Defining the stoichiometry of inositol 1,4,5-trisphosphate binding required to initiate Ca2+ release

The role of Ca2+ signalling and InsP3R in the pathogenesis of intrahepatic cholestasis of pregnancy

BACKGROUND

In order to contribute new insights for future prevention and treatment of intrahepatic cholestasis of pregnancy (ICP), and to promote positive pregnancy outcomes, we evaluated serum Ca2+ levels and inositol 1,4,5-trisphosphate receptor (InsP3R) expression in the liver tissue of a rat ICP model.

METHODS

After establishing the model by injection of oestradiol benzoate and progesterone into pregnant rats, animals were divided into normal control (n = 5) and ICP model groups (n = 5). The expression of InsP3R protein in the liver, and serum levels of Ca2+, glycocholic acid and bile acid were detected.

RESULTS

InsP3R mRNA and protein were significantly lower in the ICP model group compared to the normal group, as determined by qPCR and immunohistochemistry, respectively. Serum enzyme-linked immunosorbent assay results revealed significantly higher levels of glycocholic acid and bile acid in the ICP model group compared to the normal group, while Ca2+ levels were significantly lower. The levers of Ca2+ were significantly and negatively correlated with the levels of glycocholic acid. The observed decrease in Ca2+ was associated with an increase in total bile acids, but there was no significant correlation.

CONCLUSIONS

Our results revealed that the expression of InsP3R and serum Ca2+ levels was significantly decreased in the liver tissue of ICP model rats. Additionally, Ca2+ levels were found to be negatively correlated with the level of glycocholic acid.

Dual regulation of IP3 receptors by IP3 and PIP2 controls the transition from local to global Ca2+ signals

The spatial organization of inositol 1,4,5-trisphosphate (IP3)-evoked Ca2+ signals underlies their versatility. Low stimulus intensities evoke Ca2+ puffs, localized Ca2+ signals arising from a few IP3 receptors (IP3Rs) within a cluster tethered beneath the plasma membrane. More intense stimulation evokes global Ca2+ signals. Ca2+ signals propagate regeneratively as the Ca2+ released stimulates more IP3Rs. How is this potentially explosive mechanism constrained to allow local Ca2+ signaling? We developed methods that allow IP3 produced after G-protein coupled receptor (GPCR) activation to be intercepted and replaced by flash photolysis of a caged analog of IP3. We find that phosphatidylinositol 4,5-bisphosphate (PIP2) primes IP3Rs to respond by partially occupying their IP3-binding sites. As GPCRs stimulate IP3 formation, they also deplete PIP2, relieving the priming stimulus. Loss of PIP2 resets IP3R sensitivity and delays the transition from local to global Ca2+ signals. Dual regulation of IP3Rs by PIP2 and IP3 through GPCRs controls the transition from local to global Ca2+ signals.

A pleiotropic recurrent dominant ITPR3 variant causes a complex multisystemic disease

Inositol 1,4,5-trisphosphate (IP3) receptor type 1 (ITPR1), 2 (ITPR2), and 3 (ITPR3) encode the IP3 receptor (IP3R), a key player in intracellular calcium release. In four unrelated patients, we report that an identical ITPR3 de novo variant-NM_002224.3:c.7570C>T, p.Arg2524Cys-causes, through a dominant-negative effect, a complex multisystemic disorder with immunodeficiency. This leads to defective calcium homeostasis, mitochondrial malfunction, CD4+ lymphopenia, a quasi-absence of naïve CD4+ and CD8+ cells, an increase in memory cells, and a distinct TCR repertoire. The calcium defect was recapitulated in Jurkat knock-in. Site-directed mutagenesis displayed the exquisite sensitivity of Arg2524 to any amino acid change. Despite the fact that all patients had severe immunodeficiency, they also displayed variable multisystemic involvements, including ectodermal dysplasia, Charcot-Marie-Tooth disease, short stature, and bone marrow failure. In conclusion, unlike previously reported ITPR1-3 deficiencies leading to narrow, mainly neurological phenotypes, a recurrent dominant ITPR3 variant leads to a multisystemic disease, defining a unique role for IP3R3 in the tetrameric IP3R complex.

Functional investigation of a putative calcium-binding site involved in the inhibition of inositol 1,4,5-trisphosphate receptor activity

A wide variety of factors influence inositol 1,4,5-trisphosphate (IP 3 ) receptor (IP 3 R) activity resulting in modulation of intracellular Ca 2+ release. This regulation is thought to define the spatio-temporal patterns of Ca 2+ signals necessary for the appropriate activation of downstream effectors. The binding of both IP 3 and Ca 2+ are obligatory for IP 3 R channel opening, however, Ca 2+ regulates IP 3 R activity in a biphasic manner. Mutational studies have revealed that Ca 2+ binding to a high-affinity pocket formed by the ARM3 domain and linker domain promotes IP 3 R channel opening without altering the Ca 2+ dependency for channel inactivation. These data suggest a distinct low-affinity Ca 2+ binding site is responsible for the reduction in IP 3 R activity at higher [Ca 2+ ]. We determined the consequences of mutating a cluster of acidic residues in the ARM2 and central linker domain reported to coordinate Ca 2+ in cryo-EM structures of the IP 3 R type 3. This site is termed the "CD Ca 2+ binding site" and is well-conserved in all IP 3 R sub-types. We show that the CD site Ca 2+ binding mutants where the negatively charged glutamic acid residues are mutated to alanine exhibited enhanced sensitivity to IP 3 -generating agonists. Ca 2+ binding mutants displayed spontaneous elemental Ca 2+ events (Ca 2+ puffs) and the number of IP 3 -induced Ca 2+ puffs was significantly augmented in cells stably expressing Ca 2+ binding site mutants. When measured with "on-nucleus" patch clamp, the inhibitory effect of high [Ca 2+ ] on single channel-open probability (P o ) was reduced in mutant channels and this effect was dependent on [ATP]. These results indicate that Ca 2+ binding to the putative CD Ca 2+ inhibitory site facilitates the reduction in IP 3 R channel activation when cytosolic [ATP] is reduced and suggest that at higher [ATP], additional Ca 2+ binding motifs may contribute to the biphasic regulation of IP 3 -induced Ca 2+ release.

Calcium signaling in oocyte quality and functionality and its application

Calcium (Ca2+) is a second messenger for many signal pathways, and changes in intracellular Ca2+ concentration ([Ca2+]i) are an important signaling mechanism in the oocyte maturation, activation, fertilization, function regulation of granulosa and cumulus cells and offspring development. Ca2+ oscillations occur during oocyte maturation and fertilization, which are maintained by Ca2+ stores and extracellular Ca2+ ([Ca2+]e). Abnormalities in Ca2+ signaling can affect the release of the first polar body, the first meiotic division, and chromosome and spindle morphology. Well-studied aspects of Ca2+ signaling in the oocyte are oocyte activation and fertilization. Oocyte activation, driven by sperm-specific phospholipase PLCζ, is initiated by concerted intracellular patterns of Ca2+ release, termed Ca2+ oscillations. Ca2+ oscillations persist for a long time during fertilization and are coordinately engaged by a variety of Ca2+ channels, pumps, regulatory proteins and their partners. Calcium signaling also regulates granulosa and cumulus cells' function, which further affects oocyte maturation and fertilization outcome. Clinically, there are several physical and chemical options for treating fertilization failure through oocyte activation. Additionally, various exogenous compounds or drugs can cause ovarian dysfunction and female infertility by inducing abnormal Ca2+ signaling or Ca2+ dyshomeostasis in oocytes and granulosa cells. Therefore, the reproductive health risks caused by adverse stresses should arouse our attention. This review will systematically summarize the latest research progress on the aforementioned aspects and propose further research directions on calcium signaling in female reproduction.

Direct measurements of luminal Ca 2+ with endo-lysosomal GFP-aequorin reveal functional IP 3 receptors

Endo-lysosomes are considered acidic Ca 2+ stores but direct measurements of luminal Ca 2+ within them are limited. Here we report that the Ca 2+ -sensitive luminescent protein aequorin does not reconstitute with its cofactor at highly acidic pH but that a significant fraction of the probe is functional within a mildly acidic compartment when targeted to the endo-lysosomal system. We leveraged this probe (ELGA) to report Ca 2+ dynamics in this compartment. We show that Ca 2+ uptake is ATP-dependent and sensitive to blockers of endoplasmic reticulum Ca 2+ pumps. We find that the Ca 2+ mobilizing messenger IP 3 which typically targets the endoplasmic reticulum evokes robust luminal responses in wild type cells, but not in IP 3 receptor knock-out cells. Responses were comparable to those evoked by activation of the endo-lysosomal ion channel TRPML1. Stimulation with IP 3 -forming agonists also mobilized the store in intact cells. Super-resolution microscopy analysis confirmed the presence of IP 3 receptors within the endo-lysosomal system, both in live and fixed cells. Our data reveal a physiologically-relevant, IP 3 -sensitive store of Ca 2+ within the endo-lysosomal system.

