Valerie Reinke

Genome-wide germline-enriched and sex-biased expression profiles in Caenorhabditis elegans.

We performed a genome-wide analysis of gene expression in C. elegans to identify germline- and sex-regulated genes. Using mutants that cause defects in germ cell proliferation or gametogenesis, we identified sets of genes with germline-enriched expression in either hermaphrodites or males, or in both sexes. Additionally, we compared gene expression profiles between males and hermaphrodites lacking germline tissue to define genes with sex-biased expression in terminally differentiated somatic tissues. Cross-referencing hermaphrodite germline and somatic gene sets with in situ hybridization data demonstrates that the vast majority of these genes have appropriate spatial expression patterns. Additionally, we examined gene expression at multiple times during wild-type germline development to define temporal expression profiles for these genes. Sex- and germline-regulated genes have a non-random distribution in the genome, with especially strong biases for and against the X chromosome. Comparison with data from large-scale RNAi screens demonstrates that genes expressed in the oogenic germline display visible phenotypes more frequently than expected.

A C. elegans LSD1 Demethylase Contributes to Germline Immortality by Reprogramming Epigenetic Memory

Epigenetic information undergoes extensive reprogramming in the germline between generations. This reprogramming may be essential to establish a developmental ground state in the zygote. We show that mutants in spr-5, the Caenorhabditis elegans ortholog of the H3K4me2 demethylase LSD1/KDM1, exhibit progressive sterility over many generations. This sterility correlates with the misregulation of spermatogenesis-expressed genes and transgenerational accumulation of the histone modification dimethylation of histone H3 on lysine 4 (H3K4me2). This suggests that H3K4me2 can serve as a stable epigenetic memory, and that erasure of H3K4me2 by LSD/KDM1 in the germline prevents the inappropriate transmission of this epigenetic memory from one generation to the next. Thus, our results provide direct mechanistic insights into the processes that are required for epigenetic reprogramming between generations.

Diverse transcription factor binding features revealed by genome-wide ChIP-seq in C. elegans

Regulation of gene expression by sequence-specific transcription factors is central to developmental programs and depends on the binding of transcription factors with target sites in the genome. To date, most such analyses in Caenorhabditis elegans have focused on the interactions between a single transcription factor with one or a few select target genes. As part of the modENCODE Consortium, we have used chromatin immunoprecipitation coupled with high-throughput DNA sequencing (ChIP-seq) to determine the genome-wide binding sites of 22 transcription factors (ALR-1, BLMP-1, CEH-14, CEH-30, EGL-27, EGL-5, ELT-3, EOR-1, GEI-11, HLH-1, LIN-11, LIN-13, LIN-15B, LIN-39, MAB-5, MDL-1, MEP-1, PES-1, PHA-4, PQM-1, SKN-1, and UNC-130) at diverse developmental stages. For each factor we determined candidate gene targets, both coding and non-coding. The typical binding sites of almost all factors are within a few hundred nucleotides of the transcript start site. Most factors target a mixture of coding and non-coding target genes, although one factor preferentially binds to non-coding RNA genes. We built a regulatory network among the 22 factors to determine their functional relationships to each other and found that some factors appear to act preferentially as regulators and others as target genes. Examination of the binding targets of three related HOX factors--LIN-39, MAB-5, and EGL-5--indicates that these factors regulate genes involved in cellular migration, neuronal function, and vulval differentiation, consistent with their known roles in these developmental processes. Ultimately, the comprehensive mapping of transcription factor binding sites will identify features of transcriptional networks that regulate C. elegans developmental processes.

A Spatial and Temporal Map of C. elegans Gene Expression

The C. elegans genome has been completely sequenced, and the developmental anatomy of this model organism is described at single-cell resolution. Here we utilize strategies that exploit this precisely defined architecture to link gene expression to cell type. We obtained RNAs from specific cells and from each developmental stage using tissue-specific promoters to mark cells for isolation by FACS or for mRNA extraction by the mRNA-tagging method. We then generated gene expression profiles of more than 30 different cells and developmental stages using tiling arrays. Machine-learning-based analysis detected transcripts corresponding to established gene models and revealed novel transcriptionally active regions (TARs) in noncoding domains that comprise at least 10% of the total C. elegans genome. Our results show that about 75% of transcripts with detectable expression are differentially expressed among developmental stages and across cell types. Examination of known tissue- and cell-specific transcripts validates these data sets and suggests that newly identified TARs may exercise cell-specific functions. Additionally, we used self-organizing maps to define groups of coregulated transcripts and applied regulatory element analysis to identify known transcription factor- and miRNA-binding sites, as well as novel motifs that likely function to control subsets of these genes. By using cell-specific, whole-genome profiling strategies, we have detected a large number of novel transcripts and produced high-resolution gene expression maps that provide a basis for establishing the roles of individual genes in cellular differentiation.

