Signalling mechanisms mediating neuronal responses to guidance cues

Several families of extracellular guidance cues have been implicated in guiding neurons and axons to their appropriate destinations in the nervous system. Their receptors include single- and seven-transmembrane receptors, and their signal transduction pathways converge onto the Rho family of small GTPases, which control the cytoskeleton. A single guidance protein can use different mechanisms to regulate different kinds of motility or the motilities of different cell types. There is crosstalk between the signalling pathways initiated by distinct guidance cues. Studies of neuronal guidance mechanisms have shed light not only on neural development, but also on other processes that involve the extracellular regulation of the cytoskeleton.     

Sequence analysis and neuronal expression of fasciclin I in grasshopper and Drosophila

The fasciclin I, II, and III glycoproteins are expressed on different subsets of axon bundles (fascicles) in insect embryos and are thus candidates for surface recognition molecules involved in growth cone guidance. Here we present the sequence of grasshopper fasciclin I and the identification and sequence of the Drosophila fasciclin I homolog. In both species, fasciclin I appears to be an extrinsic membrane protein with a signal sequence but no transmembrane region; the protein comprises four homologous domains of approximately 150 amino acids each. Antibodies against Drosophila fasciclin I reveal that it is expressed on the surface of a subset of commissural axon pathways in the embryonic central nervous system and on all sensory axon pathways in the peripheral nervous system. This pattern of expression is similar to that in grasshopper.     

Netrin signaling leading to directed growth cone steering

In the developing nervous system, nerve cells and axons respond to various attractive and repulsive guidance cues while traveling to their final destination. Netrins are bifunctional guidance cues that attract several classes of axons but repel others. The response of an axon to netrins is dictated by the composition of netrin receptors on the cell surface and the internal state of the growth cone. Recent analyses have identified several signal transduction pathways that contribute to netrin-mediated guidance. A model emerges in which tyrosine phosphorylation, phosphatidylinositol signaling and regulation by Rho GTPases act in concert to trigger extension of axons and turning of growth cones in response to Netrin1.     

Ena/VASP: proteins at the tip of the nervous system

The emergence of neurites from a symmetrical cell body is an essential feature of nervous system development. Neurites are the precursors of axons and dendrites and are tipped by growth cones, motile structures that guide elongating axons in the developing nervous system. Growth cones steer the axon along a defined path to its appropriate target in response to guidance cues. This navigation involves the dynamic extension and withdrawal of actin-filled finger-like protrusions called filopodia that continuously sample their environment. Ena/VASP proteins, a conserved family of actin-regulatory proteins, are crucial for filopodia formation and function downstream of several guidance cues. Here we review recent findings into Ena/VASP function in neurite initiation, axon outgrowth and guidance.     

Notch signaling and neural connectivity

The cell surface receptor Notch contributes to the development of nearly every tissue in most metazoans by controlling the fates and differentiation of cells. Recent results have now established that Notch also regulates the connectivity of the nervous system, and does so at a variety of levels, including specification of neuronal identity, division, survival and migration, as well as axon guidance, morphogenesis of dendritic arbors and weighting of synapse strength. To these ends, Notch engages at least two signal transduction pathways, one that controls nuclear gene expression and another that directly targets the cytoskeleton. Coordinating the many functions of Notch to produce neural structure is thus a pivotal aspect of building and maintaining the nervous system.     

