MRI and histological evaluation of pulsed focused ultrasound and microbubbles treatment effects in the brain

Abstract

Magnetic resonance imaging (MRI)-guided pulsed focused ultrasound (pFUS) combined with microbubbles (MB) contrast agent infusion has been shown to transiently disrupt the blood-brain barrier (BBBD), increasing the delivery of neurotherapeutics to treat central nervous system (CNS) diseases. pFUS interaction with the intravascular MB results in acoustic cavitation forces passing through the neurovascular unit (NVU), inducing BBBD detected on contrast-enhanced MRI. Multiple pFUS+MB exposures in Alzheimer's disease (AD) models are being investigated as a method to clear amyloid plaques by activated microglia or infiltrating immune cells. Since it has been reported that pFUS+MB can induce a sterile inflammatory response (SIR) [1-5] in the rat, the goal of this study was to investigate the potential long-term effects of SIR in the brain following single and six weekly sonications by serial high-resolution MRI and pathology.Methods: Female Sprague Dawley rats weighing 217±16.6 g prior to sonication received bromo-deoxyuridine (BrdU) to tag proliferating cells in the brain. pFUS was performed at 548 kHz, ultrasound burst 10 ms and initial peak negative pressure of 0.3 MPa (in water) for 120 s coupled with a slow infusion of ~460 µL/kg (5-8×107 MB) that started 30 s before and 30 s during sonication. Nine 2 mm focal regions in the left cortex and four regions over the right hippocampus were treated with pFUS+MB. Serial high-resolution brain MRIs at 3 T and 9.4 T were obtained following a single or during the course of six weekly pFUS+MB resulting in BBBD in the left cortex and the right hippocampus. Animals were monitored over 7 to 13 weeks and imaging results were compared to histology.Results: Fewer than half of the rats receiving a single pFUS+MB exposure displayed hypointense voxels on T2*-weighted (w) MRI at week 7 or 13 in the cortex or hippocampus without differences compared to the contralateral side on histograms of T2* maps. Single sonicated rats had evidence of limited microglia activation on pathology compared to the contralateral hemisphere. Six weekly pFUS+MB treatments resulted in pathological changes on T2*w images with multiple hypointense regions, cortical atrophy, along with 50% of rats having persistent BBBD and astrogliosis by MRI. Pathologic analysis of the multiple sonicated animals demonstrated the presence of metallophagocytic Prussian blue-positive cells in the parenchyma with significantly (p<0.05) increased areas of activated astrocytes and microglia, and high numbers of systemic infiltrating CD68+ macrophages along with BrdU+ cells compared to contralateral brain. In addition, multiple treatments caused an increase in the number of hyperphosphorylated Tau (pTau)-positive neurons containing neurofibrillary tangles (NFT) in the sonicated cortex but not in the hippocampus when compared to contralateral brain, which was confirmed by Western blot (WB) (p<0.04).Conclusions: The repeated SIR following multiple pFUS+MB treatments could contribute to changes on MR imaging including persistent BBBD, co...

Blood–brain barrier opening with focused ultrasound in experimental models of Parkinson's disease

“…Moreover, transcriptomic analysis in the acute stages following sonication revealed a transient upregulation in proinflammatory cytokines and chemokines including chemokine (C‐C motif) ligand 2, chemokine (C‐C motif) ligand 3, and Tumor necrosis factor that have been found to promote the migration, proliferation, differentiation, and survival of neural progenitor cells favoring neurogenesis . The prominent presence of Bromodeoxyuridine‐positive cells in animals that survived for a week after the last of 6 sonications has been linked to enhanced neurogenesis attributed to the expression of trophic factors such as brain‐derived neurotrophic (BDNF) in the targeted brain . Concurrent with the overexpression of inflammatory markers that mostly resolved within 24 hours was the increase in angiogenetic‐related genes and astrocytic activation …”

“…31 The prominent presence of Bromodeoxyuridine-positive cells in animals that survived for a week after the last of 6 sonications has been linked to enhanced neurogenesis attributed to the expression of trophic factors such as brain-derived neurotrophic (BDNF) in the targeted brain. 32 Concurrent with the overexpression of inflammatory markers that mostly resolved within 24 hours was the increase in angiogenetic-related genes and astrocytic activation. 31 Despite the positive impact of the technique, reports on elevated microtubule-associated protein tau, phosphorylated at the Phospho-Tau (Thr231) Monoclonal Antibody epitope 32 and activated nuclear factor kappa-light-chain-enhancer of activated Bcells (Nf-κB) pathway 33 following repeated sonications, alarmed the ultrasound field and surfaced important unmet needs.…”

“…32 Concurrent with the overexpression of inflammatory markers that mostly resolved within 24 hours was the increase in angiogenetic-related genes and astrocytic activation. 31 Despite the positive impact of the technique, reports on elevated microtubule-associated protein tau, phosphorylated at the Phospho-Tau (Thr231) Monoclonal Antibody epitope 32 and activated nuclear factor kappa-light-chain-enhancer of activated Bcells (Nf-κB) pathway 33 following repeated sonications, alarmed the ultrasound field and surfaced important unmet needs. Although the uncontrolled phosphorylation of the tau protein has been linked to Alzheimer's disease, phosphorylation has to occur at specific epitopes and be proven to lead to pathological outcomes.…”

Sonoselective transfection of cerebral vasculature without blood–brain barrier disruption

“…Recent studies have revealed that, under certain conditions, opening of the BBB by FUS and MBs can induce an acute sterile inflammatory response in brain tissue (58)(59)(60). The inflammation induced by FUS has been shown to promote a wide range of beneficial effects, including immune activation and recognition Significance Focused ultrasound (FUS) is a targeted and noninvasive technique that can be used to activate gas-filled microbubbles (MBs) to oscillate within the bloodstream.…”

Focused ultrasound opening of the blood–brain barrier for treatment of Parkinson's disease

“…This question needs further exploration, especially if FUS might be envisioned for use repeatedly within the same brain territory. After experimental FUS, several types of tissue responses have been described, including localized edema, ischemia, hemorrhage, microglial activation, and other cellular changes lasting for up to several days afterward . A better understanding of the significance of such changes is needed to guide the safe and efficacious development of low‐energy sonication in human therapeutics.…”