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Transient Receptor Potential Cation Channels in Disease

Department of Physiology, Campus Gasthuisberg, KULeuven, Leuven, Belgium; and Neurosciences Institute, Division of Pathology and Neuroscience, Ninewells Hospital and Medical School, The University of Dundee, Dundee, United Kingdom

ABSTRACT

I. INTRODUCTION

II. THE TRP SUPERFAMILY

A. The ''Canonical'' TRPCs

1. Store-operated TRP(C) channels?

B. The TRPV Subfamily

C. The TRPM Subfamily

D. The TRPML Subfamily

E. The TRPP Subfamily

F. TRPA

III. THE TRP CHANNEL FAMILY AND DISEASES

A. Introductory Remarks

B. Dysfunctions in TRP Channelopathies

1. TRPC6 and focal segmental glomerulosclerosis

2. TRPM6 and hypomagnesemia with secondary hypocalcemia

3. TRPP2 and autosomal dominant polycystic kidney disease

4. TRPML1 and mucolipidosis type IV

5. TRPM7 and Guamanian amyotrophic lateral sclerosis/Parkinsonism dementia

C. TRPs and Pain

1. TRP channels in acute nociceptive pain

2. Mechanisms altering the sensitivity of ''painful'' TRP channels

3. TRPs in neuropathic pain

4. TRPs in pain related to the gastrointestinal tract

5. Neurogenic inflammation

6. TRPs in pain related to the urogenital tract

7. TRPs in pain related to bone

8. Itch

D. TRP Channels in Systemic Diseases

1. Immune system

2. Cardiovascular system

3. Respiratory system

4. Gastrointestinal system

5. Bladder

6. Reproductive system

7. Kidney

8. Skeletal muscle and bone

9. Endocrine system

10. Brain

11. Sensory systems

E. TRPs and Aging

F. TRPs and Cancer

G. Candidate Genes

H. Lessons From Knockout and Transgenic Mice

I. PIP2-TRP Connection and Its Impact on Disease

IV. CONCLUSION

GRANTS

ACKNOWLEDGMENTS

REFERENCES

	ABSTRACT

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	I. INTRODUCTION

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Calcium ions play a central role in many cellular processes including muscle contraction, transmitter release, cell proliferation, gene transcription, and cell death (42). Our knowledge of the molecular players mediating Ca²⁺ entry into cells has increased impressively during the last 10 years, not least due to the discovery of a novel superfamily of channels called "transient receptor potential," or TRP channels. TRP channels contribute to changes in the cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) either by acting as Ca²⁺ entry pathways in the plasma membrane, or via changes in membrane polarization, modulating the driving force for Ca²⁺ entry mediated by alternative pathways. TRP channels probably also form intracellular pathways for Ca²⁺ release from several cell organelles. Given the unique importance of Ca²⁺ signaling in all cell types, it is not surprising that dysfunctions in Ca²⁺ channels are causal to, or at least involved in, the pathogenesis of several diseases. With regard to the TRP channels, there are at present only a few conditions that can be referred to as a channelopathy, in which a defect in channel functioning is the direct cause of disease. However, in this review we draw upon multiple lines of evidence to provide a comprehensive description of the current state of knowledge that implicates TRP channels in the pathogenesis of several diseases. Taking into account the complexity of TRP channel regulation and their typically polymodal mechanisms of activation, we briefly first describe the main properties of the different TRP family members and subsequently discuss various diseases caused by TRP channel defects, as well as diseases and dysfunctions in which a role of TRP channels has been implicated.

Each TRP channel subunit consists of six putative transmembrane spanning segments (S1–6), a pore-forming loop between S5 and S6, and intracellularly located NH₂ and COOH termini. Assembly of channel subunits as homo- or heterotetramers results in the formation of cation-selective channels. On the basis of amino acid homology, the TRP superfamily can be divided into seven subfamilies (see Fig. 1) (73, 80, 90, 283, 284, 286, 337). The TRPC (canonical) and TRPM (melastatin) subfamilies consist of seven and eight different channels, respectively (i.e., TRPC1–7 and TRPM1–8). The TRPV (vanilloid) subfamily presently comprises six members (TRPV1–6). The most recently identified subfamily, TRPA (ankyrin), has only one mammalian member (TRPA1). The TRPP (polycystin) and TRPML (mucolipin) families, each containing three mammalian members, are relatively poorly characterized, but are attracting increasing interest because of their involvement in several human diseases (see Fig. 1). The TRPN subfamily (NOMP, No mechanopotential) in hearing-assisting sensory neurons in Drosophila and zebrafish (Danio rerio) has to date only been detected in worm, Drosophila, and zebrafish and is proposed to be a mechanostimuli sensing channel (400, 488). Currently available genome information indicates that mammals have no TRPN orthologs.

View larger version (25K): [in this window] [in a new window]   FIG. 1. Phylogenetic tree of the mammalian transient receptor potential (TRP) channel superfamily. TRPC (canonical), TRPM (melastatin), TRPV (vanilloid), TRPA (ankyrin), TRPP (polycystin), and TRPML (mucolipin) are the only identified subfamilies in mammals. TRPN (NOMP, NO-mechano-potential) has to date only been detected in worm, Drosophila, and zebrafish (400, 488).

Increases in intracellular Ca²⁺ not only arise upon Ca²⁺ influx, but also upon Ca²⁺ release from internal Ca²⁺ stores, such as the Golgi apparatus, the endoplasmic reticulum (ER) or, specifically in muscle cells, the sarcoplasmic reticulum. A number of recent studies suggest that members of the TRP superfamily may also function as intracellular Ca²⁺ release channels. TRPP2 is partially located in the ER membranes where it acts as a Ca²⁺ release channel (204). Moreover, there are indications that some bona fide plasmalemmal Ca²⁺-permeable TRP channels (e.g., TRPV1 and TRPM8) can also reside on intracellular membranes where they may function as Ca²⁺ release channels (438, 457, 458, 483, 531).

	II. THE TRP SUPERFAMILY

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The discovery of TRP channels was related to a channelopathy, albeit in an invertebrate. Phototransduction in the fruit fly, Drosophila melanogaster, involves activation of membrane cation channels leading to a depolarizing current. Drosophila photoreceptors contain the light-sensitive G protein-coupled receptor rhodopsin, whose activation results in stimulation of phospholipase C- (PLC-). Resolving components of the light-induced current (LIC) led to the identification of a Drosophila mutant displaying a transient LIC in response to light, in contrast to the sustained LIC in wild-type flies. This mutant strain was termed trp, for transient receptor potential. Mutations in this gene led to a disruption of a Ca²⁺ entry channel in the photoreceptors, indicating that TRP, the protein encoded by the trp gene, forms all, or part, of a Ca²⁺ influx channel.

