Immunoproteasome and LMP2 polymorphism in aged and Alzheimer's disease brains

Abstract

In this study, we investigated the presence and role of immunoproteasome and its LMP2 subunit polymorphism at codon 60 in Alzheimer's disease (AD). Immunoproteasome was present in brain areas such as hippocampus and cerebellum and localized in neurons, astrocytes and endothelial cells. A higher expression of immunoproteasome was found in brain of AD patients than in brain of non-demented elderly, being its expression in young brain negligible or absent. Furthermore, AD affected regions showed a partial decrease in proteasome trypsin-like activity. The study of LMP2 polymorphism (R/H) showed that it does not influence LMP2 expression (neither the mRNA nor mature protein) in brain tissue. However, control brain areas of AD patients carrying the RR genotype showed an increased proteasome activity in comparison with RH carriers. To test whether this effect of the genotype might be related to AD onset we performed a genetic study, which allowed us to exclude an association of LMP2 codon 60 polymorphism with AD onset, despite its influence on the proteasome activity in human brain.

Introduction

Alzheimer's disease (AD) is a devastating neurodegenerative disorder of the central nervous system (CNS) occurring most frequently in later stages of adulthood. AD is associated with a specific pattern of pathological changes in brain that result in neurodegeneration and progressive development of dementia. The pathological hallmarks of AD are neuronal loss accompanied by intraneuronal neurofibrillary tangles (NFTs) formed of tau-based paired helical filaments (PHFs) and extracellular senile plaques (SPs) of β-amyloid [25], [41]. Recently, it has been reported that PHFs inhibit proteasome activity and it has been suggested that this inhibition may induce neuronal damage in AD [21]. Furthermore, the ubiquitin-proteasome system is also involved in the control of the physiological maturation of the β-amyloid precursor protein (βAPP) through the modulation of the intracellular concentration of presenilins [7]. Proteasomes are multicatalytic enzyme complexes that are responsible for degradation of short-lived, damaged and antigenic proteins [39]. 26S proteasomes consist of a catalytic 20S core and either the 19S or 11S regulatory complex. 20S proteasomes are a four-ring structure with seven different subunits in each ring, arrayed as α₇β₇β₇α₇ [46]. In cells exposed to IFN-γ or tumor necrosis factor-α (TNF-α), the β subunits with catalytic activity (β1, β2, β5) of constitutive proteasomes are replaced by other catalytic subunits, respectively, LMP2, MECL-1 and LMP7 that are incorporated into an alternative proteasome form [26]. This isoform known as immunoproteasome, has an altered activity of the three catalytic sites [40], which enhance the capacity to generate antigenic epitopes and modify turnover rate of specific proteins such as tau [5], [16], [24]. Constitutive and immunoproteasomes usually coexist in cell, but the ratio between the two isoforms varies in base of cell type, tissue, condition of environment, etc. [36]. Recently, immunoproteasomes have been detected in human brains [10], [37]. Hence, we investigated its expression in brains from AD patients and age-matched controls. Furthermore, we investigated whether immunoproteasome expression was related not only to AD neuroinflammation but also to the ageing process in CNS. Moreover, evidences for chronic immune reaction in AD brains have emerged by genetic association of AD with polymorphisms of several cytokines and detection in brains of inflammatory mediators [2], [6], [31], [29]. One of these cytokines, namely TNF-α, has been shown to play a role in neuronal degeneration [48]. We previously reported that the polymorphism at codon 60 of the LMP2 gene influences the susceptibility to TNF-α-induced apoptosis [33]. This polymorphism is a non-conservative nucleotide base pair change at amino acid position 60 (in exon 3) in LMP2, resulting in two alleles, Arginine (R) or Histidine (H), which has been already associated with different autoimmune diseases [19], [45], [30]. Hence, we investigated the possible role of LMP2 codon 60 polymorphism on immunoproteasome expression, brain proteasome activity as well as on risk and age of onset of AD.