Activation of the cyclic AMP pathway promotes serotonin-induced Ca2+ oscillations in salivary glands of the blowfly Calliphora vicina

The Interactions among Hypertension, Cancer, and COVID-19: Perspectives from Ca2+/cAMP Signalling

BACKGROUND

The hypothesis that hypertension is clinically associated with an enhanced risk for developing cancer has been highlighted. However, the working principles involved in this link are still under intensive discussion. A correlation among inflammation, hypertension, and cancer could accurately describe the clinical link between these diseases. In addition, a dyshomeostasis of Ca²⁺ has been considered as a topic involved in both cancer and hypertension and inflammation. There is a strong link between Ca²⁺ signalling, e.g. enhanced Ca²⁺ signals, and inflammatory outcomes. cAMP also modulates pro- and anti-inflammatory outcomes: pharmaceuticals, which increase intracellular cAMP levels, can decrease the production of proinflammatory mediators and enhance the production of anti-inflammatory outcomes.

OBJECTIVE

This article has discussed the participation of Ca²⁺/cAMP signalling in the clinical association among inflammation, hypertension, and an enhanced risk for the development of cancer. In addition, considering coronavirus disease 2019 (COVID-19) is a rapidly evolving field, this article also reviews recent reports about the role of Ca²⁺ channel blockers for restoring Ca²⁺ signalling disruption due to COVID-19, including the relationship among COVID-19, cancer, and hypertension.

CONCLUSION

Understanding the association among these diseases could expand current pharmacotherapy, including that involving Ca²⁺ channel blockers and pharmaceuticals which rise cAMP levels.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

cAMP-Dependent Calcium Oscillations of Astrocytes: An Implication for Pathology

Astrocytes in various brain regions exhibit spontaneous intracellular calcium elevations both in vitro and in vivo; however, neither the temporal pattern underlying this activity nor its function has been fully evaluated. Here, we utilized a long-term optical imaging technique to analyze the calcium activity of more than 4000 astrocytes in acute hippocampal slices as well as in the neocortex and hippocampus of head-restrained mice. Although astrocytic calcium activity was largely sparse and irregular, we observed a subset of cells in which the fluctuating calcium oscillations repeated at a regular interval of ∼30 s. These intermittent oscillations i) depended on type 2 inositol 1,4,5-trisphosphate receptors; ii) consisted of a complex reverberatory interaction between the soma and processes of individual astrocytes; iii) did not synchronize with those of other astrocytes; iv) did not require neuronal firing; v) were modulated through cAMP-protein kinase A signaling; vi) were facilitated under pathological conditions, such as energy deprivation and epileptiform hyperexcitation; and vii) were associated with enhanced hypertrophy in astrocytic processes, an early hallmark of reactive gliosis, which is observed in ischemia and epilepsy. Therefore, calcium oscillations appear to be associated with a pathological state in astrocytes.

Physiological characterization and regulation of the contractile properties of the mosquito ventral diverticulum (crop)

In adult dipteran insects (flies), the crop is a diverticulum of the esophagus that serves as a food storage organ. The crop pumps stored contents into the alimentary canal for digestion and absorption. The pumping is mediated by peristaltic contractions of the crop musculature. In adult female mosquitoes, the crop (ventral diverticulum) selectively stores sugar solutions (e.g., nectar); proteinaceous blood meals by-pass the crop and are transferred directly to the midgut for digestion. The mechanisms that regulate crop contractions have never been investigated in mosquitoes. Here we provide the first physiological characterization of the contractile properties of the mosquito crop and explore the mechanisms that regulate crop contractions. Using an in vitro bioassay we found that the isolated crop spontaneously contracts in Ringer solution for at least 1 h and its contractions are dependent on extracellular Ca2+. Adding serotonin (5-hydroxytryptamine, 5-HT) or a membrane-permeable analog of cyclic adenosine monophosphate (cAMP) to the extracellular bath increased the frequency of crop contractions. On the other hand, adding benzethonium chloride (BzCl; a chemical that mimics the effects of myosuppressins), H-89 or Rp-cAMPS (inhibitors of protein kinase A, PKA), or carbenoxolone (an inhibitor of gap junctions) reduced the frequency of the unstimulated, spontaneous and/or 5-HT-stimulated crop contractions. Adding aedeskinin III did not detectably alter crop contraction rates. In addition to pharmacological evidence of gap junctions, we demonstrated that the crop expressed several mRNAs encoding gap junctional proteins (i.e. innexins). Furthermore, we localized immunoreactivity for innexin 2 and innexin 3 to muscle and epithelial cells of the crop, respectively. Our results 1) suggest that 5-HT and myosupressins oppositely regulate contractile activity of the mosquito crop, and 2) provide the first evidence for putative roles of cAMP, PKA, and gap junctions in modulating contractile activity of the dipteran crop.

Exchange proteins directly activated by cAMP (EPACs): Emerging therapeutic targets

Exchange proteins directly activated by cAMP (EPACs) are critical cAMP-dependent signaling pathway mediators. The discovery of EPAC proteins has significantly facilitated understanding on cAMP-dependent signaling pathway and efforts along this line open new avenues for developing novel therapeutics for cancer, diabetes, heart failure, inflammation, infections, neurological disorders and other human diseases. Over the past decade, important progress has been made in the identification of EPAC agonists, antagonists and their biological and pharmacological applications. In this review, we briefly summarize recently reported novel functions of EPACs and the discovery of their small molecule modulators. The challenges and future perspectives are also discussed.

