Asymmetric nuclear division in neural stem cells generates sibling nuclei that differ in size, envelope composition, and chromatin organization

The transmembrane protein Syndecan is required for stem cell survival and maintenance of their nuclear properties

Tissue maintenance is underpinned by resident stem cells whose activity is modulated by microenvironmental cues. Using Drosophila as a simple model to identify regulators of stem cell behaviour and survival in vivo, we have identified novel connections between the conserved transmembrane proteoglycan Syndecan, nuclear properties and stem cell function. In the Drosophila midgut, Syndecan depletion in intestinal stem cells results in their loss from the tissue, impairing tissue renewal. At the cellular level, Syndecan depletion alters cell and nuclear shape, and causes nuclear lamina invaginations and DNA damage. In a second tissue, the developing Drosophila brain, live imaging revealed that Syndecan depletion in neural stem cells results in nuclear envelope remodelling defects which arise upon cell division. Our findings reveal a new role for Syndecan in the maintenance of nuclear properties in diverse stem cell types.

Sculpting nuclear envelope identity from the endoplasmic reticulum during the cell cycle

The nuclear envelope (NE) regulates nuclear functions, including transcription, nucleocytoplasmic transport, and protein quality control. While the outer membrane of the NE is directly continuous with the endoplasmic reticulum (ER), the NE has an overall distinct protein composition from the ER, which is crucial for its functions. During open mitosis in higher eukaryotes, the NE disassembles during mitotic entry and then reforms as a functional territory at the end of mitosis to reestablish nucleocytoplasmic compartmentalization. In this review, we examine the known mechanisms by which the functional NE reconstitutes from the mitotic ER in the continuous ER-NE endomembrane system during open mitosis. Furthermore, based on recent findings indicating that the NE possesses unique lipid metabolism and quality control mechanisms distinct from those of the ER, we explore the maintenance of NE identity and homeostasis during interphase. We also highlight the potential significance of membrane junctions between the ER and NE.

Not just binary: embracing the complexity of nuclear division dynamics

Cell division presents a challenge for eukaryotic cells: how can chromosomes effectively segregate within the confines of a membranous nuclear compartment? Different organisms have evolved diverse solutions by modulating the degree of nuclear compartmentalization, ranging from complete nuclear envelope breakdown to complete maintenance of nuclear compartmentalization via nuclear envelope expansion. Many intermediate forms exist between these extremes, suggesting that nuclear dynamics during cell division are surprisingly plastic. In this review, we highlight the evolutionary diversity of nuclear divisions, focusing on two defining characteristics: (1) chromosome compartmentalization and (2) nucleocytoplasmic transport. Further, we highlight recent evidence that nuclear behavior during division can vary within different cellular contexts in the same organism. The variation observed within and between organisms underscores the dynamic evolution of nuclear divisions tailored to specific contexts and cellular requirements. In-depth investigation of diverse nuclear divisions will enhance our understanding of the nucleus, both in physiological and pathological states.

MicroRNAome profiling of breast cancer unveils hsa-miR-5683 as a tumor suppressor microRNA predicting favorable clinical outcome

BACKGROUND

Breast cancer is a heterogeneous disease with diverse molecular subtypes, underscoring a better understanding of its molecular features and underlying regulatory mechanisms. Therefore, identifying novel prognostic biomarkers and therapeutic targets is crucial for advancing the current standard of care for breast cancer patients.

METHODS

Ninety-six formalin-fixed paraffin-embedded (FFPE) breast cancer samples underwent miRNAome profiling using QIAseq microRNA library kit and sequencing on Illumina platform. Mature miRNA quantification was conducted using CLC Genomics Workbench v21.0.5, while Relapse-free survival (RFS) analysis was conducted using RStudio 2023.09.1. Gain-of-function studies were conducted using miRNA mimics, while the effects of miRNA exogenous expression on cancer hallmark were assessed using 2-dimentional (2D) proliferation assay, three-dimensional (3D) organotypic culture, and live-dead staining. TargetScan database and Ingenuity Pathway Analysis (IPA) were used for miRNA target identification.

