Alzheimer’s disease is a multifactor neurodegenerative disorder characterised by dementia, cognitive and behavioural changes and has no treatment. The turn from neuropathological and psychological description of the disease to studying the molecular, genetics, and biochemical causes of neurone degeneration enhanced the scientific knowledge on possible new treatments. Stem cells characteristics of self-renewal, pluripotency, and plasticity raised hopes of possible effective treatment.
Despite difficulties in research and debate about stem cells in therapy, success in stem cell treatment for some cancers and blood diseases makes stem cell treatment a possibility in neurodegenerative disorders. At present, there are two strategies for using stem cells in research for Alzheimer’s disease therapy. The endogenous strategy, that is to stimulate the naturally present adult brain stem and progenitor cells, and transplant replacement strategy.
Although research achieved advancements in understanding the in vitro and in vivo development of neuronal subtypes and glial cells, further research is a necessity to understand differentiation, migration and the environmental factors that affect stem cells in vivo. Coaxing endogenous adult brain stem cells to respond to brain disorders, transplanting the properly differentiating stem cells and producing neuroprotective drugs to improve the brain environment appears to be an effective strategy. This thesis aims to provide a brief review on stem cells, focusing on the possibility of stem cell research to treat Alzheimer’ disease.
Cells are the body’s key structures that can perform fundamental life processes. Each cell is a model of autonomy and self-identity; despite that, cells share and synchronize functions with other cells. Life begins when a sperm fertilizes an ovum forming one cell (zygote), which divides into two cells and so on. A blastocyst is formed of nearly150 with two types of cells, the trophoblast and the inner cell mass made of embryonic stem cells.
Adult tissues have stem cells as well having the function of differentiation into the specific tissue cell types. The human brain is a clear example where adult stem cells are not effectively functioning in response to cell injury, damage, or degeneration. The core of stem cell research is how to stimulate the existing adult stem cells to produce cell types instead of the degenerated cells. The difficulty is that adult cells are not as adaptable or flexible as those traced to the embryo’s inner cell mass (Blau and others, 2001).
Stem cell research focus is as an organism develops from one cell (the zygote), can healthy cells replace damaged adult human cells. Second, what are the possibilities of cell-base therapies of diseases, a new field of medicine called regenerative medicine. Like other rapidly evolving human biology research, stem cell research provokes questions and issues as it develops (Pursa and Hengstschläger, 2002).
Over half a century ago, there was little research on aging and associated neurodegenerative disorders; this was mainly because in the post World War II era societal effort and resources targeted the younger age groups. Advancements in medical care that led to increased longevity and redirected the interest to the older age groups. As a result conditions characterised by disturbances of cognition and behaviour started to have a fair share of medical care and research, of them neurodegenerative disorders were a major focus (Reyes, 2004).
Stem cell research is becoming popular as it evokes hopes to treat many difficult diseases that have no cure. Characteristically these diseases are caused by progressive cell damage, specifically in the nervous system, which has no regenerative potential; therefore, stem cell treatment may be the only potent solution. The key issue is how to improve the specific stem cell potential to differentiate in a way to replace the lost damaged cells’ function to cure or improve the disease (Brenner, 2005).
This thesis aims to provide a brief; yet, a comprehensive review on stem cells focusing on their potential of use in treating Alzheimer’ disease.
Stem cells, as a term, points to cells present in tissues and organs having no specific function, but characteristically have the inherent ability of transforming to specialised cells with definite function. The best-known example of stem cells in bone marrow stromal (mesenchymal) stem cells, which can differentiate to any type of blood cells, muscle, bone, and cartilage, pancreatic and neurological cells. In addition to endothelial stem cells, and haemopoietic stem cells, with the newly developed cells have their specific functions. Thus, one cell type (stem cell) gives rise (stems out) many types of cells.
Stem cells remain dormant or non-functioning until stimulated or signalled to differentiate to specialized cells. Therefore, stem cells have the function of repairing (restocking) other body cells, and when a stem cell divides, the new cell may remain as a stem cell or differentiate to a specific tissue cell. Since most tissue injuries involve loss of distinctive cell features, most repair processes include differentiation of stem cell influenced by the activation of pre-existing stem cells or progenitor (originator) cell. A progenitor cell is an intermediate between a stem cell and the fully differentiated tissue cell (Anderson and others, 2001). Stem cells can also differentiate to ectodermal, mesodermal or endodermal cells, a phenomenon known as pluripotency (Alison and others, 2002).
Characteristics of stem cells
Zech (2004 pp. 91-99) in a review article identified four criteria a cell must fulfil to be defined a stem cell. As a precondition, these cells must be capable of maintaining its cell population by self-renovating cell divisions, second, is that daughter cells stemming from one cell must have the inherent ability to differentiate to other cell type. Third, is that stem cells must replace other damaged cells if transplanted, finally, although less established, stem cells must have the competence to add differentiated offspring (progeny) in vivo yet, in the absence of injured cells. Subsequently, the question is how to identify a stem cell to isolate? Fortier (2005 pp. 415-423) reviewed the classification of stem cells and inferred there is no standard test to isolate a stem cell.
Although many authors described various markers as alkaline phosphatase; however, many cell types share these markers. Further, there are undeniable differences in stem cells’ markers of different species, which adds to his confusing problem. Many authors accept in vitro use of cell markers to test stem cells but only for haematopoietic and embryonic stem cells, which are unquestionably capable of differentiation in vivo.
About stem cells differentiation, it is important to define the spectrum and traits of differentiation. Gavin and colleagues (2005 pp. 695-698) examined stem cells’ differentiation and concerning potency (spectrum), they inferred there are six classes of stem cells. First, multipotent cells, which are cells capable of producing cells of a closely related family of cells (e.g. blood cells), second are, pluripotent stem cells that can differentiate to any cell type but are unable to form a complete organism.
The third type is totipotent stem cells that can differentiate to any type of cells besides being able to develop into a complete organism, the zygote cells are a clear example. The potential of a stem cell to differentiate to a different cell type is the plasticity of stem cells, while a progenitor cell is the one that differentiates to only one cell type, its ability to renew is the difference between a progenitor and somatic cells. Gavin et al (2005 p. 695) defined stem cell differentiation when a stem cell transforms to a specialised cell in response to specific stimuli, also stem cells have the characteristics of trans-differentiation and dedifferentiation.
Trans-differentiation as defined by Gavin et al (2005pp. 695-698) is the ability of a stem cell of one tissue (e.g. haematopoietic cell) to differentiate to another tissue cell type (e.g. a liver cell). Dedifferentiation is the regression of a differentiated cell to a less specialised cell. They explained that trans-differentiation exists in adult stem cells as shown in mesenchymal stem cells of the bone marrow, which gives hope to successful adult stem cells’ therapies. Further, they explained that dedifferentiation of adult stem cells is important to maintain their differentiation potential.
