Channels and Transporters in Astrocyte Volume Regulation in Health and Disease

Astrocytes are the second most abundant cell type in the central nervous system and serve various functions, many of which maintain homeostasis of the intracellular milieu in the face of constant change. In order to accomplish these important functions, astrocytes must regulate their cell volume. In astrocytes, cell volume regulation involves multiple channels and transporters, including AQP4, TRPV4, TRPM4, VRAC, Na⁺/K⁺ ATPase, NKCC1 and Kir4.1. AQP4 is a bidirectional water channel directly involved in astrocyte cell volume regulation. AQP4 also forms heteromultimeric complexes with other channels and transporters involved in cell volume regulation. TRPV4, a mechanosensitive channel in involved in osmotic regulation in various cell types, forms a complex with AQP4 to decrease cell volume in response to cell swelling. TRPM4 also forms a complex with AQP4 and SUR1 in response to injury resulting in cell swelling. Another complex forms between Na⁺/K⁺ ATPase, AQP4, and mGluR5 to regulate the perisynaptic space. NKCC1 is a co-transporter involved in cell volume increases either independently through cotransport of water or a functional interaction with AQPs. VRAC is implicated in regulatory volume decreases and may also functionally interact with AQP4. Although Kir4.1 colocalizes with AQP4, its role in cell volume regulation is debated. In diseases where fluid/electrolyte homeostasis is disturbed such as stroke, ischemic injury, inflammation, traumatic brain injury and hydrocephalus, cell volume regulation is challenged, sometimes past the point of recovery. Thus, a greater understanding of signaling pathways which regulate transport proteins as well as the functional and physical interactions that exist between transporters will provide a basis for the development of pharmaceutical targets to treat these prevalent and often devastating diseases.