RESULTS

Hierarchical clustering based on miRNA expression revealed distinct patterns in relation to PAM50 classification and identified miRNAs panels associated with luminal, HER2, and basal subtypes. hsa-miR-5683 emerged as a potential prognostic biomarker, showing a favorable correlation with RFS and suppressing tumorigenicity under 2D and 3D conditions in triple-negative breast cancer (TNBC) models. Findings were further extended to the MCF7 hormone receptor positive (HR+) model. Transcriptomic profiling of hsa-miR-5683 overexpressing TNBC cells revealed its potential role in key oncogenic pathways. Integration of downregulated genes and CRISPR-Cas9 perturbational effects identified ACLY, RACGAP1, AK4, MRPL51, CYB5B, MKRN1, TMEM230, NUP54, ANAPC13, PGAM1, and SOD1 as bona fide gene targets for hsa-miR-5683.

CONCLUSIONS

Our data provides comprehensive miRNA expression atlas in breast cancer subtypes and underscores the prognostic and therapeutic significance of numerous miRNAs, including hsa-miR-5683 in TNBC. The identified gene targets unravel the intricate regulatory network in TNBC progression, suggesting promising avenues for further research and targeted therapeutic interventions.

The cytokinetic midbody mediates asymmetric fate specification at mitotic exit during neural stem cell division

Asymmetric cell division (ACD) is a broadly used mechanism for generating cellular diversity. Molecules known as fate determinants are segregated during ACD to generate distinct sibling cell fates, but determinants should not be activated until fate can be specified asymmetrically. Determinants could be activated after cell division but many animal cells complete division long after mitosis ends, raising the question of how activation could occur at mitotic exit taking advantage of the unique state plasticity at this time point. Here we show that the midbody, a microtubule-rich structure that forms in the intercellular bridge connecting nascent siblings, mediates fate determinant activation at mitotic exit in neural stem cells (NSCs) of the Drosophila larval brain. The fate determinants Prospero (Pros) and Brain tumor (Brat) are sequestered at the NSC membrane at metaphase but are released immediately following nuclear division when the midbody forms, well before cell division completes. The midbody isolates nascent sibling cytoplasms, allowing determinant release from the membrane via the cell cycle phosphatase String, without influencing the fate of the incorrect sibling. Our results identify the midbody as a key facilitator of ACD that allows asymmetric fate determinant activation to be initiated before division.

The Drosophila neuroblast polarity cycle at a glance

Drosophila neural stem cells, or neuroblasts, rapidly proliferate during embryonic and larval development to populate the central nervous system. Neuroblasts divide asymmetrically to create cellular diversity, with each division producing one sibling cell that retains the neuroblast fate and another that differentiates into glia or neurons. This asymmetric outcome is mediated by the transient polarization of numerous factors to the cell cortex during mitosis. The powerful genetics and outstanding imaging tractability of the neuroblast make it an excellent model system for studying the mechanisms of cell polarity. This Cell Science at a Glance article and the accompanying poster explore the phases of the neuroblast polarity cycle and the regulatory circuits that control them. We discuss the key features of the cycle - the targeted recruitment of proteins to specific regions of the plasma membrane and multiple phases of highly dynamic actomyosin-dependent cortical flows that pattern both protein distribution and membrane structure.

Maintaining transcriptional homeostasis during cell cycle

The preservation of gene expression patterns that define cellular identity throughout the cell division cycle is essential to perpetuate cellular lineages. However, the progression of cells through different phases of the cell cycle severely disrupts chromatin accessibility, epigenetic marks, and the recruitment of transcriptional regulators. Notably, chromatin is transiently disassembled during S-phase and undergoes drastic condensation during mitosis, which is a significant challenge to the preservation of gene expression patterns between cell generations. This article delves into the specific gene expression and chromatin regulatory mechanisms that facilitate the preservation of transcriptional identity during replication and mitosis. Furthermore, we emphasize our recent findings revealing the unconventional role of yeast centromeres and mitotic chromosomes in maintaining transcriptional fidelity beyond mitosis.

