Abstract
BackgroundHuman SAMHD1 is a triphosphohydrolase that restricts the replication of retroviruses, retroelements and DNA viruses in noncycling cells. While modes of action have been extensively described for human SAMHD1, only little is known about the regulation of SAMHD1 in the mouse. Here, we characterize the antiviral activity of murine SAMHD1 with the help of knockout mice to shed light on the regulation and the mechanism of the SAMHD1 restriction and to validate the SAMHD1 knockout mouse model for the use in future infectivity studies.ResultsWe found that endogenous mouse SAMHD1 restricts not only HIV-1 but also MLV reporter virus infection at the level of reverse transcription in primary myeloid cells. Similar to the human protein, the antiviral activity of murine SAMHD1 is regulated through phosphorylation at threonine 603 and is limited to nondividing cells. Comparing the susceptibility to infection with intracellular dNTP levels and SAMHD1 phosphorylation in different cell types shows that both functions are important determinants of the antiviral activity of murine SAMHD1. In contrast, we found the proposed RNase activity of SAMHD1 to be less important and could not detect any effect of mouse or human SAMHD1 on the level of incoming viral RNA.ConclusionOur findings show that SAMHD1 in the mouse blocks retroviral infection at the level of reverse transcription and is regulated through cell cycle-dependent phosphorylation. We show that the antiviral restriction mediated by murine SAMHD1 is mechanistically similar to what is known for the human protein, making the SAMHD1 knockout mouse model a valuable tool to characterize the influence of SAMHD1 on the replication of different viruses in vivo.
Highlights
Human SAMHD1 is a triphosphohydrolase that restricts the replication of retroviruses, retroelements and DNA viruses in noncycling cells
Mouse SAMHD1 blocks HIV reporter virus infectivity at the level of reverse transcription To determine the role of muSAMHD1 during retroviral infection, we first analyzed the antiviral activity of muSAMHD1 in primary bone marrow-derived dendritic cells (BMDC) and human myeloid U937 cells, which stably express the muSAMHD1 isoforms 1 and 2 individually
Preincubation of murine BMDCs with simian immunodeficiency viruses (SIV) virus-like particles (VLP) containing the HIV-2/ SIV accessory protein Vpx did not counteract the antiviral activity of muSAMHD1 by inducing the degradation of endogenous SAMHD1
Summary
Human SAMHD1 is a triphosphohydrolase that restricts the replication of retroviruses, retroelements and DNA viruses in noncycling cells. It has been suggested that the SAMHD1 phosphomimetic mutants T592D and T592E lose their antiviral activity but are still able to hydrolyze cellular dNTPs [28, 31, 32] Based on these results an additional unknown mechanism of SAMHD1 restriction besides dNTP depletion has been proposed. Recent work by three different groups showed independently that the phosphorylation of SAMHD1 at T592 downregulates the dNTP hydrolase activity of the protein, especially at low nucleotide concentrations [33,34,35] These findings strongly suggest that the depletion of dNTPs by human SAMHD1 is the most likely mechanism of retroviral restriction
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