Postsynaptic neuroligin enhances presynaptic inputs at neuronal nicotinic synapses

Dendritic spine membrane proteome and its alterations in autistic spectrum disorder

Dendritic spines are small protrusions stemming from the dendritic shaft that constitute the primary specialization for receiving and processing excitatory neurotransmission in brain synapses. The disruption of dendritic spine function in several neurological and neuropsychiatric diseases leads to severe information-processing deficits with impairments in neuronal connectivity and plasticity. Spine dysregulation is usually accompanied by morphological alterations to spine shape, size and/or number that may occur at early pathophysiological stages and not necessarily be reflected in clinical manifestations. Autism spectrum disorder (ASD) is one such group of diseases involving changes in neuronal connectivity and abnormal morphology of dendritic spines on postsynaptic neurons. These alterations at the subcellular level correlate with molecular changes in the spine proteome, with alterations in the copy number, topography, or in severe cases in the phenotype of the molecular components, predominantly of those proteins involved in spine recognition and adhesion, reflected in abnormally short lifetimes of the synapse and compensatory increases in synaptic connections. Since cholinergic neurotransmission participates in the regulation of cognitive function (attention, memory, learning processes, cognitive flexibility, social interactions) brain acetylcholine receptors are likely to play an important role in the dysfunctional synapses in ASD, either directly or indirectly via the modulatory functions exerted on other neurotransmitter receptor proteins and spine-resident proteins.

From Hyposociability to Hypersociability-The Effects of PSD-95 Deficiency on the Dysfunctional Development of Social Behavior

Abnormal social behavior, including both hypo- and hypersociability, is often observed in neurodevelopmental disorders such as autism spectrum disorders. However, the mechanisms associated with these two distinct social behavior abnormalities remain unknown. Postsynaptic density protein-95 (PSD-95) is a highly abundant scaffolding protein in the excitatory synapses and an essential regulator of synaptic maturation by binding to NMDA and AMPA receptors. The DLG4 gene encodes PSD-95, and it is a risk gene for hypersocial behavior. Interestingly, PSD-95 knockout mice exhibit hyposociability during adolescence but hypersociability in adulthood. The adolescent hyposociability is accompanied with an NMDAR hyperfunction in the medial prefrontal cortex (mPFC), an essential part of the social brain for control of sociability. The maturation of mPFC development is delayed until young adults. However, how PSD-95 deficiency affects the functional maturation of mPFC and its connection with other social brain regions remains uncharacterized. It is especially unknown how PSD-95 knockout drives the switch of social behavior from hypo- to hyper-sociability during adolescent-to-adult development. We propose an NMDAR-dependent developmental switch of hypo- to hyper-sociability. PSD-95 deficiency disrupts NMDAR-mediated synaptic connectivity of mPFC and social brain during development in an age- and pathway-specific manner. By utilizing the PSD-95 deficiency mouse, the mechanisms contributing to both hypo- and hyper-sociability can be studied in the same model. This will allow us to assess both local and long-range connectivity of mPFC and examine how they are involved in the distinct impairments in social behavior and how changes in these connections may mature over time.

Scale-Free Dynamics of the Mouse Wakefulness and Sleep Electroencephalogram Quantified Using Wavelet-Leaders

Scale-free analysis of brain activity reveals a complexity of synchronous neuronal firing which is different from that assessed using classic rhythmic quantifications such as spectral analysis of the electroencephalogram (EEG). In humans, scale-free activity of the EEG depends on the behavioral state and reflects cognitive processes. We aimed to verify if fractal patterns of the mouse EEG also show variations with behavioral states and topography, and to identify molecular determinants of brain scale-free activity using the ‘multifractal formalism’ (Wavelet-Leaders). We found that scale-free activity was more anti-persistent (i.e., more different between time scales) during wakefulness, less anti-persistent (i.e., less different between time scales) during non-rapid eye movement sleep, and generally intermediate during rapid eye movement sleep. The scale-invariance of the frontal/motor cerebral cortex was generally more anti-persistent than that of the posterior cortex, and scale-invariance during wakefulness was strongly modulated by time of day and the absence of the synaptic protein Neuroligin-1. Our results expose that the complexity of the scale-free pattern of organized neuronal firing depends on behavioral state in mice, and that patterns expressed during wakefulness are modulated by one synaptic component.

Neuroligin 1 regulates spines and synaptic plasticity via LIMK1/cofilin-mediated actin reorganization

The C-terminal domain of NLG1 is sufficient to enhance spine and synapse number and to modulate synaptic plasticity, and it exerts these effects via its interaction with SPAR and the subsequent activation of LIMK1/cofilin-mediated actin reorganization.

