The Transient Receptor Potential Superfamily of Ion Channels

The TRPC Subfamily

The TRPC (for canonical TRP) subfamily is composed of proteins that are most highly related to Drosophila TRP. It consists of seven mammalian members, TRPC1 to 7 (5–10⇓⇓⇓⇓⇓). Members of the TRPC subfamily contain three to four ankyrin repeats in the N-terminus and a conserved TRP domain in the C-terminus (Figure 2). Human TRPC subfamily can be divided into three subgroups on the basis of amino acid homology: TRPC1, TRPC4/5, and TRPC3/6/7. TRPC2 is a pseudogene in human but seems to encode an expressed protein in rat and mouse (8–10⇓⇓). TRPC4 and TRPC5 share ∼65% homology. TRPC3, TRPC6, and TRPC7 share ∼70 to 80% homology. Members of the TRPC subfamily are highly expressed in central nervous system and to lesser degrees in many peripheral tissues, including kidney. The selectivity ratio for Ca²⁺ versus Na⁺ (P_Ca/P_Na) for TRPC channels ranges from 0.5 to 10:1. TRPC channels are involved in Ca²⁺ entry in response to activation of phospholipase C (PLC) by membrane receptors and thus play important roles in the regulation of intracellular Ca²⁺ concentration by hormones and growth factors (see Functions of TRP Channels).

The TRPV Subfamily

The TRPV subfamily is named after the first mammalian member of the subfamily, vanilloid receptor 1 (VR1). It contains six mammalian members, TRPV1 to 6. TRPV1 (VR1) was isolated by expression cloning using capsaicin, a vanilloid compound derived from “hot” pepper, as a binding ligand (11). TRPV2 to 4 were isolated by searching for expressed sequence tags (EST) with amino acid homology to TRPV1 (12–16⇓⇓⇓⇓). TRPV1 to 4 share 40 to 50% amino acid homology, are Ca²⁺-permeable nonselective cation channels (P_Ca/P_Na ∼3 to 10:1), and have steep temperature sensitivity. TRPV5 and TRPV6 were isolated by expression cloning of proteins that mediate Ca²⁺ transport in kidney and intestine, respectively (17,18⇓). They are the most highly Ca²⁺-selective TRP channels (P_Ca/P_Na >100:1). Overall, TRPV subfamily channels contain three to four ankyrin repeats in the N-terminus and a TRP domain in the C-terminus (Figure 2). TRPV channels are also present in invertebrates. Invertebrate TRPV proteins include C. elegans OSM-9 and OCR (for OSM-9/capsaicin receptor related) (19,20⇓). OSM-9 is more closely related to TRPV5/6 than TRPV1 to 4 at the amino acid level.

The TRPM Subfamily

The TRPM subfamily is named after the founding member, melastatin. Melastatin, TRPM1, is a tumor suppressor protein isolated in a screen for genes whose level of expression (inversely) correlated with the severity of metastatic potential of a melanoma cell line (21). There are eight mammalian members in the TRPM subfamily. TRPM subfamily channels lack ankyrin repeats in the N-terminus but contain the TRP domain in the C-terminus (Figure 2). The C-terminus of TRPM proteins is considerably longer than the corresponding region of other TRP. The C-terminus of several members of TRPM contains enzyme domains. These TRPM proteins are thus called “chanzymes.”

TRPM2 (initially called TRPC7 and LTRPC2) is a Ca²⁺-permeable channel that contains a C-terminal ADP-ribose pyrophosphatase domain (22–25⇓⇓⇓). ADP-ribose pyrophosphatase catalyzes the hydrolysis of nucleoside diphosphate derivatives. The domain in TRPM2, however, is an ineffective hydrolase but binds ADP-ribose and NAD (23–25⇓⇓). ADP-ribose and NAD directly activate TRPM2 to allow Ca²⁺ influx. ATP counteracts the NAD-induced activation. TRPM2 is also regulated by H₂O₂ and TNF-α (26,27⇓). Thus, TRPM2 may be important in sensing oxidative stress and in linking apoptosis with the metabolism of ADP-ribose and NAD.

