Sarah Woolner, PhD
The tissues of our bodies are extremely complicated at the cellular level, comprising different types of cells arranged with precise geometry. Within this complex system, the direction in which a cell divides is a crucial tool used to shape tissues and determine cell fate. Defects in division orientation have lethal consequences: they cause failures in embryonic development and are associated with cancer.
To coordinate cell division across a tissue, cells must be able to “read” their external cellular environment and orient their division accordingly. The mechanisms that control this remain unclear, but we know that cues from the extracellular matrix play a vital role. These cues must be fed to a cellular structure called the mitotic spindle, the positioning of which determines cell division orientation. Understanding the mechanisms used by the cell to correctly position the spindle is a key focus of our lab. In particular, we are investigating how molecular forces are balanced inside the cell to position the mitotic spindle and how these internal mechanisms are linked to the external cellular environment in order to coordinate spindle orientation across a tissue.
1. Balancing forces to position the spindle: how are spindles held level during symmetric cell divisions?
Much of our knowledge of oriented cell division has come from studies of asymmetric divisions, where polarised cells divide to produce daughter cells that differentially inherit cell fate determinants. However, we know much less about symmetric divisions – where daughter cells inherit the same fate determinants – even though these divisions are vital for expanding cell populations, maintaining stem cells and shaping tissues. For a cell to achieve a symmetric division, we know that the mitotic spindle must be held perfectly level, but the cellular mechanisms that control this are largely unknown. To address this question we are studying spindle positioning in the epithelium of the Xenopus laevis embryo, where we can investigate how cell divisions are controlled in the context of a developing tissue.
We have shown that spindles are held level during symmetric divisions by balancing two opposing, dynamic forces – a basally-directed force supplied by microtubules and myosin-10 and an apically-directed force driven by actomyosin. Loss of
this balance causes spindles to position incorrectly, leading to a loss of symmetric divisions and resulting in a disorganised epithelium. We think that the dynamic nature of this mechanism might be particularly important in the fast-changing environment of the developing embryo in order to combine accuracy of spindle positioning with the flexibility to quickly adjust division plane in response to tissue cues.
2. Reading the external environment: how do cells in a tissue know which direction to divide in?
Most investigations of spindle orientation have concentrated on regulation by internal cellular machinery. However, recent work has suggested that external mechanical cues, such as tensile or compressive forces, can also direct spindle orientation. This has very important implications for oriented cell division in tissues. For example, cells in a developing embryo undergo a variety of compressive and tensile forces during the tissue shaping movements of morphogenesis. Linking the spindle to these physical changes may help to coordinate cell division orientation across a developing tissue, essentially "telling" cells which direction to divide in. Moreover, some diseases, such as cancer, are known to alter the mechanical properties of a tissue. These changes could interfere with the control of cell division orientation, causing cells to divide in the wrong direction and potentially accelerating the disease process. It is therefore vital that we understand the mechanical cues that are important for spindle orientation and the cellular machinery that links the spindle to these cues.
To explore these issues we are using the Xenopus embryo to investigate how spindle orientation in a developing embryo is affected when the physical environment of a tissue is altered. To do this we are using two in vivo approaches: first we are physically manipulating the tissue environment using laser ablation and second we are genetically manipulating the tissue to alter actomyosin contractility. In addition, we are using an ex vivo approach to directly test how tensile stress affects spindle orientation. By combining these manipulations with gene knockdown approaches we aim to uncover the molecular machinery that allows cells to sense these physical changes and transmit them to the mitotic spindle. In particular we are focusing on the roles of cell-cell and cell-matrix adhesions and their links to the actin cytoskeleton.
Recent key publications
Jones, L.A., Villemant, C., Starborg, T., Salter, A., Goddard, G., Ruane, P., Woodman, P.G., Papalopulu, N., Woolner, S. and Allan, V.J. (2014). Dynein light intermediate chains maintain spindle bipolarity by functioning in centriole cohesion. J Cell Biol. 207, 499-516. PubMed
Nestor-Bergmann, A., Goddard, G. and Woolner, S. (2014). Force and the spindle: mechanical cues in mitotic spindle orientation. Semin Cell Dev Biol. 34, 133-9. PubMed
Woolner, S. and Papalopulu, N. (2012). Spindle position in symmetric cell division during epiboly is controlled by opposing and dynamic apicobasal forces. Dev Cell. 22, 775-787. Pubmed