The voltage-gated sodium channel TPC1 confers endolysosomal excitability

A DNA nanodevice for mapping sodium at single-organelle resolution

Cellular sodium ion (Na+) homeostasis is integral to organism physiology. Our current understanding of Na+ homeostasis is largely limited to Na+ transport at the plasma membrane. Organelles may also contribute to Na+ homeostasis; however, the direction of Na+ flow across organelle membranes is unknown because organellar Na+ cannot be imaged. Here we report a pH-independent, organelle-targetable, ratiometric probe that reports lumenal Na+. It is a DNA nanodevice containing a Na+-sensitive fluorophore, a reference dye and an organelle-targeting domain. By measuring Na+ at single endosome resolution in mammalian cells and Caenorhabditis elegans, we discovered that lumenal Na+ levels in each stage of the endolysosomal pathway exceed cytosolic levels and decrease as endosomes mature. Further, we find that lysosomal Na+ levels in nematodes are modulated by the Na+/H+ exchanger NHX-5 in response to salt stress. The ability to image subcellular Na+ will unveil mechanisms of Na+ homeostasis at an increased level of cellular detail.

The ion channels of endomembranes

The endomembrane system consists of organellar membranes in the biosynthetic pathway [endoplasmic reticulum (ER), Golgi apparatus, and secretory vesicles] as well as those in the degradative pathway (early endosomes, macropinosomes, phagosomes, autophagosomes, late endosomes, and lysosomes). These endomembrane organelles/vesicles work together to synthesize, modify, package, transport, and degrade proteins, carbohydrates, and lipids, regulating the balance between cellular anabolism and catabolism. Large ion concentration gradients exist across endomembranes: Ca2+ gradients for most endomembrane organelles and H+ gradients for the acidic compartments. Ion (Na+, K+, H+, Ca2+, and Cl-) channels on the organellar membranes control ion flux in response to cellular cues, allowing rapid informational exchange between the cytosol and organelle lumen. Recent advances in organelle proteomics, organellar electrophysiology, and luminal and juxtaorganellar ion imaging have led to molecular identification and functional characterization of about two dozen endomembrane ion channels. For example, whereas IP3R1-3 channels mediate Ca2+ release from the ER in response to neurotransmitter and hormone stimulation, TRPML1-3 and TMEM175 channels mediate lysosomal Ca2+ and H+ release, respectively, in response to nutritional and trafficking cues. This review aims to summarize the current understanding of these endomembrane channels, with a focus on their subcellular localizations, ion permeation properties, gating mechanisms, cell biological functions, and disease relevance.

Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease

Together with its β-subunit OSTM1, ClC-7 performs 2Cl-/H+ exchange across lysosomal membranes. Pathogenic variants in either gene cause lysosome-related pathologies, including osteopetrosis and lysosomal storage. CLCN7 variants can cause recessive or dominant disease. Different variants entail different sets of symptoms. Loss of ClC-7 causes osteopetrosis and mostly neuronal lysosomal storage. A recently reported de novo CLCN7 mutation (p.Tyr715Cys) causes widespread severe lysosome pathology (hypopigmentation, organomegaly, and delayed myelination and development, "HOD syndrome"), but no osteopetrosis. We now describe two additional HOD individuals with the previously described p.Tyr715Cys and a novel p.Lys285Thr mutation, respectively. Both mutations decreased ClC-7 inhibition by PI(3,5)P2 and affected residues lining its binding pocket, and shifted voltage-dependent gating to less positive potentials, an effect partially conferred to WT subunits in WT/mutant heteromers. This shift predicts augmented pH gradient-driven Cl- uptake into vesicles. Overexpressing either mutant induced large lysosome-related vacuoles. This effect depended on Cl-/H+-exchange, as shown using mutants carrying uncoupling mutations. Fibroblasts from the p.Y715C patient also displayed giant vacuoles. This was not observed with p.K285T fibroblasts probably due to residual PI(3,5)P2 sensitivity. The gain of function caused by the shifted voltage-dependence of either mutant likely is the main pathogenic factor. Loss of PI(3,5)P2 inhibition will further increase current amplitudes, but may not be a general feature of HOD. Overactivity of ClC-7 induces pathologically enlarged vacuoles in many tissues, which is distinct from lysosomal storage observed with the loss of ClC-7 function. Osteopetrosis results from a loss of ClC-7, but osteoclasts remain resilient to increased ClC-7 activity.

Coordinating activation of endo-lysosomal two-pore channels and TRP mucolipins

Two-pore channels and TRP mucolipins are ubiquitous endo-lysosomal cation channels of pathophysiological relevance. Both are Ca2+-permeable and regulated by phosphoinositides, principally PI(3,5)P2. Accumulating evidence has uncovered synergistic channel activation by PI(3,5)P2 and endogenous metabolites such as the Ca2+ mobilizing messenger NAADP, synthetic agonists including approved drugs and physical cues such as voltage and osmotic pressure. Here, we provide an overview of this coordination.

Optogenetic manipulation of lysosomal physiology and autophagy-dependent clearance of amyloid beta

Lysosomes are degradation centers of cells and intracellular hubs of signal transduction, nutrient sensing, and autophagy regulation. Dysfunction of lysosomes contributes to a variety of diseases, such as lysosomal storage diseases (LSDs) and neurodegeneration, but the mechanisms are not well understood. Altering lysosomal activity and examining its impact on the occurrence and development of disease is an important strategy for studying lysosome-related diseases. However, methods to dynamically regulate lysosomal function in living cells or animals are still lacking. Here, we constructed lysosome-localized optogenetic actuators, named lyso-NpHR3.0, lyso-ArchT, and lyso-ChR2, to achieve optogenetic manipulation of lysosomes. These new actuators enable light-dependent control of lysosomal membrane potential, pH, hydrolase activity, degradation, and Ca2+ dynamics in living cells. Notably, lyso-ChR2 activation induces autophagy through the mTOR pathway, promotes Aβ clearance in an autophagy-dependent manner in cellular models, and alleviates Aβ-induced paralysis in the Caenorhabditis elegans model of Alzheimer's disease. Our lysosomal optogenetic actuators supplement the optogenetic toolbox and provide a method to dynamically regulate lysosomal physiology and function in living cells and animals.

Optical recordings of organellar membrane potentials and the components of membrane conductance in lysosomes

The eukaryotic cell is highly compartmentalized with organelles. Owing to their function in transporting metabolites, metabolic intermediates and byproducts of metabolic activity, organelles are important players in the orchestration of cellular function. Recent advances in optical methods for interrogating the different aspects of organellar activity promise to revolutionize our ability to dissect cellular processes with unprecedented detail. The transport activity of organelles is usually coupled to the transport of charged species; therefore, it is not only associated with the metabolic landscape but also entangled with membrane potentials. In this context, the targeted expression of fluorescent probes for interrogating organellar membrane potential (Ψorg) emerges as a powerful approach, offering less-invasive conditions and technical simplicity to interrogate cellular signalling and metabolism. Different research groups have made remarkable progress in adapting a variety of optical methods for measuring and monitoring Ψorg. These approaches include using potentiometric dyes, genetically encoded voltage indicators, hybrid fluorescence resonance energy transfer sensors and photoinduced electron transfer systems. These studies have provided consistent values for the resting potential of single-membrane organelles, such as lysosomes, the Golgi and the endoplasmic reticulum. We can foresee the use of dynamic measurements of Ψorg to study fundamental problems in organellar physiology that are linked to serious cellular disorders. Here, we present an overview of the available techniques, a survey of the resting membrane potential of internal membranes and, finally, an open-source mathematical model useful to interpret and interrogate membrane-bound structures of small volume by using the lysosome as an example.

