Editor's note: "Fighting the epidemic" is a battle between humans and viruses, and a race between scientific research and time. On March 16, Li Wenhui, a senior researcher at the Institute of the Beijing Academy of Life Sciences, brought a popular popular science article "starting from scratch" at the scientific lecture on "Understanding the Future". From the mechanism of virus replication and the mechanism of invading cell receptors, we can unravel the cocoon, and lead to basic scientific knowledge of drug research and development.
The full text of Li Wenhui's speech was compiled by Zhang Qianqian, doctoral student at the Institute of Biophysics, Chinese Academy of Sciences. Guangming.com will delete, revise and publish it after authorization.
1. Types of viruses
There are many viruses in the human world, and viruses are part of the ecological environment. The virus can be spread between domestic animals or wild animals, or between humans.
We are in the vast ocean of viruses, but we are not infected with many viruses. Because the skin, as the largest organ on the surface of the human body, is not susceptible to all viruses, viruses need to have their own channels to enter the human body. This channel is the channel that connects the human body and the outside world, including the respiratory tract, digestive tract, reproductive tract, as well as mosquito bites, blood transfusion and mother-to-child transmission.
Viruses are strict intracellular parasites. The main reason is that viruses do not have their own energy system, nor their own material system. They can only provide genetic information and a small amount of protein and other substances (replication). Viruses must enter cells through specific channels, which are called receptors. After entering the cell, it replicates in the cytoplasm and nucleus respectively.
viruses can be divided into two categories, RNA viruses and DNA viruses. Most of the RNA viruses replicate in the cytoplasm, including the new coronavirus and most of the new viruses , as well as the common hepatitis C, etc.; most of the DNA viruses in replicate in the nucleus, including the common herpes virus, papillomavirus, etc. , but there are also DNA viruses replicated in the cytoplasm, such as poxvirus, that is, smallpox virus. Smallpox virus is currently the only virus that has been completely eliminated by human vaccines, and other viruses have not been completely eradicated even if there are good vaccines.
There is also a transboundary virus between these two viral worlds, which replicate through the reverse transcription process from RNA to DNA, such as HIV and HBV. The virus replicates in the most streamlined way, and the core it carries is its genetic material. Where to replicate its genetic material is the core and most critical element of the entire virus life.
2. How viruses infect humans
For RNA viruses, they replicate in the cytoplasm. The genome of the virus roughly includes two parts: one is its replicase enzyme region, and the other is its structural gene region. For example, coronaviruses, whose genome is a long positive-stranded RNA that translates into different proteins. The conservatism of each part of the genome is different. The red region on the figure indicates that the conservatism is very high, and the decreasing color indicates that the conservatism is weakened. After the RNA virus enters the cell, it can perform gene replication and gene expression in the "virus factory", producing viral structural proteins and genomic RNA. The two are assembled into complete RNA viruses and released, and then complete their own life history.
RNA virus must replicate continuously and actively in the cytoplasm before it can exist in the cell for a long time. This means that no key enzyme or key protein in the RNA virus is deleted or damaged, and the virus cannot replicate continuously. This provides multiple targets for viral inhibition. Any (key) target can be effectively inhibited and the virus can be effectively inhibited.
DNA virus is a completely different world. DNA viruses mainly exist in the nucleus and produce its structural proteins, non-structural proteins and genomes. It can be copied continuously under normal circumstances. However, there is a huge difference between DNA viruses and RNA viruses. DNA viruses can not replicate in the nucleus and can still exist for a long time.
Hepatitis B virus is a virus between RNA and DNA. Its covalently closed circular DNA (cccDNA) in the cell nucleus also exists for a long time, but its replication can be inhibited by inhibitors of RNA reverse transcriptase.
RNA viruses and DNA viruses are very different in biology. In the view of virologists, as long as RNA viruses are studied in depth enough to find suitable targets, they can be eliminated like hepatitis C.For DNA viruses, it's a much longer road.
3. The process of virus infection
No matter how DNA or RNA viruses perform at the molecular level, at the individual level, virus infection has its natural history. After the
virus infection, several outcomes will occur. One is to undergo only subclinical changes and final recovery, the other is to undergo subclinical changes to clinical diseases, and finally either recovery or death. However, due to the different characteristics of the virus itself and the differences between the host individuals, the natural history of the virus varies at the individual level. For example, some viral infections are mainly hidden infections, such as B encephalitis virus, and most infections do not have obvious clinical symptoms. Measles viruses are mainly dominant infections, and they will develop after infection, and clinical diseases are the most prominent manifestation. Another extreme example is rabies virus. As long as it is infected, patients will die if they are not vaccinated, special serum or antibody injections are not promptly administered.
4. The past and present of coronavirus
Coronavirus is a major RNA virus. The first case of avian infectious bronchitis virus isolated from avians in 1937. The first case of human coronavirus was isolated from an inflatable part of a little boy's nasal cavity in 1965. Healthy volunteers who were vaccinated with the virus were also infected and became infected.
Coronavirus is the largest known RNA virus. The coronavirus needs to enter the host cell through the cell membrane. The key to this process is the binding of the cell membrane receptor and the spike protein of the coronavirus. This binding process is equivalent to the catalytic process of an enzyme. The virus's membrane and cell membrane are first semi-fusion, and then completely fused. After the two membranes are completely fused, the virus's nucleocapsid enters the cell, thus completing the first step of virus replication, which is also a very critical step.
