Regulation of IP3 receptors by cyclic AMP

The impact of GLP-1 signalling on the energy metabolism of pancreatic islet β-cells and extrapancreatic tissues

Glucagon-like peptide-1 signalling impacts glucose homeostasis and appetite thereby indirectly affecting substrate availability at the whole-body level. The incretin canonically produces an insulinotropic effect, thereby lowering blood glucose levels by promoting the uptake and inhibiting the production of the sugar by peripheral tissues. Likewise, GLP-1 signalling within the central nervous system reduces the appetite and food intake, whereas its gastric effect delays the absorption of nutrients, thus improving glycaemic control and reducing the risk of postprandial hyperglycaemia. We review the molecular aspects of the GLP-1 signalling, focusing on its impact on intracellular energy metabolism. Whilst the incretin exerts its effects predominantly via a Gs receptor, which decodes the incretin signal into the elevation of intracellular cAMP levels, the downstream signalling cascades within the cell, acting on fast and slow timescales, resulting in an enhancement or an attenuation of glucose catabolism, respectively.

Molecular mechanisms of saliva secretion and hyposecretion

The exocrine salivary gland secretes saliva, a fundamental body component to maintain oral homeostasis. Saliva is composed of water, ions, and proteins such as amylase, mucins, and immunoglobulins that play essential roles in the digestion of food, lubrication, and prevention of dental caries and periodontitis. An increasing number of people experience saliva hyposecretion due to aging, medications, Sjögren's syndrome, and radiation therapy for head and neck cancer. However, current treatments are mostly limited to temporary symptomatic relief. This review explores the molecular mechanisms underlying saliva secretion and hyposecretion to provide insight into putative therapeutic targets for treatment. Proteins implicated in saliva secretion pathways, including Ca2+ -signaling proteins, aquaporins, soluble N-ethylmaleimide-sensitive factor attachment protein receptors, and tight junctions, are aberrantly expressed and localized in patients with saliva hyposecretion, such as Sjögren's syndrome. Analysis of studies on the mechanisms of saliva secretion and hyposecretion suggests that crosstalk between fluid and protein secretory pathways via Ca2+ /protein kinase C and cAMP/protein kinase A regulates saliva secretion. Impaired crosstalk between the two secretory pathways may contribute to saliva hyposecretion. Future research into the detailed regulatory mechanisms of saliva secretion and hyposecretion may provide information to define novel targets and generate therapeutic strategies for saliva hyposecretion.

Agonist-Induced Ca2+ Signaling in HEK-293-Derived Cells Expressing a Single IP3 Receptor Isoform

In mammals, three genes encode IP3 receptors (IP3Rs), which are involved in agonist-induced Ca2+ signaling in cells of apparently all types. Using the CRISPR/Cas9 approach for disruption of two out of three IP3R genes in HEK-293 cells, we generated three monoclonal cell lines, IP3R1-HEK, IP3R2-HEK, and IP3R3-HEK, with the single functional isoform, IP3R1, IP3R2, and IP3R3, respectively. All engineered cells responded to ACh with Ca2+ transients in an "all-or-nothing" manner, suggesting that each IP3R isotype was capable of mediating CICR. The sensitivity of cells to ACh strongly correlated with the affinity of IP3 binding to an IP3R isoform they expressed. Based on a mathematical model of intracellular Ca2+ signals induced by thapsigargin, a SERCA inhibitor, we developed an approach for estimating relative Ca2+ permeability of Ca2+ store and showed that all three IP3R isoforms contributed to Ca2+ leakage from ER. The relative Ca2+ permeabilities of Ca2+ stores in IP3R1-HEK, IP3R2-HEK, and IP3R3-HEK cells were evaluated as 1:1.75:0.45. Using the genetically encoded sensor R-CEPIA1er for monitoring Ca2+ signals in ER, engineered cells were ranged by resting levels of stored Ca2+ as IP3R3-HEK ≥ IP3R1-HEK > IP3R2-HEK. The developed cell lines could be helpful for further assaying activity, regulation, and pharmacology of individual IP3R isoforms.

Structural titration reveals Ca2+-dependent conformational landscape of the IP3 receptor

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are endoplasmic reticulum Ca2+ channels whose biphasic dependence on cytosolic Ca2+ gives rise to Ca2+ oscillations that regulate fertilization, cell division and cell death. Despite the critical roles of IP3R-mediated Ca2+ responses, the structural underpinnings of the biphasic Ca2+ dependence that underlies Ca2+ oscillations are incompletely understood. Here, we collect cryo-EM images of an IP3R with Ca2+ concentrations spanning five orders of magnitude. Unbiased image analysis reveals that Ca2+ binding does not explicitly induce conformational changes but rather biases a complex conformational landscape consisting of resting, preactivated, activated, and inhibited states. Using particle counts as a proxy for relative conformational free energy, we demonstrate that Ca2+ binding at a high-affinity site allows IP3Rs to activate by escaping a low-energy resting state through an ensemble of preactivated states. At high Ca2+ concentrations, IP3Rs preferentially enter an inhibited state stabilized by a second, low-affinity Ca2+ binding site. Together, these studies provide a mechanistic basis for the biphasic Ca2+-dependence of IP3R channel activity.

A cocktail of small molecules maintains the stemness and differentiation potential of conjunctival epithelial cells

PURPOSE

The conjunctival epithelial cells cultured with bovine serum or feeder cells were not suitable for clinical application. Therefore, we developed a novel serum-free and feeder cell-free culture system containing only a cocktail of three chemicals (3C) to expand the conjunctival epithelial cells.

METHODS

The cell proliferative ability was evaluated by counting, crystal violet staining and Ki67 immunostaining. Co-staining of K7 and MUC5AC was performed to identify goblet cells. PAS staining was used to assess the ability of cells to synthesis and secrete glycoproteins. In vivo, eye drops containing 3C was administered to verify the role of 3C in the mouse conjunctival injury model. PAS, HE and immunofluorescence staining were performed to show conjunctival epithelial repair.

RESULTS

Compared with other small molecule groups and the serum group, the cells in 3C group showed superior morphology and proliferative ability. Meanwhile, 3C maintained the well-proliferative capacity of cells even after fifth passage. The 3C group also exhibited more K7 and MUC5AC double positive cells, and the PAS staining positive areas were present in both the cytoplasm and extracellular matrix. The cell sheets treated with 3C in air-lifted culture were obviously stratified. In vivo, more goblet cells in the conjunctival epithelium were observed in the 3C group.

CONCLUSION

Overall, our culture system can expand the conjunctival epithelial cells and retain their potential to differentiate into mature goblet cells, which provided a promising source of seed cells for conjunctival reconstruction. Furthermore, this system provides new insights for the clinical treatment of ocular surface diseases.

Blockage of calcium-sensing receptor improves chronic intermittent hypoxia-induced cognitive impairment by PERK-ATF4-CHOP pathway

Obstructive sleep apnea-hypopnea syndrome (OSAHS) is involved in cognitive impairment of children. Chronic intermittent hypoxia (CIH) is considered as the critical pathophysiological mechanism of OSAHS. Calcium sensitive receptor (CaSR) mediated apoptosis in many neurological disease models by endoplasmic reticulum stress (ERS)-related pathway. However, little is known about the role of CaSR in OSAHS-induced cognitive dysfunction. In this study, we explored the effect of CaSR on CIH-induced cognitive impairment and possible mechanisms on regulation of PERK-ATF4-CHOP pathway in vivo and in vitro. CIH exposed for 9 h in PC12 cells and resulted in the cell apoptosis, simulating OSAHS-induced neuronal injury. CIH upregulated the level of CaSR, p-PERK, ATF4 and CHOP, contributing to the cell apoptosis. Treated with CaSR inhibitor (NPS-2143) or p-PERK inhibitor (GSK2656157) before CIH exposure, CIH-induced PC12 cell apoptosis was alleviated via inhibition of CaSR by downregulating p-PERK, ATF4 and CHOP. In addition, we established CIH mice model. With CIH exposure for 4 weeks in mice, more spatial memory errors were observed during 8-arm radial maze test. CIH significantly increased apoptotic cells in hippocampus via upregulating cleaved Caspase-3 and downregulating ratio of Bcl-2 to Bax. Besides, treatment of CaSR inhibitor alleviated the hippocampal neuronal apoptosis following CIH with downregulated p-PERK, ATF4 and CHOP, suggesting that CaSR contributed to CIH-induced neuronal apoptosis in hippocampus via ERS pathway. Sum up, our results demonstrated that CaSR accelerated hippocampal apoptosis via PERK-ATF4-CHOP pathway, holding a critical function on CIH-mediated cognitive impairment. Conversely, inhibition of CaSR suppressed PERK-ATF4-CHOP pathway and alleviated cognitive impairment.

