Kimberly D. Tran, Allen Ho and Rahul Jandial* Abstract
J fust a few short years ago, we still used to think that we were born with a finite number of I irreplaceable neurons. However, in recent years, there has been increasingly persuasive evidence I that suggests that neural stem cell (NSC) maintenance and differentiation continue to take ace throughout the mammal's lifetime. Studies suggest that neural stem cells not only persist to mammalian adulthood, but also play a continuous role in brain tissue repair throughout the organism's lifespan. These preliminary results further imply that NSC transplantation strategies might have therapeutic promise in treating neurodegenerative diseases often characterized by isolated or global neuronal and glial loss. The destruction ofneural circuitry in neuropathologies such as stroke, Parkinson's disease, MS, SCI prevents signals from being sent throughout the body effectively and is devastating and necessitates a cure.NSC transplantation is among one of the foremost researched fields because it offers promising therapeutic value for regenerative therapy central nervous system (CNS) diseases. Both chemotropic and exogenous cell graft mechanisms of CNS repair are under review for their therapeutic value and it is hoped that one day, these findings will be applied to human neurodegenerative disorders. The potential applications for NSC transplantations to treat both isolated and global neurodysfunction in humans are innumerable; these prospects include inherited pediatric neurodegenerative and metabolic disorders such as lysosomal storage diseases including leukodystrophies, Sandhoff disease, hypoxic-ischemic encephalopathy and adult CNS disorders characterized by neuronal or glial cell loss such as Parkinson's disease, multiple sclerosis, stroke and spinal cord injury.
Just a few short years ago, we still used to think that we were born with a finite number of irreplaceable neurons. However, in recent years, there has been increasingly persuasive evidence that suggests that neural stem cell (NSC) maintenance and differentiation continue to take place throughout the mammal's lifetime. These processes are most notably observed in cases of inflammation and ischemic or traumatic brain neural injury.1 The direct control mechanism governing neurogenesis and gliogenesis is not yet fully understood; thus, the cellular and molecular signals that are suspected to govern these processes (i.e., cytokines, chemokines, metalloproteases and adhesion molecules) are all under heavy investigation.2
There are many reasons to be hopeful that these investigations might prove fruitful for therapeutic brain repair strategies. In the animal model multiple sclerosis (MS), called chronic experimental autoimmune encephalomyelitis (EAE) and characterized by brain inflammation and global demy-elination, Picard-Riera3 and Brundin4 both showed that self-renewing progenitor cells, residing in the subventricular zone (SVZ) of the brain or in the subependymal layer of the spinal cord, could ultimately change their physiological fate in the presence of disease. Instead of traveling down the
Frontiers in Brain Repair, edited by Rahul Jandial. ©2010 Landes Bioscience and Springer Science+Business Media.
rostral migratory stream (RMS) to the olfactory bulb or to the lateral columns ofthe spinal columns5,6 progenitor cells instead migrated towards sites of demyelination and differentiated primarily into glial cells. The same alteration in cellular destiny was equally observed in experimental models of spinal cord injury (SCI) and ischemic stroke. Within one week of the traumatic event, endogenous neural stem cells residing near the injury sites had been found to migrate to the injured area borders and promoted neurogenesis in direct support of the neural tissue repair.1,2,7
These studies suggest that neural stem cells not only persist to mammalian adulthood, but also play a continuous role in brain tissue repair throughout the organism's lifespan. These preliminary results further imply that NSC transplantation strategies might have therapeutic promise in treating neurodegenerative diseases often characterized by isolated or global neuronal and glial loss. The destruction of neural circuitry in neuropathologies such as stroke, Parkinson's disease, MS, SCI prevents signals from being sent throughout the body effectively and is devastating and necessitates a cure.8
NSC transplantation is among one of the foremost researched fields because it offers promising therapeutic value for regenerative therapy central nervous system (CNS) diseases. From previous studies, NSCs have been observed to respond to CNS insults.9 However, oftentimes this endogenous NSC response is not enough to counteract the severe injury inflicted on the CNS; therefore, it is in this case that exogenous NSCs may be useful to supplement the endogenous NSC pool and aid in the preservation of existing host circuitry and also in the replacement of diseased or disordered cells.
Demyelination often plays an important role in both genetic metabolic disorders (i.e., leukodystrophies) and acquired neurodegenerative processes including traumatic, infectious, asphyxial, ischemic and inflammatory insults to the CNS. While not yet translated to human clinical trials, the ability of NSCs to remyelinate diseased neurons in host organisms has already been shown by Yandava10 and colleagues with the model shiverer (shi) mouse. The shi model suffers from a congenital tremor due to a deletion mutation of its myelin basic protein Mbp gene. When NSCs were transplanted into the ventricles of neonatal shi mice, up to 40% remyelination was observed in some recipients and in several test subjects, a visible decrease in the shi mice's characteristic tremor was observed.10 Similar results were successfully reproduced with human oligodendrocyte precursors,11 thus reconfirming NSCs' potential value to be used to globally replace cells in the CNS.
