A pilot study of poly(N-isopropylacrylamide)-g-polyethylene glycol and poly(N-isopropylacrylamide)-g-methylcellulose branched copolymers as injectable scaffolds for local delivery of neurotrophins and cellular transplants into the injured spinal cord
The authors investigated the feasibility of using injectable hydrogels, based on poly(N-isopropylacrylamide) (PNIPAAm), lightly cross-linked with polyethylene glycol (PEG) or methylcellulose (MC), to serve as injectable scaffolds for local delivery of neurotrophins and cellular transplants into the injured spinal cord. The primary aims of this work were to assess the biocompatibility of the scaffolds by evaluating graft cell survival and the host tissue immune response. The scaffolds were also evaluated for their ability to promote axonal growth through the action of released brain-derived neurotrophic factor (BDNF).
The in vivo performance of PNIPAAm-g-PEG and PNIPAAm-g-MC was evaluated using a rodent model of spinal cord injury (SCI). The hydrogels were injected as viscous liquids into the injury site and formed space-filling hydrogels. The host immune response and biocompatibility of the scaffolds were evaluated at 2 weeks by histological and fluorescent immunohistochemical analysis. Commercially available matrices were used as a control and examined for comparison.
Experiments showed that the scaffolds did not contribute to an injury-related inflammatory response. PNIPAAm-g-PEG was also shown to be an effective vehicle for delivery of cellular transplants and supported graft survival. Additionally, PNIPAAm-g-PEG and PNIPAAm-g-MC are permissive to axonal growth and can serve as injectable scaffolds for local delivery of BDNF.
Based on the results, the authors suggest that these copolymers are feasible injectable scaffolds for cell grafting into the injured spinal cord and for delivery of therapeutic factors.
Abbreviations used in this paper: BDNF = brain-derived neurotrophic factor; CGRP = calcitonin gene-related peptide; CSPG = chondroitin sulfate proteoglycan; LCST = lower critical solution temperature; MA = methacrylic anhydride; MC = methylcellulose; NIPAAm = N-isopropylacrylamide; PBS = phosphate-buffered saline; PEG = polyethylene glycol; PFA = paraformaldehyde; PNIPAAm = poly(N-isopropylacrylamide); RSF = rat skin fibroblast; SCI = spinal cord injury.
BryantSJDavis-ArehartKALuoNShoemakerRKArthurJAAnsethKS: Synthesis and characterization of photopolymerized multifunctional hydrogels: water-soluble poly(vinyl alcohol) and chondroitin sulfate macromers for chondrocyte encapsulation. Macromolecules37:6726–67332004
HimesBTNeuhuberBColemanCKushnerRSwangerSAKopenGC: Recovery of function following grafting of human bone marrow-derived stromal cells into the injured spinal cord. Neurorehabil Neural Repair20:278–2962006
HirschbergDLYolesEBelkinMSchwartzM: Inflammation after axonal injury has conflicting consequences for recovery of function: rescue of spared axons is impaired but regeneration is supported. J Neuroimmunol50:9–161994
HornKPBuschSAHawthorneALvan RooijenNSilverJ: Another barrier to regeneration in the CNS: activated macrophages induce extensive retraction of dystrophic axons through direct physical interactions. J Neurosci28:9330–93412008
IbarraACorreaDWillmsKMerchantMTGuizar-SahagúnGGrijalvaI: Effects of cyclosporin-A on immune response, tissue protection and motor function of rats subjected to spinal cord injury. Brain Res979:165–1782003
LiuYKimDHimesBTChowSYSchallertTMurrayM: Transplants of fibroblasts genetically modified to express BDNF promote regeneration of adult rat rubrospinal axons and recovery of forelimb function. J Neurosci19:4370–43871999
MadiganNNMcMahonSO'BrienTYaszemskiMJWindebankAJ: Current tissue engineering and novel therapeutic approaches to axonal regeneration following spinal cord injury using polymer scaffolds. Respir Physiol Neurobiol169:183–1992009
MitsuiTShumskyJSLeporeACMurrayMFischerI: Transplantation of neuronal and glial restricted precursors into contused spinal cord improves bladder and motor functions, decreases thermal hypersensitivity, and modifies intraspinal circuitry. J Neurosci25:9624–96362005
OudegaMGautierSEChaponPFragosoMBatesMLParelJM: Axonal regeneration into Schwann cell grafts within resorbable poly(alpha-hydroxyacid) guidance channels in the adult rat spinal cord. Biomaterials22:1125–11362001