Human endogenous retrovirus-W

It is common for viruses to incorporate pieces of their host's genome into their own, which can aid in their success. On the other hand, hosts can also keep viral DNA in their genome, which may persist if advantageous or non-deleterious. In the case of HERVs, viral DNA is integrated into the germ-line genome of a human ancestor. Later, blood samples of hominoids, Old World monkeys, New World monkeys, and prosimians were probed using a fluorescently labeled HERV-W element derived from the gorilla fosmid library. Fluorescence in situ hybridization (FISH) revealed HERV-W elements in all the primate blood samples except that of the tupaia. A proposed nomenclature suggests putting all such "class I" elements in a genus-level taxon separate from Gammaretrovirus. ==Discovery==

HERV-W was discovered because of its connection to multiple sclerosis (MS). In macrophage cell cultures of patients with MS, several retroviral-like particles with reverse transcriptase (RT) activity were detected and given the name multiple sclerosis retroviruses (MSRVs). Because of MSRV's retroviral nature, it was originally thought that MSRV had an exogenous viral origin. Using the consensus sequence for retroviral pol and "panretro" RT-PCR extensions from the pol region of MSRV (retroviral RNA), the discovery of a HERV with gag, pol, and env was made possible. The primer binding site (PBS) of this HERV was discovered to be similar to avian retroviral PBSs, which use tRNATRP. This HERV was thus named HERV-W. With in vitro transcription techniques three suggested ORFs on chromosome 3 (gag), 6 (pro), and 7 (env) were detected and further analyzed, revealing that the ORF on chromosome 7q21.2 uniquely encoded a glycosylated Env protein. Although expression of HERV-W is prevalent in the whole body, there are two tissues whose expression levels are higher than the rest. The HERV-W-derived element of chromosome 12p11.21 and 7q21.2 had 42 hits from the env gene in pancreatic islet tissues, and 224 hits (11 gag, 41 pol, 164 env) in placenta, testis, and embryotic tissues, respectively. The HERV-W element on 7q21.2 encodes for ERVWE-1, which was named syncytin-1. ==Biological function==

Upon realizing that HERV-W was prevalent in the human genome and can form viable transcripts, scientists began searching for HERV-W's biological significance. The HERV-W Env gene, expressed in a vector, was transfected into TELCeB6 and TELac2 cells, to test for virus-cell and cell-cell fusion, respectively. One-to-two days after transfection, numerous multinucleated giant cells, or syncytia, had formed, indicating the HERV-W env gene can cause homotypic and heterotypic cell-cell fusion. thus investigation began to find with which receptor HERV-W interacts. The HERV-W envelope glycoprotein could fuse parental TE671 cells (human embryo cells, identical to human rhabdomyosarcoma RD cells), and PiT-1- and PiT-2-blocked cells (PiT1/2 are retroviral (RV) receptors), but not retroviral type D receptor-blocked cells. It was concluded that HERV-W may recognize and interact with the type D mammalian retroviral receptors expressed in humans. The cytotrophoblast cells proliferate and invade maternal endometrium, which is key to implantation and placental development. Furthermore, cytotrophoblasts fuse and differentiate into multinucleated syncytiotrophoblast cells that are surrounded by maternal blood and cover the embryo. Syncytiotrophoblast help with nutrient circulation, ion exchange, and hormone synthesis, which are all key to development. These multinucleated cells appear very similar to virally induced syncytia. HERV-W's main gene expression is ERVWE-1 which is a highly fusogenic env glycoprotein, which is also called syncytin-1 because it induces the formation of syncytia (multinucleated cells). Using monoclonal, fluorescently-labeled antibodies, the Frendo Lab was able to visualize the Env-W expression at the apical membrane of the syncytiotrophoblast in first-trimester placentas. This immunosuppressive nature of syncytin-1 and syncytin-2 (HERV-FRD) may be key in creating an immunologic barrier between the mother and the fetus. Since the fetus only share half of the mother's DNA, it is critical that the mother's immune system does not attack the fetus. Analyzing 40 full-term placental tissues with immunohistochemical staining and RT in-situ PCR shows strong expression of syncytin-1 in syncytiotrophoblasts compared to cytotrophoblasts. This suggests placental micro-vesicles can modulate the mother's immune system. Today, it is still difficult to tell the exact mechanism that ERVWE-1 uses to suppress or activate the mother's immune system. ==Mechanism of expression and environmental factors==

