Inhibition of NAADP signalling on reperfusion protects the heart by preventing lethal calcium oscillations via two-pore channel 1 and opening of the mitochondrial permeability transition pore

Two-pore channels (TPCs) acts as a hub for excitation-contraction coupling, metabolism and cardiac hypertrophy signalling

Ca2+ signaling is essential for cardiac contractility and excitability in heart function and remodeling. Intriguingly, little is known about the role of a new family of ion channels, the endo-lysosomal non-selective cation "two-pore channel" (TPCs) in heart function. Here we have used double TPC knock-out mice for the 1 and 2 isoforms of TPCs (Tpcn1/2-/-) and evaluated their cardiac function. Doppler-echocardiography unveils altered left ventricular (LV) systolic function associated with a LV relaxation impairment. In cardiomyocytes isolated from Tpcn1/2-/- mice, we observed a reduction in the contractile function with a decrease in the sarcoplasmic reticulum Ca2+ content and a reduced expression of various key proteins regulating Ca2+ stores, such as calsequestrin. We also found that two main regulators of the energy metabolism, AMP-activated protein kinase and mTOR, were down regulated. We found an increase in the expression of TPC1 and TPC2 in a model of transverse aortic constriction (TAC) mice and in chronically isoproterenol infused WT mice. In this last model, adaptive cardiac hypertrophy was reduced by Tpcn1/2 deletion. Here, we propose a central role for TPCs and lysosomes that could act as a hub integrating information from the excitation-contraction coupling mechanisms, cellular energy metabolism and hypertrophy signaling.

Store-Operated Ca2+ Entry as a Putative Target of Flecainide for the Treatment of Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is a genetic disorder that may lead patients to sudden cell death through the occurrence of ventricular arrhythmias. ACM is characterised by the progressive substitution of cardiomyocytes with fibrofatty scar tissue that predisposes the heart to life-threatening arrhythmic events. Cardiac mesenchymal stromal cells (C-MSCs) contribute to the ACM by differentiating into fibroblasts and adipocytes, thereby supporting aberrant remodelling of the cardiac structure. Flecainide is an Ic antiarrhythmic drug that can be administered in combination with β-adrenergic blockers to treat ACM due to its ability to target both Nav1.5 and type 2 ryanodine receptors (RyR2). However, a recent study showed that flecainide may also prevent fibro-adipogenic differentiation by inhibiting store-operated Ca2+ entry (SOCE) and thereby suppressing spontaneous Ca2+ oscillations in C-MSCs isolated from human ACM patients (ACM C-hMSCs). Herein, we briefly survey ACM pathogenesis and therapies and then recapitulate the main molecular mechanisms targeted by flecainide to mitigate arrhythmic events, including Nav1.5 and RyR2. Subsequently, we describe the role of spontaneous Ca2+ oscillations in determining MSC fate. Next, we discuss recent work showing that spontaneous Ca2+ oscillations in ACM C-hMSCs are accelerated to stimulate their fibro-adipogenic differentiation. Finally, we describe the evidence that flecainide suppresses spontaneous Ca2+ oscillations and fibro-adipogenic differentiation in ACM C-hMSCs by inhibiting constitutive SOCE.

Lysosomal calcium loading promotes spontaneous calcium release by potentiating ryanodine receptors

Spontaneous calcium release by ryanodine receptors (RyRs) due to intracellular calcium overload results in delayed afterdepolarizations, closely associated with life-threatening arrhythmias. In this regard, inhibiting lysosomal calcium release by two-pore channel 2 (TPC2) knockout has been shown to reduce the incidence of ventricular arrhythmias under β-adrenergic stimulation. However, mechanistic investigations into the role of lysosomal function on RyR spontaneous release remain missing. We investigate the calcium handling mechanisms by which lysosome function modulates RyR spontaneous release, and determine how lysosomes are able to mediate arrhythmias by its influence on calcium loading. Mechanistic studies were conducted using a population of biophysically detailed mouse ventricular models including for the first time modeling of lysosomal function, and calibrated by experimental calcium transients modulated by TPC2. We demonstrate that lysosomal calcium uptake and release can synergistically provide a pathway for fast calcium transport, by which lysosomal calcium release primarily modulates sarcoplasmic reticulum calcium reuptake and RyR release. Enhancement of this lysosomal transport pathway promoted RyR spontaneous release by elevating RyR open probability. In contrast, blocking either lysosomal calcium uptake or release revealed an antiarrhythmic impact. Under conditions of calcium overload, our results indicate that these responses are strongly modulated by intercellular variability in L-type calcium current, RyR release, and sarcoplasmic reticulum calcium-ATPase reuptake. Altogether, our investigations identify that lysosomal calcium handling directly influences RyR spontaneous release by regulating RyR open probability, suggesting antiarrhythmic strategies and identifying key modulators of lysosomal proarrhythmic action.

Niemann-Pick C1 protein regulates platelet membrane-associated calcium ion signaling in thrombo-occlusive diseases in mice

BACKGROUND

Pathophysiologic platelet activation leads to thrombo-occlusive diseases such as myocardial infarction or ischemic stroke. Niemann-Pick C1 protein (NPC1) is involved in the regulation of lysosomal lipid trafficking and calcium ion (Ca2+) signaling, and its genetic mutation causes a lysosomal storage disorder. Lipids and Ca2+ are key players in the complex orchestration of platelet activation.

OBJECTIVES

The present study aimed to determine the impact of NPC1 on Ca2+ mobilization during platelet activation in thrombo-occlusive diseases.

METHODS

Using MK/platelet-specific knockout mice of Npc1 (Npc1Pf4∆/Pf4∆), ex vivo and in vitro approaches as well as in vivo models of thrombosis, we investigated the effect of Npc1 on platelet function and thrombus formation.

RESULTS

We showed that Npc1Pf4∆/Pf4∆ platelets display increased sphingosine levels and a locally impaired membrane-associated and SERCA3-dependent Ca2+ mobilisation compared to platelets from wildtype littermates (Npc1lox/lox). Further, we observed decreased platelet.

CONCLUSION

Our findings highlight that NPC1 regulates membrane-associated and SERCA3-dependent Ca2+ mobilization during platelet activation and that MK/platelet-specific ablation of Npc1 protects against experimental models of arterial thrombosis and myocardial or cerebral ischemia/reperfusion injury.

Timing mechanisms to control heart rhythm and initiate arrhythmias: roles for intracellular organelles, signalling pathways and subsarcolemmal Ca2

Rhythms of electrical activity in all regions of the heart can be influenced by a variety of intracellular membrane bound organelles. This is true both for normal pacemaker activity and for abnormal rhythms including those caused by early and delayed afterdepolarizations under pathological conditions. The influence of the sarcoplasmic reticulum (SR) on cardiac electrical activity is widely recognized, but other intracellular organelles including lysosomes and mitochondria also contribute. Intracellular organelles can provide a timing mechanism (such as an SR clock driven by cyclic uptake and release of Ca²⁺, with an important influence of intraluminal Ca²⁺), and/or can act as a Ca²⁺ store involved in signalling mechanisms. Ca²⁺ plays many diverse roles including carrying electric current, driving electrogenic sodium-calcium exchange (NCX) particularly when Ca²⁺ is extruded across the surface membrane causing depolarization, and activation of enzymes which target organelles and surface membrane proteins. Heart function is also influenced by Ca²⁺ mobilizing agents (cADP-ribose, nicotinic acid adenine dinucleotide phosphate and inositol trisphosphate) acting on intracellular organelles. Lysosomal Ca²⁺ release exerts its effects via calcium/calmodulin-dependent protein kinase II to promote SR Ca²⁺ uptake, and contributes to arrhythmias resulting from excessive beta-adrenoceptor stimulation. A separate arrhythmogenic mechanism involves lysosomes, mitochondria and SR. Interacting intracellular organelles, therefore, have profound effects on heart rhythms and NCX plays a central role. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Function and Role of Histamine H1 Receptor in the Mammalian Heart

Histamine can change the force of cardiac contraction and alter the beating rate in mammals, including humans. However, striking species and regional differences have been observed. Depending on the species and the cardiac region (atrium versus ventricle) studied, the contractile, chronotropic, dromotropic, and bathmotropic effects of histamine vary. Histamine is present and is produced in the mammalian heart. Thus, histamine may exert autocrine or paracrine effects in the mammalian heart. Histamine uses at least four heptahelical receptors: H₁, H₂, H₃ and H₄. Depending on the species and region studied, cardiomyocytes express only histamine H₁ or only histamine H₂ receptors or both. These receptors are not necessarily functional concerning contractility. We have considerable knowledge of the cardiac expression and function of histamine H₂ receptors. In contrast, we have a poor understanding of the cardiac role of the histamine H₁ receptor. Therefore, we address the structure, signal transduction, and expressional regulation of the histamine H₁ receptor with an eye on its cardiac role. We point out signal transduction and the role of the histamine H₁ receptor in various animal species. This review aims to identify gaps in our knowledge of cardiac histamine H₁ receptors. We highlight where the published research shows disagreements and requires a new approach. Moreover, we show that diseases alter the expression and functional effects of histamine H₁ receptors in the heart. We found that antidepressive drugs and neuroleptic drugs might act as antagonists of cardiac histamine H₁ receptors, and believe that histamine H₁ receptors in the heart might be attractive targets for drug therapy. The authors believe that a better understanding of the role of histamine H₁ receptors in the human heart might be clinically relevant for improving drug therapy.

Effects of Apocynin, a NADPH Oxidase Inhibitor, in the Protection of the Heart from Ischemia/Reperfusion Injury

Ischemia and perfusion (I/R) induce inflammation and oxidative stress, which play a notable role in tissue damage. The aim of this study was to investigate the role of an NADPH oxidase inhibitor (apocynin) in the protection of the heart from I/R injury. Hearts isolated from Wistar rats (n = 8 per group) were perfused with a modified Langendorff preparation. Left ventricular (LV) contractility and cardiovascular hemodynamics were evaluated by a data acquisition program, and infarct size was evaluated by 2,3,5-Triphenyl-2H-tetrazolium chloride (TTC) staining. Furthermore, the effect of apocynin on the pro-inflammatory cytokines (IL-1β, IL-6 and TNF-α) and anti-inflammatory cytokine (IL-10) was evaluated using an enzyme linked immunosorbent assay (ELISA). Hearts were subjected to 30 min of regional ischemia, produced by ligation of the left anterior descending (LAD) coronary artery, followed by 30 min of reperfusion. Hearts were infused with apocynin before ischemia, during ischemia or at reperfusion. To understand the potential pathways of apocynin protection of the heart, a nitric oxide donor (S-nitroso-N-acetylpenicillamine, SNAP), nitric oxide blocker (N (gamma)-nitro-L-arginine methyl ester, L-Name), nicotinic acid adenine dinucleotide phosphate (NAADP) inhibiter (Ned-K), cyclic adenosine diphosphate ribose (cADPR) agonist, or CD38 blocker (Thiazoloquin (az)olin (on)e compound, 78c) was infused with apocynin. Antioxidants were evaluated by measuring superoxide dismutase (SOD) and catalase (CAT) activity. Apocynin infusion before ischemia or at reperfusion protected the heart by normalizing cardiac hemodynamics and decreasing the infarct size. Apocynin treatment resulted in a significant (p < 0.05) decrease in pro-inflammatory cytokine levels and a significant increase (p < 0.05) in anti-inflammatory and antioxidant levels. Apocynin infusion protected the heart by improving LV hemodynamics and coronary vascular dynamics. This treatment decreased the infarct size and inflammatory cytokine levels and increased anti-inflammatory cytokine and antioxidant levels. This protection follows a pathway involving CD38, nitric oxide and acidic stores.

