Structural Determinants of the Transient Receptor Potential 1 (TRPV1) Channel Activation by Phospholipid Analogs

The transient receptor potential vanilloid 1 (TRPV1) ion channel is a polymodal protein that responds to various stimuli, including capsaicin (the pungent compound found in chili peppers), extracellular acid, and basic intracellular pH, temperatures close to 42 °C, and several lipids. Lysophosphatidic acid (LPA), an endogenous lipid widely associated with neuropathic pain, is an agonist of the TRPV1 channel found in primary afferent nociceptors and is activated by other noxious stimuli. Agonists or antagonists of lipid and other chemical natures are known to possess specific structural requirements for producing functional effects on their targets. To better understand how LPA and other lipid analogs might interact and affect the function of TRPV1, we set out to determine the structural features of these lipids that result in the activation of TRPV1. By changing the acyl chain length, saturation, and headgroup of these LPA analogs, we established strict requirements for activation of TRPV1. Among the natural LPA analogs, we found that only LPA 18:1, alkylglycerophosphate 18:1, and cyclic phosphatidic acid 18:1, all with a monounsaturated C18 hydrocarbon chain activate TRPV1, whereas polyunsaturated and saturated analogs do not. Thus, TRPV1 shows a more restricted ligand specificity compared with LPA G-protein-coupled receptors. We synthesized fatty alcohol phosphates and thiophosphates and found that many of them with a single double bond in position Δ9, 10, or 11 and Δ9 cyclopropyl group can activate TRPV1 with efficacy similar to capsaicin. Finally, we developed a pharmacophore and proposed a mechanistic model for how these lipids could induce a conformational change that activates TRPV1.     

Targeting the Transient Receptor Potential Vanilloid Type 1 (TRPV1) Assembly Domain Attenuates Inflammation-induced Hypersensitivity

The transient receptor potential channel vanilloid type 1 (TRPV1) is a non-selective cation channel expressed in sensory neurons of the dorsal root and trigeminal ganglia. TRPV1 is a polymodal channel activated by noxious heat, capsaicin, and protons. As a sensor for noxious stimuli, TRPV1 channel has been described as a key contributor to pain signaling. To form a functional channel, TRPV1 subunits must assemble into tetramers, and several studies have identified the TRPV1 C terminus as an essential element in subunit association. Here we combined biochemical assays with electrophysiology and imaging-based bimolecular fluorescence complementation (BiFC) and bioluminescence resonance energy transfer (BRET) in live cells to identify a short motif in the C-terminal tail of the TRPV1 subunit that governs channel assembly. Removing this region through early truncation or targeted deletion results in loss of subunit association and channel function. Importantly, we found that interfering with TRPV1 subunit association using a plasma membrane-tethered peptide attenuated mechanical and thermal hypersensitivity in two mouse models of inflammatory hyperalgesia. This represents a novel mechanism to disrupt TRPV1 subunit assembly and hence may offer a new analgesic tool for pain relief.     

Oligomeric State of Purified Transient Receptor Potential Melastatin-1 (TRPM1), a Protein Essential for Dim Light Vision

Transient receptor potential melastatin-1 (TRPM1) is essential for the light-induced depolarization of retinal ON bipolar cells. TRPM1 likely forms a multimeric channel complex, although almost nothing is known about the structure or subunit composition of channels formed by TRPM1 or any of its close relatives. Recombinant TRPM1 was robustly expressed in insect cells, but only a small fraction was localized to the plasma membrane. Similar intracellular localization was observed when TRPM1 was heterologously expressed in mammalian cells. TRPM1 was affinity-purified from Sf9 cells and complexed with amphipol, followed by detergent removal. In blue native gels and size exclusion chromatography, TRPM1 migrated with a mobility consistent with detergent- or amphipol-bound dimers. Cross-linking experiments were also consistent with a dimeric subunit stoichiometry, and cryoelectron microscopy and single particle analysis without symmetry imposition yielded a model with approximate 2-fold symmetrical features. Finally, electron microscopy of TRPM1-antibody complexes revealed a large particle that can accommodate TRPM1 and two antibody molecules. Taken together, these data indicate that purified TRPM1 is mostly dimeric. The three-dimensional structure of TRPM1 dimers is characterized by a small putative transmembrane domain and a larger domain with a hollow cavity. Blue native gels of solubilized mouse retina indicate that TRPM1 is present in two distinct complexes: one similar in size to the recombinant protein and one much larger. Because dimers are likely not functional ion channels, these results suggest that additional partner subunits participate in forming the transduction channel required for dim light vision and the ON pathway.     

