IRBIT Suppresses IP3 Receptor Activity by Competing with IP3 for the Common Binding Site on the IP3 Receptor

NRAS Mutant Dictates AHCYL1-Governed ER Calcium Homeostasis for Melanoma Tumor Growth

Calcium homeostasis is critical for cell proliferation, and emerging evidence shows that cancer cells exhibit altered calcium signals to fulfill their need for proliferation. However, it remains unclear whether there are oncogene-specific calcium homeostasis regulations that can expose novel therapeutic targets. Here, from RNAi screen, we report that adenosylhomocysteinase like protein 1 (AHCYL1), a suppressor of the endoplasmic reticulum (ER) calcium channel protein inositol trisphosphate receptor (IP3R), is selectively upregulated and critical for cell proliferation and tumor growth potential of human NRAS-mutated melanoma, but not for melanoma expressing BRAF V600E. Mechanistically, AHCYL1 deficiency results in decreased ER calcium levels, activates the unfolded protein response (UPR), and triggers downstream apoptosis. In addition, we show that AHCYL1 transcription is regulated by activating transcription factor 2 (ATF2) in NRAS-mutated melanoma. Our work provides evidence for oncogene-specific calcium regulations and suggests AHCYL1 as a novel therapeutic target for RAS mutant-expressing human cancers, including melanoma.

IMPLICATIONS

Our findings suggest that targeting the AHCYL1-IP3R axis presents a novel therapeutic approach for NRAS-mutated melanomas, with potential applicability to all cancers harboring RAS mutations, such as KRAS-mutated human colorectal cancers.

Multiple Regulatory Signals and Components in the Modulation of Bicarbonate Transporters

Bicarbonate transporters are responsible for the appropriate flux of bicarbonate across the plasma membrane to perform various fundamental cellular functions. The functions of bicarbonate transporters, including pH regulation, cell migration, and inflammation, are highlighted in various cellular systems, encompassing their participation in both physiological and pathological processes. In this review, we focused on recently identified modulatory signaling components that regulate the expression and activity of bicarbonate transporters. Moreover, we addressed recent advances in our understanding of cooperative systems of bicarbonate transporters and channelopathies. This current review aims to provide a new, in-depth understanding of numerous human diseases associated with the dysfunction of bicarbonate transporters.

The Discovery of Mitochondria-Endoplasmic Reticulum Contact Sites (MERCs) as Mitochondria-Associated Membranes (MAMs)

Mitochondria-endoplasmic reticulum contacts (MERCs), also called endoplasmic reticulum (ER)-mitochondria contact sites (ERMCS), are the membrane domains, where these two organelles exchange lipids, Ca2+ ions, and reactive oxygen species. This crosstalk is a major determinant of cell metabolism, since it allows the ER to control mitochondrial oxidative phosphorylation and the Krebs cycle, while conversely, it allows the mitochondria to provide sufficient ATP to control ER proteostasis. MERC metabolic signaling is under the control of tethers and a multitude of regulatory proteins. Many of these proteins have recently been discovered to give rise to rare diseases if their genes are mutated. Surprisingly, these diseases share important hallmarks and cause neurological defects, sometimes paired with, or replaced by skeletal muscle deficiency. Typical symptoms include developmental delay, intellectual disability, facial dysmorphism and ophthalmologic defects. Seizures, epilepsy, deafness, ataxia, or peripheral neuropathy can also occur upon mutation of a MERC protein. Given that most MERC tethers and regulatory proteins have secondary functions, some MERC protein-based diseases do not fit into this categorization. Typically, however, the proteins affected in those diseases have dominant functions unrelated to their roles in MERCs tethering or their regulation. We are discussing avenues to pharmacologically target genetic diseases leading to MERC defects, based on our novel insight that MERC defects lead to common characteristics in rare diseases. These shared characteristics of MERCs disorders raise the hope that they may allow for similar treatment options.

IP3 receptors: An "elementary" journey from structure to signals

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are large tetrameric channels which sit mostly in the membrane of the endoplasmic reticulum (ER) and mediate Ca2+ release from intracellular stores in response to extracellular stimuli in almost all cells. Dual regulation of IP3Rs by IP3 and Ca2+ itself, upstream "licensing", and the arrangement of IP3Rs into small clusters in the ER membrane, allow IP3Rs to generate spatially and temporally diverse Ca2+ signals. The characteristic biphasic regulation of IP3Rs by cytosolic Ca2+ concentration underpins regenerative Ca2+ signals by Ca2+-induced Ca2+-release, while also preventing uncontrolled explosive Ca2+ release. In this way, cells can harness a simple ion such as Ca2+ as a near-universal intracellular messenger to regulate diverse cellular functions, including those with conflicting outcomes such as cell survival and cell death. High-resolution structures of the IP3R bound to IP3 and Ca2+ in different combinations have together started to unravel the workings of this giant channel. Here we discuss, in the context of recently published structures, how the tight regulation of IP3Rs and their cellular geography lead to generation of "elementary" local Ca2+ signals known as Ca2+ "puffs", which form the fundamental bottleneck through which all IP3-mediated cytosolic Ca2+ signals must first pass.

S-adenosylhomocysteine hydrolase-like protein 1 (AHCYL1) inhibits lung cancer tumorigenesis by regulating cell plasticity

BACKGROUND

Lung cancer is one of the most frequently diagnosed cancers characterized by high mortality, metastatic potential, and recurrence. Deregulated gene expression of lung cancer, likewise in many other solid tumors, accounts for their cell heterogeneity and plasticity. S-adenosylhomocysteine hydrolase-like protein 1 (AHCYL1), also known as Inositol triphosphate (IP(3)) receptor-binding protein released with IP(3) (IRBIT), plays roles in many cellular functions, including autophagy and apoptosis but AHCYL1 role in lung cancer is largely unknown.

RESULTS

Here, we analyzed the expression of AHCYL1 in Non-Small Cell Lung Cancer (NSCLC) cells from RNA-seq public data and surgical specimens, which revealed that AHCYL1 expression is downregulated in tumors and inverse correlated to proliferation marker Ki67 and the stemness signature expression. AHCYL1-silenced NSCLC cells showed enhanced stem-like properties in vitro, which correlated with higher expression levels of stem markers POU5F1 and CD133. Also, the lack of AHCYL1 enhanced tumorigenicity and angiogenesis in mouse xenograft models highlighting stemness features.

CONCLUSIONS

These findings indicate that AHCYL1 is a negative regulator in NSCLC tumorigenesis by modulating cell differentiation state and highlighting AHCYL1 as a potential prognostic biomarker for lung cancer.

Synergistic effects of agonists and two-pore-domain potassium channels on secretory responses of human pancreatic duct cells Capan-1

Mechanisms of synergistic agonist stimulation and modulation of the electrochemical driving force for anion secretion are still not fully explored in human pancreatic duct epithelial cells. The first objective of this study was therefore to test whether combined agonist stimulation augments anion transport responses in the Capan-1 monolayer model of human pancreatic duct epithelium. The second objective was to test the influence of H⁺,K⁺-ATPase inhibition on anion transport in Capan-1 monolayers. The third objective was to analyze the expression and function of K⁺ channels in Capan-1, which could support anion secretion and cooperate with H⁺,K⁺-ATPases in pH and potassium homeostasis. The human pancreatic adenocarcinoma cell line Capan-1 was cultured conventionally or as polarized monolayers that were analyzed by Ussing chamber electrophysiological recordings. Single-cell intracellular calcium was assayed with Fura-2. mRNA isolated from Capan-1 was analyzed by use of the nCounter assay or RT-PCR. Protein expression was assessed by immunofluorescence and western blot analyses. Combined stimulation with different physiological agonists enhanced anion transport responses compared to single agonist stimulation. The responsiveness of Capan-1 cells to histamine was also revealed in these experiments. The H⁺,K⁺-ATPase inhibitor omeprazole reduced carbachol- and riluzole-induced anion transport responses. Transcript analyses revealed abundant TASK-2, TWIK-1, TWIK-2, TASK-5, K_Ca3.1, and KCNQ1 mRNA expression. KCNE1 mRNA and TREK-1, TREK-2, TASK-2, and KCNQ1 protein expression were also shown. This study shows that the Capan-1 model recapitulates key physiological aspects of a bicarbonate-secreting epithelium and constitutes a valuable model for functional studies on human pancreatic duct epithelium.

