Peripherally grafted human foetal dorsal root ganglion cells extend axons into the spinal cord of adult host rats by circumventing dorsal root entry zone astrocytes

The Influence of Donor Age, Nerve Growth Factor, and Cografting with Drosophila Cells on Survival of Peripherally Grafted Embryonic or Fetal Rat Dorsal Root Ganglia

Previous studies have shown that embryonic rat and human dorsal root ganglion (DRG) cells survive grafting to the cavity of extirpated adult rat DRG. Furthermore, grafted human embryonic neurons were shown to send axons peripherally and into the spinal cord, where they establish functional synaptic connections. This study analyzed the survival of orthotopically allografted rat DRG cells from embryonic stages 15 (E15) and 20 (E20), and the influence on their survival of nerve growth factor (NGF). NGF was delivered to the DRG transplants either by pump infusion or by cotransplantation of cells from Drosophila melanogaster, transgenic for human NGF. Lumbar DRGs of adult rats were removed and a collection of E15 or E20 DRGs placed in the cavity. One month after grafting the total number of DRG cells in the grafts was counted. Differentiation of subpopulations of DRG cells was estimated by counting cells immunostained for calcitonin gene-related peptide (CGRP), Griffonia simplicifolia agglutinin isolectin B4 (GSA), or heavy neurofilament protein (antibody RT97). The results show: i) similar survival of E15 and E20 grafts, with great variability in the survival of different subpopulations in E15 transplants, but a more consistent distribution of different phenotypes in E20 transplants; ii) infusion of NGF for 2 weeks increases the survival of E15 transplants, but has a negative effect on E20 transplants; iii) Drosophila cells transfected with human NGF gene survive peripheral xenografting and have a positive effect on the survival of the GSA- and CGRP-positive populations in E15 and E20 transplants; iv) Drosophila cells without the human NGF gene increase cell survival in E20 transplants. These data suggest that i) the effect of NGF is dependent on the embryonic stage of the transplants, ii) age-dependent sensitivity to NGF influences graft survival, and iii) transgenic Drosophila cells can be cotransplanted with embryonic neural tissue to the mammalian peripheral nervous system with a positive effect on the survival of neural grafts.

Human Embryonic Stem Cell-Derived Progenitors Assist Functional Sensory Axon Regeneration after Dorsal Root Avulsion Injury

Dorsal root avulsion results in permanent impairment of sensory functions due to disconnection between the peripheral and central nervous system. Improved strategies are therefore needed to reconnect injured sensory neurons with their spinal cord targets in order to achieve functional repair after brachial and lumbosacral plexus avulsion injuries. Here, we show that sensory functions can be restored in the adult mouse if avulsed sensory fibers are bridged with the spinal cord by human neural progenitor (hNP) transplants. Responses to peripheral mechanical sensory stimulation were significantly improved in transplanted animals. Transganglionic tracing showed host sensory axons only in the spinal cord dorsal horn of treated animals. Immunohistochemical analysis confirmed that sensory fibers had grown through the bridge and showed robust survival and differentiation of the transplants. Section of the repaired dorsal roots distal to the transplant completely abolished the behavioral improvement. This demonstrates that hNP transplants promote recovery of sensorimotor functions after dorsal root avulsion, and that these effects are mediated by spinal ingrowth of host sensory axons. These results provide a rationale for the development of novel stem cell-based strategies for functionally useful bridging of the peripheral and central nervous system.

Dorsal root ganglion progenitors differentiate to gamma-aminobutyric acid- and choline acetyltransferase-positive neurons

This study examined the isolation and differentiation of dorsal root ganglion progenitor cells for therapeutic use in neurodegenerative diseases. Rat embryonic dorsal root ganglia progenitors were isolated and purified using the differential adhesion method combined with cytosine arabinoside treatment. After culture in serum-free medium supplemented with B27, basic fibroblast growth factor and epidermal growth factor, these cells remained viable and survived for more than 18 months in vitro. Most cells differentiated to neurons that were immunoreactive for gamma-aminobutyric acid and choline acetyltransferase as detected by immunohistochemical staining. In addition, nerve growth factor and neurotrophic tyrosine kinase receptor expression were also observed in dorsal root ganglion progenitors and differentiated cells. K252a, an inhibitor that blocks nerve growth factor-induced signaling, inhibited cell survival, suggesting the possible existence of a nerve growth factor autocrine loop in these proliferating cells.

Forced Runx1 expression in human neural stem/progenitor cells transplanted to the rat dorsal root ganglion cavity results in extensive axonal growth specifically from spinal cord-derived neurospheres

Cell replacement therapy holds great promise for treating a wide range of human disorders. However, ensuring the predictable differentiation of transplanted stem cells, eliminating their risk of tumor formation, and generating fully functional cells after transplantation remain major challenges in regenerative medicine. Here, we explore the potential of human neural stem/progenitor cells isolated from the embryonic forebrain (hfNSPCs) or the spinal cord (hscNSPCs) to differentiate to projection neurons when transplanted into the dorsal root ganglion cavity of adult recipient rats. To stimulate axonal growth, we transfected hfNSPC- and hscNSPC-derived neurospheres, prior to their transplantation, with a Tet-Off Runx1-overexpressing plasmid to maintain Runx1 expression in vivo after transplantation. Although pronounced cell differentiation was found in the Runx1-expressing transplants from both cell sources, we observed extensive, long-distance growth of axons exclusively from hscNSPC-derived transplants. These axons ultimately reached the dorsal root transitional zone, the boundary separating peripheral and central nervous systems. Our data show that hscNSPCs have the potential to differentiate to projection neurons with long-distance axonal outgrowth and that Runx1 overexpression is a useful approach to induce such outgrowth in specific sources of NSPCs.

