Page 10 - CBAC Newsletter 2016

impairment from occurring. These results suggest that
KCNQ channels are promising targets for protection
of hippocampal function when it is affected by stress
(153).
Over activation of M-current in cultured hippocampal
neurons can mediate more K+ efflux, affecting the
pro-apoptotic process. Flupirtine and N-ethylmaleimide
(NEM) are believed to work as KCNQ channel openers
in hippocampal neurons by causing the efflux of K+ in a
dose-dependent manner, depleting the intracellular K+,
and cell death in hippocampal culture (154,155). While
both flupirtine and NEM induce cell death in
hippocampal cultures, NEM also causes cell death in
cortical cultures. The action of cell death by NEM was
inhibited by the KCNQ channel blocker, XE991 (155).
These findings suggest that M-current could play a
regulatory role in neuronal apoptosis, and flupirtine and Fenamate compounds (Fig. 2) have been used clinically
NEM can be used as research tools to study this role
(155).
Antidystonic effects have been reported for flupirtine
and retigabine in animal models with paroxysmal
dystonia. These findings show the association of
neuronal KCNQ channels in dystonia-associated
disorders. Flupiritine and RTG also exert efficacious
non-opioid analgesic effects that may help to alleviate
disorders accompanied by muscles spasms such as
dystonia (156).
5. L — 364373 ‘R-L3’
The benzodiazepine derivative L-364,373 (R-L3) (Fig.2),
an activator of IKs channels, has been reported to
shorten the action potential of cardiac myocytes in
guinea pigs (157). The effects of R-L3 on IKs channels
were also studied in preconstricted mesenteric arteries
where it produced concentration-dependent relaxation
(pEC50= 6.3 ± 0.4µM (158)) (Table1). R-L3 displays
moderate leftward shifts in the voltage dependence of
channel activation, increases the rate of channel
activation, and significantly decreases the rate of
channel deactivation (157).
R-L3 activates KCNQ channels by increasing the
amplitude of their currents (45). While it activates
KCNQ1 at concentrations up to 1 µmol/L, it blocks
the same channels at 10 µmol/L (157). Many long QT
syndrome-associated mutant KCNQ1 channels can be
affected by R-L3 similarly as the KCNQ1 WT channels,
consequently R-L3 has been shown to reverse the long
QT by increasing IKs currents (45).
6 | CBAC Center Heartbeat
The molecular determinants of R-L3 interacting with
KCNQ1 channels were revealed using molecular
modeling and mutagenesis scanning of KCNQ1 channels
with voltage-clamp analysis in Xenopus oocytes. R-L3
was found to interact with residues located at the S5
and S6 transmembranes segments of the KCNQ1
subunit (Y267, I268, L271, and G272 in S5, and F335
and I337 in S6) (45). The binding site of R-L3 may
overlap with the site where KCNE1 subunits interact with
S6, and the residues may not face the central cavity
(159). When KCNE1 is used in excess, the effect of
R-L3 in the Iks channel is abolished, which suggest that
KCNE1 prevents binding of R-L3 to the KCNQ1 subunits
by competing for a common interaction site on KCNQ1
subunits (157).
6. NSAIDs agents as openers with KCNQ channels
as an analgesic and anti-inflammatory (160). These
fenamate ligands are NSAIDs (nonsteroidal anti-
inflammatory drugs) that inhibit COX-1 and COX-2
non-selectively (161). Several derivatives of fenamate
compounds were tested with KCNQ channels, such as
mefenamic acid, flufenamic, tolfenamic, meclofenamic,
niflumic acids, diclofenac, and 4,4’-diisothiocyanatos-
tilbene-2,2’-disulfonic acid (DIDS) (Fig.2). Nifiumic,
mefenamic, fiufenamic acids, and DIDS can increase
Iks current and reduce the deactivation rate at lower
concentrations (10 µM) (160,162). These ligands pre-
sumably stabilize the open conformation of IKs
channels. Mefenamic acid activates both IKs and
KCNQ1 channels (163). It produces an approximately
20 mV leftward-shift in the activation curve of KCNQ1
currents (164). Mefenamic acid also affects the
anti-contractile mechanism by activating KCNQ channels
(160,163). At mM concentrations, mefenamic acid
relaxes the mesenteric arteries in a concentration-
dependent manner by activation of KCNQ channels
and inhibiting Ca2+-activated Cl- channels (158). The
effect of mefenamic acid was only partially blocked by
L-768673, a potent blocker on Iks channels, supporting
that the mefenamic acid effects is due to a dual action
by activating KCNQ channels and blocking
Ca2+-activated Cl- channels. Mefenamic acid
antagonizes Ca2+-activated Cl- channels at higher
concentrations (5-10 mM) (165). The effects of these
compounds on KCNQ channels suggest that they may
restore the IKs channel function of some particular
LQT-associated mutations in KCNE1 (164).
Meclofenamic acid and diclofenac have also been found
as potent activators on KCNQ2/Q3 channels (161). They
strongly potentiate KCNQ2/Q3 expressed in CHO cells,