A Role for Synaptonemal Complex in Meiotic Mismatch Repair

During meiosis a large subset of interhomolog recombination repair intermediates form within the physical context of the synaptonemal complex (SC), a protein-rich structure assembled at the interface of aligned homologous chromosomes. However, the functional relationship between SC structure and homologous recombination remains poorly defined. In prior work we determined that tripartite SC is dispensable for recombination in S. cerevisiae; SC central element proteins Ecm11 and Gmc2 instead limit the number of recombination events. Here we report that while dispensable for recombination per se, SC central element proteins influence the processing of interhomolog recombination intermediates in a manner that minimizes errors in mismatch correction. Failure to correct mis-paired bases within heteroduplex at meiotic recombination sites leads to genotypically sectored colonies (post meiotic segregation events) arising from mitotic proliferation of mismatch-containing spores. We discovered an increase in post-meiotic segregation at the THR1 locus in cells lacking Ecm11 or Gmc2, or in the SC-deficient but crossover recombination-proficient zip1[Δ21-163] mutant. High-throughput sequencing of octad meiotic products revealed a genome-wide increase in recombination events with uncorrected mismatches in ecm11 mutants relative to wild type. Meiotic cells missing Ecm11 also display longer gene conversion tracts, but tract length alone does not account for the higher frequency of uncorrected mismatches. Interestingly, the per-nucleotide mismatch frequency is elevated in ecm11 mutants when analyzing all gene conversion tracts, but is similar between wild type and ecm11 if one considers only those events with uncorrected mismatches. Our data suggest that a subset of recombination events is similarly susceptible to mismatch repair errors in both wild type and ecm11 strains, but in ecm11 mutants many more recombination events fall into this inefficient repair category. Finally, we observe elevated post-meiotic segregation at THR1 in mutants with a dual deficiency in MutSγ-mediated crossover recombination and SC assembly, but not in the mlh3 mutant, which lacks MutSγ crossovers but has abundant SC. We propose that SC structure promotes efficient mismatch repair of joint molecule recombination intermediates resolved via both MutSγ-associated and MutSγ-independent pathways, and is the molecular basis for elevated post-meiotic segregation in both MutSγ crossover-proficient (ecm11, gmc2) and MutSγ crossover-deficient (msh4, zip3) strains.

Ume6 acts as a stable platform to coordinate repression and activation of early meiosis-specific genes in Saccharomyces cerevisiae.

In response to nutrient starvation the budding yeast Saccharomyces cerevisiae abandons mitotic proliferation and embarks on a differentiation process leading through meiosis to the formation of haploid spores. This process is driven by cascading waves of meiosis-specific gene expression. The early meiosis-specific genes are repressed during mitotic proliferation by DNA-binding protein Ume6 in combination with repressors Rpd3 and Sin3. The expression of meiosis-specific transcription factor Ime1 leads to activation of the early meiosis-specific genes. We investigated the stability and promoter occupancy of Ume6 in sporulating cells and determined that it remains bound to early meiosis-specific gene promoters when those genes are activated. Further we find that repressor Rpd3 remains associated with Ume6 after the transactivator Ime1 has joined the complex and that Gcn5 and Tra1 components of the SAGA complex bind to the promoter of IME2 in an Ime1 dependent fashion to induce transcription of the early meiosis-specific genes. Our investigation supports a model whereby Ume6 provides a platform allowing recruitment of both activating and repressing factors to coordinate expression of the early meiosis-specific genes in Saccharomyces cerevisiae

Dual RNAseq analyses at soma and germline levels reveal evolutionary innovations in the elephantiasis-agent Brugia malayi, and adaptation of its Wolbachia endosymbionts

Brugia malayi is a human filarial nematode responsible for elephantiasis, a debilitating condition that is part of a broader spectrum of diseases called filariasis, including lymphatic filariasis and river blindness. Almost all filarial nematode species infecting humans live in mutualism with Wolbachia endosymbionts, present in somatic hypodermal tissues but also in the female germline which ensures their vertical transmission to the nematode progeny. These α-proteobacteria potentially provision their host with essential metabolites and protect the parasite against the vertebrate immune response. In the absence of Wolbachia wBm, B. malayi females become sterile, and the filarial nematode lifespan is greatly reduced. In order to better comprehend this symbiosis, we investigated the adaptation of wBm to the host nematode soma and germline, and we characterized these cellular environments to highlight their specificities. Dual RNAseq experiments were performed at the tissue-specific and ovarian developmental stage levels, reaching the resolution of the germline mitotic proliferation and meiotic differentiation stages. We found that most wBm genes, including putative effectors, are not differentially regulated between infected tissues. However, two wBm genes involved in stress responses are upregulated in the hypodermal chords compared to the germline, indicating that this somatic tissue represents a harsh environment to which wBm have adapted. A comparison of the B. malayi and C. elegans germline transcriptomes reveals a poor conservation of genes involved in the production of oocytes, with the filarial germline proliferative zone relying on a majority of genes absent from C. elegans. The first orthology map of the B. malayi genome presented here, together with tissue-specific expression enrichment analyses, indicate that the early steps of oogenesis are a developmental process involving genes specific to filarial nematodes, that likely result from evolutionary innovations supporting the filarial parasitic lifestyle.

