibrosis, is an anion channel devoted mainly to chloride and bicarbonate transmembrane transport, but which also regulates transport of several other ions. Moreover, it is implicated in the cell response to inflammation, and, reciprocally, cftr gene expression is modulated by inflammatory stimuli and transduction pathways. Looking for a control of CFTR expression by ionic conditions, we investigated the effect of altered extracellular bicarbonate ion concentration on CFTR expression in human pulmonary Calu-3 cells. We found that basal cftr gene transcription is enhanced when extracellular HCO(3)(-) concentration increases from 0 to 25 mmol/l. The transduction pathway controlled by these extracellular [HCO(3)(-)] variations includes cAMP production linked to the stimulation of soluble adenylyl cyclase (sAC), and nuclear accumulation of the transcription factor, CREB. Basal membrane content in CFTR protein exhibits the same variations as cftr mRNA in cells incubated in the presence of extracellular [HCO(3)(-)] between 0 and 25 mmol/l, and is also decreased by inhibiting sAC in the presence of HCO(3)(-). These results show that bicarbonate-controlled sAC stimulation must be taken into account in cell physiology and that basal CFTR expression depends on an ionic parameter.

External Ca2+ acts upstream of adenylyl cyclase SACY in the bicarbonate signaled activation of sperm motility

The HCO3(-) anion activates sperm motility, an important early step in capacitation, by increasing flagellar beat frequency through a pathway that requires the atypical adenylyl cyclase SACY and the sperm-specific C alpha2 catalytic subunit of PKA. Here we show that the accelerating action of HCO3(-) also requires the continued presence of external Ca2+ (EC50 approximately 0.5 mM), and find that Ca2+ can be replaced by Sr2+ but not by Mn2+. Ca2+ is required for HCO3(-) to elevate cAMP, but not for cAMP-AM to increase beat frequency, indicating that external Ca2+ acts before rather than after stimulation of SACY by HCO3(-). With external Ca2+ present, HCO3(-) does not alter cytosolic or near-membrane [Ca2+]. Removal of external Ca2+ initiates a slow decline in intracellular [Ca2+] and rapid block of the HCO3(-)-evoked acceleration that is not relieved upon increasing internal [Ca2+] by rapid photolysis of caged Ca2+. We also find that the rapid (t(1/2) approximately 10 s) accelerating action of HCO3(-) is slowed more than three-fold by the carbonic anhydrase inhibitor acetazolamide. It is unaltered by the broad spectrum anion transport inhibitor SITS, and is not accompanied by detectable changes in intracellular pH. We propose that external Ca2+ binds an unidentified extracellular protein that is required for HCO3(-) to engage cAMP-mediated activation of motility.

Association of soluble adenylyl cyclase with the V-ATPase in renal epithelial cells

Activation of soluble adenylyl cyclase (sAC) by bicarbonate causes local cAMP generation, indicating that sAC might act as a pH and/or bicarbonate sensor in kidney cells involved in acid-base homeostasis. Therefore, we examined the expression of sAC in renal acid-base transporting intercalated cells (IC) and compared its distribution to that of the vacuolar proton pumping ATPase (V-ATPase) under different conditions. In all IC, sAC and V-ATPase showed considerable overlap under basal conditions, but sAC staining was also found in other cellular locations in the absence of V-ATPase. In type A-IC, both sAC and V-ATPase were apically and subapically located, whereas in type B-IC, significant basolateral colocalization of sAC and the V-ATPase was seen. When apical membrane insertion of the V-ATPase was stimulated by treatment of rats with acetazolamide, sAC was also concentrated in the apical membrane of A-IC. In mice that lack a functional B1 subunit of the V-ATPase, sAC was colocalized apically in A-IC along with V-ATPase containing the alternative B2 subunit isoform. The close association between these two enzymes was confirmed by coimmunoprecipitation of sAC from kidney homogenates using anti-V-ATPase antibodies. Our data show that sAC and the V-ATPase colocalize in IC, that they are concentrated in the IC plasma membrane under conditions that "activate" these proton secretory cells, and that they are both present in an immunoprecipitated complex. This suggests that these enzymes have a close association and could be part of a protein complex that is involved in regulating renal distal proton secretion.

Particulate and soluble adenylyl cyclases participate in the sperm acrosome reaction

cAMP is important in sea urchin sperm signaling, yet the molecular nature of the adenylyl cyclases (ACs) involved remained unknown. These cells were recently shown to contain an ortholog of the mammalian soluble adenylyl cyclase (sAC). Here, we show that sAC is present in the sperm head and as in mammals is stimulated by bicarbonate. The acrosome reaction (AR), a process essential for fertilization, is influenced by the bicarbonate concentration in seawater. By using functional assays and immunofluorescence techniques we document that sea urchin sperm also express orthologs of multiple isoforms of transmembrane ACs (tmACs). Our findings employing selective inhibitors for each class of AC indicate that both sAC and tmACs participate in the sperm acrosome reaction.

A sperm-specific Na+/H+ exchanger (sNHE) is critical for expression and in vivo bicarbonate regulation of the soluble adenylyl cyclase (sAC)

We previously identified a sperm-specific Na(+)/H(+) exchanger (sNHE) principally localized to the flagellum. Disruption of the sNHE gene in mice resulted in absolute male infertility associated with a complete loss of sperm motility. Here, we show that the sNHE-null spermatozoa fail to develop the cAMP-dependent protein tyrosine phosphorylation that coincides with the functional maturation occurring upon incubation in capacitating conditions in vitro. Both the sperm motility defect and the lack of induced protein tyrosine phosphorylation are rescued by the addition of cell-permeable cAMP analogs, suggesting that cAMP metabolism is impaired in spermatozoa lacking sNHE. Our analyses of the bicarbonate-dependent soluble adenylyl cyclase (sAC) signaling pathway in sNHE-null sperm cells reveal that sNHE is required for the expression of full-length sAC, and that it is important for the bicarbonate stimulation of sAC activity in spermatozoa. Furthermore, both codependent expression and coimmunoprecipitation experiments indicate that sNHE and sAC associate with each other. Thus, these two proteins appear to be components of a signaling complex at the sperm flagellar plasma membrane. We propose that the formation of this complex efficiently modulates intracellular pH and bicarbonate levels through the rapid and effective control of sAC and sNHE activities to facilitate sperm motility regulation.

PKA, Germ Cells, and Fertility

Temporal and spatial regulation of PKA activity are essential for vigorous sperm motility and for the resumption of meiosis in oocytes, two events required for successful fertilization. Genetic mutations in mice that affect PKA signaling in germ cells lead to infertility and illustrate the importance of this pathway in mammalian reproduction.

