Sites of the NUDT9-H domain critical for ADP-ribose activation of the cation channel TRPM2

TRPM2 is a cation channel unique within the transient receptor potential family because of its gating by ADP-ribose (ADPR). ADPR gating is enabled by a cytosolic C-terminal Nudix box sequence motif embedded into a region homologous to the NUDT9 ADPR pyrophosphatase. A recently discovered splice variant of TRPM2 (TRPM2-DeltaC) lacks 34 amino acid residues in the NUDT9 domain and is insensitive to ADPR. To analyze in detail which parts of the deleted sequence (DeltaC-stretch) are critical for ADPR gating, we tested mutants that lacked 19, 25, and 29 amino acid residues in the N-terminal part or had amino acid residues substituted in the remaining C-terminal part of the DeltaC-stretch. All of these mutants displayed typical ADPR-induced currents. However, the deletion or substitution of the amino acid residue Asn-1326 immediately downstream of the DeltaC-stretch abrogated ADPR gating. We furthermore analyzed the mutation I1405E/L1406F in the Nudix box of TRPM2, because a considerably decreased AD-PRase activity of the TRPM2 NUDT9-H protein in comparison to the NUDT9 pyrophosphatase has been attributed to the reverse exchange EF --> IL. The I1405E/L1406F variant of TRPM2 failed to respond to ADPR even at concentrations up to 10 mM. We concluded that the DeltaC-stretch contains no individual amino acid residues essential for ADPR gating but may act as a spacer segment stabilizing a conformation necessary for the essential residue Asn-1326 to interact with other channel regions. Enhancing the enzymatic activity of the Nudix box abolishes the ADPR gating of TRPM2, pointing to the requirement of prolonged binding rather than degradation.

Cool (TRPM8) and hot (TRPV1) receptors in the bladder and male genital tract

PURPOSE

Overactive bladder symptoms due to various etiologies have been successfully treated with capsaicin by desensitization of the temperature sensitive vanilloid receptor TRPV1. Recently another temperature sensitive receptor, TRPM8, activated by menthol and cool temperatures (8C to 28C) was described that may be the proposed cool receptor, at least in part mediating the bladder response in the diagnostic ice water test. We defined the sites of mRNA and protein expression of TRPM8 and TRPV1 in the rat and human genitourinary tract.

MATERIALS AND METHODS

Prostate, testis, penis, bladder and dorsal root ganglion tissue was obtained from rats. Prostate, testicle, seminiferous tubules, corpus cavernosum, glans, overlying glans skin, scrotal skin and bladder were obtained from human patients. Reverse transcription-polymerase chain reaction was done using species specific primers for TRPM8 and TRPV1. Immunofluorescence staining for TRPM8 was performed in rat tissues as well as in cultured human urothelial cells.

RESULTS

TRPM8 and TRPV1 mRNA were detected in all rat tissues. Human samples demonstrated TRPM8 mRNA in prostate, testicle, seminiferous tubules, scrotal skin and bladder. No TRPM8 mRNA was identified in human corpus cavernosum, glans or overlying glans skin. Separation of layers in human bladder demonstrated mRNA for TRPM8 only in the urothelium and not in the detrusor. Immunofluorescence location of TRPM8 was found in rat prostate, DRG and bladder, and in human urothelial cells in culture. TRPV1 mRNA was detected in all human genitourinary tract tissues.

CONCLUSIONS

These results demonstrate that mRNA and protein for TRPM8 exist in multiple genitourinary organs in the rat and human, and it may be considered a possible new target, as is TRPV1, for the pharmacological treatment of detrusor overactivity or other urological disorders.

Decavanadate modulates gating of TRPM4 cation channels

We have tested the effects of decavanadate (DV), a compound known to interfere with ATP binding in ATP-dependent transport proteins, on TRPM4, a Ca(2+)-activated, voltage-dependent monovalent cation channel, whose activity is potently blocked by intracellular ATP(4-). Application of micromolar Ca(2+) concentrations to the cytoplasmic side of inside-out patches led to immediate current activation followed by rapid current decay, which can be explained by an at least 30-fold decreased apparent affinity for Ca(2+). Subsequent application of DV (10 microm) strongly affected the voltage-dependent gating of the channel, resulting in large sustained currents over the voltage range between -180 and +140 mV. The effect of DV was half-maximal at a concentration of 1.9 microm. The Ca(2+)- and voltage-dependent gating of the channel was well described by a sequential kinetic scheme in which Ca(2+) binding precedes voltage-dependent gating. The effects of DV could be explained by an action on the voltage-dependent closing step. Surprisingly, DV did not antagonize the effect of ATP(4-) on TRPM4, but caused a nearly 10-fold increase in the sensitivity of the ATP(4-) block. TRPM5, which is the most homologous channel to TRPM4, was not modulated by DV. The effect of DV was lost in a TRPM4 chimera in which the C-terminus was substituted with that of TRPM5. Deletion of a cluster in the C-terminus of TRPM4 containing positively charged amino acid residues with a high homology to part of the decavanadate binding site in SERCA pumps, completely abolished the DV effect but also accelerated desensitization. Deletion of a similar site in the N-terminus had no effects on DV responses. These results indicate that the C-terminus of TRPM4 is critically involved in mediating the DV effects. In conclusion, decavanadate modulates TRPM4, but not TRPM5, by inhibiting voltage-dependent closure of the channel.

