Author contributions: A.Y.: conception and design, experiments, data analysis and interpretation, manuscript writing, and final approval of manuscript; O.T.: conception and design, experiments, surgery, and data analysis and interpretation, S.S.: experiments and data analysis and interpretation, S.N.: experiments, surgery, and data interpretation; M.T.: experiments, surgery, and data analysis; Y.K. and Y.T.: experiments and data interpretation; K.F., C.M.H., A.M., H.J.O. and Y.T.: data analysis and interpretation; M.N. and H.O.: conception and design, data analysis and interpretation, manuscript writing, and final approval of manuscript.
Disclosure of potential conflicts of interest is found at the end of this article.
First published online in STEM CELLSEXPRESS October 25, 2011.
Previous reports of functional recovery from spinal cord injury (SCI) in rodents and monkeys after the delayed transplantation of neural stem/progenitor cells (NS/PCs) have raised hopes that stem cell therapy could be used to treat SCI in humans. More research is needed, however, to understand the mechanism of functional recovery. Oligodendrocytes derived from grafted NS/PCs remyelinate spared axons in the injured spinal cord. Here, we studied the extent of this remyelination's contribution to functional recovery following contusive SCI in mice. To isolate the effect of remyelination from other possible regenerative benefits of the grafted cells, NS/PCs obtained from myelin-deficient shiverer mutant mice (shi-NS/PCs) were used in this work alongside wild-type NS/PCs (wt-NS/PCs). shi-NS/PCs behaved like wt-NS/PCs in vitro and in vivo, with the exception of their myelinating potential. shi-NS/PC-derived oligodendrocytes did not express myelin basic protein in vitro and formed much thinner myelin sheaths in vivo compared with wt-NS/PC-derived oligodendrocytes. The transplantation of shi-NS/PCs promoted some locomotor and electrophysiological functional recovery but significantly less than that afforded by wt-NS/PCs. These findings establish the biological importance of remyelination by graft-derived cells for functional recovery after the transplantation of NS/PCs into the injured spinal cord. STEM CELLS 2011;29:1983–1994.
Traumatic spinal cord injury (SCI) results in severe and permanent neurological deficits that can include paraplegia and tetraplegia. However, there is currently no effective clinical therapeutic option to improve the functional outcome following SCI. Recent advances in stem cell biology are clearing the way toward implementing therapeutic strategies to replace lost neural cells through the transplantation of stem cells. Such strategies could be applicable to many central nervous system (CNS) disorders, and SCI may be one of the first conditions treated with stem-cell transplantation therapy. To this point, embryonic stem cells (ESCs), bone marrow mesenchymal stem cells, and glial-restricted precursor cells have all been reported to induce functional improvement following their transplantation into the injured spinal cord [1–6].
In particular, we and others have reported beneficial effects and improved functional recovery in experimental SCI models following the transplantation of neural stem/progenitor cells (NS/PCs) derived from the CNS [3, 6-10] or from pluripotent stem cells, including ESCs [11–14] and induced pluripotent stem cells (iPSCs) [15, 16]. Notably, Cummings and colleagues showed that selective ablation of grafted cells resulted in the subsequent deterioration of motor function in animals that had initially demonstrated functional motor recovery after SCI. This indicates that the long-term survival of grafted cells in the host spinal cord is very important for the maintenance of functional recovery .
Although the underlying mechanism responsible for functional recovery from SCI after stem-cell transplantation has still remained unclear [17, 18], three mechanisms have been proposed: the replacement of lost neurons to reconstruct local circuitry, the remyelination of spared, demyelinated axons by graft-derived oligodendrocytes, and the provision of trophic support that reduces the damage and creates a permissive substrate for axonal growth. In previous studies, the transplantation of NS/PCs derived from various sources into mouse SCI models [3, 6, 12, 15] resulted in the remyelination of spared axons by graft-derived oligodendrocytes and good recovery of locomotor function after SCI. These findings could imply that graft-derived oligodendrocytes play a role in functional recovery. However, a causative relationship between the actions of graft-derived oligodendrocytes, including remyelination, and the observed functional recovery has not been fully addressed.