Using whole genome sequence findings to assess gene-disease causality in cardiomyopathy and arrhythmia patients

Aim: The genetic etiologies of cardiomyopathies and arrhythmias have not been fully elucidated. Materials & methods: Research findings from genome analyses in a cardiomyopathy and arrhythmia cohort were gathered. Gene-disease relationships from two databases were compared with patient phenotypes. A literature review was conducted for genes with limited evidence. Results: Of 43 genes with candidate findings from 18 cases, 23.3% of genes had never been curated, 15.0% were curated for cardiomyopathies, 16.7% for arrhythmias and 31.3% for other conditions. 25.5% of candidate findings were curated for the patient's specific phenotype with 11.8% having definitive evidence. MYH6 and TPCN1 were flagged for recuration. Conclusion: Findings from genome sequencing in disease cohorts may be useful to guide gene-curation efforts.

Convergent activation of Ca2+ permeability in two-pore channel 2 through distinct molecular routes

TPC2 is a pathophysiologically relevant lysosomal ion channel that is activated directly by the phosphoinositide PI(3,5)P2 and indirectly by the calcium ion (Ca2+)-mobilizing molecule NAADP through accessory proteins that associate with the channel. TPC2 toggles between PI(3,5)P2-induced, sodium ion (Na+)-selective and NAADP-induced, Ca2+-permeable states in response to these cues. To address the molecular basis of polymodal gating and ion-selectivity switching, we investigated the mechanism by which NAADP and its synthetic functional agonist, TPC2-A1-N, induced Ca2+ release through TPC2 in human cells. Whereas NAADP required the NAADP-binding proteins JPT2 and LSM12 to evoke endogenous calcium ion signals, TPC2-A1-N did not. Residues in TPC2 that bind to PI(3,5)P2 were required for channel activation by NAADP but not for activation by TPC2-A1-N. The cryptic voltage-sensing region of TPC2 was required for the actions of TPC2-A1-N and PI(3,5)P2 but not for those of NAADP. These data mechanistically distinguish natural and synthetic agonist action at TPC2 despite convergent effects on Ca2+ permeability and delineate a route for pharmacologically correcting impaired NAADP-evoked Ca2+ signals.

Two-pore channel 1 and Ca2+ release-activated Ca2+ channels contribute to the acrosomal pH-dependent intracellular Ca2+ increase in mouse sperm

The acrosome is a lysosome-related vesicular organelle located in the sperm head. The acrosomal reaction (AR) is an exocytic process mediated by Ca2+ and essential for mammalian fertilization. Recent findings support the importance of acrosomal alkalinization for the AR. Mibefradil (Mib) and NNC 55-0396 (NNC) are two amphipathic weak bases that block the sperm-specific Ca2+ channel (CatSper) and induce acrosomal pH (pHa ) increase by accumulating in the acrosomal lumen of mammalian sperm. This accumulation and pHa elevation increase the intracellular Ca2+ concentration ([Ca2+ ]i ) and trigger the AR by unknown mechanisms of Ca2+ transport. Here, we investigated the pathways associated with the pHa increase-induced Ca2+ signals using mouse sperm as a model. To address these questions, we used single-cell Ca2+ imaging, the lysosomotropic agent Gly-Phe-β-naphthylamide (GPN) and pharmacological tools. Our findings show that Mib and NNC increase pHa and release acrosomal Ca2+ without compromising acrosomal membrane integrity. Our GPN results indicate that the osmotic component does not significantly contribute to acrosomal Ca2+ release caused by pHa rise. Inhibition of two-pore channel 1 (TPC1) channels reduced the [Ca2+ ]i increase stimulated by acrosomal alkalinization. In addition, blockage of Ca2+ release-activated Ca2+ (CRAC) channels diminished Ca2+ uptake triggered by pHa alkalinization. Finally, our findings contribute to understanding how pHa controls acrosomal Ca2+ efflux and extracellular Ca2+ entry during AR in mouse sperm. KEY POINTS: The acrosomal vesicle is a lysosome-related organelle located in the sperm head. The acrosome reaction (AR) is a highly regulated exocytic process mediated by Ca2+ , which is essential for fertilization. However, the molecular identity of Ca2+ transporters involved in the AR and their mechanisms to regulate Ca2+ fluxes are not fully understood. In mammalian sperm, acrosomal alkalinization induces intracellular Ca2+ concentration ([Ca2+ ]i ) increase and triggers the AR by unknown molecular mechanisms of Ca2+ transport. In this study, we explored the molecular mechanisms underlying Ca2+ signals caused by acrosomal alkalinization using mouse sperm as a model. TPC1 and CRAC channels contribute to [Ca2+ ]i elevation during acrosomal alkalinization. Our findings expand our understanding of how the acrosomal pH participates in the physiological induction of the AR.

Genome-wide structural variant analysis identifies risk loci for non-Alzheimer's dementias

We characterized the role of structural variants, a largely unexplored type of genetic variation, in two non-Alzheimer's dementias, namely Lewy body dementia (LBD) and frontotemporal dementia (FTD)/amyotrophic lateral sclerosis (ALS). To do this, we applied an advanced structural variant calling pipeline (GATK-SV) to short-read whole-genome sequence data from 5,213 European-ancestry cases and 4,132 controls. We discovered, replicated, and validated a deletion in TPCN1 as a novel risk locus for LBD and detected the known structural variants at the C9orf72 and MAPT loci as associated with FTD/ALS. We also identified rare pathogenic structural variants in both LBD and FTD/ALS. Finally, we assembled a catalog of structural variants that can be mined for new insights into the pathogenesis of these understudied forms of dementia.

TPC1 vacuole SV channel gains further shape - voltage priming of calcium-dependent gating

Across phyla, voltage-gated ion channels (VGICs) allow excitability. The vacuolar two-pore channel AtTPC1 from the tiny mustard plant Arabidopsis thaliana has emerged as a paradigm for deciphering the role of voltage and calcium signals in membrane excitation. Among the numerous experimentally determined structures of VGICs, AtTPC1 was the first to be revealed in a closed and resting state, fueling speculation about structural rearrangements during channel activation. Two independent reports on the structure of a partially opened AtTPC1 channel protein have led to working models that offer promising insights into the molecular switches associated with the gating process. We review new structure-function models and also discuss the evolutionary impact of two-pore channels (TPCs) on K⁺ homeostasis and vacuolar excitability.