Regulation of boundary cap neural crest stem cell differentiation after transplantation

Success of cell replacement therapies for neurological disorders will depend largely on the optimization of strategies to enhance viability and control the developmental fate of stem cells after transplantation. Once transplanted, stem/progenitor cells display a tendency to maintain an undifferentiated phenotype or differentiate into inappropriate cell types. Gain and loss of function experiments have revealed key transcription factors which drive differentiation of immature stem/progenitor cells toward more mature stages and eventually to full differentiation. An attractive course of action to promote survival and direct the differentiation of transplanted stem cells to a specific cell type would therefore be to force expression of regulatory differentiation molecules in already transplanted stem cells, using inducible gene expression systems which can be controlled from the outside. Here, we explore this hypothesis by employing a tetracycline gene regulating system (Tet-On) to drive the differentiation of boundary cap neural crest stem cells (bNCSCs) toward a sensory neuron fate after transplantation. We induced the expression of the key transcription factor Runx1 in Sox10-expressing bNCSCs. Forced expression of Runx1 strongly increased transplant survival in the enriched neurotrophic environment of the dorsal root ganglion cavity, and was sufficient to guide differentiation of bNCSCs toward a nonpeptidergic nociceptive sensory neuron phenotype both in vitro and in vivo after transplantation. These findings suggest that exogenous activation of transcription factors expression after transplantation in stem/progenitor cell grafts can be a constructive approach to control their survival as well as their differentiation to the desired type of cell and that the Tet-system is a useful tool to achieve this.

Long-Term Survival, Robust Neuronal Differentiation, and Extensive Migration of Human Forebrain Stem/Progenitor Cells Transplanted to the Adult Rat Dorsal Root Ganglion Cavity

Neurons in dorsal root ganglia (DRGs) transmit sensory information from peripheral tissues to the spinal cord. This pathway can be interrupted, for example, as the result of physical violence, traffic accidents, or complications during child delivery. As a consequence, the patient permanently loses sensation and often develops intractable neuropathic pain in the denervated area. Here we investigate whether human neural stem/progenitor cells (hNSPCs) transplanted to the DRG cavity can serve as a source for repairing lost peripheral sensory connections. We found that hNSPCs robustly differentiate to neurons, which survive long-term transplantation. The neuronal population in the transplants was tightly surrounded by astrocytes, suggesting their active role in neuron survival. Furthermore, 3 months after grafting hNSPCs were found in the dorsal root transitional zone (DRTZ) and within the spinal cord. The level of differentiation of transplanted cells was high in the core of the transplants whereas cells that migrated to the DRTZ and spinal cord were undifferentiated, nestin-expressing precursors. These data indicate that peripherally transplanted hNPSCs can be used for repair of dorsal root avulsion or spinal cord injuries; however, additional factors are required to guide their differentiation to the desired type of neurons. Furthermore, hNPSCs that migrate from the DRG cavity graft site to the DRTZ and spinal cord may provide trophic support for regenerating dorsal root axons, thereby allowing them to reenter the host spinal cord.

Implanted embryonic sensory neurons project axons toward adult auditory brainstem neurons in roller drum and Stoppini co-cultures

Previously we have shown in vivo the survival, migration and integration of embryonic dorsal root ganglion (DRG) neurons that were grafted into the inner ear and peripheral auditory nervous system. In order to evaluate relevant factors determining integration of sensory neurons further into the central auditory nervous system, complementary in vitro techniques are necessary. The advantages of in vitro systems are that a large number of factors including various grafts and different conditions can be efficiently examined for. Hence, we co-cultured 300 microm thick postnatal rat brainstem slices containing the cochlear nucleus including the central part of the 8th cranial nerve with mouse embryonic DRG neurons. The organotypic co-cultures were either grown on coverslips using the roller drum method described by Gähwiler or on membranes according to the interface method described by Stoppini. Neurons in the cochlear nucleus were labeled with DiI. The results demonstrate that (1) brainstem slices survive for up to 5 weeks in culture, and that (2) co-cultures of embryonic sensory neurons and brainstem show a high degree of neuronal survival, and that (3) survival and axonal outgrowth from the implanted embryonic neurons are dependent on the presence of the brainstem slice rather than on exogenous NGF and that (4) implanted embryonic neurons send axons toward neurons in the cochlear nucleus.

