Pyrimidine tract binding protein and La autoantigen interact differently with the 5' untranslated regions of lentiviruses and oncoretrovirus mRNAs.

Retrovirus genomic mRNA exhibits a several hundred nucleotides-long untranslated region (5' UTR) which encloses many control elements required for retrovirus replication. In addition, this 5' UTR contains translation regulatory elements, such as internal ribosome entry sites (IRESes) that have been described in oncoretroviruses, as well as in lentiviruses. UV cross-linking experiments suggested that the pyrimidine tract binding protein (PTB), a cellular protein known to regulate the activity of several picornaviral IRESes, binds to human T-cell leukemia virus (HTLV)-I RNA but not to lentiviral human immunodeficiency virus (HIV)-1, HIV-2 or simian immunodeficiency virus RNAs. To calculate the affinity of such RNA-protein interactions, we developed a new method based on the BIAcore technology. The absence of affinity of PTB for lentiviral RNAs was confirmed, whereas its affinity for HTLV-I RNAs was 1000-fold lower than for picornaviral RNAs. The BIAcore technology also revealed a significant affinity of the La autoantigen, previously described for its involvement in translational control of viral mRNAs, for HIV-1 and HTLV-I RNAs. Addition of recombinant PTB to in vitro translation experiments weakly enhanced translation initiation in the presence of HTLV-I IRES, suggesting that such an IRES requires additional trans-acting factors.

Alternative translation initiation of the Moloney murine leukemia virus mRNA controlled by internal ribosome entry involving the p57/PTB splicing factor.

Moloney murine leukemia virus (Mo-MuLV) genomic mRNA codes for two gag precursors by alternative initiations of translation. An AUG codon governs the synthesis of the retroviral capsid proteins precursor, whereas a CUG codon directs the synthesis of a glycosylated cell surface antigen, the gross cell surface antigen. Control of the relative synthesis of the two precursors is crucial for MuLV infectivity and pathology. Furthermore, the MuLV mRNA leader sequence is very long and should inhibit translation according to the classical scanning model. This suggests a different translation initiation mechanism allowing gag efficient expression. We demonstrate, by using bicistronic vectors expressed in COS-7 cells, that the Mo-MuLV mRNA leader drives translation initiation by internal ribosome entry. We have localized the internal ribosome entry site (IRES) between the two initiation codons. This 126 nucleotide long IRES implies an oligopyrimidine tract located 45 nucleotides upstream of AUG codon. UV cross-linking and affinity chromatography experiments show that the PTB/p57 splicing factor specifically interacts with this oligopyrimidine tract. The MuLV IRES controls alternative translation initiation by activating the capsid protein precursor expression. This gag translational enhancer could exist in other retroviruses.

Experimental study of transplacental passage of alpha interferon by two assay techniques.

Two methods of assaying alpha interferon (IFN-alpha) were compared during an experiment aimed at determining whether IFN-alpha crosses the human placenta. Human placentas, collected after delivery following a normal pregnancy to term, were catheterized on both sides: fetal and maternal. The IFN-alpha was introduced in known amounts in the maternal circulation and was assayed in the efferent fetal fluid. The following two detection methods were used: radioimmunoassay by competition with [125I]IFN-alpha and assay with a biological system in which IFN-alpha protected Madin-Darby bovine kidney cells from destruction by vesicular stomatitis virus. The results obtained by the two methods were in perfect agreement for the efferent fetal fluid samples. They showed the absence of placental transfer of IFN-alpha. The biological method was found to be more sensitive than radioimmunoassay for low IFN-alpha titers (< 10 IU/ml) but was less reproducible, probably owing to the use of twofold dilutions. The specificities of the two methods were similar and their practicalities were equivalent; the biological method, however, was less costly. The study illustrates the complementarity of the two methods, which were based on different principles. The agreement obtained between the two methods provides a clear confirmation of the experimental results.

[Study of the transplacental transfer of interferon alfa-2a on the model of isolated perfused cotyledon]. / Etude du passage transplacentaire de l'interféron alpha 2a sur le modèle du cotylédon isolé perfusé.

Interferon can be used for VIH+ pregnant women, to decrease materno-fetal contamination. Added to maternal circulation, its behaviour was studied by human placental cotyledon ex vivo perfusion. Human recombinant IFN 2a and reference substance 3H2O were injected in intervillous chamber and their behaviours in venous fetal and maternal circulations was followed. At steady state, in fetal circulation 3H2O concentration was 37% of injected rate whereas no IFN transfer rate was observed. In both venous circulations IFN amounts were lower than injected ones 56% versus of 82% for water (p < 0.05). IFN didn't cross placental filter and disappeared partially during placental contact.