Alterations in the cholinergic system of brain stem neurons in a mouse model of Rett syndrome

Abstract

Rett syndrome is an autism-spectrum disorder resulting from mutations to the X-linked gene, methyl-CpG binding protein 2 (MeCP2), which causes abnormalities in many systems. It is possible that the body may develop certain compensatory mechanisms to alleviate the abnormalities. The norepinephrine system originating mainly in the locus coeruleus (LC) is defective in Rett syndrome and Mecp2-null mice. LC neurons are subject to modulation by GABA, glutamate, and acetylcholine (ACh), providing an ideal system to test the compensatory hypothesis. Here we show evidence for potential compensatory modulation of LC neurons by post- and presynaptic ACh inputs. We found that the postsynaptic currents of nicotinic ACh receptors (nAChR) were smaller in amplitude and longer in decay time in the Mecp2-null mice than in the wild type. Single-cell PCR analysis showed a decrease in the expression of α3-, α4-, α7-, and β3-subunits and an increase in the α5- and α6-subunits in the mutant mice. The α5-subunit was present in many of the LC neurons with slow-decay nAChR currents. The nicotinic modulation of spontaneous GABAA-ergic inhibitory postsynaptic currents in LC neurons was enhanced in Mecp2-null mice. In contrast, the nAChR manipulation of glutamatergic input to LC neurons was unaffected in both groups of mice. Our current-clamp studies showed that the modulation of LC neurons by ACh input was reduced moderately in Mecp2-null mice, despite the major decrease in nAChR currents, suggesting possible compensatory processes may take place, thus reducing the defects to a lesser extent in LC neurons.

Methyl CpG Binding Protein 2 Gene Disruption Augments Tonic Currents of γ-Aminobutyric Acid Receptors in Locus Coeruleus Neurons

“…Regarding the increase in ␣6 transcript level, it is possible that deletion of Mecp2 leads to reorganization of the GABA receptor species in LC neurons by increasing ␣6 subunit expression and reducing the expression of other ␣ subunits. A similar reorganization has been found in nicotinic ACh receptors in Mecp2-null LC neurons (54). The increased ␣6 subunits may assemble the extrasynaptic receptors together with the ␦ subunit as well as synaptic GABA A Rs with ␤ and ␥ subunits.…”

Total RNA Sequencing of Rett Syndrome Autopsy Samples Identifies the M4 Muscarinic Receptor as a Novel Therapeutic Target

“…In this regard, one salient finding of our experiments was that four of the five muscarinic acetylcholine receptors (mAChRs) exhibited significantly disrupted expression in at least one brain region (CHRM1, CHRM2, CHRM3, and CHRM4). Previously, cholinergic dysfunction was associated with altered brainstem activity in Mecp2 2/y mice (Oginsky et al, 2014), and more recently has been linked to cognitive and social phenotypes in choline acetyltransferase-Mecp2 knockout mice (Zhang et al, 2016). Importantly, administration of the nonselective mAChR agonist xanomeline improved these same symptom domains in clinical trials for both Alzheimer's disease and schizophrenia (Bodick et al, 1997;Shekhar et al, 2008).…”

Disturbance of cardiac gene expression and cardiomyocyte structure predisposes Mecp2-null mice to arrhythmias

“…On the other hand, we cannot rule out the possibility that alteration of cardiac genes expression and myocardial structures could be related to multiple factors including autonomic dysfunction, mild hypoxia, mitochondrial abnormality, and oxidative stress35424344. Hypoxia predisposes neonatal mice to arrhythmia and sudden death, in association with ECG abnormalities45.…”