Bridging the gap in spinal cord injury: a novel biomaterial implant to promote central nervous system repair

Overcoming the limitations to regeneration in the central nervous system (CNS) remains the greatest challenge to spinal cord repair following injury. Two key barriers to repair are (i) the physical gap left by the injury, across which axons must extend, and (ii) the glial scar: the inhibitory milieu, perpetuated by reactive glia, filling and surrounding the site of trauma. One promising therapeutic avenue for spinal cord injury (SCI) treatment is the implantation of biomaterial scaffolds into the injury site, with the aim of stabilising the injury, bridging the SCI cavity, attenuating the glial scar and promoting regeneration. However, few biomaterials identified for this purpose possess the desired combination of material properties, including porosity, degradability and mechanical compatibility, for implantation into cord tissue. In collaboration with Spheritech Ltd, we have identified a novel biomaterial candidate with the potential to meet these requirements: Proliferate®, a poly-ε-lysine based, 3D, macroporous, biocompatible and biodegradable polymer. Here we show the potential of this construct as an SCI implant, demonstrating both its mechanical and biological viability with CNS cells and spinal cord tissue. In vitro, we showed glial and neuronal cell viability on the construct. We demonstrated a decrease in astrocytic nestin expression in astrocytes cultured on Proliferate® compared with 2D substrates, corresponding to marked changes in astrocyte morphology from flat, expansive morphologies to stellate, fibrous conformations. Myelination was also observed on the construct, but was found delayed compared to 2D surfaces. Furthermore, we found increased proliferation of embryonic spinal cord neurons on the construct, and improved adhesion to IKVAV-coated Proliferate®. These findings not only demonstrate Proliferate®’s cytocompatibility, but also show the differences in CNS cell-substrate interactions between 2D and 3D topographies. The viability of cell transplant candidates, Schwann cells and olfactory ensheathing cells, was further demonstrated on the construct, showing the capacity of Proliferate® as a potential cell transplant delivery vessel. We corroborated these in vitro findings with in vivo results, demonstrating successful Proliferate® integration in the injured spinal cord using incision, contusion and wire knife rodent SCI models. Proliferate® implants, containing parallel-guidance channels, were assessed 7 days, 3 weeks, 7 weeks, and 6 months post-implantation, and showed cellular infiltration and extensive vascularisation at all timepoints. In keeping with in vitro findings, GFAP and nestin co-expression in the glial scar surrounding IKVAV-coated implants was found reduced 7-weeks post- implantation compared with non-implanted cavities, however this effect was not maintained to 6 months. Additionally, an abundance of non-astrocytic nestin expressing cells were observed inside construct implants at all time-points. Although guidance channels were not found to consistently maintain structural integrity, intact guidance channels were observed to attract axonal extension and in some cases, myelination. Overall, our findings demonstrate the potential value of this novel construct both as an implant for SCI therapy, and as a potential 3D cell culture substrate for CNS cells.