SAMHD1 dysfunction induces IL-34 expression via NF-κB p65 in neuronal SH-SY5Y cells

Dysfunctional mutations in SAMHD1 cause Aicardi-Goutières Syndrome, an autoinflammatory encephalopathy with elevated interferon-α levels in the cerebrospinal fluid. Whether loss of function mutations in SAMHD1 trigger the expression of other cytokines apart from type I interferons in Aicardi-Goutières Syndrome is largely unclear. This study aimed to explore whether SAMHD1 dysfunction regulated the expression of IL-34, a key cytokine controlling the development and maintenance of microglia, in SH-SY5Y neural cells. We found that downregulation of SAMHD1 in SH-SY5Y cells resulted in the upregulation of IL-34 expression. The protein and mRNA levels of NF-κB p65, the transactivating subunit of a transcription factor NF-κB, were also upregulated in SAMHD1-knockdown SH-SY5Y cells. It was further found SAMHD1 knockdown in SH-SY5Y cells induced an upregulation of IL-34 expression through the canonical NF-κB-dependent pathway in which NF-κB p65, IKKα/β and the NF-κB inhibitor IκBα were phosphorylated. Moreover, knockdown of SAMHD1 in SH-SY5Y cells led to the translocation of NF-κB p65 into the nucleus and promoted NF-κB transcriptional activity. In conclusion, we found SAMHD1 dysfunction induced IL-34 expression via NF-κB p65 in neuronal SH-SY5Y cells. This finding could lay the foundation for exploring the role of IL-34-targeting microglia in the pathogenesis of Aicardi-Goutières Syndrome.

Genome-wide screens connect HD82 loss-of-function to purine analog resistance in African trypanosomes

Nucleoside analogs have been used extensively as anti-infective agents, particularly against viral infections, and have long been considered promising anti-parasitic agents. These pro-drugs are metabolized by host-cell, viral, or parasite enzymes prior to incorporation into DNA, thereby inhibiting DNA replication. Here, we report genes that sensitize African trypanosomes to nucleoside analogs, including the guanosine analog, ganciclovir. We applied ganciclovir selective pressure to a trypanosome genome-wide knockdown library, which yielded nucleoside mono- and diphosphate kinases as hits, validating the approach. The two most dominant hits to emerge, however, were Tb927.6.2800 and Tb927.6.2900, which both encode nuclear proteins; the latter of which is HD82, a SAMHD1-related protein and a putative dNTP triphosphohydrolase. We independently confirmed that HD82, which is conserved among the trypanosomatids, can sensitize Trypanosoma brucei to ganciclovir. Since ganciclovir activity depends upon phosphorylation by ectopically expressed viral thymidine kinase, we also tested the adenosine analog, ara-A, that may be fully phosphorylated by native T. brucei kinase(s). Both Tb927.6.2800 and HD82 knockdowns were resistant to this analog. Tb927.6.2800 knockdown increased sensitivity to hydroxyurea, while dNTP analysis indicated that HD82 is indeed a triphosphohydrolase with dATP as the preferred substrate. Our results provide insights into nucleoside/nucleotide metabolism and nucleoside analog metabolism and resistance in trypanosomatids. We suggest that the product of 6.2800 sensitizes cells to purine analogs through DNA repair, while HD82 does so by reducing the native purine pool.IMPORTANCEThere is substantial interest in developing nucleoside analogs as anti-parasitic agents. We used genome-scale genetic screening and discovered two proteins linked to purine analog resistance in African trypanosomes. Our screens also identified two nucleoside kinases required for pro-drug activation, further validating the approach. The top novel hit, HD82, is related to SAMHD1, a mammalian nuclear viral restriction factor. We validated HD82 and localized the protein to the trypanosome nucleus. HD82 appears to sensitize trypanosomes to nucleoside analogs by reducing native pools of nucleotides, providing insights into both nucleoside/nucleotide metabolism and nucleoside analog resistance in trypanosomatids.

Abemaciclib restricts HCMV replication by suppressing pUL97-mediated phosphorylation of SAMHD1

Human cytomegalovirus (HCMV) is a herpesvirus that causes life-threatening infections in newborns or immunosuppressed patients. For viral replication, HCMV establishes a network of cellular interactions, among others cyclin-dependent kinases (CDK). Furthermore, HCMV encodes pUL97, a viral kinase, which is a CDK-homologue. HCMV uses pUL97 in order to phosphorylate and thereby antagonize SAMHD1, an antiviral host cell factor. Since HCMV has several mechanisms to evade restriction by SAMHD1, we first analyzed the kinetics of SAMHD1-inactivation and found that phosphorylation of SAMHD1 by pUL97 occurs directly after infection of macrophages. We hence hypothesized that inhibition of this process qualifies as efficient antiviral target and FDA approved CDK-inhibitors (CDKIs) might be potent antivirals that prevent the inactivation of SAMHD1. Indeed, Abemaciclib, a 2nd generation CDKI exhibited superior IC50s against HCMV in infected macrophages and the antiviral activity largely relied on its ability to block pUL97-mediated SAMHD1-phosphorylation. Altogether, our study highlights the therapeutic potential of clinically-approved CDKIs as antivirals against HCMV, sheds light on their mode of action and establishes SAMHD1 as a valid and highly potent therapeutic target.

Graphene Oxide-Facilitated Comprehensive Analysis of Cellular Nucleic Acid Binding Proteins for Lung Cancer

Nucleic acid binding proteins (NABPs) mediate a broad range of essential cellular functions. However, it is very challenging to comprehensively extract whole cellular NABPs due to the lack of approaches with high efficiency. To this end, carbon nanomaterials, including graphene oxide (GO), carboxylated graphene (cG), and carboxylated carbon nanotube (cCNT), were utilized to extract cellular NABPs in this study through a new strategy. Our data demonstrated that GO, cG, and cCNT could extract nearly 100% cellular DNA in vitro. Conversely, their RNA extraction efficiencies were 60, 50, and 29%, respectively, partially explaining why GO has the highest NABPs yield compared to cG and cCNT. We further found that ionic bond mediated by cations between RNA and functional groups of nanomaterials facilitated RNA absorption on nanomaterials. About 2400 proteins were successfully identified from GO-enriched NABPs sample, and 88% of annotated NABPs were enriched at least 2 times compared to cell lysate, indicating the high selectivity of our strategy. The developed method was further applied to compare the NABPs in two lung cancer cell lines with different tumor progression abilities. According to label-free quantification results, 118 differentially expressed NABPs were discovered and 6 candidate NABPs, including ACAA2, GTF2I, VIM, SAMHD1, LYAR, and IGF2BP1, were successfully validated by immunoassay. The level of SAMHD1 in the serum of lung cancer patients was measured, which significantly increased upon cancer progression. Our results collectively demonstrated that GO is an ideal nanomaterial for NABPs selective extraction, which could be broadly used in varied physiological and pathophysiological settings.

