The Elimination of Cytotoxic Senile Plaques in Alzheimer's Disease Using Stem Cell Therapy - Term Paper Example

The Elimination of Cytotoxic Senile Plaques in Alzheimer's Disease Using Stem Cell Therapy Contents Introduction Materials and Methods Results Discussion of Further Work Bibliography Abstract This paper discusses the issues surrounding the elimination of cytotoxic senile plaques in Alzheimer's disease using stem cell therapy. The causes of Alzheimer's disease, in particular the production of cytotoxic senile plaques, and the techniques, and potential applications of, stem cell therapy are first discussed, and then the applications of these techniques to Alzheimer's disease will be analyzed. Introduction Alzheimer's disease is a progressive neurodegenerative disease, characterized by degeneration in intellect and a general growing disability to manage normal daily tasks or routines. The disease is caused by the loss, or atrophy, of neuronal cells, in conjunction with the deposition of cytotoxic amyloid plaques and neurofibrillary knots, with genetic factors thought to be important in the formation of the disease. It is known that the laying down of amyloid protein outside the nerve cells is one factor contributing to the onset of Alzheimer's disease, as this protein basically acts to form neuritic - cytotoxic - plaques, which begins the pathological process which leads to Alzheimer's disease. It is known that in later stage Alzheimer's disease, abnormal protein filaments - formed from tau proteins - also appear in the neurons in the brain, which normally act to stabilize the microtubules but in this case, due to the formation of paired helical filaments, destabilizes the system. What are the causes of the disease There are currently three major hypotheses put forward to explain how the disease is caused, one of which, the cholinergic hypothesis argues that acetylcholine deficiency, through a deficiency in its production, is responsible for the deterioration seen, as acetycholine is normally responsible for transmitting information in the neurons. This hypothesis has generally been dismissed by the research community, however, as treatment with acetycholinesterazes, the enzymes that break down acetylcholines, in order to stop the loss of acetylcholines, have been found to be ineffective. Other researchers have concentrated on the tau-proteins, as it is believed that abnormalities in the tau proteins lead to the symptoms seen in Alzheimer's disease, whereas others concentrate on the formation of beta amyloid deposits, as a major cause of Alzheimer's disease. It is noted, however, even by researchers working on understanding these two possible causal pathways, that the presence of cytotoxic plaques or protein tangles does not per se explain the onset of Alzheimer's disease, and so many researchers, whilst concentrating on these research avenues, keep an open mind as to the possible causes of Alzheimer's disease. Whichever research avenue research groups follow, it is generally agreed that the neuropathological changes seen in Alzheimer's disease represent an interaction between the ageing process in which normal intellectual function is retained, and changes which are specifically associated with severe cognitive deterioration (Wischik et al, 1992). Molecular analysis of these changes has tended to emphasize the distinction between neurofibrillary pathology, which is intracellular and highly correlated with cognitive deterioration, and the changes associated with the deposition of extracellular amyloid, which appears to be widespread in normal ageing (Wischik et al, 1992). Extracellular amyloid deposits consist of fibrils composed of a short 42 amino acid peptide (beta/A4) derived by abnormal proteolysis from a much larger precursor molecule (APP), and the recent demonstration of a mutation associated with APP in rare cases with familial dementia, neurofibrillary pathology in the hippocampus and atypical cortical Lewy body pathology raises the possibility that abnormal processing of APP could be linked directly with neurofibrillary pathology (Wischik et al, 1992). Neurofibrillary tangles and neuritic plaques are sites of dense accumulation of pathological paired helical filaments (PHFs), which are composed in part of an antigenically modified form of the microtubule-associated protein tau, and it has been found that the average brain tissue content of PHFs measured biochemically does not increase in the course of normal ageing but increases 10-fold relative to age-matched controls in patients with Alzheimer's disease (Wischik et al, 1992). It has also been found that there is also a substantial (three-fold) disease-related decline in normal soluble tau protein relative to age-matched controls, and it is thought that this intracellular redistribution of a protein essential for microtubule stability in cortico-cortical association circuits may play an important part in the molecular pathogenesis of dementia in Alzheimer's disease, but the role of abnormal