Altered Proteasomal Function in Sporadic Parkinson’s Disease Kevin St. P. McNaught,* Roger Belizaire,† Ole Isacson,† Peter Jenner,‡ and C. Warren Olanow* *Department of Neurology, Mount Sinai School of Medicine, New York, New York 10029; †Neuroregeneration Laboratories, Harvard Medical School and McLean Hospital, Belmont, Massachusetts 02478; and ‡Neurodegenerative Disease Research Centre, GKT School of Biomedical Sciences, King’s College, London Bridge, London SE1 1UL, United Kingdom Received August 5, 2002; accepted August 15, 2002 Parkinson’s disease (PD) is characterized patholog- ically by preferential degeneration of the dopaminer- gic neurons in the substantia nigra pars compacta (SNc). Nigral cell death is accompanied by the accu- mulation of a wide range of poorly degraded proteins and the formation of proteinaceous inclusions (Lewy bodies) in dopaminergic neurons. Mutations in the genes encoding -synuclein and two enzymes of the ubiquitin-proteasome system, parkin and ubiquitin C- terminal hydrolase L1, are associated with neurode- generation in some familial forms of PD. We now show that, in comparison to age-matched controls, -sub- units (but not -subunits) of 26/20S proteasomes are lost within dopaminergic neurons and 20S proteaso- mal enzymatic activities are impaired in the SNc in sporadic PD. In addition, while the levels of the PA700 proteasome activator are reduced in the SNc in PD, PA700 expression is increased in other brain regions such as the frontal cortex and striatum. We also found that levels of the PA28 proteasome activator are very low to almost undetectable in the SNc compared to other brain areas in both normal and PD subjects. These findings suggest that failure of the ubiquitin- proteasome system to adequately clear unwanted pro- teins may underlie vulnerability and degeneration of the SNc in both sporadic and familial PD. © 2002 Elsevier Science (USA) Key Words: Parkinson’s disease (PD); ubiquitin-pro- teasome system; -synuclein; PA700; PA28; aggresome; substantia nigra pars compacta (SNc); Lewy body in- clusion. INTRODUCTION The pathological hallmark of Parkinson’s disease (PD) is selective degeneration of the dopamine-contain- ing neurons in the substantia nigra pars compacta (SNc) (1). Nigral pathology is associated with the for- mation of intracellular inclusions (Lewy bodies) which contain a wide range of proteins, including -synuclein, ubiquitin, neurofilament, and nitrated proteins (2–5). In addition, levels of oxidatively damaged proteins are increased as evident from an elevation in the content of protein carbonyls and 4-hydroxynonenal (HNE) pro- tein adducts in the SNc in PD (6, 7). There is also evidence to suggest that there is a general increase in protein aggregation in the substantia nigra in this disorder (8). These observations have led to the sug- gestion that impaired protein clearance and/or the overwhelming production of abnormal proteins play an important role in the neurodegenerative process occur- ring in PD (9). The ubiquitin-proteasome system (UPS) is the pri- mary biochemical pathway responsible for the degra- dation of normal and abnormal intracellular proteins, and its failure leads to protein accumulation and cell death (10, 11). Several studies have demonstrated that mutations in the genes encoding two enzymes of the UPS, parkin and ubiquitin C-terminal hydrolase L1 (UCH-L1), are associated with cases of familial PD (12, 13). In addition, mutations in the gene encoding -synuclein are associated with nigral dopaminergic cell death with Lewy body formation in rare familial forms of PD (14 –16). -Synuclein is a 140-amino-acid protein and is a substrate for the UPS (12, 17–19). Mutations in -synuclein cause the protein to misfold and aggregate, resist proteolysis, and inhibit proteaso- mal function (17, 20 –22). Thus, the neurodegenerative process in -synuclein-liked familial PD could also re- late to failure of the UPS to clear mutant -synuclein and other proteins. Although these gene defects are not found in cases of sporadic PD which constitute the large majority of patients, they suggest that impair- ment of the UPS could also underlie nigral pathology in this disorder (9). Indeed, the accumulation of both nonubiquitinated and ubiquitinated proteins in the SNc and in Lewy bodies points to a primary defect in the proteasome, the central component of the UPS responsible for degrading the two groups of unwanted proteins (9, 10). Proteasomes are multicatalytic proteases found in the cytoplasm, endoplasmic reticulum, perinuclear re- gion, and nucleus of eukaryotic cells (10). The 20S proteasome is an assembly of two outer heptameric Experimental Neurology 179, 38 – 46 (2003) doi:10.1006/exnr.2002.8050 38 0014-4886/02 $35.00 © 2002 Elsevier Science (USA) All rights reserved.