Promiscuous coupling and involvement of protein kinase C and extracellular signal-regulated kinase 1/2 in the adenosine A1 receptor signalling in mammalian spermatozoa

Function and therapeutic potential of G protein-coupled receptors in epididymis

Infertility rates for both females and males have increased continuously in recent years. Currently, effective treatments for male infertility with defined mechanisms or targets are still lacking. G protein-coupled receptors (GPCRs) are the largest class of drug targets, but their functions and the implications for the therapeutic development for male infertility largely remain elusive. Nevertheless, recent studies have shown that several members of the GPCR superfamily play crucial roles in the maintenance of ion-water homeostasis of the epididymis, development of the efferent ductules, formation of the blood-epididymal barrier and maturation of sperm. Knowledge of the functions, genetic variations and working mechanisms of such GPCRs, along with the drugs and ligands relevant to their specific functions, provide future directions and a great arsenal for new developments in the treatment of male infertility.

Agonists activate different A2B adenosine receptor signaling pathways in MBA-MD-231 breast cancer cells with distinct potencies

It is well established that some receptor types including G protein-coupled receptors may transduce effects through more than one signaling pathway. This holds also true for A_2B adenosine receptors which were shown to trigger an increase in intracellular Ca²⁺ levels in addition to the canonical stimulation of adenylyl cyclase. We have recently shown that activation of A_2B receptors in the breast cancer cell line MBA-MD-231 elicits a reduction in ERK1/2 phosphorylation, an effect that might be exploited in treatment of cancer cell growth and proliferation. In this study, we investigate whether structurally divers agonists show functional selectivity for any of the signaling pathways leading to an increase of intracellular cAMP or Ca²⁺, or the reduction of ERK1/2 phosphorylation. As agonists, adenosine derivatives were used bearing different substitutions in 2- and 6-position and, in addition, a ligand with a non-nucleoside structure was tested. It was found that all the tested ligands showed similar pharmacological profiles for the three responses investigated in MBA-MD-231 cells. However, the reduction of ERK1/2 phosphorylation occurred with 40-500-fold higher potency compared to stimulation of adenylyl cyclase or increasing intracellular Ca²⁺ levels. Based on these observations, it seems possible to utilize activation of A_2B adenosine receptors expressed in certain cancers to limit cell growth and proliferation due to reduction of MAPK activity without activation of other signaling pathways potentially responsible for side effects.

Extracellular cAMP activates molecular signalling pathways associated with sperm capacitation in bovines

STUDY QUESTION

Is extracellular cAMP involved in the regulation of signalling pathways in bovine sperm capacitation?

SUMMARY ANSWER

Extracellular cAMP induces sperm capacitation through the activation of different signalling pathways that involve phospholipase C (PLC), PKC/ERK1-2 signalling and an increase in sperm Ca2+ levels, as well as soluble AC and cAMP/protein kinase A (PKA) signalling.

WHAT IS KNOWN ALREADY

In order to fertilize the oocyte, ejaculated spermatozoa must undergo a series of changes in the female reproductive tract, known as capacitation. This correlates with a number of membrane and metabolic modifications that include an increased influx of bicarbonate and Ca2+, activation of a soluble adenylyl cyclase (sAC) to produce cAMP, PKA activation, protein tyrosine phosphorylation and the development of hyperactivated motility. We previously reported that cAMP efflux by Multidrug Resistance Protein 4 (MRP4) occurs during sperm capacitation and the pharmacological blockade of this inhibits the process. Moreover, the supplementation of incubation media with cAMP abolishes the inhibition and leads to sperm capacitation, suggesting that extracellular cAMP regulates crucial signalling cascades involved in this process.