A G1-like state allows HIV-1 to bypass SAMHD1 restriction in macrophages

An unresolved question is how HIV-1 achieves efficient replication in terminally differentiated macrophages despite the restriction factor SAMHD1. We reveal inducible changes in expression of cell cycle-associated proteins including MCM2 and cyclins A, E, D1/D3 in macrophages, without evidence for DNA synthesis or mitosis. These changes are induced by activation of the Raf/MEK/ERK kinase cascade, culminating in upregulation of CDK1 with subsequent SAMHD1 T592 phosphorylation and deactivation of its antiviral activity. HIV infection is limited to these G1-like phase macrophages at the single-cell level. Depletion of SAMHD1 in macrophages decouples the association between infection and expression of cell cycle-associated proteins, with terminally differentiated macrophages becoming highly susceptible to HIV-1. We observe both embryo-derived and monocyte-derived tissue-resident macrophages in a G1-like phase at frequencies approaching 20%, suggesting how macrophages sustain HIV-1 replication in vivo Finally, we reveal a SAMHD1-dependent antiretroviral activity of histone deacetylase inhibitors acting via p53 activation. These data provide a basis for host-directed therapeutic approaches aimed at limiting HIV-1 burden in macrophages that may contribute to curative interventions.

Substrates and Inhibitors of SAMHD1

SAMHD1 hydrolyzes 2'-deoxynucleoside-5'-triphosphates (dNTPs) into 2'-deoxynucleosides and inorganic triphosphate products. In this paper, we evaluated the impact of 2' sugar moiety substitution for different nucleotides on being substrates for SAMHD1 and mechanisms of actions for the results. We found that dNTPs ((2'R)-2'-H) are only permissive in the catalytic site of SAMHD1 due to L150 exclusion of (2'R)-2'-F and (2'R)-2'-OH nucleotides. However, arabinose ((2'S)-2'-OH) nucleoside-5'-triphosphates analogs are permissive to bind in the catalytic site and be hydrolyzed by SAMHD1. Moreover, when the (2'S)-2' sugar moiety is increased to a (2'S)-2'-methyl as with the SMDU-TP analog, we detect inhibition of SAMHD1’s dNTPase activity. Our computational modeling suggests that (2'S)-2'-methyl sugar moiety clashing with the Y374 of SAMHD1. We speculate that SMDU-TP mechanism of action requires that the analog first docks in the catalytic pocket of SAMHD1 but prevents the A351-V378 helix conformational change from being completed, which is needed before hydrolysis can occur. Collectively we have identified stereoselective 2' substitutions that reveal nucleotide substrate specificity for SAMHD1, and a novel inhibitory mechanism for the dNTPase activity of SAMHD1. Importantly, our data is beneficial for understanding if FDA-approved antiviral and anticancer nucleosides are hydrolyzed by SAMHD1 in vivo.

Functional organization of human SAMHD1 and mechanisms of HIV-1 restriction

Sterile alpha motif and histidine-aspartate domain containing protein 1 (SAMHD1) is a triphosphohydrolase that catalyzes the conversion of deoxyribonucleoside triphosphate to deoxyribonucleoside and triphosphate. SAMHD1 has been a recent focus of study since it was identified as a potent human immunodeficiency virus-1 (HIV-1) restriction factor in the intrinsic antiviral immune system. Recent biochemical and biological studies have suggested that SAMHD1 restricts HIV-1 infection in non-cycling cells by limiting the pool of deoxyribonucleoside triphosphates, thereby interfering with HIV-1 reverse transcription. SAMHD1 also possesses single-stranded DNA and RNA binding activity, with reported nuclease activity, conferring additional HIV-1 restriction function. This review summarizes current knowledge regarding the structure of SAMHD1 and the regulation of its function in HIV-1 restriction.

The G1/S Specific Cyclin D2 Is a Regulator of HIV-1 Restriction in Non-proliferating Cells

Macrophages are a heterogeneous cell population strongly influenced by differentiation stimuli that become susceptible to HIV-1 infection after inactivation of the restriction factor SAMHD1 by cyclin-dependent kinases (CDK). Here, we have used primary human monocyte-derived macrophages differentiated through different stimuli to evaluate macrophage heterogeneity on cell activation and proliferation and susceptibility to HIV-1 infection. Stimulation of monocytes with GM-CSF induces a non-proliferating macrophage population highly restrictive to HIV-1 infection, characterized by the upregulation of the G1/S-specific cyclin D2, known to control early steps of cell cycle progression. Knockdown of cyclin D2, enhances HIV-1 replication in GM-CSF macrophages through inactivation of SAMHD1 restriction factor by phosphorylation. Co-immunoprecipitation experiments show that cyclin D2 forms a complex with CDK4 and p21, a factor known to restrict HIV-1 replication by affecting the function of the downstream cascade that leads to SAMHD1 deactivation. Thus, we demonstrate that cyclin D2 acts as regulator of cell cycle proteins affecting SAMHD1-mediated HIV-1 restriction in non-proliferating macrophages.

Inhibition of hepatitis B virus replication by a dNTPase-dependent function of the host restriction factor SAMHD1

SAMHD1 is a cellular protein that possesses dNTPase activity and inhibits retroviruses and DNA viruses through the depletion of cellular dNTPs. However, recent evidence suggests the existence of alternative or additional mechanisms that involve novel nuclease activities. Hepatitis B virus is a DNA virus but resembles retroviruses in that its DNA genome is synthesized via reverse transcription of an RNA transcript. SAMHD1 was shown to inhibit the expression and replication of a transfected HBV DNA. We further investigated the antiviral mechanisms in a newly developed infection assay. Our data indicated that SAMHD1 exerts a profound antiviral effect. In addition, unlike previous findings, our results demonstrate the essential role of SAMHD1 dNTPase. SAMHD1 did not affect virion-derived cccDNA and gene expression but specifically inhibited viral DNA synthesis. These results indicate that SAMHD1 inhibits HBV replication at the reverse transcription step, most likely through the depletion of cellular dNTPs.

