Recruitment of NAADP-sensitive acidic Ca2+ stores by glutamate

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.

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

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

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.

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.

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

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

The needs of a synapse—How local organelles serve synaptic proteostasis

Synaptic function crucially relies on the constant supply and removal of neuronal membranes. The morphological complexity of neurons poses a significant challenge for neuronal protein transport since the machineries for protein synthesis and degradation are mainly localized in the cell soma. In response to this unique challenge, local micro‐secretory systems have evolved that are adapted to the requirements of neuronal membrane protein proteostasis. However, our knowledge of how neuronal proteins are synthesized, trafficked to membranes, and eventually replaced and degraded remains scarce. Here, we review recent insights into membrane trafficking at synaptic sites and into the contribution of local organelles and micro‐secretory pathways to synaptic function. We describe the role of endoplasmic reticulum specializations in neurons, Golgi‐related organelles, and protein complexes like retromer in the synthesis and trafficking of synaptic transmembrane proteins. We discuss the contribution of autophagy and of proteasome‐mediated and endo‐lysosomal degradation to presynaptic proteostasis and synaptic function, as well as nondegradative roles of autophagosomes and lysosomes in signaling and synapse remodeling. We conclude that the complexity of neuronal cyto‐architecture necessitates long‐distance protein transport that combines degradation with signaling functions.

Destroy the old to build the new: Activity-dependent lysosomal exocytosis in neurons

Lysosomes are organelles that support diverse cellular functions such as terminal degradation of macromolecules and nutrient recycling. Additionally, lysosomes can fuse with the plasma membrane, a phenomenon referred to as lysosomal exocytosis, to release their contents, including hydrolytic enzymes and cargo proteins. Recently, neuronal activity has been shown to induce lysosomal exocytosis in dendrites and axons. Secreted lysosomal enzyme cathepsin B induces and stabilizes synaptic structural changes by degrading the local extracellular matrix. Extracellular matrix reorganization could also enhance the lateral diffusion of the co-released synaptic organizer Cbln1 along the surface of axons to facilitate new synapse formation. Similarly, lateral diffusion of dendritic AMPA-type glutamate receptors could be facilitated to enhance functional synaptic plasticity. Therefore, lysosomal exocytosis is a powerful way of building new cellular structures through the coordinated destruction of the old environment. Understanding the mechanisms by which lysosomal exocytosis is regulated in neurons is expected to lead to the development of new therapeutics for neuronal plasticity following spinal cord injury or neurodegenerative disease.

Genetic Inactivation of Two-Pore Channel 1 Impairs Spatial Learning and Memory

Two-pore channels (TPCs) constitute a small family of cation channels that are localized in membranes of endosomal and lysosomal compartments. Although their roles for vesicular fusion and endolysosomal trafficking have been investigated, our knowledge on their expression pattern and higher order functions in the murine brain is still limited. Western blot analysis indicated a broad expression of TPC1 in the neocortex, cerebellum and hippocampus. In order to investigate the consequences of the genetic inactivation of TPC1, we performed a set of behavioural studies with TPC1^−/− mice. TPC1^−/− mice were analysed for an altered motor coordination and grip-strength, exploratory drive and anxiety as well as learning and memory. TPC1^−/− mice did not show any differences in their exploratory drive or in their anxiety levels. There were also no differences in spontaneous activity or motor performance. However, the Morris water maze test uncovered a deficit in spatial learning and memory in TPC1^−/− mice.

Contribution of NAADP to Glutamate-Evoked Changes in Ca2+ Homeostasis in Mouse Hippocampal Neurons

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a second messenger that evokes calcium release from intracellular organelles by the engagement of calcium release channels, including members of the Transient Receptor Potential (TRP) family, such as TRPML1, the (structurally) related Two Pore Channel type 1 (TPC1) and TPC2 channels as well as Ryanodine Receptors type 1 (RYR1; Guse, 2012). NAADP evokes calcium release from acidic calcium stores of many cell types (Guse, 2012), and NAADP-sensitive Ca²⁺ stores have been described in hippocampal neurons of the rat (Bak et al., 1999; McGuinness et al., 2007). Glutamate triggers Ca²⁺-mediated neuronal excitotoxicity in inflammation-induced neurodegenerative pathologies such as Multiple Sclerosis (MS; Friese et al., 2014), and when applied extracellularly to neurons glutamate can elevate NAADP levels in these cells. Accordingly, glutamate-evoked Ca²⁺ signals from intracellular organelles were inhibited by preventing organelle acidification (Pandey et al., 2009). Analysis of reported RNA sequencing experiments of cultured hippocampal neurons revealed the abundance of Mcoln1 (encoding TRPML1), Tpcn1, and Tpcn2 (encoding TPC1 and TPC2, respectively) as potential NAADP target channels in these cells. Transcripts encoding Ryr1 were not found in contrast to Ryr2 and Ryr3. To study the contribution of NAADP signaling to glutamate-evoked calcium transients in murine hippocampal neurons we used the NAADP antagonists Ned-19 (Naylor et al., 2009) and BZ194 (Dammermann et al., 2009). Our results show that both NAADP antagonists significantly reduce glutamate-evoked calcium transients. In addition to extracellular glutamate application, we studied synchronized calcium oscillations in the cells of the neuronal cultures evoked by addition of the GABA_A receptor antagonist bicuculline. Pretreatment with Ned-19 (50 μM) or BZ194 (100 μM) led to an increase in the frequency of bicuculline-induced calcium oscillations at the cost of calcium transient amplitudes. Interestingly, Ned-19 triggered a rise in intracellular calcium concentrations 25 min after bicuculline stimulation, leading to the question whether NAADP acts as a neuroprotective messenger in hippocampal neurons. Taken together, our results are in agreement with the concept that NAADP signaling significantly contributes to glutamate evoked Ca²⁺ rise in hippocampal neurons and to the amplitude and frequency of synchronized Ca²⁺ oscillations triggered by spontaneous glutamate release events.

Nicotinic acid adenine dinucleotide phosphate activates two-pore channel TPC1 to mediate lysosomal Ca2+ release in endothelial colony-forming cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most recently discovered Ca²⁺ -releasing messenger that increases the intracellular Ca²⁺ concentration by mobilizing the lysosomal Ca²⁺ store through two-pore channels 1 (TPC1) and 2 (TPC2). NAADP-induced lysosomal Ca²⁺ release regulates multiple endothelial functions, including nitric oxide release and proliferation. A sizeable acidic Ca²⁺ pool endowed with TPC1 is also present in human endothelial colony-forming cells (ECFCs), which represent the only known truly endothelial precursors. Herein, we sought to explore the role of the lysosomal Ca²⁺ store and TPC1 in circulating ECFCs by harnessing Ca²⁺ imaging and molecular biology techniques. The lysosomotropic agent, Gly-Phe β-naphthylamide, and nigericin, which dissipates the proton gradient which drives Ca²⁺ sequestration by acidic organelles, caused endogenous Ca²⁺ release in the presence of a replete inositol-1,4,5-trisphosphate (InsP₃ )-sensitive endoplasmic reticulum (ER) Ca²⁺ pool. Likewise, the amount of ER releasable Ca²⁺ was reduced by disrupting lysosomal Ca²⁺ content. Liposomal delivery of NAADP induced a transient Ca²⁺ signal that was abolished by disrupting the lysosomal Ca²⁺ store and by pharmacological and genetic blockade of TPC1. Pharmacological manipulation revealed that NAADP-induced Ca²⁺ release also required ER-embedded InsP₃ receptors. Finally, NAADP-induced lysosomal Ca²⁺ release was found to trigger vascular endothelial growth factor-induced intracellular Ca²⁺ oscillations and proliferation, while it did not contribute to adenosine-5'-trisphosphate-induced Ca²⁺ signaling. These findings demonstrated that NAADP-induced TPC1-mediated Ca²⁺ release can selectively be recruited to induce the Ca²⁺ response to specific cues in circulating ECFCs.

N-Methyl-D-Aspartate Receptor Hypofunction in Meg-01 Cells Reveals a Role for Intracellular Calcium Homeostasis in Balancing Megakaryocytic-Erythroid Differentiation

The release of calcium ions (Ca ²⁺ ) from the endoplasmic reticulum (ER) and related store-operated calcium entry (SOCE) regulate maturation of normal megakaryocytes. The N -methyl-D-aspartate (NMDA) receptor (NMDAR) provides an additional mechanism for Ca ²⁺ influx in megakaryocytic cells, but its role remains unclear. We created a model of NMDAR hypofunction in Meg-01 cells using CRISPR-Cas9 mediated knockout of the GRIN1 gene, which encodes an obligate, GluN1 subunit of the NMDAR. We found that compared with unmodified Meg-01 cells, Meg-01- GRIN1^−/− cells underwent atypical differentiation biased toward erythropoiesis, associated with increased basal ER stress and cell death. Resting cytoplasmic Ca ²⁺ levels were higher in Meg-01- GRIN1^−/− cells, but ER Ca ²⁺ release and SOCE were lower after activation. Lysosome-related organelles accumulated including immature dense granules that may have contributed an alternative source of intracellular Ca ²⁺ . Microarray analysis revealed that Meg-01- GRIN1^−/− cells had deregulated expression of transcripts involved in Ca ²⁺ metabolism, together with a shift in the pattern of hematopoietic transcription factors toward erythropoiesis. In keeping with the observed pro-cell death phenotype induced by GRIN1 deletion, memantine (NMDAR inhibitor) increased cytotoxic effects of cytarabine in unmodified Meg-01 cells. In conclusion, NMDARs comprise an integral component of the Ca ²⁺ regulatory network in Meg-01 cells that help balance ER stress and megakaryocytic-erythroid differentiation. We also provide the first evidence that megakaryocytic NMDARs regulate biogenesis of lysosome-related organelles, including dense granules. Our results argue that intracellular Ca ²⁺ homeostasis may be more important for normal megakaryocytic and erythroid differentiation than currently recognized; thus, modulation may offer therapeutic opportunities.

Readily Releasable Stores of Calcium in Neuronal Endolysosomes: Physiological and Pathophysiological Relevance

Neurons are long-lived post-mitotic cells that possess an elaborate system of endosomes and lysosomes (endolysosomes) for protein quality control. Relatively recently, endolysosomes were recognized to contain high concentrations (400-600 μM) of readily releasable calcium. The release of calcium from this acidic organelle store contributes to calcium-dependent processes of fundamental physiological importance to neurons including neurotransmitter release, membrane excitability, neurite outgrowth, synaptic remodeling, and cell viability. Pathologically, disturbances of endolysosome structure and/or function have been noted in a variety of neurodegenerative disorders including Alzheimer's disease (AD) and HIV-1 associated neurocognitive disorder (HAND). And, dysregulation of intracellular calcium has been implicated in the neuropathogenesis of these same neurological disorders. Thus, it is important to better understand mechanisms by which calcium is released from endolysosomes as well as the consequences of such release to inter-organellar signaling, physiological functions of neurons, and possible pathological consequences. In doing so, a path forward towards new therapeutic modalities might be facilitated.

