Cryo-EM reveals a novel octameric integrase structure for betaretroviral intasome function.

Retroviral integrase catalyses the integration of viral DNA into host target DNA, which is an essential step in the life cycle of all retroviruses. Previous structural characterization of integrase-viral DNA complexes, or intasomes, from the spumavirus prototype foamy virus revealed a functional integrase tetramer, and it is generally believed that intasomes derived from other retroviral genera use tetrameric integrase. However, the intasomes of orthoretroviruses, which include all known pathogenic species, have not been characterized structurally. Here, using single-particle cryo-electron microscopy and X-ray crystallography, we determine an unexpected octameric integrase architecture for the intasome of the betaretrovirus mouse mammary tumour virus. The structure is composed of two core integrase dimers, which interact with the viral DNA ends and structurally mimic the integrase tetramer of prototype foamy virus, and two flanking integrase dimers that engage the core structure via their integrase carboxy-terminal domains. Contrary to the belief that tetrameric integrase components are sufficient to catalyse integration, the flanking integrase dimers were necessary for mouse mammary tumour virus integrase activity. The integrase octamer solves a conundrum for betaretroviruses as well as alpharetroviruses by providing critical carboxy-terminal domains to the intasome core that cannot be provided in cis because of evolutionarily restrictive catalytic core domain-carboxy-terminal domain linker regions. The octameric architecture of the intasome of mouse mammary tumour virus provides new insight into the structural basis of retroviral DNA integration.

Impact of combination antiretroviral therapy in the NOD.c3c4 mouse model of autoimmune biliary disease.

BACKGROUND & AIMS: The NOD.c3c4 mouse model develops autoimmune biliary disease characterized by spontaneous granulomatous cholangitis, antimitochondrial antibodies and liver failure. This model for primary biliary cirrhosis (PBC) has evidence of biliary infection with mouse mammary tumour virus (MMTV), suggesting that the virus may have a role in cholangitis development and progression of liver disease in this mouse model. We tested the hypothesis that MMTV infection is associated with cholangitis in the NOD.c3c4 mouse model by investigating whether antiretroviral therapy impacts on viral levels and liver disease. METHODS: NOD.c3c4 mice were treated with combination antiretroviral therapy. Response to treatment was studied by measuring MMTV RNA in the liver, liver enzyme levels in serum and liver histology using a modified Ishak score. RESULTS: Combination therapy with the reverse transcriptase inhibitors, tenofovir and emtricitabine, resulted in a significant reduction in serum liver enzyme levels, attenuation of cholangitis and decreased MMTV levels in the livers of NOD.c3c4 mice. Furthermore, treatment with the retroviral protease inhibitors, lopinavir and ritonavir, in addition to the reverse transcriptase inhibitors, resulted in further decrease in MMTV levels and attenuation of liver disease in this model. CONCLUSIONS: The attenuation of cholangitis with regimens containing the reverse transcriptase inhibitors, tenofovir and emtricitabine, and the protease inhibitors, lopinavir and ritonavir, suggests that retroviral infection may play a role in the development of cholangitis in this model.

Assembly mechanisms of RNA pseudoknots are determined by the stabilities of constituent secondary structures.

Understanding how RNA molecules navigate their rugged folding landscapes holds the key to describing their roles in a variety of cellular functions. To dissect RNA folding at the molecular level, we performed simulations of three pseudoknots (MMTV and SRV-1 from viral genomes and the hTR pseudoknot from human telomerase) using coarse-grained models. The melting temperatures from the specific heat profiles are in good agreement with the available experimental data for MMTV and hTR. The equilibrium free energy profiles, which predict the structural transitions that occur at each melting temperature, are used to propose that the relative stabilities of the isolated helices control their folding mechanisms. Kinetic simulations, which corroborate the inferences drawn from the free energy profiles, show that MMTV folds by a hierarchical mechanism with parallel paths, i.e., formation of one of the helices nucleates the assembly of the rest of the structure. The SRV-1 pseudoknot, which folds in a highly cooperative manner, assembles in a single step in which the preformed helices coalesce nearly simultaneously to form the tertiary structure. Folding occurs by multiple pathways in the hTR pseudoknot, the isolated structural elements of which have similar stabilities. In one of the paths, tertiary interactions are established before the formation of the secondary structures. Our work shows that there are significant sequence-dependent variations in the folding landscapes of RNA molecules with similar fold. We also establish that assembly mechanisms can be predicted using the stabilities of the isolated secondary structures.

