Serine racemase (SR), localized to astrocytic glia that ensheathe synapses, converts L-serine to D-serine, an endogenous ligand of the NMDA receptor. We report the activation of SR by glutamate neurotransmission involving ␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors via glutamate receptor interacting protein (GRIP) and the physiologic regulation of cerebellar granule cell migration by SR. GRIP physiologically binds SR, augmenting SR activity and D-serine release. GRIP infection of neonatal mouse cerebellum in vivo enhances granule cell migration. Selective degradation of D-serine by D-amino acid oxidase and pharmacologic inhibition of SR impede migration, whereas D-serine activates the process. Thus, in neuronal migration, glutamate stimulates Bergmann glia to form and release D-serine, which, together with glutamate, activates NMDA receptors on granule neurons, chemokinetically enhancing migration.␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor ͉ D-serine T he NMDA receptor is activated by the neurotransmitter glutamate as well as a second agonist influencing a site that can be stimulated by glycine (1). Recent studies indicate that D-serine is a major, if not principal, ligand for this ''glycine site'' (2-5). D-serine is formed by serine racemase (SR) from L-serine (5). SR and D-serine are localized to a population of protoplasmic astrocytes that ensheath synapses and possess the ␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor. SR also occurs in radial glia such as Bergmann glia that guide cerebellar granule cells migrating from the external granule layer (EGL) to the internal granule layer (IGL) during cerebellar development (6, 7). This process, one of the best-characterized instances of neuronal migration, has been perplexing, because immature granule cells lack synaptic contacts. Moreover, whereas their migration is determined by glutamate acting via NMDA receptors, the cellular interactions that mediate the neuronal migration have not heretofore been identified (7).We report that SR binds to the AMPA receptor binding protein GRIP (glutamate receptor interacting protein), leading to markedly enhanced SR activity and D-serine release from glia, actions that are elicited by AMPA receptor stimulation. Overexpression of GRIP in mice also augments NMDA receptormediated neuronal cell migration. In brain slices we demonstrate that D-serine is required for granule cell migration via chemokinetic influences on granule cells.
Materials and MethodsYeast Two-Hybrid Screening. Yeast two-hybrid screening used Y190 yeast strains, which contain HIS3 and -gal reporter genes. Full-length SR gene was subcloned into pPC97, containing GAL4 DNA binding domain. Rat hippocampus and cortex cDNA library was cloned into pPC86, containing GAL4 transactivation domain. More than one million clones were transformed and screened. All other constructs of SR and GRIP were subcloned into pPC97 or pPC86.Protein Binding Assays. SR(WT) and SR(V339G) tagged with GST were transfected into hum...