The free radical scavenger Trolox dampens neuronal hyperexcitability, reinstates synaptic plasticity, and improves hypoxia tolerance in a mouse model of Rett syndrome

Abstract

Rett syndrome (RS) causes severe cognitive impairment, loss of speech, epilepsy, and breathing disturbances with intermittent hypoxia. Also mitochondria are affected; a subunit of respiratory complex III is dysregulated, the inner mitochondrial membrane is leaking protons, and brain ATP levels seem reduced. Our recent assessment of mitochondrial function in MeCP2 (methyl-CpG-binding protein 2)-deficient mouse (Mecp2-/y) hippocampus confirmed early metabolic alterations, an increased oxidative burden, and a more vulnerable cellular redox balance. As these changes may contribute to the manifestation of symptoms and disease progression, we now evaluated whether free radical scavengers are capable of improving neuronal and mitochondrial function in RS. Acute hippocampal slices of adult mice were incubated with the vitamin E derivative Trolox for 3–5 h. In Mecp2-/y slices this treatment dampened neuronal hyperexcitability, improved synaptic short-term plasticity, and fully restored synaptic long-term potentiation (LTP). Furthermore, Trolox specifically attenuated the increased hypoxia susceptibility of Mecp2-/y slices. Also, the anticonvulsive effects of Trolox were assessed, but the severity of 4-aminopyridine provoked seizure-like discharges was not significantly affected. Adverse side effects of Trolox on mitochondria can be excluded, but clear indications for an improvement of mitochondrial function were not found. Since several ion-channels and neurotransmitter receptors are redox modulated, the mitochondrial alterations and the associated oxidative burden may contribute to the neuronal dysfunction in RS. We confirmed in Mecp2-/y hippocampus that Trolox dampens neuronal hyperexcitability, reinstates synaptic plasticity, and improves the hypoxia tolerance. Therefore, radical scavengers are promising compounds for the treatment of neuronal dysfunction in RS and deserve further detailed evaluation.

Excitatory synapses are stronger in the hippocampus of Rett syndrome mice due to altered synaptic trafficking of AMPA-type glutamate receptors

“…Neither TBS nor pairing low-frequency stimulation with postsynaptic depolarization were able to induce LTP at hippocampal CA1 synapses of Mecp2 KO mice, as shown previously with tetanic high-frequency stimulation in different MeCP2-deficient models (6)(7)(8)(9)(10). The potentiation of the amplitude and spatial spread of VSD signals observed after TBS in WT slices is absent in Mecp2 KO slices.…”

“…The visual cortex of Mecp2 KO mice shows a similar reduction of pyramidal neuron and network activity but caused by stronger inhibition due to hyperinnervation of parvalbumin interneurons (38). Neuronal activity is elevated in the hippocampus (7,8,15,(39)(40)(41) and brainstem (42-44) of Mecp2 KO mice. Enhanced synaptic transmission and spatiotemporal spread of neuronal depolarizations in CA1 is due to higher spontaneous activity of CA3 pyramidal neurons in Mecp2 KO hippocampal slices (15).…”

“…An earlier study reported that repeated tetani failed to progressively increase LTP in hippocampal slices from Mecp2 stop mice but curiously did not find evidence of enhanced synaptic transmission in naïve slices and did not explore the cellular bases of the proposed LTP saturation (10). Two reports described steeper I-O curves of fEPSPs and impaired LTP at CA3→CA1 synapses in MeCP2-deficient mice, but they did not elaborate on the bases of that enhancement (7,8). Here, we show that the A/N ratio, the I-O relationship, and rectification index of AMPAR-mediated EPSCs in naïve Mecp2 KO neurons resemble those of potentiated WT neurons and do not change after LTP induction as they normally do in WT neurons, which demonstrates that LTP is occluded by strong naïve synapses.…”

“…Long-term potentiation (LTP), a cellular correlate of learning and memory (5), is impaired at hippocampal CA1 excitatory synapses of symptomatic Mecp2 knockout (KO) mice (6,7), mice that express nonfunctional MeCP2 (Mecp2 308 ) (8), and mice with a STOP codon before Mecp2 exon 3 (Mecp2 stop ) (9,10). LTP is also impaired at excitatory synapses of layers II/III and V in the primary somatosensory cortex of Mecp2 KO mice (11,12) and Mecp2 308 mice (8) and in cortico-lateral amygdala synapses of Mecp2 KO mice (13).…”

Redox Indicator Mice Stably Expressing Genetically Encoded Neuronal roGFP: Versatile Tools to Decipher Subcellular Redox Dynamics in Neuropathophysiology

“…As a crucial control, we asked whether NADH and FAD autofluorescence may interfere with roGFP1 imaging as neuronal activity and mitochondrial metabolism also modulate the levels of these electron carriers and since similar excitation wavelengths (NADH 360 nm, FAD 445 nm) are used to monitor their tissue autofluorescence (26,32). Tests on WT slices with identical wavelengths and filter settings and similar ROIs used for roGFP imaging showed that electrical stimulation (100 Hz, 3 min) indeed increased NADH fluorescence (exc.…”