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Developmental biology - Male RNAs

Small RNA in Sperm May Ensure Embryo Implants

Changes in sperm 'small RNA' collected early for IVF, may impede egg implantation...

Louise Brown, the first person born after conception by in vitro fertilization (IVF), celebrates her 40th birthday this year. Thanks to assisted reproductive technologies like IVF and techniques developed in lab animals, research has provided live births from parents otherwise unable to conceive. New insight into genetic inheritance and the epigenetic contributions of both mother and father continues to improve these chances, and may contribute to our understanding of why some egg implantation fails.

Two studies published in the journal Developmental Cell on July 26 provides new information on unique paternal contribution to their offspring. Both articles examined paternal small RNAs to determine how these molecules contribute to epigenetic changes in the sperm of mice. Contributions by females to their offspring are easily apparent - beginning with her contribution of both nuclear and mitochondrial DNA, and the continuous neonatal exposure to various factors during gestation. But, much less is known about male contributions beyond donation of paternal DNA.
"Because of the rise in the use of assisted reproduction, it's vital to look at the differences between sperm removed directly from the testicles and those ejaculated, to investigate whether these differences may have an impact on the long-term health of the offspring."

Oliver J. Rando PhD, Prinicpal Investigator, Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Massachusetts, USA and senior author of both papers.

Past studies by Rando's and other labs suggest varying environmental conditions can affect sperm and the health of offspring. Lifestyle and exposure to hazardous toxins can induce stress which potentially affects levels and types of small RNA in sperm.

In one study, Rando's lab investigated small RNA in sperm after the two week trip from the testis, through the epididymis, then the vas deferens, and exiting as ejaculate. They found ejaculate sperm undergo dramatic changes in the RNA. They also confirm that some RNA found in sperm actually originates in the epididymis, the highly convoluted duct leading to the vas deferens. This finding provides evidence for soma-to-germline transfer in male mammals.

Mouse male reproductive organs

Male mouse reproductive system showing sperm origination in testicle and 2 week passage to ejaculate.

The second study looked at whether these small RNAs have any effect on sperm and thus on the zygote (or fertilized egg). Led by first author Colin C. Conine PhD, this team found that small RNAs in sperm are essential for normal pre-implantation development.

Specifically, sperm taken from early in that 2 week trip, from the epididymis, exhibit dramatic misregulation in RNA and a variety of other epigenetic regulators. Used to fertilize eggs, this sperm resulted in failure of embryos to implant in the uterus. However, they also found these defects can be corrected by microinjecting small RNAs from the end of the epididymal path and into the newly formed embryo.
"This research is vitally important because of the increasing use of assisted reproduction. A substantial subset of embryos are created using fertilization with testicular sperm - which has dramatically different RNA contents from ejaculated sperm.

Since we now show that even relatively subtle RNA differences between sperm from the beginning versus the end of the epididymis can seriously affect offspring, it's crucial to understand whether children born from testicular sperm will have any increased disease risks as they grow into the age of complex adult-onset disease."

Oliver J. Rando PhD

Detailed characterization of small RNA dynamics in mammalian sperm maturation
Caput epididymosomes can deliver small RNAs to testicular spermatozoa in vitro
Metabolic labeling of RNAs in intact animals tracks RNAs from epididymis to sperm

The biogenesis of the RNA payload of mature sperm is of great interest, because RNAs delivered to the zygote at fertilization can affect early development. Here, we tested the hypothesis that small RNAs are trafficked to mammalian sperm during the process of post-testicular maturation in the epididymis. By characterizing small RNA dynamics during germ cell maturation in mice, we confirm and extend prior observations that sperm undergo a dramatic switch in the RNA payload from piRNAs to tRNA fragments (tRFs) upon exiting the testis and entering the epididymis. Small RNA delivery to sperm could be recapitulated in vitro by incubating testicular spermatozoa with caput epididymosomes. Finally, tissue-specific metabolic labeling of RNAs in intact mice definitively shows that mature sperm carry RNAs that were originally synthesized in the epididymal epithelium. These data demonstrate that soma-germline RNA transfer occurs in male mammals, most likely via vesicular transport from the epididymis to maturing sperm.

Upasna Sharma, Fengyun Sun, Colin C. Conine, Brian Reichholf, Shweta Kukreja, Veronika A. Herzog, Stefan L. Ameres, Oliver J. Rando.

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Jul 31, 2018   Fetal Timeline   Maternal Timeline   News   News Archive

Sperm collected for IVF is generally taken from the widest portion of the epididymis duct. But research reveals significant difference between RNA found in the epididymis and RNA from ejaculate.
Image Credit: Mouseworks composite.

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