Presymptomatic training mitigates functional deficits in a mouse model of Rett syndrome.

Mutations in the X-linked gene MECP2 cause Rett syndrome, a progressive neurological disorder in which children develop normally for the first one or two years of life before experiencing profound motor and cognitive decline1-3. At present there are no effective treatments for Rett syndrome, but we hypothesized that using the period of normal development to strengthen motor and memory skills might confer some benefit. Here we find, using a mouse model of Rett syndrome, that intensive training beginning in the presymptomatic period dramatically improves the performance of specific motor and memory tasks, and significantly delays the onset of symptoms. These benefits are not observed when the training begins after symptom onset. Markers of neuronal activity and chemogenetic manipulation reveal that task-specific neurons that are repeatedly activated during training develop more dendritic arbors and have better neurophysiological responses than those in untrained animals, thereby enhancing their functionality and delaying symptom onset. These results provide a rationale for genetic screening of newborns for Rett syndrome, as presymptomatic intervention might mitigate symptoms or delay their onset. Similar strategies should be studied for other childhood neurological disorders.

Effects of early-life exposure to THIP on brainstem neuronal excitability in the Mecp2-null mouse model of Rett syndrome before and after drug withdrawal.

Rett syndrome (RTT) is mostly caused by mutations of the X-linked MECP2 gene. Although the causal neuronal mechanisms are still unclear, accumulating experimental evidence obtained from Mecp2-/Y mice suggests that imbalanced excitation/inhibition in central neurons plays a major role. Several approaches may help to rebalance the excitation/inhibition, including agonists of GABAA receptors (GABAAR). Indeed, our previous studies have shown that early-life exposure of Mecp2-null mice to the extrasynaptic GABAAR agonist THIP alleviates several RTT-like symptoms including breathing disorders, motor dysfunction, social behaviors, and lifespan. However, how the chronic THIP affects the Mecp2-/Y mice at the cellular level remains elusive. Here, we show that the THIP exposure in early lives markedly alleviated hyperexcitability of two types of brainstem neurons in Mecp2-/Y mice. In neurons of the locus coeruleus (LC), known to be involved in breathing regulation, the hyperexcitability showed clear age-dependence, which was associated with age-dependent deterioration of the RTT-like breathing irregularities. Both the neuronal hyperexcitability and the breathing disorders were relieved with early THIP treatment. In neurons of the mesencephalic trigeminal nucleus (Me5), both the neuronal hyperexcitability and the changes in intrinsic membrane properties were alleviated with the THIP treatment in Mecp2-null mice. The effects of THIP on both LC and Me5 neuronal excitability remained 1 week after withdrawal. Persistent alleviation of breathing abnormalities in Mecp2-/Y mice was also observed a week after THIP withdrawal. These results suggest that early-life exposure to THIP, a potential therapeutic medicine, appears capable of controlling neuronal hyperexcitability in Mecp2-/Y mice, which occurs in the absence of THIP in the recording solution, lasts at least 1 week after withdrawal, and may contribute to the RTT-like symptom mitigation.

Mitochondrial free radical overproduction due to respiratory chain impairment in the brain of a mouse model of Rett syndrome: protective effect of CNF1.

Rett syndrome (RTT) is a pervasive neurodevelopmental disorder mainly caused by mutations in the X-linked MECP2 gene associated with severe intellectual disability, movement disorders, and autistic-like behaviors. Its pathogenesis remains mostly not understood and no effective therapy is available. High circulating levels of oxidative stress markers in patients and the occurrence of oxidative brain damage in MeCP2-deficient mouse models suggest the involvement of oxidative stress in RTT pathogenesis. However, the molecular mechanism and the origin of the oxidative stress have not been elucidated. Here we demonstrate that a redox imbalance arises from aberrant mitochondrial functionality in the brain of MeCP2-308 heterozygous female mice, a condition that more closely recapitulates that of RTT patients. The marked increase in the rate of hydrogen peroxide generation in the brain of RTT mice seems mainly produced by the dysfunctional complex II of the mitochondrial respiratory chain. In addition, both membrane potential generation and mitochondrial ATP synthesis are decreased in RTT mouse brains when succinate, the complex II respiratory substrate, is used as an energy source. Respiratory chain impairment is brain area specific, owing to a decrease in either cAMP-dependent phosphorylation or protein levels of specific complex subunits. Further, we investigated whether the treatment of RTT mice with the bacterial protein CNF1, previously reported to ameliorate the neurobehavioral phenotype and brain bioenergetic markers in an RTT mouse model, exerts specific effects on brain mitochondrial function and consequently on hydrogen peroxide production. In RTT brains treated with CNF1, we observed the reactivation of respiratory chain complexes, the rescue of mitochondrial functionality, and the prevention of brain hydrogen peroxide overproduction. These results provide definitive evidence of mitochondrial reactive oxygen species overproduction in RTT mouse brain and highlight CNF1 efficacy in counteracting RTT-related mitochondrial defects.

Genetic effects of methylation diets.

DNA methylation at cytosines in CpG dinucleotides can lead to changes in gene expression and function without altering the primary sequence of the DNA. Methylation can be affected by dietary levels of methyl-donor components, such as folic acid. This may be an important mechanism for environmentally induced changes in gene expression. Recent literature supports a role for DNA-methylation changes in a number of adult-onset disorders and during development. These changes may be significant for better understanding certain birth defects (e.g., neural tube defects) and the long-term consequences of early environmental influences on gene expression (metabolic programming). Optimal "methylation diets" should be investigated as part of the prevention and treatment of all these conditions, as well as in disorders such as Rett syndrome, whose primary defects may lie in DNA methylation-dependent gene regulation.