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:

 Wang, Y. and Riechmann, V. (2008) Microtubule anchoring by cortical actin bundles prevents streaming of the oocyte cytoplasm. Mechanisms of Development 125, 142-152.

 Riechmann, V. (2007) Developmental Biology: Hippo prevents posterior patterning by preventing proliferation. Current Biology 17, R1006-1008.

 Wang, Y. and Riechmann, V. (2007) The role of the actomyosin cytoskeleton in coordination of tissue growth during Drosophila oogenesis. Current Biology 17, 1349-1355.

 Riechmann, V. and Ephrussi, A. (2004). Par-1 regulates bicoid mRNA localisation by phosphorylating Exuperantia, Development 131, 5897-5907.

 Riechmann, V., Gutierrez, G. J., Filardo, P., Nebreda, A. R. and Ephrussi, A. (2002), Par-1 regulates stability of the posterior determinant Oskar by phosphorylation. Nature Cell Biology 4, 337-342.
This article has been featured in News and Views in Nature Cell Biology. (Cell polarity: Oskar seeks PARtner for a stable relationship. Nat Cell Biol, 4, p. E117-E118) and in a Dispatch in Current Biology. (Cell polarity: Posterior Par-1 prevents proteolysis. Cur Biol 2002 12: R479-R481)

 Riechmann, V. and Ephrussi, A. (2001). Axis formation during Drosophila oogenesis. Current Opinion in Genetics and Development 4, 374-383.

 Tomancak, P., Piano, F., Riechmann, V., Gunsalus, K. C., Kemphues, K. J., Ephrussi, A. (2000). A Drosophila melanogaster homologue of Caenorhabditis elegans par-1 acts at an early step in embryonic-axis formation. Nature Cell Biology 2, 458-460.


 Complete list of publications (search Pubmed for: Riechmann-V).