Characterization of Endo-Lysosomal Cation Channels Using Calcium Imaging

Endo-lysosomes are membrane-bound acidic organelles that are involved in endocytosis, recycling, and degradation of extracellular and intracellular material. The membranes of endo-lysosomes express several Ca²⁺-permeable cation ion channels, including two-pore channels (TPC1-3) and transient receptor potential mucolipin channels (TRPML1-3). In this chapter, we will describe four different state-of-the-art Ca²⁺ imaging approaches, which are well-suited to investigate the function of endo-lysosomal cation channels. These techniques include (1) global cytosolic Ca²⁺ measurements, (2) peri-endo-lysosomal Ca²⁺ imaging using genetically encoded Ca²⁺ sensors that are directed to the cytosolic endo-lysosomal membrane surface, (3) Ca²⁺ imaging of endo-lysosomal cation channels, which are engineered in order to redirect them to the plasma membrane in combination with approaches 1 and 2, and (4) Ca²⁺ imaging by directing Ca²⁺ indicators to the endo-lysosomal lumen. Moreover, we will review useful small molecules, which can be used as valuable tools for endo-lysosomal Ca²⁺ imaging. Rather than providing complete protocols, we will discuss specific methodological issues related to endo-lysosomal Ca²⁺ imaging.

Pharmacological inhibition of lysosomal two-pore channel 2 (TPC2) confers neuroprotection in stroke via autophagy regulation

Lysosomal function and organellar Ca²⁺ homeostasis become dysfunctional in Stroke causing disturbances in autophagy, the major process for the degradation of abnormal protein aggregates and dysfunctional organelles. However, the role of autophagy in Stroke is controversial since excessive or prolonged autophagy activation exacerbates ischemic brain injury. Of note, glutamate evokes NAADP-dependent Ca²⁺ release via lysosomal TPC2 channels thus controlling basal autophagy. Considering the massive release of excitotoxins in Stroke, autophagic flux becomes uncontrolled with abnormal formation of autophagosomes causing, in turn, disruption of excitotoxins clearance and neurodegeneration. Here, a fine regulation of autophagy via a proper pharmacological modulation of lysosomal TPC2 channel has been tested in preclinical Stroke models. Primary cortical neurons were subjected to oxygen and glucose deprivation+reoxygenation to reproduce in vitro brain ischemia. Focal brain ischemia was induced in rats by transient middle cerebral artery occlusion (tMCAO). Under these conditions, TPC2 protein expression as well as autophagy and endoplasmic reticulum (ER) stress markers were studied by Western blotting, while TPC2 localization and activity were measured by immunocytochemistry and single-cell video-imaging, respectively. TPC2 protein expression and immunosignal were highly modulated in primary cortical neurons exposed to extreme hypoxic conditions causing dysfunction in organellar Ca²⁺ homeostasis, ER stress and autophagy-induced cell death. TPC2 knocking down and pharmacological inhibition by Ned-19 during hypoxia induced neuroprotection. The effect of Ned-19 was reversed by the permeable form of TPC2 endogenous agonist, NAADP-AM. Of note, Ned-19 prevented ER stress, as measured by GRP78 (78 kDa glucose-regulated protein) protein reduction and caspase 9 downregulation. In this way Ned-19 restored organellar Ca²⁺ level. Interestingly, Ned-19 reduced the infarct volume and neurological deficits in rats subjected to tMCAO and prevented hypoxia-induced cell death by blocking autophagic flux. Collectively, the pharmacological inhibition of TPC2 lysosomal channel by Ned-19 protects from focal ischemia by hampering a hyperfunctional autophagy.

Cationic amphiphilic antihistamines inhibit STAT3 via Ca2+-dependent lysosomal H+ efflux

Commonly used antihistamines and other cationic amphiphilic drugs (CADs) are emerging as putative cancer drugs. Their unique chemical structure enables CADs to accumulate rapidly inside lysosomes, where they increase lysosomal pH, alter lysosomal lipid metabolism, and eventually cause lysosomal membrane permeabilization. Here, we show that CAD-induced rapid elevation in lysosomal pH is caused by a lysosomal H+ efflux that requires P2RX4-mediated lysosomal Ca2+ release and precedes the lysosomal membrane permeabilization. The subsequent cytosolic acidification triggers the dephosphorylation, lysosomal translocation, and inactivation of the oncogenic signal transducer and activator of transcription 3 (STAT3) transcription factor. Moreover, CAD-induced lysosomal H+ efflux sensitizes cancer cells to apoptosis induced by STAT3 inhibition and acts synergistically with STAT3 inhibition in restricting the tumor growth of A549 non-small cell lung carcinoma xenografts. These findings identify lysosomal H+ efflux and STAT3 inhibition as anticancer mechanisms of CADs and reinforce the repurposing of safe and inexpensive CADs as cancer drugs with a drug combination strategy.

Lysosomal Ion Channels: What Are They Good For and Are They Druggable Targets?

Lysosomes play fundamental roles in material digestion, cellular clearance, recycling, exocytosis, wound repair, Ca²⁺ signaling, nutrient signaling, and gene expression regulation. The organelle also serves as a hub for important signaling networks involving the mTOR and AKT kinases. Electrophysiological recording and molecular and structural studies in the past decade have uncovered several unique lysosomal ion channels and transporters, including TPCs, TMEM175, TRPMLs, CLN7, and CLC-7. They underlie the organelle's permeability to major ions, including K⁺, Na⁺, H⁺, Ca²⁺, and Cl^-. The channels are regulated by numerous cellular factors, ranging from H⁺ in the lumen and voltage across the lysosomal membrane to ATP in the cytosol to growth factors outside the cell. Genetic variations in the channel/transporter genes are associated with diseases that include lysosomal storage diseases and neurodegenerative diseases. Recent studies with human genetics and channel activators suggest that lysosomal channels may be attractive targets for the development of therapeutics for the prevention of and intervention in human diseases.

Structure and Function of Plant and Mammalian TPC Channels

Two-pore channels (TPCs) belong to the family of voltage-gated tetrameric cation channels and are ubiquitously expressed in organelles of animals and plants. These channels are believed to be evolutionary intermediates between homotetrameric voltage-gated potassium/sodium channels and the four-domain, single subunit, voltage-gated sodium/calcium channels. Each TPC subunit contains 12 transmembrane segments that can be divided into two homologous copies of an S1-S6 Shaker-like 6-TM domain. A functional TPC channel assembles as a dimer - the equivalent of a voltage-gated tetrameric cation channel. The plant TPC channel is localized in the vacuolar membrane and is also called the SV channel for generating the slow vacuolar (SV) current observed long before its molecular identification. Three subfamilies of mammalian TPC channels have been defined - TPC1, 2, and 3 - with the first two being ubiquitously expressed in animals and TPC3 being expressed in some animals but not in humans. Mammalian TPC1 and TPC2 are localized to the endolysosomal membrane and their functions are associated with various physiological processes. TPC3 is localized in the plasma membrane and its physiological function is not well defined.

