Abstract
Proteasomal and mitochondrial dysfunctions are implicated in chronic neurodegenerative diseases. To investigate the impact of mitochondrial impairment on the proteasome, we treated rat cerebral cortical neurons with oligomycin, antimycin, or rotenone, which inhibit different elements of the electron transport chain. Firstly, we observed a reduction in ubiquitinated proteins and E1 activity. Secondly, we established that 26S proteasomes are disassembled with a decline in activity. Thirdly, we show, to our knowledge for the first time, that calpain activation triggers the selective processing of the 26S proteasome subunit Rpn10. Other proteasome subunits tested were not affected. Calpain also cleaved caspase 3 to an inactive fragment, thus preventing apoptosis that is an energy-dependent cell death pathway. In addition, calpain cleaved the microtubule-associated protein Tau, a major component of neurofibrillary tangles in Alzheimer disease and other tauopathies. Fourthly, we detected a rise in 20S proteasome levels and activity. Finally, we show that both acute (16 h) and long term (up to 7 days) mitochondrial impairment led to down-regulation of ubiquitinated-proteins, 26S proteasome disassembly, and a rise in 20S proteasomes. We postulate that upon mitochondrial dysfunction, ATP depletion and calpain activation contribute to the demise of protein turnover by the ubiquitin/proteasome pathway. The concomitant rise in 20S proteasomes, which seem to degrade proteins in an unregulated and energy-independent manner, in the short term may carry out the turnover of randomly unfolded oxidized proteins. However, if chronic, it could lead to neurodegeneration as regulated protein degradation by the ubiquitin/proteasome pathway is essential for neuronal survival.
Highlights
Proteasomal and mitochondrial dysfunction are implicated in neurodegeneration
In our current study with neurons, we demonstrate that low ATP levels caused by mitochondrial dysfunction correlate with impairment of the ubiquitin/proteasome pathway (UPP); there is a decline in E1 and 26S proteasome activities, both of which are energy-dependent, with a concomitant rise in 20S proteasomes
We characterize some of the mechanisms by which mitochondrial toxins affect the UPP in cortical neurons
Summary
Proteasomal and mitochondrial dysfunction are implicated in neurodegeneration. Results: Upon mitochondrial dysfunction in neurons, calpain-mediated cleavage of Rpn negatively regulates 26S proteasome stability. Impaired clearance of oxidatively modified proteins can cause their aggregation and directly promote progression of the neurodegenerative process [18] Both deleterious consequences of mitochondrial impairment, i.e. restricted ATP-generating capacity and ROS production, are likely to contribute to impaired proteasome-dependent proteolysis in neurons [10, 19, 20]. In our current study with neurons, we demonstrate that low ATP levels caused by mitochondrial dysfunction correlate with impairment of the ubiquitin/proteasome pathway (UPP); there is a decline in E1 and 26S proteasome activities, both of which are energy-dependent, with a concomitant rise in 20S proteasomes. Regulated and ATP-dependent protein degradation via the UPP is essential for long term neuronal survival
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