The COVID-19 severe disease model reveals the molecular mechanism of SARS-CoV-2 infection
The SARS-CoV-2 causing COVID-19 epidemic has been worldwide in scope since the end of 2019, with high global mortality rates, and has led to a global public health crisis. The clinical symptoms of COVID-19 are diverse, ranging from fever, malaise, and dry cough to dyspnea, from mild pneumonia to acute lung injury (ALI) and acute respiratory distress syndrome in severe cases.
Similar to SARS-CoV, SARS-CoV-2 is an enveloped single-stranded positive RNA virus belonging to the genus β-coronavirus of the coronaviridae family. Human angiotensin-converting enzyme 2 (hACE2) has been shown to be a functional receptor for SARS-CoV-2. Studies related to SARS-CoV-2 have been conducted in various countries, and some hACE2-expressing mouse models, such as hACE2 transgenic mice, AAV-hACE2 transduced mice, and Ad5-hACE2 transduced mice, have been developed. However, most models cause only mild to moderate lung injury in mice. An animal model capable of reproducing the most severe respiratory symptoms and high morbidity and mortality of COVID-19 remains an urgent need.
Recently, Qin Chengfeng/Wang Hui’s team from the Military Medical Research Institute of the Chinese Academy of Military Sciences and Wang Xiangxi’s team from the Institute of Biophysics of the Chinese Academy of Sciences published a paper entitled “Characterization and structural basis of a lethal mouse-adapted SARS-CoV-2” online in Nature Communications, publicly stating that the team successfully established a severe disease model of COVID-19 and revealed the molecular mechanism of cross-species infection of SARS-CoV-2.
First, the researchers had created a SARS-CoV-2 (MASCp6) mouse-adapted strain that has previously been shown to induce moderate lung damage in mice. Based on this, the researchers further passed the strain 30 times in succession to generate a more virulent mouse-adapted strain, eventually producing SARS-CoV-2 (named MASCp36) in the 36th generation.
After intranasal administration of varied doses to BALB/c mice of different months of age and sex, it was discovered that 9-month-old mice were particularly sensitive to and dose dependant on MASCp36 toxicity. All 9-month-old mice given high doses of MASCp36 developed normal respiratory symptoms, including hair ruffling, hunchback, and decreased activity. Male mice were also more responsive to MASCp36 than female mice. In the tiny bronchial tubes, microscopic examination reveals a considerable number of desmoplastic epithelial cells, severe necrosis of alveolar epithelial cells, and merged inflammatory cell infiltration in the alveolar wall, predominantly with neutrophils. Severe perivascular edema and scattered hemorrhage are indicative of MASCp36 infection induced necrotizing pneumonia and extensive diffuse alveolar injury.
The researchers collected lung tissue for histopathological and immunostaining investigation to better determine the pathological findings in MASCp36-infected mice. In comparison to the control group, naked-eye observation indicated severe lung damage in MASCp36-infected mice, with bilateral redness and mucus in the lungs.
Finally, the research team performed a series of in-depth studies on this model, and deep sequencing revealed that a total of 12 amino acid mutation sites were detected in MASCp36 in consecutive passages, three of which (N501Y, Q493H, and K417N) located in the S protein receptor binding region (RBD). Further experiments confirmed that this structure resulted in a significant increase in affinity between MASCp36 virus and murine-derived ACE2. The RBD of the lethal strain MASCp36 was shown to form a stable binding dense structure with murine-derived ACE2, which was very similar to the structure of the wild-type virus RBD with human-derived ACE2.
In summary, this study yielded a new mouse-adapted SARS-CoV-2 strain, MASCp36, which causes severe respiratory symptoms and mortality. The model also showed age- and sex-related mortality similar to that of severe COVID-19. During in vivo passaging, three amino acid substitutions, N501Y, Q493H, and K417N, were identified by in-depth sequencing of the MASCp36 receptor binding region (RBD). This research lays the groundwork for understanding the pathophysiology of SARS-CoV-2 and elucidating the molecular mechanisms underlying its fast adaptation and evolution.