Mechanical and systemic control of growth during Drosophila abdominal development

Project Information

GrowthControl

Grant agreement ID: 795060

DOI

10.3030/795060

Project closed

EC signature date 15 February 2018

Start date 1 March 2018

End date 29 February 2020

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 183 454,80

EU contribution

€ 183 454,80

Coordinated by

THE FRANCIS CRICK INSTITUTE LIMITED
United Kingdom

Periodic Reporting for period 1 - GrowthControl (Mechanical and systemic control of growth during Drosophila abdominal development)

Reporting period: 2018-03-01 to 2020-02-29

Summary of the context and overall objectives of the project

Many signalling pathways have been shown to influence final tissue size. However, whether and how their activities change to signal growth arrest in a timely fashion during development remains unclear. Therefore, the knowledge about the mechanisms that regulate how cell proliferation is triggered in response to different extrinsic as well as intrinsic stimuli and how transitions between different proliferative states are controlled is very limited. Not only it is important to understand how proliferation is controlled during development, but it is particularly essential to unravel how proliferation is driven in tumour cells or regenerative tissues.
We wanted to understand how mechanical forces and systemic cues contribute to the different growth and proliferative phases a tissue undergoes throughout its development, as well as to final tissue growth arrest, using the abdominal epithelium of Drosophila melanogaster as a model system. Likewise, we aimed to investigate how systemic growth signalling pathways are integrated with the physical properties of a tissue. We have explored how nutrient-sensing signalling pathways influence growth and proliferation as well as whether underlying changes in the mechanical properties of a tissue can account for changes in the proliferative state. This work showed that although nutrient-sensing pathways impact on cell growth during specific developmental phases, they do not appear to dictate proliferation arrest. Unlike what has been proposed in other organs, tissue mechanics does not seem to impact the proliferation state of cells in the abdominal epithelium.
We believe that the work developed here has furthered our fundamental understanding of developmental growth regulation, and as such drive further investigation in cancer biology, disease models and regeneration studies.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We analysed the growth and proliferation of the abdominal cells (histoblasts) during pupal development from 0h until around 34h after puparium formation (APF) using in vivo imaging. With the first set of experiments, we showed that during the first ~12h of pupal development histoblasts undergo three cleavage divisions that lack a G1 phase and as such cells reduce their size with each division. From around 14 hAPF, time at which expansion of this tissue starts, proliferation rates oscillate until around 32 hAPF time at which cells arrest in G1 and growth termination takes place. Using laser ablation experiments performed at different timepoints between 16h and 36 hAPF we showed that there is an increase in apical tension during expansion of the tissue.
Monitoring signalling activity of Insulin and TOR pathway revealed no specific spatial or temporal pattern throughout histoblasts development. We were not able to uncover any correlation with proliferation and growth parameters, or a change that coincided with the time of proliferation arrest.
We investigated the role of nutrient-sensing pathways in the different growth phases of the abdominal epithelium. While inhibition of Insulin signalling did not have an effect on abdomen development, TOR inhibition caused a delay in histoblasts expansion, as well as a reduced number of cells. Our analysis shows that TOR signalling is only required for histoblasts cell growth during the larval stage, while it is dispensable for expansion of this tissue during the its main period of pupal growth.
By changing the histoblasts’ mechanical environment we demonstrated that, unlike what has been suggested in other tissues, physical forces do not play a role in controlling proliferation rates in the abdominal epithelium.
We also developed a simple numerical model of histoblasts growth that can recapitulate the essential features we observed in the experimental data.
The results generated during our work were presented in several seminars both at the host institute as well as at international conferences. Some results obtained through implementation of this action will be included in a manuscript we have currently in preparation, and we intend to prepare another scientific publication in the next year.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We have enriched our understanding of how signalling pathways and tissue mechanics influence tissue growth. We have developed a pipeline to analyse tissue growth in vivo using live imaging and automated image analysis techniques. In particular, working closely with biophysicists, we have created a machine-learning algorithm that greatly reduces the need for manual correction of our imaging data. This is an important development since it allows the study of the timing of growth arrest by directly tracking each cell within the tissue while monitoring signalling activity as growth progresses.
We have shown that nutrient-sensing signalling activity does not show major changes during transitions between different proliferative states and that these signalling pathways do not regulate proliferation arrest in the abdominal epithelium. Although we discovered that there are changes in the tissue stress patterns during development, challenging the mechanical environment was not enough to impact proliferation rates. This is an important point since it was previously suggested, from studies in other tissues, that changes in tissue patterns could account for size control. We aim to further understand how proliferation arrest is regulated in this tissue. We will be exploring this further in the near future.
We believe we made important progress towards an understanding of tissue growth control during the development of an epithelial tissue. We believe the results here uncovered will be of major importance to the growth control and regeneration fields in general. We also hope these methodologies and results can be used for further investigation in other model organisms as well as in cancer biology and other disease models.

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