TRPM2 and TRPM7: channel/enzyme fusions to generate novel intracellular sensors

TRPM2-mediated Ca2+ signaling as a potential therapeutic target in cancer treatment: an updated review of its role in survival and proliferation of cancer cells

The transient receptor potential melastatin subfamily member 2 (TRPM2), a thermo and reactive oxygen species (ROS) sensitive Ca²⁺-permeable cation channel has a vital role in surviving the cell as well as defending the adaptability of various cell groups during and after oxidative stress. It shows higher expression in several cancers involving breast, pancreatic, prostate, melanoma, leukemia, and neuroblastoma, indicating it raises the survivability of cancerous cells. In various cancers including gastric cancers, and neuroblastoma, TRPM2 is known to conserve viability, and several underlying mechanisms of action have been proposed. Transcription factors are thought to activate TRPM2 channels, which is essential for cell proliferation and survival. In normal physiological conditions with an optimal expression of TRPM2, mitochondrial ROS is produced in optimal amounts while regulation of antioxidant expression is carried on. Depletion of TRPM2 overexpression or activity has been shown to improve ischemia-reperfusion injury in organ levels, reduce tumor growth and/or viability of various malignant cancers like breast, gastric, pancreatic, prostate, head and neck cancers, melanoma, neuroblastoma, T-cell and acute myelogenous leukemia. This updated and comprehensive review also analyzes the mechanisms by which TRPM2-mediated Ca²⁺ signaling can regulate the growth and survival of different types of cancer cells. Based on the discussion of the available data, it can be concluded that TRPM2 may be a unique therapeutic target in the treatment of several types of cancer. Video Abstract.

The role of endothelial TRP channels in age-related vascular cognitive impairment and dementia

Transient receptor potential (TRP) proteins are part of a superfamily of polymodal cation channels that can be activated by mechanical, physical, and chemical stimuli. In the vascular endothelium, TRP channels regulate two fundamental parameters: the membrane potential and the intracellular Ca²⁺ concentration [(Ca²⁺)_i]. TRP channels are widely expressed in the cerebrovascular endothelium, and are emerging as important mediators of several brain microvascular functions (e.g., neurovascular coupling, endothelial function, and blood-brain barrier permeability), which become impaired with aging. Aging is the most significant risk factor for vascular cognitive impairment (VCI), and the number of individuals affected by VCI is expected to exponentially increase in the coming decades. Yet, there are currently no preventative or therapeutic treatments available against the development and progression of VCI. In this review, we discuss the involvement of endothelial TRP channels in diverse physiological processes in the brain as well as in the pathogenesis of age-related VCI to explore future potential neuroprotective strategies.

A structural overview of the ion channels of the TRPM family

The TRPM (transient receptor potential melastatin) family belongs to the superfamily of TRP cation channels. The TRPM subfamily is composed of eight members that are involved in diverse biological functions such as temperature sensing, inflammation, insulin secretion, and redox sensing. Since the first cloning of TRPM1 in 1998, tremendous progress has been made uncovering the function, structure, and pharmacology of this family. Complete structures of TRPM2, TRPM4, and TRPM8, as well as a partial structure of TRPM7, have been determined by cryo-EM, providing insights into their channel assembly, ion permeation, gating mechanisms, and structural pharmacology. Here we summarize the current knowledge about channel structure, emphasizing general features and principles of the structure of TRPM channels discovered since 2017. We also discuss some of the key unresolved issues in the field, including the molecular mechanisms underlying voltage and temperature dependence, as well as the functions of the TRPM channels' C-terminal domains.

Bioelectric Control of Metastasis in Solid Tumors

As the leading cause of death in cancer, there is an urgent need to develop treatments to target the dissemination of primary tumor cells to secondary organs, known as metastasis. Bioelectric signaling has emerged in the last century as an important controller of cell growth, and with the development of current molecular tools we are now beginning to identify its role in driving cell migration and metastasis in a variety of cancer types. This review summarizes the currently available research for bioelectric signaling in solid tumor metastasis. We review the steps of metastasis and discuss how these can be controlled by bioelectric cues at the level of a cell, a population of cells, and the tissue. The role of ion channel, pump, and exchanger activity and ion flux is discussed, along with the importance of the membrane potential and the relationship between ion flux and membrane potential. We also provide an overview of the evidence for control of metastasis by external electric fields (EFs) and draw from examples in embryogenesis and regeneration to discuss the implications for endogenous EFs. By increasing our understanding of the dynamic properties of bioelectric signaling, we can develop new strategies that target metastasis to be translated into the clinic.

TRP Channels in Angiogenesis and Other Endothelial Functions

Angiogenesis is the growth of blood vessels mediated by proliferation, migration, and spatial organization of endothelial cells. This mechanism is regulated by a balance between stimulatory and inhibitory factors. Proangiogenic factors include a variety of VEGF family members, while thrombospondin and endostatin, among others, have been reported as suppressors of angiogenesis. Transient receptor potential (TRP) channels belong to a superfamily of cation-permeable channels that play a relevant role in a number of cellular functions mostly derived from their influence in intracellular Ca²⁺ homeostasis. Endothelial cells express a variety of TRP channels, including members of the TRPC, TRPV, TRPP, TRPA, and TRPM families, which play a relevant role in a number of functions, including endothelium-induced vasodilation, vascular permeability as well as sensing hemodynamic and chemical changes. Furthermore, TRP channels have been reported to play an important role in angiogenesis. This review summarizes the current knowledge and limitations concerning the involvement of particular TRP channels in growth factor-induced angiogenesis.

