Inhibition of human immunodeficiency virus replication by nonimmunosuppressive analogs of cyclosporin A

Facilitating the development of molecular glues: Opportunities from serendipity and rational design

Molecular glues can specifically induce interactions between two or more proteins to modulate biological functions and have been proven to be a powerful therapeutic modality in drug discovery. It plays a variety of vital roles in several biological processes, such as complex stabilization, interactome modulation and transporter inhibition, thus enabling challenging therapeutic targets to be druggable. Most known molecular glues were identified serendipitously, such as IMiDs, auxin, and rapamycin. In recent years, more rational strategies were explored with the development of chemical biology and a deep understanding of the interaction between molecular glues and proteins, which led to the rational discovery of several molecular glues. Thus, in this review, we aim to highlight the discovery strategies of molecular glues from three aspects: serendipitous discovery, screening methods and rational design principles. We expect that this review will provide a reasonable reference and insights for the discovery of molecular glues.

The Role of Cyclophilins in Inflammatory Bowel Disease and Colorectal Cancer

Cyclophilins (Cyps) is a kind of ubiquitous protein family in organisms, which has biological functions such as promoting intracellular protein folding and participating in the pathological processes of inflammation and tumor. Inflammatory bowel disease (IBD) and colorectal cancer (CRC) are two common intestinal diseases, but the etiology and pathogenesis of these two diseases are still unclear. IBD and CRC are closely associated, IBD has always been considered as one of the main risks of CRC. However, the role of Cyps in these two related intestinal diseases is rarely studied and reported. In this review, the expression of CypA, CypB and CypD in IBD, especially ulcerative colitis (UC), and CRC, their relationship with the development of these two intestinal diseases, as well as the possible pathogenesis, were briefly summarized, so as to provide modest reference for clinical researches and treatments in future.

Cyclophilin A as a target in the treatment of cytomegalovirus infections

BACKGROUND

Viruses are obligate parasites that depend on the cellular machinery of the host to regenerate and manufacture their proteins. Most antiviral drugs on the market today target viral proteins. However, the more recent strategies involve targeting the host cell proteins or pathways that mediate viral replication. This new approach would be effective for most viruses while minimizing drug resistance and toxicity.

METHODS

Cytomegalovirus replication, latency, and immune response are mediated by the intermediate early protein 2, the main protein that determines the effectiveness of drugs in cytomegalovirus inhibition. This review explains how intermediate early protein 2 can modify the action of cyclosporin A, an immunosuppressive, and antiviral drug. It also links all the pathways mediated by cyclosporin A, cytomegalovirus replication, and its encoded proteins.

RESULTS

Intermediate early protein 2 can influence the cellular cyclophilin A pathway, affecting cyclosporin A as a mediator of viral replication or anti-cytomegalovirus drug.

CONCLUSION

Cyclosporin A has a dual function in cytomegalovirus pathogenesis. It has the immunosuppressive effect that establishes virus replication through the inhibition of T-cell function. It also has an anti-cytomegalovirus effect mediated by intermediate early protein 2. Both of these functions involve cyclophilin A pathway.

CD8(+) Lymphocytes Are Required for Maintaining Viral Suppression in SIV-Infected Macaques Treated with Short-Term Antiretroviral Therapy

Infection with HIV persists despite suppressive antiretroviral therapy (ART), and treatment interruption results in rapid viral rebound. Antibody-mediated CD8(+) lymphocyte depletion in simian immunodeficiency virus (SIV)-infected rhesus macaques (RMs) shows that these cells contribute to viral control in untreated animals. However, the contribution of CD8(+) lymphocytes to maintaining viral suppression under ART remains unknown. Here, we have shown that in SIV-infected RMs treated with short-term (i.e., 8-32 week) ART, depletion of CD8(+) lymphocytes resulted in increased plasma viremia in all animals and that repopulation of CD8(+) T cells was associated with prompt reestablishment of virus control. Although the number of SIV-DNA-positive cells remained unchanged after CD8 depletion and reconstitution, the frequency of SIV-infected CD4(+) T cells before depletion positively correlated with both the peak and area under the curve of viremia after depletion. These results suggest a role for CD8(+) T cells in controlling viral production during ART, thus providing a rationale for exploring immunotherapeutic approaches in ART-treated HIV-infected individuals.

Optimization of Cyclophilin Inhibitors for Use in Antiviral Therapy

Cyclophilins are members of the Propyl Peptidase Isomerase (PPIase) family of proteins and have recently been found to be required for efficient replication and/or infectivity of several viruses. Cyclosporine A (CsA), the prototypical inhibitor of cyclophilins has shown good activity against several key viruses, including HIV‐1 and HCV, however the immunosuppressive activity of CsA precludes its use as an effective anti‐viral agent. Structural information derived from the ternary complex formed by CsA, cyclophilin A and calcineurin has allowed the design of non‐immunosuppressive derivatives of CsA that retain, and in some cases improve, antiviral activity toward hepatitis C. Chemical modification of CsA has led to compounds with improved pharmacokinetic properties and with reduced drug‐drug interaction potential. Non‐CsA derived inhibitors of cyclophilin A have recently been identified and hold promise as synthetically more tractable leads for cyclophilin‐based discovery projects.

Correlation of naturally occurring HIV-1 resistance to DEB025 with capsid amino acid polymorphisms

DEB025 (alisporivir) is a synthetic cyclosporine with inhibitory activity against human immunodeficiency virus type-1 (HIV-1) and hepatitis C virus (HCV). It binds to cyclophilin A (CypA) and blocks essential functions of CypA in the viral replication cycles of both viruses. DEB025 inhibits clinical HIV-1 isolates in vitro and decreases HIV-1 virus load in the majority of patients. HIV-1 isolates being naturally resistant to DEB025 have been detected in vitro and in nonresponder patients. By sequence analysis of their capsid protein (CA) region, two amino acid polymorphisms that correlated with DEB025 resistance were identified: H87Q and I91N, both located in the CypA-binding loop of the CA protein of HIV-1. The H87Q change was by far more abundant than I91N. Additional polymorphisms in the CypA-binding loop (positions 86, 91 and 96), as well as in the N-terminal loop of CA were detected in resistant isolates and are assumed to contribute to the degree of resistance. These amino acid changes may modulate the conformation of the CypA-binding loop of CA in such a way that binding and/or isomerase function of CypA are no longer necessary for virus replication. The resistant HIV-1 isolates thus are CypA-independent.

Liver transplantation in HCV/HIV positive patients

Since the introduction of highly active antiretroviral therapy (HAART) in 1996 for human immunodeficiency virus (HIV)-infected patients, the incidence of liver diseases secondary to co-infection with hepatitis C has increased. Although data on the outcome of liver transplantation in HIV-infected recipients is limited, the overall results to date seem to be comparable to that in non-HIV-infected recipients. Liver transplant centers are now accepting HIV-infected individuals as organ recipients. Post-transplantation HIV replication is controlled by HAART. Hepatitis C re-infection of the liver graft, however, remains an important problem because cirrhotic changes of the liver graft may be more rapid in HIV-infected recipients. Interactions between the HAART components and immunosuppressive drugs influence drug metabolism and therefore meticulous monitoring of drug blood level concentrations is required. The risk of opportunistic infection in HIV-positive transplant patients seems to be similar to that in HIV-negative transplant recipients.

