Neurobiology of the regenerating retina and its functional reconnection with the brain by means of peripheral nerve transplants in adult rats

Four steps to optic nerve regeneration

The failure of the optic nerve to regenerate after injury or in neurodegenerative disease remains a major clinical and scientific problem. Retinal ganglion cell (RGC) axons course through the optic nerve and carry all the visual information to the brain, but after injury, they fail to regrow through the optic nerve and RGC cell bodies typically die, leading to permanent loss of vision. There are at least 4 hurdles to overcome in preserving RGCs and regenerating their axons: 1) increase RGC survival, 2) overcome the inhibitory environment of the optic nerve, 3) enhance RGC intrinsic axon growth potential, and 4) optimize the mapping of RGC connections back into their targets in the brain.

Redefining tissue engineering for nanomedicine in ophthalmology

Working at the nanoscale means to completely rethink how to approach engineering in the body in general and in the eye in particular. In nanomedicine, tissue engineering is the ability to influence an environment either by adding, subtracting or manipulating that environment to allow it to be more conducive for its purpose. The goal is to function at the optimum state, or to return to that optimum state. Additive tissue engineering replaces cells or tissue, or tries to get something to grow that is no longer there. Arrestive tissue engineering tries to stop aberrant growth which, if left uncontrolled, would result in a decrease in function. Nano delivery of therapeutics can perform both additive and arrestive functions influencing the environment either way, depending on the targeting. By manipulating the environment at the nanoscale, the rate and distribution of healing can be controlled. It infers that potential applications of nanomedicine in ophthalmology include procedures, such as corneal endothelial cell transplantation, single retinal ganglion cell repair, check of retinal ganglion cell viability, building of nanofibre scaffolds, such as self-assembling peptides, to create a scaffold-like tissue-bridging structure to provide a framework for axonal regeneration in the case of optic nerve reconnection or eye transplantation, and ocular drug delivery. Examples of potential arrestive therapies include gene-related treatment modalities to inhibit intraocular neovascularization and to block retinal cell apoptosis. Looking towards the future, this review focuses on how nanoscale tissue engineering can be and is being used to influence that local environment.

Gene therapy and transplantation in the retinofugal pathway

The mature CNS has limited intrinsic capacity for repair after injury; therefore, strategies are needed to enhance the viability and regrowth of damaged neurons. Here we review gene therapy studies in the eye, aimed at improving the survival and regeneration of injured retinal ganglion cells (RGCs). To target RGCs most current methods use recombinant adeno-associated viral vectors (AAV), usually serotype-2 (AAV2), that are injected into the vitreal chamber of the eye. This vector provides long-term transduction of adult RGCs. Strong, constitutive promoters such as CMV and/or beta-actin are commonly used but cell-specific promoters have also been tested. Transgenes encoded by AAV have been selected to limit cell death, enhance growth factor expression, or promote growth cone responsiveness. We have assessed the effects of AAV vectors in adult rodent models (i) after optic nerve (ON) crush and (ii) after transplantation of peripheral nerve (PN) onto the cut ON, a procedure that induces injured RGCs to regenerate axons over longer distances. AAV-CNTF-GFP promotes RGC survival and axonal regrowth in mice after ON crush, and in rats after ON crush or PN transplantation. In rats, intravitreal injection of AAV-BDNF-GFP also increases RGC viability but does not promote regeneration. RGC viability and axonal regrowth is further enhanced when AAV-CNTF-GFP is injected into transgenic mice that over-express bcl-2. Reconstituted PN grafts containing Schwann cells that were transduced ex vivo with lentiviral (LV) vectors encoding a secretable form of CNTF support RGC axonal regrowth, however grafts containing Schwann cells transduced with LV-BDNF or LV-GDNF are less successful. We have also quantified the transduction efficiency and tropism of different AAV vectors injected intravitreally. AAV 2/2 and AAV 2/6 showed highest levels of transduction, AAV 2/8 the lowest, and each serotype displayed different transduction profiles for retinal cells. We are also studying the long-term impact of AAV2-mediated CNTF or BDNF expression on the dendritic morphology of RGCs in normal and PN grafted retinas. Analysis of regenerating RGCs intracellularly injected with lucifer yellow indicates gene-specific changes in dendritic structure that likely impact upon visual function.

