[Show abstract][Hide abstract] ABSTRACT: Two related neurodegenerative disorders, Western Pacific amyotrophic lateral sclerosis (ALS) and parkinsonism-dementia (PD), originally occurred at a high incidence on Guam, in the Kii peninsula of Japan, and in southern West New Guinea more than 50 years ago. These three foci shared a unique mineral environment characterized by the presence of severely low levels of Ca(2+) and Mg(2+), coupled with high levels of bioavailable transition metals in the soil and drinking water. Epidemiological studies suggest that genetic factors also contribute to the etiology of these disorders. Here, we report that a variant of the transient receptor potential melastatin 2 (TRPM2) gene may confer susceptibility to these diseases. TRPM2 encodes a calcium-permeable cation channel highly expressed in the brain that has been implicated in mediating cell death induced by oxidants. We found a heterozygous variant of TRPM2 in a subset of Guamanian ALS (ALS-G) and PD (PD-G) cases. This variant, TRPM2(P1018L), produces a missense change in the channel protein whereby proline 1018 (Pro(1018)) is replaced by leucine (Leu(1018)). Functional studies revealed that, unlike WT TRPM2, P1018L channels inactivate. Our results suggest that the ability of TRPM2 to maintain sustained ion influx is a physiologically important function and that its disruption may, under certain conditions, contribute to disease states.
Full-text · Article · Dec 2008 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Recent findings implicating TRPM7 and TRPM2 in oxidative stress-induced neuronal death thrust these channels into the spotlight as possible therapeutic targets for neurodegenerative diseases. In this review, we describe how the functional properties of TRPM7 and TRPM2 are interconnected with calcium (Ca(2+)) and magnesium (Mg(2+)) homeostasis, oxidative stress, mitochondrial dysfunction, and immune mechanisms, all principal suspects in neurodegeneration. We focus our discussion on Western Pacific Amyotrophic Lateral Sclerosis (ALS) and Parkinsonism Dementia (PD) because extensive studies conducted over the years strongly suggest that these diseases are ideal candidates for a gene-environment model of etiology. The unique mineral environment identified in connection with Western Pacific ALS and PD, low Mg(2+) and Ca(2+), yet high in transition metals, creates a condition that could affect the proper function of these two channels.
Preview · Article · Aug 2007 · Biochimica et Biophysica Acta
[Show abstract][Hide abstract] ABSTRACT: Guamanian amyotrophic lateral sclerosis (ALS-G) and parkinsonism dementia (PD-G) have been epidemiologically linked to an environment severely deficient in calcium (Ca2+) and magnesium (Mg2+). Transient receptor potential melastatin 7 (TRPM7) is a bifunctional protein containing both channel and kinase domains that has been proposed to be involved in the homeostatic regulation of intracellular Ca2+, Mg2+, and trace metal ion concentration. There is evidence that TRPM7 is constitutively active and that the number of available channels is dependent on intracellular free Mg2+ levels. We found a TRPM7 variant in a subset of ALS-G and PD-G patients that produces a protein with a missense mutation, T1482I. Recombinant T1482I TRPM7 exhibits the same kinase catalytic activity as WT TRPM7. However, heterologously expressed T1482I TRPM7 produces functional channels that show an increased sensitivity to inhibition by intracellular Mg2+. Because the incidence of ALS-G and PD-G has been associated with prolonged exposure to an environment severely deficient in Ca2+ and Mg2+, we propose that this variant TRPM7 allele confers a susceptibility genotype in such an environment. This study represents an initial attempt to address the important issue of gene-environment interactions in the etiology of these diseases.
