Pituicyte-derived TGFβ2 regulates Col15a1 expression in pericytes of the rat posterior pituitary

The posterior lobe (PL) of the pituitary contains a specialized neurovascular junction, where neurosecretory axon terminals closely associate with fenestrated capillaries to enable efficient hormone release into the circulation. The neurosecretory axon terminals and the fenestrated capillaries are supported by an outer basement membrane (BM) and an inner BM, respectively. Although BMs are critical structural components of this junction, their molecular composition and regulatory mechanisms remain poorly understood. We previously demonstrated that COL15A1, the α1 chain of collagen XV, localizes to the BMs surrounding fenestrated capillaries in the anterior lobe of the rat pituitary. In the present study, we examined the localization and regulation of COL15A1 in the rat PL. Immunohistochemical analyses revealed that COL15A1 is localized to the inner BMs of fenestrated capillaries. Combined in situ hybridization and immunohistochemistry demonstrated that Col15a1 is predominantly expressed in NG2-positive pericytes. Water deprivation, which induces arginine vasopressin (AVP) secretion, significantly decreased Col15a1 expression in the PL, whereas AVP stimulation did not affect its expression in primary cultured posterior and intermediate lobe (PIL) cells. We also found that TGFβ2 is produced by S100β-positive pituicytes and TGFβ receptor II is localized to NG2-positive pericytes in the PL. Furthermore, TGFβ2 induces Col15a1 expression in primary cultured PIL cells via TGFβ receptor II and SMAD2 signaling. Collectively, our results identify collagen XV as a key component of the inner BMs surrounding fenestrated capillaries and demonstrate that pericytes orchestrate the structural and functional organization of the neurovascular junction in the PL.