Neuroligins and neurexins link synaptic function to cognitive disease

Abstract

The brain processes information by transmitting signals at synapses, which connect neurons into vast networks of communicating cells. In these networks, synapses not only transmit signals but also transform and refine them. Neurexins and neuroligins are synaptic cell-adhesion molecules that connect presynaptic and postsynaptic neurons at synapses, mediate signalling across the synapse, and shape the properties of neural networks by specifying synaptic functions. In humans, alterations in genes encoding neurexins or neuroligins have recently been implicated in autism and other cognitive diseases, linking synaptic cell adhesion to cognition and its disorders.

Figure 1. Architecture of the trans-synaptic neurexin/neuroligin complex

a. Cartoon of the structure of an excitatory synapse and the putative locations of Nrxns and Nlgns in the synapse. b. Schematic diagram of the Nrxn/Nlgn junction including selected pre- and postsynaptic binding proteins: CASK, Velis, and Mints on the presynaptic side, and PSD-95 (which binds to AMPA-type glutamate receptors via its first PDZ domain, and to Nlgns via its third PDZ domain64), GKAP, and Shanks on the postsynaptic side. Note that Nrxns and CASK could be, at least in part, also postsynaptic, and that Shank may also be presynaptic (Abbreviations used: C and N = C- and N-termini; CHO = carbohydrate-attachment sequence; CaM Kinase = CaM kinase domain of CASK; E = EGF-like domain; GUK = guanylate-kinase domain; L = LNS-domain; P = PDZ-domain; S = SH3 domain). c. Alternative splicing of Nrxns and Nlgns. α-Nrxns contain five canonical splice sites (#1 to #5), and β-Nrxns two (#4 and #5). Splice site #1 is C-terminal to the first EGF-like domain, #2, #3, and #4 are at similar positions in the second, fourth and sixth LNS-domain, respectively, and #5 is between the glycosylated CHO-sequence and the transmembrane region. Most alternative splicing involves insertions of small evolutionarily conserved sequences except for splice site #5 which in Nrxn2 involves a large insert (191 residues), and in Nrxn3 involves a at least 16 variants, some of which include stop codons and thus produce secreted Nrxn3 isoforms. Nlgns contain only two sites of alternative splicing, of which site #B is only present in Nlgn1.

Figure 2. Atomic model of the trans-synaptic complex formed by neurexin-1β and Neuroligin-1

The Nrxn1β/Nlgn1 complex is shown in two orientations: Left, en face with the Nrxn1β LNS-domain on top of the Nlgn1 esterase-domain to illustrate the Nlgn1 dimer; right, in a 90° rotation to illustrate the sideways attachment of the Nrxn1β LNS-domains onto the Nlgn1 esterase-domains (structures are from the following PDB entries: 3BIW Nrxn/Nlgn, 1JXO PSD95 SH3 GuK, 1BE9 PSD95 PDZ, 3C0I CASK CAMK, 1KGD CASK GuK, 1KWA CASK PDZ; abbreviations as in Fig. 1; courtesy of D. Arac and A. Brunger, Stanford U.). The two orientations of the Nrxn1β/Nlgn1 complex illustrates the spatial arrangement and relative sizes of the Nrxn LNS-domains and the Nlgn esterase domain in a synaptic cleft. Other Nrxn and Nlgn isoforms for which no full structure is available, including α-Nrxns, would presumably have a similar arrangement except that the additional LNS-domain in α-Nrxns would occupy a larger space in the synaptic cleft.

Figure 3. Differential effects of neuroligin-1 and -2 deletions on inhibitory synapses in the somatosensory cortex

Connections of parvalbumin-positive fast-spiking interneurons (blue) and of somatostatin-positive interneurons (green) with exctitatory pyramidal neurons (pink) are shown schematically. The connectivity (as measured in paired recordings as % success) and amplitude (pA) of the inhibitory synapses of the interneurons onto the pyramidal neuron are shown for wild-type (WT), Nlgn1 KO, Nlgn2 KO, and Nlgn1/Nlgn2 double KO (DKO) mice (* = statistically significantly different from WT; modified from J. Gibson, K. Huber, and T.C. Südhof, unpublished).

Figure 4. The R451C substitution in Nlgn3 impairs Nlgn3 synthesis but enhances inhibitory synapses

a. Schematic illustration of the effect of the R451C mutation on Nlgn3 synthesis. The mutation does not alter Nlgn3 mRNA levels (1), but decreases the export of Nlgn3 from the endoplasmic reticulum (2). As a result, the concentration of R451C-mutant Nlgn3 that is exported from the Golgi complex (3) and inserted into synapses (4) is <10% of the wild-type Nlgn3 concentration. b. Despite decreasing the Nlgn3 concentration, the R451C mutation produces a synaptic gain-of-function effect in inhibitory synapses in the somatosensory cortex. The figure illustrates in two examples increased inhibitory synaptic activity in R451C mutant mice: by measurements of spontaneous ‘miniature’ synaptic events (left), or by measurements of evoked synaptic responses (right). Each example depicts representative electrophysiology traces on the left, and summary graphs on the right (modified from ref. ; * = statistically significantly different from WT). Note that Nlgn3-deficient synapses from KO mice do not exhibit this phenotype.

Figure 5

a, electron micrograph of a synapse (courtesy of Dr. X. Liu, UT Southwestern) b, time course of synaptic transmission as measured electrophysiologically. The five sequential steps are indicated, as deduced from measurements in the Calyx of Held synapse (modified from ref. 90).