The two-pore channel TPCN2 mediates NAADP-dependent Ca(2+)-release from lysosomal stores

Differential contribution of EF-hands to the Ca²⁺-dependent activation in the plant two-pore channel TPC1

Two-pore channels (TPC) have been established as components of calcium signalling networks in plants and animals. In plants, TPC1 in the vacuolar membrane is gated open upon binding of calcium in a voltage-dependent manner. Here, we analyzed the molecular mechanism of the Ca²⁺-dependent activity of TPC1 from Arabidopsis thaliana, using site-directed mutagenesis of its two canonical EF-hands. Wild-type TPC1 and TPC1-D335A with a mutated first Ca²⁺ ligand in EF-hand 1 produced channels that retained their voltage- and Ca²⁺-dependent gating characteristics, but were less sensitive at Ca²⁺ concentrations < 200 μm. Additional mutation of the first Ca²⁺ ligand in EF-hand 2 resulted in silent TPC1-D335A/D376A channels. Similarly, the single mutant TPC1-D376A could not be activated up to 1 mm Ca²⁺, indicating that the second EF-hand is essential for the Ca²⁺-dependent channel gating. Molecular modeling suggests that EF-hand 1 displays a low-affinity Ca²⁺/Mg²⁺-binding site, while EF-hand 2 represents a high-affinity Ca²⁺-binding site. Together, our data prove that EF-hand 2 is responsible for the Ca²⁺-receptor characteristics of TPC1, while EF-hand 1 is a structural site required to enable channel responses at physiological changes in Ca²⁺ concentration.

Spatial organization of intracellular Ca2+ signals

The ability of Ca(2+), the simplest of all intracellular messengers, selectively to regulate so many cellular behaviours is due largely to the complex spatiotemporal organization of intracellular Ca(2+) signals. Most signalling pathways, including those that culminate in Ca(2+) signals, comprise sequences of protein-protein interactions linked by diffusible messengers. Using specific examples to illustrate key principles, we consider the roles of both components in defining the spatial organization of Ca(2+) signals. We discuss evidence that regulation of most Ca(2+) channels by Ca(2+) contributes to controlling the duration of Ca(2+) signals, to signal integration and, via Ca(2+)-induced Ca(2+) release, to defining the spatial spread of Ca(2+) signals. We distinguish two types of protein-protein interaction: scaffolds that allow rapid local transfer of diffusible messengers between signalling proteins, and interactions that directly transfer information between signalling proteins. Store-operated Ca(2+) entry provides a ubiquitous example of the latter, and it serves also to illustrate how Ca(2+) signals can be organized at different levels of spatial organization - from interactions between proteins to interactions between organelles.

Two-pore channels form homo- and heterodimers

Two-pore channels (TPCs) have been recently identified as NAADP-regulated Ca²⁺ release channels, which are localized on the endolysosomal system. TPCs have a 12-transmembrane domain (TMD) structure and are evolutionary intermediates between the 24-TMD α-subunits of Na⁺ or Ca²⁺ channels and the transient receptor potential channel superfamily, which have six TMDs in a single subunit and form tetramers with 24 TMDs as active channels. Based on this relationship, it is predicted that TPCs dimerize to form functional channels, but the dimerization of human TPCs has so far not been studied. Using co-immunoprecipitation studies and a mass spectroscopic analysis of the immunocomplex, we show the presence of homo- and heteromeric complexes for human TPC1 and TPC2. Despite their largely distinct localization, we identified a discrete number of endosomes that coexpressed TPC1 and TPC2. Homo- and heteromerization were confirmed by a FRET study, showing that both proteins interacted in a rotational (N- to C-terminal/head-to-tail) symmetry. This is the first report describing the presence of homomultimeric TPC1 channels and the first study showing that TPCs are capable of forming heteromers.

Cyclic ADP-ribose and NAADP: fraternal twin messengers for calcium signaling

The concept advanced by Berridge and colleagues that intracellular Ca(2+)-stores can be mobilized in an agonist-dependent and messenger (IP(3))-mediated manner has put Ca(2+)-mobilization at the center stage of signal transduction mechanisms. During the late 1980s, we showed that Ca(2+)-stores can be mobilized by two other messengers unrelated to inositol trisphosphate (IP(3)) and identified them as cyclic ADP-ribose (cADPR), a novel cyclic nucleotide from NAD, and nicotinic acid adenine dinucleotide phosphate (NAADP), a linear metabolite of NADP. Their messenger functions have now been documented in a wide range of systems spanning three biological kingdoms. Accumulated evidence indicates that the target of cADPR is the ryanodine receptor in the sarco/endoplasmic reticulum, while that of NAADP is the two pore channel in endolysosomes.As cADPR and NAADP are structurally and functionally distinct, it is remarkable that they are synthesized by the same enzyme. They are thus fraternal twin messengers. We first identified the Aplysia ADP-ribosyl cyclase as one such enzyme and, through homology, found its mammalian homolog, CD38. Gene knockout in mice confirms the important roles of CD38 in diverse physiological functions from insulin secretion, susceptibility to bacterial infection, to social behavior of mice through modulating neuronal oxytocin secretion. We have elucidated the catalytic mechanisms of the Aplysia cyclase and CD38 to atomic resolution by crystallography and site-directed mutagenesis. This article gives a historical account of the cADPR/NAADP/CD38-signaling pathway and describes current efforts in elucidating the structure and function of its components.

Physiological roles of NAADP-mediated Ca2+ signaling

Nicotinic acid dinucleotide phosphate (NAADP) is unique amongst Ca(2+) mobilizing messengers in that its principal function is to mobilize Ca(2+) from acidic organelles. Early studies indicated that it was likely that NAADP activates a novel Ca(2+) release channel distinct from the well characterized Ca(2+) release channels on the (sarco)-endoplasmic reticulum (ER), inositol trisphosphate and ryanodine receptors. In this review, we discuss the emergence of a novel family of endolysosomal channels, the two-pore channels (TPCs), as likely targets for NAADP, and how molecular and pharmacological manipulation of these channels is enhancing our understanding of the physiological roles of NAADP as an intracellular Ca(2+) mobilizing messenger.

