Neuroligins Sculpt Cerebellar Purkinje-Cell Circuits by Differential Control of Distinct Classes of Synapses

Abstract

Neuroligins are postsynaptic cell-adhesion molecules that bind presynaptic neurexins and are genetically linked to autism. Neuroligins are proposed to organize synaptogenesis and/or synaptic transmission, but no systematic analysis of neuroligins in a defined circuit is available. Here, we show that conditional deletion of all neuroligins in cerebellar Purkinje cells caused loss of distal climbing-fiber synapses and weakened climbing-fiber but not parallel-fiber synapses, consistent with alternative use of neuroligins and cerebellins as neurexin ligands for the excitatory climbing-fiber vs. parallel-fiber synapses. Moreover, deletion of neuroligins increased the size of inhibitory basket/stellate-cell synapses but simultaneously severely impaired their function. Multiple neuroligin isoforms differentially contributed to climbing-fiber and basket/stellate-cell synapse functions, such that inhibitory synapse-specific neuroligin-2 was unexpectedly essential for maintaining normal climbing-fiber synapse numbers. Using systematic analyses of all neuroligins in a defined neural circuit, our data thus show that neuroligins differentially contribute to various Purkinje-cell synapses in the cerebellum in vivo.

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

“…Different from Nlgn1 deletions but commensurate with its localization to inhibitory synapses, Nlgn2 deletions impaired inhibitory but not excitatory synaptic responses (Chubykin et al, 2007; Gibson et al, 2009; Poulopoulos et al, 2009; Zhang et al, 2015; Liang et al, 2015). In cortex, Nlgn2 deletions selectively decreased synaptic transmission mediated by fast-spiking (presumably parvalbumin-positive) interneurons without affecting synaptic transmission mediated by somatostatin-positive interneurons (Gibson et al, 2009).…”

“…Constitutive Nlgn1/2/3 triple KO mice died at birth and exhibit no changes in synapse numbers or synapse ultrastructure, but displayed major impairments in synaptic transmission (Varoqueaux et al, 2006). Similarly, conditional triple KO of Nlgn1/2/3 in cerebellar Purkinje cells impaired both excitatory and inhibitory synaptic transmission without major decreases in synapse numbers (Zhang et al, 2015). However, only excitatory climbing-fiber synapses but not parallel-fiber synapses were affected, presumably because parallel-fiber synapses are maintained by cerebellin-neurexin complexes instead of neuroligin-neurexin complexes (Uemura et al, 2010).…”

“…Nlgn3 deletions had no effect on excitatory synaptic transmission in hippocampus (Etherton et al, 2011a), and caused a small selective decrease in climbing-fiber synaptic responses in cerebellum without altering parallel-fiber synaptic transmission or mGluR5-dependent LTD (Zhang et al, 2015). The most profound and informative effect of Nlgn3 deletions, however, was detected in inhibitory synapses in the hippocampus and striatum.…”

Molecular Dissection of Neuroligin 2 and Slitrk3 Reveals an Essential Framework for GABAergic Synapse Development

“…The synaptogenic properties of NL2 have been well established (Craig and Kang, 2007; Huang and Scheiffele, 2008; Krueger-Burg et al, 2017; Sudhof, 2008); however, its exact function in inhibitory synaptogenesis remains controversial (Chanda et al, 2017; Chih et al, 2005; Chubykin et al, 2007; Panzanelli et al, 2017; Poulopoulos et al, 2009; Varoqueaux et al, 2006; Zhang et al, 2015). Early investigations showed that NL2 is highly enriched at GABAergic synapses at both neuronal somatic and dendritic regions and in vitro co-culture assay has shown that NL2 is a potent synaptogenic CAM (Graf et al, 2004; Takahashi et al, 2012; Varoqueaux et al, 2004).…”

“…For example, shRNA-based knockdown of NL2 in mature neuronal cultures leads to a substantial loss of GABAergic synapses (Chih et al, 2005), while germline deletion of multiple NLs, including NL2, has no effect on inhibitory synapse density in multiple brain regions despite reducing inhibitory transmission (Chubykin et al, 2007; Varoqueaux et al, 2006). A recent elegant study using conditional KO for NL2 in cerebellar Purkinje cells failed to observe significant changes of GAD65, a presynaptic marker of inhibitory synapses (Zhang et al, 2015). Nonetheless, there was a strong reduction of inhibitory transmission (Zhang et al, 2015), indicating that NL2 is important for functional maturation, but not formation, of inhibitory synapses in Purkinje cells.…”