Determinants of hepatitis C virus p7 ion channel function and drug sensitivity identified in vitro.

Hepatitis C virus (HCV) chronically infects 170 million individuals, causing severe liver disease. Although antiviral chemotherapy exists, the current regimen is ineffective in 50% of cases due to high levels of innate virus resistance. New, virus-specific therapies are forthcoming although their development has been slow and they are few in number, driving the search for new drug targets. The HCV p7 protein forms an ion channel in vitro and is critical for the secretion of infectious virus. p7 displays sensitivity to several classes of compounds, making it an attractive drug target. We recently demonstrated that p7 compound sensitivity varies according to viral genotype, yet little is known of the residues within p7 responsible for channel activity or drug interactions. Here, we have employed a liposome-based assay for p7 channel function to investigate the genetic basis for compound sensitivity. We demonstrate using chimeric p7 proteins that neither the two trans-membrane helices nor the p7 basic loop individually determines compound sensitivity. Using point mutation analysis, we identify amino acids important for channel function and demonstrate that null mutants exert a dominant negative effect over wild-type protein. We show that, of the three hydrophilic regions within the amino-terminal trans-membrane helix, only the conserved histidine at position 17 is important for genotype 1b p7 channel activity. Mutations predicted to play a structural role affect both channel function and oligomerization kinetics. Lastly, we identify a region at the p7 carboxy terminus which may act as a specific sensitivity determinant for the drug amantadine.

Genotype-dependent sensitivity of hepatitis C virus to inhibitors of the p7 ion channel.

UNLABELLED: The hepatitis C virus (HCV) p7 protein plays a critical role during particle formation in cell culture and is required for virus replication in chimpanzees. The discovery that it displayed cation channel activity in vitro led to its classification within the "viroporin" family of virus-coded ion channel proteins, which includes the influenza A virus (IAV) M2 protein. Like M2, p7 was proposed as a potential target for much needed new HCV therapies, and this was supported by our finding that the M2 inhibitor, amantadine, blocked its activity in vitro. Since then, further compounds have been shown to inhibit p7 function but the relationship between inhibitory effects in vitro and efficacy against infectious virus is controversial. Here, we have sought to validate multiple p7 inhibitor compounds using a parallel approach combining the HCV infectious culture system and a rapid throughput in vitro assay for p7 function. We identify a genotype-dependent and subtype-dependent sensitivity of HCV to p7 inhibitors, in which results in cell culture largely mirror the sensitivity of recombinant protein in vitro; thus building separate sensitivity profiles for different p7 sequences. Inhibition of virus entry also occurred, suggesting that p7 may be a virion component. Second site effects on both cellular and viral processes were identified for several compounds in addition to their efficacy against p7 in vitro. Nevertheless, for some compounds antiviral effects were specific to a block of ion channel function. CONCLUSION: These data validate p7 inhibitors as prototype therapies for chronic HCV disease. (HEPATOLOGY 2008;48:1779-1790.).

Inhibition of hepatitis C virus p7 membrane channels in a liposome-based assay system.

Chemotherapy for patients chronically infected with hepatitis C virus (HCV) is ineffective in over 50% of cases, generating a high demand for new drug targets. The p7 protein of HCV displays membrane channel activity in vitro and is essential for replication in vivo though its precise role in the virus life cycle is unknown. p7 channel activity can be specifically inhibited by several classes of compounds, making this protein an attractive candidate for drug development, though techniques used to date in characterising this protein are unsuited to compound library screening. Here we describe an assay for the channel forming ability of p7 based on the release of a fluorescent indicator from liposomes. We show that recombinant p7 from genotype 1b HCV causes a dose-dependent release of dye when mixed with liposomes and that this property is enhanced at acidic pH. We demonstrate that this activity is due to the formation of a size-selective pore rather than non-specific disruption of liposomes and that activity can be blocked by amantadine and several other compounds, validating it as a measure of p7 channel function. This system provides the first convenient in vitro assay for exploiting p7 as a therapeutic target.