Monday August 18 2008
CEA
The target site of anti-Alzheimer drugs
CEA
Dynamic tracking of acetylcholinesterase to understand its mechanism of action – that was the objective set for researchers from Jean-Pierre Ebel Institute of Structural Biology [1] , who recently published their findings in the 4th August issue of the renowned Proceedings of the National Academy of Sciences of the USA. Working in tandem with colleagues from the Weizmann Institute of Science in Israel [2], the team was able to track structural changes in the architecture of the enzyme during its active cycle. This enzyme is one of the main targets for the development of anti-Alzheimer therapies, and an understanding of its structural dynamics is a major step forward towards engineering more efficient drug treatments.
Acetylcholinesterase enzyme is essential for nerve-based signal transmission, playing a crucial role in regulating signals for cognitive functions (including memory, judgement, language and others), which are impaired in Alzheimer’s patients. These properties have made acetylcholinesterase the target of most currently-available treatments, since its main function is the specific and very rapid breakdown of the neurotransmitter acetylcholine, which it splices into two parts, thus terminating the transmission of incoming signals. This reaction is called enzymatic cleavage.
The IBS research team managed to observe acetylcholinesterase during breakdown of the neurotransmitter using a novel method making it possible to trigger and track the cleavage reaction by X-ray diffraction-absorption. However, this could only be achieved after overcoming a number of obstacles. Firstly, before they could observe the enzyme dynamically, they had to fully control the start of the reaction. The IBS researchers approached this problem by repurposing the capabilities of the European Synchrotron (ESRF, Grenoble)[3], or more specifically, the fact that the beam power generated has the knock-on effect of causing breaks in the molecules observed. They capitalized on this property to trigger targeted cleavage in an analogue[4] of acetylcholine, thus simulating the enzymatic cut-off step. This enables the researchers to choose the moment at which they will prompt the analogue to adopt the conformation [5] that it takes naturally once it has played its role as “molecular scissors”. The synchrotron therefore played a dual role: to create the enzyme split and to observe the enzyme’s structure. The second barrier the researchers had to overcome was the fact that at room temperature, the conformational changes occurring during the reaction are extremely fast, at just a handful of milliseconds. The problem was resolved by working with cryogenic temperatures of between -120°C and 170°C, this significantly slowing the reaction enough to make it possible to analyze the different successive conformations adopted by acetylcholinesterase during one of the fastest enzymatic cleavage reactions in the natural world.
Some drugs act by trapping their target in a highly specific conformation. Identifying the full panel of conformations that an enzyme can adopt therefore makes it possible to rationally design more efficient molecules. These results open up new perspectives for developing second-generation anti-Alzheimer drugs.
When the analogue of the neurotransmitter acetylcholine (yellow bars) is broken inside the active site of acetylcholinesterase (blue bars), choline – which is one of the two cleavage products – reorients itself to the active site of the enzyme. The reorientation of choline, as illustrated by the black arrow, switches from the red position to the green position.
[1] (IBS). A CEA-CNRS-Joseph Fourier University of Grenoble joint-run institute
[2] With support from the Toulouse-based Institute of Pharmacology and Structural Biology and the Grenoble-based European Synchrotron Radiation Facility (ESRF).
[3] European synchrotron: a giant microscope using an intense-beam radiation source to observe nanoscale-objects
[4] Molecule mimicking the neurotransmitter
[5] Conformation: 3D structural arrangement of a proteinArticle reference:
Colletier, J.-P., Bourgeois, D., Sanson, B., Fournier, D., Silman, I., Sussman, J.L. & Weik, M. Shoot-and-trap: use of specific X-ray damage to study structural protein dynamics by temperature-controlled cryo-crystallography. Proc. Natl. Acad. Sci. in press.
Research teams – references:
Laboratoire de biophysique moléculaire de l’Institut de Biologie Structurale, IBS-J.P. Ebel/ CEA - CNRS- Université Joseph Fourier - 41, rue Jules Horowitz, F-38027 GRENOBLE Cedex 1