When molecular biologists go “fishing,” they don’t really expect to catch any fish. Not unless you’re talking about the protein FISH, named after its five SH3 domains. In the February 22 PNAS, Irene Griswold-Prenner and colleagues from Elan Pharmaceuticals, San Francisco, report catching this particular one while trying to net proteins that might mediate Aβ neurotoxicity. The paper is available in this week’s early online edition of the journal.
Griswold-Prenner’s fishing expedition started, like so many others, with some bait, in this case commercially available antibodies to phosphotyrosine antigens. Back in 2001, researchers from Wyeth-Ayerst (see Kajowski et al., 2001) had found that Aβ binds to a G protein coupled receptor coined β amyloid binding protein (BBP). G proteins, of course, are known to activate tyrosine kinases, so the Elan group wondered if Aβ was triggering protein phosphorylation. To test this, they used the antibodies to identify potential downstream targets of BBP or other Aβ-activated kinases.
First author Nikolay Malinin and colleagues used the antibodies to test human cortical cultures—arguably a more pathologically relevant cell-based model for Aβ toxicity than cell lines or non-human tissues. They found that one antibody—raised to a phosphorylated form of the epidermal growth factor receptor (EGFR) —detected increased phosphorylation of a single protein when cortical cultures were treated with a toxic form of Aβ. But the protein, with a molecular weight of 165 kDa, was too big to be the EGFR, and when Malinin and colleagues identified it, they found it was, in fact, the FISH adapter protein.
How might FISH play a role in Aβ-mediated toxicity? The protein is normally kept in an inactive state, brought about by intramolecular binding between an N-terminal PX domain and one of its SH3 domains. Malinin and colleagues found that expression of FISH minus the PX motif led to neurotoxicity even in the absence of Aβ. By contrast, overexpressing the PX-containing N-terminal end of the protein had the opposite effect, protecting the cortical cultures from toxic Aβ.
FISH is known to interact with the ADAMs family of disintegrins/metalloproteinases, sometimes known as “sheddases” because they cleave extracellular domains from a number of membrane proteins. The ADAMs are a self-destructive bunch, cleaving themselves in addition to their other targets. FISH is known to interact with ADAM12, 15, and 19, and this prompted the authors to examine the role of the ADAM proteins in Aβ-mediated toxicity. When Malinin exposed human cortical cultures to toxic forms of Aβ, he found that only levels of ADAM12 were reduced, suggesting that at least this metalloproteinase had been activated by FISH. Further experiments supported this idea. Deletion of the ADAM-binding domain in FISH prevented Aβ toxicity, as did expression of a metalloproteinase-deficient ADAM12.
Many other proteins and signaling pathways have been implicated in mediating Aβ toxicity; studies include work in cultured rat neurons (see ARF related news story) and in human neuroblastoma cells (see Wang et al., 2000), to quote but two examples. No consensus has emerged, however, on which players and pathways are the key ones. Where, then, do FISH and ADAM12 fit in? And how would they relate, if at all, to ADAM10, considered to be the α-secretase that generates a protective snippet of APP (see ARF related news story)?
Finding a solid in-vivo correlation between Alzheimer disease and the FISH/ADAM pathway would go a good way toward establishing their importance. As a first step toward that goal, the authors report that ADAM12 appears to be reduced in tissue from AD brain, perhaps in a sign of autocatalytic degradation, but the reduction is modest, the sample size small, and autopsy studies come with their own limitations. Nevertheless, the authors write that their findings “show an unusual pathway of Aβ-induced neurotoxicity through FISH-mediated regulation of ADAM(s) and suggest a previously undescribed strategy for AD treatment by targeting the metalloprotease activity of ADAM(s).”—Tom Fagan
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- Wang HY, Lee DH, D'Andrea MR, Peterson PA, Shank RP, Reitz AB. beta-Amyloid(1-42) binds to alpha7 nicotinic acetylcholine receptor with high affinity. Implications for Alzheimer's disease pathology. J Biol Chem. 2000 Feb 25;275(8):5626-32. PubMed.
No Available Further Reading
- Malinin NL, Wright S, Seubert P, Schenk D, Griswold-Prenner I. Amyloid-beta neurotoxicity is mediated by FISH adapter protein and ADAM12 metalloprotease activity. Proc Natl Acad Sci U S A. 2005 Feb 22;102(8):3058-63. PubMed.