Quiescent Cells Actively Replenish CENP-A Nucleosomes to Maintain Centromere Identity and Proliferative Potential

Molecular evidence for pre-chordate origins of ovarian cell types and neuroendocrine control of reproduction

Sexual reproduction in animals requires the development of oocytes, or egg cells. This process, termed oogenesis, requires complex interactions amongst germline and somatic cell types in the ovary. How did these cell types and their signaling interactions evolve? Here we use the sea star Patiria miniata as a non-chordate deuterostome representative to define the ovarian cell type toolkit in echinoderms. Sea stars continuously produce millions of new oocytes throughout their lifespan, making them a practical system to understand the mechanisms that drive oogenesis from a biomedical and evolutionary perspective. We performed scRNA-seq combined with high-resolution 3D-imaging to reveal the ovarian cell types and their spatial organization. Our data support the presence of actively dividing oogonial stem cells and granulosa-like and theca-like cells, which display similarities and possible homology with their mammalian counterparts. Lastly, our data support the existence of an endocrine signaling system between oogonial stem cells and intrinsic ovarian neurons with striking similarities to the vertebrate hypothalamic-pituitary-gonadal axis. Overall, this study provides molecular evidence supporting the possible pre-chordate origins of conserved ovarian cell types, and the presence of an intrinsic neuroendocrine system which potentially controls oogenesis and predates the formation of the hypothalamic-pituitary-gonadal axis in vertebrates.

Adaptive evolution of CENP-T modulates centromere binding

Centromeric DNA and proteins evolve rapidly despite conserved function in mediating kinetochore-microtubule attachments during cell division. This paradox is explained by selfish DNA sequences preferentially binding centromeric proteins to disrupt attachments and bias their segregation into the egg (drive) during female meiosis. Adaptive centromeric protein evolution is predicted to prevent preferential binding to these sequences and suppress drive. Here, we test this prediction by defining the impact of adaptive evolution of the DNA-binding histone fold domain of CENP-T, a major link between centromeric DNA and microtubules. We reversed adaptive changes by creating chimeric variants of mouse CENP-T with the histone fold domain from closely related species, expressed exogenously in mouse oocytes or in a transgenic mouse model. We show that adaptive evolution of mouse CENP-T reduced centromere binding, which supports robust female gametogenesis. However, this innovation is independent of the centromeric DNA sequence, as shown by comparing the binding of divergent CENP-T variants to distinct centromere satellite arrays in mouse oocytes and in somatic cells from other species. Overall, our findings support a model in which selfish sequences drive to fixation, disrupting attachments of all centromeres to the spindle. DNA sequence-specific innovations are not needed to mitigate fitness costs in this model, so centromeric proteins adapt by modulating their binding to all centromeres in the aftermath of drive.

Heat Stress Inhibits Pollen Development by Degrading mRNA Capping Enzyme ARCP1 and ARCP2

Pollen development and germination are critical for successful generation of offspring in plants, yet they are highly susceptible to heat stress (HS). However, the molecular mechanism underlying this process has not been fully elucidated. In this study, we highlight the essential roles of two mRNA capping enzymes, named Arabidopsis mRNA capping phosphatase (ARCP) 1 and 2, in regulating male and female gamete development. The transmission efficiencies of gametes carrying arcp1 arcp2 from arcp1+/- arcp2-/- and arcp1-/- arcp2+/- mutants are 30% and zero, respectively. These mutants exhibited a significant increase in misshaped pollen, with germination rates approximately half of those in wild type. ARCP1/2 exhibit RNA triphosphatase and RNA guanylyltransferase activities, which are required for proper pollen development. Through RNA-seq analysis, genes involved in pollen development/germination and HS response were identified as downregulated genes in pollen from arcp1+/- arcp2-/- mutant. Furthermore, ARCP2 protein is degraded under HS condition, and inducing the expression of ARCP2 can increase the pollen germination rate under elevated temperature. We propose that HS triggers the degradation of mRNA capping enzymes, which in turn disrupts the transcriptome that required for pollen development and pollen germination and ultimately leads to male sterility.

Starvation induces diffusion hindrance at the nanoscale in mammalian cells

Prolonged starvation leads to acute stress, inducing a state of cellular dormancy with reduced energy consumption. Our research reveals that nutrient deprivation halts the movement of large ribosomal subunits, trapping them in a gel-like structure within the cytoplasm of surviving cells. This effect is due to water efflux from cells, causing a decrease in cell volume to half the original volume. This simple physical strategy saves, in a dormant state 107 ATP per second, which is needed for normal protein production. We monitored the diffusion of GFP (radius 2.3 nm) and 40S and 60S ribosomes (radii 3.75 and 15 nm, respectively) in the cytoplasm and nucleus during starvation of HeLa cells. GFP and 40S ribosomes slowed their diffusion in the cytoplasm two and five times, respectively. 60S ribosomes exhibited only rotational diffusion. In non-starving cells, biomolecules get stuck in the gel structure of cytoplasm with sizes >100 nm. We show that the gel pore size decreased from 100 nm to 30 nm upon starvation. The diffusive transport in the cell nucleus didn't change during starvation. GFP and ribosomes had the same diffusion coefficients in non-starving and starving cells in the nucleus. This highlights the importance of nuclear transport in cancer cells during extreme stress conditions.

VitelloTag: a tool for high-throughput cargo delivery into oocytes

Delivering molecular tools into oocytes is essential for developmental and reproductive biology. Microinjection, the conventional method, is equipment intensive, often technically challenging and has a low yield, and is impractical in species with delicate oocytes or restricted spawning seasons. To overcome these limitations, we developed VitelloTag, a cost-effective, high-throughput system using vitellogenin-derived fusion proteins to enable efficient cargo delivery via receptor-mediated endocytosis. We demonstrate its utility by delivering Cas9/sgRNA complexes in two distantly related species for gene knockout.

From rest to repair: Safeguarding genomic integrity in quiescent cells

Quiescence is an important non-pathological state in which cells pause cell cycle progression temporarily, sometimes for decades, until they receive appropriate proliferative stimuli. Quiescent cells make up a significant proportion of the body, and maintaining genomic integrity during quiescence is crucial for tissue structure and function. While cells in quiescence are spared from DNA damage associated with DNA replication or mitosis, they are still exposed to various sources of endogenous DNA damage, including those induced by normal transcription and metabolism. As such, it is vital that cells retain their capacity to effectively repair lesions that may occur and return to the cell cycle without losing their cellular properties. Notably, while DNA repair pathways are often found to be downregulated in quiescent cells, emerging evidence suggests the presence of active or differentially regulated repair mechanisms. This review aims to provide a current understanding of DNA repair processes during quiescence in mammalian systems and sheds light on the potential pathological consequences of inefficient or inaccurate repair in quiescent cells.

