The adhesion GPCR ADGRV1 controls glutamate homeostasis in hippocampal astrocytes supporting neurons

Abstract

ADGRV1 is the largest member of adhesion G protein-coupled receptor (aGPCR) family. In the cell, aGPCRs serve in two major functions, namely in cell adhesion and signal transduction. Mutations in ADGRV1 were linked not only to Usher syndrome (USH), which causes deaf-blindness, but recently, also to epilepsy. While the USH defects are attributed to the loss of fiber links between membranes formed by the extracellular domain of ADGRV1, the pathomechanisms leading to epilepsy remain elusive to date. Here, we study the specific functions of ADGRV1 in astrocytes, where it is most highly expressed in the nervous system. Affinity proteomics demonstrated the interaction of ADGRV1 with proteins enriched in astrocytes. Different transcriptomes of USH2C patient-derived cells and Adgrv1-deficient mouse hippocampi compared to controls indicated dysregulation of cellular processes important in astrocyte function. Cell counts and morphometric analysis revealed reduced numbers and altered morphology of astrocytes in the hippocampus of Adgrv1-mutant mice. Monitoring the glutamate uptake in colorimetric assay and by live cell imaging of a genetic glutamate reporter consistently showed that glutamate uptake from the extracellular environment is significantly reduced in Adgrv1-deficent astrocytes. Expression analyses of key enzymes of the glutamate glutamine cycle and the glutamate metabolism indicated imbalanced glutamate homeostasis in Adgrv1-deficient astrocytes. Finally, we provide evidence that the supportive function of astrocytes in neuronal development also relies on ADGRV1 expression in astrocytes. Our data collectively provide first insights into the molecular pathophysiology associated with ADGRV1 defects in the brain, which may relate to the development of epilepsy associated with mutations in ADGRV1.