Periodic Reporting for period 1 - OPTOLEADER (Optogenetic control of leader cell mechanobiology during collective cell migration)
Reporting period: 2019-09-01 to 2021-08-31
Within the human body, the concerted and coordinated movement of cells is a recurring phenomenon of key importance for understanding both health and disease. For example, cells collectively migrate during embryo development and precisely form the body’s complex shapes; similarly, cells move cohesively to efficiently heal wounds and to maintain vital organ and tissue functions. Comparable mechanisms are at play also during cancer progression, when tumour cells employ collective migration strategies to invade healthy tissue and to drive metastasis.
These are fundamentally important biophysical processes, and in the last two decades experimental and theoretical studies have uncovered that they are driven by cell-generated forces. In this context, the mechanical interactions of specific subgroups of cells known as “leader cells” is particularly important, albeit little understood. These cells are thought to dynamically guide a collectively migrating group, by exerting forces on their neighbours and on their surroundings, by reacting to chemical cues and by having well-defined directionality. In our project, called OPTOLEADER, we aimed to understand at a fundamental level how mechanical interplay between leaders and other cells gives rise to and affects collectively migrating groups.
Our main approach was to use genetically engineered cells whose motility can be controlled by light with high precision and no damage to the cells. With these cells we can induce leader-like behaviour at will, using a microscope to both deliver the light stimulation and to observe the cells’ behaviour. We combined these experiments with a powerful technique called Traction Force Microscopy (TFM) that allows us to measure the forces exerted by the cells while they migrate, with high spatial and temporal resolution.
We employed this approach to generate leaders within cell groups and set out to discover if and how mechanical interactions allow them to guide collective migration. As is often the case, the results of the experiments revealed a much more complex and nuanced picture than was previously believed. OPTOLEADER showed that the long-held notion that leader-cells are the initiators and drivers of collective migration is an oversimplification. In fact, apparent leadership effects in migrating cells can be effectively viewed as a consequence of collective behaviours of the “followers” working together with the leaders. This coordination between the cells is a mechanical phenomenon and is brought about by the way tension is built-up and redistributed within a cohesive group of cells.
We believe that these results will shed new light on essential biophysical processes that are key to understand organism growth and development, tissue mechanics and tumour progression.
These are fundamentally important biophysical processes, and in the last two decades experimental and theoretical studies have uncovered that they are driven by cell-generated forces. In this context, the mechanical interactions of specific subgroups of cells known as “leader cells” is particularly important, albeit little understood. These cells are thought to dynamically guide a collectively migrating group, by exerting forces on their neighbours and on their surroundings, by reacting to chemical cues and by having well-defined directionality. In our project, called OPTOLEADER, we aimed to understand at a fundamental level how mechanical interplay between leaders and other cells gives rise to and affects collectively migrating groups.
Our main approach was to use genetically engineered cells whose motility can be controlled by light with high precision and no damage to the cells. With these cells we can induce leader-like behaviour at will, using a microscope to both deliver the light stimulation and to observe the cells’ behaviour. We combined these experiments with a powerful technique called Traction Force Microscopy (TFM) that allows us to measure the forces exerted by the cells while they migrate, with high spatial and temporal resolution.
We employed this approach to generate leaders within cell groups and set out to discover if and how mechanical interactions allow them to guide collective migration. As is often the case, the results of the experiments revealed a much more complex and nuanced picture than was previously believed. OPTOLEADER showed that the long-held notion that leader-cells are the initiators and drivers of collective migration is an oversimplification. In fact, apparent leadership effects in migrating cells can be effectively viewed as a consequence of collective behaviours of the “followers” working together with the leaders. This coordination between the cells is a mechanical phenomenon and is brought about by the way tension is built-up and redistributed within a cohesive group of cells.
We believe that these results will shed new light on essential biophysical processes that are key to understand organism growth and development, tissue mechanics and tumour progression.
