Protection of corticospinal tract neurons after dorsal spinal cord transection and engraftment of olfactory ensheathing cells

Convergence of cells from the progenitor fraction of adult olfactory bulb tissue to remyelinating glia in demyelinating spinal cord lesions

Background

Progenitor cells isolated from adult brain tissue are important tools for experimental studies of remyelination. Cells harvested from neurogenic regions in the adult brain such as the subependymal zone have demonstrated remyelination potential. Multipotent cells from the progenitor fraction have been isolated from the adult olfactory bulb (OB) but their potential to remyelinate has not been studied.

Methodology/Principal Findings

We used the buoyant density gradient centrifugation method to isolate the progenitor fraction and harvest self-renewing multipotent neural cells grown in monolayers from the adult green-fluorescent protein (GFP) transgenic rat OB. OB tissue was mechanically and chemically dissociated and the resultant cell suspension fractionated on a Percoll gradient. The progenitor fraction was isolated and these cells were plated in growth media with serum for 24 hrs. Cells were then propagated in N2 supplemented serum-free media containing b-FGF. Cells at passage 4 (P4) were introduced into a demyelinated spinal cord lesion. The GFP⁺ cells survived and integrated into the lesion, and extensive remyelination was observed in plastic sections. Immunohistochemistry revealed GFP⁺ cells in the spinal cord to be glial fibrillary acidic protein (GFAP), neuronal nuclei (NeuN), and neurofilament negative. The GFP⁺ cells were found among primarily P0⁺ myelin profiles, although some myelin basic protein (MBP) profiles were present. Immuno-electron microscopy for GFP revealed GFP⁺ cell bodies adjacent to and surrounding peripheral-type myelin rings.

Conclusions/Significance

We report that neural cells from the progenitor fraction of the adult rat OB grown in monolayers can be expanded for several passages in culture and that upon transplantation into a demyelinated spinal cord lesion provide extensive remyelination without ectopic neuronal differentiation.

Corticospinal neurons respond differentially to neurotrophins and myelin-associated glycoprotein in vitro

Elucidating the mechanisms that regulate the survival and outgrowth of corticospinal tract (CST) neurons and other CNS tracts will be a key component in developing novel approaches for the treatment of central nervous system (CNS) disorders, including stroke, spinal cord injury (SCI), and motor neuron disease (MND). However, the in vivo complexities of these diseases make a systematic evaluation of potential therapeutics that directly affect corticospinal regeneration or survival very challenging. Here, we use Thy1.2 transgenic mice expressing yellow fluorescent protein (YFP) in postnatal day 8 (P8) corticospinal neurons, as a source of CST neurons that have already established synapses in the spinal cord, to assess factors that influence neurite outgrowth and survival of axotomized CST neurons. After culture, YFP-positive corticospinal neurons represent an enriched neuronal population over other glia and interneurons, survive, and extend processes over time. YFP-positive CST neurons also continue to express the corticospinal markers CTIP2 and Otx1. CST neurons display different degrees of axon extension, dendritic branch length and elaboration, and neurite elongation in response to neurotrophin-3 and ciliary neurotrophic factor, and an inhibitory outgrowth response when cultured on myelin-associated glycoprotein. Some CST neurons are lost with extended culture, which provides a baseline from which we can also assess factors that enhance CST neuron survival. This assay thus allows us to assess independent aspects of CST axonal and dendritic outgrowth kinetics, which allows for the rapid and sensitive investigation of new therapies to address corticospinal neuron outgrowth in the context of CNS injury and neurodegenerative disorders.

Analysis of host-mediated repair mechanisms after human CNS-stem cell transplantation for spinal cord injury: correlation of engraftment with recovery

BACKGROUND

Human central nervous system-stem cells grown as neurospheres (hCNS-SCns) self-renew, are multipotent, and have potential therapeutic applications following trauma to the spinal cord. We have previously shown locomotor recovery in immunodeficient mice that received a moderate contusion spinal cord injury (SCI) and hCNS-SCns transplantation 9 days post-injury (dpi). Engrafted hCNS-SCns exhibited terminal differentiation to myelinating oligodendrocytes and synapse-forming neurons. Further, selective ablation of human cells using Diphtheria toxin (DT) abolished locomotor recovery in this paradigm, suggesting integration of human cells within the mouse host as a possible mechanism for the locomotor improvement. However, the hypothesis that hCNS-SCns could alter the host microenvironment as an additional or alternative mechanism of recovery remained unexplored; we tested that hypothesis in the present study.

