α-Neurexins Together with α2δ-1 Auxiliary Subunits Regulate Ca 2+ Influx through Ca v 2.1 Channels

• Published in: The Journal of neuroscience 2018-09, Vol.38 (38), p.8277-8294
• Main Authors: Brockhaus, Johannes, Schreitmüller, Miriam, Repetto, Daniele, Klatt, Oliver, Reissner, Carsten, Elmslie, Keith, Heine, Martin, Missler, Markus
• Format: Article
• Language: English
• Subjects: Animals; Axons - metabolism; Calcium - metabolism; Calcium Channels, N-Type - metabolism; Hippocampus - metabolism; Mice; Neural Cell Adhesion Molecules - genetics; Neural Cell Adhesion Molecules - metabolism; Neurons - metabolism; Presynaptic Terminals - metabolism; Synaptic Transmission - physiology
• ISSN: 0270-6474; 1529-2401
• DOI: 10.1523/JNEUROSCI.0511-18.2018

Action potential-evoked neurotransmitter release is impaired in knock-out neurons lacking synaptic cell-adhesion molecules α-neurexins (αNrxns), the extracellularly longer variants of the three vertebrate genes. Ca influx through presynaptic high-voltage gated calcium channels like the ubiquitous P/Q-type (Ca 2.1) triggers release of fusion-ready vesicles at many boutons. α2δ Auxiliary subunits regulate trafficking and kinetic properties of Ca 2.1 pore-forming subunits but it has remained unclear if this involves αNrxns. Using live cell imaging with Ca indicators, we report here that the total presynaptic Ca influx in primary hippocampal neurons of αNrxn triple knock-out mice of both sexes is reduced and involved lower Ca 2.1-mediated transients. This defect is accompanied by lower vesicle release, reduced synaptic abundance of Ca 2.1 pore-forming subunits, and elevated surface mobility of α2δ-1 on axons. Overexpression of Nrxn1α in αNrxn triple knock-out neurons is sufficient to restore normal presynaptic Ca influx and synaptic vesicle release. Moreover, coexpression of Nrxn1α together with α2δ-1 subunits facilitates Ca influx further but causes little augmentation together with a different subunit, α2δ-3, suggesting remarkable specificity. Expression of defined recombinant Ca 2.1 channels in heterologous cells validates and extends the findings from neurons. Whole-cell patch-clamp recordings show that Nrxn1α in combination with α2δ-1, but not with α2δ-3, facilitates Ca currents of recombinant Ca 2.1 without altering channel kinetics. These results suggest that presynaptic Nrxn1α acts as a positive regulator of Ca influx through Ca 2.1 channels containing α2δ-1 subunits. We propose that this regulation represents an important way for neurons to adjust synaptic strength. Synaptic transmission between neurons depends on the fusion of neurotransmitter-filled vesicles with the presynaptic membrane, which subsequently activates postsynaptic receptors. Influx of calcium ions into the presynaptic terminal is the key step to trigger vesicle release and involves different subtypes of voltage-gated calcium channels. We study the regulation of calcium channels by neurexins, a family of synaptic cell-adhesion molecules that are essential for many synapse properties. Using optical measurements of calcium influx in cultured neurons and electrophysiological recordings of calcium currents from recombinant channels, we show that a major neurexin variant facilitates calciu