NAADP-regulated two-pore channels drive phagocytosis through endo-lysosomal Ca2+ nanodomains, calcineurin and dynamin

Endolysosomal transient receptor potential mucolipins and two-pore channels: implications for cancer immunity

Past research has identified that cancer cells sustain several cancer hallmarks by impairing function of the endolysosomal system (ES). Thus, maintaining the functional integrity of endolysosomes is crucial, which heavily relies on two key protein families: soluble hydrolases and endolysosomal membrane proteins. Particularly members of the TPC (two-pore channel) and TRPML (transient receptor potential mucolipins) families have emerged as essential regulators of ES function as a potential target in cancer therapy. Targeting TPCs and TRPMLs has demonstrated significant impact on multiple cancer hallmarks, including proliferation, growth, migration, and angiogenesis both in vitro and in vivo. Notably, endosomes and lysosomes also actively participate in various immune regulatory mechanisms, such as phagocytosis, antigen presentation, and the release of proinflammatory mediators. Yet, knowledge about the role of TPCs and TRPMLs in immunity is scarce. This prompts a discussion regarding the potential role of endolysosomal ion channels in aiding cancers to evade immune surveillance and destruction. Specifically, understanding the interplay between endolysosomal ion channels and cancer immunity becomes crucial. Our review aims to comprehensively explore the current knowledge surrounding the roles of TPCs and TRPMLs in immunity, whilst emphasizing the critical need to elucidate their specific contributions to cancer immunity by pointing out current research gaps that should be addressed.

Two-pore channels regulate endomembrane tension to enable remodeling and resolution of phagolysosomes

Phagocytes promptly resolve ingested targets to replenish lysosomes and maintain their responsiveness. The resolution process requires that degradative hydrolases, solute transporters, and proteins involved in lipid traffic are delivered and made active in phagolysosomes. It also involves extensive membrane remodeling. We report that cation channels that localize to phagolysosomes were essential for resolution. Specifically, the conductance of Na+ by two-pore channels (TPCs) and the presence of a Na+ gradient between the phagolysosome lumen and the cytosol were critical for the controlled release of membrane tension that permits deformation of the limiting phagolysosome membrane. In turn, membrane deformation was a necessary step to efficiently transport the cholesterol extracted from cellular targets, permeabilizing them to hydrolases. These results place TPCs as regulators of endomembrane remodeling events that precede target degradation in cases when the target is bound by a cholesterol-containing membrane. The findings may help to explain lipid metabolism dysfunction and autophagic flux impairment reported in TPC KO mice and establish stepwise regulation to the resolution process that begins with lysis of the target.

Macrophage subpopulation identity in Drosophila is modulated by apoptotic cell clearance and related signalling pathways

In Drosophila blood, plasmatocytes of the haemocyte lineage represent the functional equivalent of vertebrate macrophages and have become an established in vivo model with which to study macrophage function and behaviour. However, the use of plasmatocytes as a macrophage model has been limited by a historical perspective that plasmatocytes represent a homogenous population of cells, in contrast to the high levels of heterogeneity of vertebrate macrophages. Recently, a number of groups have reported transcriptomic approaches which suggest the existence of plasmatocyte heterogeneity, while we identified enhancer elements that identify subpopulations of plasmatocytes which exhibit potentially pro-inflammatory behaviours, suggesting conservation of plasmatocyte heterogeneity in Drosophila. These plasmatocyte subpopulations exhibit enhanced responses to wounds and decreased rates of efferocytosis when compared to the overall plasmatocyte population. Interestingly, increasing the phagocytic requirement placed upon plasmatocytes is sufficient to decrease the size of these plasmatocyte subpopulations in the embryo. However, the mechanistic basis for this response was unclear. Here, we examine how plasmatocyte subpopulations are modulated by apoptotic cell clearance (efferocytosis) demands and associated signalling pathways. We show that loss of the phosphatidylserine receptor Simu prevents an increased phagocytic burden from modulating specific subpopulation cells, while blocking other apoptotic cell receptors revealed no such rescue. This suggests that Simu-dependent efferocytosis is specifically involved in determining fate of particular subpopulations. Supportive of our original finding, mutations in amo (the Drosophila homolog of PKD2), a calcium-permeable channel which operates downstream of Simu, phenocopy simu mutants. Furthermore, we show that Amo is involved in the acidification of the apoptotic cell-containing phagosomes, suggesting that this reduction in pH may be associated with macrophage reprogramming. Additionally, our results also identify Ecdysone receptor signalling, a pathway related to control of cell death during developmental transitions, as a controller of plasmatocyte subpopulation identity. Overall, these results identify fundamental pathways involved in the specification of plasmatocyte subpopulations and so further validate Drosophila plasmatocytes as a heterogeneous population of macrophage-like cells within this important developmental and immune model.

Loss of two-pore channel 2 function in melanoma-derived tumours reduces tumour growth in vivo but greatly increases tumour-related toxicity in the organism

BACKGROUND

Melanoma, a severe form of skin cancer, poses significant health risks due to its aggressive nature and potential for metastasis. The role of two-pore channel 2 (TPC2) in the development and progression of melanoma remains poorly understood. This study aims to investigate the impact of TPC2 knockout (KO) on melanoma-derived tumors, focusing on tumour growth and related toxicity in the organism.

