Biology subjects

Harkes, R.

Publications and source records attributed to Harkes, R..

3 recordsLinked to original sources

bioRxiv2025

Visualizing homology search in living cells

Homologous recombination (HR) is a DNA repair process that requires a homologous sequence for the repair of DNA double-stranded breaks. The search for homology is the rate limiting step for HR, but the dynamics of homology search in vivo are currently poorly understood. This is largely due to a lack of tools to visualize homology search in action in living human cells. Here, we show that RAD51 and MND1, two proteins operating in homology search, become visible in long filamentous structures several hours after double-stranded break formation. Using GFP-MND1 we can visualize these filaments in living human cells and find that they are highly dynamic, explore the nuclear space, and can resolve over time. We show that the resolution of these filaments depends on RAD54L, known for its role in RAD51-driven homology search and strand-invasion. In addition, we find that loss of cohesin also inhibits the resolution of these filaments, in accordance with a role for cohesin in HR. Thus, our data suggest that these filaments are visible intermediates of an active DNA repair process, and that MND1-GFP can be used as a tool to study the dynamics of homology search in living human cells.

cell biology↗

bioRxiv2021

Substrate rigidity modulates traction forces and stoichiometry of cell matrix adhesions.

In cell matrix adhesions, integrin receptors and associated proteins provide a dynamic coupling of the extracellular matrix (ECM) to the cytoskeleton. This allows bidirectional transmission of forces between the ECM and the cytoskeleton, which tunes intracellular signaling cascades that control survival, proliferation, differentiation, and motility. The quantitative relationships between recruitment of distinct cell matrix adhesion proteins and local cellular traction forces are not known. Here, we applied quantitative superresolution microscopy to cell matrix adhesions formed on fibronectin-stamped elastomeric pillars and developed an approach to relate the number of talin, vinculin, paxillin, and focal adhesion kinase (FAK) molecules to the local cellular traction force. We find that FAK recruitment does not show an association with traction-force application whereas a ~60 pN force increase is associated with the recruitment of one talin, two vinculin, and two paxillin molecules on a substrate of effective stiffness of 47 kPa. On a substrate with a four-fold lower effective stiffness the stoichiometry of talin:vinculin:paxillin changes to 2:12:6 for the same ~60 pN traction force. The relative change in force-related vinculin recruitment indicates a stiffness-dependent switch in vinculin function in cell matrix adhesions. Our results reveal a substrate-stiffness-dependent modulation of the relation between cellular traction-force and the molecular stoichiometry of cell-matrix adhesions.

biophysics↗

bioRxiv2021

Dynamic FRET-FLIM based screens of signal transduction pathways: a feasibility study

Fluorescence Lifetime Imaging (FLIM) is an intrinsically quantitative method to screen for protein-protein interactions and frequently used to record the outcome of signal transduction events. With new highly sensitive and photon efficient FLIM instrumentation, the technique also becomes attractive to screen, with high temporal resolution, for fast changes in Forster Resonance Energy Transfer (FRET), such as those occurring upon activation of cell signaling. We studied the effects of siRNA-mediated individual knockdown of an extensive set of 22 different phosphodiesterases (PDEs) on baseline levels and agonist-induced changes of the second messenger cAMP. Using HeLa cells stably expressing our FRET-FLIM sensor we imaged many hundreds of cells at 5 second intervals for each condition. Following segmentation of cells by the deep-learning implementation Cellpose, FLIM time traces were calculated and fitted for dynamic analysis with custom-made Python scripts. Taking advantage of the quantitative FLIM data, we found very limited effects of PDE knockdown on baseline and agonist-induced peak levels of cAMP. However, cAMP breakdown in the decay phase was significantly slower when PDE3A and, to a lesser amount, PDE10A were knocked down, identifying these isoforms as dominant in HeLa cells. In conclusion, we present a robust platform that combines photon-efficient FLIM instrumentation with systematic gene knockdown and an automated open-source analysis pipeline. Our quantitative platform provides detailed kinetic analysis of cellular signals in individual cells with unprecedented throughput.

cell biology↗