What are the different types of autophagy?

There are three primary types of autophagy in mammalian cells: Microautophagy, Macroautophagy, and Chaperone-Mediated Autophagy (CMA). All three types of autophagy culminate in delivering the cargo to the lysosome for degradation and recycling but through distinctly different mechanisms. 

• Macroautophagy - Macroautophagy is the main autophagic pathway. During macroautophagy, a double-membraned structure called an autophagosome forms around cellular material to be degraded, such as damaged organelles or protein aggregates. The autophagosome passes through the cell’s cytoplasm to reach a lysosome, a cellular structure filled with enzymes. The fusion of the autophagosome and lysosome results in the formation of an autolysosome. The enzyme acidic lysosomal hydrolase degrades the content of the autolysosome, eliminating dysfunctional cellular components, and releasing useful biomolecules back into the cell to be used as nutrients during times of starvation or stress or for building new cellular structures.

• Microautophagy - Microautophagy is a cellular process in which small portions of the cell's own cytoplasm and its contents are directly engulfed and taken up by lysosomes for degradation. During this process, the cytoplasmic contents enter the lysosome directly through an invagination (inward folding) or protrusion of the lysosomal membrane. Microautophagy plays a role in maintaining membrane homeostasis and organelle size, and supporting cell survival under limited nitrogen conditions. We have limited tools for studying microautophagy, so our understanding of its regulation and potential relevance to human health and disease remains relatively limited.

• Chaperone-mediated autophagy (CMA) - Chaperone-Mediated Autophagy (CMA) is a complex and unique form of autophagy in which specific proteins called chaperone proteins recognize and target damaged or dysfunctional proteins for degradation. These chaperone proteins deliver the targeted proteins directly to lysosomes, where they are translocated across the lysosomal membrane and broken down into their constituent parts. CMA is a more selective process that targets individual proteins. This type of autophagy is particularly important for maintaining protein quality control within cells and ensuring the removal of damaged or misfolded proteins.