Introduction: Basics of Neural Stem Cells (NSCs)

Through research, NSCs have shown great potential in the treatment of many neurological diseases. NSCs are isolated from brain tissue and are most commonly found in the hippocampus and the subventricular zone (20). NSC have the potential to regenerate and, provided with the proper microenvironment, can differentiate into neural precursor cells (NPC) and, eventually, to a variety of different cell types including neural cells (neurons, astrocytes, and oligodendrocytes), as well as erythrocytes, leukocytes, and skeletal muscle (26). Current sources of human NPC are fetal and embryonic tissue, although a process is being investigated to harvest these cells from post-natal and post-mortem neural tissues as well (20).

Two methods are being investigated for therapeutic use. The first, less invasive method is activation of dormant NSC within the person. NSC respond to environmental signals, such as dietary restriction, exercise, and hormone therapy, which can stimulate neurogenesis (14,28). In this approach, the NSC begin to divide followed by migration to the area of damaged neurons, where they develop the normal characteristics of the damaged cells and replace them. The growth factors used must be able to cross the blood brain barrier (BBB). (29)

The second method is transplantation of NSC or its progenitors from a donor into the damaged area. The hope is that the surviving NSC will differentiate into the appropriate cell type in the dysfunctional microenvironment created by the disease (10,14). Several techniques exist for isolating and identifying NSC based on the tissue or cell types that are produced. To isolating NSC that forms specialized neural cells requires observation of markers on the differentiated cells (32). Erythrocyte generating NSC can be isolated by a method that analyzes cells with fluorescent molecules that identify cell markers. 

Incentive to Use NSC
Current treatment of neurodegenerative anomalies may alleviate symptoms, but they are not curative and only provide temporary relief of symptoms. There is a dire need to find new therapeutic options. Several options are being pursued, including use of stem cells, either from activation of endogenous sources or from transplantation of donor stem cells.  The ability to self-renew, indefinite in vitro expansion, and possibility of transplantation and integration in the adult brain make embryonic stem cells (ESC), fetal NSC, adult NSC, and non-neural adult stem cells ideal candidates for treating neurodegenerative conditions. Among these, NSCs have shown promising results. Some of the many neurodegenerative conditions that NSC may have potential use for are discussed below.

Alzheimer’s Disease

Exogenous and endogenous testing of NSC has found varying results. Exogenous testing includes infusion of neural progenitor cells (NPC) has been shown to reverse the effects of age related memory loss. Flax et al. (1998), has shown that implantation of NSC into the brains of mice appear phenotypically and morphological normal (9). A study with rats had shown that infusion with nerve growth factor (NGF), a cytokine, prevented the degeneration of cholinergic neurons and may reverse age-related memory deficits (18). However, in a human clinical trial, nerve growth factor (NGF) was discontinued because of the adverse effects related to interaction with non-targeted structures (4).

Multiple sclerosis (MS) is a chronic demyelinating disease involving inflammation and scarring in the central nervous system.  Currently, the cause of MS is unknown, but it is believed that a virus causes the human host body to attack the myelin found in the CNS.  MS is more common in men than women and usually effects people between the ages of 20 and 30.  Current treatment only slows the progression of the disease, but do not heal the affected areas (21).

In 2005 neural precursor cells (NPC) taken from adult mice were intravenously injected into a special model of mice.  Once in the CNS, the NPC withstood repeated inflammatory episodes and in fact caused the immune cells to die off instead of attacking. This lead to long lasting neural protection (6,19).

MS lesions that occur in the CNS often do not repair leading to neurological damage.  This is often the result of NSC failure to grow and mature in the brain lesions.  Spinal cord samples from patients with long term MS were obtained and were found to have many lesions.  It was also seen that patients with primary progressive and secondary progressive MS had similar numbers of the needed neural stem cells, and that immature neural cells were only found in lesions that contained higher numbers of the initial neural stem cells.  This led researchers to believe that there is a decrease of specific neural stem cell in older lesions and new generations of these neural cells are damaged, while the lack of migration of nearby neural stem cells may contribute to a lack of remyelination. Consequently, future research should be focused on increasing neural stem cell migratory patterns (33).

Parkinson’s disease (PD) is an age-related disorder, with a mean age of those affecting being about 60 years (13). Approximately one million people in the United States are affectd.  Chronic neurodegeneration and loss of dopaminergic neurons results in the loss production of the neurotransmitter dopamine (16).  Clinical manifestations of PD are characterized by progressive tremor, rigidity, gait disturbance, posture instability, and bradykinesia. The primary current treatment of PD is Levodopa, also known as L-dopa, an amino acid that is the metabolic precursor of dopamine (17). Levodopa’s benefits are limited and decline after chronic administration (17). Most recently, deep brain stimulation (DBS), which involves surgical implantation of a device that sends electrical impulses to specific parts of the brain, is primarily being used to treat severe symptoms of PD3.

