HIV-1 Nef Antagonizes SERINC5 Restriction by Downregulation of SERINC5 via the Endosome/Lysosome System.

The primate lentiviral accessory protein Nef downregulates CD4 and major histocompatibility complex class I (MHC-I) from the cell surface via independent endosomal trafficking pathways to promote viral pathogenesis. In addition, Nef antagonizes a novel restriction factor, SERINC5 (Ser5), to increase viral infectivity. To explore the molecular mechanism of Ser5 antagonism by Nef, we determined how Nef affects Ser5 expression and intracellular trafficking in comparison to CD4 and MHC-I. We confirm that Nef excludes Ser5 from human immunodeficiency virus type 1 (HIV-1) virions by downregulating its cell surface expression via similar functional motifs required for CD4 downregulation. We find that Nef decreases both Ser5 and CD4 expression at steady-state levels, which are rescued by NH4Cl or bafilomycin A1 treatment. Nef binding to Ser5 was detected in living cells using a bimolecular fluorescence complementation assay, where Nef membrane association is required for interaction. In addition, Nef triggers rapid Ser5 internalization via receptor-mediated endocytosis and relocalizes Ser5 to Rab5+ early, Rab7+ late, and Rab11+ recycling endosomes. Manipulation of AP-2, Rab5, Rab7, and Rab11 expression levels affects the Nef-dependent Ser5 and CD4 downregulation. Moreover, although Nef does not promote Ser5 polyubiquitination, Ser5 downregulation relies on the ubiquitination pathway, and both K48- and K63-specific ubiquitin linkages are required for the downregulation. Finally, Nef promotes Ser5 colocalization with LAMP1, which is enhanced by bafilomycin A1 treatment, suggesting that Ser5 is targeted to lysosomes for destruction. We conclude that Nef uses a similar mechanism to downregulate Ser5 and CD4, which sorts Ser5 into a point-of-no-return degradative pathway to counteract its restriction.IMPORTANCE Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) express an accessory protein called Nef to promote viral pathogenesis. Nef drives immune escape in vivo through downregulation of CD4 and MHC-I from the host cell surface. Recently, Nef was reported to counteract a novel host restriction factor, Ser5, to increase viral infectivity. Nef downregulates cell surface Ser5, thus preventing its incorporation into virus particles, resulting in disruption of its antiviral activity. Here, we report mechanistic studies of Nef-mediated Ser5 downregulation in comparison to CD4 and MHC-I. We demonstrate that Nef binds directly to Ser5 in living cells and that Nef-Ser5 interaction requires Nef association with the plasma membrane. Subsequently, Nef internalizes Ser5 from the plasma membrane via receptor-mediated endocytosis, and targets ubiquitinated Ser5 to endosomes and lysosomes for destruction. Collectively, these results provide new insights into our ongoing understanding of the Nef-Ser5 arms race in HIV-1 infection.

TCam-2 seminoma cell line exhibits characteristic foetal germ cell responses to TGF-beta ligands and retinoic acid.

Germ cell testicular cancer is understood to arise during embryogenesis, based on the persistence of embryonic germ cell markers in carcinoma in situ and seminoma. In this study, we examine the potential of the seminoma-derived TCam-2 cell line to be used as representative in functional analyses of seminoma. We demonstrate expression of several early germ cell markers, including BLIMP1, OCT3/4, AP2γ, NANOG and KIT. Many TGF-beta superfamily receptors and downstream transcription factors are also present in these cells including the normally foetal ACTRIIA receptor, indicating potential responsiveness to TGF-beta superfamily ligands. Treatment with BMP4 or RA induces a significant increase in ACTRIA, ACTRIIA and ACTRIIB transcripts, whereas activin A decreases ACTRIB. BMP4 and RA each support TCam-2 survival and/or proliferation. In addition, despite increased KIT mRNA levels induced by BMP4, RA and activin A, activin A does not improve survival or proliferation. The capacity for BMP4 and retinoic acid to enhance foetal germ cell survival and proliferation/self-renewal has been demonstrated in mice, but not previously tested in humans. This study is the first to demonstrate a functional response in seminoma cells, using a well-characterized cell line, consistent with their foetal germ cell-like identity.