Chromosoma

Accurate chromosome segregation relies on proper centromere and kinetochore formation and phospho-regulation. We previously demonstrated that a pluripotent state confers a low fidelity of chromosome segregation, however it is unknown how a pluripotent state impacts centromere and kinetochore function. Here, we demonstrate that both centromere and kinetochore structural organization and phosphorylation in mitosis are developmentally regulated. CENP-A, CENP-C, and HEC1 protein abundance is reduced at mitotic centromeres and kinetochores of human pluripotent stem cells (hPSCs) compared to isogenic somatic cells; however, elevating their levels does not improve chromosome segregation fidelity. Rather, we find that reduced phosphorylation of kinetochores is responsible for their low fidelity. HEC1 is hypophosphorylated at kinetochores of hPSCs compared to isogenic somatic cells at Cyclin B/Cdk1 and Aurora kinase phospho-sites. Inhibiting PP2A phosphatase activity or differentiation increases HEC1 phosphorylation at hPSC kinetochores decreasing chromosome segregation errors. Thus, mitotic fidelity in non-transformed human cells depends on the developmental regulation of the kinase and phosphatase networks controlling kinetochore phosphorylation. SummaryGalaviz Sarmiento et al show that the developmental regulation of kinetochore phosphorylation governs mitotic fidelity. HEC1 is hypophosphorylated at kinetochores of hPSCs during mitosis contributing to their high rate of chromosome segregation errors. While differentiation increases HEC1 phosphorylation improving chromosome segregation fidelity.

BackgroundDynamic chromatin behavior, which is related to chromatin accessibility, plays a critical role in various genome DNA functions such as RNA transcription and DNA replication/repair. Previous studies using highly synchronized cells showed that average local chromatin motion, captured by single-nucleosome imaging and tracking on a second time scale, remained almost constant throughout G1, S, and G2 phases in living human cells, although possible effects of prolonged drug treatments for cell-cycle synchronization could not be excluded. ResultsTo avoid possible effects of prolonged drug treatment, we combined single-nucleosome imaging with Fucci probes to visualize cell-cycle progression through G1, S, and G2. Using HeLa and HCT116 cells expressing H2B-HaloTag and Fucci probes, we found that local nucleosome motion remained similar on average throughout interphase, except for elevated motion in early G1. Transcription inhibition similarly increased nucleosome motion throughout interphase. Local nucleosome motion also increased following replication stress or DNA damage. ConclusionOur findings suggest that near-constant chromatin motion supports housekeeping functions under similar physical conditions during interphase. Our findings also suggest that cells can transiently change chromatin motion to perform ad hoc tasks in response to signals from inside and outside the cell, such as DNA damage.

BackgroundMaize knobs are regions of constitutive heterochromatin that are readily identified in both meiotic and somatic chromosomes. These structures have been characterized as stable throughout the cell cycle, exhibiting late replication during the S-phase, and are composed of two specific families of highly repetitive DNA sequences: K180 and TR-1. Although widely used as cytogenetic markers due to their variability in number and chromosomal position across inbred lines, hybrids, and landraces, little is known about their chromatin structure and dynamics. In this study, we analyzed chromatin accessibility of knobs using DNS-seq data across four maize tissues representing distinct developmental stages. ResultsOur results reveal that K180 knobs exhibit tissue-specific variation in chromatin accessibility, transitioning between open and closed states during development. In contrast, the TR-1 knob of chromosome 4 remained consistently inaccessible across all tissues analyzed. A knob composed of both K180, and TR-1 further supported this observation, with only the K180 region showing dynamic accessibility. To validate these findings, we also analyzed other repetitive regions such as centromeres, which showed a uniformly closed chromatin structure similar to TR-1. These results suggest a unique developmental modulation of chromatin accessibility associated with K180 repeats. While the chromatin accessibility of knobs does not reach the levels observed at Transcription Start Sites (TSS), the comparison among different classes of repetitive DNA within maize constitutive heterochromatin provides compelling evidence for sequence-specific and tissue-specific chromatin dynamics. ConclusionsOur findings uncover a previously unrecognized property of maize knobs and establish a reference for future studies on chromatin organization and epigenetic regulation of repetitive DNA in plant genomes.

