Hydrogel-based molecular tension fluorescence microscopy for investigating receptor-mediated rigidity sensing.

Extracellular matrix (ECM) rigidity serves as a crucial mechanical cue impacting diverse biological processes. However, understanding the molecular mechanisms of rigidity sensing has been limited by the spatial resolution and force sensitivity of current cellular force measurement techniques. Here we developed a method to functionalize DNA tension probes on soft hydrogel surfaces in a controllable and reliable manner, enabling molecular tension fluorescence microscopy for rigidity sensing studies. Our findings showed that fibroblasts respond to substrate rigidity by recruiting more force-bearing integrins and modulating integrin sampling frequency of the ECM, rather than simply overloading the existing integrin-ligand bonds, to promote focal adhesion maturation. We also demonstrated that ECM rigidity positively regulates the pN force of T cell receptor-ligand bond and T cell receptor mechanical sampling frequency, promoting T cell activation. Thus, hydrogel-based molecular tension fluorescence microscopy implemented on a standard confocal microscope provides a simple and effective means to explore detailed molecular force information for rigidity-dependent biological processes.

Biomimetic nanoparticles blocking autophagy for enhanced chemotherapy and metastasis inhibition via reversing focal adhesion disassembly.

BACKGROUND: Autophagy is a conserved catabolic process, which plays an important role in regulating tumor cell motility and degrading protein aggregates. Chemotherapy-induced autophagy may lead to tumor distant metastasis and even chemo-insensitivity in the therapy of hepatocellular carcinoma (HCC). Therefore, a vast majority of HCC cases do not produce a significant response to monotherapy with autophagy inhibitors. RESULTS: In this work, we developed a biomimetic nanoformulation (TH-NP) co-encapsulating Oxaliplatin (OXA)/hydroxychloroquine (HCQ, an autophagy inhibitor) to execute targeted autophagy inhibition, reduce tumor cell migration and invasion in vitro and attenuate metastasis in vivo. The tumor cell-specific ligand TRAIL was bioengineered to be stably expressed on HUVECs and the resultant membrane vesicles were wrapped on OXA/HCQ-loaded PLGA nanocores. Especially, TH-NPs could significantly improve OXA and HCQ effective concentration by approximately 21 and 13 times in tumor tissues compared to the free mixture of HCQ/OXA. Moreover, the tumor-targeting TH-NPs released HCQ alkalized the acidic lysosomes and inhibited the fusion of autophagosomes and lysosomes, leading to effective blockade of autophagic flux. In short, the system largely improved chemotherapeutic performance of OXA on subcutaneous and orthotopic HCC mice models. Importantly, TH-NPs also exhibited the most effective inhibition of tumor metastasis in orthotopic HCCLM3 models, and in the HepG2, Huh-7 or HCCLM3 metastatic mice models. Finally, we illustrated the enhanced metastasis inhibition was attributed to the blockade or reverse of the autophagy-mediated degradation of focal adhesions (FAs) including E-cadherin and paxillin. CONCLUSIONS: TH-NPs can perform an enhanced chemotherapy and antimetastatic effect, and may represent a promising strategy for HCC therapy in clinics.

Campylobacter jejuni Triggers Signaling through Host Cell Focal Adhesions To Inhibit Cell Motility.

