Functional and morphological restoration of intracranial brachial lesion of the retinocollicular pathway by peripheral nerve autografts in adult hamsters

Induced pluripotent stem cell therapies for retinal disease

PURPOSE OF REVIEW

This review will discuss how recent advances with induced pluripotent stem (iPS) cells have brought the science of stem cell biology much closer to clinical application for patients with retinal degeneration.

RECENT FINDINGS

The ability to generate embryonic stem cells by reprogramming DNA taken from adult cells was demonstrated by the cloning of Dolly, the sheep, by somatic cell nuclear transfer, over 10 years ago. Recently, it has been shown that adult cells can be reprogrammed directly, without the need for a surrogate oocyte, through the generation of iPS cells. The method of reprogramming has since been optimized to avoid the use of retroviruses, making the process considerably safer. Last year, human iPS cells were isolated from an 80-year-old patient with neurodegenerative disease and differentiated into neurons in vitro.

SUMMARY

For stem cell therapies, the retina has the optimal combination of ease of surgical access, combined with an ability to observe transplanted cells directly through the clear ocular media. The question now is which retinal diseases are most appropriate targets for clinical trials using iPS cell approaches.

Nano neuro knitting: peptide nanofiber scaffold for brain repair and axon regeneration with functional return of vision

Nanotechnology is often associated with materials fabrication, microelectronics, and microfluidics. Until now, the use of nanotechnology and molecular self assembly in biomedicine to repair injured brain structures has not been explored. To achieve axonal regeneration after injury in the CNS, several formidable barriers must be overcome, such as scar tissue formation after tissue injury, gaps in nervous tissue formed during phagocytosis of dying cells after injury, and the failure of many adult neurons to initiate axonal extension. Using the mammalian visual system as a model, we report that a designed self-assembling peptide nanofiber scaffold creates a permissive environment for axons not only to regenerate through the site of an acute injury but also to knit the brain tissue together. In experiments using a severed optic tract in the hamster, we show that regenerated axons reconnect to target tissues with sufficient density to promote functional return of vision, as evidenced by visually elicited orienting behavior. The peptide nanofiber scaffold not only represents a previously undiscovered nanobiomedical technology for tissue repair and restoration but also raises the possibility of effective treatment of CNS and other tissue or organ trauma.

Changes in visual response properties of cat retinal ganglion cells within two weeks after axotomy

After optic nerve transection beta cells of cat retinal ganglion cells (RGCs) suffer from rapid cell death from 3 to 7 days, whereas alpha cells gradual cell death until 14 days. Here we report electrophysiological properties of Y- (morphological alpha) and X- (morphological beta) cells at 5 and 14 days after axotomy in comparison with those of intact Y- and X-cells. Most of the axotomized RGCs revealed characteristic visual response properties that enable us to classify them into Y- or X-cells. Physiological sampling ratio of X-cells sharply decreased from day 5 to 14 after axotomy, corresponding to the previous morphological results. As compared with intact RGCs, axotomized RGCs of both Y- and X-type revealed the following abnormalities: smaller receptive field centers, weaker visual responses and lower spontaneous activities. Intracellular injections of Lucifer yellow into axotomized and intact RGCs at eccentricities 0-6 mm from the area centralis revealed no sign of shrinkage in dendritic field size of either alpha or beta cells on day 5 and day 14 after axotomy, revealing that observed smaller receptive field centers of axotomized RGCs on day 5 were not due to the change of dendritic field sizes. These results suggest that the major events occurring shortly after axotomy are significant loss of synaptic inputs from afferent neurons in the retina and/or changes of membrane properties of axotomized RGCs. These events can also explain lower spontaneous activities and weaker visual responses of axotomized RGCs.

Cryosections of pre-irradiated adult rat spinal cord tissue support axonal regeneration in vitro

