Defects in sensory nerve numbers and growth in mutant Kit and Steel mice.

The roles in the nervous system of the receptor tyrosine kinase Kit and its ligand, Steel factor, are unclear. We have now found first, that sensory nerve populations are reduced in mutant Kit and Steel mice, implicating Steel-Kit interactions in neuronal development. Second, sensory axonal regeneration (which occurs independently of nerve growth factor, or NGF) is impaired, while collateral sprouting (NGF dependent) is normal. Therefore, there is a selective involvement of Kit signal transduction pathways in nerve growth; supporting this, in wild-type animals Kit was up-regulated in regenerating, but unchanged in sprouting, sensory neurons. The receptor tyrosine kinase Kit thus contrasts with the receptor tyrosine kinase trkA, which is activated by the sprouting stimulus (NGF) but not by the axonal regeneration signal.