Translocation of the Drosophila transient receptor potential-like (TRPL) channel requires both the N- and C-terminal regions together with sustained Ca2+ entry

In Drosophila photoreceptors the transient receptor potential-like (TRPL), but not the TRP channels undergo light-dependent translocation between the rhabdomere and cell body. Here we studied which of the TRPL channel segments are essential for translocation and why the TRP channels are required for inducing TRPL translocation. We generated transgenic flies expressing chimeric TRP and TRPL proteins that formed functional light-activated channels. Translocation was induced only in chimera containing both the N- and C-terminal segments of TRPL. Using an inactive trp mutation and overexpressing the Na(+)/Ca(2+) exchanger revealed that the essential function of the TRP channels in TRPL translocation is to enhance Ca(2+)-influx. These results indicate that motifs present at both the N and C termini as well as sustained Ca(2+) entry are required for proper channel translocation.     

TRP gating is linked to the metabolic state and maintenance of the Drosophila photoreceptor cells

The Drosophila light-activated channel TRP is the founding member of a large and diverse family of channel proteins that is conserved throughout evolution. In spite of much progress, the gating mechanism of TRP channels is still unknown. However, recent studies have shown multi-faceted functions of the Drosophila light-sensitive TRP channel that may shed light on TRP gating. Accordingly, metabolic stress, which leads to depletion of cellular ATP, reversibly activates the Drosophila TRP and TRPL channels in the dark in a constitutive manner. In several Drosophila mutants, constitutive activity of TRP channels lead to a rapid retinal degeneration in the dark, while genetic elimination of TRP protects the cells from degeneration. Additional studies have shown that TRPL translocates in a light-dependent manner between the signaling membranes and the cell body. This light-activated translocation is accompanied by reversible morphological changes leading to partial and reversible collapse of the microvillar signaling membranes into the cytosol, which allows turnover of signaling molecules. These morphological changes are also blocked by genetic elimination of TRP channels. The link of TRP gating to the metabolic state and maintenance of cells makes cells expressing TRP extremely vulnerable to metabolic stress via a mechanism that may underlie retinal degeneration and neuronal cell death upon malfunction.     

Mechanisms underlying stage-1 TRPL channel translocation in Drosophila photoreceptors

Background

TRP channels function as key mediators of sensory transduction and other cellular signaling pathways. In Drosophila, TRP and TRPL are the light-activated channels in photoreceptors. While TRP is statically localized in the signaling compartment of the cell (the rhabdomere), TRPL localization is regulated by light. TRPL channels translocate out of the rhabdomere in two distinct stages, returning to the rhabdomere with dark-incubation. Translocation of TRPL channels regulates their availability, and thereby the gain of the signal. Little, however, is known about the mechanisms underlying this trafficking of TRPL channels.

Methodology/Principal Findings

We first examine the involvement of de novo protein synthesis in TRPL translocation. We feed flies cycloheximide, verify inhibition of protein synthesis, and test for TRPL translocation in photoreceptors. We find that protein synthesis is not involved in either stage of TRPL translocation out of the rhabdomere, but that re-localization to the rhabdomere from stage-1, but not stage-2, depends on protein synthesis. We also characterize an ex vivo eye preparation that is amenable to biochemical and genetic manipulation. We use this preparation to examine mechanisms of stage-1 TRPL translocation. We find that stage-1 translocation is: induced with ATP depletion, unaltered with perturbation of the actin cytoskeleton or inhibition of endocytosis, and slowed with increased membrane sterol content.

Conclusions/Significance

Our results indicate that translocation of TRPL out of the rhabdomere is likely due to protein transport, and not degradation/re-synthesis. Re-localization from each stage to the rhabdomere likely involves different strategies. Since TRPL channels can translocate to stage-1 in the absence of ATP, with no major requirement of the cytoskeleton, we suggest that stage-1 translocation involves simple diffusion through the apical membrane, which may be regulated by release of a light-dependent anchor in the rhabdomere.     

