SAMHD1 is a biomarker for cytarabine response and a therapeutic target in acute myeloid leukemia.

The nucleoside analog cytarabine (Ara-C) is an essential component of primary and salvage chemotherapy regimens for acute myeloid leukemia (AML). After cellular uptake, Ara-C is converted into its therapeutically active triphosphate metabolite, Ara-CTP, which exerts antileukemic effects, primarily by inhibiting DNA synthesis in proliferating cells. Currently, a substantial fraction of patients with AML fail to respond effectively to Ara-C therapy, and reliable biomarkers for predicting the therapeutic response to Ara-C are lacking. SAMHD1 is a deoxynucleoside triphosphate (dNTP) triphosphohydrolase that cleaves physiological dNTPs into deoxyribonucleosides and inorganic triphosphate. Although it has been postulated that SAMHD1 sensitizes cancer cells to nucleoside-analog derivatives through the depletion of competing dNTPs, we show here that SAMHD1 reduces Ara-C cytotoxicity in AML cells. Mechanistically, dGTP-activated SAMHD1 hydrolyzes Ara-CTP, which results in a drastic reduction of Ara-CTP in leukemic cells. Loss of SAMHD1 activity-through genetic depletion, mutational inactivation of its triphosphohydrolase activity or proteasomal degradation using specialized, virus-like particles-potentiates the cytotoxicity of Ara-C in AML cells. In mouse models of retroviral AML transplantation, as well as in retrospective analyses of adult patients with AML, the response to Ara-C-containing therapy was inversely correlated with SAMHD1 expression. These results identify SAMHD1 as a potential biomarker for the stratification of patients with AML who might best respond to Ara-C-based therapy and as a target for treating Ara-C-refractory AML

The underpinning biology relating to multiple sclerosis disease modifying treatments during the COVID-19 pandemic

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Interaction of viral glycoproteins with cellular attachment factors

