SUN1 is required for telomere attachment to nuclear envelope and gametogenesis in mice

Telomeres, the nuclear lamina, and membrane remodeling: Orchestrating meiotic chromosome movements

Telomeres, the DNA-protein complex located at the ends of linear eukaryotic chromosomes, not only safeguard genetic information from DNA erosion and aberrant activation of the DNA damage response pathways but also play a pivotal role in sexual reproduction. During meiotic prophase I, telomeres attach to the nuclear envelope and migrate along its surface, facilitating two-dimensional DNA homology searches that ensure precise pairing and recombination of the paternal and maternal chromosomes. Recent studies across diverse model systems have revealed intricate molecular mechanisms, including modifications to telomere- and nuclear envelope-binding proteins, the nuclear lamina, and even membrane composition. Emerging evidence reveals mutations in the genes encoding these meiotic telomere and nuclear envelope-associated proteins among infertile patients. This review highlights recent advances in the field of meiotic telomere research, particularly emphasizing mammalian model systems, contextualizes these findings through comparisons with other eukaryotes, and concludes by exploring potential future research directions in the field.

Slowed epigenetic aging in Olympic champions compared to non-champions

The lifestyle patterns of top athletes are highly disciplined, featuring strict exercise regimens, nutrition plans, and mental preparation, often beginning at a young age. Recently, it was shown that physically active individuals exhibit slowed epigenetic aging and better age-related outcomes. Here, we investigate whether the extreme intensity of physical activity of Olympic champions still has a beneficial effect on epigenetic aging. To test this hypothesis, we examined the epigenetic aging of 59 Hungarian Olympic champions and of the 332 control subjects, 205 were master rowers. We observed that Olympic champions exhibit slower epigenetic aging, applying seven state-of-the-art epigenetic aging clocks. Additionally, male champions who won any medal within the last 10 years showed slower epigenetic aging compared to other male champions, while female champions exhibited the opposite trend. We also found that wrestlers had higher age acceleration compared to gymnasts, fencers, and water polo players. We identified the top 20 genes that showed the most remarkable difference in promoter methylation between Olympic champions and non-champions. The hypo-methylated genes are involved in synaptic health, glycosylation, metal ion membrane transfer, and force generation. Most of the hyper-methylated genes were associated with cancer promotion. The data suggest that rigorous and long-term exercise from adolescence to adulthood has beneficial effects on epigenetic aging.

Depletion of SUN1/2 induces heterochromatin accrual in mesenchymal stem cells during adipogenesis

Critical to the mechano-regulation of mesenchymal stem cells (MSC), Linker of the Nucleoskeleton and Cytoskeleton (LINC) complex transduces cytoskeletal forces to the nuclei. The LINC complex contains outer nuclear membrane Nesprin proteins that associate with the cytoskeleton and their inner nuclear membrane couplers, SUN proteins. Here we tested the hypothesis that severing of the LINC complex-mediated cytoskeletal connections may have different effects on chromatin organization and MSC differentiation than those due to ablation of SUN proteins. In cells cultured under adipogenic conditions, interrupting LINC complex function through dominant-negative KASH domain expression (dnKASH) increased adipogesis while heterochromatin H3K27 and H3K9 methylation was unaltered. In contrast, SUN1/2 depletion inhibited adipogenic gene expression and fat droplet formation; as well the anti-adipogenic effect of SUN1/2 depletion was accompanied by increased H3K9me3, which was enriched on Adipoq, silencing this fat locus. We conclude that releasing the nucleus from cytoskeletal constraints via dnKASH accelerates adipogenesis while depletion of SUN1/2 increases heterochromatin accrual on adipogenic genes in a fashion independent of LINC complex function. Therefore, while these two approaches both disable LINC complex functions, their divergent effects on the epigenetic landscape indicate they cannot be used interchangeably to study mechanical regulation of cell differentiation.

Targeting cyclin-dependent kinase 2 (CDK2) interactions with cyclins and Speedy 1 (Spy1) for cancer and male contraception

The review discusses progress in discovering cyclin-dependent kinase 2 (CDK2) inhibitors for cancer treatment and their potential for male contraception. It summarizes first-, second-, and third-generation CDK inhibitors and selective CDK2 inhibitors currently in clinical trials for cancer. Novel strategies to discover allosteric inhibitors, covalent inhibitors, and degraders are also discussed.

BMP and STRA8 act collaboratively to ensure correct mitotic-to-meiotic transition in the fetal mouse ovary

A successful mitosis-to-meiosis transition in germ cells is essential for fertility in sexually reproducing organisms. In mice and humans, it has been established that expression of STRA8 is crucial for meiotic onset in both sexes. Here, we show that BMP signalling is also essential, not for STRA8 induction but for correct meiotic progression in female mouse fetal germ cells. Largely in agreement with evidence from primordial germ cell-like cells (PGCLCs) in vitro, germ cell-specific deletion of BMP receptor 1A (BMPR1A; ALK3) caused aberrant retention of pluripotency marker OCT4 and meiotic progression was compromised; however, the timely onset of Stra8 and STRA8 expression was unaffected. Comparing the transcriptomes of Bmpr1a-cKO and Stra8-null models, we reveal interplay between the effects of BMP signalling and STRA8 function. Our results verify a role for BMP signalling in instructing germ cell meiosis in female mice in vivo, and shed light on the regulatory mechanisms underlying fetal germ cell development.

Heterochromatin in plant meiosis

Heterochromatin is an organizational property of eukaryotic chromosomes, characterized by extensive DNA and histone modifications, that is associated with the silencing of transposable elements and repetitive sequences. Maintaining heterochromatin is crucial for ensuring genomic integrity and stability during the cell cycle. During meiosis, heterochromatin is important for homologous chromosome synapsis, recombination, and segregation, but our understanding of meiotic heterochromatin formation and condensation is limited. In this review, we focus on the dynamics and features of heterochromatin and how it condenses during meiosis in plants. We also discuss how meiotic heterochromatin influences the interaction and recombination of homologous chromosomes during prophase I.

Dinochromosome Heterotermini with Telosomal Anchorages

Dinoflagellate birefringent chromosomes (BfCs) contain some of the largest known genomes, yet they lack typical nucleosomal micrococcal-nuclease protection patterns despite containing variant core histones. One BfC end interacts with extranuclear mitotic microtubules at the nuclear envelope (NE), which remains intact throughout the cell cycle. Ultrastructural studies, polarized light and fluorescence microscopy, and micrococcal nuclease-resistant profiles (MNRPs) revealed that NE-associated chromosome ends persisted post-mitosis. Histone H3K9me3 inhibition caused S-G2 delay in synchronous cells, without any effects at G1. Differential labeling and nuclear envelope swelling upon decompaction indicate an extension of the inner compartment into telosomal anchorages (TAs). Additionally, limited effects of low-concentration sirtinol on bulk BfCs, coupled with distinct mobility patterns in MNase-digested and psoralen-crosslinked nuclei observed on 2D gels, suggest that telomeric nucleosomes (TNs) are the primary histone structures. The absence of a nucleosomal ladder with cDNA probes, the presence of histone H2A and telomere-enriched H3.3 variants, along with the immuno-localization of H3 variants mainly at the NE further reinforce telomeric regions as the main nucleosomal domains. Cumulative biochemical and molecular analyses suggest that telomeric repeats constitute the major octameric MNRPs that provision chromosomal anchorage at the NE.