Activation of the Drosophila TRP and TRPL channels requires both Ca2+ and protein dephosphorylation

The Transient Receptor Potential (TRP) proteins constitute a large and diverse family of channel proteins, which is conserved through evolution. TRP channel proteins have critical functions in many tissues and cell types, but their gating mechanism is an enigma. In the present study patch-clamp whole-cell recordings was applied to measure the TRP- and TRP-like (TRPL)-dependent currents in isolated Drosophila ommatidia. Also, voltage responses to light and to metabolic stress were recorded from the eye in vivo. We report new insight into the gating of the Drosophila light-sensitive TRP and TRPL channels, by which both Ca2+ and protein dephosphorylation are required for channel activation. ATP depletion or inhibition of protein kinase C activated the TRP channels, while photo-release of caged ATP or application of phorbol ester antagonized channels openings in the dark. Furthermore, Mg(2+)-dependent stable phosphorylation event by ATPgammaS or protein phosphatase inhibition by calyculin A abolished activation of the TRP and TRPL channels. While a high reduction of cellular Ca2+ abolished channel activation, subsequent application of Ca2+ combined with ATP depletion induced a robust dark current that was reminiscent of light responses. The results suggest that the combined action of Ca2+ and protein dephosphorylation activate the TRP and TRPL channels, while protein phosphorylation by PKC antagonized channels openings.     

TRP gating is linked to the metabolic state and maintenance of the Drosophila photoreceptor cells

The Drosophila light-activated channel TRP is the founding member of a large and diverse family of channel proteins that is conserved throughout evolution. In spite of much progress, the gating mechanism of TRP channels is still unknown. However, recent studies have shown multi-faceted functions of the Drosophila light-sensitive TRP channel that may shed light on TRP gating. Accordingly, metabolic stress, which leads to depletion of cellular ATP, reversibly activates the Drosophila TRP and TRPL channels in the dark in a constitutive manner. In several Drosophila mutants, constitutive activity of TRP channels lead to a rapid retinal degeneration in the dark, while genetic elimination of TRP protects the cells from degeneration. Additional studies have shown that TRPL translocates in a light-dependent manner between the signaling membranes and the cell body. This light-activated translocation is accompanied by reversible morphological changes leading to partial and reversible collapse of the microvillar signaling membranes into the cytosol, which allows turnover of signaling molecules. These morphological changes are also blocked by genetic elimination of TRP channels. The link of TRP gating to the metabolic state and maintenance of cells makes cells expressing TRP extremely vulnerable to metabolic stress via a mechanism that may underlie retinal degeneration and neuronal cell death upon malfunction.     

Carvacrol is a novel inhibitor of Drosophila TRPL and mammalian TRPM7 channels

Transient receptor potential (TRP) channels are essential components of biological sensors that detect changes in the environment in response to a myriad of stimuli. A major difficulty in the study of TRP channels is the lack of pharmacological agents that modulate most members of the TRP superfamily. Notable exceptions are the thermoTRPs, which respond to either cold or hot temperatures and are modulated by a relatively large number of chemical agents. In the present study we demonstrate by patch clamp whole cell recordings from Schneider 2 and Drosophila photoreceptor cells that carvacrol, a known activator of the thermoTRPs, TRPV3 and TRPA1 is an inhibitor of the Drosophila TRPL channels, which belongs to the TRPC subfamily. We also show that additional activators of TRPV3, thymol, eugenol, cinnamaldehyde and menthol are all inhibitors of the TRPL channel. Furthermore, carvacrol also inhibits the mammalian TRPM7 heterologously expressed in HEK cells and ectopically expressed in a primary culture of CA3–CA1 hippocampal brain neurons. This study, thus, identifies a novel inhibitor of TRPC and TRPM channels. Our finding that the activity of the non-thermoTRPs, TRPL and TRPM7 channels is modulated by the same compound as thermoTRPs, suggests that common mechanisms of channel modulation characterize TRP channels.     

Regulation of Drosophila TRPC channels by protein and lipid interactions

The Drosophila TRPC channels TRP and TRPL are the founding members of the TRP superfamily of ion channels, proteins likely to be important components of calcium influx pathways. The activation of these channels in the context of fly phototransduction is one of the few in vivo models for TRPC channel activation and has served as a paradigm for understanding TRPC function. TRP and TRPL are activated by G-protein coupled PI(4,5)P₂ hydrolysis through a mechanism in which IP₃ receptor mediated calcium release seems dispensable. Recent analysis has provided compelling evidence that the accurate turnover of PI(4,5)P₂ generated lipid messengers in essential for regulating TRP and TRPL activity. TRP channels also appear to exist in the context of a macromolecular complex containing key components involved in activation such as phospholipase Cβ and protein kinase C. This complex may be important for activation. The role of these protein and lipid elements in regulating TRP and TRPL activity is discussed in this review.     

