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Chaperone-Mediated Autophagy: What Is It?

Cellular autophagy, an evolutionary-conserved intracellular breakdown mechanism based on the lysosome, is essential for maintaining homeostatic homeostasis in cells and organisms. In general, autophagy is referred to as macroautophagy. During macroautophagy, an autophagosome with a double-layer membrane structure is created to surround intracellular material.

Selective autophagy that uses chaperones instead of vesicles is known as chaperone-mediated autophagy (CMA). The essence of autophagy is the reconfiguration of the intracellular membrane, and its occurrence can be loosely divided into the following four phases:

Unfolded or improperly folded proteins are specifically targeted for lysosomal breakdown by CMA. With the aid of chaperone proteins, CMA recognizes target proteins with distinctive motifs, which is extremely selective, and transports the target protein into the lysosome for degradation.

Mechanism of CMA

• Proteins containing a specific pentapeptide motif (KFERQ) are recognized by chaperone proteins, such as heat shock homolog 70 (HSC70).
• HSC70 binds to lysosomal-associated membrane protein (LAMP-2A) on the lysosomal membrane, followed by a complex formed by Hsp70 and a their co-chaperones that targets the substrate to LAMP-2A and translocates into the lysosome after de-folding.
• LAMP-2A is a single transmembrane protein that allows unfolded proteins to translocate across the lysosomal membrane into the lysosomal lumen.
• The target proteins enter the lysosomal lumen and are degraded by lysosomal hydrolase.

Regulation of CMA

HSC70, HSP90, LAMP-2A, and autophagy-related genes are important CMA regulators. Target protein recognition and translocation are crucial processes involving HSC70, and LAMP2A is a crucial receptor that is controlled by numerous cellular signaling pathways, including mTOR and JAK/STAT. It has been demonstrated that HSP90 stabilizes LAMP-2A and facilitates target protein translocation to the lysosomal lumen. For the creation of methods that can boost CMA activity and enhance cellular function, it is critical to comprehend how CMA is regulated.

CMA for Targeted Protein Degradation

CMA targets unfolded or misfolded proteins for lysosomal degradation, in contrast to the cellular mechanism of proteolysis targeting chimeras (PROTACs), which are small bifunctional molecules that recruit specific E3 ubiquitin ligases to target proteins, leading to their ubiquitination and and subsequent proteasomal degradation.

Recent studies suggest a possible association between CMA and PROTAC. Certain target proteins of PROTAC are also substrates of CMA. In addition, induction of CMA may increase the efficiency of PROTAC-mediated protein degradation. A possible explanation for this association is that CMA prevents the accumulation of toxic or unwanted proteins in cells by targeting misfolded or damaged proteins for degradation, which may interfere with the efficiency of PROTAC-mediated degradation.

Advances in CMA Technology

In 2018, the lysosomal degradation pathway of PD-L1 and its regulator HIP1R was discovered. The peptide motif MDFSGLSLIKLKKQ on HIP1R has lysosomal targeting activity similar to the KFERQ motif, which can be applied to targeted protein degradation in lysosomes. Since the function of HIP1R depends on two sequence extensions - one that specifically recognizes PD-L1 and the other that mediates its transport to the lysosome - the authors designed the PD-LYSO peptide, which can target PD-L1 in tumor cells and transport it to the lysosome for degradation.

In 2019, a Tat-CDK5-CTM chimeric peptide was designed consisting of transmembrane peptide Tat, ligand of cyclin-dependent kinase 5 (CDK5), and CMA targeting motif CTM. This peptide can specifically bind to CDK5 and mediate its degradation, blocking the interaction between CDK5 and mouse NMDA receptor subunit NR2 (NR2B) and reducing neuronal death in the mouse brain and improving its neural function.

References

1. Wang, H., et al., HIP1R targets PD-L1 to lysosomal degradation to alter T cell-mediated cytotoxicity, Nat. Chem. Biol., 2019. 15(1): 42-50.
2. Zhou, Y.F., et al., The Peptide-Directed Lysosomal Degradation of CDK5 Exerts Therapeutic Effects against Stroke, Aging. Dis., 2019. 10(5): 1140-1145.