Interaction of calcium responsive proteins and transcriptional factors with the PHO regulon in yeasts and fungi

Phosphate and calcium ions are nutrients that play key roles in growth, differentiation and the production of bioactive secondary metabolites in filamentous fungi. Phosphate concentration regulates the biosynthesis of hundreds of fungal metabolites. The central mechanisms of phosphate transport and regulation, mediated by the master Pho4 transcriptional factor are known, but many aspects of the control of gene expression need further research. High ATP concentration in the cells leads to inositol pyrophosphate molecules formation, such as IP3 and IP7, that act as phosphorylation status reporters. Calcium ions are intracellular messengers in eukaryotic organisms and calcium homeostasis follows elaborated patterns in response to different nutritional and environmental factors, including cross-talking with phosphate concentrations. A large part of the intracellular calcium is stored in vacuoles and other organelles forming complexes with polyphosphate. The free cytosolic calcium concentration is maintained by transport from the external medium or by release from the store organelles through calcium permeable transient receptor potential (TRP) ion channels. Calcium ions, particularly the free cytosolic calcium levels, control the biosynthesis of fungal metabolites by two mechanisms, 1) direct interaction of calcium-bound calmodulin with antibiotic synthesizing enzymes, and 2) by the calmodulin-calcineurin signaling cascade. Control of very different secondary metabolites, including pathogenicity determinants, are mediated by calcium through the Crz1 factor. Several interactions between calcium homeostasis and phosphate have been demonstrated in the last decade: 1) The inositol pyrophosphate IP3 triggers the release of calcium ions from internal stores into the cytosol, 2) Expression of the high affinity phosphate transporter Pho89, a Na+/phosphate symporter, is controlled by Crz1. Also, mutants defective in the calcium permeable TRPCa7-like of Saccharomyces cerevisiae shown impaired expression of Pho89. This information suggests that CrzA and Pho89 play key roles in the interaction of phosphate and calcium regulatory pathways, 3) Finally, acidocalcisomes organelles have been found in mycorrhiza and in some melanin producing fungi that show similar characteristics as protozoa calcisomes. In these organelles there is a close interaction between orthophosphate, pyrophosphate and polyphosphate and calcium ions that are absorbed in the polyanionic polyphosphate matrix. These advances open new perspectives for the control of fungal metabolism.

Multiple Subthreshold GPCR Signals Combined by the G-Proteins Gαq and Gαs Activate the Caenorhabditis elegans Egg-Laying Muscles

Individual neurons or muscle cells express many G-protein-coupled receptors (GPCRs) for neurotransmitters and neuropeptides, yet it remains unclear how cells integrate multiple GPCR signals that all must activate the same few G-proteins. We analyzed this issue in the Caenorhabditis elegans egg-laying system, where multiple GPCRs on muscle cells promote contraction and egg laying. We genetically manipulated individual GPCRs and G-proteins specifically in these muscle cells within intact animals and then measured egg laying and muscle calcium activity. Two serotonin GPCRs on the muscle cells, Gαq-coupled SER-1 and Gαs-coupled SER-7, together promote egg laying in response to serotonin. We found that signals produced by either SER-1/Gαq or SER-7/Gαs alone have little effect, but these two subthreshold signals combine to activate egg laying. We then transgenically expressed natural or designer GPCRs in the muscle cells and found that their subthreshold signals can also combine to induce muscle activity. However, artificially inducing strong signaling through just one of these GPCRs can be sufficient to induce egg laying. Knocking down Gαq and Gαs in the egg-laying muscle cells induced egg-laying defects that were stronger than those of a SER-1/SER-7 double knockout, indicating that additional endogenous GPCRs also activate the muscle cells. These results show that in the egg-laying muscles multiple GPCRs for serotonin and other signals each produce weak effects that individually do not result in strong behavioral outcomes. However, they combine to produce sufficient levels of Gαq and Gαs signaling to promote muscle activity and egg laying.SIGNIFICANCE STATEMENT How can neurons and other cells gather multiple independent pieces of information from the soup of chemical signals in their environment and compute an appropriate response? Most cells express >20 GPCRs that each receive one signal and transmit that information through three main types of G-proteins. We analyzed how this machinery generates responses by studying the egg-laying system of C. elegans, where serotonin and multiple other signals act through GPCRs on the egg-laying muscles to promote muscle activity and egg laying. We found that individual GPCRs within an intact animal each generate effects too weak to activate egg laying. However, combined signaling from multiple GPCR types reaches a threshold capable of activating the muscle cells.

Piezo1 induces endothelial responses to shear stress via soluble adenylyl Cyclase-IP3R2 circuit

Endothelial cells (ECs) continuously sense and adapt to changes in shear stress generated by blood flow. Here, we show that the activation of the mechanosensitive channel Piezo1 by defined shear forces induces Ca2+ entry into the endoplasmic reticulum (ER) via the ER Ca2+ ATPase pump. This entry is followed by inositol trisphosphate receptor 2 (IP3R2)-elicited ER Ca2+ release into the cytosol. The mechanism of ER Ca2+ release involves the generation of cAMP by soluble adenylyl cyclase (sAC), leading to IP3R2-evoked Ca2+ gating. Depleting sAC or IP3R2 prevents ER Ca2+ release and blocks EC alignment in the direction of flow. Overexpression of constitutively active Akt1 restores the shear-induced alignment of ECs lacking Piezo1 or IP3R2, as well as the flow-induced vasodilation in endothelial restricted Piezo1 knockout mice. These studies describe an unknown Piezo1-cAMP-IP3R2 circuit as an essential mechanism activating Akt signaling and inducing adaptive changes in ECs to laminar flow.

Pathological changes in GPCR signal organisation: Opportunities for targeted therapies for triple negative breast cancer

Triple negative breast cancer (TNBC) has the poorest prognosis compared to other breast cancer subtypes, due to a historical lack of targeted therapies and high rates of relapse. Greater insight into the components of signalling pathways in TNBC tumour cells has led to the clinical evaluation, and in some cases approval, of targeted therapies. In the last decade, G protein-coupled receptors, such as the β₂-adrenoceptor, have emerged as potential new therapeutic targets. Here, we describe how the β₂-adrenoceptor accelerates TNBC progression in response to stress, and the unique signalling pathway activated by the β₂-adrenoceptor to drive the invasion of an aggressive TNBC tumour cell. We highlight evidence that supports an altered organisation of GPCRs in tumour cells, and suggests that activation of the same GPCR in a different cellular location can control unique cell responses. Finally, we speculate how the relocation of GPCRs to the "wrong" place in tumour cells presents opportunities to develop targeted anti-cancer GPCR drugs with greater efficacy and minimal adverse effects.

Functional Insights into Protein Kinase A (PKA) Signaling from C. elegans

Protein kinase A (PKA), which regulates a diverse set of biological functions downstream of cyclic AMP (cAMP), is a tetramer consisting of two catalytic subunits (PKA-C) and two regulatory subunits (PKA-R). When cAMP binds the PKA-R subunits, the PKA-C subunits are released and interact with downstream effectors. In Caenorhabditis elegans (C. elegans), PKA-C and PKA-R are encoded by kin-1 and kin-2, respectively. This review focuses on the contributions of work in C. elegans to our understanding of the many roles of PKA, including contractility and oocyte maturation in the reproductive system, lipid metabolism, physiology, mitochondrial function and lifespan, and a wide variety of behaviors. C. elegans provides a powerful genetic platform for understanding how this kinase can regulate an astounding variety of physiological responses.