Regulation of Drosophila TRPC channels by lipid messengers

The Drosophila TRPC channels TRP and TRPL are the founding members of the TRP superfamily of ion channels, which are important components of calcium influx pathways in virtually all cells. The activation of these channels in the context of fly phototransduction is one of the few in vivo models for TRPC channel activation and has served as a paradigm for understanding TRPC function. TRP and TRPL are activated by G-protein coupled PIP₂ hydrolysis through a mechanism in which IP₃ receptor mediated calcium release seems dispensable. Recent analysis has provided compelling evidence that one or more PIP₂ generated lipid messengers, as well as PIP₂ itself, are essential for regulating TRP and TRPL activity. Evidence on the role of these lipid elements in regulating TRP and TRPL activity is discussed in this review.     

Signal-dependent hydrolysis of phosphatidylinositol 4,5-bisphosphate without activation of phospholipase C: implications on gating of Drosophila TRPL (transient receptor potential-like) channel

In Drosophila, a phospholipase C (PLC)-mediated signaling cascade, couples photo-excitation of rhodopsin to the opening of the transient receptor potential (TRP) and TRP-like (TRPL) channels. A lipid product of PLC, diacylglycerol (DAG), and its metabolites, polyunsaturated fatty acids (PUFAs) may function as second messengers of channel activation. However, how can one separate between the increase in putative second messengers, change in pH, and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) depletion when exploring the TRPL gating mechanism? To answer this question we co-expressed the TRPL channels together with the muscarinic (M1) receptor, enabling the openings of TRPL channels via G-protein activation of PLC. To dissect PLC activation of TRPL into its molecular components, we used a powerful method that reduced plasma membrane-associated PI(4,5)P₂ in HEK cells within seconds without activating PLC. Upon the addition of a dimerizing drug, PI(4,5)P₂ was selectively hydrolyzed in the cell membrane without producing DAG, inositol trisphosphate, or calcium signals. We show that PI(4,5)P₂ is not an inhibitor of TRPL channel activation. PI(4,5)P₂ hydrolysis combined with either acidification or application of DAG analogs failed to activate the channels, whereas PUFA did activate the channels. Moreover, a reduction in PI(4,5)P₂ levels or inhibition of DAG lipase during PLC activity suppressed the PLC-activated TRPL current. This suggests that PI(4,5)P₂ is a crucial substrate for PLC-mediated activation of the channels, whereas PUFA may function as the channel activator. Together, this study defines a narrow range of possible mechanisms for TRPL gating.     

Open channel block by Ca2+ underlies the voltage dependence of drosophila TRPL channel

The light-activated channels of Drosophila photoreceptors transient receptor potential (TRP) and TRP-like (TRPL) show voltage-dependent conductance during illumination. Recent studies implied that mammalian members of the TRP family, which belong to the TRPV and TRPM subfamilies, are intrinsically voltage-gated channels. However, it is unclear whether the Drosophila TRPs, which belong to the TRPC subfamily, share the same voltage-dependent gating mechanism. Exploring the voltage dependence of Drosophila TRPL expressed in S2 cells, we found that the voltage dependence of this channel is not an intrinsic property since it became linear upon removal of divalent cations. We further found that Ca(2+) blocked TRPL in a voltage-dependent manner by an open channel block mechanism, which determines the frequency of channel openings and constitutes the sole parameter that underlies its voltage dependence. Whole cell recordings from a Drosophila mutant expressing only TRPL indicated that Ca(2+) block also accounts for the voltage dependence of the native TRPL channels. The open channel block by Ca(2+) that we characterized is a useful mechanism to improve the signal to noise ratio of the response to intense light when virtually all the large conductance TRPL channels are blocked and only the low conductance TRP channels with lower Ca(2+) affinity are active.     

The GTP- and Phospholipid-Binding Protein TTD14 Regulates Trafficking of the TRPL Ion Channel in Drosophila Photoreceptor Cells

Recycling of signaling proteins is a common phenomenon in diverse signaling pathways. In photoreceptors of Drosophila, light absorption by rhodopsin triggers a phospholipase Cβ-mediated opening of the ion channels transient receptor potential (TRP) and TRP-like (TRPL) and generates the visual response. The signaling proteins are located in a plasma membrane compartment called rhabdomere. The major rhodopsin (Rh1) and TRP are predominantly localized in the rhabdomere in light and darkness. In contrast, TRPL translocates between the rhabdomeral plasma membrane in the dark and a storage compartment in the cell body in the light, from where it can be recycled to the plasma membrane upon subsequent dark adaptation. Here, we identified the gene mutated in trpl translocation defective 14 (ttd14), which is required for both TRPL internalization from the rhabdomere in the light and recycling of TRPL back to the rhabdomere in the dark. TTD14 is highly conserved in invertebrates and binds GTP in vitro. The ttd14 mutation alters a conserved proline residue (P75L) in the GTP-binding domain and abolishes binding to GTP. This indicates that GTP binding is essential for TTD14 function. TTD14 is a cytosolic protein and binds to PtdIns(3)P, a lipid enriched in early endosome membranes, and to phosphatidic acid. In contrast to TRPL, rhabdomeral localization of the membrane proteins Rh1 and TRP is not affected in the ttd14 ^P75L mutant. The ttd14 ^P75L mutation results in Rh1-independent photoreceptor degeneration and larval lethality suggesting that other processes are also affected by the ttd14 ^P75L mutation. In conclusion, TTD14 is a novel regulator of TRPL trafficking, involved in internalization and subsequent sorting of TRPL into the recycling pathway that enables this ion channel to return to the plasma membrane.     

