Modulation of temperature-sensitive TRP channels

Animals sense temperature--either cold or hot--by the direct activation of temperature-sensitive members of the TRP family of ion channels, the thermo-TRPs. To date, six TRP channels--TRPV1-4, TRPM8 and TRPA1--have been reported to be directly activated by heat and to be involved in thermosensation. Temperature sensing can be modulated by phosphorylation of intracellular residues by protein kinases or by insertion of new channels into the cell membrane. In this review we provide a brief overview of the properties of thermo-TRPs, and we summarise signalling pathways involved in their regulation.     

The TRP ion channel family

Mammalian homologues of the Drosophila transient receptor potential (TRP) channel gene encode a family of at least 20 ion channel proteins. They are widely distributed in mammalian tissues, but their specific physiological functions are largely unknown. A common theme that links the TRP channels is their activation or modulation by phosphatidylinositol signal transduction pathways. The channel subunits have six transmembrane domains that most probably assemble into tetramers to form non-selective cationic channels, which allow for the influx of calcium ions into cells. Three subgroups comprise the TRP channel family; the best understood of these mediates responses to painful stimuli. Other proposed functions include repletion of intracellular calcium stores, receptor-mediated excitation and modulation of the cell cycle.     

The TRP channel and phospholipase C-mediated signaling

Drosophila photoreceptors use a phospholipase C-mediated signaling for phototransduction. This pathway begins by light activation of a G-protein-coupled photopigment and ends by activation of the TRP and TRPL channels. The Drosophila TRP protein is essential for the high Ca²⁺ permeability and constitutes the major component of the light-induced current, thereby affecting both excitation and adaptation of the photoreceptor cell. TRP is the prototype of a large and diverse multigene family whose members are sharing a structure, which is conserved through evolution from the worm Caenorhabditis elegans to humans. TRP-related channel proteins are found in a variety of cells and tissues and show a large functional diversity although the gating mechanism of Drosophila TRP and of other TRP-related channels is still unknown.     

A yeast genetic screen reveals a critical role for the pore helix domain in TRP channel gating

TRP cation channels function as cellular sensors in uni- and multicellular eukaryotes. Despite intensive study, the mechanisms of TRP channel activation by chemical or physical stimuli remain poorly understood. To identify amino acid residues crucial for TRP channel gating, we developed an unbiased, high-throughput genetic screen in yeast that uncovered rare, constitutively active mutants of the capsaicin receptor, TRPV1. We show that mutations within the pore helix domain dramatically increase basal channel activity and responsiveness to chemical and thermal stimuli. Mutation of corresponding residues within two related TRPV channels leads to comparable effects on their activation properties. Our data suggest that conformational changes in the outer pore region are critical for determining the balance between open and closed states, providing evidence for a general role for this domain in TRP channel activation.     

Coiled coils direct assembly of a cold-activated TRP channel

Transient receptor potential (TRP) channels mediate numerous sensory transduction processes and are thought to function as tetramers. TRP channel physiology is well studied; however, comparatively little is understood regarding TRP channel assembly. Here, we identify an autonomously folded assembly domain from the cold- and menthol-gated channel TRPM8. We show that the TRPM8 cytoplasmic C-terminal domain contains a coiled coil that is necessary for channel assembly and sufficient for tetramer formation. Cell biological experiments indicate that coiled-coil formation is required for proper channel maturation and trafficking and that the coiled-coil domain alone can act as a dominant-negative inhibitor of functional channel expression. Our data define an authentic TRP modular assembly domain, establish a clear role for coiled coils in ion channel assembly, demonstrate that coiled-coil assembly domains are a general feature of TRPM channels, and delineate a new tool that should be of general use in dissecting TRPM channel function.     

