The Drosophila putative histone acetyltransferase Enok maintains female germline stem cells through regulating Bruno and the niche

To RNA-binding and beyond: Emerging facets of the role of Rbfox proteins in development and disease

The RNA-binding Fox-1 homologue (Rbfox) proteins represent an ancient family of splicing factors, conserved through evolution. All members share an RNA recognition motif (RRM), and a particular affinity for the GCAUG signature in target RNA molecules. The role of Rbfox, as a splice factor, deciding the tissue-specific inclusion/exclusion of an exon, depending on its binding position on the flanking introns, is well known. Rbfox often acts in concert with other splicing factors, and forms splicing regulatory networks. Apart from this canonical role, recent studies show that Rbfox can also function as a transcription co-factor, and affects mRNA stability and translation. The repertoire of Rbfox targets is vast, including genes involved in the development of tissue lineages, such as neurogenesis, myogenesis, and erythropoeiesis, and molecular processes, including cytoskeletal dynamics, and calcium handling. A second layer of complexity is added by the fact that Rbfox expression itself is regulated by multiple mechanisms, and, in vertebrates, exhibits tissue-specific expression. The optimum dosage of Rbfox is critical, and its misexpression is etiological to various disease conditions. In this review, we discuss the contextual roles played by Rbfox as a tissue-specific regulator for the expression of many important genes with diverse functions, through the lens of the emerging data which highlights its involvement in many human diseases. Furthermore, we explore the mechanistic details provided by studies in model organisms, with emphasis on the work with Drosophila. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Turnover and Surveillance > Regulation of RNA Stability RNA Processing > Splicing Regulation/Alternative Splicing.

Tctp, a unique Ing5-binding partner, inhibits the chromatin binding of Enok in Drosophila

The MOZ/MORF histone acetyltransferase complex is highly conserved in eukaryotes and controls transcription, development, and tumorigenesis. However, little is known about how its chromatin localization is regulated. Inhibitor of growth 5 (ING5) tumor suppressor is a subunit of the MOZ/MORF complex. Nevertheless, the in vivo function of ING5 remains unclear. Here, we report an antagonistic interaction between Drosophila Translationally controlled tumor protein (TCTP) (Tctp) and ING5 (Ing5) required for chromatin localization of the MOZ/MORF (Enok) complex and H3K23 acetylation. Yeast two-hybrid screening using Tctp identified Ing5 as a unique binding partner. In vivo, Ing5 controlled differentiation and down-regulated epidermal growth factor receptor signaling, whereas it is required in the Yorkie (Yki) pathway to determine organ size. Ing5 and Enok mutants promoted tumor-like tissue overgrowth when combined with uncontrolled Yki activity. Tctp depletion rescued the abnormal phenotypes of the Ing5 mutation and increased the nuclear translocation of Ing5 and chromatin binding of Enok. Nonfunctional Enok promoted the nuclear translocation of Ing5 by reducing Tctp, indicating a feedback mechanism between Tctp, Ing5, and Enok to regulate histone acetylation. Therefore, Tctp is essential for H3K23 acetylation by controlling the nuclear translocation of Ing5 and chromatin localization of Enok, providing insights into the roles of human TCTP and ING5-MOZ/MORF in tumorigenesis.

Drosophila MESR4 Gene Ensures Germline Stem Cell Differentiation by Promoting the Transcription of bag of marbles

Ovarian germline stem cells (GSCs) of Drosophila melanogaster provide a valuable in vivo model to investigate how the adult stem cell identity is maintained and the differentiation of the daughter cells is regulated. GSCs are embedded into a specialized cellular microenvironment, the so-called stem cell niche. Besides the complex signaling interactions between the germ cells and the niche cells, the germ cell intrinsic mechanisms, such as chromatin regulation and transcriptional control, are also crucial in the decision about self-renewal and differentiation. The key differentiation regulator gene is the bag of marbles (bam), which is transcriptionally repressed in the GSCs and de-repressed in the differentiating daughter cell. Here, we show that the transcription factor MESR4 functions in the germline to promote GSC daughter differentiation. We find that the loss of MESR4 results in the accumulation of GSC daughter cells which fail to transit from the pre-cystoblast (pre-CB) to the differentiated cystoblast (CB) stage. The forced expression of bam can rescue this differentiation defect. By a series of epistasis experiments and a transcriptional analysis, we demonstrate that MESR4 positively regulates the transcription of bam. Our results suggest that lack of repression alone is not sufficient, but MESR4-mediated transcriptional activation is also required for bam expression.

