Disease Model, Drug Screen and Transplantation Trials

Adult neural stem cells that are derived from induced pluripotent stem (IPS) cells are capable of differentiate into multiple cell types, including degenerative neural cells. However, unlike embryonic stem cells, IPS cells are derived from patient's own somatic cells. This may solve ethical issues and immune-rejection problems in embryonic stem cells(1). Most of the current researches with IPS cells in neurodegenerative diseases contribute to the following three areas; Disease modeling, Drug screening and Disease therapy. By observing the disease phenotype in vitro, such as Parkinson disease, disease models will reveal the mechanisms of neurodegenerative processes. In addition, cells with disease phenotype can be targeted for drug screening and healthy neural stem cells can be used in disease therapy for cellular transplant.

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1.1 Disease models as a tool to study disease pathology
Most diseases involve multiple cells types are modeled using laboratory animals, which can be applied to our understanding of human disease pathogenesis. However, in many neurodegenerative diseases, including those affect motoneuron and astrocytes, do not naturally occur in commonly used laboratory animals. This problem is solved by developing disease models from human stem cells with disease traits that were generated directly from human patients using the technology established by Yamanaka and colleagues( 6 ).

Disease modeling by duplicating the disease phenotypes in vitro would allow researchers to study how different cell types are involved in the neurodegenerative diseases. Thus, unravel the cellular processes that may trigger genetic, as well as sporadic forms of disease. For instance, in an ALS study, Dimos et al. had taken somatic cells from an elderly patient with FALS and a SOD1 mutation, reprogram it to human induced-pluripotent stem cells (hiPSCs), and successfully directed the differentiation of hiPSCs into motor neurons expressing appropriate motor neuron markers, including ISLET and Hb9. Then, they compare the disease model of ALS with spinal muscular atrophy ( 6 ). While both ALS and SMA are neurodegenerative disease of motor neurons, only SMA motor neurons derived from hiPSCs shows disease phenotypes. This may due tothe fact that the onset age of ALS begin after middle age while the onset of SMA begin in childhood( 6 ) .

1.2 Interaction between susceptible genes and environmental factors
In modeling neurodegenerative disease, both the genetic information and environmental factors contribute to neurodegeneration, therefore it is critical to promote these conditions in vitro.

1.2a Epigenetic modification
Successful disease modeling requires the identification of cellular difference between patient and control that lead to dysfunction. However, lost of epigenetic modifications in the process of generating Parkinson disease models could be a major issue. Depending on the application of IPS cells, lost of those epigenetic features could be a good, or a bad thing. Starting fresh could be useful for neurotransplantion, since it eliminate the potential cause of the disease, if it’s not purely genetic. On the other hand, when we are trying to understand the underlying mechanism of a disease, the disease phenotype is required for modeling. Thus, without these epigenetic modifications, IPS cells derived from PD patients are less likely to be useful for modeling.

The strategy proposed by many groups of researchers is to work with PD patients who have known cause in their DNA (i.e. LRRK2 mutation), and therefore, that cause was not epigenetic(4). So the idea is to work with patients who have known genetic cause, develop cell line, induce them back into IPS cells, and then forward to dopaminergic neurons (1) . If any of these dopaminergic neurons signals a disease phenotype, we can use them to study disease mechanism and for drug screening purposes.

<span style="font-family: 'Times New Roman',Times,serif; font-size: 16px;">1.2b Circuit connections and complex brain environment
<span style="font-family: 'Times New Roman',Times,serif; font-size: 16px;">In addition to the loss of epigenetic modifications, the lack of circuit connections and complex environment of the brain could be another issue concerning the effectiveness of these IPS cells as a disease model. It is unlikely to just develop pure cultures of these IPS cells and expect a disease phenotype (3). There are much more needs to be considered, including co-cultures and various types of stress present in the brain environment. Tentative solution is to co-culture these IPS cells with glia and other neuronal cell types. In addition, apply stress to these cells, such as oxidative stress or neurotoxins (5).

<span style="font-family: 'Times New Roman',Times,serif; font-size: 16px;">1.3 Drug screen
<span style="font-family: 'Times New Roman',Times,serif; font-size: 21.3333px;">Steps in Drug Screen <span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">•Isolate NSCs that overexpress disease genes. <span style="font-family: 'Times New Roman',Times,serif;">

<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">•Grow these cells in vitro and differentiate into the cell type of interest.

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<span style="font-family: 'Times New Roman',Times,serif; font-size: 14px;">•Screen for drugs that may correct an observed disease phenotype.

