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Jonathan Sweedler What are d-amino acids doing in our brain ?

Abstract :

We are studying the roles that d-aspartate and d-glutamate (D-Asp and D-Glu) play in neurotransmission.
We have developed several approaches that allow us to measure these molecules in identified neurons, and probe their release in an activity dependent manner. Because neurochemistry is well conserved across metazoan life, we have selected the well-known physiological model Aplysia californica to probe the functions of these d-amino acids in cell to cell signaling. Specifically, using capillary electrophoresis with laser induced fluorescence and radionuclide detection, we have determined which neurons synthesize d-Asp, and have measured the formation, transport and release of d-Asp in a stimulation dependent manner. Once we determined which neurons synthesize d-Asp, we have located and characterized the enzyme responsible for its formation. The novel racemase has the ability to convert both L-Ser and L-Asp to their D-counterparts. When combined with physiological measurements, we are able to show that D-Asp acts as a neurohormone and as a neurotransmitter in our system. We have also measured the presence of a unique population of d-Glu containing neurons and are currently studying the effect of d-Glu on neurotransmission. In addition to free-d-amino acids, we also are probing the presence of d-amino acids in neuropeptides; this is a particularly challenging measurement because the post-translational modification has no associated mass change, making it difficult to probe using mass spectrometry alone. Using a combination of selective enzymatic digestion and small-volume mass spectrometry, we have been able to measure d-amino acid containing peptides in specific cells and are currently probing the changes in response caused by this modification. While the information presented emphasizes the fundamental roles of these molecules in cell-cell signaling, several aspects of d-amino acid signaling are becoming of clinical relevance, including the use of D-serine in the treatment of schizophrenia, and the fact that D-aspartate is enriched in the neuroendocrine system where it modulates the synthesis and the release of reproductive hormones. It is intriguing to speculate on what other roles in the brain are played d-amino acids.

Selected publications

Analysis of Natural D-Amino Acid-Containing Peptides in Metazoa, L. Bai, S. Sheeley, J.V. Sweedler, Bioanal. Rev. 1, 2009, 7–24.
Quantitative Peptidomics Reveal Brain Peptide Signatures of Behavior, A. Brockmann, S.P. Annangudi, T.A. Richmond, S.A. Ament, F. Xie, B.R. Southey, S.R. Rodriguez-Zas, G.E. Robinson, J.V. Sweedler, Proc. Natl. Acad. Sci., U.S.A. 106, 2009, 2383–2388.
D-Aspartate as a Putative Cell-Cell Signaling Molecule in the Aplysia californica Central Nervous System, H. Miao, S.S. Rubakhin, C.R. Scanlan, L. Wang, J.V. Sweedler, J. Neurochem. 97, 2006, 595–606.
MS-Based Discovery of Circadian Peptides, N.G. Hatcher, N. Atkins, Jr., S.P. Annangudi, A.J. Forbes, N.L. Kelleher, M.U. Gillette, J.V. Sweedler, Proc. Natl. Acad. Sci., U.S.A. 105, 2008, 12527–12532.

Jean Pierre Mothet