Adult Retinofugal Axons Regenerating Through Peripheral Nerve Grafts Can Restore the Light-induced Pupilloconstriction Reflex

Variable functional recovery and minor cell loss in the ganglion cell layer of the lizard Gallotia galloti after optic nerve axotomy

The lizard Gallotia galloti shows spontaneous and slow axon regrowth through a permissive glial scar after optic nerve axotomy. Although much of the expression pattern of glial, neuronal and extracellular matrix markers have been analyzed by our group, an estimation of the cell loss in the ganglion cell layer (GCL) and the degree of visual function recovery remained unresolved. Thus, we performed a series of tests indicative of effective visual function (pupillary light reflex, accommodation, visually elicited behavior) in 18 lizards at 3, 6, 9 and 12 months post-axotomy which were then processed for immunohistochemistry for the neuronal markers SMI-31 (neurofilaments), Tuj1 (beta-III tubulin) and SV2 (synaptic vesicles) at the last timepoint. Separately, cell loss in the GCL was estimated by comparative quantitation of DAPI(+) nuclei in control and 12 months experimental lizards. Additionally, 15 lizards were processed for electron microscopy to monitor relevant ultrastructural changes in the GCL, optic nerve and optic tract throughout regeneration. Hypertrophy of RGCs was persistent, morphology of the regenerated nerves varied from narrow to neuroma-like features and larger regenerated axons underwent remyelination by 9 months. The estimated cell loss in the GCL was 27% and two-third of the animals recovered the pupillary light reflex which involves the pretectum. Strikingly, visually elicited behavior involving the tectum was only restored in two specimens, presumably due to the higher complexity of this pathway. These preliminary results indicate that limited functional regeneration occurs spontaneously in the severely injured visual system of the lacertid G. galloti.

Neurotrophic factors and the regeneration of adult retinal ganglion cell axons

The adult central nervous system (CNS) has only a limited capacity to regenerate axons after injury. This is due to a number of factors including the presence of extrinsic inhibitory factors that limit plasticity, lack of effective trophic support, and intrinsic changes in neuronal responsiveness. In this review, we describe the expression and role of neurotrophins in retinal ganglion cells (RGCs) during development and adulthood, and the receptors and miscellaneous signaling systems that influence axonal regeneration after injury. The impact of exogenous neurotrophic factors on adult RGCs injured at different sites in the visual pathway is described for several modes of delivery, including recombinant factors, viral vectors, cell transplantation, as well as combinatorial treatments involving other pharmacotherapeutic agents. Indirect, off-target effects of neurotrophic factors on RGC axonal regeneration are also considered. There remain unresolved issues relating to optimal delivery of neurotrophic factors, and we emphasize the need to develop safe, reliable methods for the regulation of exogenous supply of these factors to the injured CNS.

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.

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.

A review of the potential to restore vision with stem cells

Vision research involving stem cells is a rapidly evolving field. Animal experiments have shown that in response to environmental cues, stem cells can repopulate damaged retinas, regrow neuronal axons, repair higher cortical pathways, and restore pupil reflexes, light responses and basic pattern recognition. Viable corneas have been grown from stem cells and transplanted into humans. Similarly, human trials to repair damaged retinas in retinitis pigmentosa and age-related macular degeneration patients have produced preliminary successes. This review attempts to place the collective contributions toward stem cell/vision research into a broader clinical model of how stem cells might ultimately be used to restore the entire visual pathway.

Injury to retinal ganglion cell axons increases polysialylated neural cell adhesion molecule (PSA-NCAM) in the adult rodent superior colliculus

The adult mammalian central nervous system (CNS) exhibits a limited regenerative response to injury. It is well established that polysialylated neural cell adhesion molecule (PSA-NCAM) contributes to nervous system plasticity. In the visual system, PSA-NCAM participates in retinal ganglion cell (RGC) axon growth during development and specifically influences RGC innervation of its principle target tissue, the superior colliculus (SC). The goals of this study were to determine whether PSA-NCAM is expressed in the normal adult mouse SC and to evaluate PSA-NCAM expression following RGC injury. In the normal rostral, but not caudal, SC we find that PSA-NCAM is present in the retinorecipient layers; however, PSA-NCAM and RGC axons do not co-localize. In the deeper collicular layers, PSA-NCAM is observed as several distinct patches that occur at the same depth along the medial-lateral axis throughout the colliculus. RGC axotomy denervates predominantly the contralateral colliculus, where increased PSA-NCAM levels are seen at 7 and 10 days after the injury. Further evaluation of the retinorecipient layers of the partially denervated SC reveals that some intact CTB-traced RGC axons (less than 5%) labeled from the ipsilateral eye do co-localize with PSA-NCAM. This study is the first characterization of PSA-NCAM expression in the normal and partially denervated adult SC and may indicate that PSA-NCAM is involved in attempted visual system remodeling after injury.

Attempts to restore visual function after optic nerve damage in adult mammals

Retinal ganglion cells (RGCs) and their axons, i.e., optic nerve (ON) fibers, provide a good experimental model for research on damaged CNS neurons and their functional ecovery. After the ON transection most RGCs undergo retrograde and anterograde degeneration but they can be rescued and regenerated by transplantation of a piece of peripheral nerve (PN). When the nerve graft was bridged to the visual center, regenerating RGC axons can restore the central visual projection. Behavioral recovery of relatively simple visual function has been proved in such PN-grafted rodents. Intravitreal injections of various neurotrophic factors and cytokines to activate intracellular signaling mechanism of RGCs and electrical stimulation to the cut end of ON have promoting effects on their survival and axonal regeneration. Axotomized RGCs in adult cats are also shown to survive and regenerate their axons through the PN graft. Among the cat RGC types, Y cells, which function as visual motion detector, tend to survive and regenerate axons better than others. X cells, which are essential for acute vision, suffer from rapid death after ON transection but they can be rescued by intravitreal application of neurotrophins accompanied with elevation of cAMP. To restore visual function in adult mammals with damaged optic pathway, the comprehensive and integrative strategies of multiple approaches will be needed, taking care of functional diversity of RGC types.

