Murine coronavirus (M-CoV) is a virus in the genus Betacoronavirus that infects mice.
Belonging to the subgenus Embecovirus, murine coronaviruses strains are enterotropic or polytrophic.
Enterotropic strains include mouse hepatitis virus (MHV) strains D, Y, RI, and DVIM, whereas polytrophic strains, such as JHM and A59, primarily cause hepatitis, enteritis, and encephalitis.
Murine coronavirus is an important pathogen in the laboratory mouse and  the laboratory rat.
It is the most studied coronavirus in animals other than humans, and has been used as an animal disease model for many virological and clinical studies.
Types
Murine hepatitis virus
Murine coronavirus was first discovered in 1949.
The researchers isolated the virus from the brain, spinal cord, liver, lung, spleen and kidneys of a rat that with encephalitis symptoms and severe myelin injury, and named it mouse virus JHM (i.e., mouse hepatitis virus JHM strain).
Now  mouse hepatitis virus (MHV) is the most studied coronavirus in animals other than humans, acting as a model organism for studying coronaviruses.
There are  more than 25 different strains of murine coronavirus.
Transmitted by fecal–oral route or respiratory tract, these viruses infect the liver of mice, and have been used as animal disease model for hepatitis.
and in people.
Transmitted in dung, the strains MHV-D, MHV-DVIM, MHV-Y and MHV-RI mainly infect the digestive tract, sometimes infecting the spleen, liver and lymphatic tissue, MHV-1, MHV-2, MHV-3, MHV-A59, MHV-S and MHV-JH M and other virus strains replicate in the respiratory tract and then spread to other organs such as the liver, lungs and brain.
MHV-JHM mainly infectes the central nervous system, and has been widely studied since 1949.
In rats these  nerve-infecting hepatitis viruses can cause acute or chronic neurological symptoms and stimulate the immunity of mice  following infection.
Infection leads to neuron demyelination, which is used as an animal disease model for the study of multiple sclerosis.
MHV-2, MHV-3 and MHV-A59 can also infect the liver, of which the first two are more toxic.
MHV-3 is the main virus strain used to study hepatitis; MHV-1 is mainly infected with the lungs.
Murine hepatitis virus is highly infectious and is one of the most common pathogens in laboratory mouse.
The symptoms of infection vary according to the type, path of infection, genotype and age of mouse.
MHV-1, MHV-S and MHV-Y are weak in viral strains, and MHV-2, MHV-3, MHV-A5 9 and MHV-JHM are more toxic, generally mild to adult mice, and high mortality in infected newborn mouse.
Infection, even if it does not cause obvious symptoms, may significantly affect the immune system of laboratory mouse and cause errors in the interpretation of experimental results.
For example, the virus can  replicate in macrophages and affect their function, and in the spleen which stimulates natural killer cells and affects T cell and B cells.
There is no vaccine to prevent and treat hepatitis virus infection in mouse, mainly because of the high mutation rate and the variety of virus strains, and the fact that vaccination may also interfere with the interpretation of experimental mice' research results, but this virus can be used as an experimental model for developing other coronavirus vaccines.
In 1991, Michael M. C. Lai Laboratory completed the whole genome sequencing of the murine hepatitis virus, with a total length of 31,000nt, making it the largest RNA virus genome known at that time.
In 2002, American virologist Ralph S Baric developed a reverse genetic system for mouse hepatitis virus, which assembled MHV cDNA and infected mouse cells, which was the first successful complete genome sequence of hepatitis B virus.
Fancy rat coronavirus
In rats Fancy rat coronavirus (RCoV) consists mainly of two virus strains, Sialodacryoadenitis virus (SDAV) and Parker's RCoV (RCoV-P), both of which cause  respiratory tract infections, and the former can also infect the eyes, Harderian gland, and salivary glands.
In the past, it was believed that the symptoms caused by the two infections were different, but in recent years, it has been argued that the symptoms of both are many, including eye nasal discharge, large salivary gland enlargement, Sialadenitis, photosensitivity, keratitis, shortness of breath and pneumonia, etc.,) which are among them.
There is no obvious difference, and it is also suggested that Parker rat coronavirus is only one type of rat salivary adenovirus.
