Remyelination of spinal cord axons by olfactory ensheathing cells and Schwann cells derived from a transgenic rat expressing alkaline phosphatase marker gene

Features of Remyelination after Transplantation of Olfactory Ensheathing Cells with Neurotrophic Factors into Spinal Cord Cysts

This paper shows for the first time that co-transplantation of human olfactory ensheathing cells with neurotrophin-3 into spinal cord cysts is more effective for activation of remyelination than transplantation of cells with brain-derived neurotrophic factor and a combination of these two factors. The studied neurotrophic factors do not affect proliferation and migration of ensheathing cells in vitro. It can be concluded that the maximum improvement of motor function in rats receiving ensheathing cells with neurotrophin-3 is largely determined by activation of remyelination.

The Vestibular Nuclei: A Cerebral Reservoir of Stem Cells Involved in Balance Function in Normal and Pathological Conditions

In this review, we explore the intriguing realm of neurogenesis in the vestibular nuclei-a critical brainstem region governing balance and spatial orientation. We retrace almost 20 years of research into vestibular neurogenesis, from its discovery in the feline model in 2007 to the recent discovery of a vestibular neural stem cell niche. We explore the reasons why neurogenesis is important in the vestibular nuclei and the triggers for activating the vestibular neurogenic niche. We develop the symbiotic relationship between neurogenesis and gliogenesis to promote vestibular compensation. Finally, we examine the potential impact of reactive neurogenesis on vestibular compensation, highlighting its role in restoring balance through various mechanisms.

Considering the Cellular Composition of Olfactory Ensheathing Cell Transplants for Spinal Cord Injury Repair: A Review of the Literature

Olfactory ensheathing cells (OECs) are specialized glia cells of the olfactory system that support the continual regeneration of olfactory neurons throughout adulthood. Owing to their pro-regenerative properties, OECs have been transplanted in animal models of spinal cord injuries (SCI) and trialed in clinical studies on SCI patients. Although these studies have provided convincing evidence to support the continued development of OEC transplantation as a treatment option for the repair of SCI, discrepancies in the reported outcome has shown that OEC transplantation requires further improvement. Much of the variability in the reparative potential of OEC transplants is due to the variations in the cell composition of transplants between studies. As a result, the optimal cell preparation is currently a subject of debate. Here we review, the characterization as well as the effect of the cell composition of olfactory cell transplantation on therapeutic outcome in SCI. Firstly, we summarize and review the cell composition of olfactory cell preparations across the different species studied prior to transplantation. Since the purity of cells in olfactory transplants might affect the study outcome we also examine the effect of the proportions of OECs and the different cell types identified in the transplant on neuroregeneration. Finally, we consider the effect of the yield of cells on neuroregeneration by assessing the cell dose of transplants on therapeutic outcome.

Human olfactory ensheathing cell-derived extracellular vesicles: miRNA profile and neuroprotective effect

BACKGROUND

Extracellular vesicle (EV)-based therapy has been identified as a leading alternative approach in several disease models. EV derived from the olfactory ensheathing cell (OEC) has been documented for its strong neuro-regenerative capacity. However, no information on its cargo that may contribute to its therapeutic effect has been available.

OBJECTIVE

To report the first miRNA profile of human OEC (hOEC) -EV, and investigate the neuroprotective effects.

METHODS

hOEC-EV was isolated and sequenced. We established in vitro experiments to assess the therapeutic potential of hOEC-EVs with respect to insulted neural progenitor cells (NPCs), and the angiogenesis effect. Secondary post-injury insults were imitated using t-BHP-mediated oxidative stress.

RESULTS

We noted a strong abundance of hOEC-EV-miRNAs, including hsa-miR148a-3p, has-miR151a-3p and several members of let-7 family. The common targets of 15 miRNAs among the top 20 miRNAs were thrombospondin 1 and cyclin dependent kinase 6. We demonstrated that hOEC-EVs promote normal NPC proliferation and differentiation to neuron-like morphologies with prolonged axons. hOEC-EVs protect cells from t-BHP mediated apoptosis. We also found that the migration rate of either NPCs or endothelial cells significantly improved with hOEC-EVs. Furthermore, in vitro tube formation assays indicated that angiogenesis, an important process for tissue repair, was significantly enhanced in human umbilical vein endothelial cells exposed to hOEC-EVs.

CONCLUSION

Our results revealed that hOEC-EVs exert neuroprotective effects by protecting cells from apoptosis and promoting in vitro biological processes that are important to neural tissue repair, including neural cell proliferation, axonal growth, and cell migration, in addition to enhancing angiogenesis. </p>.

Olfactory ensheathing cells: Unique glial cells promising for treatments of spinal cord injury

Spinal cord injury (SCI) is generally the consequence of physical damage, which may result in devastating consequences such as paraplegia or paralysis. Some certain candidates for SCI repair are olfactory ensheathing cells (OECs), which are unique glial cells located in the transition region of the peripheral nervous system and central nervous system and perform neuron regeneration in the olfactory system throughout life. Culture studies have clarified many properties of OECs, but their mechanisms of actions are not fully understood. Successful results achieved in animal models showcased that SCI treatment with OEC transplants is suitable for clinical trials. However, clinical trials are limited by difficulties like cell acquisition for autograft transplantation. Despite the improvements in both animal and clinical studies so far, there is still insufficient information about the mechanism of actions, adverse effects, proper application methods, effective subtypes, and sources of cells. This review summarizes pre-clinical and clinical literature focused on the cellular characterization of both OECs in vitro and post-transplantation. We highlight the roles and effects of OECs on (a) the injury-induced glial milieu, (b) neuronal growth/regeneration, and (c) functional recovery after injury. Due to the shown benefits of OECs with in vitro and animal studies and a limited number of clinical trials, where safety and effectivity were shown, it is necessary to conduct more studies on OECs to obtain effective and feasible treatment methods.