Simultaneous detection of reverse transcriptase and high molecular weight RNA unique to oncogenic RNA viruses.

The simultaneous assay of the reverse transcriptase and 60S to 70S RNA of oncogenic RNA viruses is described. The virus can be detected at low concentrations in biological fluids containing enzymatically active cell fragments or other contaminants. The fact that two features diagnostic of the oncornaviruses are used in the tests increases the certainty with which a positive outcome can be interpreted.

HIV protease cleaves poly(A)-binding protein.

The PABP [poly(A)-binding protein] is able to interact with the 3' poly(A) tail of eukaryotic mRNA, promoting its translation. Cleavage of PABP by viral proteases encoded by several picornaviruses and caliciviruses plays a role in the abrogation of cellular protein synthesis. We report that infection of MT-2 cells with HIV-1 leads to efficient proteolysis of PABP. Analysis of PABP integrity was carried out in BHK-21 (baby-hamster kidney) and COS-7 cells upon individual expression of the protease from several members of the Retroviridae family, e.g. MoMLV (Moloney murine leukaemia virus), MMTV (mouse mammary tumour virus), HTLV-I (human T-cell leukaemia virus type I), SIV (simian immunodeficiency virus), HIV-1 and HIV-2. Moreover, protease activity against PABP was tested in a HeLa-cell-free system. Only MMTV, HIV-1 and HIV-2 proteases were able to cleave PABP in the absence of other viral proteins. Purified HIV-1 and HIV-2 proteases cleave PABP1 directly at positions 237 and 477, separating the two first RNA-recognition motifs from the C-terminal domain of PABP. An additional cleavage site located at position 410 was detected for HIV-2 protease. These findings indicate that some retroviruses may share with picornaviruses and caliciviruses the capacity to proteolyse PABP.

Virion-associated and cellular RNA methylase activity in normal and neoplastic mammary tissue from mammary tumor virus-infected and -uninfected mice.

A comparison of cellular RNA methylase activities and patterns between normal and neoplastic mouse mammary tissue indicated the following. The rna methylases of mammary tumor tissue extracts are qualitatively different from those of normal lactating mammary tissue, based on differences in extent of methylation; the normal lactating tissue extracts have a greater capacity of methylate RNA than do the tumor extracts studied to date. There is no correlation between capacity and either the malignant state or the etiological agent. There is a qualitative effect on methylation patterns attributable to the presence of virus. Finally, both the etiological agent, mouse mammary tumor virus, and its putative nucleoprotein core, intracytoplasmic A particles, have a N-2-guanine RNA methyltransferase integrally associated with them. These conclusions are consistent with the aberrant nucleic acid methylation hypothesis, with the reservation that aberrant does not imply hypermethylation.

Experimental infection of a cat kidney cell line with the mouse mammary tumor virus.

A cat kidney cell line, CRFK-F2, was successfully inoculated in suspension and in monolayer culture with a purified mouse mammary tumor virus derived from RIII milk. The virus produced by the infected cells was identified by immunogluorescence, electron microscopy, and RNA-directed DNA polymerase assays; it was a B-type virion that did not cross-react with mouse or feline leukemia-sarcoma viruses, had spikes on its envelope, and had a RNA-directed DNA polymerase reaction that was typical of mouse mammary tumor virus. The producing cells were identified as cat cells by chromosome number, cytotoxic assays, and isoenzyme migratory patterns. A standardized method for the in vitro inoculation of cat cells is described that presently permits highly reproducible results. For the first time, the mouse mammary tumor virus is seen replicating in cells from another species, thus offering an opportunity to study the kinetics of infection of that virus.

Matrilysin (matrix metalloproteinase-7) selects for apoptosis-resistant mammary cells in vivo.

