Bryan. Carson

Characterization of Differential Toll‐like Receptor Responses below the Optical Diffraction Limit

Many membrane receptors are recruited to specific cell surface domains to form nanoscale clusters upon ligand activation. This step appears to be necessary to initiate cell signaling, including pathways in innate immune system activation. However, virulent pathogens such as Yersinia pestis (the causative agent of plague) are known to evade innate immune detection, in contrast to similar microbes (such as Escherichia coli) that elicit a robust response. This disparity has been partly attributed to the structure of lipopolysaccharides (LPS) on the bacterial cell wall, which are recognized by the innate immune receptor TLR4. It is hypothesized that nanoscale differences exist between the spatial clustering of TLR4 upon binding of LPS derived from Y. pestis and E. coli. Although optical imaging can provide exquisite details of the spatial organization of biomolecules, there is a mismatch between the scale at which receptor clustering occurs (<300 nm) and the optical diffraction limit (>400 nm). The last decade has seen the emergence of super-resolution imaging methods that effectively break the optical diffraction barrier to yield truly nanoscale information in intact biological samples. This study reports the first visualizations of TLR4 distributions on intact cells at image resolutions of <30 nm using a novel, dual-color stochastic optical reconstruction microscopy (STORM) technique. This methodology permits distinction between receptors containing bound LPS from those without at the nanoscale. Importantly, it is also shown that LPS derived from immunostimulatory bacteria result in significantly higher LPS-TLR4 cluster sizes and a nearly twofold greater ligand/receptor colocalization as compared to immunoevading LPS.

Identification of Critical Amino Acids within the Nucleoprotein of Tacaribe Virus Important for Anti-interferon Activity

Background: With the exception of Tacaribe virus, all arenavirus nucleoproteins are thought to inhibit type I interferon production. Results: Variation in nucleoprotein residues 389–392 of Tacaribe virus was characterized as a critical region regulating interferon inhibition. Conclusion: Some Tacaribe virus variants contain the important nucleoprotein residues necessary for interferon antagonism. Significance: Anti-interferon activity of nucleoproteins appears to be a conserved feature of all arenaviruses. The arenavirus nucleoprotein (NP) can suppress induction of type I interferon (IFN). This anti-IFN activity is thought to be shared by all arenaviruses with the exception of Tacaribe virus (TCRV). To identify the TCRV NP amino acid residues that prevent its IFN-countering ability, we created a series of NP chimeras between residues of TCRV NP and Pichinde virus (PICV) NP, an arenavirus NP with potent anti-IFN function. Chimera NP analysis revealed that a minimal four amino acid stretch derived from PICV NP could impart efficient anti-IFN activity to TCRV NP. Strikingly, the TCRV NP gene cloned and sequenced from viral stocks obtained through National Institutes of Health Biodefense and Emerging Infections (BEI) resources deviated from the reference sequence at this particular four-amino acid region, GPPT (GenBank KC329849) versus DLQL (GenBank NC004293), respectively at residues 389–392. When efficiently expressed in cells through codon-optimization, TCRV NP containing the GPPT residues rescued the antagonistic IFN function. Consistent with cell expression results, TCRV infection did not stimulate an IFNβ response early in infection in multiple cells types (e.g. A549, P388D1), and IRF-3 was not translocated to the nucleus in TCRV-infected A549 cells. Collectively, these data suggest that certain TCRV strain variants contain the important NP amino acids necessary for anti-IFN activity.

Magnetic-adhesive based valves for microfluidic devices used in low-resource settings.

Since the introduction of micro total analytical systems (μTASs), significant advances have been made toward development of lab-on-a-chip platforms capable of performing complex biological assays that can revolutionize public health, among other applications. However, use of these platforms in low-resource environments (e.g. developing countries) has yet to be realized as the majority of technologies used to control microfluidic flow rely on off-device hardware with non-negligible size, cost, power requirements and skill/training to operate. In this paper we describe a magnetic-adhesive based valve that is simple to construct and operate, and can be used to control fluid flow and store reagents within a microfluidic device. The design consists of a port connecting two chambers on different planes in the device that is closed by a neodymium disk magnet seated on a thin ring of adhesive. Bringing an external magnet into contact with the outer surface of the device unseats and displaces the valve magnet from the adhesive ring, exposing the port. Using this configuration, we demonstrate on-device reagent storage and on-demand transport and reaction of contents between chambers. This design requires no power or external instrumentation to operate, is extremely low cost (

Hyperspectral image correlation for monitoring membrane protein dynamics in living cells

0.20 materials cost per valve), can be used by individuals with no technical training, and requires only a hand-held magnet to actuate. Additionally, valve actuation does not compromise the integrity of the completely sealed microfluidic device, increasing safety for the operator when toxic or harmful substances are contained within. This valve concept has the potential to simplify design of μTASs, facilitating development of lab-on-a-chip systems that may be practical for use in point-of-care and low-resource settings.

