Stem cells and neurodegenerative diseases (2)
Stem cells and induced pluripotent stem cells in neurodegeneration: what to choose?
Continuation. The beginning of the article is here.
It is easiest to say that cells of human origin can be used directly to get a clearer picture of neurodegenerative diseases, but in practice this approach is not as simple as it seems. The developing in vitro scenario lacks an intact organ system, inter-organ interactions, blood supply and connective tissue. Each disease has cellular, molecular, anatomical, genotypic and phenotypic parameters characteristic only for it. Maintaining the culture of such specific cells is another difficult task that requires the development of standardized protocols, cultivation conditions and special skills. For example, as described in the following sections, Parkinson's disease requires the cultivation of dopaminergic neurons, amyotrophic lateral sclerosis requires the cultivation of glial cells, motor neurons and astrocytes, and Huntington's disease requires the cultivation of median spiny and strial neurons. Cellular technologies make it possible to meet all these requirements to a large extent.
In short, stem cells are "naive" cells of the body that have an exceptional ability to self-renewal, proliferation, differentiation and programming to transform into cells of different sprouts. They can have a fetal or embryonic origin, as well as be isolated from the tissues of an adult organism. Despite a number of ethical issues, stem cell biology is widely used in both scientific and medical fields. Stem cells can be converted into almost all types of cells, which allows them to be used in routine modeling of diseases. Monogenic diseases with a clear cellular phenotype and high penetrance (an indicator of the phenotypic manifestation of an allele in a population) are relatively easier to model compared to less penetrant diseases of late age, in the development of which many genes are involved. In the case of monogenic pathologies, the gene associated with the disease is intentionally mutated using genetic engineering methods to obtain stem cell models. Chromosomal aberration-bearing embryonic stem cells (ESCs) are used to model chromosomal diseases. Complex diseases of late age that are not detected by prenatal diagnostics are more effectively modeled on induced pluripotent stem cells of the patient himself.
Until today, animal models in vivo have been used for experimental modeling of diseases, but the data obtained in this case do not allow to reliably reproduce human diseases and therefore cannot be directly extrapolated. This is the main limitation of various studies on animal models. Only the use of human tissue samples can overcome this basic difficulty. Neurodegeneration leads to progressive loss of brain functionality as a result of irreversible gradual death of neurons and other cells of the central nervous system. In this regard, transplantation therapy can be used to restore and update damaged neural networks of the brain, as well as to replenish the dying population of neurons. The successful direction of differentiation of stem cells into neurons is described in many publications, and a huge number of protocols allows us to achieve this result.
In this regard, such diseases as cerebral ischemia, spinal muscular atrophy, spinal cord injuries, amyotrophic lateral sclerosis, Machado-Joseph disease and many others have been studied. Among other things, stem cell therapy was effectively applied to these pathologies. Embryonic stem cells are pluripotent by nature and have exceptional potential to repair brain damage and neurodegenerative changes through transplantation, but their widespread use is limited by the risk of developing tumors. Multipotent mesenchymal stem cells have also found wide application due to their immunomodulatory properties, ensuring their ability to avoid conflict with the immune system and not cause the development of a rejection reaction. Nerve progenitor cells, or nerve stem cells derived from the fetal brain, are also multipotent, but can only differentiate into nerve tissue cells. These cells are suitable for clinical use due to their characteristic reduced risk of tumor formation, but they are usually small and very difficult to isolate. In addition to naive stem cells, induced pluripotent stem cells (iPSCs) obtained by reverse programming of somatic cells are currently widely used. These cells are produced in large quantities and various iPSC lines are currently commercially available. To date, the use of Parp1 (poly(ADP-ribose) has been described in detail-polymerase-1) to produce iPSC, which also significantly reduces the risk of tumor formation. However, it has not been possible to completely eliminate the risk of teratoma formation to date. The production of iPSCs from various somatic cells of the body, including peripheral blood cells, hepatocytes, stomach cells and keratinocytes, is described in detail in the literature. To obtain iPSC, 10 µl of capillary blood obtained by pricking the finger pad is sufficient. iPSCs grown from the material of patients can be used for direct modeling of various neurodegenerative diseases of humans. Various research groups have created and described disease-specific iPSC lines. Stem cells and induced pluripotent stem cells are used not only for modeling neurodegenerative diseases, but also in transplant therapy. However, the use of iPSC also has certain limitations. Neurodegenerative diseases, as a rule, are characterized by a late onset and their symptoms begin to manifest as they age. Therefore, the creation of their animal models requires not only a lot of time, but also significant financial investments. In general, it is generally accepted, and there is also evidence that the iPSCs isolated from the patient carry disease-causing mutations and the epigenetic profile of the patient, which makes them the optimal material for creating in vitro models of diseases. However, when reprogrammed into an induced pluripotent state, somatic cells lose age-associated features and rejuvenate, acquiring a status close to that of embryonic cells. This is manifested by a loss/decrease in the expression of markers of physiological aging, an improvement in the condition of mitochondria and an increase in telomere length. Therefore, even the iPSCs obtained from the patient's cells due to the loss of age-associated phenotypes do not allow effective modeling of age-related neurodegenerative diseases. However, this problem was overcome by inducing premature aging with progerin. Progerin is a truncated transcript of lamin A (nuclear envelope protein) formed as a result of mutation of the LMNA gene. The accumulation of progerin in the nuclear membrane causes dysfunction of lamin A, which leads to violations of the organization of chromatin, the cell cycle, the maintenance of telomere length and the reaction to DNA damage. High levels of progerin are associated with aging. Under the influence of progerin, age-related phenotypes develop in iPSCs, such as degeneration of dendrites, accumulation of neuromelanin, impaired regulation of AKT, an increase in the size of mitochondria and a decrease in the number of TN-positive neurons (Tyrosine hydroxylase-positive neurons). Such bearing markers of aging of iPSCs make it possible to reproduce neurodegenerative diseases more effectively. The following sections discuss the use of stem cells and iPSCs in the study of the most common neurodegenerative diseases worldwide.
Figure 1. Schematic representation of the progress made in the field of
stem cell studies in the context of neurobiology.
Continuation: Stem cells and Alzheimer's disease
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28.02.2017