Defining new insight into atypical arrhythmia: a computational model of ankyrin-B syndrome

Abstract

Normal cardiac excitability depends on the coordinated activity of specific ion channels and transporters within specialized domains at the plasma membrane and sarcoplasmic reticulum. Ion channel dysfunction due to congenital or acquired defects has been linked to human cardiac arrhythmia. More recently, defects in ion channel-associated proteins have been associated with arrhythmia. Ankyrin-B is a multifunctional adapter protein responsible for targeting select ion channels, transporters, cytoskeletal proteins, and signaling molecules in excitable cells, including neurons, pancreatic β-cells, and cardiomyocytes. Ankyrin-B dysfunction has been linked to cardiac arrhythmia in human patients and ankyrin-B heterozygous (ankyrin-B(+/-)) mice with a phenotype characterized by sinus node dysfunction, susceptibility to ventricular arrhythmias, and sudden death ("ankyrin-B syndrome"). At the cellular level, ankyrin-B(+/-) cells have defects in the expression and membrane localization of the Na(+)/Ca(2+) exchanger and Na(+)-K(+)-ATPase, Ca(2+) overload, and frequent afterdepolarizations, which likely serve as triggers for lethal cardiac arrhythmias. Despite knowledge gathered from mouse models and human patients, the molecular mechanism responsible for cardiac arrhythmias in the setting of ankyrin-B dysfunction remains unclear. Here, we use mathematical modeling to provide new insights into the cellular pathways responsible for Ca(2+) overload and afterdepolarizations in ankyrin-B(+/-) cells. We show that the Na(+)/Ca(2+) exchanger and Na(+)-K(+)-ATPase play related, yet distinct, roles in intracellular Ca(2+) accumulation, sarcoplasmic reticulum Ca(2+) overload, and afterdepolarization generation in ankyrin-B(+/-) cells. These findings provide important insights into the molecular mechanisms underlying a human disease and are relevant for acquired human arrhythmia, where ankyrin-B dysfunction has recently been identified.

Fig. 1.

Mathematical model of the ankyrin-B-deficient (ankyrin-B^+/−) cell. A: schematic of the mouse ventricular cell model. Na⁺/Ca²⁺ exchanger (NCX) current (I_NaCa) and Na⁺-K⁺-ATPase (NKA) current (I_NaK) were altered in the model of the ankyrin-B^+/− cell (shaded boxes). I_Na, Na⁺ current; I_Na,b, background Na⁺ current; I_CaL, L-type Ca²⁺ current; I_p(Ca), sarcolemmal Ca²⁺ pump; I_Ca,b, background Ca²⁺ current; I_ns(Ca), nonspecific Ca²⁺ current; I_Kto,f, fast component of the transient outward K⁺ current; I_Kto,s, slow component of the transient outward K⁺ current; I_Kr, fast component of the delayed recifier K⁺ current; I_Ks, slow component of the delayed recifier K⁺ current; I_Kss, steady-state K⁺ current; I_Kur, ultrarapid K⁺ current; I_K1, inward rectifier K⁺ current; I_rel, Ca²⁺ release flux; I_up, Ca²⁺ uptake flux; SER, sarco(endo)plasmic reticulum Ca²⁺-ATPase; JSR, junctional sarcoplasmic reticulum (SR); NSR, network SR; I_tr, Ca²⁺ transfer flux; I_leak, Ca²⁺ leak flux. B–E: simulated action potentials (APs) in control (B and D) and ankyrin-B^+/− (C and E) cardiomyocytes from the mouse (B and C) and human (D and E) ventricular cell models [10th action potential (AP) shown at a cycle length (CL) of 1,000 ms]. F: simulated I_NaCa at a test potential of −10 mV from wild-type cell and an ankyrin-B^+/− cell compared with experimental measurements (n = 12, *P < 0.05) (17). In both the simulation and experiment, intracellular Na⁺ concentration = 20 mM, extracellular Na⁺ concentration = 145 mM, intracellular Ca²⁺ concentration ([Ca²⁺]_i) = 1 μM, and extracellular Ca²⁺ concentration = 2 mM. G: simulated and experimentally measured (n = 3, *P < 0.01) (25) NKA membrane expression in wild-type and ankyrin-B^+/− mouse ventricular cells. H: simulated and experimentally measured (n = 17, P = not significant) (28) AP duration (APD) at 90% repolarization. I: simulated and experimentally measured (n = 18, *P < 0.001) peak Ca²⁺ transients. In both the simulation and experiment, the cell was pulsed four times to test potential of +0 mV from a holding potential of −40 mV (28).