TRPM2 channel opening in response to oxidative stress is dependent on activation of poly(ADP-ribose) polymerase

1. TRPM2 (melastatin-like transient receptor potential 2 channel) is a nonselective cation channel that is activated under conditions of oxidative stress leading to an increase in intracellular free Ca(2+) concentration ([Ca(2+)](i)) and cell death. We investigated the role of the DNA repair enzyme poly(ADP-ribose) polymerase (PARP) on hydrogen peroxide (H(2)O(2))-mediated TRPM2 activation using a tetracycline-inducible TRPM2-expressing cell line. 2. In whole-cell patch-clamp recordings, intracellular adenine 5'-diphosphoribose (ADP-ribose) triggered an inward current in tetracycline-induced TRPM2-human embryonic kidney (HEK293) cells, but not in uninduced cells. Similarly, H(2)O(2) stimulated an increase in [Ca(2+)](i) (pEC(50) 4.54+/-0.02) in Fluo-4-loaded TRPM2-expressing HEK293 cells, but not in uninduced cells. Induction of TRPM2 expression caused an increase in susceptibility to plasma membrane damage and mitochondrial dysfunction in response to H(2)O(2). These data demonstrate functional expression of TRPM2 following tetracycline induction in TRPM2-HEK293 cells. 3. PARP inhibitors SB750139-B (patent number DE10039610-A1 (Lubisch et al., 2001)), PJ34 (N-(6-oxo-5,6-dihydro-phenanthridin-2-yl)-N,N-dimethylacetamide) and DPQ (3, 4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone) inhibited H(2)O(2)-mediated increases in [Ca(2+)](i) (pIC(50) vs 100 microm H(2)O(2): 7.64+/-0.38; 6.68+/-0.28; 4.78+/-0.05, respectively), increases in mitochondrial dysfunction (pIC(50) vs 300 microm H(2)O(2): 7.32+/-0.23; 6.69+/-0.22; 5.44+/-0.09, respectively) and decreases in plasma membrane integrity (pIC(50) vs 300 microm H(2)O(2): 7.45+/-0.27; 6.35+/-0.18; 5.29+/-0.12, respectively). The order of potency of the PARP inhibitors in these assays (SB750139>PJ34>DPQ) was the same as for inhibition of isolated PARP enzyme. 4. SB750139-B, PJ34 and DPQ had no effect on inward currents elicited by intracellular ADP-ribose in tetracycline-induced TRPM2-HEK293 cells, suggesting that PARP inhibitors are not interacting directly with the channel. 5. SB750139-B, PJ34 and DPQ inhibited increases in [Ca(2+)](i) in a rat insulinoma cell line (CRI-G1 cells) endogenously expressing TRPM2 (pIC(50) vs 100 microm H(2)O(2): 7.64+/-0.38; 6.68+/-0.28; 4.78+/-0.05, respectively). 6. These data suggest that oxidative stress causes TRPM2 channel opening in both recombinant and endogenously expressing cell systems via activation of PARP enzymes.