Mice lacking 1,4,5-triphosphate inositol type III receptor demonstrate inhibition of hypoxic pulmonary hypertension

Hypoxic pulmonary hypertension (HPH) is a respiratory disease characterized by increased pulmonary vascular resistance and pulmonary arterial pressure. Persistent hypoxia alters the metabolic and transport functions of endothelial cells and promotes thrombosis and inflammation. Type 3 inositol-1,4,5-trisphosphate receptor (IP3R3) controls the release of calcium ions from the endoplasmic reticulum to the cytoplasm and mitochondria and is involved in cell proliferation, migration, and protein synthesis. In this study, we investigated the role and function of IP3R3 in HPH. The results showed that the expression level of IP3R3 was increased in pulmonary artery endothelial cells (PAECs) in a rat HPH model. The pulmonary artery pressure indices of IP3R3^(-/-) mice with persistent hypoxia were significantly lower than those of HPH mice. The expression level of IP3R3 was significantly increased in hypoxia-treated PAECs. Knockdown of IP3R3 significantly inhibited the proliferation, migration and mesenchymal transition of PAECs induced by hypoxia. In conclusion, knockdown of IP3R3 can inhibit hypoxia-induced dysfunctions in PAECs, thus enabling IP3R3^(-/-) mice to avoid HPH development. IP3R3 plays a key role in HPH and can be used as a potential target for the prevention and treatment of HPH.

Resolvin D2 uses multiple Ca2+ -dependent signaling pathways to stimulate mucin secretion in rat and human conjunctival goblet cells

The mucin layer of the tear film is produced by goblet cells in the conjunctiva to protect the ocular surface and maintain homeostasis. The pro-resolving lipid mediator resolvin D2 (RvD2) biosynthesized from an omega 3 fatty acid actively terminates inflammation and regulates mucin secretion from conjunctival goblet cells. Our objective was to determine which Ca2+ -dependent signaling pathways RvD2 uses to stimulate conjunctival goblet cell function (CGC). We hypothesize that RvD2 activates multiple intracellular Ca2+ signaling pathways to stimulate CGC secretion. Rat and human CGCs were cultured from conjunctival explants. The amount of RvD2 receptor GPR18/DRV2 message and protein were determined. The intracellular concentration of Ca2+ ([Ca2+ ]i ) was measured in CGCs using a fluorescent Ca2+ dye and mucin secretion was determined by measuring protein secretion enzymatically with a lectin. Goblet cells were incubated with signaling pathway inhibitors before stimulation with RvD2 and [Ca2+ ]i or secretion was measured. In rat and human CGCs RvD2 receptor and in rat CGCs IP3 (a molecule that releases Ca2+ from intracellular organelles) receptors 1-3 were detected. In both species of CGC RvD2 increased [Ca2+ ]i similarly to RvD1. In rat CGCs, the increase in [Ca2+ ]i and secretion stimulated by RvD2 was significantly blocked by inhibitors to phospholipase (PL-) C and IP3 -receptor, but not protein kinase C. Increase in [Ca2+ ]i was blocked by the PLD inhibitor, but not the PLA2 inhibitor. Secretion was blocked by PLA2 inhibitor, but not the PLD inhibitor. An inhibitor of the epidermal growth factor receptor blocked the increase in [Ca2+ ]i by RvD2 in both species of CGCs. In CGCs RvD2 activates multiple intracellular signaling pathways that are Ca2+ -dependent, along with one Ca2+ -independent and one cAMP/protein kinase A-dependent pathway. Activation of these pathways stimulate mucin secretion from rat and human CGCs into the tear film contributing to ocular surface homeostasis and health.

A2A R mediated modulation in IP3 levels altering the [Ca2+]i through cAMP-dependent PKA signalling pathway

Stimulation of A_2A receptors (A_2A R) coupled to G_s/olf protein activates Adenylyl cyclase (AC) leading to the release of cAMP which activates the cAMP-dependent PKA phosphorylation. The possible role of A_2A R in the modulation of free cytosolic Ca²⁺ concentration ([Ca²⁺]_i) involving IP₃, cAMP and PKA was investigated in HEK 293-A_2A R. The levels of IP₃ and cAMP were observed by enzyme immunoassay detection method and [Ca²⁺]_i using Fluo-4 AM. Moreover, cAMP-dependent PKA was determined using the PKA Colorimetric Activity Kit. We observed that the cells pre-treated with A_2A R agonist NECA showed increased levels of cAMP, PKA, IP₃ and [Ca²⁺]_i levels. However, the reverse effect was observed with A_2A R antagonists (ZM241385 and caffeine). Blocking the G_αq/PLC/DAG/IP₃ pathway with neomycin, a PLC inhibitor did not affect the modulation of IP₃ and [Ca²⁺]_i levels in HEK 293-A_2A R cells. To investigate the G_αi/AC/cAMP/PKA, HEK 293-A_2A R cells pre-treated with pertussis toxin followed by forskolin in the presence of A_2A R agonist (NECA) showed no effect on cAMP levels. Further, G_αs/AC/cAMP/PKA pathway was investigated to elucidate the role of cAMP-dependent PKA in IP₃ mediated [Ca²⁺]_i modulation. In the HEK 293-A_2A R cells pre-treated with PKA inhibitor KT5720 and treated with NECA led to inhibit the IP₃ and [Ca²⁺]_i levels. The study distinctly demonstrated that A_2A R modulates IP₃ levels to release the [Ca²⁺]_i via cAMP-dependent PKA. The role of A_2A R mediated G_αs pathway inducing IP₃ mediated [Ca²⁺]_i release may open new avenues in the therapy of neurodegenerative disorder.

Spatial and temporal crosstalk between the cAMP and Ca2+ signaling systems

The ubiquitous secondary messengers, Ca²⁺ and cAMP, play a vital role in shaping a diverse array of physiological processes. More significantly, accumulating evidence over the past several decades underpin extensive crosstalk between these two canonical messengers in discrete sub-cellular nanodomains across various cell types. Within such specialized nanodomains, each messenger fine-tunes signaling to maintain homeostasis by manipulating the activities of cellular machinery accountable for the metabolism or activity of the complementary pathway. Interaction between these messengers is ensured by scaffolding proteins which tether components of the signaling machinery in close proximity. Disruption of dynamic communications between Ca²⁺ and cAMP at these loci consequently is linked to several pathological conditions. This review summarizes recent novel mechanisms underlying effective crosstalk between Ca²⁺ and cAMP in such nanodomains.

VEGF-Mediated Augmentation of Autophagic and Lysosomal Activity in Endothelial Cells Defends against Intracellular Streptococcus pyogenes

Group A Streptococcus (GAS), a deleterious human-pathogenic bacterium, causes life-threatening diseases such as sepsis and necrotic fasciitis. We recently reported that GAS survives and replicates within blood vessel endothelial cells because these cells are intrinsically defective in xenophagy. Because blood vessel endothelial cells are relatively germfree environments, specific stimulation may be required to sufficiently induce xenophagy. Here, we explored how vascular endothelial growth factor (VEGF) promoted xenophagy and lysosomal activity in endothelial cells. These effects were achieved by amplifying the activation of TFEB, a transcriptional factor crucial for lysosome/autophagy biogenesis, via cAMP-mediated calcium release. In a mouse model of local infection with GAS, the VEGF level was significantly elevated at the infection site. Interestingly, low serum VEGF levels were found in a mouse model of invasive bacteremia and in patients with severe GAS-induced sepsis. Moreover, the administration of VEGF improved the survival of GAS-infected mice. We propose a novel theory regarding GAS infection in endothelial cells, wherein VEGF concentrations in the systemic circulation play a critical role.

