Autophagy is a process by which a cell breaks down and recycles its own components. In normally functioning animal cells, autophagy occurs at a very low level. Autophagy pathways are activated when a cell is running low on nutrients. The cell breaks down already existing proteins and other cell components into their basic building block components so that they can be reused to maintain essential cellular functions. There is also evidence to suggest that autophagy can be used by the cell to break down misfolded proteins.
The induction of autophagy in Huntington disease (HD) cells results in the accelerated breakdown of huntingtinaggregates and has been shown to have neuroprotective effects. It is currently unknown whether huntingtin aggregates are the cause or result of HD, but nerve cells that build up huntingtin aggregates often die. To read more about huntingtin protein aggregation and its role in HD, click here.
The Process of Autophagy
The part (or parts) of the cell that is to be degraded is first engulfed by a double membrane to separate it from the rest of the cell; the resulting membrane-enclosed bubble of cytosol (along with all the proteins the bubble contains) becomes what is called the autophagosome. The autophagosome eventually fuses with a cellular organelle called a lysosome, a much larger membrane-enclosed bubble that contains a variety of enzymes that can break down many types of cellular components (which is why lysosomes are sometimes referred to as the "garbage disposals" of the cell). These enzymes only work in a very acidic environment, so the pH inside lysosomes is much lower than the neutral pH in the rest of the cell. This pH barrier, as well as the physical barrier of the organelle membrane, protects the rest of the cell from being degraded should the enzymes somehow leak out. Once the contents of the autophagosome are delivered to the lysosome, the lysosomal enzymes break down the new contents, which can then be recycled and reused within the cell.
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