Mutations of Gln64 in the HIV-1 gp41 N-terminal heptad repeat render viruses resistant to peptide HIV fusion inhibitors targeting the gp41 pocket

Frequency of Fusion Inhibitor Resistance Mutations Among Therapy-Naïve HIV Patients

Glycoprotein 41 (gp41) of the human immunodeficiency virus type 1 (HIV-1) protein plays a critical role in membrane fusion. Gp41 binds to proteins in the plasma membrane of CD4+ T cells, particularly the T-cell antigen receptor (TCR). These findings indicate that gp41 is involved in the assembly of HIV-1 at the plasma membrane of T cells and affects the stimulation of the TCR. To control HIV-1, new inhibitors were introduced to target the gp41 protein. However, mutations in this region might reduce their efficacy. The Gp41 region was amplified from the sera of 30 patients using nested polymerase chain reaction. The sequences were analyzed by bioinformatics tools to identify mutations and gp41 structural features. Subtyping and the interaction between fusion inhibitors and gp41 proteins were also examined. As the first report from Iran, docking analysis between fusion inhibitors and Iranian gp41 proteins showed that mutations in gp41 could not reduce the efficacy of the fusion inhibitors. Most of the patients were infected with CRF35-AD. Several post-modification positions, including glycosylation and phosphorylation sites, were identified in the gp41 protein. Our findings revealed no known multinational drug resistance to gp41 inhibitors; thus, fusion inhibitors can effectively inhibit HIV in Iranian patients. In addition, the present study introduced a new gp41 region (36-44 aa), which considerably influences the interactions between gp41 inhibitors and the gp41 protein. This region may play a pivotal role in suppressing gp41 inhibitors in CFR35-AD. Furthermore, gp41 can be considered a good target for subtyping analysis via the phylogenetic method.

ADS-J21 is a novel HIV-1 entry inhibitor targeting gp41

HIV-1 envelope glycoprotein gp41 mediates fusion between HIV-1 and host cell membranes, making inhibitors of gp41 attractive anti-HIV drugs. We previously reported an efficient HIV-1 fusion inhibitor, ADS-J1, with a Y-shaped structure. Here, we discovered a new compound, ADS-J21, with a Y-shaped structure similar to that of ADS-J1 but with a lower molecular weight. Moreover, ADS-J21 exhibited effective anti-HIV-1 activity against divergent HIV-1 strains in vitro, including several HIV-1 laboratory-adapted strains and primary isolates with different subtypes (clades A to F) and tropisms (X4 or R5). Mechanistic studies have demonstrated that ADS-J21 blocks the formation of the gp41 six-helix bundle (6-HB) by targeting conserved amino acids Lys35 and Trp32. These findings suggest that ADS-J21 can be used as a new lead compound for further optimization in the development of a small-molecule fusion inhibitor.

Adding an Artificial Tail-Anchor to a Peptide-Based HIV-1 Fusion Inhibitor for Improvement of Its Potency and Resistance Profile

Peptides derived from the C-terminal heptad repeat (CHR) of human immunodeficiency virus type 1 (HIV-1) envelope protein transmembrane subunit gp41, such as T20 (enfuvirtide), can bind to the N-terminal heptad repeat (NHR) of gp41 and block six-helix bundle (6-HB) formation, thus inhibiting HIV-1 fusion with the target cell. However, clinical application of T20 is limited because of its low potency and genetic barrier to resistance. HP23, the shortest CHR peptide, exhibits better anti-HIV-1 activity than T20, but the HIV-1 strains with E49K mutations in gp41 will become resistant to it. Here, we modified HP23 by extending its C-terminal sequence using six amino acid residues (E6) and adding IDL (Ile-Asp-Leu) to the C-terminus of E6, which is expected to bind to the shallow pocket in the gp41 NHR N-terminal region. The newly designed peptide, designated HP23-E6-IDL, was about 2- to 16-fold more potent than HP23 against a broad spectrum of HIV-1 strains and more than 12-fold more effective against HIV-1 mutants resistant to HP23. These findings suggest that addition of an anchor-tail to the C-terminus of a CHR peptide will allow binding with the pocket in the gp41 NHR that may increase the peptide's antiviral efficacy and its genetic barrier to resistance.

