Seizures in PPT1 Knock-in Mice are Associated with Inflammatory Activation of Microglia

Abstract

Infantile neuronal ceroid lipofuscinosis (INCL), the most severe form of neuronal ceroid lipofuscinoses, is caused by mutations in the lysosomal enzyme palmitoyl protein thioesterase 1 (PPT1). Typical symptoms of this disease include progressive psychomotor developmental retardation, visual failure, seizures, and premature death. Here, we investigated seizure activity and relevant pathological changes in PPT1 knock-in mice (PPT1 KI). The behavior studies in this study demonstrated that PPT1 KI mice had no significant seizure activity until 7 months of age, and local field potentials also displayed epileptiform activity at the same age. The expressional levels of Iba-1 and CD68 demonstrated by Western blot analysis, the inflammatory cytokine TNFα content measured with enzyme-linked immunosorbent assay, and the branch number and length of microglia demonstrated by immunohistochemistry (IHC) were significantly increased at age of 7 months, all of which indicate microglia activation at an age of seizure onset. The increased expressional of GFAP was seen at an earlier age of 4 month and such an increase reached its peak at age of 6 month, indicating that astrocyte activation precedes microglia. The purinergic P2X7 receptor (P2X7R) is a ATP-sensitive ionic channel that is highly expressed in microglia and fundamental to microglial activation, proliferation, cytokines release and epilepsy. We show that the ATP concentration in hippocampal tissue in PPT1 KI mice was increased using an enhanced ATP assay kit and demonstrated that the antagonist of P2X7R, A-438079 significantly reduced seizures in PPT1 KI mice. In contrast to glial cell activation and proliferation, the significant reduction of synaptic proteins GABAAR were seen in PPT1 KI mice. These results indicate that seizure in PPT1 KI mice may be associated with microglial activation involving in ATP-sensitive-P2X7R signaling and damaged inhibitory neurotransmission.

GABAAR-PPT1 palmitoylation homeostasis controls synaptic transmission and circuitry oscillation in CLN1 disease

“…Similarly, we observed considerable neuron loss in the hippocampus of 6-to 7-month-old PPT1-KI mice (Zhang et al, 2022), as well as reduced expression of NMDAR 2a/b and PSD95, which may be the underlying mechanism for impaired LTP in old mice (Fig. S9).…”

“…Further, GABAAR plays a central role in synchronising network neuronal activity, which are tightly associated with higher brain functions such as learning and memory (Nakazono et al, 2018). PPT1-KI mice carrying nonsense mutations in the CLN1/PPT1 gene exhibited reduced expression of GABAAR along with impaired gamma oscillation and seizure onset (Zhang et al, 2022). These findings indicate the importance of GABAAR in CLN1 disease.…”