Cellular tragedies, part 6
Start from the beginning
Polina Loseva, "The Attic"
The inner life of the cell is eventful no less than the human one. It is full of passions and dangers and just as inevitably ends in death. Polina Loseva understands what plots occur in the fate of cells and how their development affects you and me. This time we will talk about how to burn bridges behind you and start life anew. Radical changes are not always easy for people, but maybe cells are doing better?
"Cellular tragedies" is a large series of articles about cells, which continues to be replenished. Read other texts about the difficult life of cells: they tell about deaths and suicides, stress, shock, self-determination and old age.
Many films begin like this: a stranger gets off a train on an empty platform in a small town. There is a suitcase with personal belongings in his hand, the view is mysterious. Where did he come from? What did you run away from? Will he be able to start life from scratch in a new city? In most films, strangers cannot escape from the past – they are overtaken by pursuers, memories and old traumas. Nevertheless, the image of the mysterious fugitive remains attractive, and the Internet is replete with motivating texts and instructions for setting fire to their bridges. Ten years ago, such instructions appeared for cells.
Cellular childhood
Moving to another city and changing a profession, a person in some way returns to childhood: gives himself the opportunity to choose again where to live, who to be and with whom to communicate. For a cell, childhood is a stem, or undifferentiated, state (more about stem cells here). In this state, the cell does not have the structural features and functions characteristic of "adult" cells of organs and tissues. In addition, the stem cell has broad prospects: under the influence of external factors, it can become a cell of one of many types. Information about the proteins that a cell needs to perform all possible functions is recorded in its DNA. And all this information is available to the stem cell. But as the cell differentiates, it gradually twists the unnecessary DNA, leaving only the information that will be useful to it on the chosen path.
We have already compared this process with packing things. While a person is young, he has a large set of books that he can use. But as he gets carried away with something and specializes in it, unnecessary books are packed in boxes and wrapped with tape. At the molecular level, this means the following: the DNA strand is usually wound on histone proteins (stacked in boxes) – so it takes up less space. If you need to read information, histones easily fall off from DNA, and then stick back. But you can hang chemical groups on histones and DNA (glue them with tape), which will better fasten them. Then it will not be possible to reset histones and untwist DNA, and the information will become unavailable.
Thus, as the cell "matures", the range of possibilities narrows: the less information she can use, the fewer options she has about who to be and what to do. Similarly, our mysterious stranger, in search of inspiration, looks at a shelf that was once full of various books. But over the years, only the instructions for using a jigsaw and the "Encyclopedia of a young carpenter" remained there. And then he decides to start all over again.
Big cleaning
The return of the cell to its origins is called dedifferentiation. It is still unknown how often it occurs in the human body. With the exception of oncological diseases, when cancer cells lose the properties of their tissue, such cases can rarely be detected. But cell biologists decided to take the situation into their own hands. Since it became clear that different tissues can be grown from stem cells, the question has arisen how exactly to extract human stem cells on an "industrial scale". Taking them from living people is traumatic, sometimes dangerous (if they are brain stem cells, for example), besides there are not many of them. Taking cells from human embryos is unethical. Therefore, there was an idea to learn how to reprogram adult cells, that is, to return them to the stem state by force.
The heroes of the films usually find their favorite, but long forgotten hockey stick in the process of cleaning and decide to make up for lost opportunities by abandoning their already established life. But rebooting your life is not the easiest task. This will require radical action: pull out all the boxes, cut the tape and put it on the shelves again. In other words, you need to remove the extra labels from histones and DNA and spin everything back so that the cell gets access to all the information again. But how to do it?
The first method was proposed in the middle of the last century and consisted in transferring the nucleus from an adult cell to an egg (from which the own nucleus was previously removed). After these manipulations, under the influence of some (not fully studied) substances from the cytoplasm of the egg, the DNA of the nucleus was unwound - and we found ourselves in front of a cell that can do everything again. We can say that we sent a failed carpenter to his hometown to his mother. A responsible mother took the initiative into her own hands, cut everything, disassembled and put it in its place.
This method is convenient and quite effective, but the problem is that there are not enough "moms" for everyone. Every time we have to take eggs from female volunteers, which is not only ineffective, but also associated with ethical difficulties. It would be nice to teach the cage to do the cleaning on its own and start a new life right on the spot, without moving.
Japanese scientists led by Shinya Yamanaka suggested that some proteins may be encoded in our genes that are able to start this cleaning. They took adult skin cells and infected them with viruses carrying different genes. The viruses themselves were safe, but the genes embedded in them worked more actively than the cells' own genes. It turned out that it was enough to make four (and in some cases even one) key genes work in order for the DNA to unwind. The result was a culture of cells very similar to the cells of an embryo at an early stage of development. The resulting cells were called induced pluripotent stem cells. Induced – because scientists forced them to deploy their DNA, stem – because any cell types can be obtained from these cells, and pluripotent – because their possibilities (potency) are limitless.