In this study, we analyzed the importance of graft-derived oligodendrocytes to functional recovery after SCI by comparing the transplantation of NS/PCs from wild-type mice with NS/PCs from myelin-deficient shiverer mutant mice [19–21]. The latter have a partial deletion in the gene encoding myelin basic protein (MBP) [22–25]. The present data establish the importance of graft-derived oligodendrocytes in the functional recovery of the injured spinal cord after NS/PC transplantation. This insight will be crucial for extending stem-cell-based therapy to SCI repair.
MATERIALS AND METHODS
The methods for culture and expansion of NS/PCs were as described previously . In brief, the striata of Jcl:ICR (ICR) background homozygous shiverer mutant mice and wild-type C57BL/6J mice on embryonic day 14 were dissociated using a fire-polished glass pipette. The dissociated cells were collected by centrifugation and resuspended in culture medium followed by cell cluster (neurosphere) formation. For differentiation, the neurospheres were cultured without serum or growth factors. In vitro myelin marker assay was performed with the method modified from Stankoff et al.  (See Supporting Information Materials and Methods).
The proliferation assay was performed by measuring ATP, which indirectly reflects the number of viable cells. Population doubling times were determined using the ATP assay, as described elsewhere [28, 29]. Trophic factor analysis was also performed, as described previously  (See Supporting Information).
Adult female NOD/SCID (NOD.CB17-Prkdcscid/J) mice (8–10 week old, 20–22 g, n = 42; Charles River Laboratories, Kanagawa, Japan) were anesthetized with an intraperitoneal (i.p.) injection of ketamine (100 mg/kg) and xylazine (10 mg/kg). After laminectomy at the 10th thoracic spinal vertebra (T10), the dorsal surface of the dura mater was exposed. SCI was induced as described previously , using a commercially available SCI device (IH impactor, Precision Systems and Instrumentation, Lexington, KY, USA; http://www.presysin.com). This device creates a reliable contusion injury by rapidly applying a force-defined impact (60 kdyn) with a stainless steel-tipped impactor. All experiments were performed in accordance with the Guidelines for the Care and Use of Laboratory Animals of Keio University School of Medicine and the Guide for the Care and Use of Laboratory Animals (National Institutes of Health, Bethesda, MD, USA).
A fusion HIV-1 based lentiviral vector, expressing ffLuc (Venus fused to firefly luciferase) under the control of the elongation factor 1α (EF1α) promoter , was used to label NS/PCs prior to grafting. This vector enabled the detection of grafted cells as strong bioluminescent ffLuc signals in live SCI mice and as fluorescent Venus signals using anti-green fluorescent protein antibody in fixed spinal-cord sections. This virus was obtained as previously reported . The concentrated virus was added to the culture medium to infect primary NS/PCs derived from shiverer or wild-type mice (multiplicity of injection = 1.0). The lentivirally transduced NS/PCs (5 × 105 cells per two microliters) were transplanted into the lesion epicenter 9 days after SCI as previously reported [11, 12, 15, 16, 31-33].
The NS/PCs were injected with a glass micropipette at a rate of 1 μl/minute with a Hamilton syringe (25 μl) and a stereotaxic microinjector (KDS 310, Muromachikikai Co. Ltd., Tokyo, Japan, http://www.muromachi.com). shi-NS/PCs were transplanted into 15 mice (shi-NS/PC group), and wt-NS/PCs were transplanted into 13 mice (wt-NS/PC group). Phosphate-buffered saline (PBS; 2 μl per mouse) was injected into the lesion epicenter of control mice (control group; n = 14). After surgery, animal care was performed in the Supporting Information.
The Xenogen-IVIS spectrum cooled charge-coupled device (CCD) optical macroscopic imaging system (Caliper LifeSciences, Hopkinton, MA, USA; http://www.caliperls.com) was used for bioimaging to confirm the survival of the transplanted NS/PCs. Monitoring was performed for 6 weeks after the transplantation, as described previously with slight modification  (See Supporting Information).
