Stroke increases the expression of ACE2, the SARS-CoV-2 binding receptor, in murine lungs

The newly emerged severe acute respiratory syndrome coronavirus (SARS-CoV-2) has caused a worldwide pandemic of human respiratory disease. Angiotensin-converting enzyme (ACE) 2 is the key receptor on lung epithelial cells to facilitate initial binding and infection of SARS-CoV-2. The binding to ACE2 is mediated via the spike glycoprotein present on the viral surface. Recent clinical data have demonstrated that patients with previous episodes of brain injuries are a high-risk group for SARS-CoV-2 infection. An explanation for this finding is currently lacking. Sterile tissue injuries including stroke induce the release of several inflammatory mediators that might modulate the expression levels of signaling proteins in distant organs. Whether systemic inflammation following brain injury can specifically modulate ACE2 expression in different vital tissues has not been investigated.

For the induction of brain stroke, mice were subjected to a surgical procedure for transient interruption of blood flow in the middle cerebral artery for 45 min and sacrificed after 1 and 3 days for analysis of brain, lung, heart, and kidney tissues. Gene expression and protein levels of ACE2, ACE, IL-6 and IL1β were measured by quantitative PCR and Western blot, respectively. The level of soluble ACE2 in plasma and bronchial alveolar lavage (BAL) was measured using an immunoassay. Immune cell populations in lymphoid organs were analyzed by flow cytometry. Post-stroke pneumonia in mice was examined by bacterial cultures from lung homogenates and whole blood.

Strikingly, 1 day after surgery, we observed a substantial increase in the protein levels of ACE2 in the lungs of stroke mice compared to sham-operated mice. However, the protein levels of ACE2 were found unchanged in the heart, kidney, and brain of these animals. In addition, we found increased transcriptional levels of alveolar ACE2 after stroke. The increased expression of ACE2 was significantly associated with the severity of behavioral deficits after stroke. The higher protein levels of alveolar ACE2 persisted until 3 days of stroke. Interestingly, we found reduced levels of soluble ACE2 in plasma but not in BAL in stroke-operated mice compared to sham mice. Furthermore, stroke-induced parenchymal and systemic inflammation was evident with the increased expression of IL-6 and IL-1β. Reduced numbers of T-lymphocytes were present in the blood and spleen as an indicator of sterile tissue injury-induced immunosuppression.

We demonstrate specific augmented alveolar ACE2 levels and inflammation in murine lungs after experimental stroke. These pre-clinical findings suggest that patients with brain injuries may have increased binding affinity to SARS-CoV-2 in their lungs which might explain why stroke is a risk factor for higher susceptibility to develop COVID-19.