HIV-1 gp41 and gp160 are hyperthermostable proteins in a mesophilic environment. Characterization of gp41 mutants

Distinct antibody profiles in HLA-B∗57+, HLA-B∗57- HIV controllers and chronic progressors

OBJECTIVE

Spontaneous control of HIV replication without treatment in HIV-1 controllers (HICs) was associated with the development of an efficient T-cell response. In addition, increasing data suggest that the humoral response participates in viral clearance.

DESIGN

In-depth characterization of Ab response in HICs may help to define new parameters associated with this control.

METHODS

We assessed the levels of total and HIV-specific IgA and IgG subtypes induction and their functional potencies - that is, neutralization, phagocytosis, antibody-dependent cellular cytotoxicity (ADCC), according to the individual's major histocompatibility complex class I (HLA)-B∗57 status, and compared it with nontreated chronic progressors.

RESULTS

We found that despite an undetectable viral load, HICs displayed HIV-specific IgG levels similar to those of chronic progressors. Interestingly, our compelling multifunctional analysis demonstrates that the functional Ab profile, by itself, allowed to discriminate HLA-B∗57+ HICs from HLA-B∗57- HICs and chronic progressors.

CONCLUSION

These results show that HICs display a particular HIV-specific antibody (Ab) profile that may participate in HIV control and emphasize the relevance of multifunctional Ab response analysis in future Ab-driven vaccine studies.

Statistical optimization of culture medium to produce recombinant viral protein by Escherichia coli host for diagnostic kit to detect human immunodeficiency virus (HIV) infection

The maximal production of recombinant HIV1 gp41 by E. coli was examined in optimal culture condition and medium compositions. The culture condition such as growth, initial medium pHs, IPTG concentrations, induction times, temperature (0.5 OD, 7.6, 0.75 mM, 4.6 h, 32 °C respectively), and yeast extract (7.51 g/l), tryptone (7.26 g/l), glucose (2.45 g/l), NaCl (20.40 g/l), betaine (10.41 mM) and ampicillin (71.23 μg/ml) was optimized using statistical experimental design and response surface method (RSM). One of the main popular methods to attain high cell density in fed-batch culture is by controlling the nutrient feeding, which is often necessary for high yield in protein (0.63-0.72 mg/l) and cell (1.7-2 g/l) of the desired product in four litter fermentations.

A High Frequency of HIV-Specific Circulating Follicular Helper T Cells Is Associated with Preserved Memory B Cell Responses in HIV Controllers

Follicular helper T cells (Tfh) play an essential role in the affinity maturation of the antibody response by providing help to B cells. To determine whether this CD4⁺ T cell subset may contribute to the spontaneous control of HIV infection, we analyzed the phenotype and function of circulating Tfh (cTfh) in patients from the ANRS CO21 CODEX cohort who naturally controlled HIV-1 replication to undetectable levels and compared them to treated patients with similarly low viral loads. HIV-specific cTfh (Tet⁺), detected by Gag-major histocompatibility complex class II (MHC-II) tetramer labeling in the CD45RA⁻ CXCR5⁺ CD4⁺ T cell population, proved more frequent in the controller group (P = 0.002). The frequency of PD-1 expression in Tet⁺ cTfh was increased in both groups (median, >75%) compared to total cTfh (<30%), but the intensity of PD-1 expression per cell remained higher in the treated patient group (P = 0.02), pointing to the persistence of abnormal immune activation in treated patients. The function of cTfh, analyzed by the capacity to promote IgG secretion in cocultures with autologous memory B cells, did not show major differences between groups in terms of total IgG production but proved significantly more efficient in the controller group when measuring HIV-specific IgG production. The frequency of Tet⁺ cTfh correlated with HIV-specific IgG production (R = 0.71 for Gag-specific and R = 0.79 for Env-specific IgG, respectively). Taken together, our findings indicate that key cTfh-B cell interactions are preserved in controlled HIV infection, resulting in potent memory B cell responses that may play an underappreciated role in HIV control.

The rare patients who spontaneously control HIV replication in the absence of therapy provide a unique model to identify determinants of an effective anti-HIV immune response. HIV controllers show signs of particularly efficient antiviral T cell responses, while their humoral response was until recently considered to play only a minor role in viral control. However, emerging evidence suggests that HIV controllers maintain a significant but “silent” antiviral memory B cell population that can be reactivated upon antigenic stimulation. We report that cTfh help likely contributes to the persistence of controller memory B cell responses, as the frequency of HIV-specific cTfh correlated with the induction of HIV-specific antibodies in functional assays. These findings suggest that T follicular help may contribute to HIV control and highlight the need for inducing such help in HIV vaccine strategies that aim at eliciting persistent B cell responses.

