Regenerated synapses persist in the superior colliculus after the regrowth of retinal ganglion cell axons

Retinal ganglion cell repopulation for vision restoration in optic neuropathy: a roadmap from the RReSTORe Consortium

Retinal ganglion cell (RGC) death in glaucoma and other optic neuropathies results in irreversible vision loss due to the mammalian central nervous system's limited regenerative capacity. RGC repopulation is a promising therapeutic approach to reverse vision loss from optic neuropathies if the newly introduced neurons can reestablish functional retinal and thalamic circuits. In theory, RGCs might be repopulated through the transplantation of stem cell-derived neurons or via the induction of endogenous transdifferentiation. The RGC Repopulation, Stem Cell Transplantation, and Optic Nerve Regeneration (RReSTORe) Consortium was established to address the challenges associated with the therapeutic repair of the visual pathway in optic neuropathy. In 2022, the RReSTORe Consortium initiated ongoing international collaborative discussions to advance the RGC repopulation field and has identified five critical areas of focus: (1) RGC development and differentiation, (2) Transplantation methods and models, (3) RGC survival, maturation, and host interactions, (4) Inner retinal wiring, and (5) Eye-to-brain connectivity. Here, we discuss the most pertinent questions and challenges that exist on the path to clinical translation and suggest experimental directions to propel this work going forward. Using these five subtopic discussion groups (SDGs) as a framework, we suggest multidisciplinary approaches to restore the diseased visual pathway by leveraging groundbreaking insights from developmental neuroscience, stem cell biology, molecular biology, optical imaging, animal models of optic neuropathy, immunology & immunotolerance, neuropathology & neuroprotection, materials science & biomedical engineering, and regenerative neuroscience. While significant hurdles remain, the RReSTORe Consortium's efforts provide a comprehensive roadmap for advancing the RGC repopulation field and hold potential for transformative progress in restoring vision in patients suffering from optic neuropathies.

The basic science of optic nerve regeneration

Diverse insults to the optic nerve result in partial to total vision loss as the axons of retinal ganglion cells are destroyed. In glaucoma, axons are injured at the optic nerve head; in other optic neuropathies, axons can be damaged along the entire visual pathway. In all cases, as mammals cannot regenerate injured central nervous system cells, once the axons are lost, vision loss is irreversible. However, much has been learned about how retinal ganglion cells respond to axon injuries, and many of these crucial discoveries offer hope for future regenerative therapies. Here we review the current understanding regarding the temporal progression of axonal degeneration. We summarize known survival and regenerative mechanisms in mammals, including specific signaling pathways, key transcription factors, and reprogramming genes. We cover mechanisms intrinsic to retinal ganglion cells as well as their interactions with myeloid and glial cell populations in the retina and optic nerve that affect survival and regeneration. Finally, we highlight some non-mammalian species that are able to regenerate their retinal ganglion cell axons after injury, as understanding these successful regenerative responses may be essential to the rational design of future clinical interventions to regrow the optic nerve. In the end, a combination of many different molecular and cellular interventions will likely be the only way to achieve functional recovery of vision and restore quality of life to millions of patients around the world.

Functional and morphological alterations in a glaucoma model of acute ocular hypertension

To study short and long-term effects of acute ocular hypertension (AOHT) on inner and outer retinal layers, in adult Sprague-Dawley rats AOHT (87mmHg) was induced for 90min and the retinas were examined longitudinally in vivo with electroretinogram (ERG) recordings and optical coherent tomography (OCT) from 1 to 90 days (d). Ex vivo, the retinas were analyzed for rod (RBC) and cone (CBC) bipolar cells, with antibodies against protein kinase Cα and recoverin, respectively in cross sections, and for cones, horizontal (HZ) and ganglion (RGC) cells with antibodies against arrestin, calbindin and Brn3a, respectively in wholemounts. The inner retina thinned progressively up to 7d with no further changes, while the external retina had a normal thickness until 30d, with a 20% thinning between 30 and 90d. Functionally, the a-wave showed an initial reduction by 24h and a further reduction from 30 to 90d. All other main ERG waves were significantly reduced by 1d without significant recovery by 90d. Radial sections showed a normal population of RBCs but their terminals were reduced. The CBCs showed a progressive decrease with a loss of 56% by 30d. In wholemount retinas, RGCs diminished to 40% by 3d and to 16% by 30d without further loss. Cones diminished to 58% and 35% by 3 and 7d, respectively and further decreased between 30 and 90d. HZs showed normal values throughout the study. In conclusion, AOHT affects both the inner and outer retina, with a more pronounced degeneration of the cone than the rod pathway.

