HSP90α plays an important role in piRNA biogenesis and retrotransposon repression in mouse

HSP90, found in all kingdoms of life, is a major chaperone protein regulating many client proteins. We demonstrated that HSP90α, one of two paralogs duplicated in vertebrates, plays an important role in the biogenesis of fetal PIWI-interacting RNAs (piRNA), which act against the transposon activities, in mouse male germ cells. The knockout mutation of Hsp90α resulted in a large reduction in the expression of primary and secondary piRNAs and mislocalization of MIWI2, a PIWI homolog. Whereas the mutation in Fkbp6 encoding a co-chaperone reduced piRNAs of 28–32 nucleotides in length, the Hsp90α mutation reduced piRNAs of 24–32 nucleotides, suggesting the presence of both FKBP6-dependent and -independent actions of HSP90α. Although DNA methylation and mRNA levels of L1 retrotransposon were largely unchanged in the Hsp90α mutant testes, the L1-encoded protein was increased, suggesting the presence of post-transcriptional regulation. This study revealed the specialized function of the HSP90α isofom in the piRNA biogenesis and repression of retrotransposons during the development of male germ cells in mammals

Sequence divergence and retrotransposon insertion underlie interspecific epigenetic differences in primates

内在性レトロウイルス配列によってヒトのエピゲノムが変化してきたことを発見！ --ヒトとチンパンジーのiPS細胞の比較解析から--. 京都大学プレスリリース. 2022-10-12.Changes in the epigenome can affect the phenotype without the presence of changes in the genomic sequence. Given the high identity of the human and chimpanzee genome sequences, a substantial portion of their phenotypic divergence likely arises from epigenomic differences between the two species. In this study, the transcriptome and epigenome were determined for induced pluripotent stem cells (iPSCs) generated from human and chimpanzee individuals. The transcriptome and epigenomes for trimethylated histone H3 at lysine-4 (H3K4me3) and lysine-27 (H3K27me3) showed high levels of similarity between the two species. However, there were some differences in histone modifications. Although such regions, in general, did not show significant enrichment of interspecies nucleotide variations, gains in binding motifs for pluripotency-related transcription factors, especially POU5F1 and SOX2, were frequently found in species-specific H3K4me3 regions. We also revealed that species-specific insertions of retrotransposons, including the LTR5_Hs subfamily in human and a newly identified LTR5_Pt subfamily in chimpanzee, created species-specific H3K4me3 regions associated with increased expression of nearby genes. Human iPSCs have more species-specific H3K27me3 regions, resulting in more abundant bivalent domains. Only a limited number of these species-specific H3K4me3 and H3K27me3 regions overlap with species-biased enhancers in cranial neural crest cells, suggesting that differences in the epigenetic state of developmental enhancers appear late in development. Therefore, iPSCs serve as a suitable starting material for studying evolutionary changes in epigenome dynamics during development

Spermatogenesis arrest caused by conditional deletion of Hsp90α in adult mice

Summary It is controversial whether a functional androgen receptor (AR) on germ cells, including spermatogonia, is essential for their development into sperm and, thus, initiation and maintenance of spermatogenesis. It was recently shown that many spermatocytes underwent apoptosis in the testes of Hsp90α KO mice. We had generated Hsp90α KO mice independently and confirmed this phenotype. However, the important question of whether Hsp90α is required to maintain spermatogenesis in adult mice in which testicular maturation is already completed could not be addressed using these conventional KO mice. To answer this question, we generated a tamoxifen-inducible deletion mutant of Hsp90α and found that conditional deletion of Hsp90α in adult mice caused even more severe apoptosis in germ cells beyond the pachytene stage, leading to complete arrest of spermatogenesis and testicular atrophy. Importantly, immunohistochemical analysis revealed that AR expression in WT testis was more evident in spermatogonia than in spermatocytes, whereas its expression was aberrant and ectopic in Hsp90α KO testis, raising the possibility that an AR abnormality in primordial germ cells is involved in spermatogenesis arrest in the Hsp90α KO mice. Our results suggest that the AR, specifically chaperoned by Hsp90α in spermatogonia, is critical for maintenance of established spermatogenesis and for survival of spermatocytes in adult testis, in addition to setting the first wave of spermatogenesis before puberty

Locus- and domain-dependent control of DNA methylation at mouse B1 retrotransposons during male germ cell development

In mammals, germ cells undergo striking dynamic changes in DNA methylation during their development. However, the dynamics and mode of methylation are poorly understood for short interspersed elements (SINEs) dispersed throughout the genome. We investigated the DNA methylation status of mouse B1 SINEs in male germ cells at different developmental stages. B1 elements showed a large locus-to-locus variation in methylation; loci close to RNA polymerase II promoters were hypomethylated, while most others were hypermethylated. Interestingly, a mutation that eliminates Piwi-interacting RNAs (piRNAs), which are involved in methylation of long interspersed elements (LINEs), did not affect the level of B1 methylation, implying a piRNA-independent mechanism. Methylation at B1 loci in SINE-poor genomic domains showed a higher dependency on the de novo DNA methyltransferase DNMT3A but not on DNMT3B, suggesting that DNMT3A plays a major role in methylation of these domains. We also found that many genes specifically expressed in the testis possess B1 elements in their promoters, suggesting the involvement of B1 methylation in transcriptional regulation. Taken altogether, our results not only reveal the dynamics and mode of SINE methylation but also suggest how the DNA methylation profile is created in the germline by a pair of DNA methyltransferases