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

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

NAADP Receptors

Of the established Ca²⁺-mobilizing messengers, NAADP is arguably the most tantalizing. It is the most potent, often efficacious at low nanomolar concentrations, and its receptors undergo dramatic desensitization. Recent studies have identified a new class of calcium-release channel, the two-pore channels (TPCs), as the likely targets for NAADP regulation, even though the effect may be indirect. These channels localized at endolysosomes, where they mediate local Ca²⁺ release, and have highlighted a new role of acidic organelles as targets for messenger-evoked Ca²⁺ mobilization. Three distinct roles of TPCs have been identified. The first is to effect local Ca²⁺ release that may play a role in endolysosomal function including vesicular fusion and trafficking. The second is to trigger global calcium release by recruiting Ca²⁺-induced Ca²⁺-release (CICR) channels at lysosomal-endoplasmic reticulum (ER) junctions. The third is to regulate plasma membrane excitability by the targeting of Ca²⁺ release from appropriately positioned subplasma membrane stores to regulate plasma membrane Ca²⁺-activated channels. In this review, I discuss the role of nicotinic acid adenine nucleotide diphosphate (NAADP)-mediated Ca²⁺ release from endolysosomal stores as a widespread trigger for intracellular calcium signaling mechanisms, and how studies of TPCs are beginning to enhance our understanding of the central role of lysosomes in Ca²⁺ signaling.

Ca2+ Signaling in Cardiac Fibroblasts and Fibrosis-Associated Heart Diseases

Cardiac fibrosis is the excessive deposition of extracellular matrix proteins by cardiac fibroblasts and myofibroblasts, and is a hallmark feature of most heart diseases, including arrhythmia, hypertrophy, and heart failure. This maladaptive process occurs in response to a variety of stimuli, including myocardial injury, inflammation, and mechanical overload. There are multiple signaling pathways and various cell types that influence the fibrogenesis cascade. Fibroblasts and myofibroblasts are central effectors. Although it is clear that Ca2+ signaling plays a vital role in this pathological process, what contributes to Ca2+ signaling in fibroblasts and myofibroblasts is still not wholly understood, chiefly because of the large and diverse number of receptors, transporters, and ion channels that influence intracellular Ca2+ signaling. Intracellular Ca2+ signals are generated by Ca2+ release from intracellular Ca2+ stores and by Ca2+ entry through a multitude of Ca2+-permeable ion channels in the plasma membrane. Over the past decade, the transient receptor potential (TRP) channels have emerged as one of the most important families of ion channels mediating Ca2+ signaling in cardiac fibroblasts. TRP channels are a superfamily of non-voltage-gated, Ca2+-permeable non-selective cation channels. Their ability to respond to various stimulating cues makes TRP channels effective sensors of the many different pathophysiological events that stimulate cardiac fibrogenesis. This review focuses on the mechanisms of Ca2+ signaling in fibroblast differentiation and fibrosis-associated heart diseases and will highlight recent advances in the understanding of the roles that TRP and other Ca2+-permeable channels play in cardiac fibrosis.

Involvement of organelles and inter-organellar signaling in the pathogenesis of HIV-1 associated neurocognitive disorder and Alzheimer's disease

Endolysosomes, mitochondria, peroxisomes, endoplasmic reticulum, and plasma membranes are now known to physically and functionally interact with each other. Such findings of inter-organellar signaling and communication has led to a resurgent interest in cell biology and an increased appreciation for the physiological actions and pathological consequences of the dynamic physical and chemical communications occurring between intracellular organelles. Others and we have shown that HIV-1 proteins implicated in the pathogenesis of neuroHIV and that Alzheimer's disease both affects the structure and function of intracellular organelles. Intracellular organelles are highly mobile, and their intracellular distribution almost certainly affects their ability to interact with other organelles and to regulate such important physiological functions as endolysosome acidification, cell motility, and nutrient homeostasis. Indeed, compounds that acidify endolysosomes cause endolysosomes to exhibit a mainly perinuclear pattern while compounds that de-acidify endolysosomes cause these organelles to exhibit a larger profile as well as movement towards plasma membranes. Endolysosome pH might be an early event in the pathogenesis of neuroHIV and Alzheimer's disease and in terms of organellar biology endolysosome changes might be upstream of HIV-1 protein-induced changes to other organelles. Thus, inter-organellar signaling mechanisms might be involved in the pathogenesis of neuroHIV and other neurological disorders, and a better understanding of inter-organellar signaling might lead to improved therapeutic strategies.