Neonatal X-irradiation of central nervous system (CNS) tissue markedly reduces the glial population in the irradiated area. Previous in vivo studies have demonstrated regenerative success of adult dorsal root ganglion (DRG) neurons into the neonatally-irradiated spinal cord. The present study was undertaken to determine whether these results could be replicated in an in vitro environment. The lumbosacral spinal cord of anaesthetised Wistar rat pups, aged between 1 and 5 days, was subjected to a single dose (40 Gray) of X-irradiation. A sham-irradiated group acted as controls. Rats were allowed to reach adulthood before being killed. Their lumbosacral spinal cords were dissected out and processed for sectioning in a cryostat. Cryosections (10 microm-thick) of the spinal cord tissue were picked up on sterile glass coverslips and used as substrates for culturing dissociated adult DRG neurons. After an appropriate incubation period, cultures were fixed in 2% paraformaldehyde and immunolabelled to visualise both the spinal cord substrate using anti-glial fibrillary acidic protein (GFAP) and the growing DRG neurons using anti-growth associated protein (GAP-43). Successful growth of DRG neurites was observed on irradiated, but not on non-irradiated, sections of spinal cord. Thus, neonatal X-irradiation of spinal cord tissue appears to alter its environment such that it can later support, rather than inhibit, axonal regeneration. It is suggested that this alteration may be due, at least in part, to depletion in the number of and/or a change in the characteristics of the glial cells.

Recovery of visual behaviors in adult hamsters with the peripheral nerve graft to the sectioned optic nerve

In adult hamsters, the autologous peripheral nerve (PN) was grafted to the sectioned optic nerve to make a bridge to the superior colliculus (SC). Three behavioral tasks were used to test functional recovery of the restored retinocollicular pathway. First, change of spontaneous ambulating activity to a decrease in environmental luminance was examined in an open field. PN-grafted hamsters showed a significant increase to 186% in ambulating activity just after light off, though it was lower than that in normal hamsters (489%). Second, a classical conditioning of total body movements was tested using an increase in luminance as a conditioned stimulus (CS) paired with foot shocks. In normal hamsters the magnitude of movements during CS increased in the acquisition period and then decreased in the extinction period in both the second and the third sessions, while the magnitude remained unchanged in a blind control. PN-grafted hamsters showed an increase in the magnitude only in the third session, although it was statistically barely significant (P = 0.0619). Following section of the grafted nerve, the conditioned response disappeared completely. And third, a shuttle-box avoidance task was examined using a flickering light as CS. Normal hamsters showed improved avoidance scores, while blind controls did not. PN-grafted hamsters showed a slight increase in the score, which was similar to that in the one-eyed control. Anterogradely transported labeling of WGA-HRP, injected into the vitreous body of the grafted eye, was observed in the graft and the superficial layers of SC. These results confirm and extend our previous finding that PN-grafted hamsters can restore some visual function and further suggest that the extent of recovered visual function is as good as in one-eyed animals.

Re-establishment of visual circuitry after optic nerve regeneration

In mammals there are a few circumstances in which axotomised ganglion cell axons can regenerate. For instance, in vitro explants of retina can be encouraged to regenerate axons into appropriate culture media. Similarly, axotomised ganglion cells can regenerate into a peripheral nerve graft surgically connected to the optic nerve head, and during early development axons are able to regenerate across the retina to re-enter the optic nerve. This is certainly encouraging, but we are a long way from applying these observations to clinical practice. We need to know whether regenerating axons also retain a functional capacity for navigation. We must ask whether a regenerated projection is likely to be topographic rather than disordered. In this brief review we will look at some selected models of ganglion cell regeneration in order to examine this question of navigation in more detail. This is an important issue: the capacity to re-establish appropriate rather than random connections after ganglion cell regeneration would most likely be necessary for any meaningful return of visual function.

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

Three separate experimental models of optic nerve regeneration have been presented--along the existing pathway in the presence of antibodies to neutralise inhibitory molecules, along peripheral nerve grafts and from retinal transplants. Each offers a theoretical clinical strategy for restoration of vision, if the mechanism of re-establishment of maps and reconnection to appropriate targets during regeneration can be determined. This is the process of axon guidance, and underlines the importance of our research into the molecular determinants that guide normal development of the visual system.

Regenerative capacity of retinal ganglion cells in mammals

Retinal ganglion cells (RGCs) and their projections in the optic nerve offer a convenient model to study survival and regeneration of mammalian central nervous system (CNS) nerve cells following injury. Possible factors affecting the death of RGCs following axotomy and various approaches to rescue the axotomized RGCs are discussed. In addition, two main strategies currently used to enhance axonal regeneration of damaged RGCs are described. The first focuses on overcoming the unfavorable extrinsic CNS environment and the second concentrates on upregulating the intrinsic growth potential of RGCs. Thus, the failure or success of RGC axonal regrowth after injury depends on the complicated interplay between the extrinsic and intrinsic factors.

