Overexpression of the influenza virus polymerase can titrate out inhibition by the murine Mx1 protein.

The murine Mx1 protein is an interferon-inducible protein which confers selective resistance to influenza virus infection both in vitro and in vivo. The precise mechanism by which the murine Mx1 specifically inhibits replication of influenza virus is not known. Previously, sensitive replication systems for influenza virus ribonucleoprotein, in which a synthetic influenza virus-like ribonucleoprotein is replicated and transcribed by influenza virus proteins provided in trans, have been developed. With these systems, the antiviral activity of the murine Mx1 protein was examined. It was found that continued expression of influenza polymerase polypeptides via vaccinia virus vectors can titrate out the inhibitory action of the murine Mx1 protein. This titration of inhibitory activity also occurs when the viral PB2 protein alone is overexpressed, suggesting that an antiviral target for the murine Mx1 polypeptide is the viral PB2 protein.     

Susceptibility of inbred strains of mice to murine AIDS (MAIDS) correlates with target cell expansion and high expression of defective MAIDS virus.

Murine AIDS (MAIDS) is readily induced by the Duplan strain of defective murine leukemia virus in susceptible C57BL/6 mice. To identify mouse strains resistant to MAIDS, and to understand the genetic factors controlling susceptibility to the disease, we screened more than 20 inbred strains of mice for their susceptibility to MAIDS. For this study, mice of the Fv-1n/n, Fv-1b/b, or Fv-1n/b genotype were inoculated with stocks of defective MAIDS virus pseudotyped with N-tropic, B-tropic, or NB-tropic helper murine leukemia virus, respectively. Strains could be classified as susceptible, resistant, or moderately resistant. None of the individual H-2 haplotypes examined appears to explain resistance to MAIDS by itself. However, a very good correlation between the susceptibility or resistance phenotype and the presence or absence of defective proviral DNA and RNA in the spleen of these animals was found. Since the presence of defective proviral DNA and RNA reflects the oligoclonal proliferation of the cells infected by the defective MAIDS virus, our results strongly suggest that this target cell expansion is genetically controlled and is necessary and perhaps even sufficient for the development of the disease.     

Virions of primary human immunodeficiency virus type 1 isolates resistant to soluble CD4 (sCD4) neutralization differ in sCD4 binding and glycoprotein gp120 retention from sCD4-sensitive isolates.

Primary isolates of human immunodeficiency virus type 1 (HIV-1) are much less sensitive to neutralization by soluble CD4 (sCD4) and sCD4-immunoglobulin (Ig) chimeras (CD4-IgG) than are HIV-1 strains adapted to growth in cell culture. We demonstrated that there are significant reductions (10- to 30-fold) in the binding of sCD4 and CD4-IgG to intact virions of five primary isolates compared with sCD4-sensitive, cell culture-adapted isolates RF and IIIB. However, soluble envelope glycoproteins (gp120) derived from the primary isolate virions, directly by detergent solubilization or indirectly by recombinant DNA technology, differed in affinity from RF and IIIB gp120 by only one- to threefold. The reduced binding of sCD4 to these primary isolate virions must therefore be a consequence of the tertiary or quaternary structure of the envelope glycoproteins in their native, oligomeric form on the viral surface. In addition, the rate and extent of sCD4-induced gp120 shedding from these primary isolates was lower than that from RF. We suggest that reduced sCD4 binding and increased gp120 retention together account for the relative resistance of these primary isolates to neutralization by sCD4 and CD4-IgG and that virions of different HIV-1 isolates vary both in the mechanism of sCD4 binding and in subsequent conformational changes in their envelope glycoproteins.     

Interaction of synthetic glycophospholipids with phospholipid bilayer membranes.

A series of glycophospholipids synthesized by coupling mono-, di-, or tri-saccharides to dioleoylphosphatidylethanolamine (DOPE) by reductive amination was used to investigate the interaction of glycophospholipids with phospholipid bilayer membranes. These synthetic glycophospholipids functioned as a stabilizer for the formation of DOPE bilayer vesicles. The minimal mol% of glycophospholipid needed to stabilize the DOPE vesicles were as follows: 8% N-neuraminlactosyl-DOPE (NANL-DOPE), 20% N-maltotriosyl-DOPE (MAT-DOPE), 30% N-lactosyl-DOPE (Lac-DOPE), and 42% N-galactosyl-DOPE (Gal-DOPE). The estimated hydration number of glycophospholipid in reverse micelles was 87, 73, 46, and 14 for NANL-DOPE, MAT-DOPE, Lac-DOPE, and Gal-DOPE, respectively. Thus, the hydration intensity of the glycophospholipid was directly related to the ability to stabilize the DOPE bilayer phase for vesicle formation. Glycophospholipids also reduced the transition temperature from gel to liquid-crystalline phase (Tm) of dipalmitoylphosphatidylcholine (DPPC) bilayers. Interestingly, incorporation of NANL-DOPE induced a decrease of membrane fluidity of DPPC bilayers in the gel phase while other glycophospholipids had no effect. Also, low level of NANL-DOPE but not other glycophospholipids increased the transition temperature (TH) from liquid-crystalline to hexagonal phase of dielaidoylphosphatidylethanolamine bilayers. These results showed that NANL-DOPE with a highly hydratable headgroup which provides a strong stabilization activity for the L alpha phase of phospholipid membranes, may also be involved in specific interactions with neighboring phospholipids via its saccharide moiety.     

