Host Factor SAMHD1 Restricts DNA Viruses in Non-Dividing Myeloid Cells

SAMHD1 is a newly identified anti-HIV host factor that has a dNTP triphosphohydrolase activity and depletes intracellular dNTP pools in non-dividing myeloid cells. Since DNA viruses utilize cellular dNTPs, we investigated whether SAMHD1 limits the replication of DNA viruses in non-dividing myeloid target cells. Indeed, two double stranded DNA viruses, vaccinia and herpes simplex virus type 1, are subject to SAMHD1 restriction in non-dividing target cells in a dNTP dependent manner. Using a thymidine kinase deficient strain of vaccinia virus, we demonstrate a greater restriction of viral replication in non-dividing cells expressing SAMHD1. Therefore, this study suggests that SAMHD1 is a potential innate anti-viral player that suppresses the replication of a wide range of DNA viruses, as well as retroviruses, which infect non-dividing myeloid cells.     

Intrinsic resistance to viral infection. Mouse macrophage restriction of herpes simplex virus replication

Macrophages isolated from mice resistant to acute (lethal) infection with a neurovirulent isolate of HSV-1 express intrinsic resistance to viral infection in vitro. Bone marrow (BM), spleen (S), peritoneal (P), and thioglycolate-stimulated peritoneal (Pthio) macrophages isolated from resistant C57BL/6 Cr (B6) mice consistently restrict HSV-1 macromolecular synthesis earlier in the viral replicative cycle than do macrophages isolated from the same tissue sources from more susceptible DBA/2Cr (D2) mice. B6-BM (BM macrophages from B6 mice) restrict HSV macromolecular synthesis at least at two points in the replicative cycle: 1) before the onset of alpha-protein synthesis and 2) between the onset of gamma 1 protein and DNA synthesis. D2-BM macrophages restrict HSV replication at about the time of DNA synthesis. B6-P macrophages restrict HSV replication shortly after gamma 1 protein synthesis, and D2-P macrophages inhibit the virus slightly later, but before DNA synthesis. B6-S macrophages restrict HSV replication at about the time of DNA synthesis, and D2-S macrophages inhibit replication after the onset of gamma 2 protein synthesis. Pthio macrophages are more permissive to HSV infection than BM, P, or S macrophages: restrictions in viral replication occur at the time of DNA synthesis in B6-Pthio macrophages, and after the onset of gamma 2 protein synthesis in D2-Pthio cells. These studies demonstrate that isolated macrophages from inbred mouse strains express intrinsic resistance to HSV infection that correlates with in vivo resistance to acute (lethal) infection. Intrinsic resistance to HSV-1 infection is due to restriction of viral macromolecular synthesis. HSV replication is inhibited in macrophages at multiple points in the viral growth cycle, depending on the tissue from which the cells are isolated.     

Inhibition of CUL4A Neddylation Causes a Reversible Block to SAMHD1-Mediated Restriction of HIV-1

The deoxynucleoside triphosphohydrolase SAMHD1 restricts retroviral replication in myeloid cells. Human immunodeficiency virus type 2 (HIV-2) and a simian immunodeficiency virus from rhesus macaques (SIVmac) encode Vpx, a virion-packaged accessory protein that counteracts SAMHD1 by inducing its degradation. SAMHD1 is thought to work by depleting the pool of intracellular deoxynucleoside triphosphates but has also been reported to have exonuclease activity that could allow it to degrade the viral genomic RNA or viral reverse-transcribed DNA. To induce the degradation of SAMHD1, Vpx co-opts the cullin4a-based E3 ubiquitin ligase, CRL4. E3 ubiquitin ligases are regulated by the covalent attachment of the ubiquitin-like protein Nedd8 to the cullin subunit. Neddylation can be prevented by MLN4924, a drug that inhibits the nedd8-activating enzyme. We report that MLN4924 inhibits the neddylation of CRL4, blocking Vpx-induced degradation of SAMHD1 and maintaining the restriction. Removal of the drug several hours postinfection released the block. Similarly, Vpx-containing virus-like particles and deoxynucleosides added to the cells more than 24 h postinfection released the SAMHD1-mediated block. Taken together, these findings support deoxynucleoside triphosphate pool depletion as the primary mechanism of SAMHD1 restriction and argue against a nucleolytic mechanism, which would not be reversible.     

SAMHD1抗艾滋病病毒作用研究进展

SAMHD1蛋白全称为不育-α-基序结构域(SAM域)和组氨酸/天冬氨酸残基双联体结构域(HD域)包涵蛋白1,是由真核生物SAMHD1基因编码的蛋白质。研究表明SAMHD1蛋白对机体固有免疫有调控作用,它能够明显上调抗病毒免疫应答,介导由干扰素引起的炎症反应,参与宿主对入侵病毒的防御体系。早期的研究主要集中在其基因突变引起的Aicardi-Goutières综合征(AGS),最新研究发现SAMHD1作为一种核酸水解酶,具有代谢耗竭髓样细胞和树突状细胞内dNTP池从而抑制I型艾滋病病毒(Human immunodeficiency virus,HIV-1)cDNA合成的功能。而HIV-2和猴免疫缺陷病毒(SIVsm/mac)辅助基因编码的Vpx蛋白则能拮抗SAMHD1对病毒复制的抑制作用。近年来,有关SAMHD1蛋白的功能及其抗病毒作用机制的研究进展迅速,本文主要对此加以综述。     

Restricting HIV the SAMHD1 way: through nucleotide starvation

HIV replication is limited by cellular restriction factors, such as APOBEC and tetherin, which themselves are counteracted by viral proteins. SAMHD1 was recently identified as a novel HIV restriction factor in myeloid cells, and was shown to be blocked by the lentiviral protein Vpx. SAMHD1 limits viral replication through an original mechanism: it hydrolyses intracellular dNTPs in non-cycling cells, thus decreasing the amount of these key substrates, which are required for viral DNA synthesis. In this Progress article, we describe how SAMHD1 regulates the pool of intracellular nucleotides to control HIV replication and the innate immune response.