The Transmission of Intergenerational Epigenetic Information by Sperm microRNAs

Abstract

Epigenetic information is transmitted from one generation to the next, modulating the phenotype of offspring non-genetically in organisms ranging from plants to mammals. For intergenerational non-genetic inheritance to occur, epigenetic information must accumulate in germ cells. The three main carriers of epigenetic information-histone post-translational modifications, DNA modifications, and RNAs-all exhibit dynamic patterns of regulation during germ cell development. For example, histone modifications and DNA methylation are extensively reprogrammed and often eliminated during germ cell maturation and after fertilization during embryogenesis. Consequently, much attention has been given to RNAs, specifically small regulatory RNAs, as carriers of inherited epigenetic information. In this review, we discuss examples in which microRNAs have been implicated as key players in transmitting paternal epigenetic information intergenerationally.

Keywords: environmental exposures; epigenetic inheritance; microRNAs sperm.

Figure 1

Schematic illustrating how different paternal environmental exposures can influence inherited phenotypes of sired offspring. Environmental perturbations experienced by fathers such as changes in diet, stress, and chemical exposure can transmit non-genetically inherited phenotypes to offspring including altered metabolism, behavioral changes, and modulated susceptibility to stress and chemical exposure. The transmission of paternal environmentally modulated inherited phenotypes is thought to occur by changes in epigenetic information in sperm. Inherited information is encoded in small regulatory RNAs in the best-characterized paradigms of inter- and trans-generational epigenetic inheritance, RNA interference in C. elegans, and paramutation in plants. Accordingly, miRNAs have been identified as potential, and in some instances confirmed, carriers of paternal environmentally modulated epigenetic information transmitting inherited phenotypes to succeeding generations of progeny. However, the mechanisms underlying how an RNA in sperm transferred to the zygote upon fertilization can alter embryonic gene expression and development to generate an inherited phenotype are currently unknown.

Figure 2

Schematic representing how microinjection of RNA into zygotes can experimentally confirm causal function for RNAs, specifically miRNAs, in transmitting epigenetically inherited phenotypes to offspring in mice. First, zygotes are produced by fertilization either naturally (and then flushed from the inseminated mother) or via IVF with sperm and eggs from naive/control animals and then cultured in vitro. In parallel, RNA from environmentally exposed sperm can be purified and further fractionated to isolate specifically miRNA-size (20–24 nt) sperm RNAs. Alternatively, candidate miRNAs of interest altered in the sperm of animals exposed to the environmental condition of interest can be synthetically produced. Purified sperm RNAs or synthetic RNAs are then injected into the zygotes produced. As the control, zygotes are injected with control sperm RNA, scrambled synthetic miRNAs, or other RNAs for comparisons with the experimentally injected zygotes. The resulting embryos can be analyzed for changes in gene expression and for embryonic phenotypes or transferred to surrogate mothers for full-term development. Finally, fully developed animals are analyzed for the phenotypes transmitted naturally by the paternal environmental exposure of interest.