PO-01-124 THE CALCIUM HANDLING MACHINERY IS REMODELED IN FRIEDREICH’S ATAXIA

Abstract

Friedreich's ataxia (FA) is an inherited neurodegenerative disorder that causes progressive nervous system damage. FA is caused by a deficiency in the frataxin protein due to an expansion of GAA repeats in the first intron of the frataxin gene. Reduced frataxin protein expression is thought to negatively affect mitochondrial function, leading to increased oxidative damage. Although FA is considered a neurodegenerative disorder, it is not completely understood why most patients present with left ventricular systolic dysfunction and die from cardiac events. The objective of our study is to determine whether the abnormal calcium handling machinery is a molecular mechanism that perpetuates cardiac dysfunction in FA. We used the frataxin knock-out (KO) mouse model of FA as well as human heart samples from donors with FA and from unaffected donors. The expression of calcium handling machinery proteins was assessed with proteomics and western blot. In left ventricular myocytes from KO and WT mice, the IonOptix system was used for calcium imaging, the seahorse assay was utilized to measure oxygen consumption rate (OCR) and confocal imaging was used to quantify the mitochondrial membrane potential (Δψm), and reactive oxygen species (ROS). ECG and echocardiography were used to assess cardiac function in the mice. In the proteomic analysis and western blot, SERCA2, Ryr2 and CaMKII were significantly downregulated, and the mitochondrial calcium handling proteins voltage-dependent anion channel and mitochondrial calcium uniporter were significantly upregulated in left ventricular tissue from humans with FA and from KO mice. On the ECG, the RR, PR, QRS, and QTc intervals were significantly longer in the KO mice. Echocardiography showed that the ejection fraction and fractional shortening were significantly decreased and left ventricular wall thickness and diameter were significantly increased in the KO mice. Moreover, the calcium transient decay was significantly prolonged in ventricular myocytes from KO mice compared to WT. Additionally, the Δψm was significantly depolarized, the OCR was significantly decreased, and ROS levels were significantly elevated in left ventricular myocytes from KO compared to WT. The development of left ventricular contractile dysfunction in FA is associated with reduced expression of calcium handling proteins, abnormal calcium cycling, and mitochondrial dysfunction.