Implications of poly(N-isopropylacrylamide)-g-poly(ethylene glycol) with codissolved brain-derived neurotrophic factor injectable scaffold on motor function recovery rate following cervical dorsolateral funiculotomy in the rat

Laboratory investigation

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Object

In a follow-up study to their prior work, the authors evaluated a novel delivery system for a previously established treatment for spinal cord injury (SCI), based on a poly(N-isopropylacrylamide) (PNIPAAm), lightly cross-linked with a polyethylene glycol (PEG) injectable scaffold. The primary aim of this work was to assess the recovery of both spontaneous and skilled forelimb function following a cervical dorsolateral funiculotomy in the rat. This injury ablates the rubrospinal tract (RST) but spares the dorsal and ventral corticospinal tract and can severely impair reaching and grasping abilities.

Methods

Animals received an implant of either PNIPAAm-g-PEG or PNIPAAm-g-PEG + brain-derived neurotrophic factor (BDNF). The single-pellet reach-to-grasp task and the staircase-reaching task were used to assess skilled motor function associated with reaching and grasping abilities, and the cylinder task was used to assess spontaneous motor function, both before and after injury.

Results

Because BDNF can stimulate regenerating RST axons, the authors showed that animals receiving an implant of PNIPAAm-g-PEG with codissolved BDNF had an increased recovery rate of fine motor function when compared with a control group (PNIPAAm-g-PEG only) on both a staircase-reaching task at 4 and 8 weeks post-SCI and on a single-pellet reach-to-grasp task at 5 weeks post-SCI. In addition, spontaneous motor function, as measured in the cylinder test, recovered to preinjury values in animals receiving PNIPAAm-g-PEG + BDNF. Fluorescence immunochemistry indicated the presence of both regenerating axons and BDA-labeled fibers growing up to or within the host-graft interface in animals receiving PNIPAAm-g-PEG + BDNF.

Conclusions

Based on their results, the authors suggest that BDNF delivered by the scaffold promoted the growth of RST axons into the lesion, which may have contributed in part to the increased recovery rate.

Abbreviations used in this paper:BDA = biotinylated dextran amine; BDNF = brain-derived neurotrophic factor; LCST = lower critical solution temperature; PBS = phosphate-buffered saline; PEG = polyethylene glycol; PNIPAAm = poly(N-isopropylacrylamide); RST = rubrospinal tract; SCI = spinal cord injury.

Article Information

Address correspondence to: Lauren Conova Grous, Ph.D., Drexel University, Department of Chemical and Biological Engineering, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104. email: lgc25@drexel.edu.

Please include this information when citing this paper: published online April 12, 2013; DOI: 10.3171/2013.3.SPINE12874.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    The single-pellet reach-to-grasp task detected an accelerated improvement in percent reaching success in animals receiving scaffold + BDNF grafts (dark gray) compared with animals receiving a scaffold only control graft (light gray) 5 weeks postinjury. Immediately following SCI, both groups had fewer successful reaches from baseline values. Whiskers indicate SE. Significant post hoc t-tests, **p < 0.01.

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    The pronation and grasp components of a reach revealed an accelerated recovery rate in animals receiving scaffold + BDNF grafts compared with scaffold-only controls 5 weeks post-SCI. Upper: Graph comparing the pronation phase between scaffold-only controls (light gray) and scaffold + BDNF (dark gray) animals. Lower: Graph comparing the grasp phase between groups. 0 = absent movement; 1 = abnormal movement; 2 = normal movement. Significant post hoc t-tests, **p < 0.01 for pronation; ***p < 0.001 for grasp.

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    The staircase-reaching test detected a significant increase in the number of pellets retrieved by animals receiving scaffold + BDNF grafts (dark gray) compared with scaffold-only controls (light gray) at 4 and 8 weeks after SCI. Significant post hoc t-tests, ***p < 0.001.

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    Usage of the injured (dominant) forelimb in the cylinder showed a significant increase in dominant forelimb contacts in animals receiving scaffold + BDNF grafts (dark gray), compared with scaffold-only controls (light gray), at 4, 6, and 8 weeks following SCI. Significant post hoc t-tests, **p < 0.01; ***p < 0.001.

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    Scaffold-only controls and scaffold + BDNF animals did not have a significant difference in lesion size (p = 0.82) or a significant difference in the amount of glial scar formation around the lesions (p = 0.59). Left: An example of scaffold + BDNF tissue processed for GFAP immunocytochemistry is shown at low magnification to help visualize the lesion size and location of the lesion within the host tissue. Right: A higher-magnification image of the same section is shown to better evaluate the presence of astrocytes. Bar = 1 mm.

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    Scaffold-only controls and scaffold + BDNF animals show a similar presence of glial scar formation; scaffold + BDNF is permissive to host axonal growth. GFAP (green) stains astrocytes around the graft site, and RT-97 labels host axons. DAPI labeling (blue) was used to identify cell nuclei. A and C: GFAP staining for scaffold-only controls and scaffold + BDNF, respectively. B and D: Images obtained at a higher magnification, showing RT-97 labeling for scaffold-only controls and scaffold + BDNF, respectively. Asterisks indicate the specific area of the lesion that was taken at a higher magnification in B and D. Bar = 100 μm.

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    BDA-labeled RST axons grow within 500 μm rostral to the scaffold + BDNF host/graft interface. Upper: BDA-labeled axons that grew to within 212 ± 51 μm of the rostral host-graft interface. Arrows indicate the direction of RST axon growth. Lower: In contrast, BDA-labeled axons do not approach the scaffold-only controls host/graft interface. DAPI labeling (blue) was used to identify cell nuclei. Bar = 100 μm.

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