Infectious hematopoietic necrosis virus matrix protein inhibits host-directed gene expression and induces morphological changes of apoptosis in cell cultures

Infectious hematopoietic necrosis virus (IHNV) infection in tissue culture cells has previously been shown to result in the shutdown of host protein s ynthesis, 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 int erferon (IFN)-inducible promoter, whereas P and NV did not produce a simila r 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 immun ofluorescence confocal microscopy, fragmented nuclei were found in some cel ls expressing M protein but not in cells expressing P, NV, or beta-galactos idase protein, Electron microscopy revealed the morphological changes assoc iated with apoptosis in the M-transfected cells. Furthermore, IHNV infectio n was shown to produce DNA 'laddering" in cultured cells, Taken together, t hese data suggested at least two functions for M protein in an IHNV infecti on: down regulation of host transcription and the induction of programmed c ell death, In the course of these experiments, we also discovered that NV e xpression was associated with cell rounding, the first biological effect on cells to be attributed to the NV gene.