Biology subjects

Halstead, M. M.

Publications and source records attributed to Halstead, M. M..

7 recordsLinked to original sources

bioRxiv2024

Maternal Med12 safeguards trophoblast pluripotency and placental development

For a brief but critical period post-fertilization, the mammalian embryo is entirely dependent on maternal products inherited from the oocyte. Previous research showed that oocyte-specific loss of Med12, an X-linked gene and Mediator complex subunit, leads to female sterility despite normal folliculogenesis and ovulation. Here, we show that loss of maternal Med12 has minimal effect on the oocyte transcriptome and does not manifest in embryonic lethality until post-implantation. Implants derived from Med12-null oocytes demonstrate abnormal placentation at E9.5, with an overabundance of trophoblast giant cells (TGC). This phenotype associates with downregulation of trophoblast pluripotency markers (e.g. Cdx2) and activation of drivers of TGC identity (e.g. Stra13) in the E7.5 extraembryonic ectoderm, revealing a previously undescribed role for Med12 in trophoblast pluripotency maintenance. Notably, we find consistently low Med12 expression in embryos derived from Med12-null oocytes, potentially due to programmed paternal X chromosome inactivation (XCI). To isolate the consequences of maternal Med12 depletion, we introduced an autosomal Med12 transgene and show that embryonic expression of the transgene rescues development of Med12-null oocytes. We conclude that oocyte-specific deletion of Med12 produces a maternal-zygotic double knock-out in extraembryonic tissues due to paternal XCI, leading to loss of pluripotency in the trophoblast, placental malformation, and embryonic death.

developmental biology↗

bioRxiv2022

A comprehensive prediction of transcript isoforms in 19 chicken tissues by Oxford Nanopore long-read sequencing

To comprehensively identify and annotate transcript isoforms in the chicken genome, we generated Nanopore long-read sequencing data from a diverse set of 19 chicken tissues comprising 68 samples collected from experimental line 6 x line 7 F1 adult males and females. More than 23.8 million reads with mean read length of 790 bases and average quality of 18.2 were generated. The annotation and subsequent filtering resulted in identification of 55,382 transcripts with mean length of 1,700 bases at 40,547 loci, representing [~]1.4 transcripts per locus. Among them, we predicted 30,967 potential coding transcripts at 19,461 loci and 16,495 potential lncRNA transcripts at 15,512 loci. Compared to reference annotations, we found 52% of annotated transcripts could partially to fully match while 47% were novel and potentially transcribed from lncRNA loci. Based on our annotation, we quantified transcript expression across tissues and found brain tissues (i.e. cerebellum, cortex) expressed highest number of transcripts and loci. The further tissue specificity revealed that [~]22% of the transcripts displaying tissue specificity. Of them, the reproductive tissues (i.e. testis, ovary) contained the most tissue-specific transcripts. Despite sequencing 68 transcriptomes derived from 19 tissues, still [~]20% of Ensembl reference loci were not detected. This suggests that including additional samples from different cell types, developmental and physiological conditions, is needed to fully annotate the chicken genome. The application of Nanopore sequencing transcriptomes in this study demonstrated the usefulness of long-read data in discovering additional novel loci (e.g., lncRNA loci) and resolving complex transcripts (e.g., the longest transcript for the TTN locus).

genomics↗

bioRxiv2022

Resetting H3K4me3, H3K27ac, H3K9me3 and H3K27me3 during the maternal-to-zygotic transition and blastocyst lineage specification in bovine embryos

It remains poorly understood how histone modifications regulate changes in gene expression during preimplantation development. Using a bovine model, we profiled changes in two activating (H3K4me3 and H3K27ac) and two repressive (H3K9me3 and H3K27me3) marks in oocytes, 2-, 4- and 8-cell embryos (that developed in the presence or absence of the transcription inhibitor a-amanitin), morula, blastocysts, inner cell mass cells and trophectoderm. In oocytes, we find that broad bivalent domains of H3K4me3 and H3K27me3 mark developmental genes, and that prior to genome activation, H3K9me3 and H3K27me3 co-occupy gene bodies. During genome activation, chromatin accessibility is established before canonical H3K4me3 and H3K27ac, and although embryonic transcription is required for this active remodeling, it is dispensable for maintenance of pre-established histone marks. Finally, blastocyst lineages are defined by differential Polycomb repression and transcription factor activity. Overall, these results further support the use of bovine as a more appropriate model system than the mouse to study genome activation and cell lineage specification during human preimplantation development.

developmental biology↗

bioRxiv2021

Profiling of open chromatin in developing pig (Sus scrofa) muscle to identify regulatory regions