Outer pore architecture of a Ca2+-selective TRP channel

The TRP superfamily forms a functionally important class of cation channels related to the product of the Drosophila trp gene. TRP channels display an unusual diversity in activation mechanisms and permeation properties, but the basis of this diversity is unknown, as the structure of these channels has not been studied in detail. To obtain insight in the pore architecture of TRPV6, a Ca(2+)-selective member of the TRPV subfamily, we probed the dimensions of its pore and determined pore-lining segments using cysteine-scanning mutagenesis. Based on the permeability of the channel to organic cations, we estimated a pore diameter of 5.4 A. Mutating Asp(541), a residue involved in high affinity Ca(2+) binding, altered the apparent pore diameter, indicating that this residue lines the narrowest part of the pore. Cysteines introduced in a region preceding Asp(541) displayed a cyclic pattern of reactivity to Ag(+) and cationic methylthio-sulfanate reagents, indicative of a pore helix. The anionic methanethiosulfonate ethylsulfonate showed only limited reactivity in this region, consistent with the presence of a cation-selective filter at the outer part of the pore helix. Based on these data and on homology with the bacterial KcsA channel, we present the first structural model of a TRP channel pore. We conclude that main structural features of the outer pore, namely a selectivity filter preceded by a pore helix, are conserved between K(+) channels and TRPV6. However, the selectivity filter of TRPV6 is wider than that of K(+) channels and lined by amino acid side chains rather than main chain carbonyls.     

Regulation of TRP channel TRPM2 by the tyrosine phosphatase PTPL1

TRPM2, a member of the transient receptor potential (TRP) superfamily, is a Ca²⁺-permeable channel, which mediates susceptibility to cell death following activation by oxidative stress, TNF, or -amyloid peptide. We determined that TRPM2 is rapidly tyrosine phosphorylated after stimulation with H₂O₂ or TNF. Inhibition of tyrosine phosphorylation with the tyrosine kinase inhibitors genistein or PP2 significantly reduced the increase in [Ca²⁺]_i observed after H₂O₂ or TNF treatment in TRPM2-expressing cells, suggesting that phosphorylation is important in TRPM2 activation. Utilizing a TransSignal PDZ domain array blot to identify proteins which interact with TRPM2, we identified PTPL1 as a potential binding protein. PTPL1 is a widely expressed tyrosine phosphatase, which has a role in cell survival and tumorigenesis. Immunoprecipitation and glutathione-S-transferase pull-down assays confirmed that TRPM2 and PTPL1 interact. To examine the ability of PTPL1 to modulate phosphorylation or activation of TRPM2, PTPL1 was coexpressed with TRPM2 in human embryonic kidney-293T cells. This resulted in significantly reduced TRPM2 tyrosine phosphorylation, and inhibited the rise in [Ca²⁺]_i and the loss of cell viability, which follow H₂O₂ or TNF treatment. Consistent with these findings, reduction in endogenous PTPL1 expression with small interfering RNA resulted in increased TRPM2 tyrosine phosphorylation, a significantly greater rise in [Ca²⁺]_i following H₂O₂ treatment, and enhanced susceptibility to H₂O₂-induced cell death. Endogenous TRPM2 and PTPL1 was associated in U937-ecoR cells, confirming the physiological relevance of this interaction. These data demonstrate that tyrosine phosphorylation of TRPM2 is important in its activation and function and that inhibition of TRPM2 tyrosine phosphorylation reduces Ca²⁺ influx and protects cell viability. They also suggest that modulation of TRPM2 tyrosine phosphorylation is a mechanism through which PTPL1 may mediate resistance to cell death. transient receptor potential channels; oxidative stress     