Liver transplantation in HIV-positive patients

This article reviews the worldwide evolution of liver transplantation as a therapeutic intervention in HIV-infected patients. Since the introduction of highly active antiretroviral therapy (HAART), liver disease secondary to viral hepatitis has become a leading cause of morbidity and mortality among HIV-positive individuals. The authors contrast survival data from pilot studies in the pre-HAART era to those data emerging from more recent trials. Particular emphasis is placed on current selection criteria for HIV-positive transplant candidates. Additional consideration is given to the effect of prolonged transplant waiting time on survival outcome. The complexity of the post-transplant medication regime, including drug interactions, optimal immunosuppression and most appropriate HAART regimes, are discussed in detail. Postoperative challenges including optimal management of hepatitis B virus and recurrent hepatitis C virus post-transplant are reviewed separately. The ethical and practical arguments relating to the use of a scarce and valuable resource in this population are debated. The authors conclude with several recommendations to assist pretransplant assessment and postoperative management of such complex patients and speculate on the direction and evolution of this field in the coming years.

Inhibition of human immunodeficiency virus type 1 replication in human cells by Debio-025, a novel cyclophilin binding agent

Debio-025 is a synthetic cyclosporine with no immunosuppressive capacity but a high inhibitory potency against cyclophilin A (CypA)-associated cis-trans prolyl isomerase (PPIase) activity. A lack of immunosuppressive effects compared to that of cyclosporine was demonstrated both in vitro and in vivo. For three cyclosporines, the inhibitory potential against PPIase activity was quantitatively correlated with that against human immunodeficiency virus type 1 (HIV-1) replication. Debio-025 selectively inhibited the replication of HIV-1 in a CD4+ cell line and in peripheral blood mononuclear cells: potent activity was demonstrated against clinical isolates of various HIV-1 subtypes, including isolates with multidrug resistance to reverse transcriptase and protease inhibitors. Simian immunodeficiency virus and HIV-2 strains were generally resistant to inhibition by Debio-025; however, some notable exceptions of sensitive HIV-2 clinical isolates were detected. In two-drug combination studies, additive inhibitory effects were found between Debio-025 and 19 clinically used drugs of different classes. Clinical HIV-1 isolates that are naturally resistant to Debio-025 and that do not depend on CypA for infection were identified. Comparison of the amino acid sequences of the CypA binding domain of the capsid (CA) protein from Debio-025-sensitive and -resistant HIV-1 isolates indicated that resistance was mostly associated with an H87Q/P exchange. Mechanistically, cyclosporines competitively inhibit the binding of CypA to the HIV-1 CA protein, which is an essential interaction required for early steps in HIV-1 replication. By real-time PCR we demonstrated that early reverse transcription is reduced in the presence of Debio-025 and that late reverse transcription is almost completely blocked. Thus, Debio-025 seems to interfere with the function of CypA during the progression/completion of HIV-1 reverse transcription.

Nef enhances HIV-1 infectivity via association with the virus assembly complex

The HIV-1 accessory protein Nef enhances virus infectivity by facilitating an early post-entry step of infection. Nef acts in the virus producer cell, leading to a beneficial modification to HIV-1 particles. Nef itself is incorporated into HIV-1 particles, where it is cleaved by the viral protease during virion maturation. To probe the role of virion-associated Nef in HIV-1 infection, we generated a fusion protein consisting of the host protein cyclophilin A (CypA) linked to the amino terminus of Nef. The resulting CypA-Nef protein enhanced the infectivity of Nef-defective HIV-1 particles and was specifically incorporated into the virions via association with Gag during particle assembly. Pharmacologic or genetic inhibition of CypA-Nef binding to Gag prevented incorporation of CypA-Nef into virions and inhibited infectivity enhancement. Our results indicate that infectivity enhancement by Nef requires its association with a component of the assembling HIV-1 particle.

Analysis of human cell heterokaryons demonstrates that target cell restriction of cyclosporine-resistant human immunodeficiency virus type 1 mutants is genetically dominant

The host cell protein cyclophilin A (CypA) binds to CA of human immunodeficiency virus type 1 (HIV-1) and promotes HIV-1 infection of target cells. Disruption of the CypA-CA interaction, either by mutation of the CA residue at G89 or P90 or with the immunosuppressive drug cyclosporine (CsA), reduces HIV-1 infection. Two CA mutants, A92E and G94D, previously were identified by selection for growth of wild-type HIV-1 in cultures of CD4(+) HeLa cell cultures containing CsA. Interestingly, infection of some cell lines by these mutants is enhanced in the presence of CsA, while in other cell lines these mutants are minimally affected by the drug. Little is known about this cell-dependent phenotype of the A92E and G94D mutants, except that it is not dependent on expression of the host factor TRIM5alpha. Here, we show that infection by the A92E and G94D mutants is restricted at an early post-entry stage of the HIV-1 life cycle. Analysis of heterokaryons between CsA-dependent HeLa-P4 cells and CsA-independent 293T cells indicated that the CsA-dependent infection by A92E and G94D mutants is due to a dominant cellular restriction. We also show that addition of CsA to target cells inhibits infection by wild-type HIV-1 prior to reverse transcription. Collectively, these results support the existence of a cell-specific human cellular factor capable of restricting HIV-1 at an early post-entry step by a CypA-dependent mechanism.

Effects of immunosuppressive drugs on HIV infection: implications for solid-organ transplantation

With the advent of highly active antiretroviral therapy (HAART), HIV infection has become a chronic disease. Various end-stage organ failures have now become common co-morbidities and are primary causes of mortality in HIV-infected patients. Solid-organ transplantation therefore has been proposed to these patients, as HIV infection is not anymore considered an absolute contraindication. The initial results of organ transplantation in HIV-infected patients are encouraging with no differences in patient and graft survival compared with non-HIV-infected patients. The use of immunosuppressive drug therapy in HIV-infected patients has so far not shown major detrimental effects, and some drugs in combination with HAART have even demonstrated possible beneficial effects for specific HIV settings. Nevertheless, organ transplantation in HIV-infected patients remains a complex intervention, and more studies will be required to clarify open questions such as long-term effects of drug interactions between antiretroviral and immunosuppressive drugs, outcome of recurrent HCV infection in HIV-infected patients, incidence of graft rejection, or long-term graft and patient survival. In this article, we first review the immunological pathogenesis of HIV infection and the rationale for using immunosuppression combined with HAART. We then discuss the most recent results of solid-organ transplantation in HIV-infected patients.

A fluorescence polarization-based assay for peptidyl prolyl cis/trans isomerase cyclophilin A

Peptidyl prolyl cis/trans isomerase cyclophilin A (CypA) serves as a cellular receptor for the important immunosuppressant drug, cyclosporin A. In addition, CypA and its enzyme family have been found to play critical roles in a variety of biological processes, including protein trafficking, HIV and HCV infection/replication, and Ca(2+)-mediated intracellular signaling. For these reasons, cyclophilins have emerged as potential drug targets for several diseases. Therefore, it is extremely important to screen for novel small molecule cyclophilin inhibitors. Unfortunately, the biochemical assays reported so far are not adaptable to a high-throughput screening format. Here, we report a fluorescence polarization-based assay for human CypA that can be adapted to high-throughput screening for drug discovery. The technique is based on competition and uses a fluorescein-labeled cyclosporin A analog and purified human CypA to quantitatively measure the binding capacity of unlabeled inhibitors. Detection by fluorescence polarization allows real-time measurement of binding ratios without separation steps. The results obtained demonstrated significant correlation among assay procedures, suggesting that the application of fluorescence polarization in combination with CypA is highly advantageous for the accurate assessment of inhibitor binding.

