Green shore crab and transparent sea squirts

Yesterday I made this photo of a green shore crab Carcinus maenas between transparent sea squirts Ciona intestinalis in the Oosterschelde. Which do you think is more closely related to us?

Although these adult sea squirts, or tunicates, don’t consist of much more than translucent sacks with intestines, their larval “tadpole” stage exhibits all characteristics of the chordates. This means that they are actually more closely related to us chordate humans than this crab is!

In fact, transparent sea squirts are so similar to us that they may prove to be important for the development of more effective treatments for Alzheimer’s disease.

Alzheimer’s disease is characterized by the formation of plaques and tangles in the brain which are thought to contribute to the degradation of nerve cells, causing a host of very serious problems, such as loss of memory and speech, and ultimately death.

The drugs available today help to alleviate the symptoms, but they don’t target the plaques and tangles, and hence they cannot slow down the progression of the disease. A great deal of scientific research is therefore aimed at reducing plaque buildup, typically by studying transgenic mice or nematodes that reproduce aspects of human Alzheimer’s disease.

However, apart from ethical concerns, the use of transgenic mice is troublesome because it takes weeks before they develop plaques, making experiments time-consuming and costly. Although this problem does not arise with nematodes, they are so different from humans that they are not able to properly process the protein that produces the plaques. In order for them to make plaques, the fully processed part of the protein has to be provided.

Remarkably, Michael J. Virata and Robert W. Zeller, two scientists at San Diego State University, found that transgenic sea squirt larvae are fully capable of processing this protein and producing plaques within as little as 24 hours after fertilization.^[1] As in humans suffering from Alzheimer’s disease, the transgenic sea squirts appear to suffer from neurological problems. This is important, because it suggests that findings in sea squirts may also be relevant for humans. An experimental drug was even able to reduce both the apparent neurological problems and the amount of plaques.

While this is by no means a cure for Alzheimer’s disease in humans, these relatively simple yet chordate creatures may well provide valuable new insights into the early stages of Alzheimer’s disease.

1. Michael J. Virata & Robert W. Zeller (2010). Ascidians: an invertebrate chordate model to study Alzheimer’s disease pathogenesis. Disease Models & Mechanisms, 3, 377-385 DOI: 10.1242/dmm.003434