A model for how imbalance in the RAS pathway produces COVID-19 injury in the lungs
ACE2 is highly expressed in the lung parenchyma, particularly in type II pneumocytes (type II alveolar cells) (Zou et al, 2020). Type II cells synthesize and release pulmonary surfactant, phospholipids that lower surface tension, which is necessary to maintain alveolar structure (Andreeva et al., 2007). Type II cells also can differentiate to become type I alveolar cells (which form the structure of alveoli), a mechanism for replacement of type 1 cells that are damaged. The SARS-CoV-2 and SARS-CoV-1 viruses perturb alveoli to produce the major pathology in the lung, with increased fluid entry, cell death and inflammation along with reduction in gas exchange and levels of surfactant (Hui et al., 2005; Gralinksi & Baric, 2015; Xu et al., 2020b).
Figure 3 depicts a hypothetical framework for this process and the cell types involved. The SARS-CoV-2 virus infects alveolar pneumocytes by binding to ACE2, leading to a decrease in ANG II conversion to ACE2-derived peptides, e.g. a reduction in ANG (1-7) and its actions that counteract effects of ANG II (Figure 2) . Hence, ANG II levels increase in the alveolar microenvironment, with potential effects on multiple cell types. Below, we provide evidence in support of this framework and the role of ACE1/ACE2 imbalance in the lung injury of COVID-19.
ANG II has a pro-apoptotic action on pulmonary epithelial cells (Wang et al., 1999a; Wang et al., 1999b; Wang et al., 1999c; Wang et al., 2000; Papp et al., 2002), a response that is consistent with the pathology from SARS viruses, i.e., widespread epithelial damage and alveolar damage and cell death (Zhou et al., 2020a; Zhou et al., 2020b). In addition, ANG II promotes and Ang (1-7) suppresses epithelial-to-mesenchymal transformation (EMT), whereby epithelial cells acquire a more fibrotic phenotype (Buckley et al., 2010; Rodrigues-Díez et al., 2008; Lee at al., 2013; Shao et al., 2019), a mechanism that may contribute to the formation of pulmonary lesions. ANG II also decreases the clearance of alveolar fluid (Deng et al., 2012a; Deng et al., 2012b; Ismael-Badarneh et al., 2015). Apoptosis and EMT in alveolar epithelial cells are accompanied by an increase in secretion of pro-inflammatory cytokines (e.g., IL1-β, IL-6, IL-8, MCP-1 and TNF-α) (Aumiller et al., 2013; Ma et al., 2010; Kode et al., 2006; Pires-Neto et al., 2013; Pedersen & Ho, 2020). Epithelial cells engage in crosstalk with immune cells, in particular during infection and apoptosis (Rzepka et al., 2012; Herold et al., 2012; Chuquimia et al., 2012). These effects are compounded by the secretion of RAS components by activated myofibroblasts and epithelial cells undergoing apoptosis (Wang et al., 1999a; Wang et al., 1999b; Wang et al., 2000; Uhal et al., 2012), thereby amplifying ANG II signaling in a positive-feedback loop.
ANG II also has pro-fibrotic effects on fibroblasts that reside in interstitial spaces around alveoli (Marshall et al., 2000; Uhal et al., 2007; Uhal et al., 2012b) and increases apoptosis of endothelial cells and endothelial permeability in the surrounding capillary network (Watanabe et al., 2005; Bodor et al., 2012), which can increase fluid entry and immune infiltration into regions of the lung. ANG II also affects various types of immune cells (Forrester et al., 2018), increasing macrophage infiltration, ROS production and release of pro-inflammatory cytokines. Besides immune cells, pathological ROS production occurs in pulmonary fibroblasts, driven by ANG II stimulation and inhibited by the actions of ACE2/ANG (1-7) (Meng et al., 2015). ROS production from activated fibroblasts is a key driver of epithelial injury in models of pulmonary fibrosis (Sakai & Tager, 2013).
Crosstalk among epithelial cells, fibroblasts and immune cells suggests a role for macrophages, which are activated by pulmonary epithelial cells in injury settings and can create positive feedback that promotes inflammation (Uhal et al., 2007; Rzepka et al., 2012; Herold et al., 2012; Chuquimia et al., 2012; Bhattacharya & Westphalen, 2016). Such effects can occur in response to ANG II (Uhal et al., 2007). Ang II can also regulate the function of other immune cells (e.g., dendritic cells and neutrophils) that can promote lung injury (Grommes & Soehnlein, 2011; Florez‐Sampedro et al., 2018). Importantly, as summarized below, ANG (1-7) has opposing effects that counteract this pathology; these protective effects are blunted by the SARS-CoV-2 virus.
ANG II thus promotes a range of pro-apoptotic, inflammatory, fibrotic and edema-associated processes in the alveolar microenvironment. As cell death occurs, the epithelium, a critical component of innate immunity as a barrier to pathogens and via its formation and release of surfactant (Wright, 2003; Eisele and Anderson, 2011) becomes compromised, paving the way for secondary infection (e.g., bacterial pneumonia). As a result, immune response is further enhanced, greater inflammation ensues (Figure 3 ) and alveolar damage is increased, thereby enhancing lung injury and edema. Patients who recover may have severe tissue damage, potentially with tissue fibrosis. Indeed, studies in patients who recovered from SARS-1 document the presence of fibrosis and chronic lung damage in severe cases (Ketai et al., 2006). Lung tissue from COVID-19 patients shows evidence of a) epithelial injury and disruption, in particular of type-2 pneumocytes, b) invasion of macrophages and neutrophils and c) initiation of fibrosis (Luo et al., 2020; Tian et al., 2020; Dhama et al., 2020; He et al., 2020).