Weizmann Institute of Science scientists made bacterial cells mimic processes characteristic of multicellular organisms, and discovered a possible defense mechanism against cancer
For a population to be stable in size over time, a balance between birth and death is required; A birth rate that is too high will lead to a population explosion, while a rate that is too low will lead to a contraction. Such a balance is maintained, for example, among the approximately ten thousand billion cells that make up our body: in our adulthood, stem cells divide to renew body tissues, but after a few divisions they become mature cells that divide less and eventually die. This balance is felt only when it is violated, for example when cells divide without control and form a cancerous tumor.
If so, an equilibrium between dividing cells and mature cells is a necessary condition for the existence of any multicellular organism, but how is it maintained? In a new study whose findings were published in the scientific journal Cell, Weizmann Institute of Science scientists used unicellular organisms to better understand how multicellular organisms maintain cellular balance and defend against cancer.
Cell differentiation is a biological "specialization pathway" in which a stem cell divides into two daughter cells, with one of them taking on a defined role and acquiring properties necessary to fulfill it. When cells undergo differentiation, their new expertise benefits the multicellular creature of which they are a part, but they themselves pay a personal price: as the differentiation pathway progresses, their ability to reproduce diminishes to the point of losing this ability altogether. The slower rate of division, if any, of sorted cells makes their population vulnerable to populations of cells that divide and grow faster, and therefore may take over the tissue and its resources. For example, in certain types of blood cancer, stem cells in the bone marrow undergo a mutation that slows down their differentiation and allows them to produce more offspring. This is how the mutant cells take advantage of the natural weak point of differentiation and overcome the population of healthy cells in what is known as "mutant takeover".
Although in every cell division in our body one mutation occurs on average, most of us live in health for decades and there are countless cell divisions without experiencing "mutant takeover". From this, it can be concluded that there are effective mechanisms to deal with this danger, but it is difficult to identify them in nature in complex creatures. Therefore, the research group of Prof. Uri Alon In the Department of Molecular Biology of the Cell at the Institute for Engineering E. coli bacteria, which do not normally differentiate, so that they will undergo an artificial differentiation process and will be used to study how a population copes with "mutant takeover".
"There are distinct advantages to the ultrasonic model," explains Dr. David Glass, who led the research in Prof. Alon's laboratory. "One of them is the short generation time that makes it possible to follow the development of mutants over hundreds of generations of cells in the laboratory." In order to develop differentiating E.coli bacteria, the scientists drew inspiration from a species of blue-green bacteria called Anabina. These cyanobacteria differentiate in response to the lack of nitrogen in their environment. The differentiation process is carried out in them by cutting DNA segments, as a result of which they lose their ability to divide, but gain a survival advantage: the ability to supply nitrogen to themselves and the colony.
To simulate the differentiation process in an E.coli model, the bacteria were grown in an environment containing antibiotics and lacking an essential amino acid. Using methods of genetic engineering, several copies of a gene for resistance to antibiotics and several copies of a gene for the production of the deficient amino acid were inserted into each bacterium. Before the artificial differentiation process began, that is, when the bacteria were in a state similar to that of "stem cells", the antibiotic resistance genes were active, and the bacteria could divide and differentiate at a high rate despite the presence of the drug. When the differentiation process came into action by cutting the copies of the genes for resistance to antibiotics, the bacteria gradually lost their ability to divide and differentiate, but gained a survival advantage: the cuts led to the gradual activation of the genes for the production of the essential amino acid.
"We held a competition between 11 E. coli strains, each of which cuts DNA at a different rate, that is, differentiates at a different rate, in order to check what the best rate of differentiation is," explains Dr. Glass. "We mixed equal amounts of the bacteria together, grew them for a few days and then checked who survived. We saw that there is a very strong selection in favor of bacteria that differentiate at a moderate rate. We recognized that through a medium rate of differentiation, bacterial cultures manage to maintain an optimal balance in the population - at any given moment, a minority of the cells are 'pure stem cells' or 'fully sorted cells', and most of them are in intermediate stages of the process."
This intermediate optimal differentiation rate also characterizes various systems in the human body, where a quantitative balance is maintained between stem cells, progenitor cells that are in different stages of differentiation, and sorted cells that sometimes die and are replaced by new ones. In order for the population size to remain constant, it is important that the balance is maintained even when the environmental conditions change. To test whether the model bacteria maintain a balance even when the conditions change, the researchers grew them in 36 different combinations of the concentrations of the antibiotic and the amino acid in the growth medium.
"We saw that in all situations, except the most extreme, such as the complete absence of antibiotics, the optimal rate of differentiation of the cells remained in the medium range, and the balance in the population was preserved," explains Dr. Glass, "This means that the population balance that characterizes the differentiation model we developed is largely immune to dangers and changes in the environment". But is the population of bacteria that differentiate at an optimal rate also resistant to "mutant takeover", similar to control systems in multicellular organisms?
In order to test the bacteria's resistance to "mutant takeover", the scientists grew the differentiating bacteria over many generations. The researchers checked whether during the long growing time random mutations appeared that created bacteria that do not differentiate at all and divide without control. That is, do mutant bacteria succeed in bringing about a "mutant takeover" or are they suppressed at an early stage? The first time they performed the experiment, they were disappointed to find mutants taking over in half of the cases. "We discovered that when a genetic change occurs that breaks the connection between differentiation and the loss of the ability to divide and gaining a survival advantage, non-differentiating mutants can take over," explains Dr. Glass.
Later, the scientists using genetic engineering methods created a new bacterial strain that was resistant to the identified mutation and repeated the experiment. "We managed to grow 270 generations of differentiating bacteria without mutants taking over. Unfortunately, the war that broke out in Israel on October 7 also interrupted the experiment, and it is possible that the bacteria are even more stable," says Dr. Glass. In fact, the Eicoli model showed that when the differentiating cell stops dividing but gains a survival advantage, an optimal population balance is maintained and mutant takeover is avoided. "Many diseases, such as cancer and autoimmune diseases, are unique to multicellular organisms. When we engineer into unicellular creatures more and more characteristics of multicellular systems, we can locate weak points and look for them in human tissues as well", Dr. Glass describes the advantages of the model they created.
Beyond basic science, the new findings may also have implications for the use of bacteria in industry. "Transgenic bacteria are currently used for the large-scale production of insulin, enzymes and other substances for human use", he says, "creating a differentiated bacterial population that maintains balance, regenerates and even prevents the takeover of harmful mutations, can optimize these production processes."
Elizabeth Weisbord, Dr. Anat Baran and Dr. Avi Mayo from Prof. Alon's group in the department of molecular cell biology at the institute also participated in the study.
More of the topic in Hayadan:
