Microfiberoptic fluorescence photobleaching reveals size-dependent macromolecule diffusion in extracellular space...

Area of Science:

• Neuroscience
• Biophysics
• Cell Biology

Background:

• Diffusion in the brain extracellular space (ECS) is vital for intercellular communication, ion buffering, and transport of substances.
• Understanding diffusion dynamics in deep brain regions is challenging due to accessibility limitations.

Purpose of the Study:

• To measure macromolecular diffusion in deep brain regions in vivo.
• To investigate the factors influencing diffusion, including solute size and aquaporin-4.
• To characterize the structural properties of the deep brain ECS.

Main Methods:

• Microfiberoptic epifluorescence photobleaching technique to measure diffusion deep within mouse brain tissue.
• Utilized fluorescein isothiocyanate (FITC)-dextran as a model noninteracting molecule.
• Employed mathematical modeling to interpret diffusion data and ECS properties.

Main Results:

• Macromolecular diffusion in the brain cortex was significantly slowed (4.5-fold) and depth-independent.
• Diffusion was accelerated in mice lacking aquaporin-4, indicating its role in regulating ECS transport.
• Diffusion varied by brain region (hippocampus, thalamus) and was strongly solute size-dependent in deep brain, unlike the cortex.
• Introduced the concept of ECS "heterometricity" to explain size-dependent diffusion in deep brain.

Conclusions:

• This study provides the first in vivo data on molecular diffusion in deep brain ECS.
• Demonstrated previously unrecognized hindrance and heterometricity in deep brain ECS, affecting large macromolecule diffusion.
• Highlights the importance of ECS structure and aquaporin-4 in regulating molecular transport in the brain.