Plasticity of stem cells
Many authors challenged the view that an adult stem cell’s line of differentiation takes place in one direction that is unidirectional and irreversible. The observation that donor cells exist in non-haemopoietic recipient’s organs after bone marrow transplants lead many authors to suggest that with right conditions, adult stem cells can differentiate to cells of other tissues (Rosenthal, 2003). Zech (2004 pp. 91-99) defined plasticity of stem cells as the apparent potential of tissue-specific stem cells to differentiate to cell types different from those of the tissue of origin.
Further, Zech (2004 pp. 91-99) suggested a possible mechanism of plasticity that multipotent stem cells present in different organs have the potential to dedifferentiate into a lesser lineage-committed state. These cells have also the potential to re-differentiate on receiving the proper stimulus. Zech (2004 pp. 91-99) named this mechanism the graded propensity or. Rosenthal (2003 pp. 267-274) stated the potential plasticity of adult stem cells according to their location.
Classification and sources of stem cells
Bongso and Lee (2005 pp. 1-14) classified stem cells into four types based on their origin. Although the zygote, up to 8-cells stage and morula phase are totipotent cells; yet, these cells do not have the criteria of self-renewal and therefore, do not fulfil the stem cell criteria. The inner cell layer of the five to six days old blastocyst possesses the four criteria of stem cells, in addition to being pluripotent cells. Thus, these cells are true embryonic stem cells.
The primary cell types present in the foetus’s organs (as the neural crest, haemopoietic system…) are the foetal stem cells (Bongso and Lee, 2005). Oscar and colleagues (2004 pp. 1669-1675) showed that stem cells obtained from umbilical cord blood resemble bone marrow stem cells in the differentiation potential, but with greater potential to grow and produce larger colonies in vitro and can be expanded in long-term cultures. Umbilical cord blood stem cells have the different growth requirement than bone marrow stem cells but more important they have long telomeres (areas of repetitive DNA at the chromosomes’ ends), which are thought to play an important role in the anti-aging process.
Lindolfo and colleagues (2006 pp. 2204-2213) recognize that adult mesenchymal stem cells have the property of trans-differentiation they examined these cells’ natural distribution and biological behaviour. They collected these cells from adult mice’ various tissues and organs and bread the cells in long-term cultures. Lindolfo and colleagues (2006 pp. 2204-2213) inferred that mesenchymal stem cells are available in various adult tissue and organs but not from peripheral blood. They also inferred that both large (aorta and vena cava) vessels and small ones (renal glomerular capillaries) are suitable for long-term stem cells’ cultures.
Stem cell collection and banking
The traditional method for collecting bone marrow stem cells was by repeated bone marrow aspirations under general anaesthesia, collection of mesenchymal stem cells from peripheral blood after initial mobilization with cyclophosphamide or haemopoietic growth factor replace the bone marrow aspiration method (Mink and Armitage, 2001). Apheresis is a technique to extract blood component by a centrifuge machine based on individual cell component’s weight, the recipient receives donor’s white cells, which include a small percentage of stem cells as both are connected to the machine at the same time (Saiz et al, 2004).
Cord blood collection is either before or after the placenta delivery (In-utero or Ex-utero collection). In-utero collection is an invasive procedure performed only by an obstetrician, and in both methods, there should be no deviation from normal procedures and no jeopardy to the infant or mother’s health (New York State Council on Human Blood and Transfusion Services, 2003 pp. 3-4).
For embryonic stem cells collection, frozen human blastocyst embryos are produced by in vitro fertilization. Removal of the zona pellucida by digestion with acid tyrodes then immuno-surgery by rabbit antihuman RBCs antibodies and a guinea pig serum complement (Cowan et al, 2004). There are two main objectives for storage (banking) of stem cells; first is to control the possibility of contamination and genetic change and keep the number of cells’ passages to a minimum (Loring and Rao, 2006). Second is to control microbial contamination in both cultures and stem cell lines (Cobo et al, 2007).
Stem cell treatment: current research and debate
Although technical and ethical discussions on therapeutic uses of stem cells, (especially embryonic stem cells) might eclipse the potential of stem cell therapy, yet stem cell research has led to advances in the understanding of cell biology. At present, there are stem cells research, therapeutic applications, and promising clinical trials that in addition clarify the challenges that face stem cell therapy (Higgs, 2008).
Stem cells research: current status
In medical research applications, the use of stem cells has the benefit of providing better ways to understand how genetic abnormalities express biochemical or structural cell impairment. In addition, stem cell research better drug testing and tailoring for patients with specific genetic disorders and allow comparing drug effects on specific tissues cultured from stem cells (Hayes et al, 2006). Expectations of therapeutic uses of stem cells to repair damaged or diseased tissues are vast; using stem cells in conjunction with chemotherapeutic agents to restock destroyed immune cells is another possibility. Hayes and colleagues (2006 pp. 14-23) reported that since 1987, more than 20.000 leukaemia patients received haemopoietic stem cells through the National Marrow Donor Program based in Minneapolis, Minnesota, USA to treat leukaemia.
Higgs (2008 pp. 964-966) inferred the only cure for sickle cell anaemia is using haemopoietic stem cells or through reprogramming fully differentiated somatic cells to act like embryonic stem cells. Higgs (2008 pp. 964-966) reported nearly 250 sickle cell anaemia patients treated by haemopoietic stem cells donated by HLA identical sibling. New methods of inducing pluripotency replacing the prolonged use of c-Myc (proved an oncogenic substance) or retroviruses (proven to cause insertional mutagenesis) reduce the risk of malignant transformation of pluripotent stem cells and are important steps forwards (Higgs, 2008).
The outstanding phenomenon of stem cells’ plasticity was the trigger for many clinical trials that bring hope to manage effectively degenerative diseases like Parkinson and Alzheimer’s diseases. Other diseases as renal failure, brain tumours, diabetes (type 1), and spinal cord injuries are possible areas of progress in the foreseen future (Agius and Blundell, 2008). As clinical trials give hope, they also highlight the possible challenges that face applying stem cell therapy to treat disease, as the low cell cycle time, which results in slow copying in culture.
Other challenges include instability of stem cells in culture and the possibility of producing abnormal chromosome numbers and the possibility of stem cells’ dedifferentiation after forced differentiation in culture. Even after successful stem cell production, there are problems like immune rejection and stem cell integration in the patient’s tissue to act in coordination with the body’s natural diseased or degenerated cells still likely to occur after transplantation (Pera and Trounson, 2004).
Debate on stem cells’ research
Since the cloning of Dolly, the sheep, in Scotland in 1996, the debate on cloning grew and extended to stem cell research. Despite the potential of stem cell therapy, the debates continue focusing on embryonic stem cell as destruction of embryos is ethically and theologically unacceptable by many (Hawkins, 2001).
The four key positions of the ethical debate are should embryos be considered human individuals so that not to be destroyed or used for research purposes whatever the expected benefits are? Second, do embryos have the status (legal, ethical, or theological) as foetuses or babies; and therefore cannot be used in research? Third, if using created embryos for research is unethical, what about left over embryos from in vitro fertilization? Finally, on the opposite side, some claim that embryos are just cell clusters that are indifferent from any other cells; therefore, their use in research is justified (Greely, 2006).