Aberrant information transfer interferes with functional axon regeneration

Functional axon regeneration requires regenerating neurons to restore appropriate synaptic connectivity and circuit function. To model this process, we developed an assay in Caenorhabditis elegans that links axon and synapse regeneration of a single neuron to recovery of behavior. After axon injury and regeneration of the DA9 neuron, synapses reform at their pre-injury location. However, these regenerated synapses often lack key molecular components. Further, synaptic vesicles accumulate in the dendrite in response to axon injury. Dendritic vesicle release results in information misrouting that suppresses behavioral recovery. Dendritic synapse formation depends on dynein and jnk-1. But even when information transfer is corrected, axonal synapses fail to adequately transmit information. Our study reveals unexpected plasticity during functional regeneration. Regeneration of the axon is not sufficient for the reformation of correct neuronal circuits after injury. Rather, synapse reformation and function are also key variables, and manipulation of circuit reformation improves behavioral recovery.

Repulsive Environment Attenuation during Adult Mouse Optic Nerve Regeneration

The regenerative capacity of CNS tracts has ever been a great hurdle to regenerative medicine. Although recent studies have described strategies to stimulate retinal ganglion cells (RGCs) to regenerate axons through the optic nerve, it still remains to be elucidated how these therapies modulate the inhibitory environment of CNS. Thus, the present work investigated the environmental content of the repulsive axon guidance cues, such as Sema3D and its receptors, myelin debris, and astrogliosis, within the regenerating optic nerve of mice submitted to intraocular inflammation + cAMP combined to conditional deletion of PTEN in RGC after optic nerve crush. We show here that treatment was able to promote axonal regeneration through the optic nerve and reach visual targets at twelve weeks after injury. The Regenerating group presented reduced MBP levels, increased microglia/macrophage number, and reduced astrocyte reactivity and CSPG content following optic nerve injury. In addition, Sema3D content and its receptors are reduced in the Regenerating group. Together, our results provide, for the first time, evidence that several regenerative repulsive signals are reduced in regenerating optic nerve fibers following a combined therapy. Therefore, the treatment used made the CNS microenvironment more permissive to regeneration.

Shared and Differential Retinal Responses against Optic Nerve Injury and Ocular Hypertension

Glaucoma, one of the leading causes of blindness worldwide, affects primarily retinal ganglion cells (RGCs) and their axons. The pathophysiology of glaucoma is not fully understood, but it is currently believed that damage to RGC axons at the optic nerve head plays a major role. Rodent models to study glaucoma include those that mimic either ocular hypertension or optic nerve injury. Here we review the anatomical loss of the general population of RGCs (that express Brn3a; Brn3a⁺RGCs) and of the intrinsically photosensitive RGCs (that express melanopsin; m⁺RGCs) after chronic (LP-OHT) or acute (A-OHT) ocular hypertension and after complete intraorbital optic nerve transection (ONT) or crush (ONC). Our studies show that all of these insults trigger RGC death. Compared to Brn3a⁺RGCs, m⁺RGCs are more resilient to ONT, ONC, and A-OHT but not to LP-OHT. There are differences in the course of RGC loss both between these RGC types and among injuries. An important difference between the damage caused by ocular hypertension or optic nerve injury appears in the outer retina. Both axotomy and LP-OHT induce selective loss of RGCs but LP-OHT also induces a protracted loss of cone photoreceptors. This review outlines our current understanding of the anatomical changes occurring in rodent models of glaucoma and discusses the advantages of each one and their translational value.

Transplanted neurons integrate into adult retinas and respond to light

Retinal ganglion cells (RGCs) degenerate in diseases like glaucoma and are not replaced in adult mammals. Here we investigate whether transplanted RGCs can integrate into the mature retina. We have transplanted GFP-labelled RGCs into uninjured rat retinas in vivo by intravitreal injection. Transplanted RGCs acquire the general morphology of endogenous RGCs, with axons orienting towards the optic nerve head of the host retina and dendrites growing into the inner plexiform layer. Preliminary data show in some cases GFP(+) axons extending within the host optic nerves and optic tract, reaching usual synaptic targets in the brain, including the lateral geniculate nucleus and superior colliculus. Electrophysiological recordings from transplanted RGCs demonstrate the cells' electrical excitability and light responses similar to host ON, ON-OFF and OFF RGCs, although less rapid and with greater adaptation. These data present a promising approach to develop cell replacement strategies in diseased retinas with degenerating RGCs.