NAADP-Mediated Ca2+ Signalling

The discovery of NAADP-evoked Ca²⁺ release in sea urchin eggs and then as a ubiquitous Ca²⁺ mobilizing messenger has introduced several novel paradigms to our understanding of Ca²⁺ signalling, not least in providing a link between cell stimulation and Ca²⁺ release from lysosomes and other acidic Ca²⁺ storage organelles. In addition, the hallmark concentration-response relationship of NAADP-mediated Ca²⁺ release, shaped by striking activation/desensitization mechanisms, influences its actions as an intracellular messenger. There has been recent progress in our understanding of the molecular mechanisms underlying NAADP-evoked Ca²⁺ release, such as the identification of the endo-lysosomal two-pore channel family of cation channels (TPCs) as their principal target and the identity of NAADP-binding proteins that complex with them. The NAADP/TPC signalling axis has gained recent prominence in pathophysiology for their roles in such disease processes as neurodegeneration, tumorigenesis and cellular viral entry.

A Role for Two-Pore Channel Type 2 (TPC2)-Mediated Regulation of Membrane Contact Sites During Zebrafish Notochord Biogenesis?

We have previously shown that in the developing trunk of zebrafish embryos, two-pore channel type 2 (TPC2)-mediated Ca2+ release from endolysosomes plays a role in the formation of the skeletal slow muscle. In addition, TPC2-mediated Ca2+ signaling is required for axon extension and the establishment of synchronized activity in the primary motor neurons. Here, we report that TPC2 might also play a role in the development of the notochord of zebrafish embryos. For example, when tpcn2 was knocked down or out, increased numbers of small vacuoles were formed in the inner notochord cells, compared with the single large vacuole in the notochord of control embryos. This abnormal vacuolation was associated with embryos displaying attenuated body axis straightening. We also showed that TPC2 has a distinct pattern of localization in the notochord in embryos at ∼24 hpf. Finally, we conducted RNAseq to identify differentially expressed genes in tpcn2 mutants compared to wild-type controls, and found that those involved in actin filament severing, cellular component morphogenesis, Ca2+ binding, and structural constituent of cytoskeleton were downregulated in the mutants. Together, our data suggest that TPC2 activity plays a key role in notochord biogenesis in zebrafish embryos.

Neuromasts and Olfactory Organs of Zebrafish Larvae Represent Possible Sites of SARS-CoV-2 Pseudovirus Host Cell Entry

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the acute respiratory disease coronavirus disease 2019 (COVID-19), which has resulted in millions of deaths globally. Here, we explored the mechanism of host cell entry of a luciferase-ZsGreen spike (SARS-CoV-2)-pseudotyped lentivirus using zebrafish embryos/larvae as an in vivo model. Successful pseudovirus entry was demonstrated via the expression of the luciferase (luc) gene, which was validated by reverse transcription-PCR (RT-PCR). Treatment of larvae with chloroquine (a broad-spectrum viral inhibitor that blocks membrane fusion) or bafilomycin A1 (a specific inhibitor of vacuolar proton ATPases, which blocks endolysosomal trafficking) significantly reduced luc expression, indicating the possible involvement of the endolysosomal system in the viral entry mechanism. The pharmacological inhibition of two-pore channel (TPC) activity or use of the tpcn2^dhkz1a mutant zebrafish line also led to diminished luc expression. The localized expression of ACE2 and TPC2 in the anterior neuromasts and the forming olfactory organs was demonstrated, and the occurrence of endocytosis in both locations was confirmed. Together, our data indicate that zebrafish embryos/larvae are a viable and tractable model to explore the mechanism of SARS-CoV-2 host cell entry, that the peripheral sense organs are a likely site for viral host cell entry, and that TPC2 plays a key role in the translocation of the virus through the endolysosomal system. IMPORTANCE Despite the development of effective vaccines to combat the COVID-19 pandemic, which help prevent the most life-threatening symptoms, full protection cannot be guaranteed, especially with the emergence of new viral variants. Moreover, some resistance to vaccination remains in certain age groups and cultures. As such, there is an urgent need for the development of new strategies and therapies to help combat this deadly disease. Here, we provide compelling evidence that the peripheral sensory organs of zebrafish possess several key components required for SARS-CoV-2 host cell entry. The nearly transparent larvae provide a most amenable complementary platform to investigate the key steps of viral entry into host cells, as well as its spread through the tissues and organs. This will help in the identification of key viral entry steps for therapeutic intervention, provide an inexpensive model for screening novel antiviral compounds, and assist in the development of new and more effective vaccines.

Two-pore channels: going with the flows

In recent years, our understanding of the structure, mechanisms and functions of the endo-lysosomal TPC (two-pore channel) family have grown apace. Gated by the second messengers, NAADP and PI(3,5)P₂, TPCs are an integral part of fundamental signal-transduction pathways, but their array and plasticity of cation conductances (Na⁺, Ca²⁺, H⁺) allow them to variously signal electrically, osmotically or chemically. Their relative tissue- and organelle-selective distribution, together with agonist-selective ion permeabilities provides a rich palette from which extracellular stimuli can choose. TPCs are emerging as mediators of immunity, cancer, metabolism, viral infectivity and neurodegeneration as this short review attests.

