Maintenance of Neuron Activity by Homeostatic Alterations in Receptors and Ion Channels in a Rett Syndrome Mouse Model

Rett Syndrome (RTT) is a developmental disorder that affects numerous neuronal systems that underlie problems with breathing, movement, cognition and sleep. RTT is caused by mutations in the methyl-CpG-binding protein 2 (Mecp2) gene. MeCP2 is a ubiquitous protein that is found in all mature neurons and binds to methylated DNA to repress transcription; thus regulating protein expression levels in neurons. The mutations in Mecp2 affect a large number of proteins that are crucial for regulating neuronal activity. Despite the abnormal expression of many of these proteins, mice with a total loss of MeCP2 can live to adulthood and some people with RTT can live to a very late age as well. It is possible that mutations in the Mecp2 gene not only cause widespread defects, but also elicit neuroadaptive processes that may limit the impact of the MeCP2 dysfunction. To test this hypothesis we performed these studies in which we focused on how synaptic and membrane currents were altered to maintain normal neuronal activity in Mecp2-null mice. We show two examples from different neurons where neuroadaptations of ion channel expression allowed the neuron to remain viable. First, the properties of the nicotinic acetylcholine receptor (nAChR) current were altered in LC neurons in Mecp2-null mice. This was caused by changes in the nicotinic receptor subunit expression. Despite the changes in the nAChR current, the cholinergic modulation of LC neuron activity in WT and Mecp2-null mice were similar. Secondly, we show that the fast Na+ voltage-gated and the hyperpolarization-activated currents were altered in mesencephalic trigeminal V (Me5) propriosensory neurons. The changes in the hyperpolarization-activated current caused a smaller sag and post-inhibitory rebound. Opposite to what we expected, these cells were hyperexcitable. The hyperexcitability was due to changes in the fast Na+ voltage-gated current causing a decreased action potential threshold. Alterations in the ionic currents in Me5 neurons seem to be due to changes in subunit expression patterns. These results indicate that despite the complications caused by defects in the Mecp2 gene, neurons respond by rearranging receptor / ion channel expression. This reorganization allows neurons to remain viable despite the MeCP2 deficiency