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2-Aminoethoxydiphenyl borate (2APB) had been depicted as a universal blocker of transient receptor potential (TRP) channels. While evidence has accumulated showing that some TRP channels are indeed inhibited by 2APB, especially in heterologous expression systems, there are other TRP channels that are unaffected or affected very little by this compound. More interestingly, the thermosensitive TRPV1, TRPV2, and TRPV3 channels are activated by 2APB. This has been demonstrated both in heterologous systems and in native tissues that express these channels. A number of 2APB analogs have been examined for their effects on native store-operated channels and heterologously expressed TRPV3. These studies revealed a complex mechanism of action for 2APB and its analogs on ion channels. In this review, we have summarized the current results on 2APB-induced activation of TRPV1-3 and discussed the potential mechanisms by which 2APB may regulate TRP channels.

TRPV4 is a non-selective cation channel subunit expressed in a wide variety of tissues. TRP channels are formed by a tetrameric complex of channel subunits. The available evidence suggests that TRPV4 cannot form heteromultimers with other TRPV isoforms, and that TRPV4-containing channels are homotetramers. These channels have a characteristic outwardly rectifying current-voltage relation, and are 5–10 times more permeable for Ca than for Na. TRPV4 can be activated by a wide range of stimuli including physical (cell swelling, heat, mechanical stimulation) and chemical stimuli (endocannabinoids, arachidonic acid, and, surprisingly, 4α-phorbol esters). Activation by swelling and endocannabinoids involves cytochrome P450 epoxygenase-dependent arachidonic acid metabolism to the epoxyeicosatrienoic acids (EETs). Heat and 4α-phorbol esters also seem to share a common mechanism of activation, but the endogenous messenger involved in the response to heat has not yet been identified. Ca acting from the intracellular side can have both potentiating and inhibitory effects on channel activity and is involved in channel activation and inactivation. Given its wide expression and the variety of activatory stimuli, TRPV4 is likely to play a number of physiological roles. Studies with mice suggest a role for the channel in the regulation of body osmolarity, mechanosensation, temperature sensing, vascular regulation and, possibly, hearing.

Ca homeostasis in the body is tightly controlled, and is a balance between absorption in the intestine, excretion via the urine, and exchange from bone. Recently, the epithelial Ca channel (TRPV5) has been identified as the gene responsible for the Ca influx in epithelial cells of the renal distal convoluted tubule. TRPV5 is unique within the family of transient receptor potential (TRP) channels due to its high Ca selectivity. Ca flux through TRPV5 is controlled in three ways. First, TRPV5 gene expression is regulated by calciotropic hormones such as vitamin D and parathyroid hormone. Second, Ca transport through TRPV5 is controlled by modulating channel activity. Intracellular Ca, for example, regulates channel activity by feedback inhibition. Third, TRPV5 is controlled by mobilization of the channel through trafficking toward the plasma membrane. The newly identified anti-aging hormone Klotho regulates TRPV5 by cleaving off sugar residues from the extracellular domain of the protein, resulting in a prolonged expression of TRPV5 at the plasma membrane. Inactivation of TRPV5 in mice leads to severe hypercalciuria, which is compensated by increased intestinal Ca absorption due to augmented vitamin D levels. Furthermore, TRPV5 deficiency in mice is associated with polyuria, urine acidification, and reduced bone thickness. Somepharmaceutical compounds, such as the immunosuppressant FK506, affect the Ca balance by modulating TRPV5 gene expression. This underlines the importance of elucidating the role of TRPV5 in Ca-related disorders, thereby enhancing the possibilities for pharmacological intervention. This chapter describes a unique TRP channel and highlights its regulation and function in renal Ca reabsorption and overall Ca homeostasis.

The ion channel TRPV6 is likely to function as an epithelial calcium channel in organs with high calcium transport requirements such as the intestine, kidney, and placenta. Transcriptional regulation of TRPV6 messenger RNA (mRNA) is controlled by 1,25-dihydroxyvitamin D, which is the active hormonal form of vitamin D, and by additional calcium-dependent and vitamin D-independent mechanisms. Under physiological conditions, the conductance of the channel itself is highly calcium-selective and underlies complex inactivation mechanisms triggered by intracellular calcium and magnesium ions. There is growing evidence that transcriptional regulation of TRPV6 in certain tissues undergoing malignant transformation, such as prostate cancer, is linked to cancer progression.

TRPM2 is a cation channel enabling influx of Na and Ca, leading to depolarization and increases in the cytosolic Ca concentration ([Ca]). It is widely expressed, e.g. in many neurons, blood cells and the endocrine pancreas. Channel gating is induced by ADP-ribose (ADPR) that binds to a Nudix box motif in the cytosolic C-terminus of the channel. Endogenous ADPR concentrations in leucocytes are sufficiently high to activate TRPM2 in the presence of an increased [Ca] but probably not at resting [Ca]. Another channel activator is oxidative stress, especially hydrogen peroxide (HO) that may act through ADPR after ADPR polymers have been formed by poly(ADP-ribose) polymerases (PARPs) and hydolysed by glycohydrolases. HO-stimulated TRPM2 channels essentially contribute to insulin secretion in pancreatic β-cells and alloxan-induced diabetes mellitus. Inhibition of TRPM2 channels may be achieved by channel blockers such as flufenamic acid or the anti-fungal agents clotrimazole or econazole. Selective blockers of TRPM2 are not yet available; those would be valuable for a characterization of biological roles of TRPM2 in various tissues and as potential drugs directed against oxidative cell damage, reperfusion injury or leucocyte activation. Activation of TRPM2 may be prevented by anti-oxidants, PARP inhibitors and glycohydrolase inhibitors. In future, binding of ADPR to the Nudix box may be targeted. In light of the wide-spread expression and growing list of cellular functions of TRPM2, useful therapeutic applications are expected for future drugs that block TRPM2 channels or inhibit their activation.

