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 1

Ultrastructural analysis reveals Z-disk abnormalities in Obscn-ΔIg58/59 atria.

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Ultrastructural analysis reveals Z-disk abnormalities in Obscn-ΔIg58/59 ...
(A and B) Quantification of absolute hydroxyproline content (A) in atrial tissues did not reveal differences in fibrotic deposition between genotypes at 6 or 12 months. In contrast, when normalized to atrial tissue mass (B), hydroxyproline content was significantly reduced in Obscn-ΔIg58/59 atria at 12 months compared with age-matched wild-type, indicating that the atrial enlargement observed in Obscn-ΔIg58/59 mice at this time point is not associated with increased fibrosis; t test, ***P < 0.001; n = 5–6 animals per group; data points represent the average of 6 technical replicates per animal. (C) Representative electron micrographs of longitudinally sectioned atria depicted Z-disk streaming in Obscn-ΔIg58/59 hearts at 6 and 12 months, along with increased variability in Z-disk orientation at 12 months; scale bar: 500 nm. Z-disks are highlighted in yellow in the images on the right, which are zoomed-in areas of the electron micrographs on the left, denoted by white rectangles. (D) The percentage of images that contained Z-disk streaming was significantly increased in 6- and 12-month Obscn-ΔIg58/59 atria compared with controls; Fisher’s exact test, *P < 0.05, **P < 0.01; n = 3 animals per group, 10 ± 3 images per animal (6 months), 15 ± 3 images per animal (12 months). (E) Obscn-ΔIg58/59 atrial sarcomeres displayed significantly increased variability in the orientation of the Z-disk at 12 months as quantified by the absolute deviation of the Z-disk angle within each image; t test, ***P < 0.001; n = 3 animals per group, 2 images per animal; data points represent individual sarcomeres and are color coded by biological replicate. (F) Schematic illustrating the progressive changes in Z-disk architecture in Obscn-ΔIg58/59. While wild-type sarcomeres are properly aligned, Z-disks in Obscn-ΔIg58/59 atria are out of register (i.e., Z-disk streaming) by 6 months and nonparallel by 12 months. Figure generated with BioRender.com (License MB27PC5ZUC).