STEM CELLS IN NEUROLOGY ISABELLE COCHRANE MERIT

Transcription

1 STEM CELLS IN NEUROLOGY BY ISABELLE COCHRANE MERIT RESEARCH PAPER BASED ON PATHOLOGY LECTURES AT MEDLINK

2 Abstract Ever since their discovery, stem cells have been heralded as the ultimate cure to every disease, and have been greeted with much hope, but also much scepticism. This paper examines this claim in more depth, with particular reference to the potential use of stem cells in the treatment of neurological disorders. By studying in depth four of these disorders, strokes, Parkinson s disease multiple sclerosis and Alzheimer s, it demonstrates that indeed stem cells have great potential in this area. Many experiments have been carried out on animal models, which show that, when transplanted into the central nervous system, stem cells can prove to have very beneficial effects. However, we are still a long way from creating treatments for humans, as the field could still be said to be in its infancy. 1 Introduction 1.1 Importance of stem cells and relevance to medicine Stem cells are distinguished from other cell types by two characteristics: Firstly, they are defined as being immortal - they can regenerate during division, as seen in figure 1. Secondly, under the correct conditions they can be induced to develop into a particular cell with a specific function [1]. These two characteristics mean that stem cells are likely to play a key role in treating diseases which are as yet incurable, or whose treatment currently poses a large risk to the patient. Stem cells can be divided into two categories: Embryonic and adult stem cells [9]. The former can be taken from a fertilised egg cell 3-5 days into its development, a blastocyst. They can also be found in the placenta and umbilical chord. These stem cells can differentiate into any of the 300 different types of cell found in the human body. This huge flexibilty makes embryonic stem cells very promising for future treatments. However, there is much ethical controversy surrounding their use. Adult stem cells on the other hand can be found in the brain, the skin, the teeth, and some organs in the digestive and haematopoietic systems. The limitation presented by adult stem cells is that they can only differentiate into certain types of cells, and their capacity to divide is limited, meaning that there is less scope for potential treatments devised using these cells. However, existing stem cell treatments, such as bone marrow transplants, rely on the use of adult stem cells. Furthermore, scientists have discovered a way to genetically reprogramme adult stem cells so that they have the same properties as embryonic stem cells. These are known as induced pluripotent stem cells, and are already being used to model diseases and to test drugs. Figure 1 : When stem cells divide, they differentiate to form two new daughter cells. One will have a different function to the original cell, whereas the other will be identical to the original cell. 2

3 Although stem cell are likely to prove to be of huge benefit to medicine, it is important to remember that stem cell tratments can carry risks. For instance, with the use of allogenic stem cells (stem cells which have come from someone other than the recipient) there is still a risk of immune rejection, and if the growth of stem cells is not controlled, this can lead to the formation of tumours. It is primarily the latter reason that has proved to be a difficulty in the developmant of effective stem cell treatments. 1.2 Developments in stem cell research Scientists first observed the existence of stem cells when, in the early 1900s, they realised that red blood cells, white blood cells and platelets all arise from the division of one particular type of cell. However, it was not until much later, in 1963, that Canadian researchers Ernest A McCulloch and James E Till observed the self-renewing properties of stem cells in transplanted mouse bone marrow. Embryonic stem cells were first isolated in 1998 by James Thomson, who established the first embryonic stem cell line, which still exists today. Mounting evidence pointed towards the fact that these stem cells could differentiate into any type of human cell, giving hope that they could be used to create a variety of tissues and organs to be used in transplants, removing the need for a donor, and reducing the risk of rejection of the organ by the recipient s immune system. The next great development in stem cell research came in 2006, with the first report of mouse induced pluripotent stem cells. This led to the creation of the first human induced pluripotent stem cells in late 2007 by S. Yamanaka at Kyoto University in Japan [10]. Despite the seemingly unlimited possibilities provided by stem cells, scientists have had difficulty in developing a treatment based on their use, and although in recent years there have been many trials on animal models, few treatmens have progressed to the clinical stages. 1.3 Ethical considerations One of the greatest problems encountered by scientists working in the field of stem cell research is the opposition of certain groups to the use of embryonic stem cells on ethical and religious grounds. The main argument presented against the use of embryonic stem cells is that it is unacceptable to destroy one life despite benefitting another. It is understandable that members of the public may be misled by the name of embryonic stem cells, and believe that actual babies are destroyed in the process. In practice however, it is only excess zygotes from IVF, which would otherwise be discarded anyway, that are used for the retrieval of embryonic stem cells. Furthermore, it is only in the first few days of development that the emryo can provide totipotent stem cells, at which time in its development it does not bear any human characteristics. Therefore, those who are better informed with regards to how embryonic stem cells are found, have no doubt that the practice is ethically and morally sound. 3