Lysosomal Potassium Channels

Lysosomes are acidic membrane-bound organelles that use hydrolytic enzymes to break down material through pathways such as endocytosis, phagocytosis, mitophagy, and autophagy. To function properly, intralysosomal environments are strictly controlled by a set of integral membrane proteins such as ion channels and transporters. Potassium ion (K⁺) channels are a large and diverse family of membrane proteins that control K⁺ flux across both the plasma membrane and intracellular membranes. In the plasma membrane, they are essential in both excitable and non-excitable cells for the control of membrane potential and cell signaling. However, our understanding of intracellular K⁺ channels is very limited. In this review, we summarize the recent development in studies of K⁺ channels in the lysosome. We focus on their characterization, potential roles in maintaining lysosomal membrane potential and lysosomal function, and pathological implications.

Endo-Lysosomal Two-Pore Channels and Their Protein Partners

Two-pore channels are ion channels expressed on acidic organelles such as the various vesicles that constitute the endo-lysosomal system. They are permeable to Ca2+ and Na+ and activated by the second messenger NAADP as well as the phosphoinositide, PI(3,5)P2 and/or voltage. Here, we review the proteins that interact with these channels including recently identified NAADP receptors.

PI(3,5)P2 and NAADP: Team players or lone warriors? - New insights into TPC activation modes

NAADP (nicotinic acid adenine dinucleotide phosphate) is a second messenger, releasing Ca²⁺ from acidic calcium stores such as endosomes and lysosomes. PI(3,5)P₂ (phosphatidylinositol 3,5-bisphosphate) is a phospho-inositide, residing on endolysosomal membranes and likewise releasing Ca²⁺ from endosomes and lysosomes. Both compounds have been shown to activate endolysosomal two-pore channels (TPCs) in mammalian cells. However, their effects on ion permeability as demonstrated specifically for TPC2 differ. While PI(3,5)P₂ elicits predominantly Na⁺-selective currents, NAADP increases the Ca²⁺ permeability of the channel. What happens when both compounds are applied simultaneously was unclear until recently.

Lysosomal solute and water transport

Lysosomes mediate hydrolase-catalyzed macromolecule degradation to produce building block catabolites for reuse. Lysosome function requires an osmo-sensing machinery that regulates osmolytes (ions and organic solutes) and water flux. During hypoosmotic stress or when undigested materials accumulate, lysosomes become swollen and hypo-functional. As a membranous organelle filled with cargo macromolecules, catabolites, ions, and hydrolases, the lysosome must have mechanisms that regulate its shape and size while coordinating content exchange. In this review, we discussed the mechanisms that regulate lysosomal fusion and fission as well as swelling and condensation, with a focus on solute and water transport mechanisms across lysosomal membranes. Lysosomal H+, Na+, K+, Ca2+, and Cl- channels and transporters sense trafficking and osmotic cues to regulate both solute flux and membrane trafficking. We also provide perspectives on how lysosomes may adjust the volume of themselves, the cytosol, and the cytoplasm through the control of lysosomal solute and water transport.

Synthesis, Molecular Docking, In Vitro and In Vivo Studies of Novel Dimorpholinoquinazoline-Based Potential Inhibitors of PI3K/Akt/mTOR Pathway

A (series) range of potential dimorpholinoquinazoline-based inhibitors of the PI3K/Akt/mTOR cascade was synthesized. Several compounds exhibited cytotoxicity towards a panel of cancer cell lines in the low and sub-micromolar range. Compound 7c with the highest activity and moderate selectivity towards MCF7 cells which express the mutant type of PI3K was also tested for the ability to inhibit PI3K-(signaling pathway) downstream effectors and associated proteins. Compound 7c inhibited the phosphorylation of Akt, mTOR, and S6K at 125–250 nM. It also triggered PARP1 cleavage, ROS production, and cell death via several mechanisms. Inhibition of PI3Kα was observed at a concentration of 7b 50 µM and of 7c 500 µM and higher, that can indicate minority PI3Kα as a target among other kinases in the titled cascade for 7c. In vivo studies demonstrated an inhibition of tumor growth in the colorectal tumor model. According to the docking studies, the replacement of the triazine core in gedatolisib (8) by a quinazoline fragment, and incorporation of a (hetero)aromatic unit connected with the carbamide group via a flexible spacer, can result in more selective inhibition of the PI3Kα isoform.

Voltage-sensing phosphatase (Vsp) regulates endocytosis-dependent nutrient absorption in chordate enterocytes

Voltage-sensing phosphatase (Vsp) is a unique membrane protein that translates membrane electrical activities into the changes of phosphoinositide profiles. Vsp orthologs from various species have been intensively investigated toward their biophysical properties, primarily using a heterologous expression system. In contrast, the physiological role of Vsp in native tissues remains largely unknown. Here we report that zebrafish Vsp (Dr-Vsp), encoded by tpte gene, is functionally expressed on the endomembranes of lysosome-rich enterocytes (LREs) that mediate dietary protein absorption via endocytosis in the zebrafish mid-intestine. Dr-Vsp-deficient LREs were remarkably defective in forming endosomal vacuoles after initial uptake of dextran and mCherry. Dr-Vsp-deficient zebrafish exhibited growth restriction and higher mortality during the critical period when zebrafish larvae rely primarily on exogenous feeding via intestinal absorption. Furthermore, our comparative study on marine invertebrate Ciona intestinalis Vsp (Ci-Vsp) revealed co-expression with endocytosis-associated genes in absorptive epithelial cells of the Ciona digestive tract, corresponding to zebrafish LREs. These findings signify a crucial role of Vsp in regulating endocytosis-dependent nutrient absorption in specialized enterocytes across animal species.

The physiological role of Vsp in zebrafish is assessed, revealing Vsp expression in the mid-intestine for dietary protein absorption. A comparative study on marine invertebrate Ciona intestinalis suggests conservation of Vsp function in the GI tract.

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.

Major vacuolar TPC1 channel in stress signaling: what matters, K+, Ca2+ conductance or an ion-flux independent mechanism?

Two-pore cation channel, TPC1, is ubiquitous in the vacuolar membrane of terrestrial plants and mediates the long distance signaling upon biotic and abiotic stresses. It possesses a wide pore, which transports small mono- and divalent cations. K+ is transported more than 10-fold faster than Ca2+, which binds with a higher affinity within the pore. Key pore residues, responsible for Ca2+ binding, have been recently identified. There is also a substantial progress in the mechanistic and structural understanding of the plant TPC1 gating by membrane voltage and cytosolic and luminal Ca2+. Collectively, these gating factors at resting conditions strongly reduce the potentially lethal Ca2+ leak from the vacuole. Such tight control is impressive, bearing in mind high unitary conductance of the TPC1 and its abundance, with thousands of active channel copies per vacuole. But it remains a mystery how this high threshold is overcome upon signaling, and what type of signal is emitted by TPC1, whether it is Ca2+ or electrical one, or a transduction via protein conformational change, independent on ion conductance. Here we discuss non-exclusive scenarios for the TPC1 integration into Ca2+, ROS and electrical signaling.