Effect of cationic cholesterol derivatives on gene transfer and protein kinase C activity.

Four different cationic derivatives of cholesterol were synthesized which contain either a tertiary or a quaternary amino head group, with and without a succinyl spacer-arm. Their ability to inhibit protein kinase C (PKC) activity was measured in a detergent mixed micellar solution. Derivatives containing a quaternary amino head group were effective inhibitors (Ki approx. 12 and 59 microM) of PKC and derivatives containing a tertiary amino head group were approx. 4-20-fold less inhibitory. Liposomes containing an equimolar mixture of dioleoylphosphatidylethanolamine (DOPE) and a cationic cholesterol derivative were tested for the DNA-mediated transfection activity in mouse L929 cells. Highest activity was found with the derivative with low PKC inhibitory activity and with a succinyl spacer-arm. The transfection activity of this tertiary amine derivative, N,N-dimethylethylenediaminyl succinyl cholesterol was dependent on DOPE as a helper lipid; liposomes containing dioleoylphosphatidylcholine and this derivative had little activity. The transfection protocol of this new cationic liposome reagent was optimized with respect to the ratio of liposome/DNA, dose of the complex and time of incubation with cells. Several adherent cell lines could be efficiently transfected with this liposome reagent without any apparent cytotoxicity. However, the transfection activity was strongly inhibited by the presence of serum components.     

Central peptidergic mechanisms controlling reproductive hormone secretion: novel methodology reveals a role for the natriuretic peptides.

A variety of neural factors can influence reproductive hormone secretion by neuromodulatory actions within the hypothalamus or neuroendocrine actions within the anterior pituitary gland. Passive immunoneutralization and antagonist administration protocols have suggested physiological roles for a number of these factors; however, both experimental approaches have severe technical limitations. We have developed novel methodology utilizing cytotoxin cell targeting with neuropeptides linked to the toxic A chain of the plant cytotoxin ricin. With this methodology we can target and destroy in vivo or in vitro cells bearing receptors for that peptide. Ricin A chain conjugated to atrial natriuretic peptide (ANP), a neuropeptide known to pharmacologically inhibit luteinizing hormone-releasing hormone (LHRH) release, was injected into the cerebroventricular system of intact, cycling rats and ovariectomized rats. Cytotoxin conjugate treatment significantly lengthened the estrous cycle. In ovariectomized rats the luteinizing hormone surge induced by steroid priming was completely inhibited. LHRH content of the median eminences of these rats was not significantly altered. These data suggest that ANP binding to clearance receptors in the hypothalamus displaces the C-type natriuretic peptide (CNP) from the shared clearance receptor, making more CNP available to inhibit LHRH release. In the absence of cells bearing the clearance receptor all available CNP binds to the ANPR-B receptor and exerts its effect via an inhibitory interneuron, since LHRH fibers are spared by this treatment.     

Initiation of replication in the Chinese hamster dihydrofolate reductase domain

Two-dimensional (2-D) gel analysis of replication intermediates in the Chinese hamster dihydrofolate reductase domain has suggested that nascent chains can initiate at any of a large number of sites scattered throughout a ～50 kb “initiation locus” (although the level of initiation detected at any given site within this region was relatively low). This result contrasts markedly with data from anin vitro strand switching assay suggesting that >80% of initiations occur within a single 500 bp fragment lying within the initiation locus. In an effort to reconcile these two disparate views of the initiation reaction, we have questioned the validity of our 2-D gel data in several ways. We show here that: 1) the number of replication bubbles detected in the DHFR locus in the early S period is markedly increased when the cells are released from a synchronizing agent that inhibits initiationper se, rather than from aphidicolin, which is a chain elongation inhibitor; 2) initiation in the DHFR domain occurs only during the first 90 min of the S period, as would be expected of an early-firing origin; 3) a pulse of³H-thymidine moves through the structures observed on 2-D gels with the kinetics expected ofbonafide replication intermediates; and 4) preparations of replication intermediates that are subsequently analyzed on 2-D gels appear, by electron microscopy, to represent the typical theta structures and single-forked molecules expected of bidirectional origins of replication; no unusual structures (e.g., microbubbles) were seen.     

Murine p53 inhibits the function but not the formation of SV40 T antigen hexamers and stimulates T antigen RNA helicase activity

We have characterized the effects of p53 on several biochemical activities of simian virus 40 (SV40) large tumor (T) antigen. While p53 induced a strong inhibition of the T antigen DNA helicase activity, surprisingly, its RNA helicase activity was stimulated. This supports the liklihood that the DNA and RNA helicase activities of T antigen reflect discrete functions. p53 did not significantly affect the ATP-dependent conversion of T antigen monomers to hexamers. However, the ability of these hexamers to assemble on a DNA fragment containing the viral origin was impaired by p53. Thus, these results suggest that p53 inhibits the function but not the formation of T antigen multimers. This conclusion was further supported by the observation that the addition of a purified p53:T antigen complex was as inhihitory as free p53 to the DNA helicase activity of free T antigen. Thus our data indicates that the targets of p53 inhibition are the functional units of T antigen, namely the hexamers.