Canonical Transient Receptor Potential 6 (TRPC6), a Redox-regulated Cation Channel

This study examined the effect of H₂O₂ on the TRPC6 channel and its underlying mechanisms using a TRPC6 heterologous expression system. In TRPC6-expressing HEK293T cells, H₂O₂ significantly stimulated Ca²⁺ entry in a dose-dependent manner. Electrophysiological experiments showed that H₂O₂ significantly increased TRPC6 channel open probability and whole-cell currents. H₂O₂ also evoked a robust inward current in A7r5 vascular smooth muscle cells, which was nearly abolished by knockdown of TRPC6 using a small interfering RNA. Catalase substantially attenuated arginine vasopressin (AVP)-induced Ca²⁺ entry in cells co-transfected with TRPC6 and AVP V1 receptor. N-Ethylmaleimide and thimerosal were able to simulate the H₂O₂ response. Dithiothreitol or glutathione-reduced ethyl ester significantly antagonized the response. Furthermore, both N-ethylmaleimide- and H₂O₂-induced TRPC6 activations were only observed in the cell-attached patches but not in the inside-out patches. Moreover, 1-oleoyl-2-acetyl-sn-glycerol effect on TRPC6 was significantly greater in the presence of H₂O₂. Biotinylation assays revealed a significant increase in cell surface TRPC6 in response to H₂O₂. Similarly, in cells transfected with TRPC6-EGFP, confocal microscopy showed a significant increase in fluorescence intensity in the region of the cell membrane and adjacent to the membrane. AVP also increased the fluorescence intensity on the surface of the cells co-transfected with TRPC6-EGFP and V1 receptor, and this response was inhibited by catalase. These data indicate that H₂O₂ activates TRPC6 channels via modification of thiol groups of intracellular proteins. This cysteine oxidation-dependent pathway not only stimulates the TRPC6 channel by itself but also sensitizes the channels to diacylglycerol and promotes TRPC6 trafficking to the cell surface.     

Regulation of the Temperature-dependent Activation of Transient Receptor Potential Vanilloid 1 (TRPV1) by Phospholipids in Planar Lipid Bilayers

TRPV1 (transient receptor potential vanilloid 1) proteins are heat-activated nonselective cation channels. TRPV1 channels are polymodal in their function and exhibit multifaceted regulation with various molecular compounds. In this regard, phosphoinositides, particularly phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate, are important channel regulators. However, their effects on TRPV1 channel activity have not been conclusively determined. To characterize temperature-induced activation of TRPV1 in the presence of different phospholipids, we purified the TRPV1 protein from HEK-293 cells and incorporated it into planar lipid bilayers. In the presence of 2.5 μm phosphatidylinositol 4,5-bisphosphate, TRPV1 channels demonstrated rapid activation at 33–39 °C and achieved full channel opening at 42 °C. At this temperature range, TRPV1 heat activation exhibited steep temperature dependence (temperature coefficient (Q₁₀) of 18), and the channel openings were accompanied by large changes in entropy and enthalpy, suggesting a substantial conformation change. At a similar temperature range, another phosphoinositide, phosphatidylinositol 4-phosphate, also potentiated heat activation of TRPV1, but with much lower efficiency. Negatively charged phosphatidylglycerol could also induce heat activation of TRPV1 channels, although with a small-conductance state. Our data demonstrate that phospholipids, specifically phosphoinositides, are important regulators of TRPV1 and are required for heat-induced channel activity.     