Inhibition of IP3R3 attenuates endothelial to mesenchymal transition induced by TGF-β1 through restoring mitochondrial function

Pulmonary arterial hypertension (PAH) is a progressive disease characterized by elevated pulmonary artery pressure and right ventricular hypertrophy. Inositol 1,4,5-trisphosphate receptors (IP3Rs) release calcium ions from the endoplasmic reticulum to regulate permeability and migration of endothelial, thereby affecting PAH. In this study, We determined the expression level of IP₃R3 and its position in lung tissue from PAH rat models, and stud the effect of IP₃R3 on endothelial to mesenchymal transition (EndMT) and mitochondrial function of endothelial cells treated with TGF-β1. We observed that IP₃R3 was significantly overexpressed in the lung tissues from PAH rat models. Inhibition of IP₃R3 reduced EndMT markers, cell migration, ROS production, Ca²⁺ levels, increased mitochondrial membrane potential and mitochondrial respiratory chain complex I, III, and V activities. These results suggest that the inhibition of IP₃R3 attenuated EndMT and migration induced by TGF-β1 via restoring of mitochondrial functions, thereby suggesting a novel therapeutic opportunity for PAH.

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.

Förster Resonance Energy Transfer-Based Single-Cell Imaging Reveals Piezo1-Induced Ca2+ Flux Mediates Membrane Ruffling and Cell Survival

A mechanosensitive ion channel, Piezo1 induces non-selective cation flux in response to various mechanical stresses. However, the biological interpretation and underlying mechanisms of cells resulting from Piezo1 activation remain elusive. This study elucidates Piezo1-mediated Ca²⁺ influx driven by channel activation and cellular behavior using novel Förster Resonance Energy Transfer (FRET)-based biosensors and single-cell imaging analysis. Results reveal that extracellular Ca²⁺ influx via Piezo1 requires intact caveolin, cholesterol, and cytoskeletal support. Increased cytoplasmic Ca²⁺ levels enhance PKA, ERK, Rac1, and ROCK activity, which have the potential to promote cancer cell survival and migration. Furthermore, we demonstrate that Piezo1-mediated Ca²⁺ influx upregulates membrane ruffling, a characteristic feature of cancer cell metastasis, using spatiotemporal image correlation spectroscopy. Thus, our findings provide new insights into the function of Piezo1, suggesting that Piezo1 plays a significant role in the behavior of cancer cells.

"ADPKD-omics": determinants of cyclic AMP levels in renal epithelial cells

The regulation of cyclic adenosine monophosphate (cAMP) levels in kidney epithelial cells is important in at least 2 groups of disorders, namely water balance disorders and autosomal dominant polycystic kidney disease. Focusing on the latter, we review genes that code for proteins that are determinants of cAMP levels in cells. We identify which of these determinants are expressed in the 14 kidney tubule segments using recently published RNA-sequencing and protein mass spectrometry data ("autosomal dominant polycystic kidney disease-omics"). This includes G protein-coupled receptors, adenylyl cyclases, cyclic nucleotide phosphodiesterases, cAMP transporters, cAMP-binding proteins, regulator of G protein-signaling proteins, G protein-coupled receptor kinases, arrestins, calcium transporters, and calcium-binding proteins. In addition, compartmentalized cAMP signaling in the primary cilium is discussed, and a specialized database of the proteome of the primary cilium of cultured "IMCD3" cells is provided as an online resource (https://esbl.nhlbi.nih.gov/Databases/CiliumProteome/). Overall, this article provides a general resource in the form of a curated list of proteins likely to play roles in determination of cAMP levels in kidney epithelial cells and, therefore, likely to be determinants of progression of autosomal dominant polycystic kidney disease.

Cyclic Adenosine Monophosphate, Inositol 1,4,5-trisphosphate, Calcium, and Phosphorylated Myosin Light Chain Regulation Through M2 and M3 Muscarinic Receptors of Scleral Fibroblast Cells in Rat Myopia Model

AIM: This study aims to investigate the concentration of cyclic adenosine monophosphate (cAMP), inositol 1,4,5-trisphosphate (IP3), calcium (Ca2+), and the expression phosphorylated myosin light chain (MLC) in Rattus norvegicus scleral fibroblast cells. METHODOLOGY: This study utilized an in vitro experimental study by applying Rattus norvegicus scleral fibroblast cell culture. The cultured cells were divided into control and lens-induced myopia (LIM) groups. The control and LIM culture groups were each divided into five groups, namely, negative control, 0.1 μM acetylcholine, 0.1 μM himbacine, 0.1 μM methoctramine, and 0.1 μM 4-DAMP group. The cAMP, IP3, and Ca2+ concentration were analyzed in the 0th, 5th, 10th, 20th, and 30th. The phosphorylated MLC expression was analyzed using confocal microscope. RESULTS: In the LIM group, the highest cAMP concentration is visible at the 10th min on the himbacine group (0.304 ± 0; p = 0.043) and on the 4-DAMP group (0.346 ± 0; p = 0.043). The highest IP3 concentration is found on the LIM group at the 20th min in comparison to the control group (2503.6 ± 11 vs. 2039.2 ± 2.1; p = 0.046). The highest Ca2+ concentration belongs to the 4-DAMP treatment group from the 5th to the 30th min. The highest average phosphorylated MLC expression value in the LIM group is shown by the 0.1μM 4-DAMP treatment (184.2 ± 37.9c au). CONCLUSION: The regulation of cAMP, IP3, Ca2+, and phosphorylated MLC on the M2 and M3 muscarinic receptor of the scleral fibroblast cells of myopia animal models differs from normal animal models which may be due to interactions of M2 and M3 muscarinic receptor as compensation reaction or crosstalk on myopia induction.

Autocrine regulation of wound healing by ATP release and P2Y2 receptor activation

AIMS

Application of exogenous nucleotides can modulate wound healing via the activation of purinergic receptors. However, evidence for the release of endogenous nucleotides and the subsequent activation of purinergic receptors in this process has not been well defined. Therefore, the current study aimed to investigate wound-mediated nucleotide release and autocrine purinergic signalling during HaCaT keratinocyte wound closure following scratch injury.

MAIN METHODS

An in vitro scratch wound apparatus was employed to study wound healing over 24-h in the presence of modulators of ATP release, P2 receptors and pathways downstream of P2 receptor activation.

KEY FINDINGS

Adenosine 5'-triphosphate (ATP) was released from scratched cells. The ectonucleotidase apyrase and pharmacological inhibition of the nucleotide release hemichannel, pannexin-1, decreased wound closure over time. The non-selective P2Y receptor antagonist suramin and the selective P2Y₂ receptor antagonist AR-C118925XX, but not other P2 antagonists, decreased wound closure. AR-C118925XX decreased wound closure in a concentration-dependent fashion. However, exogenous P2Y₂ receptor agonists, ATP or uridine 5'-triphosphate, did not enhance wound closure. PCR and immunoblotting confirmed P2Y₂ receptor expression in HaCaT cells. U73122, a phospholipase C antagonist, and 2-aminoethoxydiphenylborate, an inositol 1,4,5-trisphosphate receptor-sensitive Ca²⁺-release channel antagonist, decreased wound closure consistent with P2Y₂ receptor activation. Absence of extracellular or intracellular Ca²⁺ or inhibition of intracellular Ca²⁺-release also impaired wound closure.

SIGNIFICANCE

These data describe a novel autocrine signalling mechanism in which wound-mediated release of endogenous ATP in response to mechanical scratching of HaCaT cells activates P2Y₂ receptors to facilitate wound closure.