Electrophysiology of Endolysosomal Two-Pore Channels: A Current Account

Two-pore channels TPC1 and TPC2 are ubiquitously expressed pathophysiologically relevant proteins that reside on endolysosomal vesicles. Here, we review the electrophysiology of these channels. Direct macroscopic recordings of recombinant TPCs expressed in enlarged lysosomes in mammalian cells or vacuoles in plants and yeast demonstrate gating by the Ca2+-mobilizing messenger NAADP and/or the lipid PI(3,5)P2. TPC currents are regulated by H+, Ca2+, and Mg2+ (luminal and/or cytosolic), as well as protein kinases, and they are impacted by single-nucleotide polymorphisms linked to pigmentation. Bisbenzylisoquinoline alkaloids, flavonoids, and several approved drugs demonstrably block channel activity. Endogenous TPC currents have been recorded from a number of primary cell types and cell lines. Many of the properties of endolysosomal TPCs are recapitulated upon rerouting channels to the cell surface, allowing more facile recording through conventional electrophysiological means. Single-channel analyses have provided high-resolution insight into both monovalent and divalent permeability. The discovery of small-molecule activators of TPC2 that toggle the ion selectivity from a Ca2+-permeable (NAADP-like) state to a Na+-selective (PI(3,5)P2-like) state explains discrepancies in the literature relating to the permeability of TPCs. Identification of binding proteins that confer NAADP-sensitive currents confirm that indirect, remote gating likely underpins the inconsistent observations of channel activation by NAADP.

Targeting calcium-mediated inter-organellar crosstalk in cardiac diseases

INTRODUCTION

Abnormal calcium signaling between organelles such as the sarcoplasmic reticulum (SR), mitochondria and lysosomes is a key feature of heart diseases. Calcium serves as a secondary messenger mediating inter-organellar crosstalk, essential for maintaining the cardiomyocyte function.

AREAS COVERED

This article examines the available literature related to calcium channels and transporters involved in inter-organellar calcium signaling. The SR calcium-release channels ryanodine receptor type-2 (RyR2) and inositol 1,4,5-trisphosphate receptor (IP₃R), and calcium-transporter SR/ER-ATPase 2a (SERCA2a) are illuminated. The roles of mitochondrial voltage-dependent anion channels (VDAC), the mitochondria Ca²⁺ uniporter complex (MCUC), and the lysosomal H⁺/Ca²⁺ exchanger, two pore channels (TPC), and transient receptor potential mucolipin (TRPML) are discussed. Furthermore, recent studies showing calcium-mediated crosstalk between the SR, mitochondria, and lysosomes as well as how this crosstalk is dysregulated in cardiac diseases are placed under the spotlight.

EXPERT OPINION

Enhanced SR calcium release via RyR2 and reduced SR reuptake via SERCA2a, increased VDAC and MCUC-mediated calcium uptake into mitochondria, and enhanced lysosomal calcium-release via lysosomal TPC and TRPML may all contribute to aberrant calcium homeostasis causing heart disease. While mechanisms of this crosstalk need to be studied further, interventions targeting these calcium channels or combinations thereof might represent a promising therapeutic strategy.

Nicotinic Acid Adenine Dinucleotide Phosphate Induces Intracellular Ca2+ Signalling and Stimulates Proliferation in Human Cardiac Mesenchymal Stromal Cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a newly discovered second messenger that gates two pore channels 1 (TPC1) and 2 (TPC2) to elicit endo-lysosomal (EL) Ca²⁺ release. NAADP-induced lysosomal Ca²⁺ release may be amplified by the endoplasmic reticulum (ER) through the Ca²⁺-induced Ca²⁺ release (CICR) mechanism. NAADP-induced intracellular Ca²⁺ signals were shown to modulate a growing number of functions in the cardiovascular system, but their occurrence and role in cardiac mesenchymal stromal cells (C-MSCs) is still unknown. Herein, we found that exogenous delivery of NAADP-AM induced a robust Ca²⁺ signal that was abolished by disrupting the lysosomal Ca²⁺ store with Gly-Phe β-naphthylamide, nigericin, and bafilomycin A1, and blocking TPC1 and TPC2, that are both expressed at protein level in C-MSCs. Furthermore, NAADP-induced EL Ca²⁺ release resulted in the Ca²⁺-dependent recruitment of ER-embedded InsP₃Rs and SOCE activation. Transmission electron microscopy revealed clearly visible membrane contact sites between lysosome and ER membranes, which are predicted to provide the sub-cellular framework for lysosomal Ca²⁺ to recruit ER-embedded InsP₃Rs through CICR. NAADP-induced EL Ca²⁺ mobilization via EL TPC was found to trigger the intracellular Ca²⁺ signals whereby Fetal Bovine Serum (FBS) induces C-MSC proliferation. Furthermore, NAADP-evoked Ca²⁺ release was required to mediate FBS-induced extracellular signal-regulated kinase (ERK), but not Akt, phosphorylation in C-MSCs. These finding support the notion that NAADP-induced TPC activation could be targeted to boost proliferation in C-MSCs and pave the way for future studies assessing whether aberrant NAADP signaling in C-MSCs could be involved in cardiac disorders.

Targeting Two-Pore Channels: Current Progress and Future Challenges

Two-pore channels (TPCs) are cation-permeable channels located on endolysosomal membranes and important mediators of intracellular Ca²⁺ signalling. TPCs are involved in various pathophysiological processes, including cell growth and development, metabolism, and cancer progression. Most studies of TPCs have used TPC^–/– cell or whole-animal models, or Ned-19, an indirect inhibitor. The TPC activation mechanism remains controversial, which has made it difficult to develop selective modulators. Recent studies of TPC structure and their interactomes are aiding the development of direct pharmacological modulators. This process is still in its infancy, but will facilitate future research and TPC targeting for therapeutical purposes. Here, we review the progress of current research into TPCs, including recent insights into their structures, functional roles, mechanisms of activation, and pharmacological modulators.