Xenografted fetal dorsal root ganglion, embryonic stem cell and adult neural stem cell survival following implantation into the adult vestibulocochlear nerve

Sensorineural hearing loss is a disabling condition. In the post-embryonic and adult mammalian inner ear, the regeneration of auditory hair cells, spiral ganglion neurons or their axons does not occur naturally. This decrease in excitable neurons limits the success of auditory rehabilitation. Allografts and xenografts have shown promise in the treatment of a variety of neurological diseases. Fetal dorsal root ganglion (DRG) neurons can extend functional connections in the rat spinal cord. Embryonic stem cells (ES cells) and adult neural stem cells (ANSC) have the potential to differentiate into neurons. We have implanted embryonic days (E) 13-16 fetal mouse DRGs from transgenic mouse lines that express Enhanced Green Fluorescent Protein (EGFP) or lacZ reporter genes, EGFP-expressing ES cells or lacZ-expressing ANSC into the injured vestibulocochlear nerve of adult rats and guinea pigs. Survival of the implants was assessed 2 to 4 weeks postoperatively. For further evaluation of the differentiation of the implanted ES-cells, we double labeled with the mouse-specific neuronal antibody Thy 1.2. The rats implanted with EGFP- or lacZ-expressing DRGs showed labeled DRGs after sacrifice. In addition, EGFP-positive nerve fibers were seen growing within the proximal nerve. The results from the EGFP ES cells and lacZ ANSC revealed reporter-expressing cells at the site of injection in the vestibulocochlear nerve of the host rats and guinea pigs but also within the brain stem. Thy 1.2 profiles were seen among the EGFP ES cells within the 8th cranial nerve. The findings of this study indicate that the vestibulocochlear nerve of adult rats and guinea pigs will support xenotransplants of embryonic DRG, ES cells and ANSC. This may have future clinical applicability in recreating a neuronal conduit following neuronal injury between the inner ear and the central nervous system (CNS).

The role of proteoglycans in Schwann cell/astrocyte interactions and in regeneration failure at PNS/CNS interfaces

In the dorsal root entry zone (DREZ) peripheral sensory axons fail to regenerate past the peripheral nervous system/central nervous system (PNS/CNS) interface. Additionally, in the spinal cord, central fibers that regenerate into Schwann cell (SC) bridges can enter but do not exit at the distal Schwann cell/astrocyte (AC) boundary. At both interfaces where limited mixing of the two cell types occurs, one can observe an up-regulation of inhibitory chondroitin sulfate proteoglycans (CSPGs). We treated confrontation Schwann cell/astrocyte cultures with the following: (1) a deoxyribonucleic acid (DNA) enzyme against the glycosaminoglycan (GAG)-chain-initiating enzyme, xylosyltransferase-1 (XT-1), (2) a control DNA enzyme, and (3) chondroitinase ABC (Ch'ase ABC) to degrade the GAG chains. Both techniques for reducing CSPGs allowed Schwann cells to penetrate deeply into the territory of the astrocytes. After adding sensory neurons to the assay, the axons showed different growth behaviors depending upon the glial cell type that they first encountered during regeneration. Our results help to explain why regeneration fails at PNS/CNS glial boundaries.

Strategies for repair of the deafferented spinal cord

Deafferentation of the spinal cord by interruption of the sensory fibers in the dorsal roots highlights the problem of regeneration failure in the central nervous system. The injured dorsal root axons regenerate steadily, albeit slowly, in the peripheral compartment of the dorsal root, but abruptly cease to elongate when confronted with the interface between the peripheral and central nervous system, the dorsal root transitional zone (DRTZ). The glial cells of the CNS and their products together form this regeneration barrier. Recent years have witnessed several successful approaches to, at least in part, overcome this barrier. Particularly promising results have been obtained by (1). the replacement of adult non-regenerating dorsal root ganglion neurons with corresponding cells from embryonic or fetal donors, (2). the implantation of olfactory ensheathing cells at the DRTZ, and (3). immediate intrathecal infusion of growth factors to which dorsal root ganglion cells respond. In all these instances, growth of sensory axons into the adult spinal cord, as well as return of spinal cord connectivity, have been demonstrated. These findings suggest routes towards treatment strategies for plexus avulsion, and contribute to our understanding of possibilities to overcome regeneration failure in the spinal cord.

Regeneration of primary sensory axons into the adult rat spinal cord via a peripheral nerve graft bridging the lumbar dorsal roots to the dorsal column

This study investigated the feasibility of using a peripheral nerve autograft (NAG) to promote and guide regeneration of sensory axons from the caudal lumbar dorsal roots to the rostral dorsal column following a lower thoracic cordotomy in adult rats. After a left hemicordotomy at the T13 vertebra level and ipsilateral L3 and L4 rhizotomies, a peripheral NAG (peroneal nerve) was connected to the distal roots stumps, then implanted into the left dorsal column 10 mm rostral to hemicordotomy site (n = 12). After surgery, all animals of the experimental group experienced complete anesthesia in their left hindlimb. Three months later, a slight response to nociceptive stimulation reappeared in L3 and/or L4 dermatomes in 6 of the 12 experimental animals. None of these animals exhibited self-mutilation. Nine months after surgery, we performed retrograde tracing studies by injecting horseradish peroxidase (HRP) into the left dorsal column 30 mm rostral to the NAG implantation site. In eight animals, we found HRP-stained neurons in the left L3 and/or L4 dorsal root ganglia (DRG). The mean number of HRP-stained neurons per DRG was 71 +/- 92 (range 2-259). In control groups, no HRP-stained neurons were found in L3 or L4 DRG. Histological analysis of the NAG showed evidence of axonal regeneration in all 8 animals with positive retrograde labeling of DRG neurons. However, we did not find a statistical correlation between the number of HRP-stained neurons and the degree of sensory recovery. This study demonstrates that an NAG joining dorsal roots to the dorsal column, thus shunting the original CNS-PNS junction, can support regeneration of central axons from DRG primary sensory neurons into the dorsal column over distances of at least 30 mm despite the inhibitory influence of the CNS white matter.

