Regulated Intramembrane Proteolysis of ACE2: A Potential Mechanism Contributing to COVID-19 Pathogenesis?

Abstract

Since being identified as a key receptor for SARS-CoV-2, Angiotensin converting enzyme 2 (ACE2) has been studied as one of the potential targets for the development of preventative and/or treatment options. Tissue expression of ACE2 and the amino acids interacting with the spike protein of SARS-CoV-2 have been mapped. Furthermore, the recombinant soluble extracellular domain of ACE2 is already in phase 2 trials as a treatment for SARS-CoV-2 infection. Most studies have continued to focus on the ACE2 extracellular domain, which is known to play key roles in the renin angiotensin system and in amino acid uptake. However, few also found ACE2 to have an immune-modulatory function and its intracellular tail may be one of the signaling molecules in regulating cellular activation. The implication of its immune-modulatory role in preventing the cytokine-storm, observed in severe COVID-19 disease outcomes requires further investigation. This review focuses on the regulated proteolytic cleavage of ACE2 upon binding to inducer(s), such as the spike protein of SARS-CoV, the potential of cleaved ACE2 intracellular subdomain in regulating cellular function, and the ACE2's immune-modulatory function. This knowledge is critical for targeting ACE2 levels for developing prophylactic treatment or preventative measures in SARS-CoV infections.

Keywords: ACE2; COVID-19; SARS-CoV-2; ectodomain shedding; endodomain cleavage; regulated intramembrane proteolysis.

Figure 1

Regulated intramembrane proteolysis process and proteins cleaved by ADAM17. Several proteins are targets of ADAM17 cleavage, including proteins involved in immune responses, cell adhesion and cellular development and differentiation, among others. The proteolytic activity of ADAM17 can be triggered through its phosphorylation by PKL2, PKC, and MAPKs. Activated ADAM17 cleaves several transmembrane proteins leading to the release of an extracellular domain (ECD) and the membrane retention of an intracellular cytoplasmic domain (ICD). The γ-secretase multi-protein complex removes this domain from the membrane allowing it to migrate to the cytoplasm where it can either be degraded at the proteasome or interact with other cytoplasmic proteins. Moreover, the ICD can trans-locate into the nuclei to induce transcriptional expression of several genes related to cell activation, signaling, death, proliferation and differentiation (created with BioRender.com).

Figure 2

Hypothetical model of ACE2 shedding by ADAM17 and γ-secretase after ACE2 interaction with SARS-CoV-2 and its effects on the immune response against the infection. 1). Initial interaction between SARS-CoV-2 spike protein and the transmembrane cellular receptor ACE2 may trigger proteolytic cleavage of ACE2 mediated by ADAM17, releasing the extracellular domain of this receptor (ectodomain). Other proteases such as TMPRSS2, HAT and Hepsin may also cleave ACE2 at a different catalytic site than ADAM17, thus potentially exhibiting differential outcomes. 2) Soluble ACE2 can interact with other cells through integrins acting like a cell adhesion molecule and triggering signals associated with cell proliferation and survival. 3) The cytoplasmic domain is likely released from the cellular membrane by γ-secretase, as described for other RIP proteins and it is either: 4) degraded at the proteasome, or 5) interacts with cytoplasmic proteins as it has been described for calmodulin (CAM) that can have effects on cellular activation, inflammation and cells death. 6) If CAM interacts with the transmembrane full-length ACE2, the shedding of ACE2 may be abrogated. 7) The intracellularly released ACE2 domain may also be translocated into the nuclei. At the nuclei, the endodomain may induce transcriptional expression of different genes by a still unknown mechanism that can include induction of transcription factors or formation of transcription factors-like complexes or chromatin modifications, influencing the cellular response and fate. In addition, the increased levels of IL-1β and TNFα induced during COVID-19 may increase ADAM17 phosphorylation and activity favoring a sustained shedding of ACE2. Thus, the shedding of ACE2 induced by SARS-CoV-2 infection is likely influencing the outcome of COVID-19 by mechanisms that are still unknown and that remain to be explored in order to design preventive and therapeutic approaches (created with BioRender.com).