Li Gan and colleagues published their findings in the September 21, 2006, issue of the journal Neuron, published by Cell Press.
Their experiments were prompted by previous studies showing that the cysteine protease CatB--an enzyme that snips apart proteins--closely associated with the amyloid-ß (Aß) protein that forms the amyloid plaques, a hallmark of AD. However, those studies had not determined whether CatB was "good" or "bad"--that is, whether it acted to produce Aß from a longer protein, called amyloid precursor protein (APP), or whether it broke down Aß.
In their experiments, Gan and colleagues determined that CatB was the latter--breaking down Aß, apparently to enable other enzymes to further degrade the protein for the cell's protein "garbage deposal" system.
They found that knocking out the CatB gene increased plaque deposition in a mouse model of AD in which mice expressed the human form of APP. They also found that CatB tended to accumulate within amyloid plaques and that it acted to reduce Aß levels in neurons. And they found that introducing a pathological form of Aß, called Aß1-42, into neurons increased CatB in young and middle-aged mice with human APP, but not old mice. "Thus, upregulation of CatB may represent a protective mechanism that fails with aging," wrote the researchers, and such failure may play a role in late-onset sporadic AD.
Their test tube studies showed that CatB biochemically degrades Aß