NORMAL GENOMIC

• Stein E, Zou Y, Poo M, Tessier-Lavigne M
• Binding of DCC by netrin-1 to mediate axon guidance independent of adenosine A2B receptor activation.
• Science. 2001; 291: 1976-82
• Display abstract

• Netrins stimulate and orient axon growth through a mechanism requiring receptors of the DCC family. It has been unclear, however, whether DCC proteins are involved directly in signaling or are mere accessory proteins in a receptor complex. Further, although netrins bind cells expressing DCC, direct binding to DCC has not been demonstrated. Here we show that netrin-1 binds DCC and that the DCC cytoplasmic domain fused to a heterologous receptor ectodomain can mediate guidance through a mechanism involving derepression of cytoplasmic domain multimerization. Activation of the adenosine A2B receptor, proposed to contribute to netrin effects on axons, is not required for rat commissural axon outgrowth or Xenopus spinal axon attraction to netrin-1. Thus, DCC plays a central role in netrin signaling of axon growth and guidance independent of A2B receptor activation.

• Corset V, Nguyen-Ba-Charvet KT, Forcet C, Moyse E, Chedotal A, Mehlen P
• Netrin-1-mediated axon outgrowth and cAMP production requires interaction with adenosine A2b receptor.
• Nature. 2000; 407: 747-50
• Display abstract

• The netrins, a family of laminin-related secreted proteins, are critical in controlling axon elongation and pathfinding. The DCC (for deleted in colorectal cancer) protein was proposed as a receptor for netrin-1 in the light of many observations including the inhibition of netrin-1-mediated axon outgrowth and attraction in the presence of an anti-DCC antiserum, the similitude of nervous system defects in DCC and netrin-1 knockout mice and the results of receptor swapping experiments. Previous studies have failed to show a direct interaction of DCC with netrin-1 (ref. 10), suggesting the possibility of an additional receptor or co-receptor. Here we show that DCC interacts with the membrane-associated adenosine A2b receptor, a G-protein-coupled receptor that induces cAMP accumulation on binding adenosine. We show that A2b is actually a netrin-1 receptor and induces cAMP accumulation on binding netrin-1. Finally, we show that netrin-1-dependent outgrowth of dorsal spinal cord axons directly involves A2b. Together our results indicate that the growth-promoting function of netrin-1 may require a receptor complex containing DCC and A2b.

• Hiramoto M, Hiromi Y, Giniger E, Hotta Y
• The Drosophila Netrin receptor Frazzled guides axons by controlling Netrin distribution.
• Nature. 2000; 406: 886-9
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• Netrin is a secreted protein that can act as a chemotropic axon guidance cue. Two classes of Netrin receptor, DCC and UNC-5 (refs 6-9), are required for axon guidance and are thought to mediate Netrin signals in growth cones through their cytoplasmic domains. However, in the guidance of Drosophila photoreceptor axons, the DCC orthologue Frazzled is required not in the photoreceptor neurons but instead in their targets, indicating that Frazzled also has a non-cell-autonomous function. Here we show that Frazzled can capture Netrin and 'present' it for recognition by other receptors. Moreover, Frazzled itself is actively localized within the axon through its cytoplasmic domain, and thereby rearranges Netrin protein into a spatial pattern completely different from the pattern of Netrin gene expression. Frazzled-dependent guidance of one pioneer neuron in the central nervous system can be accounted for solely on the basis of this ability of Frazzled to control Netrin distribution, and not by Frazzled signalling. We propose a model of patterning mechanism in which a receptor rearranges secreted ligand molecules, thereby creating positional information for other receptors.

• Leonardo ED et al.
• Guidance of developing axons by netrin-1 and its receptors.
• Cold Spring Harb Symp Quant Biol. 1997; 62: 467-78

• Kolodkin AL, Levengood DV, Rowe EG, Tai YT, Giger RJ, Ginty DD
• Neuropilin is a semaphorin III receptor.
• Cell. 1997; 90: 753-62
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• The semaphorin family contains a large number of phylogenetically conserved proteins and includes several members that have been shown to function in repulsive axon guidance. Semaphorin III (Sema III) is a secreted protein that in vitro causes neuronal growth cone collapse and chemorepulsion of neurites, and in vivo is required for correct sensory afferent innervation and other aspects of development. The mechanism of Sema III function, however, is unknown. Here, we report that neuropilin, a type I transmembrane protein implicated in aspects of neurodevelopment, is a Sema III receptor. We also describe the identification of neuropilin-2, a related neuropilin family member, and show that neuropilin and neuropilin-2 are expressed in overlapping, yet distinct, populations of neurons in the rat embryonic nervous system.

• Guthrie S
• Axon guidance: netrin receptors are revealed.
• Curr Biol. 1997; 7: 69-69
• Display abstract

• Netrins are molecules that guide growing axons and that are strikingly similar in sequence and in function in flies, nematodes and vertebrates. Now, members of a family of netrin receptors have been identified in all three animal groups and shown to have crucial, conserved roles in axon navigation.