SUN5, a testis-specific nuclear membrane protein, participates in recruitment and export of nuclear mRNA in spermatogenesis

SUN5, a testis-specific gene, is associated with acephalic spermatozoa syndrome (ASS). Here, we demonstrate that SUN5 is involved in mRNA export. In Sun5-knockout mice ( Sun5 -/-), poly(A) + RNA accumulates in the nuclei of germ cells, leading to reduced sperm counts, decreased sperm motility and disrupted sperm head-to-tail junctions. Additionally, in the GC-2 germ cell line with RNA interference of Sun5, heterogeneous nuclear ribonucleoproteins (hnRNPs) and poly (A) + RNA (mainly mRNA) are retained in the nucleus. Further mechanistic studies reveal that SUN5 interacts with Nxf1 (nuclear RNA export factor 1) and nucleoporin 93 (Nup93). Interference with Nup93 inhibits mRNA export. Treatment with leptomycin B to block the CRM1 pathway indicates that Sun5 regulates mRNA export through an Nxf1-dependent pathway. In Sun5 -/- mice, the binding of Nxf1 and Nup93 decreases due to loss of Sun5 function, and the process of submitting Nxf1-binding mRNPs to Nup93 is inhibited, resulting in abnormal spermatogenesis. Together, these data may elucidate a novel pathway for mRNA export in male germ cells.

Evaluation of lamin A/C mechanotransduction under different surface topography in LMNA related muscular dystrophy

Most of the single point mutations of the LMNA gene are associated with distinct muscular dystrophies, marked by heterogenous phenotypes but primarily the loss and symmetric weakness of skeletal muscle tissue. The molecular mechanism and phenotype-genotype relationships in these muscular dystrophies are poorly understood. An effort has been here to delineating the adaptation of mechanical inputs into biological response by mutant cells of lamin A associated muscular dystrophy. In this study, we implement engineered smooth and pattern surfaces of particular young modulus to mimic muscle physiological range. Using fluorescence and atomic force microscopy, we present distinct architecture of the actin filament along with abnormally distorted cell and nuclear shape in mutants, which showed a tendency to deviate from wild type cells. Topographic features of pattern surface antagonize the binding of the cell with it. Correspondingly, from the analysis of genome wide expression data in wild type and mutant cells, we report differential expression of the gene products of the structural components of cell adhesion as well as LINC (linkers of nucleoskeleton and cytoskeleton) protein complexes. This study also reveals mis expressed downstream signaling processes in mutant cells, which could potentially lead to onset of the disease upon the application of engineered materials to substitute the role of conventional cues in instilling cellular behaviors in muscular dystrophies. Collectively, these data support the notion that lamin A is essential for proper cellular mechanotransduction from extracellular environment to the genome and impairment of the muscle cell differentiation in the pathogenic mechanism for lamin A associated muscular dystrophy.

Life outside the LINC complex - Do SUN proteins have LINC-independent functions?

Sad1 and UNC84 (SUN) and Klarsicht, ANC-1, and Syne homology (KASH) proteins interact at the nuclear periphery to form the linker of nucleoskeleton and cytoskeleton (LINC) complex, spanning the nuclear envelope (NE) and connecting the cytoskeleton with the nuclear interior. It is now well-documented that several cellular functions depend on LINC complex formation, including cell differentiation and migration. Intriguingly, recent studies suggest that SUN proteins participate in cellular processes where their association with KASH proteins may not be required. Building on this recent research, we elaborate on the hypothesis that SUN proteins may perform LINC-independent functions and discuss the modalities that may allow SUN proteins to function at the INM when they are not forming LINC complex.

Rapid meiotic prophase chromosome movements in Arabidopsis thaliana are linked to essential reorganization at the nuclear envelope

Meiotic rapid prophase chromosome movements (RPMs) require connections between the chromosomes and the cytoskeleton, involving SUN (Sad1/UNC-84)-domain-containing proteins at the inner nuclear envelope (NE). RPMs remain significantly understudied in plants, with respect to their importance in the regulation of meiosis. Here, we demonstrate that Arabidopsis thaliana meiotic centromeres undergo rapid (up to 500 nm/s) and uncoordinated movements during the zygotene and pachytene stages. These centromere movements are not affected by altered chromosome organization and recombination but are abolished in the double mutant sun1 sun2. We also document the changes in chromosome dynamics and nucleus organization during the transition from leptotene to zygotene, including telomere attachment to SUN-enriched NE domains, bouquet formation, and nucleolus displacement, all of which were defective in sun1 sun2. These results establish A. thaliana as a model species for studying the functional implications of meiotic RPMs and demonstrate the mechanistic conservation of telomere-led RPMs in plants.

Mechanisms of meiosis initiation and meiotic prophase progression during spermatogenesis

Meiosis is a critical step for spermatogenesis and oogenesis. Meiosis commences with pre-meiotic S phase that is subsequently followed by meiotic prophase. The meiotic prophase is characterized by the meiosis-specific chromosomal events such as chromosome recombination and homolog synapsis. Meiosis initiator (MEIOSIN) and stimulated by retinoic acid gene 8 (STRA8) initiate meiosis by activating the meiotic genes by installing the meiotic prophase program at pre-meiotic S phase. This review highlights the mechanisms of meiotic initiation and meiotic prophase progression from the point of the gene expression program and its relevance to infertility. Furthermore, upstream pathways that regulate meiotic initiation will be discussed in the context of spermatogenic development, indicating the sexual differences in the mode of meiotic entry.

Life at the crossroads: the nuclear LINC complex and vascular mechanotransduction

Vascular endothelial cells line the inner surface of all blood vessels, where they are exposed to polarized mechanical forces throughout their lifespan. Both basal substrate interactions and apical blood flow-induced shear stress regulate blood vessel development, remodeling, and maintenance of vascular homeostasis. Disruption of these interactions leads to dysfunction and vascular pathologies, although how forces are sensed and integrated to affect endothelial cell behaviors is incompletely understood. Recently the endothelial cell nucleus has emerged as a prominent force-transducing organelle that participates in vascular mechanotransduction, via communication to and from cell-cell and cell-matrix junctions. The LINC complex, composed of SUN and nesprin proteins, spans the nuclear membranes and connects the nuclear lamina, the nuclear envelope, and the cytoskeleton. Here we review LINC complex involvement in endothelial cell mechanotransduction, describe unique and overlapping functions of each LINC complex component, and consider emerging evidence that two major SUN proteins, SUN1 and SUN2, orchestrate a complex interplay that extends outward to cell-cell and cell-matrix junctions and inward to interactions within the nucleus and chromatin. We discuss these findings in relation to vascular pathologies such as Hutchinson-Gilford progeria syndrome, a premature aging disorder with cardiovascular impairment. More knowledge of LINC complex regulation and function will help to understand how the nucleus participates in endothelial cell force sensing and how dysfunction leads to cardiovascular disease.

