University of Texas at Austin
School of Biological Sciences The Section of Molecular Cell and Developmental Biology The Institute for Cellular and Molecular Biology School of Biological Sciences / The Section of Molecular Cell and Developmental Biology / The Institute for Cellular and Molecular Biology
Research
The Papoulas lab is interested in the molecular mechanisms that control how cells form and differentiate in early animal embryos. Our research is currently focused on elucidating the molecular mechanisms of two processes that control the formation of the first polarized epithelial cell layer in early Drosophila embryos: Golgi-dependent membrane secretion and Fragile X mental retardation protein-dependent translational regulation. We study these processes using an integrated experimental approach that combines the advantages of biochemistry, genetics, molecular biology and live embryo imaging.

Introduction:

All animal embryos undergo a cleavage stage at the beginning of development when cells proliferate through cleavage divisions without growing and shift from maternal to zygotic genetic control (Davidson, 1986). The developmental fates of the many smaller cells that form during the cleavage stage progressively transition from totipotency to pluripotency before committing to form specific cell types that give rise to tissues and organ systems later in embryogenesis. This remarkable period of cellular morphogenesis and differentiation requires substantial plasma membrane growth to form the hundreds to thousands of cells that comprise cleavage stage embryos. It also requires the faithful shift from post-transcriptional regulation of maternal mRNA molecules to the transcriptional activation of the zygotic genome, referred to as the maternal to zygotic transition (MZT), to ensure that cells differentiate normally. Despite the fundamental importance of this stage in animal development, we still know relatively little about the molecular mechanisms of membrane transport and the MZT that control the cellular morphogenesis and differentiation of cleavage stage embryos.

Figure 1. Figure 1. Figure 1.
 
Figure 1. (click for details)
Figure 1. Drosophila cellularization. The cartoons below depict early (left) and late (right) nuclear cycle 14 embryos in sagittal cross-sectional view. Nuclei (n) become encapsulated during cellularization by a invaginating plasma membrane furrow to form the primary epithelium. Germ line progenitor pole cells (pc) form at the embryo’s posterior during nuclear cycle 10. Corresponding laser-scanning confocal micrographs of fixed nuclear cycle 14 embryos are shown above and reveal the subcellular localization of microtubules (green) and microfilaments (red) that are required for cleavage furrow formation.
 

In Drosophila the cleavage stage of embryogenesis occurs in two phases. First, following fertilization of the egg 13 rounds of rapid and synchronous mitotic nuclear divisions produce a syncytial blastoderm consisting of 1000s of somatic nuclei associated with the plasma membrane (Figure 1)(Foe et al., 1993). Although the MZT begins during nuclear cycle 10-11, the post-transcriptional regulation of maternal gene products is sufficient to mediate all the events of the first phase, up until nuclear cycle 14 (Edgar et al., 1986; Merrill et al., 1988; Wieschaus and Sweeton, 1988). During the interphase of nuclear cycle 14 a continuous plasma membrane furrow forms between adjacent somatic nuclei as the MZT progresses. The furrow ingresses deep into the cytoplasm before widening at its base to encapsulate individual nuclei in cells, creating the primary epithelium that ultimately gives rise to all the somatic cells of the embryo and adult (Mazumdar and Mazumdar, 2002; Schejter and Wieschaus, 1993; Schweisguth et al., 1991). It also marks the completion of the cleavage stage. This special form of cleavage furrow formation, called cellularization, takes about 1 hour to complete and has been estimated to require a 20-fold increase in plasma membrane (Foe et al., 1993). The process is thought to depend primarily on maternal gene products, but it also requires the de novo synthesis of a small number of zygotic genes (Merrill et al., 1988; Wieschaus and Sweeton, 1988). Together these gene products pause the cell cycle in interphase, stimulate membrane secretion and cytoskeletal dynamics required for cleavage furrow ingression, and mediate actomyosin-based contraction during abscission, the terminal step of cellularization. Most of the genes required for Drosophila cleavage furrow formation have not yet been identified.

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