Age-dependent aneuploidy in mammalian oocytes instigated at the second meiotic division.

Ageing severely affects the chromosome segregation process in human oocytes resulting in aneuploidy, infertility and developmental disorders. A considerable amount of segregation errors in humans are introduced at the second meiotic division. We have here compared the chromosome segregation process in young adult and aged female mice during the second meiotic division.

A cryo-fixation protocol to study the structure of the synaptonemal complex.

Genetic variability in sexually reproducing organisms results from an exchange of genetic material between homologous chromosomes. The genetic exchange mechanism is dependent on the synaptonemal complex (SC), a protein structure localized between the homologous chromosomes. The current structural models of the mammalian SC are based on electron microscopy, superresolution, and expansion microscopy studies using chemical fixatives and sample dehydration of gonads, which are methodologies known to produce structural artifacts.

Post-metaphase correction of aberrant kinetochore-microtubule attachments in mammalian eggs.

The accuracy of the two sequential meiotic divisions in oocytes is essential for creating a haploid gamete with a normal chromosomal content. Here, we have analysed the 3D dynamics of chromosomes during the second meiotic division in live mouse oocytes. We find that chromosomes form stable kinetochore-microtubule attachments at the end of prometaphase II stage that are retained until anaphase II onset.

Sexual dimorphism in the width of the mouse synaptonemal complex.

Sexual dimorphism has been used to describe morphological differences between the sexes, but can be extended to any biologically related process that varies between males and females. The synaptonemal complex (SC) is a tripartite structure that connects homologous chromosomes in meiosis. Here, aided by super-resolution microscopy techniques, we show that the SC is subject to sexual dimorphism, in mouse germ cells.

CTCF contributes in a critical way to spermatogenesis and male fertility.

The CCCTC-binding factor (CTCF) is an architectural protein that governs chromatin organization and gene expression in somatic cells. Here, we show that CTCF regulates chromatin compaction necessary for packaging of the paternal genome into mature sperm. Inactivation of Ctcf in male germ cells in mice (Ctcf-cKO mice) resulted in impaired spermiogenesis and infertility.

High density of REC8 constrains sister chromatid axes and prevents illegitimate synaptonemal complex formation.

During meiosis, cohesin complexes mediate sister chromatid cohesion (SCC), synaptonemal complex (SC) assembly and synapsis. Here, using super-resolution microscopy, we imaged sister chromatid axes in mouse meiocytes that have normal or reduced levels of cohesin complexes, assessing the relationship between localization of cohesin complexes, SCC and SC formation. We show that REC8 foci are separated from each other by a distance smaller than 15% of the total chromosome axis length in wild-type meiocytes.

The central element of the synaptonemal complex in mice is organized as a bilayered junction structure.

The synaptonemal complex transiently stabilizes pairing interactions between homologous chromosomes during meiosis. Assembly of the synaptonemal complex is mediated through integration of opposing transverse filaments into a central element, a process that is poorly understood. We have, here, analyzed the localization of the transverse filament protein SYCP1 and the central element proteins SYCE1, SYCE2 and SYCE3 within the central region of the synaptonemal complex in mouse spermatocytes using immunoelectron microscopy.

MAJIN Links Telomeric DNA to the Nuclear Membrane by Exchanging Telomere Cap.

In meiosis, telomeres attach to the inner nuclear membrane (INM) and drive the chromosome movement required for homolog pairing and recombination. Here, we address the question of how telomeres are structurally adapted for the meiotic task. We identify a multi-subunit meiotic telomere-complex, TERB1/2-MAJIN, which takes over telomeric DNA from the shelterin complex in mouse germ cells.

Bivalent separation into univalents precedes age-related meiosis I errors in oocytes.

The frequency of chromosome segregation errors during meiosis I (MI) in oocytes increases with age. The two-hit model suggests that errors are caused by the combination of a first hit that creates susceptible crossover configurations and a second hit comprising an age-related reduction in chromosome cohesion. This model predicts an age-related increase in univalents, but direct evidence of this phenomenon as a major cause of segregation errors has been lacking.

MEI4 – a central player in the regulation of meiotic DNA double-strand break formation in the mouse.

The formation of programmed DNA double-strand breaks (DSBs) at the beginning of meiotic prophase marks the initiation of meiotic recombination. Meiotic DSB formation is catalyzed by SPO11 and their repair takes place on meiotic chromosome axes. The evolutionarily conserved MEI4 protein is required for meiotic DSB formation and is localized on chromosome axes.

Merotelic attachments allow alignment and stabilization of chromatids in meiosis II oocytes.

