Repeated low-intensity focused ultrasound led to microglial profile changes at long term in TgF344-AD rats

Abstract

Alzheimers disease (AD), the most common cause of dementia, represents one of the main clinical challenges of the century as the number of patients is predicted to triple by 2050. Despite the recent approval of three monoclonal antibodies targeting Amyloid {beta} (A{beta}) aggregates by the Food and Drug Administration (FDA), immunotherapies still face challenges due to the difficulty of antibodies crossing the blood-brain barrier (BBB). This necessitates administering large doses of drugs to achieve their therapeutic effects, which is associated with significant side effects. In this context, low-intensity focused ultrasound (LiFUS) appears as an innovative and non-invasive method which, in association with intravenous injection of microbubbles (MB), leads to a transient BBB opening. This innovative strategy has been extensively studied in different preclinical models and more recently in human clinical trials, particularly in the context of AD. LiFUS+MB increases the inflammatory response at short-term, but the time course of this response is not consistent between studies, certainly due to the discrepancy between LiFUS protocols used. Moreover, the impact at longer term is understudied and the mechanisms underlying this effect are still not well understood. In our study, we therefore used the TgF344-AD rat model of AD to investigate the effect of a single or multiple exposures to LiFUS+MB in a large volume of the brain on inflammatory response, tauopathy and amyloid load, at both early and advanced stages. The ultrasound attenuation through the skull was corrected to apply a peak negative acoustic pressure of 450 kPa in all treated animals. At an advanced disease stage, single LiFUS+MB exposure induces a slight astrocyte and microglial response 24 hours post-treatment whereas chronic LiFUS treatment is associated with a transient inflammatory response predominantly affecting microglial cells, which is no longer detectable 6 weeks post-treatment. At an early stage of pathology, LiFUS seems to induce microglial reprogramming, leading to the adaptation of gene expression related to key functions such as inflammatory response, mitochondrial and energetic metabolism. In our rat model and LiFUS+MB protocol conditions, a single LiFUS exposure reduced significantly highly aggregated A{beta}42 peptide concentration. Surprisingly, multiple exposures had this opposite effect at short-term but not at longer term.