Two-pore channels for integrative Ca signaling

Two-pore channels (TPCs) are related to voltage-gated Ca(2+) and Na(+) channels. They most likely work as dimers with each of the two TPC protein subunits containing two pore-forming domains. Recent studies suggest that TPCs are expressed on the membranes of endosomes and lysosomes where they form receptors for nicotinic acid adenine dinucleotide phosphate (NAADP), the most potent Ca(2+) mobilizing messenger inside cells. Upon activation by NAADP, Ca(2+) release from endolysosomal stores through TPCs triggers cytoplasmic Ca(2+) signals. Because of discrete localizations of these acidic vesicles and their small, albeit variable, sizes, the Ca(2+) signals from endolysosomes are local and, perhaps, represent unique elementary Ca(2+) events. These localized signals can be converted into regenerative global Ca(2+) waves by triggering Ca(2+)-induced Ca(2+) release from endoplasmic reticulum. We will discuss the implications of these findings and the significance of TPCs in integrative Ca(2+) signaling in animal cells.

Ion flux and the function of endosomes and lysosomes: pH is just the start: the flux of ions across endosomal membranes influences endosome function not only through regulation of the luminal pH

The ionic nature of endosomes varies considerably in character along the endocytic pathway. Counter-ion flux across the limiting membrane of endosomes has long been considered essential for full acidification and normal endosome/lysosomal function. The proximal functions of luminal ions, however, have been difficult to assess. The recent development of transgenic mice carrying mutations in the intracellular chloride channels (ClCs) has provided a tool to uncouple Cl(-) influx from endosomal acidification. Intriguingly, many of the defects of the endo-lysomal system attributed to aberrant pH persist in the Cl(-)-deficient mice implying a direct regulatory role for Cl(-) influx in endosome function. These observations may begin to explain the abundance of endosomal ion transporters, including ClCs, sodium-proton exchangers, two-pore channels and mucolipins, that have been localized to endo-lysosomes, and the extensive changes in luminal ion composition therein. In this review, we summarize what is known regarding the mediators of endosomal ion flux, and discuss the implications of changing ionic content on endo-lysosomal function.

TRPML: transporters of metals in lysosomes essential for cell survival?

Key aspects of lysosomal function are affected by the ionic content of the lysosomal lumen and, therefore, by the ion permeability in the lysosomal membrane. Such functions include regulation of lysosomal acidification, a critical process in delivery and activation of the lysosomal enzymes, release of metals from lysosomes into the cytoplasm and the Ca(2+)-dependent component of membrane fusion events in the endocytic pathway. While the basic mechanisms of lysosomal acidification have been largely defined, the lysosomal metal transport system is not well understood. TRPML1 is a lysosomal ion channel whose malfunction is implicated in the lysosomal storage disease Mucolipidosis Type IV. Recent evidence suggests that TRPML1 is involved in Fe(2+), Ca(2+) and Zn(2+) transport across the lysosomal membrane, ascribing novel physiological roles to this ion channel, and perhaps to its relatives TRPML2 and TRPML3 and illuminating poorly understood aspects of lysosomal function. Further, alterations in metal transport by the TRPMLs due to mutations or environmental factors may contribute to their role in the disease phenotype and cell death.

Reconstitution of lysosomal NAADP-TRP-ML1 signaling pathway and its function in TRP-ML1(-/-) cells

It is well known that the mutation of TRP-ML1 (transient receptor potential-mucolipin-1) causes mucolipidosis IV, a lysosomal storage disease. Given that lysosomal nicotinic acid adenine dinucleotide phosphate (NAADP)-Ca(2+) release channel activity is associated with TRP-ML1, the present study was designed to test the hypothesis that NAADP regulates lysosome function via activation of TRP-ML1 channel activity. Using lysosomal preparations from wild-type (TRP-ML1(+/+)) human fibroblasts, channel reconstitution experiments demonstrated that NAADP (0.01-1.0 μM) produced a concentration-dependent increase in TRP-ML1 channel activity. This NAADP-induced activation of TRP-ML1 channels could not be observed in lysosomes from TRP-ML1(-/-) cells, but was restored by introducing a TRP-ML1 transgene into these cells. Microscopic Ca(2+) fluorescence imaging showed that NAADP significantly increased intracellular Ca(2+) concentration to 302.4 ± 74.28 nM (vs. 180 ± 44.13 nM of the basal) in TRP-ML1(+/+) cells, but it had no effect in TRP-ML1(-/-) cells. If a TRP-ML1 gene was transfected into TRP-ML1(-/-) cells, the Ca(2+) response to NAADP was restored to the level comparable to TRP-ML1(+/+) cells. Functionally, confocal microscopy revealed that NAADP significantly enhanced the dynamic interaction of endosomes and lysosomes and the lipid delivery to lysosomes in TRP-ML1(+/+) cells. This functional action of NAADP was abolished in TRP-ML1(-/-) cells, but restored after TRP-ML1 gene was rescued in these cells. Our results suggest that NAADP increases lysosomal TRP-ML1 channel activity to release Ca(2+), which promotes the interaction of endosomes and lysosomes and thereby regulates lipid transport to lysosomes. Failure of NAADP-TRP-ML1 signaling may be one of the important mechanisms resulting in intracellular lipid trafficking disorder and consequent mucolipidosis.

The endo-lysosomal system as an NAADP-sensitive acidic Ca(2+) store: role for the two-pore channels

Accumulating evidence suggests that the endo-lysosomal system provides a substantial store of Ca(2+) that is tapped by the Ca(2+)-mobilizing messenger, NAADP. In this article, we review evidence that NAADP-mediated Ca(2+) release from this acidic Ca(2+) store proceeds through activation of the newly described two-pore channels (TPCs). We discuss recent advances in defining the sub-cellular targeting, topology and biophysics of TPCs. We also discuss physiological roles and the evolution of this ubiquitous ion channel family.

