Alanine scanning of the hepatitis C virus core protein reveals numerous residues essential for production of infectious virus

Hepatitis C virus (HCV) is an important human pathogen affecting an estimated 3% of the world's population. Recent advances have enabled in vitro propagation of the virus and allow assembly and egress to be investigated for the first time. As a component of the virion, the HCV core protein likely functions primarily in infectious virus production, although little is known about the determinants of this activity. To investigate the roles of core in the viral life cycle, we performed a comprehensive deletion and alanine scanning mutagenesis study of this protein in the context of a genotype 2a reporter virus. We have confirmed that core protein is essential for infectious virion production and have identified numerous residues required for this role. The infectivity of several assembly-defective core mutants could be rescued by compensatory mutations identified in p7 and NS2, suggesting genetic interactions with core and highlighting the importance of these nonstructural proteins in infectious virion morphogenesis.

Functional requirements of the yellow fever virus capsid protein

Although it is known that the flavivirus capsid protein is essential for genome packaging and formation of infectious particles, the minimal requirements of the dimeric capsid protein for virus assembly/disassembly have not been characterized. By use of a trans-packaging system that involved packaging a yellow fever virus (YFV) replicon into pseudo-infectious particles by supplying the YFV structural proteins using a Sindbis virus helper construct, the functional elements within the YFV capsid protein (YFC) were characterized. Various N- and C-terminal truncations, internal deletions, and point mutations of YFC were analyzed for their ability to package the YFV replicon. Consistent with previous reports on the tick-borne encephalitis virus capsid protein, YFC demonstrates remarkable functional flexibility. Nearly 40 residues of YFC could be removed from the N terminus while the ability to package replicon RNA was retained. Additionally, YFC containing a deletion of approximately 27 residues of the C terminus, including a complete deletion of C-terminal helix 4, was functional. Internal deletions encompassing the internal hydrophobic sequence in YFC were, in general, tolerated to a lesser extent. Site-directed mutagenesis of helix 4 residues predicted to be involved in intermonomeric interactions were also analyzed, and although single mutations did not affect packaging, a YFC with the double mutation of leucine 81 and valine 88 was nonfunctional. The effects of mutations in YFC on the viability of YFV infection were also analyzed, and these results were similar to those obtained using the replicon packaging system, thus underscoring the flexibility of YFC with respect to the requirements for its functioning.

Hepatitis C virus p7 and NS2 proteins are essential for production of infectious virus

Hepatitis C virus (HCV) infection is a global health concern affecting an estimated 3% of the world's population. Recently, cell culture systems have been established, allowing recapitulation of the complete virus life cycle for the first time. Since the HCV proteins p7 and NS2 are not predicted to be major components of the virion, nor are they required for RNA replication, we investigated whether they might have other roles in the viral life cycle. Here we utilize the recently described infectious J6/JFH chimera to establish that the p7 and NS2 proteins are essential for HCV infectivity. Furthermore, unprocessed forms of p7 and NS2 were not required for this activity. Mutation of two conserved basic residues, previously shown to be important for the ion channel activity of p7 in vitro, drastically impaired infectious virus production. The protease domain of NS2 was required for infectivity, whereas its catalytic active site was dispensable. We conclude that p7 and NS2 function at an early stage of virion morphogenesis, prior to the assembly of infectious virus.

Superinfection exclusion in cells infected with hepatitis C virus

Superinfection exclusion is the ability of an established virus infection to interfere with infection by a second virus. In this study, we found that Huh-7.5 cells acutely infected with hepatitis C virus (HCV) genotype 2a (chimeric strain J6/JFH) and cells harboring HCV genotype 1a, 1b, or 2a full-length or subgenomic replicons were resistant to infection with cell culture-produced HCV (HCVcc). Replicon-containing cells became permissive for HCVcc infection after treatment with an HCV-specific protease inhibitor. With the exception of cells harboring a J6/JFH-FLneo replicon, infected or replicon-containing cells were permissive for HCV pseudoparticle (HCVpp) entry, demonstrating a postentry superinfection block downstream of primary translation. The surprising resistance of J6/JFH-FLneo replicon-containing cells to HCVpp infection suggested a defect in virus entry. This block was due to reduced expression of the HCV coreceptor CD81. Further analyses indicated that J6/JFH may be toxic for cells expressing high levels of CD81, thus selecting for a CD81(low) population. CD81 down regulation was not observed in acutely infected cells, suggesting that this may not be a general mechanism of HCV superinfection exclusion. Thus, HCV establishes superinfection exclusion at a postentry step, and this effect is reversible by treatment of infected cells with antiviral compounds.

