Pyruvate kinase splice variants in fibroblasts influence cardiac remodeling after myocardial infarction in male mice

Fibroblasts are crucial for cardiac repair after myocardial infarction (MI). In response to signaling cues, they differentiate to phenotypes with robust capacities to synthesize and secrete extracellular matrix (ECM) and signaling molecules. Although activated fibroblast phenotypes are associated with pronounced changes in metabolism, it remains unclear how the metabolic network upholds the effector functions of fibroblasts in the infarcted heart. We found that two enzymes that could facilitate a phosphoenolpyruvate cycle, i.e. pyruvate kinase muscle isoform 2 (PKM2) and phosphoenolpyruvate carboxykinase 2 (PCK2), are elevated in the heart after MI. Although Pck2 deletion had no effect on post-MI remodeling, fibroblast-specific switching of Pkm2 to Pkm1 (fbPkm2 → 1) mitigated ventricular dilation, wall thinning, and losses in ejection fraction caused by MI. Despite these salutary effects, fbPkm2 → 1 switching did not alter cardiac fibrosis in vivo, nor did it affect collagen production, cytokine or chemokine secretion, myofibroblast differentiation markers, or transcriptional regulation in vitro. Nevertheless, Pkm2 → 1 splice variant switching increased myofibroblast contractile activity as well as influenced the metabolic phenotype of fibroblasts, as shown by increased pyruvate kinase activity, higher mitochondrial respiratory capacity, and elevation in glycolytic intermediate abundance. Despite these changes, Pkm2 → 1 switching had relatively minor effects on glucose carbon fate, as determined by stable isotope-resolved metabolomics. Nevertheless, these metabolic data demonstrate that cardiac fibroblasts exhibit minimal glucose-supported de novo glycine synthesis in vitro, yet possess high hexosamine and glucuronate biosynthetic pathway activity. Collectively, these findings reveal that fibroblast PKM isoforms influence post-MI remodeling, highlighting pyruvate kinase as a potential therapeutic target.