Motor Function Analysis
Hind limb motor function was evaluated for 7 weeks after SCI using the locomotor rating test of the Basso Mouse Scale (BMS) . Well-trained investigators, blinded to the treatments, performed the behavioral analysis, determining the BMS scores at the same time each day. We also measured the motor function of each group (n = 5 each) on a rotating rod apparatus (Muromachikikai Co., Ltd., Japan), consisting of a plastic rod (3-cm diameter, 8-cm long) with a gritted surface, flanked by two large discs (40-cm diameter). At 7 weeks after SCI, mice from each group were tested by monitoring the time that each mouse spent on the rod as it rotated at 10 rpm during 2-minute sessions . Three trials were conducted, and the average and maximum numbers of seconds were recorded. Gait analysis was performed using the DigiGait Image Analysis System (Mouse Specifics, Quincy, MA, USA; http://www.mousespecifics.com). Data collection for DigiGait analysis was performed 7 weeks after injury, when each mouse demonstrated consistent weight-supported hind limb stepping on the treadmill at a speed of 8 cm/sec.
Animals were anesthetized and transcardially perfused with 4% paraformaldehyde in 0.1 M PBS, 6 weeks after transplantation. The spinal cords were removed, embedded in Optimal Cutting Temperature compound (Sakura Finetechnical Co., Ltd., Tokyo, Japan; http://www.sakuraus.com), and sectioned in the sagittal/axial plane at 12 μm on a cryostat (Leica CM3050 S, Leica Microsystems, Wetzlar, Germany; http://www.leica.com). Sections were stained with hematoxylin- eosin (HE), or Luxol Fast Blue (LFB), or processed for immunohistochemistry followed by quantitative analyses. Samples were examined on an inverted fluorescence microscope (BZ 9000; Keyence Co., Osaka, Japan; http://www.keyence.co.jp) or a confocal laser-scanning microscope (LSM 700, Carl Zeiss, Munchi, Germany; http://www.zeiss. com). We also performed immuno-electron microscopic analyses (See Supporting Information).
Electrophysiological experiments were performed immediately after injury, immediately after transplantation, and 7 weeks after injury. An electromyography (EMG)/evoked potential measuring system (Neuropack S1 MEB-9400 series, Nihon Kohden, Tokyo, Japan; http://www.nihonkohden.co.jp) was used. Mice were anesthetized with an i.p. injection of ketamine (40 mg/kg) and xylazine (4 mg/kg), as described in other paper . One electrode was injected into the spinal cord of the occipito-cervical area to induce motor-evoked potential (MEP) and the sciatic nerve to induce compound motor action potential (CMAP). For the recording of the potentials, two needle electrodes were placed in each hind limb. The active electrode was placed in the muscle belly of each limb, and the reference electrode was placed near the distal tendon of the muscle. The ground electrode was placed subcutaneously between the coil and the recording electrodes. To induce MEP, a stimulation of 0.4 mA intensity was applied at the electrode .
Throughout the experiments, the duration of the pulse was 0.2 ms. CMAPs were recorded by measuring the maximum amplitude that was achieved by stimulating the sciatic nerve with a single pulse of supramaximal intensity. The onset latency was measured as the length of time in milliseconds between the stimulus and the onset of the first wave. The amplitude (mV) was measured from the initiation point of the first wave to its highest point. Results were expressed as the MEP/CMAP ratio (%) so as to adjust for differences between individual mice [14, 36]. Ten responses were averaged and sorted for off-line analysis.
All data are presented as the mean ± SEM. An unpaired two-tailed Student's t test was used for determining significant differences between groups in the in vitro and in vivo differentiation assays, the bioimaging analysis, analyses of the Venus-positive areas, and MEP analyses. Analysis of variance (ANOVA) followed by the Tukey-Kramer test for multiple comparisons among the three transplantation groups was used for the 5-hydroxytryptamine(HT) analysis, Rota-rod and gait analysis. Repeated measures two-way ANOVA followed by the Tukey-Kramer test was used for analysis of the HE-, LFB-, and neurofilament-heavy chain (NF-H)-stained sections and BMS scores. For all statistical analyses, the significance was set at p < .05.
