Expression and Functions of Human Endogenous Retroviruses in the Placenta: An Update
Introduction
Endogenous retroviruses (ERVs) are complete or degenerate retroviral sequences which are normal components of the DNA of all cells. ERVs have been described in all vertebrates studied, and in humans, comprise about 5 per cent of the genome [1]. They are thought to be the remnants of ancient germ cell infections by now-extinct exogenous retroviruses. Phylogenetic analyses indicate that the integrations occurred early in primate evolution, with the various human endogenous retrovirus (HERV) families appearing at different times ranging from less than ten to 60 million years ago. The largest wave of insertions occurred approximately 30 million years ago, after the separation of the Old and New World monkeys [2].
HERVs are simple retroviruses consisting of gag, pol and env genes which code for the capsid components (nucleocapsid, capsid and matrix proteins), enzymes (reverse transcriptase (RT), ribonuclease H (RNaseH), integrase andprotease), and envelope protein respectively. These are flanked by regulatory regions known as long terminal repeats (LTRs) which contain a tRNA primer binding site (for priming reverse transcription), promoter and enhancer sequences, and RNA processing signals (Figure 1). HERV families are most commonly defined by the tRNA complementary to their putative primer binding site, using the one letter code for the tRNA's corresponding amino acid. So for example, the HERV-W family would be primed by tryptophan tRNA. Most HERV envelope proteins have a similar structure with surface and transmembrane portions. The transmembrane domain contains regions with potential fusogenic and immunosuppressive properties (Figure 2). The latter region is conserved amongst retroviruses and a corresponding synthetic peptide, CKS-17, has numerous immunomodulatory effects in vitro (Figure 3) [3], [4]. However, the ancient origin of HERVs has allowed the gradual accumulation of mutations which have abrogated the protein coding potential of many and, to date, there is scant evidence for the existence of infectious forms [5], [6]. Thousands of solitary LTRs are also found. These have resulted from homologous recombination between two proviral LTRs with deletion of the intervening viral genes.
The significance and consequences of the presence of HERVs in our genome have been the subject of extensive speculation and investigation. HERVs have been implicated in a wide variety of processes, both pathological, for example autoimmune disease and neoplasia, and physiological, including placental function and protection from exogenous retroviral infection [7], [8], [9], [10], [11], [12], [13]. One possible effect is mutagenesis at the insertion site and subsequent genomic instability since the latter is associated with repetitive sequences such as HERVs. HERV LTRs may affect the transcriptional regulation of neighbouring cellular genes and there is also growing evidence for the expression of retroviral proteins although their functions are incompletely understood.
The RNA expression of various HERV families has now been studied in an extensive range of cell types, tissues and diseases. From these investigations it has become apparent that the placenta is unique amongst normal tissues in both the diverse range of HERV transcripts and their high level of expression. This review will discuss the evidence for the placental expression of HERVs with particular emphasis on the envelope proteins of HERV-W and ERV3 for which interesting functional roles are emerging.
Section snippets
Placental retrovirus-like particles
One of the first hints that the placenta might be a site of HERV expression was the detection of retrovirus-like particles (RVLPs) in the villi of term placenta by electron microscopy [14]. This was corroborated by a number of further studies [15]. The percentage of placentae in which particles were detected varied quite considerably between reports, but all were consistent in their descriptions of the particle morphology, and location at the basal border of the syncytiotrophoblast where they
Cross-reactivity of trophoblast with animal retroviral antigens
Other early studies focussed on the apparent cross-reactivity between placental antigens and animal retroviruses. For example, five of six pregnant women showed cell-mediated immune reactivity to baboon endogenous virus (BaEV)-infected human fibroblasts [20]. Furthermore, an antiserum against syncytiotrophoblast plasma membrane reacted with cell lines producing BaEV and Mason–Pfizer monkey virus (M-PMV) [21]. Conversely, antibodies to BaEV and M-PMV displayed complement-dependent cytotoxicity
ERV3
ERV3 is a single copy HERV found on chromosome seven [27]. The primer binding site is most complementary to mouse arginine tRNA therefore it is alternatively known as HERV-R [28]. Its gag and pol genes have in-frame stop codons. Although env has a long ORF this is prematurely terminated in the transmembrane region, truncating the hydrophobic domain which anchors the protein in the cell membrane. Therefore, ERV3 Env may exist as a soluble molecule [29].
The transcription of ERV3 was examined in
HERV-W
Several studies have now provided evidence that HERV-W env is expressed almost exclusively in the placenta. Northern analysis of 23 normal tissues detected 8 kb and 4 kb transcripts in term placenta with much lower levels in the testis and no evidence for expression in any other tissue [38]. The more sensitive technique of quantitative PCR revealed HERV-W env mRNA in several normal tissues but confirmed restriction of high level transcription to the placenta [39]. In situ hybridization
HERV-FRD
Two recent publications have provided exciting evidence for a further placental envelope protein with possible functional importance. HERV-FRD family members were first identified in RVLPs produced by the breast carcinoma cell line T47-D by RT–PCR with degenerate pol primers and were named according to the first three amino acids of the deduced protein coding sequence [54]. A HERV-FRD env gene was subsequently identified during screening of the genome for potential complete coding Env proteins
Possible roles of HERVs in reproduction
To summarize, in addition to the abundance of HERV transcripts there is growing evidence for the expression of certain HERV proteins in the placenta. These products may have a variety of physiological roles: the envelope of HERV-W seems to be involved in cell fusion to form the syncytium and ERV3 Env is associated with cytotrophoblast differentiation.
In the Metavirus hypothesis, Villareal proposed that all mammals must express ERVs in extra-embryonic tissues to suppress local immune recognition
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