Neuronal d-serine regulates dendritic architecture in the somatosensory cortex

Abstract

d-Serine, which is synthesized by the enzyme serine racemase (SR), is a co-agonist at the N-methyl-d-aspartate receptor (NMDAR). In an animal model of NMDAR hypofunction, the constitutive SR knockout (SR−/−) mouse, pyramidal neurons in primary somatosensory cortex (S1) have reductions in the complexity, total length, and spine density of apical and basal dendrites. We wondered whether the dendritic pathology required deprivation of d-serine throughout development or reflected the loss of d-serine only in adulthood. To address this question, we used mice homozygous for floxed SR in which we bred CaMKIICre2834, which is expressed in forebrain glutamatergic neurons starting at 3–4 weeks post-partum (nSR−/−). Our prior studies demonstrated that the majority of cortical SR is expressed in glutamatergic neurons. We found that similar to SR−/− mice, pyramidal neurons in S1 of nSR−/− also had significantly reduced dendritic arborization and spine density, albeit to a lesser degree. S1 neurons of nSR−/− mice had reduced total basal dendritic length that was accompanied by less complex arborization. These characteristics were unaltered in the apical dendritic compartment. In contrast, spine density on S1 neurons was significantly reduced on apical, but not basal dendrites of nSR−/− mice. These results demonstrate that in adulthood neuronally derived d-serine, which is required for optimal activation of post-synaptic NMDAR activity, regulates pyramidal neuron dendritic arborization and spine density. Moreover, they highlight the glycine modulatory site (GMS) of the NMDAR as a potential target for therapeutic intervention in diseases characterized by synaptic deficits, like schizophrenia.

Introduction

D-Amino acids are now well established as mediators and modulators of neuronal activity in mammals [6], [37]. The discovery of substantial levels of d-serine in the mammalian forebrain sparked its interest in neurobiology [14]. Activation of the N-methyl-d-aspartate receptor (NMDAR) requires the binding of either glycine or d-serine at the glycine modulatory site (GMS) on the NR1 subunit [17]. d-Serine is enriched in corticolimbic regions of the brain, where its localization closely parallels that of NMDARs [30]. Thus, d-serine is thought to be the primary forebrain co-agonist because it is concentrated in the forebrain and elimination of synaptic d-serine reduces NMDAR-mediated currents [23], [30].

The cloning and characterization of serine racemase (SR) demonstrated that d-serine is synthesized endogenously in the mammalian brain through the conversion of l- to d-serine [36]. Inactivation of the SR gene reduces cortical d-serine levels by ∼85% [3]. Initial in vitro and immunohistochemical studies suggested that SR was present mainly in astrocytes, and therefore was the major source of d-serine in the brain [24], [30], [36]. However, recent immunohistochemical studies have suggested a more prominent neuronal SR expression, consistent with the localization of d-serine in neurons [10], [22]. Furthermore, mice with a conditional deletion of SR selectively in neurons or astrocytes demonstrated that the majority (∼65%) of SR is expressed in forebrain glutamatergic neurons, particularly in the cortex and hippocampus [5].

NMDARs have been well established to regulate dendritic elaboration and spine formation in the developing nervous system [18]. Our previous work has shown that constitutive SR−/− mice, which lack SR throughout life and display reduced NMDAR function [3], have less complex dendritic arbors and reduced spine density on pyramidal neurons in the medial prefrontal cortex [9] and primary somatosensory cortex (S1) [1]. We wondered whether the dendritic pathology required deprivation of d-serine throughout development or reflected the loss of d-serine only in adulthood, since studies of addictive drugs [28] and of the estrous cycle [21] have shown marked and rapid alterations of dendrites in adulthood.

Therefore, we utilized mice that suppress SR expression beginning at 3–4 weeks post-partum in forebrain excitatory neurons (nSR−/−) to determine whether the neuronal pool of d-serine is an important regulator of adult pyramidal cell dendritic plasticity in S1.