Dasatinib inhibits HIV-1 replication through the interference of SAMHD1 phosphorylation in CD4+ T cells

Massive activation of infected CD4+ T cells during acute HIV-1 infection leads to reservoir seeding and T-cell destruction. During T-cell activation, the antiviral effect of the innate factor SAMHD1 is neutralized through phosphorylation at T592, allowing HIV-1 infection. Dasatinib, a tyrosine kinase inhibitor currently used for treating chronic myeloid leukemia, has been described to control HIV-1 replication through its negative effect on T-cell proliferation and viral entry. We demonstrate that Dasatinib can actually interfere with SAMHD1 phosphorylation in human peripheral blood lymphocytes, preserving its antiviral activity against HIV-1. Dasatinib prevented SAMHD1 phosphorylation in vitro and ex vivo, impairing HIV-1 reverse transcription and proviral integration. This was the major mechanism of action because the presence of Vpx, which degrades SAMHD1, in HIV-1 virions impeded the inhibitory effect of Dasatinib on HIV-1 replication. In fact, infection with VSV-pseudotyped HIV-1 virions and fusion of BlaM-Vpr-containing HIV-1 viruses with activated PBMCs in the presence of Dasatinib suggested that Dasatinib was not acting at fusion level. Finally, PBMCs from patients on chronic treatment with Dasatinib showed a lower level of SAMHD1 phosphorylation in response to activating stimuli and low susceptibility to HIV-1 infection ex vivo. Consequently, Dasatinib is a compound currently used in clinic that preserves the antiviral function of SAMHD1. Using Dasatinib as adjuvant of antiretroviral therapy during early primary HIV-1 infection would contribute to reduce viral replication and spread, prevent reservoir seeding, and preserve CD4 counts and CTL responses. These events would create a more favorable virologic and immunologic environment for future interventional studies aiming at HIV-1 eradication.

SAMHD1 Gene Mutations Are Associated with Cerebral Large-Artery Atherosclerosis

BACKGROUND

To investigate whether one or more SAMHD1 gene mutations are associated with cerebrovascular disease in the general population using a Chinese stroke cohort.

METHODS

Patients with a Chinese Han background (N = 300) diagnosed with either cerebral large-artery atherosclerosis (LAA, n = 100), cerebral small vessel disease (SVD, n = 100), or other stroke-free neurological disorders (control, n = 100) were recruited. Genomic DNA from the whole blood of each patient was isolated, and direct sequencing of the SAMHD1 gene was performed. Both wild type and mutant SAMHD1 proteins identified from the patients were expressed in E. coli and purified; then their dNTPase activities and ability to form stable tetramers were analysed in vitro.

RESULTS

Three heterozygous mutations, including two missense mutations c.64C>T (P22S) and c.841G>A (p.E281K) and one splice site mutation c.696+2T>A, were identified in the LAA group with a prevalence of 3%. No mutations were found in the patients with SVD or the controls (p = 0.05). The mutant SAMHD1 proteins were functionally impaired in terms of their catalytic activity as a dNTPase and ability to assemble stable tetramers.

CONCLUSIONS

Heterozygous SAMHD1 gene mutations might cause genetic predispositions that interact with other risk factors, resulting in increased vulnerability to stroke.

Phospho-dependent Regulation of SAMHD1 Oligomerisation Couples Catalysis and Restriction

SAMHD1 restricts HIV-1 infection of myeloid-lineage and resting CD4⁺ T-cells. Most likely this occurs through deoxynucleoside triphosphate triphosphohydrolase activity that reduces cellular dNTP to a level where reverse transcriptase cannot function, although alternative mechanisms have been proposed recently. Here, we present combined structural and virological data demonstrating that in addition to allosteric activation and triphosphohydrolase activity, restriction correlates with the capacity of SAMHD1 to form “long-lived” enzymatically competent tetramers. Tetramer disruption invariably abolishes restriction but has varied effects on in vitro triphosphohydrolase activity. SAMHD1 phosphorylation also ablates restriction and tetramer formation but without affecting triphosphohydrolase steady-state kinetics. However phospho-SAMHD1 is unable to catalyse dNTP turnover under conditions of nucleotide depletion. Based on our findings we propose a model for phosphorylation-dependent regulation of SAMHD1 activity where dephosphorylation switches housekeeping SAMHD1 found in cycling cells to a high-activity stable tetrameric form that depletes and maintains low levels of dNTPs in differentiated cells.

SAMHD1 is a restriction factor that blocks infection of certain immune cells by HIV-1. It was discovered to be an enzyme that catalyses the breakdown of dNTPs, suggesting that it inhibits HIV-1 replication by reducing cellular dNTP pools to such low levels that reverse transcriptase cannot function. However, recently, alternative mechanisms have been proposed. SAMHD1 is also regulated by phosphorylation, although the effects of phosphorylation on protein function are unclear. In order to address these issues, we carried out combined structural and virological studies and have demonstrated that in addition to allosteric activation and triphosphohydrolase activity, restriction correlates with the capacity of SAMHD1 to form “long-lived” enzymatically competent tetramers. Disrupting the tetramer in various ways always abolished restriction but had differing effects on enzyme activity in vitro. SAMHD1 phosphorylation also prevented restriction and tetramer formation but without affecting enzyme catalysis under steady-state dNTP conditions. However phosphorylated SAMHD1 was unable to catalyse dNTP turnover at very low nucleotide levels that more accurately represent conditions in the cells in which restriction takes place. Based on our findings we propose a model for phosphorylation-dependent regulation of SAMHD1 activity and substantiate that degradation of dNTPs by SAMHD1 is sufficient to restrict HIV-1.

Genetics and molecular biology of brain calcification

Brain calcification is a common neuroimaging finding in patients with neurological, metabolic, or developmental disorders, mitochondrial diseases, infectious diseases, traumatic or toxic history, as well as in otherwise normal older people. Patients with brain calcification may exhibit movement disorders, seizures, cognitive impairment, and a variety of other neurologic and psychiatric symptoms. Brain calcification may also present as a single, isolated neuroimaging finding. When no specific cause is evident, a genetic etiology should be considered. The aim of the review is to highlight clinical disorders associated with brain calcification and provide summary of current knowledge of diagnosis, genetics, and pathogenesis of brain calcification.

