Localization of wild‐type and mutant neuronal ceroid lipofuscinosis CLN8 proteins in non‐neuronal and neuronal cells

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Abstract
Neuronal ceroid lipofuscinoses (NCLs) are a group of childhood‐onset neurodegenerative disorders characterized by accumulation of autofluorescent lipopigment in many tissues, especially in neurons. Mutations in the CLN8 gene underlie Northern epilepsy (progressive epilepsy with mental retardation [EPMR], OMIM 600143) and a subset of Turkish variant late infantile NCL, but the pathogenetic mechanisms have remained elusive. The CLN8 transmembrane protein is an endoplasmic reticulum (ER) resident protein that recycles between ER and ER‐Golgi intermediate compartment (ERGIC) in non‐neuronal cells. To explore the disease mechanisms, we have characterized the neuronal localization of wild‐type CLN8 protein as well as CLN8 proteins representing patient mutations. Semliki Forest virus‐mediated CLN8 protein localized in the ER of mouse hippocampal primary neurons when compared to subcellular markers by immunofluorescence analysis. We also analyzed the possible polarized targeting of CLN8 and observed basolateral targeting in polarized epithelial CaCo‐2 cells, suggesting that CLN8 may locate outside the ER or in a specialized subcompartment of the ER. We were not able, however, to demonstrate differential distribution of CLN8 between axons and dendrites of neurons. Fractionation of mouse brain tissue indicated that endogenous mouse Cln8 is observed in light membrane fractions, different from ER, which further suggested differential localization for CLN8 in polarized cells. The disease mutations did not affect intracellular localization of CLN8 in non‐neuronal or neuronal cells. Consequently, there is no obvious genotype–phenotype correlation at the level of protein localization and thus mutations most likely directly affect functionally important domains of CLN8.