Glial chemosensitivity and control of breathing in Rett syndrome

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SUMMARY Rett syndrome (RTT) (OMIM #312750) is a severe X-linked neurodevelopmental disorder caused by mutations in the methyl-CpG-binding protein 2 (MECP2). Although RTT patients suffer from many co-morbid phenotypes, wake disordered breathing has a major negative impact quality of life and is associated with high mortality rate in this patient population. Evidence from murine models of RTT suggest disordered breathing results in part from a disrupted ability to regulate breathing in response to changes in tissue CO2/H+ (i.e., central chemoreflex). The retrotrapezoid nucleus (RTN) is a critical chemosensitive brainstem nucleus, contains CO2/H+-sensitive neurons and astrocytes that together produce a CO2/H+-dependent drive to breath. Previous and preliminary data identify heteromeric Kir4.1/5.1 channels as key determinants of RTN astrocyte CO2/H+ chemosensitivity. However, homomeric Kir4.1 and heteromeric Kir4.1/5.1 are differentially CO2/H+ sensitive and regulate divergent astrocyte processes including membrane potential and clearance of neuronally released extracellular K+, and it is not clear which of these mechanisms contributes to RTN chemoreception and disordered breathing in RTT. Our previous work showed that MeCP2 deficient mice have reduced levels of Kir4.1 and 5.1 channels, diminished astrocytic Kir4.1/5.1-like currents, dysregulated extracellular K+. MeCP2 deficient mice also show a blunted ventilatory response to CO2. Similarly, preliminary results show that global deletion of astrocyte Kir4.1 channels also blunts the ventilatory response to CO2. Importantly targeted (re)expression of Kir4.1 specifically in RTN astrocytes rescued disordered breathing in both Kir4.1 cKO and MeCP2 deficient mice. Preliminary data also show that RTN astrocytes from Kir5.1 KO animals lack CO2/H+ sensitivity, thus suggesting heteromeric Kir4.1/5.1 channels regulate RTN astrocyte chemosensitivity. Recent ultrastructural studies indicate reduced astrocytic coverage of neuronal elements during sleep together with increased extracellular space; thus necessitating state-dependent changes in astrocyte ion and transmitter homeostasis, that may account for a reduced ventilatory response to CO2 during sleep. Consistent with this, preliminary results show that RTN astrocytes undergo a ~25% volume increase in the wake state as compared to sleep. Based on this, we hypothesize that Kir4.1/5.1 deficiency and compromised astrocyte chemoreception are responsible for state-dependent disordered breathing in RTT. In this proposal, we use an established mouse model of RTT, an inducible astrocyte specific Kir4.1 knockout mouse in conjunction with astrocyte targeted AAV, astrocyte volume and morphological complexity measurements, slice electrophysiology, and whole-animal plethysmography to explore the sleep-wake state-dependent contributions of Kir4.1/5.1 channels to RTN chemoreception and respiratory activity in RTT. Understanding how Kir4.1 and Kir5.1 contribute to RTN chemoreception across sleep-wake states and disordered breathing may provide mechanistic insight for targeted treatment of disordered breathing in RTT.