Bilateral NSF/BIO-BBSRC: Regulation of cell size in fission yeast

How is the size of a cell controlled? Cells come in vastly different sizes, ranging from tiny bacteria to very large neurons or protozoa, but cells of a given cell type generally display a fairly constant size. The size of cells is important for their function and livelihood, and is tightly regulated. In general, how cell size is specified by molecular components remains, however, mysterious. Are there rulers that allow the cell to determine how big it is? In actively dividing cells, growth is coordinated with cell division so that cell size is maintained. One strategy is for cells to sense that they are big enough before undergoing a cell division. Although such controls have been proposed for decades, the mechanisms for cell size sensing are still not properly understood in any cell type, not even in simpler cases such as bacteria and yeast.

We propose to study the fundamentals of cell size control in a simple model cell type, the fission yeast. These cells are ideally suited for this purpose because of their simple shape and regular division habits. The internal machinery that regulates cell division is conserved up to humans, making the findings in these simple cells generally relevant. Decades of research have identified the key molecules needed for cell size regulation. However, the central question of how these molecules are used to sense cell size remains poorly understood. We have recently suggested a new mechanism for how one of these molecules, Cdr2, is used to sense the size of the cell. We propose that Cdr2 probes the surface area of the cell, and then accumulates in dot-like structures in the middle of the cell called "nodes". From these nodes, Cdr2 can then report on the status of cell size to other proteins that tell the cell to divide. In this proposal we will study how Cdr2 senses cell size and how it reports to the cell cycle machinery. This effort involves collaboration between experts in mathematical modelling and experimentalists who will be imaging fluorescently labelled proteins inside living yeast cells. Our results will reveal how the rapid movements of Cdr2 and other molecules from one place to another in the cell allow it to sense cell size. The importance of this work is that it will provide one of the first examples of a cell size sensing mechanism. This will have broad implications into how other cells control their sizes, and ultimately provide insight into diseases such as cancer where cell size controls are altered.

The mechanistic basis of cell size control is not properly understood in any cell type. In dividing cells, cell size homeostasis can be achieved through thresholding, with cells growing to a minimal size before division. Fission yeast is a well-studied model for size control, offering many experimental advantages, including regular shapes and division patterns. Pathways of conserved protein kinases that affect cell size have been identified, but it is not generally agreed how a minimal size is sensed. Here, we will dissect cell size control in fission yeast through predictive mathematical modelling and experiments. The intrinsically quantitative nature of cell size control makes this approach highly appropriate.

We recently proposed an initial cell size control model involving Cdr2, a dose-dependent mitotic activator that regulates Cdk1/Cyclin B through Wee1 inhibition. Cdr2 localizes to a band of cortical nodes around the nucleus, with its nodal concentration scaling with cell surface area. Cdr2-encoded size information can then be transmitted across the cell to regulate the downstream cell cycle machinery. Here, we will investigate further these size scaling properties, by studying two key properties of Cdr2: its clustering to form nodes, and nodal localization to a region overlying the nucleus. We will construct mathematical models of clustering based on aggregation-fragmentation, also incorporating nuclear shuttling and inhibition by another protein Pom1. We will test model predictions by live-cell confocal, single particle TIRF and super-resolution microscopy. Finally, we will study how cell size information in the nodes is transmitted to the Spindle Pole Body where mitotic decision-making occurs. We hypothesize this takes place via concentration gradients or through signal corralling by diffusion barriers. We will test these ideas by microscopy and identification of possible barriers, such Endoplasmic Reticulum membranes.

The immediate impact of this research will be felt within the academic community, as detailed above. However, this work also has high potential to broadly impact public appreciation of basic research. The size of cells is a topic that everyone, even small school children, can readily understand and appreciate. The general question is fundamental and universal to all organisms, and we anticipate that some aspects of this work will ultimately find their way into school science textbooks.

Understanding the principles of cell size control will also influence and invigorate other areas of research, for instance in understanding how the size of intracellular structures, such as organelles, cilia and spindles, are controlled. We expect that our work will stimulate additional activity in these fields. Although the current motivation of this work is not towards addressing any particular human disease, this topic will ultimately have broad implications towards human health. Cancer, cardiomyopathy, kidney disease, developmental brain disease, obesity and aging have all been linked with abnormalities in cell growth and cell cycle regulation. In addition, cell size regulatory proteins such as Wee1 are promising drug targets for cancer therapy. Eventually an improved understanding of cell size control could therefore generate significant benefits for human health and biotechnology. However, we acknowledge that the time horizon for such enhanced understanding to percolate into new, economically important areas will probably be long term, at least 10 years from the start of the project.

BBSRC/NSF will also benefit by bringing together researchers from the US and UK to work on an important problem relevant to the BBSRC strategic priorities "International partnerships" and "Systems approaches to the biosciences", and to the NSF Division of Molecular and Cellular Biosciences (Cellular Dynamics and Function cluster, in the area of multi-scale integration). Finally, an important impact outcome will be the training of two highly interdisciplinary postdoctoral researchers.