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Autophagy means “eating of self” (from Ancient Greek “autó”= self;  and “phagein” = to devour)[1]. Although its name might sound harmful, autophagy appears to have longevity-promoting effects[2]. Here we will explain what autophagy is, how it works, its benefits, and how it plays a role in aging.

What is Autophagy?

Autophagy is the way cells break down misbehaving or nonfunctional organelles and proteins in the cell[1,2]. This means autophagy can consume organelles, such as mitochondria, peroxisomes, and the endoplasmic reticulum[1].

Mitochondria are the power plants of your cells that convert nutrients into cellular energy. Peroxisomes are small, membrane-enclosed organelles that contain enzymes involved in various metabolic reactions, including aspects of energy metabolism. The endoplasmic reticulum is a network of membranous tubules within the cytoplasm of a eukaryotic cell, continuous with the nuclear membrane. It usually has ribosomes attached and is involved in protein and lipid synthesis.

Aberrant proteins and organelles, if left unchecked, could potentially damage the cell. At the very least, they can fail to perform their needed functions[1]. One good example of this is dysfunctional mitochondria. Autophagy can selectively remove aberrant mitochondria that have no membrane potential. If not removed, these mitochondria can release reactive oxygen species which can promote apoptosis (destruction of the cell)[2].

Autophagy is also one way our cells deal with energy shortages[2]. For example, after a period of fasting, a body’s cells will respond by breaking down some of their organelles and proteins for energy.

Finally, you can also think of autophagy as a recycling mechanism. During autophagy, proteins and organelles go through catabolism, a process that breaks them down into amino acids and other small components. These small components can be reused to create new proteins and organelles[1, 2].

Summed up, autophagy does the following things:

  • clears out impaired proteins and organelles[1],

  • allows the harvest of energy when it breaks down proteins and organelles during energy shortages[2],

  • and allows recycling of the catabolised proteins and organelles into new ones[1].

How Does Autophagy Work?

Say a cell has a misfolded protein. Before it can be broken down or digested, it needs to be separated off from everything else. If this wasn’t the case, the proteases (proteins that break down other proteins) would sabotage the cell by breaking down both healthy and dysfunctional parts haphazardly.

A membrane called a phagophore, also known as the isolation membrane[2], separates the aberrant protein from everything else[1]. This double membrane wraps around the protein to form what is called an autophagosome. From there, this bubble (the autophagosome) makes its way to a lysosome[1].

The lysosome is a protease packed vesicle organelle that breaks down unwanted parts of the cell. Think of it as a membrane-bound sack of digestive proteins ready to break down anything the cell doesn’t want anymore. One useful metaphor for the lysosome is, if the cell was a city, the lysosome would be the dump and recycling center[3]. The autophagosome fuses with a nearby lysosome to form the autolysosome[1].

Our protein is now getting exposed to proteases that can digest it. Once the protein has been digested into amino acids, permeases and transporters carry off the amino acids to wherever they can be used[1]. Permeases are proteins that help substances get through membranes. Likewise, transporters are proteins that, predictably, transport things.

If you had to teach a young child how autophagy works, you’d explain that a small bubble forms around the protein (autophagosome). Then, it fuses with another bubble (a lysosome) to form a new one (an autolysosome). There, the protein breaks into smaller pieces. Other proteins aid in transporting these pieces elsewhere so that they can be used for building new structures.

In summary, the main steps of autophagy are:

  • separation of the protein/organelle into an autophagosome by the phagophore,

  • autolysosome creation via the fusion of  the autophagosome with a lysosome,

  • breakdown of protein/organelle by acid proteases found in the lysosome, and

  • shuttling of amino acids (and other products of catabolism) by permeases and transporters to be recycled [1].

Why is it beneficial?

As previously mentioned, autophagy functions as a survival mechanism when energy gets low, and it aids the development of several lower-model organisms, including yeast, Drosophila, and C. elegans[1]. In mouse models, autophagy seems to aid cytosol changes during embryonic and postnatal development[5]. It alleviates the issue of aggregated or misfolded proteins. It also rids the cell of some types of damaged organelles[1], and it serves as an innate immune system defense against both viral and bacterial pathogens[6].