Canonical and noncanonical regulators of centromere assembly and maintenance

Centromeres are specialized chromosomal domains where the kinetochores assemble during cell division to ensure accurate transmission of the genetic information to the two daughter cells. The centromeric function is evolutionary conserved and, in most organisms, centromeres are epigenetically defined by a unique chromatin containing the histone H3 variant CENP-A. The canonical regulators of CENP-A assembly and maintenance are well-known, yet some of the molecular mechanisms regulating this complex process have only recently been unveiled. We review the most recent advances on the topic, including the emergence of new and unexpected factors that favor and regulate CENP-A assembly and/or maintenance.

Topoisomerase I is an evolutionarily conserved key regulator for satellite DNA transcription

RNA Polymerase (RNAP) II transcription on non-coding repetitive satellite DNAs plays an important role in chromosome segregation, but a little is known about the regulation of satellite transcription. We here show that Topoisomerase I (TopI), not TopII, promotes the transcription of α-satellite DNAs, the main type of satellite DNAs on human centromeres. Mechanistically, TopI localizes to centromeres, binds RNAP II and facilitates RNAP II elongation. Interestingly, in response to DNA double-stranded breaks (DSBs), α-satellite transcription is dramatically stimulated in a DNA damage checkpoint-independent but TopI-dependent manner, and these DSB-induced α-satellite RNAs form into strong speckles in the nucleus. Remarkably, TopI-dependent satellite transcription also exists in mouse 3T3 and Drosophila S2 cells and in Drosophila larval imaginal wing discs and tumor tissues. Altogether, our findings herein reveal an evolutionally conserved mechanism with TopI as a key player for the regulation of satellite transcription at both cellular and animal levels.

Topoisomerase I is an Evolutionarily Conserved Key Regulator for Satellite DNA Transcription

Repetitive satellite DNAs, divergent in nucleic-acid sequence and size across eukaryotes, provide a physical site for centromere assembly to orchestrate chromosome segregation during the cell cycle. These non-coding DNAs are transcribed by RNA polymerase (RNAP) II and the transcription has been shown to play a role in chromosome segregation, but a little is known about the regulation of centromeric transcription, especially in higher organisms with tandemly-repeated-DNA-sequence centromeres. Using RNA interference knockdown, chemical inhibition and AID/IAA degradation, we show that Topoisomerase I (TopI), not TopII, promotes the transcription of α-satellite DNAs, the main type of satellite on centromeres in human cells. Mechanistically, TopI localizes to centromeres, binds RNAP II and facilitates RNAP II elongation on centromeres. Interestingly, in response to DNA double-stranded breaks (DSBs) induced by chemotherapy drugs or CRSPR/Cas9, α-satellite transcription is dramatically stimulated in a DNA damage checkpoint-independent but TopI-dependent manner. These DSB-induced α-satellite RNAs were predominantly derived from the α-satellite high-order repeats of human centromeres and forms into strong speckles in the nucleus. Remarkably, TopI-dependent satellite transcription also exists in mouse 3T3 and Drosophila S2 cells and in Drosophila larval imaginal wing discs and tumor tissues. Altogether, our findings herein reveal an evolutionally conserved mechanism with TopI as a key player for the regulation of satellite transcription at both cellular and animal levels.

Heat stress impairs centromere structure and segregation of meiotic chromosomes in Arabidopsis

Heat stress is a major threat to global crop production, and understanding its impact on plant fertility is crucial for developing climate-resilient crops. Despite the known negative effects of heat stress on plant reproduction, the underlying molecular mechanisms remain poorly understood. Here, we investigated the impact of elevated temperature on centromere structure and chromosome segregation during meiosis in Arabidopsis thaliana. Consistent with previous studies, heat stress leads to a decline in fertility and micronuclei formation in pollen mother cells. Our results reveal that elevated temperature causes a decrease in the amount of centromeric histone and the kinetochore protein BMF1 at meiotic centromeres with increasing temperature. Furthermore, we show that heat stress increases the duration of meiotic divisions and prolongs the activity of the spindle assembly checkpoint during meiosis I, indicating an impaired efficiency of the kinetochore attachments to spindle microtubules. Our analysis of mutants with reduced levels of centromeric histone suggests that weakened centromeres sensitize plants to elevated temperature, resulting in meiotic defects and reduced fertility even at moderate temperatures. These results indicate that the structure and functionality of meiotic centromeres in Arabidopsis are highly sensitive to heat stress, and suggest that centromeres and kinetochores may represent a critical bottleneck in plant adaptation to increasing temperatures.

DNA strand breaks at centromeres: Friend or foe?

Centromeres are large structural regions in the genomic DNA, which are essential for accurately transmitting a complete set of chromosomes to daughter cells during cell division. In humans, centromeres consist of highly repetitive α-satellite DNA sequences and unique epigenetic components, forming large proteinaceous structures required for chromosome segregation. Despite their biological importance, there is a growing body of evidence for centromere breakage across the cell cycle, including periods of quiescence. In this review, we provide an up-to-date examination of the distinct centromere environments at different stages of the cell cycle, highlighting their plausible contribution to centromere breakage. Additionally, we explore the implications of these breaks on centromere function, both in terms of negative consequences and potential positive effects.

A dynamic population of prophase CENP-C is required for meiotic chromosome segregation

The centromere is an epigenetic mark that is a loading site for the kinetochore during meiosis and mitosis. This mark is characterized by the H3 variant CENP-A, known as CID in Drosophila. In Drosophila, CENP-C is critical for maintaining CID at the centromeres and directly recruits outer kinetochore proteins after nuclear envelope break down. These two functions, however, happen at different times in the cell cycle. Furthermore, in Drosophila and many other metazoan oocytes, centromere maintenance and kinetochore assembly are separated by an extended prophase. We have investigated the dynamics of function of CENP-C during the extended meiotic prophase of Drosophila oocytes and found that maintaining high levels of CENP-C for metaphase I requires expression during prophase. In contrast, CID is relatively stable and does not need to be expressed during prophase to remain at high levels in metaphase I of meiosis. Expression of CID during prophase can even be deleterious, causing ectopic localization to non-centromeric chromatin, abnormal meiosis and sterility. CENP-C prophase loading is required for multiple meiotic functions. In early meiotic prophase, CENP-C loading is required for sister centromere cohesion and centromere clustering. In late meiotic prophase, CENP-C loading is required to recruit kinetochore proteins. CENP-C is one of the few proteins identified in which expression during prophase is required for meiotic chromosome segregation. An implication of these results is that the failure to maintain recruitment of CENP-C during the extended prophase in oocytes would result in chromosome segregation errors in oocytes.

