Localized and sustained release of brain-derived neurotrophic factor from injectable hydrogel/microparticle composites fosters spinal learning after spinal cord injury

Damaged axons in the adult mammalian central nervous system (CNS), including those of the spinal cord, have extremely limited endogenous capacity to regenerate. This is the result of both the intrinsic and extrinsic inhibitory factors that limit the regeneration of adult neurons. Despite attempts to limit or eliminate the extrinsic inhibitory components, regeneration of adult neurons in the CNS is still limited. Therefore, additional factors that can further enhance the intrinsic plasticity of adult neurons need to be considered. Herein, we examine the effects of brain-derived neurotrophic factor (BDNF), a known growth factor for neuronal survival and plasticity, using an in vivo delivery method for a localized and sustained delivery to the spinal cord. A highly versatile injectable biomaterial platform for the sustained delivery of BDNF was developed using a physical blend of hyaluronic acid (HA) and methylcellulose (MC), in combination with poly-lactic-co-glycolic acid (PLGA) microparticles. Contemporary studies examining the plasticity of the CNS suggest that the spinal cord is an important site for activity-dependent learning that can mediate motor function after injury or disease. Here we utilized such a learning paradigm in combination with local and sustained BDNF application (at L3-S2) to foster spinal learning after complete spinal cord injury in rodents. Our data suggest that composite biomaterial systems such as the one described herein can be utilized for the sustained and localized delivery of therapeutics following damage to the spinal cord.