Recent advances in mechanisms ensuring the pairing, synapsis and segregation of XY chromosomes in mice and humans

Sex chromosome aneuploidies are among the most common variations in human whole chromosome copy numbers, with an estimated prevalence in the general population of 1:400 to 1:1400 live births. Unlike whole-chromosome aneuploidies of autosomes, those of sex chromosomes, such as the 47, XXY aneuploidy that causes Klinefelter Syndrome (KS), often originate from the paternal side, caused by a lack of crossover (CO) formation between the X and Y chromosomes. COs must form between all chromosome pairs to pass meiotic checkpoints and are the product of meiotic recombination that occurs between homologous sequences of parental chromosomes. Recombination between male sex chromosomes is more challenging compared to both autosomes and sex chromosomes in females, as it is restricted within a short region of homology between X and Y, called the pseudo-autosomal region (PAR). However, in normal individuals, CO formation occurs in PAR with a higher frequency than in any other region, indicating the presence of mechanisms that promote the initiation and processing of recombination in each meiotic division. In recent years, research has made great strides in identifying genes and mechanisms that facilitate CO formation in the PAR. Here, we outline the most recent and relevant findings in this field. XY chromosome aneuploidy in humans has broad-reaching effects, contributing significantly also to Turner syndrome, spontaneous abortions, oligospermia, and even infertility. Thus, in the years to come, the identification of genes and mechanisms beyond XY aneuploidy is expected to have an impact on the genetic counseling of a wide number of families and adults affected by these disorders.

Telomeric function and regulation during male meiosis in mice and humans

BACKGROUND

Telomeres are unique structures situated at the ends of chromosomes. Preserving the structure and function of telomeres is essential for maintaining genomic stability and promoting genetic diversity during male meiosis in mammals.

MATERIAL-METHODS

This review compiled recent literature on the function and regulation of telomeres during male meiosis in both mice and humans, and also highlighted the critical roles of telomeres in reproductive biology and medicine.

RESULTS-DISCUSSION

Various structures, consisting of the LINC complex (SUN-KASH), SPDYA-CDK2, TTM trimer (TERB1-TERB2-MAJIN), and shelterin, are critical in controlling telomeric activities, such as nuclear envelope attachment and bouquet formation. Other than telomere-related proteins, cohesins and genes responsible for regulating telomere function are also highlighted, though the exact mechanism remains unclear. The gene-mutant mouse models with meiotic defects directly reveal the essential roles of telomeres in male meiosis. Recently reported mutant genes associated with telomere activity in clinical practice have also been illustrated in detail.

CONCLUSIONS

Proper regulation of telomere activities is essential for male meiosis progression in mice and humans.

Regulation of meiotic telomere dynamics through membrane fluidity promoted by AdipoR2-ELOVL2

The cellular membrane in male meiotic germ cells contains a unique class of phospholipids and sphingolipids that is required for male reproduction. Here, we show that a conserved membrane fluidity sensor, AdipoR2, regulates the meiosis-specific lipidome in mouse testes by promoting the synthesis of sphingolipids containing very-long-chain polyunsaturated fatty acids (VLC-PUFAs). AdipoR2 upregulates the expression of a fatty acid elongase, ELOVL2, both transcriptionally and post-transcriptionally, to synthesize VLC-PUFA. The depletion of VLC-PUFAs and subsequent accumulation of palmitic acid in AdipoR2 knockout testes stiffens the cellular membrane and causes the invagination of the nuclear envelope. This condition impairs the nuclear peripheral distribution of meiotic telomeres, leading to errors in homologous synapsis and recombination. Further, the stiffened membrane impairs the formation of intercellular bridges and the germ cell syncytium, which disrupts the orderly arrangement of cell types within the seminiferous tubules. According to our findings we propose a framework in which the highly-fluid membrane microenvironment shaped by AdipoR2-ELOVL2 underpins meiosis-specific chromosome dynamics in testes.

A report of two homozygous TERB1 protein-truncating variants in two unrelated women with primary infertility

PURPOSE

To investigate the genetic etiology of patients with female infertility.

METHODS

Whole Exome Sequencing was performed on genomic DNA extracted from the patient's blood. Exome data were filtered for damaging rare biallelic variants in genes with possible roles in reproduction. Sanger sequencing was used to validate the selected variants and segregate them in family members.

RESULTS

A novel homozygous likely pathogenic variant, c.626G>A, p.Trp209*, was identified in the TERB1 gene of the patient. Additionally, we report a second homozygous pathogenic TERB1 variant, c.1703C>G, p.Ser568*, in an infertile woman whose azoospermic brother was previously described to be homozygous for her variant.

CONCLUSIONS

Here, we report for the first time two homozygous likely pathogenic and pathogenic TERB1 variants, c.626G>A, p.Trp209* and c.1703C>G, p.Ser568*, respectively, in two unrelated women with primary infertility. TERB1 is known to play an essential role in homologous chromosome movement, synapsis, and recombination during the meiotic prophase I and has an established role in male infertility in humans. Our data add TERB1 to the shortlist of Meiosis I genes associated with human infertility in both sexes.

SUN1 facilitates CHMP7 nuclear influx and injury cascades in sporadic amyotrophic lateral sclerosis

We have recently identified the aberrant nuclear accumulation of the ESCRT-III protein CHMP7 as an initiating event that leads to a significant injury to the nuclear pore complex (NPC) characterized by the reduction of specific nucleoporins from the neuronal NPC in sporadic amyotrophic lateral sclerosis (sALS) and C9orf72 ALS/frontotemporal dementia (FTD)-induced pluripotent stem cell-derived neurons (iPSNs), a phenomenon also observed in post-mortem patient tissues. Importantly, this NPC injury is sufficient to contribute to TDP-43 dysfunction and mislocalization, a common pathological hallmark of neurodegenerative diseases. However, the molecular mechanisms and events that give rise to increased nuclear translocation and/or retention of CHMP7 to initiate this pathophysiological cascade remain largely unknown. Here, using an iPSN model of sALS, we demonstrate that impaired NPC permeability barrier integrity and interactions with the LINC complex protein SUN1 facilitate CHMP7 nuclear localization and the subsequent 'activation' of NPC injury cascades. Collectively, our data provide mechanistic insights in the pathophysiological underpinnings of ALS/FTD and highlight SUN1 as a potent contributor to and modifier of CHMP7-mediated toxicity in sALS pathogenesis.

