Epithelial-vascular co-development in the hindbrain choroid plexus

Critical to the exchange and metabolic functions served by tissues like choroid plexi and lung is the coherent development of an epithelial sheet of large surface area in tight apposition to an extensive vascular bed. Surface area is maximized and compacted through numerous villous folds of epithelium surrounding each capillary. Here, we present functional experiments in the mouse hindbrain choroid plexus that reveal Sonic hedgehog (Shh) as controlling and coordinating these disparate constituent lineages to form the frond-like choroid plexus structures critical for high-rate cerebrospinal fluid (CSF) production and detoxification. The source of Shh proved to be the choroid plexus epithelium (CPe) located in the dorsal hindbrain at midgestation. Two topographically and embryologically separable cell populations were identified as direct Hh-responders: pericytes and a subset of cells within rhombic lip (RL) neuroepithelium. Regulated by CPe-produced Shh were outgrowth of the resident vasculature and enhanced generation of CPe cells from their progenitor pool (the dorsal-most RL) to ensheath that expanding vasculature. Shh, therefore, orchestrates the co-development of separable cell lineages during morphogenesis of the hindbrain choroid plexus: adjacent perivascular cells and (indirectly) the associated vascular endothelial cells, and more distant RL cells and their production of CPe cells. Further, the molecular profile of the CPe was found to be altered indirectly in the absence of Shh, suggesting that normal CPe function, in addition to its production, depends on Shh. Distinctions among choroid plexus cell types regarding their capacity to respond autonomously to Hedgehog (Hh) signaling were further delineated. Activated Hh signaling was found to promote enhanced CPe cell production from the RL progenitor pool. This increased CPe cell production was sufficient to drive commensurate vascular expansion to underlie that overproduction of CPe. Interestingly, autonomous activation of Hh signaling in vascular endothelial cells yielded no response. Collectively, these findings not only inform on the generation of a structure, the choroid plexus, that is essential for central nervous system (CNS) health and development and for which little is known, but also have implications for the development of other tissues in which epithelial-vascular interactions are central, such as lung and kidney. Evolutionary significance is also suggested, because choroid plexus expansion provides a capacity to meet greater brain metabolic demands through enhanced production and detoxification of CSF, possibly an important permissive step for CNS enlargement and gains in circuit complexities.