The extracellular matrix molecule Reelin is required for the correct positioning of neurons during the development of the forebrain. However, the mechanism of Reelin action on neuronal migration is poorly understood. Reelin is assumed to act on neurons directly, but it may also affect the differentiation of glial cells necessary for neuronal migration. Here we show that a regular glial scaffold fails to form in vivo in the dentate gyrus of mice deficient of Reelin or Disabled 1, a neuronal adaptor protein in the Reelin signaling pathway. A subset of these defects is observed in mice that lack 1-class integrins, known to bind Reelin. Moreover, recombinant Reelin induced branching of glial processes in vitro. Our data suggest that Reelin affects glial differentiation via Disabled 1 and 1-class integrin-dependent signaling pathways. D uring cortical development, postmitotic neurons migrate along radial glial fibers from the ventricular zone toward the marginal zone, forming the characteristic layered structure of the cerebral cortex (1-3). Early-generated neurons form the deep layers, and later-generated neurons migrate through the early formed layers and are positioned more superficially. Correct layering of cortical neurons requires the expression of Reelin, an extracellular matrix molecule. Reelin is expressed by CajalRetzius (CR) cells, early-generated transient neurons that are located in the marginal zone of the developing forebrain (4-6). In the reeler mutant mouse, which does not express Reelin, the normal inside-out layering of the cortex is reversed (7,8).Reelin binds to the very low-density lipoprotein receptor and the apolipoprotein E receptor 2, and mutant mice deficient of both receptors phenocopy the reeler mutant (9). The cytoplasmic domains of both receptors bind directly to the adaptor protein Disabled 1 (Dab1). Mutant mice deficient of Dab1 show a reeler phenotype, suggesting that interaction of very low-density lipoprotein receptor and apolipoprotein E receptor 2 with Dab1 is required to mediate the Reelin signal (10-13). Cadherin-related neuronal receptors (CNRs) were reported to bind Reelin (14), and also  1 -class integrins, expressed in both neurons and glial cells (15), may act as Reelin receptors (16). Members of the integrin family have been implicated in neuronal migration in the cerebral cortex (16-18), likely by regulating the anchorage of glial endfeet and the formation of the glial scaffold (19). It is unknown to what extent Reelin acts on these various receptors and which cell types, neurons or glial cells, are involved during different developmental periods.We provide here evidence that Reelin exerts its effects, at least in part, by regulating the development of the radial glial scaffold. The long processes of radial glial cells extend from the ventricular zone to the pial surface of the cerebral wall, thereby providing a template for radially migrating neurons (3). By studying the role of Reelin in hippocampal development, we first show that a regular radial glial scaffold fai...