Homologous chromosome pairing: The linchpin of accurate segregation in meiosis

Meiosis is a specialized cell division that occurs in sexually reproducing organisms, generating haploid gametes containing half the chromosome number through two rounds of cell division. Homologous chromosomes pair and prepare for their proper segregation in subsequent divisions. How homologous chromosomes recognize each other and achieve pairing is an important question. Early studies showed that in most organisms, homologous pairing relies on homologous recombination. However, pairing mechanisms differ across species. Evidence indicates that chromosomes are dynamic and move during early meiotic stages, facilitating pairing. Recent studies in various model organisms suggest conserved mechanisms and key regulators of homologous chromosome pairing. This review summarizes these findings and compare similarities and differences in homologous chromosome pairing mechanisms across species.

3D genome remodeling and homologous pairing during meiotic prophase of mouse oogenesis and spermatogenesis

During meiosis, the chromatin and transcriptome undergo prominent switches. Although recent studies have explored the genome reorganization during spermatogenesis, the chromatin remodeling in oogenesis and characteristics of homologous pairing remain largely elusive. We comprehensively compared chromatin structures and transcriptomes at successive substages of meiotic prophase in both female and male mice using low-input high-through chromosome conformation capture (Hi-C) and RNA sequencing (RNA-seq). Compartments and topologically associating domains (TADs) gradually disappeared and slowly recovered in both sexes. We found that homologs adopted different sex-conserved pairing strategies prior to and after the leptotene-to-zygotene transition, changing from long interspersed nuclear element (LINE)-enriched compartments B to short interspersed nuclear element (SINE)-enriched compartments A. We complemented marker genes and predicted the sex-specific meiotic sterile genes for each substage. This study provides valuable insights into the similarities and distinctions between sexes in chromosome architecture, homologous pairing, and transcriptome during meiotic prophase of both oogenesis and spermatogenesis.

The scaffold nucleoporins SAR1 and SAR3 are essential for proper meiotic progression in Arabidopsis thaliana

Nuclear Pore Complexes (NPCs) are embedded in the nuclear envelope (NE), regulating macromolecule transport and physically interacting with chromatin. The NE undergoes dramatic breakdown and reformation during plant cell division. In addition, this structure has a specific meiotic function, anchoring and positioning telomeres to facilitate the pairing of homologous chromosomes. To elucidate a possible function of the structural components of the NPCs in meiosis, we have characterized several Arabidopsis lines with mutations in genes encoding nucleoporins belonging to the outer ring complex. Plants defective for either SUPPRESSOR OF AUXIN RESISTANCE1 (SAR1, also called NUP160) or SAR3 (NUP96) present condensation abnormalities and SPO11-dependent chromosome fragmentation in a fraction of meiocytes, which is increased in the double mutant sar1 sar3. We also observed these meiotic defects in mutants deficient in the outer ring complex protein HOS1, but not in mutants affected in other components of this complex. Furthermore, our findings may suggest defects in the structure of NPCs in sar1 and a potential link between the meiotic role of this nucleoporin and a component of the RUBylation pathway. These results provide the first insights in plants into the role of nucleoporins in meiotic chromosome behavior.

Building and breaking mechanical bridges between the nucleus and cytoskeleton: Regulation of LINC complex assembly and disassembly

The nucleus is physically coupled to the cytoskeleton through LINC complexes, macromolecular bridges composed of SUN and KASH proteins that span the nuclear envelope. LINC complexes are involved in a wide variety of critical cellular processes. For these processes to occur, cells regulate the composition, assembly, and disassembly of LINC complexes. Here we discuss recent studies on the regulation of the SUN-KASH interaction that forms the core of the LINC complex. These new findings encompass the stages of LINC complex assembly, from the formation of SUN-KASH heterooligomers to higher-order assemblies of LINC complexes. There is also new work on how components of the LINC complex are selectively dismantled, particularly by proteasomal degradation. It is becoming increasingly clear that LINC complexes are subject to multiple layers of regulation.

Protein target highlights in CASP15: Analysis of models by structure providers

We present an in-depth analysis of selected CASP15 targets, focusing on their biological and functional significance. The authors of the structures identify and discuss key protein features and evaluate how effectively these aspects were captured in the submitted predictions. While the overall ability to predict three-dimensional protein structures continues to impress, reproducing uncommon features not previously observed in experimental structures is still a challenge. Furthermore, instances with conformational flexibility and large multimeric complexes highlight the need for novel scoring strategies to better emphasize biologically relevant structural regions. Looking ahead, closer integration of computational and experimental techniques will play a key role in determining the next challenges to be unraveled in the field of structural molecular biology.

Dynamic regulation of LINC complex composition and function across tissues and contexts

The concept of mechanotransduction to the nucleus through a direct force transmission mechanism has fascinated cell biologists for decades. Central to such a mechanism is the linker of nucleoskeleton and cytoskeleton (LINC) complex, which spans the nuclear envelope to couple the cytoplasmic cytoskeleton to the nuclear lamina. In reality, there is not one LINC complex identity, but instead, a family of protein configurations of varied composition that exert both shared and unique functions. Regulated expression of LINC complex components, splice variants, and mechanoresponsive protein turnover mechanisms together shape the complement of LINC complex forms present in a given cell type. Disrupting specific gene(s) encoding LINC complex components therefore gives rise to a range of organismal defects. Moreover, evidence suggests that the mechanical environment remodels LINC complexes, providing a feedback mechanism by which cellular context influences the integration of the nucleus into the cytoskeleton. In particular, evidence for crosstalk between the nuclear and cytoplasmic intermediate filament networks communicated through the LINC complex represents an emerging theme in this active area of ongoing investigation.

Molecular Mechanisms for the Regulation of Nuclear Membrane Integrity

The nuclear membrane serves a critical role in protecting the contents of the nucleus and facilitating material and signal exchange between the nucleus and cytoplasm. While extensive research has been dedicated to topics such as nuclear membrane assembly and disassembly during cell division, as well as interactions between nuclear transmembrane proteins and both nucleoskeletal and cytoskeletal components, there has been comparatively less emphasis on exploring the regulation of nuclear morphology through nuclear membrane integrity. In particular, the role of type II integral proteins, which also function as transcription factors, within the nuclear membrane remains an area of research that is yet to be fully explored. The integrity of the nuclear membrane is pivotal not only during cell division but also in the regulation of gene expression and the communication between the nucleus and cytoplasm. Importantly, it plays a significant role in the development of various diseases. This review paper seeks to illuminate the biomolecules responsible for maintaining the integrity of the nuclear membrane. It will delve into the mechanisms that influence nuclear membrane integrity and provide insights into the role of type II membrane protein transcription factors in this context. Understanding these aspects is of utmost importance, as it can offer valuable insights into the intricate processes governing nuclear membrane integrity. Such insights have broad-reaching implications for cellular function and our understanding of disease pathogenesis.