TRPM3 is a Ca²⁺-permeable, nonselective cation channel whose activity is increased by hypotonicity (28). TRPM4 and TRPM5 are the only TRP channels that are permeable to monovalent cations but not Ca²⁺ (P_Ca/P_Na <0.05:1) (29,30⇓). However, they are activated by intracellular Ca²⁺ (29,30⇓). Activation of TRPM4 and TRPM5 by intracellular Ca²⁺ leads to membrane depolarization. TRPM6 and TRPM7 both contain a C-terminal protein kinase domain (31–36⇓⇓⇓⇓⇓). The role of these kinase domains in the channel function of TRPM6 and TRPM7 is not known. TRPM8 is an outward rectifying channel that can be activated by cold (37,38⇓). It was isolated by expression cloning of a menthol receptor from sensory neurons.

The TRPP Subfamily

The TRPP subfamily is so named because of its founding member, PKD2. PKD2 (also called TRPP2) was discovered as one of the gene products mutated in autosomal dominant polycystic kidney disease (39). PKD2 is a six-TM protein that shares ∼25% amino acid identity with the closely related TRPC3 and TRPC6 over the region spanning TM segments IV through VI (40). Mammalian PKD2 is a Ca²⁺-permeable, nonselective cation channel (41). It contains a Ca²⁺-binding EF-hand motif and a coiled-coil domain in the C-terminus but does not include any ankyrin repeats or a TRP domain. PKD2L1 (TRPP3) and PKD2L2 (TRPP5) are homologues of PKD2 (42,43⇓). No mutations in either PKD2L1 or PKD2L2 have been found in patients with autosomal dominant polycystic kidney disease.

The much longer PKD1 protein (TRPP1) is an 11-TM protein (44). The last six TM segments of PKD1 are homologous to PKD2 and other related TRP channels (45). PKD1 is not known to form a channel by itself but may complex with PKD2 to regulate its channel activity (46).

The TRPN Subfamily

The first member of the TRPN subfamily is Drosophila NOMPC protein. NOMPC is the gene product of mutant flies with the phenotype of “no mechanoreceptor potentials” (47,48⇓). The Drosophila NOMPC protein shares significant homology with the TRP superfamily. The N-terminus of NOMPC contains 29 ankyrin repeats (Figure 2). A NOMPC orthologue was also identified in C. elegans (48) and in zebrafish (49). No TRPN have been found in mammals.

The TRPML Subfamily

The TRPML subfamily is defined by a human protein, mucolipin 1. Mucolipin 1 (TRPML1) is a 580–amino acid long protein probably restricted to intracellular vesicles (50–52⇓⇓). TRPML1 contains two proline-rich regions, a lipase serine active site in the N-terminus and a dileucine motif suggestive of lysosomal targeting in the C-terminus. Mutations in TRPML1 cause mucolipidosis type IV (MLIV), a rare autosomal recessive lysosomal storage disease that affects brain, eyes, and gastric function. The defect in MLIV seems to be in sorting or transport of intracellular vesicles. Channel function of TRPML1 has recently been reported (53). TRPML2 and 3 were also identified (54). TRPML3 is present in the cytoplasm of hair cells and the plasma membrane of stereocilia.

Role of TRP in PLC-Dependent Ca²⁺ Influx

In virtually all eukaryotic cells, activation of PLC-coupled membrane receptors by hormones leads to an increase in intracellular Ca²⁺. Activation of PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate to generate inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). The increase in intracellular Ca²⁺ is initially produced by release of Ca²⁺ from intracellular IP₃-sensitive stores and is followed by an influx of Ca²⁺ from the extracellular space through plasma membrane Ca²⁺-permeable channels. The entry of Ca²⁺ from the extracellular space is important for refilling intracellular stores. The mechanism for Ca²⁺ entry has been studied extensively. In one mechanism, termed “store-operated Ca²⁺ entry,” the emptying of intracellular stores activates surface membrane Ca²⁺ channels via either diffusible factors or a direct interaction between IP₃ receptors and surface Ca²⁺ channels (55–57⇓⇓). These Ca²⁺-permeable channels on the cell surface are called store-operated channels (SOC). Not all Ca²⁺ entry after PLC activation is dependent on store depletion. Ca²⁺ entry may also be due to activation of surface Ca²⁺ channels by other intracellular second messengers, such as DAG.