TRP channel blamed for burning cold after a tropical fish meal

EMBO J (2012) 31 19, 3795–3808 doi:10.1038/emboj.2012.207; published online July312012Ciguatera is one of the most common forms of food poisoning, occurring after consumption of fish contaminated with ciguatoxins. New work by Vetter et al (2012) reveals the key molecular players that underlie the altered temperature sensation associated with ciguatera. In particular, they show that ciguatoxins act on sensory neurons that express TRPA1, an ion channel implicated in the detection of noxious cold.Imaging yourself in the following idyllic settings: a white sandy tropical beach, blue sea and sky; and a barbecue on which your catch of the day, a 4-kg red snapper, is being grilled for dinner. But then, just a few hours after savouring the delicious fish meal, you undergo another unforgettable but far less heavenly experience: it starts with severe nausea, vomiting and diarrhoea, followed by disturbing neurological symptoms including headache, numbness and burning of the skin. You contracted ciguatera, a food-borne disease that affects an estimated 500 000 persons each year, particularly in the tropical and sub-tropical coastal regions (Dickey and Plakas, 2010).So what causes ciguatera? The prime culprits are dinoflagellates of the genus Gambierdiscus, small microalgae that produce a group of fat-soluble toxins called ciguatoxins and grow on macroalgae in coral reefs (Yasumoto et al, 1977). Gambierdiscus-containing macroalgae serve as food for herbivorous fish, which results in the introduction of ciguatoxins into the food web. These smaller, herbivorous fish are on the menu of large, carnivorous reef fish, such as mackerel, red snapper, or barracuda, which can accumulate ciguatoxins in the fatty parts of their body over years, apparently without any distress or obvious sign of disease. However, when such a mouth-watering but toxin-loaded catch appears on your menu, just a few bites (or sips of fish broth) can be sufficient to induce ciguatera (Figure 1A). Disturbingly, affected fish looks, smells, and tastes normal, and ciguatoxins are resistant to grilling, drying, or cooking of the fish, so there is no straightforward method to predict whether your tropical culinary dream will be followed by a ciguatera nightmare. Yes, there are commercial kits available to test for the presence of the toxin in fish, but these are considered too cumbersome, unreliable, and/or expensive to be of general practical use. And yes, there are persistent rumours of tropical fishermen feeding part of their catch to the cat first…Open in a separate windowFigure 1Ciguatera and cold allodynia. (A) Ciguatoxins, polycyclic polyether toxins, are produced by dinoflagellates of the genus Gambierdiscus, which reside in macroalgae in tropical coral reefs. These Gambierdiscus-containing macroalgae are eaten by herbivorous fish, which in turn serve as food for larger carnivorous fish, which accumulate the toxins in their bodies. Human consumption of these toxin-loaded fish causes ciguatera. (B) Ciguatoxins cause cold allodynia by increasing the cold sensitivity of TRPA1-expressing nociceptor neurons. Left, sensory nerve ending of a normal TRPA1-expressing nociceptor. At temperatures in the warm or innocuous cold region, the membrane potential (E_m) is around −65 mV, and both TRPA1 and the voltage-gated Na⁺ (Na_V) channels are closed. Cooling <10°C causes sufficient TRPA1 activation to cause action potential (AP) firing, experienced as burning cold. Right, ciguatoxin causes modification of the voltage-gated Na⁺ channels, resulting in significant opening at rest and depolarization of E_m to around −55 mV. Increased neuronal excitability and depolarization-induced activation of TRPA1 result in action potential firing and burning pain at innocuously cold temperatures.Apart from the gastrointestinal torment, which mostly fades away after a day or so, one of the most striking and disturbing symptoms of ciguatera is a form of oversensitivity to cold, termed as cold allodynia (Isbister and Kiernan, 2005). For instance, ciguatera sufferers have reported that a refreshing dive in the ocean actually caused burning pain, or that drinking cool beer felt like too hot coffee. The mechanisms whereby ciguatoxins provoke this peculiar form of altered temperature sensitivity, which can last for weeks to years, were fully unknown.In their paper, Vetter et al (2012) provide the first insights into the molecular mechanisms underlying cold allodynia induced by P-CTX-1, the most potent ciguatoxin present in the Pacific Ocean. First, they