METHODS

The study utilized CHL-1 and B16 melanoma cell lines with TPC2 KO to assess the changes in proliferation dynamics. Methods included real-time monitoring of cell proliferation using the xCELLigence system, in vivo tumour growth assays in mice, histopathological analyses, inflammation marker assessment, and quantitative PCR (qPCR) for gene expression analysis RESULTS: TPC2 KO was found to significantly alter the proliferation dynamics of CHL-1 and B16 melanoma cells. The in vivo studies demonstrated reduced tumor growth in TPC2 KO cell-derived tumors. However, a notable increase in tumor-related toxicity in affected organs, such as the liver and spleen, was observed, indicating a complex role of TPC2 in melanoma pathology.

CONCLUSIONS

The loss of TPC2 function in melanoma cells leads to reduced tumour growth but exacerbates tumour-related toxicity in the organism. These findings highlight the dual role of TPC2 in melanoma progression and its potential as a therapeutic target. Further research is needed to fully understand the mechanisms underlying these effects and to explore TPC2 as a treatment target in melanoma.

Optical profiling of autonomous Ca2+ nanodomains generated by lysosomal TPC2 and TRPML1

Multiple families of Ca2+-permeable channels co-exist on lysosomal Ca2+ stores but how each family couples to its own unique downstream physiology is unclear. We have therefore investigated the Ca2+-signalling architecture underpinning different channels on the same vesicle that drive separate pathways, using phagocytosis as a physiological stimulus. Lysosomal Ca2+-channels are a major Ca2+ source driving particle uptake in macrophages, but different channels drive different aspects of Fc-receptor-mediated phagocytosis: TPC2 couples to dynamin activation, whilst TRPML1 couples to lysosomal exocytosis. We hypothesised that they are driven by discrete local plumes of Ca2+ around open channels (Ca2+ nanodomains). To test this, we optimized Ca2+-nanodomain recordings by screening panels of genetically encoded Ca2+ indicators (GECIs) fused to TPC2 to monitor the [Ca2+] next to the channel. Signal calibration accounting for the distance of the GECI from the channel mouth reveals that, during phagocytosis, TPC2 generates local Ca2+ nanodomains around itself of up to 42 µM, nearly a hundred-fold greater than the global cytosolic [Ca2+] rise. We further show that TPC2 and TRPML1, though on the same lysosomes, generate autonomous Ca2+ nanodomains of high [Ca2+] that are largely insulated from one another, a platform allowing their discrete Ca2+-decoding to promote unique respective physiologies.

Photoswitching alters fluorescence readout of jGCaMP8 Ca2+ indicators tethered to Orai1 channels

We used electrophysiology and Ca2+ channel tethering to evaluate the performance of jGCaMP8 genetically encoded Ca2+ indicators (GECIs). Orai1 Ca2+ channel-jGCaMP8 fusions were transfected into HEK 293A cells and jGCaMP8 fluorescence responses recorded by simultaneous total internal reflection fluorescence microscopy and whole-cell patch clamp electrophysiology. Noninactivating currents from the Orai1 Y80E mutant provided a steady flux of Ca2+ controlled on a millisecond time scale by step changes in membrane potential. Test pulses to -100 mV produced Orai1 Y80E-jGCaMP8f fluorescence traces that unexpectedly declined by ~50% over 100 ms before reaching a stable plateau. Testing of Orai1-jGCaMP8f using unroofed cells further demonstrated that rapid and partial fluorescence inactivation is a property of the indicator itself, rather than channel function. Photoinactivation spontaneously recovered over 5 min in the dark, and recovery was accelerated in the absence of Ca2+. Mutational analysis of residues near the tripeptide fluorophore of jGCaMP8f pointed to a mechanism: Q69M/C70V greatly increased (~90%) photoinactivation, reminiscent of fluorescent protein fluorophore cis-trans photoswitching. Indeed, 405-nm illumination of jGCaMP8f or 8m/8s/6f led to immediate photorecovery, and simultaneous illumination with 405 and 488-nm light blocked photoinactivation. Subsequent mutagenesis produced a variant, V203Y, that lacks photoinactivation but largely preserves the desirable properties of jGCaMP8f. Our results point to caution in interpreting rapidly changing Ca2+ signals using jGCaMP8 and earlier series GECIs, suggest strategies to avoid photoswitching, and serve as a starting point to produce more photostable, and thus more accurate, GECI derivatives.

Calcium Signaling in MR1-Dependent Antigen Presentation of Mycobacterium tuberculosis

MR1 is a ubiquitously expressed MHC-Ib molecule that presents microbial metabolites to MR1-restricted T cells, but there are differences in the antigen presentation pathway of an intracellular microbe compared to exogenous antigen. We have shown the importance of endosomal trafficking proteins in MR1-dependent presentation of Mycobacterium tuberculosis (Mtb). Two pore channels (TPCs) are endosomal calcium channels that regulate endosomal trafficking. Due to their location on endosomes, we hypothesized that TPCs could be required for MR1-dependent presentation of antigens derived from the intracellular microbe Mtb. We found that TPCs are critical for the presentation of Mtb by MR1; inhibition of TPCs had no effect on MR1 presentation of extracellular (exogenous) antigens, HLA-B presentation, or HLA-II presentation. Finally, we found that the calcium sensitive trafficking protein Synaptotagmin 7 was also key in the presentation of Mtb by MR1. This calcium-dependent endosomal pathway is a novel mechanism by which the immune system can sample intracellular antigens.