Two-pore channel (TPC)-mediated endolysosomal Ca²⁺ signalling regulates a variety of processes, including cell proliferation, differentiation, metabolism, viral infection, and cardiac function.

Despite the well-established model that TPCs are Ca²⁺-selective channels indirectly activated by nicotinic acid adenine dinucleotide phosphate (NAADP), it has also been proposed that TPCs as Na⁺ channels are activated directly by phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂].

3D structures of mouse TPC1 and human TPC2 were recently determined, which made it possible for structure-based virtual screening methods to identify pharmacological modulators of TPC.

Recent identification by high-throughput screens of pharmacological modulators that target TPCs will help reveal the molecular mechanisms underlying the role of endolysosomal Ca²⁺ signalling in different pathophysiological processes, and to develop new therapeutics.

Endolysosomal Ca2+ signaling in cardiovascular health and disease

An increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) regulates a plethora of functions in the cardiovascular (CV) system, including contraction in cardiomyocytes and vascular smooth muscle cells (VSMCs), and angiogenesis in vascular endothelial cells and endothelial colony forming cells. The sarco/endoplasmic reticulum (SR/ER) represents the largest endogenous Ca²⁺ store, which releases Ca²⁺ through ryanodine receptors (RyRs) and/or inositol-1,4,5-trisphosphate receptors (InsP₃Rs) upon extracellular stimulation. The acidic vesicles of the endolysosomal (EL) compartment represent an additional endogenous Ca²⁺ store, which is targeted by several second messengers, including nicotinic acid adenine dinucleotide phosphate (NAADP) and phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂], and may release intraluminal Ca²⁺ through multiple Ca²⁺ permeable channels, including two-pore channels 1 and 2 (TPC1-2) and Transient Receptor Potential Mucolipin 1 (TRPML1). Herein, we discuss the emerging, pathophysiological role of EL Ca²⁺ signaling in the CV system. We describe the role of cardiac TPCs in β-adrenoceptor stimulation, arrhythmia, hypertrophy, and ischemia-reperfusion injury. We then illustrate the role of EL Ca²⁺ signaling in VSMCs, where TPCs promote vasoconstriction and contribute to pulmonary artery hypertension and atherosclerosis, whereas TRPML1 sustains vasodilation and is also involved in atherosclerosis. Subsequently, we describe the mechanisms whereby endothelial TPCs promote vasodilation, contribute to neurovascular coupling in the brain and stimulate angiogenesis and vasculogenesis. Finally, we discuss about the possibility to target TPCs, which are likely to mediate CV cell infection by the Severe Acute Respiratory Disease-Coronavirus-2, with Food and Drug Administration-approved drugs to alleviate the detrimental effects of Coronavirus Disease-19 on the CV system.

Oxidation of Protein Kinase A Regulatory Subunit PKARIα Protects Against Myocardial Ischemia-Reperfusion Injury by Inhibiting Lysosomal-Triggered Calcium Release

BACKGROUND

Kinase oxidation is a critical signaling mechanism through which changes in the intracellular redox state alter cardiac function. In the myocardium, PKARIα (type-1 protein kinase A) can be reversibly oxidized, forming interprotein disulfide bonds in the holoenzyme complex. However, the effect of PKARIα disulfide formation on downstream signaling in the heart, particularly under states of oxidative stress such as ischemia and reperfusion (I/R), remains unexplored.

METHODS

Atrial tissue obtained from patients before and after cardiopulmonary bypass and reperfusion and left ventricular (LV) tissue from mice subjected to I/R or sham surgery were used to assess PKARIα disulfide formation by immunoblot. To determine the effect of disulfide formation on PKARIα catalytic activity and subcellular localization, live-cell fluorescence imaging and stimulated emission depletion super-resolution microscopy were performed in prkar1 knock-out mouse embryonic fibroblasts, neonatal myocytes, or adult LV myocytes isolated from "redox dead" (Cys17Ser) PKARIα knock-in mice and their wild-type littermates. Comparison of intracellular calcium dynamics between genotypes was assessed in fura2-loaded LV myocytes, whereas I/R-injury was assessed ex vivo.

RESULTS

In both humans and mice, myocardial PKARIα disulfide formation was found to be significantly increased (2-fold in humans, P=0.023; 2.4-fold in mice, P<0.001) in response to I/R in vivo. In mouse LV cardiomyocytes, disulfide-containing PKARIα was not found to impact catalytic activity, but instead led to enhanced AKAP (A-kinase anchoring protein) binding with preferential localization of the holoenzyme to the lysosome. Redox-dependent regulation of lysosomal two-pore channels by PKARIα was sufficient to prevent global calcium release from the sarcoplasmic reticulum in LV myocytes, without affecting intrinsic ryanodine receptor leak or phosphorylation. Absence of I/R-induced PKARIα disulfide formation in "redox dead" knock-in mouse hearts resulted in larger infarcts (2-fold, P<0.001) and a concomitant reduction in LV contractile recovery (1.6-fold, P<0.001), which was prevented by administering the lysosomal two-pore channel inhibitor Ned-19 at the time of reperfusion.

CONCLUSIONS

Disulfide modification targets PKARIα to the lysosome, where it acts as a gatekeeper for two-pore channel-mediated triggering of global calcium release. In the postischemic heart, this regulatory mechanism is critical for protection from extensive injury and offers a novel target for the design of cardioprotective therapeutics.