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.

Segregated cation flux by TPC2 biases Ca2+ signaling through lysosomes

Two-pore channels are endo-lysosomal cation channels with malleable selectivity filters that drive endocytic ion flux and membrane traffic. Here we show that TPC2 can differentially regulate its cation permeability when co-activated by its endogenous ligands, NAADP and PI(3,5)P₂. Whereas NAADP rendered the channel Ca²⁺-permeable and PI(3,5)P₂ rendered the channel Na⁺-selective, a combination of the two increased Ca²⁺ but not Na⁺ flux. Mechanistically, this was due to an increase in Ca²⁺ permeability independent of changes in ion selectivity. Functionally, we show that cell permeable NAADP and PI(3,5)P₂ mimetics synergistically activate native TPC2 channels in live cells, globalizing cytosolic Ca²⁺ signals and regulating lysosomal pH and motility. Our data reveal that flux of different ions through the same pore can be independently controlled and identify TPC2 as a likely coincidence detector that optimizes lysosomal Ca²⁺ signaling.

TPC2 is a lysosomal ion channel permeable to both calcium and sodium ions. Here, the authors show that TPC2 can selectively increase its calcium permeability when simultaneously challenged by both its natural activators- NAADP and PI(3,5)P₂.

The Three Two-Pore Channel Subtypes from Rabbit Exhibit Distinct Sensitivity to Phosphoinositides, Voltage, and Extracytosolic pH

Two pore channels (TPCs) are implicated in vesicle trafficking, virus infection, and autophagy regulation. As Na+- or Ca2+-permeable channels, TPCs have been reported to be activated by NAADP, PI(3,5)P2, and/or high voltage. However, a comparative study on the function and regulation of the three mammalian TPC subtypes is currently lacking. Here, we used the electrophysiological recording of enlarged endolysosome vacuoles, inside-out and outside-out membrane patches to examine the three TPCs of rabbit (Oryctolagus cuniculus, or Oc) heterologously expressed in HEK293 cells. While PI(3,5)P2 evoked Na+ currents with a potency order of OcTPC1 > OcTPC3 > OcTPC2, only OcTPC2 displayed a strict dependence on PI(3,5)P2. Both OcTPC1 and OcTPC3 were activatable by PI3P and OcTPC3 was also activated by additional phosphoinositide species. While OcTPC2 was voltage-independent, OcTPC1 and OcTPC3 showed voltage dependence with OcTPC3 depending on high positive voltages. Finally, while OcTPC2 preferred a luminal pH of 4.6−6.0 in endolysosomes, OcTPC1 was strongly inhibited by extracytosolic pH 5.0 in both voltage-dependent and -independent manners, and OcTPC3 was inhibited by pH 6.0 but potentiated by pH 8.0. Thus, the three OcTPCs form phosphoinositide-activated Na+ channels with different ligand selectivity, voltage dependence, and extracytosolic pH sensitivity, which likely are optimally tuned for function in specific endolysosomal populations.

Tonic inhibition of the chloride/proton antiporter ClC-7 by PI(3,5)P2 is crucial for lysosomal pH maintenance

The acidic luminal pH of lysosomes, maintained within a narrow range, is essential for proper degrative function of the organelle and is generated by the action of a V-type H⁺ ATPase, but other pathways for ion movement are required to dissipate the voltage generated by this process. ClC-7, a Cl^-/H⁺ antiporter responsible for lysosomal Cl^- permeability, is a candidate to contribute to the acidification process as part of this ‘counterion pathway’ The signaling lipid PI(3,5)P2 modulates lysosomal dynamics, including by regulating lysosomal ion channels, raising the possibility that it could contribute to lysosomal pH regulation. Here, we demonstrate that depleting PI(3,5)P2 by inhibiting the kinase PIKfyve causes lysosomal hyperacidification, primarily via an effect on ClC-7. We further show that PI(3,5)P2 directly inhibits ClC-7 transport and that this inhibition is eliminated in a disease-causing gain-of-function ClC-7 mutation. Together, these observations suggest an intimate role for ClC-7 in lysosomal pH regulation.

Diversity of two-pore channels and the accessory NAADP receptors in intracellular Ca2+ signaling

Intracellular Ca²⁺ signaling via changes or oscillation in cytosolic Ca²⁺ concentration controls almost every aspect of cellular function and physiological processes, such as gene transcription, cell motility and proliferation, muscle contraction, and learning and memory. Two-pore channels (TPCs) are a class of eukaryotic cation channels involved in intracellular Ca²⁺ signaling, likely present in a multitude of organisms from unicellular organisms to mammals. Accumulated evidence indicates that TPCs play a critical role in Ca²⁺ mobilization from intracellular stores mediated by the second messenger molecule, nicotinic acid adenine dinucleotide phosphate (NAADP). In recent years, significant progress has been made regarding our understanding of the structures and function of TPCs, including Cryo-EM structure determination of mammalian TPCs and characterization of a plastid TPC in a single-celled parasite.. The recent identification of Lsm12 and JPT2 as NAADP-binding proteins provides a new molecular basis for understanding NAADP-evoked Ca²⁺ signaling. In this review, we summarize basic structural and functional aspects of TPCs and highlight the most recent studies on the newly discovered TPC in a parasitic protozoan and the NAADP-binding proteins LSM12 and JPT2 as new key players in NAADP signaling.

Two-Pore Channels Regulate Inter-Organellar Ca2+ Homeostasis in Immune Cells

Two-pore channels (TPCs) are ligand-gated cation-selective ion channels that are preserved in plant and animal cells. In the latter, TPCs are located in membranes of acidic organelles, such as endosomes, lysosomes, and endolysosomes. Here, we focus on the function of these unique ion channels in mast cells, which are leukocytes that mature from myeloid hematopoietic stem cells. The cytoplasm of these innate immune cells contains a large number of granules that comprise messenger substances, such as histamine and heparin. Mast cells, along with basophil granulocytes, play an essential role in anaphylaxis and allergic reactions by releasing inflammatory mediators. Signaling in mast cells is mainly regulated via the release of Ca2+ from the endoplasmic reticulum as well as from acidic compartments, such as endolysosomes. For the crosstalk of these organelles TPCs seem essential. Allergic reactions and anaphylaxis were previously shown to be associated with the endolysosomal two-pore channel TPC1. The release of histamine, controlled by intracellular Ca2+ signals, was increased upon genetic or pharmacologic TPC1 inhibition. Conversely, stimulation of TPC channel activity by one of its endogenous ligands, namely nicotinic adenine dinucleotide phosphate (NAADP) or phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2), were found to trigger the release of Ca2+ from the endolysosomes; thereby improving the effect of TPC1 on regulated mast cell degranulation. In this review we discuss the importance of TPC1 for regulating Ca2+ homeostasis in mast cells and the overall potential of TPC1 as a pharmacological target in anti-inflammatory therapy.

