Distinct roles for ephrinB3 in the formation and function of hippocampal synapses

The transmembrane ephrinB ligands and their Eph receptor tyrosine kinases are known to regulate excitatory synaptic functions in the hippocampus. In the CA3-CA1 synapse, ephrinB ligands are localized to the post-synaptic membrane, while their cognate Eph receptors are presumed to be pre-synaptic. Interaction of ephrinB molecules with Eph receptors leads to changes in long-term potentiation (LTP), which has been reported to be mediated by reverse signaling into the post-synaptic membrane. Here, we demonstrate that the cytoplasmic domain of ephrinB3 and hence reverse signaling is not required for ephrinB dependent learning and memory tasks or for LTP of these synapses. Consistent with previous reports, we find that ephrinB3^KO null mutant mice exhibit a striking reduction in CA3-CA1 LTP that is associated with defective learning and memory tasks. We find the null mutants also show changes in both pre- and post-synaptic proteins including increased levels of synapsin and synaptobrevin and reduced levels of NMDA receptor subunits. These abnormalities are not observed in ephrinB3^lacZ reverse signaling mutants that specifically delete the ephrinB3 intracellular region, supporting a cytoplasmic domain-independent forward signaling role for ephrinB3 in these processes. We also find that both ephrinB3^KO and ephrinB3^lacZ mice show an increased number of excitatory synapses, demonstrating a cytoplasmic-dependent reverse signaling role of ephrinB3 in regulating synapse number. Together, these data suggest that ephrinB3 may act like a receptor to transduce reverse signals to regulate the number of synapses formed in the hippocampus, and that it likely acts to stimulate forward signaling to modulate a number of other proteins involved in synaptic activity and learning/memory.