Cathepsin D

Cathepsin D is a protein that in humans is encoded by the CTSD gene. It is a member of the pepsin family of endopeptidases, which are a group of enzymes involved in the breakdown of proteins into smaller peptides or amino acids. Cathepsin D is an aspartic protease that plays a crucial role in intracellular protein catabolism in lysosomes. It is involved in various cellular processes, including apoptosis (programmed cell death), autophagy (the body's way of cleaning out damaged cells to regenerate newer, healthier cells), and neurodegeneration. Due to its involvement in these critical processes, alterations in the activity or expression of cathepsin D have been linked to several diseases, including breast cancer, Alzheimer's disease, and other neurodegenerative diseases.

Function

Cathepsin D is synthesized as an inactive zymogen called preprocathepsin D, which is processed in the endoplasmic reticulum and Golgi apparatus into a slightly active proform. This proform is then transported to lysosomes, where it is activated by acidic pH and proteolytic cleavage. Once activated, cathepsin D can degrade various proteins, contributing to the general turnover of intracellular components and the specific degradation of misfolded or damaged proteins.

Clinical Significance

The role of cathepsin D has been extensively studied in the context of cancer, neurodegenerative diseases, and lysosomal storage diseases. In cancer, overexpression of cathepsin D has been correlated with increased metastasis and poor prognosis, particularly in breast cancer. It is thought to contribute to the invasive and metastatic properties of cancer cells by degrading extracellular matrix components.

In neurodegenerative diseases such as Alzheimer's disease, alterations in cathepsin D expression and activity have been observed. It is involved in the degradation of the amyloid precursor protein (APP), and its dysregulation may contribute to the accumulation of amyloid beta, a hallmark of Alzheimer's pathology.

Furthermore, mutations in the CTSD gene can lead to a rare lysosomal storage disease known as Neuronal Ceroid Lipofuscinosis (NCL), characterized by the accumulation of autofluorescent lipopigment storage material in various tissues, including the brain, leading to neurodegeneration.

Genetics

The CTSD gene is located on chromosome 11 (11p15.5) in humans. Mutations in this gene can lead to altered enzyme activity or stability, contributing to disease pathology.

Research Directions

Research into cathepsin D continues to explore its role in various diseases and its potential as a therapeutic target. Inhibitors of cathepsin D have been investigated for their potential to slow the progression of diseases such as cancer and Alzheimer's disease. However, given the enzyme's critical roles in normal cellular processes, targeting it for therapy requires careful consideration to avoid adverse effects.