An injectable, biomaterial-based therapy to promote endogenous neural progenitor cells in a hemorrhagic stroke lesion

Abstract

Stroke is the second leading cause of death and disability worldwide with prevalence and resulting costs projected to increase. There are few available treatments, and their applicability is limited to the type of stroke and the initial hours after onset. New treatments are needed to address post-stroke disability and the needs of survivors with a low quality of life. An emerging therapeutic concept is the direct injection of a biomaterial-based therapy into the stroke lesion to restore neural elements for improved function. The principal objective of this thesis was to evaluate the ability of an injectable, gelatin-based biomaterial (gelatin-hydroxyphenylpropionic acid, Gtn-HPA) incorporating epidermal growth factor (EGF) to increase the number of endogenous nestin-positive neural progenitor cells (NPCs) in the lesion. In a well-validated intracerebral hemorrhagic (ICH) stroke model in rats, NPCs were quantified and compared to ICH-only controls at 2 and 4 weeks post-injection.

At both time points, there was a ~10-fold increase in NPCs in the region of the biomaterial-treated lesion compared to the untreated control. Observations extended to 10 weeks post-injection revealed a persistence of the EGFbearing Gtn-HPA with continued infiltration of NPCs to a deeper extent into the biomaterial-filled lesion. To determine the effects of the specific biomaterial and growth factor combination, two additional groups were tested: an alternative hydrogel (RADA16 self-assembling peptides) incorporating EGF and an alternative mixture of growth factors extracted from endogenous blood platelets (platelet-rich plasma lysate) and incorporated into Gtn-HPA. Only Gtn-HPA incorporating EGF was able to increase significantly the number of NPCs in the stroke region. In a second objective, towards clinical translation, the target of the proposed therapy was characterized by examining the morphology and composition of human postmortem stroke brains.

In chronic hemorrhagic lesions, an atypical porous collagen matrix was observed--prompting further questions on the possible effect of lesion constituents on the injectable biomaterial. In a series of rheology experiments, a third objective was to determine the effects of blood elements on the gelation time and storage modulus of Gtn-HPA. Blood was found to impede gelation of the biomaterial--most likely through a catalase-promoted elimination of the catalyst that enables covalent crosslinking. These results inform the types of lesions amenable to therapy and the potential need to manage lesion constituents (e.g., by aspiration) prior to injection of the biomaterial. The results of this thesis motivate and guide further study in a large animal model to validate Gtn-HPA/EGF promotion of NPC infiltration of the ICH stroke lesion in a larger brain size and demonstrate the attendant improvement in function.