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Viktória Sz ű ts. Ionchannels and channelopaties in the heart. Action of membrane transport protein. ATP-powered pump Ion chanels Transporters 10 1 -10 3 ions/s 10 7 -10 8 ions/s 10 2 -10 4 ions/s. - PowerPoint PPT Presentation
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Ionchannels and channelopaties in the heart
Viktória Szűts
Action of membrane transport protein
ATP-powered pump Ion chanels Transporters 101-103ions/s 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 α-subunit 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.
Nerbonne et al . Circ Res. 2001;89:944-956
Molecular composition of the cardiac K-ionchannelsSelectivity filter
Membrane topology of the Kv and Kir2.x K-ionchannels
H5 H5
Voltage gated K+channel Inward rectifier K+channel
Kv channel
CO2
CO2
CO2
Kv complex
NN
CC
KChAP PSD
MiRP
Gating movi
Ionchannels are open and close changing the permeability
Abott et al Neuropharm. 2004
Assembly of different ionchannel subunits
Intracellular
Extracellular
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 membrane-spanning d-subunit by means of disulfide bonds. The α2-domain provides structural support required for channel stimulation, while the δ domain modulates the voltage-dependent activation and steady-state inactivation of the channel.
Abriel H. et al., Swiss Med Wkly 2004, 685-694. www.sm w. ch
Ionic currents and ion transporters responsible for cardiac action potential
• 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-off-
function”mechanism, while mutation in genes encoding K+ channel by “loss-off-function” mechanism
Risk factors for developing Torsade de pointes
Abriel H. et al., Swiss Med Wkly 2004, 685-694.
Genetic variants (polymorphysm or mutations)
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
LQT3 Brugada Syndrome, Cardiac conduction defect, Sick sinus syndrome
SCN5AINa
LQT7 Andersen-Tawil Syndrome Kir2.1 (KCNJ2)Ik1
LQT8 Timothy Syndrome Cav1.2 (CACNA1c)ICaL
Kir6.2IkATP
Kir3.4IkAch
Progressziv familial heart Block1Kv1.7(KCNA7),Kv1.5Ikur
LQT2LQT6, FAF
HERG (KCNH2)MiRP1 (KCNE2)
IKr
LQT1, JLN1LQT5, JLN2
KvLQT1(KCNQ1)Mink (KCNE1)
IKs
LQTKv4.3ITo1
DiseaseGenesCurrent
AF
Gene mutations in LQT1 and LQT2
LQT1LQT2
HERGKCNH2
KvLQT1KCNQ1
Molecular structure and the membrane topology of the
HERG channel
Mutations in 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
ShortQT
HERG (KCNH2)Kir2.x (KCNJ2)
KvLQT1(KCNQ1)
IKr
IK1
IKs
Kv3.1, Kv3.4
DiseaseGenesCurrent
ICaCASQ2 (Calsequestrin2) CPVT
CPVT catecholamine-induced polymorphic ventricular tachycardia
RyR2 CPVT
β1-adrenoceptor (β1-AR)
β2-adrenoceptor (β2-AR)
Risk factor, modify disease orinfluence progression of disease
Risk factor, modify disease orinfluence progression of disease
AF
ICa
IkAch
Complexity of protein-protein interaction in cardiomyocytes
Missense mutation in calsequestrin2 (CASQ2)
Associated with autosomal recessive catecholamine-induced polymorphic ventricular tachycardia (CPVT)
SyncopeSeizures orSudden death
In response to Physical activity orEmotional stress
wild type
mutant
Kir2.1 ionchannel has an autosomal dominant mutation in Andersen-Tawil Syndrome
Cardiac arrhytmiasPeriodic paralysisDysmorphic bone structure(scoliosis,low-set ears, small chin, broad forehead
Facial and sceletal features
in Andersen-Tawil syndrome
Kir2.1 ion channel mutation
GIRK mutation
ANP role
• Gene-specific mutation study• Genexpression study• Microarray, qRT-PCR• Proteomica
kir2.x mRNA in dog & human
-0.002000000.000000000.002000000.004000000.00600000
0.008000000.010000000.012000000.01400000
Kir2.1 Kir2.2 Kir2.3 Kir2.4
HUMAN
DOG
Kir2.x analysisby RT-PCR
RV LV RA LA RV LV RA LADOG HUMAN
n=12 n= 6
0
1
2
3
4
5
6
HUMAN DOG
Rel
ativ
e am
ount
of K
v1.5
LV
LA
kDa7566
Expression of Kv1.5 protein in human and dog
Co-localization of Kv2 auxillary subunit with Kv1.5 in dog left ventricular myocytes
100
Kv1.5-FITCKv2-Texas red
Kv1.5-FITC Kv2-Texas red