Functional Significance of the Adcy10-Dependent Intracellular cAMP Compartments

Administration of Bicarbonate Protects Mitochondria, Rescues Retinal Ganglion Cells, and Ameliorates Visual Dysfunction Caused by Oxidative Stress

Oxidative stress is a key factor causing mitochondrial dysfunction and retinal ganglion cell (RGC) death in glaucomatous neurodegeneration. The cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling pathway is involved in mitochondrial protection, promoting RGC survival. Soluble adenylyl cyclase (sAC) is a key regulator of the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling pathway, which is known to protect mitochondria and promote RGC survival. However, the precise molecular mechanisms connecting the sAC-mediated signaling pathway with mitochondrial protection in RGCs against oxidative stress are not well characterized. Here, we demonstrate that sAC plays a critical role in protecting RGC mitochondria from oxidative stress. Using mouse models of oxidative stress induced by ischemic injury and paraquat administration, we found that administration of bicarbonate, as an activator of sAC, protected RGCs, blocked AMP-activated protein kinase activation, inhibited glial activation, and improved visual function. Moreover, we found that this is the result of preserving mitochondrial dynamics (fusion and fission), promoting mitochondrial bioenergetics and biogenesis, and preventing metabolic stress and apoptotic cell death. Notably, the administration of bicarbonate ameliorated mitochondrial dysfunction in RGCs by enhancing mitochondrial biogenesis, preserving mitochondrial structure, and increasing ATP production in oxidatively stressed RGCs. These findings suggest that activating sAC enhances the mitochondrial structure and function in RGCs to counter oxidative stress, consequently promoting RGC protection. We propose that modulation of the sAC-mediated signaling pathway has therapeutic potential acting on RGC mitochondria for treating glaucoma and other retinal diseases.

The role of soluble adenylyl cyclase in sensing and regulating intracellular pH

Soluble adenylyl cyclase (sAC) differs from transmembrane adenylyl cyclases (tmAC) in many aspects. In particular, the activity of sAC is not regulated by G-proteins but by the prevailing bicarbonate concentrations inside cells. Therefore, sAC serves as an exquisite intracellular pH sensor, with the capacity to translate pH changes into the regulation of localization and/or activity of cellular proteins involved in pH homeostasis. In this review, we provide an overview of literature describing the regulation of sAC activity by bicarbonate, pinpointing the importance of compartmentalization of intracellular cAMP signaling cascades. In addition, examples of processes involving proton and bicarbonate transport in different cell types, in which sAC plays an important regulatory role, were described in detail.

Activating soluble adenylyl cyclase protects mitochondria, rescues retinal ganglion cells, and ameliorates visual dysfunction caused by oxidative stress

Oxidative stress is a key factor causing mitochondrial dysfunction and retinal ganglion cell (RGC) death in glaucomatous neurodegeneration. The cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling pathway is involved in mitochondrial protection, promoting RGC survival. Soluble adenylyl cyclase (sAC) is one of the key regulators of the cAMP/PKA signaling pathway. However, the precise molecular mechanisms underlying the sAC-mediated signaling pathway and mitochondrial protection in RGCs that counter oxidative stress are not well characterized. Here, we demonstrate that sAC plays a critical role in protecting RGC mitochondria from oxidative stress. Using mouse models of oxidative stress, we found that activating sAC protected RGCs, blocked AMP-activated protein kinase activation, inhibited glial activation, and improved visual function. Moreover, we found that this is the result of preserving mitochondrial dynamics (fusion and fission), promoting mitochondrial bioenergetics and biogenesis, and preventing metabolic stress and apoptotic cell death in a paraquat oxidative stress model. Notably, sAC activation ameliorated mitochondrial dysfunction in RGCs by enhancing mitochondrial biogenesis, preserving mitochondrial structure, and increasing ATP production in oxidatively stressed RGCs. These findings suggest that activating sAC enhances the mitochondrial structure and function in RGCs to counter oxidative stress, consequently promoting RGC protection. We propose that modulation of the sAC-mediated signaling pathway has therapeutic potential acting on RGC mitochondria for treating glaucoma and other retinal diseases.

