The central nervous system is lined by meninges, classically known as dura, arachnoid, and pia mater. We show the existence of a fourth meningeal layer that compartmentalizes the subarachnoid space in the mouse and human brain, designated the subarachnoid lymphatic-like membrane (SLYM). SLYM is morpho- and immunophenotypically similar to the mesothelial membrane lining of peripheral organs and body cavities, and it encases blood vessels and harbors immune cells. Functionally, the close apposition of SLYM with the endothelial lining of the meningeal venous sinus permits direct exchange of small solutes between cerebrospinal fluid and venous blood, thus representing the mouse equivalent of the arachnoid granulations. The functional characterization of SLYM provides fundamental insights into brain immune barriers and fluid transport.

Inner ear gene therapy has recently effectively restored hearing in neonatal mice, but it is complicated in adulthood by the structural inaccessibility of the cochlea, which is embedded within the temporal bone. Alternative delivery routes may advance auditory research and also prove useful when translated to humans with progressive genetic-mediated hearing loss. Cerebrospinal fluid flow via the glymphatic system is emerging as a new approach for brain-wide drug delivery in rodents as well as humans. The cerebrospinal fluid and the fluid of the inner ear are connected via a bony channel called the cochlear aqueduct, but previous studies have not explored the possibility of delivering gene therapy via the cerebrospinal fluid to restore hearing in adult deaf mice. Here, we showed that the cochlear aqueduct in mice exhibits lymphatic-like characteristics. In vivo time-lapse magnetic resonance imaging, computed tomography, and optical fluorescence microscopy showed that large-particle tracers injected into the cerebrospinal fluid reached the inner ear by dispersive transport via the cochlear aqueduct in adult mice. A single intracisternal injection of adeno-associated virus carrying solute carrier family 17, member 8 ( Slc17A8 ), which encodes vesicular glutamate transporter-3 (VGLUT3), rescued hearing in adult deaf Slc17A8 −/− mice by restoring VGLUT3 protein expression in inner hair cells, with minimal ectopic expression in the brain and none in the liver. Our findings demonstrate that cerebrospinal fluid transport comprises an accessible route for gene delivery to the adult inner ear and may represent an important step toward using gene therapy to restore hearing in humans.

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Summary statement: Our study identifies a critical developmental time window during which 27" progenitors of the adult subependymal zone, specified for generating GABAergic neurons, can 28" be reprogrammed towards glutamatergic neurogenesis. 29"Correspondence: 30"Dr. Benedikt Berninger: Phone: 0049 6131 39 21334; Email: berningb@uni-mainz.de" 31"Dr. Sergio Gascón: Phone: 0049 8921 80 71917; Email: sergio.gascon@med.uni-muenchen.de " 32" 33"All rights reserved. No reuse allowed without permission.(which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint . http://dx.doi.org/10.1101/171686 doi: bioRxiv preprint first posted online Aug. 3, 2017; 2" " Tbr2 and Tbr1 expression, hallmarking entry into the glutamatergic lineage also in vivo. 42"Intriguingly, Neurog2-expressing progenitors failed to enter the rostral migratory stream towards a glutamatergic identity, but fails to reprogram their postmitotic progeny. 48"All rights reserved. No reuse allowed without permission.