Nucleoli and centromeres play essential roles in cellular proliferation and homeostasis, and are structurally and functionally interconnected. Centromeres frequently cluster around nucleoli, and some centromere assembly factors are known to reside in the nucleoli. To investigate the spatial and temporal relationships between these nuclear domains, we examined their dynamics in living cells. We imaged HeLa cells stably expressing mCherry-NPM1 and GFP-CENP-A using time-lapse microscopy. The results show that a subset of centromeres exhibits dynamic behavior during interphase, migrating over micrometer-scale distances within two hours. On average, 40-50% of centromeres maintain an association with nucleoli throughout interphase, with some cells displaying nucleolar-centromere association and dissociation within hours. Upon entry into mitosis, nucleoli are disassembled, and NPM1 localizes to the periphery of mitotic chromosomes. Nucleolar-centromere interactions are re-established in early G1, coinciding with the assembly of new centromeres. Treatment with actinomycin D, an inhibitor of RNA polymerase I, significantly reduces nucleolar size, nucleolar-centromere interactions, and centromere dynamics. Furthermore, post-mitotic nucleolar reformation is impaired. These findings highlight the dynamic nature of centromeres in interphase nuclei and their interactions with nucleoli. This behavior is partially dependent on rDNA transcription and nucleolar integrity, underscoring the critical roles of nucleoli, centromeres, and their interaction in 4D genome organization.

Nuclear blebs are herniations of the nucleus that occur in many human conditions including aging, heart disease, muscular dystrophy, and many cancers. Nuclear blebbing causes nuclear rupture and cellular dysfunction. However, understanding the formation, stability, and identification of nuclear blebs remains an ongoing challenge. Our previous studies reveal that nuclear blebs are best hallmarked by decreased DNA density. To determine if chromatin decompaction underlies decreased DNA density in nuclear blebs, we investigated the histone composition of nuclear blebs across multiple cell lines. Time lapse and immunofluorescence imaging revealed that global histone H2B and H3 levels are decreased in the nuclear bleb relative to the nuclear body. Next, we imaged histone modification states of euchromatin and heterochromatin, which respectively track decompact and compact states of chromatin. Overall, we find that nuclear blebs display variable histone modification state across cell lines, as euchromatin does not consistently enrich nor is heterochromatin consistently depleted. Nuclear blebs did consistently show active RNA Pol II initiation is enriched relative to elongation. Thus, we find that the local histone modification state is not an essential component of nuclear blebs while transcription initiation enrichment over elongation is reproducible across cell lines and conditions. Summary statementWe measured histones and their modification states in nuclear blebs. We find that chromatin state is variable while transcription initiation is consistently enriched relative to elongation in nuclear blebs.

Understanding cis-regulatory elements (CREs) at the single cell level is fundamental to deciphering transcriptional changes during development, cell differentiation, and homeostasis. Recent studies have shown that arbitrary peak-calling thresholds complicate data interpretation and cross-study comparisons. Furthermore, due to the inherent sparsity of single-nuclei ATAC-seq (snATAC-seq) data, distinguishing between truly inaccessible regions and technical dropouts remains challenging. Our analysis of snATAC-seq experiments performed in a well-established cell line suggests that the dichotomy between accessible (open) or inaccessible (close) CREs is misleading. Thousands of accessible regions are present in a very small fraction of cells of the population but they are repeatedly identified, suggesting that they have a low accessibility or are only transiently accessible. However, depending on the detection threshold selected they could be considered as either genuine CREs or noise. To resolve this inconsistency, we propose a model where chromatin accessibility is treated as a continuum, defined by a probability of accessibility (Pa) for each accessible region across cell types and conditions. Through computational simulations, we demonstrate that snATAC-seq results can be explained by a simple "balls into bins" probability model, offering a theoretical framework for calculating Pa distributions from any snATAC-seq dataset. Furthermore, we examine how Pa distributions shift following activation of the TGF{beta} signaling pathway. This probabilistic approach removes the reliance on arbitrary thresholds and supports a more quantitative, and dynamic understanding of accessible regions function.

