Role of the Lysosomal Membrane Protein, CLN3, in the Regulation of Cathepsin D Activity.

Among Neuronal Ceroid Lipofuscinoses (NCLs), which are childhood fatal neurodegenerative disorders, the juvenile onset form (JNCL) is the most common. JNCL is caused by recessive mutations in the CLN3 gene. CLN3 encodes a lysosomal/endosomal transmembrane protein but its precise function is not completely known. We have previously reported that in baby hamster kidney (BHK) cells stably expressing myc-tagged human CLN3 (myc-CLN3), hyperosmotic conditions drastically increased myc-CLN3 mRNA and protein expression. In the present study, we analyzed the consequences of hyperosmolarity, and increased CLN3 expression on cathepsin D (CTSD) activity and prosaposin processing using BHK cells transiently or stably expressing myc-CLN3. We found that hyperosmolarity increased lysotracker staining of lysosomes, and elevated the levels of myc-CLN3 and lysosome-associated membrane protein-1 (LAMP1). Hyperosmolarity, independently of the expression level of myc-CLN3, decreased the levels of PSAP and saposin D, which are protein cofactors in sphingolipid metabolism. The lysosomal enzyme cathepsin D (CTSD) mediates the proteolytic cleavage of PSAP precursor into saposins A-D. Myc-CLN3 colocalized with CTSD and activity of CTSD decreased as myc-CLN3 expression increased, and clearly decreased under hyperosmotic conditions. Nevertheless, levels of CTSD measured by Western blotting were not altered under any studied condition. Our results suggest a direct involvement of CLN3 in the regulation of CTSD activity. J. Cell. Biochem. 118: 3883-3890, 2017.