Developmental synapse maturation is impaired in infantile neuronal ceroid lipofuscinosis Koster, Kevin P.1, Francesconi, Walter1, Berton, Fulvia1, Yoshii, Akira1, 2, 3 1University of Illinois at Chicago, Department of Anatomy and Cell Biology, 2Pediatrics and 3Neurology
Neural circuits are sculpted by the interplay between synapse formation, maintenance, and refinement. Protein palmitoylation is the reversible attachment of palmitic acid to proteins. In neurons, this posttranslational mechanism is critical for axon pathfinding, synaptic transmission, and plasticity. Depalmitoylation, the process of palmitic acid removal, is required for lysosomal proteolysis. Mutation of the depalmitoylating enzyme, palmitoyl-protein thioesterase 1 (PPT1), causes infantile neuronal ceroid lipofuscinosis (CLN1), a pediatric neurodegenerative disease characterized by visual deterioration and seizure, leading to death by five years. However, the role of protein depalmitoylation in neural circuit formation is unknown. Further, it is unclear how loss of PPT1 function leads to synaptic dysregulation and neurodegeneration in CLN1. Thus, we examined the Ppt1-/- mouse to decipher the role of protein depalmitoylation in visual cortex (VC) maturation. First, in primary cortical cultures, Ppt1-/- neurons exhibited morphologically immature dendritic protrusions, including filipodia and elongated, thin spines. Calcium imaging experiments revealed extrasynaptic Ca2+ transients in dendritic shafts of Ppt1-/- neurons, while wild-type (WT) cells had compartmentalized Ca2+ influx within spines. Next, we conducted biochemical analyses of synaptosomes prepared from VCs of developing (postnatal day 11-60), WT and Ppt1-/- mice. Ppt1-/- VCs had selective decreases in mature components of the N-methyl-D-aspartate receptor (NMDAR) protein complex; the GluN2A subunit and its scaffolding protein, PSD-95, which increase during development. In contrast, neonatal components of the NMDAR complex, GluN2B and SAP102, were unchanged. Recording of NMDAR-mediated excitatory postsynaptic currents (EPSCs) in layer II/III VC neurons confirmed a reduction in both the amplitude and decay time of the fast, GluN2A-mediated component of the EPSC in Ppt1-/- VC. Lastly, Ppt1-/- neurons had increased vulnerability to NMDA-induced excitotoxicity, which is mediated by extrasynaptic, GluN2B-rich NMDARs. Together, these findings suggest a critical role for PPT1 in NMDAR regulation and synapse maturation during development.
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