Abstract
d-Serine is a co-agonist at forebrain N-methyl-d-aspartate receptors (NMDAR) and is synthesized by serine racemase (SR). Although d-serine and SR were originally reported to be localized to glia, recent studies have provided compelling evidence that under healthy physiologic conditions both are localized primarily in neurons. However, in pathologic conditions, reactive astrocytes can also express SR and synthesize d-serine. Since cultured astrocytes exhibit features of reactive astrocytes, we have characterized d-serine synthesis and the expression of enzymes involved in its disposition in primary glial cultures. The levels of SR were quite low early in culture and increased markedly in all astrocytes with the duration in vitro. The concentration of d-serine in the culture medium increased in parallel with SR expression in the astrocytes. Microglia, identified by robust expression of Iba1, did not express SR. While the levels of glial fibrillary acidic protein (GFAP), glycine decarboxylase (GLDC) and phosphoglycerate dehydrogenase (PHGDH), the initial enzyme in the pathway converting glycine to l-serine, remained constant in culture, the expression of lipocalin-2, a marker for pan-reactive astrocytes, increased several-fold. The cultured astrocytes also expressed Complement-3a, a marker for a subpopulation of reactive astrocytes (A1). Astrocytes grown from mice with a copy number variant associated with psychosis, which have four copies of the GLDC gene, showed a more rapid production of d-serine and a reduction in glycine in the culture medium. These results substantiate the conclusion that A1 reactive astrocytes express SR and release d-serine under pathologic conditions, which may contribute to their neurotoxic effects by activating extra-synaptic NMDARs.