METHODS AND FINDINGS

Stereological quantification of human cells using a human-specific cytoplasmic marker demonstrated successful cell engraftment, survival, migration and limited proliferation in all hCNS-SCns transplanted animals. DT administration at 16 weeks post-transplant ablated 80.5% of hCNS-SCns. Stereological quantification for lesion volume, tissue sparing, descending serotonergic host fiber sprouting, chondroitin sulfate proteoglycan deposition, glial scarring, and angiogenesis demonstrated no evidence of host modification within the mouse spinal cord as a result of hCNS-SCns transplantation. Biochemical analyses supplemented stereological data supporting the absence of neural stem-cell mediated host repair. However, linear regression analysis of the number of engrafted hCNS-SCns vs. the number of errors on a horizontal ladder beam task revealed a strong correlation between these variables (r = -0.78, p<0.05), suggesting that survival and engraftment were directly related to a quantitative measure of recovery.

CONCLUSIONS

Altogether, the data suggest that the locomotor improvements associated with hCNS-SCns transplantation were not due to modifications within the host microenvironment, supporting the hypothesis that human cell integration within the host circuitry mediates functional recovery following a 9 day delayed transplant.

Towards personalized cell-replacement therapies for brain repair

The inability of the CNS to efficiently repair damage caused by trauma and neurodegenerative or demyelinating diseases has underlined the necessity for developing novel therapeutic strategies. Cell transplantation to replace lost neurons and the grafting of myelinating cells to repair demyelinating lesions are promising approaches for treating CNS injuries and demyelination. In this review, we will address the prospects of using stem cells or myelinating glial cells of the PNS, as well as olfactory ensheathing cells, in cell-replacement therapies. The recent generation of induced pluripotent stem cells from adult somatic cells by introduction of three or four genes controlling ‘stemness’ and their subsequent differentiation to desired phenotypes, constitutes a significant advancement towards personalized cell-replacement therapies.

The yellow fluorescent protein (YFP-H) mouse reveals neuroprotection as a novel mechanism underlying chondroitinase ABC-mediated repair after spinal cord injury

Chondroitinase ABC (ChABC) represents a promising therapeutic strategy for the treatment of spinal cord injury due to its potent effects on restoring function to spinal-injured adult mammals. However, there is limited mechanistic insight as to the underlying effects of ChABC treatment, where the effects are mediated, and which signaling pathways are involved in ChABC-mediated repair. Here we use a transgenic (YFP-H) mouse to demonstrate that cortical layer V projection neurons undergo severe atrophy 4 weeks after thoracic dorsal column injury and that ChABC is neuroprotective for these neurons after ICV infusion. ChABC also prevented cell atrophy after localized delivery to the spinal cord, suggesting a possible retrograde neuroprotective effect mediated at the injury site. Furthermore, neuroprotection of corticospinal cell somata coincided with increased axonal sprouting in the spinal cord. In addition, Western blot analysis of a number of kinases important in survival and growth signaling revealed a significant increase in phosphorylated ERK1 at the spinal injury site after in vivo ChABC treatment, indicating that activated ERK may play a role in downstream repair processes after ChABC treatment. Total forms of PKC and AKT were also elevated, indicating that modification of the glial scar by ChABC promotes long-lasting signaling changes at the lesion site. Thus, using the YFP-H mouse as a novel tool to study degenerative changes and repair after spinal cord injury we demonstrate, for the first time, that ChABC treatment regulates multiple signaling cascades at the injury site and exerts protective effects on axotomized corticospinal projection neurons.

Undesired effects of a combinatorial treatment for spinal cord injury--transplantation of olfactory ensheathing cells and BDNF infusion to the red nucleus

Transplantations of olfactory ensheathing cells (OECs) have been reported to promote axonal regeneration and functional recovery after spinal cord injury, but have demonstrated limited growth promotion of rat rubrospinal axons after a cervical dorsolateral funiculus crush. Rubrospinal neurons undergo massive atrophy after cervical axotomy and show only transient expression of regeneration-associated genes. Cell body treatment with brain-derived neurotrophic factor (BDNF) prevents this atrophy, stimulates regeneration-associated gene expression and promotes regeneration of rubrospinal axons into peripheral nerve transplants. Here, we hypothesized that the failure of rubrospinal axons to regenerate through a bridge of OEC transplants was due to this weak intrinsic cell body response. Hence, we combined BDNF treatment of rubrospinal neurons with transplantation of highly enriched OECs derived from the nasal mucosa and assessed axonal regeneration as well as behavioral changes after a cervical dorsolateral funiculus crush. Each treatment alone as well as their combination prevented the dieback of the rubrospinal axons, but none of them promoted rubrospinal regeneration beyond the lesion/transplantation site. Motor performance in a food-pellet reaching test and forelimb usage during vertical exploration (cylinder test) were more impaired after combining transplantation of OECs with BDNF treatment. This impaired motor performance correlated with lowered sensory thresholds in animals receiving the combinatorial therapy - which were not seen with each treatment alone. Only this combinatorial treatment group showed enhanced sprouting of calcitonin gene-related peptide-positive axons rostral to the lesion site. Hence, some combinatorial treatments, such as OECs with BDNF, may have undesired effects in the injured spinal cord.