Calcium Release Channels: Structure and Function of IP3 Receptors and Ryanodine Receptors

Ca²⁺-release channels are giant membrane proteins that control the release of Ca²⁺ from the endoplasmic and sarcoplasmic reticulum. The two members, ryanodine receptors (RyRs) and inositol-1,4,5-trisphosphate Receptors (IP₃Rs), are evolutionarily related and are both activated by cytosolic Ca²⁺. They share a common architecture, but RyRs have evolved additional modules in the cytosolic region. Their massive size allows for the regulation by tens of proteins and small molecules, which can affect the opening and closing of the channels. In addition to Ca²⁺, other major triggers include IP₃ for the IP₃Rs, and depolarization of the plasma membrane for a particular RyR subtype. Their size has made them popular targets for study via electron microscopic methods, with current structures culminating near 3Å. The available structures have provided many new mechanistic insights int the binding of auxiliary proteins and small molecules, how these can regulate channel opening, and the mechanisms of disease-associated mutations. They also help scrutinize previously proposed binding sites, as some of these are now incompatible with the structures. Many questions remain around the structural effects of post-translational modifications, additional binding partners, and the higher-order complexes these channels can make in situ. This review summarizes our current knowledge about the structures of Ca²⁺-release channels and how this informs on their function.

IP3 R attenuates oxidative stress and inflammation damage in smoking-induced COPD by promoting autophagy

Tobacco smoking is one of the most important risk factors for chronic obstructive pulmonary disease (COPD). However, the most critical genes and proteins remain poorly understood. Therefore, we aimed to investigate these hub genes and proteins in tobacco smoke-induced COPD, together with the potential mechanism(s). Differentially expressed genes (DEGs) were analysed between smokers and patients with COPD. mRNA expression and protein expression of IP3 R were confirmed in patients with COPD and extracted smoke solution (ESS)-treated human bronchial epithelial (HBE) cells. Moreover, expression of oxidative stress, inflammatory cytokines and/or autophagy-related protein was tested when IP3 R was silenced or overexpressed in ESS-treated and/or 3-MA-treated cells. A total of 30 DEGs were obtained between patients with COPD and smoker samples. IP3 R was identified as one of the key targets in tobacco smoke-induced COPD. In addition, IP3 R was significantly decreased in patients with COPD and ESS-treated cells. Loss of IP3 R statistically increased expression of oxidative stress and inflammatory cytokines in ESS-treated HBE cells, and overexpression of IP3 R reversed the above functions. Furthermore, the autophagy-related proteins (Atg5, LC3 and Beclin1) were statistically decreased, and p62 was increased by silencing of IP3 R cells, while overexpression of IP3 R showed contrary results. Additionally, we detected that administration of 3-MA significantly reversed the protective effects of IP3 R overexpression on ESS-induced oxidative stress and inflammatory injury. Our results suggest that IP3 R might exert a protective role against ESS-induced oxidative stress and inflammation damage in HBE cells. These protective effects might be associated with promoting autophagy.

Cross-Talk Between the Adenylyl Cyclase/cAMP Pathway and Ca2+ Homeostasis

Cyclic AMP and Ca²⁺ are the first second or intracellular messengers identified, unveiling the cellular mechanisms activated by a plethora of extracellular signals, including hormones. Cyclic AMP generation is catalyzed by adenylyl cyclases (ACs), which convert ATP into cAMP and pyrophosphate. By the way, Ca²⁺, as energy, can neither be created nor be destroyed; Ca²⁺ can only be transported, from one compartment to another, or chelated by a variety of Ca²⁺-binding molecules. The fine regulation of cytosolic concentrations of cAMP and free Ca²⁺ is crucial in cell function and there is an intimate cross-talk between both messengers to fine-tune the cellular responses. Cancer is a multifactorial disease resulting from a combination of genetic and environmental factors. Frequent cases of cAMP and/or Ca²⁺ homeostasis remodeling have been described in cancer cells. In those tumoral cells, cAMP and Ca²⁺ signaling plays a crucial role in the development of hallmarks of cancer, including enhanced proliferation and migration, invasion, apoptosis resistance, or angiogenesis. This review summarizes the cross-talk between the ACs/cAMP and Ca²⁺ intracellular pathways with special attention to the functional and reciprocal regulation between Orai1 and AC8 in normal and cancer cells.

In silico analysis of non-synonymous missense SNPs (nsSNPs) in CPE, GNAS genes and experimental validation in type II diabetes mellitus through Next Generation Sequencing

Non-synonymous missense SNPs (nsSNPs) in CPE and GNAS genes were investigated computationally. In silico identified nsSNPs were experimentally validated in type II diabetes mellitus (T2DM) in Pakistani Pathan population using next generation sequencing (NGS). Sixty two high-risk nsSNPs in CPE and 44 in GNAS were identified. Only 12 in GNAS were clinically significant. Thirty six high-risk nsSNPs in CPE and 08 clinically significant nsSNPs in GNAS lies in the most conserved regions. I-mutant predicted that nsSNPs decrease the proteins stability and ModPred predicted 20 and 12 post-translational modification sites in CPE and GNAS proteins respectively. Ramachandran plot showed 88.7% residues are in the most favored region of protein models. By experimentation, none of the nsSNPs were found to be associated with T2DM. In conclusion, this study differentiates the deleterious nsSNPs from the neutral ones. Although nsSNPs are not associated with T2DM, they can be targeted in other CPE and GNAS genes related disorders.

Neuronal metabotropic glutamate receptor 8 protects against neurodegeneration in CNS inflammation

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system with continuous neuronal loss. Treatment of clinical progression remains challenging due to lack of insights into inflammation-induced neurodegenerative pathways. Here, we show that an imbalance in the neuronal receptor interactome is driving glutamate excitotoxicity in neurons of MS patients and identify the MS risk-associated metabotropic glutamate receptor 8 (GRM8) as a decisive modulator. Mechanistically, GRM8 activation counteracted neuronal cAMP accumulation, thereby directly desensitizing the inositol 1,4,5-trisphosphate receptor (IP3R). This profoundly limited glutamate-induced calcium release from the endoplasmic reticulum and subsequent cell death. Notably, we found Grm8-deficient neurons to be more prone to glutamate excitotoxicity, whereas pharmacological activation of GRM8 augmented neuroprotection in mouse and human neurons as well as in a preclinical mouse model of MS. Thus, we demonstrate that GRM8 conveys neuronal resilience to CNS inflammation and is a promising neuroprotective target with broad therapeutic implications.

Calcium Signaling in the Thyroid: Friend and Foe

Simple Summary

All cells in our body are activated by several different signals. The calcium ion is one of the most versatile signaling molecules, and regulates a multitude of different events in the cells. These range from activation of muscle contraction, to the regulation of cell movement, just to name a few. In normal thyroid cells, calcium signaling is of importance for the normal physiology of the cells. In thyroid pathologies, e.g., thyroid cancer, calcium is important for the regulation of proliferation and invasion, and may also activate gene transcription programs important for cancer cell survival. In this Commentary, we summarize what is known regarding calcium in the normal thyroid, and highlight the importance of calcium signaling in thyroid pathologies.

Abstract

Calcium signaling participates in a vast number of cellular processes, ranging from the regulation of muscle contraction, cell proliferation, and mitochondrial function, to the regulation of the membrane potential in cells. The actions of calcium signaling are, thus, of great physiological significance for the normal functioning of our cells. However, many of the processes that are regulated by calcium, including cell movement and proliferation, are important in the progression of cancer. In the normal thyroid, calcium signaling plays an important role, and evidence is also being gathered showing that calcium signaling participates in the progression of thyroid cancer. This review will summarize what we know in regard to calcium signaling in the normal thyroid as, well as in thyroid cancer.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

Calcium-dependent signalling in B-cell lymphomas

Induced waves of calcium fluxes initiate multiple signalling pathways that play an important role in the differentiation and maturation of B-cells. Finely tuned transient Ca⁺² fluxes from the endoplasmic reticulum in response to B-cell receptor (BCR) or chemokine receptor activation are followed by more sustained calcium influxes from the extracellular environment and contribute to the mechanisms responsible for the proliferation of B-cells, their migration within lymphoid organs and their differentiation. Dysregulation of these well-balanced mechanisms in B-cell lymphomas results in uncontrolled cell proliferation and resistance to apoptosis. Consequently, several cytotoxic drugs (and anti-proliferative compounds) used in standard chemotherapy regimens for the treatment of people with lymphoma target calcium-dependent pathways. Furthermore, ~10% of lymphoma associated mutations are found in genes with functions in calcium-dependent signalling, including those affecting B-cell receptor signalling pathways. In this review, we provide an overview of the Ca²⁺-dependent signalling network and outline the contribution of its key components to B cell lymphomagenesis. We also consider how the oncogenic Epstein-Barr virus, which is causally linked to the pathogenesis of a number of B-cell lymphomas, can modify Ca²⁺-dependent signalling.