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<span style="font-family: 'Times New Roman',Times,serif; font-size: 16px;">1.4 Stem cell based therapy
Human neural stem cells can be readily differentiated from hESCs and hiPSCs in large quantities in a chemically defined culture system. Beside disease modeling, these undifferentiated cells can be used in cell-based therapy. Cell-based therapy is the replacement of diseased or dead cells in critical areas of the brain and spinal cord, such as respiratory centers(1). Most of the current clinical trials of cell-based therapy was done in Parkinson's disease and ALS. ** 1.4a Transplantation Trials in Parkinson's Disease ** Transplanted cells are targeted to lost midbrain and nigrostriatal dopaminergic neurons since their loss in Parkinson's disease. <span style="font-family: 宋体; font-size: 12pt;">Most c urrent transplantation trials were done in animal models and can be summarized in <span style="font-family: 宋体; font-size: 12pt;">to three key experiments: 1.Human iPS cell-derived neurons from mice fibroblasts were transplanted into healthy rat brain. Results from immunofluorescent labeling, confocal analysis and electrophysiological recordings from brain slices showed incorporation with correct morphological differentiation and function( <span style="font-family: 宋体; font-size: 12pt;">7 ). 2.The Parkinsonian rats, which had administered 6-OHDA to striatum, were given sham surgery or injection of reprogrammed mice fibroblasts. Results showed higher concentration of DA neuron markers in treated vs. control rat brains, with improvements of motor functions( <span style="font-family: 宋体; font-size: 12pt;">7 ). 3.Transplantation of iPS cells derived from human PD patients into PD rats. Results showed improved motor function, no teratoma formation, low risk of immune reaction( <span style="font-family: 宋体; font-size: 12pt;">8 )

1.4b Astrocyte replacement in ALS
In the case of ALS, the progression of the disease is subject to the interaction between motoneurons and their neighboring astrocytes. Hence, early displacement of diseased astrocytes could rescue motoneurons from further degeneration. Transplantation trials has been successfully demonstrated in a rat model of ALS in which transplanted astrocytes not only survived but also increased the life span of grafted animals( <span style="font-family: 宋体; font-size: 12pt;">6 ).

Reference:

(1)Yu J, Vodyanik MA, Smuga-Otto K, Antosiewicz-Bourget J, Frane JL, Tian S, Nie J, Jonsdottir GA, Ruotti V, Stewart R, Slukvin II, Thomson JA. Induced Pluripotent Stem Cell Lines Derived from Human Somatic Cells. Science 2007: 318(5858): 1917-1920.

(2) Brundin, P., Li, J. Y., Holton, J. L., Lindvall, O., & Revesz, T. (2008). Research in motion: the enigma of Parkinson’s disease pathology spread. Nature Reviews Neuroscience, 9(10), 741–745.

(3) D.A. Di Monte, The environment and Parkinson's disease: is the nigrostriatal system preferentially targeted by neurotoxins? Lancet Neurol. 2 (2003) 531–538

(4) J.M. Bras, A. Singleton, Genetic susceptibility in Parkinson's disease, Biochim. Biophys. Acta 1792 (7) (2009) 597–603.

(5) Takahashi K, Tanabe K, Ohnuki M, Narita M, Ichisaka T, Tomoda K, et al. Induction of pluripotent stem cells from adult human fibroblasts by defined factors. Cell 2007;131(5):861–72.

(6)A.D. Ebert, J. Yu, F.F. Rose Jr, V.B. Mattis, C.L. Lorson, J.A. Thomson, C.N. Svendsen, Induced pluripotent stem cells from a spinal muscular atrophy patient, Nature 457 (2009) 277–280.

(7)Wernig M, Zhao JP, Pruszak J, Hedlund E, Fu D, Soldner F, Broccoli V, Constantine-Paton M, Isacson O, Jaenisch R. Neurons derived from reprogrammed fibroblasts functionally integrate into the fetal brain and improve symptoms of rats with Parkinson’s disease. PNAS 2008; 105: 5856-5861.

(8)Hargus G, Cooper O, Deleidi M, Levy A, Lee K, Marlow E, Yow A, Soldner F, Hockemeyer D, Hallett PJ, Osborne T, Jaenisch R, Isacson O. Differentiated Parkinson patient-derived induced pluripotent stem cells grow in the adult rodent brain and reduce motor asymmetry in Parkinsonian rats. PNAS 2010; 107(36): 15921-15926.