Defective intracellular transport of CLN3 is the molecular basis of Batten disease (JNCL)

Abstract

Batten disease [juvenile-onset neuronal ceroid lipofuscinosis (JNCL)], the most common progressive encephalopathy of childhood, is caused by mutations in a novel lysosomal membrane protein (CLN3) with unknown function. In this study, we have confirmed the lysosomal localization of the CLN3 protein by immunoelectron microscopy by co-localizing it with soluble and membrane-associated lysosomal proteins. We have analysed the intracellular processing and localization of two mutants, 461–677del, which is present in 85% of CLN3 alleles and causes the classical JNCL, and Q295K, which is a rare mis-sense mutation associated with an atypical form of JNCL. Pulse-chase labelling and immunoprecipitation of the two mutant proteins in COS-1-cells indicated that 461–677del is synthesized as an ∼24 kDa truncated polypeptide, whereas the maturation of Q295K resembles that of the wild-type CLN3 polypeptide. Transient expression of the two mutants in BHK cells showed that 461–677del is retained in the endoplasmic reticulum, whereas Q295K was capable of reaching the lysosomal compartment. The CLN3 polypeptides were expressed further in mouse primary neurons where the wild-type CLN3 protein was localized both in the cell soma and in neuronal extensions, whereas the 461–677del mutant was arrested in the cell soma. Interestingly, co-localization of the wild-type CLN3 and Q295K proteins with a synaptic vesicle marker indicates thatthe CLN3 protein might participate in synaptic vesicle transport/transmission. The data presented here provide clear evidence for a cellular distinction between classical and atypical forms of Batten disease both in neural and non-neural cells.