Scientific arguments are no less variable and strong than ethical concerns. The fundamental issues in scientific debate are the reproducibility of therapeutic results using various preparations and derivatives of stem cells. The homogeneity of resulting cell line prepared from heterogenic stem cell preparations and the question of storage (banking) to guarantee stem cells free from infection or mutagenic potential are other arguments to consider. Further, the challenge of obtaining and preserving the stable homogenous stem cell preparations is needed for reliable research, safe and effective potential treatment (Weissman, 2002, and Hurlbut, 2007).
The political debate is important as it decides government funding to research, as happened when President Bush administration stopped funding research on embryonic stem cell lines before August 2001, leaving nearly 21 stem cells lines qualifying for Federal funding (Okie, 2006).
Societal significance and public responsibility include the duties and responsibilities of those who are engaged in stem cell research. In addition, the implications of issue like instrumentalization of human embryos on the nascent life of society. Krones and others (2006 pp. 607-617) conducted a cross sectional survey, in Germany, on 101 couples attending 2 in vitro fertilization (IVF) centres and a representative sample of health care professionals and medical ethicists and their views about embryonic stem cell research and gamete donations were compared. A clear majority of IVF couples supported the notion as related to the purpose and other independent influences.
The majority of physicians were supporting the legislation of embryonic cell production from surplus embryos. Most human geneticists and obstetricians were in favour of egg but embryo donation. Midwives and medical ethicists opposed every kind of donation and research on surplus embryo. Kornes and colleagues (2006 pp. 607-617) inferred authorities should carefully consider the variability in opinions on ethical, legal or political arguments or decision-making.
Stem cell transplantation
Definition of transplantation and sources of stem cells
Stem cell transplantation is introducing genetically matched stem cell to a recipient to replace damaged body cells. It has become a therapeutic practice for blood diseases as sickle cell anaemia, and some cancers as leukaemia, lymphomas, and multiple myeloma (Avasthi and others, 2008). From transplantation perspective, there are three types of stem cells; autologous that is self-donated. Allogeneic stem cells, which are donor’s stem cells immunologically and genetically, match the recipient’s cells. Syngeneic stem cells are those donated by a twin sibling, despite being rare, yet graft versus host disease (GVHD) is less than with analogous type.
Autologous stem cells collected from the patient the re-infused commonly by apheresis, therefore, there is no risk of immune incompatibility. However, there is still a risk that diseased cells contaminating the infusion will be re-infused. Allogeneic stem cells whether collected from a haploidentical relative or sibling or a matched unrelated donor always carry a higher risk of failure, GVHD or donor cell death. The main advantage of allogeneic stem cells is induction a graft versus malignancy effect, which is the donor cells producing their immune cells that help destroy remaining recipient’s cancer cells (Ljungman et al., 2006).
Stem cells for transplantation process
The process of stem cell transplantation occurs in six steps irrespective of the source of stem cells. First stage is patient evaluation and preparation, which includes besides complete medical work aiming to assess the patient’s health, social work up, and assessment of the patient’s psychological and emotional strengths and weakness. The second stage is eligibility, based on results of previous evaluation a decision is made about patient’s eligibility for transplantation. Further, the transplantation team decides whether the patient will be admitted although the transplantation process and following stages or when to manage the patient from the outpatient and when to admit.
Third is the conditioning treatment stage, which is treatment with high dose chemotherapy or radiation therapy to suppress the patient’s immune response and thus reduce the potential of immune rejection and or destroy a greater cancer cells volume. There is neither uniform conditioning treatment nor route of administration used in all transplants. Fourth is infusion (transplantation) of stem cells, nausea and vomiting are common if the preparation is mixed with dimethyl sulfoxide, a preservative added to prevent donor cell damage by freezing. The last two stages are recovery and rehabilitation where close patients’ follow up is emphasized to monitor early or late transplantation complications (Garrett and Yoder, 2007).
Complications of stem cell transplantation
Complications of stem cell transplantation can be early (during the infusion or recovery stages) or observed later (during the rehabilitation stage or after) (Copelan, 2006). Complications are the outcome of many interacting factors resulting from the transplantation process like GVHD, immune reactions, infections, or may be because of the conditioning treatment (Wingard, et al, 2002).
The commonest early complication is GIT mucositis secondary, in most cases, to methotrexate administered in the conditioning stage to prevent GVHD. Second common is sinusoidal obstruction syndrome (painful hepatomegaly, fluid retention, and jaundice) cause by obstruction to the hepatic circulation by sloughing sinusoidal endothelium. Chronic liver disease is a recognised risk factor and since there is no treatment for this syndrome, prophylaxis (e.g. using fludarabine instead of cyclophosphamide in the conditioning phase).
Transplantation-related infections because of immune-deficiency, secondary to instrument are reduced by adopting reduced intensity regimens instead of myeloablative regimens in the conditioning stage. Transplantation induced lung injury may be delayed for four months with a mortality rate of up to 60%, and total body irradiation in the conditioning stage is an important risk factor (Copelan, 2006).
Chronic GVHD is a serious late complication that occurs at least 100 days after transplantation. Treatment is by corticosteroids that may be for long periods up to two years; in this case monitoring corticosteroid long-term treatment complications (e.g. infection, aseptic bone necrosis, osteoporosis…) is an essential practice (Horwitz and Sullivan, 2006). Hormonal disturbances as failure of ovulation, male infertility, and hypothyroidism male also occur late. Many studies report an increase in the rate of secondary cancers after transplantation, the type and intensity of chemotherapy in the conditioning stage is a risk factor (Horwitz and Sullivan, 2006).
Graft versus host disease (GVHD)
GVHD refers to immunologically mediated set of reactions caused by genetically incongruent cells to their host, based on the National Institute of Health diagnostic criteria, GVHD there are acute and chronic categories bearing in mind the organ-function influence of the reaction (Filipovich et al, 2005).
The immuno-pathological reactions of acute GVHD occur in three phases, phase I where conditioning treatment creates an appropriate host environment, in this phase survival rate is nearly 90%. Phase II relates to donor T cell activation, differentiation, and migration, Phase III is when immune effector cells and cytokines produce an end organ damage, survival rate in these phases is affected by occurrence of other complications as infection, haemorrhage, and liver cell failure (Sun et al, 2008).
Chronic GVHD occurs in nearly 30 to 50% of long-term transplantation survivors, having a patient presentation similar to an autoimmune systemic collagen-vascular disease (Horwitz and Sullivan, 2006).
Brief review on Alzheimer’s disease
Alzheimer’s disease: A clinical perspective
Alzheimer’s disease is the commonest cause of dementia, accounting for 60 to 80% of cases of dementia (Plassman et al, 2007). In early cases there is difficulty remembering names and recent events, as the disease progresses, other symptoms like impaired judgement, disorientation and uncertainty, perplexity, behavioural changes, and problems with speaking and swallowing appear. Disorientation in time, space, and place leads to inapt social behaviour, and risky wondering. In advanced cases, patients need assistance in their daily activities like eating, bathing, and dressing. At final stages, patients fail to recognize closely related individuals, lose their ability to communicate, and become restricted to bed, eventually, the disease is fatal (Sampaio, 2007).
