Antiviral drug discovery: Pyrimidine entry inhibitors for Zika and dengue viruses.

Vector-borne diseases, constituting over 17 % of infectious diseases, are caused by parasites, viruses, and bacteria, and their prevalence is shaped by environmental and social factors. Dengue virus (DENV) and Zika virus (ZIKV), some of the most prevalent infectious agents of this type of diseases, are transmitted by mosquitoes belonging to the genus Aedes. The highest prevalence is observed in tropical regions, inhabited by around 3 billion people. DENV infects millions of people annually and constitutes an additional sanitary challenge due to the circulation of four serotypes, which has complicated vaccine development. ZIKV causes large outbreaks globally and its infection is known to lead to severe neurological diseases, including microcephaly in newborns. Besides, not only mosquito control programs have proved to be not totally effective, but also, no antiviral drugs have been developed so far. The envelope protein (E) is a major component of DENV and ZIKV virion surface. This protein plays a key role during the virus cell entry, constituting an attractive target for the development of antiviral drugs. Our previous studies have identified two pyrimidine analogs (3e and 3h) as inhibitors; however, their activity was found to be hindered by their low water solubility. In this study, we performed a low-throughput antiviral screening, revealing compound 16a as a potent DENV-2 and ZIKV inhibitor (EC50 = 1.4 µM and 2.4 µM, respectively). This work was aimed at designing molecules with improved selectivity and pharmacokinetic properties, thus advancing the antiviral efficacy of compounds for potential therapeutic use.

Evolution and activation mechanism of the flavivirus class II membrane-fusion machinery.

The flavivirus envelope glycoproteins prM and E drive the assembly of icosahedral, spiky immature particles that bud across the membrane of the endoplasmic reticulum. Maturation into infectious virions in the trans-Golgi network involves an acid-pH-driven rearrangement into smooth particles made of (prM/E)2 dimers exposing a furin site for prM cleavage into "pr" and "M". Here we show that the prM "pr" moiety derives from an HSP40 cellular chaperonin. Furthermore, the X-ray structure of the tick-borne encephalitis virus (pr/E)2 dimer at acidic pH reveals the E 150-loop as a hinged-lid that opens at low pH to expose a positively-charged pr-binding pocket at the E dimer interface, inducing (prM/E)2 dimer formation to generate smooth particles in the Golgi. Furin cleavage is followed by lid-closure upon deprotonation in the neutral-pH extracellular environment, expelling pr while the 150-loop takes the relay in fusion loop protection, thus revealing the elusive flavivirus mechanism of fusion activation.

Dengue virus NS1 protein conveys pro-inflammatory signals by docking onto high-density lipoproteins.

The dengue virus nonstructural protein 1 (NS1) is a secreted virulence factor that modulates complement, activates immune cells and alters endothelial barriers. The molecular basis of these events remains incompletely understood. Here we describe a functional high affinity complex formed between NS1 and human high-density lipoproteins (HDL). Collapse of the soluble NS1 hexamer upon binding to the lipoprotein particle leads to the anchoring of amphipathic NS1 dimeric subunits into the HDL outer layer. The stable complex can be visualized by electron microscopy as a spherical HDL with rod-shaped NS1 dimers protruding from the surface. We further show that the assembly of NS1-HDL complexes triggers the production of pro-inflammatory cytokines in human primary macrophages while NS1 or HDL alone do not. Finally, we detect NS1 in complex with HDL and low-density lipoprotein (LDL) particles in the plasma of hospitalized dengue patients and observe NS1-apolipoprotein E-positive complexes accumulating overtime. The functional reprogramming of endogenous lipoprotein particles by NS1 as a means to exacerbate systemic inflammation during viral infection provides a new paradigm in dengue pathogenesis.

Glycan-Glycan Interaction Determines Shigella Tropism toward Human T Lymphocytes.

