Gliolectin-mediated carbohydrate binding at the Drosophila midline ensuresthe fidelity of axon pathfinding

Gliolectin is a carbohydrate-binding protein (lectin) that mediates cell ad hesion in vitro and is expressed by midline glial cells in the Drosophila m elanogaster embryo. Gliolectin expression is maximal during early pathfindi ng of commissural axons across the midline (stages 12-13), a process that r equires extensive signaling and cell-cell interactions between the midline glia and extending axons. Deletion of the gliolectin locus disrupts the for mation of commissural pathways and also delays the completion of longitudin al pathfinding. The disruption in commissure formation is accompanied by re duced axon-glial contact, such that extending axons grow on other axons and form a tightly fasciculated bundle that arches over the midline. By contra st, pioneering commissural axons normally cross the midline as a distribute d array of fibers that interdigitate among the midline glia, maximizing con tact and, therefor, communication between axon and glia. Restoration of Gli olectin protein expression in the midline glia rescues the observed pathfin ding defects of null mutants in a dose-dependent manner. Hypomorphic allele s generated by ethylmethanesulfonate mutagenesis exhibit a similar phenotyp e in combination with a deletion and these defects are also rescued by tran sgenic expression of Gliolectin protein. The observed phenotypes indicate t hat carbohydrate-lectin interactions at the Drosophila midline provide the necessary surface contact to capture extending axons, thereby ensuring that combinatorial codes of positive and negative growth signals are interprete d appropriately.