Emerging Evidence for cAMP-calcium Cross Talk in Heart Atrial Nanodomains Where IP3-Evoked Calcium Release Stimulates Adenylyl Cyclases

Calcium handling is vital to normal physiological function in the heart. Human atrial arrhythmias, eg. atrial fibrillation, are a major morbidity and mortality burden, yet major gaps remain in our understanding of how calcium signaling pathways function and interact. Inositol trisphosphate (IP3) is a calcium-mobilizing second messenger and its agonist-induced effects have been observed in many tissue types. In the atria IP3 receptors (IR3Rs) residing on junctional sarcoplasmic reticulum augment cellular calcium transients and, when over-stimulated, lead to arrhythmogenesis. Recent studies have demonstrated that the predominant pathway for IP3 actions in atrial myocytes depends on stimulation of calcium-dependent forms of adenylyl cyclase (AC8 and AC1) by IP3-evoked calcium release from the sarcoplasmic reticulum. AC8 shows co-localisation with IP3Rs and AC1 appears to be nearby. These observations support crosstalk between calcium and cAMP pathways in nanodomains in atria. Similar mechanisms also appear to operate in the pacemaker region of the sinoatrial node. Here we discuss these significant advances in our understanding of atrial physiology and pathology, together with implications for the identification of potential novel targets and modulators for the treatment of atrial arrhythmias.

IP3R Channels in Male Reproduction

As a second messenger in cellular signal transduction, calcium signaling extensively participates in various physiological activities, including spermatogenesis and the regulation of sperm function. Abnormal calcium signaling is highly correlated with male infertility. Calcium signaling is mainly regulated by both extracellular calcium influx and the release of calcium stores. Inositol 1,4,5-trisphosphate receptor (IP3R) is a widely expressed channel for calcium stores. After being activated by inositol 1,4,5-trisphosphate (IP3) and calcium signaling at a lower concentration, IP3R can regulate the release of Ca²⁺ from stores into cytoplasm, and eventually trigger downstream events. The closure of the IP3R channel caused by a rise in intracellular calcium signals and the activation of the calcium pump jointly restores the calcium store to a normal level. In this review, we aim to discuss structural features of IP3R channels and the underlying mechanism of IP3R channel-mediated calcium signaling and further focus on the research progress of IP3R expression and function in the male reproductive system. Finally, we propose key directions and strategies for research of IP3R in spermatogenesis and the regulation of sperm function to provide more understanding of the function and mechanism of IP3R channel actions in male reproduction.

Redox and mTOR-dependent regulation of plasma lamellar calcium influx controls the senescence-associated secretory phenotype

IMPACT STATEMENT

Through its ability to evoke responses from cells in a paracrine fashion, the senescence-associated secretory phenotype (SASP) has been linked to numerous age-associated disease pathologies including tumor invasion, cardiovascular dysfunction, neuroinflammation, osteoarthritis, and renal disease. Strategies which limit the amplitude and duration of SASP serve to delay age-related degenerative decline. Here we demonstrate that the SASP regulation is linked to shifts in intracellular Ca2+ homeostasis and strategies which rescue redox-dependent calcium entry including enzymatic H2O2 scavenging, TRP modulation, or mTOR inhibition block SASP and TRPC6 gene expression. As Ca2+ is indispensable for secretion from both secretory and non-secretory cells, it is exciting to speculate that the expression of plasma lamellar TRP channels critical for the maintenance of intracellular Ca2+ homeostasis may be coordinately regulated with the SASP.

Insights into the Function and Evolution of Taste 1 Receptor Gene Family in the Carnivore Fish Gilthead Seabream (Sparus aurata)

A plethora of molecular and functional studies in tetrapods has led to the discovery of multiple taste 1 receptor (T1R) genes encoding G-protein coupled receptors (GPCRs) responsible for sweet (T1R2 + T1R3) and umami (T1R1 + T1R3) taste. In fish, the T1R gene family repertoires greatly expanded because of several T1R2 gene duplications, and recent studies have shown T1R2 functional divergence from canonical mammalian sweet taste perceptions, putatively as an adaptive mechanism to develop distinct feeding strategies in highly diverse aquatic habitats. We addressed this question in the carnivore fish gilthead seabream (Sparus aurata), a model species of aquaculture interest, and found that the saT1R gene repertoire consists of eight members including saT1R1, saT1R3 and six saT1R2a-f gene duplicates, adding further evidence to the evolutionary complexity of fishT1Rs families. To analyze saT1R taste functions, we first developed a stable gene reporter system based on Ca²⁺-dependent calcineurin/NFAT signaling to examine specifically in vitro the responses of a subset of saT1R heterodimers to L-amino acids (L-AAs) and sweet ligands. We show that although differentially tuned in sensitivity and magnitude of responses, saT1R1/R3, saT1R2a/R3 and saT1R2b/R3 may equally serve to transduce amino acid taste sensations. Furthermore, we present preliminary information on the potential involvement of the Gi protein alpha subunits saGαi1 and saGαi2 in taste signal transduction.

IP3-mediated Ca2+ release regulates atrial Ca2+ transients and pacemaker function by stimulation of adenylyl cyclases

Inositol trisphosphate (IP₃) is a Ca²⁺-mobilizing second messenger shown to modulate atrial muscle contraction and is thought to contribute to atrial fibrillation. Cellular pathways underlying IP₃ actions in cardiac tissue remain poorly understood, and the work presented here addresses the question whether IP₃-mediated Ca²⁺ release from the sarcoplasmic reticulum is linked to adenylyl cyclase activity including Ca²⁺-stimulated adenylyl cyclases (AC1 and AC8) that are selectively expressed in atria and sinoatrial node (SAN). Immunocytochemistry in guinea pig atrial myocytes identified colocalization of type 2 IP₃ receptors with AC8, while AC1 was located in close vicinity. Intracellular photorelease of IP₃ by UV light significantly enhanced the amplitude of the Ca²⁺ transient (CaT) evoked by electrical stimulation of atrial myocytes (31 ± 6% increase 60 s after photorelease, n = 16). The increase in CaT amplitude was abolished by inhibitors of adenylyl cyclases (MDL-12,330) or protein kinase A (H89), showing that cAMP signaling is required for this effect of photoreleased IP₃. In mouse, spontaneously beating right atrial preparations, phenylephrine, an α-adrenoceptor agonist with effects that depend on IP₃-mediated Ca²⁺ release, increased the maximum beating rate by 14.7 ± 0.5%, n = 10. This effect was substantially reduced by 2.5 µmol/L 2-aminoethyl diphenylborinate and abolished by a low dose of MDL-12,330, observations which are again consistent with a functional interaction between IP₃ and cAMP signaling involving Ca²⁺ stimulation of adenylyl cyclases in the SAN pacemaker. Understanding the interaction between IP₃ receptor pathways and Ca²⁺-stimulated adenylyl cyclases provides important insights concerning acute mechanisms for initiation of atrial arrhythmias.

