NMDA receptors (NMDARs) participate in processes ranging from neural development to learning and memory. Disorders of NMDAR signaling are linked to several neurological diseases. Accumulating evidence suggests that the endogenous co-agonist D-serine plays a prominent role in activation of synaptic NMDARs in cortex. However, there are significant gaps in our understanding of the physiological mechanisms involved in D-serine homeostasis in brain and their potential impact on NMDAR signaling. Our preliminary data suggest that SLC1A4, a neutral amino acid transporter paralog within the SLC1 solute carrier family that includes glutamate transporters, unexpectedly mediates transmembrane flux of D-serine. We will test the hypotheses that SLC1A4 is in fact the major route of sodium-dependent D-serine uptake in brain and that selective SLC1A4 inhibitors developed from a hydroxyproline pharmacophore can alter D-serine homeostasis and thereby modulate NMDAR function and synaptic plasticity. We will also characterize the structure and function of a recently identified mutation in the human gene encoding SLC1A4 that is linked to neurodevelopmental and cognitive deficits, and we will create and study a transgenic mouse model of this human disease.