Broadening horizons: the contribution of mitochondria-associated endoplasmic reticulum membrane (MAM) dysfunction in diabetic kidney disease

Diabetic kidney disease (DKD) is a global health issue that presents a complex pathogenesis and limited treatment options. To provide guidance for precise therapies, it is crucial to accurately identify the pathogenesis of DKD. Several studies have recognized that mitochondrial and endoplasmic reticulum (ER) dysfunction are key drivers of the pathogenesis of DKD. The mitochondria-associated ER membrane (MAM) is a dynamic membrane contact site (MSC) that connects the ER and mitochondria and is essential in maintaining the normal function of the two organelles. MAM is involved in various cellular processes, including lipid synthesis and transport, calcium homeostasis, mitochondrial fusion and fission, and ER stress. Meanwhile, recent studies confirm that MAM plays a significant role in the pathogenesis of DKD by regulating glucose metabolism, lipid metabolism, inflammation, ER stress, mitochondrial fission and fusion, and autophagy. Herein, this review aims to provide a comprehensive summary of the physiological function of MAMs and their impact on the progression of DKD. Subsequently, we discuss the trend of pharmaceutical studies that target MAM resident proteins for treating DKD. Furthermore, we also explore the future development prospects of MAM in DKD research, thereby providing a new perspective for basic studies and clinical treatment of DKD.

Rac1 and Rac3 GTPases and TPC2 are required for axonal outgrowth and migration of cortical interneurons

Rho GTPases, among them Rac1 and Rac3, are major transducers of extracellular signals and are involved in multiple cellular processes. In cortical interneurons, the neurons that control the balance between excitation and inhibition of cortical circuits, Rac1 and Rac3 are essential for their development. Ablation of both leads to a severe reduction in the numbers of mature interneurons found in the murine cortex, which is partially due to abnormal cell cycle progression of interneuron precursors and defective formation of growth cones in young neurons. Here, we present new evidence that upon Rac1 and Rac3 ablation, centrosome, Golgi complex and lysosome positioning is significantly perturbed, thus affecting both interneuron migration and axon growth. Moreover, for the first time, we provide evidence of altered expression and localization of the two-pore channel 2 (TPC2) voltage-gated ion channel that mediates Ca2+ release. Pharmacological inhibition of TPC2 negatively affected axonal growth and migration of interneurons. Our data, taken together, suggest that TPC2 contributes to the severe phenotype in axon growth initiation, extension and interneuron migration in the absence of Rac1 and Rac3.

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.

Endolysosomal TPCs regulate social behavior by controlling oxytocin secretion

Oxytocin (OT) is a prominent regulator of many aspects of mammalian social behavior and stored in large dense-cored vesicles (LDCVs) in hypothalamic neurons. It is released in response to activity-dependent Ca2+ influx, but is also dependent on Ca2+ release from intracellular stores, which primes LDCVs for exocytosis. Despite its importance, critical aspects of the Ca2+-dependent mechanisms of its secretion remain to be identified. Here we show that lysosomes surround dendritic LDCVs, and that the direct activation of endolysosomal two-pore channels (TPCs) provides the critical Ca2+ signals to prime OT release by increasing the releasable LDCV pool without directly stimulating exocytosis. We observed a dramatic reduction in plasma OT levels in TPC knockout mice, and impaired secretion of OT from the hypothalamus demonstrating the importance of priming of neuropeptide vesicles for activity-dependent release. Furthermore, we show that activation of type 1 metabotropic glutamate receptors sustains somatodendritic OT release by recruiting TPCs. The priming effect could be mimicked by a direct application of nicotinic acid adenine dinucleotide phosphate, the endogenous messenger regulating TPCs, or a selective TPC2 agonist, TPC2-A1-N, or blocked by the antagonist Ned-19. Mice lacking TPCs exhibit impaired maternal and social behavior, which is restored by direct OT administration. This study demonstrates an unexpected role for lysosomes and TPCs in controlling neuropeptide secretion, and in regulating social behavior.

SARS-CoV-2 Spike Protein Activates Human Lung Macrophages

COVID-19 is a viral disease caused by SARS-CoV-2. This disease is characterized primarily, but not exclusively, by respiratory tract inflammation. SARS-CoV-2 infection relies on the binding of spike protein to ACE2 on the host cells. The virus uses the protease TMPRSS2 as an entry activator. Human lung macrophages (HLMs) are the most abundant immune cells in the lung and fulfill a variety of specialized functions mediated by the production of cytokines and chemokines. The aim of this project was to investigate the effects of spike protein on HLM activation and the expression of ACE2 and TMPRSS2 in HLMs. Spike protein induced CXCL8, IL-6, TNF-α, and IL-1β release from HLMs; promoted efficient phagocytosis; and induced dysfunction of intracellular Ca2+ concentration by increasing lysosomal Ca2+ content in HLMs. Microscopy experiments revealed that HLM tracking was affected by spike protein activation. Finally, HLMs constitutively expressed mRNAs for ACE2 and TMPRSS2. In conclusion, during SARS-CoV-2 infection, macrophages seem to play a key role in lung injury, resulting in immunological dysfunction and respiratory disease.

TPC Functions in the Immune System

Two-pore channels (TPCs) are novel intracellular cation channels, which play a key role in numerous (patho-)physiological and immunological processes. In this chapter, we focus on their function in immune cells and immune reactions. Therefore, we first give an overview of the cellular immune response and the partaking immune cells. Second, we concentrate on ion channels which in the past have been shown to play an important role in the regulation of immune cells. The main focus is then directed to TPCs, which are primarily located in the membranes of acidic organelles, such as lysosomes or endolysosomes but also certain other vesicles. They regulate Ca²⁺ homeostasis and thus Ca²⁺ signaling in immune cells. Due to this important functional role, TPCs are enjoying increasing attention within the field of immunology in the last few decades but are also becoming more pertinent as pharmacological targets for the treatment of pro-inflammatory diseases such as allergic hypersensitivity. However, to uncover the precise molecular mechanism of TPCs in immune cell responses, further molecular, genetic, and ultrastructural investigations on TPCs are necessary, which then may pave the way to develop novel therapeutic strategies to treat diseases such as anaphylaxis more specifically.