In Vitro Characterization of NS/PCs Derived from shiverer Mutant Mice Embryos
For the in vitro characterization of NS/PCs derived from shiverer mutant mice, proliferation and differentiation assays were performed. shi-NS/PCs and wt-NS/PCs were cultured as neurospheres, as previously reported [26, 37]. The shi-NS/PCs formed floating cell clusters (neurospheres) within 2–3 days (Supporting Information Fig. 1A), with a morphology identical to that of the wt-NS/PC-derived neurospheres (Supporting Information Fig. 1B). The proliferation rates of the shi-NS/PCs and wt-NS/PCs were evaluated indirectly by measuring the production of ATP. There was no significant difference in the doubling time between shi-NS/PCs and wt-NS/PCs (26.7 ± 1.3 hours vs. 26.1 ± 1.3 hours) (Supporting Information Fig. 1C).
An in vitro differentiation assay revealed that both shi-NS/PCs and wt-NS/PCs differentiated into neuron-specific class III beta tubulin (Tuj-1)+ neurons, glial fibrillary acidic protein (GFAP)+ astrocytes, and 2′3′-cyclic nucleotide 3′-phosphodiesterase (CNPase)+ oligodendrocytes (Fig. 1A). There were no significant differences in the proportions of the different cell types generated between the shi-NS/PCs and wt-NS/PCs (Tuj-1, 7.7 ± 1.1% vs. 8.1 ± 0.8%; GFAP, 65.4 ± 1.9% vs. 64.9 ± 1.8%; CNPase, 9.2 ± 1.3% vs. 8.7 ± 0.6%; Fig. 1B). Next, oligodendroglial differentiation was examined via the addition of platelet-derived growth factor (PDGF)-AA and ciliary neurotrophic factor (CNTF) to promote in vitro myelination . Although the wt-NS/PCs produced O1+, CNPase+, proteolipid protein (PLP)+, and MBP+ oligodendrocytes, the shi-NS/PCs produced oligodendrocytes positive for O1, CNPase, and PLP but negative for MBP (Fig. 1C, 1D).
Grafted shi-NS/PCs and wt-NS/PCs Survived Within the Injured Spinal Cord
To identify and monitor the transplanted cells in the injured spinal cord, both shi-NS/PCs and wt-NS/PCs were transduced to express ffLuc under the EF1α promoter by lentiviral infection. Because of the stable and strong emission of ffLuc, which is a fusion protein of Venus and luciferase, we could identify the grafted cells as fluorescent Venus signals and bioluminescent luciferase signals (C.M. Hara et al., manuscript submitted for publication). Prior to the transplantation, we confirmed that ffLuc was expressed in the shi-NS/PCs by fluorescence microscopy (Fig. 2A) and was detectable by the IVIS bioimaging system. We also confirmed the ffLuc transduction into wt-NS/PCs, as previously reported . Next, shi-NS/PCs or wt-NS/PCs were transplanted into the lesion site of NOD/SCID mice 9 days after contusive SCI.
The successful transplantation of shi-NS/PCs and wt-NS/PCs could be confirmed immediately by bioluminescence imaging (BLI); the average signal intensity was (1.7 ± 0.5) × 108 photons per mouse per sec in the transplanted mice. The BLI analysis revealed that the signal intensity of the grafted cells decreased sharply within the first week after transplantation but remained at approximately 20% of the initial signal intensity in both shi-NS/PCs and wt-NS/PC groups throughout the remaining period (Fig. 2C). After 6 weeks of transplantation, the survival of both grafted shi-NS/PCs and wt-NS/PCs in and around the lesion site was detected in tissue sections by anti-Venus immunostaining (Fig. 2D). There was no significant difference in the survival rate detected by BLI between the shi-NS/PCs and wt-NS/PCs groups (21.5 ± 1.5% vs. 18.3 ± 1.4%), which was also confirmed by quantitative histological analysis of the Venus+ area (0.021 ± 0.003 mm2 vs. 0.024 ± 0.004 mm2) 6 weeks after transplantation (Fig. 2E). Next, the migration of grafted cells was analyzed by comparing Venus+ areas in axial sections of the spinal cord for each group. No significant migratory differences were revealed in and around the lesion site (Fig. 2F, 2G).
