Neuronal ceroid lipofuscinoses (NCLs) represent a group of children's inherited neurodegenerative disorders caused by mutations in at least eight different genes. Mutations in the CLN5 gene result in the Finnish variant late infantile NCL characterized by gradual loss of vision, epileptic seizures, and mental deterioration. The CLN5 gene encodes a lysosomal glycoprotein of unidentified function. In this study, we have used both transient and stable expression systems for the characterization of CLN5, focusing on the localization, processing, and intracellular trafficking. We show that CLN5 is proteolytically cleaved, and that the mature polypeptide is transported to the lysosomes. Our data provide the first evidence that soluble CLN5 protein can also undergo mannose-6-phosphate receptor-independent trafficking to the lysosomes. We studied the localization and maturation of the CLN5 carrying the previously uncharacterized vLINCL disease causing mutations in HeLa cells. All analyzed disease mutations disturb the lysosomal trafficking of the mutated CLN5 proteins. The level of lysosomal targeting does not correlate, however, to disease onset, indicating that CLN5 may also function outside lysosomes. This study furthers our understanding of the basic properties of the CLN5 protein, necessary for the characterization of the consequences of disease mutations and for the planning of future therapies for vLINCL.

BackgroundNeuronal ceroid lipofuscinoses (NCLs) comprise at least eight genetically characterized neurodegenerative disorders of childhood. Despite of genetic heterogeneity, the high similarity of clinical symptoms and pathology of different NCL disorders suggest cooperation between different NCL proteins and common mechanisms of pathogenesis. Here, we have studied molecular interactions between NCL proteins, concentrating specifically on the interactions of CLN5, the protein underlying the Finnish variant late infantile form of NCL (vLINCLFin).ResultsWe found that CLN5 interacts with several other NCL proteins namely, CLN1/PPT1, CLN2/TPP1, CLN3, CLN6 and CLN8. Furthermore, analysis of the intracellular targeting of CLN5 together with the interacting NCL proteins revealed that over-expression of PPT1 can facilitate the lysosomal transport of mutated CLN5FinMajor, normally residing in the ER and in the Golgi complex. The significance of the novel interaction between CLN5 and PPT1 was further supported by the finding that CLN5 was also able to bind the F1-ATPase, earlier shown to interact with PPT1.ConclusionWe have described novel interactions between CLN5 and several NCL proteins, suggesting a modifying role for these proteins in the pathogenesis of individual NCL disorders. Among these novel interactions, binding of CLN5 to CLN1/PPT1 is suggested to be the most significant one, since over-expression of PPT1 was shown to influence on the intracellular trafficking of mutated CLN5, and they were shown to share a binding partner outside the NCL protein spectrum.

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