CTSD and neurodegenerative disease: To further search for lysosomal alterations, we measured the activity and the protein levels of the lysosomal aspartic-type protease cathepsin D. This enzyme is involved in lysosomal-mediated degradation of unfolded or oxidized protein aggregates associated to neurodegenerative diseases (40) and slightly increased in CLN4 (28), but we did not find any significant differences between mutant mice and controls (fig.