Pharmacological targeting of catalyzed protein folding: the example of peptide bond cis/trans isomerases

Peptide bond isomerases are involved in important physiological processes that can be targeted in order to treat neurodegenerative disease, cancer, diseases of the immune system, allergies, and many others. The folding helper enzyme class of Peptidyl-Prolyl-cis/trans Isomerases (PPIases) contains the three enzyme families of cyclophilins (Cyps), FK506 binding proteins (FKBPs), and parvulins (Pars). Although they are structurally unrelated, all PPIases catalyze the cis/trans isomerization of the peptide bond preceding the proline in a polypeptide chain. This process not only plays an important role in de novo protein folding, but also in isomerization of native proteins. The native state isomerization plays a role in physiological processes by influencing receptor ligand recognition or isomer-specific enzyme reaction or by regulating protein function by catalyzing the switch between native isomers differing in their activity, e.g., ion channel regulation. Therefore elucidating PPIase involvement in physiological processes and development of specific inhibitors will be a suitable attempt to design therapies for fatal and deadly diseases.

Cyclophilin interactions with incoming human immunodeficiency virus type 1 capsids with opposing effects on infectivity in human cells

Cyclophilin A (CypA) is a peptidyl-prolyl isomerase that binds to the capsid protein (CA) of human immunodeficiency virus type 1 (HIV-1) and by doing so facilitates HIV-1 replication. Although CypA is incorporated into HIV-1 virions by virtue of CypA-Gag interactions that occur during virion assembly, in this study we show that the CypA-CA interaction that occurs following the entry of the viral capsid into target cells is the major determinant of CypA's effects on HIV-1 replication. Specifically, by using normal and CypA-deficient Jurkat cells, we demonstrate that the presence of CypA in the target and not the virus-producing cell enhances HIV-1 infectivity. Moreover, disruption of the CypA-CA interaction with cyclosporine A (CsA) inhibits HIV-1 infectivity only if the target cell expresses CypA. The effect of CsA on HIV-1 infection of human cells varies according to which particular cell line is used as a target, and CA mutations that confer CsA resistance and dependence exert their effects only if target cells, and not if virus-producing cells, are treated with CsA. The differential effects of CsA on HIV-1 infection in different human cells appear not to be caused by polymorphisms in the recently described retrovirus restriction factor TRIM5alpha. We speculate that CypA and/or CypA-related proteins affect the fate of incoming HIV-1 capsid either directly or by modulating interactions with unidentified host cell factors.

Use of cyclosporine in hematopoietic cell transplantation

Early attempts to perform successful allogeneic hematopoietic stem cell transplants were plagued by graft rejection and graft-versus-host disease, leading many investigators to lose hope that bone marrow transplantation could ever become a useful therapeutic approach in clinical medicine. Although many subsequent discoveries contributed to the successful transformation of this area of research into lifesaving therapy, there can be no doubt that the discovery of cyclosporine (CsA) constituted one of the major advances. Even today, more than 25 years after the first description of its use in humans, CsA remains one of the cornerstones of therapy for the majority of patients undergoing allogeneic hematopoietic cell transplantation (HCT). In this review we will recount the events that have established CsA as a foundation of HCT.

Liver transplantation in patients with HIV infection

1. Liver transplantation for human immunodeficiency virus (HIV)-positive patients with end-stage liver disease in the era of highly active retroviral therapy has proven to be an effective treatment. The concerns of HIV progression have not been borne out by the growing worldwide experience. 2. CD4 counts are stable and HIV viral load is controllable with medication following liver transplantation. 3. Hepatitis C virus (HCV) coinfection in HIV-positive recipients is universal, but the severity of recurrence does not appear to be different from that in HIV-negative patients with HCV liver disease. 4. Complex pharmacokinetic interactions between the calcineurin inhibitors used for immunosuppression along with protease inhibitors are present, but management directed at recognizing the need for monitoring levels does not appear to increase the risk of toxicity. 5. The degree of immunosuppression from iatrogenic drug therapy and HIV does not lead to increased risk of infectious complications.

Regulation of peptide bond cis/trans isomerization by enzyme catalysis and its implication in physiological processes

In some cases, the slow rotational movement underlying peptide bond cis/trans isomerizations is found to control the biological activity of proteins. Peptide bond cis/trans isomerases as cyclophilins, Fk506-binding proteins, parvulins, and bacterial hsp70 generally assist in the interconversion of the polypeptide substrate cis/trans isomers, and rate acceleration is the dominating mechanism of action in cells. We present evidence disputing the hypothesis that some of the molecular properties of these proteins play an auxiliary role in enzyme function.

Thermodynamic characterization of the interaction of human cyclophilin 18 with cyclosporin A

Isothermal titration calorimetry (ITC) was used to investigate thermodynamic parameters of the cyclosporin A (CsA)-cyclophilin 18 (hCyp18) association reaction. We have calculated the thermodynamic parameters (enthalpy, entropy, heat capacity, and free energy of binding) of the CsA/hCyp18 complexation. All but two methods described in the literature underestimate the affinity to hCyp18 of CsA. We found that the association constant (1.1.10(8) M(-1) at 10 degrees C) of CsA to hCyp18 is in close agreement with the reciprocal of the reported inhibitory constant of the peptidylprolyl cis/trans isomerase activity of hCyp18. Interpretation of the thermodynamic parameters in buffered solution of water, 30% glycerol and D(2)O leads to the conclusion that the highly specific binding of CsA to hCyp18 is mainly mediated through hydrogen bonding and to a lesser degree through hydrophobic interaction. Furthermore, the pH dependence of the association constant was determined and analyzed according to a single proton linkage model, resulting in a pK(a) value of 5.7 in free hCyp18 and below 4.5 in the CsA complexed form. Titration experiments using different single component buffers possessing different heats of ionization allowed us to estimate that statistically half a proton is transferred upon CsA binding from the binding interface of hCyp18 to the buffer at pH 5.5. No proton transfer was detected at pH 7.5. The thermodynamic results are discussed in relation to the published X-ray and NMR structure of the free and CsA complexed hCyp18.

Synthesis of non-immunosuppressive cyclophilin-Binding cyclosporin A derivatives as potential anti-HIV-1 drugs

Original cyclosporin A (CsA) derivatives bearing various alkylthio side chains at the sarcosine residue 3 (R configuration) and for the most potent and selective compounds a 4'-hydroxyl group at the Me-Leucine residue 4 were prepared in one or two steps from commercially available CsA. The [2-(dimethyl or diethylamino)-ethylthio-Sar](3)-[(4'-OH)MeLeu](4)-CsA derivatives 3k and 3l displayed potent in vitro anti-HIV-1 (IC(50) approximately 46 nM) and low immunosuppressive activities (IC(50)>or=1500 nM).

