PROTEIN PHOSPHATASE INHIBITORS INDUCE MODIFICATION OF SYNAPSE STRUCTURE AND TAU-HYPERPHOSPHORYLATION IN CULTURED RAT HIPPOCAMPAL-NEURONS

Protein phosphatase inhibitors, okadaic acid and Caliculin A, were use d to investigate how perturbation of phosphorylation and dephosphoryla tion processes might affect neurite and synapse structure in cultures of fetal rat hippocampal neurons, Drug treatments induced neuritic tre e modification, with retraction of the processes and the appearance of dilatations along the neurites, The characteristic dotlike pattern of immunoreactivity of synaptic vesicle proteins disappeared, Normal syn apses were extremely rare by ultrastructural observation, Vesicles of various diameters accumulated in the dilatations, as did organelles an d amorphous material, suggesting impaired axonal transport, Hyperphosp horylation of tau protein was also observed as indicated by the shift in the electrophoretic mobility of a P-32-labeled 55-kDa band and by i mmunoblot with epitope-specific tau antibody, Our results show that in hibition of protein phosphatases 1 and 2A results in a modification of the neuritic tree structure, with loss of neuronal processes, phospho rylation of a tau isoform, and a decrease in the number of synapses, T hese neuronal features are present in Alzheimer's disease (AD), Our re sults suggest that the two events might be related and provide a poten tial link between the biochemical hallmark of AD (hyperphosphorylation of tau) and a pathological finding of primary clinical relevance (the synaptic loss). (C) 1997 Wiley-Liss, Inc.