The chromosome segregation process in human oocytes is highly error-prone, generating meiosis II (MII) oocytes with unbalanced chromatids that contribute to aneuploidy in offspring. This raises questions regarding the mechanism for transmission of chromatids and how chromatids evade the error correction mechanisms in MII oocytes. Here, we analyse the behaviour of chromatids in mouse MII oocytes.

STAG3-mediated stabilization of REC8 cohesin complexes promotes chromosome synapsis during meiosis.

Cohesion between sister chromatids in mitotic and meiotic cells is promoted by a ring-shaped protein structure, the cohesin complex. The cohesin core complex is composed of four subunits, including two structural maintenance of chromosome (SMC) proteins, one α-kleisin protein, and one SA protein. Meiotic cells express both mitotic and meiosis-specific cohesin core subunits, generating cohesin complexes with different subunit composition and possibly separate meiotic functions.

Meiosis-specific cohesin mediates homolog recognition in mouse spermatocytes.

During meiosis, homologous chromosome (homolog) pairing is promoted by several layers of regulation that include dynamic chromosome movement and meiotic recombination. However, the way in which homologs recognize each other remains a fundamental issue in chromosome biology. Here, we show that homolog recognition or association initiates upon entry into meiotic prophase before axis assembly and double-strand break (DSB) formation.

ATR acts stage specifically to regulate multiple aspects of mammalian meiotic silencing.

In mammals, homologs that fail to synapse during meiosis are transcriptionally inactivated. This process, meiotic silencing, drives inactivation of the heterologous XY bivalent in male germ cells (meiotic sex chromosome inactivation [MSCI]) and is thought to act as a meiotic surveillance mechanism. The checkpoint protein ATM and Rad3-related (ATR) localizes to unsynapsed chromosomes, but its role in the initiation and maintenance of meiotic silencing is unknown.

RNF212 is a dosage-sensitive regulator of crossing-over during mammalian meiosis.

Crossing-over ensures accurate chromosome segregation during meiosis, and every pair of chromosomes obtains at least one crossover, even though the majority of recombination sites yield non-crossovers. A putative regulator of crossing-over is RNF212, which is associated with variation in crossover rates in humans. We show that mouse RNF212 is essential for crossing-over, functioning to couple chromosome synapsis to the formation of crossover-specific recombination complexes.

Synaptonemal complex components persist at centromeres and are required for homologous centromere pairing in mouse spermatocytes.

Recent studies in simple model organisms have shown that centromere pairing is important for ensuring high-fidelity meiotic chromosome segregation. However, this process and the mechanisms regulating it in higher eukaryotes are unknown. Here we present the first detailed study of meiotic centromere pairing in mouse spermatogenesis and link it with key events of the G2/metaphase I transition.

Phosphorylation of chromosome core components may serve as axis marks for the status of chromosomal events during mammalian meiosis.

Meiotic recombination and chromosome synapsis between homologous chromosomes are essential for proper chromosome segregation at the first meiotic division. While recombination and synapsis, as well as checkpoints that monitor these two events, take place in the context of a prophase I-specific axial chromosome structure, it remains unclear how chromosome axis components contribute to these processes. We show here that many protein components of the meiotic chromosome axis, including SYCP2, SYCP3, HORMAD1, HORMAD2, SMC3, STAG3, and REC8, become post-translationally modified by phosphorylation during the prophase I stage.

Meiosis in mice without a synaptonemal complex.

The synaptonemal complex (SC) promotes fusion of the homologous chromosomes (synapsis) and crossover recombination events during meiosis. The SC displays an extensive structural conservation between species; however, a few organisms lack SC and execute meiotic process in a SC-independent manner. To clarify the SC function in mammals, we have generated a mutant mouse strain (Sycp1(-/-)Sycp3(-/-), here called SC-null) in which all known SC proteins have been displaced from meiotic chromosomes.

A novel mouse synaptonemal complex protein is essential for loading of central element proteins, recombination, and fertility.

The synaptonemal complex (SC) is a proteinaceous, meiosis-specific structure that is highly conserved in evolution. During meiosis, the SC mediates synapsis of homologous chromosomes. It is essential for proper recombination and segregation of homologous chromosomes, and therefore for genome haploidization.

Age-related meiotic segregation errors in mammalian oocytes are preceded by depletion of cohesin and Sgo2.

Background: The growing trend for women to postpone childbearing has resulted in a dramatic increase in the incidence of trisomic pregnancies. Maternal age-related miscarriage and birth defects are predominantly a consequence of chromosome segregation errors during the first meiotic division (MI), which involves the segregation of replicated recombined homologous chromosomes. Despite the importance to human reproductive health, the events precipitating female age-related meiotic errors are poorly understood.