Su(H) mediated Notch signalling and the role of different her genes during zebrafish somitogenesis

Somitogenesis is the key developmental process, which divides the vertebrate body axis into segmentally repeated structures. These structures are called somites. Somites derive from the unsegmented presomitic mesoderm (PSM) that flanks the notochord to both sides. A prepatterning process, taking place in the PSM, is necessary to allow the exact spatial and temporal formation of the somites. The prepatterning is achieved by a clock and wavefront mechanism. The clock consists of the Delta-Notch (D-N) pathway, building up a genetic circuit with several cyclically expressed h/E(spl)/hey-related genes while the wave front is created by a FGF gradient, showing its highest expression in the posterior PSM. Disturbance of the clock or the mediator of the wavefront (her13.2) results in a disruption of cyclic gene expression and posterior somite border formation, while anterior somites are still formed. On the level of Delta-Notch signalling it is not clear if the escaped anterior somites are formed due to redundancy, since there are at least four notch and four delta homologues in zebrafish. Furthermore it is not known if Notch signalling is transmitted via the canonical way through Su(H) during somitogenesis or if an alternative way is used. Since there appears to be only one complete Su(H) homologue in zebrafish, the function of this gene was analyzed using morpholino oligonucleotides. The knockdown of Su(H) leads to a clear disruption of cyclic gene expression, comparable to effects in previously described D-N mutants. Beyond this, posterior somite defects were detected while anterior somites were still formed, implying that their formation is not due to redundancy between different delta or notch genes. Performing the Su(H) knockdown in the fss/tbx24 mutant it could be shown that D-N signalling is necessary for the creation and synchronization of cyclic gene expression. These results clearly suggest that the canonical way of Notch signalling is used during somitogenesis. To further specify the prepatterning process two newly identified her genes, her11 and her12, were analyzed during somitogenesis. It turned out that both genes are dynamically expressed in the PSM and are differentially regulated by D-N signalling. Functional studies suggest that her11 interacts with her1 and her7 and is involved in the fine tuning of cyclic gene expression while her12 seems to be involved in somite border formation and cyclic gene expression. It was recently shown that the D-N driven Her1 protein and the FGF activated Her13.2 protein form heterodimers in vitro. To proof a combinatorial function also in vivo, both genes were knocked down individually and in combination. The combined knockdown leads to distinct additional effects, namely the break down of cyclic gene expression right from the start and a disruption of anterior somite formation. This suggests clearly a combinatorial role for both genes in vivo during early somitogenesis

Zebrafish as a model organism for neurodegenerative disease

The zebrafish is increasingly recognized as a model organism for translational research into human neuropathology. The zebrafish brain exhibits fundamental resemblance with human neuroanatomical and neurochemical pathways, and hallmarks of human brain pathology such as protein aggregation, neuronal degeneration and activation of glial cells, for example, can be modeled and recapitulated in the fish central nervous system. Genetic manipulation, imaging, and drug screening are areas where zebrafish excel with the ease of introducing mutations and transgenes, the expression of fluorescent markers that can be detected in vivo in the transparent larval stages overtime, and simple treatment of large numbers of fish larvae at once followed by automated screening and imaging. In this review, we summarize how zebrafish have successfully been employed to model human neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease, amyotrophic lateral sclerosis, and Huntington’s disease. We discuss advantages and disadvantages of choosing zebrafish as a model for these neurodegenerative conditions

Complement as a regulator of adaptive immunity

International audienceThe complement system is an ancient and evolutionarily conserved effector system comprising in mammals over 50 circulating and membrane bound proteins. Complement has long been described as belonging to the innate immune system; however, a number of recent studies have demonstrated its key role in the modulation of the adaptive immune response. This review does not set out to be an exhaustive list of the numerous interactions of the many complement components with adaptive immunity; rather, we will focus more precisely on the role of some complement molecules in the regulation of antigen presenting cells, as well as on their direct effect on the activation of the core adaptive immune cells, B and T lymphocytes. Recent reports on the local production and activation of complement proteins also suggest a major role in the control of effector responses. The crucial role of complement in adaptive immunity is further highlighted by several examples of dysregulation of these pathways in human diseases

