A paradoxical gradient of a basal lamina-associated repellent is essential for pathfinding by the Ti1 pioneer axons in cockroach embryos.

Perturbations of in situ axon growth with proteolytic enzymes and monoclonal antibodies were used to determine the role of gradient guidance cues in the formation of the Ti1 pioneer axon trajectory in cultured cockroach embryos. Treatment with enzymes that degrade the basal lamina indicated that this substrate contains both an elastase-sensitive proximal directing cue and a collagenase-sensitive distal directing cue. The latter is shown to be a repellent of axon growth and is identical to the PROD-2 antigen that is distributed in a gradient along the proximal-distal axis of the leg with high levels in proximal regions. This means that throughout the course of their growth the axons extend in the direction of increasing levels of repellent. At a critical decision point in the trajectory the axons change both the direction of growth and the substrate to which their growth cones adhere. PROD-2 plays an essential role in both of these processes.