Although calmodulin is best known as an intracellular calcium sensor, it also possesses calcium-independent functions in unicellular organisms. This is exemplified by the budding yeast S. cerevisiae calmodulin, which binds its essential targets, the pericentrin-like protein Spc110 and type I and V myosins, without needing calcium. Whether such calcium-independent cellular functions are conserved in other yeasts and vertebrates nevertheless remains an open question. Here, we examined the calcium-independent functions of the fission yeast S. pombe calmodulin Cam1 by measuring its intracellular distribution. Using quantitative fluorescence microscopy, we assessed the intracellular localization of two cam1 mutants, where binding of Ca2+ had been compromised by mutations in their EF hands, compared to the wild type protein. Both Cam1-2V and -3V reduced their localization by 90% to the yeast microtubule-organizing center spindle pole bodies (SPB). In contrast, these two mutants did not affect the myosin-dependent localization to the equatorial division plane and to the cell tips. Replacing the endogenous cam1 with cam1-2V decreased the SPB localization of pericentrin Pcp1 by 69%, without changing the localization of either type V or I myosins. Over-expression of Pcp1 rescued the mitotic defects of cam1-2V cells at the restrictive temperature. Surprisingly, the cytokinesis of this cam1 mutant was largely normal. We concluded that fission yeast calmodulin Cam1 depends on Ca2+to be a component of SPBs, suggesting that calcium plays a critical role in the assembly of SPBs.

BackgroundGene expression changes in response to developmental and environmental cues rely on cis-regulatory sequence elements (cREs). BRG1/BRM-Associated Factors (BAF) chromatin remodeling complexes maintain chromatin accessibility at many cREs, enabling binding by transcription factors (TFs). However, cREs exhibit a broad range of sensitivity to loss of BAF function, and the basis of this variability remains unknown. ResultsTo identify the characteristics of BAF-dependent cREs, we mapped chromatin accessibility changes following acute pharmacologic BAF inhibition in GM12878 lymphoblastoid cells. We integrated these results with over 100 TF and histone modification ChIP-seq datasets and used machine learning to identify features that predict chromatin accessibility changes. We found that Activator Protein 1 (AP-1) factors and lymphoid lineage-defining TFs including RUNX3 and PU.1 predicted BAF-dependence. Strikingly, we found that cREs bearing the chromatin signature of "primed" enhancers - enriched for H3K4me1 but lacking H3K27ac - were significantly more sensitive to BAF inhibition than typical active enhancers. As primed enhancers are known to facilitate transcriptional responses to stimuli, we tested the requirement of BAF activity in these responses. Acute BAF inhibition was sufficient to prevent both chromatin and transcriptional responses to interferon gamma and dexamethasone. cREs which normally gained accessibility in response to stimulation failed to do so with BAF inhibition, and these cREs were linked to genes with suppressed transcriptional induction. ConclusionsCollectively, our results demonstrate a requirement for continuous BAF activity to enable stimulus response and suggest that defective signal responsiveness may be a pathogenic mechanism in disease states caused by loss-of-function mutations in BAF subunits.