Brain-derived Neurotrophic Factor (BDNF) Induces Sustained Intracellular Ca2+ Elevation through the Up-regulation of Surface Transient Receptor Potential 3 (TRPC3) Channels in Rodent Microglia

Microglia are immune cells that release factors, including proinflammatory cytokines, nitric oxide (NO), and neurotrophins, following activation after disturbance in the brain. Elevation of intracellular Ca²⁺ concentration ([Ca²⁺]_i) is important for microglial functions such as the release of cytokines and NO from activated microglia. There is increasing evidence suggesting that pathophysiology of neuropsychiatric disorders is related to the inflammatory responses mediated by microglia. Brain-derived neurotrophic factor (BDNF) is a neurotrophin well known for its roles in the activation of microglia as well as in pathophysiology and/or treatment of neuropsychiatric disorders. In this study, we sought to examine the underlying mechanism of BDNF-induced sustained increase in [Ca²⁺]_i in rodent microglial cells. We observed that canonical transient receptor potential 3 (TRPC3) channels contribute to the maintenance of BDNF-induced sustained intracellular Ca²⁺ elevation. Immunocytochemical technique and flow cytometry also revealed that BDNF rapidly up-regulated the surface expression of TRPC3 channels in rodent microglial cells. In addition, pretreatment with BDNF suppressed the production of NO induced by tumor necrosis factor α (TNFα), which was prevented by co-adiministration of a selective TRPC3 inhibitor. These suggest that BDNF induces sustained intracellular Ca²⁺ elevation through the up-regulation of surface TRPC3 channels and TRPC3 channels could be important for the BDNF-induced suppression of the NO production in activated microglia. We show that TRPC3 channels could also play important roles in microglial functions, which might be important for the regulation of inflammatory responses and may also be involved in the pathophysiology and/or the treatment of neuropsychiatric disorders.     

Inhibition of Transient Receptor Potential Channel 5 Reverses 5-Fluorouracil Resistance in Human Colorectal Cancer Cells

5-Fluorouracil (5-Fu) is commonly used in the chemotherapy of colorectal cancer (CRC), but resistance to 5-Fu occurs in most cases, allowing cancer progression. Suppressing ABCB1 (ATP-binding cassette, subfamily B, member 1), which is a pump overproduced in cancer cells to export cytotoxic drugs, is an attractive strategy to overcome drug resistance. In the present study, transient receptor potential channel TrpC5 was found to be overproduced at the mRNA and protein levels together with ABCB1 in 5-Fu-resistant human CRC HCT-8 (HCT-8/5-Fu) and LoVo (LoVo/5-Fu) cells. More nuclear-stabilized β-catenin accumulation was found in HCT-8/5-Fu and LoVo/5-Fu cells than in HCT-8 and LoVo cells. Suppressing TrpC5 expression with TrpC5-specific siRNA inhibited the canonical Wnt/β-catenin signal pathway, reduced the induction of ABCB1, weakened the ABCB1 efflux pump, and caused a remarkable reversal of 5-Fu resistance in HCT-8/5-Fu and LoVo/5-Fu cells. On the contrary, enforcing TrpC5 expression resulted in an activated Wnt/β-catenin signal pathway and up-regulation of ABCB1. Taken together, we demonstrated an essential role of TrpC5 in ABCB1 induction and drug resistance in human CRC cells via promoting nuclear β-catenin accumulation.     

