Human INCL fibroblasts display abnormal mitochondrial and lysosomal networks and heightened susceptibility to ROS-induced cell death

Activation of D2-like dopamine receptors improves the neuronal network and cognitive function of PPT1KI mice

Palmitoyl-protein thioesterase 1 (PPT1) is a lysosomal depalmitoylation enzyme that mediates protein posttranslational modifications. Loss-of-function mutation of PPT1 causes a failure of the lysosomal degradation of palmitoylated proteins and results in a congenital disease characterized by progressive neuronal degeneration referred to as infantile neuronal ceroid lipofuscinosis (INCL). A mouse knock-in model of PPT1 (PPT1-KI) was established by introducing the R151X mutation into exon 5 of the PPT1 gene, which exhibited INCL-like pathological lesions. We previously reported that hippocampal γ oscillations were impaired in PPT1 mice. Hippocampal γ oscillations can be enhanced by selective activation of the dopamine D4 receptor (DR4), a dopamine D2-like receptor. In this study, we investigated the changes in DR expression and the effects of dopamine and various DR agonists on neural network activity, cognition and motor function in PPT1KI mice. Cognition and motor defects were evaluated via Y-maze, novel object recognition and rotarod tests. Extracellular field potentials were elicited in hippocampal slices, and neuronal network oscillations in the gamma frequency band (γ oscillations) were induced by perfusion with kainic acid (200 nM). PPT1KI mice displayed progressive impairments in γ oscillations and hippocampus-related memory, as well as abnormal expression profiles of dopamine receptors with preserved expression of DR1 and 3, increased membrane expression of DR4 and decreased DR2 levels. The immunocytochemistry analysis revealed the colocalization of PPT1 with DR4 or DR2 in the soma and large dendrites of both WT and PPT1KI mice. Immunoprecipitation confirmed the interaction between PPT1 and DR4 or DR2. The impaired γ oscillations and cognitive functions were largely restored by the application of exogenous dopamine, the selective DR2 agonist quinpirole or the DR4 agonist A412997. Furthermore, the administration of A412997 (0.5 mg/kg, i.p.) significantly upregulated the activity of CaMKII in the hippocampus of 5-month-old PPT1KI mice. Collectively, these results suggest that the activation of D2-like dopamine receptors improves cognition and network activity in PPT1KI mice and that specific DR subunits may be potential targets for the intervention of neurodegenerative disorders, such as INCL.

Binding of hydroxychloroquine and chloroquine dimers to palmitoyl-protein thioesterase 1 (PPT1) and its glycosylated forms: a computational approach

The lysosomal enzyme palmitoyl-protein thioesterase 1 (PPT1) removes thioester-linked fatty acid groups from membrane-bound proteins to facilitate their proteolysis. A lack of PPT1 (due to gene mutations) causes the progressive death of cortical neurons and is responsible for infantile neural ceroid lipofuscinosis (INCL), a severe neurodegenerative disorder in children. Conversely, PPT1 is often over-expressed in cancer, and considered as a valid target to control tumor growth. Potent and selective inhibitors of PPT1 have been designed, in particular 4-amino-7-chloro-quinoline derivatives such as hydroxychloroquine (HCQ) and the dimeric analogues Lys05 and DC661. We have modeled the interaction of these three compounds with the enzyme, taking advantage of the PPT1 crystallographic structure. The molecules can fit into the palmitate site of the protein, with the dimeric compounds forming more stable complexes than the monomer. But the molecular modeling suggests that the most favorable binding sites are located outside the active site. Two sites centered on residues Met112 and Gln144 were identified, offering suitable cavities for drug binding. According to the calculated empirical energies of interaction (ΔE), the dimer DC661 forms the most stable complex at site Met112 of palmitate-bound PPT1. N-glycosylated forms of PPT1 were elaborated. Paucimannosidic glycans (M2FA and M3F) and a bulkier tetra-antennary complex glycan were introduced at asparagine residues N197, N212 and N232. These N-glycans do not impede drug binding, thus suggesting that all glycoforms of PPT1 can be targeted with these compounds.Communicated by Ramaswamy H. Sarma.