Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC128
Increase in atrial arrhythmic susceptibility with age in mice with atrial-specific MKK4 deletion
L. Davies1, Y. Shi1, V. Christoffels3, E. Cartwright1, X. Wang2, M. Lei1
1. Faculty of Medicine, University of Manchester, Manchester, United Kingdom. 2. Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom. 3. Department of Anatomy & Embryology, University of Amsterdam, Amsterdam, Netherlands.
Cardiovascular disease is the most common cause of death in Western countries and 50% of mortalities attributable to cardiac causes are accounted for by cardiac arrhythmias. Atrial fibrillation (AF) is the most common form of sustained arrhythmia, its incidence increases with age and it is associated with extensive structural, contractile, and electrophysiological remodelling. Current pharmacological treatment of AF is limited and understanding the molecular mechanism of such remodelling processes is essential for the development of new targeted therapeutic interventions. A pathway that is potentially involved in regulating the pathogenesis of AF is the mitogen activated protein kinase cascade (MAPK), since components of the MAPK pathway have been shown to be down-regulated in patients with permanent atrial fibrillation (Kartmann et al. 2005). The MAPK pathway is involved in the regulation of fibrosis in the ventricle in aged/hypertrophied hearts (Lui et al. 2011; Kyoi et al. 2006) and fibrosis is linked to atrial fibrillation. To establish the role of the MAPK pathway in atrial function in age, we have developed a conditional knockout (CKO) mouse model where a central component of this pathway; MKK4, has been specifically deleted from the atria (De Lange et al. 2003; Liu et al. 2009). Hearts were assessed for atrial arrhythmia, both in vivo (using ECGs performed under 2% inhalation of Isoflurane anaesthetic) and ex vivo at 3 and 12 months of age (n≥6 per group) and tissue was collected for molecular analysis. Surface ECGs of these mice at 3 months of age showed abnormal atrial excitation with reduced P amplitudes (control 0.084mV ± 0.01 vs. CKO 0.058mV± 0.01 t-test p< 0.05; mean ± SEM) but no arrhythmia; however as the mice aged they became more susceptible to atrial arrhythmia, such as atrial tachycardia (AT) and atrial ectopic beats. Ex vivo hearts from 12 month old MKK4 CKO mice were more likely to develop AF/AT with electrical programme stimulation than old control mice (37% control mice vs. 100% CKO mice). Ex vivo conduction mapping revealed longer atrial conduction times in these mice, which could make the hearts more vulnerable to re-entry arrhythmias. In old CKO mice an increase in fibrosis was detected by picro-sirius red stain of tissue sections (2.8% ± 0.2 in control vs. 6.1% ± 0.4 in CKO t-test p<0.01; mean ±SEM). The expression of fibrotic pathway components was assessed using real-time PCR and western blot analysis and CKO mice had altered patterns of expression of TGF-β1, TGF-β receptors 1 and 2, MMP2/9 and TIMP2, compared to control mice. Development of an atrial specific MKK4 knockout mouse therefore reveals a role for the MAPK pathway in protecting against atrial arrhythmia in age through regulation of fibrotic processes and could provide insight into the design of more targeted therapies for atrial fibrillation.
Where applicable, experiments conform with Society ethical requirements