A coming-of-age can be beautiful thing. Along with all the exciting responsibilities that accompany maturity comes a celebration in many cultures but perhaps most memorable to all who have been through one are the gifts. Now, it seems that sperm are no exception as during their maturation process they acquire the gift of small ncRNAs payloads that shape their future.
The lab of Oliver Rando at the University of Massachusetts has previously shown an intergenerational impact of paternal diet on offspring that is related to sperm’s ncRNAs, which include tRNA fragments (tRFs) that represent ~80% of its RNA content at maturity. Now, in two back-to-back publications, they examine the biogenesis of these small ncRNAs and their impact on sperm maturation and early embryonic development.
Sperm’s Journey to Manhood
Over an approximately two-week process, sperm vacate their childhood home in the testes to undergo a rite of passage, where they travel via the epididymis to the vas deferens. Now Sharma et al. show that on its journey to (post-testicular) manhood through the epididymis, sperm acquire an RNA payload. Here’s what they encountered when examining sperm’s journey using mice:
- Small RNA-seq of purified sperm populations at different levels of maturity revealed that the primary RNA content of sperm remodels from piRNAs to tRFs when it leaves the testes and enters the epididymis
- There are also some stranger dynamics, where a large population of miRNAs that are abundant in testicular sperm are reduced upon beginning their post-testicular travel through the proximal (caput) epididymis only to regain abundance later in the distal (cauda) epididymis
- The sperm’s new RNA payload of tRFs and miRNAs in the caput epididymis comes from small vesicles known as caput epididymosomes, as demonstrated by in vitro incubation experiments
- Finally, the whole RNA voyage from epididymis to sperm was tracked in vivo via metabolic labelling to conclusively show that mature sperm contain RNA from the epididymal epithelium
Taken together, these findings demonstrate that RNA content from somatic cells can transmit to the male mammalian germline cells via epididymosomes.
Developmental Programming of Offspring by Sperm ncRNA
To get at the function of the RNA payload, Conine et al. examined the effect on embryonic development in mice. They utilized intracytoplasmic sperm injection (ICSI) to generate zygotes from immature sperm from the proximal (caput) epididymis and mature sperm from the distal (cauda) epididymis of the same mouse. Here’s what happened:
- Single-embryo mRNA-sequencing uncovered the upregulation of ~50 genes with functions related to RNA binding and chromatin in embryos created from immature sperm
- These changes to the expression of epigenetic regulators persisted across early development and result in preimplantation defects
- When the embryos were transferred to surrogate mothers, they did not survive
- Microinjection of small (18–40 nt) RNAs purified from cauda epididymosomes into the zygotes derived from immature caput sperm rescues the developmental defects
- This size selection shows that rather than tRFs, which have them beat in size and number, miRNAs (or another similarly sized small RNA) are the key developmental regulator that is gained through epididymal transit
Overall, these findings show that miRNAs gained from cauda epididymosomes during sperm maturation play a key role in early embryonic development.
The Developmental Dimensions of Sperm Maturation
Taken together, these two studies uncover two different key changes in the RNA payload of sperm during post-testicular maturation. Senior Author Oliver Rando shares, “A substantial subset of embryos are created using fertilization with testicular sperm, which have subtly 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 impact offspring, it will be interesting to explore these effects of sperm used in assisted reproduction.”