Enzyme for spermatogenesis

The little-studied protein turned out to be indispensable for the constant production of sperm

Vasily Parfenov, Naked Science

Male mammals, unlike female individuals, produce germ cells almost all their lives from the moment of puberty. Disorders of spermatogenesis are one of the key factors of male infertility. A detailed study of its mechanisms has the potential to improve the lives of millions of people.

A scientific paper describing the discovery and a series of experiments that resulted in it was published in the journal Genes & Development (Lin et al., Histone methyltransferase DOT1L is essential for self-renewal of germline stem cells). It was prepared by scientists from the University of Pennsylvania (USA) with the participation of colleagues from China, Japan, as well as other American institutions.

The DOT1L protein is found in many eukaryotic cells and in all mammalian cells. At the same time, its role has been poorly studied. The researchers analyzed the expression patterns of DOT1L—coding genes and suggested that one of its key tasks is the regulation of meiosis. This is cell division with a decrease in the number of chromosomes by half, so germ cells are formed in animals. To test the hypothesis, scientists conducted a series of experiments on mice, disrupting the work of this protein at different stages of animal development.

Since DOT1L obviously affects not only the life cycle of germ cells, they did not focus on embryos — this is a reason for another study. Embryos with this enzyme "turned off" or "broken" simply do not survive. But the disruption of DOT1L in germ line cells (stem cells producing gametes) of already born animals does not affect their viability. At least in the short term, the experimental male mice looked perfectly healthy.

But spermatogenesis in such rodents quickly stopped, and regardless of the age of experimental exposure. If the cubs were "broken" DOT1L in gonocytes — primary stem cells, the offspring of which become spermatozoa — seed production began after puberty, but then quickly stopped. When the enzyme was blocked in adult individuals, the clinical picture was similar, only it occurred almost immediately.

The first rounds of germ line cell division were normal, but then the stem cells quickly wore out. As a result, the mice consistently lost the ability to spermatogenesis due to the gradual degradation of the entire process.

The spermatogonies ended first, the spermatocytes of the first and second orders followed, followed by spermatids and, finally, the spermatozoa themselves. Theoretically, not only the DOT1L that stopped working could be guilty of such an effect. Therefore, the scientists conducted a control experiment.

Schematic representation of the process of spermatogenesis with functioning (on the left) and with the "serviceable" DOT1L enzyme. The second row of cells from the bottom is spermatogonia (male stem cells of the germ line), after "switching off" or blocking the enzyme under study, they quickly "exhale" and are no longer able to divide. A picture from the press release of A newly identified stem cell regulator enables lifelong sperm production.

The cells of the germ line of mice were sown on a nutrient medium, but the enzyme itself was not blocked or removed in them. Instead, they received a drug that specifically interferes with the chemical activity of DOT1L in cells. The result was similar: the stem cell culture practically did not grow. And when such cells were transplanted to adult mice, they also quickly stopped spermatogenesis.

The discovery has every chance to serve for the benefit of infertility therapy. The fact is that, despite all the achievements of modern medicine, humanity still knows very few factors that ensure the longevity of stem cells. Especially if they are germline cells. In the future, the finding of scientists from the USA will allow not only to correct the functions of spermatogonia in the mammalian body, but also to transform somatic (not sexual, but related to the rest of the body) cells into stem cells.

Another equally important finding of the study was the localization of a section of the genome, for the regulation of the expression of which DOT1L is "responsible". This group of genes is called HoxC and, in turn, regulates the expression of a whole range of other genes. Moreover, as the authors of the scientific work suggest, a number of these genes are responsible for the key properties of the life cycle of stem cells. All these questions are proposed for study in the future.

Over the past couple of decades, studies of the DOT1L (DOT1-like) protein have shown that it plays an important role in the formation of chromatin. It is a complex of DNA and special proteins (mainly histones) that help to pack a giant molecule with hereditary information. During different periods of the life cycle, the compactification (packing density) of DNA varies by orders of magnitude. Also, how tightly a section of the DNA molecule is folded depends on its participation in the processes of transcription or replication.

The basis of the compact structure of DNA is the nucleosome. It consists of eight molecules of histone proteins (types H2A, H2B, H3 and H4), around which deoxyribonucleic acid is wound like a bobbin. Another type of histone (H1 — in humans and other mammals, H5 — in birds) holds DNA outside the "wound" area and restricts the nucleosome. DOT1L is involved in the formation of this formation.

It methylates histone type H3 in a certain part of the molecule, that is, it attaches a methyl group —CH ₃ to an amino acid (in this case, to lysine). As a result, the chemical properties of the histone protein change and it works differently during the formation of the nucleosome.

How exactly DOT1L affects the functioning of cells and whole organisms is not a fully understood question. It is known that this protein plays an essential role in the pathogenesis of acute lymphoblastic leukemia and other diseases caused by mutation of the MLL1 gene (KMT2A). Also published in 2020, a study by Australian scientists showed that DOT1L is critically important for the formation of humoral immunity. It is necessary for the formation of B-lymphocytes in the bone marrow and determines their fate in the organs of the lymphatic system. And last April, an Italian team of researchers proposed DOT1L as a promising target for antitumor therapy.

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