The nuclear envelope (NE) mediates transport between nucleus and cytoplasm in eukaryotic cells and protects genetic materials against cytoplasmic enzymes. Nuclear pore complexes (NPCs) regulate chromosome architecture, genome integrity, gene transcription, and cell division. In Caenorhabditis elegans embryos, depletion of NPP-3/NUP205 causes NE rupture, premature chromosome condensation, and relocalization of condensed chromosomes to the nuclear periphery, similar to responses during anoxia and quiescence. This chromosomal relocalization depends on the spindle assembly checkpoint (SAC), inner kinetochore proteins, and partially on the NE rupture repair proteins BAF-1 and LEM-2. NPP-3 depletion prolongs prophase and prometaphase, as mediated by SAC proteins MDF-1 and MDF-2. Additionally, NPP-3 depletion alters MDF-1 localization, removing it from NE and increasing its nuclear accumulation, while reducing import of kinetochore components such as KNL-1, BUB-1, and HCP-1. In 20-30 cell-stage embryos, MDF-1 foci are observed on peripheral chromosomes during prophase. Both MDF-1 and MDF-2 accumulate on chromosomes during prometaphase. The increased incidence of lagging chromosomes, DNA damage, and micronuclei upon NPP-3 and MDF-1 depletion, suggesting that peripheral chromosome localization may serve as a protective mechanism against DNA damage. These findings shed light into cellular responses to NE rupture, with potential implications for laminopathies and cancers involving nuclear envelope defects.

The spindle midzone, an array of overlapping, antiparallel microtubules, contributes to chromosome segregation and cytokinesis. As cells exit mitosis, midzone microtubules reorganize to form the midbody, the location of cell abscission. The mechanisms governing microtubule dynamics during this transition remain incompletely understood. The microtubule depolymerase, Kif2a, has been shown to contribute to midzone microtubule length control (Uehara et al., 2013), but how the depolymerase is regulated is not understood. Since CAMSAPs govern minus-end microtubule dynamics, we examined their role in midzone microtubule behavior. CAMSAP2, the major CAMSAP in HeLa cells, localized to the minus-ends of midzone microtubules and cells depleted of CAMSAP2, showed similar phenotypes as cells depleted of Kif2a, including elongated and bent midzones and enlarged asters. Next, we localized Kif2a in CAMSAP2-depleted cells and vice versa. CAMSAP2 remained present and extended along elongated midzone microtubules in Kif2a-depleted cells. In contrast Kif2a localization was no longer present at microtubule minus-ends but retained at plus-ends in CAMSAP2-depleted cells. In long-term live-cell movies of CAMSAP2-depleted cells abscission at the midbody was not detected, although two daughter cells formed. Markers for abscission including ESCRT-III component CHMP2A and Spastin were mislocalized, and midzone overlap zones, marked by PRC1, were extended. Together, our results demonstrate that CAMSAP2 is essential for midzone microtubule organization and dynamics, ultimately impacting cell abscission.

In humans, germline mutation rates are three- to four-fold higher in males than females, for largely unknown reasons. We investigated whether transcription, a well-documented source of both DNA damage and repair in somatic tissues, is associated with sex differences in germline mutations. To this end, we used expression data from male and female germline cells and phased de novo germline mutations from pedigrees. Focusing on protein-coding genes, we found no relationship between the male mutation rate and gene expression levels in the fetal germline or in adult testis tissue, despite evidence for transcriptional asymmetry. Individual stages of spermatogenesis differ in their contribution to mutation, however: expression levels in spermatogonial stem cells are significantly positively associated with paternal mutation rates, while those in primary spermatocytes are significantly negatively associated. Thus, transcription may have varying effects over male gametogenesis that are not readily detected from its cumulative effect on the total germline mutation rate. In females, by contrast, mutation rates increase significantly with transcription levels in the fetal germline, adult oocytes and adult ovary tissues, consistent with widespread transcription asymmetry. We confirm the difference between the sexes by analyzing phased mutations from three-generation pedigrees and the lack of an association in males by analyzing paternal mutations from seminiferous tubules and sperm. Thus, transcription has distinct effects on the mutation rate in the two sexes, leading to an increase in mutations in females but not males, in contrast to what one might expect from the overall paternal bias in germline mutations.

