Anna Czarna

Structures of low molecular weight inhibitors bound to MDMX and MDM2 reveal new approaches for p53-MDMX/MDM2 antagonist drug discovery

Intensive anticancer drug discovery efforts have been made to develop small molecule inhibitors of the p53-MDM2 and p53-MDMX interactions. We present here the structures of the most potent inhibitors bound to MDM2 and MDMX that are based on the new imidazo-indole scaffold. In addition, the structure of the recently reported spiro-oxindole inhibitor bound to MDM2 is described. The structures indicate how the substituents of a small molecule that bind to the three subpockets of the MDM2/X-p53 interaction should be optimized for effective binding to MDM2 and/or MDMX. While the spiro-oxindole inhibitor triggers significant ligand-induced changes in MDM2, the imidazo-indoles share similar binding modes for MDMX and MDM2, but cause only minimal induced-fit changes in the structures of both proteins. Our study includes the first structure of the complex between MDMX and a small molecule and should aid in developing efficient scaffolds for binding to MDMX and/or MDM2.

Structure of the human Mdmx protein bound to the p53 tumor suppressor transactivation domain

The Mdmx oncoprotein has only recently emerged as a critical - independent to Mdm2 - regulator of p53 activation. We have determined the crystal structure of the N-terminal domain of human Mdmx bound to a 15-residue transactivation domain peptide of human p53. The structure shows why antagonists of the Mdm2 binding to p53 are ineffective in the Mdmx-p53 interaction.

Molecular basis for the inhibition of p53 by Mdmx.

The oncoprotein Mdm2, and the recently intensely studied, homologues protein Mdmx, are principal negative regulators of the p53 tumor uppressor. The mechanisms by which they regulate the stability and activity of p53 are not fully established. We have determined the crystal structure of the N-terminal domain of Mdmx bound to a 15-residue p53 peptide. The structure reveals that although the principle features of the Mdm2-p53 interaction are preserved in the Mdmx-p53 complex, the Mdmx hydrophobic cleft on which the p53 peptide binds is significantly altered: a part of the cleft is blocked by sidechains of Met and Tyr of the p53-binding pocket of Mdmx. Thus specific inhibitors of Mdm2-p53 would not be optimal for binding to Mdmx. Our binding assays show indeed that nutlins, the newly discovered, potent antagonists of the Mdm2-p53 interaction, are notcapable to efficiently disrupt the Mdmx-p53 interaction. To achieve full activation of p53 in tumor cells, compounds that are specific for Mdmx are necessary to complement the Mdm2 specific binders.

Robust Generation of Lead Compounds for Protein–Protein Interactions by Computational and MCR Chemistry: p53/Hdm2 Antagonists†

The discovery of a lead compound is an essential process in early drug discovery, hopefully eventually resulting into a clinical candidate and a drug for the treatment of a disease. Besides affinity and selectivity for the target, however, other target unrelated compound properties are equally important for the fate of drug candidate, e.g. water solubility, lipophilicity and molecular weight since they determine important aspects such as oral bioavailability, dosing schedule and side effects. The parallel discovery and early development of several leads is therefore now pursued whenever possible, an approach that takes into account the high attrition rate of early drug discovery projects. Currently, hits as starting points for medicinal chemistry optimisations are mostly found by high-throughput screening (HTS) campaigns and to a much less extent by structure-based approaches including fragment-based and computational drug discovery. For certain target classes, however, HTS often yields very low numbers of hits.[1] For example, protein-protein interactions (PPIs) are notoriously difficult to hit with drug-like small molecules.[2] This has been assigned to the unusual structure, topology and flexibility of protein-protein interfaces.[3] The recent advancement of several drugs into clinical development clearly shows that certain PPIs, e.g., Bcl-x and XIAP, can be efficiently targeted by small molecules.[4] Here, we describe a complementary process that led to the parallel discovery of several compounds belonging to seven different scaffold classes, amenable to synthesis by efficient multicomponent reaction (MCR) chemistry in just one step, that antagonize the PPI between the transcription factor p53 and its negative regulator Hdm2.

Structures of Drosophila Cryptochrome and Mouse Cryptochrome1 Provide Insight into Circadian Function

Drosophila cryptochrome (dCRY) is a FAD-dependent circadian photoreceptor, whereas mammalian cryptochromes (CRY1/2) are integral clock components that repress mCLOCK/mBMAL1-dependent transcription. We report crystal structures of full-length dCRY, a dCRY loop deletion construct, and the photolyase homology region of mouse CRY1 (mCRY1). Our dCRY structures depict Phe534 of the regulatory tail in the same location as the photolesion in DNA-repairing photolyases and reveal that the sulfur loop and tail residue Cys523 plays key roles in the dCRY photoreaction. Our mCRY1 structure visualizes previously characterized mutations, an NLS, and MAPK and AMPK phosphorylation sites. We show that the FAD and antenna chromophore-binding regions, a predicted coiled-coil helix, the C-terminal lid, and charged surfaces are involved in FAD-independent mPER2 and FBXL3 binding and mCLOCK/mBMAL1 transcriptional repression. The structure of a mammalian cryptochrome1 protein may catalyze the development of CRY chemical probes and the design of therapeutic metabolic modulators.