Regulation of Drosophila TRPC channels by protein and lipid interactions

The Drosophila TRPC channels TRP and TRPL are the founding members of the TRP superfamily of ion channels, proteins likely to be important components of calcium influx pathways. The activation of these channels in the context of fly phototransduction is one of the few in vivo models for TRPC channel activation and has served as a paradigm for understanding TRPC function. TRP and TRPL are activated by G-protein coupled PI(4,5)P(2) hydrolysis through a mechanism in which IP(3) receptor mediated calcium release seems dispensable. Recent analysis has provided compelling evidence that the accurate turnover of PI(4,5)P(2) generated lipid messengers in essential for regulating TRP and TRPL activity. TRP channels also appear to exist in the context of a macromolecular complex containing key components involved in activation such as phospholipase Cbeta and protein kinase C. This complex may be important for activation. The role of these protein and lipid elements in regulating TRP and TRPL activity is discussed in this review.     

The Drosophila TRPL ion channel shares a Rab-dependent translocation pathway with rhodopsin

The Drosophila visual transduction cascade is embedded in the rhabdomeres of photoreceptor cells and culminates in the opening of the two ion channels, TRP and TRPL. TRPL translocates from the rhabdomeres to the cell body upon illumination and vice versa when flies are kept in the dark. Here, we studied the mechanisms underlying the light-dependent internalization of TRPL. Co-localization of TRPL and rhodopsin in endocytic particles revealed that TRPL is internalized by a vesicular transport pathway that is also utilized, at least partially, for rhodopsin endocytosis. TRPL internalization is attenuated under light conditions that result in a high rate of rhodopsin internalization and is highest in orange light that result in very little rhodopsin internalization. In line with a canonical vesicular transport pathway, we found that rab proteins, Rab5 and RabX4, are required for the internalization of TRPL into the cell body. Our results provide insight into stimulus-dependent internalization of a prominent member of the TRP superfamily.     

Linoleic acid inhibits TRP channels with intrinsic voltage sensitivity: Implications on the mechanism of linoleic acid action

Open channel block (OCB) is a process by which ions bind to the inside of a channel pore and block the flow of ions through that channel. Repulsion of the blocking ions by membrane depolarization is a known mechanism for open channel block removal. For the N-methyl-D-aspartate (NMDA) channel, this mechanism is necessary for channel activation and is involved in neuronal plasticity. Several types of Transient Receptor Potential (TRP) channels, including the Drosophila TRP and TRP-Like (TRPL) channels, also exhibit open channel block. For the Drosophila TRP and TRPL channels, removal of open channel block is necessary for the production of the physiological response to light. Recently, we have shown that lipids such as polyunsaturated fatty acids (PUFAs), represented by linoleic acid (LA), alleviate OCB under physiological conditions, from the Drosophila TRP and TRPL channels and from the mammalian NMDA channel. Here we show that OCB removal by LA is not confined to the Drosophila TRPs but also applies to mammalian TRPs such as the heat activated TRPV3 channel. TRPV3 shows OCB alleviation by LA, although it shares little amino acid sequence homology with the Drosophila TRPs. Strikingly, LA inhibits the heat-activated TRPV1 and the cold temperature-activated TRPM8 channels, which are intrinsic voltage sensitive channels and do not show OCB. Together, our findings further support the notion that lipids do not act as second messengers by direct binding to a specific site of the channels but rather act indirectly by affecting the channel-plasma membrane interface.     

