Initial Steps in Mammalian Autophagosome Biogenesis

Abstract

During the last decade, autophagy has been pointed out as a central process in cellular homeostasis with the consequent implication in most cellular settings and human diseases pathology. At present, there is significant data available about molecular mechanisms that regulate autophagy. Nevertheless, autophagy pathway itself and its importance in different cellular aspects are still not completely clear. In this article, we are focused in four main aspects: (a) Induction of Autophagy: Autophagy is an evolutionarily conserved mechanism induced by nutrient starvation or lack of growth factors. In higher eukaryotes, autophagy is a cell response to stress which starts as a consequence of organelle damage, such as oxidative species and other stress conditions. (b) Initiation of Autophagy; The two major actors in this signaling process are mTOR and AMPK. These multitasking protein complexes are capable to summarize the whole environmental, nutritional, and energetic status of the cell and promote the autophagy induction by means of the ULK1-Complex, that is the first member in the autophagy initiation. (c) ULK1-Complex: This is a highly regulated complex responsible for the initiation of autophagosome formation. We review the post-transductional modifications of this complex, considering the targets of ULK1. (d)The mechanisms involved in autophagosome formation. In this section we discuss the main events that lead to the initial structures in autophagy. The BECN1-Complex with PI3K activity and the proper recognition of PI3P are one of these. Also, the transmembrane proteins, such as VMP1 and ATG9, are critically involved. The membrane origin and the cellular localization of autophagosome biogenesis will be also considered. Hence, in this article we present an overview of the current knowledge of the molecular mechanisms involved in the initial steps of mammalian cell autophagosome biogenesis.

Keywords: AMPK; ULK1; VMP1; autophagy regulation; mTOR.

FIGURE 1

(A) Schematic overview of autophagy. UKL1 activation leads to autophagosome biogenesis. On the ER surface, the transmembrane protein VMP1 recruits a PI3K complex. The consequent PI3P subdomain is recognized by DFCP1 on the omegasome structure. Then, in the isolation membrane, WIPI proteins recruit the ATG5-ATG12-ATG16 complex which in turn make possible the lipidation of LC3 on the membrane. The formation of autophagosome, a double membrane vesicle, allows the carrying of cargo to lysosome. Eventually, cargo is degraded in the resulted autophagolysosome. ER, endoplasmic reticulum; PI3K, phosphatidylinositol 3-kinase; PI3P, phosphatidylinositol (3,4,5) triphosphate (PI3P). (B) Diagram of interrelationship among the cellular energetic and metabolic regulators, mTOR and AMPK, and the autophagy. (C) Representative scheme of the ULK1 complex proteins. Upper right number in each scheme shows the length of the amino acid chain. Described domains are showed for each protein. (D) Possible structure and interrelationship among the ULK1 complex proteins, suggested from available data. KD, kinase domain; LIR, LC3-interacting region; IDR, intrinsically disordered region; MIT, microtubule interacting and trafficking domain; HORMA, HOP1, REV7, and MAD2 domain; MIM, MIT-interacting motif; NLS, nuclear localization signal; CC, coil-coil region; LZ, leucine zipper; WF, WF finger motif. (E) Regulation of the autophagy initiation complex ULK1 by mTOR and AMPK at basal (left) and starvation (right) conditions.