Campylobacter jejuni is a major foodborne pathogen that exploits the focal adhesions of intestinal cells to promote invasion and cause severe gastritis. Focal adhesions are multiprotein complexes involved in bidirectional signaling between the actin cytoskeleton and the extracellular matrix. We investigated the dynamics of focal adhesion structure and function in C. jejuni-infected cells using a comprehensive set of approaches, including confocal microscopy of live and fixed cells, immunoblotting, and superresolution interferometric photoactivated localization microscopy (iPALM). We found that C. jejuni infection of epithelial cells results in increased focal adhesion size and altered topology. These changes resulted in a persistent modulatory effect on the host cell focal adhesion, evidenced by an increase in cell adhesion strength, a decrease in individual cell motility, and a reduction in collective cell migration. We discovered that C. jejuni infection causes an increase in phosphorylation of paxillin and an alteration of paxillin turnover at the focal adhesion, which together represent a potential mechanistic basis for altered cell motility. Finally, we observed that infection of epithelial cells with the C. jejuni wild-type strain in the presence of a protein synthesis inhibitor, a C. jejuni CadF and FlpA fibronectin-binding protein mutant, or a C. jejuni flagellar export mutant blunts paxillin phosphorylation and partially reestablishes individual host cell motility and collective cell migration. These findings provide a potential mechanism for the restricted intestinal repair observed in C. jejuni-infected animals and raise the possibility that bacteria targeting extracellular matrix components can alter cell behavior after binding and internalization by manipulating focal adhesions. IMPORTANCE Campylobacter jejuni is a major foodborne pathogen that causes severe gastritis. We investigated the dynamics of focal adhesion structure and function in C. jejuni-infected epithelial cells. Focal adhesions act as signaling complexes that connect the extracellular matrix to the intracellular cytoskeleton. The key findings of this study show that C. jejuni changes the structure (size and position), composition, and function of cellular focal adhesions using a combination of virulence factors. Mechanistically, we found that the changes in focal adhesion dynamics are dependent upon the activation of host cell signaling pathways, which affect the assembly and disassembly of cellular proteins from the focal adhesion. To summarize, we have identified a new cellular phenotype in C. jejuni-infected cells that may be responsible for the restricted intestinal repair observed in C. jejuni-infected animals.

Focal Adhesion Isolation Assay Using ECM-Coated Magnetic Beads.

Focal adhesions are force sensitive structures that dynamically alter their composition, protein-protein interactions, and signaling in response to external mechanical stimuli. These dynamic changes are critical for focal adhesion function and are required for cellular mechanosensing. Here, we describe a simple protocol that allows for isolation of the focal adhesion complex from adherent cells in culture in response to different mechanical stimuli applied at adhesion sites. By combining this assay with approaches such as proteomics or western blot analysis, one can study the force-dependent changes in focal adhesion composition, protein-protein interactions and signaling.

Transfer of HTLV-1 p8 and Gag to target T-cells depends on VASP, a novel interaction partner of p8.

The Human T-cell leukemia virus type 1 (HTLV-1) orf I-encoded accessory protein p8 is cleaved from its precursor p12, and both proteins contribute to viral persistence. p8 induces cellular protrusions, which are thought to facilitate transfer of p8 to target cells and virus transmission. Host factors interacting with p8 and mediating p8 transfer are unknown. Here, we report that vasodilator-stimulated phosphoprotein (VASP), which promotes actin filament elongation, is a novel interaction partner of p8 and important for p8 and HTLV-1 Gag cell-to-cell transfer. VASP contains an Ena/VASP homology 1 (EVH1) domain that targets the protein to focal adhesions. Bioinformatics identified a short stretch in p8 (amino acids (aa) 24-45) which may mediate interactions with the EVH1 domain of VASP. Co-immunoprecipitations confirmed interactions of VASP:p8 in 293T, Jurkat and HTLV-1-infected MT-2 cells. Co-precipitation of VASP:p8 could be significantly blocked by peptides mimicking aa 26-37 of p8. Mutational studies revealed that the EVH1-domain of VASP is necessary, but not sufficient for the interaction with p8. Further, deletion of the VASP G- and F-actin binding domains significantly diminished co-precipitation of p8. Imaging identified areas of partial co-localization of VASP with p8 at the plasma membrane and in protrusive structures, which was confirmed by proximity ligation assays. Co-culture experiments revealed that p8 is transferred between Jurkat T-cells via VASP-containing conduits. Imaging and flow cytometry revealed that repression of both endogenous and overexpressed VASP by RNA interference or by CRISPR/Cas9 reduced p8 transfer to the cell surface and to target Jurkat T-cells. Stable repression of VASP by RNA interference in chronically infected MT-2 cells impaired both p8 and HTLV-1 Gag transfer to target Jurkat T-cells, while virus release was unaffected. Thus, we identified VASP as a novel interaction partner of p8, which is important for transfer of HTLV-1 p8 and Gag to target T-cells.

A distinct talin2 structure directs isoform specificity in cell adhesion.