ClpP/ClpX deficiency impairs mitochondrial functions and mTORC1 signaling during spermatogenesis

Caseinolytic protease proteolytic subunit (ClpP) and caseinolytic protease X (ClpX) are mitochondrial matrix peptidases that activate mitochondrial unfolded protein response to maintain protein homeostasis in the mitochondria. However, the role of ClpP and ClpX in spermatogenesis remains largely unknown. In this study, we demonstrated the importance of ClpP/ClpX for meiosis and spermatogenesis with two conditional knockout (cKO) mouse models. We found that ClpP/ClpX deficiency reduced mitochondrial functions and quantity in spermatocytes, affected energy supply during meiosis and attenuated zygotene-pachytene transformation of the male germ cells. The dysregulated spermatocytes finally underwent apoptosis resulting in decreased testicular size and vacuolar structures within the seminiferous tubules. We found mTORC1 pathway was over-activated after deletion of ClpP/ClpX in spermatocytes. Long-term inhibition of the mTORC1 signaling via rapamycin treatment in vivo partially rescue spermatogenesis. The data reveal the critical roles of ClpP and ClpX in regulating meiosis and spermatogenesis.

Histone methyltransferase SUV39H1 regulates the Golgi complex via the nuclear envelope-spanning LINC complex

Cell motility is related to the higher-order structure of chromatin. Stimuli that induce cell migration change chromatin organization; such stimuli include elevated histone H3 lysine 9 trimethylation (H3K9me3). We previously showed that depletion of histone H3 lysine 9 methyltransferase, SUV39H1, suppresses directional cell migration. However, the molecular mechanism underlying this association between chromatin and cell migration remains elusive. The Golgi apparatus is a cell organelle essential for cell motility. In this study, we show that loss of H3K9 methyltransferase SUV39H1 but not SETDB1 or SETDB2 causes dispersion of the Golgi apparatus throughout the cytoplasm. The Golgi dispersion triggered by SUV39H1 depletion is independent of transcription, centrosomes, and microtubule organization, but is suppressed by depletion of any of the following three proteins: LINC complex components SUN2, nesprin-2, or microtubule plus-end-directed kinesin-like protein KIF20A. In addition, SUN2 is closely localized to H3K9me3, and SUV39H1 affects the mobility of SUN2 in the nuclear envelope. Further, inhibition of cell motility caused by SUV39H1 depletion is restored by suppression of SUN2, nesprin-2, or KIF20A. In summary, these results show the functional association between chromatin organization and cell motility via the Golgi organization regulated by the LINC complex.

Molecular insights into LINC complex architecture through the crystal structure of a luminal trimeric coiled-coil domain of SUN1

The LINC complex, consisting of interacting SUN and KASH proteins, mechanically couples nuclear contents to the cytoskeleton. In meiosis, the LINC complex transmits microtubule-generated forces to chromosome ends, driving the rapid chromosome movements that are necessary for synapsis and crossing over. In somatic cells, it defines nuclear shape and positioning, and has a number of specialised roles, including hearing. Here, we report the X-ray crystal structure of a coiled-coiled domain of SUN1's luminal region, providing an architectural foundation for how SUN1 traverses the nuclear lumen, from the inner nuclear membrane to its interaction with KASH proteins at the outer nuclear membrane. In combination with light and X-ray scattering, molecular dynamics and structure-directed modelling, we present a model of SUN1's entire luminal region. This model highlights inherent flexibility between structured domains, and raises the possibility that domain-swap interactions may establish a LINC complex network for the coordinated transmission of cytoskeletal forces.

The meiotic LINC complex component KASH5 is an activating adaptor for cytoplasmic dynein

Cytoplasmic dynein-driven movement of chromosomes during prophase I of mammalian meiosis is essential for synapsis and genetic exchange. Dynein connects to chromosome telomeres via KASH5 and SUN1 or SUN2, which together span the nuclear envelope. Here, we show that KASH5 promotes dynein motility in vitro, and cytosolic KASH5 inhibits dynein's interphase functions. KASH5 interacts with a dynein light intermediate chain (DYNC1LI1 or DYNC1LI2) via a conserved helix in the LIC C-terminal, and this region is also needed for dynein's recruitment to other cellular membranes. KASH5's N-terminal EF-hands are essential as the interaction with dynein is disrupted by mutation of key calcium-binding residues, although it is not regulated by cellular calcium levels. Dynein can be recruited to KASH5 at the nuclear envelope independently of dynactin, while LIS1 is essential for dynactin incorporation into the KASH5-dynein complex. Altogether, we show that the transmembrane protein KASH5 is an activating adaptor for dynein and shed light on the hierarchy of assembly of KASH5-dynein-dynactin complexes.

Distinct dynein complexes defined by DYNLRB1 and DYNLRB2 regulate mitotic and male meiotic spindle bipolarity

Spindle formation in male meiosis relies on the canonical centrosome system, which is distinct from acentrosomal oocyte meiosis, but its specific regulatory mechanisms remain unknown. Herein, we report that DYNLRB2 (Dynein light chain roadblock-type-2) is a male meiosis-upregulated dynein light chain that is indispensable for spindle formation in meiosis I. In Dynlrb2 KO mouse testes, meiosis progression is arrested in metaphase I due to the formation of multipolar spindles with fragmented pericentriolar material (PCM). DYNLRB2 inhibits PCM fragmentation through two distinct pathways; suppressing premature centriole disengagement and targeting NuMA (nuclear mitotic apparatus) to spindle poles. The ubiquitously expressed mitotic counterpart, DYNLRB1, has similar roles in mitotic cells and maintains spindle bipolarity by targeting NuMA and suppressing centriole overduplication. Our work demonstrates that two distinct dynein complexes containing DYNLRB1 or DYNLRB2 are separately used in mitotic and meiotic spindle formations, respectively, and that both have NuMA as a common target.

Close Ties between the Nuclear Envelope and Mammalian Telomeres: Give Me Shelter

The nuclear envelope (NE) in eukaryotic cells is essential to provide a protective compartment for the genome. Beside its role in connecting the nucleus with the cytoplasm, the NE has numerous important functions including chromatin organization, DNA replication and repair. NE alterations have been linked to different human diseases, such as laminopathies, and are a hallmark of cancer cells. Telomeres, the ends of eukaryotic chromosomes, are crucial for preserving genome stability. Their maintenance involves specific telomeric proteins, repair proteins and several additional factors, including NE proteins. Links between telomere maintenance and the NE have been well established in yeast, in which telomere tethering to the NE is critical for their preservation and beyond. For a long time, in mammalian cells, except during meiosis, telomeres were thought to be randomly localized throughout the nucleus, but recent advances have uncovered close ties between mammalian telomeres and the NE that play important roles for maintaining genome integrity. In this review, we will summarize these connections, with a special focus on telomere dynamics and the nuclear lamina, one of the main NE components, and discuss the evolutionary conservation of these mechanisms.