Unique in vivo properties of olfactory ensheathing cells that may contribute to neural repair and protection following spinal cord injury

Olfactory ensheathing cells (OECs) are specialized glial cells that guide olfactory receptor axons from the nasal mucosa into the brain where they make synaptic contacts in the olfactory bulb. While a number of studies have demonstrated that in vivo transplantation of OECs into injured spinal cord results in improved functional outcome, precise cellular mechanisms underlying this improvement are not fully understood. Current thinking is that OECs can encourage axonal regeneration, provide trophic support for injured neurons and for angiogenesis, and remyelinate axons. However, Schwann cell (SC) transplantation also results in significant functional improvement in animal models of spinal cord injury. In culture SCs and OECs share a number of phenotypic properties such as expression of the low affinity NGF receptor (p75). An important area of research has been to distinguish potential differences in the in vivo behavior of OECs and SCs to determine if one cell type may offer greater advantage as a cellular therapeutic candidate. In this review we focus on several unique features of OECs when they are transplanted into the spinal cord.

Engineering angiogenesis following spinal cord injury: a coculture of neural progenitor and endothelial cells in a degradable polymer implant leads to an increase in vessel density and formation of the blood-spinal cord barrier

Angiogenesis precedes recovery following spinal cord injury and its extent correlates with neural regeneration, suggesting that angiogenesis may play a role in repair. An important precondition for studying the role of angiogenesis is the ability to induce it in a controlled manner. Previously, we showed that a coculture of endothelial cells (ECs) and neural progenitor cells (NPCs) promoted the formation of stable tubes in vitro and stable, functional vascular networks in vivo in a subcutaneous model. We sought to test whether a similar coculture would lead to the formation of stable functional vessels in the spinal cord following injury. We created microvascular networks in a biodegradable two-component implant system and tested the ability of the coculture or controls (lesion control, implant alone, implant + ECs or implant + NPCs) to promote angiogenesis in a rat hemisection model of spinal cord injury. The coculture implant led to a fourfold increase in functional vessels compared with the lesion control, implant alone or implant + NPCs groups and a twofold increase in functional vessels over the implant + ECs group. Furthermore, half of the vessels in the coculture implant exhibited positive staining for the endothelial barrier antigen, a marker for the formation of the blood-spinal cord barrier. No other groups have shown positive staining for the blood-spinal cord barrier in the injury epicenter. This work provides a novel method to induce angiogenesis following spinal cord injury and a foundation for studying its role in repair.

Stem cell delivery by lumbar puncture as a therapeutic alternative to direct injection into injured spinal cord

Object

Using cellular transplants to treat spinal cord injury is a promising therapeutic strategy, but transplants grafted directly into the injury site can further damage the already compromised cord. To avoid additional trauma and to simplify translation to the clinic, it is advantageous to use less invasive delivery methods.

Methods

The authors compared the efficacy of intrathecal cell delivery at the lumbar region (lumbar puncture [LP]) to direct injection into a thoracic contusion injury using a mixed population of lineage-restricted neural precursor cells.

Results

Direct injection resulted in a higher volume of neural precursor cells located throughout the injury site, whereas fewer LP-delivered cells accumulated at the dorsal aspect of the injured cord. Both grafting methods were neuroprotective, resulting in reduction of injury size and greater tissue sparing compared with controls. Functional recovery was evaluated by assessing motor and bladder function. Animals that received cells via direct injection performed significantly better in the open-field locomotor test than did operated controls, while LP-treated animals showed intermediate recovery of function that did not differ statistically from that of either operated controls or directly injected animals. Bladder function, however, was significantly improved in both directly injected and LP-treated animals.

Conclusion

Grafting of stem cells via LP resulted in localized accumulation of cells at the injury site, neuroprotection, and modest recovery of function. Further optimization of the LP procedure by increasing the number of cells that are delivered and determining the optimal delivery schedule may further improve recovery to levels comparable to direct injection.

Xenografts of expanded primate olfactory ensheathing glia support transient behavioral recovery that is independent of serotonergic or corticospinal axonal regeneration in nude rats following spinal cord transection