Molecular and Physicochemical Factors Governing Solubility of the HIV gp41 Ectodomain

The HIV gp41 ectodomain (e-gp41) is an attractive target for the development of vaccines and drugs against HIV because of its crucial role in viral fusion to the host cell. However, because of the high insolubility of e-gp41, most biophysical and structural analyses have relied on the production of truncated versions removing the loop region of gp41 or the utilization of nonphysiological solubilizing conditions. The loop region of gp41 is also known as principal immunodominant domain (PID) because of its high immunogenicity, and it is essential for gp41-mediated HIV fusion. In this study we identify the aggregation-prone regions of the amino acid sequence of the PID and engineer a highly soluble mutant that preserves the trimeric structure of the wild-type e-gp41 under physiological pH. Furthermore, using a reverse mutagenesis approach, we analyze the role of mutated amino acids upon the physicochemical factors that govern solubility of e-gp41. On this basis, we propose a molecular model for e-gp41 self-association, which can guide the production of soluble e-gp41 mutants for future biophysical analyses and biotechnological applications.

Sublingual Priming with a HIV gp41-Based Subunit Vaccine Elicits Mucosal Antibodies and Persistent B Memory Responses in Non-Human Primates

Persistent B cell responses in mucosal tissues are crucial to control infection against sexually transmitted pathogens like human immunodeficiency virus 1 (HIV-1). The genital tract is a major site of infection by HIV. Sublingual (SL) immunization in mice was previously shown to generate HIV-specific B cell immunity that disseminates to the genital tract. We report here the immunogenicity in female cynomolgus macaques of a SL vaccine based on a modified gp41 polypeptide coupled to the cholera toxin B subunit designed to expose hidden epitopes and to improve mucosal retention. Combined SL/intramuscular (IM) immunization with such mucoadhesive gp41-based vaccine elicited mucosal HIV-specific IgG and IgA antibodies more efficiently than IM immunization alone. This strategy increased the number and duration of gp41-specific IgA secreting cells. Importantly, combined immunization improved the generation of functional antibodies 3 months after vaccination as detected in HIV-neutralizing assays. Therefore, SL immunization represents a promising vaccine strategy to block HIV-1 transmission.

Inhibition of HIV Entry by Targeting the Envelope Transmembrane Subunit gp41

BACKGROUND

The transmembrane subunit of the HIV envelope protein, gp41 is a vulnerable target to inhibit HIV entry. There is one fusion inhibitor T20 (brand name: Fuzeon, generic name: enfuvirtide) available by prescription. However, it has several drawbacks such as a high level of development of drug resistance, a short-half life in vivo, rapid renal clearance, low oral bioavailability, and it is only used as a salvage therapy. Therefore, investigators have been studying a variety of different modalities to attempt to overcome these limitations.

METHODS

Comprehensive literature searches were performed on HIV gp41, inhibition mechanisms, and inhibitors. The latest structural information was collected, and multiple inhibition strategies targeting gp41 were reviewed.

RESULTS

Many of the recent advances in inhibitors were peptide-based. Several creative modification strategies have also been performed to improve inhibitory efficacy of peptides and to overcome the drawbacks of T20 treatment. Small compounds have also been an area of intense research. There is a wide variety in development from those identified by virtual screens targeting specific regions of the protein to natural products. Finally, broadly neutralizing antibodies have also been important area of research. The inaccessible nature of the target regions for antibodies is a challenge, however, extensive efforts to develop better neutralizing antibodies are ongoing.

CONCLUSION

The fusogenic protein, gp41 has been extensively studied as a promising target to inhibit membrane fusion between the virus and target cells. At the same time, it is a challenging target because the vulnerable conformations of the protein are exposed only transiently. However, advances in biochemical, biophysical, structural, and immunological studies are coming together to move the field closer to an understanding of gp41 structure and function that will lead to the development of novel drugs and vaccines.

The membrane proximal external regions of gp41 from HIV-1 strains HXB2 and JRFL have different sensitivities to alanine mutation

The transmembrane subunit (gp41) of the HIV envelope protein complex (Env) mediates the viral fusion step of HIV entry. The membrane proximal external region (MPER), one of the functional domains of gp41, has been the focus of a great deal of research because it is a target for neutralizing antibodies. In this study, we examined 23 amino acid residues in the MPER (660-683) in both a CXCR4 coreceptor-utilizing strain (HXB2) and a CCR5-utilizing strain (JRFL) by alanine scanning mutagenesis. Despite the high degree of gp41 sequence conservation, the effects of alanine mutation in the MPER were different between the two strains. Most mutations in HXB2 had fusogenicity and protein expression levels not less than 50% of that of the wild type in the case of cell-cell fusion. However, ∼30% of the mutants in HXB2 showed a severe defect in fusogenicity in viral entry. Mutations in the MPER of strain JRFL had more dramatic effects than that in HXB2 in cell-cell fusion and viral entry. The fact that there are large differences in the effects of mutation between two strains suggests the potential for the interaction of the MPER with nonconserved sequences such as the fusion peptide and/or other NHR domains as well as potential long-range structural effects on the conformational changes that occur with the Env complex during membrane fusion.