Regulation of Na+/Mg2+ antiport in rat erythrocytes

In rat erythrocytes, the regulation of Na+/Mg2+ antiport by protein kinases (PKs), protein phosphatases (PPs), intracellular Mg2+, ATP and Cl- was investigated. In untreated erythrocytes, Na+/Mg2+ antiport was slightly inhibited by the PK inhibitor staurosporine, slightly stimulated by the PP inhibitor calyculin A and strongly stimulated by vanadate. PMA stimulated Na+/Mg2+ antiport. This effect was completely inhibited by staurosporine and partially inhibited by the PKC inhibitors Ro-31-8425 and BIM I. Participation of other PKs such as PKA, the MAPK cascade, PTK, CK I, CK II, CAM II-K, PI 3-K, and MLCK was excluded by use of inhibitors. Na+/Mg2+ antiport in rat erythrocytes can thus be stimulated by PKCalpha. In non-Mg2+ -loaded erythrocytes, ATP depletion reduced Mg2+ efflux and PMA stimulation in NaCl medium. A drastic activation of Na+/Mg2+ antiport was induced by Mg2+ loading which was not further stimulated by PMA. Staurosporine, Ro-31-8425, BIM I and calyculin A did not inhibit Na+/Mg2+ antiport of Mg2+ -loaded cells. Obviously, at high [Mg2+]i Na+/Mg2+ antiport is maximally stimulated. PKCalpha or PPs are not involved in stimulation by intracellular Mg2+. ATP depletion of Mg2+ -loaded erythrocytes reduced Mg2+ efflux and the affinity of Mg2+ binding sites of the Na+/Mg2+ antiporter to Mg2+. In non-Mg2+ -loaded erythrocytes Na+/Mg2+ antiport essentially depends on Cl-. Mg2+ -loaded erythrocytes were less sensitive to the activation of Na+/Mg2+ antiport by [Cl-]i.

The principle of temperature-dependent gating in cold- and heat-sensitive TRP channels

The mammalian sensory system is capable of discriminating thermal stimuli ranging from noxious cold to noxious heat. Principal temperature sensors belong to the TRP cation channel family, but the mechanisms underlying the marked temperature sensitivity of opening and closing ('gating') of these channels are unknown. Here we show that temperature sensing is tightly linked to voltage-dependent gating in the cold-sensitive channel TRPM8 and the heat-sensitive channel TRPV1. Both channels are activated upon depolarization, and changes in temperature result in graded shifts of their voltage-dependent activation curves. The chemical agonists menthol (TRPM8) and capsaicin (TRPV1) function as gating modifiers, shifting activation curves towards physiological membrane potentials. Kinetic analysis of gating at different temperatures indicates that temperature sensitivity in TRPM8 and TRPV1 arises from a tenfold difference in the activation energies associated with voltage-dependent opening and closing. Our results suggest a simple unifying principle that explains both cold and heat sensitivity in TRP channels.

[Physiological role of Mg2+ in neurons]

Physiological role of Mg ions in living cells is now in the process of reexamination. This is triggered by the finding that cytoplasmic Mg(2+) concentration measured by a fluorescent dye, mag fura-2, is surprisingly small, being not more than 1 mM. Lines of examination have revealed that Mg(2+) inhibits various species of ionc channels not only from outside but also from inside of the plasma membrane. Entry of Mg(2+) into intracellular space is revealed to take place via a Ca(2+)-channels, TRPM7; this channel may thus be called "Mg(2+)-channel". Moreover, an excess amount of intracellular Mg(2+) induced by hyper-activity of NMDA channels occasionally causes neuronal cell death as a result of inhibitory effects of Mg(2+) to mitochondrial activities.

Involvement of TRPM7 in cell growth as a spontaneously activated Ca2+ entry pathway in human retinoblastoma cells

We investigated the possible involvement of the melastatin family protein TRPM7 in Ca(2+)-mediated proliferative control of human retinoblastoma (RB) cells. The growth of RB cell was facilitated by elevating the extracellular Ca(2+) concentration with a parallel increase in the magnitude of spontaneous Ca(2+) influx. Under nystatin-perforated voltage-clamp, RB cells exhibited an outward-rectifying, spontaneous cation current (I(spont)) having Ca(2+)/Mg(2+)-inhibited but -permeating properties. Various cation channel blockers inhibiting I(spont) (Gd(3+), La(3+), LOE908, 2-APB) suppressed the spontaneous Ca(2+) influx and decelerated the growth of RB cells with similar efficacies. Excision of the RB cell membrane (inside-out) into MgATP-free solution induced a 70pS single channel activity, which was effectively inhibited by millimolar concentrations of Mg(2+) or MgATP. RT-PCR and immunocytochemical experiments revealed the expression of TRPM7 mRNA and protein in RB cells, and heterologous expression of TRPM7 in HEK293 cells reproduced the key features of I(spont). In contrast, elimination of this protein from RB cells by siRNA silencing markedly reduced I(spont) density and the magnitude of spontaneous Ca(2+) influx, which was paralleled by decreased TRPM7 immunoreactivity, decelerated cell proliferation, and retarded G(1)/S cell cycle progression. These results suggest a significant regulatory role of TRPM7 for RB cell proliferation as a spontaneously activated Ca(2+) influx pathway.

