Peripheral nerve injuries are common and associated with devastating morbidity. Direct nerve repair without a graft is rarely possible and many techniques have been used to bridge the gap. However, nerve injuries requiring long nerve grafts or constructs result in poor functional outcomes. All graft constructs (i.e. autografts, cadaveric allografts, acellular nerve allografts, and nerve conduits) have length limits for supporting regeneration. As the length of the graft construct increases, regeneration decreases until few or no axons reach the distal nerve. Our research demonstrates long nerve grafts accumulate significant quantities of senescent Schwann cells (SenSCs), with the presence of SenSCs preceding, not following, diminished axonal regeneration. Our recent data demonstrates SenSCs cause reduced axonal regenerative capacity in cell culture and in vivo. This data also suggests SenSCs may exert their effects on axons through the expression of Notch ligands. We hypothesize SenSCs impact axonal regeneration across nerve grafts. To test this hypothesis, we utilize animal models of nerve grafts that allow us to modify graft components including SenSC quantity, vascular reperfusion time, and Notch-ligand expression in both SenSCs and normal SCs. We determine how modifying these components impacts axonal regeneration across nerve grafts. The goal of this proposal is to establish the impact of SenSCs on axonal regeneration and functional recovery in long nerve grafts and provide a mechanism for reduced axonal growth due to SenSCs. We first determine the impact of SenSC quantity on axonal regeneration across nerve grafts. We then determine how improving vascular reperfusion (decreased vascular reperfusion time) affects axonal regeneration across nerve grafts and the accumulation of SenSCs. Our preliminary data identified increased expression of Notch ligands in long nerve grafts compared to short nerve grafts concomitant with decreased axonal regeneration and increased SenSC accumulation. As such, we determine the impact of Notch ligand expression by SenSCs on axonal regeneration across nerve grafts using overexpression and RNAi techniques targeting Notch ligand expression. The proposed plan will improve our understanding as to why axonal regeneration is reduced or fails in long nerve grafts and may lead to therapies, including novel therapeutic targets.
The specific aims of this proposal are:
Aim 1 : Determine the impact of SenSCs in limiting regeneration across nerve grafts;
Aim 2 : Determine the impact of SenSC-mediated Notch ligand expression on axonal growth.

Public Health Relevance

Peripheral nerve injury requiring surgical repair is a common procedure in the United States, and the frequency of these repairs has increased with ongoing military operations. Many instances of nerve injury require nerve graft surgical reconstruction to allow regeneration and restore function. In this research project, we determine the impact of cells that accumulate in nerve grafts (Senescent Schwann cells), which reduce regenerative capacity, and therapies to overcome this regenerative burden.

National Institute of Health (NIH)
National Institute of Neurological Disorders and Stroke (NINDS)
Research Project (R01)
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Clinical Neuroplasticity and Neurotransmitters Study Section (CNNT)
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Bambrick, Linda Louise
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Washington University
Schools of Medicine
Saint Louis
United States
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Hoben, Gwendolyn M; Ee, Xueping; Schellhardt, Lauren et al. (2018) Increasing Nerve Autograft Length Increases Senescence and Reduces Regeneration. Plast Reconstr Surg 142:952-961
Yan, Ying; Hunter, Daniel A; Schellhardt, Lauren et al. (2018) Nerve stepping stone has minimal impact in aiding regeneration across long acellular nerve allografts. Muscle Nerve 57:260-267
Poppler, Louis H; Ee, Xueping; Schellhardt, Lauren et al. (2016) Axonal Growth Arrests After an Increased Accumulation of Schwann Cells Expressing Senescence Markers and Stromal Cells in Acellular Nerve Allografts. Tissue Eng Part A 22:949-61
Farber, Scott J; Hoben, Gwendolyn M; Hunter, Daniel A et al. (2016) Vascularization is delayed in long nerve constructs compared with nerve grafts. Muscle Nerve 54:319-21