N-cadherin and neuroligins cooperate to regulate synapse formation in hippocampal cultures.

Cadherins and neuroligins (NLs) represent two families of cell adhesion proteins that are essential for the establishment of synaptic connections in vitro; however, it remains unclear whether these proteins act in concert to regulate synapse density. Using a combination of overexpression and knockdown analyses in primary hippocampal neurons, we demonstrate that NL1 and N-cadherin promote the formation of glutamatergic synapses through a common functional pathway. Analysis of the spatial relationship between N-cadherin and NL1 indicates that in 14-day in vitro cultures, almost half of glutamatergic synapses are associated with both proteins, whereas only a subset of these synapses are associated with N-cadherin or NL1 alone. This suggests that NL1 and N-cadherin are spatially distributed in a manner that enables cooperation at synapses. In young cultures, N-cadherin clustering and its association with synaptic markers precede the clustering of NL1. Overexpression of N-cadherin at this time point enhances NL1 clustering and increases synapse density. Although N-cadherin is not sufficient to enhance NL1 clustering and synapse density in more mature cultures, knockdown of N-cadherin at later time points significantly attenuates the density of NL1 clusters and synapses. N-cadherin overexpression can partially rescue synapse loss in NL1 knockdown cells, possibly due to the ability of N-cadherin to recruit NL2 to glutamatergic synapses in these cells. We demonstrate that cadherins and NLs can act in concert to regulate synapse formation.

Synaptic localization of neuroligin 2 in the rodent retina: comparative study with the dystroglycan-containing complex.

Several recent studies have shown that neuroligin 2 (NL2), a component of the cell adhesion neurexins-neuroligins complex, is localized postsynaptically at hippocampal and other inhibitory synapses throughout the brain. Other studies have shown that components of the dystroglycan complex are also localized at a subset of inhibitory synapses and are coexpressed with NL2 in brain. These data prompted us to undertake a comparative study between the localization of NL2 and the dystroglycan complex in the rodent retina. First, we determined that NL2 mRNA is expressed both in the inner and in the outer nuclear layers. Second, we found that NL2 is localized both in the inner and in the outer synaptic plexiform layers. In the latter, the horseshoe-shaped pattern of NL2 and its extensive colocalization with RIM2, a component of the presynaptic active zone at ribbon synapses, argue that NL2 is localized presynaptically at photoreceptor terminals. Third, comparison of NL2 and the dystroglycan complex distribution patterns reveals that, despite their coexpression in the outer plexiform layer, they are spatially segregated within distinct domains of the photoreceptor terminals, where NL2 is selectively associated with the active zone and the dystroglycan complex is distally distributed in the lateral regions. Finally, we report that the dystroglycan deficiency in the mdx(3cv) mouse does not alter NL2 localization in the outer plexiform layer. These data show that the NL2- and dystroglycan-containing complexes are differentially localized in the presynaptic photoreceptor terminals and suggest that they may serve distinct functions in retina.

ErbB4-neuregulin signaling modulates synapse development and dendritic arborization through distinct mechanisms.

Perturbations in neuregulin-1 (NRG1)/ErbB4 function have been associated with schizophrenia. Affected patients exhibit altered levels of these proteins and display hypofunction of glutamatergic synapses as well as altered neuronal circuitry. However, the role of NRG1/ErbB4 in regulating synapse maturation and neuronal process formation has not been extensively examined. Here we demonstrate that ErbB4 is expressed in inhibitory interneurons at both excitatory and inhibitory postsynaptic sites. Overexpression of ErbB4 postsynaptically enhances size but not number of presynaptic inputs. Conversely, knockdown of ErbB4 using shRNA decreases the size of presynaptic inputs, demonstrating a specific role for endogenous ErbB4 in synapse maturation. Using ErbB4 mutant constructs, we demonstrate that ErbB4-mediated synapse maturation requires its extracellular domain, whereas its tyrosine kinase activity is dispensable for this process. We also demonstrate that depletion of ErbB4 decreases the number of primary neurites and that stimulation of ErbB4 using a soluble form of NRG1 results in exuberant dendritic arborization through activation of the tyrosine kinase domain of ErbB4 and the phosphoinositide 3-kinase pathway. These findings demonstrate that NRG1/ErbB4 signaling differentially regulates synapse maturation and dendritic morphology via two distinct mechanisms involving trans-synaptic signaling and tyrosine kinase activity, respectively.