Intrinsic host restrictions to HIV-1 and mechanisms of viral escape

To replicate in their hosts, viruses have to navigate the complexities of the mammalian cell, co-opting mechanisms of cellular physiology while defeating restriction factors that are dedicated to halting their progression. Primate lentiviruses devote a relatively large portion of their coding capacity to counteracting restriction factors by encoding accessory proteins dedicated to neutralizing the antiviral function of these intracellular inhibitors. Research into the roles of the accessory proteins has revealed the existence of previously undetected intrinsic defenses, provided insight into the evolution of primate lentiviruses as they adapt to new species and uncovered new targets for the development of therapeutics. This Review discusses the biology of the restriction factors APOBEC3, SAMHD1 and tetherin and the viral accessory proteins that counteract them.

The Expression of Functional Vpx during Pathogenic SIVmac Infections of Rhesus Macaques Suppresses SAMHD1 in CD4+ Memory T Cells

For nearly 20 years, the principal biological function of the HIV-2/SIV Vpx gene has been thought to be required for optimal virus replication in myeloid cells. Mechanistically, this Vpx activity was recently reported to involve the degradation of Sterile Alpha Motif and HD domain-containing protein 1 (SAMHD1) in this cell lineage. Here we show that when macaques were inoculated with either the T cell tropic SIVmac239 or the macrophage tropic SIVmac316 carrying a Vpx point mutation that abrogates the recruitment of DCAF1 and the ensuing degradation of endogenous SAMHD1 in cultured CD4⁺ T cells, virus acquisition, progeny virion production in memory CD4⁺ T cells during acute infection, and the maintenance of set-point viremia were greatly attenuated. Revertant viruses emerging in two animals exhibited an augmented replication phenotype in memory CD4⁺ T lymphocytes both in vitro and in vivo, which was associated with reduced levels of endogenous SAMHD1. These results indicate that a critical role of Vpx in vivo is to promote the degradation of SAMHD1 in memory CD4⁺ T lymphocytes, thereby generating high levels of plasma viremia and the induction of immunodeficiency.

Primate lentiviruses, such as HIV and its SIV simian relative, encode accessory proteins that suppress cellular restriction factors interfering with efficient replication. One of these, designated Vpx, is produced in infected cells by HIV-2 and some SIV strains, which cause endemic infections in African monkeys. The primary function of Vpx has long been thought to facilitate infectivity in dendritic cells and macrophage by degrading the Sterile Alpha Motif and HD domain-containing protein 1 (SAMHD1), which restricts virus replication in these cells. Using SIVmac carrying a mutated Vpx gene with a single amino acid change that prevents it from binding to DCAF1 and subsequently mediating the degradation of SAMHD1, we show that virus infection of CD4⁺ T lymphocytes is markedly compromised both in vitro and in vivo. The SIV Vpx mutant is severely attenuated in establishing new infections in inoculated rhesus monkeys, in producing high levels of virus progeny, in degrading SAMHD1 in memory CD4⁺ T cell in infected animals, and in inducing symptomatic disease. Thus, although once considered to be only critical for optimal replication in macrophage based on earlier studies performed with cultured cells, the SIV Vpx protein is functionally important in vivo for establishing the primary infection in rhesus macaques, sustaining high levels of virus replication in CD4⁺ T lymphocytes, and promoting the onset of symptomatic immunodeficiency.

SAMHD1 has differential impact on the efficacies of HIV nucleoside reverse transcriptase inhibitors

Sterile alpha motif- and histidine/aspartic acid domain-containing protein 1 (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 here that SAMHD1 cleaves NRTI triphosphates (TPs) at significantly lower rates than dNTPs and that SAMHD1 depletion from monocytic cells affects the susceptibility of HIV-1 infections 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.

Inhibition of Hepatitis B virus replication by SAMHD1

Sterile alpha motif and HD-domain containing protein 1 (SAMHD1) is a newly identified intracellular antiviral factor. By depleting the dNTPs pool of host cells to a low level that cannot support the efficient synthesis of viral cDNA, it restricts replication of some retroviruses. As a DNA virus, Hepatitis B virus (HBV) experiences a process of reverse transcription in its life cycle akin to that of retroviruses. However, whether SAMHD1 can restrict HBV replication in liver cells is unknown. Here, we reported that SAMHD1 expression was detectable in four liver cell lines. Exogenous expression of SAMHD1 in SMMC-7721 cells restrained HBV replication. Similarly, SAMHD1 impeded HBV replication in another liver cell line, BEL-7402. Remarkably, the catalytically inactive mutant, SAMHD1 HD/AA also hampered HBV replication. Additionally, HBV replication reduced SAMHD1 expression in HepG2 cells. Moreover, it was found that IFN-α induced expression of SAMHD1 in liver cells. Together, these findings suggested that IFN-α-inducible SAMHD1 inhibited HBV replication in liver cells.

Host restriction factor SAMHD1 limits human T cell leukemia virus type 1 infection of monocytes via STING-mediated apoptosis

Human T cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T cell leukemia and HTLV-1-associated myelopathies. In addition to T cells, HTLV-1 infects cells of the myeloid lineage, which play critical roles in the host innate response to viral infection. Investigating the monocyte depletion observed during HTLV-1 infection, we discovered that primary human monocytes infected with HTLV-1 undergo abortive infection accompanied by apoptosis dependent on SAMHD1, a host restriction factor that hydrolyzes endogenous dNTPs to below the levels required for productive reverse transcription. Reverse transcription intermediates (RTI) produced in the presence of SAMHD1 induced IRF3-mediated antiviral and apoptotic responses. Viral RTIs complexed with the DNA sensor STING to trigger formation of an IRF3-Bax complex leading to apoptosis. This study provides a mechanistic explanation for abortive HTLV-1 infection of monocytes and reports a link between SAMHD1 restriction, HTLV-1 RTI sensing by STING, and initiation of IRF3-Bax driven apoptosis.

