Download Ionchannels and channelopaties in the heart

Ionchannels and channelopaties in
the heart
Viktória Szűts
Action of membrane transport protein
ATP-powered pump
101-103ions/s
Ion chanels
Transporters
107-108ions/s
102-104ions/s
• Cardiac K+ channels control the resting membrane potentials and the
amplitude, duration, refractoriness and automaticity of action potentials.
K+ channels share a similar structure, composed by four pore-forming
α-subunits assembled as tetramers or dimers forming K+ selective pores
and modulated by accessory subunits. The main K+channel pore
forming protein is not translated from a single gene as Na+ and
Ca+channels, but is made up of four separate subunits, which assembly
with ß-subunits to form the functional channel More than 80 different
K+ channels are expressed in the heart, display considerable diversity of
native K+channels.
• Ca-independent transient outward potassium current (I to1) underlies by
KCNA genes encoded Kv3.x and Kv4.x proteins.
• Delayed rectifier currents: the rapid (IKr) and slow (IKs) are encoded by
different voltage-gated K+ channel genes. IKr is produced by the αsubunit ERG (KCNH2), in co-assemblance with the ß-subunit MiRP1
(KCNE2). IKs is produced by the α-subunit KvLQT1 (KCNQ) assembly
with the accessories subunits of minK and MIPRs (KCNE1, KCNE2,
KCNE3)
• Inward rectifier current (IK1) carried by Kir 2.1, Kir 2.2 and Kir 2.3
(KCNJ2, KCNJ12 and KCNJ4) channel proteins.
Molecular composition of the cardiac K-ionchannels
Selectivity filter
Nerbonne et al . Circ Res. 2001;89:944-956
Membrane topology of the Kv and Kir2.x
K-ionchannels
Voltage gated K+channel
Inward rectifier K+channel
Kv channel
CO2
CO2
CO
2
H5
H5
Kv complex
MiRP
N
N