Paralemmin-1, a modulator of filopodia induction is required for spine maturation.

Dendritic filopodia are thought to participate in neuronal contact formation and development of dendritic spines; however, molecules that regulate filopodia extension and their maturation to spines remain largely unknown. Here we identify paralemmin-1 as a regulator of filopodia induction and spine maturation. Paralemmin-1 localizes to dendritic membranes, and its ability to induce filopodia and recruit synaptic elements to contact sites requires protein acylation. Effects of paralemmin-1 on synapse maturation are modulated by alternative splicing that regulates spine formation and recruitment of AMPA-type glutamate receptors. Paralemmin-1 enrichment at the plasma membrane is subject to rapid changes in neuronal excitability, and this process controls neuronal activity-driven effects on protrusion expansion. Knockdown of paralemmin-1 in developing neurons reduces the number of filopodia and spines formed and diminishes the effects of Shank1b on the transformation of existing filopodia into spines. Our study identifies a key role for paralemmin-1 in spine maturation through modulation of filopodia induction.

A crystal-clear interaction: relating neuroligin/neurexin complex structure to function at the synapse.

Neuronal circuits are maintained by homeostatic mechanisms controlling synapse maturation and signaling. Neuroligins (NLs) and neurexins (Nrxs) may regulate the fine balance between excitation and inhibition. In this issue of Neuron, Araç et al. and Fabrichny et al. define crystal structures of NLs bound to beta-Nrx, providing insights into their synaptic actions and clarifying structural defects associated with autism-linked mutations.

Building excitatory and inhibitory synapses: balancing neuroligin partnerships.

Processing of neural information is thought to occur by integration of excitatory and inhibitory synaptic inputs. As such, precise control mechanisms must exist to maintain an appropriate balance between each synapse type. Recent findings indicate that neuroligins and their synaptic binding partners modulate the development of both excitatory and inhibitory synapses. Here we highlight these findings and discuss a mechanism potentially involved in controlling the balance between excitation and inhibition.

New players tip the scales in the balance between excitatory and inhibitory synapses.

Synaptogenesis is a highly controlled process, involving a vast array of players which include cell adhesion molecules, scaffolding and signaling proteins, neurotransmitter receptors and proteins associated with the synaptic vesicle machinery. These molecules cooperate in an intricate manner on both the pre- and postsynaptic sides to orchestrate the precise assembly of neuronal contacts. This is an amazing feat considering that a single neuron receives tens of thousands of synaptic inputs but virtually no mismatch between pre- and postsynaptic components occur in vivo. One crucial aspect of synapse formation is whether a nascent synapse will develop into an excitatory or inhibitory contact. The tight control of a balance between the types of synapses formed regulates the overall neuronal excitability, and is thus critical for normal brain function and plasticity. However, little is known about how this balance is achieved. This review discusses recent findings which provide clues to how neurons may control excitatory and inhibitory synapse formation, with focus on the involvement of the neuroligin family and PSD-95 in this process.

Neuroligins mediate excitatory and inhibitory synapse formation: involvement of PSD-95 and neurexin-1beta in neuroligin-induced synaptic specificity.

The balance between excitatory and inhibitory synapses is a tightly regulated process that requires differential recruitment of proteins that dictate the specificity of newly formed contacts. However, factors that control this process remain unidentified. Here we show that members of the neuroligin (NLG) family, including NLG1, NLG2, and NLG3, drive the formation of both excitatory and inhibitory presynaptic contacts. The enrichment of endogenous NLG1 at excitatory contacts and NLG2 at inhibitory synapses supports an important in vivo role for these proteins in the development of both types of contacts. Immunocytochemical and electrophysiological analysis showed that the effects on excitatory and inhibitory synapses can be blocked by treatment with a fusion protein containing the extracellular domain of neurexin-1beta. We also found that overexpression of PSD-95, a postsynaptic binding partner of NLGs, resulted in a shift in the distribution of NLG2 from inhibitory to excitatory synapses. These findings reveal a critical role for NLGs and their synaptic partners in controlling the number of inhibitory and excitatory synapses. Furthermore, relative levels of PSD-95 alter the ratio of excitatory to inhibitory synaptic contacts by sequestering members of the NLG family to excitatory synapses.

