Traumatic spinal cord injury (SCI) has devastating consequences for the physical, social and vocational well-being of patients. The demographic of SCIs is shifting such that an increasing proportion of older individuals are being affected. Pathophysiologically, the initial mechanical trauma (the primary injury) permeabilizes neurons and glia and initiates a secondary injury cascade that leads to progressive cell death and spinal cord damage over the subsequent weeks. Over time, the lesion remodels and is composed of cystic cavitations and a glial scar, both of which potently inhibit regeneration. Several animal models and complementary behavioural tests of SCI have been developed to mimic this pathological process and form the basis for the development of preclinical and translational neuroprotective and neuroregenerative strategies. Diagnosis requires a thorough patient history, standardized neurological physical examination and radiographic imaging of the spinal cord. Following diagnosis, several interventions need to be rapidly applied, including haemodynamic monitoring in the intensive care unit, early surgical decompression, blood pressure augmentation and, potentially, the administration of methylprednisolone. Managing the complications of SCI, such as bowel and bladder dysfunction, the formation of pressure sores and infections, is key to address all facets of the patient's injury experience.

Once their safety is confirmed, human-induced pluripotent stem cells (hiPSCs), which do not entail ethical concerns, may become a preferred cell source for regenerative medicine. Here, we investigated the therapeutic potential of transplanting hiPSC-derived neurospheres (hiPSC-NSs) into nonobese diabetic (NOD)-severe combined immunodeficient (SCID) mice to treat spinal cord injury (SCI). For this, we used a hiPSC clone (201B7), established by transducing four reprogramming factors (Oct3/4, Sox2, Klf4, and cMyc) into adult human fibroblasts. Grafted hiPSC-NSs survived, migrated, and differentiated into the three major neural lineages (neurons, astrocytes, and oligodendrocytes) within the injured spinal cord. They showed both cell-autonomous and noncellautonomous (trophic) effects, including synapse formation between hiPSC-NS-derived neurons and host mouse neurons, expression of neurotrophic factors, angiogenesis, axonal regrowth, and increased amounts of myelin in the injured area. These positive effects resulted in significantly better functional recovery compared with vehicle-treated control animals, and the recovery persisted through the end of the observation period, 112 d post-SCI. No tumor formation was observed in the hiPSC-NS-grafted mice. These findings suggest that hiPSCs give rise to neural stem/progenitor cells that support improved function post-SCI and are a promising cell source for its treatment.stem-cell-based medicine | cell transplantation | neurotrauma | synaptic connection S tem-cell-based approaches, such as the transplantation of neural stem/progenitor cells (NS/PCs), are promising sources of therapies for various central nervous system disorders (1-3). Previous studies reported functional recovery after transplantation of NS/PCs into the injured spinal cord of rodents and nonhuman primates (4-9). Furthermore, recent studies revealed that embryonic stem cells (ESCs) can generate neural cells including NS/PCs (10-12) and oligodendrocyte precursor cells (OPCs) (13,14). Therefore, human ESC-based therapies are moving out of the laboratory and into clinical treatments for spinal cord injury (SCI) (12,13,15). However, the use of human ESC-based therapies is complicated by ethical concerns in certain countries. To avoid the problems associated with ESCs, we previously established induced pluripotent stem cells (iPSCs) from mouse fibroblasts (16, 17) and confirmed the therapeutic potential of iPSC-derived neurospheres (iPSC-NSs) for treating SCI in animal models (18).Here, aiming at human iPSC-based therapies for SCI patients, we examined the therapeutic potential of human iPSC-NSs by transplanting them into nonobese diabetic severe combined immunodeficient (NOD-SCID) SCI model mice. We used a clone from human iPSCs (hiPSCs) that we established from adult human dermal fibroblasts by the retroviral transduction of four reprogramming factors; for the clone used in this study, 201B7, the factors were Oct3/4, Sox2, Klf4, and c-Myc (19). These grafted hiPSC-NSs survived, migrated, and differentiated in...

We conclude with our perspectives on the future of treatment and research in this rapidly evolving field.

Ever since pioneering reports introduced mouse 1 and humaninduced 2-4 pluripotent stem cells (iPSCs) to the scientific community and the populace at large, there has been an increasing interest in applications for their use in the fields of biomedical research. These include cell therapy in regenerative medicine and modeling of human disease.