Identification of Significant Amino Acids in Multiple Transmembrane Domains of Human Transient Receptor Potential Ankyrin 1 (TRPA1) for Activation by Eudesmol,an Oxygenized Sesquiterpene in Hop Essential Oil

Transient receptor potential ankyrin 1 (TRPA1) is a calcium-permeable non-selective cation channel that is activated by various noxious or irritant substances in nature, including spicy compounds. Many TRPA1 chemical activators have been reported; however, only limited information is available regarding the amino acid residues that contribute to the activation by non-electrophilic activators, whereas activation mechanisms by electrophilic ligands have been well characterized. We used intracellular Ca²⁺ measurements and whole-cell patch clamp recordings to show that eudesmol, an oxygenated sesquiterpene present at high concentrations in the essential oil of hop cultivar Hallertau Hersbrucker, could activate human TRPA1. Gradual activation of inward currents with outward rectification by eudesmol was observed in human embryonic kidney-derived 293 cells expressing human TRPA1. This activation was completely blocked by a TRPA1-specific inhibitor, HC03–0031. We identified three critical amino acid residues in human TRPA1 in putative transmembrane domains 3, 4, and 5, namely threonine at 813, tyrosine at 840, and serine at 873, for activation by β-eudesmol in a systematic mutational study. Our results revealed a new TRPA1 activator in hop essential oil and provide a novel insight into mechanisms of human TRPA1 activation by non-electrophilic chemicals.     

Molecular Determinants of Phosphatidylinositol 4,5-Bisphosphate (PI(4,5)P2) Binding to Transient Receptor Potential V1 (TRPV1) Channels

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) has been recognized as an important activator of certain transient receptor potential (TRP) channels. More specifically, TRPV1 is a pain receptor activated by a wide range of stimuli. However, whether or not PI(4,5)P₂ is a TRPV1 agonist remains open to debate. Utilizing a combined approach of mutagenesis and molecular modeling, we identified a PI(4,5)P₂ binding site located between the TRP box and the S4-S5 linker. At this site, PI(4,5)P₂ interacts with the amino acid residues Arg-575 and Arg-579 in the S4-S5 linker and with Lys-694 in the TRP box. We confirmed that PI(4,5)P₂ behaves as a channel agonist and found that Arg-575, Arg-579, and Lys-694 mutations to alanine reduce PI(4,5)P₂ binding affinity. Additionally, in silico mutations R575A, R579A, and K694A showed that the reduction in binding affinity results from the delocalization of PI(4,5)P₂ in the binding pocket. Molecular dynamics simulations indicate that PI(4,5)P₂ binding induces conformational rearrangements of the structure formed by S6 and the TRP domain, which cause an opening of the lower TRPV1 channel gate.     

Structural Dynamics of the Glycine-binding Domain of the N-Methyl-d-Aspartate Receptor

N-Methyl-d-aspartate receptors mediate the slow component of excitatory neurotransmission in the central nervous system. These receptors are obligate heteromers containing glycine- and glutamate-binding subunits. The ligands bind to a bilobed agonist-binding domain of the receptor. Previous x-ray structures of the glycine-binding domain of NMDA receptors showed no significant changes between the partial and full agonist-bound structures. Here we have used single molecule fluorescence resonance energy transfer (smFRET) to investigate the cleft closure conformational states that the glycine-binding domain of the receptor adopts in the presence of the antagonist 5,7-dichlorokynurenic acid (DCKA), the partial agonists 1-amino-1-cyclobutanecarboxylic acid (ACBC) and l-alanine, and full agonists glycine and d-serine. For these studies, we have incorporated the unnatural amino acid p-acetyl-l-phenylalanine for specific labeling of the protein with hydrazide derivatives of fluorophores. The single molecule fluorescence resonance energy transfer data show that the agonist-binding domain can adopt a wide range of cleft closure states with significant overlap in the states occupied by ligands of varying efficacy. The difference lies in the fraction of the protein in a more closed-cleft form, with full agonists having a larger fraction in the closed-cleft form, suggesting that the ability of ligands to select for these states could dictate the extent of activation.     

