The importance of the endosomal pathway for SARS-CoV-2 entry

In a recent study posted to the bioRxiv* preprint server, researchers demonstrated that combined inhibition of cell-surface-based and endosomal pathways completely blocked severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) entry into mammalian cells.

Study: Dual inhibition of vacuolar ATPase and TMPRSS2 is required for complete blockade of SARS-CoV-2 entry into cells. Image Credit: Kateryna Kon/Shutterstock

Background

SARS-CoV-2 emerged in late 2019 and caused the global coronavirus disease 2019 (COVID-19) pandemic, leading to more than 460 million infections and six million deaths worldwide. An important phase in the SARS-CoV-2's infectious life cycle is the proteolytic activation of its spike (S) protein. This process allows the membrane fusion and entrance of SARS-CoV-2 into the host cell.

Two different classes of host proteases involved in the S protein activation stage are endosomal cathepsins and cell-surface transmembrane protease serine 2 (TMPRSS2). The endosomal cathepsins and TMPRSS2 lead to SARS-CoV-2 cell entry via the endosomal and cell-surface routes. 

Furthermore, TMPRSS2 is only found in some cells, whereas endosomal proteases such as cathepsin L and B are present in all cells. Previous reports showed that inhibiting endosomal proteases with lysosomotropic compounds like hydroxychloroquine or non-specific cathepsin inhibitors like aloxistatin (E64d) fails to stop SARS-CoV-2 entry in cells expressing TMPRSS2, implying that the endosomal pathway of entry is insignificant. Nevertheless, mechanism-based toxicities and poor efficacy of these molecules befuddle the current understanding of the role of the endosomal route for SARS-CoV-2 cell entry.

About the study

In the present study, the researchers assessed a panel of compounds to discover alternative pharmacological agents to expound the function of the endosomal route of host entry in SARS-CoV-2 infection. The evaluated molecules were defined through a high throughput screen and impeded the endosomal potential of hydrogen (pH) or maturation via multiple mechanisms. The inhibitors utilizing three different mechanisms to hinder endosomal pathways were employed in the study: 1) direct vacuolar-type ATPase (V-ATPase) inhibitors, 2) lysosomotropic compounds, and 3) proton shuttle compounds.

Hindrance of endosomal acidification was quantified using LysoTracker Red DND-99. Full-length SARS-CoV-2 Ss were pseudotyped to viral particles derived from the human immunodeficiency virus (HIV). SARS-CoV-2 neutralization was assessed using angiotensin-converting enzyme 2 (ACE2)-expressing HeLa (Henrietta Lacks) cells (HeLa-ACE2).

Results

The results showed that commonly employed non-specific endosomal acidification inhibitors were toxic at pH neutralization doses for SARS-CoV-2, which both complicates the comprehension of the mechanism of SARS-CoV-2 inhibition and foreshadows clinical consequences. The identified cytotoxic endosomal acidification inhibitors were 1) lysosomotropic compounds such as quinacrine, amodiaquine, and chloroquine and 2) compounds able to shuttle protons from endosomes, such as niclosamide.

By contrast, out of the three different classes of endosomal inhibitors examined, macrolide bafilomycin A1 (V-ATPase blocker) only hindered SARS-CoV-2 cell entry without developing cellular toxicity. In both authentic and pseudotyped viruses, bafilomycin A1 reduces SARS-CoV-2 infection by Alpha, Beta, and Omicron variants of concern (VOCs) with and without TMPRSS2.

In contrast, among authentic and pseudotyped cell lines, camostat, a TMPRSS2 blocker, only protected TMPRSS2-expressing cells and did not show activity against the Omicron variant. Furthermore, coupling bafilomycin A1 with camostat resulted in synergistic and complete neutralizing of SARS-CoV-2 entrance into cells expressing TMPRSS2. 

These findings highlight the relevance of the endosomal route for SARS-CoV-2 host entry. It illustrated that targeting endosomal acidification could be a potential tactic for inhibiting several SARS-CoV-2 VOCs. In addition, a complete and synergistic hindrance of SARS-CoV-2 mammalian cell entry was also demonstrated by using camostat (TMPRSS2 inhibitor) and bafilomycin A1 (endosomal inhibitor) together. 

Conclusions

The study findings demonstrate the significance of the endosomal pathway for SARS-CoV-2 entry into host cells. The specificity and safety of bafilomycin A1 in inhibiting endosomal acidification, thereby blocking multiple SARS-CoV-2 VOCs entry into mammalian cells regardless of TMPRSS2 expression, was depicted in the study.

The present findings lay the groundwork for developing effective COVID-19 intervention techniques, including inhibitors of both cell-based and endosomal SARS-CoV-2 host cell entry channels. The findings showing inhibition of endosomal pathways are beneficial in preventing SARS-CoV-2 cell entry contradict the previous reports. The reason for the previous observation might be cytotoxicity mediated by hydroxychloroquine and chloroquine, which was used to determine the role of endosomal pathways in SARS-CoV-2 infection.

The current research indicates a novel mode of protection against SARS-CoV-2 infections that should be investigated further in future studies, in which a cell-surface inhibitor and an endosomal pathway inhibitor are utilized concurrently. The present findings imply that using inhibitors that target host-dependent pathways might be resistant to SARS-CoV-2 VOCs and effectively prevent viral entrance into mammalian cells.

*Important notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
  • Icho, S. et al. (2022) "Dual inhibition of vacuolar ATPase and TMPRSS2 is required for complete blockade of SARS-CoV-2 entry into cells". bioRxiv. doi: 10.1101/2022.03.11.484006. https://www.biorxiv.org/content/10.1101/2022.03.11.484006v1

Posted in: Medical Science News | Medical Research News | Disease/Infection News

Tags: ACE2, Angiotensin, Angiotensin-Converting Enzyme 2, Cell, Chloroquine, Coronavirus, Coronavirus Disease COVID-19, covid-19, Cytotoxicity, Efficacy, Enzyme, High Throughput, HIV, Hydroxychloroquine, Immunodeficiency, Mammalian Cells, Membrane, Omicron, Pandemic, pH, Protein, Research, Respiratory, SARS, SARS-CoV-2, Serine, Severe Acute Respiratory, Severe Acute Respiratory Syndrome, Syndrome, Virus

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Written by

Shanet Susan Alex

Shanet Susan Alex, a medical writer, based in Kerala, India, is a Doctor of Pharmacy graduate from Kerala University of Health Sciences. Her academic background is in clinical pharmacy and research, and she is passionate about medical writing. Shanet has published papers in the International Journal of Medical Science and Current Research (IJMSCR), the International Journal of Pharmacy (IJP), and the International Journal of Medical Science and Applied Research (IJMSAR). Apart from work, she enjoys listening to music and watching movies.

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