Magnesium-inhibited, TRPM6/7-like channel in cardiac myocytes: permeation of divalent cations and pH-mediated regulation

Cardiac tissue expresses several TRP proteins as well as a Mg2+ -inhibited, non-selective cation current (IMIC) that bears many characteristics of TRP channel currents. We used the whole-cell voltage clamp technique in pig and rat ventricular myocytes to characterize the permeation, blockage properties and regulation of the cardiac IMIC channels in order to compare them with TRP channels, in particular with Mg2+ -sensitive TRPM6 and TRPM7. We show that removing extracellular divalent cations unmasks large inward and outward monovalent currents, which can be inhibited by intracellular Mg2+. Inward currents are suppressed upon replacing extracellular Na+ by NMDG+. Divalent cations block monovalent IMIC and, at 10-20 mm, carry measurable currents. Their efficacy sequence in decreasing outward IMIC (Ni2+ = Mg2+ > Ca2+ > Ba2+) and in inducing inward IMIC (Ni2+ >> Mg2+ = Ca2+ approximately Ba2+), and their permeabilities calculated from reversal potentials are similar to those of TRPM6 and TRPM7 channels. The trivalent cations Gd3+ and Dy3+ also block IMIC in a voltage-dependent manner (delta = 0.4-0.5). In addition they inhibit the inward current carried by divalent cations. IMIC is regulated by pH. Decreasing or increasing extracellular pH decreased and increased IMIC, respectively (pH0.5 = 6.9, nH = 0.98). Qualitatively similar results were obtained on IMIC in rat basophilic leukaemia cells. These effects in cardiac myocytes were absent in the presence of high intracellular buffering by 40 mm Hepes. Our results suggest that IMIC in cardiac cells is due to TRPM channels, most probably to TRPM6 or TRPM7 channels or to their heteromultimeres.

Overexpression of wild-type and mutant mucolipin proteins in mammalian cells: effects on the late endocytic compartment organization

Mucolipin-1 is a 65-kDa membrane protein encoded by the MCOLN1 gene, which is mutated in patients with mucolipidosis type IV (MLIV), a rare neurodegenerative lysosomal storage disorder. We studied the subcellular localization of wild-type and three different mutant forms (T232P, F408del and F465L) of mucolipin by expressing Myc-tagged proteins in HeLa cells. The overexpressed wild-type mucolipin colocalizes to late endocytic structures and induces an aberrant distribution of these compartments. F408del and F465L MLIV mutant proteins show a distribution similar to the wild-type protein, whereas T232P is retained in the endoplasmic reticulum. Among the mutants, only F408del induces a redistribution of the late endocytic compartment. These findings suggest that the overexpression of the mucolipin cation channel influences the dynamic equilibrium of late endocytic compartments.

TRPM7: Channeling the Future of Cellular Magnesium Homeostasis?

A recent paper by Schmitz and colleagues provides persuasive evidence that the ion channel transient receptor potential melastatin 7 (TRPM7) may be the long-sought regulator of magnesium (Mg) homeostasis in mammalian cells. This finding is not unexpected, because TRPM channels contain a kinase domain that allows them to participate in signal transduction pathways and regulatory networks. However, these studies introduce an exciting new twist into our understanding of Mg homeostasis; TRPM7 facilitates Mg entry into the cell, whereas other putative Mg transporters apparently operate in the opposite direction. By combining electrophysiological, biochemical, and genetic approaches, Schmitz and colleagues characterized most of the key features that demonstrate a well-defined and biologically plausible regulator of Mg homeostasis. TRPM7 genetics are well in hand, its regulation by intracellular free Mg2+ unravels the mechanisms of regulatory feedback loops, and its kinase domain modulates its sensitivity to free Mg2+. These findings are discussed in light of the indirect and descriptive information we had about Mg regulation before this rigorous characterization of TRPM7 brought it to the center of the Mg stage. Although the molecular events downstream of TRPM7 phosphorylation and dephosphorylation await in-depth elucidation, these results open up exciting perspectives in Mg research and may provide a much-needed tool with which to reexamine the role of Mg in cell proliferation and other important pathophysiologic events. Likewise, these findings will offer guidelines for research on disease states that are characterized by Mg imbalance.

Dual-function ion channel/protein kinases: novel components of vertebrate magnesium regulatory mechanisms

Although magnesium is the dominant divalent intracellular cation and is required for the function of diverse types of enzymes that participate in virtually every cellular process, the molecular mechanisms that regulate its homeostasis are poorly understood. Electrophysiologic and biochemical investigations of a novel dual-function ion channel/kinase protein have recently converged with the identification of the gene locus for an autosomal recessive form of inherited hypomagnesemia to provide new insight into vertebrate magnesium regulatory mechanisms.

