SARS-CoV-2 may hijack GPCR signaling pathways to dysregulate lung ion and fluid transport

Abstract

The tropism of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a virus responsible for the ongoing coronavirus disease 2019 (COVID-19) pandemic, toward the host cells is determined, at least in part, by the expression and distribution of its cell surface receptor, angiotensin-converting enzyme 2 (ACE2). The virus further exploits the host cellular machinery to gain access into the cells; its spike protein is cleaved by a host cell surface transmembrane serine protease 2 (TMPRSS2) shortly after binding ACE2, followed by its proteolytic activation at a furin cleavage site. The virus primarily targets the epithelium of the respiratory tract, which is covered by a tightly regulated airway surface liquid (ASL) layer that serves as a primary defense mechanism against respiratory pathogens. The volume and viscosity of this fluid layer is regulated and maintained by a coordinated function of different transport pathways in the respiratory epithelium. We argue that SARS-CoV-2 may potentially alter evolutionary conserved second-messenger signaling cascades via activation of G protein-coupled receptors (GPCRs) or by directly modulating G protein signaling. Such signaling may in turn adversely modulate transepithelial transport processes, especially those involving cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial Na⁺ channel (ENaC), thereby shifting the delicate balance between anion secretion and sodium absorption, which controls homeostasis of this fluid layer. As a result, activation of the secretory pathways including CFTR-mediated Cl^- transport may overwhelm the absorptive pathways, such as ENaC-dependent Na⁺ uptake, and initiate a pathophysiological cascade leading to lung edema, one of the most serious and potentially deadly clinical manifestations of COVID-19.

Keywords: CFTR; COVID-19; ENaC; GPCR; SARS-CoV-2.

Figure 1.

Hypothetical concept for the dysregulation of lung epithelial ion and fluid transport by SARS-CoV-2. SARS-CoV-2 binds to ACE2, is endocytosed into the cell, and uses the host cellular machinery for RNA replication and translation resulting in the production of viral proteins. These viral proteins may potentially interact with G proteins, triggering the classic GPCR pathway and activating CFTR. Activation of CFTR in turn can inhibit ENaC. Secondary mediators may in parallel or alternatively activate GPCRs from the outside (45, 46). For example, ATP may serve as secondary mediator and engage A1AR [(green arrow) which in turn stimulates CFTR; activation of CFTR in turn can inhibit ENaC]. UTP can engage P2Y6R (blue arrow) and inhibit amiloride sensitive ion transport via ENaC. In addition, UTP can lead to purinergic-mediated calcium mobilization from intracellular stores, which may combine with the influx of calcium via TRPV4 (acting as a mechano- and/or chemosensor) to increase intracellular calcium levels that can in turn activate CFTR; this activation of CFTR can cause chloride and fluid secretion, and inhibition of sodium reabsorption via ENaC (ENaC is not depicted in the left side of the figure). The potential link between TRPV4 and CFTR (47) could involve a Ca²⁺-dependent activation of adenylyl cyclase and the classical PKA pathway, whereas tyrosine phosphorylation can cause PKA-independent CFTR activation (–49). NHERF1 anchors CFTR (via ezrin, not shown) to the actin cytoskeleton. Increased levels of cAMP allow PKA and EPAC1 to promote opening and stabilization of CFTR at the plasma membrane (50), again causing chloride and fluid secretion. ACE2, angiotensin converting enzyme 2; cAMP, cyclic AMP; CFTR, cystic fibrosis transmembrane conductance regulator; ENaC, epithelial Na⁺ channel; EPAC1, exchange protein directly activated by cAMP; ER, endoplasmic reticulum; GPCR, G protein coupled-receptor; NHERF1, Na⁺/H⁺-exchanger regulatory factor isoform 1; PDZ, postsynaptic density-95 discs large, zona occludens-1 (PDZ) domain-containing protein; PKA, protein kinase A; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; TRPV4, transient receptor potential vanilloid 4.