During the research for OPTOLEADER, we initially established a repeatable and robust experimental protocol that forms the basis of all the various planned investigations of the project. As expected, this required extended periods of technique optimization and quality tests. While we verified that our approach was viable on different microscope set-ups, we then settled on a scanning confocal microscope as our most reliable option that yielded the highest quality data.
Concomitantly with these technical developments, we perfected and implemented high-throughput data analysis techniques to manage the large volumes of data generated by our experiments. We eventually combined the results of these two processes and conducted an experimental data acquisition campaign that yielded a full dataset covering Aim 1 of OPTOLEADER, relating to studying the dynamics of quasi 1D leader-follower systems.
In the later part of the fellowship, we developed a data-analysis pipeline for measuring cell-shapes, velocities and cellular force-generation. We applied this to our extensive experimental dataset and studied thereby the interplay between single-cell dynamics, collective cell dynamics and mechanical force transmission. We also worked on developing new genetically modified cell-lines that will be used to study the role of leader-cells on 2D epithelial monolayer migration.
The results of our work showed that the current understanding of leader-follower dynamics in migrating cells is an oversimplification. Our experimental techniques and theoretical analysis tools allowed us to access and quantify new variables and develop a new paradigm where leader-follower interactions are bidirectional and where apparent leadership effects are actually consequences of collective mechanical behaviours.
Concomitantly with these technical developments, we perfected and implemented high-throughput data analysis techniques to manage the large volumes of data generated by our experiments. We eventually combined the results of these two processes and conducted an experimental data acquisition campaign that yielded a full dataset covering Aim 1 of OPTOLEADER, relating to studying the dynamics of quasi 1D leader-follower systems.
In the later part of the fellowship, we developed a data-analysis pipeline for measuring cell-shapes, velocities and cellular force-generation. We applied this to our extensive experimental dataset and studied thereby the interplay between single-cell dynamics, collective cell dynamics and mechanical force transmission. We also worked on developing new genetically modified cell-lines that will be used to study the role of leader-cells on 2D epithelial monolayer migration.
The results of our work showed that the current understanding of leader-follower dynamics in migrating cells is an oversimplification. Our experimental techniques and theoretical analysis tools allowed us to access and quantify new variables and develop a new paradigm where leader-follower interactions are bidirectional and where apparent leadership effects are actually consequences of collective mechanical behaviours.
During the implementation of OPTOLEADER, our research has progressed beyond the state of the art in the study of minimal leader-follower cell systems. We have obtained the repeatable and controlled recreation of the initial instants of cellular collective motion, starting from a non-collectively moving system.
The potential societal impacts of the current state of the project are already tangible, because with our current results we are closer to understanding one of the main processes of tumour invasion: when strands of cancer cells are drawn from a tumour core by the action of a leader-cell, and led in the invasion of healthy tissue. Many current and future studies on the invasiveness of tumours, and on how to curb it, will benefit from the upcoming publication of our results on leader-cell guided migration.
After the end of the project we expect to have extended and refined our current results by applying them to more complex systems. Our next steps will broaden the societal impact of the project, since they will increase its immediate importance for studies of embryogenesis and tissue homeostasis.
We expect that OPTOLEADER will contribute to the growth of biophysics in a far-reaching sense: it will push forward our quantitative and mechanistic understanding of active matter and it will establish new and broadly applicable experimental tools for its study.
The potential societal impacts of the current state of the project are already tangible, because with our current results we are closer to understanding one of the main processes of tumour invasion: when strands of cancer cells are drawn from a tumour core by the action of a leader-cell, and led in the invasion of healthy tissue. Many current and future studies on the invasiveness of tumours, and on how to curb it, will benefit from the upcoming publication of our results on leader-cell guided migration.
After the end of the project we expect to have extended and refined our current results by applying them to more complex systems. Our next steps will broaden the societal impact of the project, since they will increase its immediate importance for studies of embryogenesis and tissue homeostasis.
We expect that OPTOLEADER will contribute to the growth of biophysics in a far-reaching sense: it will push forward our quantitative and mechanistic understanding of active matter and it will establish new and broadly applicable experimental tools for its study.