Jaagsiekte sheep retrovirus (JSRV) is a betaretrovirus which is the causative agent of a contagious lung cancer in sheep, called ovine pulmonary adenocarcinoma.

Natural history

JSRV is the virus that is the cause of the contagious lung tumors in sheep called ovine pulmonary adenocarcinoma (OPA). The disease has also been called "jaagsiekte", after the

Afrikaans Afrikaans (, ) is a West Germanic language that evolved in the Dutch Cape Colony from the Dutch vernacular of Holland proper (i.e., the Hollandic dialect) used by Dutch, French, and German settlers and their enslaved people. Afrikaans gra ...

words for "chase" (''jaag'') and "sickness" (''siekte''), to describe the respiratory distress observed in an animal out of breath from being chased, indicating the breathing difficulty experienced by infected sheep. Transmission of virus is through aerosol spread between sheep. The

exogenous In a variety of contexts, exogeny or exogeneity () is the fact of an action or object originating externally. It contrasts with endogeneity or endogeny, the fact of being influenced within a system. Economics In an economic model, an exogeno ...

infectious form of JSRV has an endogenous counterpart which is present in the genomes of all sheep and goats. The sheep

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...

has around 27 copies of endogenous retroviruses (enJSRVs) that are closely related to JSRV. Endogenous JSRV has several roles in the evolution of the domestic sheep as they are able to block the JSRV replication cycle and play a critical role in sheep

conceptus A conceptus (from Latin: ''concipere'' to conceive) is an embryo and its appendages (adnexa), the associated membranes, placenta, and umbilical cord; the products of conception or, more broadly, "the product of conception at any point between fert ...

development and

placental Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...

morphogenesis Morphogenesis (from the Greek ''morphê'' shape and ''genesis'' creation, literally "the generation of form") is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of deve ...

. Although OPA resembles human lung cancer, human lung cancer is not known to be caused by betaretroviruses. Even though a possibility of a viral cause has been eliminated in bronchoalveolar cancer, understanding the molecular mechanisms leading to the transformation of lung epithelia by JSRV may be of interest in the context of therapeutic approaches in human lung cancers in general and bronchoalveolar adenocarcinoma (BAC) in particular.

Classification

JSRV belongs to the family ''

Retroviridae A retrovirus is a type of virus that inserts a DNA copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. Once inside the host cell's cytoplasm, the virus uses its own reverse transcriptase ...

'', to the subfamily ''

Orthoretrovirinae ''Orthoretrovirinae'' is a subfamily of viruses belonging to ''Retroviridae'', a family of enveloped viruses that replicate in a host cell through the process of reverse transcription. The subfamily currently includes six genera, of which ''Lent ...

'' and the genus '' Betaretrovirus''.

Pathogenesis

JSRV is transmitted by the respiratory route and may also infect lymphocytes and myeloid cells, in addition to the lung epithelia. Expression of the JSRV Envelope protein activates signalling cascades that promote cellular proliferation and

malignant Malignancy () is the tendency of a medical condition to become progressively worse. Malignancy is most familiar as a characterization of cancer. A ''malignant'' tumor contrasts with a non-cancerous ''benign'' tumor in that a malignancy is not s ...

transformation of the cells. Initially, the tumour cells grow along the alveolar walls in a pattern reminiscent of human BAC, but subsequently become more invasive and

metastasize Metastasis is a pathogenic agent's spread from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, then, ...

to the local lymph nodes. Larger tumours may be

necrotic Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dige ...

and fibromatous at their centre. As the tumour grows, fluid production in the lung increases and this is likely to promote virus spread to other sheep. Only when the tumour reaches a size large enough to compromise lung function, do clinical signs appear. Critically, the majority of infected animals in endemic areas never show outward signs of infection, but they may be shedding virus, thus promoting inadvertent introduction of the disease into previously unaffected flocks and new geographical areas.

Genome structure

The genome of the exogenous virus is 7462 bases and has the classical "gag", "pol", "env" genome arrangement and is flanked by a

long terminal repeat A long terminal repeat (LTR) is a pair of identical sequences of DNA, several hundred base pairs long, which occur in eukaryotic genomes on either end of a series of genes or pseudogenes that form a retrotransposon or an endogenous retrovirus or ...