Centromere-specifying nucleosomes persist in aging mouse oocytes in the absence of nascent assembly

Centromeres direct genetic inheritance but are not themselves genetically encoded. Instead, centromeres are defined epigenetically by the presence of a histone H3 variant, CENP-A.1 In cultured somatic cells, an established paradigm of cell-cycle-coupled propagation maintains centromere identity: CENP-A is partitioned between sisters during replication and replenished by new assembly, which is restricted to G1. The mammalian female germ line challenges this model because of the cell-cycle arrest between pre-meiotic S phase and the subsequent G1, which can last for the entire reproductive lifespan (months to decades). New CENP-A chromatin assembly maintains centromeres during prophase I in worm and starfish oocytes,2,3 suggesting that a similar process may be required for centromere inheritance in mammals. To test this hypothesis, we developed an oocyte-specific conditional knockout (cKO) mouse for Mis18α, an essential component of the assembly machinery. We find that embryos derived from Mis18α knockout oocytes fail to assemble CENP-A nucleosomes prior to zygotic genome activation (ZGA), validating the knockout model. We show that deletion of Mis18α in the female germ line at the time of birth has no impact on centromeric CENP-A nucleosome abundance, even after 6-8 months of aging. In addition, there is no detectable detriment to fertility. Thus, centromere chromatin is maintained long-term, independent of new assembly during the extended prophase I arrest in mouse oocytes.

A Dynamic population of prophase CENP-C is required for meiotic chromosome segregation

The centromere is an epigenetic mark that is a loading site for the kinetochore during meiosis and mitosis. This mark is characterized by the H3 variant CENP-A, known as CID in Drosophila. In Drosophila, CENP-C is critical for maintaining CID at the centromeres and directly recruits outer kinetochore proteins after nuclear envelope break down. It is not known, however, if these two functions require the same CENP-C molecules. Furthermore, in Drosophila and many other metazoan oocytes, centromere maintenance and kinetochore assembly are separated by an extended prophase. Consistent with studies in mammals, CID is relatively stable and does not need to be expressed during prophase to remain at high levels in metaphase I of meiosis. Expression of CID during prophase can even be deleterious, causing ectopic localization to non-centromeric chromatin, abnormal meiosis and sterility. In contrast to CID, maintaining high levels of CENP-C requires expression during prophase. Confirming the importance of this loading, we found CENP-C prophase loading is required for multiple meiotic functions. In early meiotic prophase, CENP-C loading is required for sister centromere cohesion and centromere clustering. In late meiotic prophase, CENP-C loading is required to recruit kinetochore proteins. CENP-C is one of the few proteins identified in which expression during prophase is required for meiotic chromosome segregation. An implication of these results is that the failure to maintain recruitment of CENP-C during the extended prophase in oocytes would result in chromosome segregation errors in oocytes.

Centromere-specifying nucleosomes persist in aging mouse oocytes in the absence of nascent assembly

Centromeres direct genetic inheritance but are not themselves genetically encoded. Instead, centromeres are defined epigenetically by the presence of a histone H3 variant, CENP-A 1 . In cultured somatic cells, an established paradigm of cell cycle-coupled propagation maintains centromere identity: CENP-A is partitioned between sisters during replication and replenished by new assembly, which is restricted to G1. The mammalian female germline challenges this model because of the cell cycle arrest between pre-meiotic S-phase and the subsequent G1, which can last for the entire reproductive lifespan (months to decades). New CENP-A chromatin assembly maintains centromeres during prophase I in worm and starfish oocyte 2,3 , suggesting that a similar process may be required for centromere inheritance in mammals. However, we show that centromere chromatin is maintained long-term independent of new assembly during the extended prophase I arrest in mouse oocytes. Conditional knockout of Mis18α, an essential component of the assembly machinery, in the female germline at the time of birth has almost no impact on centromeric CENP-A nucleosome abundance nor any detectable detriment to fertility.

p97/VCP drives turnover of SUMOylated centromeric CCAN proteins and CENP-A

The centromere is a unique chromatin domain that links sister chromatids and forms the attachment site for spindle microtubules in mitosis. Centromere inheritance is largely DNA sequence-independent but strongly reliant on a self-propagating chromatin domain featuring nucleosomes containing the H3 variant CENP-A. Unlike other histones, CENP-A is maintained with unusually high stability in chromatin. Previously, we have shown that mitotic maintenance of CENP-A and other constitutive centromere-associated network (CCAN) proteins is controlled by a dynamic SUMO cycle and that the deSUMOylase SENP6 is necessary for stable maintenance of CENP-A at the centromere. Here, we discover that the removal of SENP6 leads to a rapid loss of the CCAN, followed by a delayed loss of centromeric CENP-A, indicating that the CCAN is the primary SUMO target. We found that the ATP-dependent segregase p97/VCP removes centromeric CENP-A in a SUMO-dependent manner and interacts physically with the CCAN and CENP-A chromatin. Our data suggest a direct role of p97 in removing centromeric CENP-A via SUMOylated CCAN proteins, thereby ensuring centromere homeostasis and potentially preventing ectopic CENP-A accumulation.

SUMO control of centromere homeostasis

Centromeres are unique chromosomal loci that form the anchorage point for the mitotic spindle during mitosis and meiosis. Their position and function are specified by a unique chromatin domain featuring the histone H3 variant CENP-A. While typically formed on centromeric satellite arrays, CENP-A nucleosomes are maintained and assembled by a strong self-templated feedback mechanism that can propagate centromeres even at non-canonical sites. Central to the epigenetic chromatin-based transmission of centromeres is the stable inheritance of CENP-A nucleosomes. While long-lived at centromeres, CENP-A can turn over rapidly at non-centromeric sites and even erode from centromeres in non-dividing cells. Recently, SUMO modification of the centromere complex has come to the forefront as a mediator of centromere complex stability, including CENP-A chromatin. We review evidence from different models and discuss the emerging view that limited SUMOylation appears to play a constructive role in centromere complex formation, while polySUMOylation drives complex turnover. The deSUMOylase SENP6/Ulp2 and the proteins segregase p97/Cdc48 constitute the dominant opposing forces that balance CENP-A chromatin stability. This balance may be key to ensuring proper kinetochore strength at the centromere while preventing ectopic centromere formation.

