SELECTIVE AGGREGATION OF ENDOGENOUS BETA-AMYLOID PEPTIDE AND SOLUBLE AMYLOID PRECURSOR PROTEIN IN CEREBROSPINAL-FLUID BY ZINC

Zinc added to buffered solutions of synthetic beta-amyloid peptide (A beta) has been reported to induce accelerated formation of insoluble a ggregates. This observation suggests that zinc may play a role in the formation of senile plaques, which contain A beta in Alzheimer's disea se. To test this hypothesis under conditions more representative of th e brain, we investigated the ability of zinc to induce aggregation of A beta in freshly drawn canine CSF, which contains the same sequence a s human A beta. Aggregates were separated from CSF by ultracentrifugat ion before and after incubation with zinc and assayed by quantitative western blotting and ELISA, We found that zinc induced the rapid aggre gation of endogenous A beta in CSF, with an EC50 of 120-140 mu M. The reaction was specific, because most (greater than or equal to 95%) CSF protein remained soluble under conditions where most A beta was insol uble, as assayed by scanning densitometry of Coomassie-stained gels. S taining of the precipitated material resulted in the visualization of punctate regions that were thioflavin positive or birefringent when st ained with Congo red, suggesting the formation of amyloid-related stru ctures. These results suggest that zinc could play a role in amyloid d eposition, because there is overlap between the regions of the brain w here zinc concentrations are highest and regions with the highest amyl oid content. It is surprising that zinc induced the aggregation of end ogenous soluble APP at lower concentrations than required for A beta ( EC50 80 mu M). The possibility that zinc-induced aggregation of APP ma y precede the deposition of A beta into plaques is discussed, Investig ation of aggregation of A beta in CSF will aid in assessing the biolog ical relevance of other agents that have been reported to accelerate a myloid formation.