A. The "Canonical" TRPCs

The mammalian TRP channels most closely related to Drosophila TRP are classified in the TRPC subfamily. The seven mammalian TRPC channels share a structural motif in the COOH-terminal tail, the TRP box, which is located close to the intracellular border of S6 and contains the invariant sequence EWKFAR. TRPC channels also contain three or four NH₂-terminal ankyrin repeats (349). TRPC channels are nonselective, Ca²⁺-permeable cation channels, but the permeability ratio (P_Ca/P_Na) varies significantly between different members of the family and somewhat confusingly between reports upon the same channel. For example, estimates of P_Ca/P_Na for both TRPC4 and TRPC5 expressed in heterologous systems vary between 1 and 9 (326, 347, 381). Such disparate values may reflect contamination of the TRPC4/TRPC5 currents by endogenous conductances and/or the formation of heteromultimeric channels consisting of heterologously expressed and endogenous TRPC monomers. Indeed, several TRPCs, including TRPC1, -4, and -5, can form heteromers, and the current properties are significantly different between TRPC5 and TRPC1/TRPC5 expressing cells. Similarly, TRPC3, TRPC6, and TRPC7 form heteromers (136, 172, 386, 418, 419).

In general, TRPC members can be considered as channels activated subsequent to stimulation of receptors that activate different isoforms of PLC. TRPC3, -6, and -7 are activated by diacylglycerol (DAG), independent of the stimulation of protein kinase C (PKC) (171, 255, 256, 474), suggesting that DAG mediates their physiological activation. In contrast, TRPC1, -4, and -5, which are also activated by receptor-induced PLC, are completely unresponsive to DAG (171, 475). However, the mechanism via which PLC stimulation leads to activation of these channels remains controversial. Surprisingly, one recent study reported that TRPC1 is directly activated by membrane stretch, independent of PLC activity, and that it may be the molecular correlate of the vertebrate stretch-activated cation channel MscCa (262).

Table 1 summarizes some of the key features of the members of the TRPC subfamily (for more detailed reviews, see Refs. 8, 38, 102, 111, 118, 134, 135, 200, 540, 543).

In addition to aforementioned functions, all TRPC channels as well as several members of other TRP subfamilies (e.g., TRPV6, TRPM3) have at some point been described as store-operated channels (SOCs), i.e., channels that are activated whenever intracellular Ca²⁺ stores become depleted. Actually, TRP channels were initially considered to be the long-sought molecular correlates of SOCs. However, in many cases, the classification of TRP channels as SOCs is mainly based on the results of Ca²⁺ imaging protocols, in which store-dependent Ca²⁺ influx is estimated from the rise in [Ca²⁺]_i that occurs in cells to which extracellular Ca²⁺ is readded after artificial store depletion. Such procedures are relatively prone to error, for example, due to the lack of control of the transmembrane voltage (for a more detailed discussion of the problems and pitfalls when identifying a channel as a SOC, see Ref. 308). At present, it remains questionable whether any TRP channel plays a significant physiological role as a store-dependent channel (73, 308, 351, 449). Moreover, as long as the mechanistic link between the filling state of the Ca²⁺ stores and a plasma membrane channel remains unknown, store dependence of a channel should be considered as a phenomenon, not a mechanism.

The search for proteins that form and/or regulate SOCs has recently led to the discovery of two novel types of key players: the stromal interaction molecules (STIMs) and the four transmembrane (TM) proteins ORAIs (also known as CRACMs). First, a number of studies demonstrated that downregulation of the expression of either STIM1 or ORAI1 impairs SOC activation in response to depletion of intracellular Ca²⁺ stores (242, 361, 375, 536). Second, it was found that the absence of Ca²⁺-release-activated Ca²⁺ (CRAC) channels in T cells from patients with a hereditary form of severe combined immune deficiency (SCID) arises from a homozygous missense mutation in the ORAI1 gene and that expression of wild-type ORAI1 in these cells restores CRAC channel function (116). Third, whereas heterologous expression of either ORAI1, or STIM1, alone had little, or no, effect on the amplitude of SOC currents, four independent studies demonstrated that the combined heterologous expression of these two proteins led to a dramatic (up to 100-fold) increase in the amplitude of CRAC-like store-operated Ca²⁺ channels (275, 341, 406, 535). Forth, mutations of conserved acidic residues in S1 and S3 of ORAI1 strongly impair Ca²⁺ influx, enlarge monovalent cation current, and render the channel permeable to Cs⁺, providing strong evidence that ORAI1 is a pore subunit of the CRAC channel (356, 523). Although the exact relationship between STIM1 and ORAI1 is currently still unsettled, an appealing hypothesis is that STIM1 acts as a Ca²⁺ sensor in the ER that relocates toward the membrane upon store depletion where it activates ORAI1, which constitutes all, or part of, the CRAC channel (331, 332). Certainly, more studies will be required to establish whether the CRAC channel is formed by ORAI1 alone, or rather in complex with other subunits. Moreover, the role of additional STIMs and ORAIs needs to be further investigated, and the possible interactions of these novel classes of proteins with other (channel) proteins mandates further research. In this context it is interesting to note that two recent independent studies indicate a functional interaction between STIM1 and TRPC1 (177, 251), suggesting that STIM1 may have a more general function in the regulation of Ca²⁺ influx channels, including members of the TRPC subfamily.

B. The TRPV Subfamily

The TRPV family contains six mammalian members: TRPV1-TRPV6, as well as Osm-9 from C. elegans (77) and Nanchung (Nan) from Drosophila (196) (for a review, see Refs. 40, 76, 152, 222, 235, 301, 327, 350, 443, 485). Members of the TRPV family contain three to five ankyrin repeats in their cytosolic NH₂ termini (49, 185, 190).

TRPV1-TRPV4 are all heat-activated channels that are nonselective for cations and modestly permeable to Ca²⁺, with permeability ratios (P_Ca/P_Na) between 1 and 10. In addition, TRPV1-TRPV4 also function as chemosensors for a broad array of endogenous and synthetic ligands (7, 15, 49, 71, 72, 185, 190, 271, 285, 321, 322, 430, 470, 484, 486, 496–498, 517, 518). Finally, TRPV4 is activated upon cell swelling, which is due to the cell swelling-induced formation of the endogenous ligand 5',6'-epoxyeicosatrienoic acid rather than to mechanosensing by the channel itself (484, 486). Interestingly, these different chemical and physical activatory stimuli mostly have an additive, or even supra-additive, effect on the gating of TRPV channels, which endows these channels with the ability to act as signal integrators. As discussed later in this review, this form of signal integration is of great importance to several pathological states. For example, the thermal hyperalgesia that occurs during inflammation reflects the fact that TRPV1 integrates both thermal and chemical stimuli (i.e., TRPV1-activating compounds in the inflammatory soup), leading to the sensation of noxious heat at innocuous temperatures.