NF2 regulates IP3R-mediated Ca2+ signal and apoptosis in meningiomas

Meningiomas are the most common primary intracranial tumors and account for nearly 30% of all nervous system tumors. Approximately half of meningioma patients exhibit neurofibromin 2 (NF2) gene inactivation. Here, NF2 was shown to interact with the endoplasmic reticulum (ER) calcium (Ca2+) channel inositol 1,4,5-trisphosphate receptor 1 (IP3R1) in IOMM-Lee, a high-grade malignant meningioma cell line, and the F1 subdomain of NF2 plays a critical role in this interaction. Functional assays indicated that NF2 promotes the phosphorylation of IP3R (Ser 1756) and IP3R-mediated endoplasmic reticulum (ER) Ca2+ release by binding to IP3R1, which results in Ca2+-dependent apoptosis. Knockout of NF2 decreased Ca2+ release and promoted resistance to apoptosis, which was rescued by wild-type NF2 overexpression but not by F1 subdomain deletion truncation overexpression. The effects of NF2 defects on the development of tumors were further studied in mouse models. The decreased expression level of NF2 caused by NF2 gene knockout or mutation affects the activity of the IP3R channel, which reduces Ca2+-dependent apoptosis, thereby promoting the development of tumors. We elucidated the interaction patterns of NF2 and IP3R1, revealed the molecular mechanism through which NF2 regulates IP3R1-mediated Ca2+ release, and elucidated the new pathogenic mechanism of meningioma-related NF2 variants. Our study broadens the current understanding of the biological function of NF2 and provides ideas for drug screening of NF2-associated meningioma.

A structure-based computational model of IP3R1 incorporating Ca and IP3 regulation

The inositol 1,4,5-triphosphate receptor (IP3R) mediates Ca release in many cell types and is pivotal to a wide range of cellular processes. High-resolution cryoelectron microscopy studies have provided new structural details of IP3R type 1 (IP3R1), showing that channel function is determined by the movement of various domains within and between each of its four subunits. Channel properties are regulated by ligands, such as Ca and IP3, which bind at specific sites and control the interactions between these domains. However, it is not known how the various ligand-binding sites on IP3R1 interact to control the opening of the channel. In this study, we present a coarse-grained model of IP3R1 that accounts for the channel architecture and the location of specific Ca- and IP3-binding sites. This computational model accounts for the domain-domain interactions within and between the four subunits that form IP3R1, and it also describes how ligand binding regulates these interactions. Using a kinetic model, we explore how two Ca-binding sites on the cytosolic side of the channel interact with the IP3-binding site to regulate the channel open probability. Our primary finding is that the bell-shaped open probability of IP3R1 provides constraints on the relative strength of these regulatory binding sites. In particular, we argue that a specific Ca-binding site, whose function has not yet been established, is very likely a channel antagonist. Additionally, we apply our model to show that domain-domain interactions between neighboring subunits exert control over channel cooperativity and dictate the nonlinear response of the channel to Ca concentration. This suggests that specific domain-domain interactions play a pivotal role in maintaining the channel's stability, and a disruption of these interactions may underlie disease states associated with Ca dysregulation.

Pacemaking in the lymphatic system

Lymphatic collecting vessels exhibit spontaneous phasic contractions that are critical for lymph propulsion and tissue fluid homeostasis. This rhythmic activity is driven by action potentials conducted across the lymphatic muscle cell (LMC) layer to produce entrained contractions. The contraction frequency of a lymphatic collecting vessel displays exquisite mechanosensitivity, with a dynamic range from <1 to >20 contractions per minute. A myogenic pacemaker mechanism intrinsic to the LMCs was initially postulated to account for pressure-dependent chronotropy. Further interrogation into the cellular constituents of the lymphatic vessel wall identified non-muscle cell populations that shared some characteristics with interstitial cells of Cajal, which have pacemaker functions in the gastrointestinal and lower urinary tracts, thus raising the possibility of a non-muscle cell pacemaker. However, recent genetic knockout studies in mice support LMCs and a myogenic origin of the pacemaker activity. LMCs exhibit stochastic, but pressure-sensitive, sarcoplasmic reticulum calcium release (puffs and waves) from IP3R1 receptors, which couple to the calcium-activated chloride channel Anoctamin 1, causing depolarisation. The resulting electrical activity integrates across the highly coupled lymphatic muscle electrical syncytia through connexin 45 to modulate diastolic depolarisation. However, multiple other cation channels may also contribute to the ionic pacemaking cycle. Upon reaching threshold, a voltage-gated calcium channel-dependent action potential fires, resulting in a nearly synchronous calcium global calcium flash within the LMC layer to drive an entrained contraction. This review summarizes the key ion channels potentially responsible for the pressure-dependent chronotropy of lymphatic collecting vessels and various mechanisms of IP3R1 regulation that could contribute to frequency tuning.

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.

IP3RPEP6, a novel peptide inhibitor of IP3 receptor channels that does not affect connexin-43 hemichannels

AIM

Inositol 1,4,5-trisphosphate receptors (IP3 Rs) are intracellular Ca2+ -release channels with crucial roles in cell function. Current IP3 R inhibitors suffer from off-target effects and poor selectivity towards the three distinct IP3 R subtypes. We developed a novel peptide inhibitor of IP3 Rs and determined its effect on connexin-43 (Cx43) hemichannels, which are co-activated by IP3 R stimulation.

METHODS

IP3RPEP6 was developed by in silico molecular docking studies and characterized by on-nucleus patch-clamp experiments of IP3 R2 channels and carbachol-induced IP3 -mediated Ca2+ responses in IP3 R1, 2 or 3 expressing cells, triple IP3 R KO cells and astrocytes. Cx43 hemichannels were studied by patch-clamp and ATP-release approaches, and by inhibition with Gap19 peptide. IP3RPEP6 interactions with IP3 Rs were verified by co-immunoprecipitation and affinity pull-down assays.

RESULTS

IP3RPEP6 concentration-dependently reduced the open probability of IP3 R2 channels and competitively inhibited IP3 Rs in an IC50 order of IP3 R2 (~3.9 μM) < IP3 R3 (~4.3 μM) < IP3 R1 (~9.0 μM), without affecting Cx43 hemichannels or ryanodine receptors. IP3RPEP6 co-immunoprecipitated with IP3 R2 but not with IP3 R1; interaction with IP3 R3 varied between cell types. The IC50 of IP3RPEP6 inhibition of carbachol-induced Ca2+ responses decreased with increasing cellular Cx43 expression. Moreover, Gap19-inhibition of Cx43 hemichannels significantly reduced the amplitude of the IP3 -Ca2+ responses and strongly increased the EC50 of these responses. Finally, we identified palmitoyl-8G-IP3RPEP6 as a membrane-permeable IP3RPEP6 version allowing extracellular application of the IP3 R-inhibiting peptide.

CONCLUSION

IP3RPEP6 inhibits IP3 R2/R3 at concentrations that have limited effects on IP3 R1. IP3 R activation triggers hemichannel opening, which strongly affects the amplitude and concentration-dependence of IP3 -triggered Ca2+ responses.

Cellular effects of BAPTA: Are they only about Ca2+ chelation?

Intracellular Ca2+ signals play a vital role in a broad range of cell biological and physiological processes in all eukaryotic cell types. Dysregulation of Ca2+ signaling has been implicated in numerous human diseases. Over the past four decades, the understanding of how cells use Ca2+ as a messenger has flourished, largely because of the development of reporters that enable visualization of Ca2+ signals in different cellular compartments, and tools that can modulate cellular Ca2+ signaling. One such tool that is frequently used is BAPTA; a fast, high-affinity Ca2+-chelating molecule. By making use of a cell-permeable acetoxymethyl ester (AM) variant, BAPTA can be readily loaded into the cytosol of cells (referred to as BAPTAi), where it is trapped and able to buffer changes in cytosolic Ca2+. Due to the ease of loading of the AM version of BAPTA, this reagent has been used in hundreds of studies to probe the role of Ca2+ signaling in specific processes. As such, for decades, researchers have almost universally attributed changes in biological processes caused by BAPTAi to the involvement of Ca2+ signaling. However, BAPTAi has often been used without any form of control, and in many cases has neither been shown to be retained in cells for the duration of experiments nor to buffer any Ca2+ signals. Moreover, increasing evidence points to off-target cellular effects of BAPTA that are clearly not related to Ca2+ chelation. Here, we briefly introduce Ca2+ signaling and the history of Ca2+ chelators and fluorescent Ca2+ indicators. We highlight Ca2+-independent effects of BAPTAi on a broad range of molecular targets and describe some of BAPTAi's impacts on cell functions that occur independently of its Ca2+-chelating properties. Finally, we propose strategies for determining whether Ca2+ chelation, the binding of other metal ions, or off-target interactions with cell components are responsible for BAPTAi's effect on a particular process and suggest some future research directions.