Extracellular vesicles isolated from human olfactory ensheathing cells enhance the viability of neural progenitor cells

OBJECTIVE

Acquired neurological diseases such as severe traumatic brain or spinal cord injury (SCI) cause irreversible disability. Olfactory ensheathing cell (OEC) transplantation has been trialed as a promising SCI treatment. Extracellular vesicles (EVs), which regulate cell-cell interactions, have recently garnered extensive research interests and emerged as a non-cell-based therapy in neurological disorders, including in SCI animal models. However, there have been no reports of human OEC-EVs and their beneficial effects on neuron regeneration. Here, we investigated the effects of EVs isolated from human OEC on the viability of neuronal cells.

METHODS

EVs were isolated from primary human OECs (hOECs) by serial ultracentrifugation. The hOEC-EVs were characterized by transmission electron microscopy, western blotting, and nanoparticle tracking analyses. We conducted CCK8 and lactate dehydrogenase assays to assess the cell proliferation and cytotoxicity of neural progenitor cells (NPCs) exposed to hOEC-EVs. Tert-butyl hydroperoxide (t-BHP) was utilized to mimic oxidative stress-induced cytotoxicity in NPCs.

RESULTS

The modal diameter of hOEC-derived EVs was 113.2 nm. Expressions of EV markers such as CD9, CD63, and CD81 were detected by western blotting. hOEC-derived EVs enhanced the proliferation of NPCs and ameliorated cell cytotoxicity mediated by t-BHP.

DISCUSSION

Our findings reveal a role for hOEC-derived EVs in NPC proliferation and oxidative stress-induced neuronal toxicity model. These results may be useful for developing non-cell therapy OEC-EV-based treatment in acquired nervous system disease.

Olfactory Ensheathing Cells for Spinal Cord Injury: Sniffing Out the Issues

Olfactory ensheathing cells (OECs) are glia reported to sustain the continuous axon extension and successful topographic targeting of the olfactory receptor neurons responsible for the sense of smell (olfaction). Due to this distinctive property, OECs have been trialed in human cell transplant therapies to assist in the repair of central nervous system injuries, particularly those of the spinal cord. Though many studies have reported neurological improvement, the therapy remains inconsistent and requires further improvement. Much of this variability stems from differing olfactory cell populations prior to transplantation into the injury site. While some studies have used purified cells, others have used unpurified transplants. Although both preparations have merits and faults, the latter increases the variability between transplants received by recipients. Without a robust purification procedure in OEC transplantation therapies, the full potential of OECs for spinal cord injury may not be realised.

Cell therapy for spinal cord injury with olfactory ensheathing glia cells (OECs)

The prospects of achieving regeneration in the central nervous system (CNS) have changed, as most recent findings indicate that several species, including humans, can produce neurons in adulthood. Studies targeting this property may be considered as potential therapeutic strategies to respond to injury or the effects of demyelinating diseases in the CNS. While CNS trauma may interrupt the axonal tracts that connect neurons with their targets, some neurons remain alive, as seen in optic nerve and spinal cord (SC) injuries (SCIs). The devastating consequences of SCIs are due to the immediate and significant disruption of the ascending and descending spinal pathways, which result in varying degrees of motor and sensory impairment. Recent therapeutic studies for SCI have focused on cell transplantation in animal models, using cells capable of inducing axon regeneration like Schwann cells (SchCs), astrocytes, genetically modified fibroblasts and olfactory ensheathing glia cells (OECs). Nevertheless, and despite the improvements in such cell-based therapeutic strategies, there is still little information regarding the mechanisms underlying the success of transplantation and regarding any secondary effects. Therefore, further studies are needed to clarify these issues. In this review, we highlight the properties of OECs that make them suitable to achieve neuroplasticity/neuroregeneration in SCI. OECs can interact with the glial scar, stimulate angiogenesis, axon outgrowth and remyelination, improving functional outcomes following lesion. Furthermore, we present evidence of the utility of cell therapy with OECs to treat SCI, both from animal models and clinical studies performed on SCI patients, providing promising results for future treatments.

Chronic TNFα Exposure Induces Robust Proliferation of Olfactory Ensheathing Cells, but not Schwann Cells

TNFα is persistently elevated in many injury and disease conditions. Previous reports of cytotoxicity of TNFα for oligodendrocytes and their progenitors suggest that the poor endogenous remyelination in patients with traumatic injury or multiple sclerosis may be due in part to persistent inflammation. Understanding the effects of inflammatory cytokines on potential cell therapy candidates is therefore important for evaluating the feasibility of their use. In this study, we assessed the effects of long term exposure to TNFα on viability, proliferation, migration and TNFα receptor expression of cultured rat olfactory ensheathing cells (OECs) and Schwann cells (SCs). Although OECs and SCs transplanted into the CNS produce similar myelinating phenotypes, and might be expected to have similar therapeutic uses, we report that they have very different sensitivities to TNFα. OECs exhibited positive proliferative responses to TNFα over a much broader range of concentrations than SCs. Low TNFα concentrations increased proliferation and migration of both OECs and SCs, but SC number declined in the presence of 100 ng/ml or higher concentrations of TNFα. In contrast, OECs exhibited enhanced proliferation even at high TNFα concentrations (up to 1 µg/ml) and showed no evidence of TNF cytotoxicity even at 4 weeks post-treatment. Furthermore, while both OECs and SCs expressed TNFαR1 and TNFαR2, TNFα receptor levels were downregulated in OECs after exposure to100 ng/ml TNFα for 5-7 days, but were either elevated or unchanged in SCs. These results imply that OECs may be a more suitable cell therapy candidate if transplanted into areas with persistent inflammation.

Attenuation of Oxidative Stress-Induced Cell Apoptosis in Schwann RSC96 Cells by Ocimum Gratissimum Aqueous Extract

Objectives: Cell transplantation therapy of Schwann cells (SCs) is a promising therapeutic strategy after spinal cord injury. However, challenges such as oxidative stress hinder satisfactory cell viability and intervention for enhancing SCs survival is critical throughout the transplantation procedures. Ocimum gratissimum, widely used as a folk medicine in many countries, has therapeutic and anti-oxidative properties and may protect SCs survival. Methods: We examined the protective effects of aqueous O. gratissimum extract (OGE) against cell damage caused by H₂O₂-induced oxidative stress in RSC96 Schwann cells. Results: Our results showed that the RSC96 cells, damaged by H₂O₂ oxidative stress, decreased their viability up to 32% after treatment with different concentrations of up to 300 μM H₂O₂, but OGE pretreatment (150 or 200 μg/mL) increased cell viability by approximately 62% or 66%, respectively. Cell cycle analysis indicated a high (43%) sub-G1 cell population in the H₂O₂-treated RSC96 cells compared with untreated cells (1%); whereas OGE pretreatment (150 and 200 μg/mL) of RSC96 cells significantly reduced the sub-G1 cells (7% and 8%, respectively). Furthermore, Western blot analysis revealed that OGE pretreatment inhibited H₂O₂-induced apoptotic protein caspase-3 activation and PARP cleavage, as well as it reversed Bax up-regulation and Bcl-2 down-regulation. The amelioration of OGE of cell stress and stress-induced apoptosis was proved by the HSP70 and HSP72 decrease. Conclusion: Our data suggest that OGE may minimize the cytotoxic effects of H₂O₂-induced SCs apoptosis by modulating the apoptotic pathway and could potentially supplement cell transplantation therapy.

Glial restricted precursors maintain their permissive properties after long-term expansion but not following exposure to pro-inflammatory factors

Glial restricted precursors (GRP) are a promising cellular source for transplantation therapy of spinal cord injury (SCI), capable of creating a permissive environment for axonal growth and regeneration. However, there are several issues regarding the nature of their permissive properties that remain unexplored. For example, cellular transplantation strategies for spinal cord repair require the preparation of a large number of cells, but it is unknown whether the permissive properties of GRP are maintained following the process of in vitro expansion. We used rat GRP isolated from the embryonic day 13.5 spinal cord to compare the properties of early (10-20 days) and late (120-140 days) passage GRP. We found that late passage GRP showed comparable effects on neurite outgrowth of adult rat DRG to early passage GRP in both in vitro co-culture and conditioned medium experiments. In addition, to further examine the effects of the inflammatory cascade activated in the aftermath of SCI on the microenvironment, we studied the direct effects of strong inflammatory mediators, Lipopolysaccharide and interferon gamma (LPS and IFNɤ, respectively), on the properties of GRP. We showed that exposure to these pro-inflammatory mediators altered GRP phenotype and attenuated their growth-promoting effects on neurite outgrowth in a dose dependent manner. Taken together, our data suggest that GRP maintain their growth-promoting properties following extensive in vitro passaging and underscore the importance of modulating the inflammatory environment at the injured spinal cord.

Biomaterial Approaches to Enhancing Neurorestoration after Spinal Cord Injury: Strategies for Overcoming Inherent Biological Obstacles

While advances in technology and medicine have improved both longevity and quality of life in patients living with a spinal cord injury, restoration of full motor function is not often achieved. This is due to the failure of repair and regeneration of neuronal connections in the spinal cord after injury. In this review, the complicated nature of spinal cord injury is described, noting the numerous cellular and molecular events that occur in the central nervous system following a traumatic lesion. In short, postinjury tissue changes create a complex and dynamic environment that is highly inhibitory to the process of neural regeneration. Strategies for repair are outlined with a particular focus on the important role of biomaterials in designing a therapeutic treatment that can overcome this inhibitory environment. The importance of considering the inherent biological response of the central nervous system to both injury and subsequent therapeutic interventions is highlighted as a key consideration for all attempts at improving functional recovery.