Growth cone guidance by substrate-bound laminin pathways is correlated with neuron-to-pathway adhesivity

Substrate-bound laminin pathways prepared by the method of Hammarback et al. [J.A. Hammarback, S.L. Palm, L.T. Furcht, and P.C. Letourneau (1985). J. Neurosci. Res. 13, 213-220] guided peripheral nervous system neurites (dissociated dorsal root ganglia and sympathetic ganglia) and central nervous system neurites (dissociated spinal cord and brain). Guidance of individual growth cones by 7- to 10-micron-wide laminin pathways was observed using time-lapse video microscopy. Fibronectin pathways, produced by the method used for laminin pathways, did not guide neurites. The guidance effect of laminin pathways was quantified and found to correlate with the concentration of laminin initially applied to the substratum. The concentration of laminin initially applied to the substratum also correlated with increased adhesivity of dorsal root ganglia (DRG) neurons to laminin constituting the pathways relative to uv-irradiated laminin that borders the pathways. The guidance effect of laminin pathways was blocked by anti-laminin antibodies or by laminin but not by anti-fibronectin antibodies. This study demonstrates that guidance of DRG neurites by laminin occurs at the growth cone in a manner consistent with the hypothesis of guidance by differential neuron-to-substratum adhesivity.     

Robo-Slit interactions regulate longitudinal axon pathfinding in the embryonic vertebrate brain

The early network of axons in the embryonic brain provides connectivity between functionally distinct regions of the nervous system. While many of the molecular interactions driving commissural pathway formation have been deciphered, the mechanisms underlying the development of longitudinal tracts remain unclear. We have identified here a role for the Roundabout (Robo) family of axon guidance receptors in the positioning of longitudinally projecting axons along the dorsoventral axis in the embryonic zebrafish forebrain. Using a loss-of-function approach, we established that Robo family members exhibit complementary functions in the tract of the postoptic commissure (TPOC), the major longitudinal tract in the forebrain. Robo2 acted initially to split the TPOC into discrete fascicles upon entering a broad domain of Slit1a expression in the ventrocaudal diencephalon. In contrast, Robo1 and Robo3 restricted the extent of defasciculation of the TPOC. In this way, the complementary roles of Robo family members balance levels of fasciculation and defasciculation along this trajectory. These results demonstrate a key role for Robo-Slit signaling in vertebrate longitudinal axon guidance and highlight the importance of context-specific guidance cues during navigation within complex pathways.     

The Yin and Yang of Wnt/Ryk axon guidance in development and regeneration

In the developing embryo,nascent axons navigate towards their specific targets to establish the intricate network of axonal connections linking neurons within the mature nervous system.Molecular navigational systems comprising repulsive and attractive guidance cues form chemotactic gradients along the pathway of the exploring growth cone.Axon-bound receptors detect these gradients and determine the trajectory of the migrating growth cone.In contrast to their benevolent role in the developing nervous system,repulsive guidance receptors are detrimental to the axon’s ability to regenerate after injury in the adult.In this review we explore the essential and beneficial role played by the chemorepulsive Wnt receptor,Ryk/Derailed in axon navigation in the embryonic nervous system(the Yin function).Specifically,we focus on the role of Wnt5a/Rykmediated guidance in the establishment of two major axon tracts in the mammalian central nervous system,the corticospinal tract and the corpus callosum.Recent studies have also identified Ryk as a major suppressor of axonal regeneration after spinal cord injury.Thus,we also discuss this opposing aspect of Ryk function in axonal regeneration where its activity is a major impediment to axon regrowth(the Yang function).     

Semaphorins: a new class of immunoregulatory molecules

The immune and nervous systems play distinct roles in maintaining physiological homeostasis. Recent data indicates that these systems influence one another and share many proteins and pathways that are essential for their normal function and development. Molecules originally shown to be critical for the development of proper immune responses have recently been found to function in the nervous system. Conversely, neuronal guidance cues can modulate immune functions. Although semaphorins were originally identified as axon guidance factors active during neuronal development, several recent studies have identified indispensable functions for these molecules in the immune system. This review provides an overview of the rapidly emerging functions of semaphorins and their receptors in the immune system.     

