Early micro- and nanoscale microglial responses to blood-brain barrier modulation by transcranial focused ultrasound
Elisa Gonçalves de Andrade1,2, Jared VanderZwaag1,2, Rikke Hahn Kofoed3,4,5
1Neuroscience Graduate Program, University of Victoria, Victoria, BC, Canada.
Abstract:
Modulation of the blood-brain barrier (BBB) using transcranial focused ultrasound with intravenous microbubbles (FUS) has progressed to clinical trials. Guided by magnetic resonance imaging (MRI), FUS significantly increases local BBB permeability for approximately 6-24 h in mice, with an ensuing mild inflammation. Microglia, as the immune cells of the brain, are stimulated by FUS-BBB modulation, although the timing and nature of their responses remain unclear. In this study, we characterized the cellular (i.e., density, distribution, and morphology) and subcellular (i.e., ultrastructure) microglial alterations early after FUS-BBB modulation.
Methods:
We targeted the hippocampi of adult, male mice with MRI-guided FUS in the presence of intravenous microbubbles. At 1 h and 24 h after FUS-BBB modulation, we investigated i) the density and distribution of ionized calcium-binding adaptor molecule 1 (Iba1)-positive (+) microglia; ii) the morphology of Iba1+ soma and processes; and iii) the microglial organelles and contacts using scanning electron microscopy.
Results:
Microglial density and distribution were unchanged at 1 h and 24 h. However, FUS-BBB modulation was associated with elongated microglial cell bodies at both time points. At 1 h, microglia shifted to compact shapes with stubby processes, whereas at 24 h, they were larger, with fewer processes. At the nanoscale, microglia preserved interactions with blood vessels, but they increased their contacts with swollen astrocytic endfeet at 1 h and 24 h. By 24 h after FUS-BBB, microglia contacted pre-synaptic elements and extracellular space pockets less frequently, with ultrastructural indications of changes in metabolic demand and lysosomal activity.
Conclusion:
At 1 h and 24 h after FUS-BBB, traits of microglial surveillance were maintained, despite a shift in morphology associated with injury shielding in a subset of cells. FUS-BBB modulation appears to temporarily modify lysosomal, but not phagocytic properties, of microglia and to reduce ultrastructural indications of pre-synaptic remodeling early after treatment.