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.

Effects of Schwann cell secreted factors on PC12 cell neuritogenesis and survival

We have used PC12 cells to examine the effects of factors secreted by Schwann cells that promote cell survival and neurite outgrowth, and hence are likely candidates for promoting neuronal regeneration. RT-PCR showed that primary Schwann cells produced a range of neurotrophins, excluding NT3, but this profile was different from either of two cell lines SCTM41 or PVGSCSV40T, or forskolin-expanded Schwann cells. The effects of Schwann cell conditioned media on neurite outgrowth was tested against a range of factors, and showed clear neuritogenic effects. Of the factors tested, only NGF had a significant response on neuritogenesis. Western blotting for neurofilaments showed that primary Schwann cells induced a strong response close to that of NGF. The Trk tyrosine kinase inhibitor K252a did not block the neuritogenic effects of primary Schwann cells. In contrast, K252a blocked both NGF and the SCTM41 cell effects. Schwann cell conditioned media also enhanced PC12 cell survival. Again, in contrast with NGF or SCTM41 cells, the primary Schwann cell effect was Trk tyrosine kinase independent. The Schwann cell conditioned medium contains a protein factor (greater than 12 kDa and broken down by trypsin treatment) with remarkable thermal stability (unaffected at 95 degrees C for 15 min) and the ability to bind heparin. Our results provide clear evidence that Schwann cells produce factors other than those already known to stimulate a neural phenotype in PC12 cells, and which thus have potential regeneration enhancing effects.

Reinnervation of the pretectum in adult rats by regenerated retinal ganglion cell axons: anatomical and functional studies

We have investigated the specificity of reinnervation and terminal arborization of injured retinal ganglion cell (RGC) axons in the brainstem with the object of studying in a simple situation the degree to which regenerating axons are able to replicate the characteristic patterns of terminal arborization and restore normal function. We have focussed here on the pathway that is responsible for the pupillary light reflex, which is mediated through the olivary pretectal nucleus (OPN). In adult rats, the left optic nerve was transected and a segment of peripheral nerve (PN) graft was used to bridge between the retina and different regions of the ipsilateral brainstem, including the superior colliculus. After 4-13 months, regenerated RGC axons were examined in coronal sections stained for cholera toxin B subunit. RGC axons were found extending into the ipsilateral brainstem for distances of up to 6 mm. Within the pretectum, axons innervated the OPN and the nucleus of the optic tract preferentially, and formed distinctive terminal arbors within each. Within the SC axons extended laterally into the visual layers and formed a different type of arborization. On testing the pupillary light reflex, it was found in best cases to show response amplitudes which were comparable to those recorded from control intact animals. However, unlike normals, the response amplitude tended to diminish with repeated stimulation and also appeared to deteriorate with age, although responses could still be detected in some cases as long as 15 months after grafting. These results indicate that regenerating axons can selectively reinnervate denervated nuclei, where they form typical terminal arborizations, and provide the substrates for restoring functional circuitry.

Survival and axonal regeneration of retinal ganglion cells in adult cats

Axotomized retinal ganglion cells (RGCs) in adult cats offer a good experimental model to understand mechanisms of RGC deteriorations in ophthalmic diseases such as glaucoma and optic neuritis. Alpha ganglion cells in the cat retina have higher ability to survive axotomy and regenerate their axons than beta and non-alpha or beta (NAB) ganglion cells. By contrast, beta cells suffer from rapid cell death by apoptosis between 3 and 7 days after axotomy. We introduced several methods to rescue the axotomized cat RGCs from apoptosis and regenerate their axons; transplantation of the peripheral nerve (PN), intraocular injections of neurotrophic factors, or an antiapoptotic drug. Apoptosis of beta cells can be prevented with intravitreal injections of BDNF+CNTF+forskolin or a caspase inhibitor. The injection of BDNF+CNTF+forskolin also increases the numbers of regenerated beta and NAB cells, but only slightly enhances axonal regeneration of alpha cells. Electrical stimulation to the cut end of optic nerve is effective for the survival of axotomized RGCs in cats as well as in rats. To recover function of impaired vision in cats, further studies should be directed to achieve the following goals: (1). substantial number of regenerating RGCs, (2). reconstruction of the retino-geniculo-cortical pathway, and (3). reconstruction of retinotopy in the target visual centers.

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.

Quantification of the neurodegenerative impact on the visual system following sudden retrobulbar expanding lesions - an experimental model

INTRODUCTION

Afferent disorders of the visual system are a potential consequence following orbital trauma. The aim of this study was to investigate the tolerance of neurons in the retinal ganglion cell layer to acute expanding retrobulbar lesions.

MATERIAL AND METHODS

In 42 male albino rats, intraorbital haemorrhage was simulated by transconjunctival insertion of a retrobulbar micro-balloon (filling volume 0.2-0.5 ml, duration of lesion 60 or 90 min). Neurodegeneration in the retinal ganglion cell layer was investigated by measuring the mean total neuron number and the mean neuron size.

