Identifying the mechanisms that define the mitochondrial unfolded protein response in the stressed heart

Mitochondrial dysfunction is a hallmark of cardiovascular diseases, as it contributes to oxidative stress, energy deficits, calcium dysregulation, and cardiomyocyte mortality. Thus, it is considered as a prospective therapeutic target. Currently, there are no therapies that specifically improve or maintain mitochondrial homeostasis and functions in the failing heart. The mitochondrial unfolded protein response (UPRmt) is a protective homeostatic mechanism that was initially described as a transcriptional response to damaged mitochondrial proteins. Recent reports have also demonstrated engagement of the UPRmt upon a wider range of mitochondrial insults, including increased mitochondrial ROS levels, mitochondrial membrane depolarization, and reduced mitochondrial protein import efficiency. Previously, we demonstrated UPRmt activation in the pressure-overloaded human heart, and its role in preserving cardiac function during chronic pressure overload. However, the molecular mechanisms behind UPRmt-mediated cardioprotection are poorly understood. Activating transcription factor 5 (ATF5) is the main transcriptional regulator of mammalian UPRmt. However, due to its oncogenic role in cancer, ATF5 cannot be targeted for therapeutic reasons. The aim of our study is to identify ATF5-regulated genes that could serve as novel targets to activate and/or enhance the UPRmt in pathophysiological conditions. For this, we performed RNA sequencing in stressed cardiomyocytes with activated UPRmt, with/without expression of ATF5.
Adding to the role of ATF5 in UPRmt regulation, studies in C. elegans and our preliminary evidence suggest a synergy between ATF5 and the NRF2 transcription factor in UPRmt regulation. NRF2, is the major regulator of the cellular anti-oxidant response and is known for its protective role in the stressed heart. We show that NRF2 regulates activation of the UPRmt via its interactions with the mitochondrial phosphatase PGAM5 at the outer mitochondrial membrane. Disruption of the PGAM5-NRF2 complex resulted in Nrf2 translocation to the nucleus during mitochondrial stress, enhancing the levels of UPRmt. Further studies will determine the role of ATF5 and NRF2 in the cardioprotective effects of the UPRmt.