Conditional depletion of methyl-CpG-binding protein 2 in astrocytes depresses the hypercapnic ventilatory response in mice

Abstract

Garg SK, Lioy DT, Knopp SJ, Bissonnette JM. Conditional depletion of methyl-CpG-binding protein 2 in astrocytes depresses the hypercapnic ventilatory response in mice. J Appl Physiol 119: 670 -676, 2015. First published July 23, 2015 doi:10.1152/japplphysiol.00411.2015Mice that are deficient in the transcription factor methyl-CpG-binding protein 2 (MeCP2) have a depressed hypercapnic ventilatory response (HCVR). The expression of MeCP2 can be selectively removed from astrocytes or neurons, thus offering a tool to dissect the role of this transcription factor in astrocytes from that in neurons. Studies were carried out in the progeny of mice that were a cross between those harboring a tamoxifen (TAM)-inducible Cre recombinase transgene driven by the human astrocytic glial fibrillary acidic protein (hGFAP) promoter, or Cre recombinase under control of the synapsin promoter, with mice containing a Cre-excisable exon III in the Mecp2 gene. The TAM-conditional excision of the Mecp2 exon allowed the respiratory CO2 response to be studied in the same animals before and after selective depletion of MeCP2 in astrocytes. Immunohistochemistry showed that following TAM treatment only ϳ20% of GFAP-labeled cells in the retrotrapazoid nucleus and in the raphé magnus were positive for MeCP2. The slope of the relative increase in minute ventilation as a function of 1, 3, and 5% inspired CO2 was depressed in mice with depleted astrocyte MeCP2 compared with wild-type littermates. In contrast, selective depletion of MeCP2 in neurons did not significantly affect slope. While neurons which constitute the respiratory network ultimately determine the ventilatory response to CO2, this study demonstrates that loss of MeCP2 in astrocytes alone is sufficient to result in a dramatic attenuation of the HCVR. We propose that the glial contribution to HCVR is under the control of the MeCP2 gene.astrocyte; hypercapnic ventilatory response; Rett syndrome HYDROGEN ION HOMEOSTASIS IS largely dependent on CO 2 /pH regulation of minute ventilation. Thus the cellular and molecular mechanisms that transduce changes in blood and brain parenchymal CO 2 /pH to increase the rate and depth of breathing are important areas of current study (12,17,18,21,26,27,40). While the generally accepted view holds that response to CO 2 is a property of neurons, recent work has pointed to an important contribution from astrocytes (14,15,18,21,27). Respiratory CO 2 ventilatory response is significantly depressed in mice that lack or are deficient in the X-linked transcription factor methyl-CpG-binding protein 2 (MeCP2) (6,50,58). Mutations in this nuclear protein cause the neurological disorder Rett syndrome (RTT) (3).A previous study suggested that activation of chemosensitive neurons of the retrotrapezoid nucleus (RTN) could affect CO 2 levels by both their intrinsic pH sensitivity of as well as by non-cell-autonomous activity of neighboring astrocytes (19). The extent of astrocyte contribution by release of ATP to signal RTN neurons is unsettled. Photoactivation of a...

MeCP2 deficiency results in robust Rett-like behavioural and motor deficits in male and female rats

“…Mouse models of RTT typically report normal respiratory activity under control conditions (inactive, breathing room air) up to PND 20 (63,83) followed shortly thereafter by abnormalities in post-sigh breathing (63), and later hyper- (84,85) or hypo-ventilation (62,83,86,87) with increased tidal volume (86,88) in conjunction with increased apneic frequency and duration (89–91). These respiratory abnormalities are partly recapitulated in juvenile and adult Mecp2 ZFN/y rats, which demonstrate decreased respiratory frequency, as well as Mecp2 ZFN/+ rats, which show increased frequency.…”

Proton detection and breathing regulation by the retrotrapezoid nucleus

“…Finally, whilst deleting methyl‐CpG binding protein 2 (MeCP2) selectively from astrocytes in mice attenuates the respiratory chemoreflex (Garg et al . ), only the tidal volume ( V T ) component was reduced. Such an effect could occur for numerous reasons unrelated to CO 2 detection per se (e.g.…”

“…Finally, astrocytes are less sensitive to CO 2 in Mecp2 −/− mice, a model of Rett syndrome that also features a reduced hypercapnic chemoreflex, among other deficits (Garg et al . ; Turovsky et al . ).…”

Reduced computational modelling of Kölliker‐Fuse contributions to breathing patterns in Rett syndrome

“…Moreover, the conditional depletion of MECP2 in astrocytes alone recapitulated the blunted CO 2 sensitivity phenotype (Garg et al . ). Interestingly, re‐expression of MECP2 either in glia or in GABAergic neurons alone sufficed to ameliorate the breathing phenotype in mouse models of RTT (Lioy et al .…”