Cathepsin S is a protein that in humans is encoded by the CTSS gene. [5] Transcript variants utilizing alternative polyadenylation signals exist for this gene. [5]
Cathepsin S is a member of the peptidase C1 family of cysteine cathepsins, a lysosomal cysteine protease that may participate in the degradation of antigenic proteins to peptides for presentation to the MHC class II. Cathepsin S can function as an elastase over a broad pH range in alveolar macrophages.
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Cathepsin S is a lysosomal enzyme that belongs to the papain-like protease of cysteine proteases. While a role in antigen presentation has long been recognized, it is now understood that cathepsin S has a role in itch and pain, or nociception . The nociceptive activity results from cathepsin S functioning as a signaling molecule via activation of protease-activated receptors 2 and 4 members of the G-protein coupled receptor family. [6]
Cathepsin S is expressed by antigen presenting cells including macrophages, B-lymphocytes, dendritic cells and microglia. Cathepsin S is expressed by some epithelial cells. Its expression is markedly increased in human keratinocytes following stimulation with interferon-gamma and its expression is elevated in psoriatic keratinocytes due to stimulation by proinflammatory factors. In contrast, cortical thymic epithelial cells do not express cathepsin S.
While pH optima of many lysosomal proteases are acidic, cathepsin S is an exception. This enzyme remains catalytically active under the neutral pH and has pH optimum between the pH values 6.0 and 7.5. Many lysosomal proteases are trapped inside the lysosome due to a problem with their stability. In contrast, cathepsin S remains stable and has a physiological role outside the lysosome. Immune cells, including macrophages and microglia, secrete cathepsin S in response to inflammatory mediators including lipopolysaccharides, proinflammatory cytokines and neutrophils. In vitro, cathepsin S retains some enzyme activity in the presence of 3M urea. Cathepsin S is produced as a zymogen and is activated by processing.
The activity of cathepsin S is tightly regulated by its endogenous inhibitor, cystatin C, which also has a role in antigen presentation. Cystatin A and B have a lower activity compared to cystatin C.
The active cleavage sites -(-Val-Val-Arg-)- of cathepsin S are supposed to have at least two amino acids surrounding it from each side.
While lysosomal proteases terminally degrade proteins in lysosomes, cathepsin S has own distinctive physiological role.
This enzyme has a critical role in antigen presentation. Major histocompatibility complex class II molecules interact with small peptide fragments for presentation on the surface of antigen-presenting immune cells. Cathepsin S participates in the degradation of the invariant or Ii chain that prevents loading the antigen into the complex. This degradation occurs in the lysosome. Chronologically, action of cathepsin S follows two cleavages performed by aspartyl proteases. Cathepsin S cleaves the remaining fragment of Ii (IiP1) and leaves a small part of Ii known as CLIP which stays directly associated with the complex.
Proteolytic degradation of Ii is important since it facilitates dissociation of CLIP from MHC II and then, complex can load the selected antigen. After loading the antigen, MHC II molecule moves to the cell surface. Thus, we can speculate that overexpression of cathepsin S may lead to premature degradation of Ii, occasional loading of MHC II and an autoimmune attack. Contrary, inhibition of cathepsin S will lead to a delay in degradation of Ii and loading the antigen into MHC II as well as inappropriate presence of uncleaved Li-fragments in MHC II on the cell surface. It will impair and weaken the immune response. For instance, this kind of MHC II will not be very efficient to induce proliferation of T-cells.
In macrophages, cathepsin S can be replaced by cathepsin F.
Secreted cathepsin S cleaves some extracellular matrix (ECM) proteins. Cathepsin S may be considered the most potent elastase known. The list of proposed cathepsin S substrates includes laminin, fibronectin elastin, osteocalcin and some collagens. It also cleaves chondroitin sulfate, heparan sulfate and proteoglycans of the basal membrane. Cathepsin S plays an active role in blood vessels permeability and angiogenesis due to its elastolytic and collagenolytic activities. For instance, cleavage of laminin-5 by cathepsin S leads to generation of proangiogenic peptides. The expression of cathepsin S can be triggered by proinflammatory factors secreted by tumor cells.
In tumorogenesis, cathepsin S promotes a tumor growth.
Cathepsin S expression and activity has also been shown to be upregulated in the skin of psoriasis patients. Whether it has a definitive role in causing the pathology of psoriasis is as yet unknown, however in the same study it was shown to specially cleave and activate the psoriasis-associated proinflammatory cytokine IL-36γ [7]
Cathepsin S has a role in nociception, including itch and gastrointestinal pain. The mechanism by which cathepsin S leads to itch and pain is consistent with the capacity of this cysteine protease to activate protease-activated receptors 2 and 4. [8] [6]
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Synthetic inhibitors of cathepsin S participated in numerous preclinical studies for the immune disorders including rheumatoid arthritis. Currently, at least one of them participates in a clinical trial for psoriasis. LHVS (morpholinurea-leucine-homophenylalanine-vinylsulfone-phenyl) is the most extensively studied synthetic inhibitor of cathepsin S. IC50 of LHVS is about 5 nM. Inhibition of cathepsin S by LHVS has shown to be neuroprotective after traumatic brain injury. [9] The list of commercial inhibitors also includes paecilopeptin (acetyl-Leu-Val-CHO) and some others.
Cathepsin S has been shown to be a significant prognostic factor for patients with type IV astrocytomas (glioblastoma multiforme), and its inhibition has shown improvement in survival time by mean average 5 months. This is because the cysteine enzyme can no longer act together with other proteases to break up the brain extracellular matrix. So the spread of the tumor is halted. Scientists have just announced that this enzyme predicts death, as it has been shown to be associated with both heart disease and cancer. (citation?)
