Opossum APOBEC1 is a DNA mutator with retrovirus and retroelement restriction activity

APOBEC3s (A3s) are single-stranded DNA cytosine deaminases that provide innate immune defences against retroviruses and mobile elements. A3s are specific to eutherian mammals because no direct homologs exist at the syntenic genomic locus in metatherian (marsupial) or prototherian (monotreme) mammals. However, the A3s in these species have the likely evolutionary precursors, the antibody gene deaminase AID and the RNA/DNA editing enzyme APOBEC1 (A1). Here, we used cell culture-based assays to determine whether opossum A1 restricts the infectivity of retroviruses including human immunodeficiency virus type 1 (HIV-1) and the mobility of LTR/non-LTR retrotransposons. Opossum A1 partially inhibited HIV-1, as well as simian immunodeficiency virus (SIV), murine leukemia virus (MLV), and the retrotransposon MusD. The mechanism of inhibition required catalytic activity, except for human LINE1 (L1) restriction, which was deamination-independent. These results indicate that opossum A1 functions as an innate barrier to infection by retroviruses such as HIV-1, and controls LTR/non-LTR retrotransposition in marsupials

The antiretroviral potency of APOBEC1 deaminase from small animal species

Although the role of the APOBEC3-dependent retroelement restriction system as an intrinsic immune defense against human immunodeficiency virus type1 (HIV-1) infection is becoming clear, only the rat ortholog of mammalian APOBEC1s (A1) thus far has been shown to possess antiviral activity. Here, we cloned A1 cDNAs from small animal species, and showed that similar to rat A1, both wild-type and Δvif HIV-1 infection was inhibited by mouse and hamster A1 (4- to 10-fold), whereas human A1 had negligible effects. Moreover, rabbit A1 significantly reduced the infectivity of both HIV-1 virions (>300-fold), as well as that of SIVmac, SIVagm, FIV and murine leukemia virus. Immunoblot analysis showed that A1s were efficiently incorporated into the HIV-1 virion, and their packaging is mediated through an interaction with the nucleocapsid Gag domain. Interestingly, there was a clear accumulation of particular C-T changes in the genomic RNAs of HIV-1 produced in their presence, with few G-A changes in the proviral DNA. Together, these data reveal that A1 may function as a defense mechanism, regulating retroelements in a wide range of mammalian species

Retroviruses, retroelements and their restrictions

Human retroviruses, HIV and HTLV have been recognized as important pathogens because of their association with lethal diseases such as AIDS and ATL. Considerable resources and efforts have been directed at understanding the interaction between these retroviruses and their host which may provide clues as to how the infection can be controlled or prevented. Among the key scientific successes is the identification of intracellular “restriction factors” that have evolved as obstacles to the replication of pathogens including infectious retroviruses. The discovery of APOBEC, which are strong mutagens of retroviral genomes and intracellular retroelements, began a new era of intense research activities into the spectrum of intrinsic anti-HIV activity, leading to the identification of TRIM5a, BST2/Tetherin, and SAMHD1. In response, HIV has evolved several accessory genes as weaponries to evade these intracellular restriction activities. The intracellular antiretroviral defenses evolved in response to endogenous retroelements that make up more than 40% of the entire mammalian genome, and which are regarded as ancestors of infectious retroviruses. LTR-type retroelements are present in all higher eukaryotes, representing about 8% of the human genome. Non-LTR retroelements can be found at extremely high copy numbers also, with a significant portion of mammalin genomes consisting of LINEs. Mammalian genomes are modified by LINEs through insertions, but also by the indirect replication of non-autonomous retrotransposons such as SINEs. LINEs insertion was shown to have played, and continue to play important roles in genomic evolution and somatic genome mosaicism-mediated physiology. And, because retrotransposition can confer genetic diversity that is beneficial to the host, the vertebrate intrinsic immunity has evolved to support a balance between retroelement insertions that confer beneficial and those that cause deleterious gene disruptions. The articles published in this Research Topic should serve not only as valuable references for the field, but provide future topics of research for investigators that should further our understanding of the retrovirus, retroelements and their restrictions

Ability of Small Animal Cells to Support the Postintegration Phase of Human Immunodeficiency Virus Type-1 Replication

AbstractWe examine the potential for a broad range of small animal cells, including rodent, mink, and avian cells, from multiple tissues to support postintegration steps of HIV-1 replication. These cells were engineered so as to support a stable expression of human cyclin T1 and were further transduced with HIV-1 gag and pol genes. Viral gene expression was activated by the presence of human cyclin T1, but, with the exception of mink cells, was not at the level seen in human cells. Furthermore, there were considerable defects in p24 CA release, in particular in the case of rodent cells. Fractionation of Gag proteins by sucrose floatation revealed that the Gag in human cells trafficked to membrane fractions and was processed to p24 CA and p17 MA efficiently. Confocal imaging demonstrated that Gag was localized in a punctate pattern at the plasma membrane as well as intracellular membrane trans-Golgi cisternae in these cells. In contrast, the majority of Gag in rodent cells was largely present in cytosolic complexes and remained unprocessed. Labeling with [9,10(n)-3H]myristic acid showed a similar degree of N-myristoylated Pr55gag in rodent and human cells, indicating that while N-myristoylation of Gag was essential for membrane binding, it was not sufficient to confer membrane targeting specificity. Remarkably, despite the reduced level of intracellular Gag processing, mink Mv.1.Lu cells did not appear to differ significantly from human cells in support of virion assembly and release. Analysis of reciprocal heterokaryons suggested that the cellular factor(s) required for efficient assembly and release of infectious virions is lacking in murine cells but appears to be functionally present in mink as well as human cells. Our findings confirm and extend previous reports of multiple blocks to HIV replication in nonhuman cells that are most profound in murine cells. They also raise the possibility that other small animals, such as mink, could serve as novel model systems for studying HIV-1 infection and disease