Neuroligin (NLG) 1 is important for synapse development and function, but the underlying mechanisms remain unclear. It is known that at least some aspects of NLG1 function are independent of the presynaptic neurexin, suggesting that the C-terminal domain (CTD) of NLG1 may be sufficient for synaptic regulation. In addition, NLG1 is subjected to activity-dependent proteolytic cleavage, generating a cytosolic CTD fragment, but the significance of this process remains unknown. In this study, we show that the CTD of NLG1 is sufficient to (a) enhance spine and synapse number, (b) modulate synaptic plasticity, and (c) exert these effects via its interaction with spine-associated Rap guanosine triphosphatase–activating protein and subsequent activation of LIM-domain protein kinase 1/cofilin–mediated actin reorganization. Our results provide a novel postsynaptic mechanism by which NLG1 regulates synapse development and function.

The α4β2 nicotinic acetylcholine receptor modulates autism-like behavioral and motor abnormalities in pentylenetetrazol-kindled mice

Epilepsy is associated with several psychiatric disorders, including cognitive impairment, autism and attention deficit/hyperactivity disorder (ADHD). However, the psychopathology of epilepsy is frequently unrecognized and untreated in patients. In the present study, we investigated the effects of ABT-418, a neuronal nicotinic acetylcholine receptor agonist, on pentylenetetrazol (PTZ)-kindled mice with behavioral and motor abnormalities. PTZ-kindled mice displayed impaired motor coordination (in the rotarod test), anxiety (in the elevated plus maze test) and social approach impairment (in the three-chamber social test) compared with control mice. ABT-418 treatment (0.05 mg/kg, intraperitoneally) alleviated these behavioral abnormalities in PTZ-kindled mice. Immunolabeling of tissue sections demonstrated that expression of the α4 nicotinic acetylcholine receptor subunit in the medial habenula was similar in control and PTZ-kindled mice. However, expression was significantly decreased in the piriform cortex in PTZ-kindled mice. In addition, we examined the expression of the synaptic adhesion molecule neuroligin 3 (NLG3). NLG3 expression in the piriform cortex was significantly higher in PTZ-kindled mice compared with control mice. Collectively, our findings suggest that ADHD-like or autistic-like behavioral abnormalities associated with epilepsy are closely related to the downregulation of the α4 nicotinic receptor and the upregulation of NLG3 in the piriform cortex. In summary, this study indicates that ABT-418 might have therapeutic potential for attentional impairment in epileptic patients with psychiatric disorders such as autism and ADHD.

Role of primary afferents in the developmental regulation of motor axon synapse numbers on Renshaw cells

Motor function in mammalian species depends on the maturation of spinal circuits formed by a large variety of interneurons that regulate motoneuron firing and motor output. Interneuron activity is in turn modulated by the organization of their synaptic inputs, but the principles governing the development of specific synaptic architectures unique to each premotor interneuron are unknown. For example, Renshaw cells receive, at least in the neonate, convergent inputs from sensory afferents (likely Ia) and motor axons, raising the question of whether they interact during Renshaw cell development. In other well-studied neurons, such as Purkinje cells, heterosynaptic competition between inputs from different sources shapes synaptic organization. To examine the possibility that sensory afferents modulate synaptic maturation on developing Renshaw cells, we used three animal models in which afferent inputs in the ventral horn are dramatically reduced (ER81(-/-) knockout), weakened (Egr3(-/-) knockout), or strengthened (mlcNT3(+/-) transgenic). We demonstrate that increasing the strength of sensory inputs on Renshaw cells prevents their deselection and reduces motor axon synaptic density, and, in contrast, absent or diminished sensory afferent inputs correlate with increased densities of motor axons synapses. No effects were observed on other glutamatergic inputs. We conclude that the early strength of Ia synapses influences their maintenance or weakening during later development and that heterosynaptic influences from sensory synapses during early development regulates the density and organization of motor inputs on mature Renshaw cells.

Neuroligin 2 is expressed in synapses established by cholinergic cells in the mouse brain

Neuroligin 2 is a postsynaptic protein that plays a critical role in the maturation and proper function of GABAergic synapses. Previous studies demonstrated that deletion of neuroligin 2 impaired GABAergic synaptic transmission, whereas its overexpression caused increased inhibition, which suggest that its presence strongly influences synaptic function. Interestingly, the overexpressing transgenic mouse line showed increased anxiety-like behavior and other behavioral phenotypes, not easily explained by an otherwise strengthened GABAergic transmission. This suggested that other, non-GABAergic synapses may also express neuroligin 2. Here, we tested the presence of neuroligin 2 at synapses established by cholinergic neurons in the mouse brain using serial electron microscopic sections double labeled for neuroligin 2 and choline acetyltransferase. We found that besides GABAergic synapses, neuroligin 2 is also present in the postsynaptic membrane of cholinergic synapses in all investigated brain areas (including dorsal hippocampus, somatosensory and medial prefrontal cortices, caudate putamen, basolateral amygdala, centrolateral thalamic nucleus, medial septum, vertical- and horizontal limbs of the diagonal band of Broca, substantia innominata and ventral pallidum). In the hippocampus, the density of neuroligin 2 labeling was similar in GABAergic and cholinergic synapses. Moreover, several cholinergic contact sites that were strongly labeled with neuroligin 2 did not resemble typical synapses, suggesting that cholinergic axons form more synaptic connections than it was recognized previously. We showed that cholinergic cells themselves also express neuroligin 2 in a subset of their input synapses. These data indicate that mutations in human neuroligin 2 gene and genetic manipulations of neuroligin 2 levels in rodents will potentially cause alterations in the cholinergic system as well, which may also have a profound effect on the functional properties of brain circuits and behavior.

