Going viral in the islet: mediators of SARS-CoV-2 entry beyond ACE2

Abstract

Coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Following initial infection of airway epithelia, SARS-CoV-2 invades a wide range of cells in multiple organs, including pancreatic islet cells. Diabetes is now recognised as a risk factor for severe COVID-19 outcomes, including hospitalisation and death. Additionally, COVID-19 is associated with a higher risk of new-onset diabetes and metabolic complications of diabetes. One mechanism by which these deleterious outcomes may occur is via the destruction of insulin-producing islet β cells, either directly by SARS-CoV-2 entry into β cells or indirectly due to inflammation and fibrosis in the surrounding microenvironment. While the canonical pathway of viral entry via angiotensin-converting enzyme 2 (ACE2) has been established as a major route of SARS-CoV-2 infection in the lung, it may not be solely responsible for viral entry into the endocrine pancreas. This is likely due to the divergent expression of viral entry factors among different tissues. For example, expression of ACE2 has not been unequivocally demonstrated in β cells. Thus, it is important to understand how other proteins known to be highly expressed in pancreatic endocrine cells may be involved in SARS-CoV-2 entry, with the view that these could be targeted to prevent the demise of the β cell in COVID-19. To that end, this review discusses alternate receptors of SARS-CoV-2 (CD147 and GRP78), as well as mediators (furin, TMPRSS2, cathepsin L, ADAM17, neuropilin-1, and heparan sulphate) that may facilitate SARS-CoV-2 entry into pancreatic islets independent of or in conjunction with ACE2.

Keywords: ACE2; ADAM17; CD147; COVID-19; GRP78; NRP1; SARS-CoV-2; TMPRSS2; cathepsin L; diabetes; furin; heparan sulfate; islet.

Figure 1:

ACE2-mediated SARS-CoV-2 viral entry via the membrane fusion pathway. SARS-CoV-2-spike binds to ACE2 on the target cell surface (step A), after which furin cleaves SARS-CoV-2-spike at the S1/S2 boundary (step B). Furin may also cleave the spike protein at the S1/S2 boundary during viral production, prior to virus release into the extracellular space. TMPRSS2 cleaves the S2 protein at the S2’ site (step C), which allows the insertion of hydrophobic amino acid residues in the activated S2 subunit into the plasma membrane (step D), facilitating membrane fusion between the viral envelope and the host cell plasma membrane (step E).

Figure 2:

Potential involvement of SARS-CoV-2 entry factors in islet endocrine cells under conditions of high or low ACE2 expression. ACE2 serves as the predominant SARS-CoV-2 receptor when its levels are sufficiently high for viral entry, and/or its expression is induced by pro-inflammatory cytokines. Under these circumstances, cofactors/co-receptors assist ACE2, and co-receptors further allow viral entry. If ACE2 levels are too low to permit SARS-CoV-2 entry, alternative receptors facilitate viral entry, with assistance from cofactors/co-receptors.

Figure 3:

Proposed roles for alternative receptors and mediators of viral entry into β cells based on studies in other cell types. (A) The mature virion binds to its host cell receptor, which may be CD147 (left), GRP78 (centre) or ACE2 (right). If the host cell receptor is ACE2, SARS-CoV-2-spike-ACE2 binding may be facilitated by GRP78 or HS/HSPG. ADAM17 may be involved in ACE2-mediated SARS-CoV-2 entry in several ways. On one hand, ADAM17 cleaves csACE2, releasing sACE2, which can impede SARS-CoV-2-receptor binding. On the other hand, ADAM17 can cleave and activate cytokines for release, which may upregulate viral entry mediators. On the whole, the net impact of ADAM17 on viral entry is unclear. (B) Before membrane fusion, the spike protein must be processed. After S1/S2 cleavage by furin (which can also occur during viral production), NRP1 can stabilise the SARS-Cov-2-S1-CendR motif to increase the rate of viral spike processing. S2’ cleavage may occur by another host protease in the absence of TMPRSS2. (C) Alternatively, rather than viral envelope-host cell membrane fusion, the SARS-CoV-2-receptor complex can be taken up via receptor-mediated endocytosis. CTSL in the endosome cleaves SARS-CoV-2-spike at a site distinct from the S1/S2 boundary and S2’ site. Viral entry is complete when the viral envelope fuses with the host cell surface membrane or endosomal membrane.

Figure 4:

Hypothesized interaction of ACE2, ADAM17, CD147 and GRP78 under conditions of elevated pro-inflammatory cytokines and ER stress. In patients with COVID-19, pancreatic endocrine cells are bathed in cytokines derived from the circulation. This state (A) upregulates ACE2 gene expression and (B) ER stress, which causes (C) an increase in ADAM17 mRNA levels. (D) ER stress also increases GRP78 mRNA levels, mediated by CD147. The subsequent increase in GRP78 protein results in (E) missorting and translocation of GRP78 to the cell surface, where it may (F) act as an alternative receptor or ACE2 cofactor during SARS-CoV-2 entry. (G) GRP78 is also involved in ACE2 trafficking to the plasma membrane. Additionally, GRP78 (H) protects ADAM17 from inhibition, which allows ADAM17 to (I) cleave plasma membrane bound ACE2 at an increased rate, thereby elevating soluble ACE2 levels, which may (J) bind to the spike protein and inhibit SARS-CoV-2 entry. At the same time, (K) ER stress upregulates CD147. The consequent increase in CD147 at the cell surface can (L) increase CD147-mediated SARS-CoV-2 entry.