Post-inspiratory discharges are the centrepiece of respiratory disrhythmia in a gene knockout model of Rett syndrome.

Abstract

Several respiratory disorders in humans are linked to gene defects, including Charcot–Marie–Tooth disease, Cheyne–Stokes breathing disorder, Willi–Prader syndrome, sudden infant death syndrome, congenital central hypoventilation and Rett Syndrome. In non-human species, disturbances of breathing have been reported to accompany a variety of gene deletions, as summarized by Feldman et al. (2003). Irrespective of species, it is often uncertain whether the respiratory disorder is a direct byproduct of the gene disruption, and whether the gene defect is specific for the respiratory network. In Rett syndrome, however, there is evidence that a mutation in the MECP2 gene plays a critical role in the severe respiratory disorder that accompanies the disease. Knockout mice lacking the gene also develop respiratory rhythm disturbances, characterized by Cheyne–Stokes episodes, Valsalva's manoeuvres and forceful breathing. In this issue of The Journal of Physiology,Stettner et al. (2007) provide compelling evidence that alterations in bulbar postinspiratory discharges, and a site in the pons that controls them, play important roles in rhythm disturbance produced by deleting MECP2 in mice. In most animal species with normal breathing patterns, the respiratory cycle consists of three components: inspiratory and postinspiratory discharges and a late, silent expiratory period. There are state- and activity-dependent variations in phase durations. Stettner et al. (2007) report that in an in vitro preparation, the working heart–brainstem preparation (WHBP) of knockout (KO) mice carrying the MECP2 gene mutation, there is a greater degree of variance in the inspiratory and postinspiratory phases compared to wild-type mice. In addition, there is a significant prolongation of postinspiratory discharge duration and shortening of the inspiratory and late expiratory periods. The disturbances of respiratory rhythm are similar to those recorded plethysmographically in intact MECP2-KO mice (Viermari et al. 2005) and observed in human Rett syndrome patients (Julu & Witt Engerstrom, 2005). The highly variable duration of the postinspiratory discharges in MECP2-KO mice seems to be the main factor in the respiratory disrhythmia that leads to intermittent apnoeas. The authors present evidence that there is a contribution of the Kolliker–Fuse (KF) region of the pons that enhances postinspiratory activity and the development of respiratory disrhythmia. Pressure microinjection of glutamate into the KF region produces a degree of enhanced postinspiratory activity in KO mice that is significantly greater and more prolonged than in control animals. In addition vagal, stimulus-evoked Hering–Breuer reflex activation, which suppresses respiratory discharges and has a synaptic link within the pons, is more pronounced in KO mice and fails to desensitize as in wild-type mice. Stettner and colleagues suggest that postinspiratory neurons are hyperactive in MECP2-KO mice, and offer explanations for this effect and the resultant disturbance of rhythmicity. One is that there is altered expression of NMDA receptor subunits. It seems that an up-regulation of NMDAR expression or alteration of subunits might contribute to postinspiratory hyperexcitability. Another explanation offered is that there is a MECP2-related down-regulation of GABA receptor subunits. Either or both could contribute to an imbalance that favours increased postinspiratory neuron excitability. The authors acknowledge that additional neurochemical imbalances could contribute to respiratory disrhythmia, such as defects in substance P, met-enkephalon, serotonin and noradrenergic neuromodulation. For example, Viermari et al. (2005) report that MECP2-KO mice with respiratory disrhythmia have reduced numbers of catecholaminergic neurons and levels of noradrenline (NA) and serotonin in the medulla. In their rhythmic slice preparations from KO mice, application of NA stabilizes an otherwise irregular respiratory-associated rhythm. In addition, Segawa (2001) argues that defects in dopamine neuromodulation contribute to Rett-related locomotor disturbances. The study by Stettner et al. (2007) implicates a neuroregulatory defect in the KF region as an important factor in the rhythm disturbance. Nonetheless, other potential sites in the central nervous system cannot be excluded. In the medullary slice preparation of MECP2-KO mice (Viermari et al. 2005), rhythm disturbances are detected in neuron aggregate recordings from the pre-Botzinger complex, a key region in respiratory rhythmogenesis (Feldman et al. 2003). In human Rett patients, NMRI studies show abnormalities in the size of the striatal motor system (Casanova et al. 1991), where programmes for coordinated breathing and swallowing are embedded (Grillner et al. 2005). Lesions, loss of MECP2 and loss of catecholaminergic neurons in the locus coeruleus, where respiratory rhythm can be recorded (Ballantyne et al. 2004), have also been detected post mortem in Rett syndrome patients (Itoh & Takashima, 2002). The study is important because it links a specific neural respiratory rhythm component to a gene-dependent respiratory disrhythmia. It utilizes a preparation that conserves all of the critical bulbar respiratory network components, allows precise measurements of respiratory phase and eliminates the need for general anaesthesia, which, like sleep, is known to suppress the symptoms of Rett syndrome. Whether the findings might contribute to an effective therapy, as proposed by the authors, remains to be seen, given the apparent broad-spectrum nature of the neurochemical defects in Rett syndrome.