Targeting Endolysosomal Two-Pore Channels to Treat Cardiovascular Disorders in the Novel COronaVIrus Disease 2019

Emerging evidence hints in favor of a life-threatening link between severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) and the cardiovascular system. SARS-CoV-2 may result in dramatic cardiovascular complications, whereas the severity of COronaVIrus Disease 2019 (COVID-19) and the incidence of fatalities tend to increase in patients with pre-existing cardiovascular complications. SARS-CoV-2 is internalized into the host cells by endocytosis and may then escape the endolysosomal system via endosomes. Two-pore channels drive endolysosomal trafficking through the release of endolysosomal Ca2+. Recent evidence suggested that the pharmacological inhibition of TPCs prevents Ebola virus and Middle East Respiratory Syndrome COronaVirus (MERS-CoV) entry into host cells. In this perspective, we briefly summarize the biophysical and pharmacological features of TPCs, illustrate their emerging role in the cardiovascular system, and finally present them as a reliable target to treat cardiovascular complications in COVID-19 patients.

Does lysosomal rupture evoke Ca2+ release? A question of pores and stores

Lysosomotropic agents have been used to permeabilize lysosomes and thereby implicate these organelles in diverse cellular processes. Since lysosomes are Ca²⁺ stores, this rupturing action, particularly that induced by GPN, has also been used to rapidly release Ca²⁺ from lysosomes. However, a recent study has questioned the mechanism of action of GPN and concluded that, acutely, it does not permeabilize lysosomes but releases Ca²⁺ directly from the ER instead. We therefore appraise these provocative findings in the context of the existing literature. We suggest that further work is required to unequivocally rule out lysosomes as contributors to GPN-evoked Ca²⁺ signals.

Calcium Signaling in the Heart

The aim of this chapter is to discuss evidence concerning the many roles of calcium ions, Ca²⁺, in cell signaling pathways that control heart function. Before considering details of these signaling pathways, the control of contraction in ventricular muscle by Ca²⁺ transients accompanying cardiac action potentials is first summarized, together with a discussion of how myocytes from the atrial and pacemaker regions of the heart diverge from this basic scheme. Cell signaling pathways regulate the size and timing of the Ca²⁺ transients in the different heart regions to influence function. The simplest Ca²⁺ signaling elements involve enzymes that are regulated by cytosolic Ca²⁺. Particularly important examples to be discussed are those that are stimulated by Ca²⁺, including Ca²⁺-calmodulin-dependent kinase (CaMKII), Ca²⁺ stimulated adenylyl cyclases, Ca²⁺ stimulated phosphatase and NO synthases. Another major aspect of Ca²⁺ signaling in the heart concerns actions of the Ca²⁺ mobilizing agents, inositol trisphosphate (IP₃), cADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate, (NAADP). Evidence concerning roles of these Ca²⁺ mobilizing agents in different regions of the heart is discussed in detail. The focus of the review will be on short term regulation of Ca²⁺ transients and contractile function, although it is recognized that Ca²⁺ regulation of gene expression has important long term functional consequences which will also be briefly discussed.

The protein interaction networks of mucolipins and two-pore channels

BACKGROUND

The endolysosomal, non-selective cation channels, two-pore channels (TPCs) and mucolipins (TRPMLs), regulate intracellular membrane dynamics and autophagy. While partially compensatory for each other, isoform-specific intracellular distribution, cell-type expression patterns, and regulatory mechanisms suggest different channel isoforms confer distinct properties to the cell.

SCOPE OF REVIEW

Briefly, established TPC/TRPML functions and interaction partners ('interactomes') are discussed. Novel TRPML3 interactors are shown, and a meta-analysis of experimentally obtained channel interactomes conducted. Accordingly, interactomes are compared and contrasted, and subsequently described in detail for TPC1, TPC2, TRPML1, and TRPML3.

MAJOR CONCLUSIONS

TPC interactomes are well-defined, encompassing intracellular membrane organisation proteins. TRPML interactomes are varied, encompassing cardiac contractility- and chaperone-mediated autophagy proteins, alongside regulators of intercellular signalling.

GENERAL SIGNIFICANCE

Comprising recently proposed targets to treat cancers, infections, metabolic disease and neurodegeneration, the advancement of TPC/TRPML understanding is of considerable importance. This review proposes novel directions elucidating TPC/TRPML relevance in health and disease. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Molecular regulation of MCU: Implications in physiology and disease

Ca2+ flux across the inner mitochondrial membrane (IMM) regulates cellular bioenergetics, intra-cellular cytoplasmic Ca2+ signals, and various cell death pathways. Ca2+ entry into the mitochondria occurs due to the highly negative membrane potential (ΔΨm) through a selective inward rectifying MCU channel. In addition to being regulated by various mitochondrial matrix resident proteins such as MICUs, MCUb, MCUR1 and EMRE, the channel is transcriptionally regulated by upstream Ca2+ cascade, post transnational modification and by divalent cations. The mode of regulation either inhibits or enhances MCU channel activity and thus regulates mitochondrial metabolism and cell fate.

Two-pore channels and disease

Two-pore channels (TPCs) are Ca²⁺-permeable endo-lysosomal ion channels subject to multi-modal regulation. They mediate their physiological effects through releasing Ca²⁺ from acidic organelles in response to cues such as the second messenger, NAADP. Here, we review emerging evidence linking TPCs to disease. We discuss how perturbing both local and global Ca²⁺ changes mediated by TPCs through chemical and/or molecular manipulations can induce or reverse disease phenotypes. We cover evidence from models of Parkinson's disease, non-alcoholic fatty liver disease, Ebola infection, cancer, cardiac dysfunction and diabetes. A need for more drugs targeting TPCs is identified.