Axons and glial interfaces: ultrastructural studies

At most vertebrate nerve transitional zones (TZs) there is a glial barrier which is pierced by axons passing between the CNS and PNS. Myelinated axons traverse this in individual tunnels. The same is true of larger non-myelinated axons. This holds widely among the vertebrates, for example, the large motor axons of the sea-lamprey Petromyzon (which also possess TZ specializations not found in mammals). Smaller non-myelinated axons traverse the TZ glial tunnels as fascicles and so the barriers are correspondingly less comprehensive for them. Accordingly, in nerves composed of non-myelinated axons, such as the vomeronasal or the olfactory, a TZ barrier stretching across the nerve is effectivelyabsent. The chordateAmphioxus differsfrom the vertebrates in lacking a TZ barrier throughout. Invertebrates also lack glial barriers at the TZs between ganglia and interconnecting nerve trunks. The glial barrier at the dorsal spinal root TZ (DRTZ) has considerable value for analysing protocols aimed at achieving CNS regeneration, because it provides a useful model of the gliotic reaction at sites of CNS injury. Also, it is especially amenable to morphometric analysis, and so enables objective quantification of different protocols. Being adjacent to the subarachnoid space, it is accessible for experimental intervention. The DRTZ was used to investigate the value of neurotrophin 3 (NT3) in promoting axon regeneration across the TZ barrier and into the CNS following dorsal root crush. It promoted extensive regeneration and vigorous non-myelinated axonal ensheathment. On average, around 40% of regenerating axons grew across the interface, compared with virtually none in its absence. These may have traversed the interface through loci occupied by axons prior to degeneration. Many regenerating axons became myelinated, both centrally and peripherally.

Correlation between putative inhibitory molecules at the dorsal root entry zone and failure of dorsal root axonal regeneration

The molecular mechanisms involved in preventing regenerating dorsal root axons from entering the spinal cord at the dorsal root entry zone (DREZ) are obscure. We used immunohistochemistry, in situ hybridization, and electron microscopy to study axonal regeneration after dorsal rhizotomy in adult rats and its relationship to cellular changes and the distribution of putative growth inhibitory molecules in this region. Astrocyte processes, ending as bulb-shaped expansions, grew up to 700 microm into the basal lamina tubes of injured roots, where regenerating axons were also present. Some of these axons approached or reached the DREZ but grew no further; others turned back toward the ganglion, suggesting the presence of repulsive cues in or near the DREZ. Tenascin-C mRNA and protein and CSPG stub immunoreactivity were strongly upregulated in the roots after rhizotomy, but were only weakly expressed in the DREZ. Tenascin-R immunoreactivity was confined to CNS tissue, and unaffected by rhizotomy. Large, rounded GFAP-negative, NG2-immunoreactive cells, a few of which were OX42 positive, were found in the DREZ following rhizotomy. Astrocyte processes projecting into the roots were tenascin-R and NG2 negative. Hence, only NG2-expressing cells and tenascin-R were appropriately situated to inhibit regeneration through the DREZ.

Neurobiology of human neurons (NT2N) grafted into mouse spinal cord: implications for improving therapy of spinal cord injury

Emerging data suggest that current strategies for the treatment of spinal cord injury might be improved or augmented by spinal cord grafts of neural cells, and it is possible that grafted neurons might have therapeutic potential. Thus, here we have summarized recent studies of the neurobiology of clonal human (NT2N) neurons grafted into spinal cord of immunodeficient athymic nude mice. Postmitotic human NT2N neurons derived in vitro from an embryonal carcinoma cell line (NT2) were transplanted into spinal cord of neonatal, adolescent and adult nude mice where they became integrated into the host gray and white matter, did not migrate from the graft site, and survived for > 15 months after implantation. The neuronal phenotype of the grafted NT2N cells was similar in gray and white matter regardless of host age at implantation, and some of the processes extended by the transplanted NT2N neurons became ensheathed by oligodendrocytes. However, there were consistent differences between NT2N processes traversing white versus gray matter. Most notably, NT2N processes with a trajectory in white matter extended over much longer distances (some for > 2 cm) than those confined to gray matter. Thus, NT2N neurons grafted into spinal cord of nude mice integrated into gray as well as white matter, where they exhibited and maintained the morphological and molecular phenotype of mature neurons for > 15 months after implantation. Also, the processes extended by grafted NT2N neurons differentially responded to cues restricted to gray versus white matter. Further insight into the neurobiology of grafted human NT2N neurons in the normal and injured spinal cord of experimental animals may lead to novel and more effective strategies for the treatment of spinal cord injury.

