During development, mechanical forces sculpt tissues into myriad forms. Actomyosin contractility generated within the cell has an increasingly appreciated role in this process, but how tissue forces relate to the physical properties of the extracellular matrix is still poorly understood, particularly at longer time scales and the whole tissue level. A new paper in Development addresses these issues using Drosophila leg development as a model, taking advantage of an ex vivo culturing method. We caught up with first author Amsha Proag and last author Magali Suzanne, group leader at the Centre for Integrative Biology in Toulouse, France, to hear more about the story.
Magali, can you give us your scientific biography and the questions your lab is trying to answer?
MS My main motivation is to push the limits of our knowledge, probably like many scientists. I discovered Drosophila during my PhD and was amazed by the power of this model organism. I then focused on how tissues acquire their shape or morphogenesis, an aspect that still fascinates me. I grew up surrounded by artists, and particularly sculptors, and this probably had an influence on the fascination I have for tissue shapes. When I settled my lab in 2011, we were initially working on a particular aspect of morphogenesis, the impact of cell death on the final shape of a tissue. We discovered that dying cells mechanically influence their surroundings and this led us to the biomechanics field. We are now interested more broadly in how mechanical signals are integrated at the tissue level to generate new shapes.
And Amsha, how did you come to work in the Suzanne lab, and what drives your research?
AP What first attracted me was hearing Dr Suzanne explaining some of her findings at a conference, which demonstrated how tissue shape arose from exquisite mechanical communication between cells. I remember asking her after the talk about the mechanisms behind this spatial patterning, which strongly appealed to my background in soft matter physics. The interplay between physical forces and adaptive biological behaviour remains a fascinating topic for me and a major question in developmental biology.
The interplay between physical forces and adaptive biological behaviour remains a fascinating topic
Why are cultured Drosophila leg discs a useful system to understand developmental mechanics? Was much known about the forces driving eversion before your work?
AP & MS First, the leg disc constitutes an isolated tissue that is able to pursue its development ex vivo, and is thus directly accessible to live microscopy and micro-manipulation. Second, it is small enough for multiscale investigation of morphogenesis (integrating cytoskeleton dynamics, single cell behaviour and tissue-scale mechanics).
Although little was known before our work on leg disc eversion, a few papers described the process of wing disc eversion, showing on one hand that Myosin II was important for peripodial epithelium (PE) opening and eversion, and on the other hand that extracellular matrix (ECM) was being degraded in order to allow disc eversion. The novelty in our paper, following the dynamics of both the ECM and the cell monolayer of the PE, is the discovery of their physical and mechanical uncoupling during leg disc eversion.
Can you give us the key results of the paper in a paragraph?
AP & MS The leg disc is enclosed in the PE, a thin tissue that opens, contracts and is removed to allow the leg to evert. Our work shows that the tension produced by the growing leg disc on the PE is at first mainly borne by the ECM. But as the leg elongates, the ECM and cell layer are progressively uncoupled and tension builds up in the cell monolayer. Then, each layer of the peripodial epithelium withdraws by a different mechanism. The ECM layer is opened by local proteolysis and its tension completes its removal, whereas the cell monolayer opens and is removed by Myosin-II-dependent contraction, independently of ECM degradation.
How do you think the PE cells stay alive and seemingly happy without attachment to the ECM?
AP & MS This came as a surprise at first: seeing holes form in the ECM layer while the PE cells remained cohesive told us here was something worth investigating. Indeed, single adherent cells will tend to undergo apoptosis when cultured on non-adhesive or soft substrates. However, PE cells remain attached to each other after the detachment from the ECM. In addition, apoptotic markers only appear at the free edge of the PE, where cells have lost half their intercellular adhesion as well as their contractility. Hence, we hypothesise that intercellular adhesion compensates for basal adhesion by providing sufficient survival signals, through a combination of adherens junction signalling and mechanotransduction. This in vivo behaviour recalls recent observations on cultured cell suspended monolayers and calls for characterisation as a general property of epithelia.
When doing the research, did you have any particular result or eureka moment that has stuck with you?
AP The first time I saw individual collagen structures, fibre bundles, changed my perception of the ECM basement membrane. I grasped how thin it was compared to my previous view of ECM as an amorphous gel. It made me consider other properties of the layer, such as the propensity to form holes, but also the ability to resist tension. It was also a nice demonstration of the benefits of looking at the same system using different setups.
And what about the flipside: any moments of frustration or despair?
AP Investigating a living tissue requires some patience as several technical requirements have to be met simultaneously and even successful experiments tend to bring out the complexity of the tissue. Thankfully, these challenges also act as spurs.
So what next for you after this paper?
AP I plan on developing quantitative approaches for the life sciences, mainly through automated image and data analysis. In addition, I wish to contribute to bridging biology and physics and am currently working on undergraduate teaching material to this aim.
Where will this work take the Suzanne lab?
MS Until now, we were focusing on single cell dynamics and their influence on their direct surroundings, at a local scale. The study of peripodial epithelium dynamics opens new doors in considering biomechanics at the tissue scale.
Finally, let’s move outside the lab – what do you like to do in your spare time in Toulouse?
AP A little badminton, a lot of reading. I am also involved in the Human Library organisation, which aims at removing barriers between people through conversation.
MS Toulouse is a very nice place for hiking with the Pyrenees close by. There is also a number of festivals I enjoy a lot such as the Short Film festival, the image festival ‘MAP’, or the jazz festival ‘Jazz sur son 31’.