Axonal growth of the spinal cord interneurons expressing a homophilic adhesion molecule SC1 ectopically

SC1/DM-GRASP/BEN, a cell adhesion molecule of the immunoglobulin super family, promotes the extension of neurites from neurons that express SC1 in culture, presumably by direct homophilic interactions. SC1 is specifically and transiently expressed on motoneurons during the period of their axonal growth, suggesting that it plays an important role in this growth. To explore this possibility, we expressed SC1 ectopically on the spinal cord interneurons of quail and chick embryos by in ovo electroporation at E3, when the motoneuron axonal growth starts. The axonal growth of the interneurons expressing chick SC1 was analyzed by immunohistochemistry with chick-specific anti-SC1 monoclonal antibody in quail, and by retrograde labeling with a dye in chick and quail embryos at E5. The majority of the axons of SC1-positive interneurons passed through the motor column and extended normally along the spinal cord basement membrane, but a few appeared to grow out from the cord. However, a dye back-labeling of the spinal nerves revealed that none of the interneurons were both SC1 and dye positive. These results suggest that the expression of SC1/DM-GRASP/BEN alone is insufficient for regulating the first step of the selective axonal pathfinding of motoneurons.