Ephrin-B1 Promotes Dendrite Outgrowth on Cerebellar Granule Neurons 1 M. Teresa Moreno-Flores, Ester Martı´n-Aparicio, Jesu ´s A ´ vila, Javier Dı´az-Nido, and Francisco Wandosell 2 Centro de Biologı ´a Molecular “Severo Ochoa” (CSIC-UAM), Facultad de Ciencias, Universidad Auto ´noma de Madrid, 28049 Madrid, Spain Ephrins are developmentally regulated molecules that may contribute to axonal pathfinding through their bind- ing to Eph receptor tyrosine kinases. In many cases, ephrins act as negative molecules that stimulate growth cone collapse, although some forms may promote axonal growth. Here, we have addressed the role played by eph- rin-B1 during rat postnatal cerebellar development. Eph- rin-B1 is expressed by both granule and Purkinje neurons whereas EphB is present in granule neurons in early post- natal cerebellum at a time coincident with axonal and dendrite outgrowth. Stably transfected 3T3 cells overex- pressing ephrin-B1 enhance survival and neurite growth from cultured cerebellar granule neurons, an effect that is inhibited by the presence of an excess of a soluble EphB protein. Ephrin-B1-induced neuritogenesis is correlated with an increased expression of certain neuronal-specific microtubule-associated proteins (MAPs). Cerebellar gran- ule neurons plated on stably transfected 3T3 cells over- expressing ephrin-B1 show an up-regulation of the ex- pression of axonal MAPs such as Tau and phosphorylated MAP2C compared with neurons cultured on control 3T3 cells. The level of expression of these axonal MAPs is similar to that found in neurons plated on poly- L-lysine. Interestingly, there is a noteworthy up-regulation of so- matodendritic MAPs such as high-molecular-weight MAP2 and mode II-phosphorylated MAP1B in neurons cultured on stably transfected 3T3 cells overexpressing ephrin-B1 compared with neurons plated on either control 3T3 cells or poly- L-lysine. In view of these data, we sug- gest that ephrin-B1 favors dendritogenesis of granule neurons during cerebellum development. INTRODUCTION In the developing brain, axonal growth and pathfind- ing is controlled by different cell surface molecules including the Eph family of receptor tyrosine kinases (RTK) and their ephrin ligands (Gale et al., 1996; Tessier- Lavigne and Goodman, 1996). Eph and ephrins can be subdivided into two types (type A and type B). Type A ephrins, which are attached to membranes by a glycosyl phosphatidyl inositol (GPI) moiety, bind to type A Eph receptors. In contrast, type B ephrins, which are integral membrane proteins with a single transmembrane do- main and a cytoplasmic tail, preferentially associate with type B Eph receptors (for review see Frisen et al., 1999; Wilkinson, 2000). Normally, Eph/ephrin interactions result in axonal repulsion and growth cone collapse (Drescher et al., 1995; Friedman and O’Leary, 1996; Muller et al., 1996; Nakamoto et al., 1996; Orike and Pini, 1996; Meima et al., 1997a; Orioli and Klein, 1997; Holland et al., 1998; Yue et al., 1999). However, additional functions of Eph/ephrin during neuronal development may be postulated after the analysis of some mice with targeted disruptions of genes encoding for Eph receptors. These mutants re- vealed incomplete axonal projections in important tracts such as the anterior commissure, corpus callosum (Henkemeyer et al., 1996; Orioli et al., 1996), and the tract from superior collicullus to contralateral inferior collicullus (Park et al., 1997). Indeed, Eph/ephrin inter- actions may even favor axonal growth in some cases. For instance, during development of the septohip- pocampal system, axons from lateral hippocampal neu- rons (which have a low level of EphA5 expression) project to the ventrolateral septum (which has a high level of ephrin-A2 expression), whereas axons from medial hippocampal neurons (with a high level of 1 Supplementary data for this article are available on IDEAL (www. idealibrary.com). 2 To whom correspondence should be addressed. Fax: 34 91-3974799. E-mail: FWANDOSELL@cbm.uam.es. Molecular and Cellular Neuroscience 20, 429 –446 (2002) doi:10.1006/mcne.2002.1128 MCN 1044-7431/02 $35.00 © 2002 Elsevier Science (USA) All rights reserved. 429