Transient receptor potential mucolipin 1 (TRPML1) and two-pore channels are functionally independent organellar ion channels

NAADP is a potent second messenger that mobilizes Ca(2+) from acidic organelles such as endosomes and lysosomes. The molecular basis for Ca(2+) release by NAADP, however, is uncertain. TRP mucolipins (TRPMLs) and two-pore channels (TPCs) are Ca(2+)-permeable ion channels present within the endolysosomal system. Both have been proposed as targets for NAADP. In the present study, we probed possible physical and functional association of these ion channels. Exogenously expressed TRPML1 showed near complete colocalization with TPC2 and partial colocalization with TPC1. TRPML3 overlap with TPC2 was more modest. TRPML1 and to some extent TRPML3 co-immunoprecipitated with TPC2 but less so with TPC1. Current recording, however, showed that TPC1 and TPC2 did not affect the activity of wild-type TRPML1 or constitutively active TRPML1(V432P). N-terminally truncated TPC2 (TPC2delN), which is targeted to the plasma membrane, also failed to affect TRPML1 and TRPML1(V432P) channel function or TRPML1(V432P)-mediated Ca(2+) influx. Whereas overexpression of TPCs enhanced NAADP-mediated Ca(2+) signals, overexpression of TRPML1 did not, and the dominant negative TRPML1(D471K) was without affect on endogenous NAADP-mediated Ca(2+) signals. Furthermore, the single channel properties of NAADP-activated TPC2delN were not affected by TRPML1. Finally, NAADP-evoked Ca(2+) oscillations in pancreatic acinar cells were identical in wild-type and TRPML1(-/-) cells. We conclude that although TRPML1 and TPCs are present in the same complex, they function as two independent organellar ion channels and that TPCs, not TRPMLs, are the targets for NAADP.

Membrane topology of NAADP-sensitive two-pore channels and their regulation by N-linked glycosylation

Two-pore channels (TPCs) localize to the endolysosomal system and have recently emerged as targets for the Ca(2+)-mobilizing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP). However, their membrane topology is unknown. Using fluorescence protease protection assays, we show that human TPC1 and TPC2 possess cytosolic N and C termini and therefore an even number of transmembrane regions. Fluorophores placed at position 225 or 347 in TPC1, or 339 in TPC2 were also cytosolic, whereas a fluorophore at position 628 in TPC1 was luminal. These data together with sequence similarity to voltage-gated Ca(2+) and Na(+) channels, and unbiased in silico predictions are consistent with a topology in which two homologous domains are present, each comprising 6 transmembrane regions and a re-entrant pore loop. Immunocytochemical analysis of selectively permeabilized cells using antipeptide antibodies confirmed that the C-terminal tails of recombinant TPCs are cytosolic and that residues 240-254 of TPC2 prior to putative pore 1 are luminal. Both TPC1 and TPC2 are N-glycosylated with residues 599, 611, and 616 contributing to glycosylation of TPC1. This confirms the luminal position of these residues, which immediately precede the putative pore loop of the second domain. Mutation of all three glycosylation sites in TPC1 enhances NAADP-evoked cytosolic Ca(2+) signals. Our data establish essential features of the topology of two-pore channels.

NAADP as an intracellular messenger regulating lysosomal calcium-release channels

Recent studies into the mechanisms of action of the Ca(2+)-mobilizing messenger NAADP (nicotinic acid-adenine dinucleotide phosphate) have demonstrated that a novel family of intracellular Ca(2+)-release channels termed TPCs (two-pore channels) are components of the NAADP receptor. TPCs appear to be exclusively localized to the endolysosomal system. These findings confirm previous pharmacological and biochemical studies suggesting that NAADP targets acidic Ca(2+) stores rather than the endoplasmic reticulum, the major site of action of the other two principal Ca(2+)-mobilizing messengers, InsP(3) and cADPR (cADP-ribose). Studies of the messenger roles of NAADP and the function of TPCs highlight the novel role of lysosomes and other organelles of the endocytic pathway as messenger-regulated Ca(2+) stores which also affects the regulation of the endolysosomal system.

Sea urchin eggs in the acid reign

Sea urchin eggs have been an indispensable model system for studying egg activation and ionic signalling for at least a century. Instrumental in the discovery of two Ca(2+)-mobilizing second messengers, cyclic ADP-ribose and NAADP, the sea urchin has revolutionized cell biology for all phyla. This review attempts to summarize what we currently know about egg acidic vesicles in the context of Ca(2+) signalling. The dynamics of Ca(2+) storage, Ca(2+) mobilization, proton fluxes and two-pore channels will be discussed.

Cyclic adenosine diphosphate ribose activates ryanodine receptors, whereas NAADP activates two-pore domain channels

The mechanism by which cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) mobilize intracellular Ca(2+) stores remains controversial. It is open to question whether cADPR regulates ryanodine receptors (RyRs) directly, as originally proposed, or indirectly by promoting Ca(2+) uptake into the sarco/endoplasmic reticulum by sarco/endoplasmic reticulum Ca(2+)-ATPases. Conversely, although we have proposed that NAADP mobilizes endolysosomal Ca(2+) stores by activating two-pore domain channels (TPCs), others suggest that NAADP directly activates RyRs. We therefore assessed Ca(2+) signals evoked by intracellular dialysis from a patch pipette of cADPR and NAADP into HEK293 cells that stably overexpress either TPC1, TPC2, RyR1, or RyR3. No change in intracellular Ca(2+) concentration was triggered by cADPR in either wild-type HEK293 cells (which are devoid of RyRs) or in cells that stably overexpress TPC1 and TPC2, respectively. By contrast, a marked Ca(2+) transient was triggered by cADPR in HEK293 cells that stably expressed RyR1 and RyR3. The Ca(2+) transient was abolished following depletion of endoplasmic reticulum stores by thapsigargin and block of RyRs by dantrolene but not following depletion of acidic Ca(2+) stores by bafilomycin. By contrast, NAADP failed to evoke a Ca(2+) transient in HEK293 cells that expressed RyR1 or RyR3, but it induced robust Ca(2+) transients in cells that stably overexpressed TPC1 or TPC2 and in a manner that was blocked following depletion of acidic stores by bafilomycin. We conclude that cADPR triggers Ca(2+) release by activating RyRs but not TPCs, whereas NAADP activates TPCs but not RyRs.

