An international team of researchers has uncovered tell-tale signs of Alzheimer's disease in dolphins, marking the first time that the age-related disorder has been detected in a wild animal.
Images: University of Oxford
Until very recently, scientists thought that only humans were susceptible to Alzheimer's disease. That changed back in August of this year when researchers from Kent State University detected traces of the disease in chimps, or at least, the brains of chimps who died from natural causes at zoos and research centres. A new study published this week in Alzheimer's & Dementia is now the first to find two key markers of the disease — protein plaques and tangles — in a wild animal, namely dolphins. This latest finding is further evidence that Alzheimer's is not a human-specific disease, and that other animals can be used to study the dreaded condition.
Most animals die very shortly after the end of their fertile years, but dolphins and orca whales, like humans, tend to live past their reproductive years (cool fact: Female orca whales go through menopause). This got Oxford University scientist Simon Lovestone wondering: Are dolphins, as a result of their long lives, susceptible to age-related neurological diseases such as Alzheimer's? To find out, he and his colleagues studied the brains of dolphins who had died naturally in the wild and whose bodies washed ashore along the Spanish coast.
The researchers found plaques of a protein called beta amyloid, along with the tangles of another protein called tau. Together, these proteins are the smoking guns of Alzheimer's disease in humans. In healthy brains, amyloid beta breaks down and goes away, but for people with Alzheimer's, this protein lingers, resulting in the formation of plaques between neurons. These plaques subsequently set another process in motion, whereby tau forms tangles that destabilise brain cells. Together, these two neurological disruptions produce dementia.
Importantly, this study doesn't tell us how the dolphins may be affected by their specific version of the disease, or whether they suffer from any associated memory loss or confusion. We won't know until we perform behavioural tests on captive dolphins — something the researchers do not advocate.
That said, the discovery points to potential causes of the disease, with altered insulin function being a common denominator. Insulin regulates the levels of sugar in the blood, setting off a complex chemical cascade known as insulin signalling. Changes in insulin signalling can cause diabetes in humans and other mammals. At the same time, however, extreme caloric restriction also affects insulin signalling, extending the lifespans of certain animals by significant amounts. In some animals, such as fruit flies and mice, caloric restriction can extend life by up to three times.
"We think that in humans, the insulin signalling has evolved to work in a way similar to that artificially produced by giving a mouse very few calories," said Lovestone in a statement. "That has the effect of prolonging lifespan beyond the fertile years, but it also leaves us open to diabetes and Alzheimer's disease."
Previously, scientists had shown that insulin resistance predicts the development of Alzheimer's in humans, and people with diabetes are more likely to develop Alzheimer's.
"But our study suggests that dolphins and orcas (who also have a long post fertility life span) are similar to humans in many ways; they have an insulin signalling system that makes them an interesting model of diabetes, and now we have shown that dolphin brains show signs of Alzheimer's identical to those seen in people," said Lovestone.
So while it sucks that dolphins are susceptible to Alzheimer's, the good news is that we now have a new "model" for the disease. Future comparative analyses of dolphin and human brains with Alzheimer's will tease out the various factors responsible for the disease, which will hopefully lead to new treatments.