Integrin receptors regulate normal cellular processes such as signaling, cell migration, adhesion to the extracellular matrix, and leukocyte function. Talin recruitment to the membrane is necessary for its binding to and activation of integrin. Vertebrates have two highly conserved talin homologs that differ in their expression patterns. The F1-F3 FERM subdomains of cytoskeletal proteins resemble a cloverleaf, but in talin1, its F1 subdomain and additional F0 subdomain align more linearly with its F2 and F3 subdomains. Here, we present the talin2 crystal structure, revealing that its F0-F1 di-subdomain displays another unprecedented constellation, whereby the F0-F1-F2 adopts a new cloverleaf-like arrangement. Using multiangle light scattering (MALS), fluorescence lifetime imaging (FLIM), and FRET analyses, we found that substituting the corresponding residues in talin2 that abolish lipid binding in talin1 disrupt the binding of talin to the membrane, focal adhesion formation, and cell spreading. Our results provide the molecular details of the functions of specific talin isoforms in cell adhesion.

Chlorogenic acid-enriched extract of Ilex kudingcha C.J. Tseng tea inhibits neutrophil recruitment in injured zebrafish by promoting reverse migration via the focal adhesion pathway.

Neutrophil-regulated inflammation plays crucial roles in tissue damage and repair. Dysregulation of the neutrophil response system can contribute to diseases such as cancer. Clearance of excessive neutrophils at the site of inflammation by reverse migration provides a promising strategy to mitigate the negative effects. Chlorogenic acid treatment of injured zebrafish embryos showed low-developmental toxicity. Using a transgenic zebrafish Tg (mpx: egfp) model, chlorogenic acid-enriched kudingcha extract promoted neutrophil reverse migration via phosphorylation of ERK and AKT. Using i-TRAQ analysis, differentially expressed proteins involved in focal adhesion were identified, such as: Cdc42, SRC, MLC, ITGA, and Calpain. In support of this, ERK and AKT proteins are involved in the focal adhesion pathway. Real time qPCR determined that CGA downregulates genes associated with cancer metastasis, such as: HSPA5, YWHAZ, RP17, and ITGAV. Together, these results suggest that CGA-enriched Kudingcha extract may have potential as an anticancer or anti-inflammatory therapeutic agent. PRACTICAL APPLICATIONS: Ilex kudingcha C.J Tseng, commonly referred to as the large-leaved kudingcha, is a tea variety naturally rich in chlorogenic acid. Chlorogenic acid, the ester of caffeic and quinic acids, has antioxidant, antibacterial, anticancer, and anti-inflammatory, activities. Kudingcha has several known biological functions, including: anticancer, anti-inflammatory, antidiabetic, and hypolipidemic effects. Treatment with kudingcha extract reduces the recruitment of neutrophils, potentially by inhibiting focal adhesion, and activation of cancer metastasis-related genes. Importantly, kudingcha extract could be used as an alternative nutritional supplement for anticancer or anti-inflammation via its ability to suppress neutrophil recruitment.

Traction force microscopy for understanding cellular mechanotransduction.

Under physiological and pathological conditions, mechanical forces generated from cells themselves or transmitted from extracellular matrix (ECM) through focal adhesions (FAs) and adherens junctions (AJs) are known to play a significant role in regulating various cell behaviors. Substantial progresses have been made in the field of mechanobiology towards novel methods to understand how cells are able to sense and adapt to these mechanical forces over the years. To address these issues, this review will discuss recent advancements of traction force microscopy (TFM), intracellular force microscopy (IFM), and monolayer stress microscopy (MSM) to measure multiple aspects of cellular forces exerted by cells at cell-ECM and cell-cell junctional intracellular interfaces. We will also highlight how these methods can elucidate the roles of mechanical forces at interfaces of cell-cell/cell-ECM in regulating various cellular functions. [BMB Reports 2020; 53(2): 74-81].

Spatial arrangement of LD motif-interacting residues on focal adhesion targeting domain of Focal Adhesion Kinase determine domain-motif interaction affinity and specificity.

