Trends in Neurosciences
How to build a central synapse: clues from cell culture
Introduction
Synaptogenesis involves a complex series of events, spanning neuronal differentiation, cell–cell contact and localized induction of presynaptic and postsynaptic differentiation. Synaptic specificity is determined by the developmental status of both partner cells, by neuronal and glial cues that influence competence for synaptogenesis, by long-range and local axon and dendrite guidance cues, by cell-adhesion molecules that mediate contact, and by local presentation of differentiation-inducing molecules. Although activity is a major force in sculpting circuitry during development [1] and regulates synaptic composition and strength 2, 3, 4, it is not essential for the basic assembly of synapses. Synapses form normally when neurotransmitter release is chronically blocked using clostridial neurotoxins or genetic methods 5, 6, 7. We focus here on molecular cues involved in the later stages of synaptogenesis, once appropriate axons and dendrites are brought into proximity. Studies of several major synaptogenic molecules identified for glutamatergic and/or GABAergic synapses are summarized in Table 1, and partial molecular linkages are shown in Figure 1 8, 9, 10. We also focus on aspects of recent studies that particularly illuminate how basic parameters of synapses are shaped.
For this review, we consider a ‘synapse’ to mean a functional synapse (noting that other more limited definitions of synapses, based on structure or molecular composition, can be useful in many circumstances) (Box 1). We use ‘hemi-presynapse’ or ‘presynaptic differentiation’ to refer to clusters of release-competent synaptic vesicles, and ‘hemi-postsynapse’ or ‘postsynaptic differentiation’ to refer to clusters of surface neurotransmitter receptors and associated signaling and scaffolding molecules. These hemi-synaptic elements can be combined in a bona fide synapse or induced in isolated axons or dendrites by individual synaptogenic molecules (Figure 2).
Section snippets
Competence
Neurons acquire the ability to form synapses as part of a developmental maturation process. Intrinsic limitations in competence to form synapses have been demonstrated in cell culture studies, where a difference in experience of two days can be crucial. For example, hippocampal neurons from embryonic day (E)18 rats form functional synapses in culture but, under the same conditions, E16 neurons form morphological synapses that are largely presynaptically silent, regardless of how long they are
Matching cellular partners
For synapse assembly to occur, the plasma membranes of the appropriate presynaptic and postsynaptic cells must be brought into contact. Members of the cadherin and immunoglobulin (Ig) superfamilies are thought to mediate this function. Cadherin expression patterns and some function-blocking studies support the idea that cadherins have a key role in mediating selective adhesion leading to formation of synapses between the appropriate partners 24, 25. In Drosophila, single-cell mosaic analyses
Recruiting presynaptic components
Once the cell membranes are brought into contact, the next step is to recruit the molecular assemblies that mediate transmitter release and response. Several studies in the past few years have demonstrated the surprising ability of a handful of isolated molecules to induce focal aggregation of release-competent synaptic vesicles when presented to axons of cultured neurons. Neuroligins were the first of these molecules to be identified [55] and appear to be the most potent inducers of
Size
At a typical CNS synapse composed of a single active zone, the areas of the active zone and postsynaptic density, the numbers of synaptic and docked vesicles and the volumes of the axon varicosity and spine head are all highly correlated 91, 92, 93. This size correlation suggests a coordinated regulation of presynaptic active zone and bouton size with PSD and spine size via connecting transmembrane and cytoskeletal proteins. Active zone and PSD sizes range over about two orders of magnitude 92,
Longevity and plasticity
The majority of spine synapses in the mature brain are stable for months 114, 115, presumably through continual replenishment of the synaptogenic signals. However, as exemplified by studies of ephrins and Eph receptors [116] and cadherins [117], the same molecules that function centrally in synaptogenesis also contribute to activity-dependent synaptic plasticity in more mature systems. Such plasticity can occur through synapse assembly or disassembly and through altering the strength of
Concluding remarks
A key question is whether any single factor or even single family of proteins is essential for synaptogenesis. At present, mutant mice for individual synaptogenic proteins mentioned here are viable and form synapses. Moreover, directed screens in C. elegans and Drosophila have not revealed any proteins essential for basic synapse assembly – that is, mutants completely lacking synapses have not been found 98, 99. These results, or lack of them, suggest either that the key synaptogenic factors
Acknowledgements
We thank Huaiyang Wu for preparation of cultures used in Figure 2, and YunHee Kang and other members of the Craig laboratory for helpful comments. Supported by grants from NIH, CIHR and MSFHR, and by Canada Research Chair (AMC) and NSF Predoctoral Fellow (ERG) awards.
References (146)
- et al.
AMPA receptor trafficking at excitatory synapses
Neuron
(2003) Synaptic adhesion molecules
Curr. Opin. Cell Biol.
(2003)Neurotrophins and activity-dependent plasticity of cortical interneurons
Trends Neurosci.
(1997)BDNF regulates the maturation of inhibition and the critical period of plasticity in mouse visual cortex
Cell
(1999)WNT-3, expressed by motoneurons, regulates terminal arborization of neurotrophin-3-responsive spinal sensory neurons
Neuron
(2002)Axonal remodeling and synaptic differentiation in the cerebellum is regulated by WNT-7a signaling
Cell
(2000)FGF22 and its close relatives are presynaptic organizing molecules in the mammalian brain
Cell
(2004)Thrombospondins are astrocyte-secreted proteins that promote CNS synaptogenesis
Cell
(2005)- et al.
The diversity of cadherins and implications for a synaptic adhesive code in the CNS
Neuron
(1999) N-cadherin regulates target specificity in the Drosophila visual system
Neuron
(2001)
Roles and modes of action of nectins in cell-cell adhesion
Semin. Cell Dev. Biol.
