Autophagy: Molecular mechanisms and overview

The molecular mechanism of autophagy involves several conserved Atg (autophagy-related) proteins. Various stimuli, such as nutrient starvation, lead to the formation of the phagophore, a step that involves two protein complexes:

• A Vps34 complex that contains Vps34 (the class III PI3K), Beclin1 (Atg6 in yeast), Atg14, and Vps15 (p150).
• A ULK1 complex that includes the serine/threonine kinase ULK1 (Atg1 in yeast), an essential positive regulator of autophagosome formation.

The elongation of the phagophore results in the formation of the characteristic double-membrane autophagosome. This step requires two ubiquitin-like conjugation pathways, both catalyzed by Atg7.

• The first ubiquitin-like system leads to the conjugation of Atg5-Atg12, which then forms a multimeric complex with Atg16L. The Atg5-Atg12-Atg16L complex associates with the outer membrane of the extending phagophore3,4.
• The second system results in the processing of LC3, encoded by the mammalian homolog of the yeast Atg8. Upon autophagy induction, LC3B is proteolytically cleaved by Atg4 to generate LC3-I. LC3-I is activated by Atg7 and then conjugated to phosphatidylethanolamine (PE) in the membrane to generate processed LC3-II.

Processed LC3-II is recruited onto the growing phagophore and its integration is dependent on Atg5-Atg12. Unlike Atg5-Atg12-Atg16L, LC3-II is found on both the inner and outer surfaces of the autophagosome, where it is required for the expansion and completion of the autophagic membrane. After the closure of the autophagosomal membrane, the Atg16-Atg5-Atg12 complex dissociates from the vesicle, whereas a portion of LC3-II remains covalently bound to the membrane (Figure 1). Therefore, LC3-II can be used as a marker to monitor the level of autophagy in cells.

It has been postulated that various organelles, including mitochondria, the Golgi complex, and the endoplasmic reticulum (ER), can be the origin of the autophagosomal membrane5. Recent studies demonstrate that the self-multimerization of Atg9 may facilitate membrane tethering and/or fusion6.

When the autophagosome formation is completed, LC3-II attached to the outer membrane is cleaved from PE by Atg4 and released back to the cytosol. The fusion between the autophagosome and the lysosome is thought to require the lysosomal membrane protein LAMP-1 and the small GTPase Rab7. 

After fusion, a series of acid hydrolases are involved in the degradation of the sequestered cytoplasmic cargo. The small molecules resulting from the degradation, particularly amino acids, are transported back to the cytosol for protein synthesis and maintenance of cellular functions under starvation conditions. The identification of Atg22 together with other vacuolar permeases (such as Avt3 and Avt4) as vacuolar amino acid effluxers during yeast autophagy has helped in the understanding of the mechanisms of nutrient recycling7. These permeases represent the last step in the degradation and recycling process7.

There are currently three types of autophagy in mammalian cells³:

Macroautophagy:

Macroautophagy is the main autophagic pathway and it is characterized by the delivery of cytoplasmic cargo to the lysosome through an intermediary double membrane-bound vesicle, known as an autophagosome, which fuses with the lysosome to form an autolysosome.

Microautophagy:

Microautophagy involves the direct engulfment of cytoplasmic cargo into the lysosome through the invagination of the lysosomal membrane. Microautophagy is important in the maintenance of organellar size, membrane homeostasis, and cell survival under nitrogen restriction8.   

Chaperone-mediated autophagy (CMA):

Chaperone-mediated autophagy (CMA) involves the direct translocation of cytoplasmic proteins across the lysosomal membrane in a complex with chaperone proteins that are recognized by the lysosomal membrane receptor LAMP-2A (lysosomal-associated membrane protein 2A), resulting in their unfolding and degradation.

Autophagy has been widely implicated in many pathophysiological processes, including cancer, metabolic and neurodegenerative disorders, cardiovascular and pulmonary diseases. It also has an important role in aging and exercise9.

Autophagy in cancer

Autophagy was first linked to cancer through the role of Beclin 1, which is essential for the autophagy pathway and has been mapped to tumor susceptibility10. Since then, multiple tumor-suppressor proteins have been identified that are involved in the control of the autophagy pathway (eg p53, Bcl2, PTEN, etc).

​​​The tumor cells exploit the autophagic mechanism to provide a way for them to overcome nutrient-limiting conditions and facilitate tumor growth. Studies show that autophagy can modulate the tumor microenvironment by promoting angiogenesis, supply nutrients, and modulate inflammatory response11.

Autophagy in neurodegenerative diseases

Neurodegenerative diseases are characterized by the accumulation of mutant or toxic proteins12,13. It has been shown that the autophagic pathway helps in cell survival by removing unwanted cellular organelle and protein aggregates. Disruption of autophagy-specific genes in neural cells leads to neurodegeneration14,15.

Autophagy in cardiovascular diseases

The autophagic pathway is essential for normal maintenance, repair, and adaptation of the heart tissue. Unsurprisingly, therefore, autophagic deficiencies have been associated with a variety of cardiac pathologies16.

Autophagy in infectious disease

Autophagy plays a key role in immune defense against invading bacteria and pathogens. Upon infection, autophagy regulates inflammation, antigen presentation, and micro-organism capture and degradation17.