Enhanced Hypoxia Susceptibility in Hippocampal Slices From a Mouse Model of Rett Syndrome

Fischer M, Reuter J, Gerich FJ, Hildebrandt B, Hagele S, Katschinski D, Muller M. Enhanced hypoxia susceptibility in hippocampal slices from a mouse model of Rett syndrome. J Neurophysiol 101: 1016-1032, 2009. First published December 10, 2008; doi: 10.1152/jn.91124.2008. Rett syndrome is a neurodevelopmental disorder caused by mutations in the X-chromosomal MECP2 gene encoding for the transcriptional regulator methyl CpG binding protein 2 (MeCP2). Rett patients suffer from episodic respiratory irregularities and reduced arterial oxygen levels. To elucidate whether such intermittent hypoxic episodes induce adaptation/preconditioning of the hypoxia-vulnerable hippocampal network, we analyzed its responses to severe hypoxia in adult Rett mice. The occurrence of hypoxia-induced spreading depression (HSD)-an experimental model for ischemic stroke-was hastened in Mecp2(-/y) males. The extracellular K+ rise during HSD was attenuated in Mecp2(-/y) males and the input resistance of CA1 pyramidal neurons decreased less before HSD onset. CA1 pyramidal neurons were smaller and more densely packed, but the cell swelling during HSD was unaffected. The intrinsic optical signal and the propagation of HSD were similar among the different genotypes. Basal synaptic function was intact, but Mecp2(-/y) males showed reduced paired-pulse facilitation and higher field potential/fiber volley ratios, but no increased seizure susceptibility. Synaptic failure during hypoxia was complete in all genotypes and the final degree of posthypoxic synaptic recovery indistinguishable. Cellular ATP content was normal in Mecp2(-/y) males, but their hematocrit was increased as was HIF-1 alpha expression throughout the brain. This is the first study showing that in Rett syndrome, the susceptibility of telencephalic neuronal networks to hypoxia is increased; the underlying molecular mechanisms apparently involve disturbed K+ channel function. Such an increase in hypoxia susceptibility may potentially contribute to the vulnerability of male Rett patients who are either not viable or severely disabled.