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Flipping the switch on Sex-lethal expression: Sister of Sex-lethal antagonizes Sxl-dependent alternative splicing to maintain a male-specific gene expression pattern in Drosophila (iCLIP)

ABSTRACT: In Drosophila female development is governed by a single RNA-binding protein, Sex lethal (Sxl), that controls the expression of key factors involved in dosage compensation, germline homeostasis and the establishment of female morphology and behaviour. Functional Sxl protein is only synthesized in female flies. Its expression is established in the pre-cellular, female embryo by an X-chromosome counting mechanism and maintained by a positive feedback loop with Sxl controlling alternative splicing of its own transcript. In male flies this feedback loop is not activated, keeping Sxl expression in the ‘off’ state. But what protects males against accidental triggering of the self-sustaining feedback loop and Sxl protein production snowballing out of control? Here we identify Sister of sex lethal (Ssx) as a protein that antagonizes Sxl in autoregulatory splicing by competition for the same RNA elements. Male flies mutant for ssx exhibit a low level of productive Sxl mRNA splicing and in cultured Drosophila cells, Sxl-induced changes in alternative splicing can be reverted by the expression of Ssx. In sum, this demonstrates that Ssx helps to establish a stable, male-specific gene expression pattern by protecting male flies against triggering Sxl expression.

ORGANISM(S): Drosophila melanogaster

PROVIDER: GSE98188 | GEO | 2019/01/15

REPOSITORIES: GEO

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Project description:CLAMP transcription factor is a GA binding protein that recruits the male dosage compensation complex MSL to male X-chromosome, thus upregulating transcription level twice that of female X-chromosomes. Interestingly, CLAMP is essential for survival of both male and female individuals. However, what function it plays at the autosomes, especially in females is not known. It is well establised that GA-rich sequences to which CLAMP bind have evolved from polypyrimidine tracts that regulate splicing. Also, MALDI-MS data shows that CLAMP binds to RNA binding proteins involved in sex-specific splicing like, Squid. Furthermore, as part of MSL complex CLAMP interacts with MLE, a RNA helicase which is a known component of spliceosome complex conserved from flies to humans. Thus, we hypothesized that CLAMP plays a role in sex-spcific splicing. Given that CLAMP is a transcription factor, understanding mechanism of how it regulates splicing will add to our understanding about how regulators of gene expression shape the transcriptome differentially between males and females. We tested our hypothesis by identifying CLAMP dependent female and male sex-specific splicing events by analysing RNA-seq data from control as well as clampnull mutant male and female third instar larvae (L3) using suppa based time2splice pipeline (https://github.com/ashleymaeconard/time2splice). We identified 189 and 211 CLAMP dependent splicing events in female and male L3. Out of these 139 were female sex-specific splicing whereas 161 was male-sex-specific splicing. 50 splicing events identified as CLAMP dependent were non-specific to any particular sex, and were affect in both the sexes. Interestingly, we identified splicing of master regulator of sex-determination sxl affected by absence of CLAMP in females whereas splicing of one of the sex-determination pathway effectors, tra regulated by CLAMP in males. These findings indicate CLAMP plays important role in determining sex-specific transcript diversity in Drosophila.

Project description:N6-methyladenosine (m6A) has recently emerged as a widespread and conserved RNA modification that modulates messenger RNA (mRNA) processing and activity. As most m6A studies have been conducted in cultured cells, we established Drosophila as a model system that combines molecular and genomic analyses of m6A with investigations of organismal biology. We first apply miCLIP sequencing to map m6A modifications at single-nucleotide resolution during Drosophila embryogenesis. We next characterize Drosophila components of the m6A "writer" methyltransferase complex [MTC factors, METTL3/IME4, METTL14, FL(2)D and Nito], and characterize m6A-binding properties of the YTH domain-containing proteins, YT521-B and CG6422. We complement this by generating a collection of mutants in m6A writer and reader components. While Drosophila orthologs of mammalian MTC factors that have additional roles in splicing are lethal, our mutants in METTL and YTH factors are viable but present specific developmental and behavioral defects. We note that several mammalian m6A factors were originally identified as Drosophila regulators of Sxl splicing, which generates the master determinant of female identity. While our mutants in dedicated writer and reader machinery are not required to accumulate Sxl protein in the ovary, maternal loss of m6A writers and readers collaborates with reduction of Sxl to induce female lethality. Our data indicate Sxl is directly regulated by the m6A pathway, since miCLIP data show Sxl is a substantial target of intronic m6A in early embryos, in addition to bearing exonic m6A. Consistent with this, female-specific Sxl splicing is defective in m6A pathway mutants. YT521-B appears to be the major effector of m6A for Sxl regulation, as it shows much stronger genetic interactions with Sxl than CG6422 and YT521-B overexpression can induce female-specific Sxl splicing. Overall, our transcriptomic and genetic toolkit for studying the m6A pathway in Drosophila reveals a major biological utilization in establishing sex-specific splicing.