The properties of the two other members of this subfamily, TRPV5 and TRPV6, are quite different from those of TRPV1-TRPV4. They are the only highly Ca²⁺-selective channels in the TRP family, and both are tightly regulated by [Ca²⁺]_i (314, 317, 318, 476, 477, 528). Under physiological conditions, these channels exclusively conduct Ca²⁺, but in the absence of extracellular Ca²⁺, monovalent cations permeate readily, resulting in anomalous mole fraction behavior similar to that observed in other types of Ca²⁺-selective channels (169, 318, 476, 477, 528). These properties allow TRPV5 and TRPV6 to play a crucial role as gatekeepers in epithelial Ca²⁺ transport, and as selective Ca²⁺ influx pathways in nonexcitable cells (95, 304, 305). In contrast to the other TRPVs, the temperature sensitivity of TRPV5 and TRPV6 is relatively low.

Table 2 summarizes some of the key features of the members of the TRPV subfamily.

Members of the TRPM family, on the basis of sequence homology, fall into three subgroups: TRPM1/3, TRPM4/5, and TRPM6/7, with TRPM2 and TRPM8 representing structurally distinct channels. In contrast to TRPCs and TRPVs, TRPMs do not contain ankyrin repeats within their NH₂-terminal domain. An exceptional structural feature of three TRPM members is the presence of entire functional enzymes in their COOH termini: TRPM2 contains a functional NUDT9 homology domain exhibiting ADP-ribose pyrophosphatase activity (209, 344), whereas both TRPM6 and TRPM7 contain a functional COOH-terminal -kinase (an atypical serine/threonine kinase) (162, 282, 379, 390).

TRPM channels exhibit highly variable permeability to Ca²⁺ and Mg²⁺, ranging from Ca²⁺ impermeable (TRPM4 and TRPM5) to highly Ca²⁺ and Mg²⁺ permeable (TRPM6, TRPM7 and specific splice variants of TRPM3)(144, 170, 220, 224, 266, 282, 325, 340, 344, 346, 379, 380, 389, 482, 520, 522). The gating mechanisms of the TRPM subfamily members are equally varied: TRPM2 is activated by intracellular ADP-ribose (ADPR), hydrogen peroxide, and heat, whereas reported activation mechanisms for TRPM3 include cell swelling and sphingosine; TRPM4 and TRPM5 gate upon a rise in intracellular Ca²⁺ and are further strongly activated by heating. Gating of TRPM6 and TRPM7 is regulated by intracellular levels of Mg²⁺ and MgATP. Finally, TRPM8 is activated upon cooling and by cooling agents such as menthol or icilin. As yet, functional characterization of TRPM1 has not been reported (145, 170, 208, 229, 266, 269, 311, 313, 340, 344, 379, 382, 390, 436, 437, 444, 461, 481, 482, 519).

Table 3 summarizes some of the key features of the members of the TRPM subfamily. For more detailed reviews on the TRPM family, see References 2, 69, 205, 208, 272, 369, 382.

The TRPML family consists of three mammalian members (TRPML1–3) that are relatively small proteins consisting of <600 amino acid residues. TRPML1 is widely expressed and appears to reside in late endosomes/lysosomes (30, 215, 216). It contains a nuclear localization signal and a putative late endosomal/lysosomal targeting signal (423). The loop between S1 and S2 contains a lipase domain of unknown function, although it may be speculated that this region is enzymatically active, or represents a binding site for lipids that could potentially exert a regulatory influence on TRPML1 (30). Recently, TRPML1 has been described as a H⁺ channel that may act as a H⁺ leak in lysosomes preventing overacidification in these organelles (199, 409). TRPML2 and TRPML3 are as yet not reliably functionally characterized.

Table 4 summarizes some of the key features of the members of the TRPML subfamily. For more detailed reviews, see References 30, 56, 363.

The TRPP family is very inhomogeneous and can be divided, on structurally criteria, into PKD1-like (TRPP1-like) and PKD2-like (TRPP2-like) proteins.

PKD1-like members comprise TRPP1 (previously termed PKD1), PKDREJ, PKD1L1, PKD1L2, and PKD1L3. TRPP1 consists of 11 transmembrane domains, a very long and complex 3,000 amino acid extracellular domain, and an intracellular COOH-terminal domain that interacts with the COOH terminus of TRPP2 through a coiled-coil domain (362, 455, 514). The inclusion of PKD1-like proteins within the TRP superfamily is at present somewhat tentative and rests upon a degree of structural similarity between TRP channels and the six distal transmembrane domains of at least some PKD1-like members (i.e., PKD1L2, PKD1L3, and PKDREJ) (91, 363). The NH₂-terminal domain of TRPP1 contains numerous structural motifs including several adhesive domains that are likely to participate in cell-cell and cell-matrix interactions. TRPP1-like proteins possess a large extracellular loop, containing conserved "polycystin motifs" of unknown function, between presumed S6 and S7. The latter region is homologous to the loop interposed between the putative S1 and S2 in the TRPP2-like family members (363).

The PKD2-like members structurally resemble other TRP channels in that they are predicted to have intracellular NH₂ and COOH termini, six TM-spanning domains, and a pore region. The members of this grouping comprise PKD2 (TRPP2), PKD2L1 (TRPP3), and PKD2L2 (TRPP5). All PKD2-like members possess a coiled-coil structure in their COOH terminus and form polymodal multiprotein/ion channels complexes (91). TRPP2 and TRPP3 additionally feature a Ca²⁺-binding EF-hand motif in the COOH terminus. Whether this motif provides the sensor via which Ca²⁺ exerts a regulatory influence on TRPP2 (204) is at present unclear. In heterologous expression systems TRPP2 and TRPP3 form constitutively active cation-selective channels of relatively large conductance (90, 94, 408). Both channels are permeable to Ca²⁺, with TRPP3 displaying modest selectivity towards the divalent cation (91). Interestingly, TRPP3 has been recently identified as a possible sour taste sensor in mammals (176, 184).

There is considerable evidence that TRPP1 and TRPP2 physically couple to act as a signaling complex at the plasma membrane to which TRPP2 is recruited by TRPP1 (92, 156). Their association suppresses the ability of TRPP1 to activate G proteins (93) as well as the constitutive channel activity of TRPP2 (92). Antibodies directed against an extracellular domain of TRPP1 alleviate such mutual inhibition, simultaneously enhancing channel activity of TRPP2 and G protein activation by TRPP1 (92). Such a mode of activation might mimic the physiological stimulus that activates the complex. TRPP3 and PKD2L3 form a receptor for sour taste (176, 184).

Table 4 summarizes some of the key features of the members of the TRPP subfamily. For more detailed reviews, see References 91, 203, 211, 281, 363.

F. TRPA

The TRPA family currently comprises one mammalian member, TRPA1, which is expressed in dorsal root ganglion (DRG) and trigeminal ganglion (TG) neurons and in hair cells (82, 295, 415). TRPA1 exhibits 14 NH₂-terminal ankyrin repeats (415), an unusual structural feature that may be relevant to the proposed role of the channel as a mechanosensor (221, 295).

