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Schwann Cell Senescence: A Result of Chronic Denervation?
Scott Farber, MD; Ying Yan, MD, PhD; Maryam Saheb-Al-Zamani, BA; Piyaraj Thiranansakul; Daniel A. Hunter; Susan E. Mackinnon, MD; Philip J. Johnson, PhD
Washington University in Saint Louis, Saint Louis, MO, USA
Purpose: Schwann cells (SC) provide an environment in the distal nerve stump that is essential to axonal regeneration. In the face of chronic denervation, it has been shown that axonal regeneration is significantly diminished due to decreased growth factor expression by SCs in the distal stump. Senescence is a cellular state induced by external or internal cellular stressors that causes the cell to lose its ability to proliferate and to drastically change its normal protein secretion profile. A senescent state in SCs induced by chronic denervation may cause the changes in protein expression observed in chronic denervation and may contribute to the diminished axonal regeneration in the distal stump. The current study quantifies the incidence of SC senescence over time in the distal stump of chronically denervated rat sciatic nerve. It is our hypothesis that prolonged loss of axonal contact stresses SCs into a senescent state which alters their secretion profile, creating an inhibitory environment for axonal regeneration in the distal stump.
Methods: 32 adult male Lewis rats were divided into four groups (n=8) based on endpoint analysis (1 week, 3 weeks, 5 weeks, and 7 weeks) and subjected to a chronic denervation model. The right sciatic nerve was transected 5 mm proximal to the trifurcation. The proximal stump was burned with electrocautery and then buried in the surrounding muscle. A 30mm ANA, processed by the Hudson technique was coapted to the distal stump of the transected sciatic nerve. The proximal, free end of the ANA was secured to adjacent muscle. Animals were sacrificed at the aforementioned time points and the ANAs were harvested for analysis by immunohistochemistry, qRT-PCR, and electron microscopy.
Results: Immunohistochemistry revealed a correlation between elapsed time and the amount of staining for senescent cell markers (β-galactosidase and p16). Performance of qRT-PCR further confirmed an increase in cellular senescence within the ANA as the time point increased. Electron microscopy revealed reorganization of nuclear chromatin within the nucleus of SCs at the longer time points that is characteristic of senescence.
Conclusion: Distal stump SCs undergo senescence as a result of chronic denervation. The period of chronic denervation in vivo is directly correlated to the number of senescent cells present in the distal stump. Understanding the relationship between SC senescence and diminished axonal regeneration will lead to treatments to enhance regeneration following chronic denervation.
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