Palmitoyl-protein Thioesterase 1 in Developmental Plasticity and Infantile Neuronal Ceroid Lipofuscinosis

Infantile neuronal ceroid lipofuscinosis (CLN1) is a pediatric neurodegenerative disease that affects children and results in death by age 5. Mutations in the neuronal enzyme palmitoyl-protein thioesterase 1 (PPT1) cause CLN1. PPT1 is a depalmitoylating enzyme—it removes palmitic acid from proteins that have been posttranslationally palmitoylated. Palmitoylation dynamically influences the trafficking and membrane association of proteins, especially at synapses. Neurodevelopmental plasticity of synapses and the neuronal circuits they make up is crucial for long-term brain function. We hypothesized that loss of PPT1 function compromises neurodevelopmental synaptic plasticity and thereby drives the progression of CLN1. Therefore, we studied the role of Ppt1 in the well-characterized developmental plasticity of the visual cortex in the Ppt1-/- mouse model of CLN1. N-methyl-D-aspartate (NMDA) receptors in the visual cortex undergo a developmental shift in their composition that coincides with the maturation of neuronal circuits. The GluN2B subunits that are dominant in early life are replaced by NMDA receptors containing, instead, GluN2A. I discovered that this process is idled in the CLN1 visual cortex such that the cell biological and physiological features of the immature, GluN2B-NMDA receptors persist in Ppt1-/- neurons. This, in part, leaves Ppt1-/- neurons vulnerable to excitotoxicity. In a separate line of experiments, I investigated how Ppt1 impacts a mechanism of homeostatic plasticity known as synaptic scaling, which counterbalances Hebbian plasticity. The insertion of calcium permeable AMPA receptors that constitutes synaptic scaling up was exaggerated in Ppt1-/- neurons in vitro and in vivo. Palmitoyl-proteomics showed that A kinase anchor protein 5 (Akap5) and associated signaling molecules are hyperpalmitoylated in Ppt1-/- neurons. This pathway regulates synaptic scaling and links synaptic calcium activity to neuroinflammation via activation of nuclear factor of activated T-cells (NFAT), demonstrating a direct line between these processes for the first time in CLN1. Finally, two-photon calcium imaging in awake animals revealed that Ppt1-/- visual cortical neurons are hypersynchronous, suggesting a failure to de-synchronize with age in the CLN1 brain. Taken together, this work demonstrates that depalmitoylation by Ppt1 regulates synaptic plasticity mechanisms engaged in neurodevelopment that, when disrupted, lead to circuit-wide defects that portend CLN1 symptoms.