Neuropharmacology of the Neuropsychiatric Symptoms of Dementia and Role of Pain: Essential Oil of Bergamot as a Novel Therapeutic Approach

Aging of the population makes of dementia a challenge for health systems worldwide. The cognitive disturbance is a serious but not the only issue in dementia; behavioral and psychological syndromes known as neuropsychiatric symptoms of dementia remarkably reduce the quality of life. The cluster of symptoms includes anxiety, depression, wandering, delusions, hallucinations, misidentifications, agitation and aggression. The pathophysiology of these symptoms implicates all the neurotransmitter systems, with a pivotal role for the glutamatergic neurotransmission. Imbalanced glutamatergic and GABAergic neurotransmissions, over-activation of the extrasynaptic N-methyl-D-aspartate (NMDA) receptors and alterations of the latter have been linked to the development of neuropsychiatric symptoms experienced by almost the entire demented population. Drugs with efficacy and safety for prevention or long term treatment of these disorders are not available yet. Aromatherapy provides the best evidence for positive outcomes in the control of agitation, the most resistant symptom. Demented patients often cannot verbalize pain, resulting in unrelieved symptoms and contributing to agitation. Bergamot essential oil provides extensive preclinical evidence of analgesic properties. Incidentally, the essential oil of bergamot induces anxyolitic-like effects devoid of sedation, typical of benzodiazepines, with a noteworthy advantage for demented patients. These data, together with the reported safety profile, form the rational basis for bergamot as a neurotherapeutic to be trialed for the control of behavioral and psychological symptoms of dementia.

Intracellular Ca2+ Release and Synaptic Plasticity: A Tale of Many Stores

Ca2+ is an essential trigger for most forms of synaptic plasticity. Ca2+ signaling occurs not only by Ca2+ entry via plasma membrane channels but also via Ca2+ signals generated by intracellular organelles. These organelles, by dynamically regulating the spatial and temporal extent of Ca2+ elevations within neurons, play a pivotal role in determining the downstream consequences of neural signaling on synaptic function. Here, we review the role of three major intracellular stores: the endoplasmic reticulum, mitochondria, and acidic Ca2+ stores, such as lysosomes, in neuronal Ca2+ signaling and plasticity. We provide a comprehensive account of how Ca2+ release from these stores regulates short- and long-term plasticity at the pre- and postsynaptic terminals of central synapses.

Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP) Induces Intracellular Ca2+ Release through the Two-Pore Channel TPC1 in Metastatic Colorectal Cancer Cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) gates two-pore channels 1 and 2 (TPC1 and TPC2) to elicit endo-lysosomal (EL) Ca²⁺ release. NAADP-induced EL Ca²⁺ signals may be amplified by the endoplasmic reticulum (ER) through the Ca²⁺-induced Ca²⁺ release mechanism (CICR). Herein, we aimed at assessing for the first time the role of EL Ca²⁺ signaling in primary cultures of human metastatic colorectal carcinoma (mCRC) by exploiting Ca²⁺ imaging and molecular biology techniques. The lysosomotropic agent, Gly-Phe β-naphthylamide (GPN), and nigericin, which dissipates the ΔpH which drives Ca²⁺ refilling of acidic organelles, caused massive Ca²⁺ release in the presence of a functional inositol-1,4,5-trisphosphate (InsP₃)-sensitive ER Ca²⁺ store. Liposomal delivery of NAADP induced a transient Ca²⁺ release that was reduced by GPN and NED-19, a selective TPC antagonist. Pharmacological and genetic manipulations revealed that the Ca²⁺ response to NAADP was triggered by TPC1, the most expressed TPC isoform in mCRC cells, and required ER-embedded InsP₃ receptors. Finally, NED-19 and genetic silencing of TPC1 reduced fetal calf serum-induced Ca²⁺ signals, proliferation, and extracellular signal-regulated kinase and Akt phoshorylation in mCRC cells. These data demonstrate that NAADP-gated TPC1 could be regarded as a novel target for alternative therapies to treat mCRC.

Probing Ca2+ release mechanisms using sea urchin egg homogenates

Sea urchin eggs have been extensively used to study Ca²⁺ release through intracellular Ca²⁺-permeable channels. Their amenability to homogenization yields a robust, cell-free preparation that was central to establishing the Ca²⁺ mobilizing actions of cyclic ADP-ribose and NAADP. Egg homogenates have continued to provide insight into the basic properties and pharmacology of intracellular Ca²⁺ release channels. In this chapter, we describe methods for the preparation of egg homogenates and monitoring Ca²⁺ release using fluorimetry and radiotracer flux.

Hippocampal mGluR1-dependent long-term potentiation requires NAADP-mediated acidic store Ca2+ signaling

Acidic organelles, such as endosomes and lysosomes, store Ca²⁺ that is released in response to intracellular increases in the second messenger nicotinic acid adenine dinucleotide phosphate (NAADP). In neurons, NAADP and Ca²⁺ signaling contribute to synaptic plasticity, a process of activity-dependent long-term potentiation (LTP) [or, alternatively, long-term depression (LTD)] of synaptic strength and neuronal transmission that is critical for neuronal function and memory formation. We explored the function of and mechanisms regulating acidic Ca²⁺ store signaling in murine hippocampal neurons. We found that metabotropic glutamate receptor 1 (mGluR1) was coupled to NAADP signaling that elicited Ca²⁺ release from acidic stores. In turn, this released Ca²⁺-mediated mGluR1-dependent LTP by transiently inhibiting SK-type K⁺ channels, possibly through the activation of protein phosphatase 2A. Genetically removing two-pore channels (TPCs), which are endolysosomal-specific ion channels, switched the polarity of plasticity from LTP to LTD, indicating the importance of specific receptor store coupling and providing mechanistic insight into how mGluR1 can produce both synaptic potentiation and synaptic depression.

Thinking through acidic Ca2+ stores

Glutamate signaling regulates neuronal activity and synaptic plasticity, which underlies learning and memory. In this issue of Science Signaling, Foster et al found that metabotropic glutamate receptors mediate long-term potentiation in hippocampal neurons by mobilizing acidic endolysosomal Ca²⁺ stores through the intracellular messenger NAADP.