Transplantation of autologous dorsal root ganglia into the peroneal nerve of adult rats: uni- and bidirectional axonal regrowth from the grafted DRG neurons

Previous studies have demonstrated that transplanted dorsal root ganglion neurons (DRGNs) can survive and differentiate in a variety of orthotopic and heterotopic locations. In order to develop strategies aimed at restoring the sensory function following traumatic injury to the spinal cord and to its peripheral sensory connections, we have transplanted adult autologous dorsal root ganglia (DRGs) into the peroneal nerve of adult rats. Twelve female Sprague-Dawley rats were used. A segment of the peroneal nerve was isolated by double transection and ligature to prevent undesirable reinnervation. The left fifth cervical (C5) DRG was removed from its normal location and inserted into the midportion of the isolated nerve segment. One month after the grafting procedure, a morphological study included axonal retrograde labeling with True Blue (TB) and/or Diamidino Yellow (DY) applied on each cut end of the nerve segment, cell counting, and cell measurement after staining with cresyl violet. Compared to the C5 ganglion maintained in situ, the mean number of surviving DRGNs in the transplant was 1381, corresponding to a survival rate of 20%. Both singly (TB or DY) and doubly (TB + DY) stained DRGNs were encountered. The proportion of surviving neurons that appeared to be doubly labeled was 23%. These neurons were considered as having grown two opposite axonal projections, one into the "central" part of the nerve segment and a second one into its "peripheral" part. The present results give new insights and interesting prospects concerning the possibilities of reconstructing the sensory circuitry after central and/or peripheral injuries.

Functional connections are established in the deafferented rat spinal cord by peripherally transplanted human embryonic sensory neurons

Functionally useful repair of the mature spinal cord following injury requires axon growth and the re-establishment of specific synaptic connections. We have shown previously that axons from peripherally grafted human embryonic dorsal root ganglion cells grow for long distances in adult host rat dorsal roots, traverse the interface between the peripheral and central nervous system, and enter the spinal cord to arborize in the dorsal horn. Here we show that these transplants mediate synaptic activity in the host spinal cord. Dorsal root ganglia from human embryonic donors were transplanted in place of native adult rat ganglia. Two to three months after transplantation the recipient rats were examined anatomically and physiologically. Human fibres labelled with a human-specific axon marker were distributed in superficial as well as deep laminae of the recipient rat spinal cord. About 36% of the grafted neurons were double labelled following injections of the fluorescent tracers MiniRuby into the sciatic and Fluoro-Gold into the lower lumbar spinal cord, indicating that some of the grafted neurons had grown processes into the spinal cord as well as towards the denervated peripheral targets. Electrophysiological recordings demonstrated that the transplanted human dorsal roots conducted impulses that evoked postsynaptic activity in dorsal horn neurons and polysynaptic reflexes in ipsilateral ventral roots. The time course of the synaptic activation indicated that the human fibres were non-myelinated or thinly myelinated. Our findings show that growing human sensory nerve fibres which enter the adult deafferentated rat spinal cord become anatomically and physiologically integrated into functional spinal circuits.

Cellular transplantation and spinal cord injury

Spinal cord injury is often characterized by immediate and irreversible loss of sensory and motor functions below the level of injury. Cellular transplantation in various experimental models of spinal cord injury has been used as a strategy for reducing deficits and improving functional recovery. The general strategy has been aimed at promoting regeneration of intrinsic injured axons with the development of alternative pathways that facilitate a partial functional connection. Other objectives of cellular transplantation studies have included replacement of lost cellular elements, alleviation of chronic pain, and modulation of the inflammatory response after injury. This review focuses on the cell types that have been used in spinal cord transplantation studies in the context of evolving biological perspectives, technological advances, and new therapeutic strategies and serves as a point of reference for future studies.

Functional regeneration of sensory axons into the adult spinal cord

The arrest of dorsal root axonal regeneration at the transitional zone between the peripheral and central nervous system has been repeatedly described since the early twentieth century. Here we show that, with trophic support to damaged sensory axons, this regenerative barrier is surmountable. In adult rats with injured dorsal roots, treatment with nerve growth factor (NGF), neurotrophin-3 (NT3) and glial-cell-line-derived neurotrophic factor (GDNF), but not brain-derived neurotrophic factor (BDNF), resulted in selective regrowth of damaged axons across the dorsal root entry zone and into the spinal cord. Dorsal horn neurons were found to be synaptically driven by peripheral nerve stimulation in rats treated with NGF, NT3 and GDNF, demonstrating functional reconnection. In behavioural studies, rats treated with NGF and GDNF recovered sensitivity to noxious heat and pressure. The observed effects of neurotrophic factors corresponded to their known actions on distinct subpopulations of sensory neurons. Neurotrophic factor treatment may thus serve as a viable treatment in promoting recovery from root avulsion injuries. I

Differential effects of spinal cord gray and white matter on process outgrowth from grafted human NTERA2 neurons (NT2N, hNT)

To investigate host effects on grafts of pure, postmitotic, human neurons, we assessed the morphologic and molecular phenotype of purified NTera2N (NT2N, hNT) neurons implanted into the spinal cord of athymic nude mice. NT2N neurons were implanted into both spinal cord gray matter and white matter of neonatal, adolescent, and adult mice and were evaluated at postimplantation times up to 15 months. NT2N neurons remained at the implantation site and showed process integration into all host areas, and each graft exhibited similar phenotypic features regardless of location or host age at implantation. Evidence of host oligodendrocyte ensheathment of NT2N neuronal processes was seen, and grafted NT2N neurons acquired and maintained the morphologic and molecular phenotype of mature neurons. The microenvironments of host gray matter and white matter appear to exert differential effects on implanted neuronal processes, because consistent differences were noted in the morphologies of graft processes extending into white matter versus gray matter. NT2N processes extended for long distances (>2 cm) within white matter, whereas NT2N processes located within gray matter had shorter trajectories. This suggests that NT2N neurons integrate similarly into spinal cord gray matter and white matter, but they extend processes that respond differentially to gray matter and white matter cues. Further studies of the model system described here may identify the host molecular signals that support and direct integration of grafted human neurons as well as the outgrowth of their processes in the nervous system.

