Astrocytes Contribute to Hyperexcitable Neurons in a Mouse Model of Rett Syndrome

Post by Deborah Joye

What's the science?

Rett syndrome is a rare X-linked neurological disorder that occurs in 1 in 10,000 - 15,000 children, resulting in mild to severely disabling symptoms including seizures and breathing, speech, and movement impairments. Rett syndrome is caused by loss-of-function mutations in the Mecp2 gene, which makes a protein that interacts with DNA to change how genes are expressed. Loss of Mecp2 in mice is associated with changes in brain cells that express the important inhibitory transmitter GABA. In healthy brain networks, GABA-expressing cells inhibit the excitability of neurons by releasing GABA when activated (called phasic inhibition) but also by maintaining levels of GABA in the extracellular space, creating a constant inhibitory signal known as tonic inhibition. But exactly how mutations in Mecp2 alter GABA transmission resulting in Rett syndrome symptoms is not entirely clear. This week in Journal of Neuroscience, Dong and colleagues demonstrate that a decrease in extracellular GABA in the hippocampus of a Rett syndrome mouse model reduces tonic inhibition resulting in neuronal hyperexcitability that could be targeted to slow down Rett syndrome disease progression.

How did they do it?

To compare how neural signaling might be different in Rett syndrome, the authors used a mouse model of the disease that had the Mecp2 gene knocked out and compared them to wildtype mice. To test whether tonic inhibition was altered in the absence of Mecp2, the authors used electrophysiology to measure electrical currents caused by ambient GABA in hippocampal neurons. Since altered GABA currents could be due to changes in extracellular GABA or changes in GABA receptor expression, the authors next recorded electrical currents in the presence of exogenous GABA (i.e. not produced in the brain), or with drugs that block or activate GABA receptors.

Changes in extracellular GABA could be the result of altered clearance of GABA from the extracellular space or differential release of GABA from synapses. The authors used electrophysiology to test whether neurons from Mecp2 knockout mice exhibited altered GABA release or clearance. To directly test if GABA transporter function was altered, the authors also included a condition with a specific blocker for GAT3, a GABA transporter that is preferentially expressed by astrocytes. To test whether changes in tonic GABA currents in Mecp2 knockout mice were the result of alterations to GABA transporter expression in astrocytes, the authors used a technique that specifically knocks out Mecp2 only in astrocytes and recorded electrical currents caused by tonic GABA. Finally to test whether inhibiting the astrocyte-specific GABA transporter could alleviate Rett syndrome-like symptoms in Mecp2 knockout mice the authors recorded electrical currents and behavioral measures of wildtype mice, Mecp2 knockout mice, and Mecp2 knockout mice in the presence of a GAT3 blocker.

What did they find?

The authors found that knockout of the Mecp2 gene results in reduced tonic GABA currents in hippocampal neurons. Interestingly, they found that this difference in tonic GABA currents is not present in young mice, only manifesting around 8 to 10 weeks of age. Next, the authors found that the decrease in tonic GABA currents was due to a reduction in extracellular GABA in Mecp2 knockout mice and not a difference in GABA receptor expression. The authors next demonstrate that enhanced activity of the astrocyte-specific GABA transporter GAT3 results in the reduced tonic GABA currents seen in Mecp2 knockout mice. This suggests that since the GABA transporter is clearing GABA from the extracellular space at a faster rate, there is less ambient GABA resulting in less tonic inhibition and more hyperexcitable neurons. Notably, this upregulation of GABA transport appears specific to GAT3, since the authors show that GAT1 is not altered. The authors also reveal decreases in tonic GABA current when Mecp2 is knocked out specifically in astrocytes, further suggesting that the increase in GABA clearance is due to increased uptake by astrocytes. Finally, the authors demonstrate that blockade of GAT3 function returns neuronal excitability to wildtype levels, decreases the severity of Rett syndrome-like symptoms, and lengthens survival. This suggests that targeting of the hyperexcitable phenotype by blocking increased GABA uptake by astrocytes may have therapeutic value in the treatment of Rett Syndrome

What's the impact?

This study provides a potential therapeutic mechanism for Rett syndrome by revealing that blockade of astrocyte-specific GAT3 can decrease the severity and slow the progression of Rett syndrome symptoms in mice. This important finding could have significant impacts on the clinical understanding of how Rett syndrome progresses and may result in better clinical outcomes for those living with this debilitating disease.

Dong et al., An Astrocytic Influence on Impaired Tonic Inhibition in Hippocampal CA1 Pyramidal Neurons in a Mouse Model of Rett Syndrome, Journal of Neuroscience (2020). Access the original scientific publication here.