Novel NMDA receptor splice variants

Injury to white matter oligodendrocytes is central to several important disorders including cerebral palsy, stroke and multiple sclerosis. Dr. Fern’s group and others have recently shown that NMDA receptors are present on oligodendrocyte processes and mediate injury of these myelinating processes. The pharmacological profile of NMDA receptors present in white matter is quite unique. NMDA receptors are composed of the subunits NR1, NR2A-D and NR3A-B that assemble to form a heterotetrameric complex. Importantly, the subunit composition determines the properties of the receptor complex. Two possibilities were proposed to explain the unusual profile of NMDA receptor-mediated currents in white matter: (1) novel splice variants are expressed in glia and/or (2) the major NMDA receptor complex present is composed of an uncharacterized NMDA receptor subunit stoichiometry. In this PhD project I explored these two hypotheses. NR1, NR2C and NR3A, which are thought to be the major components of NMDA receptors in oligodendrocytes, were cloned from the myelinating rat optic nerve. In addition, all known NMDA receptor subunits were cloned from the neonate rat brain. This analysis revealed that only a subset of NR1 splice variants, those lacking exon 5, is expressed in white matter. I have also cloned NR3B and identified several putative novel splice variants of this subunit in both the optic nerve and the brain. Novel splice variants of NR2B-D were also cloned. Four of these novel NR3B variant were characterized by single cell Ca2+ imaging revealing that the novel variants form function receptors. Furthermore, NR3 subunits influence NMDA receptor glutamate sensitivity and Mg2+ in an NR2-dependent manner. The results here presented reveal a previous uncharacterized wealth of NMDA receptor splice variants which modify NMDA receptor physiology.