12 Dec 2002

In Alzheimer's, as in many other neurodegenerative diseases, neurons often meet their demise after apoptotic, or programmed cell death pathways have been activated. In yesterday's early online edition of PNAS, researchers from Alnawaz Rehemtulla's lab at the University of Michigan Medical School report a method that measures such cellular attrition in real time. The technique can be applied to live animals.

First author Bharathi Laxman and colleagues devised a firefly luciferase reporter system that is inactive in normal cells but turned on in cells undergoing apoptosis. At the heart of the reporter is a chimeric protein made from the fusion of luciferase and the regulatory subunit of the estrogen receptor (ER). The ER keeps the luciferase enzymatically dormant, but once apoptosis has been activated the chimera gets cleaved and active luciferase is released and allowed to shine.

The key to the reporter is a tiny four amino acid sequence, D-E-V-D, which is inserted between the ER and luciferase domains. Caspase-3, a protease that is activated by most apoptotic pathways, recognizes and proteolytically cleaves the DEVD sequence. Laxman et al. inserted the apoptosis reporter into human glioma cells and then placed these cells under the skin of nude mice. Whole animal images showed a huge increase in bioluminescence following administration of an apoptosis-inducing ligand.

This reporter may aid in drug discovery programs using small animal models according to the authors. They note that it can also be tailored to accommodate different protease sites, providing opportunities to study other clinically important molecules.—Tom Fagan