Gene annotation for heart Gene annotation for heart rhythmrhythm

1. Control of heart rate

2. Action Potential

3. Ion channels and transporters

4. Arrhythmia

5. EC coupling

Control of heart rateControl of heart rate

Autonomic regulation of heart functionAutonomic regulation of heart function

Autonomic Regulation IIAutonomic Regulation II• Central integration of blood pressure and respiratory control• Afferents via baroreceptors, chemoreceptors etc• Integrated in brainstem centres

Autonomic Regulation IIIAutonomic Regulation IIIProteins involved in presynaptic

vesicle release

Proteins involved in signal transduction in the SA node

Effector arm

Heart Rate VariabilityHeart Rate Variability

• The heart beat is not quite regular subject to small variations • e.g. sinus arrhythmia• Indicative of health. Correlates inversely with outcome after

MI etc• Time domain:– Tachograms, SD of R-R or R-R• Frequency domain:- Potentially more revealing. • HF=vagal\respiration, LF=sympathetic\BP control

What ionic mechanisms are responsible?What ionic mechanisms are responsible?• Intrinsic rhythm set by SA node• Modulation of pacemaker depolarisation

receptor activation Gs

Adenylate cyclase

Increased cAMP

If activation

M2 receptor activation Gi\o

Adenylate cyclase

Decreased cAMP

If inhibition

G

liberationIKAch

activation

What is the intrinsic pacemaker?What is the intrinsic pacemaker?• Spontaneous activity in the absence of innervation

(intrinsic heart rate)• Actually currently quite controversial

• Two hypotheses – If current is centrally important and\or Ca2+ cycling

IIff\HCN channels\HCN channels

• Activated by hyperpolarisation• Cation but otherwise nonselective• Directly opened by cAMP• HCN1-4, mainly HCN4 in heart• Largely expressed in SA node• Ivabradine used for the treatment • of angina

Action potentialAction potential

Cardiac Action Potential ICardiac Action Potential I

Conduction systemConduction systemAP in heart regionsAnatomy

Cardiac action potential IICardiac action potential II

• IKur – Kv1.5

• IKACh – Kir3.1\3.4

• IKATP – SUR1\Kir6.1\Kir6.2 vs SUR2A\Kir6.2• Cx40 in atria. Cx43 in ventricle• SK channels

Ion Channels and Transporters

What is happening at the What is happening at the molecular level?molecular level?

Ion channels predominantly control membrane excitability

Sodium channelsSodium channels

SCN5A in the heart. Both beta subunits present.

Potassium channelsPotassium channels

Lots of genes underlying K+ channelsLots of genes underlying K+ channels

Current Molecular composition

Channel structure Function Location Reference(s)

Ito,f (Ito1) -subunit Kv4.3 and -subunit KChiP2.

Octameric complex of a tetramer of 6 TMD -subunits and 4 subunits.

Provides the rapid component of the transient outward current that contributes to early rapid repolarization during Phase 1.

Atrial and Ventricular. [23], [24], (a), (b)

Ito,s (Ito1) -subunit Kv1.4 and possibly -subunits (Kv1.2, Kv1.3 and Kv2)

A tetramer of 6 TMD subunits may coassemble with 4 -subunits.

Provides the slow component of the transient outward current that contributes to early rapid repolarization during Phase 1.

Atrial and Ventricular. [25], [26], (a), (c)

IKur -subunit Kv1.5 and -subunit Kv1.2.

A tetramer of 6 TMD subunits associates with 4 -subunits to form an octameric complex.

Plays an important role in early phase (1-2) atrial repolarization.

Atrial. [27], [28], (d), (e)

IKr -subunit Kv11.1 (HERG) and probably -subunit KCNE2.

A tetramer of 6 TMD -subunits and an unknown number of 1 TMD -subunits.

Repolarisation, outward rectifier during Phase 2 and 3.

Atrial and Ventricular. (f), (g), (h), (i)

IKs subunit Kv7.1 (KCNQ1) and subunit KCNE1.

Tetramer of 6 TMD -subunits assembles with probably two 1 TMD -subunits.