Grafted shi-NS/PCs and wt-NS/PCs Exhibited Similar Differentiation Potential
To examine the differentiation potential of shi-NS/PCs grafted into the injured spinal cord, we performed immunohistochemical analyses 6 weeks after transplantation. The shi-NS/PCs differentiated into Hu+ neurons, GFAP+ astrocytes, and APC+ oligodendrocytes (Fig. 3A). A comparison between the cell types that differentiated from the shi-NS/PCs and wt-NS/PCs in vivo showed no significant differences (Hu+, 9.1 ± 0.5% vs. 9.2 ± 1.0%; GFAP+, 54.3 ± 2.0% vs. 56.7 ± 2.4%; APC+, 33.7 ± 1.7% vs. 30.0 ± 2.2% [Fig. 3B]), similar to our findings in vitro. Thus, the shi-NS/PCs and wt-NS/PCs gave rise to similar differentiated cell types in vivo as well as in vitro.
Transplanted shi-NS/PCs Prevented Atrophic Changes in the Injured Spinal Cord and Promoted Axonal Growth After SCI
To examine the effects of shi-NS/PC transplantation on the injured spinal cord, we first evaluated the amount of atrophy, by HE staining (Fig. 4A). The transverse area of the spinal cord at the lesion epicenter was significantly larger in both the shi-NS/PC and wt-NS/PC groups than in the control group, and there was no difference between the amount of enlargement in the experimental groups, suggesting that shi-NS/PCs and wt-NS/PCs provided equivalent protection from atrophy (Fig. 4B). Next, we examined the effects of transplanted shi-NS/PCs on axonal growth after SCI by immunohistochemical analyses, with anti-NF-H (RT97) and anti-5-hydroxytryptamine (5-HT) antibodies. Although a few NF-H+ axons were observed at the rim of the epicenter in the control PBS-injected group (Fig. 4C), there were significantly more NF-H+ neuronal fibers in the shi-NS/PC and wt-NS/PC groups, at the lesion epicenter and at sites rostral and caudal to it (Fig. 4D); the same effect was seen for 5-HT+ serotonergic fibers at the lumbar enlargement, which are critical for hind limb locomotor functions [38–40] (Fig. 4E, 4F).
Grafted wt-NS/PCs, but not shi-NS/PCs, Preserved and/or Enhanced Myelination Within the Injured Spinal Cord
In the control group, contusive SCI resulted in severe demyelination at the lesion site. Both the grafted shi-NS/PCs and wt-NS/PCs showed preserved and/or enhanced myelination after SCI, compared with controls. Notably, the wt-NS/PC group showed the largest LFB+ myelination area among the three groups (Fig. 5A, 5B, 5C). Consistent with this finding, Venus+/MBP+ double-labeled myelin sheaths were seen in the wt-NS/PC group but none were observed in the shi-NS/PC group (Fig. 5D, 5E, 5F). We confirmed these findings by immuno-electron microscopy (EM). In the shi-NS/PC group, there were a few axonal profiles showing poor myelination with Venus+ labeling, whereas in the wt-NS/PC group, profiles showing mature, Venus+ myelin sheaths were present. Immuno-EM examination under higher magnification revealed nanogold-labeled Venus+ spots in the outer and inner mesaxons of the myelin cytoplasm (Fig. 5G, 5H). These findings suggest that shi-NS/PC-derived oligodendrocytes made only thin, poorly formed myelin sheathes, but those derived from wt-NS/PCs were capable of forming mature sheathes on remyelinated axons.
Transplanted wt-NS/PCs, but Not shi-NS/PCs, Enhanced Significant Locomotor Functional Recovery After SCI
We assessed locomotor functional recovery using the BMS scoring of open field walking, the rota-rod treadmill test, and gait analysis. We used PBS-injected animals as a negative control, which exhibit little difference in BMS scores compared to another negative control animals with fibroblast transplantation .
