Gliolectin is a carbohydrate-binding protein (lectin) that mediates cell adhesion in vitro and is expressed by midline glial cells in the Drosophila melanogaster embryo. Gliolectin expression is maximal during early pathfinding

The formation of a functioning nervous system requires that axons extend along appropriate pathways to reach correct terminal fields during embryonic development. Axon pathfinding in a broad range of organisms is characterized by the ability of neuronal growth cones to discriminate between appropriate and inappropriate growth substrates (Bovolenta and Mason, 1987; Landmesser et al., 1990; Marcus et al., 1995; Stoeckli and Landmesser, 1998; Stoeckli et al., 1997). In Drosophila, such discrimination is especially evident at choice points where the consequences of continued extension versus axon redirection generate distinct morphologic structures (Isbister et al., 1999; Jacobs and Goodman, 1989a; Jacobs and Goodman, 1989b; Kolodkin et al., 1992). For example, partitioning of the total Drosophila melanogaster embryonic axon pool either into commissural bundles that cross the midline or into longitudinal processes that grow in the anteriorposterior direction creates a characteristic orthogonal axon scaffold (Klämbt and Goodman, 1991a; Klämbt and Goodman, 1991b; Klämbt et al., 1991). The function of a population of glial cells at the Drosophila embryonic midline (midline glial cells) is crucial to the guidance and organization of commissural and longitudinal fibers (Hummel et al., 2000; Hummel et al., 1999). Signals originating from the midline glia allow commissural axons to cross the midline and keep longitudinal axons away from the midline (Harris et al., 1996; Seeger et al., 1993; Stein and Tessier-Lavigne, 2001). Both the amount (dose) and the type (molecular identity) of signaling can determine growth cone behavior, dictating the co-requisite establishment of an axonglia interface that facilitates high-fidelity transmission of molecular signals (Kidd et al., 1998b; Winberg et al., 1998). While the identities and functions of relevant signaling molecules and receptor families have been elegantly described, comparatively little progress has been made towards understanding whether additional components of the neuronal and midline glial cell surfaces contribute to signal transmission by regulating the axon-glia interface. At the very least, it may be necessary for exploring growth cones to adhere to the midline glial surface before sufficient signal integration or sorting ensures that the maturing axon is appropriately routed. Among Drosophila molecules that mediate cell adhesion, only the Gliolectin protein is expressed in midline glia, coincident with the extension of commissural and longitudinal axon pathways (Tiemeyer and Goodman, 1996). Originally identified in an adhesion-based cloning screen for embryonically expressed carbohydrate-binding proteins (lectins), Gliolectin binds a subset of Nacetylglucosamine-terminated Drosophila glycans. That a 4585 Development 128, 4585-4595 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV8806