Anti-HIV Host Factor SAMHD1 Regulates Viral Sensitivity to Nucleoside Reverse Transcriptase Inhibitors via Modulation of Cellular Deoxyribonucleoside Triphosphate (dNTP) Levels
ABSTRACT Newly identified anti-HIV host factor, SAMHD1, restricts replication of lentiviruses such as HIV-1, HIV-2, and SIV in macrophages by enzymatically hydrolyzing and depleting cellular dNTPs, which are the substrates of viral DNA polymerases. HIV-2 and some SIVs express Vpx, which counteracts SAMHD1 and elevates cellular dNTPs, enhancing viral replication in macrophages. Since nucleoside reverse transcriptase inhibitors (NRTIs), the most commonly used anti-HIV drugs, compete against cellular dNTPs for incorporation into proviral DNA, we tested whether SAMHD1 directly affects the efficacy of NRTIs in inhibiting HIV-1. We found that reduction of SAMHD1 levels with the use of virus-like particles (VLPs) expressing Vpx and SAMHD1 specific shRNA, subsequently elevates cellular dNTPs and significantly decreases HIV-1 sensitivity to various NRTIs in macrophages. However, VLP +Vpx treatment of activated CD4+ T cells only minimally reduced NRTI efficacy. Furthermore, our HPLC-based assay could not detect SAMHD1-mediated hydrolysis of various NRTI-triphosphates tested in this study, suggesting that the reduced sensitivity of HIV-1 to NRTIs upon SAMHD1 degradation is most likely caused by the elevation of the cellular dNTP levels.
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- "For instance, SAMHD1 expression limits the synthesis of HIV-1 provirus of macrophages in vitro    , yet this cell type can serve as a viral reservoir in vivo  . One in vitro study reported that the removal of SAMHD1 significantly reduced azidothymidine-mediated restriction of HIV-1 replication in MDM , suggesting that SAMHD1 may play a role in enhancing the effects of antiretroviral therapy during HIV-1 infection. A recent in vivo study reported that SAMHD1-deficient mice were not more susceptible to HIV-1 infection but were susceptible to a mutant virus that had a lower affinity for dNTP . "
ABSTRACT: SAMHD1 is the most recent addition to a unique group of host restriction factors that limit retroviral replication at distinct stages of the viral life cycle. SAMHD1 is a deoxynucleoside triphosphate triphosphohydrolase that degrades the intracellular pool of deoxynucleoside triphosphates available during early reverse transcription. SAMHD1 activity is blocked by the Vpx accessory function present in HIV-2 and SIVsm. Mutations in SAMHD1 are associated with the autoimmune disorder Aicardi-Goutières syndrome, thus emphasizing its role in regulation of the immune response. SAMHD1 anti-retroviral activity is modulated by post-translational modifications, cell-cycle dependent functions and cytokine-mediated changes. Innate receptors that sense retroviral DNA intermediates are the focus of intense study, and recent studies have established a link between SAMHD1 restriction, innate sensing of DNA and protective immune responses. Cell-cycle dependent regulation of SAMHD1 by phosphorylation and the increasingly broad range of viruses inhibited by SAMHD1 further emphasize the importance of these mechanisms of host restriction. This review highlights current knowledge regarding SAMHD1 regulation and its impact on innate immune signalling and retroviral restriction.Journal of Molecular Biology 10/2013; 425. DOI:10.1016/j.jmb.2013.10.022 · 4.33 Impact Factor
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ABSTRACT: SAM domain and HD domain containing protein 1 (SAMHD1) is a dGTP dependent deoxyribonucleoside triphosphate (dNTP) triphosphohydrolase that converts dNTPs into dNs and triphosphates. Therefore SAMHD1 expression, particularly in non-dividing cells, can restrict retroviral infections such as HIV/SIV by limiting cellular dNTPs, which are essential for reverse transcription. It has previously been established that dGTP acts as both an activator and a substrate of this enzyme, suggesting that phosphohydrolase activity of SAMHD1 is regulated by dGTP availability in the cell. However, we now demonstrate biochemically that the ribonucleoside triphosphate GTP is equally capable of activating SAMHD1, but GTP is not hydrolyzed by the enzyme. Activation of SAMHD1 phosphohydrolase activity was tested under physiological concentrations of dGTP or GTP found in either dividing or non-dividing cells. Since GTP is 1,000-fold more abundant than dGTP in cells, GTP was able to activate the enzyme to a greater extent than dGTP, suggesting that GTP is the primary activator of SAMHD1. Lastly, we show that SAMHD1 has the ability to hydrolyze base-modified nucleotides, indicating that the active site of SAMHD1 is not restrictive to such modifications, and is capable of regulating the levels of non-canonical dNTPs such as dUTP. This study provides further insights into the regulation of SAMHD1 in regards to allosteric activation and active site specificity.Journal of Biological Chemistry 07/2013; 289(24). DOI:10.1074/jbc.C113.493619 · 4.57 Impact Factor
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ABSTRACT: SAMHD1 limits HIV-1 replication by hydrolyzing deoxynucleoside triphosphates (dNTPs) necessary for reverse transcription. Nucleoside Reverse Transcriptase Inhibitors (NRTIs) are components of anti-HIV therapies. We report that SAMHD1 cleaves NRTI triphosphates (NRTI-TPs) at a significantly lower rate than dNTPs and that SAMHD1 depletion from monocytic cells affects susceptibility of HIV-1 infection to NRTIs in complex ways that depend not only on the relative changes in dNTP and NRTI-TP concentrations, but also on the NRTI activation pathways.Antimicrobial Agents and Chemotherapy 05/2014; 58(8). DOI:10.1128/AAC.02745-14 · 4.45 Impact Factor