Spinal cord injury leads to a permanent loss of motor and sensory function below the point of injury, for which there are minimal therapies available. Gene delivery from bridges that span the injury site have been proposed as a means to induce the expression of stimulatory growth factors, or the block the expression of inhibitory factors; however, efficient gene delivery techniques are needed and expression should be spatially patterned to address the complexity of the spinal cord.

We propose to investigate surface-mediated delivery of DNA from multiple channel bridges to further enhance gene delivery locally. Additionally, surface immobilization can be employed to tailor the channels of the bridge with genes specific for the multiple spinal cord tracts, which need different growth factors for maximal regeneration. In vitro studies have investigated DNA complexed with transfast, a lipid complex agent selected due to its non-neurotoxic properties. Surface-mediated delivery of transfast DNA complexes (1:1 ratio) from PLG led to similar transgene expression (10⁷ RLU/mg protein) compared with bolus delivery. Surface modification of PLG with ECM was also investigated as a means to both promote gene transfer and to provide a permissive support for neurite outgrowth. Coating of PLG surfaces revealed that both collagen and fibronectin efficiently bound to the PLG, when compared to laminin. Seeding of embryonic chick dorsal root ganglion (DRG) neurons on the surfaces demonstrated the most extensive neurite growth on PLG coated with fibronectin. Immobilization of transfast lipoplexes mixed within fibronectin illustrated similar transgene expression levels to surfaces coated with lipoplexes without ECM, while lipoplexes mixed within collagen did not result in transgene expression.

These observations with PLG disks were subsequently translated to PLG bridges for in vivo studies. Multiple channels bridges were fabricated using a gas foaming/particulate leaching method, which allowed for a controlled bridge porosity, pore size, channel configuration, and channel diameter. After implantation in a rat spinal cord lateral hemisection model, the bridges resulted in functional recovery of the bladder after approximately 10 days and weight bearing steps of the hind limbs with a BBB score 10.2 ± 0.7 at the ipsilateral site (n = 5). For bridges combined with gene therapy, DNA was adsorbed to the PLG surface after bridge fabrication. Using an in vivo rat spinal cord hemisection model, adsorption of DNA (100 µg) in the absence of ECM resulted in local transgene expression at the injury site for over a week. To tailor the channels with different genes, techniques were developed to pipet a mixture of DNA and ECM into the channels. Importantly, the ECM increased the viscosity of the mixture which facilitated retention within the channel. Multiple channel bridges tailored with gene therapy and ECM components provide a powerful approach to promote axonal regrowth by providing a physical support for axonal elongation and inducing the expression of stimulating neurotrophic factors, customized for each spinal cord tract.