DOT1L activity affects neural stem cell division mode and reduces differentiation and ASNS expression

Cortical neurogenesis depends on the balance between self-renewal and differentiation of apical progenitors (APs). Here, we study the epigenetic control of AP's division mode by focusing on the enzymatic activity of the histone methyltransferase DOT1L. Combining lineage tracing with single-cell RNA sequencing of clonally related cells, we show at the cellular level that DOT1L inhibition increases neurogenesis driven by a shift of APs from asymmetric self-renewing to symmetric neurogenic consumptive divisions. At the molecular level, DOT1L activity prevents AP differentiation by promoting transcription of metabolic genes. Mechanistically, DOT1L inhibition reduces activity of an EZH2/PRC2 pathway, converging on increased expression of asparagine synthetase (ASNS), a microcephaly associated gene. Overexpression of ASNS in APs phenocopies DOT1L inhibition, and also increases neuronal differentiation of APs. Our data suggest that DOT1L activity/PRC2 crosstalk controls AP lineage progression by regulating asparagine metabolism.

Nuclear elongation during spermiogenesis depends on physical linkage of nuclear pore complexes to bundled microtubules by Drosophila Mst27D

Spermatozoa in animal species are usually highly elongated cells with a long motile tail attached to a head that contains the haploid genome in a compact and often elongated nucleus. In Drosophila melanogaster, the nucleus is compacted two hundred-fold in volume during spermiogenesis and re-modeled into a needle that is thirty-fold longer than its diameter. Nuclear elongation is preceded by a striking relocalization of nuclear pore complexes (NPCs). While NPCs are initially located throughout the nuclear envelope (NE) around the spherical nucleus of early round spermatids, they are later confined to one hemisphere. In the cytoplasm adjacent to this NPC-containing NE, the so-called dense complex with a strong bundle of microtubules is assembled. While this conspicuous proximity argued for functional significance of NPC-NE and microtubule bundle, experimental confirmation of their contributions to nuclear elongation has not yet been reported. Our functional characterization of the spermatid specific Mst27D protein now resolves this deficit. We demonstrate that Mst27D establishes physical linkage between NPC-NE and dense complex. The C-terminal region of Mst27D binds to the nuclear pore protein Nup358. The N-terminal CH domain of Mst27D, which is similar to that of EB1 family proteins, binds to microtubules. At high expression levels, Mst27D promotes bundling of microtubules in cultured cells. Microscopic analyses indicated co-localization of Mst27D with Nup358 and with the microtubule bundles of the dense complex. Time-lapse imaging revealed that nuclear elongation is accompanied by a progressive bundling of microtubules into a single elongated bundle. In Mst27D null mutants, this bundling process does not occur and nuclear elongation is abnormal. Thus, we propose that Mst27D permits normal nuclear elongation by promoting the attachment of the NPC-NE to the microtubules of the dense complex, as well as the progressive bundling of these microtubules.

Region-specific heterogeneity in neuronal nuclear morphology in young, aged and in Alzheimer's disease mouse brains

Neurons in the mammalian brain exhibit enormous structural and functional diversity across different brain regions. Compared to our understanding of the morphological diversity of neurons, very little is known about the heterogeneity of neuronal nuclear morphology and how nuclear size changes in aging and diseased brains. Here, we report that the neuronal cell nucleus displays differences in area, perimeter, and circularity across different anatomical regions in the mouse brain. The pyramidal neurons of the hippocampal CA3 region exhibited the largest area whereas the striatal neuronal nuclei were the smallest. These nuclear size parameters also exhibited dichotomous changes with age across brain regions-while the neocortical and striatal neurons showed a decrease in nuclear area and perimeter, the CA3 neurons showed an increase with age. The nucleus of parvalbumin- and calbindin-positive interneurons had comparable morphological features but exhibited differences between brain regions. In the context of activity-dependent transcription in response to a novel environment, there was a decrease in nuclear size and circularity in c-Fos expressing neurons in the somatosensory cortex and hippocampal CA1 and CA3. In an APP/PS1 mutant mouse model of Alzheimer's disease (AD), the neuronal nuclear morphology varies with plaque size and with increasing distance from the plaque. The neuronal nuclear morphology in the immediate vicinity of the plaque was independent of the plaque size and the morphology tends to change away from the plaque. These changes in the neuronal nuclear size and shape at different ages and in AD may be attributed to changes in transcriptional activity. This study provides a detailed report on the differences that exist between neurons in nuclear morphology and can serve as a basis for future studies.

