Regenerative capacity of sponges, allowing them to recover complete bodies from fragments of tissue or even dissociated cells, is truly remarkable, and not many animals can match this potential. While continuously growing amount of information is available on genes involved in regeneration of model species, including cnidarians, acoels, planarians and chordates, nothing is known about molecular aspects of sponge regeneration.
Using a combination of gene candidate approach and unbiased detection of differentially expressed genes, we have investigated molecular mechanisms of regeneration in sponges. We were particularly interested in the earliest steps of this process, the wound healing, and have analysed it in detail in the emerging calcisponge model system, Sycon ciliatum. In this species, complete body can be regenerated from narrow transverse sections of the body column within 5 days of dissection.
Within three hours after dissection, over one thousand genes are differentially regulated. While many of these genes encode for novel proteins or appear to be non-coding, components of two key metazoan developmental pathways, Wnt and TGF-beta, are among the significantly and dramatically upregulated transcripts. In particular, eight (out of 22 present in Sycon) TGF-beta ligands and six (out of 21) Wnt ligands, as well as effectors (Smads, Tcf) and modulators (SFRPs) of these pathways are differentially expressed either throughout the regeneration process or concomitantly with specific morphogenetic events. In situ hybridization of selected genes allowed us to localize the transcripts to specific cells involved in the regeneration.
We have previously shown that in intact sponges many of Wnt and Tgf-beta pathway components are specifically expressed or enriched in the apical (oscular) region, supporting homology between the osculum and the head organizer of cnidarians. Thus, in both sponges and eumetazoans, “organizer genes” are also involved in regeneration, suggesting a common genetic basis of regeneration processes across the animal kingdom.