Exogenous brain-derived neurotrophic factor rescues synaptic dysfunction in Mecp2-null mice - PubMed (original) (raw)

Exogenous brain-derived neurotrophic factor rescues synaptic dysfunction in Mecp2-null mice

David D Kline et al. J Neurosci. 2010.

Abstract

Postnatal deficits in brain-derived neurotrophic factor (BDNF) are thought to contribute to pathogenesis of Rett syndrome (RTT), a progressive neurodevelopmental disorder caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). In Mecp2-null mice, a model of RTT, BDNF deficits are most pronounced in structures important for autonomic and respiratory control, functions that are severely affected in RTT patients. However, relatively little is known about how these deficits affect neuronal function or how they may be linked to specific RTT endophenotypes. To approach these issues, we analyzed synaptic function in the brainstem nucleus tractus solitarius (nTS), the principal site for integration of primary visceral afferent inputs to central autonomic pathways and a region in which we found markedly reduced levels of BDNF in Mecp2 mutants. Our results demonstrate that the amplitude of spontaneous miniature and evoked EPSCs in nTS neurons is significantly increased in Mecp2-null mice and, accordingly, that mutant cells are more likely than wild- type cells to fire action potentials in response to primary afferent stimulation. These changes occur without any increase in intrinsic neuronal excitability and are unaffected by blockade of inhibitory GABA currents. However, this synaptopathy is associated with decreased BDNF availability in the primary afferent pathway and can be rescued by application of exogenous BDNF. On the basis of these findings, we hypothesize that altered sensory gating in nTS contributes to cardiorespiratory instability in RTT and that nTS is a site at which restoration of normal BDNF signaling could help reestablish normal homeostatic controls.

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Figures

Figure 1.

Figure 1.

BDNF staining intensity in medullary cell groups is markedly decreased in P35 _Mecp2_-null mice compared to wild-type controls. A1, Representative photomicrograph showing the distribution of tyrosine hydroxylase (TH) protein in the nucleus tractus solitarius region of Mecp2 wild-type mice (Mecp2+/y). A2, A3, BDNF staining in the nucleus tractus solitarius region of Mecp2+/y and null (_Mecp2_−/y) mice, respectively. B1, Cresyl violet staining in the nucleus ambiguus region of Mecp2+/y mice. B2, B3, BDNF staining in the nucleus ambiguus region of Mecp2+/y and null (_Mecp2_−/y) mice, respectively. C1, TH protein staining in A1/C1 region of Mecp2 wild-type mice (Mecp2+/y). C2, C3, BDNF staining in the A1/C1 region of Mecp2+/y and null (_Mecp2_−/y) mice, respectively. A1/C1, A1/C1 catecholaminergic cell group; AP, area postrema; cc, central canal; DMNX, dorsal motor nucleus of the vagus nerve; NA, nucleus ambiguus; RVL, rostroventrolateral reticular nucleus. Scale bar, 200 μm.

Figure 2.

Figure 2.

TrkB-positive neurons in the mnTS receive input from BDNF-containing TS fibers. A, Representative picture demonstrating presence of BDNF protein (green) in axons of the TS in the mnTS region of P35 wild-type mice. B, Confocal images of a TrkB-immunoreactive neuron (red) surrounded by BDNF-positive varicosities (green; arrowheads) in the mnTS region. C, DiA-labeled TS axons and varicosities (green) are not immunopositive for TrkB protein (red). Scale bars: 40 μm (A), 10 μm (B), 20 μm (C).

Figure 3.

Figure 3.

_Mecp2_-null mice exhibit enhanced solitary tract evoked EPSCs. A, B, Representative tracings of TS evoked EPSCs that were recorded from a wild-type (Mecp2+/y, left) and mutant (Mecp2_−/y, right) nTS cell. The TS was stimulated at 0.5 Hz (A, overlay of 20 traces) or 20 Hz (B, average of 5 overlaying traces). C, Average amplitude for each TS-EPSC stimulated at 20 Hz. *p < 0.05, two-way RM ANOVA. Note that TS-EPSC amplitude is higher in Mecp2_−/y mice and frequency-dependent depression is observed.

Figure 4.

Figure 4.

Enhanced EPSCs in _Mecp2_−/y mice are not due to reduced inhibitory shunting. A, Application of 10 μ

m

bicuculline did not alter EPSCs amplitude evoked at 20 Hz in Mecp2+/y mice. Average amplitude for each TS-EPSC stimulated at 20 Hz for aCSF (open diamond) versus bicuculline (closed square). B, Bicuculline (10 μ

m

) did not alter EPSCs amplitude evoked at 20 Hz in _Mecp2_−/y mice. Average amplitude for each TS-EPSC stimulated at 20 Hz for aCSF (open diamond) versus bicuculline (closed square).

Figure 5.

Figure 5.

Miniature EPSCs are enhanced in null mice. A, Representative tracings of mEPSCs from a wild-type (top) and null (bottom) mouse nTS cell. Note the increased frequency and amplitude of mEPSCs in null mice. The cell was voltage clamped at −60 mV. B, The cumulative fraction of mEPSC amplitudes (2 pA bin) illustrates a rightward shift in the distribution in null mice. C, The cumulative probability of mEPSC interevent interval distribution (10 ms bin) revealed a significant shift to the left (increased frequency) in null mice.

Figure 6.

Figure 6.

Exogenous BDNF reduces exaggerated postsynaptic responses in null mice. A, Representative raw trace of evoked TS-EPSCs recorded from a Mecp2_−/y_-null cell during aCSF (left) and following exogenous BDNF (100 ng/ml, 15 min, right). B, Average amplitude for each TS-EPSC stimulated at 20 Hz for aCSF (open square) versus BDNF (closed triangle). *p < 0.05, two-way ANOVA. C, TS-EPSC amplitudes recorded at 0.1 Hz in a null nTS cell in the presence of aCSF, BDNF, and wash out. D, Representative example of TS-evoked discharge in a null nTS cell during aCSF and BDNF (overlap of 5 traces each). Note the decrease in discharge following BDNF. E, In the presence of BDNF, the cumulative fraction of null mEPSC amplitudes (left, 2 pA bin) shifted to the left, while interevent interval distribution (right, 10 ms bin) shifted to the right (decreased frequency).

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