The Drosophila gonads: models for stem cell proliferation, self-renewal, and differentiation

The male and female gonads of Drosophila melanogaster have developed into powerful model systems for both the study of stem cell behaviours, and for understanding how stem cell misregulation can lead to cancers. Using these systems, one is able to observe and manipulate the resident stem cell populations in vivo with a great deal of licence. The tractability of the testis and ovary also allow researchers to explore a range of cellular mechanisms, such as proliferation and polarity, as well as the influence exerted by the local environment through a host of highly-conserved signalling pathways. Importantly, many of the cellular behaviours and processes studied in the Drosophila testis and ovary are known to be disrupted, or otherwise misregulated, in human tumourigenic cells. Here, we review the mechanisms relating to stem cell behaviour, though we acknowledge there are many other fascinating aspects of gametogenesis, including the invasive behaviour of migratory border cells in the Drosophila ovary that, though relevant to the study of tumourigenesis, will unfortunately not be covered.

Drosophila O-GlcNAcase Mutants Reveal an Expanded Glycoproteome and Novel Growth and Longevity Phenotypes

The reversible posttranslational O-GlcNAc modification of serine or threonine residues of intracellular proteins is involved in many cellular events from signaling cascades to epigenetic and transcriptional regulation. O-GlcNAcylation is a conserved nutrient-dependent process involving two enzymes, with O-GlcNAc transferase (OGT) adding O-GlcNAc and with O-GlcNAcase (OGA) removing it in a manner that’s protein- and context-dependent. O-GlcNAcylation is essential for epigenetic regulation of gene expression through its action on Polycomb and Trithorax and COMPASS complexes. However, the important role of O-GlcNAc in adult life and health span has been largely unexplored, mainly due the lack of available model systems. Cataloging the O-GlcNAc proteome has proven useful in understanding the biology of this modification in vivo. In this study, we leveraged a recently developed oga knockout fly mutant to identify the O-GlcNAcylated proteins in adult Drosophilamelanogaster. The adult O-GlcNAc proteome revealed many proteins related to cell and organismal growth, development, differentiation, and epigenetics. We identified many O-GlcNAcylated proteins that play a role in increased growth and decreased longevity, including HCF, SIN3A, LOLA, KISMET, ATX2, SHOT, and FOXO. Interestingly, oga mutant flies are larger and have a shorter life span compared to wild type flies, suggesting increased O-GlcNAc results in increased growth. Our results suggest that O-GlcNAc alters the function of many proteins related to transcription, epigenetic modification and signaling pathways that regulate growth rate and longevity. Therefore, our findings highlight the importance of O-GlcNAc in growth and life span in adult Drosophila.

Epigenetic regulation of drosophila germline stem cell maintenance and differentiation

Gametogenesis is one of the most extreme cellular differentiation processes that takes place in Drosophila male and female germlines. This process begins at the germline stem cell, which undergoes asymmetric cell division (ACD) to produce a self-renewed daughter that preserves its stemness and a differentiating daughter cell that undergoes epigenetic and genomic changes to eventually produce haploid gametes. Research in molecular genetics and cellular biology are beginning to take advantage of the continually advancing genomic tools to understand: (1) how germ cells are able to maintain their identity throughout the adult reproductive lifetime, and (2) undergo differentiation in a balanced manner. In this review, we focus on the epigenetic mechanisms that address these two questions through their regulation of germline-soma communication to ensure germline stem cell identity and activity.

Acetyltransferase Enok regulates transposon silencing and piRNA cluster transcription

The piRNA pathway is a highly conserved mechanism to repress transposon activation in the germline in Drosophila and mammals. This pathway starts from transcribing piRNA clusters to generate long piRNA precursors. The majority of piRNA clusters lack conventional promoters, and utilize heterochromatin- and HP1D/Rhino-dependent noncanonical mechanisms for transcription. However, information regarding the transcriptional regulation of piRNA clusters is limited. Here, we report that the Drosophila acetyltransferase Enok, which can activate transcription by acetylating H3K23, is critical for piRNA production from 54% of piRNA clusters including 42AB, the major piRNA source. Surprisingly, we found that Enok not only promotes rhino expression by acetylating H3K23, but also directly enhances transcription of piRNA clusters by facilitating Rhino recruitment. Taken together, our study provides novel insights into the regulation of noncanonical transcription at piRNA clusters and transposon silencing.

