Exaptation of Bornavirus-Like Nucleoprotein Elements in Afrotherians

Co-option of a non-retroviral endogenous viral element in planthoppers

Non-retroviral endogenous viral elements (nrEVEs) are widely dispersed throughout the genomes of eukaryotes. Although nrEVEs are known to be involved in host antiviral immunity, it remains an open question whether they can be domesticated as functional proteins to serve cellular innovations in arthropods. In this study, we found that endogenous toti-like viral elements (ToEVEs) are ubiquitously integrated into the genomes of three planthopper species, with highly variable distributions and polymorphism levels in planthopper populations. Three ToEVEs display exon‒intron structures and active transcription, suggesting that they might have been domesticated by planthoppers. CRISPR/Cas9 experiments revealed that one ToEVE in Nilaparvata lugens, NlToEVE14, has been co-opted by its host and plays essential roles in planthopper development and fecundity. Large-scale analysis of ToEVEs in arthropod genomes indicated that the number of arthropod nrEVEs is currently underestimated and that they may contribute to the functional diversity of arthropod genes.

An endogenous bornavirus-like nucleoprotein in miniopterid bats retains the RNA-binding properties of the original viral protein

Endogenous bornavirus-like nucleoprotein elements (EBLNs) are sequences derived from bornaviral N genes in vertebrate genomes. Some EBLNs have been suggested to encode functional proteins in host cells; however, little is known about their evolution and functional relationship to the viral genes from which EBLNs originate. Here, we predicted functionality of EBLNs based on the properties of N as an RNA-binding protein. We showed an EBLN in miniopterid bats (miEBLN-1) has evolved under purifying selection and encodes an RNA-binding protein (miEBLN-1p) with biochemical properties similar to bornaviral N. Furthermore, we revealed miEBLN-1p interacts with host RNA-binding proteins, such as MOV10. These data suggest that miEBLN-1p has been exapted as an RNA-binding protein with similar properties to exogenous bornaviral N in miniopterid bats.

Systemic amyloidosis derived from EFEMP1 in a captive Tsushima leopard cat

In animals, most cases of systemic amyloidosis are of amyloid A type, and the other types of systemic amyloidoses are rare. This study analyzed systemic amyloidosis in a 15-year-old female Tsushima leopard cat. Amyloid deposits strongly positive for Congo red staining were observed in the arterial walls as well as the interstitium in multiple organs. Mass spectrometry-based proteomic analysis with laser microdissection of amyloid deposits identified epidermal growth factor-containing fibulin-like extracellular matrix protein 1 (EFEMP1) as a prime amyloidogenic protein candidate. Immunohistochemistry showed that the amyloid deposits were positive for the N-terminal region of EFEMP1. From these results, the present case was diagnosed as EFEMP1-derived amyloidosis. It is the first such case in an animal. EFEMP1-derived amyloidosis in humans has recently been reported as a systemic amyloidosis, and it is known as an age-related venous amyloidosis. The present case showed different characteristics from human EFEMP1-derived amyloidosis, including the amyloid deposition sites and the amyloidogenic region of the EFEMP1 protein, suggesting a different pathogenesis between Tsushima leopard cat and human EFEMP1-derived amyloidosis.

A Human Endogenous Bornavirus-Like Nucleoprotein Encodes a Mitochondrial Protein Associated with Cell Viability

Endogenous retroviruses (ERVs) are sequences in animal genomes that originated from ancient retrovirus infections; they provide genetic novelty in hosts by being coopted as functional genes or elements during evolution. Recently, we demonstrated that endogenous elements from not only from retroviruses but also nonretroviral RNA viruses are a possible source of functional genes in host animals. The remnants of ancient bornavirus infections, called endogenous bornavirus-like elements (EBLs), are present in the genomes of a wide variety of vertebrate species, and some express functional products in host cells. Previous studies have predicted that the human EBL locus derived from bornavirus nucleoprotein, termed hsEBLN-2, expresses mRNA encoding a protein, suggesting that hsEBLN-2 has acquired a cellular function during evolution. However, the detailed function of the hsEBLN-2-derived product remains to be elucidated. In this study, we show that the hsEBLN-2-derived protein E2 acts as a mitochondrial protein that interacts with mitochondrial host factors associated with apoptosis, such as HAX-1. We also demonstrate that knockdown of hsEBLN-2-derived RNA increased the levels of PARP and caspase-3 cleavage and markedly decreased cell viability. In contrast, overexpression of E2 enhanced cell viability, as well as the intracellular stability of HAX-1, under stress conditions. Our results suggest that hsEBLN-2 has been coopted as a host gene, the product of which is involved in cell viability by interacting with mitochondrial proteins. IMPORTANCE Our genomes contain molecular fossils of ancient viruses, called endogenous virus elements (EVEs). Mounting evidence suggests that EVEs derived from nonretroviral RNA viruses have acquired functions in host cells during evolution. Previous studies have revealed that a locus encoding a bornavirus-derived EVE, hsEBLN-2, which was generated approximately 43 million years ago in a human ancestor, may be linked to the development of some tumors. However, the function of hsEBLN-2 has not been determined. In this study, we found that the E2 protein, an expression product of hsEBLN-2, interacts with apoptosis-related host proteins as a mitochondrial protein and affects cell viability. This study suggests that nonretroviral RNA viral EVEs have been coopted by hosts with more diverse functions than previously thought, showing a pivotal role for RNA virus infection in evolution.