Cyclic adenosine diphosphate ribose activates ryanodine receptors, whereas NAADP activates two-pore domain channels

The mechanism by which cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) mobilize intracellular Ca(2+) stores remains controversial. It is open to question whether cADPR regulates ryanodine receptors (RyRs) directly, as originally proposed, or indirectly by promoting Ca(2+) uptake into the sarco/endoplasmic reticulum by sarco/endoplasmic reticulum Ca(2+)-ATPases. Conversely, although we have proposed that NAADP mobilizes endolysosomal Ca(2+) stores by activating two-pore domain channels (TPCs), others suggest that NAADP directly activates RyRs. We therefore assessed Ca(2+) signals evoked by intracellular dialysis from a patch pipette of cADPR and NAADP into HEK293 cells that stably overexpress either TPC1, TPC2, RyR1, or RyR3. No change in intracellular Ca(2+) concentration was triggered by cADPR in either wild-type HEK293 cells (which are devoid of RyRs) or in cells that stably overexpress TPC1 and TPC2, respectively. By contrast, a marked Ca(2+) transient was triggered by cADPR in HEK293 cells that stably expressed RyR1 and RyR3. The Ca(2+) transient was abolished following depletion of endoplasmic reticulum stores by thapsigargin and block of RyRs by dantrolene but not following depletion of acidic Ca(2+) stores by bafilomycin. By contrast, NAADP failed to evoke a Ca(2+) transient in HEK293 cells that expressed RyR1 or RyR3, but it induced robust Ca(2+) transients in cells that stably overexpressed TPC1 or TPC2 and in a manner that was blocked following depletion of acidic stores by bafilomycin. We conclude that cADPR triggers Ca(2+) release by activating RyRs but not TPCs, whereas NAADP activates TPCs but not RyRs.

NAADP receptors

Of the established Ca(2+) mobilizing messengers, NAADP is arguably the most tantalizing. It is the most potent, often efficacious at low nanomolar concentrations. Recent studies have identified a new class of calcium release channel, the two-pore channels (TPCs), as the likely targets for NAADP. These channels are endolysosomal in localization where they mediate local Ca(2+) release, and have highlighted a new role of acidic organelles as targets for messenger-evoked Ca(2+) mobilization. Three distinct roles of TPCs have been identified. The first is to effect local Ca(2+) release that may play a role in endolysosomal function including vesicular fusion and trafficking. The second is to trigger global calcium release by recruiting Ca(2+)-induced Ca(2+) release (CICR) channels at lysosomal-ER junctions. The third is to regulate plasma membrane excitability by the targeting of Ca(2+) release from appropriately positioned subplasma membrane stores to regulate plasma membrane Ca(2+)-activated channels. In this review, I discuss the role of NAADP-mediated Ca(2+) 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 Ca(2+) signaling.

Planar patch clamp approach to characterize ionic currents from intact lysosomes

Since its launch in the early 1980s, the patch clamp method has been extensively used to study ion channels in the plasma membrane, but its application to the study of intracellular ion channels has been limited. Unlike the plasma membrane, intracellular membranes are usually not stable enough to withstand mechanical manipulation by glass electrodes during seal formation and rupturing of the membrane. To circumvent these problems, we developed a method involving the immobilization of isolated organelles on a solid matrix planar glass chip. This glass chip contains a microstructured hole that supports the formation of gigaseals and subsequent electrophysiological recordings despite the high fragility of intracellular membranes. Here, we report the experimental details of this method using lysosomes, which are the smallest cellular organelles, as a model system. We demonstrate that we can record endogenous ionic currents from wild-type lysosomes, as well as from lysosomes overexpressing ion channels, and expect that this method will provide electrophysiological access to a broad range of intracellular ion channels.

TPC2 is a novel NAADP-sensitive Ca2+ release channel, operating as a dual sensor of luminal pH and Ca2+

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a molecule capable of initiating the release of intracellular Ca(2+) required for many essential cellular processes. Recent evidence links two-pore channels (TPCs) with NAADP-induced release of Ca(2+) from lysosome-like acidic organelles; however, there has been no direct demonstration that TPCs can act as NAADP-sensitive Ca(2+) release channels. Controversial evidence also proposes ryanodine receptors as the primary target of NAADP. We show that TPC2, the major lysosomal targeted isoform, is a cation channel with selectivity for Ca(2+) that will enable it to act as a Ca(2+) release channel in the cellular environment. NAADP opens TPC2 channels in a concentration-dependent manner, binding to high affinity activation and low affinity inhibition sites. At the core of this process is the luminal environment of the channel. The sensitivity of TPC2 to NAADP is steeply dependent on the luminal [Ca(2+)] allowing extremely low levels of NAADP to open the channel. In parallel, luminal pH controls NAADP affinity for TPC2 by switching from reversible activation of TPC2 at low pH to irreversible activation at neutral pH. Further evidence earmarking TPCs as the likely pathway for NAADP-induced intracellular Ca(2+) release is obtained from the use of Ned-19, the selective blocker of cellular NAADP-induced Ca(2+) release. Ned-19 antagonizes NAADP-activation of TPC2 in a non-competitive manner at 1 μM but potentiates NAADP activation at nanomolar concentrations. This single-channel study provides a long awaited molecular basis for the peculiar mechanistic features of NAADP signaling and a framework for understanding how NAADP can mediate key physiological events.