BACKGROUND: Leucine rich Aspartate motifs (LD motifs) are molecular recognition motifs on Paxillin that recognize LD-motif binding domains (LDBD) of a number of focal adhesion proteins in order to carry out downstream signaling and actin cytoskeleton remodeling. In this study, we identified structural features within LDBDs that influence their binding affinity with Paxillin LD motifs. METHODS: Various point mutants of focal adhesion targeting (FAT) domain of Focal Adhesion Kinase (FAK) were created by moving a key Lysine residue two and three helical turns in order to match the unique conformations as observed in LDBDs of two other focal adhesion proteins, Vinculin and CCM3. RESULTS: This led to identify a mutant of FAT domain of FAK, named as FAT(NV) (Asn992 of FAT domain was replaced by Val), with remarkable high affinity for LD1 (Kd = 1.5 µM vs no-binding with wild type) and LD2 peptides (Kd = 7.2 µM vs 63 µM with wild type). Consistently, the focal adhesions of MCF7 cells expressing FAK(NV) were highly stable (turnover rate = 1.25 × 10-5 µm2/s) as compared to wild type FAK transfected cells (turnover rate = 1.5 × 10-3 µm2/s). CONCLUSIONS: We observed that the relative disposition of key LD binding amino-acids at LDBD surface, hydrophobic burial of long Leucine side chains of LD-motifs and complementarity of charged surfaces are the key factors determining the binding affinities of LD motifs with LDBDs. GENERAL SIGNIFICANCE: Our study will help in protein engineering of FAT domain of FAK by modulating FAK-LD motif interactions which have implications in cellular focal adhesions and cell migration.

Development of a Fragment-Based Screening Assay for the Focal Adhesion Targeting Domain Using SPR and NMR.

The Focal Adhesion Targeting (FAT) domain of Focal Adhesion Kinase (FAK) is a promising drug target since FAK is overexpressed in many malignancies and promotes cancer cell metastasis. The FAT domain serves as a scaffolding protein, and its interaction with the protein paxillin localizes FAK to focal adhesions. Various studies have highlighted the importance of FAT-paxillin binding in tumor growth, cell invasion, and metastasis. Targeting this interaction through high-throughput screening (HTS) provides a challenge due to the large and complex binding interface. In this report, we describe a novel approach to targeting FAT through fragment-based drug discovery (FBDD). We developed two fragment-based screening assays-a primary SPR assay and a secondary heteronuclear single quantum coherence nuclear magnetic resonance (HSQC-NMR) assay. For SPR, we designed an AviTag construct, optimized SPR buffer conditions, and created mutant controls. For NMR, resonance backbone assignments of the human FAT domain were obtained for the HSQC assay. A 189-compound fragment library from Enamine was screened through our primary SPR assay to demonstrate the feasibility of a FAT-FBDD pipeline, with 19 initial hit compounds. A final total of 11 validated hits were identified after secondary screening on NMR. This screening pipeline is the first FBDD screen of the FAT domain reported and represents a valid method for further drug discovery efforts on this difficult target.

Force-Dependent Regulation of Talin-KANK1 Complex at Focal Adhesions.

KANK proteins mediate cross-talk between dynamic microtubules and integrin-based adhesions to the extracellular matrix. KANKs interact with the integrin/actin-binding protein talin and with several components of microtubule-stabilizing cortical complexes. Because of actomyosin contractility, the talin-KANK complex is likely under mechanical force, and its mechanical stability is expected to be a critical determinant of KANK recruitment to focal adhesions. Here, we quantified the lifetime of the complex of the talin rod domain R7 and the KN domain of KANK1 under shear-force geometry and found that it can withstand forces for seconds to minutes over a physiological force range up to 10 pN. Complex stability measurements combined with cell biological experiments suggest that shear-force stretching promotes KANK1 localization to the periphery of focal adhesions. These results indicate that the talin-KANK1 complex is mechanically strong, enabling it to support the cross-talk between microtubule and actin cytoskeleton at focal adhesions.

Real Microgravity Influences the Cytoskeleton and Focal Adhesions in Human Breast Cancer Cells.