STUDY DESIGN, SIZE, DURATION

Bovine sperm were selected by the wool glass column method, and washed by centrifugation in BSA-Free Tyrode's Albumin Lactate Pyruvate (sp-TALP). Pellets were resuspended then diluted for each treatment. For in vitro capacitation, 10 to 15 × 106 SPZ/ml were incubated in 0.3% BSA sp-TALP at 38.5°C for 45 min under different experimental conditions. To evaluate the role of extracellular cAMP on different events associated with sperm capacitation, 10 nM cAMP was added to the incubation medium as well as different inhibitors of enzymes associated with signalling transduction pathways: U73122 (PLC inhibitor, 10 μM), Gö6983 (PKC inhibitor, 10 μM), PD98059 (ERK-1/2 inhibitor, 30 μM), H89 and KT (PKA inhibitors, 50 μM and 100 nM, respectively), KH7 (sAC inhibitor, 10 μM), BAPTA-AM (intracellular Ca2+ chelator, 50 μM), EGTA (10 μM) and Probenecid (MRPs general inhibitor, 500 μM). In addition, assays for binding to oviductal epithelial cells and IVF were carried out to test the effect of cAMP compared with other known capacitant agents such as heparin (60 μg/ml) and bicarbonate (40 mM).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Straws of frozen bovine semen (20-25 × 106 spermatozoa/ml) were kindly provided by Las Lilas, CIALE and CIAVT Artificial Insemination Centers. The methods used in this work include western blot, immunohistochemistry, flow cytometry, computer-assisted semen analysis, live imaging of Ca2+ and fluorescence scanning. At least three independent assays with bull samples of proven fertility were carried.

MAIN RESULTS AND THE ROLE OF CHANCE

In the present study, we elucidate the molecular events induced by extracellular cAMP. Our results showed that external cAMP induces sperm capacitation, depending upon the action of PLC. Downstream, this enzyme increased ERK1-2 activation through PKC and elicited a rise in sperm Ca2+ levels (P < 0.01). Moreover, extracellular cAMP-induced capacitation also depended on the activity of sAC and PKA, and increased tyrosine phosphorylation, indicating that the nucleotide exerts a broad range of responses. In addition, extracellular cAMP-induced sperm hyperactivation and concomitantly increased the proportion of spermatozoa with high mitochondrial activity (P < 0.01). Finally, cAMP increased the in vitro fertilization rate compared to control conditions (P < 0.001).

LARGE SCALE DATA

None.

LIMITATIONS, REASONS FOR CAUTION

This is an in vitro study performed with bovine cryopreserved spermatozoa. Studies in other species and with fresh samples are needed to extrapolate these data.

WIDER IMPLICATIONS OF THE FINDINGS

These findings strongly suggest an important role of extracellular cAMP in the regulation of the signalling pathways involved in the acquisition of bull sperm fertilizing capability. The data presented here indicate that not only a rise, but also a regulation of cAMP levels is necessary to ensure sperm fertilizing ability. Thus, exclusion of the nucleotide to the extracellular space might be essential to guarantee the achievement of a cAMP tone, needed for all capacitation-associated events to take place. Moreover, the ability of cAMP to trigger such broad and complex signalling events allows us to hypothesize that cAMP is a self-produced autocrine/paracrine factor, and supports the emerging paradigm that spermatozoa do not compete but, in fact, communicate with each other. A precise understanding of the functional competence of mammalian spermatozoa is essential to generate clinical advances in the treatment of infertility and the development of novel contraceptive strategies.

STUDY FUNDING AND COMPETING INTEREST(S)

This work was supported by Consejo Nacional de Investigaciones Científicas y Técnicas [PIP0 496 to S.P.-M.], Agencia Nacional de Promoción Científica y Tecológica [PICT 2012-1195 and PICT2014-2325 to S.P.-M., and PICT 2013-2050 to C.D.], Boehringer Ingelheim Funds, and the Swedish Farmers Foundation [SLF-H13300339 to J.M.]. The authors declare there are no conflicts of interests.

