EphA4/ephrin interactions in motor axon guidance

The process by which motor axons reliably navigate to their muscle targets has puzzled developmental neurobiologists for decades. To treat neuromuscular diseases and injuries, it is essential to understand the mechanisms underlying motor axon pathfinding. It is well known that elongating motor axons use cues in their environment to form highly stereotyped and precise projections. Motor neurons in the lateral and medial divisions of the lateral motor column (LMC) project axons to dorsal and ventral limb musculature, respectively. Throughout the body axis, motor neurons in the medial motor column (MMC) innervate body wall musculature. We have investigated the involvement of EphA4, ephrin-A2, and ephrin-A5 in the development of motor axon projections to the avian hindlimb.; The Ephs are a family of receptor tyrosine kinases (RTKs) and their ligands, the ephrins. Ephs can be divided into A and B subclasses. EphA RTKs typically bind ephrin-A ligands and EphB RTKs interact with ephrin-B ligands. Members of the Eph family of RTKs and ephrins play known roles in the establishment of neural cytoarchitecture and synaptic connectivity.; During the development of dorsoventral axon projections, EphA4 localized to axons of the LMC(I) whereas ephrins were concentrated in the ventral limb mesoderm. The function of EphA4-ephrin interactions in axon pathway selection was tested using in ovo electroporation to ectopically express full-length EphA4 in LMC(m) motor neurons or ephrin-A5 in the hindlimb. Following electroporation motor axon projections were analyzed. Motor neurons in the LMC(m) that ectopically expressed EphA4 extended their axons dorsally in the limb, instead of their normal ventral projections. Ectopic expression of ephrin-A5 throughout the hindlimb prohibited entry of EphA4-positive axons. These results strongly suggest that repulsive EphA4/ephrin interactions are repulsive for LMC(I) axons.; Intriguingly, MMC neurons also express EphA4 RTK and project axons into regions of high ephrin expression. A portion of MMC motor axons project into novel ephrin-expressing territories when prematurely expressing EphA4. The function of ephrin-A5 in MMC guidance is currently being assessed by ectopic expression of ephrin-A5 in the somite. These data suggest that EphA4/ephrin interactions may exert a positive influence on MMC axons.