Notch-signalling is required for head regeneration and tentacle patterning in Hydra.

Local self-activation and long ranging inhibition provide a mechanism for setting up organising regions as signalling centres for the development of structures in the surrounding tissue. The adult hydra hypostome functions as head organiser. After hydra head removal it is newly formed and complete heads can be regenerated. The molecular components of this organising region involve Wnt-signalling and ß-catenin. However, it is not known how correct patterning of hypostome and tentacles are achieved in the hydra head and whether other signals in addition to HyWnt3 are needed for re-establishing the new organiser after head removal. Here we show that Notch-signalling is required for re-establishing the organiser during regeneration and that this is due to its role in restricting tentacle activation. Blocking Notch-signalling leads to the formation of irregular head structures characterised by excess tentacle tissue and aberrant expression of genes that mark the tentacle boundaries. This indicates a role for Notch-signalling in defining the tentacle pattern in the hydra head. Moreover, lateral inhibition by HvNotch and its target HyHes are required for head regeneration and without this the formation of the ß-catenin/Wnt dependent head organiser is impaired. Work on prebilaterian model organisms has shown that the Wnt-pathway is important for setting up signalling centres for axial patterning in early multicellular animals. Our data suggest that the integration of Wnt-signalling with Notch-Delta activity was also involved in the evolution of defined body plans in animals.

Notch signalling defines critical boundary during budding in Hydra.

Boundary formation is an important mechanism of development and has been studied in a number of bilaterian model organisms where it is often controlled by Notch, FGF and Wnt signalling. Tissue boundaries are also formed in simple pre-bilaterian animals. The boundary between parent and bud during asexual reproduction in the fresh water polyp Hydra vulgaris is an example. The Hydra homolog of the FGF-receptor FGFR (kringelchen) and some components of the Wnt signalling pathway are expressed at this boundary, but their precise functions are unknown. In this work we have discovered an important role for Notch signalling at this boundary. Notch signalling is needed to sharpen the kringelchen expression zone during the final budding stages from an initially broad band into a clear line demarcating the boundary between bud and parent. Expression of the Notch target gene HyHes and the putative matrix metalloprotease MMP-A3 was observed at the boundary shortly before the bud began to constrict and differentiate foot cells. When Notch signalling was inhibited with the presenilin inhibitor DAPT the expression pattern for kringelchen changed dramatically into a diffused pattern. The expression of both HyHes and MMP-A3 was abolished. Moreover, morphogenesis of the bud was not completed and buds did not constrict, failed to form a foot and never detached from the parent. This resulted in the formation of two-headed animals. We suggest that the function of Notch signalling during budding in Hydra is in promoting the formation of two stripes of differing gene expression, which are needed to differentiate the foot of the bud and a progressing narrowing of the mesoglea on the side of the parent.

The molecular cell death machinery in the simple cnidarian Hydra includes an expanded caspase family and pro- and anti-apoptotic Bcl-2 proteins.

The fresh water polyp Hydra belongs to the phylum Cnidaria, which diverged from the metazoan lineage before the appearance of bilaterians. In order to understand the evolution of apoptosis in metazoans, we have begun to elucidate the molecular cell death machinery in this model organism. Based on ESTs and the whole Hydra genome assembly, we have identified 15 caspases. We show that one is activated during apoptosis, four have characteristics of initiator caspases with N-terminal DED, CARD or DD domain and two undergo autoprocessing in vitro. In addition, we describe seven Bcl-2-like and two Bak-like proteins. For most of the Bcl-2 family proteins, we have observed mitochondrial localization. When expressed in mammalian cells, HyBak-like 1 and 2 strongly induced apoptosis. Six of the Bcl-2 family members inhibited apoptosis induced by camptothecin in mammalian cells with HyBcl-2-like 4 showing an especially strong protective effect. This protein also interacted with HyBak-like 1 in a yeast two-hybrid assay. Mutation of the conserved leucine in its BH3 domain abolished both the interaction with HyBak-like 1 and the anti-apoptotic effect. Moreover, we describe novel Hydra BH-3-only proteins. One of these interacted with Bcl-2-like 4 and induced apoptosis in mammalian cells. Our data indicate that the evolution of a complex network for cell death regulation arose at the earliest and simplest level of multicellular organization, where it exhibited a substantially higher level of complexity than in the protostome model organisms Caenorhabditis and Drosophila.

The Notch signaling pathway in the cnidarian Hydra.

Many of the major pathways that govern early development in higher animals have been identified in cnidarians, including the Wnt, TGFbeta and tyrosine kinase signaling pathways. We show here that Notch signaling is also conserved in these early metazoans. We describe the Hydra Notch receptor (HvNotch) and provide evidence for the conservation of the Notch signaling mode via regulated intramembrane proteolysis. We observed that nuclear translocation of the Notch intracellular domain (NID) was inhibited by the synthetic gamma-secretase inhibitor DAPT. Moreover, DAPT treatment of hydra polyps caused distinct differentiation defects in their interstitial stem cell lineage. Nerve cell differentiation proceeded normally but post-mitotic nematocyte differentiation was dramatically reduced. Early female germ cell differentiation was inhibited before exit from mitosis. From these results we conclude that gamma-secretase activity and presumably Notch signaling are required to control differentiation events in the interstitial cell lineage of Hydra.