Influenza Virus Infections in Polarized Cells

Abstract

In humans and other mammals, the respiratory tract is represented by a complex network of polarized epithelial cells, forming an apical surface facing the external environment and a basal surface attached to the basement layer. These cells are characterized by differential expression of proteins and glycans, which serve as receptors during influenza virus infection. Attachment between these host receptors and the viral surface glycoprotein hemagglutinin (HA) initiates the influenza virus life cycle. However, the virus receptor binding specificities may not be static. Sialylated N-glycans are the most well-characterized receptors but are not essential for the entry of influenza viruses, and other molecules, such as O-glycans and non-sialylated glycans, may be involved in virus-cell attachment. Furthermore, correct cell polarity and directional trafficking of molecules are essential for the orderly development of the system and affect successful influenza infection; on the other hand, influenza infection can also change cell polarity. Here we review recent advances in our understanding of influenza virus infection in the respiratory tract of humans and other mammals, particularly the attachment between the virus and the surface of the polar cells and the polarity variation of these cells due to virus infection.

Keywords: N-glycan; O-glycan; influenza A virus; polarized cell; sialic acid.

Figure 3

Illustration of possible binding interactions between influenza A virus (IAV) and those glyco-molecules other than N-glycans on the cellular surface in the human respiratory tract. The glyco-molecules can be O-glycans without sialic acid (Sia) (A), O-glycans with Sia (B), glycolipid without Sia (C), glycolipid with Sia (D), and glyco-RNA (E). Among these glyco-molecules other than N-glycans, the interactions between O-glycans and IAV are more documented in literature but those for glycolipid and glyco-RNA are still unknown [81].

Figure 1

Infection of influenza A virus (IAV) in the human upper respiratory tract (URT). (A) heterogeneous polarized pseudostratified epithelium cells in human URT targeted by influenza viruses. (B) The first step of IAV infection. Hemagglutinin (HA) of influenza virion is attached to O-glycans present on the mucin surfaces. (C) After overcoming the mucus layer barrier, IAV reaches the apical surface of the ciliated epithelium cells. HA of the virions recognizes the glycan receptor present on the cellular membrane followed by viral entry mediated by endocytosis (a). An equilibrium of the receptor-binding activities for HA and the receptor-destroying activities of NA allows a rolling movement of the virion particle on the villi of the epithelial cell (b). Following cell surface attachment, the virus enters the cell through clathrin-mediated endocytosis (CME) (c) or clathrin and caveolae-independent endocytosis (d). The virions could exhibit either spherical or filamentous morphology.

Figure 2

Illustration of N- and O-glycans associated with IAV binding. N-glycan binds to a protein through the nitrogen of an amide in Asparagine (Asn) and the first carbon of N-acetylglucosamine (GlcNAc) (left). The N-glycosylation sites are located in the “marker sequence” Asn-XaaThr/Ser without a Proline in Xaa position. The O-linkage between a glycan and a protein form between the oxygen of a Threonine (Thr) or Serine (Ser) with the first carbon of N-acetylglucosamine (GlcNAc) or N-acetylgalactosamine (GalNAc) (right).

Figure 4

Apical transport during influenza A virus infection. The viral infection cycle starts with a recognition of a glycan receptor (e.g., sialylated glycan) on the cellular membrane by viral HA. The virus enters the cell through endocytosis (Figure 1); in the acidic environment in the endosome, protons traverse the viral membrane through M2, facilitating viral fusion and release of the viral genome into the cellular cytoplasm. Then the viral ribonucleoproteins (vRNPs) are transported into the nucleus followed by transcription and replication. Next vRNPs are cotrafficked with Rab proteins and use Rab11-dependent vesicular and microtubule-based transport pathway to be transported from the nucleus to the plasma membrane through the pericentriolar recycling endosome. The membrane proteins HA, NA, and M2 proteins traffic from the Golgi to the apical membrane. Finally, the viral particle buds and is released by the action of NA to the extracellular media.