Correction of respiratory disorders in a mouse model of Rett syndrome

Abstract

Rett syndrome (RTT) is an autism spectrum disorder caused by mutations in the X-linked gene that encodes the transcription factor methyl-CpG-binding protein 2 (MeCP2). A major debilitating phenotype in affected females is frequent apneas, and heterozygous Mecp2-deficient female mice mimic the human respiratory disorder. GABA defects have been demonstrated in the brainstem of Mecp2-deficient mice. Here, using an intact respiratory network, we show that apnea in RTT mice is characterized by excessive excitatory activity in expiratory cranial and spinal nerves. Augmenting GABA markedly improves the respiratory phenotype. In addition, a serotonin 1a receptor agonist that depresses expiratory neuron activity also reduces apnea, corrects the irregular breathing pattern, and prolongs survival in MeCP2 null males. Combining a GABA reuptake blocker with a serotonin 1a agonist in heterozygous females completely corrects their respiratory defects. The results indicate that GABA and serotonin 1a receptor activity are candidates for treatment of the respiratory disorders in Rett syndrome.R ett syndrome (RTT) is an autism spectrum disorder that is caused by mutations in the X-linked gene that encodes methyl-CpG-binding protein 2 (MeCP2) (1). The role of this transcription factor is incompletely understood. Until recently, on the basis of animal studies primarily in embryonic or early neonatal brain, Mecp2 was considered a translational repressor (2, 3). Recently, however, Skene et al. (4) found in neuronal nuclei of mature (6-8 wk) WT littermates of Mecp2 null males that Mecp2 expression is ≈fivefold greater than that at birth. The amount of Mecp2 bound to DNA was proportional to the methylation density of CpG sequences. Importantly, in adult mice, it has been shown that restoring Mecp2 reverses the abnormalities in mobility, gait, hind limb clasping, tremor, breathing, and general condition that they developed during the period that the transcription factor was deficient (5). Thus, Mecp2 deficiency does not cause neuronal degeneration, nor is it necessary for the correct development of neuronal networks. Strategies aimed at pharmacological corrections of symptoms are therefore essential in treating RTT.Respiratory disorders are prominent and one of the most disturbing features of RTT (6, 7). These abnormalities are faithfully mimicked in mouse models of RTT (8). In Mecp2 null male animals studied in situ phrenic (PN) apneas were characterized by prolonged postinspiratory (post-I) activity in the central vagus nerve (9). Post-I activity is linked to the control of laryngeal adductors that control expiratory airflow and protect the lower airways from aspiration during swallowing. Active breath-hold with laryngeal closure is a common feature of RTT (6,7,10).This raises the possibility that the apneas are due to overactive brainstem expiratory neurons in Mecp2 heterozygote mice, perhaps consequent to a lack of synaptic inhibitory control. In this regard, examination of GABA synaptic inhibition in the ventrolateral medulla of Mec...

Methyl CpG Binding Protein 2 Gene Disruption Augments Tonic Currents of γ-Aminobutyric Acid Receptors in Locus Coeruleus Neurons

“…In LC neurons, our previous studies have shown that the postsynaptic GABA A and GABA B currents are both defective in Mecp2-null mice (10). Consistent with these findings, the therapeutic GABA A R activators diazepines and the reuptake blocker NO-711 improve RTT symptoms in animal models, including breathing (20,21). However, it is still unclear how the extrasynaptic GABA A Rs are affected in Mecp2-null mice, which makes this study remarkable.…”

“…(4, 13, 20, 33, 56 -58), which is likely to be due to impaired synaptic transmission and intrinsic membrane properties. The highly excitable state of some of these neurons seems to contribute to cognitive defects, motor abnormality, and breathing disturbances (9,20,56,57). Clearly, such an overexcitation in central neurons can be alleviated by GABAergic inhibition, in which excessive extrasynaptic GABA A Rs are beneficial.…”

“…These drugs may be used to correct the defects in the GABA system and relieve RTT-like symptoms. Indeed, several recent studies have shown that the breathing disorders of Mecp2-null mice can be alleviated by augmenting GABA synaptic inhibition (20,21). In contrast to the rich information of the synaptic GABA A Rs in RTT research (10,11,13,(22)(23)(24)(25), how the extrasynaptic GABA A Rs are affected by Mecp2 disruption remains unknown.…”

“…In Mecp2-null mice, several groups of neurons, including LC neurons, are hyperexcitable (4,20,23,33), which appears to contribute to the breathing disorders in Mecp2-null mice. To test whether activation of the extrasynaptic GABA A Rs can alleviate the breathing abnormalities, we studied breathing activity using plethysmography in conscious Mecp2-null mice.…”

Loss of neuronal 3D chromatin organization causes transcriptional and behavioural deficits related to serotonergic dysfunction

“…Both Drd5 (encoding for a dopamine receptor) and Tdo2 (encoding for the rate-limiting enzyme in the synthesis of serotonin tryptophan 2,3-dioxygenase) play a role in serotonin signalling and have been related to attention deficit-hyperactivity disorder and autism spectrum disorder [37][38][39][40] . The case of Rett syndrome (RS) is particularly interesting because (i) this congenital disorder associated with intellectual disability and autism has been recently related to impaired heterochromatin clustering 15 and chromocenter alterations during neuronal maturation 16 ; (ii) RS is caused by mutations in the gene encoding methyl CpG binding protein 2, one of the proteins showing abnormal distribution in H2BGFP-expressing neurons; (iii) the dysfunction of monoaminergic systems is known to be an important component of RS neuropathology 41 and 5-HT1A agonists, such as the one used in our study, prolong the life of mouse model of the disease 42 and are useful in the treatment of respiratory dysfunction in RS patients 43 ; and (iv) recent transcriptomics analyses show that Htr1a and Htr2a are downregulated in the hippocampus of mouse models of RS 44 . The transcriptional and behavioural deficits in H2BGFP mice also relate to negative symptoms in schizophrenia, such as prepulse inhibition and cognitive impairments, and reduce motivational drive.…”