C
C
KChAP

PSD
Gating movi
Ionchannels are open and close
changing the permeability
Assembly of different ionchannel subunits
Extracellular
Intracellular
Abott et al Neuropharm. 2004
Molecular assembly of ion channels
Cavα
Kvα
Kir
Activation and Inactivation of The Sodium Channel
Sodium channels are characterized by voltage-dependent
activation, rapid inactivation, and selective ion conductance.
Depolarization of the cell membrane opens the ion pore
allowing sodium to passively enter the cell down its
concentration gradient . The increase in sodium conductance
further depolarizes the membrane to near the sodium
equilibrium potential. Inactivation of the sodium channel
occurs within milliseconds, initiating a brief refractory period
during which the membrane is not excitable. The mechanism
of inactivation has been modeled as a "hinged lid" or "ball
and chain", where the intracellular loop connecting domains
III and IV of the a subunit closes the pore and prevents
passage of sodium ions.
• Voltage-Gated Calcium Channels
• Voltage-gated calcium channels are heteromultimers
composed of an α1 subunit and three auxiliary subunits,
2-δ, β and γ. The α1 subunit forms the ion pore and
possesses gating functions and, in some cases, drug
binding sites. Ten α1 subunits have been identified, which,
in turn, are associated with the activities of the six classes
of calcium channels. L-type channels have α1C (cardiac),
α1D (neuronal/endocrine), α1S (skeletal muscle), and α1F
(retinal) subunits; The α1 subunits each have four
homologous domains (I-IV) that are composed of six
transmembrane helices. The fourth transmembrane helix
of each domain contains the voltage-sensing function. The
four α1domains cluster in the membrane to form the ion
pore. The β-subunit is localized intracellularly and is
involved in the membrane trafficking of α1subunits. The
γ-subunit is a glycoprotein having four transmembrane
segments. The α2 subunit is a highly glycosylated
extracellular protein that is attached to the membranespanning d-subunit by means of disulfide bonds. The α2domain provides structural support required for channel
stimulation, while the δ domain modulates the voltagedependent activation and steady-state inactivation of the
channel.
Ionic currents and ion transporters responsible
for cardiac action potential
Abriel H. et al., Swiss Med Wkly 2004, 685-694. www.sm w. ch
• The expression and properties of these K+ channels are
altered in cardiac diseases (ie. cardiac arrhythmia,
Long QT syndrome, hypertrophyc cardiomyopathy,
Andersen syndrome, heart failure). These K+ channels
still require further investigation because they are
involved in the basic normal heart rhythm, and may be
altered in cardiac diseases.
Proposed cellular mechanism for the development of Torsade
de pointes in the long QT syndrome
• Prolonged QT interval on ECG (reflects prolonged APD)
• APD governed by a delicate balance between inward (Na+
or Ca+) and outward (K+) ionic current
• Affecting the Na+ or Ca+ channel prolong APD via“gain-offfunction”mechanism, while mutation in genes encoding K+
channel by “loss-off-function” mechanism
Risk factors
for
developing
Torsade de
pointes
Genetic variants (polymorphysm or mutations)
Abriel H. et al., Swiss Med Wkly 2004, 685-694.
Ionic current, proteins and genes associated with
inherited arrhythmias
Napolitano et al. Pharm. & ther. 2006,110:1-13
Congenital and aquired forms of long QT syndromes
Abriel H. et al., Swiss Med Wkly 2004, 685-694. www.sm w. ch
K+, Na+ channel LQT-associated genes and proteins
Current
Genes
Disease
ITo1
IKs
Kv4.3
KvLQT1(KCNQ1)
Mink (KCNE1)
LQT
LQT1, JLN1
LQT5, JLN2
IKr
HERG (KCNH2)
MiRP1 (KCNE2)
LQT2
LQT6, FAF
INa
SCN5A
Ik1
Ikur
IkAch
IkATP
ICaL
LQT3 Brugada Syndrome, Cardiac
conduction defect, Sick sinus syndrome
Kir2.1 (KCNJ2)
LQT7 Andersen-Tawil Syndrome
Kv1.7(KCNA7),Kv1.5 Progressziv familial heart Block1
AF
Kir3.4
Kir6.2
Cav1.2 (CACNA1c) LQT8 Timothy Syndrome
Gene mutations in LQT1 and LQT2
HERG
KCNH2
KvLQT1
KCNQ1
LQT2
LQT1
Molecular structure and the Mutations in HERG channel
membrane topology of the
HERG channel
Atrial fibrillation (AF):
• Rapid shortening of the AERP
• Functional changes of ion channel
• Reduction of ICaL and gene expression of
L-type Ca channel
• Increase in K+-ion channel activity of IkAch,
Ik1
• Reduction in Ito and Isus
• Reduced gene expression in Kv1.5, Kv4.3,
Kir3.1, Kir3.4, Kir6.2
Pivotal role of Ser phosphorilation
as a regulatory mechanism in
Cav1.2 mode1/mode2 gating.
Timothy’s syndrome
Current Genes
IKr
IK1
IKs
HERG (KCNH2)
Kir2.x (KCNJ2)
KvLQT1(KCNQ1)
ICa
CASQ2 (Calsequestrin2)
ICa
RyR2
β1-adrenoceptor (β1-AR)
Disease
ShortQT
CPVT catecholamine-induced
CPVT ventricular tachycardia
polymorphic
CPVT
Risk factor, modify disease or
influence progression of disease
factor, modify disease or
β2-adrenoceptor (β2-AR) Risk
influence progression of disease
IkAch
Kv3.1, Kv3.4
AF
Complexity of protein-protein interaction in
cardiomyocytes
Missense mutation in calsequestrin2 (CASQ2)
wild type
Syncope
Seizures or
Sudden death
In response to
Physical activity or
Emotional stress
mutant
Associated with autosomal recessive catecholamineinduced polymorphic ventricular tachycardia (CPVT)
Kir2.1 ionchannel has an autosomal dominant
mutation in Andersen-Tawil Syndrome
Cardiac arrhytmias
Periodic paralysis
Dysmorphic bone structure(scoliosis,
low-set ears, small chin, broad forehead
Facial and
sceletal features
in AndersenTawil syndrome
Kir2.1 ion channel mutation
GIRK mutation
ANP role
•
•
•
•
Gene-specific mutation study
Genexpression study
Microarray, qRT-PCR
Proteomica
Kir2.x analysis
by RT-PCR
kir2.x mRNA in dog & human
0.01400000
0.01200000
0.01000000
0.00800000
0.00600000
0.00400000
0.00200000
0.00000000
-0.00200000
HUMAN
DOG
Kir2.1
Kir2.2
Kir2.3
Kir2.4
Expression of Kv1.5 protein in human and dog
kDa
75
66
RV
DOG
LV
RA
LA
RV
LV
HUMAN
RA
Relative amount of Kv1.5
6
5
4
3
2
LV
1
LA
0
HUMAN
DOG
n=12
n= 6
LA
Co-localization of Kv2 auxillary subunit with
Kv1.5 in dog left ventricular myocytes
Kv1.5-FITC
Kv2-Texas
red
100 m
Kv1.5-FITC
Kv2-Texas red