Proton-mediated Conformational Changes in an Acid-sensing Ion Channel

Acid-sensing ion channels are cation channels activated by external protons and play roles in nociception, synaptic transmission, and the physiopathology of ischemic stroke. Using luminescence resonance energy transfer (LRET), we show that upon proton binding, there is a conformational change that increases LRET efficiency between the probes at the thumb and finger subdomains in the extracellular domain of acid-sensing ion channels. Additionally, we show that this conformational change is lost upon mutating Asp-238, Glu-239, and Asp-260, which line the finger domains, to alanines. Electrophysiological studies showed that the single mutant D260A shifted the EC₅₀ by 0.2 pH units, the double mutant D238A/E239A shifted the EC₅₀ by 2.5 pH units, and the triple mutant D238A/E239A/D260A exhibited no response to protons despite surface expression. The LRET experiments on D238A/E239A/D260A showed no changes in LRET efficiency upon reduction in pH from 8 to 6. The LRET and electrophysiological studies thus suggest that the three carboxylates, two of which are involved in carboxyl/carboxylate interactions, are essential for proton-induced conformational changes in the extracellular domain, which in turn are necessary for receptor activation.     

The TRPM8 Protein Is a Testosterone Receptor: I. BIOCHEMICAL EVIDENCE FOR DIRECT TRPM8-TESTOSTERONE INTERACTIONS*

The transient receptor potential ion channel of the melastatin subfamily, TRPM8, is a major cold receptor in the peripheral nervous system. Along with the sensory neurons, the TRPM8 protein is highly expressed in the prostate epithelial cells, and this expression is regulated by androgens. Here we investigated the expression and intracellular localization of the TRPM8 channel in relationship to androgens. We performed experiments using human prostate tissues obtained from healthy individuals and patients with prostate cancer at various stages of the disease as well as in cultured cells. Using an immunohistochemistry approach, we detected an intensive colocalization pattern of the TRPM8 protein with endogenous androgens in all tissues tested, suggesting possible interactions. Co-immunoprecipitation experiments performed using cultured prostate epithelial cells, prostate cancer cells, and HEK-293 cells stably expressing TRPM8 further confirmed direct binding of the steroid hormone, testosterone, to the TRPM8 protein. Applications of picomolar concentrations of testosterone to the primary human prostate cells, endogenously expressing TRPM8, elicited Ca²⁺ responses and channel currents, and those were inhibited in the presence of TRPM8 antagonist, N-(2-aminoethyl)-N-(4-(benzyloxy)-3-methoxybenzyl)thiophene-2-carboxamide hydrochloride. These results indicate that the TRPM8 channel is physically associated with testosterone and suggest that, in addition to a genomic role, testosterone plays a role in direct regulation of the TRPM8 channel function.     

Extracellular Zinc Ion Regulates Transient Receptor Potential Melastatin 5 (TRPM5) Channel Activation through Its Interaction with a Pore Loop Domain

The transient receptor potential melastatin 5 (TRPM5) channel is a monovalent cation channel activated by intracellular Ca²⁺. Expression of this channel is restricted to taste cells, the pancreas and brainstem, and is thought to be involved in controlling membrane potentials. Its endogenous ligands are not well characterized. Here, we show that extracellular application of Zn²⁺ inhibits TRPM5 activity. In whole-cell patch-clamp recordings, extracellular application of ZnCl₂ inhibited step-pulse-induced TRPM5 currents with 500 nm free intracellular Ca²⁺ in a dose-dependent manner (IC₅₀ = 4.3 μm at −80 mV). ZnSO₄ also inhibited TRPM5 activity. Extracellular application of ZnCl₂ inhibited TRPM5 activation at several temperatures. Furthermore, inhibition by 30 μm ZnCl₂ was impaired in TRPM5 mutants in which His at 896, and Glu at 926 and/or Glu at 939 in the outer pore loop were replaced with Gln. From these results, we conclude that extracellular Zn²⁺ inhibits TRPM5 channels, and the residues in the outer pore loop of TRPM5 are critically involved in the inhibition.     

