Cellular Physiology Cell Physiol Biochem 2021;55(S3):108-130 DOI: 10.33594/00000035810.33594/000000358 © 2021 The Author(s).© 2021 Published The Author(s) by and Biochemistry Published online: online: 28 28 May May 2021 2021 Cell Physiol BiochemPublished Press GmbH&Co. by Cell Physiol KG Biochem 108 Press GmbH&Co. KG, Duesseldorf SeebohmAccepted: 17et al.:May Molecular 2021 Pharmacology of TRPV Channelswww.cellphysiolbiochem.com This article is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 Interna- tional License (CC BY-NC-ND). Usage and distribution for commercial purposes as well as any distribution of modified material requires written permission. Review Beyond Hot and Spicy: TRPV Channels and their Pharmacological Modulation Guiscard Seebohma Julian A. Schreibera,b aInstitute for Genetics of Heart Diseases (IfGH), Department of Cardiovascular Medicine, University Hospital Münster, Münster, Germany, bInstitut für Pharmazeutische und Medizinische Chemie, Westfälische Wilhelms-Universität Münster, Münster, Germany Key Words TRPV • Molecular pharmacology • Capsaicin • Ion channel modulation • Medicinal chemistry Abstract Transient receptor potential vanilloid (TRPV) channels are part of the TRP channel superfamily and named after the first identified member TRPV1, that is sensitive to the vanillylamide capsaicin. Their overall structure is similar to the structure of voltage gated potassium channels (Kv) built up as homotetramers from subunits with six transmembrane helices (S1-S6). Six TRPV channel subtypes (TRPV1-6) are known, that can be subdivided into the thermoTRPV 2+ (TRPV1-4) and the Ca -selective TRPV channels (TRPV5, TRPV6). Contrary to Kv channels, TRPV channels are not primary voltage gated. All six channels have distinct properties and react to several endogenous ligands as well as different gating stimuli such as heat, pH, mechanical stress, or osmotic changes. Their physiological functions are highly diverse and subtype as well as tissue specific. In many tissues they serve as sensors for different pain stimuli (heat, pressure, pH) and contribute to the homeostasis of electrolytes, the maintenance of barrier functions and the development of macrophages. Due to their fundamental role in manifold physiological and pathophysiological processes, TRPV channels are promising targets for drug development. However, drugs targeting specific TRPV channels, that are suitable for drug therapy, are rare. Moreover, selective and potent compounds for further research at TRPV channels are often lacking. In this review different aspects of the structure, the different gating stimuli, the expression pattern, the physiological and pathophysiological roles as well as the modulating mechanisms of synthetic, natural and endogenous ligands are summarized. © 2021 The Author(s). Published by Cell Physiol Biochem Press GmbH&Co. KG The structure of TRPV channels In 1969 Cosens and Manning described a mutated Drosophila melanogaster strain, that showed abnormal electroretinogram (ERG) and impaired phototransduction. The ERG was characterized by a transient receptor potential (TRP) instead of a rather sustained potential in the wildtype [1]. Later, this mutated strain was associated with the first invertebrate TRP Dr. Julian Alexander Schreiber IfGH – Cellular Electrophysiology and Molecular Biology, Department of Cardiovascular Medicine, University Hospital Muenster Robert-Koch-Straße 45, Muenster, D-48149 (Germany) Tel. +49-251-83-58278, E-Mail [email protected] Cellular Physiology Cell Physiol Biochem 2021;55(S3):108-130 DOI: 10.33594/000000358 © 2021 The Author(s). Published by and Biochemistry Published online: 28 May 2021 Cell Physiol Biochem Press GmbH&Co. KG 109 Seebohm et al.: Molecular Pharmacology of TRPV Channels cation channel, which was cloned A B in 1989 [2]. Finally, in 1995 the C5 C3 C1C4 C7 C6 M5 the transient receptor potential M4 V1 M8 V2 canonicalfirst mammalian (TRPC) homologue family, was of M2 V4 cloned and characterized [3, 4]. M7 V3 M6 V5 From then to now 28 members M3M1 V6 of the TRP cation channel A1 P3 ML1 P5 P2 ML3 ML2 superfamily were discovered in by sequence homology into C D twomammals, groups that andcan be6 subdivided families: Group I contains the families transient potential receptor S1 S2 S3 S4 S5 S6 (TRPV1-6), ankyrin (TRPA1) and Linker TRP Helix melastatincanonical (TRPC1-7),(TRPM1-8), vanilloid while N-Linker the families polycystin (TRPP2, COOH TRPP3, TRPP5) and mucolipin (TRPML1-3) belong to Group II NH (Fig. 1A) [5, 6]. The TRPV 2 channels are named after the Fig. 1. A. Phylogenetic tree of TRP channels found in human in- first discovered member TRPV1, ankyrin (A1), melastatin (M1-8), polycystin (P2, P3, P5) and cluding the subfamilies canonical (C1, C3-C7), vanilloid (V1-6), TRPV1that is sensitive is formed to bystimulation assembling by ofthe fourvanillylamide TRPV1 subunits,capsaicin that[7]. 