Trpm2 Ablation Accelerates Protein Aggregation by Impaired ADPR and Autophagic Clearance in the Brain

TRPM2 a cation channel is also known to work as an enzyme that hydrolyzes highly reactive, neurotoxic ADP-ribose (ADPR). Although ADPR is hydrolyzed by NUT9 pyrophosphatase in major organs, the enzyme is defective in the brain. The present study questions the role of TRPM2 in the catabolism of ADPR in the brain. Genetic ablation of Trpm2 results in the disruption of ADPR catabolism that leads to the accumulation of ADPR and reduction in AMP. Trpm2^−/− mice elicit the reduction in autophagosome formation in the hippocampus. Trpm2^−/− mice also show aggregations of proteins in the hippocampus, aberrant structural changes and neuronal connections in synapses, and neuronal degeneration. Trpm2^−/− mice exhibit learning and memory impairment, enhanced neuronal intrinsic excitability, and imbalanced synaptic transmission. These results respond to long-unanswered questions regarding the potential role of the enzymatic function of TRPM2 in the brain, whose dysfunction evokes protein aggregation. In addition, the present finding answers to the conflicting reports such as neuroprotective or neurodegenerative phenotypes observed in Trpm2^−/− mice.

Beyond voltage-gated ion channels: Voltage-operated membrane proteins and cellular processes

Voltage-gated ion channels were believed to be the only voltage-sensitive proteins in excitable (and some non-excitable) cells for a long time. Emerging evidence indicates that the voltage-operated model is shared by some other transmembrane proteins expressed in both excitable and non-excitable cells. In this review, we summarize current knowledge about voltage-operated proteins, which are not classic voltage-gated ion channels as well as the voltage-dependent processes in cells for which single voltage-sensitive proteins have yet to be identified. Particularly, we will focus on the following. (1) Voltage-sensitive phosphoinositide phosphatases (VSP) with four transmembrane segments homologous to the voltage sensor domain (VSD) of voltage-gated ion channels; VSPs are the first family of proteins, other than the voltage-gated ion channels, for which there is sufficient evidence for the existence of the VSD domain; (2) Voltage-gated proton channels comprising of a single voltage-sensing domain and lacking an identified pore domain; (3) G protein coupled receptors (GPCRs) that mediate the depolarization-evoked potentiation of Ca²⁺ mobilization; (4) Plasma membrane (PM) depolarization-induced but Ca²⁺ -independent exocytosis in neurons. (5) Voltage-dependent metabolism of phosphatidylinositol 4,5-bisphosphate (PtdIns[4,5]P₂ , PIP₂ ) in the PM. These recent discoveries expand our understanding of voltage-operated processes within cellular membranes.

TRP channels in oxygen physiology: distinctive functional properties and roles of TRPA1 in O2 sensing

Transient Receptor Potential (TRP) proteins form cation channels characterized by a wide variety of activation triggers. Here, we overview a group of TRP channels that respond to reactive redox species to transduce physiological signals, with a focus on TRPA1 and its role in oxygen physiology. Our systematic evaluation of oxidation sensitivity using cysteine-selective reactive disulphides with different redox potentials reveals that TRPA1 has the highest sensitivity to oxidants/electrophiles among the TRP channels, which enables it to sense O₂. Proline hydroxylation by O₂-dependent hydroxylases also regulates the O₂-sensing function by inhibiting TRPA1 in normoxia; TRPA1 is activated by hypoxia through relief from the inhibition and by hyperoxia through cysteine oxidation that overrides the inhibition. TRPA1 enhances neuronal discharges induced by hyperoxia and hypoxia in the vagus to underlie respiratory adaptation to changes in O₂ availability. This importance of TRPA1 in non-carotid body O₂ sensors can be extended to the universal significance of redox-sensitive TRP channels in O₂ adaptation.

Sensing of redox status by TRP channels

Cellular redox status is maintained by the balance between series of antioxidant systems and production of reactive oxygen/nitrogenous species. Cells utilize this redox balance to mediate diverse physiological functions. Transient receptor potential (TRP) channels are non-selective cation channels that act as biosensors for environmental and noxious stimuli, such as capsaicin and allicin, as well as changes in temperature and conditions inside the cell. TRP channels also have an emerging role as essential players in detecting cellular redox status to regulate cellular signals mediating physiological phenomena. Reactive species activate TRP channels either directly through oxidative amino acid modifications or indirectly through second messengers. For instance, TRPA1, TRPV1 and TRPC5 channels are directly activated by oxidizing agents through cysteine modification; whereas, TRPM2 channel is indirectly activated by production of ADP-ribose. One intriguing property of several TRP channels is susceptibility to both oxidizing and reducing stimuli, suggesting TRP channels could potentially act as a bidirectional sensor for detecting deviations in redox status. In this review, we discuss the unique chemical physiologies of redox sensitive TRP channels and their physiological significance in Ca(2+) signaling.

