Retinal axon regeneration in peripheral nerve, tectal, and muscle grafts in adult rats

Vascular insufficiency, not inflammation, contributes to chronic gliosis in a rat CNS transplantation model

PURPOSE

There is considerable variability in the extent and nature of the glial response to injury and neurodegeneration. Transplantation of fetal cortical tissue onto the brain of neonatal host rats or mice results in region-specific changes dependent on where the fetal tissue is placed. These changes include chronic astrocytic and microglial gliosis, oxidative stress, and altered metabolism of a number of proteins associated with the pathogenesis of Alzheimer's disease. Such changes are only observed in heterotopic (cortex-to-midbrain) grafts and are not observed in homotopic cortex-to-cortex grafts. We investigated two possible triggers for the region-specific gliosis observed in our transplant model hypothesizing that either i) poor vascularization and lack of blood brain barrier integrity or ii) an inflammatory response initiated by the transplantation process, contributed to establishing chronic pathological changes.

METHODS

We analyzed the time course of neovascularization, blood brain barrier permeability and inflammation using a combination of immunohistochemistry, enzyme-linked immunosorbant assay and Evan's blue dye extravasation techniques.

RESULTS

Blood brain barrier permeability and altered neovascularization occurred prior to the onset of gliosis in heterotopic grafts.

CONCLUSION

These data suggest that ischemic conditions and blood brain barrier damage can be a primary mechanism that initiates chronic gliosis and associated inflammatory changes in central nervous system tissue.

Scaffolds to promote spinal cord regeneration

Substantial research effort in the spinal cord injury (SCI) field is directed towards reduction of secondary injury changes and enhancement of tissue sparing. However, pathway repair after complete transections, large lesions, or after chronic injury may require the implantation of some form of oriented bridging structure to restore tissue continuity across a trauma zone. These matrices or scaffolds should be biocompatible and create an environment that facilitates tissue growth and vascularization, and allow axons to regenerate through and beyond the implant in order to reconnect with "normal" tissue distal to the injury. The myelination of regrown axons is another important requirement. In this chapter, we describe recent advances in biomaterial technology designed to provide a terrain for regenerating axons to grow across the site of injury and/or create an environment for endogenous repair. Many different types of scaffold are under investigation; they can be biodegradable or nondegradable, natural or synthetic. Scaffolds can be designed to incorporate immobilized signaling molecules and/or used as devices for controlled release of therapeutic agents, including growth factors. These bridging structures can also be infiltrated with specific cell types deemed suitable for spinal cord repair.

Gene therapy and transplantation in CNS repair: The visual system

Normal visual function in humans is compromised by a range of inherited and acquired degenerative conditions, many of which affect photoreceptors and/or retinal pigment epithelium. As a consequence the majority of experimental gene- and cell-based therapies are aimed at rescuing or replacing these cells. We provide a brief overview of these studies, but the major focus of this review is on the inner retina, in particular how gene therapy and transplantation can improve the viability and regenerative capacity of retinal ganglion cells (RGCs). Such studies are relevant to the development of new treatments for ocular conditions that cause RGC loss or dysfunction, for example glaucoma, diabetes, ischaemia, and various inflammatory and neurodegenerative diseases. However, RGCs and associated central visual pathways also serve as an excellent experimental model of the adult central nervous system (CNS) in which it is possible to study the molecular and cellular mechanisms associated with neuroprotection and axonal regeneration after neurotrauma. In this review we present the current state of knowledge pertaining to RGC responses to injury, neurotrophic and gene therapy strategies aimed at promoting RGC survival, and how best to promote the regeneration of RGC axons after optic nerve or optic tract injury. We also describe transplantation methods being used in attempts to replace lost RGCs or encourage the regrowth of RGC axons back into visual centres in the brain via peripheral nerve bridges. Cooperative approaches including novel combinations of transplantation, gene therapy and pharmacotherapy are discussed. Finally, we consider a number of caveats and future directions, such as problems associated with compensatory sprouting and the reformation of visuotopic maps, the need to develop efficient, regulatable viral vectors, and the need to develop different but sequential strategies that target the cell body and/or the growth cone at appropriate times during the repair process.

Adult olfactory ensheathing glia promote the long-distance growth of adult retinal ganglion cell neurites in vitro

