Jean-Stéphane Joly

Summary: The central nervous system (cerebral ganglion) of adult ascidians is linked to the neural gland complex (NGC), which consists of a dorsal tubercle, a ciliated duct and a neural gland. The function of the NGC has been the subject of much debate. The recent publication of the complete genomic sequence of Ciona intestinalis provides new opportunities to examine the presence and distribution of protein families in this basal chordate. We focus here on the ascidian neuropeptide G‐protein‐coupled receptors (GPCRs), the vertebrate homologues of which are involved in homeostasis. In situ hybridization revealed that five Ciona GPCRs [vasopressin receptor, somatostatin receptor, CRH (corticotropin‐releasing hormone) receptor, angiotensin receptor and tachykinin receptor] are expressed in the NGC of adult ascidians. These findings, together with histological and ultrastructural data, provide evidence to support a role for the ascidian NGC in maintaining ionic homeostasis. We further speculate about the potential similarities between the ascidian NGC and the vertebrate choroid plexus, a neural peri‐ventricular organ.

Le terme prion a tendance à être utilisé pour désigner un nombre de plus en plus vaste de protéines souvent très divergentes. L’état actuel des connaissances permet de conclure à l’existence de ’prions’ potentiellement pathogènes chez les Mammifères uniquement. Des protéines de la même famille ont été trouvées chez les Oiseaux et les Tortues. Enfin, aucun homologue des ’prions’ n’a pu être mis en évidence chez les Poissons, en dépit de recherches approfondies. Une révision de la nomenclature de ces molécules semble donc s’imposer, à des fins de précision scientifique et d’information claire du public.

In vertebrates, the engrailed genes are expressed at early neurula stage in a narrow stripe encompassing the midbrain-hindbrain boundary (MHB), a region from which a peculiar structure, the isthmus, is formed. Knock-out experiments in mice demonstrated that these genes are essential for the development of this structure and of its derivatives. In contrast, little is known about the effect of an overexpression of engrailed genes in vertebrate development. Here we report the isolation of Ol-eng2, a medaka fish (Oryzias latipes) engrailed gene. We have monitored the effects of its widespread expression following mRNA injections in 1- and 2-cell medaka and Xenopus embryos. We found that the ectopic expression of Ol-eng2 predominantly results in an altered development of the anterior brain, including an inhibition of optic vesicle formation. No change in the patterns of mesencephalic and telencephalic markers were observed. In contrast, expressions of markers of the diencephalon were strongly repressed in injected embryos. Furthermore, the endogenous Ol-eng2, Pax2, Wnt1 and Fgf8, which are essential components of the MHB genetic cascade, were ectopically expressed in this region. Therefore, we propose that Ol-eng2 induces de novo formation of an isthmus-like structure, which correlates with the development of ectopic midbrain structures, including optic tectum. A competence of the diencephalon to change to a midbrain fate has been demonstrated in isthmic graft experiments. Our data demonstrate that this change can be mimicked by ectopic engrailed expression alone.