Tjap1/Pilt Is a cis-Golgi-Associated Protein Required for Golgi Integrity and Normal Drug Transporter Expression in Brain Microvascular Endothelial Cells In Vitro.

Background: Brain microvascular endothelial cells (BMECs) form the blood-brain barrier (BBB), a highly selective interface that restricts paracellular diffusion and regulates the transport of nutrients and drugs into the central nervous system via specialized transporters and receptors. Tight junction-associated protein 1 (Tjap1), also termed protein incorporated later into tight junctions (Pilt), has been localized to tight junctions (TJs) in epithelial cells and to the trans-Golgi network in fibroblasts; however, its expression, subcellular localization, and functional significance in BMECs are still unknown. Methods: We characterized Tjap1 subcellular localization in mouse and human BMEC cell lines as well as primary mouse BMECs by immunofluorescence with and without pharmacological Golgi disruption by treatment with Brefeldin A, Golgicide A or Pitstop 2. CRISPR/Cas9-mediated Tjap1 knockout cells were generated and examined with regard to their Golgi morphology using immunostaining. Tjap1 mRNA localization was examined by RNAscope in situ hybridization. Quantitative real-time PCR and Western blot was performed to assess the expression of BBB-associated efflux transporters, solute carrier transporters, and cellular receptors in control and Tjap1 knockout cells. Results: Tjap1 predominantly localized to the cis-Golgi compartment, co-localizing with Gm130 rather than Tgn38, and was absent from TJs in BMECs. Tjap1 knockout induced pronounced Golgi fragmentation BMECs. Importantly, Tjap1 knockout significantly downregulated mRNA-expression of Abcb1a, Abcb1b, Abcc4, Slc2a1, Slc7a1, Slc7a5 and Tfrc, while Abcg2 was upregulated. At the protein level, a decrease in the protein levels of Abcb1, Abcc4, Slc2a1, Slc7a1, and Tfrc was observed in Tjap1 knockout cEND cells. Conclusions: In BMECs, Tjap1 is a cis-Golgi-associated protein required for the structural integrity of the Golgi apparatus. Its deletion is associated with Golgi fragmentation and significant alterations in the mRNA and protein expression of drug transporters and receptors at the BBB. These findings identify Tjap1 as a candidate regulator of both Golgi architecture and the BBB transporter profile in vitro, with potential implications for modulating drug transport across the BBB.