There is growing evidence that environmental exposures to the father can affect the phenotype of his offspring and in some cases these effects can be adaptive. Indeed, in utero exposure of male rats to the endocrine disruptor vinclozolin resulted in decreased fertility and increased rates of cancer over several generations. Although classic epigenetic mechanisms such as changes in DNA methylation have been observed, to date no clear mechanism has been established for how these transgenerational effects were transmitted by spermatozoa. Epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence. While in mammals epigenetics currently refers to modifications of the genome such as DNA methylation or histone modifications, it is possible that nongenomic means of inheritance may also occur. We hypothesize that sperm prions are a novel mechanism for transgenerational inheritance in mammals. Prions are self-aggregates of proteins in highly ordered structures known as amyloids that can convert native forms of the protein to the self-aggregated state and thus are self-perpetuating. Our hypothesis is based on an established mechanism for prion-based inheritance in yeast. Many regulatory proteins in yeast contain Q/N rich prion domains that, when exposed to cell stress, switch the protein from its native fold into a self-aggregated prion resulting in changed protein function and cell phenotype that can be heritable and passed on to daughter cells. Remarkably, several mammalian regulatory proteins including the DNA binding proteins TDP43 and androgen receptor (AR) contain Q/N domains and form prions that alter normal protein function that is linked to human disease. The tumor suppressor p53 has also been shown to form prions. While the significance of the prion forms of TDP43, AR and p53 is not known, the similarities between these mammalian proteins and the yeast prions suggest a common mechanism for regulation of protein function and possibly generation of new phenotypes. We have made the exciting observation that prions are present in mouse and rat spermatozoa including the head and the centrosomal region which plays critical roles in embryonic development. Although the identity of these prions is not known, we have determined that TDP43 localizes to the sperm head and centrosome and that a proportion of TDP43 in sperm is in a prion state. We hypothesize that sperm proteins, possibly TDP43, AR, p53 and others, may be induced to form prions (or more prions) following environmental stress and that these prions are involved in the transgenerational inheritance of new phenotypes. To test our hypothesis, we will use an established model for transgenerational inheritance and expose rats in utero to vinclozolin and carry out the following studies: 1) Identify and characterize prions including TDP43 in spermatozoa from F1-F3 males;2) Determine if sperm prions are present in fertilized oocytes.
The objective of our studies is to determine if sperm prions are a mechanism of epigenetic inheritance in mammals. A completion of our aims will provide valuable information for how environmental exposures may cause phenotypic changes that can be inherited by subsequent generations. This information may also lead to the development of novel strategies to treat human disease.