Relevance of the endoplasmic reticulum-mitochondria axis in cancer diagnosis and therapy

Dynamic interactions between organelles are responsible for a variety of intercellular functions, and the endoplasmic reticulum (ER)-mitochondrial axis is recognized as a representative interorganelle system. Several studies have confirmed that most proteins in the physically tethered sites between the ER and mitochondria, called mitochondria-associated ER membranes (MAMs), are vital for intracellular physiology. MAM proteins are involved in the regulation of calcium homeostasis, lipid metabolism, and mitochondrial dynamics and are associated with processes related to intracellular stress conditions, such as oxidative stress and unfolded protein responses. Accumulating evidence has shown that, owing to their extensive involvement in cellular homeostasis, alterations in the ER-mitochondrial axis are one of the etiological factors of tumors. An in-depth understanding of MAM proteins and their impact on cell physiology, particularly in cancers, may help elucidate their potential as diagnostic and therapeutic targets for cancers. For example, the modulation of MAM proteins is utilized not only to target diverse intracellular signaling pathways within cancer cells but also to increase the sensitivity of cancer cells to anticancer reagents and regulate immune cell activities. Therefore, the current review summarizes and discusses recent advances in research on the functional roles of MAM proteins and their characteristics in cancers from a diagnostic perspective. Additionally, this review provides insights into diverse therapeutic strategies that target MAM proteins in various cancer types.

Two-pore channel-2 and inositol trisphosphate receptors coordinate Ca2+ signals between lysosomes and the endoplasmic reticulum

Lysosomes and the endoplasmic reticulum (ER) are Ca2+ stores mobilized by the second messengers NAADP and IP3, respectively. Here, we establish Ca2+ signals between the two sources as fundamental building blocks that couple local release to global changes in Ca2+. Cell-wide Ca2+ signals evoked by activation of endogenous NAADP-sensitive channels on lysosomes comprise both local and global components and exhibit a major dependence on ER Ca2+ despite their lysosomal origin. Knockout of ER IP3 receptor channels delays these signals, whereas expression of lysosomal TPC2 channels accelerates them. High-resolution Ca2+ imaging reveals elementary events upon TPC2 opening and signals coupled to IP3 receptors. Biasing TPC2 activation to a Ca2+-permeable state sensitizes local Ca2+ signals to IP3. This increases the potency of a physiological agonist to evoke global Ca2+ signals and activate a downstream target. Our data provide a conceptual framework to understand how Ca2+ release from physically separated stores is coordinated.

Detailed Analysis of ITPR1 Missense Variants Guides Diagnostics and Therapeutic Design

BACKGROUND

The ITPR1 gene encodes the inositol 1,4,5-trisphosphate (IP3 ) receptor type 1 (IP3 R1), a critical player in cerebellar intracellular calcium signaling. Pathogenic missense variants in ITPR1 cause congenital spinocerebellar ataxia type 29 (SCA29), Gillespie syndrome (GLSP), and severe pontine/cerebellar hypoplasia. The pathophysiological basis of the different phenotypes is poorly understood.

OBJECTIVES

We aimed to identify novel SCA29 and GLSP cases to define core phenotypes, describe the spectrum of missense variation across ITPR1, standardize the ITPR1 variant nomenclature, and investigate disease progression in relation to cerebellar atrophy.

METHODS

Cases were identified using next-generation sequencing through the Deciphering Developmental Disorders study, the 100,000 Genomes project, and clinical collaborations. ITPR1 alternative splicing in the human cerebellum was investigated by quantitative polymerase chain reaction.

RESULTS

We report the largest, multinational case series of 46 patients with 28 unique ITPR1 missense variants. Variants clustered in functional domains of the protein, especially in the N-terminal IP3 -binding domain, the carbonic anhydrase 8 (CA8)-binding region, and the C-terminal transmembrane channel domain. Variants outside these domains were of questionable clinical significance. Standardized transcript annotation, based on our ITPR1 transcript expression data, greatly facilitated analysis. Genotype-phenotype associations were highly variable. Importantly, while cerebellar atrophy was common, cerebellar volume loss did not correlate with symptom progression.

CONCLUSIONS

This dataset represents the largest cohort of patients with ITPR1 missense variants, expanding the clinical spectrum of SCA29 and GLSP. Standardized transcript annotation is essential for future reporting. Our findings will aid in diagnostic interpretation in the clinic and guide selection of variants for preclinical studies. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Lidocaine induces apoptosis in head and neck squamous cell carcinoma through activation of bitter taste receptor T2R14

Head and neck squamous cell carcinomas (HNSCCs) have high mortality and significant treatment-related morbidity. It is vital to discover effective, minimally invasive therapies that improve survival and quality of life. Bitter taste receptors (T2Rs) are expressed in HNSCCs, and T2R activation can induce apoptosis. Lidocaine is a local anesthetic that also activates bitter taste receptor 14 (T2R14). Lidocaine has some anti-cancer effects, but the mechanisms are unclear. Here, we find that lidocaine causes intracellular Ca2+ mobilization through activation of T2R14 in HNSCC cells. T2R14 activation with lidocaine depolarizes mitochondria, inhibits proliferation, and induces apoptosis. Concomitant with mitochondrial Ca2+ influx, ROS production causes T2R14-dependent accumulation of poly-ubiquitinated proteins, suggesting that proteasome inhibition contributes to T2R14-induced apoptosis. Lidocaine may have therapeutic potential in HNSCCs as a topical gel or intratumor injection. In addition, we find that HPV-associated (HPV+) HNSCCs are associated with increased TAS2R14 expression. Lidocaine treatment may benefit these patients, warranting future clinical studies.

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.

GRPR down-regulation inhibits spermatogenesis through Ca2+ mediated by PLCβ/IP3R signaling pathway in long-term formaldehyde-exposed rats

Formaldehyde (FA), which is known as an air pollutant, has been proven to induce male infertility. However, the underlying mechanism of FA-induced male infertility remains elusive. In this study, 24 male SD rats were exposed to different levels of FA (0, 0.5, 2.46, and 5 mg/m3) for eight consecutive weeks. Through HE staining and sperm smear, we observed that FA exposure resulted in spermatogenic injury and the sperm quality decreased in rats. The qRT-PCR and Western blot analysis further revealed that GRPR was down-regulated in testicular tissues of FA-exposed rats as well as primary spermatogenic cells. Meanwhile, ZDOCK uncovered an interaction between GRPR and PLCβ. In addition, the CCK8, Fluo 3-AM and Flow cytometry results showed that FA exposure suppressed the expression of GRPR, PLCβ and IP3R, consequently reducing the Ca2+ concentration in spermatogenic cells, inducing apoptosis and inhibiting proliferation of spermatogenic cells. Moreover, rescue experiments confirmed that promoting GRPR could improve intracellular Ca2+ concentration by upregulating PLCβ and IP3R, partially reducing the apoptosis and promoting the proliferation of FA-treated spermatogenic cells. These findings revealed that GRPR participates in spermatogenesis through Ca2+ mediated by the PLCβ/IP3R signaling pathway in FA-exposed rats.

Human IP3 receptor triple knockout stem cells remain pluripotent despite altered mitochondrial metabolism

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ER Ca2+-release channels that control a broad set of cellular processes. Animal models lacking IP3Rs in different combinations display severe developmental phenotypes. Given the importance of IP3Rs in human diseases, we investigated their role in human induced pluripotent stem cells (hiPSC) by developing single IP3R and triple IP3R knockouts (TKO). Genome edited TKO-hiPSC lacking all three IP3R isoforms, IP3R1, IP3R2, IP3R3, failed to generate Ca2+ signals in response to agonists activating GPCRs, but retained stemness and pluripotency. Steady state metabolite profiling and flux analysis of TKO-hiPSC indicated distinct alterations in tricarboxylic acid cycle metabolites consistent with a deficiency in their pyruvate utilization via pyruvate dehydrogenase, shifting towards pyruvate carboxylase pathway. These results demonstrate that IP3Rs are not essential for hiPSC identity and pluripotency but regulate mitochondrial metabolism. This set of knockout hiPSC is a valuable resource for investigating IP3Rs in human cell types of interest.

