The RET (rearranged during transfection) proto-oncogene encodes a tyrosine kinase receptor involved in both multiple endocrine neoplasia type 2 (MEN 2), an inherited cancer syndrome, and Hirschsprung disease (HSCR), a developmental defect of enteric neurons. We report here that the expression of RET receptor induces apoptosis. This pro-apoptotic effect of RET is inhibited in the presence of its ligand glial cell line-derived neurotrophic factor (GDNF). Furthermore, we present evidence that RET induces apoptosis via its own cleavage by caspases, a phenomenon allowing the liberation/exposure of a pro-apoptotic domain of RET. In addition, we report that Hirschsprung-associated RET mutations impair GDNF control of RET pro-apoptotic activity. These results indicate that HSCR may result from apoptosis of RET-expressing enteric neuroblasts.

The expression of DCC (deleted in colorectal cancer) is often markedly reduced in colorectal and other cancers. However, the rarity of point mutations identified in DCC coding sequences and the lack of a tumor predisposition phenotype in DCC hemizygous mice have raised questions about its role as a tumor suppressor. DCC also mediates axon guidance and functions as a dependence receptor; such receptors create cellular states of dependence on their respective ligands by inducing apoptosis when unoccupied by ligand. We now show that DCC drives cell death independently of both the mitochondria-dependent pathway and the death receptor͞caspase-8 pathway. Moreover, we demonstrate that DCC interacts with both caspase-3 and caspase-9 and drives the activation of caspase-3 through caspase-9 without a requirement for cytochrome c or Apaf-1. Hence, DCC defines an additional pathway for the apoptosome-independent caspase activation. V ogelstein and his colleagues (1) have shown that the development of colonic carcinoma from normal colonic epithelium is associated with the mutation of a specific set of genes. Allelic deletions (loss of heterozygosity) on chromosome 18q in more than 70% of primary colorectal tumors prompted the search for a tumor suppressor gene at that locus. This search led to the cloning of a putative cell-surface receptor, DCC (deleted in colorectal cancer) (1). DCC expression was then shown to be markedly reduced in more than 50% of colorectal tumors. Moreover, the loss of DCC expression is not restricted to colon carcinoma but has been observed in other tumor types, including carcinoma of the stomach, pancreas, esophagus, prostate, bladder, breast, male germ cell tumors, neuroblastomas, gliomas, and some leukemias (2, 3). However, proof that DCC is a tumor suppressor gene remains inconclusive (4, 5).DCC encodes an approximately 200-kDa type I membrane protein of 1,447 amino acids, which displays homology in its extracellular domain with cell adhesion molecules (2), suggesting that DCC may play a role in cell-cell or cell-matrix interactions (6). However, DCC-mediated cell aggregation has not been firmly established (7). Recently, Tessier-Lavigne and collaborators (8, 9) have suggested that DCC may function as a component of a receptor complex that mediates the effects of the axonal chemoattractant netrin-1. The role of DCC in mediating growth cone extension has been supported by the analysis of the DCC knockout mice, which display abnormal brain development (4). However, the signaling transduction of netrin-1 through DCC that results in axon outgrowth is mainly unknown. In response to netrin-1 binding, DCC has been shown to interact with other netrin-1 receptors like UNC5H (i.e., three members UNC5H1, -2, and -3) (10) or the adenosine A2b receptor shown to transduce cAMP production upon netrin-1 binding (11). Recently, it also has been proposed that Frazzled, the Drosophila ortholog of DCC, is not, in certain circumstances, a transducing receptor but rather a carrier for the cue netrin-1 that allows netr...

Neuronal growth cones are guided to their targets by attractive and repulsive guidance cues. In mammals, netrin-1 is a bifunctional cue, attracting some axons and repelling others. Deleted in colorectal cancer (Dcc) is a receptor for netrin-1 that mediates its chemoattractive effect on commissural axons, but the signalling mechanisms that transduce this effect are poorly understood. Here we show that Dcc activates mitogen-activated protein kinase (MAPK) signalling, by means of extracellular signal-regulated kinase (ERK)-1 and -2, on netrin-1 binding in both transfected cells and commissural neurons. This activation is associated with recruitment of ERK-1/2 to a Dcc receptor complex. Inhibition of ERK-1/2 antagonizes netrin-dependent axon outgrowth and orientation. Thus, activation of MAPK signalling through Dcc contributes to netrin signalling in axon growth and guidance.

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.