Murine and human iPSC-NS/PCs (induced pluripotent stem cell-derived neural stem/progenitor cells) promote functional recovery following transplantation into the injured spinal cord in rodents. However, for clinical applicability, it is critical to obtain proof of the concept regarding the efficacy of grafted human iPSC-NS/PCs (hiPSC-NS/PCs) for the repair of spinal cord injury (SCI) in a non-human primate model. This study used a pre-evaluated “safe” hiPSC-NS/PC clone and an adult common marmoset (Callithrix jacchus) model of contusive SCI. SCI was induced at the fifth cervical level (C5), followed by transplantation of hiPSC-NS/PCs at 9 days after injury. Behavioral analyses were performed from the time of the initial injury until 12 weeks after SCI. Grafted hiPSC-NS/PCs survived and differentiated into all three neural lineages. Furthermore, transplantation of hiPSC-NS/PCs enhanced axonal sparing/regrowth and angiogenesis, and prevented the demyelination after SCI compared with that in vehicle control animals. Notably, no tumor formation occurred for at least 12 weeks after transplantation. Quantitative RT-PCR showed that mRNA expression levels of human neurotrophic factors were significantly higher in cultured hiPSC-NS/PCs than in human dermal fibroblasts (hDFs). Finally, behavioral tests showed that hiPSC-NS/PCs promoted functional recovery after SCI in the common marmoset. Taken together, these results indicate that pre-evaluated safe hiPSC-NS/PCs are a potential source of cells for the treatment of SCI in the clinic.

SummaryPreviously, we described the safety and therapeutic potential of neurospheres (NSs) derived from a human induced pluripotent stem cell (iPSC) clone, 201B7, in a spinal cord injury (SCI) mouse model. However, several safety issues concerning iPSC-based cell therapy remain unresolved. Here, we investigated another iPSC clone, 253G1, that we established by transducing OCT4, SOX2, and KLF4 into adult human dermal fibroblasts collected from the same donor who provided the 201B7 clone. The grafted 253G1-NSs survived, differentiated into three neural lineages, and promoted functional recovery accompanied by stimulated synapse formation 47 days after transplantation. However, long-term observation (for up to 103 days) revealed deteriorated motor function accompanied by tumor formation. The tumors consisted of Nestin+ undifferentiated neural cells and exhibited activation of the OCT4 transgene. Transcriptome analysis revealed that a heightened mesenchymal transition may have contributed to the progression of tumors derived from grafted cells.

BackgroundThe transplantation of neural stem/progenitor cells (NS/PCs) at the sub-acute phase of spinal cord injury, but not at the chronic phase, can promote functional recovery. However, the reasons for this difference and whether it involves the survival and/or fate of grafted cells under these two conditions remain unclear. To address this question, NS/PC transplantation was performed after contusive spinal cord injury in adult mice at the sub-acute and chronic phases.ResultsQuantitative analyses using bio-imaging, which can noninvasively detect surviving grafted cells in living animals, revealed no significant difference in the survival rate of grafted cells between the sub-acute and chronic transplantation groups. Additionally, immunohistology revealed no significant difference in the differentiation phenotypes of grafted cells between the two groups. Microarray analysis revealed no significant differences in the expression of genes encoding inflammatory cytokines or growth factors, which affect the survival and/or fate of grafted cells, in the injured spinal cord between the sub-acute and chronic phases. By contrast, the distribution of chronically grafted NS/PCs was restricted compared to NS/PCs grafted at the sub-acute phase because a more prominent glial scar located around the lesion epicenter enclosed the grafted cells. Furthermore, microarray and histological analysis revealed that the infiltration of macrophages, especially M2 macrophages, which have anti-inflammatory role, was significantly higher at the sub-acute phase than the chronic phase. Ultimately, NS/PCs that were transplanted in the sub-acute phase, but not the chronic phase, promoted functional recovery compared with the vehicle control group.ConclusionsThe extent of glial scar formation and the characteristics of inflammation is the most remarkable difference in the injured spinal cord microenvironment between the sub-acute and chronic phases. To achieve functional recovery by NS/PC transplantation in cases at the chronic phase, modification of the microenvironment of the injured spinal cord focusing on glial scar formation and inflammatory phenotype should be considered.

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.