Some reports (31, 415) claim that TRPA1 can be activated by noxious cold (–17°C), although such a mode of stimulation has been disputed by others (35, 189, 295). Indirect support for a thermosensitive function of TRPA1 derives from recent reports that antisense knockout of TRPA1 alleviates cold hyperalgesia subsequent to spinal nerve ligation (192) and that the upregulation of the channel in sensory neurons following injury and inflammation contributes to cold hyperalgesia (324). Unfortunately, the recent generation of two independent TRPA1 knockout mouse models has not settled the controversy regarding whether TRPA1 is activated by noxious cold (see below). Chemical activators of TRPA1 include isothiocyanates (the pungent compounds in mustard oil, wasabi, and horseradish) (31, 189), methyl salicylate (in winter green oil) (31), cinnamaldehyde (in cinnamon) (31), allicin and diallyl disulfide (in garlic) (35, 258), acrolein (an irritant in vehicle exhaust fumes and tear gas) (35), and 9 tetra-hydrocannabinol (9THC, the psychoactive compound in marijuana) (189). Table 4 summarizes some of the key features of TRPA1. For a more detailed review, see Reference 270.

TRPN is a channel that is closely homologous to TRPA1. It is characterized by 29 ankyrin repeats within the NH₂ terminal. To date, this subfamily comprises only one member in C. elegans, Drosophila, and zebrafish (400, 488). TRPN1 probably acts as a mechanotransduction channel that is involved in hearing. Mammals apparently lack the TRPN gene (82).

	III. THE TRP CHANNEL FAMILY AND DISEASES

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A. Introductory Remarks

Defects in ion channel function are widely suspected to be the cause of various diseases, so-called channelopathies. However, of the 300 ion channels predicted in the human genome (http://www.ncbi.nlm.nih.gov/RefSeq; www.celeradiscoverysystem.com), relatively few have been directly connected to human diseases. Nonetheless, this number is constantly increasing (see the most recent reviews in Refs. 25, 191).

Channelopathies are traditionally defined as diseases coupled with identified defects in the gene encoding the channels. Following this definition, five TRP channel-related channelopathies have been identified to date. However, perturbation of physiological functions mediated by ion channels can also contribute more subtle to the genesis of several diseases. Such effects may be provoked by changes in channel abundance, or channel sensitization, or desensitization, resulting in exaggerated, or diminished, responses to various pathological stimuli. Abnormal endogenous production of various agents during the development of a disease (for example, in inflammation or autoimmune conditions) can affect channel function in a manner that contributes to the progression of the disease. Many members of the TRP superfamily are potential targets for such pathogenic factors. Abnormal regulation of ion channel function is especially interesting in all forms of inflammation and in systemic diseases, such as neurodegenerative, cardiovascular, and respiratory diseases. TRP channels are exceptional in the sense that they are often polymodal, i.e., they are activated by multiple and diverse gating stimuli and act as molecular integrators of these external and/or internal signals.

Unfortunately, our knowledge of the detailed mechanisms through which TRP channels function is still elementary. Such a situation hampers both our understanding of the mechanistic role of TRP channels in human disease and the development of drugs to target TRPs and their specific functions. The more we learn about fundamental TRP channel physiology and the potential role of TRPs in disease, the closer we will come to the development of novel therapies for various disease states. Ion channels are established targets for drugs as witnessed by a variety of therapeutically valuable agents that exert their action through, for example, members of the Cys-loop superfamily of transmitter-gated ion channels, and voltage-activated sodium, potassium, and calcium channel families. However, considering ion channels as a whole, therapeutically useful drugs have been developed for only a handful of targets. The situation is particularly critical for the TRP channel superfamily due to the paucity of selective modulators. To date, the only member of the TRP superfamily that has been targeted is TRPV1: TRPV1 antagonists and compounds that induce fast channel desensitization have been used in the treatment of pain, bladder, and gastrointestinal diseases (see below).

In general, dysregulation of TRP channel function may lead to disease by one or more of the following mechanisms.

Most TRP channels play a role in Ca²⁺ signaling. Given the universal role of Ca²⁺ as a signaling molecule, dysfunctions in Ca²⁺ signaling due to altered TRP channel function can have strong effects on a variety of cellular and systemic processes.

TRP channels can act as general polymodal cellular sensors, measuring changes in the environment to initiate adequate cell organ and behavioral response. Mistuning in these sensory inputs may cause multiple forms of cellular and somatosensory dysregulation.

Some TRP channels function as gatekeepers for the selective (re)absorption of ions such as Mg²⁺ and Ca²⁺. Dysfunction will lead to general disturbances in Mg²⁺ and Ca²⁺ homeostasis.

Some TRP channels are present on intracellular membranes, where their malfunctioning may lead to disturbed organelle function. One clear example is the dysregulation of lysosome function due to mutations in TRPML1 (30).

Several TRP channels are involved in the control of cell proliferation and growth. Dysfunctions may lead to growth disturbances, altered organogenesis, or cancer.

Some TRP channels play an important role in the trafficking of interacting proteins. Mistargetting of these binding partners may underlie a variety of pathological conditions.

TRP channels have the potential to modulate the electrical activity of excitable cells, e.g., in brain and heart. Investigating the consequences of TRP channel dysfunction on electrically complex cell functions such as generation of spontaneous electrical activity is an important challenge for future research.

In the sections that follow we describe established TRP channelopathies and draw upon a number of lines of evidence that are suggestive of the involvement of TRP channels in disease (Fig. 2). The expanding generation of TRP channel null-mutant mice and other transgenic models will allow the extrapolation of observed perturbations, at least to a certain extent, to human physiology and disease (96, 122, 123). An important caveat in such approaches is that the absence of a phenotype in knockout mice does not preclude an important function of the targeted channel, because compensatory mechanisms may mask a deficit. We will also consider the following: 1) function-based evidence (where changes in TRP channel function may be associated with symptomatology), 2) genetic evidence (where genes encoding TRP channels reside within vulnerable chromosomal regions), and 3) correlation-based evidence (where a disease state appears to be associated with changes in the abundance of individual TRP channels). Finally, the established, or potential, contribution of individual TRP channels to various disease groups is summarized in Table 5.

View larger version (27K): [in this window] [in a new window]   FIG. 2. TRPs and disease. Overview of the various extrapolations of a possible involvement of TRP channels in the pathogenesis of diseases.

Genetic defects in four TRP channels (TRPC6, TRPM6, TRPP2, and TRPML1) have been identified as the direct underlying cause of hereditary diseases. In addition, a variation in TRPM7 has been associated with an increased risk for two neurodegenerative diseases.