[EFFECT OF HYPOXIA ON SYNAPTIC TRANSMISSION BETWEEN RETINAL GANGLION CELLS AND SUPERIOR COLLICULUS NEURONS IN COCULTURE]

In this study we conducted a series of experiments to characterize the effect and define the mechanisms of hypoxia on synaptic transmission between retinal ganglion cells and superior colliculus (SC) neurons. Application of hypoxic solution leads to a long lasting potentiation (LTP) NMDA-mediated synaptic transmission. Analysis of the oxygen deficiency effect on the spontaneous and miniature postsynaptic currents (sPSC and mPSC respectively) revealed an increase in the frequency of their occurrence and the appearance of the second peak in the mPSC histogram distribution. The assessment of quantum and binomial parameters reflects the complex pre- and postsynaptic changes during the potentiation, independent of the release probability. Most likely this LTP can be caused by an increase in the total number of active synapses. Glutamatergic synaptic transmission mediated by non-NMDA activation receptor-channel complexes, responded to application of deoxygenated solution by the brief depression, which is the result of presynaptic dysfunction and associates with decrease in release probability and number of active zones. GABAergic synaptic transmission mediated by activation GABA(A)-receptor-channel complexes, responded to hypoxic action by long term depression (LTD). Analysis of sPSC and mPSC showed a significant decrease in the frequency of their occurrence and significant (P = 0.05) decrease in the quantum over a period of oxygen deficiency. In general, the effect of hypoxia-induced LTD of GABAergic synaptic transmission is based on complex changes of presynaptic (independent on the release probability) and postsynaptic (reduction sensitivity of receptors in postsynaptic membrane) mechanisms.

Protecting the retinal neurons from glaucoma: lowering ocular pressure is not enough

The retina is theater of a number of biochemical reactions allowing, within its layers, the conversion of light impulses into electrical signals. The axons of the last neuronal elements, the ganglion cells, form the optic nerve and transfer the signals to the brain. Therefore, an appropriate cellular communication, not only within the different retinal cells, but also between the retina itself and the other brain structures, is fundamental. One of the most diffuse pathologies affecting retinal function and communication, which thus reverberates in the whole visual system, is glaucoma. This insidious disease is characterized by a progressive optic nerve degeneration and sight loss which may finally lead to irreversible blindness. Nevertheless, the progressive nature of this pathology offers an opportunity for therapeutic intervention. To better understand the cellular processes implicated in the development of glaucoma useful to envision a targeted pharmacological strategy, this manuscript first examines the complex cellular and functional organization of the retina and subsequently identifies the targets sensitive to neurodegeneration. Within this context, high ocular pressure represents a key risk factor. However, recent literature findings highlight the concept that lowering ocular pressure is not enough to prevent/slow down glaucomatous damage, suggesting the importance of combining the hypotensive treatment with other pharmacological approaches, such as the use of neuroprotectants. Therefore, this important and more novel aspect is extensively considered in this review, also emphasizing the idea that the neuroprotective strategy should be extended to the entire visual system and not restricted to the retina.

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.

ERG changes in albino and pigmented mice after optic nerve transection

Optic nerve transection (ONT) triggers retinal ganglion cell (RGC) death. By using this paradigm, we have analyzed for the first time in adult albino and pigmented mice, the effects of ONT in the scotopic threshold response (STR) components (negative and positive) of the full-field electroretinogram. Two weeks after ONT, when in pigmented mice approximately 18% of the RGC population survive, the STR-implicit time decreased and the p and nSTR waves diminished approximately to 40% or 55%, in albino or pigmented, respectively, with respect to the values recorded from the non-operated contralateral eyes. These changes were maintained up to 12 weeks post-ONT, demonstrating that the ERG-STR is a useful parameter to monitor RGC functionality in adult mice.

