Search

Mutations in leucine-rich repeat kinase 2 (LRRK2) and α-synuclein lead to Parkinson's disease (PD). Disruption of protein homeostasis is an emerging theme in PD pathogenesis, making mechanisms to reduce the accumulation of misfolded proteins an attractive therapeutic strategy. We determined if activating nuclear factor erythroid 2-related factor (Nrf2), a potential therapeutic target for neurodegeneration, could reduce PD-associated neuron toxicity by modulating the protein homeostasis network. 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. Nrf2 reduced PD-associated protein toxicity by a cell-autonomous mechanism that was time-dependent. Furthermore, Nrf2 activated distinct mechanisms to handle different misfolded proteins. Nrf2 decreased steady-state levels of α-synuclein in part by increasing α-synuclein degradation. In contrast, Nrf2 sequestered misfolded diffuse LRRK2 into more insoluble and homogeneous inclusion bodies. By identifying the stress response strategies activated by Nrf2, we also highlight endogenous coping responses that might be therapeutically bolstered to treat PD., Competing Interests: The authors declare no conflict of interest.

In polyglutamine (polyQ) diseases, only certain neurons die, despite widespread expression of the offending protein. PolyQ expansion may induce neurodegeneration by impairing proteostasis, but protein aggregation and toxicity tend to confound conventional measurements of protein stability. Here, we used optical pulse labeling to measure effects of polyQ expansions on the mean lifetime of a fragment of huntingtin, the protein that causes Huntington's disease, in living neurons. We show that polyQ expansion reduced the mean lifetime of mutant huntingtin within a given neuron and that the mean lifetime varied among neurons, indicating differences in their capacity to clear the polypeptide. We found that neuronal longevity is predicted by the mean lifetime of huntingtin, as cortical neurons cleared mutant huntingtin faster and lived longer than striatal neurons. Thus, cell type-specific differences in turnover capacity may contribute to cellular susceptibility to toxic proteins, and efforts to bolster proteostasis in Huntington's disease, such as protein clearance, could be neuroprotective.

Misfolded proteins have been implicated in most of the major neurodegenerative diseases, and identifying drugs and pathways that protect neurons from the toxicity of misfolded proteins is of paramount importance. We invented a form of automated imaging and analysis called robotic microscopy that is well suited to the study of neurodegeneration. It enables the monitoring of large cohorts of individual neurons over their lifetimes as they undergo neurodegeneration. With automated analysis, multiple endpoints in neurons can be measured, including survival. Statistical approaches, typically reserved for engineering and clinical medicine, can be applied to these data in an unbiased fashion to discover whether factors contribute positively or negatively to neuronal fate and to quantify the importance of their contribution. Ultimately, multivariate dynamic models can be constructed from these data, which can provide a systems-level understanding of the neurodegenerative disease process and guide the rationale for the development of therapies.

CDK5/p35 is a cyclin-dependent kinase essential for normal neuron function. Proteolysis of the p35 subunit in vivo results in CDK5/p25 that causes neurotoxicity associated with a number of neurodegenerative diseases. Whereas the mechanism by which conversion of p35 to p25 leads to toxicity is unknown, there is common belief that CDK5/p25 is catalytically hyperactive compared to CDK5/p35. Here, we have compared the steady-state kinetic parameters of CDK5/p35 and CDK5/p25 towards both histone H1, the best known substrate for both enzymes, and the microtubule-associated protein, tau, a physiological substrate whose in vivo phosphorylation is relevant to Alzheimer's disease. We show that the kinetics of both enzymes are the same towards either substrate in vitro. Furthermore, both enzymes display virtually identical kinetics towards individual phosphorylation sites in tau monitored by NMR. We conclude that conversion of p35 to p25 does not alter the catalytic efficiency of the CDK5 catalytic subunit by using histone H1 or tau as substrates, and that neurotoxicity associated with CDK5/p25 is unlikely attributable to CDK5 hyperactivation, as measured in vitro.