Just after SCI (day 1), the animals exhibited complete hind limb paralysis (BMS score 0), followed by gradual recovery that plateaued at around 4 weeks, in the control group. Although there was no significant difference in the BMS scores among the control, shi-NS/PC, and wt-NS/PC groups at 2 weeks after SCI, mice of the wt-NS/PC group had significantly higher BMS scores than the control group mice on day 28 and thereafter. Conversely, the shi-NS/PC group showed a little functional recovery soon after transplantation but it was not statistically significant. By 6 weeks after transplantation, the BMS score of the wt-NS/PC group was significantly higher than that of the shi-NS/PC group (3.9 ± 0.2 vs. 3.0 ± 0.1; Fig. 6A). These data were consistent with the results of the rota-rod treadmill and gait analyses. In the rota-rod treadmill test, the wt-NS/PC-transplanted mice remained on the rod significantly longer than those of the control group, whereas the shi-NS/PC-transplanted mice fell off the rod with the similar timing to the control group (Fig. 6B). Gait characteristics were measured with the DigiGait Image Analysis System. Stride length was significantly greater in the wt-NS/PC group than in the control group; however, the shi-NS/PC group showed no significant difference compared with controls (Fig. 6C).
Finally, an MEP analysis was performed as an electrophysiological assessment to measure the MEP latency and amplitude and to examine signal conduction in the injured spinal cord. The latency of the MEP was measured from the onset of the stimulus to the first response to each wave. None of the mice demonstrated an MEP response immediately after SCI or immediately after NS/PC transplantation (9 days after SCI; Supporting Information Fig. 2). At 6 weeks after shi-NS/PC transplantation, we detected MEP response of which latency was significantly longer than that obtained after wt-NS/PC transplantation (15.7 ± 0.7 ms vs. 8.0 ± 0.7 ms). Furthermore, MEP amplitude of shi-NS/PC group was significantly smaller than that of wt-NS/PC group (MEP/CMAP 0.5 ± 0.1% vs. 0.8 ± 0.1%; Fig. 6F). By contrast, no MEP response was detected in the control group under our experimental conditions (Fig. 6D). These data support a critical role for signal conduction in the extent of hind limb locomotor functional recovery following NS/PC transplantation.
There is increasing interest in stem-cell transplantation for the diseased or injured CNS, including for SCI. Considering their clinical application, the precise mechanisms of action of many stem-cell populations still remain to be elucidated at the preclinical level. Theoretically, possible mechanisms of therapeutic effects of stem-cell transplantation in CNS diseases/injuries include: (a) cell replacement for lost cells after CNS damage (e.g., forming new oligodendrocytes and/or neurons) and (b) trophic support to increase survival of host neural cells and host-mediated regeneration, repair, and/or plasticity [3, 41]. While the latter mechanisms include short-term neurotrophic/neuroprotective effects, the graft-derived cells should differentiate into the appropriate cells and survive over the long-term to achieve functional recovery through the cell replacement.
Accordingly, to obtain long-term survival of graft-derived cells in the damaged host CNS and to investigate the role of graft-mediated remyelination in this study, we used a constitutively immunodeficient animal model, NOD/SCID mice, as hosts. NOD/SCID mice lack normal T-cell, B-cell, and complement responses [42, 43]. The use of NOD/SCID mice is known to result in a greater cell survival and engraftment for allograft and xenograft in comparison with conventional immunosuppressant treatment, that is, cyclosporin A or FK506, which targets T-cell activation/proliferation via disrupting calcineurin signaling. In most models using with conventional pharmacologic immunosuppression, there is a progressive loss of cells for more than 1–4 months. In the initial experiments for this study, we performed allografts using different mouse strains as grafts and recipients to investigate the effects of NS/PCs derived from myelin-deficient shiverer mutant versus wild-type mice. We maintain shiverer mutant mice with the ICR background as a homozygous closed colony. Because mice with the ICR background have been poorly characterized with respect to histopathology or behavioral assays after SCI, we selected C57BL/6J mice as the host for NS/PC transplants. However, when shi-NS/PCs with the ICR background were transplanted into C57BL/6J hosts without immunosuppression, their cell survival was much poorer than that of wt-NS/PCs derived from C57BL/6J mice. This was unexpected as the CNS is generally considered to be immune privileged. In fact, no surviving shi-NS/PCs were observed in the injured spinal cord of the C57BL/6J host at 5 weeks after transplantation, and no functional recovery was detected (Supporting Information Fig. 3). Thus, rather than using conventional pharmacologic immunosuppression, to minimize immunological barriers for long-term engraftment after SCI, we adopted NOD/SCID mice as hosts, which could be used as recipients even in the xenograft of human cells into SCI mice models [3, 41]. While NOD/SCID mice have neutrophils, macrophages, and microglia resulting in innate immune responses, we observed similar survival rate of shi-NS/PCs and wt-NS/PCs for all through 6 weeks and identical differentiation characteristics in the injured spinal cord. Moreover, we observed no evidence of progressive loss of either shi-derived or wt-derived cells.