Understanding IP3R channels: From structural underpinnings to ligand-dependent conformational landscape

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are ubiquitously expressed large-conductance Ca2+-permeable channels predominantly localized to the endoplasmic reticulum (ER) membranes of virtually all eukaryotic cell types. IP3Rs work as Ca2+ signaling hubs through which diverse extracellular stimuli and intracellular inputs are processed and then integrated to result in delivery of Ca2+ from the ER lumen to generate cytosolic Ca2+ signals with precise temporal and spatial properties. IP3R-mediated Ca2+ signals control a vast repertoire of cellular functions ranging from gene transcription and secretion to the more enigmatic brain activities such as learning and memory. IP3Rs open and release Ca2+ when they bind both IP3 and Ca2+, the primary channel agonists. Despite overwhelming evidence supporting functional interplay between IP3 and Ca2+ in activation and inhibition of IP3Rs, the mechanistic understanding of how IP3R channels convey their gating through the interplay of two primary agonists remains one of the major puzzles in the field. The last decade has seen much progress in the use of cryogenic electron microscopy to elucidate the molecular mechanisms of ligand binding, ion permeation, ion selectivity and gating of the IP3R channels. The results of these studies, summarized in this review, provide a prospective view of what the future holds in structural and functional research of IP3Rs.

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.

TRiPPing the sensors: The osmosensing pathway of Polycystin 2

Mutations to polycystin-2 (PC2), a non-selective cation permeant transient receptor potential channel, results in polycystic kidney disease (PKD). Despite the disease relevance of PC2, the physiological agonist that activates PC2 has remained elusive. As one of the earliest symptoms in PKD is a urine concentrating deficiency, we hypothesized that shifts in osmolarity experienced by the collecting duct cells would activate PC2 and loss of PC2 would prevent osmosensing. We found that mice with inducible PC2 knocked out (KO) in renal tubules had dilute urine. Hyperosmotic stimuli induced a rise in endoplasmic reticulum (ER)-mediated cytosolic calcium which was absent in PC2 KO mice and PC2 KO cells. A pathologic point mutation that prevents ion flux through PC2 inhibited the calcium rise, pointing to the centrality of PC2 in the osmotic response. To understand how an extracellular stimulus activated ER-localized PC2, we examined microtubule-ER dynamics, and found that the osmotically induced calcium increase was preceded by microtubule destabilization. This was due to a novel interaction between PC2 and the microtubule binding protein MAP4 that tethers the microtubules to the ER. Finally, disruption of the MAP4-PC2 interaction prevented incorporation of the water channel aquaporin 2 following a hyperosmotic challenge, in part explaining the dilute urine. Our results demonstrate that MAP4-dependent microtubule stabilization of ER-resident PC2 is required for PC2 to participate in the osmosensing pathway. Moreover, osmolarity represents a bona fide physiological stimulus for ER-localized PC2 and loss of PC2 in renal epithelial cells impairs osmosensing ability and urine concentrating capacity.

The ER-mitochondria interface, where Ca2+ and cell death meet

Endoplasmic reticulum (ER)-mitochondria contact sites are crucial to allow Ca²⁺ flux between them and a plethora of proteins participate in tethering both organelles together. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) play a pivotal role at such contact sites, participating in both ER-mitochondria tethering and as Ca²⁺-transport system that delivers Ca²⁺ from the ER towards mitochondria. At the ER-mitochondria contact sites, the IP₃Rs function as a multi-protein complex linked to the voltage-dependent anion channel 1 (VDAC1) in the outer mitochondrial membrane, via the chaperone glucose-regulated protein 75 (GRP75). This IP₃R-GRP75-VDAC1 complex supports the efficient transfer of Ca²⁺ from the ER into the mitochondrial intermembrane space, from which the Ca²⁺ ions can reach the mitochondrial matrix through the mitochondrial calcium uniporter. Under physiological conditions, basal Ca²⁺ oscillations deliver Ca²⁺ to the mitochondrial matrix, thereby stimulating mitochondrial oxidative metabolism. However, when mitochondrial Ca²⁺ overload occurs, the increase in [Ca²⁺] will induce the opening of the mitochondrial permeability transition pore, thereby provoking cell death. The IP₃R-GRP75-VDAC1 complex forms a hub for several other proteins that stabilize the complex and/or regulate the complex's ability to channel Ca²⁺ into the mitochondria. These proteins and their mechanisms of action are discussed in the present review with special attention for their role in pathological conditions and potential implication for therapeutic strategies.

Non-inositol 1,4,5-trisphosphate (IP3) receptor IP3-binding proteins

Conventionally, myo-D-inositol 1, 4,5-trisphosphate (IP₃) is thought to exert its second messenger effects through the gating of IP₃R Ca²⁺ release channels, located in Ca²⁺-storage organelles like the endoplasmic reticulum. However, there is considerable indirect evidence to support the concept that IP₃ might interact with other, non-IP₃R proteins within cells. To explore this possibility further, the Protein Data Bank was searched using the term "IP3". This resulted in the retrieval of 203 protein structures, the majority of which were members of the IP₃R/ryanodine receptor superfamily of channels. Only 49 of these structures were complexed with IP₃. These were inspected for their ability to interact with the carbon-1 phosphate of IP₃, since this is the least accessible phosphate group of its precursor, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂). This reduced the number of structures retrieved to 35, of which 9 were IP₃Rs. The remaining 26 structures represent a diverse range of proteins, including inositol-lipid metabolizing enzymes, signal transducers, PH domain containing proteins, cytoskeletal anchor proteins, the TRPV4 ion channel, a retroviral Gag protein and fibroblast growth factor 2. Such proteins may impact on IP₃ signalling and its effects on cell-biology. This represents an area open for exploration in the field of IP₃ signalling.

Computational investigation of IP3 diffusion

Inositol 1,4,5-trisphosphate (IP₃) plays a key role in calcium signaling. After stimulation, it diffuses from the plasma membrane where it is produced to the endoplasmic reticulum where its receptors are localized. Based on in vitro measurements, IP₃ was long thought to be a global messenger characterized by a diffusion coefficient of ~ 280 μm²s^-1. However, in vivo observations revealed that this value does not match with the timing of localized Ca²⁺ increases induced by the confined release of a non-metabolizable IP₃ analog. A theoretical analysis of these data concluded that in intact cells diffusion of IP₃ is strongly hindered, leading to a 30-fold reduction of the diffusion coefficient. Here, we performed a new computational analysis of the same observations using a stochastic model of Ca²⁺ puffs. Our simulations concluded that the value of the effective IP₃ diffusion coefficient is close to 100 μm²s^-1. Such moderate reduction with respect to in vitro estimations quantitatively agrees with a buffering effect by non-fully bound inactive IP₃ receptors. The model also reveals that IP₃ spreading is not much affected by the endoplasmic reticulum, which represents an obstacle to the free displacement of molecules, but can be significantly increased in cells displaying elongated, 1-dimensional like geometries.

Dissociation of inositol 1,4,5-trisphosphate from IP3 receptors contributes to termination of Ca2+ puffs

Ca²⁺ puffs are brief, localized Ca²⁺ signals evoked by physiological stimuli that arise from the coordinated opening of a few clustered inositol 1,4,5-trisphosphate receptors (IP₃Rs). However, the mechanisms that control the amplitude and termination of Ca²⁺ puffs are unresolved. To address these issues, we expressed SNAP-tagged IP₃R3 in HEK cells without endogenous IP₃Rs and used total internal reflection fluorescence microscopy to visualize the subcellular distribution of IP₃Rs and the Ca²⁺ puffs that they evoke. We first confirmed that SNAP-IP₃R3 were reliably identified and that they evoked normal Ca²⁺ puffs after photolysis of a caged analog of IP₃. We show that increased IP₃R expression caused cells to assemble more IP₃R clusters, each of which contained more IP₃Rs, but the mean amplitude of Ca²⁺ puffs (indicative of the number of open IP₃Rs) was unaltered. We thus suggest that functional interactions between IP₃Rs constrain the number of active IP₃Rs within a cluster. Furthermore, Ca²⁺ puffs evoked by IP₃R with reduced affinity for IP₃ had undiminished amplitude, but the puffs decayed more quickly. The selective effect of reducing IP₃ affinity on the decay times of Ca²⁺ puffs was not mimicked by exposing normal IP₃R to a lower concentration of IP₃. We conclude that distinct mechanisms constrain recruitment of IP₃Rs during the rising phase of a Ca²⁺ puff and closure of IP₃Rs during the falling phase, and that only the latter is affected by the rate of IP₃ dissociation.

