About 1% of the human genome is human endogenous retrovirus (HERV) sequences (
58). Some HERVs are transcribed, and HERV proteins as well as replication-defective virus particles have been detected in several tissues, under either pathological (
6,
48) or physiological (
32,
38) circumstances. However, the biological significance of HERV expression awaits clarification (
26,
30,
31). We have recently described a new family of HERVs, termed HERV-W (
4). A sequential multiprobe screening of a human DNA library showed that the human genome does not contain a replication-competent HERV-W provirus (F. Besème, J.-L. Blond, O. Bouton, and F. Mallet, unpublished data). The phylogenetic distribution of HERV-W sequences indicated that its ancestor entered the genomes of higher primates 25 to 40 million years ago, after the divergence of Old World and New World monkeys (
57).
We have previously shown that HERV-W expression in normal tissues, leading to transcription of mRNAs containing
gag,
pol, and
env sequences, is restricted to the placenta, allowing us to clone a cDNA containing a complete
env open reading frame (ORF) (
4). Although HERVs are frequently expressed in placental and various other tissues (
31,
58), few HERVs express complete envelope glycoproteins (Env). The loss of
env gene sequence by several HERVs (
5) or, alternatively, the selective pressure potentially exerted by evolution to maintain some HERV Env ORFs and restrict their expression in specific tissues suggests that Env may exhibit a positive role, provided adequate control and regulation of expression are supplied by the host. Indeed, a significant physiological potential resides in retroviral envelope glycoproteins (
22) and may permit several functions beneficial for the host (
26,
31), such as (i) inducing resistance to exogenous retrovirus invasion by receptor interference, (ii) conferring local immunosuppression, or (iii) allowing the formation of syncytia between neighboring cells. For example, ERV-3 envelope glycoproteins are abundantly expressed in placental tissue (
7) and have been proposed to participate in syncytiotrophoblast differentiation by fusing the underlying cytotrophoblast cell layer (
56). However, the presence of a stop codon before the membrane anchoring domain of ERV-3
env(
10) is likely to preclude a cell-cell fusion function. In contrast, the polypeptide putatively encoded by the HERV-W
env gene harbors all of the determinants (
4) exhibited by bona fide exogenous retrovirus envelopes required to promote membrane fusion, thus suggesting that HERV-W Env may be functional.
In this study, we therefore analyzed the virus-cell and cell-cell fusion properties of HERV-W Env by forcing its expression in vitro. We demonstrate that HERV-W encodes a highly fusogenic membrane glycoprotein able to induce syncytium formation upon interaction with the type D mammalian retrovirus receptor expressed in primate and pig cells. Moreover, we found that HERV-W was expressed in placenta cells, suggesting that it may be involved in normal placenta function.
DISCUSSION
Here we report the placental expression of an HERV-encoded envelope glycoprotein that exhibits all the features of retroviral envelopes necessary to promote cell-cell fusion. The persistence for more than 25 million years of an
env gene encoding a complete retroviral envelope glycoprotein in the genomes of Old World primates as well as its tissue-specific expression in human placenta suggests that evolution has retained a function of this protein that is beneficial for the host. Indeed, uncontrolled expression of fusogenic retroviral envelope glycoproteins in vivo or in vitro may cause cell death and tissue damage (
40) by a nonapoptotic process (
2). Based on in vitro studies, we demonstrate here that the HERV-W envelope glycoprotein can induce the formation of numerous syncytia upon interaction with the recently identified type D mammalian retrovirus receptor, a cell surface molecule whose gene is also transcribed in placenta cells (
53). It is therefore conceivable that HERV-W Env plays a physiological role in vivo in placenta development. During pregnancy, the syncytiotrophoblast cell layer, which comes into intimate contact with the maternal blood space, is formed by differentiation and homotypic fusion of the underlying trophoblastic cells. This process is associated with the expression of several types of endogenous retroviral particles in the placenta (
58). As such, the ERV-3 envelope glycoprotein has been suggested to play different roles, such as inducing differentiation of the cytotrophoblastic cells (
28) or, alternatively, preventing maternal immune rejection of the fetus (
56). However, the involvement of ERV-3 Env in placenta development remains questionable since a stop codon occurs before the membrane anchoring domain of ERV-3 Env (
10) and since recent results have indicated that the ERV-3
env gene is mutated in about 1% of homozygous individuals (
17). In the case of HERV-W, it will be essential to investigate the polymorphism of its
env gene(s) and promoter(s), as well as to analyze the host factors that regulate HERV-W expression in vivo, in order to better appreciate the positive selection exerted by evolution to preserve Env functional domains and expression. Nevertheless, the absence of HERV-W sequences in New World monkeys and in other nonprimate placental mammals (
57) indicates that syncytiotrophoblast differentiation may also be induced by distinct and/or complementary mechanisms.