Store-operated Ca²⁺ entry is widespread and represents the major mechanism of regulation of Ca²⁺ influx in nonexcitable cells such as renal tubule cells. Because of their prevalence and importance, considerable effort has been devoted to identifying SOC and to understanding the mechanism of activation. The discovery that Drosophila TRP proteins were PLC-dependent Ca²⁺ influx channels has fueled much of the interests and hopes for isolation of SOC. TRPC channels are the closest relatives of Drosophila TRP and all are activated by G protein–coupled receptors. A vast number of papers have reported that many TRPC channels function as SOC. Zhu et al. (7) first reported that expression of human TRPC3 in cultured human embryonic kidney (HEK) cells gives rise to agonist- and thapsigargin-activated channel activity. Because thapsigargin depletes intracellular Ca²⁺ stores by inhibiting the endoplasmic reticulum Ca²⁺-ATPase, the authors concluded that TRPC3 proteins constitute subunits of SOC. Others have also reported that reduction of TRPC1 expression using antisense oligonucleotides leads to inhibition of endogenous SOC (58). A dominant-negative form of TRPC3 inhibits store-operated Ca²⁺ entry in umbilical vein endothelial cells (59).

Kiselyov et al. (60) reported that in HEK cells that stably express human TRPC3, IP₃ activates single channels in excised inside-out patches. This activation is lost upon extensive washing of excised patches and is restored by addition of IP₃-bound IP₃ receptor. Kiselyov et al. concluded that TRPC3 is a store-operated channel that is activated via direct interaction with IP₃ receptors. Similar store-operated channel activity has been reported for TRPC4 and TRPC7 channels (61,62⇓). Recently, knockout mice that lack the TRPC4 gene have been created (61). TRPC4−/− mice exhibit impaired vasorelaxation of aortic rings; aortic endothelial cells isolated from knockout mice exhibit defective store-operated Ca²⁺ entry (61), providing support that TRPC4 channels function as SOC.

However, many publications also dispute the conclusion that TRPC proteins are SOC. For example, Zitt et al. (63) reported that TRPC3 expressed in Chinese hamster ovary (CHO) cells is not store dependent but rather is activated by DAG. Hofmann et al. (64) confirmed these findings and reported that TRPC3 and TRPC6 are activated by DAG. Drosophila TRP proteins, the earliest and most promising candidates for SOC, have now been shown definitively not to be store dependent. Mutant flies that lack the IP₃ receptor gene remain capable of generating normal sustained receptor potentials on continuous light exposure (65). It is now believed that Drosophila TRP proteins are activated by DAG, polyunsaturated fatty acids (downstream metabolites of DAG), and/or a reduction of phosphatidylinositol 4,5-bisphosphate₂ (66,67⇓). Thus, the question of whether and which TRP constitutes SOC remains unsettled. The reason(s) for the differences among these studies is not known. One possibility is that different expression systems were used in these studies. Because virtually every cell expresses some TRP proteins endogenously, expressed exogenous proteins may form heteromultimers with endogenous proteins and/or activate endogenous TRP proteins. Irrespective of the mechanism of activation (store dependent or store independent), it is clear that TRPC play an important role in PLC-dependent Ca²⁺ entry.

TRP as Thermal and Noxious Receptors

TRPV1 was first identified as a receptor that binds capsaicin. Capsaicin is an ingredient of chili peppers that produces the “hot,” spicy sensation. Capsaicin sensitivity is a functional hallmark of nociceptive sensory neurons. Consistent with this concept, TRPV1 is expressed in primary afferent sensory neurons of the dorsal root ganglion (DRG) and trigeminal ganglion (TG) (68). Binding of capsaicin activates TRPV1. TRPV1 is also activated by noxious heat (≥43°C) and low pH (5.9) (68). Gating by heat is direct, whereas low pH reduces the temperature threshold for activation. In addition, inflammatory mediators activate TRPV1 (69,70⇓). Because tissue injury produces acidosis and inflammation, TRPV1 is a polymodal detector of injury.

TRPV2 is ∼50% identical to TRPV1 but is insensitive to capsaicin and low pH (12). Instead, it is activated by heat at a higher temperature threshold than TRPV1 (≥52°C). TRPV3 is temperature sensitive in the physiologic temperature range (31 to 39°C) (13–15⇓⇓). TRPV4 was first described as a channel that was activated by hypotonicity-induced cell swelling (16). TRPV4 is also temperature sensitive but at a low activation threshold (≥25°C) (71) and is capable of integrating a large number of stimuli (72) (see below). Both TRPV3 and TRPV4 are present in sensory neurons and in keratinocytes, where they might function as thermal sensors in the skin.