Proper phosphorylation of septin 12 regulates septin 4 and soluble adenylyl cyclase expression to induce sperm capacitation

Septin-based ring complexes maintain the sperm annulus. Defective annular structures are observed in the sperm of Sept12- and Sept4-null mice. In addition, sperm capacitation, a process required for proper fertilization, is inhibited in Sept4-null mice, implying that the sperm annulus might play a role in controlling sperm capacitation. Hence, we analyzed sperm capacitation of sperm obtained from SEPT12 Ser196 phosphomimetic (S196E), phosphorylation-deficient (S196A), and SEPT4-depleted mutant mice. Capacitation was reduced in the sperm of both the Sept12 S196E- and Sept12 S196A-knock-in mice. The protein levels of septins, namely, SEPT4 and SEPT12, were upregulated, and these proteins were concentrated in the sperm annulus during capacitation. Importantly, the expression of soluble adenylyl cyclase (sAC), a key enzyme that initiates capacitation, was upregulated, and sAC was recruited to the sperm annulus following capacitation stimulation. We further found that SEPT12, SEPT4, and sAC formed a complex and colocalized to the sperm annulus. Additionally, sAC expression was reduced and disappeared in the annulus of the SEPT12 S196E- and S196A-mutant mouse sperm. In the sperm of the SEPT4-knockout mice, sAC did not localize to the annulus. Thus, our data demonstrate that SEPT12 phosphorylation status and SEPT4 activity jointly regulate sAC protein levels and annular localization to induce sperm capacitation.

Transcriptome analysis revealed potential genes involved in thermogenesis in muscle tissue in cold-exposed lambs

Cold tolerance is an important trait for sheep raised at high altitudes. Muscle tissue, comprising 30–40% of the total body mass, produces heat during cold exposure. However, little is known about the genetic mechanisms of this tissue and its role in thermogenesis in lambs. We examined genes in skeletal muscle tissue in a cold-adapted sheep breed, Altay, and a cold-intolerant sheep breed, Hu, when exposed to low air temperature. Three ewe-lambs of each breed were maintained at −5°C and three ewe-lambs of each breed were maintained at 20°C. After cold exposure for 25 days, the longissimus dorsi of each lamb was collected, and transcriptome profiles were sequenced and analyzed. The results of RNA-seq showed that the average reads among the four groups were 11.0 Gbase. The genome mapping rate averaged 88.1% and the gene mapping rate averaged 82.5%. The analysis of differentially expressed genes (DEGs) indicated that the peroxisome proliferator-activated receptors (PPAR), cAMP, and calcium signaling pathways and muscle contraction in muscle tissue were linked to thermogenesis in cold-exposed lambs. Furthermore, PCK1 (phosphoenolpyruvate carboxykinase1) increased glyceroneogenesis in cold-exposed Altay lambs, and APOC3 (apolipoprotein C3), LPL (lipoprotein lipase), and FABP4 (fatty acid binding protein 4, adipocyte) were involved in the intake and transport of free fatty acids. In Hu sheep, cAMP biosynthesis from ATP hydrolysis was regulated by ADCY10 (adenylate cyclase) and ADORA2a (adenosine A2a receptor). Skeletal muscle contraction was regulated by MYL2 (myosin light chain 2). In conclusion, cold exposure altered the expression level of genes involved in heat production in muscle tissue. Some potential mechanisms were revealed, including calcium ion transport in the calcium signaling pathway, fatty acid metabolism in the PPAR signaling pathway, and cAMP biosynthesis in the cAMP signaling pathway. This study implied that skeletal muscle plays an important role in thermoregulation in lambs.

