During nervous system development, spinal commissural axons project toward and across the ventral midline. They are guided in part by netrin-1, made by midline cells, which attracts the axons by activating the netrin receptor DCC. However, previous studies suggest that additional receptor components are required. Here, we report that the Down's syndrome Cell Adhesion Molecule (DSCAM), a candidate gene implicated in the mental retardation phenotype of Down's syndrome, is expressed on spinal commissural axons, binds netrin-1, and is necessary for commissural axons to grow toward and across the midline. DSCAM and DCC can each mediate a turning response of these neurons to netrin-1. Similarly, Xenopus spinal neurons exogenously expressing DSCAM can be attracted by netrin-1 independently of DCC. These results show that DSCAM is a receptor that can mediate turning responses to netrin-1 and support a key role for netrin/DSCAM signaling in commissural axon guidance in vertebrates.

Tumor necrosis factor-␣ (TNF␣), a potent proinflammatory cytokine, is released from cells by proteolytic cleavage of a membrane-anchored precursor. The TNF-␣ converting enzyme (TACE; a disintegrin and metalloprotease17; ADAM17) is known to have a key role in the ectodomain shedding of TNF␣ in several cell types. However, because purified ADAMs 9, 10, and 19 can also cleave a peptide corresponding to the TNF␣ cleavage site in vitro, these enzymes are considered to be candidate TNF␣ sheddases as well. In this study we used cells lacking ADAMs 9, 10, 17 (TACE), or 19 to address the relative contribution of these ADAMs to TNF␣ shedding in cell-based assays. Our results corroborate that ADAM17, but not ADAM9, -10, or -19, is critical for phorbol ester-and pervanadate-stimulated release of TNF␣ in mouse embryonic fibroblasts. However, overexpression of ADAM19 increased the constitutive release of TNF␣, whereas overexpression of ADAM9 or ADAM10 did not. This suggests that ADAM19 may contribute to TNF␣ shedding, especially in cells or tissues where it is highly expressed. Furthermore, we used mutagenesis of TNF␣ to explore which domains are important for its stimulated processing by ADAM17. We found that the cleavage site of TNF␣ is necessary and sufficient for cleavage by ADAM17. In addition, the ectodomain of TNF␣ makes an unexpected contribution to the selective cleavage of TNF␣ by ADAM17: it prevents one or more other enzymes from cleaving TNF␣ following PMA stimulation. Thus, selective stimulated processing of TNF␣ by ADAM17 in cells depends on the presence of an appropriate cleavage site as well as the inhibitory role of the TNF ectodomain toward other enzymes that can process this site. TNF␣1 is a pro-inflammatory cytokine that has a critical role in autoimmune disorders such as rheumatoid arthritis and Crohn's disease (1, 2). TNF␣ is synthesized as a trimeric type II membrane-anchored precursor referred to as pro-TNF␣ (3). Upon cleavage in the juxtamembrane domain, the mature form of TNF␣ is released from the cell and can enter the blood stream (4, 5). This proteolytic release of TNF␣ from the membrane is referred to as "protein ectodomain shedding" (6, 7). Protein ectodomain shedding also affects the function of a variety of other structurally and functionally diverse molecules on the cell surface, including cytokines and growth factors, their receptors, adhesion proteins, and other molecules, such as the amyloid precursor protein, Notch and Delta (6 -9).Because of the critical role of TNF␣ in rheumatoid arthritis, considerable efforts have been made to identify the TNF␣ convertase. ADAM17 (a disintegrin and metalloprotease 17, also referred to as TNF␣ converting enzyme or TACE) is considered to be an important, if not the major, sheddase for TNF␣ (10, 11). ADAM17 was initially purified based on its ability to process a peptide, which mimics the physiological cleavage site of TNF␣, in exactly the same position that is used by the TNF␣ converting activity in cells (see Table I and Refs. 10 and 11). A targeted de...

Cellular senescence has been viewed as a tumor suppression mechanism and also as a contributor to individual aging. Widespread shortening of 3 ′ untranslated regions (3 ′ UTRs) in messenger RNAs (mRNAs) by alternative polyadenylation (APA) has recently been discovered in cancer cells. However, the role of APA in the process of cellular senescence remains elusive. Here, we found that hundreds of genes in senescent cells tended to use distal poly(A) (pA) sites, leading to a global lengthening of 3 ′ UTRs and reduced gene expression. Genes that harbor longer 3 ′ UTRs in senescent cells were enriched in senescence-related pathways. Rras2, a member of the Ras superfamily that participates in multiple signal transduction pathways, preferred longer 3 ′ UTR usage and exhibited decreased expression in senescent cells. Depletion of Rras2 promoted senescence, while rescue of Rras2 reversed senescence-associated phenotypes. Mechanistically, splicing factor TRA2B bound to a core "AGAA" motif located in the alternative 3 ′ UTR of Rras2, thereby reducing the RRAS2 protein level and causing senescence. Both proximal and distal poly(A) signals showed strong sequence conservation, highlighting the vital role of APA regulation during evolution. Our results revealed APA as a novel mechanism in regulating cellular senescence.