Soluble adenylyl cyclase in health and disease

The role of cyclic adenosine monophosphate (cAMP) in pathophysiology of fibrosis

Fibrosis, the excessive production and disorganised deposition of extracellular matrix proteins, can occur in any organ system, disrupting functionality and causing fatality. The number, efficacy and safety of antifibrotic drugs are incredibly limited. Therapeutics which elevate intracellular cyclic adenosine monophosphate (cAMP) offer a potential solution. In this review, we present the signalling mechanisms involved in fibrosis pathophysiology, how cAMP and its effectors might interact with these pathways, and the current preclinical and clinical efforts in this field. cAMP elevating agents have the potential to be future antifibrotic drug candidates, but further studies are required, particularly to develop tissue specific therapeutics.

CFTR Inhibitors Display Antiviral Activity against Herpes Simplex Virus

The cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-dependent Cl- channel, is closely associated with multiple pathogen infections, such as SARS-CoV-2. However, whether the function of the CFTR is involved in herpes simplex virus (HSV) infection has not been reported. To evaluate the association of CFTR activity with HSV infection, the antiviral effect of CFTR inhibitors in epithelial cells and HSV-infected mice was tested in this study. The data showed that treatment with CFTR inhibitors in different concentrations, Glyh-101 (5-20 μM), CFTRi-172 (5-20 μM) and IOWH-032 (5-20 μM), or the gene silence of the CFTR could suppress herpes simplex virus 1 (HSV-1) and herpes simplex virus 2 (HSV-2) replication in human HaCaT keratinocytes cells, and that a CFTR inhibitor, Glyh-101 (10-20 μM), protected mice from HSV-1 and HSV-2 infection. Intracellular Cl- concentration ([Cl-]i) was decreased after HSV infection via the activation of adenylyl cyclase (AC)-cAMP signaling pathways. CFTR inhibitors (20 μM) increased the reduced [Cl-]i caused by HSV infection in host epithelial cells. Additionally, CFTR inhibitors reduced the activity and phosphorylation of SGK1 in infected cells and tissues (from the eye and vagina). Our study found that CFTR inhibitors can effectively suppress HSV-1 and HSV-2 infection, revealing a previously unknown role of CFTR inhibitors in HSV infection and suggesting new perspectives on the mechanisms governing HSV infection in host epithelial cells, as well as leading to potential novel treatments.

Adenylyl cyclase isoforms 5 and 6 in the cardiovascular system: complex regulation and divergent roles

Adenylyl cyclases (ACs) are crucial effector enzymes that transduce divergent signals from upstream receptor pathways and are responsible for catalyzing the conversion of ATP to cAMP. The ten AC isoforms are categorized into four main groups; the class III or calcium-inhibited family of ACs comprises AC5 and AC6. These enzymes are very closely related in structure and have a paucity of selective activators or inhibitors, making it difficult to distinguish them experimentally. AC5 and AC6 are highly expressed in the heart and vasculature, as well as the spinal cord and brain; AC6 is also abundant in the lungs, kidney, and liver. However, while AC5 and AC6 have similar expression patterns with some redundant functions, they have distinct physiological roles due to differing regulation and cAMP signaling compartmentation. AC5 is critical in cardiac and vascular function; AC6 is a key effector of vasodilatory pathways in vascular myocytes and is enriched in fetal/neonatal tissues. Expression of both AC5 and AC6 decreases in heart failure; however, AC5 disruption is cardio-protective, while overexpression of AC6 rescues cardiac function in cardiac injury. This is a comprehensive review of the complex regulation of AC5 and AC6 in the cardiovascular system, highlighting overexpression and knockout studies as well as transgenic models illuminating each enzyme and focusing on post-translational modifications that regulate their cellular localization and biological functions. We also describe pharmacological challenges in the design of isoform-selective activators or inhibitors for AC5 and AC6, which may be relevant to developing new therapeutic approaches for several cardiovascular diseases.

Epac as a tractable therapeutic target

In 1957, cyclic adenosine monophosphate (cAMP) was identified as the first secondary messenger, and the first signaling cascade discovered was the cAMP-protein kinase A (PKA) pathway. Since then, cAMP has received increasing attention given its multitude of actions. Not long ago, a new cAMP effector named exchange protein directly activated by cAMP (Epac) emerged as a critical mediator of cAMP's actions. Epac mediates a plethora of pathophysiologic processes and contributes to the pathogenesis of several diseases such as cancer, cardiovascular disease, diabetes, lung fibrosis, neurological disorders, and others. These findings strongly underscore the potential of Epac as a tractable therapeutic target. In this context, Epac modulators seem to possess unique characteristics and advantages and hold the promise of providing more efficacious treatments for a wide array of diseases. This paper provides an in-depth dissection and analysis of Epac structure, distribution, subcellular compartmentalization, and signaling mechanisms. We elaborate on how these characteristics can be utilized to design specific, efficient, and safe Epac agonists and antagonists that can be incorporated into future pharmacotherapeutics. In addition, we provide a detailed portfolio for specific Epac modulators highlighting their discovery, advantages, potential concerns, and utilization in the context of clinical disease entities.

