Abstract 314: Inactivation-Resistant GSK3αβ Dual Isoform Expression Attenuates Isoproterenol-Induced Myocardial Hypertrophy and Preserves Cardiac Function
Glycogen synthase kinase-3 (GSK-3) is implicated as an important signalling mediator in normal developmental myocardial growth and adverse hypertrophic remodelling. Upstream phosphorylation of isoform specific N-terminal serine residues - ser21(GSK-3α) and ser9(GSK-3β), respectively - inhibits kinase activity. Opposing effects of the two isoforms have recently been described in response to hypertrophic stimuli, but little is known of the overall effect of dual-isoform manipulation. We set out to characterise baseline and isoproterenol (ISO) stimulated phenotypes of mice with inactivation-resistant GSK-3αβ knockin (KI) alleles, in which ser21/9 are converted to non-phosphorylatable alanine. Between 5–15 weeks there was a non-significant trend towards higher heart weight (HW): body weight (BW) ratios in KI mice versus wild type (WT) mice. Mean left ventricular wall thickness (LVWT) at the level of the papillary muscle was comparable between genotypes (1.3±0.2mm vs 1.6±0.2mm, ns, n=8). 8–10 week old weight-matched mice received 2 weeks subcutaneous ISO (30mg/kg/d) or control saline (Con) infusions. WT hearts hypertrophied with ISO (LVWT 1.9±0.1mm vs 1.3±0.2mm Con, n=8; HW:BW ratio 6.7±0.6 vs 5.4±0.4 Con, n=10, both p< 0.05), but KI mice did not (1.4±0.1mm vs 1.6±0.2mm KI Con; HW:BW 6.2±1.0 vs 6.1±0.6 KI Con, respectively, both ns). Hypertrophy in the WT hearts was associated with reduced LV ejection fraction (41±9% vs 63±5% Con, n=6, p< 0.05) and greater interstitial fibrosis (2.1±0.5% vs 0.8±0.6%, n=6, p< 0.05). These were both attenuated in the KI mice subjected to ISO (70±13% vs 64±11% Con, and 1.3±0.7% vs 0.8±0.3% Con, respectively, both ns). GSK-3 inhibition with daily intraperitoneal 6-bromoindirubin-3′-oxime restored hypertrophic sensitivity to ISO in the KI mice (LVWT 1.8±0.2mm; HW:BW 7.3±0.5, p< 0.05 for both groups vs KI + ISO alone) but had no effect in the absence of hypertrophic stimulus (1.4±0.2mm and 5.4±0.7, respectively, for KI + Con + Bio, ns vs KI + Con alone). In conclusion, inactivation-resistant GSK-3αβ dual isoform expression does not affect age-related (eutrophic) myocardial growth in the mouse. However, it protects against pathological growth, maintaining cardiac function and attenuating interstitial fibrosis.