Capsaicin maintains TRPV1 in an open state by «pull and contact» interactions. A conformational change wave spread over the whole pore.<ref>F. Yang et al., « The conformational wave in capsaicin activation of transient receptor potential vanilloid 1 ion channel », Nat. Commun., vol. 9, no 1, Art. no 1, juill. 2018, doi: 10.1038/s41467-018-05339-6.</ref>.
Capsaicin maintains TRPV1 in an open state by «pull and contact» interactions. A conformational change wave spread over the whole pore.<ref>F. Yang et al., « The conformational wave in capsaicin activation of transient receptor potential vanilloid 1 ion channel », Nat. Commun., vol. 9, no 1, Art. no 1, juill. 2018, doi: 10.1038/s41467-018-05339-6.</ref>.
Capsaicin maintains TRPV1 in an open state. A conformational change wave spread over the whole pore.<ref>F. Yang et al., « The conformational wave in capsaicin activation of transient receptor potential vanilloid 1 ion channel », Nat. Commun., vol. 9, no 1, Art. no 1, juill. 2018, doi: 10.1038/s41467-018-05339-6.</ref>. This leads to the massive enter of Ca2+ and Na+ and the depolarization of the nerve fiber. Depolarization triggers the generation of an [https://en.wikipedia.org/wiki/Action_potential action potential] causing a painful sensation.<ref name="TRPV1"/>
Capsaicin maintains TRPV1 in an open state. A conformational change wave spread over the whole pore.<ref>F. Yang et al., « The conformational wave in capsaicin activation of transient receptor potential vanilloid 1 ion channel », Nat. Commun., vol. 9, no 1, Art. no 1, juill. 2018, doi: 10.1038/s41467-018-05339-6.</ref>. This leads to the massive enter of Ca2+ and Na+ and the depolarization of the nerve fiber. Depolarization triggers the generation of an [https://en.wikipedia.org/wiki/Action_potential action potential] causing a painful sensation.<ref name="TRPV1"/> Ca2+ has a negative retrospection effect on TRPV1 and induces a desensitization of this receptor facing capsaicin.
<ref name="TRPV1 dans les neuropathies douloureuses"> A. Danigo, L. Magy et C. Demiot , Med Sci (Paris) Volume 29, Number 6-7, Juin–Juillet 2013, p. 597-606. TRPV1 dans les neuropathies douloureuses, https://www.medecinesciences.org/en/articles/medsci/full_html/2013/08/medsci2013296-7p597/medsci2013296-7p597.html, (Consulted the : dec. 23, 2021). [Online].</ref>
====Resiniferatoxin (RTX)====
====Resiniferatoxin (RTX)====
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TRPV1 (Vanilloid Transient Receptor Potential Type 1) is a non-selective ion channel which, in response to a stimulus, induces an incoming current of cations, primarily calcium and sodium, that causes depolarization of the cell. It is part of the TRP (Transient Receptor Potential) superfamily and is the first in a subfamily of vanilloid-sensitive TRP channels: TRPVs.TRPV1 is expressed in Group A nerve fibers [1] and Group C nerve fibers [2] by sensory neurons of the dorsal and trigeminal spinal ganglia. (Gastrointestinal tract’s neurons express this receptor widely which may be involved in Crohn's disease. TRPV1 is also located on the heart peri vascularisation. It has a vasodilatation effect and protects this organ via the substance it releases.) TRPV1 is implicated in nociception, its activation by heat or by chemical substances leads to a painful sensation.[1] Functional and structural studies of TRPV1 have contributed to a clearer understanding of transmission of nociceptive stimuli mechanisms especially with low diameter fibers. TRPV1 has a major role in inflammatory and pain sensitivity and overactivation of TRPV1 increases the sensitivity to pain as well as extended pain induces overexpression of TRPV1 in nerve fibers. However, sub-expression of this receptor induces chronic pain. Which can appear with abnormal neuromodulators transport due to axonal damage.
TRPV1 can be indirectly activated by NGF (nerve growth factor) and specific molecules of the inflammatory system such as bradykinin, serotonin, histamin, prostaglandin or ATP. They increase the channel opening, by blocking the PIP2 inhibition or by decreasing the heat activation threshold.