Mitochondrial pathways to cardiac recovery: TFAM

Mitochondrial dysfunction underlines a multitude of pathologies; however, studies are scarce that rescue the mitochondria for cellular resuscitation. Exploration into the protective role of mitochondrial transcription factor A (TFAM) and its mitochondrial functions respective to cardiomyocyte death are in need of further investigation. TFAM is a gene regulator that acts to mitigate calcium mishandling and ROS production by wrapping around mitochondrial DNA (mtDNA) complexes. TFAM's regulatory functions over serca2a, NFAT, and Lon protease contribute to cardiomyocyte stability. Calcium- and ROS-dependent proteases, calpains, and matrix metalloproteinases (MMPs) are abundantly found upregulated in the failing heart. TFAM's regulatory role over ROS production and calcium mishandling leads to further investigation into the cardioprotective role of exogenous TFAM. In an effort to restabilize physiological and contractile activity of cardiomyocytes in HF models, we propose that TFAM-packed exosomes (TFAM-PE) will act therapeutically by mitigating mitochondrial dysfunction. Notably, this is the first mention of exosomal delivery of transcription factors in the literature. Here we elucidate the role of TFAM in mitochondrial rescue and focus on its therapeutic potential.

An Endosomal NAADP-Sensitive Two-Pore Ca2+ Channel Regulates ER-Endosome Membrane Contact Sites to Control Growth Factor Signaling

Membrane contact sites are regions of close apposition between organelles that facilitate information transfer. Here, we reveal an essential role for Ca²⁺ derived from the endo-lysosomal system in maintaining contact between endosomes and the endoplasmic reticulum (ER). Antagonizing action of the Ca²⁺-mobilizing messenger NAADP, inhibiting its target endo-lysosomal ion channel, TPC1, and buffering local Ca²⁺ fluxes all clustered and enlarged late endosomes/lysosomes. We show that TPC1 localizes to ER-endosome contact sites and is required for their formation. Reducing NAADP-dependent contacts delayed EGF receptor de-phosphorylation consistent with close apposition of endocytosed receptors with the ER-localized phosphatase PTP1B. In accord, downstream MAP kinase activation and mobilization of ER Ca²⁺ stores by EGF were exaggerated upon NAADP blockade. Membrane contact sites between endosomes and the ER thus emerge as Ca²⁺-dependent hubs for signaling.

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NAADP/TPC1 signaling maintains endo-lysosomal morphology

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TPC1 localizes to contacts between late endosomes and the endoplasmic reticulum

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NAADP/TPC1 signaling regulates contact site formation

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NAADP tempers EGF receptor-mediated signaling

The human two-pore channel 1 is modulated by cytosolic and luminal calcium

Two-pore channels (TPC) are intracellular endo-lysosomal proteins with only recently emerging roles in organellar signalling and involvement in severe human diseases. Here, we investigated the functional properties of human TPC1 expressed in TPC-free vacuoles from Arabidopsis thaliana cells. Large (20 pA/pF) TPC1 currents were elicited by cytosolic addition of the phosphoinositide phosphatidylinositol-(3,5)-bisphosphate (PI(3,5)P₂) with an apparent binding constant of ~15 nM. The channel is voltage-dependent, activating at positive potentials with single exponential kinetics and currents are Na⁺ selective, with measurable but low permeability to Ca²⁺. Cytosolic Ca²⁺ modulated hTPC1 in dual way: low μM cytosolic Ca²⁺ increased activity by shifting the open probability towards negative voltages and by accelerating the time course of activation. This mechanism was well-described by an allosteric model. Higher levels of cytosolic Ca²⁺ induced a voltage-dependent decrease of the currents compatible with Ca²⁺ binding in the permeation pore. Conversely, an increase in luminal Ca²⁺ decreased hTPC1 activity. Our data point to a process in which Ca²⁺ permeation in hTPC1 has a positive feedback on channel activity while Na⁺ acts as a negative regulator. We speculate that the peculiar Ca²⁺ and Na⁺ dependence are key for the physiological roles of the channel in organellar homeostasis and signalling.

Two-Pore Channels: Catalyzers of Endolysosomal Transport and Function

Two-pore channels (TPCs) have recently emerged as a novel class of non-selective cation channels in the endolysosomal system. There are two members in the human genome, TPC1 and TPC2. Studies with TPC knockout and knockdown models have revealed that these channels participate in the regulation of multiple endolysosomal trafficking pathways which when dysregulated can lead to or influence the development of a range of different diseases such as lysosomal storage, metabolic, or infectious diseases. TPCs have been demonstrated to be activated by different endogenous stimuli, PI(3,5)P₂ and NAADP, and ATP has been found to block TPC activation via mTOR. Loss of TPCs can lead to obesity and hypercholesterolemia, and to a slow-down of intracellular virus and bacterial toxin trafficking, it can affect VEGF-induced neoangiogenesis, autophagy, human hair pigmentation or the acrosome reaction in sperm. Moreover, physiological roles of TPCs in cardiac myocytes and pancreatic β cells have been postulated.

High resolution structural evidence suggests the Sarcoplasmic Reticulum forms microdomains with Acidic Stores (lysosomes) in the heart

Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP) stimulates calcium release from acidic stores such as lysosomes and is a highly potent calcium-mobilising second messenger. NAADP plays an important role in calcium signalling in the heart under basal conditions and following β-adrenergic stress. Nevertheless, the spatial interaction of acidic stores with other parts of the calcium signalling apparatus in cardiac myocytes is unknown. We present evidence that lysosomes are intimately associated with the sarcoplasmic reticulum (SR) in ventricular myocytes; a median separation of 20 nm in 2D electron microscopy and 3.3 nm in 3D electron tomography indicates a genuine signalling microdomain between these organelles. Fourier analysis of immunolabelled lysosomes suggests a sarcomeric pattern (dominant wavelength 1.80 μm). Furthermore, we show that lysosomes form close associations with mitochondria (median separation 6.2 nm in 3D studies) which may provide a basis for the recently-discovered role of NAADP in reperfusion-induced cell death. The trigger hypothesis for NAADP action proposes that calcium release from acidic stores subsequently acts to enhance calcium release from the SR. This work provides structural evidence in cardiac myocytes to indicate the formation of microdomains between acidic and SR calcium stores, supporting emerging interpretations of NAADP physiology and pharmacology in heart.