Astrocytic Calcium and cAMP in Neurodegenerative Diseases

It is commonly accepted that the role of astrocytes exceeds far beyond neuronal scaffold and energy supply. Their unique morphological and functional features have recently brough much attention as it became evident that they play a fundamental role in neurotransmission and interact with synapses. Synaptic transmission is a highly orchestrated process, which triggers local and transient elevations in intracellular Ca²⁺, a phenomenon with specific temporal and spatial properties. Presynaptic activation of Ca²⁺-dependent adenylyl cyclases represents an important mechanism of synaptic transmission modulation. This involves activation of the cAMP-PKA pathway to regulate neurotransmitter synthesis, release and storage, and to increase neuroprotection. This aspect is of paramount importance for the preservation of neuronal survival and functionality in several pathological states occurring with progressive neuronal loss. Hence, the aim of this review is to discuss mutual relationships between cAMP and Ca²⁺ signaling and emphasize those alterations at the Ca²⁺/cAMP crosstalk that have been identified in neurodegenerative disorders, such as Alzheimer's and Parkinson's disease.

Physiological roles of mammalian transmembrane adenylyl cyclase isoforms

Adenylyl cyclases (ACs) catalyze the conversion of ATP to the ubiquitous second messenger cAMP. Mammals possess nine isoforms of transmembrane ACs, dubbed AC1-9, that serve as major effector enzymes of G protein-coupled receptors (GPCRs). The transmembrane ACs display varying expression patterns across tissues, giving the potential for them to have a wide array of physiological roles. Cells express multiple AC isoforms, implying that ACs have redundant functions. Furthermore, all transmembrane ACs are activated by Gαs, so it was long assumed that all ACs are activated by Gαs-coupled GPCRs. AC isoforms partition to different microdomains of the plasma membrane and form prearranged signaling complexes with specific GPCRs that contribute to cAMP signaling compartments. This compartmentation allows for a diversity of cellular and physiological responses by enabling unique signaling events to be triggered by different pools of cAMP. Isoform-specific pharmacological activators or inhibitors are lacking for most ACs, making knockdown and overexpression the primary tools for examining the physiological roles of a given isoform. Much progress has been made in understanding the physiological effects mediated through individual transmembrane ACs. GPCR-AC-cAMP signaling pathways play significant roles in regulating functions of every cell and tissue, so understanding each AC isoform's role holds potential for uncovering new approaches for treating a vast array of pathophysiological conditions.

Expression and functions of adenylyl cyclases in the CNS

Adenylyl cyclases (ADCYs), by generating second messenger cAMP, play important roles in various cellular processes. Their expression, regulation and functions in the CNS, however, remain largely unknown. In this review, we first introduce the classification and structure of ADCYs, followed by a discussion of the regulation of mammalian ADCYs (ADCY1-10). Next, the expression and function of each mammalian ADCY isoform are summarized in a region/cell-specific manner. Furthermore, the effects of GPCR-ADCY signaling on blood-brain barrier (BBB) integrity are reviewed. Last, current challenges and future directions are discussed. We aim to provide a succinct review on ADCYs to foster new research in the future.

Gαs-Protein Kinase A (PKA) Pathway Signalopathies: The Emerging Genetic Landscape and Therapeutic Potential of Human Diseases Driven by Aberrant Gαs-PKA Signaling