Overexpression of the matrix metalloproteinase matrilysin (matrix metalloproteinase-7) in the mouse mammary gland promotes mammary hyperplasia and accelerates the onset of oncogene-induced mammary tumors. In cell culture models, acute exposure of cells coexpressing Fas and Fas ligand (FasL) to matrilysin induces apoptosis, whereas chronic exposure to matrilysin selects for apoptosis-resistant cells. We now demonstrate that matrilysin promotes resistance to apoptosis in vivo. Matrilysin expression increased apoptosis in the involuting mammary gland of mice that had undergone a single pregnancy and lactation cycle. Premature basement membrane disruption was detected in matrilysin-expressing mice, which could account for the increase in apoptosis. However, multiparous mice, in which the involuting mammary epithelial cells have been repeatedly exposed to matrilysin, show a significant decrease in apoptosis. Mammary tissue from multiparous matrilysin-expressing mice showed decreased FasL expression, suggesting that loss of FasL is at least one mechanism of matrilysin-induced resistance to apoptosis. We propose that matrilysin promotes mammary tumor formation by enhancing the selection of cells that are resistant to apoptosis.

Characterization of mouse mammary tumor viruses propagated in heterologous cells.

Mouse mammary tumor viruses (MMTV) from three different strains of mice have been used to establish productive infections in feline and mink cell lines. The virions that are released by these cells compete completely in a radioimmunoassay for the major virion surface glycoprotein of MMTV (gp52), thus demonstrating that antigenic determinants of gp52 are viral coded. Competitive molecular hybridization studies have shown that the 60 to 70 S RNA's of MMTV's propagated in feline cells contain all the nucleic acid sequences found in 60 to 70 S RNA from MMTV synthesized by murine cells. The virion buoyant densities in sucrose and cesium chloride, virion sedimentation coefficient, divalent cation requirement of the virion DNA polymerase, and morphology of MMTV's synthesized in heterologous cells are similar to those of MMTV's grown in murine cells. Cultures of MMTV-infected feline cells have continuously released between 0.1 and 1.0 microgram of virus per 10(7) cells (75-sq cm flask) per day during the 60-week observation period. No detectable feline or murine type C viruses were produced by these cultures.

Accelerated growth of mammary tumor cells in normal and athymic mice after treatment in vitro with dexamethasone.

The tumorigenicity of dexamethasone-treated tissue-cultured mammary tumor cells was compared to that of untreated mammary tumor cells. The dexamethasone-induced cells progressed faster in normal syngeneic animals than did untreated mammary tumor cells. The fact that the growth rate differential was also observed in athymic mice suggests that T-lymphocyte-mediated immunity was not a factor in the accelerated progression in vivo of the tumor cells that had been cultured in the presence of dexamethasone.

Identification, partial purification and biochemical characterization of gamma-glutamyltranspeptidase present as a membrane component in skimmed milk and milk fat-globule membranes, and in mammary-tumour virus from the milk of infected mice.

The enzyme gamma-glutamyltranspeptidase was reproducibly found to be associated with mouse milk particles; it is present in milk fat-globule membranes and mouse mammary-tumour virus of infected Swiss mice, also in particles from the milk of uninfected mice. The enzymatic activities observed range among the highest reported for mammalian tissues. The enzyme was partially purified from mouse mammary-tumour virus, and from milk fat-globule membranes. The molecule requires the presence of detergents to remain soluble, behaves as a high molecular weight component, properties characterizing integral membrane proteins. Kinetics, and the effect of competitors as well as of specific inhibitors show this enzyme to be identical to the well-known kidney gamma-glutamyltranspeptidase ((gamma-glutamyl)-peptide:amino-acid gamma-glutamyltransferase, EC 2.3.2.2). Other oncornaviruses budding from cultured cells originally expressing the enzyme in their plasma membrane also incorporate the enzyme in their structure.

Kinetic properties and stereospecificity of the monomeric dUTPase from herpes simplex virus type 1.

