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SAMHD1 specifically restricts retroviruses through its RNase activity.

ABSTRACT: Human SAMHD1 possesses dual enzymatic functions. It acts as both a dGTP-dependent triphosphohydrolase and as an exoribonuclease. The dNTPase function depletes the cellular dNTP pool, which is required for retroviral reverse transcription in differentiated myeloid cells and resting CD4(+) T cells; thus this activity mainly plays a role in SAMHD1-mediated retroviral restriction. However, a recent study demonstrated that SAMHD1 directly targets HIV-1 genomic RNA via its RNase activity, and that this function (rather than dNTPase activity) is sufficient for HIV-1 restriction. While HIV-1 genomic RNA is a potent target for SAMHD1 during viral infection, the specificity of SAMHD1-mediated RNase activity during infection by other viruses is unclear.The results of the present study showed that SAMHD1 specifically degrades retroviral genomic RNA in monocyte-derived macrophage-like cells and in primary monocyte-derived macrophages. Consistent with this, SAMHD1 selectively restricted retroviral replication, but did not affect the replication of other common non-retro RNA genome viruses, suggesting that the RNase-mediated antiviral function of SAMHD1 is limited to retroviruses. In addition, neither inhibiting reverse transcription by treatment with several reverse transcriptase inhibitors nor infection with reverse transcriptase-defective HIV-1 altered RNA levels after viral challenge, indicating that the retrovirus-specific RNase function is not dependent on processes associated with retroviral reverse transcription.The results presented herein suggest that the RNase activity of SAMHD1 is sufficient to control the replication of retroviruses, but not that of non-retro RNA viruses.

SUBMITTER: Choi J

PROVIDER: S-EPMC4450836 | biostudies-literature | 2015

REPOSITORIES: biostudies-literature

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SAMHD1 specifically restricts retroviruses through its RNase activity.

Choi Jongsu J Ryoo Jeongmin J Oh Changhoon C Hwang Sungyeon S Ahn Kwangseog K

Retrovirology 20150602

<h4>Background</h4>Human SAMHD1 possesses dual enzymatic functions. It acts as both a dGTP-dependent triphosphohydrolase and as an exoribonuclease. The dNTPase function depletes the cellular dNTP pool, which is required for retroviral reverse transcription in differentiated myeloid cells and resting CD4(+) T cells; thus this activity mainly plays a role in SAMHD1-mediated retroviral restriction. However, a recent study demonstrated that SAMHD1 directly targets HIV-1 genomic RNA via its RNase act ...[more]

PMID: 26032178

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Project description:Virus-host interactions play vital roles in viral replication and virus-induced pathogenesis. Viruses rely entirely upon host cells to reproduce progeny viruses; however, host factors positively or negatively regulate virus replication by interacting with viral proteins. The elucidation of virus-host protein interaction not only provides a better understanding of the molecular mechanisms by which host cells combat viral infections, but also facilitates the development of new anti-viral therapeutics. Identification of relevant host factors requires techniques that enable comprehensive characterization of virus-host protein interactions. In this study, we developed a proteomic approach to systematically identify human protein kinases that interact potently with viral proteins. For this purpose, we synthesized 412 full-length human protein kinases using the wheat germ cell-free protein synthesis system, and screened them for their association with a virus protein using the amplified luminescent proximity homogenous assay (AlphaScreen). Using this system, we attempted to discover a robust anti-viral host restriction mechanism targeting virus protein X (Vpx) of HIV-2. The screen identified H11/HSPB8 as a Vpx-binding protein that negatively regulates the stability and function of Vpx. Indeed, overexpression of H11/HSPB8 promoted the degradation of Vpx via the ubiquitin-proteasome pathway and inhibited its interaction with SAMHD1, a host restriction factor responsible for blocking replication of HIV. Conversely, targeted knockdown of H11/HSPB8 in human trophoblast cells, which ordinarily express high levels of this protein, restored the expression and function of Vpx, making the cells highly susceptible to viral replication. These results demonstrate that our proteomic approach represents a powerful tool for revealing virus-host interaction not yet identified by conventional methods. Furthermore, we showed that H11/HSPB8 could be a potential host regulatory factor that may prevent placental infection of HIV-2 during pregnancy.