Nucleoli and the nucleoli-centromere association are dynamic during normal development and in cancer

Centromeres are known to cluster around nucleoli in Drosophila and mammalian cells, but the significance of the nucleoli-centromere interaction remains underexplored. To determine whether the interaction is dynamic under different physiological and pathological conditions, we examined nucleolar structure and centromeres at various differentiation stages using cell culture models and the results showed dynamic changes in nucleolar characteristics and nucleoli-centromere interactions through differentiation and in cancer cells. Embryonic stem cells usually have a single large nucleolus, which is clustered with a high percentage of centromeres. As cells differentiate into intermediate states, the nucleolar number increases and the centromere association decreases. In terminally differentiated cells, including myotubes, neurons, and keratinocytes, the number of nucleoli and their association with centromeres are at the lowest. Cancer cells demonstrate the pattern of nucleoli number and nucleoli-centromere association that is akin to proliferative cell types, suggesting that nucleolar reorganization and changes in nucleoli-centromere interactions may play a role in facilitating malignant transformation. This idea is supported in a case of pediatric rhabdomyosarcoma, in which induced differentiation reduces the nucleolar number and centromere association. These findings suggest active roles of nucleolar structure in centromere function and genome organization critical for cellular function in both normal development and cancer.

Centromeres as universal hotspots of DNA breakage, driving RAD51-mediated recombination during quiescence

Centromeres are essential for chromosome segregation in most animals and plants yet are among the most rapidly evolving genome elements. The mechanisms underlying this paradoxical phenomenon remain enigmatic. Here, we report that human centromeres innately harbor a striking enrichment of DNA breaks within functionally active centromere regions. Establishing a single-cell imaging strategy that enables comparative assessment of DNA breaks at repetitive regions, we show that centromeric DNA breaks are induced not only during active cellular proliferation but also de novo during quiescence. Markedly, centromere DNA breaks in quiescent cells are resolved enzymatically by the evolutionarily conserved RAD51 recombinase, which in turn safeguards the specification of functional centromeres. This study highlights the innate fragility of centromeres, which may have been co-opted over time to reinforce centromere specification while driving rapid evolution. The findings also provide insights into how fragile centromeres are likely to contribute to human disease.

Acute injury to the mouse carotid artery provokes a distinct healing response

Treatment of vascular stenosis with angioplasty results in acute vascular damage, which may lead to restenosis. Owing to the highly complex cellularity of blood vessels, the healing response following this damage is incompletely understood. To gain further insight into this process, scRNA-seq of mouse carotid tissue after wire injury was performed. Stages of acute inflammation, resolution and remodeling were recapitulated in these data. To identify cell types which give rise to neointima, analyses focused on smooth muscle cell and fibroblast populations, and included data integration with scRNA-seq data from myocardial infarction and atherosclerosis datasets. Following carotid injury, a subpopulation of smooth muscle cells which also arises during atherosclerosis and myocardial infarction was identified. So-called stem cell/endothelial cell/monocyte (SEM) cells are candidates for repopulating injured vessels, and were amongst the most proliferative cell clusters following wire-injury of the carotid artery. Importantly, SEM cells exhibit specific transcriptional profiles which could be therapeutically targeted. SEM cell gene expression patterns could also be detected in bulk RNA-sequencing of neointimal tissue isolated from injured carotid vessels by laser capture microdissection. These data indicate that phenotypic plasticity of smooth muscle cells is highly important to the progression of lumen loss following acute carotid injury. Interference with SEM cell formation could be an innovative approach to combat development of restenosis.

Macroautophagy in quiescent and senescent cells: a pathway to longevity?

Cellular quiescence - reversible exit from the cell cycle - is an important feature of many cell types important for organismal health. Aging and cellular dysfunction compromise the survival and reactivation of quiescent cells over time. Studies suggest that autophagic processes and lysosomal function are critical to maintaining the function of quiescent cells, especially adult stem cells, throughout life. Findings also point to both pro-senescence and anti-senescence functions for macroautophagy depending on context. In this review, we will discuss these findings, unanswered questions on the role of macroautophagy and lysosomal function in quiescent and senescent cells, and the possibility for interventions that stimulate macroautophagy and lysosomes to promote quiescent cell function and tissue regeneration.

Life in lockdown: Orchestrating endoplasmic reticulum and lysosome homeostasis for quiescent cells

Cellular quiescence-reversible exit from the cell cycle-is an important feature of many cell types important for organismal health. Quiescent cells activate protective mechanisms that allow their persistence in the absence of growth and division for long periods of time. Aging and cellular dysfunction compromise the survival and re-activation of quiescent cells over time. Counteracting this decline are two interconnected organelles that lie at opposite ends of the secretory pathway: the endoplasmic reticulum and lysosomes. In this review, we highlight recent studies exploring the roles of these two organelles in quiescent cells from diverse contexts and speculate on potential other roles they may play, such as through organelle contact sites. Finally, we discuss emerging models of cellular quiescence, utilizing new cell culture systems and model organisms, that are suited to the mechanistic investigation of the functions of these organelles in quiescent cells.

The ins and outs of CENP-A: Chromatin dynamics of the centromere-specific histone

Centromeres are highly specialised chromosome domains defined by the presence of an epigenetic mark, the specific histone H3 variant called CENP-A (centromere protein A). They constitute the genomic regions on which kinetochores form and when defective cause segregation defects that can lead to aneuploidy and cancer. Here, we discuss how CENP-A is established and maintained to propagate centromere identity while subjected to dynamic chromatin remodelling during essential cellular processes like DNA repair, replication, and transcription. We highlight parallels and identify conserved mechanisms between different model organism with a particular focus on 1) the establishment of CENP-A at centromeres, 2) CENP-A maintenance during transcription and replication, and 3) the mechanisms that help preventing CENP-A localization at non-centromeric sites. We then give examples of how timely loading of new CENP-A to the centromere, maintenance of old CENP-A during S-phase and transcription, and removal of CENP-A at non-centromeric sites are coordinated and controlled by an intricate network of factors whose identity is slowly being unravelled.

Incorporation of CENP-A/CID into centromeres during early Drosophila embryogenesis does not require RNA polymerase II-mediated transcription

In many species, centromere identity is specified epigenetically by special nucleosomes containing a centromere-specific histone H3 variant, designated as CENP-A in humans and CID in Drosophila melanogaster. After partitioning of centromere-specific nucleosomes onto newly replicated sister centromeres, loading of additional CENP-A/CID into centromeric chromatin is required for centromere maintenance in proliferating cells. Analyses with cultured cells have indicated that transcription of centromeric DNA by RNA polymerase II is required for deposition of new CID into centromere chromatin. However, a dependence of centromeric CID loading on transcription is difficult to reconcile with the notion that the initial embryonic stages appear to proceed in the absence of transcription in Drosophila, as also in many other animal species. To address the role of RNA polymerase II–mediated transcription for CID loading in early Drosophila embryos, we have quantified the effects of alpha-amanitin and triptolide on centromeric CID-EGFP levels. Our analyses demonstrate that microinjection of these two potent inhibitors of RNA polymerase II–mediated transcription has at most a marginal effect on centromeric CID deposition during progression through the early embryonic cleavage cycles. Thus, we conclude that at least during early Drosophila embryogenesis, incorporation of CID into centromeres does not depend on RNA polymerase II–mediated transcription.

