Protein aggregation is a continuous cellular process and may be a result of misfolding caused by various stressors. Chaperone complexes, in conjunction with interacting ubiquitin E3 ligases, channel the misfolded protein aggregates to degradation pathways in a process termed as aggrephagy.

Now, a team of scientists led by Liang Ge from Tsinghua University have identified a chaperonin subunit CCT2 as a new type of aggrephagy receptor which specifically accelerates the autophagic clearance of solid aggregates independent of ubiquitination, providing a promising therapeutic target for neurodegenerative diseases.

The findings of the study are reported in the April 1, 2022, online edition of Cell. Ge is an associate professor at the School of Life Sciences, Tsinghua University where he studies how cells maintain homeostasis in response to stresses employing cutting-edge techniques of cell biology, biochemistry, mouse model and computational biology.

Accumulation of protein aggregates is a hallmark of many diseases ranging from neurodegenerative disorders to type 2 diabetes. Chaperones directly bind to the misfolded protein to counteract aggregation. The eukaryotic chaperone TRiC assists in correct folding of cytosolic protein substrates and prevents protein aggregation.

Speaking to Bioworld Science, Ge said that "CCT2, a subunit of TRiC, works as an aggrephagy receptor and facilitates the clearance of aggregation-prone proteins. In the presence of misfolded proteins, TRiC helps refold these proteins and prevent aggregation as the first line of defense. When excessive misfolded protein accumulates to form protein aggregates, a fraction of the TRiC subunit CCT2 becomes monomeric and switches its function to autophagy receptor as a second line of defense to maintain proteostasis. Thus, our study gives an insight into the crosstalk between chaperone system and autophagy system for proteostasis."

A big challenge for the authors in the study was to comprehensively identify new autophagy receptors that target aggrephagy. To tackle the difficulty, Ge says that they developed "an in vitro reconstitution system to recapitulate the recognition of autophagic membranes to the aggregates." This led to the finding that protein aggregates formed in the cell have different abilities to be recognized by the autophagic membrane.

The difference allowed the researchers to establish a sorting system isolating the two populations of aggregates with high and low autophagic recognition. Followed by comparative proteomics, the authors were able to identify several candidates on the aggregates which are responsible for the recognition by autophagic membranes.

Aggregation-prone proteins form phase-separated biomolecular condensates before morphing into pathogenic solid protein aggregates. These liquid droplets are cleared by autophagy.

However, little evidence exists on how solid protein aggregates are recognized and selectively cleared by aggrephagy. Surprisingly, the authors found that CCT2 specifically targeted solid aggregates instead of liquid granules for clearance. Ge added that "we were excited about this finding because so far there was no autophagy receptor reported to particularly degrade solid aggregates. Another interesting point of the work is the difference between CCT2 and known aggrephagy receptors. The known aggrephagy receptors like P62, NBR1, and TAX1BP1 associate with the ubiquitin chain on aggregate and prefer to degrade aggregates with liquidity, whereas CCT2 binds to aggregates independent of the ubiquitin and specifically clears solid aggregates, suggesting that CCT2 acts in parallel with the known receptors in clearance of solid and liquid aggregates."

Considering that the solid aggregates are pathogenic in nature, this preference for solid aggregates makes CCT2 a potential therapeutic target against neurodegenerative diseases.

Indeed, the authors found that CCT2 expression significantly reduced many neurotoxic proteins including, mutant tau, FUS and SOD1 in vitro as well as in the mouse brain. Therefore, Ge believes that enhancing the CCT2 level in patients' brain may be a potential therapy for neurodegenerative disorders.