Glutamate triggers intracellular Ca2+ oscillations and nitric oxide release by inducing NAADP- and InsP3 -dependent Ca2+ release in mouse brain endothelial cells

The neurotransmitter glutamate increases cerebral blood flow by activating postsynaptic neurons and presynaptic glial cells within the neurovascular unit. Glutamate does so by causing an increase in intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in the target cells, which activates the Ca²⁺ /Calmodulin-dependent nitric oxide (NO) synthase to release NO. It is unclear whether brain endothelial cells also sense glutamate through an elevation in [Ca²⁺ ]_i and NO production. The current study assessed whether and how glutamate drives Ca²⁺ -dependent NO release in bEND5 cells, an established model of brain endothelial cells. We found that glutamate induced a dose-dependent oscillatory increase in [Ca²⁺ ]_i , which was maximally activated at 200 μM and inhibited by α-methyl-4-carboxyphenylglycine, a selective blocker of Group 1 metabotropic glutamate receptors. Glutamate-induced intracellular Ca²⁺ oscillations were triggered by rhythmic endogenous Ca²⁺ mobilization and maintained over time by extracellular Ca²⁺ entry. Pharmacological manipulation revealed that glutamate-induced endogenous Ca²⁺ release was mediated by InsP₃ -sensitive receptors and nicotinic acid adenine dinucleotide phosphate (NAADP) gated two-pore channel 1. Constitutive store-operated Ca²⁺ entry mediated Ca²⁺ entry during ongoing Ca²⁺ oscillations. Finally, glutamate evoked a robust, although delayed increase in NO levels, which was blocked by pharmacologically inhibition of the accompanying intracellular Ca²⁺ signals. Of note, glutamate induced Ca²⁺ -dependent NO release also in hCMEC/D3 cells, an established model of human brain microvascular endothelial cells. This investigation demonstrates for the first time that metabotropic glutamate-induced intracellular Ca²⁺ oscillations and NO release have the potential to impact on neurovascular coupling in the brain.

Effects of silica nanoparticles on endolysosome function in primary cultured neurons 1

Silica nanoparticles (SiNPs) have been used as vehicles for drug delivery, molecular detection, and cellular manipulations in nanoneuromedicine. SiNPs may cause adverse effects in the brain including neurotoxicity, neuroinflammation, neurodegeneration, and enhancing levels of amyloid beta (Aβ) protein-all pathological hallmarks of Alzheimer's disease. Therefore, the extent to which SiNPs influence Aβ generation and the underlying mechanisms by which this occurs deserve investigation. Our studies were focused on the effects of SiNPs on endolysosomes which uptake, traffic, and mediate the actions of SiNPs. These organelles are also where amyloidogenesis largely originates. We found that SiNPs, in primary cultured hippocampal neurons, accumulated in endolysosomes and caused a rapid and persistent deacidification of endolysosomes. SiNPs significantly reduced endolysosome calcium stores as indicated by a significant reduction in the ability of the lysosomotropic agent glycyl-l-phenylalanine 2-naphthylamide (GPN) to release calcium from endolysosomes. SiNPs increased Aβ_1-40 secretion, whereas 2 agents that acidified endolysosomes, ML-SA1 and CGS21680, blocked SiNP-induced deacidification and increased generation of Aβ_1-40. Our findings suggest that SiNP-induced deacidification of and calcium release from endolysosomes might be mechanistically linked to increased amyloidogenesis. The use of SiNPs might not be the best nanomaterial for therapeutic strategies against Alzheimer's disease and other neurological disorders linked to endolysosome dysfunction.

Two-pore channels and disease

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

Ins and outs of Ca2+ transport by acidic organelles and cell migration

Much contemporary evidence underscores the pathophysiological importance of Ca²⁺ handling by acidic organelles such as lysosomes. Whereas our knowledge of how Ca²⁺ is released from these acidic Ca²⁺ stores (the ‘outs’) is advancing, we know relatively little about how Ca²⁺ uptake is effected (the ‘ins’). Here I highlight new work identifying animal Ca²⁺/H⁺ (CAX) exchangers that localize to acidic organelles, mediate Ca²⁺ uptake and regulate cell migration in vivo. Continued molecular definition of the acidic Ca²⁺ store toolkit provides new insight into Ca²⁺-dependent function.

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

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

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NAADP/TPC1 signaling maintains endo-lysosomal morphology

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TPC1 localizes to contacts between late endosomes and the endoplasmic reticulum

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NAADP/TPC1 signaling regulates contact site formation

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NAADP tempers EGF receptor-mediated signaling

Extracellular Remodeling by Lysosomes: An Inside-Out Mechanism of Spine Plasticity

In this issue of Neuron, Padamsey et al. (2017) demonstrate that neuronal activity triggers lysosomal fusion with the plasma membrane in dendrites. Quite unexpectedly, the lysosomes release signaling molecules that induce extracellular matrix remodeling to support long-lasting dendritic spine plasticity.

NAADP-evoked Ca2+ signals through two-pore channel-1 require arginine residues in the first S4-S5 linker

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The S4–S5 linker of TPCs harbours conserved basic residues.

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Arginine residues in the S4–S5 linker are required for activation of TPC1 by NAADP.

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The S4–S5 linker emerges as a potential common determinant for TPC activation.