proteolysis of APP in this process remains to be elucidated (Wischik et al, 1992). Immunohistochemical studies on renal dialysis cases have failed to detect evidence of neurofibrillary pathology related to aluminium accumulation in brain tissue, nevertheless it needs to be seen whether more sensitive biochemical assays of neurofibrillary pathology can demonstrate evidence of an association with aluminium (Wischik et al, 1992). The current paper looks in detail from the cytotoxic plaque formation i.e., beta amylase deposition, viewpoint, and will look, from this point on, at the role of stem cell research in potentially overcoming this disease-causing pathway. As we have seen, extracellular amyloid beta (Abeta) peptides have long been thought to be a primary cause of Alzheimer's disease, but the detection of intracellular neuronal Abeta1-42 accumulation before extracellular Abeta deposits has led to the questioning of the relevance of intracellular peptides in AD (Zhang, 2002). Zhang et al. (2002) directly addressed the issue of whether intracellular Abeta is toxic to human neurons, and through testing showed that microinjections of Abeta1--42 peptide or a cDNA-expressing cytosolic Abeta1--42 rapidly induces cell death of primary human neuron, whereas in contrast, it was found that Abeta1--40, Abeta40--1, or Abeta42--1 peptides, and cDNAs expressing cytosolic Abeta1--40 or secreted Abeta1--42 and Abeta1--40, were not toxic, with as little as a 1-pM concentration or 1500 molecules/cell of Abeta1--42 peptides being neurotoxic, and the nonfibrillized and fibrillized Abeta1--42 peptides found to be equally toxic (Zhang et al., 2002). In contrast, Abeta1--42 peptides are not toxic to human primary astrocytes, neuronal, and nonneuronal cell lines, and it has been found that the inhibition of de novo protein synthesis protects against Abeta1--42 toxicity, indicating that programmed cell death is involved in this pathway (Zhang et al., 2002). Bcl-2, Bax-neutralizing antibodies, cDNA expression of a p53R273H dominant negative mutant, and caspase inhibitors prevent Abeta1--42-mediated human neuronal cell death, which taken together, directly demonstrates that intracellular Abeta1--42 is selectively cytotoxic to human neurons through the p53--Bax cell death pathway (Zhang et al., 2002). In terms of the exact mechanism of the formation and pathology of cytotoxic plaques in Alzheimer's disease, it is known that glial activation contiguous to deposits of amyloid peptide (A) is a characteristic feature in Alzheimer's disease (Ishii et al., 2000). Ishii et al. (2000) performed complementary in vitro and in vivo experiments to study the extent kinetics, and mechanisms of microglial generation of nitric oxide (NO) induced by challenge with A, and their research showed that A fibrils dose-dependently induced a marked release of stable metabolites of NO in vivo that was strikingly similar regarding extent and temporal profile to the one in the parallel designed microglial cell culture experiments, however, costimulation with interferon , which was a prerequisite for A-induced NO generation in vitro, was not required in vivo, demonstrating that factors are present in the living brain that activate glial cells synergistically with A (Ishii et al., 2000). Therefore, the research of Ishii et al. (2000) showed that in Alzheimer's disease, deposits of A fibrils alone may be sufficient to induce a chronic release of neurotoxic microglial products, explaining the progressive neurodegeneration associated with this disease (Ishii et al., 2000). It has been noted that the observation that systemic administration of selective NOS inhibitors abolish A-induced NO generation in vivo may have implications for the therapeutic treatment of Alzheimer's disease (Ishii et al., 2000). In summary, then, Alzheimer's disease, the cause of one of the most common types of dementia, is a brain disorder affecting the elderly and is characterized by the formation of two main protein aggregates: senile plaques and neurofibrillary tangles, which are involved in the process leading to progressive neuronal degeneration and death (Maccioni et al., 2001). Neurodegeneration in Alzheimer's disease is a pathologic condition of cells rather than an accelerated way of aging. The toxic senile plaques (so-called cytotoxic cells) are generated by a deposition in the human brain of fibrils of the beta-amyloid peptide (Abeta), a fragment derived from the proteolytic processing of the amyloid precursor protein (APP) (Maccioni et al., 2001). Tau protein is the major component of paired helical filaments (PHFs), which form a compact filamentous network described as neurofibrillary tangles (NFTs), and experiments with hippocampal cells in culture have indicated a relationship between fibrillary amyloid and the cascade of molecular signals that trigger tau hyperphosphorylations (Maccioni et al., 2001). Two main protein kinases have been shown to be involved in anomalous tau phosphorylations: the cyclin-dependent kinase Cdk5 and glycogen