DNA Nanostructure-Templated Antibody Complexes Provide Insights into the Geometric Requirements of Human Complement Cascade Activation.

The classical complement pathway is activated by antigen-bound IgG antibodies. Monomeric IgG must oligomerize to activate complement via the hexameric C1q complex, and hexamerizing mutants of IgG appear as promising therapeutic candidates. However, structural data have shown that it is not necessary to bind all six C1q arms to initiate complement, revealing a symmetry mismatch between C1 and the hexameric IgG complex that has not been adequately explained. Here, we use DNA nanotechnology to produce specific nanostructures to template antigens and thereby spatially control IgG valency. These DNA-nanotemplated IgG complexes can activate complement on cell-mimetic lipid membranes, which enabled us to determine the effect of IgG valency on complement activation without the requirement to mutate antibodies. We investigated this using biophysical assays together with 3D cryo-electron tomography. Our data revealed the importance of interantigen distance on antibody-mediated complement activation, and that the cleavage of complement component C4 by the C1 complex is proportional to the number of ideally spaced antigens. Increased IgG valency also translated to better terminal pathway activation and membrane attack complex formation. Together, these data provide insights into how nanopatterning antigen-antibody complexes influence the activation of the C1 complex and suggest routes to modulate complement activation by antibody engineering. Furthermore, to our knowledge, this is the first time DNA nanotechnology has been used to study the activation of the complement system.

Super-resolution fluorescence imaging of cryosamples does not limit achievable resolution in cryoEM.

Correlated super-resolution cryo-fluorescence and cryo-electron microscopy (cryoEM) has been gaining popularity as a method to investigate biological samples with high resolution and specificity. A concern in this combined method (called SR-cryoCLEM), however, is whether and how fluorescence imaging prior to cryoEM acquisition is detrimental to sample integrity. In this report, we investigated the effect of high-dose laser light (405, 488, and 561 nm) irradiation on apoferritin samples prepared for cryoEM with excitation wavelengths commonly used in fluorescence microscopy, and compared these samples to controls that were kept in the dark. We found that laser illumination, of equal duration and intensity as used in cryo-single molecule localization microscopy (cryoSMLM) and in the presence of high concentrations of fluorescent protein, did not affect the achievable resolution in cryoEM, with final reconstructions reaching resolutions of âˆ¼ 1.8 Å regardless of the laser illumination. The finding that super-resolution fluorescence imaging of cryosamples prior to cryoEM data acquisition does not limit the achievable resolution suggests that super-resolution cryo-fluorescence microscopy and in situ structural biology using cryoEM are entirely compatible.

Charging of Vitreous Samples in Cryogenic Electron Microscopy Mitigated by Graphene.

Cryogenic electron microscopy can provide high-resolution reconstructions of macromolecules embedded in a thin layer of ice from which atomic models can be built de novo. However, the interaction between the ionizing electron beam and the sample results in beam-induced motion and image distortion, which limit the attainable resolutions. Sample charging is one contributing factor of beam-induced motions and image distortions, which is normally alleviated by including part of the supporting conducting film within the beam-exposed region. However, routine data collection schemes avoid strategies whereby the beam is not in contact with the supporting film, whose rationale is not fully understood. Here we characterize electrostatic charging of vitreous samples, both in imaging and in diffraction mode. We mitigate sample charging by depositing a single layer of conductive graphene on top of regular EM grids. We obtained high-resolution single-particle analysis (SPA) reconstructions at 2 Å when the electron beam only irradiates the middle of the hole on graphene-coated grids, using data collection schemes that previously failed to produce sub 3 Å reconstructions without the graphene layer. We also observe that the SPA data obtained with the graphene-coated grids exhibit a higher b factor and reduced particle movement compared to data obtained without the graphene layer. This mitigation of charging could have broad implications for various EM techniques, including SPA and cryotomography, and for the study of radiation damage and the development of future sample carriers. Furthermore, it may facilitate the exploration of more dose-efficient, scanning transmission EM based SPA techniques.

Switching the Mode of Drug Release from a Reaction-Coupled Low-Molecular-Weight Gelator System by Altering Its Reaction Pathway.

Low-molecular-weight hydrogels are attractive scaffolds for drug delivery applications because of their modular and facile preparation starting from inexpensive molecular components. The molecular design of the hydrogelator results in a commitment to a particular release strategy, where either noncovalent or covalent bonding of the drug molecule dictates its rate and mechanism. Herein, we demonstrate an alternative approach using a reaction-coupled gelator to tune drug release in a facile and user-defined manner by altering the reaction pathway of the low-molecular-weight gelator (LMWG) and drug components through an acylhydrazone-bond-forming reaction. We show that an off-the-shelf drug with a reactive handle, doxorubicin, can be covalently bound to the gelator through its ketone moiety when the addition of the aldehyde component is delayed from 0 to 24 h, or noncovalently bound with its addition at 0 h. We also examine the use of an l-histidine methyl ester catalyst to prepare the drug-loaded hydrogels under physiological conditions. Fitting of the drug release profiles with the Korsmeyer-Peppas model corroborates a switch in the mode of release consistent with the reaction pathway taken: increased covalent ligation drives a transition from a Fickian to a semi-Fickian mode in the second stage of release with a decreased rate. Sustained release of doxorubicin from the reaction-coupled hydrogel is further confirmed in an MTT toxicity assay with MCF-7 breast cancer cells. We demonstrate the modularity and ease of the reaction-coupled approach to prepare drug-loaded self-assembled hydrogels in situ with tunable mechanics and drug release profiles that may find eventual applications in macroscale drug delivery.