Coronavirus in rats is one of the important pathogens of laboratory rat.
It is transmitted by aerosols or indirect contact.
It is highly infectious.
Generally, the symptoms caused by young rats are more serious, and some individuals still have permanent eye damage after recovery.
The virus is also used by scientists as an animal model for coronavirus research.
Others
In 1982, researchers found a coronavirus in the brain of mice after separating the virus that caused manx shearwater in the foot of acne, infected with the tissue of ordinary petrel on the outer islands of Wales, England, which is believed to be the pathogen of ordinary petrel infection, that is, birds.
Puffinosis coronavirus (PCoV).
The experimental results show that the virus is very similar to the rat hepatitis virus, but due to the use of mice in the separation process, it cannot be excluded from viruses that originally exist in mice and are not related to ordinary petrels.
Subse studies have shown that the virus has hemagglutinin esterase (HE).
If the bird beaked chinch coronavirus is indeed an infected petrel virus, it is very small number of the coronavirus infected with birds that do not belong to gammacoronavirus or deltacoronavirus.
In 2009, the International Committee on Taxonomy of Viruses (ICTV) classified the bird-billed sparrow coronavirus as a virus strain under the mouse coronavirus.
From 2011 to 2013, researchers collected mouse samples at several locations in Zhejiang, China, and discovered three new virus strains in Longquan lesser ricefield rat, collectively known as Longquan Rl rat coronavirus (LRLV) in 2015.
Genome
Rat coronavirus is a positive-stranded single-strand RNA virus with an outer membrane with a genome length of about 31,000nt.
In addition to the four structural proteins of coronaviruses - spike protein (S), membrane protein (M), envelope protein (E) and nucleocapsid protein (N) - some mouse coronavirus surfaces also have hemagglutinin esterase (HE).
HE can bind to sialic acid on the surface of the host cell, promote viral infection, and has acetyl esterase activity, which can decompose receptors to help the virus leave.
The virus also has four auxiliary proteins, 2a, 4, 5a and I (or N2) (known as NS2, 15k, 12.6k and 7b in rat salivary adenophritis virus; the names of the coronavirus in Longquan Luosai mouse are 2a, 5a, 5b and N2), and its genes have the order 1ab-2a-HE-S-4-5a-E-M-N-I, where 5a and 5b proteins are encoded by the same mRNA, and the open reading frame of I is located in the open reading frame of capsid protein N.
These auxiliary proteins may have the function of resisting the host's immune response.
The auxiliary protein NS2 (coded by the 2a gene) has the function of 2′,5′ phosphodiesterase, which can degrade 2′,5′-oligoadenylate in the cell and avoid its activation.
Ribonuclease L in cells activates the defense mechanism for decomposing viral RNA; auxiliary protein 5a has the function of inhibiting host interferon; the types of auxiliary proteins of different virus strains may be slightly different, such as MHV-S.
There is auxiliary protein 5a, so it is less resistant to interferon.
All four auxiliary proteins are non-rat hepatitis virus infection cells that must.
The E protein is divided into the E1 and E2 glycoprotein, which are believed to serve different purposes.
Infection
When coronavirus infects the host cell, its spike protein (S) binds to the receptor on the surface of the host cell, which in turn causes infection by enabling the virus to enter the cell.
The spike protein is cut by the host's protease at all stages of the formation, transportation and infection of the new cell, so that the spike protein can cooperate in the protein domain.
The domain that helps the external membrane of the virus fuse with the cell membrane is exposed to facilitate infection.
The host cell receptor used by rat coronavirus is generally CEACAM1 (mCEACAM1).
The type of infected tissue and the time at which the spike protein is cut vary according to the virus strain.
Among them, S1 in the spike protein of MHV-A59 The cutting site of /S2 is cut by proteases such as Furin in the host cell when the virus is produced and assembled, and when the virus infects a new cell, further cutting of burst proteins in the body is also a step required for successful infection; The ocyrosin of MHV-2 does not have the cutting site of S1/S2.
It is not cut during the assembly process.
Its infection depends on the cutting of the lysoprotein on the spike protein; MHV-JHM (especially the more toxic JHM.SD and JHM-cl2) that infects nerve tissue may not need surface exposure.
