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PubMed2001

Expression of netrin-1 and its receptors DCC and UNC-5H2 after axotomy and during regeneration of adult rat retinal ganglion cells.

Netrins are a family of chemotropic factors that guide axon outgrowth during development; however, their function in the adult CNS remains to be established. We examined the expression of the netrin receptors DCC and UNC5H2 in adult rat retinal ganglion cells (RGCs) after grafting a peripheral nerve (PN) to the transected optic nerve and following optic nerve transection alone. In situ hybridization revealed that both Dcc and Unc5h2 mRNAs are expressed by normal adult RGCs. In addition, netrin-1 was found to be constitutively expressed by RGCs. Quantitative analysis using in situ hybridization demonstrated that both Dcc and Unc5h2 were down-regulated by RGCs following axotomy. In the presence of an attached PN graft, Dcc and Unc5h2 were similarly down-regulated in surviving RGCs regardless of their success in regenerating an axon. Northern blot analysis demonstrated expression of netrin-1 in both optic and sciatic nerve, and Western blot analysis revealed the presence of netrin protein in both nerves. Immunohistochemical analysis indicated that netrin protein was closely associated with glial cells in the optic nerve. These results suggest that netrin-1, DCC, and UNC5H2 may contribute to regulating the regenerative capacity of adult RGCs.

Animals↗

PubMed1997

Vertebrate homologues of C. elegans UNC-5 are candidate netrin receptors.

In the developing nervous system, migrating cells and axons are guided to their targets by cues in the extracellular environment. The netrins are a family of phylogenetically conserved guidance cues that can function as diffusible attractants and repellents for different classes of cells and axons. In vertebrates, insects and nematodes, members of the DCC subfamily of the immunoglobulin superfamily have been implicated as receptors that are involved in migration towards netrin sources. The mechanisms that direct migration away from netrin sources (presumed repulsions) are less well understood. In Caenorhabditis elegans, the transmembrane protein UNC-5 (ref. 14) has been implicated in these responses, as loss of unc-5 function causes migration defects and ectopic expression of unc-5 in some neurons can redirect their axons away from a netrin source. Whether UNC-5 is a netrin receptor or simply an accessory to such a receptor has not, however, been defined. We now report the identification of two vertebrate homologues of UNC-5 which, with UNC-5 and the product of the mouse rostral cerebellar malformation gene (rcm), define a new subfamily of the immunoglobulin superfamily, and whose messenger RNAs show prominent expression in various classes of differentiating neurons. We provide evidence that these two UNC-5 homologues, as well as the rcm gene product, are netrin-binding proteins, supporting the hypothesis that UNC-5 and its relatives are netrin receptors.

Amino Acid Sequence↗

PubMed2003

Netrin binds discrete subdomains of DCC and UNC5 and mediates interactions between DCC and heparin.

Netrins are secreted proteins that elicit both attractive and repulsive responses in migrating cells in the central and peripheral nervous systems. Netrins interact with members of two distinct families of transmembrane receptors, represented by DCC (deleted in colorectal cancer) and UNC5. A human netrin fragment (soluble netrin; sNetrin) was purified from an engineered Chinese hamster ovary cell line and used in a pull-down assay to map the interactions between netrin and its receptors. We find that sNetrin binds exclusively to the fifth fibronectin type III repeat of DCC and to each immunoglobulin repeat of UNC5. Both DCC and UNC5 bind to sNetrin with 1:1 stoichiometry in solution, and the minimal receptor fragments behave similarly to larger fragments in cross-linking experiments with purified sNetrin. We find no evidence for formation of a ternary complex between sNetrin and soluble forms of DCC and UNC5. We also find no evidence for an interaction between DCC and heparin and instead demonstrate that a loop on the fifth fibronectin type III repeat of DCC previously implicated in mediating interactions with heparin is important for sNetrin binding. Since netrin binds heparin, our results suggest that interactions between DCC and heparin are probably mediated by netrin.

Algorithms↗

PubMed2006

Protein interacting with C-kinase 1/protein kinase Calpha-mediated endocytosis converts netrin-1-mediated repulsion to attraction.