Combined Therapies in the Treatment of Neurotrauma: Polymers, Bridges and Gene Therapy in Visual System Repair

BACKGROUND

The mature central nervous system (CNS) has limited capacity for self-renewal and repair after injury or neurodegeneration, and therapeutic strategies are needed to promote the viability of damaged neurons and the regrowth of their axons. The retina and optic nerve (ON) are part of the CNS, and the visual system is widely used in experimental studies on injury and repair.

OBJECTIVE

To test various cellular and molecular approaches in attempts to replace retinal ganglion cells (RGCs) in depleted retinas or, more usually, promote the survival of endogenous injured RGCs and stimulate axonal regeneration after ON or intracranial optic tract (OT) injury.

METHODS AND RESULTS

Intraocular injections of brain-derived neurotrophic factor and ciliary neurotrophic factor (CNTF) temporarily increase RGC survival after ON injury. More sustained neuroprotection is obtained using adeno-associated viral vectors to transfect RGCs with brain-derived neurotrophic factor or CNTF genes. After ON crush, intravitreal adeno-associated viral CNTF injections also increase RGC axonal regrowth. Additional protective and growth effects are obtained after intraocular elevation of cAMP and by manipulation of protein kinase signalling pathways in RGCs. Regeneration is increased by transplanting a segment of peripheral nerve onto the cut ON. Schwann cells in peripheral nerve grafts can be genetically modified using lentiviral vectors to over-express CNTF, resulting in increased regrowth of RGC axons. After OT lesions, hydrogels have been used to bridge the injury, sometimes with the incorporation of signalling peptides or cells genetically modified to express neurotrophic factors.

CONCLUSIONS

There is now a general consensus that combinatorial approaches are needed to elicit sustained and effective regenerative responses in injured adult CNS neurons.

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.

AAV-mediated expression of CNTF promotes long-term survival and regeneration of adult rat retinal ganglion cells

We compared the effects of intravitreal injection of bi-cistronic adeno-associated viral (AAV-2) vectors encoding enhanced green fluorescent protein (GFP) and either ciliary neurotrophic factor (CNTF), brain-derived neurotrophic factor (BDNF) or growth-associated protein-43 (GAP43) on adult retinal ganglion cell (RGC) survival and regeneration following (i) optic nerve (ON) crush or (ii) after ON cut and attachment of a peripheral nerve (PN). At 7 weeks after ON crush, quantification of betaIII-tubulin immunostaining revealed that, compared to AAV-GFP controls, RGC survival was not enhanced by AAV-GAP43-GFP but was increased in AAV-CNTF-GFP (mean RGCs/retina: 17 450+/-358 s.e.m.) and AAV-BDNF-GFP injected eyes (10 200+/-4064 RGCs/retina). Consistent with increased RGC viability in AAV-CNTF-GFP and AAV-BDNF-GFP injected eyes, these animals possessed many betaIII-tubulin- and GFP-positive fibres proximal to the ON crush. However, only in the AAV-CNTF-GFP group were regenerating RGC axons seen in distal ON (1135+/-367 axons/nerve, 0.5 mm post-crush), some reaching the optic chiasm. RGCs were immunoreactive for CNTF and quantitative RT-PCR revealed a substantial increase in CNTF mRNA expression in retinas transduced with AAV-CNTF-GFP. The combination of AAV-CNTF-GFP transduction of RGCs with autologous PN-ON transplantation resulted in even greater RGC survival and regeneration. At 7 weeks after PN transplantation there were 27 954 (+/-2833) surviving RGCs/retina, about 25% of the adult RGC population. Of these, 13 352 (+/-1868) RGCs/retina were retrogradely labelled after fluorogold injections into PN grafts. In summary, AAV-mediated expression of CNTF promotes long-term survival and regeneration of injured adult RGCs, effects that are substantially enhanced by combining gene and cell-based therapies/interventions.