A long non-coding RNA (lncRNA) known as the X-inactivation specific transcript (XIST) plays a central role in X chromosome inactivation - a transcriptional process that silences one of the two X chromosomes in females to ensure dosage compensation between males and females. Much research has been conducted on how XIST regulates X chromosome transcription critical to embryonic development, but recent studies suggest a non-canonical role for XIST in regulating cancer stem cells and cellular plasticity. As cell adhesion and adhesome genes are integral to the regulation of cancer stemness, we explored the previously unrecognized link between XIST and the adhesome network. By performing gene expression and gene ontology analysis on XIST-knockdown ovarian cancer cells, our study showed that XIST loss altered adhesome gene expression and downstream adhesion pathways. Using Genotype-Tissue Expression (GTEx) and The Cancer Genome Atlas (TCGA) datasets, we identified distinct correlations between XIST lncRNA and adhesome genes across normal and cancer tissue samples, which are associated with cell stemness. Furthermore, network analysis suggests that XIST may interact with specific adhesome genes within the cell nucleus. This interaction may have significant functional implications, as demonstrated by the hazard ratio analysis of XIST and adhesome gene expression in relation to clinical outcomes. Overall, our results show that among well-annotated functional lncRNAs, XIST appears to be a modulator strongly associated with the adhesome network and cell stemness. Our findings thus support a novel link between lncRNA-mediated epigenetic regulation of cell adhesion genes, highlighting XIST as a key regulator contributing to the adhesome network. Significance StatementThis study identified that XIST, a long non-coding RNA essential for X-chromosome dosage compensation and embryonic development, plays a significant role in modulating the adhesome network. We found that XIST knockdown affected adhesion pathways in ovarian cancer cells, whereas XIST expression is strongly correlated with adhesome gene expression across all tissues. We observed that the interaction between XIST and adhesome genes changes significantly between tumors and normal tissues, and this altered interaction is associated with certain cancer outcomes. These findings reveal a possible link between lncRNA-mediated regulation and adhesome control that is associated with cell stemness signatures and the emergence of cancerous tissues.

Transcription factor IIIC (TFIIIC) is a multi-subunit protein complex that recruits RNA polymerase III (Pol III) to the majority of its target genes. Evolutionarily conserved overlap between TFIIIC binding sites and the structural maintenance of chromosomes (SMC) complexes suggested a role for TFIIIC in SMC regulation and 3D organization of eukaryotic genomes; but the evidence has remained largely correlational due to the essential role of TFIIIC in RNA Pol III transcription. Here, we directly tested the function of TFIIIC in SMC complex regulation by using auxin inducible depletion in C. elegans. We performed Hi-C and ChIP-seq analyses upon acute depletion of TFTC-3, an essential TFIIIC subunit, and RPC-1, the catalytic subunit of RNA Pol III. Our results show that TFIIIC regulates the localization of two different types of SMC complexes, cohesin and condensin. TFIIIC is also required for increased 3D contacts between distant tRNA genes located on the same or different chromosomes. Depletion of individual SMC complexes did not significantly reduce the intrachromosomal tRNA gene contacts, suggesting redundancy or an independent mechanism mediating these contacts. Together, our study demonstrates an RNA Pol III independent function for TFIIIC, regulating binding of both cohesin and condensin, as well as the 3D organization of tRNA genes.

DNA repair in biochemical and genetic experimental systems permits a precise definition of enzyme requirements and mechanistic steps. Comparing these findings to repair events at naturally occurring damage sites in multicellular organisms is essential for confirming and expanding these insights into a physiologic context. However, heterogeneity in any normal cell population increases with each cell division, and the reliable detection of replication-independent DNA damage sites and their repair has been a major barrier. Here, we examine single human colon crypts, which harbor natural cell clones, using a novel whole-genome sequencing (WGS) method to identify complex insertion-deletion (indel) in the crypt stem cells. Analysis of complex indel events likely repaired by non-homologous end joining occurring in crypt stem cells permits inferences about the in vivo repair of naturally occurring DNA damage within physiologically-relevant chromatin in normal human cells.

