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Could an Algal Toxin Cause Motor Neurone Disease?

Could an Algal Toxin Cause Motor Neurone Disease?

By Rachael Dunlop

It’s long been thought that blue-green algae might cause several brain diseases. Now a missing piece in the puzzle has been found.

Four years ago my supervisor, Dr Ken Rodgers, called me to a meeting. An ethnobotanist from the US was in town and he wanted to meet us. He’d read one of our papers and had an idea for a collaboration.

I’m what is known as a basic scientist, which means that I stand at a bench and poke cells with stuff to see how they might react. I don’t work with patients or with animal models. But years of training and poking have sharpened my skills, and what I do know back-to-front is how cells work.

So when Dr Paul Cox came to us with a neurotoxic compound found in blue-green algae (BGA) and asked us how we thought it could contribute to motor neurone disease (MND), we had an idea straight away.

Four years later the headlines read: “Scientists discover potential cause of MND”. So what happened in between?

What’s Blue-Green Algae?

Most of us would know BGA, or cyanobacteria, as a green carpet that forms on lakes and rivers. Cyanobacteria are about 3.5 billion years old as a species and have adapted to grow in fresh, salt and brackish water, in dirt, in the thermal ponds of Yellowstone National Park, and in the deserts crusts of the Middle East. Mats of cyanobacteria even cover the surface of the Inland Sea in Qatar. The oldest and most famous BGA stromatolites in the world are located in Western Australia’s Shark Bay, where they have World Heritage protection.

The earliest scientific publication reporting algal blooms appeared in the journal Nature in 1878 and described a bloom in Lake Alexandrina at the mouth of the Murray River in South Australia, which resulted in the death of sheep, horses, dogs and pigs within 24 hours. Australia holds the record for the largest freshwater BGA bloom, which occurred in the summer of 1991–92 covering 1200 km of the Barwon–Darling basin.

The Link between Blue Green Algae and Motor Neurone Disease

Although MND is relatively rare, it has a high profile as a result of well-known sufferers like Prof Steven Hawking, who has a “slow burn” form of the disease. The time from diagnosis to death is usually 1–5 years, and the disease kills by progressively paralysing the body until even swallowing and breathing becomes impossible.While about 20 genes have now been linked to MND, more than 90% of the disorder has no known cause, and there is no cure.

The neurotoxic compound that Dr Cox had found in BGA is called beta-methylamino-L-alanine (BMAA). It has been associated with MND for decades but the hypothesis for how it might cause disease was where the contention lay. Even though there is evidence that people who are exposed to BGA have an increased risk of contracting MND, and even though it was a known neurotoxin in cell culture, no one had been able to pinpoint precisely how it might cause MND. Confounding the issue was evidence that people could be exposed to food contaminated with BMAA up to 15 years before showing symptoms and without suffering any acute illness.

So the question remained: how could a seemingly innocuous compound found in algae lead to such a devastating terminal disease? As cell biologists with fresh eyes, we were able to view the puzzle from a different angle to the plant scientists. And this is how we noticed immediately that BMAA looked a lot like amino acids that humans use to make proteins. This gave us an idea.

How Can Amino Acids Be Bad?

Amino acids are the building blocks for our proteins. Humans have 20 different types that are used in different combinations to make thousands of different proteins. Some act as enzymes that convert food into energy, some form the structures of our cells, and some make the proteins that form our hair. Three-letter codes in our DNA identify specific amino acids for insertion into proteins, and this directs enzymes called tRNA synthetases to deliver them to molecules called ribosomes for synthesis into protein chains.

But plants make thousands of what are known as “non-protein amino acids”. BMAA is one of these.

Human illnesses contracted from non-protein amino acids are not a new phenomenon. Indeed BMAA is a known neurotoxin in cells in a dish. But how is it linked to MND?

BMAA and Neurodegeneration

BMAA was first identified more than 40 years ago when Cox and Dr Sandra Banack descended into the jungles of the Western Pacific Island of Guam in search of the causes for a devastating neurological disease. The condition, which is now known as ALS/PDC, was a complex combination of MND, dementia and Parkinson’s disease, and had killed nearly half of the adult indigenous Chamorro population.

With all genetic avenues exhausted and armed with clues from diseases known to be caused by plant amino acids, Cox and Banack went in search of a diet-based culprit. They found that two major food sources for the Chamorros, fruit bats and cycad flour, contained high concentrations of a previously unidentified amino acid – BMAA.

BGA is not confined to water – it also grows in terrestrial environments. On Guam it was found at the base of cycad palms.

BMAA has also been linked to clusters of neurodegenerative disease in people living in the Kii Peninsula of Japan, west Papua New Guinea and soldiers returning from the Gulf War. In addition, people living adjacent to lakes subject to frequent algal blooms and consumers of contaminated seafood including blue crabs, lobster and mussels have also had more motor neuron diseases than the general population. BMAA has also been found post mortem in the brains of North American Alzheimer’s and Parkinson’s disease patients.

Indeed, the Chamorros of Guam are not the only ones to use cycad as a staple food. Indigenous Australians have been making a sacred bread called “ngathu” from cycad kernels for thousands of years. Radiocarbon dating of pits containing cycad seed pods reveal that they may have been used for food in Australia as far back as 13,000 years ago.

