Peter K. Park

ATR inhibition selectively sensitizes G1 checkpoint-deficient cells to lethal premature chromatin condensation.

Premature chromatin condensation (PCC) is a hallmark of mammalian cells that begin mitosis before completing DNA replication. This lethal event is prevented by a highly conserved checkpoint involving an unknown, caffeine-sensitive mediator. Here, we have examined the possible involvement of the caffeine-sensitive ATM and ATR protein kinases in this checkpoint. We show that caffeines ability to inhibit ATR (but not ATM) causes PCC, that ATR (but not ATM) prevents PCC, and that ATR prevents PCC via Chk-1 regulation. Moreover, mimicking cancer cell phenotypes by disrupting normal G1 checkpoints sensitizes cells to PCC by ATR inhibition plus low-dose DNA damage. Notably, loss of p53 function potently sensitizes cells to PCC caused by ATR inhibition by a small molecule. We present a molecular model for how ATR prevents PCC and suggest that ATR represents an attractive therapeutic target for selectively killing cancer cells by premature chromatin condensation.

A Fascinating Journey into History: Exploration of the World of Isonitriles En Route to Complex Amides

We describe herein our recent explorations in the field of isonitrile chemistry. An array of broadly useful coupling methodologies has been developed for the formation of peptidyl and glycopeptidyl amide bonds. We further describe the application of these methods to the syntheses of complex systems, including the cyclic peptide cyclosporine A, constrained peptide systems, and heterocycles.

Recognition of synthetic glycopeptides by HIV-1 broadly neutralizing antibodies and their unmutated ancestors

Significance A current key goal of HIV-1 vaccine development is to learn how to induce antibodies that will neutralize many diverse HIV-1 strains. Current HIV-1 vaccines elicit strain-specific neutralizing antibodies, whereas broadly neutralizing antibodies (BnAbs) are not induced and only arise in select HIV-1 chronically infected individuals. One strategy for induction of favored antibody responses is to design and produce homogeneous immunogens with selective expression of BnAb but not dominant epitopes. In this study, we describe the binding properties of chemically synthesized variable loop 1/2 (V1V2) glycopeptides that bind both to mature HIV-1 envelope BnAbs and the receptors of their naïve B cells. These results demonstrate that such synthetic glycopeptides can be immunogens that selectively target BnAb naïve B cells. Current HIV-1 vaccines elicit strain-specific neutralizing antibodies. Broadly neutralizing antibodies (BnAbs) are not induced by current vaccines, but are found in plasma in ∼20% of HIV-1–infected individuals after several years of infection. One strategy for induction of unfavored antibody responses is to produce homogeneous immunogens that selectively express BnAb epitopes but minimally express dominant strain-specific epitopes. Here we report that synthetic, homogeneously glycosylated peptides that bind avidly to variable loop 1/2 (V1V2) BnAbs PG9 and CH01 bind minimally to strain-specific neutralizing V2 antibodies that are targeted to the same envelope polypeptide site. Both oligomannose derivatization and conformational stabilization by disulfide-linked dimer formation of synthetic V1V2 peptides were required for strong binding of V1V2 BnAbs. An HIV-1 vaccine should target BnAb unmutated common ancestor (UCA) B-cell receptors of naïve B cells, but to date no HIV-1 envelope constructs have been found that bind to the UCA of V1V2 BnAb PG9. We demonstrate herein that V1V2 glycopeptide dimers bearing Man5GlcNAc2 glycan units bind with apparent nanomolar affinities to UCAs of V1V2 BnAbs PG9 and CH01 and with micromolar affinity to the UCA of a V2 strain-specific antibody. The higher-affinity binding of these V1V2 glycopeptides to BnAbs and their UCAs renders these glycopeptide constructs particularly attractive immunogens for targeting subdominant HIV-1 envelope V1V2-neutralizing antibody-producing B cells.

Chemical synthesis of highly congested gp120 V1V2 N-glycopeptide antigens for potential HIV-1-directed vaccines.

Critical to the search for an effective HIV-1 vaccine is the development of immunogens capable of inducing broadly neutralizing antibodies (BnAbs). A key first step in this process is to design immunogens that can be recognized by known BnAbs. The monoclonal antibody PG9 is a BnAb that neutralizes diverse strains of HIV-1 by targeting a conserved carbohydrate-protein epitope in the variable 1 and 2 (V1V2) region of the viral envelope. Important for recognition are two closely spaced N-glycans at Asn(160) and Asn(156). Glycopeptides containing this synthetically challenging bis-N-glycosylated motif were prepared by convergent assembly, and were shown to be antigenic for PG9. Synthetic glycopeptides such as these may be useful for the development of HIV-1 vaccines based on the envelope V1V2 BnAb epitope.

Expanding the limits of isonitrile-mediated amidations: on the remarkable stereosubtleties of macrolactam formation from synthetic seco-cyclosporins.

The scope of isonitrile-mediated amide bond-forming reactions is further explored in this second-generation synthetic approach to cyclosporine (cyclosporin A). Both type I and type II amidations are utilized in this effort, allowing access to epimeric cyclosporins A and H from a single precursor by variation of the coupling reagents. This work lends deeper insight into the relative acylating ability of the formimidate carboxylate mixed anhydride (FCMA) intermediate, while shedding light on the far-reaching impact of remote stereochemical changes on the effective preorganization of seco-cyclosporins.

Mimicry of an HIV broadly neutralizing antibody epitope with a synthetic glycopeptide

A synthetic glycopeptide mimics a key neutralizing epitope on the HIV-1 envelope and can be used to isolate HIV-1 broadly neutralizing antibodies. Guiding anti-glycan antibodies Although it typically evades the immune system, HIV does have sites of vulnerability that can be targeted in vaccine design. One such site is a glycan near the V3 loop of the envelope protein, but antibodies recognizing this epitope are often not detected in people infected with HIV. Alam et al. designed a synthetic glycopeptide that can identify B cells targeting this epitope and also used it to immunize macaques. Bonsignori et al. used this synthetic glycopeptide and other baits to study the V3-glycan antibody responses of an HIV-infected individual that developed broadly neutralizing antibodies. They also examined viral evolution over time and found clues as to why these types of antibodies do not develop more often. These tools and findings could pave the way for a vaccine that protects against diverse strains of HIV. A goal for an HIV-1 vaccine is to overcome virus variability by inducing broadly neutralizing antibodies (bnAbs). One key target of bnAbs is the glycan-polypeptide at the base of the envelope (Env) third variable loop (V3). We have designed and synthesized a homogeneous minimal immunogen with high-mannose glycans reflective of a native Env V3-glycan bnAb epitope (Man9-V3). V3-glycan bnAbs bound to Man9-V3 glycopeptide and native-like gp140 trimers with similar affinities. Fluorophore-labeled Man9-V3 glycopeptides bound to bnAb memory B cells and were able to be used to isolate a V3-glycan bnAb from an HIV-1–infected individual. In rhesus macaques, immunization with Man9-V3 induced V3-glycan-targeted antibodies. Thus, the Man9-V3 glycopeptide closely mimics an HIV-1 V3-glycan bnAb epitope and can be used to isolate V3-glycan bnAbs.

On the Synthesis of Conformationally Modified Peptides through Isonitrile Chemistry: Implications for Dealing with Polypeptide Aggregation

A method for introducing a dimethyleneoxy constraint joining the N atoms of two consecutive amino acids in the context of a polypeptide has been developed. This constraint can profoundly affect the tendency of a polypeptide to suffer aggregation and desolubilization, and it can be readily removed under mild conditions.