Molecular Bases of PDE4D Inhibition by Memory-Enhancing GEBR Library Compounds

Selected members of the large rolipram-related GEBR family of type 4 phosphodiesterase (PDE4) inhibitors have been shown to facilitate long-term potentiation and to improve memory functions without causing emetic-like behavior in rodents. Despite their micromolar-range binding affinities and their promising pharmacological and toxicological profiles, few if any structure-activity relationship studies have been performed to elucidate the molecular bases of their action. Here, we report the crystal structure of a number of GEBR library compounds in complex with the catalytic domain of PDE4D as well as their inhibitory profiles for both the long PDE4D3 isoform and the catalytic domain alone. Furthermore, we assessed the stability of the observed ligand conformations in the context of the intact enzyme using molecular dynamics simulations. The longer and more flexible ligands appear to be capable of forming contacts with the regulatory portion of the enzyme, thus possibly allowing some degree of selectivity between the different PDE4 isoforms.

Mycobacterium tuberculosis Low Molecular Weight Phosphatases (MPtpA and MPtpB): From Biological Insight to Inhibitors

Mycobacterium tuberculosis (Mtb), the main aetiological agent of tuberculosis (TB) in humans, is estimated to cause nearly two million deaths every year. Despite their huge therapeutic value, existing antitubercular drugs have several shortcomings, such as for instance the insurgence of drug resistance, which is mostly triggered by lack of compliance during the lengthy treatment. Novel and more effective drugs against Mtb acting on new molecular targets are therefore in demand in order to reduce treatment time and address the severe issue related to the progressive loss of antibiotic efficacy. Mtb encodes for two low molecular weight tyrosine specific phosphatases (MPtpA and MPtpB) that are crucially involved in Mtb pathogenesis. While MPtpA interferes with phagosome acidification blocking its maturation, MPtpB disrupts host signal transduction cascades, causing immune response subversion in the host. The important role played by both MPtpA and MPtpB in host-pathogen interaction makes them appealing targets for TB drug discovery. Here, we provide an exhaustive review of the current knowledge on MPtpA and MPtpB characterization and role in TB pathogenesis. In particular, special emphasis is placed on all class of inhibitors that have been developed and studied to date; their binding mode, design strategies, biological activities, main pharmacophore features as well as the efforts to overcome the poor druggability of their target are summarized in detail.

Crystal Structure of Human E-Cadherin-EC1EC2 in Complex with a Peptidomimetic Competitive Inhibitor of Cadherin Homophilic Interaction

Cadherins are transmembrane cell adhesion proteins whose aberrant expression often correlates with cancer development and proliferation. We report the crystal structure of an E-cadherin extracellular fragment in complex with a peptidomimetic compound that was previously shown to partially inhibit cadherin homophilic adhesion. The structure reveals an unexpected binding mode and allows the identification of a druggable cadherin interface, thus paving the way to a future structure-guided design of cell adhesion inhibitors against cadherin-expressing solid tumors.

The X-ray structure of human P-cadherin EC1-EC2 in a closed conformation provides insight into the type I cadherin dimerization pathway

Cadherins are a large family of calcium-dependent proteins that mediate cellular adherens junction formation and tissue morphogenesis. To date, the most studied cadherins are those classified as classical, which are further divided into type I or type II depending on selected sequence features. Unlike other members of the classical cadherin family, a detailed structural characterization of P-cadherin has not yet been fully obtained. Here, the high-resolution crystal structure determination of the closed form of human P-cadherin EC1-EC2 is reported. The structure shows a novel, monomeric packing arrangement that provides a further snapshot in the yet-to-be-achieved complete description of the highly dynamic cadherin dimerization pathway. Moreover, this is the first multidomain cadherin fragment to be crystallized and structurally characterized in its closed conformation that does not carry any extra N-terminal residues before the naturally occurring aspartic acid at position 1. Finally, two clear alternate conformations are observed for the critical Trp2 residue, suggestive of a transient, metastable state. The P-cadherin structure and packing arrangement shown here provide new and valuable information towards the complete structural characterization of the still largely elusive cadherin dimerization pathway.