[2]
Structure of TRPV1
Schematic figure of the TRPV1 receptor
The TRPV1 receptor is a transmembrane protein receptor. It is made up of 839 amino acids. It’s molecular weight is 94 938Da.[3] TRPV1 exists in two states : the open state and the closed state.[4]
TRPV1 are tetrameric channel type receptors. The four subunits form a symmetry plane around a pore allowing the passage of ions.
Each TRPV1 subunits is made of one N-terminal tail, one transmembrane region, a C-terminal tail preceded by a TRP domain. The N-terminal and C-terminal region are intracellular. N and C terminal region are responsible of 70% of the total mass of TRPV1.[3]
The transmembrane region is composed of six transmembrane a helices (S1,S2,S3,S4,S5,S6). S1,S2 and S3 helices contain aromatic side chain (Y511,S512,T550).[3] A small hydrophobic domain beetween S5 and S6 with a re-entrant loop constitutes the pore allowing the passage of ions through the TRPV1 receptor.[1][3]
Threonin residue (T550) and Tyrosin residue (Y511) located on the fifth and the third transmembrane helices are very conserved. Threonin 550 and Tyrosin 511 are implicated in TRPV1 activation by vanilloids and in pain sensation.[6]
The S6 domain links the receptor to the C-terminal domain of TRPV1. The C-terminal is made of 150 amino acids and it contains TRP domain.[5].The TRP domain is made of 23-25 aminoacids with a alpha helical structure, it is found in many TRP family members.[3] TRP domain is necessary for the formation of tetrameric TRPV1.
Many amino-acids of the C-terminal domain are the target of post-translationnal modifications by kinases and phosphatases.[7]
Relation structure-function
TRPV1 is a homotetramer in which each subunit has several phosphorylation sites for PKA (protein kinase A), PKC (protein kinase C) and CaMkII (Ca2 + / calmodulin-dependent kinase II), as well as numerous glycosylation sites. These domains play a crucial role in the regulation of TRPV1 activity.
The state of the channel is modulated by two types of molecules: agents that promote its opening of the channel, called agonists, and agents that induce its closure or prevent its opening, called antagonists.
Agonists
Capsaicin
Chemical structure of capsaicin
Capsaicin is an active compound in chili.
TRPV1 receptor has a capsaicin-binding pocket formed by S3,S4 and S4-S5 linker. The capsaicin-binding pocket is surrounded by the residues Y511,S512,T550.[8]
Bound capsaicin is oriented in a « tail-up, head down » configuration. In this configuration,capsaicin is anchored into the receptor.[9]
The capsaicin cycle binds via hydrogen bounds to amino acids on the S3 helix (Y511), on the S4-S5 linker and on the S6 helix (T671). The amid group of capsaicin binds the S4 helix.Cite error: Closing </ref> missing for <ref> tag.
Capsaicin maintains TRPV1 in an open state. A conformational change wave spread over the whole pore.[10]. This leads to the massive enter of Ca2+ and Na+ and the depolarization of the nerve fiber. Depolarization triggers the generation of an action potential causing a painful sensation.[1] Ca2+ has a negative retrospection effect on TRPV1 and induces a desensitization of this receptor facing capsaicin.
Resiniferatoxin is a natural analogue of capsaicin. It is the most potent TRPV1 agonist known, with a binding affinity for TRPV1 ~500x higher than that of capsaicin.
The pocket size characteristics of the TRPV1-RTX allow the installation of RTX : The sub-pocket near Y511 is shallow in the TRPV1-RTX because Y511 and E570 are close and I569 is oriented towards the vanilloid pocket.[11]
The aromatic part of resiniferatoxin is located deeper in the sub-pocket near Y511 and is oriented almost parallel to the aromatic side chain of Y511, so it establishes a strong interaction π-π. The aromatic hydroxyl and methoxy groups of the RTX form strong hydrogen bonds with E570, R557 and S512. The ester group is linked to Y511 and T550 by hydrogen bonds.
Regulation
Sensitization
Phosphorylation of the TRPV1 receptor leads to its sensitization. Phosphorylations are either caused by PKC (IP3 signalling), by PKA (AMPc signalling), or by CamKII.[12]Cite error: Closing </ref> missing for <ref> tag. The phosphorylation of TRPV1 lead to an over-expression of TRPV1 at the membrane surface.[13] Moreover, phosphorylated TRPV1 would have a reduced channel opening threshold.[14]. As a result phosphorylated TRPV1 are more responsive to agonist and the resulting pain sensation is higher.
