The Infectivity of prM-Containing Partially Mature West Nile Virus Does Not Require the Activity of Cellular Furin-Like Proteases

Viral Pathogenesis Section, Laboratory of Viral Diseases, National Institutes of Health, 33 North Drive, Building 33, Room 2E19A.2, Bethesda, MD 20892, USA.
Journal of Virology (Impact Factor: 4.44). 08/2011; 85(22):12067-72. DOI: 10.1128/JVI.05559-11
Source: PubMed


Cleavage of the flavivirus prM protein by a cellular furin-like protease is a hallmark of virion maturation. While this cleavage
is a required step in the viral life cycle, it can be inefficient. Virions that retain uncleaved prM may be infectious. We
investigated whether cleavage by furin of prM on partially mature West Nile virus (WNV) during virus entry contributes to
infectivity. Using quantitative assays of WNV infection, we found that virions incorporating considerable amounts of uncleaved
prM protein were insensitive to treatment of cells with a potent inhibitor of furin activity. Thus, partially mature WNV does
not require furin-like proteases for infectivity.

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Available from: Swati Mukherjee, Dec 21, 2013
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    • "Early studies suggested that complete maturation is necessary to render flavivirus particles infectious (Stadler et al., 1997). However, recent studies have demonstrated that not only are virions retaining uncleaved prM infectious (Colpitts et al., 2011; Mukherjee et al., 2011; Nelson et al., 2008; Zybert et al., 2008), but that complete maturation may be detrimental to WNV replication under certain circumstances (Zybert et al., 2008). Whilst most strains of WNV contain a glycosylation site in both E and the precursor portion of prM, some non-pathogenic strains lack the E glycosylation site. "
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    ABSTRACT: The molecular basis for the increased resistance of astrocytes to a nonneuropathogenic strain of WNV, WNV-MAD78, compared to the neuropathogenic strain WNV-NY remains unclear. In this report, we demonstrate that the reduced susceptibility of astrocytes to WNV-MAD78 is due to a combination of both cellular activities as well as viral determinants. Analyses of the viral particle indicated that astrocyte-derived WNV-MAD78 particles are less infectious than those of WNV-NY. Additionally, inhibition of cellular furin-like proteases increased WNV-MAD78 infectious particle production in astrocytes, suggesting that high levels of furin-like protease activity within these cells acts in a cell- and strain-specific manner to inhibit WNV-MAD78 replication. Moreover, analysis of recombinant viruses indicated that the structural proteins of WNV-MAD78 were responsible for decreased particle infectivity and the corresponding reduction in infectious particle production compared to WNV-NY. Thus, the composition of the WNV virion is also a major determinant for viral fitness within astrocytes and may contribute to WNV propagation within the CNS. Whether the WNV-MAD78 structural genes reduce virus replication and particle infectivity through the same mechanism as the cellular furin-like protease activity, or whether these two determinants function through distinct pathways, remains to be determined.
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    • "Therefore, immature particles are presumably scored non-infectious in numerous cell lines like K562, U937, THP-1, P388D1, and human PBMCs [17], [22], [25]. However, and in line with recent results on WNV [26], we here show that immature DENV is infectious in cells expressing DC-SIGN. Binding of immature particles to DC-SIGN is presumably facilitated by sugar groups linked to position Asn69 on prM, or sugar groups linked to position Asn67 and Asn153 on E [15], [16], [26], [28]. "
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    ABSTRACT: Dengue Virus (DENV) is the most common mosquito-borne viral infection worldwide. Important target cells during DENV infection are macrophages, monocytes, and immature dendritic cells (imDCs). DENV-infected cells are known to secrete a large number of partially immature and fully immature particles alongside mature virions. Fully immature DENV particles are considered non-infectious, but antibodies have been shown to rescue their infectious properties. This suggests that immature DENV particles only contribute to the viral load observed in patients with a heterologous DENV re-infection.
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    • "This orchestrates profound rearrangements of viral glycoproteins that result in the acquisition of the mature structure of the virion [14], [15], [16]. While this cleavage is a required step in the viral life cycle, it can be inefficient; suggesting that complete maturation is not required for infectivity [17], [18]. Indeed, the presence of flaviviral ‘mosaic’ particles combining regions of mature and immature structure has been documented [19], and the degree of maturation of flavivirus particles has been related to different aspects of their interaction with antibodies [18], [20], [21], [22]. "
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    ABSTRACT: West Nile virus (WNV) is a worldwide distributed mosquito-borne flavivirus that naturally cycles between birds and mosquitoes, although it can infect multiple vertebrate hosts including horses and humans. This virus is responsible for recurrent epidemics of febrile illness and encephalitis, and has recently become a global concern. WNV requires to transit through intracellular acidic compartments at two different steps to complete its infectious cycle. These include fusion between the viral envelope and the membrane of endosomes during viral entry, and virus maturation in the trans-Golgi network. In this study, we followed a genetic approach to study the connections between viral components and acidic pH. A WNV mutant with increased resistance to the acidotropic compound NH4Cl, which blocks organelle acidification and inhibits WNV infection, was selected. Nucleotide sequencing revealed that this mutant displayed a single amino acid substitution (Lys 3 to Glu) on the highly basic internal capsid or core (C) protein. The functional role of this replacement was confirmed by its introduction into a WNV infectious clone. This single amino acid substitution also increased resistance to other acidification inhibitor (concanamycin A) and induced a reduction of the neurovirulence in mice. Interestingly, a naturally occurring accompanying mutation found on prM protein abolished the resistant phenotype, supporting the idea of a genetic crosstalk between the internal C protein and the external glycoproteins of the virion. The findings here reported unveil a non-previously assessed connection between the C viral protein and the acidic pH necessary for entry and proper exit of flaviviruses.
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