Evidence of NAAG-family tripeptide NAAG2 in the Drosophila nervous system

Annotation of Dipeptides and Tripeptides Derivatized via Dansylation Based on Liquid Chromatography-Mass Spectrometry and Iterative Quantitative Structure Retention Relationship

Small peptides such as dipeptides and tripeptides show various biological activities in organisms. However, methods for identifying dipeptides/tripeptides from complex biological samples are lacking. Here, an annotation strategy involving the derivatization of dipeptides and tripeptides via dansylation was suggested based on liquid chromatography-mass spectrometry (LC-MS) and iterative quantitative structure retention relationship (QSRR) to choose dipeptides/tripeptides by using a small number of standards. First, the LC-autoMS/MS method and initial QSRR model were built based on 25 selected grid-dipeptides and 18 test-dipeptides. To achieve high-coverage detection, dipeptide/tripeptide pools containing abundant dipeptides/tripeptides were then obtained from four dansylated biological samples including serum, tissue, feces, and soybean paste by using the parameter-optimized LC-autoMS/MS method. The QSRR model was further optimized through an iterative train-by-pick strategy. Based on the specific fragments and t_R tolerances, 198 dipeptides and 149 tripeptides were annotated. The dipeptides at lower annotation levels were verified by using authentic standards and grid-correlation analysis. Finally, variation in serum dipeptides/tripeptides of three different liver diseases including hepatitis B infection, liver cirrhosis, and hepatocellular carcinoma was characterized. Dipeptides with N-prolinyl, C-proline, N-glutamyl, and N-valinyl generally increased with disease severity. In conclusion, this study provides an efficient strategy for annotating dipeptides/tripeptides from complex samples.

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

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.