Further evidence that TRP proteins are essential thermal sensors comes from recent discovery that other TRP proteins function as receptors for cold. TRPM8, isolated by expression cloning of a menthol receptor, is activated by cold (8 to 28°C) (37,38⇓). Its activation by menthol produces the cooling sensation of the compound. TRPM8 is found in ∼10% of DRG or TG neurons. Cold temperatures below 28°C evoke large currents through TRPM8, which saturates near 8°C. Knowing that some neurons are activated only by noxious cold temperature (<17°C), Story et al. (73) searched for additional cold sensors and found a unique protein named ANKTM1. ANKTM1 is found in humans and mice and is distantly related to the invertebrate TRPN subfamily but has little sequence similarity to other mammalian TRP proteins (Table 1). It may be considered the founding member of a new subfamily, TRPA (not shown in the phylogenetic tree in Figure 1). ANKTM1 is expressed in a subset of sensory neurons that also express TRPV1.

In summary, at least six TRP proteins are involved in temperature sensing. The unique threshold for activation but overlapping range of sensitivity for these proteins allows organisms to have a combinatorial mode of thermal coding that responds to temperatures that span between 8 and 60°C. Thus, noxious cold temperatures activate ANKTM1 and TRPM8, whereas cool temperatures activate TRPM8 alone. Warm temperatures activate TRPV3 and TRPV4, whereas moderate noxious heat activates TRPV1 and high noxious heat activates TRPV1 and TRPV2.

TRP as Mechanosensors

Mechanosensation is the basis for hearing, osmolar sensing, stretch, touch, and flow sensing (74). Hearing is initiated by the bending of cilia on the inner ear hair cells in response to incoming sound waves. Bending of cilia activates “transduction channels” that convert the mechanical stimulus to electrical signals, which travel by the auditory nerve fibers to the brain. Vestibular hair cells in the semicircular canals contain similar types of transduction channels to sense movement of the head.

The best studied mechanosensitive channel is the bacterial MscL channel (75). The MscL channel is a mechanosensitive channel that is activated by lateral membrane tension. The existence of these channels in Archaebacteria highlights the ancient nature of mechanotransduction and its critical role in all cells. No MscL homologues have been identified in eukaryotes. Eukaryotic mechanosensitive channels include at least two unrelated gene families, the DEG/ENaC (degenerins/epithelial Na⁺ channels) superfamily (reviewed in ref. 74) and the TRP superfamily. The TRP superfamily of proteins contributes to all modalities of mechanosensation from flies, worms, and fish to humans. The first cloned TRP channel implicated in mechanosensation is the C. elegans OSM-9 of the TRPV subfamily. Worms with mutations in the osm-9 gene are defective in osmotic avoidance, sensitivity to nose touch, and attraction to volatile odorants (19). NOMPC, a member of TRPN, is located in mechanosensory bristles and is essential for response to touch stimuli in flies (47,48⇓). In zebrafish, NOMPC is present in sensory hair cells (49). Knockdown of NOMPC in zebrafish impairs the acoustic startle reflex.

OSM-9 and NOMPC are not present in mammals. Mammalian mechanosensitive TRP proteins include TRPV4, TRPP, and possibly TRPML3. As discussed above, TRPV4 is a mammalian TRPV member that senses temperature, osmolarity, and other mechanical stimuli (16,71,72⇓⇓). Besides sensory neurons, TRPV4 is expressed in renal distal tubules, where interstitial osmolarity fluctuates markedly. TRPV4 is expressed in osmosensing circumventricular organs of brain, mechanosensitive heart and vascular endothelial cells, and mechanosensitive inner ear hair cells (16,71,72⇓⇓). TRPV4, however, is not essential for all of these functions. TRPV4−/− mice exhibit impaired osmotic regulation and a reduced response to noxious mechanical stimuli but have no detectable defects in hearing and auditory responses and maintaining core body temperature (76). TRPV4 knockout mice have a slightly higher serum osmolarity during water restriction and a lower serum vasopressin level during hyperosmolar challenge. Urine osmolarity is not different between knockout mice and control littermates receiving dDAVP infusion, suggesting that defects in osmotic regulation reside in central osmosensation, not in the kidney tubules.