Full-text · Article · Sep 2005 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Trace metal ions such as Zn(2+), Fe(2+), Cu(2+), Mn(2+), and Co(2+) are required cofactors for many essential cellular enzymes, yet little is known about the mechanisms through which they enter into cells. We have shown previously that the widely expressed ion channel TRPM7 (LTRPC7, ChaK1, TRP-PLIK) functions as a Ca(2+)- and Mg(2+)-permeable cation channel, whose activity is regulated by intracellular Mg(2+) and Mg(2+).ATP and have designated native TRPM7-mediated currents as magnesium-nucleotide-regulated metal ion currents (MagNuM). Here we report that heterologously overexpressed TRPM7 in HEK-293 cells conducts a range of essential and toxic divalent metal ions with strong preference for Zn(2+) and Ni(2+), which both permeate TRPM7 up to four times better than Ca(2+). Similarly, native MagNuM currents are also able to support Zn(2+) entry. Furthermore, TRPM7 allows other essential metals such as Mn(2+) and Co(2+) to permeate, and permits significant entry of nonphysiologic or toxic metals such as Cd(2+), Ba(2+), and Sr(2+). Equimolar replacement studies substituting 10 mM Ca(2+) with the respective divalent ions reveal a unique permeation profile for TRPM7 with a permeability sequence of Zn(2+) approximately Ni(2+) > Ba(2+) > Co(2+) > Mg(2+) >/= Mn(2+) >/= Sr(2+) >/= Cd(2+) >/= Ca(2+), while trivalent ions such as La(3+) and Gd(3+) are not measurably permeable. With the exception of Mg(2+), which exerts strong negative feedback from the intracellular side of the pore, this sequence is faithfully maintained when isotonic solutions of these divalent cations are used. Fura-2 quenching experiments with Mn(2+), Co(2+), or Ni(2+) suggest that these can be transported by TRPM7 in the presence of physiological levels of Ca(2+) and Mg(2+), suggesting that TRPM7 represents a novel ion-channel mechanism for cellular metal ion entry into vertebrate cells.
Full-text · Article · Jan 2003 · The Journal of General Physiology
[Show abstract][Hide abstract] ABSTRACT: Capacitative Ca(2+) entry (CCE) activated by release/depletion of Ca(2+) from internal stores represents a major Ca(2+) influx mechanism in lymphocytes and other nonexcitable cells. Despite the importance of CCE in antigen-mediated lymphocyte activation, molecular components constituting this mechanism remain elusive. Here we demonstrate that genetic disruption of transient receptor potential (TRP)1 significantly attenuates both Ca(2+) release-activated Ca(2+) currents and inositol 1,4,5-trisphosphate (IP(3))-mediated Ca(2+) release from endoplasmic reticulum (ER) in DT40 B cells. As a consequence, B cell antigen receptor-mediated Ca(2+) oscillations and NF-AT activation are reduced in TRP1-deficient cells. Thus, our results suggest that CCE channels, whose formation involves TRP1 as an important component, modulate IP(3) receptor function, thereby enhancing functional coupling between the ER and plasma membrane in transduction of intracellular Ca(2+) signaling in B lymphocytes.
Full-text · Article · Apr 2002 · Journal of Experimental Medicine
[Show abstract][Hide abstract] ABSTRACT: Rat basophilic leukaemia cells (RBL-2H3-M1) were used to study the characteristics of the store-operated Ca(2+) release-activated Ca(2+) current (I(CRAC)) and the magnesium-nucleotide-regulated metal cation current (MagNuM) (which is conducted by the LTRPC7 channel). Pipette solutions containing 10 mM BAPTA and no added ATP induced both currents in the same cell, but the time to half-maximal activation for MagNuM was about two to three times slower than that of I(CRAC). Differential suppression of I(CRAC) was achieved by buffering free [Ca(2+)](i) to 90 nM and selective inhibition of MagNuM was accomplished by intracellular solutions containing 6 mM Mg.ATP, 1.2 mM free [Mg(2+)](i) or 100 microM GTP-gamma-S, allowing investigations on these currents in relative isolation. Removal of extracellular Ca(2+) and Mg(2+) caused both currents to be carried significantly by monovalent ions. In the absence or presence of free [Mg(2+)](i), I(CRAC) carried by monovalent ions inactivated more rapidly and more completely than MagNuM carried by monovalent ions. Since several studies have used divalent-free solutions on either side of the membrane to study selectivity and single-channel behaviour of I(CRAC), these experimental conditions would have favoured the contribution of MagNuM to monovalent conductance and call for caution in interpreting results where both I(CRAC) and MagNuM are activated.
Full-text · Article · Apr 2002 · The Journal of Physiology
[Show abstract][Hide abstract] ABSTRACT: Rat basophilic leukaemia cells (RBL-2H3-M1) were used to study the characteristics of the store-operated Ca2+ release-activated Ca2+ current (ICRAC) and the magnesium-nucleotide-regulated metal cation current (MagNuM) (which is conducted by the LTRPC7 channel). Pipette solutions containing 10 mm BAPTA and no added ATP induced both currents in the same cell, but the time to half-maximal activation for MagNuM was about two to three times slower than that of ICRAC. Differential suppression of ICRAC was achieved by buffering free [Ca2+]i to 90 nm and selective inhibition of MagNuM was accomplished by intracellular solutions containing 6 mm Mg.ATP, 1.2 mm free [Mg2+]i or 100 μm GTP-γ-S, allowing investigations on these currents in relative isolation. Removal of extracellular Ca2+ and Mg2+ caused both currents to be carried significantly by monovalent ions. In the absence or presence of free [Mg2+]i, ICRAC carried by monovalent ions inactivated more rapidly and more completely than MagNuM carried by monovalent ions. Since several studies have used divalent-free solutions on either side of the membrane to study selectivity and single-channel behaviour of ICRAC, these experimental conditions would have favoured the contribution of MagNuM to monovalent conductance and call for caution in interpreting results where both ICRAC and MagNuM are activated.