synthase kinase GSK3beta: Cdk5 plays a critical role in brain development and is associated with neurogenesis as revealed by studies in brain cells in culture and neuroblastoma cells, and it is thought that deregulation of this protein kinase as induced by extracellular amyloid loading results in tau hyperphosphorylations, thus triggering a sequence of molecular events that lead to neuronal degeneration (Maccioni et al., 2001). Inhibitors of Cdk5 and GSK3beta and antisense oligonucleotides exert protection against neuronal death; on the other hand, there is cumulative evidence from studies in cultured brain cells and on brains that oxidative stress constitutes a main factor in the modification of normal signaling pathways in neuronal cells, leading to biochemical and structural abnormalities and neurodegeneration as related to the pathogenesis of Alzheimer's disease (Maccioni et al., 2001). In summary, research currently focuses on the main protein aggregates responsible for neuronal death in both sporadic and familial forms of Alzheimer's disease, as well as on the alterations in the normal signaling pathways of functional neurons directly involved in neurodegeneration (Maccioni et al., 2001); as such, the current review, which looks in detail at cytotoxic cells and their elimination by stem cell treatments is extremely apt and opportune (Maccioni et al., 2001). First, before stem cell research is looked at, however, a general overview of the current knowledge of stem cells and their application will be reviewed, before proceeding on to a detailed look at the Materials/Methods and Results used by the active researchers in this field. Stem cells are essentially cells that are undifferentiated, being cells that have retained the ability to divide and differentiate in to any cell type. As such, stem cells are the hope of many researchers, in terms of them being applied to the repair of damaged cells, for the treatment of diseases, or to the growing of new organs, for organ transplants, for example. Stem cells can be harvested from adults or, more controversially, from embryos, and more recently stem cells have begun to be harvested from cord blood. Adult stem cells are undifferentiated cells that can be found located in the already differentiated cell regions of an adult human, and are generally multipotent cells (i.e., can produce only cells that are directly related to them, for example, if harvested from blood tissue, they could only reproduce red blood cells, platelets or white blood cells). Embryonic stem cells, whilst controversial, are where the hopes of most researchers lie, as embryonic stem cells, harvested from embryos at the blastocyst stage, have the potential to differentiate in to any type of cell (they are termed totipotent). Many researchers are currently excited by the prospect of using cord blood stem cells, which are collected from the placenta or umbilical cord of the fetus upon birth, for the cells thus harvested to be used in treatments for diseases - this has been shown to be an effective method for treating, for example, lymphocytic leukaemia. Much recent research time and endeavour has been channelled towards looking at the role stem cells can play in helping Alzheimer's patients. It is know that the adult mammalian brain contains neural stem cells (NSCs) with self-renewal and multilineage potential in the hippocampus and subventricular zone, however, it is also known that neurogenesis from these areas does not compensate for neuronal loss in age-related neurodegenerative disorders, such as Alzheimer's disease (Lopez-Toledano and Shelanski, 2004). In order, therefore, to test whether an impairment of neurogenesis could contribute to the pathogenesis of AD, Lopez-Toledano and Shelanski (2004) examined the effects of amyloid-beta peptide (Abeta) on the survival and neuronal differentiation of cultured NSCs from striatum and hippocampus, and showed that Abeta peptide does not impair the neurogenic rate in NSC progeny, but that it increases the total number of neurons in vitro in a dose-dependent manner (Lopez-Toledano and Shelanski, 2004). It was therefore argued that the neurogenic effect of Abeta peptide is not dependent on soluble factors released from the NSC progeny, and that neurogenesis is induced by Abeta42 and not Abeta40 or Abeta 25-35, and that their activity appears to be a property of Abeta oligomers and not fibrils (Lopez-Toledano and Shelanski, 2004). These results suggested, for the first time, that Abeta may have positive as well as deleterious actions, and that a knowledge of the mechanisms involved in the former could be valuable in exploiting the regenerative and plastic potential of the brain in preventing and treating Alzheimer's disease (Lopez-Toledano and Shelanski, 2004). This application of Abeta will be discussed in more detail later in the paper. Materials & Methods In terms of the experiments done to assess the molecular bases for Alzheimer's disease, the most interesting ones include work performed on looking