Brain-derived neurotrophic factor expression and respiratory function improve after ampakine treatment in a mouse model of Rett syndrome - PubMed (original) (raw)

Brain-derived neurotrophic factor expression and respiratory function improve after ampakine treatment in a mouse model of Rett syndrome

Michael Ogier et al. J Neurosci. 2007.

Abstract

Rett syndrome (RTT) is caused by loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). Although MeCP2 is thought to act as a transcriptional repressor of brain-derived neurotrophic factor (BDNF), Mecp2 null mice, which develop an RTT-like phenotype, exhibit progressive deficits in BDNF expression. These deficits are particularly significant in the brainstem and nodose cranial sensory ganglia (NGs), structures critical for cardiorespiratory homeostasis, and may be linked to the severe respiratory abnormalities characteristic of RTT. Therefore, the present study used Mecp2 null mice to further define the role of MeCP2 in regulation of BDNF expression and neural function, focusing on NG neurons and respiratory control. We find that mutant neurons express significantly lower levels of BDNF than wild-type cells in vitro, as in vivo, under both depolarizing and nondepolarizing conditions. However, BDNF levels in mutant NG cells can be increased by chronic depolarization in vitro or by treatment of Mecp2 null mice with CX546, an ampakine drug that facilitates activation of glutamatergic AMPA receptors. Ampakine-treated Mecp2 null mice also exhibit marked functional improvement, characterized by restoration of normal breathing frequency and minute volume. These data demonstrate that BDNF expression remains plastic in Mecp2 null mice and raise the possibility that ampakine compounds could be of therapeutic value in the treatment of RTT.

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Figures

Figure 1.

Figure 1.

Mecp2 null mutation is associated with decreased expression of specific Bdnf transcripts in nodose neurons. Bdnf transcript levels in intact NG from wild-type and Mecp2 null mice were determined using qRT-PCR. The Bdnf gene has a complex structure in which multiple promoters drive the expression of different mRNA isoforms containing alternative noncoding 5′ exons spliced to a common downstream coding exon [exon 8; nomenclature of Liu et al. (2006)]. Total Bdnf mRNA levels (Exon 8), as well as transcripts containing exons 2, 4, and 5 were markedly decreased in mutant NG compared with wild type, whereas transcripts containing exon 1 were expressed at levels that were not significantly different from wild type. Results are the mean ± SEM (n = 4). ***p < 0.001, ANOVA I with post hoc Tukey's test.

Figure 2.

Figure 2.

MeCP2 protein is expressed in nodose neurons. Left, Double immunostaining for MeCP2 (green) and β-tubulin III (red) in the newborn wild-type (Mecp2 +/y) mouse NG. nX, Vagal nerve. Right, Higher magnification of the same section shown on the left, illustrating the concentration of MeCP2-immunoreactive protein in heterochromatin foci. The inset shows that the MeCP2 antibody used in these studies does not produce any specific staining in the NG from a Mecp2 null mouse (Mecp2 −/y). The asterisk represents an anatomical landmark shared by both panels.

Figure 3.

Figure 3.

BDNF levels are depressed in P35 Mecp2 −/y NG neurons under resting and depdarizing conditions. A, C, Summary data showing that BDNF content is decreased by 40–50% in NG cultures from Mecp2 null mutants, regardless of the activity state of the cells [i.e., electrically silent (A; treated with TTX) or chronic depolarization (C, treated with KCl)]. Results show that KCl treatment can increase the BDNF level in mutant cells as in wild-type controls. B, D, Neuron survival was unaffected by either TTX (B) or KCl (D). Results are the mean ± SEM (n = 6). **p < 0.01, ANOVA I with post hoc Tukey's test.

Figure 4.

Figure 4.

Mecp2 null mice exhibit a Rett-like respiratory phenotype at 5 weeks of age (P35). Representative plethysmographic recordings from wild-type (Mecp2 +/y) and Mecp2 null (Mecp2 −/y) mice are shown. Each trace is 10 s quiet breathing in room air. The bottom graphs are frequency histograms from control (compilation of 9776 breath cycles) and mutant (compilation of 6065 breath cycles) mice showing the higher incidence of fast breaths in mutant mice compared with controls, along with a shift to higher values of minute volume/weight. BPM, Breaths per minute; MV, minute volume.

Figure 5.

Figure 5.

Chronic treatment with CX546 restores normal breathing frequency and minute volume/weight in P35 Mecp2 null mice. A, B, Representative histograms of breathing frequency (A) and minute volume/weight (B) from two mutant mice, one treated with vehicle (9227 breath cycles) and one treated with CX546 (8393 breath cycles), showing that drug treatment (40 mg/kg, b.i.d for 3 d) decreases episodes of high breathing frequency and minute volume/weight. C, D, Summary data for breathing frequency (C) and minute volume/weight (D) for all animals. Ampakine treatment completely restores wild-type frequency and minute volume/weight in mutant animals and has no effect in wild types. Results are the mean ± SEM (n = 8 for vehicle-treated wild types; n = 7 for CX546-treated wild types; n = 8 for vehicle-treated mutants; n = 9 for CX546-treated mutants). *p < 0.05; **p < 0.01, ANOVA I with post hoc Tukey's test. BPM, Breaths per minute.

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