What can sea squirts tell us about neurodegeneration?

By Christopher Vaughan

A tiny marine creature with a strange lifestyle may provide valuable insights into human neurodegenerative disorders, such as Alzheimer's disease, according to scientists at Stanford Medicine.

Botryllus schlosseri, also called a star tunicate, is humans' closest evolutionary relative among invertebrates in the sea. Attached to rocks along the coast, it appears as a tiny flower-shaped organism. Star tunicates start life as little tadpole-like creatures with two brains, swimming in the ocean. But eventually they drift down from the surface, settling into a stationary life on a rock, joining a colony of other tunicates.

As the tunicate, also known as a sea squirt, adapts to its new couch-potato lifestyle, it loses brain power: One of the two brains, its use for sea navigation now obsolete, begins to dissolve. The way the invertebrate's brain degenerates and disappears has important parallels to the way the brain degenerates in human neural disorders, said Irving Weissman, MD, director of the Institute for Stem Cell Biology and Regenerative Medicine.

In a paper published July 11 in the Proceedings of the National Academy of Sciences, Weissman and his colleagues showed that many of the genes associated with neurodegeneration in Botryllus have analogues to the genes associated with neurodegeneration in humans. What's more, the researchers said, genetic changes that build up over decades in the Botryllus colonies affect neurodegeneration in many of the same ways as age-related genetic changes affect neurodegeneration in older people.

Weissman and other scientists think that Botryllus is the modern-day representative of the beginning of the vertebrate branch of the tree of life. Every animal with a backbone, they think, evolved first from this tiny sea tunicate.