5.4.3. Trials involving iPSCs for SCIs

are most likely to maintain their effects via secretion of specific growth promoting factors that develop new olfactory axons and promote axonal regeneration following SCI [96]. At present, the olfactory ensheathing glial and fibroblastic cells are being evaluated in phase I clinical trial

Pluripotency, a special ability of a pluripotent cell to differentiate into any type of body cell, was shown to be induced in adult differentiated cells via the activity of only four embryonic transcription factors—Oct3/4, Sox2, Klf4 and c-Myc. These induced adult cells gaining the ability of pluripotency were termed as "induced pluripotent stem cells" (iPSCs) [97]. This discovery offered a very forthright procedure of how to induce pluripotency in mature cells in a basic laboratory setup, yet the scope of this cell-rewinding discovery was extraordinarily vast in biomedical sciences and regenerative medicine, which earned the principle investigator,

The reprogramming factors, also known as pluripotency markers, are the main regulators of inducing pluripotency in mature cells via a process of activating pluripotent genes expression. The induction of pluripotency can be achieved via expression of only four reprogramming factors, i.e. OCT3/4, SOX2, KLF4 and c-Myc [99]. POU class 5 homeobox 1 (POU5F1 or OCT4) is being reported to play a significant role in developing embryo and maintaining pluripotent status. It has been observed to bind an octamer motif (ATGCAAAT) of DNA, where it is involved in regulation of several genes that play important part in pluripotency. Oct4 has been observed to frequently regulate in association with Sox2 [100–102]. SRY (sex determining region Y)-box 2 (SOX2) is a transcription factor encoded by SOX2 intronless gene. It plays an important role to regulate embryonic development and determine cell fate and is usually expressed in developing embryo and neuronal cells [100]. It has been reported that expression of SOX2 is fundamental for maintaining pluripotent status of evolving embryos, whereas its downregulation is associated with mesodermal and endodermal differentiation. Embryos with no expression of SOX2 were found unable to grow and proliferate after implantation [102, 103]. Kruppel-like factor 4 (KLF4 or EZF), a zinc finger protein is a transcription factor that is involved in regular growth of the barrier properties of body skin [100]. KLF4 has been reported to have a higher expression in non-dividing cells and is associated with induction of cell cycle arrest [104, 105]. KLF4 has been shown to specifically regulate genetic stability [106, 107] and promote cellular growth and survival [108]; however, in some cases, KLF4 has been reported to induce cell death [109, 110]. c-Myc, a nuclear phosphoprotein, has been recognized to play multiple functions, including cell cycle multiplication, programmed cell death and cellular propagation, via transcriptional regulation of particular genes [100]. The efficacy of OCT3/4, SOX2 and KLF4 is shown to be enhanced by the expression of an enhancer factor c-Myc [111].

In mechanism of cellular reprogramming, three pluripotency markers (OCT3/4, SOX2 and KLF4) have been observed to greatly influence multiple genes expression in iPSCs [112]. In

(NCT01231893) for treatment of complete SCIs [44].

5.4. Induced pluripotent stem cells (iPSCs)

Yamanaka, a Nobel Prize in year 2012 [98].

5.4.2. Mechanisms of cellular reprogramming

5.4.1. Reprogramming factors

138 Essentials of Spinal Cord Injury Medicine

A preclinical study has reported that following transplantation of human iPSC-derived neurospheres into spinal cord-injured mice model, a locomotional recovery from SCI was achieved, signifying the importance of human iPSC-derived neurospheres in regenerative medicines [115]. Another study has reported the transplantation of human iPSC-derived neural stem cells into monkeys' specie that promoted locomotory function after SCI, without inducing teratomas [116]. One of a report has stated that iPSCs have the ability to generate three important functional cell types of the CNS, i.e. astrocytes, oligodendrocytes and neurons [117]. An optimization of efficient protocols that can be utilized to generate constant and longterm population of neural stem cells from ESCs and iPSCs has been demonstrated [118, 119]. These types of cells have displayed stable features, for example constant expanding properties, consistent differentiation into neurons and glia and the ability of producing efficient established neurons in vitro. Another study has reported that following transplantation of such long-term stemness-rich cells (iPSC-derived neural stem cells) into SCI mice model, improved remyelination and axonal regrowth were observed with additional support for subsistence of endogenic neurons [120].