Moderate restriction of macrophage-tropic human immunodeficiency virus type 1 by SAMHD1 in monocyte-derived macrophages

Macrophage-tropic human immunodeficiency virus type 1 (HIV-1) strains are able to grow to high titers in human monocyte-derived macrophages. However, it was recently reported that cellular protein SAMHD1 restricts HIV-1 replication in human cells of the myeloid lineage, including monocyte-derived macrophages. Here we show that degradation of SAMHD1 in monocyte-derived macrophages was associated with moderately enhanced growth of the macrophage-tropic HIV-1 strain. SAMHD1 degradation was induced by treating target macrophages with vesicular stomatitis virus glycoprotein-pseudotyped human immunodeficiency virus type 2 (HIV-2) particles containing viral protein X. For undifferentiated monocytes, HIV-2 particle treatment allowed undifferentiated monocytes to be fully permissive for productive infection by the macrophage-tropic HIV-1 strain. In contrast, untreated monocytes were totally resistant to HIV-1 replication. These results indicated that SAMHD1 moderately restricts even a macrophage-tropic HIV-1 strain in monocyte-derived macrophages, whereas the protein potently restricts HIV-1 replication in undifferentiated monocytes.

Antagonism of SAMHD1 is actively maintained in natural infections of simian immunodeficiency virus

Restriction factors are effectors of the innate immune response to viral pathogens that inhibit viral replication by operating as molecular barriers to steps of the viral life cycle. The restriction factor SAMHD1 blocks lentiviral reverse transcription in myeloid cells and resting CD4+ T cells. Many lineages of lentiviruses, including HIV-2 and other simian immunodeficiency viruses, encode accessory genes that serve to counteract host SAMHD1 restriction by causing degradation of the antiviral factor. The viral accessory protein Vpr is responsible for SAMHD1 degradation in some lineages of lentiviruses, whereas in others the related protein Vpx assumes this task. However, HIV-1 has no SAMHD1 degradation capability, leading to questions about the selective advantage of this activity. We use an evolutionary approach to examine the importance of SAMHD1 antagonism for viral fitness by studying adaptation to host SAMHD1 in natural simian immunodeficiency virus infections of African Green Monkeys. We identified multiple SAMHD1 haplotypes in African Green Monkeys and find that the vpr gene from different strains of Simian Immunodeficiency Virus has adapted to the polymorphisms of the African Green Monkey population in which it is found. Such evidence of viral adaptation to host restriction indicates that SAMHD1 antagonism is actively maintained in natural infections and that this function must be advantageous to viral fitness, despite its absence in HIV-1.

HIV accessory proteins versus host restriction factors

Primate immunodeficiency viruses, including HIV-1, are characterized by the presence of accessory genes such as vif, vpr, vpx, vpu, and nef. Current knowledge indicates that none of the primate lentiviral accessory proteins has enzymatic activity. Instead, these proteins interact with cellular ligands to either act as adapter molecules to redirect the normal function of host factors for virus-specific purposes or to inhibit a normal host function by mediating degradation or causing intracellular mislocalization/sequestration of the factors involved. This review aims at providing an update of our current understanding of how Vif, Vpu, and Vpx control the cellular restriction factors APOBEC3G, BST-2, and SAMHD1, respectively.

Assessment of interferon-related biomarkers in Aicardi-Goutières syndrome associated with mutations in TREX1, RNASEH2A, RNASEH2B, RNASEH2C, SAMHD1, and ADAR: a case-control study

BACKGROUND

Aicardi-Goutières syndrome (AGS) is an inflammatory disorder caused by mutations in any of six genes (TREX1, RNASEH2A, RNASEH2B, RNASEH2C, SAMHD1, and ADAR). The disease is severe and effective treatments are urgently needed. We investigated the status of interferon-related biomarkers in patients with AGS with a view to future use in diagnosis and clinical trials.

METHODS

In this case-control study, samples were collected prospectively from patients with mutation-proven AGS. The expression of six interferon-stimulated genes (ISGs) was measured by quantitative PCR, and the median fold change, when compared with the median of healthy controls, was used to create an interferon score for each patient. Scores higher than the mean of controls plus two SD (>2·466) were designated as positive. Additionally, we collated historical data for interferon activity, measured with a viral cytopathic assay, in CSF and serum from mutation-positive patients with AGS. We also undertook neutralisation assays of interferon activity in serum, and looked for the presence of autoantibodies against a panel of interferon proteins.

FINDINGS

74 (90%) of 82 patients had a positive interferon score (median 12·90, IQR 6·14-20·41) compared with two (7%) of 29 controls (median 0·93, IQR 0·57-1·30). Of the eight patients with a negative interferon score, seven had mutations in RNASEH2B (seven [27%] of all 26 patients with mutations in this gene). Repeat sampling in 16 patients was consistent for the presence or absence of an interferon signature on 39 of 41 occasions. Interferon activity (tested in 147 patients) was negatively correlated with age (CSF, r=-0·604; serum, r=-0·289), and was higher in CSF than in serum in 104 of 136 paired samples. Neutralisation assays suggested that measurable antiviral activity was related to interferon α production. We did not record significantly increased concentrations of autoantibodies to interferon subtypes in patients with AGS, or an association between the presence of autoantibodies and interferon score or serum interferon activity.

INTERPRETATION

AGS is consistently associated with an interferon signature, which is apparently sustained over time and can thus be used to differentiate patients with AGS from controls. If future studies show that interferon status is a reactive biomarker, the measurement of an interferon score might prove useful in the assessment of treatment efficacy in clinical trials.

FUNDING

European Union's Seventh Framework Programme; European Research Council.

SAMHD1-dependent retroviral control and escape in mice

SAMHD1 is a host restriction factor for human immunodeficiency virus 1 (HIV-1) in cultured human cells. SAMHD1 mutations cause autoimmune Aicardi-Goutières syndrome and are found in cancers including chronic lymphocytic leukaemia. SAMHD1 is a triphosphohydrolase that depletes the cellular pool of deoxynucleoside triphosphates, thereby preventing reverse transcription of retroviral genomes. However, in vivo evidence for SAMHD1's antiviral activity has been lacking. We generated Samhd1 null mice that do not develop autoimmune disease despite displaying a type I interferon signature in spleen, macrophages and fibroblasts. Samhd1(-/-) cells have elevated deoxynucleoside triphosphate (dNTP) levels but, surprisingly, SAMHD1 deficiency did not lead to increased infection with VSV-G-pseudotyped HIV-1 vectors. The lack of restriction is likely attributable to the fact that dNTP concentrations in SAMHD1-sufficient mouse cells are higher than the KM of HIV-1 reverse transcriptase (RT). Consistent with this notion, an HIV-1 vector mutant bearing an RT with lower affinity for dNTPs was sensitive to SAMHD1-dependent restriction in cultured cells and in mice. This shows that SAMHD1 can restrict lentiviruses in vivo and that nucleotide starvation is an evolutionarily conserved antiviral mechanism.

