Loss of Mitochondrial Ca ²⁺ Uniporter Limits Inotropic Reserve and Provides Trigger and Substrate for Arrhythmias in Barth Syndrome Cardiomyopathy

Abstract

Background: Barth syndrome (BTHS) is caused by mutations of the gene encoding tafazzin, which catalyzes maturation of mitochondrial cardiolipin and often manifests with systolic dysfunction during early infancy. Beyond the first months of life, BTHS cardiomyopathy typically transitions to a phenotype of diastolic dysfunction with preserved ejection fraction, blunted contractile reserve during exercise and arrhythmic vulnerability. Previous studies traced BTHS cardiomyopathy to mitochondrial formation of reactive oxygen species (ROS). Since mitochondrial function and ROS formation are regulated by excitation-contraction (EC) coupling, integrated analysis of mechano-energetic coupling is required to delineate the pathomechanisms of BTHS cardiomyopathy. Methods: We analyzed cardiac function and structure in a mouse model with global knockdown of tafazzin ( Taz -KD) compared to wild-type (WT) littermates. Respiratory chain assembly and function, ROS emission, and Ca 2+ uptake were determined in isolated mitochondria. EC coupling was integrated with mitochondrial redox state, ROS, and Ca 2+ uptake in isolated, unloaded or preloaded cardiac myocytes, and cardiac hemodynamics analyzed in vivo . Results: Taz -KD mice develop heart failure with preserved ejection fraction (>50%) and age-dependent progression of diastolic dysfunction in the absence of fibrosis. Increased myofilament Ca 2+ affinity and slowed cross-bridge cycling caused diastolic dysfunction, partly compensated by accelerated diastolic Ca 2+ decay through preactivated sarcoplasmic reticulum Ca 2+ ATPase (SERCA). Taz deficiency provoked heart-specific loss of mitochondrial Ca 2+ uniporter (MCU) protein that prevented Ca 2+ -induced activation of the Krebs cycle during β-adrenergic stimulation, oxidizing pyridine nucleotides and triggering arrhythmias in cardiac myocytes. In vivo , Taz -KD mice displayed prolonged QRS duration as a substrate for arrhythmias, and a lack of inotropic response to β-adrenergic stimulation. Cellular arrhythmias and QRS prolongation, but not the defective inotropic reserve, were restored by inhibiting Ca 2+ export via the mitochondrial Na + /Ca 2+ exchanger. All alterations occurred in the absence of excess mitochondrial ROS in vitro or in vivo . Conclusions: Downregulation of MCU, increased myofilament Ca 2+ affinity, and preactivated SERCA provoke mechano-energetic uncoupling that explains diastolic dysfunction and the lack of inotropic reserve in BTHS cardiomyopathy. Furthermore, defective mitochondrial Ca 2+ uptake provides a trigger and a substrate for ventricular arrhythmias. These insights can guide the ongoing search for a cure of this orphaned disease.

Cardiolipin Regulates Mitochondrial Ultrastructure and Function in Mammalian Cells

“…Abnormal CL biosynthesis and remodeling have been associated with altered cristae morphology and disrupted mitochondrial-shaping proteins, affecting the number, length, and organizing pattern of the cristae, and ultimately the shape of mitochondria and the progress of OXPHOS in both human patients and animal models [ 13 , 14 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 ]. Mitochondrial ATP synthase is a polymerase complex consisting of two functional domains: F0, located in the IMM, and F1, located in the mitochondrial matrix.…”

Tachycardiomyopathy entails a dysfunctional pattern of interrelated mitochondrial functions

“…The two columns on the left side summarise our results on mitochondrial functions in tachycardiomyopathy. The three columns on the right side compile the current knowledge on main mitochondrial functions in dilated cardiomyopathy (DCM) as well as in heart failure due to coronary artery disease (CAD) or pressure-overload [ 1 , 4 , 7 , 10 , 14 – 16 , 24 – 29 , 37 , 38 , 41 , 48 – 50 , 52 – 55 , 59 , 60 , 62 – 64 , 69 , 70 , 72 , 74 , 76 , 81 , 85 ]. EMID enrichment of mitochondria at intercalated discs, Redox balance ratio of reduced to oxidised nicotinamide adenine dinucleotide, ROS reactive oxygen species, TCA intermediates of the tricarboxylic acid cycle …”

Involvement of Oxidative Stress in the Development of Subcellular Defects and Heart Disease

“…Ca 2+ -handling abnormalities associated with oxidative stress are known to induce mitochondrial Ca 2+ -overload and impair mitochondrial function for the production of energy [ 109 , 110 , 111 ]. On the other hand, an increase in the mitochondrial ATPase inhibitory factor-1 due to oxidative stress has been shown to disrupt mitochondrial Ca 2+ -handling whereas a loss of mitochondrial Ca 2+ uniporter has been reported to trigger arrhythmias possibly by affecting the Ca 2+ -handling function of the sarcoplasmic reticulum [ 112 , 113 ]. It is pointed out that the sarcoplasmic reticulum, by virtue of its ability to release and accumulate Ca 2+ on a beat-to-heat basis, is known to play a major role in Ca 2+ -handling in cardiomyocytes, and has been indicated to serve as a critical target for oxidative stress.…”