Kainic acid triggers oligodendrocyte precursor cell proliferation and neuronal differentiation from striatal neural stem cells

Glutamate is an excitatory amino acid that serves important functions in mammalian brain development through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/ kainate receptor stimulation. Neural stem cells with self-renewal and multilineage potential are a useful tool to study the signals involved in the regulation of brain development. We have investigated the role played by AMPA/kainate receptors during the differentiation of neural stem cells derived from fetal rat striatum. The application of 1 and 10 microM kainic acid increased significantly the phosphorylation of the cyclic AMP response element binding protein (CREB), raised bromodeoxyuridine incorporation in O4-positive oligodendrocyte precursors, and increased the number of O1-positive cells in the cultures. Increased CREB phosphorylation and proliferation were prevented by the AMPA receptor antagonist 4-4(4-aminophenyl)-1,2-dihydro-1-methyl-2-propylcarbamoyl-6,7-methylenedioxyphthalazine (SYM 2206) and by protein kinase A and protein kinase C inhibitors. Cultures treated with 100 microM kainic acid showed decreased proliferation, a lower proportion of O1-positive cells, and apoptosis of O4-positive cells. None of these effects were prevented by SYM 2206, suggesting that kainate receptors take part in these events. We conclude that AMPA receptor stimulation by kainic acid promotes the proliferation of oligodendrocyte precursors derived from neural stem cells through a mechanism that requires the activation of CREB by protein kinase A and C. In the neurons derived from these cells, either AMPA or kainate receptor stimulation produces neuritic growth and larger cell bodies.

Premature Capacitation of Frozen-Thawed Spermatozoa from Subfertile Japanese Black Cattle

Artificial insemination (AI) subfertility is an indication of failure of AI with frozen-thawed sperm classified as normal by conventional semen examination. Recently, 8 AI-subfertile Japanese Black cattle (S1-S8) were identified using the routine AI test or in vivo fertilization test, which included AI with frozen-thawed sperm of superovulated females and subsequent non-surgical recovery of presumptive zygotes. In the present study, we assessed capacitation states and in vitro oocyte penetration of frozen-thawed sperm from these bulls to estimate causal factors of AI subfertility. Frozen-thawed sperm from 8 AI-subfertile (S1-S8) and 9 fertile (F1-F9, control) bulls were washed and then used for a chlortetracycline (CTC) staining assay and in vitro fertilization test. The CTC staining assay revealed that approximately 50% of the sperm from 4 of the AI-subfertile bulls (S5-S8) were prematurely progressing into the capacitation state immediately after washing and resuspension in a CaCl(2)-lacking medium. In contrast, most of the sperm from the fertile bulls and other AI-subfertile bulls (S1-S4) remained uncapacitated. Addition of CaCl(2) to the medium effectively promoted a spontaneous acrosome reaction in the sperm samples from the AI-subfertile bulls (S5-S8). Moreover, the in vitro fertilization test showed that rates of sperm penetration into oocytes were significantly lower in sperm samples from the AI-subfertile bulls (S5-S8) than in the control sperm samples from the fertile bulls (F2-F4 and F7-F9). It has previously been suggested that prematurely capacitated sperm undergo a spontaneous acrosome reaction possibly due to uncontrolled influx of calcium ion, and consequently they possess relatively lower in vitro fertilizing ability. It is therefore possible that premature capacitation of sperm used for AI is a causal factor of subfertility of male Japanese Black cattle and a potentially good marker for identification of subfertile bulls for removal from AI programs.

Trypanosomes and mammalian sperm: one of a kind?

Flagellar-mediated motility is an indispensable function for cell types as evolutionarily distant as mammalian sperm and kinetoplastid parasites, a large group of flagellated protozoa that includes several important human pathogens. Despite the obvious importance of flagellar motility, little is known about the signalling processes that direct the frequency and wave shape of the flagellar beat, or those that provide the motile cell with the necessary environmental cues that enable it to aim its movement. Similarly, the energetics of the flagellar beat and the problem of a sufficient ATP supply along the entire length of the beating flagellum remain to be explored. Recent proteome projects studying the flagella of mammalian sperm and kinetoplastid parasites have provided important information and have indicated a surprising degree of similarities between the flagella of these two cell types.

CO2 sensing at ocean surface mediated by cAMP in a marine diatom

Marine diatoms are known to be responsible for about a quarter of global primary production and their photosynthesis is sustained by inorganic carbon-concentrating mechanisms and/or C(4) metabolism. Activities of the inorganic carbon-concentrating mechanism are attenuated under enriched [CO(2)]; however, impacts of this factor on primary productivity and the molecular mechanisms of CO(2) responses in marine diatoms are unknown. In this study, transgenic cells were generated of the marine diatom Phaeodactylum tricornutum by the introduction of a beta-glucuronidase reporter gene under the control of an intrinsic CO(2)-responsive promoter, which is the sequence between -80 to +61 relative to the transcription start site of a chloroplastic-carbonic anhydrase gene, ptca1, obtained from P. tricornutum. The activity of the ptca1 promoter was effectively repressed in air-level CO(2) by treating cells with a 1.0 mm cAMP analog, dibutyryl cAMP, or a cAMP phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine. Deletion of the intrinsic cAMP-response element from the ptca1 promoter caused a lack of repression of the reporter gene uidA, even under elevated [CO(2)] and a null phenotype to the strong repressive effects of dibutyryl cAMP and 3-isobutyl-1-methylxanthine on the ptca1 promoter. Deletion of the cAMP-response element was also shown to cause derepression of the uidA reporter gene in the dark. These results indicate that the cytosolic cAMP level increases under elevated [CO(2)] and represses the ptca1 promoter. This strongly suggests the participation of cAMP metabolism, presumably at the cytosolic level, in controlling CO(2)-acquisition systems under elevated [CO(2)] at the ocean surface in a marine diatom.

Sperm chemotaxis in marine invertebrates--molecules and mechanisms

Sperm are attracted by chemical substances which are released by the egg. This process is called chemotaxis. Several molecules that are involved in chemotactic signaling of sperm from marine invertebrates are described and a model of the signaling pathway is presented. We discuss the motor response during chemotaxis and propose a model of the navigation strategy of sperm.

Soluble adenylyl cyclase is required for netrin-1 signaling in nerve growth cones

Growth cones at the tips of nascent and regenerating axons direct axon elongation. Netrin-1, a secreted molecule that promotes axon outgrowth and regulates axon pathfinding, elevates cyclic AMP (cAMP) levels in growth cones and regulates growth cone morphology and axonal outgrowth. These morphological effects depend on the intracellular levels of cAMP. However, the specific pathways that regulate cAMP levels in response to netrin-1 signaling are unclear. Here we show that 'soluble' adenylyl cyclase (sAC), an atypical calcium-regulated cAMP-generating enzyme previously implicated in sperm maturation, is expressed in developing rat axons and generates cAMP in response to netrin-1. Overexpression of sAC results in axonal outgrowth and growth cone elaboration, whereas inhibition of sAC blocks netrin-1-induced axon outgrowth and growth cone elaboration. Taken together, these results indicate that netrin-1 signals through sAC-generated cAMP, and identify a fundamental role for sAC in axonal development.