The body can infect the cell, that is, it can achieve membrane fusion without binding to the cell receptor, so it can infect neurostructure with little expression of mCEACAM1, and its infection may mainly depend on the cutting of its spike protein by the cell surface protease.
When rat hepatitis virus of different strains infects cells at the same time, template switching can occur while genetic replication is carried out, resulting in gene recombination, which may be important for the evolution of their diversity.
Classification and evolution
In Betacoronavirus, the murine coronavirus, together with Betacoronavirus 1, rabbit coronavirus HKU14, human coronavirus HKU1 and China Rattus coronavirus HKU24, forms an evolutionary clade A (lineage A), which is classified into Betacoronavirus by the International Committee on Taxonomy of Viruses.
The Embecovirus subgenus, in which the mouse coronavirus may be more closely related to the human coronavirus HKU1, which is a sister group.
The virus genome of this clade has genes encoded blood cell lectinin esterase (HE), while other coronaviruses have no, but many murine hepatitis virus strains (such as MHV-A59 and MHV-1) have become pseudogenes due to mutation.
That is, although there are still traces of gene sequences, they do not represent proteins, and viral strains with blood cell coagulin esterase may replicate in vitro.
This phenomenon shows that blood cell lectininsterase is not the protein necessary for rat hepatitis virus infection and replication.
Research has analyzed the structure of rat hepatitis virus stuncture protein, and found that its N-terminal domain (NTD) is similar to galectin in animal cells.
Therefore, it is proposed that the NTD of coronavirus stachyprotein is derived from the hypothesis of host animal cells, that is, the earliest coronavirus from the host.
The cell acquires a gene of lectin, which can bind to the sugar on the surface of the host cell as an infected cell.
Subsequently, the virus in branch A gets the blood cell lectininsterase to help the virus get rid of infected cells, but later the NTD of the mouse coronavirus evolved into a new structure that can be associated with the protein receptor mCEACAM1.
Combination greatly increases the binding ability of viruses and murine cells.
Because it is no longer necessary to bind to sugars, it gradually loses the function of lectinin, and further loses the blood cell lectinatesterase.
In contrast, bovine coronavirus, human coronavirus OC43, etc. are still receptors on sugars, so NTD retains the function of glutin.
Alphacoronaviruses and betacoronaviruses may all originate from bat viruses, but the subgenus Embecovirus contains many virus infected with rats (in addition to mouse coronavirus, there are also the Lucheng Rn rat coronavirus, China Rattus Coronavirus HKU24 and Myodes coronavirus 2JL14 with a large number of related virus strains found since 2015), and no bat virus has been found.
Some scholars suggest that the co-ant of branch A may be mouse virus, which is then transmitted by rats to humans and cattle.
RNA–RNA recombination
Genetic recombination can occur when at least two RNA viral genomes are present in the same infected host cell.
RNA–RNA recombination between different strains of the murine coronavirus was found to occur at a very high frequency both in tissue cultureMakino S, Keck JG, Stohlman SA, Lai MM.
High-frequency RNA recombination of murine coronaviruses.
J Virol.
1986 Mar;57(3):729–37.
PMID 3005623 and in the mouse central nervous system.Keck JG, Matsushima GK, Makino S, Fleming JO, Vannier DM, Stohlman SA, Lai MM.
In vivo RNA–RNA recombination of coronavirus in mouse brain.
J Virol.
1988 May;62(5):1810–3.
PMID 2833625 These findings suggest that RNA–RNA recombination may play a significant role in the natural evolution and neuropathogenesis of coronaviruses.
The mechanism of recombination appears to involve template switching during viral genome replication, a process referred to as copy choice recombination.
Strains
Sialodacryoadenitis virus (SDAV), which is a strain Rat coronavirus, is highly infectious coronavirus of laboratory rats, which can be transmitted between individuals by direct contact and indirectly by aerosol.
Acute infections have high morbidity and tropism for the salivary, lachrymal and harderian glands.
Rabbit enteric coronavirus causes acute gastrointestinal disease and diarrhea in young European rabbits.
Mortality rates are high.
Research
Studies are ongoing as to test the effectiveness of ivermectin for the treatment of mouse hepatitis virus (MHV).
References