The mechanism of expression for HERV-W genes is still not completely understood. The 780 bp LTR's that flank the env, pro, pol, and gag genes provide a range of regulatory sequences such as promoters, enhancers, and transcription-factor binding sites. It would be reasonable to assume that HERV-W genes could not be transcribed from HERV-W elements that have incomplete LTRs. However, using a luciferase reporter gene assay, HERV-Ws that have incomplete LTR's were still found to have promoter activity. This suggests that the transcription of HERVs can be activated not just by LTR-directed transcription but also by transcriptional leakage, This experiment was performed with human astrocytic cells and showed that TNFa has the ability to activate the ERVWE-1 promoter through an NF-κB element. Overexpression of ERVWE-1, which produces snyctin-1, would be dangerous in many adult cells. Thus, the promoter is methylated and histone modification occurs in non-placental cells to keep the expression of HERV-W low. Also, when these cells are placed in stressful environments, such as serum deprivation, similar and increased expression of HERV-W is also recorded. ==HERV-W’s role in multiple sclerosis==

Since the detection of MSRV Env protein in the plasma of multiple sclerosis patients and the realization that the protein is a member of the HERV-W family, the questions of how HERV-W was related to Multiple sclerosis and what caused transcription of HERV-W were investigated. Both the expression of MSRV in vitro with peripheral blood mononuclear cell (PBMC; such cells being critical to the immune system) cultures and in vivo in severe combined immunodeficiency (SCID) mouse models, illustrated a pro-inflammatory response. Inflammation can occur when the immune system recognizes an antigen and activates the immune response cascade. The transcribed and translated products of the HERV-W Env gene come from retroviral DNA. Thus, the human body detects these proteins as antigens and triggers the immune response. Specifically, cytokine production is elevated in the MS PBMC cultures as compared to the healthy controls and as mediated by the surface unit of the MSRV-Env protein. Finally, it was discovered—through TLR-4 signaling assays, cytokine ELISAs, OPC cell cultures, and statistical analysis—that MSRV-Env is a highly potent TLR-4 activator. This suggests a positive feedback loop where cytokines promote HERV-W transcription and then the transcription of HERV-W leads to a higher cytokine production. Comparing Gag and Env expression in control patients with patients with MS, it was found that gag and env are expressed at physiological levels in cells of the CNS under normal conditions. However, in patients with MS lesions there is a large accumulation of Gag proteins in demyelinated white matter. A contrasting study performed a micro-array to analyze HERV transcription in human brains. Using 215 brain samples derived from schizophrenia (SZ), bipolar disorder (BD), and control patients, it was found that the expression of HERV – E/F/K was weakly correlated with SZ and BD and that ERVWE-1 expression remained unaffected in SZ and BD compared to controls. It is still not known today if MSRV plays a causal or reactive role in MS. Another step in understanding the genomic origin of the HERV-W member transcribed in MS patients was made when looking into the HERV-W element of the Xq22.3. Since women are twice as likely to have MS, compared to men, and the Xq22.3 has almost a complete ORF thus a possible connection between Xq22.3 and MS was proposed. ==HERV-W and schizophrenia==

To date, not much hard evidence has been found to support a strong correlation between HERV-W transcripts and schizophrenia (SZ). One study found that 10 out of 35 individuals with recent onset schizophrenia had retroviral pol gene HERV-W transcripts and murine leukemia virus gene transcripts in cell-free cerebrospinal fluid (CSF), compared to 1 in 20 patients with chronic schizophrenia. This was significant when compared to the 22 non-inflammatory patients and the 30 healthy patients who had no retroviral transcripts. Contrasting this data, a micro-array was performed to analyze HERV transcription activity in human brains. They found a weak correlation between HERV's –K, -E, -F; and that env-W expression was constant in patients with schizophrenia and bipolar disorder (BD) compared to controls. Today, it is still hard to tell if HERVs play a causal role, are correlated with, or are just a response to, neuropsychiatric diseases. ==Drug Production==

As knowledge about the mechanism of production for HERV-W transcripts is growing, scientists are beginning to synthesize drugs that can interrupt the MSRV pathway. A humanized monoclonal antibody called GNbAc1, of the IgG4 class, binds with high specificity and affinity to the extracellular domain of the MSRV-Env protein. When performing experiments, another humanized IgG4 class antibody was used as a control. It was found through many experiments that GNbAc1 is able to antagonize all the MSRV-Env effects. ==References==