A cellular atlas of calcineurin signaling

Intracellular Ca2+ signals are temporally controlled and spatially restricted. Signaling occurs adjacent to sites of Ca2+ entry and/or release, where Ca2+-dependent effectors and their substrates co-localize to form signaling microdomains. Here we review signaling by calcineurin, the Ca2+/calmodulin regulated protein phosphatase and target of immunosuppressant drugs, Cyclosporin A and FK506. Although well known for its activation of the adaptive immune response via NFAT dephosphorylation, systematic mapping of human calcineurin substrates and regulators reveals unexpected roles for this versatile phosphatase throughout the cell. We discuss calcineurin function, with an emphasis on where signaling occurs and mechanisms that target calcineurin and its substrates to signaling microdomains, especially binding of cognate short linear peptide motifs (SLiMs). Calcineurin is ubiquitously expressed and regulates events at the plasma membrane, other intracellular membranes, mitochondria, the nuclear pore complex and centrosomes/cilia. Based on our expanding knowledge of localized CN actions, we describe a cellular atlas of Ca2+/calcineurin signaling.

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.

The evolution of organellar calcium mapping technologies

Intracellular Ca2+ fluxes are dynamically controlled by the co-involvement of multiple organellar pools of stored Ca2+. Endolysosomes are emerging as physiologically critical, yet underexplored, sources and sinks of intracellular Ca2+. Delineating the role of organelles in Ca2+ signaling has relied on chemical fluorescent probes and electrophysiological strategies. However, the acidic endolysosomal environment presents unique issues, which preclude the use of traditional chemical reporter strategies to map lumenal Ca2+. Here, we broadly address the current state of knowledge about organellar Ca2+ pools. We then outline the application of traditional probes, and their sensing paradigms. We then discuss how a new generation of probes overcomes the limitations of traditional Ca2+probes, emphasizing their ability to offer critical insights into endolysosomal Ca2+, and its feedback with other organellar pools.

The contribution of ion channels to shaping macrophage behaviour

The expanding roles of macrophages in physiological and pathophysiological mechanisms now include normal tissue homeostasis, tissue repair and regeneration, including neuronal tissue; initiation, progression, and resolution of the inflammatory response and a diverse array of anti-microbial activities. Two hallmarks of macrophage activity which appear to be fundamental to their diverse cellular functionalities are cellular plasticity and phenotypic heterogeneity. Macrophage plasticity allows these cells to take on a broad spectrum of differing cellular phenotypes in response to local and possibly previous encountered environmental signals. Cellular plasticity also contributes to tissue- and stimulus-dependent macrophage heterogeneity, which manifests itself as different macrophage phenotypes being found at different tissue locations and/or after different cell stimuli. Together, plasticity and heterogeneity align macrophage phenotypes to their required local cellular functions and prevent inappropriate activation of the cell, which could lead to pathology. To execute the appropriate function, which must be regulated at the qualitative, quantitative, spatial and temporal levels, macrophages constantly monitor intracellular and extracellular parameters to initiate and control the appropriate cell signaling cascades. The sensors and signaling mechanisms which control macrophages are the focus of a considerable amount of research. Ion channels regulate the flow of ions between cellular membranes and are critical to cell signaling mechanisms in a variety of cellular functions. It is therefore surprising that the role of ion channels in the macrophage biology has been relatively overlooked. In this review we provide a summary of ion channel research in macrophages. We begin by giving a narrative-based explanation of the membrane potential and its importance in cell biology. We then report on research implicating different ion channel families in macrophage functions. Finally, we highlight some areas of ion channel research in macrophages which need to be addressed, future possible developments in this field and therapeutic potential.

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.

TPC2 rescues lysosomal storage in mucolipidosis type IV, Niemann-Pick type C1, and Batten disease

Lysosomes are cell organelles that degrade macromolecules to recycle their components. If lysosomal degradative function is impaired, e.g., due to mutations in lysosomal enzymes or membrane proteins, lysosomal storage diseases (LSDs) can develop. LSDs manifest often with neurodegenerative symptoms, typically starting in early childhood, and going along with a strongly reduced life expectancy and quality of life. We show here that small molecule activation of the Ca²⁺‐permeable endolysosomal two‐pore channel 2 (TPC2) results in an amelioration of cellular phenotypes associated with LSDs such as cholesterol or lipofuscin accumulation, or the formation of abnormal vacuoles seen by electron microscopy. Rescue effects by TPC2 activation, which promotes lysosomal exocytosis and autophagy, were assessed in mucolipidosis type IV (MLIV), Niemann–Pick type C1, and Batten disease patient fibroblasts, and in neurons derived from newly generated isogenic human iPSC models for MLIV and Batten disease. For in vivo proof of concept, we tested TPC2 activation in the MLIV mouse model. In sum, our data suggest that TPC2 is a promising target for the treatment of different types of LSDs, both in vitro and in‐vivo.

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.

