Olfactory ensheathing glia transplantation into the injured spinal cord

Engineering biomaterial microenvironments to promote myelination in the central nervous system

Promoting remyelination and/or minimizing demyelination are key therapeutic strategies under investigation for diseases and injuries like multiple sclerosis (MS), spinal cord injury, stroke, and virus-induced encephalopathy. Myelination is essential for efficacious neuronal signaling. This myelination process is originated by oligodendrocyte progenitor cells (OPCs) in the central nervous system (CNS). Resident OPCs are capable of both proliferation and differentiation, and also migration to demyelinated injury sites. OPCs can then engage with these unmyelinated or demyelinated axons and differentiate into myelin-forming oligodendrocytes (OLs). However this process is frequently incomplete and often does not occur at all. Biomaterial strategies can now be used to guide OPC and OL development with the goal of regenerating healthy myelin sheaths in formerly damaged CNS tissue. Growth and neurotrophic factors delivered from such materials can promote proliferation of OPCs or differentiation into OLs. While cell transplantation techniques have been used to replace damaged cells in wound sites, they have also resulted in poor transplant cell viability, uncontrollable differentiation, and poor integration into the host. Biomaterial scaffolds made from extracellular matrix (ECM) mimics that are naturally or synthetically derived can improve transplanted cell survival, support both transplanted and endogenous cell populations, and direct their fate. In particular, stiffness and degradability of these scaffolds are two parameters that can influence the fate of OPCs and OLs. The future outlook for biomaterials research includes 3D in vitro models of myelination / remyelination / demyelination to better mimic and study these processes. These models should provide simple relationships of myelination to microenvironmental biophysical and biochemical properties to inform improved therapeutic approaches.

Cell migration in the developing rodent olfactory system

The components of the nervous system are assembled in development by the process of cell migration. Although the principles of cell migration are conserved throughout the brain, different subsystems may predominantly utilize specific migratory mechanisms, or may display unusual features during migration. Examining these subsystems offers not only the potential for insights into the development of the system, but may also help in understanding disorders arising from aberrant cell migration. The olfactory system is an ancient sensory circuit that is essential for the survival and reproduction of a species. The organization of this circuit displays many evolutionarily conserved features in vertebrates, including molecular mechanisms and complex migratory pathways. In this review, we describe the elaborate migrations that populate each component of the olfactory system in rodents and compare them with those described in the well-studied neocortex. Understanding how the components of the olfactory system are assembled will not only shed light on the etiology of olfactory and sexual disorders, but will also offer insights into how conserved migratory mechanisms may have shaped the evolution of the brain.

Olfactory Ensheathing Cells Express α7 Integrin to Mediate Their Migration on Laminin

The unique glia located in the olfactory system, called olfactory ensheathing cells (OECs), are implicated as an attractive choice for transplantation therapy following spinal cord injury because of their pro-regenerative characteristics. Adult OECs are thought to improve functional recovery and regeneration after injury by secreting neurotrophic factors and making cell-to-cell contacts with regenerating processes, but the mechanisms are not well understood. We show first that α7 integrin, a laminin receptor, is highly expressed at the protein level by OECs throughout the olfactory system, i.e., in the olfactory mucosa, olfactory nerve, and olfactory nerve layer of the olfactory bulb. Then we asked if OECs use the α7 integrin receptor directly to promote neurite outgrowth on permissive and neutral substrates, in vitro. We co-cultured α7^+/+ and α7^lacZ/lacZ postnatal cerebral cortical neurons with α7^+/+ or α7^lacZ/lacZ OECs and found that genotype did not effect the ability of OECs to enhance neurite outgrowth by direct contact. Loss of α7 integrin did however significantly decrease the motility of adult OECs in transwell experiments. Twice as many α7^+/+ OECs migrated through laminin-coated transwells compared to α7^+/+ OECs on poly-L-lysine (PLL). This is in contrast to α7^lacZ/lacZ OECs, which showed no migratory preference for laminin substrate over PLL. These results demonstrate that OECs express α7 integrin, and that laminin and its α7 integrin receptor contribute to adult OEC migration in vitro and perhaps also in vivo.

Differing phagocytic capacities of accessory and main olfactory ensheathing cells and the implication for olfactory glia transplantation therapies

The rodent olfactory systems comprise the main olfactory system for the detection of odours and the accessory olfactory system which detects pheromones. In both systems, olfactory axon fascicles are ensheathed by olfactory glia, termed olfactory ensheathing cells (OECs), which are crucial for the growth and maintenance of the olfactory nerve. The growth-promoting and phagocytic characteristics of OECs make them potential candidates for neural repair therapies such as transplantation to repair the injured spinal cord. However, transplanting mixed populations of glia with unknown properties may lead to variations in outcomes for neural repair. As the phagocytic capacity of the accessory OECs has not yet been determined, we compared the phagocytic capacity of accessory and main OECs in vivo and in vitro. In normal healthy animals, the accessory OECs accumulated considerably less axon debris than main OECs in vivo. Analysis of freshly dissected OECs showed that accessory OECs contained 20% less fluorescent axon debris than main OECs. However, when assayed in vitro with exogenous axon debris added to the culture, the accessory OECs phagocytosed almost 20% more debris than main OECs. After surgical removal of one olfactory bulb which induced the degradation of main and accessory olfactory sensory axons, the accessory OECs responded by phagocytosing the axon debris. We conclude that while accessory OECs have the capacity to phagocytose axon debris, there are distinct differences in their phagocytic capacity compared to main OECs. These distinct differences may be of importance when preparing OECs for neural transplant repair therapies.

