Le complexe ESCRT : du transport endosomal au développement d’organismes multicellulaires

Depuis sa découverte il y a un peu plus de 50 ans, le système endo-lysosomal a émergé comme un processeur central de différentes activités cellulaires. Cette machinerie de transport vésiculaire intervient dans des processus aussi divers que la transduction de stimuli par des récepteurs de facteurs de croissance, le recyclage et la sécrétion de molécules de signalisation ou la régulation autophagique de l’homéostasie cellulaire. En conséquence des dysfonctionnements de la machinerie de transport vésiculaire sont impliqués dans un nombre grandissant de pathologies. Dans cette revue, nous prenons comme exemple le complexe ESCRT (Endosomal Sorting Complex Required for Transport) pour illustrer les multiples fonctions exercées par une machinerie de transport endosomal qui a été conservée tout au long de l’évolution. Ici nous décrivons les principes directeurs qui ont émergé de l’étude de la fonction de ces facteurs à l’échelle du développement et de la physiologie d’organismes multicellulaires. Nous illustrerons plus particulièrement le rôle essentiel des ESCRT dans trois fonctions biologiques : la régulation endosomale de la signalisation cellulaire, l’autophagie et son importance pour la morphogénèse neuronale, et enfin la biogénèse et la fonction de vésicules extracellulaires

Three Different noggin Genes Antagonize the Activity of Bone Morphogenetic Proteins in the Zebrafish Embryo

International audienceThe dorsoventral polarity of the vertebrate embryo is established through interactions between ventrally expressed bone morphogenetic proteins and their organizer-borne antagonists Noggin, Chordin, and Follistatin. While the opposing interactions between Short Gastrulation/Chordin and Decapentaplegic/BMP4 have been evolutionarily conserved in arthropods and vertebrates, there has been up to now no functional evidence of an implication of Noggin in the early patterning of organisms other than Xenopus. We have studied the contribution of Noggin to the embryonic development of the zebrafish. While single-copy noggin genes have been characterized in several vertebrate species, we report that the zebrafish genome harbors three noggin homologues. Overexpression experiments show that Noggin1, Noggin2, and Noggin3 can antagonize ventralizing BMPs. While all three factors have similar biological activities, their embryonic expression is different. The combined expression of the three genes recapitulates the different aspects of the expression of the single-copy noggin genes of other organisms. This suggests that the three zebrafish noggin genes and the single noggin genes of other vertebrates have evolved from a common ancestor and that subsequent differential loss of tissue-specific elements in the promoters of the different zebrafish genes accounts for their more restricted spatiotemporal expression. Finally we show that noggin1 is expressed in the fish organizer and able to dorsalize the embryo, suggesting its implication in the dorsoventral patterning of the zebrafish

Fgf signalling controls the dorsoventral patterning of the zebrafish embryo

International audienceThe establishment of dorsoventral (DV) patterning in vertebrate embryos depends on the morphogenic activity of a group of Tgfβ superfamily members, the bone morphogenetic proteins (Bmps) (which specify ventral cell fates), and on their interaction with their dorsally secreted cognate inhibitors chordin and noggin. In the zebrafish, genetic analysis has revealed that Bmp2b and Bmp7, as well as their antagonist chordin, are required for proper DV patterning. The expression of Bmp genes is initially activated in the whole blastula. Well before the beginning of gastrulation, Bmp gene expression progressively disappears from the dorsal side to become restricted to the ventral part of the embryo. We show that this early restriction of Bmp gene expression, which occurs independently of noggin and chordin, is an essential step in the establishment of DV patterning. The progressive ventral restriction of Bmp gene transcripts is coincident with the spreading of Fgf activity from the dorsal side of the embryo, suggesting that Fgf signalling is implicated in dorsal downregulation of Bmp gene expression. In accordance with this, activation of the Fgf/Ras/Mapk-signalling pathway inhibits ventral Bmp gene expression, thereby causing a dorsalisation of the embryo. Conversely,inhibition of Fgf signalling causes Bmp gene expression to expand dorsally,leading to an expansion of ventral cell fates. In accordance with an important role of Fgf signalling in the DV patterning of the zebrafish, we show that loss of Fgf8 function enhances the ventralisation of chordin-deficient embryos. Our results thereby demonstrate that pre-gastrula stage Fgf-signalling is essential to delimit the expression domain of the genes encoding the functional morphogen of the dorsoventral axis of the early zebrafish embryo

Notch-mediated inhibition of neurogenesis is required for zebrafish spinal cord morphogenesis