Mitotic chromosome condensation resets chromatin to safeguard transcriptional homeostasis during interphase

Mitotic entry correlates with the condensation of the chromosomes, changes in histone modifications, exclusion of transcription factors from DNA, and the broad downregulation of transcription. However, whether mitotic condensation influences transcription in the subsequent interphase is unknown. Here, we show that preventing one chromosome to condense during mitosis causes it to fail resetting of transcription. Rather, in the following interphase, the affected chromosome contains unusually high levels of the transcription machinery, resulting in abnormally high expression levels of genes in cis, including various transcription factors. This subsequently causes the activation of inducible transcriptional programs in trans, such as the GAL genes, even in the absence of the relevant stimuli. Thus, mitotic chromosome condensation exerts stringent control on interphase gene expression to ensure the maintenance of basic cellular functions and cell identity across cell divisions. Together, our study identifies the maintenance of transcriptional homeostasis during interphase as an unexpected function of mitosis and mitotic chromosome condensation.

Nuclear envelope assembly and dynamics during development

The nuclear envelope (NE) protects but also organizes the eukaryotic genome. In this review we will discuss recent literature on how the NE disassembles and reassembles, how it varies in surface area and protein composition and how this translates into chromatin organization and gene expression in the context of animal development.

Regulation of organelle size and organization during development

During early embryogenesis, as cells divide in the developing embryo, the size of intracellular organelles generally decreases to scale with the decrease in overall cell size. Organelle size scaling is thought to be important to establish and maintain proper cellular function, and defective scaling may lead to impaired development and disease. However, how the cell regulates organelle size and organization are largely unanswered questions. In this review, we summarize the process of size scaling at both the cell and organelle levels and discuss recently discovered mechanisms that regulate this process during early embryogenesis. In addition, we describe how some recently developed techniques and Xenopus as an animal model can be used to investigate the underlying mechanisms of size regulation and to uncover the significance of proper organelle size scaling and organization.

Asymmetric chromatin retention and nuclear envelopes separate chromosomes in fused cells in vivo

Hybrid cells derived through fertilization or somatic cell fusion recognize and separate chromosomes of different origins. The underlying mechanisms are unknown but could prevent aneuploidy and tumor formation. Here, we acutely induce fusion between Drosophila neural stem cells (neuroblasts; NBs) and differentiating ganglion mother cells (GMCs) in vivo to define how epigenetically distinct chromatin is recognized and segregated. We find that NB-GMC hybrid cells align both endogenous (neuroblast-origin) and ectopic (GMC-origin) chromosomes at the metaphase plate through centrosome derived dual-spindles. Physical separation of endogenous and ectopic chromatin is achieved through asymmetric, microtubule-dependent chromatin retention in interphase and physical boundaries imposed by nuclear envelopes. The chromatin separation mechanisms described here could apply to the first zygotic division in insects, arthropods, and vertebrates or potentially inform biased chromatid segregation in stem cells.

A hybrid fly cell model to test the separation of chromosomes of different origin. Neural stem cell (NB) - ganglion mother cell (GMC) hybrids align the respective chromosomes independently, supported by NB- or GMC-derived centrosomes and their spindles.