The Membrane-Proximal Region of C-C Chemokine Receptor Type 5 Participates in the Infection of HIV-1

The initial infection and transmission of HIV-1 requires C-C chemokine receptor type 5 (CCR5). Here, we report that the membrane-proximal region (MPR, aa 22-38) of CCR5 participates in the infection of HIV-1. First, MPR-specific antibodies elicited in mice dose-dependently inhibited the infection of CCR5-tropic HIV-1. Second, substituting MPR with the same region from other co-receptors significantly impaired HIV-1 infection, while the key residues identified by alanine scanning mutagenesis formed an exposed leucine zipper-like structure. Moreover, a peptide derived from MPR could block the infection of a number of HIV-1 strains only before the formation of gp41 six-helix bundle, coincide with the early interaction between CCR5 and the gp120 protein during HIV-1 infection. These promising results ensured the potential of this previously uncharacterized domain as a starting point for the development of antiviral drugs, blocking antibodies, and HIV vaccines.

A novel HIV-1 gp41 tripartite model for rational design of HIV-1 fusion inhibitors with improved antiviral activity

OBJECTIVES

During HIV-1 fusion process, the N-terminal heptad repeat (NHR) of the HIV-1 glycoprotein 41 (gp41) interacts with the C-terminal heptad repeat (CHR) to form the fusion active six-helix bundle, thus being an effective target for the design of CHR peptide-based HIV-1 fusion inhibitors. To overcome the limitations of the simplified helix wheel model of six-helix bundle, we herein developed a novel HIV-1 gp41 NHR-CHR-NHR tripartite model for the rational design of HIV-1 fusion inhibitors with improved antiviral activities.

DESIGN

Based on the crystal structure of six-helix bundle, we evaluated the NHR-binding properties of each residue in CHR. In this new tripartite model, CHR residues were divided into three groups: major binding, nonbinding, and assistant binding sites.

METHODS

Eight CHR peptides were designed and synthesized to confirm the validity of the tripartite model. Their affinities to NHR and inhibitory activities were analyzed.

RESULTS

In this tripartite model, replacements in assistant binding sites either increased or decreased the inhibition of HIV-1 infection. We identified three peptides with mutations of the residues in CHR at the assistant binding sites in our tripartite model but nonbinding sites in the helical wheel model. These mutant peptides had anti-HIV-1 activity up to 26-fold more potent than that of C34, a CHR peptide designed on the basis of the helix wheel model.

CONCLUSION

These data verified the superiority and validity of our new tripartite model for the rational design of HIV-1 fusion inhibitors. This approach can be adapted for designing viral fusion inhibitors against other enveloped viruses with class I membrane fusion protein.

Rational improvement of gp41-targeting HIV-1 fusion inhibitors: an innovatively designed Ile-Asp-Leu tail with alternative conformations

Peptides derived from the C-terminal heptad repeat (CHR) of HIV gp41 have been developed as effective fusion inhibitors against HIV-1, but facing the challenges of enhancing potency and stability. Here, we report a rationally designed novel HIV-1 fusion inhibitor derived from CHR-derived peptide (Trp628~Gln653, named CP), but with an innovative Ile-Asp-Leu tail (IDL) that dramatically increased the inhibitory activity by up to 100 folds. We also determined the crystal structures of artificial fusion peptides N36- and N43-L6-CP-IDL. Although the overall structures of both fusion peptides share the canonical six-helix bundle (6-HB) configuration, their IDL tails adopt two different conformations: a one-turn helix with the N36, and a hook-like structure with the longer N43. Structural comparison showed that the hook-like IDL tail possesses a larger interaction interface with NHR than the helical one. Further molecular dynamics simulations of the two 6-HBs and isolated CP-IDL peptides suggested that hook-like form of IDL tail can be stabilized by its binding to NHR trimer. Therefore, CP-IDL has potential for further development as a new HIV fusion inhibitor, and this strategy could be widely used in developing artificial fusion inhibitors against HIV and other enveloped viruses.

