Transplant of embryonal nervous tissue preserves the responses of rat retinal ganglion cells after section of the optic nerve

Retinal transplantation--advantages of intact fetal sheets

Retinal transplantation aims to prevent blindness and to restore eyesight, i.e., to rescue photoreceptors or to replace damaged photoreceptors with the hope of reestablishing neural circuitry. Retinal donor tissue has been transplanted as dissociated cells or intact sheets. A promising experimental paradigm is the subretinal transplantation of sheets of fetal retina with or without its attached retinal pigment epithelium (RPE) into recipient rats with retinal degeneration. As long as healthy RPE either from the host or from the graft is present, such transplants can develop lamination resembling a normal retina. Different methods have been used to demonstrate transplant/host connectivity. In two different rat retinal degeneration models, visually evoked responses can be demonstrated in an area of the superior colliculus corresponding to the placement of the transplant in the retina. In summary, sheets of fetal retina can morphologically repair an area of a degenerated retina, and there is evidence to suggest that transplants form synaptic connections with the host and restore visual responses in blind rats.

Retinal ganglion cells regenerating through the peripheral nerve graft retain their electroretinographic responses and mediate light-induced behavior

To assess the light-induced electrical activity of rodent retinal ganglion cells (RGCs) regenerating into a peripheral nerve (PN) graft we used non-invasive recording of electroretinographic responses to the contrast-reversal of sinusoidal gratings (p-ERG). On comparing the retinas that received a PN graft and retinas with only optic nerve (ON) transection, p-ERG responses were present in grafted retinas as late as 20 months after the surgery while they completely disappeared in non-transplanted controls within 4 months of ON transection. Next, the ability of regenerating RGCs to form functional connections with their targets in the superior colliculus (SC) was tested by a light-escape task. While the bilaterally blinded animals did not improve during the test, unilaterally grafted animals (with the contralateral eye blinded) reached 26% success in the last quartile of the light-escape task. This performance was significantly better than that of blind animals (ANOVA and Student-Newman-Keuls test; p<0.05), but did not reach the level of intact rats (87%). The transplanted rats, therefore, were capable of light perception, but at a sub-normal ability. In addition, we were also able to correlate the amplitude of the p-ERG response with the visual behavioral performance for each transplanted animal. This finding indicates that there is a direct link between the RGC electrophysiological activity and the functional capacity of the regenerated visual pathway. In conclusion, the above results indicate that (a) PN grafts help to preserve the normal electroretinographic activity of injured and regenerating RGCs (b) the regenerated visual pathway is functional and capable of mediating simple visual behavior and that (c) there is a correlation between the light-evoked RGC electrical activity and visual behavior and, finally, that (d) the effect of PN graft on the electrophysiological and functional restoration of the visual pathway is long-lasting or even permanent.

The visual response of retinal ganglion cells is not altered by optic nerve transection in transgenic mice overexpressing Bcl-2

Attempts to rescue retinal ganglion cells from retrograde degeneration have had limited success, and the residual function of surviving neurons is not known. Recently, it has been found that axotomized retinal ganglion cells die by apoptotic mechanisms. We have used adult transgenic mice overexpressing the Bcl-2 protein, a powerful inhibitor of apoptosis, as a model for preventing injury-induced cell death in vivo. Several months after axotomy, the majority of retinal ganglion cells survived and exhibited normal visual responses. In control wild-type mice, the vast majority of axotomized retinal ganglion cells degenerated, and the physiological responses were abolished. These results suggest that strategies aimed at increasing Bcl-2 expression, or mimicking its function, might effectively counteract trauma-induced cell death in the central nervous system. Neuronal survival is a necessary condition in the challenge for promoting regeneration and eventually restoring neuronal function.

NADPH diaphorase expression in the rat retina after axotomy--a supportive role for nitric oxide

The large majority of mammalian retinal ganglion cells degenerate following section of their axons in the optic nerve. It has been suggested that some axotomized retina ganglion cells die because of toxic agents produced within their immediate environment. Our hypothesis was that nitric oxide might be one of the toxic factors implicated in the death of adult retinal ganglion cells post-axotomy. In the first instance, we determined whether there were any changes in the retinal expression of NADPH diaphorase both 3 and 14 days following intraorbital section of the optic nerve in adult rats. Secondly, if nitric oxide was indeed implicated in the death of ganglion cells, then trophic factors which rescue these neurons might do so by decreasing the expression of nitric oxide synthase. Recently, we found that a collicular proteoglycan purified from the major target of retinal ganglion cells, the superior colliculus, rescued a greater proportion of adult ganglion cells from axotomy-induced death than most other known trophic factors. We thus injected this proteoglycan intraocularly after section of the optic nerve and examined its effect on the expression of NADPH diaphorase in the retina. Thirdly, an inhibitor of nitric oxide synthetase was repeatedly injected into the eye following the section of the optic nerve in order to determine if such a treatment might improve the survival of retinal ganglion cells. The present results indicate that section of the optic nerve does not alter the overall levels of NADPH diaphorase within the adult rat retina. Intraocular injections of the collicular proteoglycan actually increased the number of neurons expressing NADPH diaphorase, particularly in the ganglion cell layer. Finally, inhibition of nitric oxide synthetase following axotomy resulted in increased loss of retinal ganglion cells over a 2 week period when compared with controls. Our findings indicate that, rather than being toxic, small amounts of nitric oxide may be important for the survival of a proportion of injured retina ganglion cells.

