phd student University of Toronto Toronto, Ontario, Canada
Background: S100A6, a small calcium-binding protein, is important in managing calcium storage and myocyte contractility. This protein has low basal cardiac expression and is upregulated following myocardial infarction (MI). In an acute ischemia/reperfusion injury rat model, S100A6 gene transfer improves cardiac function.
METHODS AND RESULTS: HYPOTHESIS: Delivery of S100A6 by Ultrasound-Targeted Microbubble Destruction (UTMD) after established MI model results in improved left ventricular function and prevention of adverse ventricular remodeling.
Methods: MI was induced through permanent left anterior descending (LAD) coronary artery ligation in 8-week-old Sprague Dawley (SD) male rats on day 0. On day 28, using UTMD, we delivered microbubbles (1× ) coupled with either 200 μg of human S100A6 mini-circle (MC) or Empty- MC to the left ventricle (LV), while MI group received no therapy. The three groups were monitored weekly using serial echocardiography to follow LV function. At 8-weeks post ligation tissue collected from various regions of the myocardium.
Results: On day 28 (pre-treatment), Empty and S100A6 groups showed reduced LV Ejection Fraction (LVEF; 38.86 ± 7.90 and 37.27± 7.89, respectively) compared to sham (LVEF 63.75 ± 2.38, LVFS 36.20 ± 1.29; **** p < 0.0001). there was a significant improvement in the S100 group versus Empty-MC (LVEF; 50.61 ± 9.29 and 37.91± 10.27, respectively, * p < 0.05 N = 9 ). Over-expression of S100A6 significantly increased total S100A6 levels the border zone, on day 56, (1.10 × 10^5 ± 4.09 × 10^4 copies/µg RNA vs 1.81 × 10^3 ± 2.43 × 10^3 in Empty- MC ***p < 0.001; N=4). On Day 56 ,S100A6‐MC delivery reduced TUNEL‐positive nuclei in the border zone (0.19 ± 0.10 % vs 0.81 ± 0.20 % in Empty -MC *p < 0.05, N=4) and led to a significant reduction in pro-apoptotic Bax mRNA level (1.20 ± 1.31-fold change vs 3.01 ± 1.80 in Empty -MC *p < 0.05; N=7-8). At this endpoint, cardiac-specific overexpression S100A6 reduced cardiomyocyte cross sectional area from 426.2 ± 29.1 μm² to 218.7 ± 43.8 μm² in the border zone relative to Empty-MC (**p < 0.01, n = 3). S100A6 treatment decreased fibrosis% in myocardium (19.4 ±6.2% vs 29.05 ± 8.6% in Empty-MC; *p < 0.05; N = 8) and attenuated collagen-content in the border zone (0.65±0.28 mm² vs 1.38±0.73 mm² in Empty-MC, *p < 0.05; N =6). Same region of S100A6-treated myocardium showed a suppressed β myosin heavy chain expression (0.57 ± 0.42 fold change vs 1.70 ± 0.65 in Empty‐MC, *p < 0.05), up regulation of α MHC (1.4 ± 0.55 fold change vs 0.42 ± 0.24 in Empty‐MC, *p < 0.05) and lower level of collagen II expression (1.2 ± 0.5 fold change vs 3.8± 2.5 in Empty‐MC *p < 0.05, N=6–8).
Conclusion: S100A6 overexpression decreased both hypertrophy and fibrosis and was beneficial to the preservation of cardiac function late post MI.
