A Scientific Summary

Neural stem cells (NSC) have shown great potential in the treatment of many neurological diseases. NSC are isolated from brain tissue and are most commonly found in the hippocampus and the subventricular zone26. NSC have the potential to regenerate and, provided with the proper microenvironment and differentiation conditions, 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 (24). 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 (17).

Differentiation conditions necessary for generating cells and tissues from NSC range from a wide variety of in vitro and in vivo techniques.  Fibroblast growth factors are used to grow astrocytes and neurons, while epidermal growth factors are used to expand oligodendrocytes.  Transplantation of NSC into irradiated mice can cause development of erythrocytes, leukocytes, and skeletal muscle (23). Additionally, expansion of and harvesting NSC with a tag cDNA amplification technique have utilized a population of cells called neurospheres, which are cellular aggregates grown from growth factor-dependent, clonally expanded NSC (22).  Each neurosphere contains NSC and have the potential to be used in cell therapy (22).

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, NSC have shown promising results. Some of the many neurodegenerative conditions that NSC may have potential use for are discussed below.

Alzheimer’s Disease (AD) is a form of dementia affecting as many as four million Americans, yet there are currently no diagnostic tools to determine if a living person has AD. The two types of AD are sporadic, with an age of onset in the 60s or later, and familial, the latter of which affects 11% of AD patients, but has an earlier age of onset (29). The symptoms begin with forgetfulness to loss of memories not associated with general aging and intellectual skill deterioration, erratic behavior, and loss of bodily functions4. These result from brain degeneration due to beta-amyloid plaques and neurofibrillary tangles (the hallmarks of AD) found in the cholinergic neurons.

Investigations with exogenous and endogenous methods have found varying results. Infusion of neural progenitor cells (NPC) exogenously 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 and form all three neural cell types9. 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 deficits15. However, in a human clinical trial, NGF was discontinued because of the adverse effects related to interaction with non-targeted structures4. Endogenous testing with pyrolopyrimidine compound (MS-818) was shown to stimulate biological growth activities, such as increased neurite growth. When applied to animal models, axon growth increased in mice, apoptosis of cortical neurons decreased in rats, and NSC proliferation increased in aged rats (26).

Multiple sclerosis (MS) is a chronic demyelinating disease involving inflammation and glial scarring in the CNS white matter.  The etiology is unknown, but the current theory is that a virus activates an autoimmune response to the myelin found in the CNS.  Recent data suggests genetic susceptibility in the Caucasian population (17). The age of onset of MS is between 20 and 30, with a ratio of 1.4 women being affected per 3.1 men18.  In America there are 400,000 afflicted, with 2.5 million afflicted worldwide (1). Current treatment involves prednisone in tapering treatments, β-interferon, and glatiramer aacetate (18)  all of which only slow the progression of the disease, but do not heal the affected areas. 

In 2005 neural precursor cells (NPC) taken from adult mice were intravenously injected into experimental autoimmune encephalitis (EAE) mice.  Once in the CNS, the NPC withstood repeated inflammatory episodes and in fact induced apoptosis of “blood-borne CNS-infiltrating encephalitogenic T cells (6).”  This lead to long lasting neural protection (6,16).

Parkinson’s disease (PD) is an age-related disorder, with a mean age of those affected being about 60years (11). Approximately one million people in the United States are affected.  Chronic neurodegenration and loss of dopaminergic nigrostiatal neurons in the substantia nigra pars compacta, coupled with intracytoplasmic, inclusions known as Lewy bodies, ultimately causes the loss of the capability of producing dopamine, along with possible neurodegeneration of noreepinephrine, cholinergic, and serotonin neurons (13).  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 (14).  Levodopa’s benefits are limited and decline after chronic administration (14). Most recently, the primary treatment for the symptoms of PD involves deep brain stimulation (DBS), aided by surgical implantation of a device that sends electrical impulses to specific parts of the brain (3).

Several different types of stem cells have been isolated and manipulated in vitro to differentiate into dopaminergic neurons (DAN). Thus far, variable rates of DAN differentiation have been obtained by using several techniques and supplements that include co-culture with other cells such as astrocytes and additives, such as retinoid X receptor ligands (28), fetal growth factors (25), epidermal growth factors, leukemia inhibitory factors, and interleukins-1 and –117.  The challenge remains in increasing NSC survival upon grafting into animal models (9).  Additionally, cultured NSC change their repertoire of transcription factors, making them more difficult to differentiate into specific cell types (19). Although NSC are still being investigated for their potential use for PD treatment, recent research has shown more promising possibilities of differentiation of ESCs into DAN and higher yields of surviving grafted cells (22).  Substantial progress has been made to improve cell culture protocols for differentiation of ESC to DAN in order to increase transplantability and survival, as well as decrease the possibility of developing teratomas after transplantation.

Spinal Cord Injury (SCI)
Damage to the spinal cord, either from trauma (the major cause of SCI) or disease, can lead to loss of function (sensory or motor), the extent of which relates to the area and degree of damage. In the United States, there are roughly half a million individuals who are directly affected by SCI, a majority of whom are men between the ages of 16 and 30 (23). 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 (12), 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 treatment of secondary complications and at improving quality of life. Current trial for use of autologous incubated macrophage is currently underway for individuals with SCI within 14 days. 4-aminopyridine is a drug that has been shown to improve conduction of demyelinated axons. Olfactory ensheathing glia (OEG) transplants may encourage regeneration of the spinal cord. Fetal embryonic stem cells have been experimentally shown to engraft in the spinal cord. However, because of ethical issues, current trials are using pig fetal stem cells along with antibodies to inhibit rejection (31).

For SCI, 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. Bone morphogenetic protein (BMP), a cytokine, has been shown to induce NSC/NPC toward astrocytic phenotype and increase astrocyte proliferation and reactivity (2), yet this effect was not shown to be inhibited by an antagonist to BMP, namely noggin, which did, however, show to be deleterious to the injured spinal cord (8). These results, however, were in complete contrast to those reported by Setoguchi, et al. (2004) (22). The differences may have arisen from different experimental conditions, including the species used.

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 inducing neurogenesis. 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 in which the microenvironment is manipulated to be optimal for NSC differentiation into functional nerve cells, and NSC/NCP/RG cell transplantation along with a polymer scaffold (27) to increase the survival rate of these cells. Although, RG cells must be used with caution since they show oncogenic properties (10).

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. Some risk factors of stroke include high blood pressure, cigarette smoking, diabetes, and obesity.  The more common acute, ischemic stroke is caused by blood clots that block blood vessels supplying the brain with the necessary oxygen and nutrients it needs for survival, leading to cellular death. Hemorrhagic stroke, on the other hand, occurs when a damaged blood vessel bleeds into the brain, causing a buildup of blood pressure in the injured area.  Because stroke is a neurological disorder causing brain tissue death, many areas of the body are affected, causing a wide variety of disabilities (30). 

Recent research in NSC has provided many promising and fruitful solutions by regenerating new brain cells and reversing the damage caused by stroke.  Successful implants have targeted injuries in the area of the brain called the striatum, which is linked to the loss of memory, cognitive problems, and motor coordination deficiencies characteristic of strokes.  Two clinical trials have generated new neural cells using NSC transplantation into injured intrastrial areas (20).

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.  In most TBI cases the affected individuals cannot make decisions and give consent on their own.  Occasionally, a family member must make such decisions. 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).

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.