Method for measuring the unbinding energy of strongly-bound membrane-associated proteins

Temporal image correlation provides a powerful fluorescence technique for measuring several biologically relevant parameters of molecules in living cells. These parameters include, but are not limited to local concentrations, diffusion dynamics, and aggregation states of biomolecules. However, the complex cellular environment presents several limitations, precluding high quantitative accuracy and constraining biological implementation. In order to address these issues, high speed spectral imaging was employed to compare the results of image correlation from spectrally unmixed and virtually implemented fluorescence emission filters. Of particular interest in this study is the impact of cellular autofluorescence, which is ubiquitous in fluorescence imaging of cells and tissues. Using traditional instrumentation, corrections for autofluorescence are commonly estimated as a static offset collected from a separate control specimen. While this may be sufficient in highly homogenous regions of interest, the low analyte concentrations requisite to fluctuation-based methods result in the potential for unbounded error resulting from spectral cross-talk between local autofluorescence inhomogeneities and the fluorescence signal of interest. Thus we demonstrate the importance of accurate autofluorescence characterization and discuss potential corrections using a case study focusing on fluorescence confocal spectral imaging of immune cells before and after stimulation with lipopolysaccheride (LPS). In these experiments, binding of LPS to the membrane receptor, YFP-TLR4, is observed to result in initiation of the immune response characterized by altered receptor diffusion dynamics and apparent heterogeneous aggregation states. In addition to characterizing errors resulting from autofluorescence spectral bleed-through, we present data leading to a deeper understanding of the molecular dynamics of the immune response and suggest hypotheses for future work utilizing hyperspectrally enabled multi-label fluorescence studies on this system of high biological import.

Simultaneous, dual-color STORM imaging at the cellular interface.

We describe a new method to measure the activation energy for unbinding (enthalpy ΔH*u and free energy ΔG*u) of a strongly-bound membrane-associated protein from a lipid membrane. It is based on measuring the rate of release of a liposome-bound protein during centrifugation on a sucrose gradient as a function of time and temperature. The method is used to determine ΔH*u and ΔG*u for the soluble dengue virus envelope protein (sE) strongly bound to 80:20 POPC:POPG liposomes at pH5.5. ΔH*u is determined from the Arrhenius equation whereas ΔG*u is determined by fitting the data to a model based on mean first passage time for escape from a potential well. The binding free energy ΔGb of sE was also measured at the same pH for the initial, predominantly reversible, phase of binding to a 70:30 PC:PG lipid bilayer. The unbinding free energy (20±3kcal/mol, 20% PG) was found to be roughly three times the binding energy per monomer, (7.8±0.3kcal/mol for 30% PG, or est. 7.0kcal/mol for 20% PG). This is consistent with data showing that free sE is a monomer at pH5.5, but assembles into trimers after associating with membranes. This new method to determine unbinding energies should be useful to understand better the complex interactions of integral monotopic proteins and strongly-bound peripheral membrane proteins with lipid membranes.

Insertion of Dengue E into lipid bilayers studied by neutron reflectivity and molecular dynamics simulations

Over the past decade optical approaches have been introduced that effectively break the traditional diffraction barrier. Of particular note were the introductions of Stimulated Emission and Depletion (STED) microscopy [1], Photo-Activated Localization Microscopy (PALM) [2], and the closely related Stochastic Optical Reconstruction Microscopy (STORM) [3]. Of these, STORM represents an attractive method for researchers, as it does not require highly specialized optical setups as is the case in STED, and can be more easily implemented for multi-color imaging.

Simultaneous, Dual-Color STORM Imaging of Membrane Reorganization during Early Immune Response

The envelope (E) protein of Dengue virus rearranges to a trimeric hairpin to mediate fusion of the viral and target membranes, which is essential for infectivity. Insertion of E into the target membrane serves to anchor E and possibly also to disrupt local order within the membrane. Both aspects are likely to be affected by the depth of insertion, orientation of the trimer with respect to the membrane normal, and the interactions that form between trimer and membrane. In the present work, we resolved the depth of insertion, the tilt angle, and the fundamental interactions for the soluble portion of Dengue E trimers (sE) associated with planar lipid bilayer membranes of various combinations of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol (POPG), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), and cholesterol (CHOL) by neutron reflectivity (NR) and by molecular dynamics (MD) simulations. The results show that the tip of E containing the fusion loop (FL) is located at the interface of the headgroups and acyl chains of the outer leaflet of the lipid bilayers, in good agreement with prior predictions. The results also indicate that E tilts with respect to the membrane normal upon insertion, promoted by either the anionic lipid POPG or CHOL. The simulations show that tilting of the protein correlates with hydrogen bond formation between lysines and arginines located on the sides of the trimer close to the tip (K246, K247, and R73) and nearby lipid headgroups. These hydrogen bonds provide a major contribution to the membrane anchoring and may help to destabilize the target membrane.

Imaging Innate Immune Responses using Dual Color Stochastic Reconstruction Optical Microscopy (STORM)

TLR-4 receptor reorganization in cell membranes was investigated using a novel STORM microscope. The increased resolution permits observation of receptor cluster formation following challenge with chemotypes of lipopolysaccharide.

Nuclear translocation kinetics of NF-κB in macrophages challenged with pathogens in a microfluidic platform

Over the past decade, optical approaches have been introduced that effectively break the traditional diffraction barrier. Of particular note were the introductions of Stimulated Emission and Depletion (STED) microscopy [1], Photo-Activated Localization Microscopy (PALM) [2], and the closely related Stochastic Optical Reconstruction Microscopy (STORM) [3]. Of these, the STORM/PALM approach represents an attractive method for researchers, as it does not require highly specialized optical setups as is the case in STED, and can be more easily implemented for multi-color imaging. We have implemented a dual-color direct STORM system to simultaneously image the subresolution organization/colocalization of the TLR-4 receptor, a key mediator of innate immune response and lipopolysaccharide (LPS), a bacteria-specific antigen recognized by TLR4. We show that “super-resolution” imaging approaches such as this can provide information that is not attainable with other optical methods.