Control of nuclear size by osmotic forces in Schizosaccharomyces pombe

The size of the nucleus scales robustly with cell size so that the nuclear-to-cell volume ratio (N/C ratio) is maintained during cell growth in many cell types. The mechanism responsible for this scaling remains mysterious. Previous studies have established that the N/C ratio is not determined by DNA amount but is instead influenced by factors such as nuclear envelope mechanics and nuclear transport. Here, we developed a quantitative model for nuclear size control based upon colloid osmotic pressure and tested key predictions in the fission yeast Schizosaccharomyces pombe. This model posits that the N/C ratio is determined by the numbers of macromolecules in the nucleoplasm and cytoplasm. Osmotic shift experiments showed that the fission yeast nucleus behaves as an ideal osmometer whose volume is primarily dictated by osmotic forces. Inhibition of nuclear export caused accumulation of macromolecules in the nucleoplasm, leading to nuclear swelling. We further demonstrated that the N/C ratio is maintained by a homeostasis mechanism based upon synthesis of macromolecules during growth. These studies demonstrate the functions of colloid osmotic pressure in intracellular organization and size control.

Continuous rate modeling of bacterial stochastic size dynamics

Bacterial division is an inherently stochastic process with effects on fluctuations of protein concentration and phenotype variability. Current modeling tools for the stochastic short-term cell-size dynamics are scarce and mainly phenomenological. Here we present a general theoretical approach based on the Chapman-Kolmogorov equation incorporating continuous growth and division events as jump processes. This approach allows us to include different division strategies, noisy growth, and noisy cell splitting. Considering bacteria synchronized from their last division, we predict oscillations in both the central moments of the size distribution and its autocorrelation function. These oscillations, barely discussed in past studies, can arise as a consequence of the discrete time displacement invariance of the system with a period of one doubling time, and they do not disappear when including stochasticity on either division times or size heterogeneity on the starting population but only after inclusion of noise in either growth rate or septum position. This result illustrates the usefulness of having a solid mathematical description that explicitly incorporates the inherent stochasticity in various biological processes, both to understand the process in detail and to evaluate the effect of various sources of variability when creating simplified descriptions.

Nuclear positioning during development: Pushing, pulling and flowing

The positioning of the nucleus, the central organelle of the cell, is an active and regulated process crucially linked to cell cycle, differentiation, migration, and polarity. Alterations in positioning have been correlated with cell and tissue function deficiency and genetic or chemical manipulation of nuclear position is embryonic lethal. Nuclear positioning is a precursor for symmetric or asymmetric cell division which is accompanied by fate determination of the daughter cells. Nuclear positioning also plays a key role during early embryonic developmental stages in insects, such as Drosophila, where hundreds of nuclei divide without cytokinesis and are distributed within the large syncytial embryo at roughly regular spacing. While the cytoskeletal elements and the linker proteins to the nucleus are fairly well characterised, including some of the force generating elements driving nuclear movement, there is considerable uncertainty about the biophysical mechanism of nuclear positioning, while the field is debating different force models. In this review, we highlight the current body of knowledge, discuss cell context dependent models of nuclear positioning, and outline open questions.

Aurora B-dependent polarization of the cortical actomyosin network during mitotic exit

The isotropic metaphase actin cortex progressively polarizes as the anaphase spindle elongates during mitotic exit. This involves the loss of actomyosin cortex from opposing cell poles and the accumulation of an actomyosin belt at the cell centre. Although these spatially distinct cortical remodelling events are coordinated in time, here we show that they are independent of each other. Thus, actomyosin is lost from opposing poles in anaphase cells that lack an actomyosin ring owing to centralspindlin depletion. In examining potential regulators of this process, we identify a role for Aurora B kinase in actin clearance at cell poles. Upon combining Aurora B inhibition with centralspindlin depletion, cells exiting mitosis fail to change shape and remain completely spherical. Additionally, we demonstrate a requirement for Aurora B in the clearance of cortical actin close to anaphase chromatin in cells exiting mitosis with a bipolar spindle and in monopolar cells forced to divide while flat. Altogether, these data suggest a novel role for Aurora B activity in facilitating DNA-mediated polar relaxation at anaphase, polarization of the actomyosin cortex, and cell division.

Cell biology: How does the nucleus get its membrane?

Nuclear shape and size depend on nuclear membrane availability through an unknown process. A new study of asymmetric cell division reveals that nuclear membrane is derived from the endoplasmic reticulum and that limiting nuclear membrane expansion can affect cell fate.