1. TRPC6 and focal segmental glomerulosclerosis

Six mutations of the TRPC6 gene have very recently been linked to the human proteinuric kidney disease focal and segmental glomerulosclerosis (FSGS) of the late-onset type (370, 507). In this disease, which is distinct from the early-onset form of FSGS, the podocyte foot processes and the glomerular slit diaphragms of the glomerular filter are initially well developed and functional but lose their integrity between childhood and late adulthood (207, 370, 507). Three of the identified mutations of the TRPC6 gene are missense mutations that result in enhanced signaling (gain of function) by TRPC6 via specific amino acid substitutions. The P112Q substitution enhances plasma membrane expression of heterologously expressed TRPC6 and increases both inward current responses and the [Ca²⁺]_i transient evoked by G protein-coupled receptors (GPCRs) that signal through PLC- (507). Similarly, TRPC6 R895C and E897K mutants display enhanced activity following GPCR activation, but neither the remaining missense mutations (N143S, S270T) nor the truncated mutant K874X give evidence of perturbed function (370).

Among other sites in the nephron, TRPC6 is expressed in the podocytes in the kidney glomerular filter (Fig. 3) (370, 507). Podocyte foot processes and the glomerular slit diaphragm form the glomerular filter and are an essential part of the permeability barrier in the kidney, which is defective in FSGS (108, 207). A lack of nephrin, a transmembrane protein of the immunoglobulin superfamily and a central component of the slit diaphragm, induces increased TRPC6 expression in podocytes and leads to an altered localization of TRPC6 (370). The defect in the filter function results in proteinuria and progressive kidney failure leading to end-stage renal failure (148, 207, 489). It is uncertain how mutations in TRPC6 translate into the development of FSGS, but at least for those mutations that result in gain of function there are some logical possibilities. Enhanced Ca²⁺ entry may constitute a pathogenic trigger, such as a Ca²⁺ overload of the podocyte that initiates cell death by apoptosis, or causes dysregulation that compromises the integrity of the permeability barrier (370). Notable in this regard is the renoprotective action in FSGS of agents that reduce the activity of the renin-angiotensin-aldosterone system, including angiotensin coverting enzyme (ACE) inhibitors and competitive antagonists of the angiotensin-1 (AT₁) receptor (507, 508). As mentioned above, activation of GPCRs signaling through PLC- in podocytes, for example, through activation of the AT1 receptor, evokes exaggerated [Ca²⁺]_i influx in cells expressing the TRPC6 P112Q mutant. Alternatively, mutations in TRPC6 may impair the ability of the podocyte to adapt to the normal physiological demands of maintaining the functional glomerular filter, such as responding to changes in glomerular filtration pressure. Possibilities also include a role for TRPC6 in the guidance of proteins such as nephrin and podocin, which are required to maintain the filtration barrier (508). Further functional characterization of the disease-causing mutations and their interactions with components of the glomerular filter is required to answer these questions. In addition, the development of a transgenic mouse model for FSGS, for example, by replacing wild-type TRPC6 by one of the disease-causing mutants or even by inducing overexpression of wild-type TRPC6, would be instrumental in further unraveling the role of TRPC6 in kidney function and FSGS.

View larger version (54K): [in this window] [in a new window]   FIG. 3. The glomerular filtration barrier. Two podocyte foot processes bridged by the slit membrane, the glomerular basement membrane, and the porous capillary endothelium are shown. The surfaces of the podocytes and of the endothelium are covered with a negatively charged glycocalyx containing the sialoprotein podocalyxin. The glomerular basement membrane is composed mainly of collagen IV, laminin 11, and the heparan sulfate proteoglycan agrin. The slit membrane is a porous proteinaceous membrane composed of (as far as is known) nephrin (Neph1, -2, and -3), P-cadherin, and FAT1. The slit membrane proteins are joined to the cytoskeleton by various adaptor proteins, including podocin, zonula occludens protein 1 (ZO-1; Z), CD2-associated protein (CD), and catenins. TRPC6 associates with podocin, CD, and nephrin at the slit membrane. Among the many surface receptors, only the angiotensin II (AT) type 1 receptor (AT1) is shown (for more detailed information, see Ref. 207).

Most TRP channels are involved in Ca²⁺ signaling. Much less is known about Mg²⁺, although it constitutes the third most common cation in the intracellular fluid ([Mg²⁺]_i = 0.5–1.0 mM) and the Mg²⁺ entry mechanism is incompletely understood. Mg²⁺ homeostasis demands precise regulation of the plasma concentration of Mg²⁺, which is generally within the range of 0.9–1.0 mM and dependent on a balance between intestinal absorption and renal excretion (69, 202). Heterologously expressed TRPM6 forms a Mg²⁺-permeable channel, and TRPM6 is primarily expressed in the brush-border membrane of the small intestine and in the apical membrane of the renal distal convoluted tubule (DCT), which both contain highly specialized cells responsible for Mg²⁺ absorption and reabsorption (Fig. 4) (482). TRPM6 provides an important influx pathway for Mg²⁺ and other divalent cations and is at the same time tightly regulated by the intracellular concentration of Mg²⁺ (482). One subsequent study contested this view and reported that TRPM6 can only form a functional cation channel in combination with TRPM7 (70). However, recent data confirm that TRPM6 can indeed form functional homotetrameric Mg²⁺-permeable channels, although TRPM6 + TRPM7 heterotetrameric channels are also functional (229).

View larger version (33K): [in this window] [in a new window]   FIG. 4. TRPM6 and Mg2+ reabsorption. A: the TRPM6 protein is a cation channel region with six transmembrane domains, a pore region, a long NH2 terminus conserved within the TRPM family, the TRP domain of unknown function located COOH-terminally of the ion channel domain, and a COOH-terminal kinase domain with sequence similarity to the atypical -kinases. The mutations identified in hypomagnesemia with secondary hypocalcemia (HSH) patients are shown. Note that only one single point mutation S141L (red) was identified. B: epithelial magnesium transport in intestine and kidney. The intestinal absorption demonstrates curvilinear kinetics resulting from the summation of two transport mechanisms: a saturable transcellular transport (dotted line), which is of functional importance at low intraluminal concentrations, and a paracellular passive transport (dashed line) rising linearly with intraluminal Mg2+ concentrations. TRPM6 is a component of the active transcellular pathway, but HSH patients are able to compensate for their genetic defect by high oral magnesium intake. C: in the thick ascending limb (TAL), Mg2+ is reabsorbed via the paracellular route. A specific tight juntion protein between the cells of the TAL, namely, paracellin-1, or claudin-16, permits the selective paracellular flux of Ca2+ and Mg2+. Defects in paracellin-1 lead to combined calcium and magnesium wasting. D: the distal convoluted tubule (DTC) reabsorbs Mg2+ in a transcellular fashion, consisting of an apical entry into the DCT cell through the Mg2+-selective ion channel TRPM6 and a basolateral extrusion of unknown molecular identity (69; see also Ref. 202). [Adapted from Schlingmann and Gudermann (387).]