Chiasmatic specificity in the regenerating mammalian optic nerve

The mammalian central nervous system is capable of regenerating; however, there is no evidence that the regenerating axons can navigate along their normal pathways and reestablish topographically organized projections: essential for functional return of vision. Here retinal ganglion cells in the opossum Monodelphis were birthdated with tritiated thymidine on the sixth postnatal day (P6), before being lesioned in the temporal retina at P8. Retrograde tracing with horseradish peroxidase injected into the ipsilateral optic tract at P24 showed that the temporal crescent had reformed behind the retinal lesion. By comparisons of cell and thymidine counts from lesioned and control regions of retina, it was estimated that about 40% of the normal number of ganglion cells are able to regenerate into the ipsilateral optic tract following a lesion in the temporal retina at P8. A clear line of decussation (separation of ipsilateral and contralateral projections) reformed in the lesioned temporal retina and regenerating ganglion cells labeled with DiI were turned at appropriate points on passing through the optic chiasm. This is evidence of chiasmatic specificity with regard to lesioned retinal ganglion cells regenerating into the ipsilateral optic tract.

Regeneration in the developing optic nerve: correlating observations in the opossum to other mammalian systems

Regeneration of severed axons within the central nervous system of adult mammals does not normally occur with any degree of success. During development, however, newly forming projections must send axons to distant sites and form appropriate connections with their targets: successful regeneration has been observed during this critical period. The opossum central nervous system develops during early postnatal life and has provided a useful experimental model to investigate this specialized mode of axonal regeneration in mammals. The presence of a clear decision point at the optic chiasm has also provided a useful site at which to investigate the navigational capacity of retinal ganglion cells regenerating along the optic nerve during this critical period. Regeneration failure occurs as the central nervous system progresses from this permissive, developing state to a mature, non-permissive adult state. Studies into the behaviour of glial and neuronal elements around this transition period can help elucidate some of the factors that need to be overcome if regeneration is ever to become successful in adult mammals. The regeneration characteristics of a lesioned projection are dependent upon its developmental stage and are also related to the proximity of axotomy along its pathway. A system of staging is proposed to correlate observations in the opossum optic nerve to other mammalian systems.

Light induced EEG desynchronization and behavioral arousal in rats with restored retinocollicular projection by peripheral nerve graft

Peripheral nerve (PN) was grafted to sectioned optic nerve and was bridged to the superior colliculus in adult rats. To test functional recovery of restored retinocollicular pathway, we examined cortical electroencephalogram (EEG) and behavioral arousal responses to light stimuli. In eight of 10 recording trials in PN grafted rats (n = 6) and in all of eight trials in normal rats (n = 5), cortical EEGs showed desynchronization to light stimuli. On the other hand, after bilateral sections of the optic nerve (n = 3) EEG desynchronization to light disappeared while it was induced by a white noise. Mean threshold duration of light for EEG desynchronization was significantly longer in the PN grafted rats (440 ms) than in normal rats (173 ms). In three of six trials in PN grafted rats (n = 4), and in four of eight trials in normal rats (n = 4), EEG desynchronization elicited by light stimulus was accompanied by behavioral arousal responses, whereas no behavioral arousal could be induced by light in blind rats (n = 3). These results strongly suggest that visual information processed through the restored retinocollicular pathway was further transmitted to the cerebral cortices and ultimately resulted in behavioral arousal of the PN grafted rats.

The effect of damage of the brachium of the superior colliculus in neonatal and adult hamsters and the use of peripheral nerve to restore retinocollicular projections

Using horseradish peroxidase (HRP) tracing technique, we were able to confirm the critical age in hamsters as reported previously (SO et al., 1981). Thus, following transection of the retinal fibers at the brachium of the superior colliculus (BSC) on postnatal-day 4 (P4) or later, no retinocollicular projections were observed in the adult stage. However, the retinal fibers were observed to reinnervate the superior colliculus (SC) if the BSC was cut on P3 or earlier. Physiological recording showed a close to normal retinocollicular map following a BSC damage on P0. Although retinal fibers did not reinnervate the SC following a BSC cut on or after P4, they could be observed to grow along a membrane over the damaged site. Bridging the site of BSC damage in adult hamsters using a segment of peripheral nerve (PN), retinal fibers labelled with WGA-HRP were observed to reinnervate the SC along the PN graft and visual evoked responses could be recorded in the SC showing the PN graft is effective in restoring damaged central visual pathways in adult mammals.