The Complexity and Multiplicity of the Specific cAMP Phosphodiesterase Family: PDE4, Open New Adapted Therapeutic Approaches

Cyclic nucleotides (cAMP, cGMP) play a major role in normal and pathologic signaling. Beyond receptors, cyclic nucleotide phosphodiesterases; (PDEs) rapidly convert the cyclic nucleotide in its respective 5′-nucleotide to control intracellular cAMP and/or cGMP levels to maintain a normal physiological state. However, in many pathologies, dysregulations of various PDEs (PDE1-PDE11) contribute mainly to organs and tissue failures related to uncontrolled phosphorylation cascade. Among these, PDE4 represents the greatest family, since it is constituted by 4 genes with multiple variants differently distributed at tissue, cellular and subcellular levels, allowing different fine-tuned regulations. Since the 1980s, pharmaceutical companies have developed PDE4 inhibitors (PDE4-I) to overcome cardiovascular diseases. Since, they have encountered many undesired problems, (emesis), they focused their research on other PDEs. Today, increases in the knowledge of complex PDE4 regulations in various tissues and pathologies, and the evolution in drug design, resulted in a renewal of PDE4-I development. The present review describes the recent PDE4-I development targeting cardiovascular diseases, obesity, diabetes, ulcerative colitis, and Crohn’s disease, malignancies, fatty liver disease, osteoporosis, depression, as well as COVID-19. Today, the direct therapeutic approach of PDE4 is extended by developing allosteric inhibitors and protein/protein interactions allowing to act on the PDE interactome.

PPAR-γ activation enhances myelination and neurological recovery in premature rabbits with intraventricular hemorrhage

Intraventricular hemorrhage (IVH) results in periventricular inflammation, hypomyelination of the white matter, and hydrocephalus in premature infants. No effective therapy exists to prevent these disorders. Peroxisome proliferator activated receptor-γ (PPAR-γ) agonists reduce inflammation, alleviate free radical generation, and enhance microglial phagocytosis, promoting clearance of debris and red blood cells. We hypothesized that activation of PPAR-γ would enhance myelination, reduce hydrocephalus, and promote neurological recovery in newborns with IVH. These hypotheses were tested in a preterm rabbit model of IVH; autopsy brain samples from premature infants with and without IVH were analyzed. We found that IVH augmented PPAR-γ expression in microglia of both preterm human infants and rabbit kits. The treatment with PPAR-γ agonist or PPAR-γ overexpression by adenovirus delivery further elevated PPAR-γ levels in microglia, reduced proinflammatory cytokines, increased microglial phagocytosis, and improved oligodendrocyte progenitor cell (OPC) maturation in kits with IVH. Transcriptomic analyses of OPCs identified previously unrecognized PPAR-γ-induced genes for purinergic signaling, cyclic adenosine monophosphate generation, and antioxidant production, which would reprogram these progenitors toward promoting myelination. RNA-sequencing analyses of microglia revealed PPAR-γ-triggered down-regulation of several proinflammatory genes and transcripts having roles in Parkinson's disease and amyotrophic lateral sclerosis, contributing to neurological recovery in kits with IVH. Accordingly, PPAR-γ activation enhanced myelination and neurological function in kits with IVH. This also enhanced microglial phagocytosis of red blood cells but did not reduce hydrocephalus. Treatment with PPAR-γ agonist might enhance myelination and neurological recovery in premature infants with IVH.

Cyclic AMP-binding protein Epac1 acts as a metabolic sensor to promote cardiomyocyte lipotoxicity

Cyclic adenosine monophosphate (cAMP) is a master regulator of mitochondrial metabolism but its precise mechanism of action yet remains unclear. Here, we found that a dietary saturated fatty acid (FA), palmitate increased intracellular cAMP synthesis through the palmitoylation of soluble adenylyl cyclase in cardiomyocytes. cAMP further induced exchange protein directly activated by cyclic AMP 1 (Epac1) activation, which was upregulated in the myocardium of obese patients. Epac1 enhanced the activity of a key enzyme regulating mitochondrial FA uptake, carnitine palmitoyltransferase 1. Consistently, pharmacological or genetic Epac1 inhibition prevented lipid overload, increased FA oxidation (FAO), and protected against mitochondrial dysfunction in cardiomyocytes. In addition, analysis of Epac1 phosphoproteome led us to identify two key mitochondrial enzymes of the the β-oxidation cycle as targets of Epac1, the long-chain FA acyl-CoA dehydrogenase (ACADL) and the 3-ketoacyl-CoA thiolase (3-KAT). Epac1 formed molecular complexes with the Ca2+/calmodulin-dependent protein kinase II (CaMKII), which phosphorylated ACADL and 3-KAT at specific amino acid residues to decrease lipid oxidation. The Epac1-CaMKII axis also interacted with the α subunit of ATP synthase, thereby further impairing mitochondrial energetics. Altogether, these findings indicate that Epac1 disrupts the balance between mitochondrial FA uptake and oxidation leading to lipid accumulation and mitochondrial dysfunction, and ultimately cardiomyocyte death.