Intestinal Mg²⁺ uptake in the brush-border epithelia occurs in a curvilinear manner and is regulated by two independent pathways: 1) passive paracellular absorption, which rises linearly with increasing luminal Mg²⁺ concentrations, and 2) transcellular transport, driven by secondary active transport, reaching saturation at high luminal Mg²⁺ levels (Fig. 4) (202). TRPM6 represents an essential molecular component of the active transcellular Mg²⁺ uptake at the apical membrane. The significance of saturation at high luminal Mg²⁺ concentrations is to prevent the cell being overloaded with Mg²⁺. This feedback mechanism is provided by TRPM6 itself, which is highly regulated by the intracellular concentration of Mg²⁺ (482). Electrophysiological studies on TRPM6 in heterologous expression systems revealed that both Mg²⁺ and Ca²⁺ block TRPM6 currents in a voltage-dependent manner, with higher affinity for Mg²⁺ (482). On the basolateral aspect of the epithelial cell, a yet unidentified Na⁺/Mg²⁺ exchanger accounts for onward transport of Mg²⁺ into the interstitial space and blood. The situation in HSH, where the transcellular pathway is not functional, requires utilization of the paracellular pathway to allow Mg²⁺ absorption. This effect can only be achieved by increasing the luminal Mg²⁺ concentration with a high-Mg²⁺-containing diet [up to 16-fold higher than the normal recommended daily intake (78)] to generate a stronger driving force for the passive Mg²⁺ uptake. Unfortunately, such high luminal levels of Mg²⁺ act as an osmotic laxative and frequently result in severe diarrhea.

The situation in the DCT of the nephron is different. Although most reabsorption of Mg²⁺ in the nephron occurs via a paracellular pathway in the thick ascending limb of the loop of Henle, it is the DCT that determines the final reabsorption of filtered Mg²⁺ from the lumen to the blood and thus the final urinary Mg²⁺ excretion. In the DCT, paracellular transport of Mg²⁺ does not occur, and TRPM6, located on the apical membrane of the epithelial cells, is the obligate pathway for the active reabsorption of Mg²⁺. If this pathway is defective, as in HSH, no further Mg²⁺ can be reabsorbed, resulting in the urinary Mg²⁺ leak that has often been observed in HSH patients.

Interestingly, colorectal cancer can be successfully treated with an antibody (cetuximab) directed against the epithelial growth factor receptor (EGFR) (391). However, unwanted side effects often observed in these patients are the typical HSH syndromes (i.e., hypomagnesemia and secondary hypocalcemia). It will be of high interest to investigate whether the EGFR has a modulatory influence on TRPM6 function.

3. TRPP2 and autosomal dominant polycystic kidney disease

Although the functional analysis of the TRPPs (polycystins) is not yet as developed as for most other TRP subfamilies, their involvement in human disease has been studied most extensively. Mutations in either TRPP1, or TRPP2, lead to polycystic kidney disease (PKD), which is characterized by the progressive development of large fluid-filled cysts in the kidney. Cysts can occupy much of the mass of the abnormally enlarged kidneys, thereby compressing and destroying normal renal tissue and impairing kidney function. Approximately 50% of patients with the primary form of PKD will progress to kidney failure, or end-stage renal disease (ESRD) (141). PKD is the most common inherited cause of kidney failure.

Autosomal dominant PKD (ADPKD) is by far the most prevalent, inherited, form of PKD accounting for 90% of cases. The symptoms of ADPKD usually develop between the ages of 30 and 40, but they can commence earlier, even during childhood. In most instances, ADPKD arises from mutations in the TRPP1 (PKD-1) or TRPP2 (PKD-2) genes (Fig. 5) (424). Mutations in TRPP1 (56 disease-causing mutations are known, 384 single nucleotide polymorphisms, SNPs) account for 85% of ADPKD and are associated with a more severe disease (appearing earlier) than that caused by TRPP2 mutations (45 disease causing mutations being known, 186 SNPs) (see also Ref. 514). The survival time of patients with TRPP2 mutations is longer than for those with TRPP1 defects. The cysts are not confined to the kidney, but also occur in hepatic tissue, brain tissue, and pancreatic ductal tissue. It is assumed that cystic tissue lacks functional TRPP1/TRPP2 (90, 94, 408). Other abnormalities associated with TRPP1 mutations include valvular defects in the heart, cerebral and aortic aneurysms, diverticulae in the colon, and inguinal hernia. TRPP2 mutations are also related to structural defects in the heart, e.g., defective septum formation. Mutations in TRPP2 in mice recapitulate all the cystic abnormalities, cardiac septum defects, and lead to embryonic whole body edema, renal failure, and early death (513).

View larger version (67K): [in this window] [in a new window]   FIG. 5. Model of the mechanisms of TRPP2 action. TRPP2 sequesters Id2 in the cytoplasm. Normally, phosphorylated TRPP2, associated with TRPP1, prevents nuclear translocation of Id2 by binding it to the COOH terminus. TRPP1/TRPP2 mutations allow Id2 to dissociate and trasnlocate into the nucleus (arrow) where it binds to E-protein family members. E-proteins activate growth supressor genes. If Id2 binds to E-proteins, the activity of growth suppression proteins is switched off and genes activating G1-S progression (e.g., Cdk2) are activated. This mechanism may explain the hyperproliferative feature of autosomal dominant polycystic kidney disease (ADPKD). [Adapted from Gomez (139) and Li et al. (232).]

Recently, it was shown that both basal and EGF-stimulated kidney cell proliferation are upregulated in cells that lack TRPP2, indicating that it acts as a negative regulator of cell growth. A possible explanation of that effect has been recently proposed (39, 232). TRPP1 and TRPP2, when localized in the cell membrane, inhibit the transfer of the helix-loop-helix (HLH) protein Id2, a crucial regulator of cell proliferation and differentiation, into the nucleus. Phosphorylation of the COOH terminus of TRPP1, which probably recruits TRPP2 to the cell membrane and forms a receptor-ion channel complex, is necessary for Id2 binding. When this binding is defective, for example, in ADPKD, Id2 can enter the nucleus and activate G₁-S progression. Correspondingly, renal epithelial cells from ADPKD patients demonstrate a clearly enhanced nuclear localization of Id2 (232). This could be a mechanism for the hyperproliferative phenotype in ADPKD and may cause cyst formation. Interestingly, the above mechanism involving Id2 could be important for other proliferative disorders.

TRPP2 is important for cilia movement and for the development of heart, skeletal muscle, and kidney. It probably also acts as a channel in the membranes of intracellular organelles. The current view is that TRPP2 (and possibly also TRPP3 and TRPP5) requires TRPP1, or TRPP1-like proteins, to function as plasma membrane receptors. The latter complexes may form mechanosensors in primary cilia and are involved in the development of a variety of organs, especially in tubulus formation. This mechanosensory complex is most likely indispensable for regulation of embryonic fluid movement. During embryogenesis, TRPP2 is active in node monocilia and plays a role in the establishment of left-right asymmetry (see below).

