Distribution and localisation of the water channel, AQP4, in organotypic brain slice cultures

Aquaporins (AQP) are water channels found throughout the body. In the brain, AQP1 is expressed in choroid plexus epithelial cells and ependymal cells, and AQP4 in astrocytes. Astrocytes regulate brain water homeostasis by relocalising AQP4 from intracellular vesicular pools to the cell surface, with the highest density of AQP4 found at the astrocyte endfeet covering cerebral blood vessels. This subcellular relocalisation of AQP4 has a crucial role in the regulation of AQP4 function that is independent of AQP4 expression.

The glymphatic system contributes to clearing waste products and neurotoxic agents, such as amyloid-β from the brain, and its dysfunction has been implicated in pathological conditions such as Alzheimer’s disease, haemorrhagic stroke and hydrocephalus. Due to the enrichment of AQP4 localisation on astrocyte endfeet at the blood brain barrier, AQP4 is thought to have a role in glymphatic function, especially as a lack of AQP4 has been shown to reduce fluid clearance.

Recently, organotypic brain slice cultures (OBSCs) have been used to model diverse diseases in vitro. Although we cannot model the glymphatic system in vitro, we can modulate its cellular and structural elements. However, we do not fully understand the implications of having no blood flow through cerebral blood vessels in OBSCs, and AQP4 localisation in astrocytes may be affected. To investigate this, the distribution and localisation of AQP4 were compared between acute OBSCs and those that had been cultured (with no blood flow) for 8 days. Immunohistochemistry was performed to visualise and quantify the changes in the distribution and localisation of AQP4 in astrocytes between acute and cultured OBSCs.

Understanding the distribution and localisation of AQP4 in response to the lack of blood flow in the OBSC model will aid in future research using OBSCs to investigate elements of the glymphatic system and contribute to the development of therapies to promote fluid clearance.