A Fine Balance Between Excitation and Inhibition

Researchers have clarified, for the first time, the mechanism behind a very rare brain disorder called MICPCH (microcephaly, disproportionate pontine and cerebellar hypoplasia) syndrome in animal models. Information gleaned from this study could also inform research into other, more common neurological diseases such as mental retardation, epilepsy, and autism.

MICPCH has only affected a total of 53 females and seven males worldwide so far. It is characterized by several developmental symptoms including small head size, slowed growth, cognitive delays, epilepsy, seizures, vision and hearing problems, decreased muscle tone, and autism. MICPCH is linked to irregularities, or mutations, on the X-chromosome that eventually lead to the chromosome's inactivation.

The study was published in  the journal Molecular Psychiatry.

Brain cells, or neurons, constantly communicate by sending messages to one another. There are two types of neurons in the brain: those that increase activity in other cells (excitatory neurons) and those that decrease it (inhibitory neurons). The mechanism behind keeping the balance between excitation and inhibition in the brain is very similar to that of a thermostat that is used to maintain a balanced temperature in a home. This mechanism is important because imbalances between excitation and inhibition can cause several serious disorders such as epilepsy and autism.

One of the most important molecules that maintains the balance between excitation and inhibition is a protein found within the outer membrane of neurons, called the calcium/calmodulin-dependent serine protein kinase (CASK). Mutations in the gene that produce CASK therefore lead to several neurodevelopmental disorders such as mental retardation. A lack of the protein in the brain has been found to cause MICPCH syndrome.

"The aim of the study was to understand the pathophysiology of CASK-deficiency disorders in females, such as MICPCH syndrome, which are supposed to be influenced by X-chromosome inactivation," said corresponding author Katsuhiko Tabuchi, a professor in the Department of Molecular and Cellular Physiology at the Institute of Medicine, Academic Assembly at Shinshu University in Nagano, Japan.

However, the details of CASK-deficiency consequences have thus far been difficult to study, as mice that completely lack the protein die before they are developed enough to study.

In order to understand the mechanism behind the CASK-deficiency, researchers at Shinhsu University in Japan and Kafr Elsheikh University in Egypt have used gene manipulation techniques that shut off the CASK gene through X-chromosome inactivation in female mice without lethal consequences.

They found that neurons that lack CASK have a disrupted excitation and inhibition balance. They also found that this is because of a decrease in concentration of a specific receptor on the membrane that receives signals from other neurons. When the receptor concentration was increased, the excitatory and inhibitory balance was restored again, leading the researchers to believe that the receptor plays a central role in the mechanism in CASK-deficient neurons.

In the future, the researchers hope to address the effects of a CASK-deficiency in even greater detail by looking at its effects on the neural circuitry. "We hope to highlight the effect of two different types of neurons in one brain as well as the pathophysiology of CASK-deficiency disorders at neural circuit levels," professor Tabuchi adds.

This article has been republished from materials provided by Shinshu University. Note: material may have been edited for length and content. For further information, please contact the cited source.

Reference: Takuma Mori, Enas A. Kasem, Emi Suzuki-Kouyama, Xueshan Cao, Xue Li, Taiga Kurihara, Takeshi Uemura, Toru Yanagawa and Katsuhiko Tabuchi. 2019. Deficiency of calcium/calmodulin-dependent serine protein kinase disrupts the excitatory-inhibitory balance of synapses by down-regulating GluN2B. Molecular Psychiatry. DOI: http://dx.doi.org/10.1038/s41380-018-0338-4.