Transplantation of olfactory ensheathing glial cells (OEG) may improve the outcome from spinal cord injury. Proof-of-principle studies in primates are desirable and the feasibility and efficacy of using in vitro expanded OEG should be tested. An intermediate step between the validation of rodent studies and human clinical trials is to study expanded primate OEG (POEG) xenografts in immunotolerant rodents. In this study the time course to generate purified POEG was evaluated as well as their survival, effect on damaged axons of the corticospinal and serotonergic systems, tissue sparing, and chronic locomotor recovery following transplantation. Fifty-seven nude rats underwent T9/10 spinal cord transection. Thirty-eight rats received POEG, 19 controls were injected with cell medium, and 10 received lentivirally-GFP-transfected POEG. Histological evaluation was conducted at 6 weeks, 8 weeks, 14 weeks and 23-24 weeks. Of these 57 rats, 18 were studied with 5-HT immunostaining, 16 with BDA anterograde CST labeling, and six were used for transmission electron microscopy. In grafted animals, behavioral recovery, sprouting and limited regeneration of 5-HT fibers, and increased numbers of proximal collateral processes but not regeneration of CST fibers was observed. Grafted animals had less cavitation in the spinal cord stumps than controls. Behavioral recovery peaked at three months and then declined. Five POEG-transplanted animals that had shown behavioral recovery underwent retransection and behavioral scores did not change significantly, suggesting that long tract axonal regeneration did not account for the locomotor improvement. At the ultrastructural level presumptive POEG were found to have direct contacts with astrocytes forming the glia limitans, distinct from those formed by Schwann cells. At 6 weeks GFP expression was detected in cells within the lesion site and within nerve roots but did not match the pattern of Hoechst nuclear labeling. At 3.5 months only GFP-positive debris in macrophages could be detected. Transplanted POEG support behavioral recovery via mechanisms that appear to be independent of long tract regeneration.

Peripherally-derived BDNF promotes regeneration of ascending sensory neurons after spinal cord injury

Background

The blood brain barrier (BBB) and truncated trkB receptor on astrocytes prevent the penetration of brain derived neurotrophic factor (BDNF) applied into the peripheral (PNS) and central nervous system (CNS) thus restrict its application in the treatment of nervous diseases. As BDNF is anterogradely transported by axons, we propose that peripherally derived and/or applied BDNF may act on the regeneration of central axons of ascending sensory neurons.

Methodology/Principal Findings

The present study aimed to test the hypothesis by using conditioning lesion of the sciatic nerve as a model to increase the expression of endogenous BDNF in sensory neurons and by injecting exogenous BDNF into the peripheral nerve or tissues. Here we showed that most of regenerating sensory neurons expressed BDNF and p-CREB but not p75NTR. Conditioning-lesion induced regeneration of ascending sensory neuron and the increase in the number of p-Erk positive and GAP-43 positive neurons was blocked by the injection of the BDNF antiserum in the periphery. Enhanced neurite outgrowth of dorsal root ganglia (DRG) neurons in vitro by conditioning lesion was also inhibited by the neutralization with the BDNF antiserum. The delivery of exogenous BDNF into the sciatic nerve or the footpad significantly increased the number of regenerating DRG neurons and regenerating sensory axons in the injured spinal cord. In a contusion injury model, an injection of BDNF into the footpad promoted recovery of motor functions.

Conclusions/Significance

Our data suggest that endogenous BDNF in DRG and spinal cord is required for the enhanced regeneration of ascending sensory neurons after conditioning lesion of sciatic nerve and peripherally applied BDNF may have therapeutic effects on the spinal cord injury.

Olfactory ensheathing cell transplantation following spinal cord injury: Hype or hope?

Olfactory ensheathing cells (OECs) are unique glia found only in the olfactory system that retain exceptional plasticity, and support olfactory neurogenesis and the re-targeting across the PNS:CNS boundary in the olfactory system. Because they are also relatively accessible in an adult rodent or human, OECs have become a prime candidate for cell-mediated repair following a variety of CNS lesions. A number of different labs across the world have applied OECs prepared in many different ways in several different acute and chronic models of rodent SCI, some of which have suggested surprising degrees of functional recovery. OECs can stimulate tissue sparing and neuroprotection, enhance outgrowth of both intact and lesioned axons (to different degrees), activate angiogenesis, change the response status of endogenous glia after lesion and remyelinate axons after a range of demyelinating insults. Their ability to stimulate regeneration in specific tracts is, however, limited. Despite this, the ongoing clinical use of cell preparations containing OECs has proceeded as a therapeutic approach for human spinal cord injury (SCI). Here, we review the current status of OEC research in SCI, and focus on potential mechanisms for OECs in the SCI repair response that may help to explain the biological reasons underlying the wide variation of results obtained in this promising, yet contentious, field.

