Introduction: Proteostasis is the ability of the cell to balance the requirement of folding nascent proteins and degrading terminally misfolded ones in an efficient manner. A major site of proteostasis in cardiac myocytes is the endoplasmic/sarcoplasmic reticulum (ER/SR), as it is the location of a majority of protein synthesis and folding. Our lab previously demonstrated that several adaptive genes critical for ER/SR proteostasis are downregulated during postnatal development in animal models and patients with ischemic heart disease. Despite their clinical implications, a comprehensive panel of adaptive genes responsible for maintaining cardiac myocyte proteostasis has not been fully characterized in the heart as a function of age.
Hypothesis: Compared to neonatal hearts, adult hearts are more prone to proteotoxic imbalance and cell death during ischemic injury. This imbalance is due to a suppression of the adaptive proteostasis network in the heart that progresses during postnatal development.
Methods: Cultured neonatal ventricular myocytes (NVM) and adult ventricular myocytes (AVM) were subjected to quantitative PCR and immunoblotting to examine baseline transcript and protein levels, respectively, of an adaptive gene panel known to be associated with maintaining proteostasis. These invitro data were corroborated with RNA and protein isolated from neonatal and adult mouse ventricles. Comparative viability was studied in NVM and AVM in response to simulated ischemia (sI), which is known to impair proteostasis in cultured cardiac myocytes.
Results: Compared with neonatal hearts, adult hearts and AVM displayed a significant decrease in baseline transcript and protein levels of a panel of 19 genes associated with proteostasis in cardiac myocytes. When challenged with sI, compared to NVM, AVM exhibited an impaired ability to upregulate adaptive proteostasis genes, which was associated with reduced viability.
Conclusion: Adaptive genes involved in protein folding in the heart are suppressed postnatally, leading to an imbalance in proteostasis and promotion of cell death during ischemia in the adult heart. We posit that reversal of this maladaptive genetic regulation presents a therapeutic avenue for the treatment of ischemic heart disease.