Differential Morphophysiological Characteristics of Erythrocyte Precursors and Mature Erythroid Cells in Early Postnatal Ontogenesis of Birds

In accordance with the recommendations of The International Council for Standardization in Haematology (ICSH, https://icsh.org/), this article describes the morphophysiological characteristics of the precursors and mature erythroid cells in the early period of postnatal development of birds (Gallus gallus L.) including calculation of the surface area of these cells (S, μm2 , X±SEM). Depending on cell shape, cytoplasm color, and chromatin organization in nucleus, the following types are distinguished: basophilic erythroblasts (69.60±4.01 μm2 , p≤0.05), polychromatophilic erythroblasts (65.42±2.49 μm2 , p≤0.05), and oxyphilic erythroblasts (71.10±4.43 μm2). Formation of cell pool is characteristic for erythropoiesis in birds due to mitotic proliferation of basophilic erythroblasts. There are often proerythroblasts and polychromatophilic erythroblasts. The nucleus of a polychromatophilic proerythroblast contains a large number of histone proteins; therefore, it has an intensely basophilic color with a pronounced oxyphilic hue (proteinrelated oxyphilia). The accumulation of hemoglobin in the protoplasm of these cells contributes to the gradual transition of the basophilic staining of cytoplasm to the oxyphilic one which is typical for mature red blood cells (73.95±2.10 μm2 , p≤0.05). Cell shape and the structure of erythroblast nucleus approaches to these of mature red blood cells.

XY oocytes of sex-reversed females with a Sry mutation deviate from the normal developmental process beyond the mitotic stage†

The fertility of sex-reversed XY female mice is severely impaired by a massive loss of oocytes and failure of meiotic progression. This phenomenon remains an outstanding mystery. We sought to determine the molecular etiology of XY oocyte dysfunction by generating sex-reversed females that bear genetic ablation of Sry, a vital sex determination gene, on an inbred C57BL/6 background. These mutant mice, termed XYsry− mutants, showed severe attrition of germ cells during fetal development, resulting in the depletion of ovarian germ cells prior to sexual maturation. Comprehensive transcriptome analyses of primordial germ cells (PGCs) and postnatal oocytes demonstrated that XYsry− females had deviated significantly from normal developmental processes during the stages of mitotic proliferation. The impaired proliferation of XYsry− PGCs was associated with aberrant β-catenin signaling and the excessive expression of transposable elements. Upon entry to the meiotic stage, XYsry− oocytes demonstrated extensive defects, including the impairment of crossover formation, the failure of primordial follicle maintenance, and no capacity for embryo development. Together, these results suggest potential molecular causes for germ cell disruption in sex-reversed female mice, thereby providing insights into disorders of sex differentiation in humans, such as “Swyer syndrome,” in which patients with an XY karyotype present as typical females and are infertile.

The RNA-Binding Protein DND1 Acts Sequentially as a Negative Regulator of Pluripotency and a Positive Regulator of Epigenetic Modifiers Required for Germ Cell Reprogramming

The adult spermatogonial stem cell population arises from pluripotent primordial germ cells (PGCs) that enter the fetal testis around embryonic day 10.5 (E10.5). These cells undergo rapid mitotic proliferation, then enter a prolonged period of cell cycle arrest (G1/G0) during which they transition to pro-spermatogonia. In mice homozygous for the Ter mutation in the RNA-binding protein DND1 (DND1Ter/Ter), many germ cells fail to enter G1/G0, and give rise to teratomas, tumors in which many embryonic cell types are represented. To investigate the origin of these tumors, we sequenced the transcriptome of male germ cells in DND1Ter/Ter mutants at E12.5, E13.5, and E14.5, just prior to the formation of teratomas, and correlated this information with direct targets of DND1 identified by DO-RIP-Seq. Consistent with previous results, we found that DND1 controls the down regulation of many genes associated with pluripotency and active cell cycle, including elements of the mTor, Hippo and Bmp/Nodal signaling pathways. However, DND1 targets also include genes associated with male differentiation including a large group of chromatin regulators activated in wild type but not mutant germ cells during the transition between E13.5 and E14.5. These results suggest multiple functions of DND1, and link DND1 to the initiation of epigenetic modifications in male germ cells.

