Dr Nitya Ramkumar and Prof Buzz Baum have recently been awarded a BBSRC Responsive Mode grant for their project entitled “The causes and consequences of cell division asymmetries”. We spoke with Nitya about the scientific ideas underlying this grant.

 

Nitya did her undergraduate studies in Biotechnology at the Vellore Institute of Technology in India, where she studied basic biology as well as engineering. Growing up, curiosity for science was encouraged by her parents, who asked her questions and challenged her to think and figure out how things work. After internships in both industry and research labs, she followed her heart and embarked on an academic career in biological research. She completed a PhD in Developmental Biology at the Weill Cornell Graduate School of Medical Sciences in New York, USA. Currently, she is a postdoc in the Baum Lab, where she studies cell size regulation during cell division. 

 

The overarching question we want to investigate are the mechanisms that regulate the birth size of cells during cell division. Cell size is a very fundamental part of a cell’s biology. Each cell is born with a certain size - our body is comprised of cells of a dynamic range of sizes and it is important to understand how these cells get to their individual size. We are interested in how and when this size is determined, and how growth affects size. There is also a lot of interest surrounding the size of stem cells and whether it can affect aspects of their biology. No one knows this at the moment. It is a very new, rapidly developing field. Previous work from the lab has shown that when cells go into mitosis they round up and become spherical in shape. At the end of mitosis, the sibling cells return to their normal or original shape. We assume that cells undergoing symmetric division will give rise to sibling cells with the same size at the end of mitosis. However, the cell is a 3D entity and so far not much is known about how equal sibling cells are in terms of cell volume and dry mass content, i.e. how equal is a symmetric cell division in terms of cell volume? Furthermore, we would like to know when the final cell volume of sibling cells is set during cell division. We hope to understand the contribution of different steps during division by altering them and forcing the cell to divide asymmetrically. We have selected four steps of cell division to monitor closely.

 

We are going to look at positioning of the mitotic spindle, formation of the actomyosin ring, polar relaxation and cytoplasmic flow. We are interested in the potential role each of these events may play in cell size determination. Each step is prone to some degree of error during a normal cell division. We plan to exaggerate these errors with our manipulations – for instance, forcing symmetrically dividing cells in culture to divide under confinement causes off-centering of the mitotic spindle and leads to size asymmetry. Using such manipulations, we will study the effect of the above steps on daughter cells arising from symmetric division. This will inform us about mechanisms the cell has in place to correct these errors and determine how efficient they are. We then want to apply similar strategies to cells that divide asymmetrically and see how asymmetry is set up and how the error correction mechanisms are adapted in this scenario.




To examine events in symmetric cell divisions, we will use cells that can divide in the absence of an actomyosin ring, allowing us for the first time to test whether ring formation and contraction plays any role in cell size determination. We can also misposition the spindle by making the cells very flat and big during mitosis, so the spindle ends up positioned in the far corner of the cell. For asymmetric cell divisions, we will study the sensory organ precursor cells in the fly notum, an epithelial tissue on the back of a fly which eventually gives rise to the mechanosensory bristles. This is a great system because the bristle is produced by asymmetrically dividing cells within a field of symmetrically dividing cells in the epithelium, giving us a nice internal control. It will also allow us to investigate whether there is a functional consequence of size, as we can see whether or not a functional organ is produced. 

 

 

The idea for this grant was driven by several observations from members of the lab working on different areas of cell division, which prompted us to explore the subject further. For me personally, while working on the asymmetric cell divisions in the flies, we and others noticed the size difference of the sibling cells. When I researched the literature to find out what was known about this, I essentially found very little on the topic. This made me realise that not much is known about 3D cell size in the context of cell division.

 

Until now, reliable techniques for accurately measuring volume have been lacking. Accuracy is very important in this field as differences in cell size may be hard to detect because the margin of error is too big. But now several labs have developed technologies and tools that allow us to accurately measure cell volume and the dry mass content of the cell, which helps us to characterize size with a greater degree of accuracy. For this, we will be collaborating with the lab of Matthieu Piel. 

 

Eventually I want to look at cell divisions in a mammalian context, in tissues or organs, to see whether our basic findings from cell culture and fly model systems hold true in mammals. Perhaps by initially using organoid cultures and then moving to a live system, such as mice.

 

I always wanted to write a grant application, just to get the feel of it. It is important if you want to work in academia, and it teaches you a lot of things. First, it helps you to get clarity of thought. You need to have a clear idea and a plan of action. This facilitates your day-to-day research, knowing  what experiments need to be done keeping in mind the long-term plan. Second, it gives you an oppourtuniy to exercise your writing skills, a skill fundamental to communicating science.  




The most important thing is to have clear, well-defined questions. This lays down the framework of the grant and makes writing a lot easier.