NIH Postbacs gathered in early May to share their research with the NIH community at the 2013 NIH Postbac Poster Day. Judges organized by the Office of Intramural Training & Education (OITE) evaluated each poster presentation, and postbacs scoring in the top 20 percent received an Outstanding Poster Award. The NICHD Connection is happy to announce that four NICHD postbacs received such an award. Continue below to learn about the exciting research from our winning postbacs!
Studying Human Methylation Disorders in Fish
By Brett Athans
Mentor: Dr. Brant Weinstein
Epigenetic modifications to DNA--modifications that do not disrupt the actual DNA sequence—are known to affect gene expression. DNA methylation, the placement of methyl groups at certain places in the genome, plays a crucial role in gene expression regulation in normal development as well as disease, such as cancer.
To understand how DNA methylation regulates gene expression during embryonic development, we use zebrafish as a model organism to study DNA methyltransferases (Dnmts), the enzymes that methylate cytosine residues in DNA. There are zebrafish homologs for each human Dnmt gene, making it easy to study the roles of these genes in a vertebrate model system.
One of the human Dnmt genes, called Dnmt3b, is linked to ICF-1 syndrome, a disorder with defects in immune cells and craniofacial abnormalities. We isolated zebrafish Dnmt4, a homolog of human Dnmt3b, and characterized its role in hematopoietic development. Our results showed that Dnmt4 is expressed by developing hematopoietic stem cells (HSCs) in zebrafish embryos. Loss of Dnmt4 function in zebrafish leads to a gradual reduction in HSCs and downstream blood cell lineages, including immune cells.
To develop a zebrafish model for human ICF-1 syndrome, we have used a method that can target mutations to the Dnmt4 gene. We have now obtained several fish with mutations in Dnmt4, and we are currently analyzing these mutants for blood cell and craniofacial defects.
By Ankur Narain
Mentor: Dr. Keiko Ozato
I examine how histones, proteins in the cell’s nucleus that package DNA into an ordered structure, impact the regulation of innate immunity in macrophages. Specifically, I study histone H3.3, a replacement histone found in transcriptionally active genomic regions. Using a novel knock-in mouse model, I utilized biological tags to study how H3.3 is induced and deposited onto the genome after activation of macrophages by interferon-γ (IFN-γ), an important molecule within the innate and adaptive immune system.
I have found that H3.3 protein levels remain relatively unchanged after macrophage activation, despite a significant increase in H3.3 mRNA transcript levels. This indicates that H3.3 protein has a large endogenous pool that buffers against increases in protein levels via new translation. I also found that H3.3 was deposited towards the 3’ end of genes that are activated by IFN-γ. The deposition of H3.3 was uncoupled to transcription, as H3.3 was primarily deposited after transcription of those genes had ended. This result may indicate a role for H3.3 in gene memory and reactivation of macrophages. Discovering this possible epigenetic role of H3.3 provides exciting opportunities for future research.
Sexual Activity May Promote Ovulation
By Ankita Prasad
Mentor: Dr. Enrique Schisterman
Characterizing the relationship among sexual activity, reproductive hormones, and ovulation can give us insight into female fertility. In our study, we found that women who were sexually active were less likely to have anovulatory cycles compared to women reporting no prior or current sexual activity. This relationship between sexual activity and ovulation remains even after taking into account age, race, body mass index (BMI), perceived stress level, and self-reported alcohol consumption.
We also observed significantly elevated levels of estrogen, luteal progesterone, and mid-cycle luteinizing hormone (LH), but not follicle stimulating hormone (FSH) or testosterone, in women who reported past or current sexual activity compared to women who reported never having been sexually active. These findings were from 259 women, aged 18-44, who participated in the BioCycle Study, which followed the women for ≤2 cycles and restricted participation in the study to women not attempting pregnancy or using hormonal contraception. Sexual activity, defined as vaginal intercourse, was self-reported via a daily diary.
Overall, sexual activity was associated with a lower probability of anovulation, concurrent with elevated estrogen, luteal progesterone, and mid-cycle LH concentrations, suggesting a possible correlation among sexual activity, reproductive hormones, and ovulation in humans.
Congenital Syndrome Raises Questions About Cell Fate Decisions
By Amy Ton
Mentors: Kevin Francis, Karl Pfeifer, Forbes Porter
Cells are often situated in dynamic environments that contain external signals, prodding cells to activate, to extend, or simply, to function. The encapsulating and selectively permeable cell membrane acts as an initial communicator of signals to and from the cell. Our work explores how proper cell membrane integrity changes a cell’s ability to respond to its environment.
We are interested in how membrane integrity affects the ability of stem cells to self-renew, as stem cells use their environment to determine if they should maintain their stem-cell state or differentiate into a more mature cell type. To study this, we disrupted a stem cell’s ability to synthesize cholesterol, a molecule that is highly enriched in specialized communication centers within the membrane and important for cell membrane fluidity.
We generated stem cells, called induced pluripotent stem cells, from skin cells of patients with Smith-Lemli-Opitz syndrome (SLOS), a congenital disorder characterized by defects in cholesterol biosynthesis. Additionally, we used a small molecule to disrupt cholesterol synthesis in normal human embryonic stem cells. Interestingly, when we cultured these cells in cholesterol-free conditions, the cells quickly differentiated toward neuroectodermal lineage, cells destined to become nervous system tissue. This led us to a slew of questions, such as what pathways could be driving this accelerated differentiation? Are these defects physiologically relevant? And, why are they choosing a neuroectoderm cell fate? We are currently pursuing answers to these questions—among many others.