Mechanisms underlying cell death in ischemia-like damage to the rat spinal cord in vitro

New spinal cord injury (SCI) cases are frequently due to non-traumatic causes, including vascular disorders. To develop mechanism-based neuroprotective strategies for acute SCI requires full understanding of the early pathophysiological changes to prevent disability and paralysis. The aim of our study was to identify the molecular and cellular mechanisms of cell death triggered by a pathological medium (PM) mimicking ischemia in the rat spinal cord in vitro. We previously showed that extracellular Mg²⁺ (1 mM) worsened PM-induced damage and inhibited locomotor function. The present study indicated that 1 h of PM+Mg²⁺ application induced delayed pyknosis chiefly in the spinal white matter via overactivation of poly (ADP-ribose) polymerase 1 (PARP1), suggesting cell death mediated by the process of parthanatos that was largely suppressed by pharmacological block of PARP-1. Gray matter damage was less intense and concentrated in dorsal horn neurons and motoneurons that became immunoreactive for the mitochondrial apoptosis-inducing factor (the intracellular effector of parthanatos) translocated into the nucleus to induce chromatin condensation and DNA fragmentation. Immunoreactivity to TRPM ion channels believed to be involved in ischemic brain damage was also investigated. TRPM2 channel expression was enhanced 24 h later in dorsal horn and motoneurons, whereas TRPM7 channel expression concomitantly decreased. Conversely, TRPM7 expression was found earlier (3 h) in white matter cells, whereas TRPM2 remained undetectable. Simulating acute ischemic-like damage in vitro in the presence of Mg²⁺ showed how, during the first 24 h, this divalent cation unveiled differential vulnerability of white matter cells and motoneurons, with distinct changes in their TRPM expression.

Role of ion channels and transporters in cell migration

Cell motility is central to tissue homeostasis in health and disease, and there is hardly any cell in the body that is not motile at a given point in its life cycle. Important physiological processes intimately related to the ability of the respective cells to migrate include embryogenesis, immune defense, angiogenesis, and wound healing. On the other side, migration is associated with life-threatening pathologies such as tumor metastases and atherosclerosis. Research from the last ≈ 15 years revealed that ion channels and transporters are indispensable components of the cellular migration apparatus. After presenting general principles by which transport proteins affect cell migration, we will discuss systematically the role of channels and transporters involved in cell migration.

TRPM7, the cytoskeleton and neuronal death

Ischemic stroke is one of the leading causes of disability and death in the world. Elucidation of the underlying mechanisms associated with neuronal death during this detrimental process has been of significant interest in the field of research. One principle component vital to the maintenance of cellular integrity is the cytoskeleton. Studies suggest that abnormalities at the level of this fundamental structure are directly linked to adverse effects on cellular well-being, including cell death. In recent years, evidence has also emerged regarding an imperative role for the transient receptor potential (TRP) family member TRPM7 in the mediation of excitotoxic-independent neuronal demise. In this review, we will elaborate on the current knowledge and unique properties associated with the functioning of this structure. In addition, we will deliberate the involvement of distinct mechanistic pathways during TRPM7-dependent cell death, including modifications at the level of the cytoskeleton.

A calcium-permeable non-selective cation channel in the thick ascending limb apical membrane of the mouse kidney

Non-selective cation channels have been described in the basolateral membrane of the renal tubule, but little is known about functional channels on the apical side. Apical membranes of microdissected fragments of mouse cortical thick ascending limbs were searched for ion channels using the cell-free configuration of the patch-clamp technique. A cation channel with a linear current-voltage relationship (19pS) that was permeable both to monovalent cations [P(NH4)(1.7)>P(Na) (1.0)=P(K) (1.0)] and to Ca(2+) (P(Ca)/P(Na)≈0.3) was detected. Unlike the basolateral TRPM4 Ca(2+)-impermeable non-selective cation channel, this non-selective cation channel was insensitive to internal Ca(2+), pH and ATP. The channel was already active after patch excision, and its activity increased after reduced pressure was applied via the pipette. External gadolinium (10(-5)M) decreased the channel-open probability by 70% in outside-out patches, whereas external amiloride (10(-4)M) had no effect. Internal flufenamic acid (10(-4)M) inhibited the channel in inside-out patches. Its properties suggest that the current might be supported by the TRPM7 protein that is expressed in the loop of Henle. The conduction properties of the channel suggest that it could be involved in Ca(2+) signaling.

Dependence of NMDA/GSK-3β mediated metaplasticity on TRPM2 channels at hippocampal CA3-CA1 synapses

Transient receptor potential melastatin 2 (TRPM2) is a calcium permeable non-selective cation channel that functions as a sensor of cellular redox status. Highly expressed within the CNS, we have previously demonstrated the functional expression of these channels in CA1 pyramidal neurons of the hippocampus. Although implicated in oxidative stress-induced neuronal cell death, and potentially in neurodegenerative disease, the physiological role of TRPM2 in the central nervous system is unknown. Interestingly, we have shown that the activation of these channels may be sensitized by co-incident NMDA receptor activation, suggesting a potential contribution of TRPM2 to synaptic transmission. Using hippocampal cultures and slices from TRPM2 null mice we demonstrate that the loss of these channels selectively impairs NMDAR-dependent long-term depression (LTD) while sparing long-term potentiation. Impaired LTD resulted from an inhibition of GSK-3β, through increased phosphorylation, and a reduction in the expression of PSD95 and AMPARs. Notably, LTD could be rescued in TRPM2 null mice by recruitment of GSK-3β signaling following dopamine D2 receptor stimulation. We propose that TRPM2 channels play a key role in hippocampal synaptic plasticity.