Die Interaktion von viralen Hüllproteinen mit zellulären Rezeptoren vermittelt den infektiösen Eintritt von umhüllten Viren in Zielzellen. Im Gegensatz dazu ist die Bindung von Viren an Anheftungsfaktoren nicht essentiell für die Infektion, sie kann aber den Eintritt in rezeptorpositive Zellen massiv verstärken. Die Lektine DC-SIGN und DC-SIGNR (gemeinsam abgekürzt als DC-SIGN/R) binden an glykosilierte Hüllproteine verschiedener Viren und fördern so deren Anheftung an Zielzellen. In wie weit die Bindung an Lektine den Zell- bzw. Organtropismus und die Pathogenese der einzelnen Viren im Patienten beeinflusst, ist jedoch nur unzureichend verstanden. Es wurde postuliert, dass die Lektinbindung die Ausbreitung des humanen Immundefizienz-Virus (HIV) und des Ebolavirus (EBOV) in und zwischen Patienten fördert. Das Spektrum der Lektinbindungspartner von HIV und EBOV, bzw. das Spektrum der durch Lektine gebundenen Viren ist jedoch nur unvollständig bekannt. Zudem ist unklar, ob Polymorphismen in DC-SIGNR die Infektionsverstärkung modulieren, und ob die Maus als Tiermodell zur Analyse der DC-SIGN-Funktion eingesetzt werden kann. Im Rahmen dieser Arbeit konnten bisher unbekannte Wechselwirkungen zellulärer Lektine mit viralen Hüllproteinen identifiziert werden. So verstärken DC-SIGN/R die von den Hüllproteinen des Marburgvirus und des SARS-Coronavirus (SARS-CoV) getriebene Infektion. Erstmals wurde auch das virale Pathogenspektrum von LSECtin charakterisiert. Das, auf sinusoidalen Endothelzellen in Leber und Lymphknoten exprimierte, Lektin erhöht die Infektion von Filoviren und SARS-CoV, ist aber nicht in der Lage mit HIV und den Hüllproteinen des Hepatitis-C-Virus zu interagieren. Ein Zusammenhang zwischen Polymorphismen im DC-SIGNR-Gen und dem Risiko einer HIV-Infektion ist in der Literatur dokumentiert. Es konnte jedoch gezeigt werden, dass die am häufigsten vorkommenden DC-SIGNR-Polymorphismen, zumindest in Zellkultur, keinen Einfluss auf die Interaktion von DC-SIGNR mit HIV und anderen viralen Pathogenen haben. Zur Untersuchung der Rolle von DC-SIGN bei der viralen Ausbreitung in vivo ist die Etablierung eines Mausmodells wünschenswert. Es konnte nachgewiesen werden, dass verschiedene Domänen in murinem DC-SIGN (CIRE), welches als einziges DC-SIGN-Homolog auf dendritischen Zellen der Maus exprimiert wird, eine effiziente Interaktion mit Pathogenen, die von humanem DC-SIGN erkannt werden, verhindern. Murines DC-SIGN kann daher nicht dazu verwendet werden, ein Mausmodell zur DC-SIGN-Funktion zu etablieren. Diese Ergebnisse tragen wesentlich zum Verständnis der Rolle von Lektinen in der HIV-, Filovirus- und Coronavirus-Infektion bei. Die Entwicklung eines Tiermodells für die DC-SIGN/R-Funktion ist jedoch unerlässlich, um die Bedeutung dieser Lektine für die virale Vermehrung und Pathogenese im Wirt zu klärenEngagement of a receptor is essential for infectious entry of enveloped viruses into target cells. In contrast, the interaction with so called attachment factors does not allow viral entry, but can strongly augment infection. Many lectins, like DC-SIGN or DC-SIGNR (collectively referred to as DC-SIGN/R), can act as viral attachment factors and are able to bind glycosylated viral envelope proteins and to enhance viral infection. Despite the clear phenotype in vitro, the role of DC-SIGN/R in viral spread and pathogenesis in patients is unknown. It has been postulated that lectin engagement of human immunodeficiency virus (HIV) and Ebola virus (EBOV) might promote viral spread in and between individuals. In order to elucidate the role of DC-SIGN/R and other lectins in viral dissemination and organ-tropism, this work examines possible interactions between viral glycoproteins and cellular lectins. Furthermore, it is unclear if common DC-SIGNR polymorphisms modulate augmentation of viral infection and it remains to be clarified if the mouse could serve as an animal model for DC-SIGN function. This work shows that DC-SIGN/R enhance infection mediated by the glycoproteins (GPs) of Marburg virus and SARS-coronavirus (SARS-CoV). In addition, evidence is presented that the lectin LSECtin, coexpressed with DC-SIGNR on sinusoidal endothelial cells in liver and lymph nodes, enhances infection driven by the glycoproteins (GP) of filoviruses and SARS-coronavirus, but does not interact with HIV and hepatitis-C-virus envelope proteins. An association between the DC-SIGNR genotype and the risk of HIV infection has been documented. Nevertheless, evidence was obtained that the most frequent DC-SIGNR polymorphisms do not diminish the interaction with HIV and other viral pathogens, at least in vitro. For the analysis of DC-SIGN function a mouse model for human DC-SIGN is desirable. Therefore, the murine homologue mDC-SIGN (CIRE), the only DC-SIGN homologue expressed on mouse dendritic cells, and its interactions with viral pathogens were characterized. This study shows that different domains within mDC-SIGN are blocking an efficient interaction with pathogens known to bind to human DC-SIGN. These results indicate that mDC-SIGN on mouse dendritic cells is not an adequate model for pathogen interactions with human DC-SIGN. In summary, this work sheds light on the role of lectins in HIV, filovirus and coronavirus infection, but also shows that the generation of a mouse model is essential for a better understanding of the impact of lectin engagement on viral tropism and pathogenesis

SAMHD1 … and Viral Ways around It

The SAM and HD domain-containing protein 1 (SAMHD1) is a dNTP triphosphohydrolase that plays a crucial role for a variety of different cellular functions. Besides balancing intracellular dNTP concentrations, facilitating DNA damage repair, and dampening excessive immune responses, SAMHD1 has been shown to act as a major restriction factor against various virus species. In addition to its well-described activity against retroviruses such as HIV-1, SAMHD1 has been identified to reduce the infectivity of different DNA viruses such as the herpesviruses CMV and EBV, the poxvirus VACV, or the hepadnavirus HBV. While some viruses are efficiently restricted by SAMHD1, others have developed evasion mechanisms that antagonize the antiviral activity of SAMHD1. Within this review, we summarize the different cellular functions of SAMHD1 and highlight the countermeasures viruses have evolved to neutralize the restriction factor SAMHD1

Evidence for an Activation Domain at the Amino Terminus of Simian Immunodeficiency Virus Vpx▿ †