Two-pore channels (TPCs) are two-domain members of the voltage-gated ion channel superfamily that localize to acidic organelles. Their mechanism of activation (ligands such as NAADP/PI(3,5)P₂ versus voltage) and ion selectivity (Ca²⁺ versus Na⁺) is debated. Here we report that a cluster of arginine residues in the first domain required for selective voltage-gating of TPC1 map not to the voltage-sensing fourth transmembrane region (S4) but to a cytosolic downstream region (S4-S5 linker). These residues are conserved between TPC isoforms suggesting a generic role in TPC activation. Accordingly, mutation of residues in TPC1 but not the analogous region in the second domain prevents Ca²⁺ release by NAADP in intact cells. Our data affirm the role of TPCs in NAADP-mediated Ca²⁺ signalling and unite differing models of channel activation through identification of common domain-specific residues.

Glutamate induces autophagy via the two-pore channels in neural cells

NAADP (nicotinic acid adenine dinucleotide phosphate) has been proposed as a second messenger for glutamate in neuronal and glial cells via the activation of the lysosomal Ca²⁺ channels TPC1 and TPC2. However, the activities of glutamate that are mediated by NAADP remain unclear. In this study, we evaluated the effect of glutamate on autophagy in astrocytes at physiological, non-toxic concentration. We found that glutamate induces autophagy at similar extent as NAADP. By contrast, the NAADP antagonist NED-19 or SiRNA-mediated inhibition of TPC1/2 decreases autophagy induced by glutamate, confirming a role for NAADP in this pathway. The involvement of TPC1/2 in glutamate-induced autophagy was also confirmed in SHSY5Y neuroblastoma cells. Finally, we show that glutamate leads to a NAADP-dependent activation of AMPK, which is required for autophagy induction, while mTOR activity is not affected by this treatment. Taken together, our results indicate that glutamate stimulates autophagy via NAADP/TPC/AMPK axis, providing new insights of how Ca²⁺ signalling glutamate-mediated can control the cell metabolism in the central nervous system.

Activity-Dependent Exocytosis of Lysosomes Regulates the Structural Plasticity of Dendritic Spines

Lysosomes have traditionally been viewed as degradative organelles, although a growing body of evidence suggests that they can function as Ca²⁺ stores. Here we examined the function of these stores in hippocampal pyramidal neurons. We found that back-propagating action potentials (bpAPs) could elicit Ca²⁺ release from lysosomes in the dendrites. This Ca²⁺ release triggered the fusion of lysosomes with the plasma membrane, resulting in the release of Cathepsin B. Cathepsin B increased the activity of matrix metalloproteinase 9 (MMP-9), an enzyme involved in extracellular matrix (ECM) remodelling and synaptic plasticity. Inhibition of either lysosomal Ca²⁺ signaling or Cathepsin B release prevented the maintenance of dendritic spine growth induced by Hebbian activity. This impairment could be rescued by exogenous application of active MMP-9. Our findings suggest that activity-dependent exocytosis of Cathepsin B from lysosomes regulates the long-term structural plasticity of dendritic spines by triggering MMP-9 activation and ECM remodelling.

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Back-propagating action potentials induce Ca²⁺ release from lysosomes in neurons

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Lysosomal Ca²⁺ release triggers exocytosis of the lysosomal protease Cathepsin B

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Cathepsin B maintains activity-dependent dendritic spine growth by activating MMP-9

Modulation of Calcium Entry by the Endo-lysosomal System

Endo-lysosomes are acidic organelles that besides the role in macromolecules degradation, act as intracellular Ca(2+) stores. Nicotinic acid adenine dinucleotide phosphate (NAADP), the most potent Ca(2+)-mobilizing second messenger, produced in response to agonist stimulation, activates Ca(2+)-releasing channels on endo-lysosomes and modulates a variety of cellular functions. NAADP-evoked signals are amplified by Ca(2+) release from endoplasmic reticulum, via the recruitment of inositol 1,4,5-trisphosphate and/or ryanodine receptors through a Ca(2+)-induced Ca(2+)- release (CICR) mechanism. The endo-lysosomal Ca(2+) channels activated by NAADP were recently identified as the two-pore channels (TPCs). In addition to TPCs, endo-lysosomes express another distinct family of Ca(2+)- permeable channels, namely the transient receptor potential mucolipin (TRPML) channels, functionally distinct from TPCs. TPCs belong to the voltage-gated channels, resembling voltage-gated Na(+) and Ca(2+) channels. TPCs have important roles in vesicular fusion and trafficking, in triggering a global Ca(2+) signal and in modulation of the membrane excitability. Depletion of acidic Ca(2+) stores has been shown to activate store-operated Ca(2+) entry in human platelets and mouse pancreatic β-cells. In human platelets, Ca(2+) influx in response to acidic stores depletion is facilitated by the tubulin-cytoskeleton and occurs through non-selective cation channels and transient receptor potential canonical (TRPC) channels. Emerging evidence indicates that activation of intracellular receptors, situated on endo-lysosomes, elicits canonical and non-canonical signaling mechanisms that involve CICR and activation of non-selective cation channels in plasma membrane. The ability of endo-lysosomal Ca(2+) stores to modulate the Ca(2+) release from other organelles and the Ca(2+) entry increases the diversity and complexity of cellular signaling mechanisms.