Direct interactions between bacterial and host glycans have been recently reported to be involved in the binding of pathogenic bacteria to host cells. In the case of Shigella, the Gram-negative enteroinvasive bacterium responsible for acute rectocolitis, such interactions contribute to bacterial adherence to epithelial cells. However, the role of glycans in the tropism of Shigella for immune cells whose glycosylation pattern varies depending on their activation state is unknown. We previously reported that Shigella targets activated, but not nonactivated, human CD4+ T lymphocytes. Here, we show that nonactivated CD4+ T lymphocytes can be turned into Shigella-targetable cells upon loading of their plasma membrane with sialylated glycosphingolipids (also termed gangliosides). The Shigella targeting profile of ganglioside-loaded nonactivated T cells is similar to that of activated T cells, with a predominance of injection of effectors from the type III secretion system (T3SS) not resulting in cell invasion. We demonstrate that gangliosides interact with the O-antigen polysaccharide moiety of lipopolysaccharide (LPS), the major bacterial surface antigen, thus promoting Shigella binding to CD4+ T cells. This binding step is critical for the subsequent injection of T3SS effectors, a step which we univocally demonstrate to be dependent on actin polymerization. Altogether, these findings highlight the critical role of glycan-glycan interactions in Shigella pathogenesis.IMPORTANCE Glycosylation of host cell surface varies with species and location in the body, thus contributing to species specificity and tropism of microorganisms. Cross talk by Shigella, the Gram-negative enteroinvasive bacterium responsible for bacillary dysentery, with its exclusively human host has been extensively studied. However, the molecular determinants of the step of binding to host cells are poorly defined. Taking advantage of the observation that human-activated CD4+ T lymphocytes, but not nonactivated cells, are targets of Shigella, we succeeded in rendering the refractory cells susceptible to targeting upon loading of their plasma membrane with sialylated glycosphingolipids (gangliosides) that are abundantly present on activated cells. We show that interactions between the sugar polar part of gangliosides and the polysaccharide moiety of Shigella lipopolysaccharide (LPS) promote bacterial binding, which results in the injection of effectors via the type III secretion system. Whereas LPS interaction with gangliosides was proposed long ago and recently extended to a large variety of glycans, our findings reveal that such glycan-glycan interactions are critical for Shigella pathogenesis by driving selective interactions with host cells, including immune cells.

The bright and the dark side of human antibody responses to flaviviruses: lessons for vaccine design.

Zika and dengue viruses belong to the Flavivirus genus, a close group of antigenically related viruses that cause significant arthropod-transmitted diseases throughout the globe. Although infection by a given flavivirus is thought to confer lifelong protection, some of the patient's antibodies cross-react with other flaviviruses without cross-neutralizing. The original antigenic sin phenomenon may amplify such antibodies upon subsequent heterologous flavivirus infection, potentially aggravating disease by antibody-dependent enhancement (ADE). The most striking example is provided by the four different dengue viruses, where infection by one serotype appears to predispose to more severe disease upon infection by a second one. A similar effect was postulated for sequential infections with Zika and dengue viruses. In this review, we analyze the molecular determinants of the dual antibody response to flavivirus infection or vaccination in humans. We highlight the role of conserved partially cryptic epitopes giving rise to cross-reacting and poorly neutralizing, ADE-prone antibodies. We end by proposing a strategy for developing an epitope-focused vaccine approach to avoid eliciting undesirable antibodies while focusing the immune system on producing protective antibodies only.

High-Resolution Mapping of a Repeat Protein Folding Free Energy Landscape.

A complete description of the pathways and mechanisms of protein folding requires a detailed structural and energetic characterization of the conformational ensemble along the entire folding reaction coordinate. Simulations can provide this level of insight for small proteins. In contrast, with the exception of hydrogen exchange, which does not monitor folding directly, experimental studies of protein folding have not yielded such structural and energetic detail. NMR can provide residue specific atomic level structural information, but its implementation in protein folding studies using chemical or temperature perturbation is problematic. Here we present a highly detailed structural and energetic map of the entire folding landscape of the leucine-rich repeat protein, pp32 (Anp32), obtained by combining pressure-dependent site-specific 1H-15N HSQC data with coarse-grained molecular dynamics simulations. The results obtained using this equilibrium approach demonstrate that the main barrier to folding of pp32 is quite broad and lies near the unfolded state, with structure apparent only in the C-terminal region. Significant deviation from two-state unfolding under pressure reveals an intermediate on the folded side of the main barrier in which the N-terminal region is disordered. A nonlinear temperature dependence of the population of this intermediate suggests a large heat capacity change associated with its formation. The combination of pressure, which favors the population of folding intermediates relative to chemical denaturants; NMR, which allows their observation; and constrained structure-based simulations yield unparalleled insight into protein folding mechanisms.