demonstrate that ciguatoxin-induced cold allodynia can be reproduced in a mouse model: following injection of minute quantities of P-CTX-1 into the sole of the paw, these mice exhibited clear signs of pain at moderately cool ambient temperatures, which were alleviated by warming. Next, they provide evidence that P-CTX-1 acts by sensitizing a specific subset of sensory neurons, characterized by the expression of the cation channel TRPA1. Finally, they show that pharmacological inhibition or genetic disruption of TRPA1 in mice strongly reduce the severity of cold allodynia upon P-CTX-1 injection.TRPA1 is a member of the TRP superfamily of cation channels, many of which play key roles in the sensory system as molecular sensors of temperature and of a variety of chemical ligands (Talavera et al, 2008). TRPA1 was known to be expressed in a subset of pain-sensing neurons (nociceptors), where it acts as a sensor of a wide variety of pungent/irritating substances (e.g., mustard oil) and of noxious cold (Story et al, 2003; Jordt et al, 2004; Karashima et al, 2009). TRPA1, like most other temperature-sensitive TRP channels, is voltage dependent, and thermal activation reflects a temperature-induced shift of its voltage-dependent activation curve towards more negative potentials (Voets et al, 2004; Karashima et al, 2009).Although TRPA1-expressing neurons show exquisite sensitivity to P-CTX-1, Vetter et al (2012) show that TRPA1 by itself is insensitive to P-CTX-1. Instead, P-CTX-1 responsiveness requires the combined presence of TRPA1 and of voltage-gated Na⁺ channels. In nociceptor neurons of non-intoxicated individuals, TRPA1 and the voltage-gated Na⁺ channels are largely closed at the resting membrane potential of around −65 mV, both at warm and at innocuously cool temperatures. Only upon cooling into the noxious cold range (<10°C), sufficient TRPA1 activation occurs to cause depolarization beyond the threshold for action potential firing (Figure 1B). This creates the sensation of burning cold (Karashima et al, 2009), which represents an important alarm signal, warning the body for potential cold-induced tissue damage (frostbite). However, as Vetter et al (2012) demonstrate, poisoning with P-CTX-1 strongly deregulates the cold sensitivity of the TRPA1-expressing nociceptors, such that they greatly overdo their alarm function. Reconfirming earlier work (Isbister and Kiernan, 2005), they show that P-CTX-1 acts as a potent modifier of voltage-gated Na⁺ channels in these neurons. In particular, at a concentration as low as 1 nM, P-CTX-1 causes a hyperpolarizing shift of the voltage dependence of activation of Na⁺ channels. This has a dual effect on the cold sensitivity of the TRPA1-expressing nociceptors (Figure 1B): (1) modulation of the voltage-gated Na⁺ channels increases the excitability of the neurons, an effect that is further enhanced by the inhibitory effect of P-CTX-1 on voltage-gated K⁺ channels and (2) P-CTX-1-treated voltage-gated Na⁺ channels allow Na⁺ influx at the resting membrane potential of these neurons, resulting in membrane depolarization, which in turn facilitates the voltage-sensitive activation of TRPA1 (Karashima et al, 2009). These mechanisms explain why in individuals suffering from ciguatera innocuous cooling is already sufficient to provoke action potential firing of TRPA1-expressing nociceptors, and is perceived as a burning pain.In addition to provide a molecular and cellular basis for ciguatera-associated cold allodynia, the study by Vetter et al (2012) also further strengthens the theory that TRPA1 is a relevant cold sensor in vivo, which has been disputed in several studies (Jordt et al, 2004; Knowlton et al, 2010). In particular, by using non-invasive functional MRI brain imaging, Vetter et al (2012) show for the first time significant differences in brain activity between wild-type and TRPA1-deficient mice when cooling their paw, and this difference is even more pronounced after the injection of P-CTX-1.Some important open questions remain. For example, given that P-CTX-1 is also acting on voltage-gated Na⁺ channels in TRPA1-negative sensory neurons, it is surprising that the toxin did not cause increased sensitivity to other stimuli, such as heat or mechanical stimuli. Moreover, it would be interesting to know whether TRPA1, which is also highly expressed on sensory neurons that innervate the gastrointestinal tract, plays a role in the gastrointestinal symptoms of ciguatera. If so, then TRPA1 would be an even more appealing target for the development of specific drugs to create relief from ciguatera symptoms.     

