Effect of induced dNTP pool imbalance on HIV-1 reverse transcription in macrophages

SAMHD1 shapes deoxynucleotide triphosphate homeostasis by interconnecting the depletion and biosynthesis of different dNTPs

SAMHD1 is a dNTPase that impedes replication of HIV-1 in myeloid cells and resting T lymphocytes. Here we elucidate the substrate activation mechanism of SAMHD1, which involves dNTP binding at allosteric sites and transient tetramerization. Our findings reveal that tetramerization alone is insufficient to promote dNTP hydrolysis; instead, the activation mechanism requires an inactive tetrameric intermediate with partially occupied allosteric sites. The equilibrium between inactive and active tetrameric states regulates dNTPase activity, driven by the binding and dissociation of additional allosteric dNTP ligands to the preassembled tetramer. Furthermore, catalytic efficiency, but not substrate specificity, is modulated by the identity of the dNTPs occupying the allosteric sites. We show how this allosteric regulation shapes deoxynucleotide homeostasis by balancing dNTP production and SAMHD1-catalyzed depletion. Notably, SAMHD1 exhibits a distinct functionality, which we term facilitated dNTP depletion, whereby increased biosynthesis of certain dNTPs enhances the depletion of others. The regulatory relationship between the biosynthesis and depletion of different dNTPs sheds light on the emerging role of SAMHD1 in the biology of dNTP homeostasis with implications for HIV/AIDS, innate antiviral immunity, T cell disorders, telomere maintenance and therapeutic efficacy of nucleoside analogs.

Biochemical simulation of mutation synthesis and repair during SARS-CoV-2 RNA polymerization

We biochemically simulated the mutation synthesis process of SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) complex (nsp7/nsp8/nsp12) involving two sequential mechanistic steps that occur during genomic replication: misinsertion (incorporation of incorrect nucleotides) and mismatch extension. Then, we also simulated mismatch repair process catalyzed by the viral nsp10/nsp14 ExoN complex. In these mechanistic simulations, while SARS-CoV-2 RdRp displays efficient mutation synthesis capability, the viral ExoN complex was able to effectively repair the mismatch primers generated during the mutation synthesis. Also, we observed that the delayed RNA synthesis induced by mutation synthesis process was rescued by the viral ExoN activity. Collectively, our biochemical simulations suggest that SARS-CoV-2 ExoN complex may contribute to both maintenance of proper viral genetic diversity levels and successful completion of the viral large RNA genome replication by removing mismatches generated by the viral RdRp.

Modeling of HIV-1 prophylactic efficacy and toxicity with islatravir shows non-superiority for oral dosing, but promise as a subcutaneous implant

HIV prevention with pre-exposure prophylaxis (PrEP) constitutes a major pillar in fighting the ongoing epidemic. While daily oral PrEP adherence may be challenging, long-acting (LA-)PrEP in oral or implant formulations could overcome frequent dosing with convenient administration. The novel drug islatravir (ISL) may be suitable for LA-PrEP, but dose-dependent reductions inCD 4 + T cell and lymphocyte counts were observed at high doses. We developed a mathematical model to predict ISL pro-drug levels in plasma and active intracellular ISL-triphosphate concentrations after oral vs. subcutaneous implant dosing. Using phase II trial data, we simulated antiviral effects and estimated HIV risk reduction for multiple dosages and dosing frequencies. We then established exposure thresholds where no adverse effects on immune cells were observed. Our findings suggest that implants with 56-62 mg ISL offer effective HIV risk reduction without reducing lymphocyte counts. Oral 0.1 mg daily, 3-5 mg weekly, and 10 mg biweekly ISL provide comparable efficacy, but weekly and biweekly doses may affect lymphocyte counts, while daily dosing regimen offered no advantage over existing oral PrEP. Oral 0.5-1 mg on demand provided> 90 % protection, while not being suitable for post-exposure prophylaxis. These findings suggest ISL could be considered for further development as a promising and safe agent for implantable PrEP.

Allosteric substrate activation of SAMHD1 shapes deoxynucleotide triphosphate imbalances by interconnecting the depletion and biosynthesis of different dNTPs

SAMHD1 is a dNTPase that impedes replication of HIV-1 in myeloid cells and resting T lymphocytes. Here we elucidate the substrate activation mechanism of SAMHD1 that depends on dNTP binding at allosteric sites and the concomitant tetramerization of the enzyme. The study reveals that SAMHD1 activation involves an inactive tetrameric intermediate with partial occupancy of the allosteric sites. The equilibrium between the inactive and active tetrameric states, which is coupled to cooperative binding/dissociation of at least two allosteric dNTP ligands, controls the dNTPase activity of the enzyme, which, in addition, depends on the identity of the dNTPs occupying the four allosteric sites of the active tetramer. We show how such allosteric regulation determines deoxynucleotide triphosphate levels established in the dynamic equilibria between dNTP production and SAMHD1-catalyzed depletion. Notably, the mechanism enables a distinctive functionality of SAMHD1, which we call facilitated dNTP depletion, whereby elevated biosynthesis of some dNTPs results in more efficient depletion of others. The regulatory relationship between the biosynthesis and depletion of different dNTPs sheds light on the emerging role of SAMHD1 in the biology of dNTP homeostasis with implications for HIV/AIDS, innate antiviral immunity, T cell disorders, telomere maintenance and therapeutic efficacy of nucleoside analogs.