RESULTS

Increased retrobulbar volumes of 0.3 ml for 60 min led to a decrease in a number of neurons by 14.5% and in size of the neurons by 1.7%. Prolongation of the trauma to 90 min resulted in a decrease in number of neurons by 48.3% and in reduction of size of neurons by 22.1%. In contrast, a retrobulbar filling volume of 0.5 ml for 60 min resulted in a decrease in number of neurons by 11.4% and in size of neurons by 6.7%.

CONCLUSION

The duration of a retrobulbar lesion leading to subsequent retinal ischaemia seems to be more important for neuron survival than the exerted pressure (once it is greater than a critical point for producing retinal ischaemia). The results strengthen the need for emergency treatment in cases of retrobulbar haemorrhage.

Bcl-2 overexpression does not enhance in vivo axonal regeneration of retinal ganglion cells after peripheral nerve transplantation in adult mice

Optic nerve (ON) injury in adult mammals causes retinal ganglion cell (RGC) death and subsequent visual loss. Recovery of vision requires both rescuing axotomized RGCs and inducing their axonal regeneration. Axotomized RGCs are significantly rescued by overexpression of bcl-2, an anti-apoptotic gene. However, whether bcl-2 affects axonal regeneration is controversial. In neonatal bcl-2 transgenic mice (bcl-2 mice), optic tract regeneration after tectal lesion was promoted (Chen et al., 1997), whereas ON regeneration after ON crush was not (Lodovichi et al., 2001). These conflicting results may be attributable to different environments between tectum and ON. We tested here whether bcl-2 overexpression enhances in vivo RGC axonal regeneration in adult mice through a permissive environment in the peripheral nerve (PN) graft. Four weeks after PN transplantation to the proximal ON stump, we assessed the number of surviving and regenerating RGCs by retrograde labeling. Although the survival rate in bcl-2 mice was significantly enhanced compared with that in wild-type (wt) mice, the regeneration rate was not enhanced. In both bcl-2 and wt mice, RT97 immunostaining of the PN-grafted retinas revealed some RGC axons regrowing intraretinally but repulsed at the optic disk. To circumvent this repulsive barrier, we directly transplanted the PN graft to the partially injured retina and compared regeneration rates between these mice. Here again the regeneration rate in bcl-2 mice did not exceed that in wt mice. These findings indicate that bcl-2 overexpression enhances survival but not axonal regeneration of adult RGCs even within a permissive environment.

Lens-injury-stimulated axonal regeneration throughout the optic pathway of adult rats

Axonal regrowth and restoration of visual function were studied in adult rats. The optic nerve was completely cut behind the eye. The proximal and distal nerve stumps were realigned and the meninges sutured back together. During the same surgical procedure, the lens was lesioned in order to induce secondary cellular cascades, which are known to strongly support the survival of retinal ganglion cells (RGCs) and to promote axonal regeneration. The anatomical and topographic restoration of the visual pathway was assessed neuroanatomically with the aid of anterograde and retrograde tracing using fluorescent dyes. It appeared that the axons formed growth cones at the junction of the suture soon after injury, before glial cells and extracellular matrix proteins were able to cause local scar formation. Growth cones first entered the distal optic nerve stump 3 days after injury, grew through it to reach the optic chiasm approximately 3 weeks after the lesion was made, and terminated within the retinoreceptive layers of the superior colliculus 5 weeks after lesioning. Quantification of the retrogradely labeled cell bodies within the regenerating retina revealed that up to 30% of the RGCs, which includes all major cell types, were capable of regenerating their axons along the entire visual pathway. To assess whether topography was restored, double-labeling experiments were performed, revealing only crude topographic restoration during the initial stages of regeneration. However, visual-evoked potentials could be recorded, indicating that synaptic transmission in higher visual areas was relatively intact. The data show, in principle, that cut axons can regenerate over long distances within the white matter of a central nerve like the adult optic nerve, spanning over 11 mm to the chiasm and between 12 and 15 mm to the thalamus and midbrain. The findings suggest, for the first time, that lentogenic stimulation of RGCs is sufficient to induce the formation of growth cones that can override inhibitors at the site of injury, grow through the white matter of the optic nerve, pass through the optic chiasm, and make synaptic connections within the brain.

Perforated microelectrode arrays implanted in the regenerating adult central nervous system

Adult mammalian optic nerve axons are able to regenerate, when provided with the permissive environment of an autologous peripheral nerve graft, which is usually the sciatic nerve. This study demonstrates the ability of adult rat optic nerve axons to regenerate through the preformed perforations of a polyimide electrode carrier implanted at the interface between the proximal stump of the cut optic nerve and the stump of the peripheral nerve piece used for grafting. Evidence that retinal ganglion cells regenerated their axons through the perforated electrode carrier was obtained by retrograde labeling with a fluorescent dye deposited into the sciatic nerve graft beyond the nerve-carrier-nerve junction. The number of regenerating cells could be enhanced by injecting neuroprotective drugs like aurintricarboxylic acid and cortisol intravitreally. A second line of evidence was obtained by immunohistochemical staining with antibodies to neurofilament. Third, electrical activity of the regenerating nerves was recorded after stimulating the retina with a flash of light. The results suggest that a regenerating central nerve tract may serve as an experimental model to implant artificial microdevices to monitor the physiological and topographical properties of neurites passing through the device or to stimulate them, thus interfering with their potential to grow. This study reports for the first time that the optic nerve has unique properties, which aids in the realization of these goals.

