RNF17 blocks promiscuous activity of PIWI proteins in mouse testes

PIWI proteins and their associated piRNAs protect germ cells from the activity of mobile genetic elements. Two classes of piRNAs-primary and secondary-are defined by their mechanisms of biogenesis. Primary piRNAs are processed directly from transcripts of piRNA cluster loci, whereas secondary piRNAs are generated in an adaptive amplification loop, termed the ping-pong cycle. In mammals, piRNA populations are dynamic, shifting as male germ cells develop. Embryonic piRNAs consist of both primary and secondary species and are mainly directed toward transposons. In meiotic cells, the piRNA population is transposon-poor and largely restricted to primary piRNAs derived from pachytene piRNA clusters. The transition from the embryonic to the adult piRNA pathway is not well understood. Here we show that RNF17 shapes adult meiotic piRNA content by suppressing the production of secondary piRNAs. In the absence of RNF17, ping-pong occurs inappropriately in meiotic cells. Ping-pong initiates piRNA responses against not only transposons but also protein-coding genes and long noncoding RNAs, including genes essential for germ cell development. Thus, the sterility of Rnf17 mutants may be a manifestation of a small RNA-based autoimmune reaction

Two waves of de novo methylation during mouse germ cell development

During development, mammalian germ cells reprogram their epigenomes via a genome-wide erasure and de novo rewriting of DNA methylation marks. We know little of how methylation patterns are specifically determined. The piRNA pathway is thought to target the bulk of retrotransposon methylation. Here we show that most retrotransposon sequences are modified by default de novo methylation. However, potentially active retrotransposon copies evade this initial wave, likely mimicking features of protein-coding genes. These elements remain transcriptionally active and become targets of piRNA-mediated methylation. Thus, we posit that these two waves play essential roles in resetting germ cell epigenomes at each generation

Dual functions of Macpiwi1 in transposon silencing and stem cell maintenance in the flatworm Macrostomum lignano

PIWI proteins and piRNA pathways are essential for transposon silencing and some aspects of gene regulation during animal germline development. In contrast to most animal species, some flatworms also express PIWIs and piRNAs in somatic stem cells, where they are required for tissue renewal and regeneration. Here, we have identified and characterized piRNAs and PIWI proteins in the emerging model flatworm Macrostomum lignano. We found that M. lignano encodes at least three PIWI proteins. One of these, Macpiwi1, acts as a key component of the canonical piRNA pathway in the germline and in somatic stem cells. Knockdown of Macpiwi1 dramatically reduces piRNA levels, derepresses transposons, and severely impacts stem cell maintenance. Knockdown of the piRNA biogenesis factor Macvasa caused an even greater reduction in piRNA levels with a corresponding increase in transposons. Yet, in Macvasa knockdown animals, we detected no major impact on stem cell self-renewal. These results may suggest stem cell maintenance functions of PIWI proteins in flatworms that are distinguishable from their impact on transposons and that might function independently of what are considered canonical piRNA populations

Immunohistochemical localization of peroxisomal enzymes during rat embryonic development

Peroxisomes are cytoplasmic organelles involved in a variety of metabolic pathways. Thus far, the morphological and biochemical features of peroxisomes have been extensively characterized in adult tissues. However, the existence of congenital peroxisomal disorders, primarily affecting tissue differentiation, emphasizes the importance of these organelles in the early stages of organogenesis. We investigated the occurrence and tissue distribution of three peroxisomal enzymes in rat embryos at various developmental stages. By means of a highly sensitive biotinyl–tyramide protocol, catalase, acyl-CoA oxidase, and ketoacyl-CoA thiolase were detected in embryonic tissues where peroxisomes had not thus far been recognized, i.e., adrenal and pancreatic parenchyma, choroid plexus, neuroblasts of cranial and spinal ganglia and myenteric plexus, and chondroblasts of certain skeletal structures. In other tissues, i.e., gut epithelium and neuroblasts of some CNS areas, they were identified earlier than previously. In select CNS areas, ultrastructural catalase cytochemistry allowed identification of actively proliferating organelles at early developmental stages in several cell types. Our data show that in most organs maturation of peroxisomes parallels the acquirement of specific functions, mainly related to lipid metabolism, thus supporting an involvement of the organelles in tissue differentiation

Immunohistochemical localization of peroxisomal enzymes during rat embryonic development

Peroxisomes are cytoplasmic organelles involved in a variety of metabolic pathways. Thus far, the morphological and biochemical features of peroxisomes have been extensively characterized in adult tissues. However, the existence of congenital peroxisomal disorders, primarily affecting tissue differentiation, emphasizes the importance of these organelles in the early stages of organogenesis. We investigated the occurrence and tissue distribution of three peroxisomal enzymes in rat embryos at various developmental stages. By means of a highly sensitive biotinyl-tyramide protocol, catalase, acyl-CoA oxidase, and ketoacyl-CoA thiolase were detected in embryonic tissues where peroxisomes had not thus far been recognized, i.e., adrenal and pancreatic parenchyma, choroid plexus, neuroblasts of cranial and spinal ganglia and myenteric plexus, and chondroblasts of certain skeletal structures. In other tissues, i.e., gut epithelium and neuroblasts of some CNS areas, they were identified earlier than previously. In select CNS areas, ultrastructural catalase cytochemistry allowed identification of actively proliferating organelles at early developmental stages in several cell types. Our data show that in most organs maturation of peroxisomes parallels the acquirement of specific functions, mainly related to lipid metabolism, thus supporting an involvement of the organelles in tissue differentiation

On the presence of a novel-described cell side population enriched in germ stem cell activity in the mouse testis

Phoenix, Arizona – US