Proteins associated with soluble adenylyl cyclase in sea urchin sperm flagella

Adenylyl cyclases (ACs) synthesize cAMP and are present in cells as transmembrane AC and soluble AC (sAC). In sperm, the cAMP produced regulates ion channels and it also activates protein kinase-A that in turn phosphorylates specific axonemal proteins to activate flagellar motility. In mammalian sperm, sAC localizes to the midpiece of flagella, whereas in sea urchin sperm sAC is along the entire flagellum. Here we show that in sea urchin sperm, sAC is complexed with proteins of the plasma membrane and axoneme. Immunoprecipitation shows that a minimum of 10 proteins is tightly associated with sAC. Mass spectrometry of peptides derived from these proteins shows them to be: axonemal dynein heavy chains 7 and 9, sperm specific Na+/H+ exchanger, cyclic nucleotide-gated ion channel, sperm specific creatine kinase, membrane bound guanylyl cyclase, cyclic GMP specific phosphodiesterase 5A, the receptor for the egg peptide speract, and alpha- and beta-tubulins. The sAC-associated proteins could be important in linking membrane signal transduction to energy utilisation in the regulation of flagellar motility.

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

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

Autoinhibitory regulation of soluble adenylyl cyclase

Soluble adenylyl cyclase is an evolutionarily conserved bicarbonate sensor that plays a crucial role in cAMP dependent processes that occur during mammalian fertilization. sAC protein is expressed at the highest levels in male germ cells, and is found to occur as one of two known isoforms: a truncated protein (sAC(t)) that consists almost exclusively of the two conserved catalytic domains (C1 and C2), and a full-length form (sAC(fl)) that contains an additional noncatalytic C-terminal region. Several studies suggested sAC(t) was more active than sAC(fl). We now demonstrate that the specific activity of sAC(t) is at least 10-fold higher than the specific activity of sAC(fl). Using deletion analysis and a novel genetic screen to identify activating mutations, we uncovered an autoinhibitory region just C-terminal to the C2 domain. Kinetic analysis of purified recombinant sAC revealed this autoinhibitory domain functions to lower the enzyme's V(max) without altering its affinity for substrate or regulation by any of the known modulators of sAC activity. Our results identify an additional regulatory mechanism specific to the sAC(fl) isoform.

Soluble adenylyl cyclase (sAC) is indispensable for sperm function and fertilization

We previously demonstrated that male mice deficient in the soluble adenylyl cyclase (sAC) are sterile and produce spermatozoa with deficits in progressive motility and are unable to fertilize zona-intact eggs. Here, analyses of sAC(-/-) spermatozoa provide additional insights into the functions linked to cAMP signaling. Adenylyl cyclase activity and cAMP content are greatly diminished in crude preparations of sAC(-/-) spermatozoa and are undetectable after sperm purification. HCO(3)(-) is unable to rapidly accelerate the flagellar beat or facilitate evoked Ca(2+) entry into sAC(-/-) spermatozoa. Moreover, the delayed HCO(3)(-)-dependent increases in protein tyrosine phosphorylation and hyperactivated motility, which occur late in capacitation of wild-type spermatozoa, do not develop in sAC(-/-) spermatozoa. However, sAC(-/-) sperm fertilize zona-free oocytes, indicating that gamete fusion does not require sAC. Although ATP levels are significantly reduced in sAC(-/-) sperm, cAMP-AM ester increases flagellar beat frequency, progressive motility, and alters the pattern of tyrosine phosphorylated proteins. These results indicate that sAC and cAMP coordinate cellular energy balance in wild-type sperm and that the ATP generating machinery is not operating normally in sAC(-/-) spermatozoa. These findings demonstrate that sAC plays a critical role in cAMP signaling in spermatozoa and that defective cAMP production prevents engagement of multiple components of capacitation resulting in male infertility.

Soluble adenylyl cyclase mediates nerve growth factor-induced activation of Rap1

Nerve growth factor (NGF) and the ubiquitous second messenger cyclic AMP (cAMP) are both implicated in neuronal differentiation. Multiple studies indicate that NGF signals to at least a subset of its targets via cAMP, but the link between NGF and cAMP has remained elusive. Here, we have described the use of small molecule inhibitors to differentiate between the two known sources of cAMP in mammalian cells, bicarbonate- and calcium-responsive soluble adenylyl cyclase (sAC) and G protein-regulated transmembrane adenylyl cyclases. These inhibitors, along with sAC-specific small interfering RNA, reveal that sAC is uniquely responsible for the NGF-elicited rise in cAMP and is essential for the NGF-induced activation of the small G protein Rap1 in PC12 cells. In contrast and as expected, transmembrane adenylyl cyclase-generated cAMP is responsible for Rap1 activation by the G protein-coupled receptor ligand PACAP (pituitary adenylyl cyclase-activating peptide). These results identify sAC as a mediator of NGF signaling and reveal the existence of distinct pathways leading to cAMP-dependent signal transduction.

Effects of estrogenic xenobiotics on human and mouse spermatozoa

OBJECTIVE

To investigate human sperm responsiveness to the estrogenic xenobiotic genistein and seek further information regarding the mechanism of action of estrogenic xenobiotics using mouse spermatozoa.

METHODS

Uncapacitated human spermatozoa were incubated with genistein and assessed using chlortetracycline (CTC) fluorescence. CTC was also used to evaluate mouse sperm responses to daidzein and combinations of genistein, 8-prenylnaringenin and nonylphenol. Several steroids were tested to determine structure-function relationships, and possible involvement of cAMP and G proteins in responses was also investigated.

RESULTS

Genistein significantly accelerated capacitation and acrosome loss in human spermatozoa, with 1, 10 and 100 nmol/l being equally effective. In mouse spermatozoa, daidzein produced significant responses, and combinations of xenobiotics at low concentrations were more effective than used singly. The compounds appear to act at the cell surface, and responses to three different steroids were nonidentical. A protein kinase-A inhibitor blocked responses to xenobiotics, while genistein and nonylphenol significantly stimulated cAMP production. Pertussis toxin and dideoxyadenosine blocked responses, suggesting involvement of inhibitory G proteins and membrane-associated adenylyl cyclases.

CONCLUSION

Human and mouse sperm responses to genistein are very similar, but human gametes appear to be even more sensitive. The mechanism of action may involve unregulated stimulation of cAMP production, leading to significant acrosome loss, undesirable because already acrosome-reacted cells are nonfertilizing. Xenobiotics were even more effective in combination. Since simultaneous exposure to low concentrations of multiple xenobiotics is likely to occur in animals and humans, further investigation is needed to determine whether this could impair fertility.