Virus-like insertions with sequence signatures similar to those of endogenous nonretroviral RNA viruses in the human genome

Understanding the genetics and taxonomy of ancient viruses will give us great insights into not only the origin and evolution of viruses but also how viral infections played roles in our evolution. Endogenous viruses are remnants of ancient viral infections and are thought to retain the genetic characteristics of viruses from ancient times. In this study, we used machine learning of endogenous RNA virus sequence signatures to identify viruses in the human genome that have not been detected or are already extinct. Here, we show that the k-mer occurrence of ancient RNA viral sequences remains similar to that of extant RNA viral sequences and can be differentiated from that of other human genome sequences. Furthermore, using this characteristic, we screened RNA viral insertions in the human reference genome and found virus-like insertions with phylogenetic and evolutionary features indicative of an exogenous origin but lacking homology to previously identified sequences. Our analysis indicates that animal genomes still contain unknown virus-derived sequences and provides a glimpse into the diversity of the ancient virosphere.

100-My history of bornavirus infections hidden in vertebrate genomes

Although viruses have threatened our ancestors for millions of years, prehistoric epidemics of viruses are largely unknown. Endogenous bornavirus-like elements (EBLs) are ancient bornavirus sequences derived from the viral messenger RNAs that were reverse transcribed and inserted into animal genomes, most likely by retrotransposons. These elements can be used as molecular fossil records to trace past bornaviral infections. In this study, we systematically identified EBLs in vertebrate genomes and revealed the history of bornavirus infections over nearly 100 My. We confirmed that ancient bornaviral infections have occurred in diverse vertebrate lineages, especially in primate ancestors. Phylogenetic analyses indicated that primate ancestors were infected with various bornaviral lineages during evolution. EBLs in primate genomes formed clades according to their integration ages, suggesting that bornavirus lineages infected with primate ancestors had changed chronologically. However, some bornaviral lineages may have coexisted with primate ancestors and underwent repeated endogenizations for tens of millions of years. Moreover, a bornaviral lineage that coexisted with primate ancestors also endogenized in the genomes of some ancestral bats. The habitats of these bat ancestors have been reported to overlap with the migration route of primate ancestors. These results suggest that long-term virus-host coexistence expanded the geographic distributions of the bornaviral lineage along with primate migration and may have spread their infections to these bat ancestors. Our findings provide insight into the history of bornavirus infections over geological timescales that cannot be deduced from research using extant viruses alone, thus broadening our perspective on virus-host coevolution.

Update on immunopathology of bornavirus infections in humans and animals

Knowledge on bornaviruses has expanded tremendously during the last decade through detection of novel bornaviruses and endogenous bornavirus-like elements in many eukaryote genomes, as well as by confirmation of insectivores as reservoir species for classical Borna disease virus 1 (BoDV-1). The most intriguing finding was the demonstration of the zoonotic potential of lethal human bornavirus infections caused by a novel bornavirus of different squirrel species (variegated squirrel 1 bornavirus, VSBV-1) and by BoDV-1 known as the causative agent for the classical Borna disease in horses and sheep. Whereas a T cell-mediated immunopathology has already been confirmed as key disease mechanism for infection with BoDV-1 by experimental studies in rodents, the underlying pathomechanisms remain less clear for human bornavirus infections, infection with other bornaviruses or infection of reservoir species. Thus, an overview of current knowledge on the pathogenesis of bornavirus infections focusing on immunopathology is given.

Interactions among eukaryotes, retrotransposons and riboviruses: endogenous riboviral elements in eukaryotic genomes

Riboviruses are viruses that have RNA genomes and replicate only via RNA intermediates. Although they do not require a DNA phase for replication and do not encode reverse transcriptase, the presence of DNA forms of riboviral sequences in ribovirus-infected cells has been reported since the 1970s. Additionally, heritable ribovirus-derived sequences, called riboviral endogenous viral elements (EVEs), have been found in the genomes of many eukaryotes. These are now thought to be formed by the reverse transcription machineries of retrotransposons within eukaryotic genomes sometimes referred to as selfish elements. Surprisingly, some reverse-transcribed riboviral DNAs (including EVEs) provide physiological functions for their hosts, suggesting the occurrence of novel interactions among eukaryotic genomes, retrotransposons and riboviruses, and opening the door to new avenues of investigation. Here I review current knowledge on these triangular interactions, and discuss future directions in this field.

