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Astrocyte heterogeneity is an emerging concept in which astrocytes within or between brain regions show variable morphological and/or gene expression profiles that presumably reflect different functional roles. Recent evidence indicates that retrotrapezoid nucleus (RTN) astrocytes sense changes in tissue CO2/ H+ to regulate respiratory activity; however, mechanism(s) by which they do so remain unclear. Alterations in inward K+ currents represent a potential mechanism by which CO2 /H+ signals may be conveyed to neurons. Here, we use slice electrophysiology in rats of either sex to show that RTN astrocytes intrinsically respond to CO2 /H+ by inhibition of an inward rectifying potassium (Kir ) conductance and depolarization of the membrane, while cortical astrocytes do not exhibit such CO2 /H+ -sensitive properties. Application of Ba2+ mimics the effect of CO2 /H+ on RTN astrocytes as measured by reductions in astrocyte Kir -like currents and increased RTN neuronal firing. These CO2 /H+ -sensitive currents increase developmentally, in parallel to an increased expression in Kir 4.1 and Kir 5.1 in the brainstem. Finally, the involvement of Kir 5.1 in the CO2 /H+ -sensitive current was verified using a Kir5.1 KO rat. These data suggest that Kir inhibition by CO2 /H+ may govern the degree to which astrocytes mediate downstream chemoreceptive signaling events through cell-autonomous mechanisms. These results identify Kir channels as potentially important regional CO2 /H+ sensors early in development, thus expanding our understanding of how astrocyte heterogeneity may uniquely support specific neural circuits and behaviors.

Background Rett syndrome (RTT) is an X-linked neurodevelopmental disorder predominantly caused by alterations of the methyl-CpG-binding protein 2 (MECP2) gene. The gut microbiome has been implicated in neurodevelopmental disorders such as Autism Spectrum Disorder (ASD) as a regulator of disease severity. Although the gut microbiome has been previously characterized in humans with RTT, the impact of MECP2 mutation on the composition of the gut microbiome in animal models where the host and diet can be experimentally controlled remains to be elucidated.Methods We evaluated the microbial community through 16S sequencing of fecal samples collected across postnatal development as behavioral symptoms appear and progress in a novel zinc-finger nuclease rat model of RTT. Additionally, we profiled fecal levels of fatty acids in MecP2 deficient rats. Lastly, we compared our results to predicted functional shifts in the microbiota of females with RTT compared to their mothers to further examine the translational potential of the current RTT rat model.Results We have identified microbial taxa that are differentially abundant across key timepoints in a zinc-finger nuclease rat model of RTT compared to WT. Furthermore, we have characterized functional categories of gut microbes that are similarly affected in females with RTT and female RTT rats, including similar alterations in pathways related to short chain fatty acid (SCFA) activity. Lastly, we have demonstrated that SCFA levels are decreased in the feces of RTT rats compared to WT.Limitations The current study is potentially limited by age related differences in the microbiome of RTT participants and controls as well as medication effects on the microbiome. Additionally, the current study did not assess male MeCP2-deficient rats, and it may be relevant in future studies to address potentially disparate microbial changes in male and female rats and humans with RTT.Conclusions The results of our studies establish distinct microbial community shifts that occur in RTT across developmental time points independently of diet or environmental factors. We identify p105 as a key translational timepoint at which microbial shifts most closely mirror reported microbiota communities in RTT patients. Overall, these results represent an important step in translational RTT research.

Astrocyte heterogeneity is an emerging concept in which astrocytes within or between brain regions show variable morphological and/or gene expression profiles that presumably reflect different functional roles. Recent evidence indicates that retrotrapezoid nucleus (RTN) astrocytes sense changes in tissue CO(2/)H(+) to regulate respiratory activity; however, mechanism(s) by which they do so remain unclear. Alterations in inward K(+) currents represent a potential mechanism by which CO(2)/H(+) signals may be conveyed to neurons. Here, we use slice electrophysiology in rats of either sex to show that RTN astrocytes intrinsically respond to CO(2)/H(+) by inhibition of an inward rectifying potassium (K(ir)) conductance and depolarization of the membrane, while cortical astrocytes do not exhibit such CO(2)/H(+)-sensitive properties. Application of Ba(2+) mimics the effect of CO(2)/H(+) on RTN astrocytes as measured by reductions in astrocyte K(ir)-like currents and increased RTN neuronal firing. These CO(2)/H(+)-sensitive currents increase developmentally, in parallel to an increased expression in K(ir)4.1 and K(ir)5.1 in the brainstem. Finally, the involvement of K(ir)5.1 in the CO(2)/H(+)-sensitive current was verified using a Kir5.1 KO rat. These data suggest that K(ir) inhibition by CO(2)/H(+) may govern the degree to which astrocytes mediate downstream chemoreceptive signaling events through cell-autonomous mechanisms. These results identify K(ir) channels as potentially important regional CO(2)/H(+) sensors early in development, thus expanding our understanding of how astrocyte heterogeneity may uniquely support specific neural circuits and behaviors.