Diverse CD81 proteins support hepatitis C virus infection

Hepatitis C virus (HCV) entry is dependent on CD81. To investigate whether the CD81 sequence is a determinant of HCV host range, we expressed a panel of diverse CD81 proteins and tested their ability to interact with HCV. CD81 large extracellular loop (LEL) sequences were expressed as recombinant proteins; the human and, to a low level, the African green monkey sequences bound soluble HCV E2 (sE2) and inhibited infection by retrovirus pseudotype particles bearing HCV glycoproteins (HCVpp). In contrast, mouse or rat CD81 proteins failed to bind sE2 or to inhibit HCVpp infection. However, CD81 proteins from all species, when expressed in HepG2 cells, conferred susceptibility to infection by HCVpp and cell culture-grown HCV to various levels, with the rat sequence being the least efficient. Recombinant human CD81 LEL inhibited HCVpp infectivity only if present during the virus-cell incubation, consistent with a role for CD81 after virus attachment. Amino acid changes that abrogate sE2 binding (I182F, N184Y, and F186S, alone or in combination) were introduced into human CD81. All three amino acid changes in human CD81 resulted in a molecule that still supported HCVpp infection, albeit with reduced efficiency. In summary, there is a remarkable plasticity in the range of CD81 sequences that can support HCV entry, suggesting that CD81 polymorphism may contribute to, but alone does not define, the HCV susceptibility of a species. In addition, the capacity to support viral entry is only partially reflected by assays measuring sE2 interaction with recombinant or full-length CD81 proteins.

Time- and temperature-dependent activation of hepatitis C virus for low-pH-triggered entry

Hepatitis C virus (HCV) is an important human pathogen associated with chronic liver disease. Recently, based on a genotype 2a isolate, tissue culture systems supporting complete replication and infectious virus production have been developed. In this study, we used cell culture-produced infectious HCV to analyze the viral entry pathway into Huh-7.5 cells. Bafilomycin A1 and concanamycin A, inhibitors of vacuolar ATPases, prevented HCV entry when they were present prior to infection and had minimal effect on downstream replication events. HCV entry therefore appears to be pH dependent, requiring an acidified intracellular compartment. For many other enveloped viruses, acidic pH triggers an irreversible conformational change, which promotes virion-endosomal membrane fusion. Such viruses are often inactivated by low pH. In the case of HCV, exposure of virions to acidic pH followed by return to neutral pH did not affect their infectivity. This parallels the observation made for the related pestivirus bovine viral diarrhea virus. Low pH could activate the entry of cell surface-bound HCV but only after prolonged incubation at 37 degrees C. This suggests that there are rate-limiting, postbinding events that are needed to render HCV competent for low-pH-triggered entry. Such events may involve interaction with a cellular coreceptor or other factors but do not require cathepsins B and L, late endosomal proteases that activate Ebola virus and reovirus for entry.

Construction and applications of yellow fever virus replicons

Subgenomic replicons of yellow fever virus (YFV) were constructed to allow expression of heterologous reporter genes in a replication-dependent manner. Expression of the antibiotic resistance gene neomycin phosphotransferase II (Neo) from one of these YFV replicons allowed selection of a stable population of cells (BHK-REP cells) in which the YFV replicon persistently replicated. BHK-REP cells were successfully used to trans-complement replication-defective YFV replicons harboring large internal deletions within either the NS1 or NS3 proteins. Although replicons with large deletions in either NS1 or NS3 were trans-complemented in BHK-REP, replicons that contained deletions of NS3 were trans-complemented at lower levels. In addition, replicons that retained the N-terminal protease domain of NS3 in cis were trans-complemented with higher efficiency than replicons in which both the protease and helicase domains of NS3 were deleted. To study packaging of YFV replicons, Sindbis replicons were constructed that expressed the YFV structural proteins in trans. Using these Sindbis replicons, both replication-competent and trans-complemented, replication-defective YFV replicons could be packaged into pseudo-infectious particles (PIPs). Although these results eliminate a potential role of either NS1 or full-length NS3 in cis for packaging and assembly of the flavivirus virion, they do not preclude the possibility that these proteins may act in trans during these processes.