Alpha-kinases: analysis of the family and comparison with conventional protein kinases

Alpha-kinases are a recently discovered family of protein kinases that have no detectable sequence homology to conventional protein kinases (CPKs). They include elongation factor 2 kinase, Dictyostelium myosin heavy chain kinases and many other protein kinases from diverse organisms, as revealed by various genome sequencing projects. Mammals have six alpha-kinases, including two channel-kinases-novel signaling molecules that contain an alpha-kinase domain fused to an ion-channel. Analysis of all known alpha-kinase sequences reveals the presence of several highly conserved motifs. Despite the fact that alpha-kinases have no detectable sequence identity with CPKs, the recently determined three-dimensional structure of the channel-kinase TRPM7/ChaK1 kinase domain reveals that alpha-kinases have a fold very similar to CPKs. Using the structural alignment of channel-kinase TRPM7/ChaK1 with cyclic-AMP dependent kinase, the consensus motifs of alpha-kinases and CPKs were aligned and compared. Remarkably, the majority of structural elements, sequence motifs, and the position of key amino acid residues important for catalysis appear to be very similar in alpha-kinases and CPKs. Differences between alpha-kinases and CPKs, and their possible impact on substrate recognition are discussed.

Functional characterization of a Ca(2+)-activated non-selective cation channel in human atrial cardiomyocytes

Cardiac arrhythmias, which occur in a wide variety of conditions where intracellular calcium is increased, have been attributed to the activation of a transient inward current (Iti). Iti is the result of three different [Ca]i-sensitive currents: the Na(+)-Ca2+ exchange current, a Ca(2+)-activated chloride current and a Ca(2+)-activated non-selective cationic current. Using the cell-free configuration of the patch-clamp technique, we have characterized the properties of a Ca(2+)-activated non-selective cation channel (NSC(Ca)) in freshly dissociated human atrial cardiomyocytes. In excised inside-out patches, the channel presented a linear I-V relationship with a conductance of 19 +/- 0.4 pS. It discriminated poorly among monovalent cations (Na+ and K+) and was slightly permeable to Ca2+ ions. The channel's open probability was increased by depolarization and a rise in internal calcium, for which the Kd for [Ca2+]i was 20.8 microM. Channel activity was reduced in the presence of 0.5 mM ATP or 10 microM glibenclamide on the cytoplasmic side to 22.1 +/- 16.8 and 28.5 +/- 8.6%, respectively, of control. It was also inhibited by 0.1 mM flufenamic acid. The channel shares several properties with TRPM4b and TRPM5, two members of the 'TRP melastatin' subfamily. In conclusion, the NSC(Ca) channel is a serious candidate to support the delayed after-depolarizations observed in [Ca2+] overload and thus may be implicated in the genesis of arrhythmias.

Receptor-mediated regulation of the TRPM7 channel through its endogenous protein kinase domain

TRPM7 is a ubiquitously expressed and constitutively active divalent cation-selective ion channel, whose basal activity is regulated by intracellular levels of Mg(2+) and Mg.ATP. We have investigated receptor-mediated mechanisms that may actively regulate TRPM7 activity. We here report that TRPM7 currents are suppressed by intracellular GTPgammaS, suggesting the involvement of heterotrimeric G proteins. TRPM7 currents are also inhibited by stimulating endogenous muscarinic receptors, which is mediated by G(i) because the inhibitory effect is blunted by pertussis toxin. Conversely, stimulation of endogenous G(s)-coupled beta-adrenergic receptors potentiates TRPM7 currents, whereas G(q)-coupled thrombin receptors have little effect. Consistent with the involvement of G(s)/G(i) in controlling adenylyl cyclase activity, elevations of intracellular cAMP levels enhance TRPM7 activity and prevent receptor-mediated modulation of TRPM7 activity by muscarinic and adrenergic agonists. This cAMP-dependent effect requires the functional integrity of both protein kinase A (PKA) and the endogenous kinase domain of TRPM7 because cAMP-mediated effects are abolished when treating cells with the PKA inhibitors H89 or KT5720 as well as in cells expressing phosphotransferase-deficient TRPM7 constructs. These mutant channels are also much less susceptible to GTPgammaS-mediated inhibition, suggesting that the main regulatory effect occurs through G(i)- and G(s)-mediated changes in cAMP. Taken together, our results demonstrate that TRPM7 activity is up- and down-regulated through its endogenous kinase in a cAMP- and PKA-dependent manner.

