*For medical professionals only
I have been following the research progress of the new crown virus for more than three years, and I never expected that there is still research on the mechanism of new crown infection that shocks me.
Recently, a research team led by Peter K. Jackson of Stanford University and Raul Andino of the University of California, San Francisco published an important research result in the top journal Cell [1].
They found that after the new coronavirus entered the respiratory tract, it immediately bound to ACE2 on the cilia of epithelial cells, and passed along the cilia through the periciliary layer (PCL), which is normally difficult for them to cross, to achieve infection of ciliated epithelial cells. What's more, after completing replication in the cell, the new coronavirus will make the originally short microvilli on the ciliated epithelial cells thicker and longer, pass through the PCL layer, and successfully complete the "detoxification" to infect more cells.
Jackson's team's study not only uncovered the mystery of the new coronavirus infection of respiratory cells, but also solved the reason why respiratory viruses bypass the innate immune barrier and cross back and forth to expand infection.
For me, I was blown away by the back and forth trick of the coronavirus. I don't know if you think the new crown is very powerful, anyway, my heart beat faster and my palms sweated after reading it.
▲ Screenshot of the first page of the paper
Although the new crown virus infection is a systemic disease, the main tissues and organs of the new crown virus infection are the respiratory tract.
In early 2020, scientists discovered the mechanism by which the new coronavirus infects a variety of tissue cells throughout the body. The nose is the main organ and the first stop of the new crown virus infection [2], and scientists have not yet figured out how the new crown virus passes through the mucus layer and the physical barrier of mucin around cilia, and then infects nasal epithelial cells.
It should be known that the periciliary layer (PCL) is a very powerful natural immune defense, under normal circumstances, only small particles (25nm) can pass through PCL, and the new coronavirus, a virus with a diameter of 100nm, cannot cross PCL [3].
▲ Structure of the airway, and permeability of PCL [3]
So how do huge respiratory viruses such as the new coronavirus infect nasal ciliated epithelial cells (CEC)?
Two points of information caught Jackson and his colleagues' eye.
First, ACE2 and TMPRSS2, which are necessary for new coronavirus infection cells, have cilia on them [4,5]; Second, the cilia of the cilia cells pass just through the entire PCL layer[3]. The most likely way is that the new coronavirus uses part of the cilia exposed outside the PCL as a breakthrough, and then infects the ciliated epithelial cells along the cilia.
Based on human nasal organoids, they first confirmed that human nasal ciliated epithelial cells are indeed the main entrance to the nasal epithelium of the new coronavirus infection. And they also found that there was a kinetic bottleneck in ciliated epithelial cells infected with the new coronavirus: no matter how large the infection plurality (MOI, ratio of virions and cells), only 3% of nasal ciliated epithelial cells were positive for the new coronavirus when inoculating the virus 6 and 24 hours; It is not until 48 hours after vaccination that the positivity rate increases to 80%.
▲ There is a kinetic bottleneck in ciliated epithelial cells infected with the new crown
Subsequently, with the help of various advanced microscope and fluorescence techniques, Jackson's team found that after 6 hours of inoculation with the new coronavirus, the cilia soaked in PCL did adhere to the virus, and the addition of neutralizing antibodies targeting the S protein inhibited the attachment of the new coronavirus to the cilia, and the cell infection rate also dropped significantly.
▲ The new coronavirus attaches to the cilia
In addition, inhibiting the formation of cilia also inhibits the infection of human nasal epithelial cells by the new coronavirus.
The above results show that cilia are indeed the key to nasal epithelial cells infected with the new coronavirus.
The next question is, how does the new coronavirus enter the cilia epithelial cells along the cilia?
Based on a series of molecular biology and pharmaceutical studies, Jackson's team found two ways. The first is that the new coronavirus binds directly to ACE2 and TMPRSS2, causing the new coronavirus to fuse with cilia, and the virus releases RNA into the cilia and finally into the cilia epithelial cells. The second is that after the new coronavirus binds to ACE2, with the help of ciliary dynein, it descends from the tip of the cilia to the bottom of the cilia, and then enters the ciliated epithelial cells.