An immunogen containing four tandem 10E8 epitope repeats with exposed key residues induces antibodies that neutralize HIV-1 and activates an ADCC reporter gene

After three decades of intensive research efforts, an effective vaccine against HIV-1 remains to be developed. Several broadly neutralizing antibodies to HIV-1, such as 10E8, recognize the membrane proximal external region (MPER) of the HIV-1 gp41 protein. Thus, the MPER is considered to be a very important target for vaccine design. However, the MPER segment has very weak immunogenicity and tends to insert its epitope residues into the cell membrane, thereby avoiding antibody binding. To address this complication in vaccine development, we herein designed a peptide, designated 10E8-4P, containing four copies of the 10E8 epitope as an immunogen. As predicted by structural simulation, 10E8-4P exhibits a well-arranged tandem helical conformation, with the key residues in the 10E8 epitope oriented at different angles, thus suggesting that some of these key residues may be exposed outside of the lipid membrane. Compared with a peptide containing a single 10E8 epitope (10E8-1P), 10E8-4P not only exhibited better antigenicity but also elicited neutralizing antibody response against HIV-1 pseudoviruses, whereas 10E8-1P could not induce detectable neutralizing antibody response. Importantly, antibodies elicited by 10E8-4P also possessed a strong ability to activate an antibody-dependent cell-mediated cytotoxicity (ADCC) reporter gene, thus suggesting that they may have ADCC activity. Therefore, this strategy shows promise for further optimization and application in future HIV-1 vaccine design.

Enfuvirtide-PEG conjugate: A potent HIV fusion inhibitor with improved pharmacokinetic properties

Enfuvirtide (ENF) is a clinically used peptide drug for the treatment of HIV infections, but its poor pharmacokinetic profile (T1/2 = 1.5 h in rats) and low aqueous solubility make the therapy expensive and inconvenience. In this study, we present a simple and practical strategy to address these problems by conjugating ENF with polyethylene glycol (PEG). Site-specific attachment of a 2 kDa PEG at the N-terminus of ENF resulted in an ENF-PEG (EP) conjugate with high solubility (≥3 mg/mL) and long half-life in rats (T1/2 = 16.1 h). This conjugate showed similar antiviral activity to ENF against various primary HIV-1 isolates (EC50 = 6-91 nM). Mechanistic studies suggested the sources of the antiviral potency. The conjugate bound to a functional domain of the HIV gp41 protein in a helical conformation with high affinity (Kd = 307 nM), thereby inhibiting the gp41-mediated fusion of viral and host-cell membranes. As PEG conjugation has advanced many bioactive proteins and peptides into clinical applications, the EP conjugate described here represents a potential new treatment for HIV infections that may address the unmet medical needs associated with the current ENF therapy.

Development of Small-molecule HIV Entry Inhibitors Specifically Targeting gp120 or gp41

Human immunodeficiency virus type 1 (HIV-1) envelope (Env) glycoprotein surface subunit gp120 and transmembrane subunit gp41 play important roles in HIV-1 entry, thus serving as key targets for the development of HIV-1 entry inhibitors. T20 peptide (enfuvirtide) is the first U.S. FDA-approved HIV entry inhibitor; however, its clinical application is limited by the lack of oral availability. Here, we have described the structure and function of the HIV-1 gp120 and gp41 subunits and reviewed advancements in the development of small-molecule HIV entry inhibitors specifically targeting these two Env glycoproteins. We then compared the advantages and disadvantages of different categories of HIV entry inhibitor candidates and further predicted the future trend of HIV entry inhibitor development.

