c‐Jun N‐Terminal Kinase 1 (JNK1) Is Required for Coordination of Netrin Signaling in Axon Guidance

The JNK family of MAPKs is involved in a large variety of physiological and pathological processes in brain development, such as, neural survival, migration and polarity as well as axon regeneration. However, whether JNK activation is involved in axon guidance remains unknown. Here, we provide evidence indicating the JNK pathway is required for netrin signaling in the developing nervous system. Netrin‐1 increased JNK1 in the presence of DCC or DSCAM, and the expression of both of them further enhanced netrin‐1‐induced JNK1 activity in vitro. Inhibition of JNK signaling either by expression of dominant negative MKK4, a JNK upstream activator, or a JNK inhibitor, SP600125, blocked netrin‐1‐induced JNK1 activation in HEK293 cells. Combination of the anti‐DCC function‐blocking antibody with introduction of DSCAM shRNA in primary neurons totally abolished netrin‐1‐induced JNK activation, while knockdown of DSCAM partially inhibited netrin‐1 effect. In the developing spinal cord, phospho‐JNK was strongly expressed in commissural axons before and as they cross the floor plate, and netrin‐1 stimulation dramatically increased the level of endogenous phospho‐JNK in commissural axon growth cones. Knockdown of JNK1 in ovo caused defects in spinal cord commissural axon projection and pathfinding. Our study reveals that JNK1 is important in coordination of DCC and DSCAM in netrin‐mediated attractive signaling.