Modulation of dopamine mediated phosphorylation of AMPA receptors by PSD-95 and AKAP79/150.

Communication between dopaminergic and glutamatergic synapses is critical for several functions related to cognition and emotion. Here, we examined whether dopamine receptor activity regulates phosphorylation and trafficking of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit, GluR1. We find treatment with a dopamine D1 receptor agonist enhanced GluR1 phosphorylation at Ser845, the PKA phosphorylation site, in both striatal and prefrontal cortical neurons. Enhanced phosphorylation of GluR1 also correlated with increased amounts of GluR1 on the cell surface. These effects were disrupted by expression of mutant forms of the A-kinase anchoring protein (AKAP79/150) and the postsynaptic density protein, PSD-95, that fail to target synaptic sites. Similar enhancement of the phosphorylation of GluR1 was observed in the nucleus accumbens upon stimulation of dopamine release in vivo using electrical stimulation of dopamine cell bodies in the ventral tegmental area. These results suggest in vivo stimulation of dopamine release directly influences AMPA receptor phosphorylation and together with in vitro data indicate that coupling of the AMPA receptor to AKAP79/150 and PSD-95 modulate this process.

Global mapping of the yeast genetic interaction network.

A genetic interaction network containing approximately 1000 genes and approximately 4000 interactions was mapped by crossing mutations in 132 different query genes into a set of approximately 4700 viable gene yeast deletion mutants and scoring the double mutant progeny for fitness defects. Network connectivity was predictive of function because interactions often occurred among functionally related genes, and similar patterns of interactions tended to identify components of the same pathway. The genetic network exhibited dense local neighborhoods; therefore, the position of a gene on a partially mapped network is predictive of other genetic interactions. Because digenic interactions are common in yeast, similar networks may underlie the complex genetics associated with inherited phenotypes in other organisms.

Functional, comparative and cell biological analysis of Saccharomyces cerevisiae Kre5p.

Saccharomyces cerevisiae kre5delta mutants lack beta-1,6-glucan, a polymer required for proper cell wall assembly and architecture. A functional and cell biological analysis of Kre5p was conducted to further elucidate the role of this diverged protein glucosyltransferase-like protein in beta-1,6-glucan synthesis. Kre5p was found to be a primarily soluble N-glycoprotein of approximately 200 kDa, that localizes to the endoplasmic reticulum. The terminal phenotype of Kre5p-deficient cells was observed, and revealed a severe cell wall morphological defect. KRE6, encoding a glucanase-like protein, was identified as a multicopy suppressor of a temperature-sensitive kre5 allele, suggesting that these proteins may participate in a common beta-1,6-biosynthetic pathway. An analysis of truncated versions of Kre5p indicated that all major regions of the protein are required for viability. Finally, Candida albicans KRE5 was shown to partially restore growth to S. cerevisiae kre5delta cells, suggesting that these proteins are functionally related.

Saccharomyces cerevisiae Big1p, a putative endoplasmic reticulum membrane protein required for normal levels of cell wall beta-1,6-glucan.

Deletion of Saccharomyces cerevisiae BIG1 causes an approximately 95% reduction in cell wall beta-1,6-glucan, an essential polymer involved in the cell wall attachment of many surface mannoproteins. The big1 deletion mutant grows very slowly, but growth can be enhanced if cells are given osmotic support. We have begun a cell biological and genetic analysis of its product. We demonstrate, using a Big1p-GFP fusion construct, that Big1p is an N-glycosylated integral membrane protein with a Type I topology that is located in the endoplasmic reticulum (ER). Some phenotypes of a big1Delta mutant resemble those of strains disrupted for KRE5, which encodes another ER protein affecting beta-l,6-glucan levels to a similar extent. In a big1Deltakre5Delta double mutant, both the growth and alkali-soluble beta-l,6-glucan levels were reduced as compared to either single mutant. Thus, while Big1p and Kre5p may have similar effects on beta-l,6-glucan synthesis, these effects are at least partially distinct. Residual beta-l,6-glucan levels in the big1Deltakre5Delta double mutant indicate that these gene products are unlikely to be beta-l,6-glucan synthase subunits, but rather may play some ancillary roles in beta-l,6-glucan synthase assembly or function, or in modifying proteins for attachment of beta-l,6-glucan.