Repolarisation, outward rectifier during Phase 2 and 3.

Atrial and Ventricular. [20], [21], [22], (h), (i)

IK1 Kir2.1 and perhaps Kir2.2 and Kir2.3.

Tetramer of 2 TMD subunits. Contributes to late repolarisation, late phase 3, and helps to set membrane potential.

Ventricular and Atrial [30], [31], (j), (k), (l)

IKACh Kir3.1 and Kir3.4. Tetrameric complex of 2 Kir3.1 and 2 Kir3.4 2 TMD subunits. (During development channel may be formed by a homotetramer of Kir3.4)

During late phase 3 and phase 4 activation of IKACh by acetylcholine acts to hyperpolarise the membrane potential, slow the firing rate of pacemaker cells in the SA and AV nodes and delays AV conduction.

Predominantly Atrial and nodal tissue expression.

[32], [33], (j), (m), (n), (o), (p)

IKATP (Ventricular)

Kir6.2 and SUR2A. Octameric complex formed by coassembly of 4 2 TMD pore subunits and 4 17 TMD SUR subunits.

During late phase 3 and phase 4 this channel acts to link cellular metabolism and membrane excitability.

Ventricular. [10], (q), (r), (s)

IKATP (Atrial)

Kir6.2 and SUR1. (Kir6.1?)

Octameric complex formed by coassembly of 4 2 TMD pore subunits and 4 17 TMD SUR subunits.

During late phase 3 and phase 4 this channel acts to link cellular metabolism and membrane excitability.

Atrial. [9] , [10], (q), (r), (s)

• Also SK channels and twin pore channels

K channels in Long QTK channels in Long QT

N C

H 5

Voltage-ga ted (6 -T M )

E x tra ce llu la r

In tra ce llu la r

N

C

E x tra ce llu la r

In tra ce llu la r

K C N E fam ily

alpha beta current

KCNQ1 (KvLQT1)

KCNE1 (IsK) Iks

HERG KCNE2 (MIRP1) Ikr

NaNa++\K\K++ ATPase ATPase

• Member of the P type ATPase pumps• and 3 subunits. 1 and 2

auxiliary subunits• Electrogenic 3Na+ for 2K+ but transport

rate ~4 four fold less than the Na channel (100 ions\second)

Arrhythmia

Classification of arrhythmiaClassification of arrhythmia

• Site of origin e.g. atrial, nodal, ventricular• Rate e.g. bradycardia, tachycardia• Process\Substrate e.g. fibrillation, heart

block, ectopic etc

This carefully orchestrated This carefully orchestrated activity can go wrongactivity can go wrong

Electrocardiogram (ECG)Electrocardiogram (ECG)The benchmark of clinical diagnosis is the ECG

P wave= atrial depolarisationQRS= ventricular depolarisationT wave=ventricular repolarisation

ExamplesExamples

Atrial Fibrillation Ventricular Tachycardia

Repolarisation and KRepolarisation and K++ currents currents

Excitation-contraction Excitation-contraction couplingcoupling

Cardiac excitation-contraction couplingCardiac excitation-contraction coupling

Calcium channelsCalcium channels

• Gene = CACNAx for alpha subunits (CACNA1C = Cav1.2)

• Cav1 = L-type, Cav2 = N-, P\Q and R type and Cav3 = T type

Ryanodine receptorRyanodine receptor

• RyR2 in heart• Calcium induced calcium release• LTCC and RyR2 opposed in T-

tubule• Large tetrameric complex• Protein interactions

Sodium\calcium exchangerSodium\calcium exchanger

• Major mechanism for calcium extrusion from the heart• Electrogenic – 3 Na+ for single Ca2+

• Passive coupled counter transport system• NCX1 in the heart (3 isoforms in total)• Also P type ATPase Ca2+ pump present in heart which actively

extrudes Ca2+ (PMCA)

SERCA2a and phospholambanSERCA2a and phospholamban

• Major mechanism for calcium uptake into SR• P-type ATPase that transports Ca2+ actively driven by ATP

hydrolysis• Regulated by phospholamban