Heterogeneity of the endoplasmic reticulum Ca2+ store determines colocalization with mitochondria

Contact sites between the endoplasmic reticulum (ER) and mitochondria play a pivotal role in cell signaling, and the interaction between these organelles is dynamic and finely regulated. We have studied the role of ER Ca²⁺ concentration ([Ca²⁺]_ER) in modulating this association in HeLa and HEK293 cells and human fibroblasts. According to Manders' coefficient, ER-mitochondria colocalization varied depending on the ER marker; it was the highest with ER-Tracker and the lowest with ER Ca²⁺ indicators (Mag-Fluo-4, erGAP3, and G-CEPIA1er) in both HeLa cells and human fibroblasts. Only GEM-CEPIA1er displayed a high colocalization with elongated mitochondria in HeLa cells, this ER Ca²⁺ indicator reveals low Ca²⁺ regions because this ion quenches its fluorescence. On the contrary, the typical rounded and fragmented mitochondria of HEK293 cells colocalized with Mag-Fluo-4 and, to a lesser extent, with GEM-CEPIA1er. The ablation of the three IP₃R isoforms in HEK293 cells increased mitochondria-GEM-CEPIA1er colocalization. This pattern of colocalization was inversely correlated with the rate of ER Ca²⁺ leak evoked by thapsigargin (Tg). Moreover, Tg and Histamine in the absence of external Ca²⁺ increased mitochondria-ER colocalization. On the contrary, in the presence of external Ca²⁺, both Bafilomycin A1 and Tg reduced the mitochondria-ER interaction. Notably, knocking down MCU decreased mitochondria-ER colocalization. Overall, our data suggest that the [Ca²⁺] is not homogenous within the ER lumen and that mitochondria-ER interaction is modulated by the ER Ca²⁺ leak and the [Ca²⁺]_i.

Disrupted Ca2+ homeostasis and immunodeficiency in patients with functional IP3 receptor subtype 3 defects

Calcium signaling is essential for lymphocyte activation, with genetic disruptions of store-operated calcium (Ca2+) entry resulting in severe immunodeficiency. The inositol 1,4,5-trisphosphate receptor (IP3R), a homo- or heterotetramer of the IP3R1-3 isoforms, amplifies lymphocyte signaling by releasing Ca2+ from endoplasmic reticulum stores following antigen stimulation. Although knockout of all IP3R isoforms in mice causes immunodeficiency, the seeming redundancy of the isoforms is thought to explain the absence of variants in human immunodeficiency. In this study, we identified compound heterozygous variants of ITPR3 (a gene encoding IP3R subtype 3) in two unrelated Caucasian patients presenting with immunodeficiency. To determine whether ITPR3 variants act in a nonredundant manner and disrupt human immune responses, we characterized the Ca2+ signaling capacity, the lymphocyte response, and the clinical phenotype of these patients. We observed disrupted Ca2+ signaling in patient-derived fibroblasts and immune cells, with abnormal proliferation and activation responses following T-cell receptor stimulation. Reconstitution of IP3R3 in IP3R knockout cell lines led to the identification of variants as functional hypomorphs that showed reduced ability to discriminate between homeostatic and induced states, validating a genotype-phenotype link. These results demonstrate a functional link between defective endoplasmic reticulum Ca2+ channels and immunodeficiency and identify IP3Rs as diagnostic targets for patients with specific inborn errors of immunity. These results also extend the known cause of Ca2+-associated immunodeficiency from store-operated entry to impaired Ca2+ mobilization from the endoplasmic reticulum, revealing a broad sensitivity of lymphocytes to genetic defects in Ca2+ signaling.

Missense mutations in inositol 1,4,5-trisphosphate receptor type 3 result in leaky Ca2+ channels and activation of store-operated Ca2+ entry

Mutations in all subtypes of the inositol 1,4,5-trisphosphate receptor Ca2+ release channel are associated with human diseases. In this report, we investigated the functionality of three neuropathy-associated missense mutations in IP3R3 (V615M, T1424M, and R2524C). The mutants only exhibited function when highly over-expressed compared to endogenous hIP3R3. All variants resulted in elevated basal cytosolic Ca2+ levels, decreased endoplasmic reticulum Ca2+ store content, and constitutive store-operated Ca2+ entry in the absence of any stimuli, consistent with a leaky IP3R channel pore. These variants differed in channel function; when stably over-expressed the R2524C mutant was essentially dead, V615M was poorly functional, and T1424M exhibited activity greater than that of the corresponding wild-type following threshold stimulation. These results demonstrate that a common feature of these mutations is decreased IP3R3 function. In addition, these mutations exhibit a novel phenotype manifested as a constitutively open channel, which inappropriately gates SOCE in the absence of stimulation.

Pharmacological potentiators of the calcium signaling cascade identified by high-throughput screening

Pharmacological modulators of the Ca²⁺ signaling cascade are important research tools and may translate into novel therapeutic strategies for a series of human diseases. We carried out a screening of a maximally diverse chemical library using the Ca²⁺-sensitive Cl^- channel TMEM16A as a functional readout. We found compounds that were able to potentiate UTP-dependent TMEM16A activation. Mechanism of action of these compounds was investigated by a panel of assays that looked at intracellular Ca²⁺ mobilization triggered by extracellular agonists or by caged-IP₃ photolysis, PIP₂ breakdown by phospholipase C, and ion channel activity on nuclear membrane. One compound appears as a selective potentiator of inositol triphosphate receptor type 1 (ITPR1) with a possible application for some forms of spinocerebellar ataxia. A second compound is instead a potentiator of the P2RY2 purinergic receptor, an activity that could promote fluid secretion in dry eye and chronic obstructive respiratory diseases.

TRPC3 governs the spatiotemporal organization of cellular Ca2+ signatures by functional coupling to IP3 receptors

Communication between TRPC channels and IP₃ receptors (IP₃R) is considered pivotal in the generation of spatiotemporal Ca²⁺signaling patterns. Here we revisited the role of TRPC3-IP₃R coupling for local Ca²⁺ signaling within TRPC3-harbouring micro/nanodomains using R-GECO as a reporter, fused to the channel´s C-terminus. Cytoplasmic Ca²⁺ changes at TRPC3 originated from both the entry of Ca²⁺ through the TRPC channel and Ca²⁺ mobilization via IP₃R. Local Ca²⁺ changes at TRPC3 channels expressed in HEK293 cells were predominantly biphasic with IP₃R-dependent initial Ca²⁺ transients, while exclusively monophasic signals were recorded when all three IP₃R isoforms were lacking. Abrogation of Ca²⁺ entry through TRPC3 by point mutations, which impair Ca²⁺ permeability (E630Q), cation permeation (E630K), or DAG sensitivity (G652A), promoted microdomain Ca²⁺ oscillations. Ca²⁺ signals at E630Q, E630K, and G652A channels featured initial Ca²⁺ transients along with oscillatory activity. Similarly, when extracellular Ca²⁺ was omitted, IP₃R-mediated Ca²⁺ transients and Ca²⁺ oscillations were promoted at the cytoplasmic face of wild-type TRPC3 channels. By contrast, oscillations, as well as initial Ca²⁺ transients, were virtually lacking, when the TRPC3 channels were sensitized by preexposure to low-level PLC activity. TIRF imaging provided evidence for dynamic colocalization of TRPC3 and IP₃R. We suggest that TRPC3-mediated Ca²⁺ entry controls IP₃R activity at ER-PM junctions to determine Ca²⁺ signaling signatures and enable specificity of downstream signaling.

Inositol 1,4,5-trisphosphate receptors in cardiomyocyte physiology and disease

The contraction of cardiac muscle underlying the pumping action of the heart is mediated by the process of excitation-contraction coupling (ECC). While triggered by Ca²⁺ entry across the sarcolemma during the action potential, it is the release of Ca²⁺ from the sarcoplasmic reticulum (SR) intracellular Ca²⁺ store via ryanodine receptors (RyRs) that plays the major role in induction of contraction. Ca²⁺ also acts as a key intracellular messenger regulating transcription underlying hypertrophic growth. Although Ca²⁺ release via RyRs is by far the greatest contributor to the generation of Ca²⁺ transients in the cardiomyocyte, Ca²⁺ is also released from the SR via inositol 1,4,5-trisphosphate (InsP₃) receptors (InsP₃Rs). This InsP₃-induced Ca²⁺ release modifies Ca²⁺ transients during ECC, participates in directing Ca²⁺ to the mitochondria, and stimulates the transcription of genes underlying hypertrophic growth. Central to these specific actions of InsP₃Rs is their localization to responsible signalling microdomains, the dyad, the SR-mitochondrial interface and the nucleus. In this review, the various roles of InsP₃R in cardiac (patho)physiology and the mechanisms by which InsP₃ signalling selectively influences the different cardiomyocyte cell processes in which it is involved will be presented.

This article is part of the theme issue ‘The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease’.

Conformational motions and ligand-binding underlying gating and regulation in IP3R channel

Inositol-1,4,5-trisphosphate receptors (IP3Rs) are activated by IP3 and Ca2+ and their gating is regulated by various intracellular messengers that finely tune the channel activity. Here, using single particle cryo-EM analysis we determined 3D structures of the nanodisc-reconstituted IP3R1 channel in two ligand-bound states. These structures provide unprecedented details governing binding of IP3, Ca2+ and ATP, revealing conformational changes that couple ligand-binding to channel opening. Using a deep-learning approach and 3D variability analysis we extracted molecular motions of the key protein domains from cryo-EM density data. We find that IP3 binding relies upon intrinsic flexibility of the ARM2 domain in the tetrameric channel. Our results highlight a key role of dynamic side chains in regulating gating behavior of IP3R channels. This work represents a stepping-stone to developing mechanistic understanding of conformational pathways underlying ligand-binding, activation and regulation of the channel.

