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

Regulation of ion channels by pyridine nucleotides

Recent research suggests that in addition to their role as soluble electron carriers, pyridine nucleotides [NAD(P)(H)] also regulate ion transport mechanisms. This mode of regulation seems to have been conserved through evolution. Several bacterial ion-transporting proteins or their auxiliary subunits possess nucleotide-binding domains. In eukaryotes, the Kv1 and Kv4 channels interact with pyridine nucleotide-binding β-subunits that belong to the aldo-keto reductase superfamily. Binding of NADP(+) to Kvβ removes N-type inactivation of Kv currents, whereas NADPH stabilizes channel inactivation. Pyridine nucleotides also regulate Slo channels by interacting with their cytosolic regulator of potassium conductance domains that show high sequence homology to the bacterial TrkA family of K(+) transporters. These nucleotides also have been shown to modify the activity of the plasma membrane K(ATP) channels, the cystic fibrosis transmembrane conductance regulator, the transient receptor potential M2 channel, and the intracellular ryanodine receptor calcium release channels. In addition, pyridine nucleotides also modulate the voltage-gated sodium channel by supporting the activity of its ancillary subunit-the glycerol-3-phosphate dehydrogenase-like protein. Moreover, the NADP(+) metabolite, NAADP(+), regulates intracellular calcium homeostasis via the 2-pore channel, ryanodine receptor, or transient receptor potential M2 channels. Regulation of ion channels by pyridine nucleotides may be required for integrating cell ion transport to energetics and for sensing oxygen levels or metabolite availability. This mechanism also may be an important component of hypoxic pulmonary vasoconstriction, memory, and circadian rhythms, and disruption of this regulatory axis may be linked to dysregulation of calcium homeostasis and cardiac arrhythmias.

Nicotinic acid adenine dinucleotide phosphate (NAADP) activates global and heterogeneous local Ca2+ signals from NAADP- and ryanodine receptor-gated Ca2+ stores in pulmonary arterial myocytes

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent Ca(2+)-mobilizing messenger that releases Ca(2+) from endolysosomal organelles. Recent studies showed that NAADP-induced Ca(2+) release is mediated by the two-pore channels (TPCs) TPC1 and TPC2. However, the expression of TPCs and the NAADP-induced local Ca(2+) signals have not been examined in vascular smooth muscle. Here, we found that both TPC1 and TPC2 are expressed in rat pulmonary arterial smooth muscle cells (PASMCs), with TPC1 being the major subtype. Application of membrane-permeant NAADP acetoxymethyl ester to PASMCs elicited a biphasic increase in global [Ca(2+)]i, which was independent of extracellular Ca(2+) and blocked by the NAADP antagonist Ned-19 or the vacuolar H(+)-ATPase inhibitor bafilomycin A1, indicating Ca(2+) release from acidic endolysosomal Ca(2+) stores. The Ca(2+) response was unaffected by xestospongin C but was partially blocked by ryanodine or thapsigargin. NAADP triggered heterogeneous local Ca(2+) signals, including a diffuse increase in cytosolic [Ca(2+)], Ca(2+) sparks, Ca(2+) bursts, and regenerative Ca(2+) release. The diffuse Ca(2+) increase and Ca(2+) bursts were ryanodine-insensitive, presumably arising from different endolysosomal sources. Ca(2+) sparks and regenerative Ca(2+) release were inhibited by ryanodine, consistent with cross-activation of loosely coupled ryanodine receptors. Moreover, Ca(2+) release stimulated by endothelin-1 was inhibited by Ned-19, ryanodine, or xestospongin C, suggesting that NAADP-mediated Ca(2+) signals interact with both ryanodine and inositol 1,4,5-trisphosphate receptors during agonist stimulation. Our results show that NAADP mediates complex global and local Ca(2+) signals. Depending on the physiological stimuli, these diverse Ca(2+) signals may serve to regulate different cellular functions in PASMCs.

Identification of new therapeutic targets by genome-wide analysis of gene expression in the ipsilateral cortex of aged rats after stroke

Background

Because most human stroke victims are elderly, studies of experimental stroke in the aged rather than the young rat model may be optimal for identifying clinically relevant cellular responses, as well for pinpointing beneficial interventions.

Methodology/Principal Findings

We employed the Affymetrix platform to analyze the whole-gene transcriptome following temporary ligation of the middle cerebral artery in aged and young rats. The correspondence, heat map, and dendrogram analyses independently suggest a differential, age-group-specific behaviour of major gene clusters after stroke. Overall, the pattern of gene expression strongly suggests that the response of the aged rat brain is qualitatively rather than quantitatively different from the young, i.e. the total number of regulated genes is comparable in the two age groups, but the aged rats had great difficulty in mounting a timely response to stroke. Our study indicates that four genes related to neuropathic syndrome, stress, anxiety disorders and depression (Acvr1c, Cort, Htr2b and Pnoc) may have impaired response to stroke in aged rats. New therapeutic options in aged rats may also include Calcrl, Cyp11b1, Prcp, Cebpa, Cfd, Gpnmb, Fcgr2b, Fcgr3a, Tnfrsf26, Adam 17 and Mmp14. An unexpected target is the enzyme 3-hydroxy-3-methylglutaryl-Coenzyme A synthase 1 in aged rats, a key enzyme in the cholesterol synthesis pathway. Post-stroke axonal growth was compromised in both age groups.

Conclusion/Significance

We suggest that a multi-stage, multimodal treatment in aged animals may be more likely to produce positive results. Such a therapeutic approach should be focused on tissue restoration but should also address other aspects of patient post-stroke therapy such as neuropathic syndrome, stress, anxiety disorders, depression, neurotransmission and blood pressure.

Identification of two-pore channel 2 as a novel regulator of osteoclastogenesis

Osteoclast differentiation is one of the critical steps that control bone mass levels in osteoporosis, but the molecules involved in osteoclastogenesis are still incompletely understood. Here, we show that two-pore channel 2 (TPC2) is expressed in osteoclast precursor cells, and its knockdown (TPC2-KD) in these cells suppressed RANKL-induced key events including multinucleation, enhancement of tartrate-resistant acid phosphatase (TRAP) activities, and TRAP mRNA expression levels. With respect to intracellular signaling, TPC2-KD reduced the levels of the RANKL-induced dynamic waving of Ca(2+) in RAW cells. The search for the target of TPC2 identified that nuclear localization of NFATc1 is retarded in TPC2-KD cells. Finally, TPC2-KD suppressed osteoclastic pit formation in cultures. We conclude that TPC2 is a novel critical molecule for osteoclastogenesis.

Cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate (NAADP) as messengers for calcium mobilization

Cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate were discovered >2 decades ago. That they are second messengers for mobilizing Ca(2+) stores has since been firmly established. Separate stores and distinct Ca(2+) channels are targeted, with cyclic ADP-ribose acting on the ryanodine receptors in the endoplasmic reticulum, whereas nicotinic acid adenine dinucleotide phosphate mobilizes the endolysosomes via the two-pore channels. Despite the structural and functional differences, both messengers are synthesized by a ubiquitous enzyme, CD38, whose crystal structure and catalytic mechanism have now been well elucidated. How this novel signaling enzyme is regulated remains largely unknown and is the focus of this minireview.

The Molecular Basis for Ca2+ Signalling by NAADP: Two-Pore Channels in a Complex?

NAADP is a potent Ca²⁺ mobilizing messenger in a variety of cells but its molecular mechanism of action is incompletely understood. Accumulating evidence indicates that the poorly characterized two-pore channels (TPCs) in animals are NAADP sensitive Ca²⁺-permeable channels. TPCs localize to the endo-lysosomal system but are functionally coupled to the better characterized endoplasmic reticulum Ca²⁺ channels to generate physiologically relevant complex Ca²⁺ signals. Whether TPCs directly bind NAADP is not clear. Here we discuss the idea based on recent studies that TPCs are the pore-forming subunits of a protein complex that includes tightly associated, low molecular weight NAADP-binding proteins.

Triggering of Ca2+ signals by NAADP-gated two-pore channels: a role for membrane contact sites?