▲ Two ways for the new coronavirus to enter ciliated epithelial cells
I don't know if you remember the delayed infection phenomenon introduced earlier: the virus infected more cells after 48 hours. Curiously, in Vero cells (some inactivated vaccines are cultured with this cell line), there is no such delay in new coronavirus infection. What is the reason for this?
Combined with the structure of the epidermis on the nasal cavity, Jackson's team speculates that the process by which the initial infected cells excrete the new virus may limit the spread of the virus. So they set out to study how nasal cilia epithelial cells excrete new viruses when infected.
The results are really new.
It turns out that because of the existence of PCL, the new coronavirus not only has difficulty infecting ciliated epithelial cells, but also excretes the new virus after replication in ciliated epithelial cells. The problem is that it can't get by.
So what is the new crown virus doing with this lag of a day or two?
It turns out that they are "building roads"!
A series of subsequent studies have found that the new coronavirus can actually activate p21-activated kinases 1 and 4 (PAK1 and PAK4), which regulate cytoskeletal recombination, so that the originally tiny microvilli on ciliary epithelial cells grow into giant microvilli that are as thick and long as cilia. With the help of these huge microvilli, the new coronavirus successfully crosses the PCL, completes detoxification, and infects more cells.
▲ Schematic diagram of the mechanism of new coronavirus invasion and detoxification
Jackson's team also found that there is no ACE2 and TMPRSS2 on the microvilli, so the giant microvilli is a dedicated channel for detoxification created by the new coronavirus. It is worth mentioning that the discovery of this new mechanism has also allowed scientists to discover a new way to inhibit new coronavirus infection, that is, to inhibit the formation of microvilli.
At the end of the study, Jackson's team explored why the Omicron variant is highly contagious and whether a similar mechanism exists.
The results suggest that Omicron infects the upper respiratory tract using this same mechanism, but the Omicron variant infects more rapidly than other variants, infecting 10% of cells 6 hours after inoculation and 40% at 24 hours, much higher than the 1-3% of other strains. Electron microscopy showed that there were many more Omicron strains attached to cilia than other strains. This also shows that the binding ability of the Omicron strain to ACE2 is indeed stronger than that of other strains.
▲ Omicron is indeed stronger
In general, Jackson's team's research has given us a deeper understanding of the upper respiratory tract of the new coronavirus infection, and more importantly, the study has also found targets that can prevent the new coronavirus infection, which also provides new ideas for nasal administration.
In the future, perhaps we can block the invasion of the new crown virus at the first stop of the new crown virus infection through nasal medication. This medication is not only simple, but also avoids the undesirable off-target effects of systemic therapy.
Bibliography:
[1]. Wu CT, Lidsky PV, Xiao Y, et al. SARS-CoV-2 replication in airway epithelia requires motile cilia and microvillar reprogramming. Cell. 2023;186(1):112-130.e20. doi:10.1016/j.cell.2022.11.030
[2]. Sungnak W, Huang N, Bécavin C, et al. SARS-CoV-2 entry factors are highly expressed in nasal epithelial cells together with innate immune genes. Nat Med. 2020;26(5):681-687. doi:10.1038/s41591-020-0868-6
[3]. Button B, Cai LH, Ehre C, et al. A periciliary brush promotes the lung health by separating the mucus layer from airway epithelia. Science. 2012;337(6097):937-941. doi:10.1126/science.1223012
[4]. Lee IT, Nakayama T, Wu CT, et al. ACE2 localizes to the respiratory cilia and is not increased by ACE inhibitors or ARBs. Nat Commun. 2020;11(1):5453. Published 2020 Oct 28. doi:10.1038/s41467-020-19145-6
[5]. Nakayama T, Lee IT, Jiang S, et al. Determinants of SARS-CoV-2 entry and replication in airway mucosal tissue and susceptibility in smokers. Cell Rep Med. 2021;2(10):100421. doi:10.1016/j.xcrm.2021.100421
The author of this article 丨BioTalker