Capture at the ER-mitochondrial contacts licenses IP3 receptors to stimulate local Ca2+ transfer and oxidative metabolism

Endoplasmic reticulum-mitochondria contacts (ERMCs) are restructured in response to changes in cell state. While this restructuring has been implicated as a cause or consequence of pathology in numerous systems, the underlying molecular dynamics are poorly understood. Here, we show means to visualize the capture of motile IP3 receptors (IP3Rs) at ERMCs and document the immediate consequences for calcium signaling and metabolism. IP3Rs are of particular interest because their presence provides a scaffold for ERMCs that mediate local calcium signaling, and their function outside of ERMCs depends on their motility. Unexpectedly, in a cell model with little ERMC Ca2+ coupling, IP3Rs captured at mitochondria promptly mediate Ca2+ transfer, stimulating mitochondrial oxidative metabolism. The Ca2+ transfer does not require linkage with a pore-forming protein in the outer mitochondrial membrane. Thus, motile IP3Rs can traffic in and out of ERMCs, and, when 'parked', mediate calcium signal propagation to the mitochondria, creating a dynamic arrangement that supports local communication.

Myo-D-inositol Trisphosphate Signalling in Oomycetes

Oomycetes are pathogens of plants and animals, which cause billions of dollars of global losses to the agriculture, aquaculture and forestry sectors each year. These organisms superficially resemble fungi, with an archetype being Phytophthora infestans, the cause of late blight of tomatoes and potatoes. Comparison of the physiology of oomycetes with that of other organisms, such as plants and animals, may provide new routes to selectively combat these pathogens. In most eukaryotes, myo-inositol 1,4,5 trisphosphate is a key second messenger that links extracellular stimuli to increases in cytoplasmic Ca2+, to regulate cellular activities. In the work presented in this study, investigation of the molecular components of myo-inositol 1,4,5 trisphosphate signaling in oomycetes has unveiled similarities and differences with that in other eukaryotes. Most striking is that several oomycete species lack detectable phosphoinositide-selective phospholipase C homologues, the enzyme family that generates this second messenger, but still possess relatives of myo-inositol 1,4,5 trisphosphate-gated Ca2+-channels.

The alkalinizing, lysosomotropic agent ML-9 induces a pH-dependent depletion of ER Ca2+ stores in cellulo

ML-9 elicits a broad spectrum of effects in cells, including inhibition of myosin light chain kinase, inhibition of store-operated Ca²⁺ entry and lysosomotropic actions that result in prostate cancer cell death. Moreover, the compound also affects endoplasmic reticulum (ER) Ca²⁺ homeostasis, although the underlying mechanisms remain unclear. We found that ML-9 provokes a rapid mobilization of Ca²⁺ from ER independently of IP₃Rs or TMBIM6/Bax Inhibitor-1, two ER Ca²⁺-leak channels. Moreover, in unidirectional ⁴⁵Ca²⁺ fluxes in permeabilized cells, ML-9 was able to reduce ER Ca²⁺-store content. Although the ER Ca²⁺ store content was decreased, ML-9 did not directly inhibit SERCA's ATPase activity in vitro using microsomal preparations. Consistent with its chemical properties as a cell-permeable weak alkalinizing agent (calculated pKa of 8.04), ML-9 provoked a rapid increase in cytosolic pH preceding the Ca²⁺ efflux from the ER. Pre-treatment with the weak acid 3NPA blunted the ML-9-evoked increase in intracellular pH and subsequent ML-9-induced Ca²⁺ mobilization from the ER. This experiment underpins a causal link between ML-9's impact on the pH and Ca²⁺ dynamics. Overall, our work indicates that the lysosomotropic drug ML-9 may not only impact lysosomal compartments but also have severe impacts on ER Ca²⁺ handling in cellulo.

Functional determination of calcium-binding sites required for the activation of inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate receptors (IP3Rs) initiate a diverse array of physiological responses by carefully orchestrating intracellular calcium (Ca2+) signals in response to various external cues. Notably, IP3R channel activity is determined by several obligatory factors, including IP3, Ca2+, and ATP. The critical basic amino acid residues in the N-terminal IP3-binding core (IBC) region that facilitate IP3 binding are well characterized. In contrast, the residues conferring regulation by Ca2+ have yet to be ascertained. Using comparative structural analysis of Ca2+-binding sites identified in two main families of intracellular Ca2+-release channels, ryanodine receptors (RyRs) and IP3Rs, we identified putative acidic residues coordinating Ca2+ in the cytosolic calcium sensor region in IP3Rs. We determined the consequences of substituting putative Ca2+ binding, acidic residues in IP3R family members. We show that the agonist-induced Ca2+ release, single-channel open probability (P0), and Ca2+ sensitivities are markedly altered when the negative charge on the conserved acidic side chain residues is neutralized. Remarkably, neutralizing the negatively charged side chain on two of the residues individually in the putative Ca2+-binding pocket shifted the Ca2+ required to activate IP3R to higher concentrations, indicating that these residues likely are a component of the Ca2+ activation site in IP3R. Taken together, our findings indicate that Ca2+ binding to a well-conserved activation site is a common underlying mechanism resulting in increased channel activity shared by IP3Rs and RyRs.

Modulation of Ca2+ signaling by antiapoptotic Bcl-2 versus Bcl-xL: From molecular mechanisms to relevance for cancer cell survival

Members of the Bcl-2-protein family are key controllers of apoptotic cell death. The family is divided into antiapoptotic (including Bcl-2 itself, Bcl-xL, Mcl-1, etc.) and proapoptotic members (Bax, Bak, Bim, Bim, Puma, Noxa, Bad, etc.). These proteins are well known for their canonical role in the mitochondria, where they control mitochondrial outer membrane permeabilization and subsequent apoptosis. However, several proteins are recognized as modulators of intracellular Ca²⁺ signals that originate from the endoplasmic reticulum (ER), the major intracellular Ca²⁺-storage organelle. More than 25 years ago, Bcl-2, the founding member of the family, was reported to control apoptosis through Ca²⁺ signaling. Further work elucidated that Bcl-2 directly targets and inhibits inositol 1,4,5-trisphosphate receptors (IP₃Rs), thereby suppressing proapoptotic Ca²⁺ signaling. In addition to Bcl-2, Bcl-xL was also shown to impact cell survival by sensitizing IP₃R function, thereby promoting prosurvival oscillatory Ca²⁺ release. However, new work challenges this model and demonstrates that Bcl-2 and Bcl-xL can both function as inhibitors of IP₃Rs. This suggests that, depending on the cell context, Bcl-xL could support very distinct Ca²⁺ patterns. This not only raises several questions but also opens new possibilities for the treatment of Bcl-xL-dependent cancers. In this review, we will discuss the similarities and divergences between Bcl-2 and Bcl-xL regarding Ca²⁺ homeostasis and IP₃R modulation from both a molecular and a functional point of view, with particular emphasis on cancer cell death resistance mechanisms.

Ca2+ signalling system initiated by endoplasmic reticulum stress stimulates PERK activation

The accumulation of unfolded proteins within the Endoplasmic Reticulum (ER) activates a signal transduction pathway termed the unfolded protein response (UPR), which attempts to restore ER homoeostasis. If this cannot be done, UPR signalling ultimately induces apoptosis. Ca²⁺ depletion in the ER is a potent inducer of ER stress. Despite the ubiquity of Ca²⁺ as an intracellular messenger, the precise mechanism(s) by which Ca²⁺ release affects the UPR remains unknown. Tethering a genetically encoded Ca²⁺ indicator (GCamP6) to the ER membrane revealed novel Ca²⁺ signalling events initiated by Ca²⁺ microdomains in human astrocytes under ER stress, induced by tunicamycin (Tm), an N-glycosylation inhibitor, as well as in a cell model deficient in all three inositol triphosphate receptor isoforms. Pharmacological and molecular studies indicate that these local events are mediated by translocons and that the Ca²⁺ microdomains impact (PKR)-like-ER kinase (PERK), an UPR sensor, activation. These findings reveal the existence of a Ca²⁺ signal mechanism by which stressor-mediated Ca²⁺ release regulates ER stress.