Myelin damage and repair in pathologic CNS: challenges and prospects

Injury to the central nervous system (CNS) results in oligodendrocyte cell death and progressive demyelination. Demyelinated axons undergo considerable physiological changes and molecular reorganizations that collectively result in axonal dysfunction, degeneration and loss of sensory and motor functions. Endogenous adult oligodendrocyte precursor cells and neural stem/progenitor cells contribute to the replacement of oligodendrocytes, however, the extent and quality of endogenous remyelination is suboptimal. Emerging evidence indicates that optimal remyelination is restricted by multiple factors including (i) low levels of factors that promote oligodendrogenesis; (ii) cell death among newly generated oligodendrocytes, (iii) inhibitory factors in the post-injury milieu that impede remyelination, and (iv) deficient expression of key growth factors essential for proper re-construction of a highly organized myelin sheath. Considering these challenges, over the past several years, a number of cell-based strategies have been developed to optimize remyelination therapeutically. Outcomes of these basic and preclinical discoveries are promising and signify the importance of remyelination as a mechanism for improving functions in CNS injuries. In this review, we provide an overview on: (1) the precise organization of myelinated axons and the reciprocal axo-myelin interactions that warrant properly balanced physiological activities within the CNS; (2) underlying cause of demyelination and the structural and functional consequences of demyelination in axons following injury and disease; (3) the endogenous mechanisms of oligodendrocyte replacement; (4) the modulatory role of reactive astrocytes and inflammatory cells in remyelination; and (5) the current status of cell-based therapies for promoting remyelination. Careful elucidation of the cellular and molecular mechanisms of demyelination in the pathologic CNS is a key to better understanding the impact of remyelination for CNS repair.

Voltage-dependent K+ currents contribute to heterogeneity of olfactory ensheathing cells

The olfactory nerve is permissive for axon growth throughout life. This has been attributed in part to the olfactory ensheathing glial cells that encompass the olfactory sensory neuron fascicles. Olfactory ensheathing cells (OECs) also promote axon growth in vitro and when transplanted in vivo to sites of injury. The mechanisms involved remain largely unidentified owing in part to the limited knowledge of the physiological properties of ensheathing cells. Glial cells rely for many functions on the properties of the potassium channels expressed; however, those expressed in ensheathing cells are unknown. Here we show that OECs express voltage-dependent potassium currents compatible with inward rectifier (Kir ) and delayed rectifier (KDR ) channels. Together with gap junction coupling, these contribute to the heterogeneity of membrane properties observed in OECs. The relevance of K(+) currents expressed by ensheathing cells is discussed in relation to plasticity of the olfactory nerve.

Nerve conduits for peripheral nerve surgery

Autologous nerve grafts are the current criterion standard for repair of peripheral nerve injuries when the transected nerve ends are not amenable to primary end-to-end tensionless neurorrhaphy. However, donor-site morbidities such as neuroma formation and permanent loss of function have led to tremendous interest in developing an alternative to this technique. Artificial nerve conduits have therefore emerged as an alternative to autologous nerve grafting for the repair of short peripheral nerve defects of less than 30 mm; however, they do not yet surpass autologous nerve grafts clinically. A thorough understanding of the complex biological reactions that take place during peripheral nerve regeneration will allow researchers to develop a nerve conduit with physical and biological properties similar to those of an autologous nerve graft that supports regeneration over long nerve gaps and in large-diameter nerves. In this article, the authors assess the currently available nerve conduits, summarize research in the field of developing these conduits, and establish areas within this field in which further research would prove most beneficial.

Selective stimulatory action of olfactory ensheathing glia-conditioned medium on oligodendroglial differentiation, with additional reference to signaling mechanisms

We examined the effects of conditioned medium from olfactory ensheathing glia (OEGCM) on the differentiation of oligodendrocytes in mixed cultures of early postnatal hippocampi. Differentiation was judged from the numerical density (ND) of cells immunoreactive to 2'3' cyclic nucleotide 3'phosphodiesterase (CNPase) and O4 antibodies. NDs increased according to inverted-U dose-response curves, particularly for CNPase+ cells (9-fold at optimal dilution) and these changes were blocked by inhibitors of ERK1, p38-MAPK, and PI3K. Our results raise the possibility that OEG secreted factor(s) may counteract demyelination induced by trauma, neurodegenerative diseases, and advanced age, and should stimulate novel methods to deliver these factors and/or potentiating chemicals.

CNS Schwann cells display oligodendrocyte precursor-like potassium channel activation and antigenic expression in vitro

Central nervous system (CNS) injury triggers production of myelinating Schwann cells from endogenous oligodendrocyte precursors (OLPs). These CNS Schwann cells may be attractive candidates for novel therapeutic strategies aiming to promote endogenous CNS repair. However, CNS Schwann cells have been so far mainly characterized in situ regarding morphology and marker expression, and it has remained enigmatic whether they display functional properties distinct from peripheral nervous system (PNS) Schwann cells. Potassium channels (K+) have been implicated in progenitor and glial cell proliferation after injury and may, therefore, represent a suitable pharmacological target. In the present study, we focused on the function and expression of voltage-gated K+ channels Kv(1-12) and accessory β-subunits in purified adult canine CNS and PNS Schwann cell cultures using electrophysiology and microarray analysis and characterized their antigenic phenotype. We show here that K+ channels differed significantly in both cell types. While CNS Schwann cells displayed prominent K D-mediated K+ currents, PNS Schwann cells elicited K(D-) and K(A-type) K+ currents. Inhibition of K+ currents by TEA and Ba2+ was more effective in CNS Schwann cells. These functional differences were not paralleled by differential mRNA expression of Kv(1-12) and accessory β-subunits. However, O4/A2B5 and GFAP expressions were significantly higher and lower, respectively, in CNS than in PNS Schwann cells. Taken together, this is the first evidence that CNS Schwann cells display specific properties not shared by their peripheral counterpart. Both Kv currents and increased O4/A2B5 expression were reminiscent of OLPs suggesting that CNS Schwann cells retain OLP features during maturation.

