Autophagy: Definition, Pathways & Function

Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

What is autophagy? What causes autophagy to occur in a cell? What role does it play in keeping cells and the body healthy? Read on to learn how autophagy is managed and what it does.

What Is Autophagy?

Autophagy (pronounced ought- off-uh- gee) is a strange sounding word. What does it mean? Well, 'auto' means 'self'; 'phage' refers to 'eating'. Autophagy, then, is 'self-eating'. This might sound like something from a horror movie, but it's incredibly important for helping you and your cells to stay healthy.

Autophagy takes place at the cellular level. This means that one part of the cell is eating the other parts. In cells, an organelle called the lysosome acts as the stomach. Its job is to store digestive enzymes.

The lysosome can eat a variety of other cellular organelles, such as the mitochondria and the endoplasmic reticulum (ER). It's also useful for getting rid of bacteria that have infected the cell.

Types of Autophagy

There are three types of autophagy. Macro-autophagy occurs when large things are moved via vesicle into the lysosome (macro=big). It is the most common type of autophagy and is mainly used for removing entire organelles.

In macro-autophagy, a piece of double membrane, called the isolation membrane, surrounds whatever is being eaten. The isolation membrane closes completely around the food, forming a double-membraned vesicle called the autophagosome. This then merges with the lysosome, creating the autolysosome. Within the autolysosome, whatever material is trapped gets digested by the enzymes.

An image showing the formation of the isolation membrane and how it grows into the autophagosome, which then fuses with the lysosome to make the autolysosome.
autophagy

Micro-autophagy is another type of autophagy. Despite the name, both large and small particles can be eaten during micro-autophagy. In micro-autophagy, the lysosome membrane helps to directly engulf whatever is being eaten. The lysosome membrane folds over the material and pulls it inward.

Chaperone-mediated autophagy relies on a protein to help direct and deliver the material to the lysosome. Just as a dating couple needs a chaperone to escort them somewhere, in this type of autophagy, the material needs a chaperone to escort it into the lysosome.

What is the Purpose of Autophagy?

You understand now how autophagy happens, but you might still be unsure why it happens. Why would a cell want to eat itself? That seems counterproductive.

Autophagy is actually a way for the cell to deal with stress. One signal that causes autophagy is a lack of nutrients, including protein, carbohydrates, and even oxygen. Breaking down extra organelles and proteins is a quick way for the cell to get materials it needs to continue living. Unnecessary components are broken down and recycled into more important materials.

Infection is another stress that would activate autophagy in a cell. If a cell is infected, it will try to move the bacteria into its autophagosome to break it down and kill it, stopping the infection.

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