Short-range communication between cells is required for the survival of multicellular organisms. One mechanism of chemical signaling between adjacent cells employs surface displayed ligands and receptors that only bind when two cells make physical contact. Ligand–receptor complexes that form at the cell–cell junction and physically bridge two cells likely experience mechanical forces. A fundamental challenge in this area pertains to mapping the mechanical forces experienced by ligand–receptor complexes within such a fluid intermembrane junction. Herein, we describe the development of ratiometric tension probes for direct imaging of receptor tension, clustering, and lateral transport within a model cell–cell junction. These probes employ two fluorescent reporters that quantify both the ligand density and the ligand tension and thus generate a tension signal independent of clustering. As a proof-of-concept, we applied the ratiometric tension probes to map the forces experienced by the T-cell receptor (TCR) during activation and showed the first direct evidence that the TCR-ligand complex experiences sustained pN forces within a fluid membrane junction. We envision that the ratiometric tension probes will be broadly useful for investigating mechanotransduction in juxtacrine signaling pathways.

Mechanical forces transmitted through integrin transmembrane receptors play important roles in a variety of cellular processes ranging from cell development to tumorigenesis. Despite the importance of mechanics in integrin function, the magnitude of integrin forces within adhesions remains unclear. Literature suggests a range from 1 to 50 pN, but the upper limit of integrin forces remains unknown. Herein we challenge integrins with the most mechanically stable molecular tension probe, which is comprised of the immunoglobulin 27th (I27) domain of cardiac titin flanked with a fluorophore and gold nanoparticle. Cell experiments show that integrin forces unfold the I27 domain, suggesting that integrin forces exceed ∼30–40 pN. The addition of a disulfide bridge within I27 “clamps” the probe and resists mechanical unfolding. Importantly, incubation with a reducing agent initiates SH exchange, thus unclamping I27 at a rate that is dependent on the applied force. By recording the rate of S–S reduction in clamped I27, we infer that integrins apply 110 ± 9 pN within focal adhesions of rat embryonic fibroblasts. The rates of S–S exchange are heterogeneous and integrin subtype-dependent. Nanoparticle titin tension sensors along with kinetic analysis of unfolding demonstrate that a subset of integrins apply tension many fold greater than previously reported.

Mechanical forces between cells and their extracellular matrix (ECM) are mediated by dozens of different receptors. These biophysical interactions play fundamental roles in processes ranging from cellular development to tumor progression. However, mapping the spatial and temporal dynamics of tension among various receptor–ligand pairs remains a significant challenge. To address this issue, we have developed a synthetic strategy to generate modular tension probes combining the native chemical ligation (NCL) reaction with solid phase peptide synthesis (SPPS). In principle, this approach accommodates virtually any peptide or expressed protein amenable to NCL. We generated a small library of tension probes displaying different ligands, flexible linkers, and fluorescent reporters, enabling the mapping of integrin and cadherin tension, and demonstrating the first example of long-term (∼3 days) molecular tension imaging. This approach provides a toolset to better understand mechanotransduction events fundamental to cell biology.

T cells are triggered when the T-cell receptor (TCR) encounters its antigenic ligand, the peptide-major histocompatibility complex (pMHC), on the surface of antigen presenting cells (APCs). Because T cells are highly migratory and antigen recognition occurs at an intermembrane junction where the T cell physically contacts the APC, there are long-standing questions of whether T cells transmit defined forces to their TCR complex and whether chemomechanical coupling influences immune function. Here we develop DNA-based gold nanoparticle tension sensors to provide, to our knowledge, the first pN tension maps of individual TCR-pMHC complexes during T-cell activation. We show that naïve T cells harness cytoskeletal coupling to transmit 12–19 pN of force to their TCRs within seconds of ligand binding and preceding initial calcium signaling. CD8 coreceptor binding and lymphocyte-specific kinase signaling are required for antigen-mediated cell spreading and force generation. Lymphocyte function-associated antigen 1 (LFA-1) mediated adhesion modulates TCR-pMHC tension by intensifying its magnitude to values >19 pN and spatially reorganizes the location of TCR forces to the kinapse, the zone located at the trailing edge of migrating T cells, thus demonstrating chemomechanical crosstalk between TCR and LFA-1 receptor signaling. Finally, T cells display a dampened and poorly specific response to antigen agonists when TCR forces are chemically abolished or physically “filtered” to a level below ∼12 pN using mechanically labile DNA tethers. Therefore, we conclude that T cells tune TCR mechanics with pN resolution to create a checkpoint of agonist quality necessary for specific immune response.

