Molecular mechanisms of pre- and postsynaptic EphB/ephrin-B signaling in synapse formation and function

Proper function of the central nervous system relies on precise and coordinated cell-cell interactions and communication via synaptic transmission to assemble neuronal networks. Aberrant synaptic transmission is a hallmark of neuronal disease. The EphB family of receptor tyrosine kinases and their ephrin-B ligands play critical roles in the central nervous system in axon guidance, formation of pre- and post-synaptic specializations, localization of glutamate receptors, synaptic plasticity, and disease. EphB/ephrin-B signaling has been reported to modulate these processes, but the molecular mechanisms remain poorly understood. Our laboratory has previously shown that EphBs organize the formation of both pre- and postsynaptic specializations, and interact directly with NMDA-type glutamate receptors. Therefore, I sought to investigate the molecular mechanisms for formation of presynaptic specializations and the interaction domain between EphBs and NMDA receptors. I found that EphBs can induce the formation of presynaptic specializations by trans-synaptic interactions with both ephrin-B1 and ephrin-B2. These ephrin-Bs can then recruit the machinery for neurotransmitter release through the multiple PDZ-domain containing adaptor protein syntenin-1. Furthermore, ephrin-B1 and ephrin-B2 act independently for formation of presynaptic specializations, but together to recruit syntenin-1 to synaptic sites. Based on this work and that of other laboratories, I was able to define the molecular pathway from postsynaptic EphBs to presynaptic glutamatergic vesicles. Furthermore, on the postsynaptic side of the synapse, I define a single amino acid that is necessary and sufficient to mediate the EphB-NMDAR interaction. In a novel molecular mechanism, I show that extracellular phosphorylation of this residue after ephrin-B binding is sufficient to induce the EphB-NMDAR interaction. Furthermore, I show that in the mature brain, the EphB-NMDAR interaction preferentially regulates NR2B-subunit containing NMDA receptor localization, function, and downstream gene transcription. Together, these findings impact our understanding of synapse formation and function, and highlight the EphB-NMDAR interaction as a potential target to treat neurological disease