Introduction
The emergence of a novel strain of coronavirus severe acute respiratory
syndrome coronavirus 2 (SARS COV2) from Wuhan, China has brought the
whole world down to their knees. Belonging to the β lineage of
coronaviruses, family Coronaviridae , order Nidovirales and genus
sarbecovirus, this RNA virus with the largest genome (26-32kb) has led
to a global outbreak of respiratory disease, with over 3,356,634 human
cases and 7% mortality. The worst affected region in terms of morbidity
is United States of America with 1,104,324 cases and as far as mortality
is concerned, the European region is badly affected with reported
mortality of more than 10% (≥13% reported from Italy, France and
United Kingdom). [1]
[https://www.worldometers.info/coronavirus on 01 May 2020 ].
In current scenario when specific therapeutics are not available, and
applying travel restrictions, patient isolation, and providing
supportive medical care remain the mainstay for patient management,
angiotensin convertase enzyme 2 (ACE2), a host cell surface receptor has
garnered much attention as it acts as a key receptor for attachment to
the spike glycoprotein of SARS-CoV2 to initiate viral infection. [2]
In this article, we will discuss the potential application of
recombinant human soluble ACE2 (rhACE 2) as therapy in COVID-19 patients
and the challenges that can hinder its clinical use.
ACE2, a type 1 transmembrane
glycoprotein was first discovered in the year 2000. It is present widely
in human tissues but relatively high level of expression has been
observed in respiratory epithelial cells, alveolar cells type I and II,
oral cavity, kidney, testis, small intestines, arterial and venous
endothelial cells and arterial smooth muscle cells. Glomerular tubules
show low ACE2 expression, while glomerular mesangial and glomerular
endothelial cells including Kupffer cells and hepatocytes, spleen,
thymus, lymph nodes, bone marrow, and B and T lymphocytes and
macrophages don’t possess ACE2 activity. Tissue ACE2 activity has been
observed to be higher than its plasma activity and traditional
angiotensin‐converting enzyme inhibitors (ACEIs) don’t inhibit its
function. [3]
ACE2 is an essential part of
counter regulatory axis of renin angiotensin system (RAS), and has
extensive vascular and organ protection functions in hypertension,
diabetes, cardiovascular disease, and ARDS. ACE2 has catalytic activity
against both Angiotensin II (Ang II) and Angiotensin I (Ang I) but its
affinity for Angiotensin II is 400 times higher than that for Ang I.
[3] ACE2 hydrolyzes Ang II and Ang I generating angiotensin 1-7 (Ang
1-7) and angiotensin 1-9, respectively. Ang 1–7 binds to the Mas
receptor to exert anti-inflammatory and anti-remodeling effects
antagonizing the pro-inflammatory, pro-proliferative, and fibrotic
effects caused by Angiotensin II. [3]
In SARS-COV2, ACE2 acts as co-receptors facilitating viral entry by
interaction between receptor-binding domain (RBD) of the S1 subunit on
viral spike glycoproteins with the ectodomain of ACE2. The viral
endocytosis leads to the downregulation of ACE2 receptors either through
direct effect (rapid apoptosis) or indirectly by increasing the ADAM
metallopeptidase domain 17 (ADAM 17) induced catalytic cleavage of
ectodomain of ACE2 releasing it into the circulation. [4]
The increased Ang II acts on AT1 receptor and further upregulate ADAM17
in a well characterized positive feedback loop that lead to shedding of
its regulator ACE2 and downstream extracellular signal regulated kinase
(ERK)/p38 mitogen activated protein (MAP) kinase signaling pathways.
ADAM17 also mediates the liberation of membrane bound precursors of
TNFα, IFN-γ, and IL-4 pro-inflammatory cytokines into the circulation,
justifying its alternative name i.e. tumor necrosis factor converting
enzyme (TACE) (figure 1). This cytokine storm and the reduced ACE2
activity may be associated with the disease severity in SARS-CoV2
infection. [4]
ACE2 expressed in type II alveolar epithelial cells primarily function
to prevent alveolar collapse which are critical to the gas exchange
function of the lung by producing surfactant that reduces surface
tension. Injury to these cells in SARS-COV2 infection could explain the
severe lung injury and ARDS observed in COVID-19 patients. [3]
Despite the predominance of respiratory symptoms in SARS-COV2, the acute
cardiac and kidney injuries, myocarditis, arrhythmias, and gut and liver
abnormalities occurring in COVID-19 patients are consistent with the
widespread expression of ACE2. [2]
The enzymatic conversion of the proinflammatory, profibrotic
vasoconstrictor Ang II into Ang 1-7 that exerts anti-inflammatory,
antifibrotic and cardioprotective vasodilator makes the soluble ACE2
receptor a reasonable therapeutic approach in settings where Ang II is
involved in the pathologic mechanism (like in SARS-CoV2). [1]
Recombinant soluble ACE2 has already shown potential for a vast array of
therapeutic indications. It has shown good safety and tolerability
profile in phase 1 and 2 trials done in healthy volunteers and in
patients suffering from ARDS and single doses (100 and 1200 mg/kg
rsACE2) administered intravenously in healthy volunteers had revealed a
plasma half-life of 10 hours and peak plasma concentration to be up to
20 mg/ml (223 nM) in the highest-dose cohort. [5]
In SARS-CoV2, the proposed use of soluble ACE2 (sACE2) receptor is that
it would bind to the viral S protein thereby neutralize the virus and
prevent virus mediated host cell receptor downregulation (figure 1).