TRP channels in the cardiopulmonary vasculature

Transient receptor potential (TRP) channels are expressed in almost every human tissue, including the heart and the vasculature. They play unique roles not only in physiological functions but, if over-expressed, also in pathophysiological disease states. Cardiovascular diseases are the leading cause of death in the industrialized countries. Therefore, TRP channels are attractive drug targets for more effective pharmacological treatments of these diseases. This review focuses on three major cell types of the cardiovascular system: cardiomyocytes as well as smooth muscle cells and endothelial cells from the systemic and pulmonary circulation. TRP channels initiate multiple signals in all three cell types (e.g. contraction, migration) and are involved in gene transcription leading to cell proliferation or cell death. Identification of their genes has significantly improved our knowledge of multiple signal transduction pathways in these cells. Some TRP channels are important cellular sensors and are mostly permeable to Ca(2+), while most other TRP channels are receptor activated and allow for the entry of Na(+), Ca(2+) and Mg(2+). Physiological functions of TRPA, TRPC, TRPM, TRPP and TRPV channels in the cardiovascular system, dissected by down-regulating channel activity in isolated tissues or by the analysis of gene-deficient mouse models, are reviewed. The involvement of TRPs as homomeric or heteromeric channels in pathophysiological processes in the cardiovascular system like heart failure, cardiac hypertrophy, hypertension as well as edema formation by increased endothelial permeability will be discussed.

TRPM channels in the vasculature

Recent studies show that mammalian melastatin TRPM nonselective cation channels (TRPM1-8), members of the largest and most diverse TRP subfamily, are widely expressed in the endothelium and vascular smooth muscles. When activated, these channels similarly to other TRPs permit the entry of sodium, calcium and magnesium, thus causing membrane depolarisation. Although membrane depolarisation reduces the driving force for calcium entry via TRPMs as well as other pathways for calcium entry, in smooth muscle myocytes expressing voltage-gated Ca(2+) channels the predominant functional effect is an increase in intracellular Ca(2+) concentration and myocyte contraction. This review focuses on several best documented aspects of vascular functions of TRPMs, including the role of TRPM2 in oxidant stress, regulation of endothelial permeability and cell death, the connection between TRPM4 and myogenic response, significance of TRPM7 for magnesium homeostasis, vessel injury and hypertension, and emerging evidence that the cold and menthol receptor TRPM8 is involved in the regulation of vascular tone.

Review: NAD +: a modulator of immune functions

Latterly, nicotinamide adenine dinucleotide (NAD+) has emerged as a molecule with versatile functions and of enormous impact on the maintenance of cell integrity. Besides playing key roles in almost all major aspects of energy metabolism, there is mounting evidence that NAD+ and its degradation products affect various biological activities including calcium homeostasis, gene transcription, DNA repair, and intercellular communication. This review is aimed at giving a brief insight into the life cycle of NAD+ in the cell, referring to synthesis, action and degradation aspects. With respect to their immunological relevance, the importance and function of the major NAD+ metabolizing enzymes, namely CD38/CD157, ADP-ribosyltransferases (ARTs), poly-ADP-ribose-polymerases (PARPs), and sirtuins are summarized and roles of NAD+ and its main degradation product adenosine 5'-diphosphoribose (ADPR) in cell signaling are discussed. In addition, an outline of the variety of immunological processes depending on the activity of nicotinamide phosphoribosyltransferase (Nampt), the key enzyme of the salvage pathway of NAD+ synthesis, is presented. Taken together, an efficient supply of NAD+ seems to be a crucial need for a multitude of cell functions, underlining the yet only partly revealed potency of this small molecule to influence cell fate.

Molecular identification of ancient and modern mammalian magnesium transporters

A large number of mammalian Mg2+ transporters have been hypothesized on the basis of physiological data, but few have been investigated at the molecular level. The recent identification of a number of novel proteins that mediate Mg2+ transport has enhanced our understanding of how Mg2+ is translocated across mammalian membranes. Some of these transporters have some similarity to those found in prokaryocytes and yeast cells. Human Mrs2, a mitochondrial Mg2+ channel, shares many of the properties of the bacterial CorA and yeast Alr1 proteins. The SLC41 family of mammalian Mg2+ transporters has a similarity with some regions of the bacterial MgtE transporters. The mammalian ancient conserved domain protein (ACDP) Mg2+ transporters are found in prokaryotes, suggesting an ancient origin. However, other newly identified mammalian transporters, including TRPM6/7, MagT, NIPA, MMgT, and HIP14 families, are not represented in prokaryotic genomes, suggesting more recent development. MagT, NIPA, MMgT, and HIP14 transporters were identified by differential gene expression using microarray analysis. These proteins, which are found in many different tissues and subcellular organelles, demonstrate a diversity of structural properties and biophysical functions. The mammalian Mg2+ transporters have no obvious amino acid similarities, indicating that there are many ways to transport Mg2+ across membranes. Most of these proteins transport a number of divalent cations across membranes. Only MagT1 and NIPA2 are selective for Mg2+. Many of the identified mammalian Mg2+ transporters are associated with a number of congenital disorders encompassing a wide range of tissues, including intestine, kidney, brain, nervous system, and skin. It is anticipated that future research will identify other novel Mg2+ transporters and reveal other diseases.

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.

On the putative role of transient receptor potential cation channels in asthma

The mammalian transient receptor potential (TRP) superfamily consists of 28 mammalian TRP cation channels, which can be subdivided into six main subfamilies: the TRPC ('Canonical'), TRPV ('Vanilloid'), TRPM ('Melastatin'), TRPP ('Polycystin'), TRPML ('Mucolipin') and the TRPA ('Ankyrin') groups. Increasing evidence has accumulated during the previous few years that links TRP channels to the cause of several diseases or to critically influence and/or determine their progress. This review focuses on the possible role of TRP channels in the aetiology of asthmatic lung disease.

