TRP (Transient Receptor Potential Cation Channel)
Calcium is integrated in many physiological processes like muscle contraction, hormone secretion, and intracellular signaling processes (calcium signaling). Prerequisite for this role is the 10,000-fold gradient across the plasma membrane with 2.5 mM extracellular and resting intracellular calcium ion concentration of approximately 100 nM. Calcium ATPases ( Plasma membrane calcium-transporting ATPase) and transporters manage the extrusion of calcium whereas calcium-permeable ion channels enable rapid increases in intracellular calcium up to micromolar concentrations. The biological functions of calcium are mediated by direct calcium binding of the modulated proteins or by protein–protein interaction with calcium sensor proteins (calmodulin, CaBP, calcineurin, S100, NCS, etc.). In excitable cells like neurons, heart or skeletal, or smooth muscle cells, the calcium current is mediated by voltage–gated calcium channels. The molecular identity of the ion channels mediating hormone-induced calcium entry in non-excitable cells like endothelial, epithelial, immune cells remained mysterious for a long time. While it was clear that in mammals, receptor-stimulated calcium entry depends on the phospholipase C activity, parallelly in Drosophila melanogaster, a protein named Transient Receptor Potential (TRP) was identified and described as a phospholipase C–modulated, calcium-permeable ion channel involved in phototransduction (Montell and Rubin 1989). Upcoming genome and expression profiling projects with expressed sequence tags allowed the cloning of the first mammalian TRP-homologous proteins, the ion channels of the classic TRP family (TRPC). Other approaches identified additional TRP-homologous proteins establishing the melastatin-like and vanilloid-like TRP subfamilies, TRPM and TRPV, respectively (Harteneck et al. 2000; Montell et al. 2002; Wu et al. 2010). With the identification of TRPV1 (vanilloid receptor 1, VR1) as molecular target of capsaicin, another fascinating feature of TRP channels being target of many secondary plant compounds became obvious as well as the involvement of TRP channels in sensory functions (Caterina et al. 1997). Nowadays, the TRP superfamily comprises also the mucolipins (TRPML) and the polycystines (TRPP), calcium-permeable channel proteins with similar transmembrane topology and other similarities (Montell et al. 2002; Venkatachalam and Montell 2007; Wu et al. 2010). Despite their common structure and sequence identities, the members of the TRP superfamily are involved in many different cellular functions and are integrated in a variety of physiological processes. The basis for the versatility being integrated in so many signaling cascades is founded in the fact that TRP channels as nonselective ion channels are permeable for sodium, calcium, and other divalent cations. The ratio for the main charge carriers, sodium and calcium, varies within the superfamily from highly calcium-selective ion channels (TRPV5 and TRPV6) to calcium-activated, calcium-impermeable, sodium channels (TRPM4 and TRPM5) and represents an important biophysical feature for the characterization of TRP channels in a physiological context.
For the identification, cloning, and biochemical and physiological characterization, common experimental approaches are used, whereas patch clamp as well as calcium imaging techniques provide direct evidence of the channel activity. The patch clamp techniques use small pipettes and directly measure currents carried by the channels studied, whereas calcium imaging approaches depend on intracellular calcium-binding indicator dyes showing differences in fluorescence in the presence and absence of calcium, thereby allowing the measuring of intracellular calcium concentration as a readout of channel activity. The following entry will give an introduction in the broad field of TRP channel research orientated on the TRP channel classification.
TRPA1 Channel as Signaling Molecule
TRPA1 was initially cloned starting from an up-regulated EST in many tumor cells and described as ANKTM1 based on the most significant structural property of the protein sequence, the abundance of many N-terminal ankyrine-like repeats (Basbaum et al. 2009; Patapoutian et al. 2009). This feature, ANKTM shares with the fly and worm TRP channels of the NOMPC family. The NOMPC channels of Drosophila melanogaster and Caenorhabditis elegans are involved in mechanotransduction. Based on the similarity, mammalian ANKTM1 was temporarily discussed as putative mechanosensitive ion channels probably involved in hearing (Christensen and Corey 2007). The absence of deafness in ANKTM knockout animals however showed that ANKTM1 is not involved in hearing. TRPA1 plays an important role as cold and chemo sensor in mammalian nociceptive neurons. As protein sensing cold temperatures, TRPA1 is also activated by several cooling compounds like menthol or icilin and on the other hand by many pungent structures like allylisothiocyanate (mustard oil) or allicin or methyl paraben or cinnamaldehyde. The great variety of chemical structures activating TRPA1 and thereby mediating pain and triggering inflammatory processes makes TRPA1 an important target in the pain field.
TRPC Channels as Signaling Molecules
TRPM Channels as Signaling Molecules
Melastatin, the founding member of the melastatin-like TRP family, was identified within a screen for proteins differentially regulated in melanocytes and melanoma cells (Harteneck 2005; Venkatachalam and Montell 2007; Wu et al. 2010). Analysis of clinical data showed that the presence of melastatin expression in melanoma patients inversely correlates with the severity and survival. Although melastatin is the first member of the TRPM family, its activation mechanism and physiological role is still unclear. In line with the first description as protein involved in melanocyte physiology, several reports confirmed this view. A completely unexpected function, the integration in retinal signal processing, has recently been discovered by the identification of TRPM1 expression in retinal ON bipolar cells. The critical role of TRPM1 in mammalian phototransduction is also highlighted by several reports describing TRPM1 mutations in patients suffering from congenital stationary night blindness.
