NOTCH signaling in COVID-19: a central hub controlling genes, proteins, and cells that mediate SARS-CoV-2 entry, the inflammatory response, and lung regeneration

Abstract

In the lungs of infected individuals, the downstream molecular signaling pathways induced by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) are incompletely understood. Here, we describe and examine predictions of a model in which NOTCH may represent a central signaling axis in lung infection in Coronavirus Disease 2019 (COVID-19). A pathway involving NOTCH signaling, furin, ADAM17, and ACE2 may be capable of increasing SARS-CoV-2 viral entry and infection. NOTCH signaling can also upregulate IL-6 and pro-inflammatory mediators induced to hyperactivation in COVID-19. Furthermore, if NOTCH signaling fails to turn down properly and stays elevated, airway regeneration during lung healing can be inhibited-a process that may be at play in COVID-19. With specific NOTCH inhibitor drugs in development and clinical trials for other diseases being conducted, the roles of NOTCH in all of these processes central to both infection and healing merit contemplation if such drugs might be applied to COVID-19 patients.

Keywords: ACE2; ADAM; COVID-19; Notch; SARS-CoV-2; furin; gamma-secretase inhibitor (GSI).

Figure 1

Proposed NOTCH signaling pathway effects on SARS-CoV-2 infection, modeled from Rizzo et al. (2020). SARS-CoV-2 entry is mediated by the binding of the viral spike (S) glycoprotein to ACE2 and proteolytic cleavage at the S1/S2 site of the S glycoprotein by the host protease furin. ADAM17 regulates ACE2 levels on the plasma membrane by promoting ACE2 shedding. NOTCH signaling is a positive regulator of furin and a negative regulator of ADAM17 transcript, the latter due to induction of miRNA-145. GSIs are NOTCH inhibitors that may represent a possible therapeutic strategy to inhibit viral entry into cells by reducing furin and increasing ADAM17-mediated ACE2 shedding. NOTCH precursor is cleaved by furin in the Golgi apparatus to form a heterodimer of NOTCH extracellular domain (NECD), plus the remaining cleavage product containing within the NOTCH intracellular domain (NICD). This cleavage-induced heterodimer is exported to surface expression. Upon productive ligand engagement, NOTCH is cleaved by ADAM10 or ADAM17 and later cleaved by the gamma-secretase complex. Afterwards, liberated NICD translocates into the nucleus and interacts with the transcription factor CSL and the transcriptional co-activator MAML to regulate the transcription of NOTCH target genes, including furin, and miRNA-145.

Figure 2

NOTCH signaling induces IL-6 in a positive-feedback loop. NOTCH signaling promotes the production of inflammatory cytokines including IL-6. In turn, IL-6 increases the expression of NOTCH ligands (such as Dll1 and Dll4), which can further amplify NOTCH signaling. This NOTCH : IL-6 positive-feedback loop can promote innate immune cell and adaptive immune T cell activation together with further amplified inflammatory cytokine elaboration. When these processes are highly active without sufficient downregulation in a resolution phase, they can cause inflammatory tissue damage and continued cycles of IL-6 and NOTCH upregulation and activation. While monoclonal antibodies (mAbs) that block IL-6 or its receptor represent promising therapeutic modalities, GSIs block NOTCH signaling as candidate drugs in clinical trials for other diseases, and these latter compounds could be contemplated when severe COVID-19 may be unresponsive to other therapies. In sum, it is suggested that targeting the positive feedback loop between NOTCH and IL-6 could be a potential therapeutic strategy that may have application against COVID-19.

Figure 3

Proposed role for NOTCH in lung regeneration in COVID-19: tissue structure in the proximal and distal respiratory tract. A variety of lung epithelial cells exist along the proximal‐distal axis. In proximal regions, there are five major cell populations, which include basal cells (purple), goblet cells (light pink), ciliated cells (green), club cells (yellow), and neuroendocrine cells (red). LNEPs (lineage‐negative epithelial progenitor cells) are shown in light orange color. In alveolar regions, there are mainly two types of epithelial cells, flattened alveolar type I cells (light green) and cuboidal alveolar type II cells (indigo). Septa are shown in dark orange color. Cell loss in the proximal respiratory tract due to injury triggers the expansion of basal cells and their direct segregation into N2ICD (club progenitor, light grey) and c‐myb (ciliated progenitor, light yellow) cells. However, continuous NOTCH activation during COVID-19 along with cytokine storm may result in goblet cell hyperplasia which could exacerbate disease pathogenesis and contribute to ARDS. LNEPs acquire basal cell identity and migrate towards the damaged area of the distal respiratory tract. NOTCH blocks the trans-differentiation of basal cells into alveolar cells and thus NOTCH inhibition is necessary for normal regeneration. However, continuous NOTCH activation may occur in COVID-19 and this could result in abnormal regeneration with basal cell excess and honeycomb‐like structures that may contribute to ARDS and disease severity.

Figure 4

Connecting NOTCH and SARS-CoV-2 in a molecular interaction network. This network is partially generated by STRING v11. All STRING-generated edges are shown in pink (light or dark), while other edges are drawn manually. Gamma secretase complex is represented by different subunits including APH1A, APH1B, PSEN1, PSEN2, PSENEN, and NCSTN. Other proteins included are NOTCH1, FURIN, ADAM17, ADAM10, TMPRSS2, and ACE2. The SARS-CoV2 spike protein is drawn manually. Interaction edges of some proteases (ADAM17, FURIN, TMPRSS2) with NOTCH and ACE2 are indicated by green lines, while binding of SARS-CoV-2 spike protein with its receptor ACE2 is indicated by a red line. The transcriptional (not protein) interaction of NOTCH and IL-6 is indicated by a dashed yellow line.