Therapeutischer Nutzen einer stabilisierten systemischen Redoxbalance im Rett-Syndrom: Bewertung plethysmographischer und elektrophysiologischer Parameter
von Stefanie Auel
Datum der mündl. Prüfung:2022-03-14
Betreuer:Prof. Dr. Michael Müller
Gutachter:Prof. Dr. Thomas Dresbach
EnglischRett syndrome is a neurodevelopmental disorder caused by mutations in the X-linked MECP2 gene. The MECP2 gene encodes MeCP2 (methyl-CpG-binding protein 2), which acts as a transcriptional regulator and can stimulate and inhibit various genes' transcription. The result is, among other aspects, an increased generation of reactive oxygen species (ROS), which disturb the redox balance. Since the genetic defect of Rett syndrome is localized on the X chromosome, Rett syndrome primarily affects girls with a prevalence of 1:10000 - 15000. In male embryos, the genetic defect often leads to severe neonatal encephalopathy due to the hemizygous presence of the chromosome. As a result, death usually occurs within the first year of life. Symptoms include motor impairment, respiratory dysfunction, epileptic seizures, and autism-like features. Some features of Rett syndrome, such as impaired mitochondrial function and a resulting increased systemic oxidative stress, become evident in Rett mice before the onset of the first symptoms. This suggests that early administration of antioxidants could stabilize neuronal function, thus counteracting respiratory irregularities. Therefore, the focus of these studies was the benefit of oral supplementation of a diet enriched with the antioxidant additives α-lipoic acid, N-acetyl-L-cysteine, and vitamin E. To analyze the respiratory irregularities, the respiratory behavior of the mice was assessed by whole-body plethysmography. This allowed a specific comparison of the frequency and severity of the respiratory irregularities and the respiratory patterns of the different genotypes. In addition, some mice were filmed during their whole-body plethysmography measurements to correlate different behavioral and breathing patterns. Of particular interest was whether specific behavioral patterns, such as exploratory behavior, were increasingly detectable in a group-specific manner during the measurement. The electrophysiological studies were performed on acute hippocampal tissue slices of the respective mice. Specifically, the influence of the AO diet on synaptic function, the extent of synaptic plasticity, and anoxia vulnerability were determined. Basal synaptic function and the extent of synaptic plasticity were assessed by extracellularly-recorded field potentials (fEPSP). Cerebral anoxia vulnerability was analyzed by eliciting a hypoxia-induced spreading depression. Particular focus was on the duration until HSD occurs after induction of hypoxia and the regeneration time after reoxygenation. In addition, the loss of synaptic function was documented throughout HSD to detect possible changes in the post-hypoxic recovery phase in the different mouse and treatment groups. Regarding respiratory dysfunction in MeCP2 deficiency, this work extends previous findings and confirms an early onset of symptomatology in young animals. Although the AO diet did not fundamentally improve respiratory dysfunction in all experimental groups, it somewhat reduced respiratory irregularities in the older female heterozygous animals. It had no negative effect on respiratory behavior in the wild-type control group. Thus, oral antioxidant administration could become a way to alleviate the respiratory irregularities in the Rett syndrome patients and reduce the risk of threatening cardiac consequences associated with the apneas, up to sudden cardiac death. Analysis of the respiratory irregularities also revealed a group-specific exploratory behavior, which was diminished in the diseased animals and decreased with age in the females. This suggests an early and progressive presence of autism-like features with age in the diseased animals. In addition, some of the studies in this work, particularly the respiratory disturbances, suggest the existence of a compensatory mechanism in the sense of neuronal preconditioning to low oxygen levels under the conditions of MeCP2 deficiency. The AO diet resulted in a decreased neuronal excitability, especially in wild-type females. Similarly, this feeding regime decreased short-term synaptic plasticity in wild-type mice of both sexes, so that a therapeutic benefit of oral AO application on the synaptic dysfunctions of MeCP2 mice can be rather excluded.
Keywords: rett-syndrome; MeCP2; ROS; spreading depression; redox balance