Neuroligin-3 Regulates Excitatory Synaptic Transmission and EPSP-Spike Coupling in the Dentate Gyrus In Vivo

Abstract

Neuroligin-3 (Nlgn3), a neuronal adhesion protein implicated in autism spectrum disorder (ASD), is expressed at excitatory and inhibitory postsynapses and hence may regulate neuronal excitation/inhibition balance. To test this hypothesis, we recorded field excitatory postsynaptic potentials (fEPSPs) in the dentate gyrus of Nlgn3 knockout (KO) and wild-type mice. Synaptic transmission evoked by perforant path stimulation was reduced in KO mice, but coupling of the fEPSP to the population spike was increased, suggesting a compensatory change in granule cell excitability. These findings closely resemble those in neuroligin-1 (Nlgn1) KO mice and could be partially explained by the reduction in Nlgn1 levels we observed in hippocampal synaptosomes from Nlgn3 KO mice. However, unlike Nlgn1, Nlgn3 is not necessary for long-term potentiation. We conclude that while Nlgn1 and Nlgn3 have distinct functions, both are required for intact synaptic transmission in the mouse dentate gyrus. Our results indicate that interactions between neuroligins may play an important role in regulating synaptic transmission and that ASD-related neuroligin mutations may also affect the synaptic availability of other neuroligins.

Phosphorylation Determines Whether Neuroligin-3 is at Excitatory or Inhibitory Synapses in Different Regions of the Brain

“…2E) and at excitatory synapses (Fig. 1B-C), NL3 KO caused excitatory synapse phenotypes in CA1 57,58 , dentate gyrus 59 , and stratum oriens 60 , consistent with our findings. Other studies focusing on subtype-specific phenotypes 11 and cell typespecific localization of NL3 in hippocampus 50,51 indicate more refined specificities, including at inhibitory CCK synapses, which require focused investigations into putative roles of phosphorylated NL3 at those subtypes.…”

Increased Network Inhibition in the Dentate Gyrus of Adult Neuroligin-4 Knock-Out Mice

“…Synaptic strength measured by the fEPSP slope is unchanged in Nlgn2 KO mice, but paired-pulse inhibition is strongly decreased and the population spike output is strongly increased, leading to an increase in network excitation (Jedlicka et al, 2011). Nlgn3 KO mice also exhibit a reduction in the fEPSP slope, but show a tendency toward increased, rather than decreased, paired-pulse inhibition; indicating that the unchanged population spike output is the result of a compensatory increase in the intrinsic excitability (Muellerleile et al, 2022). In the present work, we show that Nlgn4 KO mice exhibit no differences in excitatory synaptic transmission, a tendency toward decreased population spike output and an increase in paired-pulse inhibition (Fig.…”

“…Nlgn4 makes up only 3% of the total neuroligin protein in the adult mouse brain (Varoqueaux et al, 2006), so any differences in the Nlgn4 KO are expected to be weaker compared with other neuroligin KOs. Furthermore, it was previously shown that Nlgn1 protein expression was downregulated in hippocampal synaptosomes from Nlgn3 KO mice, which could help explain the prominent defects in excitatory synaptic transmission observed in these mice (Muellerleile et al, 2022). Since Nlgn4 does not form heterodimers with other neuroligins (Poulopoulos et al, 2012), its deletion is unlikely to directly affect the expression of other neuroligins and thus have a narrower impact compared with the deletion of neuroligins that form heterodimers.…”

“…Both GABAergic (Delattre et al, 2013;Hammer et al, 2015;Unichenko et al, 2018) and glycinergic (Hoon et al, 2011;Zhang et al, 2018) transmission have been shown to be continued and an E-I imbalance (Jedlicka et al, 2011). In Nlgn3 KO mice, reduced excitatory input and slightly increased inhibition is compensated by an increase in the intrinsic neuronal excitability of the granule cells, thereby preserving the E-I balance (Muellerleile et al, 2022). In Nlgn4 KO mice, excitatory inputs are unchanged, but network inhibition is increased, resulting in an E-I imbalance.…”

“…In conclusion, we provide evidence for an increased level of network inhibition in the dentate gyrus of adult Nlgn4 KO mice in vivo without any accompanying changes in the intrinsic excitability of the dentate granule cells, although whole-cell patch-clamp recordings in organotypic entorhino-hippocampal slice cultures revealed a trend toward decreased spike frequency adaptation and increased depolarization block. Intriguingly, the deletion of the fellow autism candidate gene Nlgn3 also leads to a slight increase in network inhibition in the dentate gyrus (Muellerleile et al, 2022), which suggests that increased inhibition might underlie some of the behavioral symptoms observed in Nlgn3 and Nlgn4 KO mice and in individuals with autismassociated neuroligin mutations.…”