The role of synaptic facilitation in spike coincidence detection

Using a realistic model of activity dependent dynamical synapse, which includes both depressing and facilitating mechanisms, we study the conditions in which a postsynaptic neuron efficiently detects temporal coincidences of spikes which arrive from N different presynaptic neurons at certain frequency f. A numerical and analytical treatment of that system shows that: (1) facilitation enhances the detection of correlated signals arriving from a subset of presynaptic excitatory neurons, and (2) the presence of facilitation yields to a better detection of firing rate changes in the presynaptic activity. We also observed that facilitation determines the existence of an optimal input frequency which allows the best performance for a wide (maximum) range of the neuron firing threshold. This optimal frequency can be controlled by means of facilitation parameters. Finally, we show that these results are robust even for very noisy signals and in the presence of synaptic fluctuations produced by the stochastic release of neurotransmitters.     

Binaural interaction in the brain-stem auditory evoked potential: evidence for a delay line coincidence detection mechanism

The binaural interaction component (BIC) of the brain-stem auditory evoked potential (BAEP) was studied in 13 normally hearing adults by subtracting the response to binaural clicks from the algebraic sum of monaural responses. Eight or 16 electrodes on the head and neck were referred to a non-cephalic site, the binaural stimuli were delivered either simultaneously or with an inter-aural time difference (Δt) of 0.2–1.6 msec, and masking noise was presented to the non-stimulated ear.With simultaneous binaural clicks a BIC was identifiable in every subject, the most consistent peaks being a scalp-positive potential (P1) peaking approximately 0.2 msec after wave V and a scalp negativity (N1) 0.7 msec later. Similar potentials were identifiable in 6/7 subjects with Δt fo 0.4 msec, 5/7 at 0.8 msec but only 1/7 at 1.2 msec. This suggests that the BIC may be associated with sound localization mechanisms which are sensitive to a similar range of Δt. On increasing Δt from 0.0 to 0.8 msec, the BIC was progressively delayed by approximately half the inter-aural time difference, with no suggestion of increasing temporal dispersion. This supports the notion of a ‘delay line coincidence detection’ mechanism in which the BIC represents the output of binaurally responsive neurones, probably in the superior olivary complex, which are ‘tuned’ to a particular Δt by the relative lengths of presynaptic axons relaying input from either ear.The distribution of the BIC in sagittal and coronal electrode chains was compared with that of binaural BAEP components I–VI and found to bear the closest resemblance to wave IV. It is suggested that both components may originate largely in the lateral lemnisci.     

Heteromerization of TRP channel subunits: extending functional diversity

Transient receptor potential (TRP) channels are widely found throughout the animal kingdom. By serving as cellular sensors for a wide spectrum of physical and chemical stimuli, they play crucial physiological roles ranging from sensory transduction to cell cycle modulation. TRP channels are tetrameric protein complexes. While most TRP subunits can form functional homomeric channels, heteromerization of TRP channel subunits of either the same subfamily or different subfamilies has been widely observed. Heteromeric TRP channels exhibit many novel properties compared to their homomeric counterparts, indicating that co-assembly of TRP channel subunits has an important contribution to the diversity of TRP channel functions.     

Characterization of TRPM8-like channels activated by the cooling agent icilin in the macrophage cell line RAW 264.7

Icilin is recognized as a chemical agonist of nociceptors and can activate TRPM8 channels. However, whether this agent has any effects on immune cells remains unknown. In this study, the effects of icilin on ion currents were investigated in RAW 264.7 murine macrophage-like cells. Icilin (1–100 μM) increased the amplitude of nonselective (NS) cation current (I _NS) in a concentration-dependent manner with an EC₅₀ value of 8.6 μM. LaCl₃ (100 μM) or capsazepine (30 μM) reversed icilin-induced I _NS; however, neither apamin (200 nM) nor iberiotoxin (200 nM) had any effects on it. In cell-attached configuration, when the electrode was filled with icilin (30 μM), a unique population of NS cation channels were activated with single-channel conductance of 158 pS. With the use of a long-lasting ramp pulse protocol, increasing icilin concentration produced a left shift in the activation curve of NS channels, with no change in the slope factor of the curve. The probability of channel opening enhanced by icilin was increased by either elevated extracellular Ca²⁺ or application of ionomycin (10 μM), while it was reduced by BAPTA-AM (10 μM). Icilin-stimulated activity is associated with an increase in mean open time and a reduction in mean closed time. Under current-clamp conditions, icilin caused membrane depolarization. Therefore, icilin interacts with the TRPM8-like channel to increase I _NS and depolarizes the membrane in these cells.     

The core domain as the force sensor of the yeast mechanosensitive TRP channel

Stretch-activated conductances are commonly encountered in careful electric recordings. Those of known proteins (TRP, MscL, MscS, K(2p), Kv, etc.) all share a core, which houses the ion pathway and the gate, but no recognizable force-sensing domain. Like animal TRPs, the yeast TRPY1 is polymodal, activated by stretch force, Ca(2+), etc. To test whether its S5-S6 core senses the stretch force, we tried to uncouple it from the peripheral domains by strategic peptide insertions to block the covalent core-periphery interactions. Insertion of long unstructured peptides should distort, if not disrupt, protein structures that transmit force. Such insertions between S6 and the C-terminal tail largely removed Ca(2+) activation, showing their effectiveness. However, such insertions as well as those between S5 and the N-terminal region, which includes S1-S4, did not significantly alter mechanosensitivity. Even insertions at both locations flanking the S5-S6 core did not much alter mechanosensitivity. Tryptophan scanning mutations in S5 were also constructed to perturb possible noncovalent core-periphery contacts. The testable tryptophan mutations also have little or no effects on mechanosensitivity. Boltzmann fits of the wild-type force-response curves agree with a structural homology model for a stretch-induced core expansion of ~2 nm(2) upon opening. We hypothesize that membrane tension pulls on S5-S6, expanding the core and opening the TRPY1 gate. The core being the major force sensor offers the simplest, though not the only, explanation of why so many channels of disparate designs are mechanically sensitive. Compared with the bacterial MscL, TRPY1 is much less sensitive to force, befitting a polymodal channel that relies on multiple stimuli.     

