Deficiency of mitochondrial calcium uniporter abrogates iron overload-induced cardiac dysfunction by reducing ferroptosis

Iron overload associated cardiac dysfunction remains a significant clinical challenge whose underlying mechanism(s) have yet to be defined. We aim to evaluate the involvement of the mitochondrial Ca2+ uniporter (MCU) in cardiac dysfunction and determine its role in the occurrence of ferroptosis. Iron overload was established in control (MCUfl/fl) and conditional MCU knockout (MCUfl/fl-MCM) mice. LV function was reduced by chronic iron loading in MCUfl/fl mice, but not in MCUfl/fl-MCM mice. The level of mitochondrial iron and reactive oxygen species were increased and mitochondrial membrane potential and spare respiratory capacity (SRC) were reduced in MCUfl/fl cardiomyocytes, but not in MCUfl/fl-MCM cardiomyocytes. After iron loading, lipid oxidation levels were increased in MCUfl/fl, but not in MCUfl/fl-MCM hearts. Ferrostatin-1, a selective ferroptosis inhibitor, reduced lipid peroxidation and maintained LV function in vivo after chronic iron treatment in MCUfl/fl hearts. Isolated cardiomyocytes from MCUfl/fl mice demonstrated ferroptosis after acute iron treatment. Moreover, Ca2+ transient amplitude and cell contractility were both significantly reduced in isolated cardiomyocytes from chronically Fe treated MCUfl/fl hearts. However, ferroptosis was not induced in cardiomyocytes from MCUfl/fl-MCM hearts nor was there a reduction in Ca2+ transient amplitude or cardiomyocyte contractility. We conclude that mitochondrial iron uptake is dependent on MCU, which plays an essential role in causing mitochondrial dysfunction and ferroptosis under iron overload conditions in the heart. Cardiac-specific deficiency of MCU prevents the development of ferroptosis and iron overload-induced cardiac dysfunction.

Abstract 15737: Deficiency of Mitochondrial Calcium Uniporter Protects Mouse Hearts From Iron Overload by Attenuating Ferroptosis

Iron (Fe) plays essential roles in many physiological processes. Hereditary hemochromatosis and frequent blood transfusions result in iron overload (IO) and dysfunction of iron-deposited organs including the heart. Although IO-induced cardiomyopathy remains a significant clinical challenge, the underlying mechanism is not well defined. In the present study, we aim to assess the involvement of the mitochondrial Ca uniporter (mCU) in IO-induced cardiac contractile dysfunction and ferroptosis. In a chronic IO model, after receiving Fe (dextran, i.p. 0.6 mg/g, 3 days/wk) for 6 weeks, systolic function (LVEF and LVFS) was reduced in mCU +/+ (WT) compared to mCU -/- (mCU KO). This observation was confirmed in isolated ventricular myocytes where we similarly detected a significant decrease in cell shortening in WT, but not mCU KO myocytes. We found lower Fe levels in mitochondria from mCU KO myocytes compared to WT, while observing the same level of Fe deposition in heart tissue from both groups. The mitochondrial ROS level was lower in mCU KO myocytes vs. WT. Long term cardiac dysfunction may result in myocyte cell death, however, we did not detect apoptosis (TUNEL) in either mCU KO or in WT hearts with cardiac dysfunction after chronic IO. The lipid oxidation level was increased with Fe load suggesting ferroptosis may be involved in IO-induced cardiomyopathy, which was supported by the observation that administration of the selective ferroptosis inhibitor ferrostatin-1 reduced lipid oxidation (4-HNE) and maintained heart function. To further determine the role of mCU in IO-mediated ferroptosis in the heart, we used isolated myocytes from WT and mCU KO and conducted Live/Dead cell viability assays. While Fe (FAC, 0.1-5 mM) induced ferroptosis in a dose-dependent manner, it was prevented by ferrostatin-1 (10 μM), the Fe chelator 2,2'-bipyridyl (2 mM), and MitoTEMPO (5 μM), respectively. Fe-induced ferroptosis in mCU KO myocytes did not occur while the lipid oxidation level (Liperfluo) remained low. In conclusion, mitochondrial Fe uptake, presumably mediated by mCU, accounts for the molecular/cellular mechanisms for Fe-induced myocyte ferroptosis and cardiomyopathy. Cardiac-specific deficiency of mCU prevents the development of Fe induced cardiomyopathy.