Solution structure of dengue virus capsid protein reveals another fold

Dengue virus is responsible for approximately 50-100 million infections, resulting in nearly 24,000 deaths annually. The capsid (C) protein of dengue virus is essential for specific encapsidation of the RNA genome, but little structural information on the C protein is available. We report the solution structure of the 200-residue homodimer of dengue 2 C protein. The structure provides, to our knowledge, the first 3D picture of a flavivirus C protein and identifies a fold that includes a large dimerization surface contributed by two pairs of helices, one of which has characteristics of a coiled-coil. NMR structure determination involved a secondary structure sorting approach to facilitate assignment of the intersubunit nuclear Overhauser effect interactions. The dimer of dengue C protein has an unusually high net charge, and the structure reveals an asymmetric distribution of basic residues over the surface of the protein. Nearly half of the basic residues lie along one face of the dimer. In contrast, the conserved hydrophobic region forms an extensive apolar surface at a dimer interface on the opposite side of the molecule. We propose a model for the interaction of dengue C protein with RNA and the viral membrane that is based on the asymmetric charge distribution of the protein and is consistent with previously reported results.

Flavivirus capsid is a dimeric alpha-helical protein

The capsid proteins of two flaviviruses, yellow fever virus and dengue virus, were expressed in Escherichia coli and purified to near homogeneity suitable for biochemical characterization and structure determination by nuclear magnetic resonance. The oligomeric properties of the capsid protein in solution were investigated. In the absence of nucleic acid, both proteins were predominantly dimeric in solution. Further analysis of both proteins with far-UV circular dichroism spectroscopy indicated that they were largely alpha-helical. The secondary structure elements of the dengue virus capsid were determined by chemical shift indexing of the sequence-specific backbone resonance assignments. The dengue virus capsid protein devoid of its C-terminal signal sequence was found to be composed of four alpha helices. The longest alpha helix, 20 residues, is located at the C terminus and has an amphipathic character. In contrast, the N terminus was found to be unstructured and could be removed without disrupting the structural integrity of the protein.

Structure of dengue virus: implications for flavivirus organization, maturation, and fusion

The first structure of a flavivirus has been determined by using a combination of cryoelectron microscopy and fitting of the known structure of glycoprotein E into the electron density map. The virus core, within a lipid bilayer, has a less-ordered structure than the external, icosahedral scaffold of 90 glycoprotein E dimers. The three E monomers per icosahedral asymmetric unit do not have quasiequivalent symmetric environments. Difference maps indicate the location of the small membrane protein M relative to the overlaying scaffold of E dimers. The structure suggests that flaviviruses, and by analogy also alphaviruses, employ a fusion mechanism in which the distal beta barrels of domain II of the glycoprotein E are inserted into the cellular membrane.

Cardiac catheterization reduces resource utilization in patients with chronic chest pain

The evaluation of patients with recurrent chest pain accounts for a significant proportion of the $274 billion annual cost of cardiovascular services in the United States. Our investigation examines the impact of coronary angiography on subsequent use of medical resources for evaluation of chest pain symptoms. The study seeks to determine whether a finding of noncritical coronary artery disease on cardiac catheterization leads to a reduced use of resources for subsequent evaluation and treatment of chest pain syndromes. Our study included 22 consecutive patients who had sought evaluation for chest pain symptoms, and who had persistence of symptoms after functional testing. Cardiac catheterization demonstrated angiographically mild coronary artery disease (stenosis less than 50%) in these patients. The patient cohort accounted for 22 emergency room evaluations and 41 ambulatory clinic evaluations in the 2.5 years before cardiac catheterization. In the 2.5-year period after catheterization, these patients had only 3 emergency room visits and 1 ambulatory clinic visit for chest pain evaluation (P < 0.001). There was a significant reduction in the number of prescriptions written for topical and oral nitrates (32% precatheterization vs. 5% postcatheterization, P < 0.04), but not of beta-blockers (26% vs. 21%, P = 0.53) or calcium blockers (32% vs. 32%, P = 1.0). Furthermore, most of the 21 surviving patients were found subsequently to have a noncardiac basis for their pain: pericarditis was felt to be the cause of chest pain in 4 patients, pulmonary disease in 7 patients, and gastrointestinal conditions in 8 patients. Diagnostic coronary arteriography may identify a subset of patients in whom a finding of noncritical coronary artery disease leads to a reduction in physician visits for evaluation of chest pain syndromes and reduced use of nitrates. In addition, when coronary artery disease is known to be mild, a noncardiac etiology for the chest pain can be sought. These results may decrease the use of expensive medical resources and encourage full occupational and lifestyle rehabilitation.