Though whole-genome duplication (WGD) contributes to cancer progression, the mechanism of post-WGD cell proliferation remains unclear. Here, using 6-day live-imaging, we analyzed the proliferation dynamics of more than 150 post-WGD HCT116 cell lineages. A quantitative comparison of mitotic patterns and cell fates between proliferative and non-proliferative lineages revealed that multipolar chromosome segregation in early mitosis is a key factor limiting the proliferative capacity of post-WGD progenies. Multipolar chromosome segregation suppressed post-WGD cell viability, particularly when accompanied by drastic chromosome loss or when it repeatedly occurred. Tracing proliferative lineages elucidated that they proliferated mainly by imposing the risk of multipolar chromosome segregation on one of two sub-lineages that formed after the first bipolar division. Meanwhile, a considerable proportion of proliferative lineages consisted entirely of progeny of early multipolar chromosome segregation events. Our results highlight key cellular events that determine the proliferation dynamics and diversity of post-WGD progenies, providing a fundamental reference for understanding WGD-associated bioprocesses. Summary statementLive image tracing of >150 cell lineages reveals the cross-generation dynamics of multipolar chromosome segregation that determine the fates of post-whole-genome duplication progeny cells.

Meiotic prophase-I chromosomes are organized into linear arrays of chromatin loops anchored to proteinaceous axes that define the interaction interfaces for the pairing and synapsis of homologous chromosomes. Chromatin loop size and axial chromosome length are inversely correlated and vary widely both between and within species, including between the sexes. The molecular basis of this variation remains unclear. Here, we provide evidence that the small ubiquitin-like modifier, SUMO, regulates loop-axis organization in mouse meiosis. Our analysis shows that the longer axes of oocyte chromosomes contain more SUMO per unit length than the shorter axes of spermatocyte chromosomes. In mouse models, the loss of SUMO1 results in shorter axes and longer chromatin loops. Conversely, increased SUMO1 conjugation, caused by mutation of the SENP1 isopeptidase, produces longer axes with shorter loops. Axis length positively correlates with meiotic recombination. Accordingly, Sumo1 and Senp1 mutations respectively decrease and increase crossover frequency. These findings identify SUMO as a key regulator of meiotic chromosome architecture and suggest a molecular basis for the physiological variation in chromosome length and recombination rates seen among species, sexes, individuals, and individual meiocytes. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=101 SRC="FIGDIR/small/710713v1_ufig1.gif" ALT="Figure 1"> View larger version (31K): org.highwire.dtl.DTLVardef@145c465org.highwire.dtl.DTLVardef@160c8aborg.highwire.dtl.DTLVardef@1165b76org.highwire.dtl.DTLVardef@ced5e0_HPS_FORMAT_FIGEXP M_FIG C_FIG

Crossover recombination events during meiosis repair double-strand DNA breaks and ensure accurate chromosome segregation in most organisms. For many species, the genomic distribution of crossovers is nonrandom and sexually dimorphic. While many species evolved kilobase-scale "hotspots" for crossover formation, the Caenorhabditis elegans genome lacks hotspots, and crossovers are enriched across megabase-scale domains. Further, genetic and cytological studies indicate the crossover frequency in C. elegans spermatogenesis is higher relative to oogenesis in many but not all genetic intervals. To determine the genomic features that contribute to the sexually dimorphic recombination landscape in the absence of hot spots, we defined and analyzed the recombination landscape across the whole genome in C. elegans using whole-genome sequencing and high-resolution recombination mapping in single worms bearing recombinant chromosomes from individual sperm and oocytes. We find that the spatial distribution of crossovers is sexually dimorphic on chromosomes I, II, and III, and that the global rate of double-crossover events is 4.7-fold higher in spermatocytes. Additionally, we find that pairing and synapsis may contribute to the sexually dimorphic crossover landscape. In comparison to the spermatocyte crossover landscape, a higher proportion of oocyte crossovers are formed in the domains directly adjacent to the pairing centers of each chromosome. Further, reducing the genetic dosage of the synaptonemal complex central region protein SYP-2, which is a meiotic chromosome structural protein required for homologous chromosome synapsis, reshapes the oocyte crossover landscape to resemble observations in wild-type spermatocytes. Finally, we found that spermatocyte crossovers are partially enriched in H3K36me3-marked euchromatic regions, while many oocyte crossovers are enriched in H3K27me3-marked heterochromatic regions. Taken together, our studies reveal how synaptonemal complex component dosage and local chromatin states influence crossover placement and the sex-specific regulation of meiotic recombination. Author SummaryProduction of viable eggs and sperm depends on accurate chromosome segregation during meiosis. Segregation of parental copies of homologous chromosomes requires the reciprocal exchange and physical linkage of DNA that arises through crossover recombination. Increasing evidence indicates the existence of sexual dimorphisms during meiotic recombination. In this study, we generated and analyzed high-resolution recombination maps specific to spermatogenesis and oogenesis in the nematode C. elegans, which reveals sex-specific crossover distributions and a higher rate of crossing over in sperm cells. Further, we indicate how specific chromosomal features and structures differentially affect the crossover landscape in eggs versus sperm. Our work highlights how, in a system absent of pre-defined "hotspots" for recombination, local chromatin structures, chromosomal pairing domains, and the abundance of synaptonemal complex proteins are potential drivers for establishing the observable sex differences in crossover recombination.

