Disruption of Ca
2+
i
Homeostasis and Connexin 43 Hemichannel Function in the Right Ventricle Precedes Overt Arrhythmogenic Cardiomyopathy in Plakophilin-2–Deficient Mice
Authors: Kim Joon-Chul , Pérez-Hernández Marta , Alvarado Francisco J. , Maurya Svetlana R. , Montnach Jerome , Yin Yandong , Zhang Mingliang , Lin Xianming , Vasquez Carolina , Heguy Adriana , Liang Feng-Xia , Woo Sun-Hee , Morley Gregory E. , Rothenberg Eli , Lundby Alicia , Valdivia Hector H. , Cerrone Marina , Delmar Mario 
Published: July 2019

Background:
Plakophilin-2 (PKP2) is classically defined as a desmosomal protein. Mutations in PKP2 associate with most cases of gene-positive arrhythmogenic right ventricular cardiomyopathy. A better understanding of PKP2 cardiac biology can help elucidate the mechanisms underlying arrhythmic and cardiomyopathic events consequent to PKP2 deficiency. Here, we sought to capture early molecular/cellular events that can act as nascent arrhythmic/cardiomyopathic substrates.

Methods:
We used multiple imaging, biochemical and high-resolution mass spectrometry methods to study functional/structural properties of cells/tissues derived from cardiomyocyte-specific, tamoxifen-activated, PKP2 knockout mice (PKP2cKO) 14 days post-tamoxifen injection, a time point preceding overt electrical or structural phenotypes. Myocytes from right or left ventricular free wall were studied separately.

Results:

Most properties of PKP2cKO left ventricular myocytes were not different from control; in contrast, PKP2cKO right ventricular (RV) myocytes showed increased amplitude and duration of Ca
2+
transients, increased Ca
2+
in the cytoplasm and sarcoplasmic reticulum, increased frequency of spontaneous Ca
2+
release events (sparks) even at comparable sarcoplasmic reticulum load, and dynamic Ca
2+
accumulation in mitochondria. We also observed early- and delayed-after transients in RV myocytes and heightened susceptibility to arrhythmias in Langendorff-perfused hearts. In addition, ryanodine receptor 2 in PKP2cKO-RV cells presented enhanced Ca
2+
sensitivity and preferential phosphorylation in a domain known to modulate Ca
2+
gating. RNAseq at 14 days post-tamoxifen showed no relevant difference in transcript abundance between RV and left ventricle, neither in control nor in PKP2cKO cells. Instead, we found an RV-predominant increase in membrane permeability that can permit Ca
2+
entry into the cell. Connexin 43 ablation mitigated the membrane permeability increase, accumulation of cytoplasmic Ca
2+
, increased frequency of sparks and early stages of RV dysfunction. Connexin 43 hemichannel block with GAP19 normalized [Ca
2+
]
i
homeostasis. Similarly, protein kinase C inhibition normalized spark frequency at comparable sarcoplasmic reticulum load levels.

Conclusions:

Loss of PKP2 creates an RV-predominant arrhythmogenic substrate (Ca
2+
dysregulation) that precedes the cardiomyopathy; this is, at least in part, mediated by a Connexin 43-dependent membrane conduit and repressed by protein kinase C inhibitors. Given that asymmetric Ca
2+
dysregulation precedes the cardiomyopathic stage, we speculate that abnormal Ca
2+
handling in RV myocytes can be a trigger for gross structural changes observed at a later stage.