Low-Intensity Focused Ultrasound Alters Alzheimer's Disease Pathology, In Vivo, as a Function of Ultrasound Dose and Age.

BACKGROUND/OBJECTIVES: Alzheimer's Disease (AD) and vascular dementia contribute up to ~75% of dementia cases, as determined via autopsy. AD arises in part due to the buildup of aberrant proteins (amyloid beta (Aβ) and Tau); vascular dementia is caused by reduced cerebral blood flow. Each dementia mechanisms damages brain. Bobola et al. found that their low-intensity focused ultrasound (FUS) protocol applied to the brains of the 5XFAD mouse model of AD reduced Aβ by 50% through activation of microglia. Eguchi et al. found that their own FUS protocol applied to the brains of the same mouse model reduced Aβ by 15% and increased cerebral blood flow by 50% through an increase in endothelial nitric oxide synthase (eNOS). Here, we sought to test a combined version of those two FUS protocols, expecting both a decrease in Aβ burden and an increase in eNOS.

METHODS: Using a diagnostic ultrasound probe, we applied our combined FUS protocol primarily to the left hippocampus of anesthetized 5XFAD mice, for an hour a day, for three days for younger mice and for five days for older mice. On day three or five, respectively, we harvested their brains and performed histological analysis to assess Aβ burden, microglial activation and their co-localization with Aβ, as well as the burden of eNOS within neuronal nuclei (here called intra-neuronal eNOS) and outside of neurons.

RESULTS: Relative to untreated mice, the treated younger mice had more activated microglia co-localized with Aβ and reduced Aβ burden for large plaques, as well as no change in each measure of eNOS. In contrast, the treated older AD mice had no change in activated microglia co-localized with Aβ, and no change in Aβ burden. However, relative to untreated older AD mice, FUS decreased total and extra-neuronal eNOS and increased intra-neuronal eNOS.

CONCLUSIONS: The ability of our FUS protocol to reduce Aβ burden and alter the eNOS distribution depends critically upon the age of the AD mice (more Aβ plaques for a comparable number of microglia for older mice relative to younger mice) and duration of the treatment. The observed decrease in extra-neuronal eNOS distribution in older AD mice caused by FUS raises the concern that our protocol may increase ischemia, while the increase in intra-neuronal eNOS may counteract that effect via protection of synaptic function. These findings also identify two candidate therapeutic windows for our FUS treatment protocol, each requiring more research before translation to humans. One window is early intervention to maximize Aβ plaque removal via activation of microglia. The second is later intervention to protect synaptic function if it is possible to mitigate the potential ischemic risk caused by the differential effects of FUS on eNOS.