KRAP is required for diffuse and punctate IP3-mediated Ca2+ liberation and determines the number of functional IP3R channels within clusters

KRas-induced actin-interacting protein (KRAP) has been identified as crucial for the appropriate localization and functioning of the inositol trisphosphate receptors (IP₃Rs) that mediate Ca²⁺ release from the endoplasmic reticulum. Here, we used siRNA knockdown of KRAP expression in HeLa and HEK293 cells to examine the roles of KRAP in the generation of IP₃-mediated local Ca²⁺ puffs and global, cell-wide Ca²⁺ signals. High resolution Ca²⁺ imaging revealed that the mean amplitude of puffs was strongly reduced by KRAP knockdown, whereas the Ca²⁺ flux during openings of individual IP₃R channels was little affected. In both control and KRAP knockdown cells the numbers of functional channels in the clusters underlying puff sites were stochastically distributed following a Poisson relationship, but the mean number of functional channels per site was reduced by about two thirds by KRAP knockdown. We conclude that KRAP is required for activity of IP₃R channels at puff sites and stochastically 'licenses' the function of individual channels on a one-to-one basis, rather than determining the functioning of the puff site as a whole. In addition to puff activity ('punctate' Ca²⁺ release), global, cell-wide Ca²⁺ signals evoked by higher levels of IP₃ are further composed from a discrete 'diffuse' mode of Ca²⁺ release. By applying fluctuation analysis to isolate the punctate component during global Ca²⁺ signals, we find that KRAP knockdown suppresses to similar extents punctate and diffuse Ca²⁺ release in wild-type cells and in HEK293 cells exclusively expressing type 1 and type 3 IP3Rs. Thus, KRAP appears essential for the functioning of the IP₃Rs involved in diffuse Ca²⁺ release as well as the clustered IP₃Rs that generate local Ca²⁺ puffs.

CD73-Adenosine A1R Axis Regulates the Activation and Apoptosis of Hepatic Stellate Cells Through the PLC-IP3-Ca2+/DAG-PKC Signaling Pathway

Alcohol-related liver fibrosis (ALF) is a form of alcohol-related liver disease (ALD) that generally occurs in response to heavy long-term drinking. Ecto-5'-nucleotidase (NT5E), also known as CD73, is a cytomembrane protein linked to the cell membrane via a GPI anchor that regulates the conversion of extracellular ATP to adenosine. Adenosine and its receptors are important regulators of the cellular response. Previous studies showed that CD73 and adenosine A1 receptor (A1R) were important in alcohol-related liver disease, however the exact mechanism is unclear. The aim of this study was to elucidate the role and mechanism of the CD73-A1R axis in both a murine model of alcohol and carbon tetrachloride (CCl4) induced ALF and in an in vitro model of fibrosis induced by acetaldehyde. The degree of liver injury was determined by measuring serum AST and ALT levels, H & E staining, and Masson's trichrome staining. The expression levels of fibrosis indicators and PLC-IP3-Ca2+/DAG-PKC signaling pathway were detected by quantitative real-time PCR, western blotting, ELISA, and calcium assay. Hepatic stellate cell (HSC) apoptosis was detected using the Annexin V-FITC/PI cell apoptosis detection kit. Knockdown of CD73 significantly attenuated the accumulation of α-SMA and COL1a1 damaged the histological architecture of the mouse liver induced by alcohol and CCl4. In vitro, CD73 inhibition attenuated acetaldehyde-induced fibrosis and downregulated A1R expression in HSC-T6 cells. Inhibition of CD73/A1R downregulated the expression of the PLC-IP3-Ca2+/DAG-PKC signaling pathway. In addition, silencing of CD73/A1R promoted apoptosis in HSC-T6 cells. In conclusion, the CD73-A1R axis can regulate the activation and apoptosis of HSCs through the PLC-IP3-Ca2+/DAG-PKC signaling pathway.

Binding of the erlin1/2 complex to the third intralumenal loop of IP3R1 triggers its ubiquitin-proteasomal degradation

Long-term activation of inositol 1,4,5-trisphosphate receptors (IP₃Rs) leads to their degradation by the ubiquitin–proteasome pathway. The first and rate-limiting step in this process is thought to be the association of conformationally active IP₃Rs with the erlin1/2 complex, an endoplasmic reticulum–located oligomer of erlin1 and erlin2 that recruits the E3 ubiquitin ligase RNF170, but the molecular determinants of this interaction remain unknown. Here, through mutation of IP₃R1, we show that the erlin1/2 complex interacts with the IP₃R1 intralumenal loop 3 (IL3), the loop between transmembrane (TM) helices 5 and 6, and in particular, with a region close to TM5, since mutation of amino acids D-2471 and R-2472 can specifically block erlin1/2 complex association. Surprisingly, we found that additional mutations in IL3 immediately adjacent to TM5 (e.g., D2465N) almost completely abolish IP₃R1 Ca²⁺ channel activity, indicating that the integrity of this region is critical to IP₃R1 function. Finally, we demonstrate that inhibition of the ubiquitin-activating enzyme UBE1 by the small-molecule inhibitor TAK-243 completely blocked IP₃R1 ubiquitination and degradation without altering erlin1/2 complex association, confirming that association of the erlin1/2 complex is the primary event that initiates IP₃R1 processing and that IP₃R1 ubiquitination mediates IP₃R1 degradation. Overall, these data localize the erlin1/2 complex–binding site on IP₃R1 to IL3 and show that the region immediately adjacent to TM5 is key to the events that facilitate channel opening.

Ero1α-Dependent ERp44 Dissociation From RyR2 Contributes to Cardiac Arrhythmia

BACKGROUND

Oxidative stress in cardiac disease promotes proarrhythmic disturbances in Ca²⁺ homeostasis, impairing luminal Ca²⁺ regulation of the sarcoplasmic reticulum (SR) Ca²⁺ release channel, the RyR2 (ryanodine receptor), and increasing channel activity. However, exact mechanisms underlying redox-mediated increase of RyR2 function in cardiac disease remain elusive. We tested whether the oxidoreductase family of proteins that dynamically regulate the oxidative environment within the SR are involved in this process.

METHODS

A rat model of hypertrophy induced by thoracic aortic banding (TAB) was used for ex vivo whole heart optical mapping and for Ca²⁺ and reactive oxygen species imaging in isolated ventricular myocytes (VMs).

RESULTS

The SR-targeted reactive oxygen species biosensor ERroGFP showed increased intra-SR oxidation in TAB VMs that was associated with increased expression of Ero1α (endoplasmic reticulum oxidoreductase 1 alpha). Pharmacological (EN460) or genetic Ero1α inhibition normalized SR redox state, increased Ca²⁺ transient amplitude and SR Ca²⁺ content, and reduced proarrhythmic spontaneous Ca²⁺ waves in TAB VMs under β-adrenergic stimulation (isoproterenol). Ero1α overexpression in Sham VMs had opposite effects. Ero1α inhibition attenuated Ca²⁺-dependent ventricular tachyarrhythmias in TAB hearts challenged with isoproterenol. Experiments in TAB VMs and human embryonic kidney 293 cells expressing human RyR2 revealed that an Ero1α-mediated increase in SR Ca²⁺-channel activity involves dissociation of intraluminal protein ERp44 (endoplasmic reticulum protein 44) from the RyR2 complex. Site-directed mutagenesis and molecular dynamics simulations demonstrated a novel redox-sensitive association of ERp44 with RyR2 mediated by intraluminal cysteine 4806. ERp44-RyR2 association in TAB VMs was restored by Ero1α inhibition, but not by reducing agent dithiothreitol, as hypo-oxidation precludes formation of covalent bond between RyR2 and ERp44.

CONCLUSIONS

A novel axis of intraluminal interaction between RyR2, ERp44, and Ero1α has been identified. Ero1α inhibition exhibits promising therapeutic potential by stabilizing RyR2-ERp44 complex, thereby reducing spontaneous Ca²⁺ release and Ca²⁺-dependent tachyarrhythmias in hypertrophic hearts, without causing hypo-oxidative stress in the SR.

Functional communication between IP3R and STIM2 at subthreshold stimuli is a critical checkpoint for initiation of SOCE

Stromal interaction molecules, STIM1 and STIM2, sense decreases in the endoplasmic reticulum (ER) [Ca2+] ([Ca2+]ER) and cluster in ER-plasma membrane (ER-PM) junctions where they recruit and activate Orai1. While STIM1 responds when [Ca2+]ER is relatively low, STIM2 displays constitutive clustering in the junctions and is suggested to regulate basal Ca2+ entry. The cellular cues that determine STIM2 clustering under basal conditions is not known. By using gene editing to fluorescently tag endogenous STIM2, we report that endogenous STIM2 is constitutively localized in mobile and immobile clusters. The latter associate with ER-PM junctions and recruit Orai1 under basal conditions. Agonist stimulation increases immobile STIM2 clusters, which coordinate recruitment of Orai1 and STIM1 to the junctions. Extended synaptotagmin (E-Syt)2/3 are required for forming the ER-PM junctions, but are not sufficient for STIM2 clustering. Importantly, inositol 1,4,5-triphosphate receptor (IP3R) function and local [Ca2+]ER are the main drivers of immobile STIM2 clusters. Enhancing, or decreasing, IP3R function at ambient [IP3] causes corresponding increase, or attenuation, of immobile STIM2 clusters. We show that immobile STIM2 clusters denote decreases in local [Ca2+]ER mediated by IP3R that is sensed by the STIM2 N terminus. Finally, under basal conditions, ambient PIP2-PLC activity of the cell determines IP3R function, immobilization of STIM2, and basal Ca2+ entry while agonist stimulation augments these processes. Together, our findings reveal that immobilization of STIM2 clusters within ER-PM junctions, a first response to ER-Ca2+ store depletion, is facilitated by the juxtaposition of IP3R and marks a checkpoint for initiation of Ca2+ entry.