Roughly half of our genome is composed of transposons. Activation of those transposons in the germline will result in severe DNA damages and infertility. The PIWI-interacting RNA (piRNA) pathway, which is highly conserved between mammals and flies, is a key mechanism to suppress transposon activation in the germline. Here, we identified the fly acetyltransferase Enok as a novel regulator functioning in the early steps of this pathway. We found that Enok can promote the expression of three genes involved in piRNA production by acetylating histone H3 lysine 23 (H3K23). We also demonstrated that Enok regulates the recruitment of Rhi, a factor critical for transcription initiation at piRNA-generating loci, to a subset of those loci, and therefore enhances their transcription. Our findings reveal an upstream regulator in the piRNA pathway and advance our understanding regarding the molecular mechanism of transposon silencing.

Epigenetic Regulation of ALS and CMT: A Lesson from Drosophila Models

Amyotrophic lateral sclerosis (ALS) is the third most common neurodegenerative disorder and is sometimes associated with frontotemporal dementia. Charcot–Marie–Tooth disease (CMT) is one of the most commonly inherited peripheral neuropathies causing the slow progression of sensory and distal muscle defects. Of note, the severity and progression of CMT symptoms markedly vary. The phenotypic heterogeneity of ALS and CMT suggests the existence of modifiers that determine disease characteristics. Epigenetic regulation of biological functions via gene expression without alterations in the DNA sequence may be an important factor. The methylation of DNA, noncoding RNA, and post-translational modification of histones are the major epigenetic mechanisms. Currently, Drosophila is emerging as a useful ALS and CMT model. In this review, we summarize recent studies linking ALS and CMT to epigenetic regulation with a strong emphasis on approaches using Drosophila models.

Fatty acid β-oxidation is required for the differentiation of larval hematopoietic progenitors in Drosophila

Cell-intrinsic and extrinsic signals regulate the state and fate of stem and progenitor cells. Recent advances in metabolomics illustrate that various metabolic pathways are also important in regulating stem cell fate. However, our understanding of the metabolic control of the state and fate of progenitor cells is in its infancy. Using Drosophila hematopoietic organ: lymph gland, we demonstrate that Fatty Acid Oxidation (FAO) is essential for the differentiation of blood cell progenitors. In the absence of FAO, the progenitors are unable to differentiate and exhibit altered histone acetylation. Interestingly, acetate supplementation rescues both histone acetylation and the differentiation defects. We further show that the CPT1/whd (withered), the rate-limiting enzyme of FAO, is transcriptionally regulated by Jun-Kinase (JNK), which has been previously implicated in progenitor differentiation. Our study thus reveals how the cellular signaling machinery integrates with the metabolic cue to facilitate the differentiation program.

Stem cells are special precursor cells, found in all animals from flies to humans, that can give rise to all the mature cell types in the body. Their job is to generate supplies of new cells wherever these are needed. This is important because it allows damaged or worn-out tissues to be repaired and replaced by fresh, healthy cells.

As part of this renewal process, stem cells generate pools of more specialized cells, called progenitor cells. These can be thought of as half-way to maturation and can only develop in a more restricted number of ways. For example, so-called myeloid progenitor cells from humans can only develop into a specific group of blood cell types, collectively termed the myeloid lineage.

Fruit flies, like many other animals, also have several different types of blood cells. The fly’s repertoire of blood cells is very similar to the human myeloid lineage, and these cells also develop from the fly equivalent of myeloid progenitor cells. These progenitors are found in a specialized organ in fruit fly larvae called the lymph gland, where the blood forms. These similarities between fruit flies and humans mean that flies are a good model to study how myeloid progenitor cells mature.

A lot is already known about the molecules that signal to progenitor cells how and when to mature. However, the role of metabolism – the chemical reactions that process nutrients and provide energy inside cells – is still poorly understood. Tiwari et al. set out to identify which metabolic reactions myeloid progenitor cells require and how these reactions might shape the progenitors’ development into mature blood cells.

