Early consequences of the phospholamban mutation PLN-R14del+/- in a transgenic mouse model

Aims: The heterozygous phospholamban (PLN) mutation R14del (PLN R14del+/-) is associated with a severe arrhythmogenic cardiomyopathy (ACM) developing in the adult. “Superinhibition” of SERCA2a by PLN R14del is widely assumed to underlie the pathogenesis, but alternative mechanisms such abnormal energy metabolism have also been reported. This work aims to 1) to evaluate Ca2+ dynamics and energy metabolism in a transgenic (TG) mouse model of the mutation prior to cardiomyopathy development; 2) to test whether they are causally connected. Methods and Results: Ca2+ dynamics, energy metabolism parameters, reporters of mitochondrial integrity, energy and redox homeostasis were measured in ventricular myocytes of 8-12 weeks-old, phenotypically silent, TG mice. Mutation effects were compared to pharmacological PLN antagonism and analysed during modulation of sarcoplasmic reticulum (SR) and cytosolic Ca2+ compartments. Transcripts and proteins of relevant signalling pathways were evaluated. The mutation was characterized by hyperdynamic Ca2+ handling, similar to that induced by PLN antagonism. Albeit all components of energy metabolism were depressed at rest, functional signs of mitochondrial damage or energy starvation were absent and cell energy charge was preserved. The response of mitochondrial O2 consumption to SERCA2a blockade was lost in mutant myocytes (SR-mitochondrial uncoupling) and ER-stress signalling was activated. Conclusions: 1) PLN R14del+/-loses its ability to inhibit SERCA2a, which argues against SERCA2a superinhibition as a mechanism of ACM; 2) depression of resting energy metabolism may at least partly reflect impairment of SR-mitochondrial coupling; 3) ER-stress may be an early factor in the pathogenesis.