Super-resolution shadow imaging reveals local remodeling of astrocytic microstructures and brain extracellular space after osmotic challenge

The extracellular space (ECS) plays a central role for brain physiology, shaping the time course and spread of neurochemicals, ions and nutrients that ensure proper brain homeostasis and neuronal communication. Astrocytes are the most abundant type of glia cell in the brain, whose processes densely infiltrate the brain’s parenchyma. As astrocytes are highly sensitive to changes in osmotic pressure, they are capable of exerting a potent physiological influence on the ECS.However, little is known about the spatial distribution and temporal dynamics of the ECS that surrounds astrocytes, owing mostly to a lack of appropriate techniques to visualize the ECS in live brain tissue. Mitigating this technical limitation, we applied the recent SUper-resolution SHadow Imaging technique (SUSHI) to astrocyte-labeled organotypic hippocampal brain slices, which allowed us to concurrently image the complex morphology of astrocytes and the ECS with nanoscale resolution in a live experimental setting.Focusing on ring-like astrocytic microstructures in the spongiform domain, we found them to enclose sizable pools of interstitial fluid and cellular structures like dendritic spines. Upon an experimental osmotic challenge, these microstructures remodeled and swelled up at the expense of the pools, effectively increasing the physical contact between astrocytic and cellular structures.Our study reveals novel facets of the dynamic microanatomical relationships between astrocytes, neuropil and the ECS in living brain tissue, which could be of functional relevance for neuronglia communication in a variety of (patho)physiological settings, e.g. LTP induction, epileptic seizures or acute ischemic stroke, where osmotic disturbances are known to occur.