2.1 TRPA1 and Natural Activators
A vast number of pungent natural compounds activate TRPA1 channel. The
most important activator of this class is AITC, responsible for fresh
flavor of wasabi and pungency of mustard oil(Uchida, Miura et al.,
2012). Other than AITC, wasabi also contains 6-(methyl sulfinyl)hexyl
isothiocyanate (6-MSITC) and 6-(methylthio)hexyl isothiocyanate
(6-MTITC), which act as an electrophilic activator of TRPA1(Uchida et
al., 2012).
Another prominent agonist of TRPA1 is cinnamaldehyde, found in the oil
of cinnamon (Cinnamonum verum) (Bandell et al., 2004).
Cinnamaldehyde has been reported to causes a burning or tingling
sensation and pain stimulation if taken orally, due to the activation of
TRPA1(Alenmyr, Herrmann et al., 2011).
Garlic is known for centuries for its medicinal properties including
anti-bacterial, anti-fungal, anti-cancer, anti-inflammatory effects
etc.(Bayan, Koulivand et al., 2014; Hosseini & Hosseinzadeh, 2015).
Allicin, an organo-sulfur pungent compound of garlic, is an
electrophilic activator of TRPA1(Bautista et al. 2005). Allicin also
acts as a precursor for its other derivatives like diallyl sulfide,
diallyl disulfide and diallyl trisulfide during metabolism. Allicin and
its derivatives also activates TRPV1 at lower potency as compared to
TRPA1, but efficacy of TRPV1 activation is lower than that of
capsaicin(Koizumi et al., 2009; Macpherson et al., 2005). Ajoene, an
allicin derivative, does not activate the TRPA1 channel but enhances the
activation by other electrophilic compounds including AITC and allicin.
Diethyl disulfide, an allicin related compound, found in Durian fruit
also activates TRPA1 channel to mediate the hyper thermic effects(Terada
et al., 2014).
S-alkyl-S-alkenyl disulfides, a rare class of natural products found in
the Ferula assa-foetida L . potently activates TRPA1 channel
suggesting the role of TRPA1 in the potential beneficial health claims
associated with the use of asafoetida as a spice(Shokoohinia et al.,
2013).
Phenol compounds like carvacrol and propofol activates human TRPA1(Woll
et al., 2017; Xu et al., 2006). The pungency of oregano has been
attributed to the carvacrol mediated TRPA1 activation(Xu et al., 2006).
It has been observed that intra-epidermal injections of TRPA1 agonist
carvacrol to the humans cause dose dependent pain sensations which were
reduced in the presence of TRPA1 antagonist A-967079(Schwarz, Namer et
al., 2017). Thymol, a monoterpene phenol derivative of cymene is present
in oil of the thyme plant, has bimodal action on the TRPA1 channel
similar to the menthol. Activation of TRPA1 by thymol desensitize the
channel for further exposure to AITC or thymol(Lee et al.,
2008).Monoterpenoids such as p-cymene-3-carboxylic acid,
3-amino-p-cymene(Ortar et al., 2012) and limonene(Kaimoto et al., 2016)
also activates the TRPA1 channel.
6-gingerol, a phenol derived compound, found in the essential oil of
fresh ginger and cloves, activates TRPA1(YANG et al.. 2016). Eugenol, a
phenylpropene present in cloves stimulates TRPA1(Chung et al., 2014).
Oleocanthal is a natural phenolic compound, responsible for the pungency
of extra virgin olive oil. It has been demonstrated that oleocanthal
dependent stimulation of rodent trigeminal nervous system requires
functional TRPA1(Peyrot Des Gachons et al., 2011). Piperine and its
related compounds isopiperine, isochavicine, piperanine, piperolein A,
piperolein B, and N-isobutyl-(2E,4E)-tetradeca-2,4-diamide are
responsible for the spiciness of black pepper, also activate
TRPA1(Okumura et al., 2010).
Ligustilide, found in Apium graveolens, Levisticum officinale,
Angelica sinensis , Ligusticum chuanxiong , and North American
traditional Medicine from Ligusticum portieri ), is a potent
activator and moderate inhibitor of TRPA1 channel. Its bimodal action
does not depend upon concentration, rather on its aromatization. For
example, aromatization of ligustilide produces dehydroligustilide that
has been reported to have inhibitory effects on TRPA1(Zhong et al.,
2011).
Curcumin, a principle curcuminoid of turmeric, acutely activates and
subsequently desensitize TRPA1 in HEK cells (human TRPA1 transfected
cells) and mouse sensory neurons. Curcumin showed no effects on TRPV1 or
TRPM8(Leamy, Shukla et al., 2011). TRPA1 is also activated by
non-pungent compounds like capsiate, dihydrocapsiate and
nordihydrocapsiate (all these capsinoids are found in sweet chili) to
lower potency than TRPV1(Shintaku et al., 2012). Fatty acids of royal
jelly activates TRPA1(Terada, Narukawa et al., 2011). Artepillin C
contributes to the pungency of Brazilian green propolis, activates TRPA1
channel more potently than known activator AITC(Hata et al., 2012).