Nicotinic acid adenine dinucleotide phosphate regulates skeletal muscle differentiation via action at two-pore channels

Calcium signaling is essential for the differentiation of many cell types, including skeletal muscle cells, but its mechanisms remain elusive. Here we demonstrate a crucial role for nicotinic acid adenine dinucleotide phosphate (NAADP) signaling in skeletal muscle differentiation. Although the inositol trisphosphate pathway may have a partial role to play in this process, the ryanodine signaling cascade is not involved. In both skeletal muscle precursors and C2C12, cells interfering with NAADP signaling prevented differentiation, whereas promoting NAADP signaling potentiated differentiation. Moreover, siRNA knockdown of two-pore channels, the target of NAADP, attenuated differentiation. The data presented here strongly suggest that in myoblasts, NAADP acts at acidic organelles on the recently discovered two-pore channels to promote differentiation.

Two-pore channels: Regulation by NAADP and customized roles in triggering calcium signals

NAADP is a potent regulator of cytosolic calcium levels. Much evidence suggests that NAADP activates a novel channel located on an acidic (lysosomal-like) calcium store, the mobilisation of which results in further calcium release from the endoplasmic reticulum. Here, we discuss the recent identification of a family of poorly characterized ion channels (the two-pore channels) as endo-lysosomal NAADP receptors. The generation of calcium signals by these channels is likened to those evoked by depolarisation during excitation-contraction coupling in muscle. We discuss the idea that two-pore channels can mediate a trigger release of calcium which is then amplified by calcium-induced calcium release from the endoplasmic reticulum. This is similar to the activation of voltage-sensitive calcium channels and subsequent mobilisation of sarcoplasmic reticulum calcium stores in cardiac tissue. We suggest that two-pore channels may physically interact with ryanodine receptors to account for more direct release of calcium from the endoplasmic reticulum in analogy with the conformational coupling of voltage-sensitive calcium channels and ryanodine receptors in skeletal muscle. Interaction of two-pore channels with other calcium release channels likely occurs between stores "trans-chatter" and possibly within the same store "cis-chatter". We also speculate that trafficking of two-pore channels through the endo-lysosomal system facilitates interactions with calcium entry channels. Strategic placing of two-pore channels thus provides a versatile means of generating spatiotemporally complex cellular calcium signals.

An NAADP-gated two-pore channel targeted to the plasma membrane uncouples triggering from amplifying Ca2+ signals

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a ubiquitous messenger proposed to stimulate Ca(2+) release from acidic organelles via two-pore channels (TPCs). It has been difficult to resolve this trigger event from its amplification via endoplasmic reticulum Ca(2+) stores, fuelling speculation that archetypal intracellular Ca(2+) channels are the primary targets of NAADP. Here, we redirect TPC2 from lysosomes to the plasma membrane and show that NAADP evokes Ca(2+) influx independent of ryanodine receptors and that it activates a Ca(2+)-permeable channel whose conductance is reduced by mutation of a residue within a putative pore. We therefore uncouple TPC2 from amplification pathways and prove that it is a pore-forming subunit of an NAADP-gated Ca(2+) channel.

Acidic NAADP-sensitive calcium stores in the endothelium: agonist-specific recruitment and role in regulating blood pressure

Accumulating evidence implicates nicotinic acid adenine dinucleotide phosphate (NAADP) in the control of Ca(2+)-dependent functions. Little, however, is known concerning its role in the vascular endothelium, a major regulator of blood pressure. Here, we show that NAADP acetoxymethyl ester (NAADP-AM), a cell-permeant NAADP analog, increases cytosolic Ca(2+) concentration in aortic endothelial cells. We demonstrate that these signals and those evoked by acetylcholine are blocked by disrupting acidic organelles with bafilomycin A1. In contrast, Ca(2+) signals in response to thrombin are only partially inhibited by bafilomycin A1 treatment, and those to ATP were insensitive, suggesting that recruitment of acidic stores is agonist-specific. We further show that NAADP-evoked Ca(2+) signals hyperpolarize endothelial cells and generate NO. Additionally, we demonstrate that NAADP dilates aortic rings in an endothelium- and NO-dependent manner. Finally, we show that intravenous administration of NAADP-AM into anesthetized rats decreases mean arterial pressure. Our data extend the actions of NAADP to the endothelium both in vitro and in vivo, pointing to a previously unrecognized role for this messenger in controlling blood pressure.

The ecto-enzyme CD38 is a nicotinic acid adenine dinucleotide phosphate (NAADP) synthase that couples receptor activation to Ca2+ mobilization from lysosomes in pancreatic acinar cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent Ca(2+)-mobilizing intracellular messenger and is linked to a variety of stimuli and cell surface receptors. However, the enzyme responsible for endogenous NAADP synthesis in vivo is unknown, and it has been proposed that another enzyme differing from ADP-ribosyl cyclase family members may exist. The ecto-enzyme CD38, involved in many functions as diverse as cell proliferation and social behavior, represents an important alternative. In pancreatic acinar cells, the hormone cholecystokinin (CCK) stimulates NAADP production evoking Ca(2+) signals by discharging acidic Ca(2+) stores and leading to digestive enzyme secretion. From cells derived from CD38(-/-) mice, we provide the first physiological evidence that CD38 is required for endogenous NAADP generation in response to CCK stimulation. Furthermore, CD38 expression in CD38-deficient pancreatic AR42J cells remodels Ca(2+)-signaling pathways in these cells by restoring Ca(2+) mobilization from lysosomes during CCK-induced Ca(2+) signaling. In agreement with an intracellular site for messenger synthesis, we found that CD38 is expressed in endosomes. These CD38-containing vesicles, likely of endosomal origin, appear to be proximal to lysosomes but not co-localized with them. We propose that CD38 is an NAADP synthase required for coupling receptor activation to NAADP-mediated Ca(2+) release from lysosomal stores in pancreatic acinar cells.

