A New Signal For Axon Regeneration Is Discovered

Oncomodulin, a calcium-binding protein related to calmodulin, has been identified as the signal used by activated macrophages to stimulate optic nerve regeneration by retinal ganglion cells (RGCs). The 11.7 kDa protein was first identified as the mediator via size-exclusion chromatography of proteins secreted by activated macrophages. Subsequently, the protein was found to nearly double the outgrowth of axons from cultured RGCs at low nanomolar concentrations. Receptor-ligand binding analyses revealed the presence of saturable, high-affinity binding sites for the protein on RGCs. Elevated intracellular cyclic-AMP, achieved with forskolin or 8-Br-c-AMP, was essential for both axon outgrowth and high-affinity binding. A study of the oncomodulin protein found that the N-terminus is required for binding, but the C-terminus, for the biological activity.

An investigation of the mechanism of oncomodulin action revealed that an inhibitor to calcium/calmodulin-dependent protein kinase II specifically blocks the effect of oncomodulin in the bioassay. Actinomycin-D, an inhibitor of transcription, also prevented outgrowth without altering cell viability. Injection of oncomodulin directly into the vitreous increased the level of the active form of the transcriptional activator phosphorylated c-AMP/calcium-response element binding protein. Together, these results point to a transcriptional cascade involvement in oncomodulin signaling.

In vivo studies revealed that oncomodulin mRNA is present at low levels in normal adult retina, and that it increases greatly during the inflammatory response to lens injury. Oncomodulin and 8-Br-c-AMP co-administration into the vitreous increased RGC axon regeneration by 5 to 7 fold, following optic nerve injury. (Alone, the long-acting c-AMP stimulated regeneration by about 2 fold over baseline.) Moreover, oncomodulin’s effect may not be limited to the retinal ganglion, for it also promoted neurite outgrowth from cultured sensory neurons of the dorsal root ganglion.

This study establishes oncomodulin as an important signal in at least some neural tissue regeneration. It may also function as a chemotactic agent, since the superior colliculus, which is the primary target of retinal axons, expresses oncomodulin during development.

Message posted by: Keith Markey