A C. elegans Piwi, PRG-1, Regulates 21U-RNAs during Spermatogenesis

BACKGROUND Epigenetic regulation by diverse classes of small RNAs is mediated by the highly conserved Argonaute/Piwi family of proteins. Although Argonautes are broadly expressed, the Piwi subfamily primarily functions in the germ line. Piwi proteins are associated with germline-specific ribonucleoprotein (RNP) granules in Drosophila, zebrafish, and mouse. Depending on the species and on the specific family member, Piwis play important roles in spermatogenesis and/or in maintaining germ cell and stem cell totipotency. Piwis bind to a newly discovered class of small RNAs, called piRNAs. C. elegans contains a large set of Argonaute/Piwi-related proteins, including two closely related to piwi called prg-1 and prg-2. The function of prg-1 and prg-2 and whether piRNAs exist in C. elegans is unknown. RESULTS Here, we demonstrate that the Piwi-like protein PRG-1 is localized to P granules in germ cells entering spermatogenesis and is required for successful spermatogenesis. Loss of prg-1 causes a marked reduction in expression of a subset of mRNAs expressed during spermatogenesis, and prg-1 mutant sperm exhibit extensive defects in activation and fertilization. Moreover, prg-1 activity is required for the presence of the small RNAs called 21U-RNAs. CONCLUSIONS Our data suggest that PRG-1 promotes expression, processing, or stability of 21U-RNAs, which, in turn or in concert with PRG-1, promote proper expression of spermatogenesis transcripts.

Massively parallel sequencing of the polyadenylated transcriptome of C. elegans

Using massively parallel sequencing by synthesis methods, we have surveyed the polyA+ transcripts from four stages of the nematode Caenorhabditis elegans to an unprecedented depth. Using novel statistical approaches, we evaluated the coverage of annotated features of the genome and of candidate processed transcripts, including splice junctions, trans-spliced leader sequences, and polyadenylation tracts. The data provide experimental support for >85% of the annotated protein-coding transcripts in WormBase (WS170) and confirm additional details of processing. For example, the total number of confirmed splice junctions was raised from 70,911 to over 98,000. The data also suggest thousands of modifications to WormBase annotations and identify new spliced junctions and genes not part of any WormBase annotation, including at least 80 putative genes not found in any of three predicted gene sets. The quantitative nature of the data also suggests that mRNA levels may be measured by this approach with unparalleled precision. Although most sequences align with protein-coding genes, a small fraction falls in introns and intergenic regions. One notable region on the X chromosome encodes a noncoding transcript of >10 kb localized to somatic nuclei.

Comparative analysis of the transcriptome across distant species.

The transcriptome is the readout of the genome. Identifying common features in it across distant species can reveal fundamental principles. To this end, the ENCODE and modENCODE consortia have generated large amounts of matched RNA-sequencing data for human, worm and fly. Uniform processing and comprehensive annotation of these data allow comparison across metazoan phyla, extending beyond earlier within-phylum transcriptome comparisons and revealing ancient, conserved features. Specifically, we discover co-expression modules shared across animals, many of which are enriched in developmental genes. Moreover, we use expression patterns to align the stages in worm and fly development and find a novel pairing between worm embryo and fly pupae, in addition to the embryo-to-embryo and larvae-to-larvae pairings. Furthermore, we find that the extent of non-canonical, non-coding transcription is similar in each organism, per base pair. Finally, we find in all three organisms that the gene-expression levels, both coding and non-coding, can be quantitatively predicted from chromatin features at the promoter using a ‘universal model’ based on a single set of organism-independent parameters.

Coordinate Activation of Maternal Protein Degradation during the Egg-to-Embryo Transition in C. elegans

The transition from egg to embryo occurs in the absence of transcription yet requires significant changes in gene activity. Here, we show that the C. elegans DYRK family kinase MBK-2 coordinates the degradation of several maternal proteins, and is essential for zygotes to complete cytokinesis and pattern the first embryonic axis. In mbk-2 mutants, the meiosis-specific katanin subunits MEI-1 and MEI-2 persist during mitosis and the first mitotic division fails. mbk-2 is also required for posterior enrichment of the germ plasm before the first cleavage, and degradation of germ plasm components in anterior cells after cleavage. MBK-2 distribution changes dramatically after fertilization during the meiotic divisions, and this change correlates with activation of mbk-2-dependent processes. We propose that MBK-2 functions as a temporal regulator of protein stability, and that coordinate activation of maternal protein degradation is one of the mechanisms that drives the transition from symmetric egg to patterned embryo.

A Genome-Scale Resource for In Vivo Tag-Based Protein Function Exploration in C. elegans

Understanding the in vivo dynamics of protein localization and their physical interactions is important for many problems in biology. To enable systematic protein function interrogation in a multicellular context, we built a genome-scale transgenic platform for in vivo expression of fluorescent- and affinity-tagged proteins in Caenorhabditis elegans under endogenous cis regulatory control. The platform combines computer-assisted transgene design, massively parallel DNA engineering, and next-generation sequencing to generate a resource of 14,637 genomic DNA transgenes, which covers 73% of the proteome. The multipurpose tag used allows any protein of interest to be localized in vivo or affinity purified using standard tag-based assays. We illustrate the utility of the resource by systematic chromatin immunopurification and automated 4D imaging, which produced detailed DNA binding and cell/tissue distribution maps for key transcription factor proteins.

Genome-Wide Identification of Binding Sites Defines Distinct Functions for Caenorhabditis elegans PHA-4/FOXA in Development and Environmental Response

Transcription factors are key components of regulatory networks that control development, as well as the response to environmental stimuli. We have established an experimental pipeline in Caenorhabditis elegans that permits global identification of the binding sites for transcription factors using chromatin immunoprecipitation and deep sequencing. We describe and validate this strategy, and apply it to the transcription factor PHA-4, which plays critical roles in organ development and other cellular processes. We identified thousands of binding sites for PHA-4 during formation of the embryonic pharynx, and also found a role for this factor during the starvation response. Many binding sites were found to shift dramatically between embryos and starved larvae, from developmentally regulated genes to genes involved in metabolism. These results indicate distinct roles for this regulator in two different biological processes and demonstrate the versatility of transcription factors in mediating diverse biological roles.