Sidekicks: synaptic adhesion molecules that promote lamina-specific connectivity in the retina
Cell
The immunoglobulin superfamily protein SYG-1 determines the location of specific synapses in C. elegans
Cell
Synaptic specificity is generated by the synaptic guidepost protein SYG-2 and its receptor, SYG-1
Cell
Genetic analysis of the mechanisms controlling target selection: complementary and combinatorial functions of netrins, semaphorins, and IgCAMs
Cell
Neurexins: three genes and 1001 products
Trends Genet.
Binding properties of neuroligin 1 and neurexin 1β reveal function as heterophilic cell adhesion molecules
J. Biol. Chem.
Structure and evolution of neurexin genes: insight into the mechanism of alternative splicing
Genomics
Class-specific features of neuronal wiring
Neuron
The presynaptic release apparatus is functional in the absence of dendritic contact and highly mobile within isolated axons
Neuron
Total number and distribution of inhibitory and excitatory synapses on hippocampal CA1 pyramidal cells
Neuroscience
Salient features of synaptic organisation in the cerebral cortex
Brain Res. Rev.
Ankyrin-based subcellular gradient of neurofascin, an immunoglobulin family protein, directs GABAergic innervation at purkinje axon initial segment
Cell
Neuroligin expressed in nonneuronal cells triggers presynaptic development in contacting axons
Cell
Neurexins induce differentiation of GABA and glutamate postsynaptic specializations via neuroligins
Cell
Cadherins and catenins in synapse development
Curr. Opin. Neurobiol.
Role of β-catenin in synaptic vesicle localization and presynaptic assembly
Neuron
Synaptic clustering of AMPA receptors by the extracellular immediate-early gene product Narp
Neuron
AMPA receptor-dependent clustering of synaptic NMDA receptors is mediated by Stargazin and NR2A/B in spinal neurons and hippocampal interneurons
Neuron
EphB receptors interact with NMDA receptors and regulate excitatory synapse formation
Cell
Eph/ephrin signaling in morphogenesis, neural development and plasticity
Curr. Opin. Cell Biol.
Molecular mechanisms of dendritic spine development and remodeling
Prog. Neurobiol.
Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological implications
Pharmacol. Ther.
Regulation of GABAA receptor trafficking, channel activity, and functional plasticity of inhibitory synapses
Pharmacol. Ther.
Neuroligin 2 is exclusively localized to inhibitory synapses
Eur. J. Cell Biol.
Optimizing synaptic architecture and efficiency for high-frequency transmission
Neuron
Sequential steps in clathrin-mediated synaptic vesicle endocytosis
Curr. Opin. Neurobiol.
Genetic analysis of synaptic target recognition and assembly
Trends Neurosci.
Establishing and sculpting the synapse in Drosophila and C. elegans
Curr. Opin. Neurobiol.
Synaptic activity and the construction of cortical circuits
Science
Molecular heterogeneity of central synapses: afferent and target regulation
Nat. Neurosci.
AMPA receptor trafficking and synaptic plasticity
Annu. Rev. Neurosci.
Synaptic assembly of the brain in the absence of neurotransmitter secretion
Science
Total arrest of spontaneous and evoked synaptic transmission but normal synaptogenesis in the absence of Munc13-mediated vesicle priming
Proc. Natl. Acad. Sci. U. S. A.
Synapse composition and organization following chronic activity blockade in cultured hippocampal neurons
J. Comp. Neurol.
Mechanisms of vertebrate synaptogenesis
Annu. Rev. Neurosci.
Cell-cell signaling during synapse formation in the CNS
Annu. Rev. Neurosci.
Neurotrophins induce formation of functional excitatory and inhibitory synapses between cultured hippocampal neurons
J. Neurosci.
Synaptogenesis in hippocampal cultures: evidence indicating that axons and dendrites become competent to form synapses at different stages of neuronal development
J. Neurosci.
Brain-derived neurotrophic factor mediates the activity-dependent regulation of inhibition in neocortical cultures
J. Neurosci.
Cited by (147)
Role of LRRTMs in synapse development and plasticity
2017, Neuroscience ResearchCitation Excerpt :To identify new synaptogenic proteins, Linhoff and colleagues performed the fibroblast-neuron co-culture assay in an unbiased expression screen of full-length cDNAs sourced from postnatal (P) day 11 rat brains (Linhoff et al., 2009). The fibroblast-neuron co-culture assay is a powerful tool to assess the synaptogenic activity of proteins (Craig et al., 2006; Graf et al., 2006; Scheiffele et al., 2000). A postsynaptic synapse organizer expressed in non-neuronal cells induces presynaptic differentiation in contacting axons via binding its presynaptic ligand on the co-cultured neurons (Biederer and Scheiffele, 2007; Craig et al., 2006; Graf et al., 2004; Scheiffele et al., 2000), leading to the formation of a “hemi-synapse” with a robust readout not achievable in pure neuron cultures.
Evidence for a role of glycoprotein M6a in dendritic spine formation and synaptogenesis
2016, Molecular and Cellular NeuroscienceRing finger protein 34 (RNF34) interacts with and promotes γ-aminobutyric acid type-a receptor degradation via ubiquitination of the γ2 subunit
2014, Journal of Biological ChemistryLeucine-rich repeats containing 4 protein (LRRC4) in memory, psychoneurosis, and glioblastoma
2023, Chinese Medical JournalStructure and evolution of neuronal wiring receptors and ligands
2023, Developmental Dynamics