Teasing apart the cellular and molecular pathways that regulate cell size during asymmetric neural precursor cleavages

The size of animals can vary enormously, from a blue whale that can reach a length of over 30 meters to some species of parasitic wasp that even as adults are less than a millimetre in length. However it is not just overall size that varies, the cells within our bodies also display a remarkable range of sizes. For certain cell types there is an obvious association between a cells function and its physical dimensions. Muscle cells need to be long and thin to facilitate contraction and skin cells need to be large and spread out to maximise surface area coverage. Size does matter as they say. Much progress has been made in our understanding of the cellular and molecular mechanisms that regulate how specific organs and tissues can increase in size through cell proliferation or individual cell growth. Yet in many animals cell types of different sizes are generated even during early embryogenesis at a period in which cell growth has not yet kicked in. How cell size during early embryonic divisions is regulated is much less well understood and is the focus of this proposal.

One mechanism by which cells of different sizes can be generated is known as unequal cell division. This is a cell division where, during the division, the separation of the two cells is asymmetric and generates one large daughter and one small daughter. Misregulation of this type of division has been implicated in various human diseases such as cancer and so it is imperative we gain a deeper understanding of the cellular and molecular mechanisms involved. In my lab we are interested in how neurons are generated. Neurons are a key cell type in both our brain and peripheral nervous system that not only allow us to think but also allow us to sense and respond to our environment. We study the development of neurons during embryonic development of the nematode C. elegans. Many genes and processes are conserved between this nematode and humans and so we can gain important insights into key fundamental biological process by studying this genetic model system. We study the lineage of cells that gives rise to a specific cell - known as a neuroblast - that itself generates neurons. We have discovered two unequal divisions in the precursor cells that generate this neuroblast. Intriguingly and contrary to popular belief, we have been able to show that in our system the cell size differences produced by these unequal divisions do not affect the development or production of neurons. Instead, we find that the same factors that regulate neuronal specification also regulate the unequal cleavages. This suggests that regulation of unequal division via these factors is an important way of producing cell size diversity in the absence of cell growth. Here we propose to exploit the transparency and genetic amenability of C. elegans to follow how various genetic perturbations affect this asymmetric division. We will first determine whether unequal division depends on neighbouring cells or is intrinsically programmed within the cell itself, and how robust it is to changes in overall size. We will then identify the genes directly responsible for unequal division and finally explore how the upstream regulators we identified orchestrate the complex cell biology required to achieve an unequal division plane. A deeper understanding of how cell fate and cell size are coordinated during development, particularly in situations where cell proliferation and cell growth are not possible is crucial to our understanding of how our nervous system develops and how an organism is built.

This proposal concerns the regulation of cell size during embryonic divisions in the nematode C. elegans. In particular, we will determine how cleavage divisions can generate a variety of cells of different sizes in the absence of cell growth. We study neuronal development in the C lineage, which asymmetrically generates the DVC neuroblast. We have made three important observations regarding the lineage that gives rise to the DVC neuroblast: (i) we have identified two unequal divisions in the precursors that generate the DVC neuroblast; (ii) we have found that in certain genetic backgrounds these divisions are equalised yet this has little or no effect on neuronal cell fate; (iii) we have discovered that the unequal divisions are regulated by the same factors that regulate neuronal fate specification of the DVC neuroblast. Together this suggests a paradigm shift in our thinking - rather than cell size regulating cell fate as has been previously proposed, we hypothesise that cell fate regulators are regulating unequal cleavage in order to generate cells of the correct cell size during embryonic development. To test this hypothesis we will determine the cellular principles required for these unequal cleavages by assessing autonomy and robustness to size changes using a combination of blastomere isolations, laser ablations and genetic manipulations. We will elucidate the molecular pathways involved through both forward genetic screening and candidate approaches combined with 4D-lineaging and acute temperature shifts to determine the molecular mechanisms of symmetry-breaking, polarity establishment and unequal cleavage. We will shed light into how key cell fate regulators can impinge on the mechanisms of unequal cell division, widening our understanding on how cell size diversity arises during normal embryo development.