Stem cells and neurodegenerative diseases (6)

Huntington's disease

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Huntington's disease is a fatal neurodegenerative disease that currently has no treatment. Inherited by the autosomal dominant principle, the disease is caused by an increase in the amount of the trinucleotide CAG (encoding polyglutamine) in the ITI5 gene encoding the huntingtin protein. In healthy people, the number of these repetitions usually does not exceed 36, whereas an increase in their number to 40 or more causes a predisposition to the development of the disease. The striatum, the cortex, the compact part of the substantia nigra, the reticulated part of the substantia nigra and the pale nucleus are the main regions of the brain undergoing severe degeneration. The middle spike neurons containing enkephalin and gamma-aminobutyric acid (GABA), as well as glutamatergic, GABA-ergic and parvalbuminergic neurons of the striatum are particularly severely affected. Mutated forms of the huntingtin protein are formed in the nuclei and cytoplasm of brain tissue cells. Manifested by disorders of cognitive and motor functions, as well as mental disorders, this disease leads to dysfunction of mitochondrial, synaptic and axonal transport. It causes severe transcription disorders, proteolysis (destruction of proteins) and excitotoxicity.

Despite the incurable nature of the disease, its symptoms can be alleviated with the help of gene therapy and medications. The latter provide temporary symptomatic improvement and mainly affect the motor manifestations of Huntington's disease. Tetrabenazine destroys dopamine and is used to suppress chorea (uncontrolled rapid erratic movements), but serious side effects prevent widespread use of the drug. Pouladi et al. 2013 described the advantages and disadvantages of various animal models of Huntington's disease. In most cases, rodent models were used to study the disease, but this greatly limited the success of clinical trials. At the same time, the problem is caused not only by the chosen animal model, but also by the correctness of preclinical studies.

Gene therapy involves blocking the activity and suppressing the translation of the protein product of the mutant huntingtin gene using RNA interference. However, gene therapy is effective only at the earliest stages of the disease, whereas its symptoms appear only in the later stages, when the nervous system has already suffered irreparable damage. Replenishing the population of striatum neurons through intracranial transplantation is an alternative approach to cell therapy. Fetal tissue grafts of the striatum were transplanted to monkeys and rodents with simulated Huntington's disease. Such transplants have demonstrated a low survival rate, while, due to their origin, the use of such material for transplantation raises many ethical questions. The limited availability of donors further narrows the possibilities of using this promising therapy. In this case, nerve progenitor cells derived from stem cells and iPSCs are a good alternative.

The successful differentiation of human embryonic stem cells into GABA-ergic and DARPP32-positive medium spike neurons is described in detail in the literature. When transplanted into a chemically induced mouse model of Huntington's disease, such neurons demonstrated effective interaction with the recipient's neurons and reduced the severity of motor disorders. Another study conducted by Carri et al. describes the successful differentiation of embryonic stem cells and iPSCs into medium spike neurons expressing receptors for adenosine and dopamine. However, the transplantation of nerve progenitor cells was more successful when applied to a chemically induced mouse model of the disease compared to a transgenic mouse model (R6/2). Presumably, the reason for this is the short lifespan of transgenic animals. In parallel with disease modeling and regenerative transplantation therapy, stem cells and iPSCs can be used as an in vitro tool for screening the efficacy and biosafety of drugs and small molecules that help alleviate the symptoms of diseases, as well as reduce the severity of neurodegeneration. Researchers have created cell lines specific to patients with Huntington's disease that more accurately reproduce the phenotypes characteristic of the disease.

Gene therapy of Huntington's disease involves the effect on the transcript of the mutant allele of the huntingtin gene (mHtt) using RNA interference and antisense oligonucleotides, without disrupting the transcript of the normal allele of the huntingtin HTT gene. The exact role of the huntingtin protein under normal conditions is not completely clear, however, existing data indicate that non-knockout mice that do not have it are not viable already at the embryo level. Therefore, selective exposure to the mutant allele is preferable. Antisense oligonucleotides affect complementary mRNA through a degradation mechanism mediated by RNAASE H, and are very important for researchers, as they allow targeting snips (single nucleotide polymorphisms). These snips are responsible for the difference between the normal and mutant alleles of the huntingtin gene. Experiments on monkeys and rodents with simulated Huntington's disease have demonstrated relief of symptoms of the disease. This indicates the existence of the possibility of effective correction of patients carrying iPSC mutations using antisense oligonucleotides and blocking the activity of genes, their subsequent differentiation into nerve progenitor cells and transplantation back to patients. Thus, stem cell therapy in combination with gene therapy in the future can help in the fight against an incurable disease today.

Continuation: Three-dimensional (3D) research using stem cells

Portal "Eternal youth" http://vechnayamolodost.ru 09.03.2017