Long-ing for an explanation for transgenerational epigenetic inheritance via sperm? Well at long last long non-coding RNAs (lncRNAs) are giving us some insight. Studies have shown that sperm DNA methylation and small non-coding RNA (ncRNA) content mediate the transmission of life experience to subsequent generations, especially parental stress. In fact, early life stress is one of the best studied experiences that shows transgenerational epigenetic inheritance (TEI). In mice, this is modeled with maternal separation combined with unpredictable maternal stress (MSUS) to introduce stress in developing mouse pups. This model shows inheritance of effects through as many as three generations, and many of the effects are mediated by epigenetics.
A new publication from the labs of Eric Miska (University of Cambridge, UK) and Isabelle Mansuy (University of Zurich, Switzerland) explored the MSUS model in the context of sperm RNA, where they examined the intergenerational effect. Previously, this group showed this model induces changes in sperm small ncRNA. They also found that injection of total sperm RNA from exposed males into fertilized oocytes leads to offspring with similar phenotypes as the natural offspring of exposed fathers. Many other groups have looked at the effects of small ncRNAs; however, the specific effects of long RNA (greater than 200 bp) have been not be closely examined. In their most recent study, these authors took sperm from MSUS and control males and separated it into small (<200nt) and coding and non-coding long (>200nt) before injected it into fertilized oocytes. They then characterized the behaviors of the resulting offspring.
Here’s what they found:
- Relative to non-injected controls, injection of long or small RNAs reproduces different components of phenotypes seen in naturally born offspring from MSUS males:
- Injection of long (but not small) RNA produces offspring with increased risk taking behavior, as seen in the light/dark box
- Injection of small RNA (but not long) produces offspring with increased behavioral despair, as measured by the forced swim test
- Neither RNA type on its own reproduces the behavioral changes in natural-born offspring of MSUS male mice show in the elevated plus maze, suggesting the long and small RNAs act synergistically
- Using RNA-seq, they found differential expression of many long intergenic non-coding RNAs (lincRNAs) and mRNAs involved in cell adhesion and extracellular matrix organization in the MSUS sperm vs. normally raised controls
- Fertilized zygotes deferentially express many of the same RNAs as sperm and additional ones, suggesting delivery and further downstream effects of the injected RNAs
- Fragmenting the long RNA completely removed its effects on the offspring
These results show that small and long RNAs work synergistically, and on their own can have different effects. The fact that the long RNAs must be intact to have an impact shows that they need to be full length to carry out their transcriptional functions. Going forward, CRISPR or other methods to target specific RNAs will be important for establishing the precise mechanisms of these effects. So when it comes to epigenetic inheritance, that’s the long and short of it.
Check out the full-length story in Molecular Psychiatry, October 2018.