Abstract 507: Activation of Transient Receptor Potential Canonical Channel Currents in Iron-Overloaded Cardiac Myocytes

Background: Iron (Fe) overload cardiomyopathy is the leading cause of death in hemochromatotic patients, yet the mechanistic insight is still incomplete and controversial. We investigated alterations of action potentials (APs), ionic currents, and intracellular Ca 2+ (Ca 2+ i ) in Fe-loaded mouse cardiomyocytes, as well as functional impacts of Fe overload on single-cell contraction and whole-heart arrhythmias.

Methods: Cardiomyocytes were isolated from left ventricles of mouse hearts and were superfused with Fe 3+ /8-hydroxyquinoline complex (5-100 μM). APs, L-type Ca 2+ currents (I Ca,L ), total outward K + currents (I K ), and transient receptor potential canonical (TRPC) channel currents were recorded by the patch-clamp technique. Ca 2+ i was evaluated by using Fluo-4. Cell contraction was measured by a video-based edge detection system. Arrhythmias were evaluated in Langendorff-perfused hearts under S 1 -S 2 stimulation protocol.

Results: Persistent Fe (15 μM) treatment prolonged AP duration at 90% repolarization (APD 90 : 46.8 ± 2.8 vs. 203.6 ± 63.4 ms, p<0.05), induced early and delayed afterdepolarizations (EADs: 0 % vs. 45.0 ± 15.0 %; DADs: 4.3 ± 1.4 vs. 27.0 ± 7.0 %, p<0.05, respectively) in mouse cardiomyocytes. Consistently, arrhythmia incidence was increased in Fe 3+ /8-HQ-perfused hearts. Fe treatment decreased peak I Ca,L (16.5 ± 1.7 vs.11.4 ± 1.3 pA/pF, p<0.01) and I K (59.2 ± 3.3 vs. 50.4 ± 3.0 pA/pF, p<0.01), altered Ca 2+ i transient patterns and decreased contractility (4.8 ± 0.5 vs. 3.5 ± 0.4%, p<0.01). During the late phase of Fe treatment, fast Ca 2+ waves and sustained depolarization were induced to generate a secondary (shallow) resting membrane potential (RMP: from -68.8 ± 0.6 to -25.0 ± 3.7 mV) where the myocytes became unexcitable. Gadolinium, a TRPC channel blocker, abolished fast Ca 2+ waves and reversed RMP to the deep level (-62.9 ± 3.5 mV). The involvement of TRPC activation was determined for the first time by recording TRPC current and assessing the effect of functional TRPC channel antibodies.

Conclusions: In mouse cardiomyocytes, Fe overload induced arrhythmogenic APD prolongation and EADs/DADs, aberrant Ca 2+ i dynamics, and impaired contractility. The activation of TRPC channels accounts for an important underlying mechanism.

Vagus nerve stimulation exerts cardioprotection against myocardial ischemia/reperfusion injury predominantly through its efferent vagal fibers

Vagus nerve stimulation (VNS) has been shown to exert cardioprotection against myocardial ischemia/reperfusion (I/R) injury. However, whether the cardioprotection of VNS is mainly due to direct activation through its ipsilateral efferent fibers (motor) rather than indirect effects mediated by the afferent fibers (sensory) have not been clearly understood. We hypothesized that VNS exerts cardioprotection predominantly through its efferent vagal fibers. Thirty swine (30-35 kg) were randomized into five groups: I/R no VNS (I/R), and left mid-cervical VNS with both vagal trunks intact (LC-VNS), with left vagus nerve transection (LtVNX), with right vagus nerve transection (RtVNX) and with atropine pretreatment (Atropine), respectively. VNS was applied at the onset of ischemia (60 min) and continued until the end of reperfusion (120 min). Cardiac function, infarct size, arrhythmia score, myocardial connexin43 expression, apoptotic markers, oxidative stress markers, inflammatory markers (TNF-α and IL-10) and cardiac mitochondrial function, dynamics and fatty acid oxidation (MFN2, OPA1, DRP1, PGC1α and CPT1) were determined. LC-VNS exerted cardioprotection against myocardial I/R injury via improvement of mitochondrial function and dynamics and shifted cardiac fatty acid metabolism toward beta oxidation. However, LC-VNS and LtVNX, both efferent vagal fibers are intact, produced more profound cardioprotection, particularly infarct size reduction, decreased arrhythmia score, oxidative stress and apoptosis and attenuated mitochondrial dysfunction compared to RtVNX. These beneficial effects of VNS were abolished by atropine. Our findings suggest that selective efferent VNS may potentially be effective in attenuating myocardial I/R injury. Moreover, VNS required the contralateral efferent vagal activities to fully provide its cardioprotection.

