A cytoplasmic region of the NSP4 enterotoxin of rotavirus is involved in retention in the endoplasmic reticulum

Comparative efficacy of fluorescent antibody test, immunoperoxidase test and enzyme linked immuno sorbent assay in detection of rotavirus in cell culture

Rotavirus is prevalent worldwide and has been established as a leading cause of mortality due to severe diarrhoea in neonates. Isolation of the virus is a gold standard method for confirmation of rotavirus infection in the host. Propagation of rotavirus in cell culture is a challenge as in many instances the virus does not produce detectable cytopathic effect. This study was undertaken to evaluate the efficacy of fluorescent antibody test (FAT), immunoperoxidase test (IPT) and sandwich ELISA (S-ELISA) to detect rotavirus antigen propagated in MA 104 cell line. The intensity of fluorescence and colour development for I-FAT and I-IPT was categorized and the ELISA OD values were analyzed. The overall mean of detection were 5.16 ± 0.47, 5.16 ± 0.54 and 5.66 ± 0.33 for I-FAT, I-IPT and S-ELISA, respectively. Significantly less number of samples were positive in the initial one or two passage, which increased up to 100 % from third/fourth passage onwards. The study concluded that I-FAT, I-IPT and S-ELISA were equally effective in detecting propagated rotavirus in cell line, and the former two tests are suitable for in situ demonstration of the virus while the later could be used to assay antigen in cell culture fluid.

Expression of nonstructural rotavirus protein NSP4 mimics Ca2+ homeostasis changes induced by rotavirus infection in cultured cells

Rotavirus infection modifies Ca(2+) homeostasis, provoking an increase in Ca(2+) permeation, the cytoplasmic Ca(2+) concentration ([Ca(2+)](cyto)), and total Ca(2+) pools and a decrease in Ca(2+) response to agonists. A glycosylated viral protein(s), NSP4 and/or VP7, may be responsible for these effects. HT29 or Cos-7 cells were infected by the SA11 clone 28 strain, in which VP7 is not glycosylated, or transiently transfected with plasmids coding for NSP4-enhanced green fluorescent protein (EGFP) or NSP4. The permeability of the plasma membrane to Ca(2+) and the amount of Ca(2+) sequestered in the endoplasmic reticulum released by carbachol or ATP were measured in fura-2-loaded cells at the single-cell level under a fluorescence microscope or in cell suspensions in a fluorimeter. Total cell Ca(2+) pools were evaluated as (45)Ca(2+) uptake. Infection with SA11 clone 28 induced an increase in Ca(2+) permeability and (45)Ca(2+) uptake similar to that found with the normally glycosylated SA11 strain. These effects were inhibited by tunicamycin, indicating that inhibition of glycosylation of a viral protein other than VP7 affects the changes of Ca(2+) homeostasis induced by infection. Expression of NSP4-EGFP or NSP4 in transfected cells induced the same changes observed with rotavirus infection, whereas the expression of EGFP or EGFP-VP4 showed the behavior of uninfected and untransfected cells. Increased (45)Ca(2+) uptake was also observed in cells expressing NSP4-EGFP or NSP4, as evidenced in rotavirus infection. These results indicate that glycosylated NSP4 is primarily responsible for altering the Ca(2+) homeostasis of infected cells through an initial increase of cell membrane permeability to Ca(2+).

Generation and immunogenicity of a recombinant replication-incompetent adenovirus expressing human rotavirus NSP4 protein

AIM: To investigate the efficacy of passive protection induced by the recombinant adeno-virus bearing human rotavirus NSP4 gene and to explore the clues for novel rotavirus vaccine development.

METHODS: Human rotavirus NSP4 protein was expressed in a recombinant adenovirus. AdEasy system was recruited to facilitate the preparation and the NSP4 gene was inserted into the early 1 region of the vector by homologous recombination in E.coli. The expression of NSP4 was confirmed by Western blotting. The resultant recombinant adenovirus rvAdEasyNSP4 was induced to mice by intranasal (i.n.) immunization. Specific IgG and IgA antibodies were detected in the murine serum. Murine pups born to the rvAdEasyNSP4 immunized dams were challenged with simian rotavirus SA11 strain orally 4 d after birth, and the induced diarrhea was graded.