Wasl is crucial to maintain microglial core activities during glioblastoma initiation stages

Microglia actively promotes the growth of high‐grade gliomas. Within the glioma microenvironment an amoeboid microglial morphology has been observed, however the underlying causes and the related impact on microglia functions and their tumor promoting activities is unclear. Using the advantages of the larval zebrafish model, we identified the underlying mechanism and show that microglial morphology and functions are already impaired during glioma initiation stages. The presence of pre‐neoplastic HRasV12 expressing cells induces an amoeboid morphology of microglia, increases microglial numbers and decreases their motility and phagocytic activity. RNA sequencing analysis revealed lower expression levels of the actin nucleation promoting factor wasla in microglia. Importantly, a microglia specific rescue of wasla expression restores microglial morphology and functions. This results in increased phagocytosis of pre‐neoplastic cells and slows down tumor progression. In conclusion, we identified a mechanism that de‐activates core microglial functions within the emerging glioma microenvironment. Restoration of this mechanism might provide a way to impair glioma growth

Factor associated with neutral sphingomyelinase activity mediates navigational capacity of leukocytes responding to wounds and infection:live imaging studies in zebrafish larvae

Factor associated with neutral sphingomyelinase activity (FAN) is an adaptor protein that specifically binds to the p55 receptor for TNF (TNF-RI). Our previous investigations demonstrated that FAN plays a role in TNF-induced actin reorganization by connecting the plasma membrane with actin cytoskeleton, suggesting that FAN may impact on cellular motility in response to TNF and in the context of immune inflammatory conditions. In this study, we used the translucent zebrafish larvae for in vivo analysis of leukocyte migration after morpholino knockdown of FAN. FAN-deficient zebrafish leukocytes were impaired in their migration toward tail fin wounds, leading to a reduced number of cells reaching the wound. Furthermore, FAN-deficient leukocytes show an impaired response to bacterial infections, suggesting that FAN is generally required for the directed chemotactic response of immune cells independent of the nature of the stimulus. Cell-tracking analysis up to 3 h after injury revealed that the reduced number of leukocytes is not due to a reduction in random motility or speed of movement. Leukocytes from FAN-deficient embryos protrude pseudopodia in all directions instead of having one clear leading edge. Our results suggest that FAN-deficient leukocytes exhibit an impaired navigational capacity, leading to a disrupted chemotactic response

A novel brain tumour model in zebrafish reveals the role of YAP activation in MAPK/PI3K induced malignant growth

Somatic mutations activating MAPK/PI3K signalling play a pivotal role in both tumours and brain developmental disorders. We developed a zebrafish model of brain tumour based on somatic expression of oncogenes that activate MAPK/PI3K signalling in neural progenitor cells. HRASV12 was the most effective in inducing both heterotopia and invasive tumours. Tumours, but not heterotopias, require persistent activation of phospho‑(p)ERK and express a gene signature similar to the mesenchymal glioblastoma subtype, with a strong YAP component. Application of a 8-gene signature to human brain tumours establishes that YAP activation distinguishes between mesenchymal glioblastoma and low grade glioma in a wide TCGA sample set including gliomas and glioblastomas (GBMs). This suggests that the activation of YAP may be an important event in brain tumour development, promoting malignant versus benign brain lesions. Indeed, co-expression of dominant active YAP (YAPS5A) and HRASV12 abolishes the development of heterotopias and leads to the sole development of aggressive tumours. Thus, we have developed a model proving that neurodevelopmental disorders and brain tumours may originate from the same somatic mutations activating oncogenes and established that YAP activation is a hallmark of malignant brain tumours