Progressively reduced synaptic vesicle pool size in cultured neurons derived from neuronal ceroid lipofuscinosis-1 knockout mice

The neuronal ceroid lipofuscinoses are a newly-recognized group of lysosomal storage disorders in which neurodegeneration predominates. The pathophysiological basis for this is unknown. In the current paper, we sought to determine whether neurons that lack the enzyme responsible for the infantile form of neuronal ceroid lipofuscinosis (INCL) display abnormalities in culture that could be related to the clinical disorder. Electrophysiological and fluorescent dye studies were performed using cortical neuronal cultures established from postnatal day 2 palmitoyl-protein thioesterase-1 (Ppt1) knockout mice. We found a 30% reduction in synaptic vesicle number per bouton that was progressive with time in culture as well as an elevation in lysosomal pH, whereas a number of passive and active membrane properties of the neurons were normal. The reduction in vesicle pool size was also reflected in a decrease in the frequency of miniature synaptic currents. The progressive and gradual decline in vesicle numbers and miniature event frequency we observed here may be an early indicator of synapse degeneration, in keeping with observations during competitive stimulation at the neuromuscular junction or age-related synapse elimination recently reported by others. PPT1 did not colocalize with synaptic vesicle or synapse markers, suggesting that lysosomal dysfunction leads indirectly to the synaptic abnormalities. We conclude that from an early age, neurons deficient in PPT1 enzyme activity display intrinsically abnormal properties that could potentially explain key features of the clinical disease, such as myoclonus and seizures.