There is very little information about how the genome is regulated in domestic pigs (Sus scrofa). This lack of knowledge hinders efforts to define and predict the effects of genetic variants in pig breeding programmes. In order to address this knowledge gap, we need to identify regulatory sequences in the pig genome starting with regions of open chromatin. We have optimised the Improved Protocol for the Assay for Transposase-Accessible Chromatin (Omni-ATAC-Seq) to profile regions of open chromatin in flash frozen pig muscle tissue samples. This protocol has allowed us to identify putative regulatory regions in semitendinosus muscle from 24 male piglets. We collected samples from the smallest, average, and largest sized male piglets from each litter through five developmental time points. The ATAC-Seq data were mapped to Sscrofa11.1 using Bowtie2 and Genrich was used for post-alignment peak-calling. Of the 4,661 ATAC-Seq peaks identified that represent regions of open chromatin, >50% were within 1 kb of known transcription start sites. Differential read count analysis revealed 377 ATAC-Seq defined genomic regions where chromatin accessibility differed significantly across developmental time points. We found regions of open chromatin associated with down regulation of genes involved in muscle development that were present in small sized foetal piglets but absent in large foetal piglets at day 90 of gestation. The dataset that we have generated provides: i) a resource for studies of genome regulation in pigs, and ii) contributes valuable functional annotation information to filter genetic variants for use in genomic selection in pig breeding programmes.

genomics↗

bioRxiv2020

Transcription initiation mapping in 31 bovine tissues reveals complex promoter activity, pervasive transcription, and tissue-specific promoter usage

Characterizing transcription start sites is essential for understanding the regulatory mechanisms that control gene expression. Recently, a new bovine genome assembly (ARS-UCD1.2) with high continuity, accuracy, and completeness was released; however, the functional annotation of the bovine genome lacks precise transcription start sites and contains a low number of transcripts in comparison to human and mouse. Using the RAMPAGE approach, this study identified transcription start sites at high resolution in a large collection of bovine tissues. We found several known and novel transcription start sites attributed to promoters of protein coding and lncRNA genes that were validated through experimental and in silico evidence. With these findings, the annotation of transcription start sites in cattle reached a level comparable to the mouse and human genome annotations. In addition, we identified and characterized transcription start sites for antisense transcripts derived from bidirectional promoters, potential lncRNAs, mRNAs, and pre-miRNAs. We also analyzed the quantitative aspects of RAMPAGE to produce a promoter activity atlas, reaching highly reproducible results comparable to traditional RNA-seq. Co-expression networks revealed considerable use of tissue specific promoters, especially between brain and testicle, which expressed several genes in common from alternate loci. Furthermore, regions surrounding co-expressed modules were enriched in binding factor motifs representative of each tissue. The comprehensive annotation of promoters in such a large collection of tissues will substantially contribute to our understanding of gene expression in cattle and other mammalian species, shortening the gap between genotypes and phenotypes.

genomics↗

bioRxiv2020

A comparative analysis of chromatin accessibility in cattle, pig, and mouse tissues

BackgroundAlthough considerable progress has been made towards annotating the noncoding portion of the human and mouse genomes, regulatory elements in other species, such as livestock, remain poorly characterized. This lack of functional annotation poses a substantial roadblock to agricultural research and diminishes the value of these species as model organisms. As active regulatory elements are typically characterized by chromatin accessibility, we implemented the Assay for Transposase Accessible Chromatin (ATAC-seq) to annotate and characterize regulatory elements in pigs and cattle, given a set of eight adult tissues. ResultsOverall, 306,304 and 273,594 active regulatory elements were identified in pig and cattle, respectively. 71,478 porcine and 47,454 bovine regulatory elements were highly tissue-specific and were correspondingly enriched for binding motifs of known tissue-specific transcription factors. However, in every tissue the most prevalent accessible motif corresponded to the insulator CTCF, suggesting pervasive involvement in 3-D chromatin organization. Taking advantage of a similar dataset in mouse, open chromatin in pig, cattle, and mice were compared, revealing that the conservation of regulatory elements, in terms of sequence identity and accessibility, was consistent with evolutionary distance; whereas pig and cattle shared about 20% of accessible sites, mice and ungulates only had about 10% of accessible sites in common. Furthermore, conservation of accessibility was more prevalent at promoters than at intergenic regions. ConclusionsThe lack of conserved accessibility at distal elements is consistent with rapid evolution of enhancers, and further emphasizes the need to annotate regulatory elements in individual species, rather than inferring elements based on homology. This atlas of chromatin accessibility in cattle and pig constitutes a substantial step towards annotating livestock genomes and dissecting the regulatory link between genome and phenome.

genomics↗

bioRxiv2019

Chromatin remodeling in bovine embryos indicates species-specific regulation of genome activation

The maternal-to-zygotic transition (MZT) is underpinned by wide-spread transcriptomic and epigenomic remodeling that facilitates totipotency acquisition. Factors regulating MZT vary across species and differences in timing of developmental transitions and motif enrichment at accessible chromatin between human and mouse embryos suggest a distinct regulatory circuitry. Profiling accessible chromatin in bovine preimplantation embryos--timing of developmental transitions in bovine closely resembles that in human--indicated that prior to embryonic genome activation (EGA) accessible chromatin is enriched in maternal transcription factor recognition sites, e.g., CTCF, KLFs, NFY, and SP1, echoing observations in humans and mice, and suggesting that a conserved set of maternal factors regulate chromatin remodeling prior to EGA. In contrast, open chromatin established during EGA was primarily enriched for homeobox motifs and showed remarkable similarities between cattle and humans, indicating that cattle could be a more relevant model for human preimplantation development than mice.

developmental biology↗