Use of Echinoderm Gametes and Early Embryos for Studying Meiosis and Mitosis

The early embryos of sea urchins and other echinoderms have served as experimental models for the study of cell division since the nineteenth century. Their rapid development, optical clarity, and ease of manipulation continue to offer advantages for studying spindle assembly and cytokinesis. In the absence of transgenic lines, alternative strategies must be employed to visualize microtubules and actin. Here, we describe methods to visualize actin and microtubule using either purified, recombinant proteins, or probes in in vitro-transcribed mRNAs.

Polarized Dishevelled dissolution and reassembly drives embryonic axis specification in sea star oocytes

The organismal body axes that are formed during embryogenesis are intimately linked to intrinsic asymmetries established at the cellular scale in oocytes.1 However, the mechanisms that generate cellular asymmetries within the oocyte and then transduce that polarity to organismal scale body axes are poorly understood outside of select model organisms. Here, we report an axis-defining event in meiotic oocytes of the sea star Patiria miniata. Dishevelled (Dvl) is a cytoplasmic Wnt pathway effector required for axis development in diverse species,2-4 but the mechanisms governing its function and distribution remain poorly defined. Using time-lapse imaging, we find that Dvl localizes uniformly to puncta throughout the cell cortex in Prophase I-arrested oocytes but becomes enriched at the vegetal pole following meiotic resumption through a dissolution-reassembly mechanism. This process is driven by an initial disassembly phase of Dvl puncta, followed by selective reformation of Dvl assemblies at the vegetal pole. Rather than being driven by Wnt signaling, this localization behavior is coupled to meiotic cell cycle progression and influenced by Lamp1+ endosome association and Frizzled receptors pre-localized within the oocyte cortex. Our results reveal a cell cycle-linked mechanism by which maternal cellular polarity is transduced to the embryo through spatially regulated Dvl dynamics.

An emerging role of transcription in chromosome segregation: Ongoing centromeric transcription maintains centromeric cohesion

Non-coding centromeres, which dictate kinetochore formation for proper chromosome segregation, are extremely divergent in DNA sequences across species but are under active transcription carried out by RNA polymerase (RNAP) II. The RNAP II-mediated centromeric transcription has been shown to facilitate the deposition of the centromere protein A (CENP-A) to centromeres, establishing a conserved and critical role of centromeric transcription in centromere maintenance. Our recent work revealed another role of centromeric transcription in chromosome segregation: maintaining centromeric cohesion during mitosis. Interestingly, this role appears to be fulfilled through ongoing centromeric transcription rather than centromeric transcripts. In addition, we found that centromeric transcription may not require some of the traditional transcription initiation factors, suggestive of "uniqueness" in its regulation. In this review, we discuss the novel role and regulation of centromeric transcription as well as the potential underlying mechanisms.

Loss of centromeric RNA activates the spindle assembly checkpoint in mammalian female meiosis I

The repetitive sequences of DNA centromeric regions form the structural basis for kinetochore assembly. Recently they were found to be transcriptionally active in mitosis, with their RNAs providing noncoding functions. Here we explore the role, in mouse oocytes, of transcripts generated from within the minor satellite repeats. Depletion of minor satellite transcripts delayed progression through meiosis I by activation of the spindle assembly checkpoint. Arrested oocytes had poorly congressed chromosomes, and centromeres were frequently split by microtubules. Thus, we have demonstrated that the centromeric RNA plays a specific role in female meiosis I compared with mitosis and is required for maintaining the structural integrity of centromeres. This may contribute to the high aneuploidy rates observed in female meiosis.

Separase cleaves the kinetochore protein Meikin at the meiosis I/II transition

To generate haploid gametes, germ cells undergo two consecutive meiotic divisions requiring key changes to the cell division machinery. Here, we demonstrate that the protease separase rewires key cell division processes at the meiosis I/II transition by cleaving the meiosis-specific protein Meikin. Separase proteolysis does not inactivate Meikin but instead alters its function to create a distinct activity state. Full-length Meikin and the C-terminal Meikin separase cleavage product both localize to kinetochores, bind to Plk1 kinase, and promote Rec8 cleavage, but our results reveal distinct roles for these proteins in controlling meiosis. Mutations that prevent Meikin cleavage or that conditionally inactivate Meikin at anaphase I result in defective meiosis II chromosome alignment in mouse oocytes. Finally, as oocytes exit meiosis, C-Meikin is eliminated by APC/C-mediated degradation prior to the first mitotic division. Thus, multiple regulatory events irreversibly modulate Meikin activity during successive meiotic divisions to rewire the cell division machinery at two distinct transitions.

Selective dephosphorylation by PP2A-B55 directs the meiosis I-meiosis II transition in oocytes

Meiosis is a specialized cell cycle that requires sequential changes to the cell division machinery to facilitate changing functions. To define the mechanisms that enable the oocyte-to-embryo transition, we performed time-course proteomics in synchronized sea star oocytes from prophase I through the first embryonic cleavage. Although we found that protein levels were broadly stable, our analysis reveals that dynamic waves of phosphorylation underlie each meiotic stage. We found that the phosphatase PP2A-B55 is reactivated at the meiosis I/meiosis II (MI/MII) transition, resulting in the preferential dephosphorylation of threonine residues. Selective dephosphorylation is critical for directing the MI/MII transition as altering PP2A-B55 substrate preferences disrupts key cell cycle events after MI. In addition, threonine to serine substitution of a conserved phosphorylation site in the substrate INCENP prevents its relocalization at anaphase I. Thus, through its inherent phospho-threonine preference, PP2A-B55 imposes specific phosphoregulated behaviors that distinguish the two meiotic divisions.

DNA methylation and histone variants in aging and cancer

Aging-related diseases such as cancer can be traced to the accumulation of molecular disorder including increased DNA mutations and epigenetic drift. We provide a comprehensive review of recent results in mice and humans on modifications of DNA methylation and histone variants during aging and in cancer. Accumulated errors in DNA methylation maintenance lead to global decreases in DNA methylation with relaxed repression of repeated DNA and focal hypermethylation blocking the expression of tumor suppressor genes. Epigenetic clocks based on quantifying levels of DNA methylation at specific genomic sites is proving to be a valuable metric for estimating the biological age of individuals. Histone variants have specialized functions in transcriptional regulation and genome stability. Their concentration tends to increase in aged post-mitotic chromatin, but their effects in cancer are mainly determined by their specialized functions. Our increased understanding of epigenetic regulation and their modifications during aging has motivated interventions to delay or reverse epigenetic modifications using the epigenetic clocks as a rapid readout for efficacity. Similarly, the knowledge of epigenetic modifications in cancer is suggesting new approaches to target these modifications for cancer therapy.