Proton-gated anion transport governs macropinosome shrinkage

Intracellular organelles change their size during trafficking and maturation. This requires the transport of ions and water across their membranes. Macropinocytosis, a ubiquitous form of endocytosis of particular importance for immune and cancer cells, generates large vacuoles that can be followed optically. Shrinkage of macrophage macropinosomes depends on TPC-mediated Na⁺ efflux and Cl⁻ exit through unknown channels. Relieving osmotic pressure facilitates vesicle budding, positioning osmotic shrinkage upstream of vesicular sorting and trafficking. Here we identify the missing macrophage Cl⁻ channel as the proton-activated Cl⁻ channel ASOR/TMEM206. ASOR activation requires Na⁺-mediated depolarization and luminal acidification by redundant transporters including H⁺-ATPases and CLC 2Cl⁻/H⁺ exchangers. As corroborated by mathematical modelling, feedback loops requiring the steep voltage and pH dependencies of ASOR and CLCs render vacuole resolution resilient towards transporter copy numbers. TMEM206 disruption increased albumin-dependent survival of cancer cells. Our work suggests a function for the voltage and pH dependence of ASOR and CLCs, provides a comprehensive model for ion-transport-dependent vacuole maturation and reveals biological roles of ASOR.

Zeziulia et al. identify the proton-activated Cl⁻ channel ASOR/TMEM206 as necessary for shrinkage of macropinosomes, which is needed for downstream sorting events.

Current methods to analyze lysosome morphology, positioning, motility and function

Since the discovery of lysosomes more than 70 years ago, much has been learned about the functions of these organelles. Lysosomes were regarded as exclusively degradative organelles, but more recent research has shown that they play essential roles in several other cellular functions, such as nutrient sensing, intracellular signalling and metabolism. Methodological advances played a key part in generating our current knowledge about the biology of this multifaceted organelle. In this review, we cover current methods used to analyze lysosome morphology, positioning, motility and function. We highlight the principles behind these methods, the methodological strategies and their advantages and limitations. To extract accurate information and avoid misinterpretations, we discuss the best strategies to identify lysosomes and assess their characteristics and functions. With this review, we aim to stimulate an increase in the quantity and quality of research on lysosomes and further ground-breaking discoveries on an organelle that continues to surprise and excite cell biologists.

Immunotoxicity pathway and mechanism of benzo[a]pyrene on hemocytes of Chlamys farreri in vitro

Benzo[a]pyrene (B[a]P), a typical PAHs widely existing in the marine environment, has been extensively studied for its immunotoxicity due to its persistence and high toxicity. Nevertheless, the immunotoxicity mechanism remain incompletely understood. In this study, isolated hemocytes of Chlamys farreri were exposed at three concentrations of B[a]P (5, 10 and 15 μg/mL), and the effects of B[a]P on detoxification metabolism, signal transduction, humoral immune factors, exocytosis and phagocytosis relevant proteins and immune function at 0, 6, 12, 24 h were studied. Results illustrated the AhR, ARNT and CYP1A1 were significantly induced by B[a]P at 12 h. Additionally, the content of B[a]P metabolite BPDE increased in a dose-dependent manner with pollutants. Under B[a]P stimulation, the expressions of PTK (Src, Fyn) and PLC-Ca²⁺-PKC pathway gene increased significantly, while the transcription level of AC-cAMP-PKA pathway gene decreased remarkably. Additionally, the expressions of nuclear transcription factors (CREB, NF-κB), complement system genes and C-type lectin genes up-regulated obviously. The gene expressions of phagocytosis and exocytosis related proteins were also notably affected. 5 μg/mL B[a]P could promote phagocytosis in a transitory time, but with the increase of exposure time and concentration of B[a]P, the phagocytosis, antibacterial and bacteriolytic activities gradually decreased. These results indicated that similar to vertebrates, BPDE, the metabolite of B[a]P, mediated downstream signal transduction via PTK in bivalves. The declined of the immune defense ability of hemocytes might be closely related to the inhibition of AC-cAMP-PKA pathway and the imbalance of intracellular Ca²⁺ pathway. In addition, the results manifested that complement and lectin systems play a significant role in regulating immune response. In this study, the direct relationship between detoxification metabolism and immune signal transduction in bivalves under B[a]P stress was demonstrated for the first time, which provided important information for the potential molecular mechanism of B[a]P-induced immune system disorder in bivalves.

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.

The ins and outs of virus trafficking through acidic Ca2+ stores

Many viruses exploit host-cell Ca2+ signaling processes throughout their life cycle. This is especially relevant for viruses that translocate through the endolysosomal system, where cellular infection is keyed to the microenvironment of these acidic Ca2+ stores and Ca2+-dependent trafficking pathways. As regulators of the endolysosomal ionic milieu and trafficking dynamics, two families of endolysosomal Ca2+-permeable cation channels - two pore channels (TPCs) and transient receptor potential mucolipins (TRPMLs) - have emerged as important host-cell factors in viral entry. Here, we review: (i) current evidence implicating Ca2+ signaling in viral translocation through the endolysosomal system, (ii) the roles of these ion channels in supporting cellular infection by different viruses, and (iii) areas for future research that will help define the potential of TPC and TRPML ligands as progressible antiviral agents.

Targeting the two-pore channel 2 in cancer progression and metastasis

The importance of Ca²⁺ signaling, and particularly Ca²⁺ channels, in key events of cancer cell function such as proliferation, metastasis, autophagy and angiogenesis, has recently begun to be appreciated. Of particular note are two-pore channels (TPCs), a group of recently identified Ca²⁺-channels, located within the endolysosomal system. TPC2 has recently emerged as an intracellular ion channel of significant pathophysiological relevance, specifically in cancer, and interest in its role as an anti-cancer drug target has begun to be explored. Herein, an overview of the cancer-related functions of TPC2 and a discussion of its potential as a target for therapeutic intervention, including a summary of clinical trials examining the TPC2 inhibitors, naringenin, tetrandrine, and verapamil for the treatment of various cancers is provided.

