A route for cerebrospinal fluid flow through leptomeningeal arterial-venous overlaps enables macromolecule and fluid shunting.

The flow of cerebrospinal fluid (CSF) is important for conveying brain-derived macromolecules for signaling and enabling them to be drained from the brain parenchyma. The glymphatic route is the best-characterized means of this CSF flow; however, it does not permit the movement of larger macromolecules. Here, we identify in mice an alternative route whereby intra-CSF-injected macromolecules can traverse from periarterial to perivenous spaces, with transfer occurring at sites of overlap between leptomeningeal perivascular (arteriovenous) spaces dispersed across the surface of the brain's leptomeninges. We show that intra-CSF-injected fluorescent tracers can reach the perivenous space by passing through these arteriovenous perivascular overlaps. These spaces remain functional in a mouse model of amyloidosis and are essential for clearing excess CSF volume. These anatomical structures may support brain function by allowing the drainage of brain-derived macromolecules and the shunting of excess fluid and by aiding the immune surveillance of freshly generated CSF.