Differences in gene density on chicken macrochromosomes and microchromosomes

The chicken karyotype comprises six pairs of large macrochromosomes and 33 pairs of smaller microchromosomes. Cytogenetic evidence suggests that microchromosomes may be more gene-dense than macrochromosomes. In this paper, we compare the gene densities on macrochromosomes and microchromosomes based on sequence sampling of cloned genomic DNA, and from the distribution of genes mapped by genetic linkage and physical mapping. From these different approaches we estimate that microchromosomes are twice as gene-dense as macrochromosomes and show that sequence sampling is an effective means of gene discovery in the chicken. Using this method we have also detected a conserved linkage between the genes for serotonin 1D receptor (HTR1D) and the platelet-activating factor receptor protein gene (PTAFR) on chicken chromosome 5 and human chromosome 1p34.3. Taken together with its advantages as an experimental animal, and public access to genetic and physical mapping resources, the chicken is a useful model genome for studies on the structure, function and evolution of the vertebrate genome.

The dynamics of chromosome evolution in birds and mammals

Comparative mapping, which compares the location of homologous genes in different species, is a powerful tool for studying genome evolution. Comparative maps suggest that rates of chromosomal change in mammals can vary from one to ten rearrangements per million years. On the basis of these rates we would expect 84 to 600 conserved segments in a chicken comparison with human or mouse. Here we build comparative maps between these species and estimate that numbers of conserved segments are in the lower part of this range. We conclude that the organization of the human genome is closer to that of the chicken than the mouse and by adding comparative mapping results from a range of vertebrates, we identify three possible phases of chromosome evolution. The relative stability of genomes such as those of the chicken and human will enable the reconstruction of maps of ancestral vertebrates.

Gene homologs on human chromosome 15q21-q26 and a chicken microchromosome identify a new conserved segment

The genes for insulin-like growth factor 1 receptor (IGF1R), aggrecan (AGC1), beta2-microglobulin (B2M), and an H6-related gene have been mapped to a single chicken microchromosome by genetic linkage analysis. In addition, a second H6-related gene was mapped to chicken macrochromosome 3. The Igf1r and Agc1 loci are syntenic on mouse Chr 7, together with Hmx3, an H6-like locus. This suggests that the H6-related locus, which maps to the chicken microchromosome in this study, is the homolog of mouse Hmx3. The IGF1R, AGC1, and B2M loci are located on human Chr 15, probably in the same order as found for this chicken microchromosome. This conserved segment, however, is not entirely conserved in the mouse and is split between Chr 7 (Igf1r-Agc) and 2 (B2m). This comparison also predicts that the HMX3 locus may map to the short arm of human Chr 15. The conserved segment defined by the IGF1R-AGC1-HMX3-B2M loci is approximately 21-35 Mb in length and probably covers the entire chicken microchromosome. These results suggest that a segment of human Chr 15 has been conserved as a chicken microchromosome. The significance of this result is discussed with reference to the evolution of the avian and mammalian genomes.

Superoxide dismutase mutations in an unselected cohort of Scottish amyotrophic lateral sclerosis patients

Mutations in the Cu/Zn superoxide dismutase (SOD1) gene are responsible for some cases of familial amyotrophic lateral sclerosis (ALS). We have shown that SOD1 mutations can also occur in apparently sporadic ALS. To establish how often this happens we have undertaken a study of the prevalence of SOD1 mutations in an unselected cohort of Scottish ALS patients, with both sporadic (n = 57) and familial (n = 10) disease. Single strand conformation polymorphism analysis was used to scan for new mutations, and selective restriction enzyme digestion to screen for 11 of the 20 SOD1 mutations published to date. We detected mutations in five (50%) of the familial ALS patients and also in four (7%) of the sporadic patients. One mutation, ile113thr, seems to be particularly prevalent in the Scottish population since it was detected in a total of 6/67 (9%) unrelated cases.

Muscarinic involvement in vascular and adrenal medullary responses to splanchnic nerve stimulation in conscious calves

Stimulation of the peripheral end of the right splanchnic nerve (4 Hz for 10 min) in the presence of hexamethonium caused a small but significant rise in mean aortic blood pressure which was subsequently abolished by atropine. There were also small but significant increases in the outputs of catecholamines, [Met5]-enkephalins and corticotrophin releasing factor (CRF) from the right adrenal gland. The catecholamine response was roughly halved after atropine while the outputs of enkephalins and CRF were unaffected. It is concluded that splanchnic sympathetic postganglionic neurones supplying the vasculature are completely blocked by cholinergic blockade whereas adrenal medullary responses persist in an attenuated form.