Rett syndrome (RTT) is a rare, X-linked neurodevelopmental disorder typically affecting females, resulting in a range of symptoms including autistic features, intellectual impairment, motor deterioration, and autonomic abnormalities. RTT is primarily caused by the genetic mutation of the Mecp2 gene. Initially considered a neuronal disease, recent research shows that glial dysfunction contributes to the RTT disease phenotype. In the following manuscript, we review the evidence regarding glial dysfunction and its effects on disease etiology.

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder that is predominantly caused by alterations of the methyl-CpG-binding protein 2 (MECP2) gene. Disease severity and the presence of comorbidities such as gastrointestinal distress vary widely across affected individuals. The gut microbiome has been implicated in neurodevelopmental disorders such as Autism Spectrum Disorder (ASD) as a regulator of disease severity and gastrointestinal comorbidities. Although the gut microbiome has been previously characterized in humans with RTT compared to healthy controls, the impact of MECP2 mutation on the composition of the gut microbiome in animal models where the host and diet can be experimentally controlled remains to be elucidated. By evaluating the microbial community across postnatal development as behavioral symptoms appear and progress, we have identified microbial taxa that are differentially abundant across developmental timepoints in a zinc-finger nuclease rat model of RTT compared to WT. We have additionally identified p105 as a key translational timepoint. Lastly, we have demonstrated that fecal SCFA levels are not altered in RTT rats compared to WT rats across development. Overall, these results represent an important step in translational RTT research.

Since the identification of MECP2 as the causative gene in the majority of Rett Syndrome (RTT) cases, transgenic mouse models have played a critical role in our understanding of this disease. The use of additional mammalian RTT models offers the promise of further elucidating critical early mechanisms of disease as well as providing new avenues for translational studies. We have identified significant abnormalities in growth as well as motor and behavioural function in a novel zinc-finger nuclease model of RTT utilizing both male and female rats throughout development. Male rats lacking MeCP2 (Mecp2ZFN/y) were noticeably symptomatic as early as postnatal day 21, with most dying by postnatal day 55, while females lacking one copy of Mecp2 (Mecp2ZFN/+) displayed a more protracted disease course. Brain weights of Mecp2ZFN/y and Mecp2ZFN/+ rats were significantly reduced by postnatal day 14 and 21, respectively. Early motor and breathing abnormalities were apparent in Mecp2ZFN/y rats, whereas Mecp2ZFN/+ rats displayed functional irregularities later in development. The large size of this species will provide profound advantages in the identification of early disease mechanisms and the development of appropriately timed therapeutics. The current study establishes a foundational basis for the continued utilization of this rat model in future RTT research.

Kir4.1 is an inwardly rectifying K(+) channel expressed exclusively in glial cells in the central nervous system. In glia, Kir4.1 is implicated in several functions which include extracellular K(+) homeostasis, maintenance of astrocyte resting membrane potential, cell volume regulation and facilitation of glutamate uptake. Knockout of Kir4.1 in rodent models leads to severe neurological deficits, including ataxia, seizures, sensorineural deafness, and early postnatal death. Accumulating evidence indicates that Kir4.1 plays an integral role in the central nervous system, prompting many laboratories to study the potential role that Kir4.1 plays in human disease. In this article, we review the growing evidence implicating Kir4.1 in a wide array of neurological disease. Recent literature suggests Kir4.1 dysfunction facilitates neuronal hyperexcitability and may contribute to epilepsy. Genetic screens demonstrate that mutations of KCNJ10, the gene encoding Kir4.1, causes SeSAME/EAST syndrome, which is characterized by early onset seizures, compromised verbal and motor skills, profound cognitive deficits, and salt wasting. KCNJ10 has also been linked to developmental disorders including autism. Cerebral trauma, ischemia, and inflammation are all associated with decreased astrocytic Kir4.1 current amplitude and astrocytic dysfunction. Additionally, neurodegenerative diseases such as Alzheimer’s disease and amyotrophic lateral sclerosis demonstrate loss of Kir4.1. This is particularly exciting in the context of Huntington’s disease, another neurodegenerative disease in which restoration of Kir4.1 ameliorated motor deficits, decreased medium spiny neuron hyperexcitability, and extended survival in mouse models. Understanding the expression and regulation of Kir4.1 will be critical in determining if this channel can be exploited for therapeutic benefit.