Natriuretic Peptides: The Case of Prostate Cancer

Cardiac natriuretic peptides have long been known to act as main players in the homeostatic control of blood pressure, salt and water balance. However, in the last few decades, new properties have been ascribed to these hormones. A systematic review of English articles using MEDLINE Search terms included prostate cancer, inflammation, cardiac hormones, atrial natriuretic peptide, and brain natriuretic peptide. Most recent publications were selected. Natriuretic peptides are strongly connected to the immune system, whose two branches, innate and adaptive, are finely tuned and organized to kill invaders and repair injured tissues. These peptides control the immune response and act as anti-inflammatory and immune-modulatory agents. In addition, in cancers, natriuretic peptides have anti-proliferative effects by molecular mechanisms based on the inhibition/regulation of several pathways promoting cell proliferation and survival. Nowadays, it is accepted that chronic inflammation is a crucial player in prostate cancer development and progression. In this review, we summarize the current knowledge on the link between prostate cancer and inflammation and the potential use of natriuretic peptides as anti-inflammatory and anticancer agents.

Caffeine exposure alters adenosine system and neurochemical markers during retinal development

Evidence points to beneficial properties of caffeine in the adult central nervous system, but teratogenic effects have also been reported. Caffeine exerts most of its effects by antagonizing adenosine receptors, especially A1 and A2A subtypes. In this study, we evaluated the role of caffeine on the expression of components of the adenosinergic system in the developing avian retina and the impact of caffeine exposure upon specific markers for classical neurotransmitter systems. Caffeine exposure (5-30 mg/kg by in ovo injection) to 14-day-old chick embryos increased the expression of A1 receptors and concomitantly decreased A2A adenosine receptors expression after 48 h. Accordingly, caffeine (30 mg/kg) increased [(3) H]-8-cyclopentyl-1,3-dipropylxanthine (A1 antagonist) binding and reduced [(3) H]-ZM241385 (A2A antagonist) binding. The caffeine time-response curve demonstrated a reduction in A1 receptors 6 h after injection, but an increase after 18 and 24 h. In contrast, caffeine exposure increased the expression of A2A receptors from 18 and 24 h. Kinetic assays of [(3) H]-S-(4-nitrobenzyl)-6-thioinosine binding to the equilibrative adenosine transporter ENT1 revealed an increase in Bmax with no changes in Kd , an effect accompanied by an increase in adenosine uptake. Immunohistochemical analysis showed a decrease in retinal content of tyrosine hydroxylase, calbindin and choline acetyltransferase, but not Brn3a, after 48 h of caffeine injection. Furthermore, retinas exposed to caffeine had increased levels of phosphorylated extracellular signal-regulated kinase and cAMP-response element binding protein. Overall, we show an in vivo regulation of the adenosine system, extracellular signal-regulated kinase and cAMP-response element binding protein function and protein expression of specific neurotransmitter systems by caffeine in the developing retina. The beneficial or maleficent effects of caffeine have been demonstrated by the work of different studies. It is known that during animal development, caffeine can exert harmful effects, impairing the correct formation of CNS structures. In this study, we demonstrated cellular and tissue effects of caffeine's administration on developing chick embryo retinas. Those effects include modulation of adenosine receptors (A1 , A2 ) content, increasing in cAMP response element-binding protein (pCREB) and extracellular signal-regulated kinase phosphorylation (pERK), augment of adenosine equilibrative transporter content/activity, and a reduction of some specific cell subpopulations. ENT1, Equilibrative nucleoside transporter 1.

Purinergic signalling in the reproductive system in health and disease

There are multiple roles for purinergic signalling in both male and female reproductive organs. ATP, released as a cotransmitter with noradrenaline from sympathetic nerves, contracts smooth muscle via P2X1 receptors in vas deferens, seminal vesicles, prostate and uterus, as well as in blood vessels. Male infertility occurs in P2X1 receptor knockout mice. Both short- and long-term trophic purinergic signalling occurs in reproductive organs. Purinergic signalling is involved in hormone secretion, penile erection, sperm motility and capacitation, and mucous production. Changes in purinoceptor expression occur in pathophysiological conditions, including pre-eclampsia, cancer and pain.

