Processing mechanism of GAA in choroid plexus
GAA induces epileptogenesis and neurotoxicity in the brain. As epileptic animal models have been reported to show elevated cerebral GAA levels, the processing mechanism of GAA in the brain is important for maintaining brain homeostasis. We have revealed that GAA in the cerebrospinal fluid (CSF) is removed by incorporation into the choroid plexus epithelial cells (CPxEpic), which form the blood-CSF barrier (BCSFB). However, the processing mechanism of GAA incorporated into CPxEpic remains unknown. We have reported that monocarboxylate transporter 12 (MCT12) functions as an efflux transporter of GAA and creatine, a metabolite of GAA, in the kidneys and liver. Therefore, we aimed to clarify the role of MCT12 in GAA dynamics in CPxEpic. Protein expression and localization in CPxEpic were evaluated using immunohistochemistry. Metabolic analysis was performed using high-performance liquid chromatography (HPLC) 24 h after the addition of [14C]GAA to TR-CSFB3 cells, which are conditionally immortalized rat CPxEpic. The efflux transport of [14C]creatine was evaluated in TR-CSFB3 cells after transfection with MCT12 small interfering RNA (siRNA). The CSF-to-brain parenchyma transfer of creatine was measured after intracerebroventricular injection in rats. Immunohistochemical staining revealed that MCT12-derived signals merged with those of the marker protein at the apical membrane of CPxEpic, suggesting that MCT12 is localized on the apical membrane of CPxEpic. The expression levels of guanidinoacetate N-methyltransferase (GAMT), which catalyzes the conversion of GAA to creatine, in TR-CSFB3 cells was also indicated, and GAA was considered to be metabolized to creatine after influx transport into CPxEpic, after which creatine was released into the CSF. Creatine release from TR-CSFB3 cells decreased following MCT12 knockdown. The contribution ratio of MCT12 to the release of creatine was more than 50%. The clearance of CSF-to-brain parenchyma transfer of creatine was 4.65 µL/(min·g brain), suggesting that biosynthesized creatine in CPxEpic is released into the CSF and supplied to the brain parenchyma. In CPxEpic, GAA is metabolized to creatine via GAMT. Biosynthesized creatine is then released into the CSF via MCT12 and supplied to the brain parenchyma.
Jomura R, Akanuma SI, Kubo Y, Tachikawa M, Hosoya KI. Processing mechanism of guanidinoacetate in choroid plexus epithelial cells: conversion of guanidinoacetate to creatine via guanidinoacetate N-methyltransferase and monocarboxylate transporter 12-mediated creatine release into the CSF. Fluids Barriers CNS. 2022 Jun 3;19(1):42.