Structural basis for benzothiazinone-mediated killing of Mycobacterium tuberculosis

The benzothiazinone BTZ043 is a tuberculosis drug candidate with nanomolar whole-cell activity. BTZ043 targets the DprE1 catalytic component of the essential enzyme decaprenylphosphoryl-β-D-ribofuranose-2'-epimerase, thus blocking biosynthesis of arabinans, vital components of mycobacterial cell walls. Crystal structures of DprE1, in its native form and in a complex with BTZ043, reveal formation of a semimercaptal adduct between the drug and an active-site cysteine, as well as contacts to a neighboring catalytic lysine residue. Kinetic studies confirm that BTZ043 is a mechanism-based, covalent inhibitor. This explains the exquisite potency of BTZ043, which, when fluorescently labeled, localizes DprE1 at the poles of growing bacteria. Menaquinone can reoxidize the flavin adenine dinucleotide cofactor in DprE1 and may be the natural electron acceptor for this reaction in the mycobacterium. Our structural and kinetic analysis provides both insight into a critical epimerization reaction and a platform for structure-based design of improved inhibitors.

Biological and structural characterization of the Mycobacterium smegmatis nitroreductase NfnB, and its role in benzothiazinone resistance

Tuberculosis is still a leading cause of death in developing countries, for which there is an urgent need for new pharmacological agents. The synthesis of the novel antimycobacterial drug class of benzothiazinones (BTZs) and the identification of their cellular target as DprE1 (Rv3790), a component of the decaprenylphosphoryl-β-d-ribose 2'-epimerase complex, have been reported recently. Here, we describe the identification and characterization of a novel resistance mechanism to BTZ in Mycobacterium smegmatis. The overexpression of the nitroreductase NfnB leads to the inactivation of the drug by reduction of a critical nitro-group to an amino-group. The direct involvement of NfnB in the inactivation of the lead compound BTZ043 was demonstrated by enzymology, microbiological assays and gene knockout experiments. We also report the crystal structure of NfnB in complex with the essential cofactor flavin mononucleotide, and show that a common amino acid stretch between NfnB and DprE1 is likely to be essential for the interaction with BTZ. We performed docking analysis of NfnB-BTZ in order to understand their interaction and the mechanism of nitroreduction. Although Mycobacterium tuberculosis seems to lack nitroreductases able to inactivate these drugs, our findings are valuable for the design of new BTZ molecules, which may be more effective in vivo.

Benzothiazinones kill Mycobacterium tuberculosis by blocking arabinan synthesis

New drugs are required to counter the tuberculosis (TB) pandemic. Here, we describe the synthesis and characterization of 1,3-benzothiazin-4-ones (BTZs), a new class of antimycobacterial agents that kill Mycobacterium tuberculosis in vitro, ex vivo, and in mouse models of TB. Using genetics and biochemistry, we identified the enzyme decaprenylphosphoryl-beta-d-ribose 2'-epimerase as a major BTZ target. Inhibition of this enzymatic activity abolishes the formation of decaprenylphosphoryl arabinose, a key precursor that is required for the synthesis of the cell-wall arabinans, thus provoking cell lysis and bacterial death. The most advanced compound, BTZ043, is a candidate for inclusion in combination therapies for both drug-sensitive and extensively drug-resistant TB.

Azole resistance in Mycobacterium tuberculosis is mediated by the MmpS5-MmpL5 efflux system

Tuberculosis (TB) remains the leading cause of mortality due to a bacterial pathogen, Mycobacterium tuberculosis. Moreover, the recent isolation of M. tuberculosis strains resistant to both first- and second-line antitubercular drugs (XDR-TB) threatens to make the treatment of this disease extremely difficult and becoming a threat to public health worldwide. Recently, it has been shown that azoles are potent inhibitors of mycobacterial cell growth and have antitubercular activity in mice, thus favoring the hypothesis that these drugs may constitute a novel strategy against tuberculosis disease. To investigate the mechanisms of resistance to azoles in mycobacteria, we isolated and characterized several spontaneous azoles resistant mutants from M. tuberculosis and Mycobacterium bovis BCG. All the analyzed resistant mutants exhibited both increased econazole efflux and increased transcription of mmpS5-mmpL5 genes, encoding a hypothetical efflux system belonging to the resistance-nodulation-division (RND) family of transporters. We found that the up-regulation of mmpS5-mmpL5 genes was linked to mutations either in the Rv0678 gene, hypothesized to be involved in the transcriptional regulation of this efflux system, or in its putative promoter/operator region.