Abstract
A single, maternally inherited, X-linked point mutation leading to an arginine to cysteine substitution at amino acid 451 (R451C) of Neuroligin 3 (NLGN3R451C) is a likely cause of autism in two brothers. Knockin mice expressing the Nlgn3R451C mutation in place of wild-type Nlgn3 demonstrate increased inhibitory synaptic strength in somatosensory cortex, resulting in an excitatory/inhibitory (E/I) imbalance that is potentially relevant for autism-associated behavioral deficits characteristic of these mice. We have replicated the increase in evoked inhibitory postsynaptic currents (eIPSCs) onto layer II/III cortical pyramidal neurons. We also find that increased frequency of spontaneous mIPSCs in Nlgn3R451C mice occurs in the absence of action potential-driven transmission. This suggests the E/I imbalance is due to changes at the synapse level, as opposed to the network level. Next, we use paired whole-cell recordings in an attempt to identify specific interneuron subtypes affected by the Nlgn3R451C mutation. Curiously, we observe no change in the amplitude of cell-to-cell, unitary IPSCs (uIPSCs) from parvalbumin-positive (PV) or somatostatin-positive (SOM) interneurons onto pyramidal neurons. We also observe no change in the number or density of PV and SOM interneurons in LII/III of somatosensory cortex. This effectively rules out a role for these particular interneurons in the increased inhibitory synaptic transmission, pointing to perhaps alternative interneuron subtypes. Lastly, impaired endocannabinoid signaling has been implicated in hippocampal synaptic dysfunction in Nlgn3R451C mice, but has not been investigated at cortical synapses. We find that bath application of the CB1 antagonist, AM 251 in WT mice eliminates the Nlgn3R451C increase in eIPSC amplitude and mIPSC frequency, indicating that increased inhibitory transmission in mutant mice is due, at least in part, to a loss of endocannabinoid signaling through CB1 receptors likely acting at interneurons other than PV or SOM.
Highlights
Mutations in the genes encoding the Neuroligin family of trans-synaptic cell-adhesion proteins [1,2,3] and their binding partners the Neurexins [4] are associated with autism
We measured spontaneous miniature inhibitory currents from LII/III pyramidal neurons in the presence of the sodium channel inhibitor TTX (1 μM; Fig 1A and 1B). mIPSC frequency was increased in Nlgn3R451C mice compared to WT mice (WT: 1.72 ± 0.13, Nlgn3R451C: 2.29 ± 0.22; t (69) = 2.29, P = 0.025), but no difference in mIPSC amplitude was observed between genotypes (t (69) = 0.84, P = 0.406)
Using whole-cell patch clamp electrophysiology and extracellular stimulation in LII/III we found that the relationship of evoked inhibitory postsynaptic currents (eIPSCs) amplitude to extracellular stimulus intensity (0–1 mA) is greater in pyramidal neurons from Nlgn3R451C mice compared to WT controls
Summary
Mutations in the genes encoding the Neuroligin family of trans-synaptic cell-adhesion proteins [1,2,3] and their binding partners the Neurexins [4] are associated with autism. The frequency of spontaneous inhibitory transmission onto LII/III pyramidal neurons was increased, and staining for the inhibitory marker VGAT demonstrated an increased number of putative inhibitory synaptic puncta in Nlgn3R451C mice [5]. Taken together, these findings suggested an increase in inhibitory synapse number as a possible explanation for the increased cortical inhibitory synaptic transmission. Because the frequency of spontaneous events was not measured in the presence of the Na+ channel blocker, TTX, the relative contributions of spontaneous network activity and spontaneous GABA release could not be determined, leading to difficulty in interpretation
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