Repeated ultrasound treatment of tau transgenic mice clears neuronal tau by autophagy and improves behavioral functions

Intracellular deposits of pathological tau are the hallmark of a broad spectrum of neurodegenerative disorders collectively known as tauopathies, with Alzheimer's disease, a secondary tauopathy, being further characterized by extracellular amyloid plaques. A major obstacle in developing effective treatments for tauopathiesis the presence of the blood-brain barrier, which restricts the access of therapeutic agents to the brain. An emerging technology to overcome this limitation is the application of low-intensity ultrasoundwhich, together with intravenously injected microbubbles, transiently opens the blood-brain barrier, thereby facilitating the delivery of therapeutic agents into the brain. Interestingly, even in the absence of therapeutic agents, ultrasoundhas previously been shown to reduce amyloid plaques and improve cognitive functions in amyloid-depositing mice through microglial clearance. Ultrasoundhas also been shown to facilitate the delivery of antibody fragments against pathological tau in P301L tau transgenic mice; however, the effect of ultrasoundalone has not been thoroughly investigated in a tauopathymouse model.Methods: Here, we performed repeated scanning ultrasoundtreatments over a period of 15 weeks in K3691 tau transgenic mice with an early-onset tau-related motor and memory phenotype. We used immunohistochemical and biochemical methods to analyze the effect of ultrasoundon the mice and determine the underlying mechanism of action, together with an analysis of their motor and memory functions following repeated ultrasoundtreatments.Results: Repeated ultrasoundtreatments significantly reduced tau pathology in the absence of histological damage. Associated impaired motor functions showed improvement towards the end of the treatment regime, with memory functions showing a trend towards improvement. In assessing potential clearance mechanisms, we ruled out a role for ubiquitination of tau, a prerequisite for proteasomal clearance. However, the treatment regime induced the autophagy pathway in neurons as reflected by an increase in the autophagosomemembrane marker LC3II and a reduction in the autophagic flux marker p62, along with a decrease of mTOR activity and an increase in beclin 1 levels. Moreover, there was a significant increase in the interaction of tau and p62 in the ultrasound-treated mice, suggesting removal of tau by autophagosomes.Conclusions: Our findings indicate that a neuronal protein aggregate clearance mechanism induced by ultrasound-mediated blood-brain barrieropening operates for tau, further supporting the potential of low-intensity ultrasoundto treat neurodegenerative disorders.