Two-pore channels (TPCs): Novel voltage-gated ion channels with pleiotropic functions

Two-pore channels (TPC1-3) comprise a subfamily of the eukaryotic voltage-gated ion channels (VGICs) superfamily that are mainly expressed in acidic stores in plants and animals. TPCS are widespread across the animal kingdom, with primates, mice and rats lacking TPC3, and mainly act as Ca⁺ and Na⁺ channels, although it was also suggested that they could be permeable to other ions. Nowadays, TPCs have been related to the development of different diseases, including Parkinson´s disease, obesity or myocardial ischemia. Due to this, their study has raised the interest of the scientific community to try to understand their mechanism of action in order to be able to develop an efficient drug that could regulate TPCs activity. In this review, we will provide an updated view regarding TPCs structure, function and activation, as well as their role in different pathophysiological processes.

Deviant lysosomal Ca2+ signalling in neurodegeneration. An introduction

Lysosomes are key acidic Ca²⁺ stores. The principle Ca²⁺-permeable channels of the lysosome are TRP mucolipins (TRPMLs) and NAADP-regulated two-pore channels (TPCs). Recent studies, reviewed in this collection, have linked numerous neurodegenerative diseases to both gain and loss of function of TRPMLs/TPCs, as well as to defects in acidic Ca²⁺ store content. These diseases span rare lysosomal storage disorders such as Mucolipidosis Type IV and Niemann-Pick disease, type C, through to more common ones such as Alzheimer and Parkinson disease. Cellular phenotypes, underpinned by endo-lysosomal trafficking defects, are reversed by chemical or molecular targeting of TRPMLs and TPCs. Lysosomal Ca²⁺ channels therefore emerge as potential druggable targets in combatting neurodegeneration.

Connecting Ca2+ and lysosomes to Parkinson disease

The neurodegenerative movement disorder Parkinson disease (PD) is prevalent in the aged population. However, the underlying mechanisms that trigger disease are unclear. Increasing work implicates both impaired Ca²⁺ signalling and lysosomal dysfunction in neuronal demise. Here I aim to connect these distinct processes by exploring the evidence that lysosomal Ca²⁺ signalling is disrupted in PD. In particular, I highlight defects in lysosomal Ca²⁺ content and signalling through NAADP-regulated two-pore channels in patient fibroblasts harbouring mutations in the PD-linked genes, GBA1 and LRRK2. As an emerging contributor to PD pathogenesis, the lysosomal Ca²⁺ signalling apparatus could represent a novel therapeutic target.

Exploring the biophysical evidence that mammalian two-pore channels are NAADP-activated calcium-permeable channels

Nicotinic acid adenine dinucleotide phosphate (NAADP) potently releases Ca(2+) from acidic intracellular endolysosomal Ca(2+) stores. It is widely accepted that two types of two-pore channels, termed TPC1 and TPC2, are responsible for the NAADP-mediated Ca(2+) release but the underlying mechanisms regulating their gating appear to be different. For example, although both TPC1 and TPC2 are activated by NAADP, TPC1 appears to be additionally regulated by cytosolic Ca(2+) . Ion conduction and permeability also differ markedly. TPC1 and TPC2 are permeable to a range of cations although biophysical experiments suggest that TPC2 is slightly more selective for Ca(2+) over K(+) than TPC1 and hence capable of releasing greater quantities of Ca(2+) from acidic stores. TPC1 is also permeable to H(+) and therefore may play a role in regulating lysosomal and cytosolic pH, possibly creating localised acidic domains. The significantly different gating and ion conducting properties of TPC1 and TPC2 suggest that these two ion channels may play complementary physiological roles as Ca(2+) -release channels of the endolysosomal system.

Two-Pore Channels: Lessons from Mutant Mouse Models

Recent interest in two-pore channels (TPCs) has resulted in a variety of studies dealing with the functional role and mechanism of action of these endo-lysosomal proteins in diverse physiological processes. With the availability of mouse lines harbouring mutant alleles for Tpcnl and/or Tpcn2 genes, several studies have made use of them to validate, consolidate and discover new roles for these channels not only at the cellular level but, importantly, also at the level of the whole organism. The different mutant mouse lines that have been used were derived from distinct genetic manipulation strategies, with the aim of knocking out expression of TPC proteins. However, the expression of different residual TPC sequences predicted to occur in these mutant mouse lines, together with the varied degree to which the effects on Tpcn expression have been studied, makes it important to assess the true knockout status of some of the lines. In this review we summarize these Tpcn mutant mouse lines with regard to their predicted effect on Tpcn expression and the extent to which they have been characterized. Additionally, we discuss how results derived from studies using these Tpcn mutant mouse lines have consolidated previously proposed roles for TPCs, such as mediators of NAADP signalling, endo-lysosomal functions, and pancreatic β cell physiology. We will also review how they have been instrumental in the assignment of new physiological roles for these cation channels in processes such as membrane electrical excitability, neoangiogenesis, viral infection and brown adipose tissue and heart function, revealing, in some cases, a specific contribution of a particular TPC isoform.