In this study, we used deoxyribozyme (DNAzyme) functionalized gold nanoparticles (AuNPs) to catalytically silence tumor necrosis factor-α (TNF-α) in vivo as a potential therapeutic for myocardial infarction (MI). Using primary macrophages as a model, we demonstrated 50% knockdown of TNF-α, which was not attainable using Lipofectamine-based approaches. Local injection of DNAzyme conjugated to gold particles (AuNPs) in the rat myocardium yielded TNF-α knockdown efficiencies of 50%, which resulted in significant anti-inflammatory effects and improvement in acute cardiac function following MI. Our results represent the first example showing the use of DNAzyme AuNP conjugates in vivo for viable delivery and gene regulation. This is significant as TNF-α is a multibillion dollar drug target implicated in many inflammatory-mediated disorders, thus underscoring the potential impact of DNAzyme-conjugated AuNPs.

Herein we aimed to understand how nanoscale clustering of RGD ligands alters the mechano-regulation of their integrin receptors. We combined molecular tension fluorescence microscopy with block copolymer micelle nanolithography to fabricate substrates with arrays of precisely spaced probes that can generate a 10-fold fluorescence response to pN-forces. We found that the mechanism of sensing ligand spacing is force-mediated. This strategy is broadly applicable to investigating receptor clustering and its role in mechanotransduction pathways.

Lipid vesicle encapsulation is an efficient approach to transfer quantum dots (QDs) into aqueous solutions, which is important for renewable energy applications and biological imaging. However, little is known about the molecular organization at the interface between a QD and lipid membrane. To address this issue, we investigated the properties of 3.0 nm CdSe QDs encapsulated within phospholipid membranes displaying a range of phase transition temperatures (Tm). Theoretical and experimental results indicate that the QD locally alters membrane structure, and in turn, the physical state (phase) of the membrane controls the optical and chemical properties of the QDs. Using photoluminescence, ICP-MS, optical microscopy, and ligand exchange studies, we found that the Tm of the membrane controls optical and chemical properties of lipid vesicle-embedded QDs. Importantly, QDs encapsulated within gel-phase membranes were ultrastable, providing the most photostable non-core/shell QDs in aqueous solution reported to date. Atomistic molecular dynamics simulations support these observations and indicate that membranes are locally disordered displaying greater disordered organization near the particle–solution interface. Using this asymmetry in membrane organization near the particle, we identify a new approach for site-selective modification of QDs by specifically functionalizing the QD surface facing the outer lipid leaflet to generate gold nanoparticle–QD assemblies programmed by Watson–Crick base-pairing.

Studying chemomechanical coupling at interfaces is important for fields ranging from lubrication and tribology to microfluidics and cell biology. Several polymeric macro- and microscopic systems and cantilevers have been developed to image forces at interfaces, but few materials are amenable for molecular tension sensing. To address this issue, we have developed a gold nanoparticle sensor for molecular tension-based fluorescence microscopy. As a proof of concept, we imaged the tension exerted by integrin receptors at the interface between living cells and a substrate with high spatial (<1 μm) resolution, at 100 ms acquisition times and with molecular specificity. We report integrin tension values ranging from 1 to 15 pN and a mean of ∼1 pN within focal adhesions. Through the use of a conventional fluorescence microscope, this method demonstrates a force sensitivity that is 3 orders of magnitude greater than is achievable by traction force microscopy or polydimethylsiloxane micropost arrays,(1) which are the standard in cellular biomechanics.

Upon activation, the epidermal growth factor (EGF) receptor becomes phosphorylated and triggers a vast signaling network that has profound effects on cell growth. The EGF receptor is observed to assemble into clusters after ligand binding and tyrosine kinase autophosphorylation, but the role of these assemblies in the receptor signaling pathway remains unclear. To address this question, we measured the phosphorylation of EGFR when the EGF ligand was anchored onto laterally mobile and immobile surfaces. We found that cells generated clusters of ligand–receptor complex on mobile EGF surfaces, and displayed a lower ratio of phosphorylated EGFR to EGF when compared to immobilized EGF that is unable to cluster. This result was verified by tuning the lateral assembly of ligand–receptor complexes on the surface of living cells using patterned supported lipid bilayers. Nanoscale metal lines fabricated into the supported membrane constrained lipid diffusion and EGF receptor assembly into micron and sub-micron scale corrals. Single cell analysis indicated that clustering impacts EGF receptor activation, and larger clusters (>1 μm2) of ligand–receptor complex generated lower EGF receptor phosphorylation per ligand than smaller assemblies (<1 μm2) in HCC1143 cells that were engaged to ligand-functionalized surfaces. We investigated the mechanism of EGFR clustering by treating cells with compounds that disrupt the cytoskeleton (Latrunculin B), clathrin-mediated endocytosis (Pitstop2), and inhibit EGFR activation (Gefitinib). These results help elucidate the nature of large-scale EGFR clustering, thus underscoring the general significance of receptor spatial organization in tuning biochemical function.