Intracellular calcium activates TRPM2 and its alternative spliced isoforms

Melastatin-related transient receptor potential channel 2 (TRPM2) is a Ca(2+)-permeable, nonselective cation channel that is involved in oxidative stress-induced cell death and inflammation processes. Although TRPM2 can be activated by ADP-ribose (ADPR) in vitro, it was unknown how TRPM2 is gated in vivo. Moreover, several alternative spliced isoforms of TRPM2 identified recently are insensitive to ADPR, and their gating mechanisms remain unclear. Here, we report that intracellular Ca(2+) ([Ca(2+)](i)) can activate TRPM2 as well as its spliced isoforms. We demonstrate that TRPM2 mutants with disrupted ADPR-binding sites can be activated readily by [Ca(2+)](i), indicating that [Ca(2+)](i) gating of TRPM2 is independent of ADPR. The mechanism by which [Ca(2+)](i) activates TRPM2 is via a calmodulin (CaM)-binding domain in the N terminus of TRPM2. Whereas Ca(2+)-mediated TRPM2 activation is independent of ADPR and ADPR-binding sites, both [Ca(2+)](i) and the CaM-binding motif are required for ADPR-mediated TRPM2 gating. Importantly, we demonstrate that intracellular Ca(2+) release activates both recombinant and endogenous TRPM2 in intact cells. Moreover, receptor activation-induced Ca(2+) release is capable of activating TRPM2. These results indicate that [Ca(2+)](i) is a key activator of TRPM2 and the only known activator of the spliced isoforms of TRPM2. Our findings suggest that [Ca(2+)](i)-mediated activation of TRPM2 and its alternative spliced isoforms may represent a major gating mechanism in vivo, therefore conferring important physiological and pathological functions of TRPM2 and its spliced isoforms in response to elevation of [Ca(2+)](i).

Transient receptor potential: a large family of new channels of which several are involved in cardiac arrhythmiaThis article is one of a selection of papers from the NATO Advanced Research Workshop on Translational Knowledge for Heart Health (published in part 1 of a 2-part Special Issue)

The transient receptor potential (TRP) family of ion channels comprises more than 50 cation-permeable channels expressed throughout the animal kingdom. TRPs can be grouped into 7 main subfamilies according to structural homology: the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), TRPA (ankyrin), and TRPN (NO mechanopotential). During the past 20 years, the cloning and characterization after reexpression of most members of these cation channels have led to a plethora of data and more recently to some understanding of their roles in various cells and tissues. Specifically in the heart, TRPs are known to be involved in various diseases, including hypertrophy, heart failure, and arrhythmia. The later part of this review focuses on the potential contribution of TRPs to cardiac rhythm and their potential proarrhythmic effects. Furthermore, several neurotransmitters that activate the formation of diacylglycerol could modulate cardiac rhythm or, like ATP, induce arrhythmia.

Ca2+-dependent induction of TRPM2 currents in hippocampal neurons

TRPM2 is a Ca(2+)-permeable member of the transient receptor potential melastatin family of cation channels whose activation by reactive oxygen/nitrogen species (ROS/RNS) and ADP-ribose (ADPR) is linked to cell death. While these channels are broadly expressed in the CNS, the presence of TRPM2 in neurons remains controversial and more specifically, whether they are expressed in neurons of the hippocampus is an open question. With this in mind, we examined whether functional TRPM2 channels are expressed in this neuronal population. Using a combination of molecular and biochemical approaches, we demonstrated the expression of TRPM2 transcripts and proteins in hippocampal pyramidal neurons. Whole-cell voltage-clamp recordings were subsequently carried out to assess the presence of TRPM2-mediated currents. Application of hydrogen peroxide or peroxynitrite to cultured hippocampal pyramidal neurons activated an inward current that was abolished upon removal of extracellular Ca(2+), a hallmark of TRPM2 activation. When ADPR (300 microM) was included in the patch pipette, a large inward current developed but only when depolarizing voltage ramps were continuously (1/10 s) applied to the membrane. This current exhibited a linear current-voltage relationship and was sensitive to block by TRPM2 antagonists (i.e. clotrimazole, flufenamic acid and N-(p-amylcinnamoyl)anthranilic acid (ACA)). The inductive effect of voltage ramps on the ADPR-dependent current required voltage-dependent Ca(2+) channels (VDCCs) and a rise in [Ca(2+)](i). Consistent with the need for a rise in [Ca(2+)](i), activation of NMDA receptors (NMDARs), which are highly permeable to Ca(2+), was also permissive for current development. Importantly, given the prominent vulnerability of CA1 neurons to free-radical-induced cell death, we confirmed that, with ADPR in the pipette, a brief application of NMDA could evoke a large inward current in CA1 pyramidal neurons from hippocampal slices that was abolished by the removal of extracellular Ca(2+), consistent with TRPM2 activation. Such a current was absent in interneurons of CA1 stratum radiatum. Finally, infection of cultured hippocampal neurons with a TRPM2-specific short hairpin RNA (shRNA(TRPM2)) significantly reduced both the expression of TRPM2 and the amplitude of the ADPR-dependent current. Taken together, these results indicate that hippocampal pyramidal neurons possess functional TRPM2 channels whose activation by ADPR is functionally coupled to VDCCs and NMDARs through a rise in [Ca(2+)](i).

