To get an insight into signaling cascades controlling developmental processes in Cnidarians we hunt for and make the use of so called early or immediate response genes. 

Early Response Genes

Serum Response Factor (SRF )

We have isolated the homologues of SRF from Hydra vulgaris (HvSRF) and Hydractinia echinata.
(HechSRF). Analysis of the local expression pattern of HvSRF and HechSRF by in situ hybridization reveals, that the message of this gene is predominantly expressed in the interstital cell lineage (I-cells) of Hydra and of Hydractinia. The I-cells are one of the totipotent (multipotent) stem cell pools of the animals giving rise to nerve cells, nematocytes, gland cells and germ cells. All cells of the lineage express SRF until they show a morphological visible kind of differentiation. 
The temporal expression pattern of HvSRF follows a daily rhythm possibly triggered by feeding. Shortly after feeding the message decreases and increases again later. Prolonged starvation of the animals causes an increase of the message

Northern analysis of HechSRF reveals, that the message is unchanged during embryogenesis and larval development and increases shortly after induction of metamorphosis. In situ hybridization indicates that labeled cells becomes distinguishable during larval development. They give rise to the I-cell lineage in the endoderm of the larva. During metamorphosis the labeled cells move from the endoderm into the ectoderm like the I-cells do. 

HvSRF and HechSRF are the first known genes in Cnidarians, which message is expressed predominantly in all members of the I-cell lineage, which have not yet visibly started to differentiate. We expect to get a better understanding of I-cell specific signaling cascades during developmental processes in Cnidarians. 
Work is in progress to find upstream or downstream members of the signaling cascade involving SRF. 

TIS11 (Tristetraproline family)

We have isolated fragments containing the conserved region of the homologues of TIS11 from Hydra vulgaris (HvTIS11) and Hydractinia echinata (HechTIS11).

work is in progress
 

Receptor Tyrosine Kinases (RTK)

Using the polymerase chain reaction to isolate protein-tyrosine kinase genes from the marine hydrozoa Hydractinia echinata we identified eight cDNAs coding for distinct receptor tyrosine kinases (RTK) and two cDNAs coding for non receptor tyrosine kinases (nRTK). The fragments coding for the nRTK are homologues to csk and abl . Most of the isolated RTKs are members of the family of the insulin growth factor receptors (IGRF) and the fibroblast growth factor receptors (FGFR). Among the IGFR- homologues are members of the nerve growth factor receptor subfamily (NGFR). Furthermore, we obtained a cDNA coding for a homologue of the ephrin receptor family (EPHR) so far not been described in coelenterates. Analysis of the temporal expression pattern of the isolated RTKs and nRTKs by northern blotting and/or RT-PCR reveals a developmental regulation of some kinases. Analysis of the spatial expression pattern by in situ hybridization is in progress. 
Furthermore, our work is now focused on the characterization and experimental analysis of the EPHR and the NGFR.

Eigene Beiträge:

U. Hoffmann, M Kroiher (2001) A possible role for the cnidarian homologue of serum response factor in decision making by undifferentiatied cells. Development 236, 304-315