Several different stem cells have been isolated and manipulated in vitro to differentiate into dopaminergic neurons (DAN), thus far, yielding variable rates of differentiation to DAN using several techniques and supplements that include co-culture with other cells such as astrocytes and additives (7,27).

Spinal Cord Injury (SCI)

Damage to the spinal cord, either from trauma (the major cause of SCI) or disease, can lead to varying degrees of loss of function (sensory or motor). In the United States, there are roughly half a million individuals, mostly younger men, who are directly affected by SCI (25). Damage from SCI is mostly permanent, with a very small percentage regaining full recovery. SCI often result in swelling of the spinal cord due to a variety of factors, including inflammatory cytokines, that decreases over several days or weeks (15) sometimes resulting in some functional recovery. Treatment for acute SCI involves inhibiting further damage to the spinal cord, often by realignment of the vertebral column. Treatment for chronic SCI is mostly aimed at treating secondary complications and at improving quality of life. Several trials are underway to find new treatments for SCI, including new drugs and embryonic stem cells.

With regards to NSC, researchers have been focusing on cell transplantation therapies rather than activation of endogenous NSC. Activation of endogenous NSC may prove to be fruitful once these stem cells can be coerced to differentiate into the desired progenitors. Varying results have been observed (8,23) so much more research must be done before anything can be confirmed.

During the initial trauma of SCI, various inflammatory cytokines that exhibit a neurotoxic effect, and would not allow new neurons to form, are upregulated15. Implanting NSC/NPC several days after injury showed favorable results in mice15 and also in primates, where improvements of motor function were seen and differentiation of NSC/NPC into the three neural cell types (12).

Despite all of these advances, the benefits of transplanting NSC into the injured spinal cord may not be useful in the chronic phase of SCI due to the lack of factors that will allow new cells to form. Nevertheless, there is still hope for new cases of SCI if transplantation of NSC occurs during the acute phase. Results from experimental research indicate the best approach may be a combination therapy to increase the survival rate of the implanted NSC.

Stroke

The National Institute of Neurological Disorders reports that there are an estimated 700,000 cases of stroke each year, 75% of which occur in individuals who are 65 or older (32).  A stroke results from either a blood clot that blocks blood vessels supplying blood to the brain (ischemic stroke), or by the bursting of blood vessels within the brain (hemorrhagic stroke).  Both types of strokes cause a sudden obstruction of blood flow to the brain, which leads to cellular death of brain tissue32.  Since a stroke can cause cell death in many different areas of the brain, it leads to a variety of disabilities ranging from the loss of memory, problems with thought processing, the loss of balance and coordination, and even paralysis.  The risk of having a stroke increases in those who have high blood pressure, smoke cigarettes, have diabetes, or are obese.

Although presently there is no cure for a stroke, neural stem cells (NSC) are being used to produce new brain cells to replace those that die during a stroke.  While transplantation of neuron-generating cells into injured areas is one feasible way to replace dead cells, it is not risk free and can lead to seizures and further obstruction of blood vessels (22).  A safer method to produce new brain cells is to use the Stem Cell Factor (SCF) or SDF-1a to attract NSC to areas of brain injury where they can begin to divide and produce new brain cells (11,29).

Traumatic Brain Injury

Unlike neurodegenerate diseases CNS related injuries such as brain and spinal cord injuries require immediate intervention.  Timing and methods of intervention, whether traditional or experimental, are critical to stabilize the patient and prevent any further damage.  Symptoms of a traumatic brain injury (TBI) can be mild, moderate, or severe, depending on the extent of damage to the brain.  Use of NSC, NPC, or other experimental methods is of great debate. Animal transplantation experiments in mice have shown increased survival and migration of NSC cultured as neurospheres and transplanted into mice brains one week after controlled cortical impact brain injury.  In other experiments, neuro-motor function improvements were observed in animals transplanted with NPC and fetal cortical cells 24 and 48 hours post induction of brain injury respectively.  However, these success stories in animals are difficult to extrapolate to human since it is impossible to predict the outcome.  Mobilization of NSC from other parts of the brain to the site of injury may be beneficial and may avoid possible rejection associated with allogenic NSC.   Any experimental use of NSC transplantation in a trauma setting will require the availability of NSC and timely consent of patient or family (5).

References

Acknowledgements

This review was prepared by the following graduate students in the Stem Cell Biology Class, Graduate School of Biomedical Sciences, University of Medicine and Dentistry of New Jersey: Yusra Abidi, Feryal Ahmad, Katie Fane, Rachid Hamid, Helen Liou
 (in alphabetical order).

Teaching Assistant:  Steven Greco

The review was edited by two stem cell biologists.