Spinal cord injury (SCI) refers to injuries to the spinal cord that resulted from trauma rather than disease. Symptoms of SCI can range from pain to paralysis to incontinence. SCI results in the loss of neuronal tissue which could lead to additional tissue loss, ultimately impairing sensory and motor functions. Secondary tissue damage that follows SCI involves an inflammatory cascade that includes the infiltration of neutrophils and macrophages into the site of lesion, in concert with resident microglia, stimulation of glial cells and increased expression of pro-inflammatory cytokines (1, 2). These inflammatory responses create a hostile environment for axonal regrowth (3) and therefore, often lead to the formation of a cyst at the site of injury, promoting neurological dysfunction (4).
Research using a mouse model of contusive injury has shown that pathology to white matter caused by the injury determines the extent of functional recovery (5). In particular, SCI is associated with a chronic and progressive loss of myelination of spared axons (6), which is caused by apoptosis of oligodendrocytes. Previous studies have attempted to obtain functional recovery through the antagonism of growth inhibitors, application of growth factors, cell transplantation, and vaccination strategies (7). But none of these approaches have provided full recovery in either animal models or humans. Thus, there is currently no therapeutic treatment for full functional recovery after SCI.
Stem cell therapy has the potential to provide an alternative source of cells that could ameliorate the symptoms associated with spinal cord injury, either directly or indirectly through secondary mechanisms, such as reducing inflammation that prevents the growth of spared axons after contusion. Stem cells, either embryonic or adult (from human bone marrow, neural progenitor population, adipose-derived cells, etc.) can be transplanted into the site of injury, where they can proliferate or recruit other cells such as glial cells to mediate the damage. The following figure depicts some of the options available for cell transplantation therapies for SCI.
Figure 1: Potential sources of stem cells for treatment of SCI (Thuret et al., 2006)
Figure 1: Potential sources of stem cells for treatment of SCI (Thuret et al., 2006)





1.1 Human embryonic stem cells used in rat models of SCI


Previous studies have implicated both mouse and human embryonic stem cells (ESCs), which are derived from the inner cell mass of an embryo, as having therapeutic effects on animal models of spinal cord injury (8, 9). In particular, these studies have shown that neural precursor populations from ESCs, such as oligodendrocyte progenitor cells (OPCs) enable partial recovery of SCI by protecting host neurons and promoting remyelination of damaged cells in rat models of SCI, leading to recovery of motor skills (cite 8, 9). However, these studies showed recovery when the stem cell transplantation took place within 7 days of injury and the same effects, but not replicated when the animals were treated 10 months after injury (9). This suggested that stem cell therapy in humans would be most efficient if done at an early time point after injury.
In a study by Kerr et al. (6), human ESCs have been shown to differentiate efficiently into oligodendrocyte cells, which corresponded with increased neurological responses. In particular, the study used a rat model of SCI, which was transplanted with OPCs derived from human ESCs at the two critical time-points when extensive damage to surrounding tissue takes place, that is, 3 and 24 hours post-injury. The transplanted stem cells were analyzed for migration and survivability after 8 days, post-mortem. In-vitro immunoflourescence studies detected oligodendrocyte markers within the transplanted cells, suggesting that efficient differentiation had taken place (See figure below). Furthermore, OPCs were found to survive at the site of injury and migrate outwards from the site of injection after one week. Histological analyses also confirmed that ESC-derived OPCs integrated with host cells in the spinal cord without disrupting the parenchyma, which is an important consideration in these transplants. In addition, tumour or cyst formation was not observed, which is an important consideration in the transplantation of undifferentiated stem cells. Finally, behavioural and electrophysiological assessments revealed enhanced neurological responses in contused rats that received the cell transplant versus controls.
Indirect immunofluorescent expression of markers of early neural progenitors and the oligodendroglial lineage in human embryonic stem cell derived OPCs after two weeks in PDGF- and EGF-supplemented N2B27 media. Markers of early NPCs included Nestin, Olig1, A2B5, and Sox10. Markers of early OPCs included O1, O4, and PDGFR-α. Later OPC markers included NG2, CNPase, GalC, and mature oligodendrocytes, MBP. DAPI (blue) was used to stain nuclei. (Kerr et al, 2010)
Indirect immunofluorescent expression of markers of early neural progenitors and the oligodendroglial lineage in human embryonic stem cell derived OPCs after two weeks in PDGF- and EGF-supplemented N2B27 media. Markers of early NPCs included Nestin, Olig1, A2B5, and Sox10. Markers of early OPCs included O1, O4, and PDGFR-α. Later OPC markers included NG2, CNPase, GalC, and mature oligodendrocytes, MBP. DAPI (blue) was used to stain nuclei. (Kerr et al, 2010)