Connecting Ca2+ and lysosomes to Parkinson disease

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

Release of calcium from endolysosomes increases calcium influx through N-type calcium channels: Evidence for acidic store-operated calcium entry in neurons

Neurons possess an elaborate system of endolysosomes. Recently, endolysosomes were found to have readily releasable stores of intracellular calcium; however, relatively little is known about how such 'acidic calcium stores' affect calcium signaling in neurons. Here we demonstrated in primary cultured neurons that calcium released from acidic calcium stores triggered calcium influx across the plasma membrane, a phenomenon we have termed "acidic store-operated calcium entry (aSOCE)". aSOCE was functionally distinct from store-operated calcium release and calcium entry involving endoplasmic reticulum. aSOCE appeared to be governed by N-type calcium channels (NTCCs) because aSOCE was attenuated significantly by selectively blocking NTCCs or by siRNA knockdown of NTCCs. Furthermore, we demonstrated that NTCCs co-immunoprecipitated with the lysosome associated membrane protein 1 (LAMP1), and that aSOCE is accompanied by increased cell-surface expression levels of NTCC and LAMP1 proteins. Moreover, we demonstrated that siRNA knockdown of LAMP1 or Rab27a, both of which are key proteins involved in lysosome exocytosis, attenuated significantly aSOCE. Taken together our data suggest that aSOCE occurs in neurons, that aSOCE plays an important role in regulating the levels and actions of intraneuronal calcium, and that aSOCE is regulated at least in part by exocytotic insertion of N-type calcium channels into plasma membranes through LAMP1-dependent lysosome exocytosis.

A novel Ca²⁺-mediated cross-talk between endoplasmic reticulum and acidic organelles: implications for NAADP-dependent Ca²⁺ signalling

Nicotinic acid adenine dinucleotide phosphate (NAADP) serves as the ideal trigger of spatio-temporally complex intracellular Ca(2+) signals. However, the identity of the intracellular Ca(2+) store(s) recruited by NAADP, which may include either the endolysosomal (EL) or the endoplasmic reticulum (ER) Ca(2+) pools, is still elusive. Here, we show that the Ca(2+) response to NAADP was suppressed by interfering with either EL or ER Ca(2+) sequestration. The measurement of EL and ER Ca(2+) levels by using selectively targeted aequorin unveiled that the preventing ER Ca(2+) storage also affected ER Ca(2+) loading and vice versa. This indicates that a functional Ca(2+)-mediated cross-talk exists at the EL-ER interface and exerts profound implications for the study of NAADP-induced Ca(2+) signals. Extreme caution is warranted when dissecting NAADP targets by pharmacologically inhibiting EL and/or the ER Ca(2+) pools. Moreover, Ca(2+) transfer between these compartments might be essential to regulate vital Ca(2+)-dependent processes in both organelles.

A primer of NAADP-mediated Ca(2+) signalling: From sea urchin eggs to mammalian cells

Since the discovery of the Ca(2+) mobilizing effects of the pyridine nucleotide metabolite, nicotinic acid adenine dinucleotide phosphate (NAADP), this molecule has been demonstrated to function as a Ca(2+) mobilizing intracellular messenger in a wide range of cell types. In this review, I will briefly summarize the distinct principles behind NAADP-mediated Ca(2+) signalling before going on to outline the role of this messenger in the physiology of specific cell types. Central to the discussion here is the finding that NAADP principally mobilizes Ca(2+) from acidic organelles such as lysosomes and it is this property that allows NAADP to play a unique role in intracellular Ca(2+) signalling. Lysosomes and related organelles are small Ca(2+) stores but importantly may also initiate a two-way dialogue with other Ca(2+) storage organelles to amplify Ca(2+) release, and may be strategically localized to influence localized Ca(2+) signalling microdomains. The study of NAADP signalling has created a new and fruitful focus on the lysosome and endolysosomal system as major players in calcium signalling and pathophysiology.

Two pore channel 2 differentially modulates neural differentiation of mouse embryonic stem cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) is an endogenous Ca²⁺ mobilizing nucleotide presented in various species. NAADP mobilizes Ca²⁺ from acidic organelles through two pore channel 2 (TPC2) in many cell types and it has been previously shown that NAADP can potently induce neuronal differentiation in PC12 cells. Here we examined the role of TPC2 signaling in the neural differentiation of mouse embryonic stem (ES) cells. We found that the expression of TPC2 was markedly decreased during the initial ES cell entry into neural progenitors, and the levels of TPC2 gradually rebounded during the late stages of neurogenesis. Correspondingly, TPC2 knockdown accelerated mouse ES cell differentiation into neural progenitors but inhibited these neural progenitors from committing to neurons. Overexpression of TPC2, on the other hand, inhibited mouse ES cell from entering the early neural lineage. Interestingly, TPC2 knockdown had no effect on the differentiation of astrocytes and oligodendrocytes of mouse ES cells. Taken together, our data indicate that TPC2 signaling plays a temporal and differential role in modulating the neural lineage entry of mouse ES cells, in that TPC2 signaling inhibits ES cell entry to early neural progenitors, but is required for late neuronal differentiation.

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a second messenger in muscarinic receptor-induced contraction of guinea pig trachea

Nicotinic acid adenine dinucleotide phosphate (NAADP) is increasingly being demonstrated to be involved in calcium signaling in many cell types and species. Although it has been shown to play a role in smooth muscle cell contraction in several tissues, nothing is known about its possible role in tracheal smooth muscle, a muscle type that is clinically relevant to asthma. To determine whether NAADP functions as a second messenger in tracheal smooth muscle contraction, we used the criteria set out by Sutherland for a molecule to be designated a second messenger. We report that NAADP satisfies all five criteria as follows. First, the NAADP antagonist Ned-19 inhibited contractions in tracheal rings and calcium increases in isolated smooth muscle cells induced by the muscarinic agonist carbachol. Second, NAADP increased cytosolic calcium in isolated cells when microinjected and was blocked by Ned-19. Third, tracheal homogenates could synthesize NAADP by base exchange from exogenous NADP and nicotinic acid and metabolize exogenous NAADP to nicotinic acid adenine dinucleotide by a 2'-phosphatase. Fourth, carbachol induced a rapid and transient increase in endogenous NAADP levels. Fifth, tracheal homogenates contained NAADP-binding sites of high affinity. Taken together, these data demonstrate that NAADP functions as a second messenger in tracheal smooth muscle, and therefore, steps in the NAADP signaling pathway might provide possible new drug targets.