Loss of SUN1 function in spermatocytes disrupts the attachment of telomeres to the nuclear envelope and contributes to non-obstructive azoospermia in humans

One of the most severe forms of infertility in humans, caused by gametogenic failure, is non-obstructive azoospermia (NOA). Approximately, 20-30% of men with NOA may have single-gene mutations or other genetic variables that cause this disease. While a range of single-gene mutations associated with infertility has been identified in prior whole-exome sequencing (WES) studies, current insight into the precise genetic etiology of impaired human gametogenesis remains limited. In this paper, we described a proband with NOA who experienced hereditary infertility. WES analyses identified a homozygous variant in the SUN1 (Sad1 and UNC84 domain containing 1) gene [c. 663C > A: p.Tyr221X] that segregated with infertility. SUN1 encodes a LINC complex component essential for telomeric attachment and chromosomal movement. Spermatocytes with the observed mutations were incapable of repairing double-strand DNA breaks or undergoing meiosis. This loss of SUN1 functionality contributes to significant reductions in KASH5 levels within impaired chromosomal telomere attachment to the inner nuclear membrane. Overall, our results identify a potential genetic driver of NOA pathogenesis and provide fresh insight into the role of the SUN1 protein as a regulator of prophase I progression in the context of human meiosis.

Nuclear mechanosignaling in striated muscle diseases

Mechanosignaling describes processes by which biomechanical stimuli are transduced into cellular responses. External biophysical forces can be transmitted via structural protein networks that span from the cellular membrane to the cytoskeleton and the nucleus, where they can regulate gene expression through a series of biomechanical and/or biochemical mechanosensitive mechanisms, including chromatin remodeling, translocation of transcriptional regulators, and epigenetic factors. Striated muscle cells, including cardiac and skeletal muscle myocytes, utilize these nuclear mechanosignaling mechanisms to respond to changes in their intracellular and extracellular mechanical environment and mediate gene expression and cell remodeling. In this brief review, we highlight and discuss recent experimental work focused on the pathway of biomechanical stimulus propagation at the nucleus-cytoskeleton interface of striated muscles, and the mechanisms by which these pathways regulate gene regulation, muscle structure, and function. Furthermore, we discuss nuclear protein mutations that affect mechanosignaling function in human and animal models of cardiomyopathy. Furthermore, current open questions and future challenges in investigating striated muscle nuclear mechanosignaling are further discussed.

MJL-1 is a nuclear envelope protein required for homologous chromosome pairing and regulation of synapsis during meiosis in C. elegans

Interactions between chromosomes and LINC (linker of nucleoskeleton and cytoskeleton) complexes in the nuclear envelope (NE) promote homolog pairing and synapsis during meiosis. By tethering chromosomes to cytoskeletal motors, these connections lead to processive chromosome movements along the NE. This activity is usually mediated by telomeres, but in the nematode Caenorhabditis elegans, special chromosome regions called "pairing centers" (PCs) have acquired this meiotic function. Here, we identify a previously uncharacterized meiosis-specific NE protein, MJL-1 (MAJIN-Like-1), that is essential for interactions between PCs and LINC complexes in C. elegans. Mutations in MJL-1 eliminate active chromosome movements during meiosis, resulting in nonhomologous synapsis and impaired homolog pairing. Fission yeast and mice also require NE proteins to connect chromosomes to LINC complexes. Extensive similarities in the molecular architecture of meiotic chromosome-NE attachments across eukaryotes suggest a common origin and/or functions of this architecture during meiosis.

Specific expression of alternatively spliced genes in the turkey (Meleagris gallopavo) reproductive tract revealed their function in spermatogenesis and post-testicular sperm maturation

The tissue-specific profile of alternatively spliced genes (ASGs) and their involvement in reproduction processes characteristic of turkey testis, epididymis, and ductus deferens were investigated for the first time in birds. Deep sequencing of male turkey reproductive tissue RNA samples (n = 6) was performed using Illumina RNA-Seq with 2 independent methods, rMATs and SUPPA2, for differential alternative splicing (DAS) event prediction. The expression of selected ASGs was validated using quantitative real-time reverse transcriptase-polymerase chain reaction. The testis was found to be the site of the highest number of posttranscriptional splicing events within the reproductive tract, and skipping exons were the most frequently occurring class of alternative splicing (AS) among the reproductive tract. Statistical analysis revealed 86, 229, and 6 DAS events in the testis/epididymis, testis/ductus deferens, and epididymis/ductus deferens comparison, respectively. Alternative splicing was found to be a mechanism of gene expression regulation within the turkey reproduction tract. In testis, modification was observed for spermatogenesis specific genes; the changes in 5' UTR could act as regulator of MEIG1 expression (a player during spermatocytes meiosis), and modification of 3' UTR led to diversification of CREM mRNA (modulator of gene expression related to the structuring of mature spermatozoa). Sperm tail formation can be regulated by changes in the 5' UTR of testicular SLC9A3R1 and gene silencing by producing dysfunctional variants of ODF2 in the testis and ATP1B3 in the epididymis. Predicted differentially ASGs in the turkey reproductive tract seem to be involved in the regulation of spermatogenesis, including acrosome formation and sperm tail formation and binding of sperm to the zona pellucida. Several ASGs were classified as cilia by actin and microtubule cytoskeleton organization. Such genes may play a role in the organization of sperm flagellum and post-testicular motility development. To our knowledge, this is the first functional investigation of alternatively spliced genes associated with tissue-specific processes in the turkey reproductive tract.

TSNAXIP1 is required for sperm head formation and male fertility

Purpose

TRANSLIN (TSN) and its binding partner TSNAX have been reported to contribute to a wide spectrum of biological activities including spermatogenesis. TSN accompanies specific mRNA transport in male germ cells through intercellular bridges. A testis-expressed protein TSNAXIP1 was reported to interact with TSNAX. However the role of TSNAXIP1 in spermatogenesis remained unclear. This study aimed to elucidate the role of TSNAXIP1 in spermatogenesis and male fertility in mice.

Methods

TSNAXIP1 knockout (KO) mice were generated using the CRISPR-Cas9 system. The fertility, spermatogenesis, and sperm of TSNAXIP1 KO males were analyzed.

Results

TSNAXIP1, and especially its domains, are highly conserved between mouse and human. Tsnaxip1 was expressed in testis, but not in ovary. TSNAXIP1 KO mice were generated, and TSNAXIP1 KO males were found to be sub-fertile with smaller testis and lower sperm count. Although no overt abnormalities were observed during spermatogenesis, lack of TSNAXIP1 induced sperm head malformation, resulting in a unique flower-shaped sperm head. Moreover, abnormal anchorage of the sperm neck was frequently observed in TSNAXIP1 null sperm.

Conclusion

A testis-expressed gene TSNAXIP1 has important roles in sperm head morphogenesis and male fertility. Moreover, TSNAXIP1 could be a causative gene for human infertility.

