Abstract
BackgroundThe discovery of the inherited disorders of creatine (Cr) synthesis and transport in the last few years disclosed the importance of blood Cr supply for the normal functioning of the brain. These putatively rare diseases share a common pathogenetic mechanism (the depletion of brain Cr) and similar phenotypes characterized by mental retardation, language disturbances, seizures and movement disorders. In the effort to improve our knowledge on the mechanisms regulating Cr pool inside the nervous tissue, Cr transport and synthesis and related gene transcripts were explored in primary cultures of rat cerebellar granule cells and astrocytes.MethodsCr uptake and synthesis were explored in vitro by incubating monotypic primary cultures of rat type I astrocytes and cerebellar granule cells with: a) D3-Creatine (D3Cr) and D3Cr plus β-guanidinopropionate (GPA, an inhibitor of Cr transporter), and b) labelled precursors of Guanidinoacetate (GAA) and Cr (Arginine, Arg; Glycine, Gly). Intracellular D3Cr and labelled GAA and Cr were assessed by ESI-MS/MS. Creatine transporter (CT1), L-arginine:glycine amidinotransferase (AGAT), and S-adenosylmethionine:guanidinoacetate N-methyltransferase (GAMT) gene expression was assessed in the same cells by real time PCR.ResultsD3Cr signal was extremely high in cells incubated with this isotope (labelled/unlabelled Cr ratio reached about 10 and 122, respectively in cerebellar granule cells and astrocytes) and was reduced by GPA. Labelled Arg and Gly were taken up by the cells and incorporated in GAA, whose concentration paralleled that of these precursors both in the extracellular medium and inside the cells (astrocytes). In contrast, the increase of labelled Cr was relatively much more limited since labelled Cr after precursors' supplementation did not exceed 2,7% (cerebellar granule cells) and 21% (astrocytes) of unlabelled Cr. Finally, AGAT, GAMT and SLC6A8 were expressed in both kind of cells.ConclusionsOur results confirm that both neurons and astrocytes have the capability to synthesize and uptake Cr, and suggest that at least in vitro intracellular Cr can increase to a much greater extent through uptake than through de novo synthesis. Our results are compatible with the clinical observations that when the Cr transporter is defective, intracellular Cr is absent despite the brain should be able to synthesize it. Further research is needed to fully understand to what extent our results reflect the in vivo situation.
Highlights
The discovery of the inherited disorders of creatine (Cr) synthesis and transport in the last few years disclosed the importance of blood Cr supply for the normal functioning of the brain
These putatively rare diseases are due either to defects of the enzymes devoted to Cr synthesis (L-arginine:glycine amidinotransferase (AGAT, EC 2.1.4.1) [3] and S-adenosylmethionine:guanidinoacetate N-methyltransferase (GAMT, EC 2.1.1.2) [4]) - or to defects of the Cr transporter (CT1) [5,6]
No effective treatment is available for the defect of creatine transporter (CT1): while the supplementation of the precursors of Cr, Arginine (Arg) and Glycine (Gly), restores the Cr synthesis in peripheral cells in vitro [12], it results in only a mild clinical improvement with scarce [13] or absent [14] increase of brain Cr signal in vivo
Summary
The discovery of the inherited disorders of creatine (Cr) synthesis and transport in the last few years disclosed the importance of blood Cr supply for the normal functioning of the brain. No effective treatment is available for the defect of CT1: while the supplementation of the precursors of Cr, Arginine (Arg) and Glycine (Gly), restores the Cr synthesis in peripheral cells in vitro [12], it results in only a mild clinical improvement with scarce [13] or absent [14] increase of brain Cr signal in vivo These rare metabolic disorders disclosed the importance of blood Cr supply for the normal functioning of the brain, even though AGAT and GAMT are widely expressed in the nervous tissue [15] where Cr synthesis certainly occurs [16,17]
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