Olfactory ensheathing glia: Their contribution to primary olfactory nervous system regeneration and their regenerative potential following transplantation into the injured spinal cord

Olfactory ensheathing glia (OEG) are a specialized type of glia that guide primary olfactory axons from the neuroepithelium in the nasal cavity to the brain. The primary olfactory system is able to regenerate after a lesion and OEG contribute to this process by providing a growth-supportive environment for newly formed axons. In the spinal cord, axons are not able to restore connections after an injury. The effects of OEG transplants on the regeneration of the injured spinal cord have been studied for over a decade. To date, of all the studies using only OEG as a transplant, 41 showed positive effects, while 13 studies showed limited or no effects. There are several contradictory reports on the migratory and axon growth-supporting properties of transplanted OEG. Hence, the regenerative potential of OEG has become the subject of intense discussion. In this review, we first provide an overview of the molecular and cellular characteristics of OEG in their natural environment, the primary olfactory nervous system. Second, their potential to stimulate regeneration in the injured spinal cord is discussed. OEG influence scar formation by their ability to interact with astrocytes, they are able to remyelinate axons and promote angiogenesis. The ability of OEG to interact with scar tissue cells is an important difference with Schwann cells and may be a unique characteristic of OEG. Because of these effects after transplantation and because of their role in primary olfactory system regeneration, the OEG can be considered as a source of neuroregeneration-promoting molecules. To identify these molecules, more insight into the molecular biology of OEG is required. We believe that genome-wide gene expression studies of OEG in their native environment, in culture and after transplantation will ultimately reveal unique combinations of molecules involved in the regeneration-promoting potential of OEG.

Mesenchymal stem cells derived from peripheral blood protects against ischemia

Intravenous delivery of mesenchymal stem cells (MSCs) prepared from bone marrow (BMSCs) reduces infarction volume and ameliorates functional deficits in a rat cerebral ischemia model. MSC-like multipotent precursor cells (PMSCs) have also been suggested to exist in peripheral blood. To test the hypothesis that treatment with PMSCs may have a therapeutic benefit in stroke, we compared the efficacy of systemic delivery of BMSCs and PMSCs. A permanent middle cerebral artery occlusion (MCAO) in rat was induced by intraluminal vascular occlusion with a microfilament. Rat BMSCs and PMSCs were prepared in culture and intravenously injected into the rats 6 h after MCAO. Lesion size was assessed at 6 h, and 1, 3, and 7 days using MR imaging and histology. The hemodynamic change of cerebral blood perfusion on stroke was assessed the same times using perfusion-weighted image (PWI). Functional outcome was assessed using the treadmill stress test. Both BMSCs and PMSCs treated groups had reduced lesion volume, improved regional cerebral blood flow, and functional improvement compared to the control group. The therapeutic benefits of both MSC-treated groups were similar. These data suggest that PMSCs derived from peripheral blood could be an important cell source of cell therapy for stroke.

An overview of tissue engineering approaches for management of spinal cord injuries

Severe spinal cord injury (SCI) leads to devastating neurological deficits and disabilities, which necessitates spending a great deal of health budget for psychological and healthcare problems of these patients and their relatives. This justifies the cost of research into the new modalities for treatment of spinal cord injuries, even in developing countries. Apart from surgical management and nerve grafting, several other approaches have been adopted for management of this condition including pharmacologic and gene therapy, cell therapy, and use of different cell-free or cell-seeded bioscaffolds. In current paper, the recent developments for therapeutic delivery of stem and non-stem cells to the site of injury, and application of cell-free and cell-seeded natural and synthetic scaffolds have been reviewed.

Implantation of pure cultured olfactory ensheathing cells in an animal model of parkinsonism

BACKGROUND

Implantation of neural cells has been proposed as a therapeutic strategy for repairing the injured or diseased brain. In the present study we have examined the potential of olfactory ensheathing cells (OEC) to promote brain repair after surgical implantation in a rodent model of parkinsonism.

METHODS

Neonatal OECs were implanted in the striatum after a 6-hydroxydopamine lesion of the ipsilateral substantia nigra. Amphetamine-induced rotational asymmetry scores were determined 48 hours before and 4, 6 and 8 weeks after OEC implantation. The density of immunostaining for tyrosine hydroxylase and synaptophysin in the striatum and the number of tyrosine hydroxylase-positive cells remaining in the substantia nigra were also determined.

RESULTS

Rotational asymmetry scores were similar in OEC-implanted and vehicle-treated groups at all time points examined, and at each time were similar to those observed prior to implantation. Levels of striatal tyrosine-hydroxylase and synaptophysin immunoreactivity were similar in OEC- and vehicle-treated groups. The number of tyrosine-hydroxylase-positive cells in the substantia nigra was similar in both groups indicating that severity of the lesion was similar. Visualisation of GFP-labelled OECs one week after implantation in a separate group of animals revealed the cells to be located in the area immediately surrounding the needle tract.

CONCLUSION

This study demonstrates that implantation of OECs alone is not sufficient to promote tissue repair and functional recovery in a rodent model of parkinsonism. The results add to a growing number of studies that propose a caveat for the use of pure OECs as a neurosurgical strategy for the treatment of brain disease or injury.

