Autophagy and Lysosomal Degradation

Autophagy is the regulated process by which cells engulf and deliver their own components to lysosomes for degradation, recycling materials and removing damaged structures.

Definition

Autophagy is the process by which a cell sequesters portions of its own cytoplasm or organelles in membranes and delivers them to lysosomes for degradation; lysosomal degradation is the breakdown of macromolecules by the acidic hydrolases of the lysosome.

Scope

This topic covers the lysosome as the cell's degradative compartment, the routes by which material reaches it, and the autophagy pathway in which autophagosomes capture cytoplasm and organelles, including the roles of autophagy in nutrient stress, quality control, and homeostasis.

Core questions

• What is the lysosome and how does it degrade material?
• How does an autophagosome form and capture cargo?
• How is autophagy induced by starvation and stress?
• How does autophagy contribute to quality control and homeostasis?

Key theories

Lysosome as the degradative organelle

de Duve identified the lysosome as a membrane-bound compartment of acid hydrolases that degrades macromolecules, providing the destination for autophagic and endocytic cargo.

Genetic basis of autophagy

Conserved autophagy genes, defined in yeast, direct the formation of autophagosomes and their fusion with the lysosome, establishing autophagy as a regulated pathway.

Mechanisms

Lysosomes contain acid hydrolases that degrade proteins, lipids, nucleic acids, and carbohydrates delivered by endocytosis or autophagy. In macroautophagy, a cup-shaped membrane expands to enclose cytoplasm or damaged organelles, forming a double-membrane autophagosome that fuses with a lysosome so its contents are degraded and the products recycled. A set of conserved autophagy proteins controls initiation, membrane growth, and cargo selection, and the pathway is strongly induced by nutrient starvation and other stresses.

Clinical relevance

Autophagy and lysosomal degradation provide nutrients during stress, dispose of damaged components, and maintain cellular quality control, making them central to cell homeostasis. The treatment here is descriptive and non-prescriptive.

History

de Duve's discovery of the lysosome in the mid-twentieth century revealed the cell's degradative compartment; Ohsumi's genetic dissection of autophagy in yeast in the 1990s defined the machinery and transformed autophagy into a molecularly understood pathway.

Key figures

• Christian de Duve
• Yoshinori Ohsumi

Seminal works

• ohsumi2014
• deduve1966

Frequently asked questions

What is an autophagosome?

It is a double-membrane vesicle that forms to enclose part of the cytoplasm or a damaged organelle and then fuses with a lysosome so its contents can be broken down.

When is autophagy most active?

Autophagy is strongly increased during nutrient starvation, when recycling internal components provides building blocks, and it also rises when the cell must clear damaged structures.