Stress-Induced Proteasome Sub-Cellular Translocation in Cardiomyocytes Causes Altered Intracellular Calcium Handling and Arrhythmias

The ubiquitin-proteasome system (UPS) is an essential mechanism responsible for the selective degradation of substrate proteins via their conjugation with ubiquitin. Since cardiomyocytes have very limited self-renewal capacity, as they are prone to protein damage due to constant mechanical and metabolic stress, the UPS has a key role in cardiac physiology and pathophysiology. While altered proteasomal activity contributes to a variety of cardiac pathologies, such as heart failure and ischemia/reperfusion injury (IRI), the environmental cues affecting its activity are still unknown, and they are the focus of this work. Following a recent study by Ciechanover's group showing that amino acid (AA) starvation in cultured cancer cell lines modulates proteasome intracellular localization and activity, we tested two hypotheses in human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs, CMs): (i) AA starvation causes proteasome translocation in CMs, similarly to the observation in cultured cancer cell lines; (ii) manipulation of subcellular proteasomal compartmentalization is associated with electrophysiological abnormalities in the form of arrhythmias, mediated via altered intracellular Ca2+ handling. The major findings are: (i) starving CMs to AAs results in proteasome translocation from the nucleus to the cytoplasm, while supplementation with the aromatic amino acids tyrosine (Y), tryptophan (W) and phenylalanine (F) (YWF) inhibits the proteasome recruitment; (ii) AA-deficient treatments cause arrhythmias; (iii) the arrhythmias observed upon nuclear proteasome sequestration(-AA+YWF) are blocked by KB-R7943, an inhibitor of the reverse mode of the sodium-calcium exchanger NCX; (iv) the retrograde perfusion of isolated rat hearts with AA starvation media is associated with arrhythmias. Collectively, our novel findings describe a newly identified mechanism linking the UPS to arrhythmia generation in CMs and whole hearts.

Single-Cell Analysis of Contractile Forces in iPSC-Derived Cardiomyocytes: Paving the Way for Precision Medicine in Cardiovascular Disease

Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) hold enormous potential in cardiac disease modeling, drug screening, and regenerative medicine. Furthermore, patient-specific iPSC-CMS can be tested for personalized medicine. To provide a deeper understanding of the contractile force dynamics of iPSC-CMs, we employed Atomic Force Microscopy (AFM) as an advanced detection tool to distinguish the characteristics of force dynamics at a single cell level. We measured normal (vertical) and lateral (axial) force at different pacing frequencies. We found a significant correlation between normal and lateral force. We also observed a significant force-frequency relationship for both types of forces. This work represents the first demonstration of the correlation of normal and lateral force from individual iPSC-CMs. The identification of this correlation is relevant because it validates the comparison across systems and models that can only account for either normal or lateral force. These findings enhance our understanding of iPSC-CM properties, thereby paving the way for the development of therapeutic strategies in cardiovascular medicine.

PTPMT1 protects cardiomyocytes from necroptosis induced by γ-ray irradiation through alleviating mitochondria injury

Radiation-induced heart disease (RIHD) progresses over time and may manifest decades after the initial radiation exposure, which is associated with significant morbidity and mortality. The clinical benefit of radiotherapy is always counterbalanced by an increased risk of cardiovascular events in survivors. There is an urgent need to explore the effect and the underlying mechanism of radiation-induced heart injury. Mitochondrial damage widely occurs in irradiation-induced injury, and mitochondrial dysfunction contributes to necroptosis development. Experiments were performed using induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) and rat H9C2 cells to investigate the effect of mitochondrial injury on necroptosis in irradiated cardiomyocytes and to further elucidate the mechanism underlying radiation-induced heart disease and discover possible preventive targets. After γ-ray irradiation, the expression levels of necroptosis markers were increased, along with higher oxidative stress and mitochondrial injury. These effects could be abated by overexpression of protein tyrosine phosphatase, mitochondrial 1 (PTPMT1). Inhibiting oxidative stress or increasing the expression of PTPMT1 could protect against radiation-induced mitochondrial injury and then decrease the necroptosis of cardiomyocytes. These results suggest that PTPMT1 may be a new target for the treatment of radiation-induced heart disease.NEW & NOTEWORTHY Effective strategies are still lacking for treating RIHD, with unclear pathological mechanisms. In cardiomyocytes model of radiation-induced injuries, we found γ-ray irradiation decreased the expression of PTPMT1, increased oxidative stress, and induced mitochondrial dysfunction and necroptosis in iPSC-CMs. ROS inhibition attenuated radiation-induced mitochondrial damage and necroptosis. PTPMT1 protected cardiomyocytes from necroptosis induced by γ-ray irradiation by alleviating mitochondrial injury. Therefore, PTPMT1 might be a potential strategy for treating RIHD.