Synthesis and evaluation of Glypsi(PO(2)R-N)Pro-containing pseudopeptides as novel inhibitors of the human cyclophilin hCyp-18

The human cyclophilin hCyp-18, an abundant peptidyl-prolyl cis-trans isomerase (PPIase) implicated in protein folding, controls the infection of CD4(+) T-cells by HIV-1, the pathologic agent of AIDS. Therefore, hCyp-18 is an interesting target for the development of novel anti-HIV-1 therapeutics. We focused on the design of transition-state analogue inhibitors of the PPIase activity of cyclophilin. Most experimental results reported in the literature suggest that hCyp-18 catalyzes the pyramidalization of the nitrogen of pyrrolidine via an H-bond network which results in the deconjugation of the amino acyl-prolyl peptide bond. We proposed the Glypsi(PO(2)R(1)-N)Pro motif (R = alkyl or H) as a selective transition-state analogue inhibitor of cyclophilin. This motif was inserted in Suc-Ala-Ala-Pro-Phe-pNA, a peptide substrate of hCyp-18. The pseudopeptide Suc-Ala-Glypsi(PO(2)Et-N)Pro-Phe-pNA 1b bound to hCyp-18 (K(d) = 20 +/- 5 microM) and was able to selectively inhibit its PPIase activity (IC(50) = 15 +/- 1 microM) but not hFKBP-12, another important PPIase. Deprotection of the phosphonamidate moiety resulted in a complete lack of inhibition. We previously demonstrated that reduction of the Phe-pNA moiety caused a quantitative reduction of the affinity; however, Suc-Ala-Glypsi(PO(2)Et-N)Pro-Phepsi(CH(2)-NH)pNA 7b still bound and inhibited hCyp-18, suggesting that the Glypsi(PO(2)Et-N)Pro motif plays the major role in the binding to cyclophilin. Consequently, we propose compound 1b as being a novel transition-state mimic inhibitor of hCyp-18.

Cyclophilin-mediated pathways in the effect of cyclosporin A on endothelial cells: role of vascular endothelial growth factor

The relative importance of cyclophilin (CyP) versus calcineurin (Cn)-mediated mechanisms in the effect of cyclosporin A (CsA) on endothelial cells (ECs) is largely unknown. In cultured ECs, CsA was cytotoxic/proapoptotic or cytoprotective/antiapoptotic at high or low concentrations, respectively. CsA analogs (MeVal-4-CsA and MeIle-4-CsA), which bind to CyP but do not inhibit Cn, closely reproduced the CsA effects. Based on our previous data, the role of vascular endothelial growth factor (VEGF) as a mediator of CsA-induced cytoprotection was further analyzed. The actions of CsA and CsA analogs were shifted from a protective to a cell-damaging pattern in the presence of a specific anti-VEGF monoclonal antibody (mAb). This positive interaction was further supported by a transient increase in cytosolic free calcium concentration ([Ca(2+)](i)) by VEGF after pretreatment with either CsA or MeVal-4-CsA and an increase in the expression and synthesis of VEGF receptor 2 (VEGFR2). Of functional importance, blockade of the interaction between VEGF and VEGFR2 by a VEGFR2 mAb abolished the cytoprotective effect of CsA. In addition, preconditioning with low concentrations of CsA or CsA analogs increased both cytoprotection and VEGFR2 mRNA expression when EC were exposed to higher concentrations of CsA. In summary, our results reveal that (1) the biphasic responses to CsA in EC are related to the interaction of CsA with CyP rather than with Cn and (2) VEGF is a critical factor in the cytoprotective effect of CsA, by a mechanism that involves VEGFR2.

Reversible inhibition of calcineurin by the polyphenolic aldehyde gossypol

The reversible inhibition of calcineurin (CaN), which is the only Ca(2+)/calmodulin-dependent protein Ser/Thr phosphatase, is thought to be a key functional event for most cyclosporin A (CsA)- and tacrolimus (FK506)-mediated biological effects. In addition to CaN inhibition, however, CsA and FK506 have multiple biochemical effects because of their action in a gain-of-function model that requires prior binding to immunophilic proteins. We screened a small molecule library for direct inhibitors of CaN using CaN-mediated dephosphorylation of (33)P-labeled 19-residue phosphopeptide substrate (RII phosphopeptide) as an assay and found the polyphenolic aldehyde gossypol to be a novel CaN inhibitor. Unlike CsA and FK506, gossypol does not require a matchmaker protein for reversible CaN inhibition with an IC(50) value of 15 microm. Gossypolone, a gossypol analog, showed improved inhibition of both RII phosphopeptide and p-nitrophenyl phosphate dephosphorylation with an IC(50) of 9 and 6 microm, respectively. In contrast, apogossypol hexaacetate was inactive. Gossypol acts noncompetitively, interfering with the binding site for the cyclophilin 18.CsA complex in CaN. In contrast to CsA and FK506, gossypol does not inactivate the peptidyl-prolyl-cis/trans-isomerase activity of immunophilins. Similar to CsA and FK506, T cell receptor signaling induced by phorbol 12-myristate 13-acetate/ionomycin is inhibited by gossypol in a dose-dependent manner, demonstrated by the inhibition of nuclear factor of activated T cell (NFAT) c1 translocation from the cytosol into the nucleus and suppression of NFAT-luciferase reporter gene activity.

Structural requirements for the biosynthesis of backbone cyclic peptide libraries

BACKGROUND

Combinatorial methods for the production of molecular libraries are an important source of ligand diversity for chemical biology. Synthetic methods focus on the production of small molecules that must traverse the cell membrane to elicit a response. Genetic methods enable intracellular ligand production, but products must typically be large molecules in order to withstand cellular catabolism. Here we describe an intein-based approach to biosynthesis of backbone cyclic peptide libraries that combines the strengths of synthetic and genetic methods.

RESULTS

Through site-directed mutagenesis we show that the DnaE intein from Synechocystis sp. PCC6803 is very promiscuous with respect to peptide substrate composition, and can generate cyclic products ranging from four to nine amino acids. Libraries with five variable amino acids and either one or four fixed residues were prepared, yielding between 10(7) and 10(8) transformants. The majority of randomly selected clones from each library gave cyclic products.

CONCLUSIONS

We have developed a versatile method for producing intracellular libraries of small, stable cyclic peptides. Genetic encoding enables facile manipulation of vast numbers of compounds, while low molecular weight ensures ready pharmacophore identification. The demonstrated flexibility of the method towards both peptide length and composition makes it a valuable addition to existing methods for generating ligand diversity.

