Spinal cord injury (SCI) is caused by trauma that can fracture or dislocate vertebrae, leading to nerve damage in the spinal cord, consequently resulting in a loss of muscle control, loss of sensatory perception, loss of bowel and bladder control, or of numerous other voluntary or involuntary body functions. In most SCI patients, the spinal cord is intact, but the cellular damage within it results in functional loss. SCI develops in three consecutive stages of pathophysiological events: acute phase - neuronal necrosis and axonal disruption; intermediate phase - begins within a few minutes and persists for several weeks of lesion formation. This phase is characterized by ischemia, oxidative stress, neutrophil and lymphocyte recruitment and inflammation; chronic phase - arises a few months after the initial injury and is characterized by ionic balance alteration, apoptosis of oligodendrocytes, demyelination and astroglial scar formation.
Although numerous reports have demonstrated significant improvements in medical management and clinical recuperation from SCI, to date, there is still no cure that completely allows functional recovery from SCI.
The main goals of stem cell-based therapies for SCI are regeneration and replacement of neurons and glia that undergo cell death following the injury. Although no cell transplantation strategies have been clinically approved, they are currently the most effective way to improve motor function in SCI animal models.
Current cell-based therapeutic strategies for SCI exploit:
Embryonic stem cells (ESCs) are pluripotent cells that have the capability to differentiate into most cell types, including neuronal and glial fate cells. Hence, these cells are a promising source of differentiated oligodendrocytes and motoneurons and could be used toward treatment of neurological disorders and trauma, including SCI. Following SCI, oligodendrocytes undergo apoptosis due to the inflammatory factors secreted from the damaged tissue. The loss of myelinating cells causes abnormal neuronal functionality, which can be restored by hESC-derived oligodendrocyte transplantation in animals, via activation of brain-derived neurotrophic factor (BDNF) and IL-6 signaling pathways. The beneficial effect of the transplanted ESC-derived neural precursor cells (NPCs) could be due to a neuroprotective mechanism.
Transplantation of hESC-derived NPCs with cellular matrix protein-based synthetic 3-D biodegradable scaffolds (laminin, fibronectin or collagen) could be appealing, due to the environments provided, such as an adhesive support and growth factor release, e.g. NT-3 and PDGF. This strategy has been confirmed by transplantation of collagen scaffold populated with hESC-derived NPCs into a rat model of SCI.
Clinical applications of hESC-derived neural cells are currently hindered by the ability to obtain defined and purified neural cell types in vitro and by safety concerns regarding teratoma formation. The first clinical trial involving the use of hESC-derived oligodendrocyte progenitor cells (GRNOPC1) for SCI therapy was initiated by Geron. Preclinical studies have shown that injection of GRNOPC1 significantly improved locomotor activity and kinematic scores of rodents with SCI, when administered 7 days after the injury. Histological examination of the injured spinal cords treated with GRNOPC1 showed improved axon survival and extensive remyelination around the rodent axons. Safety data from a Phase I clinical trial in 4 patients with SCI, who received one dose of 2x106 GRNOPC1 injected into the lesion site, demonstrated no complications from either the cells or the surgical procedure itself. Furthermore, there is no evidence of immune rejection of GRNOPC1, an allogeneic cell therapy, even upon withdrawal of immunosuppressive drug.
Induced pluripotent stem cells (iPSCs), like ESCs, are capable of pluripotent differentiation toward most cell types, providing a potential alternative to avoid immunological rejection, characteristic of allogeneic transplantations. However this cell type share disadvantages with ESCs, including potential teratoma formation in addition to aberrant reprogramming, which should be addressed before their clinical application.
Neural stem cells (NSCs) are multipotent cells obtained from the spinal cord, with the potential to differentiate into neurons, oligodendrocytes, and astrocytes. Demyelination is a progressive problem following SCI, and oligodendrocyte precursor cell (OPCs) transplantation presents a promising strategy to treat SCI. Ependymal stem progenitor cells, derived from the spinal cords of adult rats suffering from traumatic lesion, were propagated and differentiated in vitro into OPCs, before transplantation, which yielded functional motor recovery.
