Abstract 349: Beclin1 as a Critical Regulator of the Endosomal Degradation Pathway

Abstract

Effective cellular quality control is crucial for cellular homeostasis and cardiac health. Defects in the pathways that regulate the removal of damaged proteins and organelles contribute to heart failure. Autophagy is well known for its role in removing protein aggregates and organelles; however, we recently found that the small GTPase Rab5 and early endosomes also participate in Parkin-mediated mitochondrial clearance. Beclin1 is a scaffolding protein which can form distinct PI3K protein complexes to regulate different processes such as autophagy. Here, we have investigated the role of Beclin1 in regulating the endosomal degradation pathway in mice. We have generated inducible cardiac-specific Beclin1 KO mice to investigate the in vivo role of Beclin1 in myocytes. We found significant contractile dysfunction as early as one week after deletion of Beclin1, which led to increased mortality. Loss of Beclin1 led to rapid development of cardiac hypertrophy as characterized by increased Myh7, Nppa, Nppb , myocyte size, and heart weight/body weight. Furthermore, we found that loss of Beclin1 led to increased inflammatory markers Il6 and Tgfb1 , as well as increased DNA damage/apoptosis in the heart. Ultrastructural analysis revealed Beclin1 deficiency led to significant myocardial disarray, demonstrating the importance of Beclin1 for proper myocyte alignment. These mice also had reduced autophagic flux but increased IGFR-beta receptor protein levels, indicating defects in both the autophagy and endosomal degradation pathways. Additionally, immunofluorescence analysis revealed that Beclin1 colocalizes with Rab5 at the early endosome in cells, and coimmunoprecipitation experiments confirmed their interaction. Together, these data establish the essential role of Beclin1 in cellular homeostasis as a central regulator of endosomal activity.