Reelin Binds a3b1 Integrin and Inhibits Neuronal Migration

1999), three mutant mice show a remarkably similar cellular phenotype, suggesting that the corresponding proteins represent a biochemical pathway that mediates Lori Dulabon,*‖ Eric C. Olson,†‖ Mary G. Taglienti,‡ Scott Eisenhuth,* Barbara McGrath,* Christopher A. Walsh,† proper formation of cerebral cortical lamination. In the Jordan A. Kreidberg,‡ and E. S. Anton*§# reeler mouse, the disorganized cortex is approximately *Department of Biology and Neuroscience inverted, with early born neurons occupying abnormal Pennsylvania State University superficial positions and later born neurons adopting University Park, Pennsylvania 16802 abnormal deep positions (Caviness and Sidman, 1973). †Department of Neurology Reelin, the product of the reeler gene (D’Arcangelo et Beth Israel Deaconess Medical Center al., 1995), acts non cell autonomously (Miyata et al., Harvard Institutes of Medicine 1997), and the protein is synthesized and secreted in ‡Department of Medicine the cerebral cortex predominantly by the Cajal-Retzius Children’s Hospital cell of the marginal zone, the outermost layer of the and Department of Pediatrics developing cortex (D’Arcangelo et al., 1995; Ogawa et Harvard Medical School al., 1995). Boston, Massachusetts 02115 Mutations at a second locus, dab1, have been found to produce a phenotype indistinguishable from mutations in the reeler gene (Howell et al., 1997b; Sheldon et al., 1997; Ware et al., 1997). The Dab1 gene encodes Summary a cytoplasmic adaptor protein (Dab1) expressed by neurons in the developing cortical plate, suggesting that Dab1 represents a link in the signaling pathway that Mice that are mutant for Reelin or Dab1, or doubly receives the Reelin signal. This idea is confirmed by the mutant for the VLDL receptor (VLDLR) and ApoE reobservation that Reelin expression is normal in the dab1 ceptor 2 (ApoER2), show disorders of cerebral cortical mutant cortex (Gonzalez et al., 1997), but Dab1 protein lamination. How Reelin and its receptors regulate lamaccumulates in the reeler mouse brain (Rice et al., 1998), inar organization of cerebral cortex is unknown. We and Dab1 is phosphorylated in response to applied reshow that Reelin inhibits migration of cortical neurons combinant Reelin (Howell et al., 1999a). The phosphoand enables detachment of neurons from radial glia. tyrosine-interacting/phosphotyrosine binding (PI/PTB) Recombinant and native Reelin associate with a3b1 domain of Dab1 binds proteins that contain an NPXY integrin, which regulates neuron–glia interactions and motif (Howell et al., 1997a, 1999b; Trommsdorff et al., is required to achieve proper laminar organization. The 1998), a motif that has been implicated in clathrin-medieffect of Reelin on cortical neuronal migration in vitro ated endocytosis (Chen et al., 1990) and integrin signaland in vivo depends on interactions between Reelin ing (Law et al., 1999). and a3b1 integrin. Absence of a3b1 leads to a reducMore recently, mice with compound mutations in both tion of Dab1, a signaling protein acting downstream the VLDL receptor (VLDLR) and the ApoE receptor 2 of Reelin. Thus, Reelin may arrest neuronal migration (ApoER2) have been found to have a phenotype indistinguishable from reeler and dab1 mutants (Trommsdorff and promote normal cortical lamination by binding et al., 1999). VLDLR and ApoER2 are members of the a3b1 integrin and modulating integrin-mediated cellulow-density lipoprotein (LDL) receptor superfamily, and lar adhesion. they interact with Dab1 in two-hybrid screens through the PI/PTB domain of Dab1 and the NPXY motif of LDL superfamily members (Howell et al., 1997a; Trommsdorff Introduction et al., 1998). The NPXY motif of LDL receptor family members is essential for clathrin-mediated endocytosis Normal development of the mammalian cerebral cortex (Chen et al., 1990). Recent studies have demonstrated requires the coordinated migration of postmitotic neuthat both recombinant ApoER2 and the VLDLR bind rons from the proliferative ventricular zone to the outerReelin and that this binding leads to both the tyrosine most layer of the developing cortical plate. Migrating phosphorylation of Dab1 and, in the case of VLDLR, the neurons travel hundreds to thousands of cell body internalization of the receptor and Reelin (D’Arcangelo lengths through varying environments, migrate past preet al., 1999; Hiesberger et al., 1999). Thus, there is comviously generated neuronal cohorts of the cortical plate, pelling evidence that Reelin, VLDLR, ApoER2, and Dab1 and organize themselves into precise layers. Although function in a common signaling pathway between Cajala number of genes have recently been implicated in this Retzius cells and cortical plate neurons. process (Komuro and Rakic, 1998; Hatten, 1999; Walsh, Despite the identification of other molecules in the Reelin signaling pathway, the response of cortical neurons to the Reelin signal remains unknown. Reelin is § To whom correspondence should be addressed (e-mail: esa5@ thought to regulate cortical plate organization by initiatpsu.edu). ing the splitting of preplate into the marginal zone and ‖ These authors contributed equally to this work. the subplate, by acting as an attractant of neurons to # Present address: University of North Carolina Neuroscience Center the top of the cortical plate, or by functioning as a stop and Department of Cell and Molecular Physiology, The University signal for neuronal migration at the interface between of North Carolina School of Medicine, Chapel Hill, North Carolina