Summary We identify the leucine-rich repeat transmembrane protein LRRTM2 as a key regulator of excitatory synapse development and function. LRRTM2 localizes to excitatory synapses in transfected hippocampal neurons, and shRNA-mediated knockdown of LRRTM2 leads to a decrease in excitatory synapses without affecting inhibitory synapses. LRRTM2 interacts with PSD-95 and regulates surface expression of AMPA receptors, and lentivirus-mediated knockdown of LRRTM2 in vivo decreases the strength of evoked excitatory synaptic currents. Structure-function studies indicate that LRRTM2 induces presynaptic differentiation via the extracellular LRR domain. We identify Neurexin1 as a receptor for LRRTM2 based on affinity chromatography. LRRTM2 binds to both Neurexin 1α and Neurexin 1β, and shRNA-mediated knockdown of Neurexin1 abrogates LRRTM2-induced presynaptic differentiation. These observations indicate that an LRRTM2-Neurexin1 interaction plays a critical role in regulating excitatory synapse development.

BackgroundThe assembly of neural circuits requires the concerted action of both genetically determined and activity-dependent mechanisms. Calcium-regulated transcription may link these processes, but the influence of specific transcription factors on the differentiation of synapse-specific properties is poorly understood. Here we characterize the influence of NeuroD2, a calcium-dependent transcription factor, in regulating the structural and functional maturation of the hippocampal mossy fiber (MF) synapse.ResultsUsing NeuroD2 null mice and in vivo lentivirus-mediated gene knockdown, we demonstrate a critical role for NeuroD2 in the formation of CA3 dendritic spines receiving MF inputs. We also use electrophysiological recordings from CA3 neurons while stimulating MF axons to show that NeuroD2 regulates the differentiation of functional properties at the MF synapse. Finally, we find that NeuroD2 regulates PSD95 expression in hippocampal neurons and that PSD95 loss of function in vivo reproduces CA3 neuron spine defects observed in NeuroD2 null mice.ConclusionThese experiments identify NeuroD2 as a key transcription factor that regulates the structural and functional differentiation of MF synapses in vivo.

The establishment of neuronal circuits relies on the stabilization of functionally appropriate connections and the elimination of inappropriate ones. Here we report that postsynaptic AMPA receptors play a critical role in regulating the stability of glutamatergic synapses. Removal of surface AMPA receptors leads to a decrease in the number and stability of excitatory presynaptic inputs, whereas overexpression increases synapse number and stability. Furthermore, overexpression of AMPA receptors along with Neuroligin-1 in 293T cells is sufficient to stabilize presynaptic inputs from cortical neurons onto heterologous cells. The stabilization of presynaptic inputs by AMPA receptors is not dependent on receptor-mediated current and instead relies on structural interactions mediated by the N-terminal domain of the glutamate receptor 2 (GluR2) subunit. These observations indicate that transsynaptic signaling mediated by the extracellular domain of GluR2 regulates the stability of presynaptic terminals.GluR2 N-terminal domain | Presynaptic stabilization | Presynaptic input dynamics T he development of neural circuits is characterized by exuberant synapse formation followed by elimination of inappropriate connections (1-4). Although there is considerable evidence that activity-dependent mechanisms are involved in synapse elimination, the mechanisms responsible for regulating and maintaining stable synapses are largely unknown.Synapse formation is a dynamic process and involves the rapid recruitment of several synaptic proteins to sites of axo-dendritic contact. Transport packets containing essential components of the presynaptic active zone are highly motile before they reach synaptic sites (5). Likewise, synaptic vesicles traffic rapidly along axons before stabilizing at dendritic contact sites (6), suggesting that a dendrite-associated signal regulates presynaptic stability. At glutamatergic synapses, AMPA and NMDA receptors are recruited to the postsynaptic density in 1 h or less (7,8), raising the possibility that they may play an important role in regulating synapse stability.Electrophysiological and immunohistochemical experiments suggest that a large percentage of young synapses contain NMDA receptors but lack AMPA receptors (9, 10). AMPA receptors are recruited gradually to postsynaptic sites, resulting in an increase in the AMPA/NMDA ratio at these synapses (11-13). The increase in the AMPA/NMDA ratio correlates with the stabilization of dendritic spines.Synapse formation requires the precise apposition of presynaptic and postsynaptic elements underlying the need for bidirectional signaling. Key postsynaptic proteins such as neuroligins, synaptic cell-adhesion molecules (SynCAMs), Ephrin type B (EphB) receptors, netrin G ligands, and leucine-rich repeat transmembrane (LRRTM) proteins signal transsynaptically to induce recruitment of presynaptic components (14)(15)(16)(17)(18)(19)(20). Postsynaptic adhesion molecules also are involved in the functional maturation of presynaptic inputs (21-23). Recent work demons...

Background: The assembly of neural circuits requires the concerted action of both genetically determined and activity-dependent mechanisms. Calcium-regulated transcription may link these processes, but the influence of specific transcription factors on the differentiation of synapse-specific properties is poorly understood. Here we characterize the influence of NeuroD2, a calcium-dependent transcription factor, in regulating the structural and functional maturation of the hippocampal mossy fiber (MF) synapse. Results: Using NeuroD2 null mice and in vivo lentivirus-mediated gene knockdown, we demonstrate a critical role for NeuroD2 in the formation of CA3 dendritic spines receiving MF inputs. We also use electrophysiological recordings from CA3 neurons while stimulating MF axons to show that NeuroD2 regulates the differentiation of functional properties at the MF synapse. Finally, we find that NeuroD2 regulates PSD95 expression in hippocampal neurons and that PSD95 loss of function in vivo reproduces CA3 neuron spine defects observed in NeuroD2 null mice. Conclusion: These experiments identify NeuroD2 as a key transcription factor that regulates the structural and functional differentiation of MF synapses in vivo.