The tegument protein pp71 (UL82) of human cytomegalovirus (HCMV) has previously been shown to transactivate the major immediate-early enhancer-promoter of HCMV. Furthermore, this protein is able to enhance the infectivity of viral DNA and to accelerate the infection cycle, suggesting an important regulatory function during viral replication. To gain insight into the underlying mechanisms that are used by pp71 to exert these pleiotropic effects, we sought for cellular factors interacting with pp71 in a yeast two-hybrid screen. Here, we report the isolation of the human Daxx (hDaxx) protein as a specific interaction partner of HCMV pp71. hDaxx, which was initially described as an adapter protein involved in apoptosis regulation, has recently been identified as a nuclear protein that interacts and colocalizes with PML in the nuclear domain ND10. In order to assess whether pp71 can also be detected in ND10 structures, a vector expressing pp71 in fusion with the green fluorescent protein was used for transfection of human fibroblasts. This revealed a colocalization of pp71 with the ND10 proteins PML and Sp100. In addition, cotransfection of a hDaxx expression vector resulted in an enhanced recruitment of pp71 to ND10. Targeting of pp71 to nuclear dots could also be observed in infected human fibroblasts in the absence of de novo viral protein synthesis. Moreover, cotransfection experiments revealed that pp71-mediated transactivation of the major immediate-early enhancer-promoter was synergistically enhanced in the presence of hDaxx. These results suggest an important role of hDaxx for pp71 protein function.

MicroRNAs (miRNAs) control multiple biological processes including the innate immune responses by negative post-transcriptional regulation of gene expression. As there were no studies on the role(s) of miRNAs in viral diseases in Atlantic salmon, we aimed to identify miRNAs responding to salmonid alphavirus (SAV) infection. Their expression were studied at different time points post infection with SAV isolates associated with different mortalities. Furthermore, the genome sequences of the identified miRNAs were analysed to reveal putative cis-regulatory elements, and, finally, their putative target genes were predicted. Twenty differentially expressed miRNAs (DE miRNAs) were identified. The expression of the majority of these increased post infection with maximum levels reached after the viral load were stabilized or decreasing. On the other hand, some miRNAs (e.g. the miRNA-21 family) showed decreased expression at the early time points post infection. There were significant differences in the temporal expression of individual miRNA associated with different SAV isolates. Target gene prediction in SAV responsive immune network genes showed that seventeen of the DE miRNAs could target 24 genes (e.g. IRF3, IRF7). Applying the Atlantic salmon transcriptome as input 28 more immune network genes were revealed as putative targets (e.g. IRF5, IRF4). The majority of the predicted target genes promote inflammatory response. The upstream sequences of the miRNA genes revealed a high density of cis-regulatory sequences known as binding sites for immune network transcription factors (TFs). A high expression in the late phase could therefore be due to increased transcription promoted by immune response activated TFs. Based on the in silico target predictions, we discuss their putative roles as early promotors or late inhibitors of inflammation. We propose that the differences in expressions associated with different SAV isolates could contribute to their differences in mortality rates. This study represents the first steps in exploring miRNAs important in viral-host interaction in Atlantic salmon. We identified several miRNAs responding to SAV infection. Some likely to prohibit harmful inflammation while other may promote an early immune response. Their predicted functions need to be validated and further studied in functional assays to fully understand their roles in immune homeostasis.

Infectious pancreatic necrosis, an important problem of the salmon industry worldwide, is caused by Infectious pancreatic necrosis virus (IPNV). Fish surviving an IPNV infection become virus carriers, and the identification of infected fish is highly relevant to disease control. The target organ for IPNV diagnosis is the kidney, where the virus persists, usually with low virus loads. The current study documents a real-time reverse transcription polymerase chain reaction (real-time RT-PCR) assay that proved 100 times more sensitive than a conventional RT-PCR. Cell culture and real-time RT-PCR were compared for their ability to detect IPNV in carrier Atlantic salmon kidney samples after different preservation and storage procedures. Storage of whole tissue at −80°C for 1 month and storage of tissue homogenized in transport medium (TM) at +4°C for 1 week before investigation in cell cultures resulted in a marked reduction of virus infectivity. For detection by real-time RT-PCR, storage of whole tissue was suboptimal, whereas storage of tissue homogenized in TM did not affect virus detection. The results of the present study demonstrate that both cell culture and real-time RT-PCR are reliable tests for the detection of low amounts of IPNV in kidneys of carrier Atlantic salmon, and both methods are relatively robust against minor preservation and storage deviations, or both. Preservation of tissues in RNA stabilization solution seems only necessary when samples are to be shipped at ambient temperatures or when laboratory testing might be delayed. Independent of detection method, these results indicate that for long-term storage, samples are best kept at −80°C after homogenization in TM.