Fig 1.
Multi-scale computer models to investigate the mechanism underlying Pitx2-induced AF and the effects of class Ic AAD (flecainide).
a, The computer models incorporated Pitx2-induced electrical and structural remodeling, and flecainide interactions with ion channels. Remodeled targets (magenta circles) included gap junction, IK1, IKs, INa, ICaL, RyR and SERCA. Drug targets of flecainide (red circles) were IKr, INa and RyR. Under the control condition, heterogeneity in Pitx2 expressions (b) and AP (c) between LA and RA was included in the computer models. d, Based on experimental and clinical studies up to date, four Pitx2 deficiency-induced human atrial cell models were developed: Pitx2-1 with remodeled IK1 (green), Pitx2-2 with remodelled IKs and ICaL (magenta), Pitx2-3 with remodelled ICaL, RyR and SERCA (red) and Pitx2-4 with remodelled IK1, IKs, INa, ICaL, RyR and SERCA (blue). Abbreviations: AF–atrial fibrillation; AAD–antiarrhythmic drug; LA–left atrium; RA–right atrium; AP–action potential; RyR–ryanodine receptor; SERCA–calcium transport ATPase.
Table 1.
Review of Pitx2-insufficiency induced remodelling data and model parameters in human left atrium.
Fig 2.
Simulated action potentials (AP, Vm) and calcium transients (Cai) of left atrial (LA) and right atrial (RA) cells under controls and Pitx2-induced remodelling conditions.
At a pacing frequency of 1Hz, AP and Cai (a), RMP (b), overshoot (c), dVdtmax (d) and APD (e) under control, Pitx2-1, Pitx2-2, Pitx2-3 and Pitx2-4 conditions. Black and grey markers were used for LA and RA cells, respectively. Similar key indicators (f-j) at a pacing frequency of 2Hz were displayed. Blue arrows indicated delayed afterdepolarizations, triggered action potentials and spontaneous calcium transients. Abbreviations: RMP–resting membrane potential; dVdtmax−maximum upstroke velocity; APD–action potential duration.
Fig 3.
Antiarrhythmic effects of flecainide on action potentials (AP, Vm).
a, Comparison of APs of LA cells in the presence or absence of 2 μM Fle. The main AP parameters included RMP (b), overshoot (c), dVdtmax (d) and APD (e). Simulated effects of 2 μM Fle on all targets (f), and on INa (g), on Ikr (h) and on RyR (i) respectively. Blue arrows indicate delayed afterdepolarizations. Abbreviations: LA–left atrium; Fle–flecainide; RMP–resting membrane potential; dVdtmax−maximum upstroke velocity; APD–action potential duration; RyR–ryanodine receptor.
Fig 4.
Simulated electrical (Vm) and calcium (Cai) waves in a 1D RA-LA strand.
a, Simulated Vm and Cai waves in the drug-free settings (a) and in the presence of 2 μM flecainide (b). Comparison of CV(c) and WL (d) of LA (black) versus RA (grey) strands in the drug-free settings. In the presence of 2 μM flecainide, CV (e) and WL (f) are compared between RA and LA strands. The 1D strand contains 75 RA (#1-#75) cells and 75 LA (#76-#150) cells. Electrical waves were elicited by an extra stimulus at the RA end and propagated from RA to LA. Blue arrows indicate spontaneous delayed afterdepolarizations, triggered action potentials and calcium transients. Abbreviations: 1D –one-dimensional; RA–right atrium; LA–left atrium; CV–conduction velocity; WL–Wavelength.
Fig 5.
Simulated spontaneous ectopic activity and re-entry.
a, A 500×500 square tissue model includes normal atrial myocytes, Pitx2-4 remodelled LA cells, fibrosis, and gap junctions. Simulated spontaneous ectopic activity in the tissue model with 20% Pitx2-4 cells (b), with further increased Pitx2-4 cells (c), with enhanced cell-to-cell uncoupling (d), with increased fibrosis (e), and with increased cell-to-cell uncoupling and fibrosis (f). Re-entrant waves in the drug-free Pitx2-4 settings (g) and in the presence of 2 μM flecainide (h). Note: For the #1 scenario, the diffusion coefficient (D) was set to be 100% and the ratio of normal cells, Pitx2-4 cells and fibrosis was set to be 80:20:0. For the #2 scenario, the number of Pitx2-4 cells was increased to 40% and thereby the ratio of different cell types was 40:60:0. And D was set to be 100%. For the #3 scenario, D was reduced to 30% to model cell-to-cell uncoupling and the ratio of different cell types was set to be 80:20:0. For the #4 scenario, fibrosis was increased to 2% and thereby the ratio of different cell types was 78:20:2. And D was set to be 100%. For the #5 scenario, fibrosis was increased to 2% and D was reduced to 30%. And the ratio of different cell types was 78:20:2.
Table 2.
A quantitative summary of electrophysiology characteristics.
Fig 6.
Mechanisms underlying Pitx2 deficiency-induced AF and ionic mechanisms of anti-arrhythmic effects of flecainide in AF patients with Pitx2 down-regulation.
a, Pitx2 regulates calcium handling genes Atp2a2, Ryr2 and Sln, electrical remodelling genes Scn5a, Kcnj2, Kcnj4, Kcnj11, Kcnj12, Kcnq1 Cacna1d, Cacna2d2, and Cav1, and structural remodelling genes Gja1, Gja5 and Dsp. Pitx2 down-regulation in the LA leads calcium handling abnormities, electrical remodelling and structural remodelling, contributing to APD abbreviation, slowed atrial conduction and DAD, leading to AF triggers and substrates. b, Flecainide has block effects on RyR, INa and IKr. Flecainide can reduce spontaneous calcium waves and triggered activity, and prolong the APD, thereby suppressing Pitx2 deficiency-induced AF. Abbreviations: AF–atrial fibrillation; LA–left atrium; DAD–delayed afterdepolarization; APD–action potential duration; RyR–ryanodine receptor.