How Extracellular Chaperones Help Maintain Proteostasis

Maintaining proper proteostasis is crucial for preventing protein aggregation and the effects of misfolded proteins, which are associated with many age-related diseases. While intracellular protein degradation pathways have been well-studied, the mechanisms behind extracellular protein degradation have remained unclear.

Now, in a paper published recently in Scientific Advances, Chiba University researchers report the identification of several misfolded proteins that are substrates for alpha 2-macroglobulin (α₂M), an extracellular chaperone. The team also developed a lysosomal internalization assay for α₂M, which revealed that α₂M mediates the lysosomal degradation of extracellular misfolded proteins.

 “Thus far, no quantitative method has been available to detect the lysosomal degradation of extracellular proteins. Therefore, we established a fluorescence internalization assay to measure α₂M-mediated lysosomal degradation,” explained senior author Eisuke Itakura.

To create the assay, chaperone α₂M was tagged with red and green fluorescence proteins (RFP and GFP, or RG) that could be visually detected inside cells. When α₂M-RG was internalized into lysosomes, the fluorescence of RFP, but not GFP, was detected. This is because GFP is prone to lysosomal degradation, but RFP is quite resistant. “So, in this assay, if α₂M is inducing degradation of misfolded proteins, RFP should accumulate in the cell, producing a red fluorescence,” Itakura added. These results were also validated in red blood cell lysates.

The group also probed the significance of why multiple extracellular chaperones exist inside our body by comparing the substrate specificities of α₂M and clusterin, another extracellular chaperone. Previously, the group had reported that clusterin also plays a part in the extracellular degradation of proteins like amyloid-beta, the extracellular aggregation of which has been implicated in Alzheimer’s disease. The group found that while α₂M and clusterin had overlapping functions, their pathways were not redundant. α₂M was seen to recognize the defective proteins more prone to aggregation. According to the researchers, this finding lends credence to the theory that an array of extracellular chaperones cooperates to protect us from the spectrum of misfolded proteins likely to be found in the body.

“In the future, elucidating the molecular mechanism of protein degradation by extracellular chaperones may prove useful in treating related diseases like Alzheimer’s, Itakura said. “By degrading and removing abnormal proteins that accumulate outside cells, extracellular chaperones have the potential to be a valuable therapeutic tool.”