Bone marrow mononuclear cells increase retinal ganglion cell survival and axon regeneration in the adult rat

The central nervous system (CNS) of adult mammals generally does not regenerate, and many studies have attempted to identify factors that could increase neuroprotection and/or axonal outgrowth after CNS lesions. Using the optic nerve crush of rats as a model for CNS injury, we investigated the effect of intravitreal transplantation of syngeneic bone-marrow mononuclear cells (BMMCs) on the survival of retinal ganglion cells (RGC) and on the regeneration of optic axons. Control animals received intravitreal saline injections after lesion. Injections of BMMCs resulted in a 1.6-fold increase in the number of RGCs surviving 14 days after injury. The BMMC-treated animals also had increased numbers of axons, which grew up to 1.5 mm from the crush site, and also had reduced Müller glia activation. Analysis of mRNAs in all conditions revealed an increase in levels of fibroblast growth factor 2 (FGF-2) mRNA in treated animals 14 days after injury. To investigate whether the regenerated axons could reach the brain, we retrograde labeled the RGCs by injecting a lipophilic tracer into the superior colliculus. We also analyzed the expression of NGFI-A in the superficial layers of the superior colliculus as a possible marker of synaptic input from RGC axons. We found evidence that more RGCs were able to reach the brain after treatment and we showed that NGFI-A expression was higher in the treated animals 60 days after injury. These results demonstrate that transplant of BMMCs can increase neuroprotection and neuroregeneration after injury in a model of optic nerve crush, and these effects could be mediated by FGF-2.

Effects of optic nerve injury, glaucoma, and neuroprotection on the survival, structure, and function of ganglion cells in the mammalian retina

Glaucoma is an optic neuropathy that originates with pressure-induced damage to the optic nerve. This results in the retrograde degeneration of ganglion cells in the retina, and a progressive loss of vision. Over the past several years, a number of studies have described the structural and functional changes that characterize ganglion cell degeneration in the glaucomatous eye, and following optic nerve injury. In addition, a variety of different strategies for providing neuroprotection to the injured retina have been proposed. Many of these are based on the use of brain-derived neurotrophic factor (BDNF), a particularly potent neuroprotectant in the mammalian eye and the basis of our research in this area. Of particular importance is the fact that BDNF not only promotes ganglion cell survival following damage to the optic nerve, but also helps to preserve the structural integrity of the surviving neurons, which in turn results in enhanced visual function. The studies presented here describe these attributes, and serve as the foundation for ongoing work that suggests a need to think beyond the eye in the development of future treatment strategies.

Effects of neurotrophins on the survival and regrowth of injured retinal neurons

The focus of this short review is the role of certain neurotrophins and their receptors on the survival and regrowth of retinal ganglion cells (RGCs) whose axons are damaged in the optic nerve. Initial experiments in our laboratory documented patterns of RGC death after axotomy. Subsequent studies were designed to investigate the distribution of high-affinity neurotrophin receptors in neurons and glial cells of the retina and optic nerve. This information was used both in vitro and in vivo to study the effects of specific trophic molecules on the survival and regrowth of injured RGCs. During the course of experiments involving neurotrophin administration, an endogenous source of trophic support--independent of the exogenous administration of growth factors--was found within the eye. Several experiments were subsequently undertaken to define further this survival effect and determine its nature and source within the eye. Finally, anatomical techniques that help visualize fine axonal processes within the retina have provided insights into the specific effects of neurotrophins on the growth and branching of injured CNS axons.

Rat retinal microglial cells under normal conditions, after optic nerve section, and after optic nerve section and intravitreal injection of trophic factors or macrophage inhibitory factor

Retinal microglial cells may have a role in both degeneration and neuroprotection of retinal ganglion cells (RGC) after optic nerve (ON) section. We have used NDPase enzymohistochemistry to label adult rat retinal microglial cells and have studied these cells under normal conditions, after left ON section, and after left ON section and eye puncture or intravitreal injection of different substances: vehicle, brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), neurotrophin 3 (NT3), or macrophage inhibitory factor (MIF). Resident microglial cells are present in four layers in the adult rat retina: the nerve fiber layer (NFL), ganglion cell layer (GCL), inner plexiform layer (IPL), and outer plexiform layer (OPL). Left ON section induces microglial activation in the ipsilateral and contralateral retina as manifested by stronger staining intensity in both retinas and increased microglial cell densities in the NFL, IPL, and GCL of the ipsilateral retina. Left ON section followed by left eye puncture or intravitreal injection increases microglial cell density in both retinas and induces changes in the microglial cells of the ipsilateral retina that vary depending on the substance injected: BDNF injections delay microglial activation, possibly through retinal ganglion cell neuroprotection, whereas NT3 partially inhibits microglial activation in the NFL; MIF injections have no clear effects on microglial activation. In conclusion, retinal microglial cells become activated after an ON section and react more intensely when the eye is also punctured or injected, and this response may be altered by using neurotrophic factors, although the effects of MIF are less clear.

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.