The role of transient receptor potential cation channels in Ca2+ signaling

The 28 mammalian members of the super-family of transient receptor potential (TRP) channels are cation channels, mostly permeable to both monovalent and divalent cations, and can be subdivided into six main subfamilies: the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and the TRPA (ankyrin) groups. TRP channels are widely expressed in a large number of different tissues and cell types, and their biological roles appear to be equally diverse. In general, considered as polymodal cell sensors, they play a much more diverse role than anticipated. Functionally, TRP channels, when activated, cause cell depolarization, which may trigger a plethora of voltage-dependent ion channels. Upon stimulation, Ca2+ permeable TRP channels generate changes in the intracellular Ca2+ concentration, [Ca2+]i, by Ca2+ entry via the plasma membrane. However, more and more evidence is arising that TRP channels are also located in intracellular organelles and serve as intracellular Ca2+ release channels. This review focuses on three major tasks of TRP channels: (1) the function of TRP channels as Ca2+ entry channels; (2) the electrogenic actions of TRPs; and (3) TRPs as Ca2+ release channels in intracellular organelles.

An emerging role for NAADP-mediated Ca2+ signaling in the pancreatic β-cell

Several recent reports, including one in this journal, have reignited the debate about whether the calcium-mobilizing messenger, nicotinic adenine nucleotide diphosphate (NAADP) plays a central role in the regulation of calcium signalling in pancreatic β-cell. These studies have highlighted a role for NAADP-induced Ca(2+) mobilization not only in mediating the effects of the incretin, GLP-1 and the autocrine proliferative effects of insulin, but also possibly a fundamental role in glucose-mediated insulin secretion in the pancreatic β-cell.

Calcium- and polyphosphate-containing acidic granules of sea urchin eggs are similar to acidocalcisomes, but are not the targets for NAADP

Acidocalcisomes are acidic calcium-storage compartments described from bacteria to humans and characterized by their high content in poly P (polyphosphate), a linear polymer of many tens to hundreds of Pi residues linked by high-energy phosphoanhydride bonds. In the present paper we report that millimolar levels of short-chain poly P (in terms of Pi residues) and inorganic PPi are present in sea urchin extracts as detected using 31P-NMR, enzymatic determinations and agarose gel electrophoresis. Poly P was localized to granules randomly distributed in the sea urchin eggs, as shown by labelling with the poly-P-binding domain of Escherichia coli exopolyphosphatase. These granules were enriched using iodixanol centrifugation and shown to be acidic and to contain poly P, as determined by Acridine Orange and DAPI (4',6'-diamidino-2-phenylindole) staining respectively. These granules also contained large amounts of calcium, sodium, magnesium, potassium and zinc, as detected by X-ray microanalysis, and bafilomycin A1-sensitive ATPase, pyrophosphatase and exopolyphosphatase activities, as well as Ca2+/H+ and Na+/H+ exchange activities, being therefore similar to acidocalcisomes described in other organisms. Calcium release from these granules induced by nigericin was associated with poly P hydrolysis. Although NAADP (nicotinic acid-adenine dinucleotide phosphate) released calcium from the granule fraction, this activity was not significantly enriched as compared with the NAADP-stimulated calcium release from homogenates and was not accompanied by poly P hydrolysis. GPN (glycyl-L-phenylalanine-naphthylamide) released calcium when added to sea urchin homogenates, but was unable to release calcium from acidocalcisome-enriched fractions, suggesting that these acidic stores are not the targets for NAADP.

TPC2 proteins mediate nicotinic acid adenine dinucleotide phosphate (NAADP)- and agonist-evoked contractions of smooth muscle

Agonists such as those acting at muscarinic receptors are thought to induce contraction of smooth muscle primarily through inositol 1,4,5-trisphosphate production and release of Ca²⁺ from sarcoplasmic reticulum. However, the additional Ca²⁺-mobilizing messengers cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) may also be involved in this process, the former acting on the sarcoplasmic reticulum, the latter acting on lysosome-related organelles. In this study, we provide the first systematic analysis of the capacity of inositol 1,4,5-trisphosphate, cADPR, and NAADP to cause contraction in smooth muscle. Using permeabilized guinea pig detrusor and taenia caecum, we show that all three Ca²⁺-mobilizing messengers cause contractions in both types of smooth muscle. We demonstrate that cADPR and NAADP play differential roles in mediating contraction in response to muscarinic receptor activation, with a sizeable role for NAADP and acidic calcium stores in detrusor muscle but not in taenia caecum, underscoring the heterogeneity of smooth muscle signal transduction systems. Two-pore channel proteins (TPCs) have recently been shown to be key components of the NAADP receptor. We show that contractile responses to NAADP were completely abolished, and agonist-evoked contractions were reduced and now became independent of acidic calcium stores in Tpcn2^−/− mouse detrusor smooth muscle. Our findings provide the first evidence that TPC proteins mediate a key NAADP-regulated tissue response brought about by agonist activation of a cell surface receptor.

Characterization of two-pore channel 2 (TPCN2)-mediated Ca2+ currents in isolated lysosomes

Two-pore channels (TPCNs) have been proposed to form lysosomal Ca(2+) release channels that are activated by nicotinic acid adenine dinucleotide phosphate. Here, we employ a glass chip-based method to record for the first time nicotinic acid adenine dinucleotide phosphate -dependent currents through a two-pore channel (TPCN2) from intact lysosomes. We show that TPCN2 is a highly selective Ca(2+) channel that is regulated by intralysosomal pH. Using site-directed mutagenesis, we identify an amino acid residue in the putative pore region that is crucial for conferring high Ca(2+) selectivity. Our glass chip-based method will provide electrophysiological access not only to lysosomal TPCN channels but also to a broad range of other intracellular ion channels.

TPCs: Endolysosomal channels for Ca2+ mobilization from acidic organelles triggered by NAADP

Two-pore channels (TPCs or TPCNs) are novel members of the large superfamily of voltage-gated cation channels with slightly higher sequence homology to the pore-forming subunits of voltage-gated Ca(2+) and Na(+) channels than most other members. Recent studies demonstrate that TPCs locate to endosomes and lysosomes and form Ca(2+) release channels that respond to activation by the Ca(2+) mobilizing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP). With multiple endolysosomal targeted NAADP receptors now identified, important new insights into the regulation of endolysosomal function in health and disease will therefore be unveiled.