Cathepsins are proteases found in all animals as well as other organisms. There are approximately a dozen members of this family, which are distinguished by their structure, catalytic mechanism, and which proteins they cleave. Most of the members become activated at the low pH found in lysosomes. Thus, the activity of this family lies almost entirely within those organelles. There are, however, exceptions such as cathepsin K, which works extracellularly after secretion by osteoclasts in bone resorption. Cathepsins have a vital role in mammalian cellular turnover.
Cysteine proteases, also known as thiol proteases, are hydrolase enzymes that degrade proteins. These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad.
The cystatins are a family of cysteine protease inhibitors which share a sequence homology and a common tertiary structure of an alpha helix lying on top of an anti-parallel beta sheet. The family is subdivided as described below.
Antigen presentation is a vital immune process that is essential for T cell immune response triggering. Because T cells recognize only fragmented antigens displayed on cell surfaces, antigen processing must occur before the antigen fragment can be recognized by a T-cell receptor. Specifically, the fragment, bound to the major histocompatibility complex (MHC), is transported to the surface of the cell, a process known as presentation. If there has been an infection with viruses or bacteria, the cell will present an endogenous or exogenous peptide fragment derived from the antigen by MHC molecules. There are two types of MHC molecules which differ in the behaviour of the antigens: MHC class I molecules (MHC-I) bind peptides from the cell cytosol, while peptides generated in the endocytic vesicles after internalisation are bound to MHC class II (MHC-II). Cellular membranes separate these two cellular environments - intracellular and extracellular. Each T cell can only recognize tens to hundreds of copies of a unique sequence of a single peptide among thousands of other peptides presented on the same cell, because an MHC molecule in one cell can bind to quite a large range of peptides. Predicting which antigens will be presented to the immune system by a certain MHC/HLA type is difficult, but the technology involved is improving.
Cathepsin G is a protein that in humans is encoded by the CTSG gene. It is one of the three serine proteases of the chymotrypsin family that are stored in the azurophil granules, and also a member of the peptidase S1 protein family. Cathepsin G plays an important role in eliminating intracellular pathogens and breaking down tissues at inflammatory sites, as well as in anti-inflammatory response.
An alveolar macrophage, pulmonary macrophage, is a type of macrophage, a professional phagocyte, found in the airways and at the level of the alveoli in the lungs, but separated from their walls.
Cathepsin B belongs to a family of lysosomal cysteine proteases known as the cysteine cathepsins and plays an important role in intracellular proteolysis. In humans, cathepsin B is encoded by the CTSB gene. Cathepsin B is upregulated in certain cancers, in pre-malignant lesions, and in various other pathological conditions.
Cathepsin D is a protein that in humans is encoded by the CTSD gene. This gene encodes a lysosomal aspartyl protease composed of a protein dimer of disulfide-linked heavy and light chains, both produced from a single protein precursor. Cathepsin D is an aspartic endo-protease that is ubiquitously distributed in lysosomes. The main function of cathepsin D is to degrade proteins and activate precursors of bioactive proteins in pre-lysosomal compartments. This proteinase, which is a member of the peptidase A1 family, has a specificity similar to but narrower than that of pepsin A. Transcription of the CTSD gene is initiated from several sites, including one that is a start site for an estrogen-regulated transcript. Mutations in this gene are involved in the pathogenesis of several diseases, including breast cancer and possibly Alzheimer disease. Homozygous deletion of the CTSD gene leads to early lethality in the postnatal phase. Deficiency of CTSD gene has been reported an underlying cause of neuronal ceroid lipofuscinosis (NCL).
Cathepsin L1 is a protein that in humans is encoded by the CTSL1 gene. The protein is a cysteine cathepsin, a lysosomal cysteine protease that plays a major role in intracellular protein catabolism.
Cystatin-A is a protein that in humans is encoded by the CSTA gene.
Cystatin-B is a protein that in humans is encoded by the CSTB gene.
Cathepsin E is an enzyme that in humans is encoded by the CTSE gene. The enzyme is also known as slow-moving proteinase, erythrocyte membrane aspartic proteinase, SMP, EMAP, non-pepsin proteinase, cathepsin D-like acid proteinase, cathepsin E-like acid proteinase, cathepsin D-type proteinase) is an enzyme.
Cathepsin H is a protein that in humans is encoded by the CTSH gene.
Cystatin-F is a protein that in humans is encoded by the CST7 gene.
Legumain is a protein that in humans is encoded by the LGMN gene.
Cathepsin L2 is a protein encoded in humans by the CTSV gene.
Serpin B13 is a protein that in humans is encoded by the SERPINB13 gene.
Cathepsin F is a protein that in humans is encoded by the CTSF gene.
Asparagine endopeptidase is a proteolytic enzyme from C13 peptidase family which hydrolyses a peptide bond using the thiol group of a cysteine residue as a nucleophile. It is also known as asparaginyl endopeptidase, citvac, proteinase B, hemoglobinase, PRSC1 gene product or LGMN, vicilin peptidohydrolase and bean endopeptidase. In humans it is encoded by the LGMN gene.
Papain-like proteases are a large protein family of cysteine protease enzymes that share structural and enzymatic properties with the group's namesake member, papain. They are found in all domains of life. In animals, the group is often known as cysteine cathepsins or, in older literature, lysosomal peptidases. In the MEROPS protease enzyme classification system, papain-like proteases form Clan CA. Papain-like proteases share a common catalytic dyad active site featuring a cysteine amino acid residue that acts as a nucleophile.
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