Impaired Volume Regulation and Electrophysiology of Astrocytes In Situ in a Mouse Model for Megalencephalic...

Electrical signaling, driven by ion fluxes between intra- and extracellular compartments, is central to brain functioning. Astrocytes provide crucial support by maintaining the homeostasis of water and ions in the brain. This is disrupted in the leukodystrophy Megalencephalic Leukoencephalopathy with subcortical Cysts (MLC). Studies on cultured primary astrocytes and other isolated cell lines point to a central defect in astrocyte volume regulation in MLC. However, cell culture severely alters the properties and polarity of astrocytes. Therefore, whether astrocytes in the intact MLC brain exhibit aberrant physiology related to water and ion homeostasis remains unknown. To investigate astrocyte physiology in intact astrocytes, we performed experiments in acute brain slices from a validated MLC mouse model, the Glialcam-null mouse. We combined viral sensor delivery with two-photon microscopy to study astrocyte volume regulation and associated chloride dynamics. Cortical Glialcam-null astrocytes showed normal intracellular chloride dynamics but reduced volume recovery upon potassium-induced cell swelling. Whole-cell patch-clamp recordings revealed a modestly depolarized resting membrane potential and slower glutamate uptake in Glialcam-null astrocytes. Gap junction coupling of the astrocyte syncytium was modestly reduced, but it remained sufficient to preserve functional electrical isopotentiality. In conclusion, our findings confirm that the previously observed disturbance of astrocyte volume regulation observed in cultured cells is also observed in intact astrocytes in situ, and we uncover additional changes in astrocyte electrophysiological properties. These findings support the concept that dysfunctional astrocyte volume regulation is central to the MLC disease mechanism.