Project description:The Drosophila sex determination hierarchy consists of a splicing cascade with sex-specific transcription directing somatic sexual dimorphism. Our understanding of this pathway, and many others, is incomplete. Here we pioneer an approach to expand our knowledge of gene regulatory networks (GRNs) by leveraging natural genetic variation. This approach is generalizable to any natural population, including humans. Two studies from Drosophila female head tissue were used – the DSPR collection (alleles from 15 natural variants) and F1-hybrid collection (alleles from heterozygotes of 75 isogenic lines crossed to w1118) – in a structural equation model (SEM) analysis. We expanded the sex hierarchy GRN by adding novel links among genes in the pathway and by adding novel genes to the pathway. A link from fruitless (fru) to Sex-lethal (Sxl) was found in both populations, which is supported by the presence of fru binding sites in the Sxl locus. The splicing factors male-specific lethal 2 and Rm62 were correctly identified as downstream targets of Sxl. There were 754 additional candidate genes for an expanded sex hierarchy GRN. These candidates were enriched for genes with sex-biased splicing and many components of the spliceosome were placed in the GRN. As with other population-genetic analyses, the number of alleles limits the number of observable interactions. Network expansion was only clear in the F1-hybrid population, which has an average of twice as many alleles as the DSPR population. Independent studies of doublesex and transformer mutants support many novel connections, including evidence for a link between the sex hierarchy and metabolism, with the inclusion of Insulin-like receptor in the sex hierarchy GRN. RNA sequencing was performed on mRNA derived from adult male or female heads, for a total of 9 samples. These samples included females that produce the male isoform of dsx [w/+;DsxD/dsxm+r15 (XX)], and two dsx mutants: females [w/+; dsxm+r15/dsxd+r3 (XX)] and males [w;dsxm+r15/dsxd+r3 (XY)]. Two wild type genotypes (Berlin and Canton-S) were sequenced at the same time, but have previously been published as part of GSE50515. There were at least 3 replicates from biological samples.

Project description:The Drosophila MSL complex mediates dosage compensation by increasing transcription of the single X chromosome in males approximately two-fold. This is accomplished through recognition of the X chromosome and subsequent acetylation of histone H4K16 on X-linked genes. Initial binding to the X is thought to occur at a subset of sites. However, the consensus sequence motif of entry sites (“MSL recognition element” or MRE) is only slightly enriched on the X (~2 fold), and only a fraction of them is utilized by the MSL complex. Here we ask whether chromatin context could distinguish between utilized and non-utilized copies of the motif, by comparing their relative enrichment for histone modifications and chromosomal proteins mapped in the NHGRI modENCODE project. Through a comparative analysis of the chromatin features in male S2 cells, which contain MSL complex, and female Kc cells, which lack the complex, we find that the presence of active chromatin modifications, together with an elevated local GC content in surrounding sequence, has strong predictive value for functional MSL entry sites, independent of MSL binding. We tested these sites for function in Kc cells by RNAi knockdown of Sxl, resulting in induction of MSL complex. We show that ectopic MSL expression in Kc cells leads to H4K16 acetylation around these sites, and a relative increase in X chromosome transcription. Collectively, our results support a model in which a pre-existing active chromatin environment, coincident with H3K36me3, contributes to MSL entry site selection. The consequences of MSL targeting of the male X chromosome include increase in nucleosome lability, enrichment for H4K16 acetylation and JIL-1 kinase, and depletion of linker histone H1 on active X-linked genes. Our finding serves as a model to understand how chromatin and local sequence features are involved in the selection of functional protein binding sites in the genome. The key Drosophila female sex determinant protein, SXL, represses dosage compensation by inhibiting MSL2 translation. Loss of SXL results in the expression, stabilization, and targetting of the MSL complex in female cells. Therefore, depletion of SXL by RNA interference (RNAi) in female Kc cells will lead to a MSL2-dependent increase in transcription from the female X chromosomes, consistent with the induction of dosage compensation. In this experiment, we generated gene expression profiles of Kc cells of control (GFP), Sxl RNAi and Sxl-Msl2 RNAi experiments.

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Flipping the switch on Sex-lethal expression: Sister of Sex-lethal antagonizes Sxl-dependent alternative splicing to maintain a male-specific gene expression pattern in Drosophila (iCLIP)

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