In one of an important study, cells from a healthy man of 86 years of age were induced to generate iPSCs, from which neural stem cells were generated and transplanted into rats exhibiting immunodeficiency following SCI. After 12 weeks of C5 lateral hemi-sections, iPSCs endured and generated neuronal and glial cells, extending large number of axons from injured area to almost the complete distance of rat CNS that subsequently developed synaptic communications with host neurons [121]. Another study on human iPSC-derived neural progenitors known as IMR90 has reported functional recovery after IMR90 transplantation in SCI rat models. It was shown that iPSCs have the ability to generate functional neurons, which resulted in long-term functional recovery from SCI [122]. Altogether, several studies have reported that following transplantation of iPSC-derived neural stem/precursor cells, locomotory function was improved/recovered in spinal cord-injured animal models [115, 116, 120, 123, 124]. In contrast, other studies have reported that transplantation of clones containing human iPSC-derived neural stem/precursor cells, e.g. clone-253G1 and 836B3, has been reported to induce teratomas and suppress locomotory function after long-term follow-up. In addition, the induced teratomas were made up of undifferentiated Nestin-positive cells of human origin [125, 126]. For this purpose, one of a recent study is paving the ways to tackle such problems of teratoma formation and locomotory inhibition. In this study, it was shown that human iPSC-derived neural stem/precursor cells that were pre-treated with γ-secretase inhibitor (GSI) induce differentiation and development of neuronal cells, whereas it also recovers host neural circuitry. Moreover, the incredible results showed that following transplantation of these cells with GSI pre-treatment after SCI, tumorigenesis was prevented and locomotory function was maintained [126].

6.3. Other physiological side effects

protocols for SCIs.

SCIs

secreting recipient will raise several safety concerns.

7. Techniques for "in-vivo tracking" of transplanted cells

One of a recent study has reported that a male dominant hormone testosterone has the ability to stimulate proliferation of human adult MSCs and endothelial precursor cells, while preserving their stemness properties [139]. Transplantation of these cells in a hyper testosterones

Cellular Transplantation-Based Therapeutic Strategies for Spinal Cord Injuries: Preclinical and Clinical…

http://dx.doi.org/10.5772/intechopen.73220

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One of the most essential and obvious steps to follow after cellular transplantation is to track down the implanted cells in host tissues. To date, several studies have reported the use of in vivo tracking system where an investigator can observe and analyze transplanted cells to evaluate their extensive status including site of cell transplantation, cellular migration, proliferation, differentiation into desired cell types, long-term self-renewal and their integration within a host tissue [140]. Using MRI technique where a superparamagnetic iron oxide works as a contrast mediator, transplanted cells can be tracked down in vivo [141]. One of a study has shown that using 3D microtopographic scaffolds, reprogrammed neuronal cells were capable of colonizing damaged neural cells to replace with transplantable cells [142]. It has been reported that transplanted MSCs, labeled with established gadolinium-based MRI contrast agent, i.e. Gadoteridol, were effectively traced via in vivo tracking in a SCI mouse model. A procedure that was employed during the in vivo tracking was established on hypo-osmotic shock that induced an osmolality-contingent permeabilization and physical alterations in cellular membrane [143]. Hence the in vivo cell tracking techniques are evolving; further development in these technologies will help to optimize future cellular transplantation therapy

8. Success story and controversies of cell transplantation in patients with

Researchers in the field of cellular therapy and regenerative medicines are restraining to directly inject hESCs or iPSCs in humans, but rather more inclined to evaluate hESCs- or human iPSC-derived cell population, i.e. ODPs, HuCNS-SC, Schwann cells, olfactory ensheathing cells, umbilical cord blood mononuclear cells, autologous BMSCs and umbilical cord blood- and adipose-derived MSCs, as evident from the recent and ongoing clinical trials —Table 1. In contrast to direct transplantation of ESCs or iPSCs, direct administration of the above-mentioned derived cells is only limited to form single-specific cell progeny and also

Till date, success has been made in cellular transplantation therapies for SCIs as their usages and procedures have now reached to clinical trials; however, these procedures are still at their early stages with no further than phase I or phase I/II clinical trials [5]. Nevertheless, all the

possesses lower risk of developing teratomas in host specie [144].