Cyclic AMP efflux, via MRPs and A1 adenosine receptors, is critical for bovine sperm capacitation

Sperm capacitation has been largely associated with an increase in cAMP, although its relevance in the underlying mechanisms of this maturation process remains elusive. Increasing evidence shows that the extrusion of cAMP through multidrug resistance associated protein 4 (MRP4) regulates cell homeostasis not only in physiological but also in pathophysiological situations and studies from our laboratory strongly support this assumption. In the present work we sought to establish the role of cAMP efflux in the regulation of sperm capacitation. Sperm capacitation was performed in vitro by exposing bovine spermatozoa to bicarbonate 40 and 70 mM; cAMP; probenecid (a MRPs general inhibitor) and an adenosine type 1 receptor (A1 adenosine receptor) selective antagonist (DPCPX). Capacitation was assessed by chlortetracycline assay and lysophosphatidylcholine-induced acrosome reaction assessed by PSA-FITC staining. Intracellular and extracellular cAMP was measured by radiobinding the regulatory subunit of PKA under the same experimental conditions. MRP4 was detected by western blot and immunohistochemistry assays. Results showed that the inhibition of soluble adenylyl cyclase significantly inhibited bicarbonate-induced sperm capacitation. Furthermore, in the presence of 40 and 70 mM bicarbonate bovine spermatozoa synthesized and extruded cAMP. Interestingly, in the absence of IBMX (a PDEs inhibitor) cAMP efflux still operated in sperm cells, suggesting that cAMP extrusion would be a physiological process in the spermatozoa complementary to the action of PDE. Blockade of MRPs by probenecid abolished the efflux of the cyclic nucleotide resulting not only in the accumulation of intracellular cAMP but also in the inhibition of bicarbonate-induced sperm capacitation. The effect of probenecid was abolished by exposing sperm cells to cAMP. The high-affinity efflux pump for cAMP, MRP4 was expressed in bovine spermatozoa and localized to the midpiece of the tail as previously reported for soluble adenylyl cyclase and A1 adenosine receptor. Additionally, blockade of A1 adenosine receptor abolished not only bicarbonate-induced sperm capacitation but also that stimulated by cAMP. Present findings strongly support that cAMP efflux, presumably through MRP4, and the activation of A1 adenosine receptor regulate some events associated with bicarbonate-induced sperm capacitation, and further suggest a paracrine and/or autocrine role for cAMP.

Adenosine A1 receptor activation is arrhythmogenic in the developing heart through NADPH oxidase/ERK- and PLC/PKC-dependent mechanisms

Whether adenosine, a crucial regulator of the developing cardiovascular system, can provoke arrhythmias in the embryonic/fetal heart remains controversial. Here, we aimed to establish a mechanistic basis of how an adenosinergic stimulation alters function of the developing heart. Spontaneously beating hearts or dissected atria and ventricle obtained from 4-day-old chick embryos were exposed to adenosine or specific agonists of the receptors A(1)AR (CCPA), A(2A)AR (CGS-21680) and A(3)AR (IB-MECA). Expression of the receptors was determined by quantitative PCR. The functional consequences of blockade of NADPH oxidase, extracellular signal-regulated kinase (ERK), phospholipase C (PLC), protein kinase C (PKC) and L-type calcium channel (LCC) in combination with adenosine or CCPA, were investigated in vitro by electrocardiography. Furthermore, the time-course of ERK phosphorylation was determined by western blotting. Expression of A(1)AR, A(2A)AR and A(2B)AR was higher in atria than in ventricle while A(3)AR was equally expressed. Adenosine (100μM) triggered transient atrial ectopy and second degree atrio-ventricular blocks (AVB) whereas CCPA induced mainly Mobitz type I AVB. Atrial rhythm and atrio-ventricular propagation fully recovered after 60min. These arrhythmias were prevented by the specific A(1)AR antagonist DPCPX. Adenosine and CCPA transiently increased ERK phosphorylation and induced arrhythmias in isolated atria but not in ventricle. By contrast, A(2A)AR and A(3)AR agonists had no effect. Interestingly, the proarrhythmic effect of A(1)AR stimulation was markedly reduced by inhibition of NADPH oxidase, ERK, PLC, PKC or LCC. Moreover, NADPH oxidase inhibition or antioxidant MPG prevented both A(1)AR-mediated arrhythmias and ERK phosphorylation. These results suggest that pacemaking and conduction disturbances are induced via A(1)AR through concomitant stimulation of NADPH oxidase and PLC, followed by downstream activation of ERK and PKC with LCC as possible target.

