Synaptotoxicity in Alzheimer's Disease Involved a Dysregulation of Actin Cytoskeleton Dynamics through Cofilin 1 Phosphorylation

Amyloid-β (Aβ) drives the synaptic impairment and dendritic spine loss characteristic of Alzheimer's disease (AD), but how Aβ affects the actin cytoskeleton remains unknown and contentious. The actin-binding protein, cofilin-1 (cof1), is a major regulator of actin dynamics in dendritic spines, and is subject to phospho-regulation by multiple pathways, including the Rho-associated protein kinase (ROCK) pathway. While cof1 is implicated as a driver of the synaptotoxicity characteristic of the early phases of AD pathophysiology, questions remain about the molecular mechanisms involved. Cofilin-actin rods are observed in neurons exposed to Aβ oligomers (Aβo) and in tissue from AD patients, and others have described an increased cofilin phosphorylation (p-cof1) in AD patients. Here, we report elevated p-cof1 of the postsynaptic enriched fraction of synaptosomes from cortical samples of male APP/PS1 mice and human AD cases of either sex. In primary cortical neurons, Aβo induced rapid actin stabilization and increased p-cof1 in the postsynaptic compartment of excitatory synapses within 30 min. Fluorescence recovery after photobleaching of actin-GFP and calcium imaging in live neurons expressing active or inactive cof1 mutants suggest that cof1 phosphorylation is necessary and sufficient for Aβo-induced synaptic impairment via actin stabilization before the reported formation of cofilin-actin rods. Moreover, the clinically available and well-tolerated ROCK inhibitor, fasudil, prevented Aβo-induced actin stabilization, synaptic impairment, and synaptic loss by blocking cofilin phosphorylation. Aβo also blocked the LTP-induced insertion of the AMPAR subunit, GluA1, at the postsynaptic density, in a fasudil-sensitive manner. These data support an important role for ROCKs and cofilin in mediating Aβ-induced synaptic impairment.

SIGNIFICANCE STATEMENT We report that amyloid-β oligomers rapidly induce aberrant stabilization of F-actin within dendritic spines, which impairs synaptic strength and plasticity. Activation of the Rho-associated protein kinase (ROCK) pathway results in phosphorylation of cof1 and is sufficient to mediate Aβo-induced actin stabilization synaptic impairment and synaptic loss. Further, the ROCK inhibitor, fasudil, prevents cofilin phosphorylation, acute synaptic disruption, and synaptotoxicity in primary cortical neurons. Together, the herein presented data provide strong support for further study of the ROCK pathway as a therapeutic target for the cognitive decline and synaptotoxicity in Alzheimer's disease.

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