TGFβ signaling regulates Col15a1 expression in fenestrated endothelial cells of the rat pituitary

Fenestrated capillaries in the anterior lobe (AL) of the pituitary gland are specialized for efficient hormone exchange and are surrounded by extracellular matrix-rich basement membranes (BMs). Although several collagens have been implicated in the structural regulation of capillaries in the pituitary, the contribution of collagen XV remains unknown. In the present study, we investigated the expression, localization, and regulation of COL15A1, the α1 chain of collagen XV, in the rat pituitary. Real-time quantitative PCR (RT-qPCR) revealed that Col15a1 is highly expressed in the endothelial cell fraction isolated from rat AL. In situ hybridization showed that Col15a1 is expressed in plasmalemma vesicle-associated protein (PLVAP)-positive fenestrated endothelial cells in the AL. Immunohistochemistry demonstrated that COL15A1 is localized to BMs surrounding PLVAP-positive fenestrated capillaries in the AL, the median eminence, and the choroid plexus, but not in those surrounding continuous capillaries in the cerebrum. Because transforming growth factor-β (TGFβ) is a key regulator of collagen expression, we examined its role in Col15a1 regulation. RT-qPCR and in situ hybridization revealed that Tgfbr2 is expressed in fenestrated endothelial cells in the AL. Immunocytochemistry demonstrated that TGFβ1 stimulation induces nuclear translocation of SMAD2 in cultured endothelial cells isolated from the rat AL. RT-qPCR also showed that TGFβ1 significantly increases Col15a1 expression, which is suppressed by the TGFβ receptor I inhibitor SB431542. Together, these results demonstrate that TGFβ signaling regulates Col15a1 expression in fenestrated endothelial cells and suggest that collagen XV contributes to the structural and functional specialization of fenestrated capillaries in the pituitary.