EphA4-dependent axon guidance is mediated by the RacGAP alpha2-chimaerin

Neuronal network formation in the developing nervous system is dependent on the accurate navigation of nerve cell axons and dendrites, which is controlled by attractive and repulsive guidance cues. Ephrins and their cognate Eph receptors mediate many repulsive axonal guidance decisions by intercellular interactions resulting in growth cone collapse and axon retraction of the Eph-presenting neuron. We show that the Rac-specific GTPase-activating protein alpha2-chimaerin binds activated EphA4 and mediates EphA4-triggered axonal growth cone collapse. alpha-Chimaerin mutant mice display a phenotype similar to that of EphA4 mutant mice, including aberrant midline axon guidance and defective spinal cord central pattern generator activity. Our results reveal an alpha-chimaerin-dependent signaling pathway downstream of EphA4, which is essential for axon guidance decisions and neuronal circuit formation in vivo.     

Role of the JAK/STAT signal transduction pathway in the regulation of gene expression in CNS

Over the last 20 years the JAK/STAT signal transduction pathway has been extensively studied. An enormous amount of data on different cell signal transduction pathways is now available. The JAK/STAT signal transduction pathway is one of the intracellular signaling pathways activated by cytokines and growth factors that was first studied in the hematopoietic system, but recent data demonstrate that this signal transduction is also greatly utilized by other systems. The JAK/STAT pathway is a signaling cascade that links the activation of specific cell membrane receptors to nuclear gene expression. This review is focused on the role of JAK/STAT signal transduction pathway activation in the central nervous system (CNS).     

Vessels and nerves: marching to the same tune

The patterning of the nervous system is achieved through coordinated action of a variety of repulsive or attractive neuronal guidance factors that direct the growth of growing axons to specific pathways. Recent studies suggest that the same or similar sets of factors also guide migration of endothelial cells, helping to direct the stereotypical embryonic patterning of the developing vertebrate vascular system.     

Celsr3 and Fzd3 in axon guidance

The assembly of functional neuronal circuits depends on the correct wiring of axons and dendrites. To reach their targets, axons are guided by a variety of extracellular guidance cues, including Netrins, Ephrins, Semaphorins and Slits. Corresponding receptors in the growth cone, the dynamic structure at the tip of the growing axon, sense and integrate these positional signals, and activate downstream effectors to regulate cytoskeletal organization. In addition to the four canonical families of axon guidance cues mentioned above, some proteins that regulate planar cell polarity were recently found to be critical for axon guidance. The seven-transmembrane domain receptors Celsr3 and Fzd3, in particular, control the development of most longitudinal tracts in the central nervous system, and axon navigation in the peripheral, sympathetic and enteric nervous systems. Despite their unequivocally important role, however, underlying molecular mechanisms remain elusive. We do not know which extracellular ligands they recognize, whether they have co-receptors in the growth cone, and what their downstream effectors are. Here, we review some recent advances and discuss future trends in this emerging field.     

The plane facts of PCP in the CNS

Planar cell polarity (PCP) pathways have been defined by their ability to direct the development of obviously polarized cellular architectures. Recent studies indicate that PCP pathways also regulate aspects of cell morphology that are not restricted to the plane of the epithelium. In the developing nervous system, PCP-mediated changes in the cytoskeleton are fundamental to neuronal migration, neuronal polarity, axon guidance, and dendritic arborization, highlighting the importance of "planar polarity" genes for defining the shape of a neuron in all dimensions.     

Hierarchical guidance cues in the developing nervous system of C. elegans

During embryogenesis, the basic axon scaffold of the nervous system is formed by special axons that pioneer pathways between groups of cells. To find their way, the pioneer growth cones detect specific cues in their extracellular environment. One of these guidance cues is netrin. Observations and experimental manipulations in vertebrates and nematodes have shown that netrin is a bifunctional guidance cue that can simultaneously attract and repel axons. During the formation of this basic axon scaffold in Caenorhabditis elegans, the netrin UNC-6 is expressed by neuroglia and pioneer neurons, providing hierarchical guidance cues throughout the animal. Each cue has a characteristic role depending on the cell type, its position and the developmental stage. These roles include activities as global, decussation and labeled-pathway cues. This hierarchical model of UNC-6 netrin-mediated guidance suggests a method by which guidance cues can direct formation of basic axon scaffolds in developing nervous systems.     