4 2 Discussion 2.1 Stem cells in neurology Stem cells have for a long time been seen as a likely cure to many neurological diseases, many of which are degenerative and progressive in nature, meaning that the damage caused to neurons by these diseases is permanent and gets worse with time. Therefore, the only hope for a cure lies in being able to create neurons and implant them into the nervous system, or to somehow protect the existing neurons from the disease. Recently, scientists have been able to grow neurons and glia from stem cells in culture, which have in some cases been transplanted into animal nervous systems, but, as in other areas, we have yet to formulate this into a clinically useful therapy. For a long time, it was believed that mammals, including humans were born with a certain number of neurons, which could decrease throughout their lifetime, as a result of illness, injury, or ageing, but which could under no circumstances increase. In other words, it was believed that neurons could not regenerate [2]. This implied that there was little hope to cure people with neurological diseases such as Alzheimer s and Parkinson s disease or those with brain damage due to injury or stroke. However, it has recently been shown that there are centres of neurogenesis in the brain, and, importantly, that these centres can be triggered to produce neurons after injury [3]. Therefore, there are now two potential pathways for treatment: either to transplant stem cells grown in vitro into the Central Nervous System (CNS) or alternatively, find a way to encourage the CNS s own stem cells to regenerate. 2.2 Case Studies Different diseases of the brain and nervous system arise from different types of neurological damage. The best way to see how stem cells could be applied to a variety of neurological disorders is to study a variety of diseases and see how stem cells may be applied to each in order to provide a cure: Strokes A stroke is caused by a blockage to a cerebral artery, leading to motor and cognitive impairments due to the loss of neurons and glial cells. There is currently no treatment available, but there has been much encouraging research in this area. In one experiment, conducted by S. Kelly et al. human neural stem cells, which had been tagged with a fluorescent marker, were transplanted into rats brains in which the conditions inside the brain of a stroke sufferer had been simulated. Over a period of one month, not only did the cells survive, but a movement of new neurons to the affected area was also observed. However, it is not yet clear whether transplanting stem cells in this way is likely to restore the brain s normal function. In a further study, carried out by 4

5 Ritsuko Ikeda et al. evidence emerged that embryonic stem cells from monkeys transplanted into rats differentiated into neural and glial cells, forming connections with surrounding tissues and improving motor function [7]. There has only been one reported clinical trial of stem cells on patients suffering from a stroke. In this, patients received stem cells taken from a human teratocarcinoma cell line, which were implanted into the affected area of the brain. An increase in metabolic activity was observed in the areas where the stem cells were implanted, and there was a marked improvement in patients who exhibited this sign, which suggests graft function. However, this finding must be approached with caution, as an increase in metabolic activity can also be an indicator of inflammation. It is also difficult to determine whether this increased activity is truly caused by the graft cells metabolising, or whether it simply shows increased activity in host cells. In terms of encouraging the brain s own stem cells to regenerate to counteract the effects of a stroke, it has been found that stroke causes the brain to generate an increased number of neural stem cells, which then migrate to the site of the damage, and differentiate into the type of neuron of which most was lost [8]. However, more than 80% of neurons generated in this way die during the first few weeks after the stroke, so it is only approximately 0.2% of neurons that are replaced. There is still some uncertainty as to whether neurons formed in this way are fully functional. An effective treatment for stroke would therefore involve a way of ensuring that these neurons do not die off in this way, and a way of making these neurons perform the exact same function of the neurons destroyed by a stroke, by ensuring that they are incorporated into the brain Parkinson s Disease Parkinson s disease is caused by the loss of dopamine-containing neurons, leading to rigidity, bradykinesia, tremors, and instability on standing. There are a number of drugbased treatments for Parkinson s, which mimic the action of dopamine in the brain, but these do not stop the course of the disease, and are associated with side effects [11]. Stem cells are likely to provide a much better alternative to these treatments. Dopaminergic cells have already been created in vitro using both adult and embryonic stem cells; however, it is not yet clear whether transplanting these into the brain will restore normal function. Again, as with the treatment of stroke, the problem of integrating transplanted stem cells within the host organ, and making them operate as part of the organ is encountered. A study of how cells arrange themselves during foetal development is likely to provide us with a reliable answer to this question, as we will have a better grasp of the chemical signals that are involved in combining single cells into fully functioning organs. If a way were to be found to encourage neurons to produce these same chemical signals after the transplantation of stem cells into the 5

