Collaborative Research Centre 1027

Epithelial tissues line all the organs of our body and function as physical barriers, thus, protecting the
underlying organs from infections, and aiding in the absorption of nutrients and gases from the food. Cells
within the epithelia perform these tasks, being jammed in a steady homeostatic density with little or no
apparent movement. Interestingly, the same cells unjam and almost flow like a fluid to migrate to relatively
long distances during physiological and pathological situations such as organ development, wound healing
and cancer metastasis. During such migratory events, cells within the epithelia display spectacular
coordination in movement and often require formation of Leader-cells at the migration front. How do cells
sense and transduce guidance signals along their neighbours? How, from a seemingly uniform epithelia, a
few cells become leaders? and How are the leader cells regulated in space and time? On the basis of our
recent work on collective cell migration[1, 2], I will try to address some of these questions and summarize
our current understanding on mechano-biology of epithelial wound healing. Conceptually, I will try to
defend the role of cooperative forces in instructing wound closure and extrapolate our findings to suggest
that epithelial tissues inherently, are characterized by cellular dynamics that allow for efficient wound
healing.
Towards the end, I will focus on my ongoing work on cell-competition within epithelial tissues.
Cell-competition is an important surveillance mechanism during tissue maintenance, where misfit cells
(losers) are recognized, and removed by neighbouring healthy cells (winners). Recent studies reveal that
cell competition also has a dark side and is exploited by cancer cells to grow and expand at the expense of
the host cells[3]. For example, in the context of skin cancer originating from p53-/- mutations and UV
radiation, mutant/cancer cells use mechanical compaction to kill the surrounding host cells, however, the
underlying molecular mechanism remained elusive. I will present our findings on how losers (wild type cells)
sense mechanical compaction from the winners (p53-/- cells) and how it leads to loser cell death.
I will pitch that cooperation and conflict management within epithelial tissues not only instruct
physiological functions, for e.g. tissue- repair and Epithelial defense against cancer, but also dictate pathological
situations such as tumour formation and metastasis and is therefore central to many fundamental biological
questions.
1. Vishwakarma, M., Di Russo, J., Probst, D., Schwarz, U.S., Das, T., and Spatz, J.P. (2018). Mechanical
interactions among followers determine the emergence of leaders in migrating epithelial cell
collectives. Nat Commun 9, 3469.
2. Vishwakarma, M., Thurakkal, B., Spatz, J.P., and Das, T. (2020). Dynamic heterogeneity influences
leader-follower dynamics during epithelial wound closure. Philosophical Transactions of Royal
Society-B.
3. Vishwakarma, M., and Piddini, E. (2020). Outcompeting cancer. Nat Rev Cancer.

In the first part of the talk, I will explain the role of cell mechanics (i.e. exerted forces) and phagocytic activity during wound repair and pathogen/particle clearance. I will show the development of a novel technique that uses extensive photobleaching-induced apoptosis to study the repair response of lung epithelial tissue. This model consists of a small injury area wherein apoptotic cells are still intact.^[1] Our findings reveal that individual epithelial cells are able to clear dead cells by applying a pushing force, whilst macrophages actively phagocytose the dead cells to create an empty space. I will also demonstrate that this repair mechanism is hampered when nanoparticles such as carbon nanotubes (CNTs) are internalized by epithelial cells, which is hypothesized to be due to the increase of cellular traction force, which impedes cell migration. Moreover, I will show the phagocytic activity of macrophages during particle clearance using a particle surface.^[2] In particular, the ability of macrophages to adhere and migrate on the particle surface, and mechanically remove the particles in their vicinity will be shown. The latter is important for example to understand how macrophages sense and clear pathogens which adhere to tissue or implant surfaces.

In the second part of the talk, I will describe our effort to produce a biological laser.^[3] I will explain how we are able to engineer single biological cells to carry gain medium (e.g. fluorescent dyes) and support light amplification without the need of a conventional optical cavity. Generation of intracellular lasing activity is a powerful approach capable of supporting potential applications in the field of cell imaging and cell identification studies.

References:

1. D. Septiadi et al., Adv. Mater., 2018, 1806181
2. D. Septiadi et al., Adv. Mater., 2020, under review
3. D. Septiadi et al., Adv. Opt. Mater, 2020, accepted

Microtubules are highly dynamic polymers that are needed for intracellular transport and
chromosome segregation during cell division. They grow and shrink by addition and removal
of tubulin dimers at their extremities, while the microtubule shaft adopts a highly ordered,
crystal-like lattice structure that was for a long time not considered to be dynamic.
Using controlled in vitro systems combined with numerical simulations and observations in
living cells, we showed that tubulin dimers can spontaneously leave and be incorporated into
the lattice, leading to remarkable phenomena such as the capacity of microtubules to selfrepair
when damaged. Our observations suggest that the concept of microtubule dynamics,
previously established for microtubule ends, may need to be extended to the entire shaft.

In cells, microtubules are often linked to other filaments, and there is increasing evidence that
the manifold functions of the cytoskeleton crucially depend on the fine tuned crosstalk
between its components. During my postdoctoral work, we therefore investigated the
interplay between microtubules and a second cytoskeletal component - vimentin intermediate
filaments - on a single filament level and in in vitro dynamic networks. We revealed an
attractive interaction between individual microtubules and vimentin filaments that impacts
microtubule dynamics, suggesting that the presence of vimentin can tune microtubule
stability.