Proteins are only able to do their jobs if they fold correctly and adopt their respective and 3D structures. To ensure that nothing goes wrong with the folding process, it is strictly monitored in the cell. The consequences of flawed quality control can be seen, for example, in the deposition of misfolded proteins in neurodegenerative diseases such as Alzheimer’s.
Researchers at the Max Planck Institutes of Neurobiology and of Biochemistry report in the EMBO Journal (“Fluc-EGFP reporter mice reveal differential alterations of neuronal proteostasis in aging and disease”) that they have developed a mouse line that makes the state of protein balance visible in the mammalian brain for the first time. In this way, the processes of protein quality control (proteostasis) can now be studied in healthy and diseased neurons in more detail.
“The cellular protein quality control machinery is important for preventing protein misfolding and aggregation. Declining protein homeostasis (proteostasis) is believed to play a crucial role in age-related neurodegenerative disorders. However, how neuronal proteostasis capacity changes in different diseases is not yet sufficiently understood, and progress in this area has been hampered by the lack of tools to monitor proteostasis in mammalian models,” wrote the investigators.
“Here, we have developed reporter mice for in vivo analysis of neuronal proteostasis. The mice express EGFP-fused firefly luciferase (Fluc-EGFP), a conformationally unstable protein that requires chaperones for proper folding, and that reacts to proteotoxic stress by formation of intracellular Fluc-EGFP foci and by reduced luciferase activity. Using these mice, we provide evidence for proteostasis decline in the aging brain. Moreover, we find a marked reaction of the Fluc-EGFP sensor in a mouse model of tauopathy, but not in mouse models of Huntington’s disease.
“Mechanistic investigations in primary neuronal cultures demonstrate that different types of protein aggregates have distinct effects on the cellular protein quality control. Thus, Fluc-EGFP reporter mice enable new insights into proteostasis alterations in different diseases.”
Visualizing the state of proteostasis in mammalian brains
To study the quality control defects in the individual diseases in more detail, scientists led by Irina Dudanova, PhD, developed the new mouse line, and the state of proteostasis can reportedly be visualized in the mammalian brain for the first time.
The researchers introduced the protein that normally makes fireflies glow into the neurons of the mouse. Optimized to the body temperature of the beetle, the protein needs constant help to fold in “warmer” mammals. Only then can it adopt its correct structure and produce light. In order to precisely track the location of the luminescent protein in the cell, the scientists additionally labeled it with a dye. In this way, they showed that the protein is evenly distributed and glows in healthy neurons.
However, if the protein quality control is overstrained, the beetle protein makes clumps and no longer glows as strongly. The beetle protein, therefore, serves as a proteostasis sensor.
The researchers then crossed the newly developed mouse line with mice that represent different neurodegenerative diseases. In mice showing signs of Alzheimer’s disease, the luminescent protein formed clumps, signaling strong proteostasis disturbance. Interestingly, this was not the case in Chorea Huntington mice.
“The different results were quite surprising,” said Dudanova. “When we had a closer look at the possible reasons, we found that both the misfolded proteins themselves and their location in the cell play an important role.”
While the misfolded protein in the Alzheimer’s model forms deposits in the cell body, it clumps together in the cell nucleus in the Huntington’s mice. Accordingly, protein quality control and its capacity can vary greatly within a cell.
“This shows how complex protein quality control is and how different its alterations can be in individual neurodegenerative diseases,” explained Dudanova.
With the new mouse line, scientists now have a tool to specifically investigate this complexity, both in healthy and in diseased neurons. The researchers plan to investigate other neurodegenerative diseases and to find out whether different cell types in the brain are affected at different rates. In addition, the mouse line could help to assess the effectiveness of different therapies for neurodegenerative diseases.