Meiotic Telomere Attachment Research Articles

Homozygous Variant in KASH5 Causes Premature Ovarian Insufficiency by Disordered Meiotic Homologous Pairing.

Premature ovarian insufficiency (POI) affects 1% to 3.7% of women at reproductive age, and its etiology is heterogeneous. The linker of nucleoskeleton and cytoskeleton (LINC) complex, consisting of KASH5 and SUN1, plays an indispensable role in meiotic homolog pairing, determining the ovarian reserve. However, their roles in the pathogenesis of POI are unknown. To investigate the role of KASH5 variation in the pathogenesis of POI. Whole-exome sequencing was performed in a pedigree with 2 POI patients. The pathogenicity of identified variant was illustrated by in vitro functional studies, and its effect on ovarian function and meiosis was confirmed by histological analysis and oocyte spreads with Kash5 C-terminal deleted mice model. A homozygous splicing site variant in KASH5 (c.747G > A) was identified. In vitro studies found the variant disturbed the nuclear membrane localization of KASH5 and its binding with SUN1. Moreover, the Kash5 C-terminal deleted mice revealed defective meiotic homolog pairing and accelerated depletion of oocytes. The splicing site variant in KASH5 is responsible for POI due to defective meiotic homolog pairing and accelerated depletion of oocytes. Our study is the first to report disorganized LINC complex participating in POI pathogenesis, potentially suggesting the essential roles of meiotic telomere attachment and dynein-driven proteins for chromosome movement in ovarian function maintenance.

Electron tomography of mouse LINC complexes at meiotic telomere attachment sites with and without microtubules

Telomere movements during meiotic prophase I facilitate synapsis and recombination of homologous chromosomes. Hereby, chromosome movements depend on the dynamic attachment of meiotic telomeres to the nuclear envelope and generation of forces that actively move the telomeres. In most eukaryotes, forces that move telomeres are generated in the cytoplasm by microtubule-associated motor proteins and transduced into the nucleus through the LINC complexes of the nuclear envelope. Meiotic LINC complexes, in mouse comprised of SUN1/2 and KASH5, selectively localize to the attachment sites of meiotic telomeres. For a better understanding of meiotic telomere dynamics, here we provide quantitative information of telomere attachment sites that we have generated with the aid of electron microscope tomography (EM tomography). Our data on the number, length, width, distribution and relation with microtubules of the reconstructed structures indicate that an average number of 76 LINC complexes would be required to move a telomere attachment site.

Structural basis of meiotic telomere attachment to the nuclear envelope by MAJIN-TERB2-TERB1

Meiotic chromosomes undergo rapid prophase movements, which are thought to facilitate the formation of inter-homologue recombination intermediates that underlie synapsis, crossing over and segregation. The meiotic telomere complex (MAJIN, TERB1, TERB2) tethers telomere ends to the nuclear envelope and transmits cytoskeletal forces via the LINC complex to drive these rapid movements. Here, we report the molecular architecture of the meiotic telomere complex through the crystal structure of MAJIN-TERB2, together with light and X-ray scattering studies of wider complexes. The MAJIN-TERB2 2:2 hetero-tetramer binds strongly to DNA and is tethered through long flexible linkers to the inner nuclear membrane and two TRF1-binding 1:1 TERB2-TERB1 complexes. Our complementary structured illumination microscopy studies and biochemical findings reveal a telomere attachment mechanism in which MAJIN-TERB2-TERB1 recruits telomere-bound TRF1, which is then displaced during pachytene, allowing MAJIN-TERB2-TERB1 to bind telomeric DNA and form a mature attachment plate.

Analysis of Meiosis in SUN1 Deficient Mice Reveals a Distinct Role of SUN2 in Mammalian Meiotic LINC Complex Formation and Function

LINC complexes are evolutionarily conserved nuclear envelope bridges, composed of SUN (Sad-1/UNC-84) and KASH (Klarsicht/ANC-1/Syne/homology) domain proteins. They are crucial for nuclear positioning and nuclear shape determination, and also mediate nuclear envelope (NE) attachment of meiotic telomeres, essential for driving homolog synapsis and recombination. In mice, SUN1 and SUN2 are the only SUN domain proteins expressed during meiosis, sharing their localization with meiosis-specific KASH5. Recent studies have shown that loss of SUN1 severely interferes with meiotic processes. Absence of SUN1 provokes defective telomere attachment and causes infertility. Here, we report that meiotic telomere attachment is not entirely lost in mice deficient for SUN1, but numerous telomeres are still attached to the NE through SUN2/KASH5-LINC complexes. In Sun1−/− meiocytes attached telomeres retained the capacity to form bouquet-like clusters. Furthermore, we could detect significant numbers of late meiotic recombination events in Sun1−/− mice. Together, this indicates that even in the absence of SUN1 telomere attachment and their movement within the nuclear envelope per se can be functional.

CDK2 regulates nuclear envelope protein dynamics and telomere attachment in mouse meiotic prophase.

In most organisms, telomeres attach to the nuclear envelope at the onset of meiosis to promote the crucial processes of pairing, recombination and synapsis during prophase I. This attachment of meiotic telomeres is mediated by the specific distribution of several nuclear envelope components that interact with the attachment plates of the synaptonemal complex. We have determined by immunofluorescence and electron microscopy that the ablation of the kinase CDK2 alters the nuclear envelope in mouse spermatocytes, and that the proteins SUN1, KASH5 (also known as CCDC155) and lamin C2 show an abnormal cap-like distribution facing the centrosome. Strikingly, some telomeres are not attached to the nuclear envelope but remain at the nuclear interior where they are associated with SUN1 and with nuclear-envelope-detached vesicles. We also demonstrate that mouse testis CDK2 phosphorylates SUN1 in vitro. We propose that during mammalian prophase I the kinase CDK2 is a key factor governing the structure of the nuclear envelope and the telomere-led chromosome movements essential for homolog pairing.

