Fenestrae-associated protein Plvap regulates the rate of blood-borne proteins passage into the hypophysis
To maintain body homeostasis, endocrine systems must detect and integrate a multitude of blood-borne peripheral signals. This is mediated by specialized permeable pores in the endothelial membrane, dubbed fenestrae. Plasmalemma vesicles-associated protein (Plvap) is located in the fenestral diaphragm and is thought play a role in the selective passage of proteins through the fenestrae. However, this suggested function has yet to be demonstrated directly. Here, we studied the development of fenestrated capillaries in a major neuroendocrine interface between the blood and brain, namely the hypophysis. Using a transgenic permeability biosensor to visualize the vascular excretion of a genetically tagged plasma protein (DBP-EGFP), we show that the developmental acquisition of vascular permeability is associated with differential expression of zebrafish plvap orthologs in the hypophysis versus brain. Ultrastructural analysis of the hypophyseal vasculature revealed that plvapb mutants display deficiencies in fenestral and stomatal diaphragms as well as increased density of fenestrae, but not of caveolae. Measurements of DBP-EGFP dynamics in live plvapb mutant larvae provided a direct proof that Plvap limits the rate of blood-borne protein passage through fenestrated endothelia. Overall, we present the regulatory role of Plvap in the development of blood-borne protein detection machinery in a major neuroendocrine interface between the brain and the general circulation.