Mechanisms of synapse assembly and disassembly

The mechanisms that govern synapse formation and elimination are fundamental to our understanding of neural development and plasticity. The wiring of neural circuitry requires that vast numbers of synapses be formed in a relatively short time. The subsequent refinement of neural circuitry involves the formation of additional synapses coincident with the disassembly of previously functional synapses. There is increasing evidence that activity-dependent plasticity also involves the formation and disassembly of synapses. While we are gaining insight into the mechanisms of both synapse assembly and disassembly, we understand very little about how these phenomena are related to each other and how they might be coordinately controlled to achieve the precise patterns of synaptic connectivity in the nervous system. Here, we review our current understanding of both synapse assembly and disassembly in an effort to unravel the relationship between these fundamental developmental processes.     

Mechanisms of axon guidance: membrane dynamics and axonal transport in semaphorin signalling

The intricate geometry of neuronal networks poses many unique cell-biological problems regarding the way a growing axon responds to its environment. Several groups of ligand-receptor pairs have been identified to regulate such processes. In this study, we take class 3 semaphorins as an example and review what is known about the intracellular movements of semaphorins throughout neuronal cells, transport support structures and location of release sites. We discuss how their receptor trafficking may contribute to regulate membrane dynamics underlying growth cone motility and the physiological contribution made by class 3 semaphorins-induced acceleration of axoplasmic transport on neurite development.     

Longitudinal axon guidance

Our knowledge about molecular mechanisms underlying axon guidance along the antero-posterior axis in contrast to the dorso-ventral axis of the developing nervous system is very limited. During the past two years in vitro and in vivo studies have indicated that morphogens have a role in longitudinal axon guidance. Morphogens are secreted proteins that act in a concentration-dependent manner on susceptible groups of precursor cells and induce their differentiation to a specific cell fate. Thus, gradients of morphogens are responsible for the appropriate patterning of the nervous system during early phases of neural development. Therefore, it was surprising to find that gradients of two of these morphogens, Wnt4 and Shh, can be re-used for longitudinal axon guidance during later stages of nervous system development.     

Mechanisms of spindle assembly and size control

The spindle is crucial for cell division by allowing the faithful segregation of replicated chromosomes to daughter cells. Proper segregation is ensured only if microtubules (MTs) and hundreds of other associated factors interact to assemble this complex structure with the appropriate architecture and size. In this review, we describe the latest view of spindle organisation as well as the molecular gradients and mechanisms underlying MT nucleation and spindle assembly. We then discuss the overlapping physical and molecular constraints that dictate spindle morphology, concluding with a focus on spindle size regulation.     

Translating axon guidance cues

Plant cells practice constant vigilance using resistance (R) proteins to monitor pathogenic processes. Three papers published recently in Cell and one in Science provide support for a model in which plant cells set up surveillance of signal transduction pathways, preparing to destroy the cell if any untoward fiddling with cellular physiology is detected. The demonstration of three separate examples of such a system suggests that it is broadly used and should provoke a reexamination of microbial pathogenesis in animal cells to look for similar mechanisms.     

Molecular mechanisms of axon guidance

In order to form a functional nervous system, neurones extend axons, often over long distances, to reach their targets. This process is controlled by extracellular receptors and their ligands, several families of which have been identified. These proteins may act to either repel or attract growth cones and a given receptor may transduce either type of signal, depending on the cellular context. In addition to these archetypal axon guidance molecules, it is becoming apparent that molecules previously known for their role in patterning can also direct axonal outgrowth. The growth cone receptors do not act in isolation and combine with members of the same or other families to produce a graded response or even a complete reversal in its polarity. These signals can be further combined and/or modulated by processing of the molecule both directly at the cell surface and by the network of intracellular signalling pathways which are activated. The result is a sophisticated and dynamic set of cues that enable a growth cone to successfully navigate to its destination, modulating its response to changing environmental cues along its pathway.     

Comm sorts robo to control axon guidance at the Drosophila midline

Axon growth across the Drosophila midline requires Comm to downregulate Robo, the receptor for the midline repellent Slit. We show here that comm is required in neurons, not in midline cells as previously thought, and that it is expressed specifically and transiently in commissural neurons. Comm acts as a sorting receptor for Robo, diverting it from the synthetic to the late endocytic pathway. A conserved cytoplasmic LPSY motif is required for endosomal sorting of Comm in vitro and for Comm to downregulate Robo and promote midline crossing in vivo. Axon traffic at the CNS midline is thus controlled by the intracellular trafficking of the Robo guidance receptor, which in turn depends on the precisely regulated expression of the Comm sorting receptor.     

Epithelial axon guidance in Drosophila

Evidence is offered that the axons of developing sensory neurons in the wing of Drosophila are guided (given both location and polarity information) by the epithelium over which they grow. This guidance is effective in the absence of such potential additional cues as guidepost neurons and physical channels.     

