The long and short of antiviral activity

Only short isoforms of the pseudoenzyme PARP13 inhibit interferon responses.

Efficient Vpu-Mediated Tetherin Antagonism by an HIV-1 Group O Strain

ABSTRACT Simian immunodeficiency viruses (SIVs) use their Nef proteins to counteract the restriction factor tetherin. However, a deletion in human tetherin prevents antagonism by the Nef proteins of SIVcpz and SIVgor, which represent the ape precursors of human immunodeficiency virus type 1 (HIV-1). To promote virus release from infected cells, pandemic HIV-1 group M strains evolved Vpu as a tetherin antagonist, while the Nef protein of less widespread HIV-1 group O strains acquired the ability to target a region adjacent to this deletion. In this study, we identified an unusual HIV-1 group O strain (RBF206) that evolved Vpu as an effective antagonist of human tetherin. While both RBF206 Vpu and Nef exert anti-tetherin activity in transient-transfection assays, mainly Vpu promotes RBF206 release in infected CD4+ T cells. Although mutations distinct from the adaptive changes observed in group M Vpus (M-Vpus) were critical for the acquisition of its anti-tetherin activity, RBF206 O-Vpu potently suppresses NF-κB activation and reduces CD4 cell surface expression. Interestingly, RBF206 Vpu counteracts tetherin in a largely species-independent manner, degrading both the long and short isoforms of human tetherin. Downmodulation of CD4, but not counteraction of tetherin, by RBF206 Vpu was dependent on the cellular ubiquitin ligase machinery. Our data present the first example of an HIV-1 group O Vpu that efficiently antagonizes human tetherin and suggest that counteraction by O-Nefs may be suboptimal. IMPORTANCE Previous studies showed that HIV-1 groups M and O evolved two alternative strategies to counteract the human ortholog of the restriction factor tetherin. While HIV-1 group M switched from Nef to Vpu due to a deletion in the cytoplasmic domain of human tetherin, HIV-1 group O, which lacks Vpu-mediated anti-tetherin activity, acquired a Nef protein that is able to target a region adjacent to the deletion. Here we report an unusual exception, identifying a strain of HIV-1 group O (RBF206) whose Vpu protein evolved an effective antagonism of human tetherin. Interestingly, the adaptive changes in RBF206 Vpu are distinct from those found in M-Vpus and mediate efficient counteraction of both the long and short isoforms of this restriction factor. Our results further illustrate the enormous flexibility of HIV-1 in counteracting human defense mechanisms.

Son and Its Alternatively Spliced Isoforms Control MLL Complex-Mediated H3K4me3 and Transcription of Leukemia-Associated Genes

SON is a poorly characterized nuclear protein that is particularly abundant in hematopoietic cells/organs and embryonic stem cells. This protein was recently identified as an SR-like splicing co-factor required for proper cell cycle progression (Mol. Cell. 2011, 42:185) and maintenance of stem cell pluripotency (Nat. Cell. Biol. 2013, 15:1141). Although SON's function in RNA splicing of several genes, such as TUBG1, AKT1, OCT4 and HDAC6, was recently highlighted, SON was originally identified as a DNA-binding protein, suggesting a potential function in transcription. Our recent study found that SON represses the promoter of the miR-23a~27a~24-2 cluster in hematopoietic cells (J. Biol. Chem. 2013, 288:5381). However, whether SON is directly associated with specific chromosomal loci in mammalian genome and how SON represses gene transcription are completely unknown. In order to investigate SON's function in genome-wide DNA-binding and gene regulation, we performed chromatin immunoprecipitation-sequencing (ChIP-seq) in K562 cells using SON antibodies. Analysis of the genomic distribution of SON ChIP-seq peaks demonstrated that most of SON-binding sites are located near the promoter regions of target genes which include transcription factors, signaling mediators and cell cycle regulators. Interestingly, SON-binding regions near transcription start sites precisely overlap with the locations of tri-methylation of histone H3 lysine 4 (H3K4me3) which is associated with the open chromatin status and gene activation. Our ChIP-qPCR analyses revealed that knockdown of SON causes enhanced recruitment of the mixed lineage leukemia (MLL) complex, a writer of H3K4me3, to the SON target chromatin, significantly increasing H3K4me3 levels. SON depletion also leads to enhanced protein-protein interactions between the MLL complex components. These results indicate a repressive function of SON in H3K4me3 and MLL complex assembly. Surprisingly, our immunoprecipitation (IP) with SON antibody revealed that SON interacts with menin, an MLL complex component critical for MLL function in oncogenesis. Furthermore, SON overexpression increased the menin-SON complex and simultaneously decreased menin interaction with MLL. These findings demonstrate an inhibitory effect of menin-SON interaction on menin-MLL interaction. In addition to full-length SON (SON F), two splice variants of SON (SON B and E), which are C-terminus-truncated forms, have been predicted in genome databases. To address the functional significance of SON splice variants, we examined whether SON splice variants are differentially expressed in the condition of acute myeloid leukemia (AML). Interestingly, the expression levels of alternatively spliced SON isoforms, but not full-length SON, were significantly increased in human AML patient bone marrow and peripheral blood cells as well as in mouse AML1-ETO9a leukemic blasts. Our data also demonstrated that the short isoforms of SON retain its DNA-binding ability, thereby competing with the full-length SON for target chromatin interaction. However, unlike the full-length SON, the short isoforms lack the menin-binding ability and could not inhibit MLL complex assembly. We further demonstrated that short isoforms of SON can abrogate the full-length SON function in lowering the H3K4me3 level, and overexpression of the short isoform enhanced MLL complex assembly. Taken together, our study reveals that the MLL complex activity is competitively regulated by full-length SON and its alternatively spliced isoforms, and that target genes of MLL-menin are aberrantly controlled by overexpressed "short SON" in AML patients. Furthermore, our findings strongly suggest the significant roles of SON in transcriptional regulation of multiple genes associated with leukemia, stem cell maintenance and survival signaling through histone modification near transcription start sites. Disclosures No relevant conflicts of interest to declare.