The transitional zone and CNS regeneration

Most nerves are attached to the neuraxis by rootlets. The CNS-PNS transitional zone (TZ) is that length of rootlet containing both central and peripheral nervous tissue. The 2 tissues are separated by a very irregular but clearly defined interface, consisting of the surface of the astrocytic tissue comprising the central component of the TZ. Central to this, myelin sheaths are formed by oligodendrocytes and the supporting tissue is astrocytic. Peripheral to it, sheaths are formed by Schwann cells which are enveloped in endoneurium. The features of transitional nodes are a composite of those of central and peripheral type. The interface is penetrated only by axons. It is absent at first. It is formed by growth of processes into the axon bundle from glial cell bodies around its perimeter. These form a barrier across the bundle which fully segregates prospectively myelinated axons. Rat spinal dorsal root TZs have been used extensively to study CNS axon regeneration. The CNS part of the TZ responds to primary afferent axon degeneration and to regenerating axons in ways which constitute a satisfactory model of the gliotic tissue response which occurs in CNS lesions. It undergoes gliosis and the gliotic TZ tissue expands distally along the root. In mature animals axons can regenerate satisfactorily through the endoneurial tubes of the root but cease growth on reaching the gliotic tissue. The general objective of experimental studies is to achieve axon regeneration from the PNS through this outgrowth and into the dorsal spinal cord. Since immature tissue has a greater capacity for regeneration than that of the adult, one approach includes the transplantation of embryonic or fetal dorsal root ganglia into the locus of an extirpated adult ganglion. Axons grow centrally from the transplanted ganglion cells and some enter the cord. Other approaches include alteration of the TZ environment to facilitate axon regeneration, for example, by the application of tropic, trophic, or other molecular factors, and also by transplantation of cultured olfactory ensheathing cells (OECs) into the TZ region. OECs, by association with growing axons, facilitate their extensive regeneration into the cord. Unusually, ventral motoneuron axons may undergo some degree of unaided CNS regeneration. When interrupted in the spinal cord white matter, some grow out to the ventral rootlet TZ and thence distally in the PNS. The DRTZ is especially useful for quantitative studies on regeneration. Since the tissue is anisometric, individual parameters such as axon numbers, axon size and glial ensheathment can be readily measured and compared in the CNS and PNS environments, thereby yielding indices of regeneration across the interface for different sets of experimental conditions.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

NT-3 delivered by an adenoviral vector induces injured dorsal root axons to regenerate into the spinal cord of adult rats

Sensory axons interrupted in the dorsal roots of adult mammals are normally unable to regenerate into the spinal cord. We have investigated whether the introduction of a neurotrophin gene into the spinal cord might offer an approach to otherwise intractable spinal root injuries. The dorsal roots of the 4th, 5th, and 6th lumbar spinal nerves of adult rats were severed and reanastomosed. Fourteen to nineteen days later, adenoviral vectors containing either the LacZ or NT-3 genes were injected into the ventral horn of the lumbar spinal cord, resulting in strong expression of the transgenes in glial cells and motor neurons between 4 and 40 days after injection. When dorsal root axons were transganglionically labelled with HRP conjugated to cholera toxin subunit B, 16 to 37 days after dorsal root injury, large numbers of labelled axons could be seen to have regenerated into the cord, but only in those animals injected with vector carrying the NT-3 gene. The regenerated axons were found at the injection site, mainly in the grey matter, and had penetrated as deep as lamina V. Gene therapy with adenoviral vectors encoding a neurotrophin has therefore been shown to be capable of enhancing and directing the regeneration of a subpopulation of dorsal root axons (probably myelinated A fibres), into and through the CNS environment.