Plants, fungi and animals produce terpenes which act as protectors from
predators and foragers. These terpenes have unsaturated aldehyde
moieties that activates the TRPA1 channel. For example, sesquiterpene
isovelleral, polygodial, miogadial and miogatrial activates TRPA1. α-,
β-eudesmol and γ-eudesmol are non-electrophilic sesquiterpenes which
also activates TRPA1 channel(Escalera, Von Hehn et al., 2008; Ohara et
al., 2017; Terada et al., 2019). In a recent report by Terada et
al . in 2019, human TRPA1 is found to be activated by terpenes. These
terpenes were part of essential oil obtained from the byproduct of
daidai juice processing. Out of the total 10 terpenes tested, they found
that 5 of them (linalyl acetate, geranyl acetate, osthole, geranyl
propionate, and neryl acetate) activate human TRPA1 but not TRPV1 or
TRPM8. This study was done using transfected cell line which requires
further validation(Terada et al.. 2019).
Recently, it has been demonstrated that three natural compounds
cuminaldehyde (present in cumin), p-anisaldehyde (present in anise) and
tiglic aldehyde (present in onion/garlic) from spice’s origin activate
hTRPA1 in heterologous expression system and sensory neurons of
DRG(Legrand, Merlini, de Senarclens-Bezençon, & Michlig, 2020).
Calcium ions dependent modulation
TRPA1 channel activity is modulated by a large number of endogenous
activators including reactive oxygen species produced during
inflammation, calcium ions, prostaglandins etc. Ca2+,
one of the ubiquitous and most important regulators of the channel has
been reported to have both potentiating (at low concentrations) and
desensitizing (at high concentrations) effects. Initial studies on TRPA1
channel activity suggested the possible role of extracellular calcium in
the amplification of TRPA1 response to mustard oil or
Δ9-tetrahydrocannabinol (THC) in transfected HEK
cells(Jordt et al., 2004). These effects were mediated by calcium entry
through TRPA1 channel. Furthermore, these results were supported by
Nagata et al ., they also speculated that extracellular calcium
modulate the channel activity by binding to a site close to or within
the channel(Nagata et al., 2005). TRPA1 channel modulation by
Ca2+ is still a matter of debate. Extracellular
calcium can also inactivate the channel; however, it has been shown that
potentiation and inactivation of the TRPA1 viaCa2+ are two independent processes mediated by
subsequent elevation in intracellular calcium ion concentrations(Y. Y.
Wang, Chang, et al., 2008). The molecular mechanisms underlying
Ca2+ dependent TRPA1 activation or inactivation are
poorly understood. Initial reports demonstrated that
Ca2+ ions bind to putative N-terminal EF hand motif of
TRPA1 to activate the channel(Doerner, Gisselmann et al., 2007; Zurborg
et al., 2007), but later studies showed that mutation in this region of
TRPA1 does not affect the calcium dependent modulation of TRPA1(Nilius
et al., 2011; Y. Y. Wang et al., 2008). An alternative hypothesis
suggests that Ca2+ ions modulate TRPA1 channel
activity by binding to acidic residues at the C-terminal. Deletion of 20
amino acids at C-terminal was reportedly does not affect the
potentiation of the channel by Ca2+ ions or
thiol-reactive groups, but reduces the Ca2+ dependent
inactivation of the channel, thus supporting the
potentiation-inactivation uncoupling mechanism. Simulations studies
identified two amino acids (Asp1080 and Asp1082), critically important
for the binding of Ca2+ ions at the C-terminal (Sura
et al., 2012). Taken together, these studies indicate that the conserved
acidic residues in the C-terminus of the TRPA1 channel significantly
affect the channel modulation by Ca2+ ions. Some
conserved residues between transmembrane domain 2 (TM2) and
transmembrane domain 3 (TM3) have been identified with the help of
structural analysis and electrophysiology. These residues were E788,
Q791, Y799, N805 and E808, however E788 alone was found to be
responsible for the major regulatory effects of calcium ions(Zhao, Lin
King et al., 2020). Intriguingly, these calcium binding residues are
highly conserved among TRP channels including TRPM2, TRPM4 and
TRPM8(Autzen et al., 2018; Diver, Cheng et al., 2019; Y. Huang, Winkler
et al., 2018; Z. Zhang, Tóth et al., 2018). Calmodulin (CaM), an
intracellular calcium dependent protein has been recently reported to
play an essential role in maintaining the TRPA1 channel activity in
response to calcium ions concentrations. Notably, C-lobe of CaM directly
interacts with the 17 amino acids long non-canonical calmodulin binding
domain (CaMBD) present at the C-terminus of the TRPA1 channel. CaM acts
as a calcium sensor and mutation in either Ca2+binding sites in CaM or CaM binding sites on TRPA1 renders the
activation or desensitizing effects of calcium ions on TRPA1(Hasan,
Leeson-Payne et al., 2017).
Role of TRPA1 agonism in prevention of diet induced Obesity
Obesity is a complex disease, resulted from the accumulation of excess
fat in the body. There are many contributors to the development of
obesity including endocrine disruptors, less sleep, cessation from
smoking, less physical activity, various environmental and genetic
factors. One of the important factors for the development of obesity is
the imbalance in the energy expenditure which leads to the accumulation
of fat in the different parts of the body. TRPA1 is majorly known for
its role in pain sensation but growing evidence suggests its involvement
in metabolic functions too. In this section, we have discussed the
involvement of TRPA1 in regulating energy expenditure and thereby could
be used as a target to combat weight gain, obesity and related
complications.