Studying meiosis-specific cohesins in mouse embryonic oocytes

Shu complex SWS1-SWSAP1 promotes early steps in mouse meiotic recombination

The DNA-damage repair pathway homologous recombination (HR) requires factors that promote the activity of strand-exchange protein RAD51 and its meiosis-specific homolog DMC1. Here we show that the Shu complex SWS1-SWSAP1, a candidate for one such HR regulator, is dispensable for mouse viability but essential for male and female fertility, promoting the assembly of RAD51 and DMC1 on early meiotic HR intermediates. Only a fraction of mutant meiocytes progress to form crossovers, which are crucial for chromosome segregation, demonstrating crossover homeostasis. Remarkably, loss of the DNA damage checkpoint kinase CHK2 rescues fertility in females without rescuing crossover numbers. Concomitant loss of the BRCA2 C terminus aggravates the meiotic defects in Swsap1 mutant spermatocytes, suggesting an overlapping role with the Shu complex during meiotic HR. These results demonstrate an essential role for SWS1-SWSAP1 in meiotic progression and emphasize the complex interplay of factors that ensure recombinase function.

Homologous recombination ensures genome integrity during meiotic recombination. Here the authors reveal that factors SWS1 and SWSAP1 are critical for meiotic homologues recombination, particularly in promoting assembly of RAD51 and DMC1 on early recombination intermediates.

Chromatin Spread Preparations for the Analysis of Mouse Oocyte Progression from Prophase to Metaphase II

Chromatin spread techniques have been widely used to assess the dynamic localization of various proteins during gametogenesis, particularly for spermatogenesis. These techniques allow for visualization of protein and DNA localization patterns during meiotic events such as homologous chromosome pairing, synapsis and DNA repair. While a few protocols have been described in the literature, general chromatin spread techniques using mammalian prophase oocytes are limited and difficult due to the timing of meiosis initiation in fetal ovaries. In comparison, prophase spermatocytes can be collected from juvenile male mice with higher yields without the need for microdissection. However, it is difficult to obtain a pure synchronized population of cells at specific stages due to the heterogeneity of meiotic and post-meiotic germ cell populations in the juvenile and adult testis. For later stages of meiosis, it is advantageous to assess oocytes undergoing meiosis I (MI) or meiosis II (MII), because groups of mature oocytes can be collected from adult female mice and stimulated to resume meiosis in culture. Here, methods for meiotic chromatin spread preparations using oocytes dissected from fetal, neonatal and adult ovaries are described with accompanying video demonstrations. Chromosome missegregation events in mammalian oocytes are frequent, particularly during MI. These techniques can be used to assess and characterize the effects of different mutations or environmental exposures during various stages of oogenesis. As there are distinct differences between oogenesis and spermatogenesis, the techniques described within are invaluable to increase our understanding of mammalian oogenesis and the sexually dimorphic features of chromosome and protein dynamics during meiosis.

Cohesin loading factor Nipbl localizes to chromosome axes during mammalian meiotic prophase

BACKGROUND

Sister chromatid cohesion mediated by the cohesin complex is essential for accurate chromosome segregation during mitosis and meiosis. Loading of cohesin onto chromosomes is dependent on another protein complex called kollerin, containing Nipbl/Scc2 and Mau2/Scc4. Nipbl is an evolutionarily conserved large protein whose haploinsufficiency in humans causes a developmental disorder called Cornelia de Lange syndrome. Although the function of Nipbl homologues for chromosome cohesion in meiotic cells of non-vertebrate models has been elucidated, Nipbl has not been characterized so far in mammalian spermatocytes or oocytes.

FINDINGS

Here we describe our analyses on the expression and localization of Nipbl in nuclei of mouse spermatocytes and oocytes at different stages of meiotic prophase. In both spermatocytes and oocytes we found that Nipbl is associated with the axial/lateral element of the synaptonemal complex (AE/LE) to which cohesin also localizes. Interestingly, Nipbl in spermatocytes, but not in oocytes, dissociates from the AE/LE at mid-pachytene stage coincident with completion of DNA double-strand break repair.

CONCLUSIONS

Our data propose that cohesin loading activity is maintained during early stages of meiotic prophase in mammalian spermatocytes and oocytes.