Many of the fundamental concepts of signal transduction and kinase activity are attributed to the discovery and crystallization of cAMP-dependent protein kinase, or protein kinase A. PKA is one of the best-studied kinases in human biology, with emphasis in biochemistry and biophysics, all the way to metabolism, hormone action, and gene expression regulation. It is surprising, however, that our understanding of PKA's role in disease is largely underappreciated. Although genetic mutations in the PKA holoenzyme are known to cause diseases such as Carney complex, Cushing syndrome, and acrodysostosis, the story largely stops there. With the recent explosion of genomic medicine, we can finally appreciate the broader role of the Gαs-PKA pathway in disease, with contributions from aberrant functioning G proteins and G protein-coupled receptors, as well as multiple alterations in other pathway components and negative regulators. Together, these represent a broad family of diseases we term the Gαs-PKA pathway signalopathies. The Gαs-PKA pathway signalopathies encompass diseases caused by germline, postzygotic, and somatic mutations in the Gαs-PKA pathway, with largely endocrine and neoplastic phenotypes. Here, we present a signaling-centric review of Gαs-PKA-driven pathophysiology and integrate computational and structural analysis to identify mutational themes commonly exploited by the Gαs-PKA pathway signalopathies. Major mutational themes include hotspot activating mutations in Gαs, encoded by GNAS, and mutations that destabilize the PKA holoenzyme. With this review, we hope to incite further study and ultimately the development of new therapeutic strategies in the treatment of a wide range of human diseases. SIGNIFICANCE STATEMENT: Little recognition is given to the causative role of Gαs-PKA pathway dysregulation in disease, with effects ranging from infectious disease, endocrine syndromes, and many cancers, yet these disparate diseases can all be understood by common genetic themes and biochemical signaling connections. By highlighting these common pathogenic mechanisms and bridging multiple disciplines, important progress can be made toward therapeutic advances in treating Gαs-PKA pathway-driven disease.

A new adenylyl cyclase, putative disease-resistance RPP13-like protein 3, participates in abscisic acid-mediated resistance to heat stress in maize

In plants, 3´,5´-cyclic adenosine monophosphate (cAMP) is an important second messenger with varied functions; however, only a few adenylyl cyclases (ACs) that synthesize cAMP have been identified. Moreover, the biological roles of ACs/cAMP in response to stress remain largely unclear. In this study, we used quantitative proteomics techniques to identify a maize heat-induced putative disease-resistance RPP13-like protein 3 (ZmRPP13-LK3), which has three conserved catalytic AC centres. The AC activity of ZmRPP13-LK3 was confirmed by in vitro enzyme activity analysis, in vivo RNAi experiments, and functional complementation in the E. coli cyaA mutant. ZmRPP13-LK3 is located in the mitochondria. The results of in vitro and in vivo experiments indicated that ZmRPP13-LK3 interacts with ZmABC2, a possible cAMP exporter. Under heat stress, the concentrations of ZmRPP13-LK3 and cAMP in the ABA-deficient mutant vp5 were significantly less than those in the wild-type, and treatment with ABA and an ABA inhibitor affected ZmRPP13-LK3 expression in the wild-type. Application of 8-Br-cAMP, a cAMP analogue, increased heat-induced expression of heat-shock proteins in wild-type plants and alleviated heat-activated oxidative stress. Taken together, our results indicate that ZmRPP13-LK3, a new AC, can catalyse ATP for the production of cAMP and may be involved in ABA-regulated heat resistance.

Mucociliary Respiratory Epithelium Integrity in Molecular Defense and Susceptibility to Pulmonary Viral Infections

Mucociliary defense, mediated by the ciliated and goblet cells, is fundamental to respiratory fitness. The concerted action of ciliary movement on the respiratory epithelial surface and the pathogen entrapment function of mucus help to maintain healthy airways. Consequently, genetic or acquired defects in lung defense elicit respiratory diseases and secondary microbial infections that inflict damage on pulmonary function and may even be fatal. Individuals living with chronic and acute respiratory diseases are more susceptible to develop severe coronavirus disease-19 (COVID-19) illness and hence should be proficiently managed. In light of the prevailing pandemic, we review the current understanding of the respiratory system and its molecular components with a major focus on the pathophysiology arising due to collapsed respiratory epithelium integrity such as abnormal ciliary movement, cilia loss and dysfunction, ciliated cell destruction, and changes in mucus rheology. The review includes protein interaction networks of coronavirus infection-manifested implications on the molecular machinery that regulates mucociliary clearance. We also provide an insight into the alteration of the transcriptional networks of genes in the nasopharynx associated with the mucociliary clearance apparatus in humans upon infection by severe acute respiratory syndrome coronavirus-2.

cAMP Signaling in Pathobiology of Alcohol Associated Liver Disease

The importance of cyclic adenosine monophosphate (cAMP) in cellular responses to extracellular signals is well established. Many years after discovery, our understanding of the intricacy of cAMP signaling has improved dramatically. Multiple layers of regulation exist to ensure the specificity of cellular cAMP signaling. Hence, disturbances in cAMP homeostasis could arise at multiple levels, from changes in G protein coupled receptors and production of cAMP to the rate of degradation by phosphodiesterases. cAMP signaling plays critical roles in metabolism, inflammation and development of fibrosis in several tissues. Alcohol-associated liver disease (ALD) is a multifactorial condition ranging from a simple steatosis to steatohepatitis and fibrosis and ultimately cirrhosis, which might lead to hepatocellular cancer. To date, there is no FDA-approved therapy for ALD. Hence, identifying the targets for the treatment of ALD is an important undertaking. Several human studies have reported the changes in cAMP homeostasis in relation to alcohol use disorders. cAMP signaling has also been extensively studied in in vitro and in vivo models of ALD. This review focuses on the role of cAMP in the pathobiology of ALD with emphasis on the therapeutic potential of targeting cAMP signaling for the treatment of various stages of ALD.

