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

Abstract

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 β-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 β-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.

Functional studies on the role of Notch signaling in Hydractinia development

“…In the adult polyp, Notch was expressed in the body column, excluding the most aboral section, and at the base of the tentacles (Fig 1B). This expression pattern is similar to HyNotch expression in Hydra (Münder et al, 2013; Wenger et al, 2016). We also found that Notch is alternatively spliced to generate two main isoforms that share all major Notch domains and differ only in the first two exons, which encode EGF repeats (Fig.…”

“…DAPT treatment during regeneration led to a block in tentacle formation (Fig. 4A and B), a phenotype also known from other cnidarians (DuBuc et al, 2014; Fritz et al, 2013; Marlow et al, 2012; Münder et al, 2013). However, as shown above, the acetylated tubulin positive oral nervous system in regenerating, Notch inhibited polyps was normal (Fig 3B).…”

“…In hydrozoan cnidarians, however, neurons develop from non-epithelial, migratory stem cells, called i-cells, which are segregated during gastrulation (Gahan et al, 2016; Hager and David, 1997; Leclere et al, 2012; Miljkovic-Licina et al, 2007). Pharmacological inhibition of Notch signaling in adult Hydra , a hydrozoan cnidarian, revealed no effect on the numbers of adult neurons (Käsbauer et al, 2007; Khalturin et al, 2007), but this treatment did affect nematocyte differentiation and tentacle development (Käsbauer et al, 2007; Khalturin et al, 2007; Münder et al, 2013), defects that were also observed in Nematostella following pharmacological Notch inhibition (DuBuc et al, 2014; Fritz et al, 2013; Marlow et al, 2012). Hence, the role of Notch in tentacle morphogenesis and nematogenesis seems to be conserved in cnidarians, but its function in neural development requires further research across the phylum.…”

The Notch pathway in the annelidPlatynereis: insights into chaetogenesis and neurogenesis processes

“…Cnidarians do not have a centralized nervous system but possess neural cells. In the evolutionarily derived Hydra , a proper Delta gene is missing and the small Notch receptor found in this species is involved in head and tentacle patterning [72]. In Nematostella (a sea anemone), a proper Notch protein and a short Delta ligand are present.…”

Eph receptors and ephrin class B ligands are expressed at tissue boundaries in Hydra vulgaris

“…6 A-D). There also might be a link with Notch-signalling, which is important for both boundaries in Hydra (Münder et al, 2010;Münder et al, 2013).…”