Stress and sperm
Chronic stress affected sperm through extracellular vesicles
Alice Bakhareva, N+1
Scientists have discovered the mechanism of the epigenetic effect of stress on sperm and embryo development. In response to chronic stress, the composition of microRNAs and proteins changed in the extracellular vesicles of the epithelial cells of the epididymis, these structures in turn affected the microRNAs of spermatozoa, which was observed in the sperm of humans and mice. This effect developed over a long period after stress and affected the development of the nervous system and the response to stress in the offspring. The article was published in the journal Nature Communications (Chan et al., Reproductive tract extracellular vesicles are sufficient to transmit intergenerational stress and program neurodevelopment).
The conditions in which the germ cells developed before conception significantly affect the development of the fetus. The influence of unfavorable conditions in which the body is located, whether it is stress, diet or drugs, on sperm, has been found in a number of previous studies. However, the mechanism of this interaction remained unknown.
Stress can be mediated by molecules that are detected in sperm, for example, small non-coding RNAs. However, after spermatogenesis, there is no transcription in the germ cells, and their DNA is compactly folded to protect against damage. This means that there must be external, somatic, mechanisms that would have a long-term effect on sperm and the development of the embryo.
After spermatogenesis, germ cells continue to form in the tubules of the epididymis, where they receive somatic signals from surrounding tissues. Just here, through the epithelial cells of the appendage, the conditions in which the organism exists can affect the maturation of sperm.
One of the ways of intercellular communication is extracellular vesicles. These are small bubbles that carry proteins, lipids, or small non-coding RNAs from one cell to another. Extracellular vesicles are intermediaries between the epithelial cells of the head of the epididymis and sperm, they transmit signals important for the maturation of sperm. This is how the body adapts the state of germ cells and the subsequent development of the embryo to changing environmental conditions.
Jennifer Chan from The University of Pennsylvania and her colleagues investigated the short- and long-term effects of stress on sperm and offspring of mice. Males were put into a state of chronic stress for a month, and after one or 20 weeks they were mated with females. Then they assessed how the offspring of these animals coped with stress. At the same time intervals, sperm samples were taken from mice and a set of microRNAs was determined in them.
To compare the observed patterns in mice with humans, scientists examined sperm samples from 18 students at the University of Pennsylvania. The volunteers were tested monthly and at the same time underwent a psychological examination, during which, among other things, they determined the level of stress. According to the results of psychological tests, four students were identified whose stress level was high at the beginning of the study and significantly decreased over six months, and four whose stress level remained constant during the experiment.
Since the effect of stress, which was observed in the experiment, acted for a long time, the authors of the work suggested that chromatin rearrangements occur in the somatic cells of the epidermis of the epididymis of the testicle. To test this, posttranslational modifications of histones (proteins responsible for chromatin packaging and regulating other nuclear processes) were studied using mass spectrometry.
In heterogeneous tissues in a living organism, it is impossible to identify extracellular vesicles that secrete epithelial cells of the epididymis. Therefore, scientists reproduced the stress effect on the culture of these cells. To do this, corticosterone, a hormone that is released during stress, was added to the Wednesday for three days. At different intervals of time, samples of extracellular vesicles were taken and the composition of microRNAs and proteins was analyzed in them. Then the vesicles were labeled with a dye and injected into mice that had not previously participated in the experiment to find out the targets of extracellular vesicles of the epididymis in the body.
The researchers also tested how the impact of extracellular vesicles on sperm affects the development of the embryo. For this purpose, spermatozoa were grown on a medium to which extracellular vesicles of epithelial cells of the epididymis were added. Corticosterone was added to half of the vesicles before that for three days, and then eight days were given for the development of the response, the second half was controlled. Spermatozoa were injected into the oocytes of females, and the embryos were implanted in the same mice. In the middle of pregnancy, animals were killed and fetal tissues were studied.
Mice that were conceived a week after prolonged stress did not differ from control mice. And if the males were allowed to develop a response, their offspring reacted to stress with increased levels of corticosterone. A week after exposure, the composition of microRNAs in semen and histone modifications of the epithelium of the epididymis did not differ from the control, and after 12 weeks the differences became significant.
Surprisingly, even in such a small sample (four people in the group), significant differences were found in the composition of microRNAs in the sperm of volunteers who recovered from stress and those whose stress level was constant (p = 0.0112). In cell culture, eight days after corticosterone addition, the microRNA expression pattern and the set of extracellular vesicle proteins changed significantly compared to the control. Vesicles of the epididymis accumulated mainly in the liver and spleen, but also in the genital tract. Thus, extracellular vesicles can indeed mediate the effect of stress on sperm maturation.
In the brains of embryos that developed from sperm with stressed extracellular vesicles, the expression of genes associated with synaptic transmission and transport of mediators changed. This affects the development of the nervous system and the functioning of the adult brain. In addition, the scientists found changes in the transcriptome of the placenta – the activity of genes that are associated with the inflammatory and immune response was higher. The offspring that were obtained as a result of artificial insemination reacted to stress in the same unbalanced way as the individuals in the first experiment.
A few years ago, researchers from the same group introduced stress microRNAs into a fertilized egg, and this affected the behavior and functioning of the brain of individuals who developed from this zygote. The offspring is affected not only by the stress of males, but also of females – stress in different periods of pregnancy can both accelerate and slow down the development of cubs.
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