Cytoskeletal reorganization internalizes multiple transient receptor potential channels and blocks calcium entry into human neutrophils

Store-operated calcium entry (SOCE) is required for polymorphonuclear neutrophil (PMN) activation in response to G protein-coupled agonists. Some immunocytes express proteins homologous to the Drosophila transient receptor potential gene (trp) calcium channel. TRP proteins assemble into heterotetrameric ion channels and are known to support SOCE in overexpression systems, but the evidence that TRP proteins support SOCE and are functionally important in wild-type cells remains indirect. We therefore studied the expression and function of TRP proteins in primary human PMN. TRPC1, TRPC3, TRPC4, and TRPC6 were all expressed as mRNA as well as membrane proteins. Immunofluorescence microscopy demonstrated localization of TRPC1, TRPC3, and TRPC4 to the PMN cell membrane and their internalization after cytoskeletal reorganization by calyculin A (CalyA). Either TRPC internalization by CalyA or treatment with the inositol triphosphate receptor inhibitor 2-aminoethoxydiphenyl borane resulted in the loss of PMN SOCE. Cytochalasin D (CytoD) disrupts actin filaments, thus preventing cytoskeletal reorganization, and pretreatment with CytoD rescued PMN SOCE from inhibition by CalyA. Comparative studies of CytoD and 2-aminoethoxydiphenyl borane inhibition of PMN cationic entry after thapsigargin or platelet-activating factor suggested that SOCE occurs through both calcium-specific and nonspecific pathways. Taken together, these studies suggest that the multiple TRPC proteins expressed by human PMN participate in the formation of at least two store-operated calcium channels that have differing ionic permeabilities and regulatory characteristics.

Transcriptional regulation of the melanoma prognostic marker melastatin (TRPM1) by MITF in melanocytes and melanoma

Determining the metastatic potential of intermediate thickness lesions remains a major challenge in the management of melanoma. Clinical studies have demonstrated that expression of melastatin/TRPM1 strongly predicts nonmetastatic propensity and correlates with improved outcome, leading to a national cooperative prospective study, which is ongoing currently. Similarly, the melanocytic markers MLANA/MART1 and MITF also have been shown to lose relative expression during melanoma progression. Recent studies have revealed that MITF, an essential transcription factor for melanocyte development, directly regulates expression of MLANA. This prompted examination of whether MITF also might transcriptionally regulate TRPM1 expression. The TRPM1 promoter contains multiple MITF consensus binding elements that were seen by chromatin immunoprecipitation to be occupied by endogenous MITF within melanoma cells. Endogenous TRPM1 expression responded strongly to MITF up- or down-regulation, as did TRPM1 promoter-driven reporters. In addition, MITF and TRPM1 mRNA levels were correlated tightly across a series of human melanoma cell lines. Mice homozygously mutated in MITF showed a dramatic decrease in TRPM1 expression. Finally, the slope of TRPM1 induction by MITF was particularly steep compared with other MITF target genes, suggesting it is a sensitive indicator of MITF expression and correspondingly of melanocytic differentiation. These studies identify MITF as a major transcriptional regulator of TRPM1 and suggest that its prognostic value may be linked to MITF-mediated regulation of cellular differentiation.

Noxious heat receptors present in cold-sensory cells in rats

Noxious heat above approximately 45 degrees C applied on cold spots evokes a paradoxical cold sensation by activating cold fibers. It remains unresolved whether cold receptors respond to heat as well, or whether noxious-heat receptors and cold receptors coexist in the same fiber. Recently, noxious heat receptors (TRPV1) and cold receptors (TRPM8) have been cloned. It is controversial, however, whether TRPV1 and TRPM8 coexist in the same sensory neuron. Here, we investigate colocalization of these receptors in dorsal root ganglion (DRG) of rats. TRPV1 was expressed in 29% of TRPM8-positive cells in DRG sections. In Ca2+ imaging, noxious heat excited many of the cold-sensitive cells in culture. In a whole-cell current-clamp mode, noxious heat, capsaicin, cooling and menthol all evoked receptor potentials and impulses in a subset of DRG neurons. This colocalization of TRPV1 and TRPM8 in a DRG neuron may be the basis for the paradoxical cold sensation.