Cyclic GMP is not involved in neural induction inXenopus laevis

Summary Recent evidence indicates an important role for cell-surface mediated signal transduction in embryonic induction. We, therefore, started a systematic search to identify signal transduction pathways which are activated during embryonic induction and specifically during neural induction. We showed previously that the protein kinase C and cAMP pathways mediate neural induction inXenopus laevis. Here, we investigated whether cGMP is also involved in the early development of the nervous system. We measured the cGMP content of whole embryos at embryonic stages which mark important events in the early development of the nervous system, as well as in the developing neural tissue itself, after this was induced from ectoderm by dorsal mesoderm. No changes in cGMP content were found, either in whole embryos at different developmental stages, or in developing neural tissue from these stages. We also found no evidence for the presence of nitroprusside stimulatable guanylate cyclase in these developmental stages. A cGMP analogue, 8-Br-cGMP, was not able to induce neural tissue, either alone or in combination with known neural inducers, the phorbol ester TPA and 8-Br-cAMP. 8-Br-cGMP also had no negative influence on the neural inducing ability of dorsal mesoderm or TPA, alone or in combination with 8-Br-cAMP. We conclude that cGMP has no role in the early development of the central nervous system inXenopus laevis. This conclusion underlines the specificity of the signal transduction pathways (PKC and cAMP pathways) that do mediate neural induction.     

Wnt Signaling through the Ror Receptor in the Nervous System

The receptor tyrosine kinase-like orphan receptor (Ror) proteins are conserved tyrosine kinase receptors that play roles in a variety of cellular processes that pattern tissues and organs during vertebrate and invertebrate development. Ror signaling is required for skeleton and neuronal development and modulates cell migration, cell polarity, and convergent extension. Ror has also been implicated in two human skeletal disorders, brachydactyly type B and Robinow syndrome. Rors are widely expressed during metazoan development including domains in the nervous system. Here, we review recent progress in understanding the roles of the Ror receptors in neuronal migration, axonal pruning, axon guidance, and synaptic plasticity. The processes by which Ror signaling execute these diverse roles are still largely unknown, but they likely converge on cytoskeletal remodeling. In multiple species, Rors have been shown to act as Wnt receptors signaling via novel non-canonical Wnt pathways mediated in some tissues by the adapter protein disheveled and the non-receptor tyrosine kinase Src. Rors can either activate or repress Wnt target expression depending on the cellular context and can also modulate signal transduction by sequestering Wnt ligands away from their signaling receptors. Future challenges include the identification of signaling components of the Ror pathways and bettering our understanding of the roles of these pleiotropic receptors in patterning the nervous system.     

Synaptic information processing by astrocytes.

Glial cells were classically considered as supportive cells that do not contribute to information processing in the nervous system. However, considerable amount of evidence obtained by several groups during the last few years has demonstrated the existence of a bidirectional communication between astrocytes and neurons, which prompted a re-examination of the role of glial cells in the physiology of the nervous system. This review will discuss recent advances in the neuron-to-astrocyte communication, focusing on the recently reported properties of the synaptically evoked astrocyte Ca2+ signal that indicate that astrocytes show integrative properties for synaptic information processing. Indeed, we have recently shown that hippocampal astrocytes discriminate between the activity of different synapses, and respond selectively to different axon pathways. Furthermore, the astrocyte Ca2+ signal is modulated by the simultaneous activity of different synaptic inputs. This Ca2+ signal modulation depends on cellular intrinsic properties of the astrocytes, is bidirectionally regulated by the level of synaptic activity, and controls the spatial extension of the intracellular Ca2+ signal. Consequently, we propose that astrocytes can be considered as cellular elements involved in information processing by the nervous system.