Transplantation of olfactory ensheathing cells attenuates acute carbon monoxide poisoning-induced brain damages in rats

In this study, the therapeutic effect of olfactory ensheathing cells (OEC) transplantation on brain damage was evaluated on acute carbon monoxide (CO) poisoning rat model. Two weeks after primary culture, OECs were microinjected into hippocampus of CO poisoning rats. Survival of OECs in the host was observed and quantified. OECs survived at 2 weeks, but surviving cell number was found sharply decreased at 6 weeks and reduced to less than 10(3) at 8 weeks after transplantation. At 2 weeks after transplantation, motor function test and cerebral edema assay were performed and followed by pathological examination including hematoxylin and eosin and immunohistochemistry staining to observe the neuron injury and synapsin I and growth associated protein-43 (GAP-43) expression. Furthermore, biomarkers of oxidative stress and apoptosis related proteins in the hippocampus were detected. The results showed that CO exposure led to neurological dysfunction and cerebral edema in rats. After OEC transplantation, neurological function was significantly improved and the cerebral edema was alleviated. In addition, the numbers of neurons and Nissl bodies were increased and synapsin I and GAP-43 protein expressions were upregulated in the hippocampus. Compared with CO poisoned rats, superoxide dismutase activity and glutathione content were both increased and methane dicarboxylic aldehyde level was decreased in the hippocampus of OEC transplanted rats. Moreover, OEC transplantation reduced apoptosis induced by CO exposure. The Bcl-2 expression was significantly upregulated and Bax expression was significantly downregulated. The activity of caspase-3 and the cleaved-poly ADP-ribose polymerase expression were decreased. Taken together, our data suggest that OEC attenuates brain damages induced by acute CO poisoning within 2 weeks after transplantation.

Low-dose curcumin stimulates proliferation, migration and phagocytic activity of olfactory ensheathing cells

One of the promising strategies for neural repair therapies is the transplantation of olfactory ensheathing cells (OECs) which are the glial cells of the olfactory system. We evaluated the effects of curcumin on the behaviour of mouse OECs to determine if it could be of use to further enhance the therapeutic potential of OECs. Curcumin, a natural polyphenol compound found in the spice turmeric, is known for its anti-cancer properties at doses over 10 µM, and often at 50 µM, and it exerts its effects on cancer cells in part by activation of MAP kinases. In contrast, we found that low-dose curcumin (0.5 µM) applied to OECs strikingly modulated the dynamic morphology, increased the rate of migration by up to 4-fold, and promoted significant proliferation of the OECs. Most dramatically, low-dose curcumin stimulated a 10-fold increase in the phagocytic activity of OECs. All of these potently stimulated behavioural characteristics of OECs are favourable for neural repair therapies. Importantly, low-dose curcumin gave a transient activation of p38 kinases, which is in contrast to the high dose curcumin effects on cancer cells in which these MAP kinases tend to undergo prolonged activation. Low-dose curcumin mediated effects on OECs demonstrate cell-type specific stimulation of p38 and ERK kinases. These results constitute the first evidence that low-dose curcumin can modulate the behaviour of olfactory glia into a phenotype potentially more favourable for neural repair and thereby improve the therapeutic use of OECs for neural repair therapies.

Crucial roles for olfactory ensheathing cells and olfactory mucosal cells in the repair of damaged neural tracts

Olfactory ensheathing cells, the glial cells of the olfactory nervous system, exhibit unique growth-promoting and migratory properties that make them interesting candidates for cell therapies targeting neuronal injuries such as spinal cord injury. Transplantation of olfactory cells is feasible and safe in humans; however, functional outcomes are highly variable with some studies showing dramatic improvements and some no improvements at all. We propose that the reason for this is that the identity and purity of the cells is different in each individual study. We have shown that olfactory ensheathing cells are not a uniform cell population and that individual subpopulations of OECs are present in different regions of the olfactory nervous system, with strikingly different behaviors. Furthermore, the presence of fibroblasts and other cell types in the transplant can dramatically alter the behavior of the transplanted glial cells. Thus, a thorough characterization of the differences between olfactory ensheathing cell subpopulations and how the behavior of these cells is affected by the presence of other cell types is highly warranted.

