Zhang, B.; Juda, M.; Huang, J.; Chapski, D. J.; Arrieta, A.; Rodney, I.; Liu, J.; Leone, M.; Hsiai, T. K.; Wang, Y.; Yokota, T.

Background: Differential growth between the left (LV) and right ventricles (RV) is a cornerstone of normal heart morphogenesis after birth, leading to the relatively larger and dominant LV over RV in the adult heart regarding size and function. Yet, little is known about the factors that regulate this chamber-specific growth. Methods: We used both loss- and gain-of-function mouse models, achieved through genetic or pharmacological manipulation of IRE1alpha or Xbp1 in cardiomyocytes. We also used primary cultured neonatal cardiomyocytes to explore the roles of IRE1alpha, spliced Xbp1 (sXbp1: activated form), and newly identified sXbp1 downstream targets. In addition, we generated heart-specific mosaic mutant mouse models using CRISPR/Cas9/AAV9-based somatic mutagenesis to elucidate the roles of sXbp1 downstream targets in cardiomyocytes. Results: Pharmacological inactivation of IRE1alpha and genetic depletion of Xbp1 resulted in a smaller LV size, due to decreased cardiomyocyte proliferation and hypertrophic growth, as well as increased cardiomyocyte death. These effects were not observed in the RV. Cardiomyocyte-specific induction of IRE1alpha or sXbp1 led to increased ventricular size in both ventricles, through enhanced cardiomyocyte proliferation and hypertrophic growth in both LV and RV, and reduced apoptosis in the RV. We identified two ER resident transmembrane proteins, Vimp and Rpn2, as direct binding partners of sXbp1 in targeted gene regulation at the chromatin level. CRISPR/Cas9/AAV9-based somatic mutagenesis mouse models for Vimp and Rpn2 revealed that both genes regulate cardiomyocyte proliferation, hypertrophic growth, and death. We also observed accumulated misfolded proteins in these two mutant hearts. Conclusions: We demonstrate that the IRE1alpha-Xbp1-Vimp/Rpn2 axis regulates differential ventricular size between LV and RV during postnatal development by orchestrating cardiomyocyte proliferation, hypertrophic growth, and death through regulating protein homeostasis.