Respiratory Cilia as a Therapeutic Target of Phosphodiesterase Inhibitors

Mucociliary clearance is an essential airway defense mechanism dependent predominantly on the proper ciliary function and mucus rheology. The crucial role of cilia is evident in `a variety of respiratory diseases, as the ciliary dysfunction is associated with a progressive decline in lung function over time. The activity of cilia is under supervision of multiple physiological regulators, including second messengers. Their role is to enable a movement in coordinated metachronal waves at certain beat frequency. Ciliary function can be modulated by various stimuli, including agents from the group of beta₂ agonists, cholinergic drugs, and adenosine triphosphate (ATP). They trigger cilia to move faster in response to elevated cytoplasmic Ca²⁺ originated from intracellular sources or replenished from extracellular space. Well-known cilia-stimulatory effect of Ca²⁺ ions can be abolished or even reversed by modulating the phosphodiesterase (PDE)-mediated breakdown of cyclic adenosine monophosphate (cAMP) since the overall change in ciliary beating has been dependent on the balance between Ca²⁺ ions and cAMP. Moreover, in chronic respiratory diseases, high ATP levels may contribute to cAMP hydrolysis and thus to a decrease in the ciliary beat frequency (CBF). The role of PDE inhibitors in airway cilia-driven transport may help in prevention of progressive loss of pulmonary function often observed despite current therapy. Furthermore, administration of selective PDE inhibitors by inhalation lowers the risk of their systemic effects. Based on this review we may conclude that selective (PDE1, PDE4) or dual PDE inhibitors (PDE3/4) increase the intracellular level of cyclic nucleotides in airway epithelial cells and thus may be an important target in the development of new inhaled mucokinetic agents. Further research is required to provide evidence of their effectiveness and feasibility regarding their cilia-modulating properties.

Mechanisms of ATP to cAMP Conversion Catalyzed by the Mammalian Adenylyl Cyclase: A Role of Magnesium Coordination Shells and Proton Wires

We report a mechanism of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) conversion by the mammalian type V adenylyl cyclase revealed in molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) simulations. We characterize a set of computationally derived enzyme-substrate (ES) structures showing an important role of coordination shells of magnesium ions in the solvent accessible active site. In the lowest energy ES conformation, the coordination shell of Mg_A²⁺ does not include the O_δ1 atom of the conserved Asp440 residue. Starting from this conformation, a one-step reaction mechanism is characterized that includes proton transfer from the ribose O^3'H^3' group in ATP to Asp440 via a shuttling water molecule concerted with P^A-O^3A bond cleavage and O^3'-P^A bond formation. The energy profile of this route is consistent with the observed reaction kinetics. The computed energy profiles initiated from higher energy ES complexes are characterized by larger energy expenses to complete the reaction. Consistent with experimental data, we show that the Asp440Ala mutant of the enzyme should exhibit a reduced but retained activity. All considered reaction pathways include proton wires from the O^3'H^3' group via shuttling water molecules.

The Role of Cyclic AMP Signaling in Cardiac Fibrosis

Myocardial stress and injury invariably promote remodeling of the cardiac tissue, which is associated with cardiomyocyte death and development of fibrosis. The fibrotic process is initially triggered by the differentiation of resident cardiac fibroblasts into myofibroblasts. These activated fibroblasts display increased proliferative capacity and secrete large amounts of extracellular matrix. Uncontrolled myofibroblast activation can thus promote heart stiffness, cardiac dysfunction, arrhythmias, and progression to heart failure. Despite the well-established role of myofibroblasts in mediating cardiac disease, our current knowledge on how signaling pathways promoting fibrosis are regulated and coordinated in this cell type is largely incomplete. In this respect, cyclic adenosine monophosphate (cAMP) signaling acts as a major modulator of fibrotic responses activated in fibroblasts of injured or stressed hearts. In particular, accumulating evidence now suggests that upstream cAMP modulators including G protein-coupled receptors, adenylyl cyclases (ACs), and phosphodiesterases (PDEs); downstream cAMP effectors such as protein kinase A (PKA) and the guanine nucleotide exchange factor Epac; and cAMP signaling organizers such as A-kinase anchoring proteins (AKAPs) modulate a variety of fundamental cellular processes involved in myocardial fibrosis including myofibroblast differentiation, proliferation, collagen secretion, and invasiveness. The current review will discuss recent advances highlighting the role of cAMP and AKAP-mediated signaling in regulating pathophysiological responses controlling cardiac fibrosis.