Based on these grounds, by taking advantage of NOD/SCID mice as host, we investigated the role of graft-derived remyelination in functional recovery after NS/PC transplantation for SCI, which has been proposed as a possible mechanism [17, 44]. We found that the grafted NS/PCs were differentiated into oligodendrocytes and remyelinated spared axons in the injured spinal cord. Although some spontaneous remyelination by endogenous oligodendrocyte precursor cells (OPCs) is seen after SCI, the number of demyelinated axons increases in the chronic phase . These observations prompted studies in which myelinating Schwann cells [46, 47], olfactory ensheathing glia [48, 49], and OPCs [50, 51] were transplanted in the hope that remyelination of the remaining axonal pathways by these cells would induce symptomatic relief after SCI. In fact, these reports showed promising results in locomotor functional recovery and histological assessments, which included evidence of graft-derived remyelination. The connection between remyelination and functional recovery, however, was not directly addressed in these reports.
Here, by using the deficient myelination in shiverer mutant mice, we have obtained direct evidence that remyelination by graft-derived oligodendrocytes contributes greatly to the functional recovery after SCI. Because of their genetic lack of MBP, a major component of CNS myelin, shiverer mutant mice have been used in numerous studies as transplant hosts for engrafted wild-type donor cells, including ESCs [52, 53], Schwann cells [54, 55], NS/PCs [56, 57], glial precursors , OPCs , and iPSC-derived NS/PCs , all of which produced mature myelin that was unequivocally detected by immunohistochemistry. Furthermore, a previous study on chimeric shiverer/wild-type mice showed that the myelination defects caused by the shiverer mutation are cell-autonomous, and that the myelin sheaths elaborated by shiverer mutant oligodendrocytes do not harm wild-type sheaths . In this study, we used the shiverer mice not as hosts but as donors, to determine whether shiverer NS/PCs could promote functional recovery after SCI. To the best of our knowledge, there are no previous reports in which shiverer mouse cells have been used as grafts.
In this study, we first confirmed the similarity of shi-NS/PCs to wt-NS/PCs in vitro as well as in vivo. The shi-NS/PCs and wt-NS/PCs showed similar proliferation rates in vitro and survival rates within the injured spinal cord. Furthermore, the differentiation pattern of shi-NS/PCs was quite similar to that of wt-NS/PCs both in vitro and in vivo. A previous in vitro study demonstrated that OPCs derived from shiverer mice differentiate into mature oligodendrocytes . Consistent with this, we found that shi-NS/PCs differentiated into CNPase+ oligodendrocytes, indicating their capacity to generate a subset of myelin proteins. Furthermore, the shi-NS/PCs gave rise to O1 and PLP+ oligodendrocytes, which are thought to represent mature cells [62, 63].
A previous in vivo study also showed that the formation of a thin myelin sheath is common in the shiverer CNS . In shiverer mice, however, the CNS myelin is not uniformly compacted, and the axoglial junctions are irregular in shape, size, and distribution , suggesting that MBP plays a critical role in the compaction of CNS myelin. Consistent with their findings, our immuno-EM studies showed that shi-NS/PC-derived oligodendrocytes formed only thin and loose myelin sheaths, while wt-NS/PC-derived oligodendrocytes formed far more substantial myelin sheaths. At the chronic phase of SCI, some axons remain demyelinated and others have only thin myelin sheaths , which causes abnormal signal conduction . The thinly myelinated axons in the shi-NS/PC group could not conduct signals very well, resulting in an increased MEP latency and a smaller amplitude in the shi-NS/PC group compared with the wt-NS/PC group. Recent studies have established MEP as a sensitive technique for recording conduction in the CNS [14, 36, 68-70]. Eftekharpour et al.  demonstrated a significantly shortened latency in the spinal cord-evoked potential in adult shiverer mice after wild-type NS/PCs transplantation, suggesting that conduction differences could be owing to spinal cord myelination, all else being equal.
