New studies have underscored the importance of D-serine and its role as a coagonist for proper gating of NMDA receptors. D-serine deficiency has been implicated in major psychiatric disorders, including schizophrenia and bipolar disorder and clinical trials have been carried out based on the idea that schizophrenia represents a deficiency in D-serine. Thus the NMDA receptor hypofunction hypothesis of schizophrenia has already been attributed to a deficiency of D-serine by some workers. Despite the emerging importance of this atypical amino acid (proteins are made from L-amino acids), we still do not understand the release mechanisms or the cells that contain the enzymes necessary for D-serine synthesis (serine racemase; SR), storage and release. Part of this problem stems from the difficulty in finding antibodies that selectively label serine racemase, an important issue that will be addressed in this study. Recently, we have discovered a light-evoked release of D-serine in the retina, the only tissue site in which natural, physiological stimulation has been associated with D-serine release. This finding has elevated the vertebrate retina as one of the ideal target sites to investigate D-serine release mechanisms. A newly developed, second generation D-serine biosensor has been fabricated in my laboratory and developed through a collaboration with Dr. Stephane Marinesco, Neuroscience Institute of Lyon, France. Armed with this improved D-serine biosensor, we are now in a position to provide the most extensive analysis of D-serine localization and mechanisms of release ever carried out in a single CNS tissue site. The objectives of this proposal are to understand the cellular sites and mechanisms of D-serine release, storage and synthesis, using our new, carbon-fiber based D-serine biosensors combined with immunostaining methods using antibodies with proven specificity (comparing antibody specificity in wt vs SRKO (knockout). We will explore all of the major release mechanisms proposed for D-serine release from neurons and glial cells. In order to carry out these experiments, we plan to examine mice during the postnatal period of development, both before and after eye opening. The reason for this strategy is to see if the sources of D-serine synthesis and release change during development, as suggested by other workers. Specifically we want to see whether D-serine release mechanisms change from neuronal (ganglion cells) to glial (M?ller cells) during the course of postnatal development and, if so, does the release mechanism change from neuronal (heteroexchange through Asc-1 neutral amino acid transporters) to a synaptic release mediated by activation of Ca2+-permeable AMPA receptors. We also want to evaluate the D-serine release mechanisms in the adult to see if more than one mechanism is involved, as our preliminary results may suggest. We will also study a mouse line in which the SR promoter expresses eGFP in the cells that express SR. We will also explore the pathway of L-serine synthesis through the enzyme 3-PGDH which synthesizes L-serine and resides exclusively in brain astrocytes.
It is now clear that D-serine is essential for glutamate to gate the ion channel of NMDA receptors found in retinal ganglion cells. What is missing, however, is an understanding of the cell types that synthesize and release D-serine and whether these cell types change during the course of postnatal development. This research project is designed to clarify the cell types that express the synthesizing enzyme for D-serine, namely serine racemase, and the mechanisms by which D-serine is released in the retina, with special emphasis on the postnatal stage at which light-evoked release of D-serine (which we have already measured), can first be detected.
|Lockridge, Amber D; Baumann, Daniel C; Akhaphong, Brian et al. (2016) Serine racemase is expressed in islets and contributes to the regulation of glucose homeostasis. Islets 8:195-206|
|Gustafson, Eric G; Stevens, Eric S; Miller, Robert F (2015) Dynamic regulation of D-serine release in the vertebrate retina. J Physiol 593:843-56|
|Romero, Gabriel E; Lockridge, Amber D; Morgans, Catherine W et al. (2014) The postnatal development of D-serine in the retinas of two mouse strains, including a mutant mouse with a deficiency in D-amino acid oxidase and a serine racemase knockout mouse. ACS Chem Neurosci 5:848-54|