Olfactory ensheathing cells of hamsters, rabbits, monkeys, and mice express α-smooth muscle actin

Olfactory ensheathing cells (OECs) are the chief glial population of the mammalian olfactory nervous system, residing in the olfactory mucosa and at the surface of the olfactory bulb. We investigated the neurochemical features of OECs in a variety of mammalian species (including adult hamsters, rabbits, monkeys, and mice, as well as fetal pigs) using three biomarkers: α-smooth muscle actin (αSMA), S100β, and glial fibrillary acidic protein (GFAP). Mucosal and bulbar OECs from all five mammalian species express S100β. Both mucosal and bulbar OECs of monkeys express αSMA, yet only bulbar OECs of hamsters and only mucosal OECs of rabbits express αSMA as well. Mucosal OECs, but not bulbar OECs, also express GFAP in hamsters and monkeys; mice, by comparison, have only a sparse population of OECs expressing GFAP. Though αSMA immunostaining is not detected in OECs of adult mice, GFAP-expressing mucosal OECs isolated from adult mice do coexpress αSMA in vitro. Moreover, mucosal OECs from adult mutant mice lacking αSMA expression display perturbed cellular morphology (i.e., fewer cytoplasmic processes extending among the hundreds of olfactory axons in the olfactory nerve fascicles and nuclei having degenerative features). In sum, these findings highlight the efficacy of αSMA and S100β as biomarkers of OECs from a variety of mammalian species. These observations provide definitive evidence that mammalian OECs express the structural protein αSMA (at various levels of detection), which appears to play a pivotal role in their ensheathment of olfactory axons.

Toward defining the regenerative potential of olfactory mucosa: establishment of Schwann cell-free adult canine olfactory ensheathing cell preparations suitable for transplantation

Olfactory mucosa (OM)-derived olfactory ensheathing cells (OECs) are attractive candidates for autologous cell transplantation-based therapy of nervous system injury. However, defining the regenerative capacity of OM-derived OECs is impeded by the fact that cell cultures used for transplantation may contain significant amounts of contaminating trigeminal nerve Schwann cells that escape identification by sharing in vitro expression of OEC markers. The aim of the present study, therefore, was to quantify contaminating Schwann cells in OEC preparations and to develop a protocol for their specific depletion. Based on the observation that freshly dissociated, but not cultured, OECs and Schwann cells display differential expression of HNK-1 and p75(NTR), magnet-activated cell sorting (MACS) was used to deplete myelinating (HNK-1-positive) and nonmyelinating (p75(NTR)-positive) Schwann cells from primary cell suspensions containing HNK-1-/p75(NTR)-negative OECs. Upregulation of p75(NTR) expression in OECs during culturing allowed their subsequent MACS-based separation from fibroblasts. Immunofluorescence analysis of freshly dissociated OM prior to MACS depletion revealed that 21% of the total and 56% of all CNPase-positive cells, representing both OECs and Schwann cells, expressed the Schwann cell antigens HNK-1 or p75(NTR), indicating that freshly dissociated OM prior to culturing contained as many Schwann cells as OECs, while olfactory bulb (OB) primary cell suspensions revealed lower levels of Schwann cell contamination. Interestingly, neurite growth of neonatal rat dorsal root ganglion (DRG) neurons cocultured with OM-OECs, OB-OECs, and fibular nerve (FN) Schwann cells used as control was significantly higher in the presence of OECs than of Schwann cells. The first report on identification and specific depletion of Schwann cells from OEC preparations provides a solid basis for future efforts to fully define the regenerative potential of nasal mucosa OECs.

Repair of central nervous system lesions by transplantation of olfactory ensheathing cells

Clinical conditions affecting the central nervous system (CNS) fall into two main categories - degenerative conditions in which nerve cells are lost (Alzheimer's, Parkinson's, Huntington's disease, etc.), and traumatic insults which sever nerve fibers but leave their cell bodies and initial parts of the severed axons intact (spinal cord injury, cerebrovascular accidents, or tumors affecting fiber tracts). After injuries of this second type, the survival of the nerve cell bodies and the local sprouting at the severed ends of the proximal stumps of the axons raise the tantalizing possibility of one day learning how to induce these severed fibers to regenerate to their original targets and restore lost functions. This chapter gives an overview of current research into the strategy of transplantation of olfactory ensheathing cells into axotomizing injuries.