NAADP (nicotinic acid-adenine dinucleotide phosphate) is a potent Ca2+-mobilizing messenger implicated in many Ca2+-dependent cellular processes. It is highly unusual in that it appears to trigger Ca2+ release from acidic organelles such as lysosomes. These signals are often amplified by archetypal Ca2+ channels located in the endoplasmic reticulum. Recent studies have converged on the TPCs (two-pore channels) which localize to the endolysosomal system as the likely primary targets through which NAADP mediates its effects. 'Chatter' between TPCs and endoplasmic reticulum Ca2+ channels is disrupted when TPCs are directed away from the endolysosomal system. This suggests that intracellular Ca2+ release channels may be closely apposed, possibly at specific membrane contact sites between acidic organelles and the endoplasmic reticulum.

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 on target

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent intracellular Ca(2+)-mobilising messenger. Much evidence indicates that NAADP targets novel Ca(2+) channels located on acidic organelles but the identity of these channels has remained obscure. Recent studies have converged on a novel class of ion channels, the two-pore channels (TPCs) as likely molecular targets. The location of these channels to the endo-lysosomal system and their sensitivity to NAADP match closely those of endogenous NAADP-sensitive channels in both mammalian cells and sea urchin eggs, where the effects of NAADP were discovered. Moreover, the functional coupling of TPCs to archetypal endoplasmic reticulum (ER) Ca(2+) channels is also matched. Biophysical analysis in conjunction with site-directed mutagenesis demonstrates that TPCs are pore-forming subunits of NAADP-gated ion channels. TPCs have a unique two-repeat structure, are regulated by N-linked glycosylation and harbor an endo-lysosomal targeting motif in their N-terminus. Knockdown studies have shown TPCs to regulate smooth muscle contraction, differentiation and endothelial cell activation consistent with previous studies implicating NAADP in these processes. Thus multiple lines of evidence indicate that TPCs are the likely long sought targets for NAADP.

Pyridine nucleotide metabolites and calcium release from intracellular stores

Ca(2+) signals are probably the most common intracellular signaling elements, controlling an extensive range of responses in virtually all cells. Many cellular stimuli, often acting at cell surface receptors, evoke Ca(2+) signals by mobilizing Ca(2+) from intracellular stores. Inositol trisphosphate (IP₃) was the first messenger shown to link events at the plasma membrane to release of Ca(2+) from the endoplasmic reticulum (ER), through activation of IP₃-gated Ca(2+) release channels (IP₃ receptors). Subsequently, two additional Ca(2+) mobilizing messengers were discovered, cADPR and NAADP. Both are metabolites of pyridine nucleotides, and may be produced by the same class of enzymes, ADP-ribosyl cyclases, such as CD38. Whilst cADPR mobilizes Ca(2+) from the ER by activation of ryanodine receptors (RyRs), NAADP releases Ca(2+) from acidic stores by a mechanism involving the activation of two pore channels (TPCs).

Molecular mechanisms of endolysosomal Ca2+ signalling in health and disease

Endosomes, lysosomes and lysosome-related organelles are emerging as important Ca2+ storage cellular compartments with a central role in intracellular Ca2+ signalling. Endocytosis at the plasma membrane forms endosomal vesicles which mature to late endosomes and culminate in lysosomal biogenesis. During this process, acquisition of different ion channels and transporters progressively changes the endolysosomal luminal ionic environment (e.g. pH and Ca2+) to regulate enzyme activities, membrane fusion/fission and organellar ion fluxes, and defects in these can result in disease. In the present review we focus on the physiology of the inter-related transport mechanisms of Ca2+ and H+ across endolysosomal membranes. In particular, we discuss the role of the Ca2+-mobilizing messenger NAADP (nicotinic acid adenine dinucleotide phosphate) as a major regulator of Ca2+ release from endolysosomes, and the recent discovery of an endolysosomal channel family, the TPCs (two-pore channels), as its principal intracellular targets. Recent molecular studies of endolysosomal Ca2+ physiology and its regulation by NAADP-gated TPCs are providing exciting new insights into the mechanisms of Ca2+-signal initiation that control a wide range of cellular processes and play a role in disease. These developments underscore a new central role for the endolysosomal system in cellular Ca2+ regulation and signalling.

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.

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.

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.

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.

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.

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.

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.