CLN3 protein is targeted to neuronal synapses but excluded from synaptic vesicles: new clues to Batten disease

Batten disease (juvenile neuronal ceroid lipofuscinosis, JNCL), the most co mmon neurodegenerative disease of childhood, is caused by mutations in the CLN3 gene encoding a putative transmembrane protein. The function of CLN3 i s currently unknown but it has been shown to localize in the endosomal/lyso somal compartments of nonneuronal cells. In addition, several other intrace llular localizations have been proposed and the controversy of the reports suggests that CLN3 may have different intracellular localization in differe nt cell types. Batten disease severely affects neuronal cells but leaves ot her organs clinically unaffected, and thus it is of utmost importance to ap proach the disease mechanism by studying the expression and localization of CLN3 in the brain and neuronal cells. We have analysed here CLN3 in the mo use brain using in situ hybridization, immunohistochemical staining and wes tern blot analysis of subcellular fractions. As visual deterioration is the hallmark of Batten disease we have set up primary retinal cultures from th e mouse and analysed both endogenous mouse CLN3 and Semliki Forest virus-me diated human CLN3 localization using immunofluorescence staining and confoc al microscopy. We demonstrate that CLN3 is abundantly expressed in neuronal cells, especially in the cortex, hippocampus and cerebellum of the adult m ouse brain. Furthermore, our results indicate that in neurons CLN3 is not s olely a lysosomal protein. It is localized in the synaptosomes but, interes tingly, is not targeted to the synaptic vesicles. The novel localization of CLN3 directs attention towards molecular alterations at the synapses. This should yield important clues about the mechanisms of neurodegeneration in Batten disease.