Central neuron-glial and glial-glial interactions following axon injury

Axon injury rapidly activates microglial and astroglial cells close to the axotomized neurons. Following motor axon injury, astrocytes upregulate within hour(s) the gap junction protein connexin-43, and within one day glial fibrillary acidic protein (GFAP). Concomitantly, microglial cells proliferate and migrate towards the axotomized neuron perikarya. Analogous responses occur in central termination territories of peripherally injured sensory ganglion cells. The activated microglia express a number of inflammatory and immune mediators. When neuron degeneration occurs, microglia act as phagocytes. This is uncommon after peripheral nerve injury in the adult mammal, however, and the functional implications of the glial cell responses in this situation are unclear. When central axons are injured, the glial cell responses around the affected neuron perikarya appears to be minimal or absent, unless neuron degeneration occurs. Microglia proliferate, and astrocytes upregulate GFAP along central axons undergoing anterograde, Wallerian, degeneration. Although microglia develop into phagocytes, they eliminate the disintegrating myelin very slowly, presumably because they fail to release molecules which facilitate phagocytosis. During later stages of Wallerian degeneration, oligodendrocytes express clusterin, a glycoprotein implicated in several conditions of cell degeneration. A hypothetical scheme for glial cell activation following axon injury is discussed, implying the injured neurons initially interact with adjacent astrocytes. Subsequently, neighbouring resting microglia are activated. These glial reactions are amplified by paracrine and autocrine mechanisms, in which cytokines appear to be important mediators. The specific functional properties of the activated glial cells will determine their influence on neuronal survival, axon regeneration, and synaptic plasticity. The control of the induction and progression of these responses are therefore likely to be critical for the outcome of, for example, neurotrauma, brain ischemia and chronic neurodegenerative diseases.

Selective restoration of sensation by peripheral nerve grafts directly implanted into the contralateral C7 dorsal root ganglion: an experimental study in rat brachial plexus

OBJECTIVE

In brachial plexus injuries, when the dorsal root ganglion (DRG) is avulsed from the spinal cord there is no possibility of direct repair. Therefore, in the present report the median nerve was connected directly to the contralateral C7 DRG, to restore forepaw sensation.

METHODS

The ulnar nerve was sectioned, and a 15-mm segment of nerve was removed. The median nerve was sectioned, transposed dorsally, and repaired using a sural nerve graft directly implanted into the C7 contralateral DRG. Rats were then assessed 6 and 12 months later by behavioral, histological, and plasma extravasion techniques.

RESULTS

All animals recovered sensation in the forepaw. Retrogradely labeled sensory neurons were demonstrated in the C7 DRG, and the whole forepaw was labeled after plasma extravasion induction by median nerve antidromic stimulation. Several myelinated and positively neurofilament-stained fibers were demonstrated in the grafted median nerve.

CONCLUSION

The surgical strategy proposed might be a useful alternative to selective sensory repair in the emerging field of brachial plexus reconstruction by direct spinal cord surgery.

Human embryonic spinal cord grafts in adult rat spinal cord cavities: survival, growth, and interactions with the host

The ability of solid pieces of transplanted human embryonic spinal cord to survive, grow, and integrate with adult rat host spinal cord tissue was investigated. Unilateral cavities were surgically created at vertebral level T12-T13 in 10 athymic nude rats and 5 regular Sprague-Dawley rats. Seven of the athymic rats acutely received a human spinal cord graft, while the remaining 8 rats served as controls, with cavities alone. After 6 months the morphological outcome was evaluated with cresyl violet and with immunohistochemistry using antibodies toward human-specific neurofilament (hNF), human-specific Thy-1 (Thy-1), neurofilament, glial fibrillary acidic protein, serotonin (5-HT), and tyrosine hydroxylase (TH). The in situ morphology of the human embryonic spinal cord was also investigated and compared with grafts that were six months older. Solid human embryonic spinal cord grafts showed a 100% survival rate, grew to fill the volume of the cavity in a noninvasive manner, and expressed human specific antigens 6 months postgrafting. Thy-1 immunoreactivity (IR) was demonstrated up to 8 mm rostral to the graft suggestive of graft-derived fiber outgrowth. hNF-IR fibers and 5-HT- and TH-IR fibers traversed the graft-host border for a few hundred micrometers, respectively. Finally, our findings suggest that grafted solid pieces of human embryonic spinal cord minimize cystic deformations seen in the adult rat spinal cord with a unilateral cavity.

Transplant therapy: recovery of function after spinal cord injury

Spinal cord injuries (SCI) result in devastating loss of function and altered sensation. Presently, victims of SCI have few remedies for the loss of motor function and the altered sensation often experienced subsequent to the injury. A goal in SCI research is to improve function in both acute and chronic injuries. Among the most successful interventions is the utilization of transplanted tissues toward improved recovery. The theory is that the transplanted tissue could (1) bridge the spinal lesion and provide chemical and/or mechanical guidance for host neurons to grow across the lesion, (2) bridge the spinal lesion and provide additional cellular elements to repair the damaged circuitry, (3) provide factors that would rescue neurons that would otherwise die and/or modulate neural circuits to improve function. A variety of tissues and cells have been added to the adult mammalian spinal cord to encourage restoration of function. These include Schwann cells, motor neurons, dorsal root ganglia, adrenal tissue, hybridomas, peripheral nerves, and fetal spinal cord (FSC) tissue en bloc or as disassociated cells. It is postulated that these tissues would rescue or replace injured adult neurons, which would then integrate or promote the regeneration of the spinal cord circuitry and restore function. In some instances, host-appropriate circuitry is supplied by the transplant and functional improvement is demonstrated. In this presentation, specific examples of recent work with transplanted tissue and cells that demonstrate improved behavioral outcome are presented. New recent work describing the in vitro propagation and characterization of human fetal spinal cord multipotential progenitor cells are also described in the context of a potential resource for transplantable cells. Additionally, data from transplantation experiments of human FSC cells into nonimmunosuppressed rat spinal cord are described, and the resultant improvements in behavioral outcome reported. Lastly, directions for future SCI research are proposed.

