Intercalated Disc Ultrastructural Abnormalities Precede Histopathologic Changes in Desmoglein2 Transgenic Mice and Are Associated with Conduction Slowing and Inducible Arrhythmias
Stefania Rizzo, Elisabeth M Lodder, Arie O Verkerk, Rianne Wolswinkel, Leander Beekman, Kalliopi Pilichou, Cristina Basso, Carol A Remme, Gaetano Thiene, Connie R Bezzina. University of Padua, Padua, Italy; Academic Medical Center, University of Amsterdam, Amsterdam, Netherlands
Background: We sought to evaluate the pathological substrate of electrical instability in the pre-phenotypic stage of arrhythmogenic cardiomyopathy (AC), before myocardial cell death and fibro-fatty replacement onset.
Design: We studied transgenic mice carrying mutation of desmoglein2 (dsg2) in the adhesive extracellular domain (Tg-NS). Gross, histological and ultrastructural analyses were used in the pre-phenotypic stage of the diseases, i.e., between 2 and 6 weeks. The structure and molecular composition of the ID was assessed by electron microscopy (EM) and by immunofluorescence. Mice with cardiac over expression of wild-type dsg2 (Tg-WT) and wild-type mice served as controls. Surface ECGs, electrical epicardial mapping and patch-clamp experiments were performed to determine ventricular conduction and arrhythmia susceptibility.
Results: At gross and histologic examination, Tg-NS hearts appeared normal, with no evidence of replacement-type fibrosis. Immunofluorescence uncovered no differences in the level and localization of junctional proteins between Tg-NS/L mice and controls. Ultrastructural examination of the myocardium ruled out cardiomyopathic changes, including cell necrosis, focal myofibrillar lysis, dilated sarcoplasmatic reticulum and T-tubules, and mitochondrial clustering at this age. Widening of the intercellular spaces at the level of desmosomes/adherens junctions was seen in otherwise morphologically normal cardiomyocytes in 25% of TgNS at 2 weeks and in 100% at 6 weeks and the percentage of widened cellular junctions increased with age (from about 10% at an age of 2 weeks to 60% at 6 weeks). Morphometric analysis showed that the intercellular space was significantly widened in Tg-NS/L mice (149 ± 80 nm) compared with controls (32 ± 3.5nm), p<0.05. None of the control mice (Tg-WT, WT) displayed any intercellular space widening. QRS-prolongation and inducible ventricular arrhythmias were observed in mutant mice. A reduced action potential (AP) upstroke velocity due to a lower Na(+) current density was also observed at this stage of the disease.
Conclusions: Dsg-2 mutant mice display ultrastructural evidence of desmosomes /adherens junctions widening, before development of cardiomyopathic changes. This coincided with conduction slowing and inducible ventricular arrhythmias thus emphasizing the importance of ID integrity for proper electrical conduction.
Monday, March 4, 2013 8:15 AM
Proffered Papers: Section H1, Monday Morning