Varitint-waddler mice are spontaneously occurring mutant mice characterized by deafness and circular behavior indicative of vestibular dysfunction (54). Mutant mice show hair cell degeneration in the first few weeks after birth. Positional cloning has recently identified Mcoln3 (TRPML3) as the gene mutated in varitint-waddler mice (54). TRPML3 localizes to intracellular vesicles and plasma membrane of stereocilia in Cochlea hair cells. The physiologic role of TRPML3 in hair cells is unknown. However, its location in stereocilia raises an interesting possibility that it may function as a transduction channel in hair cells.

PKD1 and PKD2 are TRPP proteins that likely play important roles in sensing fluid flow in renal tubules. PKD1 and PKD2 proteins (also called polycystin-1 and -2) both are localized in the apical cilium of renal tubular epithelial cells (77,78⇓). Bending of the cilia in cultured epithelial cells by flow causes calcium influx (79). Nauli et al. (80) recently reported that the calcium response to bending is diminished in cells that lack cilia, in cells that lack PKD1, or in cells that are treated with a blocking antibody to PKD2. These experiments suggest that PKD1 and PKD2 are involved in calcium influx activated by flow-induced bending of the apical cilium. Defects in fluid flow sensation by cilia and Ca²⁺ influx likely play pivotal roles in cyst formation in kidney and other organs in polycystic kidney diseases.

PKD proteins are also important in embryonic development. The embryonic ventral node, which appears in the gastrulation stage of vertebrate development, consists of a dorsal layer of ectoderm over a ventral layer of cells each containing a single cilium. Normal left-right body asymmetry depends on a leftward nodal flow generated by ciliated nodal cells. McGrath et al. (81) showed that there are two types of nodal cilia: one is motile but does not contain PKD2, and the other is nonmotile but contains PKD2. The motile ciliated cells located in the center of the node generate fluid flow that is sensed by nonmotile PKD2-containing ciliated cells in the peripheral region of the node. Increase in cytosolic Ca²⁺ in the left side of the node is thought to be important for establishing left-right asymmetry. Homologues of PKD1 and PKD2 are also present in C. elegans (82,83⇓), where they are located in ciliated mechanosensory neurons of male copulatory organs and in other ciliated mechanoreceptors. Together, these reports support the important role of PKD proteins in ciliary mechanosensation.

The identity of pore-forming protein(s) in PKD2-related channel activity remains unclear. Channel activity recorded from PKD2 proteins reconstituted in lipid bilayer is strongly voltage dependent: Open probability is higher at hyperpolarized membrane potentials and reduced to near zero at positive membrane potentials (41,84⇓). This voltage dependence of single-channel open probability would predict a strongly inwardly rectifying whole-cell current if PKD2 proteins were expressed on the surface membrane and solely contributed to membrane currents. However, expression of PKD2 with or without PKD1 in cultured cells produces surface membrane whole-cell currents with nearly linear current–voltage relationships (85,86⇓). It has been shown that PKD2 protein can physically associate with TRPC1 (40). The molecular composition of pore-forming protein(s) underlying the whole-cell currents in PKD2-expressing cells versus the single-channel activity recorded in lipid bilayer deserves further investigation (87).

TRP as Taste Sensors

Salt and sour taste are detected by members of the DNG/ENaC superfamily. The ability of certain TRP proteins to integrate physical and chemical stimuli (e.g., TRPV4, OSM-9) suggests that TRP may play roles in the perception of taste. Indeed, it has been found that TRPM5 is expressed in taste buds. TRPM5 may be responsible for perception of bitter and/or sweet compounds (88).

Role of TRP in Divalent Ion Homeostasis

(Re)absorption of Ca²⁺ and Mg²⁺ ions in kidney and intestine is essential for homeostasis of these ions in the body. Transepithelial transport of Ca²⁺ and Mg²⁺ in these tissues occurs via paracellular as well as transcellular routes. Transcellular reabsorption of these ions, although accounting for a smaller portion of the total reabsorption than paracellular route, is the primary target for regulation by hormones and acid–base disturbances. Transcellular reabsorption of Ca²⁺ and Mg²⁺ is mediated by diffusion into cells through ion channels in the apical membrane followed by extrusion across basolateral membranes via pumps or exchangers. TRPV5 was initially identified as the apical Ca²⁺ entry channel in kidney and was localized to the Ca²⁺-reabsorptive distal convoluted tubules (DCT) (17). TRPV6 was identified as the apical channel responsible for intestinal absorption of Ca²⁺ (18). Recent studies, however, have found that both TRPV5 and TRPV6 are expressed in the DCT of kidney, where they may form heteromultimers (89). TRPV6 is also expressed in medullary collecting ducts (90), where Ca²⁺ reabsorption is not known to take place under physiologic conditions. The role of TRPV6 in this segment remains to be identified. Mice with targeted ablation of TRPV5 have been generated (91) and exhibit polyuria and hypercalciuria, providing support for an essential role of TRPV5 in renal Ca²⁺ reabsorption. Consistent with the phenotype of renal Ca²⁺ wasting, TRPV5 knockout animals exhibit compensatory intestinal hyperabsorption of Ca²⁺ and reduced bone thickness.

