Cytoplasmic organization and compartmentalization in Drosophila early embryo syncytium

Abstract

Syncytial nuclei found in fungi, plant endosperm, insect embryos and muscles maintain an autonomous 'nuclear-cytoplasmic region of influence', thereby generating compartments within a seemingly shared cytoplasm. Compartmentalization in the syncytial context has been found in Drosophila embryos for subcellular organelles and the plasma membrane. We characterized the cytoplasmic distribution and found that it is denser at the cortex when observed with exogenous proteins such as RFP. Diffusion of these proteins is restricted across nucleo-cytoplasmic regions and this restriction is pronounced above the nuclei as compared to around the nuclear region.To test the context of this architectural restriction on diffusion, we are using the paradigm of Bicoid gradient formation. Bicoid shows an exponential gradient in the anterior-posterior direction and nature of its gradient spread does not depend upon the presence of nuclei. We tested whether a Bicoid-like gradient would be formed by restricting diffusion as a function of protein size or attachment to the cyto-architecture. Transgenes containing tandem 5XGFP and 8XGFP with an mRNA localization sequence of Bicoid gave a linear drop of the fluorescent proteins in the anterior-posterior direction. Interestingly, transgenes expressing fluorescently tagged PH domain which binds plasma membrane regions containing PIP2 gave an exponential drop similar to Bicoid. Concomitantly, we also found that Bicoid may be enriched at the metaphase furrows. This indicates that Bicoid diffusion may result from differential association between cytoarchitecture components. Computational simulations which involve a test of dependence of Bicoid association with these components along with analysis of Bicoid spread in mutants that affect plasma membrane and cytoskeletal organization will be used to further probe the mechanisms of Bicoid diffusion across the syncytial Drosophila embryo.