Topological specificity in reinnervation of the superior colliculus by regenerated retinal ganglion cell axons in adult hamsters

In normal rodents there is a precise topology of the retinocollicular projection, the nasotemporal and ventrodorsal axes of the retina being respectively projected onto the caudorostral and mediolateral axes of the contralateral superior colliculus (SC). We evaluated the distribution of regenerated retinal ganglion cell (RGC) axon terminals in the SC of adult hamsters in which an unbranched peripheral nerve graft was directed from the retina to the contralateral SC. Responses to visual stimulation of individual RGCs were recorded from terminal arbors of their regenerated axons in the reinnervated SC. Retinal positions of these RGCs were inferred from the locations of their visual receptive fields. At some sites in the reinnervated SC, axon terminal arbors converged from widely separated RGCs. Conversely, axon terminal arbors at widely separated sites in the SC could emanate from contiguous RGCs. To assess whether any tendency for order was superimposed on the apparent disorganization of the regenerated projection, we evaluated the relative positions of pairs of RGC terminals in the SC in relation to the relative retinal locations of the corresponding pairs of RGCs. Among the 983 pairs of RGCs able to be evaluated from nine animals studied 30-60 weeks after grafting, there was a statistically significant 3/2 tendency for the more nasally situated of two RGCs to project its terminal more caudally in the SC than that of the more temporally situated RGC. A similar tendency toward appropriate organization was not found with respect to the ventrodorsal axis of the retina and the mediolateral axis of the SC.

CNTF promotes the regrowth of retinal ganglion cell axons into murine peripheral nerve grafts

Autologous peripheral nerves were transplanted onto transected optic nerves of adult mice. We examined whether intraocular CNTF injections increased retinal ganglion cell (RGC) axon regeneration, and what types of RGCs regrew axons into grafts. After temporal CNTF eye injections there were more fluorogold-labelled regenerating RGCs (mean +/- s.e.m. 342+/-113.1; n=6) than in sham eye-injected mice (133+/-27.6; n=8). Greater numbers of regenerating RGCs (1198+/-367.6; n=6) were seen in mice receiving both nasal and temporal CNTF injections. The range of soma areas in regenerate and normal retinas was similar but the average size of regenerating RGCs was greater (212 microm2 vs 111 microm2). Most regenerating RGCs had large dendritic fields. The data suggest a heterogeneous response to axotomy in adult mice, large RGCs preferentially regrowing axons into PN grafts.

Restoration of the retinofugal pathway

In a relatively short period of time covering the last 2 decades, regeneration of retinofugal axons has become one of most prominent experimental models in restorative neurobiology. There is now a significant knowledge both on the mechanisms governing retinal ganglion cell responses to transection of the optic nerve, and the subsequent cell-cell interactions accumulating in death of the neurons. In addition, retinofugal axons served as an excellent model to examine whether, and to conclude that these axons have remarkable abilities for re-growth. This last issue was of invaluable importance, because axons could regenerate in vivo, into peripheral nerve grafts, and last but not least within the white matter of the cut optic nerve. As it stands to date, the extremely complex aspects of axonal regeneration will probably be understood within the retinofugal pathway. Final elucidation of this delicate system will essentially lead to some revision of our knowledge concerning neurotraumatology and CNS-repair.

Selective innervation of retinorecipient brainstem nuclei by retinal ganglion cell axons regenerating through peripheral nerve grafts in adult rats

The pattern of axonal regeneration, specificity of reinnervation, and terminal arborization in the brainstem by axotomized retinal ganglion cell axons was studied in rats with peripheral nerve grafts linking the retina with ipsilateral regions of the brainstem, including dorsal and lateral aspects of the diencephalon and lateral aspect of the superior colliculus. Four to 13 months later, regenerated retinal projections were traced using intraocular injection of cholera toxin B subunit. In approximately one-third of the animals, regenerated retinal axons extended into the brainstem for distances of up to 6 mm. Although axons followed different patterns of ingrowth depending on their site of entry to the brainstem, within the pretectum, they innervated preferentially the nucleus of the optic tract and the olivary pretectal nucleus in which they formed two types of terminal arbors. Within the superior colliculus, axons extended laterally and formed a different terminal arbor type within the stratum griseum superficiale. In the remaining two-thirds of the animals, retinal fibers formed a neuroma-like structure at the site of entry into the brainstem, or a few fibers extended for very short distances within the neighboring neuropil. These experiments suggest that regenerated retinal axons are capable of a highly selective reinnervation pattern within adult denervated retinorecipient nuclei in which they form well defined terminal arbors that may persist for long periods of time. In addition, these studies provide the anatomical correlate for our previous functional study on the re-establishment of the pupillary light reflex in this experimental paradigm.

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.

Why does the central nervous system not regenerate after injury?

Spinal cord injuries in humans and in other mammals are never followed by regrowth. In recent years, considerable progress has been made in analyzing mechanisms that promote and inhibit regeneration. The focus of this review is changes that occur in the transition period in development when the central nervous system (CNS) changes from being able to regenerate to the adult state of failure. In our experiments we have used the neonatal opossum (Monodelphis domestica), which corresponds to a 14-day embryonic rat or mouse. The CNS isolated from an opossum pup and maintained in culture shows dramatic regeneration. Fibers grow through and beyond lesions and reform synaptic connections with their targets. Similarly, anesthetized neonatal pups attached to the mother recover the ability to walk after complete spinal cord transection. Although the CNS isolated from a 9-day-old animal will regenerate in vitro, CNS from a 12-day-old will not. This is the stage at which glial cells in the CNS develop. Present research is devoted toward molecular screening to determine which growth-promoting molecules decrease during development, which inhibitory molecules increase, and which receptors on growing axons become altered. Despite progress in many laboratories, major hurdles must be overcome before patients can hope to be treated. Nevertheless, the picture today is not as discouraging as it was: one can think of strategies for research on spinal cord injury so as to promote regeneration and restore function.

