Cell biophysics

FRET-based tension sensor

All organisms in nature are mechanosensitive. As the basic unit of structure and function of prokaryotes and eukaryotes, cells’ inherent capabilities to sense external forces and transmit them are important to understand in the context of basic biophysics, but also eventually for human health and disease. Many molecular mechanisms have been identified to play crucial roles in driving and regulating cell activities. However, further progress has been hampered by the fundamental difficulty to measure stresses in cells. Being able to determine which proteins and lipids bear load is critical to understand mechanosensitive signaling and regulatory control of local force generation. The cytoskeleton, a dynamic protein polymer network, is the most essential load-bearing and force-producing subcellular structure. In this project, we focus on forces transmitted through of one its major components—the actin network and will use actin-targeting tension sensors, developed by Prof. Brent Hoffman’s lab at Duke University, to study and understand the way forces are propagating within this subcellular structure and the mechanical loads experienced by crosslinking proteins within the actin cortex. The novel aspect of my thesis work is that I will directly apply controlled forces to cells to observe force propagation and that I will use concepts of polymer physics to interpret the data, looking for time-dependent percolation effects, formation of force chains in the semiflexible, and nonlinear polymer networks.

Representative FRET channel and bright-field channel images of ABD-TS in live NIH3T3 cells binding to actin filaments under magnetic tweezers manipulation.

Mechanical properties of suspended cell

The mechanical properties of most animal cells are dominated by the actin cortex, a~ 0.5 µm thick layer of actin filaments including a multitude of associated proteins, which is attached to the inner side of the cell membrane and encapsulates the cytoplasm. Cells can actively change their shape and volume, but osmotic pressure prohibits any substantial volume change in response to external forces.

left)The schematic of the pressurized shell used to model the suspended cell deformation. In the upper half, the meshing procedure is shown and the lower color table shows the Von miss stress distribution.
right ) 3T3 fibroblast cell suspended between two optically trapped 4 µm fibronectin-coated polystyrene beads.

We have mechanically indented suspended spherical with dual optical tweezers set-up to measure response. For small, slow indentations we find a linear elastic response. We use finite element simulation for modeling the cell as an elastic shell.

Rezvani, Samaneh, et al. “Microfluidic device for chemical and mechanical manipulation of suspended cells.” Journal of Physics D: Applied Physics 51.4 (2018): 045403.

Schlosser, Florian, Florian Rehfeldt, and Christoph F. Schmidt. “Force fluctuations in three-dimensional suspended fibroblasts.” Philosophical Transactions of the Royal Society B: Biological Sciences 370.1661 (2015): 20140028.

Mizuno, Daisuke, et al. “High-resolution probing of cellular force transmission.” Physical review letters 102.16 (2009): 168102.

Flow and mechanosensing via primary cilia in kidney epithelial cells

Many cell types are mechanosensitive, which is important for communication between cells and their environment. Mechanosensitivity involves membrane channels and coupled cytoskeletal structures. Defects in mechanosensing have been linked to human diseases, such as polycystic kidney disease (PKD). We study: (i) model systems

Mechanical structure and anchoring of the primary cilium. (A) Schematic sketch of a primary cilium with PC2 Ca2+ channels and its intracellular anchoring. (B) Differential interference contrast (DIC) micrograph of a primary cilium deflected in buffer flow of ∼4.8 µm/s. (C) DIC micrograph of a primary cilium with an attached bead deflected by an optical trap.

of substrate-supported lipid membranes with embedded channels and attached polymer networks to study basic mechanical properties and channel activation, (ii) cells that sense flow, with a focus on the role of primary cilia and TRP channels. We will use optical trapping, microrheology, electrical recording, and fluorescence microscopy.

Battle, Christopher, et al. “Intracellular and extracellular forces drive primary cilia movement.” Proceedings of the National Academy of Sciences 112.5 (2015): 1410-1415.


Mechanosensing in bone cells

Two-bead microrheology with whole cells. Differential interference contrast microscopy (DIC) image of a MLO-Y4 osteocyte-like cell suspended in culture medium by two optically trapped fibronectin-coated beads (diameter 4 µm). The beads are attached to opposing sides of the cell. Scale bar: 5 µm.

Specialized cells inside the bone matrix, the osteocytes, are the detectors of mechanical stress and strain and chemically signal to other cells in a complex regulatory network controlling the dynamic remodeling of bone. Understanding this regulatory system is medically important to fight age-related osteoporosis, but also for bone healing after injury, for implant surgery, dental surgery, and even to solve health problems humans encounter during space missions. We are developing methods to directly mechanically stimulate single osteocytes, to study their mechanical properties and measure their chemical signalling response, in particular the release of nitric oxide. (collaboration with J. Klein-Nulend, T. Smit, VUMC Amsterdam, Oxford)

Vatsa, Aviral, et al. “Bio imaging of intracellular NO production in single bone cells after mechanical stimulation.” Journal of Bone and Mineral Research 21.11 (2006): 1722-1728

Bacabac, Rommel G., et al. “Round versus flat: bone cell morphology, elasticity, and mechanosensing.” Journal of biomechanics 41.7 (2008): 1590-1598.


Protective mechanisms and mechanosensing of vascular endothelial cells


Epithelial cancer cell (PC3) with fluorescently labeled actin, showing long protrusions embedded in a pericellular matrix (not labeled) consisting mainly of hyaluronan.

The cells lining the inside of blood vessels need to protect themselves against damage by blood flow itself and the potentially dangerous molecular contents of blood (e.g. cholesterol). It is generally believed that a highly dynamic polyelectrolyte layer on the surface of the cells, the glycocalix, is responsible for this protection and its breakdown appears to stand at the beginning of many vascular and heart diseases. Regulation of the glycocalix occurs through mechanosensing again, possibly by similar mechanisms as in the bone cells. We are studying the viscoelastic properties of the glycocalix in cultured cells and we are exploring possibilities to study the dynamic properties of this extracellular matrix inside of blood vessels. (collaboration with J. Spaan, AMC Amsterdam)


Nijenhuis, Nadja, et al. “Microrheology of hyaluronan solutions: implications for the endothelial glycocalyx.” Biomacromolecules 9.9 (2008): 2390-2398.