Neural crest and olfactory system: new prospective

Sensory neurons in vertebrates are derived from two embryonic transient cell sources: neural crest (NC) and ectodermal placodes. The placodes are thickenings of ectodermal tissue that are responsible for the formation of cranial ganglia as well as complex sensory organs that include the lens, inner ear, and olfactory epithelium. The NC cells have been indicated to arise at the edges of the neural plate/dorsal neural tube, from both the neural plate and the epidermis in response to reciprocal interactions Moury and Jacobson (Dev Biol 141:243-253, 1990). NC cells migrate throughout the organism and give rise to a multitude of cell types that include melanocytes, cartilage and connective tissue of the head, components of the cranial nerves, the dorsal root ganglia, and Schwann cells. The embryonic definition of these two transient populations and their relative contribution to the formation of sensory organs has been investigated and debated for several decades (Basch and Bronner-Fraser, Adv Exp Med Biol 589:24-31, 2006; Basch et al., Nature 441:218-222, 2006) review (Baker and Bronner-Fraser, Dev Biol 232:1-61, 2001). Historically, all placodes have been described as exclusively derived from non-neural ectodermal progenitors. Recent genetic fate-mapping studies suggested a NC contribution to the olfactory placodes (OP) as well as the otic (auditory) placodes in rodents (Murdoch and Roskams, J Neurosci Off J Soc Neurosci 28:4271-4282, 2008; Murdoch et al., J Neurosci 30:9523-9532, 2010; Forni et al., J Neurosci Off J Soc Neurosci 31:6915-6927, 2011b; Freyer et al., Development 138:5403-5414, 2011; Katoh et al., Mol Brain 4:34, 2011). This review analyzes and discusses some recent developmental studies on the OP, placodal derivatives, and olfactory system.

The migration of olfactory ensheathing cells during development and regeneration

The primary olfactory nervous system is unique in that it continuously renews itself and regenerates after injury. These properties are attributed to the presence of olfactory glia, termed olfactory ensheathing cells (OECs). Evidence is now emerging that individual OEC populations exist with distinct anatomical localisations and physiological properties, but their differential roles have not been determined. Unlike other glia, OECs can migrate from the periphery into the central nervous system, and organised OEC migration can enhance axonal extension after injury. Despite this, the mechanisms regulating OEC migration are largely unknown. Here, we provide an overview of the roles of OECs in development and adulthood. We review the latest research describing the differences between individual OEC subpopulations and discuss potential regulatory mechanisms for OEC guidance and migration. Using advanced time lapse techniques, we have obtained novel insights into how OECs behave in a complex multicellular environment which we discuss here with particular focus on cell-cell interactions. Significantly, transplantation of OECs constitutes a promising novel therapy for nerve injuries, but results are highly variable and the method needs improvement. We here review the roles of transplanted OECs in neural repair of damaged neuronal tracts distinct from the primary olfactory nervous system.

Experimental reconstruction of the injured spinal cord

Injury to the spinal cord, with its pathological sequelae, results in a permanent neurological deficit. With currently available tools at hand, there is very little that clinicians can do to treat such a condition with the view of helping patients with spinal cord injury (SCI). On the other hand, in the last 20 years experimental research has brought new insights into the pathophysiology of spinal cord injury; we can divide the time course into 3 phases: primary injury (the time of traumatic impact and the period immediately afterwards), the secondary phase (cell death, inflammation, ischemia), and the chronic phase (scarring, demyelination, cyst formation). Increased knowledge about the pathophysiology of SCI can stimulate the development of new therapeutic modalities and approaches, which may be feasible in the future in clinical practice. Some of the most promising experimental therapies include: neurotrophic factors, enzymes and antibodies against inhibitory molecules (such as Nogo), activated macrophages, stem cells and bridging scaffolds. Their common goal is to reconstitute the damaged tissue in order to recover the lost function. In the current review, we focus on some of the recent developments in experimental SCI research.

Olfactory ensheathing cells reduce duration of autonomic dysreflexia in rats with high spinal cord injury

Autonomic dysreflexia is a common complication in high spinal cord injury and can result in serious consequences and death. Here we have examined the effect of acute transplantation of olfactory ensheathing cells on cardiovascular functions in rats. After T4 transection, radio-telemetric recording in conscious animals was used to study blood pressure and heart rate at rest and during autonomic dysreflexia for up to 8 weeks post-injury. Olfactory ensheathing cells from syngeneic rats were transplanted at the injury site; control animals received culture medium only. At the study end point, we examined morphometric features of sympathetic preganglionic neurons above and below the injury. T4 transection resulted in a fall in resting mean arterial pressure and an increase in resting heart rate. Colorectal distension, used to trigger autonomic dysreflexia, caused episodic hypertension and bradycardia. Although the cell transplantation had no effect on resting cardiovascular parameters, it led to a significantly faster recovery from hypertension, with the recovery time shortened by approximately 25%. The transection resulted in an increase in soma size of sympathetic preganglionic neurons above and below the injury. OEC transplantation normalised this change below the injury and increased dendritic length of preganglionic neurons above the injury, compared to controls. It has been proposed that changes in sympathetic preganglionic neurons following spinal cord transection may be related to the development of autonomic dysreflexia. Our results suggest that olfactory ensheathing cells may alter the morphology of these neurons, and hence modify their activity in the neuronal networks responsible for the dysreflexic reaction.

