Demonstration of an endocrine signaling circuit for insulin in the sponge Geodia cydonium.

The existence of an insulin-mediated cell-to-cell signaling in the sponge Geodia cydonium is demonstrated in this study by molecular biological and immunological techniques. The sequence of a sponge cDNA clone encoding preproinsulin was analyzed for the first time and determined to comprise a high homology to human preproinsulin (60-80% homology). The predicted polypeptide of preproinsulin from sponge contains two disulfide bridges which link the A- to the B-chain. The intra-A chain disulfide bridge is absent. Applying immunological and electron microscopical techniques it is shown that insulin is produced in specialized cells (spherulous cells). Experimental evidence is presented which indicates that the sponge preproinsulin (predicted Mr 11,850) is processed to insulin (Mr 5600; B-chain, Mr 3700 and A-chain, Mr 1900). Plasma membranes of sponge cells are shown to be provided with an insulin-binding receptor composed of two molecules (Mr 104,000 and Mr 98,000). Heterologous insulin (from bovine pancreas) was found to stimulate gene expression in G. cydonium cells. It is concluded that sponges are provided with an endocrine signaling circuit: signaling cells (spherulous cells), hormone (insulin), and hormone receptor bearing target cells which respond to the hormone stimulus.