Calcium-induced acrosomal exocytosis requires cAMP acting through a protein kinase A-independent, Epac-mediated pathway

Epac, a guanine nucleotide exchange factor for the small GTPase Rap, binds to and is activated by the second messenger cAMP. In sperm, there are a number of signaling pathways required to achieve egg-fertilizing ability that depend upon an intracellular rise of cAMP. Most of these processes were thought to be mediated by cAMP-dependent protein kinases. Here we report a new dependence for the cAMP-induced acrosome reaction involving Epac. The acrosome reaction is a specialized type of regulated exocytosis leading to a massive fusion between the outer acrosomal and the plasma membranes of sperm cells. Ca2+ is the archetypical trigger of regulated exocytosis, and we show here that its effects on acrosomal release are fully mediated by cAMP. Ca2+ failed to trigger acrosomal exocytosis when intracellular cAMP was depleted by an exogenously added phosphodiesterase or when Epac was sequestered by specific blocking antibodies. The nondiscriminating dibutyryl-cAMP and the Epac-selective 8-(p-chlorophenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate analogues triggered the acrosome reaction in the effective absence of extracellular Ca2+. This indicates that cAMP, via Epac activation, has the ability to drive the whole cascade of events necessary to bring exocytosis to completion, including tethering and docking of the acrosome to the plasma membrane, priming of the fusion machinery, mobilization of intravesicular Ca2+, and ultimately, bilayer mixing and fusion. cAMP-elicited exocytosis was sensitive to anti-alpha-SNAP, anti-NSF, and anti-Rab3A antibodies, to intra-acrosomal Ca2+ chelators, and to botulinum toxins but was resistant to cAMP-dependent protein kinase blockers. These experiments thus identify Epac in human sperm and evince its indispensable role downstream of Ca2+ in exocytosis.

Structures, mechanism, regulation and evolution of class III nucleotidyl cyclases

Cyclic 3',5'-guanylyl and adenylyl nucleotides function as second messengers in eukaryotic signal transduction pathways and as sensory transducers in prokaryotes. The nucleotidyl cyclases (NCs) that catalyze the synthesis of these molecules comprise several evolutionarily distinct groups, of which class III is the largest. The domain structures of prokaryotic and eukaryotic class III NCs are diverse, including a variety of regulatory and transmembrane modules. Yet all members of this family contain one or two catalytic domains, characterized by an evolutionarily ancient topological motif (betaalphaalphabetabetaalphabeta) that is preserved in several other enzymes that catalyze the nucleophilic attack of a 3'-hydroxyl upon a 5' nucleotide phosphate. Two dyad-related catalytic domains compose one catalytic unit, with the catalytic sites formed at the domain interface. The catalytic domains of mononucleotidyl cyclases (MNCs) and diguanylate cyclases (DGCs) are called cyclase homology domains (CHDs) and GGDEF domains, respectively. Prokaryotic NCs usually contain only one catalytic domain and are catalytically active as intermolecular homodimers. The different modes of dimerization in class III NCs probably evolved concurrently with their mode of binding substrate. The catalytic mechanism of GGDEF domain homodimers is not completely understood, but they are expected to have a single active site with each subunit contributing equivalent determinants to bind one GTP molecule or half a c-diGMP molecule. CHD dimers have two potential dyad-related active sites, with both CHDs contributing determinants to each site. Homodimeric class III MNCs have two equivalent catalytic sites, although such enzymes may show half-of-sites reactivity. Eukaryotic class III MNCs often contain two divergent CHDs, with only one catalytically competent site. All CHDs appear to use a common catalytic mechanism, which requires the participation of two magnesium or manganese ions for binding polyphosphate groups and nucleophile activation. In contrast, mechanisms for purine recognition and specificity are more diverse. Class III NCs are subject to regulation by small molecule effectors, endogenous domains, or exogenous protein partners. Many of these regulators act by altering the interface of the catalytic domains and therefore the integrity of the catalytic site(s). This review focuses on both conserved and divergent mechanisms of class III NC function and regulation.

Moving to the beat: a review of mammalian sperm motility regulation

Because it is generally accepted that a high percentage of poorly motile or immotile sperm will adversely affect male fertility, analysis of sperm motility is a central part of the evaluation of male fertility. In spite of its importance to fertility, poor sperm motility remains only a description of a pathology whose underlying cause is typically poorly understood. The present review is designed to bring the clinician up to date with the most current understanding of the mechanisms that regulate sperm motility and to raise questions about how aberrations in these mechanisms could be the underlying causes of this pathology.

Signaling pathways for modulation of mouse sperm motility by adenosine and catecholamine agonists

Capacitation of mammalian sperm, including alterations in flagellar motility, is presumably modulated by chemical signals encountered in the female reproductive tract. This work investigates signaling pathways for adenosine and catecholamine agonists that stimulate sperm kinetic activity. We show that 2-chloro-2'-deoxyadenosine and isoproterenol robustly accelerate flagellar beat frequency with EC50s near 10 and 0.05 microM, respectively. The several-fold acceleration is maximal by 60 sec. Although extracellular Ca2+ is required for agonist action on the flagellar beat, agonist treatment does not elevate sperm cytosolic [Ca2+] but does increase cAMP content. Acceleration does not require the conventional transmembrane adenylyl cyclase ADCY3, since it persists in sperm of ADCY3 knockout mice and in wild-type sperm in the presence of the inhibitors of conventional adenylyl cyclases SQ-22536, MDL-12330A, or 2', 5'-dideoxyadenosine. In contrast, the acceleration by these agents is absent in sperm that lack the predominant atypical adenylyl cyclase, SACY. Responses to these agonists are also absent in sperm from mice lacking the sperm-specific Calpha2 catalytic subunit of protein kinase A (PRKACA). Agonist responses also are strongly suppressed in wild-type sperm by the protein kinase inhibitor H-89. These results show that adenosine and catecholamine analogs activate sperm motility by mechanisms that require extracellular Ca2+, the atypical sperm adenylyl cyclase, cAMP, and protein kinase A.

The "soluble" adenylyl cyclase in sperm mediates multiple signaling events required for fertilization

Mammalian fertilization is dependent upon a series of bicarbonate-induced, cAMP-dependent processes sperm undergo as they "capacitate," i.e., acquire the ability to fertilize eggs. Male mice lacking the bicarbonate- and calcium-responsive soluble adenylyl cyclase (sAC), the predominant source of cAMP in male germ cells, are infertile, as the sperm are immotile. Membrane-permeable cAMP analogs are reported to rescue the motility defect, but we now show that these "rescued" null sperm were not hyperactive, displayed flagellar angulation, and remained unable to fertilize eggs in vitro. These deficits uncover a requirement for sAC during spermatogenesis and/or epididymal maturation and reveal limitations inherent in studying sAC function using knockout mice. To circumvent this restriction, we identified a specific sAC inhibitor that allowed temporal control over sAC activity. This inhibitor revealed that capacitation is defined by separable events: induction of protein tyrosine phosphorylation and motility are sAC dependent while acrosomal exocytosis is not dependent on sAC.