1.2 Neural precursor cells reduce secondary tissue damage


Various studies have shown that transplanted neural stem/precursor cell populations impart therapeutic benefits via multiple neuroprotective and immune modulatory mechanisms rather than simply cell replacement (10, 11, 12). In a study by Cusimano et al. (7) to characterize the molecular and cellular mechanisms responsible for such therapeutic plasticity, syngeneic neural stem/precursor cells were injected in a severely contused mouse spinal cord. Subsequently, the researchers studied the motor functions and secondary pathology in the animals, the fate of the transplanted cells, and level of inflammation at the site of injury. Neural stem/precursor cell injections were made at two time-points, which were subacute (7 days after injury) or early chronic (21 days after injury). Benefits to motor function were observed in the mice treated sub-acutely only. In terms of cell fate, the transplanted cells survived in an undifferentiated state around the lesion and communicated with host phagocytes via cellular-junctional coupling. These interactions correlated with enhanced expression of significant inflammatory cell transcripts. In particular, the transplanted cells reduced the ratio of ‘classically-activated’ M1-like macrophages, which promoted healing of the injury. Therefore, in this study, the results suggest that neural stem/precursor cells can alter the local inflammatory cell microenvironment into a less hostile one, promoting the healing of the injury and regeneration past the lesion.
Figure 3: Promotion of healing in the severely injured spinal cord. (A–D) Stereological quantification of volumes of GFAP+ tissue injury (A; red solid), Luxol fast blue (LFB)− demyelination (B; grey solid) and Iba1+, at the level of the injured (grey solid in C) or spared (grey solid in D) cord tissue. In the 3D renderings, the solid orange in A–D is the central canal, while the solid transparent blue in B–D is the injury volume. Shown in A–D are also representative axial images of the stainings for stereological quantifications. Images have been taken at either 600 µm above the lesion (A, B and D) or at the lesion epicentre (C), as indicated by the dashed lines. Volumes in A–D have been calculated at 56 days after the injury. Data are minimum to maximum volumes from n ≥ 3 mice per group. *P ≤ 0.05, versus PBS-treated controls. dpi = days post-injury; GFAP = glial fibrillary acidic protein. (Cusimano et al., 2012)
Figure 3: Promotion of healing in the severely injured spinal cord. (A–D) Stereological quantification of volumes of GFAP+ tissue injury (A; red solid), Luxol fast blue (LFB)− demyelination (B; grey solid) and Iba1+, at the level of the injured (grey solid in C) or spared (grey solid in D) cord tissue. In the 3D renderings, the solid orange in A–D is the central canal, while the solid transparent blue in B–D is the injury volume. Shown in A–D are also representative axial images of the stainings for stereological quantifications. Images have been taken at either 600 µm above the lesion (A, B and D) or at the lesion epicentre (C), as indicated by the dashed lines. Volumes in A–D have been calculated at 56 days after the injury. Data are minimum to maximum volumes from n ≥ 3 mice per group. *P ≤ 0.05, versus PBS-treated controls. dpi = days post-injury; GFAP = glial fibrillary acidic protein. (Cusimano et al., 2012)


1.3 Olfactory ensheathing cell transplantation


Olfactory ensheathing cells (OECs) are particular glial cells found only in the olfactory system that have special properties which make them conducive to use for cell-mediated repair following SCI (13). In particular, OECs, which play an important role in olfactory neuron turnover, retain exceptional plasticity and promote olfactory neurogenesis. OECs have been used in various models of rodent SCI and have shown varying degrees of effectiveness at functional recovery. OECs promote tissue sparing and neuro-protection, stimulate outgrowth of damaged axons and enhance angiogenesis and remyelination of damaged axons, among other beneficial effects that they impart.
In particular, a clinical study by Huang et al. tested the efficacy of intraspinal OEC transplantations in SCI patients of varying ages (14). After the transplantation surgery, various behavioural tests were used to show motor improvement, such as light touch and pin prick tests. The study concluded that OEC transplantation improved neurological function in patients with SCI over varying age groups.