(LTR) on each end. There are 4 genes that encode the viral structural proteins. They are "gag" encoding the structural internal

virion A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's ...

proteins comprising "matrix" (MA), "capsid" (CA) and "nucleocapsid"(NC); "pro", which encodes an aspartic

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...

(PR); "pol", which encodes the" RT" and "integrase"(IN) enzymes; and "env", which encodes the "surface" (SU) and "transmembrane" (TM) envelope

glycoproteins Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosy ...

. The viral proteins are synthesized initially as large precursors and are later processed into the mature proteins by

proteolytic Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called protease ...

cleavage. An additional

open reading frame In molecular biology, open reading frames (ORFs) are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a prokaryotic DNA sequence, where only one of the six possible readin ...

(ORF) was observed in the viral genome and has been called orfX and its function is undefined.

Replication cycle

The initial attachment of JSRV to its target cell is mediated through the binding of the SU subunit of the Env glycoprotein to a specific cell surface receptor molecule,"Hyal2". The entry of the JSRV core into the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...

activates reverse transcription, during which the single-stranded RNA genome is converted into a double-stranded DNA form and gets integrated as a provirus into the host. Following integration, expression of JSRV RNA from the viral promoter in the LTR is controlled by the host transcriptional machine. Following

transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...

and

translation Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. The English language draws a terminological distinction (which does not exist in every language) between ''transla ...

of the viral genome, the new progeny virus gets assembled at the plasma membrane and bud off from the host cell acquiring a lipid envelope and their "env" glycoproteins. Following release from the cell, the "Gag"-"Pro"-"Pol" polyproteins are cleaved into their mature forms by

. This step maturation is essential for the formation of

infectious An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dis ...

particles.

Receptor and entry

The cellular

receptor Receptor may refer to: * Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ...

for JSRV is hyaluronidase 2 (Hyal2), a glycophosphatidylinol(GPI)-anchored protein belonging to the

hyaluronidase Hyaluronidases are a family of enzymes that catalyse the degradation of hyaluronic acid (HA). Karl Meyer classified these enzymes in 1971, into three distinct groups, a scheme based on the enzyme reaction products. The three main types of hyal ...

family. Generally, oncogenic retroviruses cause

transformation Transformation may refer to: Science and mathematics In biology and medicine * Metamorphosis, the biological process of changing physical form after birth or hatching * Malignant transformation, the process of cells becoming cancerous * Tran ...

of host cells mostly by insertional activation of a host

protooncogene An oncogene is a gene that has the potential to cause cancer. In tumor cells, these genes are often mutated, or expressed at high levels.

into an

oncogene An oncogene is a gene that has the potential to cause cancer. In tumor cells, these genes are often mutated, or expressed at high levels.

. But JSRV is different in this aspect since its envelope glycoprotein ("env") by itself is an oncogene and this single protein was shown to be necessary and sufficient to induce lung tumors in sheep. Unlike the majority of retroviruses, JSRV entry into the host cell is pH-dependent. Thus oncogenic JSRV has borrowed features of both pH-dependent and pH-independent viruses for entry which involves both the receptor binding and a low pH for fusion transformation of host cells.

Host immune response

An important feature of JSRV infection is the absence of any specific immune response from the host. A likely explanation is that the sheep are immunologically tolerant to JSRV antigens due to the expression of closely related endogenous JSRV proteins in the

fetal A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal develo ...

thymus The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or ''T cells'' mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders. ...

during T lymphocyte development and any JSRV-reactive T cells should be recognized as ‘anti-self’ and selectively removed. Another hypothesis is that tumor cells downregulate their major histocompatibility class-I expression, possibly being the reason for the absence of any virus-specific cytotoxic T cell response (CTL).

Endogenous jaagsiekte sheep retrovirus

Integration of viral DNA into host genome

During evolution, the sheep

incorporated parts of the Jaagksiete sheep retrovirus, now known as endogenous Jaagsiekte sheep retrovirus (enJSRV). There are 27 known copies of enJSRV in the sheep genome, of which five show intact sequences found in all retroviruses. These seem to have been adopted by the sheep genome as enJSRV aids

placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mate ...

l development and provides protection against similar retroviruses. In vitro assays have found that enJSRV does this by blocking various stages of the viral replication cycle. An example of this protection is seen in ovine endometrial epithelium where the high expression of enJSVR prevents exogenous JSVR from entering the cells via blocking the common receptor to both, HYAL2. However, Jaagsiekte virus can sometimes

mutate In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitos ...

to overcome this protection, and there is evidence of this having occurred in the last 200 years. There is also indication that the endogenization of Jaagsiekte virus is still occurring today.

enJSRV mechanism in reproduction

In sheep, enJSRVs are highly expressed in the

epithelia Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...

lining different reproductive tissues, including the

vagina In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...

uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...

and

oviduct The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...

