Palmitoyl-protein thioesterase-1 deficiency mediates the activation of the unfolded protein response and neuronal apoptosis in INCL

Numerous proteins undergo modification by palmitic acid (S-acylation) for their biological functions including signal transduction, vesicular transport and maintenance of cellular architecture. Although palmitoylation is an essential modification, these proteins must also undergo depalmitoylation for their degradation by lysosomal proteases. Palmitoyl-protein thioesterase-1 (PPT1), a lysosomal enzyme, cleaves thioester linkages in S-acylated proteins and removes palmitate residues facilitating the degradation of these proteins. Thus, inactivating mutations in the PPT1 gene cause infantile neuronal ceroid lipofuscinosis (INCL), a devastating neurodegenerative storage disorder of childhood. Although rapidly progressing brain atrophy is the most dramatic pathological manifestation of INCL, the molecular mechanism(s) remains unclear. Using PPT1-knockout (PPT1-KO) mice that mimic human INCL, we report here that the endoplasmic reticulum (ER) in the brain cells of these mice is structurally abnormal. Further, we demonstrate that the level of growth-associated protein-43 (GAP-43), a palmitoylated neuronal protein, is elevated in the brains of PPT1-KO mice. Moreover, forced expression of GAP-43 in PPT1-deficient cells results in the abnormal accumulation of this protein in the ER. Consistent with these results, we found evidence for the activation of unfolded protein response (UPR) marked by elevated levels of phosphorylated translation initiation factor, eIF2 alpha, increased expression of chaperone proteins such as glucose-regulated protein-78 and activation of caspase-12, a cysteine proteinase in the ER, mediating caspase-3 activation and apoptosis. Our results, for the first time, link PPT1 deficiency with the activation of UPR, apoptosis and neurodegeneration in INCL and identify potential targets for therapeutic intervention in this uniformly fatal disease.