Mouse knockout models for HIV-1 restriction factors

Abstract

Infection of cells with human immunodeficiency virus 1 (HIV-1) is controlled by restriction factors, host proteins that counteract a variety of steps in the life cycle of this lentivirus. These include SAMHD1, APOBEC3G and tetherin, which block reverse transcription, hypermutate viral DNA and prevent progeny virus release, respectively. These and other HIV-1 restriction factors are conserved and have clear orthologues in the mouse. This review summarises studies in knockout mice lacking HIV-1 restriction factors. In vivo experiments in such animals have not only validated in vitro data obtained from cultured cells, but have also revealed new findings about the biology of these proteins. Indeed, genetic ablation of HIV-1 restriction factors in the mouse has provided evidence that restriction factors control retroviruses and other viruses in vivo and has led to new insights into the mechanisms by which these proteins counteract infection. For example, in vivo experiments in knockout mice demonstrate that virus control exerted by restriction factors can shape adaptive immune responses. Moreover, the availability of animals lacking restriction factors opens the possibility to study the function of these proteins in other contexts such as autoimmunity and cancer. Further in vivo studies of more recently identified HIV-1 restriction factors in gene targeted mice are, therefore, justified.

Fig. 1

HIV-1 restriction factors. The function of selected HIV-1 restriction factors is shown in the context of the HIV-1 life cycle. Restriction factors are indicated in black and steps in the viral life cycle are in red. Viral proteins are represented by coloured symbols, including reverse transcriptase (violet squares), integrase (green triangles) and envelope glycoprotein (orange hexagons). Viral RNA is represented by a wavy red line and viral DNA by a straight dark blue line. The functions of MOV10, Mx2 and viperin are speculative as indicated by question marks and dotted arrows. Please refer to the text for details

Fig. 2

Tetherin prevents release of virus particles from infected cells. Tetherin is shown in black with red filling. Tetherin is a type II transmembrane protein. The N-terminus is indicated (N) and bears a short cytoplasmic tail, followed by a transmembrane domain (TM), a coiled-coil region and a C-terminal glycophosphatidylinositol (GPI) anchor (circle). During budding, either the TM domain or the GPI anchor can be incorporated into virions, resulting in their “tethering” to the plasma membrane. The two different conformations by which tetherin can connect the cell membrane with the viral envelope are indicated (1 and 2). Please note that tetherin forms a dimer via coiled-coil interactions, which is not shown here for simplicity

Fig. 3

Degradation of dNTPs by SAMHD1 controls reverse transcription and may impact on other cellular processes. a Two alternative mechanisms by which SAMHD1 may inhibit reverse transcription (RT) are indicated. (Left) SAMHD1 forms a tetramer [85, 86, 192] that cleaves dNTPs—the substrates required for RT—into deoxyribonucleosides (dNs) and inorganic triphosphate (PPP). Each monomer has an active site (white circle) and dGTP (or GTP [192, 193]; black circles) is bound at an allosteric site. (Right) SAMHD1 may also counteract reverse transcription by binding and/or degrading viral nucleic acids. b By lowering the intracellular dNTP concentration ([dNTP] low), SAMHD1 not only restricts retroviruses (RVs) but also DNA viruses and perhaps endogenous retroelements (REs). Moreover, balanced dNTP pools are likely to be required for accurate DNA replication and DNA repair (green arrow). c Mutations in SAMHD1, which have been described in Aicardi-Goutières syndrome (AGS) and cancers such as chronic lymphocytic leukaemia, are predicted to disrupt the proteins’ catalytic function. As a result, dNTP concentrations in cells are elevated ([dNTP] high). This facilitates replication of retroviruses (RVs), DNA viruses and possibly retroelements (REs). Detection of REs by the innate immune system could result in chronic production of type I interferons (IFN) triggering the onset of AGS [194]. Insertion of REs into new positions in the genome could also be a source of mutations leading to the development of cancer. Moreover, imbalanced dNTP concentrations in the absence of functional SAMHD1 may impact on the fidelity of DNA replication and repair (red arrow) or on cell cycle progression, further promoting genome instability and transformation [195, 196]

Fig. 4

APOBEC3G introduces mutations into viral cDNA and prevents reverse transcription. APOBEC3G (yellow star) is incorporated into virus particles during budding and is associated with the viral core. It is, therefore, delivered into newly infected cells. APOBEC3G directly inhibits reverse transcription, for example by blocking the progression of reverse transcriptase (violet square). APOBEC3G also deaminates cytosine to uracil in minus-sense single-stranded cDNA, causing extensive incorporation of deoxyadenosine instead of deoxyguanosine during plus-strand synthesis. This introduces mutations into the viral genome that can be deleterious, a process called hypermutation