Visualizing Spatiotemporal Dynamics of Intercellular Mechanotransmission upon Wounding

Recent developments in DNA-based mechanical nanodevices

Cellular processes and functions can be regulated by mechanical forces. Nanodevices that can measure and manipulate these forces are critical tools in chemical and cellular biology. Synthetic DNA oligonucleotides have been used to develop a wide range of powerful nanodevices due to their programmable nature and precise and predictable self-assembly. In recent years, various types of DNA-based mechanical nanodevices have been engineered for studying molecular-level forces. With the help of these nanodevices, our understanding of cellular responses to physical forces has been significantly advanced. In this article, we have reviewed some recent developments in DNA-based mechanical sensors and regulators for application in the characterization of cellular biomechanics and the manipulation of cellular morphology, motion and other functions. The design principles discussed in this article can be further used to inspire other types of powerful DNA-based mechanical nanodevices.

Integration of FRET and sequencing to engineer kinase biosensors from mammalian cell libraries

The limited sensitivity of Förster Resonance Energy Transfer (FRET) biosensors hinders their broader applications. Here, we develop an approach integrating high-throughput FRET sorting and next-generation sequencing (FRET-Seq) to identify sensitive biosensors with varying substrate sequences from large-scale libraries directly in mammalian cells, utilizing the design of self-activating FRET (saFRET) biosensor. The resulting biosensors of Fyn and ZAP70 kinases exhibit enhanced performance and enable the dynamic imaging of T-cell activation mediated by T cell receptor (TCR) or chimeric antigen receptor (CAR), revealing a highly organized ZAP70 subcellular activity pattern upon TCR but not CAR engagement. The ZAP70 biosensor elucidates the role of immunoreceptor tyrosine-based activation motif (ITAM) in affecting ZAP70 activation to regulate CAR functions. A saFRET biosensor-based high-throughput drug screening (saFRET-HTDS) assay further enables the identification of an FDA-approved cancer drug, Sunitinib, that can be repurposed to inhibit ZAP70 activity and autoimmune-disease-related T-cell activation.

Src kinases relax adherens junctions between the neighbors of apoptotic cells to permit apical extrusion

Epithelia can eliminate apoptotic cells by apical extrusion. This is a complex morphogenetic event where expulsion of the apoptotic cell is accompanied by rearrangement of its immediate neighbors to form a rosette. A key mechanism for extrusion is constriction of an actomyosin network that neighbor cells form at their interface with the apoptotic cell. Here we report a complementary process of cytoskeletal relaxation that occurs when cortical contractility is down-regulated at the junctions between those neighbor cells themselves. This reflects a mechanosensitive Src family kinase (SFK) signaling pathway that is activated in neighbor cells when the apoptotic cell relaxes shortly after injury. Inhibiting SFK signaling blocks both the expulsion of apoptotic cells and the rosette formation among their neighbor cells. This reveals the complex pattern of spatially distinct contraction and relaxation that must be established in the neighboring epithelium for apoptotic cells to be extruded.

Quantifying tensile forces at cell-cell junctions with a DNA-based fluorescent probe

Cells are physically contacting with each other. Direct and precise quantification of forces at cell–cell junctions is still challenging. Herein, we have developed a DNA-based ratiometric fluorescent probe, termed DNAMeter, to quantify intercellular tensile forces. These lipid-modified DNAMeters can spontaneously anchor onto live cell membranes. The DNAMeter consists of two self-assembled DNA hairpins of different force tolerance. Once the intercellular tension exceeds the force tolerance to unfold a DNA hairpin, a specific fluorescence signal will be activated, which enables the real-time imaging and quantification of tensile forces. Using E-cadherin-modified DNAMeter as an example, we have demonstrated an approach to quantify, at the molecular level, the magnitude and distribution of E-cadherin tension among epithelial cells. Compatible with readily accessible fluorescence microscopes, these easy-to-use DNA tension probes can be broadly used to quantify mechanotransduction in collective cell behaviors.

A DNA-based fluorescent probe to quantify the magnitude and distribution of tensile forces at cell–cell junctions.

Optogenetic Control for Investigating Subcellular Localization of Fyn Kinase Activity in Single Live Cells

Previous studies with various Src family kinase biosensors showed that the nuclear kinase activities are much suppressed compared to those in the cytosol, suggesting that these kinases are regulated differently in the nucleus and in the cytosol. In this study, using Fyn as an example, we first engineered a Fyn biosensor with a light-inducible nuclear localization signal to demonstrate that the Fyn kinase activity is significantly lower in the nucleus than in the cytosol. To understand how different equilibrium states between Fyn and the corresponding phosphatases are maintained in the cytosol and nucleus, we further engineered a Fyn kinase domain with light-inducible nuclear localization signal. The results revealed that the Fyn kinase can be actively transported into the nucleus upon light activation and upregulate the biosensor signals in the nucleus. Our results suggest that there is limited transport or diffusion of Fyn kinase between the cytosol and nucleus in the cells, which is important for the maintenance of different equilibrium states of Fyn in situ.

Sensitive FRET Biosensor Reveals Fyn Kinase Regulation by Submembrane Localization

Fyn kinase plays crucial roles in hematology and T cell signaling; however, there are currently limited tools to visualize the dynamic Fyn activity in live cells. Here we developed and characterized a highly sensitive Fyn biosensor based on fluorescence resonance energy transfer (FRET) to monitor Fyn kinase activity in live cells. Our results show that Fyn kinase activity can be induced in both mouse embryonic fibroblasts (MEFs) and T cells by ligand engagement. Two different motifs were further introduced to target the biosensor at the cellular membrane microdomains in MEFs, revealing that the Fyn-tagged biosensor had 70% greater response to growth factor stimulation than the Lyn-tagged version. This suggests that the plasma membrane microdomains can be categorized into different functional subdomains. Further experiments show that while the membrane accessibility is necessary for Fyn activation, the localization of Fyn outside of its microdomains causes its hyperactivity, indicating that membrane microdomains provide a suppressive microenvironment for Fyn regulation in MEFs. Interestingly, a relatively high Fyn activity can be observed at perinuclear regions, further supporting the notion that the membrane microenvironment has a significant impact on the local molecular functions. Our work hence highlights a novel Fyn FRET biosensor for live cell imaging and its application in revealing an intricate submembrane regulation of Fyn in live MEFs.