d-Serine and Serine Racemase are Localized to Neurons in the Adult Mouse and Human Forebrain

Abstract

d-Serine, a co-agonist at the NMDA receptor (NMDAR), is synthesized from l-serine by the enzyme serine racemase (SR), which is heavily expressed in the forebrain. Although SR was originally reported to be localized exclusively to astrocytes, recent conditional knock out results demonstrate that little SR is expressed in forebrain astrocytes. As a consequence, the cellular location of its product, d-serine, in the brain is also uncertain. Immunocytochemistry now indicates that SR is expressed primarily in forebrain glutamatergic neurons with the remainder in GABAergic interneurons. We utilized SR deficient (SR–/–) mice, which have <15 % of normal d-serine levels, to validate and optimize a d-serine immunohistochemical method. Nearly all of the d-serine in neocortex and hippocampus (HP) is found in neurons, with virtually no d-serine co-localizing with two astrocyte markers. Interestingly, only a subset of the d-serine positive neurons contained SR in the neocortex and HP. Greater than half of the d-serine positive neurons were GABAergic interneurons, with a majority of these neurons containing parvalbumin and/or somatostatin. Only ∼25–40 % of interneurons expressed SR in the neocortex and HP. Finally, we demonstrate in human post-mortem neocortex that SR is found in both excitatory and inhibitory neurons, but not in S100β-containing astrocytes. In sum, these findings conclusively demonstrate that the majority of d-serine is both synthesized and stored in neurons. It will be important to determine the functional significance for the separation of synthesis and storage of d-serine in neurons, as well as the presence of this NMDAR co-agonist in GABAergic interneurons.

Enhanced astrocytic d-serine underlies synaptic damage after traumatic brain injury

“…A similar expression profile was observed for the synthesizing enzyme SRR, essential for conversion of l-serine to d-serine ( Figure 1, A and C). This expression profile supports previous studies showing that SRR and d-serine are primarily localized to neurons (12,(18)(19)(20).…”

“…NMDAR activation also requires the binding of a coagonist, such as d-serine or glycine. d-serine is considered a more likely binding partner for NMDAR in the hippocampus, since its expression patterns match NMDAR expression (12)(13)(14)(15)(16) and it has a higher affinity for the NMDAR glycine-binding site (17).…”

“…In the adult mammalian forebrain, d-serine and its synthesizing enzyme, serine racemase (SRR), are expressed by excitatory glutamatergic neurons and subpopulations of GABAergic neurons (12,(18)(19)(20). Conversely, recent studies indicate that only a small percentage of astrocytes express a negligible amount of SRR in rodent forebrain tissues (21).…”

Glycolytic flux controls d -serine synthesis through glyceraldehyde-3-phosphate dehydrogenase in astrocytes

“…SRR was first identified as a glial enzyme (44), but more recent studies using SRR knockout mice have shown that SRR is expressed primarily in neurons found in the cerebral cortex and hippocampus (27,45). On the other hand, the possibility of SRR expression in astrocytes cannot be excluded, given the following evidence: (i) astrocyte-specific knockout of SRR reduces its expression level by 15% in the brain (28); (ii) eGFP designed to express under the SRR promoter in mice is detectable in hippocampal astrocytes (29); and (iii) enhancement of immunohistochemical signal makes astrocytic SRR observable (Fig.…”

Organization, control and function of extrasynaptic NMDA receptors

“…This generated a new craze for NMDARs and laid the groundwork for a meaningful understanding of how these receptors impact cellular physiology and pathology. More recently, NMDARs have gained a new interest, as D-serine, a potential gliotransmitter released by astrocytes ( [16], but see [17]) was found to act as an endogenous co-agonist on their glycine-binding site [16,18 -21]. This discovery has made glia a possible regulator of NMDAR function and marked the emergence of an exciting cross-talk between the field of NMDARs and the expending & 2014 The Author(s) Published by the Royal Society.…”