Small molecules that inhibit Notch signaling.

The proteolytic processing of Notch receptors plays a central role in the transduction of Notch signaling, which is involved in a variety of important processes in the body. Abnormal Notch processing has been implicated in a variety of cancers. γ-Secretase is responsible for the third and last cleavage step of Notch receptors. Since γ-secretase plays an important role in Alzheimer's disease, great effort has been spent to develop γ-secretase inhibitors (GSIs). The majority of these inhibitors block γ-secretase nonselectively, which means that these compounds can be used to block Notch cleavage and thereby regulate Notch signaling. In this review we give an overview of the most-used GSIs in the Notch field, together with examples of their use. It is a huge advantage that these drug-like compounds are already optimized for γ-secretase, and some are already being used in clinical trials. However, their nonspecificity has disadvantages as well, since four Notch receptors exist with different sites of expression and different roles in cell signaling and at least four different γ-secretase proteases are involved in their cleavage. It would be worth the effort to screen many GSIs for their selectivity for the different Notch receptors and γ-secretases, in order to obtain interesting tools for further research and-in the end-to develop safer drugs.

Interaction kinetic and structural dynamic analysis of ligand binding to acetylcholine-binding protein.

The mechanism of agonist interactions with Cys-loop ligand-gated ion channels has been studied using the acetylcholine-binding protein (AChBP) from Lymnaea stagnalis as a model protein and acetylcholine, nicotine, epibatidine, and a series of substituted quinuclidines as ligands. A biosensor-based assay for direct interaction studies of immobilized AChBP and small molecule ligands was developed. It allowed the characterization of the interaction kinetics of the ligands and the structural dynamics of the protein. The interactions with AChBP were very sensitive to variations in the experimental conditions and showed several types of complexities. These could be resolved into two types of ligand-induced secondary effects with different kinetics, representing fast and slow conformational changes. The data could be rationalized in a mechanistic model, and a structural interpretation of the interaction was obtained by molecular modeling involving induced fit and loop flexibility simulations. The data suggest that AChBP exhibits ligand-induced structural dynamics, as expected for the ligand gating mechanism of Cys-loop receptors. It shows that the formation of the initial encounter complex between AChBP and ligands is very rapid, in accordance with the functional characteristics required of neurotransmission. These developed procedures will enable further exploration of the mechanism of Cys-loop receptor function and the identification of specific ligands suitable for pharmacological use.