[Virus-host coevolution: Endogenous RNA viral elements as pseudogenes]

RNA viruses do not need to take the form of DNAs, and RNAs alone complete their replication cycles. On the other hand, since the 1970s, it has been known that DNA fragments derived from RNA viruses can be detected in RNA virus-infected cells. Furthermore, in this decade, it has become clear that the eukaryotic genomes contain genetic sequences derived from non-retroviral RNA viruses. The DNA sequences derived from these RNA viruses are thought to be generatedby using a transposable mechanism of retrotransposon, such as LINE-1. Many endogenous RNA viral sequences are formed by the same mechanism as processed pseudogenes in eukaryotic cells, but the significance of the production of RNA viral "pseudogenes " in infected cells has not been elucidated. We have discovered endogenous bornavirus-like elements (EBLs), which derived from a negative-sense, single-stranded RNA virus, Bornaviruses, and have studied the evolution and function of EBLs in host animals. The analysis of EBLs provides us a clue to unravel the history of host-RNA virus coexistence. In this review, I overview about the function of endogenous RNA virus sequences, especially EBLs in mammalian genomes, and discuss the significance of endogenization of RNA viruses as viral pseudogenes in evolution.

Insights Into an Unexplored Component of the Mosquito Repeatome: Distribution and Variability of Viral Sequences Integrated Into the Genome of the Arboviral Vector Aedes albopictus

The Asian tiger mosquito Aedes albopictus is an invasive mosquito and a competent vector for public-health relevant arboviruses such as Chikungunya (Alphavirus), Dengue and Zika (Flavivirus) viruses. Unexpectedly, the sequencing of the genome of this mosquito revealed an unusually high number of integrated sequences with similarities to non-retroviral RNA viruses of the Flavivirus and Rhabdovirus genera. These Non-retroviral Integrated RNA Virus Sequences (NIRVS) are enriched in piRNA clusters and coding sequences and have been proposed to constitute novel mosquito immune factors. However, given the abundance of NIRVS and their variable viral origin, their relative biological roles remain unexplored. Here we used an analytical approach that intersects computational, evolutionary and molecular methods to study the genomic landscape of mosquito NIRVS. We demonstrate that NIRVS are differentially distributed across mosquito genomes, with a core set of seemingly the oldest integrations with similarity to Rhabdoviruses. Additionally, we compare the polymorphisms of NIRVS with respect to that of fast and slow-evolving genes within the Ae. albopictus genome. Overall, NIRVS appear to be less polymorphic than slow-evolving genes, with differences depending on whether they occur in intergenic regions or in piRNA clusters. Finally, two NIRVS that map within the coding sequences of genes annotated as Rhabdovirus RNA-dependent RNA polymerase and the nucleocapsid-encoding gene, respectively, are highly polymorphic and are expressed, suggesting exaptation possibly to enhance the mosquito's antiviral responses. These results greatly advance our understanding of the complexity of the mosquito repeatome and the biology of viral integrations in mosquito genomes.

Evolution of Immune Systems From Viruses and Transposable Elements

Virus-derived sequences and transposable elements constitute a substantial portion of many cellular genomes. Recent insights reveal the intimate evolutionary relationship between these sequences and various cellular immune pathways. At the most basic level, superinfection exclusion may be considered a prototypical virus-mediated immune system that has been described in both prokaryotes and eukaryotes. More complex immune mechanisms fully or partially derived from mobile genetic elements include CRISPR-Cas of prokaryotes and the RAG1/2 system of vertebrates, which provide immunological memory of foreign genetic elements and generate antibody and T cell receptor diversity, respectively. In this review, we summarize the current knowledge on the contribution of mobile genetic elements to the evolution of cellular immune pathways. A picture is emerging in which the various cellular immune systems originate from and are spread by viruses and transposable elements. Immune systems likely evolved from simple superinfection exclusion to highly complex defense strategies.

Systematic estimation of insertion dates of endogenous bornavirus-like elements in vesper bats

Endogenous bornavirus-like elements (EBLs) are sequences derived from bornaviruses (the family Bornaviridae) that are integrated into animal genomes. They are formed through germline insertions of segments of bornaviral transcripts into animal genomes. Because EBLs are molecular fossils of bornaviruses, they serve as precious sources of information to understand the evolutionary history of bornaviruses. Previous studies revealed the presence of many EBLs in bat genomes, especially in vesper bats, and suggested the long-term association between bats and bornaviruses. However, insertion dates of EBLs are largely unknown because of the limitations of available bat genome sequences in the public database. In this study, through a combination of database searches, PCR, and sequencing approaches, we systematically determined the gene orthologies of 13 lineages of EBLs in bats of the genus Myotis and Eptesicus and family Vespertilionidae. Using the above data, we estimated their insertion dates: the EBLs in vesper bats were inserted approximately 14.2 to 53 million years ago. These results suggest that vesper bats have been repeatedly infected by bornaviruses at different points in time during evolution. This study provides novel insights into the evolutionary history of bornaviruses and demonstrates the robustness of combining database searches, PCR, and sequencing approaches to estimate insertion dates of bornaviruses.