Unusual Reversible Oligomerization of Unfolded Dengue Envelope Protein Domain 3 at High Temperatures and Its Abolition by a Point Mutation.

We report differential scanning calorimetry (DSC) experiments between 10 and 120 °C of Dengue 4 envelope protein domain 3 (DEN4 ED3), a small 107-residue monomeric globular protein domain. The thermal unfolding of DEN4 ED3 was fully reversible and exhibited two peculiar endothermic peaks. AUC (analytical ultracentrifugation) experiments at 25 °C indicated that DEN4 ED3 was monomeric. Detailed thermodynamic analysis indicated that the two endothermic peaks separated with an increasing protein concentration, and global fitting of the DSC curves strongly suggested the presence of unfolded tetramers at temperatures around 80-90 °C, which dissociated to unfolded monomers at even higher temperatures. To further characterize this rare thermal unfolding process, we designed and constructed a DEN4 ED3 variant that would unfold according to a two-state model, typical of globular proteins. We thus substituted Val 380, the most buried residue at the dimeric interface in the protein crystal, with less hydrophobic amino acids (Ala, Ser, Thr, Asn, and Lys). All variants showed a single heat absorption peak, typical of small globular proteins. In particular, the DSC thermogram of DEN4 V380K indicated a two-state reversible thermal unfolding independent of protein concentration, indicating that the high-temperature oligomeric state was successfully abolished by a single mutation. These observations confirmed the standard view that small monomeric globular proteins undergo a two-state unfolding. However, the reversible formation of unfolded oligomers at high temperatures is a truly new phenomenon, which was fully inhibited by an accurately designed single mutation.

Exploring the Protein Folding Pathway with High-Pressure NMR: Steady-State and Kinetics Studies.

Defining the physical-chemical determinants of protein folding and stability, under normal and pathological conditions has constituted a major subfield in biophysical chemistry for over 50 years. Although a great deal of progress has been made in recent years towards this goal, a number of important questions remain. These include characterizing the structural, thermodynamic and dynamic properties of the barriers between conformational states on the protein energy landscape, understanding the sequence dependence of folding cooperativity, defining more clearly the role of solvation in controlling protein stability and dynamics and probing the high energy thermodynamic states in the native state basin and their role in misfolding and aggregation. Fundamental to the elucidation of these questions is a complete thermodynamic parameterization of protein folding determinants. In this chapter, we describe the use of high-pressure coupled to Nuclear Magnetic Resonance (NMR) spectroscopy to reveal unprecedented details on the folding energy landscape of proteins.

Evolutionarily Conserved Pattern of Interactions in a Protein Revealed by Local Thermal Expansion Properties.

The way in which the network of intramolecular interactions determines the cooperative folding and conformational dynamics of a protein remains poorly understood. High-pressure NMR spectroscopy is uniquely suited to examine this problem because it combines the site-specific resolution of the NMR experiments with the local character of pressure perturbations. Here we report on the temperature dependence of the site-specific volumetric properties of various forms of staphylococcal nuclease (SNase), including three variants with engineered internal cavities, as measured with high-pressure NMR spectroscopy. The strong temperature dependence of pressure-induced unfolding arises from poorly understood differences in thermal expansion between the folded and unfolded states. A significant inverse correlation was observed between the global thermal expansion of the folded proteins and the number of strong intramolecular hydrogen bonds, as determined by the temperature coefficient of the backbone amide chemical shifts. Comparison of the identity of these strong H-bonds with the co-evolution of pairs of residues in the SNase protein family suggests that the architecture of the interactions detected in the NMR experiments could be linked to a functional aspect of the protein. Moreover, the temperature dependence of the residue-specific volume changes of unfolding yielded residue-specific differences in expansivity and revealed how mutations impact intramolecular interaction patterns. These results show that intramolecular interactions in the folded states of proteins impose constraints against thermal expansion and that, hence, knowledge of site-specific thermal expansivity offers insight into the patterns of strong intramolecular interactions and other local determinants of protein stability, cooperativity, and potentially also of function.

