The brain as a target for inflammatory processes and neuroprotective strategies.

The importance of glial cell-propagated inflammation (i.e., neuroinflammation) disorders such as Alzheimer's disease (AD) was viewed previously as a bystander effect, or epiphenomenon, with inflammation occurring when damaged neurons elicit an activation response by glia. However, an accumulating body of evidence has challenged this earlier perspective and indicates a more active role of neuroinflammation in the pathophysiology of progressive neurodegenerative disorders such as AD, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis. This insight into pathophysiology evolved in concert with the appreciation that the brain is not as immunologically privileged as once thought. The central nervous system (CNS) has its own resident immune system, in which glial cells (microglia, astrocytes, and oligodendrocytes) not only serve supportive and nutritive roles for neurons but also engage from time to time in several "inflammatory" processes that defend the CNS from pathogens and help it to recover from stress and injury. These otherwise "normal" glial functions can sometimes result in a more severe and chronic neuroinflammatory cycle that actually promotes or propagates neurodegenerative disease. Excessive glial cell activation may thus constitute a viable target for the discovery of and development of neurodegenerative disease therapeutics. Suggestive clinical evidence in support of neuroinflammation as a drug discovery target for chronic neurodegenerative diseases, such as AD, comes from epidemiological and genetic linkage data. For example, long-term use of nonsteroidal anti-inflammatory drugs is correlated with a protective effect against AD, and certain polymorphisms in the genes for interleukin 1 and other proinflammatory mediator genes are associated with increased risk. In AD and Parkinson's disease, activated microglia and complement proteins have been identified in the brain regions most affected in these disorders. This report will briefly review selected clinical and preclinical data that reflect the prevailing approaches targeting neuroinflammation as a pathophysiological process contributing to the onset or progression of neurodegenerative diseases, as well as their neuroprotective potential.