Loss of SynDIG1 Reduces Excitatory Synapse Maturation But Not FormationIn Vivo

Abstract

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 shows 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, the 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 the 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.

SynDIG4/Prrt1 Is Required for Excitatory Synapse Development and Plasticity Underlying Cognitive Function

“…For example, we observed that loss of SynDIG4 leads to increased synapse number similar to that observed in SynDIG1 mutant mice (Chenaux et al, 2016). The increased synapse density observed in SynDIG4 −/− or SynDIG1 mutant mice could be a consequence of reduced synaptic strength.…”

“…Second, LTP induced by tetanic stimulation is abolished in both young and adult SynDIG4 −/− mice, while mEPSC amplitudes were reduced by 25% and basal fEPSP transmission was unchanged, which suggests an additional effect of SynDIG4 on synaptic plasticity beyond its role in synapse development. In contrast, such LTP is reduced only in young SynDIG1 mutant mice, likely as a consequence of reduced synaptic transmission (Chenaux et al, 2016). …”

“…The rostral two-thirds of P14 mouse brains were homogenized, and biochemical fractions were analyzed as described (Chenaux et al, 2016). …”

“…Intra- and extracellular recordings were performed using standard methods as previously described (Chenaux et al, 2016). Expression in Xenopus laevis oocytes and outside-out patch-clamp electrophysiological recordings were performed as described (Priel et al, 2006).…”

“…SynDIG1 mutant mice exhibit deficits in excitatory synapse maturation (Chenaux et al, 2016). In contrast to other AMPAR accessory proteins, SynDIG1 does not regulate AMPAR dynamics (Lovero et al, 2013), suggesting that SynDIG1 is an atypical AMPAR auxiliary factor.…”