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Sonic hedgehog (Shh) and its main receptor Patched (Ptc) are implicated in both neural development and tumorigenesis1, 2. Beside the classic morphogen activity of Shh, Shh is also a survival factor3, 4. Along this line, Ptc has been shown to function as a dependence receptor, inducing apoptosis in the absence of Shh, while its pro-apoptotic activity is blocked in Shh presence5. Here we show that, in the absence of its ligand, Ptc interacts with the adaptor protein DRAL/FHL2. DRAL/FHL2 is required for the pro-apoptotic activity of Ptc both in immortalized cells and during neural tube development in chick embryo. We demonstrate that, in the absence of Shh, Ptc recruits a protein complex that includes DRAL, the CARD containing domain proteins TUCAN or NALP1 and the apical caspase-9. Ptc triggers caspase-9 activation and enhances cell death via a caspase-9-dependent mechanism. Thus, we propose that, upon absence of its ligand Shh, the dependence receptor Ptc serves as the anchor for a caspase-activating complex that includes DRAL, a CARD domain containing protein and caspase-9.

The β-amyloid precursor protein (APP) is an orphan transmembrane receptor whose physiological role is largely unknown. APP is cleaved by proteases generating amyloid-β (Aβ) peptide, the main component of the amyloid plaques that are associated with Alzheimer’s disease. Here, we show that APP binds netrin-1, a multifunctional guidance and trophic factor. Netrin-1 binding modulates APP signaling triggering APP intracellular domain (AICD)-dependent gene transcription. Furthermore, netrin-1 binding suppresses Aβ peptide production in brain slices from Alzheimer model transgenic mice. In this mouse model, decreased netrin-1 expression is associated with increased Aβ concentration, thus supporting netrin-1 as a key regulator of Aβ production. Finally, we show that netrin-1 brain administration in Alzheimer model transgenic mice may be associated with an amelioration of the Alzheimer’s phenotype.

DCC (Deleted in Colorectal Cancer) is a putative tumor suppressor whose expression is lost in numerous cancers and whose tumor suppressor activity appears to be dependent on its ability to trigger apoptosis when disengaged by its ligand netrin-1. In this sense, netrin-1 is a survival factor that controls tumorigenesis. However, netrin-1 is also the prototypical axon guidance cue and has been shown to orient many neurons or axons, especially commissural axons, during spinal cord development. Here we show that netrin-1 is not only an attractive cue for developing commissural axons but also promotes their survival. In primary neuronal culture, in mice or in chick embryos, netrin-1 inhibits the proapoptotic activity of DCC in developing commissural neurons. Thus, adequate commissural neurons navigation requires both the attractive activity of netrin-1 and the anti-apoptotic function of this cue.apoptosis ͉ deleted in colorectal cancer ͉ nervous system development ͉ dependence receptor ͉ axon guidance T he development of colonic carcinoma from normal colonic epithelium is associated with the mutation of a specific set of genes (1). Allelic deletions [loss of heterozygosity (LOH)] on chromosome 18q in 70% of primary colorectal tumors prompted the search for a tumor suppressor gene at that locus, leading to the cloning of a putative cell-surface receptor, DCC (Deleted in Colorectal Cancer) (2). DCC expression was then shown to be markedly reduced in 50% of colorectal tumors. Moreover, the loss of DCC expression is not restricted to colon carcinoma, but has been observed in many other tumors, suggesting that DCC is a tumor suppressor (3).However, DCC is also the main component of a receptor complex that mediates the activity of the secreted axon-guidance molecule netrin-1 (4, 5). The key role of DCC and netrin-1 in mediating growth cone extension is supported by a large number of studies, such as the analysis of DCC and netrin-1 knock-out mice, which display the disorganization of many axonal tracts in the central nervous system (6).The link between a tumor suppressor in colorectal cancer and a receptor mediating axon outgrowth and turning remained unclear until recently. We proposed that the tumor suppressor function could be explained by the fact that DCC is a dependence receptor (7). DCC is functionally related to other dependence receptors such as p75 NTR , the common neurotrophin receptor, RET, Patched, UNC5H, neogenin, ALK, or TrkC (8-13). Such receptors create cellular states of dependence to their respective ligands by inducing apoptosis when unoccupied but inhibiting apoptosis in the presence of their ligands (12). We and others have shown that the in vitro expression of DCC induces apoptosis in the absence of netrin-1 (7, 14), while the presence of netrin-1 blocks this proapoptotic activity (15).The fact that DCC displays such proapoptotic activity when unbound to its ligand has led to the hypothesis that DCC normally functions to kill tumor cells that grow in an inappropriate context (e.g., local growth in a ...