In vertebrates, the receptor families deleted in colorectal cancer (DCC) and UNC5 mediate responses to the bifunctional guidance cue netrin-1. DCC mediates attraction, whereas a complex of DCC and UNC5 mediates repulsion. Thus, a primary determinant of the responsiveness of an axon to netrin-1 is the presence or absence of UNC5 family members on the cell surface. Currently, little is known about the role of receptor trafficking in regulating neuronal responses to netrin-1. We show that protein interacting with C-kinase 1 (PICK1) recruits activated protein kinase Calpha (PKCalpha) to MycUNC5A at the plasma membrane, stimulating its endocytosis. We identify two PKCalpha phosphorylation sites at serines 408 and 587, as well as dileucine internalization motifs, which are required for this endocytosis. We find that PKCalpha-stimulated internalization of UNC5A alters the functional response of developing hippocampal axons to netrin-1, preventing UNC5A-mediated growth cone collapse and converting netrin-1-stimulated chemorepulsion to attraction. To address whether this conversion in axonal response occurs in neurons expressing endogenous levels of UNC5, we show that mouse cerebellar granule axons exhibit chemorepulsion in a netrin-1 gradient and that this chemorepulsion is converted to chemoattraction after PKCalpha activation. We demonstrate that this repulsion depends on UNC5A because Unc5a-/- axons are not repelled and show this conversion depends on PICK1 because PICK1-/- axons are not converted to chemoattraction after PKCalpha activation. Together, these data provide a potential mechanism to explain how developing neurons alter their responsiveness to netrin-1 at intermediate choice points as they navigate to their targets.

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PubMed1997

Netrin-1 and DCC mediate axon guidance locally at the optic disc: loss of function leads to optic nerve hypoplasia.

Embryonic retinal ganglion cell (RGC) axons must extend toward and grow through the optic disc to exit the eye into the optic nerve. In the embryonic mouse eye, we found that immunoreactivity for the axon guidance molecule netrin-1 was specifically on neuroepithelial cells at the disk surrounding exiting RGC axons, and RGC axons express the netrin receptor, DCC (deleted in colorectal cancer). In vitro, anti-DCC antibodies reduced RGC neurite outgrowth responses to netrin-1. In netrin-1- and DCC-deficient embryos, RGC axon pathfinding to the disc was unaffected; however, axons failed to exit into the optic nerve, resulting in optic nerve hypoplasia. Thus, netrin-1 through DCC appears to guide RGC axons locally at the optic disc rather than at long range, apparently reflecting the localization of netrin-1 protein to the vicinity of netrin-1-producing cells at the optic disc.

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PubMed1999

Growth-cone attraction to netrin-1 is converted to repulsion by laminin-1.

Growing axons are guided by both diffusible and substrate-bound factors. Growth cones of retinal neurons exhibit chemoattractive turning towards the diffusible factor netrin-1 in vitro and are guided into the optic nerve head (ONH) by localized netrin-1. Here we report that, in Xenopus, laminin-1 from the extracellular matrix (ECM), converts netrin-mediated attraction into repulsion. A soluble peptide fragment of laminin-1 (YIGSR) mimics this laminin-induced conversion. Low levels of cyclic AMP in growth cones also lead to the conversion of netrin-induced attraction into repulsion, and we show that the amount of cAMP decreases in the presence of laminin-1 or YIGSR, suggesting a possible mechanism for laminin's effect. At the netrin-1-rich ONH, where axons turn sharply to leave the eye, laminin-1 is confined to the retinal surface. Repulsion from the region in which laminin and netrin are coexpressed may help to drive axons into the region where only netrin is present, providing a mechanism for their escape from the retinal surface. Consistent with this idea, YIGSR peptides applied to the developing retina cause axons to be misdirected at the ONH. These findings indicate that ECM molecules not only promote axon outgrowth, but also modify the behaviour of growth cones in response to diffusible guidance cues.

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PubMed2004

Focal adhesion kinase in netrin-1 signaling.

Netrins are a family of secreted molecules that are important for axonal outgrowth and guidance in the developing nervous system. However, the signaling mechanisms that lie immediately downstream of netrin receptors remain poorly understood. Here we report that the netrin receptor DCC (deleted in colorectal cancer) interacts with the focal adhesion kinase (FAK), a kinase implicated in regulating cell adhesion and migration. FAK was expressed in developing brains and was localized with DCC in cultured neurons. Netrin-1 induced FAK and DCC tyrosine phosphorylation. Disruption of FAK signaling abolished netrin-1-induced neurite outgrowth and attractive growth cone turning. Taken together, these results indicate a new signaling mechanism for DCC, in which FAK is activated upon netrin-1 stimulation and mediates netrin-1 function; they also identify a critical role for FAK in axon navigation.