Role of the Outer Pore Domain in Transient Receptor Potential Vanilloid 1 Dynamic Permeability to Large Cations

Transient receptor potential vanilloid 1 (TRPV1) has been shown to alter its ionic selectivity profile in a time- and agonist-dependent manner. One hallmark of this dynamic process is an increased permeability to large cations such as N-methyl-d-glucamine (NMDG). In this study, we mutated residues throughout the TRPV1 pore domain to identify loci that contribute to dynamic large cation permeability. Using resiniferatoxin (RTX) as the agonist, we identified multiple gain-of-function substitutions within the TRPV1 pore turret (N628P and S629A), pore helix (F638A), and selectivity filter (M644A) domains. In all of these mutants, maximum NMDG permeability was substantially greater than that recorded in wild type TRPV1, despite similar or even reduced sodium current density. Two additional mutants, located in the pore turret (G618W) and selectivity filter (M644I), resulted in significantly reduced maximum NMDG permeability. M644A and M644I also showed increased and decreased minimum NMDG permeability, respectively. The phenotypes of this panel of mutants were confirmed by imaging the RTX-evoked uptake of the large cationic fluorescent dye YO-PRO1. Whereas none of the mutations selectively altered capsaicin-induced changes in NMDG permeability, the loss-of-function phenotypes seen with RTX stimulation of G618W and M644I were recapitulated in the capsaicin-evoked YO-PRO1 uptake assay. Curiously, the M644A substitution resulted in a loss, rather than a gain, in capsaicin-evoked YO-PRO1 uptake. Modeling of our mutations onto the recently determined TRPV1 structure revealed several plausible mechanisms for the phenotypes observed. We conclude that side chain interactions at a few specific loci within the TRPV1 pore contribute to the dynamic process of ionic selectivity.     

Conformational changes at the agonist binding domain of the N-methyl-D-aspartic acid receptor

The conformational changes in the agonist binding domain of the glycine-binding GluN1 and glutamate-binding GluN2A subunits of the N-methyl D-aspartic acid receptor upon binding agonists of varying efficacy have been investigated by luminescence resonance energy transfer (LRET) measurements. The LRET-based distances indicate a cleft closure conformational change at the GluN1 subunit upon binding agonists; however, no significant changes in the cleft closure are observed between partial and full agonists. This is consistent with the previously reported crystal structures for the isolated agonist binding domain of this receptor. Additionally, the LRET-based distances show that the agonist binding domain of the glutamate-binding GluN2A subunit exhibits a graded cleft closure with the extent of cleft closure being proportional to the extent of activation, indicating that the mechanism of activation in this subunit is similar to that of the glutamate binding α-amino-5-methyl-3-hydroxy-4-isoxazole propionate and kainate subtypes of the ionotropic glutamate receptors.     

Amino-terminal Domain Tetramer Organization and Structural Effects of Zinc Binding in the N-Methyl-d-aspartate (NMDA) Receptor

N-Methyl-d-aspartate (NMDA) receptors mediate excitatory neurotransmission in the mammalian central nervous system. An important feature of these receptors is their capacity for allosteric regulation by small molecules, such as zinc, which bind to their amino-terminal domain (ATD). Zinc inhibition through high affinity binding to the ATD has been examined through functional studies; however, there is no direct measurement of associated conformational changes. We used luminescence resonance energy transfer to show that the ATDs undergo a cleft closure-like conformational change upon binding zinc, but no changes are observed in intersubunit distances. Furthermore, we find that the ATDs are more closely packed than the related AMPA receptors. These results suggest that the stability of the upper lobe contacts between ATDs allow for the efficient propagation of the cleft closure conformational change toward the ligand-binding domain and transmembrane segments, ultimately inhibiting the channel.     

