Biomedical subjects

Uri Frank

Publications and source records attributed to Uri Frank.

11 recordsLinked to original sources

PubMed2006

Wnt signaling in hydroid development: formation of the primary body axis in embryogenesis and its subsequent patterning.

We have studied the role the canonical Wnt pathway plays in hydroid pattern formation during embryonic development and metamorphosis. Transcripts of Wnt and Tcf were asymmetrically deposited in the oocyte and subsequent developmental stages, marking the sites of first cleavage, posterior larval pole and the upcoming head of the metamorphosed polyp. To address the function of these genes, we activated downstream events of the Wnt pathway by pharmacologically blocking GSK-3beta. These treatments rendered the polar expression of Tcf ubiquitous and induced development of ectopic axes that contained head structures. These results allow concluding that Wnt signaling controls axis formation and regional tissue fates along it, determining one single axis terminus from which later the mouth and hypostome develop. Our data also indicate Wnt functions in axis formation and axial patterning as in higher metazoans, and thus point to an ancestral role of Wnt signaling in these processes in animal evolution.

Amino Acid Sequence↗

PubMed2006

Structural but not functional conservation of an immune molecule: a tachylectin-like gene in Hydractinia.

Tachylectin-related proteins are a recently characterized group of pattern recognition molecules, functioning in the innate immunity of various animals, from the ancient sponges to vertebrates. Tachylectins are characterized by six internal tandem repeats forming beta-propeller domains. We have identified and characterized a tachylectin-related gene in the colonial marine hydroid, Hydractinia echinata. The predicted gene product, termed CTRN, contained an N-terminal signal peptide and had a well-conserved tachylectin-like structure. RT-PCR analyses revealed only post-metamorphic expression while no mRNA was detected during embryonic development or in planula larvae. Exposure of colonies to LPS under conditions known to activate an immune response in Hydractinia did not result in upregulation of the gene. In situ hybridization analysis of metamorphosed animals detected CTRN transcripts only in a small subpopulation of neurons and their precursor cells, localized in a ring-like structure around the mouth of polyps. The same ring-like structure of CTRN expressing neurons was also observed in young polyp buds, predicting the position of the future mouth. This type of expression pattern can hardly be attributed to an immune-relevant gene. Thus, despite high structural similarity to tachylectins, this cnidarian member of this group seems to be an exception to all other tachylectins identified so far as it seems to have no function in cnidarian innate immunity.

Amino Acid Sequence↗

PubMed2006

Energy integration between the solitary polyps of the clonal coral Lobophyllia corymbosa.

Clonal integration in the coral Lobophyllia corymbosa was studied from two perspectives: transfer of carbon among clonemates and allorecognition. This coral forms colonies in the early post-metamorphic stages. In later ontogeny, the tissues interconnecting polyps die, transforming the colony into a clone of solitary polyps. These polyps continue to live in close proximity but without tissue continuity. Isolated polyps labeled with radioactive carbon in the light showed oriented transfer of assimilates towards adjacent, injured polyps. No significant transfer of carbon was observed towards intact, isogeneic polyps or allogeneic polyps. Grafting of coral tissues resulted in intra-clonal fusion, but only when polyps were previously sectioned. Allogeneic sectioned grafts were always rejected. Intact polyps were unresponsive towards isogeneic and allogeneic counterparts when grafted. Our results show that isolated Lobophyllia polyps not only recognize their clonemates as such, but also help them when necessary, although no tissue continuity exists between them.

Animal Communication↗

PubMed2005

An evolutionary conserved role of Wnt signaling in stem cell fate decision.

Wnt/Frizzled/ss-catenin-based signaling systems play diverse roles in metazoan development, being involved not only in the establishment of body axes in embryogenesis but also in regulating stem cell fate in mammalian post-embryonic development. We have studied the role the canonical Wnt cascade plays in stem cell fate determination in Hydractinia, a member of the ancient metazoan phylum Cnidaria, by analyzing two key molecules in this pathway, frizzled and ss-catenin, and blocking GSK-3. Generally, frizzled was expressed in cells able to divide but absent in post-mitotic, terminally differentiated cells such as nerve cells and nematocytes. Transcripts of frizzled were identified in all embryonic stages beginning with maternal transcripts in the oocyte. Following gastrulation and in the planula larva, frizzled expression concentrated in the central endodermal mass from which the first interstitial stem cells and their derivatives arise. In post-metamorphic development, high levels of frizzled transcripts were detected in interstitial stem cells. Activating downstream events of the Wnt-cascade in the post-metamorphic life phase by blocking GSK-3 with paullones induced recruitment of nematocytes and nerve cells from the pool of interstitial stem cells. Terminal differentiation was preceded by an initial burst of proliferation of frizzled-positive i-cells. In activated i-cells, ss-catenin appeared in the cytoplasm, later in the nucleus. It was subsequently again observed in the cytoplasm and eventually faded out during terminal differentiation. Our results suggest an ancient role of Wnt signaling in stem cell fate determination.

