Effects of incorporation of immunoglobulin G and complement component C1q on uptake and degradation of Alzheimer's disease amyloid fibrils by microglia

Microglia are macrophage-like immune system cells found in the brain. They are associated with Alzheimer's Disease plaques, which contain fibrillar beta-amyloid (fA beta) and other components such as complement proteins. We have shown previously that murine microglia bind and internalize fA beta microaggregates via the type A scavenger receptor, but degradation of internalized fA beta is significantly slower than normal degradation. In this study, we compared internalization by microglia of fA beta microaggregates to that of anti-A beta-antibody-coated fA beta (IgG-fA beta) microaggregates and found that the uptake of the latter is increased by about 1.5-fold versus unmodified fA beta. The endocytic trafficking of IgG-fA beta is similar to that of fA beta microaggregates, following an endosomal/ lysosomal pathway. We also compared the internalization of fA beta microaggregates to that of complement protein, Clq-coated fA beta microaggregates, and found that the levels of uptake are also increased by about 1.5-fold. Rates of degradation of both types of modified fA beta microaggregates are unchanged compared with unmodified fA beta microaggregates. We demonstrated by blocking studies that internalization of IgG-fA beta is mediated by Fc receptors. These data suggest that, in vivo, several different microglial receptors may play a part in internalizing fA beta, but the involvement of other receptors may not increase the degradation of fA beta.