Restoring the impaired cardiac calcium homeostasis and cardiac function in iron overload rats by the combined deferiprone and N-acetyl cysteine

Intracellular calcium [Ca²⁺]_i dysregulation plays an important role in the pathophysiology of iron overload cardiomyopathy. Although either iron chelators or antioxidants provide cardioprotection, a comparison of the efficacy of deferoxamine (DFO), deferiprone (DFP), deferasirox (DFX), N-acetyl cysteine (NAC) or a combination of DFP plus NAC on cardiac [Ca²⁺]_i homeostasis in chronic iron overload has never been investigated. Male Wistar rats were fed with either a normal diet or a high iron (HFe) diet for 4 months. At 2 months, HFe rats were divided into 6 groups and treated with either a vehicle, DFO (25 mg/kg/day), DFP (75 mg/kg/day), DFX (20 mg/kg/day), NAC (100 mg/kg/day), or combined DFP plus NAC. At 4 months, the number of cardiac T-type calcium channels was increased, whereas cardiac sarcoplasmic-endoplasmic reticulum Ca²⁺ ATPase (SERCA) was decreased, leading to cardiac iron overload and impaired cardiac [Ca²⁺]i homeostasis. All pharmacological interventions restored SERCA levels. Although DFO, DFP, DFX or NAC alone shared similar efficacy in improving cardiac [Ca²⁺]i homeostasis, only DFP + NAC restored cardiac [Ca²⁺]i homeostasis, leading to restoring left ventricular function in the HFe-fed rats. Thus, the combined DFP + NAC was more effective than any monotherapy in restoring cardiac [Ca²⁺]_i homeostasis, leading to restored myocardial contractility in iron-overloaded rats.

Effects of iron overload, an iron chelator and a T-Type calcium channel blocker on cardiac mitochondrial biogenesis and mitochondrial dynamics in thalassemic mice

Although cardiac mitochondrial dysfunction is involved in the pathophysiology of iron-overload cardiomyopathy, the precise mechanisms of iron-induced mitochondrial dysfunction, and the roles of the iron chelator deferiprone and the T-type calcium channel blocker efonidipine on cardiac mitochondrial biogenesis in thalassemic mice are still unknown. β-thalassemic (HT) mice were fed with a normal diet (ND) or a high iron-diet (FE) for 90 days. Then, the FE-fed mice were treated with deferiprone (75mg/kg/day) or efonidipine (4mg/kg/day) for 30 days. The hearts were used to determine cardiac mitochondrial function, biogenesis, mitochondrial dynamics and protein expressions for oxidative phosphorylation (OXPHOS) and apoptosis. ND-fed HT mice had impaired heart rate variability (HRV), increased mitochondrial dynamic proteins and caspase-3, compared with ND-fed wild-type mice. Iron overload led to increased plasma non-transferrin bound iron, oxidative stress, and the impairments of HRV and left ventricular function, cardiac mitochondrial function and mitochondrial dynamics, and decreased complex IV in thalassemic mice. Our results suggested that deferiprone and efonidipine treatment showed similar benefit in attenuating cardiac iron deposit and oxidative stress, and improved cardiac mitochondrial function, leading to improved left ventricular function, without altering the cardiac mitochondrial biogenesis, and apoptosis proteins in iron-overload thalassemic mice.