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 super-resolution 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 with an effective stiffness of 47 kPa. On a substrate with a fourfold 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 relationship between cellular traction-force and the molecular stoichiometry of cell-matrix adhesions.

Tetraspanin CD151 and integrin α3ß1 contribute to the stabilization of integrin α6ß4-containing cell-matrix adhesions.

Tetraspanin CD151 has been suggested to regulate cell adhesion through its association with laminin-binding integrins α3ß1 and α6ß4; however, its precise function in keratinocyte adhesion remains elusive. In this study, we investigated the role of CD151 in the formation and maintenance of laminin-associated adhesions. We show that CD151, through binding to integrin α3ß1, plays a critical role in the stabilization of an adhesion structure with a distinct molecular composition of hemidesmosomes with tetraspanin features. These hybrid cell-matrix adhesions, which are formed early during cell adhesion and spreading and at later stages of cell spreading, are present in the central region of the cells. They contain the CD151-α3ß1/α6ß4 integrin complexes and the cytoskeletal linker protein plectin, but are not anchored to the keratin filaments. In contrast, hemidesmosomes, keratin filament-associated adhesions that contain integrin α6ß4, plectin, BP180 (encoded by COL17A1) and BP230 (encoded by DST), do not require CD151 for their formation or maintenance. These findings provide new insights into the dynamic and complex regulation of adhesion structures in keratinocytes and the pathogenic mechanisms underlying skin blistering diseases caused by mutations in the gene for CD151.

Syntenin-1 and ezrin proteins link activated leukocyte cell adhesion molecule to the actin cytoskeleton.

Activated leukocyte cell adhesion molecule (ALCAM) is a type I transmembrane protein member of the immunoglobulin superfamily of cell adhesion molecules. Involved in important pathophysiological processes such as the immune response, cancer metastasis, and neuronal development, ALCAM undergoes both homotypic interactions with other ALCAM molecules and heterotypic interactions with the surface receptor CD6 expressed at the T cell surface. Despite biochemical and biophysical evidence of a dynamic association between ALCAM and the actin cytoskeleton, no detailed information is available about how this association occurs at the molecular level. Here, we exploit a combination of complementary microscopy techniques, including FRET detected by fluorescence lifetime imaging microscopy and single-cell force spectroscopy, and we demonstrate the existence of a preformed ligand-independent supramolecular complex where ALCAM stably interacts with actin by binding to syntenin-1 and ezrin. Interaction with the ligand CD6 further enhances these multiple interactions. Altogether, our results propose a novel biophysical framework to understand the stabilizing role of the ALCAM supramolecular complex engaged to CD6 during dendritic cell-T cell interactions and provide novel information on the molecular players involved in the formation and signaling of the immunological synapse at the dendritic cell side.

The nanoscale architecture of force-bearing focal adhesions.

The combination of micropillar array technology to measure cellular traction forces with super-resolution imaging allowed us to obtain cellular traction force maps and simultaneously zoom in on individual focal adhesions with single-molecule accuracy. We achieved a force detection precision of 500 pN simultaneously with a mean single-molecule localization precision of 30 nm. Key to the achievement was a two-step etching process that provided an integrated spacer next to the micropillar array that permitted stable and reproducible observation of cells on micropillars within the short working distance of a high-magnification, high numerical aperture objective. In turn, we used the technology to characterize the super-resolved structure of focal adhesions during force exertion. Live-cell imaging on MCF-7 cells demonstrated the applicability of the inverted configuration of the micropillar arrays to dynamics measurements. Forces emanated from a molecular base that was localized on top of the micropillars. What appeared as a single adhesion in conventional microscopy were in fact multiple elongated adhesions emanating from only a small fraction of the adhesion on the micropillar surface. Focal adhesions were elongated in the direction of local cellular force exertion with structural features of 100-280 nm in 3T3 Fibroblasts and MCF-7 cells. The combined measure of nanoscale architecture and force exerted shows a high level of stress accumulation at a single site of adhesion.

Multicolor Localization-Based Super Resolution Microscopy.

Super Resolution (SR) microscopy has become a powerful tool to study cellular architecture at the nanometer scale. Single molecule localization microscopy (SMLM) is a method in which fluorophore labels repeatedly switch On and Off ("blink"). Their exact locations are estimated by computing the centers of individual blinks. Therefore, the image quality depends on the density of the detected labels, as well as the accuracy of the estimation of their location. Both are influenced by several factors. Here we present a step-by-step method that optimizes many of these factors to facilitate multicolor imaging.

Dynamic FRET-FLIM based screening of signal transduction pathways.