Cellular and molecular correlates of the regeneration of adult mammalian CNS axons into peripheral nerve grafts

Studies of the regeneration of CNS axons into peripheral nerve grafts have provided information crucial to our understanding of the regenerative potential of CNS neurons. Injured axons in the thalamus and corpus striatum produce regenerative sprouts within a few days of graft implantation, apparently in response to living cells in the grafts. The regenerating axons often grow directly towards the grafts, and enter Schwann cell columns where they elongate surrounded by Schwann cell processes. The regenerating CNS axons, and the Schwann cell processes along which they grow, initially express the cell adhesion molecules NCAM, and L1. The axons also express polysialic acid and, unlike regenerating peripheral axons, bind tenascin-C derived from Schwann cells. Wherever peripheral nerve grafts are implanted into the CNS they appear to promote the differential regeneration of CNS axons. Most of the axons which grow into grafts in the thalamus are derived from the thalamic reticular nucleus (TRN), whereas grafts in the striatum promote regeneration of axons from the substantia nigra pars compacta (SNpc) and grafts in the cerebellum promote regeneration from deep cerebellar nuclei (DCN) and brainstem precerebellar neurons. In contrast most thalamocortical projection neurons, striatal projection neurons and Purkinje cells in the cerebellar cortex are poor at regenerating. There are patterns to the expression of regeneration-related molecules by axons injured by nerve grafts in the CNS. Most neurons which regenerate well (e.g. TRN and DCN neurons) upregulate GAP-43, L1 and the transcription factor c-jun in response to a graft, whereas those neurons which do not regenerate well (e.g. Purkinje cells, thalamocortical and striatal projection neurons) do not upregulate these molecules. These observations suggest that some classes of CNS neurons may be intrinsically unable to regenerate axons and the repair of injuries in the brain and spinal cord may consequently require some form of gene therapy for axotomised neurons.

Does CNS Myelin Inhibit Axon Regeneration?

Membrane proteins on oligodendrocytes and CNS myelin (NI35/250) have been shown to block axon out-growth in culture, and this is thought to be one of the major reasons for severely limited regeneration of severed axons in the CNS of higher vertebrates. In a recent study, adult dorsal root ganglion (DRG) neurons, which are sensitive to these inhibitory proteins, regenerated successfully after transplantation into two white matter tracts of the rat brain without any intervention to suppress the inhibitory activity of CNS myelin. The results and implications of these two studies are considered.

Extent and duration of recovered pupillary light reflex following retinal ganglion cell axon regeneration through peripheral nerve grafts directed to the pretectum in adult rats

The functional reinnervation of the olivary pretectal nucleus (OPN) was studied in adult rats with peripheral nerve (PN) grafts bridging the interrupted retinopretectal pathway. Functional recovery was assessed quantitatively using established pupillometry techniques. The effect of intravitreal tuftsin fragment 1-3 (tuftsin 1-3) injections during the grafting procedure was also studied. A total of 53 adult rats received autologous PN grafts connecting the ocular stump of the transected optic nerve to the ipsilateral OPN. The contralateral eye was enucleated to remove the input from that eye to the OPN. A pupillary light reflex was elicited from 35 of the 53 PN-grafted animals and in the best cases, a response was obtained which compared closely to that recorded from control animals. Tuftsin 1-3 was found to increase the rate of recovery of the response. The response amplitude of PN-grafted rats was generally found to diminish with repeated stimulus presentation and also appeared to deteriorate with age. This was in contrast to control animals' responses. However, a PLR could still be elicited in 3 of the 6 animals studied 15 months after PN-grafting. These findings indicate that a near-normal PLR function can be restored using a peripheral nerve graft, but there are a number of factors that are likely to compromise optimal outcome.

Implantable bioelectric interfaces for lost nerve functions

Neuronal cells are unique within the organism. In addition to forming long-distance connections with other nerve cells and non-neuronal targets, they lose the ability to regenerate their neurites and to divide during maturation. Consequently, external violations like trauma or disease frequently lead to their disappearance and replacement by non-neuronal, and thus not properly functioning cells. The advent of microtechnology and construction of artificial implants prompted to create particular devices for specialised regions of the nervous system, in order to compensate for the loss of function. The scope of the present work is to review the current devices in connection with their applicability and functional perspectives. (1) Successful implants like the cochlea implant and peripherally implantable stimulators are discussed. (2) Less developed and not yet applicable devices like retinal or cortical implants are introduced, with particular emphasis given to the reasons for their failure to replace very complex functions like vision. (3) Material research is presented both from the technological aspect and from their biocompatibility as prerequisite of any implantation. (4) Finally, basic studies are presented, which deal with methods of shaping the implants, procedures of testing biocompatibility and modification of improving the interfaces between a technical device and the biological environment. The review ends by pointing to future perspectives in neuroimplantation and restoration of interrupted neuronal pathways.

Changes in the pupillary light reflex of pigmented royal college of surgeons rats with Age

We studied the latency and amplitude of the pupillary light reflex response of the Royal College of Surgeons rat from 10 to 52 weeks of age. The responses of these dystrophic rats were diminished compared to those of normal, non-dystrophic rats at all ages examined. This was most marked at the dimmest light intensity studied here and for the latency of dystrophic animals' responses. The latency deteriorated over the course of 52 weeks, although there was some evidence of improvement beyond 36 weeks of age. The amplitude of the dystrophic animals' responses also suggested some deterioration occurring up to 36 weeks of age, but with a substantial improvement beyond this time. In addition to these parameters, we also observed a break in the constriction phase of the pupillary light reflex that was unique to the dystrophic animals' responses. The frequency with which the anomaly occurred decreased in a light-dependent manner with age. The improvement of the pupillary light reflex at older ages, even when very few photoreceptors remain, may reflect compensatory events occurring in the inner retinal layers and/or in the central connections of the pupillary light reflex pathway. We suggest that the break in the constriction phase is a reflection of dual inputs driving the response, one of which is affected more by the degenerative events. This study provides baseline data on the effect of degeneration on function over time which can be used to evaluate the efficacy of repair strategies such as transplantation.

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.

Functional recovery of vision in regenerated optic nerve fibers

Retinal ganglion cells (RGCs) of adult mammals normally suffer from retrograde cell death after optic nerve section. However, with transplantation of a segment of peripheral nerve (PN), their axons can regenerate and regrow through the graft. When properly guided, the regenerated axons make functional synapses with the target cells in the superior colliculus. Two months after PN graft we studied the number and morphology of RGCs with regenerated axons in adult cats. Number of regenerated RGCs was a few percent of the total population and, among various RGC types, alpha cells revealed the greatest ability for axonal regeneration and ON-center RGCs tended to regenerate better than OFF-center cells. While dendritic field dimension of RGCs with regenerated axons was mostly preserved, their regenerated axons were thinner than normal optic axons and mostly unmyelinated. The RGCs with regenerated axons revealed normal physiological properties in response to visual stimuli, and were classifiable into Y, X or W cells. In accordance with morphological results, Y cells (morphological alpha cells) were most frequently sampled. In hamsters and rats it has been shown that the animals with reconstructed retinocollicular pathway by the PN graft reveal behavioral recovery of visual function. However, in the cat, trials are still in progress to reconstruct the retinogeniculate pathway. The present status of researches on optic nerve regeneration of adult mammals using the PN graft is reviewed, and some future directions discussed.

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.

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

Axotomy-induced degradation of retinal ganglion cells (RGC) can be delayed if the destructive features of activated microglial cells are pharmacologically neutralized, and prevented if the axons are permitted to regrow into transplanted autologous peripheral nerve (PN) pieces. Axotomized central nervous system neurons, whose regenerating axons are guided to their natural target areas in the brain with the aid of PN grafts, are capable of establishing synaptic contacts with normal morphological and electrophysiological properties. This study was undertaken to 1) morphometrically characterize and classify the regenerating rat RGC, 2) examine target-dependent effects on survival of subsets of neurons, and 3) investigate whether reconnected neurons are capable of restoring visual functions. In analogy to the normal rat retina, as a first step, the retrogradely labeled, regenerating RGC were categorized into five classes which are morphologically distinct and reminiscent of normal RGC correlates (called types RI, RII, RIII, Rdelta-cells, and displaced RGC). It appeared that all types of ganglion cells contributed proportionally to regeneration of axons. Transplantation of a PN graft which was not reconnected with a central target (blind-ending group) and monitoring of the extant neurons showed a progressive disappearance of the regenerating RGC, such that 6 months after surgery predominantly few, large cells survived. When the retinas were treated with macrophage/microglia inhibiting factor (MIF), and the regenerating axons were guided into the pretectum, predominantly large RGC of type RI survived. Guidance of the axons into their major natural target, the superior colliculus (SC), resulted in selective survival of many small, RII-like RGC. Calculation of the dendritic coverage factors for the major types of RGC revealed that dendrites of the most abundant, small cells of type RII overlapped uniformly and covered the retinal surface completely, whereas cells of types RI and RIII did not suffice for surface coverage. The results of this first part of the work suggest that combined suppression of axotomy-induced microglial activation and guidance of regenerating axons with a PN graft into central targets is a suitable technique to produce sufficient numbers of regenerating axons which may retrieve some functional properties. Target-specific neuronal contacts are likely involved in morphological stabilization and better survival of regenerating neurons. The second goal of this study was to analyze the functional significance of the reestablished synaptic contacts made by regenerated retinocollicular neurons. Adult rats were trained in a T- or Y-maze to obtain a food reward with the aid of visual cues. One of their optic nerves was transected and the regenerating axons were guided into the optic tract with a PN graft, to enable them to reinnervate the SC and thalamus. Postoperative testing of the animals showed a drastic improvement of visual perception. The protocol of denervation of the SC (prior to, simultaneous with, or with a delay with respect to fiber arrival) determined the performance of the animals. Rats belonging to the first two groups performed almost as well as they had before the transplantation. The functional integrity of the retina was assessed by electroretinography, which revealed typical rod spectral sensitivity at 380 and 500 nm but reduced responsiveness to illumination. In accordance, neuroanatomical assessment of the functionally relevant RGC revealed intact morphologies and multiple synaptic contacts both within the retina and within the SC. Neuroanatomical tracing of small contingents of axons throughout the regenerative pathway revealed a rough retinotopic arrangement within the graft and the area of termination. Thus, animals could discriminate between simplified vertical versus horizontal stripes, and visual evoked potentials were positive after grafting. (ABSTRACT TRUNCATED)

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.