CNPase expression in olfactory ensheathing cells

A large body of work supports the proposal that transplantation of olfactory ensheathing cells (OECs) into nerve or spinal cord injuries can promote axonal regeneration and remyelination. Yet, some investigators have questioned whether the transplanted OECs associate with axons and form peripheral myelin, or if they recruit endogenous Schwann cells that form myelin. Olfactory bulbs from transgenic mice expressing the enhanced green fluorescent protein (eGFP) under the control of the 2-3-cyclic nucleotide 3-phosphodiesterase (CNPase) promoter were studied. CNPase is expressed in myelin-forming cells throughout their lineage. We examined CNPase expression in both in situ in the olfactory bulb and in vitro to determine if OECs express CNPase commensurate with their myelination potential. eGFP was observed in the outer nerve layer of the olfactory bulb. Dissociated OECs maintained in culture had both intense eGFP expression and CNPase immunostaining. Transplantation of OECs into transected peripheral nerve longitudinally associated with the regenerated axons. These data indicate that OECs in the outer nerve layer of the olfactory bulb of CNPase transgenic mice express CNPase. Thus, while OECs do not normally form myelin on olfactory nerve axons, their expression of CNPase is commensurate with their potential to form myelin when transplanted into injured peripheral nerve.

17β-estradiol protects Schwann cells against H2O2-induced cytotoxicity and increases transplanted Schwann cell survival in a cervical hemicontusion spinal cord injury model

Schwann cell (SC) transplantation is a promising repair strategy after spinal cord injury (SCI); however, a large number of SCs do not survive following transplantation. Previous studies have shown that 17β-estradiol (E2) protects several cell types against cytotoxicity. Thus, this study evaluated the protective potential of E2 on SCs in vitro and investigated the effect of E2 on transplanted SC survival in a rat model of SCI. Primary SC cultures were found to robustly express estrogen receptors (ER) and incubation with E2 protected SCs against hydrogen peroxide-induced cell death. This protection was not inhibited by the ER antagonist ICI 182,780, suggesting that genomic signaling is not necessary for protection. In a subsequent experiment, cervical hemicontusion SCI was induced in male rats followed by sustained administration of E2 or placebo. Eight days after SCI, SCs were transplanted into the injury epicenter. E2 treatment significantly increased the number of surviving labeled transplanted SCs evaluated 7 days after transplantation. These data demonstrate that E2 protects SCs against oxidative stress and improves transplanted SC survival, which suggests that E2 administration may be an intervention of choice for enhancing survival of transplanted SCs after SCI.

Deciphering the oligodendrogenic program of neural progenitors: cell intrinsic and extrinsic regulators

In the developing and adult CNS, neural stem/progenitor cells (NSPCs) and oligodendroglial progenitor cells (OPCs) follow an oligodendrogenic process with the aim of myelinating axons. This process is to a high degree regulated by an oligodendrogenic program (OPr) composed of intrinsic and extrinsic factors that modulate the different steps required for NSPCs to differentiate into myelinating oligodendrocytes. Even though NSPCs and OPCs are present in the diseased CNS and have the capacity to generate oligodendrocytes, sparse remyelination of axons constitutes a major constraint in therapies toward multiple sclerosis (MS) and spinal cord injury (SCI). Lack of pro-oligodendrogenic factors and presence of anti-oligodendrogenic activities are thought to be the main reasons for this limitation. Thus, molecular and cellular strategies aiming at remyelination and at targeting such pro- and anti-oligodendrogenic mechanisms are currently under investigation. The present review summarizes the current knowledge on the OPr; it implements our own findings on mesenchymal stem cell-derived pro-oligodendroglial factors and on the role of p57/kip2 in oligodendroglial differentiation. Moreover, it describes molecular and cellular approaches for the development of future therapies toward remyelination.

Juvenile and adult olfactory ensheathing cells bundle and myelinate dorsal root ganglion axons in culture

Olfactory ensheathing cells (OEC), which normally associate closely with but do not myelinate axons in situ, myelinate axons in the adult mammalian spinal cord. They are of clinical interest as candidate cells for autologous transplantation but the ability of OEC to myelinate axons in vitro has been controversial. To clarify this issue, we isolated OEC from olfactory bulbs (OB) of juvenile and adult rats expressing GFP and analyzed their ability to myelinate axons. Using a well-defined assay for myelination of dorsal root ganglia (DRG) axons in culture, we found that OEC from juvenile pups associated with and then myelinated DRG axons. OEC assembled into bundles with the axons by 1week and required more than a week before myelination on axons was detected. In contrast, rat Schwann cells did not bundle axons and they formed P0(+) and MBP(+) myelin segments after as little as 1week. Most of the OEC in culture exhibited staining for calponin, a marker that was not found on Schwann cells in culture, whereas in both OEC and Schwann cell populations nearly all cells were positive for p75NTR and GFAP. These results confirm previous reports showing only subtle immunological differences between Schwann cells and OEC. Besides differences in the rate of myelination, we detected two additional functional differences in the interactions of OEC and Schwann cells with DRG axons. First, the diameter of OEC generated myelin was greater than for Schwann cell myelin on DRG axons. Second, OEC but not Schwann cells myelinated DRG axons in the absence of vitamin C. OEC isolated from adult OB were also found to bundle and myelinate DRG axons but the latter occurred only after incubation times of at least 3weeks. The results indicate that adult OEC require longer incubation times than juvenile OEC to myelinate axons and suggest that patterns of myelination by OEC and Schwann cells are distinguishable at least on axons in vitro. This article is part of a Special Issue entitled: Understanding olfactory ensheathing glia and their prospect for nervous system repair.

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.

