hERG Potassium Channels in Drug Discovery and Development - Ion Channels and Their Inhibitors - page 157

cytosolic chaperones, heat shock protein (Hsp 70 and Hsp 90), interact dynamically
with immature core-glycosylated ER-resident form of hERG , thereby forming a
transient complex. During protein synthesis, large portions of the hERG protein
are exposed to cytoplasm and thus possible binding partners for chaperones.
These partners include N-terminal Per, Arnt and Sim (PAS) domain, the putative
C-terminal ER retention signal R-G-R and the C-terminal cyclic nucleotide binding
domain (cNBD), whereas small parts of the hERG tetramer point towards ER lumen
[ 27 ]. The role of Golgi apparatus within the machinery is required for normal
protein processing. During maturation in Golgi apparatus, hERG proteins associate
with GM130/golgin-95, a Golgi-associated protein that is involved in vesicular
transport [ 25 , 28 , 29 ]. The changes in temperature have opposing effect on
activation and inactivation of channel. In voltage-dependent activation causes
hyperpolarizing shift while inactivation results in depolarizing shift. The majority
of identified trafficking-deficient mutants have been shown to be temperature-
sensitive. The temperature-dependent induction of channel folding and processing
in hERG R752W has been shown to be accompanied by a reduction in Hsp70 and
Hsp90 association as well as by reduced synthesis and trafficking process [ 30 ]. The
improved channel folding at lower temperatures might be due to an increased ER
retention time at lower temperatures. In addition, reversal of the mutant's effects on
hERG currents could be due to inhibition of proteasomal degradation at reduced
incubation temperatures [ 19 , 31 ].
Few studies have investigated the effects of a transcriptional regulator (4-phen-
ylbutyrate) on hERG channel maturation. Despite markedly enhanced synthesis of
mutant hERG R752W protein at 37 C upon application of 4-phenylbutyrate, trans-
port of the mature protein to the cell surface could not be restored, indicating that
the quality control machinery held the mutant protein in the ER, until it underwent
degradation. Furthermore, in a study of five patients with hERG R752W-associated
long QT syndrome treated with 4-phenylbutyrate (19 g/day) for 1 week; a dose
that has proven successful in cystic fibrosis transmembrane conductance regulator
(CFTR) rescue in vivo , cardiac repolarization was not significantly improved.
These results suggested that enhanced protein expression may not be sufficient
in restoring protein transport. Although data are limited, additional factors such
as prolonged ER retention time and/or interactions with molecular or pharmaco-
logical chaperones seem to be required for successful rescue of hERG LQT2
mutants [ 2 , 29 , 32 ].
Chemical chaperones such as glycerol, dimethyl sulfoxide (DMSO), and
trimethyl amine oxide (TMO) are believed to stabilize protein conformations
during their maturation and restoration of trafficking by chemical chaperones
such as glycerol has been indicated for hypomorphic hERG N470D channels.
However, hERG R752W trafficking could not be corrected by incubation with
glycerol indicating that rescue by chemical chaperones depends on the severity
of mutation in hERG , which is determined by the protein domain affected by
a mutation and by properties of introduced amino acid residue. Further inves-
tigations (e.g., mutagenesis studies on the effects of different amino acid sub-
stitutions at a certain position) need to be carried out to distinguish between