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PubMed2006

Netrin-1 induces angiogenesis via a DCC-dependent ERK1/2-eNOS feed-forward mechanism.

Netrin-1 is critical for axonal pathfinding which shares similarities with formation of vascular network. Here we report that netrin-1 induction of angiogenesis is mediated by an increase in endothelial nitric oxide (NO*) production, which occurs via a DCC-dependent, ERK1/2-eNOS feed-forward mechanism. Exposure of mature aortic endothelial cells to netrin-1 resulted in a potent, dose-dependent increase in NO* production, detected by electron spin resonance. Scavenging NO* with 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) abolished netrin-1 stimulated angiogenesis. Netrin-1-stimulated NO* production or angiogenesis was inhibited by DCC antibody, DCC small interfering RNA (siRNA), specific inhibitors (PD98059, U0126), or siRNAs for MEK1/2. PTIO attenuated ERK1/2 phosphorylation, indicating a feed-forward mechanism. Netrin-1 induced a time-dependent phosphorylation of eNOS(s1179, s116) and a rapid dephosphorylation of eNOS(t497). Only eNOS(s1179) was sensitive to U0126 or PTIO. These data characterized a mechanism whereby netrin-1 promotes angiogenesis, which may broadly relate to cardiovascular, neuronal and cancer physiology.

Animals↗

PubMed2000

Biochemical characterization of netrin-synergizing activity.

The netrin-1 protein elicits spinal commissural axon outgrowth and turning in vitro and has been shown to be required for commissural axon guidance in vivo in the developing spinal cord. Biochemical observations made during the purification of netrin-1 suggest that this ligand and its receptor, DCC, may not function alone in directing commissural axon guidance. Recombinant netrin-1 protein is approximately 10 times more active in eliciting axon outgrowth from embryonic day (E) 13 rat dorsal spinal cord explants than from E11 rat dorsal spinal cord explants (Serafini, T., Kennedy, T. E., Galko, M. J., Mirzayan, C., Jessell, T. M., and Tessier-Lavigne, M. (1994) Cell 78, 409-424) even though the starting material for the netrin purification, a high salt extract of E10 chicken brain membranes, is equally active on E13 and E11 explants. We previously reported an activity termed netrin-synergizing activity (NSA) that can potentiate the outgrowth-promoting activity of netrin-1 on E11 explants (Serafini et al.). Here we report a biochemical characterization of NSA in netrin-depleted high salt extracts of E10 chicken brain membranes. We provide evidence that NSA is composed of a denaturation-resistant basic protein(s) in the 25-35-kDa size range. We also provide evidence that the activity may be heterogeneous, splitting into three species that may be distinct or related. The results reported here should facilitate purification of this activity from a more abundant source or identification of the activity based on similarity to known proteins that share its distinctive biochemical properties.

Animals↗

PubMed2005

DCC-dependent phospholipase C signaling in netrin-1-induced neurite elongation.

Netrins, a family of secreted molecules, play important roles in axon pathfinding during nervous system development. Although phosphatidylinositol signaling has been implicated in this event, how netrin-1 regulates phosphatidylinositol signaling remains poorly understood. Here we provide evidence that netrin-1 stimulates phosphatidylinositol bisphosphate hydrolysis in cortical neurons. This event appears to be mediated by DCC (deleted in colorectal cancer), but not neogenin or Unc5h2. Netrin-1 induces phospholipase Cgamma (PLCgamma) tyrosine phosphorylation. Inhibition of PLC activity attenuates netrin-1-induced cortical neurite outgrowth. These results suggest that netrin-1 regulates phosphatidylinositol turnover and demonstrate a crucial role of PLC signaling in netrin-1-induced neurite elongation.

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PubMed2005

Netrin induces down-regulation of its receptor, Deleted in Colorectal Cancer, through the ubiquitin-proteasome pathway in the embryonic cortical neuron.

The proper regulation of temporal and spatial expression of the axon guidance cues and their receptors is critical for the normal wiring of nervous system during development. Netrins, a family of secreted guidance cues, are involved in the midline crossing of spinal commissural axons and in the guidance of cortical efferents. Axons normally lose the responsiveness to their attractants when they arrive at their targets, where the attractant is produced. However the molecular mechanism is still unknown. We investigated the molecular mechanism of down-regulation of netrin-1 signaling in the embryonic cortical neurons. Netrin-1 induced the ubiquitination and proteolytic cleavage of Deleted in Colorectal Cancer (DCC), a transmembrane receptor for netrin, in dissociated cortical neurons. A dramatic decrease of DCC level particularly on the cell surface was also observed after netrin-1 stimulation. Specific ubiquitin-proteasome inhibitors prevented the netrin-induced DCC cleavage and decrease of cell surface DCC. We suggest that the ligand-mediated down-regulation of DCC might participate in the loss of netrin-responsiveness in the developing nervous system.