Molecular characterization of a novel cell surface ADP-ribosyl cyclase from the sea urchin

The sea urchin is an extensively used model system for the study of calcium signalling by the messenger molecules NAADP and cyclic ADP-ribose. Both are synthesized by ADP-ribosyl cyclases but our molecular understanding of these enzymes in the sea urchin is limited. We have recently reported the cloning of an extended family of sea urchin ADP-ribosyl cyclases and shown that one of these enzymes (SpARC1) is active within the endoplasmic reticulum lumen. These studies suggest that production of messengers is compartmentalized. Here we characterize the properties of SpARC2. SpARC2 catalyzed both NAADP and cyclic ADP-ribose production. Unusually, the NAD surrogate, NGD was a poor substrate. In contrast to SpARC1, heterologously expressed SpARC2 localized to the plasma membrane via a glycosylphosphatidylinositol (GPI)-anchor. Transcripts for SpARC2 were readily detectable in sea urchin eggs and a majority of the endogenous membrane bound activity was found to be GPI-anchored. Our data reveal striking differences in the properties of sea urchin ADP-ribosyl cyclases and provide further evidence that messenger production may occur outside of the cytosol.

Synergistic regulation of endogenous TRPM2 channels by adenine dinucleotides in primary human neutrophils

The Ca(2+)-permeable TRPM2 channel is a dual function protein that is activated by intracellular ADPR through its enzymatic pyrophosphatase domain with Ca(2+) acting as a co-factor. Other TRPM2 regulators include cADPR, NAADP and H(2)O(2), which synergize with ADPR to potentiate TRPM2 activation. Although TRPM2 has been thoroughly characterized in overexpression or cell-line systems, little is known about the features of TRPM2 in primary cells. We here characterize the regulation of TRPM2 activation in human neutrophils and report that ADPR activates TRPM2 with an effective half-maximal concentration (EC(50)) of 1microM. Potentiation by Ca(2+) is dose-dependent with an EC(50) of 300nM. Both cADPR and NAADP activate TRPM2, albeit with lower efficacy than in the presence of subthreshold levels of ADPR (100nM), which significantly shifts the EC(50) for cADPR from 44 to 3muM and for NAADP from 95 to 1microM. TRPM2 activation by ADPR can be suppressed by AMP with an IC(50) of 10microM and cADPR-induced activation can be blocked by 8-Bromo-cADPR. We further show that 100microM H(2)O(2) enables subthreshold concentrations of ADPR (100nM) to activate TRPM2. We conclude that agonistic and antagonistic characteristics of TRPM2 as seen in overexpression systems are largely compatible with the functional properties of TRPM2 currents measured in human neutrophils, but the potencies of agonists in primary cells are significantly higher.

Role of the alpha-kinase domain in transient receptor potential melastatin 6 channel and regulation by intracellular ATP

Transient receptor potential melastatin 6 (TRPM6) plays an essential role in epithelial Mg(2+) transport. TRPM6 and its closest homologue, TRPM7, both combine a cation channel with an alpha-kinase domain. However, the role of this alpha-kinase domain in TRPM6 channel activity remains elusive. The aim of this study was to investigate the regulation of TRPM6 channel activity by intracellular ATP and the involvement of its alpha-kinase domain. We demonstrated that intracellular Na- and Mg-ATP decreased the TRPM6 current in HEK293 cells heterogeneously expressing the channel, whereas Na-CTP or Na-GTP had no effect on channel activity. Whole cell recordings in TRPM6-expressing HEK293 cells showed that deletion of the alpha-kinase domain prevented the inhibitory effect of intracellular ATP without abrogating channel activity. Mutation of the conserved putative ATP-binding motif GXG(A)XXG (G1955D) in the alpha-kinase domain of TRPM6 inhibited the ATP action, whereas this effect remained preserved in the TRPM6 phosphotransferase-deficient mutant K1804R. Mutation of the TRPM6 autophosphorylation site, Thr(1851), into either an alanine or an aspartate, resulted in functional channels that could still be inhibited by ATP. In conclusion, intracellular ATP regulates TRPM6 channel activity via its alpha-kinase domain independently of alpha-kinase activity.

Recent developments in intestinal magnesium absorption

PURPOSE OF REVIEW

Recent identification and characterization of novel Mg transporters have clarified our understanding of intestinal magnesium absorption. The predominant Mg transporters include TRPM6 and TRPM7, members of the transient receptor potential melastatin family of cation channels. Mutations of TRPM6 result in a primary disorder termed hypomagnesemia with secondary hypocalcemia.

RECENT FINDINGS

Both TRPM6 and TRPM7 channels possess an atypical alpha-kinase domain. Recent studies have shown that TRPM7 channel activity is regulated by intracellular Mg and magnesium-nucleotides and modulated via this phosphotransferase kinase. TRPM6 channel function and intestinal magnesium absorption is altered by a variety of hormones and factors. Although it is apparent that TRPM6 and TRPM7 form heteromeric ion channels, controversy surrounds the nature of this interaction. Some studies show that TRPM6 may function on its own whereas other research concludes that TRPM7 is required for effective trafficking of TRPM6 to the plasma membrane. Finally, a number of other Mg transporters have been identified in intestinal epithelial cells but the role of these proteins is unclear.

SUMMARY

The recent developments in intestinal magnesium absorption and cellular magnesium homeostasis provide a basis for understanding magnesium deficiency disorders and provide a platform for future investigations.