1.4 Bone marrow derived MSCs promotes functional recovery


Adult bone marrow derived mesenchymal stem cells (BM-MSC) have been shown to enhance anatomical and functional recovery in SCI animal models by promoting tissue sparing and axonal regeneration (15, 16). It is thought that BM-MSCs impart their therapeutic effects through the secretion of soluble factors and providing an extracellular matrix that enhances neuroprotection. Additionally, these stem cells play a role in remyelination and neural differentiation (17) (18, 19).
A recent study by Nakajima et al. used BM-MSCs to investigate the mechanism via which these cells mediate the hostile inflammatory responses created after SCI (20). The group used a rat model of SCI and intraspinally injected the animals with BM-MSCs, which were then showed to migrate within the spinal cord without differentiating into glial or neuronal cell types. The transplanted cells resulted in changed ratios of interleukins and cytokines, as well as increased numbers of alternatively activated macrophages and decreased numbers of classically activated macrophages, all of which corresponded with preservation of damaged axons, reduced scar tissue formation and increased sparing of myelination. These changes lead to functional motor recovery in the transplanted group compared to control. Therefore, Nakajima et al. concluded that acute transplantation of MSCs causes a change in the inflammatory environment at the site of lesion, which ultimately provides a permissive environment for locomotor recovery in the subacute or chronic phase after SCI.


A video of a patient who was treated with bone marrow stem cells and her story of recovering function of her core. She regained her ability to walk with the use of a walker.

1.5 Therapeutic potential of induced pluripotent stem cells in SCI
Although neural stem/progenitor cells derived from human embryonic stem cells are a promising source for cell replacement therapy in SCI models, the use of ES cells is associated with various ethical and immunological concerns, which may be overcome by using induced pluripotent stem cells (iPSCs) (21). These are created by taking somatic cells and introducing the expression of four particular transcription factors (Oct4, Sox2, Klf4 and Nanog) to induce pluripotency.
A recent study by Tsuji et al. investigated the therapeutic potential of murine iPSCs differentiated into neurons in a mouse model of SCI. iPS-derived neurospheres were first evaluated as non-tumouigenic by transplanting them into NOD/SCID mice, to ensure the production of functional neurons, astrocytes and oligodendrocytes (21). When these iPS-derived neurospheres were transplanted into the spinal cord within 9 days after injury, they were shown to differentiate into these three lineages without teratoma formation. In addition, they played a role in remyelination of damaged axons, and induced outgrowth of axons in host serotonergic fibres, enhancing motor function recovery. The findings of this study suggest that iPS-derived neurospheres may be a promising source of stem cells for transplantation therapy in cases of SCI. However, these cells must be ascertained to be ‘safe’ prior to the initiation of clinical applications. In particular, their differentiation potentials and the possibility for tumourogenicity in the context of the host cells, must be investigated to establish their safety and efficacy as transplantation therapies.

Conclusions – current state of research and future prospects


Transplantation of various types of stem cells has been shown to improve locomotion functions by enhancing spontaneous recovery in animal models of spinal cord injury (22). Although the mechanism behind this recovery are yet to be understood fully, many possible explanations have been suggested such as remyelination, recruitment of host glial cells and reduction of inflammatory damage. Moreover, which type of stem cell has the most potential in SCI cell replacement therapy has not been researched. Some of the important considerations in this area of research include the possibility of tumour formation following cell transplantation and the longevity of the cells post-transplantation. The following table summarizes the results of the various clinical trials that have been conducted since 2006 in patients with SCI.

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Results of clinical trials using stem cell therapy in patients with spinal cord injury (Sahni & Kesler, 2006)




External Links

- Stem Cell Network
- National Institute of Neurological Disorders and Stroke

References
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