Demyelinating diseases and potential repair strategies

Demyelination is associated with a number of neurological disorders including multiple sclerosis (MS), spinal cord injury and nerve compression. MS lesions often show axon loss and therefore reparative therapeutic goals include remyelination and neuroprotection of vulnerable axons. Experimental cellular transplantation has proven successful in a number of demyelination and injury models to remyelinate and improve functional outcome. Here we discuss the remyelination and neuroprotective potential of several myelin-forming cells types and their behavior in different demyelination and injury models. Better understanding of these models and current cell-based strategies for remyelination and neuroprotection offer exciting opportunities to develop strategies for clinical studies.

Olfactory ensheathing cells exert a trophic effect on the hypothalamic neurons in vitro

Olfactory ensheathing cells (OECs) constitute an usual population of glial cells sharing properties with both Schwann cells (SC) of peripheral nervous system (PNS) and astrocytes of the central nervous system (CNS). They express a high level of growth factors which play a very important role as neuronal support. Recent evidence in literature suggests that OECs may facilitate axonal regeneration in the injured nervous system. In this study, we developed an in vitro model to evaluate the neurotrophic effect of OECs on the survival and axonal outgrowth of hypothalamic neurons. Co-cultures of OECs and hypothalamus neuronal cells of postnatal rats were successfully established and cells were immunocytochemically characterized. Furthermore, some neuronal cultures were added with NGF, bFGF and GDNF to compare with the co-cultures. Our results indicate that in co-cultures of hypothalamic neurons and OECs, the number of neurons was significantly increased compared to control cultures exhibiting a dense axonal outgrowth. Moreover, we show that NGF promoted a major neuronal survival than bFGF and GDNF, while bFGF and GDNF exerted an evidence axonal and dendritic outgrowth compared to NGF. In conclusion, these data suggest that OECs have the capacity to promote the survival and axonal outgrowth of hypothalamic neurons in vitro and that bFGF, NGF and GDNF differentially support hypothalamic neurons promoting and enhancing the neuronal survival and outgrowth. Therefore, the OECs are a source of growth factors and might be considered a better approach for functional recovery and growth factors might exert a neuroprotective effect in neurodegenerative disorders.

Olfactory ensheathing cell transplantation as a strategy for spinal cord repair—what can it achieve?

Restoring function to the injured spinal cord represents one of the most formidable challenges in regenerative medicine. Glial cell transplantation is widely considered to be one of the most promising therapeutic strategies, and several differentiated glial cell types-in particular, Schwann cells and olfactory ensheathing cells (OECs)-have been proposed as transplant candidates. In this Review, we analyze evidence from animal studies for improved functional recovery following transplantation of OECs into spinal cord injuries, and examine the mechanisms by which repair might be achieved. Data obtained using various injury models support the view that OEC transplants can promote functional recovery, but accumulating anatomical evidence indicates that although axons regenerate within a transplant, they do not cross the lesion or reconnect with neurons on the opposite side to any significant extent. Consequently, it is possible that neuroprotection and promotion of sprouting from intact fibers are the main mechanisms that contribute to functional recovery. We conclude that for the foreseeable future the clinical benefits of OEC transplants alone are likely to be modest. The future potential of cell transplantation strategies will probably depend on the success with which the transplants can be combined with other, synergistic, therapies to achieve significant regeneration of axons and re-establish functionally useful connections across a spinal cord injury.

Calponin is expressed by fibroblasts and meningeal cells but not olfactory ensheathing cells in the adult peripheral olfactory system

Olfactory ensheathing cells (OECs), the principal glial cells of the peripheral olfactory system, have many phenotypic similarities with Schwann cells of the peripheral nervous system. This makes reliably distinguishing these two cells types difficult, especially following transplantation into areas of injury in the central nervous system. In an attempt to identify markers by which these two cells types can be distinguished, a recent proteomic analysis of fetal OECs and adult Schwann cells identified the actin-binding protein calponin as a potential marker expressed by OECs but not Schwann cells. Since many studies designed with the translational goal of autologous transplantation in mind have used adult OECs, this study examined the expression of calponin by adult OECs, both in vivo within the peripheral olfactory system and in vitro. Calponin colocalized with strongly fibronectin positive fibroblasts in the olfactory mucosa (OM) and meningeal cells in the olfactory bulb (OB) but not with S100beta or neuropeptide-Y positive OECs. In tissue culture, calponin was strongly expressed by fibronectin-expressing fibroblasts from OM, sciatic nerve and skin and by meningeal cells from the OB, but not by p75(NTR)- and S100beta-expressing OECs. These data, supported by Western blotting, indicate that calponin can not be used to distinguish adult OECs and Schwann cells.

Remyelination of the injured spinal cord

Contusive spinal cord injury (SCI) can result in necrosis of the spinal cord, but often long white matter tracts outside of the central necrotic core are demyelinated. One experimental strategy to improve functional outcome following SCI is to transplant myelin-forming cells to remyelinate these axons and improve conduction. This review focuses on transplantation studies using olfactory ensheathing cell (OEC) to improve functional outcome in experimental models of SCI and demyelination. The biology of the OEC, and recent experimental research and clinical studies using OECs as a potential cell therapy candidate are discussed.