Project description:HIV-1 replication is tightly regulated in host cells, and various restriction factors have important roles in inhibiting viral replication. SAMHD1, a well-known restriction factor, suppresses HIV-1 replication by hydrolyzing intracellular dNTPs, thereby limiting the synthesis of viral cDNA in quiescent cells. In this study, we revealed an additional and distinct mechanism of SAMHD1 inhibition during the postviral cDNA synthesis stage. Using immunoprecipitation and mass spectrometry analysis, we demonstrated the interaction between SAMHD1 and MX2/MxB, an interferon-induced antiviral factor that inhibits HIV-1 cDNA nuclear import. The disruption of endogenous MX2 expression significantly weakened the ability of SAMHD1 to inhibit HIV-1. The crucial region within SAMHD1 that binds to MX2 has been identified. Notably, we found that SAMHD1 can act as a sensor that recognizes and binds to the incoming HIV-1 core, subsequently delivering it to the molecular trap formed by MX2, thereby blocking the nuclear entry of the HIV-1 core structure. SAMHD1 mutants unable to recognize the HIV-1 core showed a substantial decrease in antiviral activity. Certain mutations in HIV-1 capsids confer resistance to MX2 inhibition while maintaining susceptibility to suppression by the SAMHD1-MX2 axis. Overall, our study identifies an intriguing antiviral pattern wherein two distinct restriction factors, SAMHD1 and MX2, collaborate to establish an alternative mechanism deviating from their actions. These findings provide valuable insight into the complex immune defense networks against exogenous viral infections and have implications for the development of targeted anti-HIV therapeutics.ImportanceIn contrast to most restriction factors that directly bind to viral components to exert their antiviral effects, SAMHD1, the only known deoxynucleotide triphosphate (dNTP) hydrolase in eukaryotes, indirectly inhibits viral replication in quiescent cells by reducing the pool of dNTP substrates available for viral cDNA synthesis. Our study provides a novel perspective on the antiviral functions of SAMHD1. In addition to its role in dNTP hydrolysis, SAMHD1 cooperates with MX2 to inhibit HIV-1 nuclear import. In this process, SAMHD1 acts as a sensor for incoming HIV-1 cores, detecting and binding to them, before subsequently delivering the complex to the molecular trap formed by MX2, thereby immobilizing the virus. This study not only reveals a new antiviral pathway for SAMHD1 but also identifies a unique collaboration and interaction between two distinct restriction factors, establishing a novel line of defense against HIV-1 infection, which challenges the traditional view of restriction factors acting independently. Overall, our findings further indicate the intricate complexity of the host immune defense network and provide potential targets for promoting host antiviral immune defense.

Project description:BackgroundSAM domain and HD domain containing protein 1 (SAMHD1) is a host anti-HIV-1 restriction factor known to suppress viral reverse transcription in nondividing myeloid cells by its dNTP triphosphorylase activity that depletes cellular dNTPs. However, HIV-2 and some SIV strains rapidly replicate in macrophages due to their accessory protein, viral protein X (Vpx), which proteosomally degrades SAMHD1 and elevates dNTP levels. Endogenous reverse transcription (ERT) of retroviruses is the extra-cellular reverse transcription step that partially synthesizes proviral DNAs within cell-free viral particles before the viruses infect new cells. ERT activity utilizes dNTPs co-packaged during budding from the virus-producing cells, and high ERT activity is known to enhance HIV-1 infectivity in nondividing cells. Here, since Vpx elevates cellular dNTP levels in macrophages, we hypothesize that HIV-2 should contain higher ERT activity than HIV-1 in macrophages, and that the Vpx-mediated dNTP elevation should enhance both ERT activity and infectivity of HIV-1 particles produced in macrophages.ResultsHere, we demonstrate that HIV-2 produced from human primary monocyte derived macrophages displays higher ERT activity than HIV-1 produced from macrophages. Also, HIV-1 particles produced from macrophages treated with virus like particles (VLPs) containing Vpx, Vpx (+), displayed large increases of ERT activity with the enhanced copy numbers of early, middle and late reverse transcription products within the viral particles, compared to the viruses produced from macrophages treated with Vpx (-) VLPs. Furthermore, upon the infection with an equal p24 amount to fresh macrophages, the viruses produced from the Vpx (+) VLP treated macrophages demonstrated higher infectivity than the viruses from the Vpx (-) VLP treated macrophages.ConclusionsThis finding identifies the viral ERT step as an additional step of HIV-1 replication cycle that SAMHD1 restricts in nondividing myeloid target cells.

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SAMHD1 specifically restricts retroviruses through its RNase activity.

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SAMHD1 specifically restricts retroviruses through its RNase activity.

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