Endothelial <i>Adgrl2</i> expression and alternative splicing controls the cerebrovasculature

Central nervous system development requires parallel but interrelated processes of neural circuit assembly and vascularization. Intersecting between these two processes is the cell-adhesion G-protein coupled receptor <i>Adgrl2.</i> In select neuronal populations, <i>Adgrl2</i> is localized and control the assembly of specific synaptic sites. In non-neuronal brain cells, <i>Adgrl2</i> is restricted in expression to endothelial cells. Testing for <i>Adgrl2</i> function in these cells in mice (of either sex), here we find that endothelial cell specific <i>Adgrl2</i> deletion results in an impairment in cerebrovascular integrity. To understand how it might be possible for <i>Adgrl</i>2 to function independently in neuronal and endothelial contexts, we surveyed <i>Adgrl2</i> transcripts within these cell classes. By analyzing single-cell RNA sequencing datasets, we find that <i>Adgrl2</i> mRNA is subject to robust cell type-specific alternative splicing that results in distinct isoforms being produced in neurons compared to endothelial cells. To probe the functional significance of this alternative splicing, we forced expression of the neuronal isoform of <i>Adgrl2</i> in endothelial cells. This resulted in altered cerebrovascular properties including the formation of ectopic glutamatergic synaptic contacts onto endothelial cells, indicating alterations in the cell-cell recognition process. Functionally, in direct contrast to endothelial <i>Adgrl2</i> deletion, this genetic expression switch instead enhances blood-brain barrier integrity. This overly restrictive cerebrovascular function results in dysregulation of blood to cerebrospinal fluid homeostasis, enlargement of brain ventricles, and a higher risk of hydrocephalus. Thus, alternative splicing serves as a cell type specific mechanism that provides isoform specific <i>Adgrl2</i> for discerning functions controlling neural circuit assembly and cerebrovascular homeostasis.<b>Significance statement</b> The brain's development depends on two key processes: building neural circuits and forming blood vessels. In this study, we examine <i>Adgrl2</i>, a protein involved in neural circuit formation that is also expressed in endothelial cells. Our findings reveal that <i>Adgrl2</i> functions differently in these cell types because each produces distinct versions of the protein through cell-type-specific alternative splicing. This mechanism allows the brain to repurpose the same gene for dual roles - assembling neural circuits and the cerebrovasculature.