Evolutionary Divergence in the Hedgehog Pathway

I like this paper because the findings that it reports took me completely by surprise—and serve as a constant reminder of the fallibility of my own scientific logic! A few years earlier, Rune Toftgård had invited me to give a talk at the Karolinska Institute about our analysis of the Hedgehog (HH) signaling pathway in Drosophila. During the course of my visit, Rune told me that he and his colleagues were planning to make a mouse knockout mutation of the Suppressor of fused (SUFU) gene, an exercise that I opined would be of only marginal value, given the dispensable nature of the orthologous gene in Drosophila. Indeed, my skepticism seemed well placed when we subsequently found that morpholino-mediated knockdown of SUFU has a rather subtle effect on HH signaling in zebrafish. But clearly undeterred by my advice, Toftgård and colleagues proceeded to generate a SUFU null mutation, the phenotype of which demonstrates its pivotal role in mammalian HH signaling!This PaperPick refers to “Genetic Elimination of Suppressor of Fused Reveals an Essential Repressor Function in the Mammalian Hedgehog Signaling Pathway,” by J. Svärd, K. Heby-Henricson, M. Persson-Lek, B. Rozell, M. Lauth, A. Bergström, J. Ericson, R. Toftgård, and S. Teglund, published in February 2006.Video AbstractDrs. Toftgård, Teglund, and Heby-Henricson offer a personal account of the work published in this paper and highlight areas of current research on the mechanisms of Hedgehog signal transduction

Plasticity in Zebrafish hox Expression in the Hindbrain and Cranial Neural Crest

AbstractThe anterior–posterior identities of cells in the hindbrain and cranial neural crest are thought to be determined by their Hox gene expression status, but how and when cells become committed to these identities remain unclear. Here we address this in zebrafish by cell transplantation, to test plasticity in hox expression in single cells. We transplanted cells alone, or in small groups, between hindbrain rhombomeres or between the neural crest primordia of pharyngeal arches. We found that transplanted cells regulated hox expression according to their new environments. The degree of plasticity, however, depended on both the timing and the size of the transplant. At later stages transplanted cells were more likely to be irreversibly committed and maintain their hox expression, demonstrating a progressive loss of responsiveness to the environmental signals that specify segmental identities. Individual transplanted cells also showed greater plasticity than those lying within the center of larger groups, suggesting that a community effect normally maintains hox expression within segments. We also raised experimental embryos to larval stages to analyze transplanted cells after differentiation and found that neural crest cells contributed to pharyngeal cartilages appropriate to the anterior–posterior level of the new cellular environment. Thus, consistent with models implicating hox expression in control of segmental identity, plasticity in hox expression correlates with plasticity in final cell fate

Spatiotemporal coordination of FGF and Shh signaling underlies the specification of myoblasts in the zebrafish embryo

Somitic cells give rise to a variety of cell types in response to Hh, BMP, and FGF signaling. Cell position within the developing zebrafish somite is highly dynamic: how, when, and where these signals specify cell fate is largely unknown. Combining four-dimensional imaging with pathway perturbations, we characterize the spatiotemporal specification and localization of somitic cells. Muscle formation is guided by highly orchestrated waves of cell specification. We find that FGF directly and indirectly controls the differentiation of fast and slow-twitch muscle lineages, respectively. FGF signaling imposes tight temporal control on Shh induction of slow muscles by regulating the time at which fast-twitch progenitors displace slow-twitch progenitors from contacting the Shh-secreting notochord. Further, we find a reciprocal regulation of fast and slow muscle differentiation, morphogenesis, and migration. In conclusion, robust cell fate determination in the developing somite requires precise spatiotemporal coordination between distinct cell lineages and signaling pathways

Publisher Correction: The ciliopathy protein TALPID3/KIAA0586 acts upstream of Rab8 activation in zebrafish photoreceptor outer segment formation and maintenance

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper

Prdm1- and Sox6-mediated transcriptional repression specifies muscle fibre type in the zebrafish embryo

The zebrafish u-boot (ubo) gene encodes the transcription factor Prdm1, which is essential for the specification of the primary slow-twitch muscle fibres that derive from adaxial cells. Here, we show that Prdm1 functions by acting as a transcriptional repressor and that slow-twitch-specific muscle gene expression is activated by Prdm1-mediated repression of the transcriptional repressor Sox6. Genes encoding fast-specific isoforms of sarcomeric proteins are ectopically expressed in the adaxial cells of ubotp39 mutant embryos. By using chromatin immunoprecipitation, we show that these are direct targets of Prdm1. Thus, Prdm1 promotes slow-twitch fibre differentiation by acting as a global repressor of fast-fibre-specific genes, as well as by abrogating the repression of slow-fibre-specific genes