Preclinical Assessment of Mutant Human TRIM5α as an Anti-HIV-1 Transgene

Current HIV-1 gene therapy approaches aim at stopping the viral life cycle at its earliest steps, such as entry or immediate postentry events. Among the most widely adopted strategies are CCR5 downregulation/knockout and the use of broadly neutralizing antibodies. However, the long-term efficacy and side effects are still unclear. TRIM5α is an interferon-stimulated restriction factor that can intercept incoming retroviruses within one hour of cytosolic entry and potently inhibit the infectivity of restriction-sensitive viruses. The human TRIM5α (TRIM5αhu) generally does not efficiently target HIV-1, but point mutations in its capsid-binding domain can confer anti-HIV-1 activity. Although the mechanisms by which TRIM5αhu mutants inhibit HIV-1 are relatively well understood, their characterization as potential transgenes for gene therapy is lacking. Additionally, previous reports of general immune activation by overexpression of TRIM5α have hindered its broad adoption as a potential transgene. Here we demonstrate the ability of the R332G-R335G TRIM5αhu mutant to efficiently restrict highly divergent HIV-1 strains, including Group O, as well as clinical isolates bearing cytotoxic T lymphocyte escape mutations. R332G-R335G TRIM5αhu efficiently protected human lymphocytes against HIV-1 infection, even when expressed at relatively low levels following lentiviral transduction. Most importantly, under these conditions Rhesus macaque TRIM5α (TRIM5αRh) and TRIM5αhu (wild-type or mutated) had no major effects on the NF-κB pathway. Transgenic TRIM5α did not modulate the kinetics of IκBα, JunB, and TNFAIP3 expression following TNF-α treatment. Finally, we show that human lymphocytes expressing R332G-R335G TRIM5αhu have clear survival advantages over unmodified parental cells in the presence of pathogenic, replication-competent HIV-1. These results support the relevance of R332G-R335G and other mutants of TRIM5αhu as candidate effectors for HIV-1 gene therapy.

Integrated analysis of residue coevolution and protein structures capture key protein sectors in HIV-1 proteins

HIV type 1 (HIV-1) is characterized by its rapid genetic evolution, leading to challenges in anti-HIV therapy. However, the sequence variations in HIV-1 proteins are not randomly distributed due to a combination of functional constraints and genetic drift. In this study, we examined patterns of sequence variability for evidence of linked sequence changes (termed as coevolution or covariation) in 15 HIV-1 proteins. It shows that the percentage of charged residues in the coevolving residues is significantly higher than that in all the HIV-1 proteins. Most of the coevolving residues are spatially proximal in the protein structures and tend to form relatively compact and independent units in the tertiary structures, termed as "protein sectors". These protein sectors are closely associated with anti-HIV drug resistance, T cell epitopes, and antibody binding sites. Finally, we explored candidate peptide inhibitors based on the protein sectors. Our results can establish an association between the coevolving residues and molecular functions of HIV-1 proteins, and then provide us with valuable knowledge of pathology of HIV-1 and therapeutics development.

Receptor binding domain based HIV vaccines

This paper analyzes the main trend of the development of acquired immunodeficiency syndrome (AIDS) vaccines in recent years. Designing an HIV-1 vaccine that provides robust protection from HIV-1 infection remains a challenge despite many years of effort. Therefore, we describe the receptor binding domain of gp120 as a target for developing AIDS vaccines. And we recommend some measures that could induce efficiently and produce cross-reactive neutralizing antibodies with high binding affinity. Those measures may offer a new way of the research and development of the potent and broad AIDS vaccines.

Middle East respiratory syndrome coronavirus (MERS-CoV) entry inhibitors targeting spike protein

The recent outbreak of Middle East respiratory syndrome (MERS) coronavirus (MERS-CoV) infection has led to more than 800 laboratory-confirmed MERS cases with a high case fatality rate (∼35%), posing a serious threat to global public health and calling for the development of effective and safe therapeutic and prophylactic strategies to treat and prevent MERS-CoV infection. Here we discuss the most recent studies on the structure of the MERS-CoV spike protein and its role in virus binding and entry, and the development of MERS-CoV entry/fusion inhibitors targeting the S1 subunit, particularly the receptor-binding domain (RBD), and the S2 subunit, especially the HR1 region, of the MERS-CoV spike protein. We then look ahead to future applications of these viral entry/fusion inhibitors, either alone or in combination with specific and nonspecific MERS-CoV replication inhibitors, for the treatment and prevention of MERS-CoV infection.