The high-level cell-cell fusogenicity of HERV-W Env is striking in comparison to that of the envelope glycoproteins of type C and type D mammalian retroviruses, to which HERV-W is related (
4). Indeed, when expressed individually in cell culture, in the absence of other viral components, the envelope glycoproteins of the type C and D retroviruses do not induce the formation of numerous syncytia (
8,
27,
46). In contrast to the envelope glycoproteins of exogenous retroviruses that also use the type D mammalian retrovirus receptor, the HERV-W envelope glycoprotein is highly fusogenic in vitro. Fusogenicity of retroviral envelope glycoproteins is regulated at distinct stages of the Env maturation process. First, the Env polyprotein precursor must be cleaved by a
trans-Golgi cellular protease in order to release the SU and TM Env subunits and to allow the fusion peptide, located at the amino terminus of TM, to interact with the target cell membrane during retroviral-receptor-mediated activation of Env fusogenicity (
22). Thus, fusion-competent retroviral envelope glycoproteins must be found as Env precursors as well as processed SU and TM proteins in producer cells. Analysis of HERV-W Env expression did not allow us to detect the presence of the SU and TM Env subproducts (Fig.
1). This might be due to inefficient cleavage of the HERV-W Env precursor by cellular proteases, which would prevent detection of the processed Env products by Western blot analysis. Undetected or inefficient cleavage of the MLV Env precursor has already been reported in the literature and does not necessarily imply an incapacity to mediate membrane fusion (
27,
63); yet, the high membrane fusion activity of HERV-W Env indicates that precursor processing must occur to some degree.
Second, at least for MLVs (
44,
46) and for Mason-Pfizer monkey virus, a prototype type D simian retrovirus (
8), during or shortly after budding of the viral particles, a 16-amino-acid carboxy-terminal peptide of TM, named R peptide, is cleaved by the viral protease, allowing the envelope glycoprotein to be fusion competent. Thus, the TM carboxy-terminal ends of these Env proteins exert a fusion-inhibitory effect (
60,
61), and their removal by the viral protease is necessary for the full fusion activity of the envelope glycoprotein (
8,
46). The cytoplasmic tail of HERV-W Env is 35 amino acids longer than that of type D and type C mammalian retroviruses (
4). No retroviral-protease cleavage site could be found in HERV-W Env. Thus, in contrast to those of type D and type C mammalian retroviruses, the HERV-W Env cytoplasmic tail may contain a determinant that activates, or at least does not inhibit, fusogenicity. Of note, the cytoplasmic tails of most retrovirus envelope glycoproteins contain a YXXφ tyrosine-based sorting signal (where Y is Tyr, X is any amino acid, and φ is an amino acid with a bulky hydrophobic side chain [Leu, Ile, Phe, Val, or Met]) which plays a key role in subcellular distribution and adaptin-mediated endocytosis of plasma membrane-bound glycoproteins (
11). Disruption of this motif in human T-lymphotropic virus type 1 (HTLV-1) Env and in simian immunodeficiency virus Env results in increased cell-cell fusion and/or cell surface expression (
3,
15). Interestingly, the YXXφ motif is located in the R peptide for MLVs and Mason-Pfizer monkey virus Env but is missing in HERV-W Env (
4). Thus, its removal upon cleavage of the R peptide or, alternatively, its absence in the case of HERV-W Env is likely to result in augmented Env cell surface expression and fusogenicity.
Our data suggest that the lack of infectivity of MLV viral particles generated with HERV-W Env is probably caused by an inability of these envelope glycoproteins to be incorporated on virions. MLV virions have been shown to efficiently incorporate type I glycoproteins from other viruses that harbor short cytoplasmic tails, such as vesicular stomatitis virus (
18), Rous sarcoma virus (
25), Semliki Forest virus (
52), HTLV-1 (
16), human foamy virus (
29), fowl plague virus (
21), paramyxoviruses (
20,
51), lymphocytic choriomeningitis virus (
35), and Ebola virus (
59). Interestingly, incorporation of human immunodeficiency virus envelope glycoproteins that harbor long cytoplasmic tails could be achieved only after truncation of their cytoplasmic domains (
33,
47). Similarly, the unusually long cytoplasmic tail of HERV-W Env may explain its lack of incorporation on MLV viral particles. Ongoing studies are now aiming to determine if recombinant HERV-W envelope glycoproteins with shorter cytoplasmic tails can be incorporated on MLV viral particles as well as on virions of exogenous retroviruses that may infect humans. Indeed, since retroviruses are genetically unstable organisms and since they are being used as gene delivery vectors, the outcome of such studies is critical for several reasons: (i) interaction of exogenous retroviruses such as human immunodeficiency viruses, pig endogenous retroviruses (
42), or HTLVs with HERVs may by complementation, cross-packaging, and/or recombination give rise to new viruses with altered cell tropisms and/or pathogenicities; and (ii) HERVs may provide core and envelope proteins which perhaps contribute to mobilization and dissemination of retroviral or lentiviral vectors by
trans complementation (
41). Thus, unravelling the molecular details of the fusogenic property of HERV-W Env glycoproteins and their capacity to (be) transcomplement(ed by) exogenous retroviruses could have implications in ensuring the safety of gene therapy approaches and also in the elucidation of the hitherto poorly understood biological significance of HERV-W protein expression in placenta tissue.