TRPML Channels as Signaling Molecules
Mucolipidosis Type IV is an autosomal-recessive neurodegenerative lysosomal storage disorder caused by mutations in TRPML 1 (Montell et al. 2002; Puertollano and Kiselyov 2009). TRPML1 like the other members of the mucolipin family (TRPML1-3) is described to be located as integral membrane proteins of intracellular vesicles especially the lysosomes and probably activated by protons. The lysosomes in mucolipidosis IV patients are enlarged arguing for defects of the intracellular turnover and storage of lipids such as phospholipids, sphingolipids, gangliosides, and mucopolysaccharides. Based on this observation, a contribution of mucolipin channels in the intracellular trafficking and handling of lysosomes is discussed and provides hypotheses for further research.
TRPP Channels as Signaling Molecules
The autosomal-dominant polycystic kidney disease (ADPKD) is characterized by cysts formation in the kidney, a malformation of the renal tubular structure (Montell et al. 2002; Zhou 2009). The cyst formation is linked to the disruption of the functional TRPP1/TRPP2 complex. TRPP1, a huge protein consisting of a complex N-terminal extracellular ligand binding and 11 transmembrane domains, and TRPP2, a calcium-permeable ion channel, form a receptor–effector complex implicated in various biological functions, such as cell proliferation, sperm fertilization, and mechanosensation.
TRPV Channels as Signaling Molecules
Vanilloid structures, derivates of vanillin comprising eugenol, zingerone, and capsaicin, are found in many splice plants and known for their individual characteristic flavor. Beside the use as spice, vanilloid-containing plant extracts are used as remedy in the various traditions of folk medicine. The therapeutic and experimental uses of capsaicin in pain treatment inspired research resulting in the unraveling of the molecular target of capsaicin. The molecular target, an ion channel related to Drosophila TRP, was named capsaicin or vanilloid receptor and became eponym of the subgroup or structurally related ion channels of the TRP channel superfamily (Caterina et al. 1997). The vanilloid-like TRP channels comprise six members, four proteins (TRPV1 to TRPV4) like TRPV1 are nonselective ion channels involved in thermosensation (Basbaum et al. 2009; Liedtke 2007; Talavera et al. 2008; Vriens et al. 2009; Wu et al. 2010), while two ion channels (TRPV5 and TRPV6) represent highly calcium-selective ion channels (Hoenderop and Bindels 2008; Woudenberg-Vrenken et al. 2009). Initially described as calcium transporter, TRPV5 and TRPV6 are responsible for the uptake and reuptake of calcium in the gut and kidney, respectively. Expressed in the epithelial cell layers of the gut and kidney, the activity is transcriptionally controlled in a vitamine-D3-dependent manner.
Capsaicin, the ingredient in chili pepper, causing burning and pain activates TRPV1, the heat sensor, activated by increased temperatures under physiological conditions. The capsaicin receptor being expressed in dorsal root ganglia senses increasing temperatures with a threshold of around 42 °C. The activation of TRPV1 is realized as heat and burning, the same sensation caused by capsaicin. Illustrative for the modulation of the 42 °C threshold of TRPV1 is the burning pain after the application of ethanol or acids in wounds as TRPV1 activity is dramatically enhanced at normal body temperature in the presence of increased proton concentration or in the presence of around 3% ethanol. Since the first description of TRPV1 as the molecular target of capsaicins, many pain-inducing chemicals and toxins have been described as TRPV1 ligands. The warm and heat sensors (TRPV1 to TRPV4) and the cold sensors (TRPM8 and TRPA1) represent the thermosensors of the human body and cover the complete temperature range necessary for human life (Fig. 3). As warning sensors expressed in dorsal root ganglia, the thermo-TRPs are also involved in sensation and modulation of pain and therefore interesting as molecular targets for new pain-treating drugs.
Transient receptor potential (TRP) channels are a large family of nonselective, calcium-permeable channel proteins activated and regulated by a diversity of mechanisms. TRP channels respond to intracellular stimuli such as calcium, metabolites of the arachidonic acid, or phosphatidylinositol signal transduction pathways. TRP channels sense environmental stimuli such as changes in temperature, osmolarity, and pH and represent the molecular target of pheromones, taste, and secondary plant compounds. The broad function in physiology proposed by the expression profiles of TRP channels becomes visible by the association of TRP channel mutations with hereditary diseases or the analysis of TRP channel knockout mice. The diversity of the chemical structures and the selectivity of the natural occurring compounds modulating TRP channels show the possibility for the pharmacological modulation of TRP channels and inspire to develop new synthetic structures for TRP channel interference at bench and bedside.