Introduction: Takotsubo syndrome (TTS) is a severe but reversible acute heart failure affecting predominantly post-menopausal women. Extreme adrenaline levels arising with stress cause ventricular apical akinesis with basal function preserved. This occurs due to β2AR-Gi coupling, which is dysregulated in TTS. A microRNA (miR) profile of increased miR-16 and miR-26a has been identified that distinguishes TTS from healthy control, and ST-segment elevation myocardial infarction, which it resembles at patient presentation.
Hypothesis: miR-16 and miR-26a functionally interact to predispose to, or exacerbate, the cardiodepression seen in TTS.
Methods: miRs were blindly transfected in adult rat cardiomyocytes using Lipofectamine 3000. Percentage shortening was measured using an Ionoptix system. Calcium transients were obtained using Fluo-4-AM and sarcoplasmic reticulum (SR) calcium content measured with caffeine micro-application. TTS was induced in vivo by injection of an adrenaline bolus through the jugular vein of an adult male Sprague-Dawley rat, and blood sampled at baseline, 20 min and 1 hour. N numbers displayed as n/N (cells/rats).
Results: Increased miR-16/-26a significantly reduced contractility in cardiomyocytes (miR-16=3.52±0.34% versus control=4.91±0.46%; n/N=30/6; p<0.05 and miR-26a=2.77±0.21% versus control=4.30±0.43%; n/N=50/10; p<0.01). This effect was seen in apical but not basal cardiomyocytes. miR-16/-26a did not alter β2AR-response. Inhibiting Gi with pertussis toxin prevented the decrease in contraction with both miRs. Calcium transient amplitude and SR calcium content were decreased with miR-16/-26a, although SERCA rate was unchanged. In an in vivo model of TTS, miR-16/-26a were not increased in blood within the hour following TTS induction.
Conclusions: Increased miR-16/-26a reduced contractility of apical cardiomyocytes through a Gi-dependent mechanism. Downstream pathways may include decreased calcium transient amplitude and SR calcium content. miR-16/-26a levels in blood were not increased in TTS in vivo, making it more likely these pre-exist in TTS patients. This suggests miR-16/-26a may be mechanistically involved in TTS, but further work is needed to investigate specific signalling pathways.