Platelet-Activating Factor Promotes the Development of Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a multifaceted clinicopathological syndrome characterised by excessive hepatic lipid accumulation that causes steatosis, excluding alcoholic factors. Platelet-activating factor (PAF), a biologically active lipid transmitter, induces platelet activation upon binding to the PAF receptor. Recent studies have found that PAF is associated with gamma-glutamyl transferase, which is an indicator of liver disease. Moreover, PAF can stimulate hepatic lipid synthesis and cause hypertriglyceridaemia. Furthermore, the knockdown of the PAF receptor gene in the animal models of NAFLD helped reduce the inflammatory response, improve glucose homeostasis and delay the development of NAFLD. These findings suggest that PAF is associated with NAFLD development. According to reports, patients with NAFLD or animal models have marked platelet activation abnormalities, mainly manifested as enhanced platelet adhesion and aggregation and altered blood rheology. Pharmacological interventions were accompanied by remission of abnormal platelet activation and significant improvement in liver function and lipids in the animal model of NAFLD. These confirm that platelet activation may accompany a critical importance in NAFLD development and progression. However, how PAFs are involved in the NAFLD signalling pathway needs further investigation. In this paper, we review the relevant literature in recent years and discuss the role played by PAF in NAFLD development. It is important to elucidate the pathogenesis of NAFLD and to find effective interventions for treatment.

Effect of PACAP on sweat secretion by immortalized human sweat gland cells

The process of sweating plays an important role in the human body, including thermoregulation and maintenance of the environment and health of the skin. It is known that the conditions of hyperhidrosis and anhidrosis are caused by abnormalities in sweat secretion and can result in severe skin conditions such as pruritus and erythema, which significantly reduce the patient's quality of life. However, there are many aspects of the signaling mechanisms in the process of sweating that have not been clarified, and no effective therapies or therapeutic agents have yet been discovered. Previously, it was reported that pituitary adenylate cyclase-activating polypeptide (PACAP) promotes sweating, but details of the underlying mechanism has not been clarified. We used immortalized human eccrine gland cells (NCL-SG3 cell) to investigate how sweat secretion is induced by PACAP. Intracellular Ca²⁺ levels were increased in these cells following their exposure to physiological concentrations of PACAP. Intracellular Ca²⁺ was not elevated when cells were concomitantly treated with PA-8, a specific PAC1-R antagonist, suggesting that PAC1-R is involved in the elevation of intracellular Ca²⁺ levels in response to PACAP treatment. Furthermore, immunocytochemistry experiments showed that aquaporin-5 was translocated from the cytoplasm to the cell membrane by PACAP. These results suggest that PACAP acts on eccrine sweat glands to promote sweat secretion by translocation of aquaporin-5 to the cell membrane in response to increased levels of intracellular Ca²⁺. These findings also provide a solid basis for future research initiatives to develop new therapies to treat sweating disorders.

Metabolic adaptation to the chronic loss of Ca2+ signaling induced by KO of IP3 receptors or the mitochondrial Ca2+ uniporter

Calcium signaling is essential for regulating many biological processes. Endoplasmic reticulum inositol trisphosphate receptors (IP₃Rs) and the mitochondrial Ca²⁺ uniporter (MCU) are key proteins that regulate intracellular Ca²⁺ concentration. Mitochondrial Ca²⁺ accumulation activates Ca²⁺-sensitive dehydrogenases of the tricarboxylic acid (TCA) cycle that maintain the biosynthetic and bioenergetic needs of both normal and cancer cells. However, the interplay between calcium signaling and metabolism is not well understood. In this study, we used human cancer cell lines (HEK293 and HeLa) with stable KOs of all three IP₃R isoforms (triple KO [TKO]) or MCU to examine metabolic and bioenergetic responses to the chronic loss of cytosolic and/or mitochondrial Ca²⁺ signaling. Our results show that TKO cells (exhibiting total loss of Ca²⁺ signaling) are viable, displaying a lower proliferation and oxygen consumption rate, with no significant changes in ATP levels, even when made to rely solely on the TCA cycle for energy production. MCU KO cells also maintained normal ATP levels but showed increased proliferation, oxygen consumption, and metabolism of both glucose and glutamine. However, MCU KO cells were unable to maintain ATP levels and died when relying solely on the TCA cycle for energy. We conclude that constitutive Ca²⁺ signaling is dispensable for the bioenergetic needs of both IP₃R TKO and MCU KO human cancer cells, likely because of adequate basal glycolytic and TCA cycle flux. However, in MCU KO cells, the higher energy expenditure associated with increased proliferation and oxygen consumption makes these cells more prone to bioenergetic failure under conditions of metabolic stress.

Bcl-xL acts as an inhibitor of IP3R channels, thereby antagonizing Ca2+-driven apoptosis

Anti-apoptotic Bcl-2-family members not only act at mitochondria but also at the endoplasmic reticulum, where they impact Ca²⁺ dynamics by controlling IP₃ receptor (IP₃R) function. Current models propose distinct roles for Bcl-2 vs. Bcl-xL, with Bcl-2 inhibiting IP₃Rs and preventing pro-apoptotic Ca²⁺ release and Bcl-xL sensitizing IP₃Rs to low [IP₃] and promoting pro-survival Ca²⁺ oscillations. We here demonstrate that Bcl-xL too inhibits IP₃R-mediated Ca²⁺ release by interacting with the same IP₃R regions as Bcl-2. Via in silico superposition, we previously found that the residue K87 of Bcl-xL spatially resembled K17 of Bcl-2, a residue critical for Bcl-2's IP₃R-inhibitory properties. Mutagenesis of K87 in Bcl-xL impaired its binding to IP₃R and abrogated Bcl-xL's inhibitory effect on IP₃Rs. Single-channel recordings demonstrate that purified Bcl-xL, but not Bcl-xL^K87D, suppressed IP₃R single-channel openings stimulated by sub-maximal and threshold [IP₃]. Moreover, we demonstrate that Bcl-xL-mediated inhibition of IP₃Rs contributes to its anti-apoptotic properties against Ca²⁺-driven apoptosis. Staurosporine (STS) elicits long-lasting Ca²⁺ elevations in wild-type but not in IP₃R-knockout HeLa cells, sensitizing the former to STS treatment. Overexpression of Bcl-xL in wild-type HeLa cells suppressed STS-induced Ca²⁺ signals and cell death, while Bcl-xL^K87D was much less effective in doing so. In the absence of IP₃Rs, Bcl-xL and Bcl-xL^K87D were equally effective in suppressing STS-induced cell death. Finally, we demonstrate that endogenous Bcl-xL also suppress IP₃R activity in MDA-MB-231 breast cancer cells, whereby Bcl-xL knockdown augmented IP₃R-mediated Ca²⁺ release and increased the sensitivity towards STS, without altering the ER Ca²⁺ content. Hence, this study challenges the current paradigm of divergent functions for Bcl-2 and Bcl-xL in Ca²⁺-signaling modulation and reveals that, similarly to Bcl-2, Bcl-xL inhibits IP₃R-mediated Ca²⁺ release and IP₃R-driven cell death. Our work further underpins that IP₃R inhibition is an integral part of Bcl-xL's anti-apoptotic function.

Quantal Ca2+ release mediated by very few IP3 receptors that rapidly inactivate allows graded responses to IP3

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are intracellular Ca²⁺ channels that link extracellular stimuli to Ca²⁺ signals. Ca²⁺ release from intracellular stores is "quantal": low IP₃ concentrations rapidly release a fraction of the stores. Ca²⁺ release then slows or terminates without compromising responses to further IP₃ additions. The mechanisms are unresolved. Here, we synthesize a high-affinity partial agonist of IP₃Rs and use it to demonstrate that quantal responses do not require heterogenous Ca²⁺ stores. IP₃Rs respond incrementally to IP₃ and close after the initial response to low IP₃ concentrations. Comparing functional responses with IP₃ binding shows that only a tiny fraction of a cell's IP₃Rs mediate incremental Ca²⁺ release; inactivation does not therefore affect most IP₃Rs. We conclude, and test by simulations, that Ca²⁺ signals evoked by IP₃ pulses arise from rapid activation and then inactivation of very few IP₃Rs. This allows IP₃Rs to behave as increment detectors mediating graded Ca²⁺ release.