Thesis presented July 05, 2024

Abstract:
Chirality, defined as the property of an object not being superimposable to its mirror image, is a conserved hallmark of biological systems with critical implications in various physiological processes, particularly tissue morphogenesis and embryonic development. Large-scale left-right asymmetries, including organismal body plans and directional tissue patterning, are believed to arise from the handedness of their constituting cells, commonly referred to as cellular chirality. It has been shown that in a geometrically confined environment, both individual cells and cell collectives spontaneously break symmetry and can exhibit distinctive chiral behaviors. Despite the lack of a clear molecular mechanism explaining the manifestation of this property at the cellular level, the primary chiral determinant in this phenomenon has always been linked to the actin cytoskeleton in interaction with its binding partners. In particular, several studies point at a key role for actomyosin network contractility in chiral cell alignment and cell rotation. However, the exact contribution of contractile forces to the emergence and maintenance of cellular chirality remains to be elucidated. Moreover, it has been reported that the expression of chirality in various models is closely associated with polarization, another type of symmetry breaking, which governs the organization of the cytoskeleton, biases the distribution of internal organelles, and plays an essential role in driving cellular motion. Despite the interdependence between these two symmetry-breaking phenomena, especially in terms of the implicated effectors, a clear understanding of the feedback existing between polarity and chirality is still lacking.
To begin with, our project addresses the role of contractility in establishing chirality in a minimalistic model of endothelial cell pairs. We show that cell doublets confined on disk-shaped micropatterns spontaneously display a persistent, rightward biased swirling that is strongly dependent on the contractile forces produced by the actin cytoskeleton. In particular, our experimental data demonstrate that varying cellular contractility levels not only affects the extent of rotation but also modulates the strength and the directionality of the chiral bias. Furthermore, a closer examination into the mechanics associated with the chiral rotation of doublets reveals the presence of a force asymmetry within the cell pair. Interestingly, our results indicate that the contractility level of the more contractile cell is correlated with the speed and the direction of doublet rotation, suggesting that this cell would dominate the rotational behavior of the system and predict the bias of the doublet. Secondly, by adapting the geometry of the adhesive pattern, we identify the conditions required to trigger an equivalent chiral rotation among single endothelial cells. We use this system to challenge the intrinsic nature of cellular chirality and its relationship to polarity, while shedding light on a possible implication for motility in the emergence of a persistent chiral phenotype.
Taken together, these findings demonstrate that the balance of forces produced by the cells of a pair is actively involved in driving the expression of chirality in doublets. In addition, they provide preliminary insights into the mechanisms associated with the emergence of cellular chirality, which would occur in response to their internal organelles symmetry-break and motility.

Keywords:
chirality, cytoskeleton, contractility, polarity, endothelial cell