Revising the Role of Myeloid cells in HIV Pathogenesis

Lentiviruses are characterized by their ability to infect resting cells, such as CD4 T cells, macrophages and dendritic cells (DC). Cells of myeloid lineage, which herein we include including monocytes, macrophages, and dendritic cells, play a pivotal role in HIV infection by not only promoting transmission and spread but also serving as viral reservoirs. However, the recent discovery of the HIV restriction factor SAMHD1 within myeloid cells has again led us to question the role of this lineage both in HIV transmission and pathogenesis. Herein we will summarize what the potential role of myeloid cells in HIV pathogenesis is and how recent observations have or haven't reshaped this view. Finally we highlight the idea that cells of myeloid lineage are quality rather than quantity HIV substrates. Thus, whilst is may indeed be difficult for a lentivirus like HIV to infect a resting cell like a macrophage and/or Dendritic cell, there are significant benefits in doing so, even at low frequency.

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.

Various viral pathogens such as HIV-1, herpes simplex virus (HSV) and vaccinia virus infect terminally-differentiated/non-dividing macrophages during the course of viral pathogenesis. Unlike dividing cells, non-dividing cells lack chromosomal DNA replication, do not enter the cell cycle, and harbor very low levels of cellular dNTPs, which are substrates of viral DNA polymerases. A series of recent studies revealed that the host protein SAMHD1 is dNTP triphosphohydrolase, which contributes to the poor dNTP abundance in non-dividing myeloid cells, and restricts proviral DNA synthesis of HIV-1 and other lentiviruses in macrophages, dendritic cells, and resting T cells. In this report, we demonstrate that SAMHD1 also controls the replication of large dsDNA viruses: vaccinia virus and HSV-1, in primary human monocyte-derived macrophages. SAMHD1 suppresses the replication of these DNA viruses to an even greater extent in the absence of viral genes that are involved in dNTP metabolism such as thymidine kinase. Therefore, this study supports that dsDNA viruses evolved to express enzymes necessary to increase the levels of dNTPs as a mechanism to overcome the restriction induced by SAMHD1 in myeloid cells.

Tactics used by HIV-1 to evade host innate, adaptive, and intrinsic immunities

OBJECTIVE

To review the mechanisms by which HIV evades different components of the host immune system.

DATA SOURCES

This review is based on data obtained from published articles from 1991 to 2012. To perform the PubMed literature search, the following key words were input: HIV and immune evasion.

STUDY SELECTION

Articles containing information related to HIV immune evasion were selected.

RESULTS

Although HIV is able to induce vigorous antiviral immune responses, viral replication cannot be fully controlled, and neither pre-existing infected cells nor latent HIV infection can be completely eradicated. Like many other enveloped viruses, HIV can escape recognition by the innate and adaptive immune systems. Recent findings have demonstrated that HIV can also successfully evade host restriction factors, the components of intrinsic immune system, such as APOBEC3G (apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G), TRIM5α (tripartite motif 5-α), tetherin, and SAMHD1 (SAM-domain HD-domain containing protein).

CONCLUSIONS

HIV immune evasion plays an important role in HIV pathogenesis. Fully understanding the tactics deployed by HIV to evade various components of the host immune systems will allow for the development of novel strategies aimed toward the prevention and cure of HIV/AIDS.

IL-27 inhibits HIV-1 infection in human macrophages by down-regulating host factor SPTBN1 during monocyte to macrophage differentiation

The susceptibility of macrophages to HIV-1 infection is modulated during monocyte differentiation. IL-27 is an anti-HIV cytokine that also modulates monocyte activation. In this study, we present new evidence that IL-27 promotes monocyte differentiation into macrophages that are nonpermissive for HIV-1 infection. Although IL-27 treatment does not affect expression of macrophage differentiation markers or macrophage biological functions, it confers HIV resistance by down-regulating spectrin β nonerythrocyte 1 (SPTBN1), a required host factor for HIV-1 infection. IL-27 down-regulates SPTBN1 through a TAK-1-mediated MAPK signaling pathway. Knockdown of SPTBN1 strongly inhibits HIV-1 infection of macrophages; conversely, overexpression of SPTBN1 markedly increases HIV susceptibility of IL-27-treated macrophages. Moreover, we demonstrate that SPTBN1 associates with HIV-1 gag proteins. Collectively, our results underscore the ability of IL-27 to protect macrophages from HIV-1 infection by down-regulating SPTBN1, thus indicating that SPTBN1 is an important host target to reduce HIV-1 replication in one major element of the viral reservoir.

The restriction factors of human immunodeficiency virus

Cellular proteins called "restriction factors" can serve as powerful blockades to HIV replication, but the virus possesses elaborate strategies to circumvent these barriers. First, we discuss general hallmarks of a restriction factor. Second, we review how the viral Vif protein protects the viral genome from lethal levels of cDNA deamination by promoting APOBEC3 protein degradation; how the viral Vpu, Env, and Nef proteins facilitate internalization and degradation of the virus-tethering protein BST-2/tetherin; and how the viral Vpx protein prevents the premature termination of reverse transcription by degrading the dNTPase SAMHD1. These HIV restriction and counter-restriction mechanisms suggest strategies for new therapeutic interventions.