Physiological induction of transient receptor potential canonical proteins, calcium entry channels, in human myometrium: influence of pregnancy, labor, and interleukin-1 beta

This study investigated gestational regulation of transient receptor potential canonical (TrpC) proteins, putative calcium entry channels in human myometrium, and the potential modulation of TrpC expression by IL-1 beta, a cytokine implicated in labor. Total RNA and proteins were isolated from myometrial biopsies obtained from NP women, pregnant women at term not in labor (TNL), or term active labor (TAL) and from primary cultured human myometrial smooth muscle cells incubated with IL-1 beta or IL-1 beta with or without nimesulide. Semiquantitative RT-PCR demonstrated significant up-regulation of TrpC1 in TAL and TNL (P < or = 0.01) and TrpC6 (P < or = 0.01) and TrpC7 (P < or = 0.05) in TAL samples. TrpC3 and TrpC4 mRNA expression was unaffected. Western blot demonstrated significant up-regulation of TrpC1 in TAL and TNL (P < or = 0.05) and TrpC3 (P < or = 0.01), TrpC4 (P < or = 0.05), and TrpC6 (P < or = 0.01) in TAL samples. IL-1 beta did not alter TrpC1, 3, 4, 6, or 7 mRNA expression; but IL-1 beta exclusively up-regulated TrpC3 protein expression (P < or = 0.05). TrpC3 up-regulation was unaffected by cyclooxygenase blockade. These data demonstrate physiological regulation of TrpC mRNA and protein and suggest an important role for TrpC proteins in human myometrium during labor.

Disruption of TRPM6/TRPM7 complex formation by a mutation in the TRPM6 gene causes hypomagnesemia with secondary hypocalcemia

Impaired magnesium reabsorption in patients with TRPM6 gene mutations stresses an important role of TRPM6 (melastatin-related TRP cation channel) in epithelial magnesium transport. While attempting to isolate full-length TRPM6, we found that the human TRPM6 gene encodes multiple mRNA isoforms. Full-length TRPM6 variants failed to form functional channel complexes because they were retained intracellularly on heterologous expression in HEK 293 cells and Xenopus oocytes. However, TRPM6 specifically interacted with its closest homolog, the Mg(2+)-permeable cation channel TRPM7, resulting in the assembly of functional TRPM6/TRPM7 complexes at the cell surface. The naturally occurring S141L TRPM6 missense mutation abrogated the oligomeric assembly of TRPM6, thus providing a cell biological explanation for the human disease. Together, our data suggest an important contribution of TRPM6/TRPM7 heterooligomerization for the biological role of TRPM6 in epithelial magnesium absorption.

Characterization of the mouse cold-menthol receptor TRPM8 and vanilloid receptor type-1 VR1 using a fluorometric imaging plate reader (FLIPR) assay

1. TRPM8 (CMR1) is a Ca(2+)-permeable channel, which can be activated by low temperatures, menthol, eucalyptol and icilin. It belongs to the transient receptor potential (TRP) family, and therefore is related to vanilloid receptor type-1 (VR1, TRPV1). We tested whether substances which are structurally related to menthol, or which produce a cooling sensation, could activate TRPM8, and compared the responses of TRPM8 and VR1 to these ligands. 2. The effects of 70 odorants and menthol-related substances on recombinant mouse TRPM8 (mTRPM8), expressed in HEK293 cells, were examined using a FLIPR assay. In all, 10 substances (linalool, geraniol, hydroxycitronellal, WS-3, WS-23, FrescolatMGA, FrescolatML, PMD38, CoolactP and Cooling Agent 10) were found to be agonists. 3. The EC(50) values of the agonists defined their relative potencies: icilin (0.2+/-0.1 microM)>FrescolatML (3.3+/-1.5 microM) > WS-3 (3.7+/-1.7 microM) >(-)menthol (4.1+/-1.3 microM) >frescolatMAG (4.8+/-1.1 microM) > cooling agent 10 (6+/-2.2 microM) >(+)menthol (14.4+/-1.3 microM) > PMD38 (31+/-1.1 microM) > WS-23 (44+/-7.3 microM) > Coolact P (66+/-20 microM) > geraniol (5.9+/-1.6 mM) > linalool (6.7+/-2.0 mM) > eucalyptol (7.7+/-2.0 mM) > hydroxycitronellal (19.6+/-2.2 mM). 4. Known VR1 antagonists (BCTC, thio-BCTC and capsazepine) were also able to block the response of TRPM8 to menthol (IC(50): 0.8+/-1.0, 3.5+/-1.1 and 18+/-1.1 microM, respectively). 5. The Ca(2+) response of hVR1-transfected HEK293 cells to the endogenous VR1 agonist N-arachidonoyl-dopamine was potentiated by low pH. In contrast, menthol- and icilin-activated TRPM8 currents were suppressed by low pH. 6. In conclusion, in the present study, we identified 10 new agonists and three antagonists of TRPM8. We found that, in contrast to VR1, TRPM8 is inhibited rather than potentiated by protons.