6 brain, we would be more likely to be able to incorporate the stem cells into the brain. They would align themselves into position, much as neurons do in a foetus during development. Studies involving the transplantation of foetal mesencephalic cells have shown encouraging results: these cells, which stayed in place for up to ten years, continued to replicate and replace the patient s own dopamine-containing neurons, which were still being destroyed by the disease process. Although this is not a stem cell therapy in itself, it shows that cells transplanted into the brain are a potential cure for Parkinson s. Another possible application of stem cells could be to generate neuroprotective cell lines, which can be transplanted into the brain. However, such a treatment would only be effective once it has been found out why the dopamine-producing cells are destroyed, so as to know what it is that the neurons need to be protected against Multiple Sclerosis Nervous impulses are transmitted down the axons of neurons electronically. These are normally covered by myelin, a substance that acts as an insulator. However, in multiple sclerosis, this myelin sheath disintegrates, as shown in figure 2. This distorts the impulses that are being passed from one neuron to the next, leading to a loss in motor skills, and a variety of other neurological symptoms, such as mood swings and behavioural changes. The damage to the myelin is thought to be caused by the immune system itself, meaning that this gives two windows of opportunity for the use of stem cells: Either to replace the myelin sheath, or to replace the whole immune system. The former Figure 2: Damaged myelin exposes the nerve fibre. If exposed fibres come into contact, the nerve impulses become distorted, and multiple sclerosislike symptoms ensue. autoimmune diseases. is probably a more realistic goal to aim for, but if a solution to the latter were to be found, this would benefit patients suffering from other Myelin-producing cells derived from embryonic stem cells have been shown to remyelinate cells in mouse brains and spinal cords in a study conducted by G. Nistor et al. [13]. Such a treatment would however have to be used in combination with antiinflammatory and immunosuppressant drugs, as the grafted cells do not actually address the cause of the disease, and are still susceptible to being destroyed in the inflammatory environment. Another challenge presented by multiple sclerosis is that myelin can be destroyed at any point in the nervous system, and so any grafted cells will have to be encouraged to migrate to the site of the destroyed myelin. 6

7 2.2.4 Alzheimer s In Alzheimer s disease, neurons and synapses are lost in the brain (see figure 3), causing patients to experience dementia and loss of other cognitive functions. It is one of the most widespread diseases of our time: there are an estimated 36 million people worldwide living with dementia, of which 70% are suffering from Alzheimer s [14]. On average, a person diagnosed with Alzheimer s will live for only 8-10 years after diagnosis, so finding an effective treatment would improve the quality of life and life expectancy of many. It is mainly cholinergic neurons that are targeted by the Figure 3: The loss of neurons in Alzheimer s disease leads to a considerable decrease in size, and leads to the brain taking on a porous appearance. disease, so it is specifically these neurons that need to be replaced. When stem cells are transplanted into the brain, they rely on the surrounding environment in order to differentiate. However, given the huge damage to the brain in patients with Alzheimer s, the mechanisms which help control this differentiation are unlikely to be functioning. This means that any cells transplanted into the brain of an Alzheimer s sufferer will have to have already differentiated into the desired type of neuron in an in vitro situation. This could be done by simulating the conditions inside a healthy brain, as it has been shown that stem cells will differentiate into neurons when placed in the brain. Alternatively, genetically engineering the stem cells so as to ensure that they differentiate into neurons could be an option, and would be more likely to ensure that they differentiate into the desired type of neuron. However, here another aspect of genetic engineering can be exploited before implantation: M. Tuszynski et al. (2005) have already shown that stem cells engineered to produce nerve growth factor can counteract the effects of Alzheimer s disease [15]. So stem cells need not necessarily replace diseased cells, but can also be used to stop the disease from acting upon existing neurons. 2.3 Current limitations It may seem unreasonable to believe that an organ as complicated as the human brain can be fixed simply by replacing its damaged components. The interactions between all the neurons in our body are also extremely complex, making their action difficult to replicate. Nevertheless, research has shown that this may one day become possible, despite there still being some significant obstacles to overcome. For instance, it is still difficult to reliably control the type of cell the stem cell will differentiate into. Although there is evidence that when placed in the right environment, stem cells will differentiate into neurons and glial cells, there is no mechanism to ensure this is the case every time. This is partly to do with our lack of understanding of the process by which cells differentiate. This also means that 7

