CTSC

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Introduction

Cathepsin C (CTSC, also known as CatC) is a lysosomal cysteine dipeptidyl aminopeptidase with a highly conserved tetrameric structure. The best physiological function of CTSC is to activate pro-inflammatory granule-associated serine proteases. These proteases are synthesized as inactive zymogens containing an N-terminal pro-dipeptide, its role is to maintain the zymogen in an inactive conformation and prevent premature activation, because premature activation is potentially toxic to the cells. The activation of the serine protease zymogen occurs through the cleavage of the N-terminal dipeptide by CTSC during cell maturation. The pathological deficiency in CTSC is related to Papillon-Lefèvre syndrome. However, despite the absence of active serine proteases in immune defense cells, the patient did not present significant immunodeficiency. Therefore, transitory pharmacological blockade of CTSC activity in bone marrow precursor cells may represent a potential therapeutic strategy to modulate serine protease activity under inflammatory and immunological conditions. This has prompted a large number of pharmaceutical companies and academic investigators to design and develop various CTSC inhibitors, some of which are currently being used and evaluated in preclinical/clinical trials.

CTSC biosynthesis, processing, and maturation

In mammals, CTSC is mainly expressed in lung, spleen, kidney, and myeloid cell lineages, especially in neutrophils, monocytes, mast cells, macrophages and their precursors. CTSC is initially synthesized as a 55-kDa single-chain monomeric zymogen, which contains the "exclusion" domain (Asp1-Gly119), a propeptide (Thr120-His206), a heavy chain (Leu207-Arg370) and a light chain (Asp371-Leu439), where the heavy and light chains form a papain-like structure.

After biosynthesis, the 87-residue propeptide acts as an intramolecular chaperone, responsible for folding pro-CTSC into dimers. Compared with other cysteine cathepsins, pro-CTSC does not form a mature structure through automatic processing. Studies have reported that human pro-CTSC expressed in baculovirus-transfected insect cells can be activated by CatL and S in vitro by removing the propeptide to initiate protein activation. These proteases process pro-CTSC in two steps in vitro. The first step results in the release of the exclusion domain and two peptides of 36 kDa and 33 kDa. The second step is to release the heavy chain from each peptide. High-resolution X-ray diffraction analysis of human CTSC shows that the light chain, the heavy chain, and the exclusion domain are bound together through non-covalent interactions. However, current studies have shown that pro-CTSC activation in mice does not require CatL and CatS to participate.

Figure 1. 3D structure of processed CTSC monomer (Korkmaz, B.; et al. 2018)

Biological functions of CTSC

• Maturation of neutrophil serine proteases (NSPs)

In the early stages of neutrophil maturation, NSPs are synthesized into inactive zymogens. This structure can prevent premature proteolysis in the endoplasmic reticulum which could cause cytotoxicity. CTSC's activation of NSP occurs during intracellular transport and packaging in acidic primary granules and involves removal of the N-terminal dipeptide to make the active site accessible to the substrate. Studies have reported that all NSP activities in neutrophil lysates from CTSC-deficient mice are severely reduced, which reveals that CTSC plays an important leading role in NSP activation.

• Maturation of mast cell serine proteases

The principal proteins secreted by mast cells secretory granules are serine proteases, which are released outside the cell by binding to the allergen's IgE and cationic peptides, etc. Some of these proteases have been inhibited by targeted therapies. The abundant presence of proteases is a feature of mast cells, which can lead to chronic allergic diseases driven by IgE, including asthma, rhinitis, urticaria, mastocytosis, and mast cell activation disorders. Mast cells also promote mammalian host defenses in a lesser-known way. Several mast cell proteases are fully or partially activated by CTSC from inactive zymogens. Therefore, CTSC blockade may change the function of mast cells.

Figure 2. Activation of pro-chymase by CTSC (Korkmaz, B.; et al. 2018)