Reprogramming cells: a new way to get useful stem cells

On December 18, 2008, science magazine of the United States evaluated the top ten scientific developments of the year, among which "cell reprogramming" ranked first The reason why cell reprogramming is honored is that this way opens up a new field of regenerative medicine People can bypass the ethical controversy of embryonic stem cells, directly reprogramming all kinds of cells, become all kinds of stem cells, and can grow into various tissues and organs, laying a solid foundation for regenerative medicine Rapid cell reprogramming refers to the recovery of differentiated cells to differentiated state after being reversed under specific conditions, or the formation of embryonic stem cell lines, or the further development of a new individual Prior to this, a 2007 study has indicated the significant progress in this field, that is, "induced pluripotent stem cells" (also known as stem cell like cells) become the second of the top ten scientific progress in 2007 In 2007, the research team led by James Thomson of the University of Wisconsin in the United States and Professor Tanaka Yamanaka of Kyoto University in Japan published their research results in science and cell magazine, respectively They all implanted four genes (Oct4, Sox2, c-myc, KLF4) into human skin cells with retrovirus as vectors, through gene recoding, skin Skin cells have the function of embryonic stem cells The transformed cells are called "induced pluripotent stem cells" Before the human skin stem cells were reprogrammed into stem cells, the researchers had succeeded in mouse experiments In 2006, Yamanaka and other researchers said they had found a way to get stem cells by avoiding the ethical limits of human embryonic stem cells They introduced four genes into the tail cells of mice growing in the culture dish, and got new cells with very similar appearance and function to embryonic stem cells, which are induced pluripotent stem cells Now, by inserting four genes, researchers can turn the cell back into a stem cell that can differentiate and grow again The researchers took skin cells from patients with different diseases and reprogrammed them into stem cells These stem cells can grow and divide in the laboratory, which not only allows researchers to understand the progress of the disease, but also a new way of using stem cells, so in the future, patients' own cells can be used to treat the disease Two developments before that have been remarkable One is that a genetic technique used in mice two years ago became the beginning of cell reprogramming At that time, researchers used genetic technology to completely remove the developmental "memory" of cells, make them return to the original embryonic state, and then grow into different cells In other words, adult cells can be reprogrammed to return to the embryonic state, i.e embryonic stem cells, and then they can develop into various required tissues and organs The other is more advanced The researchers used live mice to make cells change directly from one adult cell to another, breaking the rules of one-way cell development The advantage of this method is that it can directly change one cell into another cell instead of reprogramming it into a stem cell The further development of the research on the diversity of acquisition methods has also gained a new understanding of obtaining stem cells through cell reprogramming, that is, the goal of reprogramming is not necessarily to return to the embryonic state, but to make cells directly become another kind of new mature cells This is to use gene recombination technology to achieve direct transformation between different kinds of adult cells, which represents a more direct and advanced direction of regenerative medicine The team of Zhou Qiao of Harvard Medical School and Douglas Melton of Boston Children's Hospital injected three kinds of genes (Ngn3, PDX1 and MafA) into the pancreas of the diseased mice without islet β cells in vivo by injecting frozen common adenovirus As a result, about 20% exocrine cells in the pancreas were transformed into islet β cells With the increase of islet β cells, insulin secretion increased correspondingly, the level of hyperglycemia decreased and the condition of diabetes decreased However, despite the safety of adenovirus as a vector, the researchers hope to find a way to avoid using the virus to inject three genes into the human body, otherwise the risk is high, which will lead to the opposition of the US drug administration There are many other ways to reprogram stem cells For example, to reprogram mature exocrine cells in pancreas to islet β cells, we can also introduce four genes (Oct4, Sox2, c-myc, KLF4) In this way, the Massachusetts General Hospital Cancer Research Institute and the center for regenerative medicine's Mathias staetfeld research team also successfully transformed mouse pancreatic exocrine cells into islet β cells This situation is a kind of subversion to the old medical phenomenon and theory, because in the organism, the differentiated cells almost never change the direction of development, such as from muscle cells to lung cells However, direct reprogramming of such cells is simpler and safer for the treatment of certain diseases than the use of pluripotent stem cells The technology may allow scientists to speed up the type of cells they need to grow in the laboratory, using certain factors to turn one cell into another In view of the wide range of diseases in 2008, one of the important achievements of cell reprogramming is