The ephrin -B family of receptor tyrosine kinase ligands in CNS synapse development

During development, each neuron in the central nervous system must form thousands of synapses in a precise pattern. The development of this connectivity requires complex regulation such that each synapse contains the necessary molecular components, and each neurons contains the appropriate distribution of synapses. The molecular mechanisms that underlie these steps are not completely understood, but the importance of transmembrane adhesion molecules that also signal intracellularly is apparent. In particular, EphB receptors and their ephrin-B ligands are important regulators of synaptic development. However, while EphB receptors regulate postsynaptic development, the precise role of the ephrin-B ligands is less well understood. During my thesis, I have investigated how the ephrin-B family participates in the development of excitatory synapses in the central nervous system. I have found that two members of this family, ephrin-B 1 and ephrin-B2 regulate presynaptic development downstream of a trans-synaptic interaction with EphB. Knockdown of eprhin-B1 or ephrin-B2 blocks EphB-dependent presynaptic development and leads to a reduction in the number of synaptic contacts. EphB-dependent presynaptic induction can also be blocked by disrupting the intracellular adaptor syntenin-1. In contrast to the presynaptic role of ephrin-B1 and ephrin-B2, ephrin-B3 functions postsynaptically in controlling the overall number of synaptic contacts. Postsynaptic knockdown of ephrin-B3, but not ephrin-B1 or ephrin-B2, reduces the number of synapses and dendritic spines, and regulation of synapse number by ephrin-B3 requires intracellular regions that are not conserved with ephrin-B1. Ephrin-B3 expression and synapse number are highly correlated, demonstrating that ephrin-B3 is a key regulator of synapse number. Finally, whereas loss of ephrin-B3 in a small subset of neurons leads to a reduction in synapse number, loss of ephrin-B3 in all neurons, as is seen in ephrin-B3 null mice, does not affect synapse number. This suggests that ephrin-B3 regulates synapse number according to the relative, rather than absolute, level of ephrin-B3 expression. Taken together, these results demonstrate that ephrin-Bs regulate many features of synapse development, but different ephrin-Bs are specified to regulate different aspects of this process