Ventilatory response to hyperoxia in newborn mice heterozygous for the transcription factorPhox2b

Abstract

Heterozygous mutations of the transcription factor PHOX2B have been found in most patients with central congenital hypoventilation syndrome, a rare disease characterized by sleep-related hypoventilation and impaired chemosensitivity to sustained hypercapnia and sustained hypoxia. PHOX2B is a master regulator of autonomic reflex pathways, including peripheral chemosensitive pathways. In the present study, we used hyperoxic tests to assess the strength of the peripheral chemoreceptor tonic drive in Phox2bϩ/Ϫ newborn mice. We exposed 69 wild-type and 67 mutant mice to two hyperoxic tests (12-min air followed by 3-min 100% O2) 2 days after birth. Breathing variables were measured noninvasively using whole body flow plethysmography. The initial minute ventilation decrease was larger in mutant pups than in wild-type pups: Ϫ37% (SD 13) and Ϫ25% (SD 18), respectively, P Ͻ 0.0001. Furthermore, minute ventilation remained depressed throughout O2 exposure in mutants, possibly because of their previously reported impaired CO2 chemosensitivity, whereas it returned rapidly to the normoxic level in wild-type pups. Hyperoxia considerably increased total apnea duration in mutant compared with wild-type pups (P ϭ 0.0001). A complementary experiment established that body temperature was not influenced by hyperoxia in either genotype group and, therefore, did not account for genotype-related differences in the hyperoxic ventilatory response. Thus partial loss of Phox2b function by heterozygosity did not diminish the tonic drive from peripheral chemoreceptors.control of breathing; chemosensitivity; apnea CENTRAL CONGENITAL HYPOVENTILATION syndrome (CCHS or Ondine's curse) is a rare disease, generally present from birth and characterized by hypoventilation during sleep in the absence of primary neuromuscular or lung disease or of brain stem lesions (3). Throughout life, patients with CCHS have absent or markedly reduced ventilatory responses to sustained hypercapnia (29) and, to a lesser extent, to sustained hypoxia (29). These respiratory impairments are generally ascribed to impaired central integration of chemosensory inputs at the brain stem level, rather than to failure of chemoreceptor activity, which is at least partially present (15,22,37). The genetic basis for CCHS was discovered recently: most patients with CCHS have a heterozygous mutation of the PHOX2B gene (2,23,41,43). PHOX2B is a master regulator of the noradrenergic phenotype and of all neuronal relays of autonomic medullary reflex pathways (30), including peripheral chemosensitive pathways (9).Mice with a single functional Phox2b allele (i.e., Phox2bϩ/Ϫ mice) provide a unique opportunity to investigate the genotype-phenotype relationship in CCHS. Our laboratory's previous studies showed that Phox2bϩ/Ϫ mice had longer sleep apnea times than their wild-type littermates on postnatal day (P) 5 (P5) (11) and a weaker response to CO 2 on P2, followed by normalization before P10 (9). These ventilatory impairments were reminiscent of CCHS. However, the hyperpneic response t...

Breathing without CO2Chemosensitivity in ConditionalPhox2bMutants

“…Breathing variables of P2-P9 pups were measured noninvasively in unanaesthetized, unrestrained pups using whole-body flow barometric plethysmography as described previously (Matrot et al, 2005;Ramanantsoa et al, 2006). Breathing variables of P22 and adult mice were measured using a larger whole-body flow barometric plethysmograph (Dauger et al, 2003).…”

“…cre ; P2b 27Alacki mice lack CO 2 chemosensitivity at least up to postnatal day 9 We assessed breathing parameters in infant mice with wholebody plethysmography of unrestrained pups (Matrot et al, 2005;Ramanantsoa et al, 2006). In normoxia, mean ventilation (VE) of P2 Egr2 cre/ϩ ;P2b 27Alacki/ϩ pups (henceforth called mutants) was reduced by 42 Ϯ 13% (n ϭ 10) compared with control Egr2 ϩ/ϩ ; P2b 27Alacki/ϩ littermates (henceforth called controls) because of longer breath duration (TTOT) (Fig.…”

A human mutation in Phox2b causes lack of CO 2 chemosensitivity, fatal central apnea, and specific loss of parafacial neurons

“…Breathing movements were recorded noninvasively in unanaesthesized, unrestrained pups within 20 min after delivery, using whole-body flow barometric plethysmography as described (43,44). A constant bias flow (200 ml/min) prevented CO2 and water accumulation and ambient temperature drifts over time.…”

Neural control of breathing: insights from genetic mouse models

“…Arousal responses to hypoxia are not significantly different between Phox2b ϩ/ϩ and Phox2b ϩ/Ϫ pups. The ventilatory decrease caused by hyperoxia is larger in newborn Phox2b ϩ/Ϫ mutant mice than in their wild-type littermates and is magnified by longer apnea durations (66). Furthermore, compared to wild-type pups, mutant pups have a longer-lasting ventilatory decline that persists beyond the return to normoxia.…”

“…Breathing variables are measured under both baseline conditions, i.e., while the animals are breathing air and during exposure to chemical stimuli such as hypercapnia, hypoxia, or hyperoxia, to test for chemosensitivity. Peripheral chemoreceptor function can be assessed by administering pure oxygen to newborn mice to achieve physiological chemodenervation (66), taking into account the fact that repeated hyperoxia results in increasing levels of respiratory depression (48).…”