Deficiency of autophagy protein beclin 1 is implicated in tumorigenesis and neurodegenerative diseases, but the molecular mechanism remains elusive. Previous studies showed that Beclin 1 coordinates the assembly of multiple VPS34 complexes whose distinct phosphatidylinositol 3-kinase III (PI3K-III) lipid kinase activities regulate autophagy at different steps. Recent evidence suggests a function of beclin 1 in regulating multiple VPS34-mediated trafficking pathways beyond autophagy; however, the precise role of beclin 1 in autophagy-independent cellular functions remains poorly understood. Herein we report that beclin 1 regulates endocytosis, in addition to autophagy, and is required for neuron viability in vivo. We find that neuronal beclin 1 associates with endosomes and regulates EEA1/early endosome localization and late endosome formation. Beclin 1 maintains proper cellular phosphatidylinositol 3-phosphate (PI(3)P) distribution and total levels, and loss of beclin 1 causes a disruption of active Rab5 GTPase-associated endosome formation and impairment of endosome maturation, likely due to a failure of Rab5 to recruit VPS34. Furthermore, we find that Beclin 1 deficiency causes complete loss of the UVRAG-VPS34 complex and associated lipid kinase activity. Interestingly, beclin 1 deficiency impairs p40phox-linked endosome formation, which is rescued by overexpressed UVRAG or beclin 1, but not by a coiled-coil domain-truncated beclin 1 (a UVRAG-binding mutant), Atg14L or RUBICON. Thus, our study reveals the essential role for beclin 1 in neuron survival involving multiple membrane trafficking pathways including endocytosis and autophagy, and suggests that the UVRAG-beclin 1 interaction underlies beclin 1's function in endocytosis.
Beclin 1 was not only the first-described mammalian autophagy protein, but is one of the most widely-characterized players in autophagy regulation. It is implicated in multiple human disease conditions. As a core component of the essential lipid kinase complex (PI3K-III), beclin 1 has largely been characterized to date in the context of autophagy through its recruitment of additional autophagy proteins for the assembly of the PI3K-III complexes involved in the nucleation of the autophagosome. Little is known, however, about how beclin 1 regulates specific functions of PI3K-III in other membrane trafficking pathways. Furthermore, although beclin 1 has been linked to multiple neurodegenerative diseases, the function of beclin 1 in the brain remains uncharacterized. Herein, we used genetic animal models and mutant cell lines to demonstrate that beclin 1 participates in multiple organelle trafficking pathways. Beclin 1 deficiency in neurons causes severe neurodegeneration, concomitant with aberrant late endosome formation and impaired phospholipid localization. Our mechanistic study reveals the essential role for beclin 1 in neuron survival involving multiple membrane trafficking pathways including endocytosis and autophagy. Our study clarifies the physiological function of beclin 1, which leads for further understanding of its role in tumorigenesis, infectious disease and neurodegenerative disease.