Herein we report on an approach to use dip-pen nanolithography (DPN) to rapidly prototype nano- and microscale cationic polymer structures that guide the transport of lipid molecules in a fluid membrane. An atomic force microscope (AFM) tip is used to transfer poly(diallyldimethylammonium chloride) (PDAC) to a silica surface that templates the self-assembly of a phospholipid bilayer. Based on AFM and high-resolution optical microscopy data, DPN-generated PDAC structures are optically transparent and composed of up to three molecular layers of polyelectrolyte. We demonstrate that patterns can juxtapose mobile and immobile ligands in a supported lipid bilayer that simultaneously engages the epidermal growth factor receptor (EGFR) and adhesion receptors on the cell surface. In principle, DPN-generated polyelectrolyte structures have important applications in investigating supramolecular protein assemblies in living cells.

Demonstrating how surface chemistry and self-assembled monolayers (SAMs) control the macroscopic properties of materials is challenging as it often necessitates the use of specialized instrumentation. In this hands-on experiment, students directly measure a macroscopic property, the floatation of glass coverslips on water as a function of modifying the terminal surface groups of the glass. The glass surface is chemically modified by the self-assembly of monomolecular layers formed by two organosilanes, 3-aminopropyltriethoxysilane and 1-octadecyltrimethoxysilane, which change the water contact angles. These SAMs alter the ability of the modified glass to support a mass, thus demonstrating that the bulk material property can be directly controlled by molecular surface chemistry.Keywords: First-Year Undergraduate/General; Hands-On Learning/Manipulatives; High School/Introductory Chemistry; Laboratory Instruction; Materials Science; Physical Chemistry; Qualitative Analysis; Surface Science; Upper-Division Undergraduate;

DNAzymes are catalytic oligonucleotides with important applications in gene regulation, DNA computing, responsive soft materials, and ultrasensitive metal-ion sensing. The most significant challenge for using DNAzymes in vivo pertains to nontoxic delivery and maintaining function inside cells. We synthesized multivalent deoxyribozyme “10-23” gold nanoparticle (DzNP) conjugates, varying DNA density, linker length, enzyme orientation, and linker composition in order to study the role of the steric environment and gold surface chemistry on catalysis. DNAzyme catalytic efficiency was modulated by steric packing and proximity of the active loop to the gold surface. Importantly, the 10-23 DNAzyme was asymmetrically sensitive to the gold surface and when anchored through the 5′ terminus was inhibited 32-fold. This property was used to generate DNAzymes whose catalytic activity is triggered by thiol displacement reactions or by photoexcitation at λ = 532 nm. Importantly, cell studies revealed that DzNPs are less susceptible to nuclease degradation, readily enter mammalian cells, and catalytically down-regulate GDF15 gene expression levels in breast cancer cells, thus addressing some of the key limitations in the adoption of DNAzymes for in vivo work.Keywords: breast cancer; deoxyribozyme; DNAzyme; GDF15; gene regulation; gold nanoparticle; Herceptin; synthetic biology; trastuzumab

Herein we report on an approach to use dip-pen nanolithography (DPN) to rapidly prototype nano- and microscale cationic polymer structures that guide the transport of lipid molecules in a fluid membrane. An atomic force microscope (AFM) tip is used to transfer poly(diallyldimethylammonium chloride) (PDAC) to a silica surface that templates the self-assembly of a phospholipid bilayer. Based on AFM and high-resolution optical microscopy data, DPN-generated PDAC structures are optically transparent and composed of up to three molecular layers of polyelectrolyte. We demonstrate that patterns can juxtapose mobile and immobile ligands in a supported lipid bilayer that simultaneously engages the epidermal growth factor receptor (EGFR) and adhesion receptors on the cell surface. In principle, DPN-generated polyelectrolyte structures have important applications in investigating supramolecular protein assemblies in living cells.