Lentiviral-mediated transfer of CNTF to schwann cells within reconstructed peripheral nerve grafts enhances adult retinal ganglion cell survival and axonal regeneration

We recently described a method for reconstituting peripheral nerve (PN) sheaths using adult Schwann cells (SCs). Reconstructed PN tissue grafted onto the cut optic nerve supports the regeneration of injured adult rat retinal ganglion cell (RGC) axons. To determine whether genetic manipulation of such grafts can further enhance regeneration, adult SCs were transduced with lentiviral vectors encoding either ciliary neurotrophic factor (LV-CNTF) or green fluorescent protein (LV-GFP). SCs expressed transgenes for at least 4 weeks after transplantation. There were high levels of CNTF mRNA and CNTF protein in PN grafts containing LV-CNTF-transduced SCs. Mean RGC survival was significantly increased with these grafts (11,863/retina) compared with LV-GFP controls (7064/retina). LV-CNTF-transduced SCs enhanced axonal regeneration to an even greater extent (3097 vs 393 RGCs/retina in LV-GFP controls). Many regenerated axons were myelinated. The use of genetically modified, reconstituted PN grafts to bridge tissue defects may provide new therapeutic strategies for the treatment of both CNS and PNS injuries.

A new approach to CNS repair using chimeric peripheral nerve grafts

We have examined whether transplanted freeze-thawed peripheral nerve (PN) sheaths repopulated ex vivo with purified adult Schwann cells (SCs) support the regeneration of adult rat retinal ganglion cell (RGC) axons. Cultured adult SCs were derived from donor rats or from the host animals themselves. We also transplanted PN sheaths filled with neonatal SCs or donor adult olfactory ensheathing glia (OEG). 100,000 cells were injected into 1.5-cm lengths of freeze-thawed PN. After 2 days in culture, repopulated PN segments were grafted onto the transected optic nerve of adult Fischer rats. Three weeks later, 6% fluorogold (FG) was applied to distal PN. Retrogradely labeled RGCs were counted in retinal wholemounts and PN grafts were processed for immunohistochemistry. As expected, there was no RGC axon regeneration in cell-free grafts. Regrowth was also absent in neonatal SC- and adult OEG-filled grafts, which contained only small numbers of surviving donor cells. Many cells were, however, seen in adult SC repopulated PN grafts, intermingled with pan-neurofilament(+) and GAP-43(+) fibers. SCs were aligned along the grafts and were S-100(+), p75(+). Ultrastructurally, SCs were associated with myelinated and unmyelinated axons. Hundreds of FG-labeled RGCs were seen in retinas of rats with congeneic or allogeneic PN sheaths repopulated with either donor or autologous (host-derived) adult SCs. Intraocular CNTF injections significantly increased the number of regenerating RGCs in donor and autologous adult SC groups. The use of chimeric grafts to bridge CNS tissue defects could provide a clinical alternative to using multiple PN autografts, the harvesting of which would exacerbate peripheral dysfunction in already injured patients.

Optic nerve regeneration within artificial Schwann cell graft in the adult rat

We investigate whether an artificial graft made by cultured Schwann cell, extracellular matrix (ECM) and trophic factors can provide the environment for the regeneration of retinal ganglion cell (RGC) axons in adult rats. Six kinds of artificial grafts were used: ECM (control); ECM and Schwann cells; ECM, Schwann cells and either nerve growth factor, brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4); ECM, Schwann cells, BDNF and NT-4, combined with intravitreal injection of BDNF. The grafts were transplanted onto the transected optic nerve. RGC regeneration was evaluated by dil retrograde labeling, immunohistochemistry, and electron microscopy at 3 weeks post-operation. The degree of dil labeled RGC was approximately 2% for ECM alone, and 10% for ECM and Schwann cells (p < 0.01). The labeling increased to approximately 20% by administration of neurotrophins. The addition of intravitreous BDNF injection resulted in highest labeling percentage of 30%. Immunohistochemical study showed that axons were association with GAP-43 and cell adhesion molecules. Neurotrophin receptors (Trk-A and Trk-B) were detected in nerve fibers both in the retina and in the graft. Remyelination was seen by electron microscopic observation. These results demonstrate that the regeneration of RGC axons is induced with the use of cultured Schwann cells and ECM as promoting factors for regrowth. The degree of regeneration was significantly increased by neurotrophins in the grafts and in the vitreous.