Mitochondrial dysfunction, AMPK activation and peroxisomal metabolism: A coherent scenario for non-canonical 3-methylglutaconic acidurias

The occurrence of 3-methylglutaconic aciduria (3-MGA) is a well understood phenomenon in leucine oxidation and ketogenesis disorders (primary 3-MGAs). In contrast, its genesis in non-canonical (secondary) 3-MGAs, a growing-up group of disorders encompassing more than a dozen of inherited metabolic diseases, is a mystery still remaining unresolved for three decades. To puzzle out this anthologic problem of metabolism, three clues were considered: (i) the variety of disorders suggests a common cellular target at the cross-road of metabolic and signaling pathways, (ii) the response to leucine loading test only discriminative for primary but not secondary 3-MGAs suggests these latter are disorders of extramitochondrial HMG-CoA metabolism as also attested by their failure to increase 3-hydroxyisovalerate, a mitochondrial metabolite accumulating only in primary 3-MGAs, (iii) the peroxisome is an extramitochondrial site possessing its own pool and displaying metabolism of HMG-CoA, suggesting its possible involvement in producing extramitochondrial 3-methylglutaconate (3-MG). Following these clues provides a unifying common basis to non-canonical 3-MGAs: constitutive mitochondrial dysfunction induces AMPK activation which, by inhibiting early steps in cholesterol and fatty acid syntheses, pipelines cytoplasmic acetyl-CoA to peroxisomes where a rise in HMG-CoA followed by local dehydration and hydrolysis may lead to 3-MGA yield. Additional contributors are considered, notably for 3-MGAs associated with hyperammonemia, and to a lesser extent in CLPB deficiency. Metabolic and signaling itineraries followed by the proposed scenario are essentially sketched, being provided with compelling evidence from the literature coming in their support.

Acetate attenuates inflammasome activation through GPR43-mediated Ca2+-dependent NLRP3 ubiquitination

Acetate has been indicated to be elevated and to regulate inflammation in inflammatory and metabolic diseases. The inflammasome serves as a key component of immune homeostasis, and its dysregulation can lead to various inflammatory disorders. However, little is known about the effects of acetate on inflammasome activation and the underlying mechanism. Here, we demonstrate that acetate attenuates inflammasome activation via GPR43 in a Ca²⁺-dependent manner. Through binding to GPR43, acetate activates the G_q/11 subunit and subsequent phospholipase C-IP₃ signaling to decrease Ca²⁺ mobilization. In addition, acetate activates soluble adenylyl cyclase (sAC), promotes NLRP3 inflammasome ubiquitination by PKA, and ultimately induces NLRP3 degradation through autophagy. In vivo, acetate protects mice from NLRP3 inflammasome-dependent peritonitis and LPS-induced endotoxemia. Collectively, our research demonstrates that acetate regulates the NLRP3 inflammasome via GPR43 and Ca²⁺-dependent mechanisms, which reveals the mechanism of metabolite-mediated NLRP3 inflammasome attenuation and highlights acetate as a possible therapeutic strategy for NLRP3 inflammasome-related diseases.

Effects of Timolol Treatment on Pancreatic Antioxidant Enzymes in Streptozotocin-induced Diabetic Rats: An Experimental and Computational Study

BACKGROUND

The study aimed to investigate whether timolol-treatment has a beneficial effect on pentose phosphate pathway enzyme activities such as glucose-6-phosphate dehydrogenase (G6PD), 6-phosphogluconate dehydrogenase (6PGDH) enzyme activities and cAMP level in streptozotocin-induced diabetic rats in pancreatic tissues.

METHODS

Diabetes was induced by streptozotocin (STZ) in 3-month old male Wistar rats. The diabetic rats were treated with timolol (5 mg/kg body weight, for 12 weeks) while the control group received saline. Enzyme activities were determined in pancreas tissue. To support our results, we performed in silico calculations, using Protein Data Bank structures.

RESULTS

Timolol treatment of STZ-induced diabetic rats had no noteworthy effect on high blood-glucose levels. However, this treatment induced activities of G6PD and 6PGDH in diabetic rats. Timolol treatment significantly increased cAMP level in diabetic pancreatic tissue. We found that timolol cannot bind strongly to either G6PD or 6PGD, but there is a relatively higher binding affinity to adenylyl cyclase, responsible for cAMP production, serving as a regulatory signal via specific cAMP-binding proteins.

