During axotomy studies, we discovered that the A4-amyloid precursor protein (APP) participates in immune responses of the central nervous system. Since microglia constitute the main immune effector cell population of this response, we used the murine microglial cell line BV-2 to analyze immune response-related APP expression. We show that interaction of microglia with the extracellular environment, particularly components of the extracellular matrix, affects APP secretion as well as intracellular APP biogenesis and catabolism. Fibronectin enhanced APP secretion and decreased the level of cellular mature transmembrane APP, whereas laminin and collagen caused a decrease in secretion and an accumulation of cellular mature APP and APP fragments.

Our results demonstrate that APP plays a fundamental role in the regulation of microglial mobility, i.e. migration, initial target recognition, and binding. The decrease in APP secretion and the concomitant increase in cellular mature APP were accompanied by an accumulation of C-terminal APP fragments. Enrichment of APP and APP fragments is assumedly based on inhibition of catabolic processes that is caused by a disorganization of the actin microfilament network. These observations provide evidence that microglia, which are closely associated with certain amyloid deposits in the brain of Alzheimer patients, can play a key role in initial events of amyloidogenesis by initiating accumulation of APP and also of amyloidogenic APP fragments in response to physiological changes upon brain injury.

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