l-Serine-O-phosphate in the central nervous system

Abstract

l-serine-O-phosphate (l-SOP) is the immediate precursor to l-serine in the serine synthesis pathway and is also an agonist at the Group III metabotropic glutamate receptors (mGluRs). l-SOP is produced by the enzyme phosphoserine aminotransferase (PSAT) and metabolized to l-serine by phosphoserine phosphatase (PSP). Using a novel analytical procedure, we show that l-SOP is present in rat whole brain, and that in transfected cells, it is substantially more potent than l-glutamate at the mGluR4 receptor subtype. Immunocytochemical analyses showed that the distributions of PSAT and PSP in the cerebral cortex, hippocampus, and cerebellum were similar in the rat and macaque monkey brain. In the rat hippocampus, cells within the subgranular zone were co-labeled with anti-PSP and anti-PSA-NCAM, a marker for neurogenic cells. In the cerebellar cortex, Purkinje neurons expressed relatively high levels of both enzymes while robust expression of PSAT was also observed in the Bergmann glia. l-SOP released from Purkinje neurons or Bergmann glia could activate mGluR4 present on parallel fiber terminals. The presence of l-SOP in brain, its high potency at mGluR4, together with the restricted distributions of the synthetic and metabolic enzymes, suggest that l-SOP might act activate Group III metabotropic glutamate receptors in the CNS.

Introduction

l-serine has been shown to be a requisite growth factor for neurons (Savoca et al., 1995, Furuya et al., 2000), while d-serine is a co-agonist at the N-methyl-d-aspartate subtype of glutamate-gated ion channels (Mothet et al., 2000, Shleper et al., 2005), and may also act as a ligand at the GluRδ2 glutamate-like receptor (Naur et al., 2007). In mammalian cells l-serine is synthesized via the “phosphorylated pathway” which encompasses three enzymatic steps. In the first step, 3-phosphoglycerate derived from glycolysis is metabolized to phosphohydroxypyruvate by 3-phosphoglycerate dehydrogenase (de Koning et al., 2003). In the second step phosphohydroxypyruvate is converted into l-serine-O-phosphate (l-SOP), also known as l-phosphoserine, by the vitamin B6-dependent enzyme phosphoserine aminotransferase (PSAT; Hester et al., 1999). l-SOP is then dephosphorylated by phosphoserine phosphatase (PSP; Collet et al., 1999) to produce l-serine which is subsequently metabolized to glycine by the enzyme serine hydroxymethyltransferase, or isomerized to d-serine by serine racemase.

l-SOP, and its close synthetic congener 2-amino-4-phosphonobutyric acid (l-AP4), are selective agonists at the Group III metabotropic glutamate receptors that include mGluR4, mGluR6, mGluR7, and mGluR8 receptors (Nicoletti et al., 1986, Naples and Hampson, 2001, Brauner-Osborne et al., 2007, Hampson et al., 2008, Niswender et al., 2008). There is a paucity of information available about brain levels of l-SOP. Previous studies have reported values that differ considerably from one another, ranging from several hundred nanomolar to millimolar concentrations (McIlwain and Bachelard, 1971, Kataoka et al., 1991; reviewed in Klunk et al., 1991, Goodnough et al., 1995). The higher concentrations of l-SOP in some studies are roughly similar to that of free l-serine in the brain. Considering the impermeability of the blood–brain barrier to l-SOP, such a high concentration in brain would suggest that the enzymes in this pathway would be highly active and widely distributed within the CNS.

The importance of the l-SOP synthetic and metabolic enzymes in CNS development is illustrated by reports of genetic mutations in 3-phosphoglycerate dehydrogenase, PSAT, and PSP in humans. Although rare, mutations in all three enzymes produce a common series of clinical outcomes that include abnormal brain development (e.g. microcephaly), seizures, and mental retardation (de Koning et al., 2003, Hart et al., 2007). In addition, the l-SOP metabolizing enzyme PSP is enriched in proliferating neural progenitor cells and embryonic and hematopoietic stem cells (Nakano et al., 2007). It was also demonstrated that l-SOP inhibited neural stem cell/progenitor proliferation, enhanced neurogenic fate commitment, and improved neuronal survival (Nakano et al., 2007, Saxe et al., 2007). Interestingly, some of the effects of l-SOP (and exogenous l-AP4) on neural progenitor cells appear to be mediated by the Group III metabotropic glutamate receptor, mGluR4 (Iacovelli et al., 2006, Saxe et al., 2007, Nakamichi et al., 2008).

In the present study we examined the levels of l-SOP in rat brain, its relative potency at mGluR4, and the distributions of the PSAT and PSP proteins in the rat and monkey CNS. Based on the findings presented here, together with those of other studies, we hypothesize that l-SOP may activate mGluR4, and possibly other Group III mGluRs in the brain.