A comparative examination of biomarkers for olfactory ensheathing cells in cats and guinea pigs

We investigated the neurochemical characteristics of olfactory ensheathing cells (OECs) in adult cats and in adult guinea pigs. Three conventional biomarkers for OECs, p75 neurotrophin receptor (p75NTR), S100, and glial fibrillary acidic protein (GFAP), as well as two recently identified biomarkers, smooth muscle alpha-actin (SMA) and calponin, were used. We found that 1) antibodies against SMA and S100 yielded positive immunostaining of mucosal and bulbar OECs in cats and guinea pigs; 2) antibodies against GFAP gave positive immunostaining of mucosal and bulbar OECs in cats; and 3) antibodies against calponin yielded positive immunostaining of bulbar OECs in adult cats. Unexpectedly, antibodies against p75NTR failed to positively stain mucosal and bulbar OECs in cats and guinea pigs, and antibodies against GFAP and calponin failed to positively stain mucosal and bulbar OECs in guinea pigs. These findings show the importance for empirical testing of all biomarkers for OECs among different mammalian species when attempting to identify these cells in vivo, in vitro, and following intraspinal implantation.

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.

Translating basic research into clinical practice or what else do we have to learn about olfactory ensheathing cells?

Olfactory ensheathing cells (OECs) are Schwann cell-like glial cells of the olfactory system that have been shown to promote axonal regeneration and remyelination in a variety of different lesion paradigms. It is still a matter of debate in how far OECs differ from Schwann cells regarding their regenerative potential and molecular setup. The fact that OECs have been already used for transplantation in humans may imply that the need of the hour is the fine-tuning of clinical application details rather than to cross the bridge between laboratory animal and man. Considering the therapeutic transplantation of OECs, however, the basic question to date is not 'how' to translate but rather 'what' to translate into clinical practice. The aim of the present article is to provide a summary of the current literature and to define the open issues relevant for translating basic research on OECs into clinical practice.

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.

Serum-Free Medium Provides a Clinically Relevant Method to Increase Olfactory Ensheathing Cell Numbers in Olfactory Mucosa Cell Culture

There is much evidence to suggest that transplanted olfactory ensheathing cells may ameliorate the functional deficits associated with injuries to the nervous system, especially the spinal cord. For clinical implementation of this strategy it will be necessary to derive large numbers of these cells from an accessible and, preferably, autologous source, implying that olfactory mucosa would be ideal. Although olfactory ensheathing cells can be derived from olfactory mucosa, in routine culture conditions the proportion of these cells is unacceptably low for clinical purposes. This study compared the effect of culturing dissociated olfactory bulb and olfactory mucosa in two different media: one containing serum and one serum free. The results indicate that olfactory ensheathing cell proportion, and absolute cell numbers, is greatly increased in serum-free conditions. Further analysis suggests that serum-free medium has a differential effect on contaminating fibronectin-positive and p75-positive cells from olfactory bulb and olfactory mucosa. This study demonstrates that serum-free culture conditions provide a simple and useful means of deriving a sufficient number of olfactory ensheathing cells for transplantation and reveals a difference in biological behavior of the cells contained within olfactory bulb and olfactory mucosa.

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.

Cellular approaches for stimulating CNS remyelination

Myelination is critical for the normal functioning of the vertebrate nervous system. In the CNS, myelin is produced by oligodendrocytes, and the loss of oligodendrocytes and myelin results in severe functional impairment. Although spontaneous remyelination occurs in chronic demyelinating diseases such as multiple sclerosis, the repair process eventually fails, often resulting in long-term disability. Two distinct general approaches can be considered to promote myelin repair. In one the target is stimulation of the endogenous myelin repair process through delivery of growth factors, and in the second the target is augmentation of the repair process through the delivery of exogenous cells with myelination potential. In both cases, effective treatment of diseases such as multiple sclerosis requires modulation of the immune system, since demyelination is associated with specific immunological activation. Recent studies have shown that some populations of stem cells, including mesenchymal stem cells, have the capacity of promoting endogenous myelin repair and modulating the immune response, prompting an assessment of their use as therapy in demyelinating diseases such as MS. Other types of demyelinating disorders, such as the leukodystrophies, may require multiple repair strategies including both replacement of dysfunctional cells and delivery or supplementation of growth factors, immune modulators or metabolic enzymes. Here we discuss the use of stem cells for the treatment of demyelinating diseases. While the current number of stem cell-based clinical trials for demyelinating diseases is limited, this is likely to increase significantly in the next few years, and a clear understanding of the applicability, limitations and underlying mechanisms mediating stem cell repair is critical.

On the presence of neurotrophin p75 receptor on rat sympathetic cerebrovascular nerves

Although the presence of neurotrophin p75 receptor on sympathetic nerves is a well-recognised feature, there is still a scarcity of details of the distribution of the receptor on cerebrovascular nerves. This study examined the distribution of p75 receptor on perivascular sympathetic nerves of the middle cerebral artery and the basilar artery of healthy young rats using immunohistochemical methods at the laser confocal microscope and transmission electron microscope levels. Immunofluorescence methods of detection of tyrosine hydroxylase (TH) in sympathetic nerves, p75 receptor associated with the nerves, and also S-100 protein in Schwann cells were applied in conjunction with confocal microscopy, while the pre-embedding single and double immunolabelling methods (ExtrAvidin and immuno-gold-silver) were applied for the electron microscopic examination. Immunofluorescence studies revealed "punctuate" distribution of the p75 receptor on sympathetic nerves including accompanying Schwann cells. Image analysis of the nerves showed that the level of co-localization of p75 receptor and TH was low. Immunolabelling applied at the electron microscope level also showed scarce co-localization of TH (which was intra-axonal) and p75. Immunoreactivity for p75 receptor was present on the cell membrane of perivascular axons and to a greater extent on the processes of accompanying Schwann cells. Some Schwann cell processes were adjacent to each other displaying strong immunoreactivity for p75 receptor; immunoreactivity was located on the extracellular sites of the adjacent cell membranes suggesting that the receptor was involved in cross talk between these. It is likely that variability of locations of p75 receptor detected in the study reflects diverse interactions of p75 receptor with axons and Schwann cells. It might also imply a diverse role for the receptor and/or the plasticity of sympathetic cerebrovascular nerves to neurotrophin signalling.