Transgenerational inheritance of centromere identity requires the CENP-A N-terminal tail in the C. elegans maternal germ line

Centromere protein A (CENP-A) is a histone H3 variant that defines centromeric chromatin and is essential for centromere function. In most eukaryotes, CENP-A-containing chromatin is epigenetically maintained, and centromere identity is inherited from one cell cycle to the next. In the germ line of the holocentric nematode Caenorhabditis elegans, this inheritance cycle is disrupted. CENP-A is removed at the mitosis-to-meiosis transition and is reestablished on chromatin during diplotene of meiosis I. Here, we show that the N-terminal tail of CENP-A is required for the de novo establishment of centromeres, but then its presence becomes dispensable for centromere maintenance during development. Worms homozygous for a CENP-A tail deletion maintain functional centromeres during development but give rise to inviable offspring because they fail to reestablish centromeres in the maternal germ line. We identify the N-terminal tail of CENP-A as a critical domain for the interaction with the conserved kinetochore protein KNL-2 and argue that this interaction plays an important role in setting centromere identity in the germ line. We conclude that centromere establishment and maintenance are functionally distinct in C. elegans.

Centromeric transcription maintains centromeric cohesion in human cells

Centromeric transcription has been shown to play an important role in centromere functions. However, lack of approaches to specifically manipulate centromeric transcription calls into question that the proposed functions are a direct consequence of centromeric transcription. By monitoring nascent RNAs, we found that several transcriptional inhibitors exhibited distinct, even opposing, efficacies on the suppression of ongoing gene and centromeric transcription in human cells, whereas under the same conditions, total centromeric RNAs were changed to a lesser extent. The inhibitor suppressing ongoing centromeric transcription weakened centromeric cohesion, whereas the inhibitor increasing ongoing centromeric transcription strengthened centromeric cohesion. Furthermore, expression of CENP-B DNA-binding domain or CENP-B knockdown moderately increased centromeric transcription without altering gene transcription; as a result, centromeric cohesion was accordingly strengthened. Targeting of the Kox1-KRAB domain with CENP-B DB to centromeres specifically decreased centromeric transcription and weakened centromeric cohesion. Thus, based on these findings, we propose that a major function of centromeric transcription is to maintain centromeric cohesion in human cells.

Gametic specialization of centromeric histone paralogs in Drosophila virilis

In most eukaryotes, centromeric histone (CenH3) proteins mediate mitosis and meiosis and ensure epigenetic inheritance of centromere identity. We hypothesized that disparate chromatin environments in soma versus germline might impose divergent functional requirements on single CenH3 genes, which could be ameliorated by gene duplications and subsequent specialization. Here, we analyzed the cytological localization of two recently identified CenH3 paralogs, Cid1 and Cid5, in Drosophila virilis using specific antibodies and epitope-tagged transgenic strains. We find that only ancestral Cid1 is present in somatic cells, whereas both Cid1 and Cid5 are expressed in testes and ovaries. However, Cid1 is lost in male meiosis but retained throughout oogenesis, whereas Cid5 is lost during female meiosis but retained in mature sperm. Following fertilization, only Cid1 is detectable in the early embryo, suggesting that maternally deposited Cid1 is rapidly loaded onto paternal centromeres during the protamine-to-histone transition. Our studies reveal mutually exclusive gametic specialization of divergent CenH3 paralogs. Duplication and divergence might allow essential centromeric genes to resolve an intralocus conflict between maternal and paternal centromeric requirements in many animal species.

Reduce, Retain, Recycle: Mechanisms for Promoting Histone Protein Degradation versus Stability and Retention

The eukaryotic genome is packaged into chromatin. The nucleosome, the basic unit of chromatin, is composed of DNA coiled around a histone octamer. Histones are among the longest-lived protein species in mammalian cells due to their thermodynamic stability and their associations with DNA and histone chaperones. Histone metabolism plays an integral role in homeostasis. While histones are largely stable, the degradation of histone proteins is necessary under specific conditions. Here, we review the physiological and cellular contexts that promote histone degradation. We describe specific known mechanisms that drive histone proteolysis. Finally, we discuss the importance of histone degradation and regulation of histone supply for organismal and cellular fitness.

Single human oocyte transcriptome analysis reveals distinct maturation stage-dependent pathways impacted by age

Female fertility is inversely correlated with maternal age due to a depletion of the oocyte pool and a reduction in oocyte developmental competence. Few studies have addressed the effect of maternal age on the human mature oocyte (MII) transcriptome, which is established during oocyte growth and maturation, however, the pathways involved remain unclear. Here, we characterize and compare the transcriptomes of a large cohort of fully grown germinal vesicle stage (GV) and in vitro matured (IVM‐MII) oocytes from women of varying reproductive age. First, we identified two clusters of cells reflecting the oocyte maturation stage (GV and IVM‐MII) with 4445 and 324 putative marker genes, respectively. Furthermore, we identified genes for which transcript representation either progressively increased or decreased with age. Our results indicate that the transcriptome is more affected by age in IVM‐MII oocytes (1219 genes) than in GV oocytes (596 genes). In particular, we found that transcripts of genes involved in chromosome segregation and RNA splicing significantly increased representation with age, while genes related to mitochondrial activity showed a lower representation. Gene regulatory network analysis facilitated the identification of potential upstream master regulators of the genes involved in those biological functions. Our analysis suggests that advanced maternal age does not globally affect the oocyte transcriptome at GV or IVM‐MII stages. Nonetheless, hundreds of genes displayed altered transcript representation, particularly in IVM‐MII oocytes, which might contribute to the age‐related quality decline in human oocytes.

Cell Fate Decisions in the Wake of Histone H3 Deposition

An expanding repertoire of histone variants and specialized histone chaperone partners showcases the versatility of nucleosome assembly during different cellular processes. Recent research has suggested an integral role of nucleosome assembly pathways in both maintaining cell identity and influencing cell fate decisions during development and normal homeostasis. Mutations and altered expression profiles of histones and corresponding histone chaperone partners are associated with developmental defects and cancer. Here, we discuss the spatiotemporal deposition mechanisms of the Histone H3 variants and their influence on mammalian cell fate during development. We focus on H3 given its profound effect on nucleosome stability and its recently characterized deposition pathways. We propose that differences in deposition of H3 variants are largely dependent on the phase of the cell cycle and cellular potency but are also affected by cellular stress and changes in cell fate. We also discuss the utility of modern technologies in dissecting the spatiotemporal control of H3 variant deposition, and how this could shed light on the mechanisms of cell identity maintenance and lineage commitment. The current knowledge and future studies will help us better understand how organisms employ nucleosome dynamics in health, disease, and aging. Ultimately, these pathways can be manipulated to induce cell fate change in a therapeutic setting depending on the cellular context.

