viruses Article The Spike Glycoprotein of SARS-CoV-2 Binds to β1 Integrins Expressed on the Surface of Lung Epithelial Cells Eun Jeong Park 1, * , Phyoe Kyawe Myint 1 , Michael Gyasi Appiah 1 , Samuel Darkwah 1 , Siqingaowa Caidengbate 1 , Atsushi Ito 1,2 , Eri Matsuo 1 , Eiji Kawamoto 1,3 , Arong Gaowa 1 and Motomu Shimaoka 1, *   Citation: Park, E.J.; Myint, P.K.; Appiah, M.G.; Darkwah, S.; Caidengbate, S.; Ito, A.; Matsuo, E.; Kawamoto, E.; Gaowa, A.; Shimaoka, M. The Spike Glycoprotein of SARS-CoV-2 Binds to β1 Integrins Expressed on the Surface of Lung Epithelial Cells. Viruses 2021, 13, 645. https://doi.org/10.3390/v13040645 Academic Editor: Gilda Tachedjian Received: 24 March 2021 Accepted: 7 April 2021 Published: 9 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Molecular Pathobiology and Cell Adhesion Biology, Mie University Graduate School of Medicine, 2-174 Edobashi, Tsu-City 514-8507, Mie, Japan; 317DS09@m.mie-u.ac.jp (P.K.M.); 317DS06@m.mie-u.ac.jp (M.G.A.); kwekuadarkwah@gmail.com (S.D.); 320D011@m.mie-u.ac.jp (S.C.); i-atsushi@clin.medic.mie-u.ac.jp (A.I.); ematsuo@med.mie-u.ac.jp (E.M.); a-2kawamoto@med.mie-u.ac.jp (E.K.); arong-g@doc.medic.mie-u.ac.jp (A.G.) 2 Department of Cardiothoracic Surgery, Mie University Graduate School of Medicine, 2-174 Edobashi, Tsu-City 514-8507, Mie, Japan 3 Department of Emergency and Disaster Medicine, Mie University Graduate School of Medicine, 2-174 Edobashi, Tsu-City 514-8507, Mie, Japan * Correspondence: epark@med.mie-u.ac.jp (E.J.P.); shimaoka@med.mie-u.ac.jp (M.S.); Tel.: +81-59-231-6408 (E.J.P.); +81-59-231-5036 (M.S.) Abstract: The spike glycoprotein attached to the envelope of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) binds to and exploits angiotensin-converting enzyme 2 (ACE2) as an entry receptor to infect pulmonary epithelial cells. A subset of integrins that recognize the arginyl– glycyl–aspartic acid (RGD) sequence in the cognate ligands has been predicted in silico to bind the spike glycoprotein and, thereby, to be exploited for viral infection. Here, we show experimental evidence that the β1 integrins predominantly expressed on human pulmonary epithelial cell lines and primary mouse alveolar epithelial cells bind to this spike protein. The cellular β1 integrins support adhesive interactions with the spike protein independently of ACE2, suggesting the possibility that the β1 integrins may function as an alternative receptor for SARS-CoV-2, which could be targeted for the prevention of viral infections. Keywords: SARS-CoV-2; spike 1 protein; angiotensin-converting enzyme 2; integrin; cell adhesion; alveolar epithelial cells 1. Introduction Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes a potentially fatal acute pulmonary infection, leading to a cytokine storm that culminates in acute respiratory distress syndrome and other COVID-19 pathologies [1,2]. As of April 2021, the SARS-CoV-2 pandemic is reported to globally comprise more than 133 million in- fected cases, including more than 2.8 million deaths [3]. SARS-CoV-2 is a single-strand positive-sense RNA virus that on the surface displays a large spike glycoprotein (S-protein), which plays a critical role in the interaction between the virus and host cells at entry sites. Angiotensin-converting enzyme 2 (ACE2), an ecto-enzyme present on the cell surface, has been identified as a major cellular receptor for the viral spike glycoprotein, one that aids viral entry to host alveolar epithelial cells and other epithelial and endothelial cells [4]. ACE2 is regarded as a promising therapeutic target for blocking such viral entry [4,5]. However, multiple cell-surface receptors other than ACE2 may be simultaneously involved in the process of viral entry [6–9]. Integrins represent the largest family of cell-adhesion receptors. Composed of the α-and β-subunits that bind to ligands on the opposing cells and in the matrix proteins, these integrins mediate adhesive interactions across a wide range of biological processes Viruses 2021, 13, 645. https://doi.org/10.3390/v13040645 https://www.mdpi.com/journal/viruses