Degeneration of an intracellular ion channel in the primate lineage by relaxation of selective constraints

Ion channel genes are highly conserved and are rarely degenerated in the primate lineage leading to humans. So far, the only well-characterized ion channel known to be degenerated in primates is the plasma membrane transient receptor potential channel TRPC2, possibly due to changes in the pheromone signaling. Here, by analyzing the sequence data from ten primate species, we have determined the degeneration process of the TPC3 gene that encodes a member of the two-pore channel (TPC) family recently implicated in Ca(2+) release by nicotinic acid adenine dinucleotide phosphate from intracellular acidic stores in animals. We show that degeneration of TPC3 likely began in the common ancestors of Apes and Old World monkeys through a conserved inactivating mutation, followed by additional deleterious mutations resulting in the generation of a TPC3 pseudogene in the descendant catarrhine lineage. Located at a chromosome recombination hot spot, catarrhine TPC3 pseudogenes underwent a series of lineage-specific rearrangements, including exon deletion and duplication. In contrast, we identify near full-length TPC3 sequences in New World monkeys and Prosimians and show that the gene is subjected to strong purifying selection and therefore likely functional. Our data provide the first evidence for relaxed functional constraints for an intracellular ion channel in primates and shed novel insights into the evolution and regulation of Ca(2+) signaling in the primate lineage.

Purified TPC isoforms form NAADP receptors with distinct roles for Ca(2+) signaling and endolysosomal trafficking

Intracellular Ca²⁺ signals constitute key elements in signal transduction. Of the three major Ca²⁺ mobilizing messengers described, the most potent, nicotinic acid adenine dinucleotide phosphate (NAADP) is the least well understood in terms of its molecular targets [1]. Recently, we showed that heterologous expression of two-pore channel (TPC) proteins enhances NAADP-induced Ca²⁺ release, whereas the NAADP response was abolished in pancreatic beta cells from Tpcn2 gene knockout mice [2]. However, whether TPCs constitute native NAADP receptors is unclear. Here we show that immunopurified endogenous TPC complexes possess the hallmark properties ascribed to NAADP receptors, including nanomolar ligand affinity [3–5]. Our study also reveals important functional differences between the three TPC isoforms. Thus, TPC1 and TPC2 both mediate NAADP-induced Ca²⁺ release, but the subsequent amplification of this trigger Ca²⁺ by IP₃Rs is more tightly coupled for TPC2. In contrast, TPC3 expression suppressed NAADP-induced Ca²⁺ release. Finally, increased TPC expression has dramatic and contrasting effects on endolysosomal structures and dynamics, implicating a role for NAADP in the regulation of vesicular trafficking. We propose that NAADP regulates endolysosomal Ca²⁺ storage and release via TPCs and coordinates endoplasmic reticulum Ca²⁺ release in a role that impacts on Ca²⁺ signaling in health and disease [6].

A functional role for nicotinic acid adenine dinucleotide phosphate in oxytocin-mediated contraction of uterine smooth muscle from rat

Conventionally, G protein-coupled receptors are thought to increase calcium via inositol 1,4,5-trisphosphate (InsP(3)). More recent evidence shows that an alternative second messenger, nicotinic acid adenine dinucleotide phosphate (NAADP), also has a role to play, causing researchers to question established calcium releasing pathways. With the recent development, by our group, of cell-permeant NAADP (NAADP-aceteoxymethyl ester) and a selective NAADP receptor antagonist (Ned-19; 1-(3-((4-(2-fluorophenyl)piperazin-1-yl)methyl)-4-methoxyphenyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylic acid),the ability to investigate this signaling pathway has improved. Therefore, we investigated a role for NAADP in oxytocin-mediated responses in the rat uterus. Oxytocin- and NAADP-mediated effects were investigated by using contractile measurements of whole uterine strips from rat in organ baths. Responses were correlated to calcium release in cultured rat uterine smooth muscle cells measured by fluorescence microscopy. Inhibition of both oxytocin-induced contraction and calcium release by the traditional NAADP-signaling disrupter bafilomycin and the NAADP receptor antagonist Ned-19 clearly demonstrated a role for NAADP in oxytocin-induced signaling. A cell-permeant form of NAADP was able to produce both uterine contractions and calcium release. This response was unaffected by depletion of sarcoplasmic reticulum stores with thapsigargin, but was abolished by both bafilomycin and Ned-19. Crucially, oxytocin stimulated an increase in NAADP in rat uterine tissue. The present study demonstrates directly that NAADP signaling plays a role in rat uterine contractions. Moreover, investigation of this signaling pathway highlights yet another component of oxytocin-mediated signaling, stressing the need to consider the action of new components as they are discovered, even in signaling pathways that are thought to be well established.

Acidic calcium stores open for business: expanding the potential for intracellular Ca2+ signaling

Changes in cytosolic calcium concentration are crucial for a variety of cellular processes in all cells. It has long been appreciated that calcium is stored and released from intracellular calcium stores such as the endoplasmic reticulum. However, emerging evidence indicates that calcium is also dynamically regulated by a seemingly disparate collection of acidic organelles. In this paper, we review the defining features of these 'acidic calcium stores' and highlight recent progress in understanding the mechanisms of uptake and release of calcium from these stores. We also examine the nature of calcium buffering within the stores, and summarize the physiological and pathophysiological significance of these ubiquitous organelles in calcium signaling.

Transient receptor potential channelopathies

In the past years, several hereditary diseases caused by defects in transient receptor potential channels (TRP) genes have been described. This review summarizes our current knowledge about TRP channelopathies and their possible pathomechanisms. Based on available genetic indications, we will also describe several putative pathological conditions in which (mal)function of TRP channels could be anticipated.