If the transfer of nuclei required relocation and some external environment, that is, a "mom" who would sort everything out, then the Yamanaki method acted on the cell from the inside. It can be called the technology of intracellular hypnosis. It is enough to "give" the cells a certain "setting" ("Cleanliness is the key to health!", "Put things in order!") so that they do the cleaning themselves. This method attracted the attention of the scientific community so much that in 2012 it was awarded the Nobel Prize, and the technology is still being improved.
Initially, Yamanaka activated the work of genes with the help of viruses. Then scientists began to inject proteins encoded by these genes into cells. And more recently, it was proposed to use the popular CRISPR-Cas technology to selectively bind to the right genes and start their work. Finally, it turned out that you can simply slip the cell "scissors for scotch tape", that is, process it with molecules that cut off the labels from histones and DNA. And this is also enough for the cell to start a new life.
Second youth
Reprogramming draws tempting prospects: let us have a society (the human body) where there are many professionals in one area (cells, for example, skin) and few in another (brain cells). We take specialists from where they are in abundance, erase their memory (reprogram), multiply, train anew (differentiate) and throw this landing into a new area (implant the patient). But somewhere in this idyll lies a catch. The question that has been tormenting the scientific community for the second decade worries viewers of films about a mysterious stranger: is it possible to completely leave behind and forget the past life? As experiments on cell reprogramming were repeated in laboratories around the world, it became clear that this procedure gives very unstable results.
Firstly, its effectiveness is in the best cases several percent, that is, a significant part of the efforts and reagents are wasted. Secondly, not all cells are equally susceptible to hypnosis. This may depend on the laboratory, on the protocol, and on specific patients.
Anyway, when reprogrammed cells are compared with real embryonic cells, differences are often found. Converts sometimes refuse to study again (they do not differentiate into all types), then they neglect their duties (they share worse). It is believed that this may be due to the age of the patient from whom the cells were taken. Over the years, the tape sticks together tightly, and even after reprogramming, some marks on the DNA are preserved. Although in some experiments, these marks disappear if cleaning is carried out several times in a row. In addition, individual genes or proteins characteristic of the previous cell specialization may remain active. Let our former carpenter take apart all the books, put them back on the shelves and put the past out of his head, but as soon as his gaze falls on the jigsaw lying on the table, he immediately remembers everything, and the reprogramming has not happened.
To believe the reformed?
The low efficiency of reprogramming has led to the fact that it is still not used in clinics. The exception is a Japanese group of scientists who, back in 2014, managed to grow a layer of retinal pigment epithelium from reprogrammed cells and inject them into the eye of a patient with macular degeneration (a common disease in which the retinal epithelium is destroyed and visual acuity decreases). Now that three years have passed, we can say that scientists have been able to stop the loss of vision. However, they did not achieve an improvement in vision either. In other countries, however, there are no clinical studies yet. If we cannot precisely control the reprogramming of cells and subsequent differentiation, then how can we expect that newly converted stem cells will behave in the body exactly as we want?
The only way to integrate our mysterious strangers into society is to limit their contacts with others. According to this principle, a new method of diabetes treatment is arranged, which is currently undergoing clinical trials. At the moment, the technology is arranged as follows: a female volunteer gives scientists one of the embryos obtained as a result of in vitro fertilization. They extract cells from there, differentiate them into pancreatic cells, enclose them in a capsule with microscopic pores and implant them into the body. It turns out that the artificial gland can directly exchange molecules with the environment, the cells sense the amount of glucose in the blood and secrete insulin. But its inhabitants cannot go beyond the capsule, spread through the body and multiply uncontrollably in it. However, experiments with embryonic cells are prohibited in many countries (including Russia). Therefore, if this technology proves successful, it is likely that the implant capsules will be populated with induced pluripotent cells. And even if it sounds like discrimination, such "ghettos" are currently the most reliable way to treat strangers who have broken with their past.
In parallel, another idea arose: what if the cells are reprogrammed not completely, but partially? Perhaps this will help to avoid the aging of cells, including adult stem cells. In other words, not complete oblivion, but easy cleaning, but everywhere. Scientists injected mice with the same viruses as the Yamanaki group, but not into individual cells, but intravenously. The mice probably liked it. At least, they did not live 18 weeks, as the control group, but 24. However, with such experiments, you should be careful: if viruses work in the body for too long, individual cells are reprogrammed entirely and turn into tumors. But this technology has not been tested on humans yet, so we can only wait for news and envy the long-lived mice.
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