Regeneration and transplantation of the optic nerve: developing a clinical strategy

Demonstration of technical feasibility and viability of whole eye transplantation in a rodent model

INTRODUCTION

Whole eye transplantation (WET) holds promise for vision restoration in devastating/disabling visual loss (congenital or traumatic) not amenable to surgical or neuroprosthetic treatment options. The eye includes multiple tissues with distinct embryonic lineage and differential antigenicity. Anatomically and immunologically, the eye is unique due to its avascular (cornea) and highly vascular (retina) components. Our goal was to establish technical feasibility, demonstrate graft viability, and evaluate histologic changes in ocular tissues/adnexae in a novel experimental model of WET that included globe, adnexal, optic nerve (ON), and periorbital soft tissues.

METHODS

Outbred Sprague-Dawley rats (n = 5) received heterotopic vascularized WET from donors. Each WET included the entire globe, adnexa, ON, and periorbital soft tissues supplied by the common carotid artery and external jugular vein. Viability and perfusion were confirmed by clinical examination, angiography and magnetic resonance imaging (MRI). Globe, adnexal, and periorbital tissues were analyzed for histopathologic changes, and the ON was examined for neuro-regeneration at study endpoint (30 days) or Banff Grade 3 rejection in the periorbital skin (whichever was earlier).

RESULTS

Gross examination confirmed transplant viability and corneal transparency. Average operative duration was 64.0 ± 5.8 min. Average ischemia time was 26.0 ± 4.2 min. MRI revealed loss of globe volume by 36.0 ± 2.8% after transplantation. Histopathology of globe and adnexal tissues showed unique and differential patterns of inflammatory cell infiltration. The ON revealed a neurodegeneration pattern.

CONCLUSION

The present study is the first in the literature to establish an experimental model of WET. This model holds significant potential in investigating mechanistic pathways, monitoring strategies or developing management approaches involving ocular viability, immune rejection, and ON regeneration after WET.

Synergetic effects of ciliary neurotrophic factor and olfactory ensheathing cells on optic nerve reparation (complete translation)

At present, there is no effective treatment for the repair of the optic nerve after injury, or improvement of its microenvironment for regeneration. Intravitreally injected ciliary neurotrophic factor (CNTF) and olfactory ensheathing cells (OECs) promote the long-distance regrowth of severed optic nerve fibers after intracranial injury. Here, we examined the efficacy of these techniques alone and in combination, in a rat model of optic nerve injury. We injected condensed OEC suspension at the site of injury, or CNTF into the vitreous body, or both simultaneously. Retrograde tracing techniques showed that 4 weeks postoperatively, the number of surviving retinal ganglion cells and their axonal density in the optic nerve were greater in rats subjected to OEC injection only than in those receiving CNTF injection only. Furthermore, combined OEC + CNTF injection achieved better results than either monotherapy. These findings confirm that OECs are better than CNTF at protecting injured neurons in the eye, but that combined OEC and CNTF therapy is notably more effective than either treatment alone.

Emerging options for the management of age-related macular degeneration with stem cells

Age-related macular degeneration (AMD) is a devastating retinal disease that occurs in later life as the retinal pigment epithelium (RPE) cells die, with subsequent photoreceptor degeneration. In the past, RPE transplant surgeries gave evidence that AMD was potentially treatable, but it involved limited amounts of ocular tissue, and the complication rate was high. Then, stem cell transplants offered an unlimited supply of retinal precursors for endogenous repair and exogenous cell replacement. Debate continues as to which type of stem cell is most appropriate for treating AMD. The prospects include adult-derived progenitor stem cells (including progenitor cells from ocular tissues), hematopoietic stem cells, embryonic stem cells, and induced pluripotent stem cells. Now the therapy is expanding into phase I human trials. This review examines the collective research contributions toward a clinical model of AMD management with stem cells.

Standardizing optic nerve crushes with an aneurysm clip

OBJECTIVES

Despite the widespread use of optic nerve injury models to simulate central nervous system injury, model protocols vary from laboratory to laboratory, making it difficult to directly compare findings between studies.

METHODS

To standardize the optic nerve crush injury model, the commercially available Yasargil aneurysm clip, which provides a consistent clamping force, was used to produce a crush injury to the rat optic nerve. Histology was verified with hematoxylin-eosin. The number of retinal ganglion cells (RGCs) was counted by fluorescent gold dye labeling.

RESULTS

Following nerve crush injury, the density of RGCs was substantially reduced in the aneurysm clip-operated group relative to the normal and sham-operated groups, and no discernable difference was noted between the latter two control groups.

DISCUSSION

The present findings suggest that Yasargil aneurysm clip effectively produces permanent injury to the optic nerve with evidence from retrograde tracing of RGCs and may provide a standard technique for optic nerve crush studies.

