ATP and glutamate coordinate contractions in the freshwater sponge <i>Ephydatia muelleri</i>

Sponges (phylum Porifera) are an early diverging animal lineage without nervous and muscular systems, and yet they are able to produce coordinated whole-body contractions in response to disturbances. Little is known about the underlying signalling mechanisms in coordinating such responses. Previous studies demonstrated that sponges respond specifically to chemicals such as l-glutamate and γ-amino-butyric acid (GABA), which trigger and prevent contractions, respectively. Genes for purinergic P2X-like receptors are present in several sponge genomes, leading us to ask whether ATP works with glutamate to coordinate contractions in sponges as it does in other animal nervous systems. Using pharmacological approaches on the freshwater sponge Ephydatia muelleri, we show that ATP is involved in coordinating contractions. Bath application of ATP caused a rapid, sustained expansion of the excurrent canals in a dose-dependent manner. Complete contractions occurred when ATP was added in the presence of apyrase, an enzyme that hydrolyses ATP. Application of ADP, the first metabolic product of ATP hydrolysis, triggered complete contractions, whereas AMP, the subsequent metabolite, did not trigger a response. Blocking ATP from binding and activating P2X receptors with pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) prevented both glutamate- and ATP-triggered contractions, suggesting that ATP works downstream of glutamate. Bioinformatic analysis revealed two P2X receptor sequences, one of which groups with other vertebrate P2X receptors. Altogether, our results confirm that purinergic signalling by ATP is involved in coordinating contractions in the freshwater sponge.