NEW & NOTEWORTHY This study provides evidence supporting the proposal that IP₃ signaling in cardiac atria and sinoatrial node involves stimulation of Ca²⁺-activated adenylyl cyclases (AC1 and AC8) by IP₃-evoked Ca²⁺ release from junctional sarcoplasmic reticulum. AC8 and IP₃ receptors are shown to be located close together, while AC1 is nearby. Greater understanding of these novel aspects of the IP₃ signal transduction mechanism is important for future study in atrial physiology and pathophysiology, particularly atrial fibrillation.

Calcium-induced calcium release in proximity to hair cell BK channels revealed by PKA activation

Large-conductance calcium-activated potassium (BK) channels play a critical role in electrical resonance, a mechanism of frequency selectivity in chicken hair cells. We determine that BK currents are dependent on inward flow of Ca2+ , and intracellular buffering of Ca2+ . Entry of Ca2+ is further amplified locally by calcium-induced Ca2+ release (CICR) in close proximity to plasma membrane BK channels. Ca2+ imaging reveals peripheral clusters of high concentrations of Ca2+ that are suprathreshold to that needed to activate BK channels. Protein kinase A (PKA) activation increases the size of BK currents likely by recruiting more BK channels due to spatial spread of high Ca2+ concentrations in turn from increasing CICR. STORM imaging confirms the presence of nanodomains with ryanodine and IP3 receptors in close proximity to the Slo subunit of BK channels. Together, these data require a rethinking of how electrical resonance is brought about and suggest effects of CICR in synaptic release. Both genders were included in this study.

Modulators of calcium signalling at fertilization

Calcium (Ca2+) signals initiate egg activation across the animal kingdom and in at least some plants. These signals are crucial for the success of development and, in the case of mammals, health of the offspring. The mechanisms associated with fertilization that trigger these signals and the molecules that regulate their characteristic patterns vary widely. With few exceptions, a major contributor to fertilization-induced elevation in cytoplasmic Ca2+ is release from endoplasmic reticulum stores through the IP3 receptor. In some cases, Ca2+ influx from the extracellular space and/or release from alternative intracellular stores contribute to the rise in cytoplasmic Ca2+. Following the Ca2+ rise, the reuptake of Ca2+ into intracellular stores or efflux of Ca2+ out of the egg drive the return of cytoplasmic Ca2+ back to baseline levels. The molecular mediators of these Ca2+ fluxes in different organisms include Ca2+ release channels, uptake channels, exchangers and pumps. The functions of these mediators are regulated by their particular activating mechanisms but also by alterations in their expression and spatial organization. We discuss here the molecular basis for modulation of Ca2+ signalling at fertilization, highlighting differences across several animal phyla, and we mention key areas where questions remain.

The hidden role of the Sigma1 receptor in muscle cells

This review describes the very specific role of Sigma1 receptor in different types of muscle cells. Sigma1 receptor is a transmembrane protein residing in such structures like MAM. It has chaperoning activity supporting function of many proteins, particularly ion channels, including Ca2+ channels. This latter function is of particular meaning for muscle cells, due to their calcium-based/regulated metabolism. Here we discuss new reports pointing to participation of Sigma1 receptor in muscle specific processes like contraction, EC-coupling, calcium currents and in diseases like left ventricular hypertrophy, transverse aortic stenosis and hypertension-induced heart dysfunction.

PDE3 Inhibition Reduces Epithelial Mast Cell Numbers in Allergic Airway Inflammation and Attenuates Degranulation of Basophils and Mast Cells

Epithelial mast cells are generally present in the airways of patients with allergic asthma that are inadequately controlled. Airway mast cells (MCs) are critically involved in allergic airway inflammation and contribute directly to the main symptoms of allergic patients. Phosphodiesterase 3 (PDE3) tailors signaling of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), which are critical intracellular second messenger molecules in various signaling pathways. This paper investigates the pathophysiological role and disease-modifying effects of PDE3 in mouse bone marrow-derived MCs (bmMCs), human LAD2- and HMC1 mast cell lines, human blood basophils, and peripheral blood-derived primary human MCs (HuMCs). In a chronic house dust mite (HDM)-driven allergic airway inflammation mouse model, we observed that PDE3 deficiency or PDE3 inhibition (PDE3i) therapy reduced the numbers of epithelial MCs, when compared to control mice. Mouse bone marrow-derived MCs (bmMCs) and the human HMC1 and LAD2 cell lines predominantly expressed PDE3B and PDE4A. BmMCs from Pde3^−/− mice showed reduced loss of the degranulation marker CD107b compared with wild-type BmMCs, when stimulated in an immunoglobulin E (IgE)-dependent manner. Following both IgE-mediated and substance P-mediated activation, PDE3i-pretreated basophils, LAD2 cells, and HuMCs, showed less degranulation than diluent controls, as measured by surface CD63 expression. MCs lacking PDE3 or treated with the PDE3i enoximone exhibited a lower calcium flux upon stimulation with ionomycine. In conclusion PDE3 plays a critical role in basophil and mast cell degranulation and therefore its inhibition may be a treatment option in allergic disease.

The WD40 repeat protein, WDR36, orchestrates sphingosine kinase-1 recruitment and phospholipase C-β activation by Gq-coupled receptors

Sphingosine kinases (SphK) catalyse the formation of sphingosine-1-phosphate (S1P) and play important roles in the cardiovascular, nervous and immune systems. We have shown before that G_q-coupled receptors induce a rapid and long-lasting translocation of SphK1 to the plasma membrane and cross-activation of S1P receptors. Here, we further addressed G_q regulation of SphK1 by analysing the influence of the WD40 repeat protein, WDR36. WDR36 has been described as a scaffold tethering Gα_q to phospholipase C (PLC)-β and the thromboxane A₂ receptor-β (TPβ receptor). Overexpression of WDR36 in HEK-293 cells enhanced TPβ receptor-induced inositol phosphate production, as reported (Cartier et al. 2011), but significantly attenuated inositol phosphate production induced by muscarinic M₃ and bradykinin B₂ receptors. In agreement with its effect on PLCβ, WDR36 augmented TPβ receptor-induced [Ca²⁺]_i increases. Surprisingly, WDR36 also augmented M₃ receptor-induced [Ca²⁺]_i increases, which was due to increased Ca²⁺ mobilization while the Ca²⁺ content of thapsigargin-sensitive stores remained unaltered. Interestingly, overexpression of WDR36 significantly delayed SphK1 translocation by G_q-coupled M₃, B₂ and H₁ receptors in HEK-293 cells, while TPβ receptor-induced SphK1 translocation was generally slow and not altered by WDR36 in these cells. Finally, in C2C12 myoblasts, overexpression of WDR36 delayed SphK1 translocation induced by B₂ receptors. It is concluded that WDR36 reduces signalling of G_q-coupled receptors other than TPβ towards PLC and SphK1, most likely by scavenging Gα_q and PLCβ. Our results support a role of WDR36 in orchestration of G_q signalling complexes, and might help to functionally unravel its genetic association with asthma and allergy.

A model for time interval learning in the Purkinje cell

Recent experimental findings indicate that Purkinje cells in the cerebellum represent time intervals by mechanisms other than conventional synaptic weights. These findings add to the theoretical and experimental observations suggesting the presence of intra-cellular mechanisms for adaptation and processing. To account for these experimental results we propose a new biophysical model for time interval learning in a Purkinje cell. The numerical model focuses on a classical delay conditioning task (e.g. eyeblink conditioning) and relies on a few computational steps. In particular, the model posits the activation by the parallel fiber input of a local intra-cellular calcium store which can be modulated by intra-cellular pathways. The reciprocal interaction of the calcium signal with several proteins forming negative and positive feedback loops ensures that the timing of inhibition in the Purkinje cell anticipates the interval between parallel and climbing fiber inputs during training. We systematically test the model ability to learn time intervals at the 150-1000 ms time scale, while observing that learning can also extend to the multiple seconds scale. In agreement with experimental observations we also show that the number of pairings required to learn increases with inter-stimulus interval. Finally, we discuss how this model would allow the cerebellum to detect and generate specific spatio-temporal patterns, a classical theory for cerebellar function.

