Fibrinogen-like protein 2 contributes to normal murine cardiomyocyte maturation and heart development

New Findings What is the central question of this study? What is the role of fibrinogen-like protein 2 (FGL2) in murine cardiomyocyte maturation? What is the main finding and its importance? This is the first study showing both global Fgl2 knockout and cardiac-specific FGL2 deletion trigger early death and dilated cardiomyopathy. By using an adeno-associated virus (AAV)-mediated CRISPR/Cas9-based somatic mutagenesis system, it was demonstrated that cardiac-specific FGL2 depletion induces ventricular dilatation and remodelling, and disrupts the normal hypertrophic growth and polyploidization of cardiomyocytes. In addition, it was shown that modulation of signal transducer and activator of transcription 3, extracellular signal-regulated kinases 1 and 2 and fibroblast growth factor 2 signalling is associated with loss-of-FGL2-mediated cardiac dysfunction. These results suggest FGL2 is an important determinant of cardiomyocyte maturation. In the first few weeks after birth in altricial mammals, postnatal cardiomyocytes (CMs) undergo dramatic changes, including cell volume enlargement, cell cycle withdrawal and polyploidization to become mature CMs. Aberrations in this process could disrupt the essential contractility and synchronization of adult CMs, leading to various heart diseases. However, the mechanism of CM maturation is poorly understood. Fibrinogen-like protein 2 (FGL2) is an immune coagulant which participates in maturation of multiple cell types. However, little evidence exists regarding a role of FGL2 in CM maturation. In this study, we observed that global Fgl2(-/-) pups had high lethality and suffered from cardiac dysfunction before P28. To further confirm the phenotype and study the mechanisms upon FGL2 deficiency, we used an adeno-associated virus (AAV)-mediated CRISPR/Cas9-based somatic mutagenesis system to generate loss-of-function mutations of Fgl2 specifically in CMs. We designed two guide RNAs (gRNAs) exclusively targeting Fgl2 exon1 and produced Fgl2-gRNA AAV9 to deliver to neonatal Cas9 mice. Here, we demonstrated the efficient FGL2 depletion in the heart after Fgl2-gRNA AAV9 delivery. Consistent with the findings in global Fgl2(-/-) mice, we observed AAV9-mediated FGL2 depletion triggered early death and dilated cardiomyopathy. In addition, FGL2 depletion perturbed the normal hypertrophic growth and polyploidization of maturing CMs. Furthermore, we found modulation of signal transducer and activator of transcription 3, extracellular signal-regulated kinases 1 and 2 and fibroblast growth factor 2 signalling was associated with FGL2 deficiency-mediated cardiac dysfunction. Here, we demonstrate the successful depletion of FGL2 in maturing CMs in vivo and show FGL2 is an important determinant for normal CM maturation.