In this study, we did not detect any MEP response in the control group, although even the shi-NS/PC-grafted animals exhibited a low-level MEP response with the longer latency and smaller amplitude (Fig. 6D, 6E). The significant difference in the latency and amplitude between shi-NS/PC- and wt-NS/PC-grafted animals indicated that nerve conduction in the spinal cord was impaired in the mutant group, which could potentially be due to faulty remyelination by the myelin-deficient graft-derived oligodendrocytes. Conversely, the current findings suggest that the transplantation of NS/PCs, regardless of their capacity to generate myelin, improves nerve conduction enough to elicit at least a minimal signal. Furthermore, the remyelination by wt-NS/PCs-derived oligodendrocytes shortened the MEP latency compared to remyelination by myelin-deficient oligodendrocytes, putatively by enabling saltatory conduction.
The difference in the myelinating capacity of the wt- and shi-NS/PC-derived oligodendrocytes also affected the myelinated area of the injured spinal cord. The increase in the LFB+ myelinated area in the wt-NS/PC group may be attributable to remyelination as well as the prevention of demyelination. In fact, the transplantation of shi-NS/PCs also enlarged the LFB+ area, primarily by preventing demyelination, and resulted in the functional improvement observed in the early time points after transplantation in the shi-NS/PC group, as in the wt-NS/PC group.
Some studies have reported that the application of growth factors prevents demyelination after SCI [71, 72]. Functional recovery was observed soon after the treatment in those studies and this one, suggesting that trophic factors cause functional recovery early in the post-transplantation period. In fact, we confirmed that the shi-NS/PCs expressed mRNAs for many trophic factors, at levels that were almost identical to those of wt-NS/PCs (Supporting Information Fig. 4). Among these, the mRNAs encoding CNTF, growth and differentiation factor (GDF11), PDGF A (PDGFA), and vascular endothelial growth factor B showed relatively high expression levels. Other reports that discuss cell transplantation approaches for the treatment of SCI have emphasized the significance of trophic factors secreted by the grafted cells . It is possible that trophic support provided by the grafts prevents demyelination and promotes functional recovery, although our finding that the growth factor mRNAs levels were almost the same in wt- and shi-neurospheres suggests that they probably have only a partial effect, if any. Although other studies have suggested that graft-derived neurons [33, 74] and astrocytes  are important for functional recovery, similar percentages of the various cell types were generated by the wt- and shi-NS/PCs in this study.
We demonstrated that grafted NS/PCs-derived remyelination contributed to functional recovery after SCI. As myelination is a predominant contribution of grafted NS/PCs, NS/PCs can clearly target the myelination/demyelination issue and thus NS/PCs indeed hold promise for SCI treatment. Here, we elucidated one of the mechanisms that contribute to the efficacy of transplanting NS/PCs for SCI. Further comparisons of NS/PC and OPC transplantations will be needed before they can be used clinically. While not required by the Food and Drug Administration, we and others in the field believe that unraveling the regenerative mechanisms piece by piece is the quickest way to develop clinical approaches for the repair of SCI.
We thank Drs. Kunihiko Suzuki, Kinuko Suzuki, and Aileen Anderson for their critical comments on the manuscript; Tokuko Harada for tender animal care; Sachiyo Miyao and Hiroko Shimada for technical support; and Dr. Hiroko Baba (Tokyo University of Pharmacy and Life Science) for the kind gift of the shiverer mice. This work was supported by grants from the Program for the Promotion of Fundamental Studies in Health Sciences of the National Institute of Biomedical Innovation (NIBIO); a grant from the Uehara Memorial Foundation; Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT); the Project for the Realization of Regenerative Medicine and support for the core institutes for iPSC research from MEXT; the “Funding Program for World-leading Innovative R&D on Science and Technology” to H.O. from the Ministry of Health, Labor, and Welfare; the General Insurance Association of Japan; Research Fellowships for Young Scientists from the Japan Society for the Promotion of Science; Keio Gijuku Academic Development Funds; and a Grant-in-aid for the Global COE program from MEXT to Keio University.
DISCLOSURE OF POTENTIAL CONFLICTS OF INTEREST
The authors indicate no potential conflicts of interest.