Increasing nerve autograft length increases senescence and reduces regeneration

Background: Nerve grafting with an autograft is considered the gold standard. However, the functional outcomes of long (>3cm) nerve autografting are often poor. We hypothesized that a factor contributing to these outcomes is the graft microenvironment, where long compared to short autografts support axon regeneration to different extents. Methods: A rat sciatic nerve defect model was used to compare regeneration within short (2cm) and long (6cm) isografts. Axon regeneration and cell populations within grafts were assessed using histology, retrograde labeling of neurons regenerating axons, immunohistochemistry, qRT-PCR, and electron microscopy at 4 and/or 8 weeks. Results: At 8 weeks for distances of both 1 and 2cm from the proximal coaptation (equivalent regenerative distance), long isografts had reduced numbers of regenerated fibers compared to short isografts. Similarly, the number of motoneurons regenerating axons was reduced in the presence of long isografts compared to short isografts. Considering the regenerative microenvironments between short and long isografts, cell densities and general populations within both short and long isografts were similar. However, long isografts had significantly greater expression of senescence markers, which included senescence associated β-galactosidase, p21, and p16, as well as distinct chromatin changes within Schwann cells. Conclusions: This study shows that axon regeneration is reduced within long compared to short isografts, where long isografts contained an environment with an increased accumulation of senescent markers. While autografts are considered the "gold standard" for grafting, these results demonstrate that we must continue to strive for improvements in the autograft regenerative environment. Financial Disclosure Statement: The authors have the following to disclose: None Presented at: American Society for Surgery of the Hand 2016 meeting in Austin, Tx and the Plastic Surgery Research Council 2017 meeting in Durham, NC. Disclosures: None of the authors has a financial interest in any of the products, devices, or drugs mentioned in this manuscript. Acknowledgements: This work was supported in part by the National Institutes of Neurological Disorders and Stroke of the National Institutes of Health (NIH) under award number R01 NS086773 to Washington University and in part by the Plastic Surgery Foundation under award ID 346670 to Wood through Washington University. The content is solely the responsibility of the authors and does not represent the views of the NIH, the Plastic Surgery Foundation, or Washington University. Corresponding author: Matthew D. Wood, PhD, Division of Plastic and Reconstructive Surgery, Department of Surgery, Washington University School of Medicine, Campus Box 8238, 660 South Euclid Avenue, St. Louis, MO 63110, USA. woodm@wudosis.wustl.edu ©2018American Society of Plastic Surgeons