Neurons may remain immature concerning GABAergic neurotransmission in Rett syndrome

Abnormal expression of cation chloride cotransporters has been found in the cerebrospinal fluid of patients with Rett syndrome (RTT). This finding suggest an abnormal GABaergic transmission and neuronal maturation.  According to these results, neurons of RTT patients could remain “immature” regarding GABA excitability, as it happens in the physiological early developmental stages (Duarte et al, Plos One).

Reference: Duarte et al. Abnormal expression of cerebrospinal fluid cation chloride cotransporters in patients with Rett syndrome. PLoS One. 2013 Jul 19;8(7):e68851.

The methyl-CpG-binding domain of MecP2 in complex with methylated DNA

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder with an incidence of 1:10000 live female births and is one of the leading causes of mental retardation and autistic behavior in females. Loss-of-function mutations in the gene encoding methyl-CpG binding protein 2 (MeCP2) cause most cases of RTT. Individuals affected with RTT experience normal development up to the age of 6–18 months, at which time they fail to acquire new skills and enter a period of motor regression. Autistic features are a hallmark of this disorder and epilepsy is frequent. RTT patient brain does not show obvious signs of neurodegeneration, atrophy, gliosis, demyelination, or neuronal migration defects, suggesting that neurological symptoms may primarily stem from subtle defects of subcellular compartments such as dendrites, axons, or synaptic structures.

The relevance of MeCP2 for GABAergic function was previously documented in animal models. In these models, animals show deficits in brain-derived neurotrophic factor, which is thought to contribute to the pathogenesis of this disease. Neuronal Cation Chloride Cotransporters (CCCs) play a key role in GABAergic neuronal maturation, and brain-derived neurotrophic factor is implicated in the regulation of CCCs expression during development.

GABAA receptors stimulate immature neurons and inhibit mature neurons. A) In immature neurons, intracellular Cl– concentrations are higher than in mature neurons. This is due to low expression of the CCC exporter type 2 (KCC2) and high expression of the  CCC importer type 1 (NKCC1). Activation of GABAA receptors causes Cl– flux out of the cell, which makes the membrane potential more positive, leading to activation of Ca2+ channels. B) In mature neurons, intracellular Cl– concentrations are low. This is a due to high expression of KCC2 and low expression of a NKCC1. Activation of GABAA receptors causes Cl– flux into the cell, which makes the membrane potential more negative. Ca2+ channels are not activated under these conditions.

This study reports the results of the expression of the two relevant CCCs, NKCC1 and KCC2, in the cerebrospinal fluid of Rett syndrome patients and compare it with a normal control group. The presence of bumetanide sensitive NKCC1 and KCC2 was analysed in cerebrospinal fluid samples from a control pediatric population (1 day to 14 years of life) and from Rett syndrome patients (2 to 19 years of life), by immunoblot analysis. Both proteins were detected in the cerebrospinal fluid and their levels are higher in the early postnatal period. However, Rett syndrome patients showed significantly reduced levels of KCC2 and KCC2/NKCC1 ratio when compared to the control group.

This study describes a significant decrease of KCC2 in the cerebrospinal fluid of Rett patients. A major advantage of doing these in vivo studies in children with severe neurologic disorders like RTT, is that it allows to search for disturbances in the normal developmental pattern. Therefore, these findings might have implications for the understanding of RTT pathophysiology, considering that KCC2 is a neuronal specific protein with a key role for neuronal electrical function and structure, properties that are known to be altered in Mecp2 mutated neurons. Moreover, these results could bring light to new therapeutic approaches, particularly through the pharmacological manipulation of the cation chloride cotransporers. Further studies in the MECP2 knockout model and other models to study the disease process are needed to explore these possibilities.