Endolysosomal cation channels point the way towards precision medicine of cancer and infectious diseases

Infectious diseases and cancer are among the key medical challenges that humankind is facing today. A growing amount of evidence suggests that ion channels in the endolysosomal system play a crucial role in the pathology of both groups of diseases. The development of advanced patch-clamp technologies has allowed us to directly characterize ion fluxes through endolysosomal ion channels in their native environments. Endolysosomes are essential organelles for intracellular transport, digestion and metabolism, and maintenance of homeostasis. The endolysosomal ion channels regulate the function of the endolysosomal system through four basic mechanisms: calcium release, control of membrane potential, pH change, and osmolarity regulation. In this review, we put particular emphasis on the endolysosomal cation channels, including TPC2 and TRPML2, which are particularly important in monocyte function. We discuss existing endogenous and synthetic ligands of these channels and summarize current knowledge of their impact on channel activity and function in different cell types. Moreover, we summarize recent findings on the importance of TPC2 and TRPML2 channels as potential drug targets for the prevention and treatment of the emerging infectious diseases and cancer.

pH regulates potassium conductance and drives a constitutive proton current in human TMEM175

Human TMEM175, a noncanonical potassium (K+) channel in endolysosomes, contributes to their pH stability and is implicated in the pathogenesis of Parkinson's disease (PD). Structurally, the TMEM175 family exhibits an architecture distinct from canonical potassium channels, as it lacks the typical TVGYG selectivity filter. Here, we show that human TMEM175 not only exhibits pH-dependent structural changes that reduce K+ permeation at acidic pH but also displays proton permeation. TMEM175 constitutively conducts K+ at pH 7.4 but displays reduced K+ permeation at lower pH. In contrast, proton current through TMEM175 increases with decreasing pH because of the increased proton gradient. Molecular dynamics simulation, structure-based mutagenesis, and electrophysiological analysis suggest that K+ ions and protons share the same permeation pathway. The M393T variant of human TMEM175 associated with PD shows reduced function in both K+ and proton permeation. Together, our structural and electrophysiological analysis reveals a mechanism of TMEM175 regulation by pH.

Current Methods to Unravel the Functional Properties of Lysosomal Ion Channels and Transporters

A distinct set of channels and transporters regulates the ion fluxes across the lysosomal membrane. Malfunctioning of these transport proteins and the resulting ionic imbalance is involved in various human diseases, such as lysosomal storage disorders, cancer, as well as metabolic and neurodegenerative diseases. As a consequence, these proteins have stimulated strong interest for their suitability as possible drug targets. A detailed functional characterization of many lysosomal channels and transporters is lacking, mainly due to technical difficulties in applying the standard patch-clamp technique to these small intracellular compartments. In this review, we focus on current methods used to unravel the functional properties of lysosomal ion channels and transporters, stressing their advantages and disadvantages and evaluating their fields of applicability.

Lysosomal potassium channels

The lysosome is an important membrane-bound acidic organelle that is regarded as the degradative center as well as multifunctional signaling hub. It digests unwanted macromolecules, damaged organelles, microbes, and other materials derived from endocytosis, autophagy, and phagocytosis. To function properly, the ionic homeostasis and membrane potential of the lysosome are strictly regulated by transporters and ion channels. As the most abundant cation inside the cell, potassium ions (K⁺) are vital for lysosomal membrane potential and lysosomal calcium (Ca²⁺) signaling. However, our understanding about how lysosomal K⁺homeostasis is regulated and what are the functions of K⁺in the lysosome is very limited. Currently, two lysosomal K⁺channels have been identified: large-conductance Ca²⁺-activated K⁺channel (BK) and transmembrane Protein 175 (TMEM175). In this review, we summarize recent development in our understanding of K⁺ homeostasis and K⁺channels in the lysosome. We hope to guide the readers into a more in-depth discussion of lysosomal K⁺ channels in lysosomal physiology and human diseases.

Activation of endo-lysosomal two-pore channels by NAADP and PI(3,5)P2. Five things to know

Two-pore channels are ancient members of the voltage-gated ion channel superfamily that are expressed predominantly on acidic organelles such as endosomes and lysosomes. Here we review recent advances in understanding how TPCs are activated by their ligands and identify five salient features: (1) TPCs are Ca²⁺-permeable non-selective cation channels gated by NAADP. (2) NAADP activation is indirect through associated NAADP receptors. (3) TPCs are also Na⁺-selective channels gated by PI(3,5)P₂. (4) PI(3,5)P₂ activation is direct through a structurally-resolved binding site. (5) TPCs switch their ion selectivity in an agonist-dependent manner.

Whole Endosome Recording of Vesicular Neurotransmitter Transporter Currents

The analysis of organellar membrane transporters presents many technical problems. In general, their activity depends on a H⁺ electrochemical driving force (Δμ_H+). However, transport itself influences the expression of Δμ_H+ in standard radiotracer flux assays, making it difficult to disentangle the role of the chemical component ΔpH and the membrane potential Δψ. Whole endosome recording in voltage clamp circumvents many of these problems, controlling ionic conditions as well as membrane potential inside and outside the organelle . This approach has been used primarily to study the properties of endolysosomal channels, which generate substantial currents (Saito et al., J Biol Chem 282(37):27327-27333, 2007; Cang et al., Nat Chem Biol 10(6):463-469, 2014; Cang et al., Cell 152(4):778-790, 2013; Chen et al., Nat Protoc 12(8):1639-1658, 2017; Samie et al., Dev Cell 26(5):511-524, 2013; Wang et al., Cell 151(2):372-383, 2012). Electrogenic transport produces much smaller currents, but we have recently reported the detection of transport currents and an uncoupled Cl^- conductance associated with the vesicular glutamate transporters (VGLUTs) that fill synaptic vesicles with glutamate (Chang et al., eLife 7:e34896, 2018). In this protocol, we will focus on the measurement of transport currents on enlarged endosomes of heterologous mammalian cells.

Structural biology of cation channels important for lysosomal calcium release

Calcium is one of the most important second messengers in cells. The uptake and release of calcium ions are conducted by channels and transporters. Inside a eukaryotic cell, calcium is stored in intracellular organelles including the endoplasmic reticulum (ER), mitochondrion, and lysosome. Lysosomes are acid membrane-bounded organelles serving as the crucial degradation and recycling center of the cell. Lysosomes involve in multiple important signaling events, including nutrient sensing, lipid metabolism, and trafficking. Hitherto, two lysosomal cation channel families have been suggested to function as calcium release channels, namely the Two-pore Channel (TPC) family, and the Transient Receptor Potential Channel Mucolipin (TRPML) family. Additionally, a few plasma membrane calcium channels have also been found in the lysosomal membrane under certain circumstances. In this review, we will discuss the structural mechanism of the cation channels that may be important for lysosomal calcium release, primarily focusing on the TPCs and TRPMLs.