Stem cells to replace the optic nerve

Methods that exist now and that might be developed are suggested to replace retinal ganglion cells and their axons in the optic nerve, ultimately to re-establish functional vision in eyes blind from glaucoma.

Combined demyelination plus Schwann cell transplantation therapy increases spread of cells and axonal regeneration following contusion injury

Several cell populations have been shown to provide a permissive environment for axonal extension following transplantation to injury sites. The limited spread of transplanted cells from implantation sites in the mature CNS, and the superior substrate and trophic environment that they provide, likely contribute to the fact that few transplantation-based therapies have elicited axonal extension beyond the transplant. The aim of this study was to determine whether (1) regions of demyelination cranial and caudal to a spinal cord injury site would improve the spread of Schwann cells transplanted into the site of injury, and (2) whether this combination therapy was associated with improved anatomical regeneration. Three days following contusion injury, anti-galactocerebroside antibodies plus complement proteins were injected into the dorsal column cranial and caudal to the injury site, resulting in complete and well defined regions of demyelination that extended 8 mm either side of the injury site. One day later, naïve Schwann cells in suspension were injected into the contusion site. Transplanted Schwann cells homogeneously redistributed throughout the contusion site and the adjacent regions of demyelination cranial and caudal to the contusion site, providing a long-distance prospective path for repair that was free of myelin and contained transplanted cells. Animals that received demyelination plus transplantation therapy, but not untreated or single-treatment groups, exhibited robust axonal regeneration beyond the contusion site within the treated dorsal column. Axonal regeneration in these animals was not associated with an improvement in locomotor ability. These findings suggest that this combination therapy may overcome a central limitation of transplant strategies in which the permissive environment provided remains at the implantation site.

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.

Central and peripheral nerve regeneration by transplantation of Schwann cells and transdifferentiated bone marrow stromal cells

In contrast to the peripheral nervous system (PNS), little structural and functional regeneration of the central nervous system (CNS) occurs spontaneously following injury in adult mammals. The inability of the CNS to regenerate is mainly attributed to its own inhibitorial environment such as glial scar formation and the myelin sheath of oligodendrocytes. Therefore, one of the strategies to promote axonal regeneration of the CNS is to experimentally modify the environment to be similar to that of the PNS. Schwann cells are the myelinating glial cells in the PNS, and are known to play a key role in Wallerian degeneration and subsequent regeneration. Central nervous system regeneration can be elicited by Schwann cell transplantation, which provides a suitable environment for regeneration. The underlying cellular mechanism of regeneration is based upon the cooperative interactions between axons and Schwann cells involving the production of neurotrophic factors and other related molecules. Furthermore, tight and gap junctional contact between the axon and Schwann cell also mediates the molecular interaction and linking. In this review, the role of the Schwann cell during the regeneration of the sciatic (representing the PNS) and optic (representing the CNS) nerves is explained. In addition, the possibility of optic nerve reconstruction by an artificial graft of Schwann cells is also described. Finally, the application of cells not of neuronal lineage, such as bone marrow stromal cells (MSCs), in nerve regeneration is proposed. Marrow stromal cells are known as multipotential stem cells that, under specific conditions, differentiate into several kinds of cells. The strategy to transdifferentiate MSCs into the cells with a Schwann cell phenotype and the induction of sciatic and optic nerve regeneration are described.

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.

Spinal cord repair: strategies to promote axon regeneration

Neurons in the central nervous system have a remarkable capacity to regenerate their transected axons when provided with an appropriate growth environment. Advances in our understanding of axon regeneration have allowed the development of different experimental strategies to stimulate axon regeneration in animal models of spinal cord injury. Growth inhibitory proteins block axon regeneration in the CNS, and many of these proteins have been identified. Various methods that are now used to stimulate regeneration in the injured spinal cord are directed at overcoming the growth inhibitory environment of the CNS. Three general approaches tested in vivo stimulate regeneration in the spinal cord. First, antibodies that bind inhibitory proteins in myelin allow axon regeneration in the CNS. Second, methods that modulate neuronal intracellular signaling allow axons to grow directly on the inhibitory substrate of the CNS. Third, transplantation of cells to the lesioned spinal cord promotes repair. In this paper we review current advances in each of these research domains.

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.

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.

Role of Schwann cells in retinal ganglion cell axon regeneration

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

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.