CONCLUSIONS

Our data point out that timolol treatment has beneficial effects on the antioxidant defence mechanism enzymes in the pancreas of STZ-induced diabetic rats.

The Astrocytic cAMP Pathway in Health and Disease

Astrocytes are major glial cells that play critical roles in brain homeostasis. Abnormalities in astrocytic functions can lead to brain disorders. Astrocytes also respond to injury and disease through gliosis and immune activation, which can be both protective and detrimental. Thus, it is essential to elucidate the function of astrocytes in order to understand the physiology of the brain to develop therapeutic strategies against brain diseases. Cyclic adenosine monophosphate (cAMP) is a major second messenger that triggers various downstream cellular machinery in a wide variety of cells. The functions of astrocytes have also been suggested as being regulated by cAMP. Here, we summarize the possible roles of cAMP signaling in regulating the functions of astrocytes. Specifically, we introduce the ways in which cAMP pathways are involved in astrocyte functions, including (1) energy supply, (2) maintenance of the extracellular environment, (3) immune response, and (4) a potential role as a provider of trophic factors, and we discuss how these cAMP-regulated processes can affect brain functions in health and disease.

Pulsed electromagnetic fields prevented the decrease of bone formation in hindlimb-suspended rats by activating sAC/cAMP/PKA/CREB signaling pathway

Microgravity is one of the main threats to the health of astronauts. Pulsed electromagnetic fields (PEMFs) have been considered as one of the potential countermeasures for bone loss induced by space flight. However, the optimal therapeutic parameters of PEMFs have not been obtained and the action mechanism is still largely unknown. In this study, a set of optimal therapeutic parameters for PEMFs (50 Hz, 0.6 mT 50% duty cycle and 90 min/day) selected based on high-throughput screening with cultured osteoblasts was used to prevent bone loss in rats induced by hindlimb suspension, a commonly accepted animal model to simulate the space environment. It was found that hindlimb suspension for 4 weeks led to significant decreases in femoral and vertebral bone mineral density (BMD) and their maximal loads, severe deterioration in bone micro-structure, and decreases in levels of bone formation markers and increases in bone resorption markers. PEMF treatment prevented about 50% of the decreased BMD and maximal loads, preserved the microstructure of cancellous bone and thickness of cortical bone, and inhibited decreases in bone formation markers. Histological analyses revealed that PEMFs significantly alleviated the reduction in osteoblast number and inhibited the increase in adipocyte number in the bone marrow. PEMFs also blocked decreases in serum levels of parathyroid hormone and its downstream signal molecule cAMP, and maintained the phosphorylation levels of protein kinase A (PKA) and cAMP response element-binding protein (CREB). The expression level of soluble adenylyl cyclases (sAC) was also maintained. It therefore can be concluded that PEMFs partially prevented the bone loss induced by weightless environment by maintaining bone formation through signaling of the sAC/cAMP/PKA/CREB pathway. Bioelectromagnetics. 39:569-584, 2018. © 2018 Wiley Periodicals, Inc.

Total flavonoid extract of Epimedium herb increases the peak bone mass of young rats involving enhanced activation of the AC10/cAMP/PKA/CREB pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Epimedium sagittatum brevicornum Maxim. is an important traditional Chinese herb that has long been used to promote bone fracture healing and treat osteoporosis.

AIM OF THE STUDY

Achieving peak bone mass by adolescence has now been accepted to be fundamental for preventing osteoporosis in adulthood life. This study investigated the possibility of increasing peak bone mass in young rats using the total flavonoid extract of Epimedium herb (TFE).

MATERIALS AND METHODS

TFE was intragastrically administered to one-month-old Wistar rats at a low (100 mg/kg), middle (200 mg/kg) or high dose (400 mg/kg). Whole body bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry every two weeks. When BMD of any one of TFE groups was found to be significantly higher than that of the control, all rats were sacrificed, serum samples were collected for bone turnover biochemical assays, and femurs, tibiae and vertebrae were isolated and used in BMD, mechanical, micro-structural, histomorphometric and mechanistic studies.

RESULTS

Administration of TFE at middle and high doses for two months significantly increased the whole body, femoral and vertebral BMDs, and improved the bone mechanical and micro-architectural properties. The serum turnover biochemical results and the enhanced expression levels of bone-formation regulatory genes (Runx-2, OSX, and BMP-2) demonstrated that TFE administration increased bone formation but had no effect on bone resorption. The increased phosphorylation levels in femurs of PKA and CREB and expression of AC10 (the only soluble form of adenylyl cyclase) and the increased serum cAMP level after 4 h of TFE administration indicated that TFE promoted bone formation by activating the AC10/cAMP/PKA/CREB pathway in vivo.