Resistance to N-peptide fusion inhibitors correlates with thermodynamic stability of the gp41 six-helix bundle but not HIV entry kinetics

BACKGROUND

The HIV-1 envelope glycoprotein (Env) undergoes conformational changes that mediate fusion between virus and host cell membranes. These changes involve transient exposure of two heptad-repeat domains (HR1 and HR2) in the gp41 subunit and their subsequent self-assembly into a six-helix bundle (6HB) that drives fusion. Env residues and features that influence conformational changes and the rate of virus entry, however, are poorly understood. Peptides corresponding to HR1 and HR2 (N and C peptides, respectively) interrupt formation of the 6HB by binding to the heptad repeats of a fusion-intermediate conformation of Env, making the peptides valuable probes for studying Env conformational changes.

RESULTS

Using a panel of Envs that are resistant to N-peptide fusion inhibitors, we investigated relationships between virus entry kinetics, 6HB stability, and resistance to peptide fusion inhibitors to elucidate how HR1 and HR2 mutations affect Env conformational changes and virus entry. We found that gp41 resistance mutations increased 6HB stability without increasing entry kinetics. Similarly, we show that increased 6HB thermodynamic stability does not correlate with increased entry kinetics. Thus, N-peptide fusion inhibitors do not necessarily select for Envs with faster entry kinetics, nor does faster entry kinetics predict decreased potency of peptide fusion inhibitors.

CONCLUSIONS

These findings provide new insights into the relationship between 6HB stability and viral entry kinetics and mechanisms of resistance to inhibitors targeting fusion-intermediate conformations of Env. These studies further highlight how residues in HR1 and HR2 can influence virus entry by altering stability of the 6HB and possibly other conformations of Env that affect rate-limiting steps in HIV entry.

Functional characterization of two scFv-Fc antibodies from an HIV controller selected on soluble HIV-1 Env complexes: a neutralizing V3- and a trimer-specific gp41 antibody

HIV neutralizing antibodies (nAbs) represent an important tool in view of prophylactic and therapeutic applications for HIV-1 infection. Patients chronically infected by HIV-1 represent a valuable source for nAbs. HIV controllers, including long-term non-progressors (LTNP) and elite controllers (EC), represent an interesting subgroup in this regard, as here nAbs can develop over time in a rather healthy immune system and in the absence of any therapeutic selection pressure. In this study, we characterized two particular antibodies that were selected as scFv antibody fragments from a phage immune library generated from an LTNP with HIV neutralizing antibodies in his plasma. The phage library was screened on recombinant soluble gp140 envelope (Env) proteins. Sequencing the selected peptide inserts revealed two major classes of antibody sequences. Binding analysis of the corresponding scFv-Fc derivatives to various trimeric and monomeric Env constructs as well as to peptide arrays showed that one class, represented by monoclonal antibody (mAb) A2, specifically recognizes an epitope localized in the pocket binding domain of the C heptad repeat (CHR) in the ectodomain of gp41, but only in the trimeric context. Thus, this antibody represents an interesting tool for trimer identification. MAb A7, representing the second class, binds to structural elements of the third variable loop V3 and neutralizes tier 1 and tier 2 HIV-1 isolates of different subtypes with matching critical amino acids in the linear epitope sequence. In conclusion, HIV controllers are a valuable source for the selection of functionally interesting antibodies that can be selected on soluble gp140 proteins with properties from the native envelope spike.

ADS-J1 inhibits HIV-1 infection and membrane fusion by targeting the highly conserved pocket in the gp41 NHR-trimer

We previously identified a potent small-molecule human immunodeficiency virus type 1 (HIV-1) fusion inhibitor, termed ADS-J1, and hypothesized that it mainly targeted the hydrophobic pocket in the gp41 N-terminal heptad repeat (NHR) trimer. However, this hypothesis has been challenged by the fact that ADS-J1 cannot induce drug-resistance mutation in the gp41 pocket region. Therefore, we show herein that HIV-1 mutants resistant to T2635, a peptide derived from the gp41 C-terminal heptad repeat (CHR) region with pocket-binding domain (PBD), were also resistant to ADS-J1. We also show that pseudoviruses with mutations at positions 64 and 67 in the gp41 pocket region were highly resistant to ADS-J1 and C34, another CHR-peptide with PBD, but relatively sensitive to T20, a CHR-peptide without PBD. ADS-J1 could effectively bind to N36Fd, a mimic of the gp41 NHR-trimer with pocket exposed, and block binding of C34 to N36Fd trimer to form six-helix bundle (6-HB). However, ADS-J1 was less effective in binding to N36Fd trimer with mutations in the gp41 pocket region, such as N36(Q64A)Fd, N36(Q64L)Fd, N36(A67G)Fd, N36(A67S)Fd, and N36(Q66R)Fd, as well as less effective in blocking 6-HB formation between C34 and these mutant N36Fd trimers. These results confirm that ADS-J1 mainly targets the pocket region in the HIV-1 gp41 NHR trimer and suggest that it could be used as a lead for developing small-molecule HIV fusion inhibitors and as a molecule probe for studying the mechanisms of gp41-mediated membrane fusion.