The experiments in this study used fruit fly larvae that had been genetically altered so that they could no longer perform key chemical reactions needed for the breakdown of fats. In these mutant larvae, the progenitors within the lymph gland could not give rise to mature blood cells. This showed that myeloid progenitor cells need to be able to break down fats in order to develop properly.

These results highlight a previously unappreciated role for metabolism in controlling the development of progenitor cells. If this effect also occurs in humans, this knowledge could one day help medical researchers engineer replacement tissues in the lab, or even increase our own bodies’ ability to regenerate blood, and potentially other organs.

Uninvited guests: how transposable elements take advantage of Drosophila germline stem cells, and how stem cells fight back

Transposable elements (TEs) are mobile genetic parasites that spread through host genomes by replicating in germline cells. New TE copies that arise in the genomes of germline stem cells (GSCs) are of particular value, because they are potentially transmitted to multiple offspring through the plethora of gametes arising from the same progenitor GSC. However, the fidelity of GSC genomes is also of utmost importance to the host in ensuring the production of abundant and fit offspring. Here we review tactics that TEs employ to replicate in Drosophila female GSCs, as well as mechanisms those cells use to defend against TEs. We also discuss the relationship between transposition and GSC loss, which is arbitrated through reduced signaling for self renewal, increased signaling for differentiation, and DNA damage response pathways.

Sequential Regulation of Maternal mRNAs through a Conserved cis-Acting Element in Their 3' UTRs

Maternal mRNAs synthesized during oogenesis initiate the development of future generations. Some maternal mRNAs are either somatic or germline determinants and must be translationally repressed until embryogenesis. However, the translational repressors themselves are temporally regulated. We used polar granule component (pgc), a Drosophila maternal mRNA, to ask how maternal transcripts are repressed while the regulatory landscape is shifting. pgc, a germline determinant, is translationally regulated throughout oogenesis. We find that different conserved RNA-binding proteins bind a 10-nt sequence in the 3′ UTR of pgc mRNA to continuously repress translation at different stages of oogenesis. Pumilio binds to this sequence in undifferentiated and early-differentiating oocytes to block Pgc translation. After differentiation, Bruno levels increase, allowing Bruno to bind the same sequence and take over translational repression of pgc mRNA. We have identified a class of maternal mRNAs that are regulated similarly, including zelda, the activator of the zygotic genome.

Flora et al. show that pgc, a germline determinant, is translationally regulated throughout oogenesis. Different conserved RBPs bind a 10-nt sequence in the 3′ UTR to continuously repress translation throughout oogenesis. This mode of regulation applies to a class of maternal mRNAs, including zelda, the activator of the zygotic genome.

Effects of cadmium on fecundity and defence ability of Drosophila melanogaster

Cadmium (chemical symbol, Cd) is an extremely common pollutant that poses a toxicity threat to organisms. Therefore, we tested Drosophila melanogaster fecundity, Cd accumulation, and activity of two enzymes following Cd stress and used quantitative real-time polymerase chain reaction (qPCR) to quantify the mRNA expression levels of several genes involved in fecundity and defence. D. melanogaster was placed in a medium containing different concentrations of Cd (13, 26, and 52 mg L^-1), following which, inductively coupled plasma atomic emission spectroscopy showed that Cd accumulation in Drosophila increased with the increase in its dietary intake. We also observed that Cd at these concentrations significantly prolonged the mating latency in females and reduced the number of eggs laid. However, the same Cd concentrations did not affect male fecundity. Acetylcholinesterase activity was only detected at 52 mg L^-1 Cd in both sexes, whereas glutathione S-transferase activity was inhibited at 26 and 52 mg L^-1 Cd in females. The results of qPCR indicated that exposure to 13-52 mg L^-1 Cd affected the expression of reproduction-related genes, including downregulation of enok and upregulation of dally and dpp. The same level of exposure also induced transcriptional responses from three defence-related genes (hsp70, gstd2, and gstd6). Taken together, the results revealed that Cd exposure might negatively affect the expression of genes associated with D. melanogaster reproduction and trigger the transcription of defence-related genes. We suggest that further analyses of fecundity and defence responses may help develop indicators of Cd toxicity and improve our understanding of antitoxin defences.

