Masanori Tachikawa

Creatine, α-N-methyl-guanidino-acetic acid, plays a fundamental role in the storage and regeneration of high-energy phosphate in the brain. Defects in the creatine transporter gene (CRT/SLC6A8) result in a significant reduction in brain creatine levels and severe neurological symptoms such as intellectual disability. Clarifying creatine dynamics in the brain is essential to increase our understanding of CRT deficiency syndrome (CRTD) pathology and the development of CRTD therapeutics. This review comprehensively summarizes the pathophysiological roles of transporters in dynamics of creatine and related guanidine compounds in the brain barriers and brain parenchyma. Brain creatine dynamics are regulated by the cooperative actions of various influx and efflux transporters of creatine, guanidinoacetate, creatinine, and creatine biosynthetic enzymes. These transporters include CRT/SLC6A8 as a creatine/guanidinoacetate/creatinine influx transporter, MCT12/SLC16A12, and SLC22A15 for creatine efflux transport, TauT/SLC6A6, GAT2/SLC6A13, and GAT3/SLC6A11 for guanidinoacetate influx transport, and OCT3/SLC22A3 for creatinine influx transport. Transporters and creatine biosynthetic enzymes, such as arginine–glycine amidinotransferase and guanidinoacetate N-methyltransferase, exhibit cell-type specific spatio-temporal expression at the brain barrier and in neurons, astrocytes, and oligodendrocytes. To date, no effective therapeutics have been developed for the treatment of CRTD. The link between low brain creatine level and the mechanism of neurological dysfunction remains unclear. Creatine prodrugs, molecular chaperones, and adeno-associated virus-based gene therapies are potential therapeutic options for CRTD. Advanced technologies, such as omics and genetic engineering, will open new avenues for CRTD therapeutics.

The precise organization of pre- and postsynaptic terminals is crucial for normal synaptic function in the brain. In addition to its canonical role as a neurotrophin-3 receptor tyrosine kinase, postsynaptic TrkC promotes excitatory synapse organization through interaction with presynaptic receptor-type tyrosine phosphatase PTPσ. To isolate the synaptic organizer function of TrkC from its role as a neurotrophin-3 receptor, we generated mice carrying TrkC point mutations that selectively abolish PTPσ binding. The excitatory synapses in mutant mice had abnormal synaptic vesicle clustering and postsynaptic density elongation, more silent synapses, and fewer active synapses, which additionally exhibited enhanced basal transmission with impaired release probability. Alongside these phenotypes, we observed aberrant synaptic protein phosphorylation, but no differences in the neurotrophin signaling pathway. Consistent with reports linking these aberrantly phosphorylated proteins to neuropsychiatric disorders, mutant TrkC knock-in mice displayed impaired social responses and increased avoidance behavior. Thus, through its regulation of synaptic protein phosphorylation, the TrkC–PTPσ complex is crucial for the maturation, but not formation, of excitatory synapses in vivo.

We previously developed an in vitro model of the human blood–brain barrier (BBB) based on the use of endothelial cells derived from CD34⁺-hematopoietic stem cells and cultured with brain pericytes. The purpose of the present study was to provide information on the protein expression levels of the transporters, receptors, tight junction/adherence junction molecules, and transporter-associated molecules of human brain-like endothelial cells (hBLECs). The absolute protein expression levels were determined by liquid chromatography–mass spectrometry-based quantitative targeted absolute proteomics and compared with those from human brain microvessels (hBMVs). The protein levels of CD144, CD147, MRP4, Annexin A6 and caveolin-1 showed more than 3-fold abundance in hBLECs, those of MCT1, Connexin 43, TfR1, and claudin-5 showed less than 3-fold differences, and the protein levels of other drug efflux transporters and nutrient transporters were less represented in hBLECs than in hBMVs. It is noteworthy that BCRP was more expressed than MDR1 in hBLECs, as this was the case for hBMVs. These results suggest that transports mediated by MCT1, TfR1, and claudin-5-related tight junction function reflect the in vivo BBB situation. The present study provided a better characterization of hBLECs and clarified the equivalence of the transport characteristics between in vitro BBB models and in vivo BBB models using LC-MS/MS-based protein quantification.

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. 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. 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]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. 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 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. Conclusions: 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.