The inability of retinal ganglion cells (RGCs) to regenerate damaged axons through the optic nerve has dire consequences for victims of traumatic nerve injury and certain neurodegenerative diseases. Several strategies have been shown to induce appreciable regeneration in vivo, but the regrowth of axons through the entire optic nerve and on into the brain remains a major challenge. We show here that the induction of a controlled inflammatory response in the eye, when combined with elevation of intracellular cAMP and deletion of the gene encoding pten ( phosphatase and tensin homolog), enables RGCs to regenerate axons the full length of the optic nerve in mature mice; approximately half of these axons cross the chiasm, and a rare subset (ϳ1%) manages to enter the thalamus. Consistent with our previous findings, the axon-promoting effects of inflammation were shown to require the macrophage-derived growth factor Oncomodulin (Ocm). Elevation of cAMP increased the ability of Ocm to bind to its receptors in the inner retina and augmented inflammation-induced regeneration twofold. Inflammation combined with elevated cAMP and PTEN deletion increased activation of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase signaling pathways and augmented regeneration ϳ10-fold over the level induced by either pten deletion or Zymosan alone. Thus, treatments that synergistically alter the intrinsic growth state of RGCs produce unprecedented levels of axon regeneration in the optic nerve, a CNS pathway long believed to be incapable of supporting such growth.

Retinal ganglion cells (RGCs), the projection neurons of the eye, cannot regenerate their axons once the optic nerve has been injured and soon begin to die. Whereas RGC death and regenerative failure are widely viewed as being cell-autonomous or influenced by various types of glia, we report here that the dysregulation of mobile zinc (Zn 2+ ) in retinal interneurons is a primary factor. Within an hour after the optic nerve is injured, Zn 2+ increases several-fold in retinal amacrine cell processes and continues to rise over the first day, then transfers slowly to RGCs via vesicular release. Zn 2+ accumulation in amacrine cell processes involves the Zn 2+ transporter protein ZnT-3, and deletion of slc30a3, the gene encoding ZnT-3, promotes RGC survival and axon regeneration. Intravitreal injection of Zn 2+ chelators enables many RGCs to survive for months after nerve injury and regenerate axons, and enhances the prosurvival and regenerative effects of deleting the gene for phosphatase and tensin homolog (pten). Importantly, the therapeutic window for Zn 2+ chelation extends for several days after nerve injury. These results show that retinal Zn 2+ dysregulation is a major factor limiting the survival and regenerative capacity of injured RGCs, and point to Zn 2+ chelation as a strategy to promote long-term RGC protection and enhance axon regeneration.T he optic nerve has been widely used to investigate the response of CNS neurons to injury because of its accessibility, anatomy, and functional importance. Under normal circumstances, retinal ganglion cells (RGCs), the projection neurons of the eye, cannot regenerate axons after the optic nerve has been damaged and soon undergo cell death, leaving victims of traumatic or ischemic nerve injury or degenerative conditions, such as glaucoma, with permanent visual losses. Optic nerve injury leads to numerous pathological changes in RGCs and reversing some of these changes improves cell survival, although these effects are often transitory and for the most part promote little or no axon regeneration (1-10). Regeneration per se can be induced by intraocular inflammation combined with elevated cAMP (11, 12), counteracting cell-intrinsic (13-15) or cellextrinsic (16, 17) suppressors of axon growth, oncomodulin and other growth factors (18-22), or elevated physiological activity (23,24). Some of these treatments act synergistically and enable a modest number of RGCs to reestablish connections with appropriate target areas in the brain (25-27). However, although these studies show that successful regeneration can occur in principle, most RGCs eventually die after optic nerve injury, and to date only a small fraction of surviving RGCs have been induced to regenerate axons (27). These observations imply the existence of other major, as yet unknown suppressors of cell survival and regeneration. Our results point to zinc dysregulation as a critical factor.Zinc is essential for many cellular functions. Covalently bound zinc is required for the activity of numerous enzymes and t...

SUMMARY Axon regeneration in the central nervous system (CNS) requires reactivating injured neurons’ intrinsic growth state and enabling growth in an inhibitory environment. Using an inbred mouse neuronal phenotypic screen, we find that CAST/Ei mouse adult dorsal root ganglion neurons extend axons more on CNS myelin than the other eight strains tested, especially when pre-injured. Injury-primed CAST/Ei neurons also regenerate markedly in the spinal cord and optic nerve more than those from C57BL/6 mice and show greater spouting following ischemic stroke. Heritability estimates indicate that extended growth in CAST/Ei neurons on myelin is genetically determined, and two whole-genome expression screens yield the Activin transcript Inhba as most correlated with this ability. Inhibition of Activin signaling in CAST/Ei mice diminishes their CNS regenerative capacity whereas its activation in C57BL/6 animals boosts regeneration. This screen demonstrates that mammalian CNS regeneration can occur and reveals a molecular pathway that contributes to this ability.