Polycomb Repressive Complex 2 (PRC2) maintains repression of genes specific for other cell differentiation states. PRC2 binds RNA in vitro with a preference for G-rich sequences. UV-based crosslinking coupled with immunoprecipitation (CLIP) experiments have shown that PRC2 also binds RNA in cells. Recently, Guo et al reported that a stringent denaturing variant of CLIP called CLAP did not detect PRC2 RNA binding in cells. We present a reanalysis of CLAP data that supports direct interaction of PRC2 with RNA in cells. CLAP for Halo-tagged PRC2 subunits from mixed populations of human and mouse cells specifically enriched for RNA from the species in which the proteins were tagged. The lack of apparent PRC2 RNA binding in Guo and colleagues analysis stems from a scaling step that deflates enrichment scores for low-complexity CLAP samples. Our findings pave the way for studies seeking to determine the physiological roles of PRC2 RNA binding activity.

Proper connection between the sperm head and tail is critical for fertility and is mediated by the head-tail coupling apparatus (HTCA). Recent evidence suggests that the nuclear pore complex (NPC) may be important in male fertility, though a specific role at the HTCA has not been described. To investigate this, we performed a testis-specific RNAi screen targeting nucleoporins of the NPC. We identified Nup133 and Nup107 as regulators of HTCA development. We found that Nup133 and Nup107 were required to form the initial connection between the nucleus and centriole during HTCA establishment. We determined that failure to build the HTCA following Nup133 and Nup107 depletion was due to loss of nuclear envelope dynein/dynactin. Finally, we showed that loss of the NPC cytoplasmic filament component Nup358 results in the most severe centriole detachment phenotype, thus potentially functioning as the dynein anchor. Together, our data indicate that NPCs are critical regulators of early HTCA establishment and are required to recruit dynein to the nuclear envelope to bring the nucleus and centriole together during spermiogenesis.

Centromeres are epigenetically specified chromosomal sites that support kinetochore assembly and often embedded within large satellite DNA arrays. Recent telomere to telomere genome assemblies have revealed extensive variation in centromeric satellite arrays between chromosomes and between individuals, but the functional significance of this variation remains unclear. To determine how satellite array size influences centromere function, we generated hybrid mouse models in which homologous chromosomes with different array sizes are paired in meiosis I, creating array size asymmetry across each meiotic bivalent. When an extremely small array is paired with a moderate size array, we find that array size asymmetry leads to functional asymmetry in both centromere chromatin and interactions with spindle microtubules, lagging chromosomes in anaphase I, and increased aneuploidy in MII eggs. In contrast, pairing an extremely large array with a moderate array does not lead to functional centromere asymmetry. Together, these results suggest a threshold model in which centromere array size is tolerated across a broad range, but minimal arrays become functionally limiting when paired with larger arrays in meiosis.