Effects of substance P on adrenal responses to acetylcholine in conscious calves

The effect of intra-aortic infusions of substance P (SP; 10 or 20 pmol.min-1.kg-1) on adrenal responses to acetylcholine (4.5 nmol.min-1.kg-1 ia) have been investigated in functionally hypophysectomized calves given exogenous adrenocorticotropic hormone (0.7 pmol.min-1.kg-1). At the lower dose, SP had no effect on cortisol output. In contrast, SP inhibited the output of both catecholamines and enkephalins in response to acetylcholine, without affecting the output of corticotropin-releasing factor (CRF). Increasing the dose of SP to 20 pmol.min-1.kg-1 ia significantly reduced the outputs of both cortisol and CRF (P < 0.025 and 0.01 respectively). It is concluded that SP is capable of modulating both adrenal cortical and medullary responses to acetylcholine and that the latter are more sensitive to this influence than the former.

Identification of a novel exon 4 SOD1 mutation in a sporadic amyotrophic lateral sclerosis patient

We have been screening a cohort of 46 sporadic and 10 familial amyotrophic lateral sclerosis patients for mutations in the superoxide dismutase gene (SOD1) using a combination of SSCP and direct PCR sequencing. A novel missense mutation (Asp101Asn) has been detected in one sporadic patient and a previously reported mutation has been found in two familial cases.

Identification of GPCMV infected cells in vitro and in vivo with a monoclonal antibody

A monoclonal antibody was made which identifies a 160-180 kDa structural protein in guinea pig cytomegalovirus (GPCMV) infected cells by Western blot using non-reducing conditions. This protein was shown to be a virion structural protein by purification of GPCMV on a density viscosity gradient and Western blot analysis. Phosphoanacetic acid (PAA) experiments suggest that the protein is a late GPCMV protein. In vitro the monoclonal antibody labels a cytoplasmic protein in infected guinea pig embryo fibroblasts by 12 h postinfection. The monoclonal antibody also identifies GPCMV infected cells in vivo in paraffin embedded formalin fixed tissue.

Adrenal responses to the peptide PACAP in conscious functionally hypophysectomized calves

Intra-aortic infusions of pituitary adenylate cyclase-activating peptide-(1-38) (PACAP) produced a dose-related fall in aortic blood pressure over the range of 4-40 pmol.min-1.kg-1 in the presence of exogenous adrenocorticotropic hormone-(1-24) (ACTH, 2 ng.min-1.kg-1 i.v.; P < 0.01). At the higher dose there was a significant fall in adrenal vascular resistance in the absence, but not in the presence, of ACTH. PACAP also produced a dose-related increase in right adrenal cortisol output over the same range, which was significantly greater in the absence of exogenous ACTH (P < 0.01). At the higher dose, PACAP produced small but significant increases in adrenal epinephrine and norepinephrine output (P < 0.01) both in the presence and the absence of ACTH. There was also a small rise in Met5-enkephalin output, and corticotrophin-releasing factor (CRF) was released in the presence, but not in the absence, of ACTH. It is concluded that PACAP is capable of exerting potent steroidogenic and vasodilator effects in the adrenal gland in the normal conscious calf and of releasing significant amounts of catecholamines, enkephalins, and CRF from the adrenal medulla. These findings identify PACAP as a candidate neuromodulator in the adrenal gland in this species.

Autonomic control of adrenal function

Recent studies of adrenal function in conscious calves are reviewed. These have involved collecting the whole of the adrenal effluent blood from the right adrenal gland at intervals and, where necessary, prior functional hypophysectomy by destruction of the pituitary stalk under general halothane anaesthesia 3 d previously. The adrenal medulla was found to release numerous neuropeptides, in addition to catecholamines, in response to stimulation of the peripheral end of the right splanchnic nerve, which was carried out below behavioural threshold. Many of these responses were enhanced by stimulating intermittently at a relatively high frequency. Intra-aortic infusions of a relatively low dose of acetylcholine (4.5 nmol min-1 kg-1) elicited similar responses. In the adrenal cortex, agonists which either potentiated the steroidogenic response to ACTH or exerted a direct steroidogenic action included VIP, CGRP, CRF and ACh acting via muscarinic receptors. Stimulation of the peripheral end of the right splanchnic nerve strongly potentiated the steroidogenic response to ACTH and there is compelling evidence that the innervation normally plays an important part in cortisol secretion.