Evx1 is a postmitotic determinant of V0 interneuron identity in the spinalcord

Interneurons in the ventral spinal cord are essential for coordinated locom otion in vertebrates. During embryogenesis, the V0 and V1 classes of ventra l interneurons are defined by expression of the homeodomain transcription f actors Evx1/2 and En1, respectively. In this study, we show that Evx1 V0 in terneurons are locally projecting intersegmental commissural neurons. In Ev x1 mutant embryos, the majority of V0 interneurons fail to extend commissur al axons. Instead, they adopt an En1-like ipsilateral axonal projection and ectopically express En1, indicating that V0 interneurons are transfated to a V1 identity. Conversely, misexpression of Evx1 represses En1, suggesting that Evx1 may suppress the V1 interneuron differentiation program. Our fin dings demonstrate that Evx1 is a postmitotic determinant of V0 interneuron identity and reveal a critical postmitotic phase for neuronal determination in the developing spinal cord.