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.