Figure 1: oskar mRNA on the move. Time projection of a squash of ooplasm from a stage 9 oocyte imaged with TIRF microscopy. oskar mRNA (labelled with MS2-MCPGFP, shown in rainbow colours) utilises microtubules (labelled with mCherrya1-tubulin and EB1-Cherry, shown in gray with cyan tips, indicating plus ends) to take fast, long linear runs.
Figure 2: A Drosophila egg-chamber, showing colocalisation of oskar mRNA, Staufen protein and a microtubule polarity marker at the posterior of the oocyte.
The Ephrussi group aims to understand the mechanisms underlying RNA transport and localised translation – fundamental processes that promote the functional polarisation of cells during development.
Previous and current research
Intracellular RNA transport coupled with localised translation is a powerful and widespread mechanism that promotes the functional polarisation of cells, from yeast to man. Asymmetric localisation of messenger RNAs within cells has key roles in cell fate decisions, cell migration, cell morphology and function. mRNA targeting is particularly evident in large cells, such as eggs and neurons, where it allows rapid and localised deployment of protein activities in response to extrinsic signals.
An ideal model for the study of RNA transport is the large Drosophila oocyte, in which asymmetrically localised cell fate determinants specify the body axes and pattern the future embryo. During oogenesis, mRNAs encoding these embryonic axis determinants are transported to specific sites within the oocyte, where they are anchored and locally translated, thus ensuring spatial restriction of their protein products. A polarised cytoskeleton and specific motor proteins mediate mRNA transport and anchoring within the cell. We use these RNAs as models to understand how mRNA localisation and translational control are regulated in space and time.
One RNA of particular interest is oskar, which encodes the posterior determinant of Drosophila. Oskar protein is uniquely endowed with the capacity to induce germ cell formation in the embryo, which it does by nucleating formation of the germ plasm and its germline determining RNP complexes, called polar granules. How oskar mRNA is transported and anchored at the posterior pole of the oocyte and its translation regulated is one focus of research in the lab.
We are also investigating the roles of other classes of RNAs, including long non-coding RNAs and piRNAs, and of non-canonical RNA binding proteins, in Drosophila embryonic development and neurogenesis. Drosophila, with its exceptional genetic tools, is also well suited to biochemical and cell biological investigation, including live imaging, of the processes of cell polarisation, mRNA localisation and translational control.
Future projects and goals
We combine genetics, biochemistry and a broad spectrum of cell biological and imaging approaches to study:
- Polarisation of the cytoskeleton.
- The roles and regulation of cytoskeletal motors in RNA localisation.
- Assembly of transport-competent RNPs: the cis-acting RNA targeting elements and interacting proteins, how they assemble and associate with motor proteins.
- Translational regulation of localised mRNAs.
- Germ plasm assembly and function.