In vivo, transplanted adult olfactory ensheathing glia (OEG) and adult Schwann cells (SC) can support the regrowth of at least some transected axons within adult CNS neuropil. In the present study, we developed an in vitro adult rat retinal explant model to explore the influence of primary adult SC and OEG on retinal ganglion cell (RGC) neurite regrowth in the presence of glial cells endogenous to the retina. Retinal quadrants were plated RGC-side down onto aclar hats coated with either pure collagen (type 1), collagen with OEG, collagen with SCs, or collagen coated with both OEG and SCs. Regrowing retinal neurites extended onto the pure collagen substrate, largely in association with astrocytes that migrated out from the explants (mean number of neurites: 144+/-65 SEM). The additional presence of OEG (669+/-122), but not SCs (97+/-41), supported the regrowth of significantly greater numbers of RGC neurites. Furthermore, this OEG-stimulated regeneration was over significantly greater distances; >68% of neurites extended >500 microm from the explant, compared with explants plated onto SCs or collagen alone (15% and 29%, respectively). When OEG and SCs were co-cultured the number of regenerating neurites was reduced (397+/-81) compared with the pure OEG treatment. Analysis of explants on pure collagen substrates fed with media conditioned by purified OEG or SC showed no increase in neurite outgrowth compared with control treatments, suggesting that the enhanced growth in the presence of OEG is a contact-mediated effect. The observed differences between the abilities of OEG and SC to support the growth of CNS-derived fibers in the presence of astrocytes support the suggestion that OEG may be better suited for direct transplantation into CNS neuropil following injury.

Degeneration and regeneration of perivascular innervation in arterial grafts

Because the understanding of postoperative changes in arterial graft innervation is limited, this study was performed to characterize neuronal degeneration and regeneration events immunohistochemically in femoral arterial grafts transplanted to carotid arteries in rats. Specimens taken 1 day, 3 days, 7 days, 1 month, 3 months, and 5 months after surgery were assessed for vasoactive intestinal peptide, neurofilaments, growth-associated protein 43, tyrosine hydroxylase, and nitric oxide synthase isoenzymes. During neuronal degeneration, vasoactive intestinal peptide disappeared within 1 day, transmitter-synthesizing enzymes (nitric oxide synthase and tyrosine hydroxylase) had vanished by day 7, and neurofilaments (cytoskeletal markers) had essentially disappeared after 1 week. In the regeneration phase, the most robust axonal growth, as visualized by growth-associated protein 43, was observed at 1 month, followed by a gradual increase in neurotransmitter markers at 1 and 3 months, whereas the neurofilaments increased gradually up to the end of the 5-month observation period. Reinnervation proceeded from the proximal carotid (host) trunk distally to the graft. Axonal re-growth occurred mainly in arterial adventitia. Innervation density, as visually assessed, was denser in the graft than in the host. These findings suggest that 1) the main sequence of degeneration and regeneration follows that reported in other models of neuronal degeneration; 2) reinnervation of the arterial grafts comes mainly from the host arteries; and 3) the innervation density in the graft may differ from that in the host, which may suggest target-derived regulation of innervation. The latter finding may have clinical implications. It suggests that for a good outcome it would be beneficial to choose a sparsely innervated graft rather than a densely innervated one.

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.

Bcl-2 deficiency deprives peripheral nerves of neurotrophic activity against injured optic nerve

Optic nerve injury leads to retinal ganglion cell apoptosis, thus preventing fiber regeneration. Peripheral nerve grafts are known to promote survival and regeneration in injured adult mammalian central nervous system, including optic nerve, but the mechanisms of their activity remain unclear. It is likely that they attenuate the apoptotic cascade triggered by axotomy in retinal ganglion cells. The aim of this work was to examine the role of the antiapoptotic gene bcl-2 in the optic nerve regeneration induced by such grafts. Experiments were carried out on bcl-2-deficient and wild-type mice. We have reported previously that predegeneration markedly enhances neurotrophic activity of peripheral nerve grafts, so we applied both predegenerated and non-predegenerated implants to the transected optic nerves. We studied the neurotrophic effects of bcl-2-deficient grafts on wild-type and bcl-2 knock-out optic nerves, as well as wild-type grafts on both strains of mouse optic nerves. After application of fluorescent dye to the end of the graft, we counted the stained retinal ganglion cells. Predegenerated wild-type grafts promoted survival and outgrowth of retinal ganglion cells axons in both types of mice. By contrast, non-predegenerated and predegenerated bcl-2-deficient grafts induced little or no regeneration in the optic nerves. These results indicate that the lack of bcl-2 gene does not deprive retinal ganglion cells of their regenerative potential. At the same time, we found that bcl-2 knock-out dispossesses peripheral nerves of their neurotrophic activity.

Intravitreal injection of adeno-associated viral vectors results in the transduction of different types of retinal neurons in neonatal and adult rats: a comparison with lentiviral vectors

Replication-deficient viral vectors encoding the marker gene green fluorescent protein (GFP) were injected into the vitreous of newborn, juvenile (P14), and adult rats. We tested two different types of modified virus: adeno-associated viral-2-GFP (AAV-GFP) and lentiviral-GFP vectors (LV-GFP). The extent of retinal cell transduction in different-aged animals was compared 7, 21, and 70 days after eye injections. At all postinjection times, LV-GFP transduction was mostly limited to pigment epithelium and cells in sclera and choroid. In contrast, transduction of large numbers of neural retinal cells was seen 21 and 70 days after AAV-GFP injections. AAV-GFP predominantly transduced neurons, although GFP-positive Müller cells were seen. All neuronal classes were labeled, but the extent of transduction for a given class varied depending on injection age. After P0 injections about 50% of transduced cells were photoreceptors and 30-40% were amacrine or bipolar cells. After adult injections 60-70% of transduced cells were retinal ganglion cells. In adults many GFP-positive retinal axons were traced through the optic nerve/tract and terminal arbors were visualized in central targets.

