Unique Structure and Distinctive Properties of the Ancient and Ubiquitous Gamma-Type Envelope Glycoprotein

Receptor-recognition and antiviral mechanisms of retrovirus-derived human proteins

Human syncytin-1 and suppressyn are cellular proteins of retroviral origin involved in cell-cell fusion events to establish the maternal-fetal interface in the placenta. In cell culture, they restrict infections from members of the largest interference group of vertebrate retroviruses, and are regarded as host immunity factors expressed during development. At the core of the syncytin-1 and suppressyn functions are poorly understood mechanisms to recognize a common cellular receptor, the membrane transporter ASCT2. Here, we present cryo-electron microscopy structures of human ASCT2 in complexes with the receptor-binding domains of syncytin-1 and suppressyn. Despite their evolutionary divergence, the two placental proteins occupy similar positions in ASCT2, and are stabilized by the formation of a hybrid β-sheet or 'clamp' with the receptor. Structural predictions of the receptor-binding domains of extant retroviruses indicate overlapping binding interfaces and clamping sites with ASCT2, revealing a competition mechanism between the placental proteins and the retroviruses. Our work uncovers a common ASCT2 recognition mechanism by a large group of endogenous and disease-causing retroviruses, and provides high-resolution views on how placental human proteins exert morphological and immunological functions.

Comparison of Endogenous Alpharetroviruses (ALV-like) across Galliform Species: New Distant Proviruses

The Genus Alpharetrovirus contains viruses pathogenic mainly for chickens, forming the Avian Sarcoma and Leukosis Virus group (ASLV). Cells of most Galliform species, besides chickens, contain genetic elements (endogenous retroviruses, ERVs) that could recombine with other alpharetroviruses or express proteins, complementing defective ASLV, which may successfully replicate and cause disease. However, they are quite unknown, and only ALV-F, from ring-necked pheasants, has been partially published. Upon scrutiny of 53 genomes of different avian species, we found Alpharetrovirus-like sequences only in 12 different Galliformes, including six full-length (7.4-7.6 Kbp) and 27 partial sequences. Phylogenetic studies of the regions studied (LTR, gag, pol, and env) consistently resulted in five almost identical clades containing the same ERVs: Clade I (presently known ASLVs); Clade II (Callipepla spp. ERVs); Clade IIIa (Phasianus colchicus ERVs); Clade IIIb (Alectoris spp. ERVs); and Clade IV (Centrocercus spp. ERVs). The low pol identity scores suggested that each of these Clades may be considered a different species. ORF analysis revealed that putatively encoded proteins would be very similar in length and domains to those of other alpharetroviruses and thus potentially functional. This will undoubtedly contribute to better understanding the biology of defective viruses, especially in wild Galliformes, their evolution, and the danger they may represent for other wild species and the poultry industry.

Syncytin-1, syncytin-2 and suppressyn in human health and disease

In this review, we summarized the results of experimental and clinical studies about three human endogenous retroviruses and their products-syncytin-1, syncytin-2, and suppressyn in human physiology and pathophysiology. We summed up the described connection with various pathological processes and diseases, mainly with pregnancy-induced hypertensive diseases such as preeclampsia, oncogenesis, gestational trophoblastic disease, and multiple sclerosis. Supposed mechanisms of action and the potential of clinical applications are also described.

Molecular Genetics of Retrovirus Replication

Despite the availability of effective anti-HIV drug therapy, according to UNAIDS estimates, 1 [...].

An Endogenous Retrovirus Vaccine Encoding an Envelope with a Mutated Immunosuppressive Domain in Combination with Anti-PD1 Treatment Eradicates Established Tumours in Mice

Endogenous retroviruses (ERVs) account for 8% of our genome, and, although they are usually silent in healthy tissues, they become reactivated and expressed in pathological conditions such as cancer. Several studies support a functional role of ERVs in tumour development and progression, specifically through their envelope (Env) protein, which contains a region described as an immunosuppressive domain (ISD). We have previously shown that targeting of the murine ERV (MelARV) Env using virus-like vaccine (VLV) technology, consisting of an adenoviral vector encoding virus-like particles (VLPs), induces protection against small tumours in mice. Here, we investigate the potency and efficacy of a novel MelARV VLV with a mutated ISD (ISDmut) that can modify the properties of the adenoviral vaccine-encoded Env protein. We show that the modification of the vaccine's ISD significantly enhanced T-cell immunogenicity in both prime and prime-boost vaccination regimens. The modified VLV in combination with an α-PD1 checkpoint inhibitor (CPI) exhibited excellent curative efficacy against large established colorectal CT26 tumours in mice. Furthermore, only ISDmut-vaccinated mice that survived CT26 challenge were additionally protected against rechallenge with a triple-negative breast cancer cell line (4T1), showing that our modified VLV provides cross-protection against different tumour types expressing ERV-derived antigens. We envision that translating these findings and technology into human ERVs (HERVs) could provide new treatment opportunities for cancer patients with unmet medical needs.