The major risk factor is advancing age as most cases develop at 65 years, although early onset cases (as early as 30 years) rarely occur. The process of brains cells’ damage associated with Alzheimer’s disease is not yet fully vindicated. Two popular theories exist, first is the amyloid theory that explains failure of information transfer through the synapses because of beta-amyloid protein overproduction. The second theory blames Apolipoprotein E-e4 a member of three APOE gene family responsible for controlling cholesterol circulation, thus, explains the increased the plaque independent disturbance at the synapse (Ngandu and others, 2007).
There is no treatment available to cure the disease or a modifying treatment to prevent patients’ deterioration (Ngandu and others, 2007).
Neuropathology of Alzheimer’s disease
Histopathological characteristics of Alzheimer’s disease are intracellular neurofibrillary tangles (interweave) called NFTs, and extracellular deposition of β amyloid protein. Deposition of β amyloid protein occurs in the form of neuritic plaques and diffuse deposits leading to cell atrophy, reduction of synaptic transmission, and eventually cell death (Selkoe, 2002).
The β amyloid protein is the result of proteolytic cleavage of the amyloid precursor protein (APP) during normal cell metabolism,α,β, and γ secretase enzymes are responsible for the proteolytic cleavage of APP. The α-secretase enzyme action on APP results the formation of an N-terminal non-amyloidogenic soluble APP secreted in the extracellular medium and many believe it has a neuroprotective action preventing the oxidative action of the β amyloid protein. Beta-secretase enzyme works APP at its N-terminal; while γ secretase, which is composed of four membrane proteins presenilin, nicastrin, Aph-1, and Pen-2, works at the C-terminal of APP. Beta and γ secretase enzymes activity produces β amyloid protein composed of 38 to 42 amino acid residue and can form neurotoxic amyloid plaques (Robert et al, 2001, and Wolfe, 2002).
Characteristically, reactive astrocytes and microglia cells aggregate around amyloid plaques, induce an inflammatory response releasing cytokines, and chemokines, and contribute to neurone degeneration (Morgan et al, 2005).
The distribution pattern of neurofibrillary tangles (NFTs) differs from that of amyloid plaques in that NFTs are intracellular found in the degenerating cells. NFTs are composed of phosphorylated microtubules associated tau protein (a protein normally found in neuronal axons participating in cytoskeleton formation of the neurons). Thus, disruption of axonal transport occurs with subsequent progressive neuronal loss (Iqbal and colleagues, 2005).
Degeneration of brain cholinergic neurons is another fundamental feature of Alzheimer’s disease, especially remarkable in the nucleus basalis and hippocampus. This explains the cognitive impairment and non-cognitive behavioural disturbances disease characteristics (Giacobini, 2003).
Human cognitive functioning depends on synaptic density, neuropathological progression of Alzheimer’s disease include changes of synaptic density either by synaptic loss affected by β amyloid protein plaques, NFTs formation, and gliosis. The disturbed balance between synaptic reduction and emergence is another factor that changes synaptic density. One possible mechanism for this disturbed balance is accumulation of β amyloid protein precursor of the β-secretase pathway (Masliah et al, 2006).
Mechanisms of Alzheimer’s disease
In addition to the inflammatory response induced by the extracellular accumulation of the amyloid plaques, which by itself trigger neuron degeneration, NFTs disrupt axonal transport resulting in progressive neuronal loss. NFTs also disrupt neural cells’ calcium ion homeostasis, which adds to neuronal cells’ degeneration (Van Broeck et al, 2007). It is also suggested that amyloid protein accumulates in the mitochondria of Alzheimer’s patients’ brain cells, inhibiting specific enzymatic function and reducing neuronal glucose utilization (Chen and Yan, 2006).
Understanding genetics a pathway to cell biology
Familial cases of Alzheimer’ disease (AD) show an autosomal dominant mode of inheritance. The genetic mutation is near or at the sequence of amyloid protein precursor gene (APP gene) on chromosome 21 responsible for encoding the β amyloid protein. As the points of mutation are variable, several phenotype forms of the disease exist, which may vary in tendency of symptoms’ sequence, or disease progression (Theuns et al, 2006).
Familial AD is two type, Early onset familial AD (EOFAD), and late onset familial AD (LOAD), and they differ in genetic abnormalities. The genetic defects of EOFAD are on the genes encoding presenilin1 (PS1) and presenilin 2 (PS2) genes on chromosomes 1 and 14. Mutations of these 2 genes are responsible of increased level of amyloid β peptide 1-42 with greater risk of amyloid plaques deposition. On the other hand, late onset familial AD is associated with the apolipoprotein E in all its three isoforms (APOE2, 3, and 4). Apolipoprotein E is the product of corresponding alleles 2, 3, and 4 (ε2, 3, and 4). Of greater importance is having the APOE ε4 on chromosome 19, as it is associated with greater risk of developing AD (Bertram, 2007).
Theories on cell biology changes in AD suggested that in addition to genetically induced synaptic changes, tau protein changes, β amyloid protein plaques are responsible for damaging neurons, as the build up of plaques and NFTs is gradual so the disease manifestations are. This hypothesis qualified, to an extent, to explain familial forms of AD (Bertram, 2007).
Parkinson’s and Alzheimer’s diseases are the commonest neurodegenerative disorders characterised by dementia besides, in case of Parkinson’s disease, rigidity, tremors, and bradykinesia. Both have pathological similarities as loss of neurons, in Parkinson’s disease it occurs mainly in the substantia nigra instead of hippocampus and cerebral cortex in AD. In Parkinson’s disease, there is deposition of Lewy bodies, instead of amyloid plaques in AD, and Lewy neuritis (protein filaments) instead of NFTs in AD. This evoked the question of genetic basis of Parkinson’s disease similar to Alzheimer’s disease (Nussbaum and Ellis, 2003).
The observation that 10 to 15 % of Parkinson’s disease patients have positive family history for relatives of the first or second-degree suffering from the disease pointed out to study possible genetic factors of the disease (Nutt and Wooten, 2005). Mutant genes on chromosome four responsible for the control of alpha synuclein protein (in dopamine secreting nerve cells) may be the cause of autosomal dominant Parkinson’s disease. Failing genes on chromosome six, responsible for the control of parkin molecule (needed for cell self-destruction of alpha synuclein) may be responsible for autosomal recessive Parkinson’s disease.
Research suggests that in cases with positive maternal history of Parkinson’s disease, nerve cells mitochondria DNA defects are worthy of further investigation. A mutant tau gene has been recently described; this gene control controls the support system responsible for transferring nutrients through nerve cells. This gene defect may be specifically responsible for late (old age) onset Parkinson’s disease (Klein and Schlossmacher, 2006).
The search for a common genetic background for neurodegenerative Alzheimer’s and Parkinson’s disease revealed two omega class genes, GSTO1 and GSTO2 located on chromosome 10 are linked to late onset AD and Parkinson’s disease. These genes play an important role in the making of the cytosolic glutathione transferase enzymes needed to detoxify and bio-activate xenoantibiotics and reduction of methylated arsenicals (insecticides) like monomethyl arsonate (Board and Anders, 2007).