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

This study examined whether prior regenerative growth through peripheral nerve (PN) bridging grafts influenced the specificity with which lesioned adult rat retinal ganglion cell (RGC) axons grew into co-grafts of developing target tissue (fetal superior colliculus). Growth into nontarget (muscle) tissue was also examined. Autologous PN was grafted onto the transected optic nerve. After 14 days, the distal ends of the PNs were placed next to, or inserted into, embryonic tectal tissue or into autologous muscle grafts placed in frontal cortex cavities. Host retinal projections were examined 3-8 months later using anterograde and retrograde tracing techniques. In rats in which there was good apposition between PN and tectal tissue, small numbers of RGC axons were observed growing into the tectal grafts (maximum distance of 180 microm). No evidence of specific innervation of appropriate target regions within tectal grafts was detected, even though such regions (identified by acetylcholinesterase histochemistry) were often located close to the PN grafts. In rats with PN/muscle co-grafts, the extent of retinal axon outgrowth was greater (up to 465 microm from the PN tip) and labelled profiles that resembled motor endplates were seen contacting muscle fibres. Previous studies have shown that spontaneously regenerating RGC axons consistently and selectively innervate appropriate target areas in fetal tectal tissue grafted directly into optic tract lesion cavities. Together, the data suggest that exposure to a PN environment may have reduced the extent of adult retinal axon growth into fetal tectal transplants and affected the way regenerating axons responded to specific developmental cues expressed by target cells in the co-grafted tissue.

Enhanced axonal regeneration following combined demyelination plus schwann cell transplantation therapy in the injured adult spinal cord

We have treated spinal cord injured rats with demyelination plus Schwann cell transplantation and assessed neurite outgrowth in a quantifiable model of axonal regeneration. Axonal injuries of differing severity were induced in the dorsal funiculus of adult rats using a micromanipulator-controlled Scouten knife. Demyelinated regions were produced so as to overlap with the injury site by the injection of galactocerebroside antibodies plus complement one segment cranial to the axonal injury site. Schwann cells were isolated from the sciatic nerve, expanded in vitro, and transplanted into the injury site 1 day later. Animals were killed after an additional 7 days. Schwann cells were evenly distributed throughout the region of demyelination, which extended 6-7 mm cranial to the axonal injury site. The severity of axonal injury was quantified by counting degenerate axons in transverse resin sections. The degree of axonal regeneration was assessed by an electron microscopic analysis of growth cone frequency and distribution relative to the site of axonal injury. Quantification of growth cones at a distance from the site of axonal injury indicated a strong linear relationship (P < 0.001) between the number of growth cones and the number of severed axons; the ratio of growth cones to severed axons was increased by 26.5% in demyelinated plus transplanted animals compared to demyelinated animals without a transplant. Furthermore, only the demyelinated plus transplanted animals contained growth cones associated with myelin in white matter immediately outside of the region of complete demyelination. Growth cones were absent in transplanted-only animals at a distance from the site of axonal injury. These findings indicate that combined demyelination plus Schwann cell transplantation therapy enhances axonal regeneration following injury and suggests that growth cones are able to overcome myelin-associated inhibitors of neurite outgrowth in the presence of trophic support.

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.

Re-establishment of direct synaptic connections between sensory axons and motoneurons after lesions of neonatal opossum CNS (Monodelphis domestica) in culture

For functional recovery after spinal cord injury, regenerating fibres need to grow and to reform appropriate connections with their targets. The isolated central nervous system of neonatal opossums aged 1-9 days has been used to analyse the precision with which neurons become reconnected during regeneration. In culture these preparations maintain their electrical activity and show rapid outgrowth through spinal cord crushes or cuts. By recording electrically and by staining with horseradish peroxidase, we first demonstrated that direct reflex connections were already present at birth between sensory fibres in one segment and motoneurons in the same segment and in adjacent segments. As in previous experiments, 5 days after the spinal cord had been crushed, labelled sensory fibres grew across the lesion to reach the next segment (Woodward et al. (1993) J. Exp. Biol., 176, 77-88; Varga et al. (1995a) Eur. J. Neurosci., 7, 2119-2129, Varga et al. (1995b) Proc. Natl. Acad. Sci. USA, 92, 10959-10963). Beyond the lesion the labelled axons abruptly changed direction, traversed the spinal cord and terminated on labelled motoneurons in the ventral horn. In preparations that had regenerated dorsal root stimulation once again initiated ventral root reflexes. Electron micrographs revealed synapses made by labelled sensory axons on motoneurons. Double staining of growing sensory axons and radial glial fibres showed close association, suggesting guidance. These results indicate that the original pathway is re-established during repair and that appropriate connections are reformed after injury.

