Brain cells may be able to call upon a professional organizer should they find themselves hoarding junk. This organizer, like the junk it clears away, works at the molecular scale, but its effects could be momentous. By getting rid of misfolded proteins, the organizer, called Nrf2, can prevent the accumulation of toxic clumps that lead to Parkinson’s disease (PD) and other neurological conditions.
Nrf2, which stands for nuclear factor erythroid 2-related factor, is itself a protein. It has been known to coordinate certain aspects of gene expression, but beyond that, it is also something of a neat freak—as scientists based at the Gladstone Institutes recently discovered.
When these scientists activated Nfr2 in cellular models of PD, they found that they could reduce levels of disease-causing proteins to a normal, healthy range. By restoring protein homeostasis, the scientists prevented the buildup of PD-associated neuron toxicity.
Details of the work appeared December 27 in the Proceedings of the National Academy of Sciences (PNAS), in an article entitled, “Nrf2 Mitigates LRRK2- and α-Synuclein–Induced Neurodegeneration by Modulating Proteostasis.” LRRK2, or leucine-rich repeat kinase 2, and α-synuclein may occur in mutated forms that are associated with increased risks of neurodegenerative diseases.
The PNAS article describes how the scientists used both rat neurons and human neurons created from induced pluripotent stem cells. It also details how the scientists programmed the neurons to express Nrf2 and either mutant LRRK2 or α-synuclein.
Using a one-of-a-kind robotic microscope developed by Steven Finkbeiner's laboratory, the researchers tagged and tracked individual neurons over time to monitor their protein levels and overall health. They took thousands of images of the cells over the course of a week, measuring the development and demise of each one.
“Using a longitudinal imaging platform, we visualized the metabolism and location of mutant LRRK2 and α-synuclein in living neurons at the single-cell level,” wrote the authors of the study. “Nrf2 reduced PD-associated protein toxicity by a cell-autonomous mechanism that was time-dependent.”
Essentially, the Gladstone scientists found that Nrf2 turns on several housecleaning mechanisms in the cell to remove excess LRRK2 and α-synuclein. This action, the scientists emphasized was “cell autonomous,” or restricted to the mutated cells. It was distinct from the other action attributed to Nrf2, a cell-nonautonomous activation of a gene expression program known to mitigate reactive oxygen species.
The new, junk-clearing action appears to depend on two key mechanisms. Each mechanism handles different misfolded proteins.
“Nrf2 decreased steady-state levels of α-synuclein in part by increasing α-synuclein degradation,” the PNAS article indicated. “In contrast, Nrf2 sequestered misfolded diffuse LRRK2 into more insoluble and homogeneous inclusion bodies.”
For mutant LRRK2, Nrf2 drove the protein to gather into incidental clumps that can remain in the cell without damaging it. For α-synuclein, Nrf2 accelerated the breakdown and clearance of the protein, reducing its levels in the cell.
“I am very enthusiastic about this strategy for treating neurodegenerative diseases,” said Steven Finkbeiner, M.D., Ph.D., a senior investigator at Gladstone and the senior author of the PNAS paper. “We've tested Nrf2 in models of Huntington's disease, PD, and amyotrophic lateral sclerosis (ALS), and it is the most protective thing we've ever found. Based on the magnitude and the breadth of the effect, we really want to understand Nrf2 and its role in protein regulation better.”
The scientists say that Nrf2 itself may be difficult to target with a drug because it is involved in so many cellular processes, so they are now focusing on some of its downstream effects: “By identifying the stress response strategies activated by Nrf2, we also highlight endogenous coping responses that might be therapeutically bolstered to treat PD.” The scientists hope to identify other players in the protein regulation pathway that interact with Nrf2 to improve cell health and that may be easier to drug.