With the increasing number of spaceflights, it is crucial to understand the changes occurring in human cells exposed to real microgravity (r-µg) conditions. We tested the effect of r-µg on MCF-7 breast cancer cells with the objective to investigate cytoskeletal alterations and early changes in the gene expression of factors belonging to the cytoskeleton, extracellular matrix, focal adhesion, and cytokines. In the Technische Experimente unter Schwerelosigkeit (TEXUS) 54 rocket mission, we had the opportunity to conduct our experiment during 6 min of r-µg and focused on cytoskeletal alterations of MCF-7 breast cancer cells expressing the Lifeact-GFP marker protein for the visualization of F-actin as well as the mCherry-tubulin fusion protein using the Fluorescence Microscopy Analysis System (FLUMIAS) for fast live-cell imaging under r-µg. Moreover, in a second mission we investigated changes in RNA transcription and morphology in breast cancer cells exposed to parabolic flight (PF) maneuvers (31st Deutsches Zentrum für Luft- und Raumfahrt (DLR) PF campaign). The MCF-7 cells showed a rearrangement of the F-actin and tubulin with holes, accumulations in the tubulin network, and the appearance of filopodia- and lamellipodia-like structures in the F-actin cytoskeleton shortly after the beginning of the r-µg period. PF maneuvers induced an early up-regulation of KRT8, RDX, TIMP1, CXCL8 mRNAs, and a down-regulation of VCL after the first parabola. E-cadherin protein was significantly reduced and is involved in cell adhesion processes, and plays a significant role in tumorigenesis. Changes in the E-cadherin protein synthesis can lead to tumor progression. Pathway analyses indicate that VCL protein has an activating effect on CDH1. In conclusion, live-cell imaging visualized similar changes as those occurring in thyroid cancer cells in r-µg. This result indicates the presence of a common mechanism of gravity perception and sensation.

Structural basis of the target-binding mode of the G protein-coupled receptor kinase-interacting protein in the regulation of focal adhesion dynamics.

Focal adhesions (FAs) are specialized sites where intracellular cytoskeleton elements connect to the extracellular matrix and thereby control cell motility. FA assembly depends on various scaffold proteins, including the G protein-coupled receptor kinase-interacting protein 1 (GIT1), paxillin, and liprin-α. Although liprin-α and paxillin are known to competitively interact with GIT1, the molecular basis governing these interactions remains elusive. To uncover the underlying mechanisms of how GIT1 is involved in FA assembly by alternatively binding to liprin-α and paxillin, here we solved the crystal structures of GIT1 in complex with liprin-α and paxillin at 1.8 and 2.6 Å resolutions, respectively. These structures revealed that the paxillin-binding domain (PBD) of GIT1 employs distinct binding modes to recognize a single α-helix of liprin-α and the LD4 motif of paxillin. Structure-based design of protein variants produced two binding-deficient GIT1 variants; specifically, these variants lost the ability to interact with liprin-α only or with both liprin-α and paxillin. Expressing the GIT1 variants in COS7 cells, we discovered that the two PBD-meditated interactions play different roles in either recruiting GIT1 to FA or facilitating FA assembly. Additionally, we demonstrate that, unlike for the known binding mode of the FAT domain to LD motifs, the PBD of GIT1 uses different surface patches to achieve high selectivity in LD motif recognition. In summary, our results have uncovered the mechanisms by which GIT1's PBD recognizes cognate paxillin and liprin-α structures, information we anticipate will be useful for future investigations of GIT1-protein interactions in cells.

Computational and experimental analysis of bioactive peptide linear motifs in the integrin adhesome.

Therapeutic modulation of protein interactions is challenging, but short linear motifs (SLiMs) represent potential targets. Focal adhesions play a central role in adhesion by linking cells to the extracellular matrix. Integrins are central to this process, and many other intracellular proteins are components of the integrin adhesome. We applied a peptide network targeting approach to explore the intracellular modulation of integrin function in platelets. Firstly, we computed a platelet-relevant integrin adhesome, inferred via homology of known platelet proteins to adhesome components. We then computationally selected peptides from the set of platelet integrin adhesome cytoplasmic and membrane adjacent protein-protein interfaces. Motifs of interest in the intracellular component of the platelet integrin adhesome were identified using a predictor of SLiMs based on analysis of protein primary amino acid sequences (SLiMPred), a predictor of strongly conserved motifs within disordered protein regions (SLiMPrints), and information from the literature regarding protein interactions in the complex. We then synthesized peptides incorporating these motifs combined with cell penetrating factors (tat peptide and palmitylation for cytoplasmic and membrane proteins respectively). We tested for the platelet activating effects of the peptides, as well as their abilities to inhibit activation. Bioactivity testing revealed a number of peptides that modulated platelet function, including those derived from α-actinin (ACTN1) and syndecan (SDC4), binding to vinculin and syntenin respectively. Both chimeric peptide experiments and peptide combination experiments failed to identify strong effects, perhaps characterizing the adhesome as relatively robust against within-adhesome synergistic perturbation. We investigated in more detail peptides targeting vinculin. Combined experimental and computational evidence suggested a model in which the positively charged tat-derived cell penetrating part of the peptide contributes to bioactivity via stabilizing charge interactions with a region of the ACTN1 negatively charged surface. We conclude that some interactions in the integrin adhesome appear to be capable of modulation by short peptides, and may aid in the identification and characterization of target sites within the complex that may be useful for therapeutic modulation.

