👤 Jaime Cárcel-Trullols

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. © 2017 Wiley Periodicals, Inc. Show less

The fatal, primarily childhood neurodegenerative disorders, neuronal ceroid lipofuscinoses (NCLs), are currently associated with mutations in 13 genes. The protein products of these genes (CLN1 to CLN14) differ in their function and their intracellular localization. NCL-associated proteins have been localized mostly in lysosomes (CLN1, CLN2, CLN3, CLN5, CLN7, CLN10, CLN12 and CLN13) but also in the Endoplasmic Reticulum (CLN6 and CLN8), or in the cytosol associated to vesicular membranes (CLN4 and CLN14). Some of them such as CLN1 (palmitoyl protein thioesterase 1), CLN2 (tripeptidyl-peptidase 1), CLN5, CLN10 (cathepsin D), and CLN13 (cathepsin F), are lysosomal soluble proteins; others like CLN3, CLN7, and CLN12, have been proposed to be lysosomal transmembrane proteins. In this review, we give our views and attempt to summarize the proposed and confirmed functions of each NCL protein and describe and discuss research results published since the last review on NCL proteins. This article is part of a Special Issue entitled: "Current Research on the Neuronal Ceroid Lipofuscinoses (Batten Disease)". Show less

Neuronal ceroid lipofuscinoses (NCL) are lysosomal storage disorders characterized by the accumulation of lipofuscin within lysosomes. Late infantile (LINCL) and juvenile (JNCL) are their most common forms and are caused by loss-of-function mutations in tripeptidyl peptidase 1 (TPP1), a lysosomal endopeptidase, and CLN3 protein (CLN3p), whose location and function is still controversial. LINCL patients suffer more severely from NCL consequences than JNCL patients, in spite of having in common an abnormal accumulation of material with a similar composition in the lysosomes. To identify distinctive characteristics that could explain the differences in the severity of LINCL and JNCL pathologies, we compared the protein degradation mechanisms in patientś fibroblasts. Pulse-chase experiments show a significant decrease in protein degradation by macroautophagy in fibroblasts bearing TPP1 (CLN2) and CLN3p (CLN3) mutations. In CLN2 fibroblasts, LC3-II levels and other procedures indicate an impaired formation of autophagosomes, which confirms the pulse-chase experiments. This defect is linked to an accumulation of reactive oxygen species (ROS), an upregulation of the Akt-mTOR signalling pathway and increased activities of the p38α and ERK1/2 MAPKs. In CLN3 fibroblasts, LC3-II analysis indicates impairment in autophagosome maturation and there is also a defect in fluid phase endocytosis, two alterations that can be related to an observed increase of 0.5 units in lysosomal pH. CLN3 fibroblasts also accumulate ROS but to a lower extent than CLN2. TPP1 activity is completely abrogated in CLN2 and partially diminished in CLN3 fibroblasts. TPP1 cleaves small hydrophobic proteins like subunit c of mitochondrial ATP synthase and the lack or a lower activity of this enzyme can contribute to lipofuscin accumulation. These alterations in TPP1 activity lead to an increased ROS production, especially in CLN2 in which it is aggravated by a decrease in catalase activity. This could explain the earlier appearance of the symptoms in the LINCL form. Show less