Rotaviruses (RVs) are the most important cause of severe gastroenteritis in children. As a member of the reovirus family, RV has an icosahedral structure that consists of three concentric protein layers surrounding the double-stranded-RNA genome. To initiate an infection, the virion must pass through the host cell membrane by attachment to a cellular receptor, followed by delivery of its double-layered subviral particle into the cytoplasm (2
). Two major structural proteins, VP4 and VP7, are found on the outmost surfaces of the virions (11
). VP7 is a glycoprotein found rich in the endoplasmic reticulum of the infected cells and is mainly involved in virion assembly, while VP4 forms the major spike protein responsible for viral attachment and penetration into host cells (17
VP4 is processed by proteolytic cleavage into two subunits, VP5* and VP8* (14
). VP8* is believed to be mainly involved in the attachment of viruses to host cells and VP5* in the translocation of the double-shelled particles into the cytoplasm through conformation rearrangement and membrane fusion (2
), although the precise functions of the two subunits remain to be defined. Some RV strains recognize the terminal N
-acetyl neuraminic (sialic) acid (SA) residues of carbohydrates on the host cell surface for attachment (15
). However, this interaction appears nonessential for other strains (9
). RV-SA recognition has relied mainly on observation of neuraminidase sensitivity tests previously (36
), in which a strain was considered sialidase sensitive if its infection is sensitive to neuraminidase treatment. Recent reports showed that neuraminidase is capable of removing the terminal SAs without affecting subterminal SAs, and the infectivity and cell binding of two “sialidase-insensitive” RVs (Wa and DS-1) were increased following treatment of the host cells with neuraminidase (20
). Thus, the relative roles of the terminal versus the subterminal SAs in infection of RVs, particularly of the “sialidase-insensitive” human and animal RVs, remain unknown.
The sensitivity of RVs to sialidase treatment is associated with the VP8* sequence of P genotypes but not with their host origin, suggesting a linkage between the protein VP8* and viral attachment to host cells (9
). RVs of P, P, P, and P genotypes can be found in both humans and animals, and these RVs are mainly sialidase sensitive (9
). Three P types (P, P, and P) have been found to commonly cause gastroenteritis in humans, but none of them are sialidase sensitive (8
). Thus, we hypothesize that these P types of human RVs may recognize an alternative carbohydrate, such as the human histo-blood group antigen (HBGAs), similarly to human noroviruses (NoVs) (57
HBGAs are complex carbohydrates present on the surfaces of red blood cells and mucosal epithelia of the respiratory, genitourinary, and digestive tracts (10
). They are also present as free oligosaccharides in biologic fluids, such as saliva, intestinal contents, milk, and blood (38
). The biosynthesis pathway of HBGA starts with a disaccharide precursor by sequential additions of monosaccharides catalyzed by glycosyltransferases encoded by three major gene families, the ABO, Lewis, and secretor families. Each of the gene families contains silent alleles, leading to null phenotypes of the loci. For example, the FUT2-inactivated mutations are responsible for the nonsecretor phenotype found in about 20% of European and North American populations. The nonsecretor phenotype is characterized by the absence of ABH antigens in saliva and on most epithelial cells of the respiratory, genitourinary, and digestive tracts.
Human NoVs recognize different host HBGAs of individuals with different genetic makeups of the ABO, secretor, and Lewis families. The major human blood types of secretor, A/B, and Lewis bloods are determined by three unique terminal saccharides. They are α-1,2-fucose, α-N
-acetylgalactosamine/α-galactose, and α-1,3/4-fucose, respectively. These three saccharides have been shown to play an important role in binding to NoVs (57
). SA is another residue that commonly occurs on mucosal surfaces, and this residue has been suggested to be involved in binding to some NoVs (48
). We hypothesize that RVs could share common carbohydrates as receptors with NoVs because RVs and NoVs may infect the same enterocytes in the intestinal tract.
Interaction of human RV with a carbohydrate has also been suggested by a crystallographic study of RV VP8* proteins of a sialidase-sensitive animal strain (CRW-8) and a sialidase-insensitive human strain (Wa) (7
). A comparison of these two atomic structures showed that the SA binding pocket is missing for the sialidase-insensitive strain Wa. Instead, the crystal structure of WA VP8* revealed a novel groove region that is suggested to interact with carbohydrate (7
). In addition, this groove is found to be conserved in both the sialidase-insensitive human strains and the sialidase-sensitive animal strains (7
). Similar findings were reported in a separate study comparing the RRV (sialidase-sensitive) and DS-1 (sialidase-insensitive) VP8* proteins, in which a similar surface cleft was found on the DS-1 VP8* protein (43
In this study, we provided the first evidence of interaction between human RVs and HBGAs in human saliva and milk. The results were further validated by binding of VP8* to synthetic oligosaccharides representing specific HBGAs and by blocking the binding by monoclonal antibodies (MAbs) specific to these HBGAs. More importantly, we observed similar binding activities with the use of authentic RVs recovered from cell cultures. While direct evidence is still needed, our results suggest that the sialidase-insensitive human RVs may recognize human HBGAs as ligands or receptors.