Homodimerization and isoform-specific heterodimerization of neuroligins

Neuroligins are postsynaptic adhesion proteins involved in the establishment of functional synapses in the central nervous system. In rodents, four genes give rise to neuroligins that function at distinct synapses, with corresponding neurotransmitter and subtype specificities. In the present study, we examined the interactions between the different neuroligins by isolating endogenous oligomeric complexes using in situ cross-linking on primary neurons. Examining hippocampal, striatal, cerebellar and spinal cord cultures, we found that neuroligins form constitutive dimers, including homomers and, most notably, neuroligin 1/3 heteromers. Additionally, we found that neuroligin monomers are specifically retained in the secretory pathway through a cellular quality control mechanism that involves the neuroligin transmembrane domain, ensuring that dimerization occurs prior to cell surface trafficking. Lastly, we identified differences in the dimerization capacity of autism-associated neuroligin mutants, and found that neuroligin 3 R471C mutants can form heterodimers with neuroligin 1. The pervasive nature of neuroligin dimerization indicates that the unit of neuroligin function is the dimer, and raises intriguing possibilities of distinct heterodimer functions, and of interactions between native and mutant neuroligins contributing to disease phenotypes.

Glutamatergic synapse formation is promoted by α7-containing nicotinic acetylcholine receptors

Glutamate is the primary excitatory transmitter in adult brain, acting through synapses on dendritic spines and shafts. Early in development, however, when glutamatergic synapses are only beginning to form, nicotinic cholinergic excitation is already widespread; it is mediated by acetylcholine activating nicotinic acetylcholine receptors (nAChRs) that generate waves of activity across brain regions. A major class of nAChRs contributing at this time is a species containing α7 subunits (α7-nAChRs). These receptors are highly permeable to calcium, influence a variety of calcium-dependent events, and are diversely distributed throughout the developing CNS. Here we show that α7-nAChRs unexpectedly promote formation of glutamatergic synapses during development. The dependence on α7-nAChRs becomes clear when comparing wild-type (WT) mice with mice constitutively lacking the α7-nAChR gene. Ultrastructural analysis, immunostaining, and patch-clamp recording all reveal synaptic deficits when α7-nAChR input is absent. Similarly, nicotinic activation of α7-nAChRs in WT organotypic culture, as well as cell culture, increases the number of glutamatergic synapses. RNA interference demonstrates that the α7-nAChRs must be expressed in the neuron being innervated for normal innervation to occur. Moreover, the deficits persist throughout the developmental period of major de novo synapse formation and are still fully apparent in the adult. GABAergic synapses, in contrast, are undiminished in number under such conditions. As a result, mice lacking α7-nAChRs have an altered balance in the excitatory/inhibitory input they receive. This ratio represents a fundamental feature of neural networks and shows for the first time that endogenous nicotinic cholinergic signaling plays a key role in network construction.

Alternative splicing of neuroligin and its protein distribution in the outer plexiform layer of the chicken retina

Although synaptogenesis within the retina is obviously essential for vision, mechanisms responsible for the initiation and maintenance of retinal synapses are poorly understood. In addition to its scientific interest, understanding retinal synapse formation is becoming clinically relevant with ongoing efforts to develop transplantation-based approaches for the treatment of retinal degenerative disease. To extend our understanding, we have focused on the chick model system and have studied the neuroligin family of neuronal adhesion factors that has been shown to participate in synapse assembly in the brain. We identified chicken orthologs of neuroligins 1, -3, and -4, but could find no evidence of neuroligin 2. We investigated temporal and spatial patterns of mRNA and protein expression during development using standard polymerase chain reaction (RT-PCR), quantitative PCR (QPCR), laser-capture microdissection (LCM), and confocal microscopy. At the mRNA level, neuroligins were detected at the earliest period tested, embryonic day (ED)5, which precedes the period of inner retina synaptogenesis. Significant alternative splicing was observed through development. While neuroligin gene products were generally detected in the inner retina, low levels of neuroligin 1 mRNA were also detected in the photoreceptor layer. Neuroligin 3 and -4 transcripts, on the other hand, were only detected in the inner retina. At retinal synapses neuroligin 1 protein was detected in the inner plexiform layer, but its highest levels were detected in the outer plexiform layer on the tips of horizontal cell dendrites. This work lays the groundwork for future studies on the functional roles of the neuroligins within the retina.