TRPM3 is the last identified member of the TRPM subfamily and is most closely related to TRPM1. Due to alternative splicing, the TRPM3 gene encodes a large number of different variants. One splice event, affecting the pore-forming region of the channel, changes its selectivity for divalent cations. Inthis review, we give an overview of the identified TRPM3 variants and compare their functional properties.

In the current review we will summarise data from the recent literature describing molecular and functional properties of TRPM4. Together with TRPM5, these channels are up till now the only molecular candidates for a class of non-selective, Ca-impermeable cation channels which are activated by elevated Ca levels in the cytosol. Apart from intracellular Ca, TRPM4 activation is also dependent on membrane potential. Additionally, channel activity is modulated by ATP, phosphatidylinositol bisphosphate (PiP), protein kinase C (PKC) phosphorylation and heat. The molecular determinants for channel activation, permeation and modulation are increasingly being clarified, and will be discussed here in detail. The physiological role of Ca-activated non-selective cation channels is unclear, especially in the absence of gene-specific knock-out mice, but evidence indicates a role as a regulator of membrane potential, and thus the driving force for Ca entry from the extracellular medium.

TRPM5 is a cation channel that it is essential for transduction of bitter, sweet and umami tastes. Signaling of these tastes involves the activation of G protein-coupled receptors that stimulate phospholipase C (PLC) β2, leading to the breakdown of phosphatidylinositol bisphosphate (PIP) into diacylglycerol (DAG) and inositol trisphosphate (IP), and release of Ca from intracellular stores. TRPM5 forms a nonselective cation channel that is directly activated by Ca and it is likely to be the downstream target of this signaling cascade. Therefore, study of TRPM5 promises to provide insight into fundamental mechanisms of taste transduction. This review highlights recent work on the mechanisms of activation of the TRPM5 channel. Themouse TRPM5 gene encodes a protein of 1,158 amino acids that is proposed to have six transmembrane domains and to function as a tetramer. TRPM5 is structurally most closely related to the Ca-activated channel TRPM4 and it is more distantly related to the cold-activated channel TRPM8. In patch clamp recordings, TRPM5 channels are activated by micromolar concentrations of Ca and are permeable to monovalent but not divalent cations. TRPM5 channel activity is strongly regulated by voltage, phosphoinositides and temperature, and is blocked by acid pH. Study of TRPM4 and TRPM8, which show similar modes of regulation, has yielded insights into possible structural domains of TRPM5. Understanding the structural basis for TRPM5 function will ultimately allow the design of pharmaceuticals to enhance or interfere with taste sensations.

TRPM6 and TRPM7 proteins share similar molecular structures and biophysical properties. Both proteins are Mg- and Ca-permeable cation channels with the typical topology of six transmembrane domains. In addition, TRPM6 and TRPM7 function as serine/threonine kinases with kinase domains at their C-terminal tails. At present, the role of the association of kinase and channel domains in TRPM6 and TRPM7 remains elusive. TRPM6 is mainly expressed in kidney and intestine, where it might be responsible for epithelial Mg re/absorption. This hypothesis is strengthened by the identification of TRPM6 mutants in patients with a rare but severe hereditary disease called hypomagnesaemia with secondary hypocalcaemia. The aim of this review is to provide a brief but concise overview of the information currently available about TRPM6.

TRPM7 is a member of the melastatin-related subfamily of TRP channels and represents a protein that contains both an ion channel and a kinase domain. The protein is ubiquitously expressed and represents the only ion channel known that is essential for cellular viability. TRPM7 is a divalent cation-selective ion channel that is permeable to Ca and Mg, but also conducts essential metals such as Zn, Mn, and Co, as well as nonphysiologic or toxic metals such as Ni, Cd, Ba, and Sr. The channel is constitutively open but strongly downregulated by intracellular levels of Mg and MgATP and other Mg-nucleotides. Reducing the cellular levels of these regulators leads to activation of TRPM7-mediated currents that exhibit a characteristic nonlinear current-voltage relationship with pronounced outward rectification due to divalent influx at physiologically negative voltages and monovalent outward fluxes at positive voltages. TRPM7 channel activity is also actively regulated following receptor-mediated changes in cyclic AMP (cAMP) and protein kinase A activity. This regulation as well as that by Mg-nucleotides requires a functional endogenous kinase domain. The function of the kinase domain is not completely understood, but may involve autophosphorylation of TRPM7 as well as phosphorylation of other target proteins such as annexin and myosin IIA heavy chain. Based on these properties, TRPM7 is currently believed to represent a ubiquitous homeostatic mechanism that regulates Ca and Mg fluxes based on the metabolic state of the cell. Physiologically, the channel may serve as a regulated transport mechanism for these ions that could affect cell adhesion, cell growth and proliferation, and even cell death under pathological stress such as anoxia.