Testicular expression of Adora3i2 in Adora3 knockout mice reveals a role of mouse A3Ri2 and human A3Ri3 adenosine receptors in sperm

Adenosine is a candidate modulator of sperm motility in the female reproductive tract that increases sperm flagellar beat frequency in vitro. Past work suggested that this acceleration may involve equilibrative (ENT) and concentrative (CNT) nucleoside transporters. Here we show that Slc29a1 (ENT-1) is the predominant nucleoside transporter expressed in the mouse testis. Unexpectedly, the beat of Slc29a1-null sperm still accelerates in response to 2-chloro-2'-deoxyadenosine (Cl-dAdo). Moreover, in wild-type sperm neither blockade of CNTs by removal of external Na(+), nor inhibition of ENTs with nitrobenzylthioionosine, prevents acceleration of the sperm beat by Cl-dAdo. In contrast, pertussis toxin produces strong blockade, indicating involvement of a Gα(i/o)-coupled adenosine receptor. Although agonists selective for adenosine receptors A1R, A2aR, and A2bR are ineffective, A3R-selective agonists Cl-IB-MECA and IB-MECA do accelerate the beat. Consistent with this pharmacological profile, the predominant Adora transcripts in the testis are products of the nested Adora3i1 and Adora3i2 genes. Surprisingly, Cl-IB-MECA and Cl-dAdo still accelerate the beat of Adora3i1-null sperm indicating that the remaining Adora3i2 transcript produces an A3R that functions in sperm. When cloned Adora3i2 is heterologously expressed in tsA-201 cells, Cl-dAdo decreases forskolin-evoked accumulation of cAMP, indicating that Adora3i2 specifies a functional A3Ri2 adenosine receptor that couples through Gα(i). Database mining reveals that mouse Adora3i2 is expressed primarily in testis, almost exclusively in spermatids. Expression of the orthologous ADORA3i3 transcript also is most prominent in human testis; presumably producing an A3Ri3 receptor that is functional in sperm and that may be a target for development of male-directed contraceptives.

Changes in expression and activity levels of ecto-5'-nucleotidase/CD73 along the mouse female estrous cycle

AIM

Extracellular ATP and its hydrolysis product adenosine modulate various reproductive functions such as those requiring contraction, hormone synthesis and maintenance of fluid composition. Moreover, adenosine is a key molecule for sperm capacitation. Extracellular nucleotide and nucleoside levels are affected by cell surface ectonucleotidases, amongst which the ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) family is the most abundant and effective to hydrolyse ATP and ADP to AMP. In the female reproductive tract three members of this family have been recently identified: NTPDase1, NTPDase2 and NTPDase3 (Histochem. Cell Biol.131, 2009, 615). The purpose of the present study was to characterize in this system the expression profile of ecto-5'-nucleotidase (CD73), the enzyme generating adenosine from AMP.

METHODS

Immunological techniques and in situ enzymatic assays were used to characterize the ecto-5'-nucleotidase expression in the mouse female reproductive tract along the four stages of the estrous cycle, that were determined by vaginal smear examination.

RESULTS

Ecto-5'-nucleotidase was abundantly detected in the corpora lutea of the ovaries, as well as in several epithelia, such as that of oviducts, uterus and endometrial glands. Marked changes in endometrial ecto-5'-nucleotidase expression and activity along the estrous cycle are described, these being maximum at estrus phase, coinciding with optimal female sexual receptivity.

CONCLUSION

The adenosine generated thereby, besides other functions, might contribute to sperm capacitation, thus significantly influencing fertility.