Current lead natural products for the chemotherapy of human immunodeficiency virus (HIV) infection

A large variety of natural products have been described as anti-HIV agents, and for a portion thereof the target of interaction has been identified. Cyanovirin-N, a 11-kDa protein from Cyanobacterium (blue-green alga) irreversibly inactivates HIV and also aborts cell-to-cell fusion and transmission of HIV, due to its high-affinity interaction with gp120. Various sulfated polysaccharides extracted from seaweeds (i.e., Nothogenia fastigiata, Aghardhiella tenera) inhibit the virus adsorption process. Ingenol derivatives may inhibit virus adsorption at least in part through down-regulation of CD4 molecules on the host cells. Inhibition of virus adsorption by flavanoids such as (-)epicatechin and its 3-O-gallate has been attributed to an irreversible interaction with gp120 (although these compounds are also known as reverse transcriptase inhibitors). For the triterpene glycyrrhizin (extracted from the licorice root Glycyrrhiza radix) the mode of anti-HIV action may at least in part be attributed to interference with virus-cell binding. The mannose-specific plant lectins from Galanthus, Hippeastrum, Narcissus, Epipac tis helleborine, and Listera ovata, and the N-acetylgl ucosamine-specific lectin from Urtica dioica would primarily be targeted at the virus-cell fusion process. Various other natural products seem to qualify as HIV-cell fusion inhibitors: the siamycins [siamycin I (BMY-29304), siamycin II (RP 71955, BMY 29303), and NP-06 (FR901724)] which are tricyclic 21-amino-acid peptides isolated from Streptomyces spp that differ from one another only at position 4 or 17 (valine or isoleucine in each case); the betulinic acid derivative RPR 103611, and the peptides tachyplesin and polyphemusin which are highly abundant in hemocyte debris of the horseshoe crabs Tachypleus tridentatus and Limulus polyphemus, i.e., the 18-amino-acid peptide T22 from which T134 has been derived. Both T22 and T134 have been shown to block T-tropic X4 HIV-1 strains through a specific antagonism with the HIV corecept or CXCR4. A number of natural products have been reported to interact with the reverse transcriptase, i.e., baicalin, avarol, avarone, psychotrine, phloroglucinol derivatives, and, in particular, calanolides (from the tropical rainforest tree, Calophyllum lanigerum) and inophyllums (from the Malaysian tree, Calophyllum inophyllum). The natural marine substance illimaquinone would be targeted at the RNase H function of the reverse transcriptase. Curcumin (diferuloylmethane, from turmeric, the roots/rhizomes of Curcuma spp), dicaffeoylquinic and dicaffeoylt artaric acids, L-chicoric acid, and a number of fungal metabolites (equisetin, phomasetin, oteromycin, and integric acid) have all been proposed as HIV-1 integrase inhibitors. Yet, we have recently shown that L-c hicoric acid owes its anti-HIV activity to a specific interaction with the viral envelope gp120 rather than integrase. A number of compounds would be able to inhibit HIV-1 gene expression at the transcription level: the flavonoid chrysin (through inhibition of casein kinase II, the antibacter ial peptides melittin (from bee venom) and cecropin, and EM2487, a novel substance produced by Streptomyces. (ABSTRACT TRUNCATED)

Design of a Gag pentapeptide analogue that binds human cyclophilin A more efficiently than the entire capsid protein: new insights for the development of novel anti-HIV-1 drugs

Cyclophilin A (hCyp-18), a ubiquitous cytoplasmic peptidyl-prolyl cis/trans isomerase (PPIase), orchestrates HIV-1 core packaging. hCyp-18, incorporated into the virion, enables core uncoating and RNA release and consequently plays a critical role in the viral replication process. hCyp-18 specifically interacts with a single exposed loop of the Gag polyprotein capsid domain via a network of nine hydrogen bonds which mainly implicates a 7-mer fragment of the loop. As previously reported, the corresponding linear heptapeptide Ac-Val-His-Ala-Gly-Pro-Ile-Ala-NH(2) (2) binds to hCyp-18 with a low affinity (IC(50) = 850 +/- 220 microM) but a potentially useful selectivity for hCyp-18 relative to hFKBP-12, another abundant PPIase. On the basis of X-ray structures of Gag fragments:hCyp-18 complexes, we generated a series of modified peptides in order to probe the determinants of the interaction and hence to select a peptidic ligand displaying a higher affinity than the capsid domain of Gag. We synthesized a series of heptapeptides to test the energetic contribution of amino acids besides the Gly-Pro moiety. In particular the importance of the histidine residue for the interaction was underscored. We also investigated the influence of N- and C-terminal modifications. Hexapeptides containing either deaminovaline (Dav) in place of the N-terminal valine or substitution of the C-terminal alanine amide with a benzylamide group displayed increased affinities. Combination of both modifications gave the most potent competitor Dav-His-Ala-Gly-Pro-Ile-NHBn (28) which has a higher affinity for hCyp-18 (K(d) = 3 +/- 0.5 microM) than the entire capsid protein (K(d) = 16 +/- 4 microM) and a very low affinity for hFKBP-12. Some of our results strongly suggest that the title compound is not a substrate of hCyp-18 and interacts preferentially in the trans conformation.

NFAT1 enhances HIV-1 gene expression in primary human CD4 T cells

Cyclosporin A (CsA) is a potent inhibitor of the NFAT family of transcription factors that enhance T cell activation. The observation that human immunodeficiency virus type 1 (HIV-1)-positive transplant recipients have a reduced HIV-1 viral burden during treatment with CsA suggested that NFAT may play a direct role in enhancing transcription of the HIV-1 viral genome. Two sets of NFAT binding sites were identified in the HIV-1 long terminal repeat (LTR) promoter by in vitro footprinting with full-length recombinant NFAT protein, and gel shift analysis of nuclear protein from polyclonally activated primary CD4 T cells revealed specific binding of NFAT1 to the NFkappaB binding sites of the HIV-1 LTR. Activation of primary CD4 T cells transiently transfected with a HIV-1 LTR luciferase reporter plasmid, lacking the NFAT binding sites in the upstream putative negative regulatory element but maintaining the NFkappaB/NFAT sites, demonstrated increased HIV-1 gene expression when cotransfected with a NFAT1 expression vector. Moreover, CsA, FK506, and a dominant-negative NFAT1 protein independently inhibited HIV-1 LTR promoter activity in CD4 T cells stimulated with phorbol ester and calcium ionophore. In primary human CD4 T cells, CsA also inhibited promoter activity directed by multimers of binding sites for NFAT, while having no effect on NFkappaB multimer-driven promoter activity. Increasing NFAT1 levels in CD4 T cells transiently transfected with a HIV-1 provirus also increased p24 protein expression. Thus, NFAT may be a target for prevention of HIV-1 LTR-directed gene expression in human CD4 T cells.

Gag protein from human immunodeficiency virus type 1 assembles in the absence of cyclophilin A

Human immunodeficiency virus type 1 (HIV-1) replication requires coordinated activities of host and viral factors. We reported previously that interactions of the host factor cyclophilin A with HIV-1 Gag polyproteins affected Gag processing and maturation of virus particles (Streblow et al., 1998. Virology 245, 197-202). We now use in vitro translation and physical analysis of Gag structures to refine our understanding of how cyclophilin A affects HIV-1 replication. Gag assembled into oligomeric structures in vitro in the presence or absence of cyclophilin A, and proteins synthesized under the two conditions were equally susceptible to cleavage by exogenous HIV-1 protease. These and previous data show that Cyclophilin A is required at a step between Gag assembly and Gag processing/virion morphogenesis. Cyclophilin A may be required for Gag conformational changes subsequent to assembly, that are required for efficient dimerization and activation of the viral protease.

The use of two immunosuppressive drugs, cyclosporin A and tacrolimus, to inhibit virus replication and apoptosis in cells infected with feline immunodeficiency virus

An in vitro model of acute and chronic infections with feline immunodeficiency virus (FIV) was used to examine the effect of two immunosuppressive agents, cyclosporin A (CsA) and tacrolimus (also known as FK506), on the inhibition of the replication of the virus and of apoptosis. Both drugs significantly suppressed virus production in a dose-dependent manner in acutely and chronically infected cells. The ability of FK506 to inhibit virus replication was much lower than that of CsA, and was accompanied by marked antiproliferative activity. Treatment of infected cells with either CsA or FK506 did not affect the rise of free intracellular Ca2+ but did protect the cells against apoptosis. Thus, the antiviral activity of CsA and FK506 makes these compounds promising candidates for the development of drugs suitable for the treatment of AIDS.

