Scientists reveal structural secrets of antiepileptic drugs

RIKEN researchers have discovered how drugs used to treat epilepsy can interact, based on their structure, with a key protein found in synapses at the junctions of neurons. This knowledge could help develop even better drugs for the disease.

Epilepsy is a brain disorder that causes recurring seizures and can occur without warning. It is estimated that between 0.5% and 1% of the population are affected.

Fortunately, effective medications are available to treat the neurological disease, including levetiracetam and brivaracetam. But no one knows exactly how these drugs work.

Both drugs target a protein found in the small, membrane-bound sacs called synaptic vesicles that store and release neurotransmitters at the ends of neurons. This protein is known as synaptic vesicle glycoprotein 2A (SV2A).

“The exact function of SV2A is unknown, although it must play a key role in synaptic transmission,” says Atsushi Yamagata from the RIKEN Center for Biosystems Dynamics Research.

Interestingly, SV2A is also a receptor for the highly toxic botulinum neurotoxin. Botulinum neurotoxin is one of the deadliest substances known and can kill adults in concentrations as low as a few hundred nanograms. However, it is also used for a variety of cosmetic and therapeutic purposes, including the treatment of neuromuscular diseases, chronic pain and gastrointestinal diseases.

Now, Yamagata and his collaborators have used cryo-electron microscopy to determine the structures of levetiracetam and brivaracetam when bound to SV2A, both when botulinum neurotoxin is present and not (see image above). The work will be published in the journal Nature communication.

While the exact function of SV2A is still unclear, the results suggest that SV2A may function as a membrane transporter and that levetiracetam and brivaracetam may inhibit this function.

Understanding the structure “provides indirect evidence that SV2A functions as a membrane transporter,” notes Yamagata. “This function could be very important for understanding epilepsy caused by genetic mutations of SV2A.”

Obtaining a high-quality structure of SV2A using cryo-electron microscopy is challenging because the protein is quite small, but botulinum neurotoxin offered an unexpected advantage in this regard.

“Fortunately for us, when we attached a botulinum neurotoxin receptor binding domain to SV2A, we were able to use it as a marker for image analysis using cryo-electron microscopy,” explains Yamagata. “This allowed us to align the images of the SV2A particles more precisely and determine the structure using high-resolution cryo-electron microscopy.”

Yamagata and his team plan to expand this work. “We are now trying to determine the structure of SV2A in complex with full-length botulinum toxin. This could help us understand how the neurotoxin works in synapses,” says Yamagata. “We also hope to elucidate the exact function of SV2A, although this is very challenging.”