Pharmacological implications of the Ca(2+)/cAMP signaling interaction: from risk for antihypertensive therapy to potential beneficial for neurological and psychiatric disorders

In this review, we discussed pharmacological implications of the Ca²⁺/cAMP signaling interaction in the antihypertensive and neurological/psychiatric disorders therapies. Since 1975, several clinical studies have reported that acute and chronic administration of L-type voltage-activated Ca²⁺ channels (VACCs) blockers, such as nifedipine, produces reduction in peripheral vascular resistance and arterial pressure associated with an increase in plasma noradrenaline levels and heart rate, typical of sympathetic hyperactivity. Despite this sympathetic hyperactivity has been initially attributed to adjust reflex of arterial pressure, the cellular and molecular mechanisms involved in this apparent sympathomimetic effect of the L-type VACCs blockers remained unclear for decades. In addition, experimental studies using isolated tissues richly innervated by sympathetic nerves (to exclude the influence of adjusting reflex) showed that neurogenic responses were completely inhibited by L-type VACCs blockers in concentrations above 1 μmol/L, but paradoxically potentiated in concentrations below 1 μmol/L. During almost four decades, these enigmatic phenomena remained unclear. In 2013, we discovered that this paradoxical increase in sympathetic activity produced by L-type VACCs blocker is due to interaction of the Ca²⁺/cAMP signaling pathways. Then, the pharmacological manipulation of the Ca²⁺/cAMP interaction produced by combination of the L-type VACCs blockers used in the antihypertensive therapy, and cAMP accumulating compounds used in the antidepressive therapy, could represent a potential cardiovascular risk for hypertensive patients due to increase in sympathetic hyperactivity. In contrast, this pharmacological manipulation could be a new therapeutic strategy for increasing neurotransmission in psychiatric disorders, and producing neuroprotection in the neurodegenerative diseases.

Calcineurin is part of a negative feedback loop in the InsP3/Ca²⁺ signalling pathway in blowfly salivary glands

The ubiquitous InsP3/Ca(2+) signalling pathway is modulated by diverse mechanisms, i.e. feedback of Ca(2+) and interactions with other signalling pathways. In the salivary glands of the blowfly Calliphora vicina, the hormone serotonin (5-HT) causes a parallel rise in intracellular [Ca(2+)] and [cAMP] via two types of 5-HT receptors. We have shown recently that cAMP/protein kinase A (PKA) sensitizes InsP3-induced Ca(2+) release. We have now identified the protein phosphatase that counteracts the effect of PKA on 5-HT-induced InsP3/Ca(2+) signalling. We demonstrate that (1) tautomycin and okadaic acid, inhibitors of protein phosphatases PP1 and PP2A, have no effect on 5-HT-induced Ca(2+) signals; (2) cyclosporin A and FK506, inhibitors of Ca(2+)/calmodulin-activated protein phosphatase calcineurin, cause an increase in the frequency of 5-HT-induced Ca(2+) oscillations; (3) the sensitizing effect of cyclosporin A on 5-HT-induced Ca(2+) responses does not involve Ca(2+) entry into the cells; (4) cyclosporin A increases InsP3-dependent Ca(2+) release; (5) inhibition of PKA abolishes the effect of cyclosporin A on the 5-HT-induced Ca(2+) responses, indicating that PKA and calcineurin act antagonistically on the InsP3/Ca(2+) signalling pathway. These findings suggest that calcineurin provides a negative feedback on InsP3/Ca(2+) signalling in blowfly salivary glands, counteracting the effect of PKA and desensitizing the signalling cascade at higher 5-HT concentrations.

Serotonin triggers cAMP and PKA-mediated intracellular calcium waves in Malpighian tubules of Rhodnius prolixus

Rhodnius prolixus is a hematophagous insect vector of Chagas disease capable of ingesting up to 10 times its unfed body weight in blood in a single meal. The excess water and ions ingested with the meal are expelled through a rapid postprandial diuresis driven by the Malpighian tubules. Diuresis is triggered by at least two diuretic hormones, a CRF-related peptide and serotonin, which were traditionally believed to trigger cAMP as an intracellular second messenger. Recently, calcium has been suggested to act as a second messenger in serotonin-stimulated Malpighian tubules. Thus, we tested the role of calcium in serotonin-stimulated Malpighian tubules from R. prolixus. Our results show that serotonin triggers cAMP-mediated intracellular Ca(2+) waves that were blocked by incubation in Ca(2+)-free saline containing the cell membrane-permeant Ca(2+) chelator BAPTA-AM, or the PKA blocker H-89. Treatment with 8-Br-cAMP triggered Ca(2+) waves that were blocked by H-89 and BAPTA-AM. Analysis of the secreted fluid in BAPTA-AM-treated tubules showed a 75% reduction in fluid secretion rate with increased K(+) concentration, reduced Na(+) concentration. Taken together, the results indicate that serotonin triggers cAMP and PKA-mediated Ca(2+) waves that are required for maximal ion transport rate.

cAMP and Ca²⁺ signaling in secretory epithelia: crosstalk and synergism

The Ca(2+) and cAMP/PKA pathways are the primary signaling systems in secretory epithelia that control virtually all secretory gland functions. Interaction and crosstalk in Ca(2+) and cAMP signaling occur at multiple levels to control and tune the activity of each other. Physiologically, Ca(2+) and cAMP signaling operate at 5-10% of maximal strength, but synergize to generate the maximal response. Although synergistic action of the Ca(2+) and cAMP signaling is the common mode of signaling and has been known for many years, we know very little of the molecular mechanism and mediators of the synergism. In this review, we discuss crosstalk between the Ca(2+) and cAMP signaling and the function of IRBIT (IP3 receptors binding protein release with IP3) as a third messenger that mediates the synergistic action of the Ca(2+) and cAMP signaling.