Impediments to eye transplantation: ocular viability following optic-nerve transection or enucleation

Maintenance of ocular viability is one of the major impediments to successful whole-eye transplantation. This review provides a comprehensive understanding of the current literature to help guide future studies in order to overcome this hurdle. A systematic multistage review of published literature was performed. Three specific questions were addressed: (1) Is recovery of visual function following eye transplantation greater in cold-blooded vertebrates when compared with mammals? (2) Is outer retina function following enucleation and reperfusion improved compared with enucleation alone? (3) Following optic-nerve transection, is there a correlation between retinal ganglion cell (RGC) survival and either time after transection or proximity of the transection to the globe? In a majority of the studies performed in the literature, recovery of visual function can occur after whole-eye transplantation in cold-blooded vertebrates. Following enucleation (and reperfusion), outer retinal function is maintained from 4 to 9 h. RGC survival following optic-nerve transection is inversely related to both the time since transection and the proximity of transection to the globe. Lastly, neurotrophins can increase RGC survival following optic-nerve transection. This review of the literature suggests that the use of a donor eye is feasible for whole-eye transplantation.

Comparison of blood-nerve barrier disruption and matrix metalloprotease-9 expression in injured central and peripheral nerves in mice

To investigate the involvement of blood-born factors and extracellular proteases in axonal degeneration and regeneration in both PNS and CNS, we directly compared the differences of blood-nerve barrier (BNB) disruption and matrix metalloprotease-9 (MMP-9) induction between the sciatic nerve and optic nerve after crush injury in the same animal. In sciatic nerve, BNB disruption, fibrin(ogen) deposition and MMP-9 expression were observed only in the first week following injury. Neurofilament (NF) immunoreactivity dramatically decreased in the first 2 days, gradually recovered to the normal levels by day 28. In contrast, the immunoglobulin G deposits spanned from 4 h to 28 days in crushed optic nerves. Fibrin(ogen) deposition was only observed in the first 2 days, while MMP-9 induction did not occur until a week after injury but lasted for 3 weeks in the crushed optic nerves. The NF immunoreactivity did not change much until day 7 and almost completely disappeared on day 28. The decrease of NF immunoreactivity coincided with the induction of MMP-9 after optic nerve crush. These results show that BNB disruption and MMP-9 induction are differentially regulated in the PNS and CNS after injuries, and they may contribute to the different regeneration capacities of the two systems.

Early decompression of the injured optic nerve reduces axonal degeneration and improves functional outcome in the adult rat

The putative beneficial role of an early decompression of injured CNS tissue following trauma remains controversial. In this study, we approach this scientific query using a standardized injury of the optic nerve in adult rats. Adult Sprague-Dawley rats were subjected to a standardized optic nerve constriction injury by applying a loose ligature around the nerve for 5 min, 1, 6 or 24 h. All animals were sacrificed at 28 dpi. Viable axons distal to the injury were quantified using semithin sections, and regenerative fibers were studied using antisera to neurofilament and GAP43. Axonal degeneration and glial scar development were analyzed using Fluoro-Jade staining and anti-GFAP, respectively. Visual function was studied with visual evoked potentials (VEP). No significant differences were observed between 1 and 6 h of optic nerve compression. However, the number of viable axons analyzed with neurofilament and on semithin sections, decreased significantly between 6 and 24 h, paralleled by an increase in Fluoro-Jade labeled axonal debris (P < 0.001). GFAP-IR density was significantly higher (P < 0.001) in the 24 h compression group in comparison to 6 h. VEP showed preserved, but impaired visual function in animals subjected to compression up to 6 h, compared to an abolished cortical response at 24 h. Regenerative GAP43-positive sprouts were occasionally found distal to the lesion in animals subjected to compression up to 6 h, but not at 24 h. These findings suggest that early optic nerve decompression within hours after the initial trauma is beneficial for functional outcome.

Fibroblast growth factor-2 gene delivery stimulates axon growth by adult retinal ganglion cells after acute optic nerve injury

Basic fibroblast growth factor (or FGF-2) has been shown to be a potent stimulator of retinal ganglion cell (RGC) axonal growth during development. Here we investigated if FGF-2 upregulation in adult RGCs promoted axon regrowth in vivo after acute optic nerve injury. Recombinant adeno-associated virus (AAV) was used to deliver the FGF-2 gene to adult RGCs providing a sustained source of this neurotrophic factor. FGF-2 gene transfer led to a 10-fold increase in the number of axons that extended past 0.5 mm from the lesion site compared to control nerves. Detection of AAV-mediated FGF-2 protein in injured RGC axons correlated with growth into the distal optic nerve. The response to FGF-2 upregulation was supported by our finding that FGF receptor-1 (FGFR-1) and heparan sulfate (HS), known to be essential for FGF-2 signaling, were expressed by adult rat RGCs. FGF-2 transgene expression led to only transient protection of injured RGCs. Thus the effect of this neurotrophic factor on axon extension could not be solely attributed to an increase in neuronal survival. Our data indicate that selective upregulation of FGF-2 in adult RGCs stimulates axon regrowth within the optic nerve, an environment that is highly inhibitory for regeneration. These results support the hypothesis that key factors involved in axon outgrowth during neural development may promote regeneration of adult injured neurons.

Methylprednisolone Treatment Does Not Influence Axonal Regeneration or Degeneration Following Optic Nerve Injury in the Adult Rat

BACKGROUND

Methylprednisolone (MP) is often used to treat optic nerve injury. However, its effects in experimental crush injury have not been extensively evaluated.

METHODS

Adult Sprague-Dawley rats were subjected to a standardized optic nerve crush injury. Animals were treated either with 30 mg/kg MP intravenous bolus followed by subcutaneous injections every 6 hours for 48 hours, or with a drug vehicle alone.

RESULTS

The injury resulted in a partial loss of neuronal nuclei-labeled retinal neurons and a corresponding degeneration of axons distal to the injury. EDI-labeled macrophages accumulated at the site of lesion, phagocyting FJ-labeled axonal debris. Regenerative fibers expressing growth associated protein-43 were seen proximal to the lesion, but did not traverse the glial scar. Analysis of optic nerve function using visual evoked potentials showed typical signals in intact animals, which were abolished after injury in MP-treated and untreated animals.

CONCLUSIONS

We did not detect any effects of MP on retinal cell survival, macrophage activity at the site of injury, axonal degeneration/regeneration, or visual function. These experimental results provide a physiologic underpinning for the lack of efficacy demonstrated in a large trial of MP treatment of clinical optic nerve injury.

Complement Activation following Optic Nerve Crush in the Adult Rat

Activation of the complement cascade following peripheral nerve axotomy and following traumatic brain injury has been demonstrated in previous studies. This study investigates the temporal pattern of microglia/macrophages and complement activation following axotomy of sensory CNS neurons, using a standardized experimental crush injury of the optic nerve in adult rats. Numerous ED1-labeled macrophages were found at the lesion site and distal to the injury at 7 days post injury (dpi). Complement C3-mRNA was upregulated 2-28 days post lesion, indicating local synthesis of complement in the optic nerve. Furthermore, increased immunoreactivity (IR) for the end product of the complement cascade, the membrane attack complex (MAC), was detected along disintegrating axons co-labeled with anti-neurofilament distal to the injury. Double-labeling for microglia show MAC-immunoreactivity expressed in their immediate vicinity, indicating a key role of microglia/macrophages in complement activation. The complement regulator Clusterin was upregulated in astrocytes at the lesion site as well as in the distal portion of the injured optic nerve, suggesting activation of a defense response to endogenous complement attack. A crush injury of the optic nerve leads to complement activation at the site of lesion and along the distal portion of the nerve, as well as upregulation of the complement inhibitor Clusterin at least in astrocytes. Reactive microglial cells seem to have a key role in complement activation as a local source of C3. We suggest that the balance between complement activation and their regulators may have impact on axonal degeneration following optic nerve injury.

L- and D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) inhibit neurite outgrowth from SH-SY5Y cells

Gangliosides and extracellular matrix molecules influence neurite outgrowth, but the combinatorial effects of these endogenous agents on outgrowth are unclear. Exogenous gangliosides inhibit neurite outgrowth from SH-SY5Y cells stimulated with platelet-derived growth factor-BB, and different isoforms of the ceramide analog threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) stimulate (L-PDMP) or inhibit (D-PDMP) glycosphingolipid biosynthesis. In this study, we determined whether altering the endogenous ganglioside levels with PDMP in SH-SY5Y cells regulates neurite outgrowth on the outgrowth-supporting extracellular matrix molecule, laminin. In cells stimulated with 20 ng/ml platelet-derived growth factor-BB to promote outgrowth, we used image analysis to evaluate neurite outgrowth from SH-SY5Y cells grown on endogenous matrix or laminin and exposed to L- or D-PDMP. Both L- and D-PDMP decreased neurite initiation (the number of neurites/cell, the percent of neurite-bearing cells), elongation (the length of the longest neurite/cell, the total neurite length/cell), and branching (the number of branch points/neurite) from SH-SY5Y cells on endogenous matrix or laminin in a dose-dependent manner in serum-free or serum-containing medium. The inhibitory effects of each PDMP isoform were reversible. Inhibition of neurite outgrowth by L-PDMP could be mimicked by addition of exogenous gangliosides or C2-ceramide. Our analyses of neurite outgrowth in SH-SY5Y cells, a model of developing or regenerating noradrenergic neurons, demonstrate that increasing or decreasing endogenous ganglioside levels decreases neurite outgrowth. These results may indicate that SH-SY5Y cells undergo tight regulation by gangliosides, possibly through modulation of growth/trophic factor- and/or extracellular matrix-activated signaling cascades.