Trends in Neurosciences
Neuroligins and neurexins: linking cell adhesion, synapse formation and cognitive function
Introduction
Brain development and function rely on proper formation, maintenance and modification of connections between neurons. During development, axons are guided towards target dendrites by attractive and repulsive cues. Stable contacts between axons and dendrites result in the formation of functional synapses, which are highly elaborate asymmetric sites of neuron–neuron contact. The presynaptic side of the junction includes: (i) the active zone where neurotransmitter release occurs; (ii) a network of scaffolding proteins known as the cytomatrix of active zones; and (iii) a cluster of neurotransmitter-containing synaptic vesicles. Postsynaptic components include: (i) an accumulation of neurotransmitter receptors directly opposed to the active zone; and (ii) scaffolding proteins. At excitatory synapses these scaffolding proteins are known as the postsynaptic density (PSD); a distinct set of scaffolding proteins are present at inhibitory synapses. For synapses to function, all these components must be recruited and precisely aligned across the synaptic cleft, a 20-nm-wide extracellular space that separates two neurons at synaptic junctions. In view of this architecture, an appealing model of synapse assembly and maintenance invokes heterophilic presynaptic and postsynaptic transmembrane proteins that bind each other in the extracellular space and recruit additional proteins via their intracellular domains. Recently, several molecules, including synaptic cell-adhesion molecule (SynCAM), N-cadherin, neural cell-adhesion molecule (NCAM), Eph receptor tyrosine kinases, and neuroligins and neurexins, have been implicated in synapse formation and maintenance (Craig et al., in this issue). Here, we focus on the roles of two families of heterophilic adhesion molecules, neuroligins and neurexins, in trans-synaptic signaling. In particular, we address three emerging trends: (i) the potential of these two proteins to induce synapse assembly in cultured neurons; (ii) the modes of action of these proteins; and (iii) the implications of the effects of these proteins in the pathogenesis of cognitive disorders.
Section snippets
Neurexins and neuroligins
Neurexins were discovered as a result of their ability to bind α-latrotoxin, a component of black widow spider venom, which triggers massive neurotransmitter release [1]. Neurexins were hypothesized to act as cell-recognition molecules based on their structure: a single transmembrane region and an extracellular domain that is similar to laminin A, slit and agrin, proteins implicated in axon guidance and synaptogenesis 1, 2. There are three genes that encode α-neurexins and three genes that
Cell-adhesion and synapse-formation assays
Neuroligin-1 and β-neurexin, when expressed separately in heterologous cells, promote Ca2+-dependent adhesion between these cells [14]. Oligomerization of neuroligin molecules is necessary for this adhesion [13]. Remarkably, expression of neuroligins in fibroblast cells induces the formation of presynaptic terminals in axons that contact these cells 15, 16 and adhesion is required for this synaptogenic activity [13]. Such artificial hemi-synapses, composed of a fibroblast cell and neuronal
Function in neurons
Neuroligin-expressing fibroblasts can induce the formation of presynaptic terminals in contacting axons, and β-neurexin-expressing fibroblasts can induce the formation of postsynaptic sites in contacting dendrites, but can the two molecules form new synaptic sites at neuron–neuron contacts? Neuroligin overexpression in neurons has indeed been found, in several studies, to enhance the number of synapses (identified by immunostaining for presynaptic and postsynaptic proteins) (Table 1).
Presynaptic terminal assembly
The presynaptic signaling events induced by neurexins are currently unknown. Within minutes of contact between axons and dendrites, packets of active zone proteins arrive at a nascent synapse [23]. This is followed by arrival of synaptic vesicles, after which activity-dependent vesicle cycling occurs [24]. How might neurexins promote these events? One hint comes from the observation that actin can be polymerized on the cytoplasmic tail of β-neurexins via interactions with Ca2+
Neuroligins and autism
A change in the E/I-ratio has been suggested to correlate with autism and several other neurological disorders [55]. Mutations in the genes encoding neuroligin-3 and -4 on the X chromosome and the gene encoding neuroligin-4Y on the Y chromosome 8, 9 have been linked to cases of autism in several studies (Table 2), although multiple genes, in addition to neuroligins, are linked to autism. In addition, PSD95 has been implicated in a variety of disorders including autism [52]. These observations
Molecular mechanisms: future questions
The remarkable recent progress in the study of the neuroligin–neurexin interaction raises several interesting questions. For example, do different isoforms of neuroligins and neurexins confer specificity through different binding affinities or localizations? What is the sequence of events in the assembly of presynaptic and postsynaptic proteins downstream of neuroligin–neurexin and how does signaling mediate these events? How dynamic are proteins at synapses? Is there constant turnover,
Neuroligins and neurexins in the brain: what can we expect?
Perhaps the most interesting question is how neuroligins and neurexins function in the intact brain. Are neuroligins and neurexins active only during initial synapse formation or are they engaged during the modification of existing synapses in adulthood? Neuroligin-2 levels are subject to estrogen control in the brain, suggesting that regulation of expression levels has a role in neuroligin function in vivo [67]. Autistic individuals are predominantly impaired in language and social skills and
Acknowledgements
We are grateful to Meyer Jackson, Peter Scheiffele, Max Ulbrich and Xue Han for comments on the manuscript, and to J. Kirsch for support.