Channels and Transporters of the Pulmonary Lamellar Body in Health and Disease

The lamellar body (LB) of the alveolar type II (ATII) cell is a lysosome-related organelle (LRO) that contains surfactant, a complex mix of mainly lipids and specific surfactant proteins. The major function of surfactant in the lung is the reduction of surface tension and stabilization of alveoli during respiration. Its lack or deficiency may cause various forms of respiratory distress syndrome (RDS). Surfactant is also part of the innate immune system in the lung, defending the organism against air-borne pathogens. The limiting (organelle) membrane that encloses the LB contains various transporters that are in part responsible for translocating lipids and other organic material into the LB. On the other hand, this membrane contains ion transporters and channels that maintain a specific internal ion composition including the acidic pH of about 5. Furthermore, P2X4 receptors, ligand gated ion channels of the danger signal ATP, are expressed in the limiting LB membrane. They play a role in boosting surfactant secretion and fluid clearance. In this review, we discuss the functions of these transporting pathways of the LB, including possible roles in disease and as therapeutic targets, including viral infections such as SARS-CoV-2.

CLN7 is an organellar chloride channel regulating lysosomal function

Neuronal ceroid lipofuscinoses (NCLs) are a group of autosomal recessive lysosomal storage diseases. One variant form of late-infantile NCL (vLINCL) is caused by mutations of a lysosomal membrane protein CLN7, the function of which has remained unknown. Here, we identified CLN7 as a novel endolysosomal chloride channel. Overexpression of CLN7 increases endolysosomal chloride currents and enlarges endolysosomes through a Ca²⁺/calmodulin-dependent way. Human CLN7 and its yeast homolog exhibit characteristics of chloride channels and are sensitive to chloride channel blockers. Moreover, CLN7 regulates lysosomal chloride conductance, luminal pH, and lysosomal membrane potential and promotes the release of lysosomal Ca²⁺ through transient receptor potential mucolipin 1 (TRPML1). Knocking out CLN7 causes pathological features that are similar to those of patients with vLINCL, including retinal degeneration and autofluorescent lipofuscin. The pathogenic mutations in CLN7 lead to a decrease in chloride permeability, suggesting that reconstitution of lysosomal Cl⁻ homeostasis may be an effective strategy for the treatment of vLINCL.

Ion Channels and Pumps in Autophagy: A Reciprocal Relationship

Autophagy, the process of cellular self-degradation, is intrinsically tied to the degradative function of the lysosome. Several diseases have been linked to lysosomal degradative defects, including rare lysosomal storage disorders and neurodegenerative diseases. Ion channels and pumps play a major regulatory role in autophagy. Importantly, calcium signaling produced by TRPML1 (transient receptor potential cation channel, mucolipin subfamily) has been shown to regulate autophagic progression through biogenesis of autophagic-lysosomal organelles, activation of mTORC1 (mechanistic target of rapamycin complex 1) and degradation of autophagic cargo. ER calcium channels such as IP₃Rs supply calcium for the lysosome, and lysosomal function is severely disrupted in the absence of lysosomal calcium replenishment by the ER. TRPML1 function is also regulated by LC3 (microtubule-associated protein light chain 3) and mTORC1, two critical components of the autophagic network. Here we provide an overview of the current knowledge about ion channels and pumps-including lysosomal V-ATPase (vacuolar proton-ATPase), which is required for acidification and hence proper enzymatic activity of lysosomal hydrolases-in the regulation of autophagy, and discuss how functional impairment of some of these leads to diseases.

Acidic Ca2+ stores and immune-cell function

Acidic organelles act as intracellular Ca²⁺ stores; they actively sequester Ca²⁺ in their lumina and release it to the cytosol upon activation of endo-lysosomal Ca²⁺ channels. Recent data suggest important roles of endo-lysosomal Ca²⁺ channels, the Two-Pore Channels (TPCs) and the TRPML channels (mucolipins), in different aspects of immune-cell function, particularly impacting membrane trafficking, vesicle fusion/fission and secretion. Remarkably, different channels on the same acidic vesicles can couple to different downstream physiology. Endo-lysosomal Ca²⁺ stores can act under different modalities, be they acting alone (via local Ca²⁺ nanodomains around TPCs/TRPMLs) or in conjunction with the ER Ca²⁺ store (to either promote or suppress global ER Ca²⁺ release). These different modalities impinge upon functions as broad as phagocytosis, cell-killing, anaphylaxis, immune memory, thrombostasis, and chemotaxis.

Phosphoinositide regulates dynamic movement of the S4 voltage sensor in the second repeat in two-pore channel 3

The two-pore channels (TPCs) are voltage-gated cation channels consisting of single polypeptides with two repeats of a canonical 6-transmembrane unit. TPCs are known to be regulated by various physiological signals such as membrane voltage and phosphoinositide (PI). The fourth helix in the second repeat (second S4) plays a major role in detecting membrane voltage, whereas the first repeat contains a PI binding site. Therefore, each of these stimuli is detected by a unique repeat to regulate the gating of the TPC central pore. How these various stimuli regulate the dynamic structural rearrangement of the TPC molecule remain unknown. Here, we found that PI binding to the first repeat in TPC3 regulates the movement of the distally located second S4 helix, showing that the PI-binding signal is not confined to the pore gate but also transmitted to the voltage sensor. Using voltage clamp fluorometry, measurement of gating charges, and Cys-accessibility analysis, we observed that PI binding significantly potentiates the voltage dependence of the movement of the second S4 helix. Notably, voltage clamp fluorometry analysis revealed that the voltage-dependent movement of the second S4 helix occurred in two phases, of which the second phase corresponds to the transfer of the gating charges. This movement was observed in the voltage range where gate-opening occurs and was potentiated by PI. In conclusion, this regulation of the second S4 helix by PI indicates a tight inter-repeat coupling within TPC3, a feature which might be conserved among TPC family members to integrate various physiological signals.

Deciphering the Role of Ion Channels in Early Defense Signaling against Herbivorous Insects

Plants and insect herbivores are in a relentless battle to outwit each other. Plants have evolved various strategies to detect herbivores and mount an effective defense system against them. These defenses include physical and structural barriers such as spines, trichomes, cuticle, or chemical compounds, including secondary metabolites such as phenolics and terpenes. Plants perceive herbivory by both mechanical and chemical means. Mechanical sensing can occur through the perception of insect biting, piercing, or chewing, while chemical signaling occurs through the perception of various herbivore-derived compounds such as oral secretions (OS) or regurgitant, insect excreta (frass), or oviposition fluids. Interestingly, ion channels or transporters are the first responders for the perception of these mechanical and chemical cues. These transmembrane pore proteins can play an important role in plant defense through the induction of early signaling components such as plasma transmembrane potential (V_m) fluctuation, intracellular calcium (Ca²⁺), and reactive oxygen species (ROS) generation, followed by defense gene expression, and, ultimately, plant defense responses. In recent years, studies on early plant defense signaling in response to herbivory have been gaining momentum with the application of genetically encoded GFP-based sensors for real-time monitoring of early signaling events and genetic tools to manipulate ion channels involved in plant-herbivore interactions. In this review, we provide an update on recent developments and advances on early signaling events in plant-herbivore interactions, with an emphasis on the role of ion channels in early plant defense signaling.

