There is a time and pace for everything, cytokinesis is no different!
In a paper in e-Life, integrated PhD student Swati Sharma and faculty member Dr. Richa Rikhy study the process of cell division and describe how a protein named GRAF regulates the time and pace of actin ring constriction, a phenomenon that sets the stage for cell division.
The line along which a cell divides into two begins to be defined by what biologists call the cleavage furrow. This is an indentation on the cell surface which, when it deepens further, cleaves one cell into two while also distributing the cell contents amicably.
The actin cytoskeleton, a meshwork under the cell membrane that helps cells hold their shape and provides mechanical support, collaborates with molecules within the cell membrane to form the cleavage furrow. The initiation and pace of cleavage furrow formation are regulated by molecular players that activate, or inhibit, the actin cytoskeleton.
In this work, Dr. Rikhy’s group studied the role of the actin cytoskeleton and one of its modulators in the formation of the cleavage furrow. They carried out this study in the embryo of the fruit fly, Drosophila, a popular model system to delve deep into molecular details of various cellular phenomena.
The fruit fly embryo begins its development as a syncytium, where the nuclei of the cell divide within a single cell, in a common cytoplasm. After 13 such cycles of division of nucleus, individual cells begin to form, when the plasma membrane organises as a polygonal epithelial-like array around individual nuclei. The cleavage furrow helps close off these cells at the base.
How is the timing and pace of cleavage furrow formation regulated in these cells? What are the activators for this cleavage furrow formation? Are there gate keepers which control the speed of the constriction process? It is questions such as these that Sharma and Rikhy began to address beginning with this study.
Various activators of actin cytoskeleton polymerisation at the cleavage furrow constriction are known to initiate its formation. Looking for such modulators, the team found a membrane binding protein called GRAF to play a role in inhibiting cleavage furrow formation.
Their work showed that embryos that do not contain GRAF display greater constriction of the cleavage furrow, indicating that the presence of GRAF inhibited the formation of cleavage furrow.
This image shows the cleavage furrow of the cells labelled with fluorescent actin (in red) in Drosophila embryogenesis. Cells (from left to right, 3 panels) are at successive stages of constriction. Cells in the top row represent the control condition, i.e., conditions where cells have not undergone any specific treatment or disruption. In the cells in the bottom row, the levels of GRAF protein have been reduced as part of the experiment.
The schematic versions in the last panel depict the cellular locations of actin (red) and GRAF protein (green) under control and GRAF depleted conditions. In the control condition, GRAF is present along with actin in the polygonal and intermediate ring stage and is in the cytoplasm in the center of the ring during the late stages. GRAF depletion leads to loss of polygonal organisation of cells and ring hyper constriction. (Image source: Dr Richa Rikhy)
The team also found that GRAF localises at the cleavage furrow to regulate the levels and activity of actin polymerizing proteins responsible for constriction. The team suggests that in doing so, and possibly by interacting with regulators of adhesion, GRAF helps maintain adhesion, i.e., helps cells remain attached to their milieu, and prevents untimely constriction. The team also found that GRAF is present in multicellular organisms.
Based on these observations, authors Sharma and Rikhy conclude that GRAF is suitably poised to regulate the time and pace of disruption of adhesion and the onset of constriction in cleavage furrow formation.
This research was funded by the Department of Biotechnology, Government of India and IISER, Pune.
Swati Sharma and Richa Rikhy (2021). Spatiotemporal recruitment of RhoGTPase protein GRAF inhibits actomyosin ring constriction in Drosophila cellularization. eLife 10:e63535 DOI: 10.7554/eLife.63535
- With inputs from Dr. Richa Rikhy