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.

Truncated trkB receptors on nonneuronal cells inhibit BDNF-induced neurite outgrowth in vitro

The function of truncated trkB receptors during nervous system plasticity and regeneration is currently unknown. The extensive nonneuronal localization of truncated trkB-T1 receptors, coupled with their up-regulation by CNS glial cells in response to injury, has led to the speculation that these receptors may sequester BDNF and NT-4/5 to reduce their local availability and, thus, limit axonal sprouting. Conversely, trkB-T1 receptors could bind and present neurotrophins to injured axons and facilitate their regeneration in a manor analogous to that proposed for p75(NTR) receptors on Schwann cells. To address this issue, we used an in vitro coculture paradigm in which wild-type 3T3 NIH fibroblasts or two different 3T3 cell clones stably expressing trkB-T1 receptors served as monolayer substrates upon which to evaluate the effect of trkB-T1 receptors on nonneuronal cells to influence neurotrophin (NGF, BDNF, NT-3, and NT-4/5)-induced neurite outgrowth from retinoic acid (RA)-treated SY5Y neuroblastoma cells. In these experiments, BDNF and NT-4/5 produce a strong phosphorylation of trk receptors on the RA-SY5Y cells and induce differentiation of the SY5Y cells (as measured by the development of neurofilament-positive neuritic processes). This ability of the trkB ligands to stimulate neurite outgrowth is dose dependent since increasing concentrations of BDNF (5, 25, and 100 ng/ml) result in an increased percentage of SY5Y cells developing neurites and in progressively longer neurites from SY5Y cells on the control 3T3 monolayers. In these experiments, BDNF and NT-4/5 induce the strongest neurite outgrowth, followed by NT-3 and then NGF. When trkB-T1 receptors are present on the 3T3 cell substratum both BDNF- and NT-4/5-induced neurite extension from the SY5Y cells are strongly inhibited. In contrast, NGF-induced neurite growth is unaffected and NT-3-associated growth is somewhat reduced. These results suggest that the inhibitory effect of the trkB-T1 receptors on the nonneuronal cell substrates is selective for neurite outgrowth that is mediated via the trkB-kinase receptors on the neuroblastoma cells. This ability of trkB-T1 receptors on the nonneuronal substratum to inhibit BDNF-induced neurite outgrowth can be overcome by the addition of high concentrations of BDNF (1 microg/ml). Binding assays using 125I-BDNF suggest that this inhibitory effect could be mediated via binding and internalization of BDNF by the trkB-T1 receptors on the 3T3 cells. These results provide strong support for the hypothesis that the up-regulation of trkB-T1 receptors on astrocytes following CNS lesions enhances the sequestration of the trkB ligands, BDNF and NT- 4/5, at the site of reactive gliosis and, thus, contributes to the inhibition of CNS axonal regeneration from neurons expressing trkB-kinase receptors by removing their ligands from the extracellular environment.

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)

A comparison of postlesion growth of retinotectal and corticotectal axons after superior colliculus transections in neonatal rats

We examined, in neonatal rats, the postinjury response of two different axonal systems that project to a common target area in the visual system. Transections across the rostral part of the left superior colliculus (SC) were made in 2- or 6-day-old rats (P2, P6). Lesioned animals were randomly selected into short- or long-term groups. The short-term group was used to determine the efficacy of the lesion technique; 2-6 days after transections, right (contralateral) eyes were injected with horseradish peroxidase (HRP). Complete deafferentation of the SC was achieved in 73% of P2 (n = 22) and 53% of P6 (n = 10) short-term animals. In the long-term group (examined 2-7 months after transection), retinotectal and corticotectal projections were assessed in each animal by using [3H]proline and wheat germ agglutin-HRP, respectively. Examination of a series of sagittal sections revealed that the cut had extended across the entire SC in 63% of P2 (n = 19) and 55% of P6 (n = 12) long-term rats. Despite this, retinal and cortical axons were seen in appropriate layers in postlesion SC in all P2 lesioned animals. Cortical projections caudal to the cut were seen in all P6 rats; however, in these animals, the retinal projection was sparse and not always present. Differences in lesion geometry led to consistent differences in the pattern and extent of ingrowth of retinal and cortical axons into postlesion SC neuropil. The two axonal populations also followed different paths as they grew between prelesion and postlesion SC. It is likely that a number of factors influenced the patterns of postlesion growth, including the relative maturity of the axons and the neuropil into which they were growing. There was also, however, clear evidence of competitive interactions between retinal and cortical axons in postlesion SC that consistently led to greater than normal segregation of the two populations and hence restricted their terminal distributions.