One-Pot Synthesis of Defined-Length ssDNA for Multiscaffold DNA Origami.

DNA origami nanostructures generally require a single scaffold strand of specific length, combined with many small staple strands. Ideally, the length of the scaffold strand should be dictated by the size of the designed nanostructure. However, synthesizing arbitrary-length single-stranded DNA in sufficient quantities is difficult. Here, we describe a straightforward and accessible method to produce defined-length ssDNA scaffolds using PCR and subsequent selective enzymatic digestion with T7 exonuclease. This approach produced ssDNA with higher yields than other methods and without the need for purification, which significantly decreased the time from PCR to obtaining pure DNA origami. Furthermore, this enabled us to perform true one-pot synthesis of defined-size DNA origami nanostructures. Additionally, we show that multiple smaller ssDNA scaffolds can efficiently substitute longer scaffolds in the formation of DNA origami.

Grafting from a Hybrid DNA-Covalent Polymer by the Hybridization Chain Reaction.

Nucleic acid-polymer conjugates are an attractive class of materials endowed with tunable and responsive character. Herein, we exploit the dynamic character of nucleic acids in the preparation of hybrid DNA-covalent polymers with extendable grafts by the hybridization chain reaction. Addition of DNA hairpins to an initiator DNA-dextran graft copolymer resulted in the growth of the DNA grafts as evidenced by various characterization techniques over several length scales. Additionally, aggregation of the initiator DNA-graft copolymer before the hybridization chain reaction was observed resulting in the formation of kinetically trapped aggregates several hundreds of nanometers in diameter that could be disrupted by a preheating step at 60 °C prior to extension at room temperature. Materials of increasing viscosity were rapidly formed when metastable DNA hairpins were added to the initiator DNA-dextran grafted copolymer with increasing concentration of the components in the mixture. This study shows the potential for hierarchical self-assembly of DNA-grafted polymers through the hybridization chain reaction and opens the door for biomedical applications where viscosity can be used as a readout.

Squaramide-Based Supramolecular Materials for Three-Dimensional Cell Culture of Human Induced Pluripotent Stem Cells and Their Derivatives.

Synthetic hydrogel materials can recapitulate the natural cell microenvironment; however, it is equally necessary that the gels maintain cell viability and phenotype while permitting reisolation without stress, especially for use in the stem cell field. Here, we describe a family of synthetically accessible, squaramide-based tripodal supramolecular monomers consisting of a flexible tris(2-aminoethyl)amine (TREN) core that self-assemble into supramolecular polymers and eventually into self-recovering hydrogels. Spectroscopic measurements revealed that monomer aggregation is mainly driven by a combination of hydrogen bonding and hydrophobicity. The self-recovering hydrogels were used to encapsulate NIH 3T3 fibroblasts as well as human-induced pluripotent stem cells (hiPSCs) and their derivatives in 3D. The materials reported here proved cytocompatible for these cell types with maintenance of hiPSCs in their undifferentiated state essential for their subsequent expansion or differentiation into a given cell type and potential for facile release by dilution due to their supramolecular nature.

Reversible Loading of Nanoscale Elements on a Multicomponent Supramolecular Polymer System by Using DNA Strand Displacement.

Nucleic acids are excellent building blocks to enable switchable character in supramolecular polymer materials because of their inherent dynamic character and potential for orthogonal self-assembly. Herein, DNA-grafted squaramide bola-amphiphiles are used in a multicomponent supramolecular polymer system and it is shown that they can be addressed by DNAlabeled gold nanoparticles (5 and 15 nm) through sequence complementarity. These nanoparticles can be selectively erased or rewritten on-demand by means of DNA-strand displacement.

Crosslinker-Induced Effects on the Gelation Pathway of a Low Molecular Weight Hydrogel.

The use of polymeric crosslinkers is an attractive method to modify the mechanical properties of supramolecular materials, but their effects on the self-assembly of the underlying supramolecular polymer networks are poorly understood. Modulation of the gelation pathway of a reaction-coupled low molecular weight hydrogelator is demonstrated using (bio)polymeric crosslinkers of disparate physicochemical identities, providing a handle for control over materials properties.

Aromatic Gain in a Supramolecular Polymer.

The synergy of aromatic gain and hydrogen bonding in a supramolecular polymer is explored. Partially aromatic bis(squaramide) bolaamphiphiles were designed to self-assemble through a combination of hydrophobic, hydrogen-bonding, and aromatic effects into stiff, high-aspect-ratio fibers. UV and IR spectroscopy show electron delocalization and geometric changes within the squaramide ring indicative of strong hydrogen bonding and aromatic gain of the monomer units. The aromatic contribution to the interaction energy was further supported computationally by nucleus-independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) indices, demonstrating greater aromatic character upon polymerization: at least 30% in a pentamer. The aromatic gain-hydrogen bonding synergy results in a significant increase in thermodynamic stability and a striking difference in aggregate morphology of the bis(squaramide) bolamphiphile compared to isosteres that cannot engage in this effect.