Significant differences in cell-cell fusion and viral entry between strains revealed by scanning mutagenesis of the C-heptad repeat of HIV gp41

The transmembrane subunit, gp41, of the HIV envelope mediates the viral fusion step of entry into the host cell. The protein consists of an extracellular domain, a transmembrane domain, and a cytoplasmic tail. The extracellular domain contains a fusion peptide, an N-terminal heptad repeat, a loop region, a C-terminal heptad repeat (CHR), and a membrane-proximal external region. For this study, we examined each amino acid in the CHR (residues 623-659) by alanine scanning mutagenesis in two HIV strains: one CCR5-utilizing strain (JRFL) and one CXCR4-utilizing strain (HXB2). We studied the functional importance of each amino acid residue by measuring mutational effects in both cell-cell fusion and viral entry and assessing envelope expression and gp120-gp41 proteolytic processing. The transmembrane subunit of the HIV envelope, gp41, is very sensitive to subtle changes, like alanine substitution, which severely affect envelope function at multiple sites. Two important general findings are apparent when the entire data set from this study is taken into account. (1) Strain HXB2 is much more stable to mutagenesis than strain JRFL, and (2) viral entry is much more stable to mutagenesis than cell-cell fusion. These findings strengthen our notion that gp41 is a vulnerable target for therapeutic and prophylactic intervention. Further structural studies aimed at gaining a full understanding of the intermediate states that drive HIV membrane fusion are imperative.

Targeting HIV-1 gp41-induced fusion and pathogenesis for anti-viral therapy

HIV gp41 is a metastable protein whose native conformation is maintained in the form of a heterodimer with gp120. The non-covalently associated gp41/gp120 complex forms a trimer on the virus surface. As gp120 engages with HIV's receptor, CD4, and coreceptor, CXCR4 or CCR5, gp41 undergoes several conformational changes resulting in fusion between the viral and cellular membranes. Several lipophilic and amphiphilic domains have been shown to be critical in that process. While the obvious function of gp41 in viral entry is well-established its role in cellular membrane fusion and the link with pathogenesis are only now beginning to appear. Recent targeting of gp41 via fusion inhibitors has revealed an important role of this protein not only in viral entry but also in bystander apoptosis and HIV pathogenesis. Studies by our group and others have shown that the phenomenon of gp41-mediated hemifusion initiates apoptosis in bystander cells and correlates with virus pathogenesis. More interestingly, recent clinical evidence suggests that gp41 mutants arising after Enfuvirtide therapy are associated with CD4 cell increase and immunological benefits. This has in turn been correlated to a decrease in bystander apoptosis in our in vitro as well as in vivo assays. Although a great deal of work has been done to unravel HIV-1 gp41-mediated fusion mechanisms, the factors that regulate gp41-mediated fusion versus hemifusion and the mechanism by which hemifusion initiates bystander apoptosis are not fully understood. Further insight into these issues will open new avenues for drug development making gp41 a critical anti-HIV target both for neutralization and virus attenuation.

Permanent inhibition of viral entry by covalent entrapment of HIV gp41 on the virus surface

HIV entry occurs by concerted conformational changes in the envelope protein complex on the surface of the virus. This complex is made up of a trimer of heterodimers of two subunits: surface subunit, gp120, and transmembrane subunit, gp41. Conformational changes in the envelope complex allow gp41 to mediate membrane fusion leading to exposure of two gp41 regions: N-heptad repeat (NHR) and C-heptad repeat (CHR). Peptides from the NHR or the CHR have been found to inhibit HIV entry. Herein we show that we can covalently inhibit HIV viral entry by permanently trapping the gp41 intermediate on the virus surface using a covalently reactive group on inhibitory peptides. This is evidence showing that vulnerable conformational intermediates exist transiently during HIV viral entry, and the details presented herein will facilitate development of envelope as a target for therapeutics and potential chemopreventive agents that could disable the virus before contact with the host cell.