directly at cytotoxic senile plaques, and at the interaction of these plaques with tau proteins. Perez et al. (2004) report a study which looked at the interaction of amyloid (A) 25-35 with tau protein and with the peptide 1/2R (KVTSKCGSLGNIHHKPGGG), which was investigated by chromatography, electron microscopy, and surface plasmon resonance (SPR). Through this study, it was found that A 25-35 comprises the minimum region of A peptide that is able to aggregate into fibrils, and 1/2R contains residues 307-325 from the tau region involved in microtubule binding (Perez et al., 2004). The results of the chromatography showed that A 25-35 induces the aggregation of tau protein and of tau peptide 1/2R, and likewise, the results of electron microscopy showed that A 25-35 increases the tau peptide polymerization observed in the presence of polyanions like heparin. A decrease in A 25-35 aggregation induced by tau peptide was also observed by both techniques (Perez et al., 2004). It was found that no direct interaction between tau protein immobilized on the sensor surface and A25-35 could be detected by SPR, however, incubation of tau protein at room temperature produced the loss of capability of this protein for interacting with the active biosensor surface (Perez et al., 2004). Thus, it was concluded that the presence of A 25-35 during the incubation of tau protein makes more efficient this loss of interacting capability with the sensor surface, and that these results clearly indicate that A 25-35, the peptide region to which the cytotoxic properties of A can be assigned, interacts with the peptide region of tau protein involved in microtubule binding (Perez et al, 2004). This interaction produces the aggregation of tau peptide and the concomitant disassembling of A 25-35, offering thus an explanation to the lack of co-localization of neurofibrillary tangles and senile plaques in Alzheimer's disease, and suggesting the possibility that tau protein may have a protective action by preventing A from adopting the cytotoxic, aggregated form (Perez et al., 2004). This is interesting, as this study shows that one factor thought to be damaging by many researchers is actually protective in terms of another factor that is considered pathological. In terms of experiments done looking at the application of stem cells to repairing the damage caused by cytotoxic plaques, the therapeutic potential of neural stem cell transplantation has been well demonstrated in many models of focal brain damage, however, few studies have sought to determine whether neural stem cells are therapeutic in models of diffuse brain injury, such as those observed in Alzheimer's disease and global ischaemia (Wong et al., 2005). Wong et al., (2005) investigated the effects of transplanted MHP36 neural stem cells on the extent of ischaemic damage in a mouse model of global ischaemia and the effects of the immunosuppressive agent cyclosporin A (CsA); C57Bl/6J mice received an intrastriatal graft of MHP36 neural stem cells 3 days after selective neuronal damage had been induced by global ischaemia, with the experimental group being subdivided into CsA or saline controls (Wong et al., 2005). It was discovered that grafts of MHP36 neural stem cells were able to differentiate into neurons and reduce the extent of ischaemic neuronal damage, and that the reduction was particularly apparent at 4 week post-transplantation and is independent of CsA immunosuppression, during which MHP36 cells survived robustly in host ischaemic brain and migrated away from the injection tract towards the caudate nucleus and corpus callosum (Wong et al., 2005). Wong et al., (2005) presented the first study showing a therapeutic benefit of neural stem cells in a highly diffuse brain injury, further highlighting the possibilities of stem cell transplantation for all types of neurodegenerative disease (Wong et al., 2005). Results Discussion and further work As we have seen, the neuropathological bases of Alzheimer's disease are focused on two important pathophysiological mechanisms: 1) Structural damage (e.g., senile plaques, neurofibrillary tangles, neuronal loss, inflammatory processes), and 2) Loss of cholinergic neurons (and acetylcholine depletion) in the nucleus basalis of Meynert, which sends cholinergic projections to all areas of the neocortex, especially the temporal lobes and frontal and parietal association areas, and the indemnity of this system is essential for normal cognitive functioning (Lopez and Becker, 2002). At the present time, the only long term treatment available for Alzheimer's disease are acetylcholinesterase inhibitors (CEIs) (e.g., tacrine, donepezil, rivastigmine, galanthamine), although several treatments that may alter the development of neurofibrillary tangles and neuritic plaques (e.g., peripherally administered antibodies against beta amyloid proteins) are being investigated (Lopez and Becker, 2002). Nerve growth factors may have the capability of improving neuronal survival, as we shall see below, although their