Human immunodeficiency virus type 1 replication is modulated by host cyclophilin A expression levels

Human immunodeficiency virus type 1 (HIV-1) Gag and the cellular protein cyclophilin A form an essential complex in the virion core: virions produced by proviruses encoding Gag mutants with decreased cyclophilin A affinity exhibit attenuated infectivity, as do virions produced in the presence of the competitive inhibitor cyclosporine. The A224E Gag mutant has no effect on cyclophilin A affinity but renders HIV-1 replication cyclosporine resistant in Jurkat T cells. In contrast, A224E mutant virus is dead in H9 T cells, although replication is rescued by cyclosporine or by expression in cis of a Gag mutant that decreases cyclophilin A-affinity. The observation that disruption of the Gag-cyclophilin A interaction rescues A224E mutant replication in H9 cells prompted experiments which revealed that, relative to Jurkat cells, H9 cells express greater quantities of cyclophilin A. The resulting larger quantity of cyclophilin A shown to be packaged into virions produced by H9 cells is presumably disruptive to the A224E mutant virion core. Further evidence that increased cyclophilin A expression in H9 cells is of functional relevance was provided by the finding that Gag mutants with decreased cyclophilin A affinity are dead in Jurkat cells but capable of replication in H9 cells. Similarly, cyclosporine concentrations which inhibit wild-type HIV-1 replication in Jurkat cells stimulate HIV-1 replication in H9 cells. These results suggest that HIV-1 virion infectivity imposes narrow constraints upon cyclophilin A stoichiometry in virions and that infectivity is finely tuned by host cyclophilin A expression levels.

Cyclophilin a modulates processing of human immunodeficiency virus type 1 p55Gag: mechanism for antiviral effects of cyclosporin A

The molecular chaperone cyclophilin A (Cyp A) modulates human immunodeficiency virus type 1 (HIV-1) infectivity through its interactions with Gag structural proteins. The molecular mechanism for CypA in HIV-1 replication is not known. We studied chaperone effects on Gag precursor processing using cyclosporin A (CsA) to bind CypA and prevent its interaction with p55Gag. CsA treatment inhibited p55Gag processing in extracellular virus-like particles produced from COS cells. We confirmed the effect of CsA on Gag processing by examining virions produced from CEMx174 cells infected with HIV-1LAI. Particles accumulated in the presence of CsA displayed mostly immature virion morphology and lacked condensed capsids. CsA has a direct effect on HIV-1 Gag processing that implicates CypA as having an important role in the maturation of HIV-1 particles.

Cyclosporin analogs inhibit in vitro growth of Cryptosporidium parvum

Cyclosporine and nonimmunosuppressive cyclosporin (CS) analogs were demonstrated to be potent inhibitors of the growth of the intracellular parasite Cryptosporidium parvum in short-term (48-h) in vitro cultures. Fifty-percent inhibitory concentrations (IC50s) were 0.4 microM for SDZ 033-243, 1.0 microM for SDZ PSC-833, and 1.5 microM for cyclosporine. Two other analogs were less effective than cyclosporine: the IC50 of SDZ 205-549 was 5 microM, and that of SDZ 209-313 was 7 microM. These were much lower than the IC50 of 85 microM of paromomycin, a standard positive control for in vitro drug assays for this parasite. In addition, intracellular growth of excysted sporozoites that had been incubated for 1 h in cyclosporine was significantly reduced, suggesting that the drug can inhibit sporozoite invasion. The cellular activities of the CS analogs used have been characterized for mammalian cells and protozoa. The two analogs that were most active in inhibiting C. parvum, SDZ PSC-833 and SDZ 033-243, bind weakly to cyclophilin, a peptidyl proline isomerase which is the primary target of cyclosporine and CS analogs. However, they are potent modifiers of the activity of the P glycoproteins/ multidrug resistance (MDR) transporters, members of the ATP-binding cassette (ABC) superfamily. Hence, both cyclophilin and some ABC transporters may be targets for this class of drugs, although drugs that preferentially interact with the latter are more potent. Cyclosporine (0.5 microM) had no significant chemosensitizing activity. That is, it did not significantly increase sensitivity to paromomycin, suggesting that an ABC transporter is not critical in the efflux of this drug. Cyclosporine at concentrations up to 50 microM was not toxic to host Caco-2 cells in the CellTiter 96 assay. The results of this study complement those of studies of the inhibitory effect of cyclosporine and CS analogs on other apicomplexan parasites, Plasmodium falciparum, Plasmodium vivax, and Toxoplasma gondii.

Role of cyclophilin A in the uptake of HIV-1 by macrophages and T lymphocytes

Cyclophilins are a family of proteins that bind cyclosporin A (CsA) and possess peptidyl-prolyl cis-trans isomerase activity. In addition, they are secreted by activated cells and act in a cytokine-like manner, presumably via signaling through a cell surface cyclophilin receptor. More recently, host-derived cyclophilin A (CyPA) has been shown to be incorporated into HIV-1 virions and its incorporation essential for viral infectivity. Here we present evidence supporting a role for viral-associated CyPA in the early events of HIV-1 infection. We report that HIV-1 infection of primary peripheral blood mononuclear cells can be inhibited by: (i) an excess of exogenously added CyPA; (ii) a CsA analogue unable to enter the cells; (iii) neutralizing antibodies to CyPA. Taken together with our observations that recombinant CyPA-induced mobilization of calcium in immortalized, as well as primary, CD4+ T lymphocytes, and that incubation of T cells with iodinated CyPA, followed by chemical cross-linking, resulted in the formation of a high molecular mass complex on the cell surface, these results suggest that HIV-1-associated CyPA mediates an early event in viral infection via interaction with a cellular receptor. This interaction may present a target for anti-HIV therapies and vaccines.

Cells with high cyclophilin A content support replication of human immunodeficiency virus type 1 Gag mutants with decreased ability to incorporate cyclophilin A

Gag polyprotein-mediated incorporation of cellular cyclophilin A (CyPA) into virions is essential for the formation of infectious human immunodeficiency virus type 1 (HIV-1) virions. Either a point mutation in Gag (P222A) or drugs which bind CyPA decrease virion incorporation of CyPA and interfere with HIV-1 replication. We have found that lymphoid cells varied greatly in their CyPA content and that cells with high CyPA content supported the replication of P222A HIV-1 Gag mutants. These experiments demonstrated that a higher cellular CyPA content of some cells was able to compensate for the decreased binding affinity of P222A mutant Gag for CyPA, allowing virus replication to occur.

