Effects of Aβ1-42 fibrils and of the tetrapeptide Pr-IIGL on the phosphorylation state of the τ-protein and on the α7 nicotinic acetylcholine receptor in vitro

In order to investigate the possible links connecting beta-amyloid (A beta) accumulation, tau-hyperphosphorylation and nicotinic receptor expression, rat embryonic primary hippocampal cultures were incubated with amyloidogenic peptides. Exposure to 0.5 mu m fibrillar A beta(1-42) for 3 days caused retraction of dendrites, shrinkage of cell bodies and a decrease in the expression of microtubule-associated proteins 2b (MAP2b), without affecting the total number of neurons and their viability. No impact on the tau-phosphorylation sites Ser-202, Thr231/Ser235, Ser262 and Ser396/Ser404 was found. The total number of homomeric alpha 7-nicotinic receptors (alpha 7-nAChRs) and their affinity for [I-125]alpha-bungarotoxin remained unaltered. Upon incubation with the putatively protective tetrapeptide propionyl-isoleucine-isoleucine-glycine-leucine (Pr-IIGL), an analogue of the region [31-34] of A beta, cell bodies were swollen in the region of the apical dendrite. These morphological alterations, different from those elicited by A beta(1-42), did not involve MAP2 expression changes. In contrast to A beta(1-42), Pr-IIGL caused a massive hyperphosphorylation of the tau-protein at Ser-202 and at Ser396/Ser404. The total number of homomeric alpha 7-nAChRs and their affinity for [I-125]alpha-bungarotoxin were unaffected. In conclusion, the present results show a toxic effect of A beta(1-42) on the cytoskeletal structure at concentrations normally present in the brains of Alzheimer's disease patients, but raise some doubts about the role of A beta(1-42) fibrils as a direct trigger of tau-hyperphosphorylation. The tetrapeptide Pr-IIGL cannot be considered protective with regard to cell morphology. Although it prevents the A beta(1-42)-induced retraction of dendrites, it exhibits other toxic properties. The homomeric alpha 7-nAChRs were not affected either by A beta(1-42) incubation or by Pr-IIGL-induced tau-hyperphosphorylation.