Biomaterial substrate modifications that influence cell-material interactions to prime cellular responses to nonviral gene delivery.

IMPACT STATEMENT: This review summarizes how biomaterial substrate modifications (e.g. chemical modifications like natural coatings, ligands, or functional side groups, and/or physical modifications such as topography or stiffness) can prime the cellular response to nonviral gene delivery (e.g. affecting integrin binding and focal adhesion formation, cytoskeletal remodeling, endocytic mechanisms, and intracellular trafficking), to aid in improving gene delivery for applications where a cell-material interface might exist (e.g. tissue engineering scaffolds, medical implants and devices, sensors and diagnostics, wound dressings).

Profiling circRNA and miRNA of radiation-induced esophageal injury in a rat model.

Evidence has also shown that micro ribonucleic acid (miRNA) plays an important role in many cellular processes. However, it is unclear how ionizing radiation causes the miRNA and circular ribonucleic acid (circRNA) expression levels to change and how this change relates to esophageal injury. We analyzed RNA Sequencing (RNA-seq) data from normal esophageal tissue and irradiated esophageal tissues and used computational approaches to identify and characterize differentially expressed miRNAs and circRNAs. We detected 27 miRNAs and 197 circRNAs that had significantly different expression levels after ionizing radiation treatment compared with normal control.Among the 27 miRNAs, 7 miRNAs were down-regulated, and the other 20 were up-regulated. Their target genes were found to be involved in responses to wound, lipid biosynthesis, cell proliferation, cell migration, chemokine activity, hairpin binding, and the cell membrane system. We also found 197 differentially expressed circRNAs in total, of which 87 were up-regulated and 110 were down-regulated. Notably, we found that differentially expressed circRNAs were enriched in cell differentiation, epithelial cell migration, striatum development, protein binding, extracellular exosome, and focal adhesion functions. Of the related processes, sphingolipid metabolism was notable. Many of the differentially expressed circRNAs were involved in sphingolipid metabolism pathways. Cells responded to ionizing radiation (IR) using multiple pathways, which led to sphingolipid metabolism and other immune responses, ultimately leading to esophageal injury.IR-induced esophageal injury is worth studying, especially the dynamic network of circRNA and miRNA. By knowing the regulatory details of related pathways, radiation-related esophageal injury can be prevented, and the efficiency of radiation therapy can be enhanced.

Force-activatable coating enables high-resolution cellular force imaging directly on regular cell culture surfaces.

Integrin-transmitted cellular forces are crucial mechanical signals regulating a vast range of cell functions. Although various methods have been developed to visualize and quantify cellular forces at the cell-matrix interface, a method with high performance and low technical barrier is still in demand. Here we developed a force-activatable coating (FAC), which can be simply coated on regular cell culture apparatus' surfaces by physical adsorption, and turn these surfaces to force reporting platforms that enable cellular force mapping directly by fluorescence imaging. The FAC molecule consists of an adhesive domain for surface coating and a force-reporting domain which can be activated to fluoresce by integrin molecular tension. The tension threshold required for FAC activation is tunable in 10-60 piconewton (pN), allowing the selective imaging of cellular force contributed by integrin tension at different force levels. We tested the performance of two FACs with tension thresholds of 12 and 54 pN (nominal values), respectively, on both glass and polystyrene surfaces. Cellular forces were successfully mapped by fluorescence imaging on all the surfaces. FAC-coated surfaces also enable co-imaging of cellular forces and cell structures in both live cells and immunostained cells, therefore opening a new avenue for the study of the interplay of force and structure. We demonstrated the co-imaging of integrin tension and talin clustering in live cells, and concluded that talin clustering always occurs before the generation of integrin tension above 54 pN, reinforcing the notion that talin is an important adaptor protein for integrin tension transmission. Overall, FAC provides a highly convenient approach that is accessible to general biological laboratories for the study of cellular forces with high sensitivity and resolution, thus holding the potential to greatly boost the research of cell mechanobiology.

