ER, Golgi, and PM dynamics in the developing Drosophila embryo
The early Drosophila embryo provides a unique model system for studying changes in the organization and dynamics of ER and Golgi in a voluminous, multi-nucleated system during development. Drosophila embryogenesis begins with 13 rapid nuclear divisions within a common cytoplasm. The first nine of these nuclear divisions take place deep within the interior of the embryo to produce roughly 300-400 nuclei by the end of the ninth division. Development continues with the migration of these nuclei toward the periphery of the embryo. Once at the periphery, nuclei undergo four additional rounds of division (nuclear cycles 10-13) in the stage known as the syncytial blastoderm. The sharing of a common cytoplasm ceases when each nucleus becomes individually encased in plasma membrane during interphase of nuclear cycle 14. This cellularization event produces over 6000 primary epithelial cells. See movie for a surface view of nuclear divisions 6-13.
To explore how the secretory endomembrane system is organized in the multinucleated embryo, we have generated transgenic flies expressing proteins targeted to the ER lumen (lysozyme-GFP-KDEL, lyso-GFP-KDEL), Golgi membranes (galactosyltransferase-GFP, GalT-GFP) and plasma membrane (transgenic lines available upon request to Manos Mavrakis).
Time-lapse confocal imaging and photobleaching experiments in living lyso-GFP-KDEL and GalT-GFP expressing embryos showed that the ER and Golgi were found to segregate among nuclei, with each nucleus becoming surrounded by a single ER/Golgi membrane system separate from adjacent ones. The nuclear-associated units of ER and Golgi across the syncytial blastoderm produced secretory products that were delivered to the plasma membrane in a spatially restricted fashion across the embryo. The emergence of secretory membranes compartmentalized around individual nuclei in the syncytial blastoderm is likely to ensure that secretory organelles are equivalently partitioned among nuclei in preparation for cellularization and could play an important role in the establishment of localized gene and protein expression patterns within the early embryo.
The plasma membrane is the end compartment of the secretory pathway. The questions that we are currently addressing include: 1) how is maternally-loaded plasma membrane organized during the syncytial nuclear divisions, (2) what is the molecular machinery underlying PM reorganization during nuclear divisions, and (3) is lateral diffusion in the PM unrestricted across the embryo periphery.
For more information, contact Manos Mavrakis.
