Glycogen synthase kinase 3 (GSK-3) is considered a key signalling mediator of both developmental myocardial growth and adverse hypertrophic remodelling. GSK-3β inactivation, in particular, has been identified as necessary and sufficient to induce myocardial hypertrophy. However, strategies to maintain GSK-3β active, while potentially protective against pathological hypertrophy in the adult heart, are also associated with an adverse developmental phenotype. Furthermore, GSK-3α activation is associated with a profibrotic response to pathological hypertrophy. We set out to characterise baseline and isoproterenol-stimulated phenotypes of mice with targeted ser-21/9-ala mutations encoding ubiquitous expression of inactivation resistant GSK-3αβ kinases.
Methods (1) Baseline phenotype: baseline heart weight (HW) and body weight (BW) measurements from GSK-3αβser21/ser9 (WT) and GSK-3αβala21/ala9 (KI) mice were recorded in a cohort of male animals between the ages of 5 and 15 weeks to determine gross morphological development. (2) Acute β-adrenergic stimulation: isolated adult hearts from both genotypes were perfused in Langendorff mode with increasing concentrations of isoproterenol. Dose–response curves for heart rate, left ventricular developed pressure and systolic and diastolic contractile parameters were recorded. (3) Chronic β-adrenergic stimulation: age and weight-matched male mice were treated with isoproterenol (30 mg/kg per day) or vehicle (0.9% NaCl) for 14 days delivered by subcutaneous pump. These were then explanted and cardiac structure and function determined in vivo by cardiac magnetic resonance imaging under isofluorane anaesthesia (2%). Hearts were then fixed and treated with picrosirius red and isolectin-B4 for fibrosis content and capillary density, respectively. In parallel studies, hearts were snap frozen after 14 days treatment for reverse transcription PCR determination of transcript expression for procollagen IαI, IIIαI, fibronectin and β-actin.
Results (1) Cardiac growth was comparable between WT and KI mice with no difference in HW/BW ratios over the 5–15-week age range studied. (2) The acute response to isoproterenol treatment in isolated hearts of both genotypes was similar with no differences in peak, trough or EC50 measurements of haemodynamic function. (3) Chronic isoproterenol treatment in WT hearts resulted in hypertrophy, interstitial fibrosis and impaired contractile function. These findings were all significantly attenuated in KI hearts, which, in addition, demonstrated increased end-diastolic/systolic parameters, stroke index and cardiac index compared with vehicle control (see table and fig).
Conclusion Inactivation-resistant GSK-3αβ dual isoform expression is permissive of eutrophic myocardial development and potentially a physiological adaptive response to chronic adrenergic stimulation. Fibrosis and impaired cardiac function, however, are significantly attenuated.