“A BRIEF INSIGHT INTO THE RELATIONSHIP BETWIXT HUMAN ENDOGENOUS RETROVIRUSES AND MULTIPLE SCLEROSIS”
Mrs. D.P. Mane*, P. P. Mane, Ms. V. R. Akade
Tatyaraoji More College of Pharmacy, Umerga. Dist: Osmanabad (M.S.)
Correspondence Address: dhrutimane@gmail.com ;7038288289

Summary
Multiple sclerosis (MS) is a chronic, neurologic, immune-mediated inflammatory disease with an unproven etiology. At the end of the 19th century, various hypotheses were postulated that MS was caused by viruses or virus-induced pathophysiology. The pathogenicity of viruses in comparison with bacteria and fungi is predominantly high. human endogenous viruses are termed “fossil viruses” because about 1% of human genomes are composed of remnants of retroviral germline infections of human ancestors. Due to deactivating mutation and epigenetic control, these integrants are non-functional or inactive under the physiological state. However, they can be pathogenic under diseased conditions and produce viral transcripts and proteins. Epstein-Barr virus and Herpesvirus-6 family have been strongly linked with MS. Multiple sclerosis pathogeneses are associated with HERV expression. Various in-vitro and in-vivo research studies suggest that HERVs are potent modulators of the immune system. HERVs affect other immune-related genes, induce T cells and interact with the pattern recognition receptor. In this, review we give updates on the available evidence put forth on viral etiology as well as the pathophysiological mechanisms supporting it. we survey for different approaches through which viruses could cause both neuroinflammation and neurodegeneration or lead to it. Furthermore, data from animal models and well-confirmed epidemiological studies are presented while emphasis is given to existing gaps and future research directions.
Keywords: MS, HERV, neurodegeneration, and neuroinflammation.

Introduction
Multiple sclerosis is a chronic disease of the central nervous system characterized by multifocal regions of perivascular inflammation, demyelination, and gliosis. Demyelination and neurodegeneration in the MS brain are associated with a failure of the immune system. Immune system mediators like macrophages and T cells ingest the protective covering of nerves as well as they destruct the neuroglia like oligodendrocytes. In MS resulting nerve damage interrupt the communication between the CNS and the body. These inflammatory reactions form focal demyelinated plaques in the white and grey matter. Often lesions are observed in the cortex, the basal ganglia, the brain stem, and the gray matter of the spinal cord (Brownell and Hughes 1962). This inflammatory pathologic picture has prompted the postulate that immune mechanisms induced by autoantigens or viral infection mediate the tissue injury. This postulate is supported by epidemiologic data that indicate a 30 to 50% disease concordance rate among identical twins, an overrepresentation of disease in various racial groups correlated with particular major histocompatibility complex gene products, a linkage of susceptibility with certain T-cell receptor genes, and the suggestion that migration beyond a critical age results in retaining the risk rate of the country of origin. (Antel et al.,1991).  Technological advancement in the research studies in terms of neuroimaging techniques allows the evaluation of lesions in MS patients like the isolation and maintenance of myelin-reactive T-cell lines from patients with MS and control subjects, novel tissue culture, and cell biologic approaches to myelinating cells (oligodendrocytes), and the molecular biologic analyses of gene products critical for viral persistence. The new and unique viruses in MS can be searched direct (immunofluorescence, electron microscopy, tissue culture, in situ hybridization, molecular cloning techniques) or by Indirect method (serologic).
Although the evidence for a causative viral etiology for MS in humans remains inconclusive, viruses appear to play a role in modulating the neuro-immunological system of genetically susceptible individuals to cause MS. For instance, IgG antibodies against several viruses including varicella zoster virus (VZV), cytomegalovirus (CMV), measles, rubella, mumps and herpes simplex virus (HSV-1) have been identified in the cerebrospinal fluid (CSF) of patients with MS.(Mentis et al.,2017)