International audienceThe morphogenesis of the nervous system requires coordinating the specification and differentiation of neural precursor cells, the establishment of neuroepithelial tissue architecture and the execution of specific cellular movements. How these aspects of neural development are linked is incompletely understood. Here we inactivate a major regulator of embryonic neurogenesis - the Delta/Notch pathway - and analyze the effect on zebrafish central nervous system morphogenesis. While some parts of the nervous system can establish neuroepithelial tissue architecture independently of Notch, Notch signaling is essential for spinal cord morphogenesis. In this tissue, Notch signaling is required to repress neuronal differentiation and allow thereby the emergence of neuroepithelial apico-basal polarity. Notch-mediated suppression of neurogenesis is also essential for the execution of specific morphogenetic movements of zebrafish spinal cord precursor cells. In the wild-type neural tube, cells divide at the organ midline to contribute one daughter cell to each organ half. Notch signaling deficient animals fail to display this behavior and therefore form a misproportioned spinal cord. Taken together, our findings show that Notch-mediated suppression of neurogenesis is required to allow the execution of morphogenetic programs that shape the zebrafish spinal cord

The E3 Ubiquitin Ligase Mindbomb1 controls zebrafish Planar Cell Polarity

ABSTRACT Vertebrate Delta/Notch signaling involves multiple ligands, receptors and transcription factors. Delta endocytosis – a critical event for Notch activation – is however essentially controlled by the E3 Ubiquitin ligase Mindbomb1 (Mib1). Due to its position at a molecular bottleneck of the pathway, Mib1 inactivation is often used to inhibit Notch signaling. However, recent findings indicate that the importance of Mib1 extends beyond the Notch pathway. We report an essential role of Mib1 in Planar Cell Polarity (PCP). mib1 null mutants or morphants display impaired gastrulation stage Convergence Extension (CE) movements. Comparison of different mib1 mutants and functional rescue experiments indicate that Mib1 controls CE independently of Notch. In contrast, Mib1-dependent CE defects can be rescued using the PCP downstream mediator RhoA. Mib1 regulates CE through the RING Finger domains that have been implicated in substrate ubiquitination, suggesting that Mib1 may control PCP protein trafficking. Accordingly, we show that Mib1 controls the endocytosis of the PCP component Ryk and that Ryk internalization is required for CE. Numerous morphogenetic processes involve both Notch and PCP signaling. We show that Mib1, a known Notch signaling regulator, is also an essential PCP pathway component. Care should therefore be taken when interpreting Mib1 loss of function phenotypes

Myosin1D is an evolutionarily conserved regulator of animal left–right asymmetry

International audienceThe establishment of left-right (LR) asymmetry is fundamental to animal development, but the identification of a unifying mechanism establishing laterality across different phyla has remained elusive. A cilia-driven, directional fluid flow is important for symmetry breaking in numerous vertebrates, including zebrafish. Alternatively, LR asymmetry can be established independently of cilia, notably through the intrinsic chirality of the acto-myosin cytoskeleton. Here, we show that Myosin1D (Myo1D), a previously identified regulator of Drosophila LR asymmetry, is essential for the formation and function of the zebrafish LR organizer (LRO), Kupffer's vesicle (KV). Myo1D controls the orientation of LRO cilia and interacts functionally with the planar cell polarity (PCP) pathway component VanGogh-like2 (Vangl2), to shape a productive LRO flow. Our findings identify Myo1D as an evolutionarily conserved regulator of animal LR asymmetry, and show that functional interactions between Myo1D and PCP are central to the establishment of animal LR asymmetry

The ESCRT machinery regulates the secretion and long-range activity of Hedgehog.

International audienceThe conserved family of Hedgehog (Hh) proteins acts as short- and long-range secreted morphogens, controlling tissue patterning and differentiation during embryonic development. Mature Hh carries hydrophobic palmitic acid and cholesterol modifications essential for its extracellular spreading. Various extracellular transportation mechanisms for Hh have been suggested, but the pathways actually used for Hh secretion and transport in vivo remain unclear. Here we show that Hh secretion in Drosophila wing imaginal discs is dependent on the endosomal sorting complex required for transport (ESCRT). In vivo the reduction of ESCRT activity in cells producing Hh leads to a retention of Hh at the external cell surface. Furthermore, we show that ESCRT activity in Hh-producing cells is required for long-range signalling. We also provide evidence that pools of Hh and ESCRT proteins are secreted together into the extracellular space in vivo and can subsequently be detected together at the surface of receiving cells. These findings uncover a new function for ESCRT proteins in controlling morphogen activity and reveal a new mechanism for the transport of secreted Hh across the tissue by extracellular vesicles, which is necessary for long-range target induction

Report of the Second European Zebrafish Principal Investigator Meeting in Karlsruhe, Germany, March 21-24, 2012.

The second European Zebrafish Principal Investigator (PI) Meeting was held in March, 2012, in Karlsruhe, Germany. It brought together PIs from all over Europe who work with fish models such as zebrafish and medaka to discuss their latest results, as well as to resolve strategic issues faced by this research community. Scientific discussion ranged from the development of new technologies for working with fish models to progress in various fields of research such as injury and repair, disease models, and cell polarity and dynamics. This meeting also marked the establishment of the European Zebrafish Resource Centre (EZRC) at Karlsruhe that in the future will serve as an important focus and community resource for zebrafish- and medaka-based research