CLN8 protein: a post-translation modification and subcellular distribution study for understanding CLN8 physiopathology

The neuronal ceroid lipofuscinoses (NCLs) are devastating autosomal recessive neurodegenerative diseases of children, belonging to the family of lysosomal storage disorders (LSDs). NCLs show broad clinical and allelic heterogeneity, more than fourteen genes (CLN genes) and 400 mutations are known, and they are characterized by a common feature of intralysosomal accumulation of lipofuscin. Some CLN proteins have already a function (i.e., cathepsin D, PPT1, and TPP1), others proteins (CLN3, CLN5, CLN6, CLN7, and CLN8) still wait to be defined. Mutations of CLN8 cause two major clinical phenotypes: the progressive epilepsy with mental retardation (EPMR or Northern Epilepsy), a juvenile-onset phenotypic variant, and a more severe form with a late-infantile onset (LINCL). A CLN8 mutation spontaneously occurring in mice results in the motor neuron degeneration (mnd) phenotype. The CLN8 is a ubiquitous membrane protein of 286 a.a., containing an ER-retrieval signal (KKRP) and primary located at the ER. Probable functions of CLN8 implies an involvement in the lipid synthesis and/or proteolipid trafficking or as a lipid sensing. In this study, to elucidate CLN8 protein function we better characterized the subcellular localization and verified its post-translational modification. Endogenous and recombinant protein expression were examined in differentiated and undifferentiated human neuroblastoma cells and in a human epithelial cell line, that were subjected to treatments with specific drugs activating phosphorylation and/or inhibiting phosphatase. Protein localization was also assessed in fractionated lysates of cerebella from wild type and mnd mice. Data obtained indicate a possible CLN8 threonine phosphorylation site which lacks in the sequence of CLN8 mutated in the EPMR disease, thus suggesting its important functional role. Also, hints regarding CLN8 role could be uncovered by a different subcellular localization of various forms of CLN8 under physiological and pathological conditions.