Evolution of the hypoxia-sensitive cells involved in amniote respiratory reflexes

The evolutionary origins of the hypoxia-sensitive cells that trigger amniote respiratory reflexes - carotid body glomus cells, and 'pulmonary neuroendocrine cells' (PNECs) - are obscure. Homology has been proposed between glomus cells, which are neural crest-derived, and the hypoxia-sensitive 'neuroepithelial cells' (NECs) of fish gills, whose embryonic origin is unknown. NECs have also been likened to PNECs, which differentiate in situ within lung airway epithelia. Using genetic lineage-tracing and neural crest-deficient mutants in zebrafish, and physical fate-mapping in frog and lamprey, we find that NECs are not neural crest-derived, but endoderm-derived, like PNECs, whose endodermal origin we confirm. We discover neural crest-derived catecholaminergic cells associated with zebrafish pharyngeal arch blood vessels, and propose a new model for amniote hypoxia-sensitive cell evolution: endoderm-derived NECs were retained as PNECs, while the carotid body evolved via the aggregation of neural crest-derived catecholaminergic (chromaffin) cells already associated with blood vessels in anamniote pharyngeal arches.

Sensory Activation of Command Cells for Locomotion and Modulatory Mechanisms: Lessons from Lampreys

Sensorimotor transformation is one of the most fundamental and ubiquitous functions of the central nervous system (CNS). Although the general organization of the locomotor neural circuitry is relatively well understood, less is known about its activation by sensory inputs and its modulation. Utilizing the lamprey model, a detailed understanding of sensorimotor integration in vertebrates is emerging. In this article, we explore how the vertebrate CNS integrates sensory signals to generate motor behavior by examining the pathways and neural mechanisms involved in the transformation of cutaneous and olfactory inputs into motor output in the lamprey. We then review how 5-hydroxytryptamine (5-HT) acts on these systems by modulating both sensory inputs and motor output. A comprehensive review of this fundamental topic should provide a useful framework in the fields of motor control, sensorimotor integration and neuromodulation.

Spatiotemporal Pattern of Doublecortin Expression in the Retina of the Sea Lamprey

Despite the importance of doublecortin (DCX) for the development of the nervous system, its expression in the retina of most vertebrates is still unknown. The key phylogenetic position of lampreys, together with their complex life cycle, with a long blind larval stage and an active predator adult stage, makes them an interesting model to study retinal development. Here, we studied the spatiotemporal pattern of expression of DCX in the retina of the sea lamprey. In order to characterize the DCX expressing structures, the expression of acetylated α-tubulin (a neuronal marker) and cytokeratins (glial marker) was also analyzed. Tract-tracing methods were used to label ganglion cells. DCX immunoreactivity appeared initially in photoreceptors, ganglion cells and in fibers of the prolarval retina. In larvae smaller than 100 mm, DCX expression was observed in photoreceptors, in cells located in the inner nuclear and inner plexiform layers (IPLs) and in fibers coursing in the nuclear and IPLs, and in the optic nerve (ON). In retinas of premetamorphic and metamorphic larvae, DCX immunoreactivity was also observed in radially oriented cells and fibers and in a layer of cells located in the outer part of the inner neuroblastic layer (INbL) of the lateral retina. Photoreceptors and fibers ending in the outer limitans membrane (OLM) showed DCX expression in adults. Some retinal pigment epithelium cells were also DCX immunoreactive. Immunofluorescence for α-tubulin in premetamorphic larvae showed coexpression in most of the DCX immunoreactive structures. No cells/fibers were found showing DCX and cytokeratins colocalization. The perikaryon of mature ganglion cells is DCX negative. The expression of DCX in sea lamprey retinas suggests that it could play roles in the migration of cells that differentiate in the metamorphosis, in the establishment of connections of ganglion cells and in the development of photoreceptors. Our results also suggest that the radial glia and retinal pigment epithelium cells of lampreys are neurogenic. Comparison of our observations with those reported in gnathostomes reveals similarities and interesting differences probably due to the peculiar development of the sea lamprey retina.

Xenopus embryonic epidermis as a mucociliary cellular ecosystem to assess the effect of sex hormones in a non-reproductive context

Background

How important are sexual hormones beyond their function in reproductive biology has yet to be understood. In this study, we analyzed the effects of sex steroids on the biology of the embryonic amphibian epidermis, which represents an easily amenable model of non-reproductive mucociliary epithelia (MCE). MCE are integrated systems formed by multiciliated (MC), mucus-secreting (MS) and mitochondrion-rich (MR) cell populations that are shaped by their microenvironment. Therefore, MCE could be considered as ecosystems at the cellular scale, found in a wide array of contexts from mussel gills to mammalian oviduct.

Results

We showed that the natural estrogen (estradiol, E2) and androgen (testosterone, T) as well as the synthetic estrogen (ethinyl-estradiol, EE2), all induced a significant enhancement of MC cell numbers. The effect of E2, T and EE2 extended to the MS and MR cell populations, to varying degrees. They also modified the expression profile of RNA MCE markers, and induced a range of “non-typical” cellular phenotypes, with mixed identities and aberrant morphologies, as revealed by imaging analysis through biomarker confocal detection and scanning electron microscopy. Finally, these hormones also affected tadpole pigmentation, revealing an effect on the entire cellular ecosystem of the Xenopus embryonic skin.

Conclusions

This study reveals the impact in vivo, at the molecular, cellular, tissue and organism levels, of sex steroids on non-reproductive mucociliary epithelium biogenesis, and validates the use of Xenopus as a relevant model system in this field.

The sea lamprey UNC5 receptors: cDNA cloning, phylogenetic analysis and expression in reticulospinal neurons at larval and adult stages of development

UNC5 receptors mediate repulsive signaling of netrin on neurons. Although only one UNC5 receptor has been identified in invertebrates, four members of the UNC5 family have been identified in gnathostomes. Lampreys, together with mixynes, belong to the oldest branch of extant vertebrates, and their phylogenetic position near to the vertebrate root makes them an interesting model for understanding molecular evolution. Here, we cloned three sea lamprey UNC5 (UNC5L) receptors, and phylogenetic analyses indicated that the first two duplications of the ancestral UNC5 gene occurred before the separation of jawless and jawed vertebrates. UNC5 receptors play important roles during early development, but expression studies have also suggested that UNC5 receptors play roles in the mature nervous system. Here, we report the expression of the different UNC5L receptor transcripts in identified reticulospinal neurons of mature larval or adult sea lampreys detected by in situ hybridization in wholemounted brain preparations. In addition, an extensive expression of the UNC5 receptors was also observed in most brain regions of the adult lamprey. An increase in the types of identifiable reticulospinal neurons expressing the UNC5L receptors was observed in adults compared with larvae. Expression of UNC5 receptors at late developmental stages appears to be a shared characteristic of lampreys and mammals. In larvae, expression of UNC5L receptors was observed in reticulospinal neurons that when axotomized are known to be "bad regenerators." Results in lampreys and mammals suggest that the UNC5-Netrin axonal guidance system may play a role in limiting axonal regeneration after spinal cord injury.

The sea lamprey tryptophan hydroxylase: new insight into the evolution of the serotonergic system of vertebrates

Recent research has shown that at least two tryptophan hydroxylase (Tph) genes are present in gnathostome vertebrates, but it is not known when the duplication of the ancestral Tph gene took place during evolution. By their position as an out-group of gnathostomes, lampreys (agnathans) are key models to understand molecular evolution in vertebrates. Here, we report the cloning of a Tph cDNA of the sea lamprey and the pattern of Tph mRNA expression in larval and postmetamorphic (young adult) sea lampreys using in situ hybridization. Phylogenetic analysis indicated that the lamprey Tph is an orthologue of Tphs of other vertebrates and suggested that the duplication of the ancestral Tph gene occurred before the separation of agnathans and gnathostomes, although alternative hypothesis are also discussed in the present study. In the sea lamprey brain, the Tph transcript was expressed in perikarya of the pineal organ, the retina, the diencephalic and rhombencephalic nuclei reported previously with serotonin immunohistochemistry and in small cells of the spinal cord, with a pattern similar to that observed with anti-serotonin antibodies. This suggests that expression of this Tph gene is shared by all lamprey serotonergic brain populations, unlike that reported in zebrafish and mammals for their different Tph genes. However, no Tph expression was observed in peripheral serotonergic cells, which, unlike in other vertebrates, are widely distributed in lampreys. Our results suggest that the selection of Tph2 to be expressed in raphe neurons may have occurred along the line leading to gnathostomes.

Immunohistochemical localization of serotonin in the brain during natural sex change in the hermaphroditic goby Lythrypnus dalli

The neurotransmitter serotonin (5-HT) may play a central role in the inhibition of socially regulated sex change in fish because of its known modulation of both aggressive and reproductive behavior. This is the first study to use immunohistochemical techniques to examine the morphometry of serotonergic neurons at different times during sex change. Using a model species wherein sex change is socially regulated via agonistic social interactions (the bluebanded goby, Lythrypnus dalli), we sampled brains of males and females with different social status, and of females at different times during sex change. Consistent with previous studies on other teleosts, immunoreactive neurons were found in the posterior periventricular nucleus (NPPv), the nucleus of the lateral recess (NRL), the nucleus of the posterior recess (NRP) and in the raphe nucleus. We measured the total area of NPPv, NRL, NRP, and the number and mean cell area of serotonergic neurons in the raphe nucleus. There was no significant difference in any of the brain regions between males, females or sex changing fish, but there was a slight increase in the number of dorsal raphe neurons in the brain of sex changers 2h after male removal. The results show that in L. dalli the serotonergic system does not present any morphological sex and status differences, nor any dramatic modifications during sex change. These data, together with previous results, do not support the hypothesis that serotonin inhibits socially regulated sex change.