Regulation of TRPM2 by extra- and intracellular calcium

TRPM2 is a calcium-permeable nonselective cation channel that is opened by the binding of ADP-ribose (ADPR) to a C-terminal nudix domain. Channel activity is further regulated by several cytosolic factors, including cyclic ADPR (cADPR), nicotinamide adenine dinucleotide phosphate (NAADP), Ca²⁺ and calmodulin (CaM), and adenosine monophosphate (AMP). In addition, intracellular ions typically used in patch-clamp experiments such as Cs⁺ or Na⁺ can alter ADPR sensitivity and voltage dependence, complicating the evaluation of the roles of the various modulators in a physiological context. We investigated the roles of extra- and intracellular Ca²⁺ as well as CaM as modulators of ADPR-induced TRPM2 currents under more physiological conditions, using K⁺-based internal saline in patch-clamp experiments performed on human TRPM2 expressed in HEK293 cells. Our results show that in the absence of Ca²⁺, both internally and externally, ADPR alone cannot induce cation currents. In the absence of extracellular Ca²⁺, a minimum of 30 nM internal Ca²⁺ is required to cause partial TRPM2 activation with ADPR. However, 200 μM external Ca²⁺ is as efficient as 1 mM Ca²⁺ in TRPM2 activation, indicating an external Ca²⁺ binding site important for proper channel function. Ca²⁺ facilitates ADPR gating with a half-maximal effective concentration of 50 nM and this is independent of extracellular Ca²⁺. Furthermore, TRPM2 currents inactivate if intracellular Ca²⁺ levels fall below 100 nM irrespective of extracellular Ca²⁺. The facilitatory effect of intracellular Ca²⁺ is not mimicked by Mg²⁺, Ba²⁺, or Zn²⁺. Only Sr²⁺ facilitates TRPM2 as effectively as Ca²⁺, but this is due to Sr²⁺-induced Ca²⁺ release from internal stores rather than a direct effect of Sr²⁺ itself. Together, these data demonstrate that cytosolic Ca²⁺ regulates TRPM2 channel activation. Its facilitatory action likely occurs via CaM, since the addition of 100 μM CaM to the patch pipette significantly enhances ADPR-induced TRPM2 currents at fixed [Ca²⁺]_i and this can be counteracted by calmidazolium. We conclude that ADPR is responsible for TRPM2 gating and Ca²⁺ facilitates activation via calmodulin.

Lipid rafts and oxidative stress-induced cell death

Reactive oxygen species (ROS) are generated in response to a number of physiologic or pathologic conditions. In addition to ROS produced extrinsically, a cell may produce ROS as a result of normal metabolism and signaling processes. When sufficient quantities of ROS are present within the cell, this oxidative stress may have profound effects on the cell, including the induction of cell death. Various signaling pathways are initiated in response to oxidative stress, through which the cell's demise is assured. Many of these signaling pathways involve cholesterol-enriched domains of the cell membrane known as lipid rafts. These lipid rafts are platforms for initiation or transduction of the signal and may modulate protein activity through a direct change in local membrane structure or by allowing protein-protein interactions to occur with higher affinity/specificity or both. Among the examples discussed in this review are death-receptor signaling, induction of membrane-associated tyrosine kinase activation, and activation of transient receptor protein (TRP) channels. Special attention also is given to the RIP1/TRAF2 pathway, which involves the downstream activation of the stress-activated protein kinase JNK. The activation of the JNK pathway plays a key role in the induction of cellular death in response to ROS.

Transient receptor potential cation channels in disease

The transient receptor potential (TRP) superfamily consists of a large number of cation channels that are mostly permeable to both monovalent and divalent cations. The 28 mammalian TRP channels 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 expressed in almost every tissue and cell type and play an important role in the regulation of various cell functions. Currently, significant scientific effort is being devoted to understanding the physiology of TRP channels and their relationship to human diseases. At this point, only a few channelopathies in which defects in TRP genes are the direct cause of cellular dysfunction have been identified. In addition, mapping of TRP genes to susceptible chromosome regions (e.g., translocations, breakpoint intervals, increased frequency of polymorphisms) has been considered suggestive of the involvement of these channels in hereditary diseases. Moreover, strong indications of the involvement of TRP channels in several diseases come from correlations between levels of channel expression and disease symptoms. Finally, TRP channels are involved in some systemic diseases due to their role as targets for irritants, inflammation products, and xenobiotic toxins. The analysis of transgenic models allows further extrapolations of TRP channel deficiency to human physiology and disease. In this review, we provide an overview of the impact of TRP channels on the pathogenesis of several diseases and identify several TRPs for which a causal pathogenic role might be anticipated.

TRP channels in disease

"Transient receptor potential" cation channels (TRP channels) play a unique role as cell sensors, are involved in a plethora of Ca(2+)-mediated cell functions, and play a role as "gate-keepers" in many homeostatic processes such as Ca(2+) and Mg(2+) reabsorption. The variety of functions to which TRP channels contribute and the polymodal character of their activation predict that failures in correct channel gating or permeation will likely contribute to complex pathophysiological mechanisms. Dysfunctions of TRPs cause human diseases but are also involved in a complex manner to contribute and determine the progress of several diseases. Contributions to this special issue discuss channelopathias for which mutations in TRP channels that induce "loss-" or "gain-of-function" of the channel and can be considered "disease-causing" have been identified. The role of TRPs will be further elucidated in complex diseases of the intestinal, renal, urogenital, respiratory, and cardiovascular systems. Finally, the role of TRPs will be discussed in neuronal diseases and neurodegenerative disorders.