Adenine↗

PubMed2004

A fragile X mental retardation-like gene in a cnidarian.

The fragile X mental retardation syndrome in humans is caused by a mutational loss of function of the fragile X mental retardation gene 1 (FMR1). FMR1 is an RNA-binding protein, involved in the development and function of the nervous system. Despite of its medical significance, the evolutionary origin of FMR1 has been unclear. Here, we report the molecular characterization of HyFMR1, an FMR1 orthologue, from the cnidarian hydroid Hydractinia echinata. Cnidarians are the most basal metazoans possessing neurons. HyFMR1 is expressed throughout the life cycle of Hydractinia. Its expression pattern correlates to the position of neurons and their precursor stem cells in the animal. Our data indicate that the origin of the fraxile X related (FXR) protein family dates back at least to the common ancestor of cnidarians and bilaterians. The lack of FXR proteins in other invertebrates may have been due to gene loss in particular lineages.

Amino Acid Sequence↗

PubMed2004

Totipotent migratory stem cells in a hydroid.

Hydroids, members of the most ancient eumetazoan phylum, the Cnidaria, harbor multipotent, migratory stem cells lodged in interstitial spaces of epithelial cells and are therefore referred to as interstitial cells or i-cells. According to traditional understanding, based on studies in Hydra, these i-cells give rise to several cell types such as stinging cells, nerve cells, and germ cells, but not to ectodermal and endodermal epithelial cells; these are considered to constitute separate cell lineages. We show here that, in Hydractinia, the developmental potential of these migratory stem cells is wider than previously anticipated. We eliminated the i-cells from subcloned wild-type animals and subsequently introduced i-cells from mutant clones and vice versa. The mutant donors and the wild-type recipients differed in their sex, growth pattern, and morphology. With time, the recipient underwent a complete conversion into the phenotype and genotype of the donor. Thus, under these experimental conditions the interstitial stem cells of Hydractinia exhibit totipotency.

Animals↗

PubMed2004

A putative double role of a chitinase in a cnidarian: pattern formation and immunity.

Chitinases are enzymes that degrade chitin, the second most abundant polymer in nature. They are ubiquitous among living organisms where they play a role in development, food-digestion and innate immunity. We have cloned and characterized the first cnidarian chitinase cDNA from the hydroid Hydractinia. The Hydractinia chitinase exhibits a typical secreted family 18 hydrolases primary structure. In situ hybridization and RT-PCR experiments showed that it is exclusively expressed in ectodermal tissues of the animal, only following metamorphosis while undetectable in embryonic and larval stages. Most prominent expression was observed in the stolonal compartment of colonies, structures that are covered by a chitinous periderm. Chitinase mRNA was detected in new branching points along stolons and in hyperplastic stolons indicating a role of the enzyme in pattern formation and allorecognition. It was also expressed in polyps where it was mostly restricted to their basal portion. This expression pattern suggests that HyChit1 also fulfills a role in host defense, probably against fungal and nematode pathogens. Endodermal expression of HyChit1 has never been observed, suggesting that the enzyme does not participate in food-digestion.

Amino Acid Sequence↗

PubMed2004

Hydroid TNF-receptor-associated factor (TRAF) and its splice variant: a role in development.