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PubMed1997

A role for netrin-1 in the guidance of cortical efferents.

An intermediate target for axons leaving the cerebral cortex in embryonic mammals is the ganglionic eminence (GE), the embryonic precursor of the basal ganglia. The cues that direct these axons over the initial portion of their trajectory are not well understood, but could include both short-range and long-range attractants and repellents. In the present study, we provide evidence that corticofugal axons might be guided at least partly by a diffusible factor or factors originating in the lateral GE and the sulcus between the lateral and medial ridges of the GE (ISS), as well as evidence implicating the axonal chemoattractant netrin-1 in mediating these effects. Explants of lateral GE and ISS obtained from E12.5 and E13.5 mouse forebrain have a strong effect on both the outgrowth and orientation of corticofugal axons when cultured at a distance with explants of embryonic cortex in collagen gels. Netrin-1 mRNA is detected in these target tissues by in situ hybridization, and both netrin-1 protein and heterologous cells secreting netrin-1 can mimic the outgrowth-promoting effect of these target tissues in vitro. Furthermore, the growth of corticofugal axons is oriented toward an ectopic source of netrin-1 in vitro, and a function blocking anti-netrin-1 antiserum specifically abolishes the cortical axon outgrowth elicited by explants of lateral GE and the ISS in collagen gel cocultures. Taken together, these results suggest a role for netrin-1 in the attraction at a distance of early cortical axons by the GE. Thus in mammals -- as is also observed in nematodes -- the development of non-commissural projections in anterior regions of the embryo might be directed by mechanisms similar to those involved in directing the development of commissural projections in more posterior regions of the central nervous system.

Animals↗

PubMed2006

Motor axon guidance of the mammalian trochlear and phrenic nerves: dependence on the netrin receptor Unc5c and modifier loci.

Netrin signaling is important to guide migrating neurons and axons in many systems. Experiments with vertebrate CNS explants suggested netrin is bifunctional, attracting some axons and repelling others. Netrin1-expressing cells attracted spinal commissural axons and repelled trochlear cranial nerve axons in these experiments. Subsequent genetic studies demonstrated that multiple axon types, including those of the spinal commissural neurons, are attracted to netrin in vivo; however, an in vivo role for netrin signaling in trochlear nerve repulsion has not been observed. Here, we demonstrate that mice with a null mutation in the netrin receptor Unc5c on the inbred C57BL/6J (B6) genetic background have ventral/ipsilateral trochlear nerve misprojections. These misprojections are attenuated on a hybrid B6 x SJL background. In addition, B6.Unc5c(-/-) mice die as neonates of apparent respiratory distress and have incomplete phrenic nerve innervation of the diaphragm muscle. Neither the trochlear nerve misprojections nor the phrenic nerve phenotype was observed in B6 embryos lacking the netrin receptors DCC or Neogenin1, or the ligand netrin1, indicating these signaling molecules are dispensable for guidance of these axons. Like the trochlear nerve, the phrenic nerve phenotype is modified in a B6 x SJL hybrid background. To identify these modifier loci, we performed genome scans of the hybrid Unc5c(-/-) mice and found a major SJL-derived suppressor locus on Chromosome 17. Our results provide the first evidence that genes involved in netrin signaling are necessary for proper mammalian spinal motor axon development and trochlear axon guidance. In addition, they demonstrate the importance of modifier genes in vertebrate axonal guidance.

Animals↗

PubMed2005

[The epitope analysis of beta Netrin and preparation and characterization of its antibodies].

