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Cues in the extracellular environment of the nervous system guide migrating cells and axons to their targets. Netrins are cues that can function as attractants and repellents. In Caenorhabditis elegans, UNC-5, a transmembrane protein, has been implicated in certain responses, with the loss of unc-5 function leading to migration defects. A report identifies two UNC-5 vertebrate homologues, defining a new immunoglobulin subfamily. This indicates that the UNC-5 homologues and the mouse rostral cerebellar malformation gene are netrin-binding proteins.

The Deleted in colorectal cancer (DCC) gene was identified as a potential tumour-suppressor gene on human chromosome 18q, although it has not been conclusively implicated as the colorectal tumour-suppressor gene. Investigations indicate that DCC is an axonal chemoattractant netrin-1 receptor. The effects of inactivation of the mouse homologue of DCC are similar to those in netrin-1-deficient mice.

Heparan sulfate (HS) has been implicated in a wide range of cell signaling. Here we report a novel mechanism in which extracellular removal of 6-O-sulfate groups from HS by the endosulfatases, Sulf1 and Sulf2, is essential for axon guidance during development. In Sulf1/2 double knockout (DKO) mice, the corticospinal tract (CST) was dorsally displaced on the midbrain surface. In utero electroporation of Sulf1/2 into radial glial cells along the third ventricle, where Sulf1/2 mRNAs are normally expressed, rescued the CST defects in the DKO mice. Proteomic analysis and functional testing identified Slit2 as the key molecule associated with the DKO phenotype. In the DKO brain, 6-O-sulfated HS was increased, leading to abnormal accumulation of Slit2 protein on the pial surface of the cerebral peduncle and hypothalamus, which caused dorsal repulsion of CST axons. Our findings indicate that postbiosynthetic desulfation of HS by Sulfs controls CST axon guidance through fine-tuning of Slit2 presentation.

Wnt signaling plays an important role in governing cell fate decisions. Coiled-coil-DIX1 (Ccd1), Dishevelled (Dvl), and Axin are signaling proteins that regulate the canonical pathway by controlling the stability of a key signal transducer β-catenin. These proteins contain the DIX domain with a ubiquitin-like fold, which mediates their interaction in the β-catenin destruction complex through dynamic head-to-tail polymerization. Despite high sequence similarities, mammalian Ccd1 shows weaker stimulation of β-catenin transcriptional activity compared with zebrafish (z) Ccd1 in cultured cells. Here, we show that the mouse (m) Ccd1 DIX domain displays weaker ability for homopolymerization than that of zCcd1. Furthermore, X-ray crystallographic analysis of mCcd1 and zCcd1 DIX domains revealed that mCcd1 was assembled into a double-helical filament by the insertion of the β1-β2 loop into the head-to-tail interface, whereas zCcd1 formed a typical single-helical polymer similar to Dvl1 and Axin. The mutation in the contact interface of mCcd1 double-helical polymer changed the hydrodynamic properties of mCcd1 so that it acquired the ability to induce Wnt-specific transcriptional activity similar to zCcd1. These findings suggest a novel regulatory mechanism by which mCcd1 modulates Wnt signaling through auto-inhibition of dynamic head-to-tail homopolymerization.

Neuregulin-1 binds to ErbB3 and ErbB4 and regulates cancer proliferation and differentiation. Neuregulin-1 had been suggested to also react with ErbB2, but this argument becomes controversial. Here, we re-evaluated the cellular responses and ErbB2 interaction of neuregulin-1 in ErbB2 overexpressing cell lines. In a competitive ligand-binding assay, we detected significant replacement of [(35)S]-labeled neuregulin-1 with nano molar ranges of cold neuregulin-1 in L929 cells expressing ErbB2 alone and SKOV3 cells carrying sulf-1 cDNA but not in these parental cells. The concentration of neuregulin-1 significantly decreased thymidine incorporation and phosphorylation of ErbB2 (Tyr877, Tyr1396, and Tyr1121) in ErbB2-overexpressing cancer cells as well as in L929 cells expressing ErbB2. A crosslinking assay ascertained the presence of neuregulin-1 immunoreactivity in the ErbB2 immune complexes of L929 expressing ErbB2 alone. These results suggest that the higher concentrations of neuregulin-1 exert an anti-oncogenic activity to attenuate ErbB2 auto-phosphorylation potentially through its low-affinity interaction with ErbB2.