The cell biology underlying juvenile ceroid lipofuscinoses (Batten Disease)

The juvenile form of Batten Disease (JNCL) is an autosomal recessive neurodegenerative disease caused by mutations in the CLN3 protein, an integral membrane protein of unknown function. The hallmark of JNCL is the accumulation of storage material in lysosomes consisting predominantly of subunit c of mitochondrial ATP synthase. CLN3 protein is present primarily in endosomes and lysosomes in non- neuronal cells, whereas in neuronal cells it is also reported to be in presynaptic vesicles and along the neurites. The current best evidence is that it traffics through the endoplasmic reticulum and Golgi apparatus to the endosome and lysosome, sorted by a dileucine motif which binds the adaptor complexes AP-1 and AP-3. The effects of loss of CLN3 protein function on membrane trafficking were investigated in the Cln3Aexl'6 knockout mouse model and in human cell lines deficient for CLN3 due to siRNA-mediated gene silencing. Morphological analysis and immunostaining of CLN3-deficient fibroblasts and neurons revealed the presence of normal lysosomes but also an accumulation of aberrant organelles of autophagic origin. Endocytic tracer analysis indicated that the aberrant organelles accumulate due to a defect in fusion between autophagosomes and lysosomes. Phagocytic defects involving a failure to recruit late endocytic markers were also defined. Together these findings indicate a similar defect in both these cellular pathways, involving defective maturation of autophagosomes and phagosomes which is accompanied by accumulation of undigested material in CLN3-deficient cells. As well as impairments in the mechanism of endosome fusion with autophagosomes and phagosomes, defects in an additional cellular mechanism were detected in CLN3-deficient cells. The route of protein trafficking from the trans-golgi network (TGN) to endosomes is interrupted after CLN3 gene silencing, resulting in retention of the cation-independent mannose-6-phosphate receptor in the TGN accompanied by defective processing of lysosome hydrolases. This investigation of the cellular pathology underlying Batten disease reveals a role for CLN3 in two distinct but related cellular pathways: autophagyc and phagocytosis and it also plays a role in TGN to endosome trafficking. This contributes to the understanding of the cellular phenotype underlying CLN3/Batten disease and most likely other forms of neuronal ceroid lipofuscinosis.