Every living being is made up of a complicated and well organized assembly of cells, that give rise to tissue, organs and finally the macroscopic parts of the body, such as a head, arms and feet. The mechanical differences of these parts are known in everyday life. Bone is harder than even the strongest muscle, and in turn muscles are harder than fat tissue. Obviously, assemblies of cells, and their surrounding can give rise to mechanical differences on the large length-scale, which is obviously highly important for the relevant function. In turn, when opening a cell, we have only a very limited knowledge of the mechanical differences inside cells. To which extend such differences are important for the cellular organization, and might even be relevant for the correct cellular function remains unknown. PolarizeMe aims at illuminating the mechanical properties inside cells by using light, namely optical tweezers and microscopy. As model system we focus on classes of cell that are highly organized, name epithelial, or skin-, cells which show a structural differences between its basis and its top.
Such understanding of cellular mechanics is very important, as in many diseases the intracellular organization is lost, leading to severe problems. One example are kidney cells, for which it is of fundamental importance that the difference between the side facing the blood, and the side facing the urine is well established by the cell. Another important disease is represented by cancer, namely carcinomas which are of epithelial origin. Here epithelial cells loose many of their functions and start dividing and even migrating. The loss of polarity is one of the hallmarks on the transition from a healthy cell to a dangerous metastatic cancer cell. We try to understand if there is an intracellular mechanical structure that is supporting the polarity, and hence helping to maintain it even in a perturbed environment.
These studies are possible by using special properties of light, which are far beyond the everyday experience. When focused to a tiny spot, light starts applying a well defined force on all transparent objects, called a gradient force. This force finds its origin in the electromagnetic fields representing light and we use this to trap particles inside cells. We can thereby hold and manipulate such probe particles, and this is used to measure stiffness, viscosity and also forces inside cells. Another method is high resolution microscopy where we can precisely mark and track structures and small molecules inside the cell. Here we aim at understanding if there are systematic, and potentially circular material flows in the cell that would also lead to order by transporting organelles in a size dependent way. Finally, we want to understand how intracellular organelles and large structures are directionally transported inside the complicated environment of the cell.
Overall, we have the objective to give a detailed plan of the intracellular mechanical properties as well as of the intracellular forces. This will be first done in stable and polarized cells, and then in systematically perturbed cells.
In conclusion, we could significantly advance the field of intracellular mechanics, by defining the changes of mechanical properties during cell division, by providing a new mechanical fingerprint differentiation cells and by demonstrating a new statistical way to describe the active, non-equilibrium properties of cells with a new quantity, called the MBR (Mean Back Relaxation). While the assumption that epithelial cells are highly polarized was not confirmed, this project established a series of new and important findings on the search for the proposed intracellular polarization. This makes the project PolarizeMe a high success, far beyond what as originally thought we could achieve.
