Cardiomyocyte (CM) maturation is an essential step in heart development that prepares the organ for robust and sustainable contractions. However, very little is known about the contribution of CM maturation to cardiac pathogenesis, partly due to the paucity of basic knowledge about maturation. For example, although calcium plays well-established roles in cardiac stress responses, whether normal calcium oscillations are required for CM maturation has not been examined. To address this question, we created mice with mosaic inactivation of ryanodine receptor 2 (RYR2), the central channel that mediates calcium release from sarcoplasmic/endoplasmic reticulum (SR/ER), in CMs. RYR2 was depleted in a small fraction of neonatal CMs by adeno-associated virus (AAV)-mediated Cas9-based somatic mutagenesis (CASAAV). The majority of CMs intentionally retained RYR2 expression, which circumvented cardiac dysfunction and death that would result from widespread calcium transient ablation. In RYR2-depleted CMs, sarcomere expansion, transverse-tubule formation, mitochondrial metabolism and maturational hypertrophy were impaired. Strikingly, these maturation defects were associated with unfolded protein response, with the activation of three major ER stress effectors: spliced Xbp1 (XBP1s), ATF6 N-terminus (ATF6N), and ATF4. Tauroursodeoxycholic acid, an ER stress-relieving drug, partially rescued the maturation defects in RYR2-depleted CMs. Over expression (OE) of XBP1s, ATF6N or ATF4 each impaired CM maturation, with phenotypic severity ranked as ATF4>XBP1s>ATF6N. ATF4 OE in neonatal CMs triggered both ultrastructural and transcriptomic changes highly correlated with RYR2 depletion phenotypes. Interestingly, ATF4 OE altered distinct transcriptomic programs in neonatal vs. adult CMs and caused ultrastructural defects only at the neonatal stage. No terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining was detected upon RYR2 depletion or ATF4 OE, excluding the contribution of apoptosis to maturation defects. Collectively, these data indicate that RYR2-mediated calcium handling prevents an adverse and stage-specific impact of ER stress on CM maturation, which lead to CM dysfunction.
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