High affinity interaction of the p53 peptide-analogue with human Mdm2 and Mdmx

The Mdm2 and Mdmx proteins are the principal negative regulators of the p53 tumor suppressor. Reactivation of p53 activity by disrupting the Mdm2/Mdmx-p53 interactions offers new possibilities for anticancer therapeutics. Here, we present crystal structures of two complexes, a p53-like mutant peptide with the N-terminal domains of Mdm2 and Mdmx, respectively. The structures reveal that the p53 mutant peptide (amino acid sequence: LTFEHYWAQLTS) assumes virtually identical conformations in both complexes despite the different shapes of the p53-binding pockets in these two proteins, has a more extended helical nature compared to the Mdm2-bound wild-type p53 peptide, and does not disturb the native folds of Mdm2 or Mdmx. The extension of the helical structure in the mutant p53 peptide greatly improves its binding to Mdm2 and Mdmx. The fluorescence polarization assay that we have developed using this peptide indicates the affinities towards Mdm2 of 3.6 nM and for Mdmx of 6.1 nM, compared to the low micromolar binding of a similar length wild-type p53 peptide to Mdm2/Mdmx. Our assay does not require expensive non-native amino acids, and allows measurements of the interaction with both Mdm2 and Mdmx in identical conditions - without modification of experimental conditions or setups between the two proteins. The structural information presented here, coupled with the robust fluorescence polarization assay, should enable development of a simple pharmacophore model of cross-selective Mdm2-Mdmx/p53 inhibitors.

Isoquinolin‐1‐one Inhibitors of the MDM2–p53 Interaction

p53 has been at the centre of attention for drug design since the discovery of its growth‐suppressive and pro‐apoptotic activity. Herein we report the design and characterisation of a new class of isoquinolinone inhibitors of the MDM2–p53 interaction. Our identification of druglike and selective inhibitors of this protein–protein interaction included a straightforward in silico compound‐selection process, a recently reported NMR spectroscopic approach for studying the MDM2–p53 interaction, and selectivity screening assays using cells with the same genetic background. The selected inhibitors were all able to induce apoptosis and the expression of p53‐related genes, but only the isoquinolin‐1‐one‐based inhibitors stabilised p53. Our NMR experiments give a persuading explanation for these results, showing that isoquinolin‐1‐one derivates are able to dissociate the preformed MDM2–p53 complex in vitro, releasing a folded and soluble p53. The joint application of these methods provides a framework for the discovery of protein interaction inhibitors as a promising starting point for further drug design.

Quantitative analyses of cryptochrome - mBMAL1 interactions: mechanistic insights into the transcriptional regulation of the mammalian circadian clock

The mammalian cryptochromes mCRY1 and mCRY2 act as transcriptional repressors within the 24-h transcription-translational feedback loop of the circadian clock. The C-terminal tail and a preceding predicted coiled coil (CC) of the mCRYs as well as the C-terminal region of the transcription factor mBMAL1 are involved in transcriptional feedback repression. Here we show by fluorescence polarization and isothermal titration calorimetry that purified mCRY1/2CCtail proteins form stable heterodimeric complexes with two C-terminal mBMAL1 fragments. The longer mBMAL1 fragment (BMAL490) includes Lys-537, which is rhythmically acetylated by mCLOCK in vivo. mCRY1 (but not mCRY2) has a lower affinity to BMAL490 than to the shorter mBMAL1 fragment (BMAL577) and a K537Q mutant version of BMAL490. Using peptide scan analysis we identify two mBMAL1 binding epitopes within the coiled coil and tail regions of mCRY1/2 and document the importance of positively charged mCRY1 residues for mBMAL1 binding. A synthetic mCRY coiled coil peptide binds equally well to the short and to the long (wild-type and K537Q mutant) mBMAL1 fragments. In contrast, a peptide including the mCRY1 tail epitope shows a lower affinity to BMAL490 compared with BMAL577 and BMAL490(K537Q). We propose that Lys-537mBMAL1 acetylation enhances mCRY1 binding by affecting electrostatic interactions predominantly with the mCRY1 tail. Our data reveal different molecular interactions of the mCRY1/2 tails with mBMAL1, which may contribute to the non-redundant clock functions of mCRY1 and mCRY2. Moreover, our study suggests the design of peptidic inhibitors targeting the interaction of the mCRY1 tail with mBMAL1.

Rapid and efficient hydrophilicity tuning of p53/mdm2 antagonists.

The protein-protein interaction of p53 and mdm2 is an important anticancer target. The interface, however, is very hydrophobic and naturally results in very hydrophobic antagonists. We used the Orru three component reaction (O-3CR) along with a rapid and efficient, recently discovered amidation reaction to dramatically improve the water solubility of our recently discovered low molecular weight p53/mdm2 antagonists. Arrays of amides were synthesized with improved hydrophilicity and retainment and/or improvement of p53/mdm2 inhibitory activity.

Enzymatic Activity of the Staphylococcus aureus SplB Serine Protease is Induced by Substrates Containing the Sequence Trp-Glu-Leu-Gln

Proteases are of significant importance for the virulence of Staphylococcus aureus. Nevertheless, their subset, the serine protease-like proteins, remains poorly characterized. Here presented is an investigation of SplB protease catalytic activity revealing that the enzyme possesses exquisite specificity and only cleaves efficiently after the sequence Trp-Glu-Leu-Gln. To understand the molecular basis for such selectivity, we solved the three-dimensional structure of SplB to 1.8 A. Modeling of substrate binding to the protease demonstrated that selectivity relies in part on a canonical specificity pockets-based mechanism. Significantly, the conformation of residues that ordinarily form the oxyanion hole, an essential structural element of the catalytic machinery of serine proteases, is not canonical in the SplB structure. We postulate that within SplB, the oxyanion hole is only formed upon docking of a substrate containing the consensus sequence motif. It is suggested that this unusual activation mechanism is used in parallel with classical determinants to further limit enzyme specificity. Finally, to guide future development, we attempt to point at likely physiological substrates and thus the role of SplB in staphylococcal physiology.