ER Stress-Response Signaling Regulates Chamber-Specific Growth between Right and Left Ventricles during Postnatal Development.

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. We used both loss-and gain-of-function mouse models, achieved through genetic or pharmacological manipulation of IRE1α or Xbp1 in cardiomyocytes. We also used primary cultured neonatal cardiomyocytes to explore the roles of IRE1α, 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. Pharmacological inactivation of IRE1α 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 IRE1α 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 IRE1α-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. What Is New: IRE1α-Xbp1 axis is dominantly activated in the LV cardiomyocyte during the postnatal period in mouse heart.IRE1α-Xbp1 mediated ER stress signaling increases cardiomyocyte proliferation and hypertrophic growth and decreases apoptosis in the postnatal period.Activated Xbp1 directly regulates LV-specific cardiomyocyte protein homeostasis via interaction with ER membrane targeted Vimp and Rpn2.What Are the Clinical Implications?: Differential heart growth patterns between the LV and RV are critical for normal morphogenesis and function of each ventricle.Control of protein homeostasis by modulating ER stress signaling could be a potential therapeutic approach for single-chamber heart diseases.