References (92)
Conserved domain structure of beta-neurexins. Unusual cleaved signal sequences in receptor-like neuronal cell-surface proteins
J. Biol. Chem.
(1994)A splice code for trans-synaptic cell adhesion mediated by binding of neuroligin 1 to α- and β-neurexins
Neuron
(2005)Neuroligin 1: a splice site-specific ligand for beta-neurexins
Cell
(1995)Structures, alternative splicing, and neurexin binding of multiple neuroligins
J. Biol. Chem.
(1996)The structure and expression of the human neuroligin-3 gene
Gene
(2000)Neuroligin 2 is exclusively localized to inhibitory synapses
Eur. J. Cell Biol.
(2004)- et al.
Binding properties of neuroligin 1 and neurexin 1beta reveal function as heterophilic cell adhesion molecules
J. Biol. Chem.
(1997) Neuroligin expressed in nonneuronal cells triggers presynaptic development in contacting axons
Cell
(2000)Neurexins induce differentiation of GABA and glutamate postsynaptic specializations via neuroligins
Cell
(2004)Dissection of synapse induction by neuroligins: effect of a neuroligin mutation associated with autism
J. Biol. Chem.
(2005)
Neuroligins mediate excitatory and inhibitory synapse formation: involvement of PSD-95 and neurexin-1beta in neuroligin induced synaptic specificity
J. Biol. Chem.
Assembling the presynaptic active zone: a characterization of an active one precursor vesicle
Neuron
CASK and protein 4.1 support F-actin nucleation on neurexins
J. Biol. Chem.
A tripartite protein complex with the potential to couple synaptic vesicle exocytosis to cell adhesion in brain
Cell
Assembly of new individual excitatory synapses: time course and temporal order of synaptic molecule recruitment
Neuron
Birth of a synapse: not such long labor
Neuron
Silent synapses during development of thalamocortical inputs
Neuron
Postsynaptically silent synapses in single neuron cultures
Neuron
Synaptic targeting of neuroligin is independent of neurexin and SAP90/PSD95 binding
Mol. Cell. Neurosci.
Surface trafficking of receptors between synaptic and extrasynaptic membranes: and yet they do move!
Trends Neurosci.
Dendritic spines shaped by synaptic activity
Curr. Opin. Neurobiol.
Maturation of glutamatergic and GABAergic synapse composition in hippocampal neurons
Neuropharmacology
The dynamics of SAP90/PSD-95 recruitment to new synaptic junctions
Mol. Cell. Neurosci.
Activity regulates the synaptic localization of the NMDA receptor in hippocampal neurons
Neuron
Synaptic scaffolding molecule is involved in the synaptic clustering of neuroligin
Mol. Cell. Neurosci.
Synaptogenesis: a balancing act between excitation and inhibition
Curr. Biol.
X-linked mental retardation and autism are associated with a mutation in the NLGN4 gene, a member of the neuroligin family
Am. J. Hum. Genet.
Excitatory–inhibitory balance and critical period plasticity in developing visual cortex
Prog. Brain Res.
Epilepsy, hyperalgesia, impaired memory, and loss of pre- and postsynaptic GABAB responses in mice lacking GABAB(1)
Neuron
Glutamic acid decarboxylase 65 and 67 kDa proteins are reduced in autistic parietal and cerebellar cortices
Biol. Psychiatry
Neurexins: three genes and 1001 products
Trends Genet.
The structure of the ligand-binding domain of neurexin Ibeta: regulation of LNS domain function by alternative splicing
Cell
Acetylcholinesterase inhibition by fasciculin: crystal structure of the complex
Cell
Characterization of the interaction of a recombinant soluble neuroligin-1 with neurexin-1beta
J. Biol. Chem.
Mints as adaptors. Direct binding to neurexins and recruitment of munc18
J. Biol. Chem.
Association of neuronal calcium channels with modular adaptor proteins
J. Biol. Chem.
The scaffolding protein CASK mediates the interaction between rabphilin3a and beta-neurexins
FEBS Lett.
The COOH terminus of synaptotagmin mediates interaction with the neurexins
J. Biol. Chem.
The complexity of PDZ domain-mediated interactions at glutamatergic synapses: a case study on neuroligin
Neuropharmacology
CIPP, a novel multivalent PDZ domain protein, selectively interacts with Kir4.0 family members, NMDA receptor subunits, neurexins, and neuroligins
Mol. Cell. Neurosci.
Neurexins: synaptic cell surface proteins related to the alpha-latrotoxin receptor and laminin
Science
Alpha-neurexins couple Ca2+ channels to synaptic vesicle exocytosis
Nature
Postsynaptic N-methyl-D-aspartate receptor function requires alpha-neurexins
Proc. Natl. Acad. Sci. U. S. A.
Identification of a novel neuroligin in humans which binds to PSD-95 and has a widespread expression
Biochem. J.
Mutations of the X-linked genes encoding neuroligins NLGN3 and NLGN4 are associated with autism
Nat. Genet.
Neuroligin 1 is a postsynaptic cell-adhesion molecule of excitatory synapses
Proc. Natl. Acad. Sci. U. S. A.
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