Spinal Progenitor-Laden Bridges Support Earlier Axon Regeneration Following Spinal Cord Injury

Abstract

Following spinal cord injury (SCI), function is lost below the level of injury due to axon damage and demyelination. Spinal progenitors, and more broadly neural stem cells, can promote the growth of axons through multiple mechanisms, yet their poor survival following transplantation has been limiting the ability to obtain functional effects. In this study, we investigated multichannel poly(lactide-co-glycolide) bridges, which reduce inflammation and promote axon regrowth, as a support for spinal progenitor survival and function at the injury epicenter. Specifically, we hypothesized that mouse embryonic day 14 (E14) spinal progenitors expressing enhanced green fluorescent protein (EGFP) would lead to regenerative gains compared to age-matched adult progenitor controls, which are expected to have similar regenerative capacity to the endogenous progenitors. E14spinal EGFP progenitors were transplanted into a lateral T9-10 hemisection and EGFP+ cells were evident in the bridge and contralateral tissue 8 weeks postinjury, with enhanced survival of E14 compared to adult transplants. Only E14 progenitor-loaded bridges increased axon regrowth compared to blank bridges, resulting in a 3.3-fold increase in axon density (1674 v 497 axons/mm2) and 3.6-fold increase in myelination (∼30% of axons) after 8 weeks. By 6 months, NeuN+ neural bodies were increased within the bridge region of mice transplanted with E14 progenitors. Mice receiving E14 transplants exhibited modest improvements in locomotion, including an earlier ability to perform ipsilateral stepping. The combination of bridges with E14 progenitors produced synergistic reparative gains, with early axon growth and remyelination followed by latent increases in neural bodies within the bridge. Spinal cord injury (SCI) results in loss of tissue innervation below the injury. Spinal progenitors have a greater ability to repair the damage and can be injected into the injury, but their regenerative potential is hampered by their poor survival after transplantation. Biomaterials can create a cell delivery platform and generate a more hospitable microenvironment for the progenitors within the injury. In this work, polymeric bridges are used to deliver embryonic spinal progenitors to the injury, resulting in increased progenitor survival and subsequent regeneration and functional recovery, thus demonstrating the importance of combined therapeutic approaches for SCI.