We know that transplanted NSCs are able to participate in normal brain development and to either persist as undifferentiated stem cells, or to differentiate in response to local environmental cues into neurons, astroglia and oligodendrocytes.12-14 All this would be meaningless, however, ifthe exogenous cells did not integrate with host cells and instead formed their own isolated neural circuitry pathways. Fortunately, this is not the case and the ability of transplanted NSCs to functionally integrate with host neural circuitry was confirmed by electrophysiological recordings in slice cultures of engrafted brains.14,15 While researchers originally looked to neural stem cells' ability to replace dying cells as their primary mechanism oftreating neuropathologies, we now know that NSCs also express neurotrophic factors, such as glial cell line-derived neurotrophic factor (GDNF), nerve growth factor (NGF) and brain-derived neurotrophic factors (BDNF),16,17 which may play an even greater role in circuit repair by altering the local chemical environment. The NSC-regulated environment is thought to enhance host regenerative processes such as angiogenesis and migration, while inhibiting destructive processes such as apoptosis, scarring, inflammation, excitoxicity and oxidative stress. This mechanism ofpreserving existing host cells through the dispersal ofchemical signals is especially important when considering disorders that affect the entire CNS and not just one isolated region.
Both chemotropic and exogenous cell graft mechanisms ofCNS repair are under review for their therapeutic value and it is hoped that one day, these findings will be applied to human neurodegenerative disorders. The potential applications for NSC transplantations to treat both isolated and global neurodysfunction in humans are innumerable; these prospects include inherited pediatric neurode-generative and metabolic disorders such as lysosomal storage diseases including leukodystrophies, Sandhoffdisease, hypoxic-ischemic encephalopathy and adult CNS disorders characterized by neuronal or glial cell loss such as Parkinson's disease, multiple sclerosis, stroke and spinal cord injury.
When discussing stem cell transplantation methods, it is important to not only introduce the techniques and protocols, themselves, but also to determine the ideal combination of supporting factors which play an equally vital role in determining graft success. These factors include the ideal cell source for transplantation, the appropriate cell modifications before transplantation, the experimental subject animal, the transplantation method and route of cell administration and finally optimization parameters and measurable endpoints must be considered in order to test the efficacy and level of integration of exogenous NSCs within the host organism. These topics will be discussed in depth here.
There are a variety of properties that make NSCs conducive to therapeutic uses. Principally, NSCs are a homogenous, well-defined cell population able to be stored and expanded and they have been shown to self-renew through many passes without changes in morphology. NSCs can be genetically manipulated to deliver therapeutic proteins or trophic factors directly to the site of injury or can be dispersed throughout the vascular system to treat widespread neuronal damage throughout the entire body. The cells themselves are multipotent and will differentiate into neurons, astrocytes and oligodendrites in response to respective deficiencies in the host CNS. They can integrate into the developing host brain through competition and interdigitation with host cells,9 or they can selectively target and replace degenerating neural tissue in response to local neurogenic and chemotropic factors. Logistically, they are a feasible and practical solution because NSCs can easily be introduced into the host tissue and irradiation is not required prior to transplantation (as it the case in bone marrow transplants). Besides their previously described effects within the CNS, NSCs are also thought to be immunoprivileged—they lack the cytochemical markers of differentiated cells and thus do not generally trigger an immune response rejecting engrafted cells. The implications of these properties are noteworthy because NSCs theoretically have the potential to provide an unlimited source ofundifferentiated cells that could be induced to differentiate into any type of cell needed by a diseased individual.
The ideal source of neural stem cells has been a much debated topic. The most important properties considered when judging the utility of a potential NSC source are the cells' plastic and self-renewing capabilities. NSCs must be able to adapt in response to the varying environmental conditions to which they are exposed in order to restore function within the host circuitry. A NSC population must also be expandable without changes in morphology in order to generate a large number of clonally related progeny of neuronal, astrocytic, oligodendrocytic lineage.8
To date, the most research has been performed with adult and embryonic neural stem cells. While other sources such as multipotential NSCs from skin,18 bone marrow stromal cells19-22 and cells from umbilical cord blood23 have been shown to demonstrate some "stem-like" properties, we will focus on the vast majority of studies which cite the use of adult and embryonic NSCs.
In humans, adult neural stem cells (aNSC) have been shown to engage in neurogenesis and glio-genesis within restricted areas ofCNS throughout adulthood.8 In vitro, they are capable ofexpansion and develop into precursor neural cells capable of neural cell replacement and repair.24-26 They are isolated from fetal and adult human brains and can be propagated and maintained for years in stable media. In this way, they provide an important renewable stem cell source for transplantation.27,28 Their growth factor (GF)-dependent expandability, stable growth rate, self-renewal and multipotent capabilities and functional plasticity all make aNSCs a very good source ofstem cells.24,26,29,30 Vescovi and colleagues26 demonstrated that plasticity could be determined by growth factors; culturing cells in media containing leukemia inhibiting factor (LIF), brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), neurotrophin (NT)-3, NT-4, sonic hedgehog (Shh) and fibroblast-derived growth factor (FGF)-9 produced a neuronal fate in 40%-60% ofcells, while bone morphogenetic proteins (BMPs), CNTF and LIF media produced astrocytes.24,31,32 In experiments where aNSCs were transplanted intraparenchymally or intrathecally (injection into the cerebrospinal fluid around the spinal cord) into healthy rodents, aNSCs survived the transplantation into the host CNS and showed predictable patterns oftissue integration and differentiation into neuronal cells.33-35 However, in experimental models ofautoimmune, chemical, or traumatic CNS demyelination, aNSCs transplanted intraparenchymally, intracerbroventricularly, or intravenously migrated selectively to damaged sites and differentiated into myelinating oligodendrocytes.8,36-39 These results underline aNSCs' potential as a source of cell replacement in neurodegenerative disorders. Equally of note, in experimental transplantations, the introduction of aNSCs into both healthy and diseased rodents did not produce teratomas, therefore suggesting a minimal tumorigenic potential ofaNSCS40 which is an important difference from embryonic stem cells (ESCs) which we will examine next.
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