An ancestral deuterostome family of two-pore channels mediates nicotinic acid adenine dinucleotide phosphate-dependent calcium release from acidic organelles

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent and widespread calcium-mobilizing messenger, the properties of which have been most extensively described in sea urchin eggs. The molecular basis for calcium release by NAADP, however, is not clear and subject to controversy. Recent studies have provided evidence that members of the two-pore channel (TPC) family in mammals are the long sought after target channels for NAADP. Here, we show that the TPC3 gene, which has yet to be functionally characterized, is present throughout the deuterostome lineage but is a pseudogene in humans and other primates. We report the molecular cloning of the complete ancestral TPC gene family from the sea urchin and demonstrate that all three isoforms localize to acidic organelles to mediate NAADP-dependent calcium release. Our data highlight the functional divergence of this novel gene family during deuterostome evolution and provide further evidence that NAADP mediates calcium release from acidic stores through activation of TPCs.

The tissue-specific expression of TRPML2 (MCOLN-2) gene is influenced by the presence of TRPML1

Mucolipidosis type IV is a lysosomal storage disorder caused by the loss or dysfunction of the mucolipin-1 (TRPML1) protein. It has been suggested that TRPML2 could genetically compensate (i.e., become upregulated) for the loss of TRPML1. We thus investigated this possibility by first studying the expression pattern of mouse TRPML2 and its basic channel properties using the varitint-waddler (Va) model. Here, we confirmed the presence of long variant TRPML2 (TRPML2lv) and short variant (TRPML2sv) isoforms. We showed for the first time that, heterologously expressed, TRPML2lv-Va is an active, inwardly rectifying channel. Secondly, we quantitatively measured TRPML2 and TRPML3 mRNA expressions in TRPML1-/- null and wild-type (Wt) mice. In wild-type mice, the TRPML2lv transcripts were very low while TRPML2sv and TRPML3 transcripts have predominant expressions in lymphoid and kidney organs. Significant reductions of TRPML2sv, but not TRPML2lv or TRPML3 transcripts, were observed in lymphoid and kidney organs of TRPML1-/- mice. RNA interference of endogenous human TRPML1 in HEK-293 cells produced a comparable decrease of human TRPML2 transcript levels that can be restored by overexpression of human TRPML1. Conversely, significant upregulation of TRPML2sv transcripts was observed when primary mouse lymphoid cells were treated with nicotinic acid adenine dinucleotide phosphate, or N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinoline sulfonamide, both known activators of TRPML1. In conclusion, our results indicate that TRPML2 is unlikely to compensate for the loss of TRPML1 in lymphoid or kidney organs and that TRPML1 appears to play a novel role in the tissue-specific transcriptional regulation of TRPML2.

Calcium signaling via two-pore channels: local or global, that is the question

Recently, we identified, for the first time, two-pore channels (TPCs, TPCN for gene name) as a novel family of nicotinic acid adenine dinucleotide phosphate (NAADP)-gated, endolysosome-targeted calcium release channels. Significantly, three subtypes of TPCs have been characterized, TPC1-3, with each being targeted to discrete acidic calcium stores, namely lysosomes (TPC2) and endosomes (TPC1 and TPC3). That TPCs act as NAADP-gated calcium release channels is clear, given that NAADP binds to high- and low-affinity sites associated with TPC2 and thereby induces calcium release and homologous desensitization, as observed in the case of endogenous NAADP receptors. Moreover, NAADP-evoked calcium signals via TPC2 are ablated by short hairpin RNA knockdown of TPC2 and by depletion of acidic calcium stores with bafilomycin. Importantly, however, NAADP-evoked calcium signals were biphasic in nature, with an initial phase of calcium release from lysosomes via TPC2, being subsequently amplified by calcium-induced calcium release (CICR) from the endoplasmic reticulum (ER). In marked contrast, calcium release via endosome-targeted TPC1 induced only spatially restricted calcium signals that were not amplified by CICR from the ER. These findings provide new insights into the mechanisms that cells may utilize to "filter" calcium signals via junctional complexes to determine whether a given signal remains local or is converted into a propagating global signal. Essentially, endosomes and lysosomes represent vesicular calcium stores, quite unlike the ER network, and TPCs do not themselves support CICR or, therefore, propagating regenerative calcium waves. Thus "quantal" vesicular calcium release via TPCs must subsequently recruit inositol 1,4,5-trisphoshpate receptors and/or ryanodine receptors on the ER by CICR to evoke a propagating calcium wave. This may call for a revision of current views on the mechanisms of intracellular calcium signaling. The purpose of this review is, therefore, to provide an appropriate framework for future studies in this area.

Analogues of the nicotinic acid adenine dinucleotide phosphate (NAADP) antagonist Ned-19 indicate two binding sites on the NAADP receptor

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a Ca(2+)-releasing messenger. Biological data suggest that its receptor has two binding sites: one high-affinity locking site and one low-affinity opening site. To directly address the presence and function of these putative binding sites, we synthesized and tested analogues of the NAADP antagonist Ned-19. Ned-19 itself inhibits both NAADP-mediated Ca(2+) release and NAADP binding. A fluorometry bioassay was used to assess NAADP-mediated Ca(2+) release, whereas a radioreceptor assay was used to assess binding to the NAADP receptor (only at the high-affinity site). In Ned-20, the fluorine is para rather than ortho as in Ned-19. Ned-20 does not inhibit NAADP-mediated Ca(2+) release but inhibits NAADP binding. Conversely, Ned-19.4 (a methyl ester of Ned-19) inhibits NAADP-mediated Ca(2+) release but cannot inhibit NAADP binding. Furthermore, Ned-20 prevents the self-desensitization response characteristic of NAADP in sea urchin eggs, confirming that this response is mediated by a high-affinity allosteric site to which NAADP binds in the radioreceptor assay. Collectively, these data provide the first direct evidence for two binding sites (one high- and one low-affinity) on the NAADP receptor.