Synaptic organizing complexes

A number of synaptogenic factors induce presynaptic or postsynaptic differentiation when presented to axons or dendrites. Many such factors participate in bidirectional trans-synaptic adhesion complexes. Axonal neurexins interacting in an isoform-specific code with multiple dendritic partners (neuroligins, LRRTMs, or Cbln-GluRδ), and axonal protein tyrosine phosphatase receptors interacting with dendritic NGL-3, nucleate local networks of high-affinity protein-protein interactions leading to aligned presynaptic and postsynaptic differentiation. Additional secreted target-derived factors such as fibroblast growth factors and glial-derived factors such as thrombospondin bind specific axonal or dendritic receptors stimulating signal transduction mechanisms to promote selective aspects of synapse development. Together with classical adhesion molecules and controlled by transcriptional cascades, these synaptogenic adhesion complexes and secreted factors organize the molecular composition and thus functional properties of central synapses.

The postsynaptic adenomatous polyposis coli (APC) multiprotein complex is required for localizing neuroligin and neurexin to neuronal nicotinic synapses in vivo

Synaptic efficacy requires that presynaptic and postsynaptic specializations align precisely and mature coordinately. The underlying mechanisms are poorly understood, however. We propose that adenomatous polyposis coli protein (APC) is a key coordinator of presynaptic and postsynaptic maturation. APC organizes a multiprotein complex that directs nicotinic acetylcholine receptor (nAChR) localization at postsynaptic sites in avian ciliary ganglion neurons in vivo. We hypothesize that the APC complex also provides retrograde signals that direct presynaptic active zones to develop in register with postsynaptic nAChR clusters. In our model, the APC complex provides retrograde signals via postsynaptic neuroligin that interacts extracellularly with presynaptic neurexin. S-SCAM (synaptic cell adhesion molecule) and PSD-93 (postsynaptic density-93) are scaffold proteins that bind to neuroligin. We identify S-SCAM as a novel component of neuronal nicotinic synapses. We show that S-SCAM, PSD-93, neuroligin and neurexin are enriched at alpha3*-nAChR synapses. PSD-93 and S-SCAM bind to APC and its binding partner beta-catenin, respectively. Blockade of selected APC and beta-catenin interactions, in vivo, leads to decreased postsynaptic accumulation of S-SCAM, but not PSD-93. Importantly, neuroligin synaptic clusters are also decreased. On the presynaptic side, there are decreases in neurexin and active zone proteins. Further, presynaptic terminals are less mature structurally and functionally. We define a novel neural role for APC by showing that the postsynaptic APC multiprotein complex is required for anchoring neuroligin and neurexin at neuronal synapses in vivo. APC human gene mutations correlate with autism spectrum disorders, providing strong support for the importance of the association, demonstrated here, between APC, neuroligin and neurexin.

Lateral mobility of nicotinic acetylcholine receptors on neurons is determined by receptor composition, local domain, and cell type

The lateral mobility of surface receptors can define the signaling properties of a synapse and rapidly change synaptic function. Here we use single-particle tracking with Quantum Dots to follow nicotinic acetylcholine receptors (nAChRs) on the surface of chick ciliary ganglion neurons in culture. We find that both heteropentameric alpha3-containing receptors (alpha3*-nAChRs) and homopentameric alpha7-containing receptors (alpha7-nAChRs) access synaptic domains by lateral diffusion. They have comparable mobilities and display Brownian motion in extrasynaptic space but are constrained and move more slowly in synaptic space. The two receptor types differ in the nature of their synaptic restraints. Disruption of lipid rafts, PDZ-containing scaffolds, and actin filaments each increase the mobility of alpha7-nAChRs in synaptic space while collapse of microtubules has no effect. The opposite is seen for alpha3*-nAChRs where synaptic mobility is increased only by microtubule collapse and not the other manipulations. Other differences are found for regulation of alpha3*-nAChR and alpha7-nAChR mobilities in extrasynaptic space. Most striking are effects on the immobile populations of alpha7-nAChRs and alpha3*-nAChRs. Disruption of either lipid rafts or PDZ scaffolds renders half of the immobile alpha3*-nAChRs mobile without changing the proportion of immobile alpha7-nAChRs. Similar results were obtained with chick sympathetic ganglion neurons, though regulation of receptor mobility differed in at least one respect from that seen with ciliary ganglion neurons. Control of nAChR lateral mobility, therefore, is determined by mechanisms that are domain specific, receptor subtype dependent, and cell-type constrained. The outcome is a system that could tailor nicotinic signaling capabilities to specific needs of individual locations.

Synchronous and asynchronous transmitter release at nicotinic synapses are differentially regulated by postsynaptic PSD-95 proteins

The rate and timing of information transfer at neuronal synapses are critical for determining synaptic efficacy and higher network function. Both synchronous and asynchronous neurotransmitter release shape the pattern of synaptic influences on a neuron. The PSD-95 family of postsynaptic scaffolding proteins, in addition to organizing postsynaptic components at glutamate synapses, acts transcellularly to regulate synchronous glutamate release. Here we show that PSD-95 family members at nicotinic synapses on chick ciliary ganglion neurons in culture execute multiple functions to enhance transmission. Together, endogenous PSD-95 and SAP102 in the postsynaptic cell appear to regulate transcellularly the synchronous release of transmitter from presynaptic terminals onto the neuron while stabilizing postsynaptic nicotinic receptor clusters under the release sites. Endogenous SAP97, in contrast, has no effect on receptor clusters but acts transcellularly from the postsynaptic cell through N-cadherin to enhance asynchronous release. These separate and parallel regulatory pathways allow postsynaptic scaffold proteins to dictate the pattern of cholinergic input a neuron receives; they also require balancing of PSD-95 protein levels to avoid disruptive competition that can occur through common binding domains.