The autosomal recessive form of PKD (ARPKD) is a much more rare hereditary disease, with an estimated incidence of 1 in 20,000 live births (530). This disease is not related to defects in TRPP subfamily members but caused by mutations in the PKHD1 gene, which encodes for fibrocystin (also known as polyductin), a protein of >4,000 amino acids (530). Interestingly, fibrocystin colocalizes with TRPP2 at the basal bodies of primary cilia (41, 533). It will be of great interest to investigate whether TRPP2 and fibrocystin interact functionally, and whether this interaction is related to the pathology of PKD.

4. TRPML1 and mucolipidosis type IV

TRPML1 (or mucolipin 1, MLN1) is a high-conductance, nonselective,Ca²⁺-permeable cation channel encoded by the MCOLN1 gene (368). TRPML1 probably assembles into complexes that demonstrate variable single-channel conductance. Channel activity is reduced at low pH probably due to an assembly defect (56). The protein contains a serine-lipase domain in the amino part of the protein between the first two transmembrane domains that might function either enzymatically as a lipase, or as a binding domain or a transporter of lipids that might act as channel regulators (Fig. 6) (363).

View larger version (54K): [in this window] [in a new window]   FIG. 6. A model for the pathogenesis of mucolipidosis IV (MLIV). A: mutations causing MLIV (for details, see Refs. 29, 30). [Adapted from Fig. 3 in Altarescu et al. (17).] B: a model adapted from Caenorhabditis elegans. In WT, solutes and membrane are internalized and delivered to the early endosome (EE). Molecules from the endocytic and biosynthetic pathways are then delivered to late endosomes (LE). Lysosomal hydrolases and the solutes and internal vesicles destined for degradation are condensed in membrane structures budding off from these late endosomes (hybrid organelles; HO). The scission of the budding vesicles yields primary lysosomes (L). In defective TRPML1 channels, the maturation/scission of lysosomes budding from hybrid organelles is defective. These endosomes continue to receive membrane and solutes from the endocytic and biosynthetic pathways and hence increase in size (for details, see Ref. 450).

The pathological mechanisms underlying MLIV are not fully understood. Abnormal storage of amphiphilic lipids (phospholipids, gangliosides, neutral lipids, and mucopolysaccharides) and membranous materials in multiconcentric lamella together with granulated, water-soluble substances in late endosomes and lysosomes have been visualized by electron microscopy (Fig. 6) (29). However, there is considerable variability in the composition of the stored materials between different organs and tissues. Such abnormal storage has been hypothesized to be due to altered membrane fusion and fission events in the late endocytic pathway (63).

TRPML1 channel disease-causing mutations (i.e., V446l and F408) retain channel function, but unlike wild type, TRPML1 are unaffected by pH changes, suggesting that these mutations somehow affect control of clustering and complexing of TRPML1 (368). There is no defective hydrolytic activity, but rather a defective endocytic process in MLIV. An elevation of 1 pH unit was determined in the storage vacuoles of affected cells, which is typical of late endosomes and prelysosomal vacuoles (Fig. 6) (254, 264).

Defective TRPML1 channels induce changes in the endocytotic transport of membrane components, such as block of the endocytotic route to the final lysosome. In cells from MLIV patients, disturbed Ca²⁺ signaling has been described. In addition, large acidic organelles appeared consisting of late endosomes and lysosomes. It would appear that the Ca²⁺-dependent fusion between late endosome/lysosome hybrid vesicles is defective. TRPML1 could play a key role in Ca²⁺ release from the endosome/lysosome hybrid, which triggers the fusion and trafficking of these organelles. Perhaps TRPML1 is important in decreasing the intravesicular concentration of Ca²⁺ (215, 216).

The C. elegans cup-5 (coelomocytes uptake defective) mutant is the homolog of human mucolipin-1 gene. Interestingly, CUP-5 is essential for viability and required for lysosome biogenesis. Mutations in cup-5 cause the accumulation of large vacuoles in coelomocytes, resulting in increased cell death and embryonic lethality (450, 485).

Recently, a novel pathomechanism has been proposed for the development of mucolipidosis. Measurement of lysosomal pH revealed that the lysosomes in TRPML1-deficient cells obtained from patients with MLIV are overacidified. Because TRPML1 can function as a proton channel, the increased lysosomal acidification in TRPML1-deficient cells is likely to be due to a loss of function of TRPML1 channels resulting in a reduced H⁺ leakage. In addition, there is a marked reduction in lipid hydrolysis in MCOLN1^(-/–) cells, attributable to a decreased lipase activity (199). The accumulation of lipids and membranous material in intracellular organelles such as lysosomes, which is a typical characteristic of MLIV, might thus be due to decreased lipase activity (409). Alternatively, TRPMLs might be involved in correct compartmentalization. Traffic of TRPML3 to lysosomes is required for the normal function of the mucolipidosis inducing TRPML1. However, it is not clear yet whether mutants of TRPML1 causing MLIV disrupt also TRPML3 translocation and might induce a decreased number of TRPML1 channels in the patient's lysosomes (473).

5. TRPM7 and Guamanian amyotrophic lateral sclerosis/Parkinsonism dementia

TRPM7 is the closest homolog of TRPM6, with which it shares most basic functional features (293, 379). In contrast to the rather restricted expression pattern of TRPM6, TRPM7 is ubiquitously distributed in most cells and tissues, providing an ion channel pathway for entry of Mg²⁺, Ca²⁺, and trace metal ions (282). TRPM7 is therefore believed to be the molecular correlate of the ubiquitous Mg²⁺-nucleotide-regulated metal ion channel/Mg²⁺-inhibited ion channel (MagNuM/MIC) (382). Mg²⁺ influx through TRPM7 has been shown to be indispensable for cellular viability: targeted deletion of TRPM7 in DT-40 chicken B lymphocytes causes rapid cell growth arrest followed by cell death within 2–3 days (293). However, viability can be maintained and the arrest of cell proliferation reversed by supplementation of the culture medium with excess Mg²⁺ (10 mM), or reintroduction of wild type, or functional mutants, of TRPM7 (293, 390). These results illustrate the essential role of Mg²⁺ in cell cycle progression and cell proliferation (377), for which a hypothetical scheme, the "membrane magnesium mitosis" (MMM) model of proliferation control, has been proposed. "Membrane perturbation" evoked by a variety of growth factors is postulated to increase [Mg²⁺]_i via the release of Mg²⁺ from negatively charged binding sites on the inner leaflet of the plasma membrane, thus providing both free Mg²⁺ and Mg-ATP^2– for protein and DNA synthesis (377). The data obtained with TRPM7 suggest that regulated influx of Mg²⁺ through a membrane pore, rather than release from an intracellular "store" is more likely to be the pivotal event.