Nerve fibre regeneration across the peripheral-central transitional zone

Neurons cannot negotiate an elongation across the peripheral (PNS)-central nervous system (CNS) transitional zone and grow into or out of the spinal cord in the mature mammal. The astrocytic rich CNS part of the spinal nerve root is most effective in preventing regeneration even of nerve fibres from transplanted embryonic ganglion cells. Regeneration of severed nerve fibres into the spinal cord occurs when the transition zone is absent as in the immature animal. Before the establishment of a transition zone there is also new growth of neuronal processes from dorsal horn neurons distally to the injured dorsal root. Thus the experimental strategy to reestablish spinal cord to peripheral nerve connectivity has been to delete the transitional region and implant severed ventral or dorsal roots into the spinal cord. Dorsal root implantation resulted in reestablished afferent connectivity by new neuronal processes from secondary sensory neurons in the dorsal horn of the spinal cord extending into the PNS. The ability for plasticity in these cells allowed for a concurrent retention of their original rostral projection. Ventral root implantation into the spinal cord corrected deficit motor function. In a long series of experiments performed in different species, the functional restitution was demonstrated to depend on an initial regrowth of motor neuron axons through spinal cord tissue (CNS). These findings have led to the design of a new surgical strategy in cases of traumatic spinal nerve root injuries.

Attempts to facilitate dorsal column axonal regeneration in a neonatal spinal environment

The response to injury of ascending collaterals of dorsal root axons within the dorsal column (DC) was studied after neonatal spinal overhemisection (OH) made at different levels of the spinal cord. The transganglionic tracer, cholera toxin conjugated to horseradish peroxidase, and the anterograde tracer, biotinylated dextran amine, were used to label dorsal root ganglion cells with peripheral axons contributing to the sciatic nerve. There was no indication of a regenerative attempt by DC axons at acute survival times (3 days and later) after cervical injury, replicating previous work done at chronic survival periods (Lahr and Stelzner [1990] J. Comp. Neurol. 293:377-398). There was also no evidence of DC regeneration after lumbar OH injury even though immunohistochemical studies using the oligodendrocyte markers Rip and myelin basic protein showed few oligodendrocytes in the gracile fasciculus at lumbar levels at birth. Therefore, the lack of myelin in the dorsal funiculus at lumbar levels does not enhance the growth of neonatally axotomized DC axons. In addition, DC axons did not regenerate when presented with fetal spinal tissue implanted into thoracic OH lesions, even though positive control experiments showed that segmental dorsal root axons containing calcition gene-related peptide and corticospinal axons grew into these implants, replicating previous work of others. When a thoracic OH lesion, with or without a fetal spinal implant, was combined with sciatic nerve injury to attempt to stimulate an intracellular regenerative response of DRG neurons, again, no evidence of DC axonal regeneration was detected. Quantitative studies of the L4 and L5 dorsal root ganglia (DRG) showed that OH injury did not result in DRG neuronal loss. However, sciatic nerve injury did result in significant post-axotomy retrograde cell loss of DRG neurons, even in groups receiving thoracic embryonic spinal implants, and is one explanation for the minimal effect of sciatic nerve injury on DC regeneration. Although fetal tissue did not appear to rescue a significant number of DRG neurons, the quantitative analysis showed an enlargement of the largest class of DRG neuron, the class that contributes to the DC projection, in all groups receiving fetal tissue implants. This apparent trophic effect did not affect DC regeneration or neuronal survival after peripheral axotomy. Further studies are needed to determine why DC axons do not regenerate in a neonatal spinal environment or within fetal tissue implants, especially because previous work by others in both the developing and adult spinal cord shows that dorsal root axons will grow within the same type of fetal spinal implant.

Intrinsic versus extrinsic factors in determining the regeneration of the central processes of rat dorsal root ganglion neurons: the influence of a peripheral nerve graft

The relative contribution of intrinsic growth capacity versus extrinsic growth-promoting factors in determining the capacity of transected dorsal root axons to regenerate long distances was studied. L4 dorsal root axons regenerating into 4-cm peripheral nerve grafts on transected dorsal roots were counted. Few dorsal root myelinated axons regenerated to the distal end of the grafts by 10 weeks unless the sciatic nerve was also crushed. Regeneration of unmyelinated axons was also increased by peripheral lesions. Crush or transection of the dorsal roots without grafting did not alter GAP-43 mRNA expression in L4 dorsal root ganglion (DRG) cells. Grafting a peripheral nerve onto the cut end of an L4 dorsal root doubled the number of DRG cells expressing high levels of GAP-43 mRNA after a delay of several weeks. Peripheral nerve crush at the time of nerve grafting resulted in a very rapid rise in GAP-43 mRNA expression, which then declined to a steady level, twice that of controls, by 7 weeks. Thus, the rapid increase in the number of DRG neurons expressing high levels of GAP-43 mRNA after peripheral but not central axotomy correlates with the regeneration of central axons through nerve grafts. Because GAP-43 mRNA is slowly upregulated in a subpopulation of sensory neurons in response to exposure of their central axons to a peripheral nerve environment, environments favourable for axonal growth may act by increasing the intrinsic growth response of neurons. Lack of intrinsic growth capacity may contribute to the failure of dorsal root axons to regenerate into the spinal cord.