Full-text · Article · Feb 2002 · The Journal of Physiology
[Show abstract][Hide abstract] ABSTRACT: The molecular mechanisms that regulate basal or background entry of divalent cations into mammalian cells are poorly understood. Here we describe the cloning and functional characterization of a Ca2+- and Mg2+-permeable divalent cation channel, LTRPC7 (nomenclature compatible with that proposed in ref. 1), a new member of the LTRPC family of putative ion channels. Targeted deletion of LTRPC7 in DT-40 B cells was lethal, indicating that LTRPC7 has a fundamental and nonredundant role in cellular physiology. Electrophysiological analysis of HEK-293 cells overexpressing recombinant LTRPC7 showed large currents regulated by millimolar levels of intracellular Mg.ATP and Mg.GTP with the permeation properties of a voltage-independent divalent cation influx pathway. Analysis of several cultured cell types demonstrated small magnesium-nucleotide-regulated metal ion currents (MagNuM) with regulation and permeation properties essentially identical to the large currents observed in cells expressing recombinant LTRPC7. Our data indicate that LTRPC7, by virtue of its sensitivity to physiological Mg.ATP levels, may be involved in a fundamental process that adjusts plasma membrane divalent cation fluxes according to the metabolic state of the cell.
[Show abstract][Hide abstract] ABSTRACT: Receptor-mediated generation of inositol 1,4,5-trisphosphate (InsP3) initiates Ca2+ release from intracellular stores and the subsequent activation of store-operated calcium influx. InsP3 is metabolized within seconds by 5-phosphatase and 3-kinase, yielding Ins(1,4)P2 and inositol 1,3,4,5-tetrakisphosphate (InsP4), respectively. Some studies have suggested that InsP4 controls Ca2+ influx in combination with InsP3 (refs 3 and 4), but another study did not find the same result. Some of the apparent conflicts between these previous studies have been resolved; however, the physiological function of InsP4 remains elusive. Here we have investigated the function of InsP4 in Ca2+ influx in the mast cell line RBL-2H3, and we show that InsP4 inhibits InsP3 metabolism through InsP3 5-phosphatase, thereby facilitating the activation of the store-operated Ca2+ current I(CRAC) (ref. 9). Physiologically, this mechanism opens a discriminatory time window for coincidence detection that enables selective facilitation of Ca2+ influx by appropriately timed low-level receptor stimulation. At higher concentrations, InsP4 acts as an inhibitor of InsP3 receptors, enabling InsP4 to act as a potent bi-modal regulator of cellular sensitivity to InsP3, which provides both facilitatory and inhibitory feedback on Ca2+ signalling.
[Show abstract][Hide abstract] ABSTRACT: Extrait Des résultats récents qui impliquent TRPM7 et TRPM2 dans la morte neurone induite par stress oxydatif, misent ces canaux dans la lumière de jour comme objectifs thérapeutiques possibles pour des maladies neurodégénératives. Dans ce rapport, nous décrivons comment les caractéristiques fonctionnelles de TRPM7 et TRPM2 sont interconnectés avec calcium (Ca 2+) et magnésium (Mg 2+), homéostasie, stress oxydatif, dysfonction mitochondriale, et mécanismes immuns, tous des suspects douteux dans la neurodégénération. Nous focalisons notre discussion sur la Sclérose Latérale Amyotrophique (SLA) Pacifique Occidentale, et Parkinsonisme Démence (PD) parce que des études extensives qui sont fait pendant des années, suggèrent fortement que ces maladies sont des candidats idéales pour un modèle d'environnement-gène d'étiologie. L'environnement minérale unique identifié en connexion avec la SLA Pacifique Occidentale et PD -Mg 2+ et Ca 2+ basse, bien que haut dans des métaux de transition, crée une condition qui pourrait endommager le bon fonctionnement de ces deux canaux.