Like the Chamorros, indigenous Australians knew that the seeds contained a toxin and washed them in a running stream until the “poison” was removed. In 1841, explorer George Gray described the method used by indigenous Australians in south-west Western Australia to remove toxins from cycad seeds:

Having placed them in some shallow pool of water, they leave them to soak for several days. They dig in a dry sandy place, line them with rushes and fill them up with the nuts over which they sprinkle a little sand, and then cover the holes nicely over with the tops of the grass trees.

Finding the Source of BMAA on Guam

Also of interest to Cox and Banack was that the Chamorros had a voracious appetite for fruit bats. Fruit bat coconut soup was considered a delicacy on the island, where the locals described it as “like nothing you’ve ever tasted”.

The clue to bats as the source of BMAA came from the observation that the bats ate cycad seeds, and levels of BMAA were much higher in the bats than in the seeds, indicating that BMAA somehow “bioconcentrates” in the bats. So, continuing on their unconventional path, Cox and Banack literally dug around in the dirt to reveal a bright blue–green growth in the roots of the cycad trees. It turned out to be BGA.

How Is BMAA Linked to Neurodegeneration?

What was once a controversial hypothesis has rapidly gained traction as more evidence builds for the link between BMAA exposure and MND.

In addition to Guam and North American Alzheimer’s and MND patients, an elegant Google Maps analysis has found a high incidence of MND patients who have lived by lakes or water bodies that were subject to frequent algal blooms.

Somewhat frightening is the theory that BMAA can be aerosolised. Soldiers who fought in the Gulf War have an incidence of MND two to three times higher than the general population, and studies have shown that the desert crusts of Qatar – the location of an American military base – contains BMAA. it is thought that exposure comes from the dust that the soldiers inhale as they march behind trucks at night to keep warm.

Animal studies have shown that BMAA crosses the blood–brain barrier and persists in the brain, unlike other organs where it is flushed out.

And recent studies from Sweden and the US have shown that BMAA bioconcentrates up the food chain and is found in crabs, prawns, sharks and filter feeders such as mussels, where the neurotoxin concentrates in their flesh. Pink prawns from Washington Bay have been reported with levels of BMAA as high as the bats on Guam.

How BMAA Cause Could Disease

Despite a large body of epidemiological evidence, for many years the BMAA hypothesis suffered repeated blows as sceptics poked at the gaping hole in the theory – the lack of a plausible mechanism. While BMAA was a proven neurotoxin in the lab, it was not clear how it could result in neurodegeneration years after initial exposure.

To figure this out we need to look at plants again. Plants make hundreds of unusual amino acids, some of which are used as insecticides and pesticides but which also effect humans and livestock. Neurolathyrism, for example, is a permanent paralytic condition that predominantly occurs in famine-affected areas, and is caused by an amino acid called ODAP.

Indeed, ODAP was recently implicated in the 1992 death of American hiker Chris McCandless (also known as Alexander Supertramp), who ventured into the Alaskan wilderness with the aim of living a life of solitude. His emaciated body was found only 4 months later, and it’s now thought he died from starvation after becoming paralysed from the consumption of toxic seeds. His story was made into a book and film, Into the Wild.

Birdsville Disease, which causes afflicted horses to become “dejected” and stand in the sun for hours without seeking shade, is also caused by a non-protein amino acid called indospicine, which is derived from the Australian plant Birdsville indigo.

In our lab we’ve spent more than 10 years looking at non-protein amino acids, in particular the drug used to treat Parkinson’s disease, levodopa (L-DOPA). L-DOPA is very similar to an amino acid called tyrosine that mammals use to make proteins.

It turns out that L-DOPA can be “misincorporated” into our proteins, but since it’s slightly different, the resulting proteins do not fold correctly, rendering them useless. They build up as “junk” inside the cell over time and eventually choke the cell, sending it into programmed cell death. This mechanism – known as apoptosis – occurs in a variety of neurodegenerative disorders such as Alzheimer’s, Parkinson’s and, importantly, ALS/PDC.

The Final Piece of the Puzzle

When Cox read our work on L-DOPA, he struck upon an idea. Could BMAA be causing cell death by the same mechanism?

Based on our previous work with L-DOPA it was a plausible hypothesis, especially since BMAA looked a lot like an amino acid we use to make proteins called L-serine. So all we needed to confirm a mechanism for disease was show that, like L-DOPA, BMAA replaced L-serine in human protein.

This finding is was what made those headlines last September, but we also went further to show that BMAA-containing proteins do not fold properly inside cells, build-up over time and can cause the cells to die – thus a plausible mechanism for how motor neurone disease might progress.

Importantly, we have also demonstrated that if cells are exposed to L-serine at the same time as BMAA, we can stop BMAA getting into the proteins, thus preventing downstream cell death.

A Multifactorial Disease

If we’re all being exposed to BMAA, why don’t we all have MND? In humans at least, exposure to BMAA alone does not appear to be sufficient to cause disease. Like many pathologies it’s likely that MND is requires several factors to come together to trigger disease.

BMAA might be just one factor in this devastating disease, but at least we now know how it might be causing toxicity. And because we have evidence that BMAA replaces L-serine, these findings might go some way to developing a therapy.

Rachael Dunlop is a Postdoctoral fellow in the School of Medical and Molecular Biosciences at the University of Technology, Sydney.