LAMTOR1 inhibition of TRPML1‐dependent lysosomal calcium release regulates dendritic lysosome trafficking and hippocampal neuronal function

Lysosomes function not only as degradatory compartments but also as dynamic intracellular calcium ion stores. The transient receptor potential mucolipin 1 (TRPML1) channel mediates lysosomal Ca²⁺ release, thereby participating in multiple cellular functions. The pentameric Ragulator complex, which plays a critical role in the activation of mTORC1, is also involved in lysosomal trafficking and is anchored to lysosomes through its LAMTOR1 subunit. Here, we report that the Ragulator restricts lysosomal trafficking in dendrites of hippocampal neurons via LAMTOR1‐mediated tonic inhibition of TRPML1 activity, independently of mTORC1. LAMTOR1 directly interacts with TRPML1 through its N‐terminal domain. Eliminating this inhibition in hippocampal neurons by LAMTOR1 deletion or by disrupting LAMTOR1‐TRPML1 binding increases TRPML1‐mediated Ca²⁺ release and facilitates dendritic lysosomal trafficking powered by dynein. LAMTOR1 deletion in the hippocampal CA1 region of adult mice results in alterations in synaptic plasticity, and in impaired object‐recognition memory and contextual fear conditioning, due to TRPML1 activation. Mechanistically, changes in synaptic plasticity are associated with increased GluA1 dephosphorylation by calcineurin and lysosomal degradation. Thus, LAMTOR1‐mediated inhibition of TRPML1 is critical for regulating dendritic lysosomal motility, synaptic plasticity, and learning.

Endolysosomal Cation Channels and Lung Disease

Endolysosomal cation channels are emerging as key players of endolysosomal function such as endolysosomal trafficking, fusion/fission, lysosomal pH regulation, autophagy, lysosomal exocytosis, and endocytosis. Diseases comprise lysosomal storage disorders (LSDs) and neurodegenerative diseases, metabolic diseases, pigmentation defects, cancer, immune disorders, autophagy related diseases, infectious diseases and many more. Involvement in lung diseases has not been a focus of attention so far but recent developments in the field suggest critical functions in lung physiology and pathophysiology. Thus, loss of TRPML3 was discovered to exacerbate emphysema formation and cigarette smoke induced COPD due to dysregulated matrix metalloproteinase 12 (MMP-12) levels in the extracellular matrix of the lung, a known risk factor for emphysema/COPD. While direct lung function measurements with the exception of TRPML3 are missing for other endolysosomal cation channels or channels expressed in lysosome related organelles (LRO) in the lung, links between those channels and important roles in lung physiology have been established such as the role of P2X4 in surfactant release from alveolar epithelial Type II cells. Other channels with demonstrated functions and disease relevance in the lung such as TRPM2, TRPV2, or TRPA1 may mediate their effects due to plasma membrane expression but evidence accumulates that these channels might also be expressed in endolysosomes, suggesting additional and/or dual roles of these channels in cell and intracellular membranes. We will discuss here the current knowledge on cation channels residing in endolysosomes or LROs with respect to their emerging roles in lung disease.

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.

Tubular lysosomes harbor active ion gradients and poise macrophages for phagocytosis

Lysosomes are organelles that also act as cell-signaling hubs. They regulate functions ranging from antigen presentation to autophagy. Spherical lysosomes can spontaneously elongate into tubules in starving or inflamed immune cells. We describe a DNA-based reagent, denoted Tudor, that tubulates lysosomes in macrophages without triggering either an immune response or autophagy. Chemical imaging revealed that tubular lysosomes differ from vesicular ones in terms of their pH, calcium, and proteolytic activity. Tudor revealed a role for tubular lysosomes in that they enhance MMP9 secretion and phagocytosis in resting macrophages. The ability to tubulate lysosomes in resting immune cells without starving or inflaming them may help reveal new insights into how tubular lysosomes function.

Lysosomes adopt dynamic, tubular states that regulate antigen presentation, phagosome resolution, and autophagy. Tubular lysosomes are studied either by inducing autophagy or by activating immune cells, both of which lead to cell states where lysosomal gene expression differs from the resting state. Therefore, it has been challenging to pinpoint the biochemical properties lysosomes acquire upon tubulation that could drive their functionality. Here we describe a DNA-based assembly that tubulates lysosomes in macrophages without activating them. Proteolytic activity maps at single-lysosome resolution revealed that tubular lysosomes were less degradative and showed proximal to distal luminal pH and Ca²⁺ gradients. Such gradients had been predicted but never previously observed. We identify a role for tubular lysosomes in promoting phagocytosis and activating MMP9. The ability to tubulate lysosomes without starving or activating immune cells may help reveal new roles for tubular lysosomes.

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.

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.

