An investigation into the electrophysiological characteristics of neurons in the Mecp2 mosaic hippocampus

Rett syndrome (RTT) is an X-linked neurological disorder which is caused by sporadic mutations in the gene coding for methyl-CpG-binding protein-2 (MeCP2) and which predominantly affects females (Neul et al., 2010). MeCP2 is a nuclear protein that is abundant in neurons yet its exact function remains obscure. Although RTT is conventionally described as a ‘neurodevelopmental’ disorder there is increasing evidence that MeCP2 has more of a maintenance function (Guy et al., 2011, Nguyen et al., 2012). Knockout of Mecp2 in fully adult mice results in similar symptoms to the germline RTT model even though the animals have otherwise developed normally (McGraw et al., 2011, Nguyen et al., 2012). In RTT females, X-chromosome inactivation leads to a MeCP2 mosaic network of cells with each cell either expressing functional MeCP2 (MeCP2+ve) or not expressing the functional protein (MeCP2-ve). Previous studies using RTT mouse models have largely focussed on the properties of male Mecp2-null neurons whereas the electrophysiological properties of the female Mecp2-/+ mosaic hippocampus had not been fully characterised. We aimed to assess whether the absence of MeCP2 affects intrinsic and synaptic properties of CA1 hippocampal pyramidal neurons in the context of the Mecp2 mosaic hippocampus. Acute hippocampal slices were prepared from Mecp2stop/+ and WT mice. Whole-cell patch clamp recordings made from visually identified CA1 pyramidal cells were used to assess intrinsic cellular and synaptic properties. MeCP2 status was confirmed post hoc by immunocytochemistry and confocal fluorescence microscopy. A subset of mice expressing eGFP-tagged MeCP2 was also used in which eGFP indicated the MeCP2 status of recorded neurons. Analysis of a range of active and passive electrophysiological parameters (e.g. Resting membrane potential, input resistance, membrane time constant, action potential threshold, firing rate and action potential kinetics) revealed no significant differences in intrinsic properties between MeCP2-ve and MeCP2+ve cells from heterozygous mice, or between pyramidal neurons from Mecp2stop/+ and WT mice (one-way ANOVA all P>0.05). Similarly, no significant differences were observed with synaptic properties or short term synaptic plasticity (paired-pulse facilitation and frequency facilitation). The same was true when comparing intrinsic and synaptic grouped data (WT+ MeCP2+ve vs MeCP2-ve cells and WT vs MeCP2+ve + MeCP2-ve cells) suggesting there are no cell-autonomous or non-cell-autonomous effects in respect of the particular measured electrophysiological properties in the cohorts of mice tested. These data suggest that lack of MeCP2 in mice which do not display any overt RTT symptoms does not affect the salient biophysical properties of individual pyramidal cells or generate significant differences in postsynaptic properties of evoked post synaptic currents in the Mecp2 mosaic network. Clearly, the presynaptic MeCP2 status, which is unknown, will also influence responses. Given the complexity of synapses with differing combinations of pre- and postsynaptic MeCP2 status, further studies are required to address this question. In contrast to the well-characterised alterations in network long-term plasticity, the whole-cell electrophysiological properties of MeCP2+ and MeCP2- neurons had not previously been established. This study addresses that knowledge gap and provides a baseline for future investigations