Epidemiological context of Multiple Sclerosis
The prevalence of multiple sclerosis has increased in the last few decades because of the high socioeconomic burden of disease management. around 400,00 people in the United States and 2.5 million across the world have suffered from multiple sclerosis. Every week 200 people are diagnosed with MS in the United States. In young adults at age 20 to 40, it became the leading cause of non-traumatic disability. Multiple sclerosis is caused on account of the complex interaction between genetic, autoimmune, and environmental factors. 
Search methods and selection criteria
 	PubMed®, MEDLINE®, and Google Scholar were searched for References published from January 1st, 2006, to June 30th, 2020, and by the bibliographies of the above articles. For the initial screening, the search terms “multiple sclerosis”, “infectious etiology”, “virus”, or “HERV” through the Boolean operator “AND” and “OR” were applied. specific terminologies were utilized for different sections of the review based on their relevance, i.e., “genomic organization of retrovirus”, “demyelination”, “neurodegeneration”, “oligoclonal bands”, “innate immunity”, “bystander activation”. One or more names for a similar infectious agent were searched. We permit strong priority to original research articles and systematic reviews over case reports or hypothesis/ viewpoint articles, and prominence to the most recent papers. The final reference list included all the references covered in the review.
Human Endogenous Retroviruses
Human endogenous retroviruses are the footprints of ancestral retroviral infections. HERV was discovered in 1980. They are transmitted vertically through the germline and are thus inherited by successive generations in a Mendelian manner (Nelson et al.,2003). HERV has a homogenous genomic organization to present-day exogenous retroviruses such as human immunodeficiency virus (HIV) and human T cell leukemia virus (HTLV), which are composed of gag, pol, and env regions sandwiched between two long terminal repeats (LTR)
Structural organization of HERV(Genomic)  
HERV genome comprises two long terminal repeats that flank the internal part of the genes namely gag, env, and pol which are important for the function of the virus. The LTRs possess nucleotide sequence motifs that are fundamental to the regulation of retroviral gene expression (Nelson et al.,2003). These genes code for various proteins. gag codes for the proteins which are required for viral assembly i.e viral matrix and core-shell while pols codes for various enzymes which are vital for viral replication, such as reverse transcriptase, protease, ribonuclease, and integrase.  env codes for surface and transmembrane glycoproteins these proteins are required for binding of virus and cell surface membrane of a host which gives rise to host cell fusion and immunosuppression.
5’Ltr	Gag	pol	Env	3’Ltr	

THE MS-ASSOCIATED RETROVIRUS
In 1997, reporting for a collaboration of Swiss, French, and British scientists, Perron described the isolation, and partial molecular identification, of novel retrovirus particles from cell cultures of the leptomeninges, choroid plexus, and peripheral blood B lymphocytes of MS patients. They also found related genetic sequences in the CSF and cell-free plasma of non-treated patients but not from matched controls. They named the virus the MS-associated retrovirus or MSRV. Subsequently, MSRV genetic sequences were found to share close homology with the HERV-W endogenous family. (Ryan et al.,2011) Perron also confirmed that gag and env proteins encoded by the MSRV are expressed by normal cells in the central nervous system However, the expression of such proteins in neurons of normal subjects was modest in density and compatible with some physiological functions. Meanwhile, there was an accumulation of gag-encoded protein in neurons within demyelinated white matter in MS-affected brains, and prominent gag-encoded protein was found in endothelial cells within MS lesions from acute or actively demyelinating cases, a pattern that was not seen in controls. (Ryan et al.,2011)
HARV'S ROLE IN THE EVOLUTION AND SHAPING OF THE HUMAN GENOME
Transposable elements(TEs) which are part of more than 40% of our DNA are not “useless junks” and have important regulatory roles. Nowadays, growing evidence confirms that TEs had a crucial role in the shaping and evolution of vertebrates’ genomes, contributing to the establishment of lineage-specific patterns of gene expression. (feschotte et al., 2008) and (Trizzino et al.,2017) TE insertion appear to be highly conserved among mammals, showing an increased density in the proximity of cellular genes and being the main source of transcription factor binding sites and regulatory signals. (Trizzino et al.,2017) (Duperrayet al.,2013) This points out their relevance to human development and transcriptional modulation throughout evolution (van de Lagemaat et al.,2003). Due to deactivating mutation in their coding sequence and epigenetic control of various cellular mechanisms of transcriptional repression, such as histone hypermethylation these TES are non-functional or inactive under the physiological state. Despite this, many TEs are likely involved in a long-term co-evolution with the host, which exerted firstly a selective pressure against their detrimental effects and led then in some instances to their cooptation for biological processes (feschotte et al., 2012), (Hua-Van et al., 2011). Intriguingly, even the general epigenetic silencing of TEs could represent an ancestral adaptation of the harboring organism that, initially, evolved defense systems to downregulate them and, then, developed strategies to control gene expression through their exaptation (Hua-Van et al., 2011). In this view, the sporadic loss of epigenetic regulation could be not only linked to certain developmental stages and disorders but might also constitute an opportunity to boost evolvability (Hua-Van et al., 2011). Accordingly, despite controlling mechanisms, many HERV sequences still retain a residual expression capacity, leading to the production of either coding or non-coding RNA transcripts that can influence host biology. One of the most remarkable examples of the HERV impact on vertebrate physiology is represented by “syncytins,” an ensemble of Env proteins encoded by different HERV sequences in all eutherian mammals through a process of convergent evolution (45). In the case of the human genome, two env loci ERVWE1 (HERV-W, 7q21.2) and ERVWE2 (HERV-FRD, 6p24.1). this ERVWE1 (HERV-W, 7q21.2) encodes for the syncytin-1 and ERVWE2 (HERV-FRD, 6p24.1). syncytin-2, respectively. While syncytin-1 has a pivotal role in placental syncytiotrophoblast development and homeostasis (17, 18, 46, 47), syncytin-2 is thought to be involved in maternal immune tolerance to the fetal allograft. (Mangeney et al., 2007) The numerous HERV sequences dispersed in our DNA are responsible for the evolution of the primates’ genome by providing an abundant source of regulatory elements. Also, HERVs participate in this complex exchanging, allowing regulate the host genes’ activity in several ways and at different expression levels. Repetitive element insertion can genetic modification. if a HERV protein is produced, it can be able to modulate the host genic expression through the biological activities previously involved in the virus life cycle and now provide new cellular functions (Frank et al., 2017) HERV gag proteins influences the stability, localization, and translation of cellular transcripts. Gag-like protein derived from an ancient Ty3/gypsy retrotransposon has been repurposed during brain evolution to mediate communications between neural cells, having an important role in the development and plasticity of the nervous system (plath et al., 2006) and (Zhang 2015). In particular, Arc has been shown to assemble into capsids that include mRNA sequences to be transferred from a neuron to new recipient cells through extracellular vesicles, then undergoing activity-dependent translation (pastuzyn et al., 2018) Hence, the acquisition of HERV insertions could have acted as a driving force for human genome plasticity and cellular networking, and, as described below, such TE-dependent modeling and functional renewal has been particularly critical in the evolution of innate immunity.
HERV plays a role in the pivotal immune system and regards the interferon (IFN) network, as a crucial antiviral pathway for innate immunity and a fundamental effector to initiate and maintain adaptive responses. Intriguingly, Chuong and coauthors showed that HERV insertions greatly contributed to the evolution and amplification of IFN transcriptional network, dispersing independently a wide number of IFN-inducible enhancers in many mammalian genomes (Choung et al., 2016). Furthermore, due to their residual regulatory activities, HERV LTRs can act as promoters and/or enhancers after IFN-mediated stimulation (Katoh et al., 20013). This has been reported for HERVK(HML2) LTRs, which harbor two IFN-stimulated response elements (ISREs) activated by the IFN signaling, leading to increased HML2 expression in response to inflammation (Manghera et al., 2013 and 2016). In line with this, HERV-K env genes transcription can be stimulated by IFN α, encoding Env superantigens responsible for polyclonal T-cell activation (stauffer et al., 2001).

