The number of activated microglia – the brain’s resident immune cells – is dramatically increased when seizures begin to occur in a mouse model of childhood Batten disease, also known as CLN1 disease, a study has found.
Deletion of P2X7R, a receptor protein primarily present on the surface of microglia, significantly reduced the number and total duration of seizures in these mice.
These findings suggest that microglia activation contributes to the development of childhood Batten seizures, and that its suppression may provide a new therapeutic avenue for this condition, the researchers noted.
The study, “Seizures in PPT1 knock-in mice are associated with inflammatory activation of microgliawas published in the International Journal of Molecular Sciences.
Batten disease is a group of rare neurodegenerative diseases caused by mutations in at least 13 different genes. These mutations lead to the toxic accumulation of waste molecules inside lysosomes – the recycling compartments of cells – mainly in brain cells, leading to damage and death of nerve cells or neurons.
Infantile Batten disease is caused by a deficiency in the enzyme PPT1 due to mutations in the CLN1 gene and is “the fastest and most severe type” of Batten, the researchers wrote.
Mounting evidence suggests that microglia and its cell surface receptor P2X7R are involved in seizure activation.
“Activated by ATP that is released from neuronal terminals, leaks from damaged cell membrane of neurons, or leaks out of astrocytes, P2X7 mediates inflammatory activation of microglia,” the researchers wrote.
ATP is the energy storage molecule that allows all cells to function. Astrocytes are brain cells involved in neuronal support and nutrition, but whose abnormal activity has been implicated in neurological disorders.
“Activated microglia release inflammatory [molecules]which leads to neuronal inflammation, damage, and the onset of seizures,” the researchers wrote.
Levels of microglial markers in the hippocampus of the brain are significantly higher in people with seizures than in those without. The hippocampus is a region of the brain involved in memory and learning and whose shrinkage is associated with epilepsy and neurodegenerative diseases such as Batten.
However, the mechanisms underlying the onset of seizures in Batten children remain largely unclear.
Now, a team of researchers from Xinxiang Medical University in China have found that activation of P2X7R and microglia may indeed contribute to seizures in CLN1 disease.
Using an infant Batten mouse model that wears R151Xthe most common pathogen CLN1 mutation, researchers first found that seizures begin at 7 months of age – a few months after the reported development of significant motor deficits.
Moreover, the number of microglia and the levels of CD68, a marker of activated microglia, in the hippocampus were stable between the first and the sixth month of life of the animals, but showed a significant increase at 7 months.
This increase in microglia activation at the time of seizure onset was accompanied by significantly impaired inhibitory or suppressive neuronal signaling and a significant increase in neuronal death in the hippocampus. Notably, the loss of inhibitory or input neurons is associated with Batten disease and epilepsy.
No significant changes in these cells or markers were observed with age in the hippocampus of healthy mice.
At 7 months of age, the hippocampus of mice with childhood Batten-like disease also showed significantly higher levels of pro-inflammatory ATP and TNF-alpha compared to healthy mice. ATP levels were even higher when 7-month-old mice experienced seizures.
Additionally, treatment with A-438079, a P2X7R suppressor, significantly reduced the number and duration of seizures in mice with childhood Batten-like disease.
These results highlight the role of P2X7R and microglia activation in the development of seizures in CLN1 disease.
Interestingly, the pronounced activation of hippocampal astrocytes occurred earlier in time, with levels of GFAP, a marker of activated astrocytes, being significantly increased from 4 months of age and with no major differences between months 6 and 7.
Although not consistent with the timing of seizure onset in mice with childhood Batten-like disease, “the early and dramatic increase in GFAP expression” observed in this and previous studies suggests that “astrocytes play a role in neuroinflammation and microglial activation…in response to neuronal damage and death,” the researchers wrote.
Based on these findings, the team hypothesized that the release of ATP from damaged neurons and/or activated astrocytes and its subsequent binding to P2X7R on the surface of microglia is essential for activating microglia and increase the production of pro-inflammatory molecules.
These molecules promote neuroinflammation, increasing the release of ATP and glutamate, the main “excitatory” chemical messenger in the brain, which in turn causes excessive neuronal activation and seizures.
Seizures lead to more nerve cell damage and ATP release, leading to an endless cycle of neuroinflammation, seizures, neuron damage, and death.
Thus, therapeutic approaches based on suppressing microglia activation, targeting either P2X7R or ATP, “may be effective for the treatment of [infantile Batten]“, wrote the team.