Abstracts

Rationale: Rett syndrome (RTT) associated with mutations in Methyl-CpG-binding protein 2 (Mecp2) is linked to diverse neurological symptoms such as seizures, motor disabilities, and cognitive impairments. Recent studies on MeCP2 mutant mice have reported developmental dysfunction specific to GABAergic interneurons with significant deficits in presynaptic function and GABA immunoreactivity. A dysfunctional GABAergic system has been proposed as one of the many underlying mechanisms of the progressive neurodegenerative pathology. Methods: This study investigated the temporal- and location-specific alterations in the expression of GABA transporter 1 (GAT1), vesicular GABA transporter (vGAT), and glutamic acid decarboxylase 67 (GAD67) in wild type (WT), heterozygous (HET), and knockout (KO) mice in the Mecp2tm1.1Bird/+ mouse model of RTT. Immunohistochemistry co-labeling of GAT1 with vGAT identified GABAergic synapses and GAD67 with MeCP2 identified GABAergic interneurons that was quantitated for mid-sagittal sections in the frontal cortex (PrL) and hippocampal dentate gyrus (DG) at ages of 2, 7 and 14 weeks. Results: WT mice showed an age-dependent increase in the expression of co-localized synaptic transporters GAT1 and vGAT that was detected in the PrL (2 vs. 7 weeks; p=0.023 and 2 vs. 14 weeks; p=0.038) and DG (2 vs. 7 weeks; and p= 0.032 and 2 vs. 14 weeks; p= 0.008). Although HET and KO mice also showed a similar age-dependent increase in the PrL, there was a significant reduction in both GAT1 and vGAT expression in the PrL at all ages (p<0.05). In the DG the mutant mice were not significantly different from WT. GAD67-positive cell densities also showed an age-dependent increase in the frontal cortex but paradoxically a decrease in the DG in WT mice. These densities were not significantly different in the mutant mice. An associated significant increase in the number of lipofuscin deposits was detected in the forebrains of KO mice which may imply an accelerated aging process (p=0.035). Conclusions: Significantly lower expression levels of GAT1 and vGAT in the PrL may indicate deficiency in GABA recycling in the MeCP2 mutants, which may underlie the susceptibility to seizures in RTT. However, this impairment was location-specific in this study. Similar densities of GAD67-positive interneurons in the frontal cortices of WT and mutant mice may highlight this GABAergic pathophysiology to be synaptic.