Increased acid load in conditions such as high dietary protein intake and metabolic acidosis increases urinary Ca²⁺ excretion (92). A recent study reported that extracellular protons bind to a titratable amino acid, glutamate-522 (a “pH sensor”), in the extracellular loop of rabbit TRPV5 and inhibit its activity (93). A similar titratable amino acid, histidine, is present in human TRPV6, which could mediate extracellular acid regulation of TRPV5/6 heteromeric channels in the DCT. It has been estimated that inhibition of TRPV5/6 caused by a decrease in lumen pH from 6.8 to 6.4 would lead to doubling of urinary Ca²⁺ excretion (93).

Familial hypomagnesemia with secondary hypocalcemia is an autosomal recessive disease characterized by renal wasting and defective intestinal absorption of Mg²⁺ and secondary hypocalcemia. Schlingmann et al. (31) and Walder et al. (32) reported that familial hypomagnesemia is caused by mutations of TRPM6. TRPM6 proteins are expressed in kidney and intestine, supporting the hypothesis that they are Mg²⁺ channels responsible for Mg²⁺ transport in these tissues. Transcellular reabsorption of Mg²⁺ in kidney occurs in DCT. A recent paper reported that TRPM6 is localized to the apical membrane of DCT in kidney and the brush border of the small intestine (94). Expression of TRPM6 in cultured HEK cells produces a Mg²⁺-permeable channel that is regulated by intracellular Mg²⁺ (94). TRPM6 contains an atypical kinase domain in the C-terminus (31,32⇓).

TRPM7 is ∼50% identical to TRPM6 and contains an atypical kinase domain as well (33,34⇓). TRPM7 is ubiquitous and conducts divalent cations, including Mg²⁺, Ca²⁺, and trace elements such as Mn²⁺ and Co²⁺ (95,96⇓). Cultured cells in which the TRPM7 gene has been inactivated have decreased viability, which can be rescued by supplementation of extracellular Mg²⁺ (34,97⇓). Mg²⁺ uptake through TRPM7 requires a functional coupling between its channel and kinase domain (97). Chubanov et al. (98) reported that TRPM6 and TRPM7 interact with each other to increase channel currents in cultured HEK cells. The authors suggested that TRPM6 and TRPM7 heteromultimers may form the functional epithelial Mg²⁺ channels. Mg²⁺ reabsorption in kidney and intestine is regulated by many hormones. It is tempting to speculate that hormonal regulation of TRPM6 (and TRPM7) involves its intrinsic kinase domain. Identification of TRPM6 (with or without TRPM7) as epithelial Mg²⁺ channels opens the door for future investigation of Mg²⁺ transport using cellular, molecular, and whole-animal genetic approaches.

Role of TRP in Cell Growth

Several TRP proteins have been found to play an important role in cell proliferation. TRPM1 is a tumor suppressor whose level of expression is inversely correlated with melanoma aggressiveness (21). TRPV6 is expressed in intestine and kidney and mediates transepithelial Ca²⁺ transport in these tissues. TRPV6 is normally undetectable in prostate tissue but is present at high levels in prostate cancer (99). Another TRP gene, TRPM8, is also upregulated in prostate cancer and in primary tumors of breast, colon, lung, and skin origin (100). Intracellular Ca²⁺ homeostasis plays a key role in cell-cycle control. An increase in intracellular Ca²⁺ caused by upregulation of TRPV6 or TRPM8 may trigger malignant transformation of cells. Alternatively, the increase in cellular uptake of Ca²⁺ may play a permissive role for rapid growth of tumor cells. As mentioned above, uptake of Mg²⁺ through TRPM7 is essential for cell survival in the physiologic culture media (34,97⇓). How TRPM1 suppresses tumor growth is currently unknown and requires future investigation.