CONCLUSIONS

Oral administration of TFE at 200 mg/kg for two months can increase the peak bone mass of growing rats, suggesting the possibility of using total flavonoid extract of Epimedium herb to increase the peak bone mass in adolescence which is important for preventing osteoporosis in adult life.

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

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

The EGFR-ADAM17 Axis in Chronic Obstructive Pulmonary Disease and Cystic Fibrosis Lung Pathology

Chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF) share molecular mechanisms that cause the pathological symptoms they have in common. Here, we review evidence suggesting that hyperactivity of the EGFR/ADAM17 axis plays a role in the development of chronic lung disease in both CF and COPD. The ubiquitous transmembrane protease A disintegrin and metalloprotease 17 (ADAM17) forms a functional unit with the EGF receptor (EGFR), in a feedback loop interaction labeled the ADAM17/EGFR axis. In airway epithelial cells, ADAM17 sheds multiple soluble signaling proteins by proteolysis, including EGFR ligands such as amphiregulin (AREG), and proinflammatory mediators such as the interleukin 6 coreceptor (IL-6R). This activity can be enhanced by injury, toxins, and receptor-mediated external triggers. In addition to intracellular kinases, the extracellular glutathione-dependent redox potential controls ADAM17 shedding. Thus, the epithelial ADAM17/EGFR axis serves as a receptor of incoming luminal stress signals, relaying these to neighboring and underlying cells, which plays an important role in the resolution of lung injury and inflammation. We review evidence that congenital CFTR deficiency in CF and reduced CFTR activity in chronic COPD may cause enhanced ADAM17/EGFR signaling through a defect in glutathione secretion. In future studies, these complex interactions and the options for pharmaceutical interventions will be further investigated.

Cigarette smoke activates CFTR through ROS-stimulated cAMP signaling in human bronchial epithelial cells

Air pollution stimulates airway epithelial secretion through a cholinergic reflex that is unaffected in cystic fibrosis (CF), yet a strong correlation is observed between passive smoke exposure in the home and impaired lung function in CF children. Our aim was to study the effects of low smoke concentrations on cystic fibrosis transmembrane conductance regulator (CFTR) function in vitro. Cigarette smoke extract stimulated robust anion secretion that was transient, mediated by CFTR, and dependent on cAMP-dependent protein kinase activation. Secretion was initiated by reactive oxygen species (ROS) and mediated by at least two distinct pathways: autocrine activation of EP4 prostanoid receptors and stimulation of Ca2+ store-operated cAMP signaling. The response was absent in cells expressing the most common disease-causing mutant F508del-CFTR. In addition to the initial secretion, prolonged exposure of non-CF bronchial epithelial cells to low levels of smoke also caused a gradual decline in CFTR functional expression. F508del-CFTR channels that had been rescued by the CF drug combination VX-809 (lumacaftor) + VX-770 (ivacaftor) were more sensitive to this downregulation than wild-type CFTR. The results suggest that CFTR-mediated secretion during acute cigarette smoke exposure initially protects the airway epithelium while prolonged exposure reduces CFTR functional expression and reduces the efficacy of CF drugs.

The flavonol glycoside icariin promotes bone formation in growing rats by activating the cAMP signaling pathway in primary cilia of osteoblasts

Icariin, a prenylated flavonol glycoside isolated from the herb Epimedium, has been considered as a potential alternative therapy for osteoporosis. Previous research has shown that, unlike other flavonoids, icariin is unlikely to act via the estrogen receptor, but its exact mechanism of action is unknown. In this study, using rat calvarial osteoblast culture and rat bone growth models, we demonstrated that icariin promotes bone formation by activating the cAMP/protein kinase A (PKA)/cAMP response element-binding protein (CREB) pathway requiring functional primary cilia of osteoblasts. We found that icariin increases the peak bone mass attained by young rats and promotes the maturation and mineralization of rat calvarial osteoblasts. Icariin activated cAMP/PKA/CREB signaling of the osteoblasts by increasing intracellular cAMP levels and facilitating phosphorylation of both PKA and CREB. Blocking cAMP/PKA/CREB signaling with inhibitors of the cAMP-synthesizing adenylyl cyclase (AC) and PKA inhibitors significantly inhibited the osteogenic effect of icariin in the osteoblasts. Icariin-activated cAMP/PKA/CREB signaling was localized to primary cilia, as indicated by localization of soluble AC and phosphorylated PKA. Furthermore, blocking ciliogenesis via siRNA knockdown of a cilium assembly protein, IFT88, inhibited icariin-induced PKA and CREB phosphorylation and also abolished icariin's osteogenic effect. Finally, several of these outcomes were validated in icariin-treated rats. Together, these results provide new insights into icariin function and its mechanisms of action and strengthen existing ties between cAMP-mediated signaling and osteogenesis.

