👤 Leoni Hoogterp

Volume-regulated anion channels (VRACs) are central to cell volume homeostasis. They mediate swelling-induced efflux of chloride and organic osmolytes to drive regulatory volume decrease. In the brain, VRACs have been proposed to play a key role in astrocytic volume regulation. Genetic defects in astrocytic VRAC modulating proteins (MLC1, GlialCAM, Aquaporin-4, GPRC5B) cause the leukodystrophy Megalencephalic leukoencephalopathy with subcortical cysts (MLC), characterized by chronic white matter edema and myelin vacuolization. Disrupted VRAC activity in MLC-patient-derived lymphoblasts and primary astrocytes from MLC mice further supports a pathogenic link between defective VRAC activity and MLC. Here, we studied the physiological and pathological consequences of astrocyte-specific removal of the essential VRAC subunit LRRC8A. In contrast to established MLC mouse models, astrocyte specific Lrrc8a knockout mice had normal brain water content, no myelin vacuolization, and preserved expression of MLC-related proteins. At a late age they developed a mildly ataxic gait and displayed increased severity of kainate-induced seizures. Two-photon imaging in acute brain slices revealed that astrocytes lacking LRRC8A show normal volume recovery and chloride dynamics upon high potassium-induced cell swelling. Together, these findings demonstrate that astrocyte LRRC8A is not essential for volume regulation in situ and that its loss alone is insufficient to cause the chronic white matter edema typical of MLC. The mild neurological deficits indicate a physiological role for astrocyte LRRC8A, but MLC pathology likely arises from broader dysregulation of the astrocytic protein complex coordinating ion and water homeostasis. Show less

Brain oedema is a life-threatening complication of various neurological conditions. Understanding molecular mechanisms of brain volume regulation is critical for therapy development. Unique insight comes from monogenic diseases characterized by chronic brain oedema, of which megalencephalic leukoencephalopathy with subcortical cysts (MLC) is the prototype. Variants in MLC1 or GLIALCAM, encoding proteins involved in astrocyte volume regulation, are the main causes of MLC. In some patients, the genetic cause remains unknown. We performed genetic studies to identify novel gene variants in MLC patients, diagnosed by clinical and MRI features, without MLC1 or GLIALCAM variants. We determined subcellular localization of the related novel proteins in cells and in human brain tissue. We investigated functional consequences of the newly identified variants on volume regulation pathways using cell volume measurements, biochemical analysis and electrophysiology. We identified a novel homozygous variant in AQP4, encoding the water channel aquaporin-4, in two siblings, and two de novo heterozygous variants in GPRC5B, encoding the orphan G protein-coupled receptor GPRC5B, in three unrelated patients. The AQP4 variant disrupts membrane localization and thereby channel function. GPRC5B, like MLC1, GlialCAM and aquaporin-4, is expressed in astrocyte endfeet in human brain. Cell volume regulation is disrupted in GPRC5B patient-derived lymphoblasts. GPRC5B functionally interacts with ion channels involved in astrocyte volume regulation. In conclusion, we identify aquaporin-4 and GPRC5B as old and new players in genetic brain oedema. Our findings shed light on the protein complex involved in astrocyte volume regulation and identify GPRC5B as novel potentially druggable target for treating brain oedema. Show less