CTSA (Cathepsin A) encodes the enzyme cathepsin A, also known as lysosomal protective protein (PPCA). This enzyme has multiple functions in lysosomal digestion, protein activation, and cellular homeostasis. Cathepsin A is a multifunctional lysosomal enzyme that plays critical roles in protein catabolism, neuropeptide processing, and protecting other lysosomal enzymes from degradation. Beyond its well-characterized role in lysosomal storage disease (galactosialidosis), emerging research suggests cathepsin A may contribute to the pathogenesis of neurodegenerative diseases including [Alzheimer's disease](/diseases/alzheimers-disease) and [Parkinson's disease](/diseases/parkinsons-disease) through its involvement in protein aggregation, lysosomal dysfunction, and neuroinflammation[@linedale2013][@stoka2005].
Gene Overview
Protein Structure
Cathepsin A is synthesized as a 54 kDa precursor that undergoes processing to the mature enzyme:
Signal Peptide: Directs secretion to the lysosome
Propeptide: Removed during activation
Mature Enzyme: 46 kDa active form
Carboxy-terminal Fragment: Generated during processing
The enzyme has a serine protease-like catalytic triad (Ser195, Asp327, His195 in chymotrypsin numbering) and requires optimal acidic pH (4.5-5.5) for activity[@pshezhetsky1997].
Function
CTSA encodes cathepsin A (also called protective protein/cathepsin A, PPCA), a multifunctional lysosomal enzyme with several important functions:
Enzymatic Activities
Carboxypeptidase Activity: Removes C-terminal amino acids from various substrates, including:
Bradykinin
Oxytocin
Vasopressin
Various neuropeptides
Dipeptidyl-peptidase Activity: Releases dipeptides from N-termini of substrates
Esterase Activity: Minor activity on synthetic substrates
Protective Function (PPCA Activity)
The "protective protein" function is critical for lysosomal enzyme stability:
Forms a stable complex with β-galactosidase and neuraminidase
Protects these enzymes from rapid degradation in the lysosome
Essential for proper activity and localization of other lysosomal hydrolases
Mutations that disrupt this function cause secondary enzyme deficiencies
Role in Cellular Processes
Protein Catabolism: Degrades proteins in the lysosome
Neuropeptide Processing: Activates and degrades neuropeptides
Glycoprotein Processing: Removes sialic acid residues from glycoproteins
Autophagy: Involved in autophagosomal-lysosomal degradation[@linedale2013]
Expression Pattern
CTSA is ubiquitously expressed with high levels in:
Brain
Neurons: High expression in pyramidal neurons, Purkinje cells
Astrocytes: Moderate expression
Microglia: Lower expression
Oligodendrocytes: Present
Other Tissues
Liver: Highest expression (hepatocytes)
Kidney: High expression (proximal tubules)
Spleen: High expression (macrophages)
Placenta: Moderate expression
Lung: Moderate expression
Heart: Lower expression
In the brain, expression is highest in regions associated with learning and memory, including the hippocampus and cerebral cortex[@demaret2004].
Disease Associations
Lysosomal Storage Disease: Galactosialidosis
Mutations in CTSA cause Galactosialidosis (also called protective protein deficiency), a rare autosomal recessive disorder: