Ontogeny of central rhythm generation in chicks and rodents

https://doi.org/10.1016/j.resp.2006.02.004Get rights and content

Abstract

Recent studies help in understanding how the basic organization of brainstem neuronal circuits along the anterior–posterior (AP) axis is set by the Hox-dependent segmentation of the neural tube in vertebrate embryos. Neonatal respiratory abnormalities in Krox20−/−, Hoxa1−/− and kreisler mutant mice indicate the vital role of a para-facial (Krox20-dependent, rhombomere 4-derived) respiratory group, that is distinct from the more caudal rhythm generator called Pre-Bötzinger complex. Embryological studies in the chick suggest homology and conservation of this Krox20-dependent induction of parafacial rhythms in birds and mammals. Calcium imaging in embryo indicate that rhythm generators may derive from different cell lineages within rhombomeres. In mice, the Pre-Bötzinger complex is found to be distinct from oscillators producing the earliest neuronal activity, a primordial low-frequency rhythm. In contrast, in chicks, maturation of the parafacial generator is tightly linked to the evolution of this primordial rhythm. It seems therefore that ontogeny of brainstem rhythm generation involves conserved processes specifying distinct AP domains in the neural tube, followed by diverse, lineage-specific regulations allowing the emergence of organized rhythm generators at a given AP level.

Introduction

During development, the assembly of neural circuits that encode animal behaviour results from several mechanisms contributing to generate distinct neuronal cell types in appropriate number and position. Among these mechanisms, regionalization of the neural tube is conserved among vertebrates: it controls cell proliferation and exit from the cell cycle at precise location of the neural tube (Lumsden and Krumlauf, 1996, Tanabe and Jessell, 1996). Neurogenesis refers to a variety of processes by which cell types differentiate, migrate and form nuclei to eventually produce neuronal populations and their synaptic interconnections (Tanabe and Jessell, 1996). Once activity starts in primordial neurons, neuronal circuitry is refined in a use-dependent manner (Katz and Shatz, 1996) and the size of neuronal populations depends on the activity of neurotrophic factors interacting with apoptotic processes (Oppenheim, 1991). Downstream mechanisms greatly vary among vertebrate species. By considering experiments in mice and chick, the present review investigates to what extent the conserved molecular signalling operating during the segmentation of the neural tube may be responsible for the anterior–posterior subdivision of the rhythm generating networks in the brainstem.

In all vertebrates, regionalization of the brainstem (rhombencephalon) along the antero-posterior axis, leads the partitioning of the neuroepithelium into a series of cellular compartments, called rhombomeres (r1–r8, Lumsden and Krumlauf, 1996). This process takes place between embryonic days E8 and E12 in mice, between Hamburger and Hamilton (1951) stages HH9 and HH24 in chicks, and influences later differentiation and spatial distribution of neuronal patterns (Lumsden and Keynes, 1989). The branchiomotor nuclei conform to the rhombomeric pattern with a two-segment periodicity. Trigeminal motoneurons originate from r2 and r3 and send their axons to an exit point in r2. In mammals, the facial branchial nucleus originates in r4. The ventral and dorsal motor nuclei of glossopharyngeal–vagal nerves derive from r6–r8.

In rodents, respiratory groups have been located with the help of preparations isolated in vitro. A correlation can be suggested with the rhombomere related organization of the branchio-motor nuclei. Caudally, at the vagal/glossopharyngeal level, the inspiratory rhythm is generated in the pre-Bötzinger complex (PBC), and persists in coronal brainstem slices of newborn rodents, isolated in vitro (Smith et al., 1991, Gray et al., 1999, Lieske et al., 2000). At the facial level, another rhythm generator, called the para-facial respiratory group (pFRG, Onimaru and Homma, 2003), has been recently delineated by activity-dependent imaging. The pFRG is characterized by a pre-inspiratory pattern of neuronal activity that stops during inspiration and resumes afterward by a post-inhibitory rebound. In the hindbrain isolated in vitro, permanent rhythm generation requires a balanced interaction between the pFRG and the PBC, which can be disrupted by potent respiratory depressants such as opioïds (Denavit-Saubié et al., 1978, Morin-Surun et al., 1984, Morin-Surun et al., 2001, Mellen et al., 2003, Manzke et al., 2003). At the trigeminal level, ventral pontine controls of the rhythm (Borday et al., 1997) include noradrenergic neurons of the A5 group exerting a depressant effect upon the more caudal respiratory group (Di Pasquale et al., 1992).

In the following article, we consider the role of Hox genes responsible for the segmental organization of the rhombencephalon and branchio-motor nuclei. We will discuss their influence on the emergence of rhythm generators, at specific antero-posterior levels of the active hindbrain.

Section snippets

The murine parafacial controller requires expression of Krox20 in r3 and Hoxa1 in r4

Strong regulatory constraints couple Hox gene expression to the progression of embryogenesis. Therefore, the chromosomal organization of Hox genes into four clusters is highly conserved in vertebrates. In birds and mammals, the formation of territories patterned by Hox genes is accompanied by a sequential activation of these genes from 3′ to 5′ in the clusters. As a result, early structures are given an anterior identity with 3′ Hox genes as key determinants, while progressively later

Development of para-facial rhythm controlling systems in the segmented rhombencephalon of the chick

Krox20 and Hox expression, boundary formation and neurogenesis do not take place at the same time in adjacent rhombomeres. Between E9.5 and E10.5 in mice, and between HH10 and HH15-20 in chick, r3 displays a marked delay compared with even rhombomeres in the timing of neuronal differentiation and axonal outgrowth (Lumsden and Keynes, 1989, Schneider-Maunoury et al., 1993, Giudicelli et al., 2001). Therefore, heterochrony of neurogenic processes at the parafacial level of the neuraxis allows

Evolution of the low frequency primordial activity in chick and mouse

The early mechanisms of regionalization and neurogenesis involve large domains of the neural tube including a variety of neuronal types and developing circuits. It is therefore likely that the Krox20 induction targets different cell populations within r4. As a consequence, to what extent murine respiratory generators and the chick episodic generators derive from homologous or distinct progenitors remains unknown. The mature respiratory function of rhythm generators is difficult to assess in

Conclusion

It seems that a conserved signaling pathway initiated by Krox20 expression induces development of parafacial rhythm generators, thereby establishing homology of the parafacial territory in chick as in mouse. In this respect, it is important to note that we have been able to induce formation of an episodic activity in chick by mis-expressing mouse mKrox-20 (Coutinho et al., 2004). Thus, major molecular pathways defining the parafacial territory are compatible in the two species. Homology between

Acknowledgements

This work was supported by CNRS, ACI BDPI#57, E.C. grant “Brainstem Genetics” QLG2-CT-2001-01467, C.B. is supported by the FRM and L.W. by the MRT.

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    Present address: Cellular and Molecular Biology Department, Karolinska Institutet, 171 77 Stockholm, Sweden.

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