Stem cell-based therapies for spinal cord injury

Spinal cord injury (SCI) results in loss of nervous tissue and consequently loss of motor and sensory function. There is no treatment available that restores the injury-induced loss of function to a degree that an independent life can be guaranteed. Transplantation of stem cells or progenitors may support spinal cord repair. Stem cells are characterized by self-renewal and their ability to become any cell in an organism. Promising results have been obtained in experimental models of SCI. Stem cells can be directed to differentiate into neurons or glia in vitro, which can be used for replacement of neural cells lost after SCI. Neuroprotective and axon regeneration-promoting effects have also been credited to transplanted stem cells. There are still issues related to stem cell transplantation that need to be resolved, including ethical concerns. This paper reviews the current status of stem cell application for spinal cord repair.

Stem cell-based therapies for spinal cord injury

Spinal cord injury (SCI) results in loss of nervous tissue and consequently loss of motor and sensory function. There is no treatment available that restores the injury-induced loss of function to a degree that an independent life can be guaranteed. Transplantation of stem cells or progenitors may support spinal cord repair. Stem cells are characterized by self-renewal and their ability to become any cell in an organism. Promising results have been obtained in experimental models of SCI. Stem cells can be directed to differentiate into neurons or glia in vitro, which can be used for replacement of neural cells lost after SCI. Neuroprotective and axon regeneration-promoting effects have also been credited to transplanted stem cells. There are still issues related to stem cell transplantation that need to be resolved, including ethical concerns. This paper reviews the current status of stem cell application for spinal cord repair.

Derivation of high-purity oligodendroglial progenitors

Oligodendrocytes are a type of glial cells that play a critical role in supporting the central nervous system (CNS), in particular insulating axons within the CNS by wrapping them with a myelin sheath, thereby enabling saltatory conduction. They are lost, and myelin damaged - demyelination - in a wide variety of neurological disorders. Replacing depleted cell types within demyelinated areas, however, has been shown experimentally to achieve remyelination and so help restore function. One method to produce oligodendrocytes for cellular replacement therapies is through the use of progenitor or stem cells. The ability to differentiate progenitor or stem cells into high-purity fates not only permits the generation of specific cells for transplantation therapies, but also provides powerful tools for studying cellular mechanisms of development. This chapter outlines methods of generating high-purity OPCs from multipotent neonatal progenitor or human embryonic stem cells.

Single-cell printing to form three-dimensional lines of olfactory ensheathing cells

Biological laser printing (BioLP) is a unique tool capable of printing high resolution two- and three-dimensional patterns of living mammalian cells, with greater than 95% viability. These results have been extended to primary cultured olfactory ensheathing cells (OECs), harvested from adult Sprague-Dawley rats. OECs have been found to provide stimulating environments for neurite outgrowth in spinal cord injury models. BioLP is unique in that small load volumes ( approximately microLs) are required to achieve printing, enabling low numbers of OECs to be harvested, concentrated and printed. BioLP was used to form several 8 mm lines of OECs throughout a multilayer hydrogel scaffold. The line width was as low as 20 microm, with most lines comprising aligned single cells. Fluorescent confocal microscopy was used to determine the functionality of the printed OECs, to monitor interactions between printed OECs, and to determine the extent of cell migration throughout the 3D scaffold. High-resolution printing of low cell count, harvested OECs is an important advancement for in vitro study of cell interactions and functionality. In addition, these cell-printed scaffolds may provide an alternative for spinal cord repair studies, as the single-cell patterns formed here are on relevant size scales for neurite outgrowth.

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.

Cellular repair strategies for spinal cord injury

The implantation of exogenous cells or tissues has been a popular and successful strategy to overcome physical discontinuity and support axon growth in experimental models of spinal cord injury (SCI). Cellular therapies exhibit a multifarious potential for SCI restoration, providing not only a supportive substrate upon which axons can traverse the injury site, but also reducing progressive tissue damage and scarring, facilitating remyelination repair, and acting as a source for replacing and re-establishing lost neural tissue and its circuitry. The past two decades of research into cell therapies for SCI repair have seen the progressive evolution from whole tissue strategies, such as peripheral nerve grafts, to the use of specific, purified cell types from a diverse range of sources and, recently, to the employment of stem or neural precursor cell populations that have the potential to form a full complement of neural cell types. Although the progression of cell therapies from laboratory to clinical implementation has been slow, human SCI safety and efficacy trials involving several cell types within the US appear to be close at hand.

