Graded Activity of Transcription Factor Runx3 Specifies the Laminar Termination Pattern of Sensory Axons in the Developing Spinal Cord

Different functional classes of dorsal root ganglion sensory neurons project their axons to distinct target zones within the developing spinal cord. To explore the mechanisms that link sensory neuron subtype identity and axonal projection pattern, we analyzed the roles of Runx and ETS transcription factors in the laminar targeting of sensory afferents. Gain- and loss-of-function studies in chick embryos reveal that the status of Runx3 expression is a major determinant of the dorso-ventral position of termination of proprioceptive and cutaneous sensory axons. In addition, the level of expression and/or activity of Runx3 in individual proprioceptive sensory neurons appears to specify whether their axons terminate in intermediate or ventral regions. Our findings suggest that the selectivity of Runx3 expression, and its level of activity, control sensory afferent targeting in the developing spinal cord.