Trans-activation Response (TAR) RNA-binding Protein 2 Is a Novel Modulator of Transient Receptor Potential Canonical 4 (TRPC4) Protein

TRPC4 proteins function as Ca²⁺ conducting, non-selective cation channels in endothelial, smooth muscle, and neuronal cells. To further characterize the roles of TRPC4 in vivo, detailed information about the molecular composition of native channel complexes and their association with cellular signaling networks is needed. Therefore, a mouse brain cDNA library was searched for novel TRPC4-interacting proteins using a modified yeast two-hybrid assay. This screen identified Trans-activation Response RNA-binding protein 2 (Tarpb2), a protein that recruits the Dicer complex to Ago2 for microRNA processing and gene silencing. Tarbp2 was found to bind to the C terminus of TRPC4 and TRPC5 and to modulate agonist-dependent TRPC4-induced Ca²⁺ entry. A stretch of basic residues within the Tarbp2 protein is required for these actions. Tarbp2 binding to and modulation of TRPC4 occurs in the presence of endogenously expressed Dicer but is no longer detectable when the Dicer cDNA is overexpressed. Dicer activity in crude cell lysates is increased in the presence of Ca²⁺, most probably by Ca²⁺-dependent proteolytic activation of Dicer. Apparently, Tarbp2 binding to TRPC4 promotes changes of cytosolic Ca²⁺ and, thereby, leads to a dynamic regulation of Dicer activity, essentially at low endogenous Dicer concentrations.     

The TRPM8 Protein Is a Testosterone Receptor: II. FUNCTIONAL EVIDENCE FOR AN IONOTROPIC EFFECT OF TESTOSTERONE ON TRPM8*

Testosterone is a key steroid hormone in the development of male reproductive tissues and the regulation of the central nervous system. The rapid signaling mechanism induced by testosterone affects numerous behavioral traits, including sexual drive, aggressiveness, and fear conditioning. However, the currently identified testosterone receptor(s) is not believed to underlie the fast signaling, suggesting an orphan pathway. Here we report that an ion channel from the transient receptor potential family, TRPM8, commonly known as the cold and menthol receptor is the major component of testosterone-induced rapid actions. Using cultured and primary cell lines along with the purified TRPM8 protein, we demonstrate that testosterone directly activates TRPM8 channel at low picomolar range. Specifically, testosterone induced TRPM8 responses in primary human prostate cells, PC3 prostate cancer cells, dorsal root ganglion neurons, and hippocampal neurons. Picomolar concentrations of testosterone resulted in full openings of the purified TRPM8 channel in planar lipid bilayers. Furthermore, acute applications of testosterone on human skin elicited a cooling sensation. Our data conclusively demonstrate that testosterone is an endogenous and highly potent agonist of TRPM8, suggesting a role of TRPM8 channels well beyond their well established function in somatosensory neurons. This discovery may further imply TRPM8 channel function in testosterone-dependent behavioral traits.     

Zinc-induced Neurotoxicity Mediated by Transient Receptor Potential Melastatin 7 Channels

Transient receptor potential melastatin 7 (TRPM7) channels are novel Ca²⁺-permeable non-selective cation channels ubiquitously expressed. Activation of TRPM7 channels has been shown to be involved in cellular Mg²⁺ homeostasis, diseases caused by abnormal magnesium absorption, and in Ca²⁺-mediated neuronal injury under ischemic conditions. Here we show strong evidence suggesting that TRPM7 channels also play an important role in cellular Zn²⁺ homeostasis and in Zn²⁺-mediated neuronal injury. Using a combination of fluorescent Zn²⁺ imaging, small interfering RNA, pharmacological analysis, and cell injury assays, we show that activation of TRPM7 channels augmented Zn²⁺-induced injury of cultured mouse cortical neurons. The Zn²⁺-mediated neurotoxicity was inhibited by nonspecific TRPM7 blockers Gd³⁺ or 2-aminoethoxydiphenyl borate, and by knockdown of TRPM7 channels with small interfering RNA. In addition, Zn²⁺-mediated neuronal injury under oxygen-glucose deprivation conditions was also diminished by silencing TRPM7. Furthermore, we show that overexpression of TRPM7 channels in HEK293 cells increased intracellular Zn²⁺ accumulation and Zn²⁺-induced cell injury, while silencing TRPM7 by small interfering RNA attenuated the Zn²⁺-mediated cell toxicity. Thus, TRPM7 channels may represent a novel target for neurological disorders where Zn²⁺ toxicity plays an important role.