Infectious Hematopoietic Necrosis Virus Matrix Protein Inhibits Host-Directed Gene Expression and Induces Morphological Changes of Apoptosis in Cell Cultures

ABSTRACT Infectious hematopoietic necrosis virus (IHNV) infection in tissue culture cells has previously been shown to result in the shutdown of host protein synthesis, cell rounding, and cell death. We report here an investigation of the cytopathogenicity of the viral phosphoprotein (P or M1), matrix (M or M2), and nonvirion (NV) proteins in cultured fish cells. The expression of M alone potently inhibited reporter gene expression from a viral and an interferon (IFN)-inducible promoter, whereas P and NV did not produce a similar effect. Northern blot analysis further revealed a reduction in the steady-state level of reporter mRNA when the M gene was cotransfected into cells; conversely, M mRNA was not drastically reduced in the same cells. By immunofluorescence confocal microscopy, fragmented nuclei were found in some cells expressing M protein but not in cells expressing P, NV, or β-galactosidase protein. Electron microscopy revealed the morphological changes associated with apoptosis in the M-transfected cells. Furthermore, IHNV infection was shown to produce DNA “laddering” in cultured cells. Taken together, these data suggested at least two functions for M protein in an IHNV infection: down regulation of host transcription and the induction of programmed cell death. In the course of these experiments, we also discovered that NV expression was associated with cell rounding, the first biological effect on cells to be attributed to the NV gene.