Autophagy

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Autophagy is derived from the latin roots meaning "self eating," but as it occurs at a sub-cellular level, it should not be confused with self-cannibalism.

Autophagy, or autophagocytosis, is a process where cytoplasmic materials are degraded through the lysosomal machinery. The process is commonly viewed as sequestering organelles and long-lived proteins in a double-membrane vesicle inside the cell, where the contents are subsequently delivered to the lysosome for degradation. Autophagy is part of everyday normal cell growth and development where mTOR plays an important regulatory role. Its main purpose is to maintain a balance between biogenesis (production) of cell structures, and their degradation and turnover (see Bruce Alberts et al.: Molecular biology of the cell, 4th edition, Garland Publishing 2002, NCBI Bookshelf [1]). For example, a liver-cell mitochondrion lasts around ten days before it is degraded and its contents are reused.

Autophagy also plays a major role in the destruction of bacteria, viruses, and unnecessary proteins that have begun to aggregate within a cell and may potentially cause problems. It is especially prominent in insects that undergo complete metamorphosis; larval tissue is recycled to become appendages in an adult insect.

The rate of autophagy increases when the cell is subjected to nutrient deprivation, and also when it receives stimuli that result in organelle proliferation.

When autophagy involves the total destruction of the cell, it is called autophagic cell death (also known as cytoplasmic cell death or type II cell death). This is one of the main types of programmed cell death. As such, it is a regulated process of cell death in a multicellular organism, or in a colony of individual cells such as yeast (see Daniel J. Klionsky and Scott D. Emr: "Autophagy as a Regulated Pathway of Cellular Degradation", Science Vol. 290 p.1717, 1 Dec. 2000 [2]).

Autophagy can be further separated into two types: macroautophagy and microautophagy. Macroautophagy involves the formation of a membrane containing target materials moving into the lysosome/vacuole while microautophagy is the invagination of the lysosome/vacuole of target materials. Both micro and macroautophagy serve as fundamental functions in plants and metazoa (multicellular animals), as well as in other eukaryotes such as slime moulds and yeast.

Cytoplasmic autophagy is different from other methods of protein degradation, i.e. Polyubiquitination.

Autophagy selective for degradation of Peroxisomes is called Pexophagy, which can be separated in Macropexophagy and Micropexophagy.

Autophagy selective for degradation of Mitochondria is called Mitophagy, which can be separated in Macromitophagy and Micromitophagy.

Autophagy selective for degradation of bacteria and viruses is called Xenophagy, or "foreign eating", to distinguish this process from autophagy of the cell's own contents.

[edit] Components

Autolysosome: product of fusion of the mature autophagosome and lysosome in the cytoplasm, enabling degradation of autophagosome contents via lysosomal digestive enzymes.