• Chan SS et al.
• UNC-40, a C. elegans homolog of DCC (Deleted in Colorectal Cancer), is required in motile cells responding to UNC-6 netrin cues.
• Cell. 1996; 87: 187-95
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• UNC-6 netrin, a laminin-related protein secreted from neuroglia and neurons along the ventral midline, orients migrating cells and pioneering growth cones on the nematode epidermis. UNC-5, a cell surface protein expressed on motile cells and pioneer axons, orients movements away from UNC-6 sources. UNC-40, a homolog of the cell surface proteins DCC (Deleted in Colorectal Cancer) and neogenin, is also expressed on motile cells and pioneer neurons. UNC-40 acts cell autonomously to orient movement toward UNC-6 sources. For cells coexpressing UNC-5, it helps orient movement away from UNC-6 sources. Finally, UNC-40 helps determine the dorsoventral position of cells undergoing purely longitudinal migrations. Together with the recent report that DCC is a netrin receptor in vertebrates, our results suggest that UNC-40 is a component of UNC-6 receptors on motile cells.

• Keynes R, Cook GM
• Axons turn as netrins find their receptor.
• Neuron. 1996; 17: 1031-4

• Snider WD, Lichtman JW
• Are neurotrophins synaptotrophins?
• Mol Cell Neurosci. 1996; 7: 433-42

• Marti E, Bumcrot DA, Takada R, McMahon AP
• Requirement of 19K form of Sonic hedgehog for induction of distinct ventral cell types in CNS explants.
• Nature. 1995; 375: 322-5
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• The identity and patterning of ventral cell types in the vertebrate central nervous system depends on cell interactions. For example, induction of a specialized population of ventral midline cells, the floor plate, appears to require contact-mediated signalling by the underlying notochord, whereas diffusible signals from the notochord and floor plate can induce ventrolaterally positioned motor neurons. Sonic hedgehog (Shh), a vertebrate hedgehog-family member, is processed to generate two peptides (M(r) 19K and 26/27K) which are secreted by both of these organizing centres. Moreover, experiments in a variety of vertebrate embryos, and in neural explants in vitro, indicate that Shh can mediate floor-plate induction. Here we have applied recombinant Shh peptides to neural explants in serum-free conditions. High concentrations of Shh bound to a matrix induce floor plate and motor neurons, and addition of Shh to the medium leads to dose-dependent induction of motor neurons. All inducing activity resides in a highly conserved amino-terminal peptide (M(r) 19K). Moreover, antibodies that specifically recognize this peptide block induction of motor neurons by the notochord. We propose that Shh acts as a morphogen to induce distinct ventral cell types in the vertebrate central nervous system.

• Colamarino SA, Tessier-Lavigne M
• The axonal chemoattractant netrin-1 is also a chemorepellent for trochlear motor axons.
• Cell. 1995; 81: 621-9
• Display abstract

• Extending axons are guided in part by diffusible chemoattractants that lure them to their targets and by diffusible chemorepellents that keep them away from nontarget regions. Floor plate cells at the ventral midline of the neural tube express a diffusible chemoattractant, netrin-1, that attracts a group of ventrally directed axons. Here we report that floor plate cells also have a long-range repulsive effect on a set of axons, trochlear motor axons, that grow dorsally away from the floor plate in vivo. COS cells secreting recombinant netrin-1 mimic this effect, suggesting that netrin-1 is a bifunctional guidance cue that simultaneously attracts some axons to the floor plate while steering others away. This bifunctionality of netrin-1 in vertebrates mirrors the dual actions of UNC-6, a C. elegans homolog of netrin-1, which is involved in guiding both dorsal and ventral migrations in the nematode.

• Baier H, Bonhoeffer F
• Attractive axon guidance molecules.
• Science. 1994; 265: 1541-2

• Calof AL, Reichardt LF
• Response of purified chick motoneurons to myotube conditioned medium: laminin is essential for the substratum-binding, neurite outgrowth-promoting activity.
• Neurosci Lett. 1985; 59: 183-9
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• Purified motoneurons from chick, cultured on polycationic substrata treated with myotube-conditioned (MCM), respond by rapidly extending neurites. When MCM, partially purified by salt precipitation and ion-exchange chromatography, was fractionated on Sepharose CL-4B, the peak of neurite outgrowth-promoting activity (NOPA) corresponded to a peak of laminin (LA) immunoreactivity. Fractions from this peak contained a protein band that comigrated with an LA standard on sodium dodecyl sulfate gels. Antibodies to LA immunoprecipitated all motoneuron NOPA from MCM, and the specifically immunoprecipitated material comigrated with LA in both reducing and non-reducing gels. It thus appears that LA in MCM is essential for the ability of this conditioned medium to promote motoneuron neurite outgrowth.