Inflammatory processes in Alzheimer's disease.

Generation and deposition of amyloid beta peptides and neurofibrillary tangle formation are key mechanisms involved in AD pathogenesis. Recent evidence suggests that inflammatory mechanisms represent a third component which, once initiated by degeneration, may significantly contribute to disease progression and chronicity. Various neuroinflammatory mediators including complement activators and inhibitors, chemokines, cytokines, radical oxygen species and inflammatory enzymes are generated and released by microglia, astrocytes and neurons. Degeneration of aminergic brain stem nuclei such as the locus ceruleus and the nucleus baslis of Meynert may facilitate the occurrence of inflammation in their respective projection areas given the antiinflammatory and neuroprotective action of their key products norepinephrine and acetylcholine. While inflammation has been thought to arise secondary to degeneration, recent experiments demonstrated that inflammatory mediators may stimulate APP processing by upregulation of beta secretase 1 and therefore are able to establish a vicious cycle. Despite the fact that some aspects of inflammation may even exert protective effects to bystander neurons, antiinflammatory treatment strategies should therefore be considered. Non-steroidal antiinflammatory drugs have been shown to reduce the risk and delay the onset to develop AD. However, the precise molecular mechanism underlying this effect is still being debated. Several mechanisms including inhibition of cyclooxygenase 2, gamma secretase or activation of the peroxisome proliferator activated receptor gamma may alone or, more likely, in concert account for the epidemiologically observed protection.