Selective Cdk9 inhibition resolves neutrophilic inflammation and enhances cardiac regeneration in larval zebrafish.

Sustained neutrophilic inflammation is detrimental for cardiac repair and associated with adverse outcomes following myocardial infarction (MI). An attractive therapeutic strategy to treat MI is to reduce or remove infiltrating neutrophils to promote downstream reparative mechanisms. CDK9 inhibitor compounds enhance the resolution of neutrophilic inflammation; however, their effects on cardiac repair/regeneration are unknown. We have devised a cardiac injury model to investigate inflammatory and regenerative responses in larval zebrafish using heartbeat-synchronised light-sheet fluorescence microscopy. We used this model to test two clinically approved CDK9 inhibitors, AT7519 and flavopiridol, examining their effects on neutrophils, macrophages and cardiomyocyte regeneration. We found that AT7519 and flavopiridol resolve neutrophil infiltration by inducing reverse migration from the cardiac lesion. Although continuous exposure to AT7519 or flavopiridol caused adverse phenotypes, transient treatment accelerated neutrophil resolution while avoiding these effects. Transient treatment with AT7519, but not flavopiridol, augmented wound-associated macrophage polarisation, which enhanced macrophage-dependent cardiomyocyte number expansion and the rate of myocardial wound closure. Using cdk9-/- knockout mutants, we showed that AT7519 is a selective CDK9 inhibitor, revealing the potential of such treatments to promote cardiac repair/regeneration.