Presenilin-null cells have altered two-pore calcium channel expression and lysosomal calcium: implications for lysosomal function

Presenilins are necessary for calcium homeostasis and also for efficient proteolysis through the autophagy/lysosome system. Presenilin regulates both endoplasmic reticulum calcium stores and autophagic proteolysis in a γ-secretase independent fashion. The endo-lysosome system can also act as a calcium store, with calcium efflux channels being recently identified as two-pore channels 1 and 2. Here we investigated lysosomal calcium content and the channels that mediate calcium release from these acidic stores in presenilin knockout cells. We report that presenilin loss leads to a lower total lysosomal calcium store despite the buildup of lysosomes found in these cells. Additionally, we find alterations in two-pore calcium channel protein expression, with loss of presenilin preventing the formation of a high molecular weight species of TPC1 and TPC2. Finally, we find that treatments that disturb lysosomal calcium release lead to a reduction in autophagy function yet lysosomal inhibitors do not alter two-pore calcium channel expression. These data indicate that alterations in lysosomal calcium in the absence of presenilins might be leading to disruptions in autophagy.

β-Adrenergic receptor signaling increases NAADP and cADPR levels in the heart

Evidence suggests that β-Adrenergic receptor signaling increases heart rate and force through not just cyclic AMP but also the Ca(2+)-releasing second messengers NAADP (nicotinic acid adenine dinucleotide phosphate) and cADPR (cyclic ADP-ribose). Nevertheless, proof of the physiological relevance of these messengers requires direct measurements of their levels in response to receptor stimulation. Here we report that in intact Langendorff-perfused hearts β-adrenergic stimulation increased both messengers, with NAADP being transient and cADPR being sustained. Both NAADP and cADPR have physiological and therefore pathological relevance by providing alternative drug targets in the β-adrenergic receptor signaling pathway.

Domain assembly of NAADP-gated two-pore channels

TPCs (two-pore channels) have recently been identified as targets for the Ca²⁺-mobilizing messenger NAADP (nicotinic acid–adenine dinucleotide phosphate). TPCs have a unique structure consisting of cytosolic termini, two hydrophobic domains (I and II) each comprising six transmembrane regions and a pore, and a connecting cytosolic loop; however, little is known concerning how these channels are assembled. In the present paper, we report that both domain I and II of human TPCs are capable of independent insertion into membranes, whereas the loop linking the domains fails to insert. Pairs of transmembrane regions within domain I of TPC1 are also capable of insertion, consistent with sequential translational integration of hydrophobic regions. Insertion of the first two transmembrane regions, however, was inefficient, indicating possible interaction between transmembrane regions during translation. Both domains, and each pair of transmembrane regions within domain I, were capable of forming oligomers, highlighting marked redundancy in the molecular determinants driving oligomer formation. Each hydrophobic domain formed dimers upon cross-linking. The first four transmembrane regions of TPC1 also formed dimers, whereas transmembrane regions 5 and 6, encompassing the pore loop, formed both dimers and tetramers. TPCs thus probably assemble as dimers through differential interactions between transmembrane regions. The present study provides new molecular insight into the membrane insertion and oligomerization of TPCs.

NAADP mediates ATP-induced Ca2+ signals in astrocytes

Intracellular Ca(2+) signals provide astrocytes with a specific form of excitability that enables them to regulate synaptic transmission. In this study, we demonstrate that NAADP-AM, a membrane-permeant analogue of the new second messenger nicotinic acid-adenine dinucleotide phosphate (NAADP), mobilizes Ca(2+) in astrocytes and that the response is blocked by Ned-19, an antagonist of NAADP signalling. We also show that NAADP receptors are expressed in lysosome-related acidic vesicles. Pharmacological disruption of either NAADP or lysosomal signalling reduced Ca(2+) responses induced by ATP and endothelin-1, but not by bradykinin. Furthermore, ATP increased endogenous NAADP levels. Overall, our data provide evidence for NAADP being an intracellular messenger for agonist-mediated calcium signalling in astrocytes.

Nicotinic acid adenine dinucleotide phosphate (NAADP) regulates autophagy in cultured astrocytes

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent Ca(2+)-mobilizing messenger that in many cells releases Ca(2+) from the endolysosomal system. Recent studies have shown that NAADP-induced Ca(2+) mobilization is mediated by the two-pore channels (TPCs). Whether NAADP acts as a messenger in astrocytes is unclear, and downstream functional consequences have yet to be defined. Here, we show that intracellular delivery of NAADP evokes Ca(2+) signals from acidic organelles in rat astrocytes and that these signals are potentiated upon overexpression of TPCs. We also show that NAADP increases acidic vesicular organelle formation and levels of the autophagic markers, LC3II and beclin-1. NAADP-mediated increases in LC3II levels were reduced in cells expressing a dominant-negative TPC2 construct. Our data provide evidence that NAADP-evoked Ca(2+) signals mediated by TPCs regulate autophagy.