Cell-cycle phospho-regulation of the kinetochore

The kinetochore is a mega-dalton protein assembly that forms within centromeric regions of chromosomes and directs their segregation during cell division. Here we review cell cycle-mediated phosphorylation events at the kinetochore, with a focus on the budding yeast Saccharomyces cerevisiae and the insight gained from forced associations of kinases and phosphatases. The point centromeres found in the budding yeast S. cerevisiae are one of the simplest such structures found in eukaryotes. The S. cerevisiae kinetochore comprises a single nucleosome, containing a centromere-specific H3 variant Cse4^CENP-A, bound to a set of kinetochore proteins that connect to a single microtubule. Despite the simplicity of the budding yeast kinetochore, the proteins are mostly homologous with their mammalian counterparts. In some cases, human proteins can complement their yeast orthologs. Like its mammalian equivalent, the regulation of the budding yeast kinetochore is complex: integrating signals from the cell cycle, checkpoints, error correction, and stress pathways. The regulatory signals from these diverse pathways are integrated at the kinetochore by post-translational modifications, notably phosphorylation and dephosphorylation, to control chromosome segregation. Here we highlight the complex interplay between the activity of the different cell-cycle kinases and phosphatases at the kinetochore, emphasizing how much more we have to understand this essential structure.

Understanding transcription across scales: From base pairs to chromosomes

The influence of genome organization on transcription is central to our understanding of cell type specification. Higher-order genome organization is established through short- and long-range DNA interactions. Coordination of these interactions, from single atoms to entire chromosomes, plays a fundamental role in transcriptional control of gene expression. Loss of this coupling can result in disease. Analysis of transcriptional regulation typically involves disparate experimental approaches, from structural studies that define angstrom-level interactions to cell-biological and genomic approaches that assess mesoscale relationships. Thus, to fully understand the mechanisms that regulate gene expression, it is critical to integrate the findings gained across these distinct size scales. In this review, I illustrate fundamental ways in which cells regulate transcription in the context of genome organization.

Kinetochore assembly throughout the cell cycle

The kinetochore plays an essential role in facilitating chromosome segregation during cell division. This massive protein complex assembles onto the centromere of chromosomes and enables their attachment to spindle microtubules during mitosis. The kinetochore also functions as a signaling hub to regulate cell cycle progression, and is crucial to ensuring the fidelity of chromosome segregation. Despite the fact that kinetochores are large and robust molecular assemblies, they are also highly dynamic structures that undergo structural and organizational changes throughout the cell cycle. This review will highlight our current understanding of kinetochore structure and function, focusing on the dynamic processes that underlie kinetochore assembly.

Cellular Mechanisms and Regulation of Quiescence

Quiescence is a state of reversible proliferative arrest in which cells are not actively dividing and yet retain the capacity to reenter the cell cycle upon receiving an appropriate stimulus. Quiescent cells are remarkably diverse-they reside in different locations throughout the body, serve distinct roles, and are activated by a variety of signals. Despite this diversity, all quiescent cells must be able to persist in a nondividing state without compromising their proliferative potential, which requires changes to core cellular programs. How drastically different cell types are able to implement extensive changes to their gene-expression programs, metabolism, and cellular structures to induce a common cellular state is a fascinating question in cell and developmental biology. In this review, we explore the diversity of quiescent cells and highlight the unifying characteristics that define the quiescent state.

Centromeres are dismantled by foundational meiotic proteins Spo11 and Rec8

Meiotic processes are potentially dangerous to genome stability and could be disastrous if activated in proliferative cells. Here we show that two key meiosis-defining proteins, the topoisomerase Spo11 (which forms double-strand breaks) and the meiotic cohesin Rec8, can dismantle centromeres. This dismantlement is normally observable only in mutant cells that lack the telomere bouquet, which provides a nuclear microdomain conducive to centromere reassembly¹; however, overexpression of Spo11 or Rec8 leads to levels of centromere dismantlement that cannot be countered by the bouquet. Specific nucleosome remodelling factors mediate centromere dismantlement by Spo11 and Rec8. Ectopic expression of either protein in proliferating cells leads to the loss of mitotic kinetochores in both fission yeast and human cells. Hence, while centromeric chromatin has been characterized as extraordinarily stable, Spo11 and Rec8 challenge this stability and may jeopardize kinetochores in cancers that express meiotic proteins.

Centromere assembly and non-random sister chromatid segregation in stem cells

Asymmetric cell division (ACD) produces daughter cells with separate distinct cell fates and is critical for the development and regulation of multicellular organisms. Epigenetic mechanisms are key players in cell fate determination. Centromeres, epigenetically specified loci defined by the presence of the histone H3-variant, centromere protein A (CENP-A), are essential for chromosome segregation at cell division. ACDs in stem cells and in oocyte meiosis have been proposed to be reliant on centromere integrity for the regulation of the non-random segregation of chromosomes. It has recently been shown that CENP-A is asymmetrically distributed between the centromeres of sister chromatids in male and female Drosophila germline stem cells (GSCs), with more CENP-A on sister chromatids to be segregated to the GSC. This imbalance in centromere strength correlates with the temporal and asymmetric assembly of the mitotic spindle and potentially orientates the cell to allow for biased sister chromatid retention in stem cells. In this essay, we discuss the recent evidence for asymmetric sister centromeres in stem cells. Thereafter, we discuss mechanistic avenues to establish this sister centromere asymmetry and how it ultimately might influence cell fate.

Stem cell quiescence: the challenging path to activation

Quiescence is a cellular state in which a cell remains out of the cell cycle but retains the capacity to divide. The unique ability of adult stem cells to maintain quiescence is crucial for life-long tissue homeostasis and regenerative capacity. Quiescence has long been viewed as an inactive state but recent studies have shown that it is in fact an actively regulated process and that adult stem cells are highly reactive to extrinsic stimuli. This has fuelled hopes of boosting the reactivation potential of adult stem cells to improve tissue function during ageing. In this Review, we provide a perspective of the quiescent state and discuss how quiescent adult stem cells transition into the cell cycle. We also discuss current challenges in the field, highlighting recent technical advances that could help overcome some of these challenges.

