Listening to Werner Müller talk about sponges, there's no doubt about his boundless enthusiasm for these primitive creatures. Sponges may look simple, but Müller explains that they are not amorphous blobs of cells;like other creatures, sponges have a `Bauplan' or body design. Müller's team has now unravelled some of the intricacies of the sponge skeletal blueprint (p. 637).
`Sponge skeletons are made of pretty silicate-based shapes called spicules'says Müller, `and they grow very fast.' But the skeleton's raw material,silicic acid, is not abundant in oceans. Sponges therefore probably expend a lot of energy to accumulate enough silicic acid to produce their spicules, and Müller wondered where this energy comes from. Arginine kinases are enzymes that buffer cells' ATP supplies and mediate energy transfer within cells. Since arginine kinase is an energy-transferring enzyme, Müller's team suspected that it might be involved in the energy-consuming process of sponge skeletal formation. The team already knew that silicic acid influences expression of the gene for silicatein, an enzyme that is responsible for the formation of silica in skeletal spicules. Could silicic acid also trigger expression of the sponges' arginine kinase gene?
To answer this question, Müller's team used an intriguing tool:primmorphs, tiny 3D sponge stem cell cultures. They grew one set of primmorphs in aquaria with very little silicic acid and another set supplemented with high levels of silicic acid, hoping that arginine kinase gene expression would be triggered in the supplemented primmorphs. Sure enough, primmorphs exposed to high silicic acid levels showed increased arginine kinase gene expression and enzyme activity, while primmorphs exposed to low silicic acid levels did not. But for really convincing evidence that silicic acid triggers induction of the arginine kinase gene, the team needed to show that sponge cells that can't take up silicic acid do not produce arginine kinase. The team exposed primmorphs to an inhibitor that prevents cells from taking up silicic acid,and were pleased to find that these primmorphs no longer showed an increase in arginine kinase gene expression and enzyme activity. So silicic acid really is an inorganic inducer of the sponge arginine kinase gene, leading to enhanced activity of this enzyme.
But can arginine kinase, triggered by silicic acid, influence the differentiation of sponge stem cells into spicules? The team reasoned that if arginine kinase plays a role in spicule formation, they should only find spicules growing in primmorph cells exposed to high levels of silicic acid,since only these cells have high levels of arginine kinase activity. As expected, when they examined primmorph cross-sections with an electron microscope, they only saw spicules forming in primmorph cells that had been exposed to high levels of silicic acid. The team concluded that silicic acid triggers a chain reaction, beginning with gene expression leading on to high levels of arginine kinase activity, which in turn helps the sponge accomplish the energy-consuming task of skeletal formation.
Müller explains that while most animals' body designs are genetically controlled, his team's results suggest that sponge blueprints may also be inorganically controlled. When sponges appeared on the scene some 800 million years ago, oceans were rich in silicate, so it made sense for them to use this inorganic substance to build their skeletons. Somewhere along the line,silicate apparently also began regulating enzymes involved in sponge skeletal formation. According to Müller, `This is one of the first examples of an external element contributing to morphogenic processes in a multi-cellular animal.'