Molecular basis for bacterial peptidoglycan recognition by LysM domains.

Carbohydrate recognition is essential for growth, cell adhesion and signalling in all living organisms. A highly conserved carbohydrate binding module, LysM, is found in proteins from viruses, bacteria, fungi, plants and mammals. LysM modules recognize polysaccharides containing N-acetylglucosamine (GlcNAc) residues including peptidoglycan, an essential component of the bacterial cell wall. However, the molecular mechanism underpinning LysM-peptidoglycan interactions remains unclear. Here we describe the molecular basis for peptidoglycan recognition by a multimodular LysM domain from AtlA, an autolysin involved in cell division in the opportunistic bacterial pathogen Enterococcus faecalis. We explore the contribution of individual modules to the binding, identify the peptidoglycan motif recognized, determine the structures of free and bound modules and reveal the residues involved in binding. Our results suggest that peptide stems modulate LysM binding to peptidoglycan. Using these results, we reveal how the LysM module recognizes the GlcNAc-X-GlcNAc motif present in polysaccharides across kingdoms.

High-pressure fluorescence applications.

Fluorescence is the most widely used technique to study the effect of pressure on biochemical systems. The use of pressure as a physical variable sheds light into volumetric characteristics of reactions. Here we focus on the effect of pressure on protein solutions using a simple unfolding example in order to illustrate the applications of the methodology. Topics covered in this review include the relationships between practical aspects and technical limitations; the effect of pressure and the study of protein cavities; the interpretation of thermodynamic and relaxation kinetics; and the study of relaxation amplitudes. Finally, we discuss the insights available from the combination of fluorescence and other methods adapted to high pressure, such as SAXS or NMR. Because of the simplicity and accessibility of high-pressure fluorescence, the technique is a starting point that complements appropriately multi-methodological approaches related to understanding protein function, disfunction, and folding from the volumetric point of view.

Measuring residual dipolar couplings at high hydrostatic pressure: robustness of alignment media to high pressure.

Among other perturbations, high hydrostatic pressure has proven to be a mild yet efficient way to unfold proteins. Combining pressure perturbation with NMR spectroscopy allows for a residue-per-residue description of folding reactions. Accessing the full power of NMR spectroscopy under pressure involves the investigation of conformational sampling using orientational restraints such as residual dipolar couplings (RDCs) under conditions of partial alignment. The aim of this study was to identify and characterize stable and pressure resistant alignment media for measurement of RDCs at high pressure. Four alignment media were tested. A C12E5/n-hexanol alcohol mixture remains stable from 1 to 2,500 bar, whereas Pf1 phage and DNA nanotubes undergo a reversible transition between 300 and 900 bar. Phospholipid bicelles are stable only until 300 bar at ambient temperature. Hence, RDCs can be measured at high pressure, and their interpretation will provide atomic details of the structural and dynamic perturbations on unfolded or partially folded states of proteins under pressure.

Low-cost equilibrium unfolding of heme proteins using 2 µl samples.

Equilibrium unfolding experiments provide access to protein thermodynamic stability revealing basic aspects of protein structure-function relationships. A limitation of these experiments stands on the availability of large amounts of protein samples. Here we present the use of the NanoDrop for monitoring guanidinium chloride-induced unfolding by Soret absorbance of monomeric heme proteins. Unfolding experiments using 2 µl of reactant are validated by fluorescence and circular dichroism spectroscopy and supported with five heme proteins including neuroglobin, cytochrome b5, and cyanoglobin. This work guarantees 2 orders of magnitude reduction in protein expense. Promising low-cost protein unfolding experiments following other chromophores and high-throughput screenings are discussed.

Effect of internal cavities on folding rates and routes revealed by real-time pressure-jump NMR spectroscopy.

