Faculty Bibliography

BACKGROUND: HIV-1 is decorated with trimeric glycoprotein spikes that enable infection by engaging CD4 and a chemokine coreceptor, either CCR5 or CXCR4. The variable loop 3 (V3) of the HIV-1 envelope protein (Env) is the main determinant for coreceptor usage. The predominant CCR5 using (R5) HIV-1 Env has been intensively studied in function and structure, whereas the trimeric architecture of the less frequent, but more cytopathic CXCR4 using (X4) HIV-1 Env is largely unknown, as are the consequences of sequence changes in and near V3 on antigenicity and trimeric Env structure. RESULTS: Soluble trimeric gp140 Env constructs were used as immunogenic mimics of the native spikes to analyze their antigenic properties in the context of their overall 3D structure. We generated soluble, uncleaved, gp140 trimers from a prototypic T-cell line-adapted (TCLA) X4 HIV-1 strain (NL4-3) and a hybrid (NL4-3/ADA), in which the V3 spanning region was substituted with that from the primary R5 isolate ADA. Compared to an ADA (R5) gp140, the NL4-3 (X4) construct revealed an overall higher antibody accessibility, which was most pronounced for the CD4 binding site (CD4bs), but also observed for mAbs against CD4 induced (CD4i) epitopes and gp41 mAbs. V3 mAbs showed significant binding differences to the three constructs, which were refined by SPR analysis. Of interest, the NL4-3/ADA construct with the hybrid NL4-3/ADA CD4bs showed impaired CD4 and CD4bs mAb reactivity despite the presence of the essential elements of the CD4bs epitope. We obtained 3D reconstructions of the NL4-3 and the NL4-3/ADA gp140 trimers via electron microscopy and single particle analysis, which indicates that both constructs inherit a propeller-like architecture. The first 3D reconstruction of an Env construct from an X4 TCLA HIV-1 strain reveals an open conformation, in contrast to recently published more closed structures from R5 Env. Exchanging the X4 V3 spanning region for that of R5 ADA did not alter the open Env architecture as deduced from its very similar 3D reconstruction. CONCLUSIONS: 3D EM analysis showed an apparent open trimer configuration of X4 NL4-3 gp140 that is not modified by exchanging the V3 spanning region for R5 ADA.

Peptides are ideally suited to mimic natural ligands and thereby function in an antagonistic or agonistic way. Furthermore they are able to physiologically disrupt functional complexes due to their small size and specific binding properties. Proteins form homo- or heteromeric (macro)molecular complexes and intricate networks by interacting with small molecules, peptides, nucleic acids or other proteins. On average, five interaction partners have been estimated for any given cellular protein, illustrating the complexity of the formed 'interactomes' and the impact of their investigation. Many protein-protein interactions are mediated by hot-spots, which comprise only a small part of the large binding interface but account for 80% of the binding energy. Thus, these hot spots provide an 'Achilles heel' for pharmaceutical interventions aiming at the disruption of functional protein-protein complexes. Methods to select peptides for defined target structures include display technologies on phages, ribosomes or yeast, and the yeast-two-hybrid system. Once selected, these peptides can be optimized for their binding affinity using peptide arrays. Stabilization of biologically unstable peptides is achieved by the introduction of non-natural amino acids to form so-called peptidomimetics that are resistant to cellular proteases. Moreover, lipocalins and peptide aptamers represent scaffolded binding structures with unique binding characteristics and enhanced stability. In case of extracellular targets, like cell surface receptors or pathogens in patients` plasma, peptide inhibitors have direct access. Addressing intracellular targets with peptides is more difficult since short hydrophilic peptides generally do not cross plasma membranes on their own. However, intracellular uptake of peptides can be achieved by coupling to carrier systems like liposomes or nanoparticles or upon fusion to a protein transduction domain. Alternatively, peptides may be expressed within cells after transduction with viral vectors in a gene therapy setting. This review will summarize the broad potential of peptides as drugs, with special emphasis on peptides which inhibit protein-protein interactions.

The extracellular hemoglobin multimer of the planorbid snail Biomphalaria glabrata, intermediate host of the human parasite Schistosoma mansoni, is presumed to be a 1.44 MDa complex of six 240 kDa polypeptide subunits, arranged as three disulfide-bridged dimers. The complete amino acid sequence of two subunit types (BgHb1 and BgHb2), and the partial sequence of a third type (BgHb3) are known. Each subunit encompasses 13 paralogus heme domains, and N-terminally a smaller plug domain responsible for subunit dimerization. We report here the recombinant expression of different functional fragments of BgHb2 in Escherichia coli, and of the complete functional subunits BgHb1 and BgHb2 in insect cells; BgHb1 was also expressed as disulfide-bridged dimer (480 kDa). Oxygen-binding measurements of the recombinant products show a P(50) of about 7 mmHg and the absence of a significant cooperativity or Bohr effect. The covalently linked dimer of BgHb1, but not the monomer, is capable to form aggregates closely resembling native BgHb molecules in the electron microscope.

In order to propagate and persist within the host, HIV-1 subverts a variety of checkpoints of innate and adaptive viral immunosurveillance. Many of these are related to natural killer cells, which bridge innate and adaptive immunity and play a major role in defeating virus infections. HIV-1 affects cytotoxicity of natural killer cells towards infected cells and natural killer cell-mediated priming of effector cells of the adaptive immune system. Moreover, a subpopulation of natural killer cells was found sensitive to infection by HIV-1. Consequently, an efficient immune response against HIV-1 cannot be mounted in most patients. The current review highlights the molecular interplay between HIV-1 and effector cells of the host immune system with a focus on natural killer cells, and summarizes strategies of HIV-1 to escape from natural killer cell immunosurveillance. A detailed knowledge of these immune escape strategies might lead to the identification of access points for intervention in order to block infection and progression to AIDS.

During HIV-1 entry, binding of the viral envelope glycoprotein gp120 to the cellular CD4 receptor triggers conformational changes resulting in exposure of new epitopes, the highly conserved CD4-induced (CD4i) epitopes that are essential for subsequent binding to chemokine receptor CCR5 or CXCR4. Due to their functional conservation, CD4i epitopes represent attractive viral targets for HIV-1 entry inhibition. The aim of the present study was to select peptide ligands for CD4i epitopes on native dualtropic (R5X4) HIV-1 envelope (Env) glycoproteins by phage display. Using CD4-activated retroviral particles carrying Env from the R5X4 HIV-1 89.6 strain as the target, we performed screenings of random peptide phage libraries under stringent selection conditions. Selected peptides showed partial identity with amino acids in the extracellular domains of CCR5/CXCR4, including motifs rich in tyrosines and aspartates at the N terminus known to be important for gp120 binding. A synthetic peptide derivative (XD3) corresponding to the most frequently selected phages was optimized for Env binding on peptide arrays. Interestingly, the optimized peptide could bind specifically to gp120 derived from HIV-1 strains with different coreceptor usage, competed with binding of CD4i-specific monoclonal antibody (MAb) 17b, and interfered with entry of both a CCR5 (R5)-tropic and a CXCR4 (X4)-tropic Env pseudotyped virus. This peptide ligand therefore points at unique properties of CD4i epitopes shared by gp120 with different coreceptor usage and could thus serve to provide new insight into the conserved structural details essential for coreceptor binding for further drug development.