Trans-synaptic Coordination of Synapse Development and Plasticity: A Focus on Ephrin-B3 and PSD-95
The mammalian brain contains trillions of synaptic connections, each of which is an intricate, highly organized trans-synaptic complex of pre- and post-synaptic molecular machinery. These synaptic connections serve to organize neurons into functional neural circuits. Thus, neurons must possess mechanisms not only to tightly coordinate the molecular organization at individual synapses, but also to orchestrate the development of patterned neural circuits. The studies presented in this thesis focus on specific molecular mechanisms of trans-synaptic signaling that underlie synapse development, organization, and plasticity. In these studies, we use a multidisciplinary approach including multiple imaging techniques, biochemical assays, genetic models, and molecular biology to study the development and organization of synapses in vitro and in vivo. In the first study, we find that the trans-synaptic organizing protein ephrin-B3 controls synapse number by mediating a cell-cell competition for local synaptic inputs. Control of synapse number by ephrin-B3 relies on competition for its pre-synaptic ligand, EphB2. Importantly, ephrin-B3-mediated competition persisted in the absence of neuronal activity, suggesting that ephrin-B3 acts independently of the well-described activity-dependent competitive mechanisms that operate during synapse development. Next, we show that ephrin-B3 interacts with PSD-95, an important post-synaptic scaffolding protein. The ephrin-B3-PSD-95 interaction is required for anchoring PSD-95 at synapses and is negatively modulated by Erk1/2-dependent phosphorylation of ephrin-B3 and by sensory-driven activity in vivo. As control of synapse number by ephrin-B3 relies on ephrin-B3 binding to and inhibiting Erk1/2, these findings elucidate an interplay between ephrin-B3 and Erk1/2 signaling that links ephrin-B3 signaling in synapse development to its function at mature synapses where it anchors PSD-95. In the final study, we study the molecular nano-organization of excitatory synapses at baseline and during structural synaptic plasticity. We find that individual synapses contain discrete, aligned ‘nanomodules’ of pre- and post-synaptic molecules that scale with spine size in vitro and in vitro. Moreover, coordinated addition of pre- and post-synaptic nanomodules underlies NMDAR-dependent structural plasticity of dendritic spines. Together, these studies establish the trans-synaptic organizer ephrin-B3 as an important determinant of synapse number and synapse organization and highlight a remarkable trans-synaptic coordination of pre- and post-synaptic molecular nano-organization.
Henderson, Nathan T, "Trans-synaptic Coordination of Synapse Development and Plasticity: A Focus on Ephrin-B3 and PSD-95" (2018). ETD Collection for Thomas Jefferson University. AAI10933089.