Pathogenesis of Retrovirus in Multiple Sclerosis 
Retroviruses are enveloped viruses with single-strand positive RNA genomes. After the infection of the target cell, they reverse transcribe their RNA and integrate the resulting DNA product into the cellular chromosomes, forming a provirus. Occasionally, some types of retroviruses can infect germ line cells and colonize the host’s germ line by forming endogenous retroviruses. From 70 to 30 million years ago, various groups of retroviruses integrated into human germ line cells, becoming part of the human DNA and being transmitted through a Mendelian pattern over generations. Indeed up to 8% of the human genome is constituted of groups of HERVs ranging in copy number from one to many thousands. They are part of our history and evolution and although usually, they retain only a passive role in the genome, in some cases, they have been associated with diseases (Gifford et al., 2003)
The diagnostic hallmark of MS is the presence of demyelinated lesions in the white and grey matter of the CNS. The principal feature of MS is demyelination with the destruction and loss of oligodendrocytes. The lesion occurs in accounts of reactive T lymphocytes, B lymphocytes, and plasma cells. The inflammatory reaction is initiated around postcapillary venules and veins. Thus, in the initial stages of lesions, perivenous demyelination is seen and these lesions fuse to confluent demyelinated plaques, which expand at their border into the surrounding normal-appearing white matter. This gives rise to the typical perivenous extensions of established lesions, the so-called Dawson fingers. (Hassman et al., 2018).
All HERVS have identical structures as exogenous retroviruses. Two LTR are the regions bound to the genome that encode viral proteins mainly gag, env, pol, retroviral core, and the envelope. Expressions of HERVs are regulated by the promoter and enhancer regions in the LTR. 
The majority reports an association between MSRV and MS. MSRVenv and MSRVpol RNA and protein have been found expressed at higher levels in serum (Garson et al., 1998 and perron et al., 2012), PBMC (Garcia- montojo et al.,2014), and CSF (Arru et al., 2007,and Dolei et al ., 2002) of MS patients compared to healthy controls (HC) and other neurological diseases (OND) patients. Some articles have also reported the presence of this retrovirus in the brain of MS patients, in particular within lesions (perron et al., 2012), and the expression of these retroviruses correlated with disease progression.
CONCLUSION 
The etiology of MS is still ambiguous. Here, in this review, we verified all the relatable pieces of evidence which are supporting to the hypothesis that HERVs could be pathogenically linking the individual's genetic makeup and infection as a trigger of the disease. Although HERVs are retroviruses, they are passed through generations and their integration loci vary between individuals, creating genetic variability. Between MS, there is high HERV-W/MSRV expression that enhances at the time of relapses and through disease progression, confirming an association between HERVW/MSRV and MS. In the peripheral blood, these retroviruses are mainly exhibited by T-lymphocyte, B lymphocytes and monocytes, particularly after differentiation of monocytes into macrophages. In MS lesions, HERV-W/MSRV has been found in microglia. The expression of this retrovirus can lead to the production of neurotoxic elements which causes the demyelination and destruction of the oligodendrocytes. Available evidence also indicates that HERV-W/MSRV expression is increased in cells due to the increased population of exogenous viruses, such as EBV. EBV is the potential bridge between environmental triggers, and the immunopathogenic cascades leading to MS lesions as well as disease progression. 

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