Alzheimer's disease: Characterization, evolution and implications of the neuroinflammatory process

Abstract

Alzheimer's Disease (AD) is a neurodegenerative disease characterized by a progressive memory loss and severe cognition decline, associated to degradation of cholinergic neurons in many areas of central nervous system (CNS), with a dramatic reduction in neurotransmitters, specially acetylcholine. The illness progression is also accompanied by behavior changes, leading to individual incapacity and depression, and evolving to dementia and death. AD is related to cerebral aging and located loss of neurons, mainly at hippocampus and basal pro-encephalic tissue. Pathohystologically, AD is characterized by extracellular deposits of senile plaques formed by insoluble fragments of amyloid protein precursor (Aβ) and intracellular neurofibrillary tangles in the brain, constituted by fragments of hyperphosphorylated TAU protein, with a massive loss in neurons. Despite typical behavior aspects of AD installation, recent studies, especially from the last decade, have evidenced the occurrence of a complex inflammatory process in the neuronal tissue. The relevancy of neuroinflammation in the installation, progression and severity of AD, as well as the mechanisms of immune system activation and key cells in the initial shot of inflammatory cascade in CNS, as microglia and astrocytes, have been demonstrated by important reviews in the literature. Activation of microglia can lead to recruitment of astrocytes that increase the inflammatory response to the extracellular Aβ deposits. This neuroinflammatory component of AD is additionally characterized by a local acute phase response mediated by cytokines, complement cascade activation and induction of an enzymatic inflammatory system, as induced NO synthase (iNOS) and generation of cyclooxygenase 2 (COX-2). Then, astrocytes participate of the degradation and remotion of Aβ, acting as a protective barrier between Aβ deposits and neurons. The multifactorial character of the inflammatory response is characterized by the occurrence of a wide diversity of pro- and anti-inflammatory mediators, some of them being responsible for promotion of neurodegenerative mechanisms, while others could limit the advance of inflammation or exert benefit neurotrophic effects. Therefore, it includes not only a single mediator, but a set of inflammatory agents that would determine the prevalence of benefic or deletery effects during AD progression.