Mice Deficient in CHRNA7, a Subunit of the Nicotinic Acetylcholine Receptor, Produce Sperm with Impaired Motility1

In this study we investigate the role of the CHRNA7 subunit (also known as the alpha7 subunit) of the nicotinic acetylcholine receptor in mouse sperm function. We confirm by reverse-transcription-polymerase chain reaction the expression in adult mouse testis of Chrna7 mRNA and demonstrate the subunit's presence in mouse sperm by immunoblot. Alpha-bungarotoxin binds a range of nicotinic acetylcholine receptor subunits, including the CHRNA7 subunit. Localization studies using a fluorescent alpha-bungarotoxin-tetramethyl-rhodamine conjugate revealed specific binding sites on the midpiece of mouse sperm with fainter alpha-bungarotoxin binding on the remainder of the flagellum. Mice engineered with a double-null disruption of the Chrna7 gene displayed only faint fluorescence on the midpiece, suggesting that the CHRNA7 contributed the majority of the observed alpha-bungarotoxin binding sites. The location of alpha-bungarotoxin binding suggested that nicotinic acetylcholine receptors may play an ionotropic role in sperm motility. Sperm from Chrna7(-/-) mice display no difference in number, morphology, viability or spontaneous acrosome reaction rate compared with Chrna7(+/+) sperm. Studies using computer-assisted sperm analysis indicate the motility of Chrna7(-/-) sperm is significantly impaired. This impairment is characterized by significantly reduced swimming velocities, failure to maintain vigorous swimming, and lower levels of hyperactivated swimming patterns in Chrna7(-/-) sperm compared with Chrna7(+/+) sperm. This is the first genetic evidence that sperm nicotinic acetylcholine receptors are important for maintenance of normal sperm motility.

A soluble adenylyl cyclase from sea urchin spermatozoa

A previously identified, calmodulin-binding, sea urchin sperm flagellar adenylyl cyclase (AC) was cloned and sequenced and found to be a homologue of mammalian sperm soluble adenylyl cyclase (sAC). Compared to the mammalian sAC, the sea urchin sAC (susAC) has several long amino acid insertions, some of which contain protein kinase A phosphorylation sites. The enzymatic activity of susAC shows a steep pH dependency curve, the specific activity doubling when the pH is increased from 7.0 to 7.5. This suggests that like sperm dynein ATPase, the susAC is probably activated by increases in intracellular pH occurring upon spawning into seawater and also when sperm respond to contact with the egg jelly layer. The susAC is strongly activated by manganese, but has low activity in magnesium. Gene database searches identified sAC homologues in species known to have cyclic AMP-dependent sperm motility. This implies (as shown in mouse) that susAC has a role in sperm motility, most probably through axonemal protein phosphorylation or ion channel regulation.

Calcium-sensing soluble adenylyl cyclase mediates TNF signal transduction in human neutrophils

Through chemical screening, we identified a pyrazolone that reversibly blocked the activation of phagocyte oxidase (phox) in human neutrophils in response to tumor necrosis factor (TNF) or formylated peptide. The pyrazolone spared activation of phox by phorbol ester or bacteria, bacterial killing, TNF-induced granule exocytosis and phox assembly, and endothelial transmigration. We traced the pyrazolone's mechanism of action to inhibition of TNF-induced intracellular Ca2+ elevations, and identified a nontransmembrane ("soluble") adenylyl cyclase (sAC) in neutrophils as a Ca2+-sensing source of cAMP. A sAC inhibitor mimicked the pyrazolone's effect on phox. Both compounds blocked TNF-induced activation of Rap1A, a phox-associated guanosine triphosphatase that is regulated by cAMP. Thus, TNF turns on phox through a Ca2+-triggered, sAC-dependent process that may involve activation of Rap1A. This pathway may offer opportunities to suppress oxidative damage during inflammation without blocking antimicrobial function.

Regulation of CFTR channels by HCO(3)--sensitive soluble adenylyl cyclase in human airway epithelial cells

CFTR channels conduct HCO(3)(-) in addition to Cl(-) in airway epithelial cells. A defective HCO(3)(-)-transporting function of CFTR may underlie the pathogenesis of cystic fibrosis. In the present study, we have investigated whether a HCO(3)(-)-sensitive soluble adenylyl cyclase (sAC) is functionally coupled with CFTR and thus forms an autoregulatory mechanism for HCO(3)(-) transport in human airway epithelial Calu-3 cells. A reverse transcriptase-polymerase chain reaction showed that transcripts of both full-length and truncated sACs are present in Calu-3 cells. Truncated sAC protein is the predominant, if not the only, isoform expressed in Calu-3 cells. HCO(3)(-) stimulated a modest increase in cAMP production, and the increase was sensitive to 2-hydroxyestradiol (2-HE), a sAC inhibitor, but not to SQ22,536, a blocker of conventional transmembrane adenylyl cyclases. These results suggest that sAC is functional in Calu-3 cells. Adding 25 mM HCO(3)(-) to the bath stimulated CFTR-mediated whole cell currents in the absence, but not in the presence, of 2-HE. In cell-attached membrane patches, 25 or 50 mM HCO(3)(-) in the bath markedly increased the product of channel number and open probability of CFTR, and this activation was attenuated by 2-HE. These findings demonstrate that sAC signaling pathway is involved in the regulation of CFTR function in human airway epithelium and thereby provides a link between the level of intracellular HCO(3)(-)/CO(2) and the modulation of HCO(3)(-)-conductive CFTR function by cAMP/PKA.

A cyclic adenosine 3',5'-monophosphate-induced tyrosine phosphorylation of Syk protein tyrosine kinase in the flagella of boar spermatozoa

A cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein tyrosine phosphorylation is involved in the expression of fertilizing ability in mammalian spermatozoa. However, there are only limited data concerning the identification of protein tyrosine kinase (PTK) that is activated by the cAMP signaling. In this study, we have shown data supporting that boar sperm flagellum possesses a unique cAMP-protein kinase A (PKA) signaling cascade leading to phosphorylation of Syk PTK at the tyrosine residues of the activation loop. Ejaculated spermatozoa were washed and then incubated in a modified Krebs-Ringer HEPES medium (mKRH) containing polyvinyl alcohol (PVA) plus 0.1 mM cBiMPS (a cell-permeable cAMP analog), 0.25 mM sodium orthovanadate (Na3VO4) (a protein tyrosine phosphatase (PTP) inhibitor) or both at 38.5 degrees C for 180 min. Aliquots of the sperm suspensions were recovered before and after incubation and then used to detect sperm tyrosine-phosphorylated proteins by Western blotting and indirect immunofluorescence. In the Western blotting, the anti-phosphotyrosine monoclonal antibody (4G10) recognized several bands including 72-kDa protein in the protein extracts from spermatozoa that were incubated solely with cBiMPS. The tyrosine phosphorylation in these sperm proteins was dependent on cBiMPS and enhanced by the addition of Na3VO4. The 72-kDa tyrosine-phosphorylated protein was apparently reacted with the anti-phospho-Syk antibody (Tyr525/526). Indirect immunofluorescence revealed that the connecting and principal pieces of spermatozoa incubated with cBiMPS and Na3VO4 were stained with the anti-phospho-Syk antibody. However, the reactivity of the 72-kDa protein with the anti-phospho-Syk antibody was reduced by the addition of H-89 (a PKA inhibitor, 0.01-0.1 mM) to the sperm suspensions but not affected by the pretreatment of spermatozoa with BAPTA-AM (an intracellular Ca2+ chelator, 0.1 mM). Fractionation of phosphorylated proteins from the spermatozoa with a detergent Nonidet P-40 suggested that the 72-kDa tyrosine-phosphorylated protein might be a cytoskeletal component. Based on these findings, we have concluded that the cAMP-PKA signaling is linked to the Ca2+-independent tyrosine phosphorylation of Syk in the connecting and principal pieces of boar spermatozoa.

Cloning and characterization of the human soluble adenylyl cyclase

We identified the human ortholog of soluble adenylyl cyclase (hsAC) in a locus linked to familial absorptive hypercalciuria and cloned it from a human cDNA library. hsAC transcripts were expressed in multiple tissues using RT-PCR and RNA blotting. RNA blot analysis revealed a predominant 5.1-kb band in a multiple human tissue blot, but three splice transcript variants were detected using RT-PCR and confirmed by performing sequence analysis. Immunoblot analysis showed 190- and 80-kDa bands in multiple human cell lines from gut, renal, and bone origins in both cytosol and membrane fractions, including Caco-2 colorectal adenocarcinomas, HEK-293 cells, HOS cells, and primary human osteoblasts, as well as in vitro induced osteoclast-like cells. The specificity of the antiserum was verified by peptide blocking and reduction using sequence-specific small interfering RNA. Confocal immunofluorescence cytochemistry localized hsAC primarily in cytoplasm, but some labeling was observed in the nucleus and the plasma membrane. Cytoplasmic hsAC colocalized with microtubules but not with microfilaments. To test the function of hsAC, four constructs containing catalytic domains I and II (aa 1-802), catalytic domain II (aa 231-802), noncatalytic domain (aa 648-1,610), and full-length protein (aa 1-1,610) were expressed in Sf9 insect cells. Only catalytic domains I and II or full-length proteins showed adenylyl cyclase activity. Mg(2+), Mn(2+), and Ca(2+) all increased adenylyl cyclase activity in a dose-dependent manner. While hsAC had a minimal response to HCO(3)(-) in the absence of divalent cations, HCO(3)(-) robustly stimulated Mg(2+)-bound hsAC but inhibited Mn(2+)-bound hsAC in a dose-dependent manner. In summary, hsAC is a divalent cation and HCO(3)(-) sensor, and its HCO(3)(-) sensitivity is modulated by divalent cations.

Calcium Channels and Ca2+ Fluctuations in Sperm Physiology

Generating new life in animals by sexual reproduction depends on adequate communication between mature and competent male and female gametes. Ion channels are instrumental in the dialogue between sperm, its environment, and the egg. The ability of sperm to swim to the egg and fertilize it is modulated by ion permeability changes induced by environmental cues and components of the egg outer layer. Ca(2+) is probably the key messenger in this information exchange. It is therefore not surprising that different Ca(2+)-permeable channels are distinctly localized in these tiny specialized cells. New approaches to measure sperm currents, intracellular Ca(2+), membrane potential, and intracellular pH with fluorescent probes, patch-clamp recordings, sequence information, and heterologous expression are revealing how sperm channels participate in fertilization. Certain sperm ion channels are turning out to be unique, making them attractive targets for contraception and for the discovery of novel signaling complexes.

Bicarbonate activation of adenylyl cyclase via promotion of catalytic active site closure and metal recruitment

In an evolutionarily conserved signaling pathway, 'soluble' adenylyl cyclases (sACs) synthesize the ubiquitous second messenger cyclic adenosine 3',5'-monophosphate (cAMP) in response to bicarbonate and calcium signals. Here, we present crystal structures of a cyanobacterial sAC enzyme in complex with ATP analogs, calcium and bicarbonate, which represent distinct catalytic states of the enzyme. The structures reveal that calcium occupies the first ion-binding site and directly mediates nucleotide binding. The single ion-occupied, nucleotide-bound state defines a novel, open adenylyl cyclase state. In contrast, bicarbonate increases the catalytic rate by inducing marked active site closure and recruiting a second, catalytic ion. The phosphates of the bound substrate analogs are rearranged, which would facilitate product formation and release. The mechanisms of calcium and bicarbonate sensing define a reaction pathway involving active site closure and metal recruitment that may be universal for class III cyclases.

Effect of intracellular pH on depolarization-evoked calcium influx in human sperm

Human sperm are endowed with putative voltage-dependent calcium channels (VDCC) that produce measurable increases in intracellular calcium concentration ([Ca2+]i) in response to membrane depolarization with potassium. These channels are blocked by nickel, inactivate in 1–2 min in calcium-deprived medium, and are remarkably stimulated by NH4Cl, suggesting a role for intracellular pH (pHi). In a previous work, we showed that calcium permeability through these channels increases approximately onefold during in vitro “capacitation,” a calcium-dependent process that sperm require to fertilize eggs. In this work, we have determined the pHidependence of sperm VDCC. Simultaneous depolarization and pHialkalinization with NH4Cl induced an [Ca2+]iincrease that depended on the amount of NH4Cl added. VDCC stimulation as a function of pHishowed a sigmoid curve in the 6.6–7.2 pHirange, with a half-maximum stimulation at pH ∼7.00. At higher pHi(≥7.3), a further stimulation occurred. Calcium release from internal stores did not contribute to the stimulating effect of pHibecause the [Ca2+]iincrease induced by progesterone, which opens a calcium permeability pathway that does not involve gating of VDCC, was unaffected by ammonium. The ratio of pHi-stimulated-to-nonstimulated calcium influx was nearly constant at different test depolarization values. Likewise, depolarization-induced calcium influx in pHi-stimulated and nonstimulated cells was equally blocked by nickel. In our capacitating conditions pHiincreased 0.11 pH units, suggesting that the calcium influx stimulation observed during sperm capacitation might be partially caused by pHialkalinization. Additionally, a calcium permeability pathway triggered exclusively by pHialkalinization was detected.

