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Dimerization of Postsynaptic Neuroligin Drives Synaptic Assembly Via Transsynaptic Clustering of Neurexin

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Dimerization of Postsynaptic Neuroligin Drives Synaptic Assembly Via Transsynaptic Clustering of Neurexin Dimerization of postsynaptic neuroligin drives synaptic assembly via transsynaptic clustering of neurexin Seth L. Shipmana,b and Roger A. Nicolla,1 aDepartment of Cellular and Molecular Pharmacology and Physiology and bNeuroscience Graduate Program, University of California, San Francisco, CA 94158 Contributed by Roger A. Nicoll, October 9, 2012 (sent for review May 5, 2012) The transsynaptic complex of neuroligin (NLGN) and neurexin forms Like the synapse, the neuroligin/neurexin complex is itself asym- a physical connection between pre- and postsynaptic neurons that metric. Neuroligin exists natively as a dimer, whereas neurexin, in an occurs early in the course of new synapse assembly. Both neuroligin unbound state, is monomeric (10). The complex, then, is an asym- and neurexin have, indeed, been proposed to exhibit active, in- metric tetramer consisting of a neuroligin dimer and two neurexin structive roles in the formation of synapses. However, the process by molecules—the neurexin molecules brought in close proximity to which these instructive roles play out during synaptogenesis is not each other via their interaction with the neuroligin dimer (10–12). well understood. Here, we examine one aspect of postsynaptic As stated above, in vitro evidence suggests that neurexin clustering neuroligin with regard to its synaptogenic properties: its basal state may be an early step in the differentiation of an axon segment into as a constitutive dimer. We show that dimerization is required for a presynaptic terminal (13). Such clustering could be achieved by the the synaptogenic properties of neuroligin and likely serves to induce monomeric-to-dimeric conversion of neurexin upon neuroligin presynaptic differentiation via a transsynaptic clustering of neu- binding. Importantly, this clustering would be entirely dependent on rexin. Further, we introduce chemically inducible, exogenous dimer- the presence of neuroligin in a dimerized state. ization domains to the neuroligin molecule, effectively bestowing We set out to define the functional requirement of neuroligin chemical control of neuroligin dimerization. This allows us to identify dimerization with an aim to specifically test the hypothesis that the the acute requirements of neuroligin dimerization by chemically clustering of neurexin via an interaction with dimerized neuroligin manipulating the monomeric-to-dimeric conversion of neuroligin. is required for presynaptic assembly. We find strong evidence to Based on the results of the inducible dimerization experiments, we support this hypothesis and propose a model whereby postsynaptic propose a model in which dimerized neuroligin induces the mechanical neuroligin drives presynaptic differentiation via the clustering of NEUROSCIENCE clustering of presynaptic molecules as part of a requisite step in the neurexin, while also being required for some, but not all, aspects of coordinated assembly of a chemical synapse. postsynaptic assembly. electrophysiology | hippocampus | development | Results synaptic adhesion molecule Functional Neuroligin Requires an Intact Dimerization Domain. To directly test the physiological requirement for neuroligin di- he synapse is among the most complex of cellular structures, merization, we constructed mutants of neuroligin that have been Tdense with highly organized protein interactions. Although previously shown to eliminate dimerization (13, 14). We tested our knowledge of this molecular matrix is expanding rapidly, the three separate mutants that each contain alanine substitutions at the precise dynamics that govern the formation of a synapse, with its interface of the neuroligin dimer (Fig. 1 A and B). These mutants matched asymmetric sides, are still not fully understood. How- have been shown to retain surface localization and neurexin binding ever, some particularly important interactions are beginning to and, indeed, have been shown in one case to result in increases in emerge. For instance, the transsynaptic complex of presynaptic synapse density when expressed in neurons. However, the mutants neurexin and postsynaptic neuroligin (NLGN) has been pro- also display deficits in function compared with wild-type neuroligin, posed to lie at the heart of an emerging synapse (1, 2). including the lack of aggregation of neurexin-expressing cells in Independent manipulations of either neuroligin or neurexin can a nonneuronal cell-adhesion assay and the lack of an increase in result in modifications of both pre- and postsynaptic assembly, synaptic cluster size when expressed in neurons (13, 14). suggesting an instructive transsynaptic role for the