Fig. 2.

Ca²⁺ accumulation at baseline in the ankyrin-B^+/− cell. A–F: simulated steady-state AP (A), I_NaK (B), I_CaL (C), I_NaCa (D), Ca²⁺ transient (E), and JSR Ca²⁺ concentration ([Ca²⁺]_JSR; F) in control and ankyrin-B^+/− mouse ventricular cells (CL = 1,000 ms). V_m, membrane potential.

Fig. 3.

Rate dependence of the AP and Ca²⁺ transient in wild-type and ankyrin-B^+/− cells. A–D: APD (A), Ca²⁺ transient amplitude (CaT_amp; B), [Ca²⁺]_JSR (C), and concentration of Ca²⁺ bound to calsequestrin ([Ca²⁺-CSQN]; D) in control and ankyrin-B^+/− cells paced to steady state at CLs of 200, 400, 500, 750, 1,000, 1,500, and 2,000 ms.

Fig. 4.

Role of NCX and NKA in Ca²⁺ accumulation in the ankyrin-B^+/− cell. A–C: simulated steady-state AP (A), Ca²⁺ transient (B), and [Ca²⁺]_JSR (C) in control, ankyrin-B^+/−, NCX-deficient (NKA restored to normal levels), and NKA-deficient (NCX restored to normal levels) cells. D–F: summary data showing steady-state APD (D), CaT_amp (E), and [Ca²⁺]_JSR (F) in control, ankyrin-B^+/− (AnkB), NCX-deficient, and NKA-deficient cells (CL = 1,000 ms).

Fig. 5.

Isoproterenol (Iso) increases intracellular Ca²⁺ in control and ankyrin-B^+/− cells. A–F: simulated steady-state AP (A and D), Ca²⁺ transient (B and E), and I_CaL (C and F) for control (A–C) and ankyrin-B^+/− (D–F) mouse ventricular cardiomyocytes at baseline and in the presence of a saturating concentration of Iso (> 0.1 μM, CL = 1,000 ms).

Fig. 6.

Spontaneous Ca²⁺ release and afterdepolarizations in the ankyrin-B^+/− cell during rapid pacing in the presence of Iso. A–C: simulated AP (A), Ca²⁺ transient (B), and [Ca²⁺]_JSR (C) in control and ankyrin-B^+/− mouse ventricular cardiomyocytes during rapid pacing to steady state (CL = 200 ms) in the presence of Iso. Frequent spontaneous release events (arrows in B) led to abnormal repolarization (* in A) in the ankyrin-B-deficient cell. D–G: simulated AP (D), Ca²⁺ transient (E), I_NaCa (F), and I_CaL (F) in control and ankyrin-B^+/− cells during a subsequent pause after rapid pacing to steady state. Note the spontaneous release (arrows) and afterdepolarizations (*) that ultimately produced an AP.

Fig. 7.

Role of NCX and NKA in spontaneous Ca²⁺ release and afterdepolarizations in the ankyrin-B^+/− cell. A and B: simulated AP (A) and Ca²⁺ transient (B) in ankyrin-B^+/−, NCX-deficient, and NKA-deficient cells during rapid pacing to steady state (CL = 200 ms). Spontaneous Ca²⁺ release (arrows in B) and abnormal repolarization (asterisks in A) were observed in NCX-deficient and NKA-deficient cells, although with decreased frequancy and delayed onset compared with the ankyrin-B^+/− cell. C and D: summary data showing the time to first spontaneous release of Ca²⁺ from the SR (C) and number of spontaneous release events during rapid pacing (D) in ankyrin-B^+/−, NKA-deficient, and NCX-deficient cells.