The
role of ion channels in health and disease, structural bases for
function.
Our
research is directed towards understanding how ion channels operate in
health
and illness. These integral membrane proteins catalyze the selective
transfer
of ions across membranes and, like enzymes, show exquisite specificity
and
tight regulation. As a class, ion channels orchestrate electrical
signals that
allow excitation of the heart, skeletal muscle and a circulating
lymphocyte;
less sensational but equally important, ion channels mediate cellular
fluid and
electrolyte homeostasis. Remarkably, some fundamental questions remain
to be
answered. How do they open and close? What is their architecture? How
do
inherited mutations produce cardiac arrhythmia, hypertension, seizures,
or
deafness? How do drugs act on ion channels to produce beneficial
outcomes or
harmful side-effects? The laboratory uses biophysical, genetic and
biochemical
methods to pursue four current research directions:
(1) The
normal role, mechanism for disease-association, and structure of the
potassium
channel accessory subunits. MinK (encoded by KCNE1) has just 129
amino
acids and a single transmembrane domain and is active only after it
assembles
with pore-forming channel subunits. Nonetheless, MinK determines the
essential
character of key native channels. In the heart, MinK assembles with
KCNQ1 to
form IKs channels (thereby establishing conductance, gating, regulation
and
anti-arrhythmic drug sensitivity). In mutant form, MinK is associated
with
cardiac arrhythmia and deafness (due to changes in these same
attributes); its
role in the pancreas, T cells and gastrointestinal tract is still
unknown.
(Representative citations: Wang and Goldstein 1995. Neuron. 14:1303-9.
Tai and
Goldstein. 1998. Nature. 391:605-8. Chen et al. 2003. Neuron.
40:15-23.)
For
a decade it appeared that MinK was unique; recently, the laboratory
identified
a superfamily of genes encoding MinK-related peptides (MiRPs) and
demonstrated
roles for MiRP1 and MiRP2 in normal and disordered function of the
heart and
skeletal muscle, respectively. MiRP1 (encoded by KCNE2) has 123
residues and is
most like MinK (although just 27% identical). It associates with the
pore-forming subunit HERG to reconstitute the attributes of a current
in the
heart called IKr. Similarly, MiRP2 (encoded by KCNE3) is required with
the
pore-former Kv3.4 to assemble skeletal muscle channels and MiRP2
mutation is
associated with dysfunction, in this case, periodic paralysis.
(Representative
citations: Abbott et al. 1999. Cell. 97:175-186. Abbott et al. 2001.
Cell.
104:217-231. Abbott et al. 2006. FASEB J. 20:293-301). Other accessory
subunits
are also under study including KChIPs (Kim et al. 2004.
Neuron. 41:513-519.)
(2) Discovery,
cloning and function of a new superfamily of potassium channels that
produce
"background leak", the K2Ps. Leaks have been known to be central
to physiology for over 50 years but have been poorly understood (even
their
molecular nature was uncertain). The channels are found to be
widely-expressed,
numerous (17 separate gene families to date) and novel in structure as
well as
function: they bear 2 pore-forming domains in each subunit. Studies of
isolates
from yeast, mice and humans have begun to reveal their roles in the
heart and
nervous system, for example, as targets of volatile anesthetics.
(Representative citations: Ketchum et al. 1995. Nature. 376:690-5.
Goldstein et
al. 1996. PNAS. 93:13256-61. Bockenhauer et al. 2001. Nature Neurosci.
4:486-491.)
Recently,
a novel mechanism to open and close these channels was discovered:
post-translational
modification with the protein called SUMO. SUMO was previously known to
determine the activity of transcription factors in the nucleus and the
enzymes
for sumoylation and desumoylation at the plasma membrane shown to
explain the
silence of K2P1 channels. The utilization of this pathway to control
other
membrane proteins is predicted. (Representative citations: Rajan et al. 2005. Cell. 121: 37-47. Plant et al. 2005.
Curr Opin Neurobiol. 15:26-333.)
(3) Advancing the application of
genetic tools
to the function of ion channels (an approach heralded as
"proteomics") and the association of ion channels with disease to
enable diagnosis, therapy and prevention (gene-based medicine). The
laboratory’s work in these methods has helped to identify the
K2P
superfamily
and revealed the mechanism of operation of Killer RNA viruses that
impact
agriculture, commercial fermentation and fungal infections in
immuno-compromised patients (a coupled toxin-immunity system acting via
fungal
two P domain channels). Most recently, random mutation and selective
pressure
has been applied to mammalian potassium channels expressed in yeast and
bacterial cells and overproduction of material in functional form for
structural studies. (Representative citations: Ahmed et al. 1999. Cell.
99:283-291. Sesti et al. 2001. Cell. 105:637-644. Sesti et al. 2003.
Nature
Neurosci. 6:353-361.)
(4)
Diagnosis
and treatment strategies for diseases of ion channels, particularly, in
children. Methods are now being applied to disorders of cardiac
rhythm and
sudden infant death syndrome seeking to understand cause, provide
diagnostic
tools and develop therapeutic strategies and avoid untoward effects of
medications. Thus, rare inherited mutations of MiRP1 are associated
with the
arrhythmia long QT syndrome (LQTS) and sudden death while a common
single
nucleotide (SNP) polymorphism present in 1.6% of the general population
predisposes to a prevalent and equally dangerous disorder: drug-induced
LQTS
and a SNP present in 11% of African Americans predisposes to sudden
infant
death syndrome (SIDS) (Representative citations: Sesti et al. 2000.
PNAS.
10613-10618. Plant et al. 2006. J Clin Invest. 116: 430-435.)