HIV-1 resistance mechanism to an electrostatically constrained peptide fusion inhibitor that is active against T-20-resistant strains

T-20EK is a novel fusion inhibitor designed to have enhanced α-helicity over T-20 (enfuvirtide) through engineered electrostatic interactions between glutamic acid (E) and lysine (K) substitutions. T-20EK efficiently suppresses wild-type and T-20-resistant variants. Here, we selected T-20EK-resistant variants. A combination of L33S and N43K substitutions in gp41 were required for high resistance to T-20EK. While these substitutions also caused resistance to T-20, they did not cause cross-resistance to other known fusion inhibitors.

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.

Escape from human immunodeficiency virus type 1 (HIV-1) entry inhibitors

The human immunodeficiency virus (HIV) enters cells through a series of molecular interactions between the HIV envelope protein and cellular receptors, thus providing many opportunities to block infection. Entry inhibitors are currently being used in the clinic, and many more are under development. Unfortunately, as is the case for other classes of antiretroviral drugs that target later steps in the viral life cycle, HIV can become resistant to entry inhibitors. In contrast to inhibitors that block viral enzymes in intracellular compartments, entry inhibitors interfere with the function of the highly variable envelope glycoprotein as it continuously adapts to changing immune pressure and available target cells in the extracellular environment. Consequently, pathways and mechanisms of resistance for entry inhibitors are varied and often involve mutations across the envelope gene. This review provides a broad overview of entry inhibitor resistance mechanisms that inform our understanding of HIV entry and the design of new inhibitors and vaccines.

Approaches for identification of HIV-1 entry inhibitors targeting gp41 pocket

The hydrophobic pocket in the HIV-1 gp41 N-terminal heptad repeat (NHR) domain plays an important role in viral fusion and entry into the host cell, and serves as an attractive target for development of HIV-1 fusion/entry inhibitors. The peptide anti-HIV drug targeting gp41 NHR, T-20 (generic name: enfuvirtide; brand name: Fuzeon), was approved by the U.S. FDA in 2003 as the first HIV fusion/entry inhibitor for treatment of HIV/AIDS patients who fail to respond to the current antiretroviral drugs. However, because T20 lacks the pocket-binding domain (PBD), it exhibits low anti-HIV-1 activity and short half-life. Therefore, several next-generation HIV fusion inhibitory peptides with PBD have been developed. They possess longer half-life and more potent antiviral activity against a broad spectrum of HIV-1 strains, including the T-20-resistant variants. Nonetheless, the clinical application of these peptides is still limited by the lack of oral availability and the high cost of production. Thus, development of small molecule compounds targeting the gp41 pocket with oral availability has been promoted. This review describes the main approaches for identification of HIV fusion/entry inhibitors targeting the gp41 pocket and summarizes the latest progress in developing these inhibitors as a new class of anti-HIV drugs.

A bivalent recombinant protein inactivates HIV-1 by targeting the gp41 prehairpin fusion intermediate induced by CD4 D1D2 domains

Background

Most currently approved anti-HIV drugs (e.g., reverse transcriptase inhibitors, protease inhibitors and fusion/entry inhibitors) must act inside or on surface of the target cell to inhibit HIV infection, but none can directly inactivate virions away from cells. Although soluble CD4 (sCD4) can inactivate laboratory-adapted HIV-1 strains, it fails to reduce the viral loads in clinical trials because of its low potency against primary isolates and tendency to enhance HIV-1 infection at low concentration. Thus, it is essential to design a better HIV inactivator with improved potency for developing new anti-HIV therapeutics that can actively attack the virus in the circulation before it attaches to and enter into the target cell.

