Valium doesn’t calm nerves alone. It works with a partner, a transmembrane protein that sits beside the neurotransmitter receptor that valium targets. If the protein is absent, valium fails to boost the nerve-calming action of the receptor, the γ-aminobutyric acid type A (GABAA) receptor. This finding, which appeared October 10 in a paper published in Science, is significant because it could help researchers improve their understanding of sedatives. It could also help developers of neurological drugs recognize the importance of auxiliary proteins and their effects.
The Science paper, titled “Shisa7 is a GABAA receptor auxiliary subunit controlling benzodiazepine actions,” is the work of researchers based at the National Institute of Neurological Disorders and Stroke (NINDS). It describes how the NINDS team determined that a protein called Shisa7 helps regulate GABAA receptor expression and function.
“We found that Shisa7 plays a critical role in the regulation of inhibitory neural circuits and the sedative effects some benzodiazepines have on circuit activity,” said Wei Lu, PhD, a Stadtman Investigator at the NINDS and the senior author of the study. “We hope the results will help researchers design more effective treatments for a variety of neurological and neuropsychiatric disorders that are caused by problems with these circuits.”
Lu’s lab studies the genes and molecules used to control synapses—the relay points for the chemical and chemical signals that pass between neurons. In the current study, Lu and colleagues focused on the synapses that rely on GABAA receptors, ligand-gated anion channels that are of great physiological and clinical importance because they mediate fast inhibitory transmission in the mammalian brain. They are also, the scientists wrote, “thought to be determined by the channel pore–forming subunits.”
“Shisa7 controls receptor abundance at synapses and speeds up the channel deactivation kinetics,” detailed the authors of the Science paper. “Shisa7 also potently enhances the action of diazepam, a classic benzodiazepine, on GABAA receptors. Genetic deletion of Shisa7 selectively impairs GABAergic transmission and diminishes the effects of diazepam in mice.”
In 2004, Japanese researchers originally discovered that the Shisa gene played a role in the formation of frog heads and, named the gene after a mythological, large-headed, guardian figure depicted in statues throughout southern Japan. Initially, Shisa7 was thought to play a role in controlling a synapse that relies on the neurotransmitter glutamate to excite, rather than quiet, neurons.
Recent studies suggested that Shisa7 along with other Shisa genes encodes proteins that adhere to glutamate receptors. Once attached, these “auxiliary” proteins can control a receptor’s response to glutamate or its presence at synapses. Then, a few years ago, Lu’s team noticed something interesting in a scientific article on the Shisa proteins.
“We found the results striking,” said Lu. “The paper showed that Shisa7 was the only protein in this family that seemed to have no effect on the activity of an important type of glutamate receptor.”
To take a closer look, Lu and colleagues systematically examined Shisa proteins in mouse neurons. To their surprise, they found that Shisa7 appeared to play a unique and critical role in the nerve quieting GABA synapses.
The researchers used advanced microscopic techniques to spot Shisa7 tightly clustered with GABAA receptors at synapses. Genetically eliminating Shisa7 from neurons reduced the number of GABAA receptors and decreased the strength of electrical currents generated by synaptic GABAA receptor responses.
Further experiments suggested that Shisa7 proteins attached directly to GABAA receptors. Electrical recordings showed that Shisa7 hastened receptor responses to the transmitter GABA and nearly doubled the size of responses made in the presence of Valium (diazepam), suggesting the protein made the receptor more sensitive to benzodiazepines.
“These results suggest that Shisa7 directly shapes inhibitory synaptic responses under a variety of conditions, including the presence of benzodiazepines,” indicated Chris J. McBain, PhD, a co-author of the current study and a researcher at the National Institute of Child Health and Human Development.
Finally, experiments in mice supported the idea that Shisa7 also plays a role in the calming effects of benzodiazepines. For instance, in one set, the investigators tested the ability of diazepam to reduce the high anxiety mice felt when confronted with open, elevated spaces.
Here, the mice were placed in the middle of an elevated maze of two crisscrossing arms. One arm was covered and the other open. In agreement with previous studies, the researchers saw that injections of diazepam increased the time the wild-type mice chose to walk on open arms, suggesting the drug reduced anxiety. In contrast, diazepam had no effect on mice that were engineered to have no Shisa7 gene. These mice spent the same amount of time exploring the open arms regardless of whether they received the drug or a placebo.
In other experiments, the researchers found that Shisa7 also influenced the drowsiness and hypnotic effects of benzodiazepines. Mice that lacked Shisa7 were much less likely than wild-type mice to fall asleep from high levels of diazepam. Moreover, the mutant mice were dramatically better at standing up after diazepam-induced stumbles, in fact, some appeared resistant to stumbling.
“Our results shine a spotlight on the potential clinical importance of auxiliary proteins like Shisa7,” declared Lu. “Many of the neurological drugs we use today are designed to control the activity of synaptic receptors. For the first time, we show that researchers may also want to consider auxiliary proteins like Shisa7 in developing new treatments that target GABAA receptors.”