Asymmetric distribution of histones during Drosophila male germline stem cell asymmetric divisions

Mitotic chromatin marking governs asymmetric segregation of DNA damage

The faithful segregation of intact genetic material and the perpetuation of chromatin states through mitotic cell divisions are pivotal for maintaining cell function and identity across cell generations. However, most exogenous mutagens generate long-lasting DNA lesions that are segregated during mitosis. How this segregation is controlled is unknown. Here, we uncover a mitotic chromatin-marking pathway that governs the segregation of UV-induced damage in human cells. Our mechanistic analyses reveal two layers of control: histone ADP-ribosylation, and the incorporation of newly synthesized histones at UV damage sites, that both prevent local mitotic phosphorylations on histone H3 serines. Functionally, this chromatin-marking pathway drives the asymmetric segregation of UV damage in the cell progeny with potential consequences on daughter cell fate. We propose that this mechanism may help preserve the integrity of stem cell compartments during asymmetric cell divisions.

Bioimaging approaches for quantification of individual cell behavior during cell fate decisions

Tracking individual cells has allowed a new understanding of cellular behavior in human health and disease by adding a dynamic component to the already complex heterogeneity of single cells. Technically, despite countless advances, numerous experimental variables can affect data collection and interpretation and need to be considered. In this review, we discuss the main technical aspects and biological findings in the analysis of the behavior of individual cells. We discuss the most relevant contributions provided by these approaches in clinically relevant human conditions like embryo development, stem cells biology, inflammation, cancer and microbiology, along with the cellular mechanisms and molecular pathways underlying these conditions. We also discuss the key technical aspects to be considered when planning and performing experiments involving the analysis of individual cells over long periods. Despite the challenges in automatic detection, features extraction and long-term tracking that need to be tackled, the potential impact of single-cell bioimaging is enormous in understanding the pathogenesis and development of new therapies in human pathophysiology.

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.

Genetic and Non-Genetic Mechanisms Underlying Cancer Evolution

Simple Summary

Our manuscript summarizes the up-to-date data on the complex and dynamic nature of adaptation mechanisms and evolutionary processes taking place during cancer initiation, development and progression. Although for decades cancer has been viewed as a process governed by genetic mechanisms, it is becoming more and more clear that non-genetic mechanisms may play an equally important role in cancer evolution. In this review, we bring together these fundamental concepts and discuss how those tightly interconnected mechanisms lead to the establishment of highly adaptive quickly evolving cancers. Furthermore, we argue that in depth understanding of cancer progression from the evolutionary perspective may allow the prediction and direction of the evolutionary path of cancer populations towards drug sensitive phenotypes and thus facilitate the development of more effective anti-cancer approaches.

Abstract

Cancer development can be defined as a process of cellular and tissular microevolution ultimately leading to malignancy. Strikingly, though this concept has prevailed in the field for more than a century, the precise mechanisms underlying evolutionary processes occurring within tumours remain largely uncharacterized and rather cryptic. Nevertheless, although our current knowledge is fragmentary, data collected to date suggest that most tumours display features compatible with a diverse array of evolutionary paths, suggesting that most of the existing macro-evolutionary models find their avatar in cancer biology. Herein, we discuss an up-to-date view of the fundamental genetic and non-genetic mechanisms underlying tumour evolution with the aim of concurring into an integrated view of the evolutionary forces at play throughout the emergence and progression of the disease and into the acquisition of resistance to diverse therapeutic paradigms. Our ultimate goal is to delve into the intricacies of genetic and non-genetic networks underlying tumour evolution to build a framework where both core concepts are considered non-negligible and equally fundamental.

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.

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.

Differential Histone Distribution Patterns in Induced Asymmetrically Dividing Mouse Embryonic Stem Cells

Wnt3a-coated beads can induce asymmetric divisions of mouse embryonic stem cells (mESCs), resulting in one self-renewed mESC and one differentiating epiblast stem cell. This provides an opportunity for studying histone inheritance pattern at a single-cell resolution in cell culture. Here, we report that mESCs with Wnt3a-bead induction display nonoverlapping preexisting (old) versus newly synthesized (new) histone H3 patterns, but mESCs without Wnt3a beads have largely overlapping patterns. Furthermore, H4K20me2/3, an old histone-enriched modification, displays a higher instance of asymmetric distribution on chromatin fibers from Wnt3a-induced mESCs than those from non-induced mESCs. These locally distinct distributions between old and new histones have both cellular specificity in Wnt3a-induced mESCs and molecular specificity for histones H3 and H4. Given that post-translational modifications at H3 and H4 carry the major histone modifications, our findings provide a mammalian cell culture system to study histone inheritance for maintaining stem cell fate and for resetting it during differentiation.