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.

Corticospinal tract regrowth

The natural ability of the adult central nervous system of higher vertebrates to recover from injury is highly limited. This limitation is most likely due to an inhospitable environment and/or intrinsic incapacities of the neurons to re-extend their neurites after injury or axotomy. The rat corticospinal tract is the largest tract leading from brain to spinal cord and is often used as a model in developmental and regeneration studies. The extensive know-how of factors involved in the development of the corticospinal tract did provide the foundation for many studies on corticospinal tract regrowth after injury in the adult spinal cord. The results of these experiments, as discussed in this review, have led to important contributions to the further understanding of central nervous system regeneration.

Retino-geniculate axons regenerating in adult hamsters are able to form morphologically distinct terminals

Adult mammalian retinal ganglion cells (RGCs) can regenerate through peripheral nerve (PN) grafts and innervate central nervous system (CNS) targets. Previous studies have demonstrated a basic level of differentiation of such projections. To further assess the specificity and functionality of regenerated CNS connections we have developed a new model in which RGCs are directed through a PN graft into the dorsolateral geniculate body (LGBd) while preserving the visual cortex and radiations. We also describe by light microscopy that regenerating RGC axons which enter the LGBd differentiate and form subtypes of RGC terminals reminiscent of those seen in the normal LGBd. Thus the adult CNS contains cues that permit phenotypic differentiation of terminal types during regeneration.

Integration of transplanted cultured Schwann cells into the long myelinated fiber tracts of the adult spinal cord

A suspension of about 10,000 purified Schwann cells cultured from the neonatal rat sciatic nerve was transplanted into a discrete site in the upper cervical level of the corticospinal tract of one side in adult rats. From 4 days after transplantation immunostaining for p75 (low-affinity neurotrophin receptor) showed that the transplants consisted of a central mass of Schwann cells and cuffs of elongated Schwann cells along the perivascular space of curving blood vessels (most of which had been formed in response to the transplantation). Schwann cells leaving the central mass and perivascular cuffs migrated in strictly linear orientation along the rostrocaudal axis of the host corticospinal tract. According to the territory through which they migrated, the transplanted Schwann cells adopted two quite different forms: (1) The row Schwann cells, which migrated singly or in groups within the rows of host oligodendrocytic and astrocytic cell bodies, were non-process-bearing, rather cuboidal, brick-like cells (about 8 x 12 microm in size). (2) In contrast, the interfascicular Schwann cells, which migrated singly or intertwined in rope-like small groups interspersed among the axons of the host corticospinal tract, were larger, symmetrically bipolar cells, with processes about 100-120 microm long and 2 microm wide and bulging, ovoid nuclei, located in centrally placed cell bodies about 10 microm across. After about 6 weeks, the p75 immunoreactivity of the interfascicular Schwann cells had become down-regulated. However, from as early as 10 days after transplantation, immunostaining for the peripheral myelin protein, P0, semithin sections, and electron microscopy showed that these Schwann cells were not lost, but that they had myelinated the segments of the host corticospinal axons in the region of the transplant. In contrast, the row Schwann cells did not express P0 or form myelin. They retained their p75 immunoreactivity at long survivals (presumably because they were secluded from contacting the tract axons). The row Schwann cells also migrated farther than the interfascicular Schwann cells (possibly a function of their maintained p75 expression), becoming dispersed singly for at least 8 mm from the original transplant site. Our previous study of corticospinal tract lesions had shown the formation of a "closed" scar formed by hypertrophic astrocytic processes, which walled off a central astrocyte-free region and totally disrupted the normal longitudinal alignment of the tract astrocytic processes. In contrast, while the present Schwann cell transplants induced a comparable astrocytic hypertrophy over the same time course, the astrocytic processes remained able to penetrate the transplant site, which was not walled off, so that the longitudinal arrangement of the host corticospinal tract astrocytic skeleton was preserved intact across the region of the transplant. These observations show that Schwann cells can be intimately integrated into the cytoarchitecture of the myelinated adult host corticospinal tract. This integration is not a random dispersal in damaged areas: it involves direct interaction with the cell elements present in the host tract, it respects the complex and regular organization of the host tract glial cells, and it results in the formation of a precisely arranged mosaic of central and peripheral tissue.