Defective Respiratory Rhythmogenesis and Loss of Central Chemosensitivity in Phox2b Mutants Targeting Retrotrapezoid Nucleus Neurons

Abstract

The retrotrapezoid nucleus (RTN) is a group of neurons in the rostral medulla, defined here as Phox2b-, Vglut2-, neurokinin1 receptor-, and Atoh1-expressing cells in the parafacial region, which have been proposed to function both as generators of respiratory rhythm and as central respiratory chemoreceptors. The present study was undertaken to assess these two putative functions using genetic tools. We generated two conditional Phox2b mutations, which target different subsets of Phox2b-expressing cells, but have in common a massive depletion of RTN neurons. In both conditional mutants as well as in the previously described Phox2b 27Ala mutants, in which the RTN is also compromised, the respiratory-like rhythmic activity normally seen in the parafacial region of fetal brainstem preparations was completely abrogated. Rhythmic motor bursts were recorded from the phrenic nerve roots in the mutants, but their frequency was markedly reduced. Both the rhythmic activity in the RTN region and the phrenic nerve discharges responded to a low pH challenge in control, but not in the mutant embryos. Together, our results provide genetic evidence for the essential role of the Phox2b-expressing RTN neurons both in establishing a normal respiratory rhythm before birth and in providing chemosensory drive.

The paired-like homeobox 2B (PHOX2B) gene and respiratory control

“…12 Indeed, the RTN contains neurons that are vigorously activated by CO 2 /pH. 13,14 Further molecular characterization of these Phox2b-expressing neurons showed that they are glutamatergic, non-catecholaminergic, non-cholinergic, and express neurokinin1 receptors (NK1R), supporting previous work linking NK1R-expressing neurons to central chemoreception. 15 It is unclear whether the missing Phox2b-expressing neurons of the RTN are themselves chemosensors or obligate intermediates that transfer information from chemosensors to respiratory centres in the medulla 16 ; nonetheless, the overwhelming evidence supports that Phox2b-expressing neurons are essential for normal rhythmic respiratory drive and chemosensitivity.…”

“…12 En effet, le RTN contient des neurones qui sont puissamment activés par le CO 2 /pH. 13,14 Une caractérisation moléculaire plus poussée de ces neurones exprimant le PHOX2B a démontré qu'ils sont glutamatergiques, non catécholaminergiques, non cholinergiques et qu'ils expriment des récepteurs de la neurokinine-1 (NK1R), ce qui appuie les travaux antérieurs associant les neurones exprimant la NK1R à la chémoréception centrale. 15 Nous ne savons pas si les neurones exprimant le PHOX2B manquants du RTN sont des chémorécepteurs eux-mêmes ou s'ils contraignent des intermédiaires qui transfèrent les informations des chémosenseurs aux centres respiratoires de la moelle 16 ; néanmoins, une majorité des données probantes indiquent que les neurones exprimant le PHOX2B sont essentiels pour une stimulation respiratoire rythmique normale et la chémosensibilité.…”

Teashirt 3 Regulates Development of Neurons Involved in Both Respiratory Rhythm and Airflow Control

“…The RRG is composed of two interacting neuronal networks: the pre-Bötzinger complex (preBötC) (Smith et al, 1991) and the parafacial respiratory group (pFRG) (Onimaru and Homma, 2003). Required for survival at birth, the RRG is already functional at prenatal stages where it contributes to motor coordination of the respiratory apparatus and confers central chemosensitivity at embryonic stages (Thoby-Brisson and Greer, 2008;Dubreuil et al, 2009). …”

“…Important progress has been made in the analysis of mechanisms required for generating diversity in brainstem motoneurons (Guthrie, 2007), but little is known about the genetic specification of the upper airway motoneurons. Moreover, although transcription factors such as MafB, Tlx1/3, Phox2b, Egr2, and Math1 have been shown to play important roles in the specification of neurons constituting the two oscillators of the RRG (Blanchi et al, 2003;Cheng et al, 2004;Pagliardini et al, 2008;Dubreuil et al, 2009;Rose et al, 2009;Thoby-Brisson et al, 2009), it is not clear how the necessary coordination with the airway motoneurons is set up. In addition, several of these mutations lead to absence of some of the neurons involved in respiratory control, meaning that the genetic control of functional rhythmogenesis has proved hard to decipher.…”

Complexe de pré-Bötzinger et automatisme respiratoire

“…Chez l'homme, Necdin est un des gènes responsables du syndrome de Prader-Willi, un syndrome multigénique complexe accompagné des mêmes déficits respiratoires [16,40]. L'inactivation du gène Phox2b, qui est responsable du syndrome congénital d'hypoventilation centrale, altère la formation du RTN/pFRG, la réponse respiratoire à l'hypercapnie et la rythmogenèse, sans atteinte apparente des systèmes à 5-HT [27]. Les dysfonctionnements des voies aériennes supérieures et les apnées obstructives constituent les plus fréquentes des pathologies respiratoires.…”