Kinetic properties of the monomeric enzyme dUTPase from herpes simplex virus type 1 (HSV) were investigated and compared to those previously determined for homotrimeric dUTPases of bacterial and retroviral origins. The HSV and Escherichia coli dUTPases are equally potent as catalysts towards the native substrate dUTP with a kcat/K(M) of about 10(7) M(-1) s(-1) and a K(M) of 0.3 microM. However, the viral enzymes are less specific than the bacterial enzyme. The HSV and E. coli dUTPases show the same stereospecificity towards the racemic substrate analogue dUTPalphaS (2'-deoxyuridine 5'-(alpha-thio)triphosphate), suggesting that they have identical reaction mechanisms.

Characterization of deoxyuridine 5'-triphosphate nucleotidohydrolase from Trypanosoma cruzi.

We report the cloning and kinetic characterization of Trypanosoma cruzi deoxyuridine 5'-triphosphate nucleotidohydrolase (dUTPase) whose coding sequence was isolated by genetic complementation in Escherichia coli. The deduced amino acid sequence was similar to Leishmania major dUTPase although it exhibits an amino acid insertion which is sensitive to protease inactivation. The catalytically active species of the enzyme is a dimer and a detailed kinetic characterization showed that it is highly specific for dUTP and dUDP. The general observation that dUTPases from the Trypanosomatidae differ in sequence, conformation and substrate specificity suggests that a different family of dUTPases exists in certain organisms, which may be exploited as drug targets against infectious diseases.

Polyclonal and monoclonal antibodies monospecific to MMTV LTR orf protein produced in E. coli.

Monoclonal and polyclonal antibodies specific to an open reading frame of the mouse mammary tumor virus long terminal repeat were generated using an open reading frame-beta-galactosidase fusion protein produced in E. coli. Both antibodies reacted with the open reading frame-beta-galactosidase fusion protein but not with beta-galactosidase alone using an immunoblotting technique. It is concluded that these antibodies were specific for the protein encoded by the open reading frame of the mouse mammary tumor virus long terminal repeat.

Differential regulation of mouse mammary tumor virus-bacterial chloramphenicol acetyltransferase chimeric gene by human mineralocorticoid hormone-receptor complexes.

The brain tissues of the rat and mouse express two types of corticosteroid binding proteins, the glucocorticoid (GR) and aldosterone (MR) receptors. Unlike the type II (GR) receptor, type I receptor has a high affinity for aldosterone (ALDO) and corticosterone and is structurally similar to the kidney mineralocorticoid receptor (MR). The results reported in this study provide direct evidence for the interaction of dexamethasone (DEX), triamcinolone acetonide (TA), dexamethasone-21-mesylate (DXM) and 11-deoxycorticosterone (DOC) with human MR expressed in cells by transient co-transfection of a hMR expression vector. The interactions of hMR with DEX, TA, DXM, DOC, promegestone (R5020) and methyltrienelone (R1881) were measured by trans-activation of mouse mammary tumor virus long terminal repeat fused to bacterial chloramphenicol acetyltransferase (MMTV-tk-CAT) in gene co-transfection experiments and by cell free hormone binding assay. The incubation of various steroid hormones in the presence of [3H]ALDO in a competition assay with extracts prepared from HeLa cells co-transfected with hMR expression vector, showed that hMR expressed under these conditions has a high relative affinity for DEX which is similar to ALDO, TA and DOC. Incubation with DXM under these conditions showed very little competition, as was observed with R1881 and R5020. Incubation of the co-transfected cells with DEX, ALDO, DOC, R5020, TA, R1881 and DXM demonstrated that the level of trans-activation did not reflect the previously observed order of binding affinity for the hMR. The level of transactivation was always higher with DEX and TA compared to ALDO and DOC. Analysis of the binding of labeled glucocorticoid regulatory element (GRE) and hMR incubated with DEX, ALDO and DXM by gel shift analysis demonstrated that the trans-activation of MMTV-tk-CAT by hMR is a result of the interaction of hMR with GRE in the MMTV-LTR.

Pharmacological and functional characterization of human mineralocorticoid and glucocorticoid receptor ligands.

