Yeast-based assays for screening 11ß-HSD1 inhibitors.

BACKGROUND: Intracellular metabolism of glucocorticoid hormones plays an important role in the pathogenesis of metabolic syndrome and regulates, among many physiological processes, collagen metabolism in skin. At the peripheral level the concentration of active glucocorticoids is mainly regulated by the 11ß-hydroxysteroid dehydrogenase 1 (11ß-HSD1) enzyme, involved in the conversion of cortisone into the biologically active hormone cortisol. Cortisol interacts with the glucocorticoid receptor and regulates the expression of different classes of genes within the nucleus. Due to its implication in glucocorticoid metabolism, the inhibition of 11ß-HSD1 activity has become a dominant strategy for the treatment of metabolic syndrome. Moreover, inhibitors of this target enzyme can be used for development of formulations to counteract skin ageing. Here we present the construction of two yeast cell based assays that can be used for the screening of novel 11ß-HSD1 inhibitors. RESULTS: The yeast Saccharomyces cerevisiae is used as a host organism for the expression of human 11ß-HSD1 as well as a genetically encoded assay system that allows intracellular screening of molecules with 11ß-HSD1 inhibitory activity. As proof of concept the correlation between 11ß-HSD1 inhibition and fluorescent output signals was successfully tested with increasing concentrations of carbenoxolone and tanshinone IIA, two known 11ß-HSD1 inhibitors. The first assay detects a decrease in fluorescence upon 11ß-HSD1 inhibition, whereas the second assay relies on stabilization of yEGFP upon inhibition of 11ß-HSD1, resulting in a positive read-out and thus minimizing the rate of false positives sometimes associated with read-outs based on loss of signals. Specific inhibition of the ABC transporter Pdr5p improves the sensitivity of the assay strains to cortisone concentrations by up to 60 times. CONCLUSIONS: Our yeast assay strains provide a cost-efficient and easy to handle alternative to other currently available assays for the screening of 11ß-HSD1 inhibitors. These assays are designed for an initial fast screening of large numbers of compounds and enable the selection of cell permeable molecules with target inhibitory activity, before proceeding to more advanced selection processes. Moreover, they can be employed in yeast synthetic biology platforms to reconstitute heterologous biosynthetic pathways of drug-relevant scaffolds for simultaneous synthesis and screening of 11ß-HSD1 inhibitors at intracellular level.

Yeast artificial chromosomes employed for random assembly of biosynthetic pathways and production of diverse compounds in Saccharomyces cerevisiae.

BACKGROUND: Natural products are an important source of drugs and other commercially interesting compounds, however their isolation and production is often difficult. Metabolic engineering, mainly in bacteria and yeast, has sought to circumvent some of the associated problems but also this approach is impeded by technical limitations. Here we describe a novel strategy for production of diverse natural products, comprising the expression of an unprecedented large number of biosynthetic genes in a heterologous host. RESULTS: As an example, genes from different sources, representing enzymes of a seven step flavonoid pathway, were individually cloned into yeast expression cassettes, which were then randomly combined on Yeast Artificial Chromosomes and used, in a single transformation of yeast, to create a variety of flavonoid producing pathways. Randomly picked clones were analysed, and approximately half of them showed production of the flavanone naringenin, and a third of them produced the flavonol kaempferol in various amounts. This reflected the assembly of 5-7 step multi-species pathways converting the yeast metabolites phenylalanine and/or tyrosine into flavonoids, normally only produced by plants. Other flavonoids were also produced that were either direct intermediates or derivatives thereof. Feeding natural and unnatural, halogenated precursors to these recombinant clones demonstrated the potential to further diversify the type of molecules that can be produced with this technology. CONCLUSION: The technology has many potential uses but is particularly suited for generating high numbers of structurally diverse compounds, some of which may not be amenable to chemical synthesis, thus greatly facilitating access to a huge chemical space in the search for new commercially interesting compounds.

The spindle checkpoint kinase bub1 and cyclin e/cdk2 both contribute to the establishment of meiotic metaphase arrest by cytostatic factor.

In vertebrate unfertilized eggs, metaphase arrest in Meiosis II is mediated by an activity known as cytostatic factor (CSF). CSF arrest is dependent upon Mos-dependent activation of the MAPK/Rsk pathway, and Rsk activates the spindle checkpoint kinase Bub1, leading to inhibition of the anaphase-promoting complex (APC), an E3 ubiquitin ligase required for the metaphase/anaphase transition. However, it is not known whether Bub1 is required for the establishment of CSF arrest or whether other pathways also contribute. Here, we show that immunodepletion of Bub1 from egg extracts blocks the ability of Mos to establish CSF arrest, and arrest can be restored by the addition of wild-type, but not kinase-dead, Bub1. The appearance of CSF arrest at Meiosis II may result from coexpression of cyclin E/Cdk2 with the MAPK/Bub1 pathway. Cyclin E/Cdk2 was able to cause metaphase arrest in egg extracts even in the absence of Mos and could also inhibit cyclin B degradation in oocytes when expressed at anaphase of Meiosis I. Once it has been established, metaphase arrest can be maintained in the absence of MAPK, Bub1, or cyclin E/Cdk2 activity. Both pathways are independent of each other, but each appears to block activation of the APC, which is required for cyclin B degradation and the metaphase/anaphase transition.

Computer-aided design of a PDZ domain to recognize new target sequences.

PDZ domains are small globular domains that recognize the last 4-7 amino acids at the C-terminus of target proteins. The specificity of the PDZ-ligand recognition is due to side chain-side chain interactions, as well as the positioning of an alpha-helix involved in ligand binding. We have used computer-aided protein design to produce mutant versions of a Class I PDZ domain that bind to novel Class I and Class II target sequences both in vitro and in vivo, thus providing an alternative to primary antibodies in western blotting, affinity chromatography and pull-down experiments. Our results suggest that by combining different backbone templates with computer-aided protein design, PDZ domains could be engineered to specifically recognize a large number of proteins.

The mechanism of CSF arrest in vertebrate oocytes.

A cytoplasmic activity in mature oocytes responsible for second meiotic metaphase arrest was identified over 30 years ago in amphibian oocytes. In Xenopus oocytes cytostatic factor (CSF) activity is initiated by the progesterone-dependent synthesis of Mos, a MAPK kinase kinase that activates the MAPK pathway. CSF arrest is mediated by a sole MAPK target, the protein kinase p90(Rsk). Rsk phosphorylates and activates the Bub1 protein kinase, which may cause metaphase arrest due to inhibition of the anaphase-promoting complex (APC) by a conserved mechanism defined genetically in yeast and mammalian cells. CSF arrest in vertebrate oocytes by p90(Rsk) provides a link between the MAPK pathway and the spindle assembly checkpoint in the cell cycle.