Melissa Marchal and Changya Chen Will Present Their Research on May 22, 2014
The Role of Maternal Wnts and Frizzleds in Dorso-Ventral Axis Specification
Elucidating the molecular mechanisms that transform a symmetric egg into a more complex embryonic body has interested developmental biologists for some time. The asymmetric distribution of determinants within the oocyte is critical to normal developmental processes such as dorsal axis determination and cell migration. In Xenopus laevis in particular, maternal mRNAs involved in the patterning of the secondary body axis are localized to the oocyte vegetal cortex, and upon sperm entry, are subsequently translocated to the future dorsal side. This process, called cortical rotation, is required for the asymmetric activation of the Wnt/β-Catenin signaling pathway on the dorsal side of the blastula. β-catenin, a down-stream wnt effector, regulates the transcriptional activation of dorsal-specific genes at the midblastula transition. Support for the role for β-Catenin signaling in the specification of the dorso-ventral (D/V) axis was garnered from maternal mRNA depletion studies in Xenopus. Recent evidence has suggested that secreted Wnt ligands Wnt11 and Wnt5a may act together in axis formation. However, the function of other maternally expressed Wnts and their cognate receptors (frizzleds) remains uncharacterized. We propose that one or more uncharacterized wnts/fzds act in D/V axis specification. Preliminary analysis of the developmental expression patterns of all wnts and frizzleds (fzds) present in frog, found that wnts 1, 2, 5a, 5b, and 10b and fzds 1, 3, 4, 5, 6, and 7 are expressed in the oocyte, suggestive of a role in D/V patterning. Moreover, maternal fzd1-depleted embryos exhibit a ventralization phenotype and partial defects in dorsal-specific gene expression, while fzd4-depleted embryos exhibit a dorsalization phenotype and an expansion of dorsal specific markers. Further characterization of the genetic factors involved in D/V axis specification could advance the understanding of the pervasive Wnt signaling pathway and provide mechanistic insights into developmental defects.
Long-Range Chromatin Interactions in Fetal and Adult Hematopoietic Stem Cells
HSCs in fetal liver undergo rapid self-renewal divisions, which lead to a massive increase in cell number of the HSC pool. In contrast, the adult bone marrow HSCs have lower self-renewal capacity. Fetal and adult HSCs display differences in their differentiated cell output. Fetal HSCs have erythro-myeloid lineage output while adult HSCs have balanced lineage output. These differences of biological properties between fetal and adult HSCs correlate with distinct gene expression in HSCs. The transcription factor Sox17 is required for the maintenance of fetal, but not adult, HSCs. Ezh2, a core component of polycomb repressive complex 2 (PRC2), is essential for fetal, but not adult, HSCs. In contrast, Bmi1, Gfi1, Etv6, and C/EBPα are required for restriction of self-renewal in adult HSCs. Several studies have shown that the expression of Sox17 is controlled by long-range chromatin interaction. SUZ12, a PRC2 subunit, can occupy the promoter region of Sox17 and repress its expression in ES cells. In addition, the expression of Sox17 is controlled by polycomb protein and H3K27me3 during pancreatic differentiation. The transcription factor SMAD2/3 can bind to the promoter region of Sox17 and is associated with H3K27me3 depletion to activate gene expression. Ezh2 and Bmi1, aother two polycomb proteins, are also involved into HSCs regulation. It is clear that polycomb proteins are key factors of long-range chromatin interaction. In addition, the expression of Gfi1 is regulated by five regulatory regions with Scl/Tal1, Gata2, PU.1, Erg, Meis1, and Runx1 as upstream regulators. These indicate that long-range chromatin interaction is essential to the expression of key regulators in fetal and adult HSCs. We currently use 3C-based technologies to identify the long-range chromatin interactions in both fetal liver and bone marrow HSCs.