Effect of collicular proteoglycan on the survival of adult rat retinal ganglion cells following axotomy

Consistent with numerous previous studies, we have found that in adult rats 29% of cells retrogradely prelabelled by injections into retino-recipient nuclei are lost 1 week after intraorbital section of the optic nerve. This figure increases to 76% 2 weeks after axotomy. Intraocular injections of 150 ng of 480 kDa chondroitin sulphate proteoglycan purified from the superior colliculi of neonatal rats were performed every third day after axotomy. This procedure resulted in the loss of only 3 and 28% of the axotomized retinal ganglion cells 7 and 14 days respectively after optic nerve section. Intraocular injections of chondroitin sulphate type C, one of the sugar types present on the collicular proteoglycan, also resulted in a significant saving of axotomized ganglion cells (with the loss of only 48% 14 days after optic nerve lesion). These findings suggest that the collicular proteoglycan, and to a lesser extent its sugar moieties, substantially slows down the degeneration of adult retinal ganglion cells following axotomy.

Time-course and extent of retinal ganglion cell death following ablation of the superior colliculus in neonatal rats

This study has examined the deleterious effect of superior colliculus (SC) ablation on the viability of identified retinotectally projecting ganglion cells in the neonatal rat retina. The time-course and extent of lesion-induced retinal ganglion cell (rgc) death has been determined and an estimate obtained for the rate of clearance of individual dying neurons. In order to demonstrate the projection of rgcs to the SC and the subsequent death of these same neurons after SC lesions, the fluorescent dye diamidino yellow (DY) was injected into the left SC of anesthetized 2 day old Wistar rats (P2: day of birth = P0). DY retrogradely labels the nuclei of tectally projecting rgcs; if these identified rgcs subsequently die, their DY-labelled nuclei become pyknotic and can be visualized in retinal wholemounts. At P4 the rats were again anesthetized and the injected area, seen as a yellow patch in the SC, was removed by aspiration. Rats were perfused 2 to 336 hours after the lesion and retinal wholemounts of the right eye were prepared. Control rats received only DY injections and were perfused at times corresponding to the lesioned animals. In three sham-operated rats; the injected SC was reexposed at P4 but the tectal tissue was not removed. In each of the 42 rats that were analyzed, about 10% of the retina containing retrogradely labelled rgcs was counted; the number of pyknotic versus normally labelled rgcs was determined and changes in normal cell density were also assessed. Pyknotic rates in control and sham-operated rats were similar (average 0.8%, n = 11). In SC-lesioned rats, the proportion of pyknotic DY-labelled rgcs increased to about 2.5% 4 to 8 hours postlesion (PL); the peak period of death occurred at 23 hours PL (8.0%). The amount of pyknosis decreased thereafter and most dying cells had been eliminated by 50 hours PL. Phagocytosis of dying cells was a common feature of retinae in SC lesioned rats. In the long-term (336 hours) rats, counts of normal DY-labelled rgcs in corresponding regions of control and lesioned rats revealed an average decrease in rgc density of 47.3% after P4 tectal ablation. Calculations suggest a clearance time of about 3 hours for dying neonatal rgcs.

Neuronal and nonneuronal influences on retinal ganglion cell survival, axonal regrowth, and connectivity after axotomy

In contrast to the abortive regrowth that occurs when axons are interrupted in the adult mammalian CNS, exposure of injured CNS axons to the nonneuronal milieu of a peripheral nerve can lead to extensive axonal elongation. With the application of this experimental approach to the retinocollicular pathway in adult rodents, it has been possible to investigate the influences of neuron-glia and other interactions on the capacity of axotomized CNS neurons to survive injury, to elongate the distances necessary to reach specific targets, and to form connections in the CNS in adult rodents. The results of these investigations indicate that the changed glial environment provided by peripheral nerve grafts permits the guided regeneration of RGC axons to their CNS targets. Back in the CNS glial environment, regenerated axons penetrate their targets for short distances and re-form normal appearing synapses that can excite or inhibit postsynaptic neurons. Further studies will require a better understanding of intrinsic neuronal properties and of the interactions of these neurons with other neurons and with the cellular and noncellular components of the extraneural milieu.