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.     

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.     

Conservation and divergence of axon guidance mechanisms.

Analysis of axon guidance mechanisms in vertebrates, Caenorhabditis elegans, and Drosophila melanogaster has led to the identification of several signaling pathways, many of which are strikingly conserved in function. Recent studies indicate that several axon guidance mechanisms are highly conserved in all animals, whereas others, though still conserved in a general sense, show strong evolutionary divergence at a detailed mechanistic level.     

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.     

Repellent cues in axon guidance.

There is increasing evidence that axons are guided by repulsion in several regions of the developing nervous system, although this has yet to be confirmed directly in vivo. As more candidate repulsion molecules are identified, it is becoming clear that collapse of the growth cone in vitro may be mediated by more than one intracellular mechanism. The present emphasis on molecular cloning of the ligands and their receptors should enable a proper definition of their function during development.     

Bidirectional Eph-ephrin signaling during axon guidance

Ephrins are cell-surface tethered guidance cues that bind to Eph receptor tyrosine kinases in trans on opposing cells. In the developing nervous system, the Eph-ephrin signaling system controls a large variety of cellular responses including contact-mediated attraction or repulsion, adhesion or de-adhesion, and migration. Eph-ephrin signaling can be bidirectional, and is subject to modulation by ectodomain cleavage of ephrins and by Eph-ephrin endocytosis. Recent work has highlighted the importance of higher-order clustering of functional Eph-ephrin complexes and the requirement for Rho GTPases as signal transducers. Co-expression of Ephs and ephrins within the same cellular membrane can result in Eph-ephrin cis interaction or in lateral segregation into distinct domains from where they signal opposing effects on the axon.