| Summary: | Alzheimer s disease is characterized by synaptic alterations and neurodegeneration. Histopathological hallmarks represent amyloidplaques composed of amyloid-beta (Abeta) and neurofibrillary tangles containing hyperphosphorylated tau. To determine whether synaptic changes and neurodegeneration share common pathways we established an ex vivo model using organotypic hippocampal slicecultures from amyloid precursor protein transgenic mice combined with virus-mediated expression of EGFP-tagged tau constructs. Confocal high-resolution imaging, algorithm-based evaluation of spines and live imaging was employed to determine spine changes and neurodegeneration. We report that Abeta but not tau induces spine loss and shifts spine shape from mushroom to stubby through a mechanism involving NMDA receptor (NMDAR), calcineurin and GSK-3beta activation. In contrast, Abeta alone does not cause neurodegeneration but induces toxicity by phosphorylation of wt tau in a NMDAR-dependent pathway. We show thatGSK-3beta levels are elevated in APP transgenic cultures and that inhibiting GSK-3beta activity or use of phosphorylation-blocking tau mutations prevent Abeta-induced toxicity of tau. FTDP-17 tau mutants are differentially affected by Abeta. While R406W tau shows increased toxicity in the presence of Abeta, no change is observed with P301L tau. While blocking NMDAR activity abolishes toxicity of both wt and R406W tau, the inhibition of GSK-3beta only protects against toxicity of wt tau but not of R406W tau induced by Abeta. Tau aggregation does not correlate with toxicity. We propose that Abeta-induced spine pathology and tau-dependent neurodegeneration are mediated by divergent pathways downstream of NMDA receptor activation and suggest that Abeta affects wt and R406W tau toxicity by different pathways downstream of NMDAR activity.
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