Regulation of Mitochondrial Homeostasis by sAC-Derived cAMP Pool: Basic and Translational Aspects

In contrast to the traditional view of mitochondria being solely a source of cellular energy, e.g., the "powerhouse" of the cell, mitochondria are now known to be key regulators of numerous cellular processes. Accordingly, disturbance of mitochondrial homeostasis is a basic mechanism in several pathologies. Emerging data demonstrate that 3'-5'-cyclic adenosine monophosphate (cAMP) signalling plays a key role in mitochondrial biology and homeostasis. Mitochondria are equipped with an endogenous cAMP synthesis system involving soluble adenylyl cyclase (sAC), which localizes in the mitochondrial matrix and regulates mitochondrial function. Furthermore, sAC localized at the outer mitochondrial membrane contributes significantly to mitochondrial biology. Disturbance of the sAC-dependent cAMP pools within mitochondria leads to mitochondrial dysfunction and pathology. In this review, we discuss the available data concerning the role of sAC in regulating mitochondrial biology in relation to diseases.

Role of EPAC1 Signalosomes in Cell Fate: Friends or Foes?

The compartmentation of signaling processes is accomplished by the assembly of protein complexes called signalosomes. These signaling platforms colocalize enzymes, substrates, and anchoring proteins into specific subcellular compartments. Exchange protein directly activated by cAMP 1 (EPAC1) is an effector of the second messenger, 3',5'-cyclic adenosine monophosphate (cAMP) that is associated with multiple roles in several pathologies including cardiac diseases. Both EPAC1 intracellular localization and molecular partners are key players in the regulation of cell fate, which may have important therapeutic potential. In this review, we summarize the recent findings on EPAC1 structure, regulation, and pharmacology. We describe the importance of EPAC1 subcellular distribution in its biological action, paying special attention to its nuclear localization and mechanism of action leading to cardiomyocyte hypertrophy. In addition, we discuss the role of mitochondrial EPAC1 in the regulation of cell death. Depending on the cell type and stress condition, we present evidence that supports either a protective or detrimental role of EPAC1 activation.

Targeting the sAC-Dependent cAMP Pool to Prevent SARS-Cov-2 Infection

An outbreak of the novel coronavirus (CoV) SARS-CoV-2, the causative agent of COVID-19 respiratory disease, infected millions of people since the end of 2019, led to high-level morbidity and mortality and caused worldwide social and economic disruption. There are currently no antiviral drugs available with proven efficacy or vaccines for its prevention. An understanding of the underlying cellular mechanisms involved in virus replication is essential for repurposing the existing drugs and/or the discovery of new ones. Endocytosis is the important mechanism of entry of CoVs into host cells. Endosomal maturation followed by the fusion with lysosomes are crucial events in endocytosis. Late endosomes and lysosomes are characterized by their acidic pH, which is generated by a proton transporter V-ATPase and required for virus entry via endocytic pathway. The cytoplasmic cAMP pool produced by soluble adenylyl cyclase (sAC) promotes V-ATPase recruitment to endosomes/lysosomes and thus their acidification. In this review, we discuss targeting the sAC-specific cAMP pool as a potential strategy to impair the endocytic entry of the SARS-CoV-2 into the host cell. Furthermore, we consider the potential impact of sAC inhibition on CoV-induced disease via modulation of autophagy and apoptosis.