STAND, a Class of P-Loop NTPases Including Animal and Plant Regulators of Programmed Cell Death: Multiple, Complex Domain Architectures, Unusual Phyletic Patterns, and Evolution by Horizontal Gene Transfer

Using sequence profile analysis and sequence-based structure predictions, we define a previously unrecognized, widespread class of P-loop NTPases. The signal transduction ATPases with numerous domains (STAND) class includes the AP-ATPases (animal apoptosis regulators CED4/Apaf-1, plant disease resistance proteins, and bacterial AfsR-like transcription regulators) and NACHT NTPases (e.g. NAIP, TLP1, Het-E-1) that have been studied extensively in the context of apoptosis, pathogen response in animals and plants, and transcriptional regulation in bacteria. We show that, in addition to these well-characterized protein families, the STAND class includes several other groups of (predicted) NTPase domains from diverse signaling and transcription regulatory proteins from bacteria and eukaryotes, and three Archaea-specific families. We identified the STAND domain in several biologically well-characterized proteins that have not been suspected to have NTPase activity, including soluble adenylyl cyclases, nephrocystin 3 (implicated in polycystic kidney disease), and Rolling pebble (a regulator of muscle development); these findings are expected to facilitate elucidation of the functions of these proteins. The STAND class belongs to the additional strand, catalytic E division of P-loop NTPases together with the AAA+ ATPases, RecA/helicase-related ATPases, ABC-ATPases, and VirD4/PilT-like ATPases. The STAND proteins are distinguished from other P-loop NTPases by the presence of unique sequence motifs associated with the N-terminal helix and the core strand-4, as well as a C-terminal helical bundle that is fused to the NTPase domain. This helical module contains a signature GxP motif in the loop between the two distal helices. With the exception of the archaeal families, almost all STAND NTPases are multidomain proteins containing three or more domains. In addition to the NTPase domain, these proteins typically contain DNA-binding or protein-binding domains, superstructure-forming repeats, such as WD40 and TPR, and enzymatic domains involved in signal transduction, including adenylate cyclases and kinases. By analogy to the AAA+ ATPases, it can be predicted that STAND NTPases use the C-terminal helical bundle as a "lever" to transmit the conformational changes brought about by NTP hydrolysis to effector domains. STAND NTPases represent a novel paradigm in signal transduction, whereby adaptor, regulatory switch, scaffolding, and, in some cases, signal-generating moieties are combined into a single polypeptide. The STAND class consists of 14 distinct families, and the evolutionary history of most of these families is riddled with dramatic instances of lineage-specific expansion and apparent horizontal gene transfer. The STAND NTPases are most abundant in developmentally and organizationally complex prokaryotes and eukaryotes. Transfer of genes for STAND NTPases from bacteria to eukaryotes on several occasions might have played a significant role in the evolution of eukaryotic signaling systems.

Particulate Adenylate Cyclase Plays a Key Role in Human Sperm Olfactory Receptor-mediated Chemotaxis

Human sperm chemotaxis is a critical component of the fertilization process, but the molecular basis for this behavior remains unclear. Recent evidence shows that chemotactic responses depend on activation of the sperm olfactory receptor, hOR17-4. Certain floral scents, including bourgeonal, activate hOR17-4, trigger pronounced Ca(2+) fluxes, and evoke chemotaxis. Here, we provide evidence that hOR17-4 activation is coupled to a cAMP-mediated signaling cascade. Multidimensional protein identification technology was used to identify potential components of a G-protein-coupled cAMP transduction pathway in human sperm. These products included various membrane-associated adenylate cyclase (mAC) isoforms and the G(olf)-subunit. Using immunocytochemistry, specific mAC isoforms were localized to particular cell regions. Whereas mAC III occurred in the sperm head and midpiece, mAC VIII was distributed predominantly in the flagellum. In contrast, G(olf) was found mostly in the flagellum and midpiece. The observed spatial distribution patterns largely correspond to the spatiotemporal character of hOR17-4-induced Ca(2+) changes. Behavioral and Ca(2+) signaling responses of human sperm to bourgeonal were bioassayed in the presence, or absence, of the adenylate cyclase antagonist SQ22536. This specific agent inhibits particulate AC, but not soluble AC, activation. Upon incubation with SQ22536, cells ceased to exhibit Ca(2+) signaling, chemotaxis, and hyperactivation (faster swim speed and flagellar beat rate) in response to bourgeonal. Particulate AC is therefore required for induction of hOR17-4-mediated human sperm behavior and represents a promising target for future design of contraceptive drugs.

Tyrosine phosphorylation of the a kinase anchoring protein 3 (AKAP3) and soluble adenylate cyclase are involved in the increase of human sperm motility by bicarbonate

Mammalian testicular spermatozoa are immotile, thus, to reach the oocyte, they need to acquire swimming ability under the control of different factors acting during the sperm transit through the epididymis and the female genital tract. Although bicarbonate is known to physiologically increase motility by stimulating soluble adenylate cyclase (sAC) activity of mammalian spermatozoa, no extensive studies in human sperm have been performed yet to elucidate the additional molecular mechanisms involved. In this light, we investigated the effect of in vitro addition of bicarbonate to human spermatozoa on the main intracellular signaling pathways involved in regulation of motility, namely, intracellular cAMP production and protein tyrosine phosphorylation. Bicarbonate effects were compared with those of the phosphatidyl-inositol-3 kinase inhibitor, LY294002, previously demonstrated to be a pharmacological stimulus for sperm motility. Bicarbonate addition to spermatozoa results in a significant increase in sperm motility as well as in several hyperactivation parameters. This stimulatory effect of bicarbonate and LY294002 is mediated by an increase in cAMP production and tyrosine phosphorylation of the A kinase anchoring protein, AKAP3. The specificity of bicarbonate effects was confirmed by inhibition with 4,4'-di-isothiocyanostilbene-2,2'-disulfonic acid. We remark that, in human spermatozoa, bicarbonate acts primarily through activation of sAC to stimulate tyrosine phosphorylation of AKAP3 and sperm motility because both effects are blunted by the sAC inhibitor 2OH-estradiol. In conclusion, our data provide the first evidence that bicarbonate stimulates human sperm motility and hyperactivation through activation of sAC and tyrosine phosphorylation of AKAP3, finally leading to an increased recruitment of PKA to AKAP3.