Rap1-independent telomere attachment and bouquet formation in mammalian meiosis.

Attachment of telomeres to the nuclear envelope (NE) and their clustering in a chromosomal bouquet during meiotic prophase I is an evolutionary conserved event that promotes chromosome pairing and recombination. In fission yeast, bouquet formation fails when the telomeric protein Rap1 is absent or when the telomeric protein Taz1 fails to recruit Rap1 to telomeres. The mammalian Rap1 orthologue is a component of the shelterin complex and localises to telomeres through an interaction with a Taz1-like telomeric DNA binding factor, TRF2. Here, we investigated the role of mammalian Rap1 in meiotic telomere attachment and clustering by analysing spermatogenesis in Rap1-deficient mice. The results establish that the meiotic three-dimensional nuclear architecture and recombination are not affected by the absence of Rap1. Furthermore, Rap1-deficient meiotic telomeres assemble the SUN1 nuclear membrane protein, attach to the NE, and undergo bouquet formation indistinguishable from the wild-type setting. Thus, the role of Rap1 in meiosis is not conserved between fission yeast and mammals, suggesting that mammals have alternative modes for connecting telomeres to SUN proteins on the meiotic nuclear envelope.

Dynamic properties of meiosis-specific lamin C2 and its impact on nuclear envelope integrity

A hallmark of meiosis is the precise pairing and the stable physical connection (synapsis) of the homologous chromosomes. These processes are essential prerequisite for their proper segregation. Pairing of the homologs during meiotic prophase I critically depends on characteristic movements of chromosomes. These movements, in turn, require attachment of meiotic telomeres to the nuclear envelope and their subsequent dynamic repositioning. Dynamic repositioning of meiotic telomeres goes along with profound structural reorganization of the nuclear envelope. The short A-type lamin C2 is thought to play a critical role in this process due to its specific expression during meiotic prophase I and the unique localization surrounding telomere attachments. Consistent with this notion, here we provide compelling evidence that meiosis-specific lamin C2 features a significantly increased mobility compared to somatic lamins as revealed by photobleaching techniques. We show that this property can be clearly ascribed to the lack of the N-terminal head and the significantly shorter α-helical coil domain. Moreover, expression of lamin C2 in somatic cells induces nuclear deformations and alters the distribution of the endogenous nuclear envelope proteins lamin B1, LAP2, SUN1 and SUN2. Together, our data define lamin C2 as a “natural lamin deletion mutant” that confers unique properties to the nuclear envelope which would be essential for dynamic telomere repositioning during meiotic prophase I.

Dynamic properties of meiosis-specific lamin C2 and its impact on nuclear envelope integrity

A hallmark of meiosis is the precise pairing and the stable physical connection (synapsis) of the homologous chromosomes. These processes are essential prerequisite for their proper segregation. Pairing of the homologs during meiotic prophase I critically depends on characteristic movements of chromosomes. These movements, in turn, require attachment of meiotic telomeres to the nuclear envelope and their subsequent dynamic repositioning. Dynamic repositioning of meiotic telomeres goes along with profound structural reorganization of the nuclear envelope. The short A-type lamin C2 is thought to play a critical role in this process due to its specific expression during meiotic prophase I and the unique localization surrounding telomere attachments. Consistent with this notion, here we provide compelling evidence that meiosis-specific lamin C2 features a significantly increased mobility compared to somatic lamins as revealed by photobleaching techniques. We show that this property can be clearly ascribed to the lack of the N-terminal head and the significantly shorter α-helical coil domain. Moreover, expression of lamin C2 in somatic cells induces nuclear deformations and alters the distribution of the endogenous nuclear envelope proteins lamin B1, LAP2, SUN1 and SUN2. Together, our data define lamin C2 as a "natural lamin deletion mutant" that confers unique properties to the nuclear envelope which would be essential for dynamic telomere repositioning during meiotic prophase I.

SUN1 Is Required for Telomere Attachment to Nuclear Envelope and Gametogenesis in Mice

Prior to the pairing and recombination between homologous chromosomes during meiosis, telomeres attach to the nuclear envelope and form a transient cluster. However, the protein factors mediating meiotic telomere attachment to the nuclear envelope and the requirement of this attachment for homolog pairing and synapsis have not been determined in animals. Here we show that the inner nuclear membrane protein SUN1 specifically associates with telomeres between the leptotene and diplotene stages during meiotic prophase I. Disruption of Sun1 in mice prevents telomere attachment to the nuclear envelope, efficient homolog pairing, and synapsis formation in meiosis. Massive apoptotic events are induced in the mutant gonads, leading to the abolishment of both spermatogenesis and oogenesis. This study provides genetic evidence that SUN1-telomere interaction is essential for telomere dynamic movement and is required for efficient homologous chromosome pairing/synapsis during mammalian gametogenesis.

Telomere attachment and clustering during meiosis.

Telomeres are important segments of chromosomes that protect chromosome ends from nucleolytic degradation and fusion. At meiosis telomeres display an unprecedented behavior which involves their attachment and motility along the nuclear envelope. The movements become restricted to a limited nuclear sector during the so-called bouquet stage, which is widely conserved among species. Recent observations suggest that telomere clustering involves actin and/or microtubules, and is altered in the presence of impaired recombinogenic and chromosome related functions. This review aims to provide an overview of what is currently known about meiotic telomere attachment, dynamics and regulation in synaptic meiosis.