Asymmetric Histone Inheritance in Asymmetrically Dividing Stem Cells

Epigenetic mechanisms play essential roles in determining distinct cell fates during the development of multicellular organisms. Histone proteins represent crucial epigenetic components that help specify cell identities. Previous work has demonstrated that during the asymmetric cell division of Drosophila male germline stem cells (GSCs), histones H3 and H4 are asymmetrically inherited, such that pre-existing (old) histones are segregated towards the self-renewing GSC whereas newly synthesized (new) histones are enriched towards the differentiating daughter cell. In order to further understand the molecular mechanisms underlying this striking phenomenon, two key questions must be answered: when and how old and new histones are differentially incorporated by sister chromatids, and how epigenetically distinct sister chromatids are specifically recognized and segregated. Here, we discuss recent advances in our understanding of the molecular mechanisms and cellular bases underlying these fundamental and important biological processes responsible for generating two distinct cells through one cell division.

Asymmetric Centromeres Differentially Coordinate with Mitotic Machinery to Ensure Biased Sister Chromatid Segregation in Germline Stem Cells

Many stem cells utilize asymmetric cell division (ACD) to produce a self-renewed stem cell and a differentiating daughter cell. How non-genic information could be inherited differentially to establish distinct cell fates is not well understood. Here, we report a series of spatiotemporally regulated asymmetric components, which ensure biased sister chromatid attachment and segregation during ACD of Drosophila male germline stem cells (GSCs). First, sister centromeres are differentially enriched with proteins involved in centromere specification and kinetochore function. Second, temporally asymmetric microtubule activities and polarized nuclear envelope breakdown allow for the preferential recognition and attachment of microtubules to asymmetric sister kinetochores and sister centromeres. Abolishment of either the asymmetric sister centromeres or the asymmetric microtubule activities results in randomized sister chromatid segregation. Together, these results provide the cellular basis for partitioning epigenetically distinct sister chromatids during stem cell ACDs, which opens new directions to study these mechanisms in other biological contexts.

Asymmetric Inheritance of Cell Fate Determinants: Focus on RNA

During the last decade, and mainly primed by major developments in high-throughput sequencing technologies, the catalogue of RNA molecules harbouring regulatory functions has increased at a steady pace. Current evidence indicates that hundreds of mammalian RNAs have regulatory roles at several levels, including transcription, translation/post-translation, chromatin structure, and nuclear architecture, thus suggesting that RNA molecules are indeed mighty controllers in the flow of biological information. Therefore, it is logical to suggest that there must exist a series of molecular systems that safeguard the faithful inheritance of RNA content throughout cell division and that those mechanisms must be tightly controlled to ensure the successful segregation of key molecules to the progeny. Interestingly, whilst a handful of integral components of mammalian cells seem to follow a general pattern of asymmetric inheritance throughout division, the fate of RNA molecules largely remains a mystery. Herein, we will discuss current concepts of asymmetric inheritance in a wide range of systems, including prions, proteins, and finally RNA molecules, to assess overall the biological impact of RNA inheritance in cellular plasticity and evolutionary fitness.

Symmetry from Asymmetry or Asymmetry from Symmetry?

The processes of DNA replication and mitosis allow the genetic information of a cell to be copied and transferred reliably to its daughter cells. However, if DNA replication and cell division were always performed in a symmetric manner, the result would be a cluster of tumor cells instead of a multicellular organism. Therefore, gaining a complete understanding of any complex living organism depends on learning how cells become different while faithfully maintaining the same genetic material. It is well recognized that the distinct epigenetic information contained in each cell type defines its unique gene expression program. Nevertheless, how epigenetic information contained in the parental cell is either maintained or changed in the daughter cells remains largely unknown. During the asymmetric cell division (ACD) of Drosophila male germline stem cells, our previous work revealed that preexisting histones are selectively retained in the renewed stem cell daughter, whereas newly synthesized histones are enriched in the differentiating daughter cell. We also found that randomized inheritance of preexisting histones versus newly synthesized histones results in both stem cell loss and progenitor germ cell tumor phenotypes, suggesting that programmed histone inheritance is a key epigenetic player for cells to either remember or reset cell fates. Here, we will discuss these findings in the context of current knowledge on DNA replication, polarized mitotic machinery, and ACD for both animal development and tissue homeostasis. We will also speculate on some potential mechanisms underlying asymmetric histone inheritance, which may be used in other biological events to achieve the asymmetric cell fates.

