Ultrasound of gene expression
The activity of genes in a living organism can be detected using ultrasound
Mikhail Shapiro, a professor at the California Institute of Technology, and his colleagues have developed a fundamentally new method for detecting gene expression in a living organism.
The full text of the preprint of the article Ultrasound Imaging of Gene Expression in Mammalian Cells can be read on the bioRxiv – VM website.
In order to find out when a particular gene is turned on or off, researchers usually attach another one to the gene they are interested in – the so-called reporter gene. This role is played by genes whose activity is convenient to register, for example, the genes of green fluorescent protein or luciferase, which cause a characteristic glow. But such methods work well in cell culture and much worse in a living organism, since it is difficult to track fluorescence in internal organs.
Mikhail Shapiro and his colleagues have proposed a new reporter gene whose activity in the tissues of the body is easily tracked by conventional ultrasound examination. The researchers borrowed the necessary gene from bacteria, which differ in their ability to produce small air bubbles inside a bacterial cell ("gas vesicles"). Back in 2014, Shapiro's laboratory found that these bubbles are easily detected by ultrasound.
Now scientists have managed to make mammalian cells synthesize the same vesicles.
This was not an easy task, since nine genes are responsible for this work in bacteria, and most importantly, the mechanisms of information transfer from DNA to RNA and further protein synthesis in mammals (as in other eukaryotes) and bacteria differ significantly. If you simply transplant bacterial genes into the DNA of a cell, it will not be able to use them. "The translation mechanism is very different in the two types of cells," says one of the authors of the study, Arash Farhadi. "One of the biggest differences is that bacterial DNA often has many genes that are transcribed into one common RNA fragment, and it is then translated into all the corresponding proteins, whereas in eukaryotes each gene is usually on its own." Researchers were able to make bacterial DNA work in mammalian cells by using a mechanism that viruses use to insert their genome into the cell nucleus.
But even after the bacterial DNA started working in the cells and the synthesis of the necessary proteins started, air bubbles did not arise. As it turned out, it is not enough to have the right proteins for this, it is also necessary that these proteins are produced in the right ratio. "The correct ratios of proteins are programmed in clusters of bacterial genes, but when we put them in mammalian cells, we have to figure out what these ratios should be and how to get mammalian cells to produce these proteins correctly," says Farhadi. This task required several years of work.
Now the genes have worked properly, and nanobubbles filled with air really began to appear in the cells. They are detected by ultrasound, which can penetrate into the body, so the new reporter gene is applicable to study gene expression in neurons, cells of the immune system and other types of cells of the body, as well as in tumor cells. The researchers hope that in the future their method will also be used by doctors to monitor the course of gene therapy in a patient.
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