Two-Pore Channels Regulate Expression of Various Receptors and Their Pathway-Related Proteins in Multiple Ways

Two-pore channels (TPCs) constitute a small family of ion channels within membranes of intracellular acidic compartments, such as endosomes and lysosomes. They were shown to provide transient and locally restricted Ca²⁺-currents, likely responsible for fusion and/or fission events of endolysosomal membranes and thereby for intracellular vesicle trafficking. Genetic deletion of TPCs not only affects endocytosis, recycling, and degradation of various surface receptors but also uptake and impact of bacterial protein toxins and entry and intracellular processing of some types of viruses. This review points to important examples of these trafficking defects on one part but mainly focuses on the resulting impact of the TPC inactivation on receptor expression and receptor signaling. Thus, a detailed RNA sequencing analysis using TPC1-deficient fibroblasts uncovered a multitude of changes in the expression levels of surface receptors and their pathway-related signaling proteins. We refer to several classes of receptors such as EGF, TGF, and insulin as well as proteins involved in endocytosis.

Metabolomic profiling of single enlarged lysosomes

Lysosomes are critical for cellular metabolism and are heterogeneously involved in various cellular processes. The ability to measure lysosomal metabolic heterogeneity is essential for understanding their physiological roles. We therefore built a single-lysosome mass spectrometry (SLMS) platform integrating lysosomal patch-clamp recording and induced nano-electrospray ionization (nanoESI)/mass spectrometry (MS) that enables concurrent metabolic and electrophysiological profiling of individual enlarged lysosomes. The accuracy and reliability of this technique were validated by supporting previous findings, such as the transportability of lysosomal cationic amino acids transporters such as PQLC2 and the lysosomal trapping of lysosomotropic, hydrophobic weak base drugs such as lidocaine. We derived metabolites from single lysosomes in various cell types and classified lysosomes into five major subpopulations based on their chemical and biological divergence. Senescence and carcinoma altered metabolic profiles of lysosomes in a type-specific manner. Thus, SLMS can open more avenues for investigating heterogeneous lysosomal metabolic changes during physiological and pathological processes.

TPC2 promotes choroidal angiogenesis and inflammation in a mouse model of neovascular age-related macular degeneration

This study identifies the endolysosomal two-pore channel TPC2 as a potential novel drug target for age-related macular degeneration (AMD). Inhibition of TPC2 reduces two key clinical hallmarks of the AMD, retinal inflammation and formation of leaky choroi.

Age-related macular degeneration (AMD) is the most common cause of blindness among the elderly and can be classified either as dry or as neovascular (or wet). Neovascular AMD is characterized by a strong immune response and the inadequate release of cytokines triggering angiogenesis and induction of photoreceptor death. The pathomechanisms of AMD are only partly understood. Here, we identify the endolysosomal two-pore cation channel TPC2 as a key factor of neovascularization and immune activation in the laser-induced choroidal neovascularization (CNV) mouse model of AMD. Block of TPC2 reduced retinal VEGFA and IL-1β levels and diminished neovascularization and immune activation. Mechanistically, TPC2 mediates cationic currents in endolysosomal organelles of immune cells and lack of TPC2 leads to reduced IL-1β levels in areas of choroidal neovascularization due to endolysosomal trapping. Taken together, our study identifies TPC2 as a promising novel therapeutic target for the treatment of AMD.

From Pinocytosis to Methuosis-Fluid Consumption as a Risk Factor for Cell Death

The volumes of a cell [cell volume (CV)] and its organelles are adjusted by osmoregulatory processes. During pinocytosis, extracellular fluid volume equivalent to its CV is incorporated within an hour and membrane area equivalent to the cell's surface within 30 min. Since neither fluid uptake nor membrane consumption leads to swelling or shrinkage, cells must be equipped with potent volume regulatory mechanisms. Normally, cells respond to outwardly or inwardly directed osmotic gradients by a volume decrease and increase, respectively, i.e., they shrink or swell but then try to recover their CV. However, when a cell death (CD) pathway is triggered, CV persistently decreases in isotonic conditions in apoptosis and it increases in necrosis. One type of CD associated with cell swelling is due to a dysfunctional pinocytosis. Methuosis, a non-apoptotic CD phenotype, occurs when cells accumulate too much fluid by macropinocytosis. In contrast to functional pinocytosis, in methuosis, macropinosomes neither recycle nor fuse with lysosomes but with each other to form giant vacuoles, which finally cause rupture of the plasma membrane (PM). Understanding methuosis longs for the understanding of the ionic mechanisms of cell volume regulation (CVR) and vesicular volume regulation (VVR). In nascent macropinosomes, ion channels and transporters are derived from the PM. Along trafficking from the PM to the perinuclear area, the equipment of channels and transporters of the vesicle membrane changes by retrieval, addition, and recycling from and back to the PM, causing profound changes in vesicular ion concentrations, acidification, and-most importantly-shrinkage of the macropinosome, which is indispensable for its proper targeting and cargo processing. In this review, we discuss ion and water transport mechanisms with respect to CVR and VVR and with special emphasis on pinocytosis and methuosis. We describe various aspects of the complex mutual interplay between extracellular and intracellular ions and ion gradients, the PM and vesicular membrane, phosphoinositides, monomeric G proteins and their targets, as well as the submembranous cytoskeleton. Our aim is to highlight important cellular mechanisms, components, and processes that may lead to methuotic CD upon their derangement.

The Discovery of Naringenin as Endolysosomal Two-Pore Channel Inhibitor and Its Emerging Role in SARS-CoV-2 Infection

The flavonoid naringenin (Nar), present in citrus fruits and tomatoes, has been identified as a blocker of an emerging class of human intracellular channels, namely the two-pore channel (TPC) family, whose role has been established in several diseases. Indeed, Nar was shown to be effective against neoangiogenesis, a process essential for solid tumor progression, by specifically impairing TPC activity. The goal of the present review is to illustrate the rationale that links TPC channels to the mechanism of coronavirus infection, and how their inhibition by Nar could be an efficient pharmacological strategy to fight the current pandemic plague COVID-19.

Measuring Absolute Membrane Potential Across Space and Time

Membrane potential (Vmem) is a fundamental biophysical signal present in all cells. Vmem signals range in time from milliseconds to days, and they span lengths from microns to centimeters. Vmem affects many cellular processes, ranging from neurotransmitter release to cell cycle control to tissue patterning. However, existing tools are not suitable for Vmem quantification in many of these areas. In this review, we outline the diverse biology of Vmem, drafting a wish list of features for a Vmem sensing platform. We then use these guidelines to discuss electrode-based and optical platforms for interrogating Vmem. On the one hand, electrode-based strategies exhibit excellent quantification but are most effective in short-term, cellular recordings. On the other hand, optical strategies provide easier access to diverse samples but generally only detect relative changes in Vmem. By combining the respective strengths of these technologies, recent advances in optical quantification of absolute Vmem enable new inquiries into Vmem biology.