CTSD

Cathepsin D is ubiquitously distributed in lysosomes, also known as CTSD and CPSD, and is a lysosomal aspartic endoprotease, which is important for the clearance of long-lived proteins in humans. Preprocathepsin D is synthesized in the rough endoplasmic reticulum. CTSD belongs to the peptidase A1 family and possesses a similar but narrower specificity to pepsin A. CTSD exhibits main functions of degrading proteins and activating precursors of bioactive proteins in pre-lysosomal compartments. CTSD is also involved in biological procedures like antigen processing, cell proliferation, tissue renewal, and prohormone activation.

Distribution and Localization

CTSD has been reported to be synthesized in nearly all tissues and organs, and it is localized in lysosomes and most pre-lysosomal compartments. CTSD zymogen is detected in human, bovine and rat milk. Cellular release of CTSD has been described to be implicated in exocytic events. The leakage of CTSD from the cell can be compensated through endocytosis dependent on mannose-6-phosphate receptor and processes unrelated to the receptor. The nuclear localization of CTSD has also been evidenced. Cytosol localization of CTSD is connected with the proposed functions of CTSD in apoptosis.

Molecular Structure

CTSD is encoded by the CTSD gene, which encodes an aspartyl protease comprising a protein dimer of disulfide-linked heavy and light chains, both generated from a single protein precursor. The transcription of CTSD gene is commenced from several sites, one of which is a start site for an estrogen-regulated transcript. Mutations in this gene are linked to the pathogenesis of several diseases, like breast cancer and possible Alzheimer disease. The homozygous deletion of CTSD gene results in early lethality in the post-natal phase. Additionally, the deficiency of CTSD gene is also an underlying cause of neuronal ceroid lipofuscinosis (NCL). The peptide chain of CTSD can be cleaved into CTSD light chain and CTSD heavy chain, which are linked by the hydrophobic effect. The catalytic sites of CTSD consist of two critical aspartic residues of amino acid 33 and 231 located in the 14 kDa and 34kDa chains, respectively. The ultimate structure of mature CTSD is made up of 337 amino acid residues, 196 amino acid residues in the heavy chain and 141 in the light chain.

Mechanism

It is found that the hydrophobic residues of CTSD are not only a preferential factor for its cleavage function but also contribute to its recognition due to the hydrophobicity. Hydrophobic scores of neighbors (HSN) is a proposed concept to describe the hydrophobic microenvironment of CTSD recognition sites, and CTSD cleavage survey in several proteins has recommended that HSN is a sensitive indicator for judging the favorable sites of peptides in CTSD cleavage, where HSN values in the range of 0.5–1.0 represents a likely threshold and a higher HSN value means greater cleavability. Further two-dimensional gel electrophoresis conducted has shown that intact proteins with a structure consisting of all α-helices would be relatively sensitive to CTSD cleavage.

Functions

CTSD occupies a significant position in both biological and pathological processes. CTSD could be activated at a pH of 5 in hepatocyte endosomes where it destroys insulin. Apart from low pH, the activation of CTSD is also critically dependent on the protonation of its active site Asp residue. Both of the former factors cause a conformational switch in CTSD where the N terminal segment of the protease is removed from the active site as pH drops. Similar to other aspartic proteinases, CTSD could hold up to 8 amino acid residues in the binding cleft of the active sites. The physiological functions of CTSD primarily include metabolic cleavage of intracellular proteins, activation and destruction of polypeptide hormones and growth factors, activation of enzymatic precursors, handling with enzyme activators and inhibitors, processing of brain antigen and regulation of automated cell death.

Clinical Significance

The deficiency of CTSD gene is responsible for NCLs, which are accompanied with progressive loss of visual function and neurodevelopmental decline, seizure, myoclonic jerks and premature death. Knock-out of CTSD gene would induce intestinal necrosis and hemorrhage and promote apoptosis in thymus, suggesting that CTSD is essential in certain epithelial cells for tissue remodeling and renewal. However, the increase in levels of serum CTSD has also involved in certain pathological conditions. Overexpression of CTSD arouses tumorigenicity and metastasis, but it also initiates the apoptosis of tumor cells. CTSD genotype also has a strong effect on Alzheimer disease risk in male. CTSD enzymatic activity brings about hydrolytic modification of apolipoprotein B-100-containing lipoproteins, meaning that CTSD may be linked to atherosclerosis as well. CTSD as a marker for on-alcoholic steatohepatitis (NASH) also influences lipid metabolism. Inhibition of CTSD activity could dramatically improve lipid metabolism, which therefore may lead to novel treatments to combat NASH. CTSD has also been treated an independent predictor for poor prognosis in breast cancer that is associated with the incidence of clinical metastasis. The level and activities of CTSD also serve as an indicator to provide the propensity of Parkinson's disease pathogenesis. The enhancement of CTSD function either by gene and protein delivery or by chemical allosteric improvement may offer an efficacious therapy.

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