Recent studies that NEDD9 gene (neural precursor cell expressed, developmentally down-regulated 9) is a possible common gene to neurodegenerative disorders. However, a genotyping study by Chapuis and colleagues (2008, p. 2863) on 3176 cases, failed to find NEDD9 gene association with either Alzheimer’s or Parkinson’s disease, and the gene can a weak genetic determinant to the onset of the disease (Chapuis et al, 2008).
The prion (infective) theory of Alzheimer’s disease
Genetic mutations account for nearly 50% of early onset AD, and sporadic AD cases and new reports of possible gene mutations are not promising. In an article by the Nobel Prize laureate in medicine, 1997 Prusiner, Stanley, B (1998, p. 13363), defined prions as causative infectious pathogens to neurodegenerative disorders in humans and animals. In humans, prions may have a role in Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis among other neurodegenerative disorders. In animals, prions may be the cause of Bovine spongiform encephalopathy (mad cow syndrome).
All prions’ presented disorders include conversion of the normally present prion protein to a modified form through a post-translational process. Prions are transmissible particles with no nucleic acid and seem to compose of a modified protein. The sequence of the chromosomal prion protein decides the species of a certain prion in mammals. The abnormal prion protein may act as a template for the modified prion protein to refold (Prusiner, 1998).
Shorter and Lindquist (2005, p. 435) suggested the normal cellular protein has a role in maintaining long-term memory and the gene absence, in mice, may be accompanied by faulty hippocampal long-term memory effectiveness. Another research by Zhang and colleagues (2005, p. 2184), suggested expression of the normal prion protein on stem cells is a prerequisite for self-renewal and absence of this protein is accompanied by increased tendency to cell death.
Stem cell research in Alzheimer’s disease
For centuries, biologists noticed that certain animals like starfish and newt (a tailed amphibian) have the ability to renew their body parts. Man was no exception although cannot renew a part but regenerates organs like blood and skin. Human cells that can regenerate tissues were illustrious in mid 1950s through bone marrow experiments; this established the concept of availability of stem cells in our bodies (the National Academies report, 2007).
In 1998, James Thompson, of the University of Wisconsin, Madison developed the first stem cell line from the inner cell mass of early embryos. Since then, biomedical research geared and researchers pursued two main strategy lines to replace damaged brain cells. First is a to start with undifferentiated nerve cells that grow experimentally to become suitable for patient transplantation. Second is to identify trophic stimulating factors that excite the patient’s stem cells.
The first effort to treat a neurodegenerative disorder (Parkinson’s disease) by cell transplant was in 1989, by Allen and colleagues (after Reyes, 2004, p.315), who transplanted dopamine secreting cells from a patient’s suprarenal gland. However, results were limited, short lived and inconsistent. The development of stem cell technology is giving hopes, despite roadblocks, to many patients with chronic incurable disabling diseases (Reyes, 2004).
Neurogenesis in Alzheimer’s disease brain
Regeneration is a characteristic feature of all living organisms including man, which in many cases take a hypertrophic pattern rather than replacement of damaged cells. A major challenge is to understand why replacement of damaged cells occurs in certain tissues and not in others. It is a scientific fact that regeneration needs adequate blood supply, oxygenation, and nutrients, which bring macrophages to the site of damage not only to remove debris but also to release cytokines and growth factors.
These substances stimulate the division of precursor cells. Regenerating tissues takes origin from progenitor cells present at that tissue, and morphogenesis of regenerating tissue depends on physical and environmental factors as tissue microcirculation that helps recruitment of tissue-specific progenitor cells. Therefore, for regeneration and damaged cells replacement a cellular source is conditional, the current prominence of stem cells may produce working techniques to repair organs that have no potential to regenerate as the brain and islets of pancreas (Carlson, 2005).
The concept that no postnatal new neurons are added to the human brain and changes can only occur through rewiring of synaptic network has changed by results of autoradiography, proliferation biomarkers studies, and antibodies’ studies against neuronal and glial markers. These methods showed adult brain Neurogenesis exists and confirmed neuronal behaviour and integration (Jessberger and Kempermann, 2003). Areas of brain neurogenesis displayed are the supraventricular, olfactory bulb areas, and the granular cell layer of the hippocampus. Neural stem isolated in vitro from the cortex and thalamus failed in vivo to turn into the active state.
Three subtypes of neural stem cells identified the type B stem cells (astrocyte-like), cells positive for glial fibrillar acid protein, and embryonic radial glia stem cells (Gregg and Weiss, 2003). Neural stem cells can divide asymmetrically and slowly to astrocyte-like cells. Alternatively, they may rapidly divide to produce precursor cells (C-cells) (Nakatomi et al, 2002). Adult neural stem cells, in the human brain, closely associated to the angiogenic niche; thus, they are always close to blood vessels and react to the same stimuli of the endothelial stem cells (Shen et al, 2004).
Because of the complex neuro-regeneration regulation mechanisms and the altered behaviour of neural stem cell and progenitor cells in Alzheimer’s disease, it is expected that disturbed hippocampal regeneration and endogenous compensation of lost neurons be altered (Leutgeb et al, 2004). As Neurogenesis depends on the angiogenic niche, deposition of β amyloid plaques may interfere with stem cell behaviour either through loss of vascular cells or through accumulation of active microglia cells and the resulting inflammatory reaction (Monje et al, 2003).
On the other hand, Sohur and colleagues (2006, p. 1477) suggested the potential of endogenous neural precursors in other brain areas to replace damaged cells in neurodegenerative disorders. They showed that cells like astroglia and oligodendroglia are multipotent precursor cells that possess plasticity and are capable of differentiating into neurons despite their integration into some brain areas being limited.
They also showed that such cells could migrate both tangentially and radially to other brain areas and can extend axons to considerable distances. They inferred that with further research, the response of these endogenous precursor cells, their molecular and genetic control and behavioural manipulation would be more lucid. However, there are unanswered questions about neural replacement either through endogenous precursors or through transplantation of donor cells. First is to examine the multiple signals responsible for division, migration, differentiation, axon extension, survival, and integration (Sohur, 2006).
Neurogenesis and the potential of stem cell therapy
Given the facts that granular cell layer of the hippocampus is one of the brain areas where neurogenesis continues through adult life. In addition, it is an area heavily loaded with β amyloid plaques, a characteristic neuropathological feature of AD, Florian and colleagues (2008, p. 1520-1528), worked on mouse models of cerebral amyloidosis. They aimed to assess the influence of β amyloid deposition on neural stem cells and subsequently on hippocampal neurogenesis.
Results showed reduced neurogenesis in experimental animals compared to controls; they took another step to assess whether the differences are because of amyloid-induced changes at the stem cells level or other causes. Florian and others (2008, p. 1528) inferred their work shows evidence of disturbed neural stem cells biology because of amyloid environment. They also inferred that based on variations in β amyloid deposits, the effect on neurogenesis varies among the Alzheimer’s disease mouse models studied.