QTL mapping of natural variation reveals that the developmental regulator bruno reduces tolerance to P-element transposition in the Drosophila female germline

Transposable elements (TEs) are obligate genetic parasites that propagate in host genomes by replicating in germline nuclei, thereby ensuring transmission to offspring. This selfish replication not only produces deleterious mutations—in extreme cases, TE mobilization induces genotoxic stress that prohibits the production of viable gametes. Host genomes could reduce these fitness effects in two ways: resistance and tolerance. Resistance to TE propagation is enacted by germline-specific small-RNA-mediated silencing pathways, such as the Piwi-interacting RNA (piRNA) pathway, and is studied extensively. However, it remains entirely unknown whether host genomes may also evolve tolerance by desensitizing gametogenesis to the harmful effects of TEs. In part, the absence of research on tolerance reflects a lack of opportunity, as small-RNA-mediated silencing evolves rapidly after a new TE invades, thereby masking existing variation in tolerance. We have exploited the recent historical invasion of the Drosophila melanogaster genome by P-element DNA transposons in order to study tolerance of TE activity. In the absence of piRNA-mediated silencing, the genotoxic stress imposed by P-elements disrupts oogenesis and, in extreme cases, leads to atrophied ovaries that completely lack germline cells. By performing quantitative trait locus (QTL) mapping on a panel of recombinant inbred lines (RILs) that lack piRNA-mediated silencing of P-elements, we uncovered multiple QTL that are associated with differences in tolerance of oogenesis to P-element transposition. We localized the most significant QTL to a small 230-kb euchromatic region, with the logarithm of the odds (LOD) peak occurring in the bruno locus, which codes for a critical and well-studied developmental regulator of oogenesis. Genetic, cytological, and expression analyses suggest that bruno dosage modulates germline stem cell (GSC) loss in the presence of P-element activity. Our observations reveal segregating variation in TE tolerance for the first time, and implicate gametogenic regulators as a source of tolerant variants in natural populations.

Transposable elements (TEs), or “jumping genes,” are mobile fragments of selfish DNA that leave deleterious mutations and DNA damage in their wake as they spread through host genomes. Their harmful effects are known to select for resistance by the host, in which the propagation of TEs is regulated and reduced. Here, we study for the first time whether host cells might also exhibit tolerance to TEs, by reducing their harmful effects without directly controlling their movement. By taking advantage of a panel of wild-type Drosophila melanogaster that lack resistance to P-element DNA transposons, we identified a small region of the genome that influences tolerance of P-element activity. We further demonstrate that a gene within that region, bruno, strongly influences the negative effects of P-element mobilization on the fly. When bruno dosage is reduced, the fertility of females carrying mobile P-elements is enhanced. The bruno locus encodes a protein with no known role in TE regulation but multiple well-characterized functions in oogenesis. We propose that bruno function reduces tolerance of the developing oocyte to DNA damage that is caused by P-elements.

Protecting and Diversifying the Germline

Gametogenesis represents the most dramatic cellular differentiation pathways in both female and male flies. At the genome level, meiosis ensures that diploid germ cells become haploid gametes. At the epigenome level, extensive changes are required to turn on and shut off gene expression in a precise spatiotemporally controlled manner. Research applying conventional molecular genetics and cell biology, in combination with rapidly advancing genomic tools have helped us to investigate (1) how germ cells maintain lineage specificity throughout their adult reproductive lifetime; (2) what molecular mechanisms ensure proper oogenesis and spermatogenesis, as well as protect genome integrity of the germline; (3) how signaling pathways contribute to germline-soma communication; and (4) if such communication is important. In this chapter, we highlight recent discoveries that have improved our understanding of these questions. On the other hand, restarting a new life cycle upon fertilization is a unique challenge faced by gametes, raising questions that involve intergenerational and transgenerational epigenetic inheritance. Therefore, we also discuss new developments that link changes during gametogenesis to early embryonic development-a rapidly growing field that promises to bring more understanding to some fundamental questions regarding metazoan development.

