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

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Background: Guanidinoacetate (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. Our previous study 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. Our previous studies have revealed that monocarboxylate transporter 12 (MCT12) functions as an efflux transporter of GAA and creatine (Cr), a metabolite of GAA, in the kidneys and liver. Therefore, we aimed to clarify the role of MCT12 in GAA dynamics in CPxEpic.Methods: 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]Cr was evaluated in TR-CSFB3 cells after transfection with MCT12 small interfering RNA (siRNA). The CSF-to-brain parenchyma transfer of Cr was measured after intracerebroventricular injection in rats.Results: 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 Cr, in TR-CSFB3 cells was also indicated, and GAA was hypothesized to be metabolized to Cr after influx transport into CPxEpic, after which Cr was released into the CSF. Cr release from TR-CSFB3 cells decreased following MCT12 knockdown. The contribution ratio of MCT12 to the release of Cr was more than 50%. The clearance of CSF-to-brain parenchyma transfer of Cr was 4.65 µL/(min·g brain), suggesting that biosynthesized Cr in CPxEpic is released into the CSF and supplied to the brain parenchyma.Conclusions: In CPxEpic, GAA is metabolized to Cr via GAMT. Biosynthesized Cr is then released into the CSF via MCT12 and supplied to the brain parenchyma.