Rewiring the injured CNS: lessons from the optic nerve

Abstract

The optic nerve offers a number of advantages for investigating mechanisms that govern axon regeneration in the CNS. Although mature retinal ganglion cells (RGCs) normally show no ability to regenerate injured axons through the optic nerve, this situation can be partially reversed by inducing an inflammatory response in the eye. The secretion of a previously unknown growth factor, oncomodulin, along with co-factors, causes RGCs to undergo dramatic changes in gene expression and regenerate lengthy axons into the highly myelinated optic nerve. By themselves, strategies that counteract inhibitory signals associated with myelin and the glial scar are insufficient to promote extensive regeneration in this system. However, combinatorial treatments that activate neurons' intrinsic growth state and overcome inhibitory signals result in dramatic axon regeneration in vivo. Because of the ease of introducing trophic factors, soluble receptors, drugs, or viruses expressing any gene or small interfering RNA of interest into RGCs, this system is ideal for identifying intracellular signaling pathways, transcriptional cascades, and ligand-receptor interactions that enable axon regeneration to occur in the CNS.

Figure 1. Oncomodulin promotes optic nerve regeneration

Longitudinal sections of the mature rat optic nerve distal to the site of a crush injury (asterisk). Regenerating axons are visualized using an antibody to GAP-43. a: RGCs normally show no capacity to regenerate axons beyond the injury site (see Fig. 2a); intravitreal injection of polymeric microspheres causes a mild inflammatory response and stimulates a modest amount of regeneration. b: Extensive regeneration occurs when microspheres release oncomodulin and a cAMP analog. Scale bar, 250 μm (109).

Figure 2. Combinatorial treatments cause dramatic axon regeneration in the optic nerve

(a-d) GAP- 43-positive axons visualized in longitudinal sections through the optic nerve 2 wks after optic nerve (ON) crush with (b,d) or without (a,c) intravitreal macrophage activation (MΦ) (asterisk: injury site). RGCs were transfected with AAV expressing GFP alone (a,c) or C3 ribosyltransferase + GFP (b,d). (a) No regeneration is seen after ON crush. (b) Expression of C3 ribosyltransferase stimulates modest regeneration (seen in inset, showing regenerating axons at increased magnification and illumination). (c) Intravitreal macrophage activation alone increases axon regeneration. (d) Intravitreal macrophage activation combined with C3 expression causes dramatic regeneration through the injury site and beyond. Scale bar, 200 μm.