Cyclins and CDKs in the regulation of meiosis-specific events

How eukaryotic cells control their duplication is a fascinating example of how a biological system self-organizes specific activities to temporally order cellular events. During cell cycle progression, the cellular level of CDK (Cyclin-Dependent Kinase) activity temporally orders the different cell cycle phases, ensuring that DNA replication occurs prior to segregation into two daughter cells. CDK activity requires the binding of a regulatory subunit (cyclin) to the core kinase, and both CDKs and cyclins are well conserved throughout evolution from yeast to humans. As key regulators, they coordinate cell cycle progression with metabolism, DNA damage, and cell differentiation. In meiosis, the special cell division that ensures the transmission of genetic information from one generation to the next, cyclins and CDKs have acquired novel functions to coordinate meiosis-specific events such as chromosome architecture, recombination, and synapsis. Interestingly, meiosis-specific cyclins and CDKs are common in evolution, some cyclins seem to have evolved to acquire CDK-independent functions, and even some CDKs associate with a non-cyclin partner. We will review the functions of these key regulators in meiosis where variation has specially flourished.

Premeiotic pairing of homologous chromosomes during Drosophila male meiosis

In the early stages of meiosis, maternal and paternal chromosomes pair with their homologous partner and recombine to ensure exchange of genetic information and proper segregation. These events can vary drastically between species and between males and females of the same species. In Drosophila, in contrast to females, males do not form synaptonemal complexes (SCs), do not recombine, and have no crossing over; yet, males are able to segregate their chromosomes properly. Here, we investigated the early steps of homolog pairing in Drosophila males. We found that homolog centromeres are not paired in germline stem cells (GSCs) and become paired in the mitotic region before meiotic entry, similarly to females. Surprisingly, male germline cells express SC proteins, which localize to centromeres and promote pairing. We further found that the SUN/KASH (LINC) complex and microtubules are required for homolog pairing as in females. Chromosome movements in males, however, are much slower than in females and we demonstrate that this slow dynamic is compensated in males by having longer cell cycles. In agreement, slowing down cell cycles was sufficient to rescue pairing-defective mutants in female meiosis. Our results demonstrate that although meiosis differs significantly between males and females, sex-specific cell cycle kinetics integrate similar molecular mechanisms to achieve proper centromere pairing.

N6-methyladenosine modification on Hmbox1 is related to telomere dysfunction in DEHP-induced male reproductive injury

AIMS

Di (2-ethylhexyl) phthalate (DEHP), as an environmental endocrine-disrupting chemical (EDC), can induce male reproductive injury. N6-methyladenosine (m6A) plays a vital role in environmental exposure-induced diseases by regulating gene expression. Therefore, we aim to investigate the role of m6A in DEHP-induced reproductive injury.

MAIN METHODS

We established an in vivo model of mice exposed to DEHP to explore the effect of DEHP on reproductive injury and m6A. To further explore the molecular mechanism of DEHP toxicity, we built a model of GC-2 cells exposed to mono-(2-ethylhexyl) phthalate (MEHP) in vitro and further silenced Mettl3 in GC-2cells. Besides, we also conducted MeRIP-qPCR and RIP assays to identify the target genes for m6A modification.

KEY FINDINGS

DEHP induced testicular injury and senescence. And telomeres shortening, reduced levels of telomere repeat-binding factor 1 (TRF1), TRF2, protection of telomeres 1 (POT1), and telomerase reverse transcriptase (TERT) can be observed in DEHP-treated testes. MEHP also induced GC-2 cellular senescence and telomere dysfunction. Besides, increased m6A mediated by METTL3 stabilized homeobox containing 1 (Hmbox1) in an m6A-dependent manner in MEHP-exposed GC-2 cells. Mettl3 knockdown led to lower m6A modification and reduced Hmbox1 stability, resulting in further shortening of telomere length.

SIGNIFICANCE

our work uncovered that DEHP led to male reproductive injury by telomere dysfunction and m6A modified Hmbox1 contributed to maintaining telomere homeostasis in this process, suggesting that accurate regulation of m6A modification level by drugs has potential value in the treatment of DEHP-induced male reproductive injury.

CDKN2AIP is critical for spermiogenesis and germ cell development

Background

As a member of RNA-binding protein, CDKN2AIP has been shown to play a critical role in stem cell pluripotency and somatic differentiation. Recent studies indicate that Cdkn2aip is essential for spermatogonial self-renewal and proliferation through the activating Wnt-signaling pathway. However, the mechanisms of how Cdkn2aip regulate spermatogenesis is poorly characterized.

Results

We discovered that the CDKN2AIP was expressed in spermatocyte as well as spermatids and participated in spermiogenesis. Cdkn2aip^−/− mice exhibited multiple sperm head defects accompanied by age dependent germ cell loss that might be result of protamine replacement failure and impaired SUN1 expression. Loss of Cdkn2aip expression in male mice resulted in synapsis failure in 19% of all spermatocytes and increased apoptosis due to damaged DNA double-strand break (DSB) repair and crossover formation. In vitro, knockdown of Cdkn2aip was associated with extended S phase, increased DNA damage and apoptosis.

Conclusions

Our findings not only identified the importance of CDKN2AIP in spermiogenesis and germ cell development, but also provided insight upon the driving mechanism.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00861-z.

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

CONTEXT

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.

OBJECTIVE

To investigate the role of KASH5 variation in the pathogenesis of POI.

DESIGN

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.

RESULTS

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.

CONCLUSIONS

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.

Time to match; when do homologous chromosomes become closer?

In most eukaryotes, pairing of homologous chromosomes is an essential feature of meiosis that ensures homologous recombination and segregation. However, when the pairing process begins, it is still under investigation. Contrasting data exists in Mus musculus, since both leptotene DSB-dependent and preleptotene DSB-independent mechanisms have been described. To unravel this contention, we examined homologous pairing in pre-meiotic and meiotic Mus musculus cells using a three-dimensional fluorescence in situ hybridization-based protocol, which enables the analysis of the entire karyotype using DNA painting probes. Our data establishes in an unambiguously manner that 73.83% of homologous chromosomes are already paired at premeiotic stages (spermatogonia-early preleptotene spermatocytes). The percentage of paired homologous chromosomes increases to 84.60% at mid-preleptotene-zygotene stage, reaching 100% at pachytene stage. Importantly, our results demonstrate a high percentage of homologous pairing observed before the onset of meiosis; this pairing does not occur randomly, as the percentage was higher than that observed in somatic cells (19.47%) and between nonhomologous chromosomes (41.1%). Finally, we have also observed that premeiotic homologous pairing is asynchronous and independent of the chromosome size, GC content, or presence of NOR regions.

Novel bi-allelic variants in KASH5 are associated with meiotic arrest and non-obstructive azoospermia

KASH5 is an essential component of the LINC (linker of the nucleoskeleton and cytoskeleton) complex that regulates chromosome movements and nuclear envelope (NE) remodeling in mouse spermatocytes during meiosis prophase I, but its expression and function in human cells, as well as its association with male infertility are largely unknown. In this study, a novel heterozygous copy number variation (CNV) (seq [GRCh37] del(19) (19q13.33) chr19: g.49894043-49903011del) and a heterozygous loss of function variant (NM_144688: c.979_980del: p.R327Sfs*21) in human KASH5 were identified in a non-obstructive azoospermia (NOA)-affected patient and in his infertile sister by whole-exome sequencing and CNV array. Spermatogenesis in the proband was arrested at zygotene-like stage with a deficiency in homolog pairing and synapsis. KASH5 protein expression in human spermatocytes was evaluated and reported first in this study. Single-cell RNA sequencing demonstrated that the LINC complex and associated genes in human and mouse shared a similar expression pattern, indicating a conserved mechanism in the regulation of chromosome movements and NE remodeling. Kash5 knockout mouse displayed similar phenotypes, including a meiotic arrest at a zygotene-like stage and impaired pairing and synapsis. Collectively, we have identified novel rare variants within human KASH5 in patients with NOA and meiosis arrest. Our study expands the knowledge of KASH5 and associated proteins in regulating human meiosis prophase I progress and provides new insight into the genetic etiology of NOA.