Functional analysis of the relationship between Vpx and the restriction factor SAMHD1

SAMHD1 is a newly identified restriction factor that targets lentiviruses in myeloid cells and is countered by the SIV(SM)/HIV-2 Vpx protein. By analyzing a large panel of Vpx mutants, we identify several residues throughout the 3-helix bundle predicted for Vpx that impair both its functionality and its ability to degrade SAMHD1. We determine that SAMHD1 is a strictly non-shuttling nuclear protein and that as expected WT Vpx localizes with it in the nucleus. However, we also identify a functional Vpx mutant with predominant cytoplasmic distribution that colocalizes with SAMHD1 in this location, suggesting that Vpx may also retain SAMHD1 in the cell cytoplasm, prior to its entry into the nucleus. Several mutations in Vpx were shown to affect the stability of Vpx, as well as Vpx:Vpx interactions. However, no strict correlation was observed between these parameters and the functionality of Vpx, implying that neither properties is absolutely required for this function and indicating that even unstable Vpx mutants may be very efficient in inducing SAMHD1 degradation. Overall, our analysis identifies several Vpx residues required for SAMHD1 degradation and points to a very efficient and plastic mechanism through which Vpx depletes this restriction factor.

[Research progress of the anti-HIV activity of SAMHD1]

The SAM and HD domain containing protein 1 (Sterile alpha motif domain and HD domain-containing protein 1, SAMHD1) is a putative negative regulator of the antiviral innate immune response. It can significantly increase the antiviral immune response, mediates the interferon-induced inflammatory response involved in the host foreign-virus defense system. The early studies have focused on its gene mutations associated with Aicardi-Goutières syndrome (AGS), the latest study found that SAMHD1 as a potent dGTP-stimulated triphosphohydrolase restricts HIV-1 replication by hydrolyzing the majority of cellular dNTPs, thus inhibiting reverse transcription and viral complementary DNA (cDNA) synthesis. Auxiliary gene of HIV-2 and simian immunodeficiency virus (SIVsm / mac) encoding the Vpx protein can eliminate HIV-1 restriction. In recent years, the research on SAMHD1, mores forward rapidly this paper overviews the recent research progression related to the above fields.

Rules of engagement: molecular insights from host-virus arms races

Mammalian genes and genomes have been shaped by ancient and ongoing challenges from viruses. These genetic imprints can be identified via evolutionary analyses to reveal fundamental details about when (how old), where (which protein domains), and how (what are the functional consequences of adaptive changes) host-virus arms races alter the proteins involved. Just as extreme amino acid conservation can serve to identify key immutable residues in enzymes, positively selected residues point to molecular recognition interfaces between host and viral proteins that have adapted and counter-adapted in a long series of classical Red Queen conflicts. Common rules for the strategies employed by both hosts and viruses have emerged from case studies of innate immunity genes in primates. We are now poised to use these rules to transition from a retrospective view of host-virus arms races to specific predictions about which host genes face pathogen antagonism and how those genetic conflicts transform host and virus evolution.

SAMHD1-deficient CD14+ cells from individuals with Aicardi-Goutières syndrome are highly susceptible to HIV-1 infection

Myeloid blood cells are largely resistant to infection with human immunodeficiency virus type 1 (HIV-1). Recently, it was reported that Vpx from HIV-2/SIVsm facilitates infection of these cells by counteracting the host restriction factor SAMHD1. Here, we independently confirmed that Vpx interacts with SAMHD1 and targets it for ubiquitin-mediated degradation. We found that Vpx-mediated SAMHD1 degradation rendered primary monocytes highly susceptible to HIV-1 infection; Vpx with a T17A mutation, defective for SAMHD1 binding and degradation, did not show this activity. Several single nucleotide polymorphisms in the SAMHD1 gene have been associated with Aicardi-Goutières syndrome (AGS), a very rare and severe autoimmune disease. Primary peripheral blood mononuclear cells (PBMC) from AGS patients homozygous for a nonsense mutation in SAMHD1 (R164X) lacked endogenous SAMHD1 expression and support HIV-1 replication in the absence of exogenous activation. Our results indicate that within PBMC from AGS patients, CD14+ cells were the subpopulation susceptible to HIV-1 infection, whereas cells from healthy donors did not support infection. The monocytic lineage of the infected SAMHD1 -/- cells, in conjunction with mostly undetectable levels of cytokines, chemokines and type I interferon measured prior to infection, indicate that aberrant cellular activation is not the cause for the observed phenotype. Taken together, we propose that SAMHD1 protects primary CD14+ monocytes from HIV-1 infection confirming SAMHD1 as a potent lentiviral restriction factor.

Lentiviral accessory proteins play important roles in antagonizing host proteins aimed at suppressing HIV-1 replication at a cellular level. The SIV/HIV-2 protein Vpx counteracts SAMHD1, a previously unknown antiviral factor within myeloid blood cells, rendering these cells permissive to primate immunodeficiency viruses. We confirm in this study that Vpx interacts with SAMHD1 leading to ubiquitin-mediated degradation of SAMHD1, and renders CD14 positive monocytes susceptible to HIV-1 infection. We provide new insights into the ability of SAMHD1 to protect monocytic cells from HIV-1 infection by using primary cells from patients with Aicardi-Goutières syndrome (AGS) lacking endogenous SAMHD1 expression. We show that peripheral monocytic cells of AGS patients are highly permissive to HIV-1. Thus, our study demonstrates that SAMHD1 is critical for restriction of HIV-1 infection in monocytes adding SAMHD1 as a novel innate defense factor.

Aicardi-Goutières syndrome and systemic lupus erythematosus (SLE) in a 12-year-old boy with SAMHD1 mutations

Aicardi-Goutières syndrome is an early-onset encephalopathy with a presumed immune pathogenesis caused by inherited defects in nucleic acid metabolism. The clinical picture resembles a congenital viral infection despite negative investigations for common viruses. In addition to leukoencephalopathy with calcifications of basal ganglia, patients show increased levels of the antiviral cytokine interferon-α in cerebrospinal fluid. We report on a 12-year-old boy with Aicardi-Goutières syndrome and systemic lupus erythematosus (SLE) due to mutations in the SAMHD1 (sterile alpha motif domain and HD domain-containing protein 1) gene, illustrating an emerging pattern of the natural history of Aicardi-Goutières syndrome characterized by neurological disease followed by symptoms of systemic autoimmunity. Thus, Aicardi-Goutières syndrome constitutes a model disease for systemic autoimmunity triggered by the activation of the innate immune system. Recognition of the etiologic link between Aicardi-Goutières syndrome and systemic lupus erythematosus has direct implications on therapeutic management and suggests that early immune modulatory intervention can improve neurological outcome.

