Neuroinflammatory Cytokines-The Common Thread in Alzheimer's Pathogenesis

This article discusses the potential role of the cytokine cycle and its corollary as drivers of the relentless progression of Alzheimer's neuropathologies, whether they are the result of gene mutations, gene polymorphisms, and/or environmental and comorbid conditions. Based on the discovery of cytokine overexpression as an accompaniment to the dementia-related glial activation, the cytokine hypothesis was proposed. This states that in response to the negative impact on neurons of known and unknown risk factors-which include genetic inheritance, comorbid and environmental factors-microglia and astrocytes become activated and produce excess amounts of the immune-modulating cytokine interleukin-1 (IL-1) and the neuritogenic cytokine S100B, respectively. Finding that these glial events occur in fetuses and neonates with Down syndrome provided the first evidence that productive immune responses by activated glia precede rather than follow overt AD-related pathology. This finding can be added to the demonstration of IL-1 induction of amyloid β (Aβ) precursor protein and astrocyte activation with excess production of neuritogenic factor S100B. This combination suggests that IL-1 and S100B overexpression would favor the Aβ production and dystrophic neurite growth necessary for laying down neuritic Aβ plaques. This, together with demonstration of IL-1 induction of excessive production of the precursors of other features common in AD prompted a corollary to the cytokine hypothesis. The corollary states that regardless of the primary cause of the neuronal insult, the result will be chronic glial activation, which in turn will result in further neuronal injury, still more glial activation with excess cytokine expression and so on. This article discusses known causes, genetic and environmental risk factors, and comorbid conditions, and the potential contribution of glial activation with excessive cytokine expression to each.