Alzheimer’s disease is the most common form of dementia in the world, affecting more than 26 million people. Creutzfeld-Jacob disease (CJD), another affliction is far less common, but both conditions share many of the same qualities. They are fatal within a few years of diagnosis, they are incurable and they involved the crippling degeneration of the brain’s neurons. Now, a group of Yale researchers have discovered that the two diseases are also linked by a pair of critical proteins.
Look into the brain of someone with Alzheimer’s disease and you will see large, insoluble “plaques” sitting between nerve cells. They consist of a protein fragment (or “peptide”) called amyloid-beta, accumulating in its thousands. These plaques are a hallmark of the disease, but even before they have formed, amyloid-beta peptides have already begun to cluster in small soluble groups. Even at this stage, they can impair memory, degrading the connections between separate neurons.
Juha Lauren wanted to work out how exactly clusters of amyloid-beta wreak havoc in neurons before they form plaques. In particular, he was after the identity of its molecular accomplices. Many proteins work their will in a cell by attaching to other proteins called receptors. To see if amyloid-beta does the same, Lauren’s team went fishing for receptors.
They created a synthetic version of the amyloid-beta peptide and connected it to a molecule called biotin – these were their hooks. Lauren lowered them into a massive pool of different proteins found in the brains of mice; if one of those was a receptor for amyloid-beta, it should take the bait and stick to it. As a rod, he used beads covered in a molecule called avidin, which sticks very strongly to biotin. The beads attracted biotin, which was stuck to amyloid-beta, which was in turn bound to its receptor.
From hundreds of thousands of proteins, their fishing trips pulled out just one that stuck to amyloid-beta, and it’s a familiar one – the prion protein. Incorrectly folded versions of this protein (PrPSC) are the culprits behind diseases like CJD, mad cow disease and scrapie. And now it seems that the normal, correctly folded version (PrPC) plays a role in Alzheimer’s disease too, by acting as the receptor for amyloid-beta. It’s the accomplice through which amyloid-beta clusters work their damaging effects on neurons.