Given the important role of Mg²⁺ in cell proliferation and viability, it can be expected that disturbance of the cellular Mg²⁺ homeostasis, which depends on TRPM7, can have severe pathological consequences. Indeed, two diseases in humans have been linked to mutations in TRPM7: Guamanian amyotrophic lateral sclerosis (ALS-G) and Guamian Parkinsonism dementia (PD-G). These related neurodegenerative disorders are found with a relatively high incidence on the Pacific Islands Guam and Rota (352). The etiology of these disorders remains elusive, but evidence suggests a complex interplay of genetic and environmental factors (353). A TRPM7 variant, T1482I, which is located between the channel and the kinase domain, has been found in a subgroup of both ALS-G and PD-G patients but not in matched control subjects (163). Although the T1482I variant has no detectable alteration in -kinase activity, it displays a somewhat higher sensitivity to inhibition by intracellular Mg²⁺ within the physiologically relevant range. The incidence of both ALS-G and PD-G is increased in environments that are deficient in Ca²⁺ and Mg²⁺, such as the west Pacific. Thus increased sensitivity of TRPM7 to inhibition by Mg²⁺ could aggravate the Mg²⁺ homeostasis in an Mg²⁺-deficient environment, leading to a reduced intracellular Mg²⁺ concentration, which could in turn contribute to the etiology of the neurodegenerative diseases (163, 390).

At this point, it seems premature to classify ALS-G and PD-G as real TRP channelopathies. First, the functional charaterization of the T1482I variant is still quite incomplete. The difference in Mg²⁺ sensitivity between wild-type and T1482I channels is relatively mild, and the influence of this variation on the sensitivity of TRPM7 to other known regulators [e.g., Mg-ATP, cAMP, phosphatidylinositol 4,5-bisphosphate (PIP₂)] is currently unknown. Given that all ALS-G or PD-G patients that carried the T1482I allele also carried a wild-type allele, it is unclear whether the reduced Mg²⁺ sensitivity has any consequence in vivo. Second, only a relatively small number of brain samples were tested in this study and only 5 of 22 ALS-G or PD-G patients carried the T1482I variation (163). Whether the samples included close relatives is not clear. Notably, analysis of a SNP database revealed that 40% of the Japanese in Tokyo carry at least one T1482I allele (4.5% being homozygous for the mutation) (163), indicating that this mutation is also quite common in healthy subjects. However, we foresee that natural variation in the coding regions of TRP channels, leading to mild functional consequences similar to the TRPM7 T1782I variant, may play an important role in sensitivity to environmental conditions and the predisposition to certain diseases.

C. TRPs and Pain

According to the International Association for the Study of Pain (IASP), pain is an unpleasant sensory and emotional experience associated with actual, or potential, tissue damage or described in terms of such damage. Depending on its origin, pain can be classified as follows: pain caused by the stimulation of nociceptive receptors and transmitted over intact neural pathways is termed nociceptive pain; pain caused by damage to neural structures that disrupts the ability of the sensory nerves to transmit correct information to the brain is termed neuropathic pain; finally, pain with no clear physiological origin can be termed psychological pain. In addition, based on its timing, pain can be classified as acute, when the pain is directly correlated with its cause and has a clear warning function, or chronic, when pain persists much longer that the painful stimulus by which it was evoked.

1. TRP channels in acute nociceptive pain

Growing evidence implicates several members of the TRP superfamily in the detection of acute noxious thermal, mechanical, and chemical stimuli. Note that acute nociceptive pain, despite its unpleasantness, is a critical component of the body's defense system as part of a rapid warning relay that instructs the motor neurons to minimize physical harm and should not be considered as a disease condition. However, a short survey of the involvement of TRP channels in normal pain will be provided to better understand their involvement in pain related to pathological conditions. Indeed, it has become increasingly clear that the hypersensitivity and pain that occurs under various pathological conditions is often due to upregulated expression and/or increased sensitivity of TRP channels.

The first evidence for the involvement of TRP channels in the pain pathway came with the cloning of the vanilloid receptor TRPV1, which is arguably the most extensively studied member of the entire TRP superfamily (58). Its expression in DRG, TG, and nodose ganglion (NG) neurons, particularly in association with nociceptive afferent fibers, together with its activation by heat (>43°C), acid, and pungent vanilloid compounds strongly indicated that the TRPV1 plays an important role in the detection and integration of noxious stimuli (Fig. 7) (60, 442). Analysis of TRPV1 gene knockout mice confirmed that the channel contributes to the detection of acute painful chemical and thermal stimuli (59, 87). In particular, trpv1^(–/–) mice showed reduced responses to noxious heat stimuli and complete indifference to pungent vanilloids. A number of subsequent studies using gene knockout, or knockdown, strategies have highlighted the role of other TRP channels in detection of particular painful stimuli by nociceptive neurons. The trpv3^(–/–) mice exhibit deficits in innoxious and noxious heat perception (285). The trpv4^(–/–) mice present a phenotype that includes a reduced sensitivity to pressure exerted on the tail (426). Moreover, the induction of osmotic and mechanical hyperalgesia is absent in trpv4^(–/–) mice (12). In one study, trpa1^(–/–) mice displayed a reduced aversive reaction and pain in response to mustard oil, as well as reduced sensitivity to noxious cold and to punctate mechanical stimuli (214). However, in a second independent study of a different trpa1^(–/–) mouse strain, altered cold and mechanosensitivity were not evident (35). In addition, TRP channel null mutations in invertebrate model systems, such as the painless mutant of Drosophila (homologous to TRPA1) and the OSM-9 mutant of C. elegans (homologous to TRPV4) display reduced avoidance reactions to noxious thermal, chemical, and osmotic stimuli (485). Thus it seems well established that TRP channels play an important and evolutionary conserved role in the detection of noxious stimuli, which is essential for survival.

View larger version (31K): [in this window] [in a new window]   FIG. 7. Diversity of mechanisms activating TRPV1 and their relation to pathogenic stimuli. A: direct activation of TRPV1 and activation via PLC and PLA2 pathways that produce endogenous activating lipids. B: sensitization of TRPV1. Schematic diagram of some of the stimuli and intracellular pathways that contribute to the sensitization TRPV1 function in terminals of primary sensory neurons (for more information, see Ref. 131). C: a synopsis of stimuli causing TRPV1 and TRPV4 activation and their respective receptors.

In addition to their normal role as detectors of harmful stimuli, several pathological conditions lead to changes in the expression level and/or sensitivity of "pain" TRP channels. This can lead to exaggerated pain, when the experienced pain overestimates the harmfulness of the stimulus, or chronic pain, when the pain persists after the noxious stimulus has termina