Adapting to epilepsy

AP-3 is a member of the adaptor protein (AP) complex family, which regulates formation of clathrin-coated vesicles and intracellular trafficking of membrane proteins. Although the function of the ubiquitously expressed AP-3A has been elucidated, that of the neuron-specific AP-3B remains unknown. Reporting in Journal of Cell Biology, Nakatsu and colleagues show that AP-3B is important in the regulation of GABA (γ-aminobutyric acid) release and might underlie the pathogenesis of epilepsy.

The authors found that mice that lack μ3B, a subunit of AP-3B, have no abnormalities in their overall brain structure. However, these animals showed spontaneous epileptic seizures when presented with a stimulus such as positional change. When intravenous infusion of pentylenetetrazole (a GABA_A receptor antagonist) and electrical kindling were used to trigger seizures, a much lower level of stimulus was required to induce seizures in μ3B-knockout mice compared to their wild-type counterparts, indicating that the mutant animals have higher susceptibility to seizures.

Although the brain morphology of μ3B-knockout mice is basically normal, the number of synaptic vesicles per unit area is lower in the hippocampus, and the diameter of the synaptic vesicles in inhibitory synaptic terminals is also smaller. These observations prompted the authors to test synaptic function and neurotransmitter release in mutant mice. They found that the basal release of glutamate and GABA in hippocampal mini-slices was normal in the mutant mice, but that the K⁺-evoked release of GABA, but not of glutamate, was significantly reduced.

As the amounts of these neurotransmitters in the hippocampus are comparable between wild-type and mutant mice, the authors suspected that it might be the trafficking rather than metabolism of GABA that was responsible. As expected, the amount of vesicular GABA transporter (VGAT) was lower than normal in synaptosomal lysates from the hippocampus of μ3B-knockout mice, whereas the concentrations of vesicle glutamate transporters and other synaptic vesicle proteins were normal.

If the inhibitory pathway is weaker due to reduced amounts of VGAT and GABA release on stimulation, are neurons more excitable in μ3B-knockout mice? The authors found that long-term potentiation (LTP) induced by standard conditioning was intact in the mutant mice. When weak conditioning was applied, LTP was induced in the mutant but not wild-type mice. This difference disappeared when the GABA_A antagonist picrotoxin was present, indicating that weaker stimulation can induce LTP in μ3B-knockout mice, because the inhibition is weaker.

This is an interesting finding that assigns a new function to AP-3B and adds another aspect to the pathogenesis of epilepsy. μ3B-knockout mice might serve as a novel animal model of this disorder, which affects millions of people worldwide.