Comparative sequence analysis suggests a conserved gating mechanism for TRP channels

The transient receptor potential (TRP) channel superfamily plays a central role in transducing diverse sensory stimuli in eukaryotes. Although dissimilar in sequence and domain organization, all known TRP channels act as polymodal cellular sensors and form tetrameric assemblies similar to those of their distant relatives, the voltage-gated potassium (Kv) channels. Here, we investigated the related questions of whether the allosteric mechanism underlying polymodal gating is common to all TRP channels, and how this mechanism differs from that underpinning Kv channel voltage sensitivity. To provide insight into these questions, we performed comparative sequence analysis on large, comprehensive ensembles of TRP and Kv channel sequences, contextualizing the patterns of conservation and correlation observed in the TRP channel sequences in light of the well-studied Kv channels. We report sequence features that are specific to TRP channels and, based on insight from recent TRPV1 structures, we suggest a model of TRP channel gating that differs substantially from the one mediating voltage sensitivity in Kv channels. The common mechanism underlying polymodal gating involves the displacement of a defect in the H-bond network of S6 that changes the orientation of the pore-lining residues at the hydrophobic gate.     

The transient receptor potential (TRP) channel TRPC3 TRP domain and AMP-activated protein kinase binding site are required for TRPC3 activation by erythropoietin

Modulation of intracellular calcium ([Ca(2+)](i)) by erythropoietin (Epo) is an important signaling pathway controlling erythroid proliferation and differentiation. Transient receptor potential (TRP) channels TRPC3 and homologous TRPC6 are expressed on normal human erythroid precursors, but Epo stimulates an increase in [Ca(2+)](i) through TRPC3 but not TRPC6. Here, the role of specific domains in the different responsiveness of TRPC3 and TRPC6 to erythropoietin was explored. TRPC3 and TRPC6 TRP domains differ in seven amino acids. Substitution of five amino acids (DDKPS) in the TRPC3 TRP domain with those of TRPC6 (EERVN) abolished the Epo-stimulated increase in [Ca(2+)](i). Substitution of EERVN in TRPC6 TRP domain with DDKPS in TRPC3 did not confer Epo responsiveness. However, substitution of TRPC6 TRP with DDKPS from TRPC3 TRP, as well as swapping the TRPC6 distal C terminus (C2) with that of TRPC3, resulted in a chimeric TRPC6 channel with Epo responsiveness similar to TRPC3. Substitution of TRPC6 with TRPC3 TRP and the putative TRPC3 C-terminal AMP-activated protein kinase (AMPK) binding site straddling TRPC3 C1/C2 also resulted in TRPC6 activation. In contrast, substitution of the TRPC3 C-terminal leucine zipper motif or TRPC3 phosphorylation sites Ser-681, Ser-708, or Ser-764 with TRPC6 sequence did not affect TRPC3 Epo responsiveness. TRPC3, but not TRPC6, and TRPC6 chimeras expressing TRPC3 C2 showed significantly increased plasma membrane insertion following Epo stimulation and substantial cytoskeletal association. The TRPC3 TRP domain, distal C terminus (C2), and AMPK binding site are critical elements that confer Epo responsiveness. In particular, the TRPC3 C2 and AMPK site are essential for association of TRPC3 with the cytoskeleton and increased channel translocation to the cell surface in response to Epo stimulation.     

Complete RNAi rescue of neuronal degeneration in a constitutively active Drosophila TRP channel mutant

RNA interference has been widely used to reduce the quantity of the proteins encoded by the targeted genes. A constitutively active, dominant allele of trp, TrpP365, causes massive degeneration of photoreceptors through a persistent and excessive Ca2+ influx. Here we show that a substantial reduction of the TRP channel protein by RNAi in TrpP365 heterozygotes completely rescues the neuronal degeneration and significantly improves the light-elicited responses of the eye. The reduction need not be complete, suggesting that rescue of degeneration may be possible with minimal side effects arising from overdepletion of the target protein.