Cell death of melanophores in zebrafish trpm7 mutant embryos depends on melanin synthesis

Transient receptor potential melastatin 7 (TRPM7) is a broadly expressed, non-selective cation channel. Studies in cultured cells implicate TRPM7 in regulation of cell growth, spreading, and survival. However, zebrafish trpm7 homozygous mutants display death of melanophores and temporary paralysis, but no gross morphological defects during embryonic stages. This phenotype implies that melanophores are unusually sensitive to decreases in Trpm7 levels, a hypothesis we investigate here. We find that pharmacological inhibition of caspases does not rescue melanophore viability in trpm7 mutants, implying that melanophores die by a mechanism other than apoptosis. Consistent with this possibility, ultrastructural analysis of dying melanophores in trpm7 mutants reveals abnormal melanosomes and evidence of a ruptured plasma membrane, indicating that cell death occurs by necrosis. Interestingly, inhibition of melanin synthesis largely prevents melanophore cell death in trpm7 mutants. These results suggest that melanophores require Trpm7 in order to detoxify intermediates of melanin synthesis. We find that unlike TRPM1, TRPM7 is expressed in human melanoma cell lines, indicating that these cells may also be sensitized to reduction of TRPM7 levels.

TRPM2

TRPM2 is a cation channel enabling influx of Na+ and Ca2+, leading to depolarization and increases in the cytosolic Ca2+ concentration ([Ca2+]i). It is widely expressed, e.g. in many neurons, blood cells and the endocrine pancreas. Channel gating is induced by ADP-ribose (ADPR) that binds to a Nudix box motif in the cytosolic C-terminus of the channel. Endogenous ADPR concentrations in leucocytes are sufficiently high to activate TRPM2 in the presence of an increased [Ca2+]i but probably not at resting [Ca2+]i. Another channel activator is oxidative stress, especially hydrogen peroxide (H2O2) that may act through ADPR after ADPR polymers have been formed by poly(ADP-ribose) polymerases (PARPs) and hydolysed by glycohydrolases. H2O2-stimulated TRPM2 channels essentially contribute to insulin secretion in pancreatic beta-cells and alloxan-induced diabetes mellitus. Inhibition of TRPM2 channels may be achieved by channel blockers such as flufenamic acid or the anti-fungal agents clotrimazole or econazole. Selective blockers of TRPM2 are not yet available; those would be valuable for a characterization of biological roles of TRPM2 in various tissues and as potential drugs directed against oxidative cell damage, reperfusion injury or leucocyte activation. Activation of TRPM2 may be prevented by anti-oxidants, PARP inhibitors and glycohydrolase inhibitors. In future, binding of ADPR to the Nudix box may be targeted. In light of the wide-spread expression and growing list of cellular functions of TRPM2, useful therapeutic applications are expected for future drugs that block TRPM2 channels or inhibit their activation.

Unphysiologically High Magnesium Concentrations Support Chondrocyte Proliferation and Redifferentiation

The effect of unphysiologically high extracellular magnesium concentrations on chondrocytes, induced by the supplementation of magnesium sulfate, was studied using a 3-phase tissue engineering model. The experiments showed that chondrocyte proliferation and redifferentiation, on the gene and protein expression level, are enhanced. A negative influence was found during chondrogenesis where an inhibition of extracellular matrix formation was observed. In addition, a direct impact on chondrocyte metabolism, elevated magnesium concentrations also affected growth factor effectiveness by consecutive influences during chondrogenesis. All observations were dosage dependent. The results of this study indicate that magnesium may be a useful tool for cartilage tissue engineering.

TRPM channels, calcium and redox sensors during innate immune responses

Melastatin-related TRPM ion channels have emerged as novel therapeutic targets due to their potential ability to modulate the function and fate of immune cells during inflammation, innate, and adaptive immunity. Four family members, TRPM1, TRPM2, TRPM4 and TRPM7 have a strong presence in the immune system. TRPM channels regulate ion-homeostasis by sensing cellular redox status and cytoplasmic calcium levels. TRPM2 for example, is highly expressed in phagocytes. This channel is activated by intracellular ADP-ribose upon exposure to oxidative stress and induces cell death. Here we will review the functional links between TRPM-mediated ion conductance, chemotaxis, apoptosis, and innate immunity.

Cation channels of the transient receptor potential superfamily: their role in physiological and pathophysiological processes of smooth muscle cells

Smooth muscle cells (SMC) are essential components of many tissues of the body. Ion channels regulate their membrane potential, the intracellular Ca(2+) concentration ([Ca(2+)](i)) and their contractility. Among the ion channels expressed in SMC cation channels of the transient receptor potential (TRP) superfamily allow the entry of Na(+), Ca(2+) and Mg(2+). Members of the TRP superfamily are essential constituents of tonically active channels (TAC), receptor-operated channels (ROC), store-operated channels (SOC) and stretch-activated channels (SAC). This review focusses on TRP channels (TRPC1, TRPC3, TRPC4, TRPC5, TRPC6, TRPC7, TRPV2, TRPV4, TRPM4, TRPM7, TRPP2) whose physiological functions in SMC were dissected by downregulating channel activity in isolated tissues or by the analysis of gene-deficient mouse models. Their possible functional role and physiological regulation as homomeric or heteromeric channels in SMC are discussed. Moreover, TRP channels may also be responsible for pathophysiological processes involving SMC-like airway hyperresponsiveness and pulmonary hypertension. Therefore, they present important drug targets for future pharmacological interventions.