The time required to fold proteins usually increases significantly under conditions of high pressure. Taking advantage of this general property of proteins, we combined P-jump experiments with NMR spectroscopy to examine in detail the folding reaction of staphylococcal nuclease (SNase) and of some of its cavity-containing variants. The nearly 100 observables that could be measured simultaneously collectively describe the kinetics of folding as a function of pressure and denaturant concentration with exquisite site-specific resolution. SNase variants with cavities in the central core of the protein exhibit a highly heterogeneous transition-state ensemble (TSE) with a smaller solvent-excluded void volume than the TSE of the parent SNase. This heterogeneous TSE experiences Hammond behavior, becoming more native-like (higher molar volume) with increasing denaturant concentration. In contrast, the TSE of the L125A variant, which has a cavity at the secondary core, is only slightly different from that of the parent SNase. Because pressure acts mainly to eliminate solvent-excluded voids, which are heterogeneously distributed throughout structures, it perturbs the protein more selectively than chemical denaturants, thereby facilitating the characterization of intermediates and the consequences of packing on folding mechanisms. Besides demonstrating how internal cavities can affect the routes and rates of folding of a protein, this study illustrates how the combination of P-jump and NMR spectroscopy can yield detailed mechanistic insight into protein folding reactions with exquisite site-specific temporal information.

Probing the physical determinants of thermal expansion of folded proteins.

The magnitude and sign of the volume change upon protein unfolding are strongly dependent on temperature. This temperature dependence reflects differences in the thermal expansivity of the folded and unfolded states. The factors that determine protein molar expansivities and the large differences in thermal expansivity for proteins of similar molar volume are not well understood. Model compound studies have suggested that a major contribution is made by differences in the molar volume of water molecules as they transfer from the protein surface to the bulk upon heating. The expansion of internal solvent-excluded voids upon heating is another possible contributing factor. Here, the contribution from hydration density to the molar thermal expansivity of a protein was examined by comparing bovine pancreatic trypsin inhibitor and variants with alanine substitutions at or near the protein-water interface. Variants of two of these proteins with an additional mutation that unfolded them under native conditions were also examined. A modest decrease in thermal expansivity was observed in both the folded and unfolded states for the alanine variants compared with the parent protein, revealing that large changes can be made to the external polarity of a protein without causing large ensuing changes in thermal expansivity. This modest effect is not surprising, given the small molar volume of the alanine residue. Contributions of the expansion of the internal void volume were probed by measuring the thermal expansion for cavity-containing variants of a highly stable form of staphylococcal nuclease. Significantly larger (2-3-fold) molar expansivities were found for these cavity-containing proteins relative to the reference protein. Taken together, these results suggest that a key determinant of the thermal expansivities of folded proteins lies in the expansion of internal solvent-excluded voids.

Volumetric properties underlying ligand binding in a monomeric hemoglobin: a high-pressure NMR study.

The 2/2 hemoglobin of the cyanobacterium Synechococcus sp. PCC 7002, GlbN, coordinates the heme iron with two histidines and exists either with a b heme or with a covalently attached heme. The binding of exogenous ligands displaces the distal histidine and induces a conformational rearrangement involving the reorganization of internal void volumes. The formation of passageways within the resulting conformation is thought to facilitate ligand exchange and play a functional role. Here we monitored the perturbation induced by pressure on the ferric bis-histidine and cyanide-bound states of GlbN using (1)H-(15)N HSQC NMR spectroscopy. We inspected the outcome with a statistical analysis of 170 homologous 2/2 hemoglobin sequences. We found that the compression landscape of GlbN, as represented by the variation of an average chemical shift parameter, was highly sensitive to ligand swapping and heme covalent attachment. Stabilization of rare conformers was observed at high pressures and consistent with cavity redistribution upon ligand binding. In all states, the EF loop was found to be exceptionally labile to pressure, suggesting a functional role as a semi-flexible hinge between the adjacent helices. Finally, coevolved clusters presented a common pattern of compensating pressure responses. The high-pressure dissection combined with protein sequence analysis established locations with volumetric signatures relevant to residual communication of 2/2 hemoglobins. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

Structural, energetic, and dynamic responses of the native state ensemble of staphylococcal nuclease to cavity-creating mutations.

