Figure 1: A flat projection of the entire surface of a Drosophila embryo in which the position and speed of 6000 cells is followed over a 40 minute period. The head of the embryo is at the top, the center of the image is the ventral midline towards which the lateral cells are moving. Image by Matteo Rauzi
Figure 2: Zebrafish larvae 24 hours after infection with fluorescent bacteria. Normal fish survive and eventually clear the bacteria (left), but if the interferon signalling pathway is compromised (right) the bacteria proliferate and the fish die. Image by Dirk Sieger
The Leptin group studies the mechanisms and forces that determine cell shape in Drosophila and uses the zebrafish to analyse innate immune signalling.
Cell shape determination during development
The shape of a developing organism is generated by the activities of its constituent cells: growth and proliferation, movements and shape changes. We are particularly interested in shape changes.
One study concerns an extremely complex single cell, the terminal cell of the Drosophila tracheal system. It is highly branched and carries air to target tissues through an intracellular tube bounded by plasma membrane. During its rapid growth, the cell faces the task of synthesising large amounts of membrane and sorting it correctly to defined membrane domains. Extensive re-organisation of the secretory organelles precedes membrane growth. We are finding out how the cytoskeleton, small GTPases and polarity determinants direct the process, and how membrane trafficking processes contribute to building the tube.
In another project, we try to understand how the forces generated by individual cells are integrated within the supracellular organisation of the whole organism to give the tissue its final shape (see figure 1). We study the formation of the ventral furrow in the early Drosophila embryo. The cells that form the furrow are the major force generators driving invagination, but to allow furrow formation, neighbouring cells must respond and they may contribute. To understand force integration across many cell populations, we use simultaneous time-lapse imaging of multiple-angle views of the gastrulating embryo. We measure the specific shape changes in all the cells of the embryo, as well as the speed and direction of their movements. Genetic and mechanical manipulations reveal the underlying control circuits.
In vivo imaging of innate immune responses
The innate immune system provides rapid defence against pathogens and also deals with non-pathogenic stresses. Macrophages and dendritic cells, two key players in this system, patrol the body and respond to stimuli from damaged cells via extraand intracellular sensors. We aim to understand how such signals are recognised and how the appropriate subcellular and intercellular responses are triggered. We have discovered that one family of sensors, the cytoplasmic NOD-like receptors (NLRs), are particularly abundant in fish.
Fish model systems allow in vivo observation of physiological processes. Specifically, we watch pathogens and the cells that attack them. We use genetically and chemically engineered in vivo fluorescent reporters to assay immune and stress responses in real time and at high spatial and temporal resolution as the cells of the fish encounter pathogens and stress signals (see figure 2).