A repeated exposure of TRPV1 to capsaicin fails to activate the receptor. It occurs by a Ca2+-dependent mechanism that leads to a desphosphorylation by the calcineurinphosphatase of the serine and threonine residues which have been previously phosphorylated by PKA (S502 and T370). Thus, the decrease in TRPV1 phosphorylation diminish the sensitivity of the capsaicin channel and leads to a decrease in capsaicin's response by negative feedback.
The over-stimulation of TRPV1 is followed by the nerve endings' death due to calcium overload, causing analgesia. [5]
Implication of TRPV1 in the treatment of pain
In 2011 Qutenza (NeurogesX) patch containing 8% of capsaicin has been markered in France and indicated in the neuropathic pain.
The absorption through the skin of these creams generated partial desensitization of the nerve endings. This is the cause of a decrease in painful sensations.[15]
Many laboratories are conducting clinical studies on oral TRPV1 antagonists: GlaxoSmithKline, Amgen, Merk-Neurogen, Abbot, Eli-Lilly-Glenmark, AstraZeneca and Japan Tobacco. The major problem with these pain relievers is the hyperthermia generated in humans. These effects caused these studies to be stopped in phase I.[15]
↑ T. Rosenbaum et S. A. Simon, « TRPV1 Receptors and Signal Transduction », in TRP Ion Channel Function in Sensory Transduction and Cellular Signaling Cascades, W. B. Liedtke et S. Heller, Éd. Boca Raton (FL): CRC Press/Taylor & Francis, 2007
↑ 5.05.15.2G. Smutzer et R. K. Devassy, « Integrating TRPV1 Receptor Function with Capsaicin Psychophysics », Advances in Pharmacological Sciences, janv. 14, 2016
↑R. Kumar, A. Hazan, A. Basu, N. Zalcman, H. Matzner, et A. Priel, « Tyrosine Residue in the TRPV1 Vanilloid Binding Pocket Regulates Deactivation Kinetics », J. Biol. Chem., vol. 291, no 26, p. 13855‑13863, juin 2016, doi: 10.1074/jbc.M116.726372.
↑X. Yao, H.-Y. Kwan, et Y. Huang, « Regulation of TRP Channels by Phosphorylation », Neurosignals, vol. 14, no 6, p. 273‑280, 2005, doi: 10.1159/000093042
↑F. Yang et J. Zheng, « Understand spiciness: mechanism of TRPV1 channel activation by capsaicin », Protein Cell, vol. 8, no 3, p. 169‑177, mars 2017, doi: 10.1007/s13238-016-0353-7.
↑F. Yang et al., « Structural mechanism underlying capsaicin binding and activation of the TRPV1 ion channel », Nat. Chem. Biol., vol. 11, no 7, Art. no 7, juill. 2015, doi: 10.1038/nchembio.1835.
↑F. Yang et al., « The conformational wave in capsaicin activation of transient receptor potential vanilloid 1 ion channel », Nat. Commun., vol. 9, no 1, Art. no 1, juill. 2018, doi: 10.1038/s41467-018-05339-6.
↑ 11.011.1K. Elokely et al., « Understanding TRPV1 activation by ligands: Insights from the binding modes of capsaicin and resiniferatoxin », Proc. Natl. Acad. Sci., vol. 113, no 2, p. E137‑E145, janv. 2016, doi:10.1073/pnas.1517288113.
↑K. W. Ho, N. J. Ward, et D. J. Calkins, « TRPV1: a stress response protein in the central nervous system », Am. J. Neurodegener. Dis., vol. 1, no 1, p. 1‑14, avr. 2012.
↑K. W. Ho, N. J. Ward, et D. J. Calkins, « TRPV1: a stress response protein in the central nervous system », Am. J. Neurodegener. Dis., vol. 1, no 1, p. 1‑14, avr. 2012.
↑G. Bhave et al., « Protein kinase C phosphorylation sensitizes but does not activate the capsaicin receptor transient receptor potential vanilloid 1 (TRPV1) », Proc. Natl. Acad. Sci., vol. 100, no 21, p. 12480‑12485, oct. 2003, doi: 10.1073/pnas.2032100100.
↑ 15.015.1A. Danigo, L. Magy, et C. Demiot, « TRPV1 dans les neuropathies douloureuses - Des modèles animaux aux perspectives thérapeutiques », médecine/sciences, vol. 29, no 6‑7, Art. no 6‑7, juin 2013, doi: 10.1051/medsci/2013296012.
