Catalytically active or inactive sACE2?
Most reports describing the engineering of sACE2 to enhance its affinity through targeted substitutions, or to improve pharmacokinetics through Fc fusions, have deliberately eliminated ACE2 catalytic activity50,53,38. The assumption is that administration of a catalytically active decoy receptor will adversely interact with physiology to have unacceptable toxicity. Soluble decoy receptors for other viruses have also been engineered to eliminate the normal biological activity for safety and efficacy reasons66. However, there are strong arguments that ACE2 catalytic activity, even if elevated, might be beneficial for treating COVID-19 symptoms.
In the renin-angiotensin system (RAS), a set of proteases regulates the production of angiotensin peptide hormones67,68(Figure 3B). Renin is released by the kidneys when renal blood flow is low and it catalyzes the conversion of angiotensinogen in the blood to angiotensin I (Ang-I). Ang-I is in turn a substrate for angiotensin-converting enzyme (ACE or ACE1), which cleaves the peptide to produce the vasoconstrictor Ang-II that has a wide range of effects through the angiotensin II type 1 receptor (AT1R). Ang-II promotes vasoconstriction and elevated blood pressure and volume, and under conditions where Ang-II is high there can be increased inflammation, myocardial fibrosis and thrombosis. Ang-II is proteolyzed by ACE2 to produce Ang-1-769,70,71,72, which has vasodilatory properties and is cardioprotective73. Both ACE1 and ACE2 are active in lung tissue (and elsewhere) and also catalyze the conversion of kinin peptides, the consequences of which are still speculative in the context of COVID-1915,74. Genetic studies and administration of sACE2 in animal models have demonstrated that tilting the RAS system in favor of Ang-1-7 has numerous benefits for the treatment of lung injury and inflammation59,60,61. Indeed, it has been proposed that COVID-19 symptoms may in part manifest from RAS dysregulation37,68. Ang-II levels are elevated approximately 3-fold in hospitalized COVID-19 patients and are correlated with both viral load and severity of hypoxemic respiratory failure13. Catalytically active sACE2 therefore offers two potential mechanisms of action. First, it blocks receptor-binding sites on SARS-CoV-2 spikes to neutralize infection. Second, it promotes the degradation of Ang-II and production of Ang-1-7 to directly address COVID-19 symptoms. A clinical trial testing wild type sACE2 in critically ill COVID-19 patients is ongoing, but a preliminary case report offers promising signs that both mechanisms of action are at play42. This provides sACE2 a tremendous advantage over monoclonal antibodies, which are safe and effective at reducing viral load75 but do not directly address symptoms in sick patients. However, at this time no direct comparison has been made in animal models of COVID-19 to evaluate catalytically active versus inactive sACE2, alongside engineered variants with altered S affinity. The hypothesis that sACE2 will have dual mechanisms of action therefore remains unproven.
Different mutations have been introduced into sACE2 to eliminate catalytic activity, the most common being H374N and H378N to prevent coordination of an essential Zn2+ion51. Others have used alternative mutations within the ACE2 active site50,53. As a generalization, ACE2 variants with reduced catalytic activity bind slightly tighter to SARS-CoV-2 S, suggesting possible conformational coupling between the catalytic and S binding sites. We are aware of only a single report76 where mutations to inactivate catalysis were associated with a small decrease in neutralization efficacy.