Nuclear respiratory factor 1 (NRF-1) upregulates the expression and function of reduced folate carrier (RFC) at the blood-brain barrier.

Folates are important for neurodevelopment and cognitive function. Folate transport across biological membranes is mediated by three major pathways: folate receptor alpha (FRα), proton-coupled folate transporter (PCFT), and reduced folate carrier (RFC). Brain folate transport primarily occurs at the choroid plexus through FRα and PCFT; inactivation of these transport systems results in suboptimal folate levels in the cerebrospinal fluid (CSF) causing childhood neurological disorders. Our group has reported that upregulation of RFC at the blood-brain barrier (BBB) through interactions with specific transcription factors, that is, vitamin D receptor (VDR) could increase brain folate delivery. This study investigates the role of nuclear respiratory factor 1 (NRF-1) in the regulation of RFC at the BBB. Activation of NRF-1/PGC-1α signaling through treatment with its specific ligand, pyrroloquinoline quinone (PQQ), significantly induced RFC expression and transport activity in hCMEC/D3 cells. In contrast, transfection with NRF-1 or PGC-1α targeting siRNA downregulated RFC functional expression in the same cell system. Applying chromatin immunoprecipitation (ChIP) assay, we further demonstrated that PQQ treatment increased NRF-1 binding to putative NRF-1 binding sites within the SLC19A1 promoter, which encodes for RFC. Additionally, in vivo treatment of wild type mice with PQQ-induced RFC expression in isolated mouse brain capillaries. Together, these findings demonstrate that NRF-1/PGC-1α activation by PQQ upregulates RFC functional expression at the BBB and could potentially enhance brain folate uptake.

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