Role of Chromatin Modifications in Drosophila Germline Stem Cell Differentiation

During Drosophila oogenesis, germline stem cells (GSCs) self-renew and differentiate to give rise to a mature egg. Self-renewal and differentiation of GSCs are regulated by both intrinsic mechanisms such as regulation of gene expression in the germ line and extrinsic signaling pathways from the surrounding somatic niche. Epigenetic mechanisms, including histone-modifying proteins, nucleosome remodeling complexes, and histone variants, play a critical role in regulating intrinsic gene expression and extrinsic signaling cues from the somatic niche. In the GSCs, intrinsic epigenetic modifiers are required to maintain a stem cell fate by promoting expression of self-renewal factors and repressing the differentiation program. Subsequently, in the GSC daughters, epigenetic regulators activate the differentiation program to promote GSC differentiation. During differentiation, the GSC daughter undergoes meiosis to give rise to the developing egg, containing a compacted chromatin architecture called the karyosome. Epigenetic modifiers control the attachment of chromosomes to the nuclear lamina to aid in meiotic recombination and the release from the lamina for karyosome formation. The germ line is in close contact with the soma for the entirety of this developmental process. This proximity facilitates signaling from the somatic niche to the developing germ line. Epigenetic modifiers play a critical role in the somatic niche, modulating signaling pathways in order to coordinate the transition of GSC to an egg. Together, intrinsic and extrinsic epigenetic mechanisms modulate this exquisitely balanced program.

Regulation of KAT6 Acetyltransferases and Their Roles in Cell Cycle Progression, Stem Cell Maintenance, and Human Disease

The lysine acetyltransferase 6 (KAT6) histone acetyltransferase (HAT) complexes are highly conserved from yeast to higher organisms. They acetylate histone H3 and other nonhistone substrates and are involved in cell cycle regulation and stem cell maintenance. In addition, the human KAT6 HATs are recurrently mutated in leukemia and solid tumors. Therefore, it is important to understand the mechanisms underlying the regulation of KAT6 HATs and their roles in cell cycle progression. In this minireview, we summarize the identification and analysis of the KAT6 complexes and discuss the regulatory mechanisms governing their enzymatic activities and substrate specificities. We further focus on the roles of KAT6 HATs in regulating cell proliferation and stem cell maintenance and review recent insights that aid in understanding their involvement in human diseases.

The Controversy, Challenges, and Potential Benefits of Putative Female Germline Stem Cells Research in Mammals

The conventional view is that female mammals lose their ability to generate new germ cells after birth. However, in recent years, researchers have successfully isolated and cultured a type of germ cell from postnatal ovaries in a variety of mammalian species that have the abilities of self-proliferation and differentiation into oocytes, and this finding indicates that putative germline stem cells maybe exist in the postnatal mammalian ovaries. Herein, we review the research history and discovery of putative female germline stem cells, the concept that putative germline stem cells exist in the postnatal mammalian ovary, and the research progress, challenge, and application of putative germline stem cells in recent years.

Histone H1-mediated epigenetic regulation controls germline stem cell self-renewal by modulating H4K16 acetylation

Epigenetics plays critical roles in controlling stem cell self-renewal and differentiation. Histone H1 is one of the most critical chromatin regulators, but its role in adult stem cell regulation remains unclear. Here we report that H1 is intrinsically required in the regulation of germline stem cells (GSCs) in the Drosophila ovary. The loss of H1 from GSCs causes their premature differentiation through activation of the key GSC differentiation factor bam. Interestingly, the acetylated H4 lysine 16 (H4K16ac) is selectively augmented in the H1-depleted GSCs. Furthermore, overexpression of mof reduces H1 association on chromatin. In contrast, the knocking down of mof significantly rescues the GSC loss phenotype. Taken together, these results suggest that H1 functions intrinsically to promote GSC self-renewal by antagonizing MOF function. Since H1 and H4K16 acetylation are highly conserved from fly to human, the findings from this study might be applicable to stem cells in other systems.

Epigenetics plays critical roles in controlling stem cell self-renewal and differentiation. Here, Sun et al. show that H1 is intrinsically required in the regulation of germline stem cells in the Drosophila ovary by antagonizing MOF, a histone acetyltransferase specific for H4K16.

MOZ and MORF acetyltransferases: Molecular interaction, animal development and human disease

Lysine residues are subject to many forms of covalent modification and one such modification is acetylation of the ε-amino group. Initially identified on histone proteins in the 1960s, lysine acetylation is now considered as an important form of post-translational modification that rivals phosphorylation. However, only about a dozen of human lysine acetyltransferases have been identified. Among them are MOZ (monocytic leukemia zinc finger protein; a.k.a. MYST3 and KAT6A) and its paralog MORF (a.k.a. MYST4 and KAT6B). Although there is a distantly related protein in Drosophila and sea urchin, these two enzymes are vertebrate-specific. They form tetrameric complexes with BRPF1 (bromodomain- and PHD finger-containing protein 1) and two small non-catalytic subunits. These two acetyltransferases and BRPF1 play key roles in various developmental processes; for example, they are important for development of hematopoietic and neural stem cells. The human KAT6A and KAT6B genes are recurrently mutated in leukemia, non-hematologic malignancies, and multiple developmental disorders displaying intellectual disability and various other abnormalities. In addition, the BRPF1 gene is mutated in childhood leukemia and adult medulloblastoma. Therefore, these two acetyltransferases and their partner BRPF1 are important in animal development and human disease.