Activation of a novel microglial gene encoding a lysosomal membrane protein in response to neuronal apoptosis

In an attempt to understand the molecular mechanism of microglial activation in response to neuronal death or degeneration, we have employed cerebellar cell cultures prepared from P7 rats and grown in normal K(+) (5.4 mM) medium. Under this condition, glial cells respond to degeneration and cell death of granule neurons that begins to occur at 4 days in vitro (DIV). Here we describe a novel gene, granule cell death-10 (gcd-10) that is expressed in microglia and up-regulated in an early period of granule cell death. gcd-10 is homologous to the mouse lysosomal-associated multispanning membrane protein (LAPTm5) with hematopoietic origin. Immunocytochemistry and vital staining with acridine orange revealed that GCD-10 was localized at the perinuclear area of cultured microglia and COS 1 cells infected with a GCD-10-expressing adenoviral vector. In cerebellar cell cultures, however, GCD-10 was markedly up-regulated and widely distributed to the cytoplasm, which paralleled the localization of the ED1 antigen, the lysosomal marker. In vivo, gcd-10 is expressed mainly in the brain and the spleen, and was up-regulated upon nerve injury in retina 7 days after optic nerve transection. These findings suggest that gcd-10 is involved in the dynamics of lysosomal membranes associated with microglial activation both in vitro and in vivo.

Role of Schwann cells in retinal ganglion cell axon regeneration

It is a well known fact that the injured PNS can successfully regenerate, on the other hand, the CNS such as retinal ganglion cell (RGC) axons of adult mammals is incapable of regeneration. After injury, RGC axons rapidly degenerate and most cell bodies go through the process of cell death, while glial cells at the site of injury undergo a series of responses which underlie the so-called glial scar formation. However, it has become apparent that RGCs do have an intrinsic capacity to regenerate which can be elicited by experimental replacement of the inhibitory glial environment with a permissive peripheral nerve milieu. Schwann cells are a major component of the PNS and play a role in regeneration, by producing various kinds of functional substances such as diffusible neurotrophic factors, extracellular matrix and cell adhesion molecules. RGC regeneration can be induced by cooperation of these substances. The contact of RGC axons to Schwann cells based upon the structural and molecular linkages seems to be indispensable for the stable and successful regeneration. In addition to cell adhesion molecules such as NCAM and L1, data from our laboratory show that Schwann cells utilize short focal tight junctions to provide morphological stabilization of the contact with the elongating axon, as well as a small scale of gap junctions to facilitate traffic of substances between them. Moreover, our results show that modifications of functional properties in neighboring glial cells of optic nerve are induced by transplantation of Schwann cells. Astrocytes usually considered to form a glial scar guide the regenerating axons in cooperation with Schwann cells. A decrease of the oligodendrocyte marker O4 and migration of ED-1 positive macrophages is observed within the optic nerve stump. Accordingly, RGC regeneration is not a simple phenomenon of axonal elongation on the Schwann cell membrane, but is based on direct and dynamic communication between the axon and the Schwann cell, and is also accompanied by changes and responses among the glial cell populations, which may be partly associated with the mechanisms of optic nerve regeneration.

Purification of chick retinal ganglion cells for molecular analysis: combining retrograde labeling and immunopanning yields 100% purity

Retinal ganglion cells (RGCs) from embryonic and posthatch chickens were 100% purified by a novel combination of three steps: (1) Retrograde labeling by injection of the fluorescent carbocyanine tracer DiI into the optic nerve, (2) immunopanning of dissociated retinal cells with Thy1 antibodies, and (3) micro-aspiration of labeled RGCs into glass capillaries. The retina was dissected and dissociated with trypsin 12-15 h after the injection of DiI. DiI-labeled cells were identified on immunopanned dishes by fluorescence and collected for molecular analysis within 3 h after dissociation. This technique allowed the collection of up to 500 RGCs per capillary tube and 1500 labeled RGCs per retina. Extraction of RNA and molecular analysis by RT-PCR from 600 RGCs shows that expression of rare genes, such as those of neurotrophic factors, can be detected. This is the first description of a rapid and reliable technique for a 100% purification of RGCs with sufficient yield for molecular analysis of rare gene expression. The protocol can be modified for the purification of other cell types. The advantages and limitations of the three-step purification method are compared with previous RGC purification protocols.