Transfer of the HIV-1 cyclophilin-binding site to simian immunodeficiency virus from Macaca mulatta can confer both cyclosporin sensitivity and cyclosporin dependence

HIV-1 specifically incorporates the peptidyl prolyl isomerase cyclophilin A (CyPA), the cytosolic receptor for the immunosuppressant cyclosporin A (CsA). HIV-1 replication is inhibited by CsA as well as by nonimmunosuppressive CsA analogues that bind to CyPA and interfere with its virion association. In contrast, the related simian immunodeficiency virus SIVmac, which does not interact with CyPA, is resistant to these compounds. The incorporation of CyPA into HIV-1 virions is mediated by a specific interaction between the active site of the enzyme and the capsid (CA) domain of the HIV-1 Gag polyprotein. We report here that the transfer of HIV-1 CA residues 86-93, which form part of an exposed loop, to the corresponding position in SIVmac resulted in the efficient incorporation of CyPA and conferred an HIV-1-like sensitivity to a nonimmunosuppressive cyclosporin. HIV-1 CA residues 86-90 were also sufficient to transfer the ability to efficiently incorporate CyPA, provided that the length of the CyPA-binding loop was preserved. However, the resulting SIVmac mutant required the presence of cyclosporin for efficient virus replication. The results indicate that the presence or absence of a type II tight turn adjacent to the primary CyPA-binding site determines whether CyPA incorporation enhances or inhibits viral replication. By demonstrating that CyPA-binding-site residues can induce cyclosporin sensitivity in a heterologous context, this study provides direct in vivo evidence that the exposed loop between helices IV and V of HIV-1 CA not merely constitutes a docking site for CyPA but is a functional target of this cellular protein.

HIV protease substrate conformation: modulation by cyclophilin A

Cyclophilin A (CyPA), a cytosolic peptidyl-prolyl trans-cis isomerase can accelerate the trans-cis isomerization of Xxx-Pro peptide bonds. One- and two-dimensional 1H-NMR spectroscopy were used to determine that the heptapeptide Ser-Gln-Asn-Tyr-Pro-Ile-Val, a model peptide of an HIV-1 protease cleavage site in the gag polyprotein of HIV-1, is a substrate for CyPA. Experiments revealed a slow exchange about the Tyr-Pro peptide bond with 30 +/- 5% in the cis conformation (pH 1-9). While the interconversion rate is too slow to measure by kinetic NMR methods in the absence of CyPA, these methods, saturation transfer and NOE experiments, established that CyPA enhanced the rate of trans-cis interconversion, a process inhibited by cyclosporin A (CsA). With a substrate:CyPA ratio of 40:1, an interconversion rate of 2.5 s(-1) at 25 degrees C was observed.

Active-site residues of cyclophilin A are crucial for its incorporation into human immunodeficiency virus type 1 virions

Human immunodeficiency virus type 1 (HIV-1) incorporates the cellular peptidyl-prolyl cis-trans isomerase cyclophilin A (CyPA), the cytosolic receptor for the immunosuppressant cyclosporin A (CsA). CsA inhibits the incorporation of CyPA and reduces HIV-1 virion infectivity but is inactive against closely related primate lentiviruses that do not interact with CyPA. The incorporation of CyPA into HIV-1 virions is mediated by a specific interaction with a proline-containing, solvent-exposed loop in the capsid (CA) domain of the Gag polyprotein. CsA, which disrupts the interaction with CA, binds at the active site of CyPA. To test whether active-site residues are also involved in the interaction with HIV-1 CA, we used a panel of previously characterized active-site mutants of human CyPA. Expression vectors for epitope-tagged wild-type and mutant CyPA were transfected into COS-gamma cells along with HIV-1 proviral DNA, and the virions produced were analyzed for the presence of tagged proteins. Cotransfection of the wild-type expression vector led to the incorporation of readily detectable amounts of epitope-tagged CyPA into HIV-1 virions. One CyPA mutant with a substantially decreased sensitivity to CsA was incorporated with wild-type efficiency, demonstrating that the requirements for binding to CsA and to HIV-1 CA are not identical. The remaining six CyPA mutants were incorporated with markedly reduced efficiency, providing in vivo evidence that HIV-1 CA interacts with the active site of CyPA.

The hydrophobic pocket of cyclophilin is the binding site for the human immunodeficiency virus type 1 Gag polyprotein

Completion of an early step in the human immunodeficiency virus type 1 (HIV-1) life cycle requires incorporation into virions of the cellular peptidyl-prolyl isomerase cyclophilin A (CyPA) by the Gag polyprotein. Elucidation of the biochemical role of CyPA would be aided by a detailed analysis of the genetic requirements for the formation of the Gag-CyPA complex; previous experiments have demonstrated the requirement for a critical proline and the immediately preceding glycine, located within the capsid domain of Gag, but nothing is known about the necessary CyPA residues. Cyclophilins possess a hydrophobic pocket where proline-containing peptide substrates and the immunosuppressive drug cyclosporine A bind. In this study, we engineered five CyPA mutations, each of which alters a residue that contributes to the hydrophobic pocket. Compared with the wild-type protein, all of the mutants drastically reduced CyPA binding to HIV-1 Gag and similarly inhibited CyPA incorporation into virions. In addition, we demonstrated that previously reported differences between the Gag-binding properties of CyPA and CyPB are due to adventitious association involving residues in the signal sequence of CyPB and that the core domain of CyPB interacts with Gag in a fashion which is indistinguishable from that of CyPA. These studies indicate that, as with other proline-containing peptides or cyclosporine A, HIV-1 Gag directly contacts residues in the hydrophobic pocket of CyPA.

Functional gamma delta T-lymphocyte defect associated with human immunodeficiency virus infections

BACKGROUND

Antiviral cellular immune responses may influence immunological homeostasis in HIV-infected persons. Recent data indicate that V gamma 9/V delta 2 T lymphocytes display potent cytotoxic activities against human cells infected with certain viruses including HIV. Understanding the role of gamma delta T cells in the course of HIV infection may be helpful for designing novel treatment strategies for HIV-associated disorders.

MATERIALS AND METHODS

The constitutive recognition of Daudi cells and monoethyl pyrophosphate (Etpp) by peripheral blood V gamma 9/V delta 2 T cells was assessed using a proliferation assay. The cytotoxicity of Daudi-stimulated lymphocyte populations was measured by chromium release assays. The HIV infectivity for gamma delta T cell clones was determined by measuring the levels of HIV p24 in cell supernatants. The effect of in vitro HIV-infection on cytokine mRNA production by gamma delta T cell clones was assessed by PCR.

RESULTS

The constitutive proliferative responses of peripheral blood V gamma 9/V delta 2 T cells and the lytic functions of Daudi-expanded lymphoid cells from HIV+ persons were substantially diminished in comparison with those of HIV-seronegative persons. These alterations were present in asymptomatic HIV+ persons prior to substantial alpha beta CD4+ T cell loss. Productive HIV infection of gamma delta T cells in vitro had no measurable effect either on their proliferative response to Daudi stimuli or on the expression of cytokine mRNAs for IFN-gamma, IL-2, IL-4, IL-5, IL-6, IL-10, and IL-13.

CONCLUSIONS

The constitutive responsiveness of V gamma 9/V delta 2 T lymphocytes to Daudi and Etpp is severely altered in HIV+ persons. HIV infection of gamma delta T cells in vitro does not substantially change their cytokine expression or antigenic response.

