Sensory axons inhibit motor axon regeneration in vitro

During mammalian embryonic development sensory and motor axons interact as an integral part of the pathfinding process. During regeneration, however, little is known of their interactions with one another. It is thus possible that sensory axons might influence motor axon regeneration in ways not currently appreciated. To explore this possibility we have developed an organotypic model of post-natal nerve regeneration in which sensory and motor axons are color-coded by modality. Motor axons that express yellow fluorescent protein (YFP) and sensory axons that express red fluorescent protein (RFP) are blended within a three-dimensional segment of peripheral nerve. This nerve is then transected, allowing axons to interact with one another as they grow out on a collagen/laminin gel that is initially devoid of directional cues. Within hours it is apparent that sensory axons extend more rapidly than motor axons and precede them during the early stages of regeneration, the opposite of their developmental order. Motor axons thus enter an environment already populated with sensory axons, and they adhere to these axons throughout most of their course. As a result, motor axon growth is reduced dramatically. Physical delay of sensory regeneration, allowing motor axons to grow ahead, restores normal motor growth; direct axonal interactions on the gel, rather than some other aspect of the model, are thus responsible for motor inhibition. Potential mechanisms for this inhibition are explored by electroporating siRNA to the neural cell adhesion molecule (NCAM) and the L1 adhesion molecule (L1CAM) into dorsal root ganglia (DRGs) to block expression of these molecules by regenerating sensory axons. Although neither maneuver improved motor regeneration, the results were consistent with early receptor-mediated signaling among axons rather than physical adhesion as the mechanism of motor inhibition in this model.