Vpx and Vpr are related lentiviral accessory proteins that enhance virus replication in macrophages and dendritic cells. Both proteins are packaged into virions and mediate their effects in the target cell through an interaction with an E3 ubiquitin ligase that contains DCAF1 and DDB1. When introduced into primary macrophages and dendritic cells in viruslike particles, Vpx can enhance the efficiency of a subsequent infection. Here, we confirm the ability of Vpx to enhance simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) infection of macrophages up to 100-fold by using single-cycle reporter viruses and by pretreatment of the cells with Vpx-containing viruslike particles. Vpx was also active in differentiated THP-1 cells but not in other cell lines. Induction of an antiviral state in macrophages with type I interferon significantly magnified the effect of Vpx on HIV-1 infection, suggesting that Vpx helps the virus to overcome an inducible intracellular restriction. Quantitative PCR quantitation of SIV and HIV-1 reverse transcripts in newly infected macrophages showed that the block was at an early step in reverse transcription. In spite of its structural similarity, Vpr was inactive. This difference allowed us to map the functional domains of Vpx with a panel of Vpr/Vpx chimeras. Analysis of the chimeras demonstrated that the amino-terminal domain of Vpx is important for the enhancement of infection. Fine mapping of the region indicated that amino acids at positions 9, 12, and 15 to 17 were required. Although the mutants failed to enhance infection, they retained their ability to interact with DCAF1. These findings suggest that the Vpx amino terminus contains an activation domain that serves as the binding site for a cellular restriction factor

Evidence that multiple defects in murine DC-SIGN inhibit a functional interaction with pathogens

AbstractCertain viruses, bacteria, fungi and parasites target dendritic cells through the interaction with the cellular attachment factor DC-SIGN, making this C-type lectin an attractive target for therapeutic intervention. Studies on DC-SIGN function would be greatly aided by the establishment of a mouse model, however, it is unclear if the murine (m) homologue of human (h) DC-SIGN also binds to pathogens. Here, we investigated the interaction of mDC-SIGN, also termed CIRE, with the Ebolavirus glycoprotein (EBOV-GP), a ligand of hDC-SIGN. We found that mDC-SIGN neither binds EBOV-GP nor enhances infection by reporterviruses pseudotyped with EBOV-GP. Analysis of chimeras between mDC-SIGN and hDC-SIGN provided evidence that determinants in the carbohydrate recognition domain and in the neck domain of mDC-SIGN inhibit a functional interaction with EBOV-GP. Moreover, mDC-SIGN was found be monomeric, suggesting that lack of multimerization, which is believed to be required for efficient pathogen recognition by hDC-SIGN, might be one factor that prevents binding of mDC-SIGN to EBOV-GP. Our results suggest that mDC-SIGN on murine dendritic cells is not an adequate model for pathogen interactions with hDC-SIGN

The SAMHD1-mediated block of LINE-1 retroelements is regulated by phosphorylation

Abstract Background The restriction factor SAMHD1 regulates intracellular nucleotide level by degrading dNTPs and blocks the replication of retroviruses and DNA viruses in non-cycling cells, like macrophages or dendritic cells. In patients, inactivating mutations in samhd1 are associated with the autoimmune disease Aicardi-Goutières Syndrome (AGS). The accumulation of intracellular nucleic acids derived from endogenous retroelements thriving in the absence of SAMHD1 has been discussed as potential trigger of the autoimmune reaction. In vitro, SAMHD1 has been found to restrict endogenous retroelements, like LINE-1 elements (L1). The mechanism, however, by which SAMHD1 blocks endogenous retroelements, is still unclear. Results Here, we show that SAMHD1 inhibits the replication of L1 and other endogenous retroelements in cycling cells. By applying GFP- and neomycin-based reporter assays we found that the anti-L1 activity of SAMHD1 is regulated by phosphorylation at threonine 592 (T592). Similar to the block of HIV, the cofactor binding site and the enzymatic active HD domain of SAMHD1 proofed to be essential for restriction of L1 elements. However, phosphorylation at T592 did not correlate with the dNTP hydrolase activity of SAMHD1 in cycling 293T cells suggesting an alternative mechanism of regulation. Interestingly, we found that SAMHD1 binds to ORF2 protein of L1 and that this interaction is regulated by T592 phosphorylation. Together with the finding that the block is also active in cycling cells, our results suggest that the SAMHD1-mediated inhibition of L1 is similar but not identical to HIV restriction. Conclusion Our findings show conclusively that SAMHD1 restricts the replication of endogenous retroelements in vitro. The results suggest that SAMHD1 is important for maintaining genome integrity and support the idea of an enhanced replication of endogenous retroelements in the absence of SAMHD1 in vivo, potentially triggering autoimmune diseases like AGS. Our analysis also contributes to the better understanding of the activities of SAMHD1 in antiviral defense and nucleotide metabolism. The finding that the phosphorylation of SAMHD1 at T592 regulates its activity against retroelements but not necessarily intracellular dNTP level suggests that the dNTP hydrolase activity might not be the only function of SAMHD1 important for its antiviral activity and for controlling autoimmunity

Correction to: The SAMHD1-mediated block of LINE-1 retroelements is regulated by phosphorylation

The original article [1] contains an omission in the Methods