Early development of respiratory rhythm generation in mice and chicks

Abstract

Breathing in mammals starts in the foetus and acquires a vital importance at birth. The ability to produce rhythmic motor behaviours linked to respiratory function is a property of the brainstem reticular formation, which has been remarkably conserved during the evolution of vertebrates. Therefore, to understand the biological basis of the breathing behavior, we are investigating conservative developmental mechanisms orchestrating the organogenesis of the brainstem. In vertebrates, the hindbrain is one of the vesicles that appears at the anterior end of the neural tube of the embryo. Further morphogenetic subdivision ensues whereby the hindbrain neuroepithelium becomes partitioned into an iterated series of compartments called rhombomeres. The segmentation process is believed to determine neuronal fates by encoding positional information along the rostro-caudal axis. Before and at the onset of segmentation, genes encoding transcription factors such as Hox, Krox-20, kreisler, are expressed in domains corresponding to the limits of future rhombomeres. Inactivation of these genes specifically disturbs the rhombomeric pattern of the hindbrain. The presentation will address the problem of whether this primordial rhombomeric organisation influences later function of respiratory control networks in chicks and mice. Experiments were performed in embryos and after birth in transgenic mice. They show that, although expression of developmental genes and hindbrain segmentation are transient events of early embryonic development, they are important for the process of respiratory rhythm generation by brainstem neuronal networks. We have found in chick that at the end of the period of segmentation, the hindbrain contains neuronal rhythm generators that conform to the rhombomeric anatomical pattern. We have also identified a minimal rhombomeric motif allowing the post-segmental maturation of a specific (GABAergic) rhythm-promoting circuit. Furthermore, in vivo and in vitro analysis of neurons in transgenic mice revealed postnatal respiratory phenotypes associated with defects of central pontine and/or afferent respiratory control in Krox-20, Hoxa1 and kreisler mutants. Neonatal respiratory phenotypes are also induced in mice by treatment with low doses of retinoic acid that slightly change the early embryonic development of the Pons. Altogether, these experiments indicate that segmentation-related specifications of the hindbrain rhythmic neuronal network influences the respiratory patterns after birth. Therefore, early developmental processes have to be taken into account to understand normal and pathological diversity of the breathing behaviour in vertebrates.