Decreased Mitochondrial Unfolded Protein Response (UPRmt) in HFpEF

Mitochondrial dysfunction is a feature of heart failure with preserve ejection fraction (HFpEF). Central infusion of Ang II causes sympatho‐excitation and hypertension with consequent HFpEF. The UPRmt activation is a retrograde mitochondrial stress response that promote recovery of defective mitochondria. UPRmt is activated by misfolded accumulation of nuclear‐encoded mitochondrial proteins in cytosol and mitochondria. Despite this information, the mechanisms of UPRmt and mitochondrial dysfunction are largely unknown in the HFpEF heart. We hypothesized that concomitant sympathoexcitation with hypertension downregulates cardiac UPRmt leads to cardiac remodeling in HFpEF. Male Sprague‐Drawly rats (250‐300g) were subjected to central infusion of either Ang II (at 20 ng/min, 0.5 μl/h, ICV) or isotonic saline (0.5 μl/h, ICV, control) through osmotic mini‐pumps for 14 days. This leads to concomitant sympathetic overstimulation and systemic hypertension with myriad features of HFpEF. Transthoracic echocardiography and haemodynamic recordings were performed at day 14 post ICV infusion. UPRmt, mitochondrial injury, and cardiac remodeling were assessed using, whole‐tissue, cytosolic, and isolated mitochondrial protein Western immunoblots, cytochemistry, and histology. Sympatho‐excitatory effect on UPRmt was examined in vitro using norepinephrine (NE) and H9c2 cardiomyocytes. The HFpEF rats showed significant diastolic dysfunction indicated by reduced E/A (HFpEF: 1.2 ± 0.1 vs Con: 1.5 ± 0.2) with preserved left ventricular ejection fraction (HFpEF: 75 ± 3% vs Con: 77 ± 4%). Histological evaluation showed increased cardiomyocyte hypertrophy (HFpEF: 46.3 ± 5 vs Con: 36.9 ± 6) and cardiac fibrosis (HFpEF: 4.3 ± 0.2 vs Con: 2.1 ± 0.3). Measurement of ATF5 (Activating Transcription Factor 5), a key UPRmt activation marker was decreased in HFpEF cytosol and mitochondria, while mitochondrial chaperonin HSP60 (heat‐shock protein 60) was decreased in cytosol but increased in HFpEF mitochondria. Simultaneously, there was increased accumulation of oxidative phosphorylation (OXPH) Complex I, IV, and V misfolded subunits in the mitochondrial matrix. Furthermore, YME1L1, a mitochondrial matrix metalloprotease of misfolded protein degradation is reduced in HFpEF mitochondria. Concomitantly, there was increased mitochondrial ROS, reduced Mn‐SOD, and increased autophagy markers p62‐SQSTM1 and LC3B‐II in HFpEF hearts. Notably, a reduced mitochondrial biogenesis and fusion, but increased fission, indicates a lack of UPRmt stress activated mitochondrial recovery in HFpEF heart. Our in vitro data corroborated a reduced UPRmt in response to NE treatment associated with mitochondrial depolarization and increased mitochondrial ROS level. In conclusion, concomitant sympatho‐excitation and systemic hypertension contributes to reduced UPRmt, which is a unique feature of HFpEF heart. A lack of UPRmt activation fails to execute defective mitochondrial recovery in HFpEF heart conceivably driving HFpEF cardiac remodeling. This study identifies the key intermediary links of UPRmt downregulation in a potential HFpEF rat model. Therefore, boosting UPRmt can potentially have a therapeutic benefit for clinical HFpEF.