The pathophysiology of triose phosphate isomerase dysfunction in Alzheimer's disease

Alzheimer's disease (AD), the most prevalent neurodegenerative disease worldwide, has two main hallmarks: extracellular deposits of amyloid β-peptide (Aβ) and intracellular neurofibrillary tangles composed by tau protein. Most AD cases are sporadic and are not dependent on known genetic causes; aging is the major risk factor for AD. Therefore, the oxidative stress has been proposed to initiate the uncontrolled increase in Aβ production and also to mediate the Aβ's deleterious effects on brain cells, especially on neurons from the cortex and hippocampus. The production of free radicals in the presence of nitric oxide (NO) yields to the peroxynitrite generation, a very reactive agent that nitrotyrosinates the proteins irreversibly. The nitrotyrosination produces a loss of protein physiological functions, contributing to accelerate AD progression. One of the most nitrotyrosinated proteins in AD is the enzyme triosephosphate isomerase (TPI) that isomerises trioses, regulating glucose consumption by both phosphate pentose and glycolytic pathways and thereby pyruvate production. Hence, any disturbance in the glucose supply could affect the proper brain function, considering that the brain has a high rate of glucose consumption. Besides this directly affecting to the energetic metabolism of the neurons, TPI modifications, such as mutation or nitrotyrosination, increase methylglyoxal production, a toxic precursor of advanced glycated end-products (AGEs) and responsible for protein glycation. Moreover, nitro-TPI aggregates interact with tau protein inducing the intraneuronal aggregation of tau. Here we review the relationship between modified TPI and AD, highlighting the relevance of this protein in AD pathology and the consequences of protein nitro-oxidative modifications.

The physiology and pathophysiology of nitric oxide in the brain

Nitric oxide (NO) is a molecule with pleiotropic effects in different tissues. NO is synthesized by NO synthases (NOS), a family with four major types: endothelial, neuronal, inducible and mitochondrial. They can be found in almost all the tissues and they can even co-exist in the same tissue. NO is a well-known vasorelaxant agent, but it works as a neurotransmitter when produced by neurons and is also involved in defense functions when it is produced by immune and glial cells. NO is thermodynamically unstable and tends to react with other molecules, resulting in the oxidation, nitrosylation or nitration of proteins, with the concomitant effects on many cellular mechanisms. NO intracellular signaling involves the activation of guanylate cyclase but it also interacts with MAPKs, apoptosis-related proteins, and mitochondrial respiratory chain or anti-proliferative molecules. It also plays a role in post-translational modification of proteins and protein degradation by the proteasome. However, under pathophysiological conditions NO has damaging effects. In disorders involving oxidative stress, such as Alzheimer's disease, stroke and Parkinson's disease, NO increases cell damage through the formation of highly reactive peroxynitrite. The paradox of beneficial and damaging effects of NO will be discussed in this review.

Lack of oestrogen protection in amyloid-mediated endothelial damage due to protein nitrotyrosination

Amyloid beta-peptide (Abeta) cytotoxicity, the hallmark of Alzheimer's disease, implicates oxidative stress in both neurons and vascular cells, particularly endothelial cells. Consequently, antioxidants have shown neuroprotective activities against Abeta-induced cytotoxicity. Among the different antioxidants used in both in vitro and in vivo studies, 17beta-oestradiol (E2) has garnered the most attention. Oestrogen attenuated Abeta(E22Q)-induced toxicity in neurons but failed to protect endothelial cells. Here we show that E2-mediated activation of endothelial nitric oxide synthase (eNOS) increases the production of nitric oxide (NO), which, under Abeta(E22Q)-induced oxidative damage, results in the formation of peroxynitrite and increased nitration of tyrosine residues. Inhibition of eNOS prevents nitrotyrosination and permits E2-mediated protection against Abeta(E22Q) on endothelial cells. The main nitrotyrosinated proteins in the presence of E2 and Abeta(E22Q) were identified by MALDI-TOF mass spectrometry. These proteins are key players in the regulation of energy production, cytoskeletal integrity, protein metabolism and protection against oxidative stress. Our data highlight the potential damaging consequences of E2 in vascular disorders dealing with oxidative stress conditions, such as cerebral amyloid angiopathy, stroke and ischaemia-reperfusion conditions.