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Beclin1 is critical for Rab5 endosomal-mediated trafficking of plasma membrane proteins

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Proper cellular quality control is essential in all cell types but is especially important in a post-mitotic cell such as the cardiomyocyte to prevent cell death. Deficiencies in cellular quality control pathways such as autophagy or the endosomal degradation pathway are known to lead to a variety of neurodegenerative and cardiovascular diseases. Here, I focus on the regulation of plasma membrane receptor trafficking in the heart through the endosomal pathway by the scaffolding protein Beclin1. The endosomal pathway is mediated by Rab proteins, a family of GTPases. While Beclin1 is well-known as a positive regulator of autophagy, binding to the PI3-K Vps34 and autophagy protein Atg14L, its mechanistic role in the endosomal pathway is less clear. I report that Beclin1 mediates endosomal trafficking through early endosome marker Rab5. Loss of Beclin1 leads to accumulation of the EGF receptor in Rab5 positive vesicles in the cytosol. Additionally, I demonstrate that Beclin1 is required for UVRAG protein stability, a protein involved in a variety of cellular trafficking pathways, and that Beclin1, UVRAG, and Rab5 all colocalize at the early endosome. In vivo, I found that loss of Beclin1 in the heart leads to rapid heart failure and reduced survival. Furthermore, loss of Beclin1 in the heart led to increased protein levels of early endosome markers Rab5 and EEA1. Finally, restoration of UVRAG alleviated the cardiac phenotype observed after loss of Beclin1, indicating the importance of the endosomal pathway in maintaining heart function.

I also report a novel mechanism for selective removal of mitochondria by autophagy, or mitophagy, through mitochondrial outer membrane receptors FUNDC1 and BNIP3L in cardiac progenitor cells (CPCs). This pathway is distinct from the traditional mitophagy pathway mediated by PINK1/PARKIN and relies on direct binding to the autophagosome by FUNDC1 and BNIP3L. I show that FUNDC1 and BNIP3L are required for the formation of a functional, interconnected mitochondrial network in CPCs during differentiation. Also, I found that knockdown of FUNDC1 and BNIP3L decreased maximal respiration and increased CPC susceptibility to H2O2 mediated cell stress. Notably, impaired FUNDC1 and BNIP3L function reduced cell viability after injection into the myocardium post-myocardial infarction, indicating functional mitophagy is crucial for CPCs to persist in the unfavorable conditions of the injured heart. Together, these data uncover new mechanism(s) of cellular quality control in the heart and could have significant impact on the design of therapeutics targeting either the endosomal pathway or mitophagy receptors in cardiovascular disease.

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This item is under embargo until October 8, 2022.