BDNF production by olfactory ensheathing cells contributes to axonal regeneration of cultured adult CNS neurons

Olfactory ensheathing cells (OECs) are the main glial cell type that populates mammalian olfactory nerves. These cells have a great capacity to promote the regeneration of axons when transplanted into the injured adult mammalian CNS. However, little is still known about the molecular mechanisms they employ in mediating such a task. Brain-derived neurotrophic factor (BDNF) was identified as a candidate molecule in a genomic study that compared three functionally different OEC populations: Early passage OECs (OEC Ep), Late passage OECs (OEC Lp) and the OEC cell line TEG3 [Pastrana, E., Moreno-Flores, M.T., Gurzov, E.N., Avila, J., Wandosell, F., Diaz-Nido, J., 2006. Genes associated with adult axon regeneration promoted by olfactory ensheathing cells: a new role for matrix metalloproteinase 2. J. Neurosci. 26, 5347-5359]. We have here set out to determine the role played by BDNF in the stimulation of axon outgrowth by OECs. We compared the extracellular BDNF levels in the three OEC populations and show that it is produced in significant amounts by the OECs that can stimulate axon regeneration in adult retinal neurons (OEC Ep and TEG3) but it is absent from the extracellular medium of OEC Lp cells which lack this capacity. Blocking BDNF signalling impaired axonal regeneration of adult retinal neurons co-cultured with TEG3 cells and adding BDNF increased the proportion of adult neurons that regenerate their axons on OEC Lp monolayers. Combining BDNF with other extracellular proteins such as Matrix Metalloproteinase 2 (MMP2) further augmented this effect. This study shows that BDNF production by OECs plays a direct role in the promotion of axon regeneration of adult CNS neurons.

Olfactory ensheathing glia graft in combination with FK506 administration promote repair after spinal cord injury

The aim of this study was to determine whether a combination of olfactory ensheathing cell (OEC) graft with the administration of FK506, two experimental approaches that have been previously reported to exert protective/regenerative effects after spinal cord injury, promotes synergic restorative effects after complete or partial spinal cord injuries. In partial spinal cord injury, combination of an OEC graft and FK506 reduced functional deficits evaluated by the BBB score, motor-evoked potentials (MEPs) and H reflex tests, diminished cavitation, astrogliosis and increased sparing/regeneration of raphespinal fibers compared to untreated and single-treatment groups of rats. After complete spinal cord transection, the combined treatment significantly improved functional outcomes, promoted axonal regeneration caudal to the lesion, and diminished astrogliosis compared only to non-transplanted animals. Slightly, but non-significant, better functional and histological results were found in OEC-grafted animals treated with FK506 than in those given saline after spinal cord transection. Nevertheless, the combined treatment increased the percentage of rats that recovered MEPs and promoted a significant reduction in astrogliosis. In conclusion, this study demonstrates that OEC grafts combined with FK506 promote additive repair of spinal cord injuries to those exerted by single treatments, the effect being more remarkable when the spinal cord is partially lesioned.

Myelination and nodal formation of regenerated peripheral nerve fibers following transplantation of acutely prepared olfactory ensheathing cells

Transplantation of olfactory ensheathing cells (OECs) into injured spinal cord results in improved functional outcome. Mechanisms suggested to account for this functional improvement include axonal regeneration, remyelination and neuroprotection. OECs transplanted into transected peripheral nerve have been shown to modify peripheral axonal regeneration and functional outcome. However, little is known of the detailed integration of OECs at the transplantation site in peripheral nerve. To address this issue, cell populations enriched in OECs were isolated from the olfactory bulbs of adult green fluorescent protein (GFP)-expressing transgenic rats and transplanted into a sciatic nerve crush lesion which transects all axons. Five weeks to 6 months after transplantation, the nerves were studied histologically. GFP-expressing OECs survived in the lesion and distributed longitudinally across the lesion zone. The internodal regions of individual teased fibers distal to the transection site were characterized by GFP expression in the cytoplasmic and nuclear compartments of cells surrounding the axons. Immunoelectron microscopy for GFP indicated that the transplanted OECs formed peripheral type myelin. Immunostaining for sodium channel and Caspr revealed a high density of Na(v)1.6 at the newly formed nodes of Ranvier which were flanked by paranodal Caspr staining. These results indicate that transplanted OECs extensively integrate into transected peripheral nerve and form myelin on regenerated peripheral nerve fibers, and that nodes of Ranvier of these axons display proper sodium channel organization.