TNF-receptor-associated factors (TRAFs) mediate signaling via tumor necrosis factor receptor (TNFR)/TLR molecules, playing a role in cellular processes such as growth, differentiation and apoptosis. They have been most studied in the immunological context. Within the animal kingdom, TRAFs have been characterized from vertebrates, flies and worms. We have cloned and characterized the first TRAF homologue from a member of the most basal eumetazoan phylum, the Cnidaria. The cnidarian TRAF, HyTRAF1, is a typical member of its family, containing one RING finger and five zinc finger domains in its N-terminal region, followed by a TRAF domain located at the C terminus. In addition to the full-length mRNA, the gene is alternatively spliced to create a shorter isoform, HyTRAF1a, with a deletion of 35 amino acids, resulting in a protein with only four zinc fingers. This is the first described TRAF alternative splicing in invertebrates. Whereas the full-length protein is expressed in most life stages of the animal, the short isoform is exclusively found at the larval and early metamorphic stages. This stage is characterized by extensive apoptosis, suggesting that HyTRAF1a mediates a proapoptotic c-jun N-terminal kinase (JNK) signaling, similar to the murine TRAF2A isoform. Based on neighbor joining analysis of TRAF molecules across the animal kingdom, we propose that the cnidarian TRAF interacts with TNFR, rather than with TLR. Our findings suggest that TNF signaling has evolved in the common ancestor to cnidarians and bilaterians and that it has been conserved in the entire animal kingdom.

Alternative Splicing↗

PubMed2003

Allogeneic interactions in Hydractinia: is the transitory chimera beneficial?

The colonial marine hydroid, Hydractinia, exhibits four possible outcomes to allogeneic contacts: passive rejection, aggressive rejection, stable fusion and transitory fusion. In the special case of transitory fusion, Hydractinia colonies undergo tissue fusion, followed by tissue death at the original contact area, and colony separation. This type of rejection is different in several aspects from the rejection process that accompanies incompatible encounters. It has been suggested that in transitory fusion, the colonies gain immediate benefits from fusion, mainly due to size increase, without succumbing to costs associated with fusion (germ line parasitism). We report a long-term observation of repeated fusion and separation cycles in clones featuring transitory fusion that revealed a slow-down of specific growth rates following fusion, and recovery in growth rates following separation. Very rapid transfer of stained material between partners in transitory chimeras provides suggestive evidence that protection against germ line parasitism is far from being guaranteed by separation. Our data cast doubt as to whether the benefits considered for transitory fusion are sustainable and support the already made suggestion that fusion with self, rather than fusion with kin, has been the major selective force governing the evolution of allorecognition in colonial invertebrates.

Animals↗

PubMed2002

The ontogeny of allorecognition in a colonial hydroid and the fate of early established chimeras.

Colonies of the marine hydroid, Hydractinia, are able to discriminate between their own tissues and those belonging to unrelated conspecifics. We have studied the ontogeny of this allorecognition system by a series of allogeneic transplantations along a developmental gradient, including two-cell-stage embryos, 8 h morulae, planula larvae and metamorphosed polyps. Allograft acceptance of incompatible tissue was observed in all embryonic and larval stages, whereas metamorphosed polyps rejected incompatible transplanted allografts. Most of the chimeras established at the two-cell-stage, although composed of two allogeneic, incompatible entities with mismatching allorecognition loci, developed normally and remained stable through metamorphosis. The results of post metamorphic transplantation assays among the chimeras and the naive ramets, suggested that both incompatible genotypes were still represented in the chimera despite the onset of alloimmune maturation. The naive colonies always rejected each other. Chimeras established from later embryonic and larval stages did not develop into adult chimeric entities, but rather separated immediately post metamorphosis. We thus show that (1) allorecognition in this species matures during metamorphosis and (2) genetically incompatible entities may coexist in one immunologically mature, chimeric soma, provided that they were grafted early enough in ontogeny.

Animals↗

PubMedDate not supplied

Evolution of astacin-like metalloproteases in animals and their function in development.

Astacin-like metalloproteases are ubiquitous in the animal kingdom but their phylogenetic relationships and ancient functions within the Metazoa are unclear. We have cloned and characterized four astacin-like cDNAs from the marine hydroid Hydractinia echinata and performed a database search for related genes in the draft genome sequence of the sea anemone Nematostella vectensis. These sequences and those of higher animals' astacins were subjected to phylogenetic analysis revealing five clusters within the Eumetazoa. The bone morphogenetic protein-1/tolloid-like astacins were represented in all eumetazoan phyla studied. The meprins were only found in vertebrates and cnidarians. Two clusters were taxon-specific, and one cluster represented astacins, which probably evolved after the split of the Cnidaria. Interestingly, grouping of astacins according to the protease catalytic domain alone resulted in clusters of proteins with similar overall domain architecture. The Hydractinia astacins were expressed in distinct cells during metamorphosis and some also during wound healing. Previously characterized cnidarian astacins also act during development. Based on our phylogeny, however, we propose that the developmental function of most of them is not homologous to the developmental function assigned to higher animals' astacins.

Amino Acid Sequence↗