RESULTS: The recombinant adenovirus rvAdEasyNSP4 showed typical morphology under electron microscope. The transcription of NSP4 specific mRNA by rvAdEasyNSP4 was confirmed with RT-PCR in infected 293 cells, and the expression of the NSP4 protein was verified by Western blotting. After immunization for three times, the positive rates of serum IgG antibody were 28.5%, 85.7% and 100%, respectively, while after the first immunization, the titer of serum IgG reached 1:1 000. After immunization, the positive rate of IgA antibody reached the level of 71.4%. Moderate protection was achieved after simian rotavirus SA11 strain challenged in murine pups.

CONCLUSION: NSP4 protein of human rotavirus strain can be expressed by a recombinant adenovirus vector and trigger effective immune response in mice, laying a solid foundation for the development of novel rotavirus genetic engineering vaccine.

N- and C-terminal cooperation in rotavirus enterotoxin: novel mechanism of modulation of the properties of a multifunctional protein by a structurally and functionally overlapping conformational domain

Rotavirus NSP4 is a multifunctional endoplasmic reticulum (ER)-resident nonstructural protein with the N terminus anchored in the ER and about 131 amino acids (aa) of the C-terminal tail (CT) oriented in the cytoplasm. Previous studies showed a peptide spanning aa 114 to 135 to induce diarrhea in newborn mouse pups with the 50% diarrheal dose approximately 100-fold higher than that for the full-length protein, suggesting a role for other regions in the protein in potentiating its diarrhea-inducing ability. In this report, employing a large number of methods and deletion and amino acid substitution mutants, we provide evidence for the cooperation between the extreme C terminus and a putative amphipathic alpha-helix located between aa 73 and 85 (AAH73-85) at the N terminus of DeltaN72, a mutant that lacked the N-terminal 72 aa of nonstructural protein 4 (NSP4) from Hg18 and SA11. Cooperation between the two termini appears to generate a unique conformational state, specifically recognized by thioflavin T, that promoted efficient multimerization of the oligomer into high-molecular-mass soluble complexes and dramatically enhanced resistance against trypsin digestion, enterotoxin activity of the diarrhea-inducing region (DIR), and double-layered particle-binding activity of the protein. Mutations in either the C terminus, AAH73-85, or the DIR resulted in severely compromised biological functions, suggesting that the properties of NSP4 are subject to modulation by a single and/or overlapping highly sensitive conformational domain that appears to encompass the entire CT. Our results provide for the first time, in the absence of a three-dimensional structure, a unique conformation-dependent mechanism for understanding the NSP4-mediated pleiotropic properties including virus virulence and morphogenesis.

Ca2+ permeability of the plasma membrane induced by rotavirus infection in cultured cells is inhibited by tunicamycin and brefeldin A

Rotavirus infection of cultured cells induces a progressive increase in plasma membrane permeability to Ca2+. The viral product responsible for this effect is not known. We have used tunicamycin and brefeldin A to prevent glycosylation and membrane traffic and study the involvement of viral glycoproteins, NSP4 and/or VP7, in rotavirus-infected HT29 and MA104 cells. In infected cells, we observed an increase of plasma membrane Ca2+ permeability and a progressive depletion of agonist-releasable ER pools measured with fura 2 and an enhancement of total Ca2+ content measured as 45Ca2+ uptake. Tunicamycin inhibited the increase in membrane Ca2+ permeability, induced a depletion of agonist-releasable and 45Ca2+-sequestered pools. Brefeldin A inhibited the increase of Ca2+ permeability and the increase in 45Ca2+ uptake induced by infection. We propose that the glycosylated viral product NSP4 (and/or VP7) travels to the plasma membrane to form a Ca2+ channel and hence elevate Ca2+ permeability.

Single-chain variable fragment (scFv) antibodies against rotavirus NSP4 enterotoxin generated by phage display

The rotavirus non-structural NSP4 protein causes membrane destabilization as well as an increase in intracellular calcium levels in eukaryotic cells and induces diarrhea in young mice, acting as a viral enterotoxin. In this study the phage display technique was used to generate a panel of single-chain variable fragment (scFv) antibodies specific for the NSP4 protein of the human rotavirus strain Wa from a human semi-synthetic scFv library. After several rounds of panning and selection on NSP4 adsorbed to polystyrene tubes, individual scFv were isolated and characterised by fingerprinting and by sequencing the VH and VL genes. The isolated scFv antibodies specifically recognize NSP4 in enzyme immunoassay and in Western blot. Four truncated forms of the NSP4 protein were constructed which allowed us to map the binding region of the selected scFv antibodies to the C-terminal portion of NSP4. The isolated scFv antibodies constitute valuable tools to analyse the mechanisms of NSP4 functions.

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