Impaired autophagy mechanisms associated with phospholamban-R14del disease

Abstract

Abstract Funding Acknowledgements Type of funding sources: Foundation. Main funding source(s): Leducq Foundation Phospholamban (PLN) is a key modulator of sarcoplasmic reticulum Ca2+ homeostasis and cardiac contractility. The PLN variant c.40_42delAGA (p.R14del) leads to arrhythmogenic cardiomyopathy characterized by life-threatening ventricular arrhythmias and sudden cardiac death. A large number of PLN-R14del carriers have been identified worldwide, although the expanding use of genetic testing and increased awareness is unveiling a higher frequency. PLN-containing aggregates are emerging as a hallmark of this disease, and are increasingly believed to be associated with disease pathogenesis. However, the mechanism leading to the formation of aggregates is unknown. We performed an in depth molecular and cellular analysis utilizing in vitro systems to determine if these aggregates result from aberrations in autophagy, and the underlying mechanisms. Utilizing cell culture systems we studied the expression level and subcellular distribution of fundamental autophagy-related proteins and evaluated the impact of autophagy inducers and inhibitors. Expression of PLN-R14del in different cell culture systems (HEK293 and H9c2 cells) was found to result in PLN aggregate formation, which were determined to contain the autophagy marker proteins sequestosome-1 (p62) and microtubule-associated protein light chain 3 (LC3), indicating inefficient degradation by the autophagy pathway. In support of this, PLN-R14del expressing cells were shown to exhibit increased p62 and LC3-II protein levels. This was associated with significantly reduced autophagic flux, as demonstrated by examination of LC3-II protein levels after autophagy inhibition. Moreover, by detailed immunofluorescence analysis, we determined reduced co-localization of the autophagosomal protein LC3 and the lysosomal associated membrane protein 1 (LAMP1) in PLN-R14del cells, suggesting impaired autophagosome-lysosome fusion. We next examined the distribution of key proteins mediating autophagosome-lysosome membrane fusion including the Ras-related protein Rab7, the ultraviolet radiation resistance-associated gene (UVRAG) and membrane tether proteins. In PLN-R14del transfected cells, significant alterations in their co-localization with LC3 were observed, indicating their impaired recruitment to the autophagosomes. Importantly, in PLN-WT cells changes in cytosolic Ca2+ levels by thapsigargin treatment were found to recapitulate the autophagic defects of PLN-R14del, suggesting that Ca2+ alterations are a contributing factor of PLN-R14del mediated aberrant autophagy. In conclusion, our findings reveal a causal relationship between PLN-R14del and autophagy dysregulation, through inhibition of autophagosome-lysosome fusion and autophagic flux reduction. This ultimately leads to aggregate formation, an early pathological sign in human patients.