Sperm-specific protein kinase A catalytic subunit Calpha2 orchestrates cAMP signaling for male fertility

An unusual cAMP signaling system mediates many of the events that prepare spermatozoa to meet the egg. Its components include the atypical, bicarbonate-stimulated, sperm adenylyl cyclase and a cAMP-dependent protein kinase (PKA) with the unique catalytic subunit termed Calpha(2) or C(s). We generated mice that lack Calpha(2) to determine its importance in the events downstream of cAMP production. Male Calpha(2) null mice produce normal numbers of sperm that swim spontaneously in vitro. Thus, Calpha(2) has no required role in formation of a functional flagellum or the initiation of motility. In contrast, we find that Calpha(2) is required for bicarbonate to speed the flagellar beat and facilitate Ca(2+) entry channels. In addition, Calpha(2) is needed for the protein tyrosine phosphorylation that occurs late in the sequence of sperm maturation and for a negative feedback control of cAMP production, revealed here. Consistent with these specific defects in several important sperm functions, Calpha(2) null males are infertile despite normal mating behavior. These results define several crucial roles of PKA in sperm cell biology, bringing together both known and unique PKA-mediated events that are necessary for male fertility.

Angiotensin II stimulates cAMP production and protein tyrosine phosphorylation in mouse spermatozoa

Angiotensin II (AII), found in seminal plasma, has been shown to stimulate capacitation in uncapacitated mammalian spermatozoa. The present study investigated the location of AII receptors on spermatozoa and AII's mechanism of action. AT1 type receptors for AII are present on the acrosomal cap region and along the whole of the flagellum of both mouse and human spermatozoa. Because combinations of low concentrations of AII and either calcitonin or fertilization-promoting peptide (FPP), both known to regulate the adenylyl cyclase (AC)/cAMP signal transduction pathway, elicited a significant response, this study investigated the hypothesis that these peptides act on the same pathway. AII was shown to significantly stimulate cAMP production in both uncapacitated and capacitated mouse spermatozoa and this was associated with increases in protein tyrosine phosphorylation. Using an anti-phosphotyrosine antibody to visualize the location of tyrosine phosphoproteins within individual cells, AII significantly stimulated phosphorylation within 20 min in both the head, especially in the acrosomal cap region, and the flagellum, especially in the principal piece, of uncapacitated mouse spermatozoa; combined AII + FPP was stimulatory within 5 min. In addition, Western blotting revealed that AII stimulation increased phosphorylation in a number of tyrosine phosphoproteins in both uncapacitated and capacitated mouse spermatozoa, with some being altered only in the latter category of cells. These results support the hypothesis that AII stimulates AC/cAMP in mammalian spermatozoa.

Conservation of functional domain structure in bicarbonate-regulated "soluble" adenylyl cyclases in bacteria and eukaryotes

Soluble adenylyl cyclase (sAC) is an evolutionarily conserved bicarbonate sensor. In mammals, it is responsible for bicarbonate-induced, cAMP-dependent processes in sperm required for fertilization and postulated to be involved in other bicarbonate- and carbon dioxide-dependent functions throughout the body. Among eukaryotes, sAC-like cyclases have been detected in mammals and in the fungi Dictyostelium; these enzymes display extensive similarity extending through two cyclase catalytic domains and a long carboxy terminal extension. sAC-like cyclases are also found in a number of bacterial phyla (Cyanobacteria, Actinobacteria, and Proteobacteria), but these enzymes generally possess only a single catalytic domain and little, if any, homology with the remainder of the mammalian protein. Database mining through a number of recently sequenced genomes identified sAC orthologues in additional metazoan phyla (Arthropoda and Chordata) and additional bacterial phyla (Chloroflexi). Interestingly, the Chloroflexi sAC-like cyclases, a family of three enzymes from the thermophilic eubacterium, Chloroflexus aurantiacus, are more similar to eukaryotic sAC-like cyclases (i.e., mammalian sAC and Dictyostelium SgcA) than they are to other bacterial adenylyl cyclases (ACs) (i.e., from Cyanobacteria). The Chloroflexus sAC-like cyclases each possess two cyclase catalytic domains and extensive similarity with mammalian enzymes through their carboxy termini. We cloned one of the Chloroflexus sAC-like cyclases and confirmed it to be stimulated by bicarbonate. These data extend the family of organisms possessing bicarbonate-responsive ACs to numerous phyla within the bacterial and eukaryotic kingdoms.

[HCO3-]-regulated expression and activity of soluble adenylyl cyclase in corneal endothelial and Calu-3 cells

BACKGROUND

Bicarbonate activated Soluble Adenylyl Cyclase (sAC) is a unique cytoplasmic and nuclear signaling mechanism for the generation of cAMP. HCO3- activates sAC in bovine corneal endothelial cells (BCECs), increasing [cAMP] and stimulating PKA, leading to phosphorylation of the cystic fibrosis transmembrane-conductance regulator (CFTR) and increased apical Cl- permeability. Here, we examined whether HCO3- may also regulate the expression of sAC and thereby affect the production of cAMP upon activation by HCO3- and the stimulation of CFTR in BCECs.

RESULTS

RT-competitive PCR indicated that sAC mRNA expression in BCECs is dependent on [HCO3-] and incubation time in HCO3-. Immunoblots showed that 10 and 40 mM HCO3- increased sAC protein expression by 45% and 87%, respectively, relative to cells cultured in the absence of HCO3-. Furthermore, 40 mM HCO3- up-regulated sAC protein expression in Calu-3 cells by 93%. On the other hand, sAC expression in BCECs and Calu-3 cells was unaffected by changes in bath pH or osmolarity. Interestingly, BCECs pre-treated with10 microM adenosine or 10 microM forskolin, which increase cAMP levels, showed decreased sAC mRNA expression by 20% and 30%, respectively. Intracellular cAMP production by sAC paralleled the time and [HCO3-]-dependent expression of sAC. Bicarbonate-induced apical Cl- permeability increased by 78% (P < 0.01) in BCECs cultured in HCO3-. However for cells cultured in the absence of HCO3-, apical Cl- permeability increased by only 10.3% (P > 0.05).

CONCLUSION

HCO3- not only directly activates sAC, but also up-regulates the expression of sAC. These results suggest that active cellular uptake of HCO3- can contribute to the basal level of cellular cAMP in tissues that express sAC.

Discrete intracellular signaling domains of soluble adenylyl cyclase: camps of cAMP?

Soluble adenylyl cyclase can function in the nucleus, defining a nuclear microdomain of adenosine 3',5'-monophosphate (cAMP) signaling. Bundey and Insel discuss the evidence for discrete signaling microdomains of cAMP, including the nucleus and caveolae, and conclude that such microdomains may be defined by the localized, subcellular expression of adenylyl cyclase isoforms.

Bicarbonate-responsive "soluble" adenylyl cyclase defines a nuclear cAMP microdomain

Bicarbonate-responsive “soluble” adenylyl cyclase resides, in part, inside the mammalian cell nucleus where it stimulates the activity of nuclear protein kinase A to phosphorylate the cAMP response element binding protein (CREB). The existence of this complete and functional, nuclear-localized cAMP pathway establishes that cAMP signals in intracellular microdomains and identifies an alternate pathway leading to CREB activation.