An efficient production and characterization of HIV-1 gp41 ectodomain with fusion peptide in Escherichia coli system

We demonstrated a high level expression and purification of recombinant human immunodeficiency virus type 1 gp41 ectodomain (gp41e-FP) using glass bead approach with a final yield of 12±2mg/L bacterial culture. The proper folding of gp41e-FP encompassing the fusion peptide (FP) was ascertained by circular dichroism (CD) measurement and recognition by NC-1 antibody. The latter assay revealed stabilization of the gp41 coiled coil structure in the presence of liposome dispersion. The differential affinity of gp41e-FP and gp41e (devoid of FP) by NC-1 suggested an aggregated state for gp41e-FP and/or possible proximity of the fusion peptide domain to the coiled coil structure of gp41 ectodomain. Perfluorooctanoate (PFO)-PAGE electrophoresis experiment revealed the trimeric propensity of the recombinant gp41e-FP. In comparison to gp41e, the lipid mixing activity of gp41e-FP was two-fold higher suggesting a role of FP in promoting membrane fusion. The present approach to efficiently and quantitatively preparing the functional full-length recombinant gp41 ectodomain protein can be employed for structural and biomedical investigations and the extraction of other inclusion body-embedded recombinant proteins.

Comparative analysis of membrane-associated fusion peptide secondary structure and lipid mixing function of HIV gp41 constructs that model the early pre-hairpin intermediate and final hairpin conformations

Fusion between viral and host cell membranes is the initial step of human immunodeficiency virus infection and is mediated by the gp41 protein, which is embedded in the viral membrane. The approximately 20-residue N-terminal fusion peptide (FP) region of gp41 binds to the host cell membrane and plays a critical role in fusion catalysis. Key gp41 fusion conformations include an early pre-hairpin intermediate (PHI) characterized by extended coiled-coil structure in the region C-terminal of the FP and a final hairpin state with compact six-helix bundle structure. The large "N70" (gp41 1-70) and "FP-Hairpin" constructs of the present study contained the FP and respectively modeled the PHI and hairpin conformations. Comparison was also made to the shorter "FP34" (gp41 1-34) fragment. Studies were done in membranes with physiologically relevant cholesterol content and in membranes without cholesterol. In either membrane type, there were large differences in fusion function among the constructs with little fusion induced by FP-Hairpin, moderate fusion for FP34, and very rapid fusion for N70. Overall, our findings support acceleration of gp41-induced membrane fusion by early PHI conformation and fusion arrest after folding to the final six-helix bundle structure. FP secondary structure at Leu7 of the membrane-associated constructs was probed by solid-state nuclear magnetic resonance and showed populations of molecules with either beta-sheet or helical structure with greater beta-sheet population observed for FP34 than for N70 or FP-Hairpin. The large differences in fusion function among the constructs were not obviously correlated with FP secondary structure. Observation of cholesterol-dependent FP structure for fusogenic FP34 and N70 and cholesterol-independent structure for non-fusogenic FP-Hairpin was consistent with membrane insertion of the FP for FP34 and N70 and with lack of insertion for FP-Hairpin. Membrane insertion of the FP may therefore be associated with the early PHI conformation and FP withdrawal with the final hairpin conformation.

Conformational stability and membrane interaction of the full-length ectodomain of HIV-1 gp41: implication for mode of action

Membrane fusion between the human immunodeficiency virus (HIV) and the target cell plasma membrane is correlated with conformational changes in the HIV gp41 glycoprotein, which include an early exposed conformation (prehairpin) and a late low energy six helix bundle (SHB) conformation also termed hairpin. Peptides resembling regions from the exposed prehairpin have been previously studied for their interaction with membranes. Here we report on the expression, purification, SHB stability, and membrane interaction of the full-length ectodomain of the HIV gp41 and its two deletion mutants, all in their SHB-folded state. The interaction of the proteins with zwitterionic and negatively charged membranes was examined by using various biophysical methods including circular dichroism spectroscopy, differential scanning calorimetry, lipid mixing of large unilamellar vesicles, and atomic force microscopy (AFM). All experiments were done in an acidic environment in which the protein remains in its soluble trimeric state. The data reveal that all three proteins fold into a stable coiled-coil core in aqueous solution and retain a stable helical fold with reduced coiled-coil characteristics in a zwitterionic and negatively charged membrane mimetic environment. Furthermore, in contrast with the extended exposed N-terminal domain, the folded gp41 ectodomain does not induce lipid mixing of zwitterionic membranes. However, it disrupts and induces lipid mixing of negatively charged phospholipid membranes (approximately 100-fold more effective than fusion peptide alone), which are known to be expressed more in HIV-1-infected T cells or macrophages. The results support the emerging model in which one of the roles of gp41 folding into the SHB conformation is to slow down membrane disruption effects induced by early exposed gp41. However, it can further affect membrane morphology once exposed to negatively charged membranes during late stages.

