Asymmetric cell divisions can produce daughter cells of different sizes, but it is unclear whether unequal cell cleavage is important for cell fate decisions. A new paper in Development explores the role of unequal cleavages in Caenorhabditis elegans embryos. To learn more about the story behind the paper, we caught up with first author Thomas Mullan and corresponding author Richard Poole, Associate Professor of Developmental Biology at University College London, UK.

Thomas Mullan (left) and Richard Poole (right)

Richard, can you give us your scientific biography and the questions your lab is trying to answer?

RP: I studied biology (with European studies!) at the University of Sussex and then moved to Nigel Holder's, and then Steve Wilson's lab at University College London for my PhD. There I worked on the role of Eph/ephrin signalling in the polarisation and morphogenesis of somite boundaries. All the talk in Steve's lab was about left-right asymmetry in the nervous system and I became fascinated by how this develops. I moved to Columbia University to study the mechanisms of left-right asymmetric neuron specification with Oliver Hobert, switching from zebrafish to C. elegans. In Oliver's lab, I not only became interested in many aspects of neuronal development but also fell in love with the worm (as a model system). In my own lab, we focus on uncovering the cellular and molecular mechanisms that regulate embryonic neurogenesis and, more recently, plasticity and transdifferentiation during sexually dimorphic glia-to-neuron cell fate switches.

Thomas, how did you come to work in Richard's lab and what drives your research today?

TM: I joined Rich's lab as the last of my rotation projects in the first year of my PhD programme. I was really drawn to the invariant cell lineage of C. elegans and the fact that it gives you single cell resolution for investigating both molecular and cellular aspects of fate. In terms of techniques, I really enjoyed manual 4D cell lineaging of embryos – a technique I went on to use extensively. The original project was framed in terms of left-right asymmetry in the C lineage, but shifted to the unequal cleavage story we present here. Now, taking a different direction, my research today is motivated by an interest in the origins and evolution of cell types and larger structures during development.

Tell us about the background of the field that inspired your work

TM & RP: One mechanism by which to generate fate decisions in development is through asymmetric divisions, those that give rise to two daughters of different fates. These divisions are also often unequal in size. The mechanisms governing unequal cleavages are extensively studied, with the C. elegans zygote and Drosophila neuroblasts being famous example systems. Despite this, in most cases it is often unclear how differences in daughter cell size relate to fate decisions. It could be that size is a requirement of the decision itself, such as being an aspect of segregation of fate determinants. Alternatively, as different cell types have different forms and size, size it could be an outcome of a fate decision related to function. We sought to interrogate this relationship in our system.

Can you give us the key results of the paper in a paragraph?

TM & RP: In this study, we firstly identified that two successive unequal cleavages precede the expression of a key proneural transcription factor required for the specification of a specific neuroblast in C. elegans. Equalisation of both cleavages, via depletion of known regulators of unequal cleavage, did not result in defects in the specification of this neuroblast, suggesting daughter cell size does not significantly influence cell fate in this lineage. We then demonstrated that loss of the proneural transcription factor and an upstream regulator (we identified the latter via a 4D-lineage-based forward genetic screen) leads to a concomitant loss of both unequal divisions and neuroblast fate. We therefore suggest that embryonic neuroblast cell size is regulated by the same transcriptional mechanisms that regulate neuronal fate acquisition via the control of unequal cell divisions.

I was busy lineaging a set of embryos and mentioned to Rich that I'd noticed the unequal cleavages preceding neuroblast formation. I had been wondering what this could mean, whether this point of difference could be related to the acquisition of neuronal identity.

When carrying out the research, did you have any particular result or eureka moment that has stuck with you?

TM: Less a eureka moment but probably the moment that sticks with me the most is the genesis of the project. I was busy lineaging a set of embryos and mentioned to Rich that I'd noticed the unequal cleavages preceding neuroblast formation. I had been wondering what this could mean, whether this point of difference could be related to the acquisition of neuronal identity. The project, as presented, really took root from there; that small observation shaped future work and a re-evaluation of existing data. It really changed the direction of the project.

Two unequal cleavages precede expression of hlh-14, the proneural transcription factor required for the specification of the DVC neuroblast. DIC images (left) showing the daughter cells of the two cleavages. A lineage diagram (right) of the anterior half of the C lineage showing the relative cell sizes, the expression pattern of the hlh-14::gfp transgene gmIs20 and terminal cell fates, including the DVC neuron.

Two unequal cleavages precede expression of hlh-14, the proneural transcription factor required for the specification of the DVC neuroblast. DIC images (left) showing the daughter cells of the two cleavages. A lineage diagram (right) of the anterior half of the C lineage showing the relative cell sizes, the expression pattern of the hlh-14::gfp transgene gmIs20 and terminal cell fates, including the DVC neuron.

And what about the flipside: any moments of frustration or despair?

TM: I think, during the course of a PhD, when a lot of this work took place, you expect a few of these moments. Specifically related to the work in this paper, there were some frustrating moments in trying to employ temperature-sensitive alleles of spindle-related components to disrupt cleavage. These proved a frustrating technical challenge, but allowed us to explore other avenues and to use the mutant strains included in the paper.

Why did you choose to submit this paper to Development?

TM & RP: Because, as [Editor-in-Chief] James Briscoe recently emphasised, it is a not-for-profit scientist-led journal that signifies our commitment to developmental biology and the next generation of researchers!

What is next for you after this paper?

TM: I've recently started a postdoc with Chema Martín-Durán at Queen Mary University of London. I've always been interested in the origin and evolution of structures in development, and I'll be looking at heterochrony in trunk development in the larvae of two annelids, with an eye on body plan and life stage evolution. So very different questions and systems from those I've previously worked on and I'm excited for the years ahead.

Where will this story take your lab next?

RP: As with most stories, there are perhaps more questions than answers. How does a proneural transcription factor regulate the cell biological process of unequal cleavage? Does it require transcription or is this some kind of moonlighting function? What are the molecular targets and how do they regulate cleavage plane positioning? At some point we would also like to address our original question of how this neuroblast arises in a left-right asymmetric manner!

Finally, let's move outside the lab – what do you like to do in your spare time?

TM: One of my biggest interests outside the lab is language learning and I'm currently learning three: Japanese, Korean and Latin. I'm mainly only finding time for them on the daily commute currently, but it's a good use of the time. I also have a passion for history and actually find it difficult to leave a bookshop without buying a new history book, so I've got quite a backlog at the moment.

RP: In my spare time I like to fish, and amongst friends we are quite competitive. In my photo is the winning smooth-hound that I caught from a Norfolk beach last year.

T.M.: School of Biological and Behavioural Sciences, Queen Mary University of London, London E1 4NS, UK.

R.P.: Department of Cell and Developmental Biology, University College London, London WC1E 6BT, UK.

E-mail: r.poole@ucl.ac.uk

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Control of successive unequal cell divisions by neural cell fate regulators determines embryonic neuroblast cell size
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Development
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