Despite its noxious-sounding name, autophagy serves more of a protective role than a harmful one. This was observed in studies in which rodents were subjected to damage of the heart, nervous system, liver, and kidney[2]. It also seems to help protect against neurodegeneration, cancer, diabetes, and heart, liver, and autoimmune disease[1]. Autophagy can recycle aberrant mitochondria and can reduce unnecessary cell death in cells after they divide[2].

What does autophagy have to do with aging?

Autophagy and aging seem to be intimately connected. When the genes that cause autophagy are inhibited in mammalian cells, we observe degeneration. Much of this degeneration resembles the degeneration we see in aging. Aging itself often comes with reduced autophagy. When we stimulate autophagy, we seem to mitigate aging. Conversely, when we curtail it, we exacerbate aging[2].

The numerous strategies for slowing aging in model organisms also often cause autophagy to occur. This begs the question of whether these interventions work because they cause autophagy, and this might be the case. When we inhibit autophagy during some lifespan-extending therapies (such as calorie restriction, insulin growth factor pathway inhibition, spermidine, resveratrol, or rapamycin), it erodes the anti-aging effects[2].

We can also see the beneficial effects of autophagy in the study of age-related diseases, such as atherosclerosis[4]. Atherosclerosis is a circulatory disease characterized by hardening and narrowing of the arteries caused by plaque formation. Unabated, atherosclerosis can lead to heart attacks, strokes, and other forms of circulatory disease[7]. One study established a connection between reduced autophagy and the formation of the plaques responsible for atherosclerosis. Researchers bred mice to have enhanced autophagy via increasing the expression of a gene called TFEB.  The change seemed to have a protective effect against atherosclerosis caused by a Western diet. Furthermore, they showed that trehalose, a naturally occurring sugar, enhances autophagy. It reduced the signs of atherosclerosis when it was injected into the body cavities of mice[4].

Conclusion

Autophagy shows promise as an important maintenance and repair mechanism, especially in the context of aging. A number of research efforts are working on increasing the level of autophagy in cells in order to make the body work more efficiently.

Literature

[1]Glick, D., Barth, S., & Macleod, K. F. (2010). Autophagy: cellular and molecular mechanisms. The Journal of Pathology, 221(1), 3-12. doi:10.1002/path.2697

[2]Rubinsztein, D. C., Mariño, G., & Kroemer, G. (2011). Autophagy and Aging. Cell,146(5), 682-695.

[3] (2012, February20). Retrieved July 19, 2017, from https://www.youtube.com/watch?v=cj8dDTHGJBY

[4]Sergin I, Evans TD, Zhang X, Bhattacharya S, Stokes CJ, Song E, Ali S, Dehestani B, Holloway KB, Micevych PS, Javaheri A, Crowley JR, Ballabio A, Schilling JD, Epelman S, Weihl CC, Diwan A, Fan D, Zayed MA, Razani B. Exploiting macrophage autophagy-lysosomal biogenesis as a therapy for atherosclerosis. Nature Communications. June 7, 2017.

[5] Cecconi, F., & Levine, B. (2008). The Role of Autophagy in Mammalian Development: Cell Makeover Rather than Cell Death. Developmental Cell, 344-357. doi:10.1016/j.devcel.2008.08.012

[6] Wileman, T. (2013). Autophagy as a defence against intracellular pathogens. Essays in Biochemistry, 153-163. doi:10.1042/bse0550153

[7] Atherosclerosis: What role does it play? Retrieved August 17, 2017, from https://www.heart.org/HEARTORG/Conditions/Cholesterol/WhyCholesterolMatters/Atherosclerosis_UCM_305564_Article.jsp

 

 

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About the author
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Ariah Mackie

Ariah received a bachelor's in Biomolecular Engineering from the University of California in Santa Cruz in 2016. Her career interests are in regenerative medicine for aging, teaching,computational biology, genomics, and bioinformatics
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