the exploration of amyotrophic lateral sclerosis (ALS) The aigen team of Harvard University and the team of Columbia University together extracted pathogenic cells from the skin of patients with ALS, genetically modified them, and then injected the modified cells into the brain of patients, the results are encouraging - the genetically modified cells can work normally in the human body ALS is a disease of motor nerve cell damage After the disease, the muscle will gradually atrophy, and then the whole body will be paralyzed and die of respiratory failure The researchers took skin cells from two sisters with ALS (one was 83 years old, the other was 82 years old, because of the genetic nature of the disease, 2% of their ancestors had the disease), and added four genes to convert these cells back to iPS cells In fact, these four genes are the genes used by researchers to reprogram cells to induce pluripotent stem cell status, which is the * step of cell reprogramming Then, the researchers immersed the iPS cells from one of the patients in a variety of signaling molecules, trying to make these cells look like motor neurons, which are damaged cells in ALS The aim of this approach is to use iPS cells to make healthy cells that are genetically matched to the patient and replace the diseased cells, so as not to produce rejection However, there are still major obstacles to be overcome before this method can be safely applied to people Moreover, there may be troubles in the implementation For example, if reprogrammed cells become stem cells, it will be difficult to control their growth, and if not, it will easily lead to cancer Then, George Daley of the stem cell research institute of Harvard University and other researchers in the United States established 20 induced pluripotent stem cell lines, which represent 10 diseases, including adenosine deaminase severe complex immune deficiency, type III Gaucher's disease, Duchenne's muscular dystrophy, Becker's muscular dystrophy, Down's syndrome, Parkinson's disease, adolescents Diabetes mellitus, Shu dai'er syndrome, Huntington's chorea, and Laishi nain'er syndrome (carriers) The establishment of induced pluripotent stem cells for these diseases is also a method of cell reprogramming, which is the beginning of the establishment of disease stem cell like cell bank, and helps to study a variety of diseases Facing security problems, reprogramming cells is a way to change cells, which also means changing the DNA of cells, causing security risks Because inserting exogenous genes into the genome of a cell may disrupt existing genes, such as cancer prevention genes, and make the cell easy to form tumors Although the inserted gene seems to shut down automatically after reprogramming, allowing the cell's own genes to take over, scientists are still worried that the inserted gene will reactivate or have other subtle effects on the cell For example, the introduction of four genes (Oct4, Sox2, c-myc, KLF4) for cell reprogramming requires four independent viruses, each corresponding to a recombinant gene, in order to transfer the gene into the cell DNA These viruses may cause these four genes to be implanted into any part of the cell DNA, which may lead to the expression of oncogenes and cancer To avoid this danger, researchers are trying a variety of methods For example, researchers at the Whitehead Institute of biomedical research in the United States have tried to concatenate four recombinant genes and implant them into the genomes of adult experimental mice and human cells with a virus The results showed that this method could express all four recombinant genes, thus initiating cell reprogramming In the process of gene recombination, the number of required viruses is reduced from 4 to 1, which can greatly simplify the generation of induced pluripotent stem cells and increase the safety Japanese researchers, on the other hand, have found that a DNA ring called a plasmid can also load genes that initiate reprogramming of cells In addition, adenovirus is also a common vector virus in genetic engineering, which causes common cold, but does not insert itself into the cell genome The function of adenovirus is to express the inserted exogenous gene for a long time and reprogram the cell But as the cell divides, the virus is diluted to an undetectable level, leaving the original genome of the reprogrammed cell unchanged However, these relatively safe methods of inserting exogenous genes to start cell reprogramming are inefficient, which also forces researchers to explore safe and efficient methods Today, in most experiments, fewer than one out of 10000 cells can be reprogrammed successfully However, two teams in California and Spain have found that a skin cell called a keratinocyte is particularly easy to reprogram The researchers were able to reprogram about 1% of the keratinocytes in 10 days, while the rest took weeks Hair follicles are a rich source of keratinocytes, and the researchers say they have obtained a personalized cell line from a hair cell pulled from a human scalp, which is much easier to obtain than cutting a piece of skin Although researchers can bypass embryonic stem cells to obtain various kinds of stem cells, at this stage, the transformation of stem cells into specific cells and tissues is limited to the laboratory, such as the transformation of stem cells into beating heart cells Whether these cells can integrate into the patient's tissues and replace or repair the diseased cells and tissues, there are still many technologies and difficulties to be overcome.