Injectable chemokine-releasing gelatin matrices for enhancing endogenous regenerative responses in the injured rat brain

Abstract

Brain injuries acquired from hemorrhage, ischemic strokes and trauma affect millions worldwide each year and often cause irreversible loss of neural tissue that disrupts vital neurological functions. Cell transplantation has traditionally been the strategy for achieving neuroregeneration but endogenous regenerative responses such as neurogenesis and neovascularization are increasingly recognized as an elegant alternative. These responses are directed toward injured tissues via chemokine signaling such as the stromal cell-derived factor-I a (SDF-1 [alpha])/CXCR4 pathway and offer the potential to replace lost neurovascular elements. Endogenous regenerative responses are, however, not fully effective in the injured brain. Two prominent barriers are the loss of chemokine expression and disappearance of stroma following tissue loss in the brain lesion, which lead to sub-optimal engagement of endogenous regenerative responses and inability of recruited cells to infiltrate the lesion. The overall goal of this thesis was therefore to develop an injectable lesion-filling matrix that could re-establish chemokine release and stroma, thereby enhancing endogenous regenerative responses. Toward this goal, we demonstrated injectable gelatin-hydroxylphenylpropionic acid (Gtn-HPA) hydrogels as an appropriate scaffolding material that was permissive for proliferation, migration and differentiation of adult neural progenitor cells (aNPCs). We also synthesized dextran sulfate/chitosan polyelectrolyte complex nanoparticles (PCN), which could encapsulate SDF-1[alpha] efficiently and sustain its release for 4 weeks. When used in an in vitro migration assay to fill a core region that was surrounded by an aNPC-laden hydrogel construct, the resulting Gtn-HPA/SDF- 1[alpha]-PCN matrix recruited aNPCs to accumulate around and migrate into the core region. When injected into the brain lesion in a rat model of intracerebral hemorrhage, Gtn-HPA/SDF-1[alpha]-PCN matrix successfully increased the migration of endogenous neuronal precursors into the injured striatum and amplified neovascularization. Gtn-HPA/SDF-l[alpha]-PCN matrix also led to a newly formed vasculature within the lesion and supported infiltration by endogenous cells that included neutrophils expressing CXCR4 and VEGF. The neutrophil infiltrate did not spread to surrounding tissue or induce necrosis and compelled further investigation for their role in the injured brain. Importantly, Gtn-HPA/SDF-l[alpha]-PCN matrix reduced brain tissue loss and improved behavioral recovery. Overall, Gtn-HPA/SDF-l[alpha]-PCN matrix offered an opportunity to enhance endogenous regenerative responses and confer benefits to the injured rat brain.