Survival and migration of human and rat olfactory ensheathing cells in intact and injured spinal cord

Increasing evidence indicates the potential of olfactory ensheathing cells (OECs) for treating spinal cord injuries. The present study compared proliferation and migration of adult rat and human OECs transplanted into the spinal cord of athymic (immunodeficient) rats. OECs were purified from the nasal lamina propria and prelabeled with a cytoplasmic dye. After OEC injection into the thoracic spinal cord, animals were perfused 4 hr, 24 hr, and 7 days later. Both rat and human OECs showed similar migration. Cells were seen leaving the injection site after 4 hr, and by 7 days both rat and human OECs had migrated approximately 1 mm rostrally and caudally within the cord (rat: 1,400 +/- 241 microm rostral, 1,134 +/- 262 microm caudal, n = 5; human: 1,337 +/- 192 microm rostral, 1,205 +/- 148 microm caudal, n = 6). Proliferation of transplanted OECs was evident at 4 hr, but most had ceased dividing by 24 hr. In 10 animals, the spinal cord was injured by a contralateral hemisection made 5 mm rostral to the transplantation site at the time of OEC transplantation. After 7 days, macrophages were numerous both around the injury and at the transplantation site. In the injured cord, rat and human OECs migrated for shorter distances, in both rostral and caudal directions (rat: 762 +/- 118 microm rostral, 554 +/- 142 microm caudal, n = 4; human: 430 +/- 55 microm rostral, 399 +/- 161 microm caudal, n = 3). The results show that rat and human OECs rapidly stop dividing after transplantation and have a similar ability to survive and migrate within the spinal cord of immunocompromised hosts. OECs migrated less in animals with a concomitant contralateral hemisection.

La réparation nerveuse périphérique : 30 siècles de recherche

INTRODUCTION

Nerve injury compromises sensory and motor functions. Techniques of peripheral nerve repair are based on our knowledge regarding regeneration. Microsurgical techniques introduced in the late 1950s and widely developed for the past 20 years have improved repairs. However, functional recovery following a peripheral mixed nerve injury is still incomplete.

STATE OF ART

Good motor and sensory function after nerve injury depends on the reinnervation of the motor end plates and sensory receptors. Nerve regeneration does not begin if the cell body has not survived the initial injury or if it is unable to initiate regeneration. The regenerated axons must reach and reinnervate the appropriate target end-organs in a timely fashion. Recovery of motor function requires a critical number of motor axons reinnervating the muscle fibers. Sensory recovery is possible if the delay in reinnervation is short. Many additional factors influence the success of nerve repair or reconstruction. The timing of the repair, the level of injury, the extent of the zone of injury, the technical skill of the surgeon, and the method of repair and reconstruction contribute to the functional outcome after nerve injury.

CONCLUSION

This review presents the recent advances in understanding of neural regeneration and their application to the management of primary repairs and nerve gaps.

Light promotes regeneration and functional recovery and alters the immune response after spinal cord injury

BACKGROUND AND OBJECTIVES

Photobiomodulation (PBM) has been proposed as a potential therapy for spinal cord injury (SCI). We aimed to demonstrate that 810 nm light can penetrate deep into the body and promote neuronal regeneration and functional recovery.

STUDY DESIGN/MATERIALS AND METHODS

Adult rats underwent a T9 dorsal hemisection, followed by treatment with an 810 nm, 150 mW diode laser (dosage = 1,589 J/cm2). Axonal regeneration and functional recovery were assessed using single and double label tract tracing and various locomotor tasks. The immune response within the spinal cord was also assessed.

RESULTS

PBM, with 6% power penetration to the spinal cord depth, significantly increased axonal number and distance of regrowth (P < 0.001). PBM also returned aspects of function to baseline levels and significantly suppressed immune cell activation and cytokine/chemokine expression.

CONCLUSION

Our results demonstrate that light, delivered transcutaneously, improves recovery after injury and suggests that light will be a useful treatment for human SCI.

Effects of glial transplantation on functional recovery following acute spinal cord injury

Numerous efforts have been made to maximize the efficacy of treatment for spinal cord injury (SCI). Recently, oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells have been reported to remyelinate focal areas of demyelinated spinal cord in adult rats. We conducted a study to investigate the therapeutic potential of transplantation of O-2A cells in a rat model of acute SCI. SCI was induced with an NYU Impactor at T9 of rats. O-2A cells labeled with bromodeoxyuridine (BrdU) were transplanted into sites of SCI at 1 week after the induction of SCI. At 6 weeks after cell transplantation, a behavioral test showed significant functional improvement in animals that had received O-2A-cell transplants as compared to animals given cell-culture medium alone. An electrophysiological study revealed that the transplants did not improve the amplitude or latency of somatosensory evoked potentials, but a recording of motor evoked potentials showed that the latency of these potentials in the O-2A-cell-transplant group was significantly shorter than that in the group treated with cell-culture medium. Following transplantation of BrdU-labeled O-2A cells, cells positive for BrdU were detected at and near sites of SCI. Cells labeled for both BrdU and 2',3' -cyclic nucleotide-3-phosphodiesterase were also detected, showing that the transplanted O-2A cells differentiated into oligodendrocytes. By contrast, cells labeled for BrdU and glial fibrillary acidic protein, or for neuronal nuclei antigen, were not detected. Furthermore, a tract-tracing study showed that numbers of retrogradely labeled neurons increased in areas of the brain stem after O-2A-cell transplantation. The study data showed that after being transplanted into an animal with SCI, O-2A cells migrated to the area adjacent to the site of injury and differentiated into oligodendrocytes. The behavioral test and the electrophysiological and morphological studies showed that transplantation of O-2A cells may play an important role in functional recovery and the regeneration of axons after SCI.