Bridge grafts of fibroblast growth factor-4-secreting schwannoma cells promote functional axonal regeneration in the nigrostriatal pathway of the adult rat

Axons damaged in the adult mammalian central nervous system are able to regenerate when their inhibitory glial environment is replaced with a more permissive substrate. Here, we have used long oblique "bridge" grafts of fibroblast growth factor-4-transfected RN-22 schwannoma cells to allow mechanically lesioned nigrostriatal axons to regenerate back to their original target in the adult rat brain. Regenerated axons were able to leave the bridge graft to form terminal arborizations and increase the density of tyrosine hydroxylase-immunoreactive fibres within the striatum. Bridge grafting also resulted in an increase in the number of neurons within the substantia nigra pars compacta taking up the fluorescent retrograde tracer Fluoro-Gold from the striatum. Animals which had received RN-22 bridge grafts showed lower rates of amphetamine-induced rotation 10 weeks after a mechanical lesion of the nigrostriatal tract compared to lesioned controls, the magnitude of the behavioural effect being related to the number of regenerated axons, and this comparative reduction was reversed by mechanical section of the bridge graft. It is concluded that our bridge grafting strategy allowed the partial anatomical and functional regeneration of the mechanically lesioned nigrostriatal tract, an unmyelinated central axon bundle, and that bridge grafting therefore represents a realistic approach to the repair of central nervous system lesions involving axon tract damage.

Development and role of retinal glia in regeneration of ganglion cells following retinal injury

AIMS/BACKGROUND

Recent observations have shown that the glial scar resulting from a surgical lesion of the immature retina differs from elsewhere in the central nervous system, in that it permits the through growth and reconnection of regenerating axons. This study in the opossum examines in detail the development and reaction to injury of retinal glia at different developmental stages, and specifically examines the distribution of the gliosis related inhibitory molecule, chondroitin sulphate proteoglycan (CSPG), making comparisons with a control site of gliosis in the cerebral cortex.

METHODS

A linear slit was cut into the retina or cortex with a fine tungsten probe. After a variable time delay, immunocytochemistry of the resulting gliosis was employed to detect astrocytes with glial fibrillary acidic protein (GFAP), Müller cells with vimentin, and CSPG with CS-56 antibodies. GFAP was also used at different ages to examine the normal development of astrocytes in the retina of this species.

RESULTS

Astrocytes entered the retina 12 days after birth (P12), closely associated with blood vessels in the nerve fibre layer. In experiments at all ages studied, cellular continuity was re-established across the lesioned retina, which did not result in a significant astrocyte proliferation or CSPG expression. In contrast, cortical injury led to the development of a cystic cavity surrounded by astrocytes and CSPG. Müller cells expressed GFAP but not CSPG in the lesioned retina.

CONCLUSION

Successful regrowth of ganglion cells through a retinal lesion may be partly the result of the scarcity of astrocytes in the retina, which results in minimal gliosis, or of their apparent inability to express inhibitory molecules.

A critical period for axon regrowth through a lesion in the developing mammalian retina

Although the central nervous system of mature mammals is incapable of regeneration, certain elements present in the developing system must permit and promote the growth of new axons to their initial targets. We investigate whether the environment of a developing visual system is capable of supporting regeneration in the Brazilian opossum Monodelphis domestica, in which the retinofugal system develops postnatally. Retinae were lesioned up to the 16th postnatal day and analysed for regeneration after a further 7-10 days. Anterograde tracing with Dil showed axons to have regrown from the axotomized area of retina directly through the lesion. Retrograde tracing with horseradish peroxidase injected into the superior colliculus confirmed that axons from the lesioned area of retina had grown to an appropriate position in the midbrain. The proportion of retinae in which axonal continuity was restored across the lesion decreased as the visual system matured, falling to zero after the 12th postnatal day. Thus a critical period exists in the postnatal opossum in which a retinal lesion permits axon passage. Correlating these results to the known pattern of retinofugal pathway development provides an insight into factors that may restrict this critical period to the 12th postnatal day, and suggests that at least some of the axotomized neurons are regenerating.

Schwann cells and the regrowth of axons in the mammalian CNS: a review of transplantation studies in the rat visual system

1. We have used peripheral nerve transplants or cultured Schwann cells grafted in association with different types of polymer to study axonal regrowth in the rat visual system. In some instances the glia were co-grafted with fetal tectal tissue. 2. The studies have two main aims: (i) to determine whether retinal axons can be induced to regrow at a site distant from their cell soma, that is, after damage to the brachial region of the optic tract; (ii) to determine whether retinal axons exposed to Schwann cells retain the ability to recognize their appropriate target neurons in CNS tissue. 3. In brachial lesion studies, Schwann cells were placed in the lesion site in association with nitrocellulose papers, within polycarbonate tubes in the presence or absence of a supporting extracellular matrix (ECM), or within polymer hydrogel scaffolds. Autologous sciatic nerve grafts were also used. Immunohistochemical studies revealed the presence of regenerating axons within all polymer bridges. Regrowth of retinal axons was also seen, however, growth was not extensive and was limited to the proximal 1-1.5 mm of the implants. 4. In target innervation experiments, two surgical paradigms were developed. In one experiment, a segment of sciatic nerve was autografted onto the transected optic nerve in adult rats and the distal end of each graft was placed adjacent to fetal tectal (target) tissue implanted into the frontal cortex. To date, we have not been able to demonstrate selective recognition of target regions within tectal transplants by retinal axons exiting the sciatic nerve implants. 5. In the second experiment, Schwann cells were mixed with fetal tectal cells and co-grafted to the midbrain of newborn host rats. Schwann cells altered the characteristic pattern of host retinal growth into tectal grafts; in some cases axons were induced to grow away from appropriate target areas by nearby co-grafted Schwann cells. 6. In summary, Schwann cell/polymer scaffolds may provide a useful way of promoting the regrowth of damaged axons in the CNS, however: (i) in adults, at least, their effectiveness is reduced if they are located at a distance from the cell bodies giving rise to regenerating axons; (ii) in some circumstances exposure to a peripheral glial environment may affect the capacity of regenerating axons to recognize appropriate target cells in the CNS neuropil.

