Impact on virus replication

Epithelial cells line all body cavities, making up the border between the environment and the internal milieu of the organism. The apical membrane domain has evolved a rigid structure to protect the body from invasion of pathogens. Most viruses have to overcome this barrier twice when interacting with a host: first during invasion of the host and secondly when spreading to a new host. Viruses have evolved different mechanisms how to overcome this barrier: some viruses circumvent infection of epithelial cells, for example by exploiting lesions within the cell layer or by using vectors like arthropods, while others have adapted to the specialised organisation of epithelial cells for sccessful infection.

1.3. Cell polarity 43

1.3.1.1 Entry and release

For several viruses, which have adapted to infect polarised epithelial cells, it has been shown that virus entry is restricted to one side and in many cases virus release is also restricted to one side. Polar distribution of the viral receptor is the most common cause for a restricted en-try side (Compans and Herrler, 2005). Some viruses, like VSV, orHerpes simplex virus, start infection predominantly from the basolateral membrane (Fuller et al., 1984; Schelhaas et al., 2003). Those viruses have evolved mechanisms to overcome the epithelial barrier during in-vasion of the host, for example by exploiting lesions within the epithelial cell layer, like VSV.

Other viruses, likeSendai virus, can infect epithelial cells from the apical side, making direct invasion of the host possible (Tashiro et al., 1990a).

For some paramyxoviruses, the interaction with polar cells has been investigated. For most of them it has been shown that infection of epithelial cells is more efficient from the apical side, e.g. forRespiratory syncytial virus(Zhang et al., 2002) andHuman parainfluenza virus type 3 (Zhang et al., 2005).

For MV there is some controversy about its polar entry. Blau and Compans (1995) showed that MV strain Edmonston enters the polar cell lines Vero C1008 and Caco-2 predominantly from the apical side and showed a mainly apical distribution of the MV vaccine strain receptor CD46.

However, Sinn et al. (2002) reported that MV strain Edmonston enters well-differentiated hu-man airway epithelial cells predominantly from the basolateral side although even in these cells CD46 was more abundantly expressed on the apical side.

Virus release from epithelial cells is often restricted to one side as well. For HIV and VSV it has been shown that they are released specifically from the basolateral side (Owens et al., 1991;

Rodriguez-Boulan and Sabatini, 1978; Fuller et al., 1984), while influenza virus is released from the apical side (Rodriguez-Boulan and Sabatini, 1978). For the paramyxovirusSendai virusit has been demonstrated that polarised virus release can be a virulence determinant. Infection of epithelial cells of the respiratory tract bySendai virustakes place at the apical side and the virus is predominantly released to this very side. Release from the apical side does not allow the virus to reach the blood stream. Therefore, apical release restricts virus dissemination to neighbouring cells, causing only local disease. For a mutant Sendai virus, F1-R, it has been shown that release is no longer restricted to the apical side, but that release from the basolateral side is almost equally efficient (Tashiro et al., 1990a,b). Release from the basolateral side may

44 1. Introduction

enable the virus to infect tissues underneath the epithelial cell layer and to reach the blood stream as well as the lymphatic vessels for efficient spread all over the body resulting in a systemic disease (Tashiro et al., 1992a).

Except for Sendai virus (Rodriguez-Boulan and Sabatini, 1978), an apical polar release has also been shown for other paramyxoviruses, namelyRespiratory syncytial virus(Roberts et al., 1995),Human parainfluenza virus type 3(Zhang et al., 2005), andSimian virus 5 (Rodriguez-Boulan and Sabatini, 1978). MV has also been reported to be released from different cell lines as well as from well-differentiated human airway epithelial cells predominantly from the apical side (Blau and Compans, 1995; Maisner et al., 1998; Naim et al., 2000; Sinn et al., 2002).

1.3.1.2 Polar transport of proteins

For several paramyxoviruses, likeSendai virus,Respiratory syncytial virus, andHuman parain-fluenza virus type 3it has been shown that their glycoproteins are transported to the apical side of polarised epithelial cells, consistent with their preferred budding side (Rodriguez-Boulan and Pendergast, 1980; Roberts et al., 1995; Zhang et al., 2005).

However, for MV the polar distribution of the envelope glycoproteins is more complex. MV H and F are preferentially sorted to the basolateral side of epithelial cells, when expressed alone (Maisner et al., 1998) or in cells infected with a MV mutant that lacks the M protein (MV∆M) (Naim et al., 2000). This was ascribed to a tyrosine-based basolateral targeting signal within their cytoplasmic tails (Moll et al., 2001; Naim et al., 2000). In contrast, in wild type MV in-fected cells both proteins are targeted to the apical side at early times post infection (30 hours) (Naim et al., 2000). The VSV G protein, intrinsically targeted to the basolateral side, can be retargeted to the apical domain if its cytoplasmic tail is exchanged with the MV F cytoplasmic tail and is expressed together with the other MV proteins (Naim et al., 2000). It therefore has been proposed that the MV M protein redirects the intrinsically basolaterally transported H and F proteins to the apical side by interaction with their cytoplasmic tails (Naim et al., 2000). Ba-solateral expression of H and F mediates fusion with neighbouring cells, seen as syncytia (Moll et al., 2004). A basolateral distribution of H and F might therefore also mediate fusion with un-derlying cells, thus facilitating infection of unun-derlying tissues without basolateral virus release.

Syncytium formation in Sendai virus infected cells has also only been seen for virus mutants which show a basolateral expression of the F protein (F1-R mutants) (Tashiro et al., 1992b).

1.3. Cell polarity 45