SARS-COV2 and SARS CoV spike proteins share 76.5% identity in amino
acid sequences and, importantly, there is a high degree of homology
between spike proteins of SARS-COV2 and SARS-CoV. [6] Soluble ACE2
receptor was demonstrated to inhibit SARS CoV binding to cells in
culture suggesting this strategy
to be very promising against SARS-CoV2 too. [7] Apart from S protein
neutralizing agent, an additional advantage of using recombinant ACE2 in
SARS-CoV2 would be its role in treatment of pneumonia in COVID-19
patients. [6]
ACE2-Fc, an immunoadhesin format of sACE2 where sACE2 is fused to
immunoglobulin Fc domain appear more convincing and effective option by
extending the lifespan of the circulating molecule while retaining the
effector functions of the Fc domain, thereby allowing recruitment of
dendritic cells, macrophages, and natural killer cells through the CD16
receptor against viral particles or infected cells. Moreover, anin vitro study using sACE2 fused to the Fc portion of
immunoglobulin has reported successful neutralization of SARS-CoV2.
[7]
The availability of safe rhACE2 formulation gave researchers the
encouragement and motivation to rapidly start a pilot trial of rhACE2 in
patients suffering from severe COVID-19 (Clinicaltrials.gov
#NCT04287686). [6] Nevertheless, as far as rhACE2 therapy is
concerned, the results of the ongoing trials and upcoming clinical
trials conducted on large number of patients would provide the
definitive and convincing evidence for its use in COVID-19 patients.
Despite good safety profile of recombinant human ACE2, there are
research gaps that need to be addressed before it is approved for
clinical use in SARS-CoV2 infection. The challenges are that the
understanding about SARS-CoV2 pathogenesis is still in evolving and in
its naïve stage. Thereby, the possibility of virus making use of other
co-receptors/auxiliary proteins or even other mechanisms for entry into
the cells cannot be ruled out. [1] For pathophysiology of COVID 19,
till date the biochemical evidence about the RAS dysregulation in
COVID-19 is lacking, only a single study from China had reported raised
levels of Angiotensin II in all 12 COVID-19 patients as compared to
healthy controls indicating that RAS overactivation could be responsible
for severity of infection. Clinical data about the RAS metabolite levels
(i.e. Aldosterone, Angiotensin II) and sACE2 in serial patient blood
samples of COVID-19 can provide crucial information to answer questions
on disease predisposition, its progression and on the mechanisms
affected by the viral infection. [8]
In a recent study conducted by Montiel et al , the use of the
clinical-grade human ACE2 molecule - but not mouse sACE2 was found to
significantly inhibit SARS-CoV2 infections and reduction of viral load
by a factor of 1,000-5,000 was observed when virus was grown in Vero E6
cells. They further found that engineered human blood vessel organoids
and human kidney organoids can be readily infected, which can
significantly be inhibited by rhACE2 at an early stage of infection. But
they failed to perform similar studies on human lung tissue which is the
main organ affected in SARS-CoV2 infection. Hence in vitrostudies testing sACE2 efficacy against SARS-CoV2 in different human cell
lines followed by in vivo experiments should be carried out to
have better understanding of the multi-organ involvement in SARS-CoV2
infection. It would be also be interesting to note the viral
pathogenesis in ACE2 deficient individuals or similar in vitroexperiments in cell lines with reduced ACE2 expression can provide more
understanding. [9]
The cross transmission of virus between bats and/ or other intermediate
hosts and humans is the result of structural variations of the viral
spike protein and the sequence variation in animal host ACE2 receptors
may also contribute towards viral susceptibility and/or resistance.
Although many ACE2 variants have shown a similar binding affinity for
SARS‐CoV2 spike protein, ACE2 alleles, rs73635825 (S19P) and rs143936283
(E329G) have showed noticeable variations in their intermolecular
interactions with the viral spike protein. Hence, it is conceivable that
under new selection pressure as offered by SARS‐CoV2, these alleles may
undergo positive selection and can have an impact on the progression of
clinical trials testing the utility of recombinant ACE2 as COVID-19
therapy. [10] Thus, largescale and multiple tissue-level analysis of
single-cell RNA sequence would provide precise information about the
expression analysis of ACE2 in different populations and in turn help in
selecting the candidates for rhACE2 therapy.