Hallbergson and colleagues (2003, p. 1128) defined neurogenesis as a three steps process, first is proliferation that is generation of cells, second is targeted migration, and third is terminal differentiation into distinctive neural cells phenotypes. Unless the three phases are fulfilled, the process cannot be neurogenesis, in other words new hippocampal neurons should contribute to memory processing and share in the brain’s response to depression. Hallbergson and colleagues (2003, pp. 1028-1029) made two observations on the plasticity of adult neurogenesis, first is the mechanisms of regulation are not fully understood. Second adult neurogenesis is a dynamic process with different responses based on the associated environmental facto (blood supply, oxygenation, and nutrition).
Strategies of Alzheimer’s disease stem cell therapy
Neural stem cells collected multiplied in culture and developed to functional neurons make autologous stem cell transplantation a stimulating possibility. However, there are hardships to this approach including long-term culture outcome, the generation and behaviour of the neurons after transplantation. Further genetic defects and impaired functionality after transplantation are also major concerns (Snyder and Olanow, 2005).
On the other hand, as the old code of belief of no neurogenesis in the adult brain became invalid and new neurons generated in different brain parts raise another possibility. The potential of endogenous brain neurogenesis became as stirring as replacement by transplantation. However, the limitations are also challenging, lack of knowledge about human stem cells biology, stimuli for division and differentiation, besides lack of knowledge about the precise neurodegenerative diseases’ mechanism are major roadblocks (Pessina and Gribaldo, 2006).
Adult existing human brain stem cells and progenitors that have the potential to differentiate into neurons raise the expectation for new strategies (Kornblum 2007). Olstron and colleagues (2007, p. 1089) examined human adult brain stem cells and progenitors after transplantation into adult mice brains about migration, proliferation and differentiation. Their results displayed that human cells survive, show targeted migration, and differentiate after transplantation into adult mice.
Currently, there are two basic strategies to repair neurodegenerative disorders, which are endogenous stem cells repair and stem cells transplantation (Hallbergsonet al, 2003 and Olstron et al, 2007).
Given the observed plasticity of adult neural stem cell and progenitor cells, several studies suggested improving the environmental factors could initiate neurogenesis through stimulation of self-renewal of progenitor cells already present. However, studies could not elicit migration, probably because of lack of knowledge of the proper trophic factors. Another reason is the lack of knowledge of regulating mechanisms to the response of progenitor cells to environmental factors (Hallbergson et al, 2003). Pessina and Gribaldo (2006, p. 2287) showed that pluripotent adult stem cells can change to any phenotype (transdifferentiate), which raises their therapeutic potential on successful mobilization to a targeted tissue.
Mobilization of brain progenitors available to nearby damaged brain structures through use of brain derived trophic factors form the basis of recruitment strategy for endogenous neurogenesis. However, this approach needs to delivery of trophic factor in a regular and targeted manner. This may take place either systemically on condition they cross the blood-brain barrier or directly by intracranial delivery which may not be suitable for humans (Hallbergson et al, 2003).
Stem cell transplantation (replacement strategy)
Although the endogenous strategy seems convincing; yet, the major roadblock of not knowing the migratory distance of endogenous progenitor cell keeps the door open for replacement strategy to develop. The main objective of this strategy is to develop a population of defined progenitor cells in culture, designing it to perform a certain lineage, then transplanting (grafting) it to the injured brain area. The main difficulty is the non-neurogenic brain environment may not allow complete differentiation. Thus, in addition to designing the proper cell lineage, researchers need to create proper supportive brain environment (Hallbergson et al, 2003).
Sugaya and colleagues (2006, p. 87) found that human neural stem cells transplanted to rat’s brain have differentiated well and resulted in significant improvement of animals’ cognitive functions. Further, they described new technologies to help human mesenchymal stem cells obtained from the bone marrow to differentiate into neural cells. In addition, they reported using a pyrimidine derivative to increase endogenous stem cells’ proliferation. They inferred further studies are essential before promising high expectations for clinical validity.
Stem cell therapy of Alzheimer’s disease: research status
As explained by Kornblum (2007, p. 810) mobilization of the endogenous brain stem cells is a possible source of neural stem cells. The work of Nakatomi and colleagues (2002, pp. 429-441) is a significant illustration of the endogenous stem cell strategy of research. To stimulate pyramidal hippocampal progenitor cells, Nakatomi et al (2002, pp 429-441) induced temporary ischaemic injury to selective parts of rats’ hippocampus (CAI1 of Ammon’s horn); they performed intraventricular infusion of a growth factor to expand the hippocampal-progenitor cell response.
Their results showed the growth factor stimulated the growth of neural stem cells evidenced by the increased number of new neurons. In addition, there was evidence of the new neurons’ integration in the brain circuitry with significant contribution in improving the neurological defects. Of significance was the longevity of the newly regenerated cells as the new neurons survived up to six months. They inferred, based on behavioural studies, that growth factor treated animals developed better spatial orientation and memory (Nakatomi et al, 2002).
Nadareishvili and Hallenbeck (2003, pp. 2355-2356) discussed endogenous adult brain stem cell migration to the site of injury in stroke through analysing the work of Arvidsson and colleagues (2002). To induce proliferation neural progenitor cell, Arvidsson and colleagues (after Nadareishvili and Hallenbeck, 2002) performed temporary middle meningeal artery occlusion. They were labelled by a thymidine analogue for detection, results showed increased cell proliferation and migration to the damaged striatum.
Two weeks later, Arvidsson and colleagues were able to confirm that 20% of new were differentiated to mature neurons. Five weeks later, researchers were able to detect 42% of new neurons showed proper phenotypic differentiation. However, none of the new neurons migrated to the cortex, besides; survival rate of newly formed neurons was low (after Nadareishvili and Hallenbeck, 2003).
Previous studies examined endogenous brain stem cell proliferation in acute brain injury, while Alzheimer’s disease is a chronic degenerative condition. Jin and colleagues (2004, p. 343) examined the effects of chronic neurodegeneration on neurogenesis in the brains of Alzheimer’s disease (AD) patients. They traced the expression of neuronal marker proteins (doublecortin, neurogenic differentiation factor and TUC-4), which indicate development of new neurons in AD patients’ hippocampus. Their results showed increased proliferation of new neurons in hippocampal granular cell layer and CAI (main hippocampal sites of pathology in AD).
They inferred that despite of chronic degeneration, neurogenesis builds up in AD hippocampus; therefore, stimulating hippocampal neurogenesis may be an effective therapy strategy (Jin et al, 2004). Jin and colleagues (b, 2004, p.13363) reproduced the effects of AD on neurogenesis in experimental animal model, they tested increased neurogenesis in such models. Jin et al (b, 2004, p. 13363) inferred neurogenesis is more like a compensatory mechanism to chronic neurodegeneration in AD and augmenting neurogenesis in these cases may be a therapy strategy.
Thored and colleagues (2006, p. 739) studied the longevity of endogenous newly formed neurons in ischaemic brain insults. Their results showed, in response to brain ischaemia, newly formed neuroblasts maintain generation without decline for four months in experimental rats’ models. Further, they pointed to a stromal cell-derived 1 factor regulation the new neuroblasts formed to target areas. They inferred this might be a new self-renewal pathway to improve therapy after brain ischaemic insults.