Importance of bicarbonate transport in pH control during amelogenesis - need for functional studies

Dental enamel, the hardest mammalian tissue, is produced by ameloblasts. Ameloblasts show many similarities to other transporting epithelia although their secretory product, the enamel matrix, is quite different. Ameloblasts direct the formation of hydroxyapatite crystals, which liberate large quantities of protons that then need to be buffered to allow mineralization to proceed. Buffering requires a tight pH regulation and secretion of bicarbonate by ameloblasts. Many investigations have used immunohistochemical and knockout studies to determine the effects of these genes on enamel formation, but up till recently very little functional data were available for mineral ion transport. To address this, we developed a novel 2D in vitro model using HAT-7 ameloblast cells. HAT-7 cells can be polarized and develop functional tight junctions. Furthermore, they are able to accumulate bicarbonate ions from the basolateral to the apical fluid spaces. We propose that in the future, the HAT-7 2D system along with similar cellular models will be useful to functionally model ion transport processes during amelogenesis. Additionally, we also suggest that similar approaches will allow a better understanding of the regulation of the cycling process in maturation-stage ameloblasts, and the pH sensory mechanisms, which are required to develop sound, healthy enamel.

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Adenylyl cyclases (ACs) generate the second messenger cAMP from ATP. Mammalian cells express nine transmembrane AC (mAC) isoforms (AC1-9) and a soluble AC (sAC, also referred to as AC10). This review will largely focus on mACs. mACs are activated by the G-protein Gαs and regulated by multiple mechanisms. mACs are differentially expressed in tissues and regulate numerous and diverse cell functions. mACs localize in distinct membrane compartments and form signaling complexes. sAC is activated by bicarbonate with physiologic roles first described in testis. Crystal structures of the catalytic core of a hybrid mAC and sAC are available. These structures provide detailed insights into the catalytic mechanism and constitute the basis for the development of isoform-selective activators and inhibitors. Although potent competitive and noncompetitive mAC inhibitors are available, it is challenging to obtain compounds with high isoform selectivity due to the conservation of the catalytic core. Accordingly, caution must be exerted with the interpretation of intact-cell studies. The development of isoform-selective activators, the plant diterpene forskolin being the starting compound, has been equally challenging. There is no known endogenous ligand for the forskolin binding site. Recently, development of selective sAC inhibitors was reported. An emerging field is the association of AC gene polymorphisms with human diseases. For example, mutations in the AC5 gene (ADCY5) cause hyperkinetic extrapyramidal motor disorders. Overall, in contrast to the guanylyl cyclase field, our understanding of the (patho)physiology of AC isoforms and the development of clinically useful drugs targeting ACs is still in its infancy.

Hypocapnic hypothesis of Leigh disease

Leigh syndrome (LS) is a neurogenetic disorder of children caused by mutations in at least 75 genes which impair mitochondrial bioenergetics. The changes have typical localization in basal ganglia and brainstem, and typical histological picture of spongiform appearance, vascular proliferation and gliosis. ATP deprivation, free radicals and lactate accumulation are suspected to be the causes. Hypocapnic hypothesis proposed in the paper questions the energy deprivation as the mechanism of LS. We assume that the primary harmful factor is hypocapnia (decrease in pCO₂) and respiratory alkalosis (increase in pH) due to hyperventilation, permanent or in response to stress. Inside mitochondria, the pH signal of high pH/low bicarbonate ion (HCO^-₃) is transmitted by soluble adenyl cyclase (sAC) through cAMP dependent manner. The process can initiate brain lesions (necrosis, apoptosis, hypervascularity) in OXPHOS deficient cells residing at the LS area of the brain. The major message of the article is that it is not the ATP depletion but intracellular alkalization (and/or hyperoxia?) which seem to be the cause of LS. The paper includes suggestions concerning the methodology for further research on the LS mechanism and for therapeutic strategy.

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

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

Spatial resolution of cAMP signaling by soluble adenylyl cyclase

G protein–coupled receptor signaling starts at the plasma membrane and continues at endosomal stations. In this issue, Inda et al. (2016. J. Cell Biol.http://dx.doi.org/10.1083/jcb.201512075) show that different forms of adenylyl cyclase are activated at the plasma membrane versus endosomes, providing a rationale for the spatial encoding of cAMP signaling.