New insights to manage painful neuroma: Study of a nerve graft in rats

Using a long acellular nerve allograft to "cap" a damaged nerve could control axonal regeneration within the allograft and down-regulate regenerative and pain-related genes in the lesioned nerve. This was demonstrated by D. Pan et al. in an article¹ published in the PRS Journal in 2020.

A neuroma is an abnormal nerve formation that develops as a result of peripheral nerve injury, most often traumatic. It is caused by imperfect repair of the injured nerve, resulting in failure to regrow part of the proximal fibers and bulbous swelling of axonal shoots and scar tissue. Its incidence varies according to the affected area.
Symptomatic neuromas cause pain that severely affects the productivity and quality of life of patients, especially amputees, because neuromas can be stimulated by prostheses and cause patients to discontinue using them. To treat or prevent neuroma formation or recurrence, both non-operative modalities (as analgesics) and surgical interventions have been used, but without much success.

An experimental operative approach

Consequently, D. Pan et al. decided to explore an experimental operative approach. Previous research ^2-4 on rats has shown that the repair of nerve gaps using long (> 4 cm) acellular nerve allografts can promote axon regeneration for up to 5 weeks.⁵ But it was unclear whether it could last over a longer period, or whether it could change overall nerve regeneration and pain sensitization.
D. Pan et al. decided to perform a rat sciatic nerve transection and invert and ligate the distal end of the nerve to generate a terminal neuroma model. To do so, they had Thyl-GFP rats (with a Sprague-Dawley background and axons expressing green fluorescent proteins) generated as acellular nerve allograft donors for Lewis rats, allowing imaging of the axons and histomorphometric monitoring of their evolution.

Studies were carried out with three groups to assess the immediate effects of nerve injury: no treatment (control), traction neurectomy, or 5-cm acellular allograft cap attached to the proximal nerve (ANA).

Encouraging results

The growth of axons from Thyl-GFP rats was measured at postoperative days 0, 30 and 120 using a stereo-enabled macroscope with a fluorescent light source. The control group and the traction neurectomy group demonstrated evidence of green fluorescent protein-positive axon growth distal to the site of injury at postoperative days 30 and 120 and a bulbous nerve near the site of initial injury, indicative of a neuroma. For the ANA group, axon extension was observed dynamically over time, but there was a complete lack of regenerated axons in the distal segment of the acellular nerve allograft, even after 5 months.

Fig. 1. An acellular nerve allograft cap arrests axon regeneration within the integrated acellular nerve allograft tissue. Arrowhead: bulbous nerve region within the transected proximal nerve. Note its absence in the acellular nerve allograft-capped nerve.

Dorsal root ganglia harvested at 120 days from all these animals revealed elevated expression of genes associated with regeneration and pain sensitization,⁶ such as Bdnf, cfos and Galectin, among the control and traction neurectomy groups compared to uninjured nerve dorsal root ganglia. In the ANA group, however, the expression levels of these genes were not statistically different from the uninjured nerve dorsal root ganglia levels. These data suggest a direct link between arresting axon regeneration and modulating gene expression within upstream neurons, and a possible impact on pain or pain sensitization.

Using all of these data, the researchers concluded that the acellular nerve allograft treatment led to changes in gene expression suggestive of a cessation of regeneration and less risk of pain sensitization.

Further studies will be required to better evaluate pain-related behaviors in rat models. It would also benefit to determine whether this in-house approach capping only the severed proximal nerve could supersede existing commercial products connected to proximal and distal nerves, with whom axon regeneration still occurs for distances over 5 cm.

Of note, the optimized Thyl-GFP rat preclinical models used in the study carried out by D. Pan and colleagues were generated by genOway, designer and provider of multiple preclinical models in several research areas, including immuno-oncology, metabolism, cardiovascular diseases, and neuroscience.

References:

1. Pan D, Bichanich M, Wood I, et al. Long Acellular Nerve Allografts Cap Transected Nerve to Arrest Axon Regeneration and Alter Upstream Gene Expression in a Rat Neuroma Model. PRS Journal, 2020-12-11. DOI: 10.1097/PRS.0000000000008051.
2. Poppler LH, Ee X, Schellhardt L, et al. Axonal growth arrests after an increased accumulation of Schwann cells expressing senescence markers and stromal cells in acellular nerve allografts. Tissue Eng Part A 2016;22:949–961.
3. Pan D, Hunter DA, Schellhardt L, et al. The accumulation of T cells within acellular nerve allografts is length dependent and critical for nerve regeneration. Exp Neurol.2019;318:216–231.
4. Saheb-Al-Zamani M, Yan Y, Farber SJSJ, et al. Limited regeneration in long acellular nerve allografts is associated with increased Schwann cell senescence. Exp Neurol.2013;247:165–177.
5. Hong T, Wood I, Hunter DA, et al. Neuroma management: Capping nerve injuries with an acellular nerve allograft can limit axon regeneration. Hand (NY) 2019;16:157–163.
6. Dubovy P. Wallerian degeneration and peripheral nerve conditions for both axonal regeneration and neuropathic pain induction. Ann Anatomy 2011;193:267–275.