Restricted access to myeloid cells explained

The lentiviral accessory protein, Vpx, is known to counteract a restriction factor that is specific to myeloid cells, such as macrophages and dendritic cells. This review summarizes the findings in two seminal studies that identify SAMHD1 as the cellular protein that is responsible for myeloid cell restriction, and establish the existence of other types of restriction in these cells.

Expressed sequence tag analysis of human RPE/choroid for the NEIBank Project: over 6000 non-redundant transcripts, novel genes and splice variants

PURPOSE

The retinal pigment epithelium (RPE) and choroid comprise a functional unit of the eye that is essential to normal retinal health and function. Here we describe expressed sequence tag (EST) analysis of human RPE/choroid as part of a project for ocular bioinformatics.

METHODS

A cDNA library (cs) was made from human RPE/choroid and sequenced. Data were analyzed and assembled using the program GRIST (GRouping and Identification of Sequence Tags). Complete sequencing, Northern and Western blots, RH mapping, peptide antibody synthesis and immunofluorescence (IF) have been used to examine expression patterns and genome location for selected transcripts and proteins.

RESULTS

Ten thousand individual sequence reads yield over 6300 unique gene clusters of which almost half have no matches with named genes. One of the most abundant transcripts is from a gene (named "alpha") that maps to the BBS1 region of chromosome 11. A number of tissue preferred transcripts are common to both RPE/choroid and iris. These include oculoglycan/opticin, for which an alternative splice form is detected in RPE/choroid, and "oculospanin" (Ocsp), a novel tetraspanin that maps to chromosome 17q. Antiserum to Ocsp detects expression in RPE, iris, ciliary body, and retinal ganglion cells by IF. A newly identified gene for a zinc-finger protein (TIRC) maps to 19q13.4. Variant transcripts of several genes were also detected. Most notably, the predominant form of Bestrophin represented in cs contains a longer open reading frame as a result of splice junction skipping.

CONCLUSIONS

The unamplified cs library gives a view of the transcriptional repertoire of the adult RPE/choroid. A large number of potentially novel genes and splice forms and candidates for genetic diseases are revealed. Clones from this collection are being included in a large, nonredundant set for cDNA microarray construction.

Specific antiviral activity demonstrated by TGTP, a member of a new family of interferon-induced GTPases

The GTPase superfamily includes a diversity of molecules whose functions are regulated through the binding and hydrolysis of GTP. This superfamily can be segregated into families of functionally related molecules that typically share amino acid sequence similarity within and around the nucleotide-binding domains. A new family of putative GTPases, including IRG-47, LRG-47, IGTP, and TGTP/Mg21, has recently emerged that share significant sequence identity (25-40%). Expression of these molecules has been shown to be selectively induced by IFN-gamma and in some cases by IFN-alpha beta or bacterial LPS. This induction pattern implicates these putative GTPases as part of the innate defense of cells to infection, but their role in such defense has not yet been defined. We have previously described the cloning of TGTP and now confirm its intrinsic activity as a GTPase. We found that TGTP is strongly induced by endogenous IFN-alpha beta produced in response to standard lipofection of plasmid DNA or polyinosinic polycytidylic acid. The ability of endogenously produced IFN-alpha beta to efficiently induce expression of TGTP under these conditions suggested that TGTP might participate in defense against viral infection. This proposal was borne out when TGTP-transfected L cells displayed relative resistance to plaque formation by vesicular stomatitis virus but not herpes simplex virus. This observation places TGTP among a small family of innate antiviral agents and has implications for the functions of other members of this family of GTPases.

Identification of a novel GTPase, the inducibly expressed GTPase, that accumulates in response to interferon gamma

Interferon gamma is a pleiotropic cytokine that regulates many immune functions. We have identified a novel protein, inducibly expressed GTPase (IGTP), whose expression was regulated by interferon gamma in macrophages. In mouse RAW 264.7 macrophages, IGTP mRNA levels were almost undetectable but increased within 1 h of exposure to interferon gamma, peaked at very high levels within 3 h, and remained at high levels to at least 48 h; pretreatment of the cells with cycloheximide blocked the majority of mRNA accumulation. In the mouse, the mRNA was highly expressed in thymus, spleen, lung, and small intestine. Using interspecific backcross analysis, the Igtp gene was mapped to mouse chromosome 11. The IGTP cDNA encoded a putative polypeptide of Mr 48,507 and pI 7.79 that contained three consensus GTP binding motifs, GXXXXGK(S/T), DXXG, and NTKXD. Both IGTP that had been immunoprecipitated from RAW cells and a glutathione S-transferase IGTP fusion protein were able to convert GTP to GDP in vitro. Subcellular protein fractionation and Western blotting localized IGTP to the cytosol of RAW cells. In addition, the protein was homologous to proteins encoded by three previously cloned cDNAs, IRG-47, TGTP/Mg21, and LRG-47, and thus may be representative of a new family of interferon gamma-regulated GTPases.

Cloning and characterization of a novel cDNA that is IFN-gamma-induced in mouse peritoneal macrophages and encodes a putative GTP-binding protein

Macrophage activation by IFN-gamma results in a cascade of gene expression. To identify genes activated in mouse peritoneal macrophages by IFN-gamma, we created a cDNA subtraction library of IFN-gamma-induced genes. We have isolated from this subtraction library a novel cDNA clone, called Mg21, whose mRNA is absent in unstimulated mouse peritoneal macrophages and is induced to high levels within 4 h following the addition of IFN-gamma. Induction of Mg21 mRNA by IFN-gamma occurred in the presence of cycloheximide, indicating that expression of Mg21 mRNA does not require protein synthesis. A small amount of Mg21 mRNA was also induced by LPS, but not by IL-2, IL-4, IL-10, or TNF-alpha. The DNA sequence of Mg21 is 1617 nucleotides and contains an open reading frame that codes for a protein of 415 amino acids with a predicted molecular weight of 47,106 Da. The predicted amino acid sequence lacks a signal sequence or transmembrane segments, indicating that the protein is an intracellular protein. Computer search of the GenBank and EMBL databases indicates that this cDNA clone is unique but has 57% sequence identity with IRG-47, which is a mouse gene induced by IFN-gamma in pre-B and B lymphocyte cell lines. IRG-47 encodes an intracellular protein that contains three conserved protein motifs present in GTP-binding proteins. Analysis of the protein sequence of Mg21 showed that these three conserved protein motifs are also present in Mg21.