neuroligin/neu- The postsynaptic expression of wild-type neuroligin results in a rexin complex in synaptic formation. Most strikingly, in experiments profound enhancement of excitatory synaptic currents (15, 16). using a coculture system of neurons and nonneuronal cells, neu- Therefore, we evaluated the effect of expressing exogenous, di- roligin expressed in nonneuronal cells is able to induce the forma- merization-null mutants of neuroligin on this enhancement of tion of functional presynaptic terminals onto those cells from postsynaptic currents. To do so we expressed wild-type or mutated cocultured neurons (3), whereas neurexin expression in non- neuroligin using sparse biolistic transfection of organotypic hippo- neuronal cells supports the formation of postsynaptic special- campal slice cultures. This resulted in transfection of only one to a few hippocampal pyramidal cells per slice, allowing for the si- izations at the junctions of those nonneuronal cells and cocultured multaneous recording of whole-cell currents from a transfected neurons (4). Whether or not the neuroligin/neurexin complex is neuron and a neighboring control cell. Evoking action potentials in absolutely required for synapse formation is not clear, given that the Schaffer axon collaterals with an extracellular stimulating elec- dissociated hippocampal and cortical cultures from triple-knockout trode resulted in simultaneous excitatory synaptic currents in both mice lacking neuroligins 1, 2, and 3 display normal synapse density, the experimental and control cell, the relative magnitude of which although these mice do die at birth from respiratory failure as serves as a readout of the effect of the manipulation on synaptic a consequence of reduced synaptic transmission in the brainstem (5). Whereas this finding certainly suggests that the neuroligin family is not essential for synaptic formation, it remains possible Author contributions: S.L.S. and R.A.N. designed research; S.L.S. performed research; S.L.S. that compensation by another family of postsynaptic adhesion contributed new reagents/analytic tools; S.L.S. analyzed data; and S.L.S. wrote the paper. molecules such as leucine-rich repeat transmembrane (LRRTM) The authors declare no conflict of interest. – proteins (6 8) or the cerebellin/glutamate receptor delta complex 1To whom correspondence should be addressed. E-mail: [email protected]. (9) could be masking the effects of a germ-line knockout of the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. three major neuroligin subtypes. 1073/pnas.1217633109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1217633109 PNAS Early Edition | 1of6 Downloaded by guest on September 26, 2021 A C 3000 NLGN1 DM3 DM1 2000 1000 Percent Control of DM2 0 NLGN1 NLGN1DM1 NLGN1DM2 NLGN1DM3 B D 800 NLGN3 Neurexin Neuroligin DM3 DM1 600 400 Percent Control of 200 DM2 0 NLGN3 NLGN3DM1 NLGN3DM2 NLGN3DM3 Neuroligin Neurexin Fig. 1. Mutations of neuroligin affecting dimerization abolish the synaptogenic effects of postsynaptic expression. (A) Structure of the neuroligin 1/neurexin 1β extracellular domains viewed side-on, looking through the synaptic cleft. (Left) Schematic (neuroligin in blue, neurexin in purple) indicates the viewing angle. (Right) Two neuroligin molecules, shown in dark and light blue, form a dimer with each neuroligin molecule bound to neurexin, shown in purple. Calcium ions at the neuroligin/neurexin interface are shown in gray. Locations of the dimerization-inhibiting mutations are indicated in orange. Structure from Araç et al. (10). (B)AsinA, but viewed from the presynaptic side of the synapse, looking toward the postsynaptic side. (C) Postsynaptic expression of wild- type NLGN1 results in increased AMPAR-mediated EPSCs compared with control (P < 0.01, n = 11), whereas the expression of dimerization-null mutants do not (NLGN1DM1 P > 0.05, n = 15; NLGN1DM2 P > 0.05, n = 10; NLGN1DM3 P > 0.05, n = 8). Open circles represent individual pairs; closed circles indicate mean ± SEM. (D) Postsynaptic expression of wild-type NLGN3 also results in increased AMPAR-mediated EPSCs compared with control (P < 0.001, n = 12), whereas expression of dimerization-null mutants results in decreased AMPAR-mediated EPSCs (NLGN3DM1 P < 0.005, n = 9; NLGN3DM2 P < 0.05, n = 11; NLGN3DM3 P < 0.05, n = 9). As in C, open circles represent individual pairs; closed circles indicate mean ± SEM. Expression of wild-type NLGN1 and NLGN3, previously shown in Shipman et al. (25) are repeated here for clarity. Sample traces in C and D show individual paired recordings with control AMPAR-mediated currents in black and experimental in green (scale bar, 20 pA/20 ms.) strength (Fig. S1). We found that the introduction of dimerization- our findings. In the same study that found intracellular retention of null mutations into NLGN1 eliminated the neuroligin-induced en- the dimerization-null
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