Cellular transplantation strategies for spinal cord injury and translational neurobiology

Basic science advances in spinal cord injury and regeneration research have led to a variety of novel experimental therapeutics designed to promote functionally effective axonal regrowth and sprouting. Among these interventions are cell-based approaches involving transplantation of neural and non-neural tissue elements that have potential for restoring damaged neural pathways or reconstructing intraspinal synaptic circuitries by either regeneration or neuronal/glial replacement. Notably, some of these strategies (e.g., grafts of peripheral nerve tissue, olfactory ensheathing glia, activated macrophages, marrow stromal cells, myelin-forming oligodendrocyte precursors or stem cells, and fetal spinal cord tissue) have already been translated to the clinical arena, whereas others have imminent likelihood of bench-to-bedside application. Although this progress has generated considerable enthusiasm about treating what once was thought to be a totally incurable condition, there are many issues to be considered relative to treatment safety and efficacy. The following review reflects on different experimental applications of intraspinal transplantation with consideration of the underlying pathological, pathophysiological, functional, and neuroplastic responses to spinal trauma that such treatments may target along with related issues of procedural and biological safety. The discussion then moves to an overview of ongoing and completed clinical trials to date. The pros and cons of these endeavors are considered, as well as what has been learned from them. Attention is primarily directed at preclinical animal modeling and the importance of patterning clinical trials, as much as possible, according to laboratory experiences.

Freeze-dried poly(d,l-lactic acid) macroporous guidance scaffolds impregnated with brain-derived neurotrophic factor in the transected adult rat thoracic spinal cord

The effects of poly(D,L-lactic acid) macroporous guidance scaffolds (foams) with or without brain-derived neurotrophic factor (BDNF) on tissue sparing, neuronal survival, axonal regeneration, and behavioral improvements of the hindlimbs following implantation in the transected adult rat thoracic spinal cord were studied. The foams were embedded in fibrin glue containing acidic-fibroblast growth factor. One group of animals received fibrin glue with acidic-fibroblast growth factor only. The foams were prepared by a thermally induced polymer-solvent phase separation process and contained longitudinally oriented macropores connected to each other by a network of micropores. Both foams and fibrin only resulted in a similar gliotic and inflammatory response in the cord-implant interfaces. With BDNF foam, up to 20% more NeuN-positive cells in the spinal nervous tissue close to the rostral but not caudal spinal cord-implant interface survived than with control foam or fibrin only at 4 and 8 weeks after implantation. Semithin plastic sections and electron microcopy revealed that cells and axons more rapidly invaded BDNF foam than control foam. Also, BDNF foam contained almost twice as many blood vessels than control foam at 8 weeks after implantation. Tissue sparing was similar in all three implantation paradigms; approximately 42% of tissue was spared in the rostral cord and approximately 37% in the caudal cord at 8 weeks post grafting. The number of myelinated and unmyelinated axons was low and not different between the two types of foams. Many more axons were found in the fibrin only graft. Serotonergic axons were not found in any of the implants and none of the axons regenerated into the caudal spinal cord. The behavioral improvements in the hindlimbs were similar in all groups. These findings indicated that foam is well tolerated within the injured spinal cord and that the addition of BDNF promotes cell survival and angiogenesis. However, the overall axonal regeneration response is low. Future research should explore the use of poly(D,L-lactic acid) foams, with or without axonal growth-promoting factors, seeded with Schwann cells to enhance the axonal regeneration and myelination response.

LacZ-expressing olfactory ensheathing cells do not associate with myelinated axons after implantation into the compressed spinal cord

Studies have shown that implanting olfactory ensheathing cells (OECs) may be a promising therapeutic strategy to promote functional recovery after spinal cord injury. Several fundamental questions remain, however, regarding their in vivo interactions in the damaged spinal cord. We have induced a clip compression injury at the T10 level of the spinal cord in adult rats. After a delay of 1 week, OECs isolated from embryonic day 18 rats were implanted into the cystic cavity that had formed at the site of injury. Before implantation, OECs were infected with a LacZ-expressing retrovirus. At 3 weeks after implantation, LacZ-expressing OECs survived the implantation procedure and remained localized to the cystic cavity. At the electron microscopic level, the cystic cavity had clusters of LacZ-expressing OECs and numerous Schwann cells lacking LacZ expression. Although labeled OECs made no direct contact with axons, unlabeled Schwann cells were associated with either a single myelinated axon or multiple unmyelinated axons. Positively labeled OEC processes often enveloped multiple Schwann cell-axon units. These observations suggest that the role of OECs as the primary mediators of the beneficial effects on axon growth, myelination, and functional recovery after spinal cord injury may require re-evaluation.