PACAP/PAC1R signaling modulates acetylcholine release at neuronal nicotinic synapses

Neuropeptides collaborate with conventional neurotransmitters to regulate synaptic output. Pituitary adenylate cyclase-activating polypeptide (PACAP) co-localizes with acetylcholine in presynaptic nerve terminals, is released by stimulation, and enhances nicotinic acetylcholine receptor- (nAChR-) mediated responses. Such findings implicate PACAP in modulating nicotinic neurotransmission, but relevant synaptic mechanisms have not been explored. We show here that PACAP acts via selective high-affinity G-protein coupled receptors (PAC(1)Rs) to enhance transmission at nicotinic synapses on parasympathetic ciliary ganglion (CG) neurons by rapidly and persistently increasing the frequency and amplitude of spontaneous, impulse-dependent nicotinic excitatory postsynaptic currents (sEPSCs). Of the canonical adenylate cyclase (AC) and phospholipase-C (PLC) transduction cascades stimulated by PACAP/PAC(1)R signaling, only AC-generated signals are critical for synaptic modulation since the increases in sEPSC frequency and amplitude were mimicked by 8-Bromo-cAMP, blocked by inhibiting AC or cAMP-dependent protein kinase (PKA), and unaffected by inhibiting PLC. Despite its ability to increase agonist-induced nAChR currents, PACAP failed to influence nAChR-mediated impulse-independent miniature EPSC amplitudes (quantal size). Instead, evoked transmission assays reveal that PACAP/PAC(1)R signaling increased quantal content, indicating that it modulates synaptic function by increasing vesicular ACh release from presynaptic terminals. Lastly, signals generated by the retrograde messenger, nitric oxide- (NO-) are critical for the synaptic modulation since the PACAP-induced increases in spontaneous EPSC frequency, amplitude and quantal content were mimicked by NO donor and absent after inhibiting NO synthase (NOS). These results indicate that PACAP/PAC(1)R activation recruits AC-dependent signaling that stimulates NOS to increase NO production and control presynaptic transmitter output at neuronal nicotinic synapses.

Activity of nAChRs containing alpha9 subunits modulates synapse stabilization via bidirectional signaling programs

Although the synaptogenic program for cholinergic synapses of the neuromuscular junction is well known, little is known of the identity or dynamic expression patterns of proteins involved in non-neuromuscular nicotinic synapse development. We have previously demonstrated abnormal presynaptic terminal morphology following loss of nicotinic acetylcholine receptor (nAChR) alpha9 subunit expression in adult cochleae. However, the molecular mechanisms underlying these changes have remained obscure. To better understand synapse formation and the role of cholinergic activity in the synaptogenesis of the inner ear, we exploit the nAChR alpha9 subunit null mouse. In this mouse, functional acetylcholine (ACh) neurotransmission to the hair cells is completely silenced. Results demonstrate a premature, effusive innervation to the synaptic pole of the outer hair cells in alpha9 null mice coinciding with delayed expression of cell adhesion proteins during the period of effusive contact. Collapse of the ectopic innervation coincides with an age-related hyperexpression pattern in the null mice. In addition, we document changes in expression of presynaptic vesicle recycling/trafficking machinery in the alpha9 null mice that suggests a bidirectional information flow between the target of the neural innervation (the hair cells) and the presynaptic terminal that is modified by hair cell nAChR activity. Loss of nAChR activity may alter transcriptional activity, as CREB binding protein expression is decreased coincident with the increased expression of N-Cadherin in the adult alpha9 null mice. Finally, by using mice expressing the nondesensitizing alpha9 L9'T point mutant nAChR subunit, we show that increased nAChR activity drives synaptic hyperinnervation.

A neuroligin-4 missense mutation associated with autism impairs neuroligin-4 folding and endoplasmic reticulum export

Neuroligins (NLs) are postsynaptic cell-adhesion molecules essential for normal synapse function. Mutations in neuroligin-4 (NL4) (gene symbol: NLGN4) have been reported in some patients with autism spectrum disorder (ASD) and other neurodevelopmental impairments. However, the low frequency of NL4 mutations and the limited information about the affected patients and the functional consequences of their mutations cast doubt on the causal role of NL4 mutations in these disorders. Here, we describe two brothers with classical ASD who carry a single amino-acid substitution in NL4 (R87W). This substitution was absent from the brothers' asymptomatic parents, suggesting that it arose in the maternal germ line. R87 is conserved in all NL isoforms, and the R87W substitution is not observed in control individuals. At the protein level, the R87W substitution impaired glycosylation processing of NL4 expressed in HEK293 and COS cells, destabilized NL4, caused NL4 retention in the endoplasmic reticulum in non-neuronal cells and neurons, and blocked NL4 transport to the cell surface. As a result, the R87W substitution inactivated the synapse-formation activity of NL4 and abolished the functional effect of NL4 on synapse strength. Viewed together, these observations suggest that a point mutation in NL4 can cause ASD by a loss-of-function mechanism.