Nanoscale localization of proteins within focal adhesions indicates discrete functional assemblies with selective force-dependence.

Focal adhesions (FAs) are subcellular regions at the micrometer scale that link the cell to the surrounding microenvironment and control vital cell functions. However, the spatial architecture of FAs remains unclear at the nanometer scale. We used two-color and three-color super-resolution stimulated emission depletion microscopy to determine the spatial distributions and co-localization of endogenous FA components in fibroblasts. Our data indicate that adhesion proteins inside, but not outside, FAs are organized into nanometer size units of multi-protein assemblies. The loss of contractile force reduced the nanoscale co-localization between different types of proteins, while it increased this co-localization between markers of the same type. This suggests that actomyosin-dependent force exerts a nonrandom, specific, control of the localization of adhesion proteins within cell-matrix adhesions. These observations are consistent with the possibility that proteins in cell-matrix adhesions are assembled in nanoscale particles, and that force regulates the localization of the proteins therein in a protein-specific manner. This detailed knowledge of how the organization of FA components at the nanometer scale is linked to the capacity of the cells to generate contractile forces expands our understanding of cell adhesion in health and disease.

Cooperative mechanosensitivity and allostery of focal adhesion clusters.

We analyse the role of cooperative interaction between neighbouring adhesion-mechanosensor complexes by constructing an Ising-like Hamiltonian describing the free energy of cell adhesion on a substrate as a lattice of 3-state mechanosensing sites involving focal adhesion kinase (FAK). We use a Monte Carlo stochastic algorithm to find equilibrium configurations of these mechanosensors in two representative geometries: on a 1D ring representing the rim of a cell on a flat surface, and a 2D bounded surface representing the whole area of cell contact with a flat surface. The level of FAK activation depends on the pulling force applied to the individual FAK-integrin via actin-myosin contractile networks, and the details of the coupling between individual sensors in a cluster. Strong coupling is shown to make the FAK sensors experience a sharp on-off behaviour in their activation, while at low coupling the activation/autoinhibition transition occurs over a broad range of pulling force. We find that the activation/autoinhibition transition of FAK in the 2D system with strong coupling occurs with a hysteresis, the width of which depends on the rate of change of force. The effect of introducing a regulating protein (such as Src) in a limited quantity to control FAK activation is explored, and visualizations of clustering in both topologies are presented. In particular the results on the bounded 2D surface indicate that clustering of active FAK occurs preferentially at the boundary, in agreement with experimental observations of focal adhesions in cells.

The Intracranial Aneurysm Gene THSD1 Connects Endosome Dynamics to Nascent Focal Adhesion Assembly.

BACKGROUND/AIMS: We recently discovered that harmful variants in THSD1 (Thrombospondin type-1 domain-containing protein 1) likely cause intracranial aneurysm and subarachnoid hemorrhage in a subset of both familial and sporadic patients with supporting evidence from two vertebrate models. The current study seeks to elucidate how THSD1 and patient-identified variants function molecularly in focal adhesions. METHODS: Co-immunostaining and co-immunoprecipitation were performed to define THSD1 subcellular localization and interacting partners. Transient expression of patient-identified THSD1 protein variants and siRNA-mediated loss-of-function THSD1 were used to interrogate gene function in focal adhesion and cell attachment to collagen I in comparison to controls. RESULTS: THSD1 is a novel nascent adhesion protein that co-localizes with several known markers such as FAK, talin, and vinculin, but not with mature adhesion marker zyxin. Furthermore, THSD1 forms a multimeric protein complex with FAK/talin/vinculin, wherein THSD1 promotes talin binding to FAK but not to vinculin, a key step in nascent adhesion assembly. Accordingly, THSD1 promotes mature adhesion formation and cell attachment, while its rare variants identified in aneurysm patients show compromised ability. Interestingly, THSD1 also localizes at different stages of endosomes. Clathrin-mediated but not caveolae-mediated endocytosis pathway is involved in THSD1 intracellular trafficking, which positively regulates THSD1-induced focal adhesion assembly, in contrast to the traditional role of endosomes in termination of integrin signals. CONCLUSIONS: The data suggest that THSD1 functions at the interface between endosome dynamics and nascent focal adhesion assembly that is impaired by THSD1 rare variants identified from intracranial aneurysm patients.