The fibrinogen-like protein 2 contributes to normal murine cardiomyocyte maturation and heart development.

New findings What is the central question of this study? Our goal was to explore the role of 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 AAV-mediated CRISPR/Cas9-based somatic mutagenesis system, we demonstrate cardiac-specific FGL2 depletion induces ventricular dilatation and remodeling, and disrupts the normal hypertrophic growth and polyploidization of CMs. In addition, we show modulation of STAT3, ERK1/2, and FGF2 signaling is associated with loss-of-FGL2 mediated cardiac dysfunction. These results suggest FGL2 is an important determinant for CM maturation. Abstract 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 the 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 AAV-mediated CRISPR/Cas9-based somatic mutagenesis system to generate loss-of-function mutations of Fgl2 specifically in CMs. We designed two gRNAs exclusively targeting Fgl2 exon1 and produced the 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 STAT3, ERK1/2, and FGF2 signaling 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. This article is protected by copyright. All rights reserved.