Karyotype features of trematode Himasthla elongata

BACKGROUND

Trematodes have a complex life cycle with animal host changes and alternation of parthenogenetic and hermaphrodite generations. The parthenogenetic generation of the worm (rediae) from the first intermediate host Littorina littorea was used for chromosome spreads production. Karyotype description of parasitic flatworm Himasthla elongata Mehlis, 1831 (Digenea: Himasthlidae) based on fluorochrome banding and 18S rDNA mapping.

RESULTS

Chromosome spreads were obtained from cercariae embryos and redial tissue suspensions with high pressure squash method.74.4 % of the analysed spreads contained 12 chromosome pairs (2n = 24). Chromosome classification was performed according to the morphometry and nomenclature published. H. elongata spread chromosomes had a rather bead-like structure. Ideograms of DAPI-banded chromosomes contained 130 individual bands. According to flow cytometry data, the H. elongata genome contains 1.25 pg of DNA, so one band contains, on average, 9.4 Mb of DNA. Image bank captures of individual high-resolution DAPI-banded chromosomes were provided. Differential DAPI- and CMA3-staining revealed the chromatin areas that differed in AT- or GC-content. Both dyes stained chromosomes all along but with varying intensities in different areas. FISH revealed that vast majority (95.0 %) of interphase nuclei contained one signal for 18S rDNA. This corresponded to the number of nucleoli per cell detected by observations in vivo. The rDNA signal was observed on one or two homologs of chromosome 10 in 72.2 % of analysed chromosome spreads, therefore chromosome 10 possessed the main rDNA cluster and minor ones on chromosomes 3 and 6, that corresponds with AgNOR results.

CONCLUSIONS

Himasthla elongata chromosomes variations presented as image bank. Differential chromosome staining with fluorochromes and FISH used for 18S rDNA mapping let us to conclude: (1) Himasthla elongata karyotype is 2n = 24; (2) chromosome number deviates from the previously studied echinostomatids (2n = 14-22); (3). Chromosome 10 possesses the main rDNA cluster with the minor ones existing on chromosomes 3 and 6.

Histone H3 Threonine Phosphorylation Regulates Asymmetric Histone Inheritance in the Drosophila Male Germline

A long-standing question concerns how stem cells maintain their identity through multiple divisions. Previously, we reported that pre-existing and newly synthesized histone H3 are asymmetrically distributed during Drosophila male germline stem cell (GSC) asymmetric division. Here, we show that phosphorylation at threonine 3 of H3 (H3T3P) distinguishes pre-existing versus newly synthesized H3. Converting T3 to the unphosphorylatable residue alanine (H3T3A) or to the phosphomimetic aspartate (H3T3D) disrupts asymmetric H3 inheritance. Expression of H3T3A or H3T3D specifically in early-stage germline also leads to cellular defects, including GSC loss and germline tumors. Finally, compromising the activity of the H3T3 kinase Haspin enhances the H3T3A but suppresses the H3T3D phenotypes. These studies demonstrate that H3T3P distinguishes sister chromatids enriched with distinct pools of H3 in order to coordinate asymmetric segregation of "old" H3 into GSCs and that tight regulation of H3T3 phosphorylation is required for male germline activity.

Epigenetic regulation in adult stem cells and cancers

Adult stem cells maintain tissue homeostasis by their ability to both self-renew and differentiate to distinct cell types. Multiple signaling pathways have been shown to play essential roles as extrinsic cues in maintaining adult stem cell identity and activity. Recent studies also show dynamic regulation by epigenetic mechanisms as intrinsic factors in multiple adult stem cell lineages. Emerging evidence demonstrates intimate crosstalk between these two mechanisms. Misregulation of adult stem cell activity could lead to tumorigenesis, and it has been proposed that cancer stem cells may be responsible for tumor growth and metastasis. However, it is unclear whether cancer stem cells share commonalities with normal adult stem cells. In this review, we will focus on recent discoveries of epigenetic regulation in multiple adult stem cell lineages. We will also discuss how epigenetic mechanisms regulate cancer stem cell activity and probe the common and different features between cancer stem cells and normal adult stem cells.