A Rare Ovarian Tumor: Primitive Leiomyosarcoma of the Ovary

The primitive leiomyosarcoma of the ovary is rare and represents less than 1% of malignant tumors. It has a poor prognosis and frequently occurs during the postmenopausal period. We report two cases of this tumor in order to determine their epidemiological, histopathological, and evolutive aspects. Material and methods: The study material was consisted of ovariectomies fixed in 10% formalin. The sampled ovaries were subjected to usual techniques of inclusion in paraffin wax. These routine techniques were completed by immunohistochemistry assay using smooth muscle anti-actin, anti-desmin, anti-vimentin, and mitotic proliferation index (Ki-67). Results: Histological examination has shown a proliferation of fusiform cells, which were more or less fascicled in both cases. Tumor cells had a poorly-limited eosinophilic cytoplasm containing an elongated or an oval nucleus presenting an hyperchromatic or a vesicular feature. The nuclei were nucleolated. The anisocaryose was intense with more than 20 mitoses for 10 HPF. The positivity of anti-smooth muscle actin, anti-desmin, and anti-vimentin confirmed the diagnostic of leiomyosarcoma. Conclusion: The ovarian leiomyosarcoma is a rare with a poor prognosis.

Mitotic crossover promotes leukemogenesis in children born with TEL-AML1 via the generation of loss of heterozygosity at 12p

TEL-AML1 (ETV6-RUNX1) fusion gene which is formed prenatally in 1% of the newborns, is a common genetic abnormality in childhood Bcell precursor acute lymphoblastic leukemia. But only one child out of a hundred children born with this fusion gene develops leukemia (bottleneck phenomenon) later in its life, if contracts the second mutation. In other words, out of a hundred children born with TEL-AML1 only one child is at risk for leukemia development, which means that TEL-AML1 fusion gene is not sufficient for overt leukemia. There is a stringent requirement for a second genetic abnormality for leukemia development and this is the real or the ultimate cause of the leukemia <em>bottleneck</em> phenomenon. In most cases of TEL-AML1⁺ leukemia, the translocation t(12;21) is complemented with the loss of the normal TEL gene, not involved in the translocation, on the contralateral 12p. The loss of the normal TEL gene, <em>i.e.</em> loss of heterozygosity at 12p, occurs postnatally during the mitotic proliferation of TEL-AML1⁺ cell in the mitotic crossing over process. Mitotic crossing over is a very rare event with a frequency rate of 10^–6 in a 10 kb region. The exploration and identification of the environmental exposure(s) that cause(s) proliferation of the TELAML1⁺ cell in which approximately 10⁶ mitoses are generated to cause 12p loss of heterozygosity, <em>i.e.</em> TEL gene deletion, may contribute to the introduction of preventive measures for leukemia.

The neonatal marmoset monkey ovary is very primitive exhibiting many oogonia

Oogonia are characterized by diploidy and mitotic proliferation. Human and mouse oogonia express several factors such as OCT4, which are characteristic of pluripotent cells. In human, almost all oogonia enter meiosis between weeks 9 and 22 of prenatal development or undergo mitotic arrest and subsequent elimination from the ovary. As a consequence, neonatal human ovaries generally lack oogonia. The same was found in neonatal ovaries of the rhesus monkey, a representative of the old world monkeys (Catarrhini). By contrast, proliferating oogonia were found in adult prosimians (now called Strepsirrhini), which is a group of ‘lower’ primates. The common marmoset monkey (Callithrix jacchus) belongs to the new world monkeys (Platyrrhini) and is increasingly used in reproductive biology and stem cell research. However, ovarian development in the marmoset monkey has not been widely investigated. Herein, we show that the neonatal marmoset ovary has an extremely immature histological appearance compared with the human ovary. It contains numerous oogonia expressing the pluripotency factors OCT4A, SALL4, and LIN28A (LIN28). The pluripotency factor-positive germ cells also express the proliferation marker MKI67 (Ki-67), which has previously been shown in the human ovary to be restricted to premeiotic germ cells. Together, the data demonstrate the primitiveness of the neonatal marmoset ovary compared with human. This study may introduce the marmoset monkey as a non-human primate model to experimentally study the aspects of primate primitive gonad development, follicle assembly, and germ cell biology in vivo.