Notch signaling increases infection rate of SARS-CoV-2. After ligand binding and subsequent proteolysis by ADAM10/ADAM17, the liberated Notch intracellular domain (NICD) translocates into the nucleus to form transcriptional complex, which induces expression of genes including furin and miRNA-145. [1] In other words, viral activation of Notch upregulates furin and makes furin cleavage site even more efficient. Moreover, furin also cleaves immature Notch protein to mature herodimeric form for cell surface expression. [1] Notch signaling and furin form positive feedback loop.
In addition, miRNA145 inhibits ADAM17 expression level. ADAM17 mediates proteolytic cleavage and ectodomain shedding ACE2, and soluble ACE2 serves as decoy blocking viral spike protein. Activation of Notch signaling inhibits this innate antiviral mechanism. [1]
Hyperinflammation forms self-sustaining positive feedback loop. NOTCH1 can bind IL-6 promoter directly to activate its transcription, and IL-6 in turn triggers expression of NOTCH ligands such as DLL1 and DLL4 (Delta-like ligands). [1] TNF-α also upregulates expression of NOTCH1 and NOTCH4. [1]
Besides the harmful effects, Notch signaling is beneficial by aiding adaptive immune protection, immune memory generation, and antibody production. [1] Notch signaling is also helpful for tissue regeneration in the lungs after viral infection. Activation of Notch enhances proliferation of basal cells necessary for healing and regeneration of other lung cell types. [1] However, excessive Notch activation perturbs balance of cell differentiation and promotes formation of fibrosis and honeycomb-like cysts. [1]
Multiple viral factors may be involved in triggering Notch signaling. First of all, SARS-CoV-2 spike protein is known to trigger hyperinflammation by interaction with Toll-like receptors (especially TLR4). [2] TLR4 typically recognizes hydrophobic molecules such as bacterial lipopolysaccharide (LPS). Viral spike protein binding to host TLR4 produces sepsis-like inflammation, including production of IL-6 and TNF-α. This phenomenon may kick start the positive feedback loop between IL-6 and Notch.
Wang et al. found Porcine epidemic diarrhea virus (PEDV) activates Notch signaling (upregulation of mRNA levels of JAG-1, DLL4, Notch-1) primarily through ORF3. [3] ORF3 is a transmembrane ion channel that plays functions in viral replication and virulence. Perhaps ORF3 induced endoplasmic reticulum stress and subsequent Notch activation are a common feature among various coronaviruses. SARS-CoV-2 also contains ORF3a. Of note, some studies suggest SARS-CoV-2 ORF3a is a calcium ion channel, whereas SARS-CoV-1 homolog is a selective potassium channel. [4] SARS-CoV-2 ORF3a may be a more enhanced virulence factor that induces ER stress by allowing calcium ions to leak out.
Breikaa et al. suggest that 5’UTR of SARS-CoV-2 genome may interact with proteins associated with Notch2 receptor signaling. [5]
ORF7a and ORF8 are additional virulence factors that trigger hyperinflammation. They may enhance Notch signaling and positive feedback loop by upregulating IL-6. ORF8 mimics IL17a. [6]
Hypoxia from viral infection activates Notch. [5] Notch signaling also exacerbates SARS-CoV-2-associated blood coagulation. [5]
Notch activation is quite common among several different viruses such as Epstein-barr virus, influenza virus, RSV, HPV, Human T-cell leukemia virus type 1 (HTLV-1), Hepatitis C virus (HCV), HBV, HIV. [5]
Anyway, the conclusion is that Notch activation can be engineered artificially. Some mutations in the spike protein may enhance binding with TLR4. Given that the Wuhan lab and other labs have collected thousands of viral samples, they could have selected for high-virulence forms of ORF3 (high-efficiency calcium ion channel). Other virulence factors like ORF7a and ORF8 can be deliberately selected or engineered.
References:
1. Baindara P, Sarker MB, Earhart AP, Mandal SM, Schrum AG. 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. Front Cell Infect Microbiol. 2022 Aug 4;12:928704. doi: 10.3389/fcimb.2022.928704. PMID: 35992174; PMCID: PMC9386183.
pubmed.ncbi.nlm.nih.gov/3599…
2. Sahanic S, Hilbe R, Dünser C, Tymoszuk P, Löffler-Ragg J, Rieder D, Trajanoski Z, Krogsdam A, Demetz E, Yurchenko M, Fischer C, Schirmer M, Theurl M, Lener D, Hirsch J, Holfeld J, Gollmann-Tepeköylü C, Zinner CP, Tzankov A, Zhang SY, Casanova JL, Posch W, Wilflingseder D, Weiss G, Tancevski I. SARS-CoV-2 activates the TLR4/MyD88 pathway in human macrophages: A possible correlation with strong pro-inflammatory responses in severe COVID-19. Heliyon. 2023 Nov 17;9(11):e21893. doi: 10.1016/j.heliyon.2023.e21893. PMID: 38034686; PMCID: PMC10686889.
pubmed.ncbi.nlm.nih.gov/3803…
3. Wang Y, Yang S, Zhao Y, Tian S, Cao Q, Geng X, Yang M, Song X, Shang H, Liu S, Guo R, Li Y, Sun M, Hu M, Fan B, Li B. PEDV infection downregulates goblet cell differentiation through activating the Notch pathway. Vet Res. 2025 Aug 12;56(1):168. doi: 10.1186/s13567-025-01599-5. PMID: 40797223; PMCID: PMC12341102.
pubmed.ncbi.nlm.nih.gov/4079…
4. Zhang J, Ejikemeuwa A, Gerzanich V, Nasr M, Tang Q, Simard JM, Zhao RY. Understanding the Role of SARS-CoV-2 ORF3a in Viral Pathogenesis and COVID-19. Front Microbiol. 2022 Mar 9;13:854567. doi: 10.3389/fmicb.2022.854567. PMID: 35356515; PMCID: PMC8959714.
pubmed.ncbi.nlm.nih.gov/3535…
5. Breikaa RM, Lilly B. The Notch Pathway: A Link Between COVID-19 Pathophysiology and Its Cardiovascular Complications. Front Cardiovasc Med. 2021 May 26;8:681948. doi: 10.3389/fcvm.2021.681948. PMID: 34124207; PMCID: PMC8187573.
pubmed.ncbi.nlm.nih.gov/3412…
6. Wu X, Xia T, Shin WJ, Yu KM, Jung W, Herrmann A, Foo SS, Chen W, Zhang P, Lee JS, Poo H, Comhair SAA, Jehi L, Choi YK, Ensser A, Jung JU. Viral Mimicry of Interleukin-17A by SARS-CoV-2 ORF8. mBio. 2022 Apr 26;13(2):e0040222. doi: 10.1128/mbio.00402-22. Epub 2022 Mar 28. PMID: 35343786; PMCID: PMC9040823.
pubmed.ncbi.nlm.nih.gov/3534…