Peripheral olfactory ensheathing cells reduce scar and cavity formation and promote regeneration after spinal cord injury

Bridging of a lesion site and minimizing local damage to create an environment permissive for regeneration are both primary components of a successful strategy to repair spinal cord injury (SCI). Olfactory ensheathing cells (OECs) are prime candidates for autologous transplantation to bridge this gap, but little is known currently about their mechanism of action. In addition, OECs from the accessible lamina propria (LP) of the olfactory mucosa are a more viable source in humans but have yet to be tested for their ability to promote regeneration in established SCI models. Here, mouse LP-OECs expressing green fluorescent protein (GFP) transplanted directly into both rat and mouse dorsolateral spinal cord lesion sites demonstrate limited migration but interact with host astrocytes to develop a new transitional zone at the lesion border. LP-OECs also promote extensive migration of host Schwann cells into the central nervous system repair zone and stimulate angiogenesis to provide a biological scaffold for repair. This novel environment created by transplanted and host glia within the spinal cord inhibits cavity and scar formation and promotes extensive sprouting of multiple sensory and motor axons into and through the lesion site. Sixty days after rat SCI, serotonin- and tyrosine hydroxylase-positive axons sprouted across the lesion into the distal cord, although axotomized rubrospinal axons did not. Thus, even in a xenotransplant paradigm, LP-OECs work collaboratively with host glial cells to create an environment to ameliorate local damage and simultaneously promote a regenerative response in multiple axonal populations.

Lysophosphatidic acid regulates the proliferation and migration of olfactory ensheathing cells in vitro

Olfactory ensheathing cells (OECs) are a unique type of macroglia with axonal growth-promoting properties. However, our understanding of the factors that regulate OECs is at early stages. Lysophosphatidic acid (LPA) is a lipid that influences diverse functions in the nervous system. Recent studies suggest that glial cells, including astrocytes and Schwann cells, are important targets of LPA. However, the influences of LPA on OECs are not known. To address if LPA can influence OECs, we examined effects of LPA on the proliferation and migration of OECs and intracellular effector events. Initially, we observed that OECs expressed the genes for LPA1, LPA2, and LPA3 receptors. When OECs were treated with LPA, we observed stimulated proliferation and migration of OECs. Treatment of OECs with LPA also induced actin cytoskeleton reorganization and focal adhesion assembly. These effects of LPA were blocked by treatment with C3 exoenzyme or Y-27632, which inhibit Rho-GTPase and Rho-associated kinase, respectively. Effects of LPA on OEC proliferation were blocked by the MEK inhibitors PD098059 and U0126 and by the phosphotidylinositol 3-kinase (PI 3-K) inhibitors LY0294002 and wortmannin. These results show that LPA acts via Rho-GTPase, MAPK, and PI 3-K signaling cascades to influence OEC proliferation, migration, and cytoskeleton assembly.

Adult olfactory ensheathing cell transplantation for acute spinal cord injury

Cellular transplantation strategies have been explored for the treatment of spinal cord injury. In particular, olfactory nerve ensheathing cell (OEC) transplantation has been reported to improve functional outcome following injury. We investigated the effect of OEC transplantation using cells derived from adult animals on the restoration of function following a contusion injury to the spinal cord. The NYU impactor was used to create a moderate to severe spinal cord injury in 17 rats. Hoescht stained cultured OECs derived from adult rats (n = 7) or culture medium alone (n = 10) were injected into the injury site immediately following injury. Histological and functional outcomes were measured using immunohistochemistry and the Basso, Beattie, and Bresnahan (BBB) scale. All animals transplanted with OECs were found to have surviving Hoescht positive cells within the spinal cord when sacrificed 6 weeks following injury. Immunohistochemical staining of the explanted cords revealed that the surviving cells stained positively for nerve growth factor receptor. Functional outcomes were not different between the transplanted and control groups. OECs transplanted immediately following a contusion injury to the spinal cord survive during the first 6 weeks following injury. These cells do not appear to influence functional outcome during the first 6 weeks following injury. Additional studies are required in order to definitively determine the utility of this type of cellular transplantation for spinal cord injury.

Olfactory ensheathing glia transplantation: a therapy to promote repair in the mammalian central nervous system

A therapy to treat injuries to the central nervous system (CNS) is, to date, a major clinical challenge. The devastating functional consequences they cause in human patients have encouraged many scientists to search, in animal models, for a repair strategy that could, in the future, be applied to humans. However, although several experimental approaches have obtained some degree of success, very few have been translated into clinical trials. Traumatic and demyelinating lesions of the spinal cord have attracted several groups with the same aim: to find a way to promote axonal regeneration, remyelination, and functional recovery, by using a simple, safe, effective, and viable procedure. During the past decade, olfactory ensheathing glia (OEG) transplantation has emerged as a very promising experimental therapy to promote repair of spinal cords, after different types of injuries. Transplants of these cells promoted axonal regeneration and functional recovery after partial and complete spinal cord lesions. Moreover, olfactory ensheathing glia were able to form myelin sheaths around demyelinated axons. In this article, we review these recent advances and discuss to what extent olfactory ensheathing glia transplantation might have a future as a therapy for different spinal cord affections in humans.