Novel tools for genetic manipulation of follicle stem cells in the Drosophila ovary reveal an integrin-dependent transition from quiescence to proliferation

In many tissues, the presence of stem cells is inferred by the capacity of the tissue to maintain homeostasis and undergo repair after injury. Isolation of self-renewing cells with the ability to generate the full array of cells within a given tissue strongly supports this idea, but the identification and genetic manipulation of individual stem cells within their niche remain a challenge. Here we present novel methods for marking and genetically altering epithelial follicle stem cells (FSCs) within the Drosophila ovary. Using these new tools, we define a sequential multistep process that comprises transitioning of FSCs from quiescence to proliferation. We further demonstrate that integrins are cell-autonomously required within FSCs to provide directional signals that are necessary at each step of this process. These methods may be used to define precise roles for specific genes in the sequential events that occur during FSC division after a period of quiescence.

Histone acetyltransferase Enok regulates oocyte polarization by promoting expression of the actin nucleation factor spire

KAT6 histone acetyltransferases (HATs) are highly conserved in eukaryotes and have been shown to play important roles in transcriptional regulation. Here, we demonstrate that the Drosophila KAT6 Enok acetylates histone H3 Lys 23 (H3K23) in vitro and in vivo. Mutants lacking functional Enok exhibited defects in the localization of Oskar (Osk) to the posterior end of the oocyte, resulting in loss of germline formation and abdominal segments in the embryo. RNA sequencing (RNA-seq) analysis revealed that spire (spir) and maelstrom (mael), both required for the posterior localization of Osk in the oocyte, were down-regulated in enok mutants. Chromatin immunoprecipitation showed that Enok is localized to and acetylates H3K23 at the spir and mael genes. Furthermore, Gal4-driven expression of spir in the germline can largely rescue the defective Osk localization in enok mutant ovaries. Our results suggest that the Enok-mediated H3K23 acetylation (H3K23Ac) promotes the expression of spir, providing a specific mechanism linking oocyte polarization to histone modification.

Evidence for chromatin-remodeling complex PBAP-controlled maintenance of the Drosophila ovarian germline stem cells

In the Drosophila oogenesis, germline stem cells (GSCs) continuously self-renew and differentiate into daughter cells for consecutive germline lineage commitment. This developmental process has become an in vivo working platform for studying adult stem cell fate regulation. An increasing number of studies have shown that while concerted actions of extrinsic signals from the niche and intrinsic regulatory machineries control GSC self-renewal and germline differentiation, epigenetic regulation is implicated in the process. Here, we report that Brahma (Brm), the ATPase subunit of the Drosophila SWI/SNF chromatin-remodeling complexes, is required for maintaining GSC fate. Removal or knockdown of Brm function in either germline or niche cells causes a GSC loss, but does not disrupt normal germline differentiation within the germarium evidenced at the molecular and morphological levels. There are two Drosophila SWI/SNF complexes: the Brm-associated protein (BAP) complex and the polybromo-containing BAP (PBAP) complex. More genetic studies reveal that mutations in polybromo/bap180, rather than gene encoding Osa, the BAP complex-specific subunit, elicit a defect in GSC maintenance reminiscent of the brm mutant phenotype. Further genetic interaction test suggests a functional association between brm and polybromo in controlling GSC self-renewal. Taken together, studies in this paper provide the first demonstration that Brm in the form of the PBAP complex functions in the GSC fate regulation.

Chromatin modifications regulate germ cell development and transgenerational information relay

Germ cells transmit genetic, cytoplasmic and epigenetic information to the next generation. Recent reports describe the importance of chromatin modifiers and small RNAs for germ cells development in Drosophila. We also review exciting progress in our understanding of piRNAs functions, which demonstrate that this class of small RNAs is both an adaptive and inheritable epigenetic memory.