Lysosomal PPT1‐insufficiency is a common pathogenic link between INCL and JNCL

Neuronal Ceroid Lipofuscinoses (NCLs), also known as Batten disease, constitute a group of the most common inherited childhood neurodegenerative lysosomal storage disorders (LSDs). Even though mutations in >13 different genes cause various NCLs, at the cellular level, all the NCLs characteristically accumulate autofluorescent material (ceroid) in lysosomes and clinically, develop seizures, visual failure and a progressive decline in cognitive and motor functions. These findings suggest shared pathogenic mechanism(s) in these diseases. While inactivating mutations in the CLN1 gene, encoding lysosomal PPT1, cause infantile NCL (INCL), mutations in the CLN3 gene encoding a lysosomal membrane protein of unknown function underlie juvenile NCL (JNCL). PPT1 catalyzes depalmitoylation of S‐acylated proteins, required for their degradation by lysosomal hydrolases. Thus, PPT1‐deficiency causes lysosomal accumulation of S‐acylated proteins (ceroid) leading to INCL. The Cln1 −/− and Cln3 −/− mice are reliable animal models of INCL and JNCL, respectively. Here we report for the first time that Cln3‐protein requires S‐palmitoylation for its endosomal transport to the lysosomal membrane. Moreover, we found that in the brain of Cln3 −/− mice, lysosomal PPT1‐protein as well as Ppt1‐enzymatic activity are significantly lower compared with lysosomes derived from their age‐ and sex‐matched WT counterparts. Unexpectedly, we found that the lysosomes from Cln3 −/− mice accumulate significantly higher levels of S‐palmitoylated proteins, characteristically found in those from Cln1 −/− mice. Previously, it has been reported that CLN3 mutations suppress the exit of cation‐independent mannose 6‐phosphate receptor (CI‐M6PR) from trans‐Golgi network (TGN), essential for its endosomal trafficking to the late endosome/lysosome. Since CI‐M6PR transports soluble proteins including Ppt1 to the lysosome, we hypothesized that Cln3 mutations may adversely affect the transport of PPT1 to the lysosome causing Ppt1‐insufficiency. Biochemical and confocal microscopic analyses of brain tissues from Cln3 −/− mice revealed significantly higher levels of Ppt1 and CI‐M6PR in the Golgi compared to those of their WT counterparts. Collectively, our results demonstrate that lysosomal Ppt1‐insufficiency is a common pathogenic link between INCL and JNCL and suggest that a recently identified Ppt1‐mimetic, neuroprotective small molecule, NtBuHA, may have therapeutic implications for both INCL and JNCL. Support or Funding Information Intramural Research Program of the Eunice Kennedy Shriver NICHD, NIH