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Dr. Veit Riechmann

Regulation of cytoskeletal dynamics in the Drosophila egg chamber

    We aim to understand how the cytoskeleton mediates development. In particular, we are examining how the actomyosin cytoskeleton coordinates epithelial morphogenesis. In addition, we analyse the polarisation of the oocyte cytoskeleton and its role in axis determination.

    We use Drosophila oogenesis as a model to investigate cytoskeletal dynamics. Oogenesis proceeds in small units called egg chambers. These simple organs consist of two tissues: an inner germline cyst of 1 oocyte and 15 nurse cells, and an outer epithelial layer (Fig. 1).

    Two cell types are excellent models to study the cytoskeleton. First, the oocyte with its highly polarised microtubule cytoskeleton, along which the determinants of the embryonic axes are transported. Second, the outer epithelial cells, which provide an outstanding model to study epithelial polarisation.

    The understanding of the mechanisms regulating the cytoskeleton is relevant for several medical problems. For example, loss of cytoskeletal polarity is a hallmark of most tumour cells. Further, the investigation of cytoskeletal organisation during morphogenesis will greatly advance our understanding of abnormal cell behaviour in a variety of human diseases. Oogenesis provides a simple model to study the basic principles controlling cytoskeletal polarisation and morphogenesis.

Figure 1

Regulation of tissue morphogenesis by the actomyosin cytoskeleton

    We have shown how the shape of the Drosophila egg chamber is determined by counteracting forces from an inner cyst and an outer epithelium. The cyst grows rapidly leading to a dramatic volume increase (orange arrows in Fig. 1), which is compensated by cell divisions within the epithelium. We found that myosin activity is restricted to the apical surface of the epithelium facing the growing cyst (green dashed line in Fig. 1). We have demonstrated that this activity prevents epithelial deformation by balancing the force emanating from cyst growth. In addition, we have shown that cyst growth induces cell divisions in the epithelium (Wang and Riechmann, 2007).

    We are currently investigating how myosin activity is spatially controlled in the cells of the follicular epithelium. Further, we analyse the mechanisms of tension induced cell proliferation in the follicular epithelium.

Figure 2

Polarisation of the oocyte cytoskeleton and axis formation

    We have found that the actin cortex of the oocyte is composed of thick actin bundles (Fig. 2), in which the minus ends of the microtubules are embedded. Disruption of these bundles results in cortical release of the microtubule minus ends. This cortical release leads to a dramatic reorganisation of the microtubule cytoskeleton, preventing the establishment of the embryonic axis (Wang and Riechmann, 2008).

     We are further investigating the mechanisms that polarise the cytoskeleton of the oocyte and its role in the determination of the fly body axis.

Selected Publications: