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      Autophagy and the nutritional signaling pathway

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          Abstract

          During their growth and development, animals adapt to tremendous changes in order to survive. These include responses to both environmental and physiological changes and autophagy is one of most important adaptive and regulatory mechanisms. Autophagy is defined as an autolytic process to clear damaged cellular organelles and recycle the nutrients via lysosomic degradation. The process of autophagy responds to special conditions such as nutrient withdrawal. Once autophagy is induced, phagophores form and then elongate and curve to form autophagosomes. Autophagosomes then engulf cargo, fuse with endosomes, and finally fuse with lysosomes for maturation. During the initiation process, the ATG1/ULK1 (unc-51-like kinase 1) and VPS34 (which encodes a class III phosphatidylinositol (PtdIns) 3-kinase) complexes are critical in recruitment and assembly of other complexes required for autophagy. The process of autophagy is regulated by autophagy related genes (ATGs). Amino acid and energy starvation mediate autophagy by activating mTORC1 (mammalian target of rapamycin) and AMP-activated protein kinase (AMPK). AMPK is the energy status sensor, the core nutrient signaling component and the metabolic kinase of cells. This review mainly focuses on the mechanism of autophagy regulated by nutrient signaling especially for the two important complexes, ULK1 and VPS34.

          Most cited references44

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          Autophagosome formation: core machinery and adaptations.

          Eukaryotic cells employ autophagy to degrade damaged or obsolete organelles and proteins. Central to this process is the formation of autophagosomes, double-membrane vesicles responsible for delivering cytoplasmic material to lysosomes. In the past decade many autophagy-related genes, ATG, have been identified that are required for selective and/or nonselective autophagic functions. In all types of autophagy, a core molecular machinery has a critical role in forming sequestering vesicles, the autophagosome, which is the hallmark morphological feature of this dynamic process. Additional components allow autophagy to adapt to the changing needs of the cell.
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            Distinct regulation of autophagic activity by Atg14L and Rubicon associated with Beclin 1-phosphatidylinositol-3-kinase complex.

            Beclin 1, a mammalian autophagy protein that has been implicated in development, tumour suppression, neurodegeneration and cell death, exists in a complex with Vps34, the class III phosphatidylinositol-3-kinase (PI(3)K) that mediates multiple vesicle-trafficking processes including endocytosis and autophagy. However, the precise role of the Beclin 1-Vps34 complex in autophagy regulation remains to be elucidated. Combining mouse genetics and biochemistry, we have identified a large in vivo Beclin 1 complex containing the known proteins Vps34, p150/Vps15 and UVRAG, as well as two newly identified proteins, Atg14L (yeast Atg14-like) and Rubicon (RUN domain and cysteine-rich domain containing, Beclin 1-interacting protein). Characterization of the new proteins revealed that Atg14L enhances Vps34 lipid kinase activity and upregulates autophagy, whereas Rubicon reduces Vps34 activity and downregulates autophagy. We show that Beclin 1 and Atg14L synergistically promote the formation of double-membraned organelles that are associated with Atg5 and Atg12, whereas forced expression of Rubicon results in aberrant late endosomal/lysosomal structures and impaired autophagosome maturation. We hypothesize that by forming distinct protein complexes, Beclin 1 and its binding proteins orchestrate the precise function of the class III PI(3)K in regulating autophagy at multiple steps.
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              Amino acid signalling upstream of mTOR.

              Mammalian target of rapamycin (mTOR) is a conserved Ser/Thr kinase that is part of mTOR complex 1 (mTORC1), a master regulator that couples amino acid availability to cell growth and autophagy. Multiple cues modulate mTORC1 activity, such as growth factors, stress, energy status and amino acids. Although amino acids are key environmental stimuli, exactly how they are sensed and how they activate mTORC1 is not fully understood. Recently, a model has emerged whereby mTORC1 activation occurs at the lysosome and is mediated through an amino acid sensing cascade involving RAG GTPases, Ragulator and vacuolar H(+)-ATPase (v-ATPase).
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                Author and article information

                Contributors
                Journal
                Front. Agr. Sci. Eng.
                FASE
                CN10-1204/S
                Frontiers of Agricultural Science and Engineering
                Higher Education Press (4 Huixin Dongjie, Chaoyang District, Beijing 100029, China )
                2095-7505
                2095-977X
                2016
                : 3
                : 3
                : 222-230
                Affiliations
                [1 ]. State Key Lab of Animal Nutrition, Ministry of Agriculture Feed Industry Center, China Agricultural University, Beijing 100193, China
                [2 ]. Center for Autophagy Research, Department of Internal Medicine and Biochemistry, Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas, TX 75390-9113, USA
                [3 ]. College of Dental Medicine, Midwestern University, Downers Grove, IL 60515, USA
                Author notes
                maxi@cau.edu.cn
                defali@cau.edu.cn
                Article
                10.15302/J-FASE-2016106
                3284053e-3d65-4217-8262-0461e259c4b1
                Copyright @ 2016
                History
                : 21 April 2016
                : 16 June 2016
                Categories
                REVIEW

                Management,Industrial organization,Risk management,Economics
                Autophagy,ULK1 complex,VPS34 complex,AMPK,mTOR,nutrient signaling

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