We characterized the pharmacological profiles of the human mineralocorticoid and glucocorticoid receptor for 11 natural and synthetic steroids regarding binding pharmacology, intracellular localization of hormone-receptor complexes, and agonistic or antagonistic properties at the gene expression level. The sex steroid progesterone bound with an affinity (ki < 0.01 nM) even higher than that of aldosterone to the human mineralocorticoid receptor and effectively antagonized the effect of aldosterone via the human mineralocorticoid receptor in functional co-transfection assays. This indicates that progesterone has potent antimineralocorticoid properties, while its antiglucocorticoid effects were less pronounced. The partial agonistic activities of antihormones in this assay suggest a direct interaction of antihormone-receptor complexes with the response elements on the DNA. These results are supported by immunofluorescence studies, in which both unliganded human mineralocorticoid and glucocorticoid receptors were distributed throughout the cytoplasm and nucleus, whereas agonist- as well as antagonist-receptor complexes showed an exclusively nuclear localization. These results contribute to the understanding of antihormone pharmacology and increase our understanding of the role of human mineralocorticoid and glucocorticoid receptors in physiological processes during different endocrine states.

Inhibition of androgen action by dehydroepiandrosterone sulfotransferase transfected in PC-3 prostate cancer cells.

Age-dependent loss of androgen sensitivity of the rat liver is associated with a marked increase in dehydroepiandrosterone/hydroxysteroid sulfotransferase (rStd) activity. Sulfonated steroid hormones are known to be ineffective in binding receptor proteins. These observations suggest that intracellular androgen sulfonation can physiologically influence androgen action. We have examined the inhibitory effect of rStd on androgen action in the human prostate cancer-derived PC-3 cells transfected with the rat androgen receptor (AR) expression plasmid and two androgen-responsive promoter reporter constructs (murine mammary tumor long-terminal repeat ligated to chloramphenicol acetyltransferase (CAT) gene and rat probasin androgen response element (ARE) ligated to firefly luciferase (LUC) gene). These transfected cells were dependent on 5alpha-dihydrotestosterone (DHT) for the activation of both reporter genes and showed about a 200- and a 800-fold increase of CAT and LUC activity, respectively, at 10(-10) M DHT over the no-hormone control. Expression of the sulfonating enzyme in this cell transfection system via the rStd expression plasmid caused a dose-dependent decline in the reporter activity with approximately 90% inhibition of androgen action at a rStd:AR plasmid ratio of 100. From these results we conclude that irrespective of a high level of AR, changes in the Std expression can markedly alter the androgen sensitivity of target cells.

[Analysis of effectiveness of cDNA synthesis, induced using complementary primers and primers containing a noncomplementary base matrix]. / Analiz éffektivnost' sinteza kDNK, initsiirovannogo s komplementarnykh praimerov i praimerov, soderzhashchikh nekomplementarnye matritse osnovaniia.

We have studied the efficiency of DNA synthesis catalyzed by M-MLV reverse transcriptase or Thermus aquaticus DNA polymerase for primers (4-17 nucleotides long) either completely matched or possessing a single mismatched base pair at all possible positions in the primer. It has been shown that DNA synthesis efficiency depends not only on the position of mismatched base pair but on the length and primary structure of the primer. The enzyme, template, and primer concentrations determine the relative level of mismatched DNA synthesis.

[Derivatives of ddUTP, modified at the 5-position of uridine, as substrate terminators of reverse transcriptase. Hydrolysis of oligonucleotides, terminated by these analogs, by phosphodiesterase I]. / Proizvodnye ddUTP, modifitsirovannye v 5-polozhenii uridina, kak substratnye terminatory obratnykh transkriptaz. Gidroliz oligonukleotidov, terminirovannykh étimi analogami, fosfodiésterazoi I.

Synthesis of 2',3'-dideoxyuridine 5'-triphosphate analogues with fluorescent and biotin residues at C5 of uracil base was carried out. The substrate properties of these analogues were studied with AMV, M-MLV, and HIV reverse transcriptase. All 5-derivatives studied were shown to be incorporated into the 3'-terminus of oligonucleotide. The stability of oligodeoxyribonucleotides terminated with ddUTP analogues modified at the 5-position of the pyrimidine residue to the exonuclease action of phosphodiesterase I and Klenow enzyme was more than 1000 times higher than that of nonterminated oligonucleotides.