Neuroligin-1 performs neurexin-dependent and neurexin-independent functions in synapse validation

Postsynaptic neuroligins are thought to perform essential functions in synapse validation and synaptic transmission by binding to, and dimerizing, presynaptic alpha- and beta-neurexins. To test this hypothesis, we examined the functional effects of neuroligin-1 mutations that impair only alpha-neurexin binding, block both alpha- and beta-neurexin binding, or abolish neuroligin-1 dimerization. Abolishing alpha-neurexin binding abrogated neuroligin-induced generation of neuronal synapses onto transfected non-neuronal cells in the so-called artificial synapse-formation assay, even though beta-neurexin binding was retained. Thus, in this assay, neuroligin-1 induces apparent synapse formation by binding to presynaptic alpha-neurexins. In transfected neurons, however, neither alpha- nor beta-neurexin binding was essential for the ability of postsynaptic neuroligin-1 to dramatically increase synapse density, suggesting a neurexin-independent mechanism of synapse formation. Moreover, neuroligin-1 dimerization was not required for either the non-neuronal or the neuronal synapse-formation assay. Nevertheless, both alpha-neurexin binding and neuroligin-1 dimerization were essential for the increase in apparent synapse size that is induced by neuroligin-1 in transfected neurons. Thus, neuroligin-1 performs diverse synaptic functions by mechanisms that include as essential components of alpha-neurexin binding and neuroligin dimerization, but extend beyond these activities.

Postsynaptic Neuroligin1 regulates presynaptic maturation

Presynaptic nerve terminals pass through distinct stages of maturation after their initial assembly. Here we show that the postsynaptic cell adhesion molecule Neuroligin1 regulates key steps of presynaptic maturation. Presynaptic terminals from Neuroligin1-knockout mice remain structurally and functionally immature with respect to active zone stability and synaptic vesicle pool size, as analyzed in cultured hippocampal neurons. Conversely, overexpression of Neuroligin1 in immature neurons, that is within the first 5 days after plating, induced the formation of presynaptic boutons that had hallmarks of mature boutons. In particular, Neuroligin1 enhanced the size of the pool of recycling synaptic vesicles, the rate of synaptic vesicle exocytosis, the fraction of boutons responding to depolarization, as well as the responsiveness of the presynaptic release machinery to phorbol ester stimulation. Moreover, Neuroligin1 induced the formation of active zones that remained stable in the absence of F-actin, another hallmark of advanced maturation. Acquisition of F-actin independence of the active zone marker Bassoon during culture development or induced via overexpression of Neuroligin1 was activity-dependent. The extracellular domain of Neuroligin1 was sufficient to induce assembly of functional presynaptic terminals, while the intracellular domain was required for terminal maturation. These data show that induction of presynaptic terminal assembly and maturation involve mechanistically distinct actions of Neuroligins, and that Neuroligin1 is essential for presynaptic terminal maturation.

Minimal aberrant behavioral phenotypes of neuroligin-3 R451C knockin mice

Neuroligin-3 is a member of the class of cell adhesion proteins that mediate synapse development and have been implicated in autism. Mice with the human R451C mutation (NL3), identical to the point mutation found in two brothers with autism spectrum disorders, were generated and phenotyped in multiple behavioral assays with face validity to the diagnostic symptoms of autism. No differences between NL3 and their wildtype (WT) littermate controls were detected on measures of juvenile reciprocal social interaction, adult social approach, cognitive abilities, and resistance to change in a spatial habit, findings which were replicated in several cohorts of males and females. Physical and procedural abilities were similar across genotypes on measures of general health, sensory abilities, sensorimotor gating, motor functions, and anxiety-related traits. Minor developmental differences were detected between NL3 and WT, including slightly different rates of somatic growth, slower righting reflexes at postnatal days 2-6, faster homing reflexes in females, and less vocalizations on postnatal day 8 in males. Significant differences in NL3 adults included somewhat longer latencies to fall from the rotarod, less vertical activity in the open field, and less acoustic startle to high decibel tones. The humanized R451C mutation in mice did not result in apparent autism-like phenotypes, but produced detectable functional consequences that may be interpreted in terms of physical development and/or reduced sensitivity to stimuli.

