Keywords
SARS-CoV-2, Omicron, COVID-19, Lancemaside A, triterpenoid saponin,
membrane fusion
Introduction
During the ongoing coronavirus disease 2019 (COVID-19) pandemic, five
variants of concern (VOCs) of the severe acute respiratory syndrome
coronavirus 2 (SARS-CoV-2) have been identified
(Scovino, Dahab, Vieira, Freire-de-Lima,
Freire-de-Lima & Morrot, 2022). Alpha variant (lineage B.1.1.7) was
first detected in the United Kingdom in early 2020 and then four
additional variants including Beta (B.1.351), Gamma (P.1), Delta
(B.1.617.2), and Omicron (B.1.1.529) have emerged and spread to many
countries (Aleem, Akbar Samad & Slenker,
2022). Omicron, first reported to the World Health Organization (WHO)
from South Africa on November 24, 2021, now has overtaken the previous
variants and became the dominant variant around the world
(Gowrisankar, Priyanka & Banerjee, 2022;
Rahmani & Rezaei, 2022). Changes of
dominant variant over time have been attributed to enhanced
transmissibility of newly emerging variant over previous ones due to the
additional mutation on S protein and its increased affinity to human
receptor protein ACE2 (Chen, Tsung-Ning
Huang & Huang, 2022; Du et al., 2022).
These S mutations also confer resistance to antibody-based therapeutics
and evasion from infection- and/or vaccine-induced humoral immunity
(Biswas et al., 2022;
DeGrace et al., 2022;
Hoffmann et al., 2022). Thus, therapeutic
strategies targeting common mechanisms that contributes to the infection
of SARS-CoV-2 and their variants are imperative.
The entry of SARS-CoV-2 into human cells requires the binding of viral
envelope S protein to ACE2 cellular receptor and subsequent fusion
between the viral and cellular membranes, releasing the viral genome in
the cytoplasm (Hoffmann et al., 2020).
The membrane fusion is mainly mediated by S protein that is comprised of
two functional subunits termed S1 and S2, which are responsible for
receptor binding and membrane fusion, respectively
(Tang, Bidon, Jaimes, Whittaker & Daniel,
2020). Upon ACE2 binding, S protein undergoes a conformational change
from pre-fusion conformation to a post-fusion structure via its
proteolytic cleavage at S1/S2 sites by host cell proteases
(Hoffmann et al., 2020). The hydrophobic
fusion peptide (FP) of S2 subunit is exposed and inserted into host cell
membranes, then the heptad repeat 1 (HR1) and 2 (HR2) domains in the S2
subunit are bound together to form a six-helix bundle (6-HB) fusion
core, drawing the two membranes into close proximity to facilitate
virus-cell fusion (Xia et al., 2020).
Since the S2 subunit is highly conserved across the coronavirus family,
FP and HR domains are considered as a key target for the development of
pan-coronavirus fusion inhibitor (Huang,
Yang, Xu, Xu & Liu, 2020; Xia et al.,
2020). Previous studies have reported that HR2-derived peptides
targeting the HR1 domain inhibit 6-HB formation, thereby blocking viral
membrane fusion and cellular entry of human coronaviruses
(Liu et al., 2004;
Lu et al., 2014;
Xia et al., 2019). It was also
demonstrated that conjugation of cholesterol to the peptide sequence
leads to the anchoring of the peptide to the cell membrane where fusion
occurs, strongly potentiating its action
(de Vries et al., 2021;
Xia et al., 2020).
Natural products have been an important source of recent drug
development (Newman & Cragg, 2020).
Since the WHO declared COVID-19 as a pandemic, natural compounds have
provided a wide array of potential anti-SARS-CoV-2 drug candidates with
various cellular and viral targets. In line with this, we previously
reported that platycodin D (PD), a natural component of Platycodon
grandiflorum (PG), is capable of blocking SARS-CoV-2 infection
(Kim et al., 2021). The roots of PG have
been widely used to treat several respiratory diseases, such as asthma,
airway inflammation, and sore throats
(Choi, Hwang, Lee & Jeong, 2009;
Lee et al., 2020). Similarly, the roots
of Codonopsis lanceolata (CL) have been widely used as a
traditional medicinal herb in East Asian country such as Korea, China,
Japan for the treatment of several disorders including bronchitis,
cough, and allergic lung inflammation
(Hossen, Kim, Kim & Cho, 2016;
Seo et al., 2019). It contains many
biologically active compounds, including polyphenols, saponins, tannins,
triterpene, alkaloids, and steroids (Du et
al., 2018). But, whether CL extracts or their active compounds may have
inhibitory activity against viruses including SARS-CoV-2 has not been
investigated yet. In this study, we discovered that Lancemaside A (LA),
a triterpenoid saponin isolated from CL, effectively inhibits the
infection of SARS-CoV-2 and its variants including Alpha, Beta, Delta,
and Omicron. We further found that LA affects the distribution of
membrane cholesterol and blocks membrane fusion between SARS-CoV-2 and
host cells to inhibit SARS-CoV-2 infection. These findings provide the
first evidence that LA displays potent antiviral activity, particularly
against SARS-CoV-2 and suggest the potential use of LA as a natural
pan-coronavirus fusion inhibitor.
Methods