Fluorescence Lifetime Imaging (FLIM) is an intrinsically quantitative method to screen for protein-protein interactions and is 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 Förster Resonance Energy Transfer (FRET), such as those occurring upon activation of cell signaling. The second messenger cyclic adenosine monophosphate (cAMP) is rapidly formed following activation of certain cell surface receptors. cAMP is subsequently degraded by a set of phosphodiesterases (PDEs) which display cell-type specific expression and may also affect baseline levels of the messenger. To study which specific PDEs contribute most to cAMP regulation, we knocked down individual PDEs and recorded breakdown rates of cAMP levels following transient stimulation in HeLa cells stably expressing the FRET/FLIM sensor, Epac-SH189. Many hundreds of cells were recorded at 5 s intervals for each condition. FLIM time traces were calculated for every cell, and decay kinetics were obtained. cAMP clearance was significantly slower when PDE3A and, to a lesser amount, PDE10A were knocked down, identifying these isoforms as dominant in HeLa cells. However, taking advantage of the quantitative FLIM data, we found that knockdown of individual PDEs has a very limited effect on baseline cAMP levels. By combining photon-efficient FLIM instrumentation with optimized sensors, systematic gene knockdown and an automated open-source analysis pipeline, our study demonstrates that dynamic screening of transient cell signals has become feasible. The quantitative platform described here provides detailed kinetic analysis of cellular signals in individual cells with unprecedented throughput.

IHC-based Ki67 as response biomarker to tamoxifen in breast cancer window trials enrolling premenopausal women.

Window studies are gaining traction to assess (molecular) changes in short timeframes. Decreased tumor cell positivity for the proliferation marker Ki67 is often used as a proxy for treatment response. Immunohistochemistry (IHC)-based Ki67 on tissue from neo-adjuvant trials was previously reported to be predictive for long-term response to endocrine therapy for breast cancer in postmenopausal women, but none of these trials enrolled premenopausal women. Nonetheless, the marker is being used on this subpopulation. We compared pathologist assessed IHC-based Ki67 in samples from pre- and postmenopausal women in a neo-adjuvant, endocrine therapy focused trial (NCT00738777), randomized between tamoxifen, anastrozole, or fulvestrant. These results were compared with (1) IHC-based Ki67 scoring by AI, (2) mitotic figures, (3) mRNA-based Ki67, (4) five independent gene expression signatures capturing proliferation, and (5) blood levels for tamoxifen and its metabolites as well as estradiol. Upon tamoxifen, IHC-based Ki67 levels were decreased in both pre- and postmenopausal breast cancer patients, which was confirmed using mRNA-based cell proliferation markers. The magnitude of decrease of Ki67 IHC was smaller in pre- versus postmenopausal women. We found a direct relationship between post-treatment estradiol levels and the magnitude of the Ki67 decrease in tumors. These data suggest IHC-based Ki67 may be an appropriate biomarker for tamoxifen response in premenopausal breast cancer patients, but anti-proliferative effect size depends on estradiol levels.

Direct Observation of α-Synuclein Amyloid Aggregates in Endocytic Vesicles of Neuroblastoma Cells.

Aggregation of α-synuclein has been linked to both familial and sporadic Parkinson's disease. Recent studies suggest that α-synuclein aggregates may spread from cell to cell and raise questions about the propagation of neurodegeneration. While continuous progress has been made characterizing α-synuclein aggregates in vitro, there is a lack of information regarding the structure of these species inside the cells. Here, we use confocal fluorescence microscopy in combination with direct stochastic optical reconstruction microscopy, dSTORM, to investigate α-synuclein uptake when added exogenously to SH-SY5Y neuroblastoma cells, and to probe in situ morphological features of α-synuclein aggregates with near nanometer resolution. We demonstrate that using dSTORM, it is possible to follow noninvasively the uptake of extracellularly added α-synuclein aggregates by the cells. Once the aggregates are internalized, they move through the endosomal pathway and accumulate in lysosomes to be degraded. Our dSTORM data show that α-synuclein aggregates remain assembled after internalization and they are shortened as they move through the endosomal pathway. No further aggregation was observed inside the lysosomes as speculated in the literature, nor in the cytoplasm of the cells. Our study thus highlights the super-resolution capability of dSTORM to follow directly the endocytotic uptake of extracellularly added amyloid aggregates and to probe the morphology of in situ protein aggregates even when they accumulate in small vesicular compartments.

Depth-of-Focus Correction in Single-Molecule Data Allows Analysis of 3D Diffusion of the Glucocorticoid Receptor in the Nucleus.

Single-molecule imaging of proteins in a 2D environment like membranes has been frequently used to extract diffusive properties of multiple fractions of receptors. In a 3D environment the apparent fractions however change with observation time due to the movements of molecules out of the depth-of-field of the microscope. Here we developed a mathematical framework that allowed us to correct for the change in fraction size due to the limited detection volume in 3D single-molecule imaging. We applied our findings on the mobility of activated glucocorticoid receptors in the cell nucleus, and found a freely diffusing fraction of 0.49±0.02. Our analysis further showed that interchange between this mobile fraction and an immobile fraction does not occur on time scales shorter than 150 ms.

Imaging the lipid bilayer of giant unilamellar vesicles using red-to-blue light upconversion.

Red-to-blue triplet-triplet annihilation upconversion was obtained in giant unilamellar vesicles. The upconverted light was homogeneously distributed across the membrane and could be utilized for the imaging of individual giant vesicles in three dimensions. These results show the great potential of TTA-UC for imaging applications under anoxic conditions.