Presynaptic targeting of alpha4beta 2 nicotinic acetylcholine receptors is regulated by neurexin-1beta

The mechanisms involved in the targeting of neuronal nicotinic acetylcholine receptors (AChRs), critical for their functional organization at neuronal synapses, are not well understood. We have identified a novel functional association between alpha4beta2 AChRs and the presynaptic cell adhesion molecule, neurexin-1beta. In non-neuronal tsA 201 cells, recombinant neurexin-1beta and mature alpha4beta2 AChRs form complexes. alpha4beta2 AChRs and neurexin-1beta also coimmunoprecipitate from rat brain lysates. When exogenous alpha4beta2 AChRs and neurexin-1beta are coexpressed in hippocampal neurons, they are robustly targeted to hemi-synapses formed between these neurons and cocultured tsA 201 cells expressing neuroligin-1, a postsynaptic binding partner of neurexin-1beta. The extent of synaptic targeting is significantly reduced in similar experiments using a mutant neurexin-1beta lacking the extracellular domain. Additionally, when alpha4beta2 AChRs, alpha7 AChRs, and neurexin-1beta are coexpressed in the same neuron, only the alpha4beta2 AChR colocalizes with neurexin-1beta at presynaptic terminals. Collectively, these data suggest that neurexin-1beta targets alpha4beta2 AChRs to presynaptic terminals, which mature by trans-synaptic interactions between neurexins and neuroligins. Interestingly, human neurexin-1 gene dysfunctions have been implicated in nicotine dependence and in autism spectrum disorders. Our results provide novel insights as to possible mechanisms by which dysfunctional neurexins, through downstream effects on alpha4beta2 AChRs, may contribute to the etiology of these neurological disorders.

Postsynaptic scaffolds for nicotinic receptors on neurons

Complex postsynaptic scaffolds determine the structure and signaling capabilities of glutamatergic synapses. Recent studies indicate that some of the same scaffold components contribute to the formation and function of nicotinic synapses on neurons. PDZ-containing proteins comprising the PSD-95 family co-localize with nicotinic acetylcholine receptors (nAChRs) and mediate downstream signaling in the neurons. The PDZ-proteins also promote functional nicotinic innervation of the neurons, as does the scaffold protein APC and transmembrane proteins such as neuroligin and the EphB2 receptor. In addition, specific chaperones have been shown to facilitate nAChR assembly and transport to the cell surface. This review summarizes recent results in these areas and raises questions for the future about the mechanism and synaptic role of nAChR trafficking.

An activity-dependent retrograde signal induces the expression of the high-affinity choline transporter in cholinergic neurons

A well-accepted view of developing circuits is that synapses must be active to mature and persist, whereas inactive synapses remain immature and are eventually eliminated. We question this long-standing view by investigating nonfunctional cholinergic nicotinic synapses in the superior cervical ganglia (SCG) of mice with a disruption in the alpha3 nicotinic receptor (nAChR) subunit gene, a gene essential for fast synaptic transmission in sympathetic ganglia. Using imaging and electrophysiology, we show that synapses persist for at least 2-3 months without postsynaptic activity; however, the presynaptic terminals lack high-affinity choline transporters (CHTs), and as a result, they are quickly depleted of transmitter. Moreover, we demonstrate with rescue experiments that CHT is induced by signals downstream of postsynaptic activity, converting immature terminals to mature terminals capable of sustaining transmitter release in response to high-frequency or continuous firing. Importantly, postsynaptic neurons must be continually active to maintain CHT in presynaptic terminals.

Pituitary adenylate cyclase-activating polypeptide (PACAP) alters parasympathetic neuron gene expression in a time-dependent fashion

Neuropeptides, including pituitary adenylate cyclase-activating polypeptide (PACAP), can influence diverse cellular processes over a broad temporal range. In ciliary ganglion (CG) neurons, for example, PACAP binding to high-affinity PAC1 receptors triggers transduction cascades that both rapidly modulate nicotinic receptors and synapses and support long-term survival. Since PACAP/PAC1 signaling recruits intracellular messengers and effectors that potently alter transcription, we examined its activation of the transcription factor CREB and then tested for changes in gene expression. PACAP/PAC1 signaling rapidly induced prolonged CREB activation in CG neurons by a phospholipase C -independent mechanism supported by Ca2+-influx, adenylate cyclase, and effectors, including protein kinase C (PKC) and possibly PKA. Since PACAP is abundant in the CG and released from depolarized presynaptic terminals, it is well suited to regulate gene expression relevant to neuronal and synaptic development. Gene array screens conducted using RNA from CG cultures grown with PACAP for 1/4, 24, or 96 h revealed a time-dependent pattern of > 600 regulated transcripts, including several encoding proteins implicated in synaptic function, neuronal survival, and development. The results underscore rapid, neuromodulatory, and long-term, neurotrophic consequences of PAC1 signaling in CG neurons and suggest that PACAP exerts such diverse influences by altering the expression of specific gene transcripts in a time-dependent fashion.