Replication-competent HIV-1 in human alveolar macrophages and monocytes despite nucleotide pools with elevated dUTP

BACKGROUND

Although CD4⁺ memory T cells are considered the primary latent reservoir for HIV-1, replication competent HIV has been detected in tissue macrophages in both animal and human studies. During in vitro HIV infection, the depleted nucleotide pool and high dUTP levels in monocyte derived macrophages (MDM) leads to proviruses with high levels of dUMP, which has been implicated in viral restriction or reduced transcription depending on the uracil base excision repair (UBER) competence of the macrophage. Incorporated dUMP has also been detected in viral DNA from circulating monocytes (MC) and alveolar macrophages (AM) of HIV infected patients on antiretroviral therapy (ART), establishing the biological relevance of this phenotype but not the replicative capacity of dUMP-containing proviruses.

RESULTS

As compared to in vitro differentiated MDM, AM from normal donors had sixfold lower levels of dTTP and a sixfold increased dUTP/dTTP, indicating a highly restrictive dNTP pool for reverse transcription. Expression of uracil DNA glycosylase (UNG) was eightfold lower in AM compared to the already low levels in MDM. Accordingly, ~ 80% of HIV proviruses contained dUMP, which persisted for at least 14-days due to low UNG excision activity. Unlike MDM, AM expression levels of UNG and SAM and HD domain containing deoxynucleoside triphosphate triphosphohydrolase 1 (SAMHD1) increased over 14 days post-HIV infection, while dUTP nucleotidohydrolase (DUT) expression decreased. These AM-specific effects suggest a restriction response centered on excising uracil from viral DNA copies and increasing relative dUTP levels. Despite the restrictive nucleotide pools, we detected rare replication competent HIV in AM, peripheral MC, and CD4⁺ T cells from ART-treated donors.

CONCLUSIONS

These findings indicate that the potential integration block of incorporated dUMP is not realized during in vivo infection of AM and MC due to the near absence of UBER activity. In addition, the increased expression of UNG and SAMHD1 in AM post-infection is too slow to prevent integration. Accordingly, dUMP persists in integrated viruses, which based on in vitro studies, can lead to transcriptional silencing. This possible silencing outcome of persistent dUMP could promote viral latency until the repressive effects of viral dUMP are reversed.

Molecular Biology and Diversification of Human Retroviruses

Studies of retroviruses have led to many extraordinary discoveries that have advanced our understanding of not only human diseases, but also molecular biology as a whole. The most recognizable human retrovirus, human immunodeficiency virus type 1 (HIV-1), is the causative agent of the global AIDS epidemic and has been extensively studied. Other human retroviruses, such as human immunodeficiency virus type 2 (HIV-2) and human T-cell leukemia virus type 1 (HTLV-1), have received less attention, and many of the assumptions about the replication and biology of these viruses are based on knowledge of HIV-1. Existing comparative studies on human retroviruses, however, have revealed that key differences between these viruses exist that affect evolution, diversification, and potentially pathogenicity. In this review, we examine current insights on disparities in the replication of pathogenic human retroviruses, with a particular focus on the determinants of structural and genetic diversity amongst HIVs and HTLV.

Thymidylate synthase is essential for efficient HIV-1 replication in macrophages

Thymidylate synthase (TS) is a key enzyme in nucleotide biosynthesis. A study performed by our group on human monocyte-derived macrophages (MDMs) infected with HIV-1 showed that many enzymes related to the folate cycle pathway, such as TS, are upregulated in productively infected cells. Here, we suggest that TS is essential for an effective HIV-1 infection in MDMs. Indeed, a TS specific small interfering RNA (siRNA) as well as the TS specific inhibitor Raltitrexed (RTX) caused a reduction in productively infected cells. Quantitative PCR analysis showed that this treatment decreased the efficacy of the early steps of the viral cycle. The RTX inhibitory effect was counteracted by dNTP addition. These results suggest that TS is essential for the early stages of HIV-1 infection by providing optimal dNTP concentrations in MDMs. TS and its related pathway may thus be considered as a potential therapeutic target for HIV-1 treatment.