Zebrafish models of the immune response: taking it on the ChIn

The zebrafish is proving to be an extremely versatile new experimental model for unraveling the mysteries of innate immunity and has considerable promise as a system for the identification of novel modulators of this crucial biological process. A rate-limiting factor, however, is the mechanical stimulus required to induce the inflammatory response. A new chemically induced inflammation assay ('ChIn' assay) published in BMC Biology obviates this requirement and seems set to accelerate progress in the field

Корекція гемореологічних порушень у хворих на цукровий діабет з використанням низькоінтенсивного лазерного опромінення крові

In mammals, the homeodomain transcription factor Prox1 acts as the central regulator of lymphatic cell fate. Its restricted expression in a subset of cardinal vein cells leads to a switch towards lymphatic specification and hence represents a prerequisite for the initiation of lymphangiogenesis. Murine Prox1-null embryos lack lymphatic structures, and sustained expression of Prox1 is indispensable for the maintenance of lymphatic cell fate even at adult stages, highlighting the unique importance of this gene for the lymphatic lineage. Whether this pre-eminent role of Prox1 within the lymphatic vasculature is conserved in other vertebrate classes has remained unresolved, mainly owing to the lack of availability of loss-of-function mutants. Here, we re-examine the role of Prox1a in zebrafish lymphangiogenesis. First, using a transgenic reporter line, we show that prox1a is initially expressed in different endothelial compartments, becoming restricted to lymphatic endothelial cells only at later stages. Second, using targeted mutagenesis, we show that Prox1a is dispensable for lymphatic specification and subsequent lymphangiogenesis in zebrafish. In line with this result, we found that the functionally related transcription factors Coup-TFII and Sox18 are also dispensable for lymphangiogenesis. Together, these findings suggest that lymphatic commitment in zebrafish and mice is controlled in fundamentally different ways

The transcription factor SOX6 contributes to the developmental origins of obesity by promoting adipogenesis

10.1242/dev.131573Development (Cambridge, England)1436950-961GUSTO (Growing up towards Healthy Outcomes

Positive and Negative Regulation of Gli Activity by Kif7 in the Zebrafish Embryo

Loss of function mutations of Kif7, the vertebrate orthologue of the Drosophila Hh pathway component Costal2, cause defects in the limbs and neural tubes of mice, attributable to ectopic expression of Hh target genes. While this implies a functional conservation of Cos2 and Kif7 between flies and vertebrates, the association of Kif7 with the primary cilium, an organelle absent from most Drosophila cells, suggests their mechanisms of action may have diverged. Here, using mutant alleles induced by Zinc Finger Nuclease-mediated targeted mutagenesis, we show that in zebrafish, Kif7 acts principally to suppress the activity of the Gli1 transcription factor. Notably, we find that endogenous Kif7 protein accumulates not only in the primary cilium, as previously observed in mammalian cells, but also in cytoplasmic puncta that disperse in response to Hh pathway activation. Moreover, we show that Drosophila Costal2 can substitute for Kif7, suggesting a conserved mode of action of the two proteins. We show that Kif7 interacts with both Gli1 and Gli2a and suggest that it functions to sequester Gli proteins in the cytoplasm, in a manner analogous to the regulation of Ci by Cos2 in Drosophila. We also show that zebrafish Kif7 potentiates Gli2a activity by promoting its dissociation from the Suppressor of Fused (Sufu) protein and present evidence that it mediates a Smo dependent modification of the full length form of Gli2a. Surprisingly, the function of Kif7 in the zebrafish embryo appears restricted principally to mesodermal derivatives, its inactivation having little effect on neural tube patterning, even when Sufu protein levels are depleted. Remarkably, zebrafish lacking all Kif7 function are viable, in contrast to the peri-natal lethality of mouse kif7 mutants but similar to some Acrocallosal or Joubert syndrome patients who are homozygous for loss of function KIF7 alleles

Hedgehog signaling via a calcitonin receptor-like receptor can induce arterial differentiation independently of VEGF signaling in zebrafish

Multiple signaling pathways control the specification of endothelial cells (ECs) to become arteries or veins during vertebrate embryogenesis. Current models propose that a cascade of Hedgehog (Hh), vascular endothelial growth factor (VEGF), and Notch signaling acts instructively on ECs to control the choice between arterial or venous fate. Differences in the phenotypes induced by Hh, VEGF, or Notch inhibition suggest that not all of the effects of Hh on arteriovenous specification are mediated by VEGF. We establish that full derepression of the Hh pathway in ptc1;ptc2 mutants converts the posterior cardinal vein into a second arterial vessel that manifests intact arterial gene expression, intersegmental vessel sprouting, and HSC gene expression. Importantly, although VEGF was thought to be absolutely essential for arterial fates, we find that normal and ectopic arterial differentiation can occur without VEGF signaling in ptc1;ptc2 mutants. Furthermore, Hh is able to bypass VEGF to induce arterial differentiation in ECs via the calcitonin receptor-like receptor, thus revealing a surprising complexity in the interplay between Hh and VEGF signaling during arteriovenous specification. Finally, our experiments establish a dual function of Hh during induction of runx1+ HSCs