Constitutive deletion of the obscurin-Ig58/59 domains induces atrial remodeling and Ca2+-based arrhythmogenesis
Alyssa Grogan, Annie Brong, Humberto C. Joca, Liron Boyman, Aaron D. Kaplan, Christopher W. Ward, Maura Greiser, Aikaterini Kontrogianni-Konstantopoulos
Alyssa Grogan, Annie Brong, Humberto C. Joca, Liron Boyman, Aaron D. Kaplan, Christopher W. Ward, Maura Greiser, Aikaterini Kontrogianni-Konstantopoulos
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Research Article Muscle biology

Constitutive deletion of the obscurin-Ig58/59 domains induces atrial remodeling and Ca2+-based arrhythmogenesis

Abstract

Obscurin is a giant protein that coordinates diverse aspects of striated muscle physiology. Obscurin immunoglobulin domains 58/59 (Ig58/59) associate with essential sarcomeric and Ca2+ cycling proteins. To explore the pathophysiological significance of Ig58/59, we generated the Obscn-ΔIg58/59 mouse model, expressing obscurin constitutively lacking Ig58/59. Males in this line develop atrial fibrillation by 6 months, with atrial and ventricular dilation by 12 months. As Obscn-ΔIg58/59 left ventricles at 6 months exhibit no deficits in sarcomeric ultrastructure or Ca2+ signaling, we hypothesized that susceptibility to arrhythmia may emanate from the atria. Ultrastructural evaluation of male Obscn-ΔIg58/59 atria uncovered prominent Z-disk streaming by 6 months and further misalignment by 12 months. Relatedly, isolated Obscn-ΔIg58/59 atrial cardiomyocytes exhibited increased Ca2+ spark frequency and age-specific alterations in Ca2+ cycling dynamics, coinciding with arrhythmia onset and progression. Quantitative analysis of the transverse-axial tubule (TAT) network using super-resolution microscopy demonstrated significant TAT depletion in Obscn-ΔIg58/59 atria. These structural and Ca2+ signaling deficits were accompanied by age-specific alterations in the expression or phosphorylation of T-cap protein, which links transverse tubules to Z-disks, and junctophilin 2, which connects transverse tubules to the sarcoplasmic reticulum. Collectively, our work establishes the Obscn-ΔIg58/59 model as a reputable genetic model for atrial cardiomyopathy and provides mechanistic insights into atrial fibrillation and remodeling.

Authors

Alyssa Grogan, Annie Brong, Humberto C. Joca, Liron Boyman, Aaron D. Kaplan, Christopher W. Ward, Maura Greiser, Aikaterini Kontrogianni-Konstantopoulos

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Figure 2

Atrial cardiomyocytes from sedentary Obscn-ΔIg58/59 mice exhibit age-specific changes in Ca2+ cycling and SR Ca2+ content.

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Atrial cardiomyocytes from sedentary Obscn-ΔIg58/59 mice exhibit age-spe...
(A–E) Representative confocal line scan images and corresponding Ca2+ transients (A and B) depicted significantly increased Ca2+ transient amplitude (C) and rise time (D) and decreased Ca2+ decay time (E) in atrial cardiomyocytes from 6-month-old Obscn-ΔIg58/59 hearts compared with age-matched wild-type, whereas 12-month-old Obscn-ΔIg58/59 cells displayed significantly decreased Ca2+ transient amplitude (C) and prolonged Ca2+ decay (E) compared with controls, with no change in rise time (D); t test, *P < 0.05, **P < 0.01, ***P < 0.001; n = 5 animals per group (6 months), n = 3 animals per group (12 months), 9–20 cells per heart (6 months), 7–17 cells per heart (12 months); data points represent individual cells color coded by biological replicate. (F) Line profiles of the representative Ca2+ transients depicted in A and B at half-maximal fluorescence amplitude and corresponding quantifications of the SD of the TTF50 (G) and the coefficient of variation, SI (H), revealed dyssynchronous Ca2+ release in Obscn-ΔIg58/59 atria at 6 and 12 months; t test, *P < 0.05, ***P < 0.001; n = 5 animals per group (6 months), n = 3–4 animals per group (12 months), 6–24 cells per heart (6 months), 2–17 cells per heart (12 months); data points represent individual cells color coded by biological replicate. (I–L) SR Ca2+ load was measured in quiescent atrial cardiomyocytes via rapid application of 10 mmol/L caffeine preceded by steady-state 1 Hz electrical pacing. Field stimulation was subsequently restarted to ensure that all releasable Ca2+ had been depleted (I). Representative transverse confocal line scan images and caffeine-induced Ca2+ transients at 6 months (K) and 12 months (L) depicted significantly increased SR Ca2+ content (J) in atrial cardiomyocytes isolated from 6-month-old Obscn-ΔIg58/59 hearts, but not at 12 months, compared with age-matched controls; t test, *P < 0.05; n = 4–5 animals per group (6 months), n = 3–4 animals per group (12 months), 5–11 cells per heart (6 months), 2–11 cells per heart (12 months); data points represent individual cells color coded by biological replicate.