Neutral amino acid transporter ASCT1 is preferentially expressed in L-Ser-synthetic/storing glial cells in the mouse brain with transient expression in developing capillaries

Nonessential amino acid L-Ser plays an essential role in neuronal survival and differentiation, through preferential expression of the L-Ser biosynthetic enzyme 3-phosphoglycerate dehydrogenase (3PGDH), in particular in glial cells but not in neurons. To seek the molecular candidates responsible for glia-borne L-Ser transport, we performed histochemical analyses on amino acid transporter ASCT1, which prefers small neutral amino acids, such as Ala, Ser, Cys, and Thr, and mediates their obligatory exchange. At early developmental stages, neuroepithelial cells constituting the ventricular zone expressed ASCT1 mRNA and protein ubiquitously. Thereafter, ASCT1 expression was gradually downregulated in neuronal populations during the late embryonic and neonatal periods, whereas its high expression was transmitted to radial glial cells and then to astrocytes. High levels of ASCT1 were also detected in the olfactory ensheathing glia. The preferential glial expression of ASCT1 was consistent with that of 3PGDH, and their extensive colocalization was demonstrated at the cellular level. Moreover, high cellular contents of L-Ser were revealed in these glial cells by using a specific antibody to L-Ser. These results strongly suggest that a large amount of L-Ser is synthesized and stored in these glial cells and is released through ASCT1 in exchange for other extracellular substrates. In addition, we observed prominent expression of ASCT1 in capillary endothelial cells of embryonic and neonatal brains. Therefore, ASCT1 appears to be regulated to meet metabolic demands by differentiating and mature neurons through the transport of glia- and blood-borne small neutral amino acids.

Schwann cell but not olfactory ensheathing glia transplants improve hindlimb locomotor performance in the moderately contused adult rat thoracic spinal cord

Cultured adult rat Schwann cells (SCs) or olfactory ensheathing glia (OEG), or both, were transplanted in the adult Fischer rat thoracic (T9) spinal cord 1 week after a moderate contusion (10 gm, 12.5 mm, NYU impactor). Rats received either a total of 2 x 10(6) cells suspended in culture medium or culture medium only (controls). At 12 weeks after injury, all grafted animals exhibited diminished cavitation. Although in medium-injected rats 33% of spinal tissue within a 5-mm-long segment of cord centered at the injury site was spared, significantly more tissue was spared in SC (51%), OEG (43%), and SC/OEG (44%) grafted animals. All three types of glial grafts were filled with axons, primarily of spinal origin. SC grafts contained more myelinated axons than SC/OEG and OEG grafts. Both types of SC-containing grafts expressed more intense staining for glial fibrillary acidic protein and chondroitin sulfate proteoglycan compared with OEG-only grafts. Retrograde tracing demonstrated that the number of propriospinal and brainstem axons reaching 5-6 mm beyond the grafted area was significantly higher with SC and SC/OEG grafts but not with OEG-only grafts compared with controls. Corticospinal fibers terminated closer to the lesion epicenter in all grafted animals than in controls. With SC-only grafts, a modest but statistically significant improvement in hindlimb locomotor performance was detected at 8-11 weeks after injury. Thus, in addition to this functional improvement, our results show that an SC graft is more effective in promoting axonal sparing/regeneration than an SC/OEG or OEG graft in the moderately contused adult rat thoracic spinal cord.

A new millenium for spinal cord regeneration: growth-associated genes

INTRODUCTION

Neurons surviving spinal cord injury undergo extensive reorganization that may result in the formation of functional synaptic contacts. Many neurons, however, fail to activate the necessary mechanisms for successful regeneration. In this review, we discuss the implications of growth cone genes that we have correlated with successful spinal cord axonal regeneration.

METHOD

Factors that inhibit regeneration, and activation of genes that promote it are discussed.

RESULTS/DISCUSSION

The early progress n understanding mechanisms that seem to promote or inhibit regeneration in the central nervous system may have significant clinical utility in the future.

Neurotrophic factors, cellular bridges and gene therapy for spinal cord injury

Injury to the adult mammalian spinal cord results in extensive axonal degeneration, variable amounts of neuronal loss, and often severe functional deficits. Restoration of controlled function depends on regeneration of these axons through an injury site and the formation of functional synaptic connections. One strategy that has emerged for promoting axonal regeneration after spinal cord injury is the implantation of autologous Schwann cells into sites of spinal cord injury to support and guide axonal growth. Further, more recent experiments have shown that neurotrophic factors can also promote axonal growth, and, when combined with Schwann cell grafts, can further amplify axonal extension after injury. Continued preclinical development of these approaches to neural repair may ultimately generate strategies that could be tested in human injury.