Endogenous or transplanted NSCs primarily differentiate into oligodendrocytes and astrocytes following SCI, contributing to remyelination and axonal regeneration and to cell survival by neurotrophic factors secretion, respectively. In vitro-expanded NSCs maintain their self-renewing capacity and are capable of secreting neurotrophic factors; however, they exhibit reduced differentiation potential.
Fetal neural stem cells are a sub-type of NSCs. A fetal brain-derived human central nervous system stem cell population (HuCNS-SC®), purified by surface marker expression, has shown neuroprotective abilities. HuCNS-SCs are being tested in clinical trials for treating a number of neurological disorders, including SCI. HuCNS-SCs are required for locomotor recovery of spinal cord injury, possibly via differentiation and integration of the cells or continuous supply of trophic factors necessary for host cell function. Similarly, human spinal cord stem cells (NSI-566) derived from the fetal cervical-thoracic cord have been applied for treatment of multiple neurological disorders, including SCI. NSI-566 cells differentiate and structurally integrate into the segmental motor circuitry. They also express neurotrophic growth factors such as BDNF and GDNF, which facilitate recovery and protect the neurons from disease-induced degeneration.
NSCs are considered to be safer than ESCs for clinical applications, due to reduced potential of tumor formation. However, critical challenges must be overcome prior to their use in clinical applications, including purification of differentiated cells, inefficient tracking systems, and moderate cell survival following transplantation.
Bone marrow-derived mesenchymal stem cells (BM-MSCs) have been considered an ideal cell source for cell therapy of neurological lesions, due to their capacity to facilitate lesion recovery through several mechanisms including: immunomodulatory and anti-inflammatory effects, as well as secretion of neurotrophic factors. Neural crest stem cells (NCSCs) have recently been identified inside the adult bone marrow. These cells derive from the neural crest, which arises at the borders of the neural tube during embryonic development of the nervous system. NCSCs then migrate towards different organs, where they differentiate to give rise to peripheral neurons and glia, melanocytes, chondrocytes, smooth muscle cells and other cell types. Adult NCSCs have been identified in several postnatal organs. NCSCs also comprise an attractive cell candidate for cell replacement therapy. Due to their neural origin, neural crest cells are closely related to neural tube stem cells, with a close ontological relationship to the spinal cord.
Both MSCs and NCSCs accumulate inside the bone marrow stroma, at the bone epiphysis, and are consequently often referred to as bone marrow stromal cells (BMSCs). Several prospective clinical trials have demonstrated the safety of BMSC transplantation in SCI patients and have reported promotion of preliminary motor and somatosensory improvements following treatment.
Potential mechanisms of BMSC action in SCI therapy:
(1) Anti-inflammatory effect through immune response modulation by cytokine secretion and macrophage activation. This inflammatory modulation has also been observed for adipose-derived MSCs and umbilical cord blood-derived MSCs.
(2) Neuroprotective activities by increased expression of antioxidant proteins (glutathione-associated enzymes, catalase and superoxide dismutase).
(3) Neurotrophic support (promotion of axonal sparing, sprouting of new neuritis and remyelination of fibers). Formation of new myelin sheaths can be due to both recruitment of endogenous glial cells and transplantation of the BMSCs themselves.
Adult MSCs/NCSCs present an attractive cell source for combined SCI cell therapy:
There still remains an issue of the lack of reproducibility concerning use of BMSCs, which can be resolved by considering factors, such as: donor age (under 30 years), isolation protocols (defined markers) and technical design of the cell transplantation, which will aide in developing accurate cell-therapy protocols for SCI patients.
More specialized cells types have been tested for SCI treatment, such as:
Adipose-derived Schwann cells, which were transplantated into the SCI rat model and demonstrated improvement in tissue reformation and functional improvement.
Expanded autologous Schwann cells harvested from the sural nerve of patients are being assessed in a Phase I clinical trial for transplantation into the epicenter of the spinal cord injury, thereby avoiding immune rejection.
Cell-based therapies for SCI still require a better understanding of stem cell differentiation pathways and improved cell survival upon transplantation. Identifying the optimal cell source for efficient and safe cell replacement remains a major challenge that requires more investigation.