The autophagy regulators Ambra1 and Beclin 1 are required for adult neurogenesis in the brain subventricular zone

Autophagy is a conserved proteolytic mechanism required for maintaining cellular homeostasis. The role of this process in vertebrate neural development is related to metabolic needs and stress responses, even though the importance of its progression has been observed in a number of circumstances, both in embryonic and in postnatal differentiating tissues. Here we show that the proautophagic proteins Ambra1 and Beclin 1, involved in the initial steps of autophagosome formation, are highly expressed in the adult subventricular zone (SVZ), whereas their downregulation in adult neural stem cells in vitro leads to a decrease in cell proliferation, an increase in basal apoptosis and an augmented sensitivity to DNA-damage-induced death. Further, Beclin 1 heterozygosis in vivo results in a significant reduction of proliferating cells and immature neurons in the SVZ, accompanied by a marked increase in apoptotic cell death. In sum, we propose that Ambra1- and Beclin 1-mediated autophagy plays a crucial role in adult neurogenesis, by controlling the survival of neural precursor cells.In the adult mammalian brain, neural stem cells are localized in two regions: in the subventricular zone (SVZ), a layer extending along the wall of the lateral ventricle, and in the subgranular zone of the dentate gyrus in the hippocampus.¹ SVZ stem cells are strictly controlled under physiological conditions and are believed to replenish dying cells. In addition to their effect in maintaining brain homeostasis, they are also involved in neuronal replacement in response to injury.² Although several factors are known to affect neurogenesis, understanding of the mechanisms that regulate adult neurogenic niches and their metabolism is still incomplete. Macroautophagy (hereafter referred to as autophagy) is an evolutionarily conserved cellular turnover process in which bulk cytoplasmic materials, long-lived proteins or damaged organelles are sequestered and delivered to lysosomes for degradation.³ A complex crosstalk takes place between apoptosis and autophagy that determines the death or life of cells.⁴ Beclin 1 has a key role in autophagy initiation;⁵ it regulates the autophagy-promoting activity of the Class III PI 3-kinase Vps34,⁶ and is involved in the recruitment of membranes to form the key autophagy vesicles, named autophagosomes. Beclin 1 also interacts with Bcl-2,⁷ and plays an important function in the regulation of cell survival.⁸ Ambra1 (activating molecule in Beclin 1-regulated autophagy) is another modulator of autophagy, which is phosphorylated by the upstream autophagy kinase Ulk1 and acts on Ulk1 stability and function.^{9, 10} Ambra1 also interacts with Beclin 1 upon autophagic stimuli, thereby promoting the binding between Beclin 1 and its target kinase, Vps34. The binding between Ambra1 and mitochondrial Bcl-2 is also important for cell survival.¹¹ Moreover, Ambra1 is crucial for nervous system development and is expressed from early neurulation onwards, with a high specificity for the neural plate.¹²In contrast with studies on the pro-survival impact of autophagy in post-mitotic cells and in disease models, the role of autophagy in the maintenance and function of adult neural stem cells (ANSCs) is poorly understood. Here we have found that expression of upstream autophagy-regulating genes in the adult neurogenic region of SVZ, in physiological conditions, plays a crucial role in the regulation of adult neurogenesis.