NWO-TOP grant awarded to Ad IJzerman
Ad IJzerman, from the Leiden University Division of Medicinal Chemsitry has been awarded an NWO TOP grant for his research. In the average textbook on pharmacology the interaction between a drug or ligand and its receptor is often described in a cartoon-like, two-dimensional fashion (Figure 1a). In the current era of structural biology more information is being added to this picture, to the extent that we now know the 3D architecture of many soluble drug targets.
Until recently this was not yet the case for the most important class of drug targets, the so-called G protein-coupled receptors (GPCRs). These proteins are membrane bound, not soluble at all and ‘fatty’ in nature. These aspects are notoriously difficult to cope with when it comes to protein crystallization and subsequent structure determination. However, breakthroughs in protein engineering and crystallographic methods now make the structure elucidation of G protein-coupled receptors feasible, although it still represents a formidable task. We contributed to the structure elucidation of the adenosine A2A receptor in which a small molecule antagonist was co-crystallized, yielding one of the very first views on how a GPCR binds a drug molecule (Jaakola et al, Science, 2008). The adenosine A2A receptor is a promising target for future drugs, among others against Parkinson’s disease. Many pharmaceutical companies have clinical candidates in various stages of development.
The Jaakola et al Science paper, some 2.5 years after its publication, has been cited over 350 times now, showing its immediate impact on the vast GPCR research community. This is probably so, because the publication challenges quite a number of concepts and dogma’s in this field. As a consequence, it also sets the stage for a number of new research directions, which in essence constitute the basis for the awarded NWO TOP-grant proposal. Three research themes will be studied, which are outlined below.
Obtaining A2A receptor crystals suitable enough for the X-ray diffraction structure determination has been a very laborious and unpredictable process. The main bottleneck was and still is the thermal instability of the protein. The receptor is very fragile once taken out of the membrane with e.g. detergents, which, however, is necessary for crystallization. Thereby it loses its functionality (e.g. drug binding) rapidly, and tends to aggregate. The pace of new structures emerging is frustratingly slow; we need quantum leaps in methodology development before we can only think of elucidating dozens or hundreds of GPCR structures. Moreover, one structure per GPCR cannot be very telling in the end, as many types of ligands interact with GPCRs with very different consequences.
Therefore, the awarded grant aims at the much-needed methodological progress in two ways. First, we will develop methods to rapidly obtain more stable receptors by focusing on the protein itself (project 1). We have preliminary data that this can be done rapidly and exhaustively. If successful this would minimize the current hurdles with respect to receptor fragility and aggregation. Secondly, we will focus on the ligands so essential in the co-crystallization process. We have learned that some more than others act as ‘chaperones’, improving the receptor’s thermostability. This has not been explored in any systematic way, which we aim to do now (project 2). Last but not least (project 3) we will explore the opportunities and limitations in using the receptor crystal structure from a different angle, i.e. in drug discovery (‘structure-based ligand discovery’).
Figure 1. a) textbook representation of drug-receptor interaction; b) crystal structure of adenosine A2A receptor: 7 vertical helical structures (chocolate) make up the receptor’s transmembrane domain, the bound ligand (ZM241385) is shown in red, disulfide bridges in yellow and fatty acid molecules in green; c) close-up of ligand binding pocket in A2A receptor: the ligand ZM241385 is accommodated by amino acids (one-letter notation) that are either in the transmembrane domain or in the extracellular domain, grey balls: explicit water molecules.