Expression of endopeptidase-24.11 (common acute lymphoblastic leukaemia antigen CD10) in the sciatic nerve of the adult rat after lesion and during regeneration

Endopeptidase-24.11, which is identical with the common acute lymphoblastic leukaemia antigen CD10 (CALLA), is a cell surface Zn2+ metalloprotease that regulates peptide-induced responses in different tissues, including the nervous and immune systems. In the peripheral nervous system, high levels of the enzyme are present in all neonatal and early postnatal Schwann cells, while as myelination proceeds it is gradually suppressed in the majority of cells that form myelin but retained in non-myelin-forming cells in the adult animal. In the present study we have investigated the effects of transection, crush and regeneration of the adult rat sciatic nerve on the expression of the endopeptidase by Schwann cells in situ. Endopeptidase-24.11 was monitored by immunocytochemistry using the monoclonal anti-endopeptidase antibody 23B11. For comparison, a parallel study was carried out with a monoclonal antibody directed against the rat nerve growth factor receptor. We found that (i) all Schwann cells of the distal segment re-expressed endopeptidase-24.11 as early as 4 days after axotomy, the level of immunostaining reaching a maximum after 2 weeks, (ii) axonal regeneration repressed Schwann cell expression of endopeptidase-24.11, and (iii) the induction of the nerve growth factor receptor followed a similar pattern to that of endopeptidase-24.11 in the transected and crushed nerve. Enzymatic amplification of endopeptidase-24.11 cDNA from normal and axotomized adult rat sciatic nerve confirmed the expression of endopeptidase-24.11 in these tissues. Our results show that the expression of endopeptidase-24.11 in Schwann cells, as is the case with the nerve growth factor receptor, is induced by the loss of the normal axon-Schwann cell contact. The significant increase in the expression of endopeptidase-24.11 by Schwann cells after axonal damage suggests that the enzyme could play a role in axonal regeneration.

Perspectives on axonal regeneration in the mammalian CNS

In the CNS of mammals, axonal regeneration is limited by inhibitory influences of the glial and extracellular environment. Myelin-associated inhibitors of neurite growth, as well as some properties of so-called 'reactive astrocytes' which make the environment non-permissive for axonal growth, contribute to the inhibitory nature of the mammalian CNS. Furthermore, mechanisms for effective removal or neutralization of inhibitory components of cell debris are lacking in the mature mammalian CNS. However, in a permissive environment, mammalian CNS axons are able to regrow, to recognize target areas and to re-establish functional synapses with target neurones. Moreover, recent observations suggest that guiding molecules, like those required for axon guidance in the developing CNS, become expressed after lesions. Regenerating CNS axons seem to be able to recognize such guidance cues. Thus, even regenerating CNS axons of mammals might ultimately succeed in re-establishing topographically ordered functional synapses in their target regions.

Regeneration of lesioned rat optic nerve fibers is improved after neutralization of myelin-associated neurite growth inhibitors

Optic nerve axons do not regenerate after lesions in postnatal mammals, except if peripheral nerve transplants are offered as a substrate. In the present study, regeneration was assessed after intracranial freeze-crush lesions of the optic nerve, which completely interrupted all axons. In rats lesioned at the age of 8-9 days and surviving for additional 5-6 days, regenerating retinal fibers were seen to enter and partially cross the lesion site, reaching a maximum distance of 0.8 mm (mean +/- S.E.M. = 0.62 +/- 0.07 mm) in the presence of brain-derived neurotrophic factor (BDNF). Without BDNF, almost all the axons were lost due to axonal die-back. This regeneration distance could be significantly enhanced, up to 1.9 mm, by application of a monoclonal antibody (mAB-IN-1) directed against oligodendrocyte- and myelin-associated neurite growth inhibitory proteins. Similar results were obtained in rats lesioned at 16-18 days and surviving for 2 weeks: whereas fibroblast growth factor (FGF) stimulated sprouting did not exceed distances of 0.5 mm (mean = 0.38 +/- 0.07 mm), FGF and IN-1 antibody treated rats showed regenerations up to 2.3 mm (mean = 1.53 +/- 0.15 mm). The specificity of this effect was confirmed by lesions of myelin- and oligodendrocyte-free optic nerves: optic nerves were locally X-irradiated at birth, day 2, 4 and 6, a procedure which kills the dividing oligodendrocyte precursor cells. When these myelin-free nerves were lesioned at 3 weeks of age, regeneration distances between 2.5 and 3.2 mm were observed 3 weeks later.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth factors in CNS repair and regeneration

Traumatic central nervous system (CNS) injury is a significant clinical problem in the developed world. After injuries that penetrate into either the mature brain or spinal cord, damaged neurons initially begin to regrow, but this regeneration is aborted as a fibrotic scar is laid down within the wound. Reconnection of several neuronal pathways does not occur. Functional recovery from such injuries is therefore poor and morbidity severe, particularly for those patients with spinal cord damage. Although palliative measures are available to improve the quality of life, there is no accepted treatment to restore impaired sensory or motor function, so patients remain significantly and permanently debilitated. However, the rapid recent advances that have been made in our understanding of the underlying cellular and trophic pathology of such injuries offer the potential for development of novel therapies to control scarring, enhance neuron survival and stimulate axon regeneration, thereby promoting functional recovery.