AIM: To prepare and characterize anti-human beta-Netrin antibodies. METHODS: B cell dominant epitopes of human beta-Netrin C-terminal 114 amino acid sequences were predicated by the GoldKey software. One of the epitopes was synthesized and coupled with bovine serum album (BSA) by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC). The BALB/c mice were immunized with the coupled protein. The splenocytes of immunized mice were fused with Sp2/0 cells by routine method and the hybridomas were selected in HAT medium. The hybridoma cells secreting specific antibody were detected by ELISA and cloned by limiting dilution. The titer specificity, and Ig subclass of anti-beta-Netrin mAbs were characterized by ELISA, Western blot and immunocytochemical staining. In addition, New Zealand rabbits were immunized with the coupled protein to prepare polyclonal antibody against beta-Netrin. The specificity of the antiserum was verified by Western blot. RESULTS: A 16-mer peptide NH2-FRGKRTLYPES-WTDRG-COOH was the dominant epitope of the B cells. Synthesized peptide coupled with BSA was used as the immunogen to immunize BALB/c mice. Three hybridoma cell lines that stably secrete specific mAbs were obtained. The result of immunocytochemical staining showed that prepared mAb specifically recognize the antigen in the neuronal cells. The polyclonal antibody against beta-Netrin had high specificity. Western blot analysis showed that the antiserum bind with the prokaryotically expressed beta-Netrin specifically. CONCLUSION: Using the synthesized peptides as hapten, we have prepared epitope-specific mAbs and pAb against beta-Netrin successfully.

Amino Acid Sequence↗

PubMed2006

Netrins promote developmental and therapeutic angiogenesis.

Axonal guidance and vascular patterning share several guidance cues, including proteins in the netrin family. We demonstrate that netrins stimulate proliferation, migration, and tube formation of human endothelial cells in vitro and that this stimulation is independent of known netrin receptors. Suppression of netrin1a messenger RNA in zebrafish inhibits vascular sprouting, implying a proangiogenic role for netrins during vertebrate development. We also show that netrins accelerate neovascularization in an in vivo model of ischemia and that they reverse neuropathy and vasculopathy in a diabetic murine model. We propose that the attractive vascular and neural guidance functions of netrins offer a unique therapeutic potential.

Angiogenesis Inducing Agents↗

PubMed2004

Netrin requires focal adhesion kinase and Src family kinases for axon outgrowth and attraction.

Although netrins are an important family of neuronal guidance proteins, intracellular mechanisms that mediate netrin function are not well understood. Here we show that netrin-1 induces tyrosine phosphorylation of proteins including focal adhesion kinase (FAK) and the Src family kinase Fyn. Blockers of Src family kinases inhibited FAK phosphorylation and axon outgrowth and attraction by netrin. Dominant-negative FAK and Fyn mutants inhibited the attractive turning response to netrin. Axon outgrowth and attraction induced by netrin-1 were significantly reduced in neurons lacking the FAK gene. Our results show the biochemical and functional links between netrin, a prototypical neuronal guidance cue, and FAK, a central player in intracellular signaling that is crucial for cell migration.

Analysis of Variance↗

PubMed2003

Developmentally regulated expression of Netrin-1 and -3 in the embryonic mouse molar tooth germ.

The Netrins form a small, conserved family of laminin-related signaling proteins regulating axon guidance in the developing nervous system. Here, we analyzed the roles of Netrin-1 and -3 in trigeminal axon guidance to the first lower molar of the embryonic mouse. Netrin-1 showed a restricted epithelial expression domain buccal to the tooth germ, toward which the pioneer tooth axons initially appear to navigate. Later, before birth, transcripts were colocalized with nerve fibers around the bell stage tooth germ. Analysis of Netrin-1-deficient mice, however, did not reveal any obvious disturbances in the axon growth or pattern of tooth innervation. In contrast, Netrin-3 showed a prominent, distinct expression in the axon pathway and target field mesenchyme around the tooth. Hence, it is possible that Netrin-3 may regulate pioneer axon growth toward and within the embryonic tooth target field.

Animals↗

PubMed1996

Neuroglia and pioneer neurons express UNC-6 to provide global and local netrin cues for guiding migrations in C. elegans.

Netrins are laminin-related proteins that guide circumferential migrations on the ectoderm. To understand how netrin cues direct cell movements, we examined the expression of nematode netrin UNC-6 from embryo to adult. UNC-6 is expressed in 12 types of neuroglia and neurons, creating a hierarchy of netrin cues in the developing nervous system. Comparing gene expression pattern with in vivo phenotypes, we suggest how multiple netrin cues, each with a characteristic role, guide cells and axons during development. We also present the molecular analysis of selective loss-of-function and null alleles. The results indicate that the biological activities of netrins are mediated through distinct protein domains. Subtle mutations in domain VI can produce selective defects in both direction- and tissue-specific guidance. EGF-like module V-2 is essential for dorsal guidance activity; we infer this module is important for interactions between UNC-6 and the dorsal guidance receptor UNC-5.

Amino Acid Sequence↗