Viral vector-mediated gene expression in olfactory ensheathing glia implants in the lesioned rat spinal cord

Implantation of olfactory ensheathing glia (OEG) is a promising strategy to augment long-distance regeneration in the injured spinal cord. In this study, implantation of OEG following unilateral hemisection of the dorsal cervical spinal cord was combined with ex vivo gene transfer techniques. We report, to our knowledge for the first time, that purified cultures of primary OEG are capable of expressing a foreign gene following adenoviral (AdV) and lentiviral (LV) vector-mediated gene transfer. OEG implants subjected to AdV vector-mediated gene transfer expressed high levels of transgenic protein in both intact and lesioned spinal cord at 7 days after implantation. However, the levels of transgene expression gradually declined between 7 and 30 days after implantation in lesioned spinal cord. Infection with LV vectors resulted in stable transduction of primary OEG cultures and transgene expression persisted for at least 4 months after implantation. Genetic engineering of OEG opens the possibility of expressing additional neurotrophic genes and create optimal 'bridging' substrates to support spinal axon regeneration. Furthermore, stable transduction of OEG allows us to reliably study the behaviour of implanted cells and to obtain better understanding of their regeneration supporting properties.

Migration of cells into and out of peripheral nerve isografts in the peripheral and central nervous systems of the adult mouse

Peripheral nerve (PN) isografts provide a favourable environment for axon regeneration after peripheral and central nervous system (CNS) injury, but definitive information on the extent of cellular intermixing between donor and host tissues is lacking. We wished to compare migration patterns in fresh and predegenerate PN grafts, and also compare the extent of cell migration after transplantation to peripheral nervous system (PNS) versus CNS. To discern how host and donor cells interact after PN transplantation, sciatic nerve segments were transplanted from inbred adult mice into PN defects (PN-PN grafts) or into lesioned cerebral cortex of opposite gender siblings. Migrating male cells were identified using a Y-chromosome-specific probe and in situ hybridization methods, and characterized immunohistochemically. The extent of donor and host cellular intermixing was similar in fresh and predegenerate PN-PN isografts. There was substantial intermixing of donor and host cells by 8 days. Many host cells migrating into epineurial regions of grafts were immunopositive for F4/80 (macrophages). The endoneurium of grafted PN was also colonized by host cells; some were F4/80+ but many were immunostained with S-100 (Schwann cell marker). Donor S-100+ Schwann cells rapidly migrated out into proximal and distal host PN and by 12 weeks were found at least 2 mm from the grafts. Endoneurial microvessels in grafts were mostly donor-derived. By comparison, in male PN grafts to female CNS, even after 6 weeks few donor cells had migrated out into surrounding host cortex, despite the observation that almost all grafts contained regenerating axons and were thus attached to host CNS tissue.

A new type of biocompatible bridging structure supports axon regrowth after implantation into the lesioned rat optic tract

We have developed a new type of polymer/cell/matrix implant and tested whether it can promote the regrowth of retinal ganglion cell (RGC) and other axons across surgically induced tissue defects in the CNS. The constructs, which consisted of 2-2.5-mm-long polycarbonate tubes filled with lens capsule-derived extracellular matrix coated with cultured neonatal Schwann cells, were implanted into lesion cavities made in the left optic tract (OT) of 18-21-day-old rats. In one group, to promote Schwann cell proliferation and perhaps also to stimulate axon regrowth, basic fibroblast growth factor (bFGF) was added to the lens capsule matrix prior to implantation. In another group, to determine whether application of growth factors to the somata of cells enhances the regrowth of distally injured axons, the neurotrophin NT-4/5 was injected into the eye contralateral to the OT lesion. NT-4/5 and bFGF treatments were combined in some rats. After medium-term (4-10 weeks) or long-term (15-20 weeks) survivals, axon growth into implants was assessed immunohistochemically using a neurofilament (RT97) antibody. RGC axons were visualized after injection of WGA/HRP into the right eye. Viable Schwann cells were present in implants at all times after transplantation. Large numbers of RT97+ axons were consistently found within the bridging implants, often associated with the peripheral glia. Axons were traced up to 1.7 mm from the nearest CNS neuropil and there was immunohistochemical evidence of myelination by Schwann cells and by host oligodendrocytes. There were fewer RGC axons in the implants, fibers growing up to 1.6 mm from the thalamus. Neither NT-4/5 nor bFGF, alone or in combination, significantly increased the extent of RGC axon growth within the implants. A group of OT-lesioned rats was implanted with polymer tubes filled with 2-2.5-mm-long pieces of predegenerate peripheral nerve. Surprisingly, polymer/cell/matrix constructs contained comparatively greater numbers of RGC and other axons and supported more extensive axon elongation. Thus, implants of this type may potentially be useful in bridging large tissue defects in the CNS.