Widespread expression of netrin-1 by neurons and oligodendrocytes in the adult mammalian spinal cord

Netrins are a family of secreted proteins that function as chemotropic axon guidance cues during neural development. Here we demonstrate that netrin-1 continues to be expressed in the adult rat spinal cord at a level similar to that in the embryonic CNS. In contrast, netrin-3, which is also expresse d in the embryonic spinal cord, was not detected in the adult. In situ hybr idization analysis demonstrated that cells in the white matter and the gray matter of the adult spinal cord express netrin-1. Colocalization studies u sing the neuronal marker NeuN revealed that netrin-1 is expressed by multip le classes of spinal interneurons and motoneurons. Markers identifying glia l cell types indicated that netrin-1 is expressed by most, if not all, olig odendrocytes but not by astrocytes. During neural development, netrin-1 has been proposed to function as a diffusible long-range cue for growing axons . We show that in the adult spinal cord the majority of netrin-1 protein is not freely soluble but is associated with membranes or the extracellular m atrix. Fractionation of adult spinal cord white matter demonstrated that ne trin-1 was absent from fractions enriched for compact myelin but was enrich ed in fractions containing periaxonal myelin and axolemma, indicating that netrin-1 protein may be localized to the periaxonal space. These findings s uggest that in addition to its role as a long-range guidance cue for develo ping axons, netrin may have a short-range function associated with the cell surface that contributes to the maintenance of appropriate neuronal and ax on-oligodendroglial interactions in the mature nervous system.