TRP channels in Drosophila photoreceptors: the lipid connection

The light-sensitive current in Drosophila photoreceptors is mediated by transient receptor potential (TRP) channels, at least two members of which (TRP and TRPL) are activated downstream of phospholipase C (PLC) in response to light. Recent evidence is reviewed suggesting that Drosophila TRP channels are activated by one or more lipid products of PLC activity: namely diacylglycerol (DAG), its metabolites (polyunsaturated fatty acids) or the reduction in phosphatidylinositol 4,5-bisphosphate (PIP₂). The most compelling evidence for this view comes from analysis of rdgA mutants which are unable to effectively metabolise DAG due to a defect in DAG kinase. The rdgA mutation leads to constitutive activation of both TRP and TRPL channels and dramatically increases sensitivity to light in hypomorphic mutations of PLC and G protein.     

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.     

Light-regulated translocation of signaling proteins in Drosophila photoreceptors.

Illumination of Drosophila photoreceptor cells induces multi-facet responses, which include generation of the photoreceptor potential, screening pigment migration and translocation of signaling proteins which is the focus of recent extensive research. Translocation of three signaling molecules is covered in this review: (1) Light-dependent translocation of arrestin from the cytosol to the signaling membrane, the rhabdomere, determines the lifetime of activated rhodopsin. Arrestin translocates in PIP3 and NINAC myosin III dependent manner, and specific mutations which disrupt the interaction between arrestin and PIP3 or NINAC also impair the light-dependent translocation of arrestin and the termination of the response to light. (2) Activation of Drosophila visual G protein, DGq, causes a massive and reversible, translocation of the alpha subunit from the signaling membrane to the cytosol, accompanied by activity-dependent architectural changes. Analysis of the translocation and the recovery kinetics of DGq(alpha) in wild-type flies and specific visual mutants indicated that DGq(alpha) is necessary but not sufficient for the architectural changes. (3) The TRP-like (TRPL) but not TRP channels translocate in a light-dependent manner between the rhabdomere and the cell body. As a physiological consequence of this light-dependent modulation of the TRP/TRPL ratio, the photoreceptors of dark-adapted flies operate at a wider dynamic range, which allows the photoreceptors enriched with TRPL to function better in darkness and dim background illumination. Altogether, signal-dependent movement of signaling proteins plays a major role in the maintenance and function of photoreceptor cells.     

Genetic strategies to analyze primary TRP channel-expressing cells in mice

Transient receptor potential (TRP) ion channels regulate fundamental biological processes throughout the body. TRP channel dysfunction has been causally linked to a number of disease states and thus establishes these channels as promising therapeutic targets. In order to dissect the physiological role of individual TRP channels in specific tissues, a detailed understanding of the expression pattern of the different TRP channels throughout the organism is essential. We provide an overview of recent efforts to generate novel TRP channel reporter mouse strains for all 28 TRP channels encoded in the mouse genome to understand expression of these channels with a single-cell resolution in an organism-wide manner. The reporter mice will enable both the visualization and manipulation of all primary TRP channel-expressing cells allowing an unprecedented wealth in variety to investigate TRP channel function in vivo. As proof of principle, we provide preliminary results documenting TRPM5 expression throughout the entire body of juvenile and adult mice.     

Transient receptor potential-like channels are essential for calcium signaling and fluid transport in a Drosophila epithelium

Calcium signaling is an important mediator of neuropeptide-stimulated fluid transport by Drosophila Malpighian (renal) tubules. We demonstrate the first epithelial role, in vivo, for members of the TRP family of calcium channels. RT-PCR revealed expression of trp, trpl, and trpγ in tubules. Use of antipeptide polyclonal antibodies for TRP, TRPL, and TRPγ showed expression of all three channels in type 1 (principal) cells in the tubule main segment. Neuropeptide (CAP_2b)-stimulated fluid transport rates were significantly reduced in tubules from the trpl³⁰² mutant and the trpl;trp double mutant, trpl³⁰²;trp³⁴³. However, a trp null, trp³⁴³, had no impact on stimulated fluid transport. Measurement of cytosolic calcium concentrations ([Ca²⁺]_i) in tubule principal cells using an aequorin transgene in trp and trpl mutants showed a reduction in calcium responses in trpl³⁰². Western blotting of tubule preparations from trp and trpl mutants revealed a correlation between TRPL levels and CAP_2b-stimulated fluid transport and calcium signaling. Rescue of trpl³⁰² with a trpl transgene under heat-shock control resulted in a stimulated fluid transport phenotype that was indistinguishable from wild-type tubules. Furthermore, restoration of normal stimulated rates of fluid transport by rescue of trpl³⁰² was not compromised by introduction of the trp null, trp³⁴³. Thus, in an epithelial context, TRPL is sufficient for wild-type responses. Finally, a scaffolding component of the TRPL/TRP-signaling complex, INAD, is not expressed in tubules, suggesting that inaD is not essential for TRPL/TRP function in Drosophila tubules.