The KASH5 protein involved in meiotic chromosomal movements is a novel dynein activating adaptor

Dynein harnesses ATP hydrolysis to move cargo on microtubules in multiple biological contexts. Dynein meets a unique challenge in meiosis by moving chromosomes tethered to the nuclear envelope to facilitate homolog pairing essential for gametogenesis. Though processive dynein motility requires binding to an activating adaptor, the identity of the activating adaptor required for dynein to move meiotic chromosomes is unknown. We show that the meiosis-specific nuclear-envelope protein KASH5 is a dynein activating adaptor: KASH5 directly binds dynein using a mechanism conserved among activating adaptors and converts dynein into a processive motor. We map the dynein-binding surface of KASH5, identifying mutations that abrogate dynein binding in vitro and disrupt recruitment of the dynein machinery to the nuclear envelope in cultured cells and mouse spermatocytes in vivo. Our study identifies KASH5 as the first transmembrane dynein activating adaptor and provides molecular insights into how it activates dynein during meiosis.

Mechanism of initiation of meiosis in mouse germ cells

Meiosis is critical for germ cell development in multicellular organisms. Initiation of meiosis coincides with pre-meiotic S phase, which is followed by meiotic prophase, a prolonged G2 phase that ensures numerous meiosis-specific chromosome events. Meiotic prophase is accompanied by robust alterations of gene expression. In mouse germ cells, MEIOSIN and STRA8 direct cell cycle switch from mitosis to meiosis. MEIOSIN and STRA8 coordinate meiotic initiation with cell cycle, by activating the meiotic genes to have meiotic prophase program installed at S phase. This review mainly focuses on the mechanism of meiotic initiation in mouse germ cells from the viewpoint of the transcription of meiotic genes. Furthermore, signaling pathways that regulate meiotic initiation will be discussed in the context of germ cell development, pointing out the sexual differences in the mode of meiotic initiation.

LINCing Senescence and Nuclear Envelope Changes

The nuclear envelope (NE) has emerged as a nexus for cellular organization, signaling, and survival. Beyond its role as a barrier to separate the nucleoplasm from the cytoplasm, the NE's role in supporting and maintaining a myriad of other functions has made it a target of study in many cellular processes, including senescence. The nucleus undergoes dramatic changes in senescence, many of which are driven by changes in the NE. Indeed, Lamin B1, a key NE protein that is consistently downregulated in senescence, has become a marker for senescence. Other NE proteins have also been shown to play a role in senescence, including LINC (linker of nucleoskeleton and cytoskeleton) complex proteins. LINC complexes span the NE, forming physical connections between the cytoplasm to the nucleoplasm. In this way, they integrate nuclear and cytoplasmic mechanical signals and are essential not only for a variety of cellular functions but are needed for cell survival. However, LINC complex proteins have been shown to have a myriad of functions in addition to forming a LINC complex, often existing as nucleoplasmic or cytoplasmic soluble proteins in a variety of isoforms. Some of these proteins have now been shown to play important roles in DNA repair, cell signaling, and nuclear shape regulation, all of which are important in senescence. This review will focus on some of these roles and highlight the importance of LINC complex proteins in senescence.

Homozygous missense mutation in CCDC155 disrupts the transmembrane distribution of CCDC155 and SUN1, resulting in non-obstructive azoospermia and premature ovarian insufficiency in humans

Non-obstructive azoospermia (NOA) and premature ovarian insufficiency (POI) represent the most serious forms of human infertility caused by gametogenic failure. Although whole-exome sequencing (WES) has uncovered multiple monogenic causes of human infertility, our knowledge of the genetic basis of human gametogenesis defects remains at a rudimentary stage. Coiled-coil-domain-containing protein 155 (CCDC155) encodes a core component of the linker of the nucleoskeleton and cytoskeleton complex that is essential for modulating telomere-led chromosome movements during the meiotic prophase of mice. Additionally, Ccdc155 deficiency in mice causes infertility in both sexes with meiotic arrest. In this study, we applied WES to identify the pathogenic genes for 15 NOA and POI patients whose parents were consanguineous and identified a novel homozygous missense mutation in CCDC155 [c.590T>C (p.Leu197Pro)] in a pair of familial NOA and POI patients whose parents were first cousins. The affected spermatocytes were unable to complete meiotic division coupled with unresolved repair of the DNA double-strand break. This rare missense mutation with lesions in the conserved CC domain of CCDC155 blocked nuclear envelope (NE) distribution and subsequently prevented NE-specific enrichment of Sad1- and UNC84-domain-containing 1 either ex vivo or in vitro, eventually leading to disruptive NE anchoring of chromosome-induced meiotic arrest in both sexes. This study presents the first evidence of the necessity of the SUN1-CCDC155 complex during human meiosis and provides insight into the CCDC155 CC domain, thereby expanding the genetic spectrum of human NOA and POI and promoting adequate genetic counselling and appropriate fertility guidance for these patients.

How and Why Chromosomes Interact with the Cytoskeleton during Meiosis

During the early meiotic prophase, connections are established between chromosomes and cytoplasmic motors via a nuclear envelope bridge, known as a LINC (linker of nucleoskeleton and cytoskeleton) complex. These widely conserved links can promote both chromosome and nuclear motions. Studies in diverse organisms have illuminated the molecular architecture of these connections, but important questions remain regarding how they contribute to meiotic processes. Here, we summarize the current knowledge in the field, outline the challenges in studying these chromosome dynamics, and highlight distinctive features that have been characterized in major model systems.

Control of meiotic chromosomal bouquet and germ cell morphogenesis by the zygotene cilium

A hallmark of meiosis is chromosomal pairing, which requires telomere tethering and rotation on the nuclear envelope via microtubules, driving chromosome homology searches. Telomere pulling toward the centrosome forms the "zygotene chromosomal bouquet". Here, we identified the "zygotene cilium" in oocytes. This cilium provides a cable system for the bouquet machinery, extending throughout the germline cyst. Using zebrafish mutants and live manipulations, we demonstrate that the cilium anchors the centrosome to counterbalance telomere pulling. The cilium is essential for bouquet and synaptonemal complex formation, oogenesis, ovarian development, and fertility. Thus, a cilium represents a conserved player in zebrafish and mouse meiosis, which sheds light on reproductive aspects in ciliopathies, and suggests that cilia can control chromosomal dynamics.