Before this new coronavirus, there were 6 coronaviruses that could infect humans. The earliest one was the 229E virus and the latest one was MERS. The new coronavirus mainly enters the human respiratory tract. Its transmission efficiency is very high, but its mortality rate is not very high. Its important pathological characteristics are diffuse damage to the alveolar epithelium and the formation of multinucleated giant cells. This is related to the earliest step of virus invasion, that is, the entry of receptor-binding viruses.
5. How does the new coronavirus infect humans?
After the new coronavirus was discovered, researchers quickly proved that insensitive cells can be infected with the new coronavirus after transfected with the ACE2 receptor. Later, experiments have proved that blocking the ACE2 receptor can block the infection of the new coronavirus. It seems that both adequacy and necessity can prove that ACE2 is the main receptor of the new coronavirus at the cell culture level. We also identified ACE2 as an essential and adequate receptor for SARS viruses in the early stage.
But what does ACE2 look like at the human body level? First of all, it depends on its expression in each organ. ACE2 is expressed in many organs, including the lungs. The latest reports show that ACE2 is also expressed in many cells in the oral cavity, suggesting that this is related to some patients who only experience viral replication in the upper respiratory tract but do not replicate in the lower respiratory tract, thus causing serious diseases. However, this assumption has not been studied clearly yet.
In addition to ACE2, another membrane protease, TMPRSS2, can also promote infection of coronaviruses, especially SARS and the new coronavirus. The superposition of TMPRSS2 and ACE2 may be related to multiple organ damage to the new coronavirus. However, TMPRSS2 does not play a greater role in organ damage than ACE2, because TMPRSS2 is also required in HCov-229E infection, but HCov-229E is obviously a self-limiting disease. Of course, the expression of receptors and cofactors is only a factor in the occurrence and development of the disease, and other factors are also involved, which requires more research in the future. What does
look like at the crowd level? ACE2 is the short arm of the X chromosome in humans. The latest article analyzed the MAP and genome results of the Chinese genome, and found that there are more than 30 nucleotide polymorphic sites in the coding region. Interestingly, the polymorphic sites and known viral infection sites do not overlap. It means that there is no natural resistance to the new coronavirus.
6. Biomacromolecular drug treatment may be treated
Ahead I mentioned the invasion process of virus mediated by ACE2. How to block virus invasion? I would like to share some ideas about macromolecular drugs.
macromolecular drugs have many advantages over small molecule drugs, including few doses, high safety, high resistance barriers, and no influence of immune dominant epitopes. Groups with poor response to vaccination include infants and young children and the elderly. Of course, there are also limitations, such as inconvenient administration methods, complex production processes, and relatively high costs. There are also precedents in the development of
macromolecular drugs, such as RSV palizumab, and of course a series of influenza viruses and HIV monoclonal antibodies.
Anti-CoV macromolecular drugs can essentially be divided into two types. One is an antibody, including anti-ACE2 antibodies and anti-S protein antibodies, and the other is a fusion protein, including RBD-Fc fusion protein and ACE2-Fc fusion protein. The common point of their mechanism of action is to block the binding of the new coronavirus and ACE2. Of course, they have their own advantages and disadvantages.
For the new coronavirus, there are two main challenges facing macromolecular drugs. The first is whether the virus has cell transmission between cells, and can the virus avoid receptor binding and enter cells? This is impossible for hepatitis B virus. However, some viruses can be transmitted between cells. For example, HIV can be transmitted between cells. This transmission between cells will hinder the efficiency of macromolecular drugs.
For coronavirus, the most researched is the MHV of coronavirus mice. It has its own protein receptor and sialic acid receptor in MHV. They cooperate with each other to help the virus enter the cells. During the transmission stage, sialic acid can also help complete cell-cell transmission. The receptor for the new coronavirus is ACE2. It is not known whether it is involved in sialic acid and sugar molecules. However, there is no gene encoding RDE on the genome of the new coronavirus. RDE is an enzyme necessary for viruses that spread with carbohydrates. The new coronavirus does not encode RDE enzyme, so it has the probability of continuing to spread with carbohydrates, but it is not very likely.
has another question, whether there is an antibody-dependent viral infection enhancement effect (ADE). The so-called ADE effect is widely known in dengue viruses. The dengue virus that binds to non-neutralizing antibodies can enter macrophages by bypass and proliferate, and then cause a second infection, especially when the second infection is different from the first infection virus strain. The symptoms of the second infection are worse and the ADE effect occurs.
In the coronavirus, it is not known whether the new coronavirus has this ADE effect, but studies on SARS virus and MERS virus have found that low-affinity antibodies of S protein can mediate the virus to enter immune cells, but cannot replicate lively, and eventually only abortion infection occurs. Of course, it may also arouse cytokine and other reactions, which is not very clear at present. The factors of ADE depend on how strong the antibody itself is, and the activation of downstream signals, especially the degree of activation of ISG-responsive genes, etc. It is not only for antibody treatment, but also for vaccine development. (Compiled by Song Yajuan)