An endogenous adeno-associated virus element in elephants

An endogenous viral element derived from adeno-associated virus containing a nearly intact open reading frame (ORF) of the rep gene (enAAV-rep) has been identified in the genomes of various mammals including degu and African elephant. Particularly, in degu, mRNA expression of enAAV-rep has been observed specifically in the liver. Here we newly identified enAAV-rep in Asian elephant and rock hyrax, both of which are afrotherians. The enAAV-rep of African and Asian elephants appeared to be orthologous and originated from an integration event of the entire genome of AAV into the ancestral genome of elephants more than 6 million years ago, whereas that of rock hyrax appeared to have originated independently. Negative selection operating at the amino acid sequence level was detected for the ORF of enAAV-rep in elephants. As in degu, mRNA expression of enAAV-rep was specifically observed in the liver in Asian elephant. Integrations of enAAV-rep appeared to have occurred independently on the evolutionary lineages of elephants and degu, suggesting that the AAV Rep protein has been co-opted repeatedly in the mammalian liver.

Paleovirology of bornaviruses: What can be learned from molecular fossils of bornaviruses

Endogenous viral elements (EVEs) are virus-derived sequences embedded in eukaryotic genomes formed by germline integration of viral sequences. As many EVEs were integrated into eukaryotic genomes millions of years ago, EVEs are considered molecular fossils of viruses. EVEs can be valuable informational sources about ancient viruses, including their time scale, geographical distribution, genetic information, and hosts. Although integration of viral sequences is not required for replications of viruses other than retroviruses, many non-retroviral EVEs have been reported to exist in eukaryotes. Investigation of these EVEs has expanded our knowledge regarding virus-host interactions, as well as provided information on ancient viruses. Among them, EVEs derived from bornaviruses, non-retroviral RNA viruses, have been relatively well studied. Bornavirus-derived EVEs are widely distributed in animal genomes, including the human genome, and the history of bornaviruses can be dated back to more than 65 million years. Although there are several reports focusing on the biological significance of bornavirus-derived sequences in mammals, paleovirology of bornaviruses has not yet been well described and summarized. In this paper, we describe what can be learned about bornaviruses from endogenous bornavirus-like elements from the view of paleovirology using published results and our novel data.

Divergent bornaviruses from Australian carpet pythons with neurological disease date the origin of extant Bornaviridae prior to the end-Cretaceous extinction

Tissue samples from Australian carpet pythons (Morelia spilota) with neurological disease were screened for viruses using next-generation sequencing. Coding complete genomes of two bornaviruses were identified with the gene order 3'-N-X-P-G-M-L, representing a transposition of the G and M genes compared to other bornaviruses and most mononegaviruses. Use of these viruses to search available vertebrate genomes enabled recognition of further endogenous bornavirus-like elements (EBLs) in diverse placental mammals, including humans. Codivergence patterns and shared integration sites revealed an ancestral laurasiatherian EBLG integration (77 million years ago [MYA]) and a previously identified afrotherian EBLG integration (83 MYA). The novel python bornaviruses clustered more closely with these EBLs than with other exogenous bornaviruses, suggesting that these viruses diverged from previously known bornaviruses prior to the end-Cretaceous (K-Pg) extinction, 66 MYA. It is possible that EBLs protected mammals from ancient bornaviral disease, providing a selective advantage in the recovery from the K-Pg extinction. A degenerate PCR primer set was developed to detect a highly conserved region of the bornaviral polymerase gene. It was used to detect 15 more genetically distinct bornaviruses from Australian pythons that represent a group that is likely to contain a number of novel species.

The biological significance of bornavirus-derived genes in mammals

The discoveries of sequences derived from non-retroviral viruses in eukaryotic genomes have significantly expanded our knowledge about virus evolution as well as the co-evolution between viruses and eukaryotes. However, the biological functions of such sequences in the host are largely unknown. Endogenous bornavirus-like elements (EBLs) have been relatively well studied by molecular biological methods, which have provided evidence that some EBLs have been co-opted by their hosts. This review highlights the current knowledge on the biological significance of EBLs, and discusses possible functions of EBLs. Further, we highlight the importance of extensive surveillance of exogenous viruses for a better understanding of endogenous viral sequences as well as the co-evolution of viruses and eukaryotes.