Multiple cell adhesion molecules shaping a complex nicotinic synapse on neurons

Neuroligin, SynCAM, and L1-CAM are cell adhesion molecules with synaptogenic roles in glutamatergic pathways. We show here that SynCAM is expressed in the chick ciliary ganglion, embedded in a nicotinic pathway, and, as shown previously for neuroligin and L1-CAM, acts transcellularly to promote synaptic maturation on the neurons in culture. Moreover, we show that electroporation of chick embryos with dominant negative constructs disrupting any of the three molecules in vivo reduces the total amount of presynaptic SV2 overlaying the neurons expressing the constructs. Only disruption of L1-CAM and neuroligin, however, reduces the number of SV2 puncta specifically overlaying nicotinic receptor clusters. Disrupting L1-CAM and neuroligin together produces no additional decrement, indicating that they act on the same subset of synapses. SynCAM may affect synaptic maturation rather than synapse formation. The results indicate that individual neurons can express multiple synaptogenic molecules with different effects on the same class of nicotinic synapses.

Capabilities of neurexins in the chick ciliary ganglion

Transcellular interactions between neuroligins (NL) and beta-neurexin have been widely documented to promote maturation and function of both glutamatergic and GABAergic synapses. Recently it has been shown that neuroligin-1 plays a similar role at nicotinic synapses on chick ciliary ganglion neurons in culture, acting from the postsynaptic side to enhance transmitter release from adjacent cholinergic terminals and boost nicotinic input to the cells. We show here that the ciliary ganglion expresses three forms of neuroligin as well as two beta-neurexins and an alpha-neurexin. Overexpression of the beta-neurexins, but not the alpha-neurexin, can induce clustering of endogenous PSD-95 in adjacent neurons, presumably engaging neuroligin in the postsynaptic cell. The trans effects of beta-neurexins are selective; though both alpha3- and alpha7-containing nicotinic receptors are available on opposing cells, beta-neurexins induce coclustering of alpha3- but not alpha7-containing nicotinic receptors. Overexpression of other putative synaptogenic molecules, including SynCAM and L1, are ineffective at trans-clustering of PSD-95 on adjacent neurons. The beta-neurexins also exert a cis effect, coclustering presynaptic markers along with beta-neurexin in neurites juxtaposed to postsynaptic proteins, consistent with organizing presynaptic components as well. Striated muscle, the synaptic target of ciliary neurons in vivo, also expresses neuroligin. The results demonstrate that NL and neurexins are present at multiple sites in nicotinic cholinergic pathways and suggest the possibility of both cis- and trans-interactions to influence nicotinic signaling.

EphB receptors co-distribute with a nicotinic receptor subtype and regulate nicotinic downstream signaling in neurons

Activation of nicotinic acetylcholine receptors (nAChRs) on neurons engages calcium-dependent signaling pathways regulating numerous events. Receptors containing alpha7 subunits (alpha7-nAChRs) are prominent in this because of their abundance and high relative calcium permeability. We show here that EphB2 receptors are co-localized with postsynaptic alpha7-nAChRs on chick ciliary ganglion neurons and that treatment of the cells with an ephrinB1 construct to activate the EphB receptors exerts physical restraints on both classes of receptors, diminishing their dispersal after spine retraction or lipid raft disruption. Moreover, the ephrinB1/EphB receptor complex specifically enhances the ability of alpha7-nAChRs to activate the transcription factor CREB, acting through a pathway including a receptor tyrosine kinase, a Src family member, PI3 kinase, and protein kinase A most distally. The enhancement does not appear to result from a change in the alpha7-nAChR current amplitude, suggesting a downstream target. The results demonstrate a role for ephrin/EphB action in nicotinic signaling.

Induction of GABAergic postsynaptic differentiation by alpha-neurexins

Beta-neurexin and neuroligin cell adhesion molecules contribute to synapse development in the brain. The longer alpha-neurexins function at both glutamate and gamma-aminobutyric acid (GABA) synapses in coupling to presynaptic calcium channels. Binding of alpha-neurexins to neuroligins was recently reported, but the role of the alpha-neurexins in synapse development has not been well studied. Here we report that in COS cell neuron coculture assays, all three alpha-neurexins induce clustering of the GABAergic postsynaptic scaffolding protein gephyrin and neuroligin 2 but not of the glutamatergic postsynaptic scaffolding protein PSD-95 or neuroligin 1/3/4. alpha-Neurexins also induce clustering of the GABA(A) receptor gamma2 subunit. This synapse promoting activity of alpha-neurexins is mediated by the sixth LNS (laminin neurexin sex hormone-binding protein) domain and negatively modulated by upstream sequences. Although inserts at splice site 4 (S4) in beta-neurexins promote greater clustering activity for GABA than glutamate proteins in coculture assay, alpha-neurexin-specific sequences confer complete specificity for GABA proteins. We further report a developmental increase in the ratio of -S4 to +S4 forms of neurexins correlating with an increase in glutamate relative to GABA synaptogenesis and activity regulation of splicing at S4. Thus, +S4 beta-neurexins and, even more selectively, alpha-neurexins may be mediators of GABAergic synaptic protein recruitment and stabilization.