Therapeutic interventions after spinal cord injury

Spinal cord injury (SCI) can lead to paraplegia or quadriplegia. Although there are no fully restorative treatments for SCI, various rehabilitative, cellular and molecular therapies have been tested in animal models. Many of these have reached, or are approaching, clinical trials. Here, we review these potential therapies, with an emphasis on the need for reproducible evidence of safety and efficacy. Individual therapies are unlikely to provide a panacea. Rather, we predict that combinations of strategies will lead to improvements in outcome after SCI. Basic scientific research should provide a rational basis for tailoring specific combinations of clinical therapies to different types of SCI.

Olfactory ensheathing glia and spinal cord injury: basic mechanisms to transplantation

The adult CNS, unlike its counterpart the peripheral nervous system (PNS), has little ability to repair itself after traumatic injury. Therefore, neurotrauma involving the brain or spinal cord has severe and long-lasting functional consequences for injured patients, as well as a massive financial and social impact on the affected families and the community at large. In particular, spinal cord injury (SCI) has provided scientists and clinicians with a challenging problem. In attempts to improve outcomes following SCI, numerous mammalian research models have been developed. Many of these models involve either transection or contusion injuries in rodents and experimental therapies include the transplantation of a range of cell types isolated from either the PNS or CNS. The authors focus on a cell type isolated from the olfactory system; olfactory ensheathing cells (OECs). Some basic tenets of olfactory cell biology, key preclinical results suggesting a role for OECs in stimulating spinal cord repair and the strengths and limitations of this potential therapy are discussed. The current and future status of OEC transplantation in the treatment of human SCI is also considered.

Molecular reconstruction of nodes of Ranvier after remyelination by transplanted olfactory ensheathing cells in the demyelinated spinal cord

Myelin-forming glial cells transplanted into the demyelinated spinal cord can form compact myelin and improve conduction properties. However, little is known of the expression and organization of voltage-gated ion channels in the remyelinated central axons or whether the exogenous cells provide appropriate signaling for the maturation of nodes of Ranvier. Here, we transplanted olfactory ensheathing cells from green fluorescent protein (GFP)-expressing donor rats [GFP-olfactory ensheathing cells (OECs)] into a region of spinal cord demyelination and found extensive remyelination, which included the development of mature nodal, paranodal, and juxtaparanodal domains, as assessed by ultrastructural, immunocytochemical, and electrophysiological analyses. In remyelinated axons, Nav1.6 was clustered at nodes, whereas Kv1.2 was aggregated in juxtaparanodal regions, recapitulating the distribution of these channels within mature nodes of uninjured axons. Moreover, the recruitment of Nav and Kv channels to specific membrane domains at remyelinated nodes persisted for at least 8 weeks after GFP-OEC transplantation. In vivo electrophysiological recordings demonstrated enhanced conduction along the GFP-OEC-remyelinated axons. These findings indicate that, in addition to forming myelin, engrafted GFP-OECs provide an environment that supports the development and maturation of nodes of Ranvier and the restoration of impulse conduction in central demyelinated axons.

Cultures of rat olfactory ensheathing cells are contaminated with Schwann cells

Implantation of cultured olfactory ensheathing cells into the damaged spinal cord of adult rats has been reported to remyelinate central axons. This observation is curious because olfactory ensheathing cells do not myelinate axons in their native environment. We have recently determined that calponin is the first definitive phenotypic marker for olfactory ensheathing cells. Primary cultures of adult rat olfactory mucosa and olfactory bulb were immunostained for p75 neurotrophin receptor and calponin. Our results reveal that two populations of p75 neurotrophin receptor-positive cells exist in primary cultures of the olfactory mucosa and bulb: calponin-positive olfactory ensheathing cells and calponin-negative Schwann cells. As olfactory tissues likely yield a mixed glial population, the idea that olfactory ensheathing cells are capable of de novo myelin synthesis after intraspinal implantation should be re-evaluated.

Chronic transplantation of olfactory ensheathing cells promotes partial recovery after complete spinal cord transection in the rat

The goal of this study was to ascertain whether olfactory ensheathing cells (OECs) were able to promote axonal regeneration and functional recovery when transplanted 45 days after complete transection of the thoracic spinal cord in adult rats. OECs promoted partial restitution of supraspinal pathways evaluated by motor evoked potentials and modest recovery of hindlimb movements. In addition, OEC grafts reduced lumbar reflex hyperexcitability from the first month after transplantation. Histological results revealed that OECs facilitated corticospinal and raphespinal axons regrowth through the injury site and into the caudal spinal cord segments. Interestingly, raphespinal but not corticospinal fibers regenerated long distances through the gray matter and reached the lower lumbar segments (L5) of the spinal cord. However, delayed OEC grafts failed to reduce posttraumatic astrogliosis. In conclusion, the beneficial effects found in the present study further support the use of OECs for treating chronic spinal cord injuries.