The effects of cavity-creating mutations on the structural flexibility, local and global stability, and dynamics of the folded state of staphylococcal nuclease (SNase) were examined with NMR spectroscopy, MD simulations, H/D exchange, and pressure perturbation. Effects on global thermodynamic stability correlated well with the number of heavy atoms in the vicinity of the mutated residue. Variants with substitutions in the C-terminal domain and the interface between α and ß subdomains showed large amide chemical shift variations relative to the parent protein, moderate, widespread, and compensatory perturbations of the H/D protection factors and increased local dynamics on a nanosecond time scale. The pressure sensitivity of the folded states of these variants was similar to that of the parent protein. Such observations point to the capacity of the folded proteins to adjust to packing defects in these regions. In contrast, cavity creation in the ß-barrel subdomain led to minimal perturbation of the structure of the folded state, However, significant pressure dependence of the native state amide resonances, along with strong effects on native state H/D exchange are consistent with increased probability of population of excited state(s) for these variants. Such contrasted responses to the creation of cavities could not be anticipated from global thermodynamic stability or crystal structures; they depend on the local structural and energetic context of the substitutions.

Remodeling of the folding free energy landscape of staphylococcal nuclease by cavity-creating mutations.

The folding of staphylococcal nuclease (SNase) is known to proceed via a major intermediate in which the central OB subdomain is folded and the C-terminal helical subdomain is disordered. To identify the structural and energetic determinants of this folding free energy landscape, we have examined in detail, using high-pressure NMR, the consequences of cavity creating mutations in each of the two subdomains of an ultrastable SNase, Δ+PHS. The stabilizing mutations of Δ+PHS enhanced the population of the major folding intermediate. Cavity creation in two different regions of the Δ+PHS reference protein, despite equivalent effects on global stability, had very distinct consequences on the complexity of the folding free energy landscape. The L125A substitution in the C-terminal helix of Δ+PHS slightly suppressed the major intermediate and promoted an additional excited state involving disorder in the N-terminus, but otherwise decreased landscape heterogeneity with respect to the Δ+PHS background protein. The I92A substitution, located in the hydrophobic OB-fold core, had a much more profound effect, resulting in a significant increase in the number of intermediate states and implicating the entire protein structure. Denaturant (GuHCl) had very subtle and specific effects on the landscape, suppressing some states and favoring others, depending upon the mutational context. These results demonstrate that disrupting interactions in a region of the protein with highly cooperative, unfrustrated folding has very profound effects on the roughness of the folding landscape, whereas the effects are less pronounced for an energetically equivalent substitution in an already frustrated region.

Thermodynamics of cooperative DNA recognition at a replication origin and transcription regulatory site.

Binding cooperativity guides the formation of protein-nucleic acid complexes, in particular those that are highly regulated such as replication origins and transcription sites. Using the DNA binding domain of the origin binding and transcriptional regulator protein E2 from human papillomavirus type 16 as model, and through isothermal titration calorimetry analysis, we determined a positive, entropy-driven cooperativity upon binding of the protein to its cognate tandem double E2 site. This cooperativity is associated with a change in DNA structure, where the overall B conformation is maintained. Two homologous E2 domains, those of HPV18 and HPV11, showed that the enthalpic-entropic components of the reaction and DNA deformation can diverge. Because the DNA binding helix is almost identical in the three domains, the differences must lie dispersed throughout this unique dimeric ß-barrel fold. This is in surprising agreement with previous results for this domain, which revealed a strong coupling between global dynamics and DNA recognition.

Experimental snapshots of a protein-DNA binding landscape.

Protein recognition of DNA sites is a primary event for gene function. Its ultimate mechanistic understanding requires an integrated structural, dynamic, kinetic, and thermodynamic dissection that is currently limited considering the hundreds of structures of protein-DNA complexes available. We describe a protein-DNA-binding pathway in which an initial, diffuse, transition state ensemble with some nonnative contacts is followed by formation of extensive nonnative interactions that drive the system into a kinetic trap. Finally, nonnative contacts are slowly rearranged into native-like interactions with the DNA backbone. Dissimilar protein-DNA interfaces that populate along the DNA-binding route are explained by a temporary degeneracy of protein-DNA interactions, centered on "dual-role" residues. The nonnative species slow down the reaction allowing for extended functionality.