Tissue Distribution of ACE2
Detectable quantities of ACE2 protein have been found almost ubiquitously in tissues across mammalian species, using immunostaining methods. ACE2 is predominantly located in the cardiovascular system and kidney, where it probably plays a role in the maintenance of hydro electrolyte homeostasis (section “Mechanism of Viral Entry Mediated by the S Protein”). In fact, ACE2 is pervasively expressed throughout the vasculature, at the level of the arteries and veins, mainly in smooth muscle cells of the media and in the endothelium (Hamming et al., 2004; Burrell et al., 2005). Such signal from vessels also delivers part of the expression detected in specific organs. Indeed, ACE2 is evident in: coronary vessels and myocardial capillaries (Wiener et al., 2007; Garabelli et al., 2008); lung microvascular endothelial cells (Wiener et al., 2007; Chen et al., 2013); kidney interlobular arteries (Lely et al., 2004); endothelial and smooth muscle cells in the brain (Hamming et al., 2004; Kar et al., 2010). Notably, the mesangium and glomerular endothelium in the kidney, and the endothelial lining of the sinusoids in the liver are allegedly negative for ACE2 (Hamming et al., 2004). On the contrary, ACE2 is virtually absent from the lymphatic system, and human hemato-lymphoid organs (i.e., spleen, lymph nodes, and bone marrow) (Hamming et al., 2004; Li et al., 2007). In blood cells, it has been observed in platelets and macrophages, but not in B and T lymphocytes (Hamming et al., 2004; Fraga-Silva et al., 2011).
Expression of ACE2 was originally identified in rodent heart (Donoghue et al., 2000a), where it was observed to occur in both atrium an ventricle (Gembardt et al., 2005), and, cellularly, in cardiomyocytes and in specialized cells of the sinoatrial node (Burrell et al., 2005; Garabelli et al., 2008; Ferreira et al., 2011; Wang et al., 2017). In human heart, ACE2 has been found in the stromal area in spongiosa layer in aortic valves (Peltonen et al., 2011), where it is expressed in myofibroblasts and fibroblasts (Guy et al., 2008).
High levels of ACE2 protein expression have been detected in mammalian, including human, kidney (Gembardt et al., 2005; Koka et al., 2008; Reich et al., 2008; Giani et al., 2012; Mitani et al., 2014; Grobe et al., 2015; Shi et al., 2015; Larouche-Lebel et al., 2019; Alawi et al., 2020). Strong signals were reported in the brush border of the proximal tubular cells, whereas weak to moderate signals could be found in the glomeruli, Henle’s loop, distal tubules, and collecting duct (Hamming et al., 2004; Lely et al., 2004; Kamilic et al., 2010; Giani et al., 2012; Bae et al., 2015; Cao et al., 2017; Errarte et al., 2017).
In the respiratory tract of primates, positive labeling for ACE2 has been reported at multiple sites, from the nasal and oral mucosa, to the larynx, trachea, bronchi and lung (Liu et al., 2011). Whether ACE2 is expressed in human nasal and oral epithelium remains unclear, as contradictory results have been reported by studies using immunohistochemistry (Hamming et al., 2004; Bertram et al., 2012), in face of positive single-cell RNA sequencing findings (Sungnak et al., 2020). This point is of great interest to understand the role of those tissues in SARS-CoV-2 initial infection, spread and clearance (section “Links Between ACE2 and COVID-19”). In the upper respiratory tract ACE2 is expressed in the epithelial lining and lamina propria, in some muscle cells and in the salivary gland duct epithelium. In the lung, an intense signal for ACE2 protein has been consistently observed in type I and II pneumocytes in several species, including mouse, rat, cat, ferret, monkey and human (Wiener et al., 2007; van den Brand et al., 2008; Liu et al., 2011; Wong et al., 2012; Chen et al., 2013; Zhang B. N. et al., 2019). Data from rodents suggest an age- and gender-dependent pattern of expression, with a more rapid decline with age in males as compared to females (Xie et al., 2006).
Although some ACE2 signal has been observed in the liver, it appears to mainly come from small vessel endothelium, and occasionally bile duct epithelial cells, while negligible expression is observed in hepatocytes (Hamming et al., 2004; Paizis et al., 2005; Guan et al., 2020). ACE2 protein is abundantly expressed in the brush border of enterocytes of all parts of the small intestine, including the duodenum, jejunum, and ileum, but not in enterocytes of the colon. Other organs of the digestive tract, such as the stomach and colon, did not show brush border staining, but rather a positive signal in the muscolaris mucosae and the muscolaris propria (Hamming et al., 2004). In rodents, ACE2 is also expressed in both exocrine and endocrine pancreatic tissue, particularly in the islets of Langerhans (Niu et al., 2008; Fang and Yang, 2010).
ACE2 distribution is widespread in the mouse brain, from the telencephalon to the medulla. As expected, ACE2 is found in brain areas involved in the regulation of cardiovascular function and fluid balance, such as the vascular organ of lamina terminalis, subfornical organ, magnocellular neurons in the hypothalamic paraventricular nucleus, area postrema, nucleus of the solitary tract, dorsal motor nucleus of the vagus, nucleus ambiguous, and rostral ventrolateral medulla (Doobay et al., 2007). However, significant expression had also been reported in brain areas not engaged in the classical functions of the RAAS, namely the piriform cortex, hippocampus, caudate putamen, hypoglossal nucleus and primary motor cortex (Doobay et al., 2007; Lin et al., 2008; Liu et al., 2014). ACE2 immunostaining was identified in neurons as well as astrocytes (Gallagher et al., 2006; Doobay et al., 2007; Yamazato et al., 2007). Furthermore, ACE2 has been documented in the retina, predominantly in the inner nuclear layer but also in photoreceptors (Tikellis et al., 2004; Senanayake et al., 2007).
With regard to the endocrine system, ACE2 expression was found in both male and female reproductive systems. In human testis, ACE2 was localized to the Leydig and Sertoli cells, and might be involved in testicular function (Douglas et al., 2004). At present, no data about ACE2 protein expression is human ovaries is available, although evidence of expression in stroma, theca, and granulosa cells has been reported in other species (Tonellotto dos Santos et al., 2012; Barreta et al., 2015; Pereira et al., 2015).
In rodent bone, ACE2 is expressed in osteoblasts and osteoclasts, as well as in epithelial cells and fibroblasts. However, a similar expression in human samples still awaits clarification (Queiroz-Junior et al., 2019).
In human skin, ACE2 was present in the basal cell layer of the epidermis extending to the basal cell layer of hair follicles. Smooth muscle cells surrounding the sebaceous glands were also positive for ACE2. Weak cytoplasmic staining was observed in sebaceous gland cells. A strong granular staining pattern for ACE2 was seen in cells of the eccrine glands. Positive staining for ACE2 was also noted in the membrane of human fat cells in various organs, including the epicardial adipose tissue (Hamming et al., 2004; Patel et al., 2016a). Globally, ACE2 is chiefly bound to cell membranes, while negligible levels can be detected in the circulation.