5IRX) embedded into cell membrane [129]. Single subunits are coloredmucolipin in (ML1-3).magenta, B.green, Homotetrameric blue, and yellow. TRPV1 C andchannel D. Schematic (PDB ID + channelshave a similar (Fig. 1B-D) transmembrane [8, 9]. Later 5IRX) of a single TRPV1 subunit including the N-terminal domain depiction (C) and corresponding Cryo-EM structure (D; PDB ID structure like voltage gated K (NTD, orange), the ankyrin repeat domain (ARD, orange), the N- terminal linker (N-linker, orange), the pre-S1 helix (Pre-S1, or- VRL1),on, the vanilloidTRPV3 (OLMS,subfamily VRL3), was TRPV4extended (VRL2, by TRPV2 Trp12, (previously VROAC, linker (Linker, red), the inner pore domain (S5-S6, magenta), the ange), the voltage-sensing-like domain (S1-S4; purple), the S4S5- OTRPC4), TRPV5 (ECaC1, CaT2) - and TRPV6 (ECaC2, CaT1) [10– nal domain (CTD, yellow) [129]. pore helix (PH, magenta), the TRP helix (green) and the C-termi 13]. All TRPV channels possess a large cytosolic N-terminal and a smaller C-terminal region [14]. The N-terminal region forms an ankyrin repeat domain (ARD) with six ankyrin repeats, two β-sheets (N-Linker) and a pre-S1 helix (Fig. 1C) [15]. The cytosolic C-terminal region intracellularencompasses regions several areamino also acids binding forming sites one for β-sheet,modulating that proteins together and with second the two messengers. N-terminal Theβ-sheets N-terminus and the contains ARD of an binding adjacent sites subunit for calmodulin allow for (CaM) efficient and subunit ATP, that assembling both can modulate[15]. The stronglychannel activityregulated [16]. by Thephosphorylation shorter C-terminal at different region sites also of binds the N-CaM and and C-terminal other regulating regions proteins including A-kinase anchor protein (AKAP) [17, 18]. Moreover, TRPV channels are likeperformed domain by (VSLD, protein S1-S4) kinase and A (PKA)an inner as wellpore as region protein (S5-S6) kinase connected C (PKC) [19, by 20].the S4S5-linker The transmembrane region consists of six helices (S1-S6) forming the voltage sensor- also called pore turret) between the S5 and S6 helix [22]. Further, residues from the lower [21]. The ion channel pore is formed by the selectivity filter (SF) and the pore helix (PH; part of the S6 helix form an activation gate [23]. In comparison to voltage gated potassium channels, the upper gate formed by the PH / SF is shorter and the pore radius differs within Cellular Physiology Cell Physiol Biochem 2021;55(S3):108-130 DOI: 10.33594/000000358 © 2021 The Author(s). Published by and Biochemistry Published online: 28 May 2021 Cell Physiol Biochem Press GmbH&Co. KG 110 Seebohm et al.: Molecular Pharmacology of TRPV Channels the different TRPV subtypes influencing the selectivity of the channels [24]. Based on the Caion2+ selectivity the six subtypes are2+ subdivided+ ranging into two from groups: P TRPV1-4 form polyselective upcation to 10channels, (TRPV1, that TRPV3) conduct [25]. monovalent In contrast, as wellTRPV5 as divalentand TRPV6 cations are withnearly a preferenceCa2+ for Ca/Na channelsand conductivity with P ratios for Ca over Na 3 (TRPV2) over 6 (TRPV4) cations [26, 27]. selective Ca/Na >100, while monovalent cations only permeate in absence of divalent Gating stimuli of TRPV channels Although the structure of the transmembrane region is similar to the structure of voltage gated potassium channels, the voltage dependency of TRPV channels is rather low [28]. Together with another activation stimulus (agonist, pH or heat), TRPV1 as well as TRPV4 show a prominent outward rectification at highly positive voltages and slight inward rectification at negative voltages [28, 29]. Only TRPV1 exerts weak voltage dependent opening without any other stimulus [28]. TRPV3 does not show voltage dependent activation up to highly positive voltages without additional stimulation by heat or ligands [30]. However, activation of TRPV3 by 2-APB leads to similar voltage dependency as observed for TRPV1 [31]. TRPV2 channels exert also dual rectification with slightly dominant outward currents2+ under heat activationpotentials [10].[32, 33].In contrast The ion tochannels the polyselective of the two TRPVTRPV channelsgroups differ TRPV5 additionally and TRPV6 in showheat- strong inward rectification due to voltage-dependent intracellular Mg -block at positive 10 nosensitive altered gating. gating at different temperatures [34]. Thus, TRPV1-4 are also called thermoTRPV channelsWhile contributing TRPV1-4 are to sensitive temperature to heat sensing showing in Qvivo values [24]. >10,While TRPV5 TRPV4 and (threshold TRPV6 show 24- of33