Conditional ablation of neuroligin-1 in CA1 pyramidal neurons blocks LTP by a cell-autonomous NMDA receptor-independent mechanism

Abstract

Neuroligins are postsynaptic cell-adhesion molecules implicated in autism and other neuropsychiatric disorders. Despite extensive work, the role of neuroligins in synapse function and plasticity, especially NMDA receptor (NMDAR)-dependent LTP, remains unclear. To establish which synaptic functions unequivocally require neuroligins, we analyzed single and triple conditional knockout (cKO) mice for all three major neuroligin isoforms (NL1-NL3). We inactivated neuroligins by stereotactic viral expression of Cre-recombinase in hippocampal CA1 region pyramidal neurons at postnatal day 0 (P0) or day 21 (P21), and measured synaptic function, synaptic plasticity, and spine numbers in acute hippocampal slices 2–3 weeks later. Surprisingly, we find that ablation of neuroligins in newborn or juvenile mice only modestly impaired basal synaptic function in hippocampus, and caused no alteration in postsynaptic spine numbers. However, triple cKO of NL1-NL3 or single cKO of NL1 impaired NMDAR-mediated excitatory postsynaptic currents (NMDAR EPSCs), and abolished NMDAR-dependent LTP. Strikingly, the NL1 cKO also abolished LTP elicited by activation of L-type Ca2+-channels during blockade of NMDARs. These findings demonstrate that neuroligins are generally not essential for synapse formation in CA1 pyramidal neurons but shape synaptic properties and that NL1 specifically is required for LTP induced by postsynaptic Ca2+-elevations, a function which may contribute to the pathophysiological role of neuroligins in brain disorders.

γ-Protocadherins Interact with Neuroligin-1 and Negatively Regulate Dendritic Spine Morphogenesis

“…Cortical spines and synapses could not be examined in vivo , however, because of the perinatal lethal phenotype due to disrupted synapse maturation and transmission (Varoqueaux et al, 2006). Subsequent conditional ablation of Nlg1, 2, and 3 in postnatal CA1 pyramidal hippocampal neurons disrupted long-term potentiation without having any effect on spine density, indicating that Nlg1 is indeed not required for synapse and spine formation (Jiang et al, 2016). However, it has also been shown that intercellular variation of Nlg1 protein levels across cortical neurons altered synaptogenesis and spine density (Kwon et al, 2012).…”

“…To explain this observation, we asked whether the γ-Pcdhs might inhibit the postsynaptic cell adhesion molecule neuroligin-1 (Nlg1), which binds to presynaptic neurexins and has been shown to promote synaptic differentiation and dendritic spine density in vitro (Boucard et al, 2005; Chih et al, 2005; Dean et al, 2003; Scheiffele et al, 2000), and to be critical for mature synapse function in vivo (Jiang et al, 2016; Varoqueaux et al, 2006). Using a variety of biochemical and neuronal assays, we show that multiple γ-Pcdh isoforms physically interact with Nlg1 via their extracellular domains.…”

Synaptic Neurexin Complexes: A Molecular Code for the Logic of Neural Circuits

“…In acute hippocampal slices, sparse conditional deletion of Nlgn1 only decreased NMDAR- but not AMPAR-mediated synaptic responses, as monitored at Schaffer-collateral synapses (Jiang et al, 2017). In addition, the Nlgn1 deletion completely blocked postsynaptic, NMDAR-dependent LTP.…”

Optogenetic control of excitatory post-synaptic differentiation through neuroligin-1 tyrosine phosphorylation

“…Several studies relying on the expression of Nlg mutants have revealed the potential for Nlg1 to recruit both NMDA receptors (NMDARs) and AMPA receptors (AMPARs) at synapses through extracellular and intracellular interactions, respectively (Budreck et al, 2013;Giannone et al, 2013;Haas et al, 2018;Heine et al, 2008;Letellier et al, 2018;Mondin et al, 2011;Shipman and Nicoll, 2012). However, constitutive or conditional Nlg1/2/3 KO selectively affect basal NMDAR-mediated EPSCs and not AMPARs-EPSCs, and rescue experiments with truncated Nlg1 mutants suggest that the synaptic recruitment of NMDARs requires the intracellular rather than the extracellular domain of Nlg1 (Chanda et al, 2017;Chubykin et al, 2007;Jiang et al, 2017;Wu et al, 2019). Finally, while it is generally accepted that NMDAR-dependent long-term potentiation (LTP) is impaired by Nlg1 KD or KO, the issues of which Nlg1 motifs are important in this process and whether the Nlg1-NMDAR interaction is required, are unclear (Jiang et al, 2017;Kim et al, 2008;Letellier et al, 2018;Shipman and Nicoll, 2012;Wu et al, 2019).…”

“…However, constitutive or conditional Nlg1/2/3 KO selectively affect basal NMDAR-mediated EPSCs and not AMPARs-EPSCs, and rescue experiments with truncated Nlg1 mutants suggest that the synaptic recruitment of NMDARs requires the intracellular rather than the extracellular domain of Nlg1 (Chanda et al, 2017;Chubykin et al, 2007;Jiang et al, 2017;Wu et al, 2019). Finally, while it is generally accepted that NMDAR-dependent long-term potentiation (LTP) is impaired by Nlg1 KD or KO, the issues of which Nlg1 motifs are important in this process and whether the Nlg1-NMDAR interaction is required, are unclear (Jiang et al, 2017;Kim et al, 2008;Letellier et al, 2018;Shipman and Nicoll, 2012;Wu et al, 2019).…”