Roles of amyloid β-peptide-associated oxidative stress and brain protein modifications in the pathogenesis of Alzheimer's disease and mild cognitive impairment

Abstract

Oxidative stress has been implicated to play a crucial role in the pathogenesis of a number of diseases, including neurodegenerative disorders, cancer, and ischemia, just to name a few. Alzheimer disease (AD) is an age-related neurodegenerative disorder that is recognized as the most common form of dementia. AD is histopathologically characterized by the presence of extracellular amyloid plaques, intracellular neurofibrillary tangles, the presence of oligomers of amyloid β-peptide (Aβ), and synapse loss. In this review we discuss the role of Aβ in the pathogenesis of AD and also the use of redox proteomics to identify oxidatively modified brain proteins in AD and mild cognitive impairment. In addition, redox proteomics studies in in vivo models of AD centered around human Aβ(1–42) are discussed.

Introduction

Oxidative stress has been implicated to play a crucial role in the pathogenesis of a number of diseases, including neurodegenerative disorders, cancer, and ischemia [1]. Among all the body organs, the brain is particularly vulnerable to oxidative damage because of its high utilization of oxygen, increased levels of polyunsaturated fatty acid (that are readily attacked by free radicals), and relatively high levels of redox transition metal ions; in addition, the brain has relatively low levels of antioxidants [2], [3], [4], [5]. The presence of iron ion in an oxygen-rich environment can further lead to enhanced production of hydroxyl free radicals and ultimately lead to a cascade of oxidative events.

Oxidative stress occurs due to an imbalance in the pro-oxidant and antioxidant levels. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are highly reactive with biomolecules, including proteins, lipids, carbohydrate, DNA, and RNA [6]. Oxidative damage to these moieties leads to cellular dysfunctions [1], [2], [5], [7], [8], [9]. The markers of oxidative stress that are commonly used in biological samples include protein carbonyls and 3-nitrotyrosine for protein oxidation; thiobarbituric acid-reactive substances (TBARS), free fatty acid release, iso- and neuroprostane formation, 2-propen-1-al (acrolein), and 4-hydroxy-2-trans-nonenal (HNE) for lipid peroxidation; advanced glycation end products for carbohydrates; 8-OH-2′-deoxyguanosine and 8-OH-guanosine and other oxidized bases, and altered DNA repair mechanisms for DNA and RNA oxidation. Among the earliest of these changes after an oxidative insult are increased levels of toxic carbonyls, 3-nitrotyrosine (3-NT), and HNE [2], [4], [7], [10], [11], [12], [13].

Protein carbonyl groups are generated by direct oxidation of certain amino acid side chains (i.e., Lys, Arg, Pro, Thr, and His); peptide backbone scission; Michael addition reactions of His, Lys, and Cys residues with products of lipid peroxidation; or glycol oxidation of Lys amino groups [6], [14], [15], [16], [17]. Protein carbonyls are stable and hence are widely used as markers to assess the extent of oxidation of proteins under both in vivo and in vitro conditions [14], [15], [17], [18]. The levels of protein carbonyls can be determined experimentally by derivatization of the carbonyl groups with 2,4-dinitrophenylhydrazine (DNPH), followed by spectroscopic or immunochemical detection of the resulting hydrazone product [6], [15], [19].

In addition to direct effects, oxidative stress could also stimulate additional damage in brain via the overexpression of inducible nitric oxide synthase (iNOS) and the action of constitutive neuronal NOS (nNOS) that increase the production of nitric oxide (NO) via the catalytic conversion of arginine to citrulline. Nitric oxide reacts with superoxide anion (O₂⁻) at a diffusion-controlled rate to produce peroxynitrite (ONOO⁻). Peroxynitrite is highly reactive, with a half-life of less than a second, and can undergo a variety of chemical reactions depending upon its cellular environment, the presence of CO₂, and the availability of reactive targets forming modifications such as 3-NT (Fig. 1) [20], [21]. 3-Nitrotyrosine is a covalent protein modification that has been used as a marker of nitrosative stress in a variety of disease conditions [22], [23]. Peroxynitrite can also react with sulfhydryl compounds intracellularly due to the high concentration of free thiols within the cell [24]. Sulfhydryls can also react via S-nitrosylation with NO to form a nitrosothiol. Cysteine residues are preferentially nitrosylated due to favorable reaction kinetics [25], [26].

Protein oxidation could lead to aggregation or dimerization of proteins; in addition, protein oxidation can also lead to unfolding or conformational changes in the protein, thereby exposing more hydrophobic residues to an aqueous environment. This exposure may lead to a loss of structural or functional activity and protein aggregation and subsequent accumulation of the oxidized proteins as cytoplasmic inclusions, such as tau aggregation in the form of tangles and amyloid-β aggregation as senile plaques, as observed in Alzheimer disease (AD) [27], [28]. The accumulation of oxidatively modified proteins may disrupt cellular functions by alterations in protein expression and gene regulation, protein turnover, modulation of cell signaling, induction of apoptosis and necrosis, etc., which suggests that protein oxidation could have both physiological and pathological significance [6], [29], [30], [31]. In our laboratory, we used redox proteomics analyses to identify specific oxidatively modified brain proteins, carbonylated proteins, in neurodegenerative diseases and models thereof [10], [12], [13], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41].

Mild cognitive impairment (MCI) is considered an intermediate phase between normal aging and Alzheimer disease. Some researchers believe that MCI is in fact the earliest form of AD [42]. Persons with MCI have cognitive complaint, decline in cognition compared to previous years, and, most notably, no signs of dementia. Additionally, activities of daily living are not affected. These characteristics require informant confirmation for diagnosis of MCI. Accurate diagnosis can be confirmed only by medical examination to establish a level of cognitive decline [43], [44], [45]. Pathologically, MCI has also been characterized using magnetic resonance imaging technology to show measurable atrophy in the hippocampus and entorhinal cortex [46], [47]. Alzheimer disease patients have considerable neurodegeneration in these aforementioned areas. Because the hippocampus is the region of the brain primarily responsible for processing of memory, atrophy in this brain region is consistent with memory loss in AD and MCI.

An age-related neurodegenerative disorder, AD is recognized as the most common form of dementia. AD is clinically associated with cognitive impairment, loss of language and motor skills, and changes in behavior. AD is pathologically characterized by the presence of extracellular senile plaques, which consist of a core of Aβ, and intracellular neurofibrillary tangles (NFTs), and loss of synaptic connections within entorhinal cortex and progressing into the hippocampus and cortex. NFTs are composed of paired helical filaments that consist of aggregates of the hyperphosphorylated microtubule-associated protein tau [48], whereas senile plaques (SPs) are rich in Aβ [49].

In this paper, we review the involvement of Aβ and other sources of oxidative stress in AD brain and the use of redox proteomics to identify oxidatively modified brain proteins in AD and MCI. New insights into potential oxidative mechanisms underlying molecular processes in these disorders and progression from MCI to AD have emerged.