Abstract 1243: Sema3A Induces a Cardiac Ventricular Repolarization Gradient via Sympathetic Innervation Patterning
[Background] Well-organized sympathetic innervation patterning and ventricular repolarization gradient are critical for effective cardiac performance. However, their development, regulatory mechanisms and relationship remain unknown. This study was designed to investigate the expression of the neural chemorepellent Sema3A, a member of the class 3 secreted semaphorin, and its roles on cardiac sympathetic innervation patterning and ventricular repolarization gradient, using various gene modified mice models.
[Methods and Results] (1) Immunostaining for tyrosine hydroxylase and norepinephrine measurement demonstrated that sympathetic nerves illustrated epicardial-to-endocardial transmural gradients in murine ventricles. (2) The density of transient outward potassium current (Ito) and the action potential duration (APD) showed similar transmural gradient patterns. (3) Sema3A mRNA was 2.3-fold higher in the subendocardial than in subepicardial layers, and it gradually decreased with development as opposed to sympathetic innervation. (4) Heterozygous Sema3A knocked-in lacZ mice (Sema3AlacZ/+) demonstrated that Sema3A was synthesized in an endocardial-to-epicardial gradient and was abundantly expressed in Purkinje fibers. (5) In Sema3AlacZ/lacZ, the cardiac sympathetic innervation gradient was completely abolished and many aberrant nerves were observed at subendocardial layers where lacZ was expressed, while in Sema3AlacZ/+, sympathetic nerves avoided lacZ-expressing areas, confirming the importance of Sema3A in cardiac sympathetic innervation patterning. (6) Transgenic mice over-expressing Sema3A in cardiomyocytes (SemaTG) had small hearts, severe hypo-innervation, and attenuation of the epicardial-to-endocardial neural gradient in the hearts. (7) SemaTG mice presented marked APD prolongation and reduction of Ito density concordant with reduced sympathetic innervation, and they showed spontaneous ventricular arrhythmia. (8) Direct application of Sema3A did not influence APD or Ito density.
[Conclusions] These findings demonstrate that Sema3A is critical for cardiac sympathetic innervation patterning and the transmural repolarization gradient, which are critical to prevent arrhythmias.