Results

We engineered a bivalent HIV-1 inactivator, designated 2DLT, by linking the D1D2 domain of CD4 to T1144, the next generation HIV fusion inhibitor, with a 35-mer linker. The D1D2 domain in this soluble 2DLT protein could bind to the CD4-binding site and induce the formation of the gp41 prehairpin fusion-intermediate (PFI), but showed no sCD4-mediated enhancement of HIV-1 infection. The T1144 domain in 2DLT then bound to the exposed PFI, resulting in rapid inactivation of HIV-1 virions in the absence of the target cell. Beside, 2DLT could also inhibit fusion of the virus with the target cell if the virion escapes the first attack of 2DLT.

Conclusion

This bivalent molecule can serve as a dual barrier against HIV infection by first inactivating HIV-1 virions away from cells and then blocking HIV-1 entry on the target cell surface, indicating its potential for development as a new class of anti-HIV drug.

HIV-1 variants with a single-point mutation in the gp41 pocket region exhibiting different susceptibility to HIV fusion inhibitors with pocket- or membrane-binding domain

Enfuvirtide (T20), the first FDA-approved peptide HIV fusion/entry inhibitor derived from the HIV-1 gp41 C-terminal heptad-repeat (CHR) domain, is believed to share a target with C34, another well-characterized CHR-peptide, by interacting with the gp41 N-terminal heptad-repeat (NHR) to form six-helix bundle core. However, our previous studies showed that T20 mainly interacts with the N-terminal region of the NHR (N-NHR) and lipid membranes, while C34 mainly binds to the NHR C-terminal pocket region. But so far, no one has shown that C34 can induce drug-resistance mutation in the gp41 pocket region. In this study, we constructed pseudoviruses in which the Ala at the position of 67 in the gp41 pocket region was substituted with Asp, Gly or Ser, respectively, and found that these mutations rendered the viruses highly resistant to C34, but sensitive to T20. The NHR-peptide N36 with mutations of A67 exhibited reduced anti-HIV-1 activity and decreased α-helicity. The stability of six-helix bundle formed by C34 and N36 with A67 mutations was significantly lower than that formed by C34 and N36 with wild-type sequence. The combination of C34 and T20 resulted in potent synergistic anti-HIV-1 effect against the viruses with mutations in either N- or C-terminal region in NHR. These results suggest that C34 with a pocket-binding domain and T20 containing the N-NHR- and membrane-binding domains inhibit HIV-1 fusion by interacting with different target sites and the combinatorial use of C34 and T20 is expected to be effective against HIV-1 variants resistant to HIV fusion inhibitors.

Structural basis of potent and broad HIV-1 fusion inhibitor CP32M

CP32M is a newly designed peptide fusion inhibitor possessing potent anti-HIV activity, especially against T20-resistant HIV-1 strains. In this study, we show that CP32M can efficiently inhibit a large panel of diverse HIV-1 variants, including subtype B', CRF07_BC, and CRF01_AE recombinants and naturally occurring or induced T20-resistant viruses. To elucidate its mechanism of action, we determined the crystal structure of CP32M complexed with its target sequence. Differing from its parental peptide, CP621-652, the (621)VEWNEMT(627) motif of CP32M folds into two α-helix turns at the N terminus of the pocket-binding domain, forming a novel layer in the six-helix bundle structure. Prominently, the residue Asn-624 of the (621)VEWNEMT(627) motif is engaged in the polar interaction with a hydrophilic ridge that borders the hydrophobic pocket on the N-terminal coiled coil. The original inhibitor design of CP32M provides several intra- and salt bridge/hydrogen bond interactions favoring the stability of the helical conformation of CP32M and its interactions with N-terminal heptad repeat (NHR) targets. We identified a novel salt bridge between Arg-557 on the NHR and Glu-648 of CP32M that is critical for the binding of CP32M and resistance against the inhibitor. Therefore, our data present important information for developing novel HIV-1 fusion inhibitors for clinical use.