Molecular basis of semaphorin-mediated axon guidance

The semaphorin family of proteins constitute one of the major cues for axonal guidance. The prototypic member of this family is Sema3A, previously designated semD/III or collapsin-1. Sema3A acts as a diffusible, repulsive guidance cue in vivo for the peripheral projections of embryonic dorsal root ganglion neurons. Sema3A binds with high affinity to neuropilin-1 on growth cone filopodial tips. Although neuropilin-1 is required for Sema3A action, it is incapable of transmitting a Sema3A signal to the growth cone interior. Instead, the Sema3A/neuropilin-1 complex interacts with another transmembrane protein, plexin, on the surface of growth cones. Certain semaphorins, other than Sema3A, can bind directly to plexins. The intracellular domain of plexin is responsible for initiating the signal transduction cascade leading to growth cone collapse, axon repulsion, or growth cone turning. This intracellular cascade involves the monomeric G-protein, Rac1, and a family of neuronal proteins, the CRMPs. Rac1 is likely to be involved in semaphorin-induced rearrangements of the actin cytoskeleton, but how plexin controls Rac1 activity is not known. Vertebrate CRMPs are homologous to the Caenorhabditis elegans unc-33 protein, which is required for proper axon morphology in worms. CRMPs are essential for Sema3A-induced, neuropilin-plexin-mediated growth cone collapse, but the molecular interactions of growth cone CRMPs are not well defined. Mechanistic aspects of plexin-based signaling for semaphorin guidance cues may have implications for other axon guidance events and for the basis of growth cone motility.     

Neuropilin 1 and 2 control cranial gangliogenesis and axon guidance through neural crest cells

Neuropilin (NRP) receptors and their class 3 semaphorin (SEMA3) ligands play well-established roles in axon guidance, with loss of NRP1, NRP2, SEMA3A or SEMA3F causing defasciculation and errors in growth cone guidance of peripherally projecting nerves. Here we report that loss of NRP1 or NRP2 also impairs sensory neuron positioning in the mouse head, and that this defect is a consequence of inappropriate cranial neural crest cell migration. Specifically, neural crest cells move into the normally crest-free territory between the trigeminal and hyoid neural crest streams and recruit sensory neurons from the otic placode; these ectopic neurons then extend axons between the trigeminal and facioacoustic ganglia. Moreover, we found that NRP1 and NRP2 cooperate to guide cranial neural crest cells and position sensory neurons; thus, in the absence of SEMA3/NRP signalling, the segmentation of the cranial nervous system is lost. We conclude that neuropilins play multiple roles in the sensory nervous system by directing cranial neural crest cells, positioning sensory neurons and organising their axonal projections.     

The heparan sulfate proteoglycans Dally-like and Syndecan have distinct functions in axon guidance and visual-system assembly in Drosophila

Heparan sulfate proteoglycans (HSPGs), a class of glycosaminoglycan-modified proteins, control diverse patterning events via their regulation of growth-factor signaling and morphogen distribution. In C. elegans, zebrafish, and the mouse, heparan sulfate (HS) biosynthesis is required for normal axon guidance, and mutations affecting Syndecan (Sdc), a transmembrane HSPG, disrupt axon guidance in Drosophila embryos. Glypicans, a family of glycosylphosphatidylinositol (GPI)-linked HSPGs, are expressed on axons and growth cones in vertebrates, but their role in axon guidance has not been determined. We demonstrate here that the Drosophila glypican Dally-like protein (Dlp) is required for proper axon guidance and visual-system function. Mosaic studies revealed that Dlp is necessary in both the retina and the brain for different aspects of visual-system assembly. Sdc mutants also showed axon guidance and visual-system defects, some that overlap with dlp and others that are unique. dlp+ transgenes were able to rescue some sdc visual-system phenotypes, but sdc+ transgenes were ineffective in rescuing dlp abnormalities. Together, these findings suggest that in some contexts HS chains provide the biologically critical component, whereas in others the structure of the protein core is also essential.     

PDZ domains in synapse assembly and signalling

Synaptic junctions are highly specialized structures designed to promote the rapid and efficient transmission of signals from the presynaptic terminal to the postsynaptic membrane within the central nervous system. Proteins containing PDZ domains play a fundamental organizational role at both the pre- and postsynaptic plasma membranes. This review focuses on recent advances in our understanding of the mechanisms underlying the assembly of synapses in the central nervous system.     

Models of axon guidance and bundling during development

Diffusible chemoattractants and chemorepellants, together with contact attraction and repulsion, have been implicated in the establishment of connections between neurons and their targets. Here we study how such diffusible and contact signals can be involved in the whole sequence of events from bundling of axons, guidance of axon bundles towards their targets, to debundling and the final innervation of individual targets. By means of computer simulations, we investigate the strengths and weaknesses of a number of particular mechanisms that have been proposed for these processes.