Use of whole-exome sequencing to identify a novel ADCY10 mutation in a patient with nephrolithiasis

Nephrolithiasis is a prevalent condition with high morbidity, and the incidence and prevalence of nephrolithiasis have been increasing worldwide. Although dozens of monogenic reason of nephrolithiasis have been identified, the fraction of the disease caused by single genes has not been determined. In this study, employing total exon sequencing technology, we investigated two patients in south-central China with primary nephrolithiasis and identified a novel ADCY10 mutation c.2186G > A (p.G729E) and a known ADCY10 mutation c.2182G > A (p.E728K). The results of our study suggest that ADCY10 plays an important role in nephrolithiasis.

Inter-Tumor Heterogeneity-Melanomas Respond Differently to GM-CSF-Mediated Activation

Granulocyte-monocyte colony stimulating factor (GM-CSF) is used as an adjuvant in various clinical and preclinical studies with contradictory results. These were attributed to opposing effects of GM-CSF on the immune or myeloid systems of the treated patients or to lack of optimal dosing regimens. The results of the present study point to inter-tumor heterogeneity as a possible mechanism accounting for the contrasting responses to GM-CSF incorporating therapies. Employing xenograft models of human melanomas in nude mice developed in our lab, we detected differential functional responses of melanomas from different patients to GM-CSF both in vitro as well as in vivo. Whereas cells of one melanoma acquired pro metastatic features following exposure to GM-CSF, cells from another melanoma either did not respond or became less malignant. We propose that inter-melanoma heterogeneity as manifested by differential responses of melanoma cells (and perhaps also of other tumor) to GM-CSF may be developed into a predictive marker providing a tool to segregate melanoma patients who will benefit from GM-CSF therapy from those who will not.

Regulation of AMPK activity by type 10 adenylyl cyclase: contribution to the mitochondrial biology, cellular redox and energy homeostasis

The downregulation of AMP-activated protein kinase (AMPK) activity contributes to numerous pathologies. Recent reports suggest that the elevation of cellular cAMP promotes AMPK activity. However, the source of the cAMP pool that controls AMPK activity remains unknown. Mammalian cells possess two cAMP sources: membrane-bound adenylyl cyclase (tmAC) and intracellularly localized, type 10 soluble adenylyl cyclase (sAC). Due to the localization of sAC and AMPK in similar intracellular compartments, we hypothesized that sAC may control AMPK activity. In this study, sAC expression and activity were manipulated in H9C2 cells, adult rat cardiomyocytes or endothelial cells. sAC knockdown depleted the cellular cAMP content and decreased AMPK activity in an EPAC-dependent manner. Functionally, sAC knockdown reduced cellular ATP content, increased mitochondrial ROS formation and led to mitochondrial depolarization. Furthermore, sAC downregulation led to EPAC-dependent mitophagy disturbance, indicated by an increased mitochondrial mass and unaffected mitochondrial biogenesis. Consistently, sAC overexpression or stimulation with bicarbonate significantly increased AMPK activity and cellular ATP content. In contrast, tmAC inhibition or stimulation produced no effect on AMPK activity. Therefore, the sAC-EPAC axis may regulate basal and induced AMPK activity and support mitophagy, cellular energy and redox homeostasis. The study argues for sAC as a potential target in treating pathologies associated with AMPK downregulation.

The Epac1 Protein: Pharmacological Modulators, Cardiac Signalosome and Pathophysiology

The second messenger 3′,5′-cyclic adenosine monophosphate (cAMP) is one of the most important signalling molecules in the heart as it regulates many physiological and pathophysiological processes. In addition to the classical protein kinase A (PKA) signalling route, the exchange proteins directly activated by cAMP (Epac) mediate the intracellular functions of cAMP and are now emerging as a new key cAMP effector in cardiac pathophysiology. In this review, we provide a perspective on recent advances in the discovery of new chemical entities targeting the Epac1 isoform and illustrate their use to study the Epac1 signalosome and functional characterisation in cardiac cells. We summarize the role of Epac1 in different subcompartments of the cardiomyocyte and discuss how cAMP–Epac1 specific signalling networks may contribute to the development of cardiac diseases. We also highlight ongoing work on the therapeutic potential of Epac1-selective small molecules for the treatment of cardiac disorders.

