Repair of Spinal Cord Injury ; Mesenchymal Stem Cells as an Alternative for Schwann Cells

Spinal cord injury (SCI) is one of the most severe types of disabilities, which due to its negative impacts on motor control abilities, has a major impact on patient’s life and causes the person to face a wide range of constraints.1,2 The statistics show that the incidence rates of developed countries ranged from 13 to 163 per million people while the rates of non-developed countries varied from 13 to 220 per million people. In addition, the prevalence of SCI is 490 to 526 per million population among developed countries and for nondeveloped countries, reported prevalence is about 440.3 On the other hand, the cost of caring for a SCI is also huge. For example, the annual cost of spinal cord injuries is estimated to be more than $9 billion in America and a tetraplegia patient will cost over 900 000 in the first year and $ 170 000 over the next years.4 In Iran, there are more than 2000 SCI veterans due to Iraq-Iran war and more than 15 000 cases of spinal cord injuries have been diagnosed until today. According to the studies, the most common causes of spinal cord injuries include injuries during driving and sports, violent acts and war injuries, falling from heights, some diseases, and medical incidents. Complications due to spinal cord injuries include loss of complete or partial movement ability (such as walking) and sensation in the affected area and the more upper body is engaged, the more will be the complications.5,6 However, injured spinal cord axons have the ability to restore themselves, but the maximum axon growth is about one millimeter, although this restoration process is not happening at a satisfactory level. On the other hand, the glial scars formation and the lack of growth stimulating symptoms such as semaphorin and ephrin and on the contrary the existence of inhibition signals like myelin sheath components (myelinassociated glycoprotein, Nogo-A) are regarded as inhibitory factors of axon growth.7 Therefore, considering the extent and intensity of the disabilities caused by spinal cord injuries and their increased incidence, numerous efforts have been made in order to elevate the effects of mentioned damages. The effective treatment of the injury depends on the type and severity of the damage, although the high costs of treatment can affect the process of healing.8,9 The use of cell therapy is one of the solutions that have been widely considered in the last 2 decades due to the high growth rate of regenerative medicine based on tissue engineering and stem cell.10,11


Introduction
Spinal cord injury (SCI) is one of the most severe types of disabilities, which due to its negative impacts on motor control abilities, has a major impact on patient's life and causes the person to face a wide range of constraints. 1,2The statistics show that the incidence rates of developed countries ranged from 13 to 163 per million people while the rates of non-developed countries varied from 13 to 220 per million people.In addition, the prevalence of SCI is 490 to 526 per million population among developed countries and for nondeveloped countries, reported prevalence is about 440. 3 On the other hand, the cost of caring for a SCI is also huge.For example, the annual cost of spinal cord injuries is estimated to be more than $9 billion in America and a tetraplegia patient will cost over 900 000 in the first year and $ 170 000 over the next years. 4In Iran, there are more than 2000 SCI veterans due to Iraq-Iran war and more than 15 000 cases of spinal cord injuries have been diagnosed until today.According to the studies, the most common causes of spinal cord injuries include injuries during driving and sports, violent acts and war injuries, falling from heights, some diseases, and medical incidents.Complications due to spinal cord injuries include loss of complete or partial movement ability (such as walking) and sensation in the affected area and the more upper body is engaged, the more will be the complications. 5,6owever, injured spinal cord axons have the ability to restore themselves, but the maximum axon growth is about one millimeter, although this restoration process is not happening at a satisfactory level.On the other hand, the glial scars formation and the lack of growth stimulating symptoms such as semaphorin and ephrin and on the contrary the existence of inhibition signals like myelin sheath components (myelinassociated glycoprotein, Nogo-A) are regarded as inhibitory factors of axon growth. 7Therefore, considering the extent and intensity of the disabilities caused by spinal cord injuries and their increased incidence, numerous efforts have been made in order to elevate the effects of mentioned damages.The effective treatment of the injury depends on the type and severity of the damage, although the high costs of treatment can affect the process of healing. 8,9The use of cell therapy is one of the solutions that have been widely considered in the last 2 decades due to the high growth rate of regenerative medicine based on tissue engineering and stem cell. 10,11or treatment of SCI which includes embryonic tissue transplantation, use of Schwann mature cells, and embryonic, bone marrow, nervous stem cells, and olfactory ensheathing cells (OECs). 11,12In some cases gene therapy and insertion of genes such as NT-3, BDNF, NGF, which play an important role in restoring spinal cord injuries, into stem cells or mature cells such as fibroblasts is also used. 13,14Considering the progress in stem cells and restorative medicine, stem cell therapy compared to the use of mature cells is one of the most important and remarkable therapies for SCI or other injuries and diseases because of long-term self-renewal ability of stem cell and the possibility of genetic manipulation. 15,16However, access to an appropriate stem cell source is one of the most limitations and their differentiation into target cells is one of the key steps in using these cells for treatment purposes.
In general, the applied cell therapy techniques in order to repair SCI can be summarized in the following cases 17,18  In 1999, for the first time, German researchers by employing embryonic stem cells (ESCs) derived from glial precursors showed that a protective myelin sheath could be formed in the rat-damaged spinal cord. 19In addition, other studies showed that these cells could improve the rat's SCI at a very limited level.In 2000, for the first time, olfactory stem cells were used for repairing SCI in rats. 202][23][24][25][26] Here is a variety of studies due to the complexity of the spinal cord tissue and the lack of complete recognition of the effective repair process at the clinical level.However, the use of SCs is one of the options that has been considered in recent years for the treatment and repair of spinal cord injuries due to the considerable potential of these cells including [27][28][29] : • Produce growth factors, which stimulate some nerve fiber (axon) regeneration • Produce components of the extracellular matrix, which supports regenerating axons • Surround and re-insulate (re-myelinate) axons that lost their insulation after injury • Restore axonal communication upon re-myelination • Spontaneously enter the spinal cord after SCI Although, there are limitations for transplanted SCs such as (1) need to provide a neurotransmitter for extraction and transplantation of SCs, (2) the possibility of secondary damage during the isolation and receipt of primary SCs, (3) possibility of immunogenic responses, and (4) the need for long-term cell cultivation and proliferation processes.Accordingly, the use of stem cells to differentiate into SCs can reduce the risks associated with the use of mature cells in the grafting process.
Stem cells can be characterized based on their differentiation potential including totipotent, pluripotent, unipotent, and adult stem cells.Totipotent stem cells can form an entire embryo including the extraembryonic tissues.Pluripotent stem cells can trigger the three embryonic germ layers: mesoderm, endoderm, and ectoderm.Unipotent or progenitor stem cells can only differentiate into one defined cell type and adult stem cells are capable of multi-lineage differentiation in cells of only one germ layer. 30,31The differentiation potential of stem cells is related to their developmental stage so that the potential of differentiation decreases from an ESC to a specialized tissue stem cell.][34] So far, many studies have been done on the use of stem cells to repair spinal cord injuries by differentiating stem cells into SC.In these studies, various types of stem cells have been evaluated based on development stage including tissue source and iPSC.Table 1 presents different sources of stem cells that have been evaluated for SCI. 18

Mesenchymal Stem Cells as an Alternative for Schwann cell Transplantation
Bone marrow and adipose-derived stem cells (ADSCs) as mesenchymal stem cells (MSCs), which also derived from peripheral blood, placenta and umbilical cord, the lung, and the heart, are multipotent stromal cells that can differentiate to various type cells such as osteoblasts, chondrocytes, and adipocytes.They are as easily accessed source with high growth rate, low immunogenicity and a favorable ethical profile and better safety that can differentiate to all mesodermal lineage cells, therefore these properties make them an interesting source for cell therapy. 35,36MSCs can be transdifferentiated into SC-like cells in neuronal induction media. 37,38On the other hand, studies have shown that SCs that differentiated from MSCs enhance and support neurite outgrowth, axonal surviving and remyelination. 39In many studies, MSCs is considered as a brilliant cell for the treatment of central nervous system. 40,41In addition, transplantation of SCs derived from mesenchymal stem cells as a potentially useful treatment for SCI is also confirmed. 42,43Studies have shown that MSCs can produce various growth factors, neuroprotective cytokines and chemokines (Figure 1) such as vascular endothelial growth factor (VEGF), hepatocyte growth factor (HGF), fibroblast growth factor (FGF), nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF), which enhances functional benefits associated with MSC transplantation. 42,44Accordingly, MSCs are an efficient source of HGF suggested that the therapeutic effects of MSC transplantation are partly mediated by HGF secretion.This factor blocked secretion of transforming growth factor-β (TGF-β) from activated astrocyte cells and prevented expression of specific chondroitin sulfate proteoglycan (CSPG) species.Studies demonstrated that transplantation of HGF-overexpressing MSCs significantly decreased expression of neurocan and glycosaminoglycan chain deposition around hemisection lesions in the spinal cord. 42,45,46Also in animal models, HGF-MSCs showed an increase in axonal growth and improvement in functional recovery, which confirms that HGF can act as an attractive signal for the guidance of axon motor to the target tissue. 45In addition to growth factors, immunological cytokines are also involved in the process of stem cell therapy after SCI.According to the studies, transplantation of MSCs into a lesion spinal cord leads to a reduction in Tumor necrosis factor alpha (TNFα), interleukin 1 beta (IL-1β, IL-2), IL-4, IL-6, and IL-12 secretion. 47,48n addition, implantation of MSCs inhibits second-phase neuronal injury by suppressing lymphocyte and microglia effects and reduces the inflammatory reaction in the local environment after SCI. 49These results confirm that MSC administration can help to neuronal survival after lesion through cytokine release and immunomodulation.It has also been demonstrated that apoptosis-related pathways involved in SCI is affected after MSC transplantation.Accordingly, findings show that caspase-3-mediated apoptosis on neuron and oligodendrocyte cells following SCI is significantly downregulated by MSCs, which is regulated through stimulation of endogenous survival signaling pathways including PI3K/Akt, and the MAPK/ERK1/2-cascade. 50,51 Considering the contents mentioned above, it can be explained that mesenchymal cells are well suited for cell therapy of SCI.In this regard, many experiments in small and big SCI animal models have demonstrated the beneficial effects of MSCs from different sources. 52,53In addition, various studies are currently underway at various clinical stages using    Yat-Sen University, Guangzhou, Guangdong, China * FAB117-HC is a medicinal product containing human allogeneic adipose derived adult mesenchymal stem cells expanded and pulsed with H2O2 and also HC016 cells.
MSCs that their results are promising for the treatment of spinal cord injuries.Table 2 presents some clinical studies that have been performed using MCSs.

Conclusions
The main goals of stem cell-based therapies for SCI are the neuron replacement and restoration of neurological, structural, and functional of the spinal cord after injury.This type of therapy is regarded as promising methods because of their effectiveness in the treatment of SCI.However, determination of an effective and specific type of stem cell (due to the existence of different types of stem cells) for cell replacement therapy in patients, which can be used as a renewable source, is one of the key steps in this process.Moreover, some issues such as their effectiveness, ethical considerations and being as a safe option in such therapies is still a challenge and must be considered.However with regard to the potential of MSCs, it seems that these cells can be a significant option for the treatment of spinal cord injury using cell therapy.

Table 1 .
18mparison of Different Sources of Stem Cells Used for Peripheral Nerve Regeneration18