Full-length optic nerve regeneration in the absence of genetic manipulations

Abstract

The inability of mature retinal ganglion cells (RGCs) to regenerate axons after optic nerve injury can be partially reversed by manipulating cell-autonomous and/or -non-autonomous factors. Although manipulations of cell-non-autonomous factors could have higher translational potential than genetic manipulations of RGCs, they have generally produced lower levels of optic nerve regeneration. Here we report that preconditioning resulting from mild lens injury (conditioning LI, cLI) prior to optic nerve damage induces far greater regeneration than LI after nerve injury or the pro-inflammatory agent zymosan given either before or after nerve damage. Unlike zymosan-induced regeneration, cLI is unaltered by depleting mature neutrophils or T cells or blocking receptors for known inflammation-derived growth factors (Oncomodulin, SDF1, CCL5), and is only partly diminished by suppressing CCR2+ monocyte recruitment. Repeated episodes of LI lead to full-length optic nerve regeneration, and pharmacological removal of local resident macrophages with the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 enables some axons to re-innervate the brain in just 6 weeks, comparable to the results obtained with the most effective genetic manipulations of RGCs. Thus, cell-non-autonomous interventions can induce high levels of optic nerve regeneration, paving the way to uncover potent, translatable therapeutic targets for CNS repair.

Inflammatory Mediators of Axon Regeneration in the Central and Peripheral Nervous Systems

“…This phenomenon, called the conditioning lesion effect, is mediated by infiltrative monocytes that secrete oncomodulin and perhaps other factors [42,[46][47][48][49]. We now find that inflammatory conditioning also has profound effects on optic nerve regeneration [50]. As noted earlier, either lens injury or zymosan at the time of optic nerve crush leads to moderate levels of optic nerve regeneration.…”

“…As noted earlier, either lens injury or zymosan at the time of optic nerve crush leads to moderate levels of optic nerve regeneration. However, if a mild injury restricted to the lens capsule is performed two weeks prior to optic nerve crush, the resulting level of regeneration is several-fold higher than seen with either zymosan or lens injury at the time of nerve crush (Figure 5), and zymosan delivery two weeks prior to optic nerve crush has no benefit [50]. Repeated episodes of lens injury pre-and post-optic nerve crush result in even greater levels of regeneration, enabling RGCs to regenerate axons the full length of the optic nerve and into the optic chiasm within 3-4 weeks [50].…”

“…However, if a mild injury restricted to the lens capsule is performed two weeks prior to optic nerve crush, the resulting level of regeneration is several-fold higher than seen with either zymosan or lens injury at the time of nerve crush (Figure 5), and zymosan delivery two weeks prior to optic nerve crush has no benefit [50]. Repeated episodes of lens injury pre-and post-optic nerve crush result in even greater levels of regeneration, enabling RGCs to regenerate axons the full length of the optic nerve and into the optic chiasm within 3-4 weeks [50]. Blocking neutrophils had no effect on this phenomenon, while blocking monocyte entry partially diminished the effect, and suppressing microglial proliferation using the Csf-1 receptor inhibitor PLX5622 doubled the level of regeneration attained with multiple episodes of lens-injury conditioning, enabling some RGC axons to enter the suprachiasmatic and lateral geniculate nuclei after just a few weeks (Figure 5).…”

Toxic Advanced Glycation End-Products Inhibited Axonal Elongation Mediated by β-Tubulin Aggregation in Mice Optic Nerve

“…Furthermore, GA induced β-tubulin aggregation and inhibited zymosan-induced axonal elongation even in vivo. Benowitz et al reported that zymosan induces macrophage invasion and appears to release trophic factors such as oncomodulin, stromal cell-derived factor 1, and CCL5 chemokine that can promote axonal elongation [41][42][43].…”

Injury distance limits the transcriptional response to spinal injury

“…DRG neurons, which survive axotomy and mount an effective regenerative response, respond to injury with a partial loss of transcripts that distinguish subtypes and the upregulation of numerous transcripts that support axon growth . RGCs are highly vulnerable to cell death and display limited spontaneous axon growth after axotomy but can be coaxed to regenerate long axons using a variety of interventions [20][21][22][23][24][25] . Accordingly, the transcriptional response of RGCs shows strong upregulation of transcripts associated with both cell death and axon regeneration.…”