form of administration remains a problem, and it is thought that amelioration of oxidative stress and CNS inflammatory processes may slow dawn the rate of neurodegeneration (Lopez and Becker, 2002). To date, all suspected mechanisms of the metabolic cascade of Alzheimer's disease have been explored with specific and non-specific treatments, and current treatments (e.g., CEIs) still have to prove that their effects can last for long periods of time, but as we have seen in the body of this paper, with the advent of further understanding of the neurodegenerative processes that cause Alzheimer's disease, and with new treatments that may slow down the progression of the disease, or stop the disease progressing all together, such as the use of stem cells, it is hoped that one day, a cure for Alzheimer's disease will become available (Lopez and Becker, 2002). Novel therapies for Alzheimer's disease designed to prevent/eliminate Abeta deposits in the brains of Alzheimier's disease patients are being suggested all the time, but it is generally agreed that the exact causes of Alzheimer's disease are still unknown: while fibrillar Abeta deposits known as senile plaques (SPs) and intraneuronal tau fibrils known as neurofibrillary tangles (NFTs) are diagnostic of AD, more than 50% of patients with familial or sporadic AD as well as elderly Down's syndrome patients with Alzheimier's disease harbor a third type of brain amyloid known as Lewy bodies formed by intraneuronal alpha-synuclein fibrils (Trojanowski, 2002). Thus, it is still very much agreed that Alzheimier's disease is a "triple brain amyloidosis", since three different proteins (tau, alpha-synuclein) or peptide fragments (Abeta) of a larger Abeta precursor protein (APP) fibrillize and aggregate into pathological deposits of amyloid within and outside neurons in Alzheimier's disease brains (Trojanowski, 2002). Alzheimer's disease is the most common cause of severe dementia in the aging population and is caused by a loss of many different neural systems throughout the brain associated with memory; as we have seen, amongst the many neural systems affected, large cholinergic projection neurons that innervate large regions of cortex are particularly vulnerable, and thus, boosting cholinergic neuronal function and survival has been a focus of the few drugs currently available for this disorder (Ebert and Svendsen, 2005). Nerve growth factor (NGF) is the archetypical protein discovered in the 1960s that is able to both increase survival and functioning of cholinergic neurons, however, the blood-brain barrier does not allow penetration of this protein into the brain; a phase 1 clinical trial recently published in the journal Nature Medicine utilized a unique ex vivo gene therapy approach to deliver NGF directly to the basal forebrain of Alzheimer's disease patients (Ebert and Svendsen, 2005). Despite the need for further testing, this report illustrated a mild but significant therapeutic benefit of NGF for the treatment of Alzheimer's disease and provided important data concerning the safety and efficacy of ex vivo gene therapy in humans, with particular reference to Alzheimer's disease (Ebert and Svendsen, 2005). As we have seen, millions of people are affected by Alzheimer disease, and it is expected that as longevity increases as a general characteristic of the population, so will the number of patients with dementia, which, as we have seen, has led to an intense search for successful treatment strategies (Jonhagen, 2000). One area of interest, as we have discussed with reference to the work of Ebert and Svendsen (2005) is neurotrophic factors, as it is known that brain development and neuronal maintenance, as well as protective efforts, are mediated by a large number of different neurotrophic factors acting on specific receptors, and that in neurodegenerative disorders, there may be a possibility of rescuing degenerating neurons and stimulating terminal outgrowth with use of neurotrophic factors (Jonhagen, 2000). The first neurotrophic factor discovered was nerve growth factor (NGF), and as we have seen, a wealth of animal studies have shown that cholinergic neurons are NGF sensitive and NGF dependent, which is especially interesting in cognitive disorders, in which central cholinergic projections are important for cognitive function (Jonhagen, 2000). In Alzheimer disease, for example, cholinergic neurons have been shown to degenerate, which suggests that NGF may be used to pharmacologically counteract cholinergic degeneration and/or induce terminal sprouting in Alzheimer disease, and indeed, data from animal studies, and from recent clinical trials, in which NGF was infused to the lateral ventricle in patients with Alzheimer disease, have shown that NGF could be a potentially powerful treatment for Alzheimer's disease (Jonhagen, 2000). Bibliography Ebert, AD and Svendsen, CN. (2005). A new tool in the battle against Alzheimer's disease and aging: ex vivo gene therapy. 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