The Endoplasmic Reticulum-Plasma Membrane Junction: A Hub for Agonist Regulation of Ca2+ Entry

Stimulation of cell-surface receptors induces cytosolic Ca²⁺ ([Ca²⁺]_i) increases that are detected and transduced by effector proteins for regulation of cell function. Intracellular Ca²⁺ release, via endoplasmic reticulum (ER) proteins inositol 1,4,5-trisphosphate receptors (IP₃R) and ryanodine receptors (RyR), and Ca²⁺ influx, via store-operated Ca²⁺ entry (SOCE), contribute to the increase in [Ca²⁺]_i The amplitude, frequency, and spatial characteristics of the [Ca²⁺]_i increases are controlled by the compartmentalization of proteins into signaling complexes such as receptor-signaling complexes and SOCE complexes. Both complexes include protein and lipid components, located in the plasma membrane (PM) and ER. Receptor signaling initiates in the PM via phospholipase C (PLC)-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂), and culminates with the activation of IP₃R in the ER. Conversely, SOCE is initiated in the ER by Ca²⁺-sensing stromal interaction molecule (STIM) proteins, which then interact with PM channels Orai1 and TRPC1 to activate Ca²⁺ entry. This review will address how ER-PM junctions serve a central role in agonist regulation of SOCE.

VIP and muscarinic synergistic mucin secretion by salivary mucous cells is mediated by enhanced PKC activity via VIP-induced release of an intracellular Ca2+ pool

Mucin secretion by salivary mucous glands is mediated predominantly by parasympathetic acetylcholine activation of cholinergic muscarinic receptors via increased intracellular free calcium ([Ca²⁺]_i) and activation of conventional protein kinase C isozymes (cPKC). However, the parasympathetic co-neurotransmitter, vasoactive intestinal peptide (VIP), also initiates secretion, but to a lesser extent. In the present study, cross talk between VIP- and muscarinic-induced mucin secretion was investigated using isolated rat sublingual tubuloacini. VIP-induced secretion is mediated by cAMP-activated protein kinase A (PKA), independently of increased [Ca²⁺]_i. Synergistic secretion between VIP and the muscarinic agonist, carbachol, was demonstrated but only with submaximal carbachol. Carbachol has no effect on cAMP ± VIP. Instead, PKA activated by VIP releases Ca²⁺ from an intracellular pool maintained by the sarco/endoplasmic reticulum Ca²⁺-ATPase pump. Calcium release was independent of phospholipase C activity. The resultant sustained [Ca²⁺]_i increase is additive to submaximal, but not maximal carbachol-induced [Ca²⁺]_i. Synergistic mucin secretion was mimicked by VIP plus either phorbol 12-myristate 13-acetate or 0.01 μM thapsigargin, and blocked by the PKC inhibitor, Gö6976. VIP-induced Ca²⁺ release also promoted store-operated Ca²⁺ entry. Synergism is therefore driven by VIP-mediated [Ca²⁺]_i augmenting cPKC activity to enhance muscarinic mucin secretion. Additional data suggest ryanodine receptors control VIP/PKA-mediated Ca²⁺ release from a Ca²⁺ pool also responsive to maximal carbachol. A working model of muscarinic and VIP control of mucous cell exocrine secretion is presented. Results are discussed in relation to synergistic mechanisms in other secretory cells, and the physiological and therapeutic significance of VIP/muscarinic synergism controlling salivary mucous cell exocrine secretion.

Cyclic nucleotides, gut physiology and inflammation

Misregulation of gut function and homeostasis impinges on the overall well-being of the entire organism. Diarrheal disease is the second leading cause of death in children under 5 years of age, and globally, 1.7 billion cases of childhood diarrhea are reported every year. Accompanying diarrheal episodes are a number of secondary effects in gut physiology and structure, such as erosion of the mucosal barrier that lines the gut, facilitating further inflammation of the gut in response to the normal microbiome. Here, we focus on pathogenic bacteria-mediated diarrhea, emphasizing the role of cyclic adenosine 3',5'-monophosphate and cyclic guanosine 3',5'-monophosphate in driving signaling outputs that result in the secretion of water and ions from the epithelial cells of the gut. We also speculate on how this aberrant efflux and influx of ions could modulate inflammasome signaling, and therefore cell survival and maintenance of gut architecture and function.

New Insights in the IP3 Receptor and Its Regulation

The inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) is a Ca²⁺-release channel mainly located in the endoplasmic reticulum (ER). Three IP₃R isoforms are responsible for the generation of intracellular Ca²⁺ signals that may spread across the entire cell or occur locally in so-called microdomains. Because of their ubiquitous expression, these channels are involved in the regulation of a plethora of cellular processes, including cell survival and cell death. To exert their proper function a fine regulation of their activity is of paramount importance. In this review, we will highlight the recent advances in the structural analysis of the IP₃R and try to link these data with the newest information concerning IP₃R activation and regulation. A special focus of this review will be directed towards the regulation of the IP₃R by protein-protein interaction. Especially the protein family formed by calmodulin and related Ca²⁺-binding proteins and the pro- and anti-apoptotic/autophagic Bcl-2-family members will be highlighted. Finally, recently identified and novel IP₃R regulatory proteins will be discussed. A number of these interactions are involved in cancer development, illustrating the potential importance of modulating IP₃R-mediated Ca²⁺ signaling in cancer treatment.

Type 3 inositol 1,4,5-trisphosphate receptor: A calcium channel for all seasons

Inositol 1,4,5 trisphosphate receptors (ITPRs) are a family of endoplasmic reticulum Ca²⁺ channels essential for the control of intracellular Ca²⁺ levels in virtually every mammalian cell type. The three isoforms (ITPR1, ITPR2 and ITPR3) are highly homologous in amino acid sequence, but they differ considerably in terms of biophysical properties, subcellular localization, and tissue distribution. Such differences underscore the variety of cellular responses triggered by each isoform and suggest that the expression/activity of specific isoforms might be linked to particular pathophysiological states. Indeed, recent findings demonstrate that changes in expression of ITPR isoforms are associated with a number of human diseases ranging from fatty liver disease to cancer. ITPR3 is emerging as the isoform that is particularly important in the pathogenesis of various human diseases. Here we review the physiological and pathophysiological roles of ITPR3 in various tissues and the mechanisms by which the expression of this isoform is modulated in health and disease.

AMP-activated protein kinase slows D2 dopamine autoreceptor desensitization in substantia nigra neurons

Dopamine neurons in the substantia nigra zona compacta (SNC) are well known to express D2 receptors. When dopamine is released from somatodendritic sites, activation of D2 autoreceptors suppresses dopamine neuronal activity through activation of G protein-coupled K⁺ channels. AMP-activated protein kinase (AMPK) is a master enzyme that acts in somatic tissues to suppress energy expenditure and encourage energy production. We hypothesize that AMPK may also conserve energy in central neurons by reducing desensitization of D2 autoreceptors. We used whole-cell patch-clamp recordings to study the effects of AMPK activators and inhibitors on D2 autoreceptor-mediated current in SNC neurons in midbrain slices from rat pups (11-23 days post-natal). Slices were superfused with 100 μM dopamine or 30 μM quinpirole for 25 min, which evoked outward currents that decayed slowly over time. Although the AMPK activators A769662 and ZLN024 significantly slowed rundown of dopamine-evoked current, slowing of quinpirole-evoked current required the presence of a D1-like agonist (SKF38393). Moreover, the D1-like agonist also slowed the rundown of quinpirole-induced current even in the absence of an AMPK activator. Pharmacological antagonist experiments showed that the D1-like agonist effect required activation of either protein kinase A (PKA) or exchange protein directly activated by cAMP 2 (Epac2) pathways. In contrast, the effect of AMPK on rundown of current evoked by quinpirole plus SKF38393 required PKA but not Epac2. We conclude that AMPK slows D2 autoreceptor desensitization by augmenting the effect of D1-like receptors.