8 there is no way to reduce the chances of tumour formation, a considerable risk when using cells which have the ability to divide infinitely. Another difficulty is to find a way in which to encourage stem cells to connect to surrounding tissues, in such a way that will bring about the recovery of normal neurological function [5]. 2.4 Other possibilities However important stem cells prove to be in the treatment of neurological diseases, they will also have a huge role to play in helping us understand the nature of of these diseases. For instance, our understanding of the pathogenesis of Alzheimer s has been limited by the difficulty of obtaining live neurons from patients. Recently however, a team of US researchers has managed to make Alzheimer s neurons by taking skin stem cells from highrisk Alzheimer s groups. This is set to help us understand better how the disease progresses [6]. Stem cell cultures could also help us to test new drugs, by modelling the conditions inside the brain of a patient with Multiple Sclerosis, for example, and investigating which chemicals have the greatest effect on preventing demyelination of nerve axons. 3 Conclusion The effective treatment of neurological disease has for a long time been one of the greatest challenges facing modern medicine. This is due to the fact that these diseases cause permanent damage to neurons, but also because the human central nervous system is so complex, and its action so difficult to replicate. However, stem cells have provided us with a unique solution to his problem: they can differentiate into neurons when transplanted into the brain. They will also migrate to a site of damage within the CNS and, once there, will incorporate themselves into the system, restoring its function. This has been demonstrated on animal models, but never yet in a clinical trial. This means that we are still some way from seeing a functional treatment. The science is progressing fast, however, and so we may hope within the next few decades to see cures based on stem cells which surpass our current drug-based treatments and provide long-lasting symptomatic relief to the patient, and maybe even a complete cure for neurological diseases. 4 References [1]Pickrell J. (2006) Introduction to stem cells [2] Ohira K. (2011) Injury induced neurogenesis in the mammalian forebrain in Cellular and Molecular Life Sciences 68: [3]Lindvall O. (2006) Progress stem cells for the treatment of neurological disorders in Nature 441,

9 [4] Kelly S. (2004) Transplanted human foetal neural stem cells survive, migrate, and differentiate in ischemic rat cerebral cortex in PNAS [5] Lindvall O. (2004) Stem cell therapy for human neurodegenerative disorders- How to make it work in Nature 10:S42-S50 [6] Israel M. (2012) Probing sporadic and familial Alzheimer s disease using induced pluripotent stem cells in Nature 482, [7]Ikeda R. (2005) Transplantation of neural cells derived from retinoic acid-treated cynomolgus monkey embryonic stem cells successfully improved motor function of hemiplegic mice with experimental brain injury in Neurobiology of Disease 20:1, [8]Arvidsson A. (2002) Neuronal replacement form endogenous precursors in the adult brain after stroke in Nature Medicine 8, [9] Stem Cell Information from the National Institutes of Health resource for stem cell research [10] History of stem cell research from Explore Stem Cells [11]Parkinson s disease by the National Institute of Neurological Disorders and Stroke [12]Marshall J. (2007) Stem cells: from adult to embryo in New Scientist [13]Nistor G. (2004) Human embryonic stem cells differentiate into oligodendrocytes in high purity and myelinate after spinal cord transplantation in Glia 49:3, F197ADDDA.d02t04 [14] The facts on Alzheimer s disease from Alzheimer s Disease Research [15] Tuszynski M. (2005) A phase 1 clinical trial of nerve growth factor gene therapy for Alzheimer disease in Nature Medicine 11,

10 10