As growth and size of the developing brain are determined by the balance between neural cells proliferation and differentiation. Falk and colleagues (2008, p. 472), identified (Transforming growth factor beta) TGFβ as a crucial factor of the signaling pathways responsible for stem cells and progenitor cells proliferation. Further, they stated ablation of transforming growth factor, beta receptor II, Tgfbr2, a gene responsible for making TGFβ results in increased proliferation and horizontal expansion of neuroepithelial cells and enhancement of neural stem cells self-renewal. They inferred TGFβ controls the size of specific areas in the midbrain and regulation of self-renewal of neuroepithelial stem cells.
The second main strategy in therapy research of Alzheimer’s disease is stem cell transplantation. Stem cells in research studies collection are from foetal or adult experimental animals, foetal, embryonic, or adult human stem cells. Alternatively, stem cells collection is from non-neuronal origin like mesenchymal cells of bone marrow (Olstorn et al, 2007).
Transplantation research studies differ based on the expectations from stem cells used. If replacement of degenerating non-functioning cells is the objective, then the use of grafted cells is in place as the technique allows in vitro manipulation of cells to enhance survival, and integration with the host pathways. On the other hand, this may not always be necessary, as transplanted cells might be necessary to stimulate secondary effects as self-renewal or protecting brain cells at the site of tissue damage (Martino and Pluchino, 2006).
There are reports that neural transplants produced functional brain recovery in experimental models, yet the mechanism of recover is not clear. Oliveira and Hodges (2005, p. 87) responded to this question and suggested four possible mechanism for the successful action of transplanted neural cells. First is the reconstruction mechanism where transplanted cell succeeds in replacing targeted degenerated or damaged cells in the same way as neurophysiological recovery occurs. Second is that transplanted cells become capable of releasing substance that might stop degeneration, promote regeneration, or protect the healthy cells. This requires prior genetic engineering to the transplanted cells tailoring them to do any or all of these functions.
Third is, depending on the plasticity of neural stem cells, they may induce structural reorganisation through indirect stimulation of neurotrophic factors. Transplanted cells may act as a substrate for axonal growth to the damaged area either directly as with glial stem, cells being rich in developmental factor, or passively acting as a scaffold for new axons regeneration (Oliveira and Hodges, 2005). The question now is can stem cell grafting work in Alzheimer’s disease therapy.
Whereas there are more than 350 Parkinson’s disease patients received embryonic stem cells as a line of therapy (Lindvall and Bjorklund, 2004). There are similar reports on Alzheimer’s disease; however, research reports on the use of embryonic stem cell (being rich in acetyl choline, adult neural stem cells, and mesenchymal stem cells of bone marrow are available (Oliveira and Hodges, 2005).
Because of potency and plasticity, foetal stem cells would be the first choice for replacement treatment; however, because of the debate about its uses, embryonic stem cells obtained from aborted foetuses are a reasonable alternative. The embryonic stem cells have the advantage of greater in vitro expansion capability that is greater production efficiency. They have outstanding abilities to proliferate, migrate, and self-renewal.
In addition as the progresses to form a neurone, differentiation decreases while predictability of the neuronal phenotype increases. Thus, the transplant safety increases, suitable distribution of cells after transplantation, directed axon projection for longer distances, and controlled neuro-transmitter release. On the other hand, unfavourable immune reactions and tumour formation are the main disadvantages (Guillaume and Zhang, 2008).
Autologous transplantation could evade these problems, Westerlund and others (2005, p.779) collected human brain stem cells from 13 hydrocephalus patient during scheduled surgery endoscopically from the ventricular wall. They cultured the cells collected as neurospheres (non-adherent floating cells) and induced differentiation, one-week later proliferating cells showing markers for neurons appeared. Two weeks later neuronal phenotypic cells showed repetitive short-lasting action potentials. They inferred neural tissue suitable for autotransplantation could be produced from collected human brain stem cells; however, culture methodology and protocols to control phenotypic differentiation are still to develop.
Motivated by successful use of mesenchymal bone marrow stem cells in the treatment of certain leukaemias, lymphomas and blood disease, Tondreau et al (2008, p. 166) investigated the use of bone marrow stromal mesenchymal cells for transplantation replacement therapy. Their results suggested the possibility of these cells to differentiate into neuronal cells with proper gene expression and functional properties, and therefore, these cells may suitable alternative to treat neurodegenerative disorders. Despite the initial success, few studies investigated the properties and behaviour of grafted adult neural stem cells (Galvin and Jones, 2006).
Other studies reported significant longevity and plasticity of adult human brain stem cells, however in many of the studies no report was made about the nature of brain injury, which is important for survival, migration and differentiation (Walton et al, 2006). Moe and colleagues (2005, p.1182), showed that a single adult human brain stem cell can develop, in culture, to a mature functional neuronal network. Testing this finding experimentally, the transplanted cell into rats’ brains remained significantly immature with a significant number of cell death. They explained the results on the ground of rats’ immune response despite receiving immunosuppressant agents.
Oliveira and colleagues (2008, p. 50) reviewed stem cell grafts as a therapeutic tool for CNS disorders, about Alzheimer’s disease, they inferred that reconstruction of the cholinergic circuits should be the main objective for stem cell therapy. The evidence is in animal models induction of cholinergic lesions produced manifestations similar to Alzheimer’s disease, and that rats with lesions to the basal part of the forebrain improved as they received transplants into the cholinergic projection.
About transplant replacement strategy, they pointed that as most Alzheimer’s patients are elderly and the decision to do a transplant would include an intracranial procedure make the clinical decision delicate and needs reassessment. They inferred further research is still necessary before judging stem cell therapy for Alzheimer’s disease can play an important role in therapy.
The chimera issue in stem cell research for Alzheimer’s disease therapy
A recent intense debate started on the use of chimera (a hybrid organism with a mixture of cells from two different organisms of the same species) in stem cell research. Advocates claim that transplanting human pluripotent stem cells into non-human foetal or young postnatal animal is useful to observe cells’ growth and differentiation. Alternatively, others believe human stem cells transplanted into non-human blastocyte or embryo should provide a chance to disclose the full growth and differentiation potential. The ethical, more, and theological debate against these views is immense, scientifically, however, implantation of human stem cells in the early stages of non-human’ life would mix it up with the recipient’s inner cell mass developing a hybrid creature (Baylis and Fenton, 2007).
Difficulties of stem cell research to treat Alzheimer’s disease
Bryne and Howells (2003, p. 165) itemised the problems with stem cell therapy specific to the CNS as follows:
- Despite rapid progress in research, identification, enhancement and proliferation of stem cells remain problematic.
- There is a problem of integration of the transplanted stem cells in the neural network.
- There is lack of knowledge of growth and trophic factors that stimulate endogenous stem cells. In addition, research should produce information about the factors that preferentially drive transplanted stem cells to differentiate into glial or neuronal mature cells.
- There is a problem of stem cell delivery in multilocular brain problems as Alzheimer’s disease.
