Asymmetric budding of viruses in epithelial monlayers: a model system for study of epithelial polarity.

Abstract

Infection of two different lines of polarized epithelial cells grown as monolayers with several types of enveloped viruses results, for each virus type, in a characteristic asymmetric budding of virions. Influenza virus (WSN strain), simian virus 5, and Sendai virus bud exclusively from the free (apical) surface of the cells, while vesicular stomatitis virus acquires its envelope only from the basolateral plasma membrane. Because different viruses select specific domains of plasma membrane in the same cell type, virus-infected epithelial monolayers can provide an excellent model system For studies of the mechanisms that generate regional differences in the distribution of plasma membrane components of epithelial cells. Cells in secretory and transporting epithelia show a striking polarization of their plasma membranes into two distinct functional domains: one at the free surface, which may contain apical morphological differentiations such as microvilli, and another, the basolateral, that faces the internal milieu and relates to other epithelial cells and to the basal lamina. The specific location of different transport systems, receptors, and enzymes in each domain of the plasma membrane (2-5) accounts for the functional polarization of epithelia. There is, therefore, considerable interest in understanding the biogenetic mechanisms by which proteins are segregated into specific regions of the plasma membrane. In order to investigate this aspect of plasma membrane biogenesis, we have studied the production of enveloped viruses in polarized monolayers of epithelial cells. The value of these viruses as model systems for studies of plasma membrane assembly has long been recognized (6-10). A main advantage derives from the fact that they contain a small number of membrane glycoproteins that presumably are synthesized, processed, and transported to the cell surface by the same mechanisms employed by the uninfected cell for its own plasma membrane proteins (6-8). Using different enveloped viruses, we found that each virus buds asymmetrically from either the apical or basolateral surface of infected monolayers of two different kidney cell lines. Because the specificity of viral budding implies recognition of plasma membrane domains by viral components, the system of virus-infected epithelial monolayers introduced in this paper promises to be useful for studies of

Lipidic intramembranous particles

“…the apical and the basolateral membranes. Indeed it has been demonstrated that these domains appear to have different protein [66] and lipid [67] compositions. From the similar morphological appearance [68,69] and the complementarity aspect [70] it has been proposed that the 'tight junctions' elements or strands reflect inter-bilayer tubes with the lipids in the H H orientation.…”

Identification and Characterization of Three Novel Nuclear Export Signals in the Influenza A Virus Nucleoprotein

“…At a late stage of virus infection, the vRNPs exit the nucleus to assemble and bud from the apical plasma membrane of polarized cells (3). The trafficking of the vRNPs into and out of the nucleus is a tightly regulated process (7).…”

Pathogenicity of Sendai Viruses Adapted into Polarized MDCK Cells.

“…1). It has been shown previously that the SeV is released from epithelial cells in a polarized manner, predominantly at the apical surface [3]. The polarity properties of the adapted virus released from polarized MDCK cells were examined by infecting cells grown on inserts and collecting and determining the virus yields in both the apical and the basal media after predetermined intervals, in triplicate experiments.…”

“…Each virus has been observed to have a specific preference for budding direction in polarized epithelial cells. Enveloped virus is usually released preferentially by budding at the apical plasma membrane and it has been previously demonstrated that the wild type SeV is released predominantly, though not exclusively from the apical surface of polarized epithelial cells [3], in contrast with the F1-R mutant SeV virus which is released from both the apical and basolateral surfaces [25,27]. The polarized entry and release of SeV play a role in the pathogenesis of the disease.…”