. The RNA of enJSRVs is first detected in the conceptus on day 12. Experiments have found that the enJSRV

envelope An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card. Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a sh ...

regulates

trophoblast The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the p ...

growth and differentiation within the peri-implantation conceptus. It was discovered that enJSRVs are expressed in the

trophectoderm The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the ...

cells of the placenta. Their expression coincides with the key events of conceptus elongation and onset of

giant binucleate cells (BNC) differentiation. Furthermore, it was observed that an injection of

morpholino A Morpholino, also known as a Morpholino oligomer and as a phosphorodiamidate Morpholino oligomer (PMO), is a type of oligomer molecule (colloquially, an oligo) used in molecular biology to modify gene expression. Its molecular structure contain ...

s (an enJSRV envelope production inhibitor) into the uteri of pregnant sheep on day 8 of pregnancy resulted in reduced

elongation and inhibition of trophoblast giant BNC differentiation. Elongation of the sheep conceptus is an essential process as it results in the production of interferon tau (IFNT) which is a pregnancy recognition signal required for conceptus survival. This stimulates both the corpus luteum to continue to secrete progesterone and the onset of implantation. Following the injection of morpholinos, it was observed that pregnancy loss occurred 12 days later. This work supports the hypothesis that enJSRVs are crucial in sheep reproduction and placental

HYAL2

Hyaluronidase 2 (HYAL2) serves as a cell-surface receptor for both the exogenous and endogenous JSRV envelope (env). HYAL2 mRNA can be detected in the BNCs and multinucleated syncytia of sheep placentomes during pregnancy, but not in the trophectoderm cells or any cells of the endometrium.

In situ hybridization ''In situ'' hybridization (ISH) is a type of hybridization that uses a labeled complementary DNA, RNA or modified nucleic acids strand (i.e., probe) to localize a specific DNA or RNA sequence in a portion or section of tissue (''in situ'') o ...

analysis revealed that HYAL2

mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein. mRNA is created during the ...

was only detected in the binucleate cells and multi-nucleated syncytial plaques. It is hypothesised that enJSRV interactions with HYAL2 are vital for placental growth and differentiation. Whilst the cellular and molecular mechanism are still unclear, it is apparent it has a role in protecting the uterus against viral infection and placental morphogenesis. The co-expression of the enJSRV envelope and HYAL2 in the same cell types supports the hypothesis that HYAL2 binds to enJSRVs env on the binucleate cells and promotes their fusion into multi-nucleated syncytia.

Comparative physiology in humans and mice

Of interest for comparative physiology is that the presence of enJSRV envelope protein expression in the developing sheep placenta is very similar to that observed for syncytin in humans and the mouse. During the formation of the human placenta syncytiotrophoblast, by fusion of mononuclear cytotrophoblasts, human syncytins are specifically expressed. The syncytins are fusogenic when expressed

in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology ...

, supporting the hypothesis that they are involved in placental morphogenesis. These observations support the theory that an ancient

retroviral A retrovirus is a type of virus that inserts a DNA copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. Once inside the host cell's cytoplasm, the virus uses its own reverse transcriptas ...

infection had important consequences for mammalian

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

. The involvement of the betaretrovirus enJSRV in the sheep conceptus trophoblasts further argues for its involvement in sheep placentation.

Future directions and summary

Research surrounding endogenous retroviruses supports the idea that they may play critical roles in conceptus growth, placental differentiation and cell fusion in mammals. The morphological aspects of binucleate cell differentiation in

ruminant Ruminants (suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. The ...

s such as sheep are well characterised, but the mechanisms are not well defined - though evidence shows that enJSRV RNA and HYAL2 mRNA are co-expressed in the binucleate cell and multinucleated syncytiotrophoblasts throughout gestation.