Stability of a receptor-binding active human immunodeficiency virus type 1 recombinant gp140 trimer conferred by intermonomer disulfide bonding of the V3 loop: differential effects of protein disulfide isomerase on CD4 and coreceptor binding

Stable trimeric forms of human immunodeficiency virus recombinant gp140 (rgp140) are important templates for determining the structure of the glycoprotein to assist in our understanding of HIV infection and host immune response. Such information will aid the design of therapeutic drugs and vaccines. Here, we report the production of a highly stable and trimeric rgp140 derived from a HIV type 1 (HIV-1) subtype D isolate that may be suitable for structural studies. The rgp140 is functional in terms of binding to CD4 and three human monoclonal antibodies (17b, b12, and 2G12) that have broad neutralizing activities against a range of HIV-1 isolates from different subtypes. Treatment of rgp140 with protein disulfide isomerase (PDI) severely restricted 17b binding capabilities. The stable nature of the rgp140 was due to the lack of processing at the gp120/41 boundary and the presence of an intermonomer disulfide bond formed by the cysteines of the V3 loop. Further characterization showed the intermonomer disulfide bond to be a target for PDI processing. The relevance of these findings to the roles of the V3 domain and the timing of PDI action during the HIV infection process are discussed.

Secondary structure analysis of HIV-1-gp41 in solution and adsorbed to aluminum hydroxide by Fourier transform infrared spectroscopy

The formulation of human vaccines often includes adjuvants such as aluminum hydroxide that are added to enhance the immune responses to vaccine antigens. However, these adjuvants may also affect the conformation of antigenic proteins. Such structural modifications could lead to changes in antigenicity such that suboptimal protective immune responses could be generated relative to those induced by the vaccine antigens alone. Here, we used attenuated total reflectance infrared spectroscopy (ATR-FTIR) to compare the secondary structures of recombinant HIV-1-gp41 (gp41) in solution or adsorbed to aluminum hydroxide. The gp41 secondary structure content was 72% alpha-helices and 28% beta-sheets in 5 mM formate buffer p(2)H 2.5, while it was 66% beta-sheets and 34% random coil in acetonitril/(2)H(2)O (95/5:v/v). A fully reversible conformational change of gp41 in acetonitril/(2)H(2)O (95/5:v/v) was observed upon addition of either 35 mM formate p(2)H 2.5 or 0.1% (w/v) detergent (Tween 20, Hecameg, Brij 35 or beta-d-octyl-glucopyranoside). When gp41 was adsorbed to aluminum hydroxide in the presence of 0.1% (w/v) detergent, in either formate or in acetonitril/(2)H(2)O (95/5:v/v) its secondary structure remained stable and was identical to that of gp41 in 5 mM formate buffer p(2)H 2.5. The method described here could be applied for the characterization of gp41 conformers for use in immunological screening of antigens, and more generally to all antigenic proteins adsorbed to aluminum hydroxide.

Preliminary characterization of a thermostable DNA polymerase I from a mesophilic Bacillus sphaericus strain C3-41

A thermostable DNA polymerase I from a mesophilic Bacillus sphaericus strain C3-41 was characterized in this study. The polI was cloned, sequenced and over-expressed in Escherichia coli. The expressed 110 kDa fusion protein of PolI was stable at 70 degrees C for 1 h. Compared with DNA polymerase I of E. coli (TaKaRa), the relative polymerase activity of this PolI was 3.33 +/- 0.1 RFU microl(-1) at 37 degrees C using fluorescent quantitative analysis. It showed higher polymerase activity than E. coli PolI at higher temperature, with a relative activity of 3.75 +/- 0.1 RFU microl(-1) at 70 degrees C. The polI sequence analysis and the protein structure prediction indicated that this protein had a high similarly to other PolI from thermophilic micro-organisms. This information is of importance for future study for evolution of the house-keeping gene polI in entomopathogenic bacterium B. sphaericus.

Unexpected amino acid composition of modern Reptilia and its implications in molecular mechanisms of dinosaur extinction

Dinosaur extinction is a great challenge to evolutionary biology. Although accumulating evidence suggests that an abrupt change of environment, such as a long period of low temperature induced by asteroid hit or other disasters, may be responsible for dinosaur extinction, little is known about the underlying molecular mechanisms. By analyzing the amino acid compositions of 13 biological classes, we found that the charged amino acid content of modern Reptilia, the sibling of dinosaur, is strikingly different from those of other classes, which inspires us to propose a possible molecular mechanism for dinosaur extinction.