Biomaterials Approaches For Utilizing The Regenerative Potential Of The Peripheral Nerve Injury Microenvironment

Clinically available treatments are insufficient to achieve full functional recovery in large (>3cm) peripheral nerve injuries (PNI). The objectives in this thesis were 1) to study often overlooked elements of intrinsic PNI repair including release of inhibitory CSPGs and post-injury responses of inflammatory macrophages and dedifferentiated Schwann cells; 2) to create biomaterial scaf-folds featuring topographical and adhesive cues to enhance neurite outgrowth; and 3) to test the ability of those cues to direct macrophages and Schwann cells towards a pro-regenerative phe-notype. It is hypothesized that recapitulating the positive and negative cues of the PNI microenvi-ronment can better improve regeneration. The effect of a characteristic CSPG, Chondroitin Sul-fate A (CSA), was tested on neurite dynamics of dissociated chick embryo dorsal root ganglion (DRG) neurons using time lapse video microscopy. DRG growth was recorded on different ad-hesive substrates, including a novel, porcine-derived spinal cord matrix (SCM). The SCM signifi-cantly increased neurite extension, reduced neurite stalling, and mitigated CSA inhibition. Flow cytometry was used to measure changes in cell-substrate binding receptor expression in the neurons. Results showed a significant increase in Syndecan-3 receptor expression in neurons treated with CSA, suggesting a possible priming of the cells for regrowth. The CSA was success-fully immobilized within electrospun hyaluronic acid (HA) nanofibers using a methacrylation re-action. Blended electrospinning was used to create scaffolds featuring the CSA and SCM cues. Results showed significantly increased neurite outgrowth on scaffolds with the SCM and low levels of CSA. Higher incorporation of CSA maintained its inhibitory properties. Next the CSA, SCM, and HA fiber cues were tested for their effects on macrophage and Schwann cell pheno-type. It was hypothesized that one or more of the cues would accelerate the macrophages return to rest following classical activation (M1/pro-inflammatory) with lipopolysaccharide (LPS; 1μg/mL) and would accelerate the transformation of Schwann cells from an immature state fol-lowing injury to a mature/pro-myelinating one. Cell phenotypes were functionally assessed using quantified reverse transcription polymerase chain reaction (qRT-PCR), immunofluorescence, and sandwich-ELISA based antibody arrays to measure changes in mRNA expression, mor-phology, and cytokine release, respectively. Macrophages cultured with the SCM and HA fibers had significantly reduced M1 gene expression, released lower levels of M1 cytokines (IL-1a, RANTES and TFN-a) and assumed an elongated morphology indicative of M2. These cues also induced changes in the Schwann cells including significantly reduced area, increased elongation, decreased expression of immature genes (GFAP) and increased expression of mature genes (Krox20 and Oct6). These results suggest that the SCM and HA nanofibers could trigger non-neuronal cells towards regenerative programs more quickly than traditional PNI interventions. Changes induced by biomaterials have distinct benefits over the use of immunomodulatory cy-tokines and would be a novel approach to direct repair. Our collective studies offer improved in-sight into the endogenous potential of the injured peripheral nerve and offer ways to incorporate intrinsic repair cues into a biomaterial system for treating large gaps.