Aging Schwann cells in vitro

Schwann cells (SCs) can support the regeneration of lesioned fiber tracts of the peripheral and central nervous system and have been transplanted alone or in combination with synthetic nerve guides. For neuronal tissue engineering purposes, the cells must be isolated from small biopsies and expanded in vitro. In this study we analyze the impact of cell expansion on 9 different cell parameters, comparing short- and long-term cultured rat SCs, which we refer to as 'young' and 'old' or 'aged' cells, respectively. In comparison to young SCs, old SCs doubled the axonal outgrowth from dorsal root ganglion explants and displayed only one-third as much adhesion to the gray and white matter of spinal cord cryosections. In a 3-dimensional extracellular matrix the two cell populations showed very different cellular responses with regard to cell morphology and cell-cell adhesion. Cell proliferation of old SCs was independent of serum components and was not hampered by contact inhibition. In addition, population doubling times were reduced by a factor of almost three compared to those of young SCs. Despite considerable karyotype changes, with an average of 68.7 chromosomes versus 42 in native rat cells, old SCs did not show any increase in telomerase activity and loss of anchorage dependence--characteristics that are typical of tumor cells. The data also provide biological insights into which cell characteristics (proliferation and adhesion, for example) are functionally clustered and either change or remain constant with aging in vitro. Though the data indicate a lack of tumorigenic transformation coupled with increased neurite outgrowth-promoting activity after extensive SC expansion in vitro, thus suggesting better regeneration qualities, we strongly recommend that in vitro aged rat SCs (>11 passages) should not be employed for tissue engineering.

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.

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.

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

Transplantation of olfactory ensheathing cells (OECs) into the damaged rat spinal cord leads to directed elongative axonal regeneration and improved functional outcome. OECs are known to produce a number of neurotrophic molecules. To explore the possibility that OECs are neuroprotective for injured corticospinal tract (CST) neurons, we transplanted OECs into the dorsal transected spinal cord (T9) and examined primary motor cortex (M1) to assess apoptosis and neuronal loss at 1 and 4 weeks post-transplantation. The number of apoptotic cortical neurons was reduced at 1 week, and the extent of neuronal loss was reduced at 4 weeks. Biochemical analysis indicated an increase in BDNF levels in the spinal cord injury zone after OEC transplantation at 1 week. The transplanted OECs associated longitudinally with axons at 4 weeks. Thus, OEC transplantation into the injured spinal cord has distant neuroprotective effects on descending cortical projection neurons.

Defining the role of olfactory ensheathing cells in facilitating axon remyelination following damage to the spinal cord

Olfactory ensheathing cells (OECs) are unique cells that are responsible for the successful regeneration of olfactory axons throughout the life of adult mammals. More than a decade of research has shown that implantation of OECs may be a promising therapy for damage to the nervous system, including spinal cord injury. Based on this research, several clinical trials worldwide have been initiated that use autologous transplantation of olfactory tissue containing OECs into the damaged spinal cord of humans. However, research from several laboratories has challenged the widely held belief that OECs are directly responsible for myelinating axons and promoting axon regeneration. The purpose of this review is to provide a working hypothesis that integrates several current ideas regarding the mechanisms of the beneficial effects of OECs. Specifically, OECs promote axon regeneration and functional recovery indirectly by augmenting the endogenous capacity of host Schwann cells to invade the damaged spinal cord. Together with Schwann cells, OECs create a 3-dimensional matrix that provides a permissive microenvironment for successful axon regeneration in the adult mammalian central nervous system.

Identified olfactory ensheathing cells transplanted into the transected dorsal funiculus bridge the lesion and form myelin

Olfactory ensheathing cells (OECs) prepared from the olfactory bulbs of adult transgenic Sprague Dawley (SD) rats expressing green fluorescent protein (GFP) were transplanted into a dorsal spinal cord transection lesion of SD rats. Five weeks after transplantation, the cells survived within the lesion zone and oriented longitudinally along axons that bridged the transection site. Although the highest density of GFP cells was within the lesion zone, some cells distributed longitudinally outside of the lesion area. Myelinated axons spanning the lesion were observed in discrete bundles encapsulated by a cellular element. Electron micrographs of spinal cords immunostained with an anti-GFP antibody indicated that a majority of the peripheral-like myelinated axons were derived from donor OECs. Open-field locomotor behavior was significantly improved in the OEC transplantation group. Thus, transplanted OECs derived from the adult olfactory bulb can survive and orient longitudinally across a spinal cord transection site and form myelin. This pattern of repair is associated with improved locomotion.