Loss of ESRP1 blocks mouse oocyte development and leads to female infertility

Alternative splicing (AS) contributes to gene diversification, but the AS program during germline development remains largely undefined. Here, we interrupted pre-mRNA splicing events controlled by epithelial splicing regulatory protein 1 (ESRP1) and found that it induced female infertility in mice. Esrp1 deletion perturbed spindle organization, chromosome alignment and metaphase-to-anaphase transformation in oocytes. The first polar body extrusion was blocked during oocyte meiosis owing to abnormal activation of spindle assembly checkpoint and insufficiency of anaphase-promoting complex/cyclosome in Esrp1-knockout oocytes. Esrp1-knockout hampered follicular development and ovulation; eventually, premature ovarian failure occurred in six-month-old Esrp1-knockout mouse. Using single-cell RNA-seq analysis, 528 aberrant AS events of maternal mRNA transcripts were revealed and were preferentially associated with microtubule cytoskeletal organization. Notably, we found that loss of ESRP1 disturbed a comprehensive set of gene-splicing sites - including those within Trb53bp1, Rac1, Bora, Kif2c, Kif23, Ndel1, Kif3a, Cenpa and Lsm14b - that potentially caused abnormal spindle organization. Collectively, our findings provide the first report elucidating the ESRP1-mediated AS program of maternal mRNA transcripts, which may contribute to oocyte meiosis and female fertility in mice.

Asymmetric assembly of centromeres epigenetically regulates stem cell fate

Centromeres are epigenetically defined by CENP-A–containing chromatin and are essential for cell division. Previous studies suggest asymmetric inheritance of centromeric proteins upon stem cell division; however, the mechanism and implications of selective chromosome segregation remain unexplored. We show that Drosophila female germline stem cells (GSCs) and neuroblasts assemble centromeres after replication and before segregation. Specifically, CENP-A deposition is promoted by CYCLIN A, while excessive CENP-A deposition is prevented by CYCLIN B, through the HASPIN kinase. Furthermore, chromosomes inherited by GSCs incorporate more CENP-A, making stronger kinetochores that capture more spindle microtubules and bias segregation. Importantly, symmetric incorporation of CENP-A on sister chromatids via HASPIN knockdown or overexpression of CENP-A, either alone or together with its assembly factor CAL1, drives stem cell self-renewal. Finally, continued CENP-A assembly in differentiated cells is nonessential for egg development. Our work shows that centromere assembly epigenetically drives GSC maintenance and occurs before oocyte meiosis.

Alpha-satellite RNA transcripts are repressed by centromere-nucleolus associations

Although originally thought to be silent chromosomal regions, centromeres are instead actively transcribed. However, the behavior and contributions of centromere-derived RNAs have remained unclear. Here, we used single-molecule fluorescence in-situ hybridization (smFISH) to detect alpha-satellite RNA transcripts in intact human cells. We find that alpha-satellite RNA-smFISH foci levels vary across cell lines and over the cell cycle, but do not remain associated with centromeres, displaying localization consistent with other long non-coding RNAs. Alpha-satellite expression occurs through RNA polymerase II-dependent transcription, but does not require established centromere or cell division components. Instead, our work implicates centromere–nucleolar interactions as repressing alpha-satellite expression. The fraction of nucleolar-localized centromeres inversely correlates with alpha-satellite transcripts levels across cell lines and transcript levels increase substantially when the nucleolus is disrupted. The control of alpha-satellite transcripts by centromere-nucleolar contacts provides a mechanism to modulate centromere transcription and chromatin dynamics across diverse cell states and conditions.

Stable inheritance of CENP-A chromatin: Inner strength versus dynamic control

Chromosome segregation during cell division is driven by mitotic spindle attachment to the centromere region on each chromosome. Centromeres form a protein scaffold defined by chromatin featuring CENP-A, a conserved histone H3 variant, in a manner largely independent of local DNA cis elements. CENP-A nucleosomes fulfill two essential criteria to epigenetically identify the centromere. They undergo self-templated duplication to reestablish centromeric chromatin following DNA replication. More importantly, CENP-A incorporated into centromeric chromatin is stably transmitted through consecutive cell division cycles. CENP-A nucleosomes have unique structural properties and binding partners that potentially explain their long lifetime in vivo. However, rather than a static building block, centromeric chromatin is dynamically regulated throughout the cell cycle, indicating that CENP-A stability is also controlled by external factors. We discuss recent insights and identify the outstanding questions on how dynamic control of the long-term stability of CENP-A ensures epigenetic centromere inheritance.

A genetic memory initiates the epigenetic loop necessary to preserve centromere position

Centromeres are built on repetitive DNA sequences (CenDNA) and a specific chromatin enriched with the histone H3 variant CENP‐A, the epigenetic mark that identifies centromere position. Here, we interrogate the importance of CenDNA in centromere specification by developing a system to rapidly remove and reactivate CENP‐A (CENP‐A^OFF/ON). Using this system, we define the temporal cascade of events necessary to maintain centromere position. We unveil that CENP‐B bound to CenDNA provides memory for maintenance on human centromeres by promoting de novo CENP‐A deposition. Indeed, lack of CENP‐B favors neocentromere formation under selective pressure. Occasionally, CENP‐B triggers centromere re‐activation initiated by CENP‐C, but not CENP‐A, recruitment at both ectopic and native centromeres. This is then sufficient to initiate the CENP‐A‐based epigenetic loop. Finally, we identify a population of CENP‐A‐negative, CENP‐B/C‐positive resting CD4⁺ T cells capable to re‐express and reassembles CENP‐A upon cell cycle entry, demonstrating the physiological importance of the genetic memory.

Adaptations for centromere function in meiosis

The aim of mitosis is to segregate duplicated chromosomes equally into daughter cells during cell division. Meiosis serves a similar purpose, but additionally separates homologous chromosomes to produce haploid gametes for sexual reproduction. Both mitosis and meiosis rely on centromeres for the segregation of chromosomes. Centromeres are the specialized regions of the chromosomes that are attached to microtubules during their segregation. In this review, we describe the adaptations and layers of regulation that are required for centromere function during meiosis, and their role in meiosis-specific processes such as homolog-pairing and recombination. Since female meiotic divisions are asymmetric, meiotic centromeres are hypothesized to evolve quickly in order to favor their own transmission to the offspring, resulting in the rapid evolution of many centromeric proteins. We discuss this observation using the example of the histone variant CENP-A, which marks the centromere and is essential for centromere function. Changes in both the size and the sequence of the CENP-A N-terminal tail have led to additional functions of the protein, which are likely related to its roles during meiosis. We highlight the importance of CENP-A in the inheritance of centromere identity, which is dependent on the stabilization, recycling, or re-establishment of CENP-A-containing chromatin during meiosis.