Glucose and NAADP trigger elementary intracellular β-cell Ca2+ signals

Pancreatic β-cells release insulin upon a rise in blood glucose. The precise mechanisms of stimulus-secretion coupling, and its failure in Diabetes Mellitus Type 2, remain to be elucidated. The consensus model, as well as a class of currently prescribed anti-diabetic drugs, are based around the observation that glucose-evoked ATP production in β-cells leads to closure of cell membrane ATP-gated potassium (K_ATP) channels, plasma membrane depolarisation, Ca²⁺ influx, and finally the exocytosis of insulin granules. However, it has been demonstrated by the inactivation of this pathway using genetic and pharmacological means that closure of the K_ATP channel alone may not be sufficient to explain all β-cell responses to glucose elevation. We have previously proposed that NAADP-evoked Ca²⁺ release is an important step in stimulus-secretion coupling in pancreatic β-cells. Here we show using total internal reflection fluorescence (TIRF) microscopy that glucose as well as the Ca²⁺ mobilising messenger nicotinic acid adenine dinucleotide phosphate (NAADP), known to operate in β-cells, lead to highly localised elementary intracellular Ca²⁺ signals. These were found to be obscured by measurements of global Ca²⁺ signals and the action of powerful SERCA-based sequestration mechanisms at the endoplasmic reticulum (ER). Building on our previous work demonstrating that NAADP-evoked Ca²⁺ release is an important step in stimulus-secretion coupling in pancreatic β-cells, we provide here the first demonstration of elementary Ca²⁺ signals in response to NAADP, whose occurrence was previously suspected. Optical quantal analysis of these events reveals a unitary event amplitude equivalent to that of known elementary Ca²⁺ signalling events, inositol trisphosphate (IP₃) receptor mediated blips, and ryanodine receptor mediated quarks. We propose that a mechanism based on these highly localised intracellular Ca²⁺ signalling events mediated by NAADP may initially operate in β-cells when they respond to elevations in blood glucose.

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.

Deciphering the Role of Endolysosomal Ca2+ Channels in Immunity

The role of endolysosomal Ca2+ signalling in immunity has been a subject of increasing interest in recent years. Here, we discuss evolving knowledge relating to the contribution of endolysosomal Ca2+ channels that include TPCs, TRPMLs, and P2X4R in physiological processes related to innate and adaptive immunity-including phagocytosis, inflammation, cytokine/chemokine release, dendritic, natural killer, and T cell activation and migration-and we underscore the paucity of clinical studies in this field. Emerging biomedical and translational data have led to important new insights into the critical roles of these channels in immune cell function and the regulation of innate and adaptive immune responses. The evolving immunological significance of endolysosomal Ca2+ signalling warrants further investigations to better characterize the roles of these channels in immunity in order to expand our knowledge about the pathology of inflammatory and autoimmune diseases and develop endolysosomal Ca2+ channels as viable biomarkers and therapeutic and preventive targets for remodelling the immune response.

Choreographing endo-lysosomal Ca2+ throughout the life of a phagosome

The emergence of endo-lysosomes as ubiquitous Ca²⁺ stores with their unique cohort of channels has resulted in their being implicated in a growing number of processes in an ever-increasing number of cell types. The architectural and regulatory constraints of these acidic Ca²⁺ stores distinguishes them from other larger Ca²⁺ sources such as the ER and influx across the plasma membrane. In view of recent advances in the understanding of the modes of operation, we discuss phagocytosis as a template for how endo-lysosomal Ca²⁺ signals (generated via TPC and TRPML channels) can be integrated in multiple sophisticated ways into biological processes. Phagocytosis illustrates how different endo-lysosomal Ca²⁺ signals drive different phases of a process, and how these can be altered by disease or infection.

A NOVEL NOX/PHOX-CD38-NAADP-TFEB AXIS IMPORTANT FOR MACROPHAGE ACTIVATION DURING BACTERIAL PHAGOCYTOSIS

Macrophage activation in the presence of bacterial cells and molecules entails complex programs of gene expression. How such triggers elicit specific gene expression programs is incompletely understood. We previously discovered that TFEB (transcription factor EB) is a key contributor to macrophage activation during bacterial phagocytosis. However, the mechanism linking phagocytosis of bacterial cells to TFEB activation and downstream pro-inflammatory cytokine induction remained unknown. We found that macrophages lacking both TFEB and TFE3 (transcription factor E3) were unable to mount a pro-inflammatory phenotype in response to bacterial infection. The NOX/PHOX (NADPH oxidase)-dependent oxidative burst was required for nuclear translocation of TFEB during phagocytosis of Gram-positive or -negative bacteria, and reactive oxygen species (ROS) were sufficient to trigger TFEB activation in a CD38- and NAADP (nicotinic acid adenine dinucleotide phosphate)-dependent manner. Consistent with the Ca²⁺-releasing activity of NAADP, intracellular Ca²⁺ chelation and PPP3/calcineurin inhibition prevented TFEB activation by phagocytosis and ROS (reactive oxygen species), impairing the induction of pro-inflammatory cytokines such as IL6 and TNF/TNFα. Therefore, here we describe a previously unknown pathway that links phagocytosis with macrophage pro-inflammatory polarization via TFEB and related transcription factor TFE3. These findings reveal that activation of TFEB and TFE3 is a key regulatory event for the activation of macrophages, and have important implications for infections, inflammation, cancer, obesity, and atherosclerosis.

From the inside out: Ion fluxes at the centre of endocytic traffic

Endocytic traffic is a complex and elegant operation involving cargo sorting, membrane budding and tubulation, generation of force, and the formation of organellar contacts. The role of specific proteins and lipids in these processes has been studied extensively. By comparison, precious little is understood about the contribution of the endocytic fluid to these events, despite much evidence that alteration of the contents can severely affect membrane traffic along the endocytic pathway. In particular, it has long been appreciated that dissipation of ionic gradients arrests endosome-to-lysosome maturation. How cells sense inorganic ions and transmit this information have remained largely enigmatic. Herein, we review the experimental findings that reveal an intimate association between luminal ions, their transport, and endocytic traffic. We then discuss the ionic sensors and the mechanisms proposed to convert ion concentrations into protein-based trafficking events, highlighting the current paucity of convincing explanations.