17β-Estradiol reduces mitochondrial cAMP content and cytochrome oxidase activity in a phosphodiesterase 2-dependent manner

BACKGROUND AND PURPOSE

Mitochondria possess their own source of cAMP, that is, soluble adenylyl cyclase (sAC). Activation or expression of mitochondrial sAC promotes mitochondrial function. Oestrogen receptor signalling plays an essential role in the regulation of mitochondrial function. Here we aimed to determine whether 17β-estradiol may affect mitochondrial cAMP signalling.

EXPERIMENTAL APPROACH

Expression of the intra-mitochondrial proteins (Western blot), mitochondrial cAMP content (FRET-based live imaging and MS assay), mitochondrial membrane potential and cytochrome oxidase activity were analysed in H9C2 and C2C12 cells.

KEY RESULTS

A 24 h treatment with 17β-estradiol significantly reduced the basal level of mitochondrial cAMP, without affecting the intra-mitochondrial content of sAC, phosphodiesterase 2 (PDE2) or PKA and the activity of the intra-mitochondrial sAC. The effect of 17β-estradiol on mitochondrial cAMP was prevented by inhibition of a cGMP-activated PDE2 or soluble guanylyl cyclase (sGC), suggesting a role of NO signalling. Indeed, 17β-estradiol raised cellular levels of cGMP and the intra-mitochondrial expression of the catalytic subunit β of sGC was found. The 17β-estradiol-induced reduction of the mitochondrial cAMP level was accompanied by decreased cytochrome oxidase activity and mitochondrial membrane potential in a PDE2-dependent manner.

CONCLUSIONS AND IMPLICATIONS

17β-estradiol reduced the basal level of mitochondrial cAMP content and cytochrome oxidase activity in a sAC-independent but in a PDE2-dependent manner. The results suggest a role of 17β-estradiol-induced activation of NO signalling in the regulation of mitochondrial cAMP content. Our study adds a new aspect to the complex action of oestrogens on mitochondrial biology, that is relevant to hormone replacement therapy.

Protective role of soluble adenylyl cyclase against reperfusion-induced injury of cardiac cells

AIMS

Disturbance of mitochondrial function significantly contributes to the myocardial injury that occurs during reperfusion. Increasing evidence suggests a role of intra-mitochondrial cyclic AMP (cAMP) signaling in promoting respiration and ATP synthesis. Mitochondrial levels of cAMP are controlled by type 10 soluble adenylyl cyclase (sAC) and phosphodiesterase 2 (PDE2), however their role in the reperfusion-induced injury remains unknown. Here we aimed to examine whether sAC may support cardiomyocyte survival during reperfusion.

METHODS AND RESULTS

Adult rat cardiomyocytes or rat cardiac H9C2 cells were subjected to metabolic inhibition and recovery as a model of simulated ischemia and reperfusion. Cytosolic Ca²⁺, pH, mitochondrial cAMP (live-cell imaging), and cell viability were analyzed during a 15-min period of reperfusion. Suppression of sAC activity in cardiomyocytes and H9C2 cells, either by sAC knockdown, by pharmacological inhibition or by withdrawal of bicarbonate, a natural sAC activator, compromised cell viability and recovery of cytosolic Ca²⁺ homeostasis during reperfusion. Contrariwise, overexpression of mitochondria-targeted sAC in H9C2 cells suppressed reperfusion-induced cell death. Analyzing cAMP concentration in mitochondrial matrix we found that inhibition of PDE2, a predominant mitochondria-localized PDE isoform in mammals, during reperfusion significantly increased cAMP level in mitochondrial matrix, but not in cytosol. Accordingly, PDE2 inhibition attenuated reperfusion-induced cardiomyocyte death and improved recovery of the cytosolic Ca²⁺ homeostasis.

CONCLUSION

sAC plays an essential role in supporting cardiomyocytes viability during reperfusion. Elevation of mitochondrial cAMP pool either by sAC overexpression or by PDE2 inhibition beneficially affects cardiomyocyte survival during reperfusion.