O. Braissant1, E. Béard1, C. Torrent1, H. Henry1

1Clinical Chemistry, CHUV, Lausanne, Switzerland

While CNS express AGAT, GAMT and SLC6A8, the lack of SLC6A8 in astrocytes around blood-brain barrier limits the brain capacity to import creatine, and suggests that CNS has to rely on endogenous creatine synthesis. This seems contradictory with SLC6A8 deficiency, which, despite AGAT and GAMT expression, leads to creatine deficiency in CNS. We have shown that in the rat cortex, AGAT and GAMT are expressed in a dissociated way, only a few cells co-expressing both genes (Braissant and Henry, 2008, J.Inherit.Metab.Dis., 31:230-239). This suggested that to allow creatine synthesis within CNS, guanidinoacetate must be transported from AGAT- to GAMT-expressing cells, possibly through SLC6A8. 3D primary reaggregating brain cell cultures, issued from fetal rat telencephalon, were used to analyze the guanidinoacetate uptake by brain cells and their synthesis of creatine, by tandem mass spectrometry using stable isotopes. These cultures are made of all cell types found in telencephalon, and express AGAT, GAMT and SLC6A8 as the in vivo brain (Braissant et al., 2008, Eur.J.Neurosci., 27:1673-1685). We show here that guanidinoacetate is transported into brain cells, and that this uptake is competed by creatine, suggesting that in CNS, guanidinoacetate, as creatine, uses the SLC6A8 transporter. We further demonstrate that the brain cell-imported guanidinoacetate is efficiently converted into creatine. Our work demonstrates that guanidinoacetate can be efficiently imported by brain cells, which then convert it to creatine. This fits with our hypothesis of guanidinoacetate transport, by SLC6A8, from AGAT- to GAMT-expressing cells, which may explain why SLC6A8-deficient patients lack creatine in CNS.