Modification of the strength of excitatory synaptic connections is a fundamental mechanism by which neural circuits are refined during development and learning. Synapse Differentiation Induced Gene 1 (SynDIG1) has been shown to play a key role in regulating synaptic strength in vitro. Here, we investigated the role of SynDIG1 in vivo in mice with a disruption of the SynDIG1 gene rather than use an alternate loxP flanked conditional mutant that we find retains a partial protein product. The gene-trap insertion with a reporter cassette mutant mice show that the SynDIG1 promoter is active during embryogenesis in the retina with some activity in the brain, and postnatally in the mouse hippocampus, cortex, hindbrain, and spinal cord. Ultrastructural analysis of the hippocampal CA1 region shows a decrease in the average PSD length of synapses and a decrease in the number of synapses with a mature phenotype. Intriguingly, total synapse number appears to be increased in SynDIG1 mutant mice. Electrophysiological analyses show a decrease in AMPA and NMDA receptor function in SynDIG1 deficient hippocampal neurons. Glutamate stimulation of individual dendritic spines in hippocampal slices from SynDIG1 deficient mice reveals increased short-term structural plasticity. Notably, the overall levels of PSD-95 or glutamate receptors enriched in postsynaptic biochemical fractions remain unaltered; however, activity-dependent synapse development is strongly compromised upon loss of SynDIG1, supporting its importance for excitatory synapse maturation. Together, these data are consistent with a model in which SynDIG1 regulates the maturation of excitatory synapse structure and function in the mouse hippocampus in vivo.
Significance Statement: SynDIG1 is a brain specific transmembrane protein enriched at excitatory synapses. Removal of endogenous SynDIG1 in the mouse brain results in a significant reduction in excitatory synapse maturation as determined by a combination of structural and electrophysiological approaches. In contrast, biochemical analysis reveals that excitatory synapse composition is unchanged upon loss of SynDIG1. Together, these data support a model in which SynDIG1 is important for excitatory synapse maturation but is not required for synaptogenesis in vivo.
The authors declare no competing financial interests exist.
This work was supported by funds to E.D. from the National Institutes of Health (NIH) Director’s New Innovator Award Program (DP2 OD006479-01), the National Science Foundation (NSF 1322302), and a Pilot Grant from the UC Davis Academic Senate Research Program; funds to J.W.H from the NIH (R01 NS078792); funds to K.Z. from the National Science Foundation (NSF 0845285); an American Heart Association Postdoctoral Fellowship 11POST7020009 (L.M.), and the NARSAD Young Investigator Grant 20748 from the Brain & Behavior Research Foundation (L.M.)
All animal procedures followed NIH guidelines and have been approved by the Institutional Animal Care and Use Committee at UC Davis.