Mice deficient in the NAAG synthetase II gene Rimkla are impaired in a novel object recognition task

Abstract

N‐acetylaspartylglutamate (NAAG) is an abundant neuropeptide in the mammalian nervous system, synthesized by two related NAAG synthetases I and II (NAAGS‐I and ‐II) encoded by the genes Rimklb and Rimkla, respectively. NAAG plays a role in cognition and memory, according to studies using inhibitors of the NAAG hydrolase glutamate carboxypeptidase II that increase NAAG concentration. To examine consequences of reduced NAAG concentration, Rimkla‐deficient (Rimkla−/−) mice were generated. These mice exhibit normal NAAG level at birth, likely because of the intact Rimklb gene, but have significantly reduced NAAG levels in all brain regions in adulthood. In wild type mice NAAGS‐II was most abundant in brainstem and spinal cord, as demonstrated using a new NAAGS‐II antiserum. In the hippocampus, NAAGS‐II was only detectable in neurons expressing parvalbumin, a marker of GABAergic interneurons. Apart from reduced open field activity, general behavior of adult (6 months old) Rimkla−/− mice examined in different tests (dark‐light transition, optokinetic behavior, rotarod, and alternating T‐maze) was not significantly altered. However, Rimkla−/− mice were impaired in a short‐term novel object recognition test. This was also the case for mice lacking NAA synthase Nat8l, which are devoid of NAAG. Together with results from previous studies showing that inhibition of the NAAG degrading enzyme glutamate carboxypeptidase II is associated with a significant improvement in object recognition, these results suggest a direct involvement of NAAG synthesized by NAAGS‐II in the memory consolidation underlying the novel object recognition task.

N-Acetyl-Aspartyl-Glutamate in Brain Health and Disease

“…The dipeptide NAAG is synthesized from N-acetyl-aspartate (NAA) and glutamate by NAAG synthetase (NAAGS) I and II in neurons [30][31][32]. Whether NAAGSI or II are distributed differently between subcellular compartments has not been reported.…”

“…In addition, in humans, increased GCPII expression or gain-of-function mutations in the gene encoding GCPII (Folh1), which leads to decreased NAAG levels, are associated with impaired cognition [31]. Lower NAAG levels, caused by the lack of the gene encoding NAAGS I (Rimkla knock-out (KO) mice) or lack of the synthesis enzyme for its precursor NAA (Nat8l KO mice), also lead to impaired cognition [30]. On the other hand, higher NAAG levels have been reported to positively correlate with visual memory performance [31].…”

The PGRMC1 Antagonist AG-205 Inhibits Synthesis of Galactosylceramide and Sulfatide

“…SDS-PAGE and Western blotting were performed as described previously [ 38 ]. The following antibodies were used in this study: rabbit anti-CGT (dilution 1:4000; kind gift from Gerrit van Meer) [ 39 ], rabbit anti-α-tubulin (dilution 1:20,000; cat# 600-401-880, Rockland Immunochemicals, Limerick, PA, USA), and peroxidase-conjugated goat anti-rabbit IgG (dilution 1:20,000; cat# 111-035-003, Jackson ImmunoResearch Laboratories, West Grove, PA, USA).…”

The regulatory role of NAAG-mGluR3 signaling on cortical synaptic plasticity after hypoxic ischemia

“…Several studies have indicated that NAAG can attenuate excitotoxicity caused by increased Glu transmission and hence exert neuroprotective effects [ 10 – 12 ], which is determined by the metabolic processes and signaling patterns of NAAG in the brain. NAAG synthetase (NAAGS) catalyzes the biosynthesis of NAAG from N-acetyl-aspartate (NAA) and Glu (NAAGS) [ 13 , 14 ] through a mechanism that consumes Glu and helps to minimize the excitotoxic effects of Glu accumulation in pathological situations, hence reducing neuronal damage.…”

“…Further, this complex acts as a retrograde neurotransmitter and negatively regulates Glu signaling by lowering Glu release and excitotoxicity by inhibiting intra- and extrasynaptic N-methyl-D-aspartic acid receptor (NMDAR) [ 9 , 16 – 18 ]. However, NAAG acts for a short time and is quickly hydrolyzed into NAA and Glu by glutamate carboxypeptidase II (GCP-II) [ 14 , 19 ]. The use of GCP-II inhibitors such as 2-phosphonomethyl pentanedioic acid (2-PMPA) can increase NAAG levels, thus promoting the neuroprotective effect of NAAG [ 20 , 21 ].…”