Ph.D Thesis

Ph.D StudentRosenberg Dina
SubjectD-Serine Release through Asc-1 Transporter and its Role in
Synaptic Activity
DepartmentDepartment of Medicine
Supervisor PROF. Herman Wolosker
Full Thesis textFull thesis text - English Version


d-Serine is a physiologic co-agonist of NMDA receptors (NMDAR), and plays major roles in normal NMDAR activity, synaptic plasticity and neurotoxicity. D-Serine has been largely thought to be a gliotransmitter. Recent data, however, indicate that neurons are a major site of D-serine production. We found that a depolarizing agent veratridine, promotes substantial D-serine release from primary cultures, acute cortical slices and in vivo. The specificity of veratridine towards neurons was confirmed in primary astrocytic culture that does not release D-serine upon depolarization. Also, selective loading of neurons with D-[3H]serine in cortical slices did not alter the veratridine effect.  Furthermore, in cortical slices perfused with veratridine, endogenous D-serine release is 10 fold higher than glutamate receptor-evoked release. Release of D-serine from slices does not require either external or internal Ca2, suggesting a non-vesicular release mechanism. Until recently, D-serine was thought to be released mainly by vesicular exocytosis from astrocytes upon non-NMDA glutamate receptor stimulation. In contrast our data indicate that the non-vesicular D-serine efflux from neuronal pool plays a major role in D-serine dynamics.

We hypothesized that the neuronal Asc-1 transporter may be a pathway of D-serine release from neurons. A screening of Asc-1 substrates indicated d-isoleucine (d-ile) as specific enhancer of Asc-1 antiporter activity. D-Ile elicited d-serine and glycine release from Asc-1-transfected cells, primary neuronal cultures and hippocampal slices. Importantly d-ile has no effect on astrocytes, which do not express Asc-1. We show that d-ile enhances the long-term potentiation (LTP) in rat and mouse hippocampal CA1 by stimulating Asc-1-mediated d-serine release. d-Ile effects on synaptic plasticity are abolished by enzymatically depleting d-serine or by employing serine racemase knockout (SR-KO) mice, confirming its specificity and supporting the notion that LTP depends mostly on d-serine release. On the other hand, our data also disclose a role of glycine in activating synaptic NMDARs. Although acute enzymatic depletion of d-serine drastically decreases the isolated NMDAR synaptic potentials, these responses are still enhanced by d-ile. Furthermore, NMDAR synaptic potentials are preserved in SR-KO mice and are also enhanced by d-ile, indicating that glycine overlaps with d-serine binding at synaptic NMDARs.

Altogether, our results disclose a novel role of Asc-1 in regulating NMDAR-dependent synaptic activity by mediating concurrent non-vesicular release of d-serine and glycine. Our data also highlight an important role of neuron-derived d-serine and glycine, indicating that astrocytic d-serine is not the sole responsible for activating synaptic NMDARs.