Extended binding sites of cyclophilin as revealed by the interaction with HIV-1 Gag polyprotein derived oligopeptides

Oligopeptides derived from the gag polyprotein (Pr55gag) of human immunodeficiency virus type 1 (HIV-1) segment were used to evaluate the extension of the putative binding region for the complex of Pr55gag and the human cytosolic peptidyl prolyl cis/trans isomerase (PPIase) 18 kDa cyclophilin (Cyp18). Five N-terminally acetylated, C-terminally amidated oligopeptides containing one (HIV-1 Gag218-224; 1), two (HIV-1 Gag218-226 and HIV-1 Gag217-224; 2 and 3, respectively), three (HIV-1 Gag217-226; 4) or four (HIV-1 Gag213-237; 5) proline residues were synthesized. Using competition experiments with a standard substrate the binding affinities to Cypl8 of the synthesized peptides were determined. The IC50 value of 184 microM for the 25-mer peptide 5 was fivefold or more lower than those of the peptides 1-4 lacking one or more prolines. Failure of competition in assays containing enzymes of other PPIase families by millimolar concentrations of 5 revealed a Cyp18 specific interaction involving the active site of the enzyme. In its far UV circular dichroism, aqueous solutions of 5 display properties of random coil conformation, but spectra were also consistent with a small contribution of proline specific secondary structures. However, a proline-rich peptide typical of forming left-handed polyproline II helices did not compete for the active site of Cypl8. The results demonstrate that the putative binding region of HIV-1 gag polyprotein has a certain degree of binding affinity to the PPIase site of Cyp18, and may add a previously unrecognized topological component to the known subsite specificity of cyclophilins.

The human immunodeficiency virus type 1 capsid p2 domain confers sensitivity to the cyclophilin-binding drug SDZ NIM 811

Human immunodeficiency virus type 1 (HIV-1) specifically incorporates the host cell peptidyl-prolyl isomerase cyclophilin A into virions via contacts with the capsid (CA) domain of the Gag polyprotein Pr55gag. The immunosuppressant drug cyclosporin A and the nonimmunosuppressive cyclosporin A analog SDZ NIM 811 bind to cyclophilin A and inhibit its incorporation into HIV-1 virions. Both drugs inhibit the virion association of cyclophilin A and the replication of HIV-1 with a similar dose dependence. In contrast, these compounds are inactive against other primate lentiviruses which do not incorporate cyclophilin A, such as simian immunodeficiency virus (SIV). To locate determinants which confer sensitivity to SDZ NIM 811, we generated chimeric proviruses between HIV-1 and SIVmac. A hybrid SIVmac which has the CA-p2 domain of the Gag polyprotein replaced by the corresponding domain from HIV-1 replicated in an established CD4+ cell line and in human but not macaque peripheral blood mononuclear cells. The transfer of the HIV-1 CA-p2 domain to SIVmac led to the efficient incorporation of cyclophilin A, and SDZ NIM 811 effectively inhibited both the virion association of cyclophilin A and the spread of the hybrid virus in infected cultures. We conclude that the HIV-1 CA-p2 domain contains determinants which confer the necessity to interact with cyclophilin A for efficient virus replication. Furthermore, our data show that the CA-p2 domain can play a crucial role in species tropism.

Cyclosporine A-resistant human immunodeficiency virus type 1 mutants demonstrate that Gag encodes the functional target of cyclophilin A

The cellular peptidyl-prolyl isomerase cyclophilin A is incorporated into human immunodeficiency virus type 1 virions via contacts with the proline-rich domain of the Gag polyprotein. Cyclosporine A and nonimmunosuppressive analogs bind with high affinity to cyclophilin A, compete with Gag for binding to cyclophilin A, and prevent incorporation of cyclophilin A into virions; in parallel with the disruption of cyclophilin A incorporation into virions, there is a linear reduction in the initiation of reverse transcription after infection of a T cell. Passage of human immunodeficiency virus type 1 in the presence of the drug selects one of two mutations, either of which alters the proline-rich domain of Gag and is sufficient to confer drug resistance on the cloned wild-type provirus. Neither mutation alters Gag's cyclophilin A-binding properties in vitro, and cyclophilin A incorporation into drug-resistant virions is effectively disrupted by cyclosporine A, indicating that the drug-resistant mutants do not require virion-associated cyclophilin A to initiate infection. That Gag's functional dependence on cyclophilin A can be differentiated genetically from its ability to bind cyclophilin A is further demonstrated by the rescue of a mutation precluding cyclophilin A packaging by a mutation conferring cyclosporine A resistance. These experiments demonstrate that, in addition to its ability to package cyclophilin A into virions, gag encodes the functional target of cyclophilin A.

Binding of the human immunodeficiency virus type 1 Gag polyprotein to cyclophilin A is mediated by the central region of capsid and requires Gag dimerization

The cellular peptidyl-prolyl isomerase cyclophilin A (CyPA) is incorporated into human immunodeficiency virus type 1 (HIV-1) virions via direct contacts with the HIV-1 Gag polyprotein. Disruption of the Gag-CyPA interaction leads to the production of HIV-1 particles lacking CyPA; these virions are noninfectious, indicating that contacts between CyPA and Gag are necessary for HIV-1 replication. Here, we have used the yeast two-hybrid system in conjunction with an in vitro binding assay to identify the minimal domain of Gag required for binding to CyPA. Analysis of a panel of gag deletion mutants in the two-hybrid system indicated that a region spanning the central portion of the capsid (CA) domain was sufficient for interactions with CyPA, but discrepancies between results obtained in different fusion protein contexts suggested that multimerization of Gag might also be necessary for binding to CyPA. Consistent with a requirement for multimerization, the binding of Gag to CyPA in vitro required a region within the nucleocapsid (NC) domain shown previously to be important for Gag self-association. Substitution of a heterologous dimerization motif for the region from NC also promoted specific binding to CyPA, confirming that interactions with CyPA are dependent on Gag multimerization. Fusion of the heterologous dimerization motif to a 100-amino-acid domain from CA was sufficient for binding to CyPA in vitro. These results define the minimal CyPA-binding domain within Gag and provide insight into the mechanism by which CyPA is incorporated into HIV-1 virions.

Cyclophilin A is required for the replication of group M human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus SIV(CPZ)GAB but not group O HIV-1 or other primate immunodeficiency viruses

The human immunodeficiency virus type 1 (HIV-1) Gag polyprotein binds to cyclophilin A and incorporates this cellular peptidyl prolyl-isomerase into virions. Disruption of cyclophilin A incorporation, either by gag mutations or by cyclosporine A, inhibits virion infectivity, indicating that cyclophilin A plays an essential role in the HIV-1 life cycle. Using assays for packaging of cyclophilin A into virions and for viral replication sensitivity to cyclosporine A, as well as information gleaned from the alignment of Gag residues encoded by representative viral isolates, we demonstrate that of the five lineages of primate immunodeficiency viruses, only HIV-1 requires cyclophilin A for replication. Cloned viral isolates from clades A, B, and D of HIV-1 group M, as well as a phylogenetically related isolate from chimpanzee, all require cyclophilin A for replication. In contrast, the replication of two outlier (group O) HIV-1 isolates is unaffected by concentrations of cyclosporine A which disrupt cyclophilin A incorporation into virions, indicating that these viruses are capable of replicating independently of cyclophilin A. These studies identify the first phenotypic difference between HIV-1 group M and group O and are consistent with phylogenetic studies suggesting that the two HIV-1 groups were introduced into human populations via separate zoonotic transmission events.