- The progressive nature of degenerative brain diseases may interfere with the environment of the transplanted cells. This may create problems of migration to targeted area or problems of differentiation.
- There are problems of immune rejection and tumour formation.
Besides these difficulties, other roadblocks need further explanation. First is the problem of neural cell identity. Nakafuku and colleagues (2008, pp. 829-830) stated that despite the extensive research in regenerating the degenerating brain the identity of neural cells is still ambiguous. As the neural stem cells role is to replace damaged neural cells used to perform higher functions as memory and learning and integrate in the overall neural network, there are two types of neural stem cells. The quiescent ependymal and the subendymal stem cells in the subventricular zone.
Based on these facts, no single marker can characterize them in situ; further as stem cells basic identity is functional, thus, identification needs to display their properties of self-renewal and plasticity. Thus, identification of neural stem cells depends on in vitro proof with all variables that may affect the results. Nakafuku and colleagues (2008, pp. 829-830) explained as there are more than one population of neural stem cells contributing to neurogenesis, knowledge of the extent of contribution of each population becomes a necessity. The authors inferred that responding to these uncertainties as it leads to better understanding of brain function should drive therapeutic research to a successful outcome.
Taupin (2008, p. 131) reviewed the role of neuroinflammation in neurogenesis being a common factor in degenerative brain disorders. Taupin (2008, p. 131) suggested it may have a role in neurogenesis adjustment but still the significance and extent of this role are to be clarified. Many reports point to neuroinflammation as having a useful or a damaging effect, because of disturbed vascularity, on adult brain neural stem cells. Taupin (2008, p. 131) inferred that future strategies may include pro or anti-inflammatory treatments to enhance neural stem cells action.
Among the known difficulties are those of lack of finance and governments funding. Mason and Dunnill (2008, pp. 351-363) studied the cost of five major diseases in USA that could benefit from stem cell therapy; namely diabetes, late stage renal disease, stroke, Parkinson’s disease, and spinal cord injury. Their results suggested regenerative medicine if succeeded to treat these conditions would have a financial advantage. They inferred that lack of governments and insurance companies’ funding would drive researchers to pharmaceutical and devices’ companies.
The path to improving stem cell therapy for degenerative brain diseases
Although stem cell research results promise a future of stem cell strategies to treat Alzheimer’s disease, it is noteworthy to acknowledge the complexity of the disease pathology. Therefore, further research in this area is required before the disease can be successfully treated (Agius and Blundell, 2008). Neural stem cell research has advanced in the last few years; however, the clinical application of research result remains bound to better understanding of stem cell biology about proliferation, differentiation, and migration. These are essential areas to understand properly to engineer safe and effective stem cell therapy for Alzheimer’s disease (Bithell and Williams, 2005).
Improving stem cell proliferation
One of the main problems in stem cell research is although they are available in almost all human tissues and many sources, yet they are rare cells. Therefore, proliferation of these cells in culture (in vitro) is an essential step; however, there is no agreement on what to use to promote proliferation. Cytokines and growth factors are the commonest, yet, researchers cannot easily separate proliferation from differentiation, second, stem cells are available in vivo in a niche of stromal cells and endothelial cells, so the question is can researchers create a feed layer, in vitro, to support stem cells proliferation (Majka et al, 2005). Maillard and colleagues (2005, p. 945) described the Notch pathway based on gene introduction to inhibit differentiation but not proliferation.
Improving stem cell isolation
There are two problems with stem cell marking and isolation first is the larger number of niche cells that is the impurity of the stem cells specimen obtained, second is the lack of specific stem cells markers. CXCR4 is the first known stem cell marker (discovered in the late 1990s), (Majka et al, 2005). Reiss and colleagues (2002, p. 295) described CXCR4 on neuronal stem cells, later Padovan and colleagues (2003, p.) described CD133 marker on haemopoietic stem cells, endothelial progenitors and neural stem cells.
Improving differentiation, migration and proliferation
Wennersten and colleagues (2004, pp. 88-96) showed that foetal human neural stem cells can proliferate, differentiate, and migrate satisfactorily when transplanted into experimental rats’ injured brains. However, they reported the technique of stem cell proliferation still needs further development both in culture and transplantation techniques.
Improving stem cell safety
Stem cells closely interact with and are affected by the recipient physiology, therefore, looked at as biologically dynamic structures. They are cultured, proliferated, and manipulated in laboratory environment, thus safety considerations are significant. Donor assessment is the first step; the second measure is to use established standardized and controlled practices to enhance cultured stem cells safety.
The current method of culturing human stem cells on mouse fibroblast cells, as a feeder layer increases the risk of xenografting (i.e. using animal cells for human transplantation). Therefore, alternative methods sought and tested for culturing stem cells on other feeder layer add to safety. Reviewing the literature, two main questions remain unanswered, first is detailed characterisation of human stem cells. Second is to proceed with animals experiments proofs of concepts have to exist that is what these stem cells are supposed to do. Toxicity testing and assessment of proliferation and differentiation to function planned for in the experimental animals add to safety of stem cell research (The National Institutes of Health, 2001).
The need for an engineered stem cell culture
Stem cell cultures are not only important for stem cell proliferation and manipulation but also they are important for quantitative characterisation, potency, and differentiation. They are also important for objective comparison of stem cells obtained from different sources, thus developing protocols. The first step in transforming a biological discovery to a technology is process analysis. Process analysis is to quantify the different variables and to evaluate the factors that influence these factors. Looking at stem cells proliferation and differentiation as a process that needs transfer to a working technology then engineering of new culture methods is a necessity.
Because the variables in stem cell research are many and interacting, the need for quantification is great (Thomas et al, 2008). Thomas and colleagues (2008, pp. 152-158) described new automated stem cell culture systems to develop populations suitable for clinical use. They suggested that these systems provide a practical potential to production units, add significance to the output and introduce a multi-disciplinary team approach to issues of stem cell engineering in regenerative medicine.
Stem cells are precursor cells having the properties of self-renewal and multipotency. There are many sources of stem cells, embryonic (inner cell mass of the blastocyst), foetal, and adult stem cells. Stem cell therapy brings hope to many patients who are suffering from chronic degenerative diseases for which there is no current treatment. The causes and pathogenesis of brain degeneration in Alzheimer’s disease are still developing. The application of stem cell therapy in Alzheimer’s disease depends on research providing a better understanding of the complexity of adult brain circuity, the regulatory mechanisms of neurogenesis, and the signalling pathways controlling stem cells plasticity, differentiation and fate.
The knowledge that adult stem cells exist in human brain created the strategy of recruiting these cell to perform neuroregeneration, alternatively, transplantation of stem cells to replace damaged brain cells in under research. In either case, problems of stem cell survival, migration and differentiation to proper neural cell types are still hurdles to overcome. Despite its promising prospects, stem cell therapy for Alzheimer’s disease is still the lab research stage and is far from being a reality. As such research efforts on Alzheimer’s disease, brain neurophysiology in neurodegenerative disorders, improvement of stem cell lines, and developing new strategies should continue.
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