TMPRSS2 and ADAM17 interactions with ACE2 complexed with SARS-CoV-2 and B0AT1 putatively in intestine, cardiomyocytes, and kidney

Abstract

ABSTRACTCOVID-19 outcomes reflect organ-specific interplay of SARS-CoV-2 and its receptor, ACE2, with TMPRSS2 and ADAM17. Confirmed active tropism of SARS-CoV-2 in epithelial cells of intestine and kidney proximal tubule, and in aging cardiomyocytes, capriciously manifests extra-pulmonary organ-related clinical symptoms in about half of COVID-19 patients, occurring by poorly understood mechanisms. We approached this knowledge gap by recognizing a clue that these three particular cell types share a common denominator kindred of uniquely expressing the SLC6A19 neutral amino acid transporter B0AT1 protein (alternatively called NBB, B, B0) serving glutamine and tryptophan uptake. B0AT1 is a cellular trafficking chaperone partner of ACE2, shown by cryo-EM to form a thermodynamically-favored stabilized 2ACE2:2B0AT1 dimer-of-heterodimers. The gut is the body’s site of greatest magnitude expression depot of both ACE2 and B0AT1. This starkly contrasts with pulmonary pneumocyte expression of monomeric ACE2 with conspicuously undetectable B0AT1. We hypothesized that B0AT1 steers the organ-related interplay amongst ACE2, TMPRSS2, ADAM17, and SARS-CoV-2 RBD. The present study employed molecular docking modeling that indicated active site catalytic pocket residues of TMPRSS2 and ADAM17 each formed bonds ≤ 2 A with monomer ACE2 specific residues within a span R652-D713 involved in cleaving sACE2 soluble ectodomain release. These bonds are consistent with competitive binding interactions of experimental anti-SARS-CoV-2 drug small molecules including Camostat and Nafamostat. Without B0AT1, ACE2 residues K657 and N699 dominated docking bonding with TMPRSS2 or ADAM17 active sites, with ACE2 R710 and R709 contributing electrostatic attractions, but notably ACE2 S708 never closer than 16-44 A. However, in the dimer-of-heterodimers arrangement all ACE2 neck region residues were limited to TMPRSS2 or ADAM17 approaches 35 A, with the interference directly attributed to the presence of a neighboring B0AT1 subunit complexed to the partnering ACE2 subunit of 2ACE2:2B0AT1; ADAM17 failed to dock by bumping its active site pocket oriented dysfunctionally outwardly facing 1800 away. Results were the same whether the dimer-of-heterodimers was in either the “closed” or “open” conformation, or whether or not SARS-CoV-2 RBD was complexed to ACE2. The results implicate B0AT1-and in particular the 2ACE2:2B0AT1 complex-as a maJor player in the landscape of COVID-19 pathophysiology engaging TMPRSS2 and ADAM17, consistent with experimental evidence in the literature and in clinical reports. These findings provide a gateway to understanding the roles of B0AT1 relating to COVID-19 manifestations putatively assigned to intestinal and renal epithelial cells and cardiomyocytes, with underpinnings useful for considerations in public hygiene policy and drug development.