Elucidation of the signaling pathways that underpin capacitation-associated surface phosphotyrosine expression in mouse spermatozoa

Recent studies from within our laboratory have demonstrated a causal relationship between capacitation-associated surface phosphotyrosine expression and the ability of mouse spermatozoa to recognize the oocyte and engage in sperm-zona pellucida interaction. In the studies described herein we have sought to investigate the signaling pathways that underpin the tyrosine phosphorylation of sperm surface protein targets and validate the physiological significance of these pathways in relation to sperm-zona pellucida adhesion. Through selective pharmacological inhibition we have demonstrated that surface phosphotyrosine expression is unlikely to be mediated by the canonical cAMP-dependent protein kinase A (PKA) signaling cascade that has been most widely studied in relation to sperm capacitation. Rather, it appears to be primarily driven by the extracellular signal-regulated kinase (ERK) module of the mitogen-activated protein kinase (MAPK) pathway. Consistent with this notion, the main components of the ERK module (RAS, RAF1, MEK, and ERK1/2) were localized to the periacrosomal region of the head of mature mouse spermatozoa and their phosphorylation status within this region of the cell was positively modulated by capacitation. Furthermore, inhibition of several elements of this pathway suppressed sperm surface phosphotyrosine expression and induced a concomitant reduction sperm-zona pellucida interaction. Collectively, these data highlight a previously unappreciated role of the ERK module in the modification of the sperm surface during capacitation to render these cells functionally competent to engage in the process of fertilization.

The role of Mitogen activated protein kinase (MAPK) in sperm functions

The generation of mature spermatozoa in the epididymis includes the activation of the MAPK cascade in a complex manner. MAPKs are thought to be involved in the regulation of transcription and ectoplasmic specialization (ES) in the testis. MAPKs also regulate mature spermatozoa flagellar motility, hyperactivation and the acrosome reaction. Here we review the current data regarding the functions of MAPKs in spermatogenesis and in mature spermatozoa.

Determination of adenosine A1 receptor agonist and antagonist pharmacology using Saccharomyces cerevisiae: implications for ligand screening and functional selectivity

The budding yeast, Saccharomyces cerevisiae, is a convenient system for coupling heterologous G protein-coupled receptors (GPCRs) to the pheromone response pathway to facilitate empirical ligand screening and/or GPCR mutagenesis studies. However, few studies have applied this system to define GPCR-G protein-coupling preferences and furnish information on ligand affinities, efficacies, and functional selectivity. We thus used different S. cerevisiae strains, each expressing a specific human Galpha/yeast Gpa1 protein chimera, and determined the pharmacology of various ligands of the coexpressed human adenosine A(1) receptor. These assays, in conjunction with the application of quantitative models of agonism and antagonism, revealed that (-)-N(6)-(2-phenylisopropyl)adenosine was a high-efficacy agonist that selectively coupled to Gpa/1Galpha(o), Gpa1/Galpha(i1/2), and Gpa1/Galpha(i3), whereas the novel compound, 5'-deoxy-N(6)-(endo-norborn-2-yl)-5'-(2-fluorophenylthio)adenosine (VCP-189), was a lower-efficacy agonist that selectively coupled to Gpa1/Galpha(i) proteins; the latter finding suggested that VCP-189 might be functionally selective. The affinity of the antagonist, 8-cyclopentyl-1,3-dipropylxanthine, was also determined at the various strains. Subsequent experiments performed in mammalian Chinese hamster ovary cells monitoring cAMP formation/inhibition, intracellular calcium mobilization, phosphorylation of extracellular signal-regulated kinase 1 and 2 or (35)S-labeled guanosine 5'-(gamma-thio)triphosphate binding, were in general agreement with the yeast data regarding agonist efficacy estimation and antagonist affinity estimation, but revealed that the apparent functional selectivity of VCP-189 could be explained by differences in stimulus-response coupling between yeast and mammalian cells. Our results suggest that this yeast system is a useful tool for quantifying ligand affinity and relative efficacy, but it may lack the sensitivity required to detect functional selectivity of low-efficacy agonists.