Intracellular nucleotides and polyamines inhibit the Ca2+-activated cation channel TRPM4b

TRPM4b (in contrast to the short splice variant TRPM4a) is a Ca(2+)-activated but Ca(2+)-impermeable cation channel. We have studied TRPM4 currents in inside-out patches. Supramicromolar Ca(2+) concentrations applied at the inner side, [Ca(2+)](i), activated TRPM4 with an EC(50) value of 0.37 mM, a value that is much higher than that of whole-cell currents. Current amplitudes decreased above 1 mM [Ca(2+)](i), (IC(50) 9.3 mM). Sr(2+) but not Ba(2+)could partially substitute for Ca(2+). ATP, ADP, AMP and AMP-PNP all quickly and reversibly inhibited TRPM4 with IC(50) values between 2 and 19 microM (at +100 mV). Adenosine also blocked TRPM4 at 630 microM. The block at high ATP concentrations was incomplete and was not affected by the presence of free Mg(2+). ADP induced the most sensitive block with an IC(50) of 2.2 microM. For inhibition of TRPM4 by free ATP(4-), an IC(50) value of 1.7+/-0.3 microM was calculated. GTP, UTP and CTP at concentrations up to 1 mM did not induce a similar block. Spermine blocked TRPM4 currents with an IC(50) of 61 microM. In conclusion, TRPM4 is a channel that can be effectively modulated by intracellular nucleotides and polyamines.

The enemy at the gates. Ca2+ entry through TRPM7 channels and anoxic neuronal death

In brain ischemia, gating of postsynaptic glutamate receptors is thought to initiate Ca2+ overload leading to excitotoxic neuronal death. In this issue, Aarts and colleagues describe a novel mechanism, whereby gating of TRPM7, a Ca2+-permeable nonselective cation channel, mediates Ca2+ overload and demise of anoxic neurons.

A key role for TRPM7 channels in anoxic neuronal death

Excitotoxicity in brain ischemia triggers neuronal death and neurological disability, and yet these are not prevented by antiexcitotoxic therapy (AET) in humans. Here, we show that in neurons subjected to prolonged oxygen glucose deprivation (OGD), AET unmasks a dominant death mechanism perpetuated by a Ca2+-permeable nonselective cation conductance (IOGD). IOGD was activated by reactive oxygen/nitrogen species (ROS), and permitted neuronal Ca2+ overload and further ROS production despite AET. IOGD currents corresponded to those evoked in HEK-293 cells expressing the nonselective cation conductance TRPM7. In cortical neurons, blocking IOGD or suppressing TRPM7 expression blocked TRPM7 currents, anoxic 45Ca2+ uptake, ROS production, and anoxic death. TRPM7 suppression eliminated the need for AET to rescue anoxic neurons and permitted the survival of neurons previously destined to die from prolonged anoxia. Thus, excitotoxicity is a subset of a greater overall anoxic cell death mechanism, in which TRPM7 channels play a key role.

Molecular pathophysiology of mucolipidosis type IV: pH dysregulation of the mucolipin-1 cation channel

Mucolipidosis type IV (MLIV) is an autosomal recessive neurogenetic disorder characterized by developmental abnormalities of the brain and impaired neurological, ophthalmologic and gastric function. Large vacuoles accumulate in various types of cells in MLIV patients. However, the pathophysiology of the disease at the cellular level is still unknown. MLIV is caused by mutations in a recently described gene, MCOLN1, encoding mucolipin-1 (ML1), a 65 kDa protein whose function is also unknown. ML1 shows sequence homology and topological similarities with polycystin-2 and other transient receptor potential (Trp) channels. In this study, we assessed both, whether ML1 has ion channel properties, and whether disease-causing mutations in MCOLN1 have functional differences with the wild-type (WT) protein. ML1 channel function was assessed from endosomal vesicles of null (MCOLN1(-/-)) and ML1 over-expressing cells, and liposomes containing the in vitro translated protein. Evidence from both preparations indicated that WT ML1 is a multiple subconductance non-selective cation channel whose function is inhibited by a reduction of pH. The V446L and DeltaF408 MLIV causing mutations retain channel function but not the sharp inhibition by lowering pH. Atomic force imaging of ML1 channels indicated that changes in pH modified the aggregation of unitary channels. Mutant-ML1 did not change in size on reduction of pH. The data indicate that ML1 channel activity is regulated by a pH-dependent mechanism that is deficient in some MLIV causing mutations of the gene. The evidence also supports a novel role for cation channels in the acidification and normal endosomal function.