Cathespin L is a member of the cathepsin family of proteases.  It is expressed throughout the body, and it is known to be involved in various pathologies, including:

- liver fibrosis
- type I and II diabetes
- cardiac and bone diseases
- immune disorders
- kidney disorders

Michigan State University

Several cellular proteins have been reported to be involved in the early steps of beta coronavirus infections in humans.  ACE2, a membrane bound protein that is involved in the cellular entry of many beta coronaviruses, is often supported by the function of other proteins.  Some facilitate the shedding of ACE2 from the cell surface, and others cleave viral S proteins to enhance the fusion of the two membranes.  They include:

- ACE2
- ADAM10
- ADAM17
- TMPRSS2
- TMPRSS11A
- TMPRSS11D
- furin
- CTSL
- PI4KB

Cellular Proteins Related to Beta Coronavirus Infections

Cysteine cathespin proteases are a family of proteases that primarily function as endopeptidases.  They are notably found in lysosomes where they work to process and degrade biological molecules.  Their function is relevant to many physiological processes, including:

- degrading proteins in the lysosomes
- cell death (inclduing autophagy)
- the post-translational modifications of proteins
- extracellular matrix (ECM) remodeling
- immune signaling

Members of this family include Cathepsin L (L1), B, C, F, H, K, O, V (L2), X, S, and W.  

Cysteine Cathespin Proteases

Dana, D., & Pathak, S. K. (2020). A review of small molecule inhibitors and functional probes of human Cathepsin L. Molecules, 25(3), 698.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7038230/
doi: 10.3390/molecules25030698

A Review of Small Molecule Inhibitors and Functional Probes of Human Cathepsin L

ACE2 is a homologue of ACE.  Both enzymes are carboxypeptidases, meaning they cleave amino acids from the c-terminal of the peptide.  However, these homologues cleave the amino acid sequences at different points which leads to their different products.  ACE2 has four main physiological roles:

- Regulating blood pressure via its role in the renin-angiotensin-aldosterone system (RAAS)

- Limiting the amount of ACE substrate by converting Ang I into Ang 1-9

- Degrading Ang II in order to minimize its harmful effects

- Forming Ang 1-7 in order to maximize its cardioprotective effects

Angiotensin-converting enzyme 2 (ACE2)

Transmembrane protease serine 2 (TMPRSS2) is a serine protease.  Its gene is located on chromosome 21 at 21q22.3.  Androgen signaling controls its expression upstream of the gene's transcriptional start site.  

TMPRSS2

A serine protease that has a variety of physiological and pathophysiological substrates.  Dysregulation of furin has been shown to lead to the promotion of infectious diseases and certain types of cancer.  Furin can also be called paired basic amino acid cleaving enzyme (PACE).

Furin

18,263 small airway epithelia cells from nonsmokers (NS) and 16,678 cells from smokers (S) were analyzed using single-cell 10x transcriptome.  Differences in gene expression between smokers/nonsmokers as well as between cell type were all less than 10%, and there were no significant results.  Notably, there was no change in ACE2 expression between smokers and nonsmokers.  This may reflect a limitation of using single-cell RNA sequencing compared to bulk RNA sequencing as the results depend on the size and shape of the cell.  However, these results are unique in that they elucidate the gene expression of individual cell types from a mixed population of cells.  

Expression of Cellular Proteins Related to Beta Coronavirus Infections in the Small Airway Epithelium (SAE) by Cell Type

mRNA expression of proteins related to the early steps in a B-coronavirus infection, excluding AC2, were determined in the small airway epithelium (SAE) of smokers (gray circles) and nonsmokers (white circles.)  Expression levels were quantified using RNA sequencing (Illumina HiSeq 2500), and the data values are reported in FPKM (fragments per kilobase of exon per million fragments sequenced.)  Only TMPRSS2 expression was modulated by smoking: its expression was significantly increased (p < 0.01) in smokers compared to nonsmokers.  

Expression of Cellular Proteins Related to Beta Coronavirus Infections in the Small Airway Epithelium (SAE)

ADAM10 belongs to the A Disintegrin And Metalloproteinases (ADAMs) family of proteases.  ADAM10 facilitated ectodomain shedding has been shown to influence the ability of cell adhision molecules and regulate paracrine, autocrine, and juxtacrine signaling.  

ADAM10

ADAM17 belongs to the A Disintegrin And Metalloproteinases (ADAMs) family of proteases.  It regulates various cell processes through its wide range of substrates, including TNFR1, TNFR2, and IL-6R (interleukin-6 receptor.)  ADAM17 notably is involved in the activation of the epidermal growth factor receptor (EGFR) in development through the shedding of TGFα, amphiregulin, HB-EGF, and epiregulin in the presence of phorbol 12-myristate 13-acetate (PMA).  

ADAM17

TMPRSS11A is a member of the HAT/DESC subfamily of TTSPs.  It is notably expressed in the respiratory system as well as on cell surfaces.  Additionally, it has been shown to facility the entry of the virus SARS-CoV-1 into human cells.  

TMRPSS11A

HAT, or TMPRS11D, is a member of the HAT/DESC subfamily of TTSPs.  It was initially discovered in the sputum of chronic respiratory disease patients, and it is thought to have a variety of physiological roles in epithelial tissue.  Proposed functions of this protease include:

- suppressing coagulation through fibrinogenolysis
- activating the protease activated receptor (PAR)-2
- influencing cell migration and adhesion via urokinase plasminogen activator cleavage

TMPRSS11D has been shown to activate the S protein of SARS-CoV-1 by cleaving it into its S1 and S2 subunits  This facilitates endosomal entry of the virus into a host human cell.  

HAT (TMPRSS11D)

Cathespin L (CTSL)

PI4KB is a PI4K that forms phosphatidylinositol 4-phosphate (PI4P).  It is primarily located in the Golgi and trans-Golgi network, and consequently, it is involved in cellular processes including lipid transport, membrane trafficking, cytokinesis, and maintaining lysosomal identity.  PI4KB and its isoform PI4KA are common viral targets because they can be used to create PI4P rich organelles that are important to viral reproduction. 

PI4KB

BCi-NS1.1 cells were derived from a single basal cell from the large airway epithelium (LAE) of a healthy nonsmoker, and hSABCi-NS1.1 cells were derived from a single basal cell from the small airway epithelium (SAE) of a healthy nonsmoker.  Expression levels were determined using HiSeq 4000 RNA-seq data at day 0 (baseline) and after 28 days on an air-liquid interface (ALI).  Notable findings include:

- Both cell lines showed an increase in ACE2 expression after 28 days of differentiation
- ADAM10, ADAM17, FURIN, CTSL, and PI4KB expression was relatively maintained throughout differentiation in both cell lines
- TMPRSS2 was notably less expressed in undifferentiated LAE basal cells than in SAE basal cells, and their expression increased.  However, its expression was maintained in SAE basal cells.  
- TPRSS11A and TMPRSS11D expression remained low in both LAE and SAE cells throughout differentiation

Because these cell lines are immortal, they may continue to be useful in SARS-CoV-2 research - especially for early drug screening.  

ACE2 and Other Cellular Proteins Related to B-Coronavirus Infection Expression Increases in Basal Cells from the Large Airway Epithelium Throughout Differentiation

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