Depletion of 26S Proteasomes in Mouse Brain Neurons Causes Neurodegeneration and Lewy-Like Inclusions Resembling Human Pale Bodies

Abstract

Ubiquitin-positive intraneuronal inclusions are a consistent feature of the major human neurodegenerative diseases, suggesting that dysfunction of the ubiquitin proteasome system is central to disease etiology. Research using inhibitors of the 20S proteasome to model Parkinson's disease is controversial. We report for the first time that specifically 26S proteasomal dysfunction is sufficient to trigger neurodegenerative disease. Here, we describe novel conditional genetic mouse models using the Cre/loxP system to spatially restrict inactivation of Psmc1 (Rpt2/S4) to neurons of either the substantia nigra or forebrain (e.g., cortex, hippocampus, and striatum). PSMC1 is an essential subunit of the 26S proteasome and Psmc1 conditional knock-out mice display 26S proteasome depletion in targeted neurons, in which the 20S proteasome is not affected. Impairment of specifically ubiquitin-mediated protein degradation caused intraneuronal Lewy-like inclusions and extensive neurodegeneration in the nigrostriatal pathway and forebrain regions. Ubiquitin and ␣-synuclein neuropathology was evident, similar to human Lewy bodies, but interestingly, inclusion bodies contained mitochondria. We support this observation by demonstrating mitochondria in an early form of Lewy body (pale body) from Parkinson's disease patients. The results directly confirm that 26S dysfunction in neurons is involved in the pathology of neurodegenerative disease. The model demonstrates that 26S proteasomes are necessary for normal neuronal homeostasis and that 20S proteasome activity is insufficient for neuronal survival. Finally, we are providing the first reproducible genetic platform for identifying new therapeutic targets to slow or prevent neurodegeneration.

Dendritic spinopathy in transgenic mice expressing ALS/dementia-linked mutant UBQLN2

“…In neuronal cells, dynamic control of protein stability is crucial for the development, function, maintenance, and strength of synapses, and the UPS plays an important role in the regulation of synaptic proteins (13). It has been shown that depletion of the 26S proteasomes in mice leads to neurodegeneration, and therefore they are essential for normal neuronal homeostasis and survival (19). Remarkably, proteasomal impairment in individual neurons is sufficient to block activity-induced spine outgrowth (20).…”

The role of protein clearance mechanisms in organismal ageing and age-related diseases

“…In an alternative approach, conditional proteasome subunits knockout mouse models were generated since constitutive deletion is embryonically lethal 11 . Deletion of the ATPase subunit Psmc1/Rpt2 in dopaminergic neurons lead to ubiquitin and a-synuclein positive inclusions which resulted in neuronal death, thus resembling the hallmarks of PD 107 . In addition, variations in the gene PSMC4/Rpt3 correlate with the age of PD onset in patients 108 .…”

Coordinate regulation of autophagy and the ubiquitin proteasome system by MTOR

“…In cellular and animal models of these diseases, inhibition of either the proteasome or autophagy can lead to the buildup of such aggregated proteins and the death of neurons. [19][20][21][22] Rapamycin was shown by Rubinsztein and colleagues in cell and animal models to stimulate the clearance of protein aggregates and to protect neurons against the cytotoxicity caused by such aggregation-prone proteins. 23 Beneficial effects of rapamycin have also been reported in animal models of a wide range of neurodegenerative diseases, including Parkinson, Alzheimer, and Huntington diseases, as well as amyotrophic lateral sclerosis.…”