Lysosomal agents inhibit store-operated Ca2+ entry

Pharmacological manipulation of lysosome membrane integrity or ionic movements is a key strategy for probing lysosomal involvement in cellular processes. However, we have found an unexpected inhibition of store-operated Ca²⁺ entry (SOCE) by these agents. Dipeptides [glycyl-L-phenylalanine 2-naphthylamide (GPN) and L-leucyl-L-leucine methyl ester] that are inducers of lysosomal membrane permeabilization (LMP) uncoupled endoplasmic reticulum Ca²⁺-store depletion from SOCE by interfering with Stim1 oligomerization and/or Stim1 activation of Orai. Similarly, the K⁺/H⁺ ionophore, nigericin, that rapidly elevates lysosomal pH, also inhibited SOCE in a Stim1-dependent manner. In contrast, other strategies for manipulating lysosomes (bafilomycin A1, lysosomal re-positioning) had no effect upon SOCE. Finally, the effects of GPN on SOCE and Stim1 was reversed by a dynamin inhibitor, dynasore. Our data show that lysosomal agents not only release Ca²⁺ from stores but also uncouple this release from the normal recruitment of Ca²⁺ influx.

Summary: Lysosomal agents uncouple ER Ca²⁺-release from store-operated Ca²⁺ entry, predominantly by inhibiting Stim1 oligomerization and its activation of Orai.

Mechanisms and functions of calcium microdomains produced by ORAI channels, d-myo-inositol 1,4,5-trisphosphate receptors, or ryanodine receptors

With the discovery of local Ca²⁺ signals in the 1990s the concept of 'elementary Ca²⁺ signals' and 'fundamental Ca²⁺ signals' was developed. While 'elementary Ca²⁺signals' relate to optical signals gained by activity of small clusters of Ca²⁺channels, 'fundamental signals' describe such optical signals that arise from opening of single Ca²⁺channels. In this review, we discuss (i) concepts of local Ca²⁺ signals and Ca²⁺ microdomains, (ii) molecular mechanisms underlying Ca²⁺ microdomains, (iii) functions of Ca²⁺ microdomains, and (iv) mathematical modelling of Ca²⁺ microdomains. We focus on Ca²⁺ microdomains produced by ORAI channels, D-myo-inositol 1,4,5-trisphosphate receptors, or ryanodine receptors. In summary, research on local Ca²⁺ signals in different cell models aims to better understand how cells use the Ca²⁺ toolkit to produce Ca²⁺ microdomains as relevant signals for specific cellular responses, but also how local Ca²⁺ signals as building blocks merge into global Ca²⁺ signaling.

Two-pore and TRP cation channels in endolysosomal osmo-/mechanosensation and volume regulation

Two pore channels (TPCs) and mucolipins (TRPML) are the most prominent cation channels expressed in endolysosomes. Recently, roles of TPCs and TRPML2 have been revealed in regulating and detecting osmotically-driven changes in the surface-to-volume ratio of endolysosomes to promote endocytic and recycling traffic. TPCs and TRPML2 are highly expressed in macrophages and contribute to immune cell function. Here, we provide an overview of the emerging roles of these channels in innate immune cells, in particular macrophages, and highlight two models for osmo-mechanical regulation of intracellular organelle volume, trafficking, and cell homeostasis involving either TPCs or TRPML2.

Pharmacological hypothesis: TPC2 antagonist tetrandrine as a potential therapeutic agent for COVID-19

More than ten million patients worldwide have been diagnosed with coronavirus disease 19 (COVID-19) to date (WHO situation report, 1st July 2020). There is no vaccine to prevent infection with the causative organism, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), nor a cure. In the struggle to devise potentially useful therapeutics in record time, the repurposing of existing compounds is a key route of action. In this hypothesis paper, we argue that the bisbenzylisoquinoline and calcium channel blocker tetrandrine, originally extracted from the plant Stephania tetrandra and utilized in traditional Chinese medicine, may have potential in the treatment of COVID-19 and should be further investigated. We collate and review evidence for tetrandrine's putative mechanism of action in viral infection, specifically its recently discovered antagonism of the two-pore channel 2 (TPC2). While tetrandrine's particular history of use provides a very limited pharmacological dataset, there is a suggestion from the available evidence that it could be effective at doses used in clinical practice. We suggest that further research to investigate this possibility should be conducted.

Unconventional role of lysosomes in phagocytosis

Lysosomes are generally thought to be required only for the late stages of phagosome maturation, providing the proton pumps (V-ATPases) and hydrolases needed to acidify and degrade the ingested prey. A recent paper by Davis et al. (EMBO J. [2020], doi:10.15252/embj.2019104058) reports the involvement of lysosomes at a much earlier stage, namely in scission of phagosomes from the plasma membrane. Here we analyze these findings, highlighting a number of unexpected observations and unresolved questions.

NAADP-regulated two-pore channels drive phagocytosis through endo-lysosomal Ca2+ nanodomains, calcineurin and dynamin

Macrophages clear pathogens by phagocytosis and lysosomes that fuse with phagosomes are traditionally regarded as to a source of membranes and luminal degradative enzymes. Here, we reveal that endo-lysosomes act as platforms for a new phagocytic signalling pathway in which FcγR activation recruits the second messenger NAADP and thereby promotes the opening of Ca²⁺ -permeable two-pore channels (TPCs). Remarkably, phagocytosis is driven by these local endo-lysosomal Ca²⁺ nanodomains rather than global cytoplasmic or ER Ca²⁺ signals. Motile endolysosomes contact nascent phagosomes to promote phagocytosis, whereas endo-lysosome immobilization prevents it. We show that TPC-released Ca²⁺ rapidly activates calcineurin, which in turn dephosphorylates and activates the GTPase dynamin-2. Finally, we find that different endo-lysosomal Ca²⁺ channels play diverse roles, with TPCs providing a universal phagocytic signal for a wide range of particles and TRPML1 being only required for phagocytosis of large targets.