Inhibition of adenylyl cyclase 1 by ST034307 inhibits IP3-evoked changes in sino-atrial node beat rate

Atrial arrhythmias, such as atrial fibrillation (AF), are a major mortality risk and a leading cause of stroke. The IP3 signalling pathway has been proposed as an atrial-specific target for AF therapy, and atrial IP3 signalling has been linked to the activation of calcium sensitive adenylyl cyclases AC1 and AC8. We investigated the involvement of AC1 in the response of intact mouse atrial tissue and isolated guinea pig atrial and sino-atrial node (SAN) cells to the α-adrenoceptor agonist phenylephrine (PE) using the selective AC1 inhibitor ST034307. The maximum rate change of spontaneously beating mouse right atrial tissue exposed to PE was reduced from 14.5% to 8.2% (p = 0.005) in the presence of 1 μM ST034307, whereas the increase in tension generated in paced left atrial tissue in the presence of PE was not inhibited by ST034307 (Control = 14.2%, ST034307 = 16.3%; p > 0.05). Experiments were performed using isolated guinea pig atrial and SAN cells loaded with Fluo-5F-AM to record changes in calcium transients (CaT) generated by 10 μM PE in the presence and absence of 1 μM ST034307. ST034307 significantly reduced the beating rate of SAN cells (0.34-fold decrease; p = 0.003) but did not inhibit changes in CaT amplitude in response to PE in atrial cells. The results presented here demonstrate pharmacologically the involvement of AC1 in the downstream response of atrial pacemaker activity to α-adrenoreceptor stimulation and IP3R calcium release.

RyR2/IRBIT regulates insulin gene transcript, insulin content, and secretion in the insulinoma cell line INS-1

The role of ER Ca²⁺ release via ryanodine receptors (RyR) in pancreatic β-cell function is not well defined. Deletion of RyR2 from the rat insulinoma INS-1 (RyR2^KO) enhanced IP₃ receptor activity stimulated by 7.5 mM glucose, coincident with reduced levels of the protein IP₃Receptor Binding protein released with Inositol 1,4,5 Trisphosphate (IRBIT). Insulin content, basal (2.5 mM glucose) and 7.5 mM glucose-stimulated insulin secretion were reduced in RyR2^KO and IRBIT^KO cells compared to controls. INS2 mRNA levels were reduced in both RyR2^KO and IRBIT^KO cells, but INS1 mRNA levels were specifically decreased in RyR2^KO cells. Nuclear localization of S-adenosylhomocysteinase (AHCY) was increased in RyR2^KO and IRBIT^KO cells. DNA methylation of the INS1 and INS2 gene promotor regions was very low, and not different among RyR2^KO, IRBIT^KO, and controls, but exon 2 of the INS1 and INS2 genes was more extensively methylated in RyR2^KO and IRBIT^KO cells. Exploratory proteomic analysis revealed that deletion of RyR2 or IRBIT resulted in differential regulation of 314 and 137 proteins, respectively, with 41 in common. These results suggest that RyR2 regulates IRBIT levels and activity in INS-1 cells, and together maintain insulin content and secretion, and regulate the proteome, perhaps via DNA methylation.

Synaptic plasticity in hippocampal CA1 neurons of mice lacking inositol-1,4,5-trisphosphate receptor-binding protein released with IP3 (IRBIT)

In hippocampal CA1 neurons of wild-type mice, a short tetanus (15 or 20 pulses at 100 Hz) or a standard tetanus (100 pulses at 100 Hz) to a naive input pathway induces long-term potentiation (LTP) of the responses. Low-frequency stimulation (LFS; 1000 pulses at 1 Hz) 60 min after the standard tetanus reverses LTP (depotentiation [DP]), while LFS applied 60 min prior to the standard tetanus suppresses LTP induction (LTP suppression). We investigated LTP, DP, and LTP suppression of both field excitatory postsynaptic potentials and population spikes in CA1 neurons of mice lacking the inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R)-binding protein released with IP₃ (IRBIT). The mean magnitudes of LTP induced by short and standard tetanus were not different in mutant and wild-type mice. In contrast, DP and LTP suppression were attenuated in mutant mice, whereby the mean magnitude of responses after LFS or tetanus were significantly greater than in wild-type mice. These results suggest that, in hippocampal CA1 neurons, IRBIT is involved in DP and LTP suppression, but is not essential for LTP. The attenuation of DP and LTP suppression in mice lacking IRBIT indicates that this protein, released during or after priming stimulations, determines the direction of LTP expression after the delivery of subsequent stimulations.

Unbiased proteomic profiling reveals the IP3R modulator AHCYL1/IRBIT as a novel interactor of microtubule-associated protein tau

Microtubule-associated protein tau is a naturally unfolded protein that can modulate a vast array of physiological processes through direct or indirect binding with molecular partners. Aberrant tau homeostasis has been implicated in the pathogenesis of several neurodegenerative disorders, including Alzheimer's disease. In this study, we performed an unbiased high-content protein profiling assay by incubating recombinant human tau on microarrays containing thousands of human polypeptides. Among the putative tau-binding partners, we identify SAH hydrolase-like protein 1/inositol 1,4,5-trisphosphate receptor (IP3R)-binding protein (AHCYL1/IRBIT), a member of the SAH hydrolase family and a previously described modulator of IP3R activity. Using coimmunoprecipitation assays, we show that endogenous as well as overexpressed tau can physically interact with AHCYL1/IRBIT in brain tissues and cultured cells. Proximity ligation assay experiments demonstrate that tau overexpression may modify the close localization of AHCYL1/IRBIT to IP3R at the endoplasmic reticulum. Together, our experimental evidence indicates that tau interacts with AHCYL1/IRBIT and potentially modulates AHCYL1/IRBIT function.

Genetic variants in SCNN1B and AHCYL1 are associated with eggshell quality in Chinese domestic laying ducks (Anas platyrhynchos)

1. The objective of this study was to investigate the evolution of SCNN1B and AHCYL1 proteins among 10 domestic and mammalian animals, to uncover the expression patterns of SCNN1B and AHCYL1 genes in ducks, identify the genetic variants of the SCNN1B and AHCYL1 genes and analyse their effects on eggshell quality.2. Expression profiles of the SCNN1B and AHCYL1 genes in Sansui female ducks were determined using real-time fluorescence quantitative PCR to identify SNPs. The duck SCNN1B and AHCYL1 genes were amplified to identify SNPs. A total of 502 Sansui female ducks were genotyped by sequencing, and the associations between the mRNA expression/SNP genotypes and six eggshell quality indices were analysed using PASW Statistics 18.0.3. The results showed that the SCNN1B and AHCYL1 proteins are highly conserved in different mammalian or domestic animals, especially the AHCYL1 protein. The SCNN1B and AHCYL1 genes were widely expressed in different tissues of male and female ducks, and expression level in the uterus was greater than in other tissues. The expression of SCNN1B and AHCYL1 during oviposition cycle indicated that expression levels were related to the eggshell mineralisation stage.4. The mRNA expression levels of uterine SCNN1B and AHCYL1 genes were positively correlated with eggshell strength (ESS), percentage (ESP) and weight (ESW) (P<0.05), respectively. Ten novel SNPs in SCNN1B and AHCYL1 genes from Chinese domestic laying ducks were identified through PCR amplicon sequencing.5. Genetic association analysis indicated g.797509 C > T, g.797573 C > T and g.797834 C > T in SCNN1B gene and g.169244 T > A, g.169265 T > C and g.175311T > C in AHCYL1 gene had a significant effect on eggshell quality. Correlation analysis between the SNP genotype and SCNN1B and AHCYL1 genes expression in the uterus showed that the genotypes of g.797509 C>T, g.797573 C>T, g.797834 C>T, g.169244 T>A and g.175311T>C sites affected the expression of SCNN1B and AHCYL1 genes in utero (P<0.05).6. The study indicated SCNN1B and AHCYL1 as candidate genes to improve eggshell traits in ducks.

Myo-inositol improves growth performance and regulates lipid metabolism of juvenile Chinese mitten crab (Eriocheir sinensis) fed different percentage of lipid

This study evaluated the effects of dietary myo-inositol (MI) on growth performance, antioxidant status and lipid metabolism of juvenile Chinese mitten crab (Eriocheir sinensis) fed different percentage of lipid. Crabs (4·58 (sem 0·05) g) were fed four diets including a normal lipid diet (N, containing 7 % lipid and 0 mg/kg MI), N with MI supplementation (N + MI, containing 7 % lipid and 1600 mg/kg MI), a high lipid diet (H, containing 13 % lipid and 0 mg/kg MI) and H with MI supplementation (H + MI, containing 13 % lipid and 1600 mg/kg MI) for 8 weeks. The H + MI group showed higher weight gain and specific growth rate than those in the H group. The dietary MI could improve the lipid accumulations in the whole body, hepatopancreas and muscle as a result of feeding on the high dietary lipid (13 %) in crabs. Besides, the crabs fed the H + MI diets increased the activities of antioxidant enzymes but reduced the malondialdehyde content in hepatopancreas compared with those fed the H diets. Moreover, dietary MI enhanced the expression of genes involved in lipid oxidation and exportation, yet reduced lipid absorption and synthesis genes expression in the hepatopancreas of crabs fed the H diet, which might be related to the activation of inositol 1,4,5-trisphosphate receptor (IP3R)/calmodulin-dependent protein kinase kinase-β (CaMKKβ)/adenosine 5'-monophosphate-activated protein kinase (AMPK) signalling pathway. This study demonstrates that MI could increase lipid utilisation and reduce lipid deposition in the hepatopancreas of E. sinensis fed a high lipid diet through IP3R/CaMKKβ/AMPK activation. This work provides new insights into the function of MI in the diet of crustaceans.

Both IRBIT and long-IRBIT bind to and coordinately regulate Cl-/HCO3- exchanger AE2 activity through modulating the lysosomal degradation of AE2

Anion exchanger 2 (AE2) plays crucial roles in regulating cell volume homeostasis and cell migration. We found that both IRBIT and Long-IRBIT (L-IRBIT) interact with anion exchanger 2 (AE2). The interaction occurred between the conserved AHCY-homologous domain of IRBIT/L-IRBIT and the N-terminal cytoplasmic region of AE2. Interestingly, AE2 activity was reduced in L-IRBIT KO cells, but not in IRBIT KO cells. Moreover, AE2 activity was slightly increased in IRBIT/L-IRBIT double KO cells. These changes in AE2 activity resulted from changes in the AE2 expression level of each mutant cell, and affected the regulatory volume increase and cell migration. The activity and expression level of AE2 in IRBIT/L-IRBIT double KO cells were downregulated if IRBIT, but not L-IRBIT, was expressed again in the cells, and the downregulation was cancelled by the co-expression of L-IRBIT. The mRNA levels of AE2 in each KO cell did not change, and the downregulation of AE2 in L-IRBIT KO cells was inhibited by bafilomycin A1. These results indicate that IRBIT binding facilitates the lysosomal degradation of AE2, which is inhibited by coexisting L-IRBIT, suggesting a novel regulatory mode of AE2 activity through the binding of two homologous proteins with opposing functions.

Update on Calcium Signaling in Cystic Fibrosis Lung Disease

Cystic fibrosis (CF) is an autosomal recessive disorder characterized by mutations in the cystic fibrosis transmembrane conductance regulator gene, which causes multifunctional defects that preferentially affect the airways. Abnormal viscosity of mucus secretions, persistent pathogen infections, hyperinflammation, and lung tissue damage compose the classical pathological manifestation referred to as CF lung disease. Among the multifunctional defects associated with defective CFTR, increasing evidence supports the relevant role of perturbed calcium (Ca²⁺) signaling in the pathophysiology of CF lung disease. The Ca²⁺ ion is a critical player in cell functioning and survival. Its intracellular homeostasis is maintained by a fine balance between channels, transporters, and exchangers, mediating the influx and efflux of the ion across the plasma membrane and the intracellular organelles. An abnormal Ca²⁺ profile has been observed in CF cells, including airway epithelial and immune cells, with heavy repercussions on cell function, viability, and susceptibility to pathogens, contributing to proinflammatory overstimulation, organelle dysfunction, oxidative stress, and excessive cytokines release in CF lung. This review discusses the role of Ca²⁺ signaling in CF and how its dysregulation in airway epithelial and immune cells contributes to hyperinflammation in the CF lung. Finally, we provide an outlook on the therapeutic options that target the Ca²⁺ signaling to treat the CF lung disease.

The discovery and development of IP3 receptor modulators: an update

Introduction: Inositol 1,4,5-trisphosphate receptors (IP3Rs) are intracellular calcium (Ca²⁺) release channels located on the endoplasmic/sarcoplasmic reticulum. The availability of the structure of the ligand-binding domain of IP3Rs has enabled the design of compatible ligands, but the limiting step remains their actual effectiveness in a biological context.Areas covered: This article summarizes the compelling literature on both agonists and antagonists targeting IP3Rs, emphasizing their strengths and limitations. The main challenges toward the discovery and development of IP3 receptor modulators are also described.Expert opinion: Despite significant progress in recent years, the pharmacology of IP3R still has major drawbacks, especially concerning the availability of specific antag onists. Moreover, drugs specifically targeting the three different subtypes of IP3R are especially needed.

Disease-associated mutations in inositol 1,4,5-trisphosphate receptor subunits impair channel function

The inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs), which form tetrameric channels, play pivotal roles in regulating the spatiotemporal patterns of intracellular calcium signals. Mutations in IP3Rs have been increasingly associated with many debilitating human diseases such as ataxia, Gillespie syndrome, and generalized anhidrosis. However, how these mutations affect IP3R function, and how the perturbation of as-sociated calcium signals contribute to the pathogenesis and severity of these diseases remains largely uncharacterized. Moreover, many of these diseases occur as the result of autosomal dominant inheritance, suggesting that WT and mutant subunits associate in heterotetrameric channels. How the in-corporation of different numbers of mutant subunits within the tetrameric channels affects its activities and results in different disease phenotypes is also unclear. In this report, we investigated representative disease-associated missense mutations to determine their effects on IP3R channel activity. Additionally, we designed concatenated IP3R constructs to create tetrameric channels with a predefined subunit composition to explore the functionality of heteromeric channels. Using calcium imaging techniques to assess IP3R channel function, we observed that all the mutations studied resulted in severely attenuated Ca2+ release when expressed as homotetramers. However, some heterotetramers retained varied degrees of function dependent on the composition of the tetramer. Our findings suggest that the effect of mutations depends on the location of the mutation in the IP3R structure, as well as on the stoichiometry of mutant subunits assembled within the tetrameric channel. These studies provide insight into the pathogenesis and penetrance of these devastating human diseases.

IRBIT activates NBCe1-B by releasing the auto-inhibition module from the transmembrane domain

Key points

The electrogenic Na⁺/HCO₃⁻cotransporter NBCe1‐B is widely expressed in many tissues, including pancreas, submandibular gland, brain, heart, etc. NBCe1‐B has very low activity under basal condition due to auto‐inhibition, but can be fully activated by protein interaction with the IP3R‐binding protein released with inositol 1,4,5‐trisphosphate (IRBIT).

The structural components of the auto‐inhibition domain and the IRBIT‐binding domain of NBCe1‐B are finely characterized based on systematic mutations in the present study and data from previous studies.

Reducing negative charges on the cytosol side of the transmembrane domain greatly decreases the magnitude of the auto‐inhibition of NBCe1‐B.

We propose that the auto‐inhibition domain functions as a brake module that inactivates NBCe1‐B by binding to, via electrostatic attraction, the transmembrane domain; IRBIT activates NBCe1‐B by releasing the brake from the transmembrane domain via competitive binding to the auto‐inhibition domain.

Abstract

The electrogenic Na⁺/HCO₃⁻ cotransporter NBCe1‐B is widely expressed in many tissues in the body. NBCe1‐B exhibits only basal activity due to the action of the auto‐inhibition domain (AID) in its unique amino‐terminus. However, NBCe1‐B can be activated by interaction with the IP3R‐binding protein released with inositol 1,4,5‐trisphosphate (IRBIT). Here, we investigate the molecular mechanism underlying the auto‐inhibition of NBCe1‐B and its activation by IRBIT. The IRBIT‐binding domain (IBD) of NBCe1‐B spans residues 1−52, essentially consisting of two arms, one negatively charged (residues 1−24) and the other positively charged (residues 40−52). The AID mainly spans residues 40−85, overlapping with the IBD in the positively charged arm. The magnitude of auto‐inhibition of NBCe1‐B is greatly decreased by manipulating the positively charged residues in the AID or by replacing a set of negatively charged residues with neutral ones in the transmembrane domain. The interaction between IRBIT and NBCe1‐B is abolished by mutating a set of negatively charged Asp/Glu residues (to Asn/Gln) plus a set of Ser/Thr residues (to Ala) in the PEST domain of IRBIT. However, this interaction is not affected by replacing the same set of Ser/Thr residues in the PEST domain with Asp. We propose that: (1) the AID, acting as a brake, binds to the transmembrane domain via electrostatic interaction to slow down NBCe1‐B; (2) IRBIT activates NBCe1‐B by releasing the brake from the transmembrane domain.

The electrogenic Na⁺/HCO₃⁻cotransporter NBCe1‐B is widely expressed in many tissues, including pancreas, submandibular gland, brain, heart, etc. NBCe1‐B has very low activity under basal condition due to auto‐inhibition, but can be fully activated by protein interaction with the IP3R‐binding protein released with inositol 1,4,5‐trisphosphate (IRBIT).

The structural components of the auto‐inhibition domain and the IRBIT‐binding domain of NBCe1‐B are finely characterized based on systematic mutations in the present study and data from previous studies.

Reducing negative charges on the cytosol side of the transmembrane domain greatly decreases the magnitude of the auto‐inhibition of NBCe1‐B.

We propose that the auto‐inhibition domain functions as a brake module that inactivates NBCe1‐B by binding to, via electrostatic attraction, the transmembrane domain; IRBIT activates NBCe1‐B by releasing the brake from the transmembrane domain via competitive binding to the auto‐inhibition domain.

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.

Protective Role of IRBIT on Sodium Bicarbonate Cotransporter-n1 for Migratory Cancer Cells

IP₃ receptor-binding protein released with IP₃ (IRBIT) interacts with various ion channels and transporters. An electroneutral type of sodium bicarbonate cotransporter, NBCn1, participates in cell migration, and its enhanced expression is related to cancer metastasis. The effect of IRBIT on NBCn1 and its relation to cancer cell migration remain obscure. We therefore aimed to determine the effect of IRBIT on NBCn1 and the regulation of cancer cell migration due to IRBIT-induced alterations in NBCn1 activity. Overexpression of IRBIT enhanced cancer cell migration and NBC activity. Knockdown of IRBIT or NBCn1 and treatment with an NBC-specific inhibitor, S0859, attenuated cell migration. Stimulation with oncogenic epidermal growth factor enhanced the expression of NBCn1 and migration of cancer cells by recruiting IRBIT. The recruited IRBIT stably maintained the expression of the NBCn1 transporter machinery in the plasma membrane. Combined inhibition of IRBIT and NBCn1 dramatically inhibited the migration of cancer cells. Combined modulation of IRBIT and NBCn1 offers an effective strategy for attenuating cancer metastasis.

Novel homozygous variant of carbonic anhydrase 8 gene expanding the phenotype of cerebellar ataxia, mental retardation, and disequilibrium syndrome subtype 3

We report the case of an 11-year-old Syrian girl born to consanguineous parents, who presents an ataxic gait from early childhood. On clinical examination, she presented a severe static - kinetic cerebellar syndrome, walking without support is possible for short distances only. Strikingly, three consecutive MRIs did not show any sign of cerebellar abnormalities, but a brain positron emission tomography (PET) using [18F]-fluorodeoxyglucose (FDG) demonstrated a clear decrease in glucose metabolism in the cerebellum as well as the anterior and medial temporal lobe bilaterally. A clinical exome analysis identified a novel homozygous c.251A > G (p.Asn84Ser) likely pathogenic variant in the carbonic anhydrase 8 (CA8) gene. CA8 mutations cause cerebellar ataxia, mental retardation, and disequilibrium syndrome subtype 3 (CAMRQ3), a rare genetically autosomal recessive disorder, only described in four families, so far with the frequent observation of quadrupedal gait. The proband differed with other reported CA8 mutations by the absence of clear cerebellar signs on brain MRI and the presence of focal seizures. This report expands the clinical spectrum associated with mutations in CA8 and illustrates the possible discrepancy between (mild) neuro-radiological images (MRI) and (severe) clinical phenotype in young individuals. In contrast, the observation of clear cerebellar abnormal metabolic findings suggests that the FDG-PET scan may be used as an early marker for hereditary ataxia.

Activation of mouse NBCe1-B by Xenopus laevis and mouse IRBITs: Role of the variable Nt appendage of IRBITs

The IP3 receptor binding protein released with inositol 1,4,5-trisphosphate (IRBIT) plays important roles in the regulation of intracellular Ca²⁺ signaling and intracellular pH. The mammals express two IRBIT paralogs, i.e., IRBIT1 (encoded by AHCYL1) and IRBIT2 (encoded by AHCYL2). The clawed frog Xenopus laevis oocyte is widely used for biophysical studies on ion channels and transporters. It remains unknown whether endogenous IRBIT is expressed in Xenopus oocytes. Here, we cloned from frog oocyte irbit2.L and irbit2.S, orthologs of mammalian IRBIT2. When over-expressed, the frog IRBITs powerfully stimulate the electrogenic Na⁺/HCO₃^- cotransporter NBCe1-B as mouse IRBIT2-V2 does. Expression of an isolated Nt fragment of NBCe1-B containing the IRBIT-binding domain greatly decreases NBCe1-B activity in oocytes, suggesting that the basal activity of NBCe1-B contains a large component derived from the stimulation by endogenous frog IRBIT. The frog IRBITs are highly homologous to the mammalian ones in the carboxyl-terminal region, but varies greatly in the amino-terminal (Nt) appendage. Interestingly, truncation study showed that the Nt appendage of IRBIT1 and the long Nt appendage of IRBIT2-V2 modestly enhance, whereas the short Nt appendage of IRBIT2-V4 greatly inhibits the functional interaction between IRBIT and NBCe1-B. Finally, Ala-substitution of Ser68, a key phosphorylation site in the PEST domain of IRBIT, causes distinct functional consequences depending on the structural context of the Nt appendage in different IRBIT isoforms. We conclude that the Nt appendage of IRBITs is not necessary for, but plays an important regulatory role in the functional interaction between IRBIT and NBCe1-B.

Ca2+ Signaling in Exocrine Cells

Calcium (Ca²⁺) and cyclic AMP (cAMP) signaling cross talk and synergize to stimulate the cardinal functions of exocrine cells, regulated exocytosis, and fluid and electrolyte secretion. This physiological process requires the organization of the two signaling pathways into complexes at defined cellular domains and close placement. Such domains are formed by membrane contact sites (MCS). This review discusses the basic properties of Ca²⁺ signaling in exocrine cells, the role of MCS in the organization of cell signaling and in cross talk and synergism between the Ca²⁺ and cAMP signaling pathways and, finally, the mechanism by which the Ca²⁺ and cAMP pathways synergize to stimulate epithelial fluid and electrolyte secretion.

Discovery of Urinary Proteomic Signature for Differential Diagnosis of Acute Appendicitis

Acute appendicitis is one of the most common acute abdomens, but the confident preoperative diagnosis is still a challenge. In order to profile noninvasive urinary biomarkers that could discriminate acute appendicitis from other acute abdomens, we carried out mass spectrometric experiments on urine samples from patients with different acute abdomens and evaluated diagnostic potential of urinary proteins with various machine-learning models. Firstly, outlier protein pools of acute appendicitis and controls were constructed using the discovery dataset (32 acute appendicitis and 41 control acute abdomens) against a reference set of 495 normal urine samples. Ten outlier proteins were then selected by feature selection algorithm and were applied in construction of machine-learning models using naïve Bayes, support vector machine, and random forest algorithms. The models were assessed in the discovery dataset by leave-one-out cross validation and were verified in the validation dataset (16 acute appendicitis and 45 control acute abdomens). Among the three models, random forest model achieved the best performance: the accuracy was 84.9% in the leave-one-out cross validation of discovery dataset and 83.6% (sensitivity: 81.2%, specificity: 84.4%) in the validation dataset. In conclusion, we developed a 10-protein diagnostic panel by the random forest model that was able to distinguish acute appendicitis from confusable acute abdomens with high specificity, which indicated the clinical application potential of noninvasive urinary markers in disease diagnosis.

Comprehensive mapping of the cell response to E. coli infection in porcine intestinal epithelial cells pretreated with exopolysaccharide derived from Lactobacillus reuteri

Bacterial exopolysaccharides (EPSs) are known to modulate immunity. To date, a plethora of studies have reported the effect of EPSs on intestinal cells; however few works have revealed a complete picture of the signalling events in intestinal epithelial cells induced by bacterial EPSs. Here, using transcriptomics, we comprehensively mapped the biological processes in porcine intestinal epithelial cells challenged with EPS derived from Lactobacillus reuteri alone, enterotoxigenic Escherichia coli (ETEC) or ETEC after pretreatment with EPS. The Gene Ontology analysis of differentially expressed genes (DEGs) showed that ETEC is able to evoke biological processes specifically involved in cell junction reorganization, extracellular matrix degradation, and activation of the innate immune response through the activation of pattern recognition receptors, such as TLRs and CTRs. A total of 495 DEGs were induced in ETEC-challenged cells. On the other hand, EPS pretreatment was able to attenuate overexpression of the genes induced by ETEC infection. The most relevant finding of this study is that EPS has a suppressive effect on the inflammatory response evoked by ETEC infection. On the basis of high-throughput RNA-seq, this report is the first to describe the effects of EPSs derived from L. reuteri used as a pretreatment of global gene expression in porcine epithelial cells.

Depotentiation depends on IP3 receptor activation sustained by synaptic inputs after LTP induction

In CA1 neurons of guinea pig hippocampal slices, long-term potentiation (LTP) was induced in field excitatory postsynaptic potentials (EPSPs) or population spikes (PSs) by the delivery of high-frequency stimulation (HFS, 100 pulses at 100 Hz) to CA1 synapses, and was reversed by the delivery of a train of low-frequency stimulation (LFS, 1000 pulses at 2 Hz) at 30 min after HFS (depotentiation), and this effect was inhibited when test synaptic stimulation was halted for a 19-min period after HFS or for a 20-min period after LFS or applied over the same time period in the presence of an antagonist of N-methyl-D-aspartate receptors (NMDARs), group I metabotropic glutamate receptors (mGluRs), or inositol 1, 4, 5-trisphosphate receptors (IP₃Rs). Depotentiation was also blocked by the application of a Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibitor or a calcineurin inhibitor applied in the presence of test synaptic input for a 10-min period after HFS or for a 20-min period after LFS. These results suggest that, in postsynaptic neurons, the coactivation of NMDARs and group I mGluRs due to sustained synaptic activity following LTP induction results in the activation of IP₃Rs and CaMKII, which leads to the activation of calcineurin after LFS and depotentiation of CA1 synaptic responses.

The Fundamental Role of Bicarbonate Transporters and Associated Carbonic Anhydrase Enzymes in Maintaining Ion and pH Homeostasis in Non-Secretory Organs

The bicarbonate ion has a fundamental role in vital systems. Impaired bicarbonate transport leads to various diseases, including immune disorders, cystic fibrosis, tumorigenesis, kidney diseases, brain dysfunction, tooth fracture, ischemic reperfusion injury, hypertension, impaired reproductive system, and systemic acidosis. Carbonic anhydrases are involved in the mechanism of bicarbonate movement and consist of complex of bicarbonate transport systems including bicarbonate transporters. This review focused on the convergent regulation of ion homeostasis through various ion transporters including bicarbonate transporters, their regulatory enzymes, such as carbonic anhydrases, pH regulatory role, and the expression pattern of ion transporters in non-secretory systems throughout the body. Understanding the correlation between these systems will be helpful in order to obtain new insights and design potential therapeutic strategies for the treatment of pH-related disorders. In this review, we have discussed the broad prospects and challenges that remain in elucidation of bicarbonate-transport-related biological and developmental systems.

IP3 Receptor Plasticity Underlying Diverse Functions

In the body, extracellular stimuli produce inositol 1,4,5-trisphosphate (IP₃), an intracellular chemical signal that binds to the IP₃ receptor (IP₃R) to release calcium ions (Ca²⁺) from the endoplasmic reticulum. In the past 40 years, the wide-ranging functions mediated by IP₃R and its genetic defects causing a variety of disorders have been unveiled. Recent cryo-electron microscopy and X-ray crystallography have resolved IP₃R structures and begun to integrate with concurrent functional studies, which can explicate IP₃-dependent opening of Ca²⁺-conducting gates placed ∼90 Å away from IP₃-binding sites and its regulation by Ca²⁺. This review highlights recent research progress on the IP₃R structure and function. We also propose how protein plasticity within IP₃R, which involves allosteric gating and assembly transformations accompanied by rapid and chronic structural changes, would enable it to regulate diverse functions at cellular microdomains in pathophysiological states.

Cl- as a bona fide signaling ion

Cl^- is the major extracellular (Cl^-_out) and intracellular (Cl^-_in) anion whose concentration is actively regulated by multiple transporters. These transporters generate Cl^- gradients across the plasma membrane and between the cytoplasm and intracellular organelles. [Cl^-]_in changes rapidly in response to cell stimulation and influences many physiological functions, as well as cellular and systemic homeostasis. However, less appreciated is the signaling function of Cl^-. Cl^- interacts with multiple proteins to directly modify their activity. This review highlights the signaling function of Cl^- and argues that Cl^- is a bona fide signaling ion, a function deserving extensive exploration.

Molecular mechanisms of caffeine-mediated intestinal epithelial ion transports

BACKGROUND AND PURPOSE

As little is known about the effect of caffeine, one of the most widely consumed substances worldwide, on intestinal function, we aimed to study its action on intestinal anion secretion and the underlying molecular mechanisms.

EXPERIMENTAL APPROACH

Anion secretion and channel expression were examined in mouse duodenal epithelium by Ussing chambers and immunocytochemistry. Ca²⁺ imaging was also performed in intestinal epithelial cells (IECs).

KEY RESULTS

Caffeine (10 mM) markedly increased mouse duodenal short-circuit current (I_sc ), which was attenuated by a removal of either Cl^- or HCO₃ ^- , Ca²⁺ -free serosal solutions and selective blockers of store-operated Ca²⁺ channels (SOC/Ca²⁺ release-activated Ca²⁺ channels), and knockdown of Orai1 channels on the serosal side of duodenal tissues. Caffeine induced SOC entry in IEC, which was inhibited by ruthenium red and selective blockers of SOC. Caffeine-stimulated duodenal I_sc was inhibited by the endoplasmic reticulum Ca²⁺ chelator (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine), selective blockers (ruthenium red and dantrolene) of ryanodine receptors (RyR), and of Ca²⁺ -activated Cl^- channels (niflumic acid and T16A). There was synergism between cAMP and Ca²⁺ signalling, in which cAMP/PKA promoted caffeine/Ca²⁺ -mediated anion secretion. Expression of STIM1 and Orai1 was detected in mouse duodenal mucosa and human IECs. The Orai1 proteins were primarily co-located with the basolateral marker Na⁺ , K⁺ -ATPase.

CONCLUSIONS AND IMPLICATIONS

Caffeine stimulated intestinal anion secretion mainly through the RyR/Orai1/Ca²⁺ signalling pathway. There is synergism between cAMP/PKA and caffeine/Ca²⁺ -mediated anion secretion. Our findings suggest that a caffeine-mediated RyR/Orai1/Ca²⁺ pathway could provide novel potential drug targets to control intestinal anion secretion.

Consensus report of the 8 and 9th Weinman Symposia on Gene x Environment Interaction in carcinogenesis: novel opportunities for precision medicine

The relative contribution of intrinsic genetic factors and extrinsic environmental ones to cancer aetiology and natural history is a lengthy and debated issue. Gene-environment interactions (G x E) arise when the combined presence of both a germline genetic variant and a known environmental factor modulates the risk of disease more than either one alone. A panel of experts discussed our current understanding of cancer aetiology, known examples of G × E interactions in cancer, and the expanded concept of G × E interactions to include somatic cancer mutations and iatrogenic environmental factors such as anti-cancer treatment. Specific genetic polymorphisms and genetic mutations increase susceptibility to certain carcinogens and may be targeted in the near future for prevention and treatment of cancer patients with novel molecularly based therapies. There was general consensus that a better understanding of the complexity and numerosity of G × E interactions, supported by adequate technological, epidemiological, modelling and statistical resources, will further promote our understanding of cancer and lead to novel preventive and therapeutic approaches.

Modulation of Cl- signaling and ion transport by recruitment of kinases and phosphatases mediated by the regulatory protein IRBIT

IRBIT is a multifunctional protein that controls the activity of various epithelial ion transporters including NBCe1-B. Interaction with IRBIT increases NBCe1-B activity and exposes two cryptic Cl^--sensing GXXXP sites that enable regulation of NBCe1-B by intracellular Cl^- (Cl^- _in). Here, phosphoproteomic analysis revealed that IRBIT controlled five phosphorylation sites in NBCe1-B that determined both the active conformation of the transporter and its regulation by Cl^- _in Mutational analysis suggested that the phosphorylation status of Ser²³², Ser²³³, and Ser²³⁵ was regulated by IRBIT and determined whether NBCe1 transporters are in active or inactive conformations. The absence of phosphorylation at Ser²³², Ser²³³, or Ser²³⁵ produced NBCe1-B in the conformations pSer²³³/pSer²³⁵, pSer²³²/pSer²³⁵, or pSer²³²/pSer²³³, respectively. The activity of the pSer²³³/pSer²³⁵ form was similar to that of IRBIT-activated NBCe1-B, but it was insensitive to inhibition by Cl^- _in The properties of the pSer²³²/pSer²³⁵ form were similar to those of wild-type NBCe1-B, whereas the pSer²³²/pSer²³³ form was partially active, further activated by IRBIT, but retained inhibition by Cl^- _in Furthermore, IRBIT recruited the phosphatase PP1 and the kinase SPAK to control phosphorylation of Ser⁶⁵, which affected Cl^- _in sensing by the ³²GXXXP³⁶ motif. IRBIT also recruited the phosphatase calcineurin and the kinase CaMKII to control phosphorylation of Ser¹², which affected Cl^- _in sensing by the ¹⁹⁴GXXXP¹⁹⁸ motif. Ser²³², Ser²³³, and Ser²³⁵ are conserved in all NBCe1 variants and affect their activity. These findings reveal how multiple kinase and phosphatase pathways use phosphorylation sites to fine-tune a transporter, which have important implications for epithelial fluid and HCO₃ ^- secretion.

IP3 Receptor Properties and Function at Membrane Contact Sites

The inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) is a ubiquitously expressed Ca²⁺-release channel localized in the endoplasmic reticulum (ER). The intracellular Ca²⁺ signals originating from the activation of the IP₃R regulate multiple cellular processes including the control of cell death versus cell survival via their action on apoptosis and autophagy. The exact role of the IP₃Rs in these two processes does not only depend on their activity, which is modulated by the cytosolic composition (Ca²⁺, ATP, redox status, …) and by various types of regulatory proteins, including kinases and phosphatases as well as by a number of oncogenes and tumor suppressors, but also on their intracellular localization, especially at the ER-mitochondrial and ER-lysosomal interfaces. At these interfaces, Ca²⁺ microdomains are formed, in which the Ca²⁺ concentration is finely regulated by the different ER, mitochondrial and lysosomal Ca²⁺-transport systems and also depends on the functional and structural interactions existing between them. In this review, we therefore discuss the most recent insights in the role of Ca²⁺ signaling in general, and of the IP₃R in particular, in the control of basal mitochondrial bioenergetics, apoptosis, and autophagy at the level of inter-organellar contact sites.

Pathophysiological consequences of isoform-specific IP3 receptor mutations

Ca²⁺ signaling governs a diverse range of cellular processes and, as such, is subject to tight regulation. A main component of the complex intracellular Ca²⁺-signaling network is the inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R), a tetrameric channel that mediates Ca²⁺ release from the endoplasmic reticulum (ER) in response to IP₃. IP₃R function is controlled by a myriad of factors, such as Ca²⁺, ATP, kinases and phosphatases and a plethora of accessory and regulatory proteins. Further complexity in IP₃R-mediated Ca²⁺ signaling is the result of the existence of three main isoforms (IP₃R1, IP₃R2 and IP₃R3) that display distinct functional characteristics and properties. Despite their abundant and overlapping expression profiles, IP₃R1 is highly expressed in neurons, IP₃R2 in cardiomyocytes and hepatocytes and IP₃R3 in rapidly proliferating cells as e.g. epithelial cells. As a consequence, dysfunction and/or dysregulation of IP₃R isoforms will have distinct pathophysiological outcomes, ranging from neurological disorders for IP₃R1 to dysfunctional exocrine tissues and autoimmune diseases for IP₃R2 and -3. Over the past years, several IP₃R mutations have surfaced in the sequence analysis of patient-derived samples. Here, we aimed to provide an integrative overview of the clinically most relevant mutations for each IP₃R isoform and the subsequent molecular mechanisms underlying the etiology of the disease.

Ca2+ signaling and spinocerebellar ataxia

Spinocerebellar ataxia (SCA) is a neural disorder, which is caused by degenerative changes in the cerebellum. SCA is primarily characterized by gait ataxia, and additional clinical features include nystagmus, dysarthria, tremors and cerebellar atrophy. Forty-four hereditary SCAs have been identified to date, along with >35 SCA-associated genes. Despite the great diversity and distinct functionalities of the SCA-related genes, accumulating evidence supports the occurrence of a common pathophysiological event among several hereditary SCAs. Altered calcium (Ca²⁺) homeostasis in the Purkinje cells (PCs) of the cerebellum has been proposed as a possible pathological SCA trigger. In support of this, signaling events that are initiated from or lead to aberrant Ca²⁺ release from the type 1 inositol 1,4,5-trisphosphate receptor (IP₃R1), which is highly expressed in cerebellar PCs, seem to be closely associated with the pathogenesis of several SCA types. In this review, we summarize the current research on pathological hereditary SCA events, which involve altered Ca²⁺ homeostasis in PCs, through IP₃R1 signaling.

Remodeling of Ca2+ signaling in cancer: Regulation of inositol 1,4,5-trisphosphate receptors through oncogenes and tumor suppressors

The calcium ion (Ca²⁺) is a ubiquitous intracellular signaling molecule that regulates diverse physiological and pathological processes, including cancer. Increasing evidence indicates that oncogenes and tumor suppressors regulate the Ca²⁺ transport systems. Inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃Rs) are IP₃-activated Ca²⁺ release channels located on the endoplasmic reticulum (ER). They play pivotal roles in the regulation of cell death and survival by controlling Ca²⁺ transfer from the ER to mitochondria through mitochondria-associated ER membranes (MAMs). Optimal levels of Ca²⁺ mobilization to mitochondria are necessary for mitochondrial bioenergetics, whereas excessive Ca²⁺ flux into mitochondria causes loss of mitochondrial membrane integrity and apoptotic cell death. In addition to well-known functions on outer mitochondrial membranes, B-cell lymphoma 2 (Bcl-2) family proteins are localized on the ER and regulate IP₃Rs to control Ca²⁺ transfer into mitochondria. Another regulatory protein of IP₃R, IP₃R-binding protein released with IP₃ (IRBIT), cooperates with or counteracts the Bcl-2 family member depending on cellular states. Furthermore, several oncogenes and tumor suppressors, including Akt, K-Ras, phosphatase and tensin homolog (PTEN), promyelocytic leukemia protein (PML), BRCA1, and BRCA1 associated protein 1 (BAP1), are localized on the ER or at MAMs and negatively or positively regulate apoptotic cell death through interactions with IP₃Rs and regulation of Ca²⁺ dynamics. The remodeling of Ca²⁺ signaling by oncogenes and tumor suppressors that interact with IP₃Rs has fundamental roles in the pathology of cancers.

Cyclic AMP Recruits a Discrete Intracellular Ca2+ Store by Unmasking Hypersensitive IP3 Receptors

Inositol 1,4,5-trisphosphate (IP₃) stimulates Ca²⁺ release from the endoplasmic reticulum (ER), and the response is potentiated by 3′,5′-cyclic AMP (cAMP). We investigated this interaction in HEK293 cells using carbachol and parathyroid hormone (PTH) to stimulate formation of IP₃ and cAMP, respectively. PTH alone had no effect on the cytosolic Ca²⁺ concentration, but it potentiated the Ca²⁺ signals evoked by carbachol. Surprisingly, however, the intracellular Ca²⁺ stores that respond to carbachol alone could be both emptied and refilled without affecting the subsequent response to PTH. We provide evidence that PTH unmasks high-affinity IP₃ receptors within a discrete Ca²⁺ store. We conclude that Ca²⁺ stores within the ER that dynamically exchange Ca²⁺ with the cytosol maintain a functional independence that allows one store to be released by carbachol and another to be released by carbachol with PTH. Compartmentalization of ER Ca²⁺ stores adds versatility to IP₃-evoked Ca²⁺ signals.

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Cyclic AMP directly potentiates IP₃-evoked Ca²⁺ release

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The Ca²⁺ stores released by IP₃ alone or IP₃ with cAMP are functionally independent

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Cyclic AMP unmasks high-affinity IP₃ receptors in a discrete ER Ca²⁺ store

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Independent regulation of discrete Ca²⁺ stores increases signaling versatility

Molecular mechanisms of calcium signaling in the modulation of small intestinal ion transports and bicarbonate secretion

Background and Purpose:

Although Ca²⁺ signaling may stimulate small intestinal ion secretion, little is known about its critical role and the molecular mechanisms of Ca²⁺-mediated biological action.

Key Results

Activation of muscarinic receptors by carbachol(CCh) stimulated mouse duodenal I_sc, which was significantly inhibited in Ca²⁺-free serosal solution and by several selective store-operated Ca²⁺ channels(SOC) blockers added to the serosal side of duodenal tissues. Furthermore, we found that CRAC/Orai channels may represent the molecular candidate of SOC in intestinal epithelium. CCh increased intracellular Ca²⁺ but not cAMP, and Ca²⁺ signaling mediated duodenal Cl^- and HCO₃^- secretion in wild type mice but not in CFTR knockout mice. CCh induced duodenal ion secretion and stimulated PI3K/Akt activity in duodenal epithelium, all of which were inhibited by selective PI3K inhibitors with different structures. CCh-induced Ca²⁺ signaling also stimulated the phosphorylation of CFTR proteins and their trafficking to the plasma membrane of duodenal epithelial cells, which were inhibited again by selective PI3K inhibitors.

Materials and Methods

Functional, biochemical and morphological experiments were performed to examine ion secretion, PI3K/Akt and CFTR activity of mouse duodenal epithelium. Ca²⁺ imaging was performed on HT-29 cells.

Conclusions and Implications

Ca²⁺ signaling plays a critical role in intestinal ion secretion via CRAC/Orai-mediated SOCE mechanism on the serosal side of epithelium. We also demonstrated the molecular mechanisms of Ca²⁺ signaling in CFTR-mediated secretion via novel PI3K/Akt pathway. Our findings suggest new perspectives for drug targets to protect the upper GI tract and control liquid homeostasis in the small intestine.

Mutations in S-adenosylhomocysteine hydrolase (AHCY) affect its nucleocytoplasmic distribution and capability to interact with S-adenosylhomocysteine hydrolase-like 1 protein

S-adenosylhomocysteine hydrolase (AHCY) is thought to be located at the sites of ongoing AdoMet-dependent methylation, presumably in the cell nucleus. Endogenous AHCY is located both in cytoplasm and the nucleus. Little is known regarding mechanisms that drive its subcellular distribution, and even less is known on how mutations causing AHCY deficiency affect its intracellular dynamics. Using fluorescence microscopy and GFP-tagged AHCY constructs we show significant differences in the intensity ratio between nuclei and cytoplasm for mutant proteins when compared with wild type AHCY. Interestingly, nuclear export of AHCY is not affected by leptomycin B. Systematic deletions showed that AHCY has two regions, located at both sides of the protein, that contribute to its nuclear localization, implying the interaction with various proteins. In order to evaluate protein interactions in vivo we engaged in bimolecular fluorescence complementation (BiFC) based studies. We investigated previously assumed interaction with AHCY-like-1 protein (AHCYL1), a paralog of AHCY. Indeed, significant interaction between both proteins exists. Additionally, silencing AHCYL1 leads to moderate inhibition of nuclear export of endogenous AHCY.

Mitochondrial and endoplasmic reticulum calcium homeostasis and cell death

The endoplasmic reticulum (ER) and mitochondria cannot be considered as static structures, as they intimately communicate, forming very dynamic platforms termed mitochondria-associated membranes (MAMs). In particular, the ER transmits proper Ca²⁺ signals to mitochondria, which decode them into specific inputs to regulate essential functions, including metabolism, energy production and apoptosis. Here, we will describe the different molecular players involved in the transfer of Ca²⁺ ions from the ER lumen to the mitochondrial matrix and how modifications in both ER-mitochondria contact sites and Ca²⁺ signaling can alter the cell death execution program.

Splicing variation of Long-IRBIT determines the target selectivity of IRBIT family proteins

IRBIT [inositol 1,4,5-trisphosphate receptor (IP₃R) binding protein released with inositol 1,4,5-trisphosphate (IP₃)] is a multifunctional protein that regulates several target molecules such as ion channels, transporters, polyadenylation complex, and kinases. Through its interaction with multiple targets, IRBIT contributes to calcium signaling, electrolyte transport, mRNA processing, cell cycle, and neuronal function. However, the regulatory mechanism of IRBIT binding to particular targets is poorly understood. Long-IRBIT is an IRBIT homolog with high homology to IRBIT, except for a unique N-terminal appendage. Long-IRBIT splice variants have different N-terminal sequences and a common C-terminal region, which is involved in multimerization of IRBIT and Long-IRBIT. In this study, we characterized IRBIT and Long-IRBIT splice variants (IRBIT family). We determined that the IRBIT family exhibits different mRNA expression patterns in various tissues. The IRBIT family formed homo- and heteromultimers. In addition, N-terminal splicing of Long-IRBIT changed the protein stability and selectivity to target molecules. These results suggest that N-terminal diversity of the IRBIT family and various combinations of multimer formation contribute to the functional diversity of the IRBIT family.

IP3 receptor mutations and brain diseases in human and rodents

The inositol 1,4,5-trisphosphate receptor (IP₃ R) is a huge Ca²⁺ channel that is localized at the endoplasmic reticulum. The IP₃ R releases Ca²⁺ from the endoplasmic reticulum upon binding to IP₃ , which is produced by various extracellular stimuli through phospholipase C activation. All vertebrate organisms have three subtypes of IP₃ R genes, which have distinct properties of IP₃ -binding and Ca²⁺ sensitivity, and are differently regulated by phosphorylation and by their associated proteins. Each cell type expresses the three subtypes of IP₃ R in a distinct proportion, which is important for creating and maintaining spatially and temporally appropriate intracellular Ca²⁺ level patterns for the regulation of specific physiological phenomena. Of the three types of IP₃ Rs, the type 1 receptor (IP₃ R1) is dominantly expressed in the brain and is important for brain function. Recent emerging evidence suggests that abnormal Ca²⁺ signals from the IP₃ R1 are closely associated with human brain pathology. In this review, we focus on the recent advances in our knowledge of the regulation of IP₃ R1 and its functional implication in human brain diseases, as revealed by IP₃ R mutation studies and analysis of human disease-associated genes. This article is part of the mini review series "60th Anniversary of the Japanese Society for Neurochemistry".

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

IRBIT controls apoptosis by interacting with the Bcl-2 homolog, Bcl2l10, and by promoting ER-mitochondria contact

IRBIT is a molecule that interacts with the inositol 1,4,5-trisphosphate (IP₃)-binding pocket of the IP₃ receptor (IP₃R), whereas the antiapoptotic protein, Bcl2l10, binds to another part of the IP₃-binding domain. Here we show that Bcl2l10 and IRBIT interact and exert an additive inhibition of IP₃R in the physiological state. Moreover, we found that these proteins associate in a complex in mitochondria-associated membranes (MAMs) and that their interplay is involved in apoptosis regulation. MAMs are a hotspot for Ca²⁺ transfer between endoplasmic reticulum (ER) and mitochondria, and massive Ca²⁺ release through IP₃R in mitochondria induces cell death. We found that upon apoptotic stress, IRBIT is dephosphorylated, becoming an inhibitor of Bcl2l10. Moreover, IRBIT promotes ER mitochondria contact. Our results suggest that by inhibiting Bcl2l10 activity and promoting contact between ER and mitochondria, IRBIT facilitates massive Ca²⁺ transfer to mitochondria and promotes apoptosis. This work then describes IRBIT as a new regulator of cell death.

Probes for manipulating and monitoring IP3

Inositol 1,4,5-trisphosphate (IP₃) is an important second messenger produced via G-protein-coupled receptor- or receptor tyrosine kinase-mediated pathways. IP₃ levels induce Ca²⁺ release from the endoplasmic reticulum (ER) via IP₃ receptor (IP₃R) located in the ER membrane. The resultant spatiotemporal pattern of Ca²⁺ signals regulates diverse cellular functions, including fertilization, gene expression, synaptic plasticity, and cell death. Therefore, monitoring and manipulating IP₃ levels is important to elucidate not only the functions of IP₃-mediated pathways but also the encoding mechanism of IP₃R as a converter of intracellular signals from IP₃ to Ca²⁺.

The Inositol Trisphosphate/Calcium Signaling Pathway in Health and Disease

Many cellular functions are regulated by calcium (Ca2+) signals that are generated by different signaling pathways. One of these is the inositol 1,4,5-trisphosphate/calcium (InsP3/Ca2+) signaling pathway that operates through either primary or modulatory mechanisms. In its primary role, it generates the Ca2+ that acts directly to control processes such as metabolism, secretion, fertilization, proliferation, and smooth muscle contraction. Its modulatory role occurs in excitable cells where it modulates the primary Ca2+ signal generated by the entry of Ca2+ through voltage-operated channels that releases Ca2+ from ryanodine receptors (RYRs) on the internal stores. In carrying out this modulatory role, the InsP3/Ca2+ signaling pathway induces subtle changes in the generation and function of the voltage-dependent primary Ca2+ signal. Changes in the nature of both the primary and modulatory roles of InsP3/Ca2+ signaling are a contributory factor responsible for the onset of a large number human diseases.

Ahcyl2 upregulates NBCe1-B via multiple serine residues of the PEST domain-mediated association

Inositol-1,4,5-triphosphate [IP₃] receptors binding protein released with IP₃ (IRBIT) was previously reported as an activator of NBCe1-B. Recent studies have characterized IRBIT homologue S-Adenosylhomocysteine hydrolase-like 2 (AHCYL2). AHCYL2 is highly homologous to IRBIT (88%) and heteromerizes with IRBIT. The two important domains in the N-terminus of AHCYL2 are a PEST domain and a coiled-coil domain which are highly comparable to those in IRBIT. Therefore, in this study, we tried to identify the role of those domains in mouse AHCYL2 (Ahcyl2), and we succeeded in identifying PEST domain of Ahcyl2 as a regulation region for NBCe1-B activity. Site directed mutagenesis and coimmunoprecipitation assay showed that NBCe1-B binds to the N-terminal Ahcyl2-PEST domain, and its binding is determined by the phosphorylation of 4 critical serine residues (Ser151, Ser154, Ser157, and Ser160) in Ahcyl2 PEST domain. Also we revealed that 4 critical serine residues in Ahcyl2 PEST domain are indispensable for the activation of NBCe1-B using measurement of intracellular pH experiment. Thus, these results suggested that the NBCe1-B is interacted with 4 critical serine residues in Ahcyl2 PEST domain, which play an important role in intracellular pH regulation through NBCe1-B.

Keeping Our Calcium in Balance to Maintain Our Balance

Calcium is a key signaling molecule and ion involved in a variety of diverse processes in our central nervous system (CNS) which include gene expression, synaptic transmission and plasticity, neuronal excitability and cell maintenance. Proper control of calcium signaling is not only vital for neuronal physiology but also cell survival. Mutations in fundamental channels, transporters and second messenger proteins involved in orchestrating the balance of our calcium homeostasis can lead to severe neurodegenerative disorders, such as Spinocerebellar (SCA) and Episodic (EA) ataxias. Hereditary ataxias make up a remarkably diverse group of neurological disorders clinically characterized by gait ataxia, nystagmus, dysarthria, trunk and limb ataxia and often atrophy of the cerebellum. The largest family of hereditary ataxias is SCAs which consists of a growing family of 42 members. A relatively smaller family of 8 members compose the EAs. The gene mutations responsible for half of the EA members and over 35 of the SCA subtypes have been identified, and several have been found to be responsible for cerebellar atrophy, abnormal intracellular calcium levels, dysregulation of Purkinje cell pacemaking, altered cerebellar synaptic transmission